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Versions: 02 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 2002

Network Working Group                                W A Simpson, Editor
Internet Draft                                                Daydreamer
expires in six months                                           May 1994


                          IP Mobility Support
                  draft-ietf-mobileip-protocol-02.txt                     |



Status of this Memo

   This document is a submission to the Mobile-IP Working Group of the
   Internet Engineering Task Force (IETF).  Comments should be submitted
   to the mobile-ip@ossi.com mailing list.

   Distribution of this memo is unlimited.

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its Areas, and its Working Groups.  Note that
   other groups may also distribute working documents as Internet
   Drafts.

   Internet Drafts are draft documents valid for a maximum of six
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   ``working draft'' or ``work in progress.''

   Please check the 1id-abstracts.txt listing contained in the
   internet-drafts Shadow Directories on nic.ddn.mil, ds.internic.net,
   venera.isi.edu, nic.nordu.net, or munnari.oz.au to learn the current
   status of any Internet Draft.


Abstract

   This document specifies protocol enhancements that allow transparent
   routing of IP datagrams to Mobile Nodes in the Internet.  The Mobile
   Node is always identified by its Home-Address, regardless of its
   current point of attachment to the Internet.  While situated away
   from its home, a Mobile Node is also associated with a Care-Of-
   Address, which provides information about its current point of
   attachment to the Internet.  The protocol provides for registering
   the Care-Of-Address with the Home Agent.  The Home Agent tunnels
   traffic destined for the Mobile Node to the Care-Of-Address.





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1.  Introduction

   Current versions of the Internet Protocol make an implicit assumption
   that a node's attachment point remains fixed.  Datagrams are sent to
   a node based on the network and subnet number contained in the node's
   IP address.

   If a node moves while keeping its IP address unchanged, its network
   number will not reflect its new point of attachment.  The routing
   protocols will not be able to route datagrams to it correctly.

   This document defines new functions that allow a node to roam on the
   Internet, without changing its IP address.

   The following entities are defined:

   Mobile Node

      An IP host or router that changes connections from one network or   |
      subnetwork to another.

   Home Agent

      A router on a network that advertises reachability for a Mobile
      Node, maintains a registry of the current Mobility Bindings for
      that node while it is away from home, and tunnels datagrams for
      delivery to a Mobile Node.

   Foreign Agent

      A router that assists a locally reachable Mobile Node that is away
      from its home network.

   The following support services are defined:

   Agent Discovery

      Agents advertise their availability on each link.

      A newly arrived Mobile Node can send a solicitation on the link to
      learn if any prospective Agents are present.

   Care-Of-Address Assignment

      The Care-Of-Address terminates the end of a tunnel toward a Mobile
      Node.  Depending on the foreign network configuration, the Care-    |
      Of-Address may be dynamically assigned to the Mobile Node, or       |
      associated with a Foreign Agent.



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   Registration

      When the Mobile Node is away from home, it registers the Care-Of-   |
      Address with the Home Agent.

      Depending on its method of attachment, the Mobile Node will
      register either directly with a Home Agent, or through a Foreign
      Agent which forwards the registration to the Home Agent.

   Encapsulation

      Once a Mobile Node has registered a Care-Of-Address with a Home
      Agent, the Home Agent intercepts datagrams destined for the Mobile
      Node, formulates another datagram with the intercepted datagram     |
      enclosed within, and forwards the resulting datagram to the Care-   |
      Of-Address.

   Decapsulation

      At the Care-Of-Address, the enclosed datagram is extracted.         |

      When the Mobile Node has its own Care-Of-Address, it decapsulates
      its own datagrams.

      When the Care-Of-Address is associated with a Foreign Agent, the    |
      Foreign Agent decapsulates the datagrams.  If the datagram is
      addressed to a Mobile Node which the Foreign Agent is currently
      serving, it will deliver the datagram to the Mobile Node.           |

      Otherwise, the datagram MUST be silently discarded (rather than     |
      being further forwarded).  ICMP Destination Unreachable MUST NOT    |
      be sent when a Foreign Agent is unable to forward a datagram.



1.1.  Requirements

   A Mobile Node using its Home-Address shall be able to communicate
   with other nodes after having been disconnected from the Internet,
   and then reconnected at a different point.

   A Mobile Node shall continue to be capable of communicating directly
   with existing nodes that do not implement the mobility functions
   described in this document.

   A Mobile Node shall provide authentication in its registration
   messages.




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1.2.  Goals

   As few administrative messages as possible are sent between a Mobile
   Node and a Foreign Agent.  The link is likely to be bandwidth
   limited.

   The size of messages on the Mobile Node's directly attached link are
   to be kept as short as possible.  The link is likely to be bandwidth
   limited.



1.3.  Assumptions

   The protocols defined in this document place no additional
   requirements on assignment of IP addresses.  That is, a Mobile Node
   will be assigned an IP address by the organization that owns the       |
   machine, and will be able to use that IP address regardless of the     |
   current point of attachment.

   Mobile Nodes are able to change their point of attachment to the
   Internet as frequently as once per second.

   No protocol enhancements are required in hosts or routers that are
   not serving any of the mobility functions.  Similarly, no additional
   protocols are needed by a router (that is not acting as a Home Agent   |
   or a Foreign Agent) to route datagrams to or from a Mobile Node.

   The operation of this specification assumes that IP datagrams are
   routed to a destination without regard to the source of the datagram.

   If desired, the Mobile Node can tunnel to its Home Agent.  The
   definition of such tunneling mechanisms is outside the scope of this
   specification.



1.4.  Specification Language

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

   MUST      This word, or the adjective "required", means that the
             definition is an absolute requirement of the specification.

   MUST NOT  This phrase means that the definition is an absolute
             prohibition of the specification.




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   SHOULD    This word, or the adjective "recommended", means that there
             may exist valid reasons in particular circumstances to
             ignore this item, but the full implications must be
             understood and carefully weighed before choosing a
             different course.

   MAY       This word, or the adjective "optional", means that this
             item is one of an allowed set of alternatives.  An
             implementation which does not include this option MUST be
             prepared to interoperate with another implementation which
             does include the option.

   silently discard
             The implementation discards the packet without further
             processing, and without indicating an error to the sender.
             The implementation SHOULD provide the capability of logging
             the error, including the contents of the discarded packet,
             and SHOULD record the event in a statistics counter.



1.5.  Terminology

   This document frequently uses the following terms:

   Authentication Type
             This includes the algorithm and algorithm mode.  Note that
             a single algorithm (such as DES) might have several modes
             (for example, CBC and ECB).

   Correspondent Host
             The peer with which a Mobile Node is communicating.  The
             Correspondent Host may be either mobile or stationary.

