Network Working Group                                          D. Borman
Request for Comments: 2147                Berkeley Software Design, Inc.
Updates: 1883                                                   May 1997
Category: Standards Track


                   TCP and UDP over IPv6 Jumbograms

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

1.  Overview

  IPv6 supports datagrams larger than 65535 bytes long, often referred
  to as jumbograms, through use of the Jumbo Payload hop-by-hop option
  [Deering95].  The UDP protocol has a 16-bit length field that keeps
  it from being able to make use of jumbograms, and though TCP does not
  have a length field, both the MSS option and the Urgent field are
  constrained by 16-bits.  This document describes some simple changes
  that can be made to allow TCP and UDP to make use of IPv6 jumbograms.

2.  UDP Jumbograms

  To allow UDP to make use of jumbograms, either the UDP length field
  needs to be extended, or it needs to be ignored.  Since the size of
  the field can't be changed, a length of zero is used to indicate that
  it is to be ignored, and the length in the "pseudo-header" is to be
  used to determine the true length of the UDP header plus data.  This
  works because UDP length field includes the UDP header, so the
  minimum valid value for this field is 8.

  When sending a UDP packet, if and only if the length of the UDP
  header plus data is greater than 65,535, set the length field in the
  UDP header to zero.

     Note 1:  The length used in the "pseudo-header" for computing the
     UDP checksum is always the true length of the UDP header plus
     data, NOT zero [RFC-1883, Section 8.1].








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     Note 2:  An IPv6 packet that carries a UDP packet of length
     greater than 65,535 will necessarily carry a Jumbo Payload option
     in a Hop-by-Hop Options header [RFC1883, Section 4.3]).  The
     length field in the Jumbo Payload option contains the length of
     the IP packet excluding the IPv6 header, that is, it contains the
     length of all extension headers present plus the UDP header plus
     the UDP data.  The length field in the IPv6 header contains zero
     to indicate the presence of the Jumbo Payload option.

  If a UDP packet is received with a length field of zero, the length
  of the UDP packet is computed from the length field in the Jumbo
  Payload option minus the length of all extension headers present
  between the IPv6 header and the UDP header.

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, and the Urgent Pointer
  cannot reference data beyond 65535 bytes.

3.1 TCP MSS

  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, will be used to determine the actual MSS.

3.2 TCP Urgent Pointer

  The Urgent Pointer problem could be fixed by adding a TCP Urgent
  Pointer Option.  However, since it is unlikely that applications
  using jumbograms will also use Urgent Pointers, a less intrusive
  change similar to the MSS change will suffice.

  When a TCP packet is to be sent with an Urgent Pointer (i.e., the URG
  bit set), first calculate the offset from the Sequence Number to the
  Urgent Pointer.  If the offset is less than 65535, fill in the Urgent
  field and continue with the normal TCP processing.  If the offset is
  greater than 65535, and the offset is greater than or equal to the
  length of the TCP data, fill in the Urgent Pointer with 65535 and
  continue with the normal TCP processing.  Otherwise, the TCP packet
  must be split into two pieces.  The first piece contains data up to,
  but not including the data pointed to by the Urgent Pointer, and the



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  Urgent field is set to 65535 to indicate that the Urgent Pointer is
  beyond the end of this packet.  The second piece can then be sent
  with the Urgent field set normally.

     Note: The first piece does not have to include all of the data up
     to the Urgent Pointer.  It can be shorter, just as long as it ends
     within 65534 bytes of the Urgent Pointer, so that the offset to
     the Urgent Pointer in the second piece will be less than 65535
     bytes.

  For TCP input processing, when a TCP packet is received with the URG
  bit set and an Urgent field of 65535, the Urgent Pointer is
  calculated using an offset equal to the length of the TCP data,
  rather than the offset in the Urgent field.

  It should also be noted that though the TCP window is only 16-bits,
  larger windows can be used through use of the TCP Window Scale option
  [Jacobson92].

4.  Security Considerations

  There are no known security issues involved in these changes.

5.  References

  [Jacobson92] Jacobson, V., R. Braden and D. Borman, "TCP Extensions
  for High Performance", RFC 1323, LBL, ISI and Cray Research, May
  1992.

  [Deering95] Deering, S. and R. Hinden, "Internet Protocol, Version 6
  (IPv6) Specification", RFC 1883, Xerox PARC and Ipsilon Networks,
  December 1995.

Author's Address

  David A. Borman
  Berkeley Software Design, Inc.
  4719 Weston Hills Drive
  Eagan, MN 55123
  Phone: (612) 405-8194
  Mailing List: [email protected]
  Email: [email protected]









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