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In: MissingRef
LISP Working Group                                         J. N. Chiappa
Internet-Draft                              Yorktown Museum of Asian Art
Intended status: Informational                             July 15, 2013
Expires: January 16, 2014


                An Architecural Introduction to the LISP
                  Location-Identity Separation System
                    draft-ietf-lisp-introduction-01

Abstract

   LISP is an upgrade to the architecture of the IPvN internetworking
   system, one which separates location and identity (currently
   intermingled in IPvN addresses).  This is a change which has been
   identified by the IRTF as a critically necessary evolutionary
   architectural step for the Internet.  In LISP, nodes have both a
   'locator' (a name which says _where_ in the network's connectivity
   structure the node is) and an 'identifier' (a name which serves only
   to provide a persistent handle for the node).  A node may have more
   than one locator, or its locator may change over time (e.g. if the
   node is mobile), but it keeps the same identifier.

   One of the chief novelties of LISP, compared to other proposals for
   the separation of location and identity, is its approach to deploying
   this upgrade.  (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.)  LISP aims to
   achieve the near-ubiquitous deployment necessary for maximum
   exploitation of an architectural upgrade by i) minimizing the amount
   of change needed (existing hosts and routers can operate unmodified);
   and ii) by providing significant benefits to early adopters.

   This document is an introduction to the entire LISP system, for those
   who are unfamiliar with it.

Status of This Memo

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   for publication as an RFC or to translate it into languages other
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   This Internet-Draft will expire on January 16, 2014.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Prefaratory Note
   2.  Background
   3.  Deployment Philosophy
     3.1.  Economics
     3.2.  Maximize Re-use of Existing Mechanism
     3.3.  'Self-Deployment'
   4.  LISP Overview
     4.1.  Basic Approach
     4.2.  Basic Functionality
     4.3.  Mapping from EIDs to RLOCs
     4.4.  Interworking With Non-LISP-Capable Endpoints
     4.5.  Security in LISP
   5.  Initial Applications
     5.1.  Provider Independence
     5.2.  Multi-Homing
     5.3.  Traffic Engineering
     5.4.  Routing
     5.5.  Mobility
     5.6.  IP Version Reciprocal Traversal
     5.7.  Local Uses
   6.  Major Functional Subsystems
     6.1.  xTRs
       6.1.1.  Mapping Cache Performance
     6.2.  Mapping System
       6.2.1.  Mapping System Organization
       6.2.2.  Interface to the Mapping System
       6.2.3.  Indexing Sub-system
   7.  Examples of Operation
     7.1.  An Ordinary Packet's Processing
     7.2.  A Mapping Cache Miss
   8.  Design Approach
   9.  xTRs
     9.1.  When to Encapsulate
     9.2.  UDP Encapsulation Details
     9.3.  Header Control Channel
       9.3.1.  Mapping Versioning
       9.3.2.  Echo Nonces
       9.3.3.  Instances
     9.4.  Probing
     9.5.  Mapping Lifetimes and Timeouts
     9.6.  Security of Mapping Lookups
     9.7.  Mapping Gleaning in ETRs
     9.8.  Fragmentation
   10. The Mapping System
     10.1. The Mapping System Interface
       10.1.1. Map-Request Messages
       10.1.2. Map-Reply Messages
       10.1.3. Map-Register and Map-Notify Messages
     10.2. The DDT Indexing Sub-system
       10.2.1. Map-Referral Messages
     10.3. Reliability via Replication
     10.4. Security of the DDT Indexing Sub-system
     10.5. Extended Tools
     10.6. Performance of the Mapping System
   11. Deployment Mechanisms
     11.1. LISP Deployment Needs
     11.2. Internetworking Mechanism
     11.3. Proxy Devices
       11.3.1. PITRs
       11.3.2. PETRs
     11.4. LISP-NAT
     11.5. Use Through NAT Devices
       11.5.1. First-Phase NAT Support
       11.5.2. Second-Phase NAT Support
     11.6. LISP and DFZ Routing
       11.6.1. Long-term Possibilities
   12. Fault Discovery/Handling
     12.1. Handling Missing Mappings
     12.2. Outdated Mappings
       12.2.1. Outdated Mappings - Updated Mapping
       12.2.2. Outdated Mappings - Wrong ETR
       12.2.3. Outdated Mappings - No Longer an ETR
     12.3. Erroneous Mappings
     12.4. Neighbour Liveness
     12.5. Neighbour Reachability
   13. Current Improvements
     13.1. Improved NAT Support
     13.2. Mobile Device Support
     13.3. Multicast Support
     13.4. {{Any others?}}
   14. Acknowledgments
   15. IANA Considerations
   16. Security Considerations
   17. References
     17.1. Normative References
     17.2. Informative References
   Appendix A.  Glossary/Definition of Terms
   Appendix B.  Other Appendices
     B.1.  Old LISP 'Models'
     B.2.  Possible Other Appendices

1.  Prefaratory 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.

   This 'Architectural Introduction' document is primarily intended for
   those who don't know anything about LISP, and want to start learning
   about it.  It 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.
   Reading only the first part(s) 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.

