INTERNET-DRAFT A. Valko
Ericsson, Columbia University
A. Campbell, J. Gomez
<draft-valko-cellularip-00.txt> Columbia University
Expires May 1999 November 1998
Cellular IP
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this memo is unlimited.
Abstract
This document specifies a protocol that allows routing IP datagrams
to a mobile host. The protocol is intended to provide local mobility
and handoff support. It can interwork with Mobile IP [1] to provide
wide area mobility support. Four fundamental design principles of
the protocol are: (1) location information is stored in distributed
data bases (2) location information referring to a mobile host is
created and updated by regular IP datagrams originated by the said
mobile host (3) location information is stored as soft state (4)
location management for idle mobile hosts is separated from location
management of hosts that are actively transmitting or receiving data.
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Table of Contents
1. Introduction 2
1.1. Protocol Requirements . . . . . . . . . . . . . . . . . . 3
1.2. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Applicability . . . . . . . . . . . . . . . . . . . . . . 3
1.4. New Architectural Entities . . . . . . . . . . . . . . . 3
1.5. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.6. Protocol Overview . . . . . . . . . . . . . . . . . . . . 5
1.7. Location Management and Routing . . . . . . . . . . . . . 7
2. Cellular IP Functions 8
2.1. Location Management . . . . . . . . . . . . . . . . . . . 8
2.2. Routing . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3. Handoff . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4. Wide Area Mobility . . . . . . . . . . . . . . . . . . . 10
2.5. Handling Wireless Channel Black-outs . . . . . . . . . . 10
3. Protocol Details 11
3.1. Protocol Parameters . . . . . . . . . . . . . . . . . . . 11
3.2. Beacon Signal Structure . . . . . . . . . . . . . . . . . 11
3.3. Packet Formats . . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Data packet . . . . . . . . . . . . . . . . . . . 11
3.3.2. Route-update packet . . . . . . . . . . . . . . . 11
3.3.3. Paging-update packet . . . . . . . . . . . . . . . 12
3.4. Addressing . . . . . . . . . . . . . . . . . . . . . . . 13
3.5. Cellular IP Routing . . . . . . . . . . . . . . . . . . . 13
3.6. Cellular IP Gateway . . . . . . . . . . . . . . . . . . . 14
3.7. Cellular IP Mobile Host . . . . . . . . . . . . . . . . . 15
APPENDIX A. Security Issues . . . . . . . . . . . . . . . . . . . . 16
APPENDIX B. Network Planning and Performance . . . . . . . . . . . 17
APPENDIX C. Multiple Gateway Systems . . . . . . . . . . . . . . . 18
APPENDIX D. Charging . . . . . . . . . . . . . . . . . . . . . . . 18
APPENDIX E. Uplink I/F Selection . . . . . . . . . . . . . . . . . 18
References 19
Authors' Addresses 19
1. Introduction
Hosts connecting to the Internet via wireless interface are likely to
change their point of access frequently. A mechanism is required
that ensures that packets addressed to moving hosts are successfully
delivered with high probability. A change of access point during
active data transmission or reception is called a handoff. During or
immediately after a handoff, packet losses may occur due to delayed
propagation of new location information. These losses should be
minimized in order to avoid a degradation of service quality as
handoffs become more frequent.
This memo specifies Cellular IP, a protocol that provides mobility
and handoff support for frequently moving hosts. It is intended to
be used on a local level, for instance in a campus or metropolitan
area network. Cellular IP can interwork with Mobile IP [1] to
support wide area mobility, that is, mobility between Cellular IP
Networks.
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1.1. Protocol Requirements
A host connected to a Cellular IP Network must be able to send IP
datagrams to hosts outside the Cellular IP Network.
IP datagrams arriving to a Cellular IP Network, addressed to a host
connected to this Cellular IP Network, should be delivered with high
probability to the host regardless of its actual location.
IP datagrams generated by one host in the Cellular IP Network
addressed to another host in the Cellular IP Network should be
delivered to the destination without leaving the Cellular IP Network.
A mobile host migrating between Cellular IP Networks must be able to
use Mobile IP [1] for wide area mobility. Upon entering a Cellular
IP Network, it must be able to provide its home agent with a care-
of-address that ensures that its packets are routed to this Cellular
IP Network.
