A Survey of IP micro-mobility protocols

Transcription

1 A Survey of IP micro-mobility protocols Pierre Reinbold Olivier Bonaventure Infonet group, University of Namur, Belgium. March 2002 Abstract We present a detailed comparison, in a comprehensive framework, of Mobile IP and seven of the main IP Micromobility protocols. We first describe the global mobility landscape and point out the important issues that must be addressed. These are mainly Handoff management, Passive Connectivity and Paging support, Scalability and Robustness. Based on this framework, we examine in a first step Mobile IP as a Macro-mobility protocol. In a second step, we compare seven recently proposed IP Micro-mobility protocols: Hierarchical Mobile IP, Proactive Handoff, Fast Handoff, TeleMIP, Cellular IP, HAWAII, and EMA. Keywords Mobile IP, Wireless networks, micro-mobility, macro-mobility, IP mobility Introduction Today s wireless networks are largely based on circuit switching technologies and are optimized to carry voice traffic. Some extensions to these networks such as GPRS are currently being deployed to better support data traffic such as the TCP/IP protocol suite. Future broadband wireless networks such as the 4G networks for example will rely on packet switching technologies and will be entirely based on the IP protocol suite, both in the wireless and the wired parts of the network. Since IP was not designed with mobility in mind, there are several research problems that need to be solved before all-ip wireless networks are deployed. A first problem to be addressed is that inside an IP network the IP address is used to identify a given node and its location. Thus, when a mobile node moves inside the network, its IP address must change. This problem has already been addressed in several proposals such as Mobile IP [1]. Mobile IP offers a mechanism that allows mobile nodes to change their point of attachment (and thus their IP address) inside the network. However, when Mobile IP was designed, all-ip wireless networks were not taken into account and some of the mechanisms used by Mobile IP are not well-suited for such networks. For example, it can be expected that voice will remain an important service in broadband wireless networks. To efficiently support voice over IP, the network will have to provide a low delay and a low delay jitter to the voice packets. These low delays will have to be provided even while the mobile node performs a handover operation, i.e. when it moves from one base station to another. For this, the mobile node must be able to quickly change its IP address during the handover operation. Many solutions have been proposed to efficiently support mobile nodes inside IP networks. They are often called IP Micro-mobility protocols. Sometimes designed to solve very specific problems, their heterogenous characteristics and properties do not allow to easily obtain an accurate picture of the IP mobility management problems. This paper presents a comprehensive comparison, in a global framework, of Mobile IP and seven of the main IP Micro-mobility proposals. We first describe a global mobility landscape and point out the important problems to be This work was funded by the Walloon region within the ARTHUR project 1

2 addressed (section 1). Based on this framework, we examine in a first step Mobile IP as a Macro-mobility protocol (section 1.3). In a second step, we briefly describe and compare seven well-known IP Micro-mobility protocols in sections 2 and 3. We group four proposals under the label Foreign Agent () based mobility management proposals. These are extensions to Mobile IP for the micro-mobility management and the interaction between different s in the network is the basic mechanism of the mobility management for these proposals. These proposals are Hierarchical Mobile IP [13] and its extension for the regional paging [14], Fast Handoff [9], Proactive Handoff [2] (these last two have been described in a more generic approach in [8]) and finally TeleMIP [10] that is more an architecture than a real protocol. We also include in the comparison Cellular IP [3], HAWAII [24, 25] and EMA [16, 15]. All these protocols are the result of the standardization process within the IETF Mobile IP [11] and the Seamoby [12] working groups. Finally, we present our conclusions. 1 A global IP mobility framework This section focuses on the presentation of a global mobility landscape and the major issues for IP mobility to be investigated within this landscape. 1.1 The mobility landscape This paper deals with all-ip networks. These are the expected future mobile wireless networks, relying entirely on IP, from the mobile station to the gateway toward the Internet. We use a very generic and simple landscape to describe and compare the different protocols. This is a very different approach from the current evolutions of GSM networks, like GPRS, where IP is only used in the backbone. We call IP wireless domain 1 a large IP wireless access network under a single administrative authority. From an IP point of view, we can describe these networks as a set of wireless IP Points Of Attachments (IPPOA) connected to an IP backbone, with a gateway towards the Internet. This situation is illustrated in figure 1. In this picture, we have presented a wireless domain using a CDMA-like radio interface. In such networks, the base stations are grouped in so-called Radio Access Networks (RAN) under the control of a dedicated station, the Radio Network Controller. Inside a RAN, the mobiles movements are entirely managed at the radio layer and are thus transparent to the upper layers. A RAN can thus be considered as a single IPPOA or a subnet. This is obviously only an example and the actual meaning of IPPOA can vary between wireless domains using different radio interface technologies. We also assume that each Mobile Node MN is attached to a Home Network (HN), a domain from which it has obtained a static IP address: its Home Address. In this context, static means that the address validity is much more longer than the duration of a mobile movement. We call Foreign Network (FN) any other domain where the MN can connect. On figure 1, we have added an IP backbone dedicated to enable the roaming between the domains, in analogy with the current GSM/GPRS/UMTS networks. 1.2 Major mobility issues We now define some important issues for the mobility management in our mobility landscape. These will be the basis of our comparison. Handover management : From an IP point of view, the handover concerns the management of the changes of IPPOA of the mobiles during their moves. The handover management is obviously a major issue in mobility management since a MN can experience several handovers during a single session as in current mobile telephony networks. 1 We will also designate these networks by wireless domains or more simply domains. 2

