Supporting IP Multicast for Mobile Hosts. Yu Wang Weidong Chen. Southern Methodist University. May 8, 1998.

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1 Supporting IP Multicast for Mobile Hosts Yu Wang Weidong Chen Southern Methodist University May 8, 1998 Abstract IP Multicast is an ecient mechanism of delivering a large amount of data to a group of members over the Internet. Link sharing is achieved by a multicast tree that connects all group members. Mobile IP is the proposed standard for IP mobility support and provides two schemes for mobile hosts to receive multicast packets, either through tunneling from the home agent (called home subscription) or by joining multicast groups in the visited foreign network (called remote subscription). The former compromises the link sharing of IP Multicast and uses sub-optimal routing, while the latter requires the foreign agent to be a multicast router and may incur frequent modications of multicast trees due to mobile hosts. This paper introduces multicast agents for IP Multicast to mobile hosts. A multicast agent is a multicast router that serves multiple (foreign) networks. A foreign agent in the service area of a multicast agent noties the multicast agent of the multicast groups that visiting mobile hosts belong to. The multicast agent joins these multicast groups and tunnels multicast packets for these groups to the foreign agent. The foreign agent delivers the multicast packets to mobile hosts using local multicast whenever possible. Simulation results show that delivery delay and cost per multicast packet in our approach is close to the optimal in remote subscription. Our approach has a lower disruption of multicast service due to mobility when groups are sparse. 1 Introduction The trend towards tetherless communications and the advancing technology of laptop and notebook computers induce a growing demand for mobile and nomadic computing. Mobile IP [17] is the proposed standard for IP mobility support by the Internet Engineering Task Force (IETF). It provides seamless network access for mobile hosts while retaining compatibility with existing networks. There are three functional entities in Mobile IP: mobile host, home agent and foreign agent. Each mobile host is assigned a long-term IP address on a home network called a home address. Supported in part by the Texas Higher Education Coordinating Board, Advanced Technology Program Grant and by the National Science Foundation under Grant No. NCR

2 While away from home, the location of a mobile host is captured by a care-of address, which is either the IP address of a foreign agent with which the mobile host is registered or a temporary co-located IP address acquired by the mobile host in the visited foreign network. The home agent of a mobile host maintains a mobility binding between the home address and the care-of address of the mobile host. A mobile host registers with its home agent to update its mobility binding when it moves across IP subnets. All IP packets for a mobile host are routed using regular IP routing to its home agent, which then tunnels them to the care-of address of the mobile host. An increasing number of applications is relying on multicast to disseminate data to a group of receivers over a network, including multimedia services, software distribution, distributed interactive simulation and resource discovery [18]. IP Multicast [8] provides a popular model of multicast over the Internet. Each multicast group is identied by a group address. Members join and leave multicast groups as they wish using the Internet Group Management Protocol (IGMP). IP packets addressed to a group address are delivered using a multicast tree to all group members. The sender does not need to know the membership of the group. Current implementations of IP Multicast in the Internet use source specic multicast tree, e.g. DVMRP [9, 19] and MOSPF [15], where each sender of a group has its own multicast tree rooted at the sender. Protocols that use group shared tree have also been developed, including CBT [4] and PIM [11, 12], which are more scalable and independent of the underlying unicast routing protocols. Mobility raises several issues for IP Multicast. A mobile receiver will experience additional delay in receiving multicast packets when it moves into a network with no group members. A mobile sender may or may not be able to send multicast packets from a foreign network. In the case of multicast using source specic trees, it must acquire a co-located IP address and set up a multicast tree rooted at the co-located IP address before it can send out multicast packets. Frequent modications of multicast trees incur signicant routing overhead since existing multicast routing protocols have been designed for wired networks and a multicast tree cannot be changed easily or eciently. These issues, combined with the low bandwidth and higher bit error rate of the wireless link, make ecient IP Multicast a challenging task in a mobile environment. Mobile IP [17] oers two methods of multicast service for mobile hosts depending upon whether the home network or the foreign network is used. To send multicast packets using its permanent IP address, a mobile host must tunnel multicast packets to its home agent, which is assumed to be a multicast router. The home agent then sends out the packets using IP Multicast on behalf of the mobile host. To send multicast packets directly from a foreign network, a mobile host must acquire and use a co-located IP address in the foreign network as the source address of multicast packets, in which case there must be a multicast router in the foreign network. A mobile receiver also has two possibilities in Mobile IP [17]. One is called remote subscription (RS), where a mobile host joins a multicast group in the visited foreign network, assuming that there is a multicast router in the foreign network. The main advantage of remote subscription is the optimal IP Multicast routing to the current locations of mobile group members. The weakness is that it requires a multicast router in every network a mobile group member visits. In addition, a mobile group member has to join the same multicast group every time it moves, experiencing a lengthy delay especially when the group is sparse and incurring signicant multicast routing overhead. The other possibility for a mobile group member is called home subscription (HS), where a roaming mobile host joins a multicast group in its home network, assuming that the home agent is a multicast router. Each multicast packet is encapsulated as a unicast packet and then 2

