Network Working Group                                        A. Cabellos
Internet-Draft                                         UPC-BarcelonaTech
Intended status: Informational                           D. Saucez (Ed.)
Expires: March 26, 2015                                            INRIA
                                                      September 22, 2014

 An Architectural Introduction to the LISP Location-Identity Separation


   This document describes the Locator/ID Separation Protocol (LISP)
   architecture, its main operational mechanisms as well as its design

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 March 26, 2015.

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   Copyright (c) 2014 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
   ( in effect on the date of

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   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  LISP Architecture . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Design Principles . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Overview of the Architecture  . . . . . . . . . . . . . .   4
     2.3.  Data-Plane  . . . . . . . . . . . . . . . . . . . . . . .   7
       2.3.1.  LISP encapsulation  . . . . . . . . . . . . . . . . .   7
       2.3.2.  LISP Forwarding State . . . . . . . . . . . . . . . .   8
     2.4.  Control-Plane . . . . . . . . . . . . . . . . . . . . . .   9
       2.4.1.  LISP Mappings . . . . . . . . . . . . . . . . . . . .   9
       2.4.2.  Mapping System Interface  . . . . . . . . . . . . . .   9
       2.4.3.  Mapping System  . . . . . . . . . . . . . . . . . . .  10
     2.5.  Internetworking Mechanisms  . . . . . . . . . . . . . . .  13
   3.  LISP Operational Mechanisms . . . . . . . . . . . . . . . . .  13
     3.1.  Cache Management  . . . . . . . . . . . . . . . . . . . .  14
     3.2.  RLOC Reachability . . . . . . . . . . . . . . . . . . . .  14
     3.3.  ETR Synchronization . . . . . . . . . . . . . . . . . . .  15
     3.4.  MTU Handling  . . . . . . . . . . . . . . . . . . . . . .  16
   4.  Mobility  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  Multicast . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Security  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Traffic Engineering . . . . . . . . . . . . . . . . . . .  18
     7.2.  LISP for IPv6 Transition  . . . . . . . . . . . . . . . .  19
     7.3.  LISP for Network Virtualization . . . . . . . . . . . . .  19
     7.4.  LISP for Virtual Machine Mobility in Data Centers . . . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  A Brief History of Location/Identity Separation  . .  23
     A.1.  Old LISP Models . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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1.  Introduction

   There is a rough consensus that the Internet routing and addressing
   system is facing severe scalability issues [RFC4984].  Specifically,
   the growth in the size of the routing tables of the Default-Free Zone
   (DFZ) is accelerating and showing a supra-linear slope [DFZ].  The
   main driving force behind this growth is the de-aggregation of BGP
   prefixes, which results from the existing BGP multihoming and traffic
   engineering mechanisms that are used -at the time of this writing- on
   the Internet, as well as non-aggregatable address allocations.

   This issue has two profound implications, on the one hand Internet
   core routers are exposed to the network dynamics of the edge.  For
   instance this typically leads to an increased amount of BGP UPDATE
   messages (churn), which results in additional processing requirements
   of Internet core routers in order to timely compute the DFZ RIB.
   Secondly, the supra-linear growth imposes strong requirements on the
   size of the memory storing the DFZ FIB.  Both aspects lead to an
   increase on the development and production cost of high-end routers,
   and it is unclear if the semiconductor and router manufacturer
   industries will be able to cope, in the long-term, with such
   stringent requirements in a cost-effective way[RFC4984].

   Although this important scalability issue is relatively new, the
   architectural reasons behind it are well-known many years ago.
   Indeed, and as pointed out by [Chiappa], IP addresses have overloaded
   semantics.  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.

   The Locator/ID Separation Protocol (LISP), specified in [RFC6830], is
   built on top of this basic idea: decoupling the IP address overloaded
   semantics.  LISP creates two separate namespaces, EIDs (End-host
   IDentifiers) and RLOCs (Routing LOCators), both are -typically, but
   not limited to- 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- are architecturally separated and interconnected by
   LISP-capable routers.  LISP also introduces a publicly accessible
   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

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   location.  By taking advantage of such separation between location
   and identity, the Internet core is populated with RLOCs which can be
   quasi-static and highly aggregatable, hence 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].

2.  LISP Architecture

   This section presents the LISP architecture, we first detail the
   design principles of LISP and then we proceed to describe its main
   aspects: data-plane, control-plane, and internetworking mechanisms.

2.1.  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 with
      topological meaningful address and hence, can use aggregation to
      scale.  Devices are assigned with identity meaningful address that
      are independent of its topological location.

   o  Overlay architecture: Overlays route packets over the current
      Internet, allowing to deploy new protocols without changing the
      current infrastructure hence, resulting from 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 is compatible with the legacy Internet while providing
      some of the targeted benefits to early adopters.

