<pre>Internet Engineering Task Force (IETF)                         V. Fuller
Request for Comments: 6836
Category: Experimental                                      D. Farinacci
ISSN: 2070-1721                                                 D. Meyer
                                                                D. Lewis
                                                           Cisco Systems
                                                            January 2013


 <span class="h1">Locator/ID Separation Protocol Alternative Logical Topology (LISP+ALT)</span>

Abstract

   This document describes a simple distributed index system to be used
   by a Locator/ID Separation Protocol (LISP) Ingress Tunnel Router
   (ITR) or Map-Resolver (MR) to find the Egress Tunnel Router (ETR)
   that holds the mapping information for a particular Endpoint
   Identifier (EID).  The MR can then query that ETR to obtain the
   actual mapping information, which consists of a list of Routing
   Locators (RLOCs) for the EID.  Termed the Alternative Logical
   Topology (ALT), the index is built as an overlay network on the
   public Internet using the Border Gateway Protocol (BGP) and Generic
   Routing Encapsulation (GRE).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see <a href="./rfc5741#section-2">Section&nbsp;2 of RFC 5741</a>.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   <a href="http://www.rfc-editor.org/info/rfc6836">http://www.rfc-editor.org/info/rfc6836</a>.










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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.





































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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
   <a href="#section-2">2</a>. Definition of Terms .............................................<a href="#page-5">5</a>
   <a href="#section-3">3</a>. The LISP-ALT Model ..............................................<a href="#page-8">8</a>
      <a href="#section-3.1">3.1</a>. Routability of EIDs ........................................<a href="#page-8">8</a>
           <a href="#section-3.1.1">3.1.1</a>. Mechanisms for an ETR to Originate EID-Prefixes .....<a href="#page-9">9</a>
           <a href="#section-3.1.2">3.1.2</a>. Mechanisms for an ITR to Forward to EID-Prefixes ....<a href="#page-9">9</a>
           <a href="#section-3.1.3">3.1.3</a>. Map-Server Model Preferred ..........................<a href="#page-9">9</a>
      <a href="#section-3.2">3.2</a>. Connectivity to Non-LISP Sites ............................<a href="#page-10">10</a>
      <a href="#section-3.3">3.3</a>. Caveats on the Use of Data-Probes .........................<a href="#page-10">10</a>
   <a href="#section-4">4</a>. LISP+ALT: Overview .............................................<a href="#page-10">10</a>
      <a href="#section-4.1">4.1</a>. ITR Traffic Handling ......................................<a href="#page-11">11</a>
      <a href="#section-4.2">4.2</a>. EID Assignment - Hierarchy and Topology ...................<a href="#page-12">12</a>
      <a href="#section-4.3">4.3</a>. Use of GRE and BGP between LISP-ALT Routers ...............<a href="#page-14">14</a>
   <a href="#section-5">5</a>. EID-Prefix Propagation and Map-Request Forwarding ..............<a href="#page-14">14</a>
      <a href="#section-5.1">5.1</a>. Changes to ITR Behavior with LISP+ALT .....................<a href="#page-15">15</a>
      <a href="#section-5.2">5.2</a>. Changes to ETR Behavior with LISP+ALT .....................<a href="#page-15">15</a>
      <a href="#section-5.3">5.3</a>. ALT Datagram Forwarding Failure ...........................<a href="#page-16">16</a>
   <a href="#section-6">6</a>. BGP Configuration and Protocol Considerations ..................<a href="#page-16">16</a>
      <a href="#section-6.1">6.1</a>. Autonomous System Numbers (ASNs) in LISP+ALT ..............<a href="#page-16">16</a>
      <a href="#section-6.2">6.2</a>. Subsequent Address Family Identifier (SAFI) for LISP+ALT ..17
   <a href="#section-7">7</a>. EID-Prefix Aggregation .........................................<a href="#page-17">17</a>
      <a href="#section-7.1">7.1</a>. Stability of the ALT ......................................<a href="#page-18">18</a>
      <a href="#section-7.2">7.2</a>. Traffic Engineering Using LISP ............................<a href="#page-18">18</a>
      <a href="#section-7.3">7.3</a>. Edge Aggregation and Dampening ............................<a href="#page-19">19</a>
      <a href="#section-7.4">7.4</a>. EID Assignment Flexibility vs. ALT Scaling ................<a href="#page-19">19</a>
   <a href="#section-8">8</a>. Connecting Sites to the ALT Network ............................<a href="#page-20">20</a>
      <a href="#section-8.1">8.1</a>. ETRs Originating Information into the ALT .................<a href="#page-20">20</a>
      <a href="#section-8.2">8.2</a>. ITRs Using the ALT ........................................<a href="#page-21">21</a>
   <a href="#section-9">9</a>. Security Considerations ........................................<a href="#page-22">22</a>
      <a href="#section-9.1">9.1</a>. Apparent LISP+ALT Vulnerabilities .........................<a href="#page-22">22</a>
      <a href="#section-9.2">9.2</a>. Survey of LISP+ALT Security Mechanisms ....................<a href="#page-23">23</a>
      <a href="#section-9.3">9.3</a>. Use of Additional BGP Security Mechanisms .................<a href="#page-24">24</a>
   <a href="#section-10">10</a>. Acknowledgments ...............................................<a href="#page-24">24</a>
   <a href="#section-11">11</a>. References ....................................................<a href="#page-24">24</a>
      <a href="#section-11.1">11.1</a>. Normative References .....................................<a href="#page-24">24</a>
      <a href="#section-11.2">11.2</a>. Informative References ...................................<a href="#page-25">25</a>

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   This document describes the LISP+ALT system, used by an [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>]
   Ingress Tunnel Router (ITR) or MR to find the Egress Tunnel Router
   (ETR) that holds the RLOC mapping information for a particular
   Endpoint Identifier (EID).  The ALT network is built using the Border
   Gateway Protocol (BGP) [<a href="./rfc4271" title="&quot;A Border Gateway Protocol 4 (BGP-4)&quot;">RFC4271</a>], BGP multiprotocol extensions





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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   [<a href="./rfc4760" title="&quot;Multiprotocol Extensions for BGP-4&quot;">RFC4760</a>], and Generic Routing Encapsulation (GRE) [<a href="./rfc2784" title="&quot;Generic Routing Encapsulation (GRE)&quot;">RFC2784</a>] to
   construct an overlay network of devices (ALT-Routers) that operate on
   EID-Prefixes and use EIDs as forwarding destinations.

   ALT-Routers advertise hierarchically delegated segments of the EID
   namespace (i.e., prefixes) toward the rest of the ALT; they also
   forward traffic destined for an EID covered by one of those prefixes
   toward the network element that is authoritative for that EID and is
   the origin of the BGP advertisement for that EID-Prefix.  An ITR uses
   this overlay to send a LISP Map-Request (defined in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>]) to the
   ETR that holds the EID-to-RLOC mapping for a matching EID-Prefix.  In
   most cases, an ITR does not connect directly to the overlay network
   but instead sends Map-Requests via a Map-Resolver (described in
   [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>]) that does.  Likewise, in most cases, an ETR does not
   connect directly to the overlay network but instead registers its
   EID-Prefixes with a Map-Server that advertises those EID-Prefixes on
   to the ALT and forwards Map-Requests for them to the ETR.

