a fast neighbor discovery and dad scheme for fast handover in mobile.pdf

A Fast Neighbor Discovery and DAD Scheme for Fast Handover in Mobile IPv6 Networks

Byungjoo Park, Sunguk Lee, Haniph Latchman Electrical and Computer Engineering

University of Florida, Gainesville, FL32611 [email protected], [email protected], [email protected]

Abstract

In Mobile IPv6 (MIPv6), the handover process reveals numerous problems manifested by time-consuming network layer-based movement detection, non-optimized time sequencing of handover procedures and latency in configuring a new care of address with duplicate address detection (DAD). The delays associated with DAD and movement detection are unavoidable in MIPv6. To mitigate such effects we specify a more efficient fast Neighbor Discovery and DAD scheme. This approach requires each access point to execute movement detection by unicast transmission of the Stored Router Advertisement message and implementation of a modified Neighbor Cache to handle care of address configuration with fast DAD. Simulation results show that the proposed scheme can yield better performance than the Mobile IPv6, Fast Mobile IPv6 and Fast Hierarchical Mobile IPv6.

1. Introduction

Mobile IPv6 is designed to manage the movement of Mobile Nodes (MNs) between wireless IPv6 networks. The protocol provides seamless connectivity to MNs when they move from one wireless point of attachment to another in a different subnet. Mobile IPv6 notifies the correspondent(s) of a MN about its new location by binding the MN addresses. Nevertheless, the MN cannot receive IP packets on its new point of attachment until the handover finishes.

Concerning Mobility support in IPv6 (MIPv6) [1], an MN can determine its network layer movement by using Router Discovery and Neighbor Unreachability Detection. After a MN makes a new Care of Address (CoA), it must check its uniqueness by DAD.

The delay of network layer-based movement detection, non-optimized time sequencing of handover procedures and latency in configuring a new care of

address are inevitable in Mobile IPv6. These delays will cause packet disruption and increase network load. But Fast handovers for Mobile IPv6 could be reduced for real time applications and throughput sensitive application by fast movement detection scheme with L2 trigger event. Actually, the handover latency could be too long regarding real time multimedia applications. Some studies have been done to reduce handover latency, particularly in movement detection and in new CoA configuration such as Fast handovers for Mobile IPv6 [2], Optimistic Duplicate Address Detection [3], Router Advertisement Cashing in Access Point (AP) for Fast Router Discovery [4], and Advance Duplicate Address Detection [5].

In this paper, we present an Efficient Movement Detection procedure which quickly sends Router Solicitation (RS) and Router Advertisement (RA) message without Random Delay using Stored RA messages with unicast. Doing so quickly determines the uniqueness of a new CoA using the modified Neighbor cache of an access router. In particular, we focus on the delay optimization of movement detection and DAD for fast handover in Mobile IPv6 Networks. In movement detection [2], RS delays the transmission for a random amount of time (Random delay for RS, RD_RS) [6]. This time serves to alleviate congestion when many hosts start up on a link at the same time, which might happen after recovery from a power failure. Also, RA must be delayed by a random amount time (Random delay for RA, RD_RA). This time is required to prevent multiple nodes from transmitting at exactly the same time, and to prevent long-range periodic transmissions from synchronizing with each other. These random delays are the second largest delay after DAD in MIPv6 Network-Layer handovers. In DAD procedure, after generation of a CoA [1], an MN should perform DAD for testing the new CoAs uniqueness within the new link. The proposed scheme enhances processing time of DAD using lookup algorithm.

Proceedings of the International Conference on Networking, International Conference on Systems and International Conference on Mobile Communications and Learning Technologies (ICNICONSMCL06) 0-7695-2552-0/06 $20.00 2006 IEEE

2. Related Work

A. Handover Procedure in Mobile IPv6 We can define the handover procedure like as

movement detection, new CoA configuration, DAD and binding update. To process Movement Detection, an MN detects that it has moved to a new subnet by analyzing the router advertisement periodically sent by the access router (AR). The MN can also request the AR to send a router advertisement by sending a router solicitation. To initiate CoA configuration and DAD, the information contained in the router advertisement will allow the MN to create a new CoA. As specified in IPv6 [6], the MN first needs to verify the uniqueness of its link-local address on the new link. The MN performs DAD on its link-local address. Then, it may use either stateless or stateful address autoconfiguration [7] to form its new CoA.

1) Movement Detection: The primary aim of movement detection is to identify L3 handovers. In MIPv6, movement detection generally uses Neighbor Unreachability Detection to determine when the default router is no longer bi-directionally reachable, in which case the mobile node must discover a new default router on a new link.

