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In: MissingRef
LISP Working Group                                         J. N. Chiappa
Internet-Draft                              Yorktown Museum of Asian Art
Intended status: Informational                          October 21, 2013
Expires: April 24, 2014


               An Architectural Introduction to the LISP
                  Location-Identity Separation System
                    draft-ietf-lisp-introduction-03

Abstract

   LISP is an upgrade to the architecture of the IP internetworking
   system, one which separates location and identity properties
   (previously intermingled in IP addresses).  This document is an
   introductory overview of the entire LISP system, and focuses on
   describing the major concepts and functional sub-systems of LISP, and
   the interactions between them.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 24, 2014.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Prefatory Note
   2.  Part I
   3.  Initial Glossary
   4.  Background
   5.  Deployment Philosophy
     5.1.  Economics
     5.2.  Maximize Re-use of Existing Mechanism
   6.  LISP Overview
     6.1.  Basic Approach
     6.2.  Basic Functionality
     6.3.  Mapping from EIDs to RLOCs
     6.4.  Interworking With Non-LISP-Capable Endpoints
     6.5.  Security in LISP
   7.  Initial Applications
     7.1.  Provider Independence
     7.2.  Multi-Homing
     7.3.  Traffic Engineering
     7.4.  Routing
     7.5.  Mobility
     7.6.  Traversal Across Alternate IP Versions
     7.7.  Virtual Private Networks
     7.8.  Local Uses
   8.  Major Functional Subsystems
     8.1.  Data Plane - xTRs Overview
       8.1.1.  Mapping Cache Performance
     8.2.  Control Plane - Mapping System Overview
       8.2.1.  Mapping System Organization
       8.2.2.  Interface to the Mapping System
       8.2.3.  Indexing Sub-system
   9.  Examples of Operation
     9.1.  An Ordinary Packet's Processing
     9.2.  A Mapping Cache Miss
   10. Part II
   11. Design Approach
   12. xTRs
     12.1. When to Encapsulate
     12.2. UDP Encapsulation Details
     12.3. Header Control Channel
       12.3.1. Mapping Versioning
       12.3.2. Echo Nonces
       12.3.3. Instances
     12.4. Probing
     12.5. Mapping Lifetimes and Timeouts
     12.6. Mapping Gleaning in ETRs
     12.7. MTU Issues
     12.8. Security of Mapping Lookups
     12.9. xTR Mapping Cache Performance
   13. The Mapping System
     13.1. The Mapping System Interface
       13.1.1. Map-Request Messages
       13.1.2. Map-Reply Messages
       13.1.3. Map-Register and Map-Notify Messages
     13.2. The DDT Indexing Sub-system
       13.2.1. Map-Referral Messages
     13.3. Reliability via Replication
     13.4. Security of the DDT Indexing Sub-system
     13.5. Extended Capabilities
     13.6. Performance of the Mapping System
   14. Multicast Support in LISP
     14.1. Basic Concepts of Multicast Support in LISP
     14.2. Initial Multicast Support in LISP
   15. Deployment Issues and Mechanisms
     15.1. LISP Deployment Needs
     15.2. Interworking Mechanisms
       15.2.1. Proxy LISP Routers
       15.2.2. LISP-NAT
     15.3. Use Through NAT Devices
     15.4. LISP and Core Internet Routing
   16. Fault Discovery/Handling
     16.1. Handling Missing Mappings
     16.2. Outdated Mappings
       16.2.1. Outdated Mappings - Updated Mapping
       16.2.2. Outdated Mappings - Wrong ETR
       16.2.3. Outdated Mappings - No Longer an ETR
     16.3. Erroneous Mappings
     16.4. Verifying ETR Liveness
     16.5. Verifying ETR Reachability
   17. Acknowledgments
   18. IANA Considerations
   19. Security Considerations
   20. References
     20.1. Normative References
     20.2. Informative References
   Appendix A.  Glossary/Definition of Terms
   Appendix B.  Other Appendices
     B.1.  A Brief History of Location/Identity Separation
     B.2.  A Brief History of the LISP Project
     B.3.  Old LISP 'Models'
     B.4.  The ALT Mapping Indexing Sub-system
     B.5.  Early NAT Support

1.  Prefatory Note

   This document is the first of a pair which, together, form what one
   would think of as the 'architecture document' for LISP (the
   'Location-Identity Separation Protocol').  Much of what would
   normally be in an architecture document (e.g. the architectural
   design principles used in LISP, and the design considerations behind
   various components and aspects of the LISP system) is in the second
   document, the 'Architectural Perspective on LISP' document.
   [Perspective]

   This 'Architectural Introduction' document is primarily intended for
   those who unfamiliar with LISP, and want to start learning about it.
   It is intended primarily for those working _on_ LISP, but those
   working _with_ LISP, and more generally anyone who wants to know more
   about LISP, may also find this document useful.

   This document is intended to both be easy to follow, and also to give
   the reader a choice as to how much they wish to know about LISP.  It
   is structured as a series of phases, each covering the entire system,
   but with ever-increasing detail.  Reading only the first part of the
   document will give a good high-level view of the system; reading the
   complete document should provide a fairly detailed understanding of
   the entire system.

   People who just want to get an idea of how LISP works might only read
   the first part; they can stop reading either just before, or just
   after, Section 9, "Examples of Operation".  People who are going to
   go on and read the protocol specifications (perhaps to implement
   LISP) should read the entire document.

   Note: This document is a descriptive document, not a protocol
   specification.  Should it differ in any detail from any of the LISP
   protocol specification documents, they take precedence for the actual
   operation of the protocol.

2.  Part I

3.  Initial Glossary

   This initial glossary defines a few general terms which will be
   useful to have in hand when commencing reading this document.  A
   complete glossary is available in Appendix A.

   A note about style: initial usage of a term defined in the glossary
   is denoted with double quotation marks (").  Other uses of quotations
   (e.g. for quotations, euphemisms, etc) use single quotation marks
   (').

   -  Name: In this document, and in much of computer science, a 'name'
      simply refers to an identifier for an object or entity.  Names
      have both semantics (meaning) and syntax (form).  [RFC1498]
   -  Namespace: A group of "names" with matching semantics and syntax;
      they usually, but not always, refer to members of a class of
      identical objects.
   -  Mapping: In this document, a connection (or binding, to use the
      computer science term) between two names, one in each of two
      namespaces.
   -  Delegation Hierarchy: an abstract rooted tree (in the graph theory
      sense of the term) which is a virtual representation of the
      delegation of a "namespace" into smaller and smaller blocks, in a
      recursive process.
   -  Node: The general term used to describe any sort of communicating
      entity; it might be a physical or a virtual host, or a mobile
      device of some sort.  It includes both entities which forward
      packets, and entities which create or consume packets.  It was
      deliberately chosen for use in this document precisely because its
      definition is not fixed, and therefore unlikely to cause erroneous
      images in the minds of readers.
   -  Switch, Packet Switch: A packet switch, in the general meaning of
      that term.  A device which takes in packets from its interfaces
      and forwards them on, either to a next-hop switch, or to the final
      destination.  They may operate at either the network layer (e.g.
      ARPANET), or internetwork layer.  [Baran][Heart][RFC1812]
   -  Endpoint, end-end communication entity: The fate-sharing region at
      one end of an end-end communication; the collection of state
      related to both the reliable end-end communication channel, and
      the applications running there.  [Chiappa]
   -  IPvN: IPv4 ([RFC791]) or IPv6 ([RFC2460]); the two are so similar,
      in fundamental architecture, that in much discussion about their
      capabilities, limitations, etc statements about the apply equally
      to both, and to continually say 'IPv4 and IPv6' quickly becomes
      tedious.
   -  Address: In this document, and in current "IPvN" and similar
      networking suites, a "name" which has mixed semantics, in that it
      includes both identity ('who') and location ('where') semantics.
      [Atkinson]
   -  Address Block, Block: A contiguous section of a namespace, usually
      IPvN addresses; for the latter, it will normally be on a bit
      boundary, using the standard 'prefix/length' selection indication.
   -  Identifier: Here, and in current networking discussions, a "name"
      which has purely identity semantics.
   -  Locator: Originally defined as a "name" with only location
      semantics, and one that was not necessarily carried in every
      packet (as was widely assumed of "addresses") [RFC1992], it is now
      generally taken, including here, to mean a "name" with purely
      location semantics.
   -  Site: A collection of hosts, routers and networks under a single
      administrative control.
   -  LISP site: A single node, or a set of network elements in an edge
      network under the administrative control of a single organization;
      they are separated from the rest of the network by "LISP routers".
   -  LISP node: A IPvN "node" which has been enhanced with LISP
      functionality; generally this means it can process some subset of
      LISP control plane traffic.
   -  LISP router: A IPvN "switch" which has been enhanced with LISP
      functionality; a LISP node which can forward user traffic.
   -  LISP host: A IPvN host which is 'behind' (from the point of view
      of the rest of the network) a "LISP router".

4.  Background

   It has gradually been realized in the networking community that
   networks, especially large networks, should deal quite separately
   with the 'identity' and 'location' of an "endpoint" - basically,
   'who' an endpoint is, and 'where' it is.  ([RFC1498]) (A more
   detailed history of this evolution is in Appendix B.1, "A Brief
   History of Location/Identity Separation".)

   At the moment, in both IPv4 and IPv6, IP "addresses" indicate both
   where the named "node" is, as well as identify it for purposes of
   end-end communication; i.e. it has both location and identity
   properties.  However, the separation of those two properties is a
   step which has recently been identified by the IRTF as a necessary
   evolutionary architectural step for the Internet.  [RFC6115]

   The on-going LISP project is an attempt to provide a viable path
   towards this separation.  (A brief history of the LISP project can be
   found in Appendix B.2, "A Brief History of the LISP Project".)

   As an add-on to a large existing system, it has had to make certain
   compromises.  (For a good example, see [Perspective], Section
   "Residual Location Functionality in EIDs".)  However, if it reaches
   near-ubiquitous deployment, it will have two important consequences.

   First, in effectively providing separation of location and identity,
   along with providing a distributed directory of the "mappings"
   between them, 'Wheeler's Law' ('All problems in computer science can
   be solved by another level of indirection') will come into play, and
   the Internet technical community will have a new, immensely powerful,
   tool at its disposal.  The fact that the namespaces on both sides of
   the mapping are global ones maximizes the power of that tool.  (See
   [Perspective], Section "Need for a Mapping System", for more on
   this.)

   Second, because of a combination of the flexible capability built
   into LISP, and the breaking of the unification of location and
   identity names, further architectural evolution of the Internet
   becomes easily available; for example, new namespaces for location
   could be designed and deployed.  In other words, LISP is not a point
   solution to meet a particular need, but hopefully an 'escape hatch'
   which will allow further significant enhancement to the Internet's
   overall architecture.  (See [Future] for more on this.)

5.  Deployment Philosophy

   The deployment philosophy was a major driver for much of the design
   of LISP: to some degree of the architecture, and to a very large
   measure, the engineering.

   Experience over the last several decades has shown that having a
   viable 'deployment model' for a new design is absolutely key to the
   success of that design.  In general, it is comparatively easy to
   conceive of new network designs, but much harder to devise approaches
   which will actually get deployed throughout the global network.  A
   new design may be fantastic - but if it can not or will not be
   successfully deployed (for whatever factors), it is useless.

   This absolute primacy of what is hoped is a viable deployment model
   is what has lead to some painful compromises in the design; and the
   extreme focus on a viable deployment model (including economics) is
   one of the key design guides of LISP.

   LISP aims to achieve the near-ubiquitous deployment necessary for
   maximum exploitation of an architectural upgrade by i) minimizing the
   amount of change needed (most existing hosts and routers can operate
   unmodified); and ii) by providing significant benefits to early
   adopters.

5.1.  Economics

   A key factor in successful adoption is economics: does the new design
   have benefits which outweigh its costs?

   More importantly, this balance needs to hold for early adopters -
   because if they do not receive benefits to their adoption, the sphere
   of earliest adopters will not expand, and it will never get to
   widespread deployment.

   This is particularly true of architectural enhancements, which are
   far less likely to be an addition which one can 'bolt onto the side'
   of existing mechanisms, and often offer their greatest benefits only
   when widely (or ubiquitously) deployed.

   Maximizing the cost-benefit ratio obviously has two aspects.  First,
   on the cost side, by making the design as inexpensive as possible,
   which means in part making the deployment as easy as possible.
   Second, on the benefit side, by providing many new capabilities,
   which is best done not by loading the design up with lots of features
   or options (which adds complexity), but by making the addition
   powerful through deeper flexibility.  The LISP community believes
   LISP has met both of these goals.

5.2.  Maximize Re-use of Existing Mechanism

   One key part of reducing the cost of a new design is to absolutely
   minimize the amount of change _required_ to existing, deployed,
   devices: the fewer devices need to be changed, and the smaller the
   change to those that do, the lower the pain (and thus the greater the
   likelihood) of deployment.

   Designs which absolutely require 'forklift upgrades' to large amounts
   of existing gear are far less likely to succeed - because they have
   to have extremely large benefits to make their very substantial costs
   worthwhile.

   It is for this reason that LISP, in most cases, initially requires no
   changes to almost all existing devices in the Internet (both hosts
   and routers); LISP functionality needs to be added in only a few
   places (see Section 15.1, "LISP Deployment Needs", for more).

