Internet DRAFT - draft-briscoe-tcpm-inner-space
draft-briscoe-tcpm-inner-space
TCP Maintenance and Minor Extensions (tcpm) B. Briscoe
Internet-Draft BT
Updates: 793 (if approved) October 27, 2014
Intended status: Experimental
Expires: April 30, 2015
Inner Space for TCP Options
draft-briscoe-tcpm-inner-space-01
Abstract
This document describes an experimental method to extend the limited
space for control options in every segment of a TCP connection. It
can use a dual handshake so that, from the very first SYN segment,
extra option space can immediately start to be used optimistically.
At the same time a dual handshake prevents a legacy server from
getting confused and sending the control options to the application
as user-data. The dual handshake is only one strategy - a single
handshake will usually suffice once deployment has got started. The
protocol is designed to traverse most known middleboxes including
connection splitters, because it sits wholly within the TCP Data. It
also provides reliable ordered delivery for control options.
Therefore, it should allow new TCP options to be introduced i) with
minimal middlebox traversal problems; ii) with incremental deployment
from legacy servers; iii) without an extra round of handshaking delay
iv) without having to provide its own loss recovery and ordering
mechanism and v) without arbitrary limits on available space.
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
working documents as Internet-Drafts. The list of current Internet-
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."
This Internet-Draft will expire on April 30, 2015.
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Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation for Adoption Now (to be removed before
publication) . . . . . . . . . . . . . . . . . . . . . . 6
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Experiment Goals . . . . . . . . . . . . . . . . . . . . 6
1.4. Document Roadmap . . . . . . . . . . . . . . . . . . . . 7
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
2. Protocol Specification . . . . . . . . . . . . . . . . . . . 9
2.1. Protocol Interaction Model . . . . . . . . . . . . . . . 9
2.1.1. Dual 3-Way Handshake . . . . . . . . . . . . . . . . 9
2.1.2. Dual Handshake Retransmission Behaviour . . . . . . . 11
2.1.3. Continuing the Upgraded Connection . . . . . . . . . 12
2.2. Upgraded Segment Structure and Format . . . . . . . . . . 12
2.2.1. Structure of an Upgraded Segment . . . . . . . . . . 12
2.2.2. Format of the InSpace Option . . . . . . . . . . . . 14
2.3. Inner TCP Option Processing . . . . . . . . . . . . . . . 15
2.3.1. Writing Inner TCP Options . . . . . . . . . . . . . . 15
2.3.1.1. Constraints on TCP Fast Open . . . . . . . . . . 15
2.3.1.2. Option Alignment . . . . . . . . . . . . . . . . 16
2.3.1.3. Sequence Space Coverage . . . . . . . . . . . . . 16
2.3.1.4. Presence or Absence of Payload . . . . . . . . . 16
2.3.2. Reading Inner TCP Options . . . . . . . . . . . . . . 16
2.3.2.1. Reading Inner TCP Options (SYN=1) . . . . . . . . 17
2.3.2.2. Reading Inner TCP Options (SYN=0) . . . . . . . . 18
2.3.3. Forwarding Inner TCP Options . . . . . . . . . . . . 19
2.4. Exceptions . . . . . . . . . . . . . . . . . . . . . . . 20
2.5. SYN Flood Protection . . . . . . . . . . . . . . . . . . 20
3. Design Rationale . . . . . . . . . . . . . . . . . . . . . . 21
3.1. Dual Handshake and Migration to Single Handshake . . . . 21
3.2. In-Band Inner Option Space . . . . . . . . . . . . . . . 22
3.2.1. Non-Deterministic Magic Number Approach . . . . . . . 22
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3.2.2. Non-Goal: Security Middlebox Evasion . . . . . . . . 23
3.2.3. Avoiding the Start of the First Two Segments . . . . 24
3.2.4. Control Options Within Data Sequence Space . . . . . 24
3.2.5. Rationale for the Sent Payload Size Field . . . . . . 26
3.3. Rationale for the InSpace Option Format . . . . . . . . . 26
3.4. Protocol Overhead . . . . . . . . . . . . . . . . . . . . 27
4. Interaction with Pre-Existing TCP Implementations . . . . . . 29
4.1. Compatibility with Pre-Existing TCP Variants . . . . . . 29
4.2. Interaction with Middleboxes . . . . . . . . . . . . . . 31
4.3. Interaction with the Pre-Existing TCP API . . . . . . . . 31
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
6. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.1. Normative References . . . . . . . . . . . . . . . . . . 35
8.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. Protocol Extension Specifications . . . . . . . . . 36
A.1. Disabling InSpace and Generic Connection Mode Switching . 37
A.2. Dual Handshake: The Explicit Variant . . . . . . . . . . 39
A.2.1. SYN-O Structure . . . . . . . . . . . . . . . . . . . 41
A.2.2. Retransmission Behaviour - Explicit Variant . . . . . 41
A.2.3. Corner Cases . . . . . . . . . . . . . . . . . . . . 42
A.2.4. Workround if Data in SYN is Blocked . . . . . . . . . 43
A.3. Jumbo InSpace TCP Option (only if SYN=0) . . . . . . . . 44
A.4. Upgraded Segment Structure to Traverse DPI boxes . . . . 44
Appendix B. Comparison of Alternatives . . . . . . . . . . . . . 46
B.1. Implicit vs Explicit Dual Handshake . . . . . . . . . . . 46
Appendix C. Protocol Design Issues (to be Deleted before
Publication) . . . . . . . . . . . . . . . . . . . . 47
Appendix D. Change Log (to be Deleted before Publication) . . . 48
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 49
1. Introduction
TCP has become hard to extend, partly because the option space was
limited to 40B when TCP was first defined [RFC0793] and partly
because many middleboxes only forward TCP headers that conform to the
stereotype they expect.
This specification ensures new TCP capabilities can traverse most
middleboxes by tunnelling TCP options within the TCP Data as 'Inner
Options' (Figure 1). Then the TCP receiver can reconstruct the Inner
Options sent by the sender, even if the middlebox resegments the data
stream and even if it strips 'Outer' options from the TCP header that
it does not recognise. The two words 'Inner Space' are appropriate
as a name for the scheme; 'Inner' because it encapsulates options
within the TCP Data and 'Space' because the space within the TCP Data
is virtually unlimited--constrained only by the maximum segment size.
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,-----. TCP Payload ,-----.
| App |<----------------------------------------->| App |
|-----| |-----|
| | Inner Options within TCP Data | |
| |<----------------------------------------->| |
| | | |
| TCP | TCP Header and TCP header and | TCP |
| | Outer Options ,---------. Outer Options | |
| |<-------------->|Middlebox|<-------------->| |
|-----| |---------| |-----|
| IP | | IP | | IP |
: : : : : :
Figure 1: Encapsulation Approach
TCP options fall into three main categories:
a. Those that have to remain as Outer Options--typically those
concerned with transmission of each TCP segment, e.g. Timestamps
and Selective ACKnowledgements (SACK);
b. Those that are best as Inner Options--typically those concerned
with transmission of the data as a stream, e.g. the TCP
Authentication Option [RFC5925] or tcpcrypt [I-D.bittau-tcpinc];
c. Those that can be either Inner or Outer Options--typically those
used at the start of a connection which is also inherently the
start of the first segment so segmentation is not a concern.
Pressure of space is most acute in the initial segments of each half-
connection, i.e. the SYN and SYN/ACK, and particularly the SYN. Even
though Inner Space is not suitable for category (a) options, moving
all of categories (b) and (c) into Inner Space frees up plenty of
outer space in the header for category (a).
The following list of options that might be required on a SYN
illustrates how acute the problem is:
o 4B: Maximum Segment Size (MSS) [RFC0793];
o 2B: SACK-ok [RFC2018];
o 3B: Window Scale [RFC7323];
o 10B: Timestamp [RFC7323];
o 12B: Multipath TCP [RFC6824];
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o 6-18B: TCP Fast Open on a resumed connection
[I-D.ietf-tcpm-fastopen];
o 16B: TCP-AO [RFC5925];
There is probably potential for compressing together multiple options
in order to mitigate the option space problem. However, the option
space problem has to be faced, because complex special placement is
already being contemplated for options that can be larger than 40B on
their own (e.g. the key agreement options of tcpcrypt
[I-D.bittau-tcpinc]).
Given the Inner Space protocol places control options within TCP
Data, it is critical that a legacy TCP receiver is never confused
into passing this mix to an application as if it were pure data.
Naively, both ends could handshake to check they understand the
protocol, but this would introduce a round of delay and it would not
solve the shortage of space in a SYN. Instead, the client uses dual
handshakes; one suitable for an upgraded server, and the other for an
ordinary server. Then, if the client discovers that the server does
not understand the new protocol, it can abort the upgraded handshake
before the server passes corrupt data to the application. Otherwise,
if the server does understand the new protocol, the client can abort
the ordinary handshake. Either way, it has added zero extra delay.
Interworking of the dual handshake with TCP Fast Open
[I-D.ietf-tcpm-fastopen] is carefully defined so that either server
can pass data to the application as soon as the initial SYN arrives.
When control options are placed within the TCP Data they inherently
get delivered reliably and in order. Although this was not
originally recognised as part of the design brief, it offers the
significant benefit of simplifying the design of new TCP options.
Reliable ordered delivery no longer has to be individually crafted
into the design of each new TCP option.
Solving the five problems of i) option-space exhaustion; ii)
middlebox traversal; iii) legacy server confusion; iv) reliable
ordered control message delivery; and v) handshake latency; does not
come without cost:
o So that the Inner Space protocol is immune to option stripping, it
flags its presence using a magic number within the TCP Data of the
initial segment in each direction, not a conventional TCP option
in the header. This introduces a risk that payload in an ordinary
SYN or SYN/ACK might be mistaken for the Inner Space protocol (an
initial worst-case estimate of the probability is one connection
globally every 40 years). Nonetheless, the risk is zero in the
(currently common) case of an ordinary connection without payload
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during the handshake. There is also no risk of a mistake the
other way round--an upgraded connection cannot be mistaken for an
ordinary connection.
o Although the dual handshake introduces no extra latency, it
introduces extra connection processing & state, extra traffic and
extra header processing. Initial estimates put the percentage
overhead in single digits for connection processing and state, and
traffic overhead at only a few hundredths of a percent.
Nonetheless, once the most popular TCP servers have upgraded, only
a single handshake will be necessary most of the time and overhead
should drop to vanishingly small proportions.
Finally, it should be noted that the ambition of this work is more
than just an incrementally deployable, low latency way to extend TCP
option space. The aim is to move towards a more structured way for
middleboxes to interact transparently with, rather than arbitrarily
interfere with, end-system TCP stacks. This has been achieved for
connection and stream control options, but it will still be hard to
introduce new per-segment control options, which will still have to
be located within the traditional Outer TCP Options.
1.1. Motivation for Adoption Now (to be removed before publication)
It seems inevitable that ultimately more option space will be needed,
particularly given that many of the TCP options introduced recently
consume large numbers of bits in order to provide sufficient
information entropy, which is not amenable to compression.
