TCP Options and MSS
draft-ietf-tcpm-tcpmss-04

Network Working Group                              Network Working Group
Internet-Draft                                                 D. Borman
Updates: 879, 2385                                   Quantum Corporation
Intended Status: Informational                            March 27, 2012
File: draft-ietf-tcpm-tcpmss-04.txt

                          TCP Options and MSS

Abstract

   This memo discusses what value to use with the TCP MSS option, and
   updates RFC 879 and RFC 2385.

Status of This Memo

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

   This Internet-Draft is submitted in full conformance with the
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Copyright

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

1. Introduction

   There has been some confusion as to what value should be filled in

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   the TCP MSS option when using IP and TCP options.  RFC 879 [RFC879]
   stated:

        The MSS counts only data octets in the segment, it does not
        count the TCP header or the IP header.

   which is unclear about what to do about IP and TCP options, since
   they are part of the headers.  RFC 1122 [RFC1122] clarified the MSS
   option (see Appendix A), but there still seems to be some confusion.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2. The Short Statement

   When calculating the value to put in the TCP MSS option, the MTU
   value SHOULD be decreased by only the size of the fixed IP and TCP
   headers, and SHOULD NOT be decreased to account for any possible IP
   or TCP options; conversely, the sender MUST reduce the TCP data
   length to account for any IP or TCP options that it is including in
   the packets that it sends.  The rest of this document just expounds
   on that statement, and the goal is to avoid IP level fragmentation of
   TCP packets.

   The size of the fixed TCP header is 20 bytes [RFC793], the size of
   the fixed IPv4 header is 20 bytes [RFC791], and the size of the fixed
   IPv6 header is 40 bytes [RFC2460].  The determination of what MTU
   value should be used, especially in the case of multi-homed hosts, is
   beyond the scope of this document.

3. MSS in to other RFCs

   3.1 RFC 879

      RFC 879 [RFC879] discusses the MSS option and other topics.  In
      the introduction, it states:

         "THE TCP MAXIMUM SEGMENT SIZE IS THE IP MAXIMUM DATAGRAM SIZE
         MINUS FORTY."

      and in section 13, it states:

         "The definition of the MSS option can be stated:

            The maximum number of data octets that may be received by
            the sender of this TCP option in TCP segments with no TCP
            header options transmitted in IP datagrams with no IP header

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            options."

      These are both correct.  However, in the next paragraph in section
      14, it then confuses this by stating that the consequence is:

         "When TCP is used in a situation when either the IP or TCP
         headers are not minimum and yet the maximum IP datagram that
         can be received remains 576 octets then the TCP Maximum Segment
         Size option must be used to reduce the limit on data octets
         allowed in a TCP segment."

            For example, if the IP Security option (11 octets) were in
            use and the IP maximum datagram size remained at 576 octets,
            then the TCP should send the MSS with a value of 525
            (536-11)."

      That is incorrect.  The simpler and more correct statement would
      be:

         When TCP is used in a situation where either the IP or TCP
         headers are not minimum, the sender must reduce the amount of
         TCP data in any given packet by number of octets used by the IP
         and TCP options.

   3.2 RFC 2385

      RFC 2385 [RFC2385] incorrectly states in section 4.3:

         "As with other options that are added to every segment, the
         size of the MD5 option must be factored into the MSS offered to
         the other side during connection negotiation.  Specifically,
         the size of the header to subtract from the MTU (whether it is
         the MTU of the outgoing interface or IP's minimal MTU of 576
         bytes) is now at least 18 bytes larger."

      This is incorrect, the value for the MSS option is only adjusted
      by the fixed IP and TCP headers.

4. Clarification from the TCP Large Windows mailing list

   Additional clarification was sent to the TCP Large Windows mailing
   list in 1993 [Borman93].

