An Architectural Introduction to the Locator/ID Separation Protocol (LISP)
draft-ietf-lisp-introduction-15

Document Type Active Internet-Draft (lisp WG)
Authors Albert Cabellos-Aparicio  , Damien Saucez 
Last updated 2021-09-20
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Network Working Group                                        A. Cabellos
Internet-Draft                                         UPC-BarcelonaTech
Intended status: Informational                           D. Saucez (Ed.)
Expires: 24 March 2022                                             Inria
                                                       20 September 2021

  An Architectural Introduction to the Locator/ID Separation Protocol
                                 (LISP)
                    draft-ietf-lisp-introduction-15

Abstract

   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 [I-D.ietf-lisp-rfc6830bis] and
   [I-D.ietf-lisp-rfc6833bis], the protocol specifications.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on 24 March 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   4
   3.  LISP Architecture . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Design Principles . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Overview of the Architecture  . . . . . . . . . . . . . .   6
     3.3.  Data-Plane  . . . . . . . . . . . . . . . . . . . . . . .   8
       3.3.1.  LISP Encapsulation  . . . . . . . . . . . . . . . . .   9
       3.3.2.  LISP Forwarding State . . . . . . . . . . . . . . . .  10
     3.4.  Control-Plane . . . . . . . . . . . . . . . . . . . . . .  10
       3.4.1.  LISP Mappings . . . . . . . . . . . . . . . . . . . .  10
       3.4.2.  Mapping System Interface  . . . . . . . . . . . . . .  11
       3.4.3.  Mapping System  . . . . . . . . . . . . . . . . . . .  12
     3.5.  Internetworking Mechanisms  . . . . . . . . . . . . . . .  14
   4.  LISP Operational Mechanisms . . . . . . . . . . . . . . . . .  15
     4.1.  Cache Management  . . . . . . . . . . . . . . . . . . . .  15
     4.2.  RLOC Reachability . . . . . . . . . . . . . . . . . . . .  16
     4.3.  ETR Synchronization . . . . . . . . . . . . . . . . . . .  17
     4.4.  MTU Handling  . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Mobility  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Multicast . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Traffic Engineering . . . . . . . . . . . . . . . . . . .  20
     7.2.  LISP for IPv6 Co-existence  . . . . . . . . . . . . . . .  20
     7.3.  LISP for Virtual Private Networks . . . . . . . . . . . .  21
     7.4.  LISP for Virtual Machine Mobility in Data Centers . . . .  21
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     11.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Appendix A.  A Brief History of Location/Identity Separation  . .  27
     A.1.  Old LISP Models . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

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1.  Introduction

   This document introduces the Locator/ID Separation Protocol (LISP)
   architecture ([I-D.ietf-lisp-rfc6830bis],
   [I-D.ietf-lisp-rfc6833bis]), 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 Noel Chiappa [RFC4984],
   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.  On the
   one hand, routing systems require that addresses are aggregatable and
   have topological meaning, on the other hand, 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 syntactically identical to the
   current IPv4 and IPv6 addresses.  However, 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.  LISP-capable routers interconnect the two logical spaces.
   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.

   In summary:

   *  RLOCs have meaning only in the underlay network, that is the
      underlying core routing system.

   *  EIDs have meaning only in the overlay network, which is the
      encapsulation relationship between LISP-capable routers.

   *  The LISP edge maps EIDs to RLOCs

   *  Within the underlay network, RLOCs have both locator and
      identifier semantics

   *  An EID within a LISP site carries both identifier and locator
      semantics to other nodes within that site

   *  An EID within a LISP site carries identifier and limited locator
      semantics to nodes at other LISP sites (i.e., enough locator
      information to tell that the EID is external to the site)

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   The relationship described above is not unique to LISP and it is
   common to other overlay technologies.

