Network Working Group                                        A. Cabellos
Internet-Draft                                         UPC-BarcelonaTech
Intended status: Informational                           D. Saucez (Ed.)
Expires: April 26, 2015                                            INRIA
                                                        October 23, 2014

  An Architectural Introduction to the Locator/ID Separation Protocol


   This document describes the architecture of the Locator/ID Separation
   Protocol (LISP), making it easier to read the rest of the LISP
   specifications and providing a basis for discussion about the details
   of the LISP protocols.  This document is used for introductory
   purposes, more details can be found in RFC6830, the protocol

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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 April 26, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   4
   3.  LISP Architecture . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Design Principles . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Overview of the Architecture  . . . . . . . . . . . . . .   4
     3.3.  Data-Plane  . . . . . . . . . . . . . . . . . . . . . . .   7
       3.3.1.  LISP Encapsulation  . . . . . . . . . . . . . . . . .   7
       3.3.2.  LISP Forwarding State . . . . . . . . . . . . . . . .   8
     3.4.  Control-Plane . . . . . . . . . . . . . . . . . . . . . .   8
       3.4.1.  LISP Mappings . . . . . . . . . . . . . . . . . . . .   9
       3.4.2.  Mapping System Interface  . . . . . . . . . . . . . .   9
       3.4.3.  Mapping System  . . . . . . . . . . . . . . . . . . .  10
     3.5.  Interworking Mechanisms . . . . . . . . . . . . . . . . .  13
   4.  LISP Operational Mechanisms . . . . . . . . . . . . . . . . .  13
     4.1.  Cache Management  . . . . . . . . . . . . . . . . . . . .  14
     4.2.  RLOC Reachability . . . . . . . . . . . . . . . . . . . .  14
     4.3.  ETR Synchronization . . . . . . . . . . . . . . . . . . .  16
     4.4.  MTU Handling  . . . . . . . . . . . . . . . . . . . . . .  16
   5.  Mobility  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Multicast . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   8.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Traffic Engineering . . . . . . . . . . . . . . . . . . .  19
     8.2.  LISP for IPv6 Co-existence  . . . . . . . . . . . . . . .  19
     8.3.  LISP for Virtual Private Networks . . . . . . . . . . . .  20
     8.4.  LISP for Virtual Machine Mobility in Data Centers . . . .  20
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     12.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  A Brief History of Location/Identity Separation  . .  23
     A.1.  Old LISP Models . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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1.  Introduction

   This document introduces the Locator/ID Separation Protocol (LISP)
   [RFC6830] architecture, its main operational mechanisms and its
   design rationale.  Fundamentally, LISP is built following a well-
   known architectural idea: decoupling the IP address overloaded
   semantics.  Indeed and as pointed out by [Chiappa], currently IP
   addresses both identify the topological location of a network
   attachment point as well as the node's identity.  However, nodes and
   routing have fundamentally different requirements, routing systems
   require that addresses are aggregatable and have topological meaning,
   while nodes require to be identified independently of their current
   location [RFC4984].

   LISP creates two separate namespaces, EIDs (End-host IDentifiers) and
   RLOCs (Routing LOCators), both are typically syntactically identical
   to the current IPv4 and IPv6 addresses.  EIDs are used to uniquely
   identify nodes irrespective of their topological location and are
   typically routed intra-domain.  RLOCs are assigned topologically to
   network attachment points and are typically routed inter-domain.
   With LISP, the edge of the Internet (where the nodes are connected)
   and the core (where inter-domain routing occurs) can be logically
   separated and interconnected by LISP-capable routers.  LISP also
   introduces a database, called the Mapping System, to store and
   retrieve mappings between identity and location.  LISP-capable
   routers exchange packets over the Internet core by encapsulating them
   to the appropriate location.

   By taking advantage of such separation between location and identity
   LISP offers Traffic Engineering, multihoming, and mobility among
   others benefits.  Additionally, LISP's approach to solve the routing
   scalability problem [RFC4984] is that with LISP the Internet core is
   populated with RLOCs while Traffic Engineering mechanisms are pushed
   to the Mapping System.  With this RLOCs are quasi-static (i.e., low
   churn) and hence, the routing system scalable [Quoitin].

   This document describes the LISP architecture, its main operational
   mechanisms as its design rationale.  It is important to note that
   this document does not specify or complement the LISP protocol.  The
   interested reader should refer to the main LISP specifications
   [RFC6830] and the complementary documents [RFC6831],[RFC6832],
   [RFC6833],[RFC6834],[RFC6835], [RFC6836] for the protocol
   specifications along with the LISP deployment guidelines [RFC7215].

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2.  Definition of Terms

   This document describes the LISP architecture and does not define or
   introduce any new term.  The reader is referred to
   [RFC6836],[RFC7215] for the LISP definition of terms.

3.  LISP Architecture

   This section presents the LISP architecture, it first details the
   design principles of LISP and then it proceeds to describe its main
   aspects: data-plane, control-plane, and inetrworking mechanisms.

3.1.  Design Principles

   The LISP architecture is built on top of four basic design

   o  Locator/Identifier split: By decoupling the overloaded semantics
      of the current IP addresses the Internet core can be assigned
      identity meaningful addresses and hence, can use aggregation to
      scale.  Devices are assigned with identity meaningful addresses
      that are independent of their topological location.

   o  Overlay architecture: Overlays route packets over the current
      Internet, allowing deployment of new protocols without changing
      the current infrastructure hence, resulting into a low deployment

   o  Decoupled data and control-plane: Separating the data-plane from
      the control-plane allows them to scale independently and use
      different architectural approaches.  This is important given that
      they typically have different requirements.

   o  Incremental deployability: This principle ensures that the
      protocol interoperates with the legacy Internet while providing
      some of the targeted benefits to early adopters.

