Abstract

Mobile IP is designed to support uninterrupted connectivity of mobile computers as they roam from place to place. The general structure and the goals of today’s mobile IP protocol are described. The previous works to achieve fast handoff and to decrease the control traffic is given. Then, our proposed system using GPS (Global Positioning System) with its handoff mechanisms and the changes that it brings to the current system together with its improvements in handoff and traffic are outlined. The simulation results using network simulator (ns2) and C++ are presented.

1. Introduction

The Internet is expected to become a huge wired network that consists of wireless attachment points at the edges that connect the wireless mobile users inside their coverage area to the wired part of the network.

The primary aim of Mobile IP is to support mobility in addition to portability for computers without charge. Portability, which means the operability of the computer at any point of attachment except the time during which it changes its point of attachment, is achieved today. The aim of Mobile IP is to maintain these computers in almost continuous reachability even when moving.

Wireless connections are expected to be performed through radio or infrared attachments like the cellular telephone technology. However, the limited geographical coverage and the charging problems are not yet solved for these systems. The coverage problem is only solved by cellular telephone technology. However, this brings the charging of connections. By ignoring the social and economic concerns, we will work on the adaptation of this cellular idea to Mobile IP.

As wireless computing becomes more prevalent, the need for the achievement of real-time services through wireless links increases. The need for real-time services can only be solved through tiling the world into smaller cells. These smaller cells bring also transceivers of lower power for mobile nodes.

Although higher bandwidth, which is required for real-time services, increases the quality of services provided to users, small wireless cells cause frequent handoffs. Frequent handoffs bring the frequent location updates so an increase of the latency in the re-routing of packets in handoffs. Therefore, although the wireless sources improve in the future, high quality of service cannot be achieved without making the handoffs transparent to users.

Our system achieves fast handoffs by both using hierarchical structure of the network [11] and GPS (Global Positioning System) devices in the routers. According to the hierarchical structure, the network is thought to be composed of administrative or geographical domains. Each domain has a hierarchical tree of foreign agents that includes a domain foreign agent at the top. Using a GPS device, each router knows its own position. With a special advertisement messaging scheme, they make other routers inside the domain learn their positions. They will then use this position information to send the packets directed to one foreign agent to the adjacent foreign agents in addition to this foreign agent. Besides, we use this geographical information of domain foreign agents to decide whether to send the registration request to home agent or to the previous domain foreign agent of the mobile host, which brings local home agent functionality to domain foreign agents if mobile host does not go away from this domain.

This paper addresses the course project of a graduate course on High Speed Communication Networks given at UC-Berkeley by Prof. Jean Walrand in the 2000-2001 academic year.

Different Mobile IP systems have been proposed by several companies. In section 2, a brief overview of the general structure of their proposed systems and the fundamental differences between them are described. In Section 3, the previous works on achieving fast handoff and less traffic of advertisement messaging are introduced. In section 4, the basic architecture of our system is presented with the possible improvements that it brings to the current structure. In Section 5, the performance results of our simulations in Network Simulator (ns2) and C++ are presented. Section 6 highlights the areas for future work. Section 7 concludes the paper.

2.Current Mobile IP Architecture

The basic structure of today’s Mobile IP has four entities:

Mobile Host (MH): Host or router that is portable with wireless network hardware while retaining a constant IP address.

Home Agent (HA): A stationary router on a mobile host home network that keeps the current location information of this host and forwards the packets accordingly.

Foreign Agent (FA): A router that has a connection with the mobile host as it moves and that delivers the packets coming to it by having both a radio and wired Internet connection.

Corresponding Host (CH): Any host on the Internet that sends packets to the mobile host.

Figure 1: General Structure of Mobile IP

The earliest mobile IP protocols have proposed the following scheme: When a CH wants to send a packet to a MH, it assigns the destination address of the packet as the constant IP address of the MH. Then the packet goes to the HA of the MH. The HA has the location information for the MH as another IP address, which is named care-of-address. At this stage, the HA tunnels the packet by encapsulating it with the destination address as the care-of-address of the MH. The packet is received at the tunnel end point, which is either the FA or the MH and delivered to MH. When the MH moves, it informs the HA of the current location.

