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Versions: 00 01 02                                                      
Internet Engineering Task Force       A. Bansal, T. Przygienda, A. Patel
INTERNET DRAFT                                    Fore, Bell Labs/Lucent
                                                             20 Jan 1998


                            IS-IS over IPv4
                  <draft-ietf-isis-wg-over-ip-00.txt>


Status of This Memo
   This document is an Internet Draft, and can be found as
   draft-ietf-isis-wg-over-ip-02.txt in any standard internet drafts
   repository.  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
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   Please check the I-D abstract listing contained in each Internet
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   Internet Draft.


Abstract
   This draft describes an optional implementation technique within
   IS-IS [ISO90, Cal90a, Cal90b] used today by several ISPs for routing
   within their clouds.  IS-IS is an interior gateway routing protocol
   developed originally by OSI and used with IP extensions as IGP. This
   draft describes how to encapsulate IS-IS packets in IPv4 [Pos81]
   format.  Such an encapsulation has many advantages, one of those
   being the possibility to run integrated IS-IS on anything that
   understands IPv4, including avian carriers [Wai90].


1. Introduction
   Encapsulation of IS-IS as defined in ISO 10589 [ISO90, Cal90a,
   Cal90b] uses directly over Link Layer protocols as opposed to IP
   routing protocols [Moy97] that are encapsulated over IP directly.

   By defining an encapsulation of IS-IS in IP, we save on special OSI





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   encapsulation on several media types.  Such an encapsulation would
   solve fragmentation problems of large LSPs and remove the necessity
   for OSI PPP extensions [Kat92] when IS-IS is run over negotiated
   PPP links.  Additionally, on certain media, such as P2P ATM links,
   no LLC/SNAP encapsulation is necessary to provide multi-protocol
   routing, allowing for gains in efficiency.


2. Encapsulation of IS-IS and ISO 9542 packets over IP
   IS-IS is encapsulated directly over the Internet Protocol's network
   layer.  IS-IS packets are therefore encapsulated by IP and local
   data-link headers.  Within this encapsulation, IS-IS packets are
   propagated as usual, however without the appropriate link-layer
   fields but starting at NLPI.

   IS-IS does not normally provide a way to transmit packets larger
   than MTU size.  This proposal allows to use IP fragmentation when
   transmitting such packets.  If necessary, the length of IS-IS packets
   over IP can be up to 65,535 bytes (including the IP header).  The
   IS-IS packet types that are likely to be large (LSPs, CSNPs, PSNPs)
   can usually be split into several separate protocol packets, without
   loss of functionality.  This is recommended; IP fragmentation SHOULD
   be avoided whenever possible since it can lead to different problems,
   such as loss of fragments causing the retransmission of complete IP
   packets.  Following rules apply:
    -  IIHs MUST have the size of [InterfaceMTU  -  IP headersize]  (1)
       and have the DF bit set.  IIH's TTL MUST be set to 1 to prevent
       them from traveling multiple hops.

    -  SNPs on IP encapsulated interfaces MUST NOT be larger than
       the minimum of [InterfaceMTU   -   IP headersize] and respective
       originatingLSPBufferSize.

    -  LSP fragments MAY BE built with a size up to the value of
       corresponding originatingLSPBufferSize.

    -  LSP fragments MUST NOT be larger than the respective
       originatingLSPBufferSize.

___________________________________________
1. not of maximum of Buffer and DataLinkSize used normally





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    -  LSPs MUST be sent allowing IP fragmentation (DF bit not set).

    -  LSP fragments SHOULD NOT exceed the size of the minimum of
       dataLinkBlockSize and respective originatingLSPBufferSize.

   This set of rules allows to configure a network with respective
   originatingLSPBufferSize larger than some interfaces' MTUs.
   In a mixed environment, care must be taken that respective
   originatingLSPBufferSize does net exceed the MTU size of interfaces
   without IP encapsulation.
   The other important features of IS-IS in IP's IP encapsulation are:

    -  Use of IP multicast.  Some IS-IS in IP messages are multicast,
       when sent over broadcast networks.  Three distinct IP multicast
       addresses are used.  Packets sent to these multicast addresses
       should never be forwarded; they are meant to travel a single hop
       only.  To ensure that these packets will not travel multiple
       hops, their IP TTL must be set to 1.

          IPAllL1ISs

             OSI multicast value of this address was O1-80-C2-00-00-14.
             This multicast address has been assigned the IP address
             value 224.0.0.?  for IP encapsulated IS-IS. All routers
             running L1 IS-IS in IP should be prepared to receive
             packets sent to this address.  Hello packets are always
             sent to this destination.  Also, certain IS-IS in IP
             protocol packets are sent to this address during the
             flooding procedure.

          IPAllL2ISs

             OSI multicast value of this address was O1-80-C2-00-00-15.
             This multicast address has been assigned the IP address
             value 224.0.0.?  for IP encapsulated IS-IS. All routers
             running L2 IS-IS in IP should be prepared to receive
             packets sent to this address.  Hello packets are always
             sent to this destination.  Also, certain IS-IS in IP
             protocol packets are sent to this address during the
             flooding procedure.






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          IPAllIntermediateSystems

             OSI multicast value of this address was 09-00-2B-0O-00-05.
             This multicast address has been assigned the IP address
             value 224.0.0.?  for IP encapsulated IS-IS. All routers
             running IS-IS in IP should be prepared to receive packets
             sent to this address.  ISO 9542 is using this address.

    -  IS-IS in IP is IP protocol number ??.  This number has been
       requested with the Network Information Center.  IP protocol
       number assignments are documented in [RP94].

