Network Working Group                                          V. Fuller
Internet-Draft                                              D. Farinacci
Intended status: Experimental                                   D. Meyer
Expires: July 29, September 30, 2010                                     D. Lewis
                                                        January 25,
                                                          March 29, 2010

                  LISP Alternative Topology (LISP+ALT)


   This document describes a method of building an alternative, logical
   topology for managing simple mapping database to be used by the
   Locator/ID Separation Protocol (LISP) to find Endpoint Identifier
   (EID) to Routing Locator mappings
   using (RLOC) mappings.  Termed the Locator/ID Separation Protocol.  The logical network Alternative
   Logical Topology (ALT), the database is built as an overlay network
   on the public Internet using existing
   technologies and tools, specifically the Border Gateway Protocol (BGP) and
   the Generic Routing Encapsulation.  An important design goal for
   LISP+ALT is to allow for Encapsulation (GRE).  Using these proven
   protocols, the ALT can be built and deployed relatively easy deployment of an
   efficient mapping system while minimizing quickly
   without major changes to the existing
   hardware and software. routing infrastructure.

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Table of Contents

   1.  Requirements Notation  Introduction . . . . . . . . . . . . . . . . . . . . .  4
   2.  Introduction . . . .  4
   2.  Definition of Terms  . . . . . . . . . . . . . . . . . . . . .  5
   3.  Definition of Terms  . .  The LISP+ALT model . . . . . . . . . . . . . . . . . . .  6
   4.  The LISP 1.5 model . . .  8
     3.1.  Routeability of EIDs . . . . . . . . . . . . . . . . . . .  8
     4.1.  Routeability of EIDs
       3.1.1.  Mechanisms for an ETR to originate EID-prefixes  . . .  9
       3.1.2.  Mechanisms for an ITR to forward to EID-prefixes . . .  9
       3.1.3.  Map Server Model preferred . . . . . . . . . . . . .  8
     4.2. .  9
     3.2.  Connectivity to non-LISP sites . . . . . . . . . . . . . .  9
     3.3.  Caveats on the use of Data Probes  . . . . . . . . . . . .  9
   5. 10
   4.  LISP+ALT: Overview . . . . . . . . . . . . . . . . . . . . . . 10
     5.1. 11
     4.1.  ITR traffic handling . . . . . . . . . . . . . . . . . . . 11
     5.2. 12
     4.2.  EID Assignment - Hierarchy and Topology  . . . . . . . . . 11
     5.3.  LISP+ALT Router (or ALT router for short)  . . . . . . . . 12
     5.4.  ITR and ETR in a LISP+ALT Environment  . . . . . . . . . . 13
     4.3.  Use of GRE and BGP between LISP+ALT Routers  . . . . . . . 13
   6.  EID Prefix 14
   5.  EID-prefix Propagation and Map-Request Forwarding  . . . . . . 14
     6.1. 15
     5.1.  Changes to ITR behavior with LISP+ALT  . . . . . . . . . . 14
     6.2. 15
     5.2.  Changes to ETR behavior with LISP+ALT  . . . . . . . . . . 14
   7. 15
   6.  BGP configuration and protocol considerations  . . . . . . . . 16
     7.1. 17
     6.1.  Autonomous System Numbers (ASNs) in LISP+ALT . . . . . . . 16
     7.2. 17
     6.2.  Sub-Address Family Identifier (SAFI) for LISP+ALT  . . . . 16
   8.  EID-Prefix 17
   7.  EID-prefix Aggregation . . . . . . . . . . . . . . . . . . . . 17
     8.1.  Traffic engineering with LISP and LISP+ALT . . . 18
     7.1.  Stability of the ALT . . . . . 17
     8.2.  Edge aggregation and dampening . . . . . . . . . . . . . . 18
   9.  Connecting sites to the ALT network  . . . .
     7.2.  Traffic engineering using LISP . . . . . . . . . 19
     9.1.  ETRs originating information into the ALT . . . . . 18
     7.3.  Edge aggregation and dampening . . . 19
     9.2.  ITRs Using the ALT . . . . . . . . . . . . 19
     7.4.  EID assignment flexibility vs. ALT scaling . . . . . . . . 19
   8.  Connecting sites to the ALT network  . . . . . . . . . . . . . 21
     8.1.  ETRs originating information into the ALT  . . . . . . . . 21
     8.2.  ITRs Using the ALT . . . . . . . . . . . . . . . . . . . . 21
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   11. 23
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 22
     11.1. 24
     10.1. Apparent LISP+ALT Vulnerabilities  . . . . . . . . . . . . 22
     11.2. 24
     10.2. Survey of LISP+ALT Security Mechanisms . . . . . . . . . . 23
     11.3. Using existing 25
     10.3. Use of new IETF standard BGP Security mechanisms . . . . . . . . . . 23
   12. 25
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 24
   13. 26
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     13.1. 27
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     13.2. 27
     12.2. Informative References . . . . . . . . . . . . . . . . . . 25 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 28

1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Introduction

   This document describes a method of building an alternative logical
   topology for managing Endpoint identifier the LISP+ALT mapping database, to Routing Locator mappings be used by
   LISP to find EID-to-RLOC mappings.  The ALT network is built using
   the Locator/ID Separation Protocol [LISP].  This logical
   topology uses existing technology and tools, specifically the Border Gateway Protocol [RFC4271] and its (BGP, [RFC4271]), the BGP multi-protocol
   [RFC2858], along with [RFC4760], and the Generic Routing Encapsulation [RFC2784]
   protocol (GRE,
   [RFC2784]) to construct an overlay network nnetwork of devices that advertise (ALT Routers)
   which operate on EID-prefixes only.  These Endpoint Identifier Prefix Aggregators hold
   hierarchically-assigned pieces and use EIDs as forwarding

