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Versions: 00 01 draft-ietf-lisp-introduction

LISP Working Group                                         J. N. Chiappa
Internet-Draft                              Yorktown Museum of Asian Art
Intended status: Informational                             July 16, 2012
Expires: January 17, 2013


    An Introduction to the LISP Location-Identity Separation System
                   draft-chiappa-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. It is intended to be both easy to
   follow, and also give a fairly detailed understanding of the entire
   system.

Status of This Memo

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   This Internet-Draft will expire on January 17, 2013.

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Table of Contents

   1. Background
   2. Deployment Philosophy
     2.1. Economics
     2.2. Maximize Re-use of Existing Mechanism
     2.3. Self-Deployment
   3. LISP Overview
     3.1. Basic Approach
     3.2. Basic Functionality
     3.3. Mapping from EIDs to RLOCs
     3.4. Interworking With Non-LISP-Capable Endpoints
   4. Initial Applications
     4.1. Provider Independence
     4.2. Multi-Homing
     4.3. Traffic Engineering
     4.4. Mobility
     4.5. IP Version Reciprocal Traversal
     4.6. Local Uses
   5. Major Functional Subsystems
     5.1. xTRs
     5.2. Mapping System
       5.2.1. Mapping System Organization
       5.2.2. Interface to the Mapping System
       5.2.3. Indexing Subsystem
   6. Examples of Operation
     6.1. An Ordinary Packet's Processing
     6.2. A Mapping Cache Miss
   7. Design Approach
     7.1. Quick Implement-Test Loop
       7.1.1. No Desk Fixes
       7.1.2. Code Before Documentation
     7.2. Only Fix Real Problems
     7.3. No Theoretical Perfection
       7.3.1. No Ocean Boiling
     7.4. Just Enough Security
   8. xTRs
     8.1. When to Encapsulate
     8.2. UDP Encapsulation Details
     8.3. Header Control Channel
       8.3.1. Echo Nonces
       8.3.2. Instances
     8.4. Fragmentation
     8.5. Mapping Gleaning in ETRs
   9. The Mapping System
     9.1. The Indexing Subsystem
     9.2. The Mapping System Interface
       9.2.1. Map-Request Messages
       9.2.2. Map-Reply Maessages
       9.2.3. Map-Register and Map-Notify Messages
       9.2.4. Map-Referral Messages
     9.3. Reliability via Replication
     9.4. Extended Tools
     9.5. Expected Performance
   10. Deployment Mechanisms
     10.1. Internetworking Mechanism
     10.2. Proxy Devices
       10.2.1. PITRs
       10.2.2. PETRs
     10.3. LISP-NAT
     10.4. LISP and DFZ Routing
     10.5. Use Through NAT Devices
       10.5.1. First-Phase NAT Support
       10.5.2. Second-Phase NAT Support
   11. Current Improvements
     11.1. Mapping Versioning
     11.2. Replacement of ALT with DDT
       11.2.1. Why Not Use DNS
     11.3. Mobile Device Support
     11.4. Multicast Support
     11.5. {{Any others?}}
   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. Acknowledgments
   14. IANA Considerations
   15. Security Considerations
   16. References
     16.1. Normative References
     16.2. Informative References
   Appendix A. Glossary/Definition of Terms
   Appendix B. Other Appendices

1. 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.

   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. [LISP]

   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).

2. 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.

2.1. 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 just a mildly interesting piece of paper.

   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.

2.2. 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 devices in the Internet (both hosts and routers), and also
   initially reuses, whereever 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
   succeeds.

2.3. Self-Deployment

   LISP has deliberately employed a rather different deployment model,
   which we might call 'self-deployment'; 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
   initial snowball, and finds a hill of the right steepness (i.e. the
   right path for it to travel, once it starts moving), one's snowball
   is not going to go anywhere on its own. However, if one has picked
   one's spot correctly, little additional work is needed - just stand
   back and watch it go.

3. 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 [Architecture],
   Section "Namespaces-EIDs-Residual").

