Internet DRAFT - draft-eddy-rfc793bis
draft-eddy-rfc793bis
Internet Engineering Task Force W. Eddy, Ed.
Internet-Draft MTI Systems
Obsoletes: 793, 879, 6093, 6528, 6691 February 6, 2015
(if approved)
Updates: 1122 (if approved)
Intended status: Standards Track
Expires: August 10, 2015
Transmission Control Protocol Specification
draft-eddy-rfc793bis-05
Abstract
This document specifies the Internet's Transmission Control Protocol
(TCP). TCP is an important transport layer protocol in the Internet
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 and several other RFCs (TODO: list
all actual RFCs when finished).
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.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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 http://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."
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This Internet-Draft will expire on August 10, 2015.
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Table of Contents
1. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Functional Specification . . . . . . . . . . . . . . . . . . 4
3.1. Header Format . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Sequence Numbers . . . . . . . . . . . . . . . . . . . . 13
3.4. Establishing a connection . . . . . . . . . . . . . . . . 20
3.5. Closing a Connection . . . . . . . . . . . . . . . . . . 27
3.6. Precedence and Security . . . . . . . . . . . . . . . . . 29
3.7. Segmentation . . . . . . . . . . . . . . . . . . . . . . 30
3.7.1. Maximum Segment Size Option . . . . . . . . . . . . . 31
3.7.2. Path MTU Discovery . . . . . . . . . . . . . . . . . 32
3.7.3. Interfaces with Variable MSS Values . . . . . . . . . 32
3.7.4. IPv6 Jumbograms . . . . . . . . . . . . . . . . . . . 32
3.8. Data Communication . . . . . . . . . . . . . . . . . . . 32
3.9. Interfaces . . . . . . . . . . . . . . . . . . . . . . . 36
3.9.1. User/TCP Interface . . . . . . . . . . . . . . . . . 37
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3.9.2. TCP/Lower-Level Interface . . . . . . . . . . . . . . 43
3.10. Event Processing . . . . . . . . . . . . . . . . . . . . 44
3.11. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 67
4. Changes from RFC 793 . . . . . . . . . . . . . . . . . . . . 72
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 75
6. Security and Privacy Considerations . . . . . . . . . . . . . 75
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 76
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 76
8.1. Normative References . . . . . . . . . . . . . . . . . . 76
8.2. Informative References . . . . . . . . . . . . . . . . . 76
Appendix A. TCP Requirement Summary . . . . . . . . . . . . . . 77
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 80
1. Purpose and Scope
In 1981, RFC 793 [2] 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 implemented many times, 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 specification for TCP [10]. Over time, a
number of errata have been identified on RFC 793, as well as
deficiencies in security, performance, and other aspects. A number
of enhancements has grown and been documented separately. These were
never accumulated together into an update to the base specification.
The purpose of this document is to bring together all of the IETF
Standards Track changes that have been made to the basic TCP
functional specification and unify them into an update of the RFC 793
protocol specification. Some companion documents are referenced for
important algorithms that TCP uses (e.g. for congestion control), but
have not been attempted to include 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, but all TCP implementations need to implement this
specification as a common basis in order to interoperate. As 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 descibes 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 human readers to understand
aspects of the protocol design and operation. This document does not
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attempt to alter or update this informative text, and is focused only
on updating the normative protocol specification. We preserve
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, as well as in writing new
implementations.
2. Introduction
RFC 793 contains a discussion of the TCP design goals and provides
examples of its operation, including examples of connection
establishment, closing connections, and retransmitting packets to
repair losses.
This document describes the basic functionality expected in modern
implementations of TCP, and replaces the protocol specification in
RFC 793. It does not replicate or attempt to update the examples and
other discussion in 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.
TEMPORARY EDITOR'S NOTE: This is an early revision in the process of
updating RFC 793. Many planned changes are not yet incorporated.
***Please do not use this revision as a basis for any work or
reference.***
A list of changes from RFC 793 is contained in Section 4.
TEMPORARY EDITOR'S NOTE: the current revision of this document does
not yet collect all of the changes that will be in the final version.
The set of content changes planned for future revisions is kept in
Section 4.
3. Functional Specification
3.1. Header Format
TCP segments are sent as internet datagrams. The Internet Protocol
header carries several information fields, including the source and
destination host addresses [2]. A TCP header follows the internet
header, supplying information specific to the TCP protocol. This
division allows for the existence of host level protocols other than
TCP.
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TCP Header Format
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 | |U|A|P|R|S|F| |
| Offset| Reserved |R|C|S|S|Y|I| Window |
| | |G|K|H|T|N|N| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Urgent Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TCP Header Format
Note that one tick mark represents one bit position.
Figure 1
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 SYN is present). If SYN is present the sequence number is the
initial sequence number (ISN) and the first data octet is ISN+1.
Acknowledgment Number: 32 bits
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.
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Data Offset: 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
integral number of 32 bits long.
Reserved: 6 bits
Reserved for future use. Must be zero.
Control Bits: 6 bits (from left to right):
URG: Urgent Pointer field significant
ACK: Acknowledgment field significant
PSH: Push Function
RST: Reset the connection
SYN: Synchronize sequence numbers
FIN: No more data from sender
Window: 16 bits
The number of data octets beginning with the one indicated in the
acknowledgment field which the sender of this segment is willing to
accept.
Checksum: 16 bits
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a
segment contains an odd number of header and text octets to be
checksummed, the last octet is padded on the right with zeros 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 96 bit pseudo header conceptually
prefixed to the TCP header. This pseudo header contains the Source
Address, the Destination Address, the Protocol, and TCP length.
This gives the TCP protection against misrouted segments. This
information is carried in the Internet Protocol and is transferred
across the TCP/Network interface in the arguments or results of
calls by the TCP on the IP.
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+--------+--------+--------+--------+
| Source Address |
+--------+--------+--------+--------+
| Destination Address |
+--------+--------+--------+--------+
| zero | PTCL | TCP Length |
+--------+--------+--------+--------+
The TCP Length is 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.
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 be interpreted in segments
with the URG control bit set.
Options: variable
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, 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.
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 (i.e., zero).
Currently defined options include (kind indicated in octal):
Kind Length Meaning
---- ------ -------
0 - End of option list.
1 - No-Operation.
2 4 Maximum Segment Size.
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A TCP MUST be able to receive a TCP option in any segment. A TCP
MUST 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 have length fields). TCP MUST
be prepared to handle an illegal option length (e.g., zero) without
crashing; a suggested procedure is to reset the connection and log
the reason.
Specific Option Definitions
End of Option List
+--------+
|00000000|
+--------+
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.
No-Operation
+--------+
|00000001|
+--------+
Kind=1
This option code may 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.
Maximum Segment Size (MSS)
+--------+--------+---------+--------+
|00000010|00000100| max seg size |
+--------+--------+---------+--------+
Kind=2 Length=4
Maximum Segment Size Option Data: 16 bits
If this option is present, then it communicates the maximum
receive segment size at the TCP which sends this segment. This
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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. If this option is not used, any segment size is
allowed.
