INTERNET-DRAFT Zach D. Shelby
University of Oulu
Dionisios Gatzounas
Intracom
Andrew Campbell
Chieh-Yih Wan
Columbia University
July 2001
Cellular IPv6
<draft-shelby-cellularipv6-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document updates Cellular IP [6] with IPv6 capability. This
protocol will inter operate fully with Mobile IPv6 [1]. The original
is improved with an alternative method for semi-soft handoff. The
implementation is freely available for Linux from [9].
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What's Changed
The following changes have been made to the original protocol
presented in <draft-ietf-mobileip-cellularip-00>:
- Control messages use IPv6 extension headers for control
information
- The Authentication Header is used for all authentication [3]
- IPv6 Stateless Auto configuration for obtaining care-of-address
- Alternative indirect semi-soft handoff method added (section 4.1.2)
In addition, the following minor changes have been made:
- Added definitions for registration request and semi-soft handoffs
- Mobile hosts are now identified by their IPv6 care-of-address
(section 3.4)
- Added active-state-timeout as parameter (section 3.1)
- Further clarification of paging cache placement (section 2.1)
- Misc. text corrections
1. Introduction
Hosts connecting to the Internet via a 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 the
delayed propagation of new location information. These losses should
be minimized in order to avoid a degradation of service quality as
handoff become more frequent.
This memo specifies Cellular IPv6, 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 IPv6 can inter work with Mobile IPv6 [1] to sup-
port wide area mobility, that is, mobility between Cellular IPv6 Net-
works.
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1.1. Applicability
Cellular IP supports local mobility, that is, mobility inside an
access network. To provide global mobility support, Mobile IPv6 [1]
should be used in conjunction.
It is designed to support frequently migrating, rarely moving or
static hosts as well.
It is assumed that a random access L2 protocol covers the air inter-
face. Optional support for non-random access wireless interfaces to
perform semi-soft handoff is described in 4.1.2.
Throughout the draft the term Base Station is used exclusively. This
also refers to the Access Point used in WLAN and WPAN networks.
1.2. 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 a wireless interface. Referring to the latter role,
a Cellular IP node that has a wireless interface is also called
a Base Station.
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.
1.3. 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
3.8.)
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Cellular IP Network Identifier
A unique identifier assigned to Cellular IP Networks.
Control packet
Paging-update, paging-teardown and route-update packet.
Data packet
An IP packet that is not a control packet.
Downlink
Directed to a mobile host.
Downlink neighbor
All neighbors of a Cellular IP node except its Uplink neighbor
are referred to as Downlink neighbors.
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 3.8.)
Internet
A Cellular IP Network provides access to a regular IP network.
This IP network in this memo is referred to as "Internet", but
it can also be a corporate intranet, for example.
Neighbor
One Cellular IP node is said to be the neighbor of another if
they are connected directly. Neighbors are identified in a
Cellular IP node by interface and MAC address.
Paging Area
A set of Base Stations. Idle mobile hosts crossing cell
boundaries within a Paging Area do not need to transmit control
packets to update their position. (Exact definition is given in
Section 2.1.)
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
A control packet transmitted by Cellular IP mobile hosts in
order to update Paging Cache.
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Paging-update-time
Time between consecutive paging-update packets.
Paging-teardown packet
A control packet transmitted by Cellular IP mobile hosts in
order to explicitly disconnect from the Cellular IP Network.
Registration request
Type of control message used by a mobile host when it first
communicates with the Cellular IP network.
Route-timeout
Validity time of mappings in Route Cache.
Route-update packet
A control packet transmitted by Cellular IP mobile hosts in
order to update Route Cache.
Route-update-time
Time between consecutive route-update packets.
Route Cache
A cache maintained by all Cellular IP nodes, used to route
packets to mobile hosts.
Semi-soft handoff
Handoff method where traffic bound to a mobile host is bi-cast to
both the new and old BS simultaneously.
Update packet
Paging-update and route-update packet.
Uplink
Originated by a mobile host.
Uplink neighbor
The neighbor of a Cellular IP node which is the next hop on the
shortest path towards the Gateway.
1.4. Protocol Overview
The figure shown below presents a schematic view of multiple Cellular
IP Networks providing access to the Internet.
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..............................................
. .
. Internet Backbone with Mobile IPv6 .
. .
..............................................
| | |
+--+ +--+ +--+
|GW| |GW| |GW|
+--+ +--+ +--+
| | |
+-------------+ +--------------------+ +-------------+
| | | | | |
| Cellular IP | | Cellular IP | | Cellular IP |
| Network | | Network | | Network |
| | | __ __ __ | | |
+-------------+ +-|BS|---|BS|---|BS|-+ +-------------+
-- -- --
+ ... +
MH MH
In what follows, 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 Sta-
tion.
