Survey on Techniques providing Internet Connectivity to Mobile Ad Hoc Networks

Transcription

1 Survey on Techniques providing Internet Connectivity to Mobile Ad Hoc Networks Shiv Mehra and Chansu Yu, Department of Electrical and Computer Engineering Cleveland State University 2121 Euclid Avenue, SH 332, Cleveland, OH Abstract Wireless mobile users are increasing day by day and hence the need for dynamically forming a temporary network without the use of any existing infrastructure is on the rise. Such a collection of wireless nodes is called ad hoc networks, wherein each node uses other nodes as routers to route its packets. Such networks can be deployed in disastrous situations where the existing infrastructure has failed, in battlefields, in meetings where business associates share information and other such applications. There has been a lot of research on routing packets within ad hoc networks, but there are very few proposals on routing packets from the ad hoc network into the Internet. In this paper we describe different techniques proposed so far in literature in providing Internet connectivity for Mobile Ad Hoc Networks (MANET). 1 Introduction An ad-hoc network is a collection of self-organized wireless mobile hosts forming a temporary network without the aid of any established infrastructure or centralized administration stations unlike cellular wireless networks. However the existing routing protocols are unable to function on the Internet and hence providing Internet access to such networks requires gateways that can map the hierarchical routing 1 in the Internet to that in the ad hoc networks. In this paper we discuss the various techniques for providing Internet connectivity to MANETS proposed so far in literature. The techniques discussed so far pay emphasis on the following issues: Naming Convention: The addressing mechanism on the Internet is hierarchical while such an addressing scheme is not viable in ad hoc networks due to dynamic topological changes hence the integration of the two becomes a challenging task. Discovery and Registration Process: This is the process of registering with the gateway/router to gain access to the Internet. Handoffs: The process in which the mobile node switches gateways to obtain better service. 1 Internet routing is based on a two-level hierarchical routing in which an IP address divided into a network portion and a host portion. Gateways use only the network portion until an IP datagram reaches a gateway that can deliver it directly. 1

2 Mobile IP (MIP): The Mobile IP mechanism can be used for providing Internet access to ad hoc networks. An overview of the techniques is provided in Table 1. As seen the table is divided into three parts which give a brief overview of Section 2,3 and 4. The table summarizes each technique for providing Internet access to ad hoc networks proposed in literature so far, listing the special feature of each technique. Mobile IP Foreign Agent as Gateway Router (Nodes running both, the Mobile IP software as well as the MANET routing protocol gain Internet access - Section 2) Implementation Special Features Routing Protocol Section Simulation/Real Simulation in Network Simulator- 2 [8] Simulation in Network Simulator- 2 Real implementation on OS/2 and AIX Real implementation on Linux / Windows NT Real implementation on Free BSD Simulation in C++ Protocol Toolkit (CPT) [16] on Sun Ultra II Sparc Workstations MIPMANET Cell Switching Algorithm: This algorithm helps the MN to decide when to switch to a new FA s Duplicate Address Detection [6]: This algorithm helps a node to obtain a unique co-located care-ofaddress when FA is not available. Implementation of the Route Manager Program: The route manager coordinates the operations by the routing and MIP daemons on the kernels routing table. Internet Gateway as a Router (All Nodes in the MANET gain Internet access - Section 3) Cluster Gateway Model: It is a routing protocol independent gateway acting as a Service Access Point and FA Spanning MANETS across Heterogeneous Link Layers: Enables the nodes in the ad hoc network to communicate over different interfaces 2 Ad Hoc On Demand Routing Protocol Ad Hoc On Demand Routing Protocol Ad Hoc On Demand Routing Protocol Modified Version of Routing Information Protocol [13] Source Initiated Routing Protocol [9] Dynamic Source Routing Protocol [11] Wireless Internet Gateway (No support for Mobile IP Section 4) Implementation of FAMA-NCS [17] The Floor acquisition multiple access with non-persistent carrier sensing is implemented as the MAC layer protocol to eliminate the hidden terminal problem Wireless Internet Routing Protocol [15] Table 1. Overview of techniques providing Internet Access to Ad Hoc Networks. 2.1 [3] 2.2 [7] 2.3 [5] 2.4 [12] 3.1 [4] 3.2 [10] 4 [14]