   LISP separates the functions of location and identity, current
   intermingled in IPvN addresses. (This document uses the meaning for
   'address' proposed in [Atkinson], i.e. a name with mixed location and
   identity semantics.)

3.1. 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',
   and an 'identifier' (a name which serves only to provide a persistent
   handle for the node), called an 'EID'. A node may have more than one
   RLOC, or its RLOC may change over time (e.g. if the node is mobile),
   but it keeps 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 [Architecture], 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.

3.2. 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 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 one can
   view as 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 bindings from EIDs to
   RLOCS. The destination RLOC will be (initially at least) the address
   of the LISP device near the destination (the Egress Tunnel Router, or
   'ETR').

   The ITR then generates a new outer header for the original packet,
   with that header containing the 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'.

3.3. 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, to limit the amount of control traffic to
   a practicable level. (The mapping system will be discussed in more
   detail below, in Section 5.2 and Section 9)

   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. [Iannone] (This subject will
   be discussed in more detail in Section 9.5, below.)

3.4. Interworking With Non-LISP-Capable Endpoints

   The capability for 'easy' interoperation between nodes using LISP,
   and existing non-LISP-using hosts or sites (often called 'legacy'
   hosts), 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 sites. Another approach uses a device with combined LISP
   and NAT ([RFC1631]) functionality, named a LISP-NAT.

4. 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 [Architecture], in the
   "Goals of LISP" section).

4.1. 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 binding from the EIDs to the new RLOCs.

4.2. 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.

4.3. 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 visualized when the Internet was designed, so support for it
   is somewhat in the 'when the only tool you have is a hammer,
   everything looks like nail' category.

   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 5.2 below.)

4.4. 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.

4.5. 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'.

4.6. 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.

5. Major Functional Subsystems

   LISP has only two major functional subsystems - 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 8 and
   Section 9, respectively).

5.1. 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 sites may be
   sent 'vanilla'); 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 8.2), along
   with an additional IPvN header (to hold the asource 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 'vanilla' 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 indexing system (next section); and
   answering requests for mappings (ditto).

5.2. Mapping System

   The mapping database is a distributed, and potentially replicated,
   database which holds bindings between EIDs (identity) and RLOCs
   (location). To be exact, it contains bindings 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 binding 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.

5.2.1. Mapping System Organization

   The mapping system is actually split into two major functional sub-
   systems. The actual bindings themselves are held by the ETRs, and an
   ITR which needs a binding effectively gets it 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
   subsystem, an 'indexing system', itself also a distributed,
   potentally replicated database, provides information on which ETR(s)
   are authoritative sources of information about the bindings which are
   available.

5.2.2. Interface to the Mapping System

   The client interface to the mapping system from an ITR's point of
   view is not with the indexing system directly; rather, it is through
   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 uses the
   indexing system to eventually forward the Map-Request to the
   appropriate ETR. The ETR formulates reply control messages (Map-
   Reply packets), which is conveyed to the ITR. The details of the
   indexing system, etc, are thus hidden from the 'ordinary' 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, 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.

5.2.3. Indexing Subsystem

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

   Again, extensive large-scale simulations driven by lengthy recordings
   of actual traffic at several major sites, have been performed to
   verify the effectiveness of this particular indexing system. [Jakab]

6. 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 destination (at a particular ITR) is to be
   processed by LISP.

6.1. 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 open HTTP connection) as it makes its way from the source host to
   the destination.

   {{Rest to be written.}}

6.2. 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.

   {{Rest to be written.}}

7. Design Approach

   Before describing LISP's components in more detail below, it may be
   worth saying a few words about the design philosophy used in creating
   them - this may make clearer the reasons for some engineering choices
   in the mechanisms given there.

7.1. Quick Implement-Test Loop

   LISP uses a philosophy similar to that used in the early days of the
   Internet, which is to just build it, then try it and see what
   happens, and move forward from there based on what actually happens.
   The concept has been to get something up and running, and then modify
   it based on testing and experience.

