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RFC 3344

 
 
 

IP Mobility Support for IPv4

Part 3 of 4, p. 47 to 73
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3.7. Foreign Agent Considerations

   The foreign agent plays a mostly passive role in Mobile IP
   registration.  It relays Registration Requests between mobile nodes
   and home agents, and, when it provides the care-of address,
   decapsulates datagrams for delivery to the mobile node.  It SHOULD
   also send periodic Agent Advertisement messages to advertise its

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   presence as described in Section 2.3, if not detectable by link-layer
   means.

   A foreign agent MUST NOT transmit a Registration Request except when
   relaying a Registration Request received from a mobile node, to the
   mobile node's home agent.  A foreign agent MUST NOT transmit a
   Registration Reply except when relaying a Registration Reply received
   from a mobile node's home agent, or when replying to a Registration
   Request received from a mobile node in the case in which the foreign
   agent is denying service to the mobile node.  In particular, a
   foreign agent MUST NOT generate a Registration Request or Reply
   because a mobile node's registration Lifetime has expired.  A foreign
   agent also MUST NOT originate a Registration Request message that
   asks for deregistration of a mobile node; however, it MUST relay
   valid (de)Registration Requests originated by a mobile node.

3.7.1. Configuration and Registration Tables

   Each foreign agent MUST be configured with a care-of address.  In
   addition, for each pending or current registration the foreign agent
   MUST maintain a visitor list entry containing the following
   information obtained from the mobile node's Registration Request:

      -  the link-layer source address of the mobile node
      -  the IP Source Address (the mobile node's Home Address) or its
         co-located care-of address (see description of the 'R' bit in
         section 2.1.1)
      -  the IP Destination Address (as specified in 3.6.1.1)
      -  the UDP Source Port
      -  the Home Agent address
      -  the Identification field
      -  the requested registration Lifetime, and
      -  the remaining Lifetime of the pending or current registration.

   If the mobile node's Home Address is zero in the Registration Request
   message, then the foreign agent MUST follow the procedures specified
   in RFC 2794 [6].  In particular, if the foreign agent cannot manage
   pending registration request records with such a zero Home Address
   for the mobile node, the foreign agent MUST return a Registration
   Reply with Code indicating NONZERO_HOMEADDR_REQD (see [6]).

   The foreign agent MAY configure a maximum number of pending
   registrations that it is willing to maintain (typically 5).
   Additional registrations SHOULD then be rejected by the foreign agent
   with code 66.  The foreign agent MAY delete any pending Registration
   Request after the request has been pending for more than 7 seconds;
   in this case, the foreign agent SHOULD reject the Request with code
   78 (registration timeout).

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   As with any node on the Internet, a foreign agent MAY also share
   mobility security associations with any other nodes.  When relaying a
   Registration Request from a mobile node to its home agent, if the
   foreign agent shares a mobility security association with the home
   agent, it MUST add a Foreign-Home Authentication Extension to the
   Request and MUST check the required Foreign-Home Authentication
   Extension in the Registration Reply from the home agent (Sections 3.3
   and 3.4).  Similarly, when receiving a Registration Request from a
   mobile node, if the foreign agent shares a mobility security
   association with the mobile node, it MUST check the required Mobile-
   Foreign Authentication Extension in the Request and MUST add a
   Mobile-Foreign Authentication Extension to the Registration Reply to
   the mobile node.

3.7.2. Receiving Registration Requests

   If the foreign agent accepts a Registration Request from a mobile
   node, it checks to make sure that the indicated home agent address
   does not belong to any network interface of the foreign agent.  If
   not, the foreign agent then MUST relay the Request to the indicated
   home agent.  Otherwise, if the foreign agent denies the Request, it
   MUST send a Registration Reply to the mobile node with an appropriate
   denial Code, except in cases where the foreign agent would be
   required to send out more than one such denial per second to the same
   mobile node.  The following sections describe this behavior in more
   detail.

   If the foreign agent has configured one of its network interfaces
   with the IP address specified by the mobile node as its home agent
   address, the foreign agent MUST NOT forward the request again.  If
   the foreign agent serves the mobile node as a home agent, the foreign
   agent follows the procedures specified in section 3.8.2.  Otherwise,
   if the foreign agent does not serve the mobile node as a home agent,
   the foreign agent rejects the Registration Request with code 136
   (unknown home agent address).

   If a foreign agent receives a Registration Request from a mobile node
   in its visitor list, the existing visitor list entry for the mobile
   node SHOULD NOT be deleted or modified until the foreign agent
   receives a valid Registration Reply from the home agent with a Code
   indicating success.  The foreign agent MUST record the new pending
   Request as a separate part of the existing visitor list entry for the
   mobile node.  If the Registration Request requests deregistration,
   the existing visitor list entry for the mobile node SHOULD NOT be
   deleted until the foreign agent has received a successful
   Registration Reply.  If the Registration Reply indicates that the

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   Request (for registration or deregistration) was denied by the home
   agent, the existing visitor list entry for the mobile node MUST NOT
   be modified as a result of receiving the Registration Reply.

3.7.2.1. Validity Checks

   Registration Requests with an invalid, non-zero UDP checksum MUST be
   silently discarded.  Requests with non-zero bits in reserved fields
   MUST be rejected with code 70 (poorly formed request).  Requests with
   the 'D' bit set to 0, and specifying a care-of address not offered by
   the foreign agent, MUST be rejected with code 77 (invalid care-of
   address).

   Also, the authentication in the Registration Request MUST be checked.
   If the foreign agent and the mobile node share a mobility security
   association, exactly one Mobile-Foreign Authentication Extension MUST
   be present in the Registration Request, and the foreign agent 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 foreign agent MUST silently discard the Request and
   SHOULD log the event as a security exception.  The foreign agent also
   SHOULD send a Registration Reply to the mobile node with Code 67.

3.7.2.2. Forwarding a Valid Request to the Home Agent

   If the foreign agent accepts the mobile node's Registration Request,
   it MUST relay the Request to the mobile node's home agent as
   specified in the Home Agent field of the Registration Request.  The
   foreign agent MUST NOT modify any of the fields beginning with the
   fixed portion of the Registration Request up through and including
   the Mobile-Home Authentication Extension or other authentication
   extension supplied by the mobile node as an authorization-enabling
   extension for the home agent.  Otherwise, an authentication failure
   is very likely to occur at the home agent.  In addition, the foreign
   agent proceeds as follows:

      -  It MUST process and remove any Extensions following the
         Mobile-Home Authentication Extension,
      -  It MAY append any of its own non-authentication Extensions of
         relevance to the home agent, if applicable, and
      -  It MUST append the Foreign-Home Authentication Extension, if
         the foreign agent shares a mobility security association with
         the home agent.

