Internet DRAFT - draft-nishida-tcpm-agg-syn-ext
draft-nishida-tcpm-agg-syn-ext
TCPM Working Group Y. Nishida
Internet-Draft AWS
Intended status: Standards Track 5 July 2023
Expires: 6 January 2024
Aggregated Option for SYN Option Space Extension
draft-nishida-tcpm-agg-syn-ext-04
Abstract
TCP option space is scarce resource as its maximum length is limited
to 40 bytes. This limitation becomes more significant in SYN
segments as all options used in a connection should be exchanged
during SYN negotiations. This document proposes a new SYN option
negotiation scheme that can aggregate multiple TCP options in SYN
segments into a single option so that more options can be negotiate
during 3-way handshake. With its simple design, the approach does
not require fundamental changes in TCP.
Status of This Memo
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This Internet-Draft will expire on 6 January 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Aggregated Option Design . . . . . . . . . . . . . . . . . . 4
3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Predefined Aggregated Options . . . . . . . . . . . . . . 6
3.3. Option Bits Registration . . . . . . . . . . . . . . . . 7
3.4. Utilizing 3rd and 4th Segments for Further
Negotiations . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. Aggregated Option . . . . . . . . . . . . . . . . . . . . 8
5.2. Option Bits Registry for Aggregated Option . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 8
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Normative References . . . . . . . . . . . . . . . . . . . . . 8
Informative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
TCP option space is scarce resource as its maximum length is limited
to 40 bytes because the length of the Data Offset field in the TCP
header is 4 bits [RFC9293].
This limitation is a critical issue especially for SYN segments.
Because SYN segments need to contain all options expected to be used
for the connection, although a TCP endpoint can send only one SYN
segment to its peer in a connection. The only exception in the
current standards is User Timeout Option [RFC5482]. However, this is
because this option provides only advisory information and does not
need to be exchanged reliably.
As a result, the current SYN option space tends to be congested.
Many TCP connections use MSS [RFC9293], Timestamp and Window Scale
[RFC7323], SACK Permitted options [RFC2018] which already consume 19
bytes (4 + 10 + 3 + 2). In addition to these options, if a
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connection wants to use Multipath TCP [RFC8684], it requires
additional 4-12 bytes for MP_CAPABLE or 12-16 bytes for MP_JOIN
option. Similarly, TCP Fast Open [RFC7413] and TCP AO [RFC5925]
require additional 6-18 bytes and 16 bytes respectively. Moreover,
Experimental Option Format defined in [RFC6994] requires 16 bits or
32 bits ExID, which means the length of any experimental options will
be 4 bytes or 6 bytes.
If an endpoint is willing to add some of extra options in addition to
commonly used options, 40 bytes space may not be sufficient. If a
SYN segment cannot accommodate all options that an endpoint wants to
use, the endpoint needs to give up using some of them. This problem
affects the extensibility of TCP.
There have been various proposals in order to extend option space in
SYN Segments such as [I-D.eddy-tcp-loo], [I-D.yourtchenko-tcp-loic],
[I-D.touch-tcpm-tcp-syn-ext-opt], [I-D.briscoe-tcpm-inner-space] and
[I-D.allman-tcpx2-hack]. These proposals have adopted one or both of
the following two types of approach.
* Extending TCP header in SYN segment: This approach tries to
accommodate more options in a SYN segment by using payload (e.g
override Data Offset field in TCP header).
* Using Multiple SYN or SYN-like segment: This approach uses
multiple SYN segments or additional segments that can be treated
as a SYN segment (e.g sending another SYN with wrong checksum or
from different source port).
However, these kinds of approach induce some complexity as it needs
to update fundamental TCP design and have potential risks for
middlebox interventions because of it. Instead, we propose a simple
alternate approach that can aggregate multiple TCP options into a
single options. As this approach does not require drastic changes to
TCP SYN negotiation scheme, the risk for middlebox interventions will
be minimized. [I-D.boucadair-tcpm-capability-option] also proposes a
scheme to aggregate multiple options as many of these options are
basically about negotiating support with the peer before actual use
of the option. However, our approach requires less option space as
it can aggregate and condense some TCP options to create more option
space for others. Note that [I-D.boucadair-tcpm-capability-option]
specifically target controlled domains to nullify the implications of
the presence of middleboxes.
