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Versions: (draft-makela-mip4-nemo-haaro) 00 01 02 03 04 05 06 07 RFC 6521

Network Working Group                                          A. Makela
Internet-Draft                                          Aalto University
Intended status: Experimental                                J. Korhonen
Expires: April 21, 2011                           Nokia Siemens Networks
                                                        October 18, 2010


  Home Agent assisted Route Optimization between Mobile IPv4 Networks
                     draft-ietf-mip4-nemo-haaro-02

Abstract

   This document describes a Home Agent assisted Route Optimization
   functionality to IPv4 Network Mobility Protocol.  The function is
   designed to facilitate optimal routing in cases where all nodes are
   connected to a single Home Agent, thus the use case is Route
   Optimization within single organization or similar entity.  The
   functionality adds possibility to discover eligible peer nodes based
   on information received from Home Agent, Network Prefixes they
   represent, and how to establish direct tunnel between such nodes.

Status of this Memo

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

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

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

   This Internet-Draft will expire on April 21, 2011.

Copyright Notice

   Copyright (c) 2010 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
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction and motivations . . . . . . . . . . . . . . . . .  4
   2.  Terms and definitions  . . . . . . . . . . . . . . . . . . . .  6
   3.  Mobile IPv4 route optimization between mobile networks . . . .  7
     3.1.  Maintaining route optimization information . . . . . . . .  8
       3.1.1.  Advertising route-optimizable prefixes . . . . . . . .  8
       3.1.2.  Route Optimization cache . . . . . . . . . . . . . . . 10
     3.2.  Return routability procedure . . . . . . . . . . . . . . . 12
       3.2.1.  Router keys  . . . . . . . . . . . . . . . . . . . . . 13
       3.2.2.  Nonces . . . . . . . . . . . . . . . . . . . . . . . . 13
       3.2.3.  Updating Router keys and Nonces  . . . . . . . . . . . 13
     3.3.  Mobile-Correspondent Router operations . . . . . . . . . . 15
       3.3.1.  Triggering Route Optimization  . . . . . . . . . . . . 16
       3.3.2.  Mobile Router routing tables . . . . . . . . . . . . . 16
       3.3.3.  Inter-Mobile Router registration . . . . . . . . . . . 16
       3.3.4.  Inter-Mobile Router tunnels  . . . . . . . . . . . . . 19
       3.3.5.  Constructing route-optimized packets . . . . . . . . . 20
       3.3.6.  Handovers and Mobile Routers leaving network . . . . . 20
     3.4.  Convergence and synchronization issues . . . . . . . . . . 21
   4.  Data compression schemes . . . . . . . . . . . . . . . . . . . 22
     4.1.  Prefix compression . . . . . . . . . . . . . . . . . . . . 22
     4.2.  Realm compression  . . . . . . . . . . . . . . . . . . . . 24
       4.2.1.  Encoding of compressed realms  . . . . . . . . . . . . 24
       4.2.2.  Searching algorithm  . . . . . . . . . . . . . . . . . 25
       4.2.3.  Encoding example . . . . . . . . . . . . . . . . . . . 26
   5.  New Mobile IPv4 messages and extensions  . . . . . . . . . . . 28
     5.1.  Mobile router Route optimization capability  . . . . . . . 28
     5.2.  Route optimization reply . . . . . . . . . . . . . . . . . 29
     5.3.  Mobile-Correspondent authentication extension  . . . . . . 30
     5.4.  Care-of address Extension  . . . . . . . . . . . . . . . . 31
     5.5.  Route optimization prefix advertisement  . . . . . . . . . 31
     5.6.  Home-Test Init message . . . . . . . . . . . . . . . . . . 33
     5.7.  Care-of-Test Init message  . . . . . . . . . . . . . . . . 33
     5.8.  Home Test message  . . . . . . . . . . . . . . . . . . . . 34
     5.9.  Care-of test message . . . . . . . . . . . . . . . . . . . 35
   6.  Special Considerations . . . . . . . . . . . . . . . . . . . . 35
     6.1.  NATs and stateful firewalls  . . . . . . . . . . . . . . . 35
     6.2.  Handling of concurrent handovers . . . . . . . . . . . . . 37
     6.3.  Foreign Agents . . . . . . . . . . . . . . . . . . . . . . 37
     6.4.  Multiple Home Agents . . . . . . . . . . . . . . . . . . . 37
     6.5.  Mutualness of Route Optimization . . . . . . . . . . . . . 38



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     6.6.  Extensibility  . . . . . . . . . . . . . . . . . . . . . . 39
     6.7.  Load Balancing . . . . . . . . . . . . . . . . . . . . . . 39
   7.  Scalability  . . . . . . . . . . . . . . . . . . . . . . . . . 40
   8.  Example signaling scenarios  . . . . . . . . . . . . . . . . . 40
     8.1.  Registration request . . . . . . . . . . . . . . . . . . . 40
     8.2.  Route optimization with return routability . . . . . . . . 41
     8.3.  Handovers  . . . . . . . . . . . . . . . . . . . . . . . . 43
   9.  Protocol constants . . . . . . . . . . . . . . . . . . . . . . 45
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 45
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 47
     11.1. Return Routability . . . . . . . . . . . . . . . . . . . . 47
     11.2. Trust relationships  . . . . . . . . . . . . . . . . . . . 47
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 48
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 48
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 48
     13.2. Informative References . . . . . . . . . . . . . . . . . . 48
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49


































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1.  Introduction and motivations

   Traditionally, there has been no method for route optimization in
   Mobile IPv4 [RFC3344] apart from an early attempt
   [I-D.ietf-mobileip-optim].  Unlike Mobile IPv6 [RFC3775], where Route
   Optimization has been included from the start, with Mobile IPv4 route
   optimization hasn't been addressed in a generalized scope.

   Even though general route optimization may not be of interest in the
   scope of IPv4, there are still specific applications for Route
   Optimization in Mobile IPv4.  This document proposes method to
   optimize routes between networks behind mobile routers, as defined by
   NEMO [RFC5177].  Although NAT and pending shortage of IPv4 addresses
   makes widespread deployment not feasible, using Route Optimization
   only in routers is still a practical scenario.

   A particular use case concerns setting up redundant yet economical
   enterprise networks.  Recently, a trend has emerged where customers
   prefer to maintain connectivity via multiple service providers.
   Reasons include redundancy, reliability and availability issues.
   These kinds of multi-homing scenarios have traditionally been solved
   by using such technologies as multihoming BGP.  However, a more
   lightweight and economical solution is desirable.

   From service provider perspective a common topology for enterprise
   customer network consists of one to several sites (typically
   headquarters and various branch offices).  These sites are typically
   connected via various Layer 2 technologies (ATM or Frame relay PVCs),
   MPLS VPNs or Layer 3 site-to-site VPNs.  With a Service Level
   Agreement, a customer can obtain a very reliable and well supported
   intranet connectivity.  However, compared to the cost of "consumer-
   grade" broadband Internet access the SLA-guaranteed version can be
   considered very expensive.  These consumer-grade options however, are
   not reliable approach for mission-critical applications.

   Mobile IP, especially mobile routers, can be used to improve
   reliability of connectivity even when implemented over consumer-grade
   Internet access.  The customer becomes a client for a virtual service
   provider, which does not take part in the actual access technology.
   The service provider has a backend system and an IP address pool that
   it distributes to customers.  Access is provided by multiple,
   independent, possibly consumer-grade ISPs, with Mobile IP providing
   seamless handovers if service from a specific ISP fails.  The
   drawback of this solution is that it creates a star topology; All
   Mobile IP tunnels end up at the service provider hosted home agent,
   causing heavy load at the backend.  Route Optimization between mobile
   networks addresses this issue, by taking network load off the home
   agent and the backend.



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   An example network is pictured below:


           +----------------------------+
           |  Virtual Operator Backend  |
           +------------+         +-----+
           | Home Agent |         | AAA |
           +------------+---------+-----+
                        |
                      .--.
                    _(.   `)
                  _(   ISP `)_
                 (   Peering  `)
                ( `  . Point )  )
                 `--(_______)--'
           ____ /     |         \
          /           |          \
       .--.         .--.         .--.
     _(    `.     _(    `.     _(    `.
    (  ISP A )   (  ISP B )   (  ISP C )
   ( `  .  )  ) ( `  .  )  ) ( `  .  )  )
    `--(___.-'   `--(___.-'   `--(___.-'
        |     ______/    \       /
        |    /            \     /
        |   /              \   /
      +----+               +----+
      |MR A|               |MR B|
      +----+               +----+
        |                    |
       .--.                 .--.
     _(    `.             _(    `.
    ( Site A )           ( Site B )
   ( `  .  )  )         ( `  .  )  )
    `--(___.-'           `--(___.-'

            Virtual service provider architecture using NEMOv4

   In this example case, organization network consists of two sites,
   that are connected via 2 ISPs for redundancy reasons.  Mobile IP
   allows fast handovers without problems of multi-homing and BGP
   peering between each individual ISP and the organization.  The
   traffic however takes a non-optimal route through the virtual
   operator backend.

   Route optimization addresses this issue, allowing traffic between
   Sites A and B to flow through ISP B's network, or in case of a link
   failure, via the ISP peering point (such as MAE-WEST).  The backend
   will not suffer from heavy loads.



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   The primary design goal is to limit the load to the backend to
   minimum.  Additional design goals include extensibility to a more
   generalized scope, beyond the need of a single, coordinating Home
   Agent.


2.  Terms and definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Care-of Address (CoA)

        RFC 3344 [RFC3344] defines Care-of Address as the termination
        point of a tunnel toward a mobile node, for datagrams forwarded
        to the mobile node while it is away from home.  The protocol can
        use two different types of care-of address: a "foreign agent
        care-of address" which is an address of a foreign agent with
        which the mobile node is registered, and a "co-located care-of
        address", which is an externally obtained local address which
        the mobile node has associated with one of its own network
        interfaces.  However, in the case of Network Mobility, foreign
        agents are not used, so no foreign care-of addresses are used
        either.

   Correspondent Router (CR)

        RFC 3344 [RFC3344] defines a Correspondent node as a peer with
        which a mobile node is communicating.  Correspondent Router is a
        peer Mobile Router which MAY also represent one or more entire
        networks.

   Home Address (HoA)

        RFC 3344 [RFC3344] defines Home Address as an IP address that is
        assigned for an extended period of time to a mobile node.  It
        remains unchanged regardless of where the node is attached to
        the Internet.

   Home Agent (HA)

        RFC 3344 [RFC3344] defines Home Agent as a router on a mobile
        node's home network which tunnels datagrams for delivery to the
        mobile node when it is away from home, and maintains current
        location information for the mobile node.  For this application,
        the "home network" sees limited usage.