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   Specific fields within the IP header and the UDP header of the
   relayed Registration Request MUST be set as follows:

      IP Source Address

               The foreign agent's address on the interface from which
               the message will be sent.

      IP Destination Address

               Copied from the Home Agent field within the Registration
               Request.

      UDP Source Port

               <variable>

      UDP Destination Port

               434

   After forwarding a valid Registration Request to the home agent, the
   foreign agent MUST begin timing the remaining lifetime of the pending
   registration based on the Lifetime in the Registration Request.  If
   this lifetime expires before receiving a valid Registration Reply,
   the foreign agent MUST delete its visitor list entry for this pending
   registration.

3.7.2.3. Denying Invalid Requests

   If the foreign agent denies the mobile node's Registration Request
   for any reason, it SHOULD send the mobile node a Registration Reply
   with a suitable denial Code.  In such a case, the Home Address, Home
   Agent, and Identification fields within the Registration Reply are
   copied from the corresponding fields of the Registration Request.

   If the Reserved field is nonzero, the foreign agent MUST deny the
   Request and SHOULD return a Registration Reply with status code 70 to
   the mobile node.  If the Request is being denied because the
   requested Lifetime is too long, the foreign agent sets the Lifetime
   in the Reply to the maximum Lifetime value it is willing to accept in
   any Registration Request, and sets the Code field to 69.  Otherwise,
   the Lifetime SHOULD be copied from the Lifetime field in the Request.

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   Specific fields within the IP header and the UDP header of the
   Registration Reply MUST be set as follows:

      IP Source Address

               Copied from the IP Destination Address of Registration
               Request, unless the "All Agents Multicast" address was
               used.  In this case, the foreign agent's address (on the
               interface from which the message will be sent) MUST be
               used.

      IP Destination Address

               If the Registration Reply is generated by the Foreign
               Agent in order to reject a mobile node's Registration
               Request, and the Registration Request contains a Home
               Address which is not 0.0.0.0, then the IP Destination
               Address is copied from the Home Address field of the
               Registration Request.  Otherwise, if the Registration
               Reply is received from the Home Agent, and contains a
               Home Address which is not 0.0.0.0, then the IP
               Destination Address is copied from the Home Address field
               of the Registration Reply.  Otherwise, the IP Destination
               Address of the Registration Reply is set to be
               255.255.255.255.

      UDP Source Port

               434

               UDP Destination Port

               Copied from the UDP Source Port of the Registration
               Request.

3.7.3. Receiving Registration Replies

   The foreign agent updates its visitor list when it receives a valid
   Registration Reply from a home agent.  It then relays the
   Registration Reply to the mobile node.  The following sections
   describe this behavior in more detail.

   If upon relaying a Registration Request to a home agent, the foreign
   agent receives an ICMP error message instead of a Registration Reply,
   then the foreign agent SHOULD send to the mobile node a Registration
   Reply with an appropriate "Home Agent Unreachable" failure Code
   (within the range 80-95, inclusive).  See Section 3.7.2.3 for details
   on building the Registration Reply.

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3.7.3.1. Validity Checks

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

   When a foreign agent receives a Registration Reply message, it MUST
   search its visitor list for a pending Registration Request with the
   same mobile node home address as indicated in the Reply.  If no such
   pending Request is found, and if the Registration Reply does not
   correspond with any pending Registration Request with a zero mobile
   node home address (see section 3.7.1), the foreign agent MUST
   silently discard the Reply.  The foreign agent MUST also silently
   discard the Reply if the low-order 32 bits of the Identification
   field in the Reply do not match those in the Request.

   Also, the authentication in the Registration Reply MUST be checked.
   If the foreign agent and the home agent share a mobility security
   association, exactly one Foreign-Home Authentication Extension MUST
   be present in the Registration Reply, and the foreign agent MUST
   check the Authenticator value in the Extension.  If no Foreign-Home
   Authentication Extension is found, or if more than one Foreign-Home
   Authentication Extension is found, or if the Authenticator is
   invalid, the foreign agent MUST silently discard the Reply and SHOULD
   log the event as a security exception.  The foreign agent also MUST
   reject the mobile node's registration and SHOULD send a Registration
   Reply to the mobile node with Code 68.

3.7.3.2. Forwarding Replies to the Mobile Node

   A Registration Reply which satisfies the validity checks of Section
   3.8.2.1 is relayed to the mobile node.  The foreign agent MUST also
   update its visitor list entry for the mobile node to reflect the
   results of the Registration Request, as indicated by the Code field
   in the Reply.  If the Code indicates that the home agent has accepted
   the registration and the Lifetime field is nonzero, the foreign agent
   SHOULD set the Lifetime in the visitor list entry to the minimum of
   the following two values:

      -  the value specified in the Lifetime field of the Registration
         Reply, and

      -  the foreign agent's own maximum value for allowable
         registration lifetime.

   If, instead, the Code indicates that the Lifetime field is zero, the
   foreign agent MUST delete its visitor list entry for the mobile node.
   Finally, if the Code indicates that the registration was denied by

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   the home agent, the foreign agent MUST delete its pending
   registration list entry, but not its visitor list entry, for the
   mobile node.

   The foreign agent MUST NOT modify any of the fields beginning with
   the fixed portion of the Registration Reply up through and including
   the Mobile-Home Authentication Extension.  Otherwise, an
   authentication failure is very likely to occur at the mobile node.

   In addition, the foreign agent SHOULD perform the following
   additional procedures:

      -  It MUST process and remove any Extensions following the
         Mobile-Home Authentication Extension,
      -  It MAY append its own non-authentication Extensions of
         relevance to the mobile node, if applicable, and
      -  It MUST append the Mobile-Foreign Authentication Extension, if
         the foreign agent shares a mobility security association with
         the mobile node.

   Specific fields within the IP header and the UDP header of the
   relayed Registration Reply are set according to the same rules
   specified in Section 3.7.2.3.