2.2.  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 are LISP sites (e.g., an

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   Autonomous System) that use EID addresses.  EIDs are typically -but
   not limited to- IPv4 or IPv6 addresses that uniquely identify
   endhosts and are assigned and configured by the same mechanisms that
   we have at the time of this writing.  EIDs can be are typically
   Provider Independent (PI [RFC4116]) addresses and can be thought as
   they don't contain intra-domain topological information.  Because of
   this, EIDs are usually only routable in the edge.

   With LISP, LISP sites (edge) and the core of the Internet are inter-
   connected by means of LISP-capable routers (e.g., border routers).
   When they provide egress (from the core perspective) to a LISP site
   they are called Egress Tunnel Routers (ETR), Ingress Tunnel Routers
   (ITR) when they provide ingress, and xTR when they provide both.
   ITRs and ETRs exchange packets by encapsulating 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].

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   A publicly accessible and usually 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 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 can be thought as the
   equivalent of a DNS that would be accessed by ETRs to register
   mappings and by ITRs to retrieve them.

   Finally, the LISP architecture has a strong emphasis in 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
   typically require only a software upgrade.  Additionally, LISP
   requires the deployment of an independent Mapping System, this
   distributed database is a new network entity.

   In what follows we describe a simplified packet flow sequence between
   two nodes that are attached to LISP sites.  Client hostA wants to
   send a packt 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

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   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.

   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.  In order
       to do so it uses a LISP control message called Map-Request, the
       message contains EID_A 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 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 2.3.1.

   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.

2.3.  Data-Plane

   This section describes the LISP data-plane, which is specified in
   [RFC6830].  The LISP data-plane is responsible of 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 viceversa (ETR).  We first describe how packets are LISP-
   encapsulated and then we proceed to explain how ITRs cache forwarding

2.3.1.  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

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   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.

   LISP-encapsulated packets also include a LISP header (after the UDP
   header).  The LISP header is prepended by ITRs and striped by ETRs.
   It carries reachability information (see more details in Section 3.2)
   and the Instance ID field.  The Instance ID field is used to
   distinguish traffic that belongs to multiple tenants inside a LISP
   site, and that may use overlapped but logically separated 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 may contain reachability information 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
       remains unchanged.

   Finally and 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.

2.3.2.  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- to
   increase the forwarding speed of subsequent packets addressed to the
   same EID prefix.  Mappings include a (Time-to-Live) TTL (set by the
   ETR) and are expired according to this value, more details about the
   Map-Cache management can be found in Section 3.1.

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2.4.  Control-Plane

   The LISP control-plane, specified in [RFC6833], provides a standard
   interface to register, query, and retrieve mappings.  The LISP
   Mapping System, is a publicly accessible database that stores such
   mappings.  In what follows we first describe the mappings, then the
   standard interface, and finally the Mapping System architecture.

2.4.1.  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.  Such addresses are encoded
   using a general syntax called LISP Canonical Address Format (LCAF),
   specified in [I-D.ietf-lisp-lcaf].  The syntax is general enough to
   support encoding of IPv4 and IPv6 addresses and any other type of

   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.

2.4.2.  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 low
      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):

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   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.

   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.

   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 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.

2.4.3.  Mapping System

   LISP architecturally decouples control and data-plane by means of a
   standard interface.  This interface glues the data-plane, routers
   responsible of forwarding data-packets, with the LISP Mapping System,
   a publicly accessible database responsible of 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 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 discussed for the Mapping System architecture the
   four main techniques available in distributed systems, namely: 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 [I-D.lear-lisp-nerd] 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 logically centralized.  In such cases it
   is typically composed of a single Map-Server/Map-Resolver.

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   In what follows we focus on the two mapping systems that have been
   implemented and deployed (LISP-ALT and LISP+DDT).  LISP+ALT

   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.  All the
   participating nodes connect to their peers through static tunnels.
   Every ETR involved in the ALT topology advertises its EID prefixes
   making the EID routable on the overlay.

   When an ITR needs a mapping, it sends a Map-Request to a nearby ALT
   router.  The ALT routers then forward the Map-Request on the overlay
   by inspecting their ALT routing tables.  When the Map-Request reaches
   the ETR responsible for the mapping, a Map-Reply is generated and
   directly sent to the ITR's RLOC, without using the ALT overlay.  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.  The
   following figure presents a schematic representation of the DDT

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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|
+------------+     +------------+   +------------+ +------------+

      Figre 3.- An schematic representation of the DDT tree structure,
              please note that the prefixes and the structure depitected
              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 iteratively and in a
   similar way to the DNS.  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

2.5.  Internetworking Mechanisms

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

   LISP defines two entities to provide internetworking:

   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 Engress 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 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 allows to
      overcome such limitations by encapsulating packets over the
      network.  Finally, the RLOC of PETRs must be statically configured
      in ITRs.

3.  LISP Operational Mechanisms

   In this section we detail the main operational mechanisms defined in

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3.1.  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 could refresh the mapping by sending
      a new Map-Request.  Typical values for TTL defined by LISP are

   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 it a SMR to notify it that a new
      mapping is available.

