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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, Department of
Intended status: Experimental                        Communications and
Expires: May 19, 2012                                Networking (Comnet)
                                                             J. Korhonen
                                                  Nokia Siemens Networks
                                                       November 16, 2011

  Home Agent assisted Route Optimization between Mobile IPv4 Networks


   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 the possibility to discover eligible peer nodes
   based on information received from Home Agent, Network Prefixes they
   represent, and how to establish a 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 May 19, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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 . . . .  8
     3.1.  Maintaining route optimization information . . . . . . . .  9
       3.1.1.  Advertising route-optimizable prefixes . . . . . . . .  9
       3.1.2.  Route Optimization cache . . . . . . . . . . . . . . . 11
     3.2.  Return routability procedure . . . . . . . . . . . . . . . 13
       3.2.1.  Router keys  . . . . . . . . . . . . . . . . . . . . . 15
       3.2.2.  Nonces . . . . . . . . . . . . . . . . . . . . . . . . 16
       3.2.3.  Updating Router keys and Nonces  . . . . . . . . . . . 16
     3.3.  Mobile-Correspondent Router operations . . . . . . . . . . 17
       3.3.1.  Triggering Route Optimization  . . . . . . . . . . . . 17
       3.3.2.  Mobile Router routing tables . . . . . . . . . . . . . 17
       3.3.3.  Inter-Mobile Router registration . . . . . . . . . . . 18
       3.3.4.  Inter-Mobile Router tunnels  . . . . . . . . . . . . . 20
       3.3.5.  Constructing route-optimized packets . . . . . . . . . 21
       3.3.6.  Handovers and Mobile Routers leaving network . . . . . 21
     3.4.  Convergence and synchronization issues . . . . . . . . . . 22
   4.  Data compression schemes . . . . . . . . . . . . . . . . . . . 23
     4.1.  Prefix compression . . . . . . . . . . . . . . . . . . . . 24
     4.2.  Realm compression  . . . . . . . . . . . . . . . . . . . . 26
       4.2.1.  Encoding of compressed realms  . . . . . . . . . . . . 26
       4.2.2.  Searching algorithm  . . . . . . . . . . . . . . . . . 27
       4.2.3.  Encoding example . . . . . . . . . . . . . . . . . . . 28
   5.  New Mobile IPv4 messages and extensions  . . . . . . . . . . . 30
     5.1.  Mobile Router Route Optimization capability  . . . . . . . 30
     5.2.  Route optimization reply . . . . . . . . . . . . . . . . . 31
     5.3.  Mobile-Correspondent authentication extension  . . . . . . 32
     5.4.  Care-of address Extension  . . . . . . . . . . . . . . . . 33
     5.5.  Route optimization prefix advertisement  . . . . . . . . . 33
     5.6.  Home-Test Init message . . . . . . . . . . . . . . . . . . 35
     5.7.  Care-of-Test Init message  . . . . . . . . . . . . . . . . 36
     5.8.  Home Test message  . . . . . . . . . . . . . . . . . . . . 36
     5.9.  Care-of test message . . . . . . . . . . . . . . . . . . . 37
   6.  Special Considerations . . . . . . . . . . . . . . . . . . . . 38
     6.1.  NATs and stateful firewalls  . . . . . . . . . . . . . . . 38
     6.2.  Handling of concurrent handovers . . . . . . . . . . . . . 40
     6.3.  Foreign Agents . . . . . . . . . . . . . . . . . . . . . . 40

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     6.4.  Multiple Home Agents . . . . . . . . . . . . . . . . . . . 40
     6.5.  Mutualness of Route Optimization . . . . . . . . . . . . . 41
     6.6.  Extensibility  . . . . . . . . . . . . . . . . . . . . . . 42
     6.7.  Load Balancing . . . . . . . . . . . . . . . . . . . . . . 42
   7.  Scalability  . . . . . . . . . . . . . . . . . . . . . . . . . 43
   8.  Example signaling scenarios  . . . . . . . . . . . . . . . . . 43
     8.1.  Registration request . . . . . . . . . . . . . . . . . . . 43
     8.2.  Route optimization with return routability . . . . . . . . 44
     8.3.  Handovers  . . . . . . . . . . . . . . . . . . . . . . . . 46
   9.  Protocol constants . . . . . . . . . . . . . . . . . . . . . . 47
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 47
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 49
     11.1. Return Routability . . . . . . . . . . . . . . . . . . . . 50
     11.2. Trust relationships  . . . . . . . . . . . . . . . . . . . 50
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 50
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 51
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 51
     13.2. Informative References . . . . . . . . . . . . . . . . . . 51
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 52

