Internet DRAFT - draft-seamoby-paging-problem-statement
draft-seamoby-paging-problem-statement
INTERNET DRAFT James Kempf, Editor
Category: Informational Sun Microsystems
Title: draft-seamoby-paging-problem-statement-01.txt
Date: Feburary 2001
Paging Problem Statement
Status of this Memo
This document is a working group contribution for the Seamoby Working
Group.
Distribution of this memo is unlimited.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
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Copyright (C) The Internet Society 2001. All Rights Reserved.
Abstract
The IESG has requested that the Seamoby Working Group develop a
problem statement about the need for additional protocol work to
support 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.
This draft 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
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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
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 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. These steps differ depending on the
radio link, but the generic name for this process is paging.
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 relevent 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
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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
establishing 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.
Regional 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
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typically contain many radio access points.
- Reduced router state. By removing the need for routers to keep
the mobile node's binding in their binding caches, the amount of
state in routers is reduced, because the number of mobile nodes in
dormant mode may be considerably more than those that are active.
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
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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
(regional registrations). Some deployments of paging do not even use
regional 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 signalling 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
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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 consider 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.
Regional registrations or heuristics are used to establish a dormant
mode mobile's presence in a particular paging area.
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Given these two units of presence at radio and IP layers, the most
important point with regard to whether or not paging is needed in
homogeneous networks is how subnets are mapped into paging areas. To
determine whether additional IP paging protocol work is required, we
first need to determine whether we can support location of and radio
link establishment with a mobile in dormant mode given some mapping
between subnets and paging areas using Mobile IP.
Standard Mobile IP requires the mobile node to actively listen for or
solicit foreign agent or router advertisements and participate in
registering, so it is of little use if the mobile is in dormant mode.
However, network assisted handoff techniques used for fast handoff
[3] [4] allow the network to track the mobile without requiring any
signaling from the mobile. The radio signaling that occurs when a
dormant mobile crosses paging area boundaries (provided the operator
has configured regional registrations) can be used as the "L2
trigger" for network assisted handoff techniques to allow the network
to track the mobile in dormant mode, with perhaps some enhancements
to optimize the ability of the network to update the home agent
and/or any hierarchical agents about the mobile's location.
We need to examine paging area to subnet mappings in order to
determine when we can utilize the radio paging area update trigger to
trigger network assisted handoff in dormant mode. In general, the
mapping between paging areas and subnets can be arbitrary, but we
consider here 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. If it turns out that
work is needed, then more complex topologies can be investigated in
the course of actually doing the work.
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.
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 network
assisted handoff techniques for the network to track the mobile's
location. Enhancements to network-assisted handoff techniques can
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allow the network to track the mobile as it moves from paging area
(== subnet) to paging area. New Mobile IP signaling for the mobile
node to inform the network when it is going into dormant mode could
possibly optimize this. With network tracking, if a packet for the
dormant mobile comes in, it is routed by the hierarchical agent or
home agent to the last hop foreign agent or access router at the
mobile's last known position, where the radio page is performed. Once
the L2/L3 address mapping is known, the packet is delivered.
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.
In the case where a single radio paging area maps onto multiple IP
subnets, it is not possible to directly use enhanced network assisted
handoff techniques 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. In this case, some support is required at the IP level for
locating a dormant mode mobile that has moved into a different
subnet.
4.2.2 Heterogeneous Technology Networks
In a network composed of links with multiple technologies, there may
be commonalities in the corresponding radio paging protocols that
would 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.
Note, however, that the same analysis as applied to homogeneous
networks regarding the mapping between paging areas and subnets
applies here as well. For example, if there is a one-to-one mapping
between radio paging areas and subnets, then there can be only a
single technology per subnet, and movement across paging area
boundaries by a dual technology mobile in dormant mode on both
interfaces can be tracked through mobile IP network assisted handoff.
Similarly, if more than one paging area exists per subnet, even if
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the areas are from multiple technologies, the foreign agent or access
router can page in the multiple areas using the radio paging protocol
and a dual mode mobile can be tracked as it crosses paging area
boundaries. But if multiple subnets are contained within a paging
area or for more complex topologies, the lack of clear identification
between subnet boundaries at the IP layer and paging area boundaries
at radio layer requires some means of locating the mobile at the IP
layer.
An additional case to consider is when a single subnet consists of
multiple access tchnologies. 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 for intra-subnet mobility, 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.
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6.0 Recommendations
The design group recommends the following charter items for Seamboy:
1) Since the design group has identified at least one network
deployment scenerio 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 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.
8.0 References
[1] C. Perkins, editor. "IP Mobility Support", RFC 2002, October, 1966.
[2] Johnson, D., and C. Perkins, "Mobility Support in IPv6", draft-
ietf-mobileip-ipv6-13.txt, a work in progress.
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[3] Calhoun, P., et. al., "Foreign Agent Assisted Hand-off", draft-
calhoun-mobileip-proactive-fa-03.txt, a work in progress.
[4] Tsirtsis, G., Editor, "Fast Handovers for Mobile IPv6", draft-
designteam-fast-mipv6-01.txt, a work in progress.
9.0 Editor's Address
Questions about this memo can be directed to:
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
E-Mail: james.kempf@sun.com
10.0 Full Copyright Statement
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