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Internet Engineering Task Force                       C. Perkins, editor
INTERNET DRAFT                                                       IBM
                                                          11 August 1995


                          IP Mobility Support
                  draft-ietf-mobileip-protocol-12.txt


Status of This Memo

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

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  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
   months, and may be updated, replaced, or obsoleted by other documents
   at any time.  It is not appropriate to use Internet Drafts as
   reference material, or to cite them other than as a ``working draft''
   or ``work in progress.''

   To learn the current status of any Internet-Draft, please check
   the ``1id-abstracts.txt'' listing contained in the internet-drafts
   Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).


Abstract

   This document specifies protocol enhancements that allow transparent
   routing of IP datagrams to mobile nodes in the Internet.  Each
   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 a home agent.  The home agent sends packets
   destined for the mobile node through a tunnel to the care-of address.
   After arriving at the end of the tunnel, the packets are then
   delivered to the mobile node.






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                                Contents



Status of This Memo                                                    i

Abstract                                                               i

 1. Introduction                                                       1
     1.1. Requirements  . . . . . . . . . . . . . . . . . . . . . .    2
     1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . .    2
     1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . .    2
     1.4. Specification Language  . . . . . . . . . . . . . . . . .    3
     1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . .    4
     1.6. Overview of Protocol Events . . . . . . . . . . . . . . .    5

 2. Agent Discovery                                                    6
     2.1. Agent Solicitation  . . . . . . . . . . . . . . . . . . .    6
     2.2. Agent Advertisement . . . . . . . . . . . . . . . . . . .    7
     2.3. Sequence Numbers, and Rollover Handling . . . . . . . . .    7

 3. Registration                                                       9
     3.1. Authentication  . . . . . . . . . . . . . . . . . . . . .   10
     3.2. Registration Request  . . . . . . . . . . . . . . . . . .   10
     3.3. Registration Reply  . . . . . . . . . . . . . . . . . . .   12

 4. Mobility Message Extensions                                       15
     4.1. Mobile Service Extension  . . . . . . . . . . . . . . . .   16
     4.2. Mobile-Home Authentication Extension  . . . . . . . . . .   17
     4.3. Mobile-Foreign Authentication Extension . . . . . . . . .   17
     4.4. Foreign-Home Authentication Extension . . . . . . . . . .   18
     4.5. Prefix Length Extension . . . . . . . . . . . . . . . . .   19

 5. Encapsulation techniques                                          20

 6. Mobile Node Considerations                                        21
     6.1. Configuration and Registration Tables . . . . . . . . . .   21
     6.2. Registration When Away From Home  . . . . . . . . . . . .   21
     6.3. Registration with a dynamically assigned care-of address    22
     6.4. Deregistration When At Home . . . . . . . . . . . . . . .   23
     6.5. Registration Replies  . . . . . . . . . . . . . . . . . .   23
     6.6. Registration Retransmission . . . . . . . . . . . . . . .   24
     6.7. Simultaneous mobility bindings  . . . . . . . . . . . . .   24
     6.8. Mobile Routers  . . . . . . . . . . . . . . . . . . . . .   24

 7. Foreign Agent Considerations                                      26



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     7.1. Configuration and Registration Tables . . . . . . . . . .   26
     7.2. Receiving Registration Requests . . . . . . . . . . . . .   27
     7.3. Receiving Registration Replies  . . . . . . . . . . . . .   27
     7.4. Decapsulation . . . . . . . . . . . . . . . . . . . . . .   27

 8. Home Agent Considerations                                         28
     8.1. Configuration and Registration Tables . . . . . . . . . .   28
     8.2. Receiving Registration Requests . . . . . . . . . . . . .   28
     8.3. Simultaneous mobility bindings  . . . . . . . . . . . . .   30
     8.4. Registration Expiration . . . . . . . . . . . . . . . . .   30
     8.5. Encapsulation . . . . . . . . . . . . . . . . . . . . . .   30
     8.6. Broadcast packets . . . . . . . . . . . . . . . . . . . .   31
     8.7. Multicast packets . . . . . . . . . . . . . . . . . . . .   31

 9. Security Considerations                                           32
     9.1. Message Authentication Codes  . . . . . . . . . . . . . .   32
     9.2. Areas of security concern in this protocol  . . . . . . .   32
     9.3. Key management  . . . . . . . . . . . . . . . . . . . . .   32
     9.4. Picking good random numbers . . . . . . . . . . . . . . .   33
     9.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . .   33
     9.6. Replay Protection for Registration Requests . . . . . . .   33
           9.6.1. Replay Protection using Nonces  . . . . . . . . .   34
           9.6.2. Replay Protection using Timestamps  . . . . . . .   34

10. Acknowledgements                                                  35

 A. Gratuitous and Proxy ARP                                          36

 B. Link-Layer considerations                                         37
     B.1. Point-to-Point Link-Layers  . . . . . . . . . . . . . . .   38
     B.2. Multi-Point Link-Layers . . . . . . . . . . . . . . . . .   38

 C. TCP Considerations                                                38
     C.1. TCP Timers  . . . . . . . . . . . . . . . . . . . . . . .   38
     C.2. TCP Congestion Management . . . . . . . . . . . . . . . .   39

Chair's Address                                                       41

Editor's Address                                                      41












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1. Introduction

   IPv4 and earlier versions of the Internet Protocol make two implicit
   assumptions:

    -  that a node's point of attachment remains fixed, and

    -  that a node's IP address identifies the network to which it is
       attached.

   Datagrams are sent to a node based on the network number portion of
   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.  Existing
   routing protocols will be unable 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

         A host or router that changes its point of attachment from one
         network or subnetwork to another.

      Home Agent

         A router that maintains a registry of the current mobility
         bindings for that mobile node, and encapsulates datagrams for
         delivery to the mobile node while it is away from home.

      Foreign Agent

         A router that assists a locally reachable mobile node that is
         away from its home network.

      Care-of Address

         The care-of address is the IP address of the decapsulation
         (termination) end of a tunnel toward a mobile node.  Depending
         on the network configuration, the care-of address may be either
         dynamically assigned to the mobile node or associated with a
         foreign agent.






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   The following support services are defined:

      Agent Discovery

         Home agents and foreign agents advertise their availability
         on each link for which they provide service.  A newly arrived
         mobile node can send a solicitation on the link to learn if any
         prospective agents are present.

      Registration

         When the mobile node is away from home, it registers its
         care-of address with its home agent.  Depending on its method
         of attachment, the mobile node will register either directly
         with its home agent, or through a foreign agent which forwards
         the registration to the home agent.


1.1. Requirements

   A mobile node using its home address shall be able to communicate
   with other nodes despite changing its point of physical attachment.

   Implementation of the protocol described in this document shall not
   cause a mobile node to be unable to communicate with other nodes that
   do not implement these mobility functions.  No protocol enhancements
   are required in hosts or routers that are not providing any of the
   mobility functions.

   A mobile node shall provide authentication in its registration
   messages, as described in subsection 3.1.


1.2. Goals

   The link by which the mobile node is directly attached to the
   Internet is likely to be bandwidth limited, and experience a higher
   rate of errors than traditional wired networks.  Moreover, mobile
   nodes are more likely to be battery powered, and minimizing power
   consumption is important.  Therefore, only a few administrative
   messages should be sent between a mobile node and an agent, and the
   size of these messages should be kept as short as possible.


1.3. Assumptions

   The protocols defined in this document place no additional
   constraints on assignment of IP addresses.  That is, a mobile node



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   can 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.

   It is assumed that mobile nodes will not change their point of
   attachment to the Internet more frequently than once per second.

   It is assumed that IP unicast datagrams are routed based on the
   destination address in the datagram header.


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.

      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.







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1.5. Terminology

   This document frequently uses the following terms:

      Agent Advertisement

         A periodic advertisement constructed by attaching a special
         extension to a router advertisement [6] message.