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1.  Introduction and motivations

   Traditionally, there has been no method for route optimization in
   Mobile IPv4 [RFC5944] 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 a method to
   optimize routes between networks behind Mobile Routers, as defined by
   NEMO [RFC5177].  Although NAT and the pending shortage of IPv4
   addresses makes widespread deployment of end-to-end route
   optimization infeasible, using Route Optimization from a Mobile
   router to Mobile router is still a practical scenario.  Note that the
   method specified in this document is only for Route Optimization
   between Mobile Routers; any network prefix not advertised by a Mobile
   Router would still be routed via the Home Agent, although an MR could
   advertise very large address spaces, e.g. by acting as an Internet

   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 a service provider perspective a common topology for an
   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

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   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.

   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, the 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

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   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 directly 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.

   The specification in this document is meant to be experimental, with
   the primary design goal is to limit the load on the backend to
   minimum.  Additional design goals include extensibility to a more
   generalized scope, such as not requiring all MRs to be homed on the
   same Home Agent.  Experiences are mostly sought on applicability to
   real-world operations, and protocol-specific issues such as signaling
   scalability, interworking with other Mobile IP extensions not
   specifically addressed in the document and behavior of end-user
   applications over route-optimized paths.

   The aforementioned use-case is the original application.  Moving to
   standards track should be considered after enough deployment
   experience has been gathered.  Besides the aforementioned issues,
   additional elements that might require refinement based on real-world
   experiences are delivery of information on networks managed by peer
   Mobile Routers, conducting of the MR<->MR authentication, reaction
   and recovery methods to connectivity breakdowns and other break-
   before-make topology changes, keepalive timer intervals, formats of
   signaling extensions, behavior in NAT/firewalled environment and the
   prefix and realm compression algorithms.

2.  Terms and definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Care-of Address (CoA)

        RFC 5944 [RFC5944] 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

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   Correspondent Router (CR)

        RFC 5944 [RFC5944] 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

   Home Address (HoA)

        RFC 5944 [RFC5944] 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 5944 [RFC5944] 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.

   Host Network Prefix

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

   Mobility Binding

        RFC 5944 [RFC5944] 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 5944
        [RFC5944].  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.

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   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.

   | (Concatenation)

        Some formulas in this specification use the symbol "|" to
        indicate bytewise concatenation, as in A | B. This concatenation
        requires that all of the octets of the datum A appear first in
        the result, followed by all of the octets of the datum B.

      First (size, input)

        Some formulas in this specification use a functional form "First
        (size, input)" to indicate truncation of the "input" data so
        that only the first "size" bits remain to be used.

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, may inform the Home Agent of the
   mobile network prefixes they are managing (explicit mode) or the Home
   Agent already knows the prefix assignments.  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.

   The Home Agent-assisted Route Optimization is primarily intended for
   helping to optimize traffic patterns between multiple sites in a
   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 aims to maintain
   backwards compatibility; with legacy nodes or routers full
   connectivity is always preserved even though optimal routing cannot
   be guaranteed.

   The scheme requires a Mobile Router to be able to receive messages

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   from other Mobile Routers unsolicited - that is, without first
   initiating a request.  This behavior - accepting unsolicited messages
   - is similar to the registration revocation procedure [RFC3543].
   Many of the mechanisms are the 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 registering 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 of whether they are explicitly registered or
   already configured.  These are indicated with a Mobile Router Route
   Optimization capability extension; see Section 5.1.  If the Home
   Agent accepts the request for Route Optimization, this is indicated
   with a 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 the 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, a NEMO-supporting Home Agent already maintains information
   on which network prefixes are reachable behind specific Mobile
   Routers.  The only change to this functionality is that this
   information can now be distributed to other Mobile Routers upon
   request.  This request is implied by including a Route Optimization
   capability extension 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

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   reply message MUST include Route Optimization Reply extension
   (Section 5.2) to indicate that Route Optimization Capability
   extension was understood.  Furthermore, the extension also informs
   the Mobile Router whether 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 corresponding prefix(es) and their respective realm(s).

   Each network prefix can be associated with a realm[RFC4282], usually
   in the form 'organization.example.com'.  Besides the routers in the
   customer's own organization, the prefix list may also include other
   Mobile Routers, e.g. a default prefix ( pointing towards an
   Internet gateway for Internet connectivity or additional prefixes
   belonging to possible extranets.  The realm information can be used
   to make policy decisions on the Mobile Router, such as preferring
   optimization within a specific realm only.  Furthermore, the unique
   realm information can be used to differentiate between overlapping
   address spaces utilized by the same or different organizations
   concurrently and adjusting forwarding policies accordingly.