3 Section 2 discusses the first technique, wherein the Mobile IP Foreign Agent provides gateway services between the Internet and the ad hoc network. In this technique mobile nodes in the ad hoc network, which are running the Mobile IP software as well as the MANET routing protocol are the only nodes that gain access to the Internet. The gateway is a MIP foreign agent and thus this technique utilizes MIP for providing Internet access to ad hoc networks. Section 2 also gives a brief overview of the Mobile IP protocol. Section 3 gives an overview of the second technique in which, the Internet Gateway between the ad hoc network and the Internet provides Internet access to all nodes in the ad hoc network. In this technique the Internet gateway acts like a Mobile IP foreign agent when providing service for Mobile IP nodes (running MIP software as well as MANET routing protocol) and acts as a normal gateway to the Internet for the other nodes (running the MANET routing protocol only) in the ad hoc network. The basic difference between the techniques in Section 2 and 3 is that, in Section 3 the gateway supports the Mobile IP protocol while the techniques in Section 2 use the MIP protocol to provide Internet access to the ad hoc network. The gateway is a MIP foreign Agent and hence it can only provide access to nodes that run the MIP software as well as the routing software. Section 4 is similar to Section 3 where in, the Internet gateway provides Internet access to all nodes participating in the ad hoc network with an exception of no support for Mobile IP. 2 Mobile IP Foreign Agent as an Internet Gateway Overview of Mobile IP: Mobile IP is a mechanism for maintaining transparent network connectivity to mobile hosts. Mobile IP protocol enables a mobile host to be addressed by the IP address it uses in its home network (home IP address), regardless of the network to which it is currently physically attached. Mobile IP introduces the following terminologies. Mobile Node (MN) is defined a host or a router that changes its point of attachment from one subnet to the other. Home Agent (HA) is defined as router on the MN s home network that tunnels the datagrams destined for the MN to foreign network the MN is physically attached to. Foreign Agent (FA) is defined as a router on the MN s visited network that provides routing services to the registered MN. Discovery and Registration: When a MN moves out to of its home network and enters a new foreign network it needs a care-of address to communicate with the Internet. The two methods by which it can gain care-of-addresses are either through the FA or an Access Point (AP). When an MN receives an agent advertisement from a FA it registers with the FA giving its home address and its HA s address. Once the registration with the FA is done the FA contacts the concerned HA and the MN registers with the HA. The MN provides it care-of address (obtained from the FA) to the HA so that data destined for the MN can be tunneled to the MN by the HA via the FA. In case of an AP on the foreign network, the MN obtains co-located care-of-address and registers with the HA directly. The HA now has the current point of attachment of the MN and hence can tunnel the data to the FA which can then route the data to the MN. 3

4 Handoffs: When there are multiple FA s the MN can register with the one, which is closest to it, thus marinating Internet connectivity. The most important requirement for Mobile IP protocol is that the FA or AP should have link layer connectivity with the MN. In this section we describe the implementations in which Mobile IP [1] is used as a solution to provide Internet Connectivity to nodes in the Ad hoc networks. Mobile IP is a solution that helps nodes of a particular network to roam seamlessly outside their home network while still retaining network connectivity. Mobile IP Foreign Agent (FA) provides the gateway services between the Internet and the ad hoc network. In these implementations only the mobile nodes (MN) running Mobile IP software as well as the ad hoc routing protocol can gain access to the Internet. In Fig.1 MN1 and MN2 are running Mobile IP software as well the MANET routing protocol, while N1 is a running the MANET routing protocol only. The Mobile IP FA acts as gateway and provides Internet service to MN1 and MN2 only. The node N1 cannot gain Internet access even if it wishes to. The FA runs the MANET routing 4

5 protocol and MIP protocol with slight modifications so that advertisements and solicitations can be sent over multiple hops, which is not supported by basic Mobile IP. Thus if MN1 wishes to register with its HA it will first register with the FA by sending requests which are routed using the MANET routing protocol to the FA via MN2 and N1. Once FA receives the request it uses the concept of MIP to contact the HA. The routing protocol is only used to route packets within the ad hoc network. Mobile IP is gaining immense amount of popularity since it provides Internet connectivity to Mobile Hosts when they are on a move. It exploits the present IP infrastructure and hence with slight modifications it can be used to provide Internet connectivity to mobile nodes in an Ad hoc network. In ad hoc networks the MN s can be multiple path away from the FA and hence link layer connectivity, which is an important requirement for MIP, cannot be achieved. Moreover it is not feasible to have many FA s in ad hoc network so as to maintain link layer connectivity with all nodes, some slight modifications have to be made to the protocol so that the advertisements and data between the FA and MN can take over multiple hops. The techniques in this section describe the modifications necessary to achieve the communication between the FA and MN over multiple hops. 2.1 Global Connectivity for Ipv4 Mobile Ad Hoc Networks In this paper [3] the authors assume that the mobile node (MN) gains access to the Internet by obtaining a care-of-address from a Mobile IP Foreign Agent (FA), which provides the gateway services between the wired Internet and the Ad hoc network. The Ad hoc On Demand Distance Vector (AODV) [2] routing protocol is used for routing within the ad hoc network and for obtaining the routes to the FA. Route Discovery When a MN wants to send packets to a particular destination it should search its routing table in the following manner Try to locate an entry in the MANET routing table which completely matches the IP address of the destination, if found then use that route. Try to search for the destination in the ad hoc network by using normal AODV operation. Otherwise, route the packet to the FA. When a MN receives a FA-RREP it should not use that route right away, instead it should hold on to the route till it is certain that destination is not in the ad hoc network. Foreign Agent Discovery and Mobile Node registration In basic MIP the FA broadcasts agent advertisements periodically, but when in an ad hoc network such broadcasting of advertisements could lead to flooding of the network. So as to avoid flooding the MN should search for a FA in the ad hoc network, send agent solicitations and once the FA has the address of the MN it can unicast the advertisement to the MN. Thus when a MN in an ad hoc network wants to access the Internet it prepares a Rout Request Packet (RREQ) as defined in [2] to discover a FA in the ad hoc network 5