   ALT Routers advertise hierarchically-delegated segments of the Endpoint Identifier space EID
   namespace (i.e., prefixes) and their next hops toward the rest of the ALT; they also
   forward traffic destined for an EID covered by one of those prefixes
   toward the network element which is authoritative for Endpoint Identifier-to-Routing Locator that EID (i.e.
   is the origin of the advertisement of the EID-to-RLOC mapping
   for which
   applies to that prefix. EID).  Map Resolvers (MRs; see [LISP-MS]) and, in
   some cases, Ingress Tunnel routers can Routers (ITRs) use this overlay to make queries
   against and respond to send
   mapping requests made against the distributed
   Endpoint Identifier-to-Routing Locator mapping database.  Note the
   database is distributed (as described in (using [LISP]) and is stored in to the

   Note Egress Tunnel Routers (ETRs)
   that an important design goal of LISP+ALT hold the EID-to-RLOC mappings for a particular EID-prefix

   It is important to minimize note that the
   number of changes ALT does not distribute actual EID-
   to-RLOC mappings.  What it does provide is a forwarding path from an
   ITR (or MR) which requires an EID-to-RLOC mapping to existing hardware and/or software an ETR which
   holds that are
   required mapping.  The ITR/MR uses this path to deploy send an ALT
   Datagram (see Section 3) to an ETR which then responds with a Map-
   Reply containing the needed mapping system.  It information.

   One design goal for LISP+ALT is envisioned that in most
   cases to use existing technology can be used wherever
   possible.  To this end, the ALT is intended to be built using off-
   the-shelf routers which already implement and deploy LISP+
   ALT.  Since the deployment of LISP+ALT adds new required protocols (BGP
   and GRE); little, if any, LISP-specific modifications should be
   needed for such devices to be deployed on the
   network, existing devices not need changes or upgrades.  They can
   function as they ALT.  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 which are to realize an underlying already participating in the global routing system
   and robust physical
   topology. actively forwarding Internet traffic is not recommended.

   The remainder of this document is organized as follows: Section 3 2
   provides the definitions of terms used in this document.  Section 4 3
   outlines the basic LISP 1.5 model.  Section 5 4 provides a basic
   overview of the LISP Alternate Topology architecture, and Section 6 5
   describes how the ALT uses BGP to propagate Endpoint Identifier
   reachability over the overlay network. network and Section 8 6 describes other
   considerations for using BGP on the ALT.  Section 7 describes the
   construction of the ALT aggregation hierarchy, and Section 9 8
   discusses how LISP+ALT elements are connected to form the overlay


2.  Definition of Terms

   LISP+ALT operates on two name spaces and introduces a new network
   element, the LISP+ALT Router (see below).  This section provides
   high-level definitions of the LISP+ALT name spaces, network elements,
   and message types.


    Alternative Logical Topology (ALT):  The virtual overlay network
      made up of tunnels between EID Prefix Aggregators. LISP+ALT Routers.  The Border Gateway
      Protocol (BGP) runs between ALT routers Routers and is used to carry
      reachability information for EID prefixes. 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 use of it to
      forward high-bandwidth flows of Data Probes is strongly
      discouraged (see Section 3.3 for additional discussion).

    Legacy Internet:  The portion of the Internet which does not run
      LISP and does not participate in LISP+ALT.


    ALT Router:  The devices which run on the ALT.  The ALT is a static
      network built using tunnels between LISP+ALT routers. ALT Routers.  These routers
      are deployed in a hierarchy roughly-hierarchical mesh in which routers at
      each level in the this hierarchy topology are responsible for aggregating all
      EID EID-
      prefixes learned from those logically "below" them and advertising
      summary prefixes to the routers those logically "above" them.  All prefix  Prefix learning
      and propagation between levels ALT Routers is done using BGP.  A LISP+ALT router  An ALT
      Router at the lowest level, or "edge" of the ALT, learns EID EID-
      prefixes from its "client" ETRs.  See Section 4.1 3.1 for a
      description of how EID prefixes EID-prefixes are learned at the "edge" of the
      ALT.  See also Section 7 6 for details on how BGP is configured
      between the different network elements.

      The primary function of LISP+ALT routers is to provide a
      lightweight  When an ALT Router
      receives an ALT Datagram, it looks up the destination EID in its
      forwarding infrastructure for LISP control-plane
      messages (Map-Request and Map-Reply), table (composed of EID prefix routes it learned from
      neighboring ALT Routers) and forwards it to transport data
      packets when the packet has the same destination address in both logical next-hop
      on the inner (encapsulating) destination and outer destination
      addresses ((i.e., a Data Probe packet). overlay network.

    Endpoint ID (EID):  A 32-bit (for IPv4) or 128-bit (for ipv6) value
      used in to identify the ultimate source and or destination address fields of the first
      (most inner) LISP header of for a LISP-
      encapsulated packet.  See [LISP] for details.

    EID-prefix:  A packet that is emitted by
      a system contains set of EIDs delegated in its headers and LISP headers a power-of-two block.  EID-
      prefixes are
      prepended only when routed on the packet reaches an Ingress Tunnel Router
      (ITR) ALT (not on the data path global Internet) and
      are expected to the destination EID.

      In LISP+ALT, EID-prefixes MUST BE be assigned in a hierarchical manner (in power-of-two) such that
      they can be aggregated by LISP+ ALT routers.  In addition, 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 have site-local put site-local, topologically-relevant
      structure in
      how EIDs are topologically organized (subnetting) into an EID-prefix for routing
      within the site; this structure is not visible to the global
      routing system.

   EID-Prefix Aggregate: intra-site routing.

    Aggregated EID-prefixes:  A set of individual EID-prefixes said to be aggregatable that have
      been aggregated in the [RFC4632] sense.  That is, an EID-Prefix aggregate is
      defined to be a single contiguous power-of-two EID-prefix block.
      Such a block is characterized by

    Map Server (MS):   An edge ALT Router that provides a prefix registration
      function for non-ALT-connected ETRs, originates EID-prefixes into
      the ALT on behalf of those ETRs, and a length. forwards Map-Requests to
      them.  See [LISP-MS] 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 which owns the requested
      EID-prefix.  See [LISP-MS] for details.

    Ingress Tunnel Router (ITR):   A router which sends LISP Map-
      Requests or encapsulates IP datagrams with LISP headers, as
      defined in [LISP].  In this document, the term refers to any
      device implementing ITR functionality, including a Proxy-ITR (see
      [LISP-IW]).  Under some circumstances, a LISP Map Resolver may
      also originate Map-Requests (see [LISP-MS]).

    Egress Tunnel Router (ETR):   A router which sends LISP Map-Replies
      in response to LISP Map-Requests and decapsulates LISP-
      encapsulated IP datagrams for delivery to end systems, as defined
      in [LISP].  In this document, the term refers to any device
      implementing ETR functionality, including a Proxy-ETR (see
      [LISP-IW]).  Under some circumstances, a LISP Map Server may also
      respond to Map-Requests (see [LISP-MS]).

    Routing Locator (RLOC):  An  A routable IP address of an egress for a LISP tunnel
      (ETR).  It (ITR or ETR).  Interchangeably referred to as a "locator"
      in this document.  An RLOC is also the output of a an EID-to-RLOC
      mapping lookup.  An EID 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 to which 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 Aggregatable (PA) addresses.
      Note that in LISP+ALT,  Routing for RLOCs are is not
      carried by LISP+ALT routers. on the ALT.

    EID-to-RLOC Mapping:  A binding between an EID EID-prefix and the RLOC-set set of
      RLOCs that can be used to reach the EID.  The term "mapping" refers it; sometimes referred to an
      EID-to-RLOC mapping.

    EID Prefix simply
      as a "mapping".

    EID-prefix Reachability:  An EID prefix EID-prefix is said to be "reachable" if
      at least one or more of its locators are is reachable.  That is, an EID prefix EID-prefix
      is reachable if the ETR (or its proxy) that is authoritative for a given EID-to-RLOC EID-to-
      RLOC mapping is reachable.

    Default Mapping:  A Default Mapping is a mapping entry for EID-
      prefix (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 mapping cache it overrides the Default Mapping
      entry.  The Default Mapping route can be learned by configuration or
      from a Map-Reply message.

    ALT Default Route:  A Default Route in the context of LISP+ALT is a EID-
      prefix  An EID-prefix value of (or 0::/0 for
      ipv6) which is advertised
      by BGP on top of may be learned from the ALT. ALT or statically configured
      on an edge ALT Router.  The Default ALT-Default Route is used to create defines a forwarding
      path for a packet to be sent into the ALT (and ALT
      datagram) on a router which does
      not have a full ALT forwarding database.


3.  The LISP 1.5 LISP+ALT model

   As documented in [LISP], the LISP 1.5

   The LISP+ALT model uses the same basic query/response protocol machinery as LISP 1.0. that
   is documented in [LISP].  In particular, LISP+
   ALT LISP+ALT provides two mechanisms for types
   of packet that an ITR can originate to obtain EID-to-RLOC mappings
   (both mappings:

   Map-Request:  A Map-Request message is sent into the ALT to request
      an EID-to-RLOC mapping.  The ETR which 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 these techniques are described in more detail in
   Section 9.2): the Map-
      Request sent on the ALT must be an EID.  See [LISP] for the format
      of Map-Request and Map-Reply packets.

   Data Probe:  An  Alternatively, an ITR may encapsulate and send the first few
      data packets packet destined for an EID with no known RLOCs into the ALT
      as a Data Probe.  This might be done minimize packet loss and to
      probe for the mapping; 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 in this
      case, the Data Probe's inner Destination Address (DA), IP destination address,
      which is an EID, is copied to the outer DA and is IP destination address so
      that the resulting packet can be routed over the ALT.

   Map-Request:  An ITR may also send a Map-Request message into the ALT
      to request the mapping.  As in the Data Probe case, the
      authoritative ETR will respond to the ITR with a Map-Reply
      message.  Since the ALT only forwards on EID destinations, the DA
      of the Map-Request sent in to the ALT MUST be an EID.  See [LISP]
      Section 3.3 for caveats on the format usability of Map-Request and Map-Reply packets.

   ALT datagram:  A Data Probes.

   The term "ALT Datagram" is short-hand for a Map-Request or Data Probe
   to be sent into or forwarded on the ALT.

4.1.  Routeability  Note that while the outer
   header Source Address of EIDs

   As with LISP 1.0, EIDs are routable and can an ALT Datagram is currently expected to be used, unaltered, as
   the source and destination addresses in IP datagrams.  Unlike in LISP
   1.0, LISP 1.5
   an RLOC, there may be situations (e.g. for experimentation with
   caching in intermediate ALT nodes) where an EID would be used to
   force a Map-Reply to be routed back through the ALT.

3.1.  Routeability of EIDs

   A LISP EID has the same syntax as 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; instead,
   they are only routed over LISP+
   ALT provides a separate, virtual topology referred to network, known as the LISP
   Alternative Virtual Network. 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 LISP+ALT
   routers. ALT Routers.  BGP is run runs over these tunnels to propagate the
   path information needed to route forward ALT datagrams. Datagrams.  Importantly, while
   the ETRs are the source(s) of the unaggregated EID prefix data, EID-prefixes, LISP+ALT
   uses existing BGP mechanisms to aggressively aggregate this information.  Note that

3.1.1.  Mechanisms for an ETR is not required to participate (or prevented from
   participating) in LISP+ALT; originate EID-prefixes

   There are three ways that an ETR may choose to communicate originate its mappings to its serving LISP+ALT router(s) using subscription time
   static configuration or through into the

   1.  By registration with a dynamic mechanism such Map Server as that
   described documented in [LISP-MS].  An ITR may similarly use a static EID
   "default route" or other configuration as described in [LISP-MS] to
   avoid the complexity of participating in
       This is the ALT.

4.2.  Connectivity common case and is expected to non-LISP sites

   As stated above, EIDs be used as IP addresses by LISP sites are not
   routable on the public Internet.  This implies that, absent
       majority of ETRs.

   2.  Using 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 [Interworking] for

4.3.  Caveats "static route" on the use of Data Probes

   It ALT.  Where no Map-Server is worth noting that there has been
       available, an edge ALT Router may be configured with a great deal of discussion and
   controversy about "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.

3.1.2.  Mechanisms for an ITR to forward to EID-prefixes

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

   1.  Through a Map Resolver as documented in [LISP-MS].  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.

3.1.3.  Map Server Model preferred

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

3.2.  Connectivity to non-LISP sites

   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 [LISP-IW] for

3.3.  Caveats on the use of Data Probes

   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 rate-limit
   traffic to avoid congestion or DoS attacks.  There are may also other be issues involving
   caused by different latency or other differences performance characteristics
   between the ALT path
   that taken by an initial a Data Probe would take and the
   "Internet" path that taken by subsequent packets on the same flow would take once a
   mapping were is in place on an ITR.  For these and other reasons reasons, the use of Data
   Probes is not recommended at this time; they should only be
   considered experimental
   originated an ITR when explicitly configured to do so and such
   configuration should only be disabled by default in all ITR

5. enabled when performing experiments
   intended to test the viability of using Data Probes.

4.  LISP+ALT: Overview

   LISP+ALT is a hybrid push/pull architecture.  Aggregated EID prefixes EID-prefixes
   are "pushed" advertised among the LISP+ALT routers and, optionally, out ALT Routers and to those (rare) ITRs
   (which may elect that
   are directly connected via a tunnel and BGP to receive the aggregated information, as opposed to
   simply using a default mapping). ALT.  Specific
   EID-to-RLOC mappings are
   "pulled" requested by ITRs an ITR (and returned by an ETR)
   using LISP when they it sends a request either send explicit LISP requests via a Map Resolver or data
   packets on the alternate topology that result in triggered replies
   being generated by ETRs. to an
   edge ALT Router.

   The basic idea embodied in LISP+ALT is to use BGP, running over on a
   tunneled overlay network, network (the ALT), to establish reachability required to route between
   ALT datagrams over an alternate logical topology (ALT). Routers.  The ALT
   BGPRoute BGP Route Information Base (RIB) is comprised
   of EID prefixes EID-prefixes and associated next hops.  LISP+ALT routers  ALT Routers interconnect
   using eBGP BGP and propagate EID EID-prefix updates among themselves.  EID-
   prefix updates, which are information is learned over eBGP connections
   to authoritative ETRs, or by from ETRs at the "edge" of the ALT
   either through the use of the Map Server interface (the commmon
   case), static configuration.  ITRs may also
   eBGP peer with one configuration, or more LISP+ALT by BGP-speaking ETRs.

   An ITR uses the ALT to learn the best ALT router to
   use to forward path for forwarding an ALT datagram for
   Datagram destined to a particular prefix; in most
   cases, an EID-prefix.  An ITR will have normally
   use a default EID mapping pointing Map Resolver to one send its ALT Datagrams on to the ALT but may,
   in unusual circumstances, use a static ALT Default Route or more
   LISP+ALT routers. connect
   to the ALT using BGP.

   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.  In technologies,
   particularly in cases where GRE does not meet security, management,
   or other operational
   requirements, it is reasonable to use another tunneling technology
   that does.  References requirements.  References to "GRE tunnel" in
   later sections of this document should therefore not be taken as
   prohibiting or precluding the use of other, available other tunneling mechanisms.
   Note also that two
   LISP+ALT routers 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.  Use of techniques, techniques such as "eBGP multihop",
   multihop" to forward connect ALT
   datagrams through routers Routers that do not participate share a tunnel or common
   subnet is not recommended as the non-ALT Routers in between the ALT routing, is
   Routers in such a configuration may not recommended. have information necessary to
   forward ALT Datagrams destined to EID-prefixes exchanged across that
   BGP session.

   In summary, LISP+ALT uses BGP to propagate EID-prefix update
   information to facilitate forwarding build paths through ALT Routers so
   that an ALT datagram 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 ipv6 NLRI without modification
   (since an EID
   prefix EID-prefix has the same syntax as IPv4 or IPv6 ipv6 address
   prefix).  LISP+ALT
   routers eBGP peer  ALT Routers establish BGP sessions with one another,
   forming the ALT.  A LISP+ALT
   router near  An ALT Router at the edge "edge" of the topology learns EID prefixes
   EID-prefixes originated by authoritative ETRs.  This  Learning may be via eBGP with
   though the ETRs, Map Server interface, by static configuration, or through some other dynamic mechanism such as that defined in
   [LISP-MS].  A LISP+ALT router via BGP
   with the ETRs.  An ALT Router may also be configured to aggregate EID
   EID-prefixes received from ETRs or from other LISP+ALT routers that
   are topologically "downstream" from it.