The proposed approach in the draft cannot aggregate all kinds of
options. However, we believe it still will be useful especially for
newly defined experimental options as it requires at least 4 bytes
space in the option field. Also, the proposed approach can be
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combined with EDO [I-D.draft-ietf-tcpm-tcp-edo] extension or utilize
3rd segments and 4th segments like the feature negotiation scheme for
MPTCP if needed.
One example use case for the proposed approach is
[I-D.gomez-tcpm-ack-rate-request]. In order to use the feature
proposed in the document, endpoints need to exchange a 4-byte TCP
option during 3-way handshake so that they can check if the peer is
capable of the feature. However, whether an endpoint supports the
feature or not is just 1-bit information. Using 4-byte field to
carry 1-bit information looks redundant. On the other hand,
Aggregated Option can accommodate up to 18 new TCP options + 3
existing options like into a single TCP option.
Also, even if more than 1-bit information needs to be carried in a
TCP option for a certain feature, it is still possible to utilize
aggregated options in some cases. In such cases, an endpoint can
confirm that the peer supports the feature it wants to use by using
the aggregated option. After that, it can continue to negotiate
required parameters through 3rd segment and 4th segment. This type
of approach is already used in MPTCP [RFC8684]. Hence, we believe
this scheme can be applicable to many other TCP extensions.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document uses the following terms:
* Original Option format: refers to the option format as defined in
[RFC9293]
* Aggregated Option format: refers to indication of support of a TCP
option by setting the corresponding bit in the Aggregated TCP
Option.
3. Aggregated Option Design
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3.1. Option Format
The aggregated option can be used to indicate that an endpoint wants
to enable the specified features during SYN segment exchanges. This
option uses one bit in the option field for one TCP option. The
receiver of the option also uses this option to indicate that it
agrees to use the requested features or not. The format of
aggregated option format is shown in Figure 1. The option contains
1-byte field named called "Aggregated Block". Aggregated Option can
accommodate 1-3 Aggregated Blocks.
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
+--------------+---------------+---------------+- - - - - - - -
| Kind = TBD | Length = 3-5 | Block #1 | Block #2 |
+--------------+---------------+---------------+- - - - - - - -
| Block #3 |
- - - - - - - -
Figure 1: Aggregated Option format
Figure 2 shows the format of Aggregated block. Aggregated block has
1 byte length which consists of 2 bits "Group ID" (GID) field and 6
bits "Option Bits" field. The options supported by Aggregated Option
are split into 4 groups. The Group ID field is used to identify the
group identifier of the option bits in the Aggregated Option and each
single bit in Option Bits field represent a option in the group.
If all options that an endpoint wants to aggregate belong to the same
Group ID, the aggregated option needs to contain only one Aggregated
Block. Otherwise, it needs to contain multiple blocks as needed.
0 1 2 3 4 5 6 7
+----------------------+
| GID | Option Bits |
+----------------------+
Figure 2: Aggregated Block format
GID field in Aggregated Block indicate the group ID that option bits
in the block belongs to. Aggregated Blocks appear in SYN and SYN ACK
segments, however, different mappings between GID value in the option
and Group ID are used for these two segments. This is because some
implementations may copy back unknown options in SYN/ACK segments.
These mappings are used not to be confused by such cases. Figure 3
shows the mapping between GID value in the option and Group ID. For
example, GID value 1 in SYN represents group 2, while GID value 1 in
ACK segment for SYN ACK represents group 1.
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+------------------+----------------------+----------------------+
| GID value in SYN | GID value in SYN ACK | Group ID Description |
+------------------+----------------------+----------------------+
| 0 | 1 | group 1 |
| | | |
| 1 | 2 | group 2 |
| | | |
| 2 | 3 | group 3 |
| | | |
| 3 | 0 | group 4 |
+------------------+----------------------+----------------------+
Figure 3: Mapping between GID value and Group ID
The allocation of the bit in Option Bits field in each group will be
managed by the registry provided by IANA. Since an aggregated block
has 6-bits field to indicate options, one group can have 6 options at
most. As a result, the possible maximum number of options with this
format will be 4 * 6 = 24. We believe this is sufficient number for
the time being based on the current usage of option code points.