   Home-Address
             A long term IP address that is assigned to a Mobile Node.
             It remains unchanged regardless of where the node is
             attached to the Internet.  The Home-Address is intercepted
             by the Home Agent while the Mobile Node is registered with
             that Home Agent.

   Link      A communication facility or medium over which nodes can
             communicate at the link layer; underlying the network
             layer.

   Mobility Binding
             The association of a Home-Address with a Care-Of-Address,
             and the remaining LifeTime of the association.



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   Routing Prefix
             The high-order bits in an address, which are used by
             routers to locate a link for delivery of a datagram.         |

   Mobility Security Association                                          |
             The security relationship between two nodes that is used     |
             with Mobile IP protocol messages.  This relationship         |
             includes the authentication type (including algorithm and    |
             algorithm mode), the secret (such as a shared key, or        |
             appropriate public/private key pair), and possibly other     |
             information such as labelling.

   Triangle Routing
             A path followed by a datagram destined for a Mobile Node,
             when that datagram arrives first at the Home Agent, and
             then is encapsulated and tunneled by the Home Agent.



































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2.  Agent Discovery                                                       |

   To communicate with a Foreign or Home Agent, a Mobile Node must learn
   either the IP address or the link address of that Agent.

   It is assumed that a link-layer connection has been established
   between the Agent and the Mobile Node.  The method used to establish
   such a link-layer connection is not specified in this document.

   After establishing a link-layer connection that supports the
   attachment of Mobile Nodes, the node must learn if there are any
   prospective Foreign Agents available to serve it while it is away
   from home.  If the Mobile Node is returning home, it must learn if
   its Home Agent is available.

   There are often several methods of learning the availability of an     |
   Agent.  Those described here are recommended.

   Point-to-Point Link-Layers                                             |

      The Point-to-Point-Protocol (PPP) [RFC-1548] Internet Protocol      |
      Control Protocol (IPCP) [RFC-1332], negotiates the use of IP        |
      addresses.

      When the Home-Address is not accepted, but a transient IP address   |
      is dynamically assigned, that address is used as the Care-Of-       |
      Address in registration.                                            |

      When no transient IP address is dynamically assigned, but an IP     |
      address is advertised by the peer, that address is assumed to be    |
      the IP address of an Agent.                                         |

   Multi-Point Link-Layers                                                |

      Another link establishment protocol, IEEE 802.11, might yield the
      link address of an agent.  This link-layer address is used to       |
      attempt registration.

   ICMP Router Discovery                                                  |

      An Agent which is not identified by a link-layer protocol MUST      |
      implement ICMP Router Discovery [RFC-1256].  The Router             |
      Advertisements indicate whether the router is also an Agent.

   It is recommended that as few messages as possible which duplicate     |
   functionality be sent on mobile links.  This is particularly           |
   important on wireless and congested links.                             |




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   When multiple methods are in use, the Mobile Node SHOULD first
   attempt registration with routers sending Router Advertisements in     |
   preference to those sending link-layer advertisements.  This ordering  |
   maximizes the likelihood that the registration will be recognized,
   thereby minimizing the number of registration attempts.

   An Administrative Domain MAY require registration with a Foreign
   Agent even when another registration method is in use.  This facility
   is envisioned for service providers with packet filtering fire-walls,
   or visiting policies (such as accounting) which require exchanges of
   authorization.



2.1.  Authentication

   No authentication is required for the advertisement and solicitation
   process.

   These messages MAY be authenticated using a future IP Authentication
   Header, which is external to the messages described here.  Further     |
   work on authentication of advertisement and solicitation is outside    |
   of the scope of this document.                                         |

   Whenever an externally authenticated message fails authentication,
   the message is silently discarded.                                     |



2.2.  Agent Solicitation                                                  |

   Every Mobile Node is required to implement ICMP Router Solicitation.
   However, the Router Solicitation is only sent when no link-layer
   identification has been received.

   Any Foreign Agent and Home Agent which is not identified by a link-
   layer protocol MUST implement ICMP Router Solicitation.                |

   The same procedures, defaults, and constants are used as described in
   "ICMP Router Discovery Messages" [RFC-1256].



2.3.  Agent Advertisement                                                 |

   Every Mobile Node is required to correctly process ICMP Router         |
   Advertisements.




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   Any Foreign Agent and Home Agent which is not identified by a link-
   layer protocol MUST implement ICMP Router Advertisements.              |

   An Agent which is identified by a link-layer protocol SHOULD also      |
   implement Router Advertisements.  However, the Router Advertisements   |
   need not be sent, except when the site policy requires registration    |
   with the Agent, or as a response to a specific Router Solicitation.    |

   The same procedures, defaults, and constants are used as described in
   "ICMP Router Discovery Messages" [RFC-1256], except as specified
   herein.

   The Router Advertisements are extended by examining the number of      |
   advertised addresses.  When the IP total length indicates that the     |
   ICMP message is longer than needed for the number of addresses         |
   present, the remainder is interpreted as extensions.

   The Mobility Extension is required, and indicates that the router is   |
   an Agent.  Other extensions, such as the Short Encapsulation           |
   Extension indicate optionally supported features.                      |

   The Code field of the ICMP Router Advertisement is interpreted as      |
   follows:                                                               |

   0     If the Mobility Extension is present, the router supports        |
         mobility registration.  The router is participating in routing   |
         common traffic.                                                  |

   16    A Home or Foreign Agent which supports registration, but is not  |
         participating in routing common traffic.                         |

   The Mobile Node chooses a Care-Of-Address from among advertising       |
   Agents in the same fashion as it would choose a first hop router.      |
   The Care-Of-Address chosen is the most preferred Router Address        |
   listed.
















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3.  Registration

   The registration function exchanges information between Mobile Nodes
   and Home Agents.  This function creates a Mobility Binding, linking
   the Home-Address with the Care-Of-Address currently used by the
   Mobile Node.

   When assigned a transient Care-Of-Address, a Mobile Node can act
   without a Foreign Agent.  When registering or deregistering directly
   with the Home Agent, the registration process involves the exchange
   of only 2 messages.

   a)    The Mobile Node sends a Registration Request to the Home Agent,  |
         to ask the Home Agent to provide the requested service.

   b)    The Home Agent sends a Registration Reply to the Mobile Node to
         grant or deny service.

   An Administrative Domain MAY require registration through a Foreign    |
   Agent, as indicated in Agent Advertisements.

