One document matched: draft-ietf-mobileip-paging-hawaii-00.txt
Paging support for IP mobility using HAWAII
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document defines extensions to the HAWAII IP micro-mobility
protocol to enable paging. Paging facilitates efficient power
management at the mobile host by allowing the host to update the
network less frequently at the cost of providing the network with
only approximate location information. The protocol extensions
described here provide a means for the network to determine the exact
location of a mobile host before delivering packets destined to the
mobile host.
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Contents
1 Introduction 3
1.1 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Protocol Overview . . . . . . . . . . . . . . . . . . . . . 6
1.4.1 State Synchronization . . . . . . . . . . . . . . . . 8
1.4.2 Application of IP Multicasting Protocol . . . . . . . 9
1.4.3 Distributed Paging . . . . . . . . . . . . . . . . . . 9
1.4.4 Soft-State . . . . . . . . . . . . . . . . . . . . . . 10
1.4.5 Stale Paging Entry . . . . . . . . . . . . . . . . . . 10
1.5 Protocol Correctness . . . . . . . . . . . . . . . . . . . . 11
2 Detailed Protocol Operation 11
2.1 Message Formats . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Mobile Host Processing . . . . . . . . . . . . . . . . . . . 14
2.3 Base Station/Router Processing . . . . . . . . . . . . . . . 15
3 Design Implications 17
3.1 Scalability . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2 Ease of Network Management . . . . . . . . . . . . . . . . . 18
3.3 Reliability . . . . . . . . . . . . . . . . . . . . . . .. . 18
4 Paging with Mobile-IP 19
5 Security 19
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1 Introduction
Mobile-IP is the current standard for supporting macro-mobility in IP
networks [4]. Recently there have been several proposals such as
Cellular IP [6] and HAWAII [5] for supporting micro-mobility. While
these solutions enable support for high mobility users in wide-area
wireless networks, they assume that the mobile host updates the
network on every handoff. This enables the network to know the exact
location of the host, i.e., the current base station for delivering
packets to the mobile host.
On the other hand, current wide area wireless data solutions such as
General Packet Radio Service (GPRS) [1] allow the mobile host to
operate in two distinct states - an active state where the network
knows the location of the mobile host's current base station, and a
standby state where the network knows only the approximate location
of the user, such as a set of base stations on which the mobile host
resides. One of the motivations for defining the standby state is
for reducing the host's battery power consumption by allowing the
mobile host to only notify the network when it moves out of a set of
base stations. If data packets for a mobile host in standby state
arrive into the wireless access network, the network "pages" the
mobile host in this set of base stations to determine the mobile
host's current base station before delivering the data packets. In
the GPRS network, this paging functionality is performed in a
centralized fashion by a Serving GPRS Service Node (SGSN) and can be
considered as a link layer function.
We envision the next generation wireless access network as a pure
IP-based network, where base stations will be IP addressable
entities. We believe mobility, as well as the paging functionality,
should be handled at the network (IP) layer. This enables the
deployment of a homogeneous, IP-based wireless access network that is
independent of the different wireless interfaces. Wireless link
specific processing is relegated only to the base stations. Thus, we
propose extensions to the IP layer software running in routers/base
stations in the access network to support paging.
Note that HAWAII [5] is a domain-based approach for supporting
mobility that maintains the mobile host's IP address unchanged across
mobility within the domain. Since a typical HAWAII domain will cover
one or more paging areas, extending HAWAII to implement paging seems
a logical choice. HAWAII uses specialized path setup schemes which
install host-based forwarding entries in specific routers to support
intra-domain micro-mobility. In this framework, adding paging
functionality to HAWAII involves augmenting the HAWAII forwarding
functionality with paging. Thus, we extend the HAWAII protocol with
paging functionality.