   This goal explains why the document has a somewhat unusual structure.
   It is not a reference document, where all the content on a particular
   topic is grouped in one place.  (That role is filled by the various
   protocol specifications.)  It starts with a very high-level view of
   the entire system, to provide readers with a mental framework to help
   understand the more detailed material which follows.  A second pass
   over the whole system then goes into more detail; finally, individual
   sub-systems are covered in still deeper detail.

   The intent is two-fold: first, the multiple passes over the entire
   system, each one going into more detail, are intended to ease
   understanding; second, people can simply stop reading when they have
   a detailed-enough understanding for their purposes.  People who just
   want to get an idea of how LISP works might only read the first
   part(s), whereas people who are going to go on and read all the
   protocol specifications (perhaps to implement LISP) would need/want
   to 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.  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 a node (basically, 'who' a node is,
   and 'where' it is).  At the moment, in both IPv4 and IPv6, addresses
   indicate both where the named device is, as well as identify it for
   purposes of end-end communication.

   The distinction was more than a little hazy at first: the early
   Internet [RFC791], like the ARPANET before it [Heart] [NIC8246], co-
   mingled the two, 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.  However, it has taken 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.  [RFC6115]

   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 in 2012 (although work to expand and improve it
   continues, and undoubtly will for a long time to come).

3.  Deployment Philosophy

   It may seem odd to cover 'deployment philosophy' at this point in
   such a document.  However the deployment philosophy was a major
   driver for much of the design (to some degree the architecture, and
   to a very large measure, the engineering).  So, as such an important
   motivator, it is very desirable for readers to have this material in
   hand as they examine the design, so that design choices that may seem
   questionable at first glance can be better understood.

   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.  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 a viable deployment model is
   what has lead to some painful compromises in the design.

   The extreme focus on a viable deployment scheme is one of the
   novelties of LISP.

3.1.  Economics

   The key factor in successful adoption, as shown by recent experience
   in the Internet - and little appreciated to begin with, some decades
   back - 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.  One might have the world's best 'clean-slate'
   design, but if it does not have a deployment plan which is
   economically feasible, it's not good for much.

   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.  We believe LISP has met both of
   these goals.

3.2.  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 is needed in only a few places (see
   Section 11.1 for more).

   LISP also initially reuses, where-ever possible, existing protocols
   (IPv4 [RFC791] and IPv6 [RFC2460]).  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.

3.3.  'Self-Deployment'

   LISP has deliberately employed a rather different deployment model,
   which we might call 'self-deployment' (for want of a better term); it
   does not require a huge push to get it deployed, rather, it is hoped
   that once people see it and realize they can easily make good use of
   it _on their own_ (i.e. without requiring adoption by others), it
   will 'deploy itself' (hence the name of the approach).

   One can liken the problem of deploying new systems in this way to
   rolling a snowball down a hill: unless one starts with a big enough
   snowball, and finds a hill of the right steepness (i.e. the right
   path for it to travel), one's snowball is not going to go anywhere on
   its own.  However, if one has picked one's spot correctly, once
   started, little additional work is needed.

4.  LISP Overview

   LISP is an incrementally deployable architectural upgrade to the
   existing Internet infrastructure, one which provides separation of
   location and identity.  The separation is usually not perfect, for
   reasons which are driven by the deployment philosophy (above), and
   explored in a little more detail elsewhere (in [Perspective], Section
   "Namespaces-EIDs-Residual").

   LISP separates the functions of location and identity of nodes (a
   nebulous term, deliberately chosen for use in this document precisely
   because its definition is not fixed - you will not go far wrong if
   you think of a node as being something like a host), which are
   currently intermingled in IPvN addresses.  (This document uses the
   meaning for 'address' proposed in [Atkinson], i.e. a name with mixed
   location and identity semantics.)

4.1.  Basic Approach

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

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

   Technically, one should probably say that ideally, the EID names the
   node (or rather, its end-end communication stack, if one wants to be
   as forward-looking as possible), and the RLOC(s) name interface(s).
   (At the moment, in reality, the situation is somewhat more complex,
   as will be explained elsewhere (in [Perspective], Section
   "Namespaces-EIDs-Residual".)

   This second distinction, of _what_ is named by the two classes of
   name, is necessary both to enable some of the capabilities that LISP
   provides (e.g the ability to seamlessly support multiple interfaces,
   to different networks), and is also a further enhancement to the
   architecture.  Faailing to clearly recognize both interfaces and
   communication stacks as distinctly separate classes of things is
   another failing of the existing Internet architecture (again, one
   inherited from the previous generation of networking).