3 IP wireless access domain Inter domain backbone IP wireless access domain Internet Mobile node BST CDMA like RAN RNC BST IP Layer Gateway IP router IP Wireless Point of Attachment Radio Layer Radio Network Controler Base Station Transceiver BST BST BST BST Figure 1: A simple model of the future IP mobility landscape Passive connectivity and paging : The mobile devices have a very limited power capacity and their batteries must be spared by reducing the mobiles transmissions to the minimum required. This is a common problem in classical mobile telephony. An ideal solution would be that the MN emits exclusively when it has data to transmit and nothing during the rest of the time. Unfortunately, if a MN is not emitting and changes its IPPOA, it will be impossible to forward an incoming packet destined to it as we do not know where the mobile is located. A standard solution adopted in GSM networks is to divide the network in geographical areas called paging areas. When the mobile as no data to transmit (in the idle state), it emits a beacon when changing of paging area. This implies that the network only knows the approximate location (the current paging area) of the mobile. An incoming packet destined for an idle mobile triggers the network to perform a paging (a mechanism to find the exact location of this MN within its paging area) in order to deliver the packet. Enabling passive connectivity support via a paging architecture is always an additive burden for the network (at least in control traffic). This solution must thus be carefully considered with respect to efficiency concerns, number of handovers per session,... Quality of Service : Future broadband wireless networks aim at enabling their users to use multimedia applications such as Voice over IP, streaming sounds and videos. Quality of service will be extremely important for such flows and IP mobility mechanisms must be able to support it. Intra-domain traffic : Intra-domain traffic is an important part of the current traffic in wireless networks such as GSM/GPRS (the Short Messages Service for example). We can expect that the mobile users of future wireless networks will also communicate a lot between mobiles connected to the same domain. Such traffic must thus be efficiently supported. The comparison will be made with respect to these topics on the basis of performance criteria such as latency, control traffic, robustness and scalability. 1.3 Mobile IP and all-ip wireless networks In order to introduce the comparison, we present here a quick review of Mobile IP and its major drawbacks that have led to the definition of the micro-mobility approach. 3

4 1.3.1 Mobile IP Mobile IP is the oldest and probably the most widely known mobility management proposal. Its simplicity and scalability give it a growing success. Mobile IP is described in [1] (a good review paper can be found in [22]). Several extensions and enhancements are described in [19, 20, 26, 6]. In this document, we discuss the principles of Mobile IP and the discussion is applicable to both IPv6 and IPv4. In order to allow a mobile IP node to change its IPPOA in the network, Mobile IP defines two types of Mobility Agents, i.e. dedicated to the mobility management. These are : the Home Agent (HA), located in the home network of each mobile, the Foreign Agent (), located in each foreign network where a MN can connect. The basic principle is that Mobile IP uses a couple of addresses to manage user s movements. Each time the MN changes its IPPOA, it obtains a temporary address called Care-of-Address (COA) from a directly connected to this IPPOA. The presence of the in a particular subnet can be detected via advertisement messages that are extensions to ICMP router advertisement messages [7]. These messages are broadcasted at regular time intervals by the. The MN can also send advertisement messages to trigger a to transmit its advertisement message. Each time it delivers a COA to a new MN, a must add a binding for this mobile in a dedicated table called visitor s list. Once it has obtained its new COA, the MN must then inform its HA of this new address by using the registration process. When the HA is aware of the MN s current COA, it will intercept the packets received in the home network for the MN. These packets will then be encapsulated inside a packet whose destination is the. Upon reception of such encapsulated packets, the will deliver the original packet to the mobile node. This basic working is illustrated at figure 2 Correspondant Node IP address : CN IP source: CN IP dest.: MN From the Correspondent to Mobile IP within IP encapsulation From the Mobile to Correspondent R IP source: MN IP dest.: CN Foreign Agent IP Address : R Internet R R IP source: HA IP dest.: COA IP source: CN IP dest.: MN Home Network Home Agent IP Address : HA Foreign Network Mobile node IP Address : MN Care of address: COA Figure 2: Basic working of Mobile IP The micro-mobility problems Mobile IP suffers from several well-known weaknesses that have led to the definition of the macro/micro-mobility approach. In this section, we review some of these weaknesses to show the advantages of this paradigm. We also introduce the comparison by pointing out several important properties shared by all micro-mobility proposals. 4

