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Versions: 00 01 02 03 draft-ietf-idr-ix-bgp-route-server

GROW Working Group                                           E. Jasinska
Internet-Draft                                               AMS-IX B.V.
Intended status: Informational                               N. Hilliard
Expires: January 6, 2011                                            INEX
                                                               R. Raszuk
                                                           Cisco Systems
                                                            July 5, 2010

                     Internet Exchange Route Server


   The growing popularity of Internet exchange points (IXPs) brings a
   new set of requirements to interconnect participating networks.
   While bilateral exterior BGP sessions between exchange participants
   were previously the most common means of exchanging reachability
   information, the overhead associated with dense interconnection has
   caused substantial operational scaling problems for IXP participants.

   This document outlines a specification for multilateral
   interconnections at IXPs.  Multilateral interconnection describes a
   method of exchanging routing information between three or more BGP
   speakers using a single intermediate broker system, referred to as a
   route server.  Route servers are typically used on shared access
   media networks such as Internet Exchanges (IXPs), to facilitate
   simplified interconnection between multiple Internet routers on such
   a network.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   The list of current Internet-Drafts can be accessed at

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   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on January 6, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

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Table of Contents

   1.  Introduction to Multilateral Interconnection . . . . . . . . .  3
     1.1.  Specification of Requirements  . . . . . . . . . . . . . .  3
   2.  Bilateral Interconnection  . . . . . . . . . . . . . . . . . .  3
   3.  Multilateral Interconnection . . . . . . . . . . . . . . . . .  5
   4.  Technical Considerations for Route Server Implementations  . .  6
     4.1.  Client UPDATE Messages . . . . . . . . . . . . . . . . . .  6
     4.2.  Attribute Transparency . . . . . . . . . . . . . . . . . .  6
       4.2.1.  NEXT_HOP Attribute . . . . . . . . . . . . . . . . . .  6
       4.2.2.  AS_PATH Attribute  . . . . . . . . . . . . . . . . . .  6
       4.2.3.  MULTI_EXIT_DISC Attribute  . . . . . . . . . . . . . .  7
       4.2.4.  BGP Community Attributes . . . . . . . . . . . . . . .  7
     4.3.  Per-Client Prefix Filtering  . . . . . . . . . . . . . . .  7
       4.3.1.  Prefix Hiding on a Route Server  . . . . . . . . . . .  7
       4.3.2.  Mitigation Techniques  . . . . . . . . . . . . . . . .  9  Multiple Route Server RIBs . . . . . . . . . . . .  9  Advertising Multiple Paths . . . . . . . . . . . .  9
   5.  Operational Considerations for Route Server Installations  . . 10
     5.1.  Route Server Scaling . . . . . . . . . . . . . . . . . . . 10
     5.2.  NLRI Leakage Mitigation  . . . . . . . . . . . . . . . . . 11
     5.3.  Route Server Redundancy  . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction to Multilateral Interconnection

   Internet Exchange Points (IXPs) provide IP data interconnection
   facilities for their participants, typically using shared Layer-2
   networking media such as Ethernet.  The Border Gateway Protocol (BGP)
   [RFC4271], an inter-Autonomous System routing protocol, is commonly
   used to facilitate exchange of network reachability information over
   such media.

   In the case of bilateral interconnection between two exchange
   participant routers, each router must be configured with a BGP
   session to the other.  At IXPs with many participants who wish to
   implement dense interconnection, this requirement can lead both to
   large router configurations and high administrative overhead.  Given
   the growth in the number of participants at many IXPs, it has become
   operationally troublesome to implement densely meshed
   interconnections at these IXPs.

   Multilateral interconnection describes a method of interconnecting
   BGP speaking routers using a third party brokering system, commonly
   referred to as a route server and typically managed by the exchange
   fabric operator.  Each of the multilateral interconnection
   participants (usually referred to as route server clients) announces
   network reachability information to the route server using exterior
   BGP, and the route server in turn forwards this information to each
   other route server client connected to it, according to its
   configuration.  Although a route server uses BGP to exchange
   reachability information with each of its clients, it does not
   forward traffic itself and is therefore not a router.

