Multiple Gateway Cellular IP Network Manas R. Panda [email protected] E6951 Project, Dated: 05/06/02

ABSTRACT In the current Cellular IP architecture, only one gateway serves the entire CIP network. So, the gateway is single point of failure for all the mobile hosts who rely on it to be connected to the Internet. The paper proposes an extension to the current CIP architecture that allows multiple gateways in the CIP network that run simultaneously. When one gateway goes down, all the served mobile hosts are automatically switched-over to other gateways that are still active. The Mobile IP Foreign Agent, which is collocated with the gateway, is also switched over simultaneously. The architecture extension is explored and evaluated using the network simulator.

Table of Contents 1 INTRODUCTION........................................................................................................... .......................3 1.1 TERMINOLOGY..........................................................................................................................................3 1.2 PROTOCOL OVERVIEW...............................................................................................................................3 2 NEW FUNCTIONS......................................................................................................... .......................5 2.1 GATEWAY SELECTION ALGORITHM..............................................................................................................5 2.2 GATEWAY AND FOREIGN AGENT SWITCHOVER...............................................................................................5 2.3 BEACON SIGNAL STRUCTURE.....................................................................................................................6 2.4 PACKET FORMATS.....................................................................................................................................6 2.5 ROUTING.................................................................................................................................................6 2.5.1 Topology................................................................................................................... ..................6 2.5.2 Uplink Routing............................................................................................... ............................7 2.5.3 Downlink Routing........................................................................................................... ............7 3 CIMS EXTENSIONS.......................................................................................................... ...................7 4 SIMULATION.................................................................................................................. ......................8 4.1 GOALS AND METRICS AND METHODOLOGY..................................................................................................8 4.2 RESULTS..................................................................................................................................................9 5 FUTURE WORK........................................................................................................................... .........9 6 CONCLUSIONS......................................................................................................................... ............9 7 ACKNOWLEDGEMENT............................................................................................................ ..........9 8 REFERENCES................................................................................................................ .......................9

1

INTRODUCTION

Cellular IP (CIP) [1] is a protocol that provides local mobility and handoff support for frequently moving hosts. Cellular IP interworks with Mobile IP [2] to support wide area mobility. A Cellular IP network is connected to the Internet via a gateway node. All IP packets are transmitted to the gateway by hop-by-hop shortest path routing. The gateway node also serves as the Mobile IP Foreign Agent (FA). Packets arriving from the Internet to the mobile hosts are received by the gateway/FA. They are decapsulated and forwarded to the mobile host by the CIP nodes using the route-cache or paging cache. So, all the mobile hosts in a CIP network rely on a single gateway to access the Internet. This makes the gateway the single point of failure for all mobile hosts in the CIP network. The paper specifies a solution to alleviate the problem with minimum change to the CIP architecture and protocol. The solution is an extension to the current CIP architecture that allows multiple gateways in the CIP network running simultaneously. When one gateway goes down, all the served mobile hosts are automatically switchedover to other gateways that are still active. The Mobile IP FA is also switched over simultaneously. When a failed gateway comes back up, all mobile hosts that were switched over, are switched back to the gateway. This ensures that equal load on all serving gateways.

1.1 Terminology Active gateway A gateway that is up and operational. Inactive gateway A gateway that is down, and not operational. Assigned gateway Each mobile host is assigned one of the gateways in the CIP network. The assigned gateway is automatically selected using the gateway selection algorithm [Section 2.1]. Mobile host always use the assigned gateway, whenever it is active. Switched gateway If the assigned gateway is inactive, then a mobile host uses a switched gateway. The switched gateway is selected dynamically using the gateway Selection Algorithm [Section 2.1]. Serving gateway The serving gateway of a mobile host is the gateway that is used by the mobile host to access the Internet. It's either an assigned gateway or a switched gateway.

1.2 Protocol Overview This section gives an overview of the operation of a multiple gateway CIP network. Figure 1-1 shows a CIP network, which is connected to the Internet via multiple gateways. The CIP nodes inside the CIP network can be connected in any arbitrary mesh pattern. Some of the CIP nodes are connected to the Internet via a gateway. For each gateway a minimum spanning tree is constructed and the uplink neighbor information per gateway is stored in the CIP nodes. The uplink neighbor information is used to route all uplink IP packets from the mobile hosts towards the serving gateway, regardless of the destination address.