5 Handover latency and control traffic : in Mobile IP, the basic mobility management procedure is composed of two parts : the movement detection by the MN and the registration to the HA. Every time the mobile changes its IPPOA, these two steps must be accomplished to allow the MN to receive packets. However, it is the MN that initiates the process by sending a registration request once it has detected that it moved from one network to another and has obtained a new COA. This introduces two causes of latency : move detection latency : this is the time required by the MN to detect that it has changed of IPPOA. It can be high since the move detection mechanisms in Mobile IP are based on either the expiration of the lifetime in the agent advertisements 2 or on the comparison of the address prefix of two different agent advertisements. registration latency : as the HA can be located anywhere on the Internet, this process can take a very long time and sometimes be impossible to complete. This is obviously, by far, the main expected part of the total handover latency. In the case of a quickly moving mobile which changes of network rapidly, the registration process will become totally inefficient. Moreover, this mechanism produces a lot of control traffic inside the local domain and across the Internet. Quality of Service : frequent changes of point of attachment and of COA make it difficult to support Quality of Service for mobile users. With RSVP, for example, the reservations must be done again and again, each time the MN changes of COA, along the entire path, even if the largest part of this path remains unchanged. The micro-mobility approach seems to be a good solution to partially solve the last problem. It implies the utilization of two different protocols to manage the mobility: Mobile IP manages the movements of the MN between distant wireless domains and across the Internet, another protocol manages the movement of the MN inside each wireless domains. A micro-mobility protocol behaves as follows. The MN obtains a local COA when it connects to a domain. This COA remains valid while it stays in this domain and the mobile will thus make only one home registration (registration with the HA) at the time it connects to the domain. The users movements inside the domain are managed by a micromobility protocol. This is transparent to the HA and the rest of the Internet. In fact, for the HA, each wireless domain will become a Mobile IP subnet. Latency and control traffic across the whole network are thus extremely reduced. This is the main reason to adopt a micro-mobility approach. On this base, each micro-mobility proposal aims at reducing the move detection latency (by an interaction with the radio layer for example) and at optimizing the handover management inside a domain. In the case of QoS, as the network is not aware of the users movements inside a particular domain, the reservations are to be made again only when the mobile changes of domain. This reduces the control traffic and delay, as for the registration process. This is only possible if the micro-mobility protocol supports the use of RSVP [23] or other QoS mechanisms. 2 The IP micro-mobility protocols This section presents a short description of the basic principles of the different micro-mobility proposals. 2.1 based mobility management proposals Several of the micro-mobility proposals manage users mobility on the basis of interactions between s. Hierarchical Mobile IP [13], is an extension to Mobile IP that supports a hierarchy of s between the MN and the HA. Some improvements to Hierarchical Mobile IP, such as a paging mechanisms, have been proposed in [14]. Fast Handoff [9] 2 This is the lifetime indicated in the ICMP router advertisement 5

6 and Proactive Handoff [2] are two very close proposals that are based on Hierarchical Mobile IP but include improved handoff mechanisms. TeleMIP [10] adds some load balancing features in the basic principles of Hierarchical Mobile IP. The basic network model for these proposals is shown in the figure 3 A. In this type of network, we have a set of IPPOAs and each is associated with a dedicated. The s are connected to a so-called Gateway Foreign Agent (G). This network model is used by the Hierarchical Mobile IP, Proactive Handoff. TeleMIP uses the same model but allows the to be connected to several G. Figure 3 b shows a slightly more complex architecture with a multi-level hierarchy of s between the G and the leaf s. Each in the hierarchy may be in charge of a IPPOA. This type of network is described in the appendix B of the Hierarchical Mobile IP draft [13] and is used to support Fast Handoff. G G Figure 3: Network models for a based mobility management Hierarchical Mobile IP Hierarchical Mobile IP is a natural extension to Mobile IP to efficiently support the micro-mobility. After the first connection of a MN to a domain and its home registration with the address of the G as COA, the MN will perform Regional Registrations only. This type of registration is sent by the mobile to the G each time it changes of (i.e. of IPPOA). The registration contains the new local COA of the MN: the address that can be used by the G to reach the MN while it remains connected to the same. This address can be either a co-located address or the address. The routing with Hierarchical Mobile IP is then very simple. A packet destined to the MN is first intercepted by the HA and tunneled to the G. Then, the G de-capsulates and re-tunnels it towards the current local COA of the MN. Hierarchical Mobile IP also supports a multi-levels hierarchy of s between the leaf IPPOA and the G. Each in the hierarchy must maintain a binding in its visitor s list for each MN connected to a IPPOA lower in the hierarchy. These bindings are established and refreshed by the regular registration requests and replies that the mobiles exchange in the network. In this case, the regional registrations sent by a MN are only forwarded to the first that already has a binding for this MN. The upper levels of the hierarchy are not aware of the details of the mobiles movements since they do not have to change their binding. In this way, the handoff management is limited to a very small number of machines. In addition, [14] has introduced a paging support for Hierarchical Mobile IP. It relies on paging areas that are sub-trees of a same hierarchy. In each of these areas, the root of the sub-tree is called a paging Foreign Agent (P). It maintains a specific visitor s list with an idle flag set for each idle/passive mobile located currently in this area. The P is in charge of the entire paging process by performing the paging request and managing the incoming packets destined to idle MNs. This paging mechanism is not included in the other based mobility protocols discussed later in this section. 6

