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RFC 3132 - Dormant Mode Host Alerting ("IP Paging") Problem Stat


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Network Working Group                                           J. Kempf
Request for Comments: 3132                              Sun Microsystems
Category: Informational                                        June 2001

       Dormant Mode Host Alerting ("IP Paging") Problem Statement

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This memo describes paging, assesses the need for IP paging, and
   presents a list of recommendations for Seamoby charter items
   regarding work on paging.  The results are specifically directed
   toward the task undertaken by the design team, and are not meant to
   be the definitive word on paging for all time, nor to be binding on
   Seamoby or other working groups, should the situation with regard to
   IP mobility protocols or radio link support undergo a major change.

1.0 Introduction

   The IESG has requested that the Seamoby Working Group develop a
   problem statement about the need for additional protocol work to
   support alerting of dormant mode mobile hosts, commonly known as IP
   paging, for seamless IP mobility.  The paging design team interpreted
   this as direction to examine whether location of a mobile node in
   power saving mode can be supported by the existing Mobile IPv4 and
   Mobile IPv6 protocols given existing radio link protocols.

   Many existing radio link protocols and mobile systems support
   location of and radio link establishment with mobile nodes that are
   in power saving mode and hence are not actively listening for
   delivery of IP packets all the time or are not listening on the radio
   channels normally associated with delivering IP traffic to mobile
   nodes.  This alerting functionality allows mobile nodes to reduce
   power consumption and decreases signaling load on the network for
   tracking mobiles that are not actively participating in IP packet
   generation or reception.

   When a mobile is in low power consumption mode, special steps need to
   be taken to locate the mobile and alert it.  These steps differ
   depending on the radio link, but the generic name for this process is
   paging, a term that is commonly used in cellular telephony.

   In this document, after some initial definitions and material related
   to more clearly explaining what paging is, we assess the need for
   paging in existing IP mobility protocols (namely Mobile IP [1] [2]).
   We then develop a list of work items for the Seamoby working group
   related to this need.  Note that the discussion in this document and
   the conclusions regarding work items are directed toward existing IP
   mobility protocols and existing radio link protocols.  Should a major
   change occur in radio link support or the available IP mobility
   protocols, such as the introduction of a micromobility protocol for
   IP, the issues examined in this document may need to be revisited.

2.0 Definitions

   The following definitions are relevant with respect to clarifying the
   paging functionality:

      Dormant Mode - A state in which the mobile restricts its ability
      to receive normal IP traffic by reducing monitoring of radio
      channels.  This allows the mobile to save power and reduces
      signaling load on the network.

      Time-slotted Dormant Mode - A dormant mode implementation in which
      the mobile alternates between periods of not listening for any
      radio traffic and listening for traffic.  Time-slotted dormant
      mode implementations are typically synchronized with the network
      so the network can deliver traffic to the mobile during listening
      periods.  Additionally, the mobile may be restricted to listening
      on specific signaling channels that, according to current
      practice, are not typically used to carry IP traffic.

      Paging - As a consequence of a mobile-bound packet destined for a
      mobile currently in dormant mode, signaling by the network through
      radio access points directed to locating the mobile and alerting
      it to establish a last hop connection.  This messaging is in
      addition to simply delivering the packet to the mobile, i.e., last
      hop routing of packets is NOT considered to be paging.

      Paging Area - Collection of radio access points that are signaled
      to locate a dormant mode mobile node.  A paging area does not
      necessarily correspond to an IP subnet.  A dormant mode mobile
      node may be required to signal to the network when it crosses a
      paging area boundary, in order that the network can maintain a
      rough idea of where the mobile is located.

      Paging Channel - A radio channel dedicated to signaling dormant
      mode mobiles for paging purposes.  By current practice, the
      protocol used on a paging channel is usually dictated by the radio
      link protocol, although some paging protocols have provision for
      carrying arbitrary traffic (and thus could potentially be used to
      carry IP).

      Traffic Channel - The radio channel on which IP traffic to an
      active mobile is typically sent.  This channel is used by a mobile
      that is actively sending and receiving IP traffic, and is not
      continuously active in a dormant mode mobile.  For some radio link
      protocols, this may be the only channel available.

      Paging Area Registrations - Signaling from a dormant mode mobile
      node to the network when the mobile node crosses a paging area
      boundary to establish the mobile node's presence in the new paging
      area.

3.0 Discussion of Paging

   Dormant mode is advantageous to a mobile node and the network for the
   following reasons:

      - Power savings.  By reducing the amount of time the mobile is
      required to listen to the radio interface, the drain on the mobile
      node's battery is reduced.

      - Reduced signaling for location tracking.  By requiring the
      mobile to only signal when it crosses a paging area boundary
      rather than when it switches between radio access points, the
      amount of signaling for tracking the mobile is reduced because
      paging areas typically contain many radio access points.

