Network Working Group                                       T. Przygienda
Request for Comments: 2844                                          Siara
Category: Experimental                                            P. Droz
                                                                  R. Haas
                                                                      IBM
                                                                 May 2000

OSPF over ATM and Proxy-PAR

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

Abstract

   This memo specifies, 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 require
   no protocol changes and allow 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 either as
   numbered logical point-to-point links (one VC), logical NBMA networks
   (more than one VC) or Point-to-MultiPoint networks (more than one
   VC), where a solution simulating broadcast interfaces is not
   appropriate. PAR can help distribute across the ATM cloud
   configuration setup and changes of such interfaces when OSPF capable
   routers are (re-)configured.  Proxy-PAR can in turn be used to
   exchange this information between the ATM cloud and the routers
   connected to it.

1 Introduction

   Proxy-PAR and PAR have been accepted as standards by the ATM Forum in
   January 1999 [1]. A more complete overview of Proxy-PAR than in the
   section below is given in [2].

1.1 Introduction to Proxy-PAR

   Proxy-PAR [1] is an extension that allows different ATM attached
   devices (like routers) to interact with PAR-capable switches and to
   query information about non-ATM services without executing PAR
   themselves. The Proxy-PAR client side in the ATM attached device is
   much simpler in terms of implementation complexity and memory
   requirements than a complete PAR protocol stack (which includes the
   full PNNI [3] protocol stack) and should allow easy implementation,
   e.g. in existing IP routers.  In addition, clients can use Proxy-PAR
   to register the various non-ATM services and protocols they support.
   Proxy PAR has consciously been omitted as part of ILMI [4] 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 that executes Proxy-PAR does not necessarily need to
   execute ILMI or UNI signaling, although this normally will be the
   case.

   The protocol in itself does not specify how the distributed service
   registration and data delivered to the client is supposed to drive
   other protocols. Hence OSPF routers, for instance, that find
   themselves through Proxy-PAR could use this information in a
   Classical IP and ARP over ATM [5] fashion, forming a full mesh of
   point-to-point connections to interact with each other to simulate
   broadcast interfaces. For the same purpose, LANE [6] or MARS [7]
   could be used. As a byproduct, Proxy-PAR could provide the ATM
   address resolution for IP-attached devices, but such resolution can
   be achieved by other protocols under specification at the IETF as
   well, e.g. [8]. 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 [9,10,11].

1.1.1 Proxy-PAR Scopes

   Any information registered through Proxy-PAR is flooded only within a
   defined scope that is established during registration and is
   equivalent to the PNNI routing level. As no assumption 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), it cannot be summarized. This
   makes a careful handling of scopes necessary to preserve the
   scalability. More details on the usage of scope can be found in [2].

1.2 Introduction to OSPF

   OSPF (Open Shortest Path First) is an Interior Gateway Protocol (IGP)
   and described in [12] 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-circuit-capable networks
   such as Frame Relay or X.25, [13] has been made available. It
   standardizes extensions to the protocol that allow 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 of addressing a
         single physical message to all of the attached routers
         (broadcast). Neighboring routers are discovered dynamically on
         these networks using the OSPF 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

   Contrary to broadcast-simulation-based solutions such as LANE [6] or
   Classical IP and ARP over ATM [5], this document 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 the 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 capability of executing OSPF on top of
   NBMA and Point-to-MultiPoint ATM networks, and still benefit from the
   automatic discovery of OSPF neighbors. As opposed to broadcast
   networks, broadcast-simulation-based networks (such as LANE or
   Classical IP and ARP over ATM), and point-to-point networks, where an
   OSPF router dynamically discovers its neighbors by sending Hello
   packets to the All-SPFRouters multicast address, this is not the case
   on NBMA and Point-to-MultiPoint networks. On NBMA networks, the list
   of all other attached routers to the same NBMA network has to be
   manually configured or discovered by some other means: Proxy-PAR
   allows this configuration to be automated.  Also on Point-to-
   MultiPoint networks, the set of routers that are directly reachable
   can either be manually configured or dynamically discovered by
   Proxy-PAR or mechanisms such as Inverse ATMARP. In an ATM network,
   (see 8.2 in [5]) Inverse ATMARP can be used to discover the IP
   address of the router at the remote end of a given PVC, whether or
   not its ATM address is known. But Inverse ATMARP does not return, for
   instance, whether the remote router is running OSPF, unlike Proxy-
   PAR.

   Parallel to [14], which 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 as non-broadcast interfaces,
   whether 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-MultiPoint), VPN ID (the

   Virtual Private Network to which the interface belongs), address and
   mask. Layer 2 specific configurations such as the address resolution
   method, class and quality of service of circuits used, and others,
   must also be included. As a logical consequence thereof, a single,
   physical interface could encompass multiple IP subnets or even
   multiple VPNs. Contrary to layer 2 and IP addressing information,
   when running Proxy-PAR, most of the OSPF information needed to
   operate such a logical interface does not have to be configured into
   routers statically but can be provided through Proxy-PAR queries.
   This allows much more dynamic configuration of VC meshes in OSPF
   environments than, for example, Frame Relay solutions do.