The Aggregated Option MUST be only used in SYN segments. When an
endpoint receives SYN segments with Aggregated Option, it checks
Aggregated Blocks in the option. Otherwise, the segment MUST be
silently discarded. If it contains Aggregated Blocks, the options
specified in the blocks MUST be processed as well as options in
original formats. When a responder sends back a SYN ACK to the
initiator, it SHOULD send back its response with Aggregated Option.
But, it MAY uses original format of the options for the response as
long as there is enough option space.
3.2. Predefined Aggregated Options
In this proposal, group 1 is used to aggregate commonly used options
as predefined aggregated options. Hence, when new aggregated options
are registered, they will belong to the rest of groups. Figure 4
shows aggregated option format when group 1 is specified.
0 1 2 3 4 5 6 7
+------------------------------------------------+
| GID (0 or 1)| WScale | SACK | MSS |
+------------------------------------------------+
Figure 4
In order to specify group id, the GID field of this format is 0 in
SYN and 1 in SYN/ACK. The first 4 bits of 6 bytes Option Bits are
used for Window Scale Option. As the value of the shift.cnt in
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Window Scale Option is 0-14. The shift.cnt values can be stored in
the 4 bits as the same format in the original option. When this
value is 15 (all 4 bits are 1), it specifies the window scale option
is not aggregated in the segment.
The 5th bit in Option Bits field is used for SACK options. If the
bit is set, it indicate the sending endpoint supports Selective
Acknowledgement. If the bit is not set, it specifies SACK option is
not aggregated in the segment.
The 6th bit in Option Bits field is used for MSS options. If the bit
is set, it indicates that the sending endpoint uses 1460 as send MSS
which is the most common value used for MSS option. If the bit is
not set, it specifies MSS option is not aggregated in the segment.
3.3. Option Bits Registration
The allocation of the Option Bits in Aggregated Option is maintained
by IANA. If a new option can be aggregatable, one can request Option
Bit in addition to the current procedure, requesting TCP Option Kind
Number in [TCPParameters] . If an option already has assigned TCP
Option Kind Number, one can request Option bit only which will
represent the assigned option kind.
3.4. Utilizing 3rd and 4th Segments for Further Negotiations
Aggregated Option is designed to exchange 1-bit information for each
TCP extension that indicate the willingness to use the feature.
Hence, if a TCP extension wants to carry more information in the TCP
option for the extension, Aggregated Option is basically not
applicable for it.
However, it is still possible for these TCP extensions to utilize
Aggregated Option in some situations. It is based on the fact that
not all TCP extensions will be used right after SYN exchanges. For
example, SACK options are only used when there are packet losses. If
a TCP extension is not used right after SYN exchange, it is possible
to exchange additional parameters by using utilizing 3rd segments and
4th segments. This approach is already used in MPTCP [RFC8684]. As
we have a solid precedence, we believe it will not be difficult to
implement similar negotiation schemes for other features. However,
discussing negotiation schemes with 3rd and 4th segments is out of
scope of the document.
4. Security Considerations
We believe Aggregated Option maintains the same level of security as
other TCP options does.
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5. IANA Considerations
This document requests new TCP option codepoint. In addition, this
document requires new registry for the option. They are described in
the following subsections.
5.1. Aggregated Option
This document requests to add new option: Aggregated Option to the
TCP option space registry which points to this document as follows:
+------+--------+--------------------+----------------+
| Kind | Length | Meaning | Reference |
+------+--------+--------------------+----------------+
| TBD | N | Aggregated Option | This Document |
+------+--------+--------------------+----------------+
Figure 5: Aggregated Option Format
5.2. Option Bits Registry for Aggregated Option
This document also requests to create a "Aggregated Option
Identifiers" registry in IANA registries. The registry maintains
records which are mapped to the TCP Option Kind Number Records in
[TCPParameters] These records are divided into 4 groups so that each
group contains 6 records.