   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 the same higher-level domains.
   These compression mechanisms are further explained in Section 4.

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

   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

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   implicit guidance for this decision.  If the capacity of the device
   allows, the Mobile Router SHOULD use information on all advertised

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,
   information on reachability related to NAT and other divisions, and
   Return Routability procedure-related information.  The Cache is
   populated based on information received from the Home Agent in Route
   optimization prefix advertisements and in registration messages from
   Correspondent Routers.  Portions of the cache may also be configured

   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.


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


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


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


             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-addresses
             to include in the Care-of-address extension in registration
             reply.  If set to true, mandates use of UDP encapsulation.


             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 a tunnel to the Correspondent
             Router is established.


             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, it is
             calculated in-situ based on nonces and router key.  If
             configured statically, RRSTATE is permanently set to

   Care-of nonce indices

             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.

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   Home nonce indices

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

   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.  Always contains at least a single
             entry, the CR-HoA host network prefix in the form of


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


             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].  The
   same principles apply to the Mobile IPv4 version: two messages are
   sent to the 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.

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   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 the Mobile
   IP protocol to establish the KRm in a relatively lightweight fashion.
   If desired, the KRm's can be configured to Mobile Routers statically,
   or using a desired external secure key provisioning mechanism.  If
   KRm's are known to the Mobile Routers via some other mechanism, the
   Return Routability procedure can be skipped.  Such provisioning
   mechanisms are out of scope for this document.

   The main 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.

   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 Route 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 of HoTI and
   CoTI messages is set to Correspondent Router's HoA with the sources
   being MR's home and care-of-address, respectively.

   The return routability procedure begins with the Mobile Router
   sending HoTI and CoTI messages, each containing a (different) 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

   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.

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   The Home Keygen token is constructed as follows:

   Home keygen token = First (64, HMAC_SHA1 (Kcr, (home address | nonce
   | 0)))

   The Care-of Keygen token is constructed as follows:

   Care-of keygen token = First (64, HMAC_SHA1 (Kcr, (Care-of address |
   nonce | 1)))

   Note that the Care-of address in this case is the source address of
   the received CoTI message packet.  The address may have changed in-
   transit due to network address translation.  This does not affect
   registration process; subsequent registration requests are expected
   to arrive from the same translated address.

   Return Routability procedure SHOULD be initiated when the Route
   Optimization Cache's RRSTATE field for the desired Care-of Address
   with target Correspondent Router is INACTIVE.  If the state was
   INACTIVE, the state MUST be set to IN PROGRESS when Return
   Routability procedure is initiated.  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 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 SHA1 hash algorithm:

   KRm = SHA1 (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

   KRm = SHA1 (home keygen token)

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

3.2.1.  Router keys

   Each Mobile Router maintains a 'correspondent router key', Kcr, which
   MUST NOT be shared with any other entity.  Kcr is used for

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   authenticating peer Mobile Routers in the situation where a mobile
   router is acting as a CR.  This is analogous to node key, Kcn, in
   Mobile IPv6.  A Correspondent Router uses its router key to verify
   that the keygen tokens sent by a peer Mobile Router in a registration
   request are the CR's own.  The router key MUST be a random number, 16
   octets in length, generated with a good random number generator

   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
   [RFC4086].  The Mobile Router may use the 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 lifetimes have not expired yet.
   Nonce should remain valid 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
   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.

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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 the Correspondent Router's Care-of
   address by a new extension, "Care-of Address", in the registration
   reply.  The second issue is a security consideration: 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 the Home Agent to verify the claim.

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

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 when sending a packet that originates
   from a local managed network (and the network is registered as route
   optimizable) and whose destination address falls within the network
   prefixes of the Route Optimization Cache.  With a 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

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   is being performed.

   The routing table SHOULD typically contain Network Prefixes managed
   by Correspondent Routers associated with established route-optimized
   tunnel interfaces.  A default route MAY point to the reverse tunnel
   to the Home Agent if not overridden by prefix information.  The
   routing table MAY also include additional routes if required by
   tunneling implementation.

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

   If two prefixes overlap each other, e.g. and, 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 a Mobile Router and a Correspondent
   Router is desired, either the 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 Router's HoA from the desired interface.