Mobile hosts migrating inside or between Cellular IP Networks must be
able to retain their own home IP addresses regardless of location.
Hosts inside a Cellular IP Network are identified by IP addresses,
but these addresses have no location significance.
Hosts outside the Cellular IP Network must not need any updating or
enhancements in order to communicate with hosts inside the Cellular
IP Network. Nodes sending or receiving datagrams to/from the mobile
host must remain unaware of the host's location inside the Cellular
IP Network.
1.2. Assumptions
Cellular IP assumes that a random access L2 protocol covers the air
interface.
1.3. Applicability
Cellular IP is applicable to networks ranging in size from LANs to
metropolitan area networks. To provide global mobility support,
Mobile IP [1] should be used above Cellular IP.
Cellular IP is designed to support frequently migrating hosts but
with appropriate setting of protocol parameters, it can also
efficiently serve rarely moving or even static hosts.
1.4. New Architectural Entities
Cellular IP Node
A Cellular IP Network consists of interconnected Cellular IP
Nodes. The role of Nodes is twofold. They route IP packets
inside the Cellular IP Network and communicate with Mobile
Hosts via wireless interface. Referring to the latter role, a
Cellular IP Node that has a wireless interface is also called a
Base Station.
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Cellular IP Base Station
See Cellular IP Node.
Cellular IP Gateway
A Cellular IP Node that is connected to a regular IP network by
at least one of its interfaces.
Cellular IP Mobile Host
A Mobile Host that implements the Cellular IP protocol.
1.5. Terminology
Active Mobile Host
A Mobile Host is in active state if it is transmitting or
receiving IP packets. (Exact definition is given in section
3.7.)
Active-state-timeout
The time a Cellular IP Mobile Host remains in active state
without receiving IP packets.
Cellular IP Network Identifier
A unique identifier assigned to Cellular IP Networks.
Control packet
Paging-update and Route-update packet.
Data packet
An IP packet that is not a control packet.
Downlink
Directed to a Mobile Host.
Downlink interface (I/F)
All interfaces of a Cellular IP Node except its Uplink I/F are
referred to as Downlink I/Fs.
Idle Mobile Host
A Mobile Host is in idle state if it has not recently
transmitted or received IP packets. (Exact definition is given
in section 3.7.)
Internet
A Cellular IP Network provides access to a regular IP network.
This IP network in this memo is referred to as "Internet".
Paging Cache
A cache maintained by some Cellular IP Nodes, used to route
packets to Mobile Hosts.
Paging-timeout
Validity time of mappings in Paging Caches.
Paging-update packet
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A control packet transmitted by Cellular IP Mobile Hosts in
order to update Paging Cache.
Paging-update-time
Time between consecutive Paging-update packets.
Route-timeout
Validity time of mappings in Routing Caches.
Route-update packet
A control packet transmitted by Cellular IP Mobile Hosts in
order to update Routing Cache.
Route-update-time
Time between consecutive Route-update packets.
Routing Cache
A cache maintained by all Cellular IP Nodes, used to route
packets to Mobile Hosts.
Uplink
Originated by a Mobile Host.
Uplink I/F
The interface used by a Cellular IP Node to forward packets
towards the Gateway.
1.6. Protocol Overview
The figure shown below presents a schematic view of multiple Cellular
IP Networks providing access to the Mobile IP enabled Internet.
..............................................
. .
. Internet Backbone with Mobile IP .
. .
..............................................
/ | \
/ | \
+--+ +--+ +--+
|GW| |GW| |GW|
+--+ +--+ +--+
/ | \
+-------------+ +--------------------+ +-------------+
| | | | | |
| Cellular IP | | Cellular IP | | Cellular IP |
| Network | | Network | | Network |
| | | __ __ __ | | |
+-------------+ +-|BS|---|BS|---|BS|-+ +-------------+
-- -- --
+ ... +
MH MH
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In the following, we present an overview of the operation of Cellular
IP, followed by a figure illustrating the functional entities that
comprise Cellular IP.
Base Stations periodically emit beacon signals. Mobile Hosts use
these beacon signals to locate the nearest Base Station. A Mobile
Host can transmit a packet by relaying it to the nearest Base
Station.