3 tunneled by the home agent to the care-of address of the mobile host away from home. This approach, however, compromises the link sharing advantage of IP Multicast and uses suboptimal routing through the home network. To improve the multicast delivery to mobile receivers in home subscription, a Mobile Multicast protocol (called MoM) is proposed in [6, 13]. The key idea is to have home agents tunnel multicast packets to foreign agents and to have foreign agents use link-level multicast to deliver the multicast packets to visiting group members. The link-level multicast is benecial when there are multiple group members in the same foreign network. However, the very same idea also introduces a new tunnel convergence problem when visiting group members in the same group belong to dierent home networks and multiple home agents tunnel multicast packets of the same group to the same foreign agent. To overcome the problem, the MoM protocol chooses one of the home agents as the designated multicast service provider (DMSP) for each multicast group in each foreign network. Only the DMSP of a multicast group in a foreign network is allowed to tunnel a copy of multicast packets for the group to the corresponding foreign agent. This, however, introduces the need for DMSP hando when all roaming group members of the DMSP for the multicast group move out of the foreign network. In addition, the DMSP hando also aects remaining visiting group members that belong to dierent home networks. We attempt to combine the advantages of home subscription and remote subscription in Mobile IP and the link-level multicast in MoM, while avoiding their shortcomings. We focus on the issue of mobile group members because of the potentially large size of a multicast group and introduce the concept of multicast agents for ecient multicast services to mobile group members. A multicast agent is a multicast router that provides multicast services to mobile group members in multiple (foreign) networks. It maintains a list of multicast groups that have mobile members in its service area, and for each group, a list of foreign agents that have visiting mobile members belonging to the group. The multicast agent joins the multicast groups on behalf of the mobile group members in its service area. Upon receipt of multicast packets for a group, the multicast agent tunnels the multicast packets to all foreign agents that have visiting mobile hosts belonging to the group. The tunnel convergence problem is avoided since the service areas of dierent multicast agents are disjoint. A foreign agent is responsible for collecting multicast group information of visiting mobile hosts and notifying the multicast agent of any changes. Upon receiving multicast packets from the multicast agent for a multicast group, the foreign agent uses link-level multicast to deliver the multicast packets to all visiting members in the group. Simulation results demonstrate that our scheme of multicast agents has several advantages. First, it avoids possibly long multicast tunnels through home agents and takes advantage of the link-sharing of multicast routing to all multicast routers. As a result its performance is close to the optimal in the remote subscription scheme in Mobile IP in terms of the delay and the cost of delivering multicast packets to mobile hosts. Second, it reduces the disruption of multicast services due to mobility of group members, especially when group density is low. The reason is that a multicast agent provides a point of attachment to the multicast backbone that is closer to the current locations of mobile hosts. Third, it does not require every foreign agent to be a multicast router, thereby providing an incremental method of introducing multicast services in wireless networks. The rest of this paper is organized as follows. Section 2 presents multicast agents and the 3

4 proposed scheme in detail. Section 3 describes the performance results of simulation studies and compare multicast agents with home subscription and remote subscription in Mobile IP and with MoM. Section 4 discusses related work. 2 Multicast Agents This section describes the architecture of multicast agents to support multicast services to mobile hosts and presents details on group management, multicast packet delivery and hando. 2.1 Three-Layer Architecture for Multicast to Mobile Hosts IP Multicast [8] uses a two-layer architecture to provide multicast services to (xed) hosts. Locally a host must be connected to a local area network with a multicast router. The host communicates group information with the multicast router using IGMP (Internet Group Management Protocol) [10]. The multicast router is responsible for delivering multicast packets to local group members using local multicast and for sending multicast packets from local sources to group members over an internet. At the internet level, multicast routing protocols are used by multicast routers to exchange group information and to route multicast packets. In other words, a multicast router in a local area network represents the point of attachment of local hosts to the multicast backbone. In Mobile IP [17], there are two possibilities for a mobile host to attach to the multicast backbone, using either the home agent (called home subscription) or the foreign network (called remote subscription). Remote subscription is the simplest since it does not require any modication to IP Multicast. It is also the most ecient since multicast packets are delivered to the current locations of mobile group members. However, remote subscription requires a multicast router in every possible network a mobile group member may visit and may experience frequent modications to the multicast tree. In home subscription, the local multicast in IP multicast is now replaced by a tunnel between the home agent (that is a multicast router) and a mobile group member. Our approach is to use a three-layer architecture for multicast to mobile hosts and introduce multicast agents that serve as the access point by mobile hosts (via foreign agents) to the multicast backbone. The service area of a multicast agent covers multiple (foreign) networks. This avoids the requirement of a multicast router in each foreign network and provides an incremental mechanism of introducing multicast service to mobile hosts. At the internet level, multicast agents are simply multicast routers participating in multicast routing. At the local level, foreign agents still use local multicast as in IP Multicast. However, the intermediate level between multicast agents and foreign agents is responsible for propagating group information of mobile hosts from foreign agents to multicast agents and for tunneling multicast packets from multicast agents to foreign agents. The introduction of multicast agents has several advantages. First, roaming mobile hosts are kept relatively close to their access points to the multicast backbone by restricting the size of the service area of a multicast agent. Compared with home subscription, multicast agents avoid the potentially long tunnels from home agents to mobile hosts and the corresponding duplication in 4

