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Versions: (draft-lewis-lisp-interworking) 00 01 02 03 04 05 06 RFC 6832

Network Working Group                                           D. Lewis
Internet-Draft                                                  D. Meyer
Intended status: Experimental                               D. Farinacci
Expires: February 27, 2011                                     V. Fuller
                                                     Cisco Systems, Inc.
                                                         August 26, 2010


                  Interworking LISP with IPv4 and IPv6
                  draft-ietf-lisp-interworking-01.txt

Abstract

   This document describes techniques for allowing sites running the
   Locator/ID Separation Protocol (LISP) to interoperate with Internet
   sites (which may be using either IPv4, IPv6, or both) but which are
   not running LISP.  A fundamental property of LISP speaking sites is
   that they use Endpoint Identifiers (EIDs), rather than traditional IP
   addresses, in the source and destination fields of all traffic they
   emit or receive.  While EIDs are syntactically identical to IPv4 or
   IPv6 addresses, normally routes to them are not carried in the global
   routing system so an interoperability mechanism is needed for non-
   LISP-speaking sites to exchange traffic with LISP-speaking sites.
   This document introduces three such mechanisms.  The first uses a new
   network element, the LISP Proxy Ingress Tunnel Routers (PITR)
   (Section 5) to act as a intermediate LISP Ingress Tunnel Router (ITR)
   for non-LISP-speaking hosts.  Second the document adds Network
   Address Translation (NAT) functionality to LISP Ingress and LISP
   Egress Tunnel Routers (xTRs) to substitute routable IP addresses for
   non-routable EIDs.  Finally, this document introduces a Proxy Egress
   Tunnel Router (PETR) to handle cases where a LISP ITR cannot send
   packets to non-LISP sites without encapsulation.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on February 27, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  LISP Interworking Models . . . . . . . . . . . . . . . . . . .  6
   3.  Definition of Terms  . . . . . . . . . . . . . . . . . . . . .  8
   4.  Routable EIDs  . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Impact on Routing Table  . . . . . . . . . . . . . . . . . 11
     4.2.  Requirement for using BGP  . . . . . . . . . . . . . . . . 11
     4.3.  Limiting the Impact of Routable EIDs . . . . . . . . . . . 11
     4.4.  Use of Routable EIDs for sites transitioning to LISP . . . 11
   5.  Proxy Ingress Tunnel Routers . . . . . . . . . . . . . . . . . 13
     5.1.  PITR EID announcements . . . . . . . . . . . . . . . . . . 13
     5.2.  Packet Flow with PITRs . . . . . . . . . . . . . . . . . . 13
     5.3.  Scaling PITRs  . . . . . . . . . . . . . . . . . . . . . . 14
     5.4.  Impact of the PITRs placement in the network . . . . . . . 15
     5.5.  Benefit to Networks Deploying PITRs  . . . . . . . . . . . 15
   6.  LISP-NAT . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     6.1.  Using LISP-NAT with LISP-NR EIDs . . . . . . . . . . . . . 16
     6.2.  LISP Sites with Hosts using RFC 1918 Addresses Sending
           to non-LISP Sites  . . . . . . . . . . . . . . . . . . . . 17
     6.3.  LISP Sites with Hosts using RFC 1918 Addresses
           Sending Packets to Other LISP Sites  . . . . . . . . . . . 17
     6.4.  LISP-NAT and multiple EIDs . . . . . . . . . . . . . . . . 18
     6.5.  When LISP-NAT and PITRs used by the same LISP Site . . . . 18
   7.  Proxy Egress Tunnel Routers  . . . . . . . . . . . . . . . . . 19
     7.1.  Packet Flow with Proxy Egress Tunnel Routers . . . . . . . 19
   8.  Discussion of Proxy ITRs (PITRs), LISP-NAT, and Proxy-ETRs
       (PETRs)  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  How Proxy-ITRs and Proxy-ETRs Interact . . . . . . . . . . 21
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     12.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26















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1.  Introduction

   This document describes interoperation between LISP [LISP] sites
   which use non-globally-routed EIDs, and non-LISP sites.  The first is
   the use of Proxy Ingress Tunnel router (PITRs), which originate
   highly-aggregated routes to EID prefixes for non-LISP sites to use.
   It also describes the use of NAT by LISP ITRs when sending packets to
   non-LISP hosts.  Finally, it describes Proxy Egress Tunnel routers
   (PETRs) LISP for sites relying on PITRs, and which are faced with
   certain restrictions.

