Username / Password : Request For Comments

RFC Number : 2843

Title : Proxy-PAR.

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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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
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
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


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


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