Internet DRAFT - draft-ietf-tcpm-rfc793bis
draft-ietf-tcpm-rfc793bis
Internet Engineering Task Force W. Eddy, Ed.
Internet-Draft MTI Systems
Obsoletes: 793, 879, 2873, 6093, 6429, 6528, 7 March 2022
6691 (if approved)
Updates: 5961, 1011, 1122 (if approved)
Intended status: Standards Track
Expires: 8 September 2022
Transmission Control Protocol (TCP) Specification
draft-ietf-tcpm-rfc793bis-28
Abstract
This document specifies the Transmission Control Protocol (TCP). TCP
is an important transport layer protocol in the Internet protocol
stack, and has continuously evolved over decades of use and growth of
the Internet. Over this time, a number of changes have been made to
TCP as it was specified in RFC 793, though these have only been
documented in a piecemeal fashion. This document collects and brings
those changes together with the protocol specification from RFC 793.
This document obsoletes RFC 793, as well as RFCs 879, 2873, 6093,
6429, 6528, and 6691 that updated parts of RFC 793. It updates RFCs
1011 and 1122, and should be considered as a replacement for the
portions of those document dealing with TCP requirements. It also
updates RFC 5961 by adding a small clarification in reset handling
while in the SYN-RECEIVED state. The TCP header control bits from
RFC 793 have also been updated based on RFC 3168.
RFC EDITOR NOTE: If approved for publication as an RFC, this should
be marked additionally as "STD: 7" and replace RFC 793 in that role.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2022.
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than English.
Table of Contents
1. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2.2. Key TCP Concepts . . . . . . . . . . . . . . . . . . . . 6
3. Functional Specification . . . . . . . . . . . . . . . . . . 6
3.1. Header Format . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Specific Option Definitions . . . . . . . . . . . . . . . 12
3.2.1. Other Common Options . . . . . . . . . . . . . . . . 13
3.2.2. Experimental TCP Options . . . . . . . . . . . . . . 13
3.3. TCP Terminology Overview . . . . . . . . . . . . . . . . 13
3.3.1. Key Connection State Variables . . . . . . . . . . . 13
3.3.2. State Machine Overview . . . . . . . . . . . . . . . 15
3.4. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 18
3.4.1. Initial Sequence Number Selection . . . . . . . . . . 21
3.4.2. Knowing When to Keep Quiet . . . . . . . . . . . . . 23
3.4.3. The TCP Quiet Time Concept . . . . . . . . . . . . . 23
3.5. Establishing a connection . . . . . . . . . . . . . . . . 25
3.5.1. Half-Open Connections and Other Anomalies . . . . . . 28
3.5.2. Reset Generation . . . . . . . . . . . . . . . . . . 31
3.5.3. Reset Processing . . . . . . . . . . . . . . . . . . 32
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3.6. Closing a Connection . . . . . . . . . . . . . . . . . . 32
3.6.1. Half-Closed Connections . . . . . . . . . . . . . . . 35
3.7. Segmentation . . . . . . . . . . . . . . . . . . . . . . 35
3.7.1. Maximum Segment Size Option . . . . . . . . . . . . . 37
3.7.2. Path MTU Discovery . . . . . . . . . . . . . . . . . 38
3.7.3. Interfaces with Variable MTU Values . . . . . . . . . 39
3.7.4. Nagle Algorithm . . . . . . . . . . . . . . . . . . . 39
3.7.5. IPv6 Jumbograms . . . . . . . . . . . . . . . . . . . 40
3.8. Data Communication . . . . . . . . . . . . . . . . . . . 40
3.8.1. Retransmission Timeout . . . . . . . . . . . . . . . 41
3.8.2. TCP Congestion Control . . . . . . . . . . . . . . . 41
3.8.3. TCP Connection Failures . . . . . . . . . . . . . . . 42
3.8.4. TCP Keep-Alives . . . . . . . . . . . . . . . . . . . 43
3.8.5. The Communication of Urgent Information . . . . . . . 44
3.8.6. Managing the Window . . . . . . . . . . . . . . . . . 45
3.9. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 50
3.9.1. User/TCP Interface . . . . . . . . . . . . . . . . . 50
3.9.2. TCP/Lower-Level Interface . . . . . . . . . . . . . . 59
3.10. Event Processing . . . . . . . . . . . . . . . . . . . . 61
3.10.1. OPEN Call . . . . . . . . . . . . . . . . . . . . . 63
3.10.2. SEND Call . . . . . . . . . . . . . . . . . . . . . 64
3.10.3. RECEIVE Call . . . . . . . . . . . . . . . . . . . . 65
3.10.4. CLOSE Call . . . . . . . . . . . . . . . . . . . . . 67
3.10.5. ABORT Call . . . . . . . . . . . . . . . . . . . . . 68
3.10.6. STATUS Call . . . . . . . . . . . . . . . . . . . . 69
3.10.7. SEGMENT ARRIVES . . . . . . . . . . . . . . . . . . 70
3.10.8. Timeouts . . . . . . . . . . . . . . . . . . . . . . 84
4. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5. Changes from RFC 793 . . . . . . . . . . . . . . . . . . . . 89
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 96
7. Security and Privacy Considerations . . . . . . . . . . . . . 97
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 99
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 100
9.1. Normative References . . . . . . . . . . . . . . . . . . 100
9.2. Informative References . . . . . . . . . . . . . . . . . 102
Appendix A. Other Implementation Notes . . . . . . . . . . . . . 107
A.1. IP Security Compartment and Precedence . . . . . . . . . 108
A.1.1. Precedence . . . . . . . . . . . . . . . . . . . . . 108
A.1.2. MLS Systems . . . . . . . . . . . . . . . . . . . . . 109
A.2. Sequence Number Validation . . . . . . . . . . . . . . . 109
A.3. Nagle Modification . . . . . . . . . . . . . . . . . . . 109
A.4. Low Watermark Settings . . . . . . . . . . . . . . . . . 110
Appendix B. TCP Requirement Summary . . . . . . . . . . . . . . 110
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 114
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1. Purpose and Scope
In 1981, RFC 793 [16] was released, documenting the Transmission
Control Protocol (TCP), and replacing earlier specifications for TCP
that had been published in the past.
Since then, TCP has been widely implemented, and has been used as a
transport protocol for numerous applications on the Internet.
For several decades, RFC 793 plus a number of other documents have
combined to serve as the core specification for TCP [50]. Over time,
a number of errata have been filed against RFC 793. There have also
been deficiencies found and resolved in security, performance, and
many other aspects. The number of enhancements has grown over time
across many separate documents. These were never accumulated
together into a comprehensive update to the base specification.
The purpose of this document is to bring together all of the IETF
Standards Track changes and other clarifications that have been made
to the base TCP functional specification and unify them into an
updated version of RFC 793.
Some companion documents are referenced for important algorithms that
are used by TCP (e.g. for congestion control), but have not been
completely included in this document. This is a conscious choice, as
this base specification can be used with multiple additional
algorithms that are developed and incorporated separately. This
document focuses on the common basis all TCP implementations must
support in order to interoperate. Since some additional TCP features
have become quite complicated themselves (e.g. advanced loss recovery
and congestion control), future companion documents may attempt to
similarly bring these together.
In addition to the protocol specification that describes the TCP
segment format, generation, and processing rules that are to be
implemented in code, RFC 793 and other updates also contain
informative and descriptive text for readers to understand aspects of
the protocol design and operation. This document does not attempt to
alter or update this informative text, and is focused only on
updating the normative protocol specification. This document
preserves references to the documentation containing the important
explanations and rationale, where appropriate.
This document is intended to be useful both in checking existing TCP
implementations for conformance purposes, as well as in writing new
implementations.
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2. Introduction
RFC 793 contains a discussion of the TCP design goals and provides
examples of its operation, including examples of connection
establishment, connection termination, and packet retransmission to
repair losses.
This document describes the basic functionality expected in modern
TCP implementations, and replaces the protocol specification in RFC
793. It does not replicate or attempt to update the introduction and
philosophy content in Sections 1 and 2 of RFC 793. Other documents
are referenced to provide explanation of the theory of operation,
rationale, and detailed discussion of design decisions. This
document only focuses on the normative behavior of the protocol.
The "TCP Roadmap" [50] provides a more extensive guide to the RFCs
that define TCP and describe various important algorithms. The TCP
Roadmap contains sections on strongly encouraged enhancements that
improve performance and other aspects of TCP beyond the basic
operation specified in this document. As one example, implementing
congestion control (e.g. [8]) is a TCP requirement, but is a complex
topic on its own, and not described in detail in this document, as
there are many options and possibilities that do not impact basic
interoperability. Similarly, most TCP implementations today include
the high-performance extensions in [48], but these are not strictly
required or discussed in this document. Multipath considerations for
TCP are also specified separately in [59].
A list of changes from RFC 793 is contained in Section 5.
2.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [3][12] when, and only when, they appear in all capitals, as shown
here.
Each use of RFC 2119 keywords in the document is individually labeled
and referenced in Appendix B that summarizes implementation
requirements.
Sentences using "MUST" are labeled as "MUST-X" with X being a numeric
identifier enabling the requirement to be located easily when
referenced from Appendix B.
Similarly, sentences using "SHOULD" are labeled with "SHLD-X", "MAY"
with "MAY-X", and "RECOMMENDED" with "REC-X".
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For the purposes of this labeling, "SHOULD NOT" and "MUST NOT" are
labeled the same as "SHOULD" and "MUST" instances.
2.2. Key TCP Concepts
TCP provides a reliable, in-order, byte-stream service to
applications.
The application byte-stream is conveyed over the network via TCP
segments, with each TCP segment sent as an Internet Protocol (IP)
datagram.
TCP reliability consists of detecting packet losses (via sequence
numbers) and errors (via per-segment checksums), as well as
correction via retransmission.
TCP supports unicast delivery of data. Anycast applications exist
that successfully use TCP without modifications, though there is some
risk of instability due to changes of lower-layer forwarding behavior
[47].
TCP is connection-oriented, though does not inherently include a
liveness detection capability.
Data flow is supported bidirectionally over TCP connections, though
applications are free to send data only unidirectionally, if they so
choose.
TCP uses port numbers to identify application services and to
multiplex distinct flows between hosts.
A more detailed description of TCP features compared to other
transport protocols can be found in Section 3.1 of [53]. Further
description of the motivations for developing TCP and its role in the
Internet protocol stack can be found in Section 2 of [16] and earlier
versions of the TCP specification.
3. Functional Specification
3.1. Header Format
TCP segments are sent as internet datagrams. The Internet Protocol
(IP) header carries several information fields, including the source
and destination host addresses [1] [13]. A TCP header follows the IP
headers, supplying information specific to the TCP protocol. This
division allows for the existence of host level protocols other than
TCP. In early development of the Internet suite of protocols, the IP
header fields had been a part of TCP.
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This document describes the TCP protocol. The TCP protocol uses TCP
Headers.
A TCP Header, followed by any user data in the segment, is formatted
as follows, using the style from [67]:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Acknowledgment Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data | |C|E|U|A|P|R|S|F| |
| Offset| Rsrvd |W|C|R|C|S|S|Y|I| Window |
| | |R|E|G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| [Options] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| :
: Data :
: |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Note that one tick mark represents one bit position.
Figure 1: TCP Header Format
where:
Source Port: 16 bits.
The source port number.
Destination Port: 16 bits.
The destination port number.
Sequence Number: 32 bits.
The sequence number of the first data octet in this segment (except
when the SYN flag is set). If SYN is set the sequence number is
the initial sequence number (ISN) and the first data octet is
ISN+1.
Acknowledgment Number: 32 bits.
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If the ACK control bit is set, this field contains the value of the
next sequence number the sender of the segment is expecting to
receive. Once a connection is established, this is always sent.
Data Offset (DOffset): 4 bits.
The number of 32 bit words in the TCP Header. This indicates where
the data begins. The TCP header (even one including options) is an
integer multiple of 32 bits long.
Reserved (Rsrvd): 4 bits.
A set of control bits reserved for future use. Must be zero in
generated segments and must be ignored in received segments, if
corresponding future features are unimplemented by the sending or
receiving host.
The control bits are also known as "flags". Assignment is managed
by IANA from the "TCP Header Flags" registry [63]. The currently
assigned control bits are CWR, ECE, URG, ACK, PSH, RST, SYN, and
FIN.
CWR: 1 bit.
Congestion Window Reduced (see [6]).
ECE: 1 bit.
ECN-Echo (see [6]).
URG: 1 bit.
Urgent Pointer field is significant.
ACK: 1 bit.
Acknowledgment field is significant.
PSH: 1 bit.
Push Function (see the Send Call description in Section 3.9.1).
RST: 1 bit.
Reset the connection.
SYN: 1 bit.
Synchronize sequence numbers.
FIN: 1 bit.
No more data from sender.
Window: 16 bits.
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The number of data octets beginning with the one indicated in the
acknowledgment field that the sender of this segment is willing to
accept. The value is shifted when the Window Scaling extension is
used [48].
The window size MUST be treated as an unsigned number, or else
large window sizes will appear like negative windows and TCP will
not work (MUST-1). It is RECOMMENDED that implementations will
reserve 32-bit fields for the send and receive window sizes in the
connection record and do all window computations with 32 bits (REC-
1).
Checksum: 16 bits.
The checksum field is the 16 bit ones' complement of the ones'
complement sum of all 16 bit words in the header and text. The
checksum computation needs to ensure the 16-bit alignment of the
data being summed. If a segment contains an odd number of header
and text octets, alignment can be achieved by padding the last
octet with zeros on its right to form a 16 bit word for checksum
purposes. The pad is not transmitted as part of the segment.
While computing the checksum, the checksum field itself is replaced
with zeros.
The checksum also covers a pseudo header (Figure 2) conceptually
prefixed to the TCP header. The pseudo header is 96 bits for IPv4
and 320 bits for IPv6. Including the pseudo header in the checksum
gives the TCP connection protection against misrouted segments.
This information is carried in IP headers and is transferred across
the TCP/Network interface in the arguments or results of calls by
the TCP implementation on the IP layer.
+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | PTCL | TCP Length |
+--------+--------+--------+--------+
Figure 2: IPv4 Pseudo Header
Pseudo header components for IPv4:
Source Address: the IPv4 source address in network byte order
Destination Address: the IPv4 destination address in network
byte order
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zero: bits set to zero
PTCL: the protocol number from the IP header
TCP Length: the TCP header length plus the data length in
octets (this is not an explicitly transmitted quantity, but is
computed), and it does not count the 12 octets of the pseudo
header.
For IPv6, the pseudo header is defined in Section 8.1 of RFC 8200
[13], and contains the IPv6 Source Address and Destination
Address, an Upper Layer Packet Length (a 32-bit value otherwise
equivalent to TCP Length in the IPv4 pseudo header), three bytes
of zero-padding, and a Next Header value (differing from the IPv6
header value in the case of extension headers present in between
IPv6 and TCP).
The TCP checksum is never optional. The sender MUST generate it
(MUST-2) and the receiver MUST check it (MUST-3).
Urgent Pointer: 16 bits.
This field communicates the current value of the urgent pointer as
a positive offset from the sequence number in this segment. The
urgent pointer points to the sequence number of the octet following
the urgent data. This field is only to be interpreted in segments
with the URG control bit set.
Options: [TCP Option]; size(Options) == (DOffset-5)*32; present
only when DOffset > 5. Note that this size expression also
includes any padding trailing the actual options present.
Options may occupy space at the end of the TCP header and are a
multiple of 8 bits in length. All options are included in the
checksum. An option may begin on any octet boundary. There are
two cases for the format of an option:
Case 1: A single octet of option-kind.
Case 2: An octet of option-kind (Kind), an octet of option-
length, and the actual option-data octets.
The option-length counts the two octets of option-kind and option-
length as well as the option-data octets.
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Note that the list of options may be shorter than the data offset
field might imply. The content of the header beyond the End-of-
Option option MUST be header padding of zeros (MUST-69).
The list of all currently defined options is managed by IANA [62],
and each option is defined in other RFCs, as indicated there. That
set includes experimental options that can be extended to support
multiple concurrent usages [46].
A given TCP implementation can support any currently defined
options, but the following options MUST be supported (MUST-4 - note
Maximum Segment Size option support is also part of MUST-19 in
Section 3.7.2):
Kind Length Meaning
---- ------ -------
0 - End of option list.
1 - No-Operation.
2 4 Maximum Segment Size.
These options are specified in detail in Section 3.2.
A TCP implementation MUST be able to receive a TCP option in any
segment (MUST-5).
A TCP implementation MUST (MUST-6) ignore without error any TCP
option it does not implement, assuming that the option has a length
field. All TCP options except End of option list and No-Operation
MUST have length fields, including all future options (MUST-68).
TCP implementations MUST be prepared to handle an illegal option
length (e.g., zero); a suggested procedure is to reset the
connection and log the error cause (MUST-7).
Note: There is ongoing work to extend the space available for TCP
options, such as [66].
Data: variable length.
User data carried by the TCP segment.
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3.2. Specific Option Definitions
A TCP Option, in the mandatory option set, is one of: an End of
Option List Option, a No-Operation Option, or a Maximum Segment Size
Option.
An End of Option List Option is formatted as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 0 |
+-+-+-+-+-+-+-+-+
where:
Kind: 1 byte; Kind == 0.
This option code indicates the end of the option list. This might
not coincide with the end of the TCP header according to the Data
Offset field. This is used at the end of all options, not the end
of each option, and need only be used if the end of the options
would not otherwise coincide with the end of the TCP header.
A No-Operation Option is formatted as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| 1 |
+-+-+-+-+-+-+-+-+
where:
Kind: 1 byte; Kind == 1.
This option code can be used between options, for example, to align
the beginning of a subsequent option on a word boundary. There is
no guarantee that senders will use this option, so receivers MUST
be prepared to process options even if they do not begin on a word
boundary (MUST-64).
A Maximum Segment Size Option is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Length | Maximum Segment Size (MSS) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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where:
Kind: 1 byte; Kind == 2.
If this option is present, then it communicates the maximum receive
segment size at the TCP endpoint that sends this segment. This
value is limited by the IP reassembly limit. This field may be
sent in the initial connection request (i.e., in segments with the
SYN control bit set) and MUST NOT be sent in other segments (MUST-
65). If this option is not used, any segment size is allowed. A
more complete description of this option is provided in
Section 3.7.1.
Length: 1 byte; Length == 4.
Length of the option in bytes.
Maximum Segment Size (MSS): 2 bytes.
The maximum receive segment size at the TCP endpoint that sends
this segment.
3.2.1. Other Common Options
Additional RFCs define some other commonly used options that are
recommended to implement for high performance, but not necessary for
basic TCP interoperability. These are the TCP Selective
Acknowledgement (SACK) option [23][27], TCP Timestamp (TS) option
[48], and TCP Window Scaling (WS) option [48].
3.2.2. Experimental TCP Options
Experimental TCP option values are defined in [31], and [46]
describes the current recommended usage for these experimental
values.
3.3. TCP Terminology Overview
This section includes an overview of key terms needed to understand
the detailed protocol operation in the rest of the document. There
is a glossary of terms in Section 4.
3.3.1. Key Connection State Variables
Before we can discuss very much about the operation of the TCP
implementation we need to introduce some detailed terminology. The
maintenance of a TCP connection requires maintaining state for
several variables. We conceive of these variables being stored in a
connection record called a Transmission Control Block or TCB. Among
the variables stored in the TCB are the local and remote IP addresses
and port numbers, the IP security level and compartment of the
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connection (see Appendix A.1), pointers to the user's send and
receive buffers, pointers to the retransmit queue and to the current
segment. In addition, several variables relating to the send and
receive sequence numbers are stored in the TCB.
Send Sequence Variables:
SND.UNA - send unacknowledged
SND.NXT - send next
SND.WND - send window
SND.UP - send urgent pointer
SND.WL1 - segment sequence number used for last window update
SND.WL2 - segment acknowledgment number used for last window
update
ISS - initial send sequence number
Receive Sequence Variables:
RCV.NXT - receive next
RCV.WND - receive window
RCV.UP - receive urgent pointer
IRS - initial receive sequence number
The following diagrams may help to relate some of these variables to
the sequence space.
1 2 3 4
----------|----------|----------|----------
SND.UNA SND.NXT SND.UNA
+SND.WND
1 - old sequence numbers that have been acknowledged
2 - sequence numbers of unacknowledged data
3 - sequence numbers allowed for new data transmission
4 - future sequence numbers that are not yet allowed
Figure 3: Send Sequence Space
The send window is the portion of the sequence space labeled 3 in
Figure 3.
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1 2 3
----------|----------|----------
RCV.NXT RCV.NXT
+RCV.WND
1 - old sequence numbers that have been acknowledged
2 - sequence numbers allowed for new reception
3 - future sequence numbers that are not yet allowed
Figure 4: Receive Sequence Space
The receive window is the portion of the sequence space labeled 2 in
Figure 4.
There are also some variables used frequently in the discussion that
take their values from the fields of the current segment.
Current Segment Variables:
SEG.SEQ - segment sequence number
SEG.ACK - segment acknowledgment number
SEG.LEN - segment length
SEG.WND - segment window
SEG.UP - segment urgent pointer
3.3.2. State Machine Overview
A connection progresses through a series of states during its
lifetime. The states are: LISTEN, SYN-SENT, SYN-RECEIVED,
ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK,
TIME-WAIT, and the fictional state CLOSED. CLOSED is fictional
because it represents the state when there is no TCB, and therefore,
no connection. Briefly the meanings of the states are:
LISTEN - represents waiting for a connection request from any
remote TCP peer and port.
SYN-SENT - represents waiting for a matching connection request
after having sent a connection request.
SYN-RECEIVED - represents waiting for a confirming connection
request acknowledgment after having both received and sent a
connection request.
ESTABLISHED - represents an open connection, data received can be
delivered to the user. The normal state for the data transfer
phase of the connection.
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FIN-WAIT-1 - represents waiting for a connection termination
request from the remote TCP peer, or an acknowledgment of the
connection termination request previously sent.
FIN-WAIT-2 - represents waiting for a connection termination
request from the remote TCP peer.
CLOSE-WAIT - represents waiting for a connection termination
request from the local user.
CLOSING - represents waiting for a connection termination request
acknowledgment from the remote TCP peer.
LAST-ACK - represents waiting for an acknowledgment of the
connection termination request previously sent to the remote TCP
peer (this termination request sent to the remote TCP peer already
included an acknowledgment of the termination request sent from
the remote TCP peer).
TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP peer received the acknowledgment of its connection
termination request, and to avoid new connections being impacted
by delayed segments from previous connections.
CLOSED - represents no connection state at all.
A TCP connection progresses from one state to another in response to
events. The events are the user calls, OPEN, SEND, RECEIVE, CLOSE,
ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts.
The OPEN call specifies whether connection establishment is to be
actively pursued, or to be passively waited for.
A passive OPEN request means that the process wants to accept
incoming connection requests, in contrast to an active OPEN
attempting to initiate a connection.
The state diagram in Figure 5 illustrates only state changes,
together with the causing events and resulting actions, but addresses
neither error conditions nor actions that are not connected with
state changes. In a later section, more detail is offered with
respect to the reaction of the TCP implementation to events. Some
state names are abbreviated or hyphenated differently in the diagram
from how they appear elsewhere in the document.
NOTA BENE: This diagram is only a summary and must not be taken as
the total specification. Many details are not included.
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+---------+ ---------\ active OPEN
| CLOSED | \ -----------
+---------+<---------\ \ create TCB
| ^ \ \ snd SYN
passive OPEN | | CLOSE \ \
------------ | | ---------- \ \
create TCB | | delete TCB \ \
V | \ \
rcv RST (note 1) +---------+ CLOSE | \
-------------------->| LISTEN | ---------- | |
/ +---------+ delete TCB | |
/ rcv SYN | | SEND | |
/ ----------- | | ------- | V
+--------+ snd SYN,ACK / \ snd SYN +--------+
| |<----------------- ------------------>| |
| SYN | rcv SYN | SYN |
| RCVD |<-----------------------------------------------| SENT |
| | snd SYN,ACK | |
| |------------------ -------------------| |
+--------+ rcv ACK of SYN \ / rcv SYN,ACK +--------+
| -------------- | | -----------
| x | | snd ACK
| V V
| CLOSE +---------+
| ------- | ESTAB |
| snd FIN +---------+
| CLOSE | | rcv FIN
V ------- | | -------
+---------+ snd FIN / \ snd ACK +---------+
| FIN |<---------------- ------------------>| CLOSE |
| WAIT-1 |------------------ | WAIT |
+---------+ rcv FIN \ +---------+
| rcv ACK of FIN ------- | CLOSE |
| -------------- snd ACK | ------- |
V x V snd FIN V
+---------+ +---------+ +---------+
|FINWAIT-2| | CLOSING | | LAST-ACK|
+---------+ +---------+ +---------+
| rcv ACK of FIN | rcv ACK of FIN |
| rcv FIN -------------- | Timeout=2MSL -------------- |
| ------- x V ------------ x V
\ snd ACK +---------+delete TCB +---------+
-------------------->|TIME-WAIT|------------------->| CLOSED |
+---------+ +---------+
Figure 5: TCP Connection State Diagram
The following notes apply to Figure 5:
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Note 1: The transition from SYN-RECEIVED to LISTEN on receiving a
RST is conditional on having reached SYN-RECEIVED after a passive
open.
Note 2: The figure omits a transition from FIN-WAIT-1 to TIME-WAIT
if a FIN is received and the local FIN is also acknowledged.
Note 3: A RST can be sent from any state with a corresponding
transition to TIME-WAIT (see [71] for rationale). These
transitions are not explicitly shown, otherwise the diagram would
become very difficult to read. Similarly, receipt of a RST from
any state results in a transition to LISTEN or CLOSED, though this
is also omitted from the diagram for legibility.
3.4. Sequence Numbers
A fundamental notion in the design is that every octet of data sent
over a TCP connection has a sequence number. Since every octet is
sequenced, each of them can be acknowledged. The acknowledgment
mechanism employed is cumulative so that an acknowledgment of
sequence number X indicates that all octets up to but not including X
have been received. This mechanism allows for straight-forward
duplicate detection in the presence of retransmission. Numbering of
octets within a segment is that the first data octet immediately
following the header is the lowest numbered, and the following octets
are numbered consecutively.
It is essential to remember that the actual sequence number space is
finite, though large. This space ranges from 0 to 2**32 - 1. Since
the space is finite, all arithmetic dealing with sequence numbers
must be performed modulo 2**32. This unsigned arithmetic preserves
the relationship of sequence numbers as they cycle from 2**32 - 1 to
0 again. There are some subtleties to computer modulo arithmetic, so
great care should be taken in programming the comparison of such
values. The symbol "=<" means "less than or equal" (modulo 2**32).
The typical kinds of sequence number comparisons that the TCP
implementation must perform include:
(a) Determining that an acknowledgment refers to some sequence
number sent but not yet acknowledged.
(b) Determining that all sequence numbers occupied by a segment
have been acknowledged (e.g., to remove the segment from a
retransmission queue).
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(c) Determining that an incoming segment contains sequence numbers
that are expected (i.e., that the segment "overlaps" the receive
window).
In response to sending data the TCP endpoint will receive
acknowledgments. The following comparisons are needed to process the
acknowledgments.
SND.UNA = oldest unacknowledged sequence number
SND.NXT = next sequence number to be sent
SEG.ACK = acknowledgment from the receiving TCP peer (next
sequence number expected by the receiving TCP peer)
SEG.SEQ = first sequence number of a segment
SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)
SEG.SEQ+SEG.LEN-1 = last sequence number of a segment
A new acknowledgment (called an "acceptable ack"), is one for which
the inequality below holds:
SND.UNA < SEG.ACK =< SND.NXT
A segment on the retransmission queue is fully acknowledged if the
sum of its sequence number and length is less or equal than the
acknowledgment value in the incoming segment.
When data is received the following comparisons are needed:
RCV.NXT = next sequence number expected on an incoming segment,
and is the left or lower edge of the receive window
RCV.NXT+RCV.WND-1 = last sequence number expected on an incoming
segment, and is the right or upper edge of the receive window
SEG.SEQ = first sequence number occupied by the incoming segment
SEG.SEQ+SEG.LEN-1 = last sequence number occupied by the incoming
segment
A segment is judged to occupy a portion of valid receive sequence
space if
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
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or
RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
The first part of this test checks to see if the beginning of the
segment falls in the window, the second part of the test checks to
see if the end of the segment falls in the window; if the segment
passes either part of the test it contains data in the window.
Actually, it is a little more complicated than this. Due to zero
windows and zero length segments, we have four cases for the
acceptability of an incoming segment:
Segment Receive Test
Length Window
------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
Note that when the receive window is zero no segments should be
acceptable except ACK segments. Thus, it is possible for a TCP
implementation to maintain a zero receive window while transmitting
data and receiving ACKs. A TCP receiver MUST process the RST and URG
fields of all incoming segments, even when the receive window is zero
(MUST-66).
We have taken advantage of the numbering scheme to protect certain
control information as well. This is achieved by implicitly
including some control flags in the sequence space so they can be
retransmitted and acknowledged without confusion (i.e., one and only
one copy of the control will be acted upon). Control information is
not physically carried in the segment data space. Consequently, we
must adopt rules for implicitly assigning sequence numbers to
control. The SYN and FIN are the only controls requiring this
protection, and these controls are used only at connection opening
and closing. For sequence number purposes, the SYN is considered to
occur before the first actual data octet of the segment in which it
occurs, while the FIN is considered to occur after the last actual
data octet in a segment in which it occurs. The segment length
(SEG.LEN) includes both data and sequence space-occupying controls.
When a SYN is present then SEG.SEQ is the sequence number of the SYN.
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3.4.1. Initial Sequence Number Selection
A connection is defined by a pair of sockets. Connections can be
reused. New instances of a connection will be referred to as
incarnations of the connection. The problem that arises from this is
-- "how does the TCP implementation identify duplicate segments from
previous incarnations of the connection?" This problem becomes
apparent if the connection is being opened and closed in quick
succession, or if the connection breaks with loss of memory and is
then reestablished. To support this, the TIME-WAIT state limits the
rate of connection reuse, while the initial sequence number selection
described below further protects against ambiguity about what
incarnation of a connection an incoming packet corresponds to.
To avoid confusion we must prevent segments from one incarnation of a
connection from being used while the same sequence numbers may still
be present in the network from an earlier incarnation. We want to
assure this, even if a TCP endpoint loses all knowledge of the
sequence numbers it has been using. When new connections are
created, an initial sequence number (ISN) generator is employed that
selects a new 32 bit ISN. There are security issues that result if
an off-path attacker is able to predict or guess ISN values [43].
TCP Initial Sequence Numbers are generated from a number sequence
that monotonically increases until it wraps, known loosely as a
"clock". This clock is a 32-bit counter that typically increments at
least once every roughly 4 microseconds, although it is neither
assumed to be realtime nor precise, and need not persist across
reboots. The clock component is intended to ensure that with a
Maximum Segment Lifetime (MSL), generated ISNs will be unique, since
it cycles approximately every 4.55 hours, which is much longer than
the MSL.
A TCP implementation MUST use the above type of "clock" for clock-
driven selection of initial sequence numbers (MUST-8), and SHOULD
generate its Initial Sequence Numbers with the expression:
ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
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where M is the 4 microsecond timer, and F() is a pseudorandom
function (PRF) of the connection's identifying parameters ("localip,
localport, remoteip, remoteport") and a secret key ("secretkey")
(SHLD-1). F() MUST NOT be computable from the outside (MUST-9), or
an attacker could still guess at sequence numbers from the ISN used
for some other connection. The PRF could be implemented as a
cryptographic hash of the concatenation of the TCP connection
parameters and some secret data. For discussion of the selection of
a specific hash algorithm and management of the secret key data,
please see Section 3 of [43].
For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by
the data sending TCP peer, and the initial receive sequence number
(IRS) is learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCP peers
must synchronize on each other's initial sequence numbers. This is
done in an exchange of connection establishing segments carrying a
control bit called "SYN" (for synchronize) and the initial sequence
numbers. As a shorthand, segments carrying the SYN bit are also
called "SYNs". Hence, the solution requires a suitable mechanism for
picking an initial sequence number and a slightly involved handshake
to exchange the ISNs.
The synchronization requires each side to send its own initial
sequence number and to receive a confirmation of it in acknowledgment
from the remote TCP peer. Each side must also receive the remote
peer's initial sequence number and send a confirming acknowledgment.
1) A --> B SYN my sequence number is X
2) A <-- B ACK your sequence number is X
3) A <-- B SYN my sequence number is Y
4) A --> B ACK your sequence number is Y
Because steps 2 and 3 can be combined in a single message this is
called the three-way (or three message) handshake (3WHS).
A 3WHS is necessary because sequence numbers are not tied to a global
clock in the network, and TCP implementations may have different
mechanisms for picking the ISNs. The receiver of the first SYN has
no way of knowing whether the segment was an old one or not, unless
it remembers the last sequence number used on the connection (which
is not always possible), and so it must ask the sender to verify this
SYN. The three-way handshake and the advantages of a clock-driven
scheme for ISN selection are discussed in [70].
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3.4.2. Knowing When to Keep Quiet
A theoretical problem exists where data could be corrupted due to
confusion between old segments in the network and new ones after a
host reboots, if the same port numbers and sequence space are reused.
The "Quiet Time" concept discussed below addresses this and the
discussion of it is included for situations where it might be
relevant, although it is not felt to be necessary in most current
implementations. The problem was more relevant earlier in the
history of TCP. In practical use on the Internet today, the error-
prone conditions are sufficiently unlikely that it is felt safe to
ignore. Reasons why it is now negligible include: (a) ISS and
ephemeral port randomization have reduced likelihood of reuse of port
numbers and sequence numbers after reboots, (b) the effective MSL of
the Internet has declined as links have become faster, and (c)
reboots often taking longer than an MSL anyways.
To be sure that a TCP implementation does not create a segment
carrying a sequence number that may be duplicated by an old segment
remaining in the network, the TCP endpoint must keep quiet for an MSL
before assigning any sequence numbers upon starting up or recovering
from a situation where memory of sequence numbers in use was lost.
For this specification the MSL is taken to be 2 minutes. This is an
engineering choice, and may be changed if experience indicates it is
desirable to do so. Note that if a TCP endpoint is reinitialized in
some sense, yet retains its memory of sequence numbers in use, then
it need not wait at all; it must only be sure to use sequence numbers
larger than those recently used.
3.4.3. The TCP Quiet Time Concept
Hosts that for any reason lose knowledge of the last sequence numbers
transmitted on each active (i.e., not closed) connection shall delay
emitting any TCP segments for at least the agreed MSL in the internet
system that the host is a part of. In the paragraphs below, an
explanation for this specification is given. TCP implementors may
violate the "quiet time" restriction, but only at the risk of causing
some old data to be accepted as new or new data rejected as old
duplicated data by some receivers in the internet system.
TCP endpoints consume sequence number space each time a segment is
formed and entered into the network output queue at a source host.
The duplicate detection and sequencing algorithm in the TCP protocol
relies on the unique binding of segment data to sequence space to the
extent that sequence numbers will not cycle through all 2**32 values
before the segment data bound to those sequence numbers has been
delivered and acknowledged by the receiver and all duplicate copies
of the segments have "drained" from the internet. Without such an
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assumption, two distinct TCP segments could conceivably be assigned
the same or overlapping sequence numbers, causing confusion at the
receiver as to which data is new and which is old. Remember that
each segment is bound to as many consecutive sequence numbers as
there are octets of data and SYN or FIN flags in the segment.
Under normal conditions, TCP implementations keep track of the next
sequence number to emit and the oldest awaiting acknowledgment so as
to avoid mistakenly using a sequence number over before its first use
has been acknowledged. This alone does not guarantee that old
duplicate data is drained from the net, so the sequence space has
been made large to reduce the probability that a wandering duplicate
will cause trouble upon arrival. At 2 megabits/sec. it takes 4.5
hours to use up 2**32 octets of sequence space. Since the maximum
segment lifetime in the net is not likely to exceed a few tens of
seconds, this is deemed ample protection for foreseeable nets, even
if data rates escalate to 10s of megabits/sec. At 100 megabits/sec,
the cycle time is 5.4 minutes, which may be a little short, but still
within reason. Much higher data rates are possible today, with
implications described in the final paragraph of this subsection.
The basic duplicate detection and sequencing algorithm in TCP can be
defeated, however, if a source TCP endpoint does not have any memory
of the sequence numbers it last used on a given connection. For
example, if the TCP implementation were to start all connections with
sequence number 0, then upon the host rebooting, a TCP peer might re-
form an earlier connection (possibly after half-open connection
resolution) and emit packets with sequence numbers identical to or
overlapping with packets still in the network, which were emitted on
an earlier incarnation of the same connection. In the absence of
knowledge about the sequence numbers used on a particular connection,
the TCP specification recommends that the source delay for MSL
seconds before emitting segments on the connection, to allow time for
segments from the earlier connection incarnation to drain from the
system.
Even hosts that can remember the time of day and used it to select
initial sequence number values are not immune from this problem
(i.e., even if time of day is used to select an initial sequence
number for each new connection incarnation).
Suppose, for example, that a connection is opened starting with
sequence number S. Suppose that this connection is not used much and
that eventually the initial sequence number function (ISN(t)) takes
on a value equal to the sequence number, say S1, of the last segment
sent by this TCP endpoint on a particular connection. Now suppose,
at this instant, the host reboots and establishes a new incarnation
of the connection. The initial sequence number chosen is S1 = ISN(t)
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-- last used sequence number on old incarnation of connection! If
the recovery occurs quickly enough, any old duplicates in the net
bearing sequence numbers in the neighborhood of S1 may arrive and be
treated as new packets by the receiver of the new incarnation of the
connection.
The problem is that the recovering host may not know for how long it
was down between rebooting nor does it know whether there are still
old duplicates in the system from earlier connection incarnations.
One way to deal with this problem is to deliberately delay emitting
segments for one MSL after recovery from a reboot - this is the
"quiet time" specification. Hosts that prefer to avoid waiting and
are willing to risk possible confusion of old and new packets at a
given destination may choose not to wait for the "quiet time".
Implementors may provide TCP users with the ability to select on a
connection by connection basis whether to wait after a reboot, or may
informally implement the "quiet time" for all connections.
Obviously, even where a user selects to "wait," this is not necessary
after the host has been "up" for at least MSL seconds.
To summarize: every segment emitted occupies one or more sequence
numbers in the sequence space, the numbers occupied by a segment are
"busy" or "in use" until MSL seconds have passed, upon rebooting a
block of space-time is occupied by the octets and SYN or FIN flags of
any potentially still in-flight segments, and if a new connection is
started too soon and uses any of the sequence numbers in the space-
time footprint of those potentially still in-flight segments of the
previous connection incarnation, there is a potential sequence number
overlap area that could cause confusion at the receiver.
High performance cases will have shorter cycle times than those in
the megabits per second that the base TCP design described above
considers. At 1 Gbps, the cycle time is 34 seconds, only 3 seconds
at 10 Gbps, and around a third of a second at 100 Gbps. In these
higher performance cases, TCP Timestamp options and Protection
Against Wrapped Sequences (PAWS) [48] provide the needed capability
to detect and discard old duplicates.
3.5. Establishing a connection
The "three-way handshake" is the procedure used to establish a
connection. This procedure normally is initiated by one TCP peer and
responded to by another TCP peer. The procedure also works if two
TCP peers simultaneously initiate the procedure. When simultaneous
open occurs, each TCP peer receives a "SYN" segment that carries no
acknowledgment after it has sent a "SYN". Of course, the arrival of
an old duplicate "SYN" segment can potentially make it appear, to the
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recipient, that a simultaneous connection initiation is in progress.
Proper use of "reset" segments can disambiguate these cases.
Several examples of connection initiation follow. Although these
examples do not show connection synchronization using data-carrying
segments, this is perfectly legitimate, so long as the receiving TCP
endpoint doesn't deliver the data to the user until it is clear the
data is valid (e.g., the data is buffered at the receiver until the
connection reaches the ESTABLISHED state, given that the three-way
handshake reduces the possibility of false connections). It is a
trade-off between memory and messages to provide information for this
checking.
The simplest 3WHS is shown in Figure 6. The figures should be
interpreted in the following way. Each line is numbered for
reference purposes. Right arrows (-->) indicate departure of a TCP
segment from TCP peer A to TCP peer B, or arrival of a segment at B
from A. Left arrows (<--), indicate the reverse. Ellipsis (...)
indicates a segment that is still in the network (delayed). Comments
appear in parentheses. TCP connection states represent the state
AFTER the departure or arrival of the segment (whose contents are
shown in the center of each line). Segment contents are shown in
abbreviated form, with sequence number, control flags, and ACK field.
Other fields such as window, addresses, lengths, and text have been
left out in the interest of clarity.
TCP Peer A TCP Peer B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED
3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED
5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
Figure 6: Basic 3-Way Handshake for Connection Synchronization
In line 2 of Figure 6, TCP Peer A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence
number 100. In line 3, TCP Peer B sends a SYN and acknowledges the
SYN it received from TCP Peer A. Note that the acknowledgment field
indicates TCP Peer B is now expecting to hear sequence 101,
acknowledging the SYN that occupied sequence 100.
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At line 4, TCP Peer A responds with an empty segment containing an
ACK for TCP Peer B's SYN; and in line 5, TCP Peer A sends some data.
Note that the sequence number of the segment in line 5 is the same as
in line 4 because the ACK does not occupy sequence number space (if
it did, we would wind up ACKing ACKs!).
Simultaneous initiation is only slightly more complex, as is shown in
Figure 7. Each TCP peer's connection state cycles from CLOSED to
SYN-SENT to SYN-RECEIVED to ESTABLISHED.
TCP Peer A TCP Peer B
1. CLOSED CLOSED
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. SYN-RECEIVED <-- <SEQ=300><CTL=SYN> <-- SYN-SENT
4. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
5. SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
6. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
7. ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED
Figure 7: Simultaneous Connection Synchronization
A TCP implementation MUST support simultaneous open attempts (MUST-
10).
Note that a TCP implementation MUST keep track of whether a
connection has reached SYN-RECEIVED state as the result of a passive
OPEN or an active OPEN (MUST-11).
The principal reason for the three-way handshake is to prevent old
duplicate connection initiations from causing confusion. To deal
with this, a special control message, reset, is specified. If the
receiving TCP peer is in a non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP peer is in one of the synchronized states (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT), it
aborts the connection and informs its user. We discuss this latter
case under "half-open" connections below.
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TCP Peer A TCP Peer B
1. CLOSED LISTEN
2. SYN-SENT --> <SEQ=100><CTL=SYN> ...
3. (duplicate) ... <SEQ=90><CTL=SYN> --> SYN-RECEIVED
4. SYN-SENT <-- <SEQ=300><ACK=91><CTL=SYN,ACK> <-- SYN-RECEIVED
5. SYN-SENT --> <SEQ=91><CTL=RST> --> LISTEN
6. ... <SEQ=100><CTL=SYN> --> SYN-RECEIVED
7. ESTABLISHED <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED
Figure 8: Recovery from Old Duplicate SYN
As a simple example of recovery from old duplicates, consider
Figure 8. At line 3, an old duplicate SYN arrives at TCP Peer B.
TCP Peer B cannot tell that this is an old duplicate, so it responds
normally (line 4). TCP Peer A detects that the ACK field is
incorrect and returns a RST (reset) with its SEQ field selected to
make the segment believable. TCP Peer B, on receiving the RST,
returns to the LISTEN state. When the original SYN finally arrives
at line 6, the synchronization proceeds normally. If the SYN at line
6 had arrived before the RST, a more complex exchange might have
occurred with RST's sent in both directions.
3.5.1. Half-Open Connections and Other Anomalies
An established connection is said to be "half-open" if one of the TCP
peers has closed or aborted the connection at its end without the
knowledge of the other, or if the two ends of the connection have
become desynchronized owing to a failure or reboot that resulted in
loss of memory. Such connections will automatically become reset if
an attempt is made to send data in either direction. However, half-
open connections are expected to be unusual.
If at site A the connection no longer exists, then an attempt by the
user at site B to send any data on it will result in the site B TCP
endpoint receiving a reset control message. Such a message indicates
to the site B TCP endpoint that something is wrong, and it is
expected to abort the connection.
