A novel Intra-domain Mobility Management Solution in Heterogeneous

Networks using SIP

CHEN Tian, ZHOU Wen-an, MA Fei, SONG Jun-de ICN&SSME Center, Beijing University of Posts and Telecommunications, Beijing, China

echoinsilence@gmail.com; zhouwa@bupt.edu.cn; mafee2007@gmail.com; jdsong@bupt.edu.cn

Abstract

Although Session Initiation Protocol(SIP) has been the main choice of terminal mobility signaling solution in application layer, it is obvious that SIP suffers from considerable handover delay that is not suitable for real-time multimedia and frequent local movements(intra-domain handovers). By introducing Session Border Controller(SBC) and hierarchical SIP registration, this paper proposes a novel intra-domain mobility management architecture which can support seamless vertical handover, location transparency and NAT traversal. Based on the architecture, an intra-domain vertical handover procedure is proposed, using Advance Duplicate Address Detection(A-DAD) to reduce the handover delay. Performance analysis has evaluated the handover delay and indicated that the proposed handover procedure can fulfill the delay requirement of real-time applications.

1. Introduction

Over the past few years, many novel wireless access technologies and systems were increasingly emerging up to the surface and composed heterogeneous network environment, such as cellular networks(2.5G/3G) or WiMAX; local area or personal area wireless systems, comprising for example, WLAN(802.11 a/b/g) and Bluetooth. At the same time, many more advanced mobile terminal devices were appearing, equipped with multiple wireless interfaces which can be active in the same time. Thanks to the pervasive penetration of these wireless access networks featuring high data rates and these advanced terminal devices, it become possible that roaming users can access Internet anywhere,

anytime regardless of access network technologies. In fact, there is an increasingly strong demand for access to a number of IP real-time multimedia services like VoIP.

However, the issue is that how to provide seamless roaming and vertical handover to multimode terminals among different access networks to ensure service continuity and high-level QoS which bring the users satisfied service experiences. For real-time multimedia applications, Session Initiation Protocol(SIP) [1,2]is the main choice of the mobility management signaling solution in application layer to support terminal, personal and session mobility. However, normal SIP protocol can’t support seamless mobility because of considerable handover delay.

In this paper, we describe an intra-domain mobility management architecture for seamless vertical handover based on SIP. By introducing Session Border Controller(SBC) and hierarchical SIP registration mechanism, location transparency and NAT traversal can be supported. Based on this architecture, by enhancing the Access Router(AR) with Advance Duplicate Address Detection(A-DAD), a seamless vertical handover solution with low latency is proposed to support real-time multimedia services.

2. Related work

To promote the performances of mobility management solution based on SIP, four points are of importance:

1. Delay: Real-time media has very stringent delay requirements that if the handover doesn’t finish in 50-200 ms[3], the media streaming will be interrupted or even broken down. Hence, the handover delay must be short enough to prevent break an ongoing session.

2. Packet loss: The packet loss should be minimized

2009 Third International Conference on Multimedia and Ubiquitous Engineering

978-0-7695-3658-3/09 $25.00 © 2009 IEEE

DOI 10.1109/MUE.2009.54

262

2009 Third International Conference on Multimedia and Ubiquitous Engineering

978-0-7695-3658-3/09 $25.00 © 2009 IEEE

DOI 10.1109/MUE.2009.54

262

during handover. Too high packet loss can make the whole session interrupted and degrade the user experience.

3. User privacy: the mobility management solution should be able to hide the user’s actual location and movement from certain malicious callee.

4. NAT traversal: As most of the wireless access networks are currently using private IP addresses and connected through NAT devices, the support of the NAT traversal is also an important requirement.

Chahbour et al [3] recognize the delay of IP address configuration as the main contributor to degrade service quality during handover. They combine hierarchical Mobile SIP(HMSIP) with Predictive Address Reservation(PAR) to reduce the handover delay. Address acquisition and SIP registration procedures are executed in parallel before the link layer handover triggering. In the PAR scheme, there are several signaling interactions among Mobile Host(MH), current Access Point(cAP) and SIP Mobility Agent(SIP MA) during the period of the IP address configuration. It will cost more time of the handover especially when the network load is heavy.

Boyesen et al [4] describe a proactive handover solution. In the MH home network, the home registrar is implemented on a back-to-back user agent(B2BUA) that bridges session signaling and media stream between MH and any corresponding hosts(CH). The MH is equipped with different interfaces. One is chosen as main interface and the other are considered as backup interfaces. While a session is set up over the main interface, the first backup interface is keeping active. When a handover is initiated, the MH sends a new INVITE over the backup interface without network detecting time and IP address acquisition time. It’s an interesting solution that can reduce handover delay and hide the movement of MH, but the MH needs two IP address over a longer period of time. And two interfaces are active at the same time, this will consume more power.