      Correspondent

         A peer with which a mobile node is communicating.  The
         correspondent may be either mobile or stationary.

      Home Address

         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.  Datagrams addressed to the
         home address are intercepted by the home agent while the mobile
         node is registered with that home agent.

      Link

         A facility or medium over which nodes can communicate at the
         link layer.  A link underlies the network layer.

      Link-Layer Address

         The address used to identify the endpoints of the communication
         over a physical link.  Also commonly known as a MAC address.

      Mobility Agent

         Either a home agent or a foreign agent.

      Mobility Binding

         The association of a home address with a care-of address, along
         with the remaining lifetime of that association.

      Mobility Security Association

         The mobility security association between a pair of nodes is
         a collection of security contexts which may be applied to
         Mobile IP protocol messages exchanged by them.  Each context
         includes the MAC type (subsection 9.1), the secret (a shared




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         key, or appropriate public/private key pair), and the style of
         replay protection in use (subsection 9.6).

      Nonce

         A random value, different from previous choices, inserted in a
         packet to protect against replays.

      Security Parameter Index (SPI)

         The SPI[2] indicates the security context between a pair
         of nodes among those available in the Mobility Security
         Association.

      Tunnel

         The path followed by a packet while it is encapsulated.  The
         model is that, while it is encapsulated, a packet is routed to
         a knowledgeable decapsulating agent, which decapsulates the
         packet and then correctly delivers it.

      Visitor List

         The list of mobile nodes visiting a foreign agent.


1.6. Overview of Protocol Events

   The following is a rough outline of the mobile-IP protocol:

    -  Mobility agents advertise their presence via Agent Advertisements
       (see section 2).

    -  A mobile node receives these advertisements and determines
       whether it is on its home subnet or a foreign subnet.

    -  The mobile node, when it detects that it has moved to a foreign
       subnet, obtains a care-of address on the foreign subnet.  The
       care-of address can either be obtained from the advertisements,
       or by some assignment mechanism (for example, DHCP [7]).

    -  The mobile node then registers its new care-of address with its
       home agent, possibly via a foreign agent (see section 3).

    -  Packets sent to the mobile node's Home Address are received by
       the home agent and delivered (possibly through a foreign agent)
       to the mobile node via encapsulation, using the care-of address
       as the new destination (see subsection 8.5).



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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, the mobile node learns whether
   there are any agents available.  If the address of any home agent
   matches the mobile node's stored address for its home agent, the
   mobile node is at home.

   An agent which is not indicated by a link-layer protocol MUST
   implement agent advertisements.  The agent advertisements indicate
   whether the mobility agent is a home agent or a foreign agent; mobile
   nodes MUST process these agent advertisements.

   When multiple methods of agent identification are in use, the mobile
   node SHOULD first attempt registration with agents including Mobile
   Service extensions in their router advertisements in preference
   to those sending link-layer advertisements.  This order maximizes
   the likelihood that the registration will be recognized, thereby
   minimizing the number of registration attempts.

   No authentication is required for advertisement and solicitation
   messages.  They MAY be authenticated using the IP Authentication
   Header [2], which is external to the messages described here.
   Further specification of authentication of advertisement and
   solicitation is outside of the scope of this document.


2.1. Agent Solicitation

   An agent solicitation, as specified in this document, is an ICMP
   Router Solicitation (RFC 1256 [6]).  Every mobile node MUST implement
   agent solicitations and process agent advertisements, if it needs to
   obtain a care-of address in an agent advertisement.  However, the
   solicitation is only sent when no care-of address has been determined
   through a link-layer protocol or other means.  Mobility agents which
   are not identified by a link-layer protocol MUST respond to agent
   solicitations.  Mobility agents which are identified by a link-layer
   protocol SHOULD respond to agent solicitations.

   The same procedures, defaults, and constants are used for agent
   solicitation as described in RFC 1256, except that the mobile
   node may solicit more often than once every three seconds and
   MAX_SOLICITATIONS does not apply for mobile nodes that are currently
   unconnected to any foreign agent.  A mobile node MAY send a



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   solicitation once each MOBILE_SOLICITATION_INTERVAL (1 second) until
   the solicitation is answered by a mobility agent, and the mobile node
   can finally issue a registration request.


2.2. Agent Advertisement

   Agent advertisements, as specified in this document, are ICMP Router
   Advertisements (RFC 1256 [6]) that have been modified to carry the
   Mobile Service Extension (subsection 4.1).  They can be identified
   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 remaining data is interpreted as
   one or more extensions.  The extensions are described in section 4.
   Other extensions may indicate optionally supported features.

   Any mobility agent which is not indicated by a link-layer protocol
   MUST send agent advertisements.  An agent which is indicated by a
   link-layer protocol SHOULD also implement agent advertisements.
   However, the advertisements need not be sent, except when the site
   policy requires registration with the agent, or as a response to a
   specific solicitation.

   The same procedures, defaults, and constants are used in agent
   advertisements as described in RFC 1256 [6], except that:

    -  a foreign agent MUST limit the rate at which it sends agent
       advertisements; a recommended maximum rate is once per second,
       and

    -  a mobility agent that receives a Router Solicitation does not
       check that the IP Source Address is the address of a neighbor
       (i.e., an address that matches one of the router's own addresses
       on the arrival interface, under the subnet mask associated with
       that address.)

   The Code field of the agent advertisement is interpreted as follows:

      0    The mobility agent handles common traffic -- that is, IP data
           packets not necessarily related to mobile nodes.

      16   The mobility agent does not route common traffic.


2.3. Sequence Numbers, and Rollover Handling

   The sequence number in agent advertisements ranges from 0 to
   0xffff.  After booting, an agent shall use the number 0 for its first



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   advertisement.  Each subsequent advertisement shall use the sequence
   number one greater, with the exception that the sequence number
   0xffff shall be followed by sequence number 256.  In this way, mobile
   clients can distinguish reductions in sequence numbers that result
   from reboots, from reductions that result in rollover of the sequence
   number after it attains the value 0xffff.













































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3. Registration

   Registration messages exchange information between a mobile node and
   its home agent.  Registration creates or modifies a mobility binding,
   associating the mobile node's home address with a care-of address
   which can be used to reach the mobile node.

   If a mobile node itself is assigned a care-of address, it can act
   without a foreign agent, and register or deregister directly with a
   home agent by the exchange of only 2 messages:

    a) The mobile node sends a registration request to a home agent,
       asking it to provide service.

    b) The home agent sends a registration reply to the mobile node,
       granting or denying service.

   When the care-of address is associated with a foreign agent, the
   foreign agent acts as a relay between the mobile node and home agent.
   This extended registration process requires 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 to the home agent, asking
       the home agent to register the mobile node at the foreign agent's
       care-of address.

    c) The home agent sends a registration reply to the foreign agent to
       grant or deny service.

    d) The foreign agent relays the registration reply to the mobile
       node to inform it of the disposition of its request.

   The registration messages defined in subsections 3.2 and 3.3 use the
   User Datagram Protocol header [17].  A nonzero UDP checksum SHOULD be
   included in the header, and checked by each recipient.

   An administrative domain MAY require a visiting mobile node to
   register via a foreign agent (see the description of the "R" bit, in
   subsection 4.1).  Service providers may use this feature with packet
   filtering fire-walls, or visiting policies (such as accounting) which
   require exchanges of authorization.








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3.1. Authentication

   Each mobile node, foreign agent, and home agent MUST be able
   to support a mobility security association for mobile entities,
   indexed by their IP address.  See section 9.1 for requirements
   for support of authentication algorithms.  Registration messages
   between mobile node and home agent MUST be authenticated with
   the Mobile-Home Authentication Extension (subsection 4.2).  This
   extension immediately follows all non-authentication extensions,
   except those foreign agent specific extensions which may be added to
   the packet after the mobile node computes the authentication.