   A route server can be viewed as similar in function to an [RFC4456]
   route reflector, except that it operates using EBGP instead of iBGP.

1.1.  Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

2.  Bilateral Interconnection

   Bilateral interconnection describes a method of interconnecting
   routers using individual BGP sessions between each participant router
   on an IXP in order to exchange reachability information.  While
   interconnection policies vary from participant to participant, most
   IXPs have significant numbers of participants who see value in

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   interconnecting with as many other exchange participants as possible.
   In order for an IXP participant to implement a dense interconnection
   policy, it is necessary for the participant to liaise with each of
   their intended interconnection partners and if this partner agrees to
   interconnect, then both participants' routers much be configured with
   a BGP session to exchange network reachability information.  If each
   exchange participant interconnects with each other participant, a
   full mesh of BGP sessions is needed, as detailed in Figure 1.

                                AS1      AS2
                                ***      ***
                           IXP *   *    *   *
                           +---*   *----*   *---+
                           |   *   *____*   *   |
                           |    ***\    /***    |
                           |     |  \  /  |     |
                           |     |   \/   |     |
                           |     |   /\   |     |
                           |     |  /  \  |     |
                           |    ***/____\***    |
                           |   *   *    *   *   |
                           +---*   *----*   *---+
                               *   *    *   *
                                ***      ***
                                AS3      AS4

               Figure 1: Full-Mesh Interconnection at an IXP

   Figure 1 depicts an IXP platform with four connected routers,
   administered by four separate exchange participants, each with a
   locally unique autonomous system number: AS1, AS2, AS3 and AS4.  Each
   of these four participants wishes to exchange traffic with all other
   participants; this is accomplished by configuring a full mesh of BGP
   sessions on each router connected to the exchange, resulting in 6 BGP
   sessions across the IXP fabric.

   It can be calculated that the number of BGP sessions at an exchange
   has an upper bound of n*(n-1)/2, where n is the number of routers at
   the exchange.  As many exchanges have relatively large numbers of
   participating networks, the super-linear scaling requirements of
   dense interconnection tend to cause operational and administrative
   overhead at large IXPs.  In particular, new participants to an IXP
   require significant initial resourcing in order to gain value from
   their IXP connection.

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3.  Multilateral Interconnection

   Multilateral interconnection is implemented using a route server
   configured to use BGP to exchange reachability information between
   each client router.  The route server preserves the BGP NEXT_HOP
   attribute from all received NLRI UPDATE messages, and passes these
   messages with unchanged NEXT_HOP to its route server clients,
   according to its configured routing policy.  Using this method of
   exchanging NLRIs, an IXP participant router can receive an aggregated
   list of prefixes from all other route server clients using a single
   BGP session to the route server instead of depending on multiple BGP
   sessions with each other participant router.  This reduces the
   overall number of BGP sessions over an Internet exchange from
   n*(n-1)/2 to n, where n is the number of routers at the exchange.

   In practical terms, this allows dense interconnection between IXP
   participants with low administrative overhead and significantly
   simpler and smaller router configurations.  In particular, new IXP
   participants benefit from immediate and extensive interconnection,
   while existing route server participants receive reachability
   information from these new participants without necessarily having to
   adapt their configurations.

                                AS1      AS2
                                ***      ***
                           IXP *   *    *   *
                           +---*   *----*   *---+
                           |   *   *    *   *   |
                           |    ***\    /***    |
                           |        \  /        |
                           |         **         |
                           |        *  *        |
                           |     RS *  *        |
                           |        *  *        |
                           |         **         |
                           |        /  \        |
                           |    ***/    \***    |
                           |   *   *    *   *   |
                           +---*   *----*   *---+
                               *   *    *   *
                                ***      ***
                                AS3      AS4

           Figure 2: IXP-based Interconnection with Route Server

   As illustrated in Figure 2, each router on the IXP fabric requires
   only a single BGP session to the route server, from which it can

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   receive reachability information for all other routers on the IXP
   which also connect to the route server.