3.2.  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
   architectures require of explicit mechanisms to propagate
   reachability information.  LISP defines a set of mechanisms to inform
   ITRs and PITRS about the reachability of the cached RLOCs:

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   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 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 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

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

   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.

   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.

3.3.  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.

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   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

3.4.  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 defienes two mechanisms:

   Stateless:  With this mechanism ITRs fragment packets that are too
      big, typically 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.

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

4.  Mobility

   LISP can also be used to enable mobility of devices not located in
   LISP networks.  The problem with mobility of such devices is that
   their IP address changes whenever they change location, interrupting
   so flows.

   To enable mobility on such devices, the device can implement the xTR
   functionality where the IP address presented to applications is an
   EID that never changes while the IP address obtained from the network
   is used by the xTR as RLOC.  Packets are then transported on the
   network using the IP address assigned to the device by the visited
   network while at the application level IP addresses remain
   independent of the location of the device.

   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 signalling is strictly limited the Map-Server and to
   hosts with which communications are ongoing.

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5.  Multicast

   LISP also supports multicast environments, the operational changes
   required to the multicast protocols are documented in [RFC6831].

   In such scenarios, LISP creates multicast state both at the core and
   at the sites (both source and receiver).  In order to 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 that belongs to a LISP site transmits a PIM Join/
       Prune message (S-EID,G) to join a multicast group.

   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.

6.  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 must be

   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

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   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 been 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-Server that strictly respect [RFC6833]
   and the lightweight authentication mechanism proposed by LISP-Sec
   [I-D.ietf-lisp-sec] is a possibility to reduce the risk.  In more
   critical environments, stronger authentication may have to be used.

   Packets are transported encapsulated with LISP meaning that devices
   on the path between an ITR (or PITR) and an ETR (or PETR) cannot
   correctly inspect the content of packets unless they implement
   methods to strip the headers added by LISP.  Similarly, mappings
   enable triangular routing (i.e., packets of a flow cross different
   border routers depending on their direction) which means that
   intermediate boxes may have incomplete view on the traffic they
   inspect or manipulate.

   More details about security implications of LISP can be found in

7.  Use Cases

7.1.  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.

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   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 the owner of EID and is not altered by any intermediate

   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 owner of the EID and 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.

7.2.  LISP for IPv6 Transition

   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
   candidate to ease the transition to IPv4.

   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.

7.3.  LISP for Network Virtualization

   It is nowadays common to operate several virtual networks over the
   same physical infrastructure.  The current approach usually rely on
   BGP/MPLS VPNs, where BGP is used to exchange routing information and
   MPLS to segregate packets of the different logical networks.  This
   functionality could be achieved with LISP where the mappings and the
   mapping system are used instead of BGP and the LISP encapsulation is
   used to replace MPLS.

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   In virtual 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 threat the packet.

   Appart from the simplicity of managing mappings, the advantage of
   using LISP for virtual network is that it does not impose any
   requirement on the underlying network, except running IP.

7.4.  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
   subnetworks where servers are hosted as forming the EID space.  A
   LISP router is placed at the border between the backbone and each
   sub-network.  When a virtual machine is moved to another subnetwork,
   it can (temporarily) keep the address of the sub-network it was
   hosted before the move so to allow ongoing communications to subsist.
   When a subnetwork detects the presence of a host with an address that
   does not belong to the subnetwork (e.g., via a message sent by the
   hypervisor), the LISP router of the new subnetwork registers the IP
   address of the virtual machine as an EID to the Map-Server of the
   subnetwork 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, every Map-
   Server can listen on a predefined multicast address that is used as
   source address for Map-Register.  As a result, the Map-Notify sent
   back by the Map-Server will be received by all the LISP routers that
   hence automatically learn the new location of the virtual machine.

8.  Security Considerations

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

9.  IANA Considerations

   This memo includes no request to IANA.

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10.  Acknowledgements

   To Do.

11.  References

11.1.  Normative References

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

   [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.

   [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.

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

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   [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.

11.2.  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.

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

              Ermagan, V., Farinacci, D., Lewis, D., Skriver, J., Maino,
              F., and C. White, "NAT traversal for LISP", draft-ermagan-
              lisp-nat-traversal-03 (work in progress), March 2013.

              Fuller, V., Lewis, D., Ermagan, V., and A. Jain, "LISP
              Delegated Database Tree", draft-ietf-lisp-ddt-01 (work in
              progress), March 2013.

              Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", draft-ietf-lisp-lcaf-05 (work in
              progress), May 2014.

              Maino, F., Ermagan, V., Cabellos-Aparicio, A., and D.
              Saucez, "LISP-Security (LISP-SEC)", draft-ietf-lisp-sec-06
              (work in progress), April 2014.

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

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

              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].

   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.  The first Internet-Draft on LISP appeared in January,
   2007, along with a LISP mailing list at the IETF.

   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

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   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 initilly conceived, had a number of potential operating
   modes, named 'models'.  Although they are now obsolete, 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

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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