   The initial motivation in the LISP effort is to be found in the
   routing scalability problem [RFC4984], where, if LISP were to be
   completely deployed, the Internet core would be populated with RLOCs
   while Traffic Engineering mechanisms would be pushed to the Mapping
   System.  In such scenario RLOCs are quasi-static (i.e., low churn),
   hence making the routing system scalable [Quoitin], while EIDs can
   roam anywhere with no churn to the underlying global routing system.
   [RFC7215] discusses the impact of LISP on the global routing system
   during the transition period.  However, the separation between
   location and identity that LISP offers makes it suitable for use in
   additional scenarios such as Traffic Engineering (TE), multihoming,
   and mobility among others.

   This document describes the LISP architecture and its main
   operational mechanisms as well 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 [I-D.ietf-lisp-rfc6830bis] and
   [I-D.ietf-lisp-rfc6833bis], as well as the complementary documents
   [RFC6831], [RFC6832], [I-D.ietf-lisp-6834bis], [RFC6835], [RFC6836],
   [RFC7052] for the protocol specifications along with the LISP
   deployment guidelines [RFC7215].

2.  Definition of Terms

   Endpoint IDentifier (EID):  EIDs are addresses used to uniquely
      identify nodes irrespective of their topological location and are
      typically routed intra-domain.

   Routing LOcator (RLOC):  RLOCs are addresses assigned topologically
      to network attachment points and typically routed inter-domain.

   Ingress Tunnel Router (ITR):  A LISP-capable router that encapsulates
      packets from a LISP site towards the core network.

   Egress Tunnel Router (ETR):  A LISP-capable router that decapsulates
      packets from the core of the network towards a LISP site.

   xTR:  A router that implements both ITR and ETR functionalities.

   Map-Request:  A LISP signaling message used to request an EID-to-RLOC
      mapping.

   Map-Reply:  A LISP signaling message sent in response to a Map-
      Request that contains a resolved EID-to-RLOC mapping.

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   Map-Register:  A LISP signaling message used to register an EID-to-
      RLOC mapping.

   Map-Notify:  A LISP signaling message sent in response of a Map-
      Register to acknowledge the correct reception of an EID-to-RLOC
      mapping.

   This document describes the LISP architecture and does not introduce
   any new term.  The reader is referred to [I-D.ietf-lisp-rfc6830bis]
   and [I-D.ietf-lisp-rfc6833bis], [RFC6831], [RFC6832],
   [I-D.ietf-lisp-6834bis], [RFC6835], [RFC6836], [RFC7052], [RFC7215]
   for the complete 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 internetworking mechanisms.

3.1.  Design Principles

   The LISP architecture is built on top of four basic design
   principles:

   *  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 relatively opaque topologically
      meaningful addresses that are independent of their topological
      location.

   *  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
      cost.

   *  Decoupled data-plane 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 and allows for other
      data-planes to be added.  Even though the data-plane and the
      control-plane are decoupled, they are not completely isolated
      because the LISP data-plane may trigger control-plane activity.

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

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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 (RLOCs).  The edge consists of LISP sites (e.g., an
   Autonomous System) that use EID addresses.  EIDs are 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 because of this, EIDs are usually routable at the
   edge (within LISP sites) but not in the core; see Section Section 3.5
   for discussion of LISP site internetworking with non-LISP sites and
   domains in the Internet.

   LISP sites (at the edge) are connected to the interconnecting core by
   means of LISP-capable routers (e.g., border routers).  LISP sites are
   connected across the interconnecting core using tunnels between the
   LISP-capable routers.  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 which
   can perform both ITR and ETR operations.  In this context ITRs
   encapsulate packets while ETRs decapsulate them, hence LISP operates
   as an overlay on top of 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 an IPv4 or IPv6 address
   assigned to an core-facing network interface of an ITR or ETR.

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   A database which is typically distributed, 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 is conceptually similar to the DNS where it is
   organized as a distributed multi-organization network database.  With
   LISP, ETRs register mappings while ITRs retrieve them.

   Finally, the LISP architecture emphasizes incremental deployment.
   Given that LISP represents an overlay to the current Internet
   architecture, end hosts 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 space with the RLOC
   space.  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.  Please note that typical
   LISP-capable routers are xTRs (both ITR and ETR).  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.