   The MSS value to be sent in an MSS option should be equal to the
   effective MTU minus the fixed IP and TCP headers.  By ignoring both
   IP and TCP options when calculating the value for the MSS option, if
   there are any IP or TCP options to be sent in a packet, then the
   sender must decrease the size of the TCP data accordingly.  The
   reason for this can be seen in the following table:

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                        +--------------------+--------------------+
                        | MSS is adjusted    | MSS isn't adjusted |
                        | to include options | to include options |
   +--------------------+--------------------+--------------------+
   | Sender adjusts     | Packets are too    | Packets are the    |
   | packet length      | short              | correct length     |
   | for options        |                    |                    |
   +--------------------+--------------------+--------------------+
   | Sender doesn't     | Packets are the    | Packets are too    |
   | adjust packet      | correct length     | long.              |
   | length for options |                    |                    |
   +--------------------+--------------------+--------------------+

   The goal is to not send IP datagrams that have to be fragmented, and
   packets sent with the constraints in the lower right of this grid
   will cause IP fragmentation.  Since the sender doesn't know if the
   received MSS option was adjusted to include options, the only way to
   guarantee that the packets are not too long is for the data sender to
   decrease the TCP data length by the size of the IP and TCP options.
   It follows then, that since the sender will be adjusting the TCP data
   length when sending IP and TCP options, there is no need to include
   the IP and TCP option lengths in the MSS value.

   Another argument against including IP or TCP options in the
   determination of the MSS value is that the MSS is a fixed value, and
   by their very nature the length of IP and TCP options are variable,
   so the MSS value can never accurately reflect all possible IP and TCP
   option combinations, the only one that knows for sure how many IP and
   TCP options are in any given packet is the sender, hence the sender
   should be doing the adjustment to the TCP data length to account for
   any IP and TCP options.

5. Additional considerations

   5.1 Path MTU Discovery

      The TCP MSS option specifies an upper bound for the size of
      packets that can be received.  Hence setting the value in the MSS
      option too small can impact the ability for Path MTU discovery to
      find a larger Path MTU.  For more information on Path MTU
      Discovery, see:

         RFC 1191  "Path MTU Discovery" [RFC1191]
         RFC 2923 "TCP Problems with Path MTU Discovery" [RFC2923]
         RFC 4821 "Packetization Layer Path MTU Discovery" [RFC4821]

   5.2 Interfaces with Variable MSS values

      The effective MTU can sometimes vary, as when used with variable

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      compression, e.g., ROHC [RFC5795].  It is tempting for TCP to want
      to advertise the largest possible MSS, to support the most
      efficient use of compressed payloads.  Unfortunately, some
      compression schemes occasionally need to transmit full headers
      (and thus smaller payloads) to resynchronize state at their
      endpoint compressor/decompressors.  If the largest MTU is used to
      calculate the value to advertise in the MSS option, TCP
      retransmission may interfere with compressor resynchronization.

      As a result, when the effective MTU of an interface varies, TCP
      SHOULD use the smallest effective MTU of the interface to
      calculate the value to advertise in the MSS option.

   5.3 IPv6 Jumbograms

      In order to support TCP over IPv6 Jumbograms, implementations need
      to be able to send TCP segments larger than 64K.  RFC 2675
      [RFC2675] defines that a value of 65,535 is to be treated as
      infinity, and Path MTU Discovery [RFC1981] is used to determine
      the actual MSS.

6. Security Considerations

   Packets that are too long will either be fragmented or dropped.  If
   packets are fragmented, intermediary firewalls or middle boxes may
   drop the fragmented packets.  In either case, when packets are
   dropped the connection can fail; hence it is best to avoid generating
   fragments.

7. IANA Considerations

   This document has no actions for IANA.

8. References

   Informative References

      [Borman93] Borman, D., "TCP MSS & Timestamps", Message to the
      tcplw mailing list, Jan 7, 1993.

      [RFC791] Postel, J., "Internet Protocol," RFC 791, September 1981.

      [RFC793] Postel, J., "Transmission Control Protocol," RFC 793,
      September 1981.

      [RFC879] Postel, J., "The TCP Maximum Segment Size and Related
      Topics", RFC 879, ISI, November 1983.

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      [RFC1122] Braden, R., editor, "Requirements for Internet Hosts --
      Communication Layers", RFC 1122, October, 1989.