5 sender MA MA MA MA MA MA MA MA Figure 1: Routing with multicast agents tunneling multicast packets using unicast. The proximity between mobile hosts and multicast agents also reduces the disruption to multicast service caused by mobility of group members. Second, frequent modications to multicast trees are reduced since the service area of a multicast agent covers multiple (foreign) networks. Modication to a multicast tree is necessary only when a mobile group member moves into the service area of a new multicast agent that has no members in the group. Multicast agents oer the exibility of tradeo between multicast routing eciency and routing overhead due to mobility, with remote subscription and home subscription at the two ends of the spectrum. Remote subscription is the most ecient in multicast routing, but has the highest rate of multicast tree modications. In contrast, home subscription does not require any modication to the multicast tree since a mobile group member is always logically located in its home network, but it uses possibly long unicast tunnels for multicast packet delivery. By changing the size of the service area of a multicast agent, we can achieve an appropriate balance between routing eciency and routing overhead of multicast tree modications. Third, by ensuring that service areas of dierent multicast agents do not overlap, we avoid the tunnel convergence problem in MoM [6, 13] where multiple agents may tunnel multicast packets of the same group to the same foreign agent. Each foreign agent is served by only one multicast agent. Fourth, multicast agents provide an incremental mechanism of introducing multicast services in wireless networks. For example, if a foreign agent is a multicast router, it can provide multicast services directly to visiting mobile hosts, while foreign agents that are not multicast routers can still use multicast agents. 2.2 Group Management and Multicast Packet Delivery When a mobile host moves into a new foreign network, it has to register with its home agent to update its mobility binding in order to maintain its IP connectivity to the network. Similarly for IP Multicast, it has to communicate its group membership to the foreign agent. 5

6 If local broadcast/multicast is supported, the standard IGMP messages can be used. A foreign agent maintains a list of multicast groups that have members in its network. To update the list, it sends IGMP membership queries periodically (no more than once a minute [10]) to the all-hosts group A mobile host responds by sending an IGMP membership report for each group it belongs to (after a random report delay timer expires). The IGMP membership report is sent to the multicast address of the group. As mentioned in Mobile IP [17], if a mobile host uses a co-located care-of IP address, it should use this address as the source address of its IGMP membership reports. Otherwise it must use its permanent home address. If local broadcast/multicast is not supported, a mobile host has to send its IGMP membership reports directly to the IP address of the foreign agent. A foreign agent has to maintain not only a list of multicast groups, but also a list of visiting mobile hosts that belong to each group. For membership updates, the foreign agent may delete mobile hosts from the list of a group when their mobile registrations expire at the foreign agent. Whether or not local broadcast/multicast is supported can be indicated in the agent advertisement by the foreign agent. When a foreign agent detects any change in the list of multicast groups of mobile hosts through membership reports or expiration of mobile registrations, it communicates the change to the corresponding multicast agent indicating that it wants to join or leave a multicast group, which is acknowledged by the multicast agent. The multicast agent maintains a list of multicast groups that have mobile group members in its service area and for each group, a list of foreign agents that have mobile hosts in the group. The multicast agent joins these multicast groups on behalf of mobile group members and participates in multicast routing. When a multicast agent receives packets for a multicast group, it tunnels the packets to each foreign agent on the list for the multicast group. The foreign agent then uses local multicast to deliver the packets to all group members in its network. If local multicast is not supported, the foreign agent has to send a copy of a multicast packet separately to each visiting group member. 2.3 Multicast Service Hando When a mobile host that is a member of a multicast group moves from one IP subnet to another, it should immediately transmit an IGMP membership report for each group it belongs to, without waiting for a membership query from the foreign agent. There are three possibilities. One is that the new foreign network already has some members in the multicast group and the new foreign agent is already receiving multicast packets from the multicast agent for the group. The mobile host can continue to receive packets for the group. Another possibility is that the mobile host is the rst member of the group in the new foreign agent, but the multicast agent has already joined the group and is receiving packets for the group. What is needed is for the mobile host to notify the foreign agent of the group membership and for the foreign agent to notify the multicast agent so that the multicast agent can tunnel multicast packets for the group to the new foreign agent. Whenever a foreign agent detects a new group, it should immediately send a request to its multicast agent for joining the new group. The third possibility is that the mobile host is the rst member of the group in the service area of the multicast agent for the new foreign network. In addition to propagating group membership from the mobile host to the foreign agent and then to the multicast agent, the multicast agent has to join the group through some multicast routing protocol such as DVMRP [9] and MOSPF [15]. 6