All IP packets transmitted by a Mobile Host are routed from the Base
Station to the Gateway by hop-by-hop shortest path routing,
regardless of the destination address.
Cellular IP Nodes maintain Routing Cache. Packets transmitted by the
Mobile Host create and update entries in each Node's Cache. An entry
maps the Mobile Host's IP address to the interface through which the
packet entered the Node.
The chain of cached mappings referring to a single Mobile Host
constitutes a reverse path for downlink packets addressed to the same
Mobile Host. As the Mobile Host migrates, the chain always points to
its current location because its uplink packets create new mappings
and old mappings are automatically cleared after a soft state
timeout. After a migration, before the old mappings are cleared, a
Node can temporarily have mappings for the same Mobile Host to
multiple interfaces. (This causes the chain to temporarily have a
fork.)
IP packets addressed to a Mobile Host are routed by the chain of
cached mappings referring to the said Mobile Host.
To prevent its mappings from timing out, a Mobile Host can
periodically transmit control packets. Control packets are regular
IP packets with empty payloads.
Mobile Hosts that are not actively transmitting or receiving data but
want to be reachable for incoming packets, let their Routing Cache
mappings time out but maintain Paging Cache mappings. IP packets
addressed to these Mobile Hosts will be routed by Paging Caches.
Paging Caches have a longer timeout value than Routing Caches and are
not necessarily maintained in every Node.
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+--------+
|host in |
|Internet|
+--------+
| Internet
| --------------------------
+--------+ Cellular IP Network
|Cell. IP|
|Gateway |
+--------+
|
- :
| :
| : ___________ Uplink I/F
A network of | |/ (=shortest path
| +--------+ toward Gateway)
Cellular IP | |Cellular|
| |IP Node |
Nodes | +--------+
| |\___________ Downlink I/F
| : (=all other
- : interfaces)
:
|
+--------+
uplink |Cellular|
^ |IP Node |
| +--------+
| air |
| interface|
V +--------+
downlink | Mobile |
| Host |
+--------+
1.7. Location Management and Routing
Cellular IP uses two parallel cache systems to store the information
related to the location of Mobile Hosts. The two systems basically
operate in the same way. This section is intended to clarify why we
use two distinct caches.
Supposing there is just one set of cache, the following trade-off
determines the optimal time cached mappings remain valid. After a
Mobile Host performs a handoff, its path to the old Base Station will
remain valid until the cached mappings associated with this Base
Station are cleared. If in this period packets are sent to the Host,
they are routed not only to its current location, but also to the old
Base Station. This results in a waste of resources. The waste is
especially large if the Mobile Host performs a number of handoffs
within the validity time of the mappings. In this case the system
approaches a broadcasting based communication system and becomes
inefficient. This kind of waste can be minimized by selecting a
small timeout interval, typically in the order of packet time scale.
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On the other hand, in order to maintain mappings, Mobile Hosts must
send control packets with a periodicity comparable to the mappings'
validity time. If the validity time is in the order of packet time
scale, control packets must be transmitted at this time scale even by
idle Mobile Hosts which similarly results in a large load generated
by control packets making the system inefficient.
Separating the caches for active and idle Mobile Hosts allows us to
specify two optimal time scales for these operational states. More
specifically, active Hosts have mappings in Routing Caches. These
mappings remain valid for a short time, associated with the packet
time scale. Therefore Active Hosts need to send IP packets
relatively frequently; that is, when they have no data to send they
send control packets. In contrast, idle Hosts have mappings in
Paging Caches. These mappings remain in caches for longer time, in
the host mobility time scale. Therefore the frequency at which idle
Hosts must send control packets is relatively low, comparable to the
frequency of migrations. This load is not significantly higher than
explicit migration signalling would impose on the system.
2. Cellular IP Functions
2.1. Location Management
Idle mobile hosts periodically transmit Paging-update packets to keep
Paging Cache mappings up-to-date. These Paging-update packets update
Paging Cache mappings but not Routing Cache mappings. Paging-update
packets reach the Gateway and are discarded there to isolate Cellular
IP specific operations from the Internet.
As the idle Mobile Host moves, it always sends its Paging-update
packets to the nearest Base Station, forcing Paging Caches to point
at its up-to-date location. Outdated mappings are cleared after a
system specific time, paging-timeout.