   A key behavior of the separation of Locators and End-Point-IDs is
   that EID prefixes are normally not advertised into the Internet's
   Default Free Zone (DFZ).  Specifically, only RLOCs are carried in the
   Internet's DFZ.  Existing Internet sites (and their hosts) which do
   not run in the LISP protocol must still be able to reach sites
   numbered from LISP EID space.  This draft describes three mechanisms
   that can be used to provide reachability between sites that are LISP-
   capable and those that are not.

   The first mechanism uses a new network element, the LISP Proxy
   Ingress Tunnel Router (PITR) to act as a intermediate LISP Ingress
   Tunnel Router (ITR) for non-LISP-speaking hosts.  The second
   mechanism adds a form of Network Address Translation (NAT)
   functionality to Tunnel Routers (xTRs), to substitute routable IP
   addresses for non-routable EIDs.  The final network element is the
   LISP Proxy Egress Tunnel Routers (PETR), which act as an intermediate
   Egress Tunnel Router (ETR) for LISP sites which need to encapsulate
   packets LISP packets destined to non-LISP sites.

   More detailed descriptions of these mechanisms and the network
   elements involved may be found in the following sections:

   - Section 2 describes the different cases where interworking
   mechanisms are needed

   - Section 3 defines terms used throughout the document

   - Section 4 describes the relationship between the new EID prefix
   space and the IP address space used by the current Internet

   - Section 5 introduces and describes the operation of Proxy-ITRs

   - Section 6 defines how NAT is used by ETRs to translate non-routable
   EIDs into routable IP addresses.

   - Section 7 introduces and describes the operations of Proxy-ETRs




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   - Section 8 describes the relationship between asymmetric and
   Symmetric interworking mechanisms (Proxy-ITRs and Proxy-ETRs vs LISP-
   NAT)

   Note that any successful interworking model should be independent of
   any particular EID-to-RLOC mapping algorithm.  This document does not
   comment on the value of any of the particular LISP mapping systems.












































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2.  LISP Interworking Models

   There are 4 unicast connectivity cases which describe how sites can
   send packets to each other:

   1.  Non-LISP site to Non-LISP site

   2.  LISP site to LISP site

   3.  LISP site to Non-LISP site

   4.  Non-LISP site to LISP site

   Note that while Cases 3 and 4 seem similar, there are subtle
   differences due to the way packets are originated.

   The first case is the Internet as we know it today and as such will
   not be discussed further here.  The second case is documented in
   [LISP] and there are no new interworking requirements because there
   are no new protocol requirements placed on intermediate non- LISP
   routers.

   In case 3, LISP site to Non-LISP site, a LISP site can (in most
   cases) send packets to a non-LISP site because the non-LISP site
   prefixes are routable.  The non-LISP site need not do anything new to
   receive packets.  The only action the LISP site needs (with two
   possible caveats introduced below) to take is to know when not to
   LISP-encapsulate packets.  This can be achieved by using one of two
   mechanisms:

   1.  At the ITR in the source site, if the destination of an IP packet
       is found to match a prefix from the BGP routing table, then the
       site is directly reachable by the BGP core that exists and
       operates today.

   2.  Second, if (from the perspective of the ITR at the source site)
       the destination address of an IP address is not found in the EID-
       to-RLOC mapping database, the ITR could infer that it is not a
       LISP-capable site, and decide to not LISP-encapsulate the packet.

   3.  In either of the two exceptions mentioned above there could be
       some situations where (unencapsualted) packets originated by a
       LISP site may not be forwarded to a non-LISP site.  These cases
       are reviewed in section 7, (Proxy-Egress Tunnel Routers).

   Case 4, typically the most challenging, occurs when a host at a non-
   LISP site wishes to send traffic to a host at a LISP site.  If the
   source host uses a (non-globally-routable) EID as the destination IP



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   address, the packet is forwarded inside the source site until it
   reaches a router which cannot forward tin (due to lack of a default
   route), at which point the traffic is dropped.  For traffic not to be
   dropped, either some some mechanism to make this destination EID
   routable must be in place.  Section 5 (PITRs) and Section 6 (LISP-
   NAT) describe two such mechanisms.