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Assume that two user processes A and B are communicating with one
another when a failure or reboot occurs causing loss of memory to A's
TCP implementation. Depending on the operating system supporting A's
TCP implementation, it is likely that some error recovery mechanism
exists. When the TCP endpoint is up again, A is likely to start
again from the beginning or from a recovery point. As a result, A
will probably try to OPEN the connection again or try to SEND on the
connection it believes open. In the latter case, it receives the
error message "connection not open" from the local (A's) TCP
implementation. In an attempt to establish the connection, A's TCP
implementation will send a segment containing SYN. This scenario
leads to the example shown in Figure 9. After TCP Peer A reboots,
the user attempts to re-open the connection. TCP Peer B, in the
meantime, thinks the connection is open.
TCP Peer A TCP Peer B
1. (REBOOT) (send 300,receive 100)
2. CLOSED ESTABLISHED
3. SYN-SENT --> <SEQ=400><CTL=SYN> --> (??)
4. (!!) <-- <SEQ=300><ACK=100><CTL=ACK> <-- ESTABLISHED
5. SYN-SENT --> <SEQ=100><CTL=RST> --> (Abort!!)
6. SYN-SENT CLOSED
7. SYN-SENT --> <SEQ=400><CTL=SYN> -->
Figure 9: Half-Open Connection Discovery
When the SYN arrives at line 3, TCP Peer B, being in a synchronized
state, and the incoming segment outside the window, responds with an
acknowledgment indicating what sequence it next expects to hear (ACK
100). TCP Peer A sees that this segment does not acknowledge
anything it sent and, being unsynchronized, sends a reset (RST)
because it has detected a half-open connection. TCP Peer B aborts at
line 5. TCP Peer A will continue to try to establish the connection;
the problem is now reduced to the basic 3-way handshake of Figure 6.
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An interesting alternative case occurs when TCP Peer A reboots and
TCP Peer B tries to send data on what it thinks is a synchronized
connection. This is illustrated in Figure 10. In this case, the
data arriving at TCP Peer A from TCP Peer B (line 2) is unacceptable
because no such connection exists, so TCP Peer A sends a RST. The
RST is acceptable so TCP Peer B processes it and aborts the
connection.
TCP Peer A TCP Peer B
1. (REBOOT) (send 300,receive 100)
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
Figure 10: Active Side Causes Half-Open Connection Discovery
In Figure 11, two TCP Peers A and B with passive connections waiting
for SYN are depicted. An old duplicate arriving at TCP Peer B (line
2) stirs B into action. A SYN-ACK is returned (line 3) and causes
TCP A to generate a RST (the ACK in line 3 is not acceptable). TCP
Peer B accepts the reset and returns to its passive LISTEN state.
TCP Peer A TCP Peer B
1. LISTEN LISTEN
2. ... <SEQ=Z><CTL=SYN> --> SYN-RECEIVED
3. (??) <-- <SEQ=X><ACK=Z+1><CTL=SYN,ACK> <-- SYN-RECEIVED
4. --> <SEQ=Z+1><CTL=RST> --> (return to LISTEN!)
5. LISTEN LISTEN
Figure 11: Old Duplicate SYN Initiates a Reset on two Passive Sockets
A variety of other cases are possible, all of which are accounted for
by the following rules for RST generation and processing.
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3.5.2. Reset Generation
A TCP user or application can issue a reset on a connection at any
time, though reset events are also generated by the protocol itself
when various error conditions occur, as described below. The side of
a connection issuing a reset should enter the TIME-WAIT state, as
this generally helps to reduce the load on busy servers for reasons
described in [71].
As a general rule, reset (RST) is sent whenever a segment arrives
that apparently is not intended for the current connection. A reset
must not be sent if it is not clear that this is the case.
There are three groups of states:
1. If the connection does not exist (CLOSED) then a reset is sent
in response to any incoming segment except another reset. A SYN
segment that does not match an existing connection is rejected by
this means.
If the incoming segment has the ACK bit set, the reset takes its
sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment.
The connection remains in the CLOSED state.
2. If the connection is in any non-synchronized state (LISTEN,
SYN-SENT, SYN-RECEIVED), and the incoming segment acknowledges
something not yet sent (the segment carries an unacceptable ACK),
or if an incoming segment has a security level or compartment
Appendix A.1 that does not exactly match the level and compartment
requested for the connection, a reset is sent.
If the incoming segment has an ACK field, the reset takes its
sequence number from the ACK field of the segment, otherwise the
reset has sequence number zero and the ACK field is set to the sum
of the sequence number and segment length of the incoming segment.
The connection remains in the same state.
3. If the connection is in a synchronized state (ESTABLISHED,
FIN-WAIT-1, FIN-WAIT-2, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT),
any unacceptable segment (out of window sequence number or
unacceptable acknowledgment number) must be responded to with an
empty acknowledgment segment (without any user data) containing
the current send-sequence number and an acknowledgment indicating
the next sequence number expected to be received, and the
connection remains in the same state.
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If an incoming segment has a security level or compartment that
does not exactly match the level and compartment requested for the
connection, a reset is sent and the connection goes to the CLOSED
state. The reset takes its sequence number from the ACK field of
the incoming segment.
3.5.3. Reset Processing
In all states except SYN-SENT, all reset (RST) segments are validated
by checking their SEQ-fields. A reset is valid if its sequence
number is in the window. In the SYN-SENT state (a RST received in
response to an initial SYN), the RST is acceptable if the ACK field
acknowledges the SYN.
The receiver of a RST first validates it, then changes state. If the
receiver was in the LISTEN state, it ignores it. If the receiver was
in SYN-RECEIVED state and had previously been in the LISTEN state,
then the receiver returns to the LISTEN state, otherwise the receiver
aborts the connection and goes to the CLOSED state. If the receiver
was in any other state, it aborts the connection and advises the user
and goes to the CLOSED state.
TCP implementations SHOULD allow a received RST segment to include
data (SHLD-2). It has been suggested that a RST segment could
contain diagnostic data that explains the cause of the RST. No
standard has yet been established for such data.
3.6. Closing a Connection
CLOSE is an operation meaning "I have no more data to send." The
notion of closing a full-duplex connection is subject to ambiguous
interpretation, of course, since it may not be obvious how to treat
the receiving side of the connection. We have chosen to treat CLOSE
in a simplex fashion. The user who CLOSEs may continue to RECEIVE
until the TCP receiver is told that the remote peer has CLOSED also.
Thus, a program could initiate several SENDs followed by a CLOSE, and
then continue to RECEIVE until signaled that a RECEIVE failed because
the remote peer has CLOSED. The TCP implementation will signal a
user, even if no RECEIVEs are outstanding, that the remote peer has
closed, so the user can terminate their side gracefully. A TCP
implementation will reliably deliver all buffers SENT before the
connection was CLOSED so a user who expects no data in return need
only wait to hear the connection was CLOSED successfully to know that
all their data was received at the destination TCP endpoint. Users
must keep reading connections they close for sending until the TCP
implementation indicates there is no more data.
There are essentially three cases:
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1) The user initiates by telling the TCP implementation to CLOSE
the connection (TCP Peer A in Figure 12).
2) The remote TCP endpoint initiates by sending a FIN control
signal (TCP Peer B in Figure 12).
3) Both users CLOSE simultaneously (Figure 13).
Case 1: Local user initiates the close
In this case, a FIN segment can be constructed and placed on the
outgoing segment queue. No further SENDs from the user will be
accepted by the TCP implementation, and it enters the FIN-WAIT-1
state. RECEIVEs are allowed in this state. All segments
preceding and including FIN will be retransmitted until
acknowledged. When the other TCP peer has both acknowledged the
FIN and sent a FIN of its own, the first TCP peer can ACK this
FIN. Note that a TCP endpoint receiving a FIN will ACK but not
send its own FIN until its user has CLOSED the connection also.
Case 2: TCP endpoint receives a FIN from the network
If an unsolicited FIN arrives from the network, the receiving TCP
endpoint can ACK it and tell the user that the connection is
closing. The user will respond with a CLOSE, upon which the TCP
endpoint can send a FIN to the other TCP peer after sending any
remaining data. The TCP endpoint then waits until its own FIN is
acknowledged whereupon it deletes the connection. If an ACK is
not forthcoming, after the user timeout the connection is aborted
and the user is told.
Case 3: Both users close simultaneously
A simultaneous CLOSE by users at both ends of a connection causes
FIN segments to be exchanged (Figure 13). When all segments
preceding the FINs have been processed and acknowledged, each TCP
peer can ACK the FIN it has received. Both will, upon receiving
these ACKs, delete the connection.
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TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED
2. (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT
3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT
4. (Close)
TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK
5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --> CLOSED
6. (2 MSL)
CLOSED
Figure 12: Normal Close Sequence
TCP Peer A TCP Peer B
1. ESTABLISHED ESTABLISHED
2. (Close) (Close)
FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> ... FIN-WAIT-1
<-- <SEQ=300><ACK=100><CTL=FIN,ACK> <--
... <SEQ=100><ACK=300><CTL=FIN,ACK> -->
3. CLOSING --> <SEQ=101><ACK=301><CTL=ACK> ... CLOSING
<-- <SEQ=301><ACK=101><CTL=ACK> <--
... <SEQ=101><ACK=301><CTL=ACK> -->
4. TIME-WAIT TIME-WAIT
(2 MSL) (2 MSL)
CLOSED CLOSED
Figure 13: Simultaneous Close Sequence
A TCP connection may terminate in two ways: (1) the normal TCP close
sequence using a FIN handshake (Figure 12), and (2) an "abort" in
which one or more RST segments are sent and the connection state is
immediately discarded. If the local TCP connection is closed by the
remote side due to a FIN or RST received from the remote side, then
the local application MUST be informed whether it closed normally or
was aborted (MUST-12).
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3.6.1. Half-Closed Connections
The normal TCP close sequence delivers buffered data reliably in both
directions. Since the two directions of a TCP connection are closed
independently, it is possible for a connection to be "half closed,"
i.e., closed in only one direction, and a host is permitted to
continue sending data in the open direction on a half-closed
connection.
A host MAY implement a "half-duplex" TCP close sequence, so that an
application that has called CLOSE cannot continue to read data from
the connection (MAY-1). If such a host issues a CLOSE call while
received data is still pending in the TCP connection, or if new data
is received after CLOSE is called, its TCP implementation SHOULD send
a RST to show that data was lost (SHLD-3). See [24] section 2.17 for
discussion.
When a connection is closed actively, it MUST linger in the TIME-WAIT
state for a time 2xMSL (Maximum Segment Lifetime) (MUST-13).
However, it MAY accept a new SYN from the remote TCP endpoint to
reopen the connection directly from TIME-WAIT state (MAY-2), if it:
(1) assigns its initial sequence number for the new connection to
be larger than the largest sequence number it used on the previous
connection incarnation, and
(2) returns to TIME-WAIT state if the SYN turns out to be an old
duplicate.
When the TCP Timestamp options are available, an improved algorithm
is described in [41] in order to support higher connection
establishment rates. This algorithm for reducing TIME-WAIT is a Best
Current Practice that SHOULD be implemented, since timestamp options
are commonly used, and using them to reduce TIME-WAIT provides
benefits for busy Internet servers (SHLD-4).
3.7. Segmentation
The term "segmentation" refers to the activity TCP performs when
ingesting a stream of bytes from a sending application and
packetizing that stream of bytes into TCP segments. Individual TCP
segments often do not correspond one-for-one to individual send (or
socket write) calls from the application. Applications may perform
writes at the granularity of messages in the upper layer protocol,
but TCP guarantees no boundary coherence between the TCP segments
sent and received versus user application data read or write buffer
boundaries. In some specific protocols, such as Remote Direct Memory
Access (RDMA) using Direct Data Placement (DDP) and Marker PDU
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Aligned Framing (MPA) [35], there are performance optimizations
possible when the relation between TCP segments and application data
units can be controlled, and MPA includes a specific mechanism for
detecting and verifying this relationship between TCP segments and
application message data structures, but this is specific to
applications like RDMA. In general, multiple goals influence the
sizing of TCP segments created by a TCP implementation.
Goals driving the sending of larger segments include:
* Reducing the number of packets in flight within the network.
* Increasing processing efficiency and potential performance by
enabling a smaller number of interrupts and inter-layer
interactions.
* Limiting the overhead of TCP headers.
Note that the performance benefits of sending larger segments may
decrease as the size increases, and there may be boundaries where
advantages are reversed. For instance, on some implementation
architectures, 1025 bytes within a segment could lead to worse
performance than 1024 bytes, due purely to data alignment on copy
operations.
Goals driving the sending of smaller segments include:
* Avoiding sending a TCP segment that would result in an IP datagram
larger than the smallest MTU along an IP network path, because
this results in either packet loss or packet fragmentation.
Making matters worse, some firewalls or middleboxes may drop
fragmented packets or ICMP messages related to fragmentation.
* Preventing delays to the application data stream, especially when
TCP is waiting on the application to generate more data, or when
the application is waiting on an event or input from its peer in
order to generate more data.
* Enabling "fate sharing" between TCP segments and lower-layer data
units (e.g. below IP, for links with cell or frame sizes smaller
than the IP MTU).
Towards meeting these competing sets of goals, TCP includes several
mechanisms, including the Maximum Segment Size option, Path MTU
Discovery, the Nagle algorithm, and support for IPv6 Jumbograms, as
discussed in the following subsections.
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3.7.1. Maximum Segment Size Option
TCP endpoints MUST implement both sending and receiving the MSS
option (MUST-14).
TCP implementations SHOULD send an MSS option in every SYN segment
when its receive MSS differs from the default 536 for IPv4 or 1220
for IPv6 (SHLD-5), and MAY send it always (MAY-3).
If an MSS option is not received at connection setup, TCP
implementations MUST assume a default send MSS of 536 (576 - 40) for
IPv4 or 1220 (1280 - 60) for IPv6 (MUST-15).
The maximum size of a segment that TCP endpoint really sends, the
"effective send MSS," MUST be the smaller (MUST-16) of the send MSS
(that reflects the available reassembly buffer size at the remote
host, the EMTU_R [20]) and the largest transmission size permitted by
the IP layer (EMTU_S [20]):
Eff.snd.MSS =
min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize
where:
* SendMSS is the MSS value received from the remote host, or the
default 536 for IPv4 or 1220 for IPv6, if no MSS option is
received.
* MMS_S is the maximum size for a transport-layer message that TCP
may send.
* TCPhdrsize is the size of the fixed TCP header and any options.
This is 20 in the (rare) case that no options are present, but may
be larger if TCP options are to be sent. Note that some options
might not be included on all segments, but that for each segment
sent, the sender should adjust the data length accordingly, within
the Eff.snd.MSS.
* IPoptionsize is the size of any IPv4 options or IPv6 extension
headers associated with a TCP connection. Note that some options
or extension headers might not be included on all packets, but
that for each segment sent, the sender should adjust the data
length accordingly, within the Eff.snd.MSS.
The MSS value to be sent in an MSS option should be equal to the
effective MTU minus the fixed IP and TCP headers. By ignoring both
IP and TCP options when calculating the value for the MSS option, if
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there are any IP or TCP options to be sent in a packet, then the
sender must decrease the size of the TCP data accordingly. RFC 6691
[44] discusses this in greater detail.
The MSS value to be sent in an MSS option must be less than or equal
to:
MMS_R - 20
where MMS_R is the maximum size for a transport-layer message that
can be received (and reassembled at the IP layer) (MUST-67). TCP
obtains MMS_R and MMS_S from the IP layer; see the generic call
GET_MAXSIZES in Section 3.4 of RFC 1122. These are defined in terms
of their IP MTU equivalents, EMTU_R and EMTU_S [20].
When TCP is used in a situation where either the IP or TCP headers
are not fixed, the sender must reduce the amount of TCP data in any
given packet by the number of octets used by the IP and TCP options.
This has been a point of confusion historically, as explained in RFC
6691, Section 3.1.
3.7.2. Path MTU Discovery
A TCP implementation may be aware of the MTU on directly connected
links, but will rarely have insight about MTUs across an entire
network path. For IPv4, RFC 1122 recommends an IP-layer default
effective MTU of less than or equal to 576 for destinations not
directly connected, and for IPv6 this would be 1280. Using these
fixed values limits TCP connection performance and efficiency.
Instead, implementation of Path MTU Discovery (PMTUD) and
Packetization Layer Path MTU Discovery (PLPMTUD) is strongly
recommended in order for TCP to improve segmentation decisions. Both
PMTUD and PLPMTUD help TCP choose segment sizes that avoid both on-
path (for IPv4) and source fragmentation (IPv4 and IPv6).
PMTUD for IPv4 [2] or IPv6 [14] is implemented in conjunction between
TCP, IP, and ICMP protocols. It relies both on avoiding source
fragmentation and setting the IPv4 DF (don't fragment) flag, the
latter to inhibit on-path fragmentation. It relies on ICMP errors
from routers along the path, whenever a segment is too large to
traverse a link. Several adjustments to a TCP implementation with
PMTUD are described in RFC 2923 in order to deal with problems
experienced in practice [28]. PLPMTUD [32] is a Standards Track
improvement to PMTUD that relaxes the requirement for ICMP support
across a path, and improves performance in cases where ICMP is not
consistently conveyed, but still tries to avoid source fragmentation.
The mechanisms in all four of these RFCs are recommended to be
included in TCP implementations.
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The TCP MSS option specifies an upper bound for the size of packets
that can be received (see [44]). Hence, setting the value in the MSS
option too small can impact the ability for PMTUD or PLPMTUD to find
a larger path MTU. RFC 1191 discusses this implication of many older
TCP implementations setting the TCP MSS to 536 (corresponding to the
IPv4 576 byte default MTU) for non-local destinations, rather than
deriving it from the MTUs of connected interfaces as recommended.
3.7.3. Interfaces with Variable MTU Values
The effective MTU can sometimes vary, as when used with variable
compression, e.g., RObust Header Compression (ROHC) [38]. It is
tempting for a TCP implementation to advertise the largest possible
MSS, to support the most efficient use of compressed payloads.
Unfortunately, some compression schemes occasionally need to transmit
full headers (and thus smaller payloads) to resynchronize state at
their endpoint compressors/decompressors. If the largest MTU is used
to calculate the value to advertise in the MSS option, TCP
retransmission may interfere with compressor resynchronization.
As a result, when the effective MTU of an interface varies packet-to-
packet, TCP implementations SHOULD use the smallest effective MTU of
the interface to calculate the value to advertise in the MSS option
(SHLD-6).
3.7.4. Nagle Algorithm
The "Nagle algorithm" was described in RFC 896 [18] and was
recommended in RFC 1122 [20] for mitigation of an early problem of
too many small packets being generated. It has been implemented in
most current TCP code bases, sometimes with minor variations (see
Appendix A.3).
If there is unacknowledged data (i.e., SND.NXT > SND.UNA), then the
sending TCP endpoint buffers all user data (regardless of the PSH
bit), until the outstanding data has been acknowledged or until the
TCP endpoint can send a full-sized segment (Eff.snd.MSS bytes).
A TCP implementation SHOULD implement the Nagle Algorithm to coalesce
short segments (SHLD-7). However, there MUST be a way for an
application to disable the Nagle algorithm on an individual
connection (MUST-17). In all cases, sending data is also subject to
the limitation imposed by the Slow Start algorithm [8].
Since there can be problematic interactions between the Nagle
Algorithm and delayed acknowledgements, some implementations use
minor variations of the Nagle algorithm, such as the one described in
Appendix A.3.
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3.7.5. IPv6 Jumbograms
In order to support TCP over IPv6 Jumbograms, implementations need to
be able to send TCP segments larger than the 64KB limit that the MSS
option can convey. RFC 2675 [25] defines that an MSS value of 65,535
bytes is to be treated as infinity, and Path MTU Discovery [14] is
used to determine the actual MSS.
The Jumbo Payload option need not be implemented or understood by
IPv6 nodes that do not support attachment to links with a MTU greater
than 65,575 [25], and the present IPv6 Node Requirements does not
include support for Jumbograms [55].
3.8. Data Communication
Once the connection is established data is communicated by the
exchange of segments. Because segments may be lost due to errors
(checksum test failure), or network congestion, TCP uses
retransmission to ensure delivery of every segment. Duplicate
segments may arrive due to network or TCP retransmission. As
discussed in the section on sequence numbers, the TCP implementation
performs certain tests on the sequence and acknowledgment numbers in
the segments to verify their acceptability.
The sender of data keeps track of the next sequence number to use in
the variable SND.NXT. The receiver of data keeps track of the next
sequence number to expect in the variable RCV.NXT. The sender of
data keeps track of the oldest unacknowledged sequence number in the
variable SND.UNA. If the data flow is momentarily idle and all data
sent has been acknowledged then the three variables will be equal.
When the sender creates a segment and transmits it the sender
advances SND.NXT. When the receiver accepts a segment it advances
RCV.NXT and sends an acknowledgment. When the data sender receives
an acknowledgment it advances SND.UNA. The extent to which the
values of these variables differ is a measure of the delay in the
communication. The amount by which the variables are advanced is the
length of the data and SYN or FIN flags in the segment. Note that
once in the ESTABLISHED state all segments must carry current
acknowledgment information.
The CLOSE user call implies a push function (see Section 3.9.1), as
does the FIN control flag in an incoming segment.
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3.8.1. Retransmission Timeout
Because of the variability of the networks that compose an
internetwork system and the wide range of uses of TCP connections the
retransmission timeout (RTO) must be dynamically determined.
The RTO MUST be computed according to the algorithm in [10],
including Karn's algorithm for taking RTT samples (MUST-18).
RFC 793 contains an early example procedure for computing the RTO,
based on work mentioned in IEN 177 [72]. This was then replaced by
the algorithm described in RFC 1122, and subsequently updated in RFC
2988, and then again in RFC 6298.
RFC 1122 allows that if a retransmitted packet is identical to the
original packet (which implies not only that the data boundaries have
not changed, but also that none of the headers have changed), then
the same IPv4 Identification field MAY be used (see Section 3.2.1.5
of RFC 1122) (MAY-4). The same IP identification field may be reused
anyways, since it is only meaningful when a datagram is fragmented
[45]. TCP implementations should not rely on or typically interact
with this IPv4 header field in any way. It is not a reasonable way
to either indicate duplicate sent segments, nor to identify duplicate
received segments.
3.8.2. TCP Congestion Control
RFC 2914 [5] explains the importance of congestion control for the
Internet.
RFC 1122 required implementation of Van Jacobson's congestion control
algorithms slow start and congestion avoidance together with
exponential back-off for successive RTO values for the same segment.
