An architecture for seamless in various types of applications using a combination of MIP and SIP

Karl Andersson1 and Christer Åhlund2 1Department of Computer Science and Electrical Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden 2Division of Mobile Networking and Computing, Luleå University of Technology, SE-931 87 Skellefteå, Sweden {karl.andersson, christer.ahlund}@ltu.se

Abstract technologies at a certain moment and a certain place. The In the future, mobile users will have the possibility to use research question this paper tries to answer is targeted a variety of wireless access networks simultaneously towards finding an optimal mobility management making the vision “Always best connected” come true. solution for users of heterogeneous networks. The Protocol (IP) will most likely be the least common divisor allowing service providers to deploy The rest of the paper is organized as follows: Chapter 2 services in a unified way regardless of access network describes briefly two of the most widespread mobility type (being wired or wireless). The 3GPP-led management solutions, MIP and SIP. Chapter 3 describes development of the IP Multimedia Subsystem (IMS) is a the suggested architecture. Chapter 4 presents our promising step towards the vision. However, there are a evaluation framework. Chapter 5 contains information number of obstacles along this way forward. Various about related work. Chapter 6 discusses the results and deployment difficulties related to already installed base outlines future work. of user and infrastructure equipment and service providers protecting their old investments are the most 2. Mobility management solutions common obstacles. In addition to those reasons, 2.1 Mobility management at the network layer somewhat conservative regulatory authorities, short- Handling mobility management at the network layer has comings in digital rights management, multiple several advantages since applications do not need to be standardization bodies proposing non-aligned solutions, aware of mobility. If the network layer handles all as well as non-uniformed and non-optimized use of radio mobility management, applications can, in theory, be spectrum do not improve the speed of change. used as if the user was running the application in a fixed environment since the user is reachable through a fixed Our research is performed within the area of seamless IP address. The network layer is extended with a suitable real-time multimedia distribution in heterogeneous mobility management module taking care of the delivery wireless access networks with a user-centric and end-to- of datagrams to the user’s current point of attachment to end system approach. Having proposed various the Internet. This mobility management solution works extensions to Mobile IP (MIP) including multihoming both for connection oriented flows (i.e. TCP connections) and port-based handling of flows, as well as solutions for and connection less flows (i.e. UDP traffic). global connectivity in ad hoc networks we now present a combined architecture using both MIP and Session The most well-known example of mobility management Initiation Protocol (SIP) for mobility management where at the network layer is Mobile IP (MIP) which is defined MIP is used for TCP connections and SIP is used for both for IPv4 [1] and IPv6 [2]. UDP traffic carrying real-time multimedia streams of data. MIP makes use of a mobility agent located in the home network, a home agent (HA), and, in MIP for IPv4, a 1. Introduction mobility agent in the visited network, a foreign agent The Internet Protocol (IP) has been extremely successful (FA). The HA is a specialised router responsible for in delivering a widespread protocol for host-to-host forwarding datagrams aimed for the end-user at the connectivity using the basic principle “keep the network mobile node (MN). The MN is assigned a home address simple”. However, merging the Internet with the (HoA) in the same subnet as the HA. The FA is ubiquitous cellular networks has proven to be a quite responsible for assigning a care of address (CoA) for the tough problem to solve. Mobility management is handled MN and forwarding datagrams for the MN. The MN can very well by the cellular networks at the layers below the also use a co-located address CoA. In that case, the MN network layer, but it has proven to be much harder to acquires an IP address using regular mechanisms like implement efficient mobility management solutions at the DHCP. The datagrams are transported from the network and higher layers. One of the basic challenges to originating host, correspondent node (CN), to the HA and deal with when introducing mobility management on then tunnelled through an IP tunnel using IP in IP higher layers is that network layer addresses not only are encapsulation to the MN. When the MN changes it used to identifying hosts but also to finding routes current point of attachment to the Internet it sends a between hosts on the Internet. binding update (BU) message to the HA indicating its new CoA. Datagrams in the direction from the MN to the Users of heterogeneous networks with multiple access CN are sent directly from the FA to the CN. Route networks included really need a mobility management optimization techniques exist in Mobile IP enabling the solution at higher layers in order to leverage all available CN to send datagrams directly to the FA and CN without One important cornerstone in the architecture is to travelling through the HA. perform measurements continuously on all available network interfaces at each time using MIP in a Unfortunately, MIP has some serious drawbacks delaying multihomed version [6] and a policy model [7]. The global-wide deployment, indeed some of which will be decision of what interfaces to monitor is based on user solved when IPv6 is introduced, but nevertheless making preferences. Figure 1 shows the used policy-based MIP not being optimal for roll-out on today’s Internet. decision model including a policy decision point (PDP) Drawbacks include the introduction of encapsulation and a policy enforcement point (PEP), extended with a overhead when tunnelling datagrams, the necessity of policy repository (PR). deploying mobility agents, and problems with sending datagrams directly from the visited network to the CN with source addresses not being topological correct.