7 2.1.2 Fast Handoff Fast Handoff re-uses the architecture and principles of Hierarchical Mobile IP and addresses a set of remaining problems of this proposal. These are mainly the need for a fast handoff management for real-time applications and the presence of triangular routing inside the domain. In the previous section, we have seen that Hierarchical Mobile IP does not improve the Mobile IP movement detection : it relies on the ICMP messages used by Mobile IP. Fast Handoff assumes the possibility of an interaction with the radio layer to anticipate the handoff and allows the MN to perform its registration with a new through the old before the handoff actually occurs. The basic principle is that the IP layer receives the handoff events as triggers from the radio layer, these triggers are designed to inform the IP layer of the imminence of a handoff by providing the next IPPOA of the MN (the IP address of the new ). We call this interaction with the radio interface : Strong Handoff Radio Trigger (SHRT), as it contains the new IPPOA of the MN. On this basis, the protocol contains two mechanisms to perform the handoff with respect to the capabilities of the radio layer (if a mobile is able to communicate with more than one base station). A global overview of these algorithms is shown in the left side of figure 4. It is also possible to use the bicasting [1] capabilities of Mobile IP with simultaneous bindings to reduce the possibility of packet losses. The triangular routing inside the domain is limited by the use of the information found in the visitor s list. When a receives a non-encapsulated packet (i.e. coming from a MN), it consults its visitor s list to see whether it contains an entry for the destination address. If it contains one, the can directly send the packet to this address. Otherwise, it forwards the packet as normal Mobile IP packet Proactive Handoff Proactive Handoff shares many properties with Fast Handoff : it assumes the same architecture as Hierarchical Mobile IP, except the multi-levels hierarchy, and aims at providing a fast handoff mechanism by using a SHRT. The main difference is that the IP handoff is not performed by the MN with a registration request but by the two concerned. After a short negotiation exchange, the new sends a Regional Registration Request to the G on behalf of the MN. At this time, it is possible to bicast the traffic destined to this mobile to the two s. The new will then send an agent advertisement to the MN so that it can perform a normal registration, as shown in the right side of figure 4 b. Proactive Handoff also allows to use the Anchor Registration described in [13] to reduce the registration latency. 1 L 2/L 3 interaction Old Fast handoff New Proactive handoff 2 Handoff request/reply 1 L 2/L 3 interaction Old G 3 Registration with the G New 2 New agent advertisement 3 Registration with the new Registration to the new via the old Registration to the G sent by the new Figure 4: Handoff mechanisms for Fast Handoff and Proactive Handoff TeleMIP TeleMIP is described as a mobility architecture based on the same principles as Hierarchical Mobile IP. A limitation of TeleMIP is that its supports only a two-level hierarchy. Its major improvement is that there can be several Gs in the network and that the s can be connected to more than one G. This allows to select the G with some load balancing algorithm so that the burden of the mobiles management is not set on a single machine. 7

8 2.2 Cellular IP Cellular IP [3, 4, 27] aims to replace IP inside the wireless access network. A Cellular IP domain is composed of MA and one of them acts as a gateway towards the Internet and as a Mobile IP for macro-mobility. Each MA maintains a routing cache that contains the next hop to join a MN (one entry per mobile) and the next hop to join the gateway. This allows the MA to forward packets from the gateway to the MN or from the MN to the gateway. The routes are established and basically maintained by the hop-by-hop transmission of two special control packets. Upon receiving one of these packet trigger, the stations update their routing cache. These packets are : A beacon periodically flooded by the gateway inside the network. This mechanism allows each station to know which of its interfaces must be used to forward packets towards the gateway (the one from which the beacon was received). route update packets sent by the MN when it connects to the network, each time it changes of IPPOA and at regular time intervals. These packets, forwarded hop-by-hop towards the gateway, trigger the stations on their paths to update their routing cache for the concerned MN : the next hop to this mobile is the MA that has just forwarded the route update. The basic handover management in Cellular IP is called hard handoff : the MN simply transmits a route update packet to the gateway after the radio handoff is completed to establish new routes. To improve this mechanism, the protocol defines the so-called semi-soft handoff. Based on the reception of a SHRT by the MN before the occurrence of the radio handoff, the mobile can send a special packet to establish a bicasting of the traffic to the old and the new IPPOAs and thus reduce significantly the losses. Moreover, Cellular IP presents a native support for the passive connectivity with a classical paging mechanism: some stations maintain paging caches that are used to support the paging management. 2.3 HAWAII Unlike Cellular IP, HAWAII [24, 25] does not replace IP but works above IP. Each station inside the network must not only act as a classical IP router but also support specific mobility functions. The basic working of HAWAII is similar to the principles of Cellular IP : each station maintains a routing cache to manage the mobility and the hop-by-hop transmission of special packets in the network triggers the stations to update their cache. As in Cellular IP, the network is supposed to be organized as a hierarchical tree and a single gateway is located at the root of this tree. HAWAII defines two different handover mechanisms adapted to different radio access technologies (depending on whether the MN can communicate with more than one base station or not). These mechanisms present different properties and can be chosen to optimize the network with respect to packet losses, handoff latency or packet reordering. Both rely on the assumption of the reception of a SHRT by the MN. Like Cellular IP, HAWAII supports the passive connectivity with a paging mechanism. The paging areas are composed of stations belonging to the same IP multicast group. The paging requests, that must reach all the stations of an area, are transmitted to the multicast group corresponding to this area. To support efficiently Quality of Service, HAWAII defines a native integration of RSVP adapted to the user s mobility. 2.4 EMA EMA [15, 16] defines a generic framework for the mobility management within a wireless domain. Within this framework, it is possible, in theory, to work with any routing protocol to forward the packets. The authors in [15, 16] discuss the possibility of using the TORA [17, 18] ad-hoc network routing protocol with EMA. This choice seems to ensure a good scalability for the system while the EMA architecture allows to adapt TORA to the management of standard wireless access networks that have other properties than ad-hoc networks. Without any assumption on the radio access technology, EMA defines a handover mechanism completely transparent to the upper layers and even to the routing protocol. This mechanism is based on the reception by the MN of a SHRT to initiate the handover management. It starts with a three way handshake between the two IPPOAs to establish 8