   A novelty in LISP is that it uses existing IPvN addresses (initially,
   at least) for both of these kinds of names, thereby minimizing the
   deployment cost, as well as providing the ability to easily interact
   with unmodified hosts and routers.

4.2.  Basic Functionality

   The basic operation of LISP, as it currently stands, is that LISP
   augmented packet switches near the source and destination of packets
   intercept traffic, and 'enhance' the packets.

   The LISP device near the source looks up additional information about
   the destination, and then wraps the packet in an outer header, one
   which contains some of that additional information.  The LISP device
   near the destination removes that header, leaving the original,
   unmodified, packet to be processed by the destination node.

   The LISP device near the original source (the Ingress Tunnel Router,
   or 'ITR') uses the information originally in the packet about the
   identity of its ultimate destination, i.e. the destination address,
   which in LISP 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 will be (initially at least) the address
   of the LISP device near the ultimate destination (the Egress Tunnel
   Router, or 'ETR').

   {{Is it worth distinguishing between 'mapping' and 'binding'?  Should
   the document pick one term, and stick with it?}}

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

   When the packet gets to 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 device; these are normally denominated as 'xTRs'.

4.3.  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.

   Mappings are requested on need, not (generally) pre-loaded; in other
   words, mapping are distributed via a 'pull' mechanism.  Once obtained
   by an ITR, they are cached by the ITR, to limit the amount of control
   traffic to a practicable level.  (The mapping system will be
   discussed in more detail below, in Section 6.2 and Section 10)

   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 6.1.1, below.)

4.4.  Interworking With Non-LISP-Capable Endpoints

   The capability for 'easy' interoperation between nodes using LISP,
   and existing non-LISP-using hosts (often called 'legacy' hosts) or
   sites (where 'site' is usually taken to mean a collection of hosts,
   routers and networks under a single administrative control), is
   clearly crucial.

   To allow such interoperation, a number of mechanisms have been
   designed.  This multiplicity is in part because different mechanisms
   have different advantages and disadvantages (so that no single
   mechanism is optimal for all cases), but also because with limited
   field experience, it is not clear which (if any) approach will be
   preferable.

   One approach uses proxy LISP devices, called PITRs (proxy ITRs) and
   PETRs (proxy ETRs), to provide LISP functionality during interaction
   with legacy hosts.  Another approach uses a device with combined LISP
   and NAT ([RFC1631]) functionality, named a LISP-NAT.

4.5.  Security in LISP

   LISP has a subtle security philosophy; see [Perspective], Section
   "Security", where it is laid out in some detail.

   To provide a brief overview, 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.
   That means two things: First, with the limited manpower currently
   available, we cannot expect to create the complete security apparatus
   that we might see in the long term (which requires not just design,
   but also implementation, etc).  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
   thorougly analyze what LISP needs for security, and then design, in
   detail, a scheme for providing that security.  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.  Later
   on, the design can be fully specified, implemented, and deployed.

5.  Initial Applications

   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 unmodified hosts, use of LISP can still
   provide benefits for communications with 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 deploying LISP, thereby increasing the cost/benefit ratio
   needed to drive deployment, and increasing the 'self-deployment'
   aspect of LISP.

   Note also that this section only lists likely _early_ applications
   and benefits - if and once deployment becomes more widespread, other
   aspects will come into play (as described in [Perspective], in the
   "Goals of LISP" section).

5.1.  Provider Independence

   Provider independence (i.e. the ability to easily change one's
   Internet Service Provider) was probably the first place where the
   Internet engineering community finally really felt 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), the
   so-called "provider aggregated" 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 renumber, a painful undertaking.  [RFC5887]

   In theory, it ought to be possible to update the DNS entries, and
   have everyone switch to the new addresses, but in practise, addresses
   are embedded in many places, such as firewall configurations at other
   sites.

   Having separate namespaces for location and identity greatly reduces
   the problems involved with renumbering; 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.

5.2.  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 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, 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.

   Multi-homing was once somewhat esoteric, but a number of trends are
   driving an increased desirability, e.g. the wish to have multiple ISP
   links to a site for robustness; the desire to have mobile handsets
   connect up to multiple wireless systems; etc.

   Again, separation of location and identity, and the existince of a
   binding layer which can be updated fairly quickly, as provided by
   LISP, is a very useful tool for all variants of this issue.

5.3.  Traffic Engineering

   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 computationa), 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 binding 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 binding
   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 6.2 below.)

   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.

5.4.  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 so-called 'Default-Free-Zone' (DFZ) - the
   core of the Internet, the part where routes to _all_ ultimate
   destinations must be available.  LISP is expected to help with this;
   for more detail, see Section 11.6, below.

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

5.5.  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.

5.6.  IP Version Reciprocal Traversal

   Note that LISP 'automagically' allows 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
   would allow 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 heavyweight 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'.