However, this detection only occurs when the mobile node has packets to send, and in the absence of frequent Router Advertisements or indications from the link-layer, the mobile node might become unaware of an L3 handover. After a change of Link Layer connection the MN must detect any change at the IP Layer before it can signal the change to the network. In MIPv6 this uses RS and RA to detect changes of IP network prefix. This is part of the standard Router Discovery Protocol [6]. The Router Discovery Protocol of IPv6 Neighbor Discovery contains built in timers. These timers prevent a router from sending immediate responses to RS in order to prevent multiple nodes from transmitting at exactly the same time and to avoid long-range periodic transmissions from synchronizing with each other. These are significant delays since they interfere with the MIPv6 movement detection algorithm thus preventing mobility signaling for up to 1000ms [1] [6]. 2) Duplicate Address Detection (DAD): In MIPv6,

after completing movement detection an MN should generate a new CoA using IPv6 stateless address auto-configuration upon moving to the new link [6] [7].

After generation of the CoA an MN should perform DAD for testing the new CoAs uniqueness within the new link. The duration required to complete DAD is up to 1 second. This delay is inherent to MIPv6.

B. Fast Handover for Mobile IPv6 The Fast Handover Protocol is an extension of

Mobile IPv6 that allows an AR to offer services to an MN in order to anticipate a layer 3 handover. Movement anticipation is based on the layer 2 triggers. An L2 trigger is information based on the link layer protocol, below the IPv6 protocol, in order to begin the L3 handover before the L2 handover ends. An L2 trigger and link layer identification are roles of different entities [2].

1) Anticipate Handover:

Figure 1. The message sequence diagram of Fast handover mechanism for MIPv6

In anticipated handover, the MN or the current AR receives an L2 trigger indication that the MN is about to perform an L2 handover. This trigger must contain information allowing the target AR identification. If the MN receives the L2 trigger it must initiate the handover and request fast handover to its AR. The current AR then sends a valid IPv6 address for the new subnet to both the MN and the target AR for validation. Then the target AR ensures that the address is unique in its subnet and sends the validation result to the current AR. If the address is valid, the current AR forwards the authorization to the MN and target AR. When the MN establishes a connection with the new AR it can immediately use the new care of address as the source address in the outgoing packets and send a binding update to the home agent and correspondent node. To minimize the loss of packets, the previous access router (PAR) forwards all the packets intended to the MN to the new access router (NAR) [2]. These are shown in Figure 1. However, the anticipated handover must be controlled by the network since the


MN cannot send an L3 packet once it has started an L2 handover.

3. Fast Neighbor Discovery and DAD (FNDD)

In this section, we describe our proposed scheme to reduce the latency and network load resulting from movement detection and DAD for the fast handover in Mobile IPv6 networks. Figure 2 shows proposed MIPv6 with FNDD procedure.

A. Movement Detection using Stored RA In movement detection, the MN is aware of

performing handover to another AP because of the problem of channel maintenance or L3 handover. The MN achieves a scan to observe APs through probes.

Figure 2. Fast Neighbor Discovery and DAD method procedure (FNDD)

The result of the scan is a list of APs information. Authentication is completed, and then the MN sends the association request message with its MAC address. The AP grants association by sending the Association Response Message. After association is made the AP sends the stored RA to an MN in Association Response message.

Our proposed Movement detection scheme is based on FastRD [4]. In our Fast Movement Detection procedure, when the MN receives the stored RA message from an AP during an L2 handover, the MN compares the prefix of the RA message with existing prefixes in the cache. If the prefix is different, the MN is able to generate a link-local address using the

stateless address auto-configuration scheme and the prefix option allowed in RA messages. Before finishing a layer 2 handover, the MN sends the RS message by unicast signaling to the new AR using the received stored RAs source address included in the Association Response Message. Unicast signaling method is of great significance because it can reduce the network load and avoid the random delays attributed to RS and RA. We also add a mechanism to RS messages in order to process the new DAD method. An MN can help smooth handover by adding the CoA used by an MN to the RS as an option to provide interoperability with normal nodes. This take places by using a bit in reserved field and notifying the node that follows by the scheme that we offer. We name this bit as the New MN and the two options as Previous MNs CoA and previous ARs global address. This modified RS message is also named New RS (NRS). When the NAR obtains the NRS message from an MN, the AR performs DAD as well as movement detection by using the neighbor cache. In this paper, we presume that the network considered in our proposed scheme is only composed of LAN to instantly allocate the generated link local address. Figure 3 shows the format of the New Modified Router Solicitation Message (NRS).