3.2.  Overview of the Architecture

   LISP splits architecturally the core from the edge of the Internet by
   creating two separate namespaces: Endpoint Identifiers (EIDs) and
   Routing LOCators (RLOC).  The edge consists of LISP sites (e.g., an
   Autonomous System) that use EID addresses.  EIDs are typically -but
   not limited to- IPv4 or IPv6 addresses that uniquely identify
   communication end-hosts and are assigned and configured by the same
   mechanisms that exist at the time of this writing.  EIDs do not
   contain inter-domain topological information and can be thought as an

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   analogy to Provider Independent (PI [RFC4116]) addresses.  Because of
   this, EIDs are usually only routable at the edge.

   With LISP, LISP sites (edge) and the core of the Internet are
   interconnected by means of LISP-capable routers (e.g., border
   routers) using tunnels.  When packets originated from a LISP site are
   flowing towards the core network, they ingress into an encapsulated
   tunnel via an Ingress Tunnel Router (ITR).  When packets flow from
   the core network to a LISP site, they egress from an encapsulated
   tunnel to an Egress Tunnel Router (ETR).  An xTR is a router with can
   perform both ITR and ETR operations.  In this context ITRs
   encapsulate packets while ETRs decapsulate them, hence LISP operates
   as an overlay to the current Internet core.

                        /-----------------\                        ---
                        |     Mapping     |                         |
                        .     System      |                         |  Control
                       -|                 |`,                       |  Plane
                     ,' \-----------------/  .                      |
                    /                         \                    ---
    ,..,           -        _,..--..,,         `,         ,..,      |
  /     `        ,'      ,-`          `',        .      /     `     |
 /        \ +-----+    ,'                `,    +--'--+ /        \   |
 |  EID   |-| xTR |---/        RLOC        ,---| xTR |-|  EID   |   |  Data
 | Space  |-|     |---|       Space        |---|     |-| Space  |   |  Plane
 \        / +-----+   .                   /    +-----+ \        /   |
  `.    .'             `.                ,'             `.    .'    |
    `'-`                 `.,          ,.'                 `'-`     ---
  LISP Site (Edge)            Core              LISP Site (Edge)

           Figure 1.- A schema of the LISP Architecture

   With LISP, the core uses RLOCs, an RLOC is typically -but not limited
   to- an IPv4 or IPv6 address assigned to an Internet-facing network
   interface of an ITR or ETR.  Typically RLOCs are numbered from
   topologically aggregatable blocks assigned to a site at each point to
   which it attaches to the global Internet.  The topology is defined by
   the connectivity of networks, in this context RLOCs can be though as
   Provider Aggregatable addresses [RFC4116].

   A typically distributed database, called the Mapping System, stores
   mappings between EIDs and RLOCs.  Such mappings relate the identity
   of the devices attached to LISP sites (EIDs) to the set of RLOCs

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   configured at the LISP-capable routers servicing the site.
   Furthermore, the mappings also include traffic engineering policies
   and can be configured to achieve multihoming and load balancing.  The
   LISP Mapping System is conceptually similar to the DNS that would be
   accessed by ETRs to register mappings and by ITRs to retrieve them.

   Finally, the LISP architecture emphasizes a cost effective
   incremental deployment.  Given that LISP represents an overlay to the
   current Internet architecture, endhosts as well as intra and inter-
   domain routers remain unchanged, and the only required changes to the
   existing infrastructure are to routers connecting the EID with the
   RLOC space.  Such LISP capable routers, in most cases, only require a
   software upgrade.  Additionally, LISP requires the deployment of an
   independent Mapping System, such distributed database is a new
   network entity.

   The following describes a simplified packet flow sequence between two
   nodes that are attached to LISP sites.  Client hostA wants to send a
   packet to server hostB.

                            |     Mapping    |
                            |     System     |
                           .|                |-
                          ` \----------------/ `.
                        ,`                       \
                       /                          `.
                     ,'         _,..-..,,           ',
                    /         -`         `-,          \
                  .'        ,'              \          `,
                  `        '                 \           '
              +-----+     |                   | RLOC_B1+-----+
       HostA  |     |    |        RLOC         |-------|     |  HostB
       EID_A--|ITR_A|----|        Space        |       |ETR_B|--EID_B
              |     | RLOC_A1                  |-------|     |
              +-----+     |                   | RLOC_B2+-----+
                           ,                 /
                            \               /
                             `',         ,-`

               Figure 2.- Packet flow sequence in LISP

   1.  HostA retrieves the EID_B of HostB (typically querying the DNS)
       and generates an IP packet as in the Internet, the packet has
       source address EID_A and destination address EID_B.

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   2.  The packet is routed towards ITR_A in the LISP site using
       standard intra-domain mechanisms.

   3.  ITR_A upon receiving the packet queries the Mapping System to
       retrieve the locator of ETR_B that is servicing hostB's EID_B.
       In order to do so it uses a LISP control message called Map-
       Request, the message contains EID_B as the lookup key.  In turn
       it receives another LISP control message called Map-Reply, the
       message contains two locators: RLOC_B1 and RLOC_B2 along with
       traffic engineering policies: priority and weight per locator.
       ITR_A also stores the mapping in a local cache to speed-up
       forwarding of subsequent packets.

   4.  ITR_A encapsulates the packet towards RLOC_B1 (chosen according
       to the priorities/weights specified in the mapping).  The packet
       contains two IP headers, the outer header has RLOC_A1 as source
       and RLOC_B2 as destination, the inner original header has EID_A
       as source and EID_B as destination.  Furthermore ITR_A adds a
       LISP header, more details about LISP encapsulation can be found
       in Section 3.3.1.