Another application-layer solution for mobility management based on SIP is proposed by Salsano et al in [5]. Their main idea is to extend the signaling and media functionality of the SBC to manage mobility. A new entity is introduced called the mobility management server(MMS), within the SBC. The MMS/SBC performs as a regional registrar and a permanent anchor point both signaling and media stream between MHs and CHs. When the MH starts the handover procedure, it sends the handover request( a SIP REGISTER) to the MMS/SBC and simultaneously starts duplicating the RTP packets over both interfaces until it receives the relay back message(a SIP 200 OK). This solution can reduce

packet loss but don’t guarantee the handover delay within 200ms.

In Section 4 and 5, we will see how our proposed solution tries to overcome these limitations.

3. Advance Duplicate Address Detection For terminal mobility in IPv6, once the MT (Mobile

Terminal) moves to the new link, it should generate a new care-of-address using IPv6 stateless address auto-configuration with the information provided by the new Access Router(AR). To confirm this CoA(Care of address) is unique, the MH should run “Duplicate Address Detection” algorithm before it assigning the address to its interface which attaches to the new link. In the current protocol, DAD takes at least 1000 ms to detect there is no duplicate address in the link[6]. Obviously, DAD is a time consuming process, particularly to seamless handover. During DAD time, any active communications are interrupted, and this is especially unsuitable for real-time communications.

Advance Duplicate Address Detection scheme is devised to completely take off DAD time from L3 handover latency[6]. This is achieved by advance configuration of new CoA, which will be used without any concern about address collision after MN moves to new link. To put it precisely, an access router (AR) generates randomly many addresses in advance and tests their uniqueness through the existing DAD procedure (these are performed as background process). After successfully passing the DAD procedure, it puts the addresses into “Passive Proxy Cache”. AR acts as a passive proxy for addresses. It listens to neighbor discovery from other nodes in the network. If the AR hears another node performing DAD on the same address in it pool, it must silently remove that address in its cache and perform another DAD to keep the list of duplicate-free addresses constant.

According to the analysis performance in [6], the A-DAD process only takes less than 10 ms. Compare with the normal DAD scheme, Advance DAD is much more suitable for the latency-aware real-time applications.

4. Intra-domain mobility management architecture

In our solution, the Session Border Controller(SBC) is introduced to establish a SIP-based Intra-domain Mobility Management Architecture. A Session Border Controller(SBC) is a VoIP session-aware device that

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controls call admission to an IP network at the border of that network. An SBC has many functions; for example, it can provide traversal of NAT and Firewalls devices, and privacy for the users of the internal network by hiding the network structure behind it. Our architecture also integrates hierarchical registration mechanism[1] in order to reduce registration and handover delay by shortening the signaling transmission path and reducing SIP signaling overhead.

The proposed architecture is depicted in Fig. 1. An MH is equipped with multiple network interfaces; each of them is assigned and uses a different IP address when connected to different Access Networks(ANs). The MH sets up multimedia session using the SIP protocol and can support IPv6. There are several access networks overlapping in the same domain. All or part of Access Routers in the domain are enhanced by Advance Duplicate Address Detection(A-DAD).

Figure 1. SIP-based Intra-domain Mobility

Management Architecture In our architecture, the SBC is deployed in domain

border and SBCs between different domains can communicate with each other. The SIP registrar is implemented in the SBC. The SBC acts as permanent anchor point both of signaling and media. All of User Agents in the domain see only the SBC as their outbound proxy and forward the normal SIP signaling and media flows to it. Then the SBC relays them to another SBC of the domain which CH is in, and at last reach to the CH. From the media point of view the SBC acts as B2SUA(Back to Back User Agent) while from the signaling point of view it can also act as a SIP proxy with additional functions.

When the MH first enters a domain, it will send REGISTER message to the SBC/SIP registrar. After receiving the message, the SBC/SIP registrar will create the association between the MH and its point of contact. Then acting on the behalf of the MH, the SBC will forward the REGISTER message to the MH’s SIP Home Registrar after modifies the Contact header field

inserting its own address. When the MH moves in the domain, it will re-register its new IP address with the SBC/SIP registrar, but the later one needn’t forward the location update to SIP Home Registrar.

The advantages of the proposed architecture are as following:

Location privacy: The SBC acts as the outbound proxy of the MH, leaves the CH unaware of the MH’s intra-domain movements;

NAT traversal; Reduction of registration delay and signaling

overhead; Seamless vertical handover and service

continuity which will be described in next section.

5. Specification of the proposed solution

As described above, in the proposed intra-domain mobility management solution, the Session Border Controller(SBC) is the main functional entity. It manages MH handovers between different access networks providing service continuity and NAT traversal. The SBC can process both SIP protocol header fields and Session Description Protocol(SDP) bodies in order to force itself as relay for the media packets. Furthermore, to solve the NAT traversal, the User Agent in MH and the SBC need to implement the SIP protocol extension described in [7]. Some mechanism should be also introduced to keep the pinhole in the NAT open. In this way, the MH can be reached by SIP signaling and send/receive media flows ever beyond a NAT.

5.1. Session establishment

The signaling process of session establishment is shown in Fig.2(the SBC of the CH’s domain is omitted in the figure). Comparing with the normal SIP protocol, we insert SBC acting as intermediate node in both of signaling and media flows. Before relaying the INVITE request from the caller and the corresponding 200 OK response from the callee, the SBC modifies the SDP bodies of these messages in order to act as RTP proxy for the media flows in both directions.

