TCP Extended Data Offset Option
draft-ietf-tcpm-tcp-edo-01
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (tcpm WG) | |
|---|---|---|---|
| Authors | Dr. Joseph D. Touch , Wesley Eddy | ||
| Last updated | 2014-10-13 | ||
| Replaces | draft-touch-tcpm-tcp-edo | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Stream | WG state | WG Document | |
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| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
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| Send notices to | (None) |
draft-ietf-tcpm-tcp-edo-01
TCPM WG J. Touch
Internet Draft USC/ISI
Updates: 793 Wes Eddy
Intended status: Standards Track MTI Systems
Expires: April 2015 October 13, 2014
TCP Extended Data Offset Option
draft-ietf-tcpm-tcp-edo-01.txt
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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 that have evolved. This document updates RFC 793
with an optional TCP extension to that space to support the use of
multiple large options such as SACK with either TCP Multipath or TCP
AO. 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. Requirements for Extending TCP's Data Offset...................3
4. The TCP EDO Option.............................................4
5. TCP EDO Interaction with TCP...................................6
5.1. TCP User Interface........................................6
5.2. TCP States and Transitions................................7
5.3. TCP Segment Processing....................................7
5.4. Impact on TCP Header Size.................................7
5.5. Connectionless Resets.....................................8
5.6. ICMP Handling.............................................9
6. Interactions with Middleboxes..................................9
6.1. Middlebox Coexistence with EDO............................9
6.2. Middlebox Interference with EDO..........................10
7. Comparison to Previous Proposals..............................11
7.1. EDO Criteria.............................................11
7.2. Summary of Approaches....................................12
7.3. Extended Segments........................................13
7.4. TCPx2....................................................13
7.5. LO/SLO...................................................13
7.6. LOIC.....................................................14
7.7. Problems with Extending the Initial SYN..................14
8. Implementation Issues.........................................16
9. Security Considerations.......................................16
10. IANA Considerations..........................................17
11. References...................................................17
11.1. Normative References....................................17
11.2. Informative References..................................17
12. Acknowledgments..............................................19
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1. Introduction
TCP's Data Offset 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
[Ch14].
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 middlebox misbehavior.
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. 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.
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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.
4. 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. The EDO
option is organized as indicated in Figure 1 and Figure 2. When
desired, initial SYN segments (i.e., those whose ACK bit is not set)
use the EDO request option, which consists of the required Kind and
Length fields only. Depending on capability and whether EDO is
successfully negotiated, any other segments can use the EDO length
option, which adds a Header_Length field (in network-standard byte
order), indicating the length of the entire TCP header in 32-bit
words. The codepoint value of the EDO Kind is EDO-OPT.
+--------+--------+
| Kind | Length |
+--------+--------+
Figure 1 TCP EDO request option
+--------+--------+--------+--------+
| Kind | Length | Header_length |
+--------+--------+--------+--------+
Figure 2 TCP EDO length option
EDO support is determined in both directions using a single
exchange. An endpoint seeking to enable EDO support includes the EDO
request option in the initial SYN. If receiver of that SYN agrees to
support EDO, it responds with a null EDO length option in the
SYN/ACK. A null EDO length option contains the same value as the DO
field, i.e., it does not extend the TCP option space.
>> Connections using EDO MUST negotiate its availability during the
initial three-way handshake.
>> An endpoint confirming EDO support MUST respond with a null EDO
length option in its SYN/ACK.
The SYN/ACK uses the null EDO length option because it may not yet
be safe to extend the option space in the reverse direction due to
middlebox misbehavior (see Section 6.2). Extension of the SYN and
SYN/ACK space is addressed as a separate option (see Section 7.7).
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>> The EDO length option MAY be used only if confirmed when the
connection transitions to the ESTABLISHED state, e.g., a client is
enabled after receiving the null EDO length option in the SYN/ACK
and the server is enabled after seeing a null or non-null EDO length
option in the final ACK of the three-way handshake. If either of
those segments lacks the EDO length option, the connection MUST NOT
use EDO on any other segments.
>> Once enabled on a connection, all segments in both directions
MUST include the EDO length option. Segments not needing extension
MUST set the EDO length equal to the DO length.
Internet paths may vary after connection establishment, introducing
misbehaving middleboxes (see Section 6.2). Using EDO on all segments
in both directions allows this condition to be detected.
>> The EDO request option MAY occur in an initial SYN as desired
(e.g., as expressed by the user/application), but MUST NOT be
inserted in other segments. If the EDO request option is received in
other segments, it MUST be silently ignored.
>> If EDO has not been negotiated and agreed, the EDO length option
MUST be silently ignored on subsequent segments. The EDO length
option MUST NOT be sent in an initial SYN segment, and MUST be
silently ignored and not acknowledged if so received.
>> If EDO has been negotiated, any subsequent segments arriving
without the EDO length 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 Data Offset for that segment.
>> The EDO length option MUST occur within the space indicated by
the TCP Data Offset.
>> The EDO length 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 length option MUST be at least as large as the TCP Data
Offset field of the segment in which they both appear. When the EDO
length equals the DO length, the EDO option is present but it does
not extend the option space. When the EDO length is invalid, the TCP
segment MUST be silently dropped.