By default 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 a Route 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 neighbor from which the
packet arrived to the node.
The chain of cached mappings referring to a single mobile host con-
stitutes a reverse path for downlink packets addressed to the same
mobile host. As the mobile host migrates, the chain of mappings
always points to its current location because its uplink packets
create new and change old mappings.
IP packets addressed to a mobile host are routed by the chain of
cached mappings associated with the said mobile host.
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To prevent its mappings from timing out, a mobile host can periodi-
cally transmit control packets. Control packets are IPv6 packets
with Hop-by-Hop extension headers containing Cellular IP control
information.
Mobile hosts that are not actively transmitting data but want to be
reachable for incoming packets, let their Route 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 Route Caches and are not necessarily
maintained in every node.
+--------+
|host in |
|Internet|
+--------+
| Internet
| --------------------------
+--------+ Cellular IP Network
|Cell. IP|
|Gateway |
+--------+
|
- :
| :
| :___________ Uplink neighbor
A network of | | (=shortest path
| +--------+ toward Gateway)
Cellular IP | |Cellular|
| |IP node |
nodes | +--------+
| | ___________ Downlink neighbors
| :/ (=all other
- : neighbors)
:
|
+--------+
uplink |Cellular|
^ |IP node |
| +--------+
| air |
| interface|
V +--------+
downlink | Mobile |
| host |
+--------+
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1.5. 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.
When a mobile host is in active state, the network must follow its
movement from Base Station to Base Station to be able to deliver
packets without searching for the mobile host. As a consequence
active mobile hosts must notify the network about each handoff. For
idle mobile hosts exact location tracking is less important, instead
minimizing communication to save battery is of higher priority. By
deploying two caches, the granularity of location tracking can be
different for idle and active mobile hosts.
Separating the location tracking for idle and active mobile hosts
also has a performance benefit. Supposing there is just one set of
cache, for each downlink packet the entire cache must be searched to
find the destination mobile host. It is expected, however, that only
a portion of the hosts will be in active state at any given time and
that most of the packets are destined for active mobile hosts. Thus
by separating the caches for active and idle mobile hosts only a
smaller cache needs to be searched for most of the packets. This
results in faster lookups and better scalability [5].
2. Cellular IP Functions
2.1. Location Management
Cellular IP allows idle mobile hosts to roam large geographic areas
without the need to transmit location update packets at cell borders.
The network operator can group cells into Paging Areas, each compris-
ing of an arbitrary number of (typically adjacent) cells. Each Pag-
ing Area has an identifier that is unique in the given Cellular IP
Network. Each Base Station transmits its Paging Area Identifier in
its periodic beacon signals, thus enabling mobile hosts to notice
when they move into a new Paging Area.
An idle mobile host that moves into a new Paging Area must transmit a
paging-update packet. Paging-update packets are routed from the Base
Station to the Gateway using hop-by-hop routing. Selected nodes of
the Cellular IP network are equipped with Paging Cache. These nodes
monitor passing paging-update packets and update Paging Cache
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mappings to point toward the new Paging Area. Paging-update packets
reach the Gateway and are discarded there to isolate Cellular IP
specific operations from the Internet.
When the idle mobile host moves within a Paging Area, it transmits a
paging-update packet only when the system specific time, paging-
update-time expires. Outdated mappings of Paging Caches are cleared
if no update arrives before paging-timeout expires.
When an IP packet arrives at a Cellular IP node, addressed to a
mobile host for which no up-to-date Route Cache mapping is available,
the Paging Cache is used to route the packet. This is called "impli-
cit paging". If the node has no Paging Cache, it forwards the packet
to all Downlink neighbors. A node that has Paging Cache but has no
mapping in it for the destination mobile host discards the packet.
On the path from the gateway to the mobile host there may be Cellular
IP nodes with and without Paging Cache. After the paging packet
leaves the last node which has a Paging Cache it is effectively down-
link broadcast by all nodes it passes. The set of cells that are
reached by the paging packet forms a Paging Area. The number, size
and population of Paging Areas in a Cellular IP network are deter-
mined by the topology of the network and the placement of Paging
Caches. Each interface of the last downlink node with a paging cache
must belong to a separate paging area. Based on the configuration of
a Paging Area each base station (with Paging Cache configured) could
be considered an autonomous Paging Area. The other extreme case is
when a Cellular IP Network has no Paging Cache configured in which
case the complete network represents a Paging Area where paging
devolves to broadcasting throughout the network.