5.7.  Local Uses

   LISP has a number of use cases which are within purely 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, IP version traversal, and 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).

   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.

6.  Major Functional Subsystems

   LISP has only two major functional sub-systems - the collection of
   LISP packet switches (the xTRs), and the mapping system, 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 9 and
   Section 10, respectively).

6.1.  xTRs

   xTRs are fairly normal packet switches, enhanced with a little extra
   functionality in both the data and control planes, to perform LISP
   data and control functionality.

   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 'vanilla'); i.e. looking up the mapping; encapsulating the
   packet; and sending it to the ETR.  This encapsulation is done using
   UDP [RFC768] (for reasons to be explained below, in Section 9.2),
   along with an additional 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 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 Map-Request control messages; handling the returning
   Map-Replies which contain the requested information; managing the
   local 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
   neighbour reachability and liveness function (see Section 12.4);
   interacting with the mapping sub-system (next section); and answering
   requests for mappings (ditto).

6.1.1.  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.

   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.

   The first, [Iannone], was performed in the very early stages of the
   LISP effort, to verify that that approach was feasible.  First,
   packet traces of all traffic over the external connection of a large
   university (around 10,000 users) 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.  The simulations set no
   limit on the total cache size, but used a range of cache retention
   times (i.e. an entry that remained unused longer than a fixed
   retention time was discarded), from 3 minutes, up to 300 minutes.

   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 (at night, with the shortest retention time) up to
   about 100,000.  Using some estimations as to i) how many RLOCs the
   average mapping would have (since this will affect its size), and ii)
   how much memory it would take to store a mapping, 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 (again,
   depending on the time of day, cache settings, etc) from 30 per
   minute, up to 3,000 per minute (i.e. 150 per second; with the
   shortest timeout, and thus the smallest cache).  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 second, [Kim], was in general terms similar, except that it used
   data from a large ISP (taken over two days, at different times of the
   year), 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: 60
   seconds, 180 seconds, and 1,800 seconds (30 minutes), since the
   previous study had indicated that extremely long times (hours) had
   little additional benefit.

   The results were similar: cache sizes ranges from 20,000 entries with
   the shortest timeout, to roughly 60,000 with the longest; the miss
   rate ranged from very roughly 400 per minute (with the longest
   timeout) to very roughly 7,000 per minute (with the shortest),
   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 (some from an earlier study,
   [Jakab]).  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.

6.2.  Mapping System

   The mapping database is a distributed, and potentially replicated,
   database which holds mappings between EIDs (identity) and RLOCs
   (location).  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 often is) as small as a single EID.
   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 routing table came to be dominated by
   such small entries.

   A particular node may have more than one RLOC, or may change its
   RLOC(s), while keeping its singlar identity.

   The mapping contains not just the RLOC(s), but also (for each RLOC
   for any given EID) priority and weight (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.

6.2.1.  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 covers the actual mappings themselves; 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 a distributed,
   potentally 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 user interface sub-system, and indexing sub-system (which holds
   and provides the actual information).

6.2.2.  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 provied by devices
   called Map Resolvers (MRs).

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

   Similarly, the client interface to the indexing system from an ETR's
   point of view is through devices called Map Servers (MSs - admittedly
   a poorly chosen term, since their primary function is not to respond
   to queries, but it's too late to change it now).

   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 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 system are thus likewise hidden from the
   'ordinary' ETRs.

6.2.3.  Indexing Sub-system

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

   DDT replaced an earlier indexing sub-system, ALT ([Perspective],
   section "Appendices-ALT"); this swap validated the concept of having
   a separate client-interface sub-system, which would allow the actual
   indexing sub-system to be replaced without needing to modify the
   clients.

6.2.3.1.  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 tree of DDT 'nodes', starting with the 'root' DDT
   node.  Each node is responsible (authoritative?) for one or more
   blocks of the EID namespace.

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

   Just as with DNS, for reasons of performance, reliability and
   robustness, any particular node in the DDT delegation tree may be
   instantiated in more than one redundant physical server machines.
   Obviously, all the servers which instantiate a particular node in the
   tree have to have identical data about that node.

   Also, although the delegation hierarchy is a strict tree {{check - do
   all servers for the delegation of block X have to return the same
   list of servers for that block?}}, a single DDT server could be
   responsible (authoritative?) for more than one block of the EID
   namespace.

   Eventually, leaf nodes in the DDT tree assign ({{delegate? - it's all
   static configured, nothing is dynamic}}) EID namespace blocks to
   MS's, which are DDT terminal nodes; i.e. a leaf of the tree is
   reached when the delegation points to an MS instead of to another DDT
   node.

   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 that block of EID namespace.

6.2.3.2.  Use of DDT by MRs

   An MR which wants to find a mapping for a particular EID first
   interacts with the nodes of the DDT tree, discovering (by querying
   DDT nodes) the chain of delegations which cover that EID.  Eventually
   it is directed to an MS, and then to an ETR which is responsible
   {{authoritative?}} for that EID.

