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Versions: (draft-fuller-lisp-ddt) 00 01

Network Working Group                                          V. Fuller
Internet-Draft                                                  D. Lewis
Intended status: Experimental                                 V. Ermagan
Expires: September 29, 2013                                cisco Systems
                                                                 A. Jain
                                                        Juniper Networks
                                                          March 28, 2013


                      LISP Delegated Database Tree
                       draft-ietf-lisp-ddt-01.txt

Abstract

   This draft describes the LISP Delegated Database Tree (LISP-DDT), a
   hierarchical, distributed database which embodies the delegation of
   authority to provide mappings from LISP Endpoint Identifiers (EIDs)
   to Routing Locators (RLOCs).  It is a statically-defined distribution
   of the EID namespace among a set of LISP-speaking servers, called DDT
   nodes.  Each DDT node is configured as "authoritative" for one or
   more EID-prefixes, along with the set of RLOCs for Map Servers or
   "child" DDT nodes to which more-specific EID-prefixes are delegated.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   working documents as Internet-Drafts.  The list of current Internet-
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   This Internet-Draft will expire on September 29, 2013.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definition of Terms . . . . . . . . . . . . . . . . . . . . .   4
   3.  Database organization . . . . . . . . . . . . . . . . . . . .   6
     3.1.  EID-prefix tree structure and instance IDs  . . . . . . .   6
     3.2.  Configuring prefix delegation . . . . . . . . . . . . . .   7
       3.2.1.  The root DDT node . . . . . . . . . . . . . . . . . .   7
   4.  The Map-Referral message  . . . . . . . . . . . . . . . . . .   7
     4.1.  Action codes  . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Referral Set  . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Incomplete flag . . . . . . . . . . . . . . . . . . . . .   9
   5.  DDT network elements and their operation  . . . . . . . . . .   9
     5.1.  DDT node  . . . . . . . . . . . . . . . . . . . . . . . .   9
       5.1.1.  Match of a delegated prefix (or sub-prefix) . . . . .   9
       5.1.2.  Missing delegation from an authoritative prefix . . .  10
     5.2.  DDT Map Server  . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  DDT Map Resolver  . . . . . . . . . . . . . . . . . . . .  10
       5.3.1.  Queuing and sending DDT Map-Requests  . . . . . . . .  10
       5.3.2.  Receiving and following referrals . . . . . . . . . .  11
       5.3.3.  Handling referral errors  . . . . . . . . . . . . . .  13
       5.3.4.  Referral loop detection . . . . . . . . . . . . . . .  13
   6.  Psuedo Code and Decision Tree diagrams  . . . . . . . . . . .  14
     6.1.  Map Resolver processing of ITR Map-Request  . . . . . . .  14
       6.1.1.  Pseudo-code summary . . . . . . . . . . . . . . . . .  14
       6.1.2.  Decision tree diagram . . . . . . . . . . . . . . . .  14
     6.2.  Map Resolver processing of Map-Referral message . . . . .  16
       6.2.1.  Pseudo-code summary . . . . . . . . . . . . . . . . .  16
       6.2.2.  Decision tree diagram . . . . . . . . . . . . . . . .  18
     6.3.  DDT Node processing of DDT Map-Request message  . . . . .  20
       6.3.1.  Pseudo-code summary . . . . . . . . . . . . . . . . .  20
       6.3.2.  Decision tree diagram . . . . . . . . . . . . . . . .  20
   7.  Example topology and request/referral following . . . . . . .  21
     7.1.  Lookup of 10.1.1.1/32 by ITR1 . . . . . . . . . . . . . .  23
     7.2.  Lookup of 10.17.8.1/32 by ITR2  . . . . . . . . . . . . .  24
     7.3.  Lookup of 10.2.2.2/32 by ITR1 . . . . . . . . . . . . . .  25
     7.4.  Lookup of 10.16.2.1/32 by ITR2  . . . . . . . . . . . . .  25
     7.5.  Lookup of 10.16.0.1/32 (non-existant EID) by ITR2 . . . .  26
   8.  Securing the database and message exchanges . . . . . . . . .  27
     8.1.  XEID-prefix Delegation  . . . . . . . . . . . . . . . . .  27
     8.2.  DDT node operation  . . . . . . . . . . . . . . . . . . .  28



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       8.2.1.  DDT public key revocation . . . . . . . . . . . . . .  28
     8.3.  Map Server operation  . . . . . . . . . . . . . . . . . .  28
     8.4.  Map Resolver operation  . . . . . . . . . . . . . . . . .  29
   9.  Open Issues and Considerations  . . . . . . . . . . . . . . .  29
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  30
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     12.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  32
   Appendix B.  Map-Referral Message Format  . . . . . . . . . . . .  32
     B.1.  SIG section . . . . . . . . . . . . . . . . . . . . . . .  34
   Appendix C.  Encapsulated Control Message Format  . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   LISP [RFC6830] specifies an architecture and mechanism for replacing
   the addresses currently used by IP with two separate name spaces:
   relatively static Endpoint Identifiers (EIDs), used end-to-end for
   terminating transport-layer associations, and Routing Locators
   (RLOCs), which are more dynamic, are bound to topological location,
   and are used for routing and forwarding through the Internet
   infrastructure.

   LISP offers a general-purpose mechanism for mapping between EIDs and
   RLOCs.  In organizing a database of EID to RLOC mappings, this
   specification extends the definition of the EID numbering space by
   logically prepending and appending several fields for purposes of
   defining the database index key: Database-ID (DBID, 16 bits),
   Instance dentifier (IID, 32-bits), Address Family Identifier (16
   bits), and EID-prefix (variable, according to AFI value).  The
   resulting concatenation of these fields is termed an "Extended EID
   prefix" or XEID-prefix.

   The term "Extended EID" (XEID) is also used for an individual LISP
   EID that is further qualified through the use of an Instance ID.  See
   [LCAF] for further discussion of the use of Instance IDs.

   The DBID is provided for possible use in case a need evolves for
   another, higher level in the hierarchy, to allow the creation of
   multiple, separate database trees.

   LISP-DDT is a hierarchical distributed database which embodies the
   delegation of authority to provide mappings, i.e.  its internal
   structure mirrors the hierarchical delegation of address space.  It
   also provides delegation information to Map Resolvers, which use the
   information to locate EID-to-RLOC mappings.  A Map Resolver which



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   needs to locate a given mapping will follow a path through the tree-
   structured database, contacting, one after another, the DDT nodes
   along that path until it reaches the leaf DDT node(s) authoritative
   for the mapping it is seeking.

   LISP-DDT defines a new device type, the DDT node, that is configured
   as authoritative for one or more XEID-prefixes.  It also is
   configured with the set of more-specific sub-prefixes that are
   further delegated to other DDT nodes.  To delegate a sub-prefix, the
   "parent" DDT node is configured with the RLOCs of each child DDT node
   that is authoritative for the sub-prefix.  Each RLOC either points to
   a Map Server (sometimes termed a "terminal DDT node") to which an
   Egress Tunnel Routers (ETRs) has registered that sub-prefix or points
   to another DDT node in the database tree that further delegates the
   sub-prefix.  See LISP-MS [RFC6833] for a description of the
   functionality of the Map Server and Map Resolver.  Note that the
   target of a delegation must always be an RLOC (not an EID) to avoid
   any circular dependency.

   To provide a mechanism for traversing the database tree, LISP-DDT
   defines a new LISP message type, the Map-Referral, which is returned
   to the sender of a Map-Request when the receiving DDT node can refer
   the sender to another DDT node that has more detailed information.
   See Section 4 for the definition of the Map-Referral message.

   To find an EID-to-RLOC mapping, a LISP-DDT client, usually a DDT Map
   Resolver, starts by sending an Encapsulated Map-Request to a
   preconfigured DDT node RLOC.  The DDT node responds with a Map-
   Referral message that either indicates that it will find the
   requested mapping to complete processing of the request or that the
   DDT client should contact another DDT node that has more-specific
   information; in the latter case, the DDT node then sends a new
   Encapsulated Map-Request to the next DDT node and the process repeats
   in an iterative manner.