2.2 Mobility management at the application layer The above mentioned problems when introducing mobility management at the network layer has led researchers to seek solutions for mobility management at Figure 1. Policy-based decision model higher layers. Descriptions of mobility management implemented at the transport layer and the introduction of The PR is responsible for delivery of requested policy a separate mobility layer above the transport layer exist in parameters to the PDP. The PR contains information such the literature[3]. However, the idea of handling mobility as user preferences, access network performance and cost management at the application layer using the session of available access networks. The PR can obtain initiation protocol[4] (SIP) as mobility management information through measurements of the environment. protocol seems to be the most popular idea in current The PDP is the control entity that evaluates access research. networks through policy decisions. The policy decisions

are based on the parameters received from the PR. If the SIP is an end-to-end signalling protocol designed for PDP decides that a handover is motivated, the PDP initiating, maintaining, and terminating sessions on the informs the PEP to perform a handover. The PEP Internet, mainly targeted for multimedia applications, but receives policy decisions from the PDP and performs the suitable for any type of session-oriented application. In actual handover. The PEP is said to enforce the policy addition to the client side, SIP user agent (UA), it makes decision[8]. use of three types of servers: SIP proxy servers, SIP redirect servers, and SIP registrars. SIP messages are The access network performance of what interface to use carried both on top of TCP and UDP and are routed from is based on the policy value expressed in formulas (1-4) endpoint to endpoint through available servers. SIP has shown below. A detailed explanation of the formulas can inherited structures from both SMTP and HTTP making be found in [9]. To estimate a network’s capacity, the it easier to develop and deploy light-weight relative network load (RNL) is calculated in the MN. implementations when combined with email and web RNL represents a quality value for each network based client software. SIP has become the state-of-the-art protocol for signalling in both IP telephony and other on delay (or round trip time, RTT) and jitter values. zn is types of multimedia applications. SIP is also the core protocol of 3GPP IP Multimedia Subsystem (IMS)[5], the mean value of the RTT value ( rttn ) for MIP BU making its deployment to real applications even faster. It messages between the MN and its HA. x is the mean should also be mentioned that SIP is designed for n handling both pre-session mobility and mid-session value of times between arrivals of MIP BU messages at mobility. the MN, and Vn is the variance between these messages. h determines the size of the history window for the SIP has, however, some drawbacks due to its placement weighted average calculations. For example, when h = 5 in the layered protocol hierarchy. SIP can not, for the most recent value will contribute 20 per cent to the example, do anything to broken TCP connections due to calculated xn , zn and Vn values. changes of network layer addresses at handovers.

Additionally, if SIP is to be used as a general mobility 1 h − 1 (1) management solution, already existing applications need zn rtt n += zn −1 h h to be rewritten completely in order to be mobility-aware.