7 Clearly the disruption of multicast services due to mobility increases from the rst to the second and to the third possibility. By restricting the size of the service area of a multicast agent, the disruption because of the delay between a foreign agent and its multicast agent can be limited. 3 Performance Analysis This section compares multicast agents (MA) with home subscription (HS) and remote subscription (RS) in Mobile IP [17] and the MoM protocol in [7] via simulation. The delay and cost of multicast packet delivery are considered, as well as the disruption due to mobility. 3.1 Simulation Model We use a mesh network, in which each vertex is regarded as an IP subnet capable of link layer multicast. Each IP subnet has a home agent and a foreign agent that provide basic unicast and multicast services for mobile hosts. The location of a roaming mobile host is represented by its serving foreign agent. Mobility agents in our simulation model include home agent, foreign agent, and multicast agent. If two or more mobility agents are in the same IP subnet, we assume that they co-locate so that the communication cost and delay between them are considered 0. The size of the mesh network used in the simulation is We consider two sizes of the service area of a multicast agent. One is a 5 5 square with 25 IP subnets (denoted by MA5) and the other is a square with 100 IP subnets (denoted by MA10). The multicast agent is located in (or near) the center of the square. We consider one multicast group with a single source that is a xed host. All members of the multicast group are mobile hosts and the group membership is static. In any time unit, each mobile host can either stay in the same IP subnet or move into one of the neighboring IP subnets with equal probability. The initial locations of all the group members and the source host are all randomly distributed in the mesh network. The home networks of the group members are also randomly distributed in the mesh. However, the number of distinct home networks of mobile group members is limited to 20. It is assumed that mobility agents in neighboring IP subnets can communicate directly through wired links, i.e. the number of links between them is 1. The distance between two mobility agents is measured by the minimum number of links to travel from one agent to the other. A pruned source based shortest path tree is used for delivery of multicast packets from the source to all members. To add a branch to the tree, a JOIN message travels along the reverse shortest path until it reaches a branch in the multicast tree. We vary the size of the multicast group in our simulation from 2 2 to 2 12, resulting in group densities from to over 4.55 (since the size of the mesh network is 900). For each group size, we generate randomly dierent congurations of the initial locations of group members and their associated home networks and dierent source hosts. They are used for multiple runs and the averages are computed. 7

8 3.2 Delivery Cost of Multicast Packets The cost of delivering a multicast packet is measured by the total number of links that a multicast packet travels from the source to all members of a group. This includes the number of links in the multicast tree and in all the tunnels that are used to deliver the multicast packet, and the wireless link (counted as 1) between foreign agents and mobile hosts. In the home subscription (HS) scheme in Mobile IP [17], the home agent tunnels a multicast packet to each roaming mobile host that is a member. In MoM [6, 13], a designated home agent for each group in a foreign network tunnels multicast packets to the foreign agent. In our scheme, multicast agents tunnel multicast packets to each foreign agent that has group members. total delivery cost HS RS MoM MA5 MA base 2 logarithm of group size Figure 2: average delivery cost per packet Figure 2 shows the average delivery cost of a multicast packet to all group members. The delivery cost in the home subscription scheme is approximately linear to the group size and has an exponential curve with respect to the logarithm of the multicast group size. This is because home subscription uses a separate tunnel for each mobile host. The average delivery cost in MoM is much lower than that in home subscription. This is mainly due to the use of a designated home agent for each group in each foreign agent and the use of local multicast in each foreign network. In other words, a single tunnel is used for each group in each foreign agent. The remote subscription scheme in Mobile IP has the best delivery cost since it takes the maximum advantage of link sharing in the multicast tree. The access point to the multicast backbone for each mobile host is precisely the current location of the mobile host. Our approach using multicast agents is close to the optimal case of remote subscription since a multicast agent is near the mobile hosts that it serves. Even when the size of the service area of a multicast agent increases from 5 5 in MA5 to in MA10, the total delivery cost remains low. Both schemes have a much lower delivery cost than MoM. The higher delivery cost in MoM is due to the possibly long distance tunneling from home agents to the current locations of mobile hosts. To understand the impact of tunneling on multicast packet delivery, we separated the total cost into two components { one for multicast packet delivery over the multicast tree and over the wireless link using local multicast by a foreign agent and the other for multicast packet delivery over the tunnels. Figure 3 and Figure 4 show these two cost components for dierent strategies. 8