   Case 4 also applies to packets returning to the LISP site, in Case 3.











































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3.  Definition of Terms

   Endpoint ID (EID):  Endpoint ID (EID): A 32-bit (for IPv4) or 128-bit
      (for IPv6) value used in the source and destination address fields
      of the first (most inner) IP header of a packet.  The host obtains
      a destination EID the same way it obtains an destination address
      today, for example through a DNS lookup or SIP exchange.  The
      source EID is obtained via existing mechanisms used to set a
      host's "local" IP address.  An EID is allocated to a host from an
      EID-prefix block associated with the site where the host is
      located.  An EID can be used by a host to refer to other hosts.
      EIDs MUST NOT be used as LISP RLOCs.  Note that EID blocks may be
      assigned in a hierarchical manner, independent of the network
      topology, to facilitate scaling of the mapping database.  In
      addition, an EID block assigned to a site may have site-local
      structure (subnetting) for routing within the site; this structure
      is not visible to the global routing system.  When used in
      discussions with other Locator/ID separation proposals, a LISP EID
      will be called a "LEID".  Throughout this document, any references
      to "EID" refers to an LEID.

   EID-Prefix:  A power-of-2 block of EIDs which are allocated to a site
      by an address allocation authority.  EID-prefixes are associated
      with a set of RLOC addresses which make up a "database mapping".
      EID-prefix allocations can be broken up into smaller blocks when
      an RLOC set is to be associated with the smaller EID- prefix.  A
      globally routed address block (whether PI or PA) is not an EID-
      prefix.  However, a globally routed address block may be removed
      from global routing and reused as an EID-prefix.  A site that
      receives an explicitly allocated EID-prefix may not use that EID-
      prefix as a globally routed prefix assigned to RLOCs

   EID-Prefix Aggregate:  A set of EID-prefixes said to be aggregatable
      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 a prefix and a length.  Provider
      Independent (PI) Addresses: an address block assigned from a pool
      where blocks are not associated with any particular location in
      the network (e.g. from a particular service provider), and is
      therefore not topologically aggregatable in the routing system.

   Routing Locator (RLOC):  The IPv4 or IPv6 address of an egress tunnel
      router (ETR).  It is the output of a EID-to-RLOC mapping lookup.
      An EID 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 it attaches to the global Internet; where
      the topology is defined by the connectivity of provider networks,
      RLOCs can be thought of as PA addresses.  Multiple RLOCs can be



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      assigned to the same ETR device or to multiple ETR devices at a
      site.

   EID-to-RLOC Mapping:  A binding between an EID and the RLOC-set that
      can be used to reach the EID.  We use the term "mapping" in this
      document to refer to a EID-to-RLOC mapping.

   EID Prefix Reachability:  An EID prefix is said to be "reachable" if
      one or more of its locators are reachable.  That is, an EID prefix
      is reachable if the ETR (or its proxy) is reachable.

   Default Mapping:  A Default Mapping is a mapping entry for EID-prefix
      0.0.0.0/0.  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
      [LISP].

   LISP Routable (LISP-R) Site:  A LISP site whose addresses are used as
      both globally routable IP addresses and LISP EIDs.

   LISP Non-Routable (LISP-NR) Site:  A LISP site whose addresses are
      EIDs only, these EIDs are not found in the legacy Internet routing
      table.

   LISP Proxy Ingress Tunnel Router (PITR):  PITRs are used to provide
      interconnectivity between sites which use LISP EIDs and those
      which do not.  They act as gateways between those parts of the
      Internet which are not using LISP (the legacy Internet) A given
      PITR advertises one or more highly aggregated EID prefixes into
      the public Internet and acts as the ITR for traffic received from
      the public Internet.  LISP Proxy Ingress Tunnel Routers are
      described in Section 5.

   LISP Network Address Translation (LISP-NAT):  Network Address
      Translation between EID space assigned to a site and RLOC space
      also assigned to that site.  LISP Network Address Translation is
      described in Section 6.

   LISP Proxy Egress Tunnel Router (PETR):  PETRs provide a LISP
      (Routable or Non-Routable EID) site's ITRs the ability to send
      packets to non-LISP sites in cases where unencapsualted packets
      (the default mechanism) would fail to be delivered.  PETRs are
      function by having an ITR encapsulate all non-LISP destined
      traffic to a pre-configured PETR.  LISP Proxy Egress Tunnel
      Routers are described in Section 7.





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    EID Sub Namespace:  A power-of-two block of aggregatable locators
      set aside for LISP interworking.

















































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4.  Routable EIDs

   An obvious way to achieve interworking between LISP and non-LISP
   hosts is for a LISP site to simply announce EID prefixes into the
   DFZ, much like the current routing system, effectively treating them
   as "Provider Independent (PI)" prefixes.  Having a site do this is
   undesirable as it defeats one of the primary goals of LISP - to
   reduce global routing system state.