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   Host Network Prefix

        Network Prefix with the mask of /32. e.g. 192.0.2.254/32,
        consisting of a single host.

   Mobility Binding

        RFC 3344 [RFC3344] defines Mobility Binding as the association
        of Home Address with a Care-of address, along with the lifetime
        remaining for that association.

   Mobile Network Prefix  RFC 5177 [RFC5177] defines Mobile Network
        Prefix as the network prefix of the subnet delegated to a Mobile
        Router as the Mobile Network.

   Mobile Router (MR)

        Mobile Router as defined by RFC 5177 [RFC5177] and RFC 3344
        [RFC3344].  They define a Mobile Router as a mobile node that
        can be a router that is responsible for the mobility of one or
        more entire networks moving together, perhaps on an airplane, a
        ship, a train, an automobile, a bicycle, or a kayak.

   Route Optimization Cache

        Data structure maintained by Mobile Routers on possible
        destinations for Route Optimization.  Contains information (Home
        Addresses) on potential Correspondent Routers and their
        associated Mobile Networks.

   Return Routability, RR

        Procedure to bind a Mobile Router's Home Address to a Care-of
        address on a Correspondent Router with a degree of trust.


3.  Mobile IPv4 route optimization between mobile networks

   This section describes the changed functionality of Home Agent and
   Mobile Router compared to the base NEMOv4 operation defined in
   [RFC5177].  The basic premise is still the same; Mobile Routers, when
   registering to the Home Agent, either inform the Home Agent of the
   mobile network prefixes they are managing (explicit mode) or get
   prefixes assigned by Home Agent (implicit mode).  However, instead of
   prefix <-> Mobile Router mapping information only remaining on the
   Home Agent and the single Mobile Router, this information will now be
   distributed to the other Mobile Routers as well.




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   The Home Agent-assisted Route Optimization is primarily intended for
   helping to optimize traffic patterns between multiple sites in an
   single organization or administrative domain; However, extranets can
   also be reached with optimized routes, as long as all Mobile Routers
   connect to the same Home Agent.  The procedure aim to maintain
   backwards compatibility; With legacy nodes or routers full
   connectivity is always preserved even though optimal routing cannot
   be guaranteed.

   The schema requires a Mobile Router to be able to receive messages
   from Home Agent and other Mobile Routers unsolicited - that is,
   without first initiating a request.  This behavior is similar to the
   registration revocation procedure [RFC3543].  Many of the mechanisms
   are same - including the fact that advertising route optimization
   support upon registration implies capability to receive registration
   requests and return routability messages from other Mobile Routers.

   Compared to IPv6, where Mobile Node <-> Correspondent node bindings
   are maintained via Mobility Routing header and Home Address options,
   Mobile IPv4 always requires the use of tunnels.  Therefore, inter-
   mobile-router tunnel establishment has to be conducted.

3.1.  Maintaining route optimization information

   During registration, a joining Mobile Router MAY request information
   on route-optimizable network prefixes.  The Mobile Router MAY also
   allow redistribution of information on its managed network prefixes
   regardless whether they are explicit or implicit (statically
   configured or assigned by Home Agent).  These are indicated with
   Mobile Router Route Optimization capability extension, see
   Section 5.1.  If the Home Agent accepts the request for Route
   Optimization, this is indicated with Route Optimization Reply
   extension (Section 5.2) in the registration reply.

   Note that the redistribution of network prefix information from the
   Home Agent happens only during the registration signaling.  There are
   no "routing updates" from Home Agent except during re-registrations
   triggered by handovers, registration timeouts and specific
   solicitation.  The solicitation re-registration MAY occur if a
   Correspondent Router receives a registration request from a unknown
   Mobile Router (see Section 3.3.3).

3.1.1.  Advertising route-optimizable prefixes

   As noted, NEMO-supporting Home Agent already maintains, and in some
   cases assigns, information on which network prefixes are reachable
   behind specific Mobile Routers.  Only change to this functionality is
   that this information can now be distributed to other Mobile Routers



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   upon request.  This request is implied by including Route
   Optimization capability extension, Section 5.1, and setting the 'R'
   bit.

   When a Home Agent receives a registration request, standard
   authentication and authorization procedures are conducted.

   If registration is successful and the Route Optimization capability
   information extension was present in the registration request, the
   reply message MUST include Route Optimization Reply extension
   (Section 5.2) to indicate whether Route Optimization was accepted.
   Furthermore, the extension also informs Mobile Router if NAT was
   detected between Home Agent and the Mobile Router using the procedure
   in RFC 3519 [RFC3519], which is based on the discrepancy between
   requester's indicated Care-of address and packet's source address.

   The reply message MAY also include one route optimization prefix
   advertisement extension which informs the Mobile Router of existing
   mobile network prefixes and the Mobile Routers that manage them, if
   eligible for redistribution.  The networks SHOULD be included in
   order of priority, with the prefixes determined by policy as most
   desirable targets for Route Optimization listed first.  The extension
   is constructed as shown in Section 5.5.  The extension consists of a
   list where each Mobile Router, identified by Home Address, is listed
   with according prefix(es) and their respective realm(s).

   Each network prefix can be associated with a realm, usually in the
   form 'organization.example.com'.  Besides the routers in customer's
   own organization, the prefix list may also include other Mobile
   Routers, e.g.  Default prefix (0.0.0.0/0) pointing towards Internet
   gateway for Internet connectivity, and possible extranets.  The realm
   information can be used to make policy decisions on the Mobile
   Router, such as preferring optimization within specific realm only.

   In a typical scenario where Network Prefixes are allocated to Mobile
   Routers connecting to a single Home Agent, the prefixes are usually
   either continuous or at least very close to each other.  Due to these
   characteristics, an optional prefix compression mechanism is
   provided.  Another, optional, compression scheme is in use for realm
   information, where realms often share same higher-level domains.
   These compression mechanisms are further explained in Section 4.

   Upon receiving registration reply with a Route Optimization prefix
   advertisement extension, the Mobile Router SHALL insert the Mobile
   Router Home Addresses included in the extension as a host-prefixes to
   the local Route Optimization Cache if they do not already exist.  If
   present, any additional prefixes information SHALL also be inserted
   to the Route Optimization Cache.



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   The Mobile Router MAY discard entries from a desired starting point
   onwards, due to memory or other policy related constraints.  The
   intention of listing the prefixes in order of priority is to provide
   implicit guidance for this decision.  If the capacity of the device
   allows, the Mobile Router SHOULD use information on all advertised
   prefixes.

3.1.2.  Route Optimization cache

   Mobile routers supporting route optimization will maintain a Route
   Optimization Cache.

   The Route Optimization Cache contains mappings between potential
   Correspondent Router HoA's, network(s) associated with each HoA,
   network topology, and Return Routability procedure-related
   information.  The Cache is populated based on information received
   from Home Agent in Route optimization prefix advertisements, and in
   registration messages from Correspondent Routers.  Portions of the
   cache may also be configured statically.

   The Route Optimization Cache contains the following information for
   all known Correspondent Routers.  Note that some fields may contain
   multiple entries.  For example, during handovers, there may be both
   old and new CoA's listed.

   CR-HoA

             Correspondent Router's Home Address.  Primary key
             identifying each CR.

   CR-CoAs

             Correspondent Router's Care-of Address(es).  May be empty
             if none known.  Potential tunnel's destination address(es).

   MR-CoAs

             Mobile Router's Care-of Address(es) used with this
             Correspondent Router.  Tunnel's source address.

   Tunnels

             Tunnel interface(s) associated with this Correspondent
             Router.  The tunnel interface itself handles all the
             necessary operations to keep the tunnel operational, e.g.
             Sending keepalive messages required by UDP encapsulation.





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   NAT states

             A table of booleans, set for all pairs of potential MR-
             CoA's and CR-CoA's which require NAT awareness and the
             behavior is known, populated either statically or based on
             discovery.  If set to true, the MR can establish a UDP
             tunnel towards the CR, using this pair of CoA's.  A
             received advertisement can indicate this to be set
             initially false for all respective CR's CoA's.  Affects
             tunnel establishment direction, see Section 3.3.4 and the
             registration procedure in deciding which care-of-address to
             include in Care-of-address extension in registration reply.
             If the entry exists, mandates use of UDP encapsulation.

   RRSTATEs

             Return routability state for each CR-HoA and MR-CoA pair.
             States are INACTIVE, IN PROGRESS and ACTIVE.  If state is
             INACTIVE, return routability procedure must be completed
             before forwarding route-optimized traffic.  If state is IN
             PROGRESS or ACTIVE, the information concerning this
             Correspondent Router MUST NOT be removed from Route
             Optimization Cache as long as tunnel to the Correspondent
             Router is established.

   KRms

             Registration management key for each CR-HoA - MR-CoA pair.
             This field is only used if configured statically - if the
             KRm was computed using Return Routability procedure, they
             are calculated in-situ based on nonces and router key.  If
             configured statically, RRSTATE is permanently set to
             ACTIVE.

   Care-of nonce indexes  If the KRm was established with Return
             Routability procedure, contains the Care-of nonce index for
             each MR-CoA - CR-HoA pair.

   Care-of keygen token  If the KRm was established with Return
             Routability procedure, contains the Care-of keygen token
             for each MR-CoA - CR-HoA pair.

   Home nonce index  If the KRm was established with Return Routability
             procedure, contains the Home nonce index for each CR-HoA







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   Home keygen token  If the KRm was established with Return Routability
             procedure, contains the Home keygen token for each CR-HoA.

   Network Prefixes

             A list of destination network prefixes reachable via this
             Correspondent Router.  Includes network and prefix length,
             e.g. 192.0.2.0/25.  Always contains at least a single
             entry, the CR-HoA host network prefix in the form of
             192.0.2.1/32.

   Realms

             Each prefix may be associated with a realm.  May also be
             empty, if realm is not provided by advertisement or
             configuration.

   Prefix_Valid

             Boolean field for each prefix - CR-HoA pair, which is set
             to true if this prefix's owner has been confirmed.  The
             Host Network Prefix consisting of the Correspondent Router
             itself does need validation beyond Return Routability
             procedure.  For other prefixes, the confirmation is done by
             soliciting the information from HA.  Traffic for prefixes
             which have unconfirmed ownership should not be routed
             through the tunnel.

   Information that is no longer valid due to expirations or topology
   changes MAY be removed from the Route Optimization Cache as desired
   by the Mobile Router.

