RFC 2147 (rfc2147) - Page 2 of 3
TCP and UDP over IPv6 Jumbograms
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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