Extension of TCP option space requires support from both ends. This
means it will take many years before the facility is functional for
most pairs of end-points. Therefore, given the problem is already
becoming pressing, a solution needs to start being deployed now.
1.2. Scope
This experimental specification extends the TCP wire protocol. It is
independent of the dynamic rate control behaviour of TCP and it is
independent of (and thus compatible with) any protocol that
encapsulates TCP, including IPv4 and IPv6.
1.3. Experiment Goals
TCP is critical to the robust functioning of the Internet, therefore
any proposed modifications to TCP need to be thoroughly tested.
Success criteria: The experimental protocol will be considered
successful if it satisfies the following requirements in the
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consensus opinion of the IETF tcpm working group. The protocol
needs to be sufficiently well specified so that more than one
implementation can be built in order to test its function,
robustness, overhead and interoperability (with itself, with
previous version of TCP, and with various commonly deployed
middleboxes). Non-functional issues such as recommendations on
message timing also need to be tested. Various optional
extensions to the protocol are proposed in Appendix A so
experiments are also needed to determine whether these extensions
ought to remain optional, or perhaps be removed or become
mandatory.
Duration: To be credible, the experiment will need to last at least
12 months from publication of the present specification. If
successful, it would then be appropriate to progress to a
standards track specification, complemented by a report on the
experiments.
1.4. Document Roadmap
The body of the document starts with a full specification of the
Inner Space extension to TCP (Section 2). It is rather terse,
answering 'What?' and 'How?' questions, but deferring 'Why?' to
Section 3. The careful design choices made are not necessarily
apparent from a superficial read of the specification, so the Design
Rationale section is fairly extensive. The body of the document ends
with Section 4 that checks possible interactions between the new
scheme and pre-existing variants of TCP, including interaction with
partial implementations of TCP in known middleboxes.
Appendix A specifies optional extensions to the protocol that will
need to be implemented experimentally to determine whether they are
useful. And Appendix B discusses the merits of the chosen design
against alternative schemes.
1.5. Terminology
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 [RFC2119]. In this
document, these words will appear with that interpretation only when
in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
TCP Header: As defined in [RFC0793]. Even though the present
specification places TCP options beyond the Data Offset, the term
'TCP Header' is still used to mean only those fields at the head
of the segment, delimited by the TCP Data Offset.
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Inner TCP Options (or just Inner Options): TCP options placed in the
space that the present specification makes available beyond the
Data Offset.
Outer TCP Options (or just Outer Options): The TCP options in the
traditional location directly after the base TCP Header and before
the TCP Data Offset.
Prefix TCP Options: Inner Options to be processed before the Outer
Options.
Suffix TCP Options: Inner Options to be processed after the Outer
Options.
TCP options: Any TCP options, whether inner, outer or both. This
specification makes this term on its own ambiguous so it should be
qualified if it is intended to mean TCP options in a certain
location.
TCP Payload: Data to be passed to the layer above TCP. The present
specification redefines the TCP Payload so that it does not
include the Inner TCP Options, the Inner Space Option and any
Magic Number, even though they are located beyond the Data Offset.
TCP Data: The information in a TCP segment after the Data Offset,
including the TCP Payload, Inner TCP Options, the Inner Space
Option and the Magic Number defined in the present specification.
client: The process taking the role of actively opening a TCP
connection.
server: The process taking the role of TCP listener.
Upgraded Segment: A segment that will only be fully understood by a
host complying with the present specification (even though it
might appear valid to a pre-existing TCP receiver). Similarly,
Upgraded SYN, Upgraded SYN/ACK etc.
Ordinary Segment: A segment complying with pre-existing TCP
specifications but not the present specification. Similarly,
Ordinary SYN, Ordinary SYN/ACK etc.
Upgraded Connection: A connection starting with an Upgraded SYN.
Ordinary Connection: A connection starting with an Ordinary SYN.
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Upgraded Host: A host complying with the present document as well as
with pre-existing TCP specifications. Similarly Upgraded TCP
Client, Upgraded TCP Server, etc.
Legacy Host: A host complying with pre-existing TCP specifications,
but not with the present document. Similarly Legacy TCP Client,
Legacy TCP Server, etc.
Note that the term 'Ordinary' is used for segments and connections,
but the term 'Legacy' is used for hosts. This is because, if the
Inner Space protocol were widely used in future, a host that could
not open an Upgraded Connection would be considered deficient and
therefore 'Legacy', whereas an Ordinary Connection would not be
considered deficient in the future; because it will always be
legitimate to open an Ordinary Connection if extra option space is
not needed.
2. Protocol Specification
2.1. Protocol Interaction Model
2.1.1. Dual 3-Way Handshake
During initial deployment, an Upgraded TCP Client sends two
alternative SYNs: an Ordinary SYN in case the server is legacy and a
SYN-U in case the server is upgraded. The two SYNs MUST have the
same network addresses and the same destination port, but different
source ports. Once the client establishes which type of server has
responded, it continues the connection appropriate to that server
type and aborts the other without completing the 3-way handshake.
The format of the SYN-U will be described later (Section 2.2.2). At
this stage it is only necessary to know that the client can put
either TCP options or payload (or both) in a SYN-U, in the space
traditionally intended only for payload. So if the server's response
shows that it does not recognise the Upgraded SYN-U, the client is
responsible for aborting the Upgraded Connection. This ensures that
a Legacy TCP Server will never erroneously confuse the application by
passing it TCP options as if they were user-data.
Section 3.1 explains various strategies the client can use to send
the SYN-U first and defer or avoid sending the Ordinary SYN.
However, such strategies are local optimizations that do not need to
be standardized. The rules below cover the most aggressive case, in
which the client sends the SYN-U then the Ordinary SYN back-to-back
to avoid any extra delay. Nonetheless, the rules are just as
applicable if the client defers or avoids sending the Ordinary SYN.
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Table 1 summarises the TCP 3-way handshake exchange for each of the
two SYNs in the two right-hand columns, between an Upgraded TCP
Client (the active opener) and either:
1. a Legacy Server, in the top half of the table (steps 2-4), or
2. an Upgraded Server, in the bottom half of the table (steps 2-4)
Because the two SYNs come from different source ports, the server
will treat them as separate connections, probably using separate
threads (assuming a threaded server). A load balancer might forward
each SYN to separate replicas of the same logical server. Each
replica will deal with each incoming SYN independently - it does not
need to co-ordinate with the other replica.
+------+------------------+--------------------+--------------------+
| | | Ordinary | Upgraded |
| | | Connection | Connection |
+------+------------------+--------------------+--------------------+
| 1 | Upgraded Client | >SYN | >SYN-U |
| | | | |
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Legacy Server | <SYN/ACK | <SYN/ACK |
| | | | |
| 3a | Upgraded Client | Waits for response | |
| | | to both SYNs | |
| | | | |
| 3b | " | >ACK | >RST |
| | | | |
| 4 | | Cont... | |
| | | | |
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Upgraded Server | <SYN/ACK | <SYN/ACK-U |
| | | | |
| 3a | Upgraded Client | Waits for response | |
| | | to SYN-U | |
| | | | |
| 3b | " | >RST | >ACK |
| | | | |
| 4 | | | Cont... |
+------+------------------+--------------------+--------------------+
Table 1: Dual 3-Way Handshake in Two Server Scenarios
Each column of the table shows the required 3-way handshake exchange
within each connection, using the following symbols:
> means client to server;
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< means server to client;
Cont... means the TCP connection continues.
The connection that starts with an Ordinary SYN is called the
'Ordinary Connection' and the one that starts with a SYN-U is called
the 'Upgraded Connection'. An Upgraded Server MUST respond to a
SYN-U with an Upgraded SYN/ACK (termed a SYN/ACK-U and defined in
Section 2.2.2). Then the client recognises that it is talking to an
Upgraded Server. The client's behaviour depends on which response it
receives first, as follows:
o If the client first receives a SYN/ACK response on the Ordinary
Connection, it MUST wait for the response on the Upgraded
Connection. It then proceeds as follows:
* If the response on the Upgraded Connection is an Ordinary SYN/
ACK, the client MUST reset (RST) the Upgraded Connection and it
can continue with the Ordinary Connection.
* If the response on the Upgraded Connection is an Upgraded SYN/
ACK-U, the client MUST reset (RST) the Ordinary Connection and
it can continue with the Upgraded Connection.
o If the client first receives an Ordinary SYN/ACK response on the
Upgraded Connection, it MUST reset (RST) the Upgraded Connection
immediately. It can then wait for the response on the Ordinary
Connection and, once it arrives, continue as normal.
o If the client first receives an Upgraded SYN/ACK-U response on the
Upgraded Connection, it MUST reset (RST) the Ordinary Connection
immediately and continue with the Upgraded Connection.
2.1.2. Dual Handshake Retransmission Behaviour
If the client receives a response to the SYN, but a short while after
that {ToDo: duration TBA} the response to the SYN-U has not arrived,
it SHOULD retransmit the SYN-U. If latency is more important than
the extra TCP option space, in parallel to any retransmission, or
instead of any retransmission, the client MAY give up on the Upgraded
(SYN-U) Connection by sending a reset (RST) and completing the 3-way
handshake of the Ordinary Connection.
If the client receives no response at all to either the SYN or the
SYN-U, it SHOULD solely retransmit one or the other, not both. If
latency is more important than the extra TCP option space, it will
retransmit the SYN. Otherwise it will retransmit the SYN-U. It MUST
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NOT retransmit both segments, because the lack of response could be
due to severe congestion.
2.1.3. Continuing the Upgraded Connection
Once an Upgraded Connection has been successfully negotiated in the
SYN, SYN/ACK exchange, either host can allocate any amount of the TCP
Data space in any subsequent segment for extra TCP options. In fact,
the sender has to use the upgraded segment structure in every
subsequent segment of the connection that contains non-zero TCP
Payload. The sender can use the upgraded structure in a segment
carrying no user-data (e.g. a pure ACK), but it does not have to.
As well as extra option space, the facility offers other advantages,
such as reliable ordered delivery of Inner TCP Options on empty
segments and more robust middlebox traversal. If none of these
features is needed, at any point the facility can be disabled for the
rest of the connection, using the ModeSwitch TCP option in
Appendix A.1. Interestingly, the ModeSwitch options itself can be
very simple because it uses the reliable ordered delivery property of
Inner Options, rather than having to cater for the possibility that a
message to switch to disabled mode might be lost or reordered.
2.2. Upgraded Segment Structure and Format
2.2.1. Structure of an Upgraded Segment
An Upgraded Segment is structured as shown in Figure 2. Up to the
TCP Data Offset, the structure is identical to an Ordinary TCP
Segment, with a base TCP Header (BaseHdr) and the usual facility to
set the Data Offset (DO) to allow space for TCP options. These
regular TCP options are renamed by this specification to Outer TCP
Options or just Outer Options, and labelled as OuterOpts in the
figure.
