o  The Next Header value in the pseudo-header identifies the
         upper-layer protocol (e.g., 6 for TCP, or 17 for UDP).  It will
         differ from the Next Header value in the IPv6 header if there
         are extension headers between the IPv6 header and the upper-
         layer header.

      o  The Upper-Layer Packet Length in the pseudo-header is the
         length of the upper-layer header and data (e.g., TCP header
         plus TCP data).  Some upper-layer protocols carry their own




 
RFC 2460                   IPv6 Specification              December 1998


         length information (e.g., the Length field in the UDP header);
         for such protocols, that is the length used in the pseudo-
         header.  Other protocols (such as TCP) do not carry their own
         length information, in which case the length used in the
         pseudo-header is the Payload Length from the IPv6 header, minus
         the length of any extension headers present between the IPv6
         header and the upper-layer header.

      o  Unlike IPv4, when UDP packets are originated by an IPv6 node,
         the UDP checksum is not optional.  That is, whenever
         originating a UDP packet, an IPv6 node must compute a UDP
         checksum over the packet and the pseudo-header, and, if that
         computation yields a result of zero, it must be changed to hex
         FFFF for placement in the UDP header.  IPv6 receivers must
         discard UDP packets containing a zero checksum, and should log
         the error.

   The IPv6 version of ICMP [ICMPv6] includes the above pseudo-header in
   its checksum computation; this is a change from the IPv4 version of
   ICMP, which does not include a pseudo-header in its checksum.  The
   reason for the change is to protect ICMP from misdelivery or
   corruption of those fields of the IPv6 header on which it depends,
   which, unlike IPv4, are not covered by an internet-layer checksum.
   The Next Header field in the pseudo-header for ICMP contains the
   value 58, which identifies the IPv6 version of ICMP.

8.2 Maximum Packet Lifetime

   Unlike IPv4, IPv6 nodes are not required to enforce maximum packet
   lifetime.  That is the reason the IPv4 "Time to Live" field was
   renamed "Hop Limit" in IPv6.  In practice, very few, if any, IPv4
   implementations conform to the requirement that they limit packet
   lifetime, so this is not a change in practice.  Any upper-layer
   protocol that relies on the internet layer (whether IPv4 or IPv6) to
   limit packet lifetime ought to be upgraded to provide its own
   mechanisms for detecting and discarding obsolete packets.

8.3 Maximum Upper-Layer Payload Size

   When computing the maximum payload size available for upper-layer
   data, an upper-layer protocol must take into account the larger size
   of the IPv6 header relative to the IPv4 header.  For example, in
   IPv4, TCP's MSS option is computed as the maximum packet size (a
   default value or a value learned through Path MTU Discovery) minus 40
   octets (20 octets for the minimum-length IPv4 header and 20 octets
   for the minimum-length TCP header).  When using TCP over IPv6, the
   MSS must be computed as the maximum packet size minus 60 octets,





 
RFC 2460                   IPv6 Specification              December 1998


   because the minimum-length IPv6 header (i.e., an IPv6 header with no
   extension headers) is 20 octets longer than a minimum-length IPv4
   header.

8.4 Responding to Packets Carrying Routing Headers

   When an upper-layer protocol sends one or more packets in response to
   a received packet that included a Routing header, the response
   packet(s) must not include a Routing header that was automatically
   derived by "reversing" the received Routing header UNLESS the
   integrity and authenticity of the received Source Address and Routing
   header have been verified (e.g., via the use of an Authentication
   header in the received packet).  In other words, only the following
   kinds of packets are permitted in response to a received packet
   bearing a Routing header:

      o  Response packets that do not carry Routing headers.

      o  Response packets that carry Routing headers that were NOT
         derived by reversing the Routing header of the received packet
         (for example, a Routing header supplied by local
         configuration).

      o  Response packets that carry Routing headers that were derived
         by reversing the Routing header of the received packet IF AND
         ONLY IF the integrity and authenticity of the Source Address