7.1.1. No Desk Fixes

   Don't try and forsee all issues from desk analysis. (Which is not to
   say that one should not spend _some_ time on trying to forsee
   problems, but be aware that it is a 'diminishing returns' process.)
   The performance of very large, complex, physically distributed
   systems is hard to predict, so rather than try (which would
   necessarily be an incomplete exercise anyway, testing would
   inevitably be required eventually), at a certain point it's better
   just to get on with it - and you will learn a host of other lessons
   in the process, too.

7.1.2. Code Before Documentation

   This is often a corollary to the kind of style described above.
   While it probably would not have been possible in a large,
   inhomogenous group, the small, close nature of the LISP
   implementation group did allow this approach.

7.2. Only Fix Real Problems

   Don't worry about anything unless experience show it's a real
   problem. For instance, in the early stages, much was made out of the
   problem of 'what does an ITR do if it gets a packet, but does not
   (yet) have a mapping for the destination?'

   In practise, simply dropping such packets has just not proved to be a
   problem; the higher level protocol will retransmit them after a
   timeout, and the mapping is usually in place by then. So spending a
   lot of time (and its companion, energy) and mechanism (and _its_
   extremely undesirable companion, complexity) on solving this
   'problem' would not have been the most efficient approach, overall.

7.3. No Theoretical Perfection

   Attack hard problems with a number of cheap and simple mechanisms
   that co-operate and overlap. Trying to find a single mechanism that
   is all of:

   - Robust
   - Efficient
   - Fast

   is often (usually?) a fool's errand. (The analogy to the aphorism
   'Fast, Cheap, Good - Pick Any Two' should be obvious.)  However, a
   collection of simple and cheap mechanisms may effectively be able to
   meet all of these goals (see, for example, ETR Liveness/Reachability,
   Section 12.4).

   Yes, this results in a system which is not provably correct in all
   circumstances. The world, however, is full of such systems - and in
   the real world, effective robustness is more likely to result from
   having multiple, overlapping mechanisms than one single high-powered
   (and inevitably complex) one. In the world of civil engineering,
   redundancy is now accepted as a key design principle; the same should
   be true of information systems. [Salvadori]

7.3.1. No Ocean Boiling

   Don't boil the ocean to kill a single fish. This is a combination of
   7.2 (Only Fix Real Problems) and 7.3 (No Theoretical Perfection); it
   just means that spending a lot of complexity and/or overhead to deal
   with a problem that's not really a problem is not good engineering.

7.4. Just Enough Security

   How much security to have is a complex issue. It's relatively easy
   for designers to add good security, but much harder to get the users
   to jump over all the hoops necessary to use it. LISP has therefore
   adopted a position where we add 'just enough' security.

   The overall approach to security in LISP is fairly subtle, though,
   and is covered in more detail elsewhere (in [Architecture], Section
   "Security").

8. xTRs

   As mentioned above (in Section 5.1), 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.

8.1. When to Encapsulate

   An ITR knows that a destination is running 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 destination is not running LISP, and
   the packet can be forwarded normally.

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

8.2. UDP Encapsulation Details

   The UDP encapsulation used by LISP for carrying traffic from ITR to
   ETR, and many of the details of how the 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 2.1) 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 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.

8.3. Header Control Channel

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

   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.

8.3.1. Echo Nonces

   One important use 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.)

8.3.2. 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).

8.4. 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.

8.5. Mapping Gleaning in ETRs

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

   As previously mentioned, the mapping system is split into an indexing
   subsystem, which keeps track of where all the mappings are kept, and
   the mappings themsleves, the authoritative copies of which are always
   held by ETRs.

9.1. The Indexing Subsystem

   The indexing system in LISP is currently implemented by the DDT
   system. LISP initially used (for ease of getting something
   operational without having to write a lot of code) an indexing system
   called ALT, which used BGP running over virtual tunnels. [ALT] This
   proved to have a number of issues, and has now been superseded by
   DDT.

