INTERNET-DRAFT A. Valko Ericsson, Columbia University A. Campbell, J. Gomez Columbia University Expires May 1999 November 1998 Cellular IP Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id- abstracts.txt'' listing contained in the Internet- Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). 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. Valko, Campbell, Gomez Expires May 1999 [Page 1] INTERNET-DRAFT Cellular IP 18 November 1998 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. Valko, Campbell, Gomez Expires May 1999 [Page 2] INTERNET-DRAFT Cellular IP 18 November 1998 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. Valko, Campbell, Gomez Expires May 1999 [Page 3] INTERNET-DRAFT Cellular IP 18 November 1998 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 Valko, Campbell, Gomez Expires May 1999 [Page 4] INTERNET-DRAFT Cellular IP 18 November 1998 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 Valko, Campbell, Gomez Expires May 1999 [Page 5] INTERNET-DRAFT Cellular IP 18 November 1998 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. Valko, Campbell, Gomez Expires May 1999 [Page 6] INTERNET-DRAFT Cellular IP 18 November 1998 +--------+ |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. Valko, Campbell, Gomez Expires May 1999 [Page 7] INTERNET-DRAFT Cellular IP 18 November 1998 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. Valko, Campbell, Gomez Expires May 1999 [Page 8] INTERNET-DRAFT Cellular IP 18 November 1998 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 Valko, Campbell, Gomez Expires May 1999 [Page 9] INTERNET-DRAFT Cellular IP 18 November 1998 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. Valko, Campbell, Gomez Expires May 1999 [Page 10] INTERNET-DRAFT Cellular IP 18 November 1998 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: Valko, Campbell, Gomez Expires May 1999 [Page 11] INTERNET-DRAFT Cellular IP 18 November 1998 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. Valko, Campbell, Gomez Expires May 1999 [Page 12] INTERNET-DRAFT Cellular IP 18 November 1998 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: Valko, Campbell, Gomez Expires May 1999 [Page 13] INTERNET-DRAFT Cellular IP 18 November 1998 { 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 +-------------------------|----|----|---+ Valko, Campbell, Gomez Expires May 1999 [Page 14] INTERNET-DRAFT Cellular IP 18 November 1998 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 Valko, Campbell, Gomez Expires May 1999 [Page 15] INTERNET-DRAFT Cellular IP 18 November 1998 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, Valko, Campbell, Gomez Expires May 1999 [Page 16] INTERNET-DRAFT Cellular IP 18 November 1998 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 Valko, Campbell, Gomez Expires May 1999 [Page 17] INTERNET-DRAFT Cellular IP 18 November 1998 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 Valko, Campbell, Gomez Expires May 1999 [Page 18] INTERNET-DRAFT Cellular IP 18 November 1998 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 Valko, Campbell, Gomez Expires May 1999 [Page 19]