When an IP packet arrives at a Cellular IP Node, addressed to a
Mobile Host for which no up-to-date Routing Cache mapping is
available, the Paging Cache is used to route the packet. This phase
is called "implicit paging". (In the case of explicit paging, this
packet is transformed into an explicit paging packet and all Nodes
route it using Paging Caches. This solution can provide some
advantages over implicit paging, however, this is for further study.)
If the Node has no Paging Cache, it forwards the packet to all
Downlink I/Fs. A Node that has Paging Cache but has no mapping in it
for the addressed Host discards the packet.
Upon receiving the packet, the Mobile Host moves to active state and
starts updating its Routing Cache mappings. Further IP packets
addressed to the same Host will be routed by Routing Caches as long
as the Mobile Host keeps the Routing Caches updated.
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2.2. Routing
Packets transmitted by Mobile Hosts are routed to the Gateway using
regular hop-by-hop routing. Cellular IP Nodes monitor these passing
data packets and use them to create and update Routing Cache
mappings. These map Mobile Host IP addresses to Node interfaces.
Packets addressed to the Mobile Host are routed along the reverse
path, on a hop-by-hop basis, by these Routing Cache mappings.
The structure and basic operation of routing is the same as that of
location management. To clarify the duality between the two, we
summarize the operation of Paging Caches and Routing Caches in the
following table. For the reasons of separating the two functions,
see section 1.7.
-------------------------------------------------------------------
Paging Caches Routing Caches
-------------------------------------------------------------------
updated by all uplink packets (data, data and
Paging-update, Route-update) Route-update packets
scope both idle and active MHs active Mobile Hosts
purpose route downlink packets if route downlink
there is no Routing Cache entry packets
time scale mobility packet
-------------------------------------------------------------------
The Mobile Host may keep receiving data packets without sending data
for possibly long durations. To keep its Routing Cache mappings up
to date and to avoid repeated paging, Mobile Hosts in active state
that have no data to send must send periodic Route-update packets.
Like uplink data packets, Route-update packets configure Routing
Caches and ensure that the hop-by-hop route from the Gateway to the
Mobile Host remains up-to-date.
For reliability and timeliness, Paging Caches also contain Mobile
Hosts that are contained by Routing Caches. For this reason, Paging
Caches are updated by all uplink packets including data and Route-
update packets.
2.3. Handoff
Handoff is initiated by the Mobile Host. As the Host approaches a
new Base Station, it redirects its packets from the old to the new
Base Station. The first of these redirected packets will configure
Routing Caches along the way from the new Base Station to the
Gateway. (The paths leading to the old and new Base Stations may
overlap. In Nodes where the two paths are the same, the new packets
simply refresh old mappings and the handoff remains unnoticed.)
For a time equal to the timeout of Routing Cache mappings, packets
addressed to the Host will be routed to both the old and new Base
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Stations. After the timeout has elapsed the Routing Cache mappings
associated with the old Base Station will be automatically cleared.
After this time, packets addressed to the Mobile Host continue to be
delivered to the new Base Station only.
If the Mobile Host has no data packets to send at the time of
handoff, it generates and transmits a Route-update packet immediately
after moving to the new Base Station. This ensures that mappings are
created quickly with the result of minimizing the downlink packet
loss.
2.4. Wide Area Mobility
Wide area mobility occurs when the Mobile Host moves between Cellular
IP Networks. The Mobile Host can identify Cellular IP Networks by
the Cellular IP Network Identifier contained in the Base Stations'
beacon signals. The beacon signal also contains the IP address of
the Gateway. Technically, Cellular IP does not require that Mobile
Hosts register before using the Cellular IP Network. A Mobile Host
entering the service area can start transmitting Paging-update
packets configuring Paging Caches immediately. For security and
charging purposes, however, authentication and other user-related
information may need to be provided by the Mobile Host. This
information will be inserted in the payload of the first Paging-
update packet and may be repeated in a few following Paging-update
packets for reliability. Upon receiving the first Paging-update
packet, the Gateway performs admission control that may involve
technical and charging decisions. The Gateway's response is sent to
the Mobile Host in regular IP packet(s). If the request was
accepted, the response may also carry the required setting of
protocol parameters. The issues of authentication, billing and
security are for further study and are beyond the scope of this
Internet-Draft.