The basic problem with the above system is the triangle routing. Datagrams with destination address of MH have to first go to the HA even if CH and MH are near each other and both away from HA. This problem is solved with mobility binding, which is a functionality of the CH to have an entry for MH. When a CH wants to send a datagram to MH for the first time, the normal procedure of packet forwarding is implemented. Then either HA or MH informs the CH of the current care-of-address of MH so that the following packets are forwarded directly to this care-of-address. When the MH moves from one FA to another FA, again either the MH or the HA informs the CH of the current location so that CH will send the packets directly to MH avoiding triangle routing. For ordinary CH (not enhanced by mobility binding) the routing is still via HA. Also, there are proposals which address this problem.

The fundamental differences between most of the proposed Mobile IP systems are in the following areas:

(i) How does a HA know where a MH is?

(ii) How can CHs send directly to the current FA of the MH avoiding the triangular trip via HA?

The system objectives in judging these different systems are performance as non-mobile protocols, scalability and security. The first goal, achieving the performance as non-mobile protocols, requires non-triangular routing since the packets go directly to the destination address in non-mobile protocols. Additionally, this performance issue requires fast handoffs when the user moving from one cell to another since the user should not understand that he is roaming from cell to cell. The second goal, scalability, is necessary in order to avoid the bottleneck of the MH location database as the number of MHs increases. The third goal, security, is necessary in order prevent the redirection of packets to malicious eavesdroppers.

3. Previous Work on Fast Handoffs

The problem of excessive mobility management traffic and time has been addressed in various works during the past years. The main problem in the system developed to solve the triangle routing problem is that it requires a lot of signaling between the MH, HA and CHs and a lot of time to reroute packets even if MH moves inside a very small area.

One of the proposed solutions is the hierarchical mobility scheme proposed by Caceres and Padmanabhan[6]. In this system, the network consists of domains, each of which includes a domain foreign agent keeping track of all the regional movements of the MHs inside the domain. The disadvantage is that the updating procedure of the mobile node entry in the routing table of the domain foreign agent takes time, which may cause the loss of MH packets coming in between. To solve this problem, they propose the buffering of the packets coming to old FA. This can decrease the number of lost packets due to handoff. However, in the real-time case, learning the new place of the MH and sending them to the new location can take so much time that it is better to ignore them instead of causing additional overhead in the network.

Another alternative proposed by Charles Perkins[11] is to form a hierarchy of foreign agents. Every foreign agent is supposed to know its ancestors in the tree. Then the registrations due to the handoff from one FA to another will only be up to the lowest common ancestor in the tree. This prevents the trip of the registration messages to the domain foreign agent each time the MH moves. However, all FAs in the route from the domain foreign agent to the MH’s FA have to keep the MH location information and this may cause bottleneck as the number of MHs increases. In addition, when their lowest common ancestors is very high up the tree, the handoff can again take a lot of time.

The proposed scheme by Seshan et al. [7]is to assign each MH a multi-cast address. When a MH registers a FA, this FA identifies the handoff targets and makes them join to the multi-cast group. These handoff targets buffer the last packets to transmit them in case of a hand–off. This avoids the wasteful trip to HA to make it aware of the new position. However, multicast address conflicts may occur if one packet passes through another network where the same multi-cast address is used.

The solution brought to the multi-cast conflict problem by C. L. Tan et al. [5] is to assign a multi-cast address to each MH by domain foreign agent inside the domain. They propose dynamic virtual macro-cells (DVM) for each FA containing the FAs around it and assign a multicast address to each DVM in addition to each mobile in the domain. They propose the allocation of a range of multi-cast address to each domain to eliminate the multi-cast address conflict completely. However, this will require a continuous messaging between all routers to learn about the new multi-cast addresses allocated by them. Moreover, in the last proposals, an intelligent neighborhood discovery mechanism is needed to eliminate the manual configuration of handoff targets and DVM members. Also, an inter-domain handoff mechanism needs to be described for these systems since they ignore the handoffs between different domains by using hierarchical structure.

4. Proposed System

Our proposed system aiming at improving handoff for real-time applications assumes that the network contains the domains, which have a hierarchical structure of FAs, and that each router knows the positions of all routers inside its domain. This can be achieved by a special advertisement messaging and GPS (Global Positioning System) devices at each router although today’s routers do not have such a capability. Then the packets coming to MH will first go to the domain foreign agent. At this time, domain foreign agent and the other routers will cooperate with each other by using location information in order to achieve fast handoffs. Our system reaches smooth handoff goal even in inter-domain movements by using location information.

