An IPv6-Based Location Management Scheme for Client-Server ...

An IPv6-based Location Management Scheme for Client-Server Computing over Mobile Data Networks Kin Weng Ng and Victor C. M. Leung Department of Electrical and Computer Engineering University of British Columbia Vancouver, BC, V6T 1Z4, Canada kinn, vleung @ece.ubc.ca

Abstract – Mobility management is an essential component in enabling mobile hosts to move seamlessly from one location to another while maintaining the packet routing efficiency between the corresponding hosts. One of the concerns raised with the IETF Mobile IP standard is the excessive signaling generated for highly mobile computers. This paper introduces a scheme to address that issue by manipulating the inherent clientserver interaction which exists in most applications to provide the correspondent host with the current mobile host binding. To evaluate the performance of our scheme, we simulate typical Internet application sessions involving a mobile host and examine the signaling and routing costs. Results show a substantial reduction in the mobility management overhead as well as the total cost of delivering packets to the mobile host.

I. I NTRODUCTION Mobility management imposes a significant burden in terms of the amount of signaling generated, processing required, bandwidth consumed and could contribute to traffic bottlenecks. Recent work [1] have shown that mobility management occupy a significant fraction of the network traffic and processing load in Personal Communication System (PCS). The same trend is expected in mobile computing although no information is available at this time as this field is still in its infancy. As such, it is important to have a scheme that can support mobile hosts (MHs) with minimum signaling especially with the growing number of subscribers and introduction of new bandwidth intensive multimedia applications. To facilitate the bandwidth demand on the wireless medium, cell size becomes smaller in order to enlarge the radio link capacity resulting in more frequent handoffs, further aggravating the problem. Host mobility support in a computing environment is essentially an address translation problem, to be resolved at the network layer without impacting the higher level protocols. Mobile IP (MIP) allows a MH to effectively utilize two IP addresses, one for identification and the other is a location dependent address or care-of-address (COA) used for routing purposes. When a MH moves out of its home network, it acquires a COA reflecting its point-of-attachment to the Internet and register this COA with its home agent (HA). In base MIP [2], packets for a MH are routed via its home network where they This work is supported by a University of British Columbia Graduate Fellowship, and by the Natural Sciences and Engineering Research Council of Canada under Grant 0GP0044286.

are intercepted by the HA and tunneled to the MH’s COA. In the reverse direction, packets sent by the MH are delivered to the correspondent host (CH) using standard IP routing mechanisms, if the CH is a stationary host (SH). This results in a asymmetric form of routing known as “triangle routing”. The disadvantage of this approach is the potential long routes and inefficient use of network resources. To rectify this, the route optimization extensions (MIP-RO) [3] consisting of a series of binding messages used to furnish nodes with the location of MHs were introduced, as shown in Fig. 1. MIP requires its HA be notified with virtually every change in its location, which greatly increases the signaling load. This is compounded by the need to periodically renew bindings before their lifetime expires. Nevertheless, these binding messages are important as the overhead (i.e., additional encapsulation and tunneling) resulting from sub-optimum routing to a MH will further increase the packet processing cost and latency, potentially causing data loss. Despite sending binding updates to CHs in MIP-RO, inefficient routing could still result due to the delay in delivering these messages. After all, the CH is only informed of the MH’s location upon detecting non-optimum routing. The use of IPv6 [4] in MIP (MIPv6) [5] helps to reduce slightly the mobility management overhead as route optimization is now built in as a fundamental part of

2. Upon receiving packets for MH that is away, HA can deduce that the source has no binding cache for MH 9. Update binding cache for MH HA node COA MH

FA1 FA2

1. Send initial packets to MH via HA 4. HA send a binding update message to CH

5. CH create binding cache entry for MH 15. Update binding cache for MH node COA CH MH FA1 FA2

14. Send binding update to CH 16. Route packets to MH via FA2

3. HA encapsulate packets and tunnel to MH

6. Subsequent packets are routed directly to the MH

13. Send binding warning to HA to ask it to inform CH of MH’s current COA

8. Register with HA 11. Creates binding cache for forwarding node COA MH FA2

FA1

FA2 10. Leave binding cache at FA1 for smooth handoff 12. If FA1 continue to receive packets for MH, will tunnel to FA2 7. MH moved

