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Network Working Group                                          D. BormanRequest for Comments: 2147                Berkeley Software Design, Inc.Updates:1883                                                   May 1997Category: Standards TrackTCP and UDP over IPv6 JumbogramsStatus 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 theBorman                      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.comBorman                      Standards Track                     [Page 3]