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Versions: 01 02 03 rfc2113                                              
                                                               Dave Katz
INTERNET-DRAFT                                             cisco Systems
<draft-katz-router-alert-01.txt>                       November 16, 1995

                         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.  Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress."

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any Internet


   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.

   This document specifies an experimental protocol for use in the

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

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INTERNET-DRAFT            Router Alert Option          November 16, 1995

   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).

2.0  Approaches to Coupling with Routing

   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 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
   PPS in performance.

3.0  Proposal

   The goal, then, is to provide a mechanism whereby routers can
   intercept packets not addressed to them directly without incurring
   any significant performance penalty.  The proposed solution is to
   define a new IP option type with the semantic "routers should examine

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   this packet more closely" and require protocols such as RSVP to use
   this option.  This would incur 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.

3.1  Syntax

   The proposed 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

3.2  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.

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INTERNET-DRAFT            Router Alert Option          November 16, 1995

   The semantics of other values in the Value field are for further

4.0  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.  It may also be useful to
   modify protocols that use the hop-by-hop method to instead use the
   ultimate destination as the IP Destination Address and then mandate
   the use of this option.

5.0  References

   [1] Braden, B. (ed.), L. Zhang, D. Estrin, S. Herzog, S. Jamin,
       "Resource ReSerVation Protocol (RSVP)," draft-ietf-rsvp-spec-
       07.txt, 1995.

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

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