9 a soft-state tunnel between them. This tunnel is used to bicast the traffic to the MN while the network establishes one or more new routing paths to the mobile. This is achieved with the usual mechanisms of the chosen routing protocol. When the network has completed the handoff (i.e. updated the routing mechanisms to adapt it to the new point of attachment of the MN), the temporary tunnel is removed. EMA defines two different schemes to perform this procedure: Break Before Make and Make Before Break. They are used if the radio link with the old IPPOA is lost before or after the network has established a new routing path for the MN to the new IPPOA. EMA supports two types of routing: prefix routing (as in classical IP networks) and host specific routing. When its connects to the EMA domain, the mobile obtains a COA from the local subnet. In such a way, the traffic destined for this mobile node can be routed based on its prefix while it remains in the subnet. When the MN changes of subnet, specific routes are injected in the network to reach it. TORA is very well adapted to work this way. 3 Comparison of IP micro-mobility protocols In this section, we compare the different micro-mobility proposals presented in section 2 with respect to the context described in the previous section. 3.1 Handoff Comparison parameters We investigate the handoff management on the basis of the simple network model shown in figure 5 with respect to : handoff management parameters: the interaction with the radio layer, initiator of the handover management mechanism, use of traffic bicasting, etc., handoff latency: the time needed to complete the handoff inside the network, potential packet losses: the amount of lost packets due to the handoff process, involved stations: the amount of MA involved in the handoff management, i.e. that must update their routing data or process messages in the handover mechanism. For this comparison, we assume here that n gate is the average number of hops between a MN and the gateway. The delay between these two hosts is t gate msec. Similarly, n prev is the number of hops between a MN and its former IPPOA (delay: t prev msec). t cross is the average delay in msec between the MN and the so-called crossover node for a given handoff. This node is the first common network entity located on a path between the new IPPOA and the old IPPOA and on the path between the new IPPOA and the gateway. In the case of based mobility management, these concepts must be understood in terms of. For example, in a four levels hierarchy in Hierarchical Mobile IP, n gate would be equal to four. In general, we can assume that t gate t prev t cross. t HA is the average time needed to reach the HA with the classical Mobile IP registration mechanism. When investigating performance of handover mechanisms in micro-mobility, we must consider the important point of move detection. We have already seen that the micro-mobility approach reduces the registration latency as most of the registrations are limited inside the current domain. However, the detection of the occurrence of a handoff is another important source of delay for real-time applications. As the IP handover management takes place after the movement detection, this detection must be as efficient as possible. In other words, any IP handover management mechanism is useless if the movements of the MN (handovers occurrences) are detected too late and packets are already lost. In Mobile IP, the movement detection is made via two algorithms described in [1]. These algorithms are based on the ICMP router discovery messages. Handoff is detected when receiving a Mobility Agent Advertisement with a source address located in another network (beginning with a different prefix) or when the lifetime expires for the last Mobility Agent Advertisement received. With the first algorithm, the detection occurs, on average, after the time between two Agent Advertisement (twice this time in worst case). With the second algorithm, it occurs after the lifetime of the Agent Advertisement. The values of these parameters may (must) be tuned to be adapted to the local network (their 9