6 so as to obtain a care-of-address. The MN sets the destination IP address as ( All Mobility Agents i.e. FA and HA multicast group address) and broadcasts the RREQ on its interfaces. When a node not running Mobile IP software receives this RREQ it simply rebroadcasts this packet. If a node running Mobile IP software receives this packet it looks in its routing table if it has a route to any FA, if it does not have a route or has an expired route it simple rebroadcasts the packet. But in case it does have a route to the FA, it prepares a Route Reply (RREP) as defined in [2] with the exception that it appends a FA extension to the RREP and unicasts it to the requesting node. The FA extension indicates the FA IP address. If the node receiving the RREQ is the FA itself, then the FA unicast a FA-RREP as described in Section 2.3. On receiving the RREP the MN sends an Agent Solicitation to the FA, which in turn sends an Agent Advertisement to the MN. On obtaining the advertisement the MN registers with the FA and thus can now have access to the Internet. Once the MN has registered with the FA it broadcasts (just once) the FA advertisement on its interface so that other MN s wishing access to the Internet can register with the FA. The FA periodically 2 advertises their presence by broadcasting advertisement over the MANET obtaining the advertisement the MN s make a note of the FA IP address and the sequence number in order to avoid duplicate processing. FA-Route Reply and FA extension The FA-RREP is similar to the RREP described in [2] with an addition of a F bit to it. When the F bit is set it indicates that the RREP is from the FA. The MN uses this information to distinguish between the RREP from a FA and that from a normal node so as to decide whether the node is on the Internet or in the ad hoc network. The FA extension is included during a route discovery for FA s, when the source node does not know the IP address of any FA s. 2 The period between agent advertisements in basic MIP is quite less (1/3 of the advertisement lifetime). This period has to be increased so as to avoid flooding within the MANET. 6

7 Consider the example shown in the above Figure 3 where Node D is the gateway (MIP FA). Node C invokes a RREQ for node A. The RREQ propagates through the network and the node C receives a FA-RREP from the node D since node D believes that the node A is on the Internet (at its home network). After some time it receives a RREP from the node A itself. It should use this RREP to communicate with node A. Thus once the node receives FA-RREP it should wait for sometime to make sure that the destination is not within the MANET. 2.2 MIPMANET- Mobile IP for Mobile Ad Hoc Networks In paper [7] the authors propose MIPMANET protocol, which provides Internet access to ad hoc networks by using Mobile IP FA care-of-address and reverse tunneling. The authors assume that the MN requesting Internet access has a home address, which is valid on the Internet. The source node first checks the ad hoc network for the destination, if not found the node tunnels the packets to the FA. Changes to Mobile IP mechanisms The FA discovery is similar to that described in section 2 with an exception. If there are no MN s registered with the FA, then that particular FA does not broadcasts and Agent advertisements. If the FA has registered MN s with it, it can periodically 1 advertise the agent advertisements. If the number of MN s in the network is large, the agent advertisements can be broadcasted in response to agent solicitations so the nodes can cooperate with one another to minimize the number of solicitations sent, else the agent solicitations will flood the network. To transform the link layer communication between the FA and the MN (as required by MIP) to network layer communication the MIPMANET Internetworking Unit (IWU) is created. The IWU is a separate unit that can be thought to be in the FA itself or can be in a host on the same link as the foreign agent, between the ad hoc network and the FA as seen in Figure 4. below. For the FA the IWU is a visiting node, which registers with different IP addresses but with the same the link layer address. 7

