Network Working Group                                           L. Jakab
Internet-Draft                                      A. Cabellos-Aparicio
Intended status: Informational                                  F. Coras
Expires: October 10, 2011                             J. Domingo-Pascual
                                       Technical University of Catalonia
                                                                D. Lewis
                                                           Cisco Systems
                                                           April 8, 2011

             LISP Network Element Deployment Considerations


   This document discusses the different scenarios for the deployment of
   the new network elements introduced by the Locator/Identifier
   Separation Protocol (LISP).

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on October 10, 2011.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Tunnel Routers . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Customer Edge  . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Provider Edge  . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Split ITR/ETR  . . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  Inter-Service Provider Traffic Engineering . . . . . . . .  8
     2.5.  Tunnel Routers Behind NAT  . . . . . . . . . . . . . . . . 10
       2.5.1.  ITR  . . . . . . . . . . . . . . . . . . . . . . . . . 10
       2.5.2.  ETR  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     2.6.  Summary and Feature Matrix . . . . . . . . . . . . . . . . 11
   3.  Map-Resolvers and Map-Servers  . . . . . . . . . . . . . . . . 11
     3.1.  Map-Servers  . . . . . . . . . . . . . . . . . . . . . . . 11
     3.2.  Map-Resolvers  . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Proxy Tunnel Routers . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  P-ITR  . . . . . . . . . . . . . . . . . . . . . . . . . . 13
       4.1.1.  LISP+BGP . . . . . . . . . . . . . . . . . . . . . . . 14
       4.1.2.  Mapping Service Provider P-ITR Service . . . . . . . . 15
       4.1.3.  Tier 1 P-ITR Service . . . . . . . . . . . . . . . . . 15
       4.1.4.  Migration Summary  . . . . . . . . . . . . . . . . . . 17
     4.2.  P-ETR  . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   The Locator/Identifier Separation Protocol (LISP) addresses the
   scaling issues of the global Internet routing system by separating
   the current addressing scheme into Endpoint IDentifiers (EIDs) and
   Routing LOCators (RLOCs).  The main protocol specification
   [I-D.ietf-lisp] describes how the separation is achieved, which new
   network elements are introduced, and details the packet formats for
   the data and control planes.

   While the boundary between the core and edge is not strictly defined,
   one widely accepted definition places it at the border routers of
   stub autonomous systems, which may carry a partial or complete
   default-free zone (DFZ) routing table.  The initial design of LISP
   took this location as a baseline for protocol development.  However,
   the applications of LISP go beyond of just decreasing the size of the
   DFZ routing table, and include improved multihoming and ingress
   traffic engineering (TE) support for edge networks, and even
   individual hosts.  Throughout the draft we will use the term LISP
   site to refer to these networks/hosts behind a LISP Tunnel Router.
   We formally define it as:

   LISP site:  A single host or a set of network elements in an edge
      network under the administrative control of a single organization,
      delimited from other networks by LISP Tunnel Router(s).

   Since LISP is a protocol which can be used for different purposes, it
   is important to identify possible deployment scenarios and the
   additional requirements they may impose on the protocol specification
   and other protocols.  The main specification [I-D.ietf-lisp] mentions
   positioning of tunnel routers, but without an in-depth discussion.
   This document fills that gap, by exploring the most common cases.
   While the theoretical combinations of device placements are quite
   numerous, the more practical scenarios are given preference in the

   Additionally, this documents is intended as a guide for the
   operational community for LISP deployments in their networks.  It is
   expected to evolve as LISP deployment progresses, and the described
   scenarios are better understood or new scenarios are discovered.

   Each subsection considers an element type, discussing the impact of
   deployment scenarios on the protocol specification.  For definition
   of terms, please refer to the appropriate documents (as cited in the
   respective sections).

   Comments and discussions about this memo should be directed to the
   LISP working group mailing list:

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2.  Tunnel Routers

   LISP is a map-and-encap protocol, with the main goal of improving
   global routing scalability.  To achieve its goal, it introduces
   several new network elements, each performing specific functions
   necessary to separate the edge from the core.  The device that is the
   gateway between the edge and the core is called Tunnel Router (xTR),
   performing one or both of two separate functions:

   1.  Encapsulating packets originating from an end host to be
       transported over intermediary (transit) networks towards the
       other end-point of the communication

   2.  Decapsulating packets entering from intermediary (transit)
       networks, originated at a remote end host.

   The first function is performed by an Ingress Tunnel Router (ITR),
   the second by an Egress Tunnel Router (ETR).

   Section 8 of the main LISP specification [I-D.ietf-lisp] has a short
   discussion of where Tunnel Routers can be deployed and some of the
   associated advantages and disadvantages.  This section adds more
   detail to the scenarios presented there, and provides additional
   scenarios as well.

