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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].

Borman Standards Track [Page 1]  RFC 2147 TCP and UDP over IPv6 Jumbograms May 1997

    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

Borman Standards Track [Page 2]  RFC 2147 TCP and UDP over IPv6 Jumbograms May 1997

 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: ipng@sunroof.Eng.Sun.COM
 Email: dab@bsdi.com

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