RFC 2581 provided IETF Standards Track description of slow start and
congestion avoidance, along with fast retransmit and fast recovery.
RFC 5681 is the current description of these algorithms and is the
current Standards Track specification providing guidelines for TCP
congestion control. RFC 6298 describes exponential back-off of RTO
values, including keeping the backed-off value until a subsequent
segment with new data has been sent and acknowledged without
retransmission.
A TCP endpoint MUST implement the basic congestion control algorithms
slow start, congestion avoidance, and exponential back-off of RTO to
avoid creating congestion collapse conditions (MUST-19). RFC 5681
and RFC 6298 describe the basic algorithms on the IETF Standards
Track that are broadly applicable. Multiple other suitable
algorithms exist and have been widely used. Many TCP implementations
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support a set of alternative algorithms that can be configured for
use on the endpoint. An endpoint MAY implement such alternative
algorithms provided that the algorithms are conformant with the TCP
specifications from the IETF Standards Track as described in RFC
2914, RFC 5033 [7], and RFC 8961 [15] (MAY-18).
Explicit Congestion Notification (ECN) was defined in RFC 3168 and is
an IETF Standards Track enhancement that has many benefits [52].
A TCP endpoint SHOULD implement ECN as described in RFC 3168 (SHLD-
8).
3.8.3. TCP Connection Failures
Excessive retransmission of the same segment by a TCP endpoint
indicates some failure of the remote host or the Internet path. This
failure may be of short or long duration. The following procedure
MUST be used to handle excessive retransmissions of data segments
(MUST-20):
(a) There are two thresholds R1 and R2 measuring the amount of
retransmission that has occurred for the same segment. R1 and R2
might be measured in time units or as a count of retransmissions
(with the current RTO and corresponding backoffs as a conversion
factor, if needed).
(b) When the number of transmissions of the same segment reaches
or exceeds threshold R1, pass negative advice (see Section 3.3.1.4
of [20]) to the IP layer, to trigger dead-gateway diagnosis.
(c) When the number of transmissions of the same segment reaches a
threshold R2 greater than R1, close the connection.
(d) An application MUST (MUST-21) be able to set the value for R2
for a particular connection. For example, an interactive
application might set R2 to "infinity," giving the user control
over when to disconnect.
(e) TCP implementations SHOULD inform the application of the
delivery problem (unless such information has been disabled by the
application; see Asynchronous Reports section), when R1 is reached
and before R2 (SHLD-9). This will allow a remote login
application program to inform the user, for example.
The value of R1 SHOULD correspond to at least 3 retransmissions, at
the current RTO (SHLD-10). The value of R2 SHOULD correspond to at
least 100 seconds (SHLD-11).
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An attempt to open a TCP connection could fail with excessive
retransmissions of the SYN segment or by receipt of a RST segment or
an ICMP Port Unreachable. SYN retransmissions MUST be handled in the
general way just described for data retransmissions, including
notification of the application layer.
However, the values of R1 and R2 may be different for SYN and data
segments. In particular, R2 for a SYN segment MUST be set large
enough to provide retransmission of the segment for at least 3
minutes (MUST-23). The application can close the connection (i.e.,
give up on the open attempt) sooner, of course.
3.8.4. TCP Keep-Alives
A TCP connection is said to be "idle" if for some long amount of time
there have been no incoming segments received and there is no new or
unacknowledged data to be sent.
Implementors MAY include "keep-alives" in their TCP implementations
(MAY-5), although this practice is not universally accepted. Some
TCP implementations, however, have included a keep-alive mechanism.
To confirm that an idle connection is still active, these
implementations send a probe segment designed to elicit a response
from the TCP peer. Such a segment generally contains SEG.SEQ =
SND.NXT-1 and may or may not contain one garbage octet of data. If
keep-alives are included, the application MUST be able to turn them
on or off for each TCP connection (MUST-24), and they MUST default to
off (MUST-25).
Keep-alive packets MUST only be sent when no sent data is
outstanding, and no data or acknowledgement packets have been
received for the connection within an interval (MUST-26). This
interval MUST be configurable (MUST-27) and MUST default to no less
than two hours (MUST-28).
It is extremely important to remember that ACK segments that contain
no data are not reliably transmitted by TCP. Consequently, if a
keep-alive mechanism is implemented it MUST NOT interpret failure to
respond to any specific probe as a dead connection (MUST-29).
An implementation SHOULD send a keep-alive segment with no data
(SHLD-12); however, it MAY be configurable to send a keep-alive
segment containing one garbage octet (MAY-6), for compatibility with
erroneous TCP implementations.
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3.8.5. The Communication of Urgent Information
As a result of implementation differences and middlebox interactions,
new applications SHOULD NOT employ the TCP urgent mechanism (SHLD-
13). However, TCP implementations MUST still include support for the
urgent mechanism (MUST-30). Information on how some TCP
implementations interpret the urgent pointer can be found in RFC 6093
[40].
The objective of the TCP urgent mechanism is to allow the sending
user to stimulate the receiving user to accept some urgent data and
to permit the receiving TCP endpoint to indicate to the receiving
user when all the currently known urgent data has been received by
the user.
This mechanism permits a point in the data stream to be designated as
the end of urgent information. Whenever this point is in advance of
the receive sequence number (RCV.NXT) at the receiving TCP endpoint,
that TCP must tell the user to go into "urgent mode"; when the
receive sequence number catches up to the urgent pointer, the TCP
implementation must tell user to go into "normal mode". If the
urgent pointer is updated while the user is in "urgent mode", the
update will be invisible to the user.
The method employs an urgent field that is carried in all segments
transmitted. The URG control flag indicates that the urgent field is
meaningful and must be added to the segment sequence number to yield
the urgent pointer. The absence of this flag indicates that there is
no urgent data outstanding.
To send an urgent indication the user must also send at least one
data octet. If the sending user also indicates a push, timely
delivery of the urgent information to the destination process is
enhanced. Note that because changes in the urgent pointer correspond
to data being written by a sending application, the urgent pointer
can not "recede" in the sequence space, but a TCP receiver should be
robust to invalid urgent pointer values.
A TCP implementation MUST support a sequence of urgent data of any
length (MUST-31). [20]
The urgent pointer MUST point to the sequence number of the octet
following the urgent data (MUST-62).
A TCP implementation MUST (MUST-32) inform the application layer
asynchronously whenever it receives an Urgent pointer and there was
previously no pending urgent data, or whenever the Urgent pointer
advances in the data stream. The TCP implementation MUST (MUST-33)
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provide a way for the application to learn how much urgent data
remains to be read from the connection, or at least to determine
whether more urgent data remains to be read [20].
3.8.6. Managing the Window
The window sent in each segment indicates the range of sequence
numbers the sender of the window (the data receiver) is currently
prepared to accept. There is an assumption that this is related to
the currently available data buffer space available for this
connection.
The sending TCP endpoint packages the data to be transmitted into
segments that fit the current window, and may repackage segments on
the retransmission queue. Such repackaging is not required, but may
be helpful.
In a connection with a one-way data flow, the window information will
be carried in acknowledgment segments that all have the same sequence
number, so there will be no way to reorder them if they arrive out of
order. This is not a serious problem, but it will allow the window
information to be on occasion temporarily based on old reports from
the data receiver. A refinement to avoid this problem is to act on
the window information from segments that carry the highest
acknowledgment number (that is segments with acknowledgment number
equal or greater than the highest previously received).
Indicating a large window encourages transmissions. If more data
arrives than can be accepted, it will be discarded. This will result
in excessive retransmissions, adding unnecessarily to the load on the
network and the TCP endpoints. Indicating a small window may
restrict the transmission of data to the point of introducing a round
trip delay between each new segment transmitted.
The mechanisms provided allow a TCP endpoint to advertise a large
window and to subsequently advertise a much smaller window without
having accepted that much data. This, so-called "shrinking the
window," is strongly discouraged. The robustness principle [20]
dictates that TCP peers will not shrink the window themselves, but
will be prepared for such behavior on the part of other TCP peers.
A TCP receiver SHOULD NOT shrink the window, i.e., move the right
window edge to the left (SHLD-14). However, a sending TCP peer MUST
be robust against window shrinking, which may cause the "usable
window" (see Section 3.8.6.2.1) to become negative (MUST-34).
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If this happens, the sender SHOULD NOT send new data (SHLD-15), but
SHOULD retransmit normally the old unacknowledged data between
SND.UNA and SND.UNA+SND.WND (SHLD-16). The sender MAY also
retransmit old data beyond SND.UNA+SND.WND (MAY-7), but SHOULD NOT
time out the connection if data beyond the right window edge is not
acknowledged (SHLD-17). If the window shrinks to zero, the TCP
implementation MUST probe it in the standard way (described below)
(MUST-35).
3.8.6.1. Zero Window Probing
The sending TCP peer must regularly transmit at least one octet of
new data (if available) or retransmit to the receiving TCP peer even
if the send window is zero, in order to "probe" the window. This
retransmission is essential to guarantee that when either TCP peer
has a zero window the re-opening of the window will be reliably
reported to the other. This is referred to as Zero-Window Probing
(ZWP) in other documents.
Probing of zero (offered) windows MUST be supported (MUST-36).
A TCP implementation MAY keep its offered receive window closed
indefinitely (MAY-8). As long as the receiving TCP peer continues to
send acknowledgments in response to the probe segments, the sending
TCP peer MUST allow the connection to stay open (MUST-37). This
enables TCP to function in scenarios such as the "printer ran out of
paper" situation described in Section 4.2.2.17 of [20]. The behavior
is subject to the implementation's resource management concerns, as
noted in [42].
When the receiving TCP peer has a zero window and a segment arrives
it must still send an acknowledgment showing its next expected
sequence number and current window (zero).
The transmitting host SHOULD send the first zero-window probe when a
zero window has existed for the retransmission timeout period (SHLD-
29) (Section 3.8.1), and SHOULD increase exponentially the interval
between successive probes (SHLD-30).
3.8.6.2. Silly Window Syndrome Avoidance
The "Silly Window Syndrome" (SWS) is a stable pattern of small
incremental window movements resulting in extremely poor TCP
performance. Algorithms to avoid SWS are described below for both
the sending side and the receiving side. RFC 1122 contains more
detailed discussion of the SWS problem. Note that the Nagle
algorithm and the sender SWS avoidance algorithm play complementary
roles in improving performance. The Nagle algorithm discourages
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sending tiny segments when the data to be sent increases in small
increments, while the SWS avoidance algorithm discourages small
segments resulting from the right window edge advancing in small
increments.
3.8.6.2.1. Sender's Algorithm - When to Send Data
A TCP implementation MUST include a SWS avoidance algorithm in the
sender (MUST-38).
The Nagle algorithm from Section 3.7.4 additionally describes how to
coalesce short segments.
The sender's SWS avoidance algorithm is more difficult than the
receiver's, because the sender does not know (directly) the
receiver's total buffer space RCV.BUFF. An approach that has been
found to work well is for the sender to calculate Max(SND.WND), the
maximum send window it has seen so far on the connection, and to use
this value as an estimate of RCV.BUFF. Unfortunately, this can only
be an estimate; the receiver may at any time reduce the size of
RCV.BUFF. To avoid a resulting deadlock, it is necessary to have a
timeout to force transmission of data, overriding the SWS avoidance
algorithm. In practice, this timeout should seldom occur.
The "usable window" is:
U = SND.UNA + SND.WND - SND.NXT
i.e., the offered window less the amount of data sent but not
acknowledged. If D is the amount of data queued in the sending TCP
endpoint but not yet sent, then the following set of rules is
recommended.
Send data:
(1) if a maximum-sized segment can be sent, i.e., if:
min(D,U) >= Eff.snd.MSS;
(2) or if the data is pushed and all queued data can be sent now,
i.e., if:
[SND.NXT = SND.UNA and] PUSHED and D <= U
(the bracketed condition is imposed by the Nagle algorithm);
(3) or if at least a fraction Fs of the maximum window can be sent,
i.e., if:
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[SND.NXT = SND.UNA and]
min(D,U) >= Fs * Max(SND.WND);
(4) or if the override timeout occurs.
Here Fs is a fraction whose recommended value is 1/2. The override
timeout should be in the range 0.1 - 1.0 seconds. It may be
convenient to combine this timer with the timer used to probe zero
windows (Section 3.8.6.1).
3.8.6.2.2. Receiver's Algorithm - When to Send a Window Update
A TCP implementation MUST include a SWS avoidance algorithm in the
receiver (MUST-39).
The receiver's SWS avoidance algorithm determines when the right
window edge may be advanced; this is customarily known as "updating
the window". This algorithm combines with the delayed ACK algorithm
(Section 3.8.6.3) to determine when an ACK segment containing the
current window will really be sent to the receiver.
The solution to receiver SWS is to avoid advancing the right window
edge RCV.NXT+RCV.WND in small increments, even if data is received
from the network in small segments.
Suppose the total receive buffer space is RCV.BUFF. At any given
moment, RCV.USER octets of this total may be tied up with data that
has been received and acknowledged but that the user process has not
yet consumed. When the connection is quiescent, RCV.WND = RCV.BUFF
and RCV.USER = 0.
Keeping the right window edge fixed as data arrives and is
acknowledged requires that the receiver offer less than its full
buffer space, i.e., the receiver must specify a RCV.WND that keeps
RCV.NXT+RCV.WND constant as RCV.NXT increases. Thus, the total
buffer space RCV.BUFF is generally divided into three parts:
|<------- RCV.BUFF ---------------->|
1 2 3
----|---------|------------------|------|----
RCV.NXT ^
(Fixed)
1 - RCV.USER = data received but not yet consumed;
2 - RCV.WND = space advertised to sender;
3 - Reduction = space available but not yet
advertised.
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The suggested SWS avoidance algorithm for the receiver is to keep
RCV.NXT+RCV.WND fixed until the reduction satisfies:
RCV.BUFF - RCV.USER - RCV.WND >=
min( Fr * RCV.BUFF, Eff.snd.MSS )
where Fr is a fraction whose recommended value is 1/2, and
Eff.snd.MSS is the effective send MSS for the connection (see
Section 3.7.1). When the inequality is satisfied, RCV.WND is set to
RCV.BUFF-RCV.USER.
Note that the general effect of this algorithm is to advance RCV.WND
in increments of Eff.snd.MSS (for realistic receive buffers:
Eff.snd.MSS < RCV.BUFF/2). Note also that the receiver must use its
own Eff.snd.MSS, making the assumption that it is the same as the
sender's.
3.8.6.3. Delayed Acknowledgements - When to Send an ACK Segment
A host that is receiving a stream of TCP data segments can increase
efficiency in both the Internet and the hosts by sending fewer than
one ACK (acknowledgment) segment per data segment received; this is
known as a "delayed ACK".
A TCP endpoint SHOULD implement a delayed ACK (SHLD-18), but an ACK
should not be excessively delayed; in particular, the delay MUST be
less than 0.5 seconds (MUST-40). An ACK SHOULD be generated for at
least every second full-sized segment or 2*RMSS bytes of new data
(where RMSS is the MSS specified by the TCP endpoint receiving the
segments to be acknowledged, or the default value if not specified)
(SHLD-19). Excessive delays on ACKs can disturb the round-trip
timing and packet "clocking" algorithms. More complete discussion of
delayed ACK behavior is in Section 4.2 of RFC 5681 [8], including
recommendations to immediately acknowledge out-of-order segments,
segments above a gap in sequence space, or segments that fill all or
part of a gap, in order to accelerate loss recovery.
Note that there are several current practices that further lead to a
reduced number of ACKs, including generic receive offload (GRO) [73],
ACK compression, and ACK decimation [29].
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3.9. Interfaces
There are of course two interfaces of concern: the user/TCP interface
and the TCP/lower level interface. We have a fairly elaborate model
of the user/TCP interface, but the interface to the lower level
protocol module is left unspecified here, since it will be specified
in detail by the specification of the lower level protocol. For the
case that the lower level is IP we note some of the parameter values
that TCP implementations might use.
3.9.1. User/TCP Interface
The following functional description of user commands to the TCP
implementation is, at best, fictional, since every operating system
will have different facilities. Consequently, we must warn readers
that different TCP implementations may have different user
interfaces. However, all TCP implementations must provide a certain
minimum set of services to guarantee that all TCP implementations can
support the same protocol hierarchy. This section specifies the
functional interfaces required of all TCP implementations.
Section 3.1 of [54] also identifies primitives provided by TCP, and
could be used as an additional reference for implementers.
The following sections functionally characterize a USER/TCP
interface. The notation used is similar to most procedure or
function calls in high level languages, but this usage is not meant
to rule out trap type service calls.
The user commands described below specify the basic functions the TCP
implementation must perform to support interprocess communication.
Individual implementations must define their own exact format, and
may provide combinations or subsets of the basic functions in single
calls. In particular, some implementations may wish to automatically
OPEN a connection on the first SEND or RECEIVE issued by the user for
a given connection.
In providing interprocess communication facilities, the TCP
implementation must not only accept commands, but must also return
information to the processes it serves. The latter consists of:
(a) general information about a connection (e.g., interrupts,
remote close, binding of unspecified remote socket).
(b) replies to specific user commands indicating success or
various types of failure.
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3.9.1.1. Open
Format: OPEN (local port, remote socket, active/passive [,
timeout] [, DiffServ field] [, security/compartment] [local IP
address,] [, options]) -> local connection name
If the active/passive flag is set to passive, then this is a call
to LISTEN for an incoming connection. A passive open may have
either a fully specified remote socket to wait for a particular
connection or an unspecified remote socket to wait for any call.
A fully specified passive call can be made active by the
subsequent execution of a SEND.
A transmission control block (TCB) is created and partially filled
in with data from the OPEN command parameters.
Every passive OPEN call either creates a new connection record in
LISTEN state, or it returns an error; it MUST NOT affect any
previously created connection record (MUST-41).
A TCP implementation that supports multiple concurrent connections
MUST provide an OPEN call that will functionally allow an
application to LISTEN on a port while a connection block with the
same local port is in SYN-SENT or SYN-RECEIVED state (MUST-42).
On an active OPEN command, the TCP endpoint will begin the
procedure to synchronize (i.e., establish) the connection at once.
The timeout, if present, permits the caller to set up a timeout
for all data submitted to TCP. If data is not successfully
delivered to the destination within the timeout period, the TCP
endpoint will abort the connection. The present global default is
five minutes.
The TCP implementation or some component of the operating system
will verify the user's authority to open a connection with the
specified DiffServ field value or security/compartment. The
absence of a DiffServ field value or security/compartment
specification in the OPEN call indicates the default values must
be used.
TCP will accept incoming requests as matching only if the
security/compartment information is exactly the same as that
requested in the OPEN call.
The DiffServ field value indicated by the user only impacts
outgoing packets, may be altered en route through the network, and
has no direct bearing or relation to received packets.
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A local connection name will be returned to the user by the TCP
implementation. The local connection name can then be used as a
short-hand term for the connection defined by the <local socket,
remote socket> pair.
The optional "local IP address" parameter MUST be supported to
allow the specification of the local IP address (MUST-43). This
enables applications that need to select the local IP address used
when multihoming is present.
A passive OPEN call with a specified "local IP address" parameter
will await an incoming connection request to that address. If the
parameter is unspecified, a passive OPEN will await an incoming
connection request to any local IP address, and then bind the
local IP address of the connection to the particular address that
is used.
For an active OPEN call, a specified "local IP address" parameter
will be used for opening the connection. If the parameter is
unspecified, the host will choose an appropriate local IP address
(see RFC 1122 section 3.3.4.2).
If an application on a multihomed host does not specify the local
IP address when actively opening a TCP connection, then the TCP
implementation MUST ask the IP layer to select a local IP address
before sending the (first) SYN (MUST-44). See the function
GET_SRCADDR() in Section 3.4 of RFC 1122.
At all other times, a previous segment has either been sent or
received on this connection, and TCP implementations MUST use the
same local address that was used in those previous segments (MUST-
45).
A TCP implementation MUST reject as an error a local OPEN call for
an invalid remote IP address (e.g., a broadcast or multicast
address) (MUST-46).
3.9.1.2. Send
Format: SEND (local connection name, buffer address, byte count,
PUSH flag (optional), URGENT flag [,timeout])
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This call causes the data contained in the indicated user buffer
to be sent on the indicated connection. If the connection has not
been opened, the SEND is considered an error. Some
implementations may allow users to SEND first; in which case, an
automatic OPEN would be done. For example, this might be one way
for application data to be included in SYN segments. If the
calling process is not authorized to use this connection, an error
is returned.
A TCP endpoint MAY implement PUSH flags on SEND calls (MAY-15).
If PUSH flags are not implemented, then the sending TCP peer: (1)
MUST NOT buffer data indefinitely (MUST-60), and (2) MUST set the
PSH bit in the last buffered segment (i.e., when there is no more
queued data to be sent) (MUST-61). The remaining description
below assumes the PUSH flag is supported on SEND calls.
If the PUSH flag is set, the application intends the data to be
transmitted promptly to the receiver, and the PUSH bit will be set
in the last TCP segment created from the buffer.
The PSH bit is not a record marker and is independent of segment
boundaries. The transmitter SHOULD collapse successive bits when
it packetizes data, to send the largest possible segment (SHLD-
27).
If the PUSH flag is not set, the data may be combined with data
from subsequent SENDs for transmission efficiency. When an
application issues a series of SEND calls without setting the PUSH
flag, the TCP implementation MAY aggregate the data internally
without sending it (MAY-16). Note that when the Nagle algorithm
is in use, TCP implementations may buffer the data before sending,
without regard to the PUSH flag (see Section 3.7.4).
An application program is logically required to set the PUSH flag
in a SEND call whenever it needs to force delivery of the data to
avoid a communication deadlock. However, a TCP implementation
SHOULD send a maximum-sized segment whenever possible (SHLD-28),
to improve performance (see Section 3.8.6.2.1).
New applications SHOULD NOT set the URGENT flag [40] due to
implementation differences and middlebox issues (SHLD-13).
If the URGENT flag is set, segments sent to the destination TCP
peer will have the urgent pointer set. The receiving TCP peer
will signal the urgent condition to the receiving process if the
urgent pointer indicates that data preceding the urgent pointer
has not been consumed by the receiving process. The purpose of
urgent is to stimulate the receiver to process the urgent data and
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to indicate to the receiver when all the currently known urgent
data has been received. The number of times the sending user's
TCP implementation signals urgent will not necessarily be equal to
the number of times the receiving user will be notified of the
presence of urgent data.