2.1.  Customer Edge

   LISP was designed with deployment at the core-edge boundary in mind,
   which can be approximated as the set of DFZ routers belonging to non-
   transit ASes.  For the purposes of this document, we will consider
   this boundary to be consisting of the routers connecting LISP sites
   to their upstreams.  As such, this is the most common expected
   scenario for xTRs, and this document considers it the reference
   location, comparing the other scenarios to this one.

                                ISP1    ISP2
                                 |        |
                                 |        |
                               +----+  +----+
                            |  +----+  +----+  |
                            |                  |
                            |     LISP site    |

                    Figure 1: xTRs at the customer edge

   From the LISP site perspective the main advantage of this type of

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   deployment (compared to the one described in the next section) is
   having direct control over its ingress traffic engineering.  This
   makes it is easy to set up and maintain active/active, active/backup,
   or more complex TE policies, without involving third parties.

   Being under the same administrative control, reachability information
   of all ETRs is easier to synchronize, because the necessary control
   traffic can be allowed between the locators of the ETRs.  A correct
   synchronous global view of the reachability status is thus available,
   and the Loc-Status-Bits can be set correctly in the LISP data header
   of outgoing packets.

   By placing the tunnel router at the edge of the site, existing
   internal network configuration does not need to be modified.
   Firewall rules, router configurations and address assignments inside
   the LISP site remain unchanged.  This helps with incremental
   deployment and allows a quick upgrade path to LISP.  For larger sites
   with many external connections, distributed in geographically diverse
   PoPs, and complex internal topology, it may however make more sense
   to both encapsulate and decapsulate as soon as possible, to benefit
   from the information in the IGP to choose the best path (see
   Section 2.3 for a discussion of this scenario).

   Another thing to consider when placing tunnel routers are MTU issues.
   Since encapsulating packets increases overhead, the MTU of the end-
   to-end path may decrease, when encapsulated packets need to travel
   over segments having close to minimum MTU.  Some transit networks are
   known to provide larger MTU than the typical value of 1500 bytes of
   popular access technologies used at end hosts (e.g., IEEE 802.3 and
   802.11).  However, placing the LISP router connecting to such a
   network at the customer edge could possibly bring up MTU issues,
   depending on the link type to the provider as opposed to the
   following scenario.

2.2.  Provider Edge

   The other location at the core-edge boundary for deploying LISP
   routers is at the Internet service provider edge.  The main incentive
   for this case is that the customer does not have to upgrade the CE
   router(s), or change the configuration of any equipment.
   Encapsulation/decapsulation happens in the provider's network, which
   may be able to serve several customers with a single device.  For
   large ISPs with many residential/business customers asking for LISP
   this can lead to important savings, since there is no need to upgrade
   the software (or hardware, if it's the case) at each client's
   location.  Instead, they can upgrade the software (or hardware) on a
   few PE routers serving the customers.  This scenario is depicted in
   Figure 2.

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                  +----------+        +------------------+
                  |   ISP1   |        |       ISP2       |
                  |          |        |                  |
                  |  +----+  |        |  +----+  +----+  |
                  +--|xTR1|--+        +--|xTR2|--|xTR3|--+
                     +----+              +----+  +----+
                        |                  |       |
                        |                  |       |
                        +--<[LISP site]>---+-------+

                          Figure 2: xTR at the PE

   While this approach can make transition easy for customers and may be
   cheaper for providers, the LISP site looses one of the main benefits
   of LISP: ingress traffic engineering.  Since the provider controls
   the ETRs, additional complexity would be needed to allow customers to
   modify their mapping entries.

   The problem is aggravated when the LISP site is multihomed.  Consider
   the scenario in Figure 2: whenever a change to TE policies is
   required, the customer contacts both ISP1 and ISP2 to make the
   necessary changes on the routers (if they provide this possibility).
   It is however unlikely, that both ISPs will apply changes
   simultaneously, which may lead to inconsistent state for the mappings
   of the LISP site (e.g., weights for the same priority don't sum 100).
   Since the different upstream ISPs are usually competing business
   entities, the ETRs may even be configured to compete, either to
   attract all the traffic or to get no traffic.  The former will happen
   if the customer pays per volume, the latter if the connectivity has a
   fixed price.  A solution could be to have the mappings in the Map-
   Server(s), and have their operator give control over the entries to
   customer, much like in today's DNS.

   Additionally, since xTR1, xTR2, and xTR3 are in different
   administrative domains, locator reachability information is unlikely
   to be exchanged among them, making it difficult to set Loc-Status-
   Bits correctly on encapsulated packets.

