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RFC 3344

 
 
 

IP Mobility Support for IPv4

Part 2 of 4, p. 17 to 47
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2. Agent Discovery

   Agent Discovery is the method by which a mobile node determines
   whether it is currently connected to its home network or to a foreign
   network, and by which a mobile node can detect when it has moved from
   one network to another.  When connected to a foreign network, the
   methods specified in this section also allow the mobile node to
   determine the foreign agent care-of address being offered by each
   foreign agent on that network.

   Mobile IP extends ICMP Router Discovery [10] as its primary mechanism
   for Agent Discovery.  An Agent Advertisement is formed by including a
   Mobility Agent Advertisement Extension in an ICMP Router
   Advertisement message (Section 2.1).  An Agent Solicitation message
   is identical to an ICMP Router Solicitation, except that its IP TTL
   MUST be set to 1 (Section 2.2).  This section describes the message
   formats and procedures by which mobile nodes, foreign agents, and
   home agents cooperate to realize Agent Discovery.

   Agent Advertisement and Agent Solicitation may not be necessary for
   link layers that already provide this functionality.  The method by
   which mobile nodes establish link-layer connections with prospective
   agents is outside the scope of this document (but see Appendix B).
   The procedures described below assume that such link-layer
   connectivity has already been established.

   No authentication is required for Agent Advertisement and Agent
   Solicitation messages.  They MAY be authenticated using the IP
   Authentication Header [22], which is unrelated to the messages
   described in this document.  Further specification of the way in
   which Advertisement and Solicitation messages may be authenticated is
   outside of the scope of this document.

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2.1. Agent Advertisement

   Agent Advertisements are transmitted by a mobility agent to advertise
   its services on a link.  Mobile nodes use these advertisements to
   determine their current point of attachment to the Internet.  An
   Agent Advertisement is an ICMP Router Advertisement that has been
   extended to also carry an Mobility Agent Advertisement Extension
   (Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section
   2.1.2), One-byte Padding Extension (Section 2.1.3), or other
   Extensions that might be defined in the future.

   Within an Agent Advertisement message, ICMP Router Advertisement
   fields of the message are required to conform to the following
   additional specifications:

      -  Link-Layer Fields

         Destination Address

               The link-layer destination address of a unicast Agent
               Advertisement MUST be the same as the source link-layer
               address of the Agent Solicitation which prompted the
               Advertisement.

      -  IP Fields

         TTL      The TTL for all Agent Advertisements MUST be set
                  to 1.

         Destination Address

               As specified for ICMP Router Discovery [10], the IP
               destination address of an multicast Agent Advertisement
               MUST be either the "all systems on this link" multicast
               address (224.0.0.1) [11] or the "limited broadcast"
               address (255.255.255.255).  The subnet-directed broadcast
               address of the form <prefix>.<-1> cannot be used since
               mobile nodes will not generally know the prefix of the
               foreign network.  When the Agent Advertisement is unicast
               to a mobile node, the IP home address of the mobile node
               SHOULD be used as the Destination Address.

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      -  ICMP Fields

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

                   0 The mobility agent handles common traffic -- that
                     is, it acts as a router for IP datagrams not
                     necessarily related to mobile nodes.
                  16 The mobility agent does not route common traffic.
                     However, all foreign agents MUST (minimally)
                     forward to a default router any datagrams received
                     from a registered mobile node (Section 4.2.2).

         Lifetime

               The maximum length of time that the Advertisement is
               considered valid in the absence of further
               Advertisements.

         Router Address(es)

               See Section 2.3.1 for a discussion of the addresses that
               may appear in this portion of the Agent Advertisement.

         Num Addrs

               The number of Router Addresses advertised in this
               message.  Note that in an Agent Advertisement message,
               the number of router addresses specified in the ICMP
               Router Advertisement portion of the message MAY be set to
               0.  See Section 2.3.1 for details.

   If sent periodically, the nominal interval at which Agent
   Advertisements are sent SHOULD be no longer than 1/3 of the
   advertisement Lifetime given in the ICMP header.  This interval MAY
   be shorter than 1/3 the advertised Lifetime.  This allows a mobile
   node to miss three successive advertisements before deleting the
   agent from its list of valid agents.  The actual transmission time
   for each advertisement SHOULD be slightly randomized [10] in order to
   avoid synchronization and subsequent collisions with other Agent

   Advertisements that may be sent by other agents (or with other Router
   Advertisements sent by other routers).  Note that this field has no
   relation to the "Registration Lifetime" field within the Mobility
   Agent Advertisement Extension defined below.

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2.1.1. Mobility Agent Advertisement Extension

   The Mobility Agent Advertisement Extension follows the ICMP Router
   Advertisement fields.  It is used to indicate that an ICMP Router
   Advertisement message is also an Agent Advertisement being sent by a
   mobility agent.  The Mobility Agent Advertisement Extension is
   defined as follows:

    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        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Registration Lifetime      |R|B|H|F|M|G|r|T|   reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  zero or more Care-of Addresses               |
   |                              ...                              |

      Type     16

      Length   (6 + 4*N), where 6 accounts for the number of bytes in
               the Sequence Number, Registration Lifetime, flags, and
               reserved fields, and N is the number of care-of addresses
               advertised.

