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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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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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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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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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
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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.
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.
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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.
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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.
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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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