When the mobile host receives the paging packet, it moves to active
state and creates its Route Cache mappings by sending a route-update
packet. Subsequent IP packets addressed to the same host will be
routed by Route Caches as long as the mobile host keeps the Route
Caches updated.
2.2. Routing
Packets transmitted by mobile hosts are routed to the Gateway using
shortest path hop-by-hop routing. Cellular IP nodes monitor these
passing data packets and use them to create and update Route Cache
mappings. These map mobile host IP addresses to Downlink neighbors
of the Cellular IP node. Packets addressed to the mobile host are
routed along the reverse path, on a hop-by-hop basis, by these Route
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Cache mappings.
The structure and basic operation of routing is similar to that of
location management. To clarify the duality between the two, we sum-
marize the operation of Paging Caches and Route Caches in the follow-
ing table. For the reasons of separating the two functions, see Sec-
tion 1.5.
---------------------------------------------------------------------
Paging Caches Route Caches
---------------------------------------------------------------------
refreshed by all uplink packets (data, data and
paging-update, route-update) route-update packets
updated by all update packets route-update packets
(paging-update, route-update)
updated when moving to a new Paging moving to a new
Area, or after cell, or after
paging-update-time route-update-time
scope both idle and active MHs active mobile hosts
purpose route downlink packets if route downlink
there is no Route Cache entry packets
---------------------------------------------------------------------
The mobile host may keep receiving data packets without sending data
for possibly long durations. To keep its Route 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 update Route Caches and
ensure that the hop-by-hop route from the Gateway to the mobile host
does not time out.
In addition, active mobile hosts must transmit a route-update packet
when they cross cell borders. This is required because the Route
Cache mappings associated with the new Base Station can only be
created by authenticated route-update packets. Data packets are not
required to carry authentication information and hence can refresh,
but not modify Route Cache mappings.
For reliability and timeliness, Paging Caches also contain mobile
hosts that are contained by Route Caches. For this reason, Paging
Caches are updated by all uplink update packets and refreshed by all
uplink packets including data packets as well.
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2.3. Handoff
Handoff is initiated by the mobile host. As an active host
approaches a new Base Station, it transmits a route-update packet and
redirects its packets from the old to the new Base Station. The
route-update packet will configure Route 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
coincide, the route-update packet simply refreshes the old mapping
and the handoff remains unnoticed.)
An idle mobile host, moving to a new Base Station, transmits a
paging-update packet only if the new Base Station is in a new Paging
Area. During handoffs between Base Stations within the same Paging
Area idle mobile hosts may remain silent, as paging is performed
within the entire Paging Area.
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. After successful authentication to the Cellular IP net-
work the mobile host will invoke the IPv6 Stateless Address Auto con-
figuration mechanism to generate a temporary Mobile IPv6 care-of
address in the visited Cellular IP network. This address will be
assembled by pre pending the IPv6 subnet prefix advertised by Cellu-
lar IP beacon signals to the mobile host's interface identifier [8].
To ensure that the configured Mobile IPv6 care-of address is likely
to be unique, the mobile host may run a duplicate address detection
algorithm before assigning the new Mobile IPv6 address on its inter-
face. For security and charging purposes, authentication and other
user-related information may need to be provided by the mobile host,
when it first contacts a Cellular IP Network.
This information will be included in the first route-update or
paging-update packet and may be repeated in a few subsequent route-
update or paging-update packets for reliability. Upon receiving the
first route-update or 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
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the required setting for protocol parameters. Upon successful admis-
sion, the mobile host should send binding update messages to its Home
Agent and its correspondent nodes notifying them about its new point
of attachment [1].
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. Alternatively, as a performance optimization the host may
send a paging-teardown packet to clear Cache mappings from both Route
and Paging Caches.
2.5. Security
Cellular IP control packets (paging-update, route-update and paging-
teardown packets) carry mandatory Authentication Headers [3]. This
prevents malicious mobile hosts from changing location information
related to other mobile hosts using a spoofed source address.
Data security issues are not discussed in this document. We note
that any further authentication or encryption can be performed in
addition to control packet authentication built into Cellular IP.
3. Protocol Details
3.1. Protocol Parameters
The following parameters shall be set by network management. The
values listed here are for information only. Note that in the most
typical case a mobile host that is in active state will regularly
transmit data packets and hence route-update packets will need to be
transmitted at handoffs only.