   The registration request's source address and Care-of address field
   are set to the address of the desired outgoing interface on the
   Mobile Router.  The address MAY be same as the Care-of address used
   with the Home Agent.  The Home Agent field is set to the Home Agent
   of the Mobile Router.  The registration request MUST be sent to (have
   a destination address of) the Home Address of the Correspondent
   Router.  The registration request MUST include a Mobile-Correspondent
   Authentication extension defined in Section 5.3 and SHOULD include a
   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 Authenticator field is hashed with the key KRm.

   The Correspondent Router replies to the request with a Registration
   Reply.  The registration reply MUST include a 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

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   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, the Registration
   Request MUST specify UDP encapsulation.  It is RECOMMENDED to always
   use UDP encapsulation to facilitate detection 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 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 then check it's
   Route Optimization Cache for whether the Mobile Router exists is
   associated with the prefixes included in the request (Prefixes are
   present and Flag HA is true for each prefix).  Note that the
   viewpoint is always local; the Correspondent Router compares CR-HoA
   entries against the MR's Home Address - from the CR's perspective the
   Mobile Router is also a "Correspondent Router".

   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 the 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".
   The registration MAY also be rejected completely.  This verification
   is done to protect against Mobile Routers claiming to represent
   arbitrary networks; however, since the Home Agent is assumed to
   provide trusted information, it can authorize the Mobile Router's
   claim.  If the environment itself is considered trusted, the
   Correspondent Router can, as a policy, accept registrations 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 the Route Optimization Cache updated.  The reply MUST include a
   list of eligible care-of-addresses (see Section 5.4), with which the
   Mobile Router may establish a tunnel.  The reply MUST also include
   Mobile-Correspondent Authentication extension (See Section 5.3).

   Upon receiving the registration reply, the Mobile Router MUST check

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   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 indicate
   that 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 pending, the attempt should
   be accepted with reply code "concurrent registration" (Value TBA_S1).
   If receiving such an indication, the recipient SHOULD consider the
   registration a success, but only act on it once the peer has
   completed it's own registration.

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

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   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 its own

   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
   per 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
   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

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   "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 join its home link, 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 to their Home Address.  The Correspondent
   Router(s), upon accepting this request and sending the reply, will
   check whether 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 on the
   order of ten seconds.  In the case of UDP encapsulation, it is
   RECOMMENDED to use same port numbers for both registration signaling
   and tunneled traffic if possible.  The initial keepalive message sent
   by the MR will verify that direct connectivity exists between MR and
   CR - if the keepalive fails, the MR SHOULD attempt alternate paths.

   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, it is 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 also to 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 that at
   least some of the traffic between nodes inside mobile networks is

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   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

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

   MR B registers and receives information on mobile network A being
   reachable via MR A.

   MR C registers and receives information on both of the other mobile

   If a node in mobile network C is about to send traffic to mobile
   network A, the route optimization is straightforward; MR C already
   has network A in its Route Optimization Cache.  Thus, packet
   transmission 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 sends traffic to mobile network C, MR B
   has no information on network C. No route optimization is triggered.
   However, when the node in network C replies and the reply reaches MR
   C, route optimization happens as above.  Further examples 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.

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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 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 the 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.  If the prefixes are equal, for example in the case where
   same prefix appears in multiple realms, then one octet is still
   encoded, consisting completely of padding from the master prefix.

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

   Example of compression and transmitted data, where network prefixes, and 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, 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
  ^                                                                   ^
  +---------------------------- encoded ------------------------------+
                                                                ^     ^
                                                                +-pad-+, 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
                                          ^               ^
                                          +--- encoded ---+
                                          ^             ^
                                          +-- padding --+, 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
                                        ^               ^
                                        +--- encoded ---+
                                        ^           ^
                                        +- padding -+

                   Figure 1: Prefix Compression Example

   First prefix,, 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., 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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   copied from master prefix, but bit 26 is changed to 1.  The final
   notation in binary is "1001", or 0x09. 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  |
   |   |  28    |0| 0xc0 0x00 0x02 0x00 |
   |  |  26    |1| 0x09                |
   | |  25    |1| 0x05                |

   It should be noted that in this case the order of prefix transmission
   would not affect compression efficiency.  If prefix
   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

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 a maximum of 128 strings, after
   that, a rollover MUST occur and existing contents will be
   overwritten.  Thus, when adding the 129th string into the dictionary,

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   the first entry of the dictionary MUST be overwritten and the index
   of the new 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 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 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

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

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

   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

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  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 registration requests.  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 a 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 other Mobile Routers.

