draft-hilliard-ix-bgp-route-server-operations-01.xml

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<rfc category="info"
     docName="draft-hilliard-ix-bgp-route-server-operations-01"
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  <!-- ***** FRONT MATTER ***** -->

  <front>

    <title abbrev="IX BGP Route Server Operations">
      Internet Exchange Route Server Operations
    </title>

    <author initials="N" surname="Hilliard" fullname="Nick Hilliard">
      <organization>INEX</organization>
      <address>
        <postal>
          <street>4027 Kingswood Road</street>
          <city>Dublin</city> 
          <code>24</code>
          <country>IE</country>
        </postal>
        <email>nick@inex.ie</email>
      </address>
    </author>

    <author initials="E" surname="Jasinska" fullname="Elisa Jasinska">
      <organization>Limelight Networks</organization>
      <address>
        <postal>
          <street>222 South Mill Avenue</street>
          <city>Tempe</city> 
          <region>AZ</region>
          <code>85281</code>
          <country>US</country>
        </postal>
        <email>elisa@llnw.com</email>
      </address>
    </author>

    <author initials="R" surname="Raszuk" fullname="Robert Raszuk">
      <organization>NTT MCL Inc.</organization>
      <address>
        <postal>
        <street>101 S Ellsworth Avenue Suite 350</street>
        <city>San Mateo</city>
        <region>CA</region>
        <code>94401</code>
        <country>US</country>
        </postal>
        <email>robert@raszuk.net</email>
      </address>
    </author>

    <author initials="N" surname="Bakker" fullname="Niels Bakker">
      <organization>AMS-IX B.V.</organization>
      <address>
        <postal>
          <street>Westeinde 12</street>
          <city>Amsterdam</city> 
          <region>NH</region>
          <code>1017 ZN</code>
          <country>NL</country>
        </postal>
        <email>niels.bakker@ams-ix.net</email>
      </address>
    </author>

    <date month="April" year="2012" />
    <area>Routing</area>
    <workgroup>GROW Working Group</workgroup>
    <keyword>I-D</keyword>
    <keyword>Internet-Draft</keyword>
    <keyword>GROW</keyword>

    <abstract>
      <t>
        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.
      </t>
      <t>
        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.
      </t> 
      <t>
        This document describes operational considerations for multilateral
        interconnections at IXPs.
      </t>
    </abstract>

  </front>

  <middle>

     <section title="Introduction">
      <t>
        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)
        <xref target="RFC4271" /> is normally used to facilitate exchange of
        network reachability information over these media.
      </t>
      <t>
        As bilateral interconnection between IXP participants requires
        operational and administrative overhead, BGP route servers <xref
        target="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 <xref target="RFC4456" /> route reflector.
      </t>
      <t>
        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.
      </t>

      <section title="Notational Conventions">
        <t>
          The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
          "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
          "OPTIONAL" in this document are to be interpreted as described in
          <xref target="RFC2119" />.
        </t>
      </section>

    </section>

    <section title="Bilateral BGP Sessions">
      <t>
        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 <xref target="ixp_interconnection"
        />.
      </t>
      <figure title="Full-Mesh Interconnection at an IXP" anchor="ixp_interconnection">
        <preamble></preamble>
        <artwork align="center">
     ___      ___
    /   \    /   \
 ..| AS1 |..| AS2 |..
:   \___/____\___/   :
:     | \    / |     :
:     |  \  /  |     :
: IXP |   \/   |     :
:     |   /\   |     :
:     |  /  \  |     :
:    _|_/____\_|_    :
:   /   \    /   \   :
 ..| AS3 |..| AS4 |..
    \___/    \___/
        </artwork>
        <postamble></postamble>
      </figure>
      <t>
        <xref target="ixp_interconnection" /> 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.
      </t>
      <t>
        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.
      </t>
    </section>


    <section title="Multilateral Interconnection">
      <t>
        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 <xref target="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.
      </t>
      <t>
        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.
      </t>
      <t>
        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.
       </t>

      <figure title="IXP-based Interconnection with Route Server" anchor="rs_interconnection">
        <preamble></preamble>
        <artwork align="center">
     ___      ___
    /   \    /   \
 ..| AS1 |..| AS2 |..
:   \___/    \___/   :
:      \      /      :
:       \    /       :
:        \__/        :
: IXP   /    \       :
:      |  RS  |      :
:       \____/       :
:        /  \        :
:       /    \       :
:    __/      \__    :
:   /   \    /   \   :
 ..| AS3 |..| AS4 |..
    \___/    \___/
        </artwork>
        <postamble></postamble>
      </figure>
      <t>
        As illustrated in <xref target="rs_interconnection" />, 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.
      </t>
    </section>


    <section title="Operational Considerations for Route Server Installations" anchor="ops_considerations">

      <section title="Path Hiding" anchor="path_hiding">
        <t>
          "Path hiding" is a term used in <xref
          target="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.
        </t>
        <t>
          There are several approaches which may be used to mitigate against
          the effect of path hiding; these are described in <xref
          target="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.
        </t>
      </section>