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-------------------------------------------------------------------
Name Meaning Typical Value
-------------------------------------------------------------------
route-update-time Maximal inter-arrival time 3 sec
of packets updating the
Route Cache
route-timeout Validity of Route 9 sec
Cache mappings
paging-update-time Maximal inter-arrival time 3 min
of packets updating the
Paging Cache
paging-timeout Validity of Paging 9 min
Cache mappings
-------------------------------------------------------------------
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.
Beacons are sent in an IPv6 packet with Destination Options header
[7]. This is multicast to FF02:0:0:0:0:0:0:1 (All nodes multicast).
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=1 | Option Type=B |Opt Data Len=? |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Subnet Prefix (8 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BS ID | Paging ID | Cellular IP Network ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| |
+ Gateway IP Address (16 octets) +
| |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Layer 2 Parameters (Variable) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
8-bit type identifier. B=Beacon type.
Option Data Length
Length of extension header option.
Subnet Prefix
IPv6 subnet prefix advertisement. A 64 bit global IPv6 address
prefix must be used (not site or link-local since this would
result in the generation of a locally scoped care-of-address).
BS ID
Identifier of the advertising base station.
Paging Area ID
The ID of the current paging area.
Cellular IP Network ID
Unique ID of the current Cellular IP Network.
CU
Currently unused field.
Gateway IP
IP address of the Cellular IP network's Gateway.
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Layer 2 Parameters
Variable length field using the type-length-value (TLV) format
for giving any needed L2 parameters to a MT.
All parameters can be configured by network management.
3.3. Packet Formats
3.3.1. Data packet
Cellular IP forwards regular IP packets without modification, segmen-
tation, encapsulation or tunneling.
3.3.2. Route-update packet
A route-update packet is an IPv6 packet with a Hop-by-Hop Options
extension header [7].
- the source address is the IP address of the sending mobile host;
- the destination address is the Gateway; and
- the Hop-by-Hop option is of Route-update type.
The route-update fields will be held inside a type-length-value (TLV)
field inside the extension header.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len=1 | Option Type=R |Opt Data Len=? |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp (8 octets) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CU |S|I| CU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Control information (variable length) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
8-bit type identifier. R=Route-update type.
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Option Data Length
Length of extension header option.
Timestamp
Contains a timestamp used to determine the order in which update
packets are sent. The timestamp field is formatted as specified
by the Network Time Protocol [2]. The low-order 32 bits of the
NTP format represent fractional seconds, and those bits which are
not available from a time source should be generated from a good
source of randomness. Mobile hosts must ensure that the 64 bit
value of timestamps is strictly increasing in consecutive control
packets.
CU
Currently Unused. Must be set to 0.
S flag
Set to 1 to indicate semi-soft handoff. Default value is 0. Any
Cellular IP node that does not support semi-soft handoffs may
ignore this bit. (See Section 4.1.1)
I flag
Used to indicate an indirect semi-soft handoff. Only a Base
Station will recognize this flag and act accordingly.
Control Information
This field uses the same TLV structure as the Hop-by-Hop options.
Currently only one is defined:
Registration request
Used when a mobile host enters the Cellular IP Network.
3.3.3. Paging-update packet
A paging-update packet is an IPv6 packet with a Hop-by-Hop Options
extension header where
- the source address is the IP address of the sending mobile host;
- the destination address is the Gateway; and
- the Hop-by-Hop option is of Paging-update type.
The option of the paging-update packet carries control information in
the same format as the route-update packet. The S and I flags must be
0 for paging-update packets.
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3.3.4. Paging-teardown packet
A paging-teardown packet is an IPv6 packet with a Hop-by-Hop Options
extension header where
- the source address is the IP address of the sending mobile host;
- the destination address is the Gateway; and
- the Hop-by-Hop option is of Paging-teardown type.
The payload of the paging-teardown packet carries control information
in the same format as the route-update packet. The S and I flags must
be 0 for paging-teardown packets.
3.4. Addressing
Cellular IP requires no address space allocation beyond what is
present in IPv6. Mobile hosts are identified by their care-of
addresses.
3.5. Security
Each Cellular IP Network has a secret network key of arbitrary length
known to all Cellular IP nodes. The network key is kept secret from
mobile hosts and other nodes outside the Cellular IP Network, how-
ever. Upon initial registration the Gateway must authenticate and
possibly authorize the mobile host. This initial authentication and
authorization can be based on any known symmetric or asymmetric
method. After authentication the Gateway concatenates the key of the
network and the IP address of the mobile host and calculates the PID
of the mobile host by an MD5 Hash similarly as in [4]:
PID := MD5(network key, IP address of MH)
Then it acquires the public key of the mobile host from a trusted
party, encrypts the PID and sends it to the mobile host. This way
the mobile host and the Cellular IP network have a shared secret. The
PID remains the same during handoff and can be easily computed by
each Base Station.