   It is important to note that the ALT does not distribute actual
   EID-to-RLOC mappings.  What it does provide is a forwarding path from
   an ITR (or MR) that requires an EID-to-RLOC mapping to an ETR that
   holds that mapping.  The ITR/MR uses this path to send an ALT
   Datagram (see <a href="#section-3">Section 3</a>) to an ETR, which then responds with a
   Map-Reply containing the needed mapping information.

   One design goal for LISP+ALT is to use existing technology wherever
   possible.  To this end, the ALT is intended to be built using
   off-the-shelf routers that already implement the required protocols
   (BGP and GRE); little, if any, LISP-specific modifications should be
   needed for such devices to be deployed on the ALT (see <a href="#section-7">Section 7</a> for
   aggregation requirements).  Note, though, that organizational and
   operational considerations suggest that ALT-Routers be both logically
   and physically separate from the "native" Internet packet transport
   system; deploying this overlay on those routers that are already
   participating in the global routing system and actively forwarding
   Internet traffic is not recommended.

   This specification is experimental, and there are areas where further
   experience is needed to understand the best implementation strategy,
   operational model, and effects on Internet operations.  These areas
   include:

   o  application effects of on-demand route map discovery

   o  tradeoff in connection setup time vs. ALT design and performance
      when using a Map Request instead of carrying initial user data in
      a Data-Probe




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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   o  best practical ways to build ALT hierarchies

   o  effects of route leakage from ALT to the current Internet,
      particularly for LISP-to-non-LISP interworking

   o  effects of exceptional situations, such as denial-of-service (DoS)
      attacks

   Experimentation, measurements, and deployment experience on these
   aspects is appreciated.  While these issues are conceptually well-
   understood (e.g., an ALT lookup causes potential delay for the first
   packet destined to a given network), the real-world operational
   effects are much less clear.

   The remainder of this document is organized as follows: <a href="#section-2">Section 2</a>
   provides the definitions of terms used in this document.  <a href="#section-3">Section 3</a>
   outlines the LISP-ALT model, where EID-Prefixes are advertised using
   BGP on an overlay network (the "ALT") and Map-Requests are forwarded
   across it.  <a href="#section-4">Section 4</a> provides a basic overview of the LISP
   Alternative Logical Topology architecture, and <a href="#section-5">Section 5</a> describes
   how the ALT uses BGP to propagate EID reachability over the overlay
   network.  <a href="#section-6">Section 6</a> describes other considerations for using BGP on
   the ALT.  <a href="#section-7">Section 7</a> describes the construction of the ALT aggregation
   hierarchy, and <a href="#section-8">Section 8</a> discusses how LISP-ALT elements are
   connected to form the overlay network.  <a href="#section-9">Section 9</a> discusses security
   considerations relevant to LISP+ALT.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Definition of Terms</span>

   This section provides high-level definitions of LISP concepts and
   components involved with and affected by LISP+ALT.

    Alternative Logical Topology (ALT):  The virtual overlay network
      made up of tunnels between LISP-ALT Routers.  The Border Gateway
      Protocol (BGP) runs between ALT-Routers and is used to carry
      reachability information for EID-Prefixes.  The ALT provides a way
      to forward Map-Requests (and, if supported, Data-Probes) toward
      the ETR that "owns" an EID-Prefix.  As a tunneled overlay, its
      performance is expected to be quite limited, so using it to
      forward high-bandwidth flows of Data-Probes is strongly
      discouraged (see <a href="#section-3.3">Section 3.3</a> for additional discussion).

    ALT-Router:  The device that runs on the ALT.  The ALT is a static
      network built using tunnels between ALT-Routers.  These routers
      are deployed in a roughly hierarchical mesh in which routers at
      each level in the topology are responsible for aggregating
      EID-Prefixes learned from those logically "below" them and
      advertising summary prefixes to those logically "above" them.



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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


      Prefix learning and propagation between ALT-Routers is done using
      BGP.  An ALT-Router at the lowest level, or "edge" of the ALT,
      learns EID-Prefixes from its "client" ETRs.  See <a href="#section-3.1">Section 3.1</a> for a
      description of how EID-Prefixes are learned at the "edge" of the
      ALT.  See also <a href="#section-6">Section 6</a> for details on how BGP is configured
      between the different network elements.  When an ALT-Router
      receives an ALT Datagram, it looks up the destination EID in its
      forwarding table (composed of EID-Prefix routes it learned from
      neighboring ALT-Routers) and forwards it to the logical next hop
      on the overlay network.

    Endpoint ID (EID):  A 32-bit (for IPv4) or 128-bit (for IPv6) value
      used to identify the ultimate source or destination for a LISP-
      encapsulated packet.  See [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] for details.

    EID-Prefix:  A set of EIDs delegated in a power-of-two block.
      Information about EID-Prefixes is exchanged among ALT-Routers (not
      on the global Internet) using BGP, and EID-Prefixes are expected
      to be assigned in a hierarchical manner such that they can be
      aggregated by ALT-Routers.  Such a block is characterized by a
      prefix and a length.  Note that while the ALT routing system
      considers an EID-Prefix to be an opaque block of EIDs, an end site
      may put site-local, topologically relevant structure (subnetting)
      into an EID-Prefix for intra-site routing.

    Aggregated EID-Prefixes:  A set of individual EID-Prefixes that have
      been aggregated in the [<a href="./rfc4632" title="&quot;Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan&quot;">RFC4632</a>] sense.

    Map-Server (MS):   An edge ALT-Router that provides a registration
      function for non-ALT-connected ETRs, originates EID-Prefixes into
      the ALT on behalf of those ETRs, and forwards Map-Requests to
      them.  See [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>] for details.

    Map-Resolver (MR):   An edge ALT-Router that accepts an Encapsulated
      Map-Request from a non-ALT-connected ITR, decapsulates it, and
      forwards it on to the ALT toward the ETR that owns the requested
      EID-Prefix.  See [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>] for details.

    Ingress Tunnel Router (ITR):   A router that sends LISP Map-Requests
      or encapsulates IP datagrams with LISP headers, as defined in
      [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].  In this document, "ITR" refers to any device
      implementing ITR functionality, including a Proxy-ITR (see
      [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>]).  Under some circumstances, a LISP Map-Resolver may
      also originate Map-Requests (see [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>]).







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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


    Egress Tunnel Router (ETR):   A router that sends LISP Map-Replies
      in response to LISP Map-Requests and decapsulates LISP-
      encapsulated IP datagrams for delivery to end-systems, as defined
      in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].  In this document, "ETR" refers to any device
      implementing ETR functionality, including a Proxy-ETR (see
      [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>]).  Under some circumstances, a LISP Map-Server may also
      respond to Map-Requests (see [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>]).