10 Gateway Crossover Station Node tcross tgate Internet Base stations coverage area New Base IP POA Station tprev Base stations coverage area Previous Base IP POA Station Mobile s movement Figure 5: A simple model to compare handoff mechanisms default values are 30 min. for the lifetime and 7-10 min. for the rate of Agent Advertisement [7]). We will call this latency t mip. In the case of protocols relying on interaction with the radio layer, we call dt trigger the time between the reception of the radio trigger (ex. SHRT) and the actual radio handoff i.e. the moment when the radio link between the MN and its old IPPOA is removed. This time interval depends obviously on the radio technology, the load, the local topology of the network, the MN movements, etc. In figure 5, dt trigger may represent the time that the mobile crosses the overlapping area, going from point a to point b, if, for example, the trigger is sent when the mobile is at point a. For each proposal, we have defined the uncertainty time. During this time interval, after the radio link with the old IPPOA is deleted, the packets destined to the MN may be lost or incorrectly routed by the network. This parameter is a very important since it reflects the efficiency of a handoff management mechanism with respect to the packet losses. In this comparison, we do not take into account the time needed by the MN to reach the stations on the wireless interface. This time, which can be long, is not relevant for our comparison. Moreover, we also neglect the time needed to transfer packets inside a RAN for the based mobility management proposals. We consider that this is a layer 2 characteristic. However, it can be large in the case of protocols assuming interaction between layers since RAN can cover a very large geographic area. In our comparison, we identify two type of radio technologies: TDMA-like, allowing a MN to communicate only with one base station and CDMA-like, allowing the MN to communicate with more than one base station Handoff comparison The Hierarchical Mobile IP handoff mechanisms are designed to limit the handoff management at a local level while the MN remains in the same hierarchy 3. When changing of IPPOA, the MN must issue a registration request. This registration request must only reach the first with an existing binding for this mobile. This is obviously the crossover node and the time to reach it is thus t cross. When receiving a regional registration request for a MN for which it already has an entry in its visitor s list, the crossover node must send a binding update with a zero lifetime to the previous address of this MN to remove the old route: this is called de-registration. As t cross is the average time to reach the crossover, the total time to reach the crossover node and to remove the old route is 2t cross. When changing of hierarchy, the MN must perform a classical Mobile IP registration with its HA, the latency is thus 2t HA. The uncertainty time is t mip + t cross in the first case. In the case of a registration with the home agent, it is more difficult to evaluate this time interval. Indeed, Hierarchical Mobile IP allows to use the soft handoff mechanisms described in [21] to ensure that the losses occur only during the time needed to reach the previous. If we assume a handoff between two hierarchies belonging to the same domain, the uncertainty time will be t mip + t prev. The handover will involve 2n cross s in the first case and n gate + 1 MA (s in the new hierarchy and the HA) in the second case. Fast Handoff tries to reduce the handoff latency by using a SHRT to detect the mobile movements. When a handoff is almost happening, the network triggers the new to send an Agent Advertisement to the MN via the old before 3 We assume here Hierarchical Mobile IP with multi-levels hierarchy. 10

11 the end of the radio handoff. The MN can thus register with the new with classical Mobile IP mechanisms while still having an IP binding to the old. Three working modes for Fast Handoff are defined in [8]. If the old receives the SHRT and initiates the IP handoff management, the mechanism is called source triggered handoff. If it is the new, it is called target triggered handoff. Finally, if the MN initiates the handoff after receiving a SHRT, we will speak of mobile initiated handoff. Furthermore, the different can exchange Agent Solicitation and Advertisement messages before handoff so that the old can send directly an Agent Advertisement referencing the new to the MN. This allows to skip the Agent Advertisement Request/Reply process between the new and the MN. The uncertainty time is thus reduced to the time needed to complete the registration. With Fast Handoff, the mobile must receive an Agent Advertisement before making a registration with the new, all these messages passing by the old. As the Advertisement can be sent by the old, this leads to a total IP handoff latency of t prev +2t cross in the case of source triggered or mobile initiated handoff, and 2t prev +2t cross with target triggered handoff. The packet losses may be avoided if the crossover is reached by the registration request before the radio handover is completed 4. It is the case when dt trigger is greater than or equal to the IP handoff latency minus the time to receive the registration reply (t cross ). Proactive Handoff also uses a SHRT to anticipate the handoff but in a different way : the handoff is entirely managed by the s. When a detects that handoff is happening, it sends a Handoff Request to the other concerned (it can be the old or the new one). This agent replies with a Handoff Reply and if this reply is positive, the new sends a Regional Registration Request for the mobile to the G. In this way, a can establish an IP binding with a MN before having a radio link with it. The exchange of handoff request/reply messages allows the s to establish a tunnel between them to bicast the packets to the MN. If this occurs before the radio handoff, it is possible to avoid packet losses. This is the case when dt trigger is greater than or equal to 2t prev, the time requested to transfer the request from the old to the new and the reply in the opposite direction. TeleMIP has exactly the same handoff characteristics as Hierarchical Mobile IP with a two level hierarchy since it uses the same mechanisms. With Cellular IP, the handoff mechanism triggers the MN to send a packet that is forwarded hop-by-hop towards the gateway and that must be acknowledged. The latency is thus 2t gate (time to reach the gateway and to receive its acknowledgment) and n gate stations are involved in the handoff process (all stations on the path to the gateway). We can expect that no packet loss will occur with semi-soft handoff if the crossover node receives the semi-soft handoff packet before radio handoff is completed. We can see that dt trigger must be greater or equal to t cross. In the case of hard handoff, the uncertainty time is t mip + t cross losses since packets are lost from the time the mobile changes of station to the time the route update message reaches the crossover node. The HAWAII handoff mechanism is based on an exchange between the old and the new IPPOA. The total latency is thus 2t prev. The forwarding scheme implies a uncertainty time equal to t mip + t prev because packets are lost until the update message reaches the old base station. The non forwarding scheme is faster since the packets are correctly forwarded as soon as the crossover station is aware of the handoff (this is similar to the hard handoff in Cellular IP). In this case, assuming that the MN can be connected with two base stations, the packet losses can be avoided if dt trigger is greater than or equal to t cross. In HAWAII, the only stations located on the path between the two concerned IPPOAs perform a routing update. This very local handoff management involves n prev stations. From a theoretical point of view, EMA defines mechanisms sufficient to avoid packet losses if the handoff can be anticipated. As soon as the tunnel is established, it can be used to bicast the traffic. We can thus avoid losses if dt trigger is greater or equal to the time needed for the three way handshake : 3t prev. The total handoff latency is the time needed to perform the three way handshake followed by the network routing convergence time. Indeed, TORA will build new routes to the MN each time it changes its point of attachment since the three way handshake is finished when the MN injects its new TORA height in the network. Many stations will be aware of the handoff and perform routing updates in addition to those located between the concerned IPPOA and the MN itself. We call N(TORA) the average number of stations involve in the handoff management in a TORA network (this number depends on the network topology [17]). We assume that the network convergence is done in T(TORA)msec. It is obviously difficult to compare this time to the others but we focus here on the EMA handoff management. 4 We consider that the Hierarchical Mobile IP mechanisms are used inside a single hierarchy. In the case of a handoff between two different hierarchies, t cross becomes t HA. 11

