IP Multimedia Services: Analysis of Mobile IP and SIP Interactions in 3G Networks

IPV6: BASIS FOR THE NEXT GENERATION NETWORKS IP Multimedia Services: Analysis of Mobile IP and SIP Interactions in 3G Networks Stefano M. Faccin, Nokia Research Center Poornima Lalwaney, Nokia Mobile Phones Basavaraj Patil, Nokia Networks ABSTRACT Third-generation cellular s have been designed to provide a variety of IP data services. Both IPv4 and IPv6 are supported in order to provide future-proof solutions. Mobility is supported through both cellular-specific and IP mechanisms. Mobile IP is becoming a key technology for managing mobility wireless s. At the same time, the Session Initiation Protocol is the key to realizing and provisioning services in IP-based cellular s. The need for mobility of future real-time service independent of terminal mobility requires SIP to seamlessly interwork with Mobile IP operations. In this article we investigate the issues related to interworking between SIP and Mobile IP, with a focus on IPv6 and the applicability to 3G s being standardized in 3GPP and 3GPP2. INTRODUCTION In the past few years the evolution of cellular s has reflected the success and growth the Internet has experienced in the last decade, thus leading to s where IP connectivity is provided to mobile nodes. The result is thirdgeneration (3G) s where IP services such as voice over IP (VoIP) and instant messaging (IM) are provided to mobile nodes in addition to connectivity. 3G s have introduced support of IP mobility through Mobile IP (MIP) [1, 2]. In particular, cdma2000 s specified by 3G Partnership Project 2 (3GPP2) [3] deploy MIP to support terminal mobility between points of attachment to the. MIP, in particular MIPv6, can also be supported in 3GPP s [4]. The Session Initiation Protocol (SIP) [5] defined by the Internet Engineering Task Force (IETF) is key to service provisioning in 3G s beyond plain IP connectivity. 3GPP has defined and standardized a infrastructure called the IP multimedia subsystem (IMS) [6, 7] based on SIP for supporting a multitude of services to 3G users. Examples are VoIP, IM, and streaming. 3GPP2 is also adopting the IMS for cdma2000 s, adapting the concepts from 3GPP to align SIP service provisioning in the two s. However, there are substantial differences between 3GPP and 3GPP2 packet core s that have an impact on how SIP services can be provided through the IMS. In particular, 3GPP2 s use MIP to support terminal mobility. The adoption of SIP and MIP in 3G s introduces the need for SIP and the IMS to interwork with MIP. This article analyzes the issues related to the adoption of SIP services in 3G s, in particular the implications of adopting SIP in the presence of MIP and the interactions between the two protocols within the scope of the IMS, focusing on IPv6. The article also analyzes aspects related to the transition from IPv4 to IPv6, interworking between the two protocols, and possible scenarios for 3G s. BACKGROUND This section provides an introduction to the MIP and SIP protocols, identifying the features relevant to their interworking in 3G s. Also, the status of standardization is described. A. MOBILE IP MIP has been designed for supporting host mobility on the Internet. In order for a mobile node (MN) to move across different connection points while maintaining connectivity with other nodes on the Internet (indicated as correspondent node, s), the MN needs to maintain the IEEE Communications Magazine January 2004 0163-6804/04/$20.00 2004 IEEE 113

Current cellular s providing IP services adopt IPv4. However, the size of such s and the number of services available do not compare to the wide deployment of IPv4 in the general Internet. same address. Two versions of MIP have been defined depending on the IP version used in the : MIPv4 [1] for IPv4 s and MIPv6 [2] for IPv6. In MIP, an MN maintains a fixed IP address called the home address (HoA). As the MN roams into a visited, it also obtains a care-of address (CoA) valid on the new link (MIPv6); in MIPv4, it registers with a foreign agent (FA) that becomes the point of contact for the MN. Subsequently, the MN updates its home agent (HA), which is a router on the home that can forward packets meant for the MN s HoA to the MN s current point of attachment (i.e., the CoA of the FA). This allows the MN to remain always on always reachable at its HoA. MIPv6 also supports direct peer-to-peer communication, called route optimization [2], between the MN and its s without having to traverse the HA. In this way, the MN uses the HoA for communication with a and the CoA for routing purposes. Since MIP operates at the layer, any change of CoA is transparent to the transport protocols and applications. Hence, all applications in the MN and can ignore the mobility of the MN and do not have to deal with change of