    Routing Locator (RLOC):  A routable IP address for a LISP Tunnel
      Router (ITR or ETR).  Interchangeably referred to as a "locator"
      in this document.  An RLOC is also the output of an EID-to-RLOC
      mapping lookup; an EID-Prefix maps to one or more RLOCs.
      Typically, RLOCs are numbered from topologically aggregatable
      blocks that are assigned to a site at each point where it attaches
      to the global Internet; where the topology is defined by the
      connectivity of provider networks, RLOCs can be thought of as
      Provider-Assigned (PA) addresses.  Routing for RLOCs is not
      carried on the ALT.

    EID-to-RLOC Mapping:  A binding between an EID-Prefix and the set of
      RLOCs that can be used to reach it; sometimes simply referred to
      as a "mapping".

    EID-Prefix Reachability:  An EID-Prefix is said to be "reachable" if
      at least one of its Locators is reachable.  That is, an EID-Prefix
      is reachable if the ETR that is authoritative for a given
      EID-to-RLOC mapping is reachable.

    Default Mapping:  A mapping entry for EID-Prefix 0.0.0.0/0 (::/0 for
      IPv6).  It maps to a Locator-Set used for all EIDs in the
      Internet.  If there is a more-specific EID-Prefix in the
      map-cache, it overrides the Default Mapping entry.  The Default
      Mapping entry can be learned by configuration or from a Map-Reply
      message.

    ALT Default Route:  An EID-Prefix value of 0.0.0.0/0 (or ::/0 for
      IPv6) that may be learned from the ALT or statically configured on
      an edge ALT-Router.  The ALT Default Route defines a forwarding
      path for a packet to be sent into the ALT on a router that does
      not have a full ALT forwarding database.











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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  The LISP-ALT Model</span>

   The LISP-ALT model uses the same basic query/response protocol that
   is documented in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].  In particular, LISP+ALT provides two
   types of packets that an ITR can originate to obtain EID-to-RLOC
   mappings:

   Map-Request:  A Map-Request message is sent into the ALT to request
      an EID-to-RLOC mapping.  The ETR that owns the mapping will
      respond to the ITR with a Map-Reply message.  Since the ALT only
      forwards on EID destinations, the destination address of the
      Map-Request sent on the ALT must be an EID.

   Data-Probe:  Alternatively, an ITR may encapsulate and send the first
      data packet destined for an EID with no known RLOCs into the ALT
      as a Data-Probe.  This might be done to minimize packet loss and
      to probe for the mapping.  As above, the authoritative ETR for the
      EID-Prefix will respond to the ITR with a Map-Reply message when
      it receives the data packet over the ALT.  As a side-effect, the
      encapsulated data packet is delivered to the end-system at the ETR
      site.  Note that the Data-Probe's inner IP destination address,
      which is an EID, is copied to the outer IP destination address so
      that the resulting packet can be routed over the ALT.  See
      <a href="#section-3.3">Section 3.3</a> for caveats on the usability of Data-Probes.

   The term "ALT Datagram" is shorthand for a Map-Request or Data-Probe
   to be sent into or forwarded on the ALT.  Note that such packets use
   an RLOC as the outer-header source IP address and an EID as the
   outer-header destination IP address.

   Detailed descriptions of the LISP packet types referenced by this
   document may be found in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Routability of EIDs</span>

   A LISP EID has the same syntax as an IP address and can be used,
   unaltered, as the source or destination of an IP datagram.  In
   general, though, EIDs are not routable on the public Internet; LISP+
   ALT provides a separate, virtual network, known as the LISP
   Alternative Logical Topology (ALT) on which a datagram using an EID
   as an IP destination address may be transmitted.  This network is
   built as an overlay on the public Internet using tunnels to
   interconnect ALT-Routers.  BGP runs over these tunnels to propagate
   path information needed to forward ALT Datagrams.  Importantly, while
   the ETRs are the source(s) of the unaggregated EID-Prefixes, LISP+ALT
   uses existing BGP mechanisms to aggregate this information.





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<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>.  Mechanisms for an ETR to Originate EID-Prefixes</span>

   There are three ways that an ETR may originate its mappings into
   the ALT:

   1.  By registration with a Map-Server, as documented in [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>].
       This is the common case and is expected to be used by the
       majority of ETRs.

   2.  Using a "static route" on the ALT.  Where no Map-Server is
       available, an edge ALT-Router may be configured with a "static
       EID-Prefix route" pointing to an ETR.

   3.  Edge connection to the ALT.  If a site requires fine-grained
       control over how its EID-Prefixes are advertised into the ALT, it
       may configure its ETR(s) with tunnel and BGP connections to edge
       ALT-Routers.

<span class="h4"><a class="selflink" id="section-3.1.2" href="#section-3.1.2">3.1.2</a>.  Mechanisms for an ITR to Forward to EID-Prefixes</span>

   There are three ways that an ITR may send ALT Datagrams:

   1.  Through a Map-Resolver, as documented in [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>].  This is the
       common case and is expected to be used by the majority of ITRs.

   2.  Using a "default route".  Where a Map-Resolver is not available,
       an ITR may be configured with a static ALT Default Route pointing
       to an edge ALT-Router.

   3.  Edge connection to the ALT.  If a site requires fine-grained
       knowledge of what prefixes exist on the ALT, it may configure its
       ITR(s) with tunnel and BGP connections to edge ALT-Routers.

<span class="h4"><a class="selflink" id="section-3.1.3" href="#section-3.1.3">3.1.3</a>.  Map-Server Model Preferred</span>

   The ALT-connected ITR and ETR cases are expected to be rare, as the
   Map-Server/Map-Resolver model is simpler for an ITR/ETR operator to
   use and also provides a more general service interface to not only
   the ALT but to other mapping databases that may be developed in the
   future.











<span class="grey">Fuller, et al.                Experimental                      [Page 9]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-10" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  Connectivity to Non-LISP Sites</span>

   As stated above, EIDs used as IP addresses by LISP sites are not
   routable on the public Internet.  This implies that, absent a
   mechanism for communication between LISP and non-LISP sites,
   connectivity between them is not possible.  To resolve this problem,
   an "interworking" technology has been defined; see [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>] for
   details.

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  Caveats on the Use of Data-Probes</span>

   It is worth noting that there has been a great deal of discussion and
   controversy about whether Data-Probes are a good idea.  On the one
   hand, using them offers a method of avoiding the "first packet drop"
   problem when an ITR does not have a mapping for a particular
   EID-Prefix.  On the other hand, forwarding data packets on the ALT
   would require that it either be engineered to support relatively high
   traffic rates, which is not generally feasible for a tunneled
   network, or that it be carefully designed to aggressively rate-limit
   traffic to avoid congestion or DoS attacks.  There may also be issues
   caused by different latency or other performance characteristics
   between the ALT path taken by an initial Data-Probe and the
   "Internet" path taken by subsequent packets on the same flow once a
   mapping is in place on an ITR.  For these reasons, the use of
   Data-Probes is not recommended at this time; they should only be
   originated from an ITR when explicitly configured to do so, and such
   configuration should only be enabled when performing experiments
   intended to test the viability of using Data-Probes.

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  LISP+ALT: Overview</span>

   LISP+ALT is a hybrid push/pull architecture.  Aggregated EID-Prefixes
   are advertised among the ALT-Routers and to those (rare) ITRs that
   are directly connected via a tunnel and BGP to the ALT.  Specific
   EID-to-RLOC mappings are requested by an ITR (and returned by an ETR)
   using LISP when it sends a request either via a Map-Resolver or to an
   edge ALT-Router.