Figure 1-1 Cellular IP Network Base Stations periodically emit beacon signals. Mobile hosts use these beacon signals to locate the nearest Base Station. The beacon signal is also used by the mobile Hosts to determine if the serving gateway is still active. If not, the Mobile host initiates a gateway switchover. All active gateways serve the mobile hosts simultaneously. The mobile hosts are partitioned by their IP addresses, such that each mobile host is served by a single gateway at any point and all gateways serve almost same number of mobile hosts. The CIP nodes and mobile hosts automatically select the serving gateway using a simple gateway selection algorithm. Each mobile has an assigned gateway, which it tries to use all the time. If the assigned gateway is inactive, the served mobile hosts are uniformly distributed

to all other active gateways using the gateway selection algorithm. When the assigned gateway becomes active again, the mobile hosts are switched back to it. The Mobile IP Foreign Agent is collocated with the gateway. So, the serving gateway is also the FA for a mobile host. Whenever a gateway is switched, the FA is also switched with it. There is no change in the basic routing and location management functions using the route cache and paging cache, and is described in Cellular IP Internet draft [1].

2

NEW FUNCTIONS

2.1 Gateway Selection Algorithm The gateway used by a mobile host at a particular time is automatically determined by using a hash function on the mobile host IP address. A mod function is used for simplicity. Each mobile host has an assigned gateway, which is a mod N function is used on the host IP address, where N is the total number of provisioned gateways in the CIP network. For example, if the CIP network has three gateways, gw0 and gw1, then gw1 is selected for host 142.6.160.11 since 2382798865 mod 2 = 1. If a gateway goes down, the next active gateway in the gateway list is chosen. The list is circular; hence after the last gateway in the list, we go to the beginning of the list. In the above example, if gw1 goes down, gw0 is selected for host 142.6.160.11 assuming gw0 is active. In this case, the mobile host is said to be using a switched gateway. The gateway used by a mobile host at any point, either assigned or switched, is called the serving gateway. The order of the gateways in the list is important, and should be established by the network management. It can either be assigned or determined by the order in which the gateways are powered up. The algorithm is used by mobile host to select a serving gateway when sending route-update and pageupdate messages. The algorithm is also used by the CIP nodes to determine the serving gateway for mobile hosts, when forwarding data packets for which there is no route-cache.

2.2 Gateway and Foreign Agent Switchover The mobile host is responsible for initiating the gateway switchover. The FA, which is collocated with the gateway, is also switched over simultaneously. Whenever a gateway goes down and comes back up, the Base Stations are informed via the gateway broadcast packets (section 3.3.1). This information is relayed to the mobile hosts by the Base Station beacon. The mobile host uses the gateway state in the beacon signal (section 3.1) to initiate the gateway switchover. The mobile host initiate switchover in the following situations: Gateway Goes Down: The mobile host keeps track of the gateway it is currently using, which is called the serving gateway. For each beacon it listens, it checks the active flag in the gateway list to verify if the serving gateway is still active. If it is not, the mobile host initiates a gateway switchover. Gateway Comes Back Up: When a gateway that is down comes back up, all mobile hosts are switched back to the gateway immediately, so that the gateway selection algorithm is consistent between the CIP nodes and mobile hosts. If the mobile host is using a switched gateway, it checks the gateway list in the beacon to see if the assigned gateway is active again. If it is, it initiates a switchover to go back to the assigned gateway and FA.

The switchover works like this. The mobile host selects a new serving gateway address from the beacon gateway list using the gateway selection algorithm. The mobile host then sends a route-update packet to the new serving gateway to update the route-cache and paging-cache at the CIP nodes. The payload of the packet carries authentication and control information. A Mobile IP registration request is included in the control information. The new gateway becomes the Foreign Agent for the mobile host, and forwards the registration to the home agent, specifying the gateway's IP address as care-of-address.

2.3 Beacon Signal Structure Cellular IP Base Stations periodically transmit beacon signals, which allow mobile hosts to identify an available Base Station and determine the state of the serving gateway. Information elements carried by the beacon signal are: - Layer2 parameters related to Base Station - Cellular IP Network identifier. - ID of the Paging Area. - (New) Total number of gateways in CIP network. - (New) IP addresses of all provisioned gateways in the network, with active flag indicating if the gateway is up or down.

2.4 Packet Formats There is no change in the existing formats of data or control packets for multiple gateway support. Except for the gateway Broadcast control packets, there is no new control packet required for multiple gateway support.