8 The IWU is introduced so that minimal changes can be made to the basic Mobile IP protocol. All packets destined to the FA from the ad hoc network are forwarded by the IWU and all packets destined to a node in the ad hoc network from the FA are sent out through the IWU. Thus the IWU is a link between the ad hoc network and the FA. Changes to AODV mechanisms In order for cooperation between Mobile IP and AODV few changes in the AODV protocol are also necessary. When the agent advertisements from FA are broadcasted, no reverse routes to the FA are setup by AODV. Thus when a node receives an advertisement it has to conduct a route discovery for the FA. But if the AODV protocol would have setup reverse routes to the FA while broadcasting the advertisements, then the MN s could easily use this route to reach the FA. Thus the AODV protocol was modified to enable set up reverse routes when broadcasting FA advertisements i.e. it should treat broadcasting messages as RREQ. Functioning of MIPMANET Once the changes defined in section 2.2 are made the MN s in ad hoc networks can gain Internet access by registering with the FA. When a node in an ad hoc network want access to the Internet, it has to obtain a care-of address from a FA, hence it has to locate the FA. Once a FA is located, the MN registers with the FA and can now gain access to the Internet. Foreign Agent Discovery The MN broadcasts an agent solicitation message over the network. When a FA receives the agent solicitation it can either broadcast the agent advertisement or unicast it to the MN. The approach chosen depends on the number of MN s in the network. If the number of MN s in the network are small the it is a better idea to unicast the advertisement to the MN since broadcasting it would lead to flooding the ad hoc network, but if there are large number of MN s then unicasting advertisement to each MN will lead to flooding the network and hence broadcasting the advertisement is a better option. 8

9 Registration and Route Discovery The registration procedure is the same as defined in Mobile IP [1] except that, the registration in MIPMANET is over multiple hops. In ordinary Mobile IP the FA transmits the packets for the MN based on the link layer address, but in MIPMANET the FA may not have the link layer address since the MN may be multiple hops away and thus uses the routing protocol to send packets destined to the MN. The route discovery method in MIPMANET is quite similar to that described in section 2.1 Handoffs considerations in MIPMANET When the MN receives agent advertisements from more than one FA it should be able to decide when to switch to the new FA. Thus the authors have proposed a MIPMANET Cell Switching (MMCS) Algorithm to perform the switching operation. As defined by the algorithm the MN should switch to a new FA, when the new FA is at least two hops closer to the MN than the current FA to which the MN is registered with, for a two consecutive agent advertisement. 2.3 Internet Connectivity to Ad hoc Mobile Networks In paper [5] the authors propose the cooperation of Mobile IP protocol and AODV protocol so as to discover multihop paths between the MN s and FA s. It also gives insight on a method for Duplicate Address Detection [6] (DAD) whereby a node can obtain a unique co-located care-of-address when FA is not available. The protocol is implemented in the NS-2 simulator. Care-of-Address As specified by the Mobile IP protocol a MN can acquire care-of-address either through a FA or through a gateway 3, which advertises network prefixes. To obtain a co-located care-of-address the node should be able to receive the network prefix so that it can select its address. Once a node receives a network prefix it chooses a random unique identifier to append to the network prefix. This is the address for which the node will perform the DAD and is called the requested address. It then chooses any arbitrary temporary address, which has the same network prefix and prepares an Address Request (AREQ) packet and then broadcasts this request to its neighbors. Once a node receives the AREQ it first creates a reverse route to the temporary address and then checks its own IP address with that of the requested address. If the address matches then the node prepares an Address Route Reply (AREP) stating that the requested IP address is already in use and unicasts the AREP on the reverse route to the source node. If the IP address does not match then the node simply rebroadcasts the AREQ on to its interfaces. If the source node does not receive the AREP in a specified amount of time, it assumes that the address is unique and begins to use it, else if it receives an AREP then the node again chooses a random identifier and repeats the entire process. 3 This gateway is a node that is configured to advertise network prefixes. 9

10 Foreign Agent Discovery and Registration The procedure of obtaining a care-of-address from a FA and registering with the FA is similar to that described in section 2.1. Handoffs When there are multiple FA s and the MN receives Agent Advertisements from more than one FA, the MN has to decide when to switch from one FA to the other. In order to achieve this, authors of paper [5] have used the modified version of MIPMANET Cell Switching (MMCS) algorithm as described in section 2.2. The MMCS decides to switch FA s when the MN is at least two hops closer to the new FA than to its current FA, for two consecutive agent advertisement. This restriction is applied so that the MN does not oscillate between FA s. But as seen in the Figure 4 below if a MN (node B) is equidistant from two FA s (FA1 and FA2) and if the restriction applies, then the MN would keep re-registering with the FA s frequently. The authors of [5] have eliminated the restriction two consecutive agent advertisement to avoid the frequent reregistrations. 2.4 Ad Hoc Networking with Mobile IP In paper [12] the authors describe a method by which the Mobile IP daemon (mipd) and the MANET routing protocol daemon (routed) modify the kernels routing table so as to accomplish the task of providing Internet connectivity to ad hoc networks. The routing daemon routed is invoked during boot time to manage the network routing table. routed uses the Routing Information Protocol [13] to maintain the kernel routing table. The mipd daemon handles Mobile IP registrations messages and manages registration entry and its lifetime for mobile nodes. In this version of the Mobile IP protocol when a MN registers with a FA, a new entry called the visitor entry is made in the FA s routing table and is tagged with a RTF_VISITOR flag. Similarly when the MN registers with the HA a location entry is created in the HA s routing table and is flagged with the RTF_LOCATION flag. The protocol was implemented on a real ad hoc test bed. The routing protocol used was the modified version of RIP. 10