3.2. Registration Request

   A mobile node sends a registration request message so that its home
   agent can create or modify a mobility binding for that mobile node
   (with a new lifetime).  The request may be relayed to the home agent
   by the foreign agent from which the mobile node is accepting service,
   or it may be sent directly in case the mobile node has received a
   care-of address by some other means (e.g, DHCP [7]).

   IP fields:

      Source        For registering with a foreign agent whose IP
                    address is known, the source address of Registration
                    Request from the mobile node to the foreign agent
                    is the IP address of the interface from which the
                    packet is sent.  For registering without a foreign
                    agent, the source address on the registration
                    request MUST be its care-of address.

      Destination   When the IP address is unknown (e.g., the agent was
                    discovered via a link-layer protocol), the "All
                    Mobility Agents" multicast address (224.0.0.11) is
                    used.  The link-layer unicast address is used to
                    deliver the datagram to the correct agent.

                    Any Registration Request sent from the mobile node
                    to a foreign agent should have the destination
                    address of the foreign agent, as learned from the
                    source address of the Agent Advertisement from which
                    the mobile node discovered its proposed care-of
                    address.  For registering without a foreign agent,
                    the destination address should be the address that
                    the mobile node uses for its home agent.





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   UDP fields:

      Source Port        variable

      Destination Port   434

   The UDP header is followed by the Mobile-IP 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      |S|B|D|M|G|rsvd |          Lifetime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Care-of Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type              1, for version 1 of this protocol

      S                 If the 'S' bit is set, the mobile client is
                        requesting that the home agent retain its
                        prior mobility bindings, as described in
                        subsection 6.7.

      B                 If the 'B' bit is set, the mobile client
                        requests that the home agent send to it, all
                        broadcasts on the home network, as described in
                        subsection 8.6.

      D                 If the 'D' bit is set, the mobile client will
                        itself decapsulate datagrams which are sent to
                        the care-of address.

      M                 If the 'M' bit is set, the mobile node asks its
                        home agent to use minimal encapsulation [15].

      G                 If the 'G' bit is set, the mobile node asks its
                        home agent to use GRE encapsulation [9].





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      Lifetime          The number of seconds remaining before the
                        registration is considered expired.  A value of
                        zero indicates a request for deregistration.  A
                        value of all ones indicates infinity.

      Home Address      The IP address of the mobile node.

      Home Agent        The IP address of a home agent.

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

      Identification    A 64-bit number, constructed by the mobile node,
                        useful for matching requests with replies, and
                        for protecting against replay attacks (see
                        subsections 9.4, 9.6).


3.3. Registration Reply

   The registration reply message is returned by a home agent to a
   mobile node which has sent a registration request (subsection 3.2)
   message.  If the mobile node is accepting service from a foreign
   agent, that foreign agent will receive the reply from the home
   agent and subsequently relay it to the mobile node.  The reply
   message contains the necessary codes to inform the mobile node about
   the status of its request, along with the lifetime granted by the
   home agent, which MAY be smaller than the original request.  If a
   foreign agent receives a registration reply with status code greater
   than or equal to 128, it must use the home agent field from the
   registration reply when it relays the request to the mobile node.
   See subsection 8.2 for details regarding the selection of the reply
   identification.

   Mobility agents MUST NOT increase the lifetime selected by the mobile
   node in the registration request.  If the lifetime of the reply is
   greater than the original request, the excess time MUST be ignored.
   When the lifetime of the reply is smaller than the original request,
   another registration SHOULD occur before the smaller lifetime
   expires.












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   IP fields:

      Source        copied from the destination address of the
                    Registration Request to which the agent is replying

      Destination   copied from the source address of the Registration
                    Request to which the agent is replying

   UDP fields:

      Source Port        variable

      Destination Port   variable, depending upon the source port of the
                         request

   A foreign agent that has received a registration request message must
   save the IP source address and the UDP source port from that message
   so that it will be able to send the subsequent registration reply
   message to the correct UDP port on the mobile node (subsection 7.1).

   The UDP header is followed by the Mobile-IP 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                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Identification                        |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type             3

      Code             One of the following codes:

                        0 service will be provided
                        1 service will be provided; simultaneous
                          mobility bindings unsupported







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                       Service denied by the foreign agent:

                       64 reason unspecified
                       65 administratively prohibited
                       66 insufficient resources
                       67 mobile node failed authentication
                       68 home agent failed authentication
                       69 requested lifetime too long
                       70 poorly formed request
                       71 poorly formed reply
                       80 home network unreachable (ICMP error)
                       81 home agent host unreachable (ICMP error)
                       82 home agent port unreachable (ICMP error)
                       88 home agent unreachable (other ICMP error)

                       Service denied by the home agent:

                       128 reason unspecified
                       129 administratively prohibited
                       130 insufficient resources
                       131 mobile node failed authentication
                       132 foreign agent failed authentication
                       133 identification mismatch
                       134 poorly formed request
                       135 too many simultaneous mobility bindings
                       136 unknown home agent address

                       Up-to-date values of the Code field are specified
                       in the most recent "Assigned Numbers" [18].

      Lifetime         The seconds remaining before the registration is
                       considered expired.  A value of zero confirms a
                       request for deregistration.  A value of all ones
                       indicates infinity.

      Home Address     The IP address of the mobile node.

      Home Agent       The IP address of a home agent.

      Identification   The registration identification is copied from
                       the request message, for use by the mobile
                       node in matching its reply with an outstanding
                       request.








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4. Mobility Message Extensions

   Each message begins with a short fixed part, followed by one or more
   mobility message extensions, in type-length-value format.  These
   extensions may apply to agent advertisement messages (subsection 2.2)
   and registration messages (section 3).

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |    Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      Type

         Current values are assigned as follows:

         16   Mobile Service
         19   Prefix Lengths
         32   Mobile-Home Authentication
         33   Mobile-Foreign Authentication
         34   Foreign-Home Authentication

         Up-to-date values are specified in the most recent "Assigned
         Numbers" [18].

      Length

         Indicates the length (in bytes) of the data field.  The length
         does not include the Type and Length bytes.

      Data

         This field is zero or more bytes in length and contains the
         value(s) for this extension.  The format and length of the data
         field is determined by the type and length fields.

   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.

   When an extension numbered in the range 0-127 is encountered but not
   recognized, the packet containing the extension must be dropped.
   When an extension numbered in the range 128-255 is encountered which
   is not recognized, that particular extension is ignored, but the rest
   of the extensions and packet data can still be processed.  The length
   field of the extension is used to skip the data field in searching
   for the next extension.



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4.1. Mobile Service Extension

   The Mobile Service Extension is used to indicate that a router
   advertisement message is actually an agent advertisement being sent
   by a mobility agent (see subsection 2.2).  When foreign agents cannot
   accept new requests for service from mobile clients, they will set
   the Busy bit; if the Busy bit is turned off, the agent may attract
   new mobile clients.  An agent which wishes to serve as a foreign
   agent, sets the 'F' bit in the flags field of the extension; likewise
   an agent which wishes to serve as a home agent sets the 'H' bit.
   Any home agent must always be prepared to serve its mobile clients.
   Thus, it is an error to have the 'B' bit set without also having the
   'F' bit set, since only foreign agents are permitted to be too busy
   to service new requests.  When the 'R' bit is set to 1, the mobile
   node SHOULD register through the foreign agent, even when the mobile
   node has been assigned another care-of address.

    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     |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Lifetime            |R|B|H|F|M|G|     reserved      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  zero or more Care-of Addresses               |
   |                              ...                              |


      Type              16

      Length            (6 + 4*N), where N is the number of
                        care-of addresses advertised.

      Sequence number   The count of advertisement messages sent since
                        the agent was initialized (see section 2.2).

      Lifetime          The longest lifetime (measured in seconds)
                        that the agent is willing to accept in any
                        registration request.  A value of all ones
                        indicates infinity.