4.  Technical Considerations for Route Server Implementations

4.1.  Client UPDATE Messages

   A route server MUST accept all UPDATE messages transmitted to it from
   each of its clients for inclusion in its Adj-RIB-In.  These UPDATE
   messages may subsequently not be included in the route server's Loc-
   RIB or Loc-RIBs, due to filters configured for the purposes of
   implementing policy routing.  The route server SHOULD perform one or
   more BGP Decision Processes to select routes for subsequent
   advertisement to its clients, taking into account possible
   configuration to provide multiple NLRI paths to a particular client
   as described in Section or multiple Loc-RIBs as described in
   Section  The route server SHOULD forward UPDATE messages
   where appropriate from its Loc-RIB or Loc-RIBs to its clients.

4.2.  Attribute Transparency

   As a route server primarily performs a brokering service,
   modification of attributes could cause route server clients to alter
   their BGP best-path selection process for received prefix
   reachability information, thereby changing the intended routing
   policies of exchange participants.  Therefore, contrary to what is
   specified in section 5.1 of [RFC4271], route servers should not
   generally modify BGP attributes received from route server clients
   before distributing them to their other route server clients.

4.2.1.  NEXT_HOP Attribute

   The BGP NEXT_HOP attribute defines the IP address of the router used
   as the next hop to the destinations listed in the Network Layer
   Reachability Information field of the UPDATE message.  As the route
   server does not participate in the actual routing of traffic, the
   NEXT_HOP attribute MUST be passed unmodified to the route server
   clients, similar to the "third party" next hop feature described in

4.2.2.  AS_PATH Attribute

   AS_PATH is a mandatory transitive attribute which identifies the
   autonomous systems through which routing information carried in the
   UPDATE message has passed.

   As a route server does not participate in the process of forwarding

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   data between client routers, and because modification of the AS_PATH
   attribute could affect route server client best-path calculations,
   the route server SHOULD NOT either prepend its own AS number to the
   AS_PATH segment or modify the AS_PATH segment in any other way.

4.2.3.  MULTI_EXIT_DISC Attribute

   The multi-exit discriminator is an optional non-transitive attribute
   intended to be used on external (inter-AS) links to discriminate
   among multiple exit or entry points to the same neighboring AS.  If
   applied to an NLRI UPDATE sent to a route server, the attribute
   SHOULD be treated as a transitive attribute (contrary to section
   5.1.4 of [RFC4271]) and the route server SHOULD NOT modify the value
   of this attribute.

4.2.4.  BGP Community Attributes

   The BGP COMMUNITIES and Extended Communities attributes are optional
   transitive attributes intended for labeling information carried in
   BGP UPDATE messages.  If applied to an NLRI UPDATE sent to a route
   server, these attributes SHOULD NOT not generally be modified or
   removed, except in the case where the attributes are intended for
   processing by the route server itself.

4.3.  Per-Client Prefix Filtering

4.3.1.  Prefix Hiding on a Route Server

   While IXP participants often use route servers with the intention of
   interconnecting with as many other route server participants as
   possible, there are several circumstances where control of prefix
   distribution on a per-client basis is important for ensuring that the
   desired interconnection policy is met.

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                                AS1      AS2
                                ***      ***
                           IXP *   *    *   *
                           +---*   *----*   *---+
                           |   *   *    *   *   |
                           |    ***\    /***    |
                           |        \  /  |     |
                           |         \/   |     |
                           |         /\   |     |
                           |        /  \  |     |
                           |    ***/____\***    |
                           |   *   *    *   *   |
                           +---*   *----*   *---+
                               *   *    *   *
                                ***      ***
                                AS3      AS4

               Figure 3: Filtered Interconnection at an IXP

   Using the example in Figure 3, AS1 does not directly exchange prefix
   information with either AS2 or AS3 at the IXP, but only interconnects
   with AS4.

   In the traditional bilateral interconnection model, prefix filtering
   to a third party exchange participant is accomplished either by not
   engaging in a bilateral interconnection with that participant or else
   by implementing outbound prefix filtering on the BGP session towards
   that participant.  However, in a multilateral interconnection
   environment, only the route server can perform outbound prefix
   filtering in the direction of the route server client; other route
   server clients do not have direct control over what prefix filters
   the route server employs towards any particular client.