   The basic idea embodied in LISP+ALT is to use BGP, running on a
   tunneled overlay network (the ALT), to establish reachability between
   ALT-Routers.  The ALT BGP Routing Information Base (RIB) is comprised
   of EID-Prefixes and associated next hops.  ALT-Routers interconnect
   using BGP and propagate EID-Prefix updates among themselves.
   EID-Prefix information is learned from ETRs at the "edge" of the ALT
   either through the use of the Map-Server interface (the common case),
   by static configuration, or by BGP-speaking ETRs.





<span class="grey">Fuller, et al.                Experimental                     [Page 10]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-11" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   Map-Resolvers learns paths through the ALT to Map-Servers for
   EID-Prefixes.  An ITR will normally use a Map-Resolver to send its
   ALT Datagrams on to the ALT but may, in unusual cases (see
   <a href="#section-3.1.2">Section 3.1.2</a>), use a static ALT Default Route or connect to the ALT
   using BGP.  Likewise, an ETR will normally register its prefixes in
   the mapping database using a Map-Server or can sometimes (see
   <a href="#section-3.1.1">Section 3.1.1</a>) connect directly to the ALT using BGP.  See [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>]
   for details on Map-Servers and Map-Resolvers.

   Note that while this document specifies the use of Generic Routing
   Encapsulation (GRE) as a tunneling mechanism, there is no reason that
   parts of the ALT cannot be built using other tunneling technologies,
   particularly in cases where GRE does not meet security, management,
   or other operational requirements.  References to "GRE tunnel" in
   later sections of this document should therefore not be taken as
   prohibiting or precluding the use of other tunneling mechanisms.
   Note also that two ALT-Routers that are directly adjacent (with no
   layer-3 router hops between them) need not use a tunnel between them;
   in this case, BGP may be configured across the interfaces that
   connect to their common subnet, and that subnet is then considered to
   be part of the ALT topology.  The use of techniques such as "eBGP
   multihop" to connect ALT-Routers that do not share a tunnel or common
   subnet is not recommended, as the non-ALT routers in between the
   ALT-Routers in such a configuration may not have information
   necessary to forward ALT Datagrams destined to EID-Prefixes exchanged
   across that BGP session.

   In summary, LISP+ALT uses BGP to build paths through ALT-Routers so
   that an ALT Datagram sent into the ALT can be forwarded to the ETR
   that holds the EID-to-RLOC mapping for that EID-Prefix.  This
   reachability is carried as IPv4 or IPv6 Network Layer Reachability
   Information (NLRI) without modification (since an EID-Prefix has the
   same syntax as an IPv4 or IPv6 address prefix).  ALT-Routers
   establish BGP sessions with one another, forming the ALT.  An
   ALT-Router at the "edge" of the topology learns EID-Prefixes
   originated by authoritative ETRs.  Learning may be through the
   Map-Server interface, by static configuration, or via BGP with the
   ETRs.  An ALT-Router may also be configured to aggregate EID-Prefixes
   received from ETRs or from other LISP-ALT Routers that are
   topologically "downstream" from it.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  ITR Traffic Handling</span>

   When an ITR receives a packet originated by an end-system within its
   site (i.e., a host for which the ITR is the exit path out of the
   site) and the destination EID for that packet is not known in the
   ITR's map-cache, the ITR creates either a Map-Request for the
   destination EID or the original packet encapsulated as a Data-Probe



<span class="grey">Fuller, et al.                Experimental                     [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   (see <a href="#section-3.3">Section 3.3</a> for caveats on the usability of Data-Probes).  The
   result, known as an ALT Datagram, is then sent to an ALT-Router (see
   also [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>] for non-ALT-connected ITRs, noting that Data-Probes
   cannot be sent to a Map-Resolver).  This "first-hop" ALT-Router uses
   EID-Prefix routing information learned from other ALT-Routers via BGP
   to guide the packet to the ETR that "owns" the prefix.  Upon receipt
   by the ETR, normal LISP processing occurs: the ETR responds to the
   ITR with a LISP Map-Reply that lists the RLOCs (and, thus, the ETRs
   to use) for the EID-Prefix.  For Data-Probes, the ETR also
   decapsulates the packet and transmits it toward its destination.

   Upon receipt of the Map-Reply, the ITR installs the RLOC information
   for a given prefix into a local mapping database.  With these mapping
   entries stored, additional packets destined to the given EID-Prefix
   are routed directly to an RLOC without use of the ALT, until either
   the entry's Time to Live (TTL) has expired or the ITR can otherwise
   find no reachable ETR.  Note that a current mapping may exist that
   contains no reachable RLOCs; this is known as a Negative Cache Entry,
   and it indicates that packets destined to the EID-Prefix are to be
   dropped.

   Full details on Map-Request/Map-Reply processing may be found in
   [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>].

   Traffic routed on to the ALT consists solely of ALT Datagrams, i.e.,
   Map-Requests and Data-Probes (if supported).  Given the relatively
   low performance expected of a tunneled topology, ALT-Routers (and
   Map-Resolvers) should aggressively rate-limit the ingress of ALT
   Datagrams from ITRs and, if possible, should be configured to not
   accept packets that are not ALT Datagrams.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  EID Assignment - Hierarchy and Topology</span>

   The ALT database is organized in a hierarchical manner with
   EID-Prefixes aggregated on power-of-2 block boundaries.  Where a LISP
   site has multiple EID-Prefixes that are aligned on a power-of-2 block
   boundary, they should be aggregated into a single EID-Prefix for
   advertisement.  The ALT network is built in a roughly hierarchical,
   partial mesh that is intended to allow aggregation where clearly
   defined hierarchical boundaries exist.  Building such a structure
   should minimize the number of EID-Prefixes carried by LISP+ALT nodes
   near the top of the hierarchy.

   Routes on the ALT do not need to respond to changes in policy,
   subscription, or underlying physical connectivity, so the topology
   can remain relatively static and aggregation can be sustained.
   Because routing on the ALT uses BGP, the same rules apply for
   generating aggregates; in particular, an ALT-Router should only be



<span class="grey">Fuller, et al.                Experimental                     [Page 12]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-13" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   configured to generate an aggregate if it is configured with BGP
   sessions to all of the originators of components (more-specific
   prefixes) of that aggregate.  Not all of the components need to be
   present for the aggregate to be originated (some may be holes in the
   covering prefix, and some may be down), but the aggregating router
   must be configured to learn the state of all of the components.

   Under what circumstances the ALT-Router actually generates the
   aggregate is a matter of local policy: in some cases, it will be
   statically configured to do so at all times with a "static discard"
   route.  In other cases, it may be configured to only generate the
   aggregate prefix if at least one of the components of the aggregate
   is learned via BGP.