   5.  The encapsulated packet is forwarded by the Internet core as a
       normal IP packet, making the EID invisible from the Internet

   6.  Upon reception of the encapsulated packet by ETR_B, it
       decapsulates the packet and forwards it to hostB.

3.3.  Data-Plane

   This section provides a high-level description of the LISP data-
   plane, which is specified in detail in [RFC6830].  The LISP data-
   plane is responsible for encapsulating and decapsulating data packets
   and caching the appropriate forwarding state.  It includes two main
   entities, the ITR and the ETR, both are LISP capable routers that
   connect the EID with the RLOC space (ITR) and vice versa (ETR).

3.3.1.  LISP Encapsulation

   ITRs encapsulate data packets towards ETRs.  LISP data packets are
   encapsulated using UDP (port 4341).  A particularity of LISP is that
   UDP packets should include a zero checksum [RFC6935] [RFC6936] that
   it is not verified in reception, LISP also supports non-zero
   checksums that may be verified.  This decision was made because the
   typical transport protocols used by the applications already include
   a checksum, by neglecting the additional UDP encapsulation checksum
   xTRs can forward packets more efficiently.

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   LISP-encapsulated packets also include a LISP header (after the UDP
   header and before the original IP header).  The LISP header is
   prepended by ITRs and striped by ETRs.  It carries reachability
   information (see more details in Section 4.2) and the Instance ID
   field.  The Instance ID field is used to distinguish traffic to/from
   different tenant address spaces at the LISP site and that may use
   overlapped but logically separated EID addressing.

   Overall, LISP encapsulated data packets carry 4 headers [RFC6830]
   ("outer" to "inner"):

   1.  Outer IP header containing RLOCs as source and destination
       addresses.  This header is originated by ITRs and stripped by

   2.  UDP header (port 4341) with zero checksum.  This header is
       originated by ITRs and stripped by ETRs.

   3.  LISP header that contains various forwarding-plane features (such
       as reachability) and an Instance ID field.  This header is
       originated by ITRs and stripped by ETRs.

   4.  Inner IP header containing EIDs as source and destination
       addresses.  This header is created by the source end-host and is
       left unchanged by LISP data plane processing on the ITR and ETR.

   Finally, in some scenarios Recursive and/or Re-encapsulating tunnels
   can be used for Traffic Engineering and re-routing.  Re-encapsulating
   tunnels are consecutive LISP tunnels and occur when an ETR removes a
   LISP header and then acts as an ITR to prepend another one.  On the
   other hand, Recursive tunnels are nested tunnels and are implemented
   by using multiple LISP encapsulations on a packet.  Typically such
   functions are implemented by Reencapsulating Tunnel Routers (RTRs).

3.3.2.  LISP Forwarding State

   ITRs retrieve from the LISP Mapping System mappings between EID
   prefixes and RLOCs that are used to encapsulate packets.  Such
   mappings are stored in a local cache -called the Map-Cache- for
   subsequent packets addressed to the same EID prefix.  Mappings
   include a (Time-to-Live) TTL (set by the ETR).  More details about
   the Map-Cache management can be found in Section 4.1.

3.4.  Control-Plane

   The LISP control-plane, specified in [RFC6833], provides a standard
   interface to register, request, and resolve mappings.  The LISP
   Mapping System is a database that stores such mappings.  The

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   following first describes the mappings, then the standard interface
   to the Mapping System, and finally its architecture.

3.4.1.  LISP Mappings

   Each mapping includes the bindings between EID prefix(es) and set of
   RLOCs as well as traffic engineering policies, in the form of
   priorities and weights for the RLOCs.  Priorities allow the ETR to
   configure active/backup policies while weights are used to load-
   balance traffic among the RLOCs (on a per-flow basis).

   Typical mappings in LISP bind EIDs in the form of IP prefixes with a
   set of RLOCs, also in the form of IPs.  IPv4 and IPv6 addresses are
   encoded using the appropriate Address Family Identifier (AFI)
   [RFC3232].  However LISP can also support more general address
   encoding by means of the ongoing effort around the LISP Canonical
   Address Format (LCAF) [I-D.ietf-lisp-lcaf].

   With such a general syntax for address encoding in place, LISP aims
   to provide flexibility to current and future applications.  For
   instance LCAFs could support MAC addresses, geo-coordinates, ASCII
   names and application specific data.

3.4.2.  Mapping System Interface

   LISP defines a standard interface between data and control planes.
   The interface is specified in [RFC6833] and defines two entities:

   Map-Server:  A network infrastructure component that learns mappings
      from ETRs and publishes them into the LISP Mapping System.
      Typically Map-Servers are not authoritative to reply to queries
      and hence, they forward them to the ETR.  However they can also
      operate in proxy-mode, where the ETRs delegate replying to queries
      to Map-Servers.  This setup is useful when the ETR has limited
      resources (i.e., CPU or power).

   Map-Resolver:  A network infrastructure component that interfaces
      ITRs with the Mapping System by proxying queries and -in some
      cases- responses.

   The interface defines four LISP control messages which are sent as
   UDP datagrams (port 4342):

   Map-Register:  This message is used by ETRs to register mappings in
      the Mapping System and it is authenticated using a shared key
      between the ETR and the Map-Server.

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   Map-Notify:  When requested by the ETR, this message is sent by the
      Map-Server in response to a Map-Register to acknowledge the
      correct reception of the mapping and convey the latest Map-Server
      state on the EID to RLOC mapping.

   Map-Request:  This message is used by ITRs or Map-Resolvers to
      resolve the mapping of a given EID.

   Map-Reply:  This message is sent by Map-Servers or ETRs in response
      to a Map-Request and contains the resolved mapping.  Please note
      that a Map-Reply may contain a negative reply if, for example, the
      queried EID is not part of the LISP EID space.  In such cases the
      ITR typically forwards the traffic natively (non encapsulated) to
      the public Internet, this behavior is defined to support
      incremental deployment of LISP.