attachment. Whereas MIPv4 has been a standard for some years, MIPv6 is currently becoming a standard and will therefore be ready soon for adoption in 3G s. SESSION INITIATION PROTOCOL SIP [5] is currently the most popular protocol for establishing peer-to-peer IP communication. SIP specifications define an architecture of user agents and servers (proxy server, redirect server, registrar) that support communications between SIP peers through user tracking, call routing, and so on. In SIP, each user is uniquely identified by a SIP universal resource indicator (URI), which is used as the identifier to address the called user when sending session initiation requests. However, an IP address is associated with the user in order to route SIP signaling from a SIP registrar. A SIP user registers with the SIP registrar to indicate its presence in the and its willingness to receive incoming session initiation requests from other users. A typical session in SIP begins with a user sending an message to a peer through SIP proxies. When the recipient accepts the request and the initiator is notified, the actual data flow begins, usually taking a path other than the one taken by the SIP signaling messages. An message typically carries a description of the session parameters. In particular, each media component of the SIP session (e.g., audio and video) is described in terms of quality of service (QoS) parameters. The user can modify the parameters regarding an existing session, say, adding or removing media components or modifying the current QoS (e.g., the codecs or the actual QoS if local policies are applied) using a re- message. SIP also supports personal mobility by allowing a user to reregister with a SIP registrar on changing its point of attachment to the, in particular on changing IP address. A user could also change point of attachment during an active session provided the user reinvites the session providing the new parameters (in particular the new address). IP IN 3G NETWORKS IPv4, the current dominant basis of the Internet, was the first version of IP adopted in cellular s. However, the importance of IPv6 has been recognized, and IPv6 connectivity is also provided in 3G s. Due to the wide availability of IPv4 s, the transition and interworking between IPv4 and IPv6 is an aspect that 3G s need to consider. Thanks to the large address space available, IPv6 allows for the concept of always on s by enabling a large amount of devices to have IP addresses assigned permanently or for long periods of time. The key point for the deployment of IP in cellular s is that end-to-end communications enable services to be deployed without the complexity due to the deployment of middleboxes such as address translators (NAT) [8] and applicationlevel gateways (ALGs) [9] otherwise required to interwork IP versions and public/private address spaces. This particularly applies to services such as VoIP and streaming that are especially of interest to 3G s. Since IPv6 addresses are globally routable, with IPv6 the end-to-end communication model of the Internet is applicable to 3G s. Although IPv4 is the most common version of IP currently used by IP s, 3G s can be considered new players for Internet access and IP-based services provisioning. Current cellular s providing IP services adopt IPv4. However, the size of such s and the number of services available do not compare to the wide deployment of IPv4 in the general Internet. 3G s do not necessarily have the same burden of other IP s of an existing IPv4 infrastructure that needs to be upgraded to IPv6. Hence, the option of adopting IPv6, especially for new services, ensures that these s will not have to worry about the transition between IPv4 and IPv6. MOBILE IP IN 3G NETWORKS 3G s have introduced support of IP mobility through MIP. This section describes how MIP is and can be used in cdma2000 and General Packet Radio Service/Universal Mobile Telecommunications System (GPRS/UMTS). MOBILE IP IN CDMA2000 Code-division multiple access (CDMA) s, in particular cdma2000 s specified by 3GPP2, deploy MIPv4 to support terminal mobility between points of attachment to the (Fig. 1). The MN gains IP connectivity through a radio and a packet data switching node (PDSN). The PDSN is the actual point of attachment of the MN to the IP and acts as an FA to support IP mobility in order to allow the MS to move from one PDSN to another. In such a scenario, the MIPv4 HA is the anchor point for the MS traffic, especially if reverse tunnel- 114 IEEE Communications Magazine January 2004