9 multicast delivery cost HS RS MoM MA5 MA base 2 logarithm of group size Figure 3: average delivery cost per packet using multicast tunneling delivery cost HS RS MoM MA5 MA base 2 logarithm of group size Figure 4: average delivery cost per packet using tunnels In Figure 3, the multicast tree for the home subscription scheme is the smallest since the number of distinct home networks for mobile group members is limited to 20 in the simulation. The other strategies make better use of multicast delivery, including the local multicast in a foreign agent, the number of which can be as large as the size of the mesh network. The multicast delivery cost of MoM lies between those of MA5 and MA10. This is due to the fact that the number of distinct home networks of mobile group members is limited to 20, between the number of multicast agents in MA5 and that in MA10. The remote subscription scheme has the largest multicast delivery tree since it extends to the current location of each mobile group member. However, the delivery cost using multicast is only a small portion of the total cost since the tunneling delivery cost dominates as the group size increases (as shown in Figure 4). The tunneling cost in home subscription increases linearly with the number of mobile group members. (The relative cost may be even higher since a multicast packet has to be encapsulated as a unicast packet before being tunneled to a mobile group member if the care-of address of the member 9

10 is the foreign agent address.) The tunneling cost in MoM levels o when the group density is high. The reason is that the tunneling cost in MoM depends upon the number of foreign agents, not the number of mobile group members. For each multicast group and for each foreign agent, there is only one home agent, namely the DMSP, that tunnels multicast packets of the group to the foreign agent. In the multicast agent scheme, tunneling occurs within the service area of a multicast agent, as supposed to the entire network in MoM. The remote subscription scheme does not use tunneling at all. 3.3 Delivery Delay of Multicast Packets The delay of delivering a multicast packet to a group member is the length of the path from the source node to the group member. For each packet we consider the maximum and the average delay of delivering the packet to all group members. In the home subscription scheme of Mobile IP [17], the path from the source to a mobile group member includes the path in the multicast tree from the source to the home agent and the tunnel from the home agent to the current location of the mobile group member. The delivery path in MoM [6, 13] is similar except that a designated home agent is responsible for tunneling packets of a group to a foreign agent and that local multicast is used in the foreign network. The ideal case for the delivery path is in the remote subscription scheme of Mobile IP [17], which includes the path in the multicast tree from the source to the serving foreign agent and the wireless link from the serving foreign agent to a mobile group member. The delivery path in multicast agents includes the path in the multicast tree from the source to the serving multicast agent, the tunnel from the serving multicast agent to a foreign agent, and the wireless link from the foreign agent to a mobile group member. 90 maximum delay HS RS MoM MA5 MA base 2 logarithm of group size Figure 5: maximum delivery delay per multicast packet Figure 5 and Figure 6 show the maximum and the average delivery delay for a multicast packet to reach all group members. The maximum delays for home subscription and MoM are almost the same and very high because both schemes multicast packets to the home agents which then tunnel the packets to the current locations of mobile group members. The remote subscription scheme has the smallest delays because it multicasts packets directly to the current locations 10

11 average delay HS RS MoM MA5 MA base 2 logarithm of group size Figure 6: average delivery delay per multicast packet of mobile group members. Our multicast agent scheme multicasts packets to multicast agents, which are located close to the current locations of mobile hosts. As a result its performance on delays is close to the ideal case of remote subscription. Increasing the size of the service area of a multicast agent (from 5 5 in MA5 to in MA10) increases the delay slightly, proportional to the increment in the radius of the service area. 3.4 Disruption of Multicast Service due to Mobility When a mobile group member moves from one IP subnet to another, it may experience disruption of multicast service. The disruption starts when the mobile group member loses contact with the previous foreign agent. Included in the disruption is the agent discovery time between a mobile host losing contact with the previous foreign agent and establishing contact with the new foreign agent. Since the agent discovery time is the same regardless what multicast scheme is used for mobile hosts, we do not consider it in our simulation studies. Instead, we try to measure the delay from when a mobile host can communicate with the new foreign agent to the time multicast packets can be sent correctly to the new location of the mobile host. In the following we describe how disruption is calculated for each scheme. In the home subscription scheme of Mobile IP [17], a mobile host joins multicast groups at its home agent. The home agent is responsible for tunneling all multicast packets to each individual mobile host away from home that is a group member. When a mobile host moves into a new IP subnet, it has to register with the home agent to update its care-of address. The disruption is computed as the length of the path that the registration message from the mobile host travels, namely the wireless link from the mobile host to the new foreign agent and the path from the new foreign agent to the home agent. In the remote subscription scheme of Mobile IP [17], a mobile host joins multicast groups at the serving foreign agent. When a mobile group member moves into a new IP subnet, there are two possible cases. One is that the mobile group member is not the rst member of the group in the new IP subnet. The mobile group member can receive the multicast packets in the new IP subnet immediately. The other case is that the mobile group member is the rst member of 11