11 MIPD and ROUTED Internetworking The mipd was modified to enable unicasting of messages between the FA and the MN and a modified version of routed was made to operate on the FA so as to enable the transfer of packets between the FA and MN over multiple hops. The routed was modified to pick up agent advertisements from the loopback interface 4 and include it as an extension to the route table entry for itself, thus the agent advertisements piggybacked with the route table entry could reach all the routed process on all nodes in the ad hoc network. The routed process on a mobile node is required to relay the agent advertisements periodically to the local node following which the MN can register with the FA. The MN sets a default route to the FA and tags it with the RTF_INDIRECT flag which indicates that the FA is only an indirect gateway on the default route while the direct gateway can be located by looking up the routing table for the FA. When the nodes wishes to send packets it looks up for route in its routing table, if found with a route flagged with RTF_INDIRECT it performs a second lookup so as to determine the link layer address to which the packets needed to be forwarded. Route Manager Program (rtmgrd) Both the processes routed as well as mipd update the entries in the routing table. If the two processes request a route entry to the same destination via the different gateways then either one of the routes can be entered in the routing table but not both. The authors have thus implemented a Route Manager program (rtmgrd), which decides on routes and manages the routing table. The routed has the cuurent-updated topology of the network and hence the routes requested by it should be given priority over the routes requested by mipd. The routed and mipd are modified to relay their route manipulation request to the rtmgrd, hence the route manager decides the appropriate action to be taken based on the following rules A gateway supplied by the routed is always superseded by a mipd-supplied gateway to the same destination. A route containing a RTF_VISITOR flag is always preserved if existed. A route to the destination is added to the routing table when either one of the routed or mipd has requested a route. to a destination be created. 3 Internet Gateway as a Router In this technique the Internet gateway acts as router and provides Internet access to all nodes within the ad hoc network. It acts as a Mobile IP FA for the nodes running Mobile IP software as well as the MANET routing protocol and acts as a normal gateway for the other nodes (running the MANET routing protocol only) thus providing Internet access to all nodes in the ad hoc network 4 The loopback interface enables a client and a server on the same host to communicate with each other using TCP/IP. Any IP datagram sent to the loopback interface must not appear on the network. 11

12 . In this section we describe 2 techniques, which have specially designed gateways to provide Internet access to the MANETS. In the first technique the authors have designed a special gateway, which advertises its presence and coordinates the entire operation of the MANET. The protocol structure is quite similar to that of MIP. In the second technique the nodes can communicate over different interfaces with the same IP address. The gateway communicates with a router over a wireless link and thus gains Internet access. 3.1 A Protocol Independent Internet Gateway for MANET S In paper [4] the authors propose an Internet gateway that works together with the existing ad hoc routing protocols as well as Mobile IP to provide seamless Internet access for mobile nodes. It provides Internet access to nodes in the ad hoc network by acting as a Service Access Point and a Mobile IP Foreign Agent. The proposed gateway functions independently of the underlying ad hoc routing protocol. Such a gateway is named as the Cluster Gateway (CG). This implementation assumes the routing protocol to have the following capabilities. Self-Healing Routes: When route failures occur, the routing protocol must be able to dynamically adapt (or repair) the routes by finding an alternate route if one exists. Support for asymmetric network environment: Routing protocol must be able to generate valid routes in a wireless network with asymmetric links. Support Lightweight implementation: The routing protocol must be able to conserve processing power and memory usage. Policy Routing: The protocol must facilitate the generation of routes optimized to application specific metrics. The proposed CG has the capability to map addresses from one network to the other network i.e. it is able to understand both, the hierarchical routing/nature of the Internet as well as the particular ad hoc routing protocol. The technique was implemented on a real test bed with Linux and Windows NT machines and the routing protocol used is Source- Initiated Adaptive Routing Algorithm [9]. The Cluster Gateway Model and Services The CG has information of every node in the ad hoc network, since all nodes have to register with the CG irrespective of whether it desires Internet access or not. In order to make the CG independent of the routing protocol, it consists of two modules viz. 12