4.1.  ITR traffic handling

   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
   mapping cache, the ITR encapsulates the packet in creates either a LISP header, copying Map-Request for the
   destination address (EID) to EID or the outer destination address
   (RLOC), and transmits it through a GRE tunnel to original packet encapsulated as a LISP+ALT router in Data Probe
   (see Section 3.3 for caveats on the usability of Data Probes).  The
   result, known as an ALT Datagram, is then sent to an ALT Router (see
   also [LISP-MS] for non-ALT-connected ITRs, noting that
   an ITR cannot send Data Probes
   cannot be sent to a Map-Server). Map-Resolver).  This "first hop"
   LISP+ALT router ALT Router uses
   EID-prefix routing information learned from other LISP+ALT routers ALT Routers via BGP
   to guide the packet to the ETR which "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.  The EID-prefix.  For Data Probes, the ETR also de-encapsulates
   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 EID-prefix
   are routed directly to a viable ETR an RLOC without use of the ALT, until either
   the entry's TTL has expired, or the ITR can otherwise find no
   reachable ETR.  Note that a valid current mapping (not timed-out) may exist that contains
   no reachable RLOCs (i.e. all paths to that ETR are
   down); in RLOCs; this case, is known as a Negative Cache Entry and it
   indicates that packets destined to the EID prefix EID-prefix are dropped,
   not routed through the ALT. to be dropped.

   Full details on Map-Request/Map-Reply processing may be found in

   Traffic routed over on to the ALT therefore consists of:

   o  EID prefix Map-Requests, and

   o  data packets destined for those EID prefixes while solely of ALT Datagrams, i.e.
   Map-Requests and Data Probes (if supported).  Given the ITR awaits
      map replies

5.2. relatively
   low performance expected of a tuneled 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.

4.2.  EID Assignment - Hierarchy and Topology

   EID-prefixes will are expected to be allocated to a LISP site by Internet
   Multiple  Where a site has multiple allocations may not be in which are aligned
   on a power-of-2 blocks.  But when they
   are, block boundary, they will should be aggregated into a single, advertised EID-prefix.
   single EID-prefix for advertisement.  The ALT network is built in a tree-structured hierarchy
   roughly hierarchical, partial mesh which is intended to allow
   aggregation at merge points in the tree. 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 will do not need to change due respond to subscription changes in policy,
   subscription, or policy
   reasons, 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, a LISP+ALT
   router ALT Router should only be
   configured to generate an aggregate if it is configured with BGP
   sessions to all of the originators of components
   (more-specifics (more-specific
   prefixes) of that aggregate; not aggregate.  Not all of the components of 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.

   As an example, consider ETRs that are originating EID prefixes for,,, and  An ALT
   router should only be configured to generate an aggregate for 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.

   Under what circumstances the ALT router 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 [LISP]) 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 [LISP-MS].  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 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 which connects to the ALT using BGP must maintain BGP sessions
   with all of the ALT routers Routers that are configured to originate an
   aggregate which covers that prefix.  See also [LISP-MS] for an
   example 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 [LISP-MS] 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,,, and  An ALT
   Router should only be configured to generate an aggregate for 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 receives an ALT Datagram destined for, a non-
   LISP destination covered by the aggregate, it returns a Negative Map-
   Reply with action "Natively-Forward".  If it receives an ALT Datagram
   destined for but the configured LISP prefix 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 proceeds, a guide to how
   to best build the ALT network will be developed.

5.3.  LISP+ALT Router (or ALT router for short)

   A LISP+ALT Router has the following functionality:

   1.  It runs, at a minimum, the eBGP part

4.3.  Use of the BGP protocol.

   2.  It supports a separate RIB which uses next-hop GRE tunnel
       interfaces for forwarding ALT datagrams.

   3.  It can act as a "proxy-ITR" to support non-LISP sites.

   4.  It can act as an ETR, or as a recursive or re-encapsulating ITR
       to reduce mapping tables in site-based LISP routers.

5.4.  ITR and ETR in a LISP+ALT Environment

   An ITR using BGP between LISP+ALT may have additional functionality as follows:

   1.  If it Routers

   The ALT network is also acting as a LISP+ALT Router, it sends built using GRE tunnels between ALT datagrams
       on the Routers.  BGP best path computed GRE tunnel for
   sessions are configured over those tunnels, with each EID prefix.

   2.  When ALT Router
   acting solely as a ITR, it sends ALT datagrams directly to a
       configured LISP+ALT router.

   An ETR using LISP+ALT may also behave slightly differently:

   1.  If it is also acting as a LISP+ALT router, it advertises its
       configured EID-prefixes into BGP for distribution through the

   2.  It receives ALT datagrams only from its "upstream" LISP+ALT
       routers over the GRE tunnel(s) configured to it/them.  It
       responds with Map-Replies for the EID prefixes that it "owns".

5.5.  Use of GRE and BGP between LISP+ALT Routers

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

   LISP+ALT's use of GRE and BGP reduces provider Operational Expense
   (OPEX) facilities deployment and operation of
   LISP because no new protocols need to be either defined defined, implemented, or
   used on the overlay topology.  Also, 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 are local in
   scope, no coordination used to establish that tunnel is needed for their assignment; that the attached routers
   be reachable by each other; any addressing
   scheme (including plan, including private addressing)
   addressing, can therefore be used for tunnel

6.  EID Prefix ALT tunnels.

5.  EID-prefix Propagation and Map-Request Forwarding

   As described in Section 9.2, 8.2, an ITR may send either a Map-Request or
   a data probe sends an ALT Datagram to find a given
   EID-to-RLOC mapping.  The ALT provides the infrastructure that allows
   these requests to reach the authoritative ETR.

   Note that, that under normal circumstances, circumstances Map-Replies are not sent over
   the ALT - an ETR sends a Map-Reply to the source RLOC learned from
   the original Map-Request.  There may be scenarios, perhaps to
   encourage caching of EID-to-RLOC mappings by ALT routers, 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 sent returned to it; these the "first-hop" ALT
   Router.  These cases will not be supported by initial LISP+ALT
   implementations but may be subject to future experimentation.