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Figure 2. The session establishment

procedure 5.2. Off-call handover

The “off-call” handover procedure is performed after the initial registration procedure while the terminal is not engaged in any call. When the MH detects a new access network, it will establish the L2 connection with the new Access Router(AR) over the corresponding interface and obtain a new IP address. Once the MH notices the old link signal strength is going down, it will send a re-REGISTER request with the new IP address directed to the SBC over the new wireless interface. The SBC processes the request in order to update the previously stored contact information associated with the MH and then sends back the 200 OK message. In another word, the off-call handover, which shown in Fig.3, is the same as the local re-registration procedure.

Figure 3. The off-call handover procedure

5.3. Mid-call handover

The mid-call handover takes place when the MH identifies the need for handoff during an ongoing VoIP session. As described in Section 2, the real-time multimedia services require handover delay between 50 ms-200 ms to avoid service degradation or interruption. SIP can’t offer such delays even by using hierarchical registration mechanism. In this paper, we propose a new intra-domain handover procedure based on A-DAD to support seamless handover, which is depicted in Fig. 4(the SBC of the CH’s domain is omitted in the figure).

First, we assume that the MH is engaged in an ongoing session using a certain access network interface(Interface 1) and its other interfaces listen to the other networks. Whenever the MH moves towards another access network, the corresponding interface (Interface 2) will generate and report a trigger(Link Detect event) to the upper-layer. Then through Interface 2, the MH sends Router Solicitation(RtSol) message to all-router multicast address(FF02::2) including the NCoA-request option which indicates a request for a unique address.

Whenever the new AR receives the RtSol it will reply with the modified Router Advertisement(RtAdv) message which contains the duplicate-free IP address from the AR’s passive proxy cache.

On reception of RtAdv message, the MH configures the new IP address to Interface 2, but it does not send location update to SBC. The MH needs to keep the ongoing session through Interface 1 until there is an L2 event reporting form this interface indicating the decreased QoS. Whenever receiving such L2 handover trigger, the MH will immediately send re-REGISTER message to SBC.

As soon as the SBC receives the REGISTER request, it will transfer the ongoing media session flows to the new interface(Interface 2) of the MH on both direction, corresponding to the session ID contained in the REGISTER message body. It will also update location information associated with the MH and send back the 200 OK message. At last, the MH releases the old link resource on Interface 1.

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Figure 4. The mid-call handover procedure

5.4. Delay analysis

As the off-call handover needn’t consider delay problem, we focus the delay analysis on mid-call handover. Generally, the handover delay thandover consists of three parts: network discovery time tnetwork_discovery; IP address configuration time tconfig_IP_addr; SIP signaling sending and processing time tSIP_register [4]. Corresponding to the proposed handover procedure, the network discovery delay means the time from the lower-layer detecting the new link to generating and reporting this event trigger to upper-layer. A low-layer mechanism is needed to support the event service in order to inform the higher layer that the link condition changes and some action has to be taken. We suggest using IEEE 802.21 Media Independent Handover(MIH) Services as such mechanism. However, the implementation of MIH is out of the scope of this paper and we still focus on reducing the other parts of the handover delay.

If the old link degrades gradually, IP address configuration delay can be completely omitted, because the proposed handover procedure performs as “soft” handover. If the old link is suddenly lost before the MH obtains a new IP address, tconfig_IP_addr can be reduced to 10 ms by using A-DAD[7].

The last part of the handover delay tSIP_register is the time from the new REGISTER message is sent until the new interface receives its first packets. According to the analysis result in [4], the roundtrip time for the SIP message within a local network is around 1 ms and the tSIP_register is less than 40 ms. Thus, the maximum amount of tconfig_IP_addr and tSIP_register is 50 ms, within the limit of 200 ms described earlier and also leave time to network discovery.

6. Conclusion

In this paper, we have presented a solution for intra-domain seamless vertical handover between heterogeneous networks based on SIP. We established a novel intra-domain mobility management architecture with hierarchical SIP registration mechanism. The proposed architecture not supports terminal mobility only, but also provides location transparency and NAT traversal by using the so called Session Border Controllers. Although the hierarchical SIP registration can reduce intra-domain handover delay by reducing the delay concerned with the location update, it is still not fulfill the strict requirement of handover delay for real-time application. Hence, combining the hierarchical SIP registration and Advance Duplicate Address Detection, we proposed a vertical handover procedure based on our architecture to support seamless handover.

For the SBC processes all of signaling and media flows of the users in a domain, it will become a hot spot and a bottleneck of the system capacity. Thus, our future work will study the effects of higher load on the SBC with respect to handover delay. Acknowledgement This paper is supported in part by (a) 863 program of China (Grant No:2007AA01Z204), (b) the National Key Technology R&D Program of China(Grant No: 2006BAH02A01), (c) Sino-Sweden IMT-advanced Project(Grant No: 2008DFA11950). ICN&SSME Center of Beijing University of Posts and Telecommunication is grateful acknowledged for the financial support.

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