The first segment in each direction (i.e. the SYN or the SYN/ACK) is
identifiable as upgraded by the presence of the 4-octet Magic Number
A (MagicA) at the start of the TCP Data. The probability that an
Upgraded Server will mistake arbitrary data at the beginning of the
payload of an Ordinary Segment for the Magic Number has to be allowed
for, but it is vanishingly small (see Section 3.2.1). Once an
Upgraded Connection has been negotiated during the SYN - SYN/ACK
exchange, a magic number is not needed to identify Upgraded Segments,
because both ends know that the protocol requires the sender to use
the upgraded format on all subsequent segments with non-zero TCP
Data. Aside from the magic number, the structure of the rest of an
Upgraded Segment is effectively the same whether a) SYN=1 or b)
SYN=0.
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| SOO |
a) SYN=1 ,--------->|
| DO | 1 | Len | InOO | SPS |
,------------------>,------>,------->,-------------------->,------->|
+--------+----------+-------+--------+----------+----------+--------+
| BaseHdr| OuterOpts| MagicA| InSpace|PrefixOpts|SuffixOpts| Payload|
+--------+----------+-------+--------+----------+----------+--------+
| '----------.----------' |
| Inner Options |
`-----------------------.-----------------------'
TCP Data
b) SYN=0
| DO | Len | InOO | SPS |
,------------------>,------->,---------------------->,------->|
+--------+----------+--------+-----------------------+--------+
| BaseHdr| OuterOpts| InSpace| Inner Options | Payload|
+--------+----------+--------+-----------------------+--------+
`----------------.------------------------'
TCP Data
All offsets are specified in 4-octet (32-bit) words, except SPS,
which is in octets.
Figure 2: The Structure of an Upgraded Segment (not to scale)
Unlike an Ordinary TCP Segment, the Payload of an Upgraded Segment
does not start straight after the TCP Data Offset. Instead, Figure 2
shows that space is provided for additional Inner TCP Options before
the TCP Payload. The size of this space is termed the Inner Options
Offset (InOO). The TCP receiver reads the InOO field from the Inner
Option Space (InSpace) option defined in Section 2.2.2.
The InSpace Option is located in a standardized location so that the
receiver can find it:
o On a segment with SYN=1, an Upgraded TCP Sender MUST locate the
InSpace Option straight after the magic number, specifically 4 *
(DO + 1) octets from the start of the segment.
o On a segment with SYN=0, an Upgraded TCP Sender MUST locate the
InSpace Option at the beginning of the TCP Data, specifically 4 *
DO octets from the start of the segment.
Because the InSpace Option is only ever located in a standardized
location it does not need to follow the RFC 793 format of a TCP
option. Therefore, although we call InSpace an 'option', we do not
describe it as a 'TCP option'.
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The Sent Payload Size (SPS) is also read from within the InSpace
Option. If the byte-stream has been resegmented, it allows the
receiver to step from one InSpace Option to the next even if the
InSpace Options are no longer at the start of each segment (see
Section 2.3).
On a segment with SYN=1 (i.e. a SYN or SYN/ACK) the Suffix Options
Offset (SOO) is also read from within the InSpace Option. It
delineates the end of the Prefix TCP Options (PrefixOpts in the
figure) and the start of the Suffix TCP Options (SuffixOpts). When
SYN=1, the receiver processes PrefixOpts before OuterOpts, then
SuffixOpts afterwards. When SYN=0, the receiver processes the Outer
Options before the Inner Options. Full details of option processing
are given in Section 2.3.
2.2.2. Format of the InSpace Option
The internal structure of the InSpace Option for an Upgraded SYN or
SYN/ACK segment (SYN=1) is defined in Figure 3a) and for a segment
with SYN=0 in Figure 3b).
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
a) SYN = 1
+-------------------------------+---------------------------+---+
| Sent Payload Size (SPS) |Inner Options Offset (InOO)|Len|
+-------------------------------+---------------------------+---+
| Magic Number B |Suffix Options Offset (SOO)|CU |
+-------------------------------+---------------------------+---+
b) SYN = 0
+-------------------------------+---------------------------+---+
| Sent Payload Size (SPS) |Inner Options Offset (InOO)|Len|
+-------------------------------+---------------------------+---+
Figure 3: InSpace Option Format
The fields are defined as follows (see Section 3.3 for the rationale
behind these format choices):
Option Length (Len): The 2-bit Len field specifies the length of the
InSpace Option in 4-octets words (see Section 3.3 for rationale).
For this experimental specification:
When SYN=1: the sender MUST use Len=2;
When SYN=0: the sender MUST use Len=1.
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Sent Payload Size (SPS): In this 16-bit field the sender MUST record
the size in octets of the TCP Payload when it was sent. This
specification defines the TCP Payload as solely the user-data to
be passed to the application. This excludes Inner TCP options,
the InSpace Option and any magic number.
Inner Options Offset (InOO): This 14-bit field defines the total
size of the Inner TCP Options in 4-octet words.
The following fields are only defined on a segment with SYN=1 (i.e. a
SYN or SYN/ACK):
Magic Number B: The sender MUST fill this 16-bit field with Magic
Number B {ToDo: Value TBA} to further reduce the chance that a
receiver will mistake the end of an arbitrary Ordinary Payload for
the InSpace Option.
Suffix Options Offset (SOO): The 14-bit SOO field defines an
additional offset in 4-octet words from the start of the Inner
Options that identifies the extent of the Prefix Options (see
Section 2.3.2).
Currently Unused (CU): The sender MUST fill the CU field with zeros
and they MUST be ignored and forwarded unchanged by other nodes,
even if their value is different.
2.3. Inner TCP Option Processing
2.3.1. Writing Inner TCP Options
2.3.1.1. Constraints on TCP Fast Open
If an Upgraded TCP Client uses a TCP Fast Open (TFO) cookie
[I-D.ietf-tcpm-fastopen] in an Upgraded SYN-U, it MUST place the TFO
option within the Inner TCP Options, beyond the Data Offset.
This rule is specific to TFO, but it can be generalised to any
capability similar to TFO as follows: An Upgraded TCP Client MUST NOT
place any TCP option in the Outer TCP Options of a SYN if it might
cause a TCP server to pass user-data directly to the application
before its own 3-way handshake completes.
If a client uses TCP Fast Open cookies on both the parallel
connection attempts of a dual handshake, an Upgraded Server will
deliver the TCP Payload to the application twice before the client
aborts the Ordinary Connection. This is not a problem, because
[I-D.ietf-tcpm-fastopen] requires that TFO is only used for
applications that are robust to duplicate requests.
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2.3.1.2. Option Alignment
If the end of the last Inner TCP Option does not align on a 4-octet
boundary, the sender MUST append sufficient no-op TCP options. On a
SYN=1 segment, the end of the Prefix TCP Options MUST be similarly
aligned.
If a block-mode transformation (e.g. compression or encryption) is
being used, the sender might have to add some padding options to
align the end of the Inner Options with the end of a block. Any
future encryption specification will need to carefully define this
padding in order not to weaken the cipher.
2.3.1.3. Sequence Space Coverage
TCP's sequence number and acknowledgement number space MUST include
all the TCP Data, i.e. the InSpace Option, any Inner Options, and any
magic number as well as the TCP Payload. Similarly, the sender MUST
NOT transmit any form of TCP Data unless the advertised receive
window is sufficient. These rules have significant implications,
which are discussed in Section 3.2.4.
2.3.1.4. Presence or Absence of Payload
Whenever the sender includes non-zero user-data payload in a segment,
it MUST also include an InSpace Option, whether or not there are any
Inner Options.
If the sender includes no user-data in a segment (e.g. pure ACKs,
RSTs) it MAY include an InSpace Option but it does not have to.
{ToDo: Consider whether there is any reason to preclude Inner Options
on a RST, FIN or FIN-ACK.}
Once a sender has included the InSpace Option and possibly other
Inner Options on a segment with no TCP Payload, while it has no
further user-data to send it SHOULD NOT repeat the same set of
control options on subsequent segments. Thus, in a sequence of pure
ACKs, any particular set of Inner Options will only appear once, and
other pure ACKs will be empty. The only envisaged exception to this
rule would be infrequent repetition (i.e. tens of minutes to hours)
of the same control options, which might be necessary to provide a
heartbeat or keep-alive capability.
2.3.2. Reading Inner TCP Options
The rules for reading Inner TCP Options are divided between the
following two subsections, depending on whether SYN=1 or SYN=0.
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2.3.2.1. Reading Inner TCP Options (SYN=1)
This subsection applies when TCP receives a segment with SYN=1, i.e.
when the server receives a SYN or the client receives a SYN/ACK.
Before processing any TCP options, unless the size of the TCP Data is
less than 8 octets, an Upgraded Receiver MUST determine whether the
segment is an Upgraded Segment by checking that all the following
conditions apply:
o The first 4 octets of the segment match Magic Number A;
o The value of the Length field of the InSpace Option is 2;
o The value of Magic Number B in the InSpace Option is correct;
o The value of the Sent Payload Size matches the size of the TCP
Payload.
If all these conditions pass, the receiver MAY walk the sequence of
Inner TCP Options, using the length of each to check that the sum of
their lengths equals InOO. The receiver then concludes that the
received segment is an Upgraded Segment.
The receiver then processes the TCP Options in the following order:
1. Any Prefix TCP options (PrefixOpts in Figure 2)
2. Any Outer TCP options (OuterOpts in Figure 2);
3. Any Suffix TCP options (SuffixOpts in Figure 2)
The receiver removes the magic number, the InSpace Option and each
TCP Option from the TCP Data as it processes each. This frees up
receive buffer, so the receiver increases its local value of the
receive window accordingly. Once only the TCP Payload remains, the
receiver holds it ready to pass to the application. It then returns
the appropriate Upgraded Acknowledgement to progress the dual
handshake (see Section 2.1.1).
If any of the above tests to find the InSpace Option fails:
1. the receiver concludes that the received segment is an Ordinary
Segment. It MUST then proceed by processing any Outer TCP
options in the TCP Header in the normal order (OuterOpts in
Figure 2).
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2. If some previous control message causes the TCP receiver to alter
the TCP Data (e.g. decompression, decryption), it reruns the
above tests to check if the altered TCP Data now looks like an
Upgraded Segment.
3. If it finds an InSpace Option, it suspends processing the Outer
TCP Options and instead processes and removes TCP Options in the
following order:
1. Any Prefix Inner Options;
2. Any remaining Outer TCP Options;
3. Any Suffix Inner Options.
4. If it does not find an InSpace Option, it continues processing
the remaining Outer TCP Options as normal.
For the avoidance of doubt the above rules imply that, as long as an
InSpace Option has not been found in the segment, the receiver might
rerun the tests for it multiple times if multiple Outer TCP Options
alter the TCP Data. However, once the receiver has found an InSpace
Option, it MUST NOT rerun the tests for an Upgraded Segment in the
same segment.
If the receiver has not found an InSpace Option after processing all
the Outer Options, it returns the appropriate Ordinary
Acknowledgement to progress the dual handshake (see Section 2.1.1).
As normal, it holds any TCP Payload ready to pass to the application.
2.3.2.2. Reading Inner TCP Options (SYN=0)
This subsection applies once the TCP connection has successfully
negotiated to use the upgraded InSpace structure.
