Internet Engineering Task Force                    T. Przygienda/P. Droz
INTERNET DRAFT                                                  Fore/IBM
                                                         30 October 1997

                      OSPF over ATM and Proxy PAR

Status of This Memo

   This document is an Internet Draft, and can be found as
   draft-ietf-ospf-atm-00.txt in any standard internet drafts
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   This draft specifes for OSPF implementors and users mechanisms
   describing how the protocol operates in ATM networks over PVC and SVC
   meshes with the presence of Proxy PAR. These recommendations do not
   require any protocol changes and allow for simpler, more efficient
   and cost- effective network designs.  It is recommended that OSPF
   implementations should be able to support logical interfaces, each
   consisting of one or more virtual circuits and used as numbered
   logical point-to-point links (one VC) or logical NBMA networks (more
   than one VC) where a solution simulating broadcast interfaces is not
   appropriate.  Proxy PAR can help to distribute configuration changes
   of such interfaces when OSPF capable routers are reconfigured on the
   ATM cloud.

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1. Introduction

1.1. Introduction to Proxy PAR

   Proxy PAR [CPS96, PD97] is an extension allowing for different ATM
   attached devices to interact with PAR capable switches and obtain
   information about non-ATM services without executing PAR [Ca96] which
   is an extension of PNNI [AF96b] themselves.  The client side is much
   simpler in terms of implementation complexity and memory requirements
   than a complete PAR stack and should allow for easy implementation in
   e.g.  existing IP routers.  Additionally, clients can use Proxy PAR
   to register different non-ATM services and protocols they support.
   Proxy PAR has consciously not been included as part of ILMI due to
   the complexity of PAR information passed in the protocol and the
   fact that it is intended for integration of non-ATM protocols and
   services only.  A device executing Proxy PAR does not necessarily
   need to execute ILMI or UNI signaling although this normally will be
   the case.  The context or reference model is therefore aligned with
   the one included in [AF96a].

   The protocol in itself does not specify how the distributed service
   registration and data delivered to the client is supposed to be
   driving other protocols so OSPF routers finding themselves through
   proxy PAR could use this information in e.g.  RFC1577 [Lau94]
   fashion, forming a full mesh of point- to-point connections to
   interact with each other to simulate broadcast interfaces.  For the
   same purpose LANE [AF95] or MARS [Arm96] could be used.  Contrary
   to such solutions, this RFC elaborates on how to handle virtual
   OSPF interfaces over ATM such as NBMA, point-to-multipoint or
   point-to-point and allow for their auto-configuration in presence of
   Proxy PAR. One advantage is the circumvention of server solutions
   that often present single points of failure or hold large amounts of
   configuration information.  The other main benefit is the possibility
   to execute OSPF on top of partially meshed VC topologies.

   As a by-product, Proxy PAR could provide the ATM address resolution
   for IP attached devices but such resolution can be achieved by other
   protocols under specification in IETF as well, e.g.  [CH97a, CH97b].
   Last but not least, it should be mentioned here that the protocol
   coexists with and complements the ongoing work in IETF on server
   detection via ILMI extensions [Dav97] and opaque LSAs [CH97a, CH97b].

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1.1.1. Proxy PAR scopes

   Any Proxy PAR registration is carried only within a defined scope
   that is set during registration and is equivalent to the PNNI routing
   level.  Since no assumptions except scope values can be made about
   the information distributed (e.g.  IP addresses bound to NSAPs
   are not assumed to be aligned with them in any respect such as
   encapsulation or functional mapping), registration information cannot
   be summarized.  This makes a careful handling of scopes necessary to
   preserve the scalability.

1.2. Introduction to OSPF

   OSPF (Open Shortest Path First) is an Interior Gateway Protocol (IGP)
   and described in [Moy94, Moy97] from which most of the following
   paragraphs has been taken almost literally.  OSPF distributes routing
   information between routers belonging to a single Autonomous System.
   The OSPF protocol is based on link-state or SPF technology.  It was
   developed by the OSPF working group of the Internet Engineering
   Task Force.  It has been designed expressly for the TCP/IP internet
   environment, including explicit support for IP subnetting, and
   the tagging of externally-derived routing information.  OSPF also
   utilizes IP multicast when sending/receiving the updates.  In
   addition, much work has been done to produce a protocol that responds
   quickly to topology changes, yet involves small amounts of routing
   protocol traffic.

