rfc3132.txt

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   the network to maintain a rough idea of where the mobile is,
   otherwise, the amount of signaling involved in tracking the mobile
   and power drain on the mobile is not reduced.

   However, as the description in the previous section indicates, for
   radio links without paging support, the network always has an *exact*
   idea of where the mobile is located.  When the mobile moves into
   range of a new radio access point, it re-registers with the access
   point in that cell allowing the new access point to contact the old
   and deliver any buffered traffic.  Additionally, the new access point
   at that time may choose to deliver a foreign agent advertisement (for
   Mobile IPv4) or router advertisement (for Mobile IPv6) to the mobile
   if the mobile node has changed subnets, so that the mobile can
   perform Mobile IP re-registration in order to make sure its IP
   routing is current.  There is absolutely no ambiguity in the mobile's



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   location as far as the network is concerned, and so the network can
   continue to route packets to the mobile node while the mobile is in
   dormant mode with assurance (modulo buffer overflows and timeouts at
   the radio access point) that the packets will be delivered to the
   mobile the next time it wakes up from dormant mode.

   As a consequence, IP paging provides no advantages for radio link
   protocols in which the radio link does not have support for paging.

4.2 IP Paging for Radio Links with Paging Support

   In radio links that do support paging, there are two cases to
   consider: networks of radio links having a homogeneous radio
   technology and networks of radio links having heterogeneous radio
   technologies.  We examine whether Mobile IP can support dormant mode
   location for both these cases.

4.2.1 Homogeneous Technology Networks

   For homogeneous technology networks, the primary issue is whether
   signaling involved in Mobile IP is enough to provide support for
   locating dormant mode mobile nodes.  Subnets constitute the unit of
   signaling for presence in IP.  When a mobile node moves from one
   subnet to another, Mobile IP signaling is required to change the
   mobile's care-of address.  This signaling establishes the mobile's
   presence in the new subnet.  Paging areas constitute the unit of
   signaling for dormant mode mobile presence at the radio level.
   Paging area registrations or heuristics are used to establish a
   dormant mode mobile's presence in a particular paging area.

   If paging area registrations can always serve to trigger Mobile IP
   registrations, there is no need for an IP paging protocol because the
   network (specifically the home or hierarchical agent) will always
   have an up-to-date picture of where the mobile is and can always
   route packets to the mobile.  The key determining factor with regard
   to whether paging area registrations can be used in this fashion is
   how subnets are mapped into paging areas.  If it is always possible
   to map the two such that a paging area registration can serve as a
   transport for a Mobile IP registration, or some other technique (such
   as network assisted handoff [3] [4]) can be used to transfer the
   Mobile IP registration, then no IP paging protocol is needed.

   In general, the mapping between paging areas and subnets can be
   arbitrary, but we consider initially a smooth subset relationship, in
   which paging areas are subsets of subnets or vice versa.  Network
   topologies in which one subnet is split between two or more paging
   areas are therefore eliminated.  The restriction is arbitrary, but by




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   starting here, we can discover whether additional work is needed.  We
   also consider a case where paging area registrations in the radio
   layer protocol are always done.  This is also optimistic.

   There are three cases:

      1) The topological boundaries of the paging area and subnet are
         identical.

      2) Multiple paging areas are part of the same subnet.

      3) Multiple subnets are part of the same paging area.

      Each case is considered in the following subsections.

4.2.1.1 Subnet and Paging Area Boundaries Identical

   In the case where radio paging areas map one to one onto IP subnets
   (and hence Mobile IPv4 foreign agents or IPv6 access routers), it is
   possible to use radio link paging together with Mobile IP handoff
   techniques for the network to track the mobile's location.  If the
   paging area update protocol supports sending arbitrary packet data
   over the paging channel, the access router or foreign agent can send
   a router advertisement or foreign agent advertisement to the mobile
   as part of the signal that the mobile has entered the new paging
   area, and the mobile can send a Mobile IP registration as part of the
   paging area update.  For other cases, enhancements to Mobile IP
   network-assisted handoff techniques can allow the network to track
   the mobile as it moves from paging area (== subnet) to paging area.
   Other uses of the Mobile IP registration protocol are also possible
   depending on the level of paging support for packet data.  As a
   consequence, the home or hierarchical agent has complete knowledge of
   routes to the mobile and can route packets to the foreign agent or
   access router.  Radio layer paging may be needed at the foreign agent
   or access router in order to re-establish a traffic channel with the
   mobile, but no IP paging is required.

4.2.1.2 Multiple Paging Areas Map into One Subnet

   The case where multiple radio paging areas map to a single IP subnet
   is the same as above, with the exception that the last hop Mobile
   IPv4 foreign agent or IPv6 access router for the subnet performs
   paging in multiple paging areas to locate the mobile.








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4.2.1.3 Multiple Subnets Map into One Paging Area

   In the case where a single radio paging area maps onto multiple IP
   subnets, it is not possible to directly use Mobile IP handoff between
   last hop access routers or foreign agents to track the mobile's
   location as it moves, because the mobile does not signal its location
   when it changes subnets.  Within the set of subnets that span the
   paging area, the mobile's movement is invisible to the L2 paging
   system, so a packet delivered to the mobile's last known location may
   result in a page that is answered in a different subnet.

