|
|
|
1. Introduction 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. 1.1. 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. 1.2. New Architectural Entities Cellular IP Node Cellular IP Base Station 1.3. Terminology Active Mobile Host Active-state-timeout Cellular IP Network Identifier Control packet Data packet Downlink Downlink neighbor Idle Mobile Host Internet Neighbor Paging Area Paging Cache Paging-timeout Paging-update packet Paging-update-time Paging-teardown packet Route-timeout Route-update packet Route-update-time Route Cache Update packet Uplink Uplink neighbor 1.4. Protocol Overview The figure shown below presents a schematic view of multiple Cellular IP Networks providing access to the Internet.
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. To prevent its mappings from timing out, a mobile host can periodically
transmit control packets. Control packets are ICMP packets with specific
authentication payloads.
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. 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 the cells into Paging Areas, each comprising an arbitrary
number of (typically adjacent) cells. Each Paging 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. 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 "implicit 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. 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 Cache mappings.
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. 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.) 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. 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 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. After successful authentication to the Cellular IP network 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. 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 information. This prevents malicious
mobile hosts from changing location information related to other mobile hosts
using a spoofed source address. The details of the authentication mechanism can
be found in section 3.5. 3.1. Protocol Parameters The following parameters shall be set by network management. The values listed here are for information only. Note that most of the time an active mobile host will transmit data packets and route- update packets will need to be sent less frequently than 1 in every second.
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; All parameters can be configured by network management. As an alternative, in Appendix A we present an example algorithm for automatically distributing the Cellular IP Network Identifier, the IP address of the Gateway and the Paging Area IDs to Base Stations. 3.3. Packet Formats 3.3.1. Data packet 3.3.2. Route-update packet - the source address is the IP address of the sending mobile host; The payload of the route-update packet carries authentication and control information in the following format:
Timestamp CU S flag AType Auth. Length Authentication Alternatively the Authentication Header [3] could also be used for authenticating control packets. This issue is for further study. Control information is encoded in the following Type-Length-Value format:
Type Length Data Currently the following type of control information is defined (details are for further study): Registration request 3.3.3. Paging-update packet - the source address is the IP address of the sending mobile host; The payload of the paging-update packet carries authentication and control information in the same format as the route-update packet. The S flag must be 0 for paging-update packets. 3.3.4. Paging-teardown packet - the source address is the IP address of the sending mobile host; The payload of the paging-teardown packet carries authentication and control information in the same format as the route-update packet. The S flag must be 0 for paging-teardown packets. 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. 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, however. 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 creating a short hash from the (PID, timestamp, packet content) triple that is placed into the transmitted packets. The validity of each packet can be easily checked by any Base Station even immediately 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 3.6.2 Uplink Routing { IP-address, interface, MAC address, expiration time, timestamp } 5-tuples, called mappings. The IP 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. Then the packet is forwarded uplink. 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 validated first. Packets with invalid authentication 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 IP 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 IP address already
exists, then it is replaced by the new 5-tuple, if the timestamp is newer,
otherwise the packet is dropped. If no mapping exists for the source IP address
then the mapping is added to 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). 3.6.3 Downlink Routing 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 Controller.
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 information field and are immediately dropped. If the packet
carries control information, for instance a registration request, it is
interpreted and processed by the Gateway controller. 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 any IP packet. If it does not receive or send 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. 4.1. 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]. 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 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. 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. 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. 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 implementations. 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. 6. Intellectual Property Right Notice This is to affirm that Telefonaktiebolaget LM Ericsson and its subsidiaries, in accordance with corporate policy, will offer patent licensing for submissions rightfully made by its employees which are adopted or recommended as a standard by your organization as follows: If part(s) of a submission by Ericsson employees is (are) included in a standard and Ericsson has patents and/or patent application(s) that are essential to implementation of such included part(s) in said standard, Ericsson is prepared to grant - on the basis of reciprocity (grantback) - a license on such included part(s) on reasonable, non- discriminatory terms and conditions. Ericsson has filed patent applications that might possibly become essential to the implementation of this contribution. References [1] "IP Mobility Support," C. Perkins, ed., IETF RFC 2002, October 1996. [2] "Network Time Protocol (Version 3): Specification, Implementation and Analysis," D. Mills, IETF RFC 1305, March 1992. [3] "IP Authentication Header," R. Atkinson, IETF RFC 1826, August 1995. [4] "IP Authentication using Keyed MD5," P. Metzger, W. Simpson, IETF RFC 1828, August 1995. [5] "Cellular IP Performance," A. T. Cambell, S. Kim, J. Gomez,
C-Y. Wan, Z. Turanyi, A. Valko
|
|
Copyright© Comet Group in Columbia
University 1999 |