Padding: variable
The TCP header padding is used to ensure that the TCP header ends
and data begins on a 32 bit boundary. The padding is composed of
zeros.
3.2. Terminology
Before we can discuss very much about the operation of the TCP we
need to introduce some detailed terminology. The maintenance of a
TCP connection requires the remembering of 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 socket numbers, the
security and precedence of the connection, 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.
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Send Sequence Space
1 2 3 4
----------|----------|----------|----------
SND.UNA SND.NXT SND.UNA
+SND.WND
1 - old sequence numbers which have been acknowledged
2 - sequence numbers of unacknowledged data
3 - sequence numbers allowed for new data transmission
4 - future sequence numbers which are not yet allowed
Send Sequence Space
Figure 2
The send window is the portion of the sequence space labeled 3 in
Figure 2.
Receive Sequence Space
1 2 3
----------|----------|----------
RCV.NXT RCV.NXT
+RCV.WND
1 - old sequence numbers which have been acknowledged
2 - sequence numbers allowed for new reception
3 - future sequence numbers which are not yet allowed
Receive Sequence Space
Figure 3
The receive window is the portion of the sequence space labeled 2 in
Figure 3.
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
SEG.PRC - segment precedence value
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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 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.
FIN-WAIT-1 - represents waiting for a connection termination
request from the remote TCP, or an acknowledgment of the
connection termination request previously sent.
FIN-WAIT-2 - represents waiting for a connection termination
request from the remote TCP.
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.
LAST-ACK - represents waiting for an acknowledgment of the
connection termination request previously sent to the remote TCP
(this termination request sent to the remote TCP already included
an acknowledgment of the termination request sent from the remote
TCP).
TIME-WAIT - represents waiting for enough time to pass to be sure
the remote TCP received the acknowledgment of its connection
termination request.
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,
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ABORT, and STATUS; the incoming segments, particularly those
containing the SYN, ACK, RST and FIN flags; and timeouts.
The state diagram in Figure 4 illustrates only state changes,
together with the causing events and resulting actions, but addresses
neither error conditions nor actions which are not connected with
state changes. In a later section, more detail is offered with
respect to the reaction of the TCP to events.
NOTA BENE: this diagram is only a summary and must not be taken as
the total specification.
+---------+ ---------\ 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
+---------+ +---------+ +---------+
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|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 |
+---------+ +---------+
note 1: The transition from SYN-RCVD to LISTEN on receiving a RST is
conditional on having reached SYN-RCVD after a passive open.
note 2: An unshown transition exists from FIN-WAIT-1 to TIME-WAIT if
a FIN is received and the local FIN is also acknowledged.
TCP Connection State Diagram
Figure 4
3.3. 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 very 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 which the TCP must
perform include:
(a) Determining that an acknowledgment refers to some sequence
number sent but not yet acknowledged.
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(b) Determining that all sequence numbers occupied by a segment
have been acknowledged (e.g., to remove the segment from a
retransmission queue).
(c) Determining that an incoming segment contains sequence numbers
which are expected (i.e., that the segment "overlaps" the receive
window).
In response to sending data the TCP 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 (next sequence
number expected by the receiving TCP)
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 segments,
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
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A segment is judged to occupy a portion of valid receive sequence
space if
RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
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 be possible for a TCP to
maintain a zero receive window while transmitting data and receiving
ACKs. However, even when the receive window is zero, a TCP must
process the RST and URG fields of all incoming segments.
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
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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.
Initial Sequence Number Selection
The protocol places no restriction on a particular connection being
used over and over again. A connection is defined by a pair of
sockets. 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 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 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 crashes and loses all knowledge of the
sequence numbers it has been using. When new connections are
created, an initial sequence number (ISN) generator is employed which
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.
The recommended ISN generator is based on the combination of a
(possibly fictitious) 32 bit clock whose low order bit is incremented
roughly every 4 microseconds, and a pseudorandom hash function (PRF).
The clock component is intended to insure 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.
TCP SHOULD generate its Initial Sequence Numbers with the expression:
ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
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").
F() MUST NOT be computable from the outside, 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 has 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 [8].
For each connection there is a send sequence number and a receive
sequence number. The initial send sequence number (ISS) is chosen by
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the data sending TCP, and the initial receive sequence number (IRS)
is learned during the connection establishing procedure.
For a connection to be established or initialized, the two TCPs 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 ISN's.
The synchronization requires each side to send it's own initial
sequence number and to receive a confirmation of it in acknowledgment
from the other side. Each side must also receive the other side'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.
A three way handshake is necessary because sequence numbers are not
tied to a global clock in the network, and TCPs may have different
mechanisms for picking the ISN's. The receiver of the first SYN has
no way of knowing whether the segment was an old delayed 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 are discussed in [3].
Knowing When to Keep Quiet
To be sure that a TCP does not create a segment that carries a
sequence number which may be duplicated by an old segment remaining
in the network, the TCP must keep quiet for a maximum segment
lifetime (MSL) before assigning any sequence numbers upon starting up
or recovering from a crash in which 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 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.
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The TCP Quiet Time Concept
This specification provides that hosts which "crash" without
retaining any 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 Maximum Segment Lifetime (MSL)
in the internet system of which the host is a part. 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 by some receivers in the internet
system.
TCPs 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
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, TCPs 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 very
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 l0's of megabits/sec. At 100 megabits/sec, the
cycle time is 5.4 minutes which may be a little short, but still
within reason.
The basic duplicate detection and sequencing algorithm in TCP can be
defeated, however, if a source TCP does not have any memory of the
sequence numbers it last used on a given connection. For example, if
the TCP were to start all connections with sequence number 0, then
upon crashing and restarting, a TCP might re-form an earlier
connection (possibly after half-open connection resolution) and emit
packets with sequence numbers identical to or overlapping with
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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 which 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 on a particular connection. Now suppose, at this
instant, the host crashes, recovers, and establishes a new
incarnation of the connection. The initial sequence number chosen is
S1 = ISN(t) -- 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
crashed 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 crash- this is the "quiet
time" specification. Hosts which prefer to avoid waiting are willing
to risk possible confusion of old and new packets at a given
destination may choose not to wait for the "quite time".
Implementors may provide TCP users with the ability to select on a
connection by connection basis whether to wait after a crash, or may
informally implement the "quite 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 crashing a
block of space-time is occupied by the octets and SYN or FIN flags of
the last emitted segment, if a new connection is started too soon and
uses any of the sequence numbers in the space-time footprint of the
last segment of the previous connection incarnation, there is a
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potential sequence number overlap area which could cause confusion at
the receiver.
3.4. Establishing a connection
The "three-way handshake" is the procedure used to establish a
connection. This procedure normally is initiated by one TCP and
responded to by another TCP. The procedure also works if two TCP
simultaneously initiate the procedure. When simultaneous attempt
occurs, each TCP receives a "SYN" segment which 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
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
doesn't deliver the data to the user until it is clear the data is
valid (i.e., the data must be buffered at the receiver until the
connection reaches the ESTABLISHED state). The three-way handshake
reduces the possibility of false connections. It is the
implementation of a trade-off between memory and messages to provide
information for this checking.
