RFC 2147 (rfc2147) - Page 2 of 3

TCP and UDP over IPv6 Jumbograms

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 [RFC 1883, 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