IDR Working Group                                            E. Jasinska
Internet-Draft                                        Limelight Networks
Intended status: Standards Track                             N. Hilliard
Expires: September 27, 2012                                         INEX
                                                               R. Raszuk
                                                            NTT MCL Inc.
                                                               N. Bakker
                                                             AMS-IX B.V.
                                                          March 26, 2012

                     Internet Exchange Route Server


   This document outlines a specification for multilateral
   interconnections at Internet exchange points (IXPs).  Multilateral
   interconnection is a method of exchanging routing information between
   three or more exterior 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
   exchange points (IXPs), to facilitate simplified interconnection
   between multiple Internet routers.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 27, 2012.

Copyright Notice

   Copyright (c) 2012 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

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   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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 Simplified BSD License.

Table of Contents

   1.  Introduction to Multilateral Interconnection . . . . . . . . .  3
     1.1.  Specification of Requirements  . . . . . . . . . . . . . .  3
   2.  Technical Considerations for Route Server Implementations  . .  4
     2.1.  Client UPDATE Messages . . . . . . . . . . . . . . . . . .  4
     2.2.  Attribute Transparency . . . . . . . . . . . . . . . . . .  4
       2.2.1.  NEXT_HOP Attribute . . . . . . . . . . . . . . . . . .  4
       2.2.2.  AS_PATH Attribute  . . . . . . . . . . . . . . . . . .  4
       2.2.3.  MULTI_EXIT_DISC Attribute  . . . . . . . . . . . . . .  5
       2.2.4.  Communities Attributes . . . . . . . . . . . . . . . .  5
     2.3.  Per-Client Policy Control in Multilateral
           Interconnection  . . . . . . . . . . . . . . . . . . . . .  5
       2.3.1.  Path Hiding on a Route Server  . . . . . . . . . . . .  6
       2.3.2.  Implementing Per-Client Policy Control . . . . . . . .  7  Multiple Route Server RIBs . . . . . . . . . . . .  7  Advertising Multiple Paths . . . . . . . . . . . .  7
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     6.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     6.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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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.

   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 Internet exchange
   point participants.

   Multilateral interconnection is 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 IXP
   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.
   Certain adaptions to [RFC4271] are required to enable an EBGP router
   to operate as a route server, which are outlined in Section 2 of this

   The term "route server" is often in a different context used to
   describe a BGP node whose purpose is to accept BGP feeds from
   multiple clients for the purpose of operational analysis and
   troubleshooting.  A system of this form may alternatively be known as
   a "route collector" or a "route-views server".  This document uses
   the term "route server" exclusively to describe multilateral peering
   brokerage systems.

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

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2.  Technical Considerations for Route Server Implementations

2.1.  Client UPDATE Messages

   A route server MUST accept all UPDATE messages received from each of
   its clients for inclusion in its Adj-RIB-In.  These UPDATE messages
   MAY by omitted from the route server's Loc-RIB or Loc-RIBs, due to
   filters configured for the purposes of implementing routing policy.
   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.

2.2.  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. of [RFC4271], route servers SHOULD NOT by
   default (unless explicitly configured) update well-known BGP
   attributes received from route server clients before redistributing
   them to their other route server clients.  Optional recognized and
   unrecognized BGP attributes, whether transitive or non-transitive,
   SHOULD NOT be updated by the route server (unless enforced by local
   IX operator configuration) and SHOULD be passed on to other route
   server clients.

2.2.1.  NEXT_HOP Attribute

   The NEXT_HOP, a well-known mandatory BGP 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 section 5.1.3. of [RFC4271].

2.2.2.  AS_PATH Attribute

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

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   As a route server does not participate in the process of forwarding
   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 prepend its own AS number to the AS_PATH
   segment nor modify the AS_PATH segment in any other way.

2.2.3.  MULTI_EXIT_DISC Attribute

   MULTI_EXIT_DISC 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 (contrary to
   section 5.1.4 of [RFC4271]) SHOULD be propagated to other route
   server clients and the route server SHOULD NOT modify the value of
   this attribute.

2.2.4.  Communities Attributes

   The BGP COMMUNITIES ([RFC1997]) and Extended Communities ([RFC4360])
   attributes are attributes intended for labeling information carried
   in BGP UPDATE messages.  Transitive as well as non-transitive
   Communities attributes applied to an NLRI UPDATE sent to a route
   server SHOULD NOT be modified, processed or removed.  However, if
   such an attribute is intended for processing by the route server
   itself, it MAY be modified or removed.

2.3.  Per-Client Policy Control in Multilateral Interconnection

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

   The control of path distribution on a per-client basis can lead to a
   path being hidden from the route server client.  We refer to this as
   "path hiding", which is described in Section 2.3.1.

   Route server implementations SHOULD implement one of the methods
   described in Section 2.3.2, for the operator to be able to allow the
   control of path distribution on a per-client basis without the
   occurrence of "path hiding".