The simplest three-way handshake is shown in Figure 5 below. 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 A to TCP B, or arrival of a
segment at B from A. Left arrows (<--), indicate the reverse.
Ellipsis (...) indicates a segment which is still in the network
(delayed). An "XXX" indicates a segment which is lost or rejected.
Comments appear in parentheses. TCP 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.
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TCP A TCP 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
Basic 3-Way Handshake for Connection Synchronization
Figure 5
In line 2 of Figure 5, TCP A begins by sending a SYN segment
indicating that it will use sequence numbers starting with sequence
number 100. In line 3, TCP B sends a SYN and acknowledges the SYN it
received from TCP A. Note that the acknowledgment field indicates
TCP B is now expecting to hear sequence 101, acknowledging the SYN
which occupied sequence 100.
At line 4, TCP A responds with an empty segment containing an ACK for
TCP B's SYN; and in line 5, TCP 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 ACK's!).
Simultaneous initiation is only slightly more complex, as is shown in
Figure 6. Each TCP cycles from CLOSED to SYN-SENT to SYN-RECEIVED to
ESTABLISHED.
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TCP A TCP 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
Simultaneous Connection Synchronization
Figure 6
The principle 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, has been devised. If
the receiving TCP is in a non-synchronized state (i.e., SYN-SENT,
SYN-RECEIVED), it returns to LISTEN on receiving an acceptable reset.
If the TCP 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 A TCP 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. SYN-SENT <-- <SEQ=400><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
8. ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK> --> ESTABLISHED
Recovery from Old Duplicate SYN
Figure 7
As a simple example of recovery from old duplicates, consider
Figure 7. At line 3, an old duplicate SYN arrives at TCP B. TCP B
cannot tell that this is an old duplicate, so it responds normally
(line 4). TCP A detects that the ACK field is incorrect and returns
a RST (reset) with its SEQ field selected to make the segment
believable. TCP B, on receiving the RST, returns to the LISTEN
state. When the original SYN (pun intended) 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.
Half-Open Connections and Other Anomalies
An established connection is said to be "half-open" if one of the
TCPs 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 crash 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, and the recovery procedure is
mildly involved.
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
receiving a reset control message. Such a message indicates to the
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site B TCP that something is wrong, and it is expected to abort the
connection.
Assume that two user processes A and B are communicating with one
another when a crash occurs causing loss of memory to A's TCP.
Depending on the operating system supporting A's TCP, it is likely
that some error recovery mechanism exists. When the TCP 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. In an attempt to establish the connection, A's TCP
will send a segment containing SYN. This scenario leads to the
example shown in Figure 8. After TCP A crashes, the user attempts to
re-open the connection. TCP B, in the meantime, thinks the
connection is open.
TCP A TCP B
1. (CRASH) (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> -->
Half-Open Connection Discovery
Figure 8
When the SYN arrives at line 3, TCP 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 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 B aborts at line 5. TCP A will
continue to try to establish the connection; the problem is now
reduced to the basic 3-way handshake of Figure 5.
An interesting alternative case occurs when TCP A crashes and TCP B
tries to send data on what it thinks is a synchronized connection.
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This is illustrated in Figure 9. In this case, the data arriving at
TCP A from TCP B (line 2) is unacceptable because no such connection
exists, so TCP A sends a RST. The RST is acceptable so TCP B
processes it and aborts the connection.
TCP A TCP B
1. (CRASH) (send 300,receive 100)
2. (??) <-- <SEQ=300><ACK=100><DATA=10><CTL=ACK> <-- ESTABLISHED
3. --> <SEQ=100><CTL=RST> --> (ABORT!!)
Active Side Causes Half-Open Connection Discovery
Figure 9
In Figure 10, we find the two TCPs A and B with passive connections
waiting for SYN. An old duplicate arriving at TCP 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 B accepts
the reset and returns to its passive LISTEN state.
TCP A TCP 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
Old Duplicate SYN Initiates a Reset on two Passive Sockets
Figure 10
A variety of other cases are possible, all of which are accounted for
by the following rules for RST generation and processing.
Reset Generation
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As a general rule, reset (RST) must be sent whenever a segment
arrives which 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. In
particular, SYNs addressed to a non-existent connection are
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
which does not exactly match the level and compartment requested
for the connection, a reset is sent.
If our SYN has not been acknowledged and the precedence level of
the incoming segment is higher than the precedence level requested
then either raise the local precedence level (if allowed by the
user and the system) or send a reset; or if the precedence level
of the incoming segment is lower than the precedence level
requested then continue as if the precedence matched exactly (if
the remote TCP cannot raise the precedence level to match ours
this will be detected in the next segment it sends, and the
connection will be terminated then). If our SYN has been
acknowledged (perhaps in this incoming segment) the precedence
level of the incoming segment must match the local precedence
level exactly, if it does not a reset must be 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 elicit only an empty
acknowledgment segment containing the current send-sequence number
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and an acknowledgment indicating the next sequence number expected
to be received, and the connection remains in the same state.
If an incoming segment has a security level, or compartment, or
precedence which does not exactly match the level, and
compartment, and precedence 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.
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.
3.5. 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 he is told that the other side 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
other side has CLOSED. We assume that the TCP will signal a user,
even if no RECEIVEs are outstanding, that the other side has closed,
so the user can terminate his side gracefully. A TCP 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 his data was received at the
destination TCP. Users must keep reading connections they close for
sending until the TCP says no more data.
There are essentially three cases:
1) The user initiates by telling the TCP to CLOSE the connection
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2) The remote TCP initiates by sending a FIN control signal
3) Both users CLOSE simultaneously
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, 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
has both acknowledged the FIN and sent a FIN of its own, the first
TCP can ACK this FIN. Note that a TCP receiving a FIN will ACK
but not send its own FIN until its user has CLOSED the connection
also.
Case 2: TCP receives a FIN from the network
If an unsolicited FIN arrives from the network, the receiving TCP
can ACK it and tell the user that the connection is closing. The
user will respond with a CLOSE, upon which the TCP can send a FIN
to the other TCP after sending any remaining data. The TCP 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. When all segments preceding the
FINs have been processed and acknowledged, each TCP can ACK the
FIN it has received. Both will, upon receiving these ACKs, delete
the connection.
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TCP A TCP 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
Normal Close Sequence
Figure 11
TCP A TCP 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
Simultaneous Close Sequence
Figure 12
3.6. Precedence and Security
The intent is that connection be allowed only between ports operating
with exactly the same security and compartment values and at the
higher of the precedence level requested by the two ports.
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The precedence and security parameters used in TCP are exactly those
defined in the Internet Protocol (IP) [2]. Throughout this TCP
specification the term "security/compartment" is intended to indicate
the security parameters used in IP including security, compartment,
user group, and handling restriction.
A connection attempt with mismatched security/compartment values or a
lower precedence value must be rejected by sending a reset.
Rejecting a connection due to too low a precedence only occurs after
an acknowledgment of the SYN has been received.