The PID can be used to authenticate (and optionally to encrypt) IP
packets over the air interface. Authentication is performed by
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creating a short hash from the (PID, timestamp, packet content) tri-
ple that is placed into the transmitted packets. The validity of
each packet can be easily checked by any Base Station even immedi-
ately after a handoff and without prior communication with the mobile
host or with the old Base Station.
In addition to authenticating control packets, PID can optionally
also be used to provide security for data packets transmitted over
the wireless link. To this avail, any known shared secret based
security mechanism can be used where PID serve as the shared secret.
3.6. 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 3.7.
3.6.1 Topology
In uplink direction (toward the Gateway), packets are routed in the
Cellular IP Network on a hop-by-hop basis. The neighbor to which a
node will forward a packet toward the Gateway is referred to as the
node's Uplink neighbor. The Uplink neighbor at each node may be
designated by network management. Alternatively, a simplified shor-
test path algorithm can select Uplink neighbors instead of manual
configuration. (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
neighbors and automatically reconfigures them if necessary after a
topology change is described in Appendix A.
A node's neighbors other than the Uplink neighbor are called Downlink
neighbors.
3.6.2 Uplink Routing
A packet arriving at a node from one of its Downlink neighbors is
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assumed to be coming from a mobile host. The packet is first used to
update the node's Route and Paging Caches and is then forwarded to
the node's Uplink neighbor.
To update the Caches, the node reads the packet type, port number and
the source IP address. Paging-update packets update the Paging Cache
only. Route-update packets update both Route and Paging Caches.
Data packets only refresh the soft state of both caches, but do not
change it. Both types of caches consist of
{ IPv6 address, interface, MAC address, expiration time, timestamp }
5-tuples, called mappings. The IPv6 address is the address of the
mobile host the mapping corresponds to. The interface and the MAC
address denote the Downlink neighbor toward the mobile host. The
timestamp field contains the timestamp of the control packet that has
established the mapping.
When a data packet arrives from a Downlink neighbor, the Route Cache
entry of the source IP address is searched first. If the data packet
is coming from the same neighbor as indicated by the cache entry then
it is sent from the direction where the mobile host was last seen. In
that case the mapping is only refreshed: the expiration time is set
to the current time + route-timeout. If the node has Paging Cache,
then the expiration time of the mapping in the Paging Cache is set to
current time + paging-timeout as well.
If the data packet arrived from a different neighbor than that is in
its mapping or no mapping exists for the IP address, then the packet
is dropped.
When an update packet arrives from a Downlink neighbor then the
authentication is first validated. Packets with invalid authentica-
tion must be dropped and the event should be logged as a potential
tampering attempt. For valid packets the node creates the following
5-tuple:
{ the newly arrived packet's source IPv6 address,
the interface through which it arrived,
the source MAC address of the arrived packet,
current time + route-timeout,
the timestamp in the arrived update packet }
This mapping is used to update Route Cache, if the incoming packet is
a route-update packet. If a valid mapping for the source IPv6
address already exists, then it is replaced by the new 5-tuple, if
the timestamp is newer, otherwise the packet is dropped. If no map-
ping exists for the source IP address then the mapping is added to
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the Route Cache. The Paging Cache is updated in the same way, but
using paging-timeout instead of route-timeout. If the node has no
Paging Cache then only the Route Cache is updated. If the incoming
packet is a paging-update, then only the Paging Cache is updated (if
any).
If the packet is a paging-teardown packet and the authentication
information is valid, then mappings of the mobile host with timestamp
earlier than the timestamp of the packet are removed from both the
Route and the Paging Cache.
After cache modifications the control packet is forwarded to the
Uplink neighbor.
3.6.3 Downlink Routing
A packet arriving to a Cellular IP node from the Uplink neighbor is
assumed to be addressed to a mobile host. The node first checks if
the destination IP address has a valid mapping in the Route Cache.
If such a mapping exists, the packet is forwarded to the Downlink
neighbor found in the mapping.
If the Route Cache contains no mapping for the destination IP address
and the node has no Paging Cache, then the packet is broadcast on all
interfaces of the node except the interface of the Uplink neighbor.
If the node has Paging Cache and there is a mapping for the destina-
tion IP address, then the packet is forwarded to the neighbor found
in that mapping.