   Also, again like DNS, MRs cache information about the delegations in
   the DDT 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 do a complete tree-walk of the DDT tree 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.

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

7.  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, 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.

7.1.  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) 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 is also a border router for the site), the border router looks
   up the desination address (an EID) in its local mapping cache.  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).  The destination
   address in the outer header is set by the ITR to the RLOC of the
   destination ETR.

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

   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.

7.2.  A Mapping Cache Miss

   If a host sends a packet, and it gets to the ITR, and the ITR both i)
   determines that it needs to perform LISP processing on the user data
   packet, but ii) does not yet have a mapping cache entry which covers
   that destination EID, then more complex processing ensues.

   It 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 see if it has the RLOC for the ETR for that
   destination EID.  If not, it will query the DDT system to find the
   RLOC of the ETR.  When it has the RLOC, it will send the Map-Request
   on to the 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.  (Typically, like many ARP
   implementations, the original user packet will have been discarded,
   not cached waiting for the mapping to be found.  When the host
   retransmits the packet, the mapping will be there, and the packet
   will be forwarded.)

8.  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.

   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.

9.  xTRs

   As mentioned above (in Section 6.1), xTRs are the basic data-handling
   devices in LISP.  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.

9.1.  When to Encapsulate

   An ITR knows that an ultimate destination is 'running' LISP (remember
   that the 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 normally.

   Note that the ITR cannot simply depend on the appearance, or non-
   appearance, of the destination in the routing tables in the DFZ, as a
   way to tell if an ultimate destination is a LISP node or not, because
   mechanisms to allow interoperation of LISP sites and 'legacy' sites
   necessarily involve advertising LISP sites' EIDs into the DFZ.

9.2.  UDP Encapsulation Details

   The UDP encapsulation used by LISP for carrying traffic from ITR to
   ETR, and many of the details of how it works, were all chosen for
   very practical reasons.

   Use of UDP (instead of, say, a LISP-specific protocol number) was
   driven by the fact that many devices filter out 'unknown' protocols,
   so adopting a non-UDP encapsulation would have made the initial
   deployment of LISP harder - and our goal (see Section 3.1) was to
   make the deployment as easy as possible.

   The UDP source port in the encapsulated packet is a hash of the
   original source and ultimate destination; 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 an intolerable load, for no
   benefit.

9.3.  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 8-bit 'flags'
   field, and it also includes two data fields (one 24 bits, one 32),
   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.

9.3.1.  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.
   [Versioning] 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 multiplex 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.
   Some felt that this was non-optimal, and that a better approach would
   have been to have 'fingerprints' which were computed from the current
   mapping data (i.e. a hash).  It is not clear that the ordering buys
   much (if anything), and the potential for mishaps with manually
   configured version numbers is self-evident.

9.3.2.  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 correspondent 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 lack of a response is not necessarily _proof_ that
   something has gone wrong - but it stronly 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 12.5 for more about Echo Nonces.)

9.3.3.  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).

9.4.  Probing

   RLOC-Probing (see [LISP], 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.

9.5.  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.)

9.6.  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 what what have
   been actually delegated to it).

   Very briefly, the ITR provided a One-Time Key with its query; this
   key is used by both the MS (to verify the EID block that it has
   delegated to the ETR), and indirectly by the ETR (to verify 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).

9.7.  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. {{Is this still there?
   Check the latest version of the spec.}} 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.

9.8.  Fragmentation

   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.

   One, called the 'stateful' approach, 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.

   It is not clear at this point which approach is preferable.

10.  The Mapping System

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

   To recap, the mapping system is split into an indexing sub-system,
   which keeps track of where all the mappings are kept, and the
   mappings themselves, the authoritative copies of which are always
   held by ETRs.

10.1.  The Mapping System Interface

   As mentioned in Section 6.2.2, 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
   the details, see [MapInterface].

10.1.1.  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.
   [AFI] The inclusion of the AFI allows the mapping system interface
   (as embodied in these control packets) a great deal of flexibility.
   (See [Perspective], Section "Namespaces" for more on this.)

   Other important fields are the source EID (and its AFI), and one or
   more RLOCs for the source EID, along with their AFIs.  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 9.7).

   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.

10.1.2.  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.)

   For each RLOC in the entry, there is the RLOC, its AFI (of course),
   priority and weight fields (see Section 6.2), and multicast priority
   and weight fields.

10.1.2.1.  Solicit-Map-Request Messages

   "Solicit-Map-Request" (SMR) messages are actually not another message
   type, but a sub-type of Map-Reply messages.  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. {{Need to
   say more about rate limiting, probably in security section?  Ref to
   that from here.}}

10.1.3.  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 9.3.1), 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.  [Mobility]

   Map-Notify messages have the exact same contents as Map-Register
   messages; they are purely acknowledgements.