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1.1 Goals
We have the following design goals:
o Achieve efficient power consumption at the mobile host by
limiting the location update traffic and using paging to locate
the mobile host when necessary.
o Perform paging in a scalable fashion. This involves pushing the
paging functionality closer to the base station.
o Perform paging in a distributed fashion. This involves being
able to page from any base station/router in the access network.
This eliminates single points of failure for enhanced
reliability.
o Support for different types of paging areas such as fixed,
hierarchical, and user-defined paging areas.
1.2 Assumptions
We assume that HAWAII operates as the micro-mobility protocol in the
access portion of the wireless network. We propose extensions to
HAWAII to support paging functionality. In Section 4, we discuss how
paging functionality can be applied to a basic Mobile-IP network,
albeit without some of the scalability and reliability advantages
that paging with HAWAII provides.
We also assume that there is link-level paging support on the
wireless link. This entails that a mobile host is able to detect
paging requests and identify its current paging area. There are
several ways in which this may be implemented. A typical solution,
used in current cellular networks, is to have the base stations send
paging requests on separate paging channels and send beacons with
base station and paging area identities periodically on a broadcast
channel. A mobile client monitoring these paging and broadcast
channels can then detect paging requests and changes in paging area.
Another solution is to let the mobile host query the base station by
sending link layer point to point messages.
Note that the paging functionality proposed in this draft is
necessary only if updating the network on every handoff of the mobile
host is expensive, for example, in terms of signaling load or battery
power consumption; mobile devices for which this is not an issue or
for devices that use wireless link protocols such as WaveLAN which
have no link-level paging support can simply utilize the basic HAWAII
proposal without the paging extension described in this draft.
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1.3 Terminology
Domain
A division of the wireless access network. It consists of one or
more routers and multiple base stations. It will appear as a
subnet to routers external to the domain.
Domain Root Router
The gateway router into a domain is called the domain root router.
Active State
In active state, the mobile host updates the network on every
handoff. Thus, the network tracks the current base station of the
mobile host.
Standby State
In standby state, the mobile host reduces battery power
consumption by listening to only selective broadcast channels.
Furthermore, the mobile host updates the network of its location
only when it crosses a set of base stations, known as the paging
area.
Paging Area
The granularity to which the mobile user is tracked in standby
state. It consists of a set of base stations, typically defined
by a network administrator. In this draft, we identify these base
stations by a IP Multicast Group Address (MGA).
Routing Entry
A routing entry in the base stations and routers in the domain
specifies where to forward a packet for a given mobile host. It
is established by the HAWAII protocol. A routing entry for a
mobile host is present in selected routers/base stations when the
mobile host is in active state.
Paging Entry
A paging entry in the base stations and routers in the domain
specifies which set of base stations to page for a given mobile
host. It is established by the HAWAII protocol. A paging entry
for a mobile host is present in selected routers/base stations
when the mobile host is in active or standby state.
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1.4 Protocol Overview
In this section, we present the protocol design for the paging
functionality within the HAWAII framework. We believe that mobile
devices would mostly operate in standby state, with brief periods in
active state. In the standby state, the mobile host conserves
significant battery power. The mobile host can switch to active
state immediately after receiving notification from the network that
data packets are destined for it.
Like today's cellular networks, it is not desirable for the mobile
host to update its location every time the mobile host moves to a
different base station. Therefore the network is not able to
maintain exact location information for the mobile host and instead
maintains only approximate location information. Location
information can be maintained in one place in the network such as the
domain root router (DRR). However, such a centralized approach
introduces a single point of failure and results in scalability
concerns. Therefore we favor a distributed approach. On the other
hand, maintaining paging information in every element in the access
network is also not desirable. This would require flooding the
location information to the entire HAWAII domain, which wastes
bandwidth and processing resources. Thus, our solution endeavors to
maintain the location information for a given mobile host only in
selective routers and base stations.
The approximate location information can be represented by a set of
base stations called the Paging Area (PA). We do not assume any
specific way of defining PAs. Our goal is to support fixed,
hierarchical, and even personalized PAs. In order to efficiently
search the PA for the mobile host, we exploit the use of IP multicast
routing protocol for distributing paging request to a set of base
stations.