   In existing radio link protocols, there is a clear distinction
   between those protocols that support dormant mode only and those that
   support dormant mode with paging.  Radio link protocols that do not
   support paging have no paging areas, no dedicated paging channel, and
   no radio link protocol specifically directed towards locating a
   dormant mode mobile, while radio link protocols that do support
   paging have these features.  Although generalizations always run the
   risk of being contradicted by specific exceptions, the following
   comparison of existing radio link protocol support for these two
   cases may be instructive.

3.1 Dormant Mode Support Only

   In radio link protocols that only support dormant mode, a dormant
   mode mobile node typically operates in time slotted mode and there is
   only one radio channel available, namely the traffic channel.  The
   mobile node periodically wakes up, and, synchronously, the radio
   access point in the network with which the mobile node is associated
   delivers any IP packets that have arrived while the mobile node was
   asleep.  Radio access points are required to buffer incoming packets
   for dormant mode mobiles; exactly how many packets and how long they
   are buffered are implementation dependent.

   If the mobile node happens to move out of range of the access point
   with which it was associated, while it is in dormant mode, it
   discovers this when it awakens and reassociates with a new access
   point.  The new access point then contacts the old access point over
   the wired backbone, the old access point sends any buffered packets,
   and the new access point delivers them to the mobile.

   Radio link protocols with dormant mode support only are typically
   wireless LAN protocols in unlicensed spectrum in which the mobile
   node is not charged for using a traffic channel, and hence there is
   no need for conserving spectrum usage.

3.2 Dormant Mode with Paging Support

   In radio link protocols with support for paging, the radio link
   typically supports more than one channel.  A dormant mode mobile node
   may operate in time slotted mode, periodically waking up to listen to
   the paging channel, or it may simply listen to the paging channel
   continuously.  The important point is that the mobile does not listen
   to nor transmit on a traffic channel while in dormant mode.

   The radio access points are grouped into paging areas, and the radio
   link protocol supports periodic signaling between the mobile and the
   network only when the mobile crosses a paging area boundary, for the
   purpose of giving the network a rough idea of the mobile's location
   (paging area registrations).  Some deployments of paging do not even
   use paging area registrations.  They use heuristics to determine
   where the mobile is located when a packet arrives, in which case, no
   signaling is required while the mobile is in dormant mode.

   An incoming packet is directed to the paging area where the mobile
   last reported, or the paging area is determined by heuristics.  The
   network performs a radio link page by sending out a signal on the
   paging channel.  The signal may be repeated until the mobile answers
   or a timeout occurs.  In the former case, the packet is delivered, in
   the latter, the mobile is assumed to be unreachable.

   Radio link protocols with paging support tend to be in licensed
   spectrum where the network operator has an interest in reducing the
   amount of signaling over traffic channels.  Such reduction frees
   traffic channel spectrum for revenue-producing use, and avoids
   charging the customer for signaling overhead.

4.0 Is IP Paging Necessary?

   In this section, we consider whether IP paging support is necessary.
   We first consider radio link protocols that have no support for
   paging.  We then examine radio link protocols that have paging
   support.  As discussed in the introduction, the focus is on whether
   the existing IETF mobility protocol, namely Mobile IP, requires
   enhancement.  We also briefly discuss the relationship between paging
   and a potential future micromobility protocol.

4.1 IP Paging for Dormant Mode Only Radio Links

   One possible justification for IP paging is for radio links that do
   not support paging.  The reasoning is that an IP paging protocol
   could allow location of a dormant mode mobile in radio networks that
   do not support paging in the radio protocol.

   An important point to keep in mind when considering this possibility
   is that, for radio links that do support paging, paging is typically
   used to locate mobiles for which the network has a rough idea of
   where the mobile is located.  More specifically, in order to conserve
   signaling between the network and the mobile and to reduce power
   drain on the mobile, the mobile only updates the network about its
   location when it crosses a paging area boundary (if even then), which
   is far less frequent than when it crosses a radio access point
   boundary.  If IP paging is to be of any use to radio link protocols
   that do not support paging, it must also be the case that it allows
   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

   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

   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.

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

   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

   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.

4.2.2 Heterogeneous Technology Networks

   In a network composed of links with multiple technologies, the
   problems identified above become multiplied.  Using Mobile IP becomes
   even more cumbersome, because the subnet to which the initial packet
   is delivered, besides not being in the same subnet on which the
   dormant mode mobile is located, may be on a radio network which the
   user would actually not prefer to use in their current location.
   This could happen, for example, if the mobile moved inside a building
   and radio coverage on one interface became weak or nonexistent, or if
   the user had a choice of a cheaper or higher bandwidth connection.
   The mobile may actually no longer be listening or reachable on the
   paging channel of the old network, so when the old access router or
   foreign agent pages on the old radio network, the mobile, which is
   now listening only for pages on the new network, may not answer, even
   though it is reachable on the new network.  Arranging for pages in
   multiple radio networks is a possibility, but without an L3 paging
   protocol to abstract away from the L2 details, the details of each L2
   protocol must be handled separately.