Once the registration is accepted, the Mobile Host can send a Mobile
IP registration message to its home agent, specifying the Gateway's
IP address as care-of-address. (Alternatively, the Gateway can
register at the Home Agent on behalf of the Mobile Host.)
The Mobile Host may leave the service area at any time without prior
notice. Mappings associated to the Host will be cleared after the
timeout.
2.5. Handling Wireless Channel Black-outs
Due to conditions in the wireless channel, Mobile Hosts may become
temporarily disconnected. A host that reappears after a black-out
can continue operation normally regardless of whether it reappeared
in the same cell or in another one. The first packets transmitted
(data or control) will configure or re-configure mappings in Routing
and/or Paging Caches. The network does not notice the black-out
except for the Base Station that discards packets addressed to the
unreachable Mobile Host.
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3. Protocol Details
3.1. Protocol Parameters
The following parameters shall be set by network management. The
values listed here are for information only. Consideration of
selecting the proper values are discussed in Appendix B.
-------------------------------------------------------------------
Name Meaning Typical Value
-------------------------------------------------------------------
route-update-time Inter-arrival time 100 ms
of Route-update packets
route-timeout Validity of Routing 300 ms
Cache mappings
paging-update-time Inter-arrival time 1 min
of Paging-update packets
paging-timeout Validity of Paging 3 min
Cache mappings
active-state-timeout Time the Mobile Host 10 sec
remains in active state
without receiving data
-------------------------------------------------------------------
3.2. Beacon Signal Structure
Cellular IP Base Stations must periodically transmit beacon signals
to allow for Mobile Hosts to identify an available Base Station.
Information elements carried by the beacon signal are:
- Layer2 parameters related to the Base Station;
- the Cellular IP Network Identifier; and
- the IP address of the Gateway.
3.3. Packet Formats
3.3.1. Data packet
Cellular IP forwards regular IP packets without modification,
segmentation, encapsulation or tunnelling.
3.3.2. Route-update packet
A Route-update packet is an IP packet of which
- the source address is the IP address of the sending Mobile Host;
- the destination address is the Gateway; and
- the protocol type is IPPROTO_CELLIPRU.
The payload of the Route-update packet may be empty. Optionally,
control information may be carried in the Route-update packet's
payload, encoded in the following Type-Length-Value format:
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0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+-+-+-+-+-+-+-+-
| Type | Length | Data ... | Type ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+-+-+-+-+-+-+-+-
Type Indicates the particular type of control information.
Length Indicates the length (in bytes) of the following data
field within. The length does not include the Type and
Length bytes.
Data This field may be zero or more bytes in length. The
meaning, format and length of the data field is
determined by the Type and Length fields.
Currently the following types of control information are defined
(details are for further study):
Registration request
Used when a Mobile Host enters the Cellular IP Network.
Authentication
Must be used when the Registration request field is present and
may be used at other times, too. For further study.
3.3.3. Paging-update packet
A Paging-update packet is an IP packet of which
- the source address is the IP address of the sending Mobile Host;
- the destination address is the Gateway; and
- the protocol type is IPPROTO_CELLIPPU.
The payload of the Paging-update packet may be empty. Optionally,
control information may be carried in the Paging-update packet's
payload, encoded in the following Type-Length-Value format:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+-+-+-+-+-+-+-+-
| Type | Length | Data ... | Type ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- -+-+-+-+-+-+-+-+-
Type Indicates the particular type of control information.
Length Indicates the length (in bytes) of the following data
field within. The length does not include the Type and
Length bytes.
Data This field may be zero or more bytes in length. The
meaning, format and length of the data field is
determined by the Type and Length fields.
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Currently the following types of control information are defined
(details are for further study):
Registration request
Used when a Mobile Host enters the Cellular IP Network.
Authentication
Must be used when the Registration request field is present and
may be used at other times, too. For further study.
3.4. Addressing
Cellular IP requires no address space allocation beyond what is
present in IP. Mobile Hosts are identified by their home IP
addresses.
3.5. Cellular IP Routing
Cellular IP Nodes need only to implement the algorithm described in
this section. They do not need regular IP routing capability. This
section describes the routing algorithm in Cellular IP Nodes other
than the Gateway. The extra functions required only in the Cellular
IP Gateway are described in section 3.6.