   An ALT-Router must not generate an aggregate that includes a
   non-LISP-speaking hole unless it can be configured to return a
   Negative Map-Reply with action="Natively-Forward" (see [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>]) if
   it receives an ALT Datagram that matches that hole.  If it receives
   an ALT Datagram that matches a LISP-speaking hole that is currently
   not reachable, it should return a Negative Map-Reply with
   action="drop".  Negative Map-Replies should be returned with a short
   TTL, as specified in [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>].  Note that an off-the-shelf,
   non-LISP-speaking router configured as an aggregating ALT-Router
   cannot send Negative Map-Replies, so such a router must never
   originate an aggregate that includes a non-LISP-speaking hole.

   This implies that two ALT-Routers that share an overlapping set of
   prefixes must exchange those prefixes if either is to generate and
   export a covering aggregate for those prefixes.  It also implies that
   an ETR that connects to the ALT using BGP must maintain BGP sessions
   with all of the ALT-Routers that are configured to originate an
   aggregate that covers that prefix and that each of those ALT-Routers
   must be explicitly configured to know the set of EID-Prefixes that
   make up any aggregate that it originates.  See also [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>] for an
   example of other ways that prefix origin consistency and aggregation
   can be maintained.

   As an example, consider ETRs that are originating EID-Prefixes for
   10.1.0.0/24, 10.1.64.0/24, 10.1.128.0/24, and 10.1.192.0/24.  An
   ALT-Router should only be configured to generate an aggregate for
   10.1.0.0/16 if it has BGP sessions configured with all of these ETRs,
   in other words, only if it has sufficient knowledge about the state
   of those prefixes to summarize them.  If the Router originating
   10.1.0.0/16 receives an ALT Datagram destined for 10.1.77.88, a
   non-LISP destination covered by the aggregate, it returns a Negative
   Map-Reply with action "Natively-Forward".  If it receives an ALT





<span class="grey">Fuller, et al.                Experimental                     [Page 13]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-14" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   Datagram destined for 10.1.128.199 but the configured LISP prefix
   10.1.128.0/24 is unreachable, it returns a Negative Map-Reply with
   action "drop".

   Note: Much is currently uncertain about the best way to build the ALT
   network; as testing and prototype deployment proceed, a guide to how
   to best build the ALT network will be developed.

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Use of GRE and BGP between LISP-ALT Routers</span>

   The ALT network is built using GRE tunnels between ALT-Routers.  BGP
   sessions are configured over those tunnels, with each ALT-Router
   acting as a separate Autonomous System (AS) "hop" in a Path Vector
   for BGP.  For the purposes of LISP+ALT, the AS-path is used solely as
   a shortest-path determination and loop-avoidance mechanism.  Because
   all next hops are on tunnel interfaces, no IGP is required to resolve
   those next hops to exit interfaces.

   LISP+ALT's use of GRE and BGP facilitates deployment and operation of
   LISP because no new protocols need to be defined, implemented, or
   used on the overlay topology; existing BGP/GRE tools and operational
   expertise are also re-used.  Tunnel address assignment is also easy:
   since the addresses on an ALT tunnel are only used by the pair of
   routers connected to the tunnel, the only requirement of the IP
   addresses used to establish that tunnel is that the attached routers
   be reachable by each other; any addressing plan, including private
   addressing, can therefore be used for ALT tunnels.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  EID-Prefix Propagation and Map-Request Forwarding</span>

   As described in <a href="#section-8.2">Section 8.2</a>, an ITR sends an ALT Datagram to a given
   EID-to-RLOC mapping.  The ALT provides the infrastructure that allows
   these requests to reach the authoritative ETR.

   Note that under normal circumstances Map-Replies are not sent over
   the ALT; an ETR sends a Map-Reply to one of the ITR RLOCs learned
   from the original Map-Request.  See Sections <a href="#section-6.1.2">6.1.2</a> and <a href="#section-6.2">6.2</a> of
   [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>] for more information on the use of the Map-Request 'ITR
   RLOC Address' field.  Keep in mind that the 'ITR RLOC Address' field
   supports multiple RLOCs in multiple address families, so a Map-Reply
   sent in response to a Map-Request is not necessarily sent back to the
   Map-Request RLOC source.

   There may be scenarios, perhaps to encourage caching of EID-to-RLOC
   mappings by ALT-Routers, where Map-Replies could be sent over the ALT
   or where a "first-hop" ALT-Router might modify the originating RLOC
   on a Map-Request received from an ITR to force the Map-Reply to be




<span class="grey">Fuller, et al.                Experimental                     [Page 14]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-15" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   returned to the "first-hop" ALT-Router.  These cases will not be
   supported by initial LISP+ALT implementations but may be subject to
   future experimentation.

   ALT-Routers propagate path information via BGP ([<a href="./rfc4271" title="&quot;A Border Gateway Protocol 4 (BGP-4)&quot;">RFC4271</a>]) that is
   used by ITRs to send ALT Datagrams toward the appropriate ETR for
   each EID-Prefix.  BGP is run on the inter-ALT-Router links, and
   possibly between an edge ("last-hop") ALT-Router and an ETR or
   between an edge ("first-hop") ALT-Router and an ITR.  The ALT BGP RIB
   consists of aggregated EID-Prefixes and their next hops toward the
   authoritative ETR for that EID-Prefix.

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Changes to ITR Behavior with LISP+ALT</span>

   As previously described, an ITR will usually use the Map-Resolver
   interface and will send its Map Requests to a Map-Resolver.  When an
   ITR instead connects via tunnels and BGP to the ALT, it sends ALT
   Datagrams to one of its "upstream" ALT-Routers; these are sent only
   to obtain new EID-to-RLOC mappings -- RLOC probe and cache TTL
   refresh Map-Requests are not sent on the ALT.  As in basic LISP, it
   should use one of its RLOCs as the source address of these queries;
   it should not use a tunnel interface as the source address, as doing
   so will cause replies to be forwarded over the tunneled topology and
   may be problematic if the tunnel interface address is not routed
   throughout the ALT.  If the ITR is running BGP with the LISP-ALT
   Router(s), it selects the appropriate ALT-Router based on the BGP
   information received.  If it is not running BGP, it uses a statically
   configured ALT Default Route to select an ALT-Router.

<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  Changes to ETR Behavior with LISP+ALT</span>

   As previously described, an ETR will usually use the Map-Server
   interface (see [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>]) and will register its EID-Prefixes with its
   configured Map-Servers.  When an ETR instead connects using BGP to
   one or more ALT-Routers, it announces its EID-Prefix(es) to those
   ALT-Routers.

   As documented in [<a href="./rfc6830" title="&quot;The Locator/ID Separation Protocol (LISP)&quot;">RFC6830</a>], when an ETR generates a Map-Reply message
   to return to a querying ITR, it sets the outer-header IP destination
   address to one of the requesting ITR's RLOCs so that the Map-Reply
   will be sent on the underlying Internet topology, not on the ALT;
   this avoids any latency penalty (or "stretch") that might be incurred
   by sending the Map-Reply via the ALT, reduces load on the ALT, and
   ensures that the Map-Reply can be routed even if the original ITR
   does not have an ALT-routed EID.  For details on how an ETR selects
   which ITR RLOC to use, see <a href="./rfc6830#section-6.1.5">Section&nbsp;6.1.5 of [RFC6830]</a>.