3.2.  Return routability procedure

   The purpose of return routability procedure is to establish Care-of-
   Address <-> Home Address bindings in a trusted manner.  The return
   routability procedure for Mobile IPv6 is described in [RFC3775].
   Same principles apply to the Mobile IPv4 version: Two messages are
   sent to Correspondent Router's Home Address, one via Home Agent using
   Mobile Router's Home Address, and the other directly from the Mobile
   Router CoA, with two responses coming through same routes.
   Registration management key is derived from token information carried
   on these messages.  This registration management key (KRm) can then
   be used to authenticate registration requests (comparable to Binding
   Updates in Mobile IPv6).

   The Return Routability procedure is a method provided by Mobile IP
   protocol to establish the KRm in a relatively lightweight fashion.



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   If desired, the KRms can be configured to Mobile Routers statically,
   or using an desired external secure key provisioning mechanism.  If
   KRm's are known to the Mobile Routers via some other mechanism,
   Return Routability procedure can be skipped.  Such provisioning
   mechanisms are out of scope for this document.

   Assumption on traffic patterns is that the Mobile Router that
   initiates the RR procedure can always send outbound messages, even
   when behind NAT or firewall.  This basic assumption made for NAT
   Traversal in [RFC3519] is also applicable here.  In case the
   Correspondent Router is behind such obstacles, it receives these
   messages via the reverse tunnel to CR's Home Address, thus any
   problem regarding the CR's connectivity is addressed during the
   registration to the Home Agent.

3.2.1.  Router keys

   Each Mobile Router maintains a 'correspondent router key', Kcr, which
   is MUST NOT be shared with any other entity.  Kcr is used for
   authenticating peer Mobile Routers in the situation where mobile
   router is acting as a CR.  This is analogous to node key, Kcn, in
   Mobile IPv6.  Correspondent Router uses router key to verify that the
   keygen tokens sent by Mobile Router in registration request are its
   own.  The router key MUST be a random number, 16 octets in length.

   The Mobile Router MAY generate a new key at any time to avoid
   persistent key storage.  If desired, it is RECOMMENDED to expire the
   keys in conjunction with nonces, see Section 3.2.3.

3.2.2.  Nonces

   Each Mobile Router also maintains one or more indexed nonces.  Nonces
   should be generated periodically with a good random number generator.
   The Mobile Router may use same nonces with all Mobile Routers.
   Nonces may be of any length, with the RECOMMENDED length being 64
   bits.

3.2.3.  Updating Router keys and Nonces

   The router keys and nonce updating guidelines are similar to ones in
   Mobile IPv6.  Mobile Routers keep both the current nonce and small
   set of valid previous nonces whose lifetime have not expired yet.
   Nonce should be kept acceptable for at least MAX_TOKEN_LIFETIME (see
   Section 9) seconds after it has first been used in constructing a
   return routability response.  However, the correspondent router MUST
   NOT accept nonces beyond MAX_NONCE_LIFETIME seconds (see Section 9)
   after the first use.  As the difference between these two constants
   is 30 seconds, a convenient way to enforce the above lifetimes is to



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   generate a new nonce every 30 seconds.  The node can then continue to
   accept keygen tokens that have been based on the last 8
   (MAX_NONCE_LIFETIME / 30) nonces.  This results in keygen tokens
   being acceptable MAX_TOKEN_LIFETIME to MAX_NONCE_LIFETIME seconds
   after they have been sent to the mobile node, depending on whether
   the token was sent at the beginning or end of the first 30 second
   period.  Note that the correspondent node may also attempt to
   generate new nonces on demand, or only if the old nonces have been
   used.  This is possible, as long as the correspondent node keeps
   track of how long a time ago the nonces were used for the first time,
   and does not generate new nonces on every return routability request.

   If Kcr is being updated, the update SHOULD be done at the same time
   as nonce is updated.  This way, nonce indexes can be used to refer to
   both Kcr's and nonces.

   The Return Routability procedure consists of four Mobile IP messages:
   Home Test Init, Care-of Test Init, Home Test and Care-of Test.  They
   are constructed as shown in Section 5.6 through Section 5.9.  If the
   Mobile Router has included the Mobile Router optimization capability
   extension in its Registration Request, it MUST be able to accept
   Return Routability messages.  The messages are delivered as Mobile IP
   signaling packets.  The destination address is set to Correspondent
   Router's HoA.

   The return routability procedure begins with the Mobile Router
   sending HoTI and CoTI messages, each containing a 64-bit random
   value, the cookie.  The cookie is used to bind specific signaling
   exchange together.

   Upon receiving the HoTI or CoTI message the Correspondent Router MUST
   have a secret Kcr and nonce.  If it does not have this material yet,
   it MUST produce it before continuing with the return routability
   procedure.

   Correspondent Router responds to HoTI and CoTI messages by
   constructing HoT and CoT messages, respectively, as replies.  The HoT
   message contains home init cookie, current home nonce index and home
   keygen token.  The CoT message contains care-of init cookie, current
   care-of nonce index and care-of keygen token.

   Return Routability procedure SHOULD be initiated only when the Route
   Optimization Cache's RRSTATE field for the desired Care-of Address
   with target Correspondent Router is INACTIVE.  When Return
   Routability procedure is initiated, the state MUST be set to IN
   PROGRESS.  In case of handover occurring, the Mobile Router SHOULD
   only send a CoTI message to obtain a new care-of keygen token; The
   home keygen token may still be valid.  If the reply to a registration



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   indicates that one or both of the tokens has expired, the RRSTATE
   MUST be set to INACTIVE.  The Return Routability procedure may then
   be restarted as needed.

   Upon completion of Return Routability procedure, the Routing
   Optimization Cache's RRSTATE field is set to ACTIVE, allowing for
   registration requests to be sent.  The Mobile Router will establish a
   registration management key KRm by default using MD5 hash algorithm:

   KRm = MD5 (home keygen token | care-of keygen token)

   When de-registering (by setting time to zero), care-of keygen token
   is not used.  Instead the Registration management key is generated as
   follows:

   KRm = MD5 (home keygen token)

   Like in Mobile IPv6, the Correspondent Router does not maintain any
   state for the Mobile Router until after receiving a registration
   request.

3.3.  Mobile-Correspondent Router operations

   This section deals with the operation of Mobile and Correspondent
   Routers performing route optimization.  Note that in the context of
   this document all routers work as both Mobile Router and
   Correspondent Router.  The term "Mobile Router" applies to the router
   initiating the Route Optimization procedure, and "Correspondent
   Router" indicates the peer router.

   Especially compared to Mobile IPv6 route optimization there are two
   issues that are different regarding IPv4.  First of all, since Mobile
   IPv4 always uses tunnels, there must be a tunnel established between
   MR and CR's Care-of addresses.  The Correspondent Router learns of
   Mobile Router's Care-of address as it is provided by the Registration
   Request.  The Mobile Router learns Correspondent Router's Care-of
   address by a new extension, "Care-of Address", in registration reply.
   Second issue is rising from security standpoint: In a registration
   request, the Mobile Router claims to represent an arbitrary IPv4
   network.  If the CR has not yet received this information (HoA <->
   Network prefix), it SHOULD perform a re-registration to Home Agent to
   verify the claim.

   Additional aspect is that Mobile Router MAY use different Care-of-
   Address for different Correspondent Routers (and Home Agent).  This
   is useful in situations where network provides only partial-mesh
   connectivity, and specific interfaces must be used to reach specific
   destinations.  In addition, this allows for load balancing.



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3.3.1.  Triggering Route Optimization

   Since each Mobile Router knows the eligible route-optimizable
   networks, the route optimization between all Correspondent Routers
   can be established at any time; However a better general practice is
   to conduct Route Optimization on-demand only.  It is RECOMMENDED to
   start Route optimization only be started when receiving a packet
   where destination address is in a locally managed prefix (and the
   prefix is registered as route optimizable) and source address exists
   in the network prefixes of Route Optimization Cache.  With small
   number of Mobile Routers, such on-demand behavior may not be
   necessary and full-mesh route-optimization may be in place
   constantly.

3.3.2.  Mobile Router routing tables

   Each Mobile Router maintains a routing table.  In a typical
   situation, the Mobile Router has one or more interface(s) to the
   local networks, one or more interface(s) to wide-area networks (such
   as provided by ISPs), and a tunnel interface to the Home Agent.
   Additional tunnel interfaces become activated as Route Optimization
   is being performed.

   The routing table SHOULD typically contain Network Prefixes managed
   by Correspondent Routers associated with established route-optimized
   tunnel interfaces.  In addition, host-routes to Correspondent
   Routers' Care-of addresses SHOULD be associated with the assigned to
   the physical interfaces assigned with corresponding MR-CoA.  If the
   tunneling method does not require such host-routes, these can be
   omitted.  A default route MAY point to the reverse tunnel to the Home
   Agent if not overridden by prefix information.

   The route for the Home Address of Correspondent Router SHOULD also be
   pointing towards the optimized tunnel.

   If two prefixes overlap each other, e.g. 192.0.2.128/25 and
   192.0.2.128/29, the standard longest match rule for routing is in
   effect.  However, overlapping private address SHOULD be considered an
   error situation.  Any aggregation for routes in private address space
   SHOULD be conducted only at HA.

3.3.3.  Inter-Mobile Router registration

   If route optimization between Mobile Router and Correspondent Router
   is desired, either Return Routability procedure must have been
   performed ( See Section 3.2), or key KRm must be pre-shared between
   the Mobile and Correspondent Router.  If either condition applies, a
   Mobile Router MAY send a registration request to the Correspondent



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   Router's HoA from desired interface.

   The registration request's source address and Care-of address field
   are set to the address of desired outgoing interface on the Mobile
   Router.  The address MAY be same as the Care-of address used with
   Home Agent.  The registration request MUST include Mobile-
   Correspondent Authentication extension defined in Section 5.3 and
   SHOULD include Mobile Network Request Extension defined in [RFC5177].
   If present, the Mobile Network Request Extension MUST contain the
   network prefixes, as if registering in explicit mode.  If timestamps
   are used, the Correspondent Router MUST check the identification
   field for validity.  The registration request MUST include Home
   Address.  The Authenticator field is hashed with the key KRm.

   The Correspondent Router relies to the request with a Registration
   Reply.  The registration reply MUST include Mobile-Correspondent
   Authentication extension defined in Section 5.3 and, if Mobile
   Network Request Extension was present in the request, a Mobile
   Network Acknowledgement extension.

   The encapsulation can be set as desired, except in the case where the
   Route Optimization Cache Entry has NAT entries for the Correspondent
   Router, or the Mobile Router itself is known to be behind NAT or
   firewall.  If either of the conditions apply, registration request
   MUST specify UDP encapsulation.  It is RECOMMENDED to always use UDP
   encapsulation to facilitate detecting of path failures via keepalive
   mechanism.