Mobile IP in 3GPP2 MN Visited MSC/CLR AAA visited Radio access PDSN MN gains IP connectivity at the PDSN through L2 mechanisms User traffic to HoA (MIPv4) User traffic tunneled to MN (MIPv4) Mobile IP in 3GPP Visited 3GPP SS7 IP User traffic to MN (MIPv6) Home 3GPP GGSN AAA home HLR Home agent Home MN gains IP connectivity at the GGSN through L2 mechanisms MIPv6 can be easily adopted in cdma2000 by allowing user authentication and address allocation as it currently happens in Simple IPv6, and then enabling the MN to perform MIPv6 update procedure. MN Radio access SGSN IP Home agent Home mobile IP User traffic without route optimization User traffic with route optimization Figure 1. Support of Mobile IP in 3G s. ing from the FA is utilized. The MN gains connectivity with the FA based on layer 2 mechanisms, after which the MN performs MIPv4 registration with the FA and in turn with the HA. The MN updates the new PDSN address in its registration update. cdma2000 currently supports IPv6 without inter-pdsn mobility (simple IPv6 service) [3]. cdma2000 is expected to adopt MIPv6 when the IETF specifications are completed. However, MIPv6 can easily be adopted in cdma2000 by allowing user authentication and address allocation as currently done in Simple IPv6, and then enabling the MN to perform the MIPv6 update procedure. MOBILE IP IN GPRS/UMTS GPRS and UMTS s specified in 3GPP do not explicitly support MIP. However, IP mobility, in particular MIPv6, can be used to support terminal mobility for multi-access s, s that support both cellular access through GPRS/UMTS and, say, wireless LAN (WLAN) access (Fig. 1). MIPv6 is in fact transparent to the mechanisms GPRS/UMTS define to provide IP connectivity to an MN. When MIPv6 is used in GPRS (Fig. 1), the MN gets local IP connectivity at the GGSN. The address obtained by the GGSN is used as CoA. The MN then sends a MIPv6 binding update (BU) message to the HA and any to enable route optimization. IP MULTIMEDIA SERVICES AND SIP IN 3G NETWORKS SIP is the protocol of choice for the support of IP multimedia services in the IMS [6] of the 3GPP UMTS. The IMS allows peer-to-peer connectivity for services such as VoIP and rich calls (i.e., calls with multiple media including data transfer). 3GPP has defined a specific architecture for the IMS (Fig. 2). The main elements are the proxy call service control function (P-CSCF), the interrogating CSCF (I-CSCF), and the serving CSCF (S-CSCF). The P-CSCF acts as a SIP proxy between the MN and the I-CSCF/S-CSCF. The S-CSCF implements the actual SIP registrar functionality and session control, including service triggering. After gaining IP connectivity through the packet (e.g., GPRS in 3GPP), the MN registers [7] with the S-CSCF (Fig. 3). The registration request is forwarded via the P-CSCF. S- CSCF authenticates the MN and retrieves the user profile. After successful authentication, the MN is registered and ready to set up or receive SIP calls. To set up a SIP call, the MN sends an IEEE Communications Magazine January 2004 115

SIP is the protocol of choice for the support of IP multimedia services in the Internet IMS of 3GPP UMTS. The IMS allows peer-to-peer connectivity for services such as VoIP and rich calls (i.e. calls with multiple media including data transfer). MN Radio access Visited SIP signaling User plane for Mobile IPv4 User plane for IP or Mobile IPv6 with optimized routing Figure 2. SIP infrastructure in the IMS. P-CSCF Packet core to the SIP infrastructure and addressed to the correspondent node. The P-CSCF forwards the request to the S-CSCF based on the setup service route information. The P-CSCF verifies that the request was coming on a valid security association, whereas the S-CSCF trusts the requests coming from the P-CSCF since they are in the same trust domain [10]. The call setup requires a series of round-trips in order to allow the two end nodes to exchange information on the media components of the call (e.g. voice, video) and to set up the corresponding quality of service. It is relevant to note that the MN always exchanges SIP signaling with the P-CSCF [7], and direct communication with the S-CSCF is not allowed. There are two main reasons for this: first, the P-CSCF is the endpoint of the security association with the user and takes care of integrity protection of SIP signaling. Second, the SIP signaling exchanged between the user and the P- CSCF is compressed in order to optimize the usage of wireless resources, and the P-CSCF handles SIP signaling compression (SigComp) [11]. In addition, this allows the visited service provider to monitor and apply policies and local services to the SIP services the MN accesses, since the P- CSCF (if in the visited ) can inspect all SIP signaling exchanged by the MN and SIP infrastructure. The IETF SIP specifications define SIP extensions on top of the basic SIP RFC, therefore 3GPP has specified a profile of SIP extensions to be mandatorily implemented by IMS s, i.e. which options of SIP are used and how. In particular, since in 3GPP mobility is supported by the packet core functions such as GPRS, SIP mobility is not supported in