GROW Working Group                                           N. Hilliard
Internet-Draft                                                      INEX
Intended status: Informational                               E. Jasinska
Expires: December 09, 2013                         Microsoft Corporation
                                                               R. Raszuk
                                                                  NTT I3
                                                               N. Bakker
                                                             AMS-IX B.V.
                                                           June 07, 2013

               Internet Exchange Route Server Operations


   The popularity of Internet exchange points (IXPs) brings new
   challenges to interconnecting networks.  While bilateral eBGP
   sessions between exchange participants were historically the most
   common means of exchanging reachability information over an IXP, the
   overhead associated with this interconnection method causes serious
   operational and administrative scaling problems for IXP participants.

   Multilateral interconnection using Internet route servers can
   dramatically reduce the administrative and operational overhead of
   IXP participation and these systems used by many IXP participants as
   a preferred means of exchanging routing information.

   This document describes operational considerations for multilateral
   interconnections at IXPs.

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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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 09, 2013.

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Copyright Notice

   Copyright (c) 2013 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
   ( in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Bilateral BGP Sessions  . . . . . . . . . . . . . . . . . . .   3
   3.  Multilateral Interconnection  . . . . . . . . . . . . . . . .   4
   4.  Operational Considerations for Route Server Installations . .   5
     4.1.  Path Hiding . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Route Server Scaling  . . . . . . . . . . . . . . . . . .   6
       4.2.1.  Tackling Scaling Issues . . . . . . . . . . . . . . .   6  View Merging and Decomposition  . . . . . . . . .   6  Destination Splitting . . . . . . . . . . . . . .   7  NEXT_HOP Resolution . . . . . . . . . . . . . . .   8
     4.3.  Prefix Leakage Mitigation . . . . . . . . . . . . . . . .   8
     4.4.  Route Server Redundancy . . . . . . . . . . . . . . . . .   8
     4.5.  AS_PATH Consistency Check . . . . . . . . . . . . . . . .   9
     4.6.  Export Routing Policies . . . . . . . . . . . . . . . . .   9
       4.6.1.  BGP Communities . . . . . . . . . . . . . . . . . . .   9
       4.6.2.  Internet Routing Registry . . . . . . . . . . . . . .   9
       4.6.3.  Client-accessible Databases . . . . . . . . . . . . .  10
     4.7.  Layer 2 Reachability Problems . . . . . . . . . . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Internet exchange points (IXPs) provide IP data interconnection
   facilities for their participants, typically using shared Layer-2

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   networking media such as Ethernet.  The Border Gateway Protocol (BGP)
   [RFC4271] is normally used to facilitate exchange of network
   reachability information over these media.

   As bilateral interconnection between IXP participants requires
   operational and administrative overhead, BGP route servers
   [I-D.ietf-idr-ix-bgp-route-server] are often deployed by IXP
   operators to provide a simple and convenient means of interconnecting
   IXP participants with each other.  A route server redistributes
   prefixes received from its BGP clients to other clients according to
   a pre-specified policy, and it can be viewed as similar to an eBGP
   equivalent of an iBGP [RFC4456] route reflector.

   Route servers at IXPs require careful management and it is important
   for route server operators to thoroughly understand both how they
   work and what their limitations are.  In this document, we discuss
   several issues of operational relevance to route server operators and
   provide recommendations to help route server operators provision a
   reliable interconnection service.

1.1.  Notational Conventions

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

2.  Bilateral BGP Sessions

   Bilateral interconnection is a method of interconnecting routers
   using individual BGP sessions between each participant router on an
   IXP, in order to exchange reachability information.  If an IXP
   participant wishes to implement an open interconnection policy - i.e.
   a policy of interconnecting with as many other IXP participants as
   possible - it is necessary for the participant to liaise with each of
   their intended interconnection partners.  Interconnection can then be
   implemented bilaterally by configuring a BGP session on both
   participants' routers to exchange network reachability information.
   If each exchange participant interconnects with each other
   participant, a full mesh of BGP sessions is needed, as shown in
   Figure 1.