      R                 Foreign agent registration required bit.

      B                 Busy bit.  The foreign agent is not willing to
                        accept any more registrations.  Only valid if
                        F=1.

      H                 Agent offers service as a home agent.



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      F                 Agent offers service as a foreign agent.

      M                 Agent offers minimal encapsulation (see [15]).

      G                 Agent offers GRE encapsulation (see [9]).

      reserved          Sent as zero; ignored on reception.

      Care of Address   a foreign agent's care-of addresses


4.2. Mobile-Home Authentication Extension

   This extension must be present in all registration requests and
   replies, and is intended to eliminate problems[3] which result from
   the uncontrolled propagation of remote redirects in the Internet.
   See subsection 9.1 for information about support requirements
   for message authentication codes, etc.  The location of the
   authentication extension marks the end of the authenticated data.

    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    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type            32

      Length          The number of data bytes in the extension.

      SPI             Security Parameter Index (4 bytes)

      Authenticator   (variable length) A value computed from a stream
                      of bytes including the shared secret, the UDP
                      payload (that is, the registration request
                      or reply data), all prior extensions in their
                      entirety, and the type and length of this
                      extension, but not including the Authenticator
                      field itself nor the UDP header.


4.3. Mobile-Foreign Authentication Extension

   This extension may be found in registration requests and replies
   where a security association exists between the mobile node and a




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   foreign agent.  See subsection 9.1 for information about support
   requirements for message authentication codes, etc.

    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    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type            33

      SPI             Security Parameter Index (4 bytes)

      Length          The number of data bytes in the extension.

      Authenticator   (variable length) A value computed from a stream
                      of bytes including the shared secret, the UDP
                      payload (that is, the registration request
                      or reply data), all prior extensions in their
                      entirety, and the type and length of this
                      extension, but not including the Authenticator
                      field itself nor the UDP header.


4.4. Foreign-Home Authentication Extension

   This extension may be found in registration requests and replies
   where a security association exists between the foreign agent and
   a home agent.  See subsection 9.1 for information about support
   requirements for message authentication codes, etc.

    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    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type            34

      SPI             Security Parameter Index (4 bytes)

      Length          The number of data bytes in the extension.





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      Authenticator   (variable length) A value computed from a stream
                      of bytes including the shared secret, the UDP
                      payload (that is, the registration request
                      or reply data), all prior extensions in their
                      entirety, and the type and length of this
                      extension, but not including the Authenticator
                      field itself nor the UDP header.


4.5. Prefix Length Extension

   The Prefix Lengths extension may be found in agent advertisements
   (see subsection 2.2).  This extension allows a mobile node to
   determine the subnet prefixes used by the agent for the router
   interfaces listed in the agent's advertisements.

    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     | Prefix Length |      ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type             19

      Length           2

      Prefix length    (multiple prefixes may be specified) The number
                       of leading bits which define the network number
                       of an advertised router interface.

   The prefix length may be used by mobile nodes to determine whether or
   not a newly detected care-of address should cause the initiation of
   a registration request using that new care-of address.  The mobile
   node can compare the local IP addresses known from several received
   advertisements, mask off the number of bits if the advertised prefix
   lengths are the same, and determine whether or not the foreign agents
   are advertising over the same local medium.  If the mobile node
   determines that they are, then it may wish to avoid submitting a new
   registration request.

   Note that extreme caution is indicated in the use and interpretation
   of this extension.  In the case of wireless interfaces, it is
   almost impossible for two different foreign agents to provide
   identical coverage in space, so that they cannot claim to have
   wireless interfaces situated on the same subnetwork.  In the case
   of wired interfaces, a mobile node connecting to a new point of
   attachment to another is likely to send in a new registration
   request no matter whether the new advertisement is on the same



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   medium as the last recorded advertisement.  And, finally, in areas
   with dense populations of foreign agents it would seem unwise
   to require the propagation via routing protocols of the subnet
   prefixes associated with each individual wireless foreign agent;
   such a strategy could lead to quick depletion of available space
   for routing tables, unwarranted increases in the time required for
   processing routing updates, and longer decision times for route
   selection if routes (which are almost always unnecessary) are stored
   for wireless "subnets".  Moreover, in the latter case, there is no
   expected improvement from the use of subnet prefixes for the wireless
   interfaces in the foreign agents.


5. Encapsulation techniques

   Support for IP in IP encapsulation [14] is required in home
   agents and foreign agents, and any mobile node which can accept a
   dynamically assigned care-of address.  Minimal encapsulation [15] and
   GRE encapsulation [9] are alternate encapsulation methods which may
   optionally be supported by mobility agents.  Minimal encapsulation
   may only be used when an original datagram is not a fragment.

   A foreign agent which is capable of using minimal decapsulation
   includes the 'M' bit (subsection 4.1) in its agent advertisements.
   A mobile node, after receiving this indication in an agent
   advertisement, indicates the capability of decapsulating the minimal
   header at the care-of address by setting the 'M' bit (subsection 3.2)
   in its registration request.  A mobile node MUST NOT set this bit
   unless its foreign agent has advertised support for it.  The use of
   the minimal header is entirely at the discretion of the home agent.

   Similar considerations hold for use of GRE encapsulation and setting
   the 'G' bit (subsections 3.2, 4.1)


















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6. Mobile Node Considerations

   A mobile node listens for agent advertisements 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 detects such a change in its network connectivity, it
   should initiate the registration process.  When it is away from home,
   the mobile node's (de)registration request allows its home agent to
   create or modify a mobility binding, (subsections 3.2, 7.3).  When it
   is at home, the mobile node's registration request allows its home
   agent to erase any previous mobility binding (subsection 6.4).  A
   mobile node operates without the support of mobility functions when
   it is at home.

   Appendix B discusses the interaction of this mobility specification
   with some link layer implementations for media which may be used with
   mobile nodes.

   A mobile node MUST NOT register with a new foreign agent because
   it has received an ICMP Redirect from the foreign agent that is
   currently providing service to it.


6.1. Configuration and Registration Tables

   A mobile node must be configured with its IP home address, and a
   mobility security association for each home agent.  In addition, a
   mobile node may be configured with the IP address of one or more of
   its home agents.

   For each pending registration, the mobile host needs the following:

    - link-layer address of foreign agent, if applicable
    - care-of address
    - registration identification
    - lifetime


6.2. Registration When Away From Home

   In the absence of link-layer indications of changes in point of
   attachment, agent advertisements from new agents do not necessarily
   affect a current registration.  In the absence of link-layer
   indications, a mobile node MUST NOT attempt to register more
   often than once per second.  A mobile node may register with a
   different agent when transport-layer protocols indicate excessive
   retransmissions.  Within these constraints, the mobile node MAY
   register again at any time.



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   If a mobile node detects two successive values of the sequence number
   in the advertisements from its foreign agent, the second of which is
   less than the first and inside the range 0 to 255, the mobile node
   MUST register again.  If the second value is less than the first, but
   greater than or equal to 256, the mobile node may assume that the
   sequence number has rolled over past its maximum value (0xffff), and
   that there is no need to re-register (see subsection 2.2).

   If the mobile node does not know the address of any of its home
   agents, it may send a registration request to the directed broadcast
   address of the home network.  In this case, any registration
   reply that is returned to the mobile node will contain a valid
   address for a home agent, so that the mobile node can re-issue
   the registration request with the correct home agent address if
   necessary.  This registration reply MUST contain a lifetime of 0.
   Note that those home agents which are not satisfied with the mobile
   node's authentication extension may choose not to reply to such a
   registration request.  This may indicate that several or all the home
   agents on the home network should share the same key for the mobile
   node.