      Sequence Number

               The count of Agent Advertisement messages sent since the
               agent was initialized (Section 2.3.2).

      Registration Lifetime

               The longest lifetime (measured in seconds) that this
               agent is willing to accept in any Registration Request.
               A value of 0xffff indicates infinity.  This field has no
               relation to the "Lifetime" field within the ICMP Router
               Advertisement portion of the Agent Advertisement.

      R        Registration required.  Registration with this foreign
               agent (or another foreign agent on this link) is required
               even when using a co-located care-of address.

      B        Busy.  The foreign agent will not accept registrations
               from additional mobile nodes.

      H        Home agent.  This agent offers service as a home agent on
               the link on which this Agent Advertisement message is
               sent.

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      F        Foreign agent.  This agent offers service as a foreign
               agent on the link on which this Agent Advertisement
               message is sent.

      M        Minimal encapsulation.  This agent implements receiving
               tunneled datagrams that use minimal encapsulation [34].

      G        GRE encapsulation.  This agent implements receiving
               tunneled datagrams that use GRE encapsulation [16].

      r        Sent as zero; ignored on reception.  SHOULD NOT be
               allocated for any other uses.

      T        Foreign agent supports reverse tunneling [27].

      reserved
               Sent as zero; ignored on reception.

      Care-of Address(es)

               The advertised foreign agent care-of address(es) provided
               by this foreign agent.  An Agent Advertisement MUST
               include at least one care-of address if the 'F' bit is
               set.  The number of care-of addresses present is
               determined by the Length field in the Extension.

   A home agent MUST always be prepared to serve the mobile nodes for
   which it is the home agent.  A foreign agent may at times be too busy
   to serve additional mobile nodes; even so, it must continue to send
   Agent Advertisements, so that any mobile nodes already registered
   with it will know that they have not moved out of range of the
   foreign agent and that the foreign agent has not failed.  A foreign
   agent may indicate that it is "too busy" to allow new mobile nodes to
   register with it, by setting the 'B' bit in its Agent Advertisements.
   An Agent Advertisement message MUST NOT have the 'B' bit set if the
   'F' bit is not also set.  Furthermore, at least one of the 'F' bit
   and the 'H' bit MUST be set in any Agent Advertisement message sent.

   When a foreign agent wishes to require registration even from those
   mobile nodes which have acquired a co-located care-of address, it
   sets the 'R' bit to one.  Because this bit applies only to foreign
   agents, an agent MUST NOT set the 'R' bit to one unless the 'F' bit
   is also set to one.

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2.1.2. Prefix-Lengths Extension

   The Prefix-Lengths Extension MAY follow the Mobility Agent
   Advertisement Extension.  It is used to indicate the number of bits
   of network prefix that applies to each Router Address listed in the
   ICMP Router Advertisement portion of the Agent Advertisement.  Note
   that the prefix lengths given DO NOT apply to care-of address(es)
   listed in the Mobility Agent Advertisement Extension.  The Prefix-
   Lengths Extension is defined as follows:

    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 (Prefix-Lengths Extension)

      Length   N, where N is the value (possibly zero) of the Num Addrs
               field in the ICMP Router Advertisement portion of the
               Agent Advertisement.

      Prefix Length(s)

               The number of leading bits that define the network number
               of the corresponding Router Address listed in the ICMP
               Router Advertisement portion of the message.  The prefix
               length for each Router Address is encoded as a separate
               byte, in the order that the Router Addresses are listed
               in the ICMP Router Advertisement portion of the message.

   See Section 2.4.2 for information about how the Prefix-Lengths
   Extension MAY be used by a mobile node when determining whether it
   has moved.  See Appendix E for implementation details about the use
   of this Extension.

2.1.3. One-byte Padding Extension

   Some IP protocol implementations insist upon padding ICMP messages to
   an even number of bytes.  If the ICMP length of an Agent
   Advertisement is odd, this Extension MAY be included in order to make
   the ICMP length even.  Note that this Extension is NOT intended to be
   a general-purpose Extension to be included in order to word- or
   long-align the various fields of the Agent Advertisement.  An Agent
   Advertisement SHOULD NOT include more than one One-byte Padding
   Extension and if present, this Extension SHOULD be the last Extension
   in the Agent Advertisement.

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   Note that unlike other Extensions used in Mobile IP, the One-byte
   Padding Extension is encoded as a single byte, with no "Length" nor
   "Data" field present.  The One-byte Padding Extension is defined as
   follows:

    0 1 2 3 4 5 6 7
   +-+-+-+-+-+-+-+-+
   |     Type      |
   +-+-+-+-+-+-+-+-+

   Type 0 (One-byte Padding Extension)

2.2. Agent Solicitation

   An Agent Solicitation is identical to an ICMP Router Solicitation
   with the further restriction that the IP TTL Field MUST be set to 1.

2.3. Foreign Agent and Home Agent Considerations

   Any mobility agent which cannot be discovered by a link-layer
   protocol MUST send Agent Advertisements.  An agent which can be
   discovered 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 (i.e., when
   the 'R' bit is set), or as a response to a specific Agent
   Solicitation.  All mobility agents MUST process packets that they
   receive addressed to the Mobile-Agents multicast group, at address
   224.0.0.11.  A mobile node MAY send an Agent Solicitation to
   224.0.0.11.  All mobility agents SHOULD respond to Agent
   Solicitations.

