RFC 2711 (rfc2711) - Page 2 of 6

IPv6 Router Alert Option

RFC 2711                IPv6 Router Alert Option            October 1999


   routed without further datagram parsing.  Hosts originating IPv6
   datagrams are required to include this option in certain
   circumstances.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC-2119].

2.0  Approach

   The goal is to provide an efficient mechanism whereby routers can
   know when to intercept datagrams not addressed to them without having
   to extensively examine every datagram.  The described solution is to
   define a new IPv6 Hop-by-Hop Header option having the semantic
   "routers should examine this datagram more closely" and require
   protocols such as RSVP to use this option.  This approach incurs
   little or no performance penalty on the forwarding of normal
   datagrams.  Not including this option tells the router that there is
   no need to closely examine the contents of the datagram.

2.1  Syntax

   The router alert option has the following format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0|0 0 1 0 1|0 0 0 0 0 0 1 0|        Value (2 octets)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      length = 2

      The first three bits of the first byte are zero and the value 5 in
      the remaining five bits is the Hop-by-Hop Option Type number.
      [RFC-2460] specifies the meaning of the first three bits.  By
      zeroing all three, this specification requires that nodes not
      recognizing this option type should skip over this option and
      continue processing the header and that the option must not change
      en route.

      There MUST only be one option of this type, regardless of value,
      per Hop-by-Hop header.












Partridge & Jackson         Standards Track