TCP Extended Data Offset Option
draft-ietf-tcpm-tcp-edo-11
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Joseph Touch
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Wesley Eddy
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draft-touch-tcpm-tcp-edo
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TCPM WG J. Touch
Internet Draft Independent consultant
Updates: 793 Wes Eddy
Intended status: Standards Track MTI Systems
Expires: April 2022 October 12, 2021
TCP Extended Data Offset Option
draft-ietf-tcpm-tcp-edo-11.txt
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
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This Internet-Draft will expire on April 12, 2022.
Copyright Notice
Copyright (c) 2021 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
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Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Abstract
TCP segments include a Data Offset field to indicate space for TCP
options but the size of the field can limit the space available for
complex options such as SACK and Multipath TCP and can limit the
combination of such options supported in a single connection. This
document updates RFC 793 with an optional TCP extension to that
space to support the use of multiple large options. It also explains
why the initial SYN of a connection cannot be extending a single
segment.
Table of Contents
1. Introduction...................................................3
2. Conventions used in this document..............................3
3. Motivation.....................................................3
4. Requirements for Extending TCP's Data Offset...................4
5. The TCP EDO Option.............................................4
5.1. EDO Supported.............................................5
5.2. EDO Extension.............................................5
5.3. The two EDO Extension variants............................8
6. TCP EDO Interaction with TCP...................................9
6.1. TCP User Interface........................................9
6.2. TCP States and Transitions................................9
6.3. TCP Segment Processing...................................10
6.4. Impact on TCP Header Size................................10
6.5. Connectionless Resets....................................11
6.6. ICMP Handling............................................11
7. Interactions with Middleboxes.................................12
7.1. Middlebox Coexistence with EDO...........................12
7.2. Middlebox Interference with EDO..........................13
8. Comparison to Previous Proposals..............................14
8.1. EDO Criteria.............................................14
8.2. Summary of Approaches....................................15
8.3. Extended Segments........................................16
8.4. TCPx2....................................................16
8.5. LO/SLO...................................................17
8.6. LOIC.....................................................17
8.7. Problems with Extending the Initial SYN..................18
9. Implementation Issues.........................................19
10. Security Considerations......................................20
11. IANA Considerations..........................................20
12. References...................................................20
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12.1. Normative References....................................20
12.2. Informative References..................................20
13. Acknowledgments..............................................22
1. Introduction
TCP's Data Offset (DO)is a 4-bit field, which indicates the number
of 32-bit words of the entire TCP header [RFC793]. This limits the
current total header size to 60 bytes, of which the basic header
occupies 20, leaving 40 bytes for options. These 40 bytes are
increasingly becoming a limitation to the development of advanced
capabilities, such as when SACK [RFC2018][RFC6675] is combined with
either Multipath TCP [RFC6824], TCP-AO [RFC5925], or TCP Fast Open
[RFC7413].
This document specifies the TCP Extended Data Offset (EDO) option,
and is independent of (and thus compatible with) IPv4 and IPv6. EDO
extends the space available for TCP options, except for the initial
SYN and SYN/ACK. This document also explains why the option space of
the initial SYN segments cannot be extended as individual segments
without severe impact on TCP's initial handshake and the SYN/ACK
limitation that results from potential middlebox misbehavior.
Multiple other TCP extensions are being considered in the TCPM
working group in order to address the case of SYN and SYN/ACK
segments [Bo14][Br14][To18]. Some of these other extensions can work
in conjunction with EDO (e.g., [To18]).
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [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.
In this document, the characters ">>" preceding an indented line(s)
indicates a compliance requirement statement using the key words
listed above. This convention aids reviewers in quickly identifying
or finding the explicit compliance requirements of this RFC.
3. Motivation
TCP supports headers with a total length of up to 15 32-bit words,
as indicated in the 4-bit Data Offset field [RFC793]. This accounts
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for a total of 60 bytes, of which the default TCP header fields
occupy 20 bytes, leaving 40 bytes for options.
TCP connections already use this option space for a variety of
capabilities. These include Maximum Segment Size (MSS) [RFC793],
Window Scale (WS) [RFC7323], Timestamp (TS) [RFC7323], Selective
Acknowledgement (SACK) [RFC2018][RFC6675], TCP Authentication Option
(TCP-AO) [RFC5925], Multipath TCP (MP-TCP)_[RFC6824], and TCP User
Timeout [RFC5482]. Some options occur only in a SYN or SYN/ACK (MSS,
WS), and others vary in size when used in SYN vs. non-SYN segments.