   The same procedures, defaults, and constants are used in Agent
   Advertisement messages and Agent Solicitation messages as specified
   for ICMP Router Discovery [10], except that:

   -  a mobility agent MUST limit the rate at which it sends broadcast
      or multicast Agent Advertisements; the maximum rate SHOULD be
      chosen so that the Advertisements do not consume a significant
      amount of network bandwidth, AND

   -  a mobility agent that receives a Router Solicitation MUST NOT
      require 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 of the router).

   -  a mobility agent MAY be configured to send Agent Advertisements
      only in response to an Agent Solicitation message.

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   If the home network is not a virtual network, then the home agent for
   any mobile node SHOULD be located on the link identified by the
   mobile node's home address, and Agent Advertisement messages sent by
   the home agent on this link MUST have the 'H' bit set.  In this way,
   mobile nodes on their own home network will be able to determine that
   they are indeed at home.  Any Agent Advertisement messages sent by
   the home agent on another link to which it may be attached (if it is
   a mobility agent serving more than one link), MUST NOT have the 'H'
   bit set, unless the home agent also serves as a home agent (to other
   mobile nodes) on that other link.  A mobility agent MAY use different
   settings for each of the 'R', 'H', and 'F' bits on different network
   interfaces.

   If the home network is a virtual network, the home network has no
   physical realization external to the home agent itself.  In this
   case, there is no physical network link on which to send Agent
   Advertisement messages advertising the home agent.  Mobile nodes for
   which this is the home network are always treated as being away from
   home.

   On a particular subnet, either all mobility agents MUST include the
   Prefix-Lengths Extension or all of them MUST NOT include this
   Extension.  Equivalently, it is prohibited for some agents on a given
   subnet to include the Extension but for others not to include it.
   Otherwise, one of the move detection algorithms designed for mobile
   nodes will not function properly (Section 2.4.2).

2.3.1. Advertised Router Addresses

   The ICMP Router Advertisement portion of the Agent Advertisement MAY
   contain one or more router addresses.  An agent SHOULD only put its
   own addresses, if any, in the advertisement.  Whether or not its own
   address appears in the Router Addresses, a foreign agent MUST route
   datagrams it receives from registered mobile nodes (Section 4.2.2).

2.3.2. Sequence Numbers and Rollover Handling

   The sequence number in Agent Advertisements ranges from 0 to 0xffff.
   After booting, an agent MUST use the number 0 for its first
   advertisement.  Each subsequent advertisement MUST use the sequence
   number one greater, with the exception that the sequence number
   0xffff MUST be followed by sequence number 256.  In this way, mobile
   nodes can distinguish a reduction in the sequence number that occurs
   after a reboot from a reduction that results in rollover of the
   sequence number after it attains the value 0xffff.

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2.4. Mobile Node Considerations

   Every mobile node MUST implement Agent Solicitation.  Solicitations
   SHOULD only be sent in the absence of Agent Advertisements and when a
   care-of address has not been determined through a link-layer protocol
   or other means.  The mobile node uses the same procedures, defaults,
   and constants for Agent Solicitation as specified for ICMP Router
   Solicitation messages [10], except that the mobile node MAY solicit
   more often than once every three seconds, and that a mobile node that
   is currently not connected to any foreign agent MAY solicit more
   times than MAX_SOLICITATIONS.

   The rate at which a mobile node sends Solicitations MUST be limited
   by the mobile node.  The mobile node MAY send three initial
   Solicitations at a maximum rate of one per second while searching for
   an agent.  After this, the rate at which Solicitations are sent MUST
   be reduced so as to limit the overhead on the local link.  Subsequent
   Solicitations MUST be sent using a binary exponential backoff
   mechanism, doubling the interval between consecutive Solicitations,
   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 [10].

   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.

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   If there is more than one advertised address, the mobile node SHOULD
   pick the first address for its initial registration attempt.  If the
   registration attempt fails with a status Code indicating rejection by
   the foreign agent, the mobile node MAY retry the attempt with each
   subsequent advertised address in turn.

   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.

   A mobile node MUST ignore reserved bits in Agent Advertisements, as
   opposed to discarding such advertisements.  In this way, new bits can
   be defined later, without affecting the ability for mobile nodes to
   use the advertisements even when the newly defined bits are not
   understood.

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.

   If formerly reserved bits require some kind of monitoring/enforcement
   at the foreign link, foreign agents implementing the new
   specification for the formerly reserved bits can set the 'R' bit.
   This has the effect of forcing the mobile node to register through
   the foreign agent, so the foreign agent could then monitor/enforce
   the policy.

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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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.

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

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   addition, if the home network is using ARP [36], 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.

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

      -  discover its home address, if the mobile node is not configured
         with this information.

      -  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

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      -  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.

      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.

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   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) [37].  A nonzero UDP checksum SHOULD be
   included in the header, and MUST be checked by the recipient.  A zero
   UDP checksum SHOULD be accepted by the recipient.  The behavior of
   the mobile node and the home agent with respect to their mutual
   acceptance of packets with zero UDP checksums SHOULD be defined as
   part of the mobility security association which exists between them.