IMS. In fact, in GPRS/UMTS the MS never changes the point of attachment to the, and therefore never changes the IP address. IMS IN 3GPP2 3GPP has completed specifying the architecture and interfaces for IMS Release 5 [6], and 3GPP2 is currently in the process of adopting the same IP I-CSCF Home S-CSCF Home agent IMS architecture. Since the IMS has been designed considering the GPRS/UMTS core packet, the interworking between the IMS and the cdma2000 core packet needs careful consideration. For the simple IP scenario, the IMS interworking with the cdma2000 packet core does not introduce any issues. Interworking with MIP for both cdma2000 and GPRS/UMTS is described in the next section. IP VERSION FOR IMS 3GPP has selected IPv6 as the IP version supported by the IMS in order to benefit from the advantages of IPv6. In contrast, in 3GPP2 the question whether only IPv6 is supported by the IMS has just been settled, and IPv4 is allowed. Several parties support IPv6 only for the same reasons IPv6 was chosen for the IMS in 3GPP. However, wide deployment of IPv4 3GPP2 s can be seen as an obstacle to an IPv6-only IMS solution, since operators are interested in reusing their investment in architecture. Hence, for 3GPP2 s IMS deployment over IPv4 and possibly MIPv4 needs to be studied. Issues related to deployment of IMS over IPv4 are discussed later in this article. SIP INTERWORKING WITH MOBILE IPV4 When connecting to an IPv4 with or without MIPv4, the MN obtains an IPv4 address that may be public or private. NATs [8] are needed to support private addresses and allow the MN to connect to an IPv6 SIP infrastructure. Traditionally, the NAT needs to modify the external IP header to convert between IPv4 and IPv6 headers and addresses. However, with SIP the NAT needs to also modify the packet payload, in particular the SIP header fields and SDP session description. In fact, the MN provides an IPv4 address in the 116 IEEE Communications Magazine January 2004

Visited Home MS Packet core P-CSCF S-CSCF SIP registration MIP binding update 401 unauthorized 401 unauthorized SIP SIP SIP session setup 183 session progress 183 session progress 183 session progress [...] 3GPP has selected IPv6 as the IP version supported by the IMS in order to benefit from the advantages of IPv6. In contrast, in 3GPP2 the question whether only IPv6 is supported by the IMS has just been settled and IPv4 is allowed. Figure 3. SIP registration and session establishment in the IMS. SIP messages (e.g., at registration in call setup), and the address needs to be replaced with an IPv6 address for the IMS to be able process the messages. The port numbers may also need to be mapped. Traditionally, a NAT/NAPT does not modify the packet payload. This requires the use of an ALG [9] able to interpret the SIP signaling to relate the IP packets to specific SIP transactions and sessions so that the correct address translation can take place. Although in theory the ALG can be deployed separate from the IMS elements, in reality this is not feasible. In fact, the IMS [6] specifies the use of a security association between the MN and the P-CSCF in order to protect the integrity of SIP messages. The P- CSCF or MN would reject any modification to the packets made by an ALG, since the integrity verification fails. Hence, in order to support IPv4/IPv6 conversion the ALG needs to be implemented as part of the IMS, and needs the ability to instruct the NAT on how addresses should be mapped. Possible solutions have been studied in the literature [12]. If MIPv4 is used, further issues arise. The location of P-CSCF and S-CSCF with respect to the FA and HA has implications on the way SIP and MIPv4 interact. The IMS assumes that the P-CSCF is located in the visited (i.e., in the same of the PDSN/FA). In 3GPP2, however, the P-CSCF can be located in either the visited (i.e., the same of the PDSN/FA) or the home, where the HA is located. When the P-CSCF is located in the visited, with MIPv4 in 3GPP2 packets destined to the MN are routed through the HA and tunneled to the MN, whereas packets from the MN may be sent directly to the destination or tunneled through the HA. This implies that SIP signaling with the P-CSCF follows triangular routing. In fact, packets from the P- CSCF to the MN are first routed back to the home and then tunneled to the visited, while packets from the MN may be routed directly to the P-CSCF. This causes considerable inefficiency. Due to the issues mentioned above, adoption of IMS services over MIPv4 introduces complexity and inefficiency. As described in the following, provisioning of IMS services over IPv6 and MIPv6 allows optimization. SIP INTERWORKING WITH MOBILE IPV6 The adoption of SIP services with MIP introduces a set of issues. In particular, the type of addresses used for SIP services, and architectural implications of the SIP service infrastructure and of MIP need to be considered. These and other relevant issues are discussed below. ADDRESSING CONSIDERATIONS As described earlier, both SIP and MIP support mobility of the MN. However, the two types of mobility are rather different. This section analyzes the differences and describes why MIP mobility is preferred over SIP mobility. Moreover, IP address usage is described. When MIP is used, the MN has two addresses: the HoA and CoA. MIP supports node mobility by allowing applications to be unaware of a change in node address. Therefore, the addresses used by the MN for SIP communica- IEEE Communications Magazine January 2004 117