10.2.  The DDT Indexing Sub-system

   As previously mentioned Section 6.2.3, the indexing sub-system in
   LISP is currently the DDT system.

   The overall operation is fairly simple; an MR which needs a mapping
   starts at a server for the root DDT node (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 not complex; the MR
   sends the DDT server a Map-Request control message (which looks
   almost exactly like the Map-Request which an ITR sends to an MR).
   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 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.  The Map-Referral also gives the RLOCs
   of all the machines which are DDT servers for that block. and the MR
   can then send Map-Requests to any one (or all) of them.

   Control flags in the Map-Referral indicate to the querying MR whether
   the referral is to another DDT node, an MS, or an ETR.  If the
   former, the MR then sends the Map-Request to the child DDT node,
   repeating the process.

   If the latter, 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, so in such cases the Map-
   Reply will come from the MS, not the ETR. {{I think this case has
   been mentioned already; check.}})

   Delegations are cached in the MRs, so that once an MR has received
   information about a delegation, it will not need to look that up
   again.  Once it has been in operation for a short while, it will only
   need to ask for 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 10.6), significant amounts of modeling and
   performance measurement have been performed, to verify that DDT has
   (and will continue to have) acceptable performance.

10.2.1.  Map-Referral Messages

   Map-Referral messages look almost identical to Map-Reply messages
   (which is felt to be an advantage by some people, although having a
   more generic record-based format would probably be better in the long
   run, as ample experience with DNS has shown), except that the RLOCs
   potentially name either i) other DDT nodes (children in the
   delegation tree), or ii) terminal MSs.

10.3.  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.

   The deployed DDT system actually uses anycast [RFC4786], along with
   replicated servers, to improve both performance and robustness.

   There are generally no mechanisms specified yet to ensure coherence
   between multiple copies of any particular data item, etc - 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.

10.4.  Security of the DDT Indexing Sub-system

   LISP provides an advanced model for securing the mapping indexing
   system, in line with the overall LISP security philosophy.

   Briefly, securing the mapping indexing system is broken 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
   nodes/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.  This requires very little configuration in the
   clients, and is fairly secure.

   The interface between the DDT nodes/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 DNS-SEC ([RFC4033]), where configuration
   complexity in the servers has been a major stumbling block to
   deployment.

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

10.5.  Extended Tools

   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 'source-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.

10.6.  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.  [Jakab] The basic structure and
   concepts of DDT are identical to those of TREE, so the performance
   simulation work done for that design applies aequally 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
   lower nodes 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.

11.  Deployment 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.

11.1.  LISP Deployment Needs

   As mentioned earlier (Section 3.2), 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.

   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 small amount of LISP-specific support
   infrastructure, such as MRs, MSs, the DDT hierarchy, etc but much of
   this will either i) already be deployed, and if the new site can make
   arrangements to use it, it need do nothing else, or ii) those
   functions it must provide may be co-located in other LISP devices
   (again, either new devices, or new software on existing ones).

11.2.  Internetworking Mechanism

   One aspect which has received a lot of attention are the mechanisms
   previously referred to (in Section 4.4) to allow interoperation of
   LISP sites with so-called 'legacy' sites which are not running LISP
   (yet).

   To briefly refresh what was said there, there are two main approaches
   to such interworking: proxy nodes (PITRs and PETRs), and an
   alternative mechanism using device with combined NAT and LISP
   functionality; these are described in more detail here.

11.3.  Proxy Devices

   PITRs (proxy ITRs) serve as ITRs for traffic _from_ legacy hosts to
   nodes using LISP.  PETRs (proxy ETRs) serve as ETRs for LISP traffic
   _to_ legacy hosts (for cases where a LISP device 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.

11.3.1.  PITRs

   PITRs (proxy ITRs) serve as ITRs for traffic _from_ legacy hosts to
   nodes using LISP.  To do that, 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.

   As mentioned previously (Section 9.1), 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 DFZ) by checking to see if a LISP mapping exists for that
   ultimate destination.

   This technique obviously has an impact on routing table in the DFZ,
   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.

   A PITR may cover a group of EID blocks with a single EID
   advertisement, 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.)

   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.

11.3.2.  PETRs

   PETRs (proxy ETRs) serve as ETRs for LISP traffic _to_ legacy hosts,
   for cases where a LISP device 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 paired with 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.

11.4.  LISP-NAT

   A LISP-NAT device, 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).  [Interworking]

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

11.5.  Use Through NAT Devices

   Like them or not (and NAT devices have many egregious issues - some
   inherent in the nature of the process of mapping addresses; others,
   such as the brittleness due to non-replicated critical state, caused
   by the way NATs were introduced, as stand-alone 'invisible' boxes),
   NATs are both ubiquitous, and here to stay for a long time to come.

   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.