Note that TCP modules which operate only at the default value of
precedence will still have to check the precedence of incoming
segments and possibly raise the precedence level they use on the
connection.
The security parameters may be used even in a non-secure environment
(the values would indicate unclassified data), thus hosts in non-
secure environments must be prepared to receive the security
parameters, though they need not send them.
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.
For efficiency and performance reasons, it is desirable to send large
segments that contain as many bytes of payload data as possible.
However, packets that are too long will either be fragmented or
dropped within the network. Some firewalls or middleboxes may drop
fragmented packets. In either case, when packets are dropped, the
connection can fail; hence, it is best for a TCP implementation to
avoid generating fragments.
To enable a TCP sender to maximize the size of segments that it
sends, without generating fragments, TCP includes the Maximum Segment
Size option to convey endpoint information, and TCP implementations
also support Path MTU Discovery to discover the limits and
capabilites of intermediate networks.
When TCP is used in a situation where either the IP or TCP headers
are not minimum, 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.
The rationale for this is explained in RFC 6691.
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3.7.1. Maximum Segment Size Option
TCP MUST implement both sending and receiving the Maximum Segment
Size option.
TCP SHOULD send an MSS (Maximum Segment Size) option in every SYN
segment when its receive MSS differs from the default 536, and MAY
send it always.
If an MSS option is not received at connection setup, TCP MUST assume
a default send MSS of 536 (576-40).
The maximum size of a segment that TCP really sends, the "effective
send MSS," MUST be the smaller of the send MSS (which reflects the
available reassembly buffer size at the remote host) and the largest
size permitted by the IP layer:
Eff.snd.MSS =
min(SendMSS+20, MMS_S) - TCPhdrsize - IPoptionsize
where:
o SendMSS is the MSS value received from the remote host, or the
default 536 if no MSS option is received.
o MMS_S is the maximum size for a transport-layer message that TCP
may send.
o TCPhdrsize is the size of the fixed TCP header; this is normally
20, but may be larger if TCP options are to be sent.
o IPoptionsize is the size of any IP options that TCP will pass to
the IP layer with the current message.
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
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
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
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where MMS_R is the maximum size for a transport-layer message that
can be received (and reassembled). TCP obtains MMS_R and MMS_S from
the IP layer; see the generic call GET_MAXSIZES in Section 3.4 of RFC
1122.
3.7.2. Path MTU Discovery
The TCP MSS option specifies an upper bound for the size of packets
that can be received. Hence, setting the value in the MSS option too
small can impact the ability for Path MTU Discovery to find a larger
path MTU. For more information on Path MTU Discovery, see:
o "Path MTU Discovery" [RFC1191]
o "TCP Problems with Path MTU Discovery" [RFC2923]
o "Packetization Layer Path MTU Discovery" [RFC4821]
3.7.3. Interfaces with Variable MSS Values
The effective MTU can sometimes vary, as when used with variable
compression, e.g., RObust Header Compression (ROHC) [RFC5795]. It is
tempting for TCP to want 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, TCP
SHOULD use the smallest effective MTU of the interface to calculate
the value to advertise in the MSS option.
3.7.4. IPv6 Jumbograms
In order to support TCP over IPv6 jumbograms, implementations need to
be able to send TCP segments larger than 64K. RFC 2675 [RFC2675]
defines that a value of 65,535 is to be treated as infinity, and Path
MTU Discovery [RFC1981] is used to determine the actual MSS.
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 (after a timeout) to ensure delivery of every segment.
Duplicate segments may arrive due to network or TCP retransmission.
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As discussed in the section on sequence numbers the TCP 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, as does the FIN control
flag in an incoming segment.
Retransmission Timeout
NOTE: TODO this needs to be updated in light of 1122 4.2.2.15 and
errata 573; this will be done as part of RFC 1122 incorporation into
this document.
Because of the variability of the networks that compose an
internetwork system and the wide range of uses of TCP connections the
retransmission timeout must be dynamically determined. One procedure
for determining a retransmission timeout is given here as an
illustration.
An Example Retransmission Timeout Procedure
Measure the elapsed time between sending a data octet with a
particular sequence number and receiving an acknowledgment that
covers that sequence number (segments sent do not have to match
segments received). This measured elapsed time is the Round Trip
Time (RTT). Next compute a Smoothed Round Trip Time (SRTT) as:
SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)
and based on this, compute the retransmission timeout (RTO) as:
RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]
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where UBOUND is an upper bound on the timeout (e.g., 1 minute),
LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
factor (e.g., 1.3 to 2.0).
The Communication of Urgent Information
As a result of implementation differences and middlebox interactions,
new applications SHOULD NOT employ the TCP urgent mechanism.
However, TCP implementations MUST still include support for the
urgent mechanism. Details can be found in RFC 6093 [7].
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 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, that TCP
must tell the user to go into "urgent mode"; when the receive
sequence number catches up to the urgent pointer, the TCP 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 a urgent field which 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.
A TCP MUST support a sequence of urgent data of any length. [3]
A TCP MUST inform the application layer asynchronously whenever it
receives an Urgent pointer and there was previously no pending urgent
data, or whenvever the Urgent pointer advances in the data stream.
There MUST be a way for the application to learn how much urgent data
remains to be read from the connection, or at least to determine
whether or not more urgent data remains to be read. [3]
Managing the Window
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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.
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 TCPs. 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 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 dictates that TCPs
will not shrink the window themselves, but will be prepared for such
behavior on the part of other TCPs.
The sending TCP must be prepared to accept from the user and send at
least one octet of new data even if the send window is zero. The
sending TCP must regularly retransmit to the receiving TCP even when
the window is zero. Two minutes is recommended for the
retransmission interval when the window is zero. This retransmission
is essential to guarantee that when either TCP has a zero window the
re-opening of the window will be reliably reported to the other.
When the receiving TCP 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 sending TCP packages the data to be transmitted into segments
which 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).
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The window management procedure has significant influence on the
communication performance. The following comments are suggestions to
implementers.
Window Management Suggestions
Allocating a very small window causes data to be transmitted in
many small segments when better performance is achieved using
fewer large segments.
One suggestion for avoiding small windows is for the receiver to
defer updating a window until the additional allocation is at
least X percent of the maximum allocation possible for the
connection (where X might be 20 to 40).
Another suggestion is for the sender to avoid sending small
segments by waiting until the window is large enough before
sending data. If the user signals a push function then the data
must be sent even if it is a small segment.
Note that the acknowledgments should not be delayed or unnecessary
retransmissions will result. One strategy would be to send an
acknowledgment when a small segment arrives (with out updating the
window information), and then to send another acknowledgment with
new window information when the window is larger.
The segment sent to probe a zero window may also begin a break up
of transmitted data into smaller and smaller segments. If a
segment containing a single data octet sent to probe a zero window
is accepted, it consumes one octet of the window now available.
If the sending TCP simply sends as much as it can whenever the
window is non zero, the transmitted data will be broken into
alternating big and small segments. As time goes on, occasional
pauses in the receiver making window allocation available will
result in breaking the big segments into a small and not quite so
big pair. And after a while the data transmission will be in
mostly small segments.
