Internet Engineering Task Force                    T. Przygienda/P. Droz
INTERNET DRAFT                                             Bell Labs/IBM
                                                            5 March 1998

                      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-01.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
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   ``working draft'' or ``work in progress.''
   Please check the I-D abstract listing contained in each Internet
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   Internet Draft.

   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.  PAR can help to distribute configuration changes of
   such interfaces when OSPF capable routers are reconfigured on the ATM

1. Introduction

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1.1. Introduction to PAR

   PNNI Augmented Routing (PAR) [For98] is an extension to PNNI [AF96b]
   routing to allow information about non-ATM services to be distributed
   in an ATM network as part of the PNNI topology.  The content and
   format of the information is specified by PAR but is transparent to
   PNNI routing.  A PAR-capable device, one that implements PNNI and the
   PAR extension, is able to create PAR PTSEs that describe the non-ATM
   services located on or behind that device.  Because this information
   is flooded by PNNI routing, PAR-capable devices are also able to
   examine the PAR PTSEs in the topology database that were originated
   by other nodes to obtain information on desired services reachable
   through the ATM network.  An important example of how PAR can be used
   is provided by overlay routing on ATM backbones.  If the routers
   are PAR-capable, they can create PTSEs to advertise the routing
   protocol supported on the given interface (e.g., OSPF, RIP, or BGP),
   along with their IP address and subnet, and other protocol-specific
   details.  The PAR-capable routers can also automatically learn about
   "compatible" routers (e.g., supporting the same routing protocol,
   in the same IP subnet) active in the same ATM network.  In this
   manner the overlay routing network can be established automatically
   on an ATM backbone.  The mechanism is dynamic, and does not require
   configuration.  One potential drawback of PAR is that a device must
   implement PNNI in order to participate.  Therefore an additional set
   of optional protocols called Proxy PAR has been defined to allow a
   client that is not PAR-capable to interact with a server that is
   PAR-capable and thus obtain the PAR capabilities.  The server acts as
   a proxy for the client in the operation of PAR. The client is able to
   register its own services, and query the server to obtain information
   on compatible services available in the ATM network.  A key feature
   of PAR and Proxy PAR is the ability to provide VPN support in a
   simple yet very effective manner.  All PAR information is tagged
   with a VPN ID and can therefore be filtered on that basis.  This can
   be used for example, in a service provider network.  Each customer
   can be provided with a unique VPN ID that is part of all Proxy PAR
   registrations and queries.  Usage of the correct VPN ID can easily
   be enforced at the Proxy PAR server.  In this way the services of a
   given customer will be available only to clients in that customer's

1.1.1. Overview of PNNI Augmented Routing (PAR)

   PNNI Augmented Routing (PAR) is an extension to PNNI to allow the
   flooding of information about non-ATM devices.  PAR uses a new PTSE

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   type to carry this non-ATM-related information.  The current version
   of PAR specifies IGs for the flooding of IPv4-related protocol
   information such as OSPF or BGP. In addition PAR also allows the use
   of the System Capabilities IG, which can be used to carry proprietary
   or experimental information.

   PAR supports extensive filtering possibilities, which allow the
   implementation of virtual private networks (VPN). As PAR is a
   PNNI extension, it can reuse existing PNNI routing level scopes.
   In addition, PAR provides filtering in terms of a VPN ID, IP
   address, including a subnet mask, as well as protocol flags.  The
   correct filtering according to these parameters is part of a PAR

1.1.2. Overview of Proxy PAR

   Proxy PAR is a protocol that allows for different ATM attached
   devices to interact with PAR-capable switches and obtain information
   about non-ATM services without executing PAR themselves.  The client
   side is much simpler in terms of implementation complexity and memory
   requirements than a complete PAR instance and should allow easy
   implementation in, for example, existing IP routers.  Clients can use
   Proxy PAR to register different non-ATM services and protocols they
   support.  This protocol has deliberately not been included as part of
   ILMI [AF96a] owing 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 will
   normally be the case.
   The protocol does not specify how the distributed service
   registration and data delivered to the client are supposed to drive
   other protocols.  For example, OSPF routers finding themselves
   through Proxy PAR could use this information to form a full mesh of
   P2P VCs and communicate using RFC1483 [Hei93] encapsulation.  In
   terms of the discovery of other devices such as IP routers, Proxy PAR
   is an alternative to LANE [AF95] or MARS [Arm96].  It is expected
   that the guidelines defining how a certain protocol can make use of
   Proxy PAR and PAR should come from the group or standardization body
   that is responsible for the particular protocol.

   PAR and Proxy PAR have the ability to provide ATM address resolution
   for IP attached devices, but such resolution can also be achieved by
   other protocols under specification in IETF e.g.  [CH97a, CH97b].
   However, the main purpose of the protocol is to allow the automatic

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   detection of devices over an ATM cloud in a distributed fashion, not
   relying on a broadcast facility.  Finally, it should be mentioned
   that the protocol complements and coexists with server detection via
   ILMI extensions.

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
       (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.

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       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

   Contrary to broadcast-simulation based solutions such as LANE [AF95]
or RFC1577 [Lau94], 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.
   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.  The different interactions are described
   in sections 2.2.1, 2.2.2 and 2.2.3.