11.5.1.  First-Phase NAT Support

   The first mechanism used by LISP to operate through a NAT device only
   worked with some NATs, those which were configurable to allow inbound
   packet traffic to reach a configured host.

   A pair of new LISP control messages, LISP Echo-Request and Echo-
   Reply, 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.

   The fact that this mechanism did not support all NATs, and also
   required manual configuration of the NAT, meant that this was not a
   good solution; in addition, since LISP expects all incoming data
   traffic to be on a specific port, it was not possible to have
   multiple ETRs behind a single NAT (which normally would have only one
   global address to share, meaning port mapping would have to be used,
   except that... )

11.5.2.  Second-Phase NAT Support

   For a more comprehensive approach to support of LISP xTR deployment
   behind NAT devices, a fairly extensive supplement to LISP, LISP NAT
   Traversal, has been designed.  [LISP-NAT]

   A new class of LISP device, the LISP Re-encapsulating Tunnel Router
   (RTR), passes traffic through the NAT, both to and from the xTR.
   (Inbound traffic has to go through the RTR as well, since otherwise
   multiple xTRs could not operate behind a single NAT, for the
   'specified port' reason in the section above.)

   (Had the Map-Reply included a port number, this could have been
   avoided - although of course it would be possible to define a new
   RLOC type which included protocol and port, to allow other
   encapsulation techniques.)

   Two new LISP control messages (Info-Request and Info-Reply) allow an
   xTR to detect if it is behind a NAT device, and also discover the
   global IP address and UDP port assigned by the NAT to the xTR.  A
   modification to LISP Map-Register control messages allows the xTR to
   initialize mapping state in the NAT, in order to use the RTR.

   This mechanism addresses cases where the xTR is behind a NAT, but the
   xTR's associated MS is on the public side of the NAT; this
   limitation, that MS's must be in the 'public' part of the Internet,
   seems reasonable.

11.6.  LISP and DFZ Routing

   One of LISP's original motivations was to try and control the growth
   of the size of the so-called 'Default-Free-Zone' (DFZ), the core of
   the Internet, 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.

   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 DFZ 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.

11.6.1.  Long-term Possibilities

   At a later stage of the deployment, a more aggressive approach
   becomes available: taking part of the DFZ, one for which all 'stub'
   sites connected to it have deployed LISP, and removing all 'EID
   routes' (used for backwards compatability with 'legacy' sites); only
   RLOC routes would remain in the routing table in that part of the
   Internet backbone.

   Obviously there would be a boundary between the two parts of the DFZ,
   and the routers on the border would have to (effectively) become
   PITRs, and inject routes to all of the LISP sites 'behind' them into
   the 'legacy' DFZ (to coin a name for the part of the DFZ which, for
   reasons of interoperability with legacy sites, still carries EID
   routes).

   Note that it is likely not feasible to have the 'RLOC only' part of
   the DFZ in the 'middle' of the DFZ; that would require (effectively)
   EID routes to be removed from BGP on crossing the boundary _into_ the
   RLOC DFZ, but re-created on crossing the boundary _out_ of the RLOC
   DFZ.  This is likely to be impractical, leading to the suggestion of
   a simpler boundary between the RLOC-only part of the DFZ, and the
   'legacy' DFZ.

   The mechanism for detecting which routes are 'EID routes' and which
   are 'RLOC routes' (required for the boundary routers to be able to
   filter out the 'EID routes') would also need to be worked out; the
   most likely appears to be something involving BGP attributes.

12.  Fault Discovery/Handling

   LISP is, in terms of its functionality, a fairly simple system: the
   list of failure modes is thus not extensive.

12.1.  Handling Missing Mappings

   Handling of missing mappings is fairly simple: 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 'default PITR'.

   A number of PITRs advertise all EID blocks into the backbone routing,
   so that any ITRs which are temporarily missing a mapping can forward
   the traffic to these default PITRs via normal transmission methods,
   where they are encapsulated and passed on.

12.2.  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 traffic is being sent to 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

12.2.1.  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.
   [Versioning]

12.2.2.  Outdated Mappings - Wrong ETR

   If an ITR is holding a seriously 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).

   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.

12.2.3.  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 device at all.  In such cases, one might get an ICMP Destination
   Unreachable 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 [LISP], 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 9.3.2) 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.

12.3.  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.

12.4.  Neighbour Liveness

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

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

   Note that the two terms (liveness and reachability) are _not_
   synonmous (although a lot of LISP documentation confuses them).
   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.

   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.

12.5.  Neighbour Reachability

   A more significant issue than whether a particular ETR E is up or not
   is, as mentioned above, that although ETR E may be up, attached to
   the network, etc, an issue in the network between a source ITR I and
   E may prevent traffic from I from getting to E. (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 devices, reliance on it as a sole mechanism would
   produce a fair amount of overhead.)