As each segment with SYN=0 arrives, the receiver immediately
processes any Outer TCP options.
As the receiver buffers TCP Data, it uses TCP's regular mechanisms to
fill any gaps due to reordering or loss so that it can work its way
along the ordered byte-stream. As the receiver encounters each set
of Inner Options, it MUST process them in the order they were sent,
as illustrated in Figure 4a) in Section 3.2.4. The receiver MUST
remove the InSpace Option and Inner TCP Options from the TCP Data as
it processes them, adding to the receive window accordingly. Once
only the TCP Payload remains the receiver passes it to the
application.
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It uses each InSpace Option to calculate the extent of the associated
Inner Options (using InOO), and the amount of payload data before the
next InSpace Option (using Sent Payload Size). The receiver MUST NOT
locate InSpace Options by assuming there is one at the start of the
TCP Data in every segment, because resegmentation might invalidate
this assumption.
Therefore, the receiver processes the Inner Options in the order they
were sent, which is not necessarily the order in which they are
received. And if an Inner Option applies to the data stream, the
receiver applies it at the point in the data stream where the sender
inserted it. As a consequence, the receiver always processes the
Inner Options after the Outer Options.
The Inner Options are deliberately placed within the byte-stream so
that the sender can transform them along with the payload data, e.g.
to compress or encrypt them. A previous control message might have
required the TCP receiver to alter the byte-stream before passing it
to the application, e.g. decompression or decryption. If so, the
TCP receiver applies transformations progressively, to one sent
segment at a time, in the following order:
1. The receiver MUST apply any transformations to the byte-stream up
to the end of the next set of Inner Options, i.e. over the extent
of the next Sent Payload Size, InSpace Option and any Inner
Options.
2. The receiver MUST then process and remove the InSpace Option and
any Inner Options (which might change the way it transforms the
next segment, e.g. a rekey option).
3. Having established the extent of the next sent segment, The
receiver returns to step 1.
2.3.3. Forwarding Inner TCP Options
Middleboxes exist that process some aspects of the TCP Header.
Although the present specification defines a new location for Inner
TCP Options beyond the Data Offset, this is intended for the
exclusive use of the destination TCP implementation. Therefore:
o A middlebox MUST treat any octets beyond the Data Offset as
immutable user-data. Legacy Middleboxes already do not expect to
find options beyond the Data Offset anyway.
o A middlebox MUST NOT defer data in a segment with SYN=1 to a
subsequent segment.
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A TCP implementation is not necessarily aware whether it is deployed
in a middlebox or in a destination, e.g. a split TCP connection might
use a regular off-the-shelf TCP implementation. Therefore, a
general-purpose TCP that implements the present specification will
need a configuration switch to disable any search for options beyond
the Data Offset and to enable immediate forwarding of data in a SYN.
2.4. Exceptions
{ToDo: Define behaviour of forwarding or receiving nodes if the
structure or format of an Upgraded Segment is not as specified.}
If an Upgraded TCP Receiver receives an InSpace Option with a Length
it does not recognise as valid, it MUST drop the packet and
acknowledge the octets up to the start of the unrecognised option.
Values of Sent Payload Size greater than 2^16 - 25 (=65,511) octets
in a regular (non-jumbo) InSpace Option MUST be treated as the
distance to the next InSpace option, but they MUST NOT be taken as
indicative of the size of the TCP Payload when it was sent. This is
because the TCP Payload in a regular IPv6 packet cannot be greater
than (2^16 -1 - 20 - 4) octets (given the minimum TCP header is 20
octets and the minimum InSpace Option is 4 octets). A Sent Payload
Size of 0xFFFF octets MAY be used to minimise the occurrence of empty
InSpace options without permanently disabling the Inner Space
protocol for the rest of the connection.
If the size of the payload is greater than 65,511 octets, the sender
MUST use a Jumbo InSpace Option (Appendix A.3).
2.5. SYN Flood Protection
An implementation of the Inner Space protocol MUST support the
EchoCookie TCP option [I-D.briscoe-tcpm-echo-cookie]. To indicate
its support for EchoCookie, an Ordinary Client would send an empty
EchoCookie TCP option on the SYN. Support for the Inner Space
protocol makes this redundant. Therefore an Inner Space client MUST
NOT send an empty EchoCookie TCP option on a SYN-U.
The EchoCookie TCP option replaces the SYN Cookie mechanism
[RFC4987], which only has sufficient space to hold the result of one
TCP option negotiation (the MSS), and then only a subset of the
possible values (see the discussion under Security Considerations
Section 6).
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3. Design Rationale
This section is informative, not normative.
3.1. Dual Handshake and Migration to Single Handshake
In traditional [RFC0793] TCP, the space for options is limited to 40B
by the maximum possible Data Offset. Before a TCP sender places
options beyond that, it has to be sure that the receiver will
understand the upgraded protocol, otherwise it will confuse and
potentially crash the application by passing it TCP options as if
they were payload data.
The Dual Handshake (Section 2.1.1) ensures that a Legacy TCP Server
will never pass on TCP options as if they were user-data. If a SYN
carries TCP Data, a TCP server typically holds it back from the
application until the 3-way handshake completes. This gives the
client the opportunity to abort the Upgraded Connection if the
response from the server shows it does not recognise an Upgraded SYN.
The strategy of sending two SYNs in parallel is not essential to the
Alternative SYN approach. It is merely an initial strategy that
minimises latency when the client does not know whether the server
has been upgraded. Evolution to a single SYN with greater option
space could proceed as follows:
o Clients could maintain a white-list of upgraded servers discovered
by experience and send just the Upgraded SYN-U in these cases.
o Then, for white-listed servers, the client could send an Ordinary
SYN only in the rare cases when an attempt to use an Upgraded
Connection had previously failed (perhaps a mobile client
encountering a new blockage on a new path to a server that it had
previously accessed over a good path).
o In the longer term, once it can be assumed that most servers are
upgraded and the risk of having to fall back to legacy has dropped
to near-zero, clients could send just the Upgraded SYN first,
without maintaining a white-list, but still be prepared to send an
Ordinary SYN in the rare cases when that might fail.
There is concern that, although dual handshake approaches might well
eventually migrate to a single handshake, they do not scale when
there are numerous choices to be made simultaneously. For instance:
o trying IPv6 then IPv4 [RFC6555];
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o and trying SCTP and TCP in parallel
[I-D.wing-tsvwg-happy-eyeballs-sctp];
o and trying ECN and non-ECN in parallel;
o and so on.
Nonetheless, it is not necessary to try every possible combination of
N choices, which would otherwise require 2^N handshakes (assuming
each choice is between two options). Instead, a selection of the
choices could be attempted together. At the extreme, two handshakes
could be attempted, one with all the new features, and one without
all the new features.
3.2. In-Band Inner Option Space
3.2.1. Non-Deterministic Magic Number Approach
This section justifies the magic number approach by contrasting it
with a more 'conventional' approach. A conventional approach would
use a regular (Outer) TCP option to point to the dividing line within
the TCP Data between the extra Inner Options and the TCP Payload.
This 'conventional' approach cannot provide extra option space over a
path on which a middlebox strips TCP options that it does not
recognise. [Honda11] quantifies the prevalence of such paths. It
reports on experiments conducted in 2010-2011 that found unknown
options were stripped from the SYN-SYN/ACK exchange on 14% of paths
to port 80 (HTTP), 6% of paths to port 443 (HTTPS) and 4% of paths to
port 34343 (unassigned). Further analysis found that the option-
stripping middleboxes fell into two main categories:
o about a quarter appeared to actively remove options that they did
not recognise (perhaps assuming they might be indicative of an
attack?);
o the rest were some type of higher layer proxy that split the TCP
connection, unwittingly failing to pass unknown options between
the two connections.
In contrast, the magic number approach ensures that not only are the
Inner Options tucked away beyond the Data Offset, but the option that
gives the extent of the Inner Options is also beyond the Data Offset
(see Section 2.2.1). This ensures that all the TCP Headers and
options up to the Data Offset are completely indistinguishable from
an Ordinary Segment. It is very unusual for a middlebox not to
forward TCP Data unchanged, so it will be highly likely (but not
certain--see Appendix A.2.4) to forward the extra Inner Options.
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The downside of the magic number approach is that it is slightly non-
deterministic, quantified as follows:
o The probability that an Upgraded SYN=1 segment will be mistaken
for an Ordinary Segment is precisely zero.
o In the currently common case of a SYN with zero payload, the
probability that it will be mistaken for an Upgraded Segment is
also precisely zero.
o However, there will be a very small probability (roughly 2^{-66}
or 1 in 74 billion billion (74 * 10^18)) that payload data in an
Ordinary SYN=1 segment could be mistaken for an Upgraded SYN or
SYN/ACK, if it happens to contain a pattern in exactly the right
place that matches the correct Sent Payload Size, Length and Magic
Numbers of an InSpace Option. {ToDo: Estimate how often a
collision will occur globally. Rough estimate: 1 connection
collision globally every 40 years.}
The above probability is based on the assumptions that:
o the magic numbers will be chosen randomly (in reality they will
not--for instance, a magic number that looked just like the start
of an HTTP connection would be rejected)
o data at the start of Ordinary SYN=1 segments is random (in reality
it is not--the first few bytes of most payloads are very
predictable).
Therefore even though 2^{-66} is a vanishingly small probability, the
actual probability of a collision will be much lower.
If a collision does occur, it will result in TCP removing a number of
32-bit words of data from the start of a byte-stream before passing
it to the application.
3.2.2. Non-Goal: Security Middlebox Evasion
The purpose of locating control options within the TCP Data is not to
evade security. Security middleboxes can be expected to evolve to
examine control options in the new inner location. Instead, the
purpose is to traverse middleboxes that block new TCP options
unintentionally--as a side effect of their main purpose--merely
because their designers were too careless to consider that TCP might
evolve. This category of middleboxes tends to forward the TCP
Payload unaltered.
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By sitting within the TCP Data, the Inner Space protocol should
traverse enough existing middleboxes to reach critical mass and prove
itself useful. In turn, this will open an opportunity to introduce
integrity protection for the TCP Data (which includes Inner Options).
Whereas today, no operating system would introduce integrity
protection of Outer TCP options, because in too many cases it would
fail and abort the connection. Once the integrity of Inner Options
is protected, it will raise the stakes. Any attempt to meddle with
control options within the TCP Data will not just close off the
theoretical potential benefit of a protocol advance that no-one knows
they want yet; it will fail integrity checks and therefore completely
break any communication. It is unlikely that a network operator will
buy a middlebox that does that.
Then middlebox designers will be on the back foot. To completely
block communications they will need a sound justification. If they
block an attack, that will be fine. But if they want to block
everything abnormal, they will have to block the whole communication,
or nothing. So the operator will want to choose middlebox vendors
who take much more care to ensure their policies track the latest
protocol advances--to avoid costly support calls.