   The Correspondent Router first checks the registration request's
   authentication against Kcr and nonce indexes negotiated during Return
   Routability procedure.  This ensures that the registration request is
   coming from a correct Mobile Router.  If the check fails, an
   appropriate registration reply code is sent (see Section
   (Section 10).  If the failure is due to nonce index expiring, the
   Mobile Router sets RRSTATE for the CR to INACTIVE.  Return
   routability procedure MAY then be initiated again.

   If the check passes, the Correspondent Router MUST check whether the
   Mobile Router already exists in it's Route Optimization Cache and is
   associated with the prefixes included in the request (Prefixes are
   present and Flag HA is true for each prefix).

   If the check against the cache fails, the Correspondent Router SHOULD
   send a re-registration request to Home Agent with the 'S'
   (solicitation) bit set, thus obtaining the latest information on
   Network Prefixes managed by incoming Mobile Router.  If, even after
   this update, the prefixes still don't match, the reply's Mobile
   Network Acknowledgement code MUST be set to "MOBNET_UNAUTHORIZED".



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   The registration can also be rejected completely.  This verification
   is done to protect against Mobile Routers claiming to represent
   arbitrary networks; However, since Home Agent is assumed to provide
   trusted information, it can authorize Mobile Router's claim.  If the
   environment itself is considered trusted, the Correspondent Router
   can, as a policy, accept registrations from without this check;
   however, this is NOT RECOMMENDED as a general practice.

   If the prefixes match, the Correspondent Router MAY accept the
   registration.  If the CR chooses to accept, the CR MUST check if a
   tunnel to the Mobile Router already exists.  If the tunnel does NOT
   exist or has wrong endpoints (CoAs), a new tunnel MUST be established
   and Route Optimization Cache updated.  The reply MUST include a list
   of eligible care-of-addresses for the tunnel in Section 5.4, with
   which the Mobile Router may establish a tunnel with.  The reply MUST
   also include Mobile-Correspondent Authentication
   extensionSection 5.3.

   Upon receiving the registration reply, the Mobile Router MUST check
   if a tunnel to the Correspondent Router already exists.  If the
   tunnel does NOT exist, or has wrong endpoints (CoAs), a new tunnel
   MUST be established and Route Optimization Cache updated.  This is
   covered in detail in Section 3.3.4.

   The Correspondent Router's routing table MUST be updated to include
   the Mobile Router's networks are reachable via the direct tunnel to
   the Mobile Router.

   After the tunnel is established, the Mobile Router MAY update it's
   routing tables to reach all Correspondent Router's Prefixes via the
   tunnel, although it is RECOMMENDED to wait for the Correspondent
   Router to perform it's own, explicit registration.  This is primarily
   a policy decision depending on the network environment.  See
   Section 6.5.

   Due to the fact that the route optimization procedures may occur
   concurrently at two Mobile Routers, each working as each other's
   Correspondent Router, there may be a situation where two routers are
   attempting to establish separate tunnels between them at the same
   time.  If a router with a smaller Home Address (meaning a normal 32-
   bit integer comparison treating IPv4 addresses as 32-bit unsigned
   integers) receives a registration request (in CR role) while its own
   registration request (sent in MR role) is still pending, the attempt
   should be rejected with reply code "concurrent registration".  If
   receiving such an indication, the recipient should not attempt to re-
   register again until a grace period has passed without route
   optimization occurring.




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3.3.4.  Inter-Mobile Router tunnels

   Inter-Mobile Router tunnel establishment follows establishing
   standard reverse tunnels to the Home Agent.  The registration request
   to Correspondent Router includes information on the desired
   encapsulation.  It is RECOMMENDED to use UDP encapsulation.  In the
   cases of GRE [RFC2784], IP over IP [RFC2003] or minimal encapsulation
   [RFC2004] no special considerations regarding the reachability are
   necessary; The tunnel has no stateful information; The packets are
   simply encapsulated within the GRE, IP, or minimal header.

   The tunnel origination point for the Correspondent Router is its
   Care-of Address, not the Home Address where the registration requests
   were sent.  This is different from creation of the Reverse Tunnel to
   Home Agent, which reuses the channel from registration signaling.

   Special considerations rise from using UDP encapsulation, especially
   in cases where one of the Mobile Routers is located behind NAT or
   firewall.A deviation from RFC 3519 [RFC3519] is that keepalives
   should be sent both from ends of the tunnel to detect path failures
   after the initial keepalive has been sent - this allows both MR and
   CR to detect path failures.

   The initial UDP keepalive SHOULD be sent by the MR.  Only after first
   keepalive is successfully completed, SHOULD the tunnel be considered
   eligible for traffic.  If reply to the initial keepalive is not
   received, the MR may opt to attempt sending the keepalive with other
   Care-of addresses provided by the registration reply to check whether
   they provide better connectivity, or if all of these fail, perform a
   re-registration via alternative interface, or deregister completely.
   See Section 6.1.  Once the initial keepalive packet has reached the
   CR and reply has been sent, the CR MAY start sending it's own
   keepalives.

   The original specification for UDP encapsulation suggests a keepalive
   interval default of 110 seconds.  However, to provide fast response
   time and switching to alternate paths, it is RECOMMENDED, if power
   and other constraints allow, to use considerably shorter periods,
   adapting to the perceived latency as needed.  However, the maximum
   amount of keepalives should at no point exceed MAX_UPDATE_RATE times
   in second.  The purpose of keepalive is not to keep NAT or firewall
   mappings in place, but serve as a mechanism to provide fast response
   in case of path failures.

   If both the Mobile Router and the Correspondent Router are behind
   separate NATs, route optimization cannot be performed between them.
   Possibilities to set up mutual tunneling when both routers are behind
   NAT, are outside the scope of this document.  However, some of these



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   issues are addressed in Section 6.1.

   The designations "MR" and "CR" only apply to the initial tunnel-
   establishment phase.  Once a tunnel is established between two
   routers, either of them can opt to either tear down the tunnel or
   perform a handover.  Signaling messages have to be authenticated with
   valid KRm.

3.3.5.  Constructing route-optimized packets

   All packets received by the Mobile Router are forwarded using normal
   routing rules according to the routing table.  There are no special
   considerations when constructing the packets, the tunnel interface's
   own processes will encapsulate any packet automatically.

3.3.6.  Handovers and Mobile Routers leaving network

   Handovers and connection breakdowns can be categorized as either
   ungraceful or graceful, also known as "break-before-make" (bbm) and
   "make-before-break" (mbb) situations.

   As with establishment, the "Mobile Router" discussed here is the
   router wishing to change connectivity state, "Correspondent Router"
   being the peer.

   When a Mobile Router wishes to leave network, it SHOULD, in addition
   to sending the registration request to the Home Agent with lifetime
   set to zero, also send such a request to all known Correspondent
   Routers.  The Correspondent Router(s), upon accepting this request
   and sending the reply, will check if it's Route Optimization Cache
   contains any prefixes associated with the requesting Mobile Router.
   These entries should be removed and routing table updated accordingly
   (traffic for the prefixes will be forwarded via the Home Agent
   again).  The tunnel MUST then be destroyed.  A short grace period
   SHOULD be used to allow possible in-transit packets to be received
   correctly.

   In the case of a handover, the Correspondent Router simply needs to
   update the tunnel's destination to the Mobile Router's new Care-of
   Address.  Mobile Router SHOULD keep accepting packets from both old
   and new care-of Addresses for a short grace period, typically in the
   order of ten seconds.  In the case of UDP encapsulation, the port
   numbers SHOULD be reused if possible.

   If the Mobile Router was unable to send the re-registration request
   before handover, it MUST send it immediately after handover has been
   completed and tunnel with the Home Agent is established.  Since
   Care-of Address(es) changing invalidates the Krm, at it is



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   RECOMMENDED to conduct partial Return Routability by sending CoTI
   message via the new Care-of-Address and obtaining new care-of keygen
   token.  In all cases, necessary tokens have to also be acquired if
   the existing ones have expired.

   If a reply is not received for a registration request to a
   Correspondent Router, any routes to the network prefixes managed by
   the Correspondent Router MUST be removed from the routing table, thus
   causing the user traffic to be forwarded via the Home Agent.

3.4.  Convergence and synchronization issues

   The information the Home Agent maintains on Mobile Network prefixes
   and the Mobile Routers' Route Optimization Caches do not need to be
   explicitly synchronized.  This is based on the assumption is that at
   least some of the traffic between nodes inside mobile networks is
   always bidirectional.  If using on-demand route optimization, this
   also implies that when a node in a mobile network talks to a node in
   another mobile network, if the initial packet does not trigger Route
   Optimization, the reply packet will.

   Consider a situation with three mobile networks, A, B, C handled by
   three Mobile Routers, MR A, MR B and MR C respectively.  If they
   register to a Home Agent in this order, the situation goes as
   follows:

   MR A registers; Receives no information on other networks from HA, as
   no other MR has registered yet.

   MR B registers; Receives information on mobile network A being
   reachable via MR A.

   MR C registers; Receives information on both of the other mobile
   networks.

   If a node in mobile network C receives traffic from mobile network A,
   the route optimization is straightforward; MR C already has network A
   in its Route Optimization Cache.  Thus, packet reception triggers
   Route Optimization towards MR A. When MR C registers to MR A (after
   Return Routability procedure is completed), MR A does not have
   information on mobile network C; Thus it will perform a re-
   registration to the Home Agent on-demand.  This allows MR A to verify
   that MR C is indeed managing network C.

   If a node in mobile network B receives to traffic from mobile network
   C, MR B has no information on network C. No route optimization is
   triggered.  However, when the node in network B replies and the reply
   reaches MR C, route optimization happens as above.  Further examples



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   of signaling are in Section 8.

   Even in the very rare case of completely unidirectional traffic from
   an entire network, the re-registrations to the Home Agent caused by
   timeouts will eventually cause convergence.  However, this should be
   treated as a special case.

   Note that all Mobile Routers are connected to same Home Agent.  For
   possibilities concerning multiple Home Agents, see Section 6.4


4.  Data compression schemes

   This section defines the two compression formats used in Route
   Optimization Prefix Advertisement extensions.

4.1.  Prefix compression

   The prefix-compression is based on the idea that prefixes usually
   share common properties.  The scheme is simple delta-compression.  In
   the prefix information advertisement, Section 5.5, the D bit
   indicates whether receiving a "master" or a "delta" prefix.  This,
   combined with the Prefix Length information, allows for compression
   and decompression of prefix information.

   If D=0, what follows in the "Prefix" field are bits 1..n of the a new
   master prefix, where n is PLen.  This is rounded up to nearest full
   octet.  Thus, prefix lengths of /4 and /8 take 1 octet, /12 and /16
   take 2 octets, /20 and /24 three, and larger than that full 4 octets.