   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 2.5 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]
   can ignore the fact that all routers on the specific NBMA subnet are

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   available in its configuration since it only needs to maintain VCs to
   the DR and BDR.

   Traditionally, router configuration for a NBMA network provides
the list of all neighboring routers to allow for proper protocol
operation.  For stability purposes, the user may choose to provide a
list of neighbors through such static means but additionally enable the
operation of Proxy PAR protocol to complete the list.  It is left to
specific router implementations whether the manual configuration is used
in addition to the information provided by Proxy PAR, used as filter of
the dynamic information or whether a concurrent mode of operation is
prohibited.  In any case it should be obvious that allowing for more
flexibility may facilitate operation but provides more possibilities for
misconfiguration as well.

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 to OSPF. 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 or pre-determined SVC meshes.  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, LH96].

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

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   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.

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.

   It should be emphasized here that since the registration information
   can be used by other routers to resolve IP addresses against NSAPs as
   explained in section 2.4 already, whole IP address of the router must
   be registered.  It is not enough to just indicate the subnet up to
   the mask length but all address bits must be provided.

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.

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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.

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. IP address to NSAP Resolution Using Proxy PAR

   As a byproduct of Proxy PAR presence, an OSPF implementation
could use the information in registrations for the resolution of IP
addresses to ATM NSAPs on a subnet without having to use static data or
mechanisms such as ATMARP [LH96].  This again should allow for drastic
simplification of number of mechanisms involved in operation of OSPF
over ATM to provide an IP overlay.

2.5. Connection Setup Mechanisms

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

2.5.1. 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.

2.5.2. 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

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          +           +                             +
          |   +---+   |                             |
   +--+   |---|RTA|---|          +-------+          |   +--+
   |H1|---|   +---+   |          | ATM   |          |---|H2|
   +--+   |           |   +---+  | Cloud |  +---+   |   +--+
          |LAN Y      |---|RTB|-------------|RTC|---|
          +           |   +---+  | PPAR  |  +---+   |
                      +          +-------+          +

 Figure 1:  Simple Topology with Router B and Router C operating across
                   NBMA ATM interfaces with Proxy PAR

   detects that the VC initiated by the other side is bidirectional, it
   is free to close its own VC and use the detected one.  Observe that
   this behavior operates correctly in case OSPF over Demand Circuits
   extensions are used [Moy95] over SVC capable interfaces.

   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 (1) to send packets if their endpoints
   are adequate and must accept hello packets arriving on any of the VCs
   belonging to the logical interface even if OSPF operating on such an
   interface is not aware of their existence.  An OSPF implementation
   may not accept or close connections being initiated by another router
   that has either not been discovered by Proxy PAR or whose Proxy PAR
   registration is indicating that it is not adjacent.
   As an example consider the topology in figure 2.5.2 where router
   RTB and RTC are connected to a common ATM cloud offering Proxy PAR
   services.  Assuming that RTB's OSPF implementation is aware of SVCs
   initiated on the interface and RTC only makes minimal use of Proxy
   PAR information the following sequence could develop illustrating
   some of the cases described above:

    1. RTC and RTB register with ATM cloud as Proxy PAR capable and
       discover each other as adjacent OSPF routers.

1. in case they are aware of their existence

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    2. RTB sends a hello which forces it to establish a SVC connection
       to RTC.

    3. RTC sends a hello to RTB but disregards the already existing VC
       and establishes a new VC to RTB to deliver the packet.

    4. RTB sees a new bi-directional VC and assuming here that RTC's
       OSPF Id is higher, closes the VC originated in step 2.

    5. Host H1 sends data to H2 and RTB establishes a new data SVC
       between itself and RTC.

    6. RTB sends a Hello to RTC and decides to do it using the newly
       establish data SVC. RTC must accept the hello despite the minimal

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

4. Security Consideration
   Several aspects are to be considered when talking about security of
operating OSPF over ATM and/or Proxy PAR. The security of registered
information handed to the ATM cloud must be guaranteed by the underlying
PNNI protocol.  Extensions to PNNI are available and given their
implementation spoofing of registrations and/or denial-of-service issues
can be addressed [PB97].  The registration itself through proxy PAR is
not secured and appropriate mechanisms are for further study.  However,
even if the security at the ATM layer is not guaranteed, OSPF security
mechanisms can be used to verify that detected neighbors are authorized
to interact with the entity discovering them.

   [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.

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   [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

   [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.

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

   [For98] ATM Forum.  PNNI Augmented Routing (PAR) Version 1.0.  ATM
           Forum PNNI-RA-PAR-01.04, 1998.

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

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

   [LH96]  M. Laubach and J. Halpern.  Classical IP and ARP over ATM.
           Internet Draft, 1996.

   [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.

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   [PB97]  T. Przygienda and C. Bullard.  Baseline Text for PNNI Peer
           Authentication and Cryptographic Data Integrity.  ATM Forum
           97-0472, July 1997.

Authors' Addresses

Tony Przygienda
Bell Labs, Lucent Technologies
101 Crawfords Corner Road
Holmdel, NJ 07733-3030

Patrick Droz
IBM Research Division
Saumerstrasse 4
8803 Ruschlikon

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