3.2.3. Avoiding the Start of the First Two Segments
Some middleboxes discard a segment sent to a well-known port
(particularly port 80) if the TCP Data does not conform to the
expected app-layer protocol (particularly HTTP). Often such
middleboxes only parse the start of the app-layer header (e.g. Web
filters only continue until they find the URL being accessed, or DPI
boxes only continue until they have identified the application-layer
protocol).
The segment structure defined in Section 2.2.1 would not traverse
such middleboxes. An alternative segment structure that avoids the
start of the first two segments in each direction is defined in
Appendix A.4. It is not mandatory to implement in the present
specification. However, it is hoped that it will be included in some
experimental implementations so that it can be decided whether it is
worth making mandatory.
3.2.4. Control Options Within Data Sequence Space
Including Inner Options within TCP's sequence space gives the sender
a simple way to ensure that control options will be delivered
reliably and in order to the remote TCP, even if the control options
are on segments without user-data. By using TCP's existing stream
delivery mechanisms, it adds no extra protocol processing, no extra
packets and no extra bits.
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The sender can even choose to place control options on a segment
without user-data, e.g. to reliably re-key TCP-level encryption on a
connection currently sending no data in one direction. The sender
can even add an InSpace Option without further Inner Options. Then
it can ensure that the segment will automatically be delivered
reliably and in order to the remote TCP, even though it carries no
user-data or other TCP control options, e.g. for a test probe, a
tail-loss probe or a keep-alive.
Figure 4a) illustrates control options arriving reliably and in order
at the receiving TCP stack in comparison with the traditional
approach shown in Figure 4b), in which control options are outside
the sequence space. In the traditional approach, during a period
when the remote TCP is sending no user-data, the local TCP may
receive control options E, B and D without ever knowing that they are
out of order, and without ever knowing that C is missing.
a) __ ____ _______ _ __
|__|____|_______|_| |__| control
:E : D : C :B: :A :
________________: : : : :__________________: :
|________________| |__________________| data
b) __
|__| E
|_|__ B __
|____|D |__|A control
\ / \ /
________________\/__________________\/
|________________||__________________| data
!
!drop
____!__
|_______|C
Figure 4: Control options a) inside vs. b) outside TCP sequence
space`
By including Inner Options within the sequence space, each control
option is automatically bound to the start of a particular byte in
the data stream, which makes it easy to switch behaviour at a
specific point mid-stream (e.g. re-keying or switching to a different
control mode). With traditional TCP options, a bespoke reliable and
ordered binding to the data stream would have to be developed for
each TCP option that needs this capability (e.g. co-ordinating use
of new keys in TCP-AO [RFC5925] or tcpcrypt [I-D.bittau-tcpinc]).
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Including Inner Options in sequence also allows the receiver to tell
the sender the exact point at which it encountered an unrecognised
TCP option using only TCP's pre-existing byte-granularity
acknowledgement scheme.
Middleboxes exist that rewrite TCP sequence and acknowledgement
numbers, and they also rewrite options that refer to sequence numbers
(at least those known when the middlebox was produced, such as SACK,
but not any introduced afterwards). If Inner Options were not
included in sequence, the number of bytes beyond the TCP Data Offset
in each segment would not match the sequence number increment between
segments. Then, such middleboxes could unintentionally corrupt the
user-data and options by 'normalising' sequence or acknowledgement
numbering. Fortunately, including Inner Options in sequence improves
robustness against such middleboxes.
3.2.5. Rationale for the Sent Payload Size Field
A middlebox that splits a TCP connection can coalesce and/or divide
the original segments. Segmentation offload hardware introduces
similar resegmentation. Inclusion of the Sent Payload Size field in
the InSpace Option makes the scheme robust against such
resegmentation.
The Sent Payload Size is not strictly necessary on a SYN (SYN=1,
ACK=0) because a SYN is never resegmented. However, for simplicity,
the layout for a SYN is made the same as for a SYN/ACK. This future-
proofs the protocol against the possibility that SYNs might be
resegmented in future. And it makes it easy to introduce the
alternative segment structure of Appendix A.4 if it is needed.
3.3. Rationale for the InSpace Option Format
The format of the InSpace Option (Figure 3) does not necessarily have
to comply with the RFC 793 format for TCP options, because it is not
intended to ever appear in a sequence of TCP options. In particular,
it does not need an Option Kind, because the option is always in a
known location. In effect the magic number serves as a multi-octet
Option Kind for the first InSpace Option, and the location of each
subsequent options is always known as an offset from the previous
one, using InOO and Sent Payload Size fields.
Other aspects of the layout are justified as follows:
Length: Whatever the size of the InSpace Option, the right-hand edge
of the Length field is always located 4 octets from the start of
the option, so that the receiver can find it to determine the
layout of the rest of the option. The option is always a multiple
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of 4 octets long, so that any subsequent Inner TCP Options comply
with TCP's option alignment requirements.
Sent Payload Size: This field is 16 bits wide, which is reasonable
given segment size cannot exceed the limits set by the Total
Length field in the IPv4 header and the Payload Length field in
the IPv6 header, both of which are 16 bits wide.
If the sender were to use a jumbogram [RFC2675], it could use the
Jumbo InSpace Option defined in Appendix A.3, which offers a
32-bit Sent Payload Size field. The Jumbo InSpace Option is not
mandatory to implement for the present experimental specification.
Even if it is implemented, it is only defined when SYN=0, given
use of a jumbogram for a SYN or SYN/ACK would significantly exceed
other limits that TCP sets for these segments.
InSpace Options Offset The 14-bit field is in units of 4-octet
words, in order to restrict Inner Options to no less than the size
of a maximum sized segment (given 4 * 2^14 = 2^16 octets).
When SYN=1 the layout of the InSpace Option is extended to include:
Suffix Options Offset: The SOO field is the same 14-bit width as the
InOO field, and for the same reason. Both the SOO and InOO fields
are aligned 2 bits to the left of a word boundary so that they can
be used directly in units of octets by masking out the 2-bit field
to the right.
Magic Number B: The 32-bit size of Magic Number A is not enough to
reduce the probability of mistaking the start of an Ordinary SYN
Payload for the start of the Inner Space protocol. A 64-bit magic
number could have been provided by using the next 4-octet word,
but this would be unnecessarily large. Therefore, when SYN=1, 16
more bits of magic number are provided within the InSpace Option.
Otherwise, these 16-bits would only have to be used for padding to
align with the next 4-octet word boundary anyway.
3.4. Protocol Overhead
The overhead of the Inner Space protocol is quantified as follows:
Dual Handshake:
Latency:
Upgraded Server : zero;
Legacy Server: worst latency of the dual handshakes.
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Connection Rate: The typical connection rate will inflate by P*D,
where:
P [0-100%] is the proportion of connections that use extra
option space;
D [0-100%] is the proportion of these that use a dual
handshake (the remainder use a single handshake, e.g. by
caching knowledge of upgraded servers).
For example, if P=80% and D=10%, the connection rate will
inflate by 8%. P is difficult to predict. D is likely to be
small, and in the longer term it should reduce to the
proportion of connections to remaining legacy servers, which
are likely to be the less frequently accessed ones. In the
worst case if both P & D are 100%, the maximum that the
connection rate can inflate by is 100% (i.e. to twice present
levels).
Connection State: Connection state on servers and middleboxes
will inflate by P*D/R, where
R is the average hold time of connection state measured in
round trip times
This is because a server or middlebox only holds dual
connection state for one round trip, until the RST on one of
the two connections. For example, keeping P & D as they were
in the above example, if R = 3 round trips {ToDo: TBA},
connection state would inflate by 2.7%. In the longer term, any
extra connection state would be focused on legacy servers, with
none on upgraded servers. Therefore, if memory for dual
handshake flow state was a problem, upgrading the server to
support the Inner Space protocol would solve the problem.
Network Traffic: The network traffic overhead is 2*H*P*D/J
counting in bytes or 2*P*D/K counting in packets, where
H is 88B for IPv4 or 108B for IPv6 (assuming the Ordinary SYN
and SYN/ACK have a TCP header packed to the maximum of 60B
with TCP options, they have no TCP payload, their IP headers
have no extensions and the InSpace Option in the SYN-U and
SYN/ACK-U is 8B);
J is the average number of bytes per TCP connection (in both
directions)
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K is the average number of packets per TCP connection (in both
directions);
For example, keeping and P & D as they were in the above
example, if J = 50KiB for IPv4 and K = 70 packets (ToDo: TBA),
traffic overhead would be 0.03% counting in bytes or 0.2%
counting in packets.
Processing: {ToDo: Implementation tests}
InSpace Option on every non-empty SYN=0 segment:
Network Traffic: The traffic overhead is P*Q*4/F, where
Q is the proportion of Inner Space connections that leave the
protocol enabled after the initial handshake;
F is the average frame size in bytes (assuming one segment per
frame).
This is because the InSpace option adds 4B per segment. For
example, keeping P as it was in the above example and taking
Q=10% and F=750B, the traffic overhead is 0.04%. It is as
difficult to predict Q as it is to predict P.
Processing: {ToDo: Implementation tests}
4. Interaction with Pre-Existing TCP Implementations
4.1. Compatibility with Pre-Existing TCP Variants
A TCP option MUST by default only be used as an Outer Option, unless
it is explicitly specified that it can (or must) be used as an Inner
Option. The following list of pre-existing TCP options can be
located as Inner Options:
o Maximum Segment Size (MSS) [RFC0793];
o SACK-ok [RFC2018];
o Window Scale [RFC7323];
o Multipath TCP [RFC6824], except the Data ACK part of the Data
Sequence Signal (DSS) option;
o TCP Fast Open [I-D.ietf-tcpm-fastopen];
o The tcpcrypt CRYPT option [I-D.bittau-tcpinc].
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The following MUST NOT be located as Inner Options:
o Timestamp [RFC7323];
o SACK [RFC2018];
o The Data ACK part of the DSS option of Multipath TCP [RFC6824];
o TCP-AO [RFC5925];
o The tcpcrypt MAC option [I-D.bittau-tcpinc] as long as it covers
the TCP header.
{ToDo: The above list is not authoritative. Many of the above
schemes involve multiple different types of TCP option, and all the
types need to be separately assessed.}
The Inner Space protocol supports TCP Fast Open, by constraining the
client to obey the rules in Section 2.3.1.1).
All the sub-types of the MPTCP option [RFC6824] except one could be
located as Inner Options. That is, MP_CAPABLE, MP_JOIN, ADD_ADDR(2),
REMOVE_ADDR, MP_PRIO, MP_FAIL, MP_FASTCLOSE. The Data Sequence
Signal (DSS) of MPTCP consists of four separable parts: i) the Data
ACK; ii) the mapping between the Data Sequence Number and the Subflow
Sequence Number over a Data-Level Length; iii) the Checksum; and iv)
the DATA_FIN flag. If MPTCP were re-factored to take advantage of
the Inner Space protocol, all these parts except the Data ACK could
be located as Inner Options (the Checksum would not be necessary).
The MPTCP Data ACK has to remain as an Outer Option otherwise there
would be a risk of flow control deadlock, as pointed out in
[Raiciu12]. For instance, a Web client might pipeline multiple
requests that fill a Web server's receive buffer, while the Web
server might be busy sending a large response to the first request
before it reads the second request. If the Data ACK were an Inner
Option, the Web client would have to stop acknowledging the first
response from the server (due to lack of receive window). Then the
server would not be able to move on to the next request--a classic
deadlock.