   A mobile node SHOULD NOT request a lifetime for its registration that
   exceeds the lifetime learned in an agent advertisement.  When the
   method by which the care-of address is learned does not include a
   lifetime, the default router advertisement lifetime (1800 seconds)
   may be used.  The lifetime MAY be modified by the home agent in its
   reply.  A mobile node SHOULD register again before the lifetime of
   its registration expires.  A mobile node MAY ask a home agent to
   terminate forwarding service to a particular care-of address, by
   sending a registration with a lifetime of zero (see subsection 8.2).

   The mobile node SHOULD construct its registration identification by
   concatenating another value of its own choice to the most recent
   nonce received from its home agent.  This value in the low order 32
   bits of the identification can be another nonce, or a duplicate of
   the nonce received from the home agent (see subsection 9.6.1).


6.3. Registration with a dynamically assigned care-of address

   In cases where a mobile node away from home is able to dynamically
   acquire a transient IP address (e.g.  via DHCP [7]), the mobile node
   can operate indepently of any foreign agent, using the transient
   address as the care-of address.  This feature MUST NOT be used unless
   the mobile node has mechanisms to detect changes in its link-layer
   connectivity, and can initiate acquisition of a new transient address
   each time such a change occurs.  The lifetime of such a registration
   is chosen by the mobile node.



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   When the mobile node is away from home and detects a foreign agent
   advertisement that has the "R" bit (registration required) set in the
   Mobile Service Extension (subsection 4.1), the mobile node SHOULD
   register through an appropriate foreign agent, even when it has
   obtained a dynamically assigned care-of address.


6.4. Deregistration When At Home

   When a mobile node is attached to its home link, it will no longer
   need any forwarding service from its home agent.  A deregistration
   procedure SHOULD be used between the mobile node and its home
   agent.  The deregistration 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 bindings for the
       mobile node.  The care-of address is set to the mobile node's
       home address.

    b) The home agent sends a registration reply to the mobile node to
       indicate the success or failure of the mobile node's attempted
       deregistration.

   A mobile node on its home network need not register again with a home
   agent when a change of sequence number occurs, or the advertisement
   lifetime expires, or even when the home agent crashes, since it isn't
   seeking service from the home agent.


6.5. Registration Replies

   To be accepted, the reply must match the registration identification
   of its most recent registration request to the sender; otherwise, the
   message is silently discarded.  If nonces are in use, the mobile node
   records the first 32 bits for use in its next registration request;
   otherwise, if timestamps are in use, the entire 64 bit field may be
   used for identification (see subsection 9.6).

   When a reply is received which has a code indicating rejection by
   the foreign agent, the Mobile-Home Authenticator will be missing or
   invalid.  If a later authenticated reply is received, and if the
   previous registration is remembered, that later reply supersedes the
   unauthenticated reply.  Otherwise, when a reply is received with
   an invalid authenticator, the message is silently discarded.  The
   mobile node is not required to issue any message in response to a
   registration reply.



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6.6. Registration Retransmission

   When no reply has been received within a reasonable time,
   another registration request is transmitted.  When timestamps
   are used, a new registration identification is chosen for each
   retransmission; thus it counts as a new registration.  When nonces
   are used, the unanswered request is retransmitted unchanged;
   thus the retransmission does not count as a new registration (see
   subsection 9.6).  In this way a retransmission will not require the
   home agent to resynchronize with the mobile node by issuing another
   nonce.

   The maximum time until a new registration request is sent SHOULD be
   no greater than the requested lifetime of the registration request.
   The minimum value SHOULD be large enough to account for the size
   of the packets, twice the round trip time for transmission at the
   link speed, and at least an additional 100 milliseconds to allow
   for processing the packets before responding.  Some circuits add
   another 200 milliseconds of satellite delay.  The minimum time
   between registration requests MUST NOT be less than 1 second.  Note
   that retransmissions with nonces do not count as new registration
   requests.  Each successive wait SHOULD be at least twice the previous
   wait, as long as that is less than the maximum.


6.7. Simultaneous mobility bindings

   Multiple simultaneous mobility bindings are likely to be useful when
   a mobile node moves within range of multiple cellular systems.  IP
   explicitly allows duplication of datagrams.  When the home agent
   allows simultaneous bindings, it will encapsulate a separate copy of
   each arriving datagram to each care-of address, and the mobile node
   will receive multiple copies of its datagrams.

   In order to request this optional capability, the mobile node sends
   the registration request with the 'S' bit set to 1.  The return code
   in the registration reply indicates whether or not previous bindings
   were maintained.  When the need for multiple mobility bindings has
   passed, the mobile node SHOULD register again, to remove the other
   bindings.


6.8. Mobile Routers

   A mobile node can be a router, which 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.
   The nodes connected to a network served by the mobile router may



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   themselves be fixed nodes or mobile nodes or routers.  In this
   subsection, such networks are called "mobile networks".

   A mobile router may provide a care-of address to mobile nodes
   connected to the mobile network.  In this case, when a correspondent
   host sends a packet to the mobile node, the actions described in the
   next paragraph should occur.

   Normal IP procedures will route the packet addressed to the mobile
   node from the correspondent host to the mobile node's home agent.
   This home agent's binding for the mobile node causes it to tunnel the
   packet to the mobile router.  Normal IP procedures will then route
   the packet from this home agent to the mobile router's home agent.
   That home agent's binding for the mobile router causes the packet
   to be doubly tunneled to the mobile router's care-of address.  For
   the sake of discussion, assume there is a foreign agent available at
   that care-of address.  The mobile router's foreign agent will then
   detunnel the packet and use its visitor list entry to deliver the
   packet to the mobile router.  The mobile router will then detunnel
   the packet and use its visitor list entry to deliver the packet
   finally to the mobile node.

   If a fixed node is connected to a mobile network then either of two
   methods may be used to cause packets from correspondent hosts to be
   routed to the fixed node.

   A home agent may be configured that has a permanent registration for
   the fixed node that indicates the mobile router's address as the
   fixed host's care-of address.  The mobile router's home agent will
   usually be used for this purpose.  The home agent is then responsible
   for advertising connectivity using normal routing protocols to
   the fixed node.  Any packets sent to the fixed node will thus use
   recursive tunneling as described above.

   Alternatively, the mobile router may advertise connectivity to the
   fixed node using normal routing protocols through its own home agent.
   This method avoids the need for recursive tunneling of packets.














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7. Foreign Agent Considerations

   The foreign agent is passive and has a minimal role; it relays
   registration requests between the home agent and the mobile node,
   and decapsulates datagrams for delivery to the mobile node.  It may
   advertise its services to prospective mobile clients as described in
   subsections 2.2 and 4.1.

   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 that asks for deregistration of a mobile
   node; however, it MUST relay valid deregistration requests originated
   by the mobile node.

   The foreign agent MUST NOT advertise to other routers in its routing
   domain, nor to any other mobile node, the presence of a mobile
   router.  The foreign agent SHOULD be able to serve as a default
   router for the mobile nodes registered with it.


7.1. Configuration and Registration Tables

   Each foreign agent will need a care-of address.  In addition, for
   each pending or current registration, the foreign agent will need a
   visitor list entry containing:

    - Media address of mobile node
    - home address
    - home agent
    - lifetime

   For each pending registration, a foreign agent must also store
   the low-order 32 bits of the registration identification, as sent
   by the mobile node.  (The high-order 32 bits may differ in the
   registration reply.  See subsection 9.6).  In addition, the foreign
   agent must store the source port from which the mobile node's
   registration request was sent, so that the foreign agent can properly
   return the eventual registration reply.  As with any host on the
   internet, a foreign agent may also maintain a security association
   for each pending or current registrant, and use it to authenticate
   the registration requests and replies of the mobile node or its
   home agent (subsections 4.3, 4.4).  The foreign agent may use an
   available security association with the home agent to compute the
   authentication data for the foreign-home authentication extension.
   Even if a foreign agent implements authentication, it might not use
   authentication with each registration, because of the key management
   difficulties.