MH

Fig. 1. Operation of Mobile IP with route optimization

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II. P ROPOSED S CHEME Following the premise, EMIPv6-RF eliminates the MIP practice of sending registration, binding renewals, and explicit binding messages to CH during a communication session. In addition, EMIPv6-RF restricts registration to the home network only when there is mobility across network boundaries, essentially localizing mobility management signaling for visiting MHs to the confines of the particular network which they are located. To achieve this, a redirection agent (RA) located at the border router (BR) of each mobile data network is introduced to assist in the location tracking and routing operation. Furthermore, each access level router incorporates a local agent (LA) capable of carrying out router advertisement oper-

ations, maintaining information of MHs currently residing in its cell, provide buffering and packet forwarding service for MHs which have recently moved to other locations. The buffer at the LA also serves as a fault tolerance mechanism, to provide buffering for incoming packets should the MH get disconnected from the network. These agents carry out the bulk of the signaling in EMIPv6-RF in order to limit MH transmission over the wireless medium so as to converse power. Due to the limited signaling carried out within EMIPv6-RF, a paging mechanism is put in place for locating MHs when needed. A. Signaling for Host Mobility The migration detection and acquisition of COA procedures are assumed to operate the same way as that in MIPv6, using the IPv6 neighbor discovery [7] and address autoconfiguration [8] protocols. When a MH enters a foreign network, it informs the HA using the registration procedure illustrated in Fig. 2. The proposed scheme differs from MIP in that this is the only time a MH registers with its home network while it remains powered on and in the same foreign network. Therefore, a HA only knows which foreign network its MHs are located but might not know the precise location. There is also no lifetime associated with the MH binding created at the LA. As such, explicit messaging is required to remove the binding whenever the MH moves to another location. This responsibility fits in well with the smooth handoff operation [3] which is to be adopted in EMIPv6-RF, that allows the LA to maintain a binding for their former mobile visitors to ease handoff especially when a MH crosses cell boundaries while in the midst of a data transfer. The previous LA upon receiving such notification is expected to send an acknowledgment, which contain authentication of the MH’s identity and its service profile to the current LA. This step replaces what would otherwise have been carried out through the registration acknowledgment message from the MH’s HA in MIP. The previous LA obtain these information either directly from the HA if it was the initial location of the MH in that network, or from its predecessor. B. Routing Operation Each time a MH send packets to a CH during a communication session, the RA intercepts and caches the location inforLA in foreign network

MH

RA in foreign network

MH’s HA

beacon registration request to HA dest src dest opt COA HA MH pay load

LA creates a binding cache entry for visiting MH and awaits confirmation before rendering services

time

the protocol rather than being added on as an optional set of extensions. The IPv6 features of interest to mobility support are the destination option header which allows both the MH’s home address and COA to be included in each packet originating from the MH, and the routing header which replaces encapsulation when routing directly to the MH’s COA. In a way this integration allows both routing and location tracking to be performed together. This concept is used in the proposed scheme and forms an integral part in achieving our goals. In order to reduce mobility management signaling, there are two premise in which we based our scheme. First, based on the usage of current Internet applications rarely does the situation arise whereby a mobile user is not the party initiating a session. A MH simply provides the mobile user with a platform to carry out the exchange of commands inherent in a client-server application. The client-server protocol [6] is the dominant mode in today’s computing environment and operates on a simple request-response basis. This makes the use of IPv6 appropriate for propagating MH binding updates without the need for additional explicit binding updates. Furthermore, almost all Internet applications today involve retrieving objects from servers or databases, which in most cases are SHs. A MH-to-MH communication session is rare, unlike in PCS, because peer-to-peer Internet applications are still relatively uncommon. Even in application like Internet telephony, user machines are usually connected via stationary servers. This reinforces the former notion that a MH is seldom at the receiving end of an unsolicited session. As a result the frequency in which a MH has to register with its HA can be significantly reduced. The requirement that a home network has to know the precise location of their MHs at all times can be relaxed thereby diminishing the role of the HA and shifts the emphasis to simply ensuring that communicating parties are aware of each others point-ofattachment to the Internet during a session. The remainder of this paper is organized as follows. Section II describes the operation of the proposed mobility management scheme, hereby referred to as Enhanced Mobile IPv6 with Redirection Forwarding (EMIPv6-RF). Section III cover the models used for simulation, with the results presented in Section IV. Concluding remarks are given in Section V.