        ___      ___
       /   \    /   \
    ..| AS1 |..| AS2 |..
   :   \___/____\___/   :
   :     | \    / |     :
   :     |  \  /  |     :

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   : 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 of them
   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.

   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 large numbers of participating networks, the amount of
   administrative and operation overhead required to implement an open
   interconnection scales quadratically.  New participants to an IXP
   require significant initial resourcing in order to gain value from
   their IXP connection, while existing exchange participants need to
   commit ongoing resources in order to benefit from interconnecting
   with these new participants.

3.  Multilateral Interconnection

   Multilateral interconnection is implemented using a route server
   configured to use BGP to distribute network layer reachability
   information (NLRI) among all client routers.  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, as
   described in [I-D.ietf-idr-ix-bgp-route-server].  Using this method
   of exchanging NLRI messages, 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 BGP
   sessions with each other router at the exchange.  This reduces the
   overall number of BGP sessions at an Internet exchange from n*(n-1)/2
   to n, where n is the number of routers at the exchange.

   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.

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

4.  Operational Considerations for Route Server Installations

4.1.  Path Hiding

   "Path hiding" is a term used in [I-D.ietf-idr-ix-bgp-route-server] to
   describe the process whereby a route server may mask individual paths
   by applying conflicting routing policies to its Loc-RIB.  When this
   happens, route server clients receive incomplete information from the
   route server about network reachability.

   There are several approaches which may be used to mitigate against
   the effect of path hiding; these are described in
   [I-D.ietf-idr-ix-bgp-route-server].  However, the only method which
   does not require explicit support from the route server client is for
   the route server itself to maintain a individual Loc-RIB for each
   client which is the subject of conflicting routing policies.

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4.2.  Route Server Scaling

   While deployment of multiple Loc-RIBs on the route server presents a
   simple way to avoid the path hiding problem noted in Section 4.1,
   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] BGP 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 paths received by the route server and N
   is the number of route server clients which require a unique Loc-RIB.
   As this is a super-linear scaling relationship, large route servers
   may derive benefit from deploying per-client Loc-RIBs only where they
   are required.

   Regardless of any Loc-RIB optimization technique is implemented, the
   route server's control plane bandwidth requirements will scale
   according to O(P * N), where P is the total number of unique paths
   received by the route server and N is the total number of route
   server clients.  In the case where P_avg (the arithmetic mean number
   of unique paths received per route server client) remains roughly
   constant even as the number of connected clients increases, this
   relationship can be rewritten as O((P_avg * N) * N) or O(N^2).  This
   quadratic upper bound on the network traffic requirements indicates
   that the route server model will not scale to arbitrarily large

   This scaling analysis presents problems in three key areas: route
   processor CPU overhead associated with BGP decision process
   calculations, the memory requirements for handling many different BGP
   path entries, and the network traffic bandwidth required to
   distribute these prefixes from the route server to each route server

4.2.1.  Tackling Scaling Issues

   The network traffic scaling issue presents significant difficulties
   with no clear solution - ultimately, each client must receive a
   UPDATE for each unique prefix received by the route server.  However,
   there are several potential methods for dealing with the CPU and
   memory resource requirements of route servers.  View Merging and Decomposition

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   View merging and decomposition, outlined in [RS-ARCH], describes a
   method of optimising memory and CPU requirements where multiple route
   server clients are subject to exactly the same routing policies.  In
   this situation, the multiple Loc-RIB views required by each client
   are merged into a single view.

   There are several variations of this approach.  If the route server
   operator has prior knowledge of interconnection relationships between
   route server clients, then the operator may configure separate Loc-
   RIBs only for route server clients with unique outbound routing
   policies.  As this approach requires prior knowledge of
   interconnection relationships, the route server operator must depend
   on each client sharing their interconnection policies, either in a
   internal provisioning database controlled by the operator, or else in
   an external data store such as an Internet Routing Registry Database.