12 Table 1: Comparative chart for handoff parameters Protocol Handoff type Radio Layer Hierarchical Mobile IP Fast Handoff Handoff initiator Traffic bicasting Move Detection Latency Total IP Latency Uncertainty time Stations involved L2 trigger Inside a hierarchy - no MN no t mip 2t cross t mip + t cross 2n cross Between hierarchies - no MN no t mip 2t HA t mip + t prev n gate + 1 a Source Trigger CDMA SHRT MN or s yes radio layer 2t cross + t prev max(0, t prev + t cross dt trigger) Target Trigger CDMA SHRT MN or s yes radio layer 2t cross + t prev max(0,2t prev + t cross dt trigger) Mobile initiated CDMA SHRT MN or s yes radio layer 2t cross + t prev max(0, t prev + t cross dt trigger) 2n cross 2n cross 2n cross 12 Proactive Handoff TeleMIP Cellular IP HAWAII EMA (using soft-state tunnels) With Anchor Registration CDMA SHRT MN or s yes radio layer 4t prev max(0,2t prev dt trigger) 2 (old and new ) With Regional Registration dt trigger) CDMA SHRT MN or s yes radio layer 2t prev + 2t max(0,2t prev gate 3 (old and new, G) Between IPPOA - no MN no t mip 2t gate t mip + t gate 2 (G, ) Between G - no MN no t mip 2t HA t mip + t HA 3 (G,, HA) Semi-soft handoff CDMA SHRT MN yes radio layer 2t gate max(0, t cross dt trigger) Hard handoff TDMA no MN no t mip 2t gate t mip + t cross n gate Forwarding scheme TDMA SHRT MN no radio layer 2t prev t mip + t prev n prev Non-forwarding scheme CDMA SHRT MN yes radio layer 2t prev max(0, t cross dt trigger) Make Before Break CDMA SHRT MN yes radio layer 3t prev + T(TORA) Break Before Make TDMA SHRT MN no radio layer 3t prev + T(TORA) max(0,3t prev dt trigger) max(0,3t prev dt trigger) n gate n prev n prev + N(TORA) n prev + N(TORA) a Using smooth handoff

13 3.1.3 Conclusion It is clear that the handover management will obviously remain the most important point for the micro-mobility. The micro-mobility approach within the Mobile IP framework allows to make only one home registration when connecting to a new domain. Inside the domain, we have two sources of latency : the move detection latency and the IP routing update latency. Protocols that remain totally independent of the radio layer rely all on a Mobile IP-like move detection mechanism based on timers and broadcasted IP messages. This requires an established radio link with the new IPPOA. For TDMA-like radio technologies, it means that the radio handoff is already done and that further latency in IP handoff may cause packet losses. In all cases, we can expect that an IP handoff process based on this move detection will always begin after the radio handoff. In the worst case, it may only be finished after the radio handoff and this could cause packet losses. Hierarchical Mobile IP, TeleMIP and Cellular IP (with hard handoff) are concerned by this. To avoid such losses, the other protocols assume the possibility of what we have called a Strong Handoff Radio Trigger (SHRT). This trigger is sent by the radio layer before the radio handoff that contains the new IPPOA of the concerned mobile. On this basis, it is possible to anticipate the radio handoff and, maybe, complete the IP handoff before, so that no packet losses occur. For This, the SHRT must be received sufficiently in advance in time before the radio handoff, depending on the IP routing update algorithm. This method makes thus three important assumptions : it is possible to receive a radio handoff trigger before the actual radio handoff, this trigger provides the new IPPOA of the MN, the trigger arrives sufficiently in advance in time before the radio handoff. If it seems to be possible to receive a radio trigger before the actual radio handoff, the assumption that it can provide the new IPPOA of the MN is very strong. It seems more realistic to assume that we can receive a radio trigger very soon before the handoff, based on power measurements, but that this trigger is only an indication of the imminence of a handoff, containing no new IPPOA. 3.2 Passive connectivity and Paging Only a few proposals explicitly include the support of these features: Hierarchical Mobile IP with its paging extension [14], Cellular IP and HAWAII. These protocols use the classical cellular telephony concepts of location area and paging. As in mobile telephony networks, the stations are grouped in paging areas and the network must perform a paging to find the actual IPPOA of the MN. In Hierarchical Mobile IP and Cellular IP, these paging areas must be sub-trees of a single hierarchy. A major difference between Hierarchical Mobile IP, Cellular IP and HAWAII is their paging algorithm. In Cellular IP, the arrival of a packet destined for an idle MN triggers the paging from the gateway. This paging request is propagated inside the network by stations with paging caches in charge of the concerned paging area. The stations that are to perform the paging requests are thus defined by the network manager and only these machines will maintain paging information. The situation is exactly the same with Hierarchical Mobile IP where the root of the sub-tree in each paging area, called Paging, is responsible for the paging inside this area. HAWAII defines an algorithm to dynamically balance the load of paging among the stations of the network. Based on the current load of each router, a particular station is chosen to perform each paging. The paging information is thus distributed throughout the network to ensure that any station can perform a paging. The passive connectivity is extremely valuable unless the mobile devices have infinite capacity batteries. The paging is a well known and very efficient solution to this problem. It seems to us that a micro-mobility proposal should include an IP paging support, at least as an option. 3.3 Intra-network traffic In this section we focus on the traffic between the MNs connected to the same wireless network. This kind of communication is a large part of today s GSM communications (both voice and short messages services) and we can expect 13