   Optional Mobile Router HoA

             Solicited Mobile Router's Home Address.  This field is only
             included if flag 'S' is set.

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  1

   O         The 'O' flag in Mobile Router Route 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
   [RFC5944], 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  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 field, by default constructed with First(128,
             HMAC_SHA1 (KRm, 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

   Length    Total length of the packet.  When processing the
             information structures, if Length octets have been reached,
             this is an indication that the final information structure
             was reached as well.

   Sub-Type  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, setting 'R'-flag of the Mobile Router
   Route Optimization capability extension.  Implicit prioritization of
   prefixes is caused by the order of extensions.

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   The extension contains a sequence of information structures.  An
   information structure may consist of either an MR HoA or a network
   prefix.  Any network prefixes following an MR HoA are owned by that
   MR.  An MR HoA MUST be first in sequence, since you cannot have
   prefixes without an MR.

     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  1

   Length    Total length of the packet.  When processing the
             information structures, if Length octets have been reached,
             this is an indication that the final information structure
             was reached as well.

   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

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

   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

   This message is sent from Mobile Router to Correspondent Router when
   performing Return Routability procedure.  The source and destination
   IP addresses are set to MR's Home Address and CR's Home Address,
   respectively.  The UDP source port MAY be randomly chosen.  The UDP
   destination port is 434.
     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.

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   Home Init Cookie

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

5.7.  Care-of-Test Init message

   This message is sent from Mobile Router to Correspondent Router when
   performing Return Routability procedure.  The source and destination
   IP addresses are set to MR's Care-of-Address and CR's Home Address,
   respectively.  The UDP source port MAY be randomly chosen.  The UDP
   destination port is 434.
     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                     |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               |

   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

   This message is sent from Correspondent Router to Mobile Router when
   performing Return Routability procedure as a reply to Home-Test Init
   message.  The source and destination IP addresses, as well as UDP
   ports, are reversed from the Home-Test Init message the message is
   constructed for.  As such, the UDP source port is always 434.

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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      |   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

   Home Keygen Token

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

5.9.  Care-of test message

   This message is sent from Correspondent Router to Mobile Router when
   performing Return Routability procedure as a reply to Care-of-test
   Init message.  The source and destination IP addresses, as well as
   UDP ports, are reversed from the Care-of-test Init message the
   message is constructed for.  As such, the UDP source port is always

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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      |   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

   Care-of Keygen Token

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

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
   prefix advertisement extension (Section 5.5).  Home Agent may also

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   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 needs to 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.

   If both the Mobile Router and the Correspondent Router are behind two

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   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

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

   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

   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

   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] |
+----------+ +--------+ +------+ +--------+ +----------+
   |            |          |         |       |
   x------------O==========O=========O------>|  Mobile Router A is
   |            |          |         |       |  unaware of network C,
   |            |          |         |       |  thus, nothing happens
   |            |          |         |       |
   |<-----------O==========O=========O-------x  Mobile Router C
   |            |          |         |       |  notices packet to
   |            |          |         |       |  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= = = ==>|       |  Optional mutual registration
   |            |          |         |       |  from MR A to MR C (same
   |            |          |         |       |  procedure as above,
   |            |          |         |       |  multiple packets),
   |            |          |         |       |  possible keepalive checks
   |            |          |         |       |
   |<-----------O=-=-=-==-=-=-=-==-=-O-------x  Packet from Node C -> A
   |            |          |         |       |  routed to direct tunnel

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   |            |          |         |       |  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
   |            |          |         |       |  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.  This diagram considers the
   case where MR C gets immediate notification of lost connectivity e.g.
   due to link status indication.  MR A would eventually notice the
   breakdown due to keepalive messages failing.
  ======= 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=-=-=-==-=-=-=->x   |       | 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
   |            |          |         |       |