13 CG Application Module: It is responsible for routing packets between the Internet and the ad hoc network. CG Node Support Module: It is responsible for connecting the node to the CG application i.e. it is responsible for registration with the CG. Thus by having two separate modules the services offered by the CG are kept independent of the routing protocol. The two services provided by the CG are the Service Access Point (SAP) and the Mobile IP Service. In the SAP mode the CG acts as a simple Internet Gateway and performs NAT (Network Address Translation) for all outgoing packets from the node in order to assure proper routing to the Global Internet. In the Mobile IP service mode the CG acts as a normal Mobile IP FA for a MN in the ad hoc network. Lifecycle of a node session with a CG A node can exploit the capabilities of the MANET routing protocol to reach a destination in the ad hoc network. However to gain access to the Internet the node has to register with the CG so as to route packets to the Internet. In order to register with the CG the node has to know about the existence of the CG. Thus the CG should broadcast Agent Advertisements on the network so as to indicate its presence. Once the node receives an advertisement it tries to register with the CG for the service it requires. The node can then reach destinations on the Internet on successful registration by using the CG as a router. The CG has the capability to determine whether the node is internal to the ad hoc network or external to the ad hoc network. Thus the node has an option to either look for a destination within the ad hoc network or ask the CG for the location of the node (i.e. whether the node is on the Internet or in the ad hoc network). There is a requirement of translation of routing-protocol messages to the CG-messages. The CG node Support Module as described in section 3.1 above does this translation. The CG-Advertisement message, CG-Solicitation message and CG-Registration Request are quite similar to the Mobile IP Advertisement message, Solicitation message and Registration Request as defined in [1] respectively. A few CG extensions are appended to the Mobile IP messages so as to support the CG model. 3.2 Supporting Hierarchy and Heterogeneous Interfaces in MANETS In paper [10] the authors introduce the support of heterogeneous interfaces to achieve scalability in MANETS and internetworking of MANETS with the Internet. It also supports the migration of mobile nodes from the Internet in and out of MANETS via Mobile IP. The routing protocol used is Dynamic Source Routing protocol and the implemented test bed is as shown below. 13

14 The nodes T1 through T5, E1 and E2 form the ad hoc network. The Nodes T1 - T5 are mobile while E1 and E2 are fixed nodes and are called end systems. E1 and E2 ensure connectivity within the network. Nodes T1-T5 and E1 have a single interface, which is used in the ad hoc network while E2 has two interfaces, one to participate in the ad hoc network and the other for participating outside the network. The other interface communicates with the router at the field office, which is connected to the Internet. Thus nodes wanting Internet access use E2 as the gateway to the Internet. Flat Addressing and Heterogeneous Interface Handling On the Internet the nodes are topologically at a fixed location and hence hierarchical addressing is used. But in case of ad hoc networks, nodes move randomly thus hierarchical addressing is not feasible. The authors of this paper have proposed a Flat Addressing technique viable for MANETS. In this addressing scheme the address of every node on the MANET serves as an identifier only and does not convey any information about where a node is topologically located with respect to any other node. Thus every node is assigned an IP address from a single IP subnet, creating an illusion to the outer world that the ad hoc network is a normal IP subnet. Thus the gateway receives packets for the node by means of normal IP routing and then uses DSR to route packets to the appropriate destination within the ad hoc network. To support heterogeneous interfaces in the ad hoc network the nodes assign unique Index Identifiers to their interfaces and these identifiers are meaningless to other nodes. 14

15 When a source node S wants to discover a route to destination D it will broadcast its request on the interface, which is to be used in the DSR protocol. From the Figure 8 above, it can be seen that S and D have different interfaces represented by a rectangle and circle respectively. Source node S will broadcast its request on interface 1. The intermediate node I will receive the request on its interface 1, will set the forward route as S/1, I/2 and rebroadcast it on its interface 2. The destination node D will obtain the forward as well as reverse route in the packet. Forward Route: - S/1 I/2 D Reverse Route: - D/1 I/1 S In the next two sub-sections we give a brief overview of the two modes in which node E2 can function. Internet Connectivity Gateway Operation The nodes in the ad hoc network are assigned IP addresses from the same subnet thus packets from the Internet destined for nodes in the ad hoc network can reach the gateway through normal IP routing. Once the gateways receive the packets they use the DSR protocol to send packets to the required destination. When a node within an ad hoc network wishes to send packets to a node on the Internet it generates a route request as described in [11]. Assuming that the destination is within the ad hoc network and it has a valid IP address on the Internet, the source node may receive two replies, one from the destination itself (which is presently in the ad hoc network) and the other (proxy reply) from the gateway which may advertise itself as the as the second-to-last node to the destination. To differentiate between the two replies, a GATEWAY INTERFACE INDEX is used in the proxy reply from the gateway to that from the normal route replies. The node receiving the request can be configured to use the route provided by the normal route reply. In case the node is not within the ad hoc network and a proxy reply is obtained from the gateway, the node sends the packets to the gateway that in turn routes the packets to the destination using normal IP routing. 15