The TCP-AO has to be located as an Outer Option to prevent the
possibility of flow-control deadlock (because it would consume
receive window on pure ACKs).
All sub-options of the tcpcrypt CRYPT option could be located as
Inner Options. However, as long as the tcpcrypt MAC option covers
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the TCP header and Outer Options, it has to be located as an Outer
Option for the same deadlock reason as TCP-AO.
An Upgraded Server can support SYN Cookies [RFC4987] for Ordinary
Connections. For Upgraded Connections Section 2.5 defines a new
EchoCookie TCP option that is a prerequisite for InSpace
implementations, and provides sufficient space for the more extensive
connection state requirements of an InSpace server.
{ToDo: TCP States and Transitions, Connectionless Resets, ICMP
Handling, Forward-Compatibility.}
4.2. Interaction with Middleboxes
The interaction with the assumptions about TCP made by middleboxes is
covered extensively elsewhere:
o Section 2.3.3 specifies forwarding behaviour for Inner Options;
o The following sections explain the Inner Space protocol approach
to middlebox traversal:
* Section 3.2.1 justifies the magic number approach;
* Section 3.2.2 explains why the protocol will remain robust as
middlboxes evolve;
* Section 3.2.4 justifies including Inner Options in sequence;
* Section 3.2.5) explains how the protocol will remain robust to
resegmentation.
4.3. Interaction with the Pre-Existing TCP API
An aim of the Inner Space protocol is for legacy applications to
continue to just work without modification. Therefore it is expected
that the dual handshaking logic and any maintenance of a cached
white-list of servers that support the Inner Space protocol will be
implemented beneath the well-known socket interface.
Inner Space implementations will need to comply with the following
behaviours to ensure that legacy applications continue to receive
predictable behaviour from the socket interface:
Querying local port (TCP client): If an application calls
"getsockname()" while the TCP client behind the socket is engaged
in a dual TCP handshake, the call SHOULD block until the local TCP
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has aborted one of the connections so it knows which of the two
ports will continue to be used.
Binding to an explicit port: If an application specifies that it
wants the TCP client to use a specific port, the Inner Space
capability MUST be disabled, because the dual handshake has to try
two ports. Use of a specific port might be necessary, for example
in a port-testing application or if the application wants to
explicitly control all the handshaking logic of the Inner Space
protocol itself.
Logging: The dual handshake will show up as a specific signature in
logs of network activity. Log formats might not be able to record
two local ports against one socket, so logs might contain
unexpected or erroneous data. Even if logs correctly track both
connection attempts, log analysis software might not expect to see
one socket attempt to use two different ports. {ToDo: All this
needs to be turned into a predictability requirement.}
Note that Inner Space has no impact on queries for the remote port
from a TCP server. If an application calls "getpeername()" while the
TCP server behind the socket is (unwittingly) engaged in a dual
handshake, it will return the port of the remote client, even though
this connection might subsequently be aborted. This is because a TCP
server is not aware of whether it is part of a dual handshake.
It would be appropriate to enable the Inner Space protocol on a per-
host or per-user basis. The necessary configuration switch does not
need to be standardised, but it might allow the following three
states:
Enabled: The stack will enable Inner Space on any TCP connection
that that needs Inner Space for its TCP options. The stack might
still disable the Inner Space protocol autonomously after the
initial handshake if it is not needed.
Forwarding: The Forwarding mode is for TCP implementations on
middleboxes that implement split TCP connections, as discussed in
Section 2.3.3. Forwarding mode is similar to Disabled, except it
forwards data in SYN without deferring it until the incoming
connection is established.
Disabled: Inner Space is not enabled by default on any connections,
except those that specifically request it.
The socket API might also need to be extended for future applications
that want to control the Inner Space protocol explicitly. Experience
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will determine the best API, so these ideas are merely informational
suggestions at this stage:
Enabling/disabling Inner Space: As well as the above per-host or
per-user switches, the extended API might need to allow an
application to disable Inner Options on a per-socket basis (e.g.
for testing). A socket might need to be opened in one of three
possible Inner Space modes: i) Enabled; ii) Enabled initially but
can be disabled autonomously by the stack if redundant; iii)
Enabled initially, then disables itself after the SYN/ACK; and iv)
Disabled. It also ought to be possible for an application to
disable Inner Options on-demand mid-connection.
Querying support for Inner Space: An application might need to be
able to determine whether the host supports Inner Space and in
which mode it is enabled on a particular socket. For instance, an
application might need to choose different socket options
depending on whether Inner Space is enabled to make the necessary
space available.
Latency vs Efficiency: A socket that prefers efficient use of
connection state over latency might use the optional explicit
variant of the dual handshake (Appendix B). It is unlikely that a
new option specific to Inner Space would be needed to express this
preference, as many operating systems already offer a similar
socket option.
Logging: Log formats and log analysis software might need to be
extended to distinguish between the deliberate RST within the dual
handshake and an unexpected connection RST.
5. IANA Considerations
This specification requires IANA to allocate values from the TCP
Option Kind name-space against the following names:
o "Inner Option Space Upgraded (InSpaceU)"
o "Inner Option Space Ordinary (InSpaceO)"
o "ModeSwitch"
Early implementation before the IANA allocation MUST follow [RFC6994]
and use experimental option 254 and respective Experiment IDs:
o 0xUUUU (16 bits);
o 0xOOOO (16 bits);
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o 0xMMMM (16 bits);
{ToDo: Values TBA and register them with IANA} then migrate to the
assigned option after allocation.
6. Security Considerations
Certain cryptographic functions have different coverage rules for the
TCP Header and TCP Payload. Placing some TCP options beyond the Data
Offset could mean that they are treated differently from regular TCP
options. This is a deliberate feature of the protocol, but
application developers will need to be aware that this is the case.
A malicious host can send bogus SYN segments with a spoofed source IP
address (a SYN flood attack). The Inner Space protocol does not
alter the feasibility of this attack. However, the extra space for
TCP options on a SYN allows the attacker to include more TCP options
on a SYN than before, so it can make a server do more option
processing before replying with a SYN/ACK. To mitigate this problme,
a server under stress could deprioritise SYNs with longer option
fields to focus its resources on SYNs that require less processing.
Each SYN in a SYN flood attack causes a TCP server to consume memory.
The Inner Space protocol allows a potentially large amount of TCP
option state to be negotiated during the SYN exchange, which could
exhaust the TCP server's memory. The EchoCookie TCP option (see
Section 2.5) allows the server to place this state in a cookie and
send it on the SYN/ACK to the purported address of the client--rather
than hold it in memory.
Then, as long as the client returns the cookie on the acknowledgement
and the server verifies it, the server can recover its full record of
all the TCP options it negotiated and continue the connection without
delay. On the other hand, the server's responses to SYNs from
spoofed addresses will scatter to those spoofed addresses and the
server will not have consumed any memory while waiting in vain for
them to reply. See the Security Considerations in
[I-D.briscoe-tcpm-echo-cookie] for how the EchoCookie facility
protects against reflection and amplification attacks.
7. Acknowledgements
The idea of this approach grew out of discussions with Joe Touch
while developing draft-touch-tcpm-syn-ext-opt, and with Jana Iyengar
and Olivier Bonaventure. The idea that it is architecturally
preferable to place a protocol extension within a higher layer, and
code its location into upgraded implementations of the lower layer,
was originally articulated by Rob Hancock. {ToDo: Ref?} The following
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people provided useful review comments: Joe Touch, Yuchung Cheng,
John Leslie, Mirja Kuehlewind, Andrew Yourtchenko, Costin Raiciu,
Marcelo Bagnulo Braun, Julian Chesterfield and Jaime Garcia.
Bob Briscoe's contribution is part-funded by the European Community
under its Seventh Framework Programme through the Trilogy 2 project
(ICT-317756) and the Reducing Internet Transport Latency (RITE)
project (ICT-317700). The views expressed here are solely those of
the author.
8. References
8.1. Normative References
[I-D.ietf-tcpm-fastopen]
Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", draft-ietf-tcpm-fastopen-10 (work in
progress), September 2014.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6994] Touch, J., "Shared Use of Experimental TCP Options", RFC
6994, August 2013.
8.2. Informative References
[Honda11] Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and H. Tokuda, "Is it Still Possible to
Extend TCP?", Proc. ACM Internet Measurement Conference
(IMC'11) 181--192, November 2011.
[I-D.bittau-tcpinc]
Bittau, A., Boneh, D., Hamburg, M., Handley, M., Mazieres,
D., and Q. Slack, "Cryptographic protection of TCP Streams
(tcpcrypt)", draft-bittau-tcpinc-01 (work in progress),
July 2014.
[I-D.briscoe-tcpm-echo-cookie]
Briscoe, B., "The Echo Cookie TCP Option", draft-briscoe-
tcpm-echo-cookie-00 (work in progress), October 2014.
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[I-D.wing-tsvwg-happy-eyeballs-sctp]
Wing, D. and P. Natarajan, "Happy Eyeballs: Trending
Towards Success with SCTP", draft-wing-tsvwg-happy-
eyeballs-sctp-02 (work in progress), October 2010.
[Iyengar10]
Iyengar, J., Ford, B., Ailawadi, D., Amin, S., Nowlan, M.,
Tiwari, N., and J. Wise, "Minion--an All-Terrain Packet
Packhorse to Jump-Start Stalled Internet Transports",
Proc. Int'l Wkshp on Protocols for Future, Large-scale &
Diverse Network Transports (PFLDnet'10) , November 2010.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
Dual-Stack Hosts", RFC 6555, April 2012.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, January 2013.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, "TCP Extensions for High Performance", RFC
7323, September 2014.
[Raiciu12]
Raiciu, C., Paasch, C., Barre, S., Ford, A., Honda, M.,
Duchene, F., Bonaventure, O., and M. Handley, "How Hard
Can It Be? Designing and Implementing a Deployable
Multipath TCP", Proc. USENIX Symposium on Networked
Systems Design and Implementation , April 2012.
Appendix A. Protocol Extension Specifications
This appendix specifies protocol extensions that are OPTIONAL while
the specification is experimental. If an implementation includes an
extension, this section gives normative specification requirements.
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However, if the extension is not implemented, the normative
requirements can be ignored.
{Temporary note: The IETF may wish to consider making some of these
extensions mandatory to implement if early testing shows they are
useful or even necessary. Or it may wish to make at least the
receiving side mandatory to implement to ensure that two-ended
experiments are more feasible.}
A.1. Disabling InSpace and Generic Connection Mode Switching
This appendix is normative. It is separated from the body of the
specification because it is OPTIONAL to implement while the Inner
Space protocol is experimental. It defines the new ModeSwitch TCP
option illustrated in Figure 5. This option provides a facility to
disable the Inner Space protocol for the remainder of a connection.