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   1.  HostA retrieves the EID_B of HostB, typically querying the DNS
       and obtaining an A or AAAA record.  Then it generates an IP
       packet as in the Internet, the packet has source address EID_A
       and destination address EID_B.

   2.  The packet is forwarded 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.
       Note that a Map-Reply can contain more locators if needed.  ITR_A
       can cache the mapping in a local storage 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_B1 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 over the interconnecting
       core as a normal IP packet, making the EID invisible from the
       core.

   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 [I-D.ietf-lisp-rfc6830bis].
   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).

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3.3.1.  LISP Encapsulation

   ITRs encapsulate data packets towards ETRs.  LISP data packets are
   encapsulated using UDP (port 4341), the source port is usually
   selected by the ITR using a 5-tuple hash of the inner header (so to
   be consistent in case of multi-path solutions such as ECMP [RFC2992])
   and ignored on reception.  LISP data packets are often encapsulated
   in UDP packets that include a zero checksum [RFC6935] [RFC6936] that
   may not be verified when it is received, because LISP data packets
   typically include an inner transport protocol header with a non-zero
   checksum.  The use of UDP zero checksums over IPv6 for all tunneling
   protocols like LISP is subject to the applicability statement in
   [RFC6936].  If LISP data packets are encapsulated in UDP packets with
   non-zero checksums, the outer UDP checksums are verified when the UDP
   packets are received, as part of normal UDP processing.

   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 works on 4 headers, the inner header the source
   constructed, and the 3 headers a LISP encapsulator prepends ("outer"
   to "inner"):

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

   2.  UDP header (port 4341), usually 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 Re-encapsulating and/or Recursive tunnels
   are useful to choose a specified path in the underlay network, for
   instance to avoid congestion or failure.  Re-encapsulating tunnels
   are consecutive LISP tunnels and occur when a decapsulator (an ETR
   action) removes a LISP header and then acts as an encapsultor (an ITR

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   action) to prepend another one.  On the other hand, Recursive tunnels
   are nested tunnels and are implemented by using multiple LISP
   encapsulations on a packet.  Such functions are implemented by
   Reencapsulating Tunnel Routers (RTRs).  An RTR can be thought of as a
   router that first acts as an ETR by decapsulating packets and then as
   an ITR by encapsulating them towards another locator, more
   information can be found at [I-D.ietf-lisp-rfc6830bis] and
   [I-D.ietf-lisp-rfc6833bis].

3.3.2.  LISP Forwarding State

   In the LISP architecture, ITRs keep just enough information to route
   traffic flowing through them.  Meaning that, ITRs retrieve from the
   LISP Mapping System mappings between EID-prefixes (blocks of EIDs)
   and RLOCs that are used to encapsulate packets.  Such mappings are
   stored in a local cache called the LISP Map-Cache for subsequent
   packets addressed to the same EID prefix.  Note that, in case of
   overlapping EID-prefixes, following a single request, the ITR may
   receive a set of mappings, covering the requested EID-prefix and all
   more-specifics (cf., Section 5.5 [I-D.ietf-lisp-rfc6833bis]).
   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 [I-D.ietf-lisp-rfc6833bis],
   provides a standard interface to register and request mappings.  The
   LISP Mapping System is a database that stores such mappings.  The
   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 IP addresses.  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) [RFC8060].

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   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 [I-D.ietf-lisp-rfc6833bis] 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 (e.g., 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.

   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.  In some cases a Map-Notify can
      be sent to the previous RLOCs when an EID is registered by a new
      set of RLOCs.

   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.

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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
   LCAF, 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, as a result of the local mapping
   cache available at ITR, the Mapping System does not need to operate
   at line-rate.