   After forwarding a valid Registration Reply to the mobile node, the
   foreign agent MUST update its visitor list entry for this
   registration as follows.  If the Registration Reply indicates that
   the registration was accepted by the home agent, the foreign agent
   resets its timer of the lifetime of the registration to the Lifetime
   granted in the Registration Reply; unlike the mobile node's timing of
   the registration lifetime as described in Section 3.6.2.2, the
   foreign agent considers this lifetime to begin when it forwards the
   Registration Reply message, ensuring that the foreign agent will not
   expire the registration before the mobile node does.  On the other
   hand, if the Registration Reply indicates that the registration was
   rejected by the home agent, the foreign agent deletes its visitor
   list entry for this attempted registration.

3.8. Home Agent Considerations

   Home agents play a reactive role in the registration process.  The
   home agent receives Registration Requests from the mobile node
   (perhaps relayed by a foreign agent), updates its record of the
   mobility bindings for this mobile node, and issues a suitable
   Registration Reply in response to each.

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   A home agent MUST NOT transmit a Registration Reply except when
   replying to a Registration Request received from a mobile node.  In
   particular, the home agent MUST NOT generate a Registration Reply to
   indicate that the Lifetime has expired.

3.8.1. Configuration and Registration Tables

   Each home agent MUST be configured with an IP address and with the
   prefix size for the home network.  The home agent MUST be configured
   with the mobility security association of each authorized mobile node
   that it is serving as a home agent.

   When the home agent accepts a valid Registration Request from a
   mobile node that it serves as a home agent, the home agent MUST
   create or modify the entry for this mobile node in its mobility
   binding list containing:

      -  the mobile node's home address
      -  the mobile node's care-of address
      -  the Identification field from the Registration Reply
      -  the remaining Lifetime of the registration

   The home agent MAY optionally offer the capability to dynamically
   associate a home address to a mobile node upon receiving a
   Registration Request from that mobile node.  The method by which a
   home address is allocated to the mobile node is beyond the scope of
   this document, but see [6].  After the home agent makes the
   association of the home address to the mobile node, the home agent
   MUST put that home address into the Home Address field of the
   Registration Reply.

   The home agent MAY also maintain mobility security associations with
   various foreign agents.  When receiving a Registration Request from a
   foreign agent, if the home agent shares a mobility security
   association with the foreign agent, the home agent MUST check the
   Authenticator in the required Foreign-Home Authentication Extension
   in the message, based on this mobility security association.
   Similarly, when sending a Registration Reply to a foreign agent, if
   the home agent shares a mobility security association with the
   foreign agent, the home agent MUST include a Foreign-Home
   Authentication Extension in the message, based on this mobility
   security association.

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3.8.2. Receiving Registration Requests

   If the home agent accepts an incoming Registration Request, it MUST
   update its record of the the mobile node's mobility binding(s) and
   SHOULD send a Registration Reply with a suitable Code.  Otherwise
   (the home agent denies the Request), it SHOULD send a Registration
   Reply with an appropriate Code specifying the reason the Request was
   denied.  The following sections describe this behavior in more
   detail.  If the home agent does not support broadcasts (see section
   4.3), it MUST ignore the 'B' bit (as opposed to rejecting the
   Registration Request).

3.8.2.1. Validity Checks

   Registration Requests with an invalid, non-zero UDP checksum MUST be
   silently discarded by the home agent.

   The authentication in the Registration Request MUST be checked.  This
   involves the following operations:

      a) The home agent MUST check for the presence of an
         authorization-enabling extension, and perform the indicated
         authentication.  Exactly one authorization-enabling extension
         MUST be present in the Registration Request; and the home agent
         MUST either check the Authenticator value in the extension or
         verify that the authenticator value has been checked by another
         agent with which it has a security association.  If no
         authorization-enabling extension is found, or if more than one
         authorization-enabling extension is found, or if the
         Authenticator is invalid, the home agent MUST reject the mobile
         node's registration and SHOULD send a Registration Reply to the
         mobile node with Code 131.  The home agent MUST then discard
         the Request and SHOULD log the error as a security exception.

      b) The home agent MUST check that the registration Identification
         field is correct using the context selected by the SPI within
         the authorization-enabling extension.  See Section 5.7 for a
         description of how this is performed.  If incorrect, the home
         agent MUST reject the Request and SHOULD send a Registration
         Reply to the mobile node with Code 133, including an
         Identification field computed in accordance with the rules
         specified in Section 5.7.  The home agent MUST do no further
         processing with such a Request, though it SHOULD log the error
         as a security exception.

      c) If the home agent shares a mobility security association with
         the foreign agent, the home agent MUST check for the presence
         of a valid Foreign-Home Authentication Extension.  Exactly one

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         Foreign-Home Authentication Extension MUST be present in the
         Registration Request in this case, and the home agent MUST
         check the Authenticator value in the Extension.  If no
         Foreign-Home Authentication Extension is found, or if more than
         one Foreign-Home Authentication Extension is found, or if the
         Authenticator is invalid, the home agent MUST reject the mobile
         node's registration and SHOULD send a Registration Reply to the
         mobile node with Code 132.  The home agent MUST then discard
         the Request and SHOULD log the error as a security exception.

   In addition to checking the authentication in the Registration
   Request, home agents MUST deny Registration Requests that are sent to
   the subnet-directed broadcast address of the home network (as opposed
   to being unicast to the home agent).  The home agent MUST discard the
   Request and SHOULD returning a Registration Reply with a Code of 136.
   In this case, the Registration Reply will contain the home agent's
   unicast address, so that the mobile node can re-issue the
   Registration Request with the correct home agent address.

   Note that some routers change the IP destination address of a
   datagram from a subnet-directed broadcast address to 255.255.255.255
   before injecting it into the destination subnet.  In this case, home
   agents that attempt to pick up dynamic home agent discovery requests
   by binding a socket explicitly to the subnet-directed broadcast
   address will not see such packets.  Home agent implementors should be
   prepared for both the subnet-directed broadcast address and
   255.255.255.255 if they wish to support dynamic home agent discovery.

3.8.2.2. Accepting a Valid Request

   If the Registration Request satisfies the validity checks in Section
   3.8.2.1, and the home agent is able to accommodate the Request, the
   home agent MUST update its mobility binding list for the requesting
   mobile node and MUST return a Registration Reply to the mobile node.