   If D=1, what follows in the "Prefix" field are bits m..PLen of the
   prefix, where m is the first changed bit of previous master prefix,
   with padding from master prefix filling the field to full octet.
   Maximum value of Plen-m is 8 (that is, delta MUST fit into one
   octet).  If this is not possible, a new master prefix has to be
   declared.

   Determining the order of prefix transmission should be based on
   saving maximum space during transmission.

   Example of compression and transmitted data, where network prefixes
   192.0.2.0/28, 192.0.2.64/26 and 192.0.2.128/25 are transmitted are
   illustrated in Figure 1.  Because of the padding to full octets,
   redundant information is also sent.  The bit-patterns being
   transmitted are:






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  =+= shows the prefix mask
  --- shows the master prefix for delta coded prefixes
  192.0.2.0/28, D=0

  0                   1                     2                     3
  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 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|0|0|0|0|0|0|0|0|
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+
  ^                                                                   ^
  +---------------------------- encoded ------------------------------+
                                                                ^     ^
                                                                +-pad-+
  192.0.2.64/26, D=1

  0                   1                     2                     3
  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 2
 +-------------------------------------------------------------+-+-+-+-+
 |1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|0|1|0|0|0|0|0|0|
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+-+-+
                                          ^               ^
                                          +--- encoded ---+
                                          ^             ^
                                          +-- padding --+
  192.0.2.128/25, D=1

  0                   1                     2                     3
  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 2
 +-------------------------------------------------------------+-+-+-+-+
 |1|1|0|0|0|0|0|0|.|0|0|0|0|0|0|0|0|.|0|0|0|0|0|0|1|0|.|1|0|0|0|0|0|0|0|
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+-+-+-+-+-+-+-+
                                        ^               ^
                                        +--- encoded ---+
                                        ^           ^
                                        +- padding -+


                   Figure 1: Prefix Compression Example

   First prefix, 192.0.2.0/28, is considered a master prefix and is
   transmitted in full.  The PLen of 28 bits determines that all four
   octets must be transmitted.  If the prefix would have been e.g.
   192.0.2.0/24, three octets would have sufficed since 24 bits fit into
   3 octets.

   For the following prefixes, the D=1.  Thus, they are deltas of the
   previous prefix where D was zero.




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   192.0.2.64/26 includes bits 19-26 (full octet).  Bits 19-25 are
   copied from master prefix, but bit 26 is changed to 1.  The final
   notation in binary is "1001", or 0x09.

   192.0.2.128/25 includes bits 18-25 (full octet).  Bits 18-24 are
   copied from master prefix, but bit 25 is changed to 1.  The final
   notation in binary is "101", or 0x05.

   The final encoding thus becomes:

   +----------------+--------+-+---------------------+
   |     Prefix     |  Plen  |D| Transmitted Prefix  |
   +----------------+--------+-+---------------------+
   | 192.0.2.0/28   |  28    |0| 0xc0 0x00 0x02 0x00 |
   | 192.0.2.64/26  |  26    |1| 0x09                |
   | 192.0.2.128/25 |  25    |1| 0x05                |
   +----------------+--------+-+---------------------+

   It should be noted that in this case the order of prefix transmission
   would not affect compression efficiency.  If prefix 192.0.2.128/25
   would have been considered the master prefix and the others as deltas
   instead, the resulting encoding still fits into one octet for the
   subsequent prefixes.  There would be no need to declare a new master
   prefix.

4.2.  Realm compression

4.2.1.  Encoding of compressed realms

   In order to reduce the size of messages, the system introduces a
   realm compression scheme, which reduces the size of realms in a
   message.  The compression scheme is a simple dynamically updated
   dictionary based algorithm, which is designed to compress arbitrary
   length text strings.  In this scheme, an entire realm, a single label
   or a list of labels may be replaced with an index to a previous
   occurrence of the same string stored in the dictionary.  The realm
   compression defined in this specification was inspired by the RFC
   1035 [RFC1035] DNS domain name label compression.  Our algorithm is,
   however, improved to gain more compression.

   When compressing realms, the dictionary is first reset and does not
   contain a single string.  The realms are processed one by one so the
   algorithm does not expect to see them all or the whole message at
   once.  The state of the compressor is the current content of the
   dictionary.  The realms are compressed label by label or as a list of
   labels.  The dictionary can hold maximum 128 strings.  Thus, when
   adding the 129th string into the dictionary, the dictionary MUST
   first be reset to the initial state (i.e.  Emptied) and the index of



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   the string will become 0.

   The encoding of an index to the dictionary or an uncompressed run of
   octets representing a single label has purposely been made simple and
   the whole encoding works on an octet granularity.  The encoding of an
   uncompressed label takes the form of a one octet:

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+-+-+-+-=================-+-+-+-+
   |0|   LENGTH    | 'length' octets long string.. |
   +-+-+-+-+-+-+-+-+-+-+-+-=================-+-+-+-+

   This encoding allows label lengths from 1 to 127 octets.  A label
   length of zero (0) is not allowed.  The "label length" tag octet is
   then followed by up to 127 octets of the actual encoded label string.

   The index to the dictionary (the "label index" tag octet) takes the
   form of a one octet:

    0
    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |1|   INDEX     |
   +-+-+-+-+-+-+-+-+

   The above encodings do not allow generating an output octet value of
   zero (0).  The encapsulating Mobile IPv4 extension makes use of this
   property and uses the value of zero (0) to mark the end of compressed
   realm or to indicate an empty realm.  It is also possible to encode
   the complete realm using only "label length" tags.  In this case no
   compression takes place.  This allows the sender to skip compression,
   for example to reduce computation requirements when generating
   messages.  However, the receiver MUST always be prepared to receive
   compressed realms.

4.2.2.  Searching algorithm

   When compressing the input realm, the dictionary is searched for a
   matching string.  If no match could be found, the last label is
   removed from the right-hand side of the used input realm.  The search
   is repeated until the whole input realm has been processed.  If no
   match was found at all, then the first label of the original input
   realm is encoded using the "label length" tag and the label is
   inserted into the dictionary.  The previously described search is
   repeated with the remaining part of the input realm, if any.  If
   nothing remains, the realm encoding is complete.




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   When a matching string is found in the dictionary the matching part
   of the input realm is encoded using the "label index" tag.  The
   matching part of the input realm is removed and the search is
   repeated with the remaining part of the input realm, if any.  If
   nothing remains, the octet value of zero (0) is inserted to mark the
   end of encoded realm.

   The search algorithm also maintains the "longest non-matching string"
   for each input realm.  Each time the search in dictionary fails and a
   new label gets encoded using the "label length" tag and inserted into
   the dictionary, the "longest non-matching string" is concatenated by
   this label including the separating "." (dot, i.e.  Hexadecimal
   0x2e).  When a match is found in the dictionary the "longest non-
   matching string" is reset (i.e.  Emptied).  Once the whole input
   realm has been processed and encoded, all possible suffixes longer
   than one label are taken from the string and inserted into the
   dictionary.

4.2.3.  Encoding example

   This section shows an example how to encode a set of realms using the
   specified realm compression algorithm.  For example, a message might
   need to compress the realms "foo.example.com", "bar.foo.example.com",
   "buz.foo.example.org", "example.com" and "bar.example.com.org".  The
   following example shows the processing of input realms on the left
   side and the contents of the dictionary on the right hand side.  The
   example uses hexadecimal representation of numbers.

   COMPRESSOR:                                 DICTIONARY:
   1) Input "foo.example.com"
   Search("foo.example.com")
   Search("foo.example")
   Search("foo")
   Encode(0x03,'f','o','o')                    0x00 "foo"
     +-> "longest non-matching string" = "foo"
   Search("example.com")
   Search("example")
   Encode(0x07,'e','x','a','m','p','l','e')    0x01 "example"
     +-> "longest non-matching string" = "foo.example"
   Search("com")
   Encode(0x03,'c','o','m')                    0x02 "com"
     +-> "longest non-matching string" = "foo.example.com"
                                               0x03 "foo.example.com"
                                               0x04 "example.com"
   Encode(0x00)
   2) Input "bar.foo.example.com"
   Search("bar.foo.example.com")
   Search("bar.foo.example")



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   Search("bar.foo"
   Search("bar")
   Encode(0x03,'b','a','r')                    0x05 "bar"
     +-> "longest non-matching string" = "bar"
   Search("foo.example.com") -> match to 0x03
   Encode(0x83)
     +-> "longest non-matching string" = NUL
   Encode(0x00)
   3) Input "buz.foo.example.org"
   Search("buz.foo.example.org")
   Search("buz.foo.example")
   Search("buz.foo")
   Search("buz")
   Encode(0x03,'b','u','z')                    0x06 "buz"
     +-> "longest non-matching string" = "buz"
   Search("foo.example.org")
   Search("foo.example")
   Search("foo") -> match to 0x00
   Encode(0x80)
     +-> "longest non-matching string" = NUL
   Search("example.org")
   Search("example") -> match to 0x01
   Encode(0x81)
     +-> "longest non-matching string" = NUL
   Search("org")
   Encode(0x03,'o','r','g')                    0x07 "org"
     +-> "longest non-matching string" = "org"
   Encode(0x00)
   4) Input "example.com"
   Search("example.com") -> match to 0x04
   Encode(0x84)
   Encode(0x00)
   5) Input "bar.example.com.org"
   Search("bar.example.com.org")
   Search("bar.example.com")
   Search("bar.example")
   Search("bar") -> match to 0x05
   Encode(0x85)
   Search("example.com.org")
   Search("example.com") -> match to 0x04
   Encode(0x84)
   Search("org") -> match to 0x07
   Encode(0x87)
   Encode(0x00)

   As can be seen from the example, due the greedy approach of encoding
   matches, the search algorithm and the dictionary update function is
   not the most optimal one.  However, we do not claim the algorithm



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   would be the most efficient.  It functions efficiently enough for
   most inputs.  In this example, the original input realm data was 79
   octets and the compressed output excluding the end mark is 35 octets.


5.  New Mobile IPv4 messages and extensions

   This section describes the construction of all new information
   elements.

5.1.  Mobile router Route optimization capability

   This skippable extension MAY be sent by a Mobile Router to a Home
   Agent in the registration request message.

     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      |    Length     |   Sub-type    |A|R|S|O| Rsvd  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                 Optional Mobile Router HoA                    ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type      TBA_T1.  Skippable; If Home Agent does not support route
             optimization advertisements, it can ignore this request and
             simply not include any information in the reply.  "Short"
             extension format.

   Sub_Type  TBA_ST1_1

   Reserved  Set to zero, MUST be ignored on reception.