   Compared to the customer edge scenario, deploying LISP at the
   provider edge might have the advantage of diminishing potential MTU
   issues, because the tunnel router is closer to the core, where links
   typically have higher MTUs than edge network links.

2.3.  Split ITR/ETR

   In a simple LISP deployment, xTRs are located at the border of the
   LISP site (see Section 2.1).  In this scenario packets are routed
   inside the domain according to the EID.  However, more complex

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   networks may want to route packets according to the destination RLOC.
   This would enable them to choose the best egress point.

   The LISP specification separates the ITR and ETR functionality and
   considers that both entities can be deployed in separated network
   equipment.  ITRs can be deployed closer to the host (i.e., access
   routers).  This way packets are encapsulated as soon as possible, and
   packets exit the network through the best egress point in terms of
   BGP policy.  In turn, ETRs can be deployed at the border routers of
   the network, and packets are decapsulated as soon as possible.
   Again, once decapsulated packets are routed according to the EID, and
   can follow the best path according to internal routing policy.

   In the following figure we can see an example.  The Source (S)
   transmits packets using its EID and in this particular case packets
   are encapsulated at ITR_1.  The encapsulated packets are routed
   inside the domain according to the destination RLOC, and can egress
   the network through the best point (i.e., closer to the RLOC's AS).
   On the other hand, inbound packets are received by ETR_1 which
   decapsulates them.  Then packets are routed towards S according to
   the EID, again following the best path.

      |                                       |
      |       +-------+                   +-------+         +-------+
      |       | ITR_1 |---------+         | ETR_1 |-RLOC_A--| ISP_A |
      |       +-------+         |         +-------+         +-------+
      |  +-+        |           |             |
      |  |S|        |    IGP    |             |
      |  +-+        |           |             |
      |       +-------+         |         +-------+         +-------+
      |       | ITR_2 |---------+         | ETR_2 |-RLOC_B--| ISP_B |
      |       +-------+                   +-------+         +-------+
      |                                       |

                     Figure 3: Split ITR/ETR Scenario

   This scenario has a set of implications:

   o  The site must carry at least partial BGP routes in order to choose
      the best egress point, increasing the complexity of the network.
      However, this is usually already the case for LISP sites that
      would benefit from this scenario.

   o  If the site is multihomed to different ISPs and any of the
      upstream ISPs is doing uRPF filtering, this scenario may become
      impractical.  ITRs need to determine the exit ETR, for setting the

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      correct source RLOC in the encapsulation header.  This adds
      complexity and reliability concerns.

   o  In LISP, ITRs set the reachability bits when encapsulating data
      packets.  Hence, ITRs need a mechanism to be aware of the liveness
      of ETRs.

   o  ITRs encapsulate packets and in order to achieve efficient
      communications, the MTU of the site must be large enough to
      accommodate this extra header.

   o  In this scenario, each ITR is serving fewer hosts than in the case
      when it is deployed at the border of the network.  It has been
      shown that cache hit ratio grows logarithmically with the amount
      of users [cache].  Taking this into account, when ITRs are
      deployed closer to the host the effectiveness of the mapping cache
      may be lower (i.e., the miss ratio is higher).  Another
      consequence of this is that the site will transmit a higher amount
      of Map-Requests, increasing the load on the distributed mapping

2.4.  Inter-Service Provider Traffic Engineering

   With LISP, two LISP sites can route packets among them and control
   their ingress TE policies.  Typically, LISP is seen as applicable to
   stub networks, however the LISP protocol can also be applied to
   transit networks recursively.

   Consider the scenario depicted in Figure 4.  Packets originating from
   the LISP site Stub1, client of ISP_A, with destination Stub4, client
   of ISP_B, are LISP encapsulated at their entry point into the ISP_A's
   network.  The external IP header now has as the source RLOC an IP
   from ISP_A's address space (R_A1, R_A2, or R_A3) and destination RLOC
   from ISP_B's address space (R_B1 or R_B2).  One or more ASes separate
   ISP_A from ISP_B. With a single level of LISP encapsulation, Stub4
   has control over its ingress traffic.  However, ISP_B only has the
   current tools (such as BGP prefix deaggregation) to control on which
   of his own upstream or peering links should packets enter.  This is
   either not feasible (if fine-grained per-customer control is
   required, the very specific prefixes may not be propagated) or
   increases DFZ table size.

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    Stub1 ...   +-------+      ,-''     `--.      +-------+   ... Stub3
             \  |   R_A1|----,'             `. ---|R_B1   |  /
              --|   R_A2|---(     Transit     )   |       |--
    Stub2 .../  |   R_A3|-----.             ,' ---|R_B2   |  \... Stub4
                +-------+      `--.     _.-'      +-------+
          ...     ISP_A            `--''            ISP_B     ...