   In this case, the Reply Code will be either 0 if the home agent
   supports simultaneous mobility bindings, or 1 if it does not.  See
   Section 3.8.3 for details on building the Registration Reply message.

   The home agent updates its record of the mobile node's mobility
   bindings as follows, based on the fields in the Registration Request:

      -  If the Lifetime is zero and the Care-of Address equals the
         mobile node's home address, the home agent deletes all of the
         entries in the mobility binding list for the requesting mobile
         node.  This is how a mobile node requests that its home agent
         cease providing mobility services.

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      -  If the Lifetime is zero and the Care-of Address does not equal
         the mobile node's home address, the home agent deletes only the
         entry containing the specified Care-of Address from the
         mobility binding list for the requesting mobile node.  Any
         other active entries containing other care-of addresses will
         remain active.

      -  If the Lifetime is nonzero, the home agent adds an entry
         containing the requested Care-of Address to the mobility
         binding list for the mobile node.  If the 'S' bit is set and
         the home agent supports simultaneous mobility bindings, the
         previous mobility binding entries are retained.  Otherwise, the
         home agent removes all previous entries in the mobility binding
         list for the mobile node.

   In all cases, the home agent MUST send a Registration Reply to the
   source of the Registration Request, which might indeed be a different
   foreign agent than that whose care-of address is being
   (de)registered.  If the home agent shares a mobility security
   association with the foreign agent whose care-of address is being
   deregistered, and that foreign agent is different from the one which
   relayed the Registration Request, the home agent MAY additionally
   send a Registration Reply to the foreign agent whose care-of address
   is being deregistered.  The home agent MUST NOT send such a Reply if
   it does not share a mobility security association with the foreign
   agent.  If no Reply is sent, the foreign agent's visitor list will
   expire naturally when the original Lifetime expires.

   The home agent MUST NOT increase the Lifetime above that specified by
   the mobile node in the Registration Request.  However, it is not an
   error for the mobile node to request a Lifetime longer than the home
   agent is willing to accept.  In this case, the home agent simply
   reduces the Lifetime to a permissible value and returns this value in
   the Registration Reply.  The Lifetime value in the Registration Reply
   informs the mobile node of the granted lifetime of the registration,
   indicating when it SHOULD re-register in order to maintain continued
   service.  After the expiration of this registration lifetime, the
   home agent MUST delete its entry for this registration in its
   mobility binding list.

   If the Registration Request duplicates an accepted current
   Registration Request, the new Lifetime MUST NOT extend beyond the
   Lifetime originally granted.  A Registration Request is a duplicate
   if the home address, care-of address, and Identification fields all
   equal those of an accepted current registration.

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   In addition, if the home network implements ARP [36], and the
   Registration Request asks the home agent to create a mobility binding
   for a mobile node which previously had no binding (the mobile node
   was previously assumed to be at home), then the home agent MUST
   follow the procedures described in Section 4.6 with regard to ARP,
   proxy ARP, and gratuitous ARP.  If the mobile node already had a
   previous mobility binding, the home agent MUST continue to follow the
   rules for proxy ARP described in Section 4.6.

3.8.2.3. Denying an Invalid Request

   If the Registration Reply does not satisfy all of the validity checks
   in Section 3.8.2.1, or the home agent is unable to accommodate the
   Request, the home agent SHOULD return a Registration Reply to the
   mobile node with a Code that indicates the reason for the error.  If
   a foreign agent was involved in relaying the Request, this allows the
   foreign agent to delete its pending visitor list entry.  Also, this
   informs the mobile node of the reason for the error such that it may
   attempt to fix the error and issue another Request.

   This section lists a number of reasons the home agent might reject a
   Request, and provides the Code value it should use in each instance.
   See Section 3.8.3 for additional details on building the Registration
   Reply message.

   Many reasons for rejecting a registration are administrative in
   nature.  For example, a home agent can limit the number of
   simultaneous registrations for a mobile node, by rejecting any
   registrations that would cause its limit to be exceeded, and
   returning a Registration Reply with error code 135.  Similarly, a
   home agent may refuse to grant service to mobile nodes which have
   entered unauthorized service areas by returning a Registration Reply
   with a Code of 129.

   Requests with non-zero bits in reserved fields MUST be rejected with
   code 134 (poorly formed request).

3.8.3. Sending Registration Replies

   If the home agent accepts a Registration Request, it then MUST update
   its record of the mobile node's mobility binding(s) and SHOULD send a
   Registration Reply with a suitable Code.  Otherwise (the home agent
   has denied the Request), it SHOULD send a Registration Reply with an
   appropriate Code specifying the reason the Request was denied.  The
   following sections provide additional detail for the values the home
   agent MUST supply in the fields of Registration Reply messages.

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3.8.3.1. IP/UDP Fields

   This section provides the specific rules by which home agents pick
   values for the IP and UDP header fields of a Registration Reply.

      IP Source Address
               Copied from the IP Destination Address of Registration
               Request, unless a multicast or broadcast address was
               used.  If the IP Destination Address of the Registration
               Request was a broadcast or multicast address, the IP
               Source Address of the Registration Reply MUST be set to
               the home agent's (unicast) IP address.

      IP Destination Address
               Copied from the IP Source Address of the Registration
               Request.

      UDP Source Port
               Copied from the UDP Destination Port of the Registration
               Request.

      UDP Destination Port
               Copied from the UDP Source Port of the Registration
               Request.

   When sending a Registration Reply in response to a Registration
   Request that requested deregistration of the mobile node (the
   Lifetime is zero and the Care-of Address equals the mobile node's
   home address) and in which the IP Source Address was also set to the
   mobile node's home address (this is the normal method used by a
   mobile node to deregister when it returns to its home network), the
   IP Destination Address in the Registration Reply will be set to the
   mobile node's home address, as copied from the IP Source Address of
   the Request.

   In this case, when transmitting the Registration Reply, the home
   agent MUST transmit the Reply directly onto the home network as if
   the mobile node were at home, bypassing any mobility binding list
   entry that may still exist at the home agent for the destination
   mobile node.  In particular, for a mobile node returning home after
   being registered with a care-of address, if the mobile node's new
   Registration Request is not accepted by the home agent, the mobility
   binding list entry for the mobile node will still indicate that
   datagrams addressed to the mobile node should be tunneled to the
   mobile node's registered care-of address; when sending the
   Registration Reply indicating the rejection of this Request, this
   existing binding list entry MUST be ignored, and the home agent MUST
   transmit this Reply as if the mobile node were at home.