   Conceptually, this is similar to the way that a client of the Domain
   Name System (DNS) follows referrals (DNS responses that contain only
   NS records) from a series of DNS servers until it finds an answer.

2.  Definition of Terms

   Extended EID (XEID):   a LISP EID, optionally extended with a non-
      zero Instance ID (IID) if the EID is intended for use in a context
      where it may not be a unique value, such as on a Virtual Private
      Network where [RFC1918] address space is used.  See "Using
      Virtualization and Segmentation with LISP" in [RFC6830] for more
      discussion of Instance IDs.




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   XEID-prefix:   a LISP EID-prefix with 16-bit LISP-DDT DBID (provided
      to allow the definition of multiple databases; currently always
      zero in this version of DDT, with other values reserved for future
      use), 32-bit IID and 16-bit AFI prepended.  An XEID-prefix is used
      as a key index into the database.

   DDT node:   a network infrastructure component responsible for
      specific XEID-prefix and for delegation of more-specific sub-
      prefixes to other DDT nodes.

   DDT client:   a network infrastructure component that sends Map-
      Request messages and implements the iterative following of Map-
      Referral results.  Typically, a DDT client will be a Map Resolver
      but it is also possible for an ITR to implement DDT client
      functionality.

   DDT Map Server:   a DDT node that also implements Map Server
      functionality (forwarding Map-Requests and/or returning Map-
      Replies if offering proxy Map-Reply service) for a subset of its
      delegated prefixes.

   DDT Map Resolver:   a network infrastructure element that accepts a
      Map-Request, adds the XEID to its pending request list, then
      queries one or more DDT nodes for the requested EID, following
      returned referrals until it receives one with action code MS-ACK
      (or an error indication).  MS-ACK indicates that the Map-Request
      has been sent to a Map Server that will forward it to an ETR that,
      in turn, will provide a Map-Reply to the original sender.  A DDT
      Map Resolver maintains both a cache of Map-Referral message
      results containing RLOCs for DDT nodes responsible for XEID-
      prefixes of interest (termed the "referral cache") a pending
      request list of XEIDs that are being resolved through iterative
      querying of DDT nodes.

   Encapsulated Map-Request:   a LISP Map-Request carried within an
      Encapsulated Control Message, which has an additional LISP header
      prepended.  Sent to UDP destination port 4342.  The "outer"
      addresses are globally-routable IP addresses, also known as RLOCs.
      Used by an ITR when sending to a Map Resolver and by a Map Server
      when forwarding a Map-Request to an ETR as documented in LISP-MS
      [RFC6833].

   DDT Map-Request:   an Encapsulated Map-Request sent by a DDT client
      to a DDT node.  The "DDT-originated" flag is set in the
      encapsulation header indicating that the DDT node should return
      Map-Referral messages if the Map-Request EID matches a delegated
      XEID-prefix known to the DDT node.  Section 5.3.1 describes how
      DDT Map-Requests are sent.



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   Authoritative XEID-prefix:   an XEID-prefix delegated to a DDT node
      and for which the DDT node may provide further delegations of
      more-specific sub-prefixes.

   Map-Referral:   a LISP message sent by a DDT node in response to a
      DDT Map-Request for an XEID that matches a configured XEID-prefix
      delegation.  A non-negative Map-Referral includes a "referral", a
      set of RLOCs for DDT nodes that have more information about the
      sub-prefix; a DDT client "follows the referral" by sending another
      DDT Map-Request to one of those RLOCs to obtain either an answer
      or another referral to DDT nodes responsible for a more-specific
      XEID-prefix.  See Section 5.1 and Section 5.3.2 for details on the
      sending and processing of Map-Referral messages.

   negative Map-Referral:   a Map-Referral sent in response to a DDT
      Map-Request that matches an authoritative XEID-prefix but for
      which there is no delegation configured (or no ETR registration if
      sent by a DDT Map-Server).

   Pending Request List:   the set of outstanding requests for which a
      DDT Map Resolver has received encapsulated Map-Requests from a DDT
      client for an XEID.  Each entry in the list contains additional
      state needed by the referral following process, including the
      requestor(s) of the XEID (typically, one or more ITRs), saved
      information about the last referral received and followed
      (matching XEID-prefix, action code, RLOC set, index of last RLOC
      queried in the RLOC set), and any [LISP-SEC] information that was
      included in the DDT client Map-Request.  An entry in the list may
      be interchangeably termed a "pending request list entry" or simply
      a "pending request".

   For definitions of other terms, notably Map-Request, Map-Reply,
   Ingress Tunnel Router (ITR), Egress Tunnel Router (ETR), Map Server,
   and Map Resolver, please consult the LISP specification [RFC6830] and
   the LISP Mapping Service specification [RFC6833].

3.  Database organization

3.1.  EID-prefix tree structure and instance IDs

   LISP-DDT defines a tree structure that is indexed by a binary
   encoding of five fields, in order of significance: DBID (16 bits),
   Instance Identifier (IID, 32 bits), Address Family Identifier (AFI,
   16 bits), and EID-prefix (variable, according to AFI value).  The
   fields are concatenated, with the most significant fields as listed
   above.  The index into this structure is also referred to as an
   Extended EID-prefix (XEID-prefix).




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   It is important to note that LISP-DDT does not store actual EID-to-
   RLOC mappings; it is, rather, a distributed index that can be used to
   find the devices (Map Servers and their registered EIDs) that can be
   queried with LISP to obtain those mappings.  Changes to EID-to-RLOC
   mappings are made on the ETRs which define them, not to any DDT node
   configuration.  DDT node configuration changes are only required when
   branches of the database hierarchy are added, removed, or modified.

3.2.  Configuring prefix delegation

   Every DDT node is configured with one or more XEID-prefixes for which
   it is authoritative along with a list of delegations of XEID-prefixes
   to other DDT nodes.  A DDT node is required to maintain a list of
   delegations for all sub-prefixes of its authoritative XEID-prefixes;
   it also may list "hints", which are prefixes that it knows about that
   belong to its parents, to the root, or to any other point in the
   XEID-prefix hierarchy.  A delegation (or hint) consists of an XEID-
   prefix, a set of RLOCs for DDT nodes that have more detailed
   knowledge of the XEID-prefix, and accompanying security information.
   Those RLOCs are returned in Map-Referral messages when the DDT node
   receives a DDT Map-Request with an xEID that matches a delegation.  A
   DDT Map Server will also have a set of sub-prefixes for which it
   accepts ETR mapping registrations and for which it will forward (or
   answer, if it provides proxy Map-Reply service) Map-Requests.  For
   details of security infomation in Map-Referrals see Section 8.

3.2.1.  The root DDT node

   The root DDT node is the logical "top" of the database hierarchy:
   DBID=0, IID=0, AFI=0, EID-prefix=0/0.  A DDT Map-Request that matches
   no configured XEID-prefix will be referred to the root node.  The
   root node in a particular instantiation of LISP-DDT must therefore be
   configured with delegations for at least all defined IIDs and AFIs.

   To aid in defining a "sub-root" DDT node that is responsible for all
   EID-prefixes within multiple IIDs (say, for using LISP to create
   virtual networks that use overlapping address space), it may be
   useful to implement configuration language that allows for a range of
   IIDs to be delegated together.  Additional configuration shorthand
   for delegating a range of IIDs (and all of the EIDs under them) may
   also be helpful.

4.  The Map-Referral message

   A Map-Referral message is sent by a DDT node to a DDT client in
   response to a DDT Map-Request message.  The message consists of an
   action code along with delegation information about the XEID-prefix
   that matches the XEID requested.



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   See Appendix B for a detailed layout of the Map-Referral message
   fields.

4.1.  Action codes

   The action codes are as follows:

   NODE-REFERRAL (0):  indicates that the replying DDT node has
      delegated an XEID-prefix that matches the requested XEID to one or
      more other DDT nodes.  The Map-Referral message contains a "map-
      record" with additional information, most significantly the set of
      RLOCs to which the prefix has been delegated, that is used by a
      DDT Map Resolver to "follow" the referral.

