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Versions: (draft-touch-tcpm-tcp-edo) 00 01 02 03         Standards Track
          04 05 06 07 08 09 10                                          
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

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
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   The list of current Internet-Drafts can be accessed at

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   This Internet-Draft will expire on April 13, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document. Code Components extracted from this
   document must include Simplified BSD License text as described in

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   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.


   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

   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",
   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

   >> 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

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

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)

   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

   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

   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

   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

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

   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

   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

   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

   This document was prepared using 2-Word-v2.0.template.dot.

Authors' Addresses

   Joe Touch
   4676 Admiralty Way
   Marina del Rey, CA 90292-6695 USA

   Phone: +1 (310) 448-9151
   Email: touch@isi.edu

   Wesley M. Eddy
   MTI Systems

   Email: wes@mti-systems.com

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