If no remote socket was specified in the OPEN, but the connection
is established (e.g., because a LISTENing connection has become
specific due to a remote segment arriving for the local socket),
then the designated buffer is sent to the implied remote socket.
Users who make use of OPEN with an unspecified remote socket can
make use of SEND without ever explicitly knowing the remote socket
address.
However, if a SEND is attempted before the remote socket becomes
specified, an error will be returned. Users can use the STATUS
call to determine the status of the connection. Some TCP
implementations may notify the user when an unspecified socket is
bound.
If a timeout is specified, the current user timeout for this
connection is changed to the new one.
In the simplest implementation, SEND would not return control to
the sending process until either the transmission was complete or
the timeout had been exceeded. However, this simple method is
both subject to deadlocks (for example, both sides of the
connection might try to do SENDs before doing any RECEIVEs) and
offers poor performance, so it is not recommended. A more
sophisticated implementation would return immediately to allow the
process to run concurrently with network I/O, and, furthermore, to
allow multiple SENDs to be in progress. Multiple SENDs are served
in first come, first served order, so the TCP endpoint will queue
those it cannot service immediately.
We have implicitly assumed an asynchronous user interface in which
a SEND later elicits some kind of SIGNAL or pseudo-interrupt from
the serving TCP endpoint. An alternative is to return a response
immediately. For instance, SENDs might return immediate local
acknowledgment, even if the segment sent had not been acknowledged
by the distant TCP endpoint. We could optimistically assume
eventual success. If we are wrong, the connection will close
anyway due to the timeout. In implementations of this kind
(synchronous), there will still be some asynchronous signals, but
these will deal with the connection itself, and not with specific
segments or buffers.
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In order for the process to distinguish among error or success
indications for different SENDs, it might be appropriate for the
buffer address to be returned along with the coded response to the
SEND request. TCP-to-user signals are discussed below, indicating
the information that should be returned to the calling process.
3.9.1.3. Receive
Format: RECEIVE (local connection name, buffer address, byte
count) -> byte count, urgent flag, push flag (optional)
This command allocates a receiving buffer associated with the
specified connection. If no OPEN precedes this command or the
calling process is not authorized to use this connection, an error
is returned.
In the simplest implementation, control would not return to the
calling program until either the buffer was filled, or some error
occurred, but this scheme is highly subject to deadlocks. A more
sophisticated implementation would permit several RECEIVEs to be
outstanding at once. These would be filled as segments arrive.
This strategy permits increased throughput at the cost of a more
elaborate scheme (possibly asynchronous) to notify the calling
program that a PUSH has been seen or a buffer filled.
A TCP receiver MAY pass a received PSH flag to the application
layer via the PUSH flag in the interface (MAY-17), but it is not
required (this was clarified in RFC 1122 section 4.2.2.2). The
remainder of text describing the RECEIVE call below assumes that
passing the PUSH indication is supported.
If enough data arrive to fill the buffer before a PUSH is seen,
the PUSH flag will not be set in the response to the RECEIVE. The
buffer will be filled with as much data as it can hold. If a PUSH
is seen before the buffer is filled the buffer will be returned
partially filled and PUSH indicated.
If there is urgent data the user will have been informed as soon
as it arrived via a TCP-to-user signal. The receiving user should
thus be in "urgent mode". If the URGENT flag is on, additional
urgent data remains. If the URGENT flag is off, this call to
RECEIVE has returned all the urgent data, and the user may now
leave "urgent mode". Note that data following the urgent pointer
(non-urgent data) cannot be delivered to the user in the same
buffer with preceding urgent data unless the boundary is clearly
marked for the user.
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To distinguish among several outstanding RECEIVEs and to take care
of the case that a buffer is not completely filled, the return
code is accompanied by both a buffer pointer and a byte count
indicating the actual length of the data received.
Alternative implementations of RECEIVE might have the TCP endpoint
allocate buffer storage, or the TCP endpoint might share a ring
buffer with the user.
3.9.1.4. Close
Format: CLOSE (local connection name)
This command causes the connection specified to be closed. If the
connection is not open or the calling process is not authorized to
use this connection, an error is returned. Closing connections is
intended to be a graceful operation in the sense that outstanding
SENDs will be transmitted (and retransmitted), as flow control
permits, until all have been serviced. Thus, it should be
acceptable to make several SEND calls, followed by a CLOSE, and
expect all the data to be sent to the destination. It should also
be clear that users should continue to RECEIVE on CLOSING
connections, since the remote peer may be trying to transmit the
last of its data. Thus, CLOSE means "I have no more to send" but
does not mean "I will not receive any more." It may happen (if
the user level protocol is not well-thought-out) that the closing
side is unable to get rid of all its data before timing out. In
this event, CLOSE turns into ABORT, and the closing TCP peer gives
up.
The user may CLOSE the connection at any time on their own
initiative, or in response to various prompts from the TCP
implementation (e.g., remote close executed, transmission timeout
exceeded, destination inaccessible).
Because closing a connection requires communication with the
remote TCP peer, connections may remain in the closing state for a
short time. Attempts to reopen the connection before the TCP peer
replies to the CLOSE command will result in error responses.
Close also implies push function.
3.9.1.5. Status
Format: STATUS (local connection name) -> status data
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This is an implementation dependent user command and could be
excluded without adverse effect. Information returned would
typically come from the TCB associated with the connection.
This command returns a data block containing the following
information:
- local socket,
remote socket,
local connection name,
receive window,
send window,
connection state,
number of buffers awaiting acknowledgment,
number of buffers pending receipt,
urgent state,
DiffServ field value,
security/compartment,
and transmission timeout.
Depending on the state of the connection, or on the implementation
itself, some of this information may not be available or
meaningful. If the calling process is not authorized to use this
connection, an error is returned. This prevents unauthorized
processes from gaining information about a connection.
3.9.1.6. Abort
Format: ABORT (local connection name)
This command causes all pending SENDs and RECEIVES to be aborted,
the TCB to be removed, and a special RESET message to be sent to
the remote TCP peer of the connection. Depending on the
implementation, users may receive abort indications for each
outstanding SEND or RECEIVE, or may simply receive an ABORT-
acknowledgment.
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3.9.1.7. Flush
Some TCP implementations have included a FLUSH call, which will
empty the TCP send queue of any data that the user has issued SEND
calls for but is still to the right of the current send window.
That is, it flushes as much queued send data as possible without
losing sequence number synchronization. The FLUSH call MAY be
implemented (MAY-14).
3.9.1.8. Asynchronous Reports
There MUST be a mechanism for reporting soft TCP error conditions
to the application (MUST-47). Generically, we assume this takes
the form of an application-supplied ERROR_REPORT routine that may
be upcalled asynchronously from the transport layer:
- ERROR_REPORT(local connection name, reason, subreason)
The precise encoding of the reason and subreason parameters is not
specified here. However, the conditions that are reported
asynchronously to the application MUST include:
- * ICMP error message arrived (see Section 3.9.2.2 for
description of handling each ICMP message type, since some
message types need to be suppressed from generating reports to
the application)
- * Excessive retransmissions (see Section 3.8.3)
- * Urgent pointer advance (see Section 3.8.5)
However, an application program that does not want to receive such
ERROR_REPORT calls SHOULD be able to effectively disable these
calls (SHLD-20).
3.9.1.9. Set Differentiated Services Field (IPv4 TOS or IPv6 Traffic
Class)
The application layer MUST be able to specify the Differentiated
Services field for segments that are sent on a connection (MUST-
48). The Differentiated Services field includes the 6-bit
Differentiated Services Code Point (DSCP) value. It is not
required, but the application SHOULD be able to change the
Differentiated Services field during the connection lifetime
(SHLD-21). TCP implementations SHOULD pass the current
Differentiated Services field value without change to the IP
layer, when it sends segments on the connection (SHLD-22).
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The Differentiated Services field will be specified independently
in each direction on the connection, so that the receiver
application will specify the Differentiated Services field used
for ACK segments.
TCP implementations MAY pass the most recently received
Differentiated Services field up to the application (MAY-9).
3.9.2. TCP/Lower-Level Interface
The TCP endpoint calls on a lower level protocol module to actually
send and receive information over a network. The two current
standard Internet Protocol (IP) versions layered below TCP are IPv4
[1] and IPv6 [13].
If the lower level protocol is IPv4 it provides arguments for a type
of service (used within the Differentiated Services field) and for a
time to live. TCP uses the following settings for these parameters:
DiffServ field: The IP header value for the DiffServ field is
given by the user. This includes the bits of the DiffServ Code
Point (DSCP).
Time to Live (TTL): The TTL value used to send TCP segments MUST
be configurable (MUST-49).
- Note that RFC 793 specified one minute (60 seconds) as a
constant for the TTL, because the assumed maximum segment
lifetime was two minutes. This was intended to explicitly ask
that a segment be destroyed if it cannot be delivered by the
internet system within one minute. RFC 1122 changed this
specification to require that the TTL be configurable.
- Note that the DiffServ field is permitted to change during a
connection (Section 4.2.4.2 of RFC 1122). However, the
application interface might not support this ability, and the
application does not have knowledge about individual TCP
segments, so this can only be done on a coarse granularity, at
best. This limitation is further discussed in RFC 7657 (sec
5.1, 5.3, and 6) [51]. Generally, an application SHOULD NOT
change the DiffServ field value during the course of a
connection (SHLD-23).
Any lower level protocol will have to provide the source address,
destination address, and protocol fields, and some way to determine
the "TCP length", both to provide the functional equivalent service
of IP and to be used in the TCP checksum.
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When received options are passed up to TCP from the IP layer, a TCP
implementation MUST ignore options that it does not understand (MUST-
50).
A TCP implementation MAY support the Time Stamp (MAY-10) and Record
Route (MAY-11) options.
3.9.2.1. Source Routing
If the lower level is IP (or other protocol that provides this
feature) and source routing is used, the interface must allow the
route information to be communicated. This is especially important
so that the source and destination addresses used in the TCP checksum
be the originating source and ultimate destination. It is also
important to preserve the return route to answer connection requests.
An application MUST be able to specify a source route when it
actively opens a TCP connection (MUST-51), and this MUST take
precedence over a source route received in a datagram (MUST-52).
When a TCP connection is OPENed passively and a packet arrives with a
completed IP Source Route option (containing a return route), TCP
implementations MUST save the return route and use it for all
segments sent on this connection (MUST-53). If a different source
route arrives in a later segment, the later definition SHOULD
override the earlier one (SHLD-24).
3.9.2.2. ICMP Messages
TCP implementations MUST act on an ICMP error message passed up from
the IP layer, directing it to the connection that created the error
(MUST-54). The necessary demultiplexing information can be found in
the IP header contained within the ICMP message.
This applies to ICMPv6 in addition to IPv4 ICMP.
[36] contains discussion of specific ICMP and ICMPv6 messages
classified as either "soft" or "hard" errors that may bear different
responses. Treatment for classes of ICMP messages is described
below:
Source Quench
TCP implementations MUST silently discard any received ICMP Source
Quench messages (MUST-55). See [11] for discussion.
Soft Errors
For IPv4 ICMP these include: Destination Unreachable -- codes 0, 1,
5; Time Exceeded -- codes 0, 1; and Parameter Problem.
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For ICMPv6 these include: Destination Unreachable -- codes 0, 3;
Time Exceeded -- codes 0, 1; and Parameter Problem -- codes 0, 1,
2.
Since these Unreachable messages indicate soft error conditions,
TCP implementations MUST NOT abort the connection (MUST-56), and it
SHOULD make the information available to the application (SHLD-25).
Hard Errors
For ICMP these include Destination Unreachable -- codes 2-4.
These are hard error conditions, so TCP implementations SHOULD
abort the connection (SHLD-26). [36] notes that some
implementations do not abort connections when an ICMP hard error is
received for a connection that is in any of the synchronized
states.
Note that [36] section 4 describes widespread implementation behavior
that treats soft errors as hard errors during connection
establishment.
3.9.2.3. Source Address Validation
RFC 1122 requires addresses to be validated in incoming SYN packets:
An incoming SYN with an invalid source address MUST be ignored
either by TCP or by the IP layer (MUST-63) (Section 3.2.1.3 of
[20]).
A TCP implementation MUST silently discard an incoming SYN segment
that is addressed to a broadcast or multicast address (MUST-57).
This prevents connection state and replies from being erroneously
generated, and implementers should note that this guidance is
applicable to all incoming segments, not just SYNs, as specifically
indicated in RFC 1122.
3.10. Event Processing
The processing depicted in this section is an example of one possible
implementation. Other implementations may have slightly different
processing sequences, but they should differ from those in this
section only in detail, not in substance.
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The activity of the TCP endpoint can be characterized as responding
to events. The events that occur can be cast into three categories:
user calls, arriving segments, and timeouts. This section describes
the processing the TCP endpoint does in response to each of the
events. In many cases the processing required depends on the state
of the connection.
Events that occur:
User Calls
- OPEN
SEND
RECEIVE
CLOSE
ABORT
STATUS
Arriving Segments
- SEGMENT ARRIVES
Timeouts
- USER TIMEOUT
RETRANSMISSION TIMEOUT
TIME-WAIT TIMEOUT
The model of the TCP/user interface is that user commands receive an
immediate return and possibly a delayed response via an event or
pseudo interrupt. In the following descriptions, the term "signal"
means cause a delayed response.
Error responses in this document are identified by character strings.
For example, user commands referencing connections that do not exist
receive "error: connection not open".
Please note in the following that all arithmetic on sequence numbers,
acknowledgment numbers, windows, et cetera, is modulo 2**32 (the size
of the sequence number space). Also note that "=<" means less than
or equal to (modulo 2**32).
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A natural way to think about processing incoming segments is to
imagine that they are first tested for proper sequence number (i.e.,
that their contents lie in the range of the expected "receive window"
in the sequence number space) and then that they are generally queued
and processed in sequence number order.
When a segment overlaps other already received segments we
reconstruct the segment to contain just the new data, and adjust the
header fields to be consistent.
Note that if no state change is mentioned the TCP connection stays in
the same state.
3.10.1. OPEN Call
CLOSED STATE (i.e., TCB does not exist)
- Create a new transmission control block (TCB) to hold
connection state information. Fill in local socket identifier,
remote socket, DiffServ field, security/compartment, and user
timeout information. Note that some parts of the remote socket
may be unspecified in a passive OPEN and are to be filled in by
the parameters of the incoming SYN segment. Verify the
security and DiffServ value requested are allowed for this
user, if not return "error: DiffServ value not allowed" or
"error: security/compartment not allowed." If passive enter
the LISTEN state and return. If active and the remote socket
is unspecified, return "error: remote socket unspecified"; if
active and the remote socket is specified, issue a SYN segment.
An initial send sequence number (ISS) is selected. A SYN
segment of the form <SEQ=ISS><CTL=SYN> is sent. Set SND.UNA to
ISS, SND.NXT to ISS+1, enter SYN-SENT state, and return.
- If the caller does not have access to the local socket
specified, return "error: connection illegal for this process".
If there is no room to create a new connection, return "error:
insufficient resources".
LISTEN STATE
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- If the OPEN call is active and the remote socket is specified,
then change the connection from passive to active, select an
ISS. Send a SYN segment, set SND.UNA to ISS, SND.NXT to ISS+1.
Enter SYN-SENT state. Data associated with SEND may be sent
with SYN segment or queued for transmission after entering
ESTABLISHED state. The urgent bit if requested in the command
must be sent with the data segments sent as a result of this
command. If there is no room to queue the request, respond
with "error: insufficient resources". If the remote socket was
not specified, then return "error: remote socket unspecified".
SYN-SENT STATE
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
- Return "error: connection already exists".
3.10.2. SEND Call
CLOSED STATE (i.e., TCB does not exist)
- If the user does not have access to such a connection, then
return "error: connection illegal for this process".
- Otherwise, return "error: connection does not exist".
LISTEN STATE
- If the remote socket is specified, then change the connection
from passive to active, select an ISS. Send a SYN segment, set
SND.UNA to ISS, SND.NXT to ISS+1. Enter SYN-SENT state. Data
associated with SEND may be sent with SYN segment or queued for
transmission after entering ESTABLISHED state. The urgent bit
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if requested in the command must be sent with the data segments
sent as a result of this command. If there is no room to queue
the request, respond with "error: insufficient resources". If
the remote socket was not specified, then return "error: remote
socket unspecified".
SYN-SENT STATE
SYN-RECEIVED STATE
- Queue the data for transmission after entering ESTABLISHED
state. If no space to queue, respond with "error: insufficient
resources".
ESTABLISHED STATE
CLOSE-WAIT STATE
- Segmentize the buffer and send it with a piggybacked
acknowledgment (acknowledgment value = RCV.NXT). If there is
insufficient space to remember this buffer, simply return
"error: insufficient resources".
- If the urgent flag is set, then SND.UP <- SND.NXT and set the
urgent pointer in the outgoing segments.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
- Return "error: connection closing" and do not service request.
3.10.3. RECEIVE Call
CLOSED STATE (i.e., TCB does not exist)
- If the user does not have access to such a connection, return
"error: connection illegal for this process".
- Otherwise return "error: connection does not exist".
LISTEN STATE
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SYN-SENT STATE
SYN-RECEIVED STATE
- Queue for processing after entering ESTABLISHED state. If
there is no room to queue this request, respond with "error:
insufficient resources".
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
- If insufficient incoming segments are queued to satisfy the
request, queue the request. If there is no queue space to
remember the RECEIVE, respond with "error: insufficient
resources".
- Reassemble queued incoming segments into receive buffer and
return to user. Mark "push seen" (PUSH) if this is the case.
- If RCV.UP is in advance of the data currently being passed to
the user notify the user of the presence of urgent data.
- When the TCP endpoint takes responsibility for delivering data
to the user that fact must be communicated to the sender via an
acknowledgment. The formation of such an acknowledgment is
described below in the discussion of processing an incoming
segment.
CLOSE-WAIT STATE
- Since the remote side has already sent FIN, RECEIVEs must be
satisfied by data already on hand, but not yet delivered to the
user. If no text is awaiting delivery, the RECEIVE will get an
"error: connection closing" response. Otherwise, any remaining
data can be used to satisfy the RECEIVE.
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
- Return "error: connection closing".
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3.10.4. CLOSE Call
CLOSED STATE (i.e., TCB does not exist)
- If the user does not have access to such a connection, return
"error: connection illegal for this process".
- Otherwise, return "error: connection does not exist".
LISTEN STATE
- Any outstanding RECEIVEs are returned with "error: closing"
responses. Delete TCB, enter CLOSED state, and return.
SYN-SENT STATE
- Delete the TCB and return "error: closing" responses to any
queued SENDs, or RECEIVEs.
SYN-RECEIVED STATE
- If no SENDs have been issued and there is no pending data to
send, then form a FIN segment and send it, and enter FIN-WAIT-1
state; otherwise queue for processing after entering
ESTABLISHED state.
ESTABLISHED STATE
- Queue this until all preceding SENDs have been segmentized,
then form a FIN segment and send it. In any case, enter FIN-
WAIT-1 state.
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
- Strictly speaking, this is an error and should receive an
"error: connection closing" response. An "ok" response would
be acceptable, too, as long as a second FIN is not emitted (the
first FIN may be retransmitted though).
CLOSE-WAIT STATE
- Queue this request until all preceding SENDs have been
segmentized; then send a FIN segment, enter LAST-ACK state.
CLOSING STATE
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LAST-ACK STATE
TIME-WAIT STATE
- Respond with "error: connection closing".
3.10.5. ABORT Call
CLOSED STATE (i.e., TCB does not exist)
- If the user should not have access to such a connection, return
"error: connection illegal for this process".
- Otherwise return "error: connection does not exist".
LISTEN STATE
- Any outstanding RECEIVEs should be returned with "error:
connection reset" responses. Delete TCB, enter CLOSED state,
and return.
SYN-SENT STATE
- All queued SENDs and RECEIVEs should be given "connection
reset" notification, delete the TCB, enter CLOSED state, and
return.
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
- Send a reset segment:
o <SEQ=SND.NXT><CTL=RST>
- All queued SENDs and RECEIVEs should be given "connection
reset" notification; all segments queued for transmission
(except for the RST formed above) or retransmission should be
flushed, delete the TCB, enter CLOSED state, and return.
CLOSING STATE LAST-ACK STATE TIME-WAIT STATE
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- Respond with "ok" and delete the TCB, enter CLOSED state, and
return.
3.10.6. STATUS Call
CLOSED STATE (i.e., TCB does not exist)
- If the user should not have access to such a connection, return
"error: connection illegal for this process".
- Otherwise return "error: connection does not exist".
LISTEN STATE
- Return "state = LISTEN", and the TCB pointer.
SYN-SENT STATE
- Return "state = SYN-SENT", and the TCB pointer.
SYN-RECEIVED STATE
- Return "state = SYN-RECEIVED", and the TCB pointer.
ESTABLISHED STATE
- Return "state = ESTABLISHED", and the TCB pointer.
FIN-WAIT-1 STATE
- Return "state = FIN-WAIT-1", and the TCB pointer.
FIN-WAIT-2 STATE
- Return "state = FIN-WAIT-2", and the TCB pointer.
CLOSE-WAIT STATE
- Return "state = CLOSE-WAIT", and the TCB pointer.
CLOSING STATE
- Return "state = CLOSING", and the TCB pointer.
LAST-ACK STATE
- Return "state = LAST-ACK", and the TCB pointer.
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TIME-WAIT STATE
- Return "state = TIME-WAIT", and the TCB pointer.
3.10.7. SEGMENT ARRIVES
3.10.7.1. CLOSED State
If the state is CLOSED (i.e., TCB does not exist) then
all data in the incoming segment is discarded. An incoming
segment containing a RST is discarded. An incoming segment not
containing a RST causes a RST to be sent in response. The
acknowledgment and sequence field values are selected to make the
reset sequence acceptable to the TCP endpoint that sent the
offending segment.
If the ACK bit is off, sequence number zero is used,
- <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the ACK bit is on,
- <SEQ=SEG.ACK><CTL=RST>
Return.
3.10.7.2. LISTEN State
If the state is LISTEN then
first check for an RST
- An incoming RST segment could not be valid, since it could not
have been sent in response to anything sent by this incarnation
of the connection. An incoming RST should be ignored. Return.
second check for an ACK
- Any acknowledgment is bad if it arrives on a connection still
in the LISTEN state. An acceptable reset segment should be
formed for any arriving ACK-bearing segment. The RST should be
formatted as follows:
o <SEQ=SEG.ACK><CTL=RST>
- Return.