   A paging protocol that unifies paging across multiple radio
   technologies therefore looks attractive.  There may be commonalities
   in the corresponding radio paging protocols that allow a mapping to
   be established between the radio protocols and an abstract IP paging
   protocol.  For example, assume we have a common paging area
   identifier defined at the IP layer that is mapped to each radio
   paging protocol by the access points.  An IP paging message
   containing the identifier is sent to multiple access points, where
   the appropriate radio paging message is sent based on the particular
   technology implemented by the access points.  The results are then
   returned by the radio paging responses, mapped back into IP by the
   access points, and delivered back to the origin of the page.

   An additional case to consider is when a single subnet consists of
   multiple radio access technologies.  A wireless access point usually
   provides L2 bridge behavior to the wired link with which it is
   connected.  If two access points with incompatible technologies and
   non-overlapping cells are connected to the same subnet, a mobile node
   with interfaces to both technologies would need paging from both
   technologies.  If reachability can be established simply by ARP or
   neighbor discovery, no IP paging is needed.  However, note that ARP
   or neighbor discovery requires that a functional traffic channel be
   available to the mobile, since these protocols are typically
   implemented for wired networks in which a single channel exists on
   which all IP traffic is delivered.  If the mobile is currently in the
   sleep phase of a time-slotted dormant mode, or if it is listening to
   a paging channel it will fail to respond to these requests.  In this
   case, some means of triggering a radio page from IP is necessary to
   find the mobile.  Modifying ARP or neighbor discovery to utilize a
   paging channel if available is a possible, if somewhat messy,
   alternative, but a dedicated location protocol may be somewhat
   cleaner.

4.3 Paging and Micromobility

   If the Seamoby Working Group decides that an IP micromobility
   protocol is necessary, then the above analysis is no longer complete.
   A micromobility protocol may require some type of paging support.
   The design team does not want to include any further discussion of
   paging and micromobility at this point, because it is not clear
   whether micromobility will be pursued by Seamoby and hence such
   discussion would be premature.

5.0 What Exactly is the Problem?

   While the above analysis has identified situations in which location
   of a mobile in dormant mode may require some action at the IP layer,
   it is important keep in mind what the problem is.  The problem to be
   solved is the location of a mobile node because it has moved while in
   dormant mode.  IP paging is one solution to the problem, there may be
   others.

6.0 Recommendations

   The design group recommends the following charter items for Seamboy:

      1) Since the design group has identified several network
         deployment scenarios where existing Mobile IP technology cannot
         find a mobile in dormant mode, protocol work is necessary to
         define a way for the network to find a mobile that is currently
         in dormant mode.

      2) The work defined above should be pursued in a way that is
         maximally consistent with Mobile IP and other existing IETF
         protocols.  The work should also generate recommendations about
         how to achieve the best match between existing radio paging
         protocols and IP.

      3) If the Seamoby working group decides to pursue a micromobility
         protocol that requires paging, the Seamoby group should
         undertake the design of a new paging protocol within the
         context of that work.

      4) There is some evidence that cellular operators' deployments of
         paging are highly variable, and may, in fact, be suboptimal in
         many cases with respect to supporting IP.  The Seamoby working
         group should write a BCP which explains how to perform IP
         subnet to paging area mapping and which techniques to use when,
         so network designers in wireless networks have a guide when
         they are setting up their networks.

7.0 Acknowledgements

         The editor would like to thank the Seamoby paging design team
         for helping formulate the first draft of the document.  Jari
         Malinen contributed text to Section 4.2. Hesham Soliman, Karim
         El-Malki, and Behcet Sarikaya contributed critical commentary
         on the first draft, which was important in sharpening the
         reasoning about what can and can't be expected in the absence
         of radio layer paging support and how Mobile IP might be used
         to support dormant mode location.

8.0 References

   [1]  Perkins, C., Editor, "IP Mobility Support", RFC 2002, October
        1996.

   [2]  Johnson, D., and C. Perkins, "Mobility Support in IPv6", Work in
        Progress.

   [3]  El Malki, K. et. al., "Low Latency Handoff in Mobile IPv4", Work
        in Progress.

   [4]  Tsirtsis, G., Editor, "Fast Handovers for Mobile IPv6", Work in
        Progress.

   [5]  Perkins, C. and D. Johnson, "Route Optimization in Mobile IP",
        Work in Progress.

   [6]  Holma, H. and A. Toskala, "WCDMA for UMTS: Radio Access for
        Third Generation Mobile Communication", John Wiley and Sons, New
        York, 2000.

9.0  Editor's Address

   James Kempf
   Sun Labs California
   Sun Microsystems, Inc.
   901 San Antonio Rd., UMPK15-214
   Palo Alto, CA, 94303
   USA

   Phone: +1 650 786 5890
   Fax:   +1 650 786 6445
   EMail: james.kempf@sun.com

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