In uplink direction (toward the Gateway), packets are routed in the
Cellular IP Network on a hop-by-hop basis. The interface through
which a Node will forward a packet toward the Gateway is referred to
as the Node's Uplink I/F. The Uplink I/F at each Node may be
designated by network management. Alternatively, a simplified
shortest path algorithm can select Uplink I/Fs. (A regular shortest
path algorithm is also applicable but is more complex than required
since it determines routes to all nodes in the network.) A simple
algorithm that configures Uplink I/Fs and automatically reconfigures
them if necessary after a topology change is described in Appendix E.
A Node's interfaces other than the Uplink I/F are called Downlink
I/Fs. A packet arriving to the Node through one of the Downlink I/Fs
is assumed to be coming from a Mobile Host. The packet is first used
to update the Node's Routing and Paging Caches and is then forwarded
through the Node's Uplink I/F.
To update the Caches, the Node reads the packet type (IPPROTO) and
the source IP address. Paging-update packets update the Paging Cache
only. Route-update and data packets update both Routing and Paging
Caches. Both types of caches consist of
{ IP-address, interface, expiration time }
triplets, called mappings. To update the Routing Cache, the Node
creates the following triplet:
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{ the newly arrived packet's source IP address,
the interface through which it arrived,
current time + route-timeout
}
If a mapping existed in the Routing Cache with the same IP address
and the same interface, it is replaced by the new triplet. If such a
triplet did not exist, the new triplet is inserted in the cache. The
Paging Cache is updated in the same way, using paging-timeout instead
of route-timeout. If the Node has no Paging Cache then only the
Routing Cache is updated by Route-update and data packets and no
cache is updated by Paging-update packets.
A packet arriving to a Cellular IP Node through the Uplink I/F is
assumed to be addressed to a Mobile Host. The Node first checks if
the destination IP address has a valid mapping in the Routing Cache.
If such mapping(s) exist(s), the packet is forwarded to all
interfaces to which valid Routing Cache mappings were found.
If there are no valid Routing Cache mappings for the destination
address and the Node has a Paging Cache, the packet is routed
according to the Paging Cache as follows. It is forwarded to all
interfaces to which the destination IP address has valid Paging Cache
mapping. If the Node has Paging Cache but there are no valid
mappings, the packet is discarded.
If there are no valid Routing Cache mappings for the destination, and
the Node has no Paging Cache, the packet is forwarded to all Downlink
I/Fs.
3.6. Cellular IP Gateway
The following figure is a schematic view of a Cellular IP Gateway.
The Gateway can logically be divided into three building blocks: a
regular Cellular IP Node, a Gateway Packet Filter and a Gateway
Controller.
IP network
===================
|
+------------------------------|--------+
| | |
| +----------+ +-------------+ |
| | Gateway |__________| Gateway | |
| |Controller| |Packet Filter| |
| +----------+ +-------------+ |
| | _______|____Uplink I/F
| |/ |
| +-------------+ |
| Cellular IP | Cellular IP | |
| Gateway | Node | |
| +-------------+ |
| | | |\__|____Downlink I/Fs
+-------------------------|----|----|---+
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Uplink packets update the Routing and/or Paging Caches in the
Cellular IP Node block and are forwarded towards the Gateway filter.
The Gateway filter reads the destination IP address. If this is the
Gateway's address, the packet is forwarded to the Gateway controller.
Most of these packets are Route-update and Paging-update packets with
empty payload and are immediately dropped. If the packet carries
control information, for instance a registration request, it is
interpreted and processed by the Gateway controller.
If the destination address is not the Gateway's, the packet is
forwarded to the Internet. (This means that a packet sent from a
Mobile Host to another Mobile Host in the same Cellular IP Network
goes through the destination Home Agent. However, this is not the
case if route optimization is used. To operate efficiently even
without Mobile IP route optimization, the Gateway Packet Filter can
also check if the destination address of an uplink packet has a valid
mapping in any of the Gateway's caches. If a mapping is found, the
packet is "turned back" and is treated as a downlink packet.)
Packets arriving to the Gateway Packet Filter from the Internet can
be of the following types:
If the destination address is the Gateway and the packet is
tunnelled, it must be sent using Mobile IP. The packet is then
detunnelled and forwarded to the Cellular IP Node.