   Many of the existing mechanisms to create distributed systems have
   been explored and considered for the Mapping System architecture:
   graph-based databases in the form of LISP+ALT [RFC6836], hierarchical
   databases in the form of LISP-DDT [RFC8111], monolithic databases in
   the form of LISP-NERD [RFC6837], flat databases in the form of LISP-
   DHT [I-D.cheng-lisp-shdht], [Mathy], and a multicast-based database
   [I-D.curran-lisp-emacs].  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).

3.4.3.1.  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 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.

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3.4.3.2.  LISP-DDT

   LISP-DDT [RFC8111] 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, 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.

                           /---------\
                           |         |
                           | 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

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   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].

   The DDT Mapping System relies on manual configuration.  That is Map-
   Resolvers are configured with the set of available DDT root nodes
   while DDT nodes are 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 changes).

3.5.  Internetworking 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 routing system beyond the local LISP domain.  As
   a result LISP requires an internetworking mechanism to allow LISP
   sites to speak with non-LISP sites and vice versa.  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 packet from a source not in
      a LISP site (a non-EID), 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 Default-Free Zone (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

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      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.

   Additionally, LISP also defines mechanisms to operate with private
   EIDs [RFC1918] by means of LISP-NAT [RFC6832].  In this case the xTR
   replaces a private EID source address with a routable one.  At the
   time of this writing, work is ongoing to define NAT-traversal
   capabilities, that is xTRs behind a NAT using non-routable RLOCs.

   PITRs, PETRs and, LISP-NAT enable incremental deployment of LISP, by
   providing significant flexibility in the placement of the boundaries
   between the LISP and non-LISP portions of the network, and making it
   easy to change those boundaries over time.

4.  LISP Operational Mechanisms

   This section details the main operational mechanisms defined in LISP.

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
   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:

   Record Time-To-Live (TTL):  Each mapping record contains a TTL set by
      the ETR, upon expiration of the TTL the ITR can't use the mapping
      until it is refreshed by sending a new Map-Request.

   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.  Further uses of
      SMRs are documented in [I-D.ietf-lisp-rfc6833bis].

   Map-Versioning:  This optional mechanism piggybacks in the LISP

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      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.  Further details are available in
      [I-D.ietf-lisp-6834bis].

   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

   In most cases LISP operates with a pull-based Mapping System (e.g.,
   DDT), this results in an edge to edge pull architecture.  In such
   scenario the network state is stored in the control-plane while the
   data-plane pulls it on demand.  This has consequences concerning the
   propagation of xTRs reachability/liveness information since pull
   architectures require explicit mechanisms to propagate this
   information.  As a result 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 as such
   they must not be used over the public Internet and should be coupled
   with Map-Versioning to prevent race conditions where LSB are
   interpreted as referring to different RLOCs than intended.

   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.  The echo-nonce mechanism
   must be used in trusted environments only, not over the public
   Internet.

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   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.

   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 when it cannot tell the
   difference between a failed bidirectional path or the return path is
   not used (a unidirectional path).

   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 IGP 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:

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   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.  ITRs will send ICMP Too Big
      messages to inform the sources about the effective MTU.
      Additionally ITRs can use mechanisms such as PMTUD [RFC1191] or
      PLPMTUD [RFC4821] to keep track of the MTU towards the locators.

   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 is suitable
   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.  This functionality is similar to Network Mobility
   [RFC3963].  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.  Nevertheless, LISP can be
   used to enable seamless IP mobility when LISP is directly implemented
   in the endpoint or when the endpoint roams to an attached xTR.  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.  This functionality is
   similar to Mobile IP ([RFC5944] and [RFC6275]).

   Whenever the device changes of RLOC, the xTR updates the RLOC of its
   local mapping and registers it to its Map-Server, typically with a
   low TTL value (1min).  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.  In the mobility case the EID-prefix can
   be as small as a full /32 or /128 (IPv4 or IPv6 respectively)
   depending on the specific use-case (e.g., subnet mobility vs single
   VM/Mobile node mobility).

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   The decoupled identity and location provided by LISP allows it to
   operate with other layer 2 and layer 3 mobility solutions.

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
   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 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 (S-EID, G) 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.