   MS-REFERRAL (1):  indicates that the replying DDT node has delegated
      an XEID-prefix that matches the requested XEID to one or more DDT
      Map Servers.  It contains the same additional information as a
      NODE-REFERRAL but is handled slightly differently by the receiving
      DDT client (see Section 5.3.2).

   MS-ACK (2):  indicates that a replying DDT Map Server received a DDT
      Map-Request that matches an authoritative XEID-prefix for which is
      has one or more registered ETRs.  This means that the request can
      be forwarded to one of those ETRs to provide an answer to the
      querying ITR.

   MS-NOT-REGISTERED (3):  indicates that the replying DDT Map Server
      received a Map-Request for one of its configured XEID-prefixes
      which has no ETRs registered.

   DELEGATION-HOLE (4):  indicates that the requested XEID matches a
      non-delegated sub-prefix of the XEID space.  This is a non-LISP
      "hole", which has not been delegated to any DDT Map Server or ETR.
      See Section 5.1.2 for details.

   NOT-AUTHORITATIVE (5):  indicates that the replying DDT node received
      a Map-Request for an XEID-request for which it is not
      authoritative.  This can occur if a cached referral has become
      invalid due to a change in the database hierarchy.

4.2.  Referral Set

   For "positive" action codes (NODE-REFERRAL, MS-REFERRAL, MS-ACK), a
   DDT node includes in the Map-Referral message a list of RLOCs for all
   DDT nodes that are authoritative for the XEID-prefix being returned;
   a DDT Map Resolver uses this information to contact one of those DDT
   nodes as it "follows" a referral.




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4.3.  Incomplete flag

   A DDT node sets the "Incomplete" flag in a Map-Referral message if
   the Referral Set is incomplete; this is intended to prevent a DDT Map
   Resolver from caching a referral with incomplete information.  A DDT
   node must set the "incomplete" flag in the following cases:

   o  If it is setting action code MS-ACK or MS-NOT-REGISTERED but does
      not have configuration for other "peer" DDT nodes that are also
      authoritative for the matched XEID-prefix.

   o  If it is setting action code NOT-AUTHORITATIVE.

5.  DDT network elements and their operation

   As described above, DDT introduces a new network element, the "DDT
   node", extends the functionality of Map Servers and Map Resolvers to
   send and receive Map-Referral messages.  The operation of each of
   these devices is described as follows.

5.1.  DDT node

   When a DDT node receives a DDT Map-Request, it compares the requested
   XEID against its list of XEID-prefix delegations and its list of
   authoritative XEID-prefixes and acts as follows:

5.1.1.  Match of a delegated prefix (or sub-prefix)

   If the requested XEID matches one of the DDT node's delegated
   prefixes, then a Map-Referral message is returned with the matching
   more-specific XEID-prefix and the set of RLOCs for the referral
   target DDT nodes including associated security information (see
   Section 8 for details on security).  If the delegation is known to be
   a DDT Map Server, then the Map-Referral message is sent with action
   code MS-REFERRAL to indicate to the receiver that LISP-SEC
   information (if saved in the pending request) should be included in
   the next DDT Map-Request; otherwise, the action code NODE-REFERRAL is
   used.

   Note that a matched delegation does not have to be for a sub-prefix
   of an authoritative prefix; in addition to being configured to
   delegate sub-prefixes of an authoritative prefix, a DDT node may also
   be configured with other XEID-prefixes for which it can provide
   referrals to DDT nodes anywhere in the database hierarchy.  This
   capability to define "shortcut hints" is never required to be
   configured but may be a useful heuristic for reducing the number of
   iterations needed to find an EID, particular for private network
   deployments.



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5.1.2.  Missing delegation from an authoritative prefix

   If the requested XEID did not match a configured delegation but does
   match an authoritative XEID-prefix, then the DDT node returns a
   negative Map-Referral that uses the least-specific XEID-prefix that
   does not match any XEID-prefix delegated by the DDT node.  The action
   code is set to DELEGATION-HOLE; this indicates that the XEID is not a
   LISP destination.

   If the requested XEID did not match either a configured delegation or
   an authoritative XEID-prefix, then the request is dropped and a
   negative Map-Referral with action code NOT-AUTHORITATIVE is returned.

5.2.  DDT Map Server

   When a DDT Map Server receives a DDT Map-Request, its operation is
   similar to that of a DDT node with additional processing as follows:

   o  If the requested XEID matches a registered XEID-prefix, then the
      Map-Request is forwarded to one of the destination ETR RLOCs (or
      the Map Server sends a Map-Reply, if it is providing proxy Map-
      Reply service) and a Map-Referral with the MS-ACK action is
      returned to the sender of the DDT Map-Request.

   o  If the requested XEID matches a configured XEID-prefix for which
      no ETR registration has been received then a negative Map-Referral
      with action code MS-NOT-REGISTERED is returned to the sender of
      the DDT Map-Request.

5.3.  DDT Map Resolver

   Just as any other Map Resolver, a DDT Map Resolver accepts Map-
   Requests from its clients (typically, ITRs) and ensures that those
   Map-Requests are forwarded to the correct ETR, which generates Map-
   Replies.  Unlike a Map Resolver that uses the ALT mapping system (see
   [RFC6836]), however, a DDT Map Resolver uses an iterative process of
   following referrals to find the correct ETR to answer a Map-Request;
   this requires a DDT Map Resolver to maintain additional state: a Map-
   Referral cache and pending request list of XEIDs that are going
   through the iterative referral process.

5.3.1.  Queuing and sending DDT Map-Requests









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   When a DDT Map Resolver receives an encapsulated Map-Request, it
   first performs a longest-match search for the XEID in its referral
   cache.  If no match is found or if a negative cache entry is found,
   then the destination is not in the database; a negative Map-Reply is
   returned and no further processing is performed by the DDT Map
   Resolver.

   If a match is found, the DDT Map Resolver creates a pending request
   list entry for the XEID and saves the original Map-Request (minus the
   encapsulation header) along with other information needed to track
   progress through the iterative referral process; the "referral XEID-
   prefix" is also initialized to the null value since no referral has
   yet been received.  The Map Resolver then creates a DDT Map-Request
   (which is an encapsulated Map-Request with the "DDT-originated" flag
   set in the message header) for the XEID but without any
   authentication data that may have been included in the original Map-
   Request.  It sends the DDT Map-Request to one of the RLOCs in the
   chosen referral cache entry.  The referral cache is initially
   populated with one or more statically-configured entries; additional
   entries are added when referrals are followed, as described below.  A
   DDT Map Resolver is not absolutely required to cache referrals but it
   doing so decreases latency and reduces lookup delays.

   Note that in normal use on the public Internet, the statically-
   configured initial referral cache for a DDT Map Resolver should
   include a "default" entry with RLOCs for one or more DDT nodes that
   can reach the DDT root node.  If a Map Resolver does not have such
   configuration, it will return a Negative Map-Reply if it receives a
   query for an EID outside the subset of the mapping database known to
   it.  While this may be desirable on private network deployments or
   during early transition to LISP when few sites are using it, this
   behavior is not appropriate when LISP is in general use on the
   Internet.

5.3.2.  Receiving and following referrals

   After sending a DDT Map-Request, a DDT Map Resolver expects to
   receive a Map-Referral response.  If none occurs within the timeout
   period, the DDT Map Resolver retransmits the request, sending to the
   next RLOC in the referral cache entry if one is available.  If the
   maximum number of retransmissions has occurred and all RLOCs have
   been tried, then the pending request list entry is dequeued.

   A DDT Map Resolver processes a received Map-Referral message
   according to each action code:

   NODE-REFERRAL:  The DDT Map Resolver checks for a possible referral
      loop as as described in Section 5.3.4.  If no loop is found, the



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      DDT Map Resolver saves the prefix returned in the Map-Referral
      message in the referral cache, updates the saved prefix and saved
      RLOCs in the pending request list entry, and follows the referral
      by sending a new DDT Map-Request to one of the DDT node RLOCs
      listed in the Referral Set; security information saved with the
      original Map-Request is not included.

   MS-REFERRAL:  The DDT Map Resolver follows an MS-REFERRAL in the same
      manner as a NODE-REFERRAL except that that security information
      saved with the original Map-Request is included in the new Map-
      Request sent to a Map Server (see Section 8 for details on
      security).