   LISP also initially re-uses, where-ever possible, existing protocols.
   The 'initially' must be stressed - careful attention has also long
   been paid to the long-term future (see [Future]), and larger changes
   become feasible as deployment increases.

6.  LISP Overview

   LISP is an incrementally deployable architectural upgrade to the
   existing Internet infrastructure, one which provides separation of
   location and identity.  It thus starts to separate the names used for
   identity and location of nodes, which are currently unified in "IPvN"
   "addresses".

   The separation into names with purely location and purely identity
   semantics is usually - but not necessarily - not perfect, for reasons
   which are driven by the deployment philosophy (above), and explored
   in more detail elsewhere (in [Perspective], Section "Namespaces-EIDs-
   Residual").

6.1.  Basic Approach

   In LISP, the first key concept is that nodes have both an
   'identifier' (a name which serves only to provide a persistent handle
   for the node), called an "EID" (short for 'endpoint identifier'), and
   an associated 'locator' (a name which says _where_ the node is, in
   the network's connectivity structure), called an "RLOC" (short for
   'routing locator').

   A node may be associated with more than one RLOC, or the RLOC may
   change over time (e.g. if the node is mobile), but it would normally
   always have the same EID.

   The second key concept is that if one wants to be as forward-looking
   as possible, conceptually one should think of the two kinds of names
   (EIDs and RLOCs) as naming _different classes of entities_.

   EIDs name nodes - or rather, their end-end communication entities
   (see [Chiappa] for more).  RLOC(s), on the other hand, name
   interfaces, i.e. places to which the system of routers sends packets.
   (These will usually be on the "LISP routers", in the early stages of
   LISP deployment; see below for more.)

   This distinction, the formal recognition of _different_ kinds of
   entities ("endpoints" and interfaces), and their association with the
   two different classes of names, is also important.  Clearly
   recognizing interfaces and endpoints as distinctly separate classes
   of objects is another improvement to the existing Internet
   architecture.

   An important insight in LISP is that it initially uses existing IPvN
   addresses for both of these kinds of names, as opposed to some
   similar earlier deployment proposals for separation of location and
   identity (e.g.  [RFC1992]), which proposed using a new namespace for
   locators.  This choice minimized LISP's deployment cost, as well as
   providing the ability to easily interact with un-modified hosts and
   routers.

   The capability to use namespaces other than IPvN addresses for both
   kinds of names is already built in, which is expected to greatly
   increase the long-term benefits, flexibility, and power of the LISP
   "mapping" layer.  [AFI][LCAF]

6.2.  Basic Functionality

   The basic operation of LISP, as it currently stands, is quite simple.
   LISP augmented packet switches, "LISP routers", near the source and
   destination of packets intercept traffic, and 'enhance' the packets
   for the trip between the LISP switches.

   The LISP router near the original source (the Ingress Tunnel Router,
   or "ITR") looks up additional information about the destination of
   the packet, and then wraps the packet in an outer header, one which
   contains some of that additional information.

   The LISP router near the destination, the (the Egress Tunnel Router,
   or "ETR") removes that header, leaving the original, un-modified,
   packet to be sent on to the original destination node.

   The overall processing is shown below, in Figure 1:

                  (to be added)

           Figure 1: Basic LISP Packet Flow

   To retrieve that additional information, the ITR uses the information
   in the original packet about the identity of its ultimate
   destination, i.e. the destination address; in LISP, this is the EID
   of the ultimate destination.  It uses the destination EID to look up
   the current location (the RLOC) of that EID.

   The lookup is performed through a "mapping system", which is the
   heart of LISP: it is a distributed directory of "mappings" from EIDs
   to RLOCs.  The destination RLOC(s) will normally be the address(es)
   of the ETR(s) near the ultimate destination.

   The ITR then generates a new outer header for the original packet,
   with that header containing the ETR's RLOC as the wrapped packet's
   destination, and the ITR's own address (i.e. the RLOC usually
   associated with the original source) as the wrapped packet's source,
   and sends it off.

   When the packet arrives at the ETR, that outer header is stripped
   off, and the original packet is forwarded to the original ultimate
   destination for normal processing.

   Return traffic is handled similarly, often (depending on the
   network's configuration) with the original ITR and ETR switching
   roles.  The ETR and ITR functionality is usually co-located in a
   single LISP router; these are normally denominated as "xTRs".

6.3.  Mapping from EIDs to RLOCs

   The "mappings" from EIDs to RLOCs are provided by a distributed, and
   potentially replicated, database, the "mapping database", which is
   the heart of LISP.  (Here, and in other places in LISP, the
   replication is not a deep architectural concept, simply an
   engineering device to obtain reliability via potential redundancy.)

   Entities which need mappings get them from the "mapping system",
   which is a collection of sub-systems through which clients can find
   and obtain mappings.  (The mapping system will be discussed in more
   detail below, in Section 8.2, "Control Plane - Mapping System
   Overview" and Section 13, "The Mapping System".)

   Mappings are are normally distributed via a 'pull' mechanism; in
   other words, they are generally not pre-loaded, but requested on
   demand.  Once obtained by an ITR, they are cached by the ITR, for
   performance reasons.

   Extensive studies, including large-scale simulations driven by
   lengthy recordings of actual traffic at several major sites, have
   been performed to verify that this 'pull and cache' approach is
   viable, in practical engineering terms.  (This subject will be
   discussed in more detail in Section 12.9, "xTR Mapping Cache
   Performance", below, including references to the studies.)

6.4.  Interworking With Non-LISP-Capable Endpoints

   It is clearly crucial to provide the capability for 'easy'
   interoperation between "LISP hosts" - i.e. they are behind xTRs, and
   their EIDs are in the mapping database - and existing non-LISP-using
   hosts (often called 'legacy' hosts) or legacy "sites".

   To allow such interoperation, a number of mechanisms have been
   designed.  One approach uses proxy LISP routers, called "PITRs"
   (proxy ITRs) and "PETRs" (proxy ETRs), to provide LISP functionality
   during interaction with legacy hosts.  Another approach uses a router
   with combined LISP and NAT ([RFC1631]) functionality, named a LISP-
   NAT.

   (See Section 15.2.1, "Proxy LISP Routers", and Section 15.2.2, "LISP-
   NAT", respectively, for details of each, and their respective
   advantages and disadvantages.)

6.5.  Security in LISP

   To provide a brief overview of security in LISP, it is definitely
   understood that LISP needs to be highly _securable_, especially in
   the long term; over time, the attacks mounted by 'bad guys' are
   becoming more and more sophisticated.  So LISP, like DNS, needs to be
   _capable_ of providing 'the very best' security there is.

   At the same time, there is a conflicting goal: it must be deployable
   at a a viable cost.  That means two things: First, as an experiment,
   we cannot expect to create the complete security apparatus which we
   might see in the finished product, including both design and
   implementation.  Second, security needs to be flexible, so that we
   don't overload the users with more security than they need at any
   point.

   To accomplish these divergent goals, the approach taken is to first
   analyze what LISP needs for security.  [Threats].  Then, steps can be
   taken to ensure that the appropriate 'hooks' (such as packet fields)
   are included at an early stage, when doing so is still easy.  Over
   time, additional mechanisms will be fully specified, implemented, and
   deployed.

   LISP does already include a number of security mechanisms; in
   particular, requesting mappings can be secured (see Section 12.8,
   "Security of Mapping Lookups"), as can registering of xTRs (see
   Section 13.1.3, "Map-Register and Map-Notify Messages"); the key
   database of the mapping system is also secured (see Section 13.4,
   "Security of the DDT Indexing Sub-system").

   The existing security mechanisms, and their configuration (which is
   mostly manual at this point) currently in LISP are felt to be
   adequate for the needs of the on-going early stages of deployment;
   experience will indicate when improvements are required (within the
   constraints of the conflicting goal given above).

   For more on LISP's security philosophy; see [Perspective], Section
   "Security", where it is laid out in some detail.

7.  Initial Applications

   {{Reorder the whole section in popularity order?}}

   As previously mentioned, it is felt that LISP will provide even the
   earliest adopters with some useful capabilities, and that these
   capabilities will drive early LISP deployment.

   It is very imporant to note that even when used only for
   interoperation with existing un-modified hosts, use of LISP can still
   provide benefits to the site which has deployed it - and, perhaps
   even more importantly, can do so _to both sides_.  This
   characteristic acts to further enhance the utility for early adopters
   of LISP. .

   Note also that this section only lists some early applications and
   benefits.  See [Perspective], in the Section "Goals of LISP", for a
   more extensive discussion of some of what LISP might ultimately
   provide.

7.1.  Provider Independence

   Provider independence (i.e. the ability to easily change one's
   Internet Service Provider) is a good example of the utility of
   separating location and identity.

   The problem is simple: for the global routing to scale, addresses
   need to be aggregated; i.e. things which are close in the overall
   network's connectivity need to have closely related addresses (so-
   called "provider aggregatable" addresses).  [RFC4116] However, if
   this principle is followed, it means that when an entity switches
   providers (i.e. it moves to a different 'place' in the network), it
   has to re-number, a painful undertaking.  [RFC5887]

   Having separate namespaces for location and identity greatly reduces
   the problems involved with re-numbering; an organization which moves
   retains its EIDs (which are how most other parties refer to its
   nodes), but is allocated new RLOCs, and the mapping system can
   quickly provide the updated mapping from the EIDs to the new RLOCs.

7.2.  Multi-Homing

   Multi-homing is another place where the value of separation of
   location and identity became apparent.  There are several different
   sub-flavours of the multi-homing problem - e.g. depending on whether
   one wants open TCP connections to keep working, etc - and other axes
   as well (e.g. site multi-homing versus host multi-homing).

   In particular, for the 'keep open connections up' case, without
   separation of location and identity, with most currently deployed
   implementations, the only currently feasible approach is to use
   provider-independent addressses - which moves the problem into the
   global routing system, with attendant costs.  This approach is also
   not really feasible for host multi-homing.

7.3.  Traffic Engineering

   {{Needs a fix - not sure what.}}

   Traffic engineering (TE) [RFC3272], desirable though this capability
   is in a global network, is currently somewhat problematic to provide
   in the Internet.  The problem, fundamentally, is that this capability
   was not forseen when the Internet was designed, so the support for it
   via 'hacks' is neither clean, nor flexible.

   TE is, fundamentally, a routing issue.  However, the current Internet
   routing architecture, which is basically the Baran design of fifty
   years ago [Baran] (a single large, distributed computation), is ill-
   suited to provide TE.  The Internet seems a long way from adopting a
   more-advanced routing architecture, although the basic concepts for
   such have been known for some time.  [RFC1992]

   Although the identity-location mapping layer is thus a poor place,
   architecturally, to provide TE capabilities, it is still an
   improvement over the current routing tools available for this purpose
   (e.g. injection of more-specific routes into the global routing
   table).

   In addition, instead of the entire network incurring the costs
   (through the routing system overhead), when using a mapping layer to
   provide TE, the overhead is limited to those who are actually
   communicating with that particular destination.

   LISP includes a number of features in the mapping system to support
   TE. (described in Section 8.2, "Control Plane - Mapping System
   Overview", below); more details about using LISP for TE can be found
   in [LISP-TE].

   Also, a number of academic papers have explored how LISP can be used
   to do TE, and how effective it can be.  See the online LISP
   Bibliography ([Bibliography]) for information about them.

7.4.  Routing

   Multi-homing and Traffic Engineering are both, in some sense, uses of
   LISP for routing, but there are many other routing-related uses for
   LISP.

   One of the major original motivations for the separation of location
   and identity in general, and thus LISP, was to reduce the growth of
   the routing tables in the "Internet core", the part where routes to
   _all_ ultimate destinations must be available.  LISP is expected to
   help with this; for more detail, see Section 15.4, "LISP and Core
   Internet Routing", below.

   LISP may also have more local applications in which it can help with
   routing; see, for instance, [CorasBGP].

7.5.  Mobility

   Mobility is yet another place where separation of location and
   identity is obviously a key part of a clean, efficient and high-
   functionality solution.  Considerable experimentation has been
   completed on doing mobility with LISP.

   The mobility provided by LISP allows active sessions to survive moves
   (provided of course that there is not a period of inaccessability
   which exceeds a timeout).  LISP mobility also will typically have
   better packet 'stretch' (i.e. increase in path length) compared to
   traditional mobility schemes, which use a 'home agent'.

7.6.  Traversal Across Alternate IP Versions

   Note that LISP inherently supports intermixing of various IP versions
   for packet carriage; IPv4 packets might well be carried in IPv6, or
   vice versa, depending on the network's configuration.

   This capability allows an 'island' of operation of one type to be
   automatically tunneled over a stretch of infrastucture which only
   supports the other type.

   While the machinery of LISP may seem too heavy-weight to be good for
   such a mundane use, this is not intended as a 'sole use' case for
   deployment of LISP.  Rather, it is something which, if LISP is being
   deployed anyway (for its other advantages), is an added benefit that
   one gets 'for free'.

7.7.  Virtual Private Networks

   L2 and L3 {{Need to add text here - This used to be part of 'Local'
   below, but we decided this was so important it deserved its own
   section.  Maybe move this up further, as it seems to be the most
   important 'early adopter' application?}}

   This includes support of VPN's for segmentation and multi-tenancy
   (i.e. a spatially separated private VPN whose components are joined
   together using the public Internet as a backbone).