   Conversely, the route server implementation itself may implement
   internal view decomposition by creating virtual Loc-RIBs based on a
   single in-memory master Loc-RIB, with delta differences for each
   prefix subject to different routing policies.  This allows a more
   granular and flexible approach to the problem of Loc-RIB scaling, at
   the expense of requiring a more complex in-memory Loc-RIB structure.

   Whatever method of view merging and decomposition is chosen on a
   route server, pathological edge cases can be created whereby they
   will scale no better than fully non-optimised per-client Loc-RIBs.
   However, as most route server clients connect to a route server for
   the purposes of reducing overhead, rather than implementing complex
   per-client routing policies, edge cases tend not to arise in
   practice.  Destination Splitting

   Destination splitting, also described in [RS-ARCH], describes a
   method for route server clients to connect to multiple route servers
   and to send non-overlapping sets of prefixes to each route server.
   As each route server computes the best path for its own set of
   prefixes, the quadratic scaling requirement operates on multiple
   smaller sets of prefixes.  This reduces the overall computational and
   memory requirements for managing multiple Loc-RIBs and performing the
   best-path calculation on each.  In order for this method to perform
   well, destination splitting would require significant co-ordination
   between the route server operator and each route server client.  In
   practice, this level of close co-ordination between IXP operators and
   their participants tends not to occur, suggesting that the approach
   is unlikely to be of any real use on production IXPs.

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Internet-Draft       IX BGP Route Server Operations            June 2013  NEXT_HOP Resolution

   As route servers are usually deployed at IXPs which use flat layer 2
   networks, recursive resolution of the NEXT_HOP attribute is generally
   not required, and can be replaced by a simple check to ensure that
   the NEXT_HOP value for each prefix is a network address on the IXP
   LAN's IP address range.

4.3.  Prefix Leakage Mitigation

   Prefix leakage occurs when a BGP client unintentionally distributes
   NLRI UPDATE messages to one or more neighboring BGP routers.  Prefix
   leakage of this form to a route server can cause serious 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 prefix 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 properly.

   If a route server operator implements per-client inbound prefix
   filtering, then it is RECOMMENDED that the operator also builds in
   mechanisms to automatically compare the Adj-RIB-In received from each
   client with the inbound prefix lists configured for those clients.
   Naturally, it is the responsibility of the route server client to
   ensure that their stated prefix list is compatible with what they
   announce to an IXP route server.  However, many network operators do
   not carefully manage their published routing policies and it is not
   uncommon to see significant variation between the two sets of
   prefixes.  Route server operator visibility into this discrepancy can
   provide significant advantages to both operator and client.

4.4.  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 or operating system
   for each route server on the IXP fabric; however, it is RECOMMENDED
   that where an operator provisions more than a single server on the
   same shared Layer-2 domain, each route server implementation be

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   configured equivalently and in such a manner that the path
   reachability information from each system is identical.

4.5.  AS_PATH Consistency Check

   [RFC4271] requires that every BGP speaker which advertises a route to
   another external BGP speaker prepends its own AS number as the last
   element of the AS_PATH sequence.  Therefore the leftmost AS in an
   AS_PATH attribute should be equal to the autonomous system number of
   the BGP speaker which sent the UPDATE message.

   As [I-D.ietf-idr-ix-bgp-route-server] suggests that route servers
   should not modify the AS_PATH attribute, a consistency check on the
   AS_PATH of an UPDATE received by a route server client would normally
   fail.  It is therefore RECOMMENDED that route server clients disable
   the AS_PATH consistency check towards the route server.