If the destination address is not the Gateway and the packet is an
IPv6 packet containing a routing header, it must be sent using
Mobile IP. The packet is then forwarded to the Cellular IP Node,
unchanged.
If the destination address is not the Gateway and the packet does
not contain a routing header, it is a regular IP packet addressed
to a Mobile Host of which this Cellular IP Network is the home
network. The packet is then forwarded to the Cellular IP Node,
unchanged.
The Gateway's Cellular IP Node block treats these packets as
determined by the Cellular IP Routing algorithm (section 3.5). The
packet is routed according to the Routing Cache if valid mapping(s)
exist(s) for the destination address and is routed according to the
Paging Cache otherwise. Though in Cellular IP Nodes it is optional
to have Paging Cache, it is recommended that the Gateway's Cellular
IP Node have one. This way, packets addressed to Hosts currently not
connected to the Cellular IP Network do not enter the network and
load it in vain but are immediately discarded in the Gateway when
neither Routing, nor Paging Cache mapping is found for the
destination address. (It may be advantageous to also generate a
warning message in this case and send it back to the packet's source
address.)
3.7. Cellular IP Mobile Host
While connected to a Cellular IP Network, a Mobile Host must be in
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one of two states: 'active' or 'idle'. The Host moves from idle to
active state when it receives any IP packet. If it does not receive
more IP packets, it remains in active state for a time equal to
active-state-timeout. Any IP packet received in active state
restarts the active state timer. When the timer elapses, the Host
returns to idle state.
When the Host moves from idle to active state, it must transmit a
Route-update packet. At the same time, a timer is initiated from a
value equal to route-update-time. If the timer expires without any
data packet being transmitted from the Host, again a Route-update
packet is transmitted and the timer is re-initiated. Any IP packet
transmitted before the timer expires, resets the timer to route-
update-time. This ensures that while the Mobile Host is in active
state, the largest interval between two transmitted packets is never
longer than route-update-time. The mechanism also ensures that if
data packets are transmitted with sufficient frequency, no Route-
update packets will be generated.
In idle state, the Mobile Host must transmit Paging-update packets
periodically, at intervals of paging-update-time. Similarly to the
Route-update packet timer, the paging-update timer is reset if a data
packet is transmitted. (We recall that a transmitted IP packet does
not make the Mobile Host go to active state.)
Regardless of which state the Host is in, it must immediately
transmit an IP packet whenever it connects to a new base station.
This typically happens at migration, but is also the case after a
wireless channel black-out or when the Host enters the Cellular IP
Network. The packet transmitted this way is a Route-update packet if
the Host is active and a Paging-update packet if the Host is idle.
(If the Host has a data packet queued and ready for transmission, it
can send that packet instead of a control packet.) A packet
transmitted this way also resets the appropriate control packet
timer.
Appendix A. Security Issues
A Cellular IP Network is a single administrative domain. It is
connected to the Internet through a Gateway that may eventually also
serve as a firewall. Hence security issues only need to be
considered at the wireless interface.
The security of a Cellular IP system will be determined by the
wireless link. Cellular IP does not assume one specific wireless
link protocol. If the wireless link protocol does not include
encryption, a malicious user can listen to the traffic of other users
even without being connected to the Cellular IP network. By
transmitting packets with a false source address, a host can also
imitate another host and thus creating false traffic. These security
issues appear in all wireless IP systems and are not specific to
Cellular IP, however, they must also be dealt with in Cellular IP.
A security problem specific to Cellular IP is that a malicious host,
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by transmitting packets with a false source address, can redirect
packets addressed to another user. In normal circumstances, this
will not prevent the real addressee from receiving the packet, since
the malicious host will only add new routing entries but not remove
existing route entries. However, this and other attacks will need to
be addressed in an operational Cellular IP Network.
The following is a list of possible security protection mechanisms.
Encrypted wireless link.
This is probably the only strategy that can give full
protection. For high security, the encryption code must be
user-specific. The code can be agreed upon when the Mobile
Host enters the network. This, however, allows malicious hosts
to listen to the code decision procedure. To prevent this, the
Gateway can obtain the code (or part of it) from the Mobile
Host's home agent.