7.8.  Local Uses

   LISP has a number of use cases which are within purely
   organizationally-local contexts, i.e. not in the larger Internet.
   These fall into two categories: uses seen on the Internet (above),
   but here on a private (and usually small scale) setting; and
   applications which do not have a direct analog in the larger
   Internet, and which apply only to local deployments.

   Among the former are multi-homing and IP version traversal. {{This
   was marked to be deleted - why?  The next part doesn't make sense
   without this first?}}

   Among the latter class, non-Internet applications which have no
   analog on the Internet, are the following example applications:
   virtual machine mobility in data centers; other non-IP EID types such
   as local network MAC addresses, or application specific data.

   Several of the applications listed in this section are the ones which
   have been most popular for LISP in practise; these include virtual
   networks, and virtual machine mobility.

   These often show a synergistic tendency, in that a site which
   installs LISP to do one, often finds that then becomes a small matter
   to use it for the second.  Given all the things which LISP can do, it
   is hoped that this synergistic effect will continue to expand LISP's
   uses.

   {{Preceeding paragraphs should probably get moved up into VPN
   section?}}

8.  Major Functional Subsystems

   LISP has only two major functional sub-systems - the collection of
   LISP "packet switches" (the xTRs), which form the 'data plane' of
   LISP; and the "mapping system", the most important part of the
   'control plane', which manages the "mapping database".

   The purpose and operation of each is described at a high level below,
   and then, later on, in a fair amount of detail, in separate sections
   on each (Sections Section 12, "xTRs", and Section 13, "The Mapping
   System", respectively).

8.1.  Data Plane - xTRs Overview

   xTRs are packet switches which have been augmented with extra
   functionality in both the data and control planes.  The data plane
   functions in ITRs include deciding which packets need to be given
   LISP processing (since packets to non-LISP hosts may be sent as they
   are); i.e. looking up the mapping; encapsulating (wrapping) the
   packet; and sending it to the ETR.

   This encapsulation is done using UDP [RFC768] (for reasons to be
   explained below, in Section 12.2, "UDP Encapsulation Details"), along
   with an additional outer IPvN header (to hold the source and
   destination RLOCs).  To the extent that traffic engineering features
   are in use for a particular EID, the ITRs implement them as well.

   In the ETR, the data plane simply decapsulates (unwraps) the packets,
   and forwards the now-normal packets to the ultimate destination.

   Control plane functions in ITRs include: asking for {EID->RLOC}
   mappings via request control messages (Map-Request packets); handling
   the returning reply control messages (Map-Reply packets), which
   contain the requested information; managing the local "mapping cache"
   of "mappings"; checking for the "reachability" and "liveness" of
   their neighbour ETRs; and checking for outdated mappings and
   requesting updates.

   In the ETR, control plane functions include participating in the
   reachability and liveness function (see Section 16.4, "Verifying ETR
   Liveness"); interacting with the mapping sub-system to let it know
   what mapping this ETR can provide (see Section 8.2.2, "Interface to
   the Mapping System"); and answering requests from ITRs for those
   mappings (ditto).

8.1.1.  Mapping Cache Performance

   As mentioned, studies have been performed to verify that caching
   mappings in ITRs is viable, in practical engineering terms.  These
   studies not only verified that such caching is feasible, but also
   provided some insight for designing ITR "mapping caches".

   Briefly, they took lengthy traces of all packets leaving a large
   site, over a period of a week or so, and used those to drive
   simulations which showed how many mappings would be required.  It
   also allowed analysis of how much control traffic (for loading needed
   mappings) would result, using various cache sizes and replacement
   algorithms.

   A more extended look at the results us given below, in Section 12.9,
   "xTR Mapping Cache Performance".

   Obviously, these studies are all snapshots of a particular point in
   time, and as the Internet continues its life-cycle they will
   increasingly become out-dated.  However, they are useful because they
   provide an insight into how well LISP can be expected to perform, and
   scale, over time.

8.2.  Control Plane - Mapping System Overview

   The mapping system's entire purpose is to give ITRs on-demand access
   to the mapping database, which is a distributed, and potentially
   replicated, database which holds mappings between EIDs (identity) and
   RLOCs (location), along with needed ancillary data (e.g. lifetimes).

   To be exact, it contains mappings between EID "blocks" and RLOCs (the
   block size is given explicitly, as part of the syntax).  Support for
   blocks is both for minimizing the administrative configuration
   overhead, as well as for operational efficiency; e.g. when a group of
   EIDs are behind a single xTR.

   However, the block may be, and sometimes is, as small as a single
   EID.  However, since mappings are only loaded upon demand, if smaller
   blocks become predominant, then the increased size of the overall
   database is far less problematic than if the Internet's routing
   tables came to be dominated by such small entries.

   A particular EID (or EID block) may have more than one RLOC, or may
   change its RLOC(s), while keeping its basic identity.

   Also, in general, throughout LISP, anyplace a name (EID, RLOC, etc)
   appears in a control packet, the packet format also includes an
   Address Family Identifier (AFI) for that name.  [AFI] The inclusion
   of the AFI allows LISP (and in particular, the mapping system
   interface, as embodied in those control packets) a great deal of
   flexibility.  (See [Perspective], Section "Namespaces" for more on
   this.)

   Finally, the mapping from an EID (or EID block) contains not just the
   RLOC(s), but also (for each RLOC for any given EID entry) priority
   and weight fields (to allow allocation of load between several RLOCs
   at a given priority); this allows a certain amount of traffic
   engineering to be accomplished with LISP.

8.2.1.  Mapping System Organization

   The "mapping system" is actually split into what are effectively
   three major functional sub-systems (although the latter two are
   closely integrated, and appear to most entities in the LISP system as
   a single sub-system).

   The first is the actual mappings themselves, collectively the
   "mapping database"; they are held by the ETRs, and an ITR which needs
   a mapping gets it (effectively) directly from the ETR.  This co-
   location of the authoritative version of the mappings, and the
   forwarding functionality which it describes, is an instance of fate-
   sharing.  [Clark]

   To find the appropriate ETR(s) to query for the mapping, the second
   two sub-systems form an 'indexing system', itself also based on a
   distributed, potentially replicated database.  It provides
   information on which ETR(s) are authoritative sources for the various
   {EID -> RLOC} mappings which are available.  The two sub-systems
   which form it are the client interface sub-system, and "indexing sub-
   system" (which holds and provides the actual information).

8.2.2.  Interface to the Mapping System

   The client interface to the indexing system from an ITR's point of
   view is not with the indexing sub-system directly; rather, it is
   through the client-interface sub-system, which is provided by LISP
   nodes called Map-Resolvers (MRs) and Map-Servers (MSs).

   ITRs send request control messages (Map-Request packets) to an MR.
   (This interface is probably the most important standardized interface
   in LISP - it is the key to the entire system.)

   The MR then uses the indexing sub-system to allow it to forward the
   Map-Request to an appropriate Map-Server (MS), which in turn sends
   the Map-Request on to the appropriate ETR.  The latter is
   authoritative for the actual contents of all mappings for those EID
   namespace blocks which have been delegated to it.

   The ETR then formulates reply control messages (Map-Reply packets),
   which are sent to the ITR.  The details of the indexing sub-system
   are thus hidden from the ITRs.

   (Note that in some cases, it is desirable for the MS to reply on
   behalf of the ETR, in so-called 'proxy' mode.  This behaviour can be
   selected when the ETR registers with the MR, described immediately
   below.)

   Similarly, the client interface to the indexing system from an ETR's
   point of view is through LISP nodes called Map-Servers (MSs).  ETRs
   send registration control messages (Map-Register packets) to an MS,
   which makes the information about the mappings which the ETR
   indicates it is authoritative for available to the indexing sub-
   system.

   The MS formulates a reply control message (the Map-Notify packet),
   which confirms the registration, and is returned to the ETR.  The
   details of the indexing sub-system are thus likewise hidden from the
   'ordinary' ETRs.

   The fact that the details of the indexing sub-system are entirely
   hidden from xTRs gives considerably flexibility to this aspect of
   LISP.  As long as any potential indexing sub-system can track where
   mappings are, it could potentially be used; this would allow the
   actual indexing sub-system to be replaced without needing to modify
   the clients - as has happened once already (see below).

8.2.3.  Indexing Sub-system

   The current indexing sub-system is the Delegated Database Tree (DDT),
   which is very similar to DNS ([DDT], [RFC1034]).  Unlike DNS, the
   actual mappings are not handled by DDT; DDT, as the indexing sub-
   system, merely identifies the ETRs which hold the actual mappings.

   DDT replaces an earlier indexing sub-system, ALT (Appendix B.4, "The
   ALT Mapping Indexing Sub-system"); this swap validated the concept of
   having a client-interface sub-system between the indexing sub-system,
   and the clients.

8.2.3.1.  DDT Overview

   Conceptually, DDT is fairly simple: like DNS, in DDT the delegation
   of the EID namespace ([Perspective], Section "Namespaces-XEIDs") is
   instantiated as a "delegation hierarchy", a tree of "DDT vertices",
   starting with the 'root' DDT vertex.  Each vertex is responsible for
   a "block" of the EID namespace.

   The 'root' vertex is reponsible for the entire namespace; any DDT
   vertex can 'delegate' part(s) of its block of the namespace to child
   DDT vertex(s).  The child vertex(s) can in turn further delegate
   (necessarily smaller) blocks of namespace to their children, through
   as many levels as are needed (for operational, administrative, etc,
   needs).

   Just as with DNS, any particular vertex in the DDT delegation tree
   may be instantiated in one or more "DDT servers".  Multiple
   (redundant) servers for a given vertex would be used for reasons of
   performance, reliability and robustness.  Obviously, all the servers
   which instantiate a particular vertex in the tree have to have
   identical data about that vertex; if they do not, when a Map-Request
   is sent to one that does not have consistent information with its
   other sibling(s), incorrect results will be returned.

   Also, although the delegation hierarchy is a strict tree, a single
   DDT server could be authoritative for more than one block of the EID
   namespace (i.e. it could be a server for more than one vertex).

   Eventually, leaf vertices in the delegation hierarchy statically
   delegate EID namespace blocks to MS's, which are DDT terminal
   servers; i.e. a leaf of the tree is reached when the delegation
   points to an MS instead of to another DDT vertex. {{Straighten out.}}

   The MS is in direct communication with the ETR(s) which both i) are
   authoritative for the mappings for that block, and ii) handle traffic
   to all nodes in that block of EID namespace.

8.2.3.2.  Use of DDT by MRs

   An MR which wants to find a mapping for a particular EID first
   interacts with the "DDT servers" which instantiate the "vertices" of
   the LISP "delegation hierarchy" tree, discovering (by querying the
   servers for information about DDT vertices) the chain of delegations
   which cover that EID.  Eventually it is directed to an MS, which is
   the 'door' to an ETR which is authoritative for that EID.

   Also, again like DNS, MRs cache information they receive about the
   delegations in the delegation tree.  This means that once an MR has
   been in operation for while, it will usually have much of the
   delegation information cached locally (especially the top levels of
   the delegation tree).  This allows them, when passed a request for a
   mapping by an ITR, to usually forward the mapping request to the
   appropriate MS without having to interact with all the DDT servers on
   the path down the delegation tree, in order to find any particular
   mappping.

   Thus, a typical resolution cycle would usually involve looking at
   some locally cached delegation information, perhaps loading some
   missing delegation entries into their delegation cache, and finally
   sending the Map-Request to the appropriate MS.

   It should also be noted that the delegation tree is fairly static,
   since it reflects namespace allocations, which are themselves fairly
   static.  This stability has several important consequences.  First,
   it increases the performance of the mapping system, since the sub-
   system almost never needs to be re-queried for information about
   intermediate vertices.  Second, it is not necessary to include a
   mechanism to find out-dated delegations.  [LISP-TREE]

   This contrasts with the _mappings_, which may change at a high rate -
   changes which have no impact on the indexing sub-system.  LISP is
   designed to make sure that changes in the mappings are detected and
   acted upon fairly quickly; this allows LISP to provide a number of
   capabilities, such as mobility.

9.  Examples of Operation

   To aid in comprehension, a few examples are given of user packets
   traversing the LISP system.  The first shows the processing of a
   typical user packet which is LISP forwarded, i.e. what the vast
   majority of user packets will see.  The second shows what happens
   when the first packet to a previously-unseen ultimate destination (at
   a particular ITR) is to be processed by LISP.

9.1.  An Ordinary Packet's Processing

   This case follows the processing of a typical user packet (for
   instance, a normal TCP data or acknowledgment packet associated with
   an already-open TCP connection) - i.e. not the first packet sent from
   a given source to a given destination - as it makes its way from the
   original source host to the ultimate destination.

   When the packet has made its way through the local site to an ITR,
   which in this case is a border router for the site, the border router
   looks up the destination address - an EID - in its local "mapping
   cache".  For EIDs which are IPvN addresses, this lookup usually uses
   the usual IPvN 'longest prefix match' algorithm.

   It finds a mapping, which instructs it to wrap the packet in an outer
   header - an IP packet, containing a UDP packet which contains a LISP
   header - and then the user's original packet (see Section 12.2, "UDP
   Encapsulation Details", for the reasons for this particular choice).
   The destination address in the outer header is set by the ITR to the
   RLOC of the destination ETR.

   The encapsulated packet is then sent off through the Internet, using
   normal Internet routing.

