rfc3344.txt

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   is also set to one.








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RFC 3344              IP Mobility Support for IPv4           August 2002


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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RFC 3344              IP Mobility Support for IPv4           August 2002


   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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RFC 3344              IP Mobility Support for IPv4           August 2002


   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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RFC 3344              IP Mobility Support for IPv4           August 2002


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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RFC 3344              IP Mobility Support for IPv4           August 2002


   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.







Perkins                     Standards Track