16 Internet Connectivity Mobile IP FA Operation When a mobile node enters an ad hoc network and needs to access the Internet, it needs to register with a foreign agent. The services of the FA are provided by the gateway. In order to register with the FA the mobile node (MN) broadcasts a Mobile IP Agent Solicitation message piggybacked on a route request to the limited broadcast address , since the MN does not have the address of the FA. When the FA receives the route request, it sends a Mobile IP Agent Advertisement piggybacked on the route reply. Once the MN receives the advertisement it registers with the FA and can obtain Internet connectivity. 4 Wireless Internet Gateways (WINGS) No Mobile IP support In paper [14] the authors propose the concept of Wireless Internet Gateways (WINGS), which acts as an IP router that enables connecting the ad hoc networks to the Internet. This implementation does not support Mobile IP and is solely meant to provide Internet access to nodes in the ad hoc network. The routing protocol implemented is Wireless Internet Routing Protocol (WIRP) [15], which is a modified version of Routing Information Protocol. In order to eliminate the hidden terminal problem, the MAC layer protocol used is the FAMA-NCS [17] (Floor Acquisition Multiple Access with Nonpersistent Carrier Sensing), this being the distinguishing feature of WINGS when compared to traditional gateways. In order to reduce the control traffic to maintain the routing tables, the routing protocol (WIRP) interacts with the link layer protocol (FAMA- NCS). FAMA-NCS The concept of FAMA-NCS is similar IEEE [18]. It uses the non-persistent carrier sensing technique in order to sense the channel. When sender has data to send it transmits a Request to Send (RTS) to the receiver. Once the receiver receives the RTS it sends a CTS (Clear to send) signal. The CTS lasts for longer duration of time than the RTS so that the hidden senders, which did not hear the RTS being acknowledged, get to know about the channel being busy. Thus they can back off for enough duration of time and collision free transmissions can take place. The FAMA-NCS is based on the single channel and asynchronous transmissions. Since the CTS last for a longer duration the channel in the vicinity of the receiver is occupied i.e. it occupies the floor (channel) and hence the name floor acquisition multiple access. The drawback of CSMA/CD (Carrier Sense Multiple Access with collision detection) is that it does not eliminate the hidden terminal problem in wireless networks. Protocols based on collision avoidance and carrier sensing or packet sensing do not provide floor acquisition in networks with hidden terminals [19]. To overcome these two drawbacks the FAMA-NCS is used as the MAC layer protocol. 16

17 WIRP The WIRP is the modified version of RIP and it runs on top of UDP. It was specifically designed for networks, which are prone to dynamic topological changes. The function of WIRP can be classified into three categories described in the following subsections. Reliable Transmission and Updates In this implementation it is required to update the WING of topological changes in the network and hence periodically updates are sent to the gateways. These updates have to be acknowledged and hence when an error free message is obtained it is acknowledged by indicating that there is good connectivity the neighbor and the neighbor has processed the update message. When an acknowledgement is not received the message is again retransmitted and the WING keeps a Message Retransmission List (MRL), which indicates the neighbors, who have not acknowledged. Neighbor Discovery Mechanism This mechanism helps the WING to check connectivity with its neighbors. To achieve this goal WIRP and FAMA-NCS interoperate with one another by sharing a Neighbor Information Table (NIT) and a Subnet Activity Table (SAT). The NIT has a table has a flag and counter associated with each neighbor entry. FAMA-NCS sets the flag associated with a particular neighbor entry whenever it hears a packet with that neighbor and the WIRP scans the table periodically and sets the flag back to 0 and increments the counter. The SAT table has a flag associated with each subnet entry that the FAMA-NCS is attached to. Whenever the FAMA-NCS sends a packet to that particular subnet it sets the flag and WIRP periodically scans the table and resets the flag. This operation of setting and resetting the flag is done so as to decide whether the retransmissions of messages are required or not. Wireless Internet Routing The WING s communicates to its neighbors a hierarchical routing tree in an incremental fashion. The hierarchical tree reported by each WING consists of all the WING s preferred shortest paths to each known IP network and IP host, where an IP host is typically a WING. WIRP implements Dijkstra s shortest path algorithm distributed over a hierarchical graph representing the connectivity of the WING s own IP network. The algorithm used for this purpose is a modification of the path-finding algorithm (PFA) [15]. 5 Conclusion and Future Work We have discussed various techniques proposed in literature so far of providing Internet connectivity to ad hoc networks. The techniques were broadly classified into 3 categories based on the service they provided. In category 1 the Mobile IP FA acts like an Internet gateway and provides Internet Access to only those nodes in the ad hoc network, which are running the Mobile IP software. In the second category the Internet gateway provides 17