   On arrival at the destination ETR, the ETR will notice that it is
   listed as the destination in the outer header.  It will examine the
   packet, detect that it is a LISP packet, and unwrap it.  It will then
   examine the header of the user's original packet, and forward it
   internally, through the local site, to the ultimate destination.

   At the ultimate destination, the packet will be processed, and may
   produce a return packet, which follows the exact same process in
   reverse - with the exception that the roles of the ITR and ETR are
   swapped.

9.2.  A Mapping Cache Miss

   If a host sends a packet, and it gets to the ITR, and the ITR
   determines that it does not yet have a "mapping cache" entry which
   covers that destination EID, then additional processing ensues; it
   has to look up the mapping in the mapping system (as previously
   described in Section 6.2, "Basic Functionality").

   The overall processing is shown below, in Figure 2:

                        Mapping System

                    -----            -----
                   |     |    3     |     |
     Map Resolver  |     | -------> |     |  Map Server
                   |     |          |     |
                    -----            -----
                      ^                |
         Key:         |                |
                      |                |
     -- = Control     |                |
     == = Data        |                |
                   2  |       5        |  4
                      |      ---       |
     Host A           |    /     \     |            Host B
                      |  |_       \    V
     -----          -----          \ -----          -----
    |     |   1    |     |    6     |     |   7    |     |
    |     | =====> | ITR | =======> | ETR | =====> |     |
    |     |        |     |          |     |        |     |
     -----          -----            -----          -----

           Figure 2: Packet Flow With Missing Mapping

   1.  Source-EID sends packet (to Dest-EID) to ITR
   2.  ITR sends Map-Request to Map Resolver
   3.  Map-Resolver delivers Map-Request to Map-Server
   4.  Map-Server delivers Map-Request to ETR
   5.  ETR returns Map-Reply to ITR; ITR caches EID-to-RLOC(s) mapping
   6.  ITR uses mapping to encapsulate to ETR; sends user packet to ETR
   7.  ETR decapsulates packet, delivers to Dest-EID

   The ITR first sends a Map-Request packet, giving the destination EID
   it needs a mapping for, to its MR.  The MR will look in its cache of
   delegation information to find the vertex which is the most specific
   in the delegation tree for that destination EID .  If it does not
   have the address of an appropriate MS, it will query the DDT system,
   recursively if need be, in order to eventually find the address of
   such an MS.

   When it has the MS's address, it will send the Map-Request on to the
   MS, which then usually sends it on to an appropriate ETR.  The ETR
   sends a Map-Reply to the ITR which needs the mapping; from then on,
   processing of user packets through that ITR to that ultimate
   destination proceeds as above.

   Often the original user packet will have been discarded, and not
   queued waiting for the mapping to be returned.  When the host
   retransmits such a packet, the mapping will be there, and the packet
   will be forwarded.  Alternatively, it might have been queued, or
   perhaps it was forwarded using a PITR.  (Section 6.4, "Interworking
   With Non-LISP-Capable Endpoints")

10.  Part II

11.  Design Approach

   Before describing LISP's components in more detail below, it it worth
   pointing out that what may seem, in some cases, like odd (or poor)
   design approaches do in fact result from the application of a
   thought-through, and consistent, design philosophy used in creating
   them. {{Subjective: maybe JMH, Dino can help with better words?}}

   This design philosophy is covered in detail in in [Perspective],
   Section "Design"), and readers who are interested in the 'why' of
   various mechanisms should consult that; reading it may make clearer
   the reasons for some engineering choices in the mechanisms given
   here.

12.  xTRs

   As mentioned above (in Section 8.1, "Data Plane - xTRs Overview"),
   xTRs are the basic data-handling nodes in LISP, and, as such, form
   the LISP data plane - although of necessity they are also involved in
   some control plane functions.  This section explores some advanced
   topics related to xTRs.

   Careful rules have been specified for both TTL and ECN [RFC3168] to
   ensure that passage through xTRs does not interfere with the
   operation of these mechanisms.  In addition, care has been taken to
   ensure that 'traceroute' works when xTRs are involved.

12.1.  When to Encapsulate

   An ITR knows that an ultimate destination is 'running' LISP (remember
   that the actual destination machine itself probably knows nothing
   about LISP), and thus that it should perform LISP processing on a
   packet (including potential encapsulation) if it has an entry in its
   local "mapping cache" that covers the destination EID.

   Conversely, if the cache contains a 'negative' entry (indicating that
   the ITR has previously attempted to find a mapping that covers this
   EID, and it has been informed by the mapping system that no such
   mapping exists), it knows the ultimate destination is not running
   LISP, and the packet can be forwarded natively (i.e. not LISP-
   encapsulated).

   Note that the ITR cannot simply depend on the appearance, or non-
   appearance, of the destination in the routing tables in the "Internet
   core", as a way to tell if an ultimate destination is a LISP node or
   not.  That is because mechanisms to allow interoperation of LISP
   sites and 'legacy' sites necessarily involve advertising LISP sites'
   EIDs into the Internet core; in other words, LISP sites which need to
   interoperate with 'legacy' nodes will appear in the Internet core
   routing tables, along with non-LISP sites.

12.2.  UDP Encapsulation Details

   Use of UDP (instead of, say, a LISP-specific protocol number) was
   driven by the fact that many routers filter out 'unknown' protocols,
   so adopting a non-UDP encapsulation would have made the initial
   deployment of LISP harder.

   The UDP source port in the encapsulated packet is a 5-way hash of the
   original source and ultimate destination in the inner header, along
   with the ports, and the protocol.

   This is because many ISPs use multiple parallel paths (so-called
   'Equal Cost Multi-Path'), and load-share across them.  Using such a
   hash in the source-port in the outer header both allows LISP traffic
   to be load-shared, and also ensures that packets from individual
   connections are delivered in order (since most ISPs try to ensure
   that packets for a particular {source, source port, destination,
   destination port} tuple flow along a single path, and do not become
   disordered).

   The UDP checksum is zero because the inner packet usually already has
   a end-end checksum, and the outer checksum adds no value.  [Saltzer]
   In most exising hardware, computing such a checksum (and checking it
   at the other end) would also present a major load, for no benefit.

12.3.  Header Control Channel

   LISP provides a multiplexed channel in the encapsulation header.  It
   is mostly (but not entirely) used for control purposes.  (See
   [Perspective], Section "Architecture-Piggyback" for a longer
   discussion of the architectural implications of performing control
   functions with data traffic.)

   The general concept is that the header starts with an 'flags' field,
   and it also includes two data fields, the contents and meaning of
   which vary, depending on which flags are set.  This allows these
   fields to be multiplexed among a number of different low-duty-cycle
   functions, while minimizing the space overhead of the LISP
   encapsulation header.

12.3.1.  Mapping Versioning

   One important use of the multiplexed control channel is mapping
   versioning; i.e. the discovery of when the mapping cached in an ITR
   is outdated.  To allow an ITR to discover this, identifying sequence
   numbers are applied to different versions of a mappping.  [RFC6834]
   This allows an ITR to easily discover when a cached mapping has been
   updated by a more recent variant.

   Version numbers are available in control messages (Map-Replies), but
   the initial concept is that to limit control message overhead, the
   versioning mechanism should primarily use the multiplexed user data
   header control channel.

   Versioning can operate in both directions: an ITR can advise an ETR
   what version of a mapping it is currently using (so the ETR can
   notify it if there is a more recent version), and ETRs can let ITRs
   know what the current mapping version is (so the ITRs can request an
   update, if their copy is outdated).

   At the moment version numbers are manually assigned, and ordered.

12.3.2.  Echo Nonces

   Another important use of the header control channel is for a
   mechanism known as the Nonce Echo, which is used as an efficient
   method for ITRs to check the reachability of "neighbour ETRs".

   Basically, an ITR which wishes to ensure that an ETR is up, and
   "reachable", sends a nonce to that ETR, carried in the encapsulation
   header; when that ETR (acting as an ITR) sends some other user data
   packet back to the ITR (acting in turn as an ETR), that nonce is
   carried in the header of that packet, allowing the original ITR to
   confirm that its packets are reaching that ETR.

   Note that a lack of a response is not necessarily _proof_ that
   something has gone wrong - but it strongly suggests that something
   has, so other actions (e.g. a switch to an alternative ETR, if one is
   listed; a direct probe; etc) are advised.

   (See Section 16.5, "Verifying ETR Reachability", for more about Echo
   Nonces.)

12.3.3.  Instances

   Another use of these header fields is for 'Instances' - basically,
   support for VPN's across backbones.  [RFC4026] Since there is only
   one destination UDP port used for carriage of user data packets, and
   the source port is used for multiplexing (above), there is no other
   way to differentiate among different destination address namespaces
   (which are often overlapped in VPNs).

12.4.  Probing

   RLOC-Probing (see [RFC6830], Section 6.3.2.  "RLOC-Probing Algorithm"
   for details) is a mechanism method that an ITR can use to determine
   with certainty that an ETR is up and reachable from the ITR.  As a
   side-benfit, it gives a rough RTT estimates.

   It is quite a simple mechanism - an ITR simply sends a specially
   marked Map-Request directly to the ETR it wishes information about;
   that ETR sends back a specially marked Map-Reply.  A Map-Request and
   Map-Reply are used, rather than a special probing control-message
   pair, because as a side-benefit the ITR can discover if the mapping
   has been updated since it cached it.

   The probing mechanism is rather heavy-weight and expensive (compared
   to mechanisms like the Echo-Nonce), since it costs a control message
   from each side, so it should only be used sparingly.  However, it has
   the advantages of providing information quickly (a single RTT), and
   being a simple, direct, robust way of doing so.

   If the number of active neighbour ETRs of the ITR is large, use of
   RLOC-Probing to check on their reachability will result in
   considerable control traffic; such control traffic has to be spread
   out to prevent a load peak.

   Obviously, if RLOC-Probing is the only mechanism being used to detect
   unreachable neighbour ETRs, the rate at which RLOC-Probing is done
   will control the timeliness of the detection of loss of reachability.
   There is thus a tradeoff between overhead and responsiveness,
   particular when an ITR has a large fanout of neighbour ETRs.

   A further observation is that unless what are likely unreasonable
   amounts of RLOC Probing are being done, Echo Nonce will generally
   provide faster notification of loss of reachability (unless there is
   little or no bi-directional traffic between the ITR and ETR). {{ENs
   help reduce the amount of probing when both are in use}}

12.5.  Mapping Lifetimes and Timeouts

   Mappings come with a Time-To-Live, which indicate how long the
   creator of the mapping expects them to be useful for.  The TTL may
   also indicate that the mapping should not be cached at all, or it can
   indicate that it has no particular lifetime, and the recipient can
   chose how long to store it.

   Mappings might also be discarded before the TTL expires, depending on
   what strategies the ITR is using to maintain its cache; if the
   maximum cache size is fixed, or the ITR needs to reclaim memory,
   mappings which have not been used 'recently' may be discarded.
   (After all, there is no harm in so doing; a future reference will
   merely cause that mapping to be reloaded.)

   {{Contents may change before TTL expires?}}

12.6.  Mapping Gleaning in ETRs

   As an optimization to the mapping acquisition process, ETRs are
   allowed to 'glean' mappings from incoming user data packets, and also
   from incoming Map-Request control messages.  This is not secure, and
   so any such mapping must be 'verified' by sending a Map-Request to
   get an authoritative mapping.  (See further discussion of the
   security implications of this in [Perspective], Section "Security-
   xTRs".)

   The value of gleaning is that most communications are two-way, and so
   if host A is sending packets to host B (therefore needing B's
   EID->RLOC mapping), very likely B will soon be sending packets back
   to A (and thus needing A's EID->RLOC mapping).  Without gleaning,
   this would sometimes result in a delay, and the dropping of the first
   return packet; this is felt to be very undesirable.

12.7.  MTU Issues

   Several mechanisms have been proposed for dealing with packets which
   are too large to transit the path from a particular ITR to a given
   ETR.

   In one, called the 'stateful' approach, the ITR keeps a per-ETR
   record of the maximum size allowed, and sends an ICMP Too Big message
   to the original source host when a packet which is too large is seen.

   In the other, referred to as the 'stateless' approach, for IPv4
   packets without the 'DF' bit set, too-large packets are fragmented,
   and then the fragments are forwarded; all other packets are
   discarded, and an ICMP Too Big message returned.

12.8.  Security of Mapping Lookups

   LISP provides an optional mechanism to secure the obtaining of
   mappings by an ITR.  [LISP-SEC] It provides protection against
   attackers generating spurious Map-Reply messages (including replaying
   old Map-Replies), and also against 'over-claiming' attacks (where a
   malicious ETR by claims EID-prefixes which are larger than what have
   been actually delegated to it).

   In summary, the ITR provides a One-Time Key with its Map-Request;
   this key is used by both the MS (to sign an affirmation that it has
   delegated that EID block to that ETR), and indirectly by the ETR (to
   sign the mapping that it is returning to the ITR).

   The specification for LISP-SEC suggests that the ITR-MR stage be
   cryptographically protected, and indicates that the existing
   mechanisms for securing the ETR-MS stage are used to protect Map-
   Rquests also.  It does assume that the channel from the MR to the MS
   is secure (otherwise an attacker could obtain the OTK from the Map-
   Request and use it to forge a reply).