4.1.  Impact on Routing Table

   If EID prefixes are announced into the DFZ, the impact is similar to
   the case in which LISP has not been deployed, because these EID
   prefixes will be no more aggregatable than existing PI addressing.
   Such a mechanism is not viewed as a viable long term solution, but
   may be a viable short term way for a site to transition a portion of
   its address space to EID space without changing its existing routing
   policy.

4.2.  Requirement for using BGP

   Non-LISP sites today use BGP to, among other things, enable ingress
   traffic engineering.  Relaxing this requirement is another primary
   design goal of LISP.

4.3.  Limiting the Impact of Routable EIDs

   Two schemes are proposed to limit the impact of having EIDs announced
   in the current global Internet routing table:

   1.  Section 5 discusses the LISP Proxy Tunnel Router, an approach
       that provides ITR functionality to bridge LISP-capable and non-
       LISP-capable sites.

   2.  Section 6 discusses another approach, LISP-NAT, in which NAT
       [RFC2993] is combined with ITR functionality to limit the the
       impact of routable EIDs on the Internet routing infrastructure.

4.4.  Use of Routable EIDs for sites transitioning to LISP

   A primary design goal for LISP (and other Locator/ID separation
   proposals) is to facilitate topological aggregation of namespace used
   by the path computation, and, thus, decrease global routing system
   overhead.  Another goal is to achieve the benefits of improved
   aggregation as soon as possible.  Individual sites advertising their
   own routes for LISP EID prefixes into the global routing system is
   therefore not recommended.




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   That being said, single homed sites (or multi-homed sites that are
   not leaking more specific exceptions) and that are already using
   provider-aggregated prefixes can use these prefixes as LISP EIDs
   without adding state to the routing system.  In other words, such
   sites do not cause additional prefixes to be advertised.  For such
   sites, connectivity to a non-LISP sites does not require interworking
   machinery because the "PA" EIDs are already routable (they are
   effectively LISP-R type sites).  Their EIDs are found in the LISP
   mapping system, and their (aggregate) PA prefix(es) are found in the
   DFZ Internet.

   The continued announcements of an existing site's Provider
   Independent (or "PI") prefix(es) is of course under control of that
   site.  Some period of transition, where a site is is found both in
   the LISP mapping system, and as a discrete prefix in the Internet
   routing system, may be a viable transition strategy.  Care should be
   taken not to advertise additional more specific LISP EID prefixes
   into the DFZ.

































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5.  Proxy Ingress Tunnel Routers

   Proxy Ingress Tunnel Routers (PITRs) allow for non-LISP sites to send
   packets to LISP-NR sites.  A PITR is a new network element that
   shares many characteristics with the LISP ITR.  PITRs allow non-LISP
   sites to send packets to LISP-NR sites without any changes to
   protocols or equipment at the non-LISP site.  PITRs have two primary
   functions:

   Originating EID Advertisements:  PITRs advertise highly aggregated
      EID-prefix space on behalf of LISP sites to so that non-LISP sites
      can reach them.

   Encapsulating Legacy Internet Traffic:  PITRs also encapsulate non-
      LISP Internet traffic into LISP packets and route them towards
      their destination RLOCs.

5.1.  PITR EID announcements

   A key part of PITR functionality is to advertise routes for highly-
   aggregated EID prefixes into part of the global routing system.
   Aggressive aggregation is performed to minimize the number of new
   announced routes.  In addition, careful placement of PITRs can
   greatly reduce the advertised scope of these new routes.  To this
   end, PITRs should be deployed close to non-LISP-speaking rather than
   close to LISP sites.  Such deployment not only limits the scope of
   EID-prefix route advertisements, it also also allows traffic
   forwarding load to be spread among many PITRs.

5.2.  Packet Flow with PITRs

   What follows is an example of the path a packet would take when using
   a PITR.  In this example, the LISP-NR site is given the EID prefix
   240.0.0.0/24.  For the purposes of this example, this prefix and no
   covering aggregate is present in the global routing system.  In other
   words, without the Proxy-ITR announcing 240.0.0.0/24, a packet with
   this destination were to reach a router in the "Default Free Zone",
   it would be dropped.

   A full protocol exchange example follows:

   1.  The source host makes a DNS lookup EID for destination, and gets
       240.1.1.1 in return.

   2.  The source host has a default route to customer Edge (CE) router
       and forwards the packet to the CE.





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   3.  The CE has a default route to its Provider Edge (PE) router, and
       forwards the packet to the PE.