In order to manage router and link failures gracefully, we use
soft-state mechanisms for maintaining paging state. We now
illustrate the basic mechanism for paging using a simple tree-based
topology for the case when packets arrive at the domain root router
from some external host for a mobile host that is in standby state.
Selected routing and paging entries denoted by the letters R and P
are shown adjacent to the routers in Figure 1. These entries are
prepended with a message number label indicating which message was
responsible for establishing the entry. The letters A,B, and C
denote the different interfaces. Let us assume that the mobile host
powered up at the old base station and established routing and paging
entries as denoted by label (0). Also, let the paging area be
configured to consist of the two base stations, OLD BS and NEW BS,
assigned to a multicast group address (MGA) of 239.0.0.1. The
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multicast routing protocols would build a multicast tree with the OLD
BS and NEW BS as the leafs and Router 1 as a node in the tree (the
corresponding multicast routing entries are shown with a suffix M in
the figure). At this time, packets arriving for the mobile host at
router 0 will get delivered to the mobile host through the OLD BS
using the routing entries just established.
| @
(0)R,P:1.1.1.1->B,239.0.0.1 | @ 2
(1)P :1.1.1.1->B,239.0.0.1 ------v--
| A |
| |
| B |
--------- ROUTER 0
| @
| @ 2
| @
------v-- ROUTER 1
(0)M :239.0.0.1->B,C | A |
(0)R,P:1.1.1.1->B,239.0.0.1 | | (5)R,P:1.1.1.1->C,239.0.0.1
(1)P :1.1.1.1->B,239.0.0.1 | B C |
---------<$$$$$$$
* / @ \ * $
3 * / @ \ * 3 $ 5
OLD BS * / 6 @ \ * $ NEW BS
--v-- @ --v--
(0)M:239.0.0.1->A / A \ @ / A \ (0)M:239.0.0.1->A
(0)R,P:1.1.1.1->B, | | @ | | (0)R:Default->A
239.0.0.1 \ B / @ \ B / (4)R,P:1.1.1.1->B,
(1)P :1.1.1.1->B, ----- @ $$>----- 239.0.0.1
239.0.0.1 * @ $ 4 *
3 * * * @ $ * * * 3
* * * * * @ $ * * * * *
-v--
M:Multicast entry MOBILE / \ @: data packets
R:Routing entry HOST \ / *: page request
P:Paging entry ---- $: page response
IP:1.1.1.1
Figure 1: Illustration of paging in HAWAII
Now, let label (1) denote the timeout event where the mobile host and
the routers in the network enter the standby state because of lack of
refreshes from the mobile host. At this time, the routing entries
for the mobile host are timed out in the routers and the base
stations and only the paging entries remain. Furthermore, let the
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mobile host now move to NEW BS. Note that the network is unaware of
this movement since the host is in standby state.
Consider data packets arriving for the mobile host at router 0 (label
2). Router 0 would look up its HAWAII entries and find that a paging
entry exists for the host. Since the router does not have any
entries for MGA 239.0.0.1, it simply forwards the packet along
interface B to Router 1. Router 1 looks up the paging entry for the
mobile host and finds that the MGA 239.0.01 multicast routing entry
exists and has two interfaces associated with it. Thus, router 1
buffers the data packets for the mobile host and initiates a paging
request (label 3).
The mobile host responds to the paging request to the new base
station (label 4) which adds routing and paging entries for the host
and sends a paging response to the initiator of paging request,
Router 1. On receiving message 5, Router 1 updates its routing and
paging entries for the host. It then forwards the buffered data
packets to the mobile host (label 6).
Note that Router 0 would only be updated later by a paging refresh
message from Router 1 (until then it will continue forwarding packets
for the mobile host correctly to Router 0 since it is not part of the
multicast tree for MGA 239.0.0.1).