1 h − 1 (2) 3. Suggested architecture x δ += x n n n −1 Our solution is based on the fact that mobility hh management at the network layer makes TCP 1 2 h −1 V δ x )( +−= V (3) connections not to break and mobility management n n n n−1 h h solutions at the application layer makes real-time applications like multimedia sessions handled optimally. zRNL += V (4) Hence, we decided to base our architecture on a nnn combination of MIP and SIP where MIP is responsible for handling TCP traffic and measurements of available The variables h,⎯z 0, x 0, and V0 are initialized with the networks and SIP is responsible for handling real-time following values: UDP traffic. h = some positive integer, e.g. h = 5 higher layers are allowed to take advantage of information from lower layers, our architecture follows z = rtt 0 0 the schemes outlined in [10] and [11] enabling the SIP

user agent in the MN to subscribe to changes of its

V0 = 0 current CoA. The SIP Proxy server functionality in the HA subscribes in a similar way to changes in changes of x = defined MIP registration intervals CoA for MNs. 0 It should also be pointed out that when the performance The variable δ is calculated as n δ nnn −1 ntt >−= }0:{ of the selected access network degrades (i.e. the policy where − tt is the time difference between consecu- value for the currently selected access network is growing nn −1 compared to the policy value for other available access tive MIP BU messages received at the MN. networks) a vertical handover is scheduled. This way, the vertical handover is normally executed before the The vertical handover, i.e. handover between different network connection is lost. wireless access technologies, is performed at the network layer where MIP is sending a BU message to the HA (and 4. Evaluation framework possibly to connected correspondent nodes, CNs). Together with our industrial partner, we have a test bed Information about the handover is also propagated, including UMTS, WLAN (802.11b), and WiMAX through cross-layer communication, to the application (802.16-2004) access networks. Figure 3 shows the layer in the mobile node (MN) where a SIP re-INVITE configuration of the test bed. message is sent to SIP peers in any ongoing real-time multimedia session followed by the SIP 200 OK and SIP ACK messages. Meanwhile, the HA responds to the MN with an appropriate binding ack (BA) message and, through an extension of the HA with SIP proxy server functionality, sends a corresponding SIP re-REGISTER SIP Registrar UMTS to the SIP registrar indicating the new CoA of the MN followed by the SIP handshake messages (see figure 2). This requires the HA to store SIP addresses and user credentials for each connected MN.

This way, handover will occur at the network layer enabling connection oriented traffic to reach the destination using MIP. In parallel the SIP peer will be noticed about the handover by using re-INVITE with the SIP Peer new CoA address indicated in the Contact field. Hence, Figure 3: Test bed configuration real-time traffic can be sent directly between peers avoiding suboptimal paths (i.e. routed via the HA). It The UMTS network covers most of the test area, while should be noted that this is beneficial even when using the WiMAX network covers parts of the test area. WLAN route optimization in MIPv6 and that the network layer hotspots are available at certain places. will make the handover before the network connection is lost completely, i.e. when performance of an access Both servers and clients are executing on laptops running technology degrades. the Linux distribution Fedora core 4[12], with kernel

2.6.11 as operating system. Network layer mobility management on MN and HA has been implemented in- house in a Java environment (Java™ 2 Standard Edition v 1.5.0, J2SE). The Linux kernel has been compiled to support the universal TUN/TAP device driver, PPP (point-to-point protocol), IP advanced routing, and IP policy routing. In addition to that, the open source packages openVPN and iproute2 were installed. Finally, iptables are used to mark datagrams for policy routing. The SIP registrar is implemented using the SIP Express Router software[13]. The Java Media Framework, version 2.1.1e, is used for development of a multimedia application sending audio and video over real-time protocol (RTP) in both directions simultaneously (MN- SIP Peer and SIP Peer-MN).

5. Related work The idea of using SIP as an application layer mobility solution is wide spread in the research community Figure 2: Message sequence diagram at handover including [14], and [15]. The idea of using a combination of MIP and SIP is also to be found in various papers [16], Cross-layer communication is currently a hot topic in [17], [18], and [19]. To our knowledge, the integration of computer networking research. With the assumption that the described policy-based decision model using delay and jitter measurements from the network layer with a IP Networks in the MobileCity Testbed, International combined MIP- and SIP-based mobility management Conference on Testbeds and Research Infrastructures for solution for UMTS, WLAN, and WiMAX access the DEvelopment of NeTworks and COMmunities, networks is new. February 2005.