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3.8.3.2. Registration Reply Fields

   This section provides the specific rules by which home agents pick
   values for the fields within the fixed portion of a Registration
   Reply.

   The Code field of the Registration Reply is chosen in accordance with
   the rules specified in the previous sections.  When replying to an
   accepted registration, a home agent SHOULD respond with Code 1 if it
   does not support simultaneous registrations.

   The Lifetime field MUST be copied from the corresponding field in the
   Registration Request, unless the requested value is greater than the
   maximum length of time the home agent is willing to provide the
   requested service.  In such a case, the Lifetime MUST be set to the
   length of time that service will actually be provided by the home
   agent.  This reduced Lifetime SHOULD be the maximum Lifetime allowed
   by the home agent (for this mobile node and care-of address).

   If the Home Address field of the Registration Request is nonzero, it
   MUST be copied into the Home Address field of the Registration Reply
   message.  Otherwise, if the Home Address field of the Registration
   Request is zero as specified in section 3.6, the home agent SHOULD
   arrange for the selection of a home address for the mobile node, and
   insert the selected address into the Home Address field of the
   Registration Reply message.  See [6] for further relevant details in
   the case where mobile nodes identify themselves using an NAI instead
   of their IP home address.

   If the Home Agent field in the Registration Request contains a
   unicast address of this home agent, then that field MUST be copied
   into the Home Agent field of the Registration Reply.  Otherwise, the
   home agent MUST set the Home Agent field in the Registration Reply to
   its unicast address.  In this latter case, the home agent MUST reject
   the registration with a suitable code (e.g., Code 136) to prevent the
   mobile node from possibly being simultaneously registered with two or
   more home agents.

3.8.3.3. Extensions

   This section describes the ordering of any required and any optional
   Mobile IP Extensions that a home agent appends to a Registration
   Reply.  The following ordering MUST be followed:

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

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      b) If present, any non-authentication Extensions used by the
         mobile node (which may or may not also be used by the foreign
         agent),

      c) The Mobile-Home Authentication Extension,

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

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

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

4. Routing Considerations

   This section describes how mobile nodes, home agents, and (possibly)
   foreign agents cooperate to route datagrams to/from mobile nodes that
   are connected to a foreign network.  The mobile node informs its home
   agent of its current location using the registration procedure
   described in Section 3.  See the protocol overview in Section 1.7 for
   the relative locations of the mobile node's home address with respect
   to its home agent, and the mobile node itself with respect to any
   foreign agent with which it might attempt to register.

4.1. Encapsulation Types

   Home agents and foreign agents MUST support tunneling datagrams using
   IP in IP encapsulation [32].  Any mobile node that uses a co-located
   care-of address MUST support receiving datagrams tunneled using IP in
   IP encapsulation.  Minimal encapsulation [34] and GRE encapsulation
   [16] are alternate encapsulation methods which MAY optionally be
   supported by mobility agents and mobile nodes.  The use of these
   alternative forms of encapsulation, when requested by the mobile
   node, is otherwise at the discretion of the home agent.

4.2. Unicast Datagram Routing

4.2.1. Mobile Node Considerations

   When connected to its home network, a mobile node operates without
   the support of mobility services.  That is, it operates in the same
   way as any other (fixed) host or router.  The method by which a
   mobile node selects a default router when connected to its home

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   network, or when away from home and using a co-located care-of
   address, is outside the scope of this document.  ICMP Router
   Advertisement [10] is one such method.

   When registered on a foreign network, the mobile node chooses a
   default router by the following rules:

      -  If the mobile node is registered using a foreign agent care-of
         address, it MAY use its foreign agent as a first-hop router.
         The foreign agent's MAC address can be learned from Agent
         Advertisement.  Otherwise, the mobile node MUST choose its
         default router from among the Router Addresses advertised in
         the ICMP Router Advertisement portion of that Agent
         Advertisement message.

      -  If the mobile node is registered directly with its home agent
         using a co-located care-of address, then the mobile node SHOULD
         choose its default router from among those advertised in any
         ICMP Router Advertisement message that it receives for which
         its externally obtained care-of address and the Router Address
         match under the network prefix.  If the mobile node's
         externally obtained care-of address matches the IP source
         address of the Agent Advertisement under the network prefix,
         the mobile node MAY also consider that IP source address as
         another possible choice for the IP address of a default router.
         The network prefix MAY be obtained from the Prefix-Lengths
         Extension in the Router Advertisement, if present.  The prefix
         MAY also be obtained through other mechanisms beyond the scope
         of this document.

   While they are away from the home network, mobile nodes MUST NOT
   broadcast ARP packets to find the MAC address of another Internet
   node.  Thus, the (possibly empty) list of Router Addresses from the
   ICMP Router Advertisement portion of the message is not useful for
   selecting a default router, unless the mobile node has some means not
   involving broadcast ARP and not specified within this document for
   obtaining the MAC address of one of the routers in the list.
   Similarly, in the absence of unspecified mechanisms for obtaining MAC
   addresses on foreign networks, the mobile node MUST ignore redirects
   to other routers on foreign networks.

4.2.2. Foreign Agent Considerations

   Upon receipt of an encapsulated datagram sent to its advertised
   care-of address, a foreign agent MUST compare the inner destination
   address to those entries in its visitor list.  When the destination
   does not match the address of any mobile node currently in the
   visitor list, the foreign agent MUST NOT forward the datagram without

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   modifications to the original IP header, because otherwise a routing
   loop is likely to result.  The datagram SHOULD be silently discarded.
   ICMP Destination Unreachable MUST NOT be sent when a foreign agent is
   unable to forward an incoming tunneled datagram.  Otherwise, the
   foreign agent forwards the decapsulated datagram to the mobile node.

   The foreign agent MUST NOT advertise to other routers in its routing
   domain, nor to any other mobile node, the presence of a mobile router
   (Section 4.5) or mobile node in its visitor list.

   The foreign agent MUST route datagrams it receives from registered
   mobile nodes.  At a minimum, this means that the foreign agent must
   verify the IP Header Checksum, decrement the IP Time To Live,
   recompute the IP Header Checksum, and forward such datagrams to a
   default router.