1.4.1 State Synchronization
As mentioned earlier, the mobile host operates in two states, active
and standby. This can be modeled by a state machine with three
states: active, standby and null, with null representing a powered
off mobile host. Base stations and routers in the access network
need to implement an analogous state machine so that the mobile host
is paged in standby state and packets are delivered directly to the
mobile host in active state. The state of the network must reflect
the state of the mobile host. If the mobile host is in standby
state, the state of the mobile host in the network also needs to be
in standby state so that paging can be initiated; otherwise, if the
mobile host's state in the base station/routers is in active state,
the mobile host will not be paged, which may result in packets being
misrouted to the wrong base station. Note that, even if the network
is in standby state with respect to a mobile host that is in active
state, packets will still get delivered correctly; however, this
would result in an unnecessary page.
Thus, we would like the network to go into standby state for the
mobile host exactly when, or just before, the mobile host goes into
standby state. Similarly, it is preferable for the network to go
into null state only after the mobile host goes into null state.
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Although this synchronization is not tight, it guarantees that the
mobile host will be reachable as long as it is powered up.
Table 1: Router operation
-----------------------------------------------------------------
Routing Paging State Operation
entry entry
-----------------------------------------------------------------
Yes Yes Active Regular forwarding
Yes No Active No paging support (basic HAWAII)
No Yes Standby Paging processing (details: see Figure 5)
No No Null Forward if default route exists, else drop
-----------------------------------------------------------------
We distinguish two types of entries in the network components such as
base stations/routers for maintaining the mobile host's state
machine: a routing entry and a paging entry. The operation of the
router or base station with respect to these entries is shown in
Table 1.
1.4.2 Application of IP Multicasting Protocol
We need to maintain the current PA for each mobile host and
distribute paging requests to the base stations in the PA. Instead of
unicasting the paging request to the set of base stations in the PA,
IP multicast is used to distribute the paging request. Thus, each PA
in a given domain is configured with a multicast group address and
each base station in a given PA joins that multicast group. Since
the multicast group is within the HAWAII domain, we use the range of
addresses that are allocated for administratively scoped IP
Multicast [2].
Fixed or hierarchical PAs can be statically configured with different
multicast group addresses. In order to support user-defined paging
areas, base stations may have to join multicast groups in a dynamic
fashion. This is a subject for further study.
1.4.3 Distributed Paging
The HAWAII paging entry for each mobile host is maintained at a base
station and each router on the path from the base station to the
Domain Root Router. Every router/base station is capable of
initiating paging by buffering incoming packets and sending a paging
request to the multicast group. However, the paging processing rules
(discussed later) ensure that only one node in the network initiates
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paging for a particular mobile host at a given time. We also ensure
that paging is initiated from routers with the up-to-date paging
entries for the mobile host by enforcing that paging is initiated
only if packets arrives from the interface to the DRR. In order to
push the paging load further down towards the base stations, the
router that has multiple interfaces for the PA's MGA initiates the
paging. If no such router exists, the packet will eventually reach a
base station, which will then assume the paging responsibility.
1.4.4 Soft-State
The notion of ``soft-state'' refers to state established within
routers that needs to be periodically refreshed; otherwise, it is
removed automatically when a preset timer associated with that state
expires. In addition to maintain routing information as soft state,
the HAWAII paging entries within the routers are also maintained as
soft-state. This increases the robustness of the protocol to router
and link failures.
Our protocol uses four types of control messages: requests,
responses, updates, and refreshes, to query, establish and maintain
the paging soft-state. Paging request messages are triggered inside
the network for locating the mobile user, which then responds with a
paging response. Paging updates are sent by the mobile host during
the crossing of a paging area in standby state. These messages are
explicitly acknowledged by the recipient. Paging refresh messages
are sent periodically by mobile hosts. Aggregate paging refresh
messages are sent periodically by base stations and routers in a
hop-by-hop manner to the router upstream of the mobile hosts. As we
shall see in the following sections, paging messages are sent to only
selected routers in the domain, resulting in very little overhead
associated with maintaining soft-state.