               Figure 4: Inter-Service provider TE scenario

   A solution for this is to apply LISP recursively.  ISP_A and ISP_B
   may reach a bilateral agreement to deploy their own private mapping
   system.  ISP_A then encapsulates packets destined for the prefixes of
   ISP_B, which are listed in the shared mapping system.  Note that in
   this case the packet is double-encapsulated.  ISP_B's ETR removes the
   outer, second layer of LISP encapsulation from the incoming packet,
   and routes it towards the original RLOC, the ETR of Stub4, which does
   the final decapsulation.

   If ISP_A and ISP_B agree to share a private distributed mapping
   database, both can control their ingress TE without the need of
   disaggregating prefixes.  In this scenario the private database
   contains RLOC-to-RLOC bindings.  The convergence time on the TE
   policies updates is expected to be fast, since ISPs only have to
   update/query a mapping to/from the database.

   This deployment scenario includes two important recommendations.
   First, it is intended to be deployed only between two ISPs (ISP_A and
   ISP_B in Figure 4).  If more than two ISPs use this approach, then
   the xTRs deployed at the participating ISPs must either query
   multiple mapping systems, or the ISPs must agree on a common shared
   mapping system.  Second, the scenario is only recommended for ISPs
   providing connectivity to LISP sites, such that source RLOCs of
   packets to be reencapsulated belong to said ISP.  Otherwise the
   participating ISPs must register prefixes they do not own in the
   above mentioned private mapping system.  Failure to follow these
   recommendations may lead to operational and security issues when
   deploying this scenario.

   Besides these recommendations, the main disadvantages of this
   deployment case are:

   o  Extra LISP header is needed.  This increases the packet size and,
      for efficient communications, it requires that the MTU between
      both ISPs can accommodate double-encapsulated packets.

   o  The ISP ITR must encapsulate packets and therefore must know the
      RLOC-to-RLOC binding.  These bindings are stored in a mapping

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      database and may be cached in the ITR's mapping cache.  Cache
      misses lead to an extra lookup latency, unless NERD
      [I-D.lear-lisp-nerd] is used for the lookups.

   o  The operational overhead of maintaining the shared mapping

2.5.  Tunnel Routers Behind NAT

   NAT in this section refers to IPv4 network address and port

2.5.1.  ITR

   Packets encapsulated by an ITR are just UDP packets from a NAT
   device's point of view, and they are handled like any UDP packet,
   there are no additional requirements for LISP data packets.

   Map-Requests sent by an ITR, which create the state in the NAT table
   have a different 5-tuple in the IP header than the Map-Reply
   generated by the authoritative ETR.  Since the source address of this
   packet is different from the destination address of the request
   packet, no state will be matched in the NAT table and the packet will
   be dropped.  To avoid this, the NAT device has to do the following:

   o  Send all UDP packets with source port 4342, regardless of the
      destination port, to the RLOC of the ITR.  The most simple way to
      achieve this is configuring 1:1 NAT mode from the external RLOC of
      the NAT device to the ITR's RLOC (Called "DMZ" mode in consumer
      broadband routers).

   o  Rewrite the ITR-AFI and "Originating ITR RLOC Address" fields in
      the payload.

   This setup supports a single ITR behind the NAT device.

2.5.2.  ETR

   An ETR placed behind NAT is reachable from the outside by the
   Internet-facing locator of the NAT device.  It needs to know this
   locator (and configure a loopback interface with it), so that it can
   use it in Map-Reply and Map-Register messages.  Thus support for
   dynamic locators for the mapping database is needed in LISP

   Again, only one ETR behind the NAT device is supported.

   An implication of the issues described above is that LISP sites with

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   xTRs can not be behind carrier based NATs, since two different sites
   would collide on the port forwarding.

2.6.  Summary and Feature Matrix

          Feature                         CE    PE    Split   Rec.
          Control of ingress TE            x     -      x      x
          No modifications to existing
             int. network infrastructure   x     x      -      -
          Loc-Status-Bits sync             x     -      x      x
          MTU/PMTUD issues minimized       -     x      -      x

3.  Map-Resolvers and Map-Servers

3.1.  Map-Servers

   The Map-Server learns EID-to-RLOC mapping entries from an
   authoritative source and publishes them in the distributed mapping
   database.  These entries are learned through authenticated Map-
   Register messages sent by authoritative ETRs.  Also, upon reception
   of a Map-Request, the Map-Server verifies that the destination EID
   matches an EID-prefix for which it is responsible for, and then re-
   encapsulates and forwards it to a matching ETR.  Map-Server
   functionality is described in detail in [I-D.ietf-lisp-ms].