It also provides a general-purpose facility for a TCP connection to
co-ordinate between the endpoints before switching into a yet-to-be-
defined mode.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+---------------+---------------+-----------+-+-+
| ModeSwitch | Length=3 |Flags (CU) |I|R|
+---------------+---------------+-----------+-+-+
Figure 5: The ModeSwitch TCP Option
The Option Kind is ModeSwitch, the value of which is to be allocated
by IANA {ToDo: Value TBA}. ModeSwitch MUST be used only as an Inner
Option, because it uses the reliable ordered delivery property of
Inner Options. Therefore implementation of the Inner Space protocol
is REQUIRED for an implementation of ModeSwitch. Nonetheless,
ModeSwitch is a generic facility for switching a connection between
yet-to-be-defined modes that do not have to relate to extra option
space.
The sender MUST set the option Length to 3 (octets). The Length
field MUST be forwarded unchanged by other nodes, even if its value
is different.
The Flags field is available for defining modes of the connection.
Only two connection modes are currently defined. The first 6 bits of
the Flags field are Currently Unused (CU) and the sender MUST set
them to zero. The CU flags MUST be ignored and forwarded unchanged
by other nodes, even if their value is non-zero.
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The two 1-bit connection mode flags that are currently defined have
the following meanings:
o R: Request flag if 1. Request mode is a special mode that allows
the hosts to co-ordinate a change to any other mode(s);
o I: Inner Space mode: Enabled if 1, Disabled if 0.
The default Inner Space mode at the start of a connection is I=1,
meaning Inner Space is in enabled mode.
The procedure for changing a mode or modes is as follows:
o The host that wants to change modes (the requester) sends a
ModeSwitch message as an Inner Option with R=1 and with the other
flag(s) set to the mode(s) it wants to change to. The requester
does not change modes yet.
o The responder echoes the mode flag(s) it is willing to change to,
with the request flag R=0.
o The half-connection from the responder changes to the mode(s) it
confirms directly after the end of the segment that echoes its
confirmation, i.e. after the last octet of the TCP Payload
following the ModeSwitch option that echoes its confirmation.
Therefore it sends the segment carrying the confirmation in the
prior mode(s) of the connection.
o Once the requester receives the responder's confirmation message,
it re-echoes its confirmation of the responder's confirmation,
with the mode(s) set to those that both hosts agree on and R=0.
o The half-connection from the requester changes to the mode(s) it
confirms directly after the end of the segment that re-echoes its
confirmation. Therefore it sends the segment carrying the
confirmation in the prior mode(s) of the connection.
o The responder can refuse a request to change into a mode in any
one of three ways:
* either implicitly by never confirming it;
* or explicitly by sending a message with R=0 and the opposite
mode;
* or explicitly be sending a counter-request to switch to the
opposite mode (that the connection is already in) with R=1.
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The regular TCP sequence numbers and acknowledgement numbers of
requests or confirmations can be used to disambiguate overlapping
requests or responses.
Once a host switches to Disabled mode, it MUST NOT send any further
InSpace Options. Therefore it can send no further Inner Options and
it cannot switch back to Enabled mode for the rest of the connection.
To temporarily reduce InSpace overhead without permanently disabling
the protocol, the sender can use a value of 0xFFFF in the Sent
Payload Size (see Section 2.4).
A.2. Dual Handshake: The Explicit Variant
This appendix is normative. It is separated from the body of the
specification because it is OPTIONAL to implement while the Inner
Space protocol is experimental. It is not mandatory to implement
because it will be more useful once the Inner Space protocol has
become accepted widely enough that fewer middleboxes will discard SYN
segments carrying this option (see Appendix B for when best to deploy
it). It only works if both ends support it, but it can be deployed
one end at a time, so there is no need for support in early
experimental implementations.
{Temporary note: The choice between the explicit handshake in the
present section or the handshake in Section 2.1.1 is a tradeoff
between robustness against middlebox interference and minimal server
state. During the IETF review process, one might be chosen as the
only variant to go forward, at which point the other will be deleted.
Alternatively, the IETF could require a server to understand both
variants and a client could be implemented with either, or both. If
both, the application could choose which to use at run-time. Then we
will need a section describing the necessary API.}
This explicit dual handshake is similar to that in Section 2.1.1,
except the SYN that the Upgraded Client sends on the Ordinary
Connection is explicitly distinguishable from the SYN that would be
sent by a Legacy Client. Then, if the server actually is an Upgraded
Server, it can reset the Ordinary Connection itself, rather than
creating connection state for at least a round trip until the client
resets the connection.
For an explicit dual handshake, the TCP client still sends two
alternative SYNs: a SYN-O intended for Legacy Servers and a SYN-U
intended for Upgraded Servers. The two SYNs MUST have the same
network addresses and the same destination port, but different source
ports. Once the client establishes which type of server has
responded, it continues the connection appropriate to that server
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type and aborts the other. The SYN intended for Upgraded Servers
includes additional options within the TCP Data (the SYN-U defined as
before in Section 2.2.1).
Table 2 summarises the TCP 3-way handshake exchange for each of the
two SYNs in the two right-hand columns, between an Upgraded TCP
Client (the active opener) and either:
1. a Legacy Server, in the top half of the table (steps 2-4), or
2. an Upgraded Server, in the bottom half of the table (steps 2-4)
The table uses the same layout and symbols as Table 1, which has
already been explained in Section 2.1.1.
+------+------------------+--------------------+--------------------+
| | | Ordinary | Upgraded |
| | | Connection | Connection |
+------+------------------+--------------------+--------------------+
| 1 | Upgraded Client | >SYN-O | >SYN-U |
| | | | |
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Legacy Server | <SYN/ACK | <SYN/ACK |
| | | | |
| 3a | Upgraded Client | Waits for response | |
| | | to both SYNs | |
| | | | |
| 3b | " | >ACK | >RST |
| | | | |
| 4 | | Cont... | |
| | | | |
| /\/\ | /\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ | /\/\/\/\/\/\/\/\/\ |
| 2 | Upgraded Server | <RST | <SYN/ACK-U |
| | | | |
| 3 | Upgraded Client | | >ACK |
| | | | |
| 4 | | | Cont... |
+------+------------------+--------------------+--------------------+
Table 2: Explicit Variant of Dual 3-Way Handshake in Two Server
Scenarios
As before, an Upgraded Server MUST respond to a SYN-U with a SYN/ACK-
U. Then, the client recognises that it is talking to an Upgraded
Server.
Unlike before, an Upgraded Server MUST respond to a SYN-O with a RST.
However, the client cannot rely on this behaviour, because a
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middlebox might be stripping Outer TCP Options which would turn the
SYN-O into a regular SYN before it reached the server. Then the
handshake would effectively revert to the implicit variant.
Therefore the client's behaviour still depends on which SYN-ACK
arrives first, so its response to SYN-ACKs has to follow the rules
specified for the implicit handshake variant in Section 2.1.1.
The rules for processing TCP options are also unchanged from those in
Section 2.3.
A.2.1. SYN-O Structure
The SYN-O is merely a SYN with an extra InSpaceO Outer TCP Option as
shown in Figure 6. It merely identifies that the SYN is opening an
Ordinary Connection, but explicitly identifies that the client
supports the Inner Space protocol.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---------------+---------------+
| Kind=InSpaceO | Length=2 |
+---------------+---------------+
Figure 6: An InSpaceO TCP Option Flag
An InSpaceO TCP Option has Option Kind InSpaceO with value {ToDo:
Value TBA} and MUST have Length = 2 octets.
To use this option, the client MUST place it with the Outer TCP
Options. A Legacy Server will just ignore this TCP option, which is
the normal behaviour for an option that TCP does not recognise
[RFC0793].
A.2.2. Retransmission Behaviour - Explicit Variant
If the client receives a RST on one connection, but a short while
after that {ToDo: duration TBA} the response to the SYN-U has not
arrived, it SHOULD retransmit the SYN-U. If latency is more
important than the extra TCP option space, in parallel to any
retransmission, or instead of any retransmission, the client MAY send
a SYN without any InSpace TCP Option, in case this is the cause of
the black-hole. However, the presence of the RST implies that the
SYN with the InSpaceO TCP Option (the SYN-O) probably reached the
server, therefore it is more likely (but not certain) that the lack
of response on the other connection is due to transmission loss or
congestion loss.
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If the client receives no response at all to either the SYN-O or the
SYN-U, it SHOULD solely retransmit one or the other, not both. If
latency is more important than the extra TCP option space, it SHOULD
send a SYN without an InSpaceO TCP Option. Otherwise it SHOULD
retransmit the SYN-U. It MUST NOT retransmit both segments, because
the lack of response could be due to severe congestion.
A.2.3. Corner Cases
There is a small but finite possibility that the Explicit Dual
Handshake might encounter the cases below. The Implicit Handshake
(Section 2.1.1) is robust to these possibilities, but the Explicit
Handshake is not, unless the following additional rules are followed:
Both successful: This could occur if one load-sharing replica of a
server is upgraded, while another is not. This could happen in
either order but, in both cases, the client aborts the last
connection to respond:
* The client completes the Ordinary Handshake (because it
receives a SYN/ACK), but then, before it has aborted the
Upgraded Connection, it receives a SYN/ACK-U on it. In this
case, the client MUST abort the Upgraded Connection even though
it would work. Otherwise the client will have opened both
connections, one with Inner TCP Options and one without. This
could confuse the application.
* The client completes the Upgraded Connection after receiving a
SYN/ACK-U, but then it receives a SYN/ACK in response to the
SYN-O. In this case, the client MUST abort the connection it
initiated with the SYN-O.
Both aborted: The client might receive a RST in response to its SYN-
O, then an Ordinary SYN/ACK on its Upgraded Connection in response
to its SYN-U. This could occur i) if a split connection middlebox
actively forwards unknown options but holds back or discards data
in a SYN; or ii) if one load-sharing replica of a server is
upgraded, while another is not.
Whatever the likely cause, the client MUST still respond with a
RST on its Upgraded Connection. Otherwise, its Inner TCP Options
will be passed as user-data to the application by a Legacy Server.
If confronted with this scenario where both connections are
aborted, the client will not be able to include extra options on a
SYN, but it might still be able to set up a connection with extra
option space on all the other segments in both directions using
the approach in Appendix A.2.4. If that doesn't work either, the
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client's only recourse is to retry a new dual handshake on
different source ports, or ultimately to fall-back to sending an
Ordinary SYN.
A.2.4. Workround if Data in SYN is Blocked
If a path either holds back or discards data in a SYN-U, but there is
evidence that the server is upgraded from a RST response to the SYN-
O, the strategy below might at least allow a connection to use extra
option space on all the segments except the SYN.
It is assumed that the symptoms described in the 'both aborted' case
(Appendix A.2.3) have occurred, i.e. the server has responded to the
SYN-O with a RST, but it has responded to the SYN-U with an Ordinary
SYN/ACK not a SYN/ACK-U, so the client has had to RST the Upgraded
Connection as well. In this case, the client SHOULD attempt the
following (alternatively it MAY give up and fall back to opening an
Ordinary TCP connection).