   When the Care-Of-Address is associated with Foreign Agent, the
   Foreign Agent acts as a relay between the Mobile Node and Home Agent.
   The extended registration process involves the exchange of 4
   messages:

   a)    The Mobile Node sends a Registration Request to the prospective  |
         Foreign Agent to begin the registration process.

   b)    The Foreign Agent relays the request by sending a Registration   |
         Request to the Home Agent, to ask the Home Agent to provide the
         requested service.

   c)    The Home Agent sends a Registration Reply to the Foreign Agent
         to grant or deny service.

   d)    The Foreign Agent sends a copy of the Registration Reply to the
         Mobile Node to inform it of the disposition of its request.



3.1.  Authentication

   Each Mobile Node, Foreign Agent, and Home Agent MUST support an
   internal table holding a list of IP addresses, and the Mobility        |
   Security Association for each address.

   Mobile Node to Home Agent registration messages are required to be



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   authenticated with the Mobile-Home Authentication Extension.  The
   Mobile Node and Home Agent MUST support authentication using keyed     |
   MD5 and key sizes of 128 bits or greater, with manual key              |
   distribution.  Additional authentication algorithms, algorithm modes,  |
   and key distribution methods MAY also be supported.

   In addition, the Foreign Agent SHOULD support authentication using     |
   keyed MD5 and key sizes of 128 bits or greater, with manual key        |
   distribution.  Additional authentication algorithms, algorithm modes,  |
   and key distribution methods MAY also be supported.

   Only one Mobility Security Association exists between any given pair   |
   of participating nodes at any given time.

   Whenever a Mobility Security Association exists between a pair of      |
   nodes, all registration messages between these nodes MUST be           |
   authenticated, using the appropriate authentication extension.



3.2.  UDP                                                                 |

   The Registration messages defined herein use the User Datagram         |
   Protocol header [RFC-768].  The UDP well-known port <TBD> is used.     |

   The UDP checksum is required.  Any mobility message with an incorrect  |
   or zero UDP checksum is silently discarded.                            |
























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3.3.  Registration Request                                                |

   The UDP Header is followed by the fields shown below:

    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      |     Code      |           LifeTime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home-Address                         |      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |
   |                        Care-Of-Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Reserved                    |  Prefix-Size  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                           TimeStamp                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

   IP fields:

   Source           The Home-Address of the Mobile Node.

   Destination      The IP address of the Agent, when known.

                    When the IP address is unknown (the agent was
                    discovered via a link-layer protocol), the "all
                    Mobile Agents" multicast address.  The link-layer
                    unicast address is used to deliver the datagram to
                    the correct Agent.

   UDP fields:

   Source Port      variable

   Destination Port <TBD>

   MobileIP fields:

   Type                                                                   |

                    1 when sent by the Mobile Node
                    2 when sent by the Foreign Agent



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   Code             Optional capabilities:

                      0 - remove prior registrations
                      1 - retain prior registrations


   LifeTime         The seconds remaining before the registration is
                    considered expired.  A value of zero indicates a
                    request for de-registration.  A value of all ones
                    indicates infinity.

                    The LifeTime SHOULD NOT be set to greater than the
                    LifeTime learned in an Agent Advertisement.

   Home Agent       The IP address of the Home Agent.                     |

   Home-Address     The Home IP address of the Mobile Node.

   Care-Of-Address  The IP address for the decapsulation end of a         |
                    tunnel.                                               |

   Reserved         Sent as zero; ignored on reception.

   Prefix-Size      The size of the left-justified bit-mask that is
                    applied to the Home-Address to determine the IP
                    subnet routing-prefix.  Ranges from 0 to 30.  Set to
                    zero by Mobile Nodes which are not routers.

   TimeStamp        64 bits.  A sequence number assigned by the Mobile
                    Node.  A Network Time Protocol [RFC-1305] value is    |
                    preferred, but the elapsed time since system
                    startup, or any other monotonically increasing
                    counter MAY be used.  The value MUST NOT be the same
                    as an immediately preceeding request.

   The Mobile-Home Authentication Extension is required, and immediately  |
   follows all non-authentication extensions.

   Authenticator    A hash value taken over a stream of bytes consisting
                    of the shared secret between the Mobile Node and
                    Home Agent, followed by (concatenated with) the
                    fields in the message beginning with the Code field,  |
                    including all prior extensions, and the Type and      |
                    Length of this extension, but not including the
                    Authenticator field itself, and finally the shared    |
                    secret again.

   The Mobile-Foreign or Foreign-Home Authentication Extension is         |



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   optional, and immediately follows the Mobile-Home Authentication
   Extension.                                                             *

   When forwarded by a Foreign Agent, fields and extensions are copied    |
   from the Registration Request without modification.














































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3.4.  Registration Reply

   The UDP Header is followed by the fields shown below:

    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      |     Code      |           LifeTime            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home-Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Reserved                    |  Prefix-Size  |      *
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                           TimeStamp                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

   IP fields:

   The Source and Destination of the Request message are swapped for the  |
   Reply message.

   Note that the Source of the original Registration Request must be      |
   saved in order for the Foreign Agent to return the reply to the
   correct Mobile Node.

   UDP fields:

   The Source Port and Destination Port of the Request message are        |
   swapped for the Reply message.

   Note that the Source Port of the original Registration Request must    |
   be saved in order for the Foreign Agent to return the reply to the
   correct Mobile Node port.

   MobileIP fields:

   Type             3

   Code             One of the following codes:

                      0  service will be provided.

                    denied by Foreign Agent,
                     16  reason unspecified.



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                     17  administratively prohibited.
                     18  insufficient resources.
                     19  Mobile Node failed authentication.
                     20  Home Agent failed authentication.
                     21  Request LifeTime too long.                       |

                    denied by Home Agent,
                     32  reason unspecified.
                     33  administratively prohibited.
                     34  insufficient resources.
                     35  Mobile Node failed authentication.
                     36  Foreign Agent failed authentication.

                    Up-to-date values of the Code field are specified in
                    the most recent "Assigned Numbers" RFC [2].

   LifeTime         The seconds remaining before the registration is
                    considered expired.  A value of zero confirms a
                    request for de-registration.  A value of all ones
                    indicates infinity.                                   |

                    May be modified by the Home Agent.

   Home-Address     Copied from the Request message.                      |

   Reserved         Copied from the Request message.

   Prefix-Size      Copied from the Request message.                      |

   TimeStamp        Copied from the Request message.                      |

   The Mobile-Home Authentication Extension is required, and immediately  |
   follows all non-authentication extensions.

   Authenticator    A hash value taken over a stream of bytes consisting
                    of the shared secret between the Mobile Node and
                    Home Agent, followed by (concatenated with) all of
                    the fields in the message beginning with the Code
                    field, including all prior extensions, and the Type   |
                    and Length of this extension, but not including the
                    Authenticator field itself, and finally the shared    |
                    secret again.