   4.  The PE has route to 240.0.0.0/24 and the next hop is the PITR.

   5.  The PITR has or acquires a mapping for 240.1.1.1 and LISP
       encapsulates the packet.  The outer IP header now has a
       destination address of one of the destination EID's RLOCs.  The
       outer source address of this encapsulated packet is the PITR's
       RLOC.

   6.  The PITR looks up the RLOC, and forwards LISP packet to the next
       hop, after which, it is forwarded by other routers to the ETR's
       RLOC.

   7.  The ETR decapsulates the packet and delivers the packet to the
       240.1.1.1 host in the destination LISP site.

   8.  Packets from host 240.1.1.1 will flow back through the LISP
       site's ITR.  Such packets are not encapsulated because the ITR
       knows that the destination (the original source) is a non-LISP
       site.  The ITR knows this because it can check the LISP mapping
       database for the destination EID, and on a failure determine that
       the destination site is not LISP enabled.

   9.  Packets are then routed natively and directly to the destination
       (original source) site.

   Note that in this example the return path is asymmetric, so return
   traffic will not go back through the PITR.  This is because the
   LISP-NR site's ITR will discover that the originating site is not a
   LISP site, and not encapsulate the returning packet (see [LISP] for
   details of ITR behavior).

   The asymmetric nature of traffic flows allows the PITR to be
   relatively simple - it will only have to encapsulate LISP packets.

5.3.  Scaling PITRs

   PITRs attract traffic by announcing the LISP EID namespace into parts
   of the non-LISP-speaking global routing system.  There are several
   ways that a network could control how traffic reaches a particular
   PITR to prevent it from receiving more traffic than it can handle:

   1.  The PITR's aggregate routes might be selectively announced,
       giving a coarse way to control the quantity of traffic attracted
       by that PITR.  For example, some of the routes being announced
       might be tagged with a BGP community and their scope of



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       announcement limited by the routing policy of the provider.

   2.  The same address might be announced by multiple PITRs in order to
       share the traffic using IP Anycast.  The asymmetric nature of
       traffic flows through the Proxy ITR means that operationally,
       deploying a set PITRs would be very similar to existing Anycasted
       services like DNS caches.  Multiple Proxy ITRs could advertise
       the same BGP Next Hop IP address as their RLOC, and traffic would
       be attracted to the nearest Next Hop according to the the
       network's IGP.

5.4.  Impact of the PITRs placement in the network

   There are several approaches that a network could take in placing
   PITRs.  Placing the PITR near the source of traffic allows for the
   communication between the non-LISP site and the LISP site to have the
   least "stretch" (i.e. the least number of forwarding hops when
   compared to an optimal path between the sites).

   Some proposals, for example CRIO [CRIO], have suggested grouping
   PITRs near an arbitrary subset of ETRs and announcing a 'local'
   subset of EID space.  This model cannot guarantee minimum stretch if
   the EID prefix route advertisement points are changed (such a change
   might occur if a site adds, removes, or replaces one or more of its
   ISP connections).

5.5.  Benefit to Networks Deploying PITRs

   When packets destined for LISP-NR sites arrive and are encapsulated
   at a Proxy-ITR, a new LISP packet header is pre-pended.  This causes
   the packet's destination to be set to the destination ETRs RLOC.
   Because packets are thus routed towards RLOCs, it can potentially
   better follow the Proxy-ITR network's traffic engineering policies
   (such as closest exit routing).  This also means that providers which
   are not default-free and do not deploy Proxy-ITRs end up sending more
   traffic to expensive transit links (assuming their upstreams have
   deployed Proxy-ITRs) rather than to the ETR's RLOC addresses, to
   which they may well have cheaper and closer connectivity to (via, for
   example, settlement-free peering).  A corollary to this would be that
   large transit providers, deploying PITRs may attract more traffic,
   and therefore more revenue, from their customers.










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6.  LISP-NAT

   LISP Network Address Translation (LISP-NAT) is a limited form of NAT
   [RFC2993].  LISP-NAT is designed to enable the interworking of non-
   LISP sites and LISP-NR sites by ensuring that the LISP-NR's site
   addresses are always routable.  LISP-NAT accomplishes this by
   translating a host's source address from an 'inner' (LISP-NR EID)
   value to an 'outer' (LISP-R) value and keeping this translation in a
   table that it can reference for subsequent packets.

   In addition, existing RFC 1918 [RFC1918] sites can use LISP-NAT to
   talk to both LISP or non-LISP sites.

   The basic concept of LISP-NAT is that when transmitting a packet, the
   ITR replaces a non-routable EID source address with a routable source
   address, which enables packets to return to the site.