<span class="grey">Fuller, et al.                Experimental                     [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  ALT Datagram Forwarding Failure</span>

   Intermediate ALT-Routers forward ALT Datagrams using normal,
   hop-by-hop routing on the ALT overlay network.  Should an ALT-Router
   not be able to forward an ALT Datagram, whether due to an unreachable
   next hop, TTL exceeded, or other problem, it has several choices:

   o  If the ALT-Router understands LISP, as is the case for a
      Map-Resolver or Map-Server, it may respond to a forwarding failure
      by returning a Negative Map-Reply, as described in <a href="#section-4.2">Section 4.2</a> and
      [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>].

   o  If the ALT-Router does not understand LISP, it may attempt to
      return an ICMP message to the source IP address of the packet that
      cannot be forwarded.  Since the source address is an RLOC, an
      ALT-Router would send this ICMP message using "native" Internet
      connectivity, not via the ALT overlay.

   o  A non-LISP-capable ALT-Router may also choose to silently drop the
      non-forwardable ALT Datagram.

   [<a id="ref-RFC6830">RFC6830</a>] and [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>] define how the source of an ALT Datagram
   should handle each of these cases.  The last case, where an ALT
   Datagram is silently discarded, will generally result in several
   retransmissions by the source, followed by treating the destination
   as unreachable via LISP when no Map-Reply is received.  If a problem
   on the ALT is severe enough to prevent ALT Datagrams from being
   delivered to a specific EID, this is probably the only sensible way
   to handle this case.

   Note that the use of GRE tunnels should prevent MTU problems from
   ever occurring on the ALT; an ALT Datagram that exceeds an
   intermediate MTU will be fragmented at that point and will be
   reassembled by the target of the GRE tunnel.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  BGP Configuration and Protocol Considerations</span>

<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>.  Autonomous System Numbers (ASNs) in LISP+ALT</span>

   The primary use of BGP today is to define the global Internet routing
   topology in terms of its participants, known as Autonomous Systems.
   LISP+ALT specifies the use of BGP to create a global overlay network
   (the ALT) for finding EID-to-RLOC mappings.  While related to the
   global routing database, the ALT serves a very different purpose and
   is organized into a very different hierarchy.  Because LISP+ALT does
   use BGP, however, it uses ASNs in the paths that are propagated among
   ALT-Routers.  To avoid confusion, LISP+ALT should use newly assigned




<span class="grey">Fuller, et al.                Experimental                     [Page 16]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-17" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   AS numbers that are unrelated to the ASNs used by the global routing
   system.  Exactly how this new space will be assigned and managed will
   be determined during the deployment of LISP+ALT.

   Note that the ALT-Routers that make up the "core" of the ALT will not
   be associated with any existing core-Internet ASN because the ALT
   topology is completely separate from, and independent of, the global
   Internet routing system.

<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>.  Subsequent Address Family Identifier (SAFI) for LISP+ALT</span>

   As defined by this document, LISP+ALT may be implemented using BGP
   without modification.  Given the fundamental operational difference
   between propagating global Internet routing information (the current
   dominant use of BGP) and creating an overlay network for finding
   EID-to-RLOC mappings (the use of BGP as proposed by this document),
   it may be desirable to assign a new SAFI [<a href="./rfc4760" title="&quot;Multiprotocol Extensions for BGP-4&quot;">RFC4760</a>] to prevent
   operational confusion and difficulties, including the inadvertent
   leaking of information from one domain to the other.  The use of a
   separate SAFI would make it easier to debug many operational problems
   but would come at a significant cost: unmodified, off-the-shelf
   routers that do not understand the new SAFI could not be used to
   build any part of the ALT network.  At present, this document does
   not request the assignment of a new SAFI; additional experimentation
   may suggest the need for one in the future.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  EID-Prefix Aggregation</span>

   To facilitate EID-Prefix aggregation, the ALT BGP topology is
   provisioned in a hierarchical manner; the fact that all inter-node
   links are tunnels means that topology can be constrained to follow
   the EID-Prefix assignment hierarchy.  Redundant links are provisioned
   to compensate for node and link failures.  A basic assumption is that
   as long as the routers are up and running, the underlying Internet
   will provide alternative routes to maintain tunnel and BGP
   connectivity among ALT-Routers.

   Note that, as mentioned in <a href="#section-4.2">Section 4.2</a>, the use of BGP by LISP+ALT
   requires that information only be aggregated where all active more-
   specific prefixes of a generated aggregate prefix are known.  This is
   no different than the way that BGP route aggregation works in the
   existing global routing system: a service provider only generates an
   aggregate route if it is configured to learn all prefixes that make
   up that aggregate.







<span class="grey">Fuller, et al.                Experimental                     [Page 17]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-18" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  Stability of the ALT</span>

   It is worth noting that LISP+ALT does not directly propagate
   EID-to-RLOC mappings.  What it does is provide a mechanism for an ITR
   to communicate with the ETR that holds the mapping for a particular
   EID-Prefix.  This distinction is important when considering the
   stability of BGP on the ALT network as compared to the global routing
   system.  It also has implications for how site-specific EID-Prefix
   information may be used by LISP but not propagated by LISP+ALT (see
   <a href="#section-7.2">Section 7.2</a> below).

   RLOC prefixes are not propagated through the ALT, so their
   reachability is not determined through the use of LISP+ALT.  Instead,
   reachability of RLOCs is learned through the LISP ITR-ETR exchange.
   This means that link failures or other service disruptions that may
   cause the reachability of an RLOC to change are not known to the ALT.
   Changes to the presence of an EID-Prefix on the ALT occur much less
   frequently: only at subscription time or in the event of a failure of
   the ALT infrastructure itself.  This means that "flapping" (frequent
   BGP updates and withdrawals due to prefix state changes) is not
   likely and mapping information cannot become "stale" due to slow
   propagation through the ALT BGP mesh.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  Traffic Engineering Using LISP</span>

   Since an ITR learns an EID-to-RLOC mapping directly from the ETR that
   owns it, it is possible to perform site-to-site Traffic Engineering
   by setting the preference and/or weight fields, and by including
   more-specific EID-to-RLOC information in Map-Reply messages.

   This is a powerful mechanism that can conceivably replace the
   traditional practice of routing prefix deaggregation for Traffic
   Engineering purposes.  Rather than propagating more-specific
   information into the global routing system for local or regional
   optimization of traffic flows, such more-specific information can be
   exchanged, through LISP (not LISP+ALT), on an as-needed basis between
   only those ITRs/ETRs (and, thus, site pairs) that need it.  Such an
   exchange of "more-specifics" between sites facilitates Traffic
   Engineering by allowing richer and more fine-grained policies to be
   applied without advertising additional prefixes into either the ALT
   or the global routing system.

   Note that these new Traffic Engineering capabilities are an attribute
   of LISP and are not specific to LISP+ALT; discussion is included here
   because the BGP-based global routing system has traditionally used
   propagation of more-specific routes as a crude form of Traffic
   Engineering.