   To cope with the needs of NBMA and demand circuits capable networks
   such as Frame Relay or X.25, [Moy95] has been made available that
   standardizes extensions to the protocol allowing for efficient
   operation over on-demand circuits.

   OSPF supports three types of networks today:

    -  Point-to-point networks:  A network that joins a single pair
       of routers.  Point- to-point networks can either be numbered
       or unnumbered in the latter case the interfaces do not have IP
       addresses nor masks.  Even when numbered, both sides of the link
       do not have to agree on the IP subnet.

    -  Broadcast networks:  Networks supporting many (more than two)
       attached routers, together with the capability to address
       a single physical message to all of the attached routers

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       (broadcast).  Neighboring routers are discovered dynamically
       on these nets using OSPF's Hello Protocol.  The Hello Protocol
       itself takes advantage of the broadcast capability.  The protocol
       makes further use of multicast capabilities, if they exist.  An
       Ethernet is an example of a broadcast network.

    -  Non-broadcast networks:  Networks supporting many (more than
       two) attached routers, but having no broadcast capability.
       Neighboring routers are maintained on these nets using
       OSPF's Hello Protocol.  However, due to the lack of broadcast
       capability, some configuration information is necessary for the
       correct operation of the Hello Protocol.  On these networks, OSPF
       protocol packets that are normally multicast need to be sent to
       each neighboring router, in turn.  An X.25 Public Data Network
       (PDN) is an example of a non-broadcast network.

       OSPF runs in one of two modes over non-broadcast networks.  The
       first mode, called non-broadcast multi-access (NBMA), simulates
       the operation of OSPF on a broadcast network.  The second mode,
       called Point-to-MultiPoint, treats the non-broadcast network as a
       collection of point-to-point links.  Non-broadcast networks are
       referred to as NBMA networks or Point-to-MultiPoint networks,
       depending on OSPF's mode of operation over the network.

2. OSPF over ATM

2.1. Model

   Parallel to [dR94] that describes the recommended operation of
   OSPF over Frame Relay networks, a similar model is assumed where
   the underlying ATM network can be used to model single VCs as
   point-to-point interfaces or collections of VCs can be accessed as an
   non-broadcast interface in NBMA or point-to-multipoint mode.  Such a
   VC or collection of VCs is called a logical interface and specified
   through its type (either point-to-point, NBMA or point-to-point),
   IP instance (presenting an incarnation of IP with its own address
   spaces), address and mask.  Layer 2 specific configuration such as
   address resolution method, class and quality of service of used
   circuits and other must be also included.  As logical consequence
   thereof, a single, physical interface could encompass multiple IP
   subnets or even multiple, independent IP instances.  In contrary to
   layer 2 and IP addressing information, when running Proxy PAR, most
   of the OPSF information needed to operate such a logical interface

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   does not have to be configured into routers statically but can be
   provided through Proxy PAR queries.  This allows for much more
   dynamic configuration of VC meshes in OSPF environments than e.g.  in
   Frame Relay solutions.

2.2. OSPF Configuration Interaction with Proxy PAR

   To achieve the goal of simplification of VC mesh reconfiguration,
   Proxy PAR allows the router to learn automatically most of the
   configuration that has to be provided to OSPF. Non-broadcast
   and point-to-point interface information can be learned across
   an ATM cloud as described in the ongoing sections.  It is up to
   the implementation to possibly allow for a mixture of Proxy PAR
   autoconfiguration and manual configuration of neighbor information.
   Moreover, manual configuration could e.g.  override or complement
   information derived from a proxy PAR client.  Additionally, OSPF
   extensions to handle on-demand circuits [Moy95] can be used to allow
   for graceful tearing down of VCs not carrying any OSPF traffic over
   prolonged periods of time.

   Even after autoconfiguration of interfaces has been provided, the
   problem of VC setups in an ATM network is unsolved since none of the
   normally used mechanisms such as RFC1577 [Lau94] or LANE [AF95] are
   assumed to be present.  Section 2.3 describes the behavior of OSPF
   routers to allow for router connectivity necessary.

