rfc2002.txt

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   up to a maximum interval.  The maximum interval SHOULD be chosen
   appropriately based upon the characteristics of the media over which
   the mobile node is soliciting.  This maximum interval SHOULD be at
   least one minute between Solicitations.

   While still searching for an agent, the mobile node MUST NOT increase
   the rate at which it sends Solicitations unless it has received a
   positive indication that it has moved to a new link.  After
   successfully registering with an agent, the mobile node SHOULD also
   increase the rate at which it will send Solicitations when it next
   begins searching for a new agent with which to register.  The
   increased solicitation rate MAY revert to the maximum rate, but then
   MUST be limited in the manner described above.  In all cases, the
   recommended solicitation intervals are nominal values.  Mobile nodes
   MUST randomize their solicitation times around these nominal values
   as specified for ICMP Router Discovery [4].





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RFC 2002                  IP Mobility Support               October 1996


   Mobile nodes MUST process received Agent Advertisements.  A mobile
   node can distinguish an Agent Advertisement message from other uses
   of the ICMP Router Advertisement message by examining the number of
   advertised addresses and the IP Total Length field.  When the IP
   total length indicates that the ICMP message is longer than needed
   for the number of advertised addresses, the remaining data is
   interpreted as one or more Extensions.  The presence of a Mobility
   Agent Advertisement Extension identifies the advertisement as an
   Agent Advertisement.

   When multiple methods of agent discovery are in use, the mobile node
   SHOULD first attempt registration with agents including Mobility
   Agent Advertisement Extensions in their advertisements, in preference
   to those discovered by other means.  This preference maximizes the
   likelihood that the registration will be recognized, thereby
   minimizing the number of registration attempts.

2.4.1. Registration Required

   When the mobile node receives an Agent Advertisement with the 'R' bit
   set, the mobile node SHOULD register through the foreign agent, even
   when the mobile node might be able to acquire its own co-located
   care-of address.  This feature is intended to allow sites to enforce
   visiting policies (such as accounting) which require exchanges of
   authorization.

2.4.2. Move Detection

   Two primary mechanisms are provided for mobile nodes to detect when
   they have moved from one subnet to another.  Other mechanisms MAY
   also be used.  When the mobile node detects that it has moved, it
   SHOULD register (Section 3) with a suitable care-of address on the
   new foreign network.  However, the mobile node MUST NOT register more
   frequently than once per second on average, as specified in Section
   3.6.3.
















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RFC 2002                  IP Mobility Support               October 1996


2.4.2.1. Algorithm 1

   The first method of move detection is based upon the Lifetime field
   within the main body of the ICMP Router Advertisement portion of the
   Agent Advertisement.  A mobile node SHOULD record the Lifetime
   received in any Agent Advertisements, until that Lifetime expires.
   If the mobile node fails to receive another advertisement from the
   same agent within the specified Lifetime, it SHOULD assume that it
   has lost contact with that agent.  If the mobile node has previously
   received an Agent Advertisement from another agent for which the
   Lifetime field has not yet expired, the mobile node MAY immediately
   attempt registration with that other agent.  Otherwise, the mobile
   node SHOULD attempt to discover a new agent with which to register.

2.4.2.2. Algorithm 2

   The second method uses network prefixes.  The Prefix-Lengths
   Extension MAY be used in some cases by a mobile node to determine
   whether or not a newly received Agent Advertisement was received on
   the same subnet as the mobile node's current care-of address.  If the
   prefixes differ, the mobile node MAY assume that it has moved.  If a
   mobile node is currently using a foreign agent care-of address, the
   mobile node SHOULD NOT use this method of move detection unless both
   the current agent and the new agent include the Prefix-Lengths
   Extension in their respective Agent Advertisements; if this Extension
   is missing from one or both of the advertisements, this method of
   move detection SHOULD NOT be used.  Similarly, if a mobile node is
   using a co-located care-of address, it SHOULD not use this method of
   move detection unless the new agent includes the Prefix-Lengths
   Extension in its Advertisement and the mobile node knows the network
   prefix of its current co-located care-of address.  On the expiration
   of its current registration, if this method indicates that the mobile
   node has moved, rather than re-registering with its current care-of
   address, a mobile node MAY choose instead to register with a the
   foreign agent sending the new Advertisement with the different
   network prefix.  The Agent Advertisement on which the new
   registration is based MUST NOT have expired according to its Lifetime
   field.