The suggestion here is that the TCP implementations need to
actively attempt to combine small window allocations into larger
windows, since the mechanisms for managing the window tend to lead
to many small windows in the simplest minded implementations.
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
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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 TCPs might use.
3.9.1. User/TCP Interface
The following functional description of user commands to the TCP 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
TCPs 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.
TCP User Commands
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 (e.g., SVCs, UUOs,
EMTs).
The user commands described below specify the basic functions the
TCP 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 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 foreign socket).
(b) replies to specific user commands indicating success or
various types of failure.
Open
Format: OPEN (local port, foreign socket, active/passive [,
timeout] [, precedence] [, security/compartment] [, options])
-> local connection name
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We assume that the local TCP is aware of the identity of the
processes it serves and will check the authority of the process
to use the connection specified. Depending upon the
implementation of the TCP, the local network and TCP
identifiers for the source address will either be supplied by
the TCP or the lower level protocol (e.g., IP). These
considerations are the result of concern about security, to the
extent that no TCP be able to masquerade as another one, and so
on. Similarly, no process can masquerade as another without
the collusion of the TCP.
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 foreign socket to wait for a
particular connection or an unspecified foreign 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.
On an active OPEN command, the TCP 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
will abort the connection. The present global default is five
minutes.
The TCP or some component of the operating system will verify
the users authority to open a connection with the specified
precedence or security/compartment. The absence of precedence
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 and only
if the precedence is equal to or higher than the precedence
requested in the OPEN call.
The precedence for the connection is the higher of the values
requested in the OPEN call and received from the incoming
request, and fixed at that value for the life of the
connection.Implementers may want to give the user control of
this precedence negotiation. For example, the user might be
allowed to specify that the precedence must be exactly matched,
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or that any attempt to raise the precedence be confirmed by the
user.
A local connection name will be returned to the user by the
TCP. The local connection name can then be used as a short
hand term for the connection defined by the <local socket,
foreign socket> pair.
Send
Format: SEND (local connection name, buffer address, byte
count, PUSH flag, URGENT flag [,timeout])
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. If the calling
process is not authorized to use this connection, an error is
returned.
If the PUSH flag is set, the data must be transmitted promptly
to the receiver, and the PUSH bit will be set in the last TCP
segment created from the buffer. If the PUSH flag is not set,
the data may be combined with data from subsequent SENDs for
transmission efficiency.
New applications SHOULD NOT set the URGENT flag [7] due to
implementation differences and middlebox issues.
If the URGENT flag is set, segments sent to the destination TCP
will have the urgent pointer set. The receiving TCP 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 to indicate to the receiver when all the currently known
urgent data has been received. The number of times the sending
user's TCP 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 foreign socket was specified in the OPEN, but the
connection is established (e.g., because a LISTENing connection
has become specific due to a foreign segment arriving for the
local socket), then the designated buffer is sent to the
implied foreign socket. Users who make use of OPEN with an
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unspecified foreign socket can make use of SEND without ever
explicitly knowing the foreign socket address.
However, if a SEND is attempted before the foreign socket
becomes specified, an error will be returned. Users can use
the STATUS call to determine the status of the connection. In
some implementations the TCP 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 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. 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. 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.
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 which should be returned to the
calling process.
Receive
Format: RECEIVE (local connection name, buffer address, byte
count) -> byte count, urgent flag, push flag
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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.
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.
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
allocate buffer storage, or the TCP might share a ring buffer
with the user.
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
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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 other
side 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 gives up.
The user may CLOSE the connection at any time on his own
initiative, or in response to various prompts from the TCP
(e.g., remote close executed, transmission timeout exceeded,
destination inaccessible).
Because closing a connection requires communication with the
foreign TCP, connections may remain in the closing state for a
short time. Attempts to reopen the connection before the TCP
replies to the CLOSE command will result in error responses.
Close also implies push function.
Status
Format: STATUS (local connection name) -> status data
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,
foreign socket,
local connection name,
receive window,
send window,
connection state,
number of buffers awaiting acknowledgment,
number of buffers pending receipt,
urgent state,
precedence,
security/compartment,
and transmission timeout.
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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.
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 TCP on the other side 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.
TCP-to-User Messages
It is assumed that the operating system environment provides a
means for the TCP to asynchronously signal the user program.
When the TCP does signal a user program, certain information is
passed to the user. Often in the specification the information
will be an error message. In other cases there will be
information relating to the completion of processing a SEND or
RECEIVE or other user call.
The following information is provided:
Local Connection Name Always
Response String Always
Buffer Address Send & Receive
Byte count (counts bytes received) Receive
Push flag Receive
Urgent flag Receive
3.9.2. TCP/Lower-Level Interface
The TCP calls on a lower level protocol module to actually send and
receive information over a network. One case is that of the ARPA
internetwork system where the lower level module is the Internet
Protocol (IP) [2].
If the lower level protocol is IP it provides arguments for a type of
service and for a time to live. TCP uses the following settings for
these parameters:
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Type of Service = Precedence: given by user, Delay: normal,
Throughput: normal, Reliability: normal; or binary XXX00000, where
XXX are the three bits determining precedence, e.g. 000 means
routine precedence.
Time to Live = one minute, or 00111100.
Note that the assumed maximum segment lifetime is two minutes.
Here we explicitly ask that a segment be destroyed if it cannot
be delivered by the internet system within one minute.
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.
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.
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.
The activity of the TCP 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 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
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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 are given as 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).
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 stays in the same
state.
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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,
foreign socket, precedence, security/compartment, and user
timeout information. Note that some parts of the foreign
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 precedence requested are allowed for
this user, if not return "error: precedence not allowed" or
"error: security/compartment not allowed." If passive enter
the LISTEN state and return. If active and the foreign socket
is unspecified, return "error: foreign socket unspecified"; if
active and the foreign 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
If active and the foreign 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 Foreign socket was not specified,
then return "error: foreign socket unspecified".
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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".
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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 foreign 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
Foreign socket was not specified, then return "error: foreign
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.
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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
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 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 text already on hand, but not yet delivered to the
user. If no text is awaiting delivery, the RECEIVE will get a
"error: connection closing" response. Otherwise, any remaining
text can be used to satisfy the RECEIVE.
CLOSING STATE
LAST-ACK STATE
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TIME-WAIT STATE
Return "error: connection closing".
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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 a
"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
LAST-ACK STATE
TIME-WAIT STATE
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Respond with "error: connection closing".
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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:
<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
Respond with "ok" and delete the TCB, enter CLOSED state, and
return.
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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.
TIME-WAIT STATE
Return "state = TIME-WAIT", and the TCB pointer.
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SEGMENT ARRIVES
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 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.
If the state is LISTEN then
first check for an RST
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:
<SEQ=SEG.ACK><CTL=RST>
Return.
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.
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
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If the SEG.PRC is greater than the TCB.PRC then if allowed
by the user and the system set TCB.PRC<-SEG.PRC, if not
allowed send a reset and return.