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7.2. Receiving Registration Requests

   If the foreign agent is able to satisfy an incoming registration
   request, then it relays the request to the home agent.  Otherwise, it
   denies the request by sending a registration reply to the mobile node
   with an appropriate code.  If the request is being denied because
   the requested lifetime is too long, the foreign agent replaces
   it with an acceptable value for the lifetime in the registration
   reply containing the rejection code.  If the foreign agent denies a
   registration request, the home agent field in the registration reply
   should be copied from the request message.  The foreign agent must
   maintain a list of pending requests, which includes the IP source
   address and UDP source port, in order that a correctly addressed
   reply can be returned to the mobile node.  If a foreign agent
   receives a deregistration request from a mobile node in its visitor
   list, the visitor list entry SHOULD NOT be purged until the home
   agent sends back a status indicating success.


7.3. Receiving Registration Replies

   A registration reply should be silently discarded unless its
   identification matches a pending registration request.  If the
   registration reply is sent from the home agent with a status code
   indicating a successful registration, then the foreign agent updates
   its visitor list accordingly.  If the foreign agent receives an ICMP
   error instead of a registration reply in response to the registration
   request, then it returns an appropriate "Home Agent Unreachable"
   failure code (within the range 80-95, inclusive) to the mobile node.


7.4. Decapsulation

   Every foreign agent which receives an encapsulated packet sent to
   its advertised care-of address MUST compare the inner destination
   address to those entries in its visitor list.  When the destination
   does not match any node currently in the visitor list, the foreign
   agent MUST NOT forward the datagram without modifications to the
   original IP header, because otherwise a routing loop is likely to
   result.  The datagram SHOULD be silently discarded.  ICMP Destination
   Unreachable MUST NOT be sent when a foreign agent is unable to
   forward an incoming tunneled datagram.









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8. Home Agent Considerations

   The home agent has primary responsibility for processing and
   coordinating mobility services.  Packets destined for mobile clients
   will arrive at a home agent that advertises connectivity to the home
   network containing the addresses of those mobile clients.  The home
   agent will then encapsulate the packet and deliver it to the care-of
   address most recently reported by the mobile client.

   Often, the home agent will advertise connectivity to a home network
   which does not correspond to any particular physical medium (e.g,
   extent of Ethernet cabling).  This is described by saying that the
   mobile clients have addresses on a virtual home network.

   The home agent for a given mobile node SHOULD be located on the link
   identified by the home address, if the home network is not merely a
   virtual network.  In this case, the home agent MUST send out agent
   advertisements with the 'H' bit (see subsection 4.1) set, so that
   mobile nodes on their home network will be able to determine that
   they are indeed at home.


8.1. Configuration and Registration Tables

   Each home agent will need an IP address, and the prefix size for the
   home network, if the home network is not a virtual network.  The
   home agent will need to know home address and mobility security
   association of each authorized mobile node.  When an authorized
   mobile node becomes registered, the home agent will create or modify
   its mobility binding list entry containing:

    - care-of address
    - registration identification
    - lifetime


8.2. Receiving Registration Requests

   Upon receipt of a registration request (subsection 3.2), the
   home agent grants or denies the service requested, by sending a
   registration reply (subsection 3.2) to the sender of the request with
   the appropriate code set.  The home agent sends the registration
   reply back to the same UDP port from which it was sent.  If service
   permission is granted, the home agent will update its mobility
   binding list with the care-of address of the tunnel.  The home
   agent MAY impose a shorter lifetime than was requested for in the
   Registration Request message.  If the Registration Request duplicates




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   an accepted current Registration Request, the new lifetime MUST NOT
   extend beyond the lifetime originally granted.

   The request is validated by checking the registration identification
   (see subsection 9.6), and the Mobile-Home Authentication Extension
   (subsection 4.2) using the context selected by the security parameter
   index.  Other authentication extensions are also validated when
   present.  When the registration request is valid, the home agent
   may select a new nonce for use by the mobile node upon its next
   registration request, and include it in the first 32 bits of the
   identification field of the registration reply.  The low order 32
   bits of that field remain unmodified for use by the mobile node
   in matching the registration reply with one of its outstanding
   registration requests.

   When a registration request is invalid, a registration reply is
   sent with the appropriate error code.  This reply will be used by a
   foreign agent to delete its pending request list entry, if a foreign
   agent was involved in relaying the registration request.  If the
   request was invalid because of the use of an unexpected value in the
   identification field of the registration request, the home agent
   SHOULD use the high-order bits of the current identification to
   provide a new identification value for the mobile node.  In this
   case, the home agent MAY report an authentication exception to its
   network management support software.  The registration reply status
   code in this case is 133.  If the registration request was invalid
   because of an invalid authenticator value, the home agent MUST issue
   an authentication exception.  The registration reply status code is
   then 131.

   If the registration request is sent to the directed broadcast
   address of the home network, any home agent may deny the registration
   request, returning status code 136.  In this case, the registration
   reply will contain the home agent's address, so that the mobile node
   can re-issue the registration request with the correct home agent
   address.

   A mobile node requests termination of service by indicating a
   lifetime of zero.  If the Code field is 1, the home agent removes
   the mobility binding for that care-of address from its forwarding
   list.  Otherwise, if the Code field is 0, the home agent removes the
   mobility bindings for all foreign agents associated with that mobile
   node from its mobility binding list.  On termination, no reply is
   sent to additional associated foreign agents.  The entries in their
   visitor lists are allowed to expire naturally.






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8.3. Simultaneous mobility bindings

   When a home agent supports the optional capability of multiple
   simultaneous mobility bindings, any datagrams forwarded are
   simply duplicated, and a copy is sent to each care-of address.
   If the home agent is unable to fulfill requests for simultaneous
   bindings but is otherwise willing to approve the mobile node's
   registration request, it returns the appropriate status (1) in
   the registration reply (subsection 3.3) to the mobile node.  When
   the mobile node makes future registration requests, it will then
   be able to determine whether it can expect simultaneous service
   at multiple care-of addresses.  A home agent can limit the number
   of simultaneous registrations for a mobile client, by rejecting
   any registrations that would cause its limit to be exceeded, and
   returning a registration reply with error code 135.


8.4. Registration Expiration

   If the lifetime for a given mobility binding expires before the home
   agent has received another registration request, then that binding is
   erased from the mobility binding list.  No special registration reply
   is sent to the foreign agents.  The entries in the visitor lists will
   expire naturally, and probably at the same time.  When a mobility
   binding's lifetime expires, the home agent drops it regardless of
   whether or not simultaneous bindings are supported.


8.5. Encapsulation

   Every home agent must examine the IP header of all arriving
   traffic to see if it contains a destination address equal to the
   home address of any of its mobile nodes.  Packets with matching
   destination addresses are encapsulated and delivered to the indicated
   care-of address found in the associated mobility binding.  If the
   mobile node is at home, the home agent will simply forward the
   datagram directly to it; however, in this case, it is expected
   that the datagram will never be received by the home agent.  See
   section 5) about methods of encapsulation that may be used.
   Maintenance of "soft tunnel state" (described in [14]) effectively
   reduces transmission errors in the tunnel.

   Suppose an encapsulated datagram arrives at the home agent, that is
   to be delivered to one of its mobile clients.  If the destination of
   the inner header is not that same the mobile client, the home agent
   may recursively encapsulated it for delivery to its care-of address.
   Otherwise, the home agent may simply alter the outer destination
   to the care-of address, unless the care-of address is the same as



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   the origination point of the encapsulated datagram.  In the latter
   case, if the home agent receives a datagram for one of its mobile
   clients, and the packet's IP source address is identical to the
   care-of address contained in the mobility binding list, the home
   agent MUST discard that packet.  If the packet were forwarded back to
   the care-of address, a loop might result.