   If the same prefix is sent to a route server from multiple route
   server clients with different BGP attributes, and traditional best-
   path route selection is performed on that list of prefixes, then the
   route server will select a single best-path prefix for propagation to
   all connected clients.  If, however, the route server has been
   configured to filter the calculated best-path prefix from reaching a
   particular route server client, then that client will receive no
   reachability information for that prefix from the route server,
   despite the fact that the route server has received alternative
   reachability information for that prefix from other route server
   clients.  This phenomenon is referred to as "prefix hiding".

   For example, in Figure 3, if the same prefix were sent to the route
   server via AS2 and AS4, and the route via AS2 was preferred according
   to BGP's traditional best-path selection, but AS2 was filtered by

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   AS1, then AS1 would never receive this prefix, even though the route
   server had previously received a valid alternative path via AS4.
   This happens because the best-path selection is performed only once
   on the route server for all clients.

   It is noted that prefix hiding will only occur on route servers which
   employ per-client prefix filtering; if an IXP operator deploys a
   route server without prefix filtering, then prefix hiding does not
   occur, as all paths are considered equally valid from the point of
   view of the route server.

   There are several techniques which may be employed to prevent the
   prefix hiding problem from occurring.  Route server implementations
   SHOULD implement at least one one method to prevent prefix hiding.

4.3.2.  Mitigation Techniques  Multiple Route Server RIBs

   The most portable means of preventing the route server prefix hiding
   problem is by using a route server BGP implementation which performs
   the per-client best-path calculation for each set of prefixes which
   results after the route server's client filtering policies have been
   taken into consideration.  This can be implemented by using per-
   client Loc-RIBs, with prefix filtering implemented between the Adj-
   RIB-In and the per-client Loc-RIB.  Live implementations will usually
   optimise this by maintaining prefixes not subject to filtering
   policies in a global Loc-RIB, with per-client Loc-RIBs stored as

   This problem mitigation technique is highly portable, as it makes no
   assumptions about the feature capabilities of the route server
   clients.  Advertising Multiple Paths

   The prefix distribution model described above assumes standard BGP
   session encoding where the route server sends a single path to its
   client for any given prefix.  This path is selected using the BGP
   path selection decision process described in [RFC4271].  If, however,
   it were possible for the route server to send more than a single path
   to a route server client, then this would alleviate the requirement
   for route server clients to depend on receiving a single best path to
   a particular prefix; consequently the prefix hiding problem described
   in Section 4.3.1 would disappear.

   This document discusses two methods which describe how such increased
   path diversity could be implemented.

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Internet-Draft             IX BGP Route Server                 July 2010  Diverse BGP Path Approach

   The Diverse BGP Path proposal as defined in
   [I-D.ietf-grow-diverse-bgp-path-dist] is a simple way to distribute
   multiple prefix paths from a route server to a route server client by
   using a separate BGP session between the route server and the route
   server client for each different path.

   The number of paths which may be distributed to a client is
   constrained by the number of BGP sessions which the route server and
   the route server client are willing to establish with each other.
   The distributed paths may be established both from the global BGP
   Loc-RIB on the route server in addition to any per-client Loc-RIB.
   As there may be more potential paths to a given prefix than
   configured BGP sessions, this method is not guaranteed to eliminate
   the prefix hiding problem in all situations.  Add-Paths Approach

   The [I-D.ietf-idr-add-paths] Internet draft proposes a different
   approach to multiple path propagation, by allowing a BGP speaker to
   forward multiple paths for the same prefix on a single BGP session.
   As [RFC4271] specifies that a BGP listener must implement an implicit
   withdraw when it receives an UPDATE message for a prefix which
   already exists in its Adj-RIB-In, this approach requires explicit
   support for the feature both on the route server and on its clients.
   Furthermore, if the add-paths capability is negotiated
   bidirectionally between the route server and a route server client,
   and the route server client propagates multiple paths for the same
   prefix to the route server, then this could potentially cause the
   propagation of inactive, invalid or suboptimal paths to the route
   server, thereby causing loss of reachability to other route server