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 an
   authorization-enabling extension, e.g. the Mobile-Home Authentication
   Extension (Section 3.5.2).  This extension MUST be the first
   authentication extension; other foreign agent-specific extensions 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 home agent by the foreign agent
   through which the mobile node is registering, or it may be sent
   directly to the home agent in the case in which the mobile node is
   registering a co-located care-of address.

   IP fields:

      Source Address Typically the interface address from which the
      message is sent.

      Destination Address Typically that of the foreign agent or the
      home agent.

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   See Sections 3.6.1.1 and 3.7.2.2 for details.  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|r|T|x|          Lifetime             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Home Address                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Home Agent                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Care-of Address                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Identification                        +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Extensions ...
   +-+-+-+-+-+-+-+-

      Type     1 (Registration Request)

      S        Simultaneous bindings.  If the 'S' bit is set, the mobile
               node is requesting that the home agent retain its prior
               mobility bindings, as described in Section 3.6.1.2.

      B        Broadcast datagrams.  If the 'B' bit is set, the mobile
               node requests that the home agent tunnel to it any
               broadcast datagrams that it receives on the home network,
               as described in Section 4.3.

      D        Decapsulation by mobile node.  If the 'D' bit is set, the
               mobile node will itself decapsulate datagrams which are
               sent to the care-of address.  That is, the mobile node is
               using a co-located care-of address.

      M        Minimal encapsulation.  If the 'M' bit is set, the mobile
               node requests that its home agent use minimal
               encapsulation [34] for datagrams tunneled to the mobile
               node.

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      G        GRE encapsulation.  If the 'G' bit is set, the mobile
               node requests that its home agent use GRE encapsulation
               [16] for datagrams tunneled to the mobile node.

      r        Sent as zero; ignored on reception.  SHOULD NOT be
               allocated for any other uses.

      T        Reverse Tunneling requested; see [27].

      x        Sent as zero; ignored on reception.

      Lifetime

               The number of seconds remaining before the registration
               is considered expired.  A value of zero indicates a
               request for deregistration.  A value of 0xffff indicates
               infinity.

      Home Address

               The IP address of the mobile node.

      Home Agent

               The IP address of the mobile node's home agent.

      Care-of Address

               The IP address for the end of the tunnel.

      Identification

               A 64-bit number, constructed by the mobile node, used for
               matching Registration Requests with Registration Replies,
               and for protecting against replay attacks of registration
               messages.  See Sections 5.4 and 5.7.

      Extensions

               The fixed portion of the Registration Request is followed
               by one or more of the Extensions listed in Section 3.5.
               An authorization-enabling extension MUST be included in
               all Registration Requests.  See Sections 3.6.1.3 and
               3.7.2.2 for information on the relative order in which
               different extensions, when present, MUST be placed in a
               Registration Request message.

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3.4. Registration Reply

   A mobility agent returns a Registration Reply message to a mobile
   node which has sent a Registration Request (Section 3.3) message.  If
   the mobile node is requesting 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.

   The foreign agent MUST NOT increase the Lifetime selected by the
   mobile node in the Registration Request, since the Lifetime is
   covered by an authentication extension which enables authorization by
   the home agent.  Such an extension contains authentication data which
   cannot be correctly (re)computed by the foreign agent.  The home
   agent MUST NOT increase the Lifetime selected by the mobile node in
   the Registration Request, since doing so could increase it beyond the
   maximum Registration Lifetime allowed by the foreign agent.  If the
   Lifetime received in the Registration Reply is greater than that in
   the Registration Request, the Lifetime in the Request MUST be used.
   When the Lifetime received in the Registration Reply is less than
   that in the Registration Request, the Lifetime in the Reply MUST be
   used.

   IP fields:

      Source Address       Typically copied from the destination address
                           of the Registration Request to which the
                           agent is replying.  See Sections 3.7.2.3 and
                           3.8.3.1 for complete details.

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

   UDP fields:

      Source Port          <variable>

      Destination Port     Copied from the source port of the
                           corresponding Registration Request (Section
                           3.7.1).

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   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 (Registration Reply)

      Code     A value indicating the result of the Registration
               Request.  See below for a list of currently defined Code
               values.

      Lifetime

               If the Code field indicates that the registration was
               accepted, the Lifetime field is set to the number of
               seconds remaining before the registration is considered
               expired.  A value of zero indicates that the mobile node
               has been deregistered.  A value of 0xffff indicates
               infinity.  If the Code field indicates that the
               registration was denied, the contents of the Lifetime
               field are unspecified and MUST be ignored on reception.

      Home Address

               The IP address of the mobile node.

      Home Agent

               The IP address of the mobile node's home agent.

      Identification

               A 64-bit number used for matching Registration Requests
               with Registration Replies, and for protecting against
               replay attacks of registration messages.  The value is

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               based on the Identification field from the Registration
               Request message from the mobile node, and on the style of
               replay protection used in the security context between
               the mobile node and its home agent (defined by the
               mobility security association between them, and SPI value
               in the authorization-enabling extension).  See Sections
               5.4 and 5.7.

      Extensions

               The fixed portion of the Registration Reply is followed
               by one or more of the Extensions listed in Section 3.5.
               An authorization-enabling extension MUST be included in
               all Registration Replies returned by the home agent.  See
               Sections 3.7.2.2 and 3.8.3.3 for rules on placement of
               extensions to Reply messages.