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>> The EDO request option SHOULD be aligned on a 16-bit boundary and
the EDO length 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,
where EDO would be the first option processed, at which point the
EDO length 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 length 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 length 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.
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 length option to operate
correctly.
5. TCP EDO Interaction with TCP
The following subsections describe how EDO interacts with the TCP
specification [RFC793].
5.1. 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.
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>> Due to the potential impacts of legacy middleboxes (discussed in
Section 6), 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.
5.2. TCP States and Transitions
TCP EDO does not alter the existing TCP state or state transition
mechanisms.
5.3. TCP Segment Processing
TCP EDO alters segment processing during the TCP option processing
step. Once detected, the TCP EDO length option overrides the TCP
Data Offset field for all subsequent option processing. Option
processing continues at the next option (if present) after the EDO
length option.
5.4. Impact on TCP Header Size
The TCP EDO request option increases SYN header length by a minimum
of 2 bytes. Currently popular SYN options total 19 bytes, which
leaves more than enough room for the EDO request:
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 option would result in a total of 21 bytes of SYN
option space. Subsequent segments would use 19 bytes of option space
without any SACK blocks 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 [Ch14].
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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 increases the SYN option space use to 37 bytes;
with Multipath TCP the use is 33 bytes. When Multipath TCP is
enabled with the typical options, later segments might require 39
bytes without SACK, thus effectively disabling the SACK option
unless EDO is also supported on at least non-SYN segments.
The full combination of the above options (49 bytes including EDO)
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 [Ni14]. Even using "TS-
permitted", the total space is still too large to support in the
initial SYN without SYN option space extension [Br14][To14].
The EDO 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 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.
5.5. 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 option is mandatory on all TCP segments once negotiated,
except the SYN and SYN/ACK of the three-way handshake to establish
its support and the RST. A RST may lack the context to know that EDO
is active on a connection.
>> The EDO length 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 option, the
transmitted RST MUST NOT include the EDO length option.
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5.6. 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
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.
6. 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
6.1. 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-
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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.
6.2. 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 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].
Middleboxes that silently remove options they do not implement have
been observed [Ho11]. Such boxes interfere with the use of the EDO
length option in the SYN and SYN/ACK segments because extended
option space would be misinterpreted as user data if the EDO option
were removed, and this cannot be avoided. This is one reason that
SYN and SYN/ACK extension requires alternate mechanisms (see Section
7.7). Further, if such middleboxes become present on a path they
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could cause similar misinterpretation on segments exchanged in the
ESTABLISHED and subsequent states. As a result, this document
requires that the EDO length option be avoided on the SYN/ACK and
that this option needs to be used on all segments once successfully
negotiated.
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, such
as IPsec, TCP-AO, etc. It is useful to note that such protection
helps find only non-compliant components.
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.
7. 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.
7.1. 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.
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].
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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.
7.2. 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.
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.
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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.
7.3. 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.
7.4. 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.
7.5. LO/SLO
The TCP Long Option (LO, [Ed08]) is very similar to EDO, except that
presence of LO results in ignoring the existing 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.
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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.
7.6. 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
options or require a separate connection attempt (either concurrent
or subsequent).
7.7. 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
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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 [Ch14]. 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.
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 [Br14][To14].
Option space cannot be extended in outer layer headers, e.g., IPv4
or IPv6. These layers typically try to avoid extensions altogether,
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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.
8. 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.
9. Security Considerations
It is meaningless to have the Data Offset further exceed the
position of the EDO data offset option.
>> When the EDO length option is present, the EDO length 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.
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This also makes it more difficult to use the Data Offset field as a
covert channel.
10. 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.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
11.2. Informative References
[Al06] Allman, M., "TCPx2: Don't Fence Me In", draft-allman-
tcpx2-hack-00 (work in progress), May 2006.
[Br14] Briscoe, B., "Extended TCP Option Space in the Payload of
an Alternative SYN", draft-briscoe-tcpm-syn-op-sis-02
(work in progress), September 2014.
[Ch14] Cheng, Y., Chu, J., and A. Jain, "TCP Fast Open", draft-
ietf-tcpm-fastopen-10, September 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.
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[Ni14] Nishida, Y., "A-PAWS: Alternative Approach for PAWS",
draft-nishida-tcpm-apaws-01 (work in progress), June 2014.
[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.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, June 2010.
[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.
[To14] Touch, J., T. Faber, "TCP SYN Extended Option Space Using
an Out-of-Band Segment", draft-touch-tcpm-tcp-syn-ext-opt-
01 (work in progress), September 2014.
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[Yo11] Yourtchenko, A., "Introducing TCP Long Options by Invalid
Checksum", draft-yourtchenko-tcp-loic-00 (work in
progress), April 2011.
12. 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 document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Joe Touch
USC/ISI
4676 Admiralty Way
Marina del Rey, CA 90292-6695 USA
Phone: +1 (310) 448-9151
Email: touch@isi.edu
Wesley M. Eddy
MTI Systems
US
Email: wes@mti-systems.com
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