12.9.  xTR Mapping Cache Performance

   As mentioned previously (Section 8.1.1 "Mapping Cache Performance"),
   a substantial amount of simulation work has been performed to
   predict, and understand, the performance of the "mapping cache" in
   xTRs.

   For a comprehensive survey of this work, see [Perspective], Section
   "Mapping Cache Performance", and the references; full details are too
   lengthy to include here.

   Briefly, however, the first, [Iannone], was performed in the very
   early stages of the LISP effort, to verify that that caching approach
   was feasible.

   Packet traces of all traffic over the external connection of a large
   university over a week-long period were collected; simulations driven
   by these recording were then performed.  A variety of control
   settings on the cache were used, to study the effects of varying the
   settings.

   First, the simulation gave the cache sizes that would result from
   such a cache design: it showed that the resulting cache sizes ranged
   from 7,500 entries, up to about 100,000 (depending on factors such as
   traffic and entry retention time).  Using some estimations as to how
   much memory mapping entries would use, this indicated cache sizes of
   between roughly 100 Kbytes and a few Mbytes.

   Of more interest, in a way, were the results regarding two important
   measurements of the effectiveness of the cache: i) the hit ratio
   (i.e. the share of references which could be satisified by the
   cache), and ii) the miss _rate_ (since control traffic overhead is
   one of the chief concerns when using a cache).  These results were
   also encouraging: miss (and hence lookup) rates ranged from 30 per
   minute, up to 3,000 per minute.

   Significantly, this was substantially lower than the amount of
   observed DNS traffic, which ranged from 1,800 packets per minute up
   to 15,000 per minute.  The results overall showed that using a
   demand-loaded cache was an entirely plausible design approach: both
   cache size, and the control plane traffic load, were definitely
   feasible.

   The second, [Kim], was in general terms similar, except that it used
   data from a large ISP, one with about three times as many users as
   the previous study.  It used the same cache design philosophy (the
   cache size was not fixed), but slightly different, lower, retention
   time values.

   The results were similar: cache sizes ranges from 20,000 entries to
   roughly 60,000; the miss rate ranged from very roughly 400 per minute
   to very roughly 7,000 per minute, similar to the previous results.

   Finally, a third study, [CorasCache], examined the effect of using a
   fixed size cache, and a purely Least Recently Used (LRU) cache
   eviction algorithm (i.e. no timeouts).  It also tried to verify that
   models of the performance of such a cache (using previous theoretical
   work on caches) produced results that conformed with actual empirical
   measurements.

   It used yet another set of packet traces; using a cache size of
   around 50,000 entries produced a miss rate of around 1x10-4; again,
   definitely viable, and in line with the results of the other studies.

13.  The Mapping System

   As discussed already in Section 8.2, "Control Plane - Mapping System
   Overview", the LISP "mapping system" is an important part of LISP's
   control plane: it i) maintains the database of "mappings" between
   EIDs, and the RLOCs at which they are to be found, and ii) provides
   those mappings to ITRs which request them, so that the ITRs can send
   traffic for a given EID to the correct RLOC(s) for that EID.

   RFC 1034 ("DNS Concepts and Facilities") has this to say about the
   DNS name to IP address database and mapping system:

     "The sheer size of the database and frequency of updates suggest
     that it must be maintained in a distributed manner, with local
     caching to improve performance. Approaches that attempt to
     collect a consistent copy of the entire database will become more
     and more expensive and difficult, and hence should be avoided."

   and this observation applies equally to the LISP mapping database and
   mapping system.

   To briefly recap, the mapping system is split into three parts: i) an
   "indexing sub-system", which keeps track of where all the mappings
   are kept; ii) the interface to the indexing system (which remains the
   same, even if the actual indexing system is changed); and iii) the
   mappings themselves (collectively, the "mapping database"), the
   authoritative copies of which are always held by ETRs.

13.1.  The Mapping System Interface

   As mentioned in Section 8.2.2, "Interface to the Mapping System",
   both of the inferfaces to the mapping system (from ITRs, and ETRs)
   are standardized, so that the more numerous xTRs do not have to be
   modified when the mapping indexing sub-system is changed.

   (This precaution has already allowed the mapping system to be
   upgraded during LISP's evolution, when ALT was replaced by DDT.)

   This section describes the interfaces in a little more detail; for
   details, see [RFC6833].

13.1.1.  Map-Request Messages

   The Map-Request message contains a number of fields, the two most
   important of which are the requested EID block identifier (remember
   that individual mappings may cover a block of EIDs, not just a single
   EID), and the Address Family Identifier (AFI) for that EID block.

   Other important fields are the source EID (and its AFI), and one or
   more RLOCs for the source EID, along with their AFIs. {{Not quite
   right, Dino will clarify. - Also two sets of RLOCs.}} Multiple RLOCs
   are included to ensure that at least one is in a form which will
   allow the reply to be returned to the requesting ITR, and the source
   EID is used for a variety of functions, including 'gleaning' (see
   Section 12.6, " Mapping Gleaning in ETRs").

   Finally, the message includes a long nonce, for simple, efficient
   protection against offpath attackers (see [Perspective], Section
   "Security-xTRs" for more), and a variety of other fields and control
   flag bits.

13.1.2.  Map-Reply Messages

   The Map-Reply message looks similar, except it includes the mapping
   entry for the requested EID(s), which contains one or more RLOCs and
   their associated data.  (Note that the reply may cover a larger block
   of the EID namespace than the request; most requests will be for a
   single EID, the one which prompted the query.)

   If there are no mappings available at all for the EID(s) requested, a
   'Negative Map-Reply' message will be returned.  This is a Map-Reply
   message with flag bits set to indicate that fact.

   For each RLOC in the entry, there is the RLOC, its AFI, priority and
   weight fields (see Section 8.2, "Control Plane - Mapping System
   Overview"), and multicast priority and weight fields (see Section 14,
   "Multicast Support in LISP" for more about multicast support in
   LISP).

13.1.2.1.  Solicit-Map-Request Messages

   "Solicit-Map-Request" (SMR) messages are actually not another message
   type, but a variant of Map-Request messages. {{Look at how probe is
   handled, do similar here - take out 'not xxx', say what they are.}}
   They include a special flag which indicates to the recipient that it
   _should_ send a new Map-Request message, to refresh its mapping,
   because the ETR has detected that the one it is using is out-dated.

   SMR's, like most other control traffic, is rate-limited.

13.1.3.  Map-Register and Map-Notify Messages

   The Map-Register message contains authentication information, and a
   number of mapping records, each with an individual Time-To-Live
   (TTL).  Each of the records contains an EID (potentially, a block of
   EIDs) and its AFI, a version number for this mapping (see
   Section 12.3.1, "Mapping Versioning"), and a number of RLOCs and
   their AFIs.

   Each RLOC entry also includes the same data as in the Map-Replies
   (i.e. priority and weight); this is because in some circumstances it
   is advantageous to allow the MS to proxy reply on the ETR's behalf to
   Map-Request messages, and the MS needs this information when it does
   so (see [Mobility]).

   Map-Notify messages have the exact same contents as Map-Register
   messages; they are purely acknowledgements (although planned LISP
   functionality extensions may give them other functions as well).

   The entire interaction can be authenticated by use of a shared key,
   configured in the MS and ETR.  Although the protocol does already
   allow for replacement of the encryption algorithm, it does not
   support automated key management (although it appears to fall under
   the exclusions in [RFC4107]).

   {{Deregistering??}}

13.2.  The DDT Indexing Sub-system

   As previously mentioned in Section 8.2.3, "Indexing Sub-system", the
   "indexing sub-system" in LISP is currently the DDT system.

   The overall functioning is conceptually fairly simple; an MR which
   needs a "mapping" starts at a server for the root "DDT vertex" (there
   will normally be more than one such server available, for both
   performance and robustness reasons), and through a combination of
   cached delegation information, and repetitive querying of a sequence
   of DDT servers, works its way down the delegation tree until it
   arrives at an MS which is authoritative (responsible?) for the block
   of EID namespace which holds the destination EID in question.

   The interaction between MRs and DDT servers is as follow.  The MR
   sends to the DDT server a Map-Request control message.  The DDT
   server uses its data (which is configured, and static) to see whether
   it is directly peered to an MS which can answer the request, or if it
   has a child (or children, if replicated) which is responsible for
   that portion of the EID namespace.

   If it has children configured which are responsible, it will reply to
   the MR with another kind of LISP control message, a Map-Referral
   message, which provides information about the delegation of the block
   containing the requested EID.  This step is secured; see
   Section 13.4, "Security of the DDT Indexing Sub-system", for more.

   The Map-Referral also gives the addresses of DDT servers for that
   block. and the MR can then send Map-Requests to any one (or all) of
   them.  In addition, the Map-Referral includes keying material for the
   children, which allows any information provided by them to be
   cryptographically verified.

   Control flags in the Map-Referral indicate to the querying MR whether
   the referral is to another DDT server, an MS, or an ETR. {{All three?
   Check}} If the former, the MR then sends the Map-Request to the child
   DDT server, repeating the process.

   If the second, the MR then interacts with that MS, and usually the
   block's ETR(s) as well, to cause a mapping to be sent to the ITR
   which queried the MR for it.  (Recall that some MS's provide Map-
   Replies on behalf of an associated ETR, in so-called 'proxy mode', so
   in such cases the Map-Reply will come from the MS, not the ETR. )

   Delegations are cached in the MRs, so that once an MR has received
   information about a delegation, it usually will not need to look that
   up again.  Once it has been in operation for a short while, there
   will usually only be a limited amount of delegation information which
   is has not yet asked about - probably only the last stage in a
   delegation to a 'leaf' MS.

   As describe below (Section 13.6, "Performance of the Mapping
   System"), an extensive modeling and performance evaluation has
   verified that DDT provides acceptable performance, as well as
   scalability.  [LISP-TREE]

13.2.1.  Map-Referral Messages

   Map-Referral messages look almost identical to Map-Reply messages,
   except that the RLOCs potentially name either i) the DDT servers for
   other DDT vertices (children in the delegation tree), or ii) terminal
   MSs.

13.3.  Reliability via Replication

   Everywhere throughout the mapping system, robustness to operational
   failures is obtained by replicating data in multiple instances of any
   particular node (of whatever type).  Map-Resolvers, Map-Servers, DDT
   nodes, ETRs - all of them can be replicated, and the protocol
   supports this replication.

   {{About replication - we don't talk about how that rep occurs}}
   {{Reliablity through rep is much sturdier - provide good ref}}

   There are generally no mechanisms specified yet to ensure coherence
   between multiple copies of any particular data item (e.g. the copies
   of delegation data for a particular block of namespace, in DDT
   sibling servers) - this is currently a manual responsibility.

   If and when LISP protocol adoption proceeds, an automated layer to
   perform this functionality can 'easily' be layered on top of the
   existing mechanisms.

   The deployed DDT system actually uses anycast [RFC4786], along with
   replicated servers, to improve both performance and robustness. {{Not
   just DDT, other places as well.}}

13.4.  Security of the DDT Indexing Sub-system

   In summary, securing the mapping indexing system is divided into two
   parts: the interface between the clients of the system (MR's) and the
   mapping indexing system itself, and the interaction between the DDT
   servers which make it up.

   The client interface provides only a single model, using the
   'canonical' public-private key system (starting from a trust anchor),
   in which the child's public key is provided by the parent, along with
   the delegation.  When the child returns any data, it can sign the
   data, and the requestor can use that signature to verify the data.
   This requires very little configuration in the clients.

   The interface between the DDT servers allows for choices between a
   number of different options, allowing the operators to trade off
   among configuration complexity, security level, etc.  This is based
   on experience with DNSSEC ([RFC4033]), where configuration complexity
   has been a major stumbling block to deployment.

   See [Perspective], Section "Security-Mappings" for more.

13.5.  Extended Capabilities

   In addition to the priority and weight data items in mappings, LISP
   offers other tools to enhance functionality, particularly in the
   traffic engineering area.

   One is 'requestor-specific mappings', i.e. the ETR may return
   different mappings to the enquiring ITR, depending on the identity of
   the ITR.  This allows very fine-tuned traffic engineering, far more
   powerful than routing-based TE. {{Policy-based?}}

13.6.  Performance of the Mapping System

   Prior to the creation of DDT, a large study of the performance of the
   previous mapping system, ALT ([ALT]), along with a proposed new
   design called TREE (which used DNS to hold delegation information)
   provided considerable insight into the likely performance of the
   mapping systems at larger scale.  (See [LISP-TREE], in particular
   Section V, "Mapping System Comparison".)

   The basic structure and concepts of DDT are identical to those of
   TREE, so the performance simulation work done for that design applies
   equally to DDT.

   In that study, as with earlier LISP performance analyses, extensive
   large-scale simulations were driven by lengthy recordings of actual
   traffic at several major sites; one was the site in the first study
   ([Iannone]), and the other was an even large university, with roughly
   35,000 users.

   The results showed that a system like DDT, which caches information
   about delegations, and allows the MR to communicate directly with the
   servers for the lower vertices on the delegation hierarchy based on
   cached delegation information, would have good performance, with
   average resolution times on the order of the MR to MS RTT.  This
   verified the effectiveness of this particular type of indexing
   system.

   A more recent study, [Saucez], has measured actual resolution times
   in the deployed LISP network; it took measurements from a variety of
   locations in the Internet, with respect to a number of different
   target EIDs.  Average measured resolution delays ranged from roughly
   175 msec to 225 msec, depending on the location.