   LISP+ALT routers

   ALT Routers propagate mapping path information for use via BGP ([RFC4271]) that is
   used by ITRs (when
   sending to send ALT datagrams) using eBGP [RFC4271]. eBGP Datagrams toward the appropriate ETR for
   each EID-prefix.  BGP is run on the
   inter-LISP+ALT router inter-ALT Router links, and
   possibly between an edge ("last hop") LISP+ALT router ALT Router and an ETR or
   between an edge ("first hop")
   LISP+ALT router ALT Router and an ITR.  The ALT eBGP BGP RIB
   consists of aggregated
   EID prefixes EID-prefixes and their next hops toward the
   authoritative ETR for that EID prefix.

6.1. EID-prefix.

5.1.  Changes to ITR behavior with LISP+ALT

   When using LISP+ALT,

   As previously described, an ITR sends ALT datagrams to one of will usually use the Map Resolver
   interface and will send its
   "upstream" LISP+ALT routers; these are sent only to 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 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 explicitly 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 explicitly routed
   throughout the ALT.  If the ITR is running BGP with the LISP+ALT
   router(s), it selects the appropriate LISP+ALT router ALT Router based on the BGP
   information received.  If it is not running BGP, it uses static
   configuration a
   statically-configued ALT Default Route to select a LISP+ALT router; in the general case, this
   will effectively be an "EID-prefix default route".

6.2. ALT Router.

5.2.  Changes to ETR behavior with LISP+ALT


   As previously described, an ETR will usually use the Map Server
   interface (see [LISP-MS]) and will register its EID-prefixes with its
   configured Map Servers.  When an ETR instead connects using BGP to
   one or more LISP+ALT router(s), ALT Routers, it
   simply announces its EID-prefix EID-prefix(es) to those LISP+ALT routers. ALT
   Routers.  Note that when an ETR generates a Map-Reply message to
   return to a querying ITR, it sends it to the ITR's source-RLOC (i.e.,
   on the underlying Internet topology, not on the ALT; this avoids any
   latency penalty (or "stretch") that might be incurred by routing over
   the ALT).


6.  BGP configuration and protocol considerations


6.1.  Autonomous System Numbers (ASNs) in LISP+ALT

   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
   mapping database which, while 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 LISP+ALT routers.
   ALT Routers.  To avoid confusion, it needs to be stressed that that
   these LISP+ALT ASNs use a new numbering space that is unrelated to
   the ASNs used by the global routing system.  Exactly how this new
   space will be assigned and managed will be determined during experimental the
   deployment of LISP+ALT.

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


6.2.  Sub-Address Family Identifier (SAFI) for LISP+ALT

   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, current
   dominant use of BGP) and managing the global EID-to-RLOC database creating an overlay network for finding EID-
   to-RLOC mappings (the use of BGP proposed by this document), it may
   be desirable to assign a new SAFI [RFC2858] [RFC4760] to prevent operational
   confusion and difficulties, including the inadvertent leaking of
   information from one domain to the other.  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 which
   do not understand the new SAFI could not be used to build any part of
   the ALT network.  At present, this document does not require request the
   assignment of a new SAFI but the authors anticipate that SAFI; additional experimentation may suggest the
   need for one in the future.

8.  EID-Prefix

7.  EID-prefix Aggregation

   The ALT BGP peering topology should be arranged in a tree-like
   fashion (with some meshiness), with redundancy to deal with node and
   link failures.  A basic assumption is that as long as the routers are
   up and running, the underlying topology Internet will provide alternative
   routes to maintain BGP connectivity among LISP+ALT routers. ALT Routers.

   Note that, as mentioned in Section 5.2, 4.2, the use of BGP by LISP+ALT
   requires that information can only be aggregated where all active
   more-specific more-
   specific prefixes of a generated aggregate prefix are known.  This implies, for example, that if a given set of prefixes is used by
   multiple, ALT networks, those networks must interconnect and share
   information about all of the prefixes if either were to generate an
   aggregate prefix that covered all of them.  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 to all prefixes that
   make up that aggregate.

8.1.  Traffic engineering with LISP and LISP+ALT

7.1.  Stability of the ALT

   It is worth noting that LISP+ALT does not directly propagate EID-to-
   RLOC mappings.  What it does is provide a mechanism for a LISP an ITR to
   commonicate with the ETR that holds the mapping for a particular EID EID-
   prefix.  This distinction is important for several reasons.  First, it means
   that when considering the reachability stability
   of RLOCs is learned through BGP on the LISP ITR-ETR
   exchange 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 Section 7.2

   RLOC prefixes are not propagated through the ALT so "flapping" their
   reachability is not determined through use of state information 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
   nor can and mapping information cannot become "stale" by due to slow
   propagation through the ALT BGP mesh.  Second, by deferring

7.2.  Traffic engineering using LISP

   Since an ITR learns an EID-to-RLOC mapping
   to an ITR-ETR exchange, directly from the ETR that
   owns it, it is possible to perform site-to-site traffic engineering through a combination of
   by setting the preference
   and and/or weight fields fields, and by returning including
   more-specific EID-to-RLOC information in LISP 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 regional-
   optimization of traffic flows, such more-
   specific 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; should it.  Should a
   receiving ITR decide that it does not wish to store such more-specific more-
   specific information, it has the option of discarding it as long as a
   shorter, covering EID prefix EID-prefix exists.  Not
   only does this greatly improve the scalability  Such an exchange of the global routing
   system but it also allows improved "more-
   specifics" between sites facilitates traffic engineering techniques engineering, by allowing
   richer and more fine-grained policies to be applied.