<span class="grey">Fuller, et al.                Experimental                     [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>.  Edge Aggregation and Dampening</span>

   Normal BGP best common practices apply to the ALT network.  In
   particular, first-hop ALT-Routers will aggregate EID-Prefixes and
   dampen changes to them in the face of excessive updates.  Since
   EID-Prefix assignments are not expected to change as frequently as
   global routing BGP prefix reachability, such dampening should be very
   rare and might be worthy of logging as an exceptional event.  It is
   again worth noting that the ALT carries only EID-Prefixes, used to
   construct a BGP path to each ETR (or Map-Server) that originates each
   prefix; the ALT does not carry reachability information about RLOCs.
   In addition, EID-Prefix information may be aggregated as the topology
   and address assignment hierarchy allow.  Since the topology is all
   tunneled and can be modified as needed, reasonably good aggregation
   should be possible.  In addition, since most ETRs are expected to
   connect to the ALT using the Map-Server interface, Map-Servers will
   implement a natural "edge" for the ALT where dampening and
   aggregation can be applied.  For these reasons, the set of prefix
   information on the ALT can be expected to be both better aggregated
   and considerably less volatile than the actual EID-to-RLOC mappings.

<span class="h3"><a class="selflink" id="section-7.4" href="#section-7.4">7.4</a>.  EID Assignment Flexibility vs. ALT Scaling</span>

   There are major open questions regarding how the ALT will be deployed
   and what organization(s) will operate it.  In a simple,
   non-distributed world, centralized administration of EID-Prefix
   assignment and ALT network design would facilitate a well-aggregated
   ALT routing system.  Business and other realities will likely result
   in a more complex, distributed system involving multiple levels of
   prefix delegation, multiple operators of parts of the ALT
   infrastructure, and a combination of competition and cooperation
   among the participants.  In addition, the re-use of existing IP
   address assignments, both Provider-Independent ("PI") and Provider-
   Assigned ("PA"), to avoid renumbering when sites transition to LISP
   will further complicate the processes of building and operating
   the ALT.

   A number of conflicting considerations need to be kept in mind when
   designing and building the ALT.  Among them are:

   1.  Target ALT routing state size and level of aggregation.  As
       described in <a href="#section-7.1">Section 7.1</a>, the ALT should not suffer from the same
       performance constraints or stability issues as does the Internet
       global routing system, so some reasonable level of deaggregation
       and an increased number of EID-Prefixes beyond what might be
       considered ideal should be acceptable.  That said, measures, such
       as tunnel rehoming to preserve aggregation when sites move from
       one mapping provider to another and implementing aggregation at



<span class="grey">Fuller, et al.                Experimental                     [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


       multiple levels in the hierarchy to collapse deaggregation at
       lower levels, should be taken to reduce unnecessary explosion of
       ALT routing state.

   2.  Number of operators of parts of the ALT and how they will be
       organized (hierarchical delegation vs. shared administration).
       This will determine not only how EID-Prefixes are assigned but
       also how tunnels are configured and how EID-Prefixes can be
       aggregated between different parts of the ALT.

   3.  Number of connections between different parts of the ALT.
       Tradeoffs will need to be made among resilience, performance, and
       placement of aggregation boundaries.

   4.  EID-Prefix portability between competing operators of the ALT
       infrastructure.  A significant benefit for an end site to adopt
       LISP is the availability of EID space that is not tied to a
       specific connectivity provider; it is important to ensure that an
       end site doesn't trade lock-in to a connectivity provider for
       lock-in to a provider of its EID assignment, ALT connectivity, or
       Map-Server facilities.

   This is, by no means, an exhaustive list.

   While resolving these issues is beyond the scope of this document,
   the authors recommend that existing distributed resource structures,
   such as the IANA/Regional Internet Registries and the ICANN/Domain
   Registrar, be carefully considered when designing and deploying the
   ALT infrastructure.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Connecting Sites to the ALT Network</span>

<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>.  ETRs Originating Information into the ALT</span>

   EID-Prefix information is originated into the ALT by three different
   mechanisms:

   Map-Server:  In most cases, a site will configure its ETR(s) to
      register with one or more Map-Servers (see [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>]) and does not
      participate directly in the ALT.

   BGP:  For sites requiring complex control over their EID-Prefix
      origination into the ALT, an ETR may connect to the LISP+ALT
      overlay network by running BGP to one or more ALT-Routers over
      tunnel(s).  The ETR advertises reachability for its EID-Prefixes
      over these BGP connection(s).  The edge ALT-Router(s) that
      receive(s) these prefixes then propagate(s) them into the ALT.




<span class="grey">Fuller, et al.                Experimental                     [Page 20]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-21" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


      Here, the ETR is simply a BGP peer of ALT-Router(s) at the edge of
      the ALT.  Where possible, an ALT-Router that receives EID-Prefixes
      from an ETR via BGP should aggregate that information.

   Configuration:  One or more ALT-Routers may be configured to
      originate an EID-Prefix on behalf of the non-BGP-speaking ETR that
      is authoritative for a prefix.  As in the case above, the ETR is
      connected to ALT-Router(s) using GRE tunnel(s), but rather than
      BGP being used, the ALT-Router(s) are configured with what are in
      effect "static routes" for the EID-Prefixes "owned" by the ETR.
      The GRE tunnel is used to route Map-Requests to the ETR.

   Note:  In all cases, an ETR may register to multiple Map-Servers or
      connect to multiple ALT-Routers for the following reasons:

      *  redundancy, so that a particular ETR is still reachable even if
         one path or tunnel is unavailable.

      *  to connect to different parts of the ALT hierarchy if the ETR
         "owns" multiple EID-to-RLOC mappings for EID-Prefixes that
         cannot be aggregated by the same ALT-Router (i.e., are not
         topologically "close" to each other in the ALT).

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>.  ITRs Using the ALT</span>

   In the common configuration, an ITR does not need to know anything
   about the ALT, since it sends Map-Requests to one of its configured
   Map-Resolvers (see [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>]).  There are two exceptional cases:

   Static default:  If a Map-Resolver is not available but an ITR is
      adjacent to an ALT-Router (either over a common subnet or through
      the use of a tunnel), it can use an ALT Default Route to cause all
      ALT Datagrams to be sent to that ALT-Router.  This case is
      expected to be rare.

   Connection to ALT:  A site with complex Internet connectivity may
      need more fine-grained distinction between traffic to LISP-capable
      and non-LISP-capable sites.  Such a site may configure each of its
      ITRs to connect directly to the ALT, using a tunnel and BGP
      connection.  In this case, the ITR will receive EID-Prefix routes
      from its BGP connection to the ALT-Router and will LISP-
      encapsulate and send ALT Datagrams through the tunnel to the
      ALT-Router.  Traffic to other destinations may be forwarded
      (without LISP encapsulation) to non-LISP next-hop routers that the
      ITR knows.






<span class="grey">Fuller, et al.                Experimental                     [Page 21]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-22" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


      In general, an ITR that connects to the ALT does so only to
      ALT-Routers at the "edge" of the ALT (typically two for
      redundancy).  There may, though, be situations where an ITR would
      connect to other ALT-Routers to receive additional, shorter-path
      information about a portion of the ALT of interest to it.  This
      can be accomplished by establishing GRE tunnels between the ITR
      and the set of ALT-Routers with the additional information.  This
      is a purely local policy issue between the ITR and the ALT-Routers
      in question.