Security is a fundamental building block for the provisioning of IP services. Security mechanisms have been defined for MIPv6, in particular to allow the authentication of the MN with the Home Agent to send BU messages. tions is the HoA. However, the MN s current point of attachment corresponds to the CoA, so to avoid tunneling of SIP signaling through the HA, the CoA should be used to exchange SIP signaling. An additional aspect to consider is the IP address used by the MN as source address in IP packets containing the SIP messages sent to the P-CSCF, and the security mechanisms required to ensure SIP signaling security. The IMS has defined a security mechanism to verify that the source IP address of SIP messages from the MN corresponds to the IP address in the SIP messages. Hence, this requires the MN to use the same address (i.e., either the HoA or CoA) for the source address and the address provided at the SIP level. Based on these considerations, several scenarios are possible for MN usage of addresses: The MN provides the CoA at SIP registration and for session establishment, and uses the CoA as source address. Although this satisfies the security requirements for MN addressing and allows direct optimized routing of both SIP signaling and SIP session media, this corresponds to using SIP mobility. In fact, the MN needs to reregister the new CoA with the SIP proxy every time it changes its point of attachment to the. In particular, if the MN is involved in an active SIP session with other nodes, it needs to re-invite the nodes providing the new CoA. In real-time communications (e.g., VoIP) this would cause disruptions in the communications (loss of RTP packets while the re- procedure is completed) since no mechanism is available to avoid any communication break due to SIP mobility. Also, the adoption of SIP mobility is application-specific and does not support mobility for other applications not based on SIP. Therefore, this option is not recommended. The MN provides both the CoA and HoA in SIP signaling. This requires changes to current SIP standards, and defies the reason MIP was introduced: to make applications unaware of the node mobility. The MN provides the HoA at SIP registration and for session establishment, and uses the HoA as source address. In this way, MIP mobility is used, and the MN does not need to reregister or re-invite other nodes when it changes CoA. When the MN is involved in an active SIP session, the MN updates the other nodes with the new CoA through MIP signaling. With MIPv6, this requires the SIP proxy (P-CSCF) to support MIPv6 and route optimization in order to allow the SIP application in the SIP proxy to be unaware of the MN change of address. The requirement is legitimate since, according to IPv6 specifications, every IPv6 node must support MIPv6. Hence, the MN can regard the SIP proxy as a MIPv6 and send BU messages. The scenario can be supported in a nonoptimized way if the P-CSCF is not MIPv6-capable. In this case, SIP signaling is sent by the MN to the P-CSCF through the HA, since route optimization cannot be supported, and reverse tunneling is used instead. In conclusion, adoption of MIP mobility is recommended for MIP and SIP interworking. ARCHITECTURAL CONSIDERATIONS From an architectural point of view, with MIPv6 the location of P-CSCF and S-CSCF with respect to the HA needs to be considered for SIP and MIP interactions. When MIPv6 is adopted in cdma2000, a possible scenario foresees that the P-CSCF is located in the same of the HA. This corresponds to the scenario where the nodes are both located in the home. Interworking between SIP and MIP in this scenario is rather straightforward. In a second case for MIPv6, the P-CSCF is located in the visited, while the HA is located in the home. In such scenario, MIPv6 can be supported transparent to SIP. The MN can, in fact, consider the P-CSCF as a MIPv6 and perform route optimization for SIP signaling. In this way the SIP signaling between the MN and the P-CSCF is routed directly without requiring tunneling through the HA. SIP REGISTRATION AND SESSION SETUP Figure 4 describes a SIP session setup for the IMS in 3G s in the presence