   The following values are defined for use within the Code field.
   Registration successful:

      0 registration accepted
      1 registration accepted, but simultaneous mobility
       bindings unsupported

   Registration 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
      72 requested encapsulation unavailable
      73 reserved and unavailable
      77 invalid care-of address
      78 registration timeout
      80 home network unreachable (ICMP error received)
      81 home agent host unreachable (ICMP error received)
      82 home agent port unreachable (ICMP error received)
      88 home agent unreachable (other ICMP error received)

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   Registration 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 registration 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" [40].

3.5. Registration Extensions

3.5.1. Computing Authentication Extension Values

   The Authenticator value computed for each authentication Extension
   MUST protect the following fields from the registration message:

      -  the UDP payload (that is, the Registration Request or
         Registration Reply data),

      -  all prior Extensions in their entirety, and

      -  the Type, Length, and SPI of this Extension.

   The default authentication algorithm uses HMAC-MD5 [23] to compute a
   128-bit "message digest" of the registration message.  The data over
   which the HMAC is computed is defined as:

      -  the UDP payload (that is, the Registration Request or
         Registration Reply data),

      -  all prior Extensions in their entirety, and

      -  the Type, Length, and SPI of this Extension.

   Note that the Authenticator field itself and the UDP header are NOT
   included in the computation of the default Authenticator value.  See
   Section 5.1 for information about support requirements for message
   authentication codes, which are to be used with the various
   authentication Extensions.

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   The Security Parameter Index (SPI) within any of the authentication
   Extensions defines the security context which is used to compute the
   Authenticator value and which MUST be used by the receiver to check
   that value.  In particular, the SPI selects the authentication
   algorithm and mode (Section 5.1) and secret (a shared key, or
   appropriate public/private key pair) used in computing the
   Authenticator.  In order to ensure interoperability between different
   implementations of the Mobile IP protocol, an implementation MUST be
   able to associate any SPI value with any authentication algorithm and
   mode which it implements.  In addition, all implementations of Mobile
   IP MUST implement the default authentication algorithm (HMAC-MD5)
   specified above.

3.5.2. Mobile-Home Authentication Extension

   Exactly one authorization-enabling extension MUST be present in all
   Registration Requests, and also in all Registration Replies generated
   by the Home Agent.  The Mobile-Home Authentication Extension is
   always an authorization-enabling for registration messages specified
   in this document.  This requirement is intended to eliminate problems
   [2] which result from the uncontrolled propagation of remote
   redirects in the Internet.  The location of the 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         4 plus the number of bytes in the Authenticator.

      SPI            Security Parameter Index (4 bytes).  An opaque
                     identifier (see Section 1.6).

      Authenticator  (variable length) (See Section 3.5.1.)

3.5.3. Mobile-Foreign Authentication Extension

   This Extension MAY be included in Registration Requests and Replies
   in cases in which a mobility security association exists between the
   mobile node and the foreign agent.  See Section 5.1 for information
   about support requirements for message authentication codes.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Length    |         SPI  ....
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          ... SPI (cont.)          |       Authenticator ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Type           33

      Length         4 plus the number of bytes in the Authenticator.

      SPI            Security Parameter Index (4 bytes).  An opaque
                     identifier (see Section 1.6).

      Authenticator  (variable length) (See Section 3.5.1.)

3.5.4. Foreign-Home Authentication Extension

   This Extension MAY be included in Registration Requests and Replies
   in cases in which a mobility security association exists between the
   foreign agent and the home agent.  See Section 5.1 for information
   about support requirements for message authentication codes.

    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

      Length         4 plus the number of bytes in the Authenticator.

      SPI            Security Parameter Index (4 bytes).  An opaque
                     identifier (see Section 1.6).

      Authenticator  (variable length) (See Section 3.5.1.)

3.6. Mobile Node Considerations

   A mobile node MUST be configured with a netmask and a mobility
   security association for each of its home agents.  In addition, a
   mobile node MAY be configured with its home address, and the IP

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   address of one or more of its home agents; otherwise, the mobile node
   MAY discover a home agent using the procedures described in Section
   3.6.1.2.

   If the mobile node is not configured with a home address, it MAY use
   the Mobile Node NAI extension [6] to identify itself, and set the
   Home Address field of the Registration Request to 0.0.0.0.  In this
   case, the mobile node MUST be able to assign its home address after
   extracting this information from the Registration Reply from the home
   agent.

   For each pending registration, the mobile node maintains the
   following information:

      -  the link-layer address of the foreign agent to which the
         Registration Request was sent, if applicable,
      -  the IP destination address of the Registration Request,
      -  the care-of address used in the registration,
      -  the Identification value sent in the registration,
      -  the originally requested Lifetime, and
      -  the remaining Lifetime of the pending registration.

   A mobile node SHOULD initiate a registration whenever it detects a
   change in its network connectivity.  See Section 2.4.2 for methods by
   which mobile nodes MAY make such a determination.  When it is away
   from home, the mobile node's Registration Request allows its home
   agent to create or modify a mobility binding for it.  When it is at
   home, the mobile node's (de)Registration Request allows its home
   agent to delete any previous mobility binding(s) for it.  A mobile
   node operates without the support of mobility functions when it is at
   home.