                    Note that the Care-Of-Address and Home Agent are not  |
                    present in the message.  This provides a separate
                    calculation value for mutual authentication from the
                    Home Agent to the Mobile Node.




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   The Mobile-Foreign or Foreign-Home Authentication Extension is         |
   optional, and immediately follows the Mobile-Home Authentication
   Extension.

   When forwarded by a Foreign Agent, fields and extensions are copied    |
   from the Registration Reply without modification.













































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4.  Mobility Message Extensions                                           |

   To promote extensibility, each message begins with a short fixed       |
   part, which is followed by one or more extensions in Type-Length-      |
   Value format.

   Extensions allow variable amounts of information to be carried within  |
   each datagram.  The end of the list of Extensions is indicated by the  |
   Total Length of the IP datagram.                                       |


    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |   Extension   |    Length     |    Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-


   Extension        Current values are assigned as follows:               |

                     16     Mobility
                     32     Mobile-Home Authentication
                     33     Mobile-Foreign Authentication
                     34     Foreign-Home Authentication
                     64     Minimal Encapsulation
                     65     GRE Encapsulation

                    Up-to-date values are specified in the most recent    |
                    "Assigned Numbers" RFC [2].                           |

   Length           Indicates the length of the Data field.  The Length   |
                    does not include the Extension and Length bytes.      |

   Data             This field is zero or more bytes and contains the     |
                    value(s) for this Extension.  The format and length   |
                    of the Data field is determined by the Extension and  |
                    Length fields.                                        |

   When an extension is encountered which is not recognized, it is        |
   ignored.  The length field is used to skip the data field in           |
   searching for the next extension.                                      |










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4.1.  Mobility Extension                                                  |


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Extension   |    Length     |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|  Reserved   |
   +-+-+-+-+-+-+-+-+


   Extension        16                                                    |

   Length           3                                                     |

   Sequence Number  Contains the number of advertisement messages sent    |
                    since the node was initialized.  This number MUST     |
                    include this advertisement.

                    When this value decreases, the Mobile Node MUST       |
                    assume that any current registration has been lost.   |
                    This field cannot roll over in less than 2**16        |
                    seconds, and rollover is unambiguously indicated by   |
                    the value zero.                                       |

   R                Registration required bit.  When this bit is set to   |
                    1, registration with the Foreign Agent is required,   |
                    even when the Mobile Node has acquired a transient    |
                    Care-Of-Address.                                      |

   Reserved         Sent as zero; ignored on reception.                   |



4.2.  Authentication Extensions                                           |


    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       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |
   |   Extension   |    Length     |        Authenticator                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |


   Extension                                                              |





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                    32 Mobile-Home
                    33 Mobile-Foreign
                    34 Foreign-Home


   Length           The number of data bytes in the Extension (16 when    |
                    MD5 is used).                                         |

   Authenticator    Variable length (128 bits for MD5).                   |

                    For Mobile-Home authentication, the value differs     |
                    depending on the direction the message is sent.       |
                    These calculations are defined in the Registration    |
                    Request and Reply messages.                           |

                    For Mobile-Foreign and Foreign-Home authentication,   |
                    a hash value taken over a stream of bytes consisting  |
                    of the shared secret, followed by (concatenated       |
                    with) the Source, the Destination, the remaining      |
                    fields in the message beginning with the UDP header,  |
                    including all prior extensions, and the Type and      |
                    Length of this extension, but not including the       |
                    Authenticator field itself, and finally the shared    |
                    secret again.                                         |



4.3.  Minimal Encapsulation Extension                                     |


    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Extension   |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Extension        64                                                    |

   Length           0                                                     |










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5.  Forwarding Datagrams to the Mobile Node                               |

5.1.  IP in IP Encapsulation                                              |

   Support for IP in IP encapsulated tunneling is required.  Use of       |
   other tunneling methods is optional.                                   |

   The full IP fragmentation header is inserted before the datagram's IP
   header:

                                          +---------------------------+
                                          |      Outer IP Header      |   |
      +---------------------------+       +---------------------------+
      |         IP Header         |       |         IP Header         |   |
      +---------------------------+ ====> +---------------------------+
      |                           |       |                           |
      |         IP Payload        |       |         IP Payload        |   |
      |                           |       |                           |
      +---------------------------+       +---------------------------+

   The format of the IP header is as described in [RFC-791].  The outer   |
   IP header Source and Destination addresses identify the "endpoints"    |
   of the tunnel.  The inner IP header Source and Destination addresses   |
   identify the sender and recipient of the datagram.                     |

   The Protocol field in the IP header is replaced by protocol number     |
   <TBD> for the encapsulation protocol.                                  |

   The Destination field in the IP header is replaced by the Care-Of-     |
   Address of the Mobile Node.                                            |

   If the encapsulating agent is not the original source of the           |
   datagram, the Source field in the IP header is replaced by the IP      |
   address of the encapsulating agent.                                    |

   When the Home Agent encapsulates the datagram, it sets the IP Time To  |
   Live (TTL) field to be the same as the original datagram.              |

   When decapsulating, the outer IP TTL minus one is inserted into the    |
   inner IP TTL.  Thus, IP hops are counted, but the actual routers       |
   interior to the tunnel are not identified.










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5.2.  Minimal Encapsulation                                               |

   A minimal forwarding header is defined for datagrams which are not     |
   fragmented prior to tunneling.  When a datagram is already fragmented  |
   prior to tunneling, IP in IP is used.                                  |

   The minimal header is inserted between the datagram's IP header and    |
   the rest of the datagram:

      +---------------------------+       +---------------------------+   |
      |         IP Header         |       |     Modified IP Header    |   |
      +---------------------------+ ====> +---------------------------+   |
      |                           |       |     Forwarding Header     |   |
      |         IP Payload        |       +---------------------------+   |
      |                           |       |                           |   |
      +---------------------------+       |         IP Payload        |   |
                                          |                           |   |
                                          +---------------------------+   |

   A Foreign Agent which is capable of decapsulating the minimal header   |
   will include the Minimal Encapsulation Extension in its Router         |
   Advertisements.

   A Mobile Node indicates the capability of decapsulating the minimal    |
   header at the Care-Of-Address by the inclusion of the Minimal          |
   Encapsulation Extension in its Registration Request.

   The Minimal Encapsulation Extension is not included in the             |
   Registration Reply.  The use of the minimal header is entirely at the  |
   discretion of the Home Agent.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Protocol    |S|  Reserved   |        Header Checksum        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Home-Address                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Correspondent Source Address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Protocol         Copied from the Protocol field in the original IP     *
                    header.