   Please note that the inner and outer multicast addresses are in
   general different, unless in specific cases where the underlay
   provider implements a tight control on the overlay.  LISP
   specifications already support all PIM modes [RFC6831].
   Additionally, LISP can support as well non-PIM mechanisms in order to
   maintain multicast state.

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   When multicast sources and receivers are active at LISP sites, and
   the core network between the sites does not provide multicast
   support, a signal-free mechanism can be used to create an overlay
   that will allow multicast traffic to flow between sites and connect
   the multicast trees at the different sites [RFC8378].  Registrations
   from the different receiver sites will be merged at the mapping
   system to assemble a multicast-replication-list inclusive of all
   Routing Locators (RLOCs) that lead to receivers for a particular
   multicast group or multicast channel.  The replication list for each
   specific multicast entry is maintained as a database mapping entry in
   the LISP mapping system.

7.  Use Cases

7.1.  Traffic Engineering

   A LISP site can strictly impose via which ETRs the traffic must
   enters the LISP site 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 (or set of ETRs) 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 support a different mapping policy for
   each remote site.

7.2.  LISP for IPv6 Co-existence

   LISP encapsulations allows to transport packets using EIDs from a
   given address family (e.g., IPv6) with packets from other address
   families (e.g., IPv4).  The absence of correlation between the
   address family of RLOCs and EIDs makes LISP a candidate to allow,
   e.g., IPv6 to be deployed when all of the core network may not have
   IPv6 enabled.

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   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 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 which virtual network a packet belongs and
   tags or labels are used for that purpose.  When using 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 usage of LISP for virtual private networks does not
   introduce additional requirements on the underlying network, as long
   as it runs 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 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 keep (temporarily)
   the address it had before the move so to continue without a transport
   layer connection reset.  When an xTR detects a source address
   received on a subnet to be an address not assigned to the subnet, it
   registers the address to the Mapping System.

   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.

8.  Security Considerations

   This section describes the security considerations associated to the
   LISP protocol.

   While in a push mapping 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

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   (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
   thought carefully so to not overload the slow path and rate limiting
   should be used as specified in [I-D.ietf-lisp-rfc6830bis] and
   [I-D.ietf-lisp-rfc6833bis].

   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 [I-D.ietf-lisp-rfc6830bis] and
   [I-D.ietf-lisp-rfc6833bis] 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 information, such as
   reachabilty of locators, directly into data plane packets.  In
   environments that are not fully trusted, like the open Internet,
   control information gleaned from data-plane packets must not be used
   or must be verified before using it.

   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
   [I-D.ietf-lisp-rfc6833bis] and the 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.  The way security is implemented for a given mapping
   system strongly depends on the architecture of the mapping system
   itself and the threat model assumed for the deployment.  Thus, the
   mapping system security has to be discussed in the relevant documents
   proposing the mapping system architecture.

   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.

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   Like other map-and-encap mechanisms, LISP enables triangular routing
   (i.e., packets of a flow cross different border routers depending on
   their direction).  This means that intermediate boxes may have
   incomplete view on the traffic they inspect or manipulate.  Moreover,
   LISP-encapsulated packets are routed based on the outer IP address
   (i.e., the RLOC), and can be delivered to an ETR that is not
   responsible of the destination EID of the packet or even to a network
   element that is not an ETR.  The mitigation consists in applying
   appropriate filtering techniques on the network elements that can
   potentially receive un-expected LISP-encapsulated packets

   More details about security implications of LISP are discussed in
   [RFC7835].

9.  IANA Considerations

   This memo includes no requests to IANA.

10.  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
   efforts.

   The authors would also like to thank Dino Farinacci, Fabio Maino,
   Luigi Iannone, Sharon Barkai, Isidoros Kouvelas, Christian Cassar,
   Florin Coras, Marc Binderberger, Alberto Rodriguez-Natal, Ronald
   Bonica, Chad Hintz, Robert Raszuk, Joel M.  Halpern, Darrel Lewis,
   David Black.