If the node has Paging Cache, but there is no mapping for the desti-
nation IP address, then the packet is dropped.
3.7. 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 Con-
troller.
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IP network
===================
|
+------------------------------|--------+
| | |
| +----------+ +-------------+ |
| | Gateway |__________| Gateway | |
| |Controller| |Packet Filter| |
| +----------+ +-------------+ |
| |________|___Uplink neighbor
| | |
| +-------------+ |
| Cellular IP |Cellular IP | |
| Gateway | node | |
| +-------------+ |
| | | | |
+-------------------------|----|----|---+
<----Downlink neighbors
Uplink packets update the Route 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 control packets with empty control informa-
tion field and are immediately dropped. If the packet carries con-
trol information, for instance a registration request, it is inter-
preted and processed by the Gateway controller.
If the destination address is not the Gateway's, the packet is for-
warded 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 map-
ping in any of the Gateway's caches. If a mapping is found, the
packet is "turned back" and is treated as a downlink packet.)
All packets arriving from the Internet are forwarded normally to the
Cellular IP node block. The Gateway's Cellular IP node block treats
these packets as determined by the Cellular IP Routing algorithm
(Section 3.6) according to the mappings in Route and Paging Cache.
It is optional whether Cellular IP Nodes have Paging Cache configured
or not. However, it is recommended that at least the Gateway's Cellu-
lar IP node has a Paging Cache configured. This ensures that packets
addressed to hosts currently not connected to the Cellular IP Network
do not enter the network and do not load it in vain but are
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immediately discarded in the Gateway when neither Route, nor Paging
Cache mapping is found for the destination address. (It may be
advantageous to also generate an ICMP message in this case and send
it back to the packet's source address.)
3.8. Cellular IP Mobile Host
While connected to a Cellular IP Network, a mobile host must be in
one of two states: 'active' or 'idle'. The host moves from idle to
active state when it receives or wishes to send a data packet.
Active state is maintained as long as the host is transmitting or
receiving data packets. When the host has not received or transmit-
ted any data packets for some time (the value of this timer may be
implementation-specific) then it 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, which will probably be typical.
If the host is in active state, it must immediately transmit a
route-update 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. A
packet transmitted this way also resets the route-update packet
timer.
In idle state, the mobile host must transmit paging-update packets
periodically, at intervals of paging-update-time. In addition, the
host must transmit a paging-update packet when it connects to a new
Base Station which has a different Paging Area ID from the previous
Base Station. (When connecting to a Base Station that belongs to the
same Paging Area as the previous one, the host need not transmit
paging-update packet.) Similarly to the route-update packet timer,
the paging-update timer is reset if a data packet is transmitted.
The mobile host must ensure that the 64 bit value of timestamps is
strictly increasing in consecutive control packets.
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The mobile host processes all IPv6 packets which it receives. If an
IPv6 packet carries a Routing extension header then it is processed
normally to cause the packet to be delivered to the upper layers (the
home address of the mobile host is included in the Routing header as
the final destination of the packet). Alternatively, the packet is
received encapsulated into an IPv6 tunnel header. In this case the
mobile host performs IPv6 decapsulation to extract the original IPv6
packet and then sends a Mobile IPv6 binding update message to the
packet sender.
4. Extensions to Cellular IP
4.1. Handoff Extensions
4.1.1. Semi-soft Handoff
When a mobile host switches to a new Base Station it sends a route-
update packet to make the chain of cache bindings to point to the new
Base Station. Packets that are traveling on the old path will be
delivered to the old Base Station and will be lost. Although this
loss may be small it can potentially degrade TCP throughput. This
kind of handoff, when the mobile switches all at once to the new Base
Station is called "hard" handoff. For performance details of hard
handoff in a Cellular IP network see [5].
To improve the performance of loss sensitive applications, another
type of handoff may be introduced, called "semi-soft" handoff. Dur-
ing semi-soft handoff a mobile host may be in contact with either of
the old and new Base Stations and receive packets from them. Packets
intended to the mobile host are sent to both Base Stations, so when
the mobile host eventually moves to the new location it can continue
to receive packets without interruption.
To initiate semi-soft handoff, the moving mobile host transmits a
route-update packet to the new Base Station and continues to listen
to the old one. The S flag is set in this route-update packet to
indicate semi-soft handoff. Semi-soft route-update packets create
new mappings in the Route and Paging Cache similarly to regular
route-update packets. When the semi-soft route-update packet reaches
the cross-over node where the old and new path meet (note that the
cross-over node already has a mapping for the mobile host), the new
mapping is added to the cache instead of replacing the old one.