   MS-ACK:  This is returned by a DDT Map Server to indicate that it has
      one or more registered ETRs that can answer a Map-Request for the
      XEID.  If the pending request did not include saved LISP-SEC
      information or if that information was already included in the
      previous DDT Map-Request (sent by the DDT Map Resolver in response
      to either an MS-REFERRAL or a previous MS-ACK referral), then the
      pending request for the XEID is complete and is dequeued.
      Otherwise, LISP-SEC information is required and has not yet been
      sent to the authoritative DDT Map-Server; the DDT Map Resolver re-
      sends the DDT Map-Request with LISP-SEC information included and
      the pending request queue entry remains until another Map-Referral
      with MS-ACK action code is received.  If the "incomplete" flag is
      not set, the prefix is saved in the referral cache.

   MS-NOT-REGISTERED:  The DDT Map Server qurieed could not process the
      request because it did not have any ETRs registered for a
      matching, authoritative XEID-prefix.  If the DDT Map Resolver has
      not yet tried all of the RLOCs saved with the pending request,
      then it sends a Map-Request to the next RLOC in that list.  If all
      RLOCs have been tried, then the destination XEID is not registered
      and is unreachable.  The DDT Map Resolver returns a negative Map-
      Reply to the original Map-Request sender; this Map-Reply contains
      the non-registered XEID-prefix with TTL value of one minute.  A
      negative referral cache entry is created for the prefix (also with
      TTL of one minute) and the pending request is dequeued.

   DELEGATION-HOLE:  The DDT Map Server queried did not have an XEID-
      prefix defined that matched the requested XEID so it does not
      exist in the mapping database.  The DDT Map Resolver returns a
      negative Map-Reply to the original Map-Request sender; this Map-
      Reply will indicate the least-specific XEID-prefix matching the
      requested XEID for which no delegations exist and will have a TTL
      value of 15 minutes.  A negative referral cache entry is created
      for the prefix (also with TTL of 15 minutes) and the pending
      request is dequeued.



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   NOT-AUTHORITATIVE:  The DDT Map Server queried is not authoritative
      for the requested XEID.  This can occur if a cached referral has
      become invalid due to a change in the database hierarchy.  If the
      DDT Map Resolver receiving this message can determine that it is
      using old cached information, it may choose to delete that cached
      information and re-try the original Map-Request, starting from its
      "root" cache entry.  If this action code is received in response
      to a query that was not to cached referral information, then it
      indicates a database synchronization problem or configuration
      error.  The pending request list entry that caused this answer is
      removed, with no answer returned to the original requestor.

5.3.3.  Handling referral errors

   Other states are possible, such as a misconfigured DDT node (acting
   as a proxy Map Server, for example) returning a Map-Reply to the DDT
   Map Resolver; they should be considered errors and logged as such.
   It is not clear exactly what else the DDT Map Resolver should do in
   such cases; one possibility is to remove the pending request list
   entry and send a negative Map-Reply to the original Map-Request
   sender.  Alternatively, if a DDT Map Resolver detects unexpected
   behavior by a DDT node, it could mark that node as unusable in its
   referral cache and update the pending request to try a different DDT
   node if more than one is listed in the referral cache.  In any case,
   any prefix contained in a Map-Referral message that causes a referral
   error (including a referral loop) is not saved in the DDT Map-
   Resolver referral cache.

5.3.4.  Referral loop detection

   In response to a Map-Referral message with action code NODE-REFERRAL
   or MS-REFERRAL, a DDT Map Resolver is directed to query a new set of
   DDT node RLOCs that are expected to have more-specific XEID-prefix
   information for the requested XEID.  To prevent a possible "iteration
   loop" (following referrals back-and-forth among a set of DDT nodes
   without ever finding an answer), a DDT Map Resolver saves the last
   received referral XEID-prefix for each pending request and checks
   that a newly received NODE-REFERRAL or MS-REFERRAL message contains a
   more-specific referral XEID-prefix; an exact or less-specific match
   of the saved XEID-prefix indicates a referral loop.  If a loop is
   detected, the DDT Map Resolver handles the request as described in
   Section 5.3.3.  Otherwise, the Map Resolver saves the most recently
   received referral XEID-prefix with the pending request when it
   follows the referral.

   As an extra measure to prevent referral loops, it is probably also
   wise to limit the total number of referrals for any request to some
   reasonable number; the exact value of that number will be determined



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   during experimental deployment of LISP-DDT but is bounded by the
   maximum length of the XEID.

   Note that when a DDT Map Resolver adds an entry to its lookup queue
   and sends an initial Map-Request for an XEID, the queue entry has no
   previous referral XEID-prefix; this means that the first DDT node
   contacted by a DDT Map Resolver may provide a referral to anywhere in
   the DDT hierarchy.  This, in turn, allows a DDT Map Resolver to use
   essentially any DDT node RLOCs for its initial cache entries and
   depend on the initial referral to provide a good starting point for
   Map-Requests; there is no need to configure the same set of root DDT
   nodes on all DDT Map Resolvers.

6.  Psuedo Code and Decision Tree diagrams

   To aid in implementation, each of the major DDT Map Server and DDT
   Map Resolver functions are described below, first using simple
   "psuedo-code" and then in the form of a decision tree.

6.1.  Map Resolver processing of ITR Map-Request

6.1.1.  Pseudo-code summary

   if ( request pending i.e., (ITR,EID) of request same ) {
       replace old request with new & use new request nonce
        for future requests
   } else if ( no match in refcache ) {
       return negative map-reply to ITR
   } else if ( match type delegation hole ) {
       return negative map-reply to ITR
   } else if ( match type ms-ack ) {
       fwd DDT request to map-server
   } else {
       store & fwd DDT request w/o OTK to node delegation
   }



6.1.2.  Decision tree diagram

   +------------+
   | Is Request | Yes
   |            |----> Replace old request with
   |  Pending?  |      new nonce for future requests
   +------------+
         |
         |No
         |



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         V
   +------------+
   | Match In   | No
   | Referral   |----> Send Negative Map Reply
   | cache?     |      (not a likely event as root
   +------------+      configured on every MR)
         |
         |Yes
         |
         V
   +------------+
   | Match Type | Yes
   | Delegation |----> Send Negative Map Reply
   | Hole ?     |
   +------------+
         |
         |No
         |
         V
   +------------+
   | Match Type | Yes
   | MS-ACK?    |----> Forward DDT Map-request to Map-Server
   |            |
   +------------+
         |
         |No
         |
         V
   Store request & Fwd DDT Request w/o OTK
    to DDT node delegation





















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6.2.  Map Resolver processing of Map-Referral message

6.2.1.  Pseudo-code summary

   if ( no request pending matched by referral nonce ) {
       silently drop
   }

   if ( pfx in referral less specific than last referral used ) {
       if ( gone through root ) {
           silently drop
       } else {
           goto root
       }
   }

   switch (map_referral_type) {

       case NOT_AUTHORITATIVE :
           if ( gone through root ) {
               return negative map-reply to ITR
           } else {
               goto root
           }

       case DELEGATION_HOLE:
           cache & send negative map-reply to ITR

       case MS_REFERRAL:
           if ( referral equal to last used ) {
               if ( gone through root ) {
                   return negative map-reply to ITR
               } else {
                   goto root
               }
           } else {
               cache & follow the referral
           }

       case NODE_REFERRAL:
           if ( referral equal to last used ) {
               if ( gone through root ) {
                   return negative map-reply to ITR
               } else {
                   goto root
               }
           } else {
               cache & follow the referral



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           }

       case MS_ACKNOWLEDGEMENT:
           if ( OTK stripped ) {
               if ( incomplete ) {
                   resend request with OTK
               } else {
                   cache & resend request with OTK
               }
           }

       case MS_NOT_REGISTERED:
           if { all map-server delegations not tried } {
               follow delegations not tried
              if ( !incomplete ) {
                   cache
               }
           } else {
               send negative map-reply to ITR
               if { !incomplete } {
                   cache
               }
           }

       case DEFAULT:
           drop

       }
   }






















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6.2.2.  Decision tree diagram


















