13.  Current Improvements

   In line with the philosophies laid out in Section 8, LISP is
   something of a moving target.  This section discusses some of the
   contemporaneous improvements being made to LISP.

13.1.  Improved NAT Support

13.2.  Mobile Device Support

   Mobility is an obvious capability to provide with LISP.  Doing so is
   relatively simple, if the mobile host is prepared to act as its own
   ETR.  It obtains a local 'temporary use' address, and registers that
   address as its RLOC.  Packets to the mobile host are sent to its
   temporary address, wherever that may be, and the mobile host first
   unwraps them (acting as an ETR), and the processes them normally
   (acting as a host).

   (Doing mobility without having the mobile host act as its ETR is
   difficult, even if ETRs are quite common.  The reason is that if the
   ETR and mobile host are not integrated, during the step from the ETR
   to the mobile host, the packets must contain the mobile host's EID,
   and this may not be workable.  If there is a local router between the
   ETR and mobile host, for instance, it is unlikely to know how to get
   the packets to the mobile host.)

   If the mobile host migrates to a site which is itself a LISP site,
   things get a little more complicated.  The 'temporary address' it
   gets is itself an EID, requiring mapping, and wrapping for transit
   across the rest of the Internet.  A 'double encapsulation' is thus
   required at the other end; the packets are first encapsulated with
   the mobile node's temporary address as their RLOC, and then this has
   to be looked up in a second lookup cycle (see Section 9.1), and then
   wrapped again, with the site's RLOC as their destination.

   This results in slight loss in maximum packet size, due to the
   duplicated headers, but on the whole it is considerably simpler than
   the alternative, which would be to re-wrap the packet at the site's
   ETR, when it is discovered that the ultimate destination's EID was
   not 'native' to the site.  This would require that the mobile node's
   EID effectively have two different mappings, depending on whether the
   lookup was being performed outside the LISP site, or inside.

   {{Also probably need to mention briefly how the other end is notified
   when mappings are updated, and about proxy-Map-Replies.}} [Mobility]

13.3.  Multicast Support

   Multicast may seem an odd thing to support with LISP, since LISP is
   all about separating identity from location, but although a multicast
   group in some sense has an identity, it certainly does not have _a_
   location.

   However, multicast is important to some users of the network, for a
   number of reasons: doing multiple unicast streams is inefficient; it
   is easy to use up all the upstream bandwidth, and without multicast a
   server can also be saturated fairly easily in doing the unicast
   replication.  So it is important for LISP to 'play nicely' with
   multicast; work on multicast support in LISP is fairly advanced,
   although not far-ranging.

   Briefly, 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, LISP's mapping capability is used:
   it is just applied to the source, not the destination (as with most
   LISP activity); the inner source is the EID, and the outer source is
   the EID's RLOC.

   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 approach currently used by LISP requires no packet format changes
   to existing multicast protocols.  See [Multicast] for more;
   additional LISP multicast issues are discussed in [LISP], Section 12.

13.4.  {{Any others?}}

14.  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) Vina Ermagan,
   Vince Fuller and Vasileios Lakafosis for their careful review of, and
   helpful suggestions for, this document.  (If I have missed anyone in
   this list, I apologize most profusely.)  A very special thank you
   goes to Joel Halpern, who, 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 Vince Fuller and Dino Farinacci 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.

15.  IANA Considerations

   This document makes no request of the IANA.

16.  Security Considerations

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

17.  References

17.1.  Normative References

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

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

   [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.

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

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

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

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

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

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

   [DDT]           V. Fuller, D. Lewis, and D. Farinacci, "LISP
                   Delegated Database Tree", draft-ietf-lisp-ddt-00
                   (work in progress), October 2012.

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

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

   [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.

   [LISP-NAT]      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-07
                   (work in progress), April 2012.

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

   [Deployment]    L. Jakab, A. Cabellos-Aparicio, F. Coras, J. Domingo-
                   Pascual, and D. Lewis, "LISP Network Element
                   Deployment Considerations",
                   draft-ietf-lisp-deployment-08 (work in progress),
                   June 2013.

17.2.  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.

   [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.

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

   [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.

   [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.

   [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.

   [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'.

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

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

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

   [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>.

   [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.

   [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.

   [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>.

   [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.

   [Jakab]         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.

   [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.

Appendix A.  Glossary/Definition of Terms

   -  Address
   -  Locator
   -  EID
   -  RLOC
   -  ITR
   -  ETR
   -  xTR
   -  PITR
   -  PETR
   -  MR
   -  MS
   -  DFZ

Appendix B.  Other Appendices

B.1.  Old LISP 'Models'

   LISP, as initilly conceived, had a number of potential operating
   modes, named 'models'.  Although they are now obsolete, one
   occasionally sees mention of them, so they are briefly described
   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.2.  Possible Other Appendices

   -- Location/Identity Separation Brief History
   -- LISP History

Author's Address

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

   EMail: jnc@mit.edu


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