1.4.5 Stale Paging Entry
The protocol ensures that the latest (up-to-date) paging entries are
maintained along the path from the DRR to one base station in the PA.
Thus, packets arriving for the mobile host from outside the domain
will be correctly delivered. However, stale paging entries may exist
in internal routers for several reasons such as outdated refresh
messages, topology or routing changes, etc. In order to avoid paging
using stale paging entries for packets originating inside the domain
and destined for a mobile host in standby state, these packets will
first be forwarded along the default route to the DRR. The DRR always
has the latest paging entry and forwards the packet along the path to
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a base station. These packets will then trigger paging from a router
with the latest paging entry and deliver it to the mobile host.
1.5 Protocol Correctness
Our paging protocol maintains the following invariants.
1. Latest paging entries are maintained for each mobile host along
the path from DRR to one base station in PA.
2. Paging is initiated from routers with paging entry for the
mobile host only if packets arrive from the interface to the
DRR.
3. Response to a paging request is sent to the paging initiator in
a hop-by-hop manner from the mobile host's current base station.
This sets up routing and paging entries along the path from the
mobile host to the paging initiator.
4. State of the base station/routers with the mobile host is
``synchronized'' in the sense that its routing entries time out
before a mobile host goes into standby and its paging entries
exist as long as the mobile host is not powered off.
These invariants guarantee the correctness of the paging protocol.
Invariant 4 ensures that a mobile host's paging entry and not its
routing entry is used when the mobile host is in the standby state.
Invariants 1 and 2 imply that the router/ base station initiating the
paging has the latest (up-to-date) paging entry. Invariant 3
guarantees that the routing path is set up after paging for packet
delivery to the mobile host.
2 Detailed Protocol Operation
In this section, we describe the protocol processing details of
paging for HAWAII. We assume that the HAWAII update message (type 1)
is extended to include the multicast group address (MGA)
corresponding to the PA. We now describe four new message types and
their respective formats in the HAWAII protocol corresponding to the
paging request, update, response, and refresh messages. We then
present the processing at the mobile host and finally, the protocol
processing at the base stations/routers.
2.1 Message Formats
The format for the paging request is shown below. It is initiated by
a router or base station satisfying invariant 2 in Section 1.5.
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Type | Seq No | Scheme +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Host Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Paging Initiator Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
Version 1
Type 5 (paging request)
Scheme 1 (fixed PA)
Seq No Sequence number of paging request
Mobile host Address Home address or Care-of address
Paging Initiator Address Router/base station initiating paging
The format of paging update and response messages sent by base
station/routers is shown next. Paging updates (type 6) are sent
hop-by-hop to the DRR when the mobile host crosses a paging area.
Paging responses (type 7) are sent hop-by-hop to the initiator of the
paging request in response.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Type | Reason | Scheme +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Host Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metric | Routing Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Old Base Station |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New Base Station |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MGA for current PA |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
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Version 1
Type 6 (paging update), 7 (paging response)
Scheme 1 (fixed PA)
Mobile host Address Home address or Care-of address
Metric Distance to the mobile host in hops
Routing Lifetime Soft state timer value
Old Base Station Old Base Station IP address for Type 2
0.0.0.0 for Type 1 (power up)
MGA for current PA intra-domain MGA for host's current PA
Timestamp Timestamp formatted as in
Network Time Protocol [3].
Extensions Authentication field
Wireless link specific fields, for study
The format for a paging refresh message is shown next. The message
could contain multiple entries as part of an aggregate refresh when
sent by base stations and routers to their upstream router. The
maximum message size is constrained to 4KB.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Type | Reason | Size +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Host Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MGA[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp[1] +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
...
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Host Address[N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MGA[N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp[N] +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extensions ...