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                               +------------+
                               | Is Request | No
                               |  Pending?  |----> Silently drop
                               +------------+
                                     | Yes
                                     V
                       +------------------------------+ Yes
                       | Pfx less specific than last? |----> Silently drop
                       +------------------------------+
                                     |No
                                     V
          +---------------------------------------------------+
          |             What is Map-Referral Type?            |--UNKNOWN-+
          +---------------------------------------------------+          |
            |        |         |       |         |          |            V
            |        |         |       |         |       DEL_HOLE      DROP
            |        |         |       |      MS_ACK        |
            |        |         |       |         |          V
            |        |     MS_REF   NODE_REF     |      Cache & return
            |        |         |       |         V      negative map-reply
            |        |         |       |    +---------+
            |   NOT_AUTH       |       |    | Was OTK | Yes
            |        |         |       |    |Stripped?|----> Done
            |        |         V       V    +---------+
            |        |       +------------+      | No
            |        |   Yes | Pfx equal  |      V
   MS_NOT_REGISTERED |   +---| to last    |  +------------+
            |        |   |   | used?      |  | Incomplete | Yes
            |        |   |   +------------+  | bit set?   |---> Resend DDT
            |        V   V          |No      +------------+     request
            |  +------------+       |               |No         with OTK
            |  |  Gone      |       V               |
            |  |  Through   |   Cache & follow      V
            |  |  Root?     |   the referral     Cache & resend DDT
            |  +------------+                    request with OTK
            |    |No      |Yes
            |    |        |
            |    V        V
            |  Goto root  Send negative map-reply
            V
    +-----------+ Yes                         +-----------+ Yes
    | Other MS  |-----Follow other MS-------->|Incomplete |----> Dont cache
    | not tried?|                             |bit set?   |
    |           |----Send negative map-reply->|           |----> Cache
    +-----------+ No                          +-----------+ No






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6.3.  DDT Node processing of DDT Map-Request message

6.3.1.  Pseudo-code summary

   if ( I am not authoritative ) {
       send map-referral NOT_AUTHORITATIVE with
        incomplete bit set and ttl 0
   } else if ( delegation exists ) {
       if ( delegated map-servers ) {
           send map-referral MS_REFERRAL with
             ttl 'Default_DdtNode_Ttl'
       } else {
           send map-referral NODE_REFERRAL with
             ttl 'Default_DdtNode_Ttl'
       }
   } else {
       if ( eid in site) {
           if ( site registered ) {
               forward map-request to etr
               if ( map-server peers configured ) {
                   send map-referral MS_ACKNOWLEDGEMENT with
                    ttl 'Default_Registered_Ttl'
               } else {
                   send map-referral MS_ACKNOWLEDGEMENT with
                    ttl 'Default_Registered_Ttl' and incomplete bit set
               }
           } else {
               if ( map-server peers configured ) {
                   send map-referral MS_NOT_REGISTERED with
                    ttl 'Default_Configured_Not_Registered_Ttl'
               } else {
                   send map-referral MS_NOT_REGISTERED with
                    ttl 'Default_Configured_Not_Registered_Ttl'
                    and incomplete bit set
               }
           }
       } else {
           send map-referral DELEGATION_HOLE with
            ttl 'Default_Negative_Referral_Ttl'
       }
   }



6.3.2.  Decision tree diagram

   +------------+
   |    Am I    | No



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   |  Authori-  |----> Return NOT_AUTHORITATIVE
   |   tative?  |       Incomplete = 1
   +------------+       ttl = Default_DdtNode_Ttl
         |
         |Yes
         |
         V
   +------------+     +------------+
   | Delegation | Yes | Delegations| Yes
   |   Exists?  |---->| are map    |----> Return MS_REFERRAL
   |            |     | servers?   |       ttl = Default_DdtNode_Ttl
   +------------+     +------------+
         |                  \ No
         |No                 +--> Return NODE_REFERRAL
         |                        ttl = Default_DdtNode_Ttl
         V
   +------------+     +------------+                  +------------+
   | EID in     | Yes | Site       | Yes              | Map-server |
   |  Site      |---->| Registered?|----> Forward---->| peers      |
   | Config?    |     |            |      Map-request | configured?|
   +------------+     +------------+      to ETR      +------------+
         |                |                          |       |
         |                |No                      No|       |Yes
         |                |                          |       |
         |                |                          V       V
         |                |                Return MS_ACK    Return MS_ACK
         |                V                with INC=1
         |         +------------+          ttl=Default_Registered_Ttl
         |         | Map-server | Yes
         |         | peers      |----> Return MS_NOT_REGISTERED
         |         | configured?|      ttl = Default_Negative_Referral_Ttl
         |         +------------+
         |                \ No
         |No               +--> Return MS_NOT_REGISTERED
         |                      Incomplete = 1
         V                      ttl = Default_Negative_Referral_Ttl
   Return DELEGATION_HOLE
    ttl = Default_Negative_Referral_Ttl



7.  Example topology and request/referral following

   To show how referrals are followed to find the RLOCs for a number of
   EIDs, consider the following example EID topology for DBID=0, IID=0,
   AFI=1, and EID=0/0





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        +---------------------+  +---------------------+
        |  root1: 192.0.2.1   |  |  root2: 192.0.2.2   |
        | authoritative: 0/0  |  | authoritative: 0/0  |
        +---------------------+  +---------------------+
                   |         \   /        |
                   |          \ /         |
                   |          / \         |
                   |         /   \        |
                   |        |     |       |
                   V        V     V       V
   +--------------------------+  +--------------------------+
   |  DDT node1: 192.0.2.11   |  |  DDT node2: 192.0.2.12   |
   | authoritative: 10.0.0.0/8|  | authoritative: 10.0.0.0/8|
   +--------------------------+  +--------------------------+
                   |         \   /        |
                   |          \ /         |
                   |          / \         |
                   |         /   \        |
                   |        |     |       |
                   V        V     V       V
   +--------------------------+  +---------------------------+
   | Map-Server1: 192.0.2.101 |  |  DDT node3: 192.0.2.201   |
   |authoritative: 10.0.0.0/12|  |authoritative: 10.16.0.0/12|
   | site1: 10.1.0.0/16       |  +---------------------------+
   | site2: 10.2.0.0/16       |     |                    |
   +--------------------------+     |                    |
                                    |                    |
                                    |                    |
                                    V                    V
             +---------------------------+   +---------------------------+
             | Map-Server2: 192.0.2.211  |   | Map-Server3: 192.0.2.221  |
             |authoritative: 10.16.0.0/16|   |authoritative: 10.17.0.0/16|
             | site3: 10.16.1.0/24       |   | site5: 10.17.8.0/24       |
             | site4: 10.16.2.0/24       |   | site6: 10.17.9.0/24       |
             +---------------------------+   +---------------------------+
















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   DDT nodes are configured for this "root" at IP addresses 192.0.2.1
   and 192.0.2.2.  DDT Map Resolvers are configured with default
   referral cache entries to these addresses.

   The root DDT nodes delegate 10.0.0/8 to two DDT nodes with IP
   addresses 192.0.2.11 and 192.0.2.12.

   The DDT nodes for 10.0.0.0/8 delegate 10.0.0.0/12 to a DDT Map Server
   with RLOC 192.0.2.101

   The DDT Map Server for 10.0.0.0/12 is configured to allow ETRs to
   register the sub-prefixes 10.1.0.0/16 and 10.2.0.0/16

   The DDT nodes for 10.0.0.0/8 also delegate 10.16.0.0/12 to a DDT node
   with RLOC 192.0.2.201

   The DDT node for 10.16.0.0/12 is further configured to delegate
   10.16.0.0/16 to a DDT Map Server with RLOC 192.0.2.211 and 10.17.0.0/
   16 to a DDT Map Server with RLOC 192.0.2.221

   The DDT Map Server for 10.16.0.0/16 is configured to allow ETRs to
   register the sub-prefixes 10.16.1.0/24 and 10.16.2.0/24

   The DDT Map Server for 10.17.0.0/16 is configured to allow ETRs to
   register the sub-prefixes 10.17.8.0/24 and 10.17.9.0/24

7.1.  Lookup of 10.1.1.1/32 by ITR1

   The first example shows a DDT Map Resolver following a delegation
   from the root to a DDT node followed by another delegation to a DDT
   Map Server.

