Network Working Group                                            D. Katz
Request for Comments: 2113                                 cisco Systems
Category: Standards Track                                  February 1997

                         IP Router Alert Option

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   This memo describes a new IP Option type that alerts transit routers
   to more closely examine the contents of an IP packet.  This is useful
   for, but not limited to, new protocols that are addressed to a
   destination but require relatively complex processing in routers
   along the path.

1.0  Introduction

   A recent trend in routing protocols is to loosely couple new routing
   functionality to existing unicast routing.  The motivation for this
   is simple and elegant -- it allows deployment of new routing
   functionality without having to reinvent all of the basic routing
   protocol functions, greatly reducing specification and implementation
   complexity.

   The downside of this is that the new functionality can only depend on
   the least common denominator in unicast routing, the next hop toward
   the destination.  No assumptions can be made about the existence of
   more richly detailed information (such as a link state database).

   It is also desirable to be able to gradually deploy the new
   technology, specifically to avoid having to upgrade all routers in
   the path between source and destination.  This goal is somewhat at
   odds with the least common denominator information available, since a
   router that is not immediately adjacent to another router supporting
   the new protocol has no way of determining the location or identity
   of other such routers (unless something like a flooding algorithm is
   implemented over unicast forwarding, which conflicts with the
   simplicity goal).







 
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