11.  References

11.1.  Normative References

   [I-D.ietf-lisp-6834bis]
              Iannone, L., Saucez, D., and O. Bonaventure, "Locator/ID
              Separation Protocol (LISP) Map-Versioning", Work in
              Progress, Internet-Draft, draft-ietf-lisp-6834bis-09, 31
              August 2021, <https://www.ietf.org/archive/id/draft-ietf-
              lisp-6834bis-09.txt>.

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   [I-D.ietf-lisp-rfc6830bis]
              Farinacci, D., Fuller, V., Meyer, D., Lewis, D., and A.
              Cabellos-Aparicio, "The Locator/ID Separation Protocol
              (LISP)", Work in Progress, Internet-Draft, draft-ietf-
              lisp-rfc6830bis-36, 18 November 2020,
              <https://www.ietf.org/internet-drafts/draft-ietf-lisp-
              rfc6830bis-36.txt>.

   [I-D.ietf-lisp-rfc6833bis]
              Farinacci, D., Maino, F., Fuller, V., and A. Cabellos-
              Aparicio, "Locator/ID Separation Protocol (LISP) Control-
              Plane", Work in Progress, Internet-Draft, draft-ietf-lisp-
              rfc6833bis-30, 18 November 2020, <https://www.ietf.org/
              internet-drafts/draft-ietf-lisp-rfc6833bis-30.txt>.

   [I-D.ietf-lisp-sec]
              Maino, F., Ermagan, V., Cabellos-Aparicio, A., and D.
              Saucez, "LISP-Security (LISP-SEC)", Work in Progress,
              Internet-Draft, draft-ietf-lisp-sec-22, 12 January 2021,
              <https://www.ietf.org/internet-drafts/draft-ietf-lisp-sec-
              22.txt>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/info/rfc1191>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
              <https://www.rfc-editor.org/info/rfc2992>.

   [RFC3232]  Reynolds, J., Ed., "Assigned Numbers: RFC 1700 is Replaced
              by an On-line Database", RFC 3232, DOI 10.17487/RFC3232,
              January 2002, <https://www.rfc-editor.org/info/rfc3232>.

   [RFC3963]  Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
              Thubert, "Network Mobility (NEMO) Basic Support Protocol",
              RFC 3963, DOI 10.17487/RFC3963, January 2005,
              <https://www.rfc-editor.org/info/rfc3963>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <https://www.rfc-editor.org/info/rfc4821>.

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   [RFC4984]  Meyer, D., Ed., Zhang, L., Ed., and K. Fall, Ed., "Report
              from the IAB Workshop on Routing and Addressing",
              RFC 4984, DOI 10.17487/RFC4984, September 2007,
              <https://www.rfc-editor.org/info/rfc4984>.

   [RFC5944]  Perkins, C., Ed., "IP Mobility Support for IPv4, Revised",
              RFC 5944, DOI 10.17487/RFC5944, November 2010,
              <https://www.rfc-editor.org/info/rfc5944>.

   [RFC6275]  Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility
              Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July
              2011, <https://www.rfc-editor.org/info/rfc6275>.

   [RFC6831]  Farinacci, D., Meyer, D., Zwiebel, J., and S. Venaas, "The
              Locator/ID Separation Protocol (LISP) for Multicast
              Environments", RFC 6831, DOI 10.17487/RFC6831, January
              2013, <https://www.rfc-editor.org/info/rfc6831>.

   [RFC6832]  Lewis, D., Meyer, D., Farinacci, D., and V. Fuller,
              "Interworking between Locator/ID Separation Protocol
              (LISP) and Non-LISP Sites", RFC 6832,
              DOI 10.17487/RFC6832, January 2013,
              <https://www.rfc-editor.org/info/rfc6832>.

   [RFC6835]  Farinacci, D. and D. Meyer, "The Locator/ID Separation
              Protocol Internet Groper (LIG)", RFC 6835,
              DOI 10.17487/RFC6835, January 2013,
              <https://www.rfc-editor.org/info/rfc6835>.