   A foreign agent MUST NOT use broadcast ARP for a mobile node's MAC
   address on a foreign network.  It may obtain the MAC address by
   copying the information from an Agent Solicitation or a Registration
   Request transmitted from a mobile node.  A foreign agent's ARP cache
   for the mobile node's IP address MUST NOT be allowed to expire before
   the mobile node's visitor list entry expires, unless the foreign
   agent has some way other than broadcast ARP to refresh its MAC
   address associated with the mobile node's IP address.

   Each foreign agent SHOULD support the mandatory features for reverse
   tunneling [27].

4.2.3. Home Agent Considerations

   The home agent MUST be able to intercept any datagrams on the home
   network addressed to the mobile node while the mobile node is
   registered away from home.  Proxy and gratuitous ARP MAY be used in
   enabling this interception, as specified in Section 4.6.

   The home agent must examine the IP Destination Address of all
   arriving datagrams to see if it is equal to the home address of any
   of its mobile nodes registered away from home.  If so, the home agent
   tunnels the datagram to the mobile node's currently registered care-
   of address or addresses.  If the home agent supports the optional
   capability of multiple simultaneous mobility bindings, it tunnels a
   copy to each care-of address in the mobile node's mobility binding
   list.  If the mobile node has no current mobility bindings, the home
   agent MUST NOT attempt to intercept datagrams destined for the mobile
   node, and thus will not in general receive such datagrams.  However,
   if the home agent is also a router handling common IP traffic, it is
   possible that it will receive such datagrams for forwarding onto the

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   home network.  In this case, the home agent MUST assume the mobile
   node is at home and simply forward the datagram directly onto the
   home network.

   For multihomed home agents, the source address in the outer IP header
   of the encapsulated datagram MUST be the address sent to the mobile
   node in the home agent field of the registration reply.  That is, the
   home agent cannot use the the address of some other network interface
   as the source address.

   See Section 4.1 regarding methods of encapsulation that may be used
   for tunneling.  Nodes implementing tunneling SHOULD also implement
   the "tunnel soft state" mechanism [32], which allows ICMP error
   messages returned from the tunnel to correctly be reflected back to
   the original senders of the tunneled datagrams.

   Home agents MUST decapsulate packets addressed to themselves, sent by
   a mobile node for the purpose of maintaining location privacy, as
   described in Section 5.5.  This feature is also required for support
   of reverse tunneling [27].

   If the Lifetime for a given mobility binding expires before the home
   agent has received another valid Registration Request for that mobile
   node, then that binding is deleted from the mobility binding list.
   The home agent MUST NOT send any Registration Reply message simply
   because the mobile node's binding has expired.  The entry in the
   visitor list of the mobile node's current foreign agent will expire
   naturally, probably at the same time as the binding expired at the
   home agent.  When a mobility binding's lifetime expires, the home
   agent MUST delete the binding, but it MUST retain any other (non-
   expired) simultaneous mobility bindings that it holds for the mobile
   node.

   When a home agent receives a datagram, intercepted for one of its
   mobile nodes registered away from home, the home agent MUST examine
   the datagram to check if it is already encapsulated.  If so, special
   rules apply in the forwarding of that datagram to the mobile node:

      -  If the inner (encapsulated) Destination Address is the same as
         the outer Destination Address (the mobile node), then the home
         agent MUST also examine the outer Source Address of the
         encapsulated datagram (the source address of the tunnel).  If
         this outer Source Address is the same as the mobile node's
         current care-of address, the home agent MUST silently discard
         that datagram in order to prevent a likely routing loop.  If,
         instead, the outer Source Address is NOT the same as the mobile
         node's current care-of address, then the home agent SHOULD
         forward the datagram to the mobile node.  In order to forward

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         the datagram in this case, the home agent MAY simply alter the
         outer Destination Address to the care-of address, rather than
         re-encapsulating the datagram.

      -  Otherwise (the inner Destination Address is NOT the same as the
         outer Destination Address), the home agent SHOULD encapsulate
         the datagram again (nested encapsulation), with the new outer
         Destination Address set equal to the mobile node's care-of
         address.  That is, the home agent forwards the entire datagram
         to the mobile node in the same way as any other datagram
         (encapsulated already or not).

4.3. Broadcast Datagrams

   When a home agent receives a broadcast datagram, it MUST NOT forward
   the datagram to any mobile nodes in its mobility binding list other
   than those that have requested forwarding of broadcast datagrams.  A
   mobile node MAY request forwarding of broadcast datagrams by setting
   the 'B' bit in its Registration Request message (Section 3.3).  For
   each such registered mobile node, the home agent SHOULD forward
   received broadcast datagrams to the mobile node, although it is a
   matter of configuration at the home agent as to which specific
   categories of broadcast datagrams will be forwarded to such mobile
   nodes.

   If the 'D' bit was set in the mobile node's Registration Request
   message, indicating that the mobile node is using a co-located care-
   of address, the home agent simply tunnels appropriate broadcast IP
   datagrams to the mobile node's care-of address.  Otherwise (the 'D'
   bit was NOT set), the home agent first encapsulates the broadcast
   datagram in a unicast datagram addressed to the mobile node's home
   address, and then tunnels this encapsulated datagram to the foreign
   agent.  This extra level of encapsulation is required so that the
   foreign agent can determine which mobile node should receive the
   datagram after it is decapsulated.  When received by the foreign
   agent, the unicast encapsulated datagram is detunneled and delivered
   to the mobile node in the same way as any other datagram.  In either
   case, the mobile node must decapsulate the datagram it receives in
   order to recover the original broadcast datagram.

4.4. Multicast Datagram Routing

   As mentioned previously, a mobile node that is connected to its home
   network functions in the same way as any other (fixed) host or
   router.  Thus, when it is at home, a mobile node functions
   identically to other multicast senders and receivers.  This section
   therefore describes the behavior of a mobile node that is visiting a
   foreign network.

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   In order to receive multicasts, a mobile node MUST join the multicast
   group in one of two ways.  First, a mobile node MAY join the group
   via a (local) multicast router on the visited subnet.  This option
   assumes that there is a multicast router present on the visited
   subnet.  If the mobile node is using a co-located care-of address, it
   SHOULD use this address as the source IP address of its IGMP [11]
   messages.  Otherwise, it MAY use its home address.

   Alternatively, a mobile node which wishes to receive multicasts MAY
   join groups via a bi-directional tunnel to its home agent, assuming
   that its home agent is a multicast router.  The mobile node tunnels
   IGMP messages to its home agent and the home agent forwards multicast
   datagrams down the tunnel to the mobile node.  For packets tunneled
   to the home agent, the source address in the IP header SHOULD be the
   mobile node's home address.