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2.3.1.  Path Hiding on a Route Server

                                ___      ___
                               /   \    /   \
                            ..| AS1 |..| AS2 |..
                           :   \___/    \___/   :
                           :       \    / |     :
                           :        \  /  |     :
                           : IXP     \/   |     :
                           :         /\   |     :
                           :        /  \  |     :
                           :    ___/____\_|_    :
                           :   /   \    /   \   :
                            ..| AS3 |..| AS4 |..
                               \___/    \___/

     Figure 1: Per-Client Policy Controlled Interconnection at an IXP

   Using the example in Figure 1, 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, per-client policy
   control to a third party exchange participant is accomplished either
   by not engaging in a bilateral interconnection with that participant
   or else by implementing outbound filtering on the BGP session towards
   that participant.  However, in a multilateral interconnection
   environment, only the route server can perform outbound filtering in
   the direction of the route server client; route server clients depend
   on the route server to perform their outbound filtering for them.

   Assuming a traditional best path selection, when the same prefix is
   advertised to a route server from multiple route server clients, the
   route server will select a single best path for propagation to all
   connected clients.  If, however, the route server has been configured
   to filter the calculated best path from reaching a particular route
   server client, then that client will not receive a path for that
   prefix, although alternate paths received by the route server might
   have been policy compliant for that client.  This phenomenon is
   referred to as "path hiding".

   For example, in Figure 1, 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 AS1's policy prevents
   AS2's path from being accepted, then AS1 would never receive a path
   to this prefix, even though the route server had previously received
   a valid alternative path via AS4.  This happens because the best path

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   selection is performed only once on the route server for all clients.

   Path hiding will only occur on route servers which employ per-client
   policy control; if an IXP operator deploys a route server without the
   possibility for policy control, then path hiding does not occur, as
   all paths are considered equally valid from the point of view of the
   route server.

2.3.2.  Implementing Per-Client Policy Control

   In this section, we describe the alternatives to provide per-client
   policy control while preventing the occurrence of path hiding.  Multiple Route Server RIBs

   The most portable means to allow for per-client policy control
   without the occurrence of path hiding, is by using a route server BGP
   implementation which performs the per-client best path calculation
   for each set of paths to a prefix, which results after the route
   server's client policies have been taken into consideration.  This
   can be implemented by using per-client Loc-RIBs, with path filtering
   implemented between the Adj-RIB-In and the per-client Loc-RIB.
   Implementations MAY optimize this by maintaining paths not subject to
   filtering policies in a global Loc-RIB, with per-client Loc-RIBs
   stored as deltas.

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

   The path 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 route server clients would no longer
   depend on receiving a single best path to a particular prefix;
   consequently, the path hiding problem described in Section 2.3.1
   would disappear.

   We present two methods which describe how such increased path
   diversity could be implemented.  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

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   multiple prefix paths from a route server to a route server client by
   using a separate BGP session from the route server to a 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 server and the
   client are willing to establish with each other.  The distributed
   paths may be established 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 path hiding problem in all
   situations.  Furthermore, this method may significantly increase the
   number of BGP sessions handled by the route server, which may
   negatively impact its performance.  BGP ADD-PATH 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.

   If the ADD-PATH 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 clients.  For this reason, ADD-
   PATH implementations on a route server SHOULD enforce send-only mode
   with the route server clients, which would result in negotiating
   receive-only mode from the client to the route server.

3.  Security Considerations

   The path hiding problem outlined in section Section 2.3.1 can be used
   in certain circumstances to proactively block third party path
   announcements from other route server clients.

4.  IANA Considerations

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

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5.  Acknowledgments

   The authors would like to thank Ryan Bickhart, Steven Bakker, Chris
   Hall, Bruno Decraene and Pierre Francois 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

6.  References

6.1.  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-06 (work in
              progress), November 2011.

              Walton, D., Chen, E., Retana, A., and J. Scudder,
              "Advertisement of Multiple Paths in BGP",
              draft-ietf-idr-add-paths-06 (work in progress),
              September 2011.

   [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
              Communities Attribute", RFC 1997, August 1996.

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

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

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, February 2006.

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

6.2.  Informative References

   [RFC1863]  Haskin, D., "A BGP/IDRP Route Server alternative to a full
              mesh routing", RFC 1863, October 1995.

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   [RFC4223]  Savola, P., "Reclassification of RFC 1863 to Historic",
              RFC 4223, October 2005.

   [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
   Limelight Networks
   2220 W 14th St
   Tempe, AZ  85281


   Nick Hilliard
   4027 Kingswood Road
   Dublin  24


   Robert Raszuk
   NTT MCL Inc.
   101 S Ellsworth Avenue Suite 350
   San Mateo, CA  94401


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   Niels Bakker
   AMS-IX B.V.
   Westeinde 12
   Amsterdam, NH  1017 ZN


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