   A         Advertise my networks.  If 'A' bit is set, the Home Agent
             is allowed to advertise the networks managed by this Mobile
             Router to other Mobile Routers.  This also indicates that
             the Mobile Router is capable of receiving route
             optimization binding updates.  In effect, this allows the
             Mobile Router to work in Correspondent Router role.

   R         Request mobile network information.  If 'R' bit is set, the
             Home Agent MAY respond with information about mobile
             networks in the same domain.

   S         Soliciting prefixes managed by specific Mobile Router.  The
             Mobile Router is specified in the Optional Mobile Router
             HoA field.




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   O         Explicitly specifying the requesting Router is only able to
             initiate outgoing connections, not accept any incoming
             ones, due to NAT device, stateful firewall, or similar
             issue on any interface.  This is reflected by the Home
             Agent in the reply, and distributed in Prefix
             Advertisements to outer Mobile Routers.

   Optional Mobile Router HoA

             Solicited Mobile Router's Home Address.

5.2.  Route optimization reply

   This non-skippable extension MUST be sent by a Home Agent to a Mobile
   Router in the registration reply message, if Mobile Router indicated
   support for Route Optimization in registration message and Home Agent
   supports Route Optimization.

     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      |    Length     |    Sub-Type   |O|N|S|   Code  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type      TBA_T2 (Non-skippable), "short" extension format

   Sub-Type  TBA_ST2_1.

   O         The 'O' flag in Mobile Router Optimization capability
             extension was set during registration.

   N         Presence of NAT was detected by Home Agent.  This informs
             the Mobile Router that it is located behind NAT.  The
             detection procedure is specified in RFC 3519 [RFC3519], and
             is based on the discrepancy between registration packet's
             source address and indicated Care-of Address.  The Mobile
             Router can use this information to make decisions about
             Route Optimization strategy.

   S         Responding to a solicitation.  If 'S' bit was present in
             Mobile router Route optimization capability extension
             (Section 5.1), this is set, otherwise unset.

   The Reply code indicates whether Route Optimization has been
   accepted.  Values of 0..15 indicate assent and values 16..63 indicate
   Route Optimization is not done.




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   0         Will do Route Optimization

   16        Route Optimization declined, reason unspecified.

5.3.  Mobile-Correspondent authentication extension

   Mobile-Correspondent authentication extension is included in
   registration requests sent from Mobile Router to Correspondent
   Router.  The existence of this extension indicates that the message
   is not destined to a Home Agent, but another Mobile Router.  The
   format is similar to the other Authentication Extensions defined in
   [RFC3344], with SPIs replaced by Nonce Indexes.

     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      |    Length     | Sub-Type      |    Reserved   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Home Nonce Index         |     Care-of Nonce Index       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Authenticator...                         ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The Home Nonce Index field tells the Correspondent Router which nonce
   value to use when producing the home keygen token.  The Care-of Nonce
   Index field is ignored in requests to remove a binding.  Otherwise,
   it tells the Correspondent Router which nonce value to use when
   producing the Care-of Keygen Token.

   Type      TBA_T2 (non-skippable).  "Short" extension format.

   Sub-Type  TBA_ST_2_2

   Reserved  Set to zero, MUST be ignored on reception.

   Home Nonce Index

             Home Nonce Index in use.

   Care-of Nonce Index

             Care-of Index in use.

   Authenticator

             Authenticator field, by default constructed with HMAC_MD5
             (KRm2, Protected Data)



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   The protected data, just like on other cases where Authenticator is
   used, consists of

   o  the UDP payload (i.e., the Registration Request or Registration
      Reply data),

   o  all prior Extensions in their entirety, and

   o  the Type, Length, and Nonce Indexes of this Extension.

5.4.  Care-of address Extension

   The Care-of Address extension is added to a registration reply sent
   by the Correspondent Router to inform the Mobile Router of the
   upcoming tunnel endpoint.
     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      |    Length     |    Sub-type   |   Reserved    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       1-n Times the following information structure
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Care-of Address                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type      TBA_T2 (Non-skippable), "short" extension format

   Sub-Type  TBA_ST_2_3

   Care-of Address

             Care-of address(es) which may be used for tunnel with
             Mobile Router, in order of priority.  Multiple CoA's MAY be
             listed to facilitate faster NAT traversal.

5.5.  Route optimization prefix advertisement

   This non-skippable extension MAY be sent by a Home Agent to a Mobile
   Router in the registration reply message.  The extension is only
   included when explicitly requested by the Mobile Router in the
   registration request message.  Implicit prioritization of prefixes is
   caused by the order of extensions.









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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |    Sub-type   |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     1-n times the following information structure
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |D|M| Plen/Info |  Optional Mobile Router HoA, 4 octets         ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~               |  Optional Prefix, 1,2,3 or 4 octets           ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                 Realm  (1-n characters)                       ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type      TBA_T3 (Non-skippable), "long" extension format

   Sub-Type  TBA_ST3_1

   D         Delta.  If D=1, the prefix is a delta from last Prefix
             where D=0.  MUST be zero on first information structure,
             MAY be zero or one on subsequent information structures.
             If D=1, the Prefix field is one octet in length.  See
             Section 4.1 for details.

   M         Mobile Router HoA bit.  If M=1, the next field is Mobile
             Router HoA, and Prefix and Realm are omitted.  If M=0, the
             next field is Prefix followed by Realm, and Mobile Router
             HoA is omitted.  For the first information structure, M
             MUST be set to 1.  If M=1, the D bit is set to zero and
             ignored upon reception.

   PLen/Info This field is interpreted differently depending on whether
             M is set or not.  If M=0, this indicates the length of the
             prefix advertised. 6 bits, allows for values from 0 to 63,
             of which 33-63 are illegal.  If M=1, the Information field
             can be set to zero to indicate no specific information, or
             to 1 to indicate "outbound connections only".  This
             indicates that the target Mobile Router can only initiate,
             not receive, connections on any of it's interfaces (apart
             from the reverse tunnel to HA).  This is set if the Mobile
             Router has explicitly requested it by the 'O' flag in
             Mobile router Route optimization capability extension
             (Section 5.1).

   Mobile router HoA

             Mobile Router's Home address.  All prefixes in the
             following information structures where M=0 are maintained



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             by this Mobile Router.  This field is present only when M =
             1.

   Prefix    The IPv4 prefix advertised.  If D=0, the field length is
             Plen bits, rounded up to nearest full octet.  Least-
             significant bits starting off Plen (and are zeros) are
             omitted.  If D=1, field length is one octet.  This field is
             present only when M = 0.

   Realm     The Realm that is associated with the advertised Mobile
             Router HoA and prefix.  If empty, MUST be set to '\0'.  For
             realm encoding and optional compression scheme, refer to
             Section 4.2.  This field is present only when M = 0.

5.6.  Home-Test Init message

     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      |   Reserved    |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                          Home Init Cookie                     |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type      TBA_MIP1

   Reserved  Set to zero, MUST be ignored on reception.

   Home Init Cookie

             64-bit field which contains a random value, the Home Init
             Cookie.

5.7.  Care-of-Test Init message

     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      |   Reserved    |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                       Care-of Init Cookie                     |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   Type      TBA_MIP2

   Reserved  Set to zero, MUST be ignored on reception.

   Care-of Init Cookie

             64-bit field which contains a random value, the Care-of
             Init Cookie.

5.8.  Home Test message

     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      |   Reserved    |         Nonce Index           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                    Home Init Cookie                           +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                    Home Keygen Token                          +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type      TBA_MIP3

   Reserved  Set to zero, MUST be ignored on reception.

   Nonce Index

             This field will be echoed back by the Mobile Router to the
             Correspondent Router in a subsequent registration request's
             authentication extension.

   Home Init Cookie

             64-bit field which contains a random value, the Home Init
             Cookie.

   Home Keygen Token

             This field contains the 64 bit home keygen token used in
             the Return Routability procedure.  Generated from cookie +
             nonce.






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5.9.  Care-of test message

     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      |   Reserved    |         Nonce Index           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                    Care-of Init Cookie                        +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                    Care-of Keygen Token                       +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type      TBA_MIP4

   Reserved  Set to zero, MUST be ignored on reception.

   Care-of Nonce Index

             This field will be echoed back by the Mobile Router to the
             Correspondent Router in a subsequent registration requests'
             authentication extension.

   Care-of Init Cookie

             64-bit field which contains a random value, the Home Init
             Cookie.

   Care-of Keygen Token

             This field contains the 64 bit home keygen token used in
             the Return Routability procedure.


6.  Special Considerations

6.1.  NATs and stateful firewalls

   Mechanisms described in MIP NAT traversal [RFC3519] allow the Home
   Agent to work with Mobile Routers situated behind a NAT device or a
   stateful firewall.  Furthermore, the Home Agent may also detect
   whether NAT device is located between the Mobile Node and the HA.
   Mobile Router may also explicitly state it is behind a NAT or
   firewall on all interfaces, and this information is passed on to the
   other Mobile Routers with the information field in Route optimization



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   prefix advertisement extension (Section 5.5).  Home Agent may also
   detect presence of NAT and informs the registering Mobile Router with
   the 'N' flag in Route Optimization Reply extension (Section 5.2).  In
   the case of one or both of the routers is known to be behind NAT or
   similarly impaired (not being able to accept incoming connections),
   the tunnel establishment procedure SHOULD take this into account.

   In the case where Mobile Router is behind NAT (or firewall) and
   Correspondent Router is not, the Mobile Router will, when tunnel has
   been established, send keepalive messages (ICMP echo requests)
   through the tunnel.  Until a reply has been received, the tunnel
   SHOULD NOT be considered active.  Once reply has been received, NAT
   mapping is in place and traffic can be sent.

   Source address may change due to NAT in CoTI and Registration Request
   messages.  This does not affect the process - the hash values are
   calculated by the translated address, and the Registration Request
   will also appear from the same translated address.

   Unlike in communication with the Home Agent, in the case of Route
   Optimization the path used for signaling is not used for tunneled
   packets, as signaling always uses Home Addresses, and MR <-> CR
   tunnel is from CoA to CoA.  It is assumed that even though port
   numbers may change, NAT processing rarely allocates more than one
   external IP address to a single internal address, thus the IP address
   seen in the Registration Request and Tunnel packets remains the same.
   However, the UDP source port number may be different in Registration
   Request and incoming tunnel packets due to port translation.  This
   must not cause an error situation - the Correspondent Router MUST be
   able to accept tunneling packets from a different UDP source port
   than what was used in the Registration Request.