   The rules for multicast datagram delivery to mobile nodes in this
   case are identical to those for broadcast datagrams (Section 4.3).
   Namely, if the mobile node is using a co-located care-of address (the
   'D' bit was set in the mobile node's Registration Request), then the
   home agent SHOULD tunnel the datagram to this care-of address;
   otherwise, the home agent MUST first encapsulate the datagram in a
   unicast datagram addressed to the mobile node's home address and then
   MUST tunnel the resulting datagram (nested tunneling) to the mobile
   node's care-of address.  For this reason, the mobile node MUST be
   capable of decapsulating packets sent to its home address in order to
   receive multicast datagrams using this method.

   A mobile node that wishes to send datagrams to a multicast group also
   has two options:  (1) send directly on the visited network; or (2)
   send via a tunnel to its home agent.  Because multicast routing in
   general depends upon the IP source address, a mobile node which sends
   multicast datagrams directly on the visited network MUST use a co-
   located care-of address as the IP source address.  Similarly, a
   mobile node which tunnels a multicast datagram to its home agent MUST
   use its home address as the IP source address of both the (inner)
   multicast datagram and the (outer) encapsulating datagram.  This
   second option assumes that the home agent is a multicast router.

4.5. Mobile Routers

   A mobile node can be a router that is responsible for the mobility of
   one or more entire networks moving together, perhaps on an airplane,
   a ship, a train, an automobile, a bicycle, or a kayak.  The nodes
   connected to a network served by the mobile router may themselves be
   fixed nodes or mobile nodes or routers.  In this document, such
   networks are called "mobile networks".

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   A mobile router MAY act as a foreign agent and provide a foreign
   agent care-of address to mobile nodes connected to the mobile
   network.  Typical routing to a mobile node via a mobile router in
   this case is illustrated by the following example:

      a) A laptop computer is disconnected from its home network and
         later attached to a network port in the seat back of an
         aircraft.  The laptop computer uses Mobile IP to register on
         this foreign network, using a foreign agent care-of address
         discovered through an Agent Advertisement from the aircraft's
         foreign agent.

      b) The aircraft network is itself mobile.  Suppose the node
         serving as the foreign agent on the aircraft also serves as the
         default router that connects the aircraft network to the rest
         of the Internet.  When the aircraft is at home, this router is
         attached to some fixed network at the airline's headquarters,
         which is the router's home network.  While the aircraft is in
         flight, this router registers from time to time over its radio
         link with a series of foreign agents below it on the ground.
         This router's home agent is a node on the fixed network at the
         airline's headquarters.

      c) Some correspondent node sends a datagram to the laptop
         computer, addressing the datagram to the laptop's home address.
         This datagram is initially routed to the laptop's home network.

      d) The laptop's home agent intercepts the datagram on the home
         network and tunnels it to the laptop's care-of address, which
         in this example is an address of the node serving as router and
         foreign agent on the aircraft.  Normal IP routing will route
         the datagram to the fixed network at the airline's
         headquarters.

      e) The aircraft router and foreign agent's home agent there
         intercepts the datagram and tunnels it to its current care-of
         address, which in this example is some foreign agent on the
         ground below the aircraft.  The original datagram from the
         correspondent node has now been encapsulated twice:  once by
         the laptop's home agent and again by the aircraft's home agent.

      f) The foreign agent on the ground decapsulates the datagram,
         yielding a datagram still encapsulated by the laptop's home
         agent, with a destination address of the laptop's care-of
         address.  The ground foreign agent sends the resulting datagram
         over its radio link to the aircraft.

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      g) The foreign agent on the aircraft decapsulates the datagram,
         yielding the original datagram from the correspondent node,
         with a destination address of the laptop's home address.  The
         aircraft foreign agent delivers the datagram over the aircraft
         network to the laptop's link-layer address.

   This example illustrated the case in which a mobile node is attached
   to a mobile network.  That is, the mobile node is mobile with respect
   to the network, which itself is also mobile (here with respect to the
   ground).  If, instead, the node is fixed with respect to the mobile
   network (the mobile network is the fixed node's home network), then
   either of two methods may be used to cause datagrams from
   correspondent nodes to be routed to the fixed node.

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

   Alternatively, the mobile router MAY advertise connectivity to the
   entire mobile network using normal IP routing protocols through a
   bi-directional tunnel to its own home agent.  This method avoids the
   need for nested tunneling of datagrams.

4.6. ARP, Proxy ARP, and Gratuitous ARP

   The use of ARP [36] requires special rules for correct operation when
   wireless or mobile nodes are involved.  The requirements specified in
   this section apply to all home networks in which ARP is used for
   address resolution.

   In addition to the normal use of ARP for resolving a target node's
   link-layer address from its IP address, this document distinguishes
   two special uses of ARP:

      -  A Proxy ARP [39] is an ARP Reply sent by one node on behalf of
         another node which is either unable or unwilling to answer its
         own ARP Requests.  The sender of a Proxy ARP reverses the
         Sender and Target Protocol Address fields as described in [36],
         but supplies some configured link-layer address (generally, its
         own) in the Sender Hardware Address field.  The node receiving
         the Reply will then associate this link-layer address with the
         IP address of the original target node, causing it to transmit
         future datagrams for this target node to the node with that
         link-layer address.

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      -  A Gratuitous ARP [45] is an ARP packet sent by a node in order
         to spontaneously cause other nodes to update an entry in their
         ARP cache.  A gratuitous ARP MAY use either an ARP Request or
         an ARP Reply packet.  In either case, the ARP Sender Protocol
         Address and ARP Target Protocol Address are both set to the IP
         address of the cache entry to be updated, and the ARP Sender
         Hardware Address is set to the link-layer address to which this
         cache entry should be updated.  When using an ARP Reply packet,
         the Target Hardware Address is also set to the link-layer
         address to which this cache entry should be updated (this field
         is not used in an ARP Request packet).

         In either case, for a gratuitous ARP, the ARP packet MUST be
         transmitted as a local broadcast packet on the local link.  As
         specified in [36], any node receiving any ARP packet (Request
         or Reply) MUST update its local ARP cache with the Sender
         Protocol and Hardware Addresses in the ARP packet, if the
         receiving node has an entry for that IP address already in its
         ARP cache.  This requirement in the ARP protocol applies even
         for ARP Request packets, and for ARP Reply packets that do not
         match any ARP Request transmitted by the receiving node [36].