8.2. 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

7.3.  Edge aggregation and dampening

   Note also that normal

   Normal BGP best common practices apply to the ALT network.  In
   particular, first-hop ALT routers Routers will aggregate EID prefixes and
   dampen changes to them in the face of excessive updates.  Since EID EID-
   prefix assignments are not expected to change with anywhere as frequently as global
   routing BGP prefix reachability on the Internet, reachability, such dampening should be very rare rare,
   and might be worthy of logging as an exceptional event.  It is again
   worth noting that the ALT carries only EID
   prefixes, along with BGP-generated EID-prefixes, used to
   construct BGP paths to their owning ETRs; it does not carry
   reachability 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 ETRs that source
   those prefixes as advertisements travel over the logical topology;
   this set of prefix information is considerablly on the ALT can be expected to be both
   better aggregated and considerably less volatile than the actual EID-to-RLOC EID-
   to-RLOC mappings.


7.4.  EID assignment flexibility vs. ALT scaling

   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, re-use of existing IP address
   assignments, both "PI" and "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 Section 7.1, the ALT should not suffer from some of
       the performance constraints or stability issues as the Internet
       global routing system, so some reasonable level of deaggregation
       and 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
       multiple levels in the hierarchy to collapse de-aggregation 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.  Trade-
       offs 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, and 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.

8.  Connecting sites to the ALT network


8.1.  ETRs originating information into the ALT

   EID prefix

   EID-prefix information is originated into the ALT by two three different

   eBGP:  An ETR usually participates

   Map Server:  In most cases, a site will configure its ETR(s) to
      register with one or more Map Servers (see [LISP-MS]), and does
      not participate directly in the ALT.

   BGP:  For a site requiring complex control over their EID-prefix
      origination into the ALT, an ETR may connect to the LISP+ALT
      overlay network by running eBGP BGP to one or more LISP+ALT router(s) ALT Router(s) over
      tunnel(s).  The ETR advertises reachability for its EID prefixes EID-prefixes
      over these
      eBGP BGP connection(s).  The LISP+ALT router(s) edge ALT Router(s) that
      receive(s) these prefixes then propagate(s) them into the ALT.
      Here the ETR is simply an eBGP BGP peer of LISP+ALT router(s) ALT Router(s) at the edge of
      the ALT.  Where possible, a LISP+ALT router an ALT Router that receives EID prefixes EID-prefixes
      from an ETR via eBGP BGP should aggregate that information.

   Configuration:  One or more LISP+ALT router(s) ALT Router(s) may be configured to
      originate an EID prefix 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 LISP+ALT router(s) ALT Router(s) using GRE tunnel(s) but rather than BGP
      being used, the LISP+ALT router(s) ALT Router(s) are configured with what are in
      effect "static routes" for the EID prefixes EID-prefixes "owned" by the ETR.
      The GRE tunnel is used to route Map-Requests to the ETR.
      Note that the LISP+ALT router could also serve as a proxy for its
      TCP-connected ETRs.

   Note:  in both all cases, an ETR may have connections register to multiple Map Servers or
      connect to multiple
      LISP+ALT routers ALT Routers for the following reasons:

      *  redundancy, so that a particular ETR is still reachable through
         the ALT 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 EID-prefixes that
         cannot be aggregated by the same LISP+ALT router ALT Router (i.e. are not
         topologically "close" to each other in the ALT).


8.2.  ITRs Using the ALT

   In order to source Map-Requests to the ALT or common configuration, an ITR does not need to route a Data Probe
   packet over know anything
   about the ALT, each ITR participating in the ALT establishes a
   connection since it sends Map-Requests to one or more LISP+ALT routers.  These connections can be
   either eBGP or TCP (as described above).

   In the case in which the ITR of its configured
   Map-Resolvers (see [LISP-MS]).  There are two exceptional cases:

   Static default:  If a Map Resolver is running eBGP, the peer LISP+ALT
   routers use these connections to advertise highly aggregated EID-
   prefixes to the peer ITRs.  The not available but an ITR then installs the received
   prefixes into a forwarding table that is used to to send LISP Map-
      adjacent to an ALT Router (either over a common subnet or through
      the appropriate LISP+ALT router.  In most cases, use of a LISP+ tunnel), it can use an ALT router will send a default mapping Default Route route to its client ITRs so
      cause all ALT Datagrams to be sent that
   they can send request for any EID prefix into the ALT.

   In the ALT Router.  This case in which the ITR is connected
      expected to some set of LISP+ALT
   routers without eBGP, the ITR sends Map-Requests be rare.

   Connection to any ALT:  A site with complex Internet connectivity needs
      may need more fine-grained distinction between traffic to LISP-
      capable and non-LISP-capable sites.  Such a site may configure
      each of its
   connected LISP+ALT routers.

   An ITR may also choose ITRs to send the first few data packets over the
   ALT connect directly to minimize packet loss the ALT, using a tunnel
      and reduce mapping latency. BGP connection.  In this case, the data packet serves as a mapping probe (Data Probe) and the
   ETR which receives the data packet (over the ALT) responds with a
   Map-Reply is sent ITR will receive EID-prefix
      routes from its BGP connection to the ITR's source-RLOC using ALT Router and will LISP-
      encapsulate and send ALT Datagrams through the underlying
   topology.  Note tunnel to the ALT
      Router.  Traffic to other destinations may be forwarded (without
      LISP encapsulation) to non-LISP next-hop routers that the use of Data Probes is discouraged at this
   time (see Section 4.3). ITR

      In general, an ITR will establish connections that connects to the ALT does so only to LISP+ALT
   routers to ALT
      Routers at the "edge" of the ALT (typically two for redundancy) but
   there may also redundancy).
      There may, though, be situations where an ITR would connect to
   LISP+ALT routers 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 LISP+ALT routers ALT Routers with the additional information.  This is a
      purely local policy issue between the ITR and the LISP+ALT routers ALT Routers in


   As described in [LISP-MS], 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 Section 3.3).