Packets sent to the mobile host are transmitted to both Downlink
neighbors. When the mobile host eventually makes the move then the
packets will already be underway to the new Base Station and the
handoff can be performed with minimal packet loss. After migration
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the mobile host sends a route-update packet to the new Base Station
with the S bit cleared. This route-update packet will remove all map-
pings in Route Cache except for the ones pointing to the new Base
Station. The semi-soft handoff is then complete.
If the path to the new Base Station is longer than to the old Base
Station or it takes non negligible time to switch to the new Base
Station, then some packets may not reach the mobile host. To over-
come the problem, packets sent to the new Base Station can be delayed
during the semi-soft handoff. This way a few packets may be
delivered twice to the mobile host, but in many cases this results in
better performance than a few packets lost. Introduction of packet
delay can be best performed in the Cellular IP node that has multiple
mappings for the mobile host as a result of a semi-soft route-update
packet. Packets that belong to flows that require low delay but can
tolerate occasional losses should not be delayed. For performance
details of semi-soft handoff in a Cellular IP network see [5].
4.1.2. Indirect Semi-soft Handoff
Not all wireless technologies have simultaneous connection capabil-
ity. e.g. They cannot listen to the current BS while sending a
route-update packet to the new BS (as required in 4.1.1). For this
situation an alternative indirect technique is used. It is assumed
the network can obtain the IP address of the new BS. This is the
case in many cellular networks.
When the mobile decides to make a handoff, instead of sending a
route-update packet to the new BS directly (as it cannot), it sends
the packet to the current BS. This packet will have as a destination
IP address, the IP address of the new BS. Unlike in section 4.1.1.
the I flag will be set to indicate indirect semi-soft handoff. The
current BS will forward this uplink to the Gateway normally. The
Gateway then uses normal IP routing to deliver the packet to the new
BS. When the new BS receives the indirect handoff packet, a semi-soft
route update packet is created with the IP address of the mobile
host. It is then forwarded upstream. The algorithm then proceeds to
work as in 4.1.1, just as if the packet had originated through the
new BS. A security association is assumed for the new BS.
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4.2. Multiple Gateway Networks
Cellular IP requires that a mobile host be using exactly one Gateway
at a time. This requirement comes from the fact that the Gateway
relays its packets both up and downlink. It is also required to make
uplink routing unambiguous. The Cellular IP Network can have multi-
ple Gateways as long as a single host still uses just one Gateway at
any time. (The host can change Gateway, involving a Mobile IPv6 loca-
tion 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.
4.3. 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.
5. Security Considerations
A Cellular IP Network is a single administrative domain. It is con-
nected to the Internet through a Gateway that may eventually also
serve as a firewall. Hence security issues only need to be con-
sidered at the wireless interface.
The security of a Cellular IP system will be determined by the wire-
less link. Security issues relating to wireless links are not
specific to Cellular IP, and are out of the scope of Cellular IP,
even though they must be dealt with in practical Cellular IP imple-
mentations.
A security problem specific to Cellular IP is the security of the
control packets, which can be solved by the authentication mechanism
described in Section 3.5.
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6. Intellectual Property Right Notice
Ericsson has filed patent applications that might possibly become
essential to the implementation of this contribution.
References
[1] "Mobility Support in IPv6," D. Johnson, C. Perkins, Work in
Progress, <draft-ietf-mobileip-ipv6-14.txt>, July 2000.
[2] "Network Time Protocol (Version 3): Specification, Implementation
and Analysis," D. Mills, IETF RFC 1305, March 1992.
[3] "IP Authentication Header," S. Kent, R. Atkinson, IETF RFC 2402,
November 1998.
[4] "IP Authentication using Keyed MD5," P. Metzger, W. Simpson, IETF
RFC 1828, August 1995.
[5] A. T. Campbell, Gomez, J., Kim, S., Turanyi, Z., Wan, C-Y. and A, Valko
"Design, Implementation and Evaluation of Cellular IP", IEEE Personal
Communications, June/July 2000.
[6] A. G. Valko, "Cellular IP - A New Approach to Internet Host Mobility,"
ACM Computer Communication Review, January 1999.
[7] "Internet Protocol, Version 6 (IPv6) Specification," IETF RFC
2460, December 1998.
[8] "IPv6 stateless address autoconfiguration," Susan Thomson, Thomas
Narten, IETF RFC 2462, December 1998.
[9] "Cellular IPv6 home page", <http://cipv6.intranet.gr>, Intracom Greece,
July 2001.