   The Map-Server is provided by a Mapping Service Provider (MSP).  A
   MSP can be any of the following:

   o  EID registrar.  Since the IPv4 address space is nearing
      exhaustion, IPv4 EIDs will come from already allocated Provider
      Independent (PI) space.  The registrars in this case remain the
      current five Regional Internet Registries (RIRs).  In the case of
      IPv6, the possibility of reserving a /16 block as EID space is
      currently under consideration [I-D.meyer-lisp-eid-block].  If
      granted by IANA, the community will have to determine the body
      responsible for allocations from this block, and the associated
      policies.  For already allocated IPv6 prefixes the principles from
      IPv4 should be applied.

   o  Third parties.  Participating in the LISP mapping system is
      similar to participating in global routing or DNS: as long as
      there is at least another already participating entity willing to
      forward the newcomer's traffic, there is no barrier to entry.
      Still, just like routing and DNS, LISP mappings have the issue of
      trust, with efforts underway to make the published information
      verifiable.  When these mechanisms will be deployed in the LISP

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      mapping system, the burden of providing and verifying trust should
      be kept away from MSPs, which will simply host the secured
      mappings.  This will keep the low barrier of entry to become an
      MSP for third parties.

   In all cases, the MSP configures its Map-Server(s) to publish the
   prefixes of its clients in the distributed mapping database and start
   encapsulating and forwarding Map-Requests to the ETRs of the AS.
   These ETRs register their prefix(es) with the Map-Server(s) through
   periodic authenticated Map-Register messages.  In this context, for
   some LISP end sites, there is a need for mechanisms to:

   o  Automatically distribute EID prefix(es) shared keys between the
      ETRs and the EID-registrar Map-Server.

   o  Dynamically obtain the address of the Map-Server in the ETR of the

   The Map-Server plays a key role in the reachability of the EID-
   prefixes it is serving.  On the one hand it is publishing these
   prefixes into the distributed mapping database and on the other hand
   it is encapsulating and forwarding Map-Requests to the authoritative
   ETRs of these prefixes.  ITRs encapsulating towards EIDs under the
   responsibility of a failed Map-Server will be unable to look up any
   of their covering prefixes.  The only exception are the ITRs that
   already contain the mappings in their local cache.  In this case ITRs
   can reach ETRs until the entry expires (typically 24 hours).  For
   this reason, redundant Map-Server deployments are desirable.  A set
   of Map-Servers providing high-availability service to the same set of
   prefixes is called a redundancy group.  ETRs are configured to send
   Map-Register messages to all Map-Servers in the redundancy group.  To
   achieve fail-over (or load-balancing, if desired), current known BGP
   practices can be used on the LISP+ALT BGP overlay network.

   Additionally, if a Map-Server has no reachability for any ETR serving
   a given EID block, it should not originate that block into the
   mapping system.

3.2.  Map-Resolvers

   A Map-Resolver a is a network infrastructure component which accepts
   LISP encapsulated Map-Requests, typically from an ITR, and finds the
   appropriate EID-to-RLOC mapping by either consulting its local cache
   or by consulting the distributed mapping database.  Map-Resolver
   functionality is described in detail in [I-D.ietf-lisp-ms].

   Anyone with access to the distributed mapping database can set up a
   Map-Resolver and provide EID-to-RLOC mapping lookup service.  In the

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   case of the LISP+ALT mapping system, the Map-Resolver needs to become
   part of the ALT overlay so that it can forward packets to the
   appropriate Map-Servers.  For more detail on how the ALT overlay
   works, see [I-D.ietf-lisp-alt]

   For performance reasons, it is recommended that LISP sites use Map-
   Resolvers that are topologically close to their ITRs.  ISPs
   supporting LISP will provide this service to their customers,
   possibly restricting access to their user base.  LISP sites not in
   this position can use open access Map-Resolvers, if available.
   However, regardless of the availability of open access resolvers, the
   MSP providing the Map-Server(s) for a LISP site should also make
   available Map-Resolver(s) for the use of that site.

   In medium to large-size ASes, ITRs must be configured with the RLOC
   of a Map-Resolver, operation which can be done manually.  However, in
   Small Office Home Office (SOHO) scenarios a mechanism for
   autoconfiguration should be provided.

   One solution to avoid manual configuration in LISP sites of any size
   is the use of anycast RLOCs for Map-Resolvers similar to the DNS root
   server infrastructure.  Since LISP uses UDP encapsulation, the use of
   anycast would not affect reliability.  LISP routers are then shipped
   with a preconfigured list of well know Map-Resolver RLOCs, which can
   be edited by the network administrator, if needed.