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the SEG.PRC is less than the TCB.PRC then continue.
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:
<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 foreign socket was not fully
specified), then the unspecified fields should be filled in
now.
fourth other text or control
Any other control or text-bearing segment (not containing
SYN) must have an ACK and thus would be discarded by the ACK
processing. 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. So you are
unlikely to get here, but if you do, drop the segment, and
return.
If the state is SYN-SENT then
first check the ACK bit
If the ACK bit is set
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>
and discard the segment. Return.
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If SND.UNA < SEG.ACK =< SND.NXT then the ACK is
acceptable. (TODO: in processing Errata ID 3300, it was
noted that some stacks in the wild that do not send data
on the SYN are just checking that SEG.ACK == SND.NXT ...
think about whether anything should be said about that
here)
second check the RST bit
If the RST bit is set
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 and precedence
If the security/compartment in the segment does not exactly
match the security/compartment in the TCB, send a reset
If there is an ACK
<SEQ=SEG.ACK><CTL=RST>
Otherwise
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If there is an ACK
The precedence in the segment must match the precedence
in the TCB, if not, send a reset
<SEQ=SEG.ACK><CTL=RST>
If there is no ACK
If the precedence in the segment is higher than the
precedence in the TCB then if allowed by the user and the
system raise the precedence in the TCB to that in the
segment, if not allowed to raise the prec then send a
reset.
<SEQ=0><ACK=SEG.SEQ+SEG.LEN><CTL=RST,ACK>
If the precedence in the segment is lower than the
precedence in the TCB continue.
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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 it the segment did not contain a RST.
If the SYN bit is on and the security/compartment and
precedence are 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 which 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
<SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
and send it. Data or controls which were queued for
transmission may be included. If there are other controls
or text in the segment then continue processing at the sixth
step below where the URG bit is checked, otherwise return.
Otherwise enter SYN-RECEIVED, form a SYN,ACK segment
<SEQ=ISS><ACK=RCV.NXT><CTL=SYN,ACK>
and send it. Set the variables:
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.
fifth, if neither of the SYN or RST bits is set then drop the
segment and return.
Otherwise,
first check sequence number
SYN-RECEIVED STATE
ESTABLISHED STATE
FIN-WAIT-1 STATE
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FIN-WAIT-2 STATE
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
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 should be processed.
There are four cases for the acceptability test for 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
If the RCV.WND is zero, no segments will be acceptable, but
special allowance should be made to accept valid ACKs, URGs
and RSTs.
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>
After sending the acknowledgment, drop the unacceptable
segment and return.
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
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if the segment then begins at RCV.NXT. Segments with higher
beginning sequence numbers should be held for later
processing.
second check the RST bit,
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, all segments on
the retransmission queue should be removed. And in
the active OPEN case, enter the CLOSED state and
delete the TCB, and return.
ESTABLISHED
FIN-WAIT-1
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.
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 and precedence
SYN-RECEIVED
If the security/compartment and precedence in the segment
do not exactly match the security/compartment and
precedence in the TCB then send a reset, and return.
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ESTABLISHED
FIN-WAIT-1
FIN-WAIT-2
CLOSE-WAIT
CLOSING
LAST-ACK
TIME-WAIT
If the security/compartment and precedence in the segment
do not exactly match the security/compartment and
precedence 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.
Note this check is placed following the sequence check to
prevent a segment from an old connection between these ports
with a different security or precedence from causing an
abort of the current connection.
fourth, check the SYN bit,
SYN-RECEIVED
ESTABLISHED STATE
FIN-WAIT STATE-1
FIN-WAIT STATE-2
CLOSE-WAIT STATE
CLOSING STATE
LAST-ACK STATE
TIME-WAIT STATE
TODO: need to incorporate RFC 1122 4.2.2.20(e) here
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,
if the ACK bit is off drop the segment and return
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if the ACK bit is on
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,
<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
which are thereby entirely acknowledged are removed.
Users should receive positive acknowledgments for
buffers which 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
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In addition to the processing for the ESTABLISHED
state, if our FIN 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.
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,
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.
CLOSE-WAIT STATE
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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,
ESTABLISHED STATE
FIN-WAIT-1 STATE
FIN-WAIT-2 STATE
Once in the ESTABLISHED state, it is possible to deliver
segment text to user RECEIVE buffers. Text from segments
can be moved into buffers until either the buffer is full
or the segment is empty. If the segment empties and
carries an PUSH flag, then the user is informed, when the
buffer is returned, that a PUSH has been received.
When the TCP takes responsibility for delivering the data
to the user it must also acknowledge the receipt of the
data.
Once the TCP 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.
Please note the window management suggestions in section
3.7.
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.
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,
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.
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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USER TIMEOUT
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.
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3.11. Glossary
1822 BBN Report 1822, "The Specification of the Interconnection of
a Host and an IMP". The specification of interface between a
host and the ARPANET.
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.
ARPANET message
The unit of transmission between a host and an IMP in the
ARPANET. The maximum size is about 1012 octets (8096 bits).
ARPANET packet
A unit of transmission used internally in the ARPANET between
IMPs. The maximum size is about 126 octets (1008 bits).
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 destination address, usually the network and host
identifiers.
FIN
A control bit (finis) occupying one sequence number, which
indicates that the sender will send no more data or control
occupying sequence space.
fragment
A portion of a logical unit of data, in particular an
internet fragment is a portion of an internet datagram.
FTP
A file transfer protocol.
header
Control information at the beginning of a message, segment,
fragment, packet or block of data.
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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.
IMP
The Interface Message Processor, the packet switch of the
ARPANET.
internet address
A source or destination address specific to the host level.
internet datagram
The unit of data exchanged between an internet module and the
higher level protocol together with the internet header.
internet fragment
A portion of the data of an internet datagram with an
internet header.
IP
Internet Protocol.
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.
leader
Control information at the beginning of a message or block of
data. In particular, in the ARPANET, the control information
on an ARPANET message at the host-IMP interface.
left sequence
This is the next sequence number to be acknowledged by the
data receiving TCP (or the lowest currently unacknowledged
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sequence number) and is sometimes referred to as the left
edge of the send window.
local packet
The unit of transmission within a local network.
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. The options are used
primarily in testing situations; for example, to carry
timestamps. Both the Internet Protocol and TCP provide for
options fields.
packet
A package of data with a header which may or may not be
logically complete. More often a physical packaging than a
logical packaging of data.
port
The portion of a socket that specifies which logical input or
output channel of a process is associated with the data.
process
A program in execution. A source or destination of data from
the point of view of the TCP 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 is expecting
to receive.
receive window
This represents the sequence numbers the local (receiving)
TCP is willing to receive. Thus, the local TCP 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 of this range
are considered duplicates 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.
RTP
Real Time Protocol: A host-to-host protocol for communication
of time critical information.
