RFC 2113 (rfc2113) - Page 2 of 4
IP Router Alert Option
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RFC 2113 Router Alert Option February 1997 One obvious approach to leveraging unicast routing is to do hop-by- hop forwarding of the new protocol packets along the path toward the ultimate destination. Each system that implements the new protocol would be responsible for addressing the packet to the next system in the path that understood it. As noted above, however, it is difficult to know the next system implementing the protocol. The simple, degenerate case is to assume that every system along the path implements the protocol. This is a barrier to phased deployment of the new protocol, however. RSVP [1] finesses the problem by instead putting the address of the ultimate destination in the IP Destination Address field, and then asking that every RSVP router make a "small change in its ... forwarding path" to look for the specific RSVP packet type and pull such packets out of the mainline forwarding path, performing local processing on the packets before forwarding them on. This has the decided advantage of allowing automatic tunneling through routers that don't understand RSVP, since the packets will naturally flow toward the ultimate destination. However, the performance cost of making this Small Change may be unacceptable, since the mainline forwarding path of routers tends to be highly tuned--even the addition of a single instruction may incur penalties of hundreds of packets per second in performance. 2.0 Router Alert Option The goal, then, is to provide a mechanism whereby routers can intercept packets not addressed to them directly, without incurring any significant performance penalty. This document defines a new IP option type, Router Alert, for this purpose. The Router Alert option has the semantic "routers should examine this packet more closely". By including the Router Alert option in the IP header of its protocol message, RSVP can cause the message to be intercepted while causing little or no performance penalty on the forwarding of normal data packets. Routers that support option processing in the fast path already demultiplex processing based on the option type field. If all option types are supported in the fast path, then the addition of another option type to process is unlikely to impact performance. If some option types are not supported in the fast path, this new option type will be unrecognized and cause packets carrying it to be kicked out into the slow path, so no change to the fast path is necessary, and no performance penalty will be incurred for regular data packets. Katz Standards Track