Acknowledgments
The authors would like to appreciate Mohamed Boucadair for his
insightful comments on this document.
Contributors
The contents in this document are the individual contributions from
the authors and do not relate to the authors' positions at their
affiliations.
References
Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
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[RFC6994] Touch, J., "Shared Use of Experimental TCP Options",
RFC 6994, DOI 10.17487/RFC6994, August 2013,
<https://www.rfc-editor.org/rfc/rfc6994>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9293] Eddy, W., Ed., "Transmission Control Protocol (TCP)",
STD 7, RFC 9293, DOI 10.17487/RFC9293, August 2022,
<https://www.rfc-editor.org/rfc/rfc9293>.
Informative References
[I-D.allman-tcpx2-hack]
Allman, M., "TCPx2: Don't Fence Me In", Work in Progress,
Internet-Draft, draft-allman-tcpx2-hack-00, 8 May 2006,
<https://datatracker.ietf.org/doc/html/draft-allman-tcpx2-
hack-00>.
[I-D.boucadair-tcpm-capability-option]
Boucadair, M. and C. Jacquenet, "TCP Capability Option",
Work in Progress, Internet-Draft, draft-boucadair-tcpm-
capability-option-01, 8 December 2016,
<https://datatracker.ietf.org/doc/html/draft-boucadair-
tcpm-capability-option-01>.
[I-D.briscoe-tcpm-inner-space]
Briscoe, B., "Inner Space for TCP Options", Work in
Progress, Internet-Draft, draft-briscoe-tcpm-inner-space-
01, 27 October 2014,
<https://datatracker.ietf.org/doc/html/draft-briscoe-tcpm-
inner-space-01>.
[I-D.draft-ietf-tcpm-tcp-edo]
Touch, J. D. and W. Eddy, "TCP Extended Data Offset
Option", Work in Progress, Internet-Draft, draft-ietf-
tcpm-tcp-edo-13, 22 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
tcp-edo-13>.
[I-D.eddy-tcp-loo]
Eddy, W. and A. Langley, "Extending the Space Available
for TCP Options", Work in Progress, Internet-Draft, draft-
eddy-tcp-loo-04, 1 July 2008,
<https://datatracker.ietf.org/doc/html/draft-eddy-tcp-loo-
04>.
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[I-D.gomez-tcpm-ack-rate-request]
Gomez, C. and J. Crowcroft, "TCP ACK Rate Request Option",
Work in Progress, Internet-Draft, draft-gomez-tcpm-ack-
rate-request-06, 12 October 2022,
<https://datatracker.ietf.org/doc/html/draft-gomez-tcpm-
ack-rate-request-06>.
[I-D.touch-tcpm-tcp-syn-ext-opt]
Touch, J. D. and T. Faber, "TCP SYN Extended Option Space
Using an Out-of-Band Segment", Work in Progress, Internet-
Draft, draft-touch-tcpm-tcp-syn-ext-opt-12, 22 October
2022, <https://datatracker.ietf.org/doc/html/draft-touch-
tcpm-tcp-syn-ext-opt-12>.
[I-D.yourtchenko-tcp-loic]
Yourtchenko, A., "Introducing TCP Long Options by Invalid
Checksum", Work in Progress, Internet-Draft, draft-
yourtchenko-tcp-loic-00, 11 April 2011,
<https://datatracker.ietf.org/doc/html/draft-yourtchenko-
tcp-loic-00>.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018,
DOI 10.17487/RFC2018, October 1996,
<https://www.rfc-editor.org/rfc/rfc2018>.
[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
RFC 5482, DOI 10.17487/RFC5482, March 2009,
<https://www.rfc-editor.org/rfc/rfc5482>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/rfc/rfc5925>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/rfc/rfc7323>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/rfc/rfc7413>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/rfc/rfc8684>.
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[TCPParameters]
"Transmission Control Protocol (TCP) Parameters", n.d.,
<https://www.iana.org/assignments/tcp-parameters/tcp-
parameters.xhtml#tcp-parameters-1>.
Author's Address
Yoshifumi Nishida
Amazon Web Services
440 Terry Ave N
Seattle, WA 98109
United States of America
Email: nsd.ietf@gmail.com
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