5.  Operational Considerations for Route Server Installations

5.1.  Route Server Scaling

   While deployment of multiple Loc-RIBs on the route server presents a
   simple way of avoid the prefix hiding problem noted in Section 4.3.1,
   this approach requires significantly more computing resources on the
   route server than where a single Loc-RIB is deployed for all clients.
   As the [RFC4271] Decision Process must be applied to all Loc-RIBs
   deployed on the route server, both CPU and memory requirements on the
   host computer scale approximately according to O(P * N), where P is
   the total number of unique prefixes received by the route server and
   N is the number of route server clients which require a unique Loc-

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   RIB.  As this is a super-linear scaling relationship, large route
   server deployments may derive benefit from only deploying per-client
   Loc-RIBs where they are required.

   Regardless of any Loc-RIB optimization implemented, the route
   server's network bandwidth requirements will continue to bounded
   above by a relationship of order O(P * N), where P is the total
   number of unique prefixes received by the route server and N is the
   total number of route server clients.  In the case where P_avg, the
   mean number of unique prefixes received per route server client,
   remains roughly constant according as the number of connected
   clients, this relationship can be rewritten as O((P_avg * N) * N) or
   O(N^2).  This polynomial upper bound on the network traffic
   requirements indicates that the route server model will not
   functionally scale to arbitrarily large sizes.

5.2.  NLRI Leakage Mitigation

   NLRI leakage occurs when a BGP client unintentionally distributes
   NLRI UPDATE messages to one or more neighboring BGP routers.  NLRI
   leakage of this form to a route server can cause connectivity
   problems at an IXP if each route server client is configured to
   accept all prefix UPDATE messages from the route server.  It is
   therefore RECOMMENDED when deploying route servers that, due to the
   potential for collateral damage caused by NLRI leakage, route server
   operators deploy NLRI leakage mitigation measures in order to prevent
   unintentional prefix announcements or else limit the scale of any
   such leak.  Although not foolproof, per-client inbound prefix limits
   can restrict the damage caused by prefix leakage in many cases.  Per-
   client inbound prefix filtering on the route server is a more
   deterministic and usually more reliable means of preventing prefix
   leakage, but requires more administrative resources to maintain

5.3.  Route Server Redundancy

   As the purpose of an IXP route server implementation is to provide a
   reliable reachability brokerage service, it is RECOMMENDED that
   exchange operators who implement route server systems provision
   multiple route servers on each shared Layer-2 domain.  There is no
   requirement to use the same BGP implementation for each route server
   on the IXP fabric; however, it is RECOMMENDED that where an operators
   provisions more than a single server on the same shared Layer-2
   domain, each route server implementation be configured equivalently
   and in such a manner that the path reachability information from each
   system is identical.

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6.  Security Considerations

   On route server installations which do not employ prefix-hiding
   mitigation techniques, the prefix hiding problem outlined in section
   Section 4.3.1 can be used in certain circumstances to proactively
   block third party prefix announcements from other route server

7.  IANA Considerations

   The new set of mechanism for route servers does not require any new
   allocations from IANA.

8.  Acknowledgments

   The authors would like to thank Niels Bakker and Ryan Bickhart for
   their valuable input.

   In addition, the authors would like to acknowledge the developers of
   BIRD, OpenBGPD and Quagga, whose open source BGP implementations
   include route server capabilities which are compliant with this

9.  References

9.1.  Normative References

              Raszuk, R., Fernando, R., Patel, K., McPherson, D., and K.
              Kumaki, "Distribution of diverse BGP paths.",
              draft-ietf-grow-diverse-bgp-path-dist-01 (work in
              progress), June 2010.

              Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP",
              draft-ietf-idr-add-paths-03 (work in progress),
              February 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2283]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 2283,
              February 1998.

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   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

9.2.  Informative References

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              January 2007.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065, August 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

Authors' Addresses

   Elisa Jasinska
   AMS-IX B.V.
   Westeinde 12
   Amsterdam, NH  1017 ZN

   Email: elisa.jasinska@ams-ix.net

   Nick Hilliard
   4027 Kingswood Road
   Dublin  24

   Email: nick@inex.ie

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   Robert Raszuk
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134

   Email: raszuk@cisco.com

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