4.1 Structure of the Network

Figure 2: Structure of the Internet

The Internet is composed of administrative or geographical domains as shown in Figure 2.

The structure of each domain is given in Figure 3. Every domain has a hierarchy of foreign agents that includes a domain foreign agent at the top. Every FA knows its ancestors and its children in the tree.

Figure 3: Structure of the domain

4.2 Handoff Mechanisms

4.2.1 Intra-Domain Handoffs

Our system uses the hierarchical structure in order to eliminate the wasteful trip to HA for each movement of the MH inside the domain. Each domain has one domain foreign agent (DFA), which is the ancestor of all other routers inside this domain.

The communication between the MH and FA is achieved through the wireless network hardware of the MH and one of the radio interfaces of the FA. Each FA broadcasts a beacon packet with a beacon period. When a MH wants to register to a FA, it registers with the DFA and sends the address of the DFA to its HA. Therefore, when a MH moves inside this domain, it does not need to inform HA of its current FA.

Switching from one FA to another can still be a problem inside the domain for real-time services since this kind of services is not tolerable to delay. To solve this problem, we propose to send each datagram directed to a specific FA to all the FAs adjacent to this FA since the MH is expected to move to one of these FAs if it changes FA. While the current FA of the MH is forwarding packets to the MH, the adjacent FAs buffer the last packets in order to forward these packets to the MH if a handoff occurs.

Our system involves two ways of achieving smooth handoff by sending the packets to all the FAs adjacent to the FA of the MH without needing multi-cast address.

First way of Fast Intra-Domain Handoff:

According to the first way, the following exchange of messages takes place:

  • DFA takes the encapsulated packet coming from HA or CH and decapsulates it.
  • DFA looks at the destination address and finds from its visitor list the care-of-address of MH.
  • DFA finds the adjacent FAs of the MH’s FA from the location-IP address table. Then from its routing table, it decides to which of its branches it should send these packets in order to cover all of the adjacent FAs, and sends these packets to these branches by encapsulating them.
  • When the routers at the end of these branches take the packets, they decapsulate the packet, check the location-IP address to find the adjacent FAs of the MH’s FA and decides to which of its branches it should send these packets, and send them with encapsulation.

By applying this method at each of the routers taking the encapsulated packets, we achieve the multi-cast forwarding of datagrams without allocating any multi-cast address.

Second way of Fast Intra-Domain Handoff:

According to the second way, the following exchange of messages occurs:

  • DFA takes the encapsulated packet coming from HA or CH and decapsulates it.
  • DFA looks at the destination address and finds from its visitor list the FA that the MH is connected to and sends the packet to that FA.
  • The FA taking the encapsulated packet decapsulates the packet and checks whether the destination address, the constant IP address of the MH, is inside its visitor list.
  • FA decides whether to send the packet over the radio link, send the packets to adjacent FAs or ignore them.
  • If MH is inside its visitor list, it sends the packet via radio link.
  • If MH is not inside its visitor list but inside its cache list, this means that MH has just left this FA. If the destination address of this cache list entry is “all ones”, this means that FA does not know the new care-of-address of the MH. Since MH has moved to one of its adjacent routers, it finds its adjacent routers from the location-IP address table and sends the coming packets to its adjacent FAs. If the destination address is not “all ones”, this means that FA knows the new care-of-address of the MH and send the incoming packet only to this address.
  • If MH is neither inside its visitor list not inside its cache list, it buffers the packet.

Comparison of Two Ways of Intra-Domain Handoffs:

The advantage of the first way is that it provides the minimum delay for the reception of the next packet from the new FA. The MH only waits for the discovery of the new FA and the registration time to DFA. Then it immediately starts to receive packets from the new FA. The disadvantage of the first method is that it sends the packets to all of the FAs adjacent to the current FA all the time. This increases the traffic in the domain even if the MH does not move at all.

The advantage of the second way is that it does not create traffic if the MH does not move. It only creates extra traffic when FA does not know the new location of the MH. However, if the number of links between the old and new FAs is very high, the forwarding of packets from the old to new FA can take time and can increase the real-time delay.

The second way of intra-domain handoffs can also be improved with appropriate additional hardware. For instance, MH may order FA to begin to send the data to the adjacent FAs when it starts to receive weak signal from FA. Moreover, MH can send the location direction of its movement in order that its packets come to FAs in that direction.

Performance of Intra-Domain Handoff Mechanisms: