A SIP-based Method for Intra-Domain Handoffs Dimitra Vali OTE Research Hellenic Telecommunications Organization - OTE S.A. Athens, Greece dvali@oteresearch.gr Sarantis Paskalis, Alexandros Kaloxylos, Lazaros Merakos Department of Informatics and Telecommunications University of Athens Athens, Greece {paskalis, agk, merakos}@di.uoa.gr Abstract- The mobility management issue in IP access networks can be dealt with from various perspectives. A possible approach includes the use of the Session Initiation Protocol (SIP) and its associate entities for mobility signaling. Following this approach, existing SIP functionality in a network can be reused for mobility management purposes, providing efficient utilization of resources. In this paper we propose the Hierarchical Mobile SIP (HMSIP) framework, a SIP-based intra-domain mobility management approach that builds upon the general framework of SIP mobility management. The HMSIP Agent is introduced as a border network entity providing the necessary mobility functionality. Detailed location and mobility management procedures are presented. The scheme is further enhanced with additional functionality for the support of multiple HMSIP Agents across the border of a domain, maintaining a unique contact address throughout a session within the domain. Keywords: SIP, handoff, micro-mobility. I. INTRODUCTION IP networks tend to become the ubiquitous infrastructure for every telecommunication exchange. One of the most important challenges IP networks face is mobility support. Mobile IP (MIP) [1] and Mobile IPv6 (MIPv6) [2] are the results of IETF mobility support efforts. The Mobile IP protocols enhance the network (IP) layer, so that IP hosts can change location and retain their communicating sessions. In addition, various micro-mobility approaches (e.g. Cellular IP [3], HAWAII [4], HMIPv6 [5], IDMP [6]) have also been developed for enhancing the Mobile IP performance in high mobility environments. Micro-mobility schemes enable fast intra-domain handoff by constraining mobility related signaling inside the boundaries of the domain. Mobility in IP networks can be alternatively supported by application layer mobility protocols that rely on higher layer signaling to achieve the sought results. Some of these efforts include the use of the Session Initiation Protocol (SIP) [7]. For consistency reasons, we will refer to those schemes as “Mobile SIP (MSIP)” (in accordance to “Mobile IP (MIP)”). Motivation and description of Mobile SIP functionality can be found in [8], [9] and [10]. According to these proposals, SIP can be used to provide terminal mobility to Internet multimedia applications, with the appropriate SIP extensions to the base SIP specification. Specifically, SIP signaling is used after the handoff for the end-to-end session re-establishment of SIP on- going sessions between the communicating users. The newly acquired IP address of the mobile host is communicated to the corresponding host via SIP methods and subsequent data are sent to the new location of the user. The major argument for using Mobile SIP to achieve terminal mobility in SIP environments is the reuse of existing SIP infrastructure (i.e. SIP proxies, SIP registrars, etc.) for the functionality required by mobile hosts. The deployment of Mobile IP leads to some extent in a duplicated network functionality and stored user data. Both SIP and Mobile IP have their own mechanism for registration/location update and their own database for storing location info. Additionally, the use of Mobile SIP could also compensate for the current lack of wide deployment of Mobile IP. However, Mobile SIP needs to be optimized for fast handoffs in order to handle mobility in IP networks efficiently. Without optimization, the communication re-establishment after a handoff requires end-to-end signaling exchange that results in long update delays and high signaling overhead in the backbone network. This is especially critical in cases where mobility of resource reserving QoS flows is entailed. The mobility related end-to-end signaling exchange results in end- to-end reservation re-establishment, long handoff delay and bandwidth waste through over-reservation. Therefore, micro- mobility enhancements are also necessary in SIP environments (similar to Mobile IP environments) to handle intra-domain handoff efficiently. Mobile IP-based micro-mobility schemes if applied will also lead to duplication of functionality in the deployed networks. Hence, enhancing the Mobile SIP approach to cater for micro-mobility seems to be the most efficient solution for enabling fast handoff in SIP networks. An RTP [11] translator optimization for Mobile SIP is proposed in [9] to accomplish this task, which caters, however, only for RTP flows and induces several modifications to the standard RTP discard behavior of the mobile terminal. In this paper, we introduce a SIP-based micro-mobility scheme, named Hierarchical Mobile SIP (HMSIP) that seeks to efficiently handle micro-mobility in SIP environments. Our approach assumes the existence of one or more SIP-based domain border entities (named HMSIP Agents), which are responsible for handling the intra-domain mobility locally. HMSIP Agent functionality may be centralized or distributed across numerous border entities. Our work specifically focuses on the specification of efficient mobility procedures that enable seamless mobile host handoff between HMSIP Agents belonging to the same domain. The procedures described are also sufficient for the treatment of the centralized HMSIP Agent case. In our proposed scheme, SIP signaling is the preferred means of micro-mobility signaling. If the corresponding host does not support SIP, the sessions will not be disrupted, when the mobile host moves within its roaming domain. If the corresponding host supports SIP, it will also be