2.5 Routing The CIP nodes do not need regular IP routing capability. Packets are routed between the serving gateway and mobile host is done using the base Cellular IP algorithms specified in [1]. There is no change in the basic routing algorithms. However, some of the state information regarding the topology needs to be extended to support multiple gateways. This section highlights those extensions only.

2.5.1 Topology Each gateway has a shortest path tree that connects all the CIP nodes in the network, which have physical path to it. The shortest path tree is realized as uplink neighbor information that is stored in the CIP nodes. So, for N gateways in the CIP network, each CIP node has N uplink neighbor information, one per gateway. For each gateway the CIP node maintains an active flag, which indicates if the gateway is up (active=1) or down (active=0). So, the uplink neighbor information for each gateway is stored as the following 4-tuple record: { gateway IP-address, active flag, interface, MAC address } There is one such record per gateway. If values { gateway IP-address, active flag } are passed in the Beacon signal if the nodes are Base Station. The uplink neighbor information are created and updated by topology changes. The topology changes are propagated to the CIP nodes using the following gateway broadcast protocols: Gateway Beacon Broadcast The gateway beacon broadcast control packet, as described in appendix A of the Cellular IP draft [1], creates and updates the uplink neighbor in the CIP nodes. Each gateway periodically broadcasts these control packets into the CIP network to update the uplink neighbor information in the CIP nodes. In order to minimize the load of control packets in the CIP network, the gateway beacon are broadcasted less frequently (30 seconds). The broadcast from all gateways are done in a staggered manner.

Since, gateway beacon broadcast are infrequent, it cannot be used to detect the gateway failure. So, we need explicit gateway down broadcast. Gateway Down Broadcast When a gateway fails, the CIP node directly connected to the gateway know about it. It sends a gateway down broadcast packet on all interfaces except those connected to the Internet. The packet uses the ICMP packet format and contains the gateway IP address that is unreachable. A CIP node receiving a gate down broadcast packet follows the step below: 1. It drops the packet if the gateway is already marked inactive (active flag = 0) in the uplink neighbor information. In this case no further processing is needed. This will avoid any broadcast loop. 2. It marks the gateway indicated in the packet inactive (active flag = 0) in the uplink neighbor information. 3. After a random delay, the node broadcasts the packet through all its interfaces, except the air interface(s) and the interface the packet came from. If the node is a Base Station, the subsequent Beacons have the gateway marked as inactive. So, all the mobile hosts who are it as the serving gateway initiate a gateway switchover.

2.5.2 Uplink Routing The only change in uplink routing is selection of the serving gateway for the mobile host, and forwarding packets to the uplink neighbor for the particular gateway. There is no change to the route cache and paging cache management. When a packet comes from a mobile host, it is first used to update the node's route cache and paging cache using the existing algorithm. In order to forward the packet, the node uses the gateway selection algorithm to find the serving gateway for the mobile host. Then the packet is forwarded to the node's uplink neighbor of the serving gateway.

2.5.3 Downlink Routing There is no change in downlink routing for multiple gateways.

3

CIMS EXTENSIONS

The multiple gateway solution specified in this document is implemented using the ns-2 [3] network simulator. The Columbia IP Micro-Mobility Software (CIMS) [4], which provides a micro-mobility extension for the ns-2, is further extended to support multiple gateways. The following changes are made to CIMS to support multiple gateways. cip.h - Add gateway list to the CIP header, so that the gateway list is sent in the beacon to mobile host. - Add gateway list CIPBSAgent. - Add the serving gateway in CIPMHAgent. cip-reg.cc - Add gateway-add, gateway-up and gateway-down commands to CIPBSAgent. - CIPBSAgent sends up-to-date gateway list to the mobile hosts in the beacon. - CIPMHAgent gets the gateway list from the beacon and checks if the serving gateway has changed. When the serving gateway is changed, the TCL script sends a route-update message. cip-mgw.h (new) - Define the gateway list and methods to manipulate the gateway list. cip-mgw.cc (new) - Implements methods to manipulate the gateway list.