8.6. Broadcast packets

   When a home agent receives a broadcast packet, it transmits the
   packet to only those mobile nodes on its mobility binding list that
   have requested broadcast service.  Mobile nodes request encapsulated
   delivery of broadcast packets by setting the 'B' bit in their
   Registration Request packets (subsection 3.2).  If the mobile node is
   using its own dynamically-assigned care-of address, as indicated by
   the 'D' bit in its Registration Request packet (subsection 3.2), the
   home agent simply tunnels each received broadcast IP datagram to this
   care-of address.  Otherwise, when the mobile node registered through
   a foreign agent, the home agent first encapsulates the broadcast
   datagram in a unicast datagram addressed to the mobile node's home
   address, and then tunnels this encapsulated datagram to the foreign
   agent.  This extra level of encapsulation is required so that foreign
   agent can determine which mobile node should receive the packet after
   it is decapsulated.  When received by the foreign agent, the unicast
   encapsulated datagram is detunneled and delivered to the mobile
   node in the same way as any other datagram.  In either case, the
   mobile node must decapsulate the datagram it receives to recover the
   original broadcast datagram.


8.7. Multicast packets

   The rules regarding multicast packets to mobile clients are much the
   same as those relevant to multicast to other clients.
















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9. 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.


9.1. Message Authentication Codes

   Home agents and mobile nodes MUST be able to perform authentication.
   The default algorithm is keyed MD5 [19], with a key size of 128
   bits.  The default mode of operation is to both precede and follow
   the data to be hashed, by the 128-bit key; that is, MD5 is to be
   used in suffix+prefix mode.  The foreign agent SHOULD also support
   authentication using keyed MD5 and key sizes of 128 bits or greater,
   with manual key distribution.  More authentication algorithms,
   algorithm modes, key distribution methods, and key sizes MAY also be
   supported.


9.2. Areas of security concern in this protocol

   The registration protocol described in this document will result
   in a mobile node's traffic being tunneled to its care-of address.
   This tunneling feature could be a significant vulnerability if the
   registration were not authentic.  Such remote redirection, for
   instance as performed by the mobile registration protocol, is widely
   understood to be a security problem in the current Internet([3]).
   Moreover, the Address Resolution Protocol (ARP) is not authenticated,
   and can potentially be used to steal another host's traffic.  The use
   of "Gratuitous ARP" (see Appendix A) brings with it all of the risks
   associated with the use of ARP.


9.3. Key management

   This specification requires 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,
   messages with the foreign agent are not all required to be
   authenticated.  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.



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9.4. Picking good random numbers

   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 authorized parties) and pseudo-random.
   If nonces are used in connection with replay protection, they must
   also be selected carefully.  Eastlake, et.al. ([8]) provides more
   information on generating pseudo-random numbers.


9.5. Privacy

   Users who have sensitive data that they do not wish others to see
   should use mechanisms outside the scope of this document (such as
   encryption) to provide appropriate protection.  Users concerned about
   traffic analysis should consider appropriate use of link encryption.
   If absolute location privacy is desired, the Mobile Node can create a
   tunnel to its Home Agent.  Then, packets destined for correspondent
   hosts will appear to emanate from the Home Network, and it may be
   more difficult to pinpoint the location of the mobile node.


9.6. Replay Protection for Registration Requests

   The Identification field is used to let the home agent verify that a
   registration message has been freshly generated by the mobile node,
   not replayed by an attacker from some previous registration.  The
   exact method of using the field depends upon the mobile security
   association defined between the mobile node and home agent.  Two
   methods are described here:  using random "nonce" values (preferred),
   and another method using timestamps.  A mobile node and its home
   agent must agree on the use of replay protection, because if a home
   agent expects only a nonce, it is unlikely to accept the mobile
   node's time value.

   Whatever method is used, the low order 32 bits of the identification
   MUST be copied unchanged from the registration request to the reply.
   The foreign agent uses those bits to match registration requests with
   corresponding replies.  The mobile node MUST verify that the low
   order 32 bits of any registration reply are identical to the bits it
   sent in the registration request.





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   The Identification in a new registration request MUST NOT be the same
   as in an immediately preceding request, and SHOULD NOT repeat during
   the lifetime of the selected security context between the mobile node
   and the home agent.  Retransmission as in subsection 6.6 is allowed.


9.6.1. Replay Protection using Nonces

   The basic principle of nonce replay protection is that Node A
   includes a new random number in every message to node B, and checks
   that Node B returns that same number in its next message to node
   A. Both messages use a cryptographic checksum to protect against
   alteration by an attacker.  At the same time Node B can send its own
   nonces in all messages to Node A (to be echoed by node A), so that it
   too can verify that it is receiving fresh messages.

   The home agent may be expected to have resources for computing
   pseudo-random numbers useful as nonces[8].  It inserts a new nonce
   as the high-order 32 bits of the identification field of every
   registration reply.  The home agent copies the low-order 32 bits of
   the Identification from the registration request message.  When the
   mobile node receives an authenticated registration reply from the
   home agent, it saves the high order 32 bits of the identification for
   use as the high-order 32 bits of its next registration request.

   The mobile node is responsible for generating the low order 32
   bits of the Identification in each registration request.  Ideally
   it should generate its own random nonces.  However it may use any
   expedient method, including duplication of the random value sent by
   the home agent.  The method chosen is of concern only to the mobile
   node, because it is the node that checks for valid values in the
   registration reply.  The high-order and low-order 32 bits of the
   identification chosen SHOULD both differ from their previous values.
   The home agent needs a new high order value and the mobile node needs
   a new low-order value for replay protection.  The foreign agent needs
   a new low-order value to correctly match registration replies with
   pending requests (see subsection 7.1).

   If a registration message is rejected because of an invalid nonce,
   the reply always provides the mobile node with a new nonce to
   be used in the next registration.  Thus the nonce protocol is
   self-synchronizing.


9.6.2. Replay Protection using Timestamps

   The basic principle of timestamp replay protection is that the node
   generating a message inserts the current time of day, and the node



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   receiving the message checks that this timestamp is sufficiently
   close to its own time of day.  Obviously the two nodes must have
   adequately synchronized time of day clocks.  As usual all messages
   are protected against tampering by a cryptographic checksum.

   If timestamps are used, the mobile node sets the Identification
   field to a 64-bit value formatted as specified by the Network Time
   Protocol [13].  The low-order 32 bits of the NTP format represent
   fractional seconds, and those bits which are not available from a
   time source SHOULD be generated from a good source of randomness.

   If the timestamp in a registration request that has passed
   authentication is close enough to the home agent's time of day, the
   home agent copies the entire Identification into the registration
   reply.  If the timestamp is unacceptable, the home agent copies only
   the low order 32 bits into the registration reply, and supplies the
   high order 32 bits from its own time of day.  The error code in the
   registration reply indicates an identification mismatch.  The mobile
   node MUST verify that the low order 32 bits of the identification
   in the registration reply are identical to those in the rejected
   registration attempt, before using the high order bits for clock
   resynchronization.  Time tolerances and resynchronization details are
   specific to a particular mobile security association.


10. Acknowledgements

   Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
   and John Ioannidis (Columbia), for forming the working group,
   chairing it, and putting so much effort into its early development.

   Thanks also to Kannan Alaggapan and Greg Minshall for their
   contributions to the group while performing the duties of
   chairperson.

















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   Thanks to the active members of the Mobile-IP working group,
   particularly those who contributed text, including (in alphabetical
   order)

    - Ran Atkinson (Naval Research Lab),
    - Dave Johnson (Carnegie Mellon University),
    - Frank Kastenholz (FTP Software)
    - Anders Klemets (KTH)
    - Chip Maguire (KTH - also, John Ioannidis's advisor and early
      contributor)
    - Andrew Myles (Macquarie University),
    - John Penners (US West),
    - Al Quirt (Bell Northern Research),
    - Yakov Rekhter (IBM), and
    - Fumio Teraoka (Sony).

   Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
   produced the first drafts for of this document, reflecting the
   discussions of the Working Group.

   Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), and Frank
   Kastenholz (FTP Software) for their generous support in hosting
   interim Working Group meetings.

   Implementors may note that Anders Klemets has an implementation
   of the protocol specified here for mobile nodes, foreign agents,
   and home agents running under SunOS v4.1.3.  He is willing to
   provide it to people wishing to perform beta testing.  Contact
   him at <klemets@sics.se> if you would like a copy.  There
   is also a version of mobile-IP which was developed by Vipul
   Gupta <vgupta@cs.binghamton.edu> at the State University of
   New York Binghamton.  It is available via anonymous ftp from
   anchor.cs.binghamton.edu in pub/Linux-MobileIP/Linux-MobileIP.tar.gz.


A. 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.

   Mobile nodes do not need a separate "virtual" IP address block; this
   would require a small network to have an extra router between the
   mobile and non-mobile nodes, which is an unacceptable expense.

   This section details the special care to be taken when using ARP [16]
   with nodes on the same link as a mobile node.



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   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
   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, but 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.  With gratuitous ARP, the source IP address is the home
   address of the mobile node, the link-layer address is the source
   link-layer address for the interface used, the target IP address is
   the all-systems multicast address, and the target link-layer address
   is the general broadcast address.

   When there is a physical link which corresponds to the home network,
   a gratuitous ARP is issued by the home agent on behalf of a mobile
   node whenever the home agent receives a valid registration.  That
   should cause the remaining nodes to associate the home address of the
   mobile node with the link-layer address of the home agent which is
   now serving the mobile node.

   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, immediately before sending its deregistration request to
   the home agent.

   Although the gratuitous ARP can be lost, this is not different from
   the usual ARP Reply problems, which are outside the scope of this
   document.  A home agent may repeat the gratuitous ARP a small number
   of times.


B. Link-Layer considerations

   The mobile node primarily uses link-layer mechanisms to decide that
   its point of attachment has changed.  Such indications include
   the Down/Testing/Up interface status [11], and changes in cell or
   administration.  The mechanisms will be specific to the particular
   link-layer technology, and are outside the scope of this document.







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B.1. Point-to-Point Link-Layers

   The Point-to-Point-Protocol (PPP) [20] and its Internet Protocol
   Control Protocol (IPCP) [12], negotiates the use of IP addresses.

   The mobile node SHOULD first attempt to specify its home address.
   This allows an unrouted link to function correctly.

   When the home address is not accepted by the peer, but a transient
   IP address is dynamically assigned, that address MAY be used as the
   care-of address for registration.  When the peer specifies its own IP
   address, that address MUST NOT be assumed to be the care-of address
   of a foreign agent or the IP address of a home agent.

   When router advertisements are received which contain the Mobile
   Service Extension, registration with the agent SHOULD take place as
   usual.  If the link is bandwidth limited, this method is preferred
   over use of the transient care-of address.  The encapsulation will
   be removed by the peer, allowing header compression techniques to
   function correctly [10].


B.2. Multi-Point Link-Layers

   Another link establishment protocol, IEEE 802.11 [1], might yield the
   link address of an agent.  This link-layer address SHOULD be used to
   attempt registration.

   The receipt of an agent's address via a router advertisement
   supersedes that obtained via IEEE 802.11.


C. TCP Considerations

C.1. 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 may cause
   retransmissions or timeouts, even when the link and network is
   actually operating properly with greater than usual delays because
   of the medium 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 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



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   mobile nodes should be sensitive to the possibility of timer-related
   difficulties.


C.2. TCP Congestion Management

   Mobility nodes are likely to use media which have low bandwidth and
   are more likely to introduce errors, effectively causing more packets
   to be dropped.  This introduces a conflict with the mechanisms for
   congestion management found in modern versions of TCP. Now, when
   a packet is dropped, the correspondent's TCP implementation is
   likely to react as if there were a source of network congestion,
   and initiate the slow-start mechanisms [5] designed for controlling
   that problem.  However, those mechanisms are inappropriate for
   overcoming errors introduced by the links themselves, and have the
   effect of magnifying the discontinuity introduced by the dropped
   packet.  This problem has been analyzed by Caceres, et. al.([4]);
   there is no easy solution available, and certainly no solution likely
   to be installed soon on all correspondents.  While this problem has
   nothing to do with any of the specifications in this document, it
   does illustrate that providing performance transparency to mobile
   nodes involves understanding mechanisms outside the network layer.
   It also indicates the need to avoid designs which systematically drop
   packets; such designs might otherwise be considered favorably when
   making engineering tradeoffs.


References

    [1] Draft Standard, Wireless LAN MAC and PHY Specifications, Rev.
        D1.  IEEE Document P802.11/D1-94/12, Dec 1994.

    [2] R. Atkinson.  IP Authentication Header.  RFC 1826, August 1995.

    [3] S.M. Bellovin.  Security Problems in the TCP/IP Protocol Suite.
        ACM Computer Communications Review, 19(2), March 1989.

    [4] Ramon Caceres and Liviu Iftode.  The Effects of Mobility on
        Reliable Transport Protocols.  In Proceedings of the 14th
        International Conference on Distributed Computing Systems, June
        1994.

    [5] Douglas E. Comer.  Internetworking with TCP/IP, volume 1.
        Prentice Hall, 1991.

    [6] S. Deering.  Router Discovery.  RFC 1256, September 1991.





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    [7] R. Droms.  Dynamic Host Configuration Protocol.  RFC 1541,
        October 1993.

    [8] D.E. Eastlake, S.D. Crocker, and J.I. Schiller.  Randomness
        Requirements for Security.  RFC 1750, December 1994.

    [9] S. Hanks, T. Li, D. Farinacci, and P. Traina.  Generic Routing
        Encapsulation (GRE).  RFC 1701, October 1994.

   [10] V. Jacobson.  Compressing TCP/IP Headers for Low-Speed Serial
        Links.  RFC 1144, February 1990.

   [11] K. McCloghrie and F. Kastenholz.  Evolution of the Interfaces
        Group MIP-II.  RFC 1573, January 1994.

   [12] G. McGregor.  The PPP Internet Procotol Control Protocol (IPCP).
        RFC 1332, May 1992.

   [13] D. Mills.  Network Time Protocol (Version 3).  RFC 1305, March
        1992.

   [14] C. Perkins.  IP Encapsulation within IP.  Internet Draft -- work
        in progress, October 1995.

   [15] C. Perkins.  Minimal Encapsulation within IP.  Internet Draft --
        work in progress, July 1995.

   [16] D. Plummer.  An Ethernet Address Resolution Protocol.  RFC 826,
        November 1982.

   [17] J. Postel.  User Datagram Protocol.  RFC 768, August 1980.

   [18] J. Reynolds and J. Postel.  Assigned Numbers.  RFC 1700, October
        1994.

   [19] R. Rivest.  The MD5 Message-Digest Algorithm.  RFC 1321, April
        1992.

   [20] W. Simpson (Editor).  The Point-to-Point Protocol (PPP).  RFC
        1661, July 1994.











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Chair's Addresses

   The working group can be contacted via the current chairs:


        Jim Solomon                       Tony Li
        Motorola, Inc.                    cisco systems
        1301 E. Algonquin Rd.             170 W. Tasman Dr.
        Schaumburg, IL  60196             San Jose, CA  95134

        Work:   +1-708-576-2753           Work:   +1-408-526-8186
        E-mail: solomon@comm.mot.com      E-mail: tli@cisco.com



Editor's Address

   Questions about this memo can also be directed to:

          Charles Perkins
          Room J1-A25
          T. J. Watson Research Center
          IBM Corporation
          30 Saw Mill River Rd.
          Hawthorne, NY  10532

          Work:   +1-914-784-7350
          Fax:    +1-914-784-7007
          E-mail: perk@watson.ibm.com






















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