The client sends an 'Alternative SYN-U' by including an InSpaceU
Outer TCP Option (Figure 7). This Alternative SYN-U merely flags
that the client is attempting to open an Upgraded Connection. The
client MUST NOT include any Inner Options or InSpace Option or Magic
Number. If the previous aborted SYN/ACK-U acknowledged the data that
the client sent within the original SYN-U, the client SHOULD resend
the TCP Payload data in the Alternative SYN-U, otherwise it might as
well defer it to the first data segment.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---------------+---------------+
| Kind=InSpaceU | Length=2 |
+---------------+---------------+
Figure 7: An InSpaceU Flag TCP option
An InSpaceU Flag TCP Option has Option Kind InSpaceU with value
{ToDo: Value TBA} and MUST have Length = 2 octets.
To use this option, the client MUST place it with the Outer TCP
Options. A Legacy Server will just ignore this TCP option, which is
the normal behaviour for an option that TCP does not recognise
[RFC0793]. Because the client has received a RST from the server in
response to the SYN-O it can assume that the server is upgraded. So
the client probably only needs to send a single Alternative SYN-U in
this repeat attempt. Nonetheless, the RST might have been spurious.
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Therefore the client MAY also send an Ordinary SYN in parallel, i.e.
using the Implicit Dual Handshake (Section 2.1.1).
If an Upgraded Server receives a SYN carrying the InSpaceU option, it
MUST continue the rest of the connection as if it had received a full
SYN-U (Section 2.2), i.e. by processing any Outer Options in the
SYN-U and responding with a SYN/ACK-U.
A.3. Jumbo InSpace TCP Option (only if SYN=0)
This appendix is normative. It is separated from the body of the
specification because it is OPTIONAL to implement while the Inner
Space protocol is experimental. In experimental implementations, it
will be sufficient to implement the required behaviour for when the
Length of a received InSpace Option is not recognised (Section 2.4).
If the IPv6 Jumbo extension header is used, the SentPayloadSize field
will need to be 4 octets wide, not 2 octets. This section defines
the format of the InSpace Option necessary to support jumbograms.
If sending a jumbogram, a sender MUST use the InSpace Option format
defined in Figure 8. All the fields have the same meanings as
defined in Section 2.2.2, except InOO and SentPayloadSize use more
bits.
When reading a segment, the Jumbo InSpace Option could be present in
a packet that is not a jumbogram (e.g. due to resegmentation).
Therefore a receiver MUST use the Jumbo InSpace Option to work along
the stream irrespective of whether arriving packets are jumbo sized
or not.
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
+-----------------------------------------------------------+---+
| Inner Options Offset (InOO) |Len|
+-----------------------------------------------------------+---+
| Sent Payload Size (SPS) |
+---------------------------------------------------------------+
Figure 8: InSpace Option for a Jumbo Data-UNJH
A.4. Upgraded Segment Structure to Traverse DPI boxes
This appendix is normative. It is separated from the body of the
specification because it is OPTIONAL to implement while the Inner
Space protocol is experimental. If a receiver has implemented the
Inner Space protocol but not this extension, no mechanism is provided
for it to ask the sender to fall-back to the base Inner Space
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protocol if it is sent a segment formatted according to this
extension. However, it will at least fall-back naturally to regular
TCP behaviour because of the dual handshake.
In experiments conducted between 2010 and 2011, [Honda11] reported
that 7 of 142 paths (about 5%) blocked access to port 80 if the
payload was not parsable as valid HTTP. This variant of the
specification has been defined in case experiments prove that it
significantly improves traversal of such deep packet inspection (DPI)
boxes.
This variant starts the TCP Data with the expected app-layer headers
on the first two segments in each direction:
SYN=1: The structure in Figure 9a) is used on a SYN or SYN/ACK. The
sender locates the 4-octet Magic Number A at the end of the
segment. The sender right-aligns the 8-octet InSpace Option just
before Magic Number A. Then it right-aligns the Inner Options
against the InSpace Option, all after the end of the TCP Payload.
The start of the Inner Options is therefore 4 * (InOO +3) octets
before the end of the segment, where InOO is read from within the
InSpace Option.
A receiver implementation will check whether Magic Number A is
present at the end of the segment if it does not first find it at
the start of the segment. Although the InnerOptions are located
at the end of the TCP Payload, they are considered to be applied
before the first octet of the TCP Payload.
SYN=0: The structure of the first non-SYN segment that contains any
TCP Data is shown in Figure 9b).
The receiver will find the second InSpace Option (InSpace#2)
located SPS#1 octets from the start of the segment, where SPS#1 is
the value of Sent Payload Size that was read from the InSpace
Option in the previous (SYN=1) segment that started the half-
connection. Although the Inner Options are shifted, as for the
first segment, they are still considered to be applied at the
start of the TCP Data in this second segment.
From the second InSpace Option onwards, the structure of the stream
reverts to that already defined in Section 2.2.1. So the value of
Sent Payload Size (SPS#2) in the second InSpace Option (InSpace #2)
defines the length of any remaining TCP Payload before the end of the
first data segment, as shown.
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TCP Data
.------------------------'----------------------.
| Inner Options |
a) SYN=1 | .----------'----------. |
+--------+----------+--------+----------+----------+---------+------+
| BaseHdr| OuterOpts| Payload|PrefixOpts|SuffixOpts|InSpace#1|MagicA|
+--------+----------+--------+----------+----------+---------+------+
| DO | | SOO | | | 1 |
`------------------>| `--------->| | Len |<-----'
| | | InOO |<--------' |
|<--------------------' |
b) First SYN=0 segment in either direction
+--------+----------+----------+---------+---------------+----------+
| BaseHdr| OuterOpts| Payload |InSpace#2| Inner Options | Payload |
+--------+----------+----------+---------+---------------+----------+
| DO | SPS#1 | Len | InOO | SPS#2 |
`------------------>`--------->`-------->`-------------->`--------->|
All offsets are specified in 4-octet (32-bit) words, except SPS,
which is in octets.
Figure 9: Segment Structures to Traverse DPI boxes (not to scale)
It is recognised that having to work from the end of the first
segment makes processing more involved. Experimental implementation
of this approach will determine whether the extra complexity improves
DPI box traversal sufficiently to make it worthwhile.
Appendix B. Comparison of Alternatives
B.1. Implicit vs Explicit Dual Handshake
In the body of this specification, two variants of the dual handshake
are defined:
1. The implicit dual handshake (Section 2.1.1) starting with just an
Ordinary SYN (no InSpaceO flag option) on the Ordinary
Connection;
2. The explicit dual handshake (Appendix A.2) starting with a SYN-O
(InSpaceO flag option) on the Ordinary Connection.
Both schemes double up connection state (for a round trip) on the
Legacy Server. But only the implicit scheme doubles up connection
state (for a round trip) on the Upgraded Server as well. On the
other hand, the explicit scheme risks delay accessing a Legacy Server
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if a middlebox discards the SYN-O (it is possible that some firewalls
will discard packets with unrecognised TCP options {ToDo: ref?}).
Table 3 summarises these points.
+----------------------------------+---------------+----------------+
| | SYN | SYN-L |
| | (Implicit) | (Explicit) |
+----------------------------------+---------------+----------------+
| Minimum state on Upgraded Server | - | + |
| | | |
| Minimum risk of delay to Legacy | + | - |
| Server | | |
+----------------------------------+---------------+----------------+
Table 3: Comparison of Implicit vs. Explicit Dual Handshake on the
Ordinary Connection
There is no need for the IETF to choose between these. If the
specification allows either or both, the tradeoff can be left to
implementers at build-time, or to the application at run-time.
Initially clients might choose the Implicit Dual Handshake to
minimise delays due to middlebox interference. But later, perhaps
once more middleboxes support the scheme, clients might choose the
Explicit scheme, to minimise state on Upgraded Servers.
Appendix C. Protocol Design Issues (to be Deleted before Publication)
This appendix is informative, not normative. It records outstanding
issues with the protocol design that will need to be resolved before
publication.
Option alignment following re-segmentation: If the byte-stream is
resegmented (e.g. by a connection splitter), the TCP options
within the stream will not necessarily align on 4-octet word
boundaries within the new segments.
Ossifies reliable ordered delivery into TCP design: At present it is
theoretically possible to implement a variant of TCP that provides
partial reliability. Inner Space as it stands would prevent a
future partial reliable TCP, but not if out-of-order delivery were
added, as discussed below.
Ideally Outer Options in Inner: Ideally enable Outer Options to be
located beyond the Data Offset: i) without consuming receive
window ii) either without consuming sequence space or, if
otherwise, must be robust to middlebox correction; iii) delivered
immediately on reception, not in sent order. Could use the Minion
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[Iyengar10] variant (or a similar variant) of the consistent
overhead byte-stuffing (COBS) encoding.
Appendix D. Change Log (to be Deleted before Publication)
A detailed version history can be accessed at
<http://datatracker.ietf.org/doc/draft-briscoe-tcpm-inner-space/
history/>
From briscoe-...-inner-space-00 to briscoe-...-inner-space-01:
Technical changes:
* Corrected DO to 4 * DO (twice)
* Confirmed that receive window applies to Inner Options
* Generalised the cause of decryption/decompression from a
previous TCP option to any previouis control message
* Added requirement for a middlebox not to defer data on SYN
* Latency of dual handshake is worst of two
* Completed "Interaction with Pre-Existing TCP Implementations"
section, covering other TCP variants, TCP in middleboxes and
the TCP API. Shifted some TCP options to Outer only, because
of RWND deadlock problem
* Added two outstanding issues: i) ossifies reliable ordered
delivery; ii) Ideally Outer in Inner.
Editorial changes:
* Removed section on Echo TCP option to a separate I-D that is
mandatory to implement for inner-space, and shifted some SYN
flood discussion in Security Considerations
* Clarifications throughout
* Acknowledged more review comments
From draft-briscoe-tcpm-syn-op-sis-02 to draft-briscoe-tcpm-inner-
space-00:
The Inner Space protocol is a development of a proposal called the
SynOpSis (Sister SYN options) protocol. Most of the elements of
Inner Space were in SynOpSis, such as the implicit and explicit
dual handshakes; the use of a magic number to flag the existence
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of the option; the various header offsets; and the option
processing rules.
The main technical differences are: Inner Space extends option
space on any segment, not just the SYN; this advance requires the
introduction of the Sent Payload Size field and a general
rearrangement and simplification of the protocol format; the
option processing rules have been extended to assure compatibility
with TFO and one degree of recursion has been introduced to cater
for encryption or compression of Inner Options; The Echo option
has been added to provide a SYN-cookie-like capability. Also, the
default protocol has been pared down to the bare bones and
optional extensions relegated to appendices.
The main editorial differences are: The emphasis of the Abstract
and Introduction has expanded from a focus on just extra space
using the dual handshake to include much more comprehensive
middlebox traversal. A comprehensive Design Rationale section has
been added.
Author's Address
Bob Briscoe
BT
B54/77, Adastral Park
Martlesham Heath
Ipswich IP5 3RE
UK
Phone: +44 1473 645196
Email: bob.briscoe@bt.com
URI: http://bobbriscoe.net/
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