of MIPv6. Several options are possible for the MN to send BU messages to the. In the basic scenario (option 1), the MN receives packets from the tunneled though the HA, and initiates the route optimization procedure. This implies that traffic will be routed through the HA before being routed directly to the MN, even if for a limited amount of time. This can have implications on quality of service (QoS), since QoS is initially established only for the route from the MN to the HA and to the, whereas QoS for the optimized route is not established. A second scenario (option 2) introduces an optimization where the MN sends a BU to the immediately after setting up the SIP call, before any traffic is received from the. This requires slight modifications to the implementation of the MN, but benefits from route optimization since the beginning of the communication. However, packets from the can still be received through the HA before the BU procedure is completed. An additional optimization is to send the BU message while the SIP session is still being set up (e.g., after QoS setup has taken place) to ensure that the is willing and able to accept the SIP session. Again, this requires modification of the MN implementation, but ensures that the route between the MN and the is optimized from the beginning of communication. SECURITY CONSIDERATIONS Security is a fundamental building block in the provisioning of IP services. Security mechanisms have been defined for MIPv6 ([2]), in particular to allow authentication of the MN with the HA to send BU messages. In addition, MIPv6 has defined the return routability (RR) test as a mandatory mechanism to be implemented by MIPv6 nodes to support route optimization. With the RR test, when an MN initiates the procedure, the challenges the MN to prove that it owns the HoA and is currently located at the CoA. The challenges are sent to the CoA and 118 IEEE Communications Magazine January 2004

MS Packet core P-CSCF S-CSCF HA IP address acquisition and Mobile IP procedures SIP registration SIP session setup Visited MIP binding update 401 unauthorized SIP 401 unauthorized SIP Home Standardization of IMS support of SIP services in 3GPP2 for cdma2000 is proceeding. The implications of supporting both IPv6 and IPv4 for SIP services need to be studied further. 183 session progress COMET 180 ringing ACK 183 session progress COMET 180 ringing ACK 183 session progress COMET 180 ringing ACK Option 1 Downlink user plane (tunneled by HA) Downlink user plane (reverse tunneling through HA) Option 2 MIP binding update Downlink user plane Uplink user plane Figure 4. IMS SIP procedures with Mobile IPv6. HoA separately, and contain parameters for creating a session key. The MN sends a combined response in a BU and secures it using the session key. This allows the to create a binding cache associating the HoA andcoa. The RR test needs to be re-executed at a certain frequency (a few minutes) in order to ensure that no session hijacking takes place and the CoA registered for the MN is still valid, since the MN can change CoA rather often. In previous sections we describe how SIP is used with MIPv6, and conclude that the MN needs to see the P-CSCF as a and perform BU procedures. According to MIPv6 specifications, this implies that the RR test is executed between the MN and the P-CSCF at regular intervals. This, however, introduces signaling overhead, since the SIP registration may last for a long period of time with the MN never changing CoA (e.g., idle MN). In order to avoid the overhead introduced by the RR test, this article proposes to reuse the security associations set up at SIP registration between the MN and the P- CSCF. The security association is currently used to integrity protect the SIP signaling, but it can be used also to authenticate the MIPv6 BU mes- IEEE Communications Magazine January 2004 119

MIP and SIP are two key protocols for 3G cellular s. Interworking between SIP, IPv4/IPv6, and MIPv4/IPv6 has not been defined completely. sages between the MN and the P-CSCF. Hence, security for MIPv6 support in the IMS can be achieved without signaling overhead. FUTURE WORK Standardization of IMS support of SIP services in 3GPP2 for cdma2000 is proceeding. The implications of supporting both IPv6 and IPv4 for SIP services need to be studied further. Further studies need to be performed on QoS and security issues. CONCLUSION MIP and SIP are two key protocols for 3G cellular s. Interworking between SIP, IPv4/IPv6, and MIPv4/IPv6 has not been defined completely. This article introduces possible scenarios and related issues in the deployment of SIP over MIP. Solutions have been described for use of MN addresses and a mobility procedure when SIP is used. Security considerations have been analyzed and optimizations proposed. ACKNOWLEDGMENT The authors thank Krisztian