9.  IANA Considerations

   This document makes no request of the IANA.


10.  Security Considerations

   LISP+ALT shares many of the security characteristics of BGP.  Its
   security mechanisms are comprised of existing technologies in wide
   operational use today.  Securing LISP+ALT is much simpler than today, so securing BGP.

   Compared to BGP, LISP+ALT routers are not topologically bound,
   allowing them to the ALT should be put in locations away from mostly a matter
   of applying the vulnerable AS
   border (unlike eBGP speakers).

11.1. same technology that is used to secure the BGP-based
   global routing system (see Section 10.3 below).

10.1.  Apparent LISP+ALT Vulnerabilities

   This section briefly lists of the apparent known potential vulnerabilities of LISP+

   Mapping Integrity:  Can an attacker insert bogus mappings to black-
      hole (create a DoS) Denial-of-Service, or DoS attack) or intercept LISP
      data-plane packets?

   LISP+ALT router

   ALT Router Availability:  Can an attacker DoS the LISP+ALT
      routers ALT Routers
      connected to a given ETR? without access to  If a site's ETR cannot advertise its
      EID-to-RLOC mappings,
      a the site is essentially unavailable.

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

   EID Map-Request Exploits for Reconnaissance:  Can an attacker learn
      about a LISP destination sites' site's TE policy by sending legitimate mapping
      requests messages and then observing the RLOC mapping replies?  Is this
      information useful in attacking or subverting peer relationships?
      Note that any public LISP 1.0 has a mapping database will have similar data-plane data-
      plane reconnaissance issue.

   Scaling of LISP+ALT router 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 denial-of-service attacks.  For this reason,
      all components of the ALT (ETRs and ALT routers) Routers) should be
      prepared to rate-limit traffic (ALT datagrams) Datagrams) that could be
      received across the ALT.

   UDP Map-Reply from ETR:  Since Map-Replies packets are sent directly from the
      ETR to the ITR's RLOC, ITR's RLOC, the ITR's RLOC may be vulnerable to various
      types of DoS attacks (this is a general property of LISP, not an
      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 [LISP-IW] much more so, accidental
      leaking or malicious advertisement of an EID-prefix into the ITR's RLOC may
      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 vulnerable
      to various types of DoS attacks.

11.2. achieved by much more strict prefix filtering
      and authentication on the global routing system.

10.2.  Survey of LISP+ALT Security Mechanisms

   Explicit peering:  The devices themselves can both prioritize
      incoming packets 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 Protected TCP BGP/TCP Connections:  HMAC is used to verify
      message integrity and authenticity, making it nearly impossible
      for third party devices to either insert or modify messages.

   Message Sequence Numbers and Nonce Values in Messages:  This allows
      for devices
      an ITR to verify that the mapping-reply packet was Map-Reply from an ETR is in response to the mapping-request
      a Map-Request originated by that they sent.

11.3.  Using existing ITR (this is a general property
      of LISP; LISP+ALT does not change this behavior).

10.3.  Use of new IETF standard BGP Security mechanisms

   LISP+ALT's use of BGP allows for the ALT to take advantage of BGP
   security features designed for existing Internet BGP use.

   For example, should either sBGP S-BGP [I-D.murphy-bgp-secr] or soBGP
   [I-D.white-sobgparchitecture] become widely deployed it expected that
   LISP+ALT could use these mechanisms to provide authentication of EID-
   to-RLOC mappings, and EID origination.


11.  Acknowledgments


   The authors would like to specially thank J. Noel Chiappa who was a
   key contributer to the design of the LISP-CONS mapping database (many
   ideas described in this document were developed during
   detailed discussions with Scott Brim from which made their way into LISP+ALT) and Darrel Lewis, who made many
   insightful comments on earlier versions of this document.  Additional
   thanks are due has continued
   to provide invaluable insight as the LISP effort has evolved.  Others
   who have provided valuable contributions include John Zwiebel, Hannu Flinck and
   Flinck, Amit Jain who offered many helpful
   suggestions for the -02 version.

13. Jain, John Scudder, and Scott Brim.

12.  References


12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use

   [LISP]     Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-06.txt (work in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997. progress), January 2010.

   [LISP-MS]  Fuller, V. and D. Farinacci, "LISP Map Server",
              draft-ietf-lisp-ms-04.txt (work in progress),
              October 2009.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              March 2000.

   [RFC2858]  Bates, T., Rekhter, Y., Chandra, R., and D. Katz,
              "Multiprotocol Extensions for BGP-4", RFC 2858, June 2000.

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

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, August 2006.


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

12.2.  Informative References

              Murphy, S., "BGP Security Analysis",
              draft-murphy-bgp-secr-04 (work in progress),
              November 2001.

              White, R., "Architecture and Deployment Considerations for
              Secure Origin BGP (soBGP)",
              draft-white-sobgparchitecture-00 (work in progress),
              May 2004.


   [LISP-IW]  Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking LISP with IPv4 and ipv6",
              draft-ietf-lisp-interworking-01.txt (work in progress),
              January 2010.

   [LISP]     Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-interworking-02.txt (work in progress), January
              February 2010.

   [LISP-MS]  Fuller, V. and D. Farinacci, "LISP Map Server",
              draft-ietf-lisp-ms-04.txt (work in progress),
              October 2009.

Authors' Addresses

   Vince Fuller
   Tasman Drive
   San Jose, CA  95134


   Dino Farinacci
   Tasman Drive
   San Jose, CA  95134


   Dave Meyer
   Tasman Drive
   San Jose, CA  95134


   Darrel Lewis
   Tasman Drive
   San Jose, CA  95134