   Since Mobile Routers may have multiple interfaces connecting to
   several different networks, it might be possible that specific Mobile
   Routers may only be able to perform Route Optimization using specific
   Care-of-address pairs, obtained from specific networks, for example
   in a case where two Mobile Routers have an interface behind same NAT.
   Similar case may be applicable to nested NATs.  In such cases, Mobile
   Router MAY attempt to detect eligible Care-of-Address pairs by
   performing a registration and attempting to establish a tunnel
   (sending keepalives) with each Care-of-Address listed in the
   Registration Reply's Care-of-Address extension.  The eligible pairs
   should be recorded in Route Optimization cache.  If tunnel cannot be
   established with any CoA's, the Mobile Router MAY attempt to repeat
   the procedure with alternative interfaces.  The above information on
   network topology can also be configured to the Mobile Routers either
   statically or via some external feedback mechanism.




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   If both the Mobile Router and the Correspondent Router are behind two
   separate NATs, some sort of proxy or hole-punching technique may be
   applicable.  This is out of scope of this document.

6.2.  Handling of concurrent handovers

   If both Mobile Router and Correspondent Router move at the same time,
   this causes no issues from signaling perspective, as all requests are
   always sent from a Care-of-Address to Home Addresses.  Thus, the
   recipient will always receive the request and can send the reply.
   This applies even in break-before-make situations where both MR and
   CR get disconnected at same time - once the connectivity is restored,
   one end-point of the signaling messages is always the Home Address of
   respective router, and it is up to the Home Agent to provide
   reachability.

6.3.  Foreign Agents

   Since Foreign Agents have been dropped from Network Mobility for
   Mobile IPv4 work, they are not considered here.

6.4.  Multiple Home Agents

   Mobile Routers can negotiate and perform route optimization without
   the assistance of Home Agent - if they can discover each others
   existence and thus know where to send registration messages.  This
   document only addresses a logically single Home Agent that
   distributes network prefix information to the Mobile Routers.
   Problems arise from possible trust relationships; In this document
   the Home Agent serves as a way to provide verification that a
   specific network is managed by a specific router.

   If Route Optimization is desired between nodes attached to separate
   Home Agents, there are several possibilities.  Note that standard
   high availability redundancy protocols, such as VRRP, can be
   utilized; However, in such case the Home Agent is still a single
   logical entity even if consisting of more than a single node.

   Several possibilities exist for achieving Route Optimization between
   Mobile Routers attached to separate Home Agents, such as a new
   discovery/probing protocol, routing protocol between Home Agents or
   DNS SRV records, or a common AAA architecture.  There already is a
   framework for HA to retrieve information from AAA so it can be
   considered as the most viable possibility.  See Section 6.6 for
   information on possibility to generalize the method.

   Any discovery/probing protocols are out of scope for this document.




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6.5.  Mutualness of Route Optimization

   The procedure as specified is asymmetric; That is, if bidirectional
   route optimization is desired while maintaining consistency, the
   route optimization (RR check and registration) has to be performed in
   both directions, but this is not strictly necessary.  This is
   primarily a policy decision depending on how often the mobile
   prefixes are reconfigured.

   Consider the case where two networks, A and B, are handled by Mobile
   Routers A and B respectively.  If the routers are set up in such a
   fashion that Route Optimization is triggered when a packet is
   received from a Network Prefix in Route Optimization Cache, the
   following occurs if a node in network A starts sending ICMP echo
   requests (pinging) a node in network B.

   MR B sees the incoming ICMP echo request packet, which is travelling
   inside the reverse tunnel to the Home Agent.  MR B sees that the
   destination is in network B, and furthermore, source is in network A
   which exists in the cache.  This triggers Route Optimization
   processing.  Until RO is active, the ping packets (echo requests and
   replies) are routed via the reverse tunnel.

   MR B completes RR procedure and registration with MR A, which thus
   becomes a Correspondent Router for MR B. A tunnel is created between
   the routers.  MR A updates its routing tables so that network B is
   reachable via MR A <-> MR B tunnel.

   The traffic pattern is now that packets from network B to network A
   are sent over the direct tunnel, but the packets from A to B are
   transmitted via the Home Agent and reverse tunnels.  MR A now
   performs its own registration towards MR B. Upon completion, MR A
   notices that a tunnel to MR B already exists, but updates its routing
   table so that network B is now reachable via the MR A <-> MR B
   tunnel.  From this point onward, traffic is bidirectional.

   In this scenario, if MR A does NOT perform a separate route
   optimization (RR check and registration), but instead simply updates
   its routing table to reach network B via the tunnel, problems may
   arise if MR B has started to manage another network B' before the
   information has propagated to MR A. The end result is that MR B
   starts to receive packets for network B' via the Home Agent and for
   network B via direct tunnel.  If Reverse Path checking or similar
   mechanism is in use on MR B, packets from network A could be black
   holed.

   Whether to perform this mutual registration or not thus depends on
   the situation, and whether Mobile Routers are going to start managing



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   additional Network Prefixes during operation.

6.6.  Extensibility

   The design considerations include several mechanisms which might not
   be strictly necessary if Route Optimization would only be desired
   between individual customer sites in a managed network.  The
   registration procedure (with the optional Return Routability part),
   which allows for Correspondent Routers to learn Mobile Router's
   Care-of Addresses is not strictly necessary; The CoA's could have
   been provided by HA directly.

   However, this approach allows the method to be extended to a more
   generic route optimization.  The primary driver for having Home Agent
   to work as a centralized information distributer is to provide Mobile
   Routers with the knowledge of not only the other routers, but to
   provide information on which networks are managed by which routers.

   The Home Agent provides the information on all feasible nodes with
   which it is possible to establish Route Optimization.  If
   representing a whole Mobile Network is not necessary, in effect the
   typical Mobile Node <-> Correspondent Node situation, the mechanisms
   in this document work just as well - only problem is discovering if
   the target Correspondent Node can provide Route Optimization
   capability.  This can be performed by not including any prefixes in
   the information extension, just the HoA address of Mobile Router.

   In addition, with Route Optimization for single node, checks on
   whether a Mobile Router is allowed to represent specific networks are
   unnecessary since there are none.

   Correspondent node/router discovery protocols (whether they are based
   on probing or a centralized directory beyond the single Home Agent)
   are outside the scope of this document.

6.7.  Load Balancing

   The design simply provides possibility to create optimal paths
   between Mobile Routers; It doesn't dictate what should be the user
   traffic using these paths.  One possible approach in helping
   facilitate load balancing and utilizing all available paths is
   presented in [I-D.ietf-mip4-multiple-tunnel-support], which
   effectively allows for multiple Care-of addresses for a single Home
   Address.  In addition, per-tunnel load balancing is possible by using
   separate Care-of-Addresses for separate tunnels.






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7.  Scalability

   Home Agent assisted Route Optimization scalability issues stem from
   the general Mobile IPv4 architecture which is based on tunnels.
   Creating, maintaining and destroying tunnel interfaces can cause load
   on the Mobile Routers.  However, the MRs can always fall back to
   normal, reverse tunnelled routing if resource constraints are
   apparent.

   If there is a large number of optimization-capable prefixes,
   maintaining state for all of these may be an issue also, due to
   limits on routing table sizes.

   Registration responses from Home Agent to Mobile Router may provide
   information on large number of network prefixes.  If thousands of
   networks are involved, the registration reply messages are bound to
   grow very large.  The prefix- and realm compression mechanisms
   defined in Section 4 mitigates this problem to an extent.  There
   will, however, be some practical upper limit after which point some
   other delivery mechanism for the prefix information will be needed.


8.  Example signaling scenarios

8.1.  Registration request

   The following example signaling assumes that there are three Mobile
   Routers, MR A, B, C, each managing network prefixes A, B, and C. At
   the beginning, no networks are registered to the Home Agent.  Any AAA
   processing at the Home Agent is omitted from the diagram.





















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  +--------+ +--------+ +--------+ +--------------+
  | [MR A] | | [MR B] | | [MR C] | | [Home Agent] |
  +--------+ +--------+ +--------+ +--------------+
     |          |          |          |
     x------------------------------->|  Registration request,
     |          |          |          |  includes Mobile Router
     |          |          |          |  route optimization
     |          |          |          |  capability extension
     |          |          |          |
     |<-------------------------------x  Registration response,
     |          |          |          |  no known networks from HA
     |          |          |          |  in response
     |          |          |          |
     |          x-------------------->|  Registration request, similar
     |          |          |          |  to the one sent by MR A
     |          |          |          |
     |          |<--------------------x  Registration reply, includes
     |          |          |          |  network A in route optimization
     |          |          |          |  prefix advertisement extension
     |          |          |          |
     |          |          x--------->|  Registration request, similar
     |          |          |          |  the one sent by MR A
     |          |          |          |
     |          |          |<---------x  Registration reply, includes
     |          |          |          |  networks A and B in route
     |          |          |          |  optimization prefix
     |          |          |          |  advertisement extension.
     |          |          |          |  Network B is sent in
     |          |          |          |  compressed form.
     |          |          |          |


8.2.  Route optimization with return routability

   The following example signaling has same network setup as in
   Section 8.1 - Three mobile routers, each corresponding to their
   respective network.  Node A is in network A and Node C is in network
   C.

   At the beginning, no mobile routers know KRm's of each other.  If the
   KRm's would be pre-shared or provisioned with some other method, the
   Return Routability messages can be omitted.  Signaling in Section 8.1
   has occurred, thus MR A is not aware of the other networks, and MR C
   is aware of networks A and B.