   As described in [<a href="./rfc6833" title="&quot;Locator/ID Separation Protocol (LISP) Map-Server Interface&quot;">RFC6833</a>], Map-Resolvers do not accept or forward
   Data-Probes; in the rare scenario that an ITR does support and
   originate Data-Probes, it must do so using one of the exceptional
   configurations described above.  Note that the use of Data-Probes is
   discouraged at this time (see <a href="#section-3.3">Section 3.3</a>).

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Security Considerations</span>

   LISP+ALT shares many of the security characteristics of BGP.  Its
   security mechanisms are comprised of existing technologies in wide
   operational use today, so securing the ALT should be mostly a matter
   of applying the same technology that is used to secure the BGP-based
   global routing system (see <a href="#section-9.3">Section 9.3</a> below).

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>.  Apparent LISP+ALT Vulnerabilities</span>

   This section briefly lists the known potential vulnerabilities of
   LISP+ALT.

   Mapping integrity:  Potential for an attacker to insert bogus
      mappings to black-hole (create a DoS attack) or intercept LISP
      data-plane packets.

   ALT-Router availability:  Can an attacker DoS the ALT-Routers
      connected to a given ETR?  If a site's ETR cannot advertise its
      EID-to-RLOC mappings, the site is essentially unavailable.

   ITR mapping/resources:  Can an attacker force an ITR or ALT-Router to
      drop legitimate mapping requests by flooding it with random
      destinations for which it will generate large numbers of
      Map-Requests and fill its map-cache?  Further study is required to
      see the impact of admission control on the overlay network.









<span class="grey">Fuller, et al.                Experimental                     [Page 22]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-23" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


   EID Map-Request exploits for reconnaissance:  Can an attacker learn
      about a LISP site's TE policy by sending legitimate mapping
      requests and then observing the RLOC mapping replies?  Is this
      information useful in attacking or subverting peer relationships?
      Note that any public LISP mapping database will have similar
      data-plane reconnaissance issues.

   Scaling of ALT-Router resources:  Paths through the ALT may be of
      lesser bandwidth than more "direct" paths; this may make them more
      prone to high-volume DoS attacks.  For this reason, all components
      of the ALT (ETRs and ALT-Routers) should be prepared to rate-limit
      traffic (ALT Datagrams) that could be received across the ALT.

   UDP Map-Reply from ETR:  Since Map-Replies are sent directly from the
      ETR to the ITR's RLOC, the ITR's RLOC may be vulnerable to various
      types of DoS attacks (this is a general property of LISP, not a
      LISP+ALT vulnerability).

   More-specific prefix leakage:  Because EID-Prefixes on the ALT are
      expected to be fairly well-aggregated and EID-Prefixes propagated
      out to the global Internet (see [<a href="./rfc6832" title="&quot;Interworking between Locator/ID Separation Protocol (LISP) and Non-LISP Sites&quot;">RFC6832</a>]) much more so,
      accidental leaking or malicious advertisement of an EID-Prefix
      into the global routing system could cause traffic redirection
      away from a LISP site.  This is not really a new problem, though,
      and its solution can only be achieved by much more strict prefix
      filtering and authentication on the global routing system.
      <a href="#section-9.3">Section 9.3</a> describes an existing approach to solving this
      problem.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>.  Survey of LISP+ALT Security Mechanisms</span>

   Explicit peering:  The devices themselves can prioritize incoming
      packets as well as potentially do key checks in hardware to
      protect the control plane.

   Use of TCP to connect elements:  This makes it difficult for third
      parties to inject packets.

   Use of HMAC to protect BGP/TCP connections:  Hashed Message
      Authentication Code (HMAC) [<a href="./rfc5925" title="&quot;The TCP Authentication Option&quot;">RFC5925</a>] is used to verify the
      integrity and authenticity of TCP connections used to exchange BGP
      messages, making it nearly impossible for third-party devices to
      either insert or modify messages.

   Message sequence numbers and nonce values in messages:  This allows
      an ITR to verify that the Map-Reply from an ETR is in response to
      a Map-Request originated by that ITR (this is a general property
      of LISP; LISP+ALT does not change this behavior).



<span class="grey">Fuller, et al.                Experimental                     [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h3"><a class="selflink" id="section-9.3" href="#section-9.3">9.3</a>.  Use of Additional BGP Security Mechanisms</span>

   LISP+ALT's use of BGP allows it to take advantage of BGP security
   features designed for existing Internet BGP use.  This means that
   LISP+ALT can and should use technology developed for adding security
   to BGP (in the IETF SIDR working group or elsewhere) to provide
   authentication of EID-Prefix origination and EID-to-RLOC mappings.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Acknowledgments</span>

   The authors would like to specially thank J. Noel Chiappa, who was a
   key contributor to the design of the Content distribution Overlay
   Network Service for LISP (LISP-CONS) mapping database (many ideas
   from which made their way into LISP+ALT) and who has continued to
   provide invaluable insight as the LISP effort has evolved.  Others
   who have provided valuable contributions include John Zwiebel, Hannu
   Flinck, Amit Jain, John Scudder, Scott Brim, and Jari Arkko.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  References</span>

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>.  Normative References</span>

   [<a id="ref-RFC2784">RFC2784</a>]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", <a href="./rfc2784">RFC 2784</a>,
              March 2000.

   [<a id="ref-RFC4271">RFC4271</a>]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", <a href="./rfc4271">RFC 4271</a>, January 2006.

   [<a id="ref-RFC4632">RFC4632</a>]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", <a href="https://www.rfc-editor.org/bcp/bcp122">BCP 122</a>, <a href="./rfc4632">RFC 4632</a>, August 2006.

   [<a id="ref-RFC4760">RFC4760</a>]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", <a href="./rfc4760">RFC 4760</a>,
              January 2007.

   [<a id="ref-RFC6830">RFC6830</a>]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", <a href="./rfc6830">RFC 6830</a>,
              January 2013.

   [<a id="ref-RFC6833">RFC6833</a>]  Fuller, V. and D. Farinacci, "Locator/ID Separation
              Protocol (LISP) Map-Server Interface", <a href="./rfc6833">RFC 6833</a>,
              January 2013.







<span class="grey">Fuller, et al.                Experimental                     [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc6836">RFC 6836</a>                        LISP+ALT                    January 2013</span>


<span class="h3"><a class="selflink" id="section-11.2" href="#section-11.2">11.2</a>.  Informative References</span>

   [<a id="ref-RFC5925">RFC5925</a>]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", <a href="./rfc5925">RFC 5925</a>, June 2010.

   [<a id="ref-RFC6832">RFC6832</a>]  Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking between Locator/ID Separation Protocol
              (LISP) and Non-LISP Sites", <a href="./rfc6832">RFC 6832</a>, January 2013.

Authors' Addresses

   Vince Fuller

   EMail: vaf@vaf.net


   Dino Farinacci
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: farinacci@gmail.com


   Dave Meyer
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: dmm@1-4-5.net


   Darrel Lewis
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: darlewis@cisco.com










Fuller, et al.                Experimental                     [Page 25]
</pre>