4.6.  Export Routing Policies

   Policy filtering is commonly implemented on route servers to provide
   prefix distribution control mechanisms for route server clients.  A
   route server "export" policy is a policy which affects prefixes sent
   from the route server to a route server client.  Several different
   strategies are commonly used for implementing route server export

4.6.1.  BGP Communities

   Prefixes sent to the route server are tagged with specific [RFC1997]
   or [RFC4360] BGP community attributes, based on pre-defined values
   agreed between the operator and all client.  Based on these community
   tags, prefixes may be propagated to all other clients, a subset of
   clients, or none.  This mechanism allows route server clients to
   instruct the route server to implement per-client export routing

   As both standard and extended BGP communities values are restricted
   to 6 octets, the route server operator should take care to ensure
   that the predefined BGP community values mechanism used on their
   route server is compatible with [RFC4893] 4-octet autonomous system

4.6.2.  Internet Routing Registry

   Internet Routing Registry databases (IRRDBs) may be used by route
   server operators to implement construct per-client routing policies.
   [RFC2622] Routing Policy Specification Language (RPSL) provides an
   comprehensive grammar for describing interconnection relationships,

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   and several toolsets exist which can be used to translate RPSL policy
   description into route server configurations.

4.6.3.  Client-accessible Databases

   Should the route server operator not wish to use either BGP community
   tags or the public IRRDBs for implementing client export policies,
   they may implement their own routing policy database system for
   managing their clients' requirements.  A database of this form SHOULD
   allow a route server client operator to update their routing policy
   and provide a mechanism for allowing the client to specify whether
   they wish to exchange all their prefixes with any other route server
   client.  Optionally, the implementation may allow a client to specify
   unique routing policies for individual prefixes over which they have
   routing policy control.

4.7.  Layer 2 Reachability Problems

   Layer 2 reachability problems on an IXP can cause serious operational
   problems for IXP participants which depend on route servers for
   interconnection.  Ethernet switch forwarding bugs have occasionally
   been observed to cause non-commutative reachability.  For example,
   given a route server and two IXP participants, A and B, if the two
   participants can reach the route server but cannot reach each other,
   then traffic between the participants may be dropped until such time
   as the layer 2 forwarding problem is resolved.  This situation does
   not tend to occur in bilateral interconnection arrangements, as the
   routing control path between the two hosts is usually (but not
   always, due to IXP inter-switch connectivity load balancing
   algorithms) the same as the data path between them.

   Problems of this form can be dealt with using [RFC5881] bidirectional
   forwarding detection.  However, as this is a bilateral protocol
   configured between routers, and as there is currently no means for
   automatic configuration of BFD between route server clients, BFD does
   not currently provide an optimal means of handling the problem.

5.  Security Considerations

   On route server installations which do not employ path hiding
   mitigation techniques, the path hiding problem outlined in section
   Section 4.1 can be used in certain circumstances to proactively block
   third party prefix announcements from other route server clients.

6.  IANA Considerations

   There are no IANA considerations.

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7.  Acknowledgments

   The authors would like to thank Chris Hall, Ryan Bickhart and Steven
   Bakker 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

8.  References

8.1.  Normative References

              Jasinska, E., Hilliard, N., Raszuk, R., and N. Bakker,
              "Internet Exchange Route Server", draft-ietf-idr-ix-bgp-
              route-server-02 (work in progress), February 2013.

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

8.2.  Informative References

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

   [RFC2622]  Alaettinoglu, C., Villamizar, C., Gerich, E., Kessens, D.,
              Meyer, D., Bates, T., Karrenberg, D., and M. Terpstra,
              "Routing Policy Specification Language (RPSL)", RFC 2622,
              June 1999.

   [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.

   [RFC4893]  Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
              Number Space", RFC 4893, May 2007.

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

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   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, June

   [RS-ARCH]  Govindan, R., Alaettinoglu, C., Varadhan, K., and D.
              Estrin, "A Route Server Architecture for Inter-Domain
              Routing", 1995,

Authors' Addresses

   Nick Hilliard
   4027 Kingswood Road
   Dublin  24


   Elisa Jasinska
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052


   Robert Raszuk
   NTT I3
   101 S Ellsworth Avenue Suite 350
   San Mateo, CA  94401


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


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