   ======= Traffic inside Mobile IP tunnel to/from HA
   =-=-=-= Traffic inside Mobile IP tunnel between MRs
   ------- Traffic outside Mobile IP tunnel



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 +----------+ +--------+ +------+ +--------+ +----------+
 | [Node A] | | [MR A] | | [HA] | | [MR C] | | [Node C] |
 +----------+ +--------+ +------+ +--------+ +----------+
    |            |          |         |       |
    |<-----------O==========O=========O-------x  Mobile Router A is
    |            |          |         |       |  unaware of network C,
    |            |          |         |       |  thus, nothing happens
    |            |          |         |       |
    x------------O==========O=========O------>|  Mobile Router C
    |            |          |         |       |  notices packet from
    |            |          |         |       |  Net A - begins RO
    |            |          |         |       |
    |            |          |         |       |  Return Routability
    |            |          |         |       |  (If no preshared KRms)
    |            |          |         |       |
    |            |<=========O---------x       |  CoTI
    |            |<=========O=========x       |  HoTI
    |            |          |         |       |
    |            x==========O-------->|       |  CoT
    |            x==========O========>|       |  HoT
    |            |          |         |       |
    |            |          |         |       |  KRm between MR A <-> C
    |            |          |         |       |  established
    |            |          |         |       |
    |            |<=========O---------x       |  Registration request
    |            |          |         |       |
    |            x--------->|         |       |  Registration request
    |            |          |         |       |  to HA due to MR A
    |            |          |         |       |  being unaware of
    |            |          |         |       |  network C.
    |            |          |         |       |  Solicit bit set.
    |            |          |         |       |
    |            |<---------x         |       |  Registration reply,
    |            |          |         |       |  contains info on Net A
    |            |          |         |       |
    |            x==========O-------->|       |  Registration reply,
    |            |          |         |       |  includes MR A's CoA in
    |            |          |         |       |  Care-of-Address
    |            |          |         |       |  extension
    |            |          |         |       |
    |<-----------O=-=-=-==-=-=-=-==-=-O-------x  Packet from Node C -> A
    |            |          |         |       |  routed to direct tunnel
    |            |          |         |       |  at MR C, based on
    |            |          |         |       |  MR C now knowing MR A's
    |            |          |         |       |  CoA and tunnel being up
    |            |          |         |       |
    x------------O=-=-=-==-=-=-=-==-=-O------>|  Packet from Node A -> C
    |            |          |         |       |  routed to direct tunnel



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    |            |          |         |       |  at MR A, based on MR A
    |            |          |         |       |  now knowing MR C's CoA
    |            |          |         |       |  and tunnel being up



8.3.  Handovers

   In this example signaling, MR C changes care-of address while Route
   Optimization between MR A is operating and data is being transferred.
   Both cases where the handover is graceful ("make before break") and
   ungraceful ("break before make") occur in similar fashion, except in
   the graceful version no packets get lost.






































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   ======= Traffic inside Mobile IP tunnel to/from HA
   =-=-=-= Traffic inside Mobile IP tunnel between MRs
   ------- Traffic outside Mobile IP tunnel

 +----------+ +--------+ +------+ +--------+ +----------+
 | [Node A] | | [MR A] | | [HA] | | [MR C] | | [Node C] |
 +----------+ +--------+ +------+ +--------+ +----------+
    |            |          |         |       |
    x------------O=-=-=-==-=-=-=-==-=-O------>| Nodes A and C
    |<-----------O=-=-=-==-=-=-=-==-=-O-------x exchanging traffic
    |            |          |         |       |
    |            |          xxxxxxxxxxx       | Break occurs: MR C
    |            |          |         |       | loses connectivity to
    |            |          |         |       | current attachment point
    |            |          |         |       |
    x------------O=-=-=-==-=-=-=->    |       | Traffic from A -> C
    |            |          |         |       | lost and
    |            |          |   x<=-=-O-------x vice versa
    |            |          |         |       |
    |            |          |<--------x       | MR C finds a new
    |            |          |         |       | point of attachment,
    |            |          |         |       | registers to HA, clears
    |            |          |         |       | routing tables
    |            |          |         |       |
    |            |          x-------->|       | Registration reply
    |            |          |         |       |
    x------------O=-=-=-==-=-=-=->    |       | Traffic from A -> C
    |            |          |         |       | lost
    |<-----------O==========O=========O-------| Traffic from C -> A
    |            |          |         |       | sent via HA
    |            |          |         |       |
    |            |<=========O---------x       | Registration request,
    |            |          |         |       | reusing still active KRm
    |            |          |         |       |
    |            x==========O-------->|       | Registration reply
    |            |          |         |       |
    x------------O=-=-=-==-=-=-=-==-=-O------>| Traffic from A -> C
    |            |          |         |       | forwarded again
    |<-----------O=-=-=-==-=-=-=-==-=-O-------x Traffic from C -> A
    |            |          |         |       | switches back to direct
    |            |          |         |       | tunnel
    |            |          |         |       |









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9.  Protocol constants
      MAX_NONCE_LIFETIME              240 seconds
      MAX_TOKEN_LIFETIME              210 seconds
      MAX_RR_BINDING_LIFETIME         420 seconds
      MAX_UPDATE_RATE                 5 times


10.  IANA Considerations

   IANA has assigned rules for the existing registries "Mobile IP
   Messages" and "Mobile IPv4 numbers" in RFC 3344 [RFC3344].  Numbering
   spaces for Mobile IP messages and for Extensions that may appear in
   Mobile IP control messages (those sent to and from UDP port number
   434) should be modified.

   New Mobile IP control message extension and message type values are
   needed for the messages and extensions listed in Section 5.  The
   Route Optimization authentication processing requires four new
   message type numbers.  In addition, there is a skippable extension
   which requires it's own type number.  The rest of the new extensions
   are non-skippable, and grouped under two new types as subtypes.
   Other type is for extensions in "short" format and other for single
   extension in "long" extension format.

   New Mobile IP registration reply code values are needed for responses
   from Correspondent Routers.  The Route Optimization requires three
   new reply codes.  In addition, a new allocation guideline for
   "Correspondent Router reply codes" are needed.

   The new MIP message types are listed below:

                 +----------+---------------------------+
                 | Value    | Name                      |
                 +----------+---------------------------+
                 | TBA_MIP1 | Home-Test Init message    |
                 | TBA_MIP2 | Care-of-Test Init message |
                 | TBA_MIP3 | Home Test message         |
                 | TBA_MIP4 | Care-of Test message      |
                 +----------+---------------------------+

              Table 1: New Values for Mobile IP Message types

   The new MIP control message extension types are listed below:








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     +-----------------+---------------------------------------------+
     | Value           | Name                                        |
     +-----------------+---------------------------------------------+
     | TBA_T1, 128-255 | Mobile router Route optimization indication |
     | TBA_T2, 0-127   | Route Optimization Extensions               |
     | TBA_T3, 0-127   | Route Optimization data                     |
     +-----------------+---------------------------------------------+

     Table 2: New Values and Names for Extensions in Mobile IP Control
                                 messages

   Three new number spaces have been created for the Values and Names
   for the Sub-Type for Route Optimization-related Extensions.  This
   number spaces are initially defined to hold the following entries,
   allocated by this document:

       +-----------+-----------------------------------------------+
       | Value     | Name                                          |
       +-----------+-----------------------------------------------+
       | TBA_ST1_1 | Mobile router Route optimization capability   |
       | TBA_ST2_1 | Route optimization reply                      |
       | TBA_ST2_2 | Mobile-Correspondent authentication extension |
       | TBA_ST2_3 | Care-of address Extension                     |
       | TBA_ST3_1 | Route optimization prefix advertisement       |
       +-----------+-----------------------------------------------+

     Table 3: New Values and Names for the Sub-type Route Optimization
                                 Extension

   Note to RFC Editor: this section may be removed on publication as an
   RFC.

   Three new registration reply codes have been created for Code Values
   for Mobile IP Registration Reply Messages.  Following values are
   added:

                 +--------+-----------------------------+
                 | Value  | Name                        |
                 +--------+-----------------------------+
                 | TBA_C1 | Expired Home nonce Index    |
                 | TBA_C2 | Expired Care-of nonce Index |
                 | TBA_C3 | Expired nonces              |
                 | TBA_C4 | Concurrent registration     |
                 +--------+-----------------------------+







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11.  Security Considerations

   There are two primary security issues: Other relates to return
   routability check, which establishes that a specific Care-of address
   is, indeed, managed by a specific Home Address.  Other issue is trust
   relationships and arbitrary router claiming to represent arbitrary
   network.

   The end-user traffic can be protected using normal IPSec mechanisms.

11.1.  Return Routability

   The Return Routability check's security has been vetted with Mobile
   IPv6.  There are no large differences apart from requiring a separate
   ICMP message for connectivity check, and replay attack protection,
   which in this case uses Mobile IPv4 timestamps in registration
   request's identification field instead of sequence numbers.

   The Return Routability procedure does not establish any kind of state
   information on the Correspondent router, mitigating Denial of Service
   attacks.  State information is only maintained after a Registration
   request has been accepted.

11.2.  Trust relationships

   The network of trust relationships in Home Agent assisted Route
   Optimization solve the issues where arbitrary Correspondent Router
   can trust an arbitrary Mobile Router that it is indeed the proper
   route to reach an arbitrary mobile network.

   It is assumed that all Mobile Routers have a trust relationship with
   the Home Agent.  Thus, they trust information provided by Home Agent.

   The Home Agent provides information matching Home Addresses and
   network prefixes.  Each Mobile Router trusts this information.

   Mobile Routers may perform Return Routability procedure between each
   other.  This creates a trusted association between Mobile Router Home
   Address and Care-of Address.  The Mobile Router also claims to
   represent a specific network.  This information is not trustworthy as
   such.

   The claim can be verified by checking the Home Address <-> network
   prefix information received, either earlier, or due to on-demand
   request, from the Home Agent.  If they match, the Mobile Router's
   claim is authentic.  If the network is considered trusted, a policy
   decision can be made to skip this check.  Exact definitions on
   situations where such decision can be made are out of scope of this



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   document.  The RECOMMENDED general practice is to perform the check.


12.  Acknowledgements

   Thanks to Jyrki Soini and Kari Laihonen for initial reviews.This work
   was supported by TEKES as part of the Future Internet program of
   TIVIT (Finnish Strategic Centre for Science, Technology and
   Innovation in the field of ICT).


13.  References

13.1.  Normative References

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.

   [RFC2004]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
              October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              March 2000.

   [RFC3344]  Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
              August 2002.

   [RFC3519]  Levkowetz, H. and S. Vaarala, "Mobile IP Traversal of
              Network Address Translation (NAT) Devices", RFC 3519,
              April 2003.

   [RFC5177]  Leung, K., Dommety, G., Narayanan, V., and A. Petrescu,
              "Network Mobility (NEMO) Extensions for Mobile IPv4",
              RFC 5177, April 2008.

13.2.  Informative References

   [I-D.ietf-mip4-multiple-tunnel-support]
              Gundavelli, S., Leung, K., Tsirtsis, G., Soliman, H., and
              A. Petrescu, "Flow Binding Support for Mobile IPv4",
              draft-ietf-mip4-multiple-tunnel-support-00 (work in
              progress), August 2010.

   [I-D.ietf-mobileip-optim]



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              Perkins, C. and D. Johnson, "Route Optimization in Mobile
              IP", September 2001.

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

   [RFC3543]  Glass, S. and M. Chandra, "Registration Revocation in
              Mobile IPv4", RFC 3543, August 2003.

   [RFC3775]  Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
              in IPv6", RFC 3775, June 2004.


Authors' Addresses

   Antti Makela
   Aalto University
   P.O. BOX 13000
   FIN-00076 Aalto
   FINLAND

   Phone: +358 9 451 5590
   Email: antti.makela@tkk.fi


   Jouni Korhonen
   Nokia Siemens Networks
   Linnoitustie 6
   FI-02600 Espoo
   FINLAND

   Email: jouni.nospam@gmail.com



















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