+-+-+-+-+-+-+-+-
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Version 1
Type 8 (paging refresh)
Size Number of mobile host entries
Reason 0 (normal)
1 (triggered refresh due to failure)
Mobile host Address Host-entry address
Timestamp Host-entry timestamp
Extensions Authentication field
2.2 Mobile Host Processing
Figure 2 shows the state diagram at the mobile host for maintaining
the ACTIVE, STANDBY, and NULL states. The mobile host can transmit
and receive data only in ACTIVE state. In order to transit into
ACTIVE state, either the mobile host sends a regular HAWAII
registration or the mobile host responds to a paging request. While
in ACTIVE state, the mobile host sends HAWAII registrations at least
once every Tactive time units. The mobile host goes into STANDBY
from ACTIVE state when the mobile host is idle for time Tactive.
While in STANDBY state, the mobile hosts sends paging updates at
least once every Tstandby time units or if it crosses a PA.
Startup: send
++++++++ power up update ++++++++ ____ Timeout:
+ +---------------> + +/ | resend power up
+ NULL0 + + NULL1 + | update
+ + <---------------+ + <--/
++++++++ Give up resends ++++++++
Power ^ ----------------------| | ^
down | | Get ack | |
| | w/ active Get ack | | Crossing PA:
Every | | w/ standby | | send paging update
Tactive, | | | |
send routing| v Idle for v |
refresh +++++++++ time Tactive ++++++++++ ____ Every
|----\+ +---------------> + +/ | Tstandby,
| + ACTIVE + + STANDBY + | send paging
\--> + + <---------------+ + <--/ refresh
+++++++++ Paging response ++++++++++
or routing update
Figure 2: Client State Diagram
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Thus two timers, Tactive and Tstandby, denote the idle time for
transitions between the ACTIVE to STANDBY and STANDBY to NULL states
respectively. These timers will be configured based on usage
patterns and battery power consumption statistics for wide-area
wireless data devices. Default values for these timers in HAWAII are
30 seconds and 30 minutes respectively.
2.3 Base Station/Router Processing
The processing of power up update message is similar to processing
basic HAWAII power up update message in [5]. While basic HAWAII
power up update messages establish only routing entries, in this
case, we extend it so that both routing and paging entries with the
multicast group address (MGA) are established.
The pseudo-code for processing paging update message is shown in
Figure 3. Paging update messages are sent along the ``default''
route. They are sent when the mobile host crosses a PA. The
processing is similar to the processing of a power up update message.
The only difference is that this message only sets up paging entries.
Note that the notation of the paging entry is similar to the one used
in explaining Figure 1. It consists of the mobile host address (MH
IP ADDRESS), the multicast group address (MGA), and the forwarding
interface.
--------------------------------------------------------------------
Figure 3: HAWAII paging update message processing
--------------------------------------------------------------------
1. Receive the message from neighbor on Interface A
Message contains {MH IP ADDRESS,MGA, TIMESTAMP}
2. If TIMESTAMP is greater than current paging entry timestamp then
3. If I am the Domain Root Router
Add/Update paging entry to be
(MH IP ADDRESS -> MGA, Interface A)
set timer Tstandby
Generate an acknowledgement
else
Add/Update paging entry to be
(MH IP ADDRESS -> MGA, Interface A)
set timer Tstandby
Forward paging update to upstream neighbor along default route
endif
endif
--------------------------------------------------------------------
The pseudo-code for packet forwarding in HAWAII with paging support
is shown in Figure 4. As in basic HAWAII, if a routing entry exists,
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packets are forwarded using the entry. If the routing entry does not
exist but a paging entry exists, paging is initiated only when the
packet arrives on the interface from the DRR and the node is a base
station or part of a multicast tree with at least two branches.
Otherwise it is forwarded to the DRR. This helps push the burden of
paging towards the base station, thereby reducing the load at the
DRR.