Authors' Addresses
Zach D. Shelby
University of Oulu
Center for Wireless Communications
PO Box 4500
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90014 Oulu, Finland
phone: +358 40 779 6297
email: zach.shelby@ee.oulu.fi
Dionisios D. Gatzounas
INTRACOM S.A.
Development Programmes Department
Panepistimiou 254
26443 Patras
GREECE
phone: +30 61 465168
fax: +30 61 465070
email: dgat@intracom.gr
Andrew T. Campbell, Chieh-Yih Wan
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,wan}@comet.columbia.edu
Appendix A. Uplink Neighbor Selection
This algorithm selects the Uplink neighbor of all nodes of a Cellular
IPv6 Network and reconfigures them if necessary after a change of
topology. An Uplink neighbor is identified by the interface through
which it is accessible from the node and its corresponding MAC
address. The algorithm also distributes the Cellular IP Network
Identifier, the IP address of the Gateway and the Paging Area IDs to
the Base Stations.
The Gateway periodically creates a control packet called a "Gateway
broadcast packet". Packet uses a hop-by-hop extension header. The
Gateway broadcast packet contains
- the Cellular IP Network Identifier;
- the IP address of the Gateway;
- a sequence number increased each time by the Gateway;
- a Paging Area ID field initially set to the ID of the Gateway; and
- the advertised IPv6 subnet prefix.
The Gateway broadcasts the packet on all of its interfaces except
those connected to the Internet. A Cellular IP node receiving a
Gateway broadcast packet follows the steps below:
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1) It drops the packet if the sequence number is lower or equal to
the sequence number of one of the previously received Gateway
broadcast packets. In this case no further processing is needed.
2) It stores the sequence number of the Gateway broadcast packet for
later comparison.
3) It stores the Cellular IP Network Identifier and the IP address of
the Gateway.
4) It stores the interface through which the packet arrived together
with source MAC address of the packet (if any) to identify the
Uplink neighbor. All other interface/MAC address combinations
will denote Downlink neighbors.
5) If the node has a Paging Cache, it overwrites the value of the
Paging Area ID field in the packet by its own ID.
6) The value of the (possibly overwritten) Paging Area ID field is
stored as the Paging Area ID of the node. This value will be used
in beacon signals if the node is a Base Station.
7) It stores the Cellular IP Network Identifier and the IP address of
the Gateway. These values will be used in beacon signals if the
node is a Base Station.
8) It stores the advertised IPv6 subnet prefix. This value will be
used in beacon signals if the node is a Base Station.
9) After a short random delay, the node broadcasts the packet through
all of its interfaces, except the air interface(s) and the
interface of the Uplink neighbor.
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Table of Contents
1. Introduction ................................................... 2
1.1. Applicability ................................................ 3
1.2. New Architectural Entities ................................... 3
1.3. Terminology .................................................. 3
1.4. Protocol Overview ............................................ 5
1.5. Location Management and Routing .............................. 8
2. Cellular IP Functions .......................................... 8
2.1. Location Management .......................................... 8
2.2. Routing ...................................................... 9
2.3. Handoff ...................................................... 11
2.4. Wide Area Mobility ........................................... 11
2.5. Security ..................................................... 12
3. Protocol Details ............................................... 12
3.1. Protocol Parameters .......................................... 12
3.2. Beacon Signal Structure ...................................... 13
3.3. Packet Formats ............................................... 15
3.3.1. Data packet ................................................ 15
3.3.2. Route-update packet ........................................ 15
3.3.3. Paging-update packet ....................................... 16
3.3.4. Paging-teardown packet ..................................... 17
3.4. Addressing ................................................... 17
3.5. Security ..................................................... 17
3.6. Cellular IP Routing .......................................... 18
3.6.1 Topology .................................................... 18
3.6.2 Uplink Routing .............................................. 18
3.6.3 Downlink Routing ............................................ 20
3.7. Cellular IP Gateway .......................................... 20
3.8. Cellular IP Mobile Host ...................................... 22
4. Extensions to Cellular IP ...................................... 23
4.1. Handoff Extensions ........................................... 23
4.1.1. Semi-soft Handoff .......................................... 23
4.1.2. Indirect Semi-soft Handoff ................................. 24
4.2. Multiple Gateway Networks .................................... 25
4.3. Charging ..................................................... 25
5. Security Considerations ........................................ 25
6. Intellectual Property Right Notice ............................. 26
References ........................................................ 26
Authors' Addresses ................................................ 26
Appendix A. Uplink Neighbor Selection ............................. 27
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