rfc2002.txt

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        home network    |    /          routing delivers each to its
        via standard    |  |_           destination.  In this figure,
        IP routing.   +----+            the foreign agent is the
                      |host|            mobile node's default router.
                      +----+

1.8. Specification Language

   In this document, several words are used to signify the requirements
   of the specification.  These words are often capitalized.

      MUST       This word, or the adjective "required", means that
                 the definition is an absolute requirement of the
                 specification.

      MUST NOT   This phrase means that the definition is an absolute
                 prohibition of the specification.

      SHOULD     This word, or the adjective "recommended", means
                 that, in some circumstances, valid reasons may exist
                 to ignore this item, but the full implications must
                 be understood and carefully weighed before choosing
                 a different course.  Unexpected results may result
                 otherwise.

      MAY        This word, or the adjective "optional", means that this
                 item is one of an allowed set of alternatives.  An
                 implementation which does not include this option MUST
                 be prepared to interoperate with another implementation
                 which does include the option.









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      silently discard
                 The implementation discards the datagram without
                 further processing, and without indicating an error
                 to the sender.  The implementation SHOULD provide the
                 capability of logging the error, including the contents
                 of the discarded datagram, and SHOULD record the event
                 in a statistics counter.

1.9. Message Format and Protocol Extensibility

   Mobile IP defines a set of new control messages, sent with UDP [17]
   using well-known port number 434.  Currently, the following two
   message types are defined:

      1  Registration Request
      3  Registration Reply

   Up-to-date values for the message types for Mobile IP control
   messages are specified in the most recent "Assigned Numbers" [20].

   In addition, for Agent Discovery, Mobile IP makes use of the existing
   Router Advertisement and Router Solicitation messages defined for
   ICMP Router Discovery [4].

   Mobile IP defines a general Extension mechanism to allow optional
   information to be carried by Mobile IP control messages or by ICMP
   Router Discovery messages.  Each of these Extensions (with one
   exception) is encoded in the following Type-Length-Value format:

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
   |     Type      |    Length     |    Data ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

      Type     Indicates the particular type of Extension.

      Length   Indicates the length (in bytes) of the data field within
               this Extension.  The length does NOT include the Type and
               Length bytes.

      Data     The particular data associated with this Extension.  This
               field may be zero or more bytes in length.  The format
               and length of the data field is determined by the type
               and length fields.






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   Extensions allow variable amounts of information to be carried within
   each datagram.  The end of the list of Extensions is indicated by the
   total length of the IP datagram.

   Two separately maintained sets of numbering spaces, from which
   Extension Type values are allocated, are used in Mobile IP:

    -  The first set consists of those Extensions which may appear only
       in Mobile IP control messages (those sent to and from UDP port
       number 434).  Currently, the following Types are defined for
       Extensions appearing in Mobile IP control messages:

          32  Mobile-Home Authentication
          33  Mobile-Foreign Authentication
          34  Foreign-Home Authentication

    -  The second set consists of those extensions which may appear only
       in ICMP Router Discovery messages [4].  Currently, Mobile IP
       defines the following Types for Extensions appearing in ICMP
       Router Discovery messages:

           0  One-byte Padding (encoded with no Length nor Data field)
          16  Mobility Agent Advertisement
          19  Prefix-Lengths

   Each individual Extension is described in detail in a separate
   section later in this document.  Up-to-date values for these
   Extension Type numbers are specified in the most recent "Assigned
   Numbers" [20].

   Due to the separation (orthogonality) of these sets, it is
   conceivable that two Extensions that are defined at a later date
   could have identical Type values, so long as one of the Extensions
   may be used only in Mobile IP control messages and the other may be
   used only in ICMP Router Discovery messages.

   When an Extension numbered in either of these sets within the range 0
   through 127 is encountered but not recognized, the message containing
   that Extension MUST be silently discarded.  When an Extension
   numbered in the range 128 through 255 is encountered which is not
   recognized, that particular Extension is ignored, but the rest of the
   Extensions and message data MUST still be processed.  The Length
   field of the Extension is used to skip the Data field in searching
   for the next Extension.







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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 [4] 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 [1], 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.

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:




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    -  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 [4], the IP
                   destination address of an Agent Advertisement MUST
                   be either the "all systems on this link" multicast
                   address (224.0.0.1) [5] 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.

    -  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.






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          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 1/3 of the advertisement Lifetime
   given in the ICMP header.  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 [4] 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.

2.1.1. Mobility Agent Advertisement Extension