18 Internet access to all nodes in the ad hoc network and acts as Mobile IP FA for nodes running the MIP software. In third category we discuss the Wireless Internet Gateway, which provides Internet Access to the ad hoc network but does not support Mobile IP. Based on the survey conducted we conclude that there are namely two distinct problems discussed below, that are to be addressed in order to provide Internet Connectivity to ad hoc networks. Gateway Discovery The gateway is a boundary between the Internet and the ad hoc network which connects the ad hoc network to the Internet. Discovery of such a gateway by the node within the ad hoc network is important so as to obtain Internet connectivity. Addressing The addressing mechanism on the Internet is hierarchical while such an addressing scheme is not viable in ad hoc networks due to dynamic topological changes hence the integration of the two becomes a challenging task. Paper [5] describes the Duplicate Address Detection, while paper [10] describes the flat addressing technique. Paper [7] assumes that the mobile node has a home address that is valid on the Internet. Our future work consists of proposing a technique to connect the ad hoc network to the Internet based on the survey we conducted in this paper. 6 References [1] C. Perkins ed., "IP Mobility Support", IETF RFC [2] Charles E. Perkins, Elizabeth M. Belding-Royer, and Samir Das. "Ad Hoc On Demand Distance Vector (AODV) Routing." IETF Internet draft, draft-ietfmanet-aodv-11.txt, June 2002 (Work in Progress). [3] Elizabeth M. Belding-Royer, Yuan Sun, and Charles E. Perkins. "Global Connectivity for IPv4 Mobile Ad hoc Networks."IETF Internet Draft, draft-royermanet-globalv4-00.txt, November 2001 (Work in Progress). [4] A. Striegel, R. Ramanujan, J. Bonney, "A Protocol Independent Internet Gateway for Ad Hoc Wireless Networks," in Proc. of Local Computer Networks (LCN) 2001, Tampa, Florida, Nov [5] Yuan Sun, Elizabeth M. Belding-Royer, and Charles E. Perkins. "Internet Connectivity for Ad hoc Mobile Networks." International Journal of Wireless Information Networks special issue on Mobile Ad hoc Networks, 9(2), April [6] Charles E. Perkins, Jari T. Malinen, Ryuji Wakikawa, Elizabeth M. Belding- Royer, and Yuan Sun. "Ad hoc Address Autoconfiguration". IETF Internet Draft, draft-ietf-manet-autoconf-01.txt, November 2001 (Work in Progress). 18

19 [7] U. J"onsson, F. Alriksson, T. Larsson, P. Johansson, and G. Maguire. MIPMANET - mobile IP for mobile ad-hoc networks. In Proceedings of Workshop on Mobile Ad Hoc Networking (MobiHOC'00), Boston, MA, USA, August [8] K. Fall and K. Varadhan. Ns Manual. The VINIT Project. [9] R.Ramanujan, S. Takkella, J. Bonney, K. Thurber, Source-Initiated Adaptive Routing Algorithm (SARA) for Autonomous Wireless Local Area Networks, Proc. of the 23rd IEEE Conference on Computer Networks, Oct [10] J. Broch, D. Maltz, and D. Johnson, Supporting Hierarchy and Heterogeneous Interfaces in Multi-Hop Wireless Ad Hoc Networks, Proc. I-SPAN, June [11] Josh Broch, David B. Johnson, and David A. Maltz. The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks. InternetDraft, draftietfmanetdsr01.txt, December Work in progress. [12] H. Lei and C. Perkins, Ad Hoc Networking with Mobile IP, Proc. of EPMCC, [13] C. Hedrick, Routing Information Protocol, RFC 1058, Jun [14] J.J. Garcia-Luna-Aceves, C.L. Fullmer, E. Madruga, D.Beyer, and T. Frivold, Wireless Internet Gateways (WINGS), Proc. of MILCOM'97, Oct [15] S. Murthy and J.J. Garcia-Luna-Aceves, An Efficient Routing Protocol for Wireless Networks, ACM Mobile Networks and Applications Journal, Special issue on Routing in Mobile Communication Networks, Vol. 1, No. 2, [16] D. Beyer, B. Nguyen, The C++ Protocol Toolkit: Overview, Rooftop communications Technical Manual, December Also see Research and Development page. [17] C. L. Fullmer and J. J. Garcia-Luna-Aceves. Floor acquisition multiple access (FAMA) for packet radio networks. In SIGCOMM, pages , Cambridge, MA, [18] K. Biba, A Hybrid Wireless MAC Protocol Supporting Asynchronous and SyncronousMSDU Delivery Services, Tech. Rep. Paper /91-92, IEEE Working Group, [19] C. L. Fullmer and J.J. Garcia-Luna-Aceves, Solutions to Hidden Terminal Problems in Wireless Networks, in Proceedings of ACM SIGCOMM 97, ACM,