Authentication
The Mobile Host can be required to provide authentication
information upon entering the Cellular IP Network. If it has
no security binding with the network, the Gateway will use the
Mobile Host's home agent to check the validity of the
authentication.
Packet filtering in Gateway
To ensure that Mobile Hosts that have not registered
successfully can not use the Cellular IP Network, the Gateway
can filter regular data packets and discard those that do not
belong to an authorized user.
Appendix B. Network Planning and Performance
To adapt the system to actual traffic and mobility characteristics,
the operator of a Cellular IP Cellular IP Network can set the
following system parameters:
route-timeout
Will typically be a small multiple of the route-update-time.
route-update-time
Will typically be on the packet time scale. Higher values
would result in less frequent Route-update packet
transmissions, but it also increases the route-timeout. This
extends the time a route is valid after the Mobile Host moves
away and hence increases network load.
paging-timeout
Will typically be a small multiple of the paging-update-time.
paging-update-time
Will typically be on the host mobility time scale. Higher
values would result in less frequent Paging-update packets, but
it also increases the paging-timeout. This extends the time
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Paging Cache mappings associated with the old location remain
valid after the Mobile Host moves away and hence increases the
cost of paging.
active-state-timeout
The value should be such that short pauses between bursts do
not cause the Mobile Host to go idle. Too high a value would
result in transmitting Route-update packets in vain for a long
time.
Paging Cache population
Paging Caches need not be maintained in all nodes. The
operator is free to select the nodes that maintain Paging
Caches and will typically select nodes with many downlink I/Fs.
Appendix C. Multiple Gateway Systems
Cellular IP requires that a Mobile Host be using exactly one Gateway
at a time. This requirement comes from the fact that the Gateway
serves as the Mobile Host's Foreign Agent and it relays its packets
both up and downlink. It is also required to make uplink routing
unambiguous. The Cellular IP Network can have multiple Gateways as
long as a single Host still uses just one Gateway at any time. (The
Host can change Gateway, involving a Mobile IP location updating.)
In a Network with multiple Gateways, Nodes must be able to determine
which Gateway a given Mobile Host is using. Assignment of Gateways
can, for instance, be based on geographical partitioning of the
network, or on partitioning the Mobile Hosts' address space. This
issue is for further study.
Appendix D. Charging
Cellular IP Network providers can charge Cellular IP Mobile users for
connectivity or for transmitted data or both. Charging information
is best collected in the Gateway. The Gateway receives all control
packets and can determine the time a Mobile Host was connected to the
network. It can also measure through traffic in both directions.
Appendix E. Uplink I/F Selection
This algorithm selects Uplink I/Fs in all Nodes of a Cellular IP
Network and reconfigures them if necessary after a change of
topology.
The Gateway periodically creates a control packet called a
"Gateway broadcast packet". The Gateway broadcast packet contains
a sequence number increased each time by the Gateway. The Gateway
transmits the packet through all of its interfaces except those
connected to the Internet. A Cellular IP Node receiving a Gateway
broadcast packet sets as Uplink I/F the interface through which
the packet arrived and denotes all other interfaces as Downlink
I/Fs, including the air interface if there is one. The Node
stores the sequence number of the Gateway broadcast packet. After
a short random delay, the Node forwards the packet through all of
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its Downlink I/Fs, except the air interface. The Node ignores
further Gateway broadcast packets with the same sequence number,
but repeats the procedure if one arrives with a higher sequence
number.
References
[1] "IP Mobility Support," C. Perkins, ed., IETF RFC 2002, October
1996.
Authors' Addresses
Andras G. Valko
Ericsson Traffic Analysis and Network Performance Laboratory
Center for Telecommunications Research, Columbia University
H-1300 Bp.3.P.O.Box 197, Hungary
phone: +36 1 437 7774
fax : +36 1 437 7219
email: andras.valko@lt.eth.ericsson.se, andras@comet.columbia.edu
Andrew T. Campbell, Javier Gomez
Department of Electrical Engineering, Columbia University
Rm. 801 Schapiro Research Building
530 W. 120th Street, New York, N.Y. 10027
phone: (212) 854 3109
fax : (212) 316 9068
email: [campbell,javierg]@comet.columbia.edu
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