   ITR1 sends an Encapsulated Map-Request for 10.1.1.1 to one of its
   configured (DDT) Map Resolvers.  The DDT Map Resolver proceeds as
   follows:

   1.  Send DDT Map-Request (for 10.1.1.1) to one of the root DDT nodes,
       192.0.2.1 or 192.0.2.2

   2.  Receive (and save in referral cache) Map-Referral for EID-prefix
       10.0.0.0/8, action code NODE-REFERRAL, RLOC set (192.0.2.11,
       192.0.2.12)

   3.  Send DDT Map-Request to 192.0.2.11 or 192.0.2.12

   4.  Receive (and save in referral cache) Map-Referral for EID-prefix
       10.0.0.0/12, action code MS-REFERRAL, RLOC set (192.0.2.101)




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   5.  Send DDT Map-Request to 192.0.2.101; if the ITR-originated
       Encapsulated Map-Request had a LISP-SEC signature, it is included

   6.  DDT Map Server at 192.0.2.101 decapsulates the DDT Map-Request
       and forwards to a registered site1 ETR for 10.1.0.0/16

   7.  DDT Map Server at 192.0.2.101 sends a Map-Referral message for
       EID-prefix 10.1.0.0/16, action code MS-ACK to the DDT Map
       Resolver

   8.  DDT Map Resolver receives Map-Referral message and dequeues the
       pending request for 10.1.1.1

   9.  site1 ETR for 10.1.0.0/16 receives Map-Request forwarded by DDT
       Map Server and sends Map-Reply to ITR1

7.2.  Lookup of 10.17.8.1/32 by ITR2

   The next example shows a three-level delegation: root to first DDT
   node, first DDT node to second DDT node, second DDT node to DDT Map
   Server.

   ITR2 sends an Encapsulated Map-Request for 10.17.8.1 to one of its
   configured (DDT) Map Resolvers, which are different from those for
   ITR1.  The DDT Map Resolver proceeds as follows:

   1.   Send DDT Map-Request (for 10.17.8.1) to one of the root DDT
        nodes, 192.0.2.1 or 192.0.2.2

   2.   Receive (and save in referral cache) Map-Referral for EID-prefix
        10.0.0.0/8, action code NODE-REFERRAL, RLOC set (192.0.2.11,
        192.0.2.12)

   3.   Send DDT Map-Request to 192.0.2.11 or 192.0.2.12

   4.   Receive (and save in referral cache) Map-Referral for EID-prefix
        10.16.0.0/12, action code NODE-REFERRAL, RLOC set (192.0.2.201)

   5.   Send DDT Map-Request to 192.0.2.201

   6.   Receive (and save in referral cache) Map-Referral for EID-prefix
        10.17.0.0/16, action code MS-REFERRAL, RLOC set (192.0.2.221)

   7.   Send DDT Map-Request to 192.0.2.221; if the ITR-originated
        Encapsulated Map-Request had a LISP-SEC signature, it is
        included





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   8.   DDT Map Server at 192.0.2.221 decapsulates the DDT Map-Request
        and forwards to a registered site5 ETR for 10.17.8.0/24

   9.   DDT Map Server at 192.0.2.221 sends a Map-Referral message for
        EID-prefix 10.17.8.0/24, action code MS-ACK, to the DDT Map
        Resolver

   10.  DDT Map Resolver receives Map-Referral(MS-ACK) message and
        dequeues the pending request for 10.17.8.1

   11.  site5 ETR for 10.17.8.0/24 receives Map-Request forwarded by DDT
        Map Server and sends Map-Reply to ITR2

7.3.  Lookup of 10.2.2.2/32 by ITR1

   This example shows how a DDT Map Resolver uses a saved referral cache
   entry to skip the referral process and go directly to a DDT Map
   Server for a prefix that is similar to one previously requested.

   In this case, ITR1 uses the same Map Resolver used in example
   Section 7.1.  It sends an Encapsulated Map-Request for 10.2.2.2 to
   that (DDT) Map Resolver.  The DDT Map-Resolver finds an MS-REFERRAL
   cache entry for 10.0.0.0/12 with RLOC set (192.0.2.101) and proceeds
   as follows:

   1.  Send DDT Map-Request (for 10.2.2.2) to 192.0.2.101; if the ITR-
       originated Encapsulated Map-Request had a LISP-SEC signature, it
       is included

   2.  DDT Map Server at 192.0.2.101 decapsulates the DDT Map-Request
       and forwards to a registered site2 ETR for 10.2.0.0/16

   3.  DDT Map Server at 192.0.2.101 sends a Map-Referral message for
       EID-prefix 10.2.0.0/16, action code MS-ACK to the DDT Map
       Resolver

   4.  DDT Map Resolver receives Map-Referral(MS-ACK) and dequeues the
       pending request for 10.2.2.2

   5.  site2 ETR for 10.2.0.0/16 receives Map-Request and sends Map-
       Reply to ITR1

7.4.  Lookup of 10.16.2.1/32 by ITR2

   This example shows how a DDT Map Resolver uses a saved referral cache
   entry to start the referral process at a non-root, intermediate DDT
   node for a prefix that is similar to one previously requested.




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   In this case, ITR2 asks the same Map Resolver used in example
   Section 7.2.  It sends an Encapsulated Map-Request for 10.16.2.1 to
   that (DDT) Map Resolver, which finds a NODE-REFERRAL cache entry for
   10.16.0.0/12 with RLOC set (192.0.2.201).  It proceeds as follows:

   1.  Send DDT Map-Request (for 10.16.2.1) to 192.0.2.201

   2.  Receive (and save in referral cache) Map-Referral for EID-prefix
       10.16.0.0/16, action code MS-REFERRAL, RLOC set (192.0.2.211)

   3.  Send DDT Map-Request to 192.0.2.211; if the ITR-originated
       Encapsulated Map-Request had a LISP-SEC signature, it is included

   4.  DDT Map Server at 192.0.2.211 decapsulates the DDT Map-Request
       and forwards to a registered site4 ETR for 10.16.2.0/24

   5.  DDT Map Server at 192.0.2.211 sends a Map-Referral message for
       EID-prefix 10.16.2.0/24, action code MS-ACK to the DDT Map
       Resolver

   6.  DDT Map Resolver receives Map-Referral(MS-ACK) and dequeues the
       pending request for 10.16.2.1

   7.  site4 ETR for 10.16.2.0/24 receives Map-Request and sends Map-
       Reply to ITR2

7.5.  Lookup of 10.16.0.1/32 (non-existant EID) by ITR2

   This example uses the cached MS-REFERRAL for 10.16.0.0/16 learned
   above to start the lookup process at the DDT Map-Server at
   192.0.2.211.  The DDT Map Resolver proceeds as follows:

   1.  Send DDT Map-Request (for 10.16.0.1) to 192.0.2.211; if the ITR-
       originated Encapsulated Map-Request had a LISP-SEC signature, it
       is included

   2.  DDT Map Server at 192.0.2.211, which is authoritative for
       10.16.0.0/16, does not have a matching delegation for 10.16.0.1.
       It respondes with a Map-Referral message for 10.16.0.0/24, action
       code DELEGATION-HOLE to the DDT Map Resolver.  The prefix
       10.16.0.0/24 is used because it is the least-specific prefix that
       does match the requested EID but does not match one of configured
       delegations (10.16.1.0/24 and 10.16.2.0/24).

   3.  DDT Map Resolver receives the delegation, adds a negative
       referral cache entry for 10.16.0.0/24, dequeues the pending
       request for 10.16.0.1, and returns a negative Map-Reply to ITR2.




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8.  Securing the database and message exchanges

   This section specifies the DDT security architecture that provides
   data origin authentication, data integrity protection, and XEID-
   prefix delegation.  Global XEID-prefix authorization is out of the
   scope of this document.