14.  Multicast Support in LISP

   Multicast ([RFC3170], [RFC5110]) , since LISP is all about separating
   identity from location, and although a multicast group in some sense
   has an identity, it certainly does not have _a_ location.

   {{Say something about sources.}}

   Multicast is am important requirement, for a number of reasons: doing
   multiple unicast streams is inefficient, as it is easy to use up all
   the upstream bandwidth; without multicast a server can also be
   saturated fairly easily in doing the unicast replication; etc.

   Since it is important for LISP to work well with multicast; doing so
   has been a significant focus in LISP throughout its entire
   development.

   Further very significant improvements to multicast support in LISP
   are in progress; see [Improvements], Section "Multicast" for more on
   them.

14.1.  Basic Concepts of Multicast Support in LISP

   This section introduces some of the basic principles of multicast
   support in LISP.

   Since group addresses name distributed collective entities, in
   general they cannot have a single RLOC (although they may, after
   future improvements in multicast support in LISP, have multiple
   RLOCs); also, since they usually refer to collections of entities,
   they aren't really EIDs either.

   A multicast source at a LISP site may not be able to become the root
   of a distribution tree in the core if it uses its EID as its identity
   for that distribution tree (i.e. a distribution tree (S-EID, G));
   that is because there may not be a route to its EID in the core
   (assuming that its section of the core even supports multicast; not
   all parts of the core do).

   Therefore, outside the LISP site, multicast state for the
   distribution tree (S-RLOC, G) needs to be built instead, where S-RLOC
   is the RLOC of the ITR that the multicast source inside the LISP site
   will be sending its traffic through.

   Multicast LISP requires no packet format changes to existing
   multicast packets (both control, and user data).  The initial
   multicast support in LISP uses existing multicast control mechanisms
   exclusively; improvements currently being worked on provide LISP-
   specific control mechanisms (see [Improvements], Section "Multicast",
   for more).

14.2.  Initial Multicast Support in LISP

   Readers who wish to fully understand multicast support need to
   consult the appropriate specifications: LISP multicast issues are
   discussed in [RFC6830], Section 11; and see [RFC6831] for the full
   details of current multicast support in LISP.

   In the current simple operating mode (covered in [RFC6831]),
   destination group addresses are not mapped; only the source address
   (when the original source is inside a LISP site) needs to be mapped,
   both during distribution tree setup, as well as actual traffic
   delivery.

   In other words, while LISP's mapping capability is used, at this
   stage it is only applied to the source, not the destination (as with
   most LISP activity).  Thus, in LISP-encapsulated multicast packets in
   this mode, the inner source is the EID, and the outer source is the
   ITR's RLOC; both inner and outer destinations are the group's
   multicast address.

   Note that this does mean that if the group is using separate source-
   specific trees for distribution, there isn't a separate distribution
   tree outside the LISP site for each different source of traffic to
   the group from inside the LISP site; they are all lumped together
   under a single source, the RLOC.

   The issue of encapsulation is complex, because if the rest of the
   group outside the LISP site includes some members which are at other
   LISP sites (i.e. packets to them have to be encapsulated), and some
   members at legacy sites (i.e. encapsulated packets would not be
   understood), there is no simple answer.  (The situation becomes even
   more complex when one considers that as hosts leave and joint the
   group, it may switch back and forth between 'mixed' and
   'homogenous'.)

   This issue is too complex to fully cover here; see Section 9.2.,
   "LISP Sites with Mixed Address Families", in [RFC6831], for complete
   coverage of this issue.

   Basically, there are multicast equivalents of some of the legacy
   interoperability mechanisms used for unicast; mPITRs and mPETRs
   (multicast-capable PITRs and PETRs) etc.  When 'mixed' groups are a
   possibility, two choices are available: i) send two copies (one
   encapsulated, and one not) of all traffic, or ii) employ mPETRs to
   distribute non-encapsulated copies to 'legacy' group members.

15.  Deployment Issues and Mechanisms

   This section discusses several deployment issues in more detail.
   With LISP's heavy emphasis on practicality, much work has gone into
   making sure it works well in the real-world environments most people
   have to deal with.

15.1.  LISP Deployment Needs

   As mentioned earlier (Section 5.2, "Maximize Re-use of Existing
   Mechanism"), LISP requires no change to almost all existing hosts and
   routers.  Obviously, however, one must deploy _something_ to run
   LISP!  Exactly what that has to be will depend greatly on the details
   of the site's existing networking gear, and choices it makes for how
   to achieve LISP deployment.

   The primary requirement is for one or more xTRs.  These may be
   existing routers, just with new software loads, or it may require the
   deployment of new devices.

   LISP also requires a certain amount of LISP-specific support
   infrastructure, such as MRs, MSs, the DDT hierarchy, etc.  However,
   much of this will either i) {{for the case where you are adding a new
   site using existing LISP infrastructure}} already be deployed, and if
   the new site can make arrangements to use it, it need do nothing
   else; or ii) those functions the site must provide may be co-located
   in other LISP devices (again, either new devices, or new software on
   existing ones).

15.2.  Interworking Mechanisms

   One aspect which has received a lot of attention are the mechanisms
   previously referred to (in Section 6.4, "Interworking With Non-LISP-
   Capable Endpoints") to allow interoperation of LISP sites with so-
   called 'legacy' sites which are not running LISP (yet).

   There are two main approaches to such interworking: proxy routers
   (PITRs and PETRs), and an alternative mechanism using a router with
   combined NAT and LISP functionality; these are described in more
   detail here.

15.2.1.  Proxy LISP Routers

   PITRs (proxy ITRs) serve as ITRs for traffic _from_ legacy hosts to
   nodes in LISP sites.  PETRs (proxy ETRs) serve as ETRs for LISP
   traffic _to_ legacy hosts (for cases where a LISP node cannot send
   packets directly to such hosts, without encapsulation).

   Note that return traffic _to_ a legacy host from a LISP-using node
   does not necessarily have to pass through an ITR/PETR pair - the
   original packets can usually just be sent directly to the ultimate
   destination.  However, for some kinds of LISP operation (e.g. mobile
   nodes), this is not possible; in these situations, the PETR is
   needed.

15.2.1.1.  PITRs

   To serve as ITRs for traffic _from_ legacy hosts to nodes in LISP
   sites, PITRs they have to advertise into the existing legacy backbone
   Internet routing the availability of whatever ranges of EIDs (i.e. of
   nodes using LISP) they are proxying for, so that legacy hosts will
   know where to send traffic to those LISP nodes.

   This technique obviously has an impact on routing table in the
   "Internet core", but it is not clear yet exactly what that impact
   will be; it is very dependent on the collected details of many
   individual deployment decisions. {{Check on text elsewhere for
   effects on routing table size, specifically advertizement of large
   blocks.}}

   A PITR may cover a group of EID blocks with a single EID
   advertisement to the core, in order to reduce the number of routing
   table entries added.  (In fact, at the moment, aggressive aggregation
   of EID announcements is performed, precisely to to minimize the
   number of new announced routes added by this technique.) {{BGP tools
   can be used to restrict the direction and scope of these
   advertisements.}}

   At the same time, if a site does traffic engineering with LISP
   instead of fine-grained BGP announcement, that will help keep table
   sizes down (and this is true even in the early stages of LISP
   deployment).  The same is true for multi-homing. {{Maybe mixing two
   concepts?  LISP TE tools will still apply to traffic between PITR and
   LISP site.}}

   {{Maybe reword, as we changed the target section.}} As mentioned
   previously (Section 12.1, "When to Encapsulate"), an ITR at another
   LISP site can avoid using a PITR (i.e. it can detect that a given
   ultimate destination is not a legacy host, if a PITR is advertising
   it into the "Internet core") by checking to see if a LISP mapping
   exists for that ultimate destination.

15.2.1.2.  PETRs

   PETRs (proxy ETRs) serve as ETRs for LISP traffic _to_ legacy hosts,
   for cases where a LISP node cannot send packets to such hosts without
   encapsulation.  That typically happens for one of two reasons.

   First, it will happen in places where some device is implementing
   Unicast Reverse Path Forwarding (uRPF), to prevent a variety of
   negative behaviour; originating packets with the original source's
   EID in the source address field will result in them being filtered
   out and discarded.

   Second, it will happen when a LISP site wishes to send packets to a
   non-LISP site, and the path in between does not support the
   particular IP protocol version used by the original source along its
   entire length.  Use of a PETR on the other side of the 'gap' will
   allow the LISP site's packet to 'hop over' the gap, by utilizing
   LISP's built-in support for mixed protocol encapsulation.

   PETRs are generally used by specific ITRs, which have the location of
   their PETRs configured into them.  In other words, unlike normal
   ETRS, PETRs do not have to register themselves in the mapping
   database, on behalf of any legacy sites they serve.

   Also, allowing an ITR to always send traffic leaving a site to a PETR
   does avoid having to chose whether or not to encapsulate packets; it
   can just always encapsulate packets, sending them to the PETR if it
   has no specific mapping for the ultimate destination.  However, this
   is not advised: as mentioned, it is easy to tell if something is a
   legacy destination.

15.2.2.  LISP-NAT

   A LISP-NAT router, as previously mentioned, combines LISP and NAT
   functionality, in order to allow a LISP site which is internally
   using addresses which cannot be globally routed to communicate with
   non-LISP sites elsewhere in the Internet.  (In other words, the
   technique used by the PITR approach simply cannot be used in this
   case.)

   To do this, a LISP-NAT performs the usual NAT functionality, and
   translates a host's source address(es) in packets passing through it
   from an 'inner' value to an 'outer' value, and storing that
   translation in a table, which it can use to similarly process
   subsequent packets (both outgoing and incoming).  [RFC6832]

   There are two main cases where this might apply:
   -  Sites using non-routable global addresses
   -  Sites using private addresses [RFC1918]

15.3.  Use Through NAT Devices

   NATs are both ubiquitous, and here to stay for a long time to come.
   [RFC1631] Thus, in the actual Internet of today, having any new
   mechanisms function well in the presence of NATs (i.e. with LISP xTRs
   behind a NAT device) is absolutely necessary.

   LISP has produced a variety of mechanisms to do this.  An
   experimental mechanism to support them had major limitations; it, and
   its limitations, are described in Appendix B.5, "Early NAT Support".
   A more recent proposed mechanism, which avoids those limitations, is
   described in [Improvements], Section "Improved NAT Support".

15.4.  LISP and Core Internet Routing

   One of LISP's original motivations was to try and control the growth
   of the size of routing tables in the Internet core, the part where
   routes to _all_ destinations must be available.  As LISP becomes more
   widely deployed, it can help with this issue, in a variety of ways.
   {{Give ref for why large rout tables bad.}}

   {{Does applications make forward ref to this section?}}

   In covering this topic, one must recognize that conditions in various
   stages of LISP deployment (in terms of ubiquity) will have a large
   influence.  [Deployment] introduced useful terminology for this
   progression, in addition to some coverage of the topic (see Section
   5, "Migration to LISP"):

     The loosely defined terms of "early transition phase", "late
     transition phase", and "LISP Internet phase" refer to time periods
     when LISP sites are a minority, a majority, or represent all edge
     networks respectively.

   In the early phases of deployment, two primary effects will allow
   LISP to have a positive impact on the routing table growth:

   -  Using LISP for traffic engineering instead of BGP
   -  Aggregation of smaller PI sites into a single PITR advertisement

   The first is fairly obvious (doing TE with BGP requires injecting
   more-specific routes into the "Internet core" routing tables,
   something doing TE with LISP avoids); the second is not guaranteed to
   happen (since it requires coordination among a number of different
   parties), and only time will tell if it does happen.

   {{Add xref to text moved to "Improvments" document.}}

16.  Fault Discovery/Handling

   The structure of LISP gives rise to a moderate number of of failure
   modes.

16.1.  Handling Missing Mappings

   To handling missing mappings, the ITR calls for the mapping, and in
   the meantime can either discard traffic to that ultimate destination
   (as many ARP implementations do) [RFC826], or, if dropping the
   traffic is deemed undesirable, it can forward them via a PITR.

16.2.  Outdated Mappings

   If a mapping changes once an ITR has retrieved it, that may result in
   traffic to the EIDs covered by that mapping failing.  There are three
   cases to consider:

   -  When the ETR to which traffic is being sent is still a valid ETR
      for that EID, but the mapping has been updated (e.g. to change the
      priority of various ETRs)
   -  When the ETR traffic is being sent to is still an ETR, but no
      longer a valid ETR for that EID
   -  When the ETR traffic is being sent to is no longer an ETR
   -  {{No longer an ETR, but still a LISP node - another case to
      consider.}}

16.2.1.  Outdated Mappings - Updated Mapping

   A 'mapping versioning' system, whereby mappings have version numbers,
   and ITRs are notified when their mapping is out of date, has been
   added to detect this, and the ITR responds by refreshing the mapping.
   [RFC6834]

16.2.2.  Outdated Mappings - Wrong ETR

   If an ITR is holding an outdated cached mapping, it may send packets
   to an ETR which is no longer an ETR for that EID.

   It might be argued that if the ETR is properly managing the lifetimes
   on its mapping entries, this 'cannot happen', but it is a wise design
   methodology to assume that 'cannot happen' events will in fact happen
   (as they do, due to software errors, or, on rare occasions, hardware
   faults), and ensure that the system will handle them properly (if,
   perhaps not in the most expeditious, or 'clean' way - they are, after
   all, very unlikely to happen). {{Make less run on, easier to
   understand.}}

   ETRs can easily detect cases where this happpens, after they have un-
   wrapped a user data packet; in response, they send a Solicit-Map-
   Request to the source ITR to cause it to refresh its mapping.