12 the group in the new IP subnet. After a multicast router in the new foreign network receives the membership report from the mobile host, it tries to join the multicast group by attaching itself to the source-based shortest path multicast tree. The disruption in the latter case is computed as the length of the branch added to the multicast tree as the result plus 1 for the wireless link over which membership reports from the mobile host travel. The hando of a mobile group member in MoM [6, 13] is more involved. As far as multicast is concerned, each home agent maintains for each multicast group three lists: a list of roaming mobile hosts that are members of the group; a list of foreign agents at which roaming mobile group members are; and a list of foreign agents for which the home agent serves as the DMSP (designated multicast service provider). Each foreign agent also maintains three lists for each multicast group: a list of visiting mobile hosts that are members of the group; a list of home agents to which the visiting group members belong; and a list of home agents that are currently serving as the DMSP for the group. Maintaining multiple DMSP for each group reduces the disruption through duplication and buering. Since all of the schemes that we discuss can be improved by using some buering or caching mechanism to minimize disruption, we will not include any buering in our comparisons and will consider MoM in which every foreign agent maintains only one DMSP for each group. When a mobile group member moves into a new IP subnet, there are two cases. If there are already group members in the new IP subnet, the new mobile group member will not experience any disruption in multicast service. If the mobile group member is the rst member of the group in the new IP subnet, its home agent will become the DMSP for the group in the IP subnet. The disruption for the mobile group member is calculated as the length of the path that the registration message from the mobile group member travels to its home agent. Due to the use of DMSP for tunneling, a leaving mobile group member may aect the multicast service to the other group members that remain in the old IP subnet when the home agent the leaving member is the DMSP for the group for the old foreign network, and the leaving member is the last roaming member of the group in the old foreign network for the home agent. In this situation, the home agent that is the current DMSP will notify the old foreign agent so that the old foreign agent can select a new DMSP for the group. The disruption for each remaining group member is calculated as the length of the path from the old foreign agent to a newly selected DMSP. In our simulation, the new DMSP is selected using the FIFO among the home agents of the remaining group members. For our multicast scheme, when a mobile group member moves into a new IP subnet, there are several cases. 12

13 If the mobile group member is not the rst member of the group in the new IP subnet, it will not experience any disruption. If the mobile group member is the rst member of the group in the new IP subnet, but not the rst member of the group in the service area of the multicast agent for the new IP subnet, the disruption is calculated as the length of the path from the new foreign agent to its multicast agent plus 1 for the wireless link. If the mobile group is the rst member of the group in the service area of the multicast agent for the new IP subnet, the disruption is calculated as the sum of the disruption in the second case plus the disruption from the multicast agent joining the new group overall disruption HS RS MoM MA5 MA base 2 logarithm of group size Figure 7: disruption per group member per movement Figure 7 shows the disruption of multicast service per group member per movement for dierent schemes. Both home subscription and MoM have a high disruption. This is because the hando of multicast service involves contacting the home agent of a mobile host, which can be far away from the current location of the mobile host. The disruption in MoM decreases with the increasing group density due to the sharing of DMSP by visiting group members of the same group in the same foreign network. The higher the group density, the lower the possibility for the change of DMSP in MoM, hence the lower disruption. Compared with home subscription and MoM, remote subscription and multicast agents have a much lower disruption. The reason is that the access point to the multicast backbone by mobile hosts is much closer to the locations of mobile hosts in remote subscription and multicast agents. Even though remote subscription and multicast agents need to modify multicast trees when mobile hosts move, such modications are often incremental since geographic proximity frequently implies network proximity. When the group size (or density) is small, there is a very low probability of nding other group members when a mobile host moves into a new IP subnet. The JOIN message from a new IP subnet may have to traverse a long path before reaching the existing multicast tree. Since the service area of a multicast agent consists of multiple IP subnets, the probability of nding other group members in the service area of the multicast agent is much higher than in a new IP 13