registration reply received by MH src dest

rout opt

pay HA MH MH load

LA verifies identity of MH and HA

RA caches the COA address included in the destination option header of the MH’s message registration reply from HA to MH rout opt pay HA COA MH load

src dest

Fig. 2. Timeline showing exchange of messages for registration

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mation of the MH for a predetermined lifetime. This is possible since all IPv6 packets originating from the MH will contain both its COA and home address through use of the destination option header. When the CH replies to a MH, it uses the routing header option for packets destined to the COA. As the packets arrive at the RA, they are checked to determine if they are directed to the location specified in RA’s binding cache. Should the destination address for the MH differ from that indicated by the RA’s binding, the packets are encapsulated and forwarded appropriately. When the packets arrived at the LA, the COA address in the packet’s header is checked against the entries in the LA’s visitor list before they are transmitted to the MH. If there is a forwarding cache instead, the packets are tunneled accordingly. This operation takes advantage of the constant exchange of packets between corresponding hosts in a communication session to update the RA, thereby providing it with a more up-to-date COA for a MH than what the CH might have. The routing operation is shown in Fig. 3. C. Paging Operation It is unlikely that a CH will initiate a session to a MH. However should such a scenario occur, paging may be required to determine the whereabouts of a MH. A sequential paging mechanism is proposed. When the packets reaches the destination as indicated in the packet header, the LA at that router will originate a paging message to its cell and surrounding cells in a stepwise manner based on the separation distance from the LA initiating the paging request until the MH is located. While waiting for the MH to reply to the page, the packets are buffered at the LA. Upon receiving a reply, a forwarding cache is created at the LA, and the RA is informed of the MH’s whereabouts using a binding update message to handle future packets directed at that MH. The packets are then tunneled to the MH by the LA, as shown in Fig. 4.

mobile network architecture with the coverage area partitioned into 81 square shaped cells. When a MH departs from a cell, it is assumed with equal probability that any one of the four neighboring cells is selected as the next location the MH moves to [10]. A MH generally resides in each cell it visits for a specific duration before moving to another. The MH mobility can therefore be modeled in terms of its cell residence time, which is assumed to have a gamma distribution [11]. Values generated using this distribution represents the duration the MH resides in its current location. The performance is measured by the amount of processing produced from mobility management signaling and routing of packets to the MH. These costs are based on the amount of messages transferred on the radio path and the fixed network (i.e., the radio bandwidth occupied, the distance traveled and capacity of leased lines used on the fixed network). The cost metrics used in the simulation are given in Table I, based on the values cited in [12, 13]. Several variations of these values are used in the simulation to ensure that the results obtained are not a direct consequence of these parameters, by randomly increasing or decreasing each metric from their default value. The statistical studies of Internet traffic patterns in [14–16] are used to generate the application traffic characteristics between the corresponding hosts. Data messages generated by hosts are fragmented into maximum transfer unit (MTU) size packets of 512 bytes, which is the historically defined fragmentation size for wide-area TCP traffic. Each packet is then appended with the necessary IPv4 or IPv6 headers. Following [12], an upper bound of 50 bytes is assumed for all control messages including those used for mobility management as they generally contain a small and fixed amount of information. Data traffic tend to be bursty in nature with packet interarrival times following a “packet-train” model [14]. The

III. S IMULATION M ODEL

Internet

The proposed scheme is compared against the three MIP variations and evaluated by simulating applications such as FTP, TELNET and WWW between a MH and a SH. A logical tree structure [9] as shown in Fig. 5 is used to model the

Internet

Subnet Access router with LA dest dest opt

MH

src COA CH MH pay load

LA1 3. setup forwarding cache at LA1, then tunnel packets to MH

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1. packets arrived are buffered while paging if MH is located in this cell

Fig. 4. Paging operation

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Binding cache updated or created for MH id location MH COA

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cells are paged in sequential manner according to distance from the cell pointed to by the MH’s HA

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cell from which registration was last performed

4. notify RA of MH’s COA

RA

Tunnel based on its binding cache for COA if its binding cache entry differs from that in the packet

src dest

rout opt

CH COA MH pay load CH replying to MH

Fig. 3. Basic routing operation of the proposed scheme

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Fig. 5. Simulation network model

packet-train model describes network traffic as a collection of packet streams traveling between pairs of node, delimited by a silence time between streams that can vary between 500 milliseconds to 50 seconds. In addition, successive packets within a train are separated by less than 4 milliseconds, with an average of 20 packets in each train. This model is used to simulate the packet transfer operation, particularly for applications like FTP which involves bulk transfer of data. IV. R ESULTS

AND

R ELATED D ISCUSSION

Fig. 6 shows the total cost comprising of both signaling and routing in a WWW session, with varying MH cell residence time. The WWW session lasts an average of about 33.4 minutes. The total cost represents the load incurred by the network to deliver the same amount of traffic to the MH using the four schemes. For proper comparison, we have ignored the periodic registration and binding update required in MIP as it has a direct impact on the amount of signaling generated depending on the lifetime specified. The result show a clear advantage in EMIPv6-RF when the MH is highly mobile, largely due to the reduction in mobility management signaling overhead. However, Fig. 6 in itself does not reflect the routing efficiency of the various schemes. The higher total cost exhibited by MIPRO compared to MIPv6 can be attributed to the additional processing at the FA for control traffic and binding messages sent to the CH by the HA, assuming both schemes exhibit optimum routing. In addition, the use of encapsulation at the CH in MIPRO when forwarding directly to the MH’s COA also has some bearing on the cost incurred. To determine the effect of paging in EMIPv6-RF, Fig. 7 shows the signaling and routing cost separately for the case when a CH initiates a FTP session to a MH. The session arrival is modeled as a Poisson distribution [14]. Despite possibly having to page for the MH at the start of the session, the average signaling cost for the FTP session lasting an average of 10.6 minutes is still lower for EMIPv6-RF compared to MIP. Looking back at Fig. 6, it is obvious that signaling has a profound impact on the overall cost. Nevertheless, EMIPv6-RF is seen to exhibit higher routing cost. This can be attributed to the additional processing carried out at the RA for packets destined for the MH, in terms of checking its binding cache for that TABLE I N ETWORK COST METRICS

Definition Bandwidth of wireless link Bandwidth of wired network To access MH binding cache Process mobility management signaling Time to acquire a wireless channel Processing a tunneled packet Latency through the Internet Router processing time To generate control message

Value 1 Mbps 1 Gbps 10 ms 3 ms 20 ms 7 ms 300 ms 3 ms 5 ms

MH. Using IPv6 also results in higher transmission costs because of the larger packet header. As expected, the routing cost is highest in base MIP because of triangle routing. However, it generates the least mobility management overhead among the MIP variations. The outcome of the routing cost is reflected in the average packet delay shown in Fig. 8. To prove that the higher average routing cost and packet delay in EMIPv6-RF is not attributed to sub-optimal routing but due to paging and additional processing at RA, Fig. 8 also shows the average packet delay for a MH initiated FTP session. In this case the packet delay for EMIPv6-RF is only slightly higher than for MIP-RO and MIPv6, approximately equivalent to the additional cost of looking up a binding cache. Routing cost for MIP-RO is higher than MIPv6 because of the latency in providing the CH with a binding update thereby resulting in some sub-optimum routed packets. Furthermore, initial packets to the MH in MIP-RO is sent using triangle routing. When the MH is highly mobile, router-to-router forwarding is required in MIP-RO, MIPv6 and EMIPv6-RF which accounts for the higher routing cost. Finally, Fig. 9 shows the effect of the MH running multiple concurrent sessions in terms of the percentage difference in “savings” for routing cost per packet relative to the base MIP scheme. The more simultaneous sessions in operation at a MH, the lower the routing cost per packet for EMIPv6-RF (i.e., bigger discrepancy compared to base MIP which more or less has constant routing cost despite varying cell residence time as it uses triangle routing). This is because the MH’s binding at the RA is updated more frequently by the multiple traffic streams to one or more CHs, thereby enabling more optimum routing for packets destined to the MH in the case of high MH mobility. It can be assumed that the time it takes for a packet from a MH to reach the RA of the mobile data network which the MH is located is smaller than for a packet from the CH to reach the RA. As such, the RA would possess a more up-to-date binding of the MH compared to the CH because of continuous data and explicit acknowledgment packets sent by the MH in response to the previous batch of packets received from the CH, while waiting for the next burst of packets en route to the MH. This gain is obtained only when there is mobility involved. V. C ONCLUSION MIP requires the home network be aware of the location of their MHs at all times as there is no connection setup phase unlike in PCS, thereby resulting in excessive signaling. However, as indicated in Section I this is not needed based on the way things operate in a computing environment. Also by virtue of the request-response interaction in a client-server application, it is unnecessary for a MH to send explicit binding update messages. Eventually, the MH would have to reply to the CH where it can then furnished the CH with a binding cache. Simulation results show a reduction in the total processing costs to deliver packets to a MH for EMIPv6-RF in comparison to MIP, without any significant adverse effects in routing efficiency and is fairly consistent for all applications tested. Nevertheless,

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there is a tradeoff for this improvement in terms of a slight increase in the routing cost due to additional processing required for each packet.

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