16.2.3.  Outdated Mappings - No Longer an ETR

   In another case for what can happen if an ITR uses an outdated
   mapping, the destination of traffic from an ITR might no longer be a
   LISP node at all.  In such cases, one might get an ICMP Destination
   Unreachable (Port Unreachble subtype) error message.  However, one
   cannot depend on that - and in any event, that would provide an
   attack vector, so it should be used with care.  (See [RFC6830],
   Section 6.3, "Routing Locator Reachability" for more about this.)

   The following mechanism will work, though.  Since the destination is
   not an ETR, the echoing reachability detection mechanism (see
   Section 12.3.2, "Echo Nonces") will detect a problem.  At that point,
   the backstop mechanism, Probing, will kick in.  Since the destination
   is still not an ETR, that will fail, too.

   At that point, traffic will be switched to a different ETR, or, if
   none are available, a reload of the mapping may be initiated.

16.3.  Erroneous Mappings

   Again, this 'should not happen', but a good system should deal with
   it.  However, in practise, should this happen, it will produce one of
   the prior two cases (the wrong ETR, or something that is not an ETR),
   and will be handled as described there.

16.4.  Verifying ETR Liveness

   The ITR, like all packet switches, needs to detect, and react, when
   its neighbour ceases operation.  As LISP traffic is effectively
   always uni-directional (from ITR to ETR), this could be somewhat
   problematic.

   Solving a related problem, "neighbour ETR" "reachability" below)
   subsumes handling this fault mode, however.

   Note that the two terms - "liveness" and "reachability" - are _not_
   synonmous (although they are often confused).  Liveness is a property
   of a node - it is either up and functioning, or it is not.
   Reachability is only a property of a particular _pair_ of nodes.
   {{Really property of path - if only one path, property of pair,
   otherwise of path.}}

   If packets sent from a first node to a second are successfully
   received at the second, it is 'reachable' from the first.  However,
   the second node may at the very same time _not_ be reachable from
   some other node.  Reachability is _always_ a ordered pairwise
   property, and of a specified ordered pair.

16.5.  Verifying ETR Reachability

   A more significant issue than whether a particular ETR is up or not
   is, as mentioned above, that although the ETR may be up, attached to
   the network, etc, an issue in the network, between a source ITR, and
   the ETR, may prevent traffic from the ITR from getting to the ETR.
   (Perhaps a routing problem, or perhaps some sort of access control
   setting.)

   The one-way nature of LISP traffic makes this situation hard to
   detect in a way which is economic, robust and fast.  Two out of the
   three are usually not to hard, but all three at the same time - as is
   highly desirable for this particular issue - are harder.

   In line with the LISP design philosophy ([Perspective], Section
   "Design-Theoretical"), this problem is attacked not with a single
   mechanism (which would have a hard time meeting all those three goals
   simultaneously), but with a collection of simpler, cheaper
   mechanisms, which collectively will usually meet all three.

   They are reliance on the underlying routing system (which can of
   course only reliably provide a negative reachabilty indication, not a
   positive one), the echo nonce (which depends on some return traffic
   from the destination xTR back to the source xTR), and finally direct
   'pinging', in the case where no positive echo is returned.

   (The last is not the first choice, as due to the large fan-out
   expected of LISP router, reliance on it as a sole mechanism would
   produce a fair amount of overhead.)

17.  Acknowledgments

   The author would like to start by thanking all the members of the
   core LISP group for their willingness to allow him to add himself to
   their effort, and for their enthusiasm for whatever assistance he has
   been able to provide.

   He would also like to thank (in alphabetical order) Michiel Blokzijl,
   Peter Chiappa, Vina Ermagan, Dino Farinacci, Vince Fuller and
   Vasileios Lakafosis for their review of, and helpful suggestions for,
   this document.  (If I have missed anyone in this list, I apologize
   most profusely.)

   A special thank you goes to Joel Halpern, who almost invariably, when
   asked, promptly returned comments on intermediate versions of this
   document.  Grateful thanks go also to Darrel Lewis for his help with
   material on non-Internet uses of LISP, and to Dino Farinacci and
   Vince Fuller for answering detailed questions about some obscure LISP
   topics.

   A final thanks is due to John Wrocklawski for the author's
   organizational affiliation, and to Vince Fuller for help with XML.
   This memo was created using the xml2rfc tool.

   I would like to dedicate this document to the memory of my parents,
   who gave me so much, and whom I can no longer thank in person, as I
   would have so much liked to be able to.

18.  IANA Considerations

   This document makes no request of the IANA.

19.  Security Considerations

   This memo does not define any protocol and therefore creates no new
   security issues.

20.  References

20.1.  Normative References

   [AFI]            IANA, "Address Family Indicators (AFIs)", Address
                    Family Numbers, January 2011, <http://www.iana.org/
                    assignments/address-family-numbers>.

   [RFC768]         J. Postel, "User Datagram Protocol", RFC 768,
                    August 1980.

   [RFC791]         J. Postel, "Internet Protocol", RFC 791,
                    September 1981.

   [RFC2460]        S. Deering and R. Hinden, "Internet Protocol,
                    Version 6 (IPv6) Specification", RFC 2460,
                    December 1998.

   [RFC6830]        D. Farinacci, V. Fuller, D. Meyer, and D. Lewis,
                    "The Locator/ID Separation Protocol (LISP)",
                    RFC 6830, January 2013.

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

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

   [RFC6833]        V. Fuller and D. Farinacci, "Locator/ID Separation
                    Protocol (LISP) Map-Server Interface", RFC 6833,
                    January 2013.

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

   [Perspective]    J. N. Chiappa, "An Architectural Perspective on the
                    LISP Location-Identity Separation System",
                    draft-ietf-lisp-perspective-00 (work in progress),
                    February 2013.

   [Improvements]   J. N. Chiappa, "An Overview of On-Going Improvements
                    to the LISP Location-Identity Separation System",
                    draft-chiappa-lisp-improvements-00 (work in
                    progress), September 2013.

   [DDT]            V. Fuller, D. Lewis, and D. Farinacci, "LISP
                    Delegated Database Tree", draft-ietf-lisp-ddt-01
                    (work in progress), March 2013.

   [LISP-SEC]       F. Maino, V. Ermagan, A. Cabellos-Aparicio,
                    D. Saucez, and O. Bonaventure, "LISP-Security (LISP-
                    SEC)", draft-ietf-lisp-sec-04 (work in progress),
                    October 2012.

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

   [Mobility]       D. Farinacci, V. Fuller, D. Lewis, and D. Meyer,
                    "LISP Mobility Architecture", draft-meyer-lisp-mn-08
                    (work in progress), April 2012.

   [Deployment]     L. Jakab, A. Cabellos-Aparicio, F. Coras,
                    J. Domingo-Pascual, and D. Lewis, "LISP Network
                    Element Deployment Considerations",
                    draft-ietf-lisp-deployment-09 (work in progress),
                    July 2013.

   [Threats]        D. Saucez, L. Iannone, and O. Bonaventure, "LISP
                    Threats Analysis", draft-ietf-lisp-threats-08 (work
                    in progress), October 2013.

   [LCAF]           D. Farinacci, D. Meyer, and J. Snijders, "LISP
                    Canonical Address Format (LCAF)",
                    draft-ietf-lisp-lcaf-03 (work in progress),
                    September 2013.

   [LISP-TE]        D. Farinacci, P. Lahiri, and M. Kowal, "LISP Traffic
                    Engineering Use-Cases", draft-farinacci-lisp-te-03
                    (work in progress), July 2013.

20.2.  Informative References

   [NIC8246]        A. McKenzie and J. Postel, "Host-to-Host Protocol
                    for the ARPANET", NIC 8246, Network Information
                    Center, SRI International, Menlo Park, CA,
                    October 1977.

   [NSAP]           International Organization for Standardization,
                    "Information Processing Systems - Open Systems
                    Interconnection - Basic Reference Model", ISO
                    Standard 7489.1984, 1984.

   [IEN19]          J. F. Shoch, "Inter-Network Naming, Addressing, and
                    Routing", IEN (Internet Experiment Note) 19,
                    January 1978.

   [RFC826]         D. Plummer, "Ethernet Address Resolution Protocol",
                    RFC 826, November 1982.

   [RFC1034]        P. V. Mockapetris, "Domain Names - Concepts and
                    Facilities", RFC 1034, November 1987.

   [RFC1498]        J. H. Saltzer, "On the Naming and Binding of Network
                    Destinations", RFC 1498, (Originally published in:
                    'Local Computer Networks', edited by P. Ravasio et
                    al., North-Holland Publishing Company, Amsterdam,
                    1982, pp. 311-317.), August 1993.

   [RFC1631]        K. Egevang and P. Francis, "The IP Network Address
                    Translator (NAT)", RFC 1631, May 1994.

   [RFC1812]        F. Baker, "Requirements for IP Version 4 Routers",
                    RFC 1812, June 1995.

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

   [RFC1992]        I. Castineyra, J. N. Chiappa, and M. Steenstrup,
                    "The Nimrod Routing Architecture", RFC 1992,
                    August 1996.

   [RFC3168]        K. Ramakrishnan, S. Floyd, and D. Black, "The
                    Addition of Explicit Congestion Notification (ECN)
                    to IP", RFC 3168, September 2001.

   [RFC3170]        B. Quinn and K. Almeroth, "IP Multicast
                    Applications: Challenges and Solutions", RFC 3170,
                    September 2001.

   [RFC3272]        D. Awduche, A. Chiu, A. Elwalid, I. Widjaja, and
                    X. Xiao, "Overview and Principles of Internet
                    Traffic Engineering", RFC 3272, May 2002.

   [RFC4026]        L. Andersson and T. Madsen, "Provider Provisioned
                    Virtual Private Network (VPN) Terminology",
                    RFC 4026, March 2005.

   [RFC4033]        R. Arends, R. Austein, M. Larson, D. Massey, and
                    S. Rose, "DNS Security Introduction and
                    Requirements", RFC 4033, March 2005.

   [RFC4107]        S. Bellovin and R. Housley, "Guidelines for
                    Cryptographic Key Management", RFC 4107, June 2005.

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

   [RFC4786]        J. Abley and K. Lindqvist, "Operation of Anycast
                    Services", RFC 4786, December 2006.

   [RFC4984]        D. Meyer, L. Zhang, and K. Fall, "Report from the
                    IAB Workshop on Routing and Addressing", RFC 4984,
                    September 2007.

   [RFC5110]        P. Savola, "Overview of the Internet Multicast
                    Routing Architecture", RFC 5110, January 2008.

   [RFC5887]        B. Carpenter, R. Atkinson, and H. Flinck,
                    "Renumbering Still Needs Work", RFC 5887, May 2010.

   [RFC6115]        T. Li, Ed., "Recommendation for a Routing
                    Architecture", RFC 6115, February 2011.

                    (Perhaps the most ill-named RFC of all time; it
                    contains nothing that could truly be called a
                    'routing architecture'.)

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

   [LISP0]          D. Farinacci, V. Fuller, and D. Oran, "Locator/ID
                    Separation Protocol (LISP)", draft-farinacci-lisp-00
                    (work in progress), January 2007.

   [Future]         J. N. Chiappa, "Potential Long-Term Developments
                    With the LISP System",
                    draft-chiappa-lisp-evolution-00 (work in progress),
                    October 2012.

   [Baran]          P. Baran, "On Distributed Communications Networks",
                    IEEE Transactions on Communications Systems Vol.
                    CS-12 No. 1, pp. 1-9, March 1964.

   [Chiappa]        J. N. Chiappa, "Endpoints and Endpoint Names: A
                    Proposed Enhancement to the Internet Architecture",
                    Personal draft (work in progress), 1999,
                    <http://www.chiappa.net/~jnc/tech/endpoints.txt>.

   [Clark]          D. D. Clark, "The Design Philosophy of the DARPA
                    Internet Protocols", in 'Proceedings of the
                    Symposium on Communications Architectures and
                    Protocols SIGCOMM '88', pp. 106-114, 1988.

   [Saltzer]        J. H. Saltzer, D. P. Reed, and D. D. Clark, "End-To-
                    End Arguments in System Design", ACM TOCS, Vol 2,
                    No. 4, pp 277-288, November 1984.

   [Heart]          F. E. Heart, R. E. Kahn, S. M. Ornstein,
                    W. R. Crowther, and D. C. Walden, "The Interface
                    Message Processor for the ARPA Computer Network",
                    Proceedings AFIPS  1970 SJCC, Vol. 36, pp. 551-567.

   [Iannone]        L. Iannone and O. Bonaventure, "On the Cost of
                    Caching Locator/ID Mappings", in 'Proceedings of the
                    3rd International Conference on emerging Networking
                    EXperiments and Technologies (CoNEXT'07)', ACM, pp.
                    1-12, December 2007.

   [Kim]            J. Kim, L. Iannone, and A. Feldmann, "A Deep Dive
                    Into the LISP Cache and What ISPs Should Know About
                    It", in 'Proceedings of the 10th International IFIP
                    TC 6 Conference on Networking - Volume Part I
                    (NETWORKING '11)', IFIP, pp. 367-378, May 2011.