   In DDT, the EID namespace(s) are instantiated as a tree of DDT nodes.
   Starting with the root node(s), which have 'reponsibility' for the
   entire namespace, portions of the namespace are delegated to child
   nodes, in a recursive process extending through as many levels as are
   needed. Eventually, leaf nodes in the DDT tree delegate namespace
   blocks to ETRs.

   MRs obtain information about delegations by interrogating DDT nodes,
   and caching the results. aThis allows them, when passed a request for
   a mapping by an ITR, to forward the mapping request to the
   appropriate ETR (perhaps after loading some missing delegation
   entries into their delegation cache).

9.2. The Mapping System Interface

   As mentioned in Section 5.2.2, 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
   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.

9.2.1. 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 [Architecture], 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 8.5).

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

9.2.2. Map-Reply Maessages

   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 5.2), and multicast priority
   and weight fields.

9.2.3. 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 11.1), 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.

9.2.4. 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.

   There are also optional authentication fields; see [Architecture],
   Section "Security-Mappings" for more.

9.3. 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.

   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.

9.4. 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 are '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.

9.5. Expected Performance

   {{To be written.}}

10. 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.

10.1. Internetworking Mechanism

   One aspect which has received a lot of attention are the mechanisms
   previously referred to (in Section 3.4) 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.

10.2. 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 sites, without encapsulation).

   Note that return traffic _to_ a legacy site 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
   destination. However, for some kinds of LISP operation (e.g. mobile
   nodes), this is not possible; in these situations, the PETR is
   needed.

10.2.1. 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 8.1), an ITR at another LISP site
   can avoid using a PITR (i.e. it can detect that a given destination
   is not a legacy site, if a PITR is advertising it into the DFZ) by
   checking to see if a LISP mapping exists for that 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, it 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.

10.2.2. PETRs

   PETRs (proxy ETRs) serve as ETRs for LISP traffic _to_ legacy hosts,
   for cases where a LISP device cannot send packets to sites 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 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 source along its entir
   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 destination. However, this is not
   advised: as mentioned, it is easy to tell if something is a legacy
   destination.

10.3. 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]

10.4. LISP and DFZ Routing

   {{To be written.}}

10.5. 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.

10.5.1. 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... )

10.5.2. 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. [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. Current Improvements

   In line with the philosophies laid out in Section 7, LISP is
   something of a moving target. This section discusses some of the
   contemporaneous improvements being made to LISP.

11.1. Mapping Versioning

   As mentioned, LISP has been under development for a considerable
   time. One early addition to LISP (it is already part of the base
   specification) is mapping versioning; i.e. the application of
   identifying sequence numbers 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 (see Section 8.3).

   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.

11.2. Replacement of ALT with DDT

   As mentioned in Section 9.2, an interface is provided to allow
   replacement of the indexing subsystem. LISP initially used an
   indexing system called ALT. [ALT] ALT was relatively easy to
   construct from existing tools (GRE, BGP, etc), but it had a number of
   issues that made it unsuitable for large-scale use. ALT is now being
   superseded by DDT.

   As indicated previously (Section 9.5), the basic structure and
   operation of DDT is identical to that of TREE, so the extensive
   simulation work done for TREE applies equally to DDT, as do the
   conclusions drawn about TREE's superiority to ALT. [Jakab]

   {{Briefly synopsize results}}

11.2.1. Why Not Use DNS

   One obvious question is 'Since DDT is so similar to DNS, why not
   simply use DNS?'  In particular, people are familiar with the DNS,
   how to configure it, etc - would it not thus be preferable to use it?
   To completely answer this would take more space that available here,
   but, briefly, there were two main reasons, and one lesser one.

   First, the syntax of DNS names did not lend itself to looking up
   names in other syntaxes (e.g. bit fields). This is a problem which
   has been previously encountered, e.g. in reverse address lookups.
   [RFC5855]

   Second, as an existing system, the interfaces between DNS (should it
   have been used as an indexing subsystem for LISP) would not be
   'tuneable' to be optimal for LISP. For instance, if it were desired
   to have the leaf node in an indexing lookup directly contact the ETR
   on behalf of the node doing the lookup (thereby avoiding a round-trip
   delay), that would not be easy without modifications to the DNS code.
   Obviously, with a 'custom' system, this issue does not arise.