--------------------------------------------------------------------
Figure 4: HAWAII packet forwarding processing in the BS and router
--------------------------------------------------------------------
If (there is no routing entry for MH IP address)
If (paging table has an entry for MH) // has paging entry
Entry contains {MH IP address -> MGA, Interface A}
Let Interface B be the interface of the default route
if (packet is from Interface B or I am the Domain root Router)
if ((there is 0 or 1 interface in multicast routing entry)
and I am not a base station)
route the packet to interface A
else
buffer the packet
send a paging request message to the MGA
increase the retry counter and set timer for paging retry
endif
else
forward the packet along the default route to DRR
endif
else // no routing or paging entries
If (I am not the Domain Root Router)
forward the packet along the default route to DRR
else
discard the packet
endif
endif
else // has routing entry
route the packet using the routing entry
endif
--------------------------------------------------------------------
The pseudo-code for processing a paging response message is shown in
Figure 5. The paging response is sent to the initiator of the paging
request. It is sent hop-by-hop and routing entries are set up along
the way.
--------------------------------------------------------------------
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Figure 5: HAWAII paging response processing for fixed paging area
--------------------------------------------------------------------
1. Receive the message from neighbor on Interface A
Message contains {MH IP ADDRESS,MGA, TIMESTAMP}
2. If TIMESTAMP is greater than current paging entry timestamp then
3. If I am the paging initiator
Look up pending paging response for this MH
Add/Update routing entry to be {MH IP ADDRESS -> Interface A},
set timer Tactive
Generate an acknowledgement
else
Add/Update routing entry to be {MH IP ADDRESS -> Interface A}
set timer Tactive
Forward the paging response packet towards paging initiator
endif
endif
--------------------------------------------------------------------
The soft-state paging refresh messages are sent independently by each
of the nodes on a hop by hop basis. The mobile host refreshes the
base station at least every Tstandby seconds in the STANDBY state.
The base stations and routers send HAWAII routing and paging
refreshes to their upstream routers (determined based on their
default route to the domain root router) every TR1 and TR2 seconds
respectively. We require that TR1 and TR2 be more frequent than
Tactive and Tstandby in order for the protocol to be robust across
message losses. Also, typically Tstandby would be much longer than
TR2 in order to conserve the limited wireless bandwidth. When the
refresh message is received, the expiry timer corresponding to the
refresh entry is updated. The processing of the paging refresh
message is very similar to the processing of the routing refresh
message in HAWAII [5].
3 Design Implications
In this section, we illustrate the advantages of our protocol for
paging by studying the implications on scalability, ease of network
management and reliability.
3.1 Scalability
Paging entries for a given mobile host are only present along one
path from a base station to the DRR. The closer a router is to the
DRR, more paging entries and more refresh/update messages it will
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process. On the other hand, the farther a router is from the DRR,
the probability of paging being initiated is higher. This is because
of the rule that the router initiating a paging should be on the
multicast tree for the given paging area and have at least two
branches; since paging areas are typically localized, such a router
would be closer to the paging area and farther from the DRR. Thus,
the protocol distributes the processing load due to paging among the
different routers in the domain. Given that HAWAII is shown to be
scalable in [5] for large-sized domains, we believe that the addition
of paging functionality will not impact the scalability of HAWAII
adversely.
3.2 Ease of Network Management
In today's cellular network, every update with respect to location
management needs to be propagated to the Mobile Switching Center
(MSC) or the Serving GPRS Service Node (SGSN). In HAWAII, if a new
base station is installed due to a cell split, the base station just
creates/joins the appropriate multicast group. If the base station
changes to use a different algorithm to determine the PA, the base
station can just regroup into different PAs, and then join the
corresponding multicast groups. These changes are transparent to
other routers in the domain; the multicast routing protocol will
automatically compute the new multicast tree for each of the PAs.