3.4.3.  Mapping System

   LISP architecturally decouples control and data-plane by means of a
   standard interface.  This interface glues the data-plane, routers
   responsible for forwarding data-packets, with the LISP Mapping
   System, a database responsible for storing mappings.

   With this separation in place the data and control-plane can use
   different architectures if needed and scale independently.  Typically
   the data-plane is optimized to route packets according to
   hierarchical IP addresses.  However the control-plane may have
   different requirements, for instance and by taking advantage of the
   LCAFs, the Mapping System may be used to store non-hierarchical keys
   (such as MAC addresses), requiring different architectural approaches
   for scalability.  Another important difference between the LISP
   control and data-planes is that, and as a result of the local mapping
   cache available at ITR, the Mapping System does not need to operate
   at line-rate.

   The LISP WG has explored application of the following distributed
   system techniques to the Mapping System architecture: graph-based
   databases in the form of LISP+ALT [RFC6836], hierarchical databases
   in the form of LISP-DDT [I-D.ietf-lisp-ddt], monolithic databases in
   the form of LISP-NERD [RFC6837] and flat databases in the form of
   LISP-DHT [I-D.cheng-lisp-shdht],[I-D.mathy-lisp-dht].  Furthermore it
   is worth noting that, in some scenarios such as private deployments,
   the Mapping System can operate as logically centralized.  In such
   cases it is typically composed of a single Map-Server/Map-Resolver.

   The following focuses on the two mapping systems that have been
   implemented and deployed (LISP-ALT and LISP+DDT).

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   The LISP Alternative Topology (LISP+ALT) [RFC6836] was the first
   Mapping System proposed, developed and deployed on the LISP pilot
   network.  It is based on a distributed BGP overlay participated by
   Map-Servers and Map-Resolvers.  The nodes connect to their peers
   through static tunnels.  Each Map-Server involved in the ALT topology
   advertises the EID-prefixes registered by the serviced ETRs, making
   the EID routable on the ALT topology.

   When an ITR needs a mapping it sends a Map-Request to a Map-Resolver
   that, using the ALT topology, forwards the Map-Request towards the
   Map-Server responsible for the mapping.  Upon reception the Map-
   Server forwards the request to the ETR that in turn, replies directly
   to the ITR using the native Internet core.  LISP-DDT

   LISP-DDT [I-D.ietf-lisp-ddt] is conceptually similar to the DNS, a
   hierarchical directory whose internal structure mirrors the
   hierarchical nature of the EID address space.  The DDT hierarchy is
   composed of DDT nodes forming a tree structure, the leafs of the tree
   are Map-Servers.  On top of the structure there is the DDT root node
   [DDT-ROOT], which is a particular instance of a DDT node and that
   matches the entire address space.  As in the case of DNS, DDT
   supports multiple redundant DDT nodes and/or DDT roots.  Finally,
   Map-Resolvers are the clients of the DDT hierarchy and can query
   either the DDT root and/or other DDT nodes.

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                          |         |
                          | DDT Root|
                          |   /0    |
                    ,-'`       |       `'.,
                 -'`           |           `-
             /-------\     /-------\    /-------\
             |  DDT  |     |  DDT  |    |  DDT  |
             | Node  |     | Node  |    | Note  |  ...
             |  0/8  |     |  1/8  |    |  2/8  |
             \-------/     \-------/    \-------/
           _.                _.            . -..,,,_
         -`                -`              \        ````''--
  +------------+     +------------+   +------------+ +------------+
  | Map-Server |     | Map-Server |   | Map-Server | | Map-Server |
  | EID-prefix1|     | EID-prefix2|   | EID-prefix3| | EID-prefix4|
  +------------+     +------------+   +------------+ +------------+

        Figure 3.- A schematic representation of the DDT tree structure,
                please note that the prefixes and the structure depicted
                should be only considered as an example.

   The DDT structure does not actually index EID-prefixes but eXtended
   EID-prefixes (XEID).  An XEID-prefix is just the concatenation of the
   following fields (from most significant bit to less significant bit):
   Database-ID, Instance ID, Address Family Identifier and the actual
   EID-prefix.  The Database-ID is provided for possible future
   requirements of higher levels in the hierarchy and to enable the
   creation of multiple and separate database trees.

   In order to resolve a query LISP-DDT operates in a similar way to the
   DNS but only supports iterative lookups.  DDT clients (usually Map-
   Resolvers) generate Map-Requests to the DDT root node.  In response
   they receive a newly introduced LISP-control message: a Map-Referral.
   A Map-Referral provides the list of RLOCs of the set of DDT nodes
   matching a configured XEID delegation.  That is, the information
   contained in the Map-Referral points to the child of the queried DDT
   node that has more specific information about the queried XEID-
   prefix.  This process is repeated until the DDT client walks the tree
   structure (downwards) and discovers the Map-Server servicing the
   queried XEID.  At this point the client sends a Map-Request and
   receives a Map-Reply containing the mappings.  It is important to
   note that DDT clients can also cache the information contained in
   Map-Referrals, that is, they cache the DDT structure.  This is used
   to reduce the mapping retrieving latency[Jakab].

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   The DDT Mapping System relies on manual configuration.  That is Map-
   Resolvers are manually configured with the set of available DDT root
   nodes while DDT nodes are manually configured with the appropriate
   XEID delegations.  Configuration changes in the DDT nodes are only
   required when the tree structure changes itself, but it doesn't
   depend on EID dynamics (RLOC allocation or traffic engineering policy

3.5.  Interworking Mechanisms

   EIDs are typically identical to either IPv4 or IPv6 addresses and
   they are stored in the LISP Mapping System, however they are usually
   not announced in the Internet global routing system.  As a result
   LISP requires an inetrworking mechanism to allow LISP sites to speak
   with non-LISP sites and vice versa.  LISP inetrworking mechanisms are
   specified in [RFC6832].