-

Implement gateway selection algorithm.

classifier-cip.h - Store rootnode (uplink neighbor) id per gateway. - Add a flag that tells if the node is a gateway or not. - Add gateway list to the CIP header, which is used for uplink routing. classifier-cip.cc - Add gateway-add, gateway-up, gateway-down, get-rootnodeid, set-rootnodeid commands. - The gateway selection algorithm is run on the gateway list to determine the serving gateway for a mobile host. The uplink packet is forwarded to the rootnode of the serving gateway. - Dump the CIP_PAGE and CIP_ROUTE control packets at the gateway. Do not forward them to Internet. cip-mgw-init.tcl (modified cipInit.tcl) - Split MakeGW procedure into two; MakeGW and gwConnectToInternet. MakeGW sets up a gateway and add it to the list. gwConnectToInternet connects a gateway to Internet corresponding host. - Add GWdown and GWup procedure to inform the node and its CIP classifier about gateways going down and coming back up. - Add gateway list to node and classifiers in cipEnabledNode procedure. - cipConnectToRoot procedure is modified to set the rootnodeid per gateway. - Add RouteUpdate procedure to send route-update message, when the gateway switchover occurs. cip-mgw.tcl (modified cip-test.tcl) - This script demonstrates multiple gateway support in CIP. - Defines the topology with 2 gateways, 5 CIP nodes, 4 Base Stations, 1 mobile host and 2 corresponding hosts (CH). - Defines the minimum spanning tree for each gateway. - Added gw-down and gw-up procedure to inform all nodes and classifiers about gateways going down and coming back up. - Mobile IP FA assignment is simulated by switching the CBR traffic from CH, from one gateway to the other.

4

SIMULATION

The extension to CIMS is used for the simulation of multiple gateway solution for CIP.

4.1 Goals and Metrics and Methodology The goals of the simulation are following: - Verify the multiple gateway solution as specified in this document. - Verify gateway switchover with mobile host handoff. - Evaluate the gateway switchover performance. Gateway switchover performance metrics are: - Packet loss - Gateway Switchover Delay Due to limited time and incomplete support of CIP protocol in CIMS, the switchover performance is not supported in the first phase. The simulation is performed using the network topology shown in Figure 1-1. The topology construction and dynamics are hard-coded in the simulation scripts. The topology used for simulation has a single CIP network with 2 gateways, 5 CIP nodes and 4 Base Stations. The corresponding host on the Internet is

considered just a hop away from all gateways. A single mobile host is used for simulation. A Constant Bit Rate (CBR) UDP traffic stream is run from the corresponding host and the mobile host. To start with, the mobile host uses GW1 as the serving gateway. The gateway GW1 is taken down during simulation, at which point the mobile host is switched to GW0. After sometime GW1 comes back and the mobile host is switched back to GW1. During the simulation, the mobile host is moving from base station BS0 to BS3 and handed-over appropriately. Some of the short cuts taken for the simulation are: 1. All topology is assigned. The Gateway Beacon Protocol and Gateway Down Protocol are not implemented. 2. The simulation does not support the broadcast protocols. So, all gateway dynamics are assigned to the nodes and CIP classifiers. 3. Since Mobile IP and FA are not supported in CIMS, it was not possible to switch the CBR flow from one gateway to the other, when gateway goes down. So, two corresponding host are used with their own CBR traffic. The CBR is started and stopped, when the gateway is switched.

4.2 Results The multiple gateway architecture, gateway selection algorithm and gateway switchover functionality is verified by the CIMS extension software. The gateway switchover did not impact the mobile host handoff adversely.

5

FUTURE WORK

The work can be extended by evaluating the gateway switchover performance. The performance metrics are switchover delay and packet loss during switchover. The switchover performance is depended upon how soon the gateway up and down information is conveyed to the Base Station, which is then relayed to the mobile host in the beacon. So, to do the evaluation, a good model for gateway topology changes needs to be used. The current can certainly benefit from a good implementation of Gateway Beacon Protocol and Gateway Down Protocol.

6

CONCLUSIONS

The paper proposes a lightweight extension to the CIP architecture to allow multiple gateways in the CIP network. The mobile host initiates the gateway switchover, when the serving gateway goes down. The gateway selection algorithm uses the current snapshot of gateway states, and uniformly distributes the serving gateways to the mobile hosts. The architecture extension is implement using the CIMS.

7

ACKNOWLEDGEMENT

The author would like to acknowledge Hung-yu Wie and Sanghyo Kim for their valuable inputs.

8

REFERENCES

[1] Cellular IP, Internet Draft, draft-valko-cellularip-01.txt, A Valko, A Campbell et. al, Oct 1999. [2] IP Mobility Support, C. Perkins, IETF RFC 2002, October 1996. [3] The Network Simulator - ns-2, http://www.isi.edu/nsnam/ns/. [4] Columbia IP Micro-Mobility Software, http://www.comet.columbia.edu/micromobility/