   There are other conditions under which the mobile node SHOULD
   (re)register with its foreign agent, such as when the mobile node
   detects that the foreign agent has rebooted (as specified in Section
   2.4.4) and when the current registration's Lifetime is near
   expiration.

   In the absence of link-layer indications of changes in point of
   attachment, Agent Advertisements from new agents SHOULD NOT cause a
   mobile node to attempt a new registration, if its current
   registration has not expired and it is still also receiving Agent
   Advertisements from the foreign agent with which it is currently
   registered.  In the absence of link-layer indications, a mobile node
   MUST NOT attempt to register more often than once per second.

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   A mobile node MAY register with a different agent when transport-
   layer protocols indicate excessive retransmissions.  A mobile node
   MUST NOT consider reception of an ICMP Redirect from a foreign agent
   that is currently providing service to it as reason to register with
   a new foreign agent.  Within these constraints, the mobile node MAY
   register again at any time.

   Appendix D shows some examples of how the fields in registration
   messages would be set up in some typical registration scenarios.

3.6.1. Sending Registration Requests

   The following sections specify details for the values the mobile node
   MUST supply in the fields of Registration Request messages.

3.6.1.1. IP Fields

   This section provides the specific rules by which mobile nodes pick
   values for the IP header fields of a Registration Request.

   IP Source Address:

      -  When registering on a foreign network with a co-located care-of
         address, the IP source address MUST be the care-of address.

      -  Otherwise, if the mobile node does not have a home address, the
         IP source address MUST be 0.0.0.0.

      -  In all other circumstances, the IP source address MUST be the
         mobile node's home address.

   IP Destination Address:

      -  When the mobile node has discovered the agent with which it is
         registering, through some means (e.g., link-layer) that does
         not provide the IP address of the agent (the IP address of the
         agent is unknown to the mobile node), then the "All Mobility
         Agents" multicast address (224.0.0.11) MUST be used.  In this
         case, the mobile node MUST use the agent's link-layer unicast
         address in order to deliver the datagram to the correct agent.

      -  When registering with a foreign agent, the address of the agent
         as learned from the IP source address of the corresponding
         Agent Advertisement MUST be used.  This MAY be an address which
         does not appear as an advertised care-of address in the Agent
         Advertisement.  In addition, when transmitting this
         Registration Request message, the mobile node MUST use a link-

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         layer destination address copied from the link-layer source
         address of the Agent Advertisement message in which it learned
         this foreign agent's IP address.

      -  When the mobile node is registering directly with its home
         agent and knows the (unicast) IP address of its home agent, the
         destination address MUST be set to this address.

      -  If the mobile node is registering directly with its home agent,
         but does not know the IP address of its home agent, the mobile
         node may use dynamic home agent address resolution to
         automatically determine the IP address of its home agent
         (Section 3.6.1.2).  In this case, the IP destination address is
         set to the subnet-directed broadcast address of the mobile
         node's home network.  This address MUST NOT be used as the
         destination IP address if the mobile node is registering via a
         foreign agent, although it MAY be used as the Home Agent
         address in the body of the Registration Request when
         registering via a foreign agent.

   IP Time to Live:

      -  The IP TTL field MUST be set to 1 if the IP destination address
         is set to the "All Mobility Agents" multicast address as
         described above.  Otherwise a suitable value should be chosen
         in accordance with standard IP practice [38].

3.6.1.2. Registration Request Fields

   This section provides specific rules by which mobile nodes pick
   values for the fields within the fixed portion of a Registration
   Request.

   A mobile node MAY set the 'S' bit in order to request that the home
   agent maintain prior mobility binding(s).  Otherwise, the home agent
   deletes any previous binding(s) and replaces them with the new
   binding specified in the Registration Request.  Multiple simultaneous
   mobility bindings are likely to be useful when a mobile node using at
   least one wireless network interface moves within wireless
   transmission range of more than one foreign agent.  IP explicitly
   allows duplication of datagrams.  When the home agent allows
   simultaneous bindings, it will tunnel a separate copy of each
   arriving datagram to each care-of address, and the mobile node will
   receive multiple copies of datagrams destined to it.

   The mobile node SHOULD set the 'D' bit if it is registering with a
   co-located care-of address.  Otherwise, the 'D' bit MUST NOT be set.

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   A mobile node MAY set the 'B' bit to request its home agent to
   forward to it, a copy of broadcast datagrams received by its home
   agent from the home network.  The method used by the home agent to
   forward broadcast datagrams depends on the type of care-of address
   registered by the mobile node, as determined by the 'D' bit in the
   mobile node's Registration Request:

      -  If the 'D' bit is set, then the mobile node has indicated that
         it will decapsulate any datagrams tunneled to this care-of
         address itself (the mobile node is using a co-located care-of
         address).  In this case, to forward such a received broadcast
         datagram to the mobile node, the home agent MUST tunnel it to
         this care-of address.  The mobile node de-tunnels the received
         datagram in the same way as any other datagram tunneled
         directly to it.