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   x------------O=-=-=-==-=-=-=->x   |       | Traffic from A -> C
   |            |          |         |       | lost (reverts to routing
   |            |          |         |       | via HA if enough keepalives
   |            |          |         |       | fail)
   |            |          |         |       |
   |<-----------O==========O=========O-------| Traffic from C -> A
   |            |          |         |       | sent via HA
   |            |          |         |       |
   |            O<=========O---------x       | CoTI message
   |            |          |         |       | (partial RR check)
   |            |          |         |       |
   |            x==========O-------->|       | CoT message
   |            |          |         |       |
   |            |<=========O---------x       | Registration request,
   |            |          |         |       | reusing newly calculated KRm
   |            |          |         |       |
   |            x==========O-------->|       | Registration reply
   |            |          |         |       |
   |            O<=-=-=-=-=-=-=-=-=-=x       | First keepalive check if using
   |            |          |         |       | UDP encapsulation, also
   |            x=-=-=-=-=-=-=-=-=-=>|       | creates holes in firewalls
   |            |          |         |       |
   |            |          |         |       |
   x------------O=-=-=-==-=-=-=-==-=-O------>| Traffic from A -> C
   |            |          |         |       | forwarded directly again
   |            |          |         |       |
   |<-----------O=-=-=-==-=-=-=-==-=-O-------x Traffic from C -> A
   |            |          |         |       | switches back to direct
   |            |          |         |       | tunnel
   |            |          |         |       |

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

10.  IANA Considerations

   IANA has assigned rules for the existing registries "Mobile IP
   Message Types" and "Extensions to Mobile IP Registration Messages",
   specified in RFC 5944 [RFC5944].  New Mobile IP message types and
   extension code allocations are needed for the messages and extensions
   listed in section 5.

   The Route Optimization authentication processing requires four new
   message type numbers.  The new Mobile IP Message types are listed

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   below, in Table 1.  They should preferably be allocated from a
   contiguous range.

                 | 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

   Three new registration message extension types are required and
   listed in Table 2.  The first type, TBA_T1 is skippable and should be
   allocated from range 128-255.  The other two, TBA_T2 and TBA_T3 are
   non-skippable and should be allocated from range 0-127, TBA_T2 being
   of the "short" format and TBA_T3 being of the "long" format.  None of
   the messages are permitted for notification messages.

     | 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
                           Registration messages

   In addition, the registry "Code Values for Mobile IP Registration
   Reply Messages" needs to be modified.  A new success code, TBA_S1,
   should be allocated as follows:

   TBA_S1    Concurrent registration (pre-accept)

   In addition, a new allocation range needs to be created as "Error
   Codes from the Correspondent Node", subject to policy of Expert
   Review [RFC5226].  Suggested range is 201-210.  Three new
   registration reply codes are to be allocated from this range.  They
   are specified in Table 3, below:

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                 | Value  | Name                        |
                 | TBA_C1 | Expired Home nonce Index    |
                 | TBA_C2 | Expired Care-of nonce Index |
                 | TBA_C3 | Expired nonces              |

   Table 3: New Values for Code Values for Mobile IP Registration Reply

   Three new number spaces are required for the subtypes of the
   extensions in Table 2.  A new registry, named "Route Optimization
   Types and Subtypes" is created with an allocation policy of RFC
   Required [RFC5226].  The registration entries include Type, Subtype
   and Name.  Type and Subtype have range of 0-255.  Types are
   references to registration message extension types.  Subtypes are
   allocated initially as in Table 4, below:

   | Type   | Subtype | Name                                          |
   | TBA_T1 | 0       | Reserved                                      |
   | TBA_T1 | 1       | Mobile router Route optimization capability   |
   | TBA_T2 | 0       | Reserved                                      |
   | TBA_T2 | 1       | Route optimization reply                      |
   | TBA_T2 | 2       | Mobile-Correspondent authentication extension |
   | TBA_T2 | 3       | Care-of address extension                     |
   | TBA_T3 | 0       | Reserved                                      |
   | TBA_T3 | 1       | Route optimization prefix advertisement       |

     Table 4: Initial Values and Names for registry Route Optimization
                            Types and Subtypes

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

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

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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

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

12.  Acknowledgements

   Thanks to Alexandru Petrescu for constructive comments and support.
   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).

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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.

   [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.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5944]  Perkins, C., "IP Mobility Support for IPv4, Revised",
              RFC 5944, November 2010.

13.2.  Informative References

              Gundavelli, S., Leung, K., Tsirtsis, G., Soliman, H., and
              A. Petrescu, "Flow Binding Support for Mobile IPv4",
              draft-ietf-mip4-multiple-tunnel-support-02 (work in
              progress), August 2011.

              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

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              Mobile IPv4", RFC 3543, August 2003.

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

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

Authors' Addresses

   Antti Makela
   Aalto University, Department of Communications and Networking (Comnet)
   P.O. BOX 13000
   FIN-00076 Aalto

   Email: antti.makela@aalto.fi

   Jouni Korhonen
   Nokia Siemens Networks
   Linnoitustie 6
   FI-02600 Espoo

   Email: jouni.nospam@gmail.com

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