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third check for a SYN
- If the SYN bit is set, check the security. If the security/
compartment on the incoming segment does not exactly match the
security/compartment in the TCB then send a reset and return.
o <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
- Set RCV.NXT to SEG.SEQ+1, IRS is set to SEG.SEQ and any other
control or text should be queued for processing later. ISS
should be selected and a SYN segment sent of the form:
o <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
- SND.NXT is set to ISS+1 and SND.UNA to ISS. The connection
state should be changed to SYN-RECEIVED. Note that any other
incoming control or data (combined with SYN) will be processed
in the SYN-RECEIVED state, but processing of SYN and ACK should
not be repeated. If the listen was not fully specified (i.e.,
the remote socket was not fully specified), then the
unspecified fields should be filled in now.
fourth other data or control
- This should not be reached. Drop the segment and return. Any
other control or data-bearing segment (not containing SYN) must
have an ACK and thus would have been discarded by the ACK
processing in the second step, unless it was first discarded by
RST checking in the first step.
3.10.7.3. SYN-SENT State
If the state is SYN-SENT then
first check the ACK bit
- If the ACK bit is set
o If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send a reset
(unless the RST bit is set, if so drop the segment and
return)
+ <SEQ=SEG.ACK><CTL=RST>
o and discard the segment. Return.
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o If SND.UNA < SEG.ACK =< SND.NXT then the ACK is acceptable.
Some deployed TCP code has used the check SEG.ACK == SND.NXT
(using "==" rather than "=<", but this is not appropriate
when the stack is capable of sending data on the SYN,
because the TCP peer may not accept and acknowledge all of
the data on the SYN.
second check the RST bit
- If the RST bit is set
o A potential blind reset attack is described in RFC 5961 [9].
The mitigation described in that document has specific
applicability explained therein, and is not a substitute for
cryptographic protection (e.g. IPsec or TCP-AO). A TCP
implementation that supports the RFC 5961 mitigation SHOULD
first check that the sequence number exactly matches RCV.NXT
prior to executing the action in the next paragraph.
o If the ACK was acceptable then signal the user "error:
connection reset", drop the segment, enter CLOSED state,
delete TCB, and return. Otherwise (no ACK), drop the
segment and return.
third check the security
- If the security/compartment in the segment does not exactly
match the security/compartment in the TCB, send a reset
o If there is an ACK
+ <SEQ=SEG.ACK><CTL=RST>
o Otherwise
+ <SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
- If a reset was sent, discard the segment and return.
fourth check the SYN bit
- This step should be reached only if the ACK is ok, or there is
no ACK, and the segment did not contain a RST.
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- If the SYN bit is on and the security/compartment is acceptable
then, RCV.NXT is set to SEG.SEQ+1, IRS is set to SEG.SEQ.
SND.UNA should be advanced to equal SEG.ACK (if there is an
ACK), and any segments on the retransmission queue that are
thereby acknowledged should be removed.
- If SND.UNA > ISS (our SYN has been ACKed), change the
connection state to ESTABLISHED, form an ACK segment
o <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
- and send it. Data or controls that were queued for
transmission MAY be included. Some TCP implementations
suppress sending this segment when the received segment
contains data that will anyways generate an acknowledgement in
the later processing steps, saving this extra acknowledgement
of the SYN from being sent. If there are other controls or
text in the segment then continue processing at the sixth step
under Section 3.10.7.4 where the URG bit is checked, otherwise
return.
- Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
o <SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
- and send it. Set the variables:
o SND.WND <- SEG.WND
SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACK
If there are other controls or text in the segment, queue them
for processing after the ESTABLISHED state has been reached,
return.
- Note that it is legal to send and receive application data on
SYN segments (this is the "text in the segment" mentioned
above. There has been significant misinformation and
misunderstanding of this topic historically. Some firewalls
and security devices consider this suspicious. However, the
capability was used in T/TCP [22] and is used in TCP Fast Open
(TFO) [49], so is important for implementations and network
devices to permit.
fifth, if neither of the SYN or RST bits is set then drop the
segment and return.
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3.10.7.4. Other States
Otherwise,
first check sequence number
- SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
o Segments are processed in sequence. Initial tests on
arrival are used to discard old duplicates, but further
processing is done in SEG.SEQ order. If a segment's
contents straddle the boundary between old and new, only the
new parts are processed.
o In general, the processing of received segments MUST be
implemented to aggregate ACK segments whenever possible
(MUST-58). For example, if the TCP endpoint is processing a
series of queued segments, it MUST process them all before
sending any ACK segments (MUST-59).
o There are four cases for the acceptability test for an
incoming segment:
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Segment Receive Test
Length Window
------- ------- -------------------------------------------
0 0 SEG.SEQ = RCV.NXT
0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
>0 0 not acceptable
>0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
o In implementing sequence number validation as described
here, please note Appendix A.2.
o If the RCV.WND is zero, no segments will be acceptable, but
special allowance should be made to accept valid ACKs, URGs
and RSTs.
o If an incoming segment is not acceptable, an acknowledgment
should be sent in reply (unless the RST bit is set, if so
drop the segment and return):
+ <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
o After sending the acknowledgment, drop the unacceptable
segment and return.
o Note that for the TIME-WAIT state, there is an improved
algorithm described in [41] for handling incoming SYN
segments, that utilizes timestamps rather than relying on
the sequence number check described here. When the improved
algorithm is implemented, the logic above is not applicable
for incoming SYN segments with timestamp options, received
on a connection in the TIME-WAIT state.
o In the following it is assumed that the segment is the
idealized segment that begins at RCV.NXT and does not exceed
the window. One could tailor actual segments to fit this
assumption by trimming off any portions that lie outside the
window (including SYN and FIN), and only processing further
if the segment then begins at RCV.NXT. Segments with higher
beginning sequence numbers SHOULD be held for later
processing (SHLD-31).
- second check the RST bit,
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o RFC 5961 [9] section 3 describes a potential blind reset
attack and optional mitigation approach. This does not
provide a cryptographic protection (e.g. as in IPsec or TCP-
AO), but can be applicable in situations described in RFC
5961. For stacks implementing the RFC 5961 protection, the
three checks below apply, otherwise processing for these
states is indicated further below.
+ 1) If the RST bit is set and the sequence number is
outside the current receive window, silently drop the
segment.
+ 2) If the RST bit is set and the sequence number exactly
matches the next expected sequence number (RCV.NXT), then
TCP endpoints MUST reset the connection in the manner
prescribed below according to the connection state.
+ 3) If the RST bit is set and the sequence number does not
exactly match the next expected sequence value, yet is
within the current receive window, TCP endpoints MUST
send an acknowledgement (challenge ACK):
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
After sending the challenge ACK, TCP endpoints MUST drop
the unacceptable segment and stop processing the incoming
packet further. Note that RFC 5961 and Errata ID 4772
contain additional considerations for ACK throttling in
an implementation.
o SYN-RECEIVED STATE
+ If the RST bit is set
* If this connection was initiated with a passive OPEN
(i.e., came from the LISTEN state), then return this
connection to LISTEN state and return. The user need
not be informed. If this connection was initiated
with an active OPEN (i.e., came from SYN-SENT state)
then the connection was refused, signal the user
"connection refused". In either case, the
retransmission queue should be flushed. And in the
active OPEN case, enter the CLOSED state and delete
the TCB, and return.
o ESTABLISHED
FIN-WAIT-1
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FIN-WAIT-2
CLOSE-WAIT
+ If the RST bit is set then, any outstanding RECEIVEs and
SEND should receive "reset" responses. All segment
queues should be flushed. Users should also receive an
unsolicited general "connection reset" signal. Enter the
CLOSED state, delete the TCB, and return.
o CLOSING STATE
LAST-ACK STATE
TIME-WAIT
+ If the RST bit is set then, enter the CLOSED state,
delete the TCB, and return.
- third check security
o SYN-RECEIVED
+ If the security/compartment in the segment does not
exactly match the security/compartment in the TCB then
send a reset, and return.
o ESTABLISHED
FIN-WAIT-1
FIN-WAIT-2
CLOSE-WAIT
CLOSING
LAST-ACK
TIME-WAIT
+ If the security/compartment in the segment does not
exactly match the security/compartment in the TCB then
send a reset, any outstanding RECEIVEs and SEND should
receive "reset" responses. All segment queues should be
flushed. Users should also receive an unsolicited
general "connection reset" signal. Enter the CLOSED
state, delete the TCB, and return.
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o Note this check is placed following the sequence check to
prevent a segment from an old connection between these port
numbers with a different security from causing an abort of
the current connection.
- fourth, check the SYN bit,
o SYN-RECEIVED
+ If the connection was initiated with a passive OPEN, then
return this connection to the LISTEN state and return.
Otherwise, handle per the directions for synchronized
states below.
ESTABLISHED STATE
FIN-WAIT STATE-1
FIN-WAIT STATE-2
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
+ If the SYN bit is set in these synchronized states, it
may be either a legitimate new connection attempt (e.g.
in the case of TIME-WAIT), an error where the connection
should be reset, or the result of an attack attempt, as
described in RFC 5961 [9]. For the TIME-WAIT state, new
connections can be accepted if the timestamp option is
used and meets expectations (per [41]). For all other
cases, RFC 5961 provides a mitigation with applicability
to some situations, though there are also alternatives
that offer cryptographic protection (see Section 7). RFC
5961 recommends that in these synchronized states, if the
SYN bit is set, irrespective of the sequence number, TCP
endpoints MUST send a "challenge ACK" to the remote peer:
+ <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
+ After sending the acknowledgement, TCP implementations
MUST drop the unacceptable segment and stop processing
further. Note that RFC 5961 and Errata ID 4772 contain
additional ACK throttling notes for an implementation.
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+ For implementations that do not follow RFC 5961, the
original RFC 793 behavior follows in this paragraph. If
the SYN is in the window it is an error, send a reset,
any outstanding RECEIVEs and SEND should receive "reset"
responses, all segment queues should be flushed, the user
should also receive an unsolicited general "connection
reset" signal, enter the CLOSED state, delete the TCB,
and return.
+ If the SYN is not in the window this step would not be
reached and an ACK would have been sent in the first step
(sequence number check).
- fifth check the ACK field,
o if the ACK bit is off drop the segment and return
o if the ACK bit is on
+ RFC 5961 [9] section 5 describes a potential blind data
injection attack, and mitigation that implementations MAY
choose to include (MAY-12). TCP stacks that implement
RFC 5961 MUST add an input check that the ACK value is
acceptable only if it is in the range of ((SND.UNA -
MAX.SND.WND) =< SEG.ACK =< SND.NXT). All incoming
segments whose ACK value doesn't satisfy the above
condition MUST be discarded and an ACK sent back. The
new state variable MAX.SND.WND is defined as the largest
window that the local sender has ever received from its
peer (subject to window scaling) or may be hard-coded to
a maximum permissible window value. When the ACK value
is acceptable, the processing per-state below applies:
+ SYN-RECEIVED STATE
* If SND.UNA < SEG.ACK =< SND.NXT then enter ESTABLISHED
state and continue processing with variables below set
to:
- SND.WND <- SEG.WND
SND.WL1 <- SEG.SEQ
SND.WL2 <- SEG.ACK
* If the segment acknowledgment is not acceptable, form
a reset segment,
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- <SEQ=SEG.ACK><CTL=RST>
* and send it.
+ ESTABLISHED STATE
* If SND.UNA < SEG.ACK =< SND.NXT then, set SND.UNA <-
SEG.ACK. Any segments on the retransmission queue
that are thereby entirely acknowledged are removed.
Users should receive positive acknowledgments for
buffers that have been SENT and fully acknowledged
(i.e., SEND buffer should be returned with "ok"
response). If the ACK is a duplicate (SEG.ACK =<
SND.UNA), it can be ignored. If the ACK acks
something not yet sent (SEG.ACK > SND.NXT) then send
an ACK, drop the segment, and return.
* If SND.UNA =< SEG.ACK =< SND.NXT, the send window
should be updated. If (SND.WL1 < SEG.SEQ or (SND.WL1
= SEG.SEQ and SND.WL2 =< SEG.ACK)), set SND.WND <-
SEG.WND, set SND.WL1 <- SEG.SEQ, and set SND.WL2 <-
SEG.ACK.
* Note that SND.WND is an offset from SND.UNA, that
SND.WL1 records the sequence number of the last
segment used to update SND.WND, and that SND.WL2
records the acknowledgment number of the last segment
used to update SND.WND. The check here prevents using
old segments to update the window.
+ FIN-WAIT-1 STATE
* In addition to the processing for the ESTABLISHED
state, if the FIN segment is now acknowledged then
enter FIN-WAIT-2 and continue processing in that
state.
+ FIN-WAIT-2 STATE
* In addition to the processing for the ESTABLISHED
state, if the retransmission queue is empty, the
user's CLOSE can be acknowledged ("ok") but do not
delete the TCB.
+ CLOSE-WAIT STATE
* Do the same processing as for the ESTABLISHED state.
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+ CLOSING STATE
* In addition to the processing for the ESTABLISHED
state, if the ACK acknowledges our FIN then enter the
TIME-WAIT state, otherwise ignore the segment.
+ LAST-ACK STATE
* The only thing that can arrive in this state is an
acknowledgment of our FIN. If our FIN is now
acknowledged, delete the TCB, enter the CLOSED state,
and return.
+ TIME-WAIT STATE
* The only thing that can arrive in this state is a
retransmission of the remote FIN. Acknowledge it, and
restart the 2 MSL timeout.
- sixth, check the URG bit,
o ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
+ If the URG bit is set, RCV.UP <- max(RCV.UP,SEG.UP), and
signal the user that the remote side has urgent data if
the urgent pointer (RCV.UP) is in advance of the data
consumed. If the user has already been signaled (or is
still in the "urgent mode") for this continuous sequence
of urgent data, do not signal the user again.
o CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT
+ This should not occur, since a FIN has been received from
the remote side. Ignore the URG.
- seventh, process the segment text,
o ESTABLISHED STATE
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FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
+ Once in the ESTABLISHED state, it is possible to deliver
segment data to user RECEIVE buffers. Data from segments
can be moved into buffers until either the buffer is full
or the segment is empty. If the segment empties and
carries a PUSH flag, then the user is informed, when the
buffer is returned, that a PUSH has been received.
+ When the TCP endpoint takes responsibility for delivering
the data to the user it must also acknowledge the receipt
of the data.
+ Once the TCP endpoint takes responsibility for the data
it advances RCV.NXT over the data accepted, and adjusts
RCV.WND as appropriate to the current buffer
availability. The total of RCV.NXT and RCV.WND should
not be reduced.
+ A TCP implementation MAY send an ACK segment
acknowledging RCV.NXT when a valid segment arrives that
is in the window but not at the left window edge (MAY-
13).
+ Please note the window management suggestions in
Section 3.8.
+ Send an acknowledgment of the form:
* <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
+ This acknowledgment should be piggybacked on a segment
being transmitted if possible without incurring undue
delay.
o CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
+ This should not occur, since a FIN has been received from
the remote side. Ignore the segment text.
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- eighth, check the FIN bit,
o Do not process the FIN if the state is CLOSED, LISTEN or
SYN-SENT since the SEG.SEQ cannot be validated; drop the
segment and return.
o If the FIN bit is set, signal the user "connection closing"
and return any pending RECEIVEs with same message, advance
RCV.NXT over the FIN, and send an acknowledgment for the
FIN. Note that FIN implies PUSH for any segment text not
yet delivered to the user.
+ SYN-RECEIVED STATE
ESTABLISHED STATE
* Enter the CLOSE-WAIT state.
+ FIN-WAIT-1 STATE
* If our FIN has been ACKed (perhaps in this segment),
then enter TIME-WAIT, start the time-wait timer, turn
off the other timers; otherwise enter the CLOSING
state.
+ FIN-WAIT-2 STATE
* Enter the TIME-WAIT state. Start the time-wait timer,
turn off the other timers.
+ CLOSE-WAIT STATE
* Remain in the CLOSE-WAIT state.
+ CLOSING STATE
* Remain in the CLOSING state.
+ LAST-ACK STATE
* Remain in the LAST-ACK state.
+ TIME-WAIT STATE
* Remain in the TIME-WAIT state. Restart the 2 MSL
time-wait timeout.
- and return.
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3.10.8. Timeouts
USER TIMEOUT
- For any state if the user timeout expires, flush all queues,
signal the user "error: connection aborted due to user timeout"
in general and for any outstanding calls, delete the TCB, enter
the CLOSED state and return.
RETRANSMISSION TIMEOUT
- For any state if the retransmission timeout expires on a
segment in the retransmission queue, send the segment at the
front of the retransmission queue again, reinitialize the
retransmission timer, and return.
TIME-WAIT TIMEOUT
- If the time-wait timeout expires on a connection delete the
TCB, enter the CLOSED state and return.
4. Glossary
ACK
A control bit (acknowledge) occupying no sequence space,
which indicates that the acknowledgment field of this segment
specifies the next sequence number the sender of this segment
is expecting to receive, hence acknowledging receipt of all
previous sequence numbers.
connection
A logical communication path identified by a pair of sockets.
datagram
A message sent in a packet switched computer communications
network.
Destination Address
The network layer address of the endpoint intended to receive
a segment.
FIN
A control bit (finis) occupying one sequence number, which
indicates that the sender will send no more data or control
occupying sequence space.
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flush
To remove all of the contents (data or segments) from a store
(buffer or queue).
fragment
A portion of a logical unit of data, in particular an
internet fragment is a portion of an internet datagram.
header
Control information at the beginning of a message, segment,
fragment, packet or block of data.
host
A computer. In particular a source or destination of
messages from the point of view of the communication network.
Identification
An Internet Protocol field. This identifying value assigned
by the sender aids in assembling the fragments of a datagram.
internet address
A network layer address.
internet datagram
A unit of data exchanged between internet hosts, together
with the internet header that allows the datagram to be
routed from source to destination.
internet fragment
A portion of the data of an internet datagram with an
internet header.
IP
Internet Protocol. See [1] and [13].
IRS
The Initial Receive Sequence number. The first sequence
number used by the sender on a connection.
ISN
The Initial Sequence Number. The first sequence number used
on a connection, (either ISS or IRS). Selected in a way that
is unique within a given period of time and is unpredictable
to attackers.
ISS
The Initial Send Sequence number. The first sequence number
used by the sender on a connection.
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left sequence
This is the next sequence number to be acknowledged by the
data receiving TCP endpoint (or the lowest currently
unacknowledged sequence number) and is sometimes referred to
as the left edge of the send window.
module
An implementation, usually in software, of a protocol or
other procedure.
MSL
Maximum Segment Lifetime, the time a TCP segment can exist in
the internetwork system. Arbitrarily defined to be 2
minutes.
octet
An eight bit byte.
Options
An Option field may contain several options, and each option
may be several octets in length.
packet
A package of data with a header that may or may not be
logically complete. More often a physical packaging than a
logical packaging of data.
port
The portion of a connection identifier used for
demultiplexing connections at an endpoint.
process
A program in execution. A source or destination of data from
the point of view of the TCP endpoint or other host-to-host
protocol.
PUSH
A control bit occupying no sequence space, indicating that
this segment contains data that must be pushed through to the
receiving user.
RCV.NXT
receive next sequence number
RCV.UP
receive urgent pointer
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RCV.WND
receive window
receive next sequence number
This is the next sequence number the local TCP endpoint is
expecting to receive.
receive window
This represents the sequence numbers the local (receiving)
TCP endpoint is willing to receive. Thus, the local TCP
endpoint considers that segments overlapping the range
RCV.NXT to RCV.NXT + RCV.WND - 1 carry acceptable data or
control. Segments containing sequence numbers entirely
outside this range are considered duplicates or injection
attacks and discarded.
RST
A control bit (reset), occupying no sequence space,
indicating that the receiver should delete the connection
without further interaction. The receiver can determine,
based on the sequence number and acknowledgment fields of the
incoming segment, whether it should honor the reset command
or ignore it. In no case does receipt of a segment
containing RST give rise to a RST in response.
SEG.ACK
segment acknowledgment
SEG.LEN
segment length
SEG.SEQ
segment sequence
SEG.UP
segment urgent pointer field
SEG.WND
segment window field
segment
A logical unit of data, in particular a TCP segment is the
unit of data transferred between a pair of TCP modules.
segment acknowledgment
The sequence number in the acknowledgment field of the
arriving segment.
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segment length
The amount of sequence number space occupied by a segment,
including any controls that occupy sequence space.
segment sequence
The number in the sequence field of the arriving segment.
send sequence
This is the next sequence number the local (sending) TCP
endpoint will use on the connection. It is initially
selected from an initial sequence number curve (ISN) and is
incremented for each octet of data or sequenced control
transmitted.
send window
This represents the sequence numbers that the remote
(receiving) TCP endpoint is willing to receive. It is the
value of the window field specified in segments from the
remote (data receiving) TCP endpoint. The range of new
sequence numbers that may be emitted by a TCP implementation
lies between SND.NXT and SND.UNA + SND.WND - 1.
(Retransmissions of sequence numbers between SND.UNA and
SND.NXT are expected, of course.)
SND.NXT
send sequence
SND.UNA
left sequence
SND.UP
send urgent pointer
SND.WL1
segment sequence number at last window update
SND.WL2
segment acknowledgment number at last window update
SND.WND
send window
socket (or socket number, or socket address, or socket identifier)
An address that specifically includes a port identifier, that
is, the concatenation of an Internet Address with a TCP port.
Source Address
The network layer address of the sending endpoint.
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SYN
A control bit in the incoming segment, occupying one sequence
number, used at the initiation of a connection, to indicate
where the sequence numbering will start.
TCB
Transmission control block, the data structure that records
the state of a connection.
TCP
Transmission Control Protocol: A host-to-host protocol for
reliable communication in internetwork environments.
TOS
Type of Service, an obsoleted IPv4 field. The same header
bits currently are used for the Differentiated Services field
[4] containing the Differentiated Services Code Point (DSCP)
value and the 2-bit ECN codepoint [6].
Type of Service
See "TOS".
URG
A control bit (urgent), occupying no sequence space, used to
indicate that the receiving user should be notified to do
urgent processing as long as there is data to be consumed
with sequence numbers less than the value indicated by the
urgent pointer.
urgent pointer
A control field meaningful only when the URG bit is on. This
field communicates the value of the urgent pointer that
indicates the data octet associated with the sending user's
urgent call.
