rfc3132.txt

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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.





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RFC 3132      Dormant Mode Host Alerting Problem Statement     June 2001


   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.



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RFC 3132      Dormant Mode Host Alerting Problem Statement     June 2001


      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.








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RFC 3132      Dormant Mode Host Alerting Problem Statement     June 2001


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.



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RFC 3132      Dormant Mode Host Alerting Problem Statement     June 2001


   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