   Each DDT node is configured with one or more public/private key
   pair(s) that are used to digitally sign referral records for XEID-
   prefix(es) that the DDT node is authoritative for.  In other words,
   each public/private key pair is associated with the combination of a
   DDT node and the XEID-prefix that it is authoritative for.  Every DDT
   node is also configured with the public keys of its children DDT
   nodes.  By including public keys of target child DDT nodes in the
   Map-Referral records, and signing each record with the DDT node's
   private key, a DDT node can securely delegate sub-prefixes of its
   authoritative XEID-prefixes to its children DDT nodes.

   Map Resolvers are configured with one or more trusted public keys
   referred to as trust anchors.  Trust anchors are used to authenticate
   the DDT security infrastructure.  Map Resolvers can discover a DDT
   node's public key either by having it configured as a trust anchor,
   or by obtaining it from the node's parent as part of a signed Map-
   Referral.  When a public key is obtained from a node's parent, it is
   considered trusted if it is signed by a trust anchor, or if it is
   signed by a key that was previously trusted.  Typically, in a Map
   Resolver, the root DDT node public keys should be configured as trust
   anchors.  Once a Map Resolver authenticates a public key it locally
   caches the key along with the associated DDT node RLOC and XEID-
   prefix for future use.

8.1.  XEID-prefix Delegation

   In order to delegate XEID sub-prefixes to its children, a parent DDT
   node signs its Map-Referrals.  Every signed Map-Referral also
   includes the public keys associated with each child DDT node.  Such a
   signature indicates that the parent node is delegating the specified
   XEID -prefix to a given child DDT node.  The signature is also
   authenticating the public keys associated with the children nodes,
   and authorizing them to be used by the children DDT nodes to provide
   origin authentication and integrity protection for further
   delegations and mapping information of the XEID-prefix allocated to
   the DDT node.

   As a result, for a given XEID-prefix, a Map Resolver can form an
   authentication chain from a configured trust anchor (typically the
   root DDT node) to the leaf nodes (Map Servers).  Map Resolvers
   leverage this authentication chain to verify the Map-Referral



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   signatures while walking the DDT tree until they reach a Map Server
   authoritative for the given XEID-prefix.

8.2.  DDT node operation

   Upon receiving a Map-Request, the DDT node responds with a Map-
   Referral as specified in Section 5.  For every record present in the
   Map-Referral, the DDT node also includes the public keys associated
   with the record's XEID-prefix and the RLOCs of the children DDT
   nodes.  Each record contained in the Map-Referral is signed using the
   DDT node's private key.

8.2.1.  DDT public key revocation

   The node that owns a public key can also revoke that public key.  For
   instance if a parent node advertises a public key for one of its
   child DDT nodes, the child DDT node can at a later time revoke that
   key.  Since DDT nodes do not keep track of the Map Resolvers that
   query them, revocation is done in a pull model, where the Map
   Resolver is informed of the revocation of a key only when it queries
   the node that owns that key.  If the parent DDT is configured to
   advertise this key, the parent node must also be signaled to remove
   the key from the records it advertises for the child DDT node; this
   is necessary to avoid further distribution of the revoked key.

   To securely revoke a key, the DDT node creates a new Record for the
   associated XEID-prefix and locator, including the revoked key with
   the R bit set.  The DDT node must also include a signature in the
   Record that covers this record; this is computed using the private
   key corresponding to the key being revoked.  Such a record is termed
   a "revocation record".  By including this record in its Map-
   Referrals, the DDT node informs querying Map Resolvers about the
   revoked key.  A digital signature computed with a revoked key can
   only be used to authenticate the revocation, and should not be used
   to validate any data.  To prevent a compromised key from revoking
   other valid keys, a given key can only be used to sign a revocation
   for that specific key; it cannot be used to revoke other keys.  This
   prevents the use of a compromised key to revoke other valid keys as
   described in [RFC5011].  A revocation record must be advertised for a
   period of time equal to or greater than the TTL value of the Record
   that initially advertisied the key, starting from the time that the
   advertisement of the key was stopped by removal from the parent DDT
   node.

8.3.  Map Server operation

   Similar to a DDT node, a Map Server is configured with one (or more)
   public/private key pairs that it must use to sign Map-Referrals.



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   However unlike DDT nodes, Map Servers do not delegate prefixes and as
   a result they do not need to include keys in the Map-Referrals they
   generate.

8.4.  Map Resolver operation

   Upon receiving a Map-Referral, the Map Resolver must first verify the
   signature(s) by using a trust anchor, or a previously authenticated
   public key, associated with the DDT node sending the Map-Referral.
   If multiple authenticated keys are associated with the DDT node
   sending this Map-Referral, the Key Tag field of the signature can be
   used to select the right public key for verifying the signature.  If
   the key tag matches more than one key associated with that DDT node,
   the Map Resolver must try verifying the signature with all matching
   keys.  For every matching key that is found the Map Resolver must
   also verify that the key is authoritative for the XEID-prefix in the
   Map-Referral record.  If such a key is found, the Map Resolver must
   use it to verify the associated signature in the record.  If no
   matching key is found, or if none of the matching keys is
   authoritative for the XEID-prefix in the Map-Referral record, or if
   such a key is found but the signature is not valid the Map-Referral
   record is considered corrupted and must be discarded.  This may be
   due to expired keys.  The Map Resolver can try other siblings of this
   node if there is an alternative node authoritative for the same
   prefix.  If not, the Map Resolver can query the DDT node's parent to
   retrieve a valid key.  It is good practice to use a counter or timer
   to avoid repeating this process if the resolver cannot verify the
   signature after several trials.

   Once the signature is verified, the Map Resolver has verified the
   XEID-prefix delegation in the Map-Referral, and authenticated the
   public keys of the children DDT nodes.  The Map Resolver must add
   these keys to the authenticated keys associated with each child DDT
   node and XEID-prefix.  These keys are considered valid for the
   duration specified in the record's TTL field.

9.  Open Issues and Considerations

   There are a number of issues with the organization of the mapping
   database that need further investigation.  Among these are:

   o  Unlike in LISP-ALT (see [RFC6836]), DDT does not currently define
      a mechanism for propagating ETR-to-Map Server registration state.
      This requires DDT Map Servers to suppress returning negative Map-
      Reply messages for defined but unregistered XEID-prefixes to avoid
      loss of connectivity during partial ETR registration failures.
      Suppressing these messages may cause a delay for an ITR obtaining
      a mapping entry when such a failure is occurring.



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   o  Defining an interface to implement interconnection and/or
      interoperability with other mapping databases, such as LISP+ALT.

   o  Additional key structures for use with LISP-DDT, such as to
      support additional EID formats as defined in [LCAF].

   o  Authentication of delegations between DDT nodes.

   o  Possibility of a new, more general format for the Map-Referral
      messages to facilitate the use of LISP-DDT with additional DBID/
      IID/EID combinations.  Currently-defined packet formats should be
      considered to be preliminary and provisional until this issue has
      received greater attention.

   o  Management of the DDT Map Resolver referral cache, in particular,
      detecting and removing outdated entries.

   The authors expect that experimentation on the LISP pilot network
   will help answer open questions surrounding these and other issues.

10.  IANA Considerations

   This document makes no request of the IANA.

11.  Security Considerations

   Section 8 describes a DDT security architecture that provides data
   origin authentication, data integrity protection, and XEID-prefix
   delegation within the DDT Infrastructure.

   Global XEID-prefix authorization is beyond the scope of this
   document, but the SIDR working group [RFC6480] is developing an
   infrastructure to support improved security of Internet routing.
   Further work is required to determine if SIDR's public key
   infrastructure (PKI) and the distributed repository system it uses
   for storing and disseminating PKI data objects may also be used by
   DDT devices to verifiably assert that they are the legitimate holders
   of a set of XEID prefixes.

   DDT security and [LISP-SEC] complement each other in securing the DDT
   infrastructure, Map-Referral messages and the Map-Request/Map-Reply
   protocol.  In addition LISP-SEC can use the DDT public key
   infrastructure to secure the transport of LISP-SEC key material (the
   One-Time Key) from a Map-Resolver to the corresponding Map-Server.
   For this reason, when LISP-SEC is deployed in conjunction with a
   LISP-DDT mapping database and the path between Map-Resolver and Map-
   Server needs to be protected, DDT security should be enabled as well.