   LISP defines two entities to provide inetrworking:

   Proxy Ingress Tunnel Router (PITR):  PITRs provide connectivity from
      the legacy Internet to LISP sites.  PITRs announce in the global
      routing system blocks of EID prefixes (aggregating when possible)
      to attract traffic.  For each incoming data-packet, the PITR LISP-
      encapsulates it towards the RLOC(s) of the appropriate LISP site.
      The impact of PITRs in the routing table size of the DFZ is, in
      the worst-case, similar to the case in which LISP is not deployed.
      EID-prefixes will be aggregated as much as possible both by the
      PITR and by the global routing system.

   Proxy Egress Tunnel Router (PETR):  PETRs provide connectivity from
      LISP sites to the legacy Internet.  In some scenarios, LISP sites
      may be unable to send encapsulated packets with a local EID
      address as a source to the legacy Internet.  For instance when
      Unicast Reverse Path Forwarding (uRPF) is used by Provider Edge
      routers, or when an intermediate network between a LISP site and a
      non-LISP site does not support the desired version of IP (IPv4 or
      IPv6).  In both cases the PETR overcomes such limitations by
      encapsulating packets over the network.  There is no specified
      provision for the distribution of PETR RLOC addresses to the ITRs.

4.  LISP Operational Mechanisms

   This section details the main operational mechanisms defined in LISP.

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4.1.  Cache Management

   LISP's decoupled control and data-plane, where mappings are stored in
   the control-plane and used for forwarding in the data plane, requires
   of a local cache in ITRs to reduce signaling overhead (Map-Request/
   Map-Reply) and increase forwarding speed.  The local cache available
   at the ITRs, called Map-Cache, is used by the router to LISP-
   encapsulate packets.  The Map-Cache is indexed by (Instance ID, EID-
   prefix) and contains basically the set of RLOCs with the associated
   traffic engineering policies (priorities and weights).

   The Map-Cache, as any other cache, requires cache coherence
   mechanisms to maintain up-to-date information.  LISP defines three
   main mechanisms for cache coherence:

   Time-To-Live (TTL):  Each mapping contains a TTL set by the ETR, upon
      expiration of the TTL the ITR has to refresh the mapping by
      sending a new Map-Request.  Typical values for TTL defined by LISP
      are 24h.

   Solicit-Map-Request (SMR):  SMR is an explicit mechanism to update
      mapping information.  In particular a special type of Map-Request
      can be sent on demand by ETRs to request refreshing a mapping.
      Upon reception of a SMR message, the ITR must refresh the bindings
      by sending a Map-Request to the Mapping System.

   Map-Versioning:  This optional mechanism piggybacks in the LISP
      header of data-packets the version number of the mappings used by
      an xTR.  This way, when an xTR receives a LISP-encapsulated packet
      from a remote xTR, it can check whether its own Map-Cache or the
      one of the remote xTR is outdated.  If its Map-Cache is outdated,
      it sends a Map-Request for the remote EID so to obtain the newest
      mappings.  On the contrary, if it detects that the remote xTR Map-
      Cache is outdated, it sends a SMR to notify it that a new mapping
      is available.

   Finally it is worth noting that in some cases an entry in the map-
   cache can be proactively refreshed using the mechanisms described in
   the section below.

4.2.  RLOC Reachability

   The LISP architecture is an edge to edge pull architecture, where the
   network state is stored in the control-plane while the data-plane
   pulls it on demand.  On the contrary BGP is a push architecture,
   where the required network state is pushed by means of BGP UPDATE
   messages to BGP speakers.  In push architectures, reachability
   information is also pushed to the interested routers.  However pull

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   architectures require explicit mechanisms to propagate reachability
   information.  LISP defines a set of mechanisms to inform ITRs and
   PITRS about the reachability of the cached RLOCs:

   Locator Status Bits (LSB): LSB is a passive technique, the LSB field
   is carried by data-packets in the LISP header and can be set by a
   ETRs to specify which RLOCs of the ETR site are up/down.  This
   information can be used by the ITRs as a hint about the reachability
   to perform additional checks.  Also note that LSB does not provide
   path reachability status, only hints on the status of RLOCs.

   Echo-nonce: This is also a passive technique, that can only operate
   effectively when data flows bi-directionally between two
   communicating xTRs.  Basically, an ITR piggybacks a random number
   (called nonce) in LISP data packets, if the path and the probed
   locator are up, the ETR will piggyback the same random number on the
   next data-packet, if this is not the case the ITR can set the locator
   as unreachable.  When traffic flow is unidirectional or when the ETR
   receiving the traffic is not the same as the ITR that transmits it
   back, additional mechanisms are required.

   RLOC-probing: This is an active probing algorithm where ITRs send
   probes to specific locators, this effectively probes both the locator
   and the path.  In particular this is done by sending a Map-Request
   (with certain flags activated) on the data-plane (RLOC space) and
   waiting in return a Map-Reply, also sent on the data-plane.  The
   active nature of RLOC-probing provides an effective mechanism to
   determine reachability and, in case of failure, switching to a
   different locator.  Furthermore the mechanism also provides useful
   RTT estimates of the delay of the path that can be used by other
   network algorithms.

   Additionally, LISP also recommends inferring reachability of locators
   by using information provided by the underlay, in particular:

   It is worth noting that RLOC probing and Echo-nonce can work
   together.  Specifically if a nonce is not echoed, an ITR could RLOC-
   probe to determine if the path is up because the return bidirectional
   path may have failed or the return path is not used, that is there is
   only a unidirectional path.

   ICMP signaling: The LISP underlay -the current Internet- uses the
   ICMP protocol to signal unreachability (among other things).  LISP
   can take advantage of this and the reception of a ICMP Network
   Unreachable or ICMP Host Unreachable message can be seen as a hint
   that a locator might be unreachable, this should lead to perform
   additional checks.