   S                Source field present bit, which indicates that the
                    Correspondent Source Address field is present.



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                      0  not present.
                      1  present.


   Reserved         Sent as zero; ignored on reception.

   Header Checksum  The 16-bit one's complement of the one's complement
                    sum of the encapsulation header.  For computing the
                    checksum, the Checksum field is set to 0.

   Home-Address     Copied from the Destination field in the original IP
                    header.

   Correspondent Source Address
                    Copied from the Source field in the original IP
                    header.  Present only if the S-bit is set.

   The Protocol field in the IP header is replaced by protocol number
   <TBD> for the encapsulation protocol.

   The Destination field in the IP header is replaced by the Care-Of-
   Address of the Mobile Node.

   If the encapsulating agent is not the original source of the
   datagram, the Source field in the IP header is replaced by the IP
   address of the encapsulating agent.

   Finally, the Don't Fragment bit is set in the IP header.

   When decapsulating a datagram, the fields in the forwarding header
   are restored to the IP header, and the forwarding header is removed
   from the datagram.



5.3.  Tunneling Management                                                |

   It is possible that one of the routers along the tunnel interior       |
   might encounter an error while processing the datagram, causing it to  |
   return an IP ICMP error message to the source end of the tunnel.  The  |
   three types of ICMP errors that can occur in this circumstance are:    |

    - Datagram too big.
    - Time Exceeded.
    - Destination Unreachable.

   Unfortunately, ICMP only requires IP routers to return 8 bytes (64     |



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   bits) of the datagram beyond the IP header.  This is not enough to     |
   include the encapsulated header, so it is not generally possible for   |
   the Home Agent to immediately reflect the ICMP message from the        |
   interior of a tunnel back to the source host.

   However, by carefully maintaining "soft state" about its tunnels, a    |
   Home Agent can return accurate ICMP messages in most cases.  The Home  |
   Agent SHOULD maintain at least the following soft state information    |
   about each tunnel:                                                     |

    - MTU of the tunnel.
    - TTL (path length) of the tunnel
    - Reachability of the end of the tunnel.

   The Home Agent uses the ICMP messages it receives from the interior    |
   of a tunnel to update the soft state information for that tunnel.      |
   When subsequent datagrams arrive that would transit the tunnel, the    |
   router checks the soft state for the tunnel.  If the datagram would    |
   violate the state of the tunnel (such as, the TTL is less than the     |
   tunnel TTL) the Home Agent sends an ICMP error message back to the     |
   source, but also forwards the datagram into the tunnel.

   Using this technique, the ICMP error messages sent by Home Agents      |
   will not always match up one-to-one with errors encountered within     |
   the tunnel, but they will accurately reflect the state of the          |
   network.

























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6.  Mobile Node Considerations                                            |

   A Mobile Node listens for Beacons at all times that it has a link
   connection.  In this manner, it can learn that its Foreign Agent has
   changed, or that it has arrived home.

   Whenever a Mobile Node changes its point of attachment to the
   Internet, it must initiate the registration process.  If it is away
   from home, it must register with a Foreign Agent.  If it is returning
   home, it must deregister with its Home Agent.

   A Mobile Node will operate without the support of mobility functions
   when it is at home.



6.1.  Configuration and Registration Tables

   Each Mobile Node will need:

    - Home-Address
    - Prefix-Size
    - one or more Home Agents                                             |

   For each pending registration:                                         |

    - Media Address of Agent
    - Care-Of-Address
    - TimeStamp used
    - LifeTime

   For each Mobility Security Association:                                |

    - Authentication Type
    - Authentication Key



6.2.  Registration When Away From Home

   If a Mobile Node detects a change in the Incarnation Number of a
   Foreign Agent with which it is registered, it SHOULD re-register with
   that Agent.

   A Mobile Node SHOULD re-register with its Foreign Agent(s) before the
   LifeTime of its registration expires.  The Mobile Node MAY re-
   register with its Foreign Agent(s) at any time.




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   A Mobile Node can ask the Home Agent to terminate forwarding service
   through a particular Care-Of-Address, by sending a registration with
   a LifeTime of zero.



6.3.  Registration without a Foreign Agent

   In cases where a Mobile Node away from home is able to dynamically     *
   acquire a transient IP address, the Mobile Node can serve without a
   Foreign Agent, using the transient address as the Care-Of-Address.
   Thus, the registration function and the tunnel decapsulation function
   can be co-located in a single node.  This eliminates the need to
   deploy separate entities as Foreign Agents.

   The direct registration process involves the exchange of only two
   messages:

   a)    The Mobile Node sends a Registration Request to the Home Agent,  |
         to ask the Home Agent to provide the requested service.

   b)    The Home Agent sends a Registration Reply to the Mobile Node to
         grant or deny service.

   All communication between the Mobile Node and its Home Agent is
   direct, and there is no need to use the Agent Solicitation, Agent
   Advertisement, and Registration Request.                               |

   It is assumed that such a Mobile Node has mechanisms to detect
   changes in its link-layer connectivity, and to initiate acquisition
   of a new transient address each time such a change occurs.  The
   mechanisms will be specific to the particular link-layer technology,
   and are outside the scope of this document.



6.4.  De-registration When At Home

   At times, a Mobile Node will attach itself to its home link.  Since a
   Mobile Node that is at home needs no forwarding, a de-registration
   procedure MAY be used between the Mobile Node and its Home Agent.

   The de-registration process involves the exchange of only two
   messages:

   a)    The Mobile Node sends a Registration Request directly to its     |
         Home Agent, with the LifeTime set to zero, and the Code field
         set to 0, to indicate that the Home Agent remove all related



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         entries.

   b)    The Home Agent sends a Registration Reply to the Mobile Node to
         grant or deny service.

   In this special case, for Authenticator calculation, the Care-Of-      |
   Address is set to the Home-Address.

   This procedure is specified for the sake of convenience.  The Mobile
   Node is not required to register with its Home Agent.  It MAY de-
   register with each Foreign Agent, or it MAY allow its Mobility
   Bindings to simply expire.

   It is not necessary to re-register with a Home Agent when a change of
   Incarnation Number occurs, or the Advertisement LifeTime expires,
   since the Mobile Node is not seeking tunneling service.



6.5.  Registration Replies

   When a Mobile Node receives a Registration Reply which has a
   TimeStamp which is not the same as the TimeStamp of its most recent
   Registration Request to the putative sender, the message is silently
   discarded.

