RFC 2711 (rfc2711) - Page 3 of 6

IPv6 Router Alert Option

RFC 2711                IPv6 Router Alert Option            October 1999


      Value:  A 2 octet code in network byte order with the following
      values:

         0        Datagram contains a Multicast Listener Discovery
                  message [RFC-2710].
         1        Datagram contains RSVP message.
         2        Datagram contains an Active Networks message.
         3-65535  Reserved to IANA for future use.

      Alignment requirement: 2n+0

      Values are registered and maintained by the IANA.  See section 5.0
      for more details.

2.2  Semantics

   The option indicates that the contents of the datagram may be
   interesting to the router.  The router's interest and the actions
   taken by employing Router Alert MUST be specified in the RFC of the
   protocol that mandates or allows the use of Router Alert.

   The final destination of the IPv6 datagram MUST ignore this option
   upon receipt to prevent multiple evaluations of the datagram.
   Unrecognized value fields MUST be silently ignored and the processing
   of the header continued.

   Routers that recognize the option will examine datagrams carrying it
   more closely to determine whether or not further processing is
   necessary.  The router only needs to parse the packet in sufficient
   detail to decide whether the packet contains something of interest.
   The value field can be used by an implementation to speed processing
   of the datagram within the transit router.

   Observe that further processing can involve protocol layers above
   IPv6.  E.g., for RSVP messages, the datagram will have to undergo UDP
   and RSVP protocol processing.  Once the datagram leaves the IPv6
   layer, there is considerable ambiguity about whether the router is
   acting as an IPv6 host or an IPv6 router.  Precisely how the router
   handles the contents is value-field specific.  However, if the
   processing required for the datagram involves examining the payload
   of the IPv6 datagram, then the interim router is performing a host
   function and SHOULD interpret the data as a host.









Partridge & Jackson         Standards Track