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12.  References

12.1.  Normative References

   [LCAF]     Farinacci, D., Meyer, D., and J. Snijders, "LISP Canonical
              Address Format", draft-ietf-lisp-lcaf-02.txt (work in
              progress), March 2013.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, September 2007.

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

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

12.2.  Informative References

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

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

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, February 2012.

   [RFC6836]  Fuller, V., Farinacci, D., Meyer, D., and D. Lewis,
              "Locator/ID Separation Protocol Alternative Logical
              Topology (LISP+ALT)", RFC 6836, January 2013.





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Appendix A.  Acknowledgments

   The authors with to express their thanks to Damien Saucez, Lorand
   Jakab, Olivier Bonaventure, Albert Cabellos-Asparicio, and FLorin
   Coras for work on LISP-TREE and LISP iterable mappings that inspired
   the hierarchical database structure and lookup iteration approach
   described in this document.  Thanks also go to Dino Farinacci and
   Isidor Kouvelas for their implementation work; to Selina Heimlich and
   Srin Subramanian for testing; to Fabio Maino for work on security
   processing; and to Job Snijders, Glen Wiley, Neel Goyal, and Mike
   Gibbs for work on operational considerations and initial deployment
   of a prototype database infrastructure.  Special thanks go to Jesper
   Skriver, Andrew Partan, and Noel Chiappa; all of whom have
   participated in (and put up with) seemingly endless hours of
   discussion of mapping database ideas, concepts, and issues.

Appendix B.  Map-Referral Message Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Type=6 |                Reserved               | Record Count  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Nonce . . .                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         . . . Nonce                           |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   |                          Record  TTL                          |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   R   | Referral Count| EID mask-len  | ACT |A|I|     Reserved        |
   e   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   c   |SigCnt |   Map Version Number  |            EID-AFI            |
   o   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   r   |                          EID-prefix ...                       |
   d   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  /|    Priority   |    Weight     |  M Priority   |   M Weight    |
   | L +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | o |        Unused Flags         |R|         Loc/LCAF-AFI          |
   | c +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  \|                             Locator ...                       |
   +-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   ACT: The "action" field of the mapping record in a Map-Referral
   message encodes 6 action types.  The values for the action types are:

   NODE-REFERRAL (0):  Sent by a DDT node with a child delegation which
      is authoritative for the EID.



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   MS-REFERRAL (1):  Sent by a DDT node that has information about Map
      Server(s) for the EID but it is not one of the Map Servers listed,
      i.e.  the DDT-Node sending the referral is not a Map Server.

   MS-ACK (2):  Sent by a DDT Map Server that has one or more ETR
      registered for the EID.

   MS-NOT-REGISTERED (3):  Sent by a DDT Map Server that is configured
      for the EID-prefix but for which no ETRs are registered.

   DELEGATION-HOLE (4):  Sent by an intermediate DDT node with
      authoritative configuration covering the requested EID but without
      any child delegation for the EID.  Also sent by a DDT Map Server
      with authoritative configuration covering the requested EID but
      for which no specific site ETR is configured.

    NOT-AUTHORITATIVE (5):  Sent by a DDT node that does not have
      authoritative configuration for the requested EID.  The EID-prefix
      returned MUST be the original requested EID and the TTL MUST be
      set to 0.  However, if such a DDT node has a child delegation
      covering the requested EID, it may choose to return NODE-REFERRAL
      or MS-REFERRAL as appropriate.  A DDT Map Server with site
      information may choose to return of type MS-ACK or MS-NOT-
      REGISTERED as appropriate.

   Incomplete: The "I" bit indicates that a DDT node's referral-set of
   locators is incomplete and the receiver of this message should not
   cache the referral.  A DDT sets the "incomplete" flag, the TTL, and
   the Action Type field as follows:

   -------------------------------------------------------------------
    Type (Action field)          Incomplete Referral-set   TTL values
   -------------------------------------------------------------------
     0    NODE-REFERRAL              NO         YES           1440

     1    MS-REFERRAL                NO         YES           1440

     2    MS-ACK                     *          *             1440

     3    MS-NOT-REGISTERED          *          *             1

     4    DELEGATION-HOLE            NO         NO            15

     5    NOT-AUTHORITATIVE          YES        NO            0
   -------------------------------------------------------------------






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   *: The "Incomplete" flag setting on Map Server originated referral of
      MS-REFERRAL and MS-NOT-REGISTERED types depend on whether the Map
      Server has the full peer Map Server configuration for the same
      prefix and has encoded the information in the mapping record.
      Incomplete bit is not set when the Map Server has encoded the
      information, which means the referral-set includes all the RLOCs
      of all Map Servers that serve the prefix.  It is set when the Map
      Server has not encoded the Map Server set information.

   SigCnt: Indicates the number of signatures (sig section) present in
   the Record.  If SigCnt is larger than 0, the signature information
   captured in a sig section as described in Appendix B.1 will be
   appended to the end of the record.  The number of sig sections at the
   end of the Record must match the SigCnt.

   Loc/LCAF-AFI: If this is a Loc AFI, keys are not included in the
   record.  If this is a LCAF AFI, the contents of the LCAF depend on
   the Type field of the LCAF.  Security material are stored in LCAF
   Type 11.  DDT nodes and Map Servers can use this LCAF Type to include
   public keys associated with their Child DDT nodes for a XEID-prefix
   referral record.  LCAF types and formats are defined in [LCAF].

   All the field descriptions are equivalent to those in the Map-Reply
   message, as defined in LISP [RFC6830].  Note, though, that the set of
   RLOCs correspond to the DDT node to be queried as a result of the
   referral not the RLOCs for an actual EID-to-RLOC mapping.

B.1.  SIG section

   If SigCnt field in the Map-Referral is not 0, the signature
   information is included at the end of captured in a sig section as
   described below.  SigCnt counts the number of sig sections that
   appear at the end of the Record.

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /|                      Original Record TTL                      |
     / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /  |                      Signature Expiration                     |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   s   |                      Signature Inception                      |
   i   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   g   |            Key Tag            |           Sig Length          |
   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \   | Sig-Algorithm |    Reserved   |            Reserved           |
    \  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ ~                             Signature                         ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   Original Record TTL: The original Record TTL for this record that is
   covered by the signature.  Record TTL is in minutes.

   Key Tag: An identifier to specify which key is used for this
   signature if more than one valid key exists for the signing DDT node.

   Sig Length: The length of the Signature field.

   Sig-Algorithm: The identifier of the cryptographic algorithm used for
   the signature.  Default value is RSA-SHA1.

   Reserved: This field must be set to 0 on transmit and must be ignored
   on receipt.

   Signature Expiration and Inception: Specify the validity period for
   the signature.  Each specify a date and time in the form of a 32-bit
   unsigned number of seconds elapsed since 1 January 1970 00:00:00 UTC,
   ignoring leap seconds, in network byte order.

   Signature: Contains the cryptographic signature that covers the
   entire record.  The Record TTL and the sig fields are set to zero for
   the purpose of computing the Signature

Appendix C.  Encapsulated Control Message Format



























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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |                       IPv4 or IPv6 Header                     |
   OH  |                      (uses RLOC addresses)                    |
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = xxxx      |       Dest Port = 4342        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   LH  |Type=8 |S|D|                Reserved                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |                       IPv4 or IPv6 Header                     |
   IH  |                  (uses RLOC or EID addresses)                 |
     \ |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     / |       Source Port = xxxx      |       Dest Port = yyyy        |
   UDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     \ |           UDP Length          |        UDP Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   LCM |                      LISP Control Message                     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   "D" is the "DDT-originated" flag and is set by a DDT client to
   indicate that the receiver can and should return Map-Referral
   messages as appropriate.

Authors' Addresses

   Vince Fuller

   Email: vaf@vaf.net


   Darrel Lewis
   cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   Email: darlewis@cisco.com








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   Vina Ermagan
   cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   Email: vermagan@cisco.com


   Amit Jain
   Juniper Networks
   1133 Innovation Way
   Sunnyvale, CA  94089
   USA

   Email: atjain@juniper.net


































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