   When a Reply is received which has a Code indicating information from
   the Foreign Agent, the Mobile-Home Authenticator will be missing or    |
   invalid.  However, if no other reply has as yet been received, the
   reason for denial SHOULD be accepted, and result in an appropriate
   action.  If a later authenticated reply is received, that reply
   supercedes the unauthenticated reply.

   When a Reply is received which has a Code indicating that
   authentication failed with the Home Agent, the reason for denial
   SHOULD result in an appropriate action.

   Otherwise, when a Reply is received with an invalid Authenticator,
   the message is silently discarded.

   When the LifeTime of the reply is greater than the original request,
   the excess time SHOULD be ignored.  When the LifeTime of the reply is
   smaller than the original request, re-registration SHOULD occur
   before the LifeTime expires.

   The Mobile Node is not required to issue any message in reply to a
   Registration Reply.




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6.6.  Simultaneous Registrations

   Under normal circumstances, sending a new Registration Request         |
   removes other unexpired registrations for a Mobile Node from the Home
   Agent.

   An optional capability is to allow multiple simultaneous
   registrations.  For example, this is particularly useful when a
   Mobile Node is on a border between multiple cellular systems.

   In order to request simultaneous registrations, the Mobile Node sends  |
   the Registration Request with a Code set to 1.

   The return Code in the Registration Reply is the same.  No error
   occurs if the Home Agent is unable to fulfill the request.

   IP explicitly allows duplication of datagrams.  When the Home Agent
   is able to fulfill the request, the Home Agent will encapsulate a
   copy of each arriving datagram to each Care-Of-Address, and the
   Mobile Node will receive multiple copies of its datagrams.



7.  Foreign Agent Considerations

   It is the intent that Foreign Agent involvement be as minimal as
   possible.  The role of the Foreign Agent is passive, passing
   registration requests to the Home Agent, and decapsulating tunneled
   datagrams to pass to the Mobile Node.

   When no Mobility Security Association exists, this also reduces the    |
   risks resulting from absence of authentication from Foreign Agent
   messages.

   The Foreign Agent MUST NOT originate a Request or Reply that has not
   been prompted by the Mobile Node.  No Request or Reply is generated
   to indicate that the service LifeTime has expired.

   A Foreign Agent MUST NOT originate a message which revokes the
   registration of a different Foreign Agent.  A Foreign Agent SHOULD
   forward such revocations without modification when such revocation
   messages originated from an appropriate Mobile Node or Home Agent.     *



7.1.  Configuration and Registration Tables

   Each Foreign Agent will need:



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    - Care-Of-Address

   For each pending or current registration, the Foreign Agent will need
   a Visitor List:

    - Media Address of Mobile
    - Home-Address
    - Prefix-Size
    - Home Agent
    - LifeTime

   A Foreign Agent that has implemented and is using authentication will  |
   also need to have the Mobility Security Association information for
   each pending or current authenticated registration.  Even if a
   Foreign Agent implements authentication, it might not use
   authentication with each registration, because of the key management
   difficulties.



7.2.  Receiving Registration Requests                                     |

   Upon receipt of a Registration Request, the Foreign Agent may:         |

   -  immediately deny service to the Mobile Node, by sending a
      Registration Reply with the appropriate Code set.

   -  request permission from the Home Agent to provide service to the
      Mobile Node, by sending a Registration Request.                     |

   If the Foreign Agent is unable to satisfy the request for some         |
   reason, such as the Mobile Node proposes a Lifetime longer than the    |
   Foreign Agent has advertised, then the Foreign Agent sends a           |
   Registration Reply with an appropriate Code, and does not forward the  |
   request to the Home Agent.

   The Foreign Agent must maintain a list of pending Requests, which      *
   includes the IP Source Address and UDP Source Port, in order that the
   Reply can be returned to the Mobile Node.                              *



7.3.  Receiving Registration Replies

   A Registration Reply which does not relate to a pending Registration
   Request, or to a currently registered Mobile Node, is silently
   discarded.



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   If the Registration Reply granted permission to provide service to
   the Mobile Node, then the Foreign Agent updates its Visitor List
   accordingly.



8.  Home Agent Considerations

   It is the intent that the Home Agent have primary responsibility for
   processing and coordinating services.

   The Home Agent for a given Mobile Node SHOULD be located on the link
   identified by the Home-Address.  This link MAY be virtual.



8.1.  Configuration and Registration Tables

   Each Home Agent will need:

    - an IP Address

   For each authorized Mobile Node, the Home Agent will need:

    - Home-Address                                                        *
    - Prefix-Size

   For each registered Mobile Node, the Home Agent will need a
   Forwarding List:

    - Home-Address
    - Prefix-Size
    - Care-Of-Address
    - LifeTime

   For each Mobility Security Association:                                |

    - Authentication Type
    - Authentication Key



8.2.  Receiving Requests from the Foreign Agent                           |

   Upon receipt of a Registration Request from the Foreign Agent, the     |
   Home Agent grants or denies the service requested by sending a
   Registration Reply to the sender of the request, with the appropriate
   Code set.



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   When a Registration Request has an invalid Authenticator for the
   Mobile Node, a Reply is sent to the Foreign Agent, in order that the
   Foreign Agent can clear its pending request list.

   If permission is granted for the Foreign Agent to provide service to
   the Mobile Node, the Home Agent will update its Forwarding List with
   the Home-Address of the Mobile Node, and the Care-Of-Address of the
   tunnel.

   The Home Agent MAY shorten the LifeTime of the request.

   If the Request asks for termination of service by indicating a
   LifeTime of zero, the Home Agent removes the Mobility Binding for
   that Care-Of-Address from its Forwarding List.



8.3.  Receiving Requests from the Mobile Node                             |

   Upon receipt of a Registration Request from the Mobile Node, the Home  |
   Agent grants or denies the service requested by sending a
   Registration Reply to the sender of the request, with the appropriate
   Code set.

   In this special case, for Authenticator calculation, the Care-Of-
   Address is a copy of the Home-Address.

   The Home Agent MAY shorten the LifeTime of the request.                |

   If the Request asks for termination of service by indicating a
   LifeTime of zero, and the Code field set to 0, the Home Agent removes
   the Mobility Bindings for all Foreign Agents associated with that
   Mobile Node from its Forwarding List.

   No special Reply is sent to associated Foreign Agents.  The entries
   in their Visiting Lists are allowed to expire naturally.



8.4.  Simultaneous Registrations

   When a Home Agent supports the optional capability of multiple
   simultaneous registrations, any datagrams forwarded are simply
   duplicated, and a copy is sent to each Care-Of-Address.

   The return Code in the Registration Reply is the same.  No error
   occurs if the Home Agent is unable to fulfill the request, and
   earlier entries in the Forwarding List are removed.