Each of these options consumes space, where some options consuming
as much space as available (SACK) and other desired combinations can
easily exceed the currently available space. For example, it is not
currently possible to use TCP-AO with both TS and MP-TCP in the same
non-SYN segment, i.e., to combine accurate round-trip estimation,
authentication, and multipath support in the same connection - even
though these options can be negotiated during a SYN exchange (10 for
TS, 16 for TCP-AO, and 12 for MP-TCP).
TCP EDO is intended to overcome this limitation for non-SYN
segments, as well as to increase the space available for SACK
blocks. Further discussion of the impact of EDO and existing options
is discussed in Section 6.4. Extending SYN segments is much more
complicated, as discussed in Section 8.7.
4. Requirements for Extending TCP's Data Offset
The primary goal of extending the TCP Data Offset field is to
increase the space available for TCP options in all segments except
the initial SYN.
An important requirement of any such extension is that it not impact
legacy endpoints. Endpoints seeking to use this new option should
not incur additional delay or segment exchanges to connect to either
new endpoints supporting this option or legacy endpoints without
this option. We call this a "backward downgrade" capability.
An additional consideration of this extension is avoiding user data
corruption in the presence of popular network devices, including
middleboxes. Consideration of middlebox misbehavior can also
interfere with extension in the SYN/ACK.
5. The TCP EDO Option
TCP EDO extends the option space for all segments except the initial
SYN (i.e., SYN set and ACK not set) and SYN/ACK response. EDO is
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indicated by the TCP option codepoint of EDO-OPT and has two types:
EDO Supported and EDO Extension, as discussed in the following
subsections.
5.1. EDO Supported
EDO capability is determined in both directions using a single
exchange of the EDO Supported option (Figure 1). When EDO is desired
on a given connection, the SYN and SYN/ACK segments include the EDO
Supported option, which consists of the two required TCP option
fields: Kind and Length. The EDO Supported option is used only in
the SYN and SYN/ACK segments and only to confirm support for EDO in
subsequent segments.
+--------+--------+
| Kind | Length |
+--------+--------+
Figure 1 TCP EDO Supported option
An endpoint seeking to enable EDO includes the EDO Supported option
in the initial SYN. If receiver of that SYN agrees to use EDO, it
responds with the EDO Supported option in the SYN/ACK. The EDO
Supported option does not extend the TCP option space.
>> Connections using EDO MUST negotiate its availability during the
SYN exchange of the initial three-way handshake.
>> An endpoint confirming and agreeing to EDO use MUST respond with
the EDO Supported option in its SYN/ACK.
The SYN/ACK uses only the EDO Supported option (and not the EDO
Extension option, below) because it may not yet be safe to extend
the option space in the reverse direction due to potential middlebox
misbehavior (see Section 7.2). Extension of the SYN and SYN/ACK
space is addressed as a separate option (see Section 8.7).
5.2. EDO Extension
When EDO is successfully negotiated, all other segments use the EDO
Extension option, of which there are two variants (Figure 2 and
Figure 3). Both variants are considered equivalent and either
variant can be used in any segment where the EDO Extension option is
required. Both variants add a Header_Length field (in network-
standard byte order), indicating the length of the entire TCP header
in 32-bit words. Figure 3 depicts the longer variant, which includes
an additional Segment_Length field, which is identical to the TCP
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pseudoheader TCP Length field and used to detect when segments have
been altered in ways that would interfere with EDO (discussed
further in Section 5.3).
+--------+--------+--------+--------+
| Kind | Length | Header_Length |
+--------+--------+--------+--------+
Figure 2 TCP EDO Extension option - simple variant
+--------+--------+--------+--------+
| Kind | Length | Header_Length |
+--------+--------+--------+--------+
| Segment_Length |
+--------+--------+
Figure 3 TCP EDO Extension option - with segment length verification
>> Once enabled on a connection, all segments in both directions
MUST include the EDO Extension option. Segments not needing
extension MUST set the EDO Extension option Header Length field
equal to the Data Offset length.
>> The EDO Extension option MAY be used only if confirmed when the
connection transitions to the ESTABLISHED state, e.g., a client is
enabled after receiving the EDO Supported option in the SYN/ACK and
the server is enabled after seeing the EDO Extension option in the
final ACK of the three-way handshake. If either of those segments
lacks the appropriate EDO option, the connection MUST NOT use any
EDO options on any other segments.
Internet paths may vary after connection establishment, introducing
misbehaving middleboxes (see Section 7.2). Using EDO on all segments
in both directions allows this condition to be detected.
>> The EDO Supported option MAY occur in an initial SYN as desired
(e.g., as expressed by the user/application) and in the SYN/ACK as
confirmation, but MUST NOT be inserted in other segments. If the EDO
Supported option is received in other segments, it MUST be silently
ignored.
