RFC 2113 (rfc2113) - Page 2 of 4
IP Router Alert Option
Alternative Format: Original Text Document
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