Kiss for valuable input on the article. REFERENCES [1] C. Perkins, IP Mobility Support for IPv4, IETF RFC 3344. [2] D. Johnson, C. Perkins, and J. Arkko, Mobility Support in IPv6, draft-ietfmobileip-ipv6-21.txt, IETF Mobile IP WGr, Feb. 26, 2003. [3] 3GPP2 X.P0011.C, Wireless IP Network Standard, Sept. 2003. [4] TS 23.060, General Packet Radio Service (GPRS) Service description; Stage 2, 5.6.0, 3GPP Rel. 5. [5] J. Rosenberg et al., SIP: Session Initiation Protocol, RFC 3261, June 2002. [6] TS 22.228, IP multimedia subsystem; Stage 1, 5.6.0, 3GPP Rel. 5. [7] TS 23.228, IP multimedia subsystem; Stage 2, 5.9.0, 3GPP Rel. 5. [8] P. Srisuresh and K. Egevang, Traditional IP Network Address Translator, RFC 3022, Jan. 2001. [9] J. Rosenberg, Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for the Session Initiation Protocol (SIP), draft-rosenberg-sipping-ice-01, IETF SIPPING WG, June 30, 2003. [10] J. Peterson, A Privacy Mechanism for the Session Initiation Protocol (SIP), Nov. 2002, RFC 3323. [11] Z. Liu, R. Price, Applying Signaling Compression (Sig- Comp) to the Session Initiation Protocol (SIP), draftietf-rohc-sigcomp-sip-00.txt, IETF Robust Header Compression (ROHC) WG, June 17, 2003. [12] K. Kiss, G. Baikó, and B. Bertényi, Multimedia Sessions Between 3G Wireless and Internet Users, ICC 2001, June 2001. BIOGRAPHIES STEFANO M. FACCIN (stefano.faccin@nokia.com) received his degree in computer science from Universita di Udine, Italy in 1993, and his Master s degree in computer science and telecommunications from the Politecnico di Torino, Italy, in 1995. He is assistant research manager with Nokia Research Center, Irving, Texas, leading a team of researchers working on IP architectures for wireless mobile systems. He is responsible for development, validation, and standardization of new technologies. Prior to this he joined Nokia Research Center in 1998 as a senior research engineer, working on the development of 3G standards for IP mobile s focused on 3GPP and IETF. His interests include wireless data and voice architectures, VoIP, IP mobility, and security. Prior to Nokia, from 1994 to 1997 he worked in CSELT, the Telecom Italia research center, as a research engineer investigating architectures for 3G cellular systems and focusing on security aspects. He has authored and co-authored several papers in international conferences and journals, and co-authored a book, IP in Wireless Networks (Prentice Hall, January 2003). He has two granted patents and 43 patents pending. POORNIMA LALWANEY (poornima.lalwaney@nokia.com) received her Ph.D. degree in electrical and computer engineering from the University of Massachusetts at Amherst in 1995 and a Masters degree in electrical communications engineering from the Indian Institute of Science, Bangalore, in 1989. She is a technology manager at Nokia Mobile Phones, San Diego, California, where she is responsible for the development of IP software technologies in Nokia s CDMA terminals. Her interests are in mobile and fixed wireless Internet architectures, technologies, and applications. Prior to Nokia she was with Motorola s Broadband Communications Sector (formerly General Instrument Corporation) where she worked on data ing architecture issues for cable modems and contributed to the standards on VoIP over cable access s. Her areas of interest include fixed and mobile broadband data ing architectures, mobile ad hoc s, VoIP, performance modeling and simulation, and wavelength-division multiplexing based optical interconnection s. She has published several papers in international conferences and journals, and has two patents granted and four pending patent applications. She has served on technical program committees and chaired sessions at several IEEE ComSoc conferences. BASAVARAJ PATIL (basavaraj.patil@nokia.com) is a native of India and currently works in the United States. He has a Bachelor s degree in electronics and communication from Karnatak University, Dharwar, India (1988), and a Master s in computer science from the University of Texas at Arlington (1993). He has worked in the computer and telecommunication industry since 1989. He worked for ICL India from 1989 to 1991 as a hardware engineer in Bangalore. He worked at the U.S. Department of Energy s Superconduciting Super Collider Laboratory, Waxhachie, Texas. After receiving his Master s in 1993 he worked for Nortel Networks until 2000. At Nortel he worked in the cdma business unit and the Advanced Technology division. Since February 2000 he has worked for Nokia Networks, Irving, Texas. He is involved with various aspects of 3G packet data s as well as IETF standards. He is currently the co-chair of the MIP6 and PANA working groups in the IETF. He is co-author of the book IP in Wireless Networks (Prentice Hall, January 2003). 120 IEEE Communications Magazine January 2004