   The use of anycast also helps improving mapping lookup performance.
   Large MSPs can increase the number and geographical diversity of
   their Map-Resolver infrastructure, using a single anycasted RLOC.
   Once LISP deployment is advanced enough, very large content providers
   may also be interested running this kind of setup, to ensure minimal
   connection setup latency for those connecting to their network from
   LISP sites.

   While Map-Servers and Map-Resolvers implement different
   functionalities within the LISP mapping system, they can coexist on
   the same device.  For example, MSPs offering both services, can
   deploy a single Map-Resolver/Map-Server in each PoP where they have a

4.  Proxy Tunnel Routers

4.1.  P-ITR

   Proxy Ingress Tunnel Routers (P-ITRs) are part of the non-LISP/LISP
   transition mechanism, allowing non-LISP sites to reach LISP sites.
   They announce via BGP certain EID prefixes (aggregated, whenever

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   possible) to attract traffic from non-LISP sites towards EIDs in the
   covered range.  They do the mapping system lookup, and encapsulate
   received packets towards the appropriate ETR.  Note that for the
   reverse path LISP sites can reach non-LISP sites simply by not
   encapsulating traffic.  See [I-D.ietf-lisp-interworking] for a
   detailed description of P-ITR functionality.

   The success of new protocols depends greatly on their ability to
   maintain backwards compatibility and inter-operate with the
   protocol(s) they intend to enhance or replace, and on the incentives
   to deploy the necessary new software or equipment.  A LISP site needs
   an interworking mechanism to be reachable from non-LISP sites.  A
   P-ITR can fulfill this role, enabling early adopters to see the
   benefits of LISP, similar to tunnel brokers helping the transition
   from IPv4 to IPv6.  A site benefits from new LISP functionality
   (proportionally with existing global LISP deployment) when going
   LISP, so it has the incentives to deploy the necessary tunnel
   routers.  In order to be reachable from non-LISP sites it has two
   options: keep announcing its prefix(es) with BGP (see next
   subsection), or have a P-ITR announce prefix(es) covering them.

   If the goal of reducing the DFZ routing table size is to be reached,
   the second option is preferred.  Moreover, the second option allows
   LISP-based ingress traffic engineering from all sites.  However, the
   placement of P-ITRs greatly influences performance and deployment
   incentives.  The following subsections present the LISP+BGP
   transition strategy and then possible P-ITR deployment scenarios.
   They use the loosely defined terms of "early transition phase", "late
   transition phase", and "LISP Internet phase", which refer to time
   periods when LISP sites are a minority, a majority, or represent all
   edge networks respectively.

4.1.1.  LISP+BGP

   For sites wishing to go LISP with their PI prefix the least
   disruptive way is to upgrade their border routers to support LISP,
   register the prefix into the LISP mapping system, but keep announcing
   it with BGP as well.  This way LISP sites will reach them over LISP,
   while legacy sites will be unaffected by the change.  The main
   disadvantage of this approach is that no decrease in the DFZ routing
   table size is achieved.  Still, just increasing the number of LISP
   sites is an important gain, as an increasing LISP/non-LISP site ratio
   will slowly decrease the need for BGP-based traffic engineering that
   leads to prefix deaggregation.  That, in turn, may lead to a decrease
   in the DFZ size in the late transition phase.

   This scenario is not limited to sites that already have their
   prefixes announced with BGP.  Newly allocated EID blocks could follow

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   this strategy as well during the early LISP deployment phase,
   depending on the cost/benefit analysis of the individual networks.
   Since this leads to an increase in the DFZ size, one of the following
   scenarios should be preferred for new allocations.

4.1.2.  Mapping Service Provider P-ITR Service

   In addition to publishing their clients' registered prefixes in the
   mapping system, MSPs with enough transit capacity can offer them
   P-ITR service as a separate service.  This service is especially
   useful for new PI allocations, to sites without existing BGP
   infrastructure, that wish to avoid BGP altogether.  The MSP announces
   the prefix into the DFZ, and the client benefits from ingress traffic
   engineering without prefix deaggregation.  The downside of this
   scenario is path stretch, which may be greater than 1.

   Routing all non-LISP ingress traffic through a third party which is
   not one of its ISPs is only feasible for sites with modest amounts of
   traffic (like those using the IPv6 tunnel broker services today),
   especially in the first stage of the transition to LISP, with a
   significant number of legacy sites.  When the LISP/non-LISP site
   ratio becomes high enough, this approach can prove increasingly

   Compared to LISP+BGP, this approach avoids DFZ bloat caused by prefix
   deaggregation for traffic engineering purposes, resulting in slower
   routing table increase in the case of new allocations and potential
   decrease for existing ones.  Moreover, MSPs serving different clients
   with adjacent aggregable prefixes may lead to additional decrease,
   but quantifying this decrease is subject to future research study.