6. Discussion and future work [10] A. E. Yegin, M.M. Tariq, A. Yokote, G. Fu, C. The results from previous work with the policy-based Williams, and A. Takeshita, Mobile IP API, draft-yokote- decision model are promising regarding service mobileip-api-02.txt, June 2003 continuity and minimization of packet losses during vertical handovers. We are now in the phase of adding [11] S. Chakrabarti, E. Nordmark, Extension to Sockets new functionality to the prototype and will in near future API for Mobile IPv6, RFC 4584, July 2006 present results from simulations of the suggested architecture and perform a proof of concept in a live [12] http://fedora.redhat.com prototype in the test bed. [13] http://www.iptel.org We intend to further develop the mobility management architecture and the policy based model for network [14] H. Schulzrinne, and E. Wedlund, Application layer selection. Areas of future work include improved quality mobility using SIP, ACM Mobile Comp. and Commun. of service handling including usage of metrics originating Rev., vol. 4, no. 3, July 2000 from RTP and RTCP data as well as handling of simultaneous handovers at the MN and SIP Peer. We also [15] A. Dutta, S. Madhani, W. Chen, O. Altintas, H. intend to introduce a new point of view on network Schulzrinne, Optimized Fast-Handoff Schemes for management with a focus on service surveillance and Application Layer Mobility Management, The Eighth service quality and to investigate applications and Annual International Conference on Mobile Computing services deployed over open overlay networks. and Networking, MobiCom 2002, September 2002

Acknowledgments [16] K.D. Wong, A. Dutta, J. Burns, R. Jain, K. Young, The work presented in this article is based on results from H. Schulzrinne, A multilayered mobility management the HybriNet@Skellefteå [20] project supported by scheme for auto-configured wireless IP networks, IEEE Skellefteå Kraft. Parts of the prototype have been Wireless Communications, Oct 2003 developed with support from TietoEnator Telecom&Media R&D Services. [17] H. Lee, S. W. Lee, D.-H. Cho, Mobility Management Based on the Integration of Mobile IP and References Session Initiation Protocol in Next Generation Mobile [1] C. E. Perkins (ed.), IP Mobility Support for IPv4, Data Networks, IEEE Vehicular Technology Conference, RFC 3344, August 2002. Oct 2003

[2] Johnson, D., Perkins, C., and J. Arkko, Mobility [18] S. M. Faccin, P. Lalwaney, B. Patil, IP Multimedia Support in IPv6, RFC 3775, June 2004. Services: Analysis of Mobile IP and SIP Interactions in 3G Networks, IEEE Communications Magazine, January [3] W. M. Eddy, At what layer does mobility belong?, 2004 IEEE Communications Magazine, October 2004 [19] R. Good, N. Ventura, A Multilayered Hybrid [4] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Architecture to Support Vertical Handover between Johnston, J. Peterson, R. Sparks, M. Handley, E. IEEE802.11 and UMTS, International Conference On Schooler, SIP, Session Initiation Protocol, RFC 3261, Communications And Mobile Computing, July 2006 June 2002. [20] http://www.hybrinet.org [5] http://www.3gpp.org

[6] C. Åhlund, R. Brännström, A. Zaslavsky, M-MIP: Extended Mobile IP to Maintain Multiple Connections to Overlapping Wireless Access Networks, International Conference on Networking, April 2005.

[7] R.Yavatkar, D. Pendarakis and R. Guerin, A Framework for Policy-based Admission Control, RFC 2753, January 2000.

[8] K. Murray, R. Mathur, and D. Pesch, Intelligent access and mobility management in heterogeneous wireless networks using policy. In ACM 1st International Workshop on Information and Communication technologies, pages 181-186, 2003.

[9] C. Åhlund, R. Brännström, A. Zaslavsky, Running Variance Metric for evaluating performance of Wireless