>> If EDO has not been negotiated and agreed, the EDO Extension
option MUST be silently ignored on subsequent segments. The EDO
Extension option MUST NOT be sent in an initial SYN segment or
SYN/ACK, and MUST be silently ignored and not acknowledged if so
received.
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>> If EDO has been negotiated, any subsequent segments arriving
without the EDO Extension option MUST be silently ignored. Such
events MAY be logged as warning errors and logging MUST be rate
limited.
When processing a segment, EDO needs to be visible within the area
indicated by the Data Offset field, so that processing can use the
EDO Header_length to override the field for that segment.
>> The EDO Extension option MUST occur within the space indicated by
the TCP Data Offset.
>> The EDO Extension option indicates the total length of the
header. The EDO Header_length field MUST NOT exceed that of the
total segment size (i.e., TCP Length).
>> The EDO Header Length MUST be at least as large as the TCP Data
Offset field of the segment in which they both appear. When the EDO
Header Length equals the Data Offset length, the EDO Extension
option is present but it does not extend the option space. When the
EDO Header Length is invalid, the TCP segment MUST be silently
dropped.
>> The EDO Supported option SHOULD be aligned on a 16-bit boundary
and the EDO Extension option SHOULD be aligned on a 32-bit boundary,
in both cases for simpler processing.
For example, a segment with only EDO would have a Data Offset of 6
or 7 (depending on the EDO Extension variant used), where EDO would
be the first option processed, at which point the EDO Extension
option would override the Data Offset and processing would continue
until the end of the TCP header as indicated by the EDO
Header_length field.
There are cases where it might be useful to process other options
before EDO, notably those that determine whether the TCP header is
valid, such as authentication, encryption, or alternate checksums.
In those cases, the EDO Extension option is preferably the first
option after a validation option, and the payload after the Data
Offset is treated as user data for the purposes of validation.
>> The EDO Extension option SHOULD occur as early as possible,
either first or just after any authentication or encryption, and
SHOULD be the last option covered by the Data Offset value.
Other options are generally handled in the same manner as when the
EDO option is not active, unless they interact with other options.
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One such example is TCP-AO [RFC5925], which optionally ignores the
contents of TCP options, so it would need to be aware of EDO to
operate correctly when options are excluded from the HMAC
calculation.
>> Options that depend on other options, such as TCP-AO [RFC5925]
(which may include or exclude options in MAC calculations) MUST also
be augmented to interpret the EDO Extension option to operate
correctly.
5.3. The two EDO Extension variants
There are two variants of the EDO Extension option; one includes a
copy of the TCP segment length, copied from the TCP pseduoheader
[RFC793]. The Segment_Length field is added to the longer variant to
detect when segments are incorrectly and inappropriately merged by
middleboxes or TCP offload processing but without consideration for
the additional option space indicated by the EDO Header_Length
field. Such effects are described in further detail in Section 7.2.
>> An endpoint MAY use either variant of the EDO Extension option
interchangeably.
When the longer, 6-byte variant is used, the Segment_Length field is
used to check whether modification of the segment was performed
consistent with knowledge of the EDO option. The Segment_Length
field will detect any modification of the length of the segment,
such as might occur when segments are split or merged, that occurs
without also updating the Segment Length field as well. The Segment
Length field thus helps endpoints detects devices that merge or
split TCP segments without support for EDO. Devices that merge or
split TCP segments that support EDO would update the Segment Length
field as needed but would also ensure that the user data is handled
separately from the extended option space indicate by EDO.
>> When an endpoint creates a new segment using the 6-byte EDO
Extension option, the Segment_Length field is initialized with a
copy of the segment length from the TCP pseudoheader.
>> When an endpoint receives a segment using the 6-byte EDO
Extension option, it MUST validate the Segment_Length field with the
length of the segment as indicated in the TCP pseudoheader. If the
segment lengths do not match, the segment MUST be discarded and an
error SHOULD be logged in a rate-limited manner.
>> The 6-byte EDO Extension variant SHOULD be used where middlebox
or TCP offload support could merge or split TCP segments without
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consideration for the EDO option. Because these conditions could
occur at either endpoint or along the network path, the 6-byte
variant SHOULD be preferred until sufficient evidence for safe use
of the 4-byte variant is determined by the community.
The field will not detect other modification of the TCP user data;
such modifications would need more complex detection mechanisms,
such as checksums or hashes. When these are used, as with IPsec or
TCP-AO, the 4-byte variant is sufficient.
>> The 4-byte EDO Extension variant is sufficient when EDO is used
in conjunction with other mechanisms that provide integrity
protection, such as IPsec or TCP-AO.