   Consider the following example.  Suppose we have a network in which
   there are two paging areas, PA(1) and PA(2).  Within each, there are
   many subnets.  Consider a mobile that moves from PA(1) to PA(2), and
   enters PA(2) at subnet X.  Using the paging area registration, it
   signals the network that it has moved, and suppose that the paging
   area registration contains a Mobile IP registration.  The agent
   handling the L2 paging protocol sends the registration to the
   home/hierarchical agent (or perhaps it simply gets routed).  The
   home/hierarchical agent now knows that the mobile has a CoA in subnet
   X, as does the mobile.  After the mobile has completed the paging
   area registration/Mobile IP registration, it goes back to sleep.

   But the mobile does not stop in subnet X, it keeps moving while in
   dormant mode, when it is doing no signaling (L2, mobile IP or other)
   to the network.  It moves from subnet X where it originally entered
   the paging area clear to the other side of the paging area, in a
   completely different subnet, subnet Y.

   Suppose a packet comes into the home/hierarchical agent for this
   mobile.  Because the home/hierarchical agent believes the mobile is
   in subnet X, it sends the packet to the access router or foreign
   agent for subnet X.  The packet gets to the access router or foreign
   agent, and the access router or foreign agent performs a radio page
   for the mobile in subnet X.  Since the mobile isn't in subnet X, it
   wakes up in subnet Y because the radio page propagates throughout the
   paging area.  It does a mobile IP re-registration because it sees
   that it is in a new subnet, but the packet at the access router or
   foreign agent in subnet X can't get to the mobile.

   Without any further support, the access router or foreign agent in
   subnet X drops the packet.  The only way to get the packet to the
   mobile node from the access router or foreign agent is for the mobile
   node to send a binding update to the access router or foreign agent
   when it wakes up in the new subnet.  Once the access router or
   foreign agent has the new binding, it can forward the packet.  Some
   smooth handoff techniques depend on sending binding updates to
   foreign agents [5], so arranging for the mobile node to send a



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   binding update would be possible.  In IPv6, it becomes less
   attractive because of the need for security on the binding update.
   In either case, the result would be yet more Mobile IP signaling
   before the packet could be delivered, increasing the amount of
   latency experienced by the mobile.

   While it may be possible with enhancements to Mobile IP to handle the
   case, the enhancements would probably introduce more latency and
   signaling into the initial connection between the mobile and the
   network when the mobile awakes from dormant mode.  An IP paging
   protocol between the home or hierarchical agent and a paging agent in
   the paging area would serve to reduce the amount of latency involved
   in delivering the initial packet.  With IP paging, the arrival of the
   packet at the home/hierarchical agent results in an IP page to a
   paging agent in the last reported paging area.  The paging agent
   performs an L2 page to the mobile.  The mobile answers the page with
   a mobile IP registration to the home/hierarchical agent and the
   home/hierarchical agent sends the packet.  The home/hierarchical
   agent and the mobile already have a security association, so there is
   no need to negotiate one, and buffering of the first packet and any
   further incoming packets prior to the mobile IP registration is
   handled by the home/hierarchical agent rather than a router at the
   edge, so the edge routers can be simpler.  Finally, the
   home/hierarchical agent can start routing to the mobile as soon as
   the registration comes in.

4.1.2.4 More Complex Homogeneous Network Cases

   Up until now, the discussion has not identified any case where the
   problem of locating and delivering the first packet to a dormant mode
   mobile could not be handled by Mobile IP with enhancements.  IP
   paging serves as a promising optimization in the multiple subnets to
   single paging area case, but in principle additional Mobile IP
   signaling (potentially lots in the case of IPv6 if a security
   association is needed) could handle the problem.  However, the
   examples examined in the above sections are really best-case.  In
   practice, the mapping of subnets to paging areas is likely to be far
   less clear cut, and the use of paging area registrations far less
   common than has been assumed in these cases.

   Requiring network operators to make paging areas and subnets conform
   to a subset relationship that would allow mobile IP signaling to do
   double duty as paging area updates is unrealistic.  In practice,
   paging areas often overlap and there is often not even a clear subset
   relationship between paging areas themselves.  Some radio protocols,
   such as wCDMA [6], allow different mobile terminals in the same
   geographical area to have different paging area identifiers.  Working
   through each case and trying to identify whether Mobile IP needs



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   enhancement would probably result in a much more complex result than
   having a simple IP paging protocol that allows a home/hierarchical
   agent to notify an L2 agent in the paging area when a new packet
   comes in.

   Finally, requiring operators to always turn on paging area
   registrations is unacceptable, and using Mobile IP registrations
   won't work if paging area registrations are not done.  The above
   description is ideal with regard to signaling between the mobile node
   in dormant mode and the network.  Anecdotal evidence indicates that
   most operators do not turn on paging area registrations, they use
   heuristics to determine where to page for the mobile.  If the
   operator does not turn on paging area registrations, there is no way
   for the mobile to report its position when it changes paging area,
   hence no L2 vehicle for potential dormant mode use of Mobile IP.