TCP Options and Maximum Segment Size (MSS)
RFC 6691

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Internet Engineering Task Force (IETF)                         D. Borman
Request for Comments: 6691                           Quantum Corporation
Updates: 879, 2385                                             July 2012
Category: Informational
ISSN: 2070-1721

               TCP Options and Maximum Segment Size (MSS)

Abstract

   This memo discusses what value to use with the TCP Maximum Segment
   Size (MSS) option, and updates RFC 879 and RFC 2385.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6691.

Copyright Notice

   Copyright (c) 2012 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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Borman                        Informational                     [Page 1]
RFC 6691                   TCP Options and MSS                 July 2012

1.  Introduction

   There has been some confusion as to what value to use with the TCP
   MSS option when using IP and TCP options.  RFC 879 [RFC879] states:

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

   This statement 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, which is discussed in 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 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.

Borman                        Informational                     [Page 2]
RFC 6691                   TCP Options and MSS                 July 2012

   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 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: