rfc3344.txt

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      Foreign Agent

         A router on a mobile node's visited network which provides
         routing services to the mobile node while registered.  The
         foreign agent detunnels and delivers datagrams to the mobile
         node that were tunneled by the mobile node's home agent.  For
         datagrams sent by a mobile node, the foreign agent may serve as
         a default router for registered mobile nodes.

   A mobile node is given a long-term IP address on a home network.
   This home address is administered in the same way as a "permanent" IP
   address is provided to a stationary host.  When away from its home
   network, a "care-of address" is associated with the mobile node and
   reflects the mobile node's current point of attachment.  The mobile
   node uses its home address as the source address of all IP datagrams
   that it sends, except where otherwise described in this document for
   datagrams sent for certain mobility management functions (e.g., as in
   Section 3.6.1.1).

1.6. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4].

   In addition, this document frequently uses the following terms:

      Authorization-enabling extension

            An authentication which makes a (registration) message
            acceptable to the ultimate recipient of the registration
            message.  An authorization-enabling extension MUST contain
            an SPI.

            In this document, all uses of authorization-enabling
            extension refer to authentication extensions that enable the
            Registration Request message to be acceptable to the home
            agent.  Using additional protocol structures specified
            outside of this document, it may be possible for the mobile
            node to provide authentication of its registration to the




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            home agent, by way of another authenticating entity within
            the network that is acceptable to the home agent (for
            example, see RFC 2794 [6]).

      Agent Advertisement

            An advertisement message constructed by attaching a special
            Extension to a router advertisement [10] message.

      Authentication

            The process of verifying (using cryptographic techniques,
            for all applications in this specification) the identity of
            the originator of a message.

      Care-of Address

            The termination point of a tunnel toward a mobile node, for
            datagrams forwarded to the mobile node while it is away from
            home.  The protocol can use two different types of care-of
            address:  a "foreign agent care-of address" is an address of
            a foreign agent with which the mobile node is registered,
            and a "co-located care-of address" is an externally obtained
            local address which the mobile node has associated with one
            of its own network interfaces.

      Correspondent Node

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

      Foreign Network

            Any network other than the mobile node's Home Network.

      Gratuitous ARP

            An ARP packet sent by a node in order to spontaneously cause
            other nodes to update an entry in their ARP cache [45].  See
            section 4.6.

      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.





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      Home Network

            A network, possibly virtual, having a network prefix
            matching that of a mobile node's home address.  Note that
            standard IP routing mechanisms will deliver datagrams
            destined to a mobile node's Home Address to the mobile
            node's Home Network.

      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 an endpoint of some
            communication over a physical link.  Typically, the Link-
            Layer address is an interface's Media Access Control (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

            A collection of security contexts, between a pair of nodes,
            which may be applied to Mobile IP protocol messages
            exchanged between them.  Each context indicates an
            authentication algorithm and mode (Section 5.1), a secret (a
            shared key, or appropriate public/private key pair), and a
            style of replay protection in use (Section 5.7).

      Node

            A host or a router.

      Nonce

            A randomly chosen value, different from previous choices,
            inserted in a message to protect against replays.





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      Security Parameter Index (SPI)

            An index identifying a security context between a pair of
            nodes among the contexts available in the Mobility Security
            Association.  SPI values 0 through 255 are reserved and MUST
            NOT be used in any Mobility Security Association.

      Tunnel

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

      Virtual Network

            A network with no physical instantiation beyond a router
            (with a physical network interface on another network).  The
            router (e.g., a home agent) generally advertises
            reachability to the virtual network using conventional
            routing protocols.

      Visited Network

            A network other than a mobile node's Home Network, to which
            the mobile node is currently connected.

      Visitor List

            The list of mobile nodes visiting a foreign agent.

1.7. Protocol Overview

   The following support services are defined for Mobile IP:

      Agent Discovery

            Home agents and foreign agents may 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



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            directly with its home agent, or through a foreign agent
            which forwards the registration to the home agent.

      silently discard

            The implementation discards the datagram 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 datagram,
            and SHOULD record the event in a statistics counter.

   The following steps provide a rough outline of operation of the
   Mobile IP protocol:

      -  Mobility agents (i.e., foreign agents and home agents)
         advertise their presence via Agent Advertisement messages
         (Section 2).  A mobile node may optionally solicit an Agent
         Advertisement message from any locally attached mobility agents
         through an Agent Solicitation message.

      -  A mobile node receives these Agent Advertisements and
         determines whether it is on its home network or a foreign
         network.

      -  When the mobile node detects that it is located on its home
         network, it operates without mobility services.  If returning
         to its home network from being registered elsewhere, the mobile
         node deregisters with its home agent, through exchange of a
         Registration Request and Registration Reply message with it.

      -  When a mobile node detects that it has moved to a foreign
         network, it obtains a care-of address on the foreign network.
         The care-of address can either be determined from a foreign
         agent's advertisements (a foreign agent care-of address), or by
         some external assignment mechanism such as DHCP [13] (a co-
         located care-of address).

      -  The mobile node operating away from home then registers its new
         care-of address with its home agent through exchange of a
         Registration Request and Registration Reply message with it,
         possibly via a foreign agent (Section 3).

      -  Datagrams sent to the mobile node's home address are
         intercepted by its home agent, tunneled by the home agent to
         the mobile node's care-of address, received at the tunnel
         endpoint (either at a foreign agent or at the mobile node
         itself), and finally delivered to the mobile node (Section
         4.2.3).



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      -  In the reverse direction, datagrams sent by the mobile node are
         generally delivered to their destination using standard IP
         routing mechanisms, not necessarily passing through the home
         agent.

   When away from home, Mobile IP uses protocol tunneling to hide a
   mobile node's home address from intervening routers between its home
   network and its current location.  The tunnel terminates at the
   mobile node's care-of address.  The care-of address must be an
   address to which datagrams can be delivered via conventional IP
   routing.  At the care-of address, the original datagram is removed
   from the tunnel and delivered to the mobile node.

   Mobile IP provides two alternative modes for the acquisition of a
   care-of address:

      a) A "foreign agent care-of address" is a care-of address provided
         by a foreign agent through its Agent Advertisement messages.