Figure 3. New Router Solicitation Message (NRS)

B. CoA configuration and DAD After movement detection in Standard MIPv6 an

MN has to perform DAD. This DAD procedure takes up to 1 second to complete [3] [6]. Hence we propose a new DAD scheme. The proposed method is to use the neighbor cache of an AR with sufficient buffer size to allow many nodes to attach simultaneously. Firstly, the MN allocates this generated link-local address in its interface without DAD.

An MN cannot instantly allocate the generated link-local address from the existing method. Since there is little probability of address duplication the address can be allocated in advance and all other RA messages from the AR are ignored to prevent duplication. In our proposed scheme, we assume that the network is composed of Local Area Network (LAN) with unique


MAC address in case of IEEE-802.11 networks. Hence, the probability of address duplication is almost zero. We need to consider how a neighbor cache and the Neighbor Unreachability Detection (NUD) procedure perform DAD. As stated [6], a neighbor cache contains one entry for each neighbor to which the node has recently sent messages. Each entry of a neighbor cache may be generated by the RS, the Neighbor Solicitation (NS) or the Unsolicited Neighbor Advertisement (NA) in the case of router. The entries generated in this way are maintained in the state of stale until traffic is sent to the neighbor. Since the neighbor cache has the list of all hosts of the link, which the AR manages, we can compare the entry of the neighbor cache with an MNs MAC address, which is included in each NRS message transmitted from MN to AR. Figure 4 show RFC 2462 [7] operation of DAD and FNDD operation of DAD.

Figure 4. Duplicate Address Detection Procedures. (a) RFC 2462 and (b) FNDD

DAD must be specially managed. If the addresses of all nodes are generated on the link by stateless auto-configuration, we dont have to consider DAD. However, if the link-local address of a node is changed manually in the middle of use or generated by some exceptional process the ARs can not maintain the entry of the node in the neighbor cache. We propose that ARs should be able to receive solicited multicast NS messages for normal DAD of exceptional nodes. Since the solicited NS message is sent by multicast, the ARs can receive this message by modification of its interface [7]. The lookup in the neighbor cache on NAR is the DAD procedure. The DAD using lookup algorithm [9] [10] consumes an extremely short amount of time, typically a few micro second units, such as Longest Prefix Matching speeds in routing table. In the Patricia Trie search, the address lookup delay could be represented to multiply the number of lookup by the delay for access and comparison operations in RAM. This Patricia Trie has the worst performance in line per minute (LPM). We use this

algorithm in order to show the lookup time on the worst performance. Under the present circumstance, since a memory access requires from 60 to 100 nsec [9] and a comparison requires 10nsec in DRAM [10], we can use the delay value for access and comparison operations in RAM as 70 and 110nsec. In Patricia Trie case, since lookup requires memory access of 48 times in worst case, the number of lookup value is 48. Hence, Lookup delay is 5.28 sec in worst case. Also, the use of neighbor cache for movement detection and DAD gives an additional advantage of obtaining alternative addresses because addresses are managed in the neighbor cache. This acquisition of alternative addresses does not consume much time. This is merely the problem of implementation.

Figure 5. New Router Advertisement Message (NRA)

If the NAR receives the NRS message from an MN, it can check the entry of the Neighbor Cache for movement detection and DAD. If there is no duplication, an NRS message should also be included in the Neighbor Cache. If not, it must find an alternative address. This alternative address can be chosen in the pre-configured table made by the router. This alternative address is inserted as a new entry in the Neighbor Cache. If this procedure is completed, the ARs can unicast the RA message to the MNs Link-Local Address of the destination address. This RA can also be modified like the RS message by adding a 2-bit D-flag to the Reserved flag and including the New MAC address and New link-local address as options in the option field in case of address duplication. We name this new RA message NRA. Figure 5 shows the format of Modified Router Advertisement Message (NRA).

Table 1 defines the D-bits. When an MN receives an NRA the MN has to be operated by D-bits.

D-flag Mean00 Must change MAC address. (Non-802 case) 01 Can allocate a new CoA and a link-local


address. 10 Must change the link-local address assigned

into the alternative Address. 11 Can not use.

Table 1. The D-flag of NRA message We again consider that the MN immediately

allocates a link-local address in the new interface. This MN can not communicate until the completion of DAD using lookup. However, since 802.11 assigns unique MAC addresses, the time required to complete DAD is irrelevant in the case where multiple ARs have the same link-local address. The problem is how the AR notifies the NRA message to this node when the two nodes of the same address exist. This can be settled by multicasting the NRA. The other IPv6 nodes do not care about this NRA message, because they can neglect the option of the message which is not known well.