   [RFC6836]  Fuller, V., Farinacci, D., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol Alternative Logical
              Topology (LISP+ALT)", RFC 6836, DOI 10.17487/RFC6836,
              January 2013, <https://www.rfc-editor.org/info/rfc6836>.

   [RFC6837]  Lear, E., "NERD: A Not-so-novel Endpoint ID (EID) to
              Routing Locator (RLOC) Database", RFC 6837,
              DOI 10.17487/RFC6837, January 2013,
              <https://www.rfc-editor.org/info/rfc6837>.

   [RFC6935]  Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
              UDP Checksums for Tunneled Packets", RFC 6935,
              DOI 10.17487/RFC6935, April 2013,
              <https://www.rfc-editor.org/info/rfc6935>.

   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

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   [RFC7052]  Schudel, G., Jain, A., and V. Moreno, "Locator/ID
              Separation Protocol (LISP) MIB", RFC 7052,
              DOI 10.17487/RFC7052, October 2013,
              <https://www.rfc-editor.org/info/rfc7052>.

   [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, DOI 10.17487/RFC7215, April
              2014, <https://www.rfc-editor.org/info/rfc7215>.

   [RFC7835]  Saucez, D., Iannone, L., and O. Bonaventure, "Locator/ID
              Separation Protocol (LISP) Threat Analysis", RFC 7835,
              DOI 10.17487/RFC7835, April 2016,
              <https://www.rfc-editor.org/info/rfc7835>.

   [RFC8060]  Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format (LCAF)", RFC 8060, DOI 10.17487/RFC8060,
              February 2017, <https://www.rfc-editor.org/info/rfc8060>.

   [RFC8111]  Fuller, V., Lewis, D., Ermagan, V., Jain, A., and A.
              Smirnov, "Locator/ID Separation Protocol Delegated
              Database Tree (LISP-DDT)", RFC 8111, DOI 10.17487/RFC8111,
              May 2017, <https://www.rfc-editor.org/info/rfc8111>.

   [RFC8378]  Moreno, V. and D. Farinacci, "Signal-Free Locator/ID
              Separation Protocol (LISP) Multicast", RFC 8378,
              DOI 10.17487/RFC8378, May 2018,
              <https://www.rfc-editor.org/info/rfc8378>.

11.2.  Informative References

   [I-D.cheng-lisp-shdht]
              Cheng, L. and J. Wang, "LISP Single-Hop DHT Mapping
              Overlay", Work in Progress, Internet-Draft, draft-cheng-
              lisp-shdht-04, 15 July 2013, <http://www.ietf.org/
              internet-drafts/draft-cheng-lisp-shdht-04.txt>.

   [I-D.curran-lisp-emacs]
              Brim, S., Farinacci, D., Meyer, D., and J. Curran, "EID
              Mappings Multicast Across Cooperating Systems for LISP",
              Work in Progress, Internet-Draft, draft-curran-lisp-emacs-
              00, 9 November 2007,
              <http://tools.ietf.org/html/draft-curran-lisp-emacs-00>.

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   [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.

   [Mathy]    Mathy, L., Iannone, L., and O. Bonaventure, "LISP-DHT:
              Towards a DHT to map identifiers onto locators. The ACM
              ReArch, Re-Architecting the Internet. Madrid (Spain)",
              December 2008.

   [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 architecture 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 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
   updates.

   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).  The LISP WG was rechartered in 2018 to
   continue work on the LISP base protocol and produce standard-track
   documents.

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A.1.  Old LISP Models

   LISP, as initially conceived, had a number of potential operating
   modes, named 'models'.  Although they are no used anymore, one
   occasionally sees mention of them, so they are briefly described
   here.

   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
      needed.

Authors' Addresses

   Albert Cabellos
   UPC-BarcelonaTech
   c/ Jordi Girona 1-3
   08034 Barcelona Catalonia
   Spain

   Email: acabello@ac.upc.edu

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

   Email: damien.saucez@inria.fr

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