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Versions: 00 01                                                         
Network Working Group                                      Juha Heinanen
INTERNET DRAFT                                           Telecom Finland
Expires January 1998                                           July 1997

          Intra-area IP unicast among routers over legacy ATM

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.

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   ftp.isi.edu (US West Coast).


   This document describes how IP unicast can be efficiently implemented
   among routers belonging to the same area of a routing domain, where
   the connectivity is provided by a legacy ATM network as defined by
   the ATM Forum or ITU.  This proposal is designed to be complementary
   to IP multicast solutions such as the one described in [1].

1. Introduction

   This document describes how a set of routers (such as the access/edge
   routers of an ISP or enterprise) connected to a legacy ATM network
   can in a dynamic and plug-and-play fashion optimize ATM connections
   for efficient forwarding of unicast IP packets.  The method can be
   used in situations where the number of routers is so large that a
   full mesh of point-to-point ATM VCs is not practical from technical
   or economic reasons.  In addition, it can be applied to smaller
   router networks to automate the setup of a full mesh of ATM
   connectivity between the routers.

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INTERNET DRAFT                                               April, 1997

   The set of routers must belong to the same area of a link state
   routing protocol, such as OSPF or IS-IS, that floods topology and
   reachability information to every router in the area.

   This proposal only deals with IP unicast, but it complements and can
   be used in conjunction with IP multicast solutions such as the one
   described in [1].

2. Router configuration and behavior

   As introduced above, this document defines a method of dynamically
   managing ATM connectivity among a set of routers that belong to the
   same area of a routing domain, where a link state protocol, such as
   OSPF or IS-IS, is used to exchange topology and reachability

   Before the dynamic management of ATM VCs can begin, the routers of
   the area must be manually configured to exchange routing information
   among themselves.  There must thus be enough initial connectivity so
   that at least one IP path exists from each router to each other
   router in the area.  This initial connectivity is also used to
   forward IP packets when dynamic ATM VCs don't exist.

   Note that the initial connectivity doesn't necessarily need to be
   implemented over ATM and that not all routers of the area need to be
   ATM attached.  Furthermore, even if a router is ATM attached, it
   doesn't need to participate in the dynamic management of ATM VCs.
   The ATM routers of an area can thus be upgraded one at a time to
   support the method described in this document.

   Once an ATM attached router R becomes operational, it may start to
   measure, how many bytes it has received during the past M seconds,
   whose final hop router S within the area is also ATM attached.  If
   the number of bytes is less than N, R forwards the packets according
   to its routing table.  When the number of bytes equals or exceeds N,
   and R doesn't yet have a dynamic ATM VC to S, R creates such a VC and
   starts to forward S bound packets directly.

   The dynamic ATM VC is unidirectional that allows R to manage it
   autonomously without coordination with S.  Also, in order to keep the
   number of routing peers small and in order to avoid frequent changes
   in topology information, R doesn't use the dynamic ATM VC for the
   exchange of routing information nor does R advertise the dynamic ATM
   VC to its routing peers.

   ATM traffic categories and traffic parameters of the dynamic VCs are
   decided locally by the network administrator.  The default traffic
   category is UBR with the peak cell rate set to the link rate and the

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INTERNET DRAFT                                               April, 1997

   minimal acceptable cell rate set to zero.  Since the dynamic ATM VCs
   are only used to carry IP packets, it is recommended that the packets
   are "VC multiplexed" [2] to the AAL 5 payload without an LLC/SNAP
   header.  See [3] and Appendix D.3.2 of [4] for details on coding of
   the signalling messages.

   Once the dynamic ATM VC from R to S has been created, R starts to
   measure the traffic along it.  When R detects that during the past K
   seconds the number of bytes along the dynamic ATM VC to S has fallen
   below L, it deletes the dynamic ATM VC and returns to the initial
   mode of operation that was described above.

   The values of the constants K, L, M, and N control the rate of
   dynamic ATM VC creation and deletion.  They are assigned by the
   network administrator and may differ from one ATM attached router to

   Setting the VC creation and deletion limits N and L to zero, turns
   off the measurement process and causes the router to create a dynamic
   VC to every other participating router.  That can be the default in
   small router networks that want to use this method automate the setup
   of a full mesh of ATM VCs.

   If a router doesn't want to set up any dynamic ATM VCs, the VC
   creation limit N is set to a positive value and the measurement
   interval M is set to zero.  Finally, if a router doesn't want to be a
   destination of dynamic ATM VCs, it doesn't make its ATM address
   available to the other routers for the purpose of this application.

   Note that if a router is not capable in measuring traffic, it can
   still participate as a destination of dynamic ATM VCs and can itself
   set up dynamic VCs non-selectively to every other router.

3. Address resolution

   Since all the routers belong to the same area of a link state routing
   domain, they learn each others' router IDs and the IP address
   prefixes that are reachable via each router.  In order to dynamically
   create an ATM VC from one router to another, the source router also
   needs to learn the ATM address of the destination router.

   A router that wants to participate as a source in the dynamic
   management of ATM VCs, makes its ATM address known to the other
   routers of the area by including in its link state advertisements a
   field that contains an ATM address of the advertising router.  In
   OSPF, this ATM address information is carried in an Address
   Resolution Advertizement [5] with area-local flooding scope and with
   a service type.  The service type identifies the possible uses of the

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INTERNET DRAFT                                               April, 1997

   advertised ATM address which includes the dynamic creation of ATM VCs
   according the method described in this document.

   A field similar to Opaque LSA could be easily defined for IS-IS.
   Futher, it could be possible to use a well-known discretionary non-
   transitive attribute of BGP to carry the address resolution
   information, but the use of inter-domain routing protocols is outside
   the scope of this document.

4. Discussion

   The method proposed in this document allows efficient interconnection
   of a set of routers over a legacy ATM network.  After small amount of
   manual configuration, the routers will automatically optimize direct
   connectivity among themselves based on dynamic traffic load.  Network
   administrators can control the number of ATM VCs created by the
   method taking into account scalability and cost.

   The number of ATM VCs could be further reduced if legacy ATM networks
   would support signalling of multipoint-to-point VCs.  The method
   would also benefit from the capability to renegotiate the traffic
   parameters of active ATM VCs.  Both of these capabilities are
   currently under study in the ATM Forum or the ITU.

5. Security Considerations

   Since the method described in this document allows data paths to be
   established that bypass the normal hop-by-hop control path, the
   location of any access filters should be decided carefully.  To
   ensure proper enforcement of filter policies, filters should be moved
   to the edges of an area so that they me be applied on entry or exit
   from the short-cut data path.


   [1] Farinacci, D., Meyer, D., and Rekhter, Y., "Intra-LIS IP
       multicast among routers over ATM".
       draft-ietf-ion-intralis-multicast-00.txt, April 1997.

   [2] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation
       Layer 5".  RFC 1483, July 1993.

   [3] Perez, M, et al., "ATM Signalling Support for IP over ATM".  RFC
       1755, February 1995.

   [4] The ATM Forum, "ATM User-Network Interface Specification Version
       3.1".  September 1994.

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INTERNET DRAFT                                               April, 1997

   [5] Coltun, R., Heinanen, J., Cai Y., "The OSPF Address Resolution
       Advertisement Option". Internet Draft, August 1997.


   I would like to thank Rob Coltun and Lou Berger of Fore Systems for
   their comments on earlier versions of this document.

Author Information

   Juha Heinanen
   Telecom Finland
   PO Box 777
   33101 Tampere

   Phone +358 400 500 958
   Email jh@tele.fi

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