      -  If the 'D' bit is NOT set, then the mobile node has indicated
         that it is using a foreign agent care-of address, and that the
         foreign agent will thus decapsulate arriving datagrams before
         forwarding them to the mobile node.  In this case, to forward
         such a received broadcast datagram to the mobile node, the home
         agent MUST first encapsulate the broadcast datagram in a
         unicast datagram addressed to the mobile node's home address,
         and then MUST tunnel this resulting datagram to the mobile
         node's care-of address.

         When decapsulated by the foreign agent, the inner datagram will
         thus be a unicast IP datagram addressed to the mobile node,
         identifying to the foreign agent the intended destination of
         the encapsulated broadcast datagram, and will be delivered to
         the mobile node in the same way as any tunneled datagram
         arriving for the mobile node.  The foreign agent MUST NOT
         decapsulate the encapsulated broadcast datagram and MUST NOT
         use a local network broadcast to transmit it to the mobile
         node.  The mobile node thus MUST decapsulate the encapsulated
         broadcast datagram itself, and thus MUST NOT set the 'B' bit in
         its Registration Request in this case unless it is capable of
         decapsulating datagrams.

   The mobile node MAY request alternative forms of encapsulation by
   setting the 'M' bit and/or the 'G' bit, but only if the mobile node
   is decapsulating its own datagrams (the mobile node is using a co-
   located care-of address) or if its foreign agent has indicated
   support for these forms of encapsulation by setting the corresponding
   bits in the Mobility Agent Advertisement Extension of an Agent
   Advertisement received by the mobile node.  Otherwise, the mobile
   node MUST NOT set these bits.

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   The Lifetime field is chosen as follows:

      -  If the mobile node is registering with a foreign agent, the
         Lifetime SHOULD NOT exceed the value in the Registration
         Lifetime field of the Agent Advertisement message received from
         the foreign agent.
         When the method by which the care-of address is learned does
         not include a Lifetime, the default ICMP Router Advertisement
         Lifetime (1800 seconds) MAY be used.

      -  The mobile node MAY ask a home agent to delete a particular
         mobility binding, by sending a Registration Request with the
         care-of address for this binding, with the Lifetime field set
         to zero (Section 3.8.2).

      -  Similarly, a Lifetime of zero is used when the mobile node
         deregisters all care-of addresses, such as upon returning home.

   The Home Address field MUST be set to the mobile node's home address,
   if this information is known.  Otherwise, the Home Address MUST be
   set to zeroes.

   The Home Agent field MUST be set to the address of the mobile node's
   home agent, if the mobile node knows this address.  Otherwise, the
   mobile node MAY use dynamic home agent address resolution to learn
   the address of its home agent.  In this case, the mobile node MUST
   set the Home Agent field to the subnet-directed broadcast address of
   the mobile node's home network.  Each home agent receiving such a
   Registration Request with a broadcast destination address MUST reject
   the mobile node's registration and SHOULD return a rejection
   Registration Reply indicating its unicast IP address for use by the
   mobile node in a future registration attempt.

   The Care-of Address field MUST be set to the value of the particular
   care-of address that the mobile node wishes to (de)register.  In the
   special case in which a mobile node wishes to deregister all care-of
   addresses, it MUST set this field to its home address.

   The mobile node chooses the Identification field in accordance with
   the style of replay protection it uses with its home agent.  This is
   part of the mobility security association the mobile node shares with
   its home agent.  See Section 5.7 for the method by which the mobile
   node computes the Identification field.

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3.6.1.3. Extensions

   This section describes the ordering of any mandatory and any optional
   Extensions that a mobile node appends to a Registration Request.
   This following ordering MUST be followed:

      a) The IP header, followed by the UDP header, followed by the
         fixed-length portion of the Registration Request, followed by

      b) If present, any non-authentication Extensions expected to be
         used by the home agent (which may or may not also be useful to
         the foreign agent), followed by

      c) An authorization-enabling extension, followed by

      d) If present, any non-authentication Extensions used only by the
         foreign agent, followed by

      e) The Mobile-Foreign Authentication Extension, if present.

   Note that items (a) and (c) MUST appear in every Registration Request
   sent by the mobile node.  Items (b), (d), and (e) are optional.
   However, item (e) MUST be included when the mobile node and the
   foreign agent share a mobility security association.

3.6.2. Receiving Registration Replies

   Registration Replies will be received by the mobile node in response
   to its Registration Requests.  Registration Replies generally fall
   into three categories:

      - the registration was accepted,
      - the registration was denied by the foreign agent, or
      - the registration was denied by the home agent.

   The remainder of this section describes the Registration Reply
   handling by a mobile node in each of these three categories.

3.6.2.1. Validity Checks

   Registration Replies with an invalid, non-zero UDP checksum MUST be
   silently discarded.

   In addition, the low-order 32 bits of the Identification field in the
   Registration Reply MUST be compared to the low-order 32 bits of the
   Identification field in the most recent Registration Request sent to
   the replying agent.  If they do not match, the Reply MUST be silently
   discarded.