2.2.1. Autoconfiguration of non-broadcast interfaces

   Proxy PAR allows to autoconfigure the list of all routers residing
   on the same IP network in the same IP instance by simply querying
   the Proxy PAR server.  Each router can easily obtain the list of
   all OSPF routers on the same subnet with their router priorities
   and ATM address bindings.  This is the precondition for OSPF to
   work properly across such logical NBMA interfaces.  Note that the
   memberlist, when learned through Proxy PAR queries, can dynamically
   change with PNNI (in)stability and general ATM network behavior.  It
   maybe preferable for an implementation to withdraw list membership
   e.g.  much slower than detect new members.  Relying on OSPF mechanism
   to discover lack of reachability in the overlaying logical IP network
   could alleviate the risk of thrashing DR elections and excessive
   information flooding.  Once the DR registration is completed and the
   router has not been elected DR or BDR, an implementation of [Moy95]

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   can ignore the fact that all routers on the specific NBMA subnet are
   available in its configuration since it only needs to maintain VCs to
   the DR and BDR.

2.2.2. Autoconfiguration of point-to-multipoint interfaces

   Point-to-Multipoint interfaces in ATM networks only make sense if
   no VCs can be dynamically set up since an SVC-capable ATM network
   normally presents a NBMA cloud.  This is e.g.  the case if the
   intended use of the network is only to execute OSPF in presence of a
   partial PVC or SPVC mesh.  Such a collection could be modeled using
   the point-to-multipoint OSPF interface and the neighbor detection
   could be provided by Proxy PAR or other means.  In Proxy PAR case
   the router queries for all OSPF routers on the same network in the
   same IP instance but it installs in the interface configuration
   only routers that are already reachable through preset PVCs.  The
   underlying assumption is that a router understands the remote NSAP of
   a PVC and can compare it with appropriate Proxy PAR registrations.
   If the remote NSAP of the PVC is unknown, alternative autodiscovery
   mechanisms have to be used e.g.  inverse ARP [BB92].

2.2.3. Autoconfiguration of numbered point-to-point interfaces

   OSPF point-to-point links do not necessarily have an IP address
   assigned and even when having one, the mask is undefined.  As a
   precondition to successfully register a service with Proxy PAR, IP
   address and mask is required.  Therefore, if a router desires to use
   Proxy PAR to advertise the local end of a point-to- point link to the
   router it intends to form an adjacency with, an IP address has to
   be provided and a netmask set or a default of (this
   gives as the default case a subnet with 2 routers on it) assumed.  To
   allow the discovery of the remote end of the interface, IP address
   of the remote side has to be provided and a netmask set or a default
   of assumed.  Obviously the discovery can only be
   successfull when both sides of the interface are configured with the
   same network mask and are within the same IP network.  The situation
   where more than two possible neighbors are discovered through
   queries and the interface type is set to point-to-point presents a
   configuration error.

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2.2.4. Autoconfiguration of unnumbered point-to-point interfaces

   For reasons given already in [dR94] using unnumbered point-to-point
   interfaces with Proxy PAR is not a very attractive alternative
   since the lack of an IP address prevents efficient registration and
   retrieval of configuration information.  Relying on the numbering
   method based on MIB entries generates conflicts with the dynamic
   nature of creation of such entries and is beyond the scope of this

2.3. Proxy PAR Interaction with OSPF Configuration

   To allow other routers to discover an OSPF interface automatically,
   the IP address, mask, Area ID, interface type and router priority
   information given must be registered with the Proxy PAR server at an
   appropriate scope.  A change in any of these parameters has to force
   a reregistration with Proxy PAR.

2.3.1. Registration of non-broadcast interfaces

   For an NBMA interface the appropriate parameters are available and
   can be registered through Proxy PAR without further complications.

2.3.2. Registration of point-to-multipoint interfaces

   In case of a point-to-multipoint interface the router registers its
   information in the same fashion as in the NBMA case except that the
   interface type is modified accordingly.

2.3.3. Registration of point-to-point interfaces

   In case of point-to-point numbered interfaces the address mask is not
   specified in the OSPF configuration.  If the router has to use Proxy
   PAR to advertise its capability, a mask must be defined or a default
   value of used.