Furthermore, by having a pure IP-based solution for mobility
management, the routers in the wireless access network are shielded
from details specific to the wireless interface. On the other hand,
the use of specialized components such as the MSCs or the SGSNs for
each wireless link protocols implies that each of these components
must be managed in a separate manner, thereby increasing the cost and
complexity of deployment.
3.3 Reliability
Paging can be initiated by any router/base station along the path to
the DRR. Therefore unlike a centralized approach, there is no single
point of failure with respect to paging.
Link and router failures are handled through the soft-state refresh
mechanism in HAWAII. The HAWAII daemon running at each router would
detect these failures and update its default route/paging entry.
This will trigger an immediate soft-state routing and paging refresh
messages for all its host entries to a new uplink router. This will
result in further propagation of soft-state refresh messages until a
router that has pre-existing entries for the affected mobile hosts is
notified (this will be the domain root router in the worst case).
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4 Paging with Mobile-IP
In this draft, we considered how paging support can be added to
HAWAII. Paging support can also be added to basic Mobile-IP using a
similar approach. However, some of the scalability and reliability
advantages of paging with HAWAII may no longer be possible.
In a Mobile-IP network employing foreign agents, paging can be
initiated by the previous foreign agent where the tunnel from the
home agent for a given mobile host is terminated. Packets arriving
at the foreign agent from the home agent are first buffered by the
foreign agent. Paging is then performed by the foreign agent by
multicasting to all foreign agents in the paging area for the mobile
host. Upon receiving a paging response from the mobile host, packets
are forwarded from this foreign agent to the new foreign agent of the
mobile host. Also, a tunnel to the new foreign agent from the home
agent can be established at this time. One issue with this approach
that needs to be addressed is the impact of foreign agent failures.
While it is understandable that if a foreign agent attached to a base
station fails, mobile hosts using that base station are denied
service, in this case, the mobile host could be at a different base
station but is still denied service because the host's foreign agent
at a previous base station, that is responsible for initiating the
paging request, is unavailable. Thus, the foreign agent becomes a
single point of failure in the case of paging and could impact
service to mobile hosts even if the host is no longer served directly
by the foreign agent.
In a Mobile-IP network without foreign agents, paging has to be
initiated from the home agent. This entails that Mobile-IP
registrations be extended to included paging areas. Furthermore,
paging areas can no longer have a administratively scoped multicast
address since the home agent may not belong to the same
administrative domain as the foreign agents. Thus, the scalability
of home agents in supporting a large number of mobile hosts becomes
an issue.
5 Security
This protocol has been defined as an extension to the HAWAII
protocol [5]. The security model of the HAWAII protocol directly
applies for the messages from the mobile host. Regarding the paging
messages which are generated and processed only from within a given
administrative domain, simple mechanisms such as password protection
should suffice.
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References
[1] Digital cellular telecommunication system, General Packet Radio
Service, Service description - Stage 2, GSM 03.60 Version 6.0,
ETSI, 1998.
[2] D. Meyer, ``Administratively Scoped IP Multicast,'' Request for
Comment 2365, July 1998.
[3] D. Mills, "Network Time Protocol (Version 3): Specification,
Implementation and Analysis", RFC 1305, Mar 1992.
[4] C.E. Perkins, ``IP Mobility Support,'' Request for Comments 2002,
Oct 1996.
[5] R. Ramjee, T. La Porta, S. Thuel, K. Varadhan, L. Salgarelli,
``IP micro-mobility support using HAWAII,'' Internet Draft, Work
in Progress, June 1999.
[6] A. Valko, A. Campbell, and J. Gomez, ``Cellular IP,'' Internet
Draft, Work in Progress, November 1998.
Authors' Addresses
R. Ramjee, T. La Porta
Bell Labs, Lucent Technologies,
101 Crawfords Corner Road,
Holmdel, NJ 07733 (USA)
Phone: 732-949-3306
Fax: 732-949-4513
Email: {ramjee,tlp}@bell-labs.com
L. Li
Department of Computer Science,
Cornell University
Email: lili@cs.cornell.edu
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