Therefore, our nodes can discard this NRA message as the nodes confirm the MAC address included in the NRA message option. After all, the NRA message is only sent to MN which sent the NRS message. Despite the required random delay of 500 msec to send RA we can solve this problem by FastRA [8].

4. Simulation Results

To evaluate the performance of the proposed mechanism, we used a simulation network as shown in Figure 6. A simulator is implemented by using NS-2 to evaluate the packet delay and packet loss of Standard MIPv6, Fast MIPv6, Fast-Hierarchical and Proposed Mobile IPv6 with FNDD. In this simulation network we used ten units, including home agent (HA), correspondent node (CN), Router, AR and MN. For the simulation network we used wired links that have bandwidth of 5Mps and link delay of 35ms, 40ms, and 280ms respectively. Routers active like Mobility Anchor Point (MAP).

Figure 6. Simulation Network Topology

To measure packet delay time during handover, we use UDP traffic that constantly send packet from CN to MN without ACK. Mobile Node moves to AR6 with speed 10m/s.

Fig. 7, Fig. 8 and Fig. 9 compare packet delay during handover between ARs with an L2 trigger event occurring at 245m.

Figure 7. Packet Delay when mobile node moves from AR 1 to AR 2

Figure 8. Packet Delay when mobile node moves from AR 2 to AR 3

In Fig. 7 and Fig. 8, our proposed FNDD packet delay shows good performance compared with other protocols during handover between ARs connected MAP1.


Figure 9. Packet Delay when mobile node moves from AR3 to AR4

In Fig. 9, during handover in different MAPs (From AR3 to AR4), the FNDD handover delay increased compared with same MAP (From AR1 to AR2); however, the handover delay shows good performance compared with other protocols.

Figure 10. Latency of Handover Delay in each protocol

The overall latency between the time when an MN received the last packet from its PAR and the time when the MN received the first packet from its NAR is depicted in Figure 10. In MIPv6, the latency of handover is similar for all handover cases between ARs whether the MN switches MAPs or not, since the MN can receive a new packet from NAR by sending BU message to HA. The blocking and releasing packet time of FMIPv6 and FHMIPv6 protocol are almost same. Therefore the FMIPv6 has a similar latency of handover with the FHMIPv6. The proposed MIPv6 with FNDD enhances the latency value for handover between ARs and between MAPs by fast movement detection and DAD. Conclusively, our proposed scheme represents prominent results when compared to others.

5. Conclusions

In this paper we proposed a fast handover mechanism using fast neighbor discovery and DAD for fast moving mobile nodes. The use of a modified Neighbor Cache with look up algorithm has merits, such as a quicker DAD checking speed, which solves the shortcomings of conventional DAD when a router has more than two links. We also can take alternative addresses by managing addresses in the network. Through fast movement detection and DAD, we could supply faster smooth handover with the RS message. By measuring the packet delay under the fast handover scheme, we confirmed that the proposed FNDD mechanism has much lower packet delay time than existing mechanisms, which culminates in less packet delay time.

References

[1] D.Johnson, C. Perkins, J. Arkko, Mobility Support in IPv6, draft-ietf-mobileip-ipv6-24.txt,june 2003

[2] Koodli, R., Fast Handovers for Mobile IPv6, RFC 4068, July 2005.

[3] N. Moore, Optimistic Duplicate Address Detection, draft-ietf-ipv6-optimistic-dad-05.txt, Feb 2005.

[4] JinHyoeck Choi, DongYun Shin. Fast Router Discovery with RA, draft-jinchoi-mobileip-frd-00.txt, June 2002.

[5] Y. Han, Y. Choi, S. Park, Advance Duplicate Address Detection, draft-han-mobileip-adad-00.txt, Feb 2003

[6] Narten, T., Nordmark, E., Neighbor Discovery for IP version 6 (IPv6), RFC 2461, December 1998

[7] S. Thomson, T. Narten, IPv6 Stateless Address Auto-configuration, RFC 2462, Dec. 1998

[8] J. Kempf, M. Khalil, B. Pentland. Ipv6 Fast Router Advertisement, draft-mkhalil-ipv6-fastra-04-txt, Oct. 2003.

[9] V. Srinivasan, G. Varghese, Fast Address Lookups Using Controlled Prefix Expansion, ACM Transactions on Computer System, Vol.17, Feb.1999.

[10] R. Kawabe, S. Ata, M. Murata On Performance Prediction of Address Lookup Algorithms of IP Routers through Simulation and Analysis Techniques, IEEE International Conference on Communications 2002 (ICC 2002), April 2002.


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