4.1.3.  Tier 1 P-ITR Service

   The ideal location for a P-ITR is on the traffic path, as close to
   non-LISP site as possible, to minimize or completely eliminate path
   stretch.  However, this location is far away from the networks that
   most benefit from the P-ITR services (i.e., LISP sites, destinations
   of encapsulated traffic) and have the most incentives to deploy them.
   But the biggest challenge having P-ITRs close to the traffic source
   is the large number of devices and their wide geographical diversity
   required to have a good coverage, in addition to considerable transit
   capacity.  Tier 1 service providers fulfill these requirements and
   have clear incentives to deploy P-ITRs: to attract more traffic from
   their customers.  Since a large fraction is multihomed to different
   providers with more than one active link, they compete with the other
   providers for traffic.

   To operate the P-ITR service, the ISP announces an aggregate of all

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   known EID prefixes (a mechanism will be needed to obtain this list)
   downstream to their customers with BGP.  First, the performance
   concerns of the MSP P-ITR service described in the previous section
   are now addressed, as P-ITRs are on-path, eliminating path stretch
   (except when combined with LISP+BGP, see below).  Second, thanks to
   the direction of the announcements, the DFZ routing table size is not

   The main downside of this approach is non-global coverage for the
   announced prefixes, caused by the downstream direction of the
   announcements.  As a result, a LISP site will be only reachable from
   customers of service providers running P-ITRs, unless one of the
   previous approaches is used as well.  Due to this issue, it is
   unlikely that existing BGP speakers migrating to LISP will withdraw
   their announcements to the DFZ, resulting in a combination of this
   approach with LISP+BGP.  At the same time, smaller new LISP sites
   still depend on MSP for global reachability.  The early transition
   phase thus will keep the status quo in the DFZ routing table size,
   but offers the benefits of increasingly better ingress traffic
   engineering to early adopters.

   As the number of LISP destinations increases, traffic levels from
   those non-LISP, large multihomed clients who rely on BGP path length
   for provider selection (such as national/regional ISPs), start to
   shift towards the Tier 1 providing P-ITRs.  The competition is then
   incentivised to deploy their own service, thus improving global P-ITR
   coverage.  If all Tier 1 providers have P-ITR service, the LISP+BGP
   and MSP alternatives are not required for global reachability of LISP
   sites.  Still, LISP+BGP users may still want to keep announcing their
   prefixes for security reasons (i.e., preventing hijacking).  DFZ size
   evolution in this phase depends on that choice, and the aggregability
   of all LISP prefixes.  As a result, it may decrease or stay at the
   same level.

   For performance reasons, and to simplify P-ITR implementations, it is
   desirable to minimize the number of non-aggregable EID prefixes.  In
   IPv6 this can be easily achieved if a large prefix block is reserved
   as LISP EID space [I-D.meyer-lisp-eid-block].  If the EID space is
   not fragmented, new LISP sites will not cause increase in the DFZ
   size, unless they do LISP+BGP.

   To summarize, the main benefits of this scenario are stopping the
   increase and potentially decreasing the size of the DFZ routing
   tables, while keeping path stretch close to 1, with the cost of not
   having global coverage of one's prefixes.

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4.1.4.  Migration Summary

   The following table presents the expected effects of the different
   transition scenarios during a certain phase on the DFZ routing table

     Phase            | LISP+BGP     | MSP               | Tier 1
     Early transition | no change    | slowdown increase | no change
     Late transition  | may decrease | slowdown increase | may decrease
     LISP Internet    |              considerable decrease

   It is expected that a combination of these scenarios will exist
   during the migration period, in particular existing sites choosing
   LISP+BGP, new small sites choosing MSP, and competition between Tier
   1 providers bringing optimized service.  If all Tier 1 ISPs have
   P-ITR service in place, the other scenarios can be deprecated,
   greatly reducing DFZ size.

4.2.  P-ETR

   In contrast to P-ITRs, P-ETRs are not required for the correct
   functioning of all LISP sites.  There are two cases, where they can
   be of great help:

   o  LISP sites with unicast reverse path forwarding (uRPF)
      restrictions, and

   o  LISP sites without native IPv6 communicating with LISP nodes with
      IPv6-only locators.