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   Underlay routing: Both BGP and IBGP carry reachability information,
   LISP-capable routers that have access to underlay routing information
   can use it to determine if a given locator or path are reachable.

4.3.  ETR Synchronization

   All the ETRs that are authoritative to a particular EID-prefix must
   announce the same mapping to the requesters, this means that ETRs
   must be aware of the status of the RLOCs of the remaining ETRs.  This
   is known as ETR synchronization.

   At the time of this writing LISP does not specify a mechanism to
   achieve ETR synchronization.  Although many well-known techniques
   could be applied to solve this issue it is still under research, as a
   result operators must rely on coherent manual configuration

4.4.  MTU Handling

   Since LISP encapsulates packets it requires dealing with packets that
   exceed the MTU of the path between the ITR and the ETR.  Specifically
   LISP defines two mechanisms:

   Stateless:  With this mechanism the effective MTU is assumed from the
      ITR's perspective.  If a payload packet is too big for the
      effective MTU, and can be fragmented, the payload packet is
      fragmented on the ITR, such that reassembly is performed at the
      destination host.

   Stateful:  With this mechanism ITRs keep track of the MTU of the
      paths towards the destination locators by parsing the ICMP Too Big
      packets sent by intermediate routers.  Additionally ITRs will send
      ICMP Too Big messages to inform the sources about the effective

   In both cases if the packet cannot be fragmented (IPv4 with DF=1 or
   IPv6) then the ITR drops it and replies with a ICMP Too Big message
   to the source.

5.  Mobility

   The separation between locators and identifiers in LISP was initially
   proposed for traffic engineering purpose where LISP sites can change
   their attachment points to the Internet (i.e., RLOCs) without
   impacting endpoints or the Internet core.  In this context, the
   border routers operate the xTR functionality and endpoints are not
   aware of the existence of LISP.  However, this mode of operation does
   not allow seamless mobility of endpoints between different LISP sites
   as the EID address might not be routable in a visited site.

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   Nevertheless, LISP can be used to enable seamless IP mobility when
   LISP is directly implemented in the endpoint.  Each endpoint is then
   an xTR and the EID address is the one presented to the network stack
   used by applications while the RLOC is the address gathered from the
   network when it is visited.

   Whenever the device changes of RLOC, the ITR updates the RLOC of its
   local mapping and registers it to its Map-Server.  To avoid the need
   of a home gateway, the ITR also indicates the RLOC change to all
   remote devices that have ongoing communications with the device that
   moved.  The combination of both methods ensures the scalability of
   the system as signaling is strictly limited the Map-Server and to
   hosts with which communications are ongoing.

6.  Multicast

   LISP also supports transporting IP multicast packets sent from the
   EID space, the operational changes required to the multicast
   protocols are documented in [RFC6831].

   In such scenarios, LISP may create multicast state both at the core
   and at the sites (both source and receiver).  When signaling is used
   create multicast state at the sites, LISP routers unicast encapsulate
   PIM Join/Prune messages from receiver to source sites.  At the core,
   ETRs build a new PIM Join/Prune message addressed to the RLOC of the
   ITR servicing the source.  An simplified sequence is shown below

   1.  An end-host willing to join a multicast channel sends an IGMP
       report.  Multicast PIM routers at the LISP site propagate PIM
       Join/Prune messages (S-EID, G) towards the ETR.

   2.  The join message flows to the ETR, upon reception the ETR builds
       two join messages, the first one unicast LISP-encapsulates the
       original join message towards the RLOC of the ITR servicing the
       source.  This message creates multicast state at the source site.
       The second join message contains as destination address the RLOC
       of the ITR servicing the source (S-RLOC, G) and creates multicast
       state at the core.

   3.  Multicast data packets originated by the source (S-EID, G) flow
       from the source to the ITR.  The ITR LISP-encapsulates the
       multicast packets, the outter header includes its own RLOC as the
       source (S-RLOC) and the original multicast group address (G) as
       the destination.  Please note that multicast group address are
       logical and are not resolved by the mapping system.  Then the
       multicast packet is transmitted through the core towards the
       receiving ETRs that decapsulates the packets and sends them using
       the receiver's site multicast state.

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   LISP also support non-PIM mechanisms to maintain multicast state.

7.  Security

   LISP uses a pull architecture to learn mappings.  While in a push
   system, the state necessary to forward packets is learned
   independently of the traffic itself, with a pull architecture, the
   system becomes reactive and data-plane events (e.g., the arrival of a
   packet for an unknown destination) may trigger control-plane events.
   This on-demand learning of mappings provides many advantages as
   discussed above but may also affect the way security is enforced.

   Usually, the data-plane is implemented in the fast path of routers to
   provide high performance forwarding capabilities while the control-
   plane features are implemented in the slow path to offer high
   flexibility and a performance gap of several order of magnitude can
   be observed between the slow and the fast paths.  As a consequence,
   the way data-plane events are notified to the control-plane must be
   though carefully so to not overload the slow path and rate limiting
   should be used as specified in [RFC6830].

   Care must also be taken so to not overload the mapping system (i.e.,
   the control plane infrastructure) as the operations to be performed
   by the mapping system may be more complex than those on the data-
   plane, for that reason [RFC6830] recommends to rate limit the sending
   of messages to the mapping system.

   To improve resiliency and reduce the overall number of messages
   exchanged, LISP offers the possibility to leak control informations,
   such as reachabilty of locators, directly into data plane packets.
   In environments that are not fully trusted, control informations
   gleaned from data-plane packets should be verified before using them.