SEG.ACK
segment acknowledgment
SEG.LEN
segment length
SEG.PRC
segment precedence value
SEG.SEQ
segment sequence
SEG.UP
segment urgent pointer field
SEG.WND
segment window field
segment
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A logical unit of data, in particular a TCP segment is the
unit of data transfered between a pair of TCP modules.
segment acknowledgment
The sequence number in the acknowledgment field of the
arriving segment.
segment length
The amount of sequence number space occupied by a segment,
including any controls which 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 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 which the remote
(receiving) TCP is willing to receive. It is the value of
the window field specified in segments from the remote (data
receiving) TCP. The range of new sequence numbers which may
be emitted by a TCP 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
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An address which specifically includes a port identifier,
that is, the concatenation of an Internet Address with a TCP
port.
Source Address
The source address, usually the network and host identifiers.
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.
TCB.PRC
The precedence of the connection.
TCP
Transmission Control Protocol: A host-to-host protocol for
reliable communication in internetwork environments.
TOS
Type of Service, an Internet Protocol field.
Type of Service
An Internet Protocol field which indicates the type of
service for this internet fragment.
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 in 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 which
indicates the data octet associated with the sending user's
urgent call.
4. 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 the
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informational text with background and rationale has not 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.
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, 1565, 1571,
1572, 2296, 2297, 2298, 2748, 2749, 2934, 3213, 3300, 3301). Some
errata were not applicable due to other changes (Errata IDs: 572,
575, 1569, 3602). TODO: 3305
Changes to the specification of the Urgent Pointer described in RFC
1122 and 6093 were incorporated. See RFC 6093 for detailed
discussion of why these changes were necessary.
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.
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.
The -00 revision of this document was merely a proposal and rough
plan for updating RFC 793.
The -01 revision of this document 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 this document incorporates errata that have been
verified:
Errata ID 573: Reported by Bob Braden (note: This errata 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
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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
Errata ID 3300: Reported by Botong Huang
Errata ID 3301: 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.
There is an errata 3305 currently reported that need to be
verified, held, or rejected by the ADs; it is addressing the same
issue as draft-gont-tcpm-tcp-seq-validation and was not attempted
to be applied to this document.
Not related to RFC 793 content, this revision also makes small tweaks
to the introductory text, fixes indentation of the pseudoheader
diagram, and notes that the Security Considerations should also
include privacy, when this section is written.
The -03 revision of this document 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 this document includes the ISN generation changes
from RFC 6528.
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The -05 revision of this document incorporates MSS requirements and
definitions from RFC 879, 1122, and 6691, as well as option-handling
requirements from RFC 1122.
TODO: Incomplete list of planned changes - these need to be added to
and made more specific, as the document proceeds:
1. incorporate 1122 additions
2. point to major additional docs like 1323bis and 5681
3. incorporate relevant parts of 3168 (ECN)
4. incorporate Fernando's new number-checking fixes (if past the
IESG in time)
5. point to PMTUD?
6. point to 5461 (soft errors)
7. mention 5961 state machine option
8. mention 6161 (reducing TIME-WAIT)
9. incorporate 6429 (ZWP/persist)
10. look at Tony Sabatini suggestion for describing DO field
11. clearly specify treatment of reserved bits (see TCPM thread on
EDO draft April 25, 2014)
12. look at possible mention of draft-minshall-nagle (e.g. as in
Linux)
13. make sure that clarifications in RFC 1011 are captured
14. per TCPM discussion, discussion of checking reserved bits may
need to be altered from 793
15. MSL acronymn is defined multiple times
5. IANA Considerations
This memo includes no request to IANA. Existing IANA registries for
TCP parameters are sufficient.
TODO: check whether entries pointing to 793 and other documents
obsoleted by this one should be updated to point to this one instead.
6. Security and Privacy Considerations
TODO
See RFC 6093 [7] for discussion of security considerations related to
the urgent pointer field.
Editor's Note: Scott Brim mentioned that this should include a
PERPASS/privacy review.
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7. 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:
Michael Scharf
Yoshifumi Nishida
Pasi Sarolahti
On the TCPM mailing list, and at the IETF 88 meeting in Vancouver,
helpful comments, critiques, and reviews were received from (listed
alphebetically): David Borman, Yuchung Cheng, Martin Duke, Kevin
Lahey, Kevin Mason, Matt Mathis, Hagen Paul Pfeifer, Anthony
Sabatini, Joe Touch, Reji Varghese, Lloyd Wood, and Alex Zimmermann.
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.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[2] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[3] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[4] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[5] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
2923, September 2000.
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[6] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[7] Gont, F. and A. Yourtchenko, "On the Implementation of the
TCP Urgent Mechanism", RFC 6093, January 2011.
[8] Gont, F. and S. Bellovin, "Defending against Sequence
Number Attacks", RFC 6528, February 2012.
[9] Borman, D., "TCP Options and Maximum Segment Size (MSS)",
RFC 6691, July 2012.
[10] Duke, M., Braden, R., Eddy, W., Blanton, E., and A.
Zimmermann, "A Roadmap for Transmission Control Protocol
(TCP) Specification Documents", RFC 7414, February 2015.
Appendix A. TCP Requirement Summary
This section is adapted from RFC 1122.
TODO: this needs to be seriously redone, to use 793bis section
numbers instead of 1122 ones, and all 1122 requirements need to be
reflected in 793bis text.
