RFC 2113 (rfc2113) - Page 3 of 4

IP Router Alert Option

RFC 2113                  Router Alert Option              February 1997


   Routers that do not support option processing in the fast path will
   cause packets carrying this new option to be forwarded through the
   slow path, so no change to the fast path is necessary and no
   performance penalty will be incurred for regular data packets.

2.1  Syntax

   The Router Alert option has the following format:

                 +--------+--------+--------+--------+
                 |10010100|00000100|  2 octet value  |
                 +--------+--------+--------+--------+

   Type:
     Copied flag:  1 (all fragments must carry the option)
     Option class: 0 (control)
     Option number: 20 (decimal)

   Length: 4

   Value:  A two octet code with the following values:
     0 - Router shall examine packet
     1-65535 - Reserved

2.2  Semantics

   Hosts shall ignore this option.  Routers that do not recognize this
   option shall ignore it.  Routers that recognize this option shall
   examine packets carrying it more closely (check the IP Protocol
   field, for example) to determine whether or not further processing is
   necessary.  Unrecognized value fields shall be silently ignored.

   The semantics of other values in the Value field are for further
   study.

3.0  Impact on Other Protocols

   For this option to be effective, its use must be mandated in
   protocols that expect routers to perform significant processing on
   packets not directly addressed to them.  Currently such protocols
   include RSVP [1] and IGMP [2].

4.0 Security Considerations

   If the Router Alert option is not set and should be set, the behavior
   of the protocol using the Router Alert, e.g., RSVP or IGMPv2, will be
   adversely affected since the protocol relies on the use of the Router
   Alert option.



Katz                        Standards Track