6. TCP EDO Interaction with TCP
The following subsections describe how EDO interacts with the TCP
specification [RFC793].
6.1. TCP User Interface
The TCP EDO option is enabled on a connection using a mechanism
similar to any other per-connection option. In Unix systems, this is
typically performed using the 'setsockopt' system call.
>> Implementations can also employ system-wide defaults, however
systems SHOULD NOT activate this extension by default to avoid
interfering with legacy applications.
>> Due to the potential impacts of legacy middleboxes (discussed in
Section 7), a TCP implementation supporting EDO SHOULD log any
events within an EDO connection when options that are malformed or
show other evidence of tampering arrive. An operating system MAY
choose to cache the list of destination endpoints where this has
occurred with and block use of EDO on future connections to those
endpoints, but this cache MUST be accessible to users/applications
on the host. Note that such endpoint assumptions can vary in the
presence of load balancers where server implementations vary behind
such balancers.
6.2. TCP States and Transitions
TCP EDO does not alter the existing TCP state or state transition
mechanisms.
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6.3. TCP Segment Processing
TCP EDO alters segment processing during the TCP option processing
step. Once detected, the TCP EDO Extension option overrides the TCP
Data Offset field for all subsequent option processing. Option
processing continues at the next option (if present) after the EDO
Extension option.
6.4. Impact on TCP Header Size
The TCP EDO Supported option increases SYN header length by a
minimum of 2 bytes but could increase it by more depending on 32-bit
word alignment. Currently popular SYN options total 19 bytes, which
leaves more than enough room for the EDO Supported option:
o SACK permitted (2 bytes in SYN, optionally 2 + 8N bytes after)
[RFC2018][RFC6675]
o Timestamp (10 bytes) [RFC7323]
o Window scale (3 bytes) [RFC7323]
o MSS option (4 bytes) [RFC793]
Adding the EDO Supported option would result in a total of 21 bytes
of SYN option space.
Subsequent segments would use 10 bytes of option space without any
SACK blocks (TS only; WS and MSS are used only in SYN and SYN/ACK)
or allow up to 3 SACK blocks before needing to use EDO; with EDO,
the number of SACK blocks or additional options would be
substantially increased. There are also other options that are
emerging in the SYN, including TCP Fast Open, which uses another 6-
18 (typically 10) bytes in the SYN/ACK of the first connection and
in the SYN of subsequent connections [RFC7413].
TCP EDO can also be negotiated in SYNs with either of the following
large options:
o TCP-AO (authentication) (16 bytes) [RFC5925]
o Multipath TCP (12 bytes in SYN and SYN/ACK, 20 after) [RFC6824]
Including TCP-AO with TS, WS, SACK increases the SYN option space
use to 35 bytes; with Multipath TCP the use is 31 bytes. When
Multipath TCP is enabled with the typical options, later segments
would require 30 bytes without SACK, thus limiting the SACK option
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to one block unless EDO is also supported on at least non-SYN
segments.
The full combination of the above options (47 bytes for TS, WS, MSS,
SACK, TCP-AO, and MPTCP) does not fit in the existing SYN option
space and (as noted) that space cannot be extended within a single
SYN segment. There has been a proposal to change TS to a 2 byte "TS
permitted" signal in the initial SYN, provided it can be safely
enabled during the connection later or might be avoided completely
[Ni15]. Even using "TS-permitted", the total space is still too
large to support in the initial SYN without SYN option space
extension [Bo14][Br14][To18].
The EDO Extension option has negligible impact on other headers
because it can either come first or just after security information,
and in either case the additional 4 or 6 bytes are easily
accommodated within the TCP Data Offset length. Once the EDO option
is processed, the entirety of the remainder of the TCP segment is
available for any remaining options.
6.5. Connectionless Resets
A RST may arrive during a currently active connection or may be
needed to cleanup old state from an abandoned connection. The latter
occurs when a new SYN is sent to an endpoint with matching existing
connection state, at which point that endpoint responds with a RST
and both ends remove stale information.
The EDO Extension option is mandatory on all TCP segments once
negotiated, i.e., except in the SYN and SYN/ACK (which establish
support) and the RST. A RST may lack the context to know that EDO is
active on a connection.
>> The EDO Extension option MAY occur in a RST when the endpoint has
connection state that has negotiated EDO. However, unless the RST is
generated by an incoming segment that includes an EDO Extension
option, the transmitted RST MUST NOT include the EDO Extension
option.
6.6. ICMP Handling
ICMP responses are intended to include the IP and the port fields of
TCP and UDP headers of typical TCP/IP and UDP/IP packets [RFC792].
This includes the first 8 data bytes of the original datagram,
intended to include the transport port numbers used for connection
demultiplexing. Later specifications encourage returning as much of
the original payload as possible [RFC1812]. In either case, legacy
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options or new options in the EDO extension area might or might not
be included, and so options are generally not assumed to be part of
ICMP processing anyway.
7. Interactions with Middleboxes
Middleboxes are on-path devices that typically examine or modify
packets in ways that Internet routers do not [RFC3234]. This
includes parsing transport headers and/or rewriting transport
segments in ways that may affect EDO.
There are several cases to consider:
- Typical NAT/NAPT devices, which modify only IP address and/or TCP
port number fields (with associated TCP checksum updates)
- Middleboxes that try to reconstitute TCP data streams, such as
for deep-packet inspection for virus scanning
- Middleboxes that modify known TCP header fields
- Middleboxes that rewrite TCP segments
7.1. Middlebox Coexistence with EDO
Middleboxes can coexist with EDO when they either support EDO or
when they ignore its impact on segment structure.
NATs and NAPTs, which rewrite IP address and/or transport port
fields, are the most common form of middlebox and are not affected
by the EDO option.
Middleboxes that support EDO would be those that correctly parse the
EDO option. Such boxes can reconstitute the TCP data stream
correctly or can modify header fields and/or rewrite segments
without impact to EDO.
Conventional TCP proxies terminate the TCP connection in both
directions and thus operate as TCP endpoints, such as when a client-
middlebox and middlebox-server each have separate TCP connections.
They would support EDO by following the host requirements herein on
both connections. The use of EDO on one connection is independent of
its use on the other in this case.
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7.2. Middlebox Interference with EDO
Middleboxes that do not support EDO cannot coexist with its use when
they modify segment boundaries or do not forward unknown (e.g., the
EDO) options.
So-called "transparent" rewriting proxies, which inappropriately and
incorrectly modify TCP segment boundaries, might mix option
information with user data if they did not support EDO. Such devices
might also interfere with other TCP options such as TCP-AO. There
are three types of such boxes:
o Those that process received options and transmit sent options
separately, i.e., although they rewrite segments, they behave as
TCP endpoints in both directions.
o Those that split segments, taking a received segment and emitting
two or more segments with revised headers.
o Those that join segments, receiving multiple segments and
emitting a single segment whose data is the concatenation of the
components.
In all three cases, EDO is either treated as independent on
different sides of such boxes or not. If independent, EDO would
either be correctly terminated in either or both directions or
disabled due to lack of SYN/ACK confirmation in either or both
directions. Problems would occur only when TCP segments with EDO are
combined or split while ignoring the EDO option. In the split case,
the key concern is if the split happens within the option extension
space or if EDO is silently copied to both segments without copying
the corresponding extended option space contents. However, the most
comprehensive study of these cases indicates that "although
middleboxes do split and coalesce segments, none did so while
passing unknown options" [Ho11].
Note that the second and third types of middlebox behaviors listed
above may create syndromes similar to TCP transmit and receive
hardware offload engines that incorrectly modify segments with
unknown options.
Middleboxes that silently remove options that they do not implement
have been observed [Ho11]. Such boxes interfere with the use of the
EDO Extension option in the SYN and SYN/ACK segments because
extended option space would be misinterpreted as user data if the
EDO Extension option were removed, and this cannot be avoided. This
is one reason that SYN and SYN/ACK extension requires alternate
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mechanisms (see Section 8.7). It is also the reason for the 6-byte
EDO Extension variant (see Section 5.3), which can detect such
merging or splitting of segments. Further, if such middleboxes
become present on a path they could cause similar misinterpretation
on segments exchanged in the ESTABLISHED and subsequent states. As a
result, this document requires that the EDO Extension option be
avoided on the SYN/ACK and that this option needs to be used on all
segments once successfully negotiated and encourages use of the 6-
byte EDO Extension variant.
Deep-packet inspection systems that inspect TCP segment payloads or
attempt to reconstitute the data stream would incorrectly include
option data in the reconstituted user data stream, which might
interfere with their operation.
>> It can be important to detect misbehavior that could cause EDO
space to be misinterpreted as user data. In such cases, EDO SHOULD
be used in conjunction with an integrity protection mechanism. This
includes the 6-byte EDO Extension variant or stronger mechanisms
such as IPsec, TCP-AO, etc. It is useful to note that such
protection only helps non-compliant components and enable avoidance
(e.g., disabling EDO), but integrity protection alone cannot correct
the misinterpretation of EDO space as user data.
This situation is similar to that of ECN and ICMP support in the
Internet. In both cases, endpoints have evolved mechanisms for
detecting and robustly operating around "black holes". Very similar
algorithms are expected to be applicable for EDO.