RFC EDITOR'S NOTE: 793bis in the heading below should be replaced by
the number of this RFC
| | | | |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
|RFC1122 |T|D|Y|O|O|t
FEATURE |SECTION | | | |T|T|e
-------------------------------------------------|--------|-|-|-|-|-|--
| | | | | | |
Push flag | | | | | | |
Aggregate or queue un-pushed data |4.2.2.2 | | |x| | |
Sender collapse successive PSH flags |4.2.2.2 | |x| | | |
SEND call can specify PUSH |4.2.2.2 | | |x| | |
If cannot: sender buffer indefinitely |4.2.2.2 | | | | |x|
If cannot: PSH last segment |4.2.2.2 |x| | | | |
Notify receiving ALP of PSH |4.2.2.2 | | |x| | |1
Send max size segment when possible |4.2.2.2 | |x| | | |
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| | | | | | |
Window | | | | | | |
Treat as unsigned number |4.2.2.3 |x| | | | |
Handle as 32-bit number |4.2.2.3 | |x| | | |
Shrink window from right |4.2.2.16| | | |x| |
Robust against shrinking window |4.2.2.16|x| | | | |
Receiver's window closed indefinitely |4.2.2.17| | |x| | |
Sender probe zero window |4.2.2.17|x| | | | |
First probe after RTO |4.2.2.17| |x| | | |
Exponential backoff |4.2.2.17| |x| | | |
Allow window stay zero indefinitely |4.2.2.17|x| | | | |
Sender timeout OK conn with zero wind |4.2.2.17| | | | |x|
| | | | | | |
Urgent Data | | | | | | |
Pointer indicates first non-urgent octet |4.2.2.4 |x| | | | |
Arbitrary length urgent data sequence |4.2.2.4 |x| | | | |
Inform ALP asynchronously of urgent data |4.2.2.4 |x| | | | |1
ALP can learn if/how much urgent data Q'd |4.2.2.4 |x| | | | |1
| | | | | | |
TCP Options | | | | | | |
Receive TCP option in any segment |4.2.2.5 |x| | | | |
Ignore unsupported options |4.2.2.5 |x| | | | |
Cope with illegal option length |4.2.2.5 |x| | | | |
Implement sending & receiving MSS option |4.2.2.6 |x| | | | |
Send MSS option unless 536 |4.2.2.6 | |x| | | |
Send MSS option always |4.2.2.6 | | |x| | |
Send-MSS default is 536 |4.2.2.6 |x| | | | |
Calculate effective send seg size |4.2.2.6 |x| | | | |
| | | | | | |
TCP Checksums | | | | | | |
Sender compute checksum |4.2.2.7 |x| | | | |
Receiver check checksum |4.2.2.7 |x| | | | |
| | | | | | |
ISN Selection | | | | | | |
Include a clock-driven ISN generator component |4.2.2.9 |x| | | | |
Secure ISN generator with a PRF component | N/A | |x| | | |
| | | | | | |
Opening Connections | | | | | | |
Support simultaneous open attempts |4.2.2.10|x| | | | |
SYN-RCVD remembers last state |4.2.2.11|x| | | | |
Passive Open call interfere with others |4.2.2.18| | | | |x|
Function: simultan. LISTENs for same port |4.2.2.18|x| | | | |
Ask IP for src address for SYN if necc. |4.2.3.7 |x| | | | |
Otherwise, use local addr of conn. |4.2.3.7 |x| | | | |
OPEN to broadcast/multicast IP Address |4.2.3.14| | | | |x|
Silently discard seg to bcast/mcast addr |4.2.3.14|x| | | | |
| | | | | | |
Closing Connections | | | | | | |
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RST can contain data |4.2.2.12| |x| | | |
Inform application of aborted conn |4.2.2.13|x| | | | |
Half-duplex close connections |4.2.2.13| | |x| | |
Send RST to indicate data lost |4.2.2.13| |x| | | |
In TIME-WAIT state for 2xMSL seconds |4.2.2.13|x| | | | |
Accept SYN from TIME-WAIT state |4.2.2.13| | |x| | |
| | | | | | |
Retransmissions | | | | | | |
Jacobson Slow Start algorithm |4.2.2.15|x| | | | |
Jacobson Congestion-Avoidance algorithm |4.2.2.15|x| | | | |
Retransmit with same IP ident |4.2.2.15| | |x| | |
Karn's algorithm |4.2.3.1 |x| | | | |
Jacobson's RTO estimation alg. |4.2.3.1 |x| | | | |
Exponential backoff |4.2.3.1 |x| | | | |
SYN RTO calc same as data |4.2.3.1 | |x| | | |
Recommended initial values and bounds |4.2.3.1 | |x| | | |
| | | | | | |
Generating ACK's: | | | | | | |
Queue out-of-order segments |4.2.2.20| |x| | | |
Process all Q'd before send ACK |4.2.2.20|x| | | | |
Send ACK for out-of-order segment |4.2.2.21| | |x| | |
Delayed ACK's |4.2.3.2 | |x| | | |
Delay < 0.5 seconds |4.2.3.2 |x| | | | |
Every 2nd full-sized segment ACK'd |4.2.3.2 |x| | | | |
Receiver SWS-Avoidance Algorithm |4.2.3.3 |x| | | | |
| | | | | | |
Sending data | | | | | | |
Configurable TTL |4.2.2.19|x| | | | |
Sender SWS-Avoidance Algorithm |4.2.3.4 |x| | | | |
Nagle algorithm |4.2.3.4 | |x| | | |
Application can disable Nagle algorithm |4.2.3.4 |x| | | | |
| | | | | | |
Connection Failures: | | | | | | |
Negative advice to IP on R1 retxs |4.2.3.5 |x| | | | |
Close connection on R2 retxs |4.2.3.5 |x| | | | |
ALP can set R2 |4.2.3.5 |x| | | | |1
Inform ALP of R1<=retxs<R2 |4.2.3.5 | |x| | | |1
Recommended values for R1, R2 |4.2.3.5 | |x| | | |
Same mechanism for SYNs |4.2.3.5 |x| | | | |
R2 at least 3 minutes for SYN |4.2.3.5 |x| | | | |
| | | | | | |
Send Keep-alive Packets: |4.2.3.6 | | |x| | |
- Application can request |4.2.3.6 |x| | | | |
- Default is "off" |4.2.3.6 |x| | | | |
- Only send if idle for interval |4.2.3.6 |x| | | | |
- Interval configurable |4.2.3.6 |x| | | | |
- Default at least 2 hrs. |4.2.3.6 |x| | | | |
- Tolerant of lost ACK's |4.2.3.6 |x| | | | |
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| | | | | | |
IP Options | | | | | | |
Ignore options TCP doesn't understand |4.2.3.8 |x| | | | |
Time Stamp support |4.2.3.8 | | |x| | |
Record Route support |4.2.3.8 | | |x| | |
Source Route: | | | | | | |
ALP can specify |4.2.3.8 |x| | | | |1
Overrides src rt in datagram |4.2.3.8 |x| | | | |
Build return route from src rt |4.2.3.8 |x| | | | |
Later src route overrides |4.2.3.8 | |x| | | |
| | | | | | |
Receiving ICMP Messages from IP |4.2.3.9 |x| | | | |
Dest. Unreach (0,1,5) => inform ALP |4.2.3.9 | |x| | | |
Dest. Unreach (0,1,5) => abort conn |4.2.3.9 | | | | |x|
Dest. Unreach (2-4) => abort conn |4.2.3.9 | |x| | | |
Source Quench => slow start |4.2.3.9 | |x| | | |
Time Exceeded => tell ALP, don't abort |4.2.3.9 | |x| | | |
Param Problem => tell ALP, don't abort |4.2.3.9 | |x| | | |
| | | | | | |
Address Validation | | | | | | |
Reject OPEN call to invalid IP address |4.2.3.10|x| | | | |
Reject SYN from invalid IP address |4.2.3.10|x| | | | |
Silently discard SYN to bcast/mcast addr |4.2.3.10|x| | | | |
| | | | | | |
TCP/ALP Interface Services | | | | | | |
Error Report mechanism |4.2.4.1 |x| | | | |
ALP can disable Error Report Routine |4.2.4.1 | |x| | | |
ALP can specify TOS for sending |4.2.4.2 |x| | | | |
Passed unchanged to IP |4.2.4.2 | |x| | | |
ALP can change TOS during connection |4.2.4.2 | |x| | | |
Pass received TOS up to ALP |4.2.4.2 | | |x| | |
FLUSH call |4.2.4.3 | | |x| | |
Optional local IP addr parm. in OPEN |4.2.4.4 |x| | | | |
-------------------------------------------------|--------|-|-|-|-|-|--
FOOTNOTES: (1) "ALP" means Application-Layer program.
Author's Address
Wesley M. Eddy (editor)
MTI Systems
US
Email: wes@mti-systems.com
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