5. Changes from RFC 793
This document obsoletes RFC 793 as well as RFC 6093 and 6528, which
updated 793. In all cases, only the normative protocol specification
and requirements have been incorporated into this document, and some
informational text with background and rationale may not have been
carried in. The informational content of those documents is still
valuable in learning about and understanding TCP, and they are valid
Informational references, even though their normative content has
been incorporated into this document.
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The main body of this document was adapted from RFC 793's Section 3,
titled "FUNCTIONAL SPECIFICATION", with an attempt to keep formatting
and layout as close as possible.
The collection of applicable RFC Errata that have been reported and
either accepted or held for an update to RFC 793 were incorporated
(Errata IDs: 573, 574, 700, 701, 1283, 1561, 1562, 1564, 1571, 1572,
2297, 2298, 2748, 2749, 2934, 3213, 3300, 3301, 6222). Some errata
were not applicable due to other changes (Errata IDs: 572, 575, 1565,
1569, 2296, 3305, 3602).
Changes to the specification of the Urgent Pointer described in RFCs
1011, 1122, and 6093 were incorporated. See RFC 6093 for detailed
discussion of why these changes were necessary.
The discussion of the RTO from RFC 793 was updated to refer to RFC
6298. The RFC 1122 text on the RTO originally replaced the 793 text,
however, RFC 2988 should have updated 1122, and has subsequently been
obsoleted by 6298.
RFC 1011 [19] contains a number of comments about RFC 793, including
some needed changes to the TCP specification. These are expanded in
RFC 1122, which contains a collection of other changes and
clarifications to RFC 793. The normative items impacting the
protocol have been incorporated here, though some historically useful
implementation advice and informative discussion from RFC 1122 is not
included here. The present document updates RFC 1011, since this is
now the TCP specification rather than RFC 793, and the comments noted
in 1011 have been incorporated.
RFC 1122 contains more than just TCP requirements, so this document
can't obsolete RFC 1122 entirely. It is only marked as "updating"
1122, however, it should be understood to effectively obsolete all of
the RFC 1122 material on TCP.
The more secure Initial Sequence Number generation algorithm from RFC
6528 was incorporated. See RFC 6528 for discussion of the attacks
that this mitigates, as well as advice on selecting PRF algorithms
and managing secret key data.
A note based on RFC 6429 was added to explicitly clarify that system
resource management concerns allow connection resources to be
reclaimed. RFC 6429 is obsoleted in the sense that this
clarification has been reflected in this update to the base TCP
specification now.
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The description of congestion control implementation was added, based
on the set of documents that are IETF BCP or Standards Track on the
topic, and the current state of common implementations.
RFC EDITOR'S NOTE: the content below is for detailed change tracking
and planning, and not to be included with the final revision of the
document.
This document started as draft-eddy-rfc793bis-00, that was merely a
proposal and rough plan for updating RFC 793.
The -01 revision of this draft-eddy-rfc793bis incorporates the
content of RFC 793 Section 3 titled "FUNCTIONAL SPECIFICATION".
Other content from RFC 793 has not been incorporated. The -01
revision of this document makes some minor formatting changes to the
RFC 793 content in order to convert the content into XML2RFC format
and account for left-out parts of RFC 793. For instance, figure
numbering differs and some indentation is not exactly the same.
The -02 revision of draft-eddy-rfc793bis incorporates errata that
have been verified:
Errata ID 573: Reported by Bob Braden (note: This errata report
basically is just a reminder that RFC 1122 updates 793. Some of
the associated changes are left pending to a separate revision
that incorporates 1122. Bob's mention of PUSH in 793 section 2.8
was not applicable here because that section was not part of the
"functional specification". Also, the 1122 text on the
retransmission timeout also has been updated by subsequent RFCs,
so the change here deviates from Bob's suggestion to apply the
1122 text.)
Errata ID 574: Reported by Yin Shuming
Errata ID 700: Reported by Yin Shuming
Errata ID 701: Reported by Yin Shuming
Errata ID 1283: Reported by Pei-chun Cheng
Errata ID 1561: Reported by Constantin Hagemeier
Errata ID 1562: Reported by Constantin Hagemeier
Errata ID 1564: Reported by Constantin Hagemeier
Errata ID 1565: Reported by Constantin Hagemeier
Errata ID 1571: Reported by Constantin Hagemeier
Errata ID 1572: Reported by Constantin Hagemeier
Errata ID 2296: Reported by Vishwas Manral
Errata ID 2297: Reported by Vishwas Manral
Errata ID 2298: Reported by Vishwas Manral
Errata ID 2748: Reported by Mykyta Yevstifeyev
Errata ID 2749: Reported by Mykyta Yevstifeyev
Errata ID 2934: Reported by Constantin Hagemeier
Errata ID 3213: Reported by EugnJun Yi
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Errata ID 3300: Reported by Botong Huang
Errata ID 3301: Reported by Botong Huang
Errata ID 3305: Reported by Botong Huang
Note: Some verified errata were not used in this update, as they
relate to sections of RFC 793 elided from this document. These
include Errata ID 572, 575, and 1569.
Note: Errata ID 3602 was not applied in this revision as it is
duplicative of the 1122 corrections.
Not related to RFC 793 content, this revision also makes small tweaks
to the introductory text, fixes indentation of the pseudo header
diagram, and notes that the Security Considerations should also
include privacy, when this section is written.
The -03 revision of draft-eddy-rfc793bis revises all discussion of
the urgent pointer in order to comply with RFC 6093, 1122, and 1011.
Since 1122 held requirements on the urgent pointer, the full list of
requirements was brought into an appendix of this document, so that
it can be updated as-needed.
The -04 revision of draft-eddy-rfc793bis includes the ISN generation
changes from RFC 6528.
The -05 revision of draft-eddy-rfc793bis incorporates MSS
requirements and definitions from RFC 879 [17], 1122, and 6691, as
well as option-handling requirements from RFC 1122.
The -00 revision of draft-ietf-tcpm-rfc793bis incorporates several
additional clarifications and updates to the section on segmentation,
many of which are based on feedback from Joe Touch improving from the
initial text on this in the previous revision.
The -01 revision incorporates the change to Reserved bits due to ECN,
as well as many other changes that come from RFC 1122.
The -02 revision has small formatting modifications in order to
address xml2rfc warnings about long lines. It was a quick update to
avoid document expiration. TCPM working group discussion in 2015
also indicated that we should not try to add sections on
implementation advice or similar non-normative information.
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The -03 revision incorporates more content from RFC 1122: Passive
OPEN Calls, Time-To-Live, Multihoming, IP Options, ICMP messages,
Data Communications, When to Send Data, When to Send a Window Update,
Managing the Window, Probing Zero Windows, When to Send an ACK
Segment. The section on data communications was re-organized into
clearer subsections (previously headings were embedded in the 793
text), and windows management advice from 793 was removed (as
reviewed by TCPM working group) in favor of the 1122 additions on
SWS, ZWP, and related topics.
The -04 revision includes reference to RFC 6429 on the ZWP condition,
RFC1122 material on TCP Connection Failures, TCP Keep-Alives,
Acknowledging Queued Segments, and Remote Address Validation. RTO
computation is referenced from RFC 6298 rather than RFC 1122.
The -05 revision includes the requirement to implement TCP congestion
control with recommendation to implement ECN, the RFC 6633 update to
1122, which changed the requirement on responding to source quench
ICMP messages, and discussion of ICMP (and ICMPv6) soft and hard
errors per RFC 5461 (ICMPv6 handling for TCP doesn't seem to be
mentioned elsewhere in standards track).
The -06 revision includes an appendix on "Other Implementation Notes"
to capture widely-deployed fundamental features that are not
contained in the RFC series yet. It also added mention of RFC 6994
and the IANA TCP parameters registry as a reference. It includes
references to RFC 5961 in appropriate places. The references to TOS
were changed to DiffServ field, based on reflecting RFC 2474 as well
as the IPv6 presence of traffic class (carrying DiffServ field)
rather than TOS.
The -07 revision includes reference to RFC 6191, updated security
considerations, discussion of additional implementation
considerations, and clarification of data on the SYN.
The -08 revision includes changes based on:
describing treatment of reserved bits (following TCPM mailing list
thread from July 2014 on "793bis item - reserved bit behavior"
addition a brief TCP key concepts section to make up for not
including the outdated section 2 of RFC 793
changed "TCP" to "host" to resolve conflict between 1122 wording
on whether TCP or the network layer chooses an address when
multihomed
fixed/updated definition of options in glossary
moved note on aggregating ACKs from 1122 to a more appropriate
location
resolved notes on IP precedence and security/compartment
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added implementation note on sequence number validation
added note that PUSH does not apply when Nagle is active
added 1122 content on asynchronous reports to replace 793 section
on TCP to user messages
The -09 revision fixes section numbering problems.
The -10 revision includes additions to the security considerations
based on comments from Joe Touch, and suggested edits on RST/FIN
notification, RFC 2525 reference, and other edits suggested by
Yuchung Cheng, as well as modifications to DiffServ text from Yuchung
Cheng and Gorry Fairhurst.
The -11 revision includes a start at identifying all of the
requirements text and referencing each instance in the common table
at the end of the document.
The -12 revision completes the requirement language indexing started
in -11 and adds necessary description of the PUSH functionality that
was missing.
The -13 revision contains only changes in the inline editor notes.
The -14 revision includes updates with regard to several comments
from the mailing list, including editorial fixes, adding IANA
considerations for the header flags, improving figure title
placement, and breaking up the "Terminology" section into more
appropriately titled subsections.
The -15 revision has many technical and editorial corrections from
Gorry Fairhurst's review, and subsequent discussion on the TCPM list,
as well as some other collected clarifications and improvements from
mailing list discussion.
The -16 revision addresses several discussions that rose from
additional reviews and follow-up on some of Gorry Fairhurst's
comments from revision 14.
The -17 revision includes errata 6222 from Charles Deng, update to
the key words boilerplate, updated description of the header flags
registry changes, and clarification about connections rather than
users in the discussion of OPEN calls.
The -18 revision includes editorial changes to the IANA
considerations, based on comments from Richard Scheffenegger at the
IETF 108 TCPM virtual meeting.
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The -19 revision includes editorial changes from Errata 6281 and 6282
reported by Merlin Buge. It also includes WGLC changes noted by
Mohamed Boucadair, Rahul Jadhav, Praveen Balasubramanian, Matt Olson,
Yi Huang, Joe Touch, and Juhamatti Kuusisaari.
The -20 revision includes text on congestion control based on mailing
list and meeting discussion, put together in its final form by Markku
Kojo. It also clarifies that SACK, WS, and TS options are
recommended for high performance, but not needed for basic
interoperability. It also clarifies that the length field is
required for new TCP options.
The -21 revision includes slight changes to the header diagram for
compatibility with tooling, from Stephen McQuistin, clarification on
the meaning of idle connections from Yuchung Cheng, Neal Cardwell,
Michael Scharf, and Richard Scheffenegger, editorial improvements
from Markku Kojo, notes that some stacks suppress extra
acknowledgments of the SYN when SYN-ACK carries data from Richard
Scheffenegger, and adds MAY-18 numbering based on note from Jonathan
Morton.
The -22 revision includes small clarifications on terminology (might
versus may) and IPv6 extension headers versus IPv4 options, based on
comments from Gorry Fairhurst.
The -23 revision has a fix to indentation from Michael Tuexen and
idnits issues addressed from Michael Scharf.
The -24 revision incorporates changes after Martin Duke's AD review,
including further feedback on those comments from Yuchung Cheng and
Joe Touch. Important changes for review include (1) removal of the
need to check for the PUSH flag when evaluating the SWS override
timer expiration, (2) clarification about receding urgent pointer,
and (3) de-duplicating handling of the RST checking between step 4
and step 1.
The -25 revision incorporates changes based on the GENART review from
Francis Dupont, SECDIR review from Kyle Rose, and OPSDIR review from
Sarah Banks.
The -26 revision incorporates changes stemming from the IESG reviews,
and INTDIR review from Bernie Volz.
The -27 revision fixes a few small editorial incompatibilities that
Stephen McQuistin found related to automated code generation.
The -28 revision addresses some COMMENTs from Ben Kaduk's IESG
review.
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Some other suggested changes that will not be incorporated in this
793 update unless TCPM consensus changes with regard to scope are:
1. Tony Sabatini's suggestion for describing DO field
2. Per discussion with Joe Touch (TAPS list, 6/20/2015), the
description of the API could be revisited
3. Reducing the R2 value for SYNs has been suggested as a possible
topic for future consideration.
Early in the process of updating RFC 793, Scott Brim mentioned that
this should include a PERPASS/privacy review. This may be something
for the chairs or AD to request during WGLC or IETF LC.
6. IANA Considerations
In the "Transmission Control Protocol (TCP) Header Flags" registry,
IANA is asked to make several changes described in this section.
RFC 3168 originally created this registry, but only populated it with
the new bits defined in RFC 3168, neglecting the other bits that had
previously been described in RFC 793 and other documents. Bit 7 has
since also been updated by RFC 8311.
The "Bit" column is renamed below as the "Bit Offset" column, since
it references each header flag's offset within the 16-bit aligned
view of the TCP header in Figure 1. The bits in offsets 0 through 4
are the TCP segment Data Offset field, and not header flags.
IANA should add a column for "Assignment Notes".
IANA should assign values indicated below.
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TCP Header Flags
Bit Name Reference Assignment Notes
Offset
--- ---- --------- ----------------
4 Reserved for future use (this document)
5 Reserved for future use (this document)
6 Reserved for future use (this document)
7 Reserved for future use [RFC8311] [1]
8 CWR (Congestion Window Reduced) [RFC3168]
9 ECE (ECN-Echo) [RFC3168]
10 Urgent Pointer field is significant (URG) (this document)
11 Acknowledgment field is significant (ACK) (this document)
12 Push Function (PSH) (this document)
13 Reset the connection (RST) (this document)
14 Synchronize sequence numbers (SYN) (this document)
15 No more data from sender (FIN) (this document)
FOOTNOTES:
[1] Previously used by Historic [RFC3540] as NS (Nonce Sum).
This TCP Header Flags registry should also be moved to a sub-registry
under the global "Transmission Control Protocol (TCP) Parameters
registry (https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml).
The registry's Registration Procedure should remain Standards Action,
but the Reference can be updated to this document, and the Note
removed.
7. Security and Privacy Considerations
The TCP design includes only rudimentary security features that
improve the robustness and reliability of connections and application
data transfer, but there are no built-in cryptographic capabilities
to support any form of confidentiality, authentication, or other
typical security functions. Non-cryptographic enhancements (e.g.
[9]) have been developed to improve robustness of TCP connections to
particular types of attacks, but the applicability and protections of
non-cryptographic enhancements are limited (e.g. see section 1.1 of
[9]). Applications typically utilize lower-layer (e.g. IPsec) and
upper-layer (e.g. TLS) protocols to provide security and privacy for
TCP connections and application data carried in TCP. Methods based
on TCP options have been developed as well, to support some security
capabilities.
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In order to fully provide confidentiality, integrity protection, and
authentication for TCP connections (including their control flags)
IPsec is the only current effective method. For integrity protection
and authentication, the TCP Authentication Option (TCP-AO) [39] is
available, with a proposed extension to also provide confidentiality
for the segment payload. Other methods discussed in this section may
provide confidentiality or integrity protection for the payload, but
for the TCP header only cover either a subset of the fields (e.g.
tcpcrypt [57]) or none at all (e.g. TLS). Other security features
that have been added to TCP (e.g. ISN generation, sequence number
checks, and others) are only capable of partially hindering attacks.
Applications using long-lived TCP flows have been vulnerable to
attacks that exploit the processing of control flags described in
earlier TCP specifications [34]. TCP-MD5 was a commonly implemented
TCP option to support authentication for some of these connections,
but had flaws and is now deprecated. TCP-AO provides a capability to
protect long-lived TCP connections from attacks, and has superior
properties to TCP-MD5. It does not provide any privacy for
application data, nor for the TCP headers.
The "tcpcrypt" [57] Experimental extension to TCP provides the
ability to cryptographically protect connection data. Metadata
aspects of the TCP flow are still visible, but the application stream
is well-protected. Within the TCP header, only the urgent pointer
and FIN flag are protected through tcpcrypt.
The TCP Roadmap [50] includes notes about several RFCs related to TCP
security. Many of the enhancements provided by these RFCs have been
integrated into the present document, including ISN generation,
mitigating blind in-window attacks, and improving handling of soft
errors and ICMP packets. These are all discussed in greater detail
in the referenced RFCs that originally described the changes needed
to earlier TCP specifications. Additionally, see RFC 6093 [40] for
discussion of security considerations related to the urgent pointer
field, that has been deprecated.
Since TCP is often used for bulk transfer flows, some attacks are
possible that abuse the TCP congestion control logic. An example is
"ACK-division" attacks. Updates that have been made to the TCP
congestion control specifications include mechanisms like Appropriate
Byte Counting (ABC) [30] that act as mitigations to these attacks.
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Other attacks are focused on exhausting the resources of a TCP
server. Examples include SYN flooding [33] or wasting resources on
non-progressing connections [42]. Operating systems commonly
implement mitigations for these attacks. Some common defenses also
utilize proxies, stateful firewalls, and other technologies outside
the end-host TCP implementation.
The concept of a protocol's "wire image" is described in RFC 8546
[56], which describes how TCP's cleartext headers expose more
metadata to nodes on the path than is strictly required to route the
packets to their destination. On-path adversaries may be able to
leverage this metadata. Lessons learned in this respect from TCP
have been applied in the design of newer transports like QUIC [60].
Additionally, based partly on experiences with TCP and its
extensions, there are considerations that might be applicable for
future TCP extensions and other transports that the IETF has
documented in RFC 9065 [61], along with IAB recommendations in RFC
8558 [58] and [68].
There are also methods of "fingerprinting" that can be used to infer
the host TCP implementation (operating system) version or platform
information. These collect observations of several aspects such as
the options present in segments, the ordering of options, the
specific behaviors in the case of various conditions, packet timing,
packet sizing, and other aspects of the protocol that are left to be
determined by an implementer, and can use those observations to
identify information about the host and implementation.
8. Acknowledgements
This document is largely a revision of RFC 793, which Jon Postel was
the editor of. Due to his excellent work, it was able to last for
three decades before we felt the need to revise it.
Andre Oppermann was a contributor and helped to edit the first
revision of this document.
We are thankful for the assistance of the IETF TCPM working group
chairs, over the course of work on this document:
Michael Scharf
Yoshifumi Nishida
Pasi Sarolahti
Michael Tuexen
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During the discussions of this work on the TCPM mailing list, in
working group meetings, and via area reviews, helpful comments,
critiques, and reviews were received from (listed alphabetically by
last name): Praveen Balasubramanian, David Borman, Mohamed Boucadair,
Bob Briscoe, Neal Cardwell, Yuchung Cheng, Martin Duke, Francis
Dupont, Ted Faber, Gorry Fairhurst, Fernando Gont, Rodney Grimes, Yi
Huang, Rahul Jadhav, Markku Kojo, Mike Kosek, Juhamatti Kuusisaari,
Kevin Lahey, Kevin Mason, Matt Mathis, Stephen McQuistin, Jonathan
Morton, Matt Olson, Tommy Pauly, Tom Petch, Hagen Paul Pfeifer, Kyle
Rose, Anthony Sabatini, Michael Scharf, Greg Skinner, Joe Touch,
Michael Tuexen, Reji Varghese, Bernie Volz, Tim Wicinski, Lloyd Wood,
and Alex Zimmermann.
Joe Touch provided additional help in clarifying the description of
segment size parameters and PMTUD/PLPMTUD recommendations. Markku
Kojo helped put together the text in the section on TCP Congestion
Control.
This document includes content from errata that were reported by
(listed chronologically): Yin Shuming, Bob Braden, Morris M. Keesan,
Pei-chun Cheng, Constantin Hagemeier, Vishwas Manral, Mykyta
Yevstifeyev, EungJun Yi, Botong Huang, Charles Deng, Merlin Buge.
9. References
9.1. Normative References
[1] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[2] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[3] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[4] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
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[5] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[6] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[7] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC5033, August 2007,
<https://www.rfc-editor.org/info/rfc5033>.
[8] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[9] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>.
[10] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.
[11] Gont, F., "Deprecation of ICMP Source Quench Messages",
RFC 6633, DOI 10.17487/RFC6633, May 2012,
<https://www.rfc-editor.org/info/rfc6633>.
[12] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[13] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[14] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
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[15] Allman, M., "Requirements for Time-Based Loss Detection",
BCP 233, RFC 8961, DOI 10.17487/RFC8961, November 2020,
<https://www.rfc-editor.org/info/rfc8961>.
9.2. Informative References
[16] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[17] Postel, J., "The TCP Maximum Segment Size and Related
Topics", RFC 879, DOI 10.17487/RFC0879, November 1983,
<https://www.rfc-editor.org/info/rfc879>.
[18] Nagle, J., "Congestion Control in IP/TCP Internetworks",
RFC 896, DOI 10.17487/RFC0896, January 1984,
<https://www.rfc-editor.org/info/rfc896>.
[19] Reynolds, J. and J. Postel, "Official Internet protocols",
RFC 1011, DOI 10.17487/RFC1011, May 1987,
<https://www.rfc-editor.org/info/rfc1011>.
[20] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[21] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
<https://www.rfc-editor.org/info/rfc1349>.
[22] Braden, R., "T/TCP -- TCP Extensions for Transactions
Functional Specification", RFC 1644, DOI 10.17487/RFC1644,
July 1994, <https://www.rfc-editor.org/info/rfc1644>.
[23] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/info/rfc2018>.
[24] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner,
J., Heavens, I., Lahey, K., Semke, J., and B. Volz, "Known
TCP Implementation Problems", RFC 2525,
DOI 10.17487/RFC2525, March 1999,
<https://www.rfc-editor.org/info/rfc2525>.
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[25] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, DOI 10.17487/RFC2675, August 1999,
<https://www.rfc-editor.org/info/rfc2675>.
[26] Xiao, X., Hannan, A., Paxson, V., and E. Crabbe, "TCP
Processing of the IPv4 Precedence Field", RFC 2873,
DOI 10.17487/RFC2873, June 2000,
<https://www.rfc-editor.org/info/rfc2873>.
