rfc3344.txt

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   Since the length field is 16 bits wide, a the extension data can
   exceed 256 bytes in length.

1.11.  Short Extension Format

   This format is compatible with the skippable extensions defined in
   section 1.9.  It is not applicable for extensions which require more
   than 256 bytes of data; for such extensions, use the format described
   in section 1.10.

     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      |    Sub-Type   |    Data ....
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Short Extension format requires that the following fields be
   specified as the first fields of the extension:



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      Type     is the type, which describes a collection of extensions
               having a common data type.

      Sub-Type is a unique number given to each member in the aggregated
               type.

      Length   8-bit unsigned integer.  Length of the extension, in
               bytes, excluding the extension Type and the extension
               Length fields.  This field MUST be set to 1 plus the
               total length of the data field.

      Data     is the data associated with this extension.  This
               specification does not place any additional structure on
               the subtype data.

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