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IP Router Alert Option

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2113.
Author Dave Katz
Last updated 2013-03-02 (Latest revision 1996-12-18)
RFC stream Legacy stream
Intended RFC status (None)
Stream Legacy state (None)
Consensus boilerplate Unknown
RFC Editor Note (None)
IESG IESG state Became RFC 2113 (Proposed Standard)
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INTERNET-DRAFT                                                 Dave Katz
Expiration: June 1997                                      cisco Systems
File: draft-katz-router-alert-03.txt                    December 2, 1996

                         IP Router Alert Option

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on (US East Coast),
   (Europe), (US West Coast), or (Pacific

   IESG Note:

      Internet Engineering Steering Group comment from the co-Area
      Director for Transport Services:  This is a Proposed Standard.  It
      is a matter of individual choice as to how to implement the
      specification. Implementation as a temporary feature or in a test
      version of the product may be appropriate.


   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.

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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

   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).

   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.

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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.

   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  |

     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

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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

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  References

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

   [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

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