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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).

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 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 [Page 2]  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.

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 If the Router Alert option is set when it should not be set, it is
 likely that the flow will experience a performance penalty, as a
 packet whose Router Alert option is set will not go through the
 router's fastpath and will be processed in the router more slowly
 than if the option were not set.

5.0 References

 [1] Braden, B. (ed.), L. Zhang, D. Estrin, S. Herzog, S. Jamin,
     "Resource ReSerVation Protocol (RSVP)," work in progress, March
     1996.

 [2] Fenner, W., "Internet Group Management Protocol, Version 2
     (IGMPv2)," work in progress, October 1996.

Author's Address

    Dave Katz
    cisco Systems
    170 W. Tasman Dr.
    San Jose, CA  95134-1706  USA

    Phone:  +1 408 526 8284
    Email:  dkatz@cisco.com

Katz Standards Track [Page 4]