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   Also, the Registration Reply MUST be checked for presence of an
   authorization-enabling extension.  For all Registration Reply
   messages containing a Status Code indicating status from the Home
   Agent, the mobile node MUST check for the presence of an
   authorization-enabling extension, acting in accordance with the Code
   field in the Reply.  The rules are as follows:

      a) If the mobile node and the foreign agent share a mobility
         security association, exactly one Mobile-Foreign Authentication
         Extension MUST be present in the Registration Reply, and the
         mobile node MUST check the Authenticator value in the
         Extension.  If no Mobile-Foreign Authentication Extension is
         found, or if more than one Mobile-Foreign Authentication
         Extension is found, or if the Authenticator is invalid, the
         mobile node MUST silently discard the Reply and SHOULD log the
         event as a security exception.

      b) If the Code field indicates that service is denied by the home
         agent, or if the Code field indicates that the registration was
         accepted by the home agent, exactly one Mobile-Home
         Authentication Extension MUST be present in the Registration
         Reply, and the mobile node MUST check the Authenticator value
         in the Extension.  If the Registration Reply was generated by
         the home agent but no Mobile-Home Authentication Extension is
         found, or if more than one Mobile-Home Authentication Extension
         is found, or if the Authenticator is invalid, the mobile node
         MUST silently discard the Reply and SHOULD log the event as a
         security exception.

   If the Code field indicates an authentication failure, either at the
   foreign agent or the home agent, then it is quite possible that any
   authenticators in the Registration Reply will also be in error.  This
   could happen, for example, if the shared secret between the mobile
   node and home agent was erroneously configured.  The mobile node
   SHOULD log such errors as security exceptions.

3.6.2.2. Registration Request Accepted

   If the Code field indicates that the request has been accepted, the
   mobile node SHOULD configure its routing table appropriately for its
   current point of attachment (Section 4.2.1).

   If the mobile node is returning to its home network and that network
   is one which implements ARP, the mobile node MUST follow the
   procedures described in Section 4.6 with regard to ARP, proxy ARP,
   and gratuitous ARP.

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   If the mobile node has registered on a foreign network, it SHOULD
   re-register before the expiration of the Lifetime of its
   registration.  As described in Section 3.6, for each pending
   Registration Request, the mobile node MUST maintain the remaining
   lifetime of this pending registration, as well as the original
   Lifetime from the Registration Request.  When the mobile node
   receives a valid Registration Reply, the mobile node MUST decrease
   its view of the remaining lifetime of the registration by the amount
   by which the home agent decreased the originally requested Lifetime.
   This procedure is equivalent to the mobile node starting a timer for
   the granted Lifetime at the time it sent the Registration Request,
   even though the granted Lifetime is not known to the mobile node
   until the Registration Reply is received.  Since the Registration
   Request is certainly sent before the home agent begins timing the
   registration Lifetime (also based on the granted Lifetime), this
   procedure ensures that the mobile node will re-register before the
   home agent expires and deletes the registration, in spite of possibly
   non-negligible transmission delays for the original Registration
   Request and Reply that started the timing of the Lifetime at the
   mobile node and its home agent.

3.6.2.3. Registration Request Denied

   If the Code field indicates that service is being denied, the mobile
   node SHOULD log the error.  In certain cases the mobile node may be
   able to "repair" the error.  These include:

      Code 69:  (Denied by foreign agent, Lifetime too long)

         In this case, the Lifetime field in the Registration Reply will
         contain the maximum Lifetime value which that foreign agent is
         willing to accept in any Registration Request.  The mobile node
         MAY attempt to register with this same agent, using a Lifetime
         in the Registration Request that MUST be less than or equal to
         the value specified in the Reply.

      Code 133:  (Denied by home agent, Identification mismatch)

         In this case, the Identification field in the Registration
         Reply will contain a value that allows the mobile node to
         synchronize with the home agent, based upon the style of replay
         protection in effect (Section 5.7).  The mobile node MUST
         adjust the parameters it uses to compute the Identification
         field based upon the information in the Registration Reply,
         before issuing any future Registration Requests.

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      Code 136:  (Denied by home agent, Unknown home agent address)

         This code is returned by a home agent when the mobile node is
         performing dynamic home agent address resolution as described
         in Sections 3.6.1.1 and 3.6.1.2.  In this case, the Home Agent
         field within the Reply will contain the unicast IP address of
         the home agent returning the Reply.  The mobile node MAY then
         attempt to register with this home agent in future Registration
         Requests.  In addition, the mobile node SHOULD adjust the
         parameters it uses to compute the Identification field based
         upon the corresponding field in the Registration Reply, before
         issuing any future Registration Requests.

3.6.3. Registration Retransmission

   When no Registration Reply has been received within a reasonable
   time, another Registration Request MAY be 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 (Section
   5.7).  In this way a retransmission will not require the home agent
   to resynchronize with the mobile node by issuing another nonce in the
   case in which the original Registration Request (rather than its
   Registration Reply) was lost by the network.

   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 messages, twice the round trip time for transmission to the home
   agent, and at least an additional 100 milliseconds to allow for
   processing the messages before responding.  The round trip time for
   transmission to the home agent will be at least as large as the time
   required to transmit the messages at the link speed of the mobile
   node's current point of attachment.  Some circuits add another 200
   milliseconds of satellite delay in the total round trip time to the
   home agent.  The minimum time between Registration Requests MUST NOT
   be less than 1 second.  Each successive retransmission timeout period
   SHOULD be at least twice the previous period, as long as that is less
   than the maximum as specified above.



(page 47 continued on part 3)

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