8. Comparison to Previous Proposals
EDO is the latest in a long line of attempts to increase TCP option
space [Al06][Ed08][Ko04][Ra12][Yo11]. The following is a comparison
of these approaches to EDO, based partly on a previous summary
[Ra12]. This comparison differs from that summary by using a
different set of success criteria.
8.1. EDO Criteria
Our criteria for a successful solution are as follows:
o Zero-cost fallback to legacy endpoints.
o Minimal impact on middlebox compatibility.
o No additional side-effects.
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Zero-cost fallback requires that upgraded hosts incur no penalty for
attempting to use EDO. This disqualifies dual-stack approaches,
because the client might have to delay connection establishment to
wait for the preferred connection mode to complete. Note that the
impact of legacy endpoints that silently reflect unknown options are
not considered, as they are already non-compliant with existing TCP
requirements [RFC793].
Minimal impact on middlebox compatibility requires that EDO works
through simple NAT and NAPT boxes, which modify IP addresses and
ports and recompute IPv4 header and TCP segment checksums.
Middleboxes that reject unknown options or that process segments in
detail without regard for unknown options are not considered; they
process segments as if they were an endpoint but do so in ways that
are not compliant with existing TCP requirements (e.g., they should
have rejected the initial SYN because of its unknown options rather
than silently relaying it).
EDO also attempts to avoid creating side-effects, such as might
happen if options were split across multiple TCP segments (which
could arrive out of order or be lost) or across different TCP
connections (which could fail to share fate through firewalls or
NAT/NAPTs).
These requirements are similar to those noted in [Ra12], but EDO
groups cases of segment modification beyond address and port - such
as rewriting, segment drop, sequence number modification, and option
stripping - as already in violation of existing TCP requirements
regarding unknown options, and so we do not consider their impact on
this new option.
8.2. Summary of Approaches
There are three basic ways in which TCP option space extension has
been attempted:
1. Use of a TCP option.
2. Redefinition of the existing TCP header fields.
3. Use of option space in multiple TCP segments (split across
multiple segments).
A TCP option is the most direct way to extend the option space and
is the basis of EDO. This approach cannot extend the option space of
the initial SYN.
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Redefining existing TCP header fields can be used to either contain
additional options or as a pointer indicating alternate ways to
interpret the segment payload. All such redefinitions make it
difficult to achieve zero-impact backward compatibility, both with
legacy endpoints and middleboxes.
Splitting option space across separate segments can create
unintended side-effects, such as increased delay to deal with path
latency or loss differences.
The following discusses three of the most notable past attempts to
extend the TCP option space: Extended Segments, TCPx2, LO/SLO, and
LOIC. [Ra12] suggests a few other approaches, including use of TCP
option cookies, reuse/overload of other TCP fields (e.g., the URG
pointer), or compressing TCP options. None of these is compatible
with legacy endpoints or middleboxes.
8.3. Extended Segments
TCP Extended Segments redefined the meaning of currently unused
values of the Data Offset (DO) field [Ko04]. TCP defines DO as
indicating the length of the TCP header, including options, in 32-
bit words. The default TCP header with no options is 5 such words,
so the minimum currently valid DO value is 5 (meaning 40 bytes of
option space). This document defines interpretations of values 0-4:
DO=0 means 48 bytes of option space, DO=1 means 64, DO=2 means 128,
DO=3 means 256, and DO=4 means unlimited (e.g., the entire payload
is option space). This variant negotiates the use of this capability
by using one of these invalid DO values in the initial SYN.
Use of this variant is not backward-compatible with legacy TCP
implementations, whether at the desired endpoint or on middleboxes.
The variant also defines a way to initiate the feature on the
passive side, e.g., using an invalid DO during the SYN/ACK when the
initial SYN had a valid DO. This capability allows either side to
initiate use of the feature but is also not backward compatible.
8.4. TCPx2
TCPx2 redefines legacy TCP headers by basically doubling all TCP
header fields [Al06]. It relies on a new transport protocol number
to indicate its use, defeating backward compatibility with all
existing TCP capabilities, including firewalls, NATs/NAPTs, and
legacy endpoints and applications.
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8.5. LO/SLO
The TCP Long Option (LO, [Ed08]) is very similar to EDO, except that
presence of LO results in ignoring the existing Data Offset (DO)
field and that LO is required to be the first option. EDO considers
the need for other fields to be first and declares that the EDO is
the last option as indicated by the DO field value. Like LO, EDO is
required in every segment once negotiated.
The TCP Long Option draft also specified the SYN Long Option (SLO)
[Ed08]. If SLO is used in the initial SYN and successfully
negotiated, it is used in each subsequent segment until all of the
initial SYN options are transmitted.