14 subnet. The disruption is therefore limited by the radius of the service area in most situations. Consequently multicast agents have a lower disruption than remote subscription when the group is small. As the group size increases, the multicast tree in remote subscription covers more IP subnets. Even if there is no other group member in a new IP subnet, the JOIN message does not have to travel far to reach the multicast tree. On the other hand, multicast agents serve as the leaf nodes of a multicast tree. When the new IP subnet has no other members, the membership report has to travel from the new visiting group member to the foreign agent and then to the multicast agent. When the average length of the path that JOIN messages travel in remote subscription is less than the radius of the service area, remote subscription has a lower disruption. When the group size is so large that the probability of nding other group members in a new IP subnet approaches 1, the dierences between remote subscription and multicast agents become insignicant as far as disruption is concerned. In our calculation of the disruption of multicast services so far, sending a message along a path has the same cost as adding a branch of the same length to a multicast tree. However, existing multicast routing protocols such as DVMRP [9, 19] and MOSPF [15] are designed for xed hosts in mind. Modifying a multicast tree is not an easy task. To understand the impact of the cost of multicast tree modication, we re-calculated the disruption assuming that adding a branch to a multicast tree is 1.5 times the cost of sending a message along a path of the same length. The result is shown in Figure 8. Compared with Figure 7, multicast agents have a lower disruption than remote subscription for a longer range of low density groups. Notice that home subscription and MoM are not aected since mobile hosts join multicast groups at their home agents overall disruption HS RS MoM MA5 MA base 2 logarithm of group size Figure 8: disruption per group member per movement when adding a link to a multicast tree is 1.5 times the cost of sending a message over a link 4 Related Work Several researchers have considered reliable multicast to mobile hosts. In [1, 2], several methods are presented that deliver multicast packets to mobile hosts under dierent semantics, including 14

15 at least once, at most once and exactly once. Associated with each multicast group is a set of mobile support stations (MSSs) that represent the aggregate location information of the group members. A multicast packet to the group is propagated to the corresponding set of MSSs, which are responsible for sending the packet to visiting group members. To guarantee the at least once and exactly once semantics, the sender to a multicast group must maintain the list of mobile hosts that belong to the group. In [5], a reliable multicast protocol called RelM is proposed for mobile networks. A hierarchical model is employed to support exactly-once delivery of multicast packets to all members of a multicast group. Multiple mobile support stations are grouped together and assigned a supervisor host. Reliable multicast to mobile hosts is carried out in two stages. In the wired network, supervisor hosts join multicast groups on behalf of mobile group members. Multicast packets from a source are delivered reliably to all supervisor hosts that have mobile group members in their domains. In the wireless networks, each supervisor host is responsible for reliable delivery of multicast packets to all mobile group members. It keeps a list of all visiting group members and delivers multicast packets to each visiting member individually. It also handles hando when a mobile group member moves from one mobile support station to another or moves across the domains of dierent supervisor hosts. In [16], algorithms for scheduling multicast transmissions of byte streams to mobile hosts in a highway environment are presented. When a mobile host roams from one cell to another, it may lose packets due to fading or the delay in joining the multicast group in the new cell. The schedule algorithm determines when to retransmit some data and when to transmit new data so that the eective bandwidth can be maximized. This paper considers providing the best eort multicast service to mobile hosts using IP Multicast. An early paper on IP Multicast for mobile hosts is [3], which is based upon the Columbia Mobile*IP [14] for IP mobility support. The Columbia Mobile*IP assumes that all mobile hosts belong to the same virtual subnet on a campus network. On the other hand, the physical link layer connectivity is only shared by mobile hosts in the same cell. Consequently the main focus in [3] is to provide an abstraction of link-layer connectivity among all mobile support routers and mobile hosts to be consistent with them sharing IP addresses in the same virtual subnet. A virtual multicast tunnel is created to link all the mobile support stations within a campus. Outgoing and incoming multicast packets are sent via the multicast tunnel to all the mobile support stations, which then send them to mobile hosts over their wireless interfaces. Our approach is based upon the proposed standard for IP mobility by IETF called Mobile IP [17]. The two methods of multicast in Mobile IP, namely home subscription and remote subscription, represent the two extremes in multicast routing to mobile hosts. In home subscription, the multicast tree for a group reaches all the home agents of mobile group members and does not have to be modied. But unicast tunneling is used by home agents to send multicast packets to all mobile hosts away from home. This causes sub-optimal routing, compromises the link sharing of IP Multicast, and leads a potentially long disruption of multicast services when a mobile group member is far away from home. In remote subscription, the multicast tree for a group reaches the current locations of mobile group members, which is the ideal case for routing eciency and link sharing. However, the multicast tree may be changed frequently when mobile group members roam from one cell to another. The approach in MoM [7, 13] attempts to reduce the duplicate tunneling in home subscription by choosing a designated home agent for tunneling multicast packets of a group to a foreign agent. 15