   Proxy-PAR queries can also be issued with a subnet address set to
   0.0.0.0, instead of a specific subnet address. This type of query
   returns information on all OSPF routers available in all subnets
   within the scope specified in the query. This can be used for
   instance when the IP addressing information has not been configured.

2.2 Configuration of OSPF interfaces 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, for instance, override or
   complement information derived from a Proxy-PAR client. In addition,
   OSPF extensions to handle on-demand circuits [13] can be used to
   allow the graceful tearing down of VCs not carrying any OSPF traffic
   over prolonged periods of time.  The various 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 because none of
   the normally used mechanisms such as Classical IP and ARP over ATM
   [5] or LANE [6] are assumed to be present.  Section 2.5 describes the
   behavior of OSPF routers necessary to allow for router connectivity.

2.2.1 Autoconfiguration of Non-Broadcast Multiple-Access (NMBA)
Interfaces

   Proxy-PAR allows the autoconfiguation of the list of all routers
   residing on the same IP network in the same VPN 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
   corresponding ATM addresses. This is the precondition for OSPF to

   work properly across such logical NBMA interfaces. Note that this
   member list, when learned through Proxy-PAR queries, can dynamically
   change with PNNI (in)stability and general ATM network behavior.
   Relying on an OSPF mechanism to discover a lack of reachability in
   the overlaying logical IP network could alleviate the risk of
   thrashing DR elections and excessive information flooding. Once the
   DR election has been completed and the router has not been elected DR
   or BDR, an implementation of [13] can ignore the fact that all
   routers on the specific NBMA subnet are available in its
   configuration because it only needs to maintain VCs to the DR and
   BDR. Note that this information can serve other purposes, such as the
   forwarding of data packets (see section 2.4).

   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 also enable the
   operation of Proxy-PAR protocol to complete the list.  It is left up
   to specific router implementations to determine whether to use the
   manual configuration in addition to the information provided by
   Proxy-PAR, to use the manual configuration to filter 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 set up dynamically because an SVC-capable ATM network
   normally presents a NBMA cloud to OSPF. This is for example the case
   if OSPF executes over a network composed of a partial PVC or SPVC
   mesh or predetermined SVC meshes. Such a network could be modeled
   using the Point-to-MultiPoint OSPF interface and the neighbor
   detection could be provided by Proxy-PAR or other means. In the
   Proxy-PAR case the router queries for all OSPF routers on the same
   network in the same VPN but it installs in the interface
   configuration only routers that are already reachable through
   existing PVCs. The underlying assumption is that a router knows the
   remote ATM address of a PVC and can compare it with appropriate
   Proxy-PAR registrations. If the remote ATM address of the PVC is
   unknown, it can be discovered by such mechanisms as Inverse ARP [15].

   Proxy-PAR provides a true OSPF neighbor detection mechanism, whereas
   a mechanism like Inverse ARP only returns addresses of directly
   reachable routers (which are not necessarily running OSPF), in the
   Point-to-Multi-Point environment.

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 if they do, the mask is undefined. As a
   precondition to successfully register a service with Proxy-PAR, an IP
   address and a mask are required. Therefore, if a router desires to
   use Proxy-PAR to advertise the local end of a point-to-point link to
   the router with which it intends to form an adjacency, an IP address
   has to be provided as well as a netmask set or a default of
   255.255.255.252 (this gives as the default case a subnet with two
   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 255.255.255.252 assumed. Obviously the
   discovery can only be successful 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.

   Sending multicast Hello packets on the point-to-point links allows
   OSPF neighbors to be discovered automatically. On the other hand,
   using Proxy-PAR instead avoids sending Hello messages to routers that
   are not necessarily running OSPF.

2.2.4 Autoconfiguration of Unnumbered Point-to-Point Interfaces

   For reasons given in [14], the use of unnumbered point-to-point
   interfaces with Proxy-PAR is not a very attractive alternative
   because 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
   work.

2.3 Registration of OSPF interfaces with Proxy-PAR

   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 because the registration
   information can be used by other routers to resolve IP addresses
   against NSAPs as explained in section 2.4, the entire IP address of
   the router must be registered. It is not sufficient to indicate the
   subnet up to the mask length; all address bits must be provided.

2.3.1 Registration of Non-Broadcast Multiple-Access 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 the 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 Numbered Point-to-Point Interfaces

   In the 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 255.255.255.252 used.

2.3.4 Registration of Unnumbered Point-to-Point Interfaces

   Owing 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 [5]. This again should allow a drastic
   simplification of the number of mechanisms involved in operating OSPF
   over ATM to provide an IP overlay.

   From a system perspective, the OSPF component, the Proxy-PAR client,
   the IP to NSAP address resolution table, and the ATM circuit manager
   can be depicted as in Figure 1. Figure 1 shows an example of
   component interactions triggered by a Proxy-PAR query from the
   Proxy-PAR client.