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8.5.  Registration Expiration

   If the LifeTime for a given Mobile Node expires before the Home Agent
   has received a re-registration request, then the associated Mobility
   Binding is erased from the Forwarding List.

   No special Registration Reply is sent to the Foreign Agents.  The
   entries in the Visiting Lists will expire naturally, and probably at
   the same time.










































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A.  Mobile Networks

   A Mobile Node can be a router, which is responsible for the mobility
   of an entire network moving together, such as on an airplane, a ship,
   a train, an automobile, a bicycle, or a kayak.  Provision for a
   Routing-Prefix in registration messages allows such a Mobile Router
   to register with a Foreign or Home Agent.

   Every Foreign Agent MUST be capable of passing all arriving
   encapsulated traffic for the routing-prefix to the correct Mobile
   Router.  The Foreign Agent SHOULD NOT advertise the presence of the
   Mobile Router to other routers in its routing domain.

   When a transient IP address has been assigned, the Mobile Router can
   act as its own Foreign Agent, and register directly with the Home
   Agent, as described above.  Such a Mobile Router MAY advertise to
   other routers in the foreign routing domain.

   The Mobile Router continues to participate in its home routing domain
   through the tunnel to the Home Agent.

   When the Mobile Router returns home, and de-registers with the Home
   Agent, it MAY participate in routing with other routers in its home
   routing domain.

   DISCUSSION:

   Dissatisfaction has been expressed that this restricts the roaming
   net to a single contiguous subnet.

   Language changes have been requested for "location privacy".



B.  Gratuitous and Proxy ARP                                              |

   Many people will use their computers for extended periods of time on
   a single link, whether or not it is at their Home Network.  When
   doing so, they will expect the same level of service from their
   infrastructure as they receive today on the Home Network.  Special
   care has to be taken with handling ARP Requests from other nodes on
   the same link.

   A problem can arise if a Mobile Node which has previously answered an
   ARP Request moves away from the link, leaving behind a stale entry in
   another node's ARP cache.  For example, if a router which forwards
   datagrams into the Home Network has a stale ARP cache entry for the
   Mobile Node, any datagrams arriving through that router for the



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   Mobile Node will be lost.  Thus, it is important that ARP caches of
   nodes populating the link be updated as soon as possible.

   A gratuitous ARP is an ARP Reply that is broadcast to all nodes on a
   link, which is not in response to any ARP Request.  When an ARP Reply
   is broadcast, all hosts are required to update their local ARP
   caches, whether or not the ARP Reply was in response to an ARP
   Request they had issued [RFC-826].

   Therefore, a reasonably good solution is that a gratuitous proxy ARP
   is issued by the Home Agent on behalf of a Mobile Node whenever the
   Home Agent receives a valid registration.  The gratuitous proxy ARP
   will indicate that all remaining nodes should associate the Home-
   Address of the Mobile Node with the link-layer address of the Home
   Agent that is now serving the Mobile Node.

   For this purpose, the source IP address would be the Home-Address,
   the source link-layer address would be for the interface used, the
   target IP address would be the all-systems multicast address, and the
   target link-layer address would be the general broadcast.

   The gratuitous ARP SHOULD NOT be repeated.  Another proxy ARP will be
   sent in response to further Mobile Node registration requests, or
   Correspondent Host ARP Requests.

   While the Mobile Node is away from its Home Network, the Home Agent
   performs proxy ARP Replies for the Mobile Node.

   When a Mobile Node returns to its Home Network, it SHOULD issue a
   gratuitous ARP on its own behalf, just before de-registering itself
   from the Home Agent.

   After a Mobile Node de-registers, the Home Agent SHOULD issue ICMP
   Redirects when it receives a datagram from a Correspondent Host that
   could be sent directly to the Mobile Node.

   DISCUSSION

   This has pretty much the same set of problems (compounded by broken
   proxy ARP implementations) as gratuitous ARP. I would suggest to
   remove this as well.

   1. The block of addresses (routing prefix) given to a HA shall be
      used *exclusively* for mobile hosts. A non-mobile host should
      not be assigned an address out of this block. If a mobile
      host is assigned address out of this block, then it may
      adversely impact its operations with mobile hosts.




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   2. Communication from CH to MH goes always through the HA associated
      with the MH (regardless of whether CH and MH are on a common subnet or not),
      unless triangular route elimination is employed.

   3. From (2) it follows that the only
      IP <-> Link Layer address mapping an MH has to perform is for the
      MH's first hop router (usually the FA). In the case of a separate
      FA, MH learns FA's address as part of the registration, so MH
      doesn't need to do any ARP. In the case of an MH acting as its own
      tunnel end-point the MH acquires the IP address of its first hop
      router by means outside of the document (e.g. via DHCP), and that is
      the only IP address that MH may require ARP. So, a MH should be
      constrained NOT to use ARP if the MH  doesn't act as its own tunnel
      end-point, and to use ARP to resolve ONLY the address of its first
      hop router if the MH acts as its own tunnel end-point.

   Specifically, it is well know that in real life packets (including
   ARP packets) can be lost. Thus a node that has a stale ARP cache
   may not receive the gratuitous ARP, and thus wouldn't purge its
   ARP entry. Since the gratuitous ARP mechanism is inherently
   unreliable and has unpredictable behaviour (you don't know
   whether a host would or wouldn't be able to receive such an ARP),
   it would be unwise to build any dependencies on it. As such
   all the text on gratuitous ARP should be removed from the document.

   COUNTER:

   Having a separate address block for mobile hosts would require a
   person with a home network to have a router between the laptop
   (which is mobile) and the non-mobile servers.  This is bad.

   In fact, you can't STOP someone from doing this.  You CAN indicate
   what features need to be in the MH and HA to enter and leave gracefully.




C.  TCP Timers

   Most hosts and routers which implement TCP/IP do not permit easy       |
   configuration of the TCP Timer values.  When high-delay (e.g. SATCOM)  |
   or low-bandwidth (e.g. High-Frequency Radio) links are in use, the     |
   default TCP Timer values in many systems will cause retransmissions    |
   or timeouts when the link and network is actually operating properly,  |
   though with greater than usual delays because of the media in use.     |
   This can cause an inability to create or maintain connections over     |
   such links, and can also cause unneeded retransmissions which consume  |
   already scarce bandwidth.  Vendors are encouraged to make TCP Timers   |



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   more configurable.  Vendors of systems designed for the mobile         |
   computing markets should pick default timer values more suited to      |
   low-bandwidth, high-delay links.  Users of Mobile Nodes should be      |
   sensitive to the possibility of timer-related difficulties.















































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Security Considerations

   The mobile computing environment is potentially very different from
   the ordinary computing environment.  In many cases, mobile computers
   will be connected to the network via wireless links.  Such links are
   particularly vulnerable to passive eavesdropping, active replay
   attacks, and other active attacks.

