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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].
+
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+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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+Borman                      Standards Track                     [Page 3]
+