   Finally, DNS security, while robust, is fairly complex. Doing DDT
   offered an opportunity to provide a more nuanced security model.
   (See [Architecture], Section "Security" for more about this.)

11.3. 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, whereever 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 8.1), 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 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]

11.4. 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 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 isa 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.

11.5. {{Any others?}}

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 the
   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

   {{To be written.}}

12.2.3. Outdated Mappings - No Longer an ETR

   If the destination of traffic from an ITR is no longer an ETR, one
   might get an ICMP Destination Unreachable error message. However,
   one cannot depend on that. The following mechanism will work,
   though.

   Since the destination is not an ETR, the echoing reachability
   detection mechanism (see Section 8.3.1) 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 re-map may be requested.

12.3. Erroneous mappings

   {{To be written.}}

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 (Section 7.3), 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), 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. Acknowledgments

   The author would like thank 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 especially Joel Halpern for their careful
   review of, and helpful suggestions for, this document. Grateful
   thanks also to Darrel Lewis for his help with material on non-
   Internet uses of LISP, and to Vince Fuller for help with XML.

   A final thanks is due to John Wrocklawski for the author's
   organizational affiliation. This memo was created using the xml2rfc
   tool

14. IANA Considerations

   This document makes no request of the IANA.

15. Security Considerations

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

16. References

16.1. 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.

   [Architecture]  J.N. Chiappa, "The Architecture of the LISP Location-
                   Identity Separation System",
                   draft-chiappa-lisp-architecture-00 (work in
                   progress), July 2012.

   [DDT]           V. Fuller, D. Lewis, and D. Farinacci, "LISP
                   Delegated Database Tree", draft-fuller-lisp-ddt-01
                   (work in progress), March 2012.

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

   [Interworking]  D. Lewis, D. Meyer, D. Farinacci, and V. Fuller,
                   "Interworking LISP with IPv4 and IPv6",
                   draft-ietf-lisp-interworking-06 (work in progress),
                   March 2012.

   [LISP]          D. Farinacci, V. Fuller, D. Meyer, and D. Lewis,
                   "Locator/ID Separation Protocol (LISP)",
                   draft-ietf-lisp-23 (work in progress), May 2012.

   [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,
                   "LISP for Multicast Environments",
                   draft-ietf-lisp-multicast-14 (work in progress),
                   February 2012.

   [NAT]           V. Ermagan, D. Farinacci, D. Lewis, J. Skriver,
                   F. Maino, and C. White, "NAT traversal for LISP",
                   draft-ermagan-lisp-nat-traversal-01 (work in
                   progress), March 2012.

   [Versioning]    L. Iannone, D. Saucez, and O. Bonaventure, "LISP
                   Mapping Versioning",
                   draft-ietf-lisp-map-versioning-09 (work in progress),
                   March 2012.

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

16.2. 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.

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

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

   [RFC5855]       J. Abley and T. Manderson, "Nameservers for IPv4 and
                   IPv6 Reverse Zones", RFC 5855, May 2010.

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

   [ALT]           D. Farinacci, V. Fuller, D. Meyer, and D. Lewis,
                   "LISP Alternative Topology (LISP-ALT)",
                   draft-ietf-lisp-alt-10 (work in progress),
                   December 2011.

   [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.

   [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.

   [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.

   [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.

   [Salvadori]     M. Salvadori and M. Levy, "Why Buildings Fall Down",
                   W. W. Norton, New York, pg. 81, 1992.

Appendix A. Glossary/Definition of Terms

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

Appendix B. Other Appendices

   Possible appendices:

   -- Location/Identity Separation Brief History
   -- LISP History
   -- Old models (LISP 1, LISP 1.5, etc)

Author's Address

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

   EMail: jnc@mit.edu


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