16 But it still inherits the disadvantages of home subscription, including sub-optimal routing and possibly long distance tunneling from home. In addition, the hando of one mobile group member from one cell to another may aect multicast services to those that remain in the previous cell. We introduce the concept of multicast agents to serve as access points to the multicast backbone for mobile hosts. They are located close to the current locations of mobile group members to avoid long distance tunneling of multicast packets and to take advantage of the link sharing of IP Multicast in wired networks as much as possible. The proximity between multicast agents and mobile group members also reduces the disruption to multicast services caused by mobility. On the other hand, multicast agents are not identical to foreign agents, and in fact can serve multiple foreign networks. This avoids the high infrastructure requirement of remote subscription and the frequent modications of multicast trees. In terms of multicast delivery delay and cost, our scheme is close to the optimal in remote subscription. In terms of disruption due to mobility, our scheme is much better than home subscription and MoM and is better than remote subscription when groups are sparse. Performance results show that increasing the service area of a multicast agent from 25 subnets to 100 subnets (or reducing the number of multicast agents from 36 to 9 in a mesh network of 900 subnets) only causes small degradation in performance. This indicates that multicast agents provide an ecient incremental solution for deploying multicast services in wireless networks. References [1] A. Acharya and B.R. Badrinath. Delivering multicast messages in network with mobile hosts. In Intl. Conference on Distributed Computing Systems, May [2] A. Acharya and B.R. Badrinath. A framework for delivering multicast messages in networks with mobile hosts. ACM/Baltzer Journal of Mobile Networks and Applications, 1(2):199{ 219, October [3] A. Acharya, A. Bakre, and B.R. Badrinath. IP multicast extensions for mobile internetworking. In Proceedings of IEEE Infocom'96, March [4] T. Ballardie, P. Francis, and J. Crowcroft. Core based trees (CBT): An architecture for scalable inter-ddomain multicast routing. In ACM SIGCOMM Symposium on Communications Architecture & Protocols, September [5] K. Brown and S. Singh. RelM: Reliable multicast in mobile networks. Journal of Computer Communications, to appear. [6] V. Chikarmane. Network support for mobile hosts in a TCP/IP intternetwork. Master's thesis, Department of Computer Science, University of Saskatchewan, August [7] V. Chikarmane, R. Bunt, and C. Williamson. Mobile IP-based multicast as a service for mobile hosts. In Proceedings of the 2nd Intl. Workshop on Services in Distributed and Networked Enviroments, BC, Canada, June [8] S. E. Deering and D.R. Cheriton. Multicast routing in datagram internetworks and extended LANs. ACM Transactions on Computer Systems, 8(2):85{110, May

17 [9] S.E. Deering. Multicast routing in internetworks and extended LANs. In ACM SIGCOMM Symposium on Communications Architecture & Protocols, August [10] S.E. Deering. Host extensions for IP multicasting, August RFC [11] S.E. Deering, D. Estrin, D. Farinacci, and V. Jacobson. Protocol independent multicast (PIM) dense mode protocol: Specication. Internet Draft, March [12] S.E. Deering, D. Estrin, D. Farinacci, V. Jacobson, G. Liu, L. Wei, P. Sharma, and A. Helmy. Protocol independent multicast-sparse mode (PIM-SM): Protocol specication. Internet Draft, IETF, September [13] T. Harrison, C. Williamson, W. Mackrell, and R. Bunt. Mobile multicast (MoM) protocol: Multicast support for mobile hosts. In ACM International Conference on Mobile Computing and Networking (Mobicom), September [14] J. Ioannidis, D. Duchamp, and G. Maguire. IP-based protocols for mobile internetworking. In ACM SIGCOMM Symposium on Communications Architecture & Protocols, pages 235{ 245, September [15] J. Moy. Multicast extensions to OSPF. Network Working Group RFC 1584, March [16] M. Nagy and S. Singh. Multicast scheduling algorithms in mobile networks. University of South Carolina, July [17] C. Perkins. IP mobileity support. RFC 2002, October [18] M. Schwartz, A. Emtage, B. Kahle, and B. Neuman. A comparison of internet resource discovery approaches. Computing Systems: The Journal of the USENIX Association, 5(4):461{ 493, [19] D. Waitzman, C. Partridge, and S.E. Deering. Distance vector multicast routing protocol. Network Working Group, RFC 1075, November