[27] Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
Extension to the Selective Acknowledgement (SACK) Option
for TCP", RFC 2883, DOI 10.17487/RFC2883, July 2000,
<https://www.rfc-editor.org/info/rfc2883>.
[28] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>.
[29] Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
Sooriyabandara, "TCP Performance Implications of Network
Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
December 2002, <https://www.rfc-editor.org/info/rfc3449>.
[30] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting (ABC)", RFC 3465, DOI 10.17487/RFC3465, February
2003, <https://www.rfc-editor.org/info/rfc3465>.
[31] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727,
DOI 10.17487/RFC4727, November 2006,
<https://www.rfc-editor.org/info/rfc4727>.
[32] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[33] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>.
[34] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>.
[35] Culley, P., Elzur, U., Recio, R., Bailey, S., and J.
Carrier, "Marker PDU Aligned Framing for TCP
Specification", RFC 5044, DOI 10.17487/RFC5044, October
2007, <https://www.rfc-editor.org/info/rfc5044>.
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[36] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
DOI 10.17487/RFC5461, February 2009,
<https://www.rfc-editor.org/info/rfc5461>.
[37] StJohns, M., Atkinson, R., and G. Thomas, "Common
Architecture Label IPv6 Security Option (CALIPSO)",
RFC 5570, DOI 10.17487/RFC5570, July 2009,
<https://www.rfc-editor.org/info/rfc5570>.
[38] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>.
[39] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[40] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093, DOI 10.17487/RFC6093,
January 2011, <https://www.rfc-editor.org/info/rfc6093>.
[41] Gont, F., "Reducing the TIME-WAIT State Using TCP
Timestamps", BCP 159, RFC 6191, DOI 10.17487/RFC6191,
April 2011, <https://www.rfc-editor.org/info/rfc6191>.
[42] Bashyam, M., Jethanandani, M., and A. Ramaiah, "TCP Sender
Clarification for Persist Condition", RFC 6429,
DOI 10.17487/RFC6429, December 2011,
<https://www.rfc-editor.org/info/rfc6429>.
[43] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February
2012, <https://www.rfc-editor.org/info/rfc6528>.
[44] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, DOI 10.17487/RFC6691, July 2012,
<https://www.rfc-editor.org/info/rfc6691>.
[45] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, DOI 10.17487/RFC6864, February 2013,
<https://www.rfc-editor.org/info/rfc6864>.
[46] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/info/rfc6994>.
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[47] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<https://www.rfc-editor.org/info/rfc7094>.
[48] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
[49] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[50] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414,
DOI 10.17487/RFC7414, February 2015,
<https://www.rfc-editor.org/info/rfc7414>.
[51] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015,
<https://www.rfc-editor.org/info/rfc7657>.
[52] Fairhurst, G. and M. Welzl, "The Benefits of Using
Explicit Congestion Notification (ECN)", RFC 8087,
DOI 10.17487/RFC8087, March 2017,
<https://www.rfc-editor.org/info/rfc8087>.
[53] Fairhurst, G., Ed., Trammell, B., Ed., and M. Kuehlewind,
Ed., "Services Provided by IETF Transport Protocols and
Congestion Control Mechanisms", RFC 8095,
DOI 10.17487/RFC8095, March 2017,
<https://www.rfc-editor.org/info/rfc8095>.
[54] Welzl, M., Tuexen, M., and N. Khademi, "On the Usage of
Transport Features Provided by IETF Transport Protocols",
RFC 8303, DOI 10.17487/RFC8303, February 2018,
<https://www.rfc-editor.org/info/rfc8303>.
[55] Chown, T., Loughney, J., and T. Winters, "IPv6 Node
Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
January 2019, <https://www.rfc-editor.org/info/rfc8504>.
[56] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.
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[57] Bittau, A., Giffin, D., Handley, M., Mazieres, D., Slack,
Q., and E. Smith, "Cryptographic Protection of TCP Streams
(tcpcrypt)", RFC 8548, DOI 10.17487/RFC8548, May 2019,
<https://www.rfc-editor.org/info/rfc8548>.
[58] Hardie, T., Ed., "Transport Protocol Path Signals",
RFC 8558, DOI 10.17487/RFC8558, April 2019,
<https://www.rfc-editor.org/info/rfc8558>.
[59] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.
[60] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[61] Fairhurst, G. and C. Perkins, "Considerations around
Transport Header Confidentiality, Network Operations, and
the Evolution of Internet Transport Protocols", RFC 9065,
DOI 10.17487/RFC9065, July 2021,
<https://www.rfc-editor.org/info/rfc9065>.
[62] IANA, "Transmission Control Protocol (TCP) Parameters,
https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml", 2019.
[63] IANA, "Transmission Control Protocol (TCP) Header Flags,
https://www.iana.org/assignments/tcp-header-flags/tcp-
header-flags.xhtml", 2019.
[64] Gont, F., "Processing of IP Security/Compartment and
Precedence Information by TCP", Work in Progress,
Internet-Draft, draft-gont-tcpm-tcp-seccomp-prec-00, 29
March 2012, <http://www.ietf.org/internet-drafts/draft-
gont-tcpm-tcp-seccomp-prec-00.txt>.
[65] Gont, F. and D. Borman, "On the Validation of TCP Sequence
Numbers", Work in Progress, Internet-Draft, draft-gont-
tcpm-tcp-seq-validation-04, 11 March 2019,
<http://www.ietf.org/internet-drafts/draft-gont-tcpm-tcp-
seq-validation-04.txt>.
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[66] Touch, J. and W. Eddy, "TCP Extended Data Offset Option",
Work in Progress, Internet-Draft, draft-ietf-tcpm-tcp-edo-
10, 19 July 2018, <http://www.ietf.org/internet-drafts/
draft-ietf-tcpm-tcp-edo-10.txt>.
[67] McQuistin, S., Band, V., Jacob, D., and C. Perkins,
"Describing Protocol Data Units with Augmented Packet
Header Diagrams", Work in Progress, Internet-Draft, draft-
mcquistin-augmented-ascii-diagrams-08, 5 May 2021,
<https://www.ietf.org/archive/id/draft-mcquistin-
augmented-ascii-diagrams-08.txt>.
[68] Thomson, M. and T. Pauly, "Long-term Viability of Protocol
Extension Mechanisms", Work in Progress, Internet-Draft,
draft-iab-use-it-or-lose-it-02, 23 August 2021,
<https://www.ietf.org/archive/id/draft-iab-use-it-or-lose-
it-02.txt>.
[69] Minshall, G., "A Proposed Modification to Nagle's
Algorithm", Work in Progress, Internet-Draft, draft-
minshall-nagle-01, June 1999,
<https://datatracker.ietf.org/doc/html/draft-minshall-
nagle-01>.
[70] Dalal, Y. and C. Sunshine, "Connection Management in
Transport Protocols", Computer Networks Vol. 2, No. 6, pp.
454-473, December 1978.
[71] Faber, T., Touch, J., and W. Yui, "The TIME-WAIT state in
TCP and Its Effect on Busy Servers", Proceedings of IEEE
INFOCOM pp. 1573-1583, March 1999.
[72] Postel, J., "Comments on Action Items from the January
Meeting", IEN 177, March 1981,
<https://www.rfc-editor.org/ien/ien177.txt>.
[73] "Segmentation Offloads", Linux Networking Documentation ,
<https://www.kernel.org/doc/html/latest/networking/
segmentation-offloads.html>.
Appendix A. Other Implementation Notes
This section includes additional notes and references on TCP
implementation decisions that are currently not a part of the RFC
series or included within the TCP standard. These items can be
considered by implementers, but there was not yet a consensus to
include them in the standard.
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A.1. IP Security Compartment and Precedence
The IPv4 specification [1] includes a precedence value in the (now
obsoleted) Type of Service field (TOS) field. It was modified in
[21], and then obsoleted by the definition of Differentiated Services
(DiffServ) [4]. Setting and conveying TOS between the network layer,
TCP implementation, and applications is obsolete, and replaced by
DiffServ in the current TCP specification.
RFC 793 required checking the IP security compartment and precedence
on incoming TCP segments for consistency within a connection, and
with application requests. Each of these aspects of IP have become
outdated, without specific updates to RFC 793. The issues with
precedence were fixed by [26], which is Standards Track, and so this
present TCP specification includes those changes. However, the state
of IP security options that may be used by MLS systems is not as
apparent in the IETF currently.
Resetting connections when incoming packets do not meet expected
security compartment or precedence expectations has been recognized
as a possible attack vector [64], and there has been discussion about
amending the TCP specification to prevent connections from being
aborted due to non-matching IP security compartment and DiffServ
codepoint values.
A.1.1. Precedence
In DiffServ the former precedence values are treated as Class
Selector codepoints, and methods for compatible treatment are
described in the DiffServ architecture. The RFC 793/1122 TCP
specification includes logic intending to have connections use the
highest precedence requested by either endpoint application, and to
keep the precedence consistent throughout a connection. This logic
from the obsolete TOS is not applicable for DiffServ, and should not
be included in TCP implementations, though changes to DiffServ values
within a connection are discouraged. For discussion of this, see RFC
7657 (sec 5.1, 5.3, and 6) [51].
The obsoleted TOS processing rules in TCP assumed bidirectional (or
symmetric) precedence values used on a connection, but the DiffServ
architecture is asymmetric. Problems with the old TCP logic in this
regard were described in [26] and the solution described is to ignore
IP precedence in TCP. Since RFC 2873 is a Standards Track document
(although not marked as updating RFC 793), current implementations
are expected to be robust to these conditions. Note that the
DiffServ field value used in each direction is a part of the
interface between TCP and the network layer, and values in use can be
indicated both ways between TCP and the application.
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A.1.2. MLS Systems
The IP security option (IPSO) and compartment defined in [1] was
refined in RFC 1038 that was later obsoleted by RFC 1108. The
Commercial IP Security Option (CIPSO) is defined in FIPS-188
(withdrawn by NIST in 2015), and is supported by some vendors and
operating systems. RFC 1108 is now Historic, though RFC 791 itself
has not been updated to remove the IP security option. For IPv6, a
similar option (CALIPSO) has been defined [37]. RFC 793 includes
logic that includes the IP security/compartment information in
treatment of TCP segments. References to the IP "security/
compartment" in this document may be relevant for Multi-Level Secure
(MLS) system implementers, but can be ignored for non-MLS
implementations, consistent with running code on the Internet. See
Appendix A.1 for further discussion. Note that RFC 5570 describes
some MLS networking scenarios where IPSO, CIPSO, or CALIPSO may be
used. In these special cases, TCP implementers should see section
7.3.1 of RFC 5570, and follow the guidance in that document.
A.2. Sequence Number Validation
There are cases where the TCP sequence number validation rules can
prevent ACK fields from being processed. This can result in
connection issues, as described in [65], which includes descriptions
of potential problems in conditions of simultaneous open, self-
connects, simultaneous close, and simultaneous window probes. The
document also describes potential changes to the TCP specification to
mitigate the issue by expanding the acceptable sequence numbers.
In Internet usage of TCP, these conditions are rarely occurring.
Common operating systems include different alternative mitigations,
and the standard has not been updated yet to codify one of them, but
implementers should consider the problems described in [65].
A.3. Nagle Modification
In common operating systems, both the Nagle algorithm and delayed
acknowledgements are implemented and enabled by default. TCP is used
by many applications that have a request-response style of
communication, where the combination of the Nagle algorithm and
delayed acknowledgements can result in poor application performance.
A modification to the Nagle algorithm is described in [69] that
improves the situation for these applications.
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This modification is implemented in some common operating systems,
and does not impact TCP interoperability. Additionally, many
applications simply disable Nagle, since this is generally supported
by a socket option. The TCP standard has not been updated to include
this Nagle modification, but implementers may find it beneficial to
consider.
A.4. Low Watermark Settings
Some operating system kernel TCP implementations include socket
options that allow specifying the number of bytes in the buffer until
the socket layer will pass sent data to TCP (SO_SNDLOWAT) or to the
application on receiving (SO_RCVLOWAT).
In addition, another socket option (TCP_NOTSENT_LOWAT) can be used to
control the amount of unsent bytes in the write queue. This can help
a sending TCP application to avoid creating large amounts of buffered
data (and corresponding latency). As an example, this may be useful
for applications that are multiplexing data from multiple upper level
streams onto a connection, especially when streams may be a mix of
interactive / real-time and bulk data transfer.
Appendix B. TCP Requirement Summary
This section is adapted from RFC 1122.
Note that there is no requirement related to PLPMTUD in this list,
but that PLPMTUD is recommended.
| | | | |S| |
| | | | |H| |F
| | | | |O|M|o
| | |S| |U|U|o
| | |H| |L|S|t
| |M|O| |D|T|n
| |U|U|M| | |o
| |S|L|A|N|N|t
| |T|D|Y|O|O|t
FEATURE | ReqID | | | |T|T|e
-------------------------------------------------|--------|-|-|-|-|-|--
| | | | | | |
Push flag | | | | | | |
Aggregate or queue un-pushed data | MAY-16 | | |x| | |
Sender collapse successive PSH flags | SHLD-27| |x| | | |
SEND call can specify PUSH | MAY-15 | | |x| | |
If cannot: sender buffer indefinitely | MUST-60| | | | |x|
If cannot: PSH last segment | MUST-61|x| | | | |
Notify receiving ALP of PSH | MAY-17 | | |x| | |1
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Send max size segment when possible | SHLD-28| |x| | | |
| | | | | | |
Window | | | | | | |
Treat as unsigned number | MUST-1 |x| | | | |
Handle as 32-bit number | REC-1 | |x| | | |
Shrink window from right | SHLD-14| | | |x| |
- Send new data when window shrinks | SHLD-15| | | |x| |
- Retransmit old unacked data within window | SHLD-16| |x| | | |
- Time out conn for data past right edge | SHLD-17| | | |x| |
Robust against shrinking window | MUST-34|x| | | | |
Receiver's window closed indefinitely | MAY-8 | | |x| | |
Use standard probing logic | MUST-35|x| | | | |
Sender probe zero window | MUST-36|x| | | | |
First probe after RTO | SHLD-29| |x| | | |
Exponential backoff | SHLD-30| |x| | | |
Allow window stay zero indefinitely | MUST-37|x| | | | |
Retransmit old data beyond SND.UNA+SND.WND | MAY-7 | | |x| | |
Process RST and URG even with zero window | MUST-66|x| | | | |
| | | | | | |
Urgent Data | | | | | | |
Include support for urgent pointer | MUST-30|x| | | | |
Pointer indicates first non-urgent octet | MUST-62|x| | | | |
Arbitrary length urgent data sequence | MUST-31|x| | | | |
Inform ALP asynchronously of urgent data | MUST-32|x| | | | |1
ALP can learn if/how much urgent data Q'd | MUST-33|x| | | | |1
ALP employ the urgent mechanism | SHLD-13| | | |x| |
| | | | | | |
TCP Options | | | | | | |
Support the mandatory option set | MUST-4 |x| | | | |
Receive TCP option in any segment | MUST-5 |x| | | | |
Ignore unsupported options | MUST-6 |x| | | | |
Include length for all options except EOL+NOP | MUST-68|x| | | | |
Cope with illegal option length | MUST-7 |x| | | | |
Process options regardless of word alignment | MUST-64|x| | | | |
Implement sending & receiving MSS option | MUST-14|x| | | | |
IPv4 Send MSS option unless 536 | SHLD-5 | |x| | | |
IPv6 Send MSS option unless 1220 | SHLD-5 | |x| | | |
Send MSS option always | MAY-3 | | |x| | |
IPv4 Send-MSS default is 536 | MUST-15|x| | | | |
IPv6 Send-MSS default is 1220 | MUST-15|x| | | | |
Calculate effective send seg size | MUST-16|x| | | | |
MSS accounts for varying MTU | SHLD-6 | |x| | | |
MSS not sent on non-SYN segments | MUST-65| | | | |x|
MSS value based on MMS_R | MUST-67|x| | | | |
Pad with zero | MUST-69|x| | | | |
| | | | | | |
TCP Checksums | | | | | | |
Sender compute checksum | MUST-2 |x| | | | |
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Receiver check checksum | MUST-3 |x| | | | |
| | | | | | |
ISN Selection | | | | | | |
Include a clock-driven ISN generator component | MUST-8 |x| | | | |
Secure ISN generator with a PRF component | SHLD-1 | |x| | | |
PRF computable from outside the host | MUST-9 | | | | |x|
| | | | | | |
Opening Connections | | | | | | |
Support simultaneous open attempts | MUST-10|x| | | | |
SYN-RECEIVED remembers last state | MUST-11|x| | | | |
Passive Open call interfere with others | MUST-41| | | | |x|
Function: simultan. LISTENs for same port | MUST-42|x| | | | |
Ask IP for src address for SYN if necc. | MUST-44|x| | | | |
Otherwise, use local addr of conn. | MUST-45|x| | | | |
OPEN to broadcast/multicast IP Address | MUST-46| | | | |x|
Silently discard seg to bcast/mcast addr | MUST-57|x| | | | |
| | | | | | |
Closing Connections | | | | | | |
RST can contain data | SHLD-2 | |x| | | |
Inform application of aborted conn | MUST-12|x| | | | |
Half-duplex close connections | MAY-1 | | |x| | |
Send RST to indicate data lost | SHLD-3 | |x| | | |
In TIME-WAIT state for 2MSL seconds | MUST-13|x| | | | |
Accept SYN from TIME-WAIT state | MAY-2 | | |x| | |
Use Timestamps to reduce TIME-WAIT | SHLD-4 | |x| | | |
| | | | | | |
Retransmissions | | | | | | |
Implement exponential backoff, slow start, and | MUST-19|x| | | | |
congestion avoidance | | | | | | |
Retransmit with same IP ident | MAY-4 | | |x| | |
Karn's algorithm | MUST-18|x| | | | |
| | | | | | |
Generating ACKs: | | | | | | |
Aggregate whenever possible | MUST-58|x| | | | |
Queue out-of-order segments | SHLD-31| |x| | | |
Process all Q'd before send ACK | MUST-59|x| | | | |
Send ACK for out-of-order segment | MAY-13 | | |x| | |
Delayed ACKs | SHLD-18| |x| | | |
Delay < 0.5 seconds | MUST-40|x| | | | |
Every 2nd full-sized segment or 2*RMSS ACK'd | SHLD-19| |x| | | |
Receiver SWS-Avoidance Algorithm | MUST-39|x| | | | |
| | | | | | |
Sending data | | | | | | |
Configurable TTL | MUST-49|x| | | | |
Sender SWS-Avoidance Algorithm | MUST-38|x| | | | |
Nagle algorithm | SHLD-7 | |x| | | |
Application can disable Nagle algorithm | MUST-17|x| | | | |
| | | | | | |
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Connection Failures: | | | | | | |
Negative advice to IP on R1 retxs | MUST-20|x| | | | |
Close connection on R2 retxs | MUST-20|x| | | | |
ALP can set R2 | MUST-21|x| | | | |1
Inform ALP of R1<=retxs<R2 | SHLD-9 | |x| | | |1
Recommended value for R1 | SHLD-10| |x| | | |
Recommended value for R2 | SHLD-11| |x| | | |
Same mechanism for SYNs | MUST-22|x| | | | |
R2 at least 3 minutes for SYN | MUST-23|x| | | | |
| | | | | | |
Send Keep-alive Packets: | MAY-5 | | |x| | |
- Application can request | MUST-24|x| | | | |
- Default is "off" | MUST-25|x| | | | |
- Only send if idle for interval | MUST-26|x| | | | |
- Interval configurable | MUST-27|x| | | | |
- Default at least 2 hrs. | MUST-28|x| | | | |
- Tolerant of lost ACKs | MUST-29|x| | | | |
- Send with no data | SHLD-12| |x| | | |
- Configurable to send garbage octet | MAY-6 | | |x| | |
| | | | | | |
IP Options | | | | | | |
Ignore options TCP doesn't understand | MUST-50|x| | | | |
Time Stamp support | MAY-10 | | |x| | |
Record Route support | MAY-11 | | |x| | |
Source Route: | | | | | | |
ALP can specify | MUST-51|x| | | | |1
Overrides src rt in datagram | MUST-52|x| | | | |
Build return route from src rt | MUST-53|x| | | | |
Later src route overrides | SHLD-24| |x| | | |
| | | | | | |
Receiving ICMP Messages from IP | MUST-54|x| | | | |
Dest. Unreach (0,1,5) => inform ALP | SHLD-25| |x| | | |
Abort on Dest. Unreach (0,1,5) =>nn | MUST-56| | | | |x|
Dest. Unreach (2-4) => abort conn | SHLD-26| |x| | | |
Source Quench => silent discard | MUST-55|x| | | | |
Abort on Time Exceeded => | MUST-56| | | | |x|
Abort on Param Problem => | MUST-56| | | | |x|
| | | | | | |
Address Validation | | | | | | |
Reject OPEN call to invalid IP address | MUST-46|x| | | | |
Reject SYN from invalid IP address | MUST-63|x| | | | |
Silently discard SYN to bcast/mcast addr | MUST-57|x| | | | |
| | | | | | |
TCP/ALP Interface Services | | | | | | |
Error Report mechanism | MUST-47|x| | | | |
ALP can disable Error Report Routine | SHLD-20| |x| | | |
ALP can specify DiffServ field for sending | MUST-48|x| | | | |
Passed unchanged to IP | SHLD-22| |x| | | |
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ALP can change DiffServ field during connection| SHLD-21| |x| | | |
ALP generally changing DiffServ during conn. | SHLD-23| | | |x| |
Pass received DiffServ field up to ALP | MAY-9 | | |x| | |
FLUSH call | MAY-14 | | |x| | |
Optional local IP addr parm. in OPEN | MUST-43|x| | | | |
| | | | | | |
RFC 5961 Support: | | | | | | |
Implement data injection protection | MAY-12 | | |x| | |
| | | | | | |
Explicit Congestion Notification: | | | | | | |
Support ECN | SHLD-8 | |x| | | |
| | | | | | |
Alternative Congestion Control: | | | | | | |
Implement alternative conformant algorithm(s) | MAY-18 | | |x| | |
-------------------------------------------------|--------|-|-|-|-|-|-
FOOTNOTES: (1) "ALP" means Application-Layer Program.
Author's Address
Wesley M. Eddy (editor)
MTI Systems
United States of America
Email: wes@mti-systems.com
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