LO is backward compatible, as is SLO; in both cases, endpoints not
supporting the option would not respond with the option, and in both
cases the initial SYN is not itself extended.
SLO does modify the three-way handshake because the connection isn't
considered completely established until the first data byte is
acknowledged. Legacy TCP can establish a connection even in the
absence of data. SLO also changes the semantics of the SYN/ACK; for
legacy TCP, this completes the active side connection establishment,
where in SLO an additional data ACK is required. A connection whose
initial SYN options have been confirmed in the SYN/ACK might still
fail upon receipt of additional options sent in later SLO segments.
This case - of late negotiation fail - is not addressed in the
specification.
8.6. LOIC
TCP Long Options by Invalid Checksum is a dual-stack approach that
uses two initial SYNS to initiate all updated connections [Yo11].
One SYN negotiates the new option and the other SYN payload contains
only the entire options. The negotiation SYN is compliant with
existing procedures, but the option SYN has a deliberately incorrect
TCP checksum (decremented by 2). A legacy endpoint would discard the
segment with the incorrect checksum and respond to the negotiation
SYN without the LO option.
Use of the option SYN and its incorrect checksum both interfere with
other legacy components. Segments with incorrect checksums will be
silently dropped by most middleboxes, including NATs/NAPTs. Use of
two SYNs creates side-effects that can delay connections to upgraded
endpoints, notably when the option SYN is lost or the SYNs arrive
out of order. Finally, by not allowing other options in the
negotiation SYN, all connections to legacy endpoints either use no
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options or require a separate connection attempt (either concurrent
or subsequent).
8.7. Problems with Extending the Initial SYN
The key difficulty with most previous proposals is the desire to
extend the option space in all TCP segments, including the initial
SYN, i.e., SYN with no ACK, typically the first segment of a
connection, as well as possibly the SYN/ACK. It has proven difficult
to extend space within the segment of the initial SYN in the absence
of prior negotiation while maintaining current TCP three-way
handshake properties, and it may be similarly challenging to extend
the SYN/ACK (depending on asymmetric middlebox assumptions).
A new TCP option cannot extend the Data Offset of a single TCP
initial SYN segment and cannot extend a SYN/ACK in a single segment
when considering misbehaving middleboxes. All TCP segments,
including the initial SYN and SYN/ACK, may include user data in the
payload data [RFC793], and this can be useful for some proposed
features such as TCP Fast Open [RFC7413]. Legacy endpoints that
ignore the new option would process the payload contents as user
data and send an ACK. Once ACK'd, this data cannot be removed from
the user stream.
The Reserved TCP header bits cannot be redefined easily, even though
three of the six total bits have already been redefined (ECE/CWR
[RFC3168] and NS [RFC3540]). Legacy endpoints have been known to
reflect received values in these fields; this was safely dealt with
for ECN but would be difficult here [RFC3168].
TCP initial SYN (SYN and not ACK) segments can use every other TCP
header field except the Acknowledgement number, which is not used
because the ACK field is not set. In all other segments, all fields
except the three remaining Reserved header bits are actively used.
The total amount of available header fields, in either case, is
insufficient to be useful in extending the option space.
The representation of TCP options can be optimized to minimize the
space needed. In such cases, multiple Kind and Length fields are
combined, so that a new Kind would indicate a specific combination
of options, whose order is fixed and whose length is indicated by
one Length field. Most TCP options use fields whose size is much
larger than the required Kind and Length components, so the
resulting efficiency is typically insufficient for additional
options.
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The option space of an initial SYN segment might be extended by
using multiple initial segments (e.g., multiple SYNs or a SYN and
non-SYN) or based on the context of previous or parallel
connections. This method may also be needed to extend space in the
SYN/ACK in the presence of misbehaving middleboxes. Because of their
potential complexity, these approaches are addressed in separate
documents [Bo14][Br14][To18].
Option space cannot be extended in outer layer headers, e.g., IPv4
or IPv6. These layers typically try to avoid extensions altogether,
to simplify forwarding processing at routers. Introducing new shim
layers to accommodate additional option space would interfere with
deep-packet inspection mechanisms that are in widespread use.
As a result, EDO does not attempt to extend the space available for
options in TCP initial SYNs. It does extend that space in all other
segments (including SYN/ACK), which has always been trivially
possible once an option is defined.
9. Implementation Issues
TCP segment processing can involve accessing nonlinear data
structures, such as chains of buffers. Such chains are often
designed so that the maximum default TCP header (60 bytes) fits in
the first buffer. Extending the TCP header across multiple buffers
may necessitate buffer traversal functions that span boundaries
between buffers. Such traversal can also have a significant
performance impact, which is additional rationale for using TCP
option space - even extended option space - sparingly.