   In the first case, uRPF filtering is applied at their upstream PE
   router.  When forwarding traffic to non-LISP sites, an ITR does not
   encapsulate packets, leaving the original IP headers intact.  As a
   result, packets will have EIDs in their source address.  Since we are
   discussing the transition period, we can assume that a prefix
   covering the EIDs belonging to the LISP site is advertised to the
   global routing tables by a P-ITR, and the PE router has a route
   towards it.  However, the next hop will not be on the interface
   towards the CE router, so non-encapsulated packets will fail uRPF

   To avoid this filtering, the affected ITR encapsulates packets
   towards the locator of the P-ETR for non-LISP destinations.  Now the
   source address of the packets, as seen by the PE router is the ITR's
   locator, which will not fail the uRPF check.  The P-ETR then
   decapsulates and forwards the packets.

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   The second use case is IPv4-to-IPv6 transition.  Service providers
   using older access network hardware, which only supports IPv4 can
   still offer IPv6 to their clients, by providing a CPE device running
   LISP, and P-ETR(s) for accessing IPv6-only non-LISP sites and LISP
   sites, with IPv6-only locators.  Packets originating from the client
   LISP site for these destinations would be encapsulated towards the
   P-ETR's IPv4 locator.  The P-ETR is in a native IPv6 network,
   decapsulating and forwarding packets.  For non-LISP destination, the
   packet travels natively from the P-ETR.  For LISP destinations with
   IPv6-only locators, the packet will go through a P-ITR, in order to
   reach its destination.

   For more details on P-ETRs see the [I-D.ietf-lisp-interworking]

   P-ETRs can be deployed by ISPs wishing to offer value-added services
   to their customers.  As is the case with P-ITRs, P-ETRs too may
   introduce path stretch.  Because of this the ISP needs to consider
   the tradeoff of using several devices, close to the customers, to
   minimize it, or few devices, farther away from the customers,
   minimizing cost instead.

   Since the deployment incentives for P-ITRs and P-ETRs are different,
   it is likely they will be deployed in separate devices, except for
   the CDN case, which may deploy both in a single device.

   In all cases, the existence of a P-ETR involves another step in the
   configuration of a LISP router.  CPE routers, which are typically
   configured by DHCP, stand to benefit most from P-ETRs.  To enable
   autoconfiguration of the P-ETR locator, a DHCP option would be

   As a security measure, access to P-ETRs should be limited to
   legitimate users by enforcing ACLs.

5.  Security Considerations

   Security implications of LISP deployments are to be discussed in
   separate documents.  [I-D.saucez-lisp-security] gives an overview of
   LISP threat models, while securing mapping lookups is discussed in

6.  IANA Considerations

   This memo includes no request to IANA.

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7.  Acknowledgements

   Many thanks to Margaret Wasserman for her contribution to the IETF76
   presentation that kickstarted this work.  The authors would also like
   to thank Damien Saucez, Luigi Iannone, Joel Halpern, Vince Fuller,
   Dino Farinacci, Terry Manderson, Noel Chiappa, and everyone else who
   provided input.

8.  References

8.1.  Normative References

              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-10 (work in progress), March 2011.

              Fuller, V., Farinacci, D., Meyer, D., and D. Lewis, "LISP
              Alternative Topology (LISP+ALT)", draft-ietf-lisp-alt-06
              (work in progress), March 2011.

              Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking LISP with IPv4 and IPv6",
              draft-ietf-lisp-interworking-01 (work in progress),
              August 2010.

              Fuller, V. and D. Farinacci, "LISP Map Server",
              draft-ietf-lisp-ms-07 (work in progress), March 2011.

              Maino, F., Ermagan, V., Cabellos-Aparicio, A., Saucez, D.,
              and O. Bonaventure, "LISP-Security (LISP-SEC)",
              draft-maino-lisp-sec-00 (work in progress), March 2011.

              Saucez, D., Iannone, L., and O. Bonaventure, "LISP
              Security Threats", draft-saucez-lisp-security-03 (work in
              progress), March 2011.

8.2.  Informative References

              Lear, E., "NERD: A Not-so-novel EID to RLOC Database",
              draft-lear-lisp-nerd-08 (work in progress), March 2010.

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              Iannone, L., Lewis, D., Meyer, D., and V. Fuller, "LISP
              EID Block", draft-meyer-lisp-eid-block-02 (work in
              progress), March 2011.

   [cache]    Jung, J., Sit, E., Balakrishnan, H., and R. Morris, "DNS
              performance and the effectiveness of caching", 2002.

Authors' Addresses

   Lorand Jakab
   Technical University of Catalonia
   C/Jordi Girona, s/n
   BARCELONA  08034


   Albert Cabellos-Aparicio
   Technical University of Catalonia
   C/Jordi Girona, s/n
   BARCELONA  08034


   Florin Coras
   Technical University of Catalonia
   C/Jordi Girona, s/n
   BARCELONA  08034


   Jordi Domingo-Pascual
   Technical University of Catalonia
   C/Jordi Girona, s/n
   BARCELONA  08034


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


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