14 that it will remain an important class of traffic in future wireless networks. The effective support of this type of traffic is thus an important concern. With Hierarchical Mobile IP, Proactive Handoff and TeleMIP, this type of traffic will be directed to the HA of the destination. Even with the use of the route optimization extension [21], it will pass through the gateway. Only Fast Handoff defines a routing mechanism that re-uses the informations in the visitor s list to directly route the traffic whenever possible. If a receives a non-encapsulated packet, it looks in its visitor s list for a binding for the destination. If it found such binding, it sends the packet directly. Otherwise, it follows the classical Hierarchical Mobile IP forwarding scheme. With Cellular IP, all the traffic coming from a MN must pass through the gateway, even if the MN is communicating with another host in the same wireless network. This type of routing increases unduly the load on the gateway and the neighboring stations. HAWAII suffers from the same problem. TORA allows EMA networks to manage the intra-network traffic efficiently if the network implements all the features of TORA. The traffic between two mobiles inside the same domain constitutes today an important part of the wireless communications. This kind of traffic must be efficiently managed by the micro-mobility proposals. Fast Handoff mechanism seems to be a very good solution for this. 3.4 Scalability and robustness Current mobile networks support millions of connected users communicating at the same time. We can expect that future large wireless access networks will have the same constrains in terms of users load. For example, a commercial router acting as GGSN in a GPRS network is able to manage 90,000 simultaneous user contexts [5]. These facts are to be related to the increasing load of today s Internet routers: routing tables containing a few hundreds of thousands entries have become a performance problem. The based mobility management proposals, Cellular IP and HAWAII rely on a tree-like wireless access network. A dedicated machine acts as a gateway and is the root of this tree. A direct consequence is that stations close to the gateway are more loaded than the leaf stations. This increasing load is due to traffic processing and soft table handling in memory. The gateway is hence the more heavily loaded station in the network, processing all updates and maintaining tables entries for all the MNs within the network. This table may thus contain so many of entries that its handling by a single machine may become almost impossible. These architectures are weak since they rely on specific routers such as the gateway and the surrounding stations. In the case of Cellular IP and Hierarchical Mobile IP with regional paging, the situation is even worse since only a few stations maintain the paging information, making the network extremely vulnerable to a crash of these stations. HAWAII distributes the paging information inside the network and assigns dynamically the paging processing. This increases its robustness but at the cost of a greater load on the routers memory. On the other hand, Cellular IP basically manages link failure or station crashes with two kinds of refresh mechanisms: the beacon periodically transmitted by the gateway and the routing refreshes sent by the MN. The based mobility management proposals and HAWAII work on the top of IP and benefit from the existing IP recovery mechanisms. We can also mention the special case of TeleMIP which defines an architecture where load-balancing is possible between several Gs. The nodes in Hierarchical Mobile IP, Fast Handoff, Proactive Handoff and TeleMIP are classical Mobile IP. Cellular IP defines stations working with advanced layer two switch capabilities, HAWAII assumes classical IP routers with extended features. HAWAII stations must hence act as IP routers (including maintaining a routing table and actually routing the traffic) in addition to the management of the mobility (and support an IP multicast protocol). Finally, in the case of semi-soft handoff, Cellular IP base stations must support delay device mechanisms. EMA relies on TORA to manage the mobility but aims also at providing a classical prefix-based routing by establishing both subnet and destination specific routes. This seems to be a good compromise with respect to the size of the tables in each station. However, TORA is designed to be an ad-hoc network protocol and provides more than one route to each destination. Each node situated on a route towards a given host must maintain information about this route (its height with respect to this destination). In large networks, the route multiplicity may become a problem because many nodes will maintain redundant informations about MNs or subnets. Moreover, this information will mainly be useless since the largest part of the network is fixed and wired in contrast with ad-hoc wireless network where the availability of more than one route is an extremely valuable feature. On this basis, the tables to be maintained by 14