1. RFC 2843
Network Working Group                                           P. Droz
Request for Comments: 2843                                          IBM
Category: Informational                                   T. Przygienda
                                                               May 2000


Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.


   Proxy-PAR is a minimal version of PAR (PNNI Augmented Routing) that
   gives ATM-attached devices the ability to interact with PNNI devices
   without the necessity to fully support PAR. Proxy-PAR is designed as
   a client/server interaction, of which the client side is much simpler
   than the server side to allow fast implementation and deployment.

   The purpose of Proxy-PAR is to allow non-ATM devices to use the
   flooding mechanisms provided by PNNI for registration and automatic
   discovery of services offered by ATM attached devices.  The first
   version of PAR primarily addresses protocols available in IPv4. But
   it also contains a generic interface to access the flooding of PNNI.
   In addition, Proxy-PAR-capable servers provide filtering based on VPN
   IDs [1], IP protocols and address prefixes. This enables, for
   instance, routers in a certain VPN running OSPF to find OSPF
   neighbors on the same subnet. The protocol is built using a
   registration/query approach where devices can register their services
   and query for services and protocols registered by other clients.

1 Introduction

   In June of 1996, the ATM Forum accepted the "Proxy-PAR contribution
   as minimal subset of PAR" as a work item of the Routing and
   Addressing (RA) working group, which was previously called the PNNI
   working group [2].  The PAR [3] specification provides a detailed
   description of the protocol including state machines and packet

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   The intention of this document is to provide general information
   about Proxy-PAR. For the detailed protocol description we refer the
   reader to [3].

   Proxy-PAR is a protocol that allows various ATM-attached devices (ATM
   and non-ATM devices) to interact with PAR-capable switches to
   exchange 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. This should allow an easy implementation on existing IP
   devices such as IP routers. Additionally, clients can use Proxy-PAR
   to register various non-ATM services and the protocols they support.
   The protocol has deliberately been omitted from ILMI [4] because of
   the complexity of PAR information passed in the protocol and the fact
   that it is intended for the integration of non-ATM protocols and
   services only. A device executing Proxy-PAR does not necessarily need
   to execute ILMI or UNI signalling, although this will normally be the

   The protocol does not specify how a client should make use of the
   obtained information to establish connectivity. For example, OSPF
   routers finding themselves through Proxy-PAR could establish a full
   mesh of P2P VCs by means of RFC2225 [5], or use RFC1793 [6] to
   interact with each other.  LANE [7] or MARS [8] could be used for the
   same purpose. It is expected that the guidelines defining how a
   certain protocol can make use of Proxy-PAR should be produced by the
   appropriate working group or standardization body responsible for the
   particular protocol. An additional RFC [9] describing how to run OSPF
   together with Proxy-PAR is published together with this document.

   The protocol has the ability to provide ATM address resolution for
   IP-attached devices, but such resolutions can also be achieved by
   other protocols under specification in the IETF, e.g. [10]. Again,
   the main purpose of the protocol is to allow the automatic detection
   of devices over an ATM cloud in a distributed fashion, omitting the
   usual pitfalls of server-based solutions. Last but not least, it
   should be mentioned here as well that the protocol complements and
   coexists with the work done in the IETF on server detection via ILMI
   extensions [11,12,13].

2 Proxy-PAR Operation and Interaction with PNNI

   The protocol is asymmetric and consists of a discovery and
   query/registration part. The discovery is very similar to the
   existing PNNI Hello protocol and is used to initiate and maintain
   communication between adjacent clients and servers. The registration
   and update part execute after a Proxy-PAR adjacency has been
   established. The client can register its own services by sending

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   registration messages to the server. The client obtains information
   it is interested in by sending query messages to the server. When the
   client needs to change its set of registered protocols, it has to
   re-register with the server. The client can withdraw all registered
   services by registering a null set of services. It is important to
   note that the server side does not push new information to the
   client, neither does the server keep any state describing which
   information the client received. It is the responsibility of the
   client to update and refresh its information and to discover new
   clients or update its stored information about other clients by
   issuing queries and registrations at appropriate time intervals. This
   simplifies the protocol, but assumes that the client will not store
   and request large amounts of data. The main responsibility of the
   server is to flood the registered information through the PNNI cloud
   such that potential clients can discover each other. The Proxy-PAR
   server side also provides filtering functions to support VPNs and IP
   subnetting. It is assumed that services advertised by Proxy-PAR will
   be advertised by a relatively small number of clients and be fairly
   stable, so that polling and refreshing intervals can be relatively

   The Proxy-PAR extensions rely on appropriate flooding of information
   by the PNNI protocol. When the client side registers or re-registers
   a new service through Proxy-PAR, it associates an abstract membership
   scope with the service. The server side maps this membership scope
   into a PNNI routing level that restricts the flooding. This allows
   changes of the PNNI routing level without reconfiguration of the
   client. In addition, the server can set up the mapping table such
   that a client can flood information only to a certain level. Nodes
   within the PNNI network take into account the associated scope of the
   information when it is flooded.  It is thus possible to exploit the
   PNNI routing hierarchy by announcing different protocols on different
   levels of the hierarchy, e.g. OSPF could be run inside certain peer
   groups, whereas BGP could be run between the set of peer -groups
   running OSPF. Such an alignment or mapping of non-ATM protocols to
   the PNNI hierarchy can drastically enhance the scalability and
   flexibility of Proxy-PAR service. Figure 1 helps visualize such a
   scenario. For this topology the following registrations are issued:

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    | | PNNI peer group    # PPAR capable  @ PNNI capable  * Router
    +-+                      switch          switch

                   Level 40
                   |                           |
                   |                           |
                   |      @ ---- @ ---- @      |
                   |      |             |      |
                   +----- | ----------- | -----+
                          |             |
           Level 60       |             |
           +------------- | ---+    +-- | --------------+
           |              |    |    |   |               |
      R1* ------#-P1------@    |    |   @---------P3-#------- * R3
           |              |    |    |   |               |
      R2* ------#-P2------+    |    |   +---------P4-#------- * R4
           |                   |    |                   |
           +-------------------+    +-------------------+

       Figure 1: OSPF and BGP scalability with Proxy-PAR autodetection
                               (ATM topology).

      1. R1 registers OSPF protocol as running on the IP interface and subnet 1.1.1/24 with scope 60

      2. R2 registers OSPF protocol as running on the IP interface and subnet 1.1.1/24 with scope 60

      3. R3 registers OSPF protocol as running on the IP interface and subnet 1.1.2/24 with scope 60

      4. R4 registers OSPF protocol as running on the IP interface and subnet 1.1.2/24 with scope 60


      1. R1 registers BGP4 protocol as running on the IP interface and subnet 1.1/16 with scope 40 within AS101

      2. R3 registers BGP4 protocol as running on the IP interface and subnet 1.1/16 with scope 40 within AS100

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   For simplicity the real PNNI routing level have been specified, which
   are 60 and 40. Instead of these two values the clients would use an
   abstract membership scope "local" and "local+1". In addition, all
   registered information would be part of the same VPN ID.

   Table 1 describes the resulting distribution and visibility of
   registrations and whether the routers not only see but also utilize
   the received information. After convergence of protocols and the
   building of necessary adjacencies and sessions, the overlying IP
   topology is illustrated in Figure 2.

                     AS101         DMZ      AS100
                   #########                ##########
                           #                #
               |           #   |            #            |
               +-- R1 ---------+            #       R4 --+
               |           #   |            #            |
               |           #   | BGP4 on    #    OSPF on |
               | OSPF on   #   | subnet     #     subnet |
               | subnet    #   | 1.1/16     #   1.1.2/24 |
               | 1.1.1/24  #   |            #            |
               |           #   +------------------- R3 --+
               +-- R2      #   |            #            |
               |           #                #
                   #########                ##########

       Figure 2: OSPF and BGP scalability with Proxy-PAR autodetection
                                (IP topology).

   Expressing the above statements differently, one can say that if the
   scope of the Proxy-PAR information indicates that a distribution
   beyond the boundaries of the peer group is necessary, the leader of a
   peer group collects such information and propagates it into a higher
   layer of the PNNI hierarchy. As no assumptions except scope values
   can normally be made about the information distributed (e.g. IP
   addresses bound to AESAs are not assumed to be aligned with them in
   any respect), such information cannot be summarized. This makes a
   careful handling of scopes necessary to preserve the scalability of
   the approach as described above.

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RFC 2843                       Proxy-PAR                        May 2000

                       Reg#   1. 2. 3. 4. 5. 6.
                      R1      R  U        R  U
                      R2      U  R        Q  Q
                      R3            R  U  R  U
                      R4            U  R  Q  Q

                        R registered
                        Q seen through query
                        U used (implies Q)

        Table 1: Flooding scopes of Proxy-PAR registrations.

3 Proxy-PAR Protocols

3.1 Hello Protocol

   The Proxy-PAR Hello Protocol is closely related to the Hello protocol
   specified in [2]. It uses the same packet header and version
   negotiation methods. For the sake of simplicity, states that are
   irrelevant to Proxy-PAR have been removed from the original PNNI
   Hello protocol. The purpose of the Proxy-PAR Hello protocol is to
   establish and maintain a Proxy-PAR adjacency between the client and
   server that supports the exchange of registration and query messages.
   If the protocol is executed across multiple, parallel links between
   the same server and client pair, individual registration and query
   sessions are associated with a specific link. It is the
   responsibility of the client and server to assign registration and
   query sessions to the various communication instances. Proxy-PAR can
   be run in the same granularity as ILMI [4] to support virtual links
   and VP tunnels.

   In addition to the PNNI Hello, the Proxy-PAR Hellos travelling from
   the server to the client inform the client about the lifetime the
   server assigns to registered information. The client has to retrieve
   this interval from the Hello packet and set its refresh interval to a
   value below the obtained time interval in order to avoid the aging
   out of registered information by the server.

3.2 Registration/Query Protocol

   The registration and query protocols enable the client to announce
   and learn about protocols supported by the clients. All
   query/register operations are initiated by the clients. The server
   never tries to push information to the client. It is the client's
   responsibility to register and refresh the set of protocols supported

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   and to re-register them when changes occur. In the same sense, the
   client must query the information from the server at appropriate time
   intervals if it wishes to obtain the latest information. It is
   important to note that neither client nor server is supposed to cache
   any state information about the information stored by the other side.

   Registered information is associated with an ATM address and scope
   inside the PNNI hierarchy. From the IP point of view, all information
   is associated with a VPN ID, IP address, subnet mask, and IP protocol
   family. In this context, each VPN refers to a completely separated IP
   address space. For example <A,,, OSPF>
   describes an OSPF interface in VPN A. In addition to the IP scope
   further parameters can be registered that contain more detailed
   information about the protocol itself. In the above example this
   would be OSPF-specific information such as the area ID or router
   priority.  However, Proxy-PAR server takes only the ATM and IP-
   specific information into account when retrieving information that
   was queried. Protocol specific information is never looked at by a
   Proxy-PAR server.

3.2.1 Registration Protocol

   The registration protocol enables a client to register the protocols
   and services it supports. All protocols are associated with a
   specific AESA and membership scope in the PNNI hierarchy.  As the
   default scope, implementations should choose the local scope of the
   PNNI peer group. In this way, manual configuration can be avoided
   unless information has to cross PNNI peer group boundaries. PNNI is
   responsible for the correct flooding either in the local peer group
   or across the hierarchy.

   The registration protocol is aligned with the standard initial
   topology database exchange protocol used in link-state routing
   protocols as far as possible. It uses a window size of one. A single
   information element is registered at a time and must be acknowledged
   before a new registration packet can be sent. The protocol uses '
   initialization' and 'more' bits in the same manner PNNI and OSPF do.
   Any registration on a link unconditionally overwrites all
   registration data previously received on the same link. By means of a
   return code the server indicates to the client whether the
   registration was successful.

   Apart form the IP-related information, the protocol also offers a
   generic interface to the PNNI flooding. By means of so-called System
   Capabilities Information Groups other information can be distributed
   that can be used for proprietary or experimental implementations.

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3.2.2 Query Protocol

   The client uses the query protocol to obtain information about
   services registered by other clients. The client requests services
   registered within a specific membership scope, VPN and IP address
   prefix. It is always the client's task to request information, the
   server never makes an attempt to push information to the client. If
   the client needs to filter the returned data based on service-
   specific information, such as BGP AS, it must parse and interpret the
   received information. The server never looks beyond the IP scope.

   The more generic interface to the flooding is supported in a similar
   manner as the registration protocol.

4 Supported Protocols

   Currently the protocols indicated in Table 2 have been included.
   Furthermore, for protocols marked 'yes', additional information has
   been specified that is beneficial for their operation. Many of the
   protocols do not need additional information; it is sufficient to
   know they are supported and to which addresses they are bound.