      [RFC1191] Mogul, J.C., Deering, S.E., "Path MTU discovery" RFC
      1191, November 1990.

      [RFC1981] McCann, J., Deering, S. and Mogul, J.,, "Path MTU
      Discovery for IP Version 6", RFC 1981, August 1986.

      [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", RFC 2119, March 1997.

      [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP
      MD5 Signature Option", RFC 2385, August 1988.

      [RFC2460] Deering, S., Hinden, R., "Internet Protocol, Version 6
      (IPv6) Specification", RFC 2460, December, 1998.

      [RFC2675] Borman, D., Deering, S., Hinden, R., "IPv6 Jumbograms",
      RFC 2675, August, 1999.

      [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
      2923, September 2000.

      [RFC4821] Mathis, M., Heffner, J., "Packetization Layer Path MTU
      Discovery", RFC 4821, March 2007.

      [RFC5795] Sandlund, K., Gelletier, G. and Jonsson, L-E., "The
      RObust Header Compression (ROHC) Framework", RFC 5795, March 2010.

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Appendix A: Details from RFC 793 and RFC 1122

   RFC 793 [RFC793] defines the MSS option:

        Maximum Segment Size Option Data:  16 bits
          If this option is present, then it communicates the maximum
          receive segment size at the TCP which sends this segment.
          This field must only be sent in the initial connection request
          (i.e., in segments with the SYN control bit set).  If this
          option is not used, any segment size is allowed.

   RFC 1122 [RFC1122] provides additional clarification in section
   4.2.2.6, pages 85-86.  First, it changes the default behavior when
   the MSS option is not present:

        If an MSS option is not received at connection setup, TCP
        MUST assume a default send MSS of 536 (576-40) [TCP:4].

   Then it clarifies how to determine the value to use in the MSS
   option:

        The MSS value to be sent in an MSS option must be less than
        or equal to:

           MMS_R - 20

        where MMS_R is the maximum size for a transport-layer
        message that can be received (and reassembled).  TCP obtains
        MMS_R and MMS_S from the IP layer; see the generic call
        GET_MAXSIZES in Section 3.4.

   What is implied, but not explicitly stated, is that the 20 is the
   size of the fixed TCP header.  The definition of MMS_R is found in
   section 3.3.2 on page 57:

        MMS_R is given by:

           MMS_R = EMTU_R - 20

        since 20 is the minimum size of an IP header.

   and on page 56, also section 3.3.2:

        We designate the largest datagram size that can be reassembled
        by EMTU_R ("Effective MTU to receive"); this is sometimes
        called the "reassembly buffer size".  EMTU_R MUST be greater
        than or equal to 576, SHOULD be either configurable or
        indefinite, and SHOULD be greater than or equal to the MTU of

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        the connected network(s).

   What should be noted here is that EMTU_R is the largest datagram size
   that can be reassembled, not the largest datagram size that can be
   received without fragmentation.  Taking this literally, since most
   modern TCP/IP implementations can reassemble a full 64K IP packet,
   implementations should be using 65535 - 20 - 20, or 65495 for the MSS
   option.  But there is more to it than that, in RFC 1122 on page 86 it
   also states:

        The choice of TCP segment size has a strong effect on
        performance.  Larger segments increase throughput by
        amortizing header size and per-datagram processing
        overhead over more data bytes; however, if the packet
        is so large that it causes IP fragmentation, efficiency
        drops sharply if any fragments are lost [IP:9].

   Since it is guaranteed that any IP datagram that is larger than the
   MTU of the connected network will have to be fragmented to be
   received, implementations ignore the "greater than or" part of
   "SHOULD be greater than or equal to the MTU of the connected
   network(s)".  Thus, the MSS value to be sent in an MSS option must be
   less than or equal to:

        EMTU_R - FixedIPhdrsize - FixedTCPhdrsize

   where FixedTCPhdrsize is 20, and FixedIPhdrsize is 20 for IPv4 and 40
   for IPv6.

Authors' Addresses

   David Borman
   Quantum Corporation
   Mendota Heights, MN 55120

   Email: david.borman@quantum.com

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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