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2.3.4. Registration of unnumbered point-to-point interfaces

   Due to the lack of a configured IP address and difficulties generated
   by this fact as described earlier, registration of unnumbered
   point-to-point interfaces is not covered in this document.

2.4. Connection setup mechanisms

   This sections describes OSPF behavior in an ATM network under
   different assumptions in terms of signaling capabilities and preset

2.4.1. OSPF in PVC environments

   In environments where only partial PVCs (or SPVCs) meshes are
   available and modeled as point-to-multipoint interfaces, the routers
   see reachable routers through autodiscovery provided by Proxy PAR.
   This leads to expected OSPF behavior.  In cases where a full mesh of
   PVCs is present, such an interface should preferably be modeled as
   broadcast and Proxy PAR discovery should be superfluous.

2.4.2. OSPF in SVC environments

   In SVC-capable environments the routers can initiate VCs after having
   discovered the appropriate neighbors, preferably driven by the need
   to send data such as Hello-packets.  Since this can lead to race
   conditions where both sides can open a VC and it is desirable to
   minimize this valuable resource, if the router with lower Router ID
   detects that the VC initiated by the other side is bidirectional, it
   is free to close its own VC and use the detected one.  The existence
   of VCs used for OSPF exchanges is orthogonal to the number and type
   of VCs the router chooses to use within the logical interface to
   forward data to other routers.  OSPF implementations are free to use
   any of these VCs to send packets if their endpoints are adequate and
   must accept hello packets arriving on any of the VCs belonging to the
   logical interface.

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3. Acknowledgments

   Comments and contributions from several sources, especially Rob
   Coltun and John Moy are included in this work.

4. Security Consideration

   Security issues are not discussed in this memo.


   [AF95]  ATM-Forum.  LAN Emulation over ATM 1.0.  ATM Forum
           af-lane-0021.000, January 1995.

   [AF96a] ATM-Forum.  Interim Local Management Interface (ILMI)
           Specification 4.0.  ATM Forum 95-0417R8, June 1996.

   [AF96b] ATM-Forum.  Private Network-Network Interface Specification
           Version 1.0.  ATM Forum af-pnni-0055.000, March 1996.

   [Arm96] G. Armitage.  Support for Multicast over UNI 3.0/3.1 based
           ATM Networks, RFC 2022.  Internet Engineering Task Force,
           November 1996.

   [BB92]  T. Bradley and C. Brown.  Inverse Address Resolution
           Protocol, RFC 1293.  Internet Engineering Task Force, January

   [Ca96]  R. Callon and al.  An Overview of Pnni Augmented Routing.
           ATM Forum 96-0354, April 1996.

   [CH97a] R. Coltun and J. Heinanen.  Opaque LSA in OSPF.  Internet
           Draft, 1997.

   [CH97b] R. Coltun and J. Heinanen.  The OSPF Address Resolution
           Advertisement Option.  Internet Draft, 1997.

   [CPS96] R. Coltun, T. Przygienda, and S. Shew.  MIPAR: Minimal PNNI
           Augmented Routing.  ATM Forum 96-0838, June 1996.

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   [Dav97] M. Davison.  Simple ILMI-Based Server Discovery.  Internet
           Draft, 1997.

   [dR94]  O. deSouza and M. Rodrigues.  Guidelines for Running OSPF
           Over Frame Relay Networks, RFC 1586.  Internet Engineering
           Task Force, March 1994.

   [Lau94] M. Laubach.  Classical IP and ARP over ATM, RFC 1577.
           Internet Engineering Task Force, January 1994.

   [Moy94] J. Moy.  OSPFv2, RFC 1583.  Internet Engineering Task Force,
           March 1994.

   [Moy95] J. Moy.  Extending OSPF to Support Demand Circuits, RFC 1793.
           Internet Engineering Task Force, April 1995.

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

   [PD97]  T. Przygienda and P. Droz.  Proxy PAR.  ATM Forum 97-0495,
           97-0705, 97-0882, July 1997.

Authors' Addresses

Tony Przygienda
FORE Systems
6905 Rockledge Drive
Suite 800
Bethesda, MD 20817

Patrick Droz
IBM Research Division
Saumerstrasse 4
8803 Ruschlikon

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