      <section title="Route Server Scaling" anchor="ops_scaling"> 
        <t>
          While deployment of multiple Loc-RIBs on the route server presents a
          simple way to avoid the path hiding problem noted in <xref
          target="path_hiding" />, this approach requires significantly more
          computing resources on the route server than where a single Loc-RIB
          is deployed for all clients. As the <xref target="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.
        </t>
        <t>
          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
          sizes.
        </t>
        <t>
          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
          client.
        </t>

        <section title="Tackling Scaling Issues">
          <t>
            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.
          </t>

          <section title="View Merging and Decomposition"> 
            <t>
              View merging and decomposition, outlined in <xref
              target="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.
            </t>
            <t>
              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.
            </t>
            <t>
              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.
      	    </t>
            <t>
              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. 
            </t>
          </section>

          <section title="Destination Splitting">
            <t>
              Destination splitting, also described in <xref
              target="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.
            </t>
          </section>

          <section title="NEXT_HOP Resolution">
            <t>
              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.
            </t>
          </section>
        </section>
      </section>

      <section title="Prefix Leakage Mitigation" anchor="ops_leakage_mitigation"> 
        <t>
          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.
        </t>
        <t>
          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.
        </t>
      </section>

      <section title="Route Server Redundancy" anchor="ops_use_two_of_em"> 
        <t>
          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 configured equivalently and in such a manner
          that the path reachability information from each system is
          identical.
        </t>
      </section>

      <section title="AS_PATH Consistency Check"> 
        <t>
          <xref target="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.
        </t>
        <t>
          As <xref target="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.
        </t>
      </section>

      <section title="Export Routing Policies"> 
        <t>
          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 policies.
        </t>

        <section title="BGP Communities">
          <t>
            Prefixes sent to the route server are tagged with specific <xref
            target="RFC1997" /> or <xref target="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
            policies.
          </t>
          <t>
            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 <xref
            target="RFC4893" /> 4-octet autonomous system numbers.
          </t>
        </section>

        <section title="Internet Routing Registry">
	      <t>
            Internet Routing Registry databases (IRRDBs) may be used by route
            server operators to implement construct per-client routing
            policies. <xref target="RFC2622" /> Routing Policy Specification
            Language (RPSL) provides an comprehensive grammar for describing
            interconnection relationships, and several toolsets exist which
            can be used to translate RPSL policy description into route server
            configurations.
          </t>
        </section>

        <section title="Client-accessible Databases">
      	  <t>
            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. 
          </t>
        </section>
      </section>

      <section title="Layer 2 Reachability Problems"> 
        <t>
          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.
        </t>
        <t>
          Problems of this form can be dealt with using <xref
          target="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.
        </t>
      </section>
    </section>

    <section title="Security Considerations">
      <t>
        On route server installations which do not employ path hiding
        mitigation techniques, the path hiding problem outlined in section
        <xref target="path_hiding" /> can be used in certain circumstances
        to proactively block third party prefix announcements from other route
        server clients.
      </t>
    </section>

    <section title="IANA Considerations">
      <t>
        There are no IANA considerations.
      </t>
    </section>


    <section title="Acknowledgments">
      <t>
        The authors would like to thank Chris Hall, Ryan Bickhart and Steven
        Bakker for their valuable input.
      </t>
      <t>
        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
        document.
      </t>
    </section>

  </middle>

  <back>
    <references title="Normative References">
      <?rfc include="reference.I-D.ietf-idr-ix-bgp-route-server"?>
      <?rfc include="reference.RFC.2119"?> <!-- key words -->
    </references>

    <references title="Informative References">
      <?rfc include="reference.RFC.1997"?> <!-- BGP Communities -->
      <?rfc include="reference.RFC.2622"?> <!-- RPSL -->
      <?rfc include="reference.RFC.4271"?> <!-- BGP-4 -->
      <?rfc include="reference.RFC.4360"?> <!-- Extended Communities -->
      <?rfc include="reference.RFC.4456"?> <!-- Route Reflector IBGP -->
      <?rfc include="reference.RFC.4893"?> <!-- BGP Support for Four-octet AS Number Space -->
      <?rfc include="reference.RFC.5226"?> <!-- IANA Considerations Section Guidelines -->
      <?rfc include="reference.RFC.5881"?> <!-- BFD -->
      <reference anchor="RS-ARCH"
                 target="http://www.cs.usc.edu/research/95-603.ps.Z">
        <front>
          <title>A Route Server Architecture for Inter-Domain Routing</title>
          <author initials="R" surname="Govindan" fullname="Ramesh Govindan">
            <organization>The University of Southern California</organization>
          </author>
          <author initials="C" surname="Alaettinoglu" fullname="Cengiz Alaettinoglu">
            <organization>The University of Southern California</organization>
          </author>
          <author initials="K" surname="Varadhan" fullname="Kannan Varadhan">
            <organization>The University of Southern California</organization>
          </author>
          <author initials="D" surname="Estrin" fullname="Deborah Estrin">
            <organization>The University of Southern California</organization>
          </author>
          <date year="1995" />
        </front>
      </reference>
    </references>
  </back>

</rfc>