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RFC 2002                  IP Mobility Support               October 1996


2.4.3. Returning Home

   A mobile node can detect that it has returned to its home network
   when it receives an Agent Advertisement from its own home agent.  If
   so, it SHOULD deregister with its home agent (Section 3).  Before
   attempting to deregister, the mobile node SHOULD configure its
   routing table appropriately for its home network (Section 4.2.1).  In
   addition, if the home network is using ARP [16], the mobile node MUST
   follow the procedures described in Section 4.6 with regard to ARP,
   proxy ARP, and gratuitous ARP.

2.4.4. Sequence Numbers and Rollover Handling

   If a mobile node detects two successive values of the sequence number
   in the Agent Advertisements from the foreign agent with which it is
   registered, the second of which is less than the first and inside the
   range 0 to 255, the mobile node SHOULD register again.  If the second
   value is less than the first but is greater than or equal to 256, the
   mobile node SHOULD assume that the sequence number has rolled over
   past its maximum value (0xffff), and that reregistration is not
   necessary (Section 2.3).

3. Registration

   Mobile IP registration provides a flexible mechanism for mobile nodes
   to communicate their current reachability information to their home
   agent.  It is the method by which mobile nodes:

    -  request forwarding services when visiting a foreign network,

    -  inform their home agent of their current care-of address,

    -  renew a registration which is due to expire, and/or

    -  deregister when they return home.

   Registration messages exchange information between a mobile node,
   (optionally) a foreign agent, and the home agent.  Registration
   creates or modifies a mobility binding at the home agent, associating
   the mobile node's home address with its care-of address for the
   specified Lifetime.










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RFC 2002                  IP Mobility Support               October 1996


   Several other (optional) capabilities are available through the
   registration procedure, which enable a mobile node to:

    -  maintain multiple simultaneous registrations, so that a copy of
       each datagram will be tunneled to each active care-of address

    -  deregister specific care-of addresses while retaining other
       mobility bindings, and

    -  discover the address of a home agent if the mobile node is not
       configured with this information.

3.1. Registration Overview

   Mobile IP defines two different registration procedures, one via a
   foreign agent that relays the registration to the mobile node's home
   agent, and one directly with the mobile node's home agent.  The
   following rules determine which of these two registration procedures
   to use in any particular circumstance:

    -  If a mobile node is registering a foreign agent care-of address,
       the mobile node MUST register via that foreign agent.

    -  If a mobile node is using a co-located care-of address, and
       receives an Agent Advertisement from a foreign agent on the
       link on which it is using this care-of address, the mobile node
       SHOULD register via that foreign agent (or via another foreign
       agent on this link) if the 'R' bit is set in the received Agent
       Advertisement message.

    -  If a mobile node is otherwise using a co-located care-of address,
       the mobile node MUST register directly with its home agent.

    -  If a mobile node has returned to its home network and is
       (de)registering with its home agent, the mobile node MUST
       register directly with its home agent.

   Both registration procedures involve the exchange of Registration
   Request and Registration Reply messages (Sections 3.3 and 3.4).  When
   registering via a foreign agent, the registration procedure requires
   the following four messages:

      a)   The mobile node sends a Registration Request to the
           prospective foreign agent to begin the registration process.

      b)   The foreign agent processes the Registration Request and then
           relays it to the home agent.




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RFC 2002                  IP Mobility Support               October 1996


      c)   The home agent sends a Registration Reply to the foreign
           agent to grant or deny the Request.

      d)   The foreign agent processes the Registration Reply and then
           relays it to the mobile node to inform it of the disposition
           of its Request.

   When the mobile node instead registers directly with its home agent,
   the registration procedure requires only the following two messages:

         a)   The mobile node sends a Registration Request to the home
              agent.

         b)   The home agent sends a Registration Reply to the mobile
              node, granting or denying the Request.

   The registration messages defined in Sections 3.3 and 3.4 use the
   User Datagram Protocol (UDP) [17].  A nonzero UDP checksum SHOULD be
   included in the header, and MUST be checked by the recipient.

3.2. Authentication

   Each mobile node, foreign agent, and home agent MUST be able to
   support a mobility security association for mobile entities, indexed
   by their SPI and IP address.  In the case of the mobile node, this
   must be its Home Address.  See Section 5.1 for requirements for
   support of authentication algorithms.  Registration messages between
   a mobile node and its home agent MUST be authenticated with the
   Mobile-Home Authentication Extension (Section 3.5.2).  This Extension
   immediately follows all non-authentication Extensions, except those
   foreign agent-specific Extensions which may be added to the message
   after the mobile node computes the authentication.

3.3. Registration Request

   A mobile node registers with its home agent using a Registration
   Request message so that its home agent can create or modify a
   mobility binding for that mobile node (e.g., with a new lifetime).
   The Request may be relayed to the h