   While a mobile node is registered on a foreign network, its home
   agent uses proxy ARP [39] to reply to ARP Requests it receives that
   seek the mobile node's link-layer address.  When receiving an ARP
   Request, the home agent MUST examine the target IP address of the
   Request, and if this IP address matches the home address of any
   mobile node for which it has a registered mobility binding, the home
   agent MUST transmit an ARP Reply on behalf of the mobile node.  After
   exchanging the sender and target addresses in the packet [39], the
   home agent MUST set the sender link-layer address in the packet to
   the link-layer address of its own interface over which the Reply will
   be sent.

   When a mobile node leaves its home network and registers a binding on
   a foreign network, its home agent uses gratuitous ARP to update the
   ARP caches of nodes on the home network.  This causes such nodes to
   associate the link-layer address of the home agent with the mobile
   node's home (IP) address.  When registering a binding for a mobile
   node for which the home agent previously had no binding (the mobile
   node was assumed to be at home), the home agent MUST transmit a
   gratuitous ARP on behalf of the mobile node.  This gratuitous ARP
   packet MUST be transmitted as a broadcast packet on the link on which
   the mobile node's home address is located.  Since broadcasts on the
   local link (such as Ethernet) are typically not guaranteed to be
   reliable, the gratuitous ARP packet SHOULD be retransmitted a small
   number of times to increase its reliability.

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   When a mobile node returns to its home network, the mobile node and
   its home agent use gratuitous ARP to cause all nodes on the mobile
   node's home network to update their ARP caches to once again
   associate the mobile node's own link-layer address with the mobile
   node's home (IP) address.  Before transmitting the (de)Registration
   Request message to its home agent, the mobile node MUST transmit this
   gratuitous ARP on its home network as a local broadcast on this link.
   The gratuitous ARP packet SHOULD be retransmitted a small number of
   times to increase its reliability, but these retransmissions SHOULD
   proceed in parallel with the transmission and processing of its
   (de)Registration Request.

   When the mobile node's home agent receives and accepts this
   (de)Registration Request, the home agent MUST also transmit a
   gratuitous ARP on the mobile node's home network.  This gratuitous
   ARP also is used to associate the mobile node's home address with the
   mobile node's own link-layer address.  A gratuitous ARP is
   transmitted by both the mobile node and its home agent, since in the
   case of wireless network interfaces, the area within transmission
   range of the mobile node will likely differ from that within range of
   its home agent.  The ARP packet from the home agent MUST be
   transmitted as a local broadcast on the mobile node's home link, and
   SHOULD be retransmitted a small number of times to increase its
   reliability; these retransmissions, however, SHOULD proceed in
   parallel with the transmission and processing of its (de)Registration
   Reply.

   While the mobile node is away from home, it MUST NOT transmit any
   broadcast ARP Request or ARP Reply messages.  Finally, while the
   mobile node is away from home, it MUST NOT reply to ARP Requests in
   which the target IP address is its own home address, unless the ARP
   Request is unicast by a foreign agent with which the mobile node is
   attempting to register or a foreign agent with which the mobile node
   has an unexpired registration.  In the latter case, the mobile node
   MUST use a unicast ARP Reply to respond to the foreign agent.  Note
   that if the mobile node is using a co-located care-of address and
   receives an ARP Request in which the target IP address is this care-
   of address, then the mobile node SHOULD reply to this ARP Request.
   Note also that, when transmitting a Registration Request on a foreign
   network, a mobile node may discover the link-layer address of a
   foreign agent by storing the address as it is received from the Agent
   Advertisement from that foreign agent, but not by transmitting a
   broadcast ARP Request message.

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   The specific order in which each of the above requirements for the
   use of ARP, proxy ARP, and gratuitous ARP are applied, relative to
   the transmission and processing of the mobile node's Registration
   Request and Registration Reply messages when leaving home or
   returning home, are important to the correct operation of the
   protocol.

   To summarize the above requirements, when a mobile node leaves its
   home network, the following steps, in this order, MUST be performed:

      -  The mobile node decides to register away from home, perhaps
         because it has received an Agent Advertisement from a foreign
         agent and has not recently received one from its home agent.

      -  Before transmitting the Registration Request, the mobile node
         disables its own future processing of any ARP Requests it may
         subsequently receive requesting the link-layer address
         corresponding to its home address, except insofar as necessary
         to communicate with foreign agents on visited networks.

      -  The mobile node transmits its Registration Request.

      -  When the mobile node's home agent receives and accepts the
         Registration Request, it performs a gratuitous ARP on behalf of
         the mobile node, and begins using proxy ARP to reply to ARP
         Requests that it receives requesting the mobile node's link-
         layer address.  In the gratuitous ARP, the ARP Sender Hardware
         Address is set to the link-layer address of the home agent.
         If, instead, the home agent rejects the Registration Request,
         no ARP processing (gratuitous nor proxy) is performed by the
         home agent.

   When a mobile node later returns to its home network, the following
   steps, in this order, MUST be performed:

      -  The mobile node decides to register at home, perhaps because it
         has received an Agent Advertisement from its home agent.

      -  Before transmitting the Registration Request, the mobile node
         re-enables its own future processing of any ARP Requests it may
         subsequently receive requesting its link-layer address.

      -  The mobile node performs a gratuitous ARP for itself.  In this
         gratuitous ARP, the ARP Sender Hardware Address is set to the
         link-layer address of the mobile node.

      -  The mobile node transmits its Registration Request.

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      -  When the mobile node's home agent receives and accepts the
         Registration Request, it stops using proxy ARP to reply to ARP
         Requests that it receives requesting the mobile node's link-
         layer address, and then performs a gratuitous ARP on behalf of
         the mobile node.  In this gratuitous ARP, the ARP Sender
         Hardware Address is set to the link-layer address of the mobile
         node.  If, instead, the home agent rejects the Registration
         Request, the home agent MUST NOT make any change to the way it
         performs ARP processing (gratuitous nor proxy) for the mobile
         node.  In this latter case, the home agent should operate as if
         the mobile node has not returned home, and continue to perform
         proxy ARP on behalf of the mobile node.



(page 73 continued on part 4)

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