       Note:  For development purposes, IP Protocol number 9 (Private
       IGP protocol number) [RP94] will be used until an official number
       is granted.

    -  All IS-IS in IP routing protocol packets are sent using the
       normal service TOS value of binary 0000 defined in [Alm92].

    -  Routing protocol packets are sent with IP precedence set to
       Internetwork Control.  IS-IS in IP protocol packets should be
       given precedence over regular IP data traffic, in both sending
       and receiving.  Setting the IP precedence field in the IP
       header to Internetwork Control [Pos81] may help implement this
       objective.


3. Internal Encapsulation
   On point-to-point links no MAC addresses are used by IS-IS. Therefore
   the Intradomain Routeing Protocol Discriminator or ISO 9542 Network
   Layer Protocol Identifier starts directly after the IP header.  For
   P2P link running PPP, the Payload format will consist of PPP header,
   followed by the IP header and NLPID as indicated in figure 1.


           +------------+-----------+----------------+
           | PPP Header | IP Header | NLPID|ISIS PDU |
           +------------+-----------+----------------+



            Figure 1: Encapsulation of ISIS frames over PPP




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   For P2P ATM links using VC muxing, the payload format must not
   include the PPP header as indicated in figure 2.


           +-------------+-----------+-----------------+
           | AAL5 Header | IP Header | NLPID|ISIS  PDU |
           +-------------+-----------+-----------------+



            Figure 2: Encapsulation of ISIS frames over ATM


   In the case of broadcast media, MAC addresses are used for adjacency
   identification.  It is rather painful from the implementation
   perspective to assume that an IS-IS must have access to MAC headers
   when receiving frames.  On the other hand, introducing an artificial
   `bridging' encapsulation header preceding the Intradomain Routeing
   Protocol Discriminator within the routed frame was perceived as
   a very bulky solution.  As yet another approach, based on the
   observation that ethernets always have at least one IP address,
   MAC address necessary for adjacency maintenance could be built
   algorithmically using the source address of the IP packet received.
   However, this would necessitate e.g.  the allocation of a 2-byte
   prefix within the 802.2/3 MAC space.  Given the difficulties
   surrounding each of these solutions, no special encapsulation is
   defined for the ethernet case and the protocol implementation is
   either required to received the complete frame, including layer 2
   encapsulation or obtain the appropriate MAC address from the source
   IP address using an interface to the lower layers of the IP stack.

4. Interoperability with Devices without IP Encapsulation

   An interoperability solution for devices using IP encapsulation and
   OSI encapsulation of ISIS frames would be only useful if it could
   significantly ease the migration path in the existing networks.
   Given the fact that graceful migration is not a paramount issue for
   existing networks and any solution would invariable lead to the
   problem of partitioning of broadcast media, such a solution is not
   defined.






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5. Acknowledgments

   The encapsulation description part has been "borrowed" from a
   well-known RFC [Moy97] with the author's consent.  Tony Li, Mike
   Shand, Henk Smit, Rajesh Varadarajan, Jeff Swinton, Stacy Smith gave
   many constructive comments.

6. Security Consideration

   ISIS security applies to the work presented.  No specific security
   issues with the proposed solutions are known.  Things like IPSec may
   influence the work in strange and unknown ways ;-)


References

   [Alm92]  P. Almquist.  Type of Service in the Internet Protocol
            Suite.  INTERNET-RFC, Internet Engineering Task Force, July
            1992.

   [Cal90a] R. Callon.  OSI ISIS Intradomain Routing Protocol.
            INTERNET-RFC, Internet Engineering Task Force, February
            1990.

   [Cal90b] R. Callon.  Use of OSI ISIS for Routing in TCP/IP and Dual
            Environments.  INTERNET-RFC, Internet Engineering Task
            Force, December 1990.

   [Hei93]  J. Heinanen.  Rfc 1483, Multiprotocol Encapsulation over ATM
            Adaptation Layer 5.  Internet Engineering Task Force, July
            1993.

   [ISO90]  ISO.  Information Technology - Telecommunications and
            Information Exchange between Systems - Intermediate System
            to Intermediate System Routing Exchange Protocol for
            Use in Conjunction with the Protocol for Providing the
            Connectionless-Mode Network Service.  ISO, 1990.

   [Kat92]  D. Katz.  Rfc 1377, The PPP OSI Network Layer Control
            Protocol (OSINLCP).  Internet Engineering Task Force,
            November 1992.

   [MD90]   J. Mogul and S. Deering.  Rfc 1191, Path MTU Discovery.
            Internet Engineering Task Force, November 1990.


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   [Moy97]  J. Moy.  OSPFv2, RFC 2178.  Internet Engineering Task Force,
            July 1997.

   [Pos81]  J. Postel.  Rfc 791, rfc Internet Protocol.  Internet
            Engineering Task Force, September 1981.

   [RP94]   J. Reynolds and J. Postel.  Rfc 1700, Assigned Numbers.
            Internet Engineering Task Force, October 1994.

   [Wai90]  D. Waitzman.  Rfc 1149, standard for the Transmission of
            IP Datagrams on Avian Carriers.  Internet Engineering Task
            Force, April 1990.


Authors' Addresses

Tony Przygienda
Bell Labs, Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733-3030
prz@dnrc.bell-labs.com

Ajay Patel
Bell Labs, Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733-3030
ajayp@dnrc.bell-labs.com

Atul Bansal
FORE Systems,
1595 Spring Hill Rd
Vienna, VA 22181
bansal@fore.com













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