   To include other information in an experimental manner the generic
   information element can be used to carry such information.

5 VPN Support

   To implement virtual private networks all information distributed via
   PAR can be scoped under a VPN ID [1]. Based on this ID, individual
   VPNs can be separated. Inside a certain VPN further distinctions can
   be made according to IP-address-related information and/or protocol

   In most cases the best VPN support can be provided when Proxy-PAR is
   used between the client and server because in this way it is possible
   to hide the real PNNI topology from the client. The PAR capable
   server translates from the abstract membership scope into the real
   PNNI routing level. In this way the real PNNI topology is hidden from
   the client and the server can apply restrictions in the PNNI scope.
   The server can for instance have a mapping such that the membership
   scope "global" is mapped to the highest level peer group to which a
   particular VPN has access. Thus the membership scopes can be seen as
   hierarchical structuring inside a certain VPN. With such mappings a
   network provider can also change the mapping without having to
   reconfigure the clients.

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   For more secure VPN implementations it will also be necessary to
   implement VPN ID filters on the server side. In this way a client can
   be restricted to a certain set (typically one) of VPN IDs.  The
   server will then allow queries and registrations only from the
   clients that are in the allowed VPNs. In this way it is possible to
   avoid an attached client from finding devices that are outside of its
   own VPN.  There is even room for further restriction in terms of not
   allowing wildcard queries by a client. In terms of security, some of
   the protocols have their own methods, so PAR is only used for the
   discovery of the counterparts. For instance OSPF has an
   authentication that can be used during the OSPF operation. Hence even
   in the case where two wrong partners find each other, they will not
   communicate because they will not be able to authenticate each other.

                       Protocol    Additional Info

                       OSPF              yes
                       BGP4              yes
                       MOSPF             yes
                       BBN SPF IGP
                       DNS               yes

   Table 2: Additional protocol information carried in PAR and PPAR.

   The VPN ID used by PAR and Proxy-PAR is aligned with the VPN ID used
   by other protocols from the ATM Forum and IETF. The VPN ID is
   structured into two parts, namely the 3-byte-long OUI plus a 4-byte

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6 Interoperation with ILMI based Server Discovery

   PAR can be used to complement the server discovery via ILMI as
   specified in [11,12,13]. It can be used to provide the flooding of
   information across the PNNI network. For this purpose a server has to
   register with a PAR-capable device.  This can be achieved via Proxy-
   PAR or a direct PAR interaction.  Manual configuration would also be
   possible. For instance the ATMARP server could register its service
   via Proxy-PAR. A direct interaction with PAR will be required in
   order to provide an appropriate flooding scope.

   A PAR-capable device that has the additional MIB variables in the
   Service Registry MIB can set these variables when getting information
   via PAR. All required information is either contained in PAR or is
   static, such as the IP version.

7 Security Consideration

   The Proxy-PAR protocol itself does not have its own security
   concepts.  As PAR is an extension of PNNI, it has all the security
   features that come with PNNI. In addition, the protocol is mainly
   used for automatic discovery of peers for certain protocols.  After
   the discovery process the security concepts of the individual
   protocol are used for the bring-up. As explained in the section about
   VPN support, the only security considerations are on the server side,
   where access filters for VPN IDs can be implemented and restrictive
   membership scope mappings can be configured.

8 Conclusion

   This document describes the basic functions of Proxy-PAR, which has
   been specified within the ATM Forum body. The main purpose of the
   protocol is to provide automatic detection and configuration of non-
   ATM devices over an ATM cloud.

   In the future, support for further protocols and address families may
   be added to widen the scope of applicability of Proxy-PAR.

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9 Bibliography

   [1]  Fox, B. and B. Gleeson, "Virtual Private Networks Identifier",
        RFC 2685, September 1999.

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

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

   [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

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

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

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

   [9]  Droz, P., Haas, R. and T. Przygienda, "OSPF over ATM and Proxy
        PAR", RFC 2844, May 2000.

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

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

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

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

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Authors' Addresses

   Patrick Droz
   IBM Research
   Zurich Research Laboratory
   Saumerstrasse 4
   8803 Ruschlikon

   EMail: dro@zurich.ibm.com

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

   EMail: prz@siara.com

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Full Copyright Statement

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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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  1. RFC 2843