Although EDO can be large enough to consume the entire segment, it
is important to leave space for data so that the TCP connection can
make forward progress. It would be wise to limit EDO to consuming no
more than MSS-4 bytes of the IP segment, preferably even less (e.g.,
MSS-128 bytes).
When using the ExID variant for testing and experimentation, either
TCP option codepoint (253, 254) is valid in sent or received
segments.
Implementers need to be careful about the potential for offload
support interfering with this option. The EDO data needs to be
passed to the protocol stack as part of the option space, not
integrated with the user segment, to allow the offload to
independently determine user data segment boundaries and combine
them correctly with the extended option data. Some legacy hardware
receive offload engines may present challenges in this regard, and
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may be incompatible with EDO where they incorrectly attempt to
process segments with unknown options. Such offload engines are part
of the protocol stack and updated accordingly. Issues with incorrect
resegmentation by an offload engine can be detected in the same way
as middlebox tampering.
10. Security Considerations
It is meaningless to have the Data Offset further exceed the
position of the EDO data offset option.
>> When the EDO Extension option is present, the EDO Extension
option SHOULD be the last non-null option covered by the TCP Data
Offset, because it would be the last option affected by Data Offset.
This also makes it more difficult to use the Data Offset field as a
covert channel.
11. IANA Considerations
We request that, upon publication, this option be assigned a TCP
Option codepoint by IANA, which the RFC Editor will replace EDO-OPT
in this document with codepoint value.
The TCP Experimental ID (ExID) with a 16-bit value of 0x0ED0 (in
network standard byte order) has been assigned for use during
testing and preliminary experiments.
12. References
12.1. Normative References
[RFC793] 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.
12.2. Informative References
[Al06] Allman, M., "TCPx2: Don't Fence Me In", draft-allman-
tcpx2-hack-00 (work in progress), May 2006.
[Bo14] Borman, D., "TCP Four-Way Handshake", draft-borman-
tcp4way-00 (work in progress), October 2014.
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[Br14] Briscoe, B., "Inner Space for TCP Options", draft-briscoe-
tcpm-inner-space-01 (work in progress), October 2014.
[Ed08] Eddy, W. and A. Langley, "Extending the Space Available
for TCP Options", draft-eddy-tcp-loo-04 (work in
progress), July 2008.
[Ho11] Honda, M., Nishida, Y., Raiciu, C., Greenhalgh, A.,
Handley, M., and H. Tokuda, "Is it still possible to
extend TCP", Proc. ACM Sigcomm Internet Measurement
Conference (IMC), 2011, pp. 181-194.
[Ko04] Kohler, E., "Extended Option Space for TCP", draft-kohler-
tcpm-extopt-00 (work in progress), September 2004.
[Ni15] Nishida, Y., "A-PAWS: Alternative Approach for PAWS",
draft-nishida-tcpm-apaws-02 (work in progress), Oct. 2015.
[Ra12] Ramaiah, A., "TCP option space extension", draft-ananth-
tcpm-tcpoptext-00 (work in progress), March 2012.
[RFC792] Postel, J., "Internet Control Message Protocol", RFC 792,
September 1981.
[RFC1812] Baker, F. (Ed.), "Requirements for IP Version 4 Routers,"
RFC 1812, June 1995.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC3234] Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
Issues", RFC 3234, February 2002.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces", RFC
3540, June 2003.
[RFC5482] Eggert, L., and F. Gont, "TCP User Timeout Option", RFC
5482, March 2009.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
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[RFC6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP", RFC
6675, August 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
(Ed.), "TCP Extensions for High Performance", RFC 7323,
September 2014.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, December 2014.
[To21] Touch, J., T. Faber, "TCP SYN Extended Option Space Using
an Out-of-Band Segment", draft-touch-tcpm-tcp-syn-ext-opt
(work in progress), Oct. 2019.
[Yo11] Yourtchenko, A., "Introducing TCP Long Options by Invalid
Checksum", draft-yourtchenko-tcp-loic-00 (work in
progress), April 2011.
13. Acknowledgments
The authors would like to thank the IETF TCPM WG for their feedback,
in particular: Oliver Bonaventure, Bob Briscoe, Ted Faber, John
Leslie, Pasi Sarolahti, Richard Scheffenegger, and Alexander
Zimmerman.
This work is partly supported by USC/ISI's Postel Center.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Joe Touch
Manhattan Beach, CA 90266 USA
Phone: +1 (310) 560-0334
Email: touch@strayalpha.com
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Wesley M. Eddy
MTI Systems
US
Email: wes@mti-systems.com
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