   [CorasCache]     F. Coras, A. Cabellos-Aparicio, and J. Domingo-
                    Pascual, "An Analytical Model for the LISP Cache
                    Size", in 'Proceedings of the 11th International
                    IFIP TC 6 Networking Conference: Part I', IFIP, pp.
                    409-420, May 2012.

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

   [Saucez]         D. Saucez, L. Iannone, and B. Donnet, "A First
                    Measurement Look at the Deployment and Evolution of
                    the Locator/ID Separation Protocol", in 'ACM SIGCOMM
                    Computer Communication Review', Vol. 43 No. 2, pp.
                    37-43, April 2013.

   [CorasBGP]       F. Coras, D. Saucez, L. Jakab, A. Cabellos-Aparicio,
                    and J. Domingo-Pascual, "Implementing a BGP-free ISP
                    Core with LISP", in 'Proceedings of the Global
                    Communications Conference (GlobeCom)', IEEE, pp.
                    2772-2778, December 2012.

   [Atkinson]       R. Atkinson, "Revised draft proposed definitions",
                    RRG list message, Message-Id: 808E6500-97B4-4107-
                    8A2F-36BC913BE196@extremenetworks.com, 11 June 2007,
                    <http://www.ietf.org/mail-archive/web/ram/current/
                    msg01470.html>.

   [Bibliography]   J. N. Chiappa (editor), "LISP (Location/Identity
                    Separation Protocol) Bibliography", Personal
                    site (work in progress), July 2013, <http://
                    www.chiappa.net/~jnc/tech/lisp/LISPbiblio.html>.

Appendix A.  Glossary/Definition of Terms

   -  EID, Enpoint Identifier: Originally defined as a name for an
      "endpoint", one with purely identity semantics, and globally
      unique, and with syntax of relatively short fixed length.
      [Chiappa] It is used in the LISP work to mean the "identifier" of
      a "node"; it is the input to an EID->RLOC lookup in the "mapping
      system"; it is usually an "IPvN" "address".  The source and
      destination addresses of the _innermost_ header in a LISP packet
      are usually EIDs.
   -  RLOC, Routing Locator: a LISP-specific term meaning the "locator"
      associated with an entity identified by an EID; as such, it is
      often the output of an EID->RLOC lookup in the "mapping system";
      it is usually an "IPvN" address, and of an "ETR".  The source and
      adestination addresses of the _outermost_ header in a LISP packet
      are usually RLOCs.
   -  ITR, Ingress Tunnel Router: a "LISP router" at the border of a
      "LISP site" which takes user packets sent to it from inside the
      LISP site, encapsulates in a LISP header, and then sends them
      across the Internet to an "ETR"; in other words, the start of a
      'tunnel' from the ITR to an ETR.
   -  ETR: Egress Tunnel Router: a "LISP router" at the border of a
      "LISP site" which decapsulates user packets which arrive at it
      encapsulated in a LISP header, and sends them on towards their
      ultimate destination; in other words, the end of the 'tunnel' from
      an "ITR" to the ETR.
   -  Neighbour ETR: Although an "ITR" and "ETR" may be separated by
      many actual physical hops, _at the LISP level_, they are direct
      neighbours; so any ETR which an ITR sends traffic to is a
      'neighbour ETR' of that ITR.
   -  xTR: An xTR refers to a "LISP router" which functions both as an
      "ITR" and an "ETR" (which is typical), when the discussion
      involves packet flows in both directions through the router, which
      results in it alternately functioning as an ITR and then as an
      ETR.
   -  Reachable; Reachability; Neighbour ETR Reachability: The ability
      of an "ITR" to be able to send packets to a "neighbour ETR", or
      the property of an ITR to be able to send such packets.
   -  Liveness: Whether a LISP "node" of any kind is 'up' and operating,
      or not; or the property of a LISP node to be in such a state.
   -  MR, Map Resolver: A LISP "node" to which "ITRs' send requests for
      "mappings".  See Section 8.2.2, "Interface to the Mapping System",
      for more.
   -  MS, Map Server: A LISP "node" with which "ETRs" register
      "mappings", to indicate their availability to handle incoming
      traffic to the "EIDs" covered in those mappings.  See
      Section 8.2.2, "Interface to the Mapping System" for more.
   -  Mapping System: The entire ensemble of data and mechanisms which
      allow clients - usually "ITRs" - to find "mappings" (from EIDs to
      RLOCs).  It includes both the "mapping database", and also
      everything used to gain access to it - the MRs, the "indexing sub-
      system", etc.  See Section 8.2.1, "Mapping System Organization"
      for more.
   -  Mapping Database: The term 'mapping database' refers to the entire
      collection of {EID->RLOC} "mappings" spread throughout the entire
      LISP system.  It is a subset of the "mapping system".  See
      Section 8.2, "Control Plane - Mapping System Overview", for more.
   -  Mapping Cache: A collection of copies of {EID->RLOC} "mappings"
      retained in an ITR; not the entire "mapping database", but just
      the subset of it that an ITR needs in order to be able to properly
      handle the user data traffic which is flowing through it.
   -  Indexing Sub-system: the entire ensemble of data and mechanisms
      which allows "MRs" to find out which "ETR(s)" hold the mappping
      for a given "EID" or "EID block".  It includes both the data on
      "namespace" delegations, as well as the nodes which hold that
      data, and the protocols used to interact with those nodes.  See
      Section 8.2.1, "Mapping System Organization" for more.
   -  DDT Vertex; Vertex: a node (in the graph theory sense of the term)
      in the (abstract) LISP namespace "delegation hierarchy".
   -  DDT Server: an actual machine, which one can send packets to, in
      the DDT server hierarchy - which is, hopefully, a one-to-one
      projection of the LISP address "delegation hierarchy" (although of
      course a single "DDT vertex" may turn into several sibling
      servers).  Some documents refer to these as 'DDT nodes' but this
      document does not use that term, to prevent confusion with "DDT
      vertex".
   -  PITR: Proxy ITR; an "ITR" which is used for interworking between a
      LISP-speaking "node" or "site", and legacy nodes or sites; in
      general, it acts like a normal ITR, but does so on behalf of LISP
      nodes which are receiving packets from a legacy node.  See
      Section 15.2.1.1, "PITRs", for more.
   -  PETR: Proxy ETR; an "ETR" which is used for interworking between a
      LISP-speaking "node" or "site", and legacy nodes or sites; in
      general, it acts like a normal ETR, but does so on behalf of LISP
      nodes which are sending packets to a legacy node.  See
      Section 15.2.1.2, "PETRs" for more.
   -  RTR: Re-encapsulating Tunnel Router; a data plane 'anchor point'
      used by a LISP-speaking node to perform functions that can only be
      be performed in the core of the network.  One use is for LISP-
      speaking node behind a NAT device to send and receive traffic
      through the NAT device; see [Improvements], Section "Improved NAT
      Support" for more.
   -  Internet core: That part of the Internet in which there are no
      'default' entries in routing tables, but where the routing tables
      hold entries for every single reachable destination in the
      Internet.  (Sometimes referred to colloquially as the 'DFZ', or
      'Default Free Zone'.)

Appendix B.  Other Appendices

B.1.  A Brief History of Location/Identity Separation

   It was only gradually realized in the networking community that
   networks (especially large networks) should deal quite separately
   with the identity and location of a node; the distinction between the
   two was more than a little hazy at first.

   The ARPANET had no real acknowledgment of the difference between the
   two.  [Heart] [NIC8246] The early Internet also co-mingled the two
   ([RFC791]), although there was recognition in the early Internet work
   that there were two different things going on.  [IEN19]

   This likely resulted not just from lack of insight, but also the fact
   that extra mechanism is needed to support this separation (and in the
   early days there were no resources to spare), as well as the lack of
   need for it in the smaller networks of the time.  (It is a truism of
   system design that small systems can get away with doing two things
   with one mechanism, in a way that usually will not work when the
   system gets much larger.)

   The ISO protocol architecture took steps in this direction [NSAP],
   but to the Internet community the necessity of a clear separation was
   definitively shown by Saltzer.  [RFC1498] Later work expanded on
   Saltzer's, and tied his separation concepts into the fate-sharing
   concepts of Clark.  [Clark], [Chiappa]

   The separation of location and identity is a step which has recently
   been identified by the IRTF as a critically necessary evolutionary
   architectural step for the Internet.  [RFC6115] However, it has taken
   quite some time for this requirement to be generally accepted by the
   Internet engineering community at large, although it seems that this
   may finally be happening.

   Unfortunately, although the development of IPv6 presented a golden
   opportunity to learn from this particular failing of IPv4, that
   design failed to recognize the need for separation of location and
   identity.

B.2.  A Brief History of the LISP Project

   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.  [LISP0]

   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).

B.3.  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
   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.

B.4.  The ALT Mapping Indexing Sub-system

   LISP initially used an indexing sub-system called ALT.  [ALT] ALT re-
   purposed a number of existing mechanisms to provide an indexing
   system, which allowed an experimental LISP initial deployment to
   become operational without having to write a lot of code, ALT was
   relatively easily constructed from basically unmodified existing
   mechanisms; it used BGP running over virtual tunnels using GRE.

   ALT proved to have a number of issues which made it unsuitable for
   large-scale use, and it has now been superseded by DDT.  A complete
   list of these is not possible here, but the issues mostly were of two
   kinds: technical issues which would have arisen at large scale, and
   practical operational issues which appeared even in the experimental
   deployment.

   The biggest operational issues was the effort involved in
   configuring, and maintain the configuration, of the virtual tunnels
   over which ALT ran (including assigning the addresses for the ends,
   etc); also, managing the multiple disjoint routing tables required
   was difficult and confusing (even for those who were very familiar
   with ALT).  Debugging faults in ALT was also difficult; and finally,
   because of ALT's nature, administrative issues (who pays for what,
   who controls what, etc) were problematic.

   However, ALT would have had significant technical issues had it been
   used at a larger scale.

   The most severe (and fundamental) issue was that since all traffic on
   the ALT had to transit the 'root' of the ALT tree, those locations
   would have become traffic 'hot-spots' in a large scale deployment.

   In addition, optimal performance would have required that the ALT
   overall topology be restrained to follow the EID namespace
   allocation; however, it was not clear that this was feasible.  In any
   event, even optimal performance was still less than that in
   alternatives.  The ALT was also very vulnerable to misconfiguration.

   See [LISP-TREE] for more about these issues: the basic structure and
   operation of DDT is identical to that of TREE, so the conclusions
   drawn there about TREE's superiority to ALT apply equally to DDT.

   In particular, the big advantage of DDT over the ALT, in performance
   terms, is that it allows MRs to interact _directly_ with distant DDT
   servers (as opposed to the ALT, which _always_ required mediation
   through intermediate servers); caching of information about those
   distant servers allows DDT to make extremely effective use of this
   capability.

   The ALT did have some useful properties which its replacement, DDT,
   did not, e.g. the ability to forward data directly to the
   destination, over the ALT, when no mapping was available yet for the
   destination.  However, these were minor, and heavily outweighed by
   its problems.

   A recent study, [Saucez], measured actual resolution times in the
   deployed LISP network during the changeover from ALT to DDT, allowing
   direct comparison of the performance of the two systems.  The study
   took measurements from a variety of locations in the Internet, with
   respect to a number of different target EIDs.  The results indicate
   that the performance was almost identical; there was more variance
   with DDT (perhaps due to the effects of caching), but the greatly
   improved scalability of DDT as compared to ALT made that effect
   acceptable.

B.5.  Early NAT Support

   The first mechanism used by LISP to support operation through a NAT
   device, described here, has now been superseded by the more general
   mechanism proposed in [NAT-Traversal].  That mechanism is, however,
   based heavily on this mechanism.  The initial mechanism had some
   serious limitations, which is why that particular form of it has been
   dropped.

   First, it only worked with some NATs, those which were configurable
   to allow inbound packet traffic to reach a configured host.  The NAT
   had to be configured to know of the ETR.

   Second, since NATs share addresses by using ports, it was only
   possible to have a single LISP node behind any given NAT device.
   That is because LISP expects all incoming data traffic to be on a
   specific port, so it was not possible to have multiple ETRs behind a
   single NAT (which normally would have only one global IP address to
   share).  Even looking at the sort host and port would not necessarily
   help, because some source ITR could be sending packets to both ETRs,
   so packets to either ETR could also have the identical source host/
   port.  In short, there was no way for a NAT with multiple ETRs behind
   it to know which ETR the packet was for.

   To support operation behind a NAT, there was a pair of new LISP
   control messages, LISP Echo-Request and Echo-Reply, which allowed the
   ETR to discover its temporary global address.  The Echo-Request was
   sent to the configured Map-Server, and it replied with an Echo-Reply
   which included the source address from which the Echo Request was
   received (i.e. the public global address assigned to the ETR by the
   NAT).  The ETR could then insert that address in any Map-Reply
   control messages which it sent to correspondent ITRs.

   Echo-Request and Echo-Reply have been replaced by Info-Request and
   Info-Reply in the replacement, [NAT-Traversal], where they perform
   very similar functions; the main change is the addition of the {{xxx
   - probably the port, etc to allow multiple XTRs behind a NAT}}.

Author's Address

   J. Noel Chiappa
   Yorktown Museum of Asian Art
   Yorktown, Virginia
   USA

   EMail: jnc@mit.edu


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