   Mappings are the centrepiece of LISP and all precautions must be
   taken to avoid them to be manipulated or misused by malicious
   entities.  Using trustable Map-Servers that strictly respect
   [RFC6833] and the lightweight authentication mechanism proposed by
   LISP-Sec [I-D.ietf-lisp-sec] reduces the risk of attacks to the
   mapping integrity.  In more critical environments, secure measures
   may be needed.

   As with any other tunneling mechanism, middleboxes on the path
   between an ITR (or PITR) and an ETR (or PETR) must implement
   mechanisms to strip the LISP encapsulation to correctly inspect the
   content of LISP encapsulated packets.

   Like other map-and-encap mechanisms, LISP enables triangular routing
   (i.e., packets of a flow cross different border routers depending on

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   their direction).  This means that intermediate boxes may have
   incomplete view on the traffic they inspect or manipulate.

   More details about security implications of LISP are discussed in

8.  Use Cases

8.1.  Traffic Engineering

   BGP is the standard protocol to implement inter-domain routing.  With
   BGP, routing informations are propagated along the network and each
   autonomous system can implement its own routing policy that will
   influence the way routing information are propagated.  The direct
   consequence is that an autonomous system cannot precisely control the
   way the traffic will enter the network.

   As opposed to BGP, a LISP site can strictly impose via which ETRs the
   traffic must enter the network even though the path followed to reach
   the ETR is not under the control of the LISP site.  This fine control
   is implemented with the mappings.  When a remote site is willing to
   send traffic to a LISP site, it retrieves the mapping associated to
   the destination EID via the mapping system.  The mapping is sent
   directly by an authoritative ETR of the EID and is not altered by any
   intermediate network.

   A mapping associates a list of RLOCs to an EID prefix.  Each RLOC
   corresponds to an interface of an ETR that is able to correctly
   forward packets to EIDs in the prefix.  Each RLOC is tagged with a
   priority and a weight in the mapping.  The priority is used to
   indicates which RLOCs should be preferred to send packets (the least
   preferred ones being provided for backup purpose).  The weight
   permits to balance the load between the RLOCs with the same priority,
   proportionally to the weight value.

   As mappings are directly issued by the authoritative ETR of the EID
   and are not altered while transmitted to the remote site, it offers
   highly flexible incoming inter-domain traffic engineering with even
   the possibility for a site to issue a different mapping for each
   remote site, implementing so precise routing policies.

8.2.  LISP for IPv6 Co-existence

   LISP encapsulations permits to transport packets using EIDs from a
   given address family (e.g., IPv6) with packets with addresses
   belonging to another address family (e.g., IPv4).  The absence of
   correlation between the address family of RLOCs and EIDs makes LISP a

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   candidate to allow, e.g., IPv6 to be deployed when all of the core
   network may not have IPv6 enabled.

   For example, two IPv6-only data centers could be interconnected via
   the legacy IPv4 Internet.  If their border routers are LISP capable,
   sending packets between the data center is done without any form of
   translation as the native IPv6 packets (in the EID space) will be
   LISP encapsulated and transmitted over the IPv4 legacy Internet by
   the mean of IPv4 RLOCs.

8.3.  LISP for Virtual Private Networks

   It is common to operate several virtual networks over the same
   physical infrastructure.  In such virtual private networks, it is
   essential to distinguish to which virtual network a packet belongs
   and tags or labels are used for that purpose.  With LISP, the
   distinction can be made with the Instance ID field.  When an ITR
   encapsulates a packet from a particular virtual network (e.g., known
   via the VRF or VLAN), it tags the encapsulated packet with the
   Instance ID corresponding to the virtual network of the packet.  When
   an ETR receives a packet tagged with an Instance ID it uses the
   Instance ID to determine how to treat the packet.

   The main advantage of using LISP for virtual networks, on top of the
   simplicity of managing the mappings, is that it does not impose any
   requirement on the underlying network, as long as it is running IP.

8.4.  LISP for Virtual Machine Mobility in Data Centers

   A way to enable seamless virtual machine mobility in data center is
   to conceive the datacenter backbone as the RLOC space and the subnet
   where servers are hosted as forming the EID space.  A LISP router is
   placed at the border between the backbone and each subnet.  When a
   virtual machine is moved to another subnet, it can (temporarily) keep
   the address of the subnet it was hosted before the move so to allow
   ongoing communications to subsist.  When a subnet detects the
   presence of a host with an address that does not belong to the subnet
   (e.g., via a message sent by the hypervisor or traffic inspection),
   the LISP router of the new subnet registers the IP address of the
   virtual machine as an EID to the Map-Server of the subnet and
   associates its own address as RLOC.

   To inform the other LISP routers that the machine moved and where,
   and then to avoid detours via the initial subnetwork, mechanisms such
   as the Solicit-Map-Request messages are used.

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9.  Security Considerations

   This document does not specify any protocol or operational practices
   and hence, does not have any security considerations.

10.  IANA Considerations

   This memo includes no request to IANA.

11.  Acknowledgements

   This document was initiated by Noel Chiappa and much of the core
   philosophy came from him.  The authors acknowledge the important
   contributions he has made to this work and thank him for his past

   The authors would also like to thank Dino Farinacci, Fabio Maino,
   Luigi Iannone, Sharon Barakai, Isidoros Kouvelas, Christian Cassar,
   Florin Coras, Marc Binderberger, Alberto Rodriguez-Natal, Ronald
   Bonica, Chad Hintz, Robert Raszuk, Joel M.  Halpern, Darrel Lewis, as
   well as every people acknowledged in [RFC6830].

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3232]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
              an On-line Database", RFC 3232, January 2002.

   [RFC4116]  Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
              Gill, "IPv4 Multihoming Practices and Limitations", RFC
              4116, July 2005.

   [RFC4984]  Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
              Workshop on Routing and Addressing", RFC 4984, September

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830, January

   [RFC6831]  Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
              Locator/ID Separation Protocol (LISP) for Multicast
              Environments", RFC 6831, January 2013.

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   [RFC6832]  Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking between Locator/ID Separation Protocol
              (LISP) and Non-LISP Sites", RFC 6832, January 2013.

   [RFC6833]  Fuller, V. and D. Farinacci, "Locator/ID Separation
              Protocol (LISP) Map-Server Interface", RFC 6833, January

   [RFC6834]  Iannone, L., Saucez, D., and O. Bonaventure, "Locator/ID
              Separation Protocol (LISP) Map-Versioning", RFC 6834,
              January 2013.

   [RFC6835]  Farinacci, D. and D. Meyer, "The Locator/ID Separation
              Protocol Internet Groper (LIG)", RFC 6835, January 2013.

   [RFC6836]  Fuller, V., Farinacci, D., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol Alternative Logical
              Topology (LISP+ALT)", RFC 6836, January 2013.

   [RFC6837]  Lear, E., "NERD: A Not-so-novel Endpoint ID (EID) to
              Routing Locator (RLOC) Database", RFC 6837, January 2013.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935, April 2013.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, April 2013.

   [RFC7215]  Jakab, L., Cabellos-Aparicio, A., Coras, F., Domingo-
              Pascual, J., and D. Lewis, "Locator/Identifier Separation
              Protocol (LISP) Network Element Deployment
              Considerations", RFC 7215, April 2014.

12.2.  Informative References

   [Chiappa]  Chiappa, J., "Endpoints and Endpoint names: A Propose
              Enhancement to the Internet Architecture,
    ", 1999.

              LISP DDT ROOT, , "", August 2013.

   [DFZ]      Huston, Geoff., "Growth of the BGP Table - 1994 to Present
    ", August 2013.

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              Cheng, L. and J. Wang, "LISP Single-Hop DHT Mapping
              Overlay", draft-cheng-lisp-shdht-04 (work in progress),
              July 2013.

              Fuller, V., Lewis, D., Ermagan, V., and A. Jain, "LISP
              Delegated Database Tree", draft-ietf-lisp-ddt-02 (work in
              progress), October 2014.

              Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", draft-ietf-lisp-lcaf-06 (work in
              progress), October 2014.

              Maino, F., Ermagan, V., Cabellos-Aparicio, A., and D.
              Saucez, "LISP-Security (LISP-SEC)", draft-ietf-lisp-sec-07
              (work in progress), October 2014.

              Saucez, D., Iannone, L., and O. Bonaventure, "LISP Threats
              Analysis", draft-ietf-lisp-threats-10 (work in progress),
              July 2014.

              Mathy, L., Iannone, L., and O. Bonaventure, ""LISP-DHT:
              Towards a DHT to map identifiers onto locators" draft-
              mathy-lisp-dht-00 (work in progress)", April 2008.

   [Jakab]    Jakab, L., Cabellos, A., Saucez, D., and O. Bonaventure,
              "LISP-TREE: A DNS Hierarchy to Support the LISP Mapping
              System, IEEE Journal on Selected Areas in Communications,
              vol. 28, no. 8, pp. 1332-1343", October 2010.

   [Quoitin]  Quoitin, B., Iannone, L., Launois, C., and O. Bonaventure,
              ""Evaluating the Benefits of the Locator/Identifier
              Separation" in Proceedings of 2Nd ACM/IEEE International
              Workshop on Mobility in the Evolving Internet
              Architecture", 2007.

Appendix A.  A Brief History of Location/Identity Separation

   The LISP system for separation of location and identity resulted from
   the discussions of this topic at the Amsterdam IAB Routing and
   Addressing Workshop, which took place in October 2006 [RFC4984].

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   A small group of like-minded personnel from various scattered
   locations within Cisco, spontaneously formed immediately after that
   workshop, to work on an idea that came out of informal discussions at
   the workshop and on various mailing lists.  The first Internet-Draft
   on LISP appeared in January, 2007.

   Trial implementations started at that time, with initial trial
   deployments underway since June 2007; the results of early experience
   have been fed back into the design in a continuous, ongoing process
   over several years.  LISP at this point represents a moderately
   mature system, having undergone a long organic series of changes and

   LISP transitioned from an IRTF activity to an IETF WG in March 2009,
   and after numerous revisions, the basic specifications moved to
   becoming RFCs at the start of 2013 (although work to expand and
   improve it, and find new uses for it, continues, and undoubtly will
   for a long time to come).

A.1.  Old LISP Models

   LISP, as initially conceived, had a number of potential operating
   modes, named 'models'.  Although they are note used anymore, one
   occasionally sees mention of them, so they are briefly described

   LISP 1:  EIDs all appear in the normal routing and forwarding tables
      of the network (i.e. they are 'routable');this property is used to
      'bootstrap' operation, by using this to load EID->RLOC mappings.
      Packets were sent with the EID as the destination in the outer
      wrapper; when an ETR saw such a packet, it would send a Map-Reply
      to the source ITR, giving the full mapping.

   LISP 1.5:  Similar to LISP 1, but the routability of EIDs happens on
      a separate network.

   LISP 2:  EIDs are not routable; EID->RLOC mappings are available from
      the DNS.

   LISP 3:  EIDs are not routable; and have to be looked up in in a new
      EID->RLOC mapping database (in the initial concept, a system using
      Distributed Hash Tables).  Two variants were possible: a 'push'
      system, in which all mappings were distributed to all ITRs, and a
      'pull' system in which ITRs load the mappings they need, as

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Internet-Draft              LISP Introduction               October 2014

Authors' Addresses

   Albert Cabellos
   c/ Jordi Girona 1-3
   Barcelona, Catalonia  08034


   Damien Saucez (Ed.)
   2004 route des Lucioles BP 93
   Sophia Antipolis Cedex  06902


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