   The registration protocol described here will result in a host's
   traffic being source routed to its mobile location.  Such traffic
   redirection could be a significant vulnerability when the
   registration were not authentic.  Also, source routing is widely
   understood to be a security problem in the current Internet.
   [Bellovin89] The Address Resolution Protocol (ARP) is not
   authenticated, and can potentially be used to steal another host's
   traffic.  The use of "Gratuitous ARP" as described in this
   specification increases the risks of ARP because ARP is not
   authenticatable.

   This specification includes a strong authentication mechanism (keyed
   MD5) which precludes many potential attacks based on the Mobile IP
   registration protocol.  However, because key distribution is
   difficult in the absence of a network key management protocol, not
   all messages with the Foreign Agent are authenticated.
   Vulnerabilities remain in the registration protocol whenever a
   registration message is not authenticated.  For example, in a
   commercial environment it might be important to authenticate all
   messages between the Foreign Agent and the Home Agent, so that
   billing is possible, and service providers don't provide service to
   users that are not legitimate customers of that service provider.

   The strength of any authentication mechanism is dependent on several
   factors, including the innate strength of the authentication
   algorithm, the secrecy of the key used, the strength of the key used,
   and the quality of the particular implementation.  This specification
   requires implementation of keyed MD5 for authentication, but does not
   preclude the use of other authentication algorithms and modes.  For
   keyed MD5 authentication to be useful, the 128-bit key must be both
   secret (that is, known only to authorised parties) and pseudo-random.
   RFC-XXXX provides more information on generating pseudo-random
   numbers.

   Users who have sensitive data that they do not wish others to see
   should use mechanisms outside the scope of this specification (such
   as encryption) to provide appropriate protection.  Users concerned
   about traffic analysis should consider appropriate use of link
   encryption.




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References

   [1]   "V.L. Voydock & S.T. Kent, "Security Mechanisms in High-level    |
         Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983."

         Also, the [Bellovin89] reference is:                             |

         "Steven M. Bellovin, "Security Problems in the TCP/IP Protocol   |
         Suite", ACM Computer Communications Review, Vol. 19, No. 2,      |
         March 1989."                                                     |

         RFC-1256, RFC-1305, RFC-1321, and RFC-1548 here.



Acknowledgments

   Special thanks to John Ioannidis (Columbia), for his inspiration and
   experimentation which began this most recent round of IP mobility
   development.

   Many thanks to Charlie Perkins (IBM), who tirelessly proposed common
   definitions and summaries, without which we may still have
   uncomparable proposals with different terminologies.  Charlie also
   coalesed the Home and Foreign Agent objects.

   Security details are primarily the work of Randall Atkinson (NRL).

   Tunnel soft state was originally developed for the "IP Address
   Encapsulation (IPAE)" specification, by Robert E. Gilligan, Erik
   Nordmark, and Bob Hinden (all of Sun Microsystems).

   Much of the text of this specification is derived from earlier drafts
   by Charlie Kunzinger (IBM), the former Working Group Editor, who
   never put his name on the document.

   Thanks to the verbose members of the Working Group, particularly
   those who contributed text, including Dave Johnson (Carnegie Mellon
   University), Tony Li (Cisco Systems), Andrew Myles (Macquarie
   University), John Penners (US West), Fumio Taraoka (Sony), and John
   Zao (Harvard).

   Finally, the Editor wishes to thank Phil Karn (Qualcomm), whose
   decade of IP mobility experimentation in the amateur radio community,
   and widespread freeware dissemination of his KA9Q software, provided
   the impetus and availability for many thousands throughout the world
   to join the Internet community.




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Chair's Address

   The working group can be contacted via the current chairs:

      Stephen Deering                 Greg Minshall
      3333 Coyote Hill Road
      Palo Alto, CA  94304

      415-812-4839                    617-873-4153

      Deering@PARC.Xerox.com          minshall@wc.novell.com



Editor's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Daydreamer
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

      Bill.Simpson@um.cc.umich.edu
          bsimpson@MorningStar.com

























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                           Table of Contents


     1.     Introduction ..........................................    1
        1.1       Requirements ....................................    2
        1.2       Goals ...........................................    3
        1.3       Assumptions .....................................    3
        1.4       Specification Language ..........................    3
        1.5       Terminology .....................................    4

     2.     Agent Discovery .......................................    6
        2.1       Authentication ..................................    7
        2.2       Agent Solicitation ..............................    7
        2.3       Agent Advertisement .............................    7

     3.     Registration ..........................................    9
        3.1       Authentication ..................................    9
        3.2       UDP .............................................10|
        3.3       Registration Request ............................11|
        3.4       Registration Reply ..............................   14

     4.     Mobility Message Extensions ...........................17|
        4.1       Mobility Extension ..............................18|
        4.2       Authentication Extensions .......................18|
        4.3       Minimal Encapsulation Extension .................19|

     5.     Forwarding Datagrams to the Mobile Node ...............20|
        5.1       IP in IP Encapsulation ..........................20|
        5.2       Minimal Encapsulation ...........................21|
        5.3       Tunneling Management ............................22|

     6.     Mobile Node Considerations ............................   24
        6.1       Configuration and Registration Tables ...........   24
        6.2       Registration When Away From Home ................   24
        6.3       Registration without a Foreign Agent ............   25
        6.4       De-registration When At Home ....................   25
        6.5       Registration Replies ............................   26
        6.6       Simultaneous Registrations ......................   27

     7.     Foreign Agent Considerations ..........................   27
        7.1       Configuration and Registration Tables ...........   27
        7.2       Receiving Registration Requests .................28|
        7.3       Receiving Registration Replies ..................   28

     8.     Home Agent Considerations .............................   29
        8.1       Configuration and Registration Tables ...........   29
        8.2       Receiving Requests from the Foreign Agent .......29|
        8.3       Receiving Requests from the Mobile Node .........30|



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        8.4       Simultaneous Registrations ......................   30
        8.5       Registration Expiration .........................   31

     APPENDICES ...................................................   32

     A.     Mobile Networks .......................................   32

     B.     Gratuitous and Proxy ARP ..............................   32

     C.     TCP Timers ............................................   34

     SECURITY CONSIDERATIONS ......................................   36

     REFERENCES ...................................................   37

     ACKNOWLEDGEMENTS .............................................   37

     CHAIR'S ADDRESS ..............................................   38

     EDITOR'S ADDRESS .............................................   38


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