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Versions: 00 RFC 2147

Network Working Group                    Internet Engineering Task Force
Internet-Draft                                        IPNG Working Group
Updates: 1883                                               David Borman
                                          Berkeley Software Design, Inc.
                                                             August 1996


                    TCP and UDP over IPv6 Jumbograms
                    <draft-borman-jumbograms-00.txt>

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   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 reference
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   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
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   ftp.isi.edu (US West Coast).

1. Overview

   IPv6 supports datagrams larger than 65535 bytes long, called
   jumbograms.  The UDP protocol has a 16 length field that keeps it
   from being able to make use of jumbograms, and though TCP does not
   have a length field, it does have an MSS option that has a 65535 byte
   length limitation.  This document describes some simple changes that
   can be made to TCP and UDP over IPv6 to allow them to take advantage
   of jumbograms.

2. UDP Jumbograms

   Because the UDP length field includes the UDP header, the minimum
   valid value for this field is 8.

      To send a UDP packet with a UDP length > 65535, set the UDP length
      field to zero, and the IPv6 length field to correctly reflect the
      length of the UDP packet.

      If a UDP packet arrives with a length field of zero, the UDP



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Internet-Draft      TCP and UDP over IPv6 Jumbograms         August 1996


      length should be ignored, and the length field from the pseudo
      header should be used.

3. TCP jumbograms

   Because there is no length field in the TCP header, there is nothing
   limiting the length of an individual TCP packet.  However, the MSS
   value that is negotiated at the beginning of the connection limits
   the largest TCP packet that can be sent.

      When determining what MSS value to send, if the MTU of the
      directly attached interface is greater than 65535, then set the
      MSS value to 65535.

      When an MSS value of 65535 is received, it is to be treated as
      infinity.  MTU discovery code, starting with the MTU of the
      outgoing interface, should be used to determine the actual MSS.

4. Security Considerations

   There are no known security issues involved in these changes.

Author's Address

   David A. Borman
   Berkeley Software Design, Inc.
   4719 Weston Hills Drive
   Eagan, MN 55123
   Phone: (612) 405-8194
   Mailing List: ipng@sunroof.Eng.Sun.COM
   Email: dab@bsdi.com






















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