   There are two main cases that involve LISP-NAT:

   1.  Hosts at LISP sites that use non-routable global EIDs speaking to
       non-LISP sites using global addresses.

   2.  Hosts at LISP sites that use RFC 1918 private EIDs speaking to
       other sites, who may be either LISP or non-LISP.

   Note that LISP-NAT is not needed in the case of LISP-R (routable
   global EIDs) sources.  This case occurs when a site is announcing its
   prefix into both the LISP mapping system as well as the Internet DFZ.
   This is because the LISP-R source's address is routable, and return
   packets will be able to natively reach the site.

6.1.  Using LISP-NAT with LISP-NR EIDs

   LISP-NAT allows a host with a LISP-NR EID to send packets to non-LISP
   hosts by translating the LISP-NR EID to a globally unique address (a
   LISP-R EID).  This globally unique address may be a either a PI or PA
   address.

   An example of this translation follows.  For this example, a site has
   been assigned a LISP-NR EID of 220.1.1.0/24.  In order to utilize
   LISP-NAT, the site has also been provided the PA EID of
   128.200.1.0/24, and uses the first address (128.200.1.1) as the
   site's RLOC.  The rest of this PA space (128.200.1.2 to
   128.200.1.254) is used as a translation pool for this site's hosts
   who need to send packets to non-LISP hosts.

   The translation table might look like the following:




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          Site NR-EID    Site R-EID      Site's RLOC    Translation Pool
          ==============================================================
          220.1.1.0/24   128.200.1.0/24  128.200.1.1    128.200.1.2-254

                    Figure 1: Example Translation Table

   The Host 220.1.1.2 sends a packet destined for a non-LISP site to its
   default route (the ITR).  The ITR receives the packet, and determines
   that the destination is not a LISP site.  How the ITR makes this
   determination is up to the ITRs implementation of the EID-to-RLOC
   mapping system used (see, for example [LISP-ALT]).

   The ITR then rewrites the source address of the packet from 220.1.1.2
   to 128.200.1.2, which is the first available address in the LISP-R
   EID space available to it.  The ITR keeps this translation in a table
   in order to reverse this process when receiving packets destined to
   128.200.1.2.

   Finally, when the ITR forwards this packet without encapsulating it,
   it uses the entry in its LISP-NAT table to translate the returning
   packets' destination IPs to the proper host.

6.2.  LISP Sites with Hosts using RFC 1918 Addresses Sending to non-LISP
      Sites

   In the case where hosts using RFC 1918 addresses desire to send
   packets to non-LISP hosts, the LISP-NAT implementation acts much like
   an existing IPv4 NAT device.  The ITR providing the NAT service must
   use LISP-R EIDs for its global address pool as well as providing all
   the standard NAT functions required today.

   The source of the packet must be translated to a LISP-R EID in a
   manner similar to Section 6, and this packet must be forwarded to the
   ITR's next hop for the destination, without LISP encapsulation.

6.3.  LISP Sites with Hosts using RFC 1918 Addresses   Sending Packets
      to Other LISP Sites

   LISP-NAT allows a host with an RFC 1918 address to send packets to
   LISP hosts by translating the RFC 1918 address to a LISP EID.  After
   translation, the communication between source and destination ITR and
   ETRs continues as described in [LISP].

   An example of this translation and encapsulation follows.  For this
   example, a host has been assigned a RFC 1918 address of 192.168.1.2.
   In order to utilize LISP-NAT, the site also has been provided the
   LISP-R EID prefix of 192.0.2.0/24, and uses the first address
   (192.0.2.1) as the site's RLOC.  The rest of this PA space (192.0.2.2



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   to 192.0.2.254) is used as a translation pool for this site's hosts
   who need to send packets to both non-LISP and LISP hosts.

   The Host 192.168.1.2 sends a packet destined for a non-LISP site to
   its default route (the ITR).  The ITR receives the packet and
   determines that the destination is a LISP site.  How the ITR makes
   this determination is up to the ITRs implementation of the EID/RLOC
   mapping system.

   The ITR then rewrites the source address of the packet from
   192.168.1.2 to 192.0.2.2, which is the first available address in the
   LISP EID space available to it.  The ITR keeps this translation in a
   table in order to reverse this process when receiving packets
   destined to 192.0.2.2.

   The ITR then LISP encapsulates this packet (see [LISP] for details).
   The ITR uses the site's RLOC as the LISP outer header's source and
   the translation address as the LISP inner header's source.  Once it
   decapsulates returning traffic, it uses the entry in its LISP-NAT
   table to translate the returning packet's destination IP address and
   then forward to the proper host.

6.4.  LISP-NAT and multiple EIDs

   When a site has two addresses that a host might use for global
   reachability, care must be chosen on which EID is found in DNS.  For
   example, whether applications such as DNS use the LISP-R EID or the
   LISP-NR EID.  This problem exists for NAT in general, but the
   specific issue described above is unique to LISP.  Using PITRs can
   mitigate this problem, since the LISP-NR EID can be reached in all
   cases.

6.5.  When LISP-NAT and PITRs used by the same LISP Site

   With LISP-NAT, there are two EIDs possible for a given host, the
   LISP-R EID and the LISP-NR EID.  When a site has two addresses that a
   host might use for global reachability, name-to-address directories
   may need to be modified.

   This problem, global addressability, exists for NAT in general, but
   the specific issue described above is unique to location/identity
   separation schemes.  Some of these have suggested running a separate
   DNS instance for new types of EIDs.  This solves the problem but
   introduces complexity for the site.  Alternatively, using PITRs can
   mitigate this problem, because the LISP-NR EID can be reached in all
   cases.





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7.  Proxy Egress Tunnel Routers

   Proxy Egress Tunnel Routers (PETRs) allow for LISP sites to send
   packets to non-LISP sites in the case where the access network does
   not allow for the LISP site send packets with the source address of
   the site's EID(s).  A PETR is a new network element that,
   conceptually, acts as an ETR for traffic destined to non-LISP sites.
   This also has the effect of allowing an ITR avoid having to decide
   whether to encapsulate packets or not - it can always encapsulate
   packets.  An ITR would encapsulate packets destined for LISP sites
   (no change here) and these would be routed directly to the
   corespondent site's ETR.  All other packets (those destined to non-
   LISP sites) will be sent to the originating site's PETR.

   There are two primary reasons why sites would want to utilize a PETR:

   Avoiding strict uRPF failures:  Some provider's access networks
      require the source of the packets emitted to be within the
      addressing scope of the access networks. (see section 9)

   Traversing a different IP Protocol:  A LISP site may want to transmit
      packets to a non-LISP site where the some of the intermediate
      network does not support the particular IP protocol desired (v4 or
      v6).  PETRs can allow this LISP site's data to 'hop over' this by
      utilizing LISP's support for mixed protocol encapsulation.

7.1.  Packet Flow with Proxy Egress Tunnel Routers

   Packets from a LISP site can reach a non-LISP site with the aid of a
   Proxy-ETR (or PETR).  An ITR is simply configured to send all non-
   LISP traffic, which it normally would have forwarded natively (non-
   encapsulated), to a PETR.  In the case where the ITR uses the Map-
   Resolver interface the ITR will encapsulate packets that match its
   Negative Map-Cache to the configured Proxy-ETR(s).  In the case where
   the ITR is connected to the mapping system directly it would
   encapsulate all packets to the configured Proxy-ETR that are cache
   misses.  Note that this outer encapsulation to the Proxy-ETR may be
   in an IP protocol other than the (inner) encapsulated data.  Routers
   then use the LISP (outer) header's destination address to route the
   packets toward the configured Proxy-ETR.

   A PETR should verify the (inner) source EID of the packet at time of
   decapsulation in order to verify that this is from a configured LISP
   site.  This is to prevent spoofed inner sources from being
   encapsulated through the Proxy-ETR.

   What follows is an example of the path a packet would take when using
   a PETR.  In this example, the LISP-NR (or LISP-R) site is given the



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   EID prefix 240.2.0.0/24, and it is trying to reach host at a non-LISP
   site with the IP prefix of 192.0.2.0/24.  For the purposes of this
   example, the destination is a non-LISP site and 192.0.2.0/24 is found
   in the Internet's routing system.

   A full protocol exchange example follows:

   1.  The source host makes a DNS lookup for the destination, and gets
       192.0.2.100 (a host in a non-LISP site) in return.

   2.  The source host has a default route to customer Edge (CE) router
       and forwards the packet towards the CE.

   3.  The CE is a LISP ITR, and is configured to encapsulate traffic
       destined for non-LISP sites to a Proxy-ETR.

   4.  The Proxy ETR decapsulates the LISP packet and forwards the
       original packet to its next hop.

   5.  The packet is then routed natively and directly to the
       destination (non-LISP) site 192.0.2.0/24.

   Note that in this example the return path is asymmetric, so return
   traffic will not go back through the Proxy-ETR.  This means that in
   order to reach LISP-NR sites, non-LISP sites must still use Proxy
   ITRs.

























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8.  Discussion of Proxy ITRs (PITRs), LISP-NAT, and Proxy-ETRs (PETRs)

   In summary, there are three mechanisms for interworking LISP with
   non-LISP Sites (for both IPv4 and IPv6).  In the LISP-NAT option the
   LISP site can manage and control the interworking on its own.  In the
   PITR case, we the site is not required to manage the advertisement of
   it's EID prefix into the DFZ, with the cost of potentially adding
   stretch to the connections of non-LISP sites sending packets to the
   LISP site.  The third option is Proxy-ETRs, which are optionally used
   by sites relying on PITRs case to mitigate two caveats for LISP sites
   sending packets to non-LISP sites.  This means Proxy-ETRs are not
   usually expected to be deployed by themselves, rather they will be
   used to assist LISP-NR sites which are already using PITRs.

8.1.  How Proxy-ITRs and Proxy-ETRs Interact

   There is a subtle difference between Symmetrical (LISP-NAT) vs
   Asymmetrical (Proxy-ITR and Proxy-ETR) Interworking techniques.
   Operationally, Proxy-ITRs (PITRs) and Proxy-ETRs (PETRs) can (and
   likely should) be decoupled since Proxy-ITRs are best deployed
   closest to non-LISP sites, and Proxy-ETRs are best located close to
   the LISP sites they are decapsulating for.  This asymmetric placement
   of the two network elements minimizes the stretch imposed on each
   direction of the packet flow, while still allowing for coarsely
   aggregated announcements of EIDs into the Internet's routing table.


























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9.  Security Considerations

   Like any router or LISP ITR, PITRs will have the opportunity to
   inspect traffic at the time that they encapsulate.  The location of
   these devices in the network can have implications for discarding
   malicious traffic on behalf of ETRs which request this behavior (via
   the drop action bit in Map-Reply packets for an EID or EID prefix).

   As with traditional NAT, LISP-NAT will obscure the actual host
   LISP-NR EID behind the LISP-R addresses used as the NAT pool.

   When LISP sites send packets to non-LISP sites (these non-LISP sites
   rely on PITRs to enable Interworking), packets will have the Site's
   EID as its source IP address.  These EIDs may not be recognized by
   their Internet Service Provider's Unicast Reverse Path Forwarding
   (uRPF) rules enabled on the Provider Edge Router.  Several options
   are available to the service provider.  For example they could enable
   a less strict version of uRPF, where they only look for the existence
   of the the EID prefix in the routing table.  Another, more secure,
   option is to add a static route for the customer on the PE router,
   but not redistribute this route into the provider's routing table.
   Finally, Proxy-ETRs can enable LISP sites to bypass this uRPF check
   by encapsulating all of their egressing traffic destined to non-LISP
   sites to the Proxy-ETR (thus ensuring the outer IP source address is
   the site's RLOC).


























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10.  Acknowledgments

   Thanks goes to Christian Vogt, Lixia Zhang, Robin Whittle, Michael
   Menth, and Xuewei Wang, and Noel Chiappa who have made insightful
   comments with respect to LISP Interworking and transition mechanisms.

   A special thanks goes to Scott Brim for his initial brainstorming of
   these ideas and also for his careful review.











































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11.  IANA Considerations

   This document creates no new requirements on IANA namespaces
   [RFC2434].















































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12.  References

12.1.  Normative References

   [LISP]     Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol (LISP)",
              draft-ietf-lisp-06 (work in progress), January 2010.

   [LISP-ALT]
              Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "LISP
              Alternative Topology (LISP+ALT)",
              draft-ietf-lisp-alt-03.txt (work in progress),
              Febuary 2010.

   [LISP-MS]  Farinacci, D. and V. Fuller, "LISP Map Server",
              draft-ietf-lisp-ms-03.txt (work in progress), Feb 2010.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

12.2.  Informative References

   [CRIO]     Zhang, X., Francis, P., Wang, J., and K. Yoshida, "CRIO:
              Scaling IP Routing with the Core Router-Integrated
              Overlay".

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              November 2000.














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Authors' Addresses

   Darrel Lewis
   Cisco Systems, Inc.

   Email: darlewis@cisco.com


   David Meyer
   Cisco Systems, Inc.

   Email: dmm@cisco.com


   Dino Farinacci
   Cisco Systems, Inc.

   Email: dino@cisco.com


   Vince Fuller
   Cisco Systems, Inc.

   Email: vaf@cisco.com



























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