2.5 Connection Setup Mechanisms

   This section describes the OSPF behavior in an ATM network under
   various assumptions in terms of signaling capabilities and preset
   connectivity.

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 a network should preferably be modeled as NBMA.
   Note that in such a case, PVCs failures will translate into not-so-
   obvious routing failures.

        __________                      _________
       |          |                    |         |
       |   OSPF   |<-------------------|Proxy-PAR|<---(Proxy-PAR query)
       |__________|  notify            | client  |
            ^        neighbor changes  |_________|
            |                               |
   send and |                               | maintain Proxy-PAR
   receive  |                               | entries in table
   OSPF msg |                               |
            |                               |
            |                               |
        ____V____                       ____V_____
       |   ATM   |                     |          |
       | circuit |-------------------->|IP to NSAP|
       | manager | check               |  table   |
       |_________| IP to NSAP bindings |__________|

   Figure 1: System perspective of typical components interactions.

2.5.2 OSPF in SVC Environments

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

     Figure 2: Simple topology with Router B and Router C operating
               across NBMA ATM interfaces with Proxy-PAR.

   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. This can lead to race conditions
   where both sides can open a VC simultaneously. It is generally
   desirable to avoid wasting this valuable resource: if the router with
   lower IP address (i.e., the IP address of the OSPF interface
   registered 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. Note that this either requires the OSPF implementation
   to be aware of the VCs used to send and receive Hello messages, or
   the component responsible of managing VCs to be aware of the usage of
   particular VCs.

   Observe that this behavior operates correctly in case OSPF over
   Demand Circuits extensions are used [13] over SVC capable interfaces.

   Most of the time, it is possible to avoid the setup of redundant VCs
   by delaying the sending of the first OSPF Hello from the router with
   the lower IP address by an amout of time greater than the interval
   between the queries from the Proxy-PAR client to the server. Chances
   are that the router with the higher IP address opens the VC (or use
   an already existing VC) and sends the OSPF Hello first if its
   interval between queries is shorter than the Hello delay of the
   router with the lower IP address. As this interval can vary depending
   on particular needs and implementations, the race conditions
   described above can still be expected to happen, albeit presumably
   less often.

   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 (in case they are aware of their
   existence) to send packets if their end points 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 ignore
   connections being initiated by another router that has not been
   discovered by Proxy-PAR. In any case, the OSPF implementation will
   ignore a neighbor whose Proxy-PAR registration indicates that it is
   not adjacent.

   As an example consider the topology in Figure 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 that 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.

      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 bidirectional VC and, assuming here that RTC's
IP address 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 so using the newly
         establish data SVC. RTC must accept the Hello despite the
         minimal implementation.

3 Acknowledgments

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

4 Security Considerations

   Several aspects are to be considered in the context of the 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. The registration itself through Proxy-
   PAR is not secured, and are thus appropriate mechanisms 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.

5 Bibliography

   [1]  ATM Forum, "PNNI Augmented Routing (PAR) Version 1.0."  ATM
        Forum af-ra-0104.000, January 1999.

   [2]  Droz, P. and T. Przygienda, "Proxy-PAR", RFC 2843, May 2000.

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

   [4]  ATM-Forum, "Interim Local Management Interface, (ILMI)
        Specification 4.0." ATM Forum af-ilmi-0065.000, September 1996.

   [5]  Laubach, J., "Classical IP and ARP over ATM", RFC 2225, April
        1998.

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

   [7]  Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
        Networks", RFC 2022, November 1996.

   [8]  Coltun, R., "The OSPF Opaque LSA Option", RFC 2328, July 1998.

   [9]  Davison, M., "ILMI-Based Server Discovery for ATMARP", RFC 2601,
        June 1999.

   [10] Davison, M., "ILMI-Based Server Discovery for MARS", RFC 2602,
        June 1999.

   [11] Davison, M., "ILMI-Based Server Discovery for NHRP", RFC 2603,
        June 1999.

   [12] Moy, J., "OSPF Version 2", RFC 2328, April 1998.

   [13] Moy, J., "Extending OSPF to Support Demand Circuits", RFC 1793,
        April 1995.

   [14] deSouza, O. and M. Rodrigues, "Guidelines for Running OSPF Over
        Frame Relay Networks", RFC 1586, March 1994.

   [15] Bradley, A. and C. Brown, "Inverse Address Resolution Protocol",
        RFC 2390, September 1999.

Authors' Addresses

   Tony Przygienda
   Siara Systems Incorporated
   1195 Borregas Avenue
   Sunnyvale, CA 94089
   USA

   EMail: prz@siara.com

   Patrick Droz
   IBM Research
   Zurich Research Laboratory
   Saumerstrasse 4
   8803 Ruschlikon
   Switzerland

   EMail: dro@zurich.ibm.com

   Robert Haas
   IBM Research
   Zurich Research Laboratory
   Saumerstrasse 4
   8803 Ruschlikon
   Switzerland

   EMail: rha@zurich.ibm.com

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