Requirements for vertical handover of multimedia sessions using SIP

Let us consider a terminal (hereafter named "Mobile Host" or MH), possibly equipped with different network interfaces (i.e. a subset of WiFi, Bluetooth, GPRS, 3G, fixed Ethernet, WiMax). A user will be able to access the Internet using the same MH. This seems to be the trend for the foreseeable future, i.e. we will be surrounded by a heterogeneous network where several access networks (ANs) supplement each other. Each interface of the MH will receive an IP address from the corresponding AN. Therefore the mobile host will have a set of different IP addresses and will have to select which one to use when running multimedia sessions with correspondent terminals. While the mobile host moves, the "selected" interface may become not available due to loss of signal, or could suffer high packet loss or packet delay. Under these circumstances, the MH would like to switch to another interface (using a different IP address) keeping the running sessions active, and might adjust the service quality to be optimal depending on circumstances. Even with a single interface the connected access network can become not available anymore and the terminal could connect to another Access Network (in this case on the same technology), which provides a different IP address. If the switch to the new AN is fast enough, the MH could also be interested in keeping the running session active. This problem has been addressed with different approaches. One approach is based on "network level" mobility solutions like Mobile IP or MobileIPv6. Another approach is based on "application level" mobility solution. The main advantage of application level mobility solutions is that they do not require any support at the network level from the different access networks, which only needs to provide plain IP connectivity. Application level mobility can conduct flexible mobility management, which is another advantage. This document details the issues and the requirements regarding an "application level" mobility solution, in particular considering the solution that exploits the SIP protocol. The aim of SIP based application level mobility is to keep active a multimedia session that has been established with SIP, while the terminal switches from one network interface to another or it changes IP address on the same interface.

This section presents a few terms used throughout the document. Vertical handover: A vertical handover refers to changing a MH's point of attachment between different types of access networks, e.g. GPRS and WiFi. The feature of a vertical handover is that the transmission rate and latency may well differ significantly before and after handover, respectively. Network level mobility: Network level mobility refers to managing an MH's mobility at the network layer. Mobile IP (MIP) and Mobile IPv6 (MIPv6) are well-known as network level mobility. One of the merits of network level mobility is making the the mobility of an MH transparent to an application by keeping the same IP address. This type of mobility management is useful to run existing applications. However, an ubiquitous deployment of MIP/MIPv6 is needed if an MH's mobility is globally supported. In addition, it is difficult for an application to recognize an available bandwidth and delay before or during handoff and to adjust a transmitting rate or buffer space to a target network since network level mobility makes the MH's mobility transparent to the upper layer. Application level mobility: Application level mobility refers to managing an MH's mobility at the application layer. SIP mobility is well-known as application level mobility. Application lavel mobility has its merits in terms of its ease of deployment. This does not require whole networks to be changed. Therefore, the mobility support can be more and more rapidly deployable using application level mobility. Furthermore, SIP has an affinity for applications, which can make handoff more flexible. The feature can fit heterogeneous networks where an available bandwidth and delay may well differ significantly among access networks.

The figure below shows a Mobile Host that wants to communicate with a "Correspondent Host" (CH). The Mobile Host can connect to different Access Networks (AN1, AN2, AN3 are represented in the figure). The different ANs could have different wireless or wired techologies and difference bandwidth/delay, and the Mobile Host could be connected to more than one Access Network at the same time if it has more than one physical network interface. Note that the Access Networks can provide public or private addresses to the mobile host (in most typical scenarios the Access Networks are likely to provide private IP addresses). For example in the figure below AN 1 and AN 3 provide a private address (as shown by the NAT box), while AN2 provides a public address. Similarly, the Correspondent Host can have a public address (like CH 1 in the figure) or a private IP address (like CH 2 in the figure).

The goal of the handover mechanism is to let the MH roam among different Access Networks in a seamless way. The mobility management mechanism should consider the roaming of the MH both "off call" and during an active call. The MH should be able to dynamically choose among the available ANs the one that better suits its needs (e.g. perceived quality of media flows and cost) in a given moment. It is important to notice that this draft does not address the criteria and tools for selection of the "best" access network, it only details the issues and the requirements regarding the mobility management and handover execution mechanims.

In this session we discuss a set of requirements that a mobility management solution based on SIP should have. The requirements are divided into two types, i.e., mandatory requirements and optional requirements. Mandatory Requirements The solution should be as fast as possible. The goal is to provide a "seamless" handover with no perception from the user point of view. If it is not possible to provide a truly seamless solution, the impairment should be minimized. The handover solution should not require a support in the different access network. The access networks are only required to provide IP connectivity (either with public or private addresses) for the forwarding of signalling SIP packets and media RTP packets. Correspondent Host (CH) (which in general are not moving, but they are communicating with a moving terminal) should be an basic User Agent (UA) which only has basic SIP capabilities. If this requirement is not fulfilled there is the need to change all SIP terminals to support the handovers of Mobile Host. The handover solution should be compatible with NATted networks, i.e. it should interoperate gracefully with NAT traversal mechanisms for SIP signaling and for session media flows. Optional Desirable Requirements The solution may support a "forward handover" (i.e. in which all the procedure is performed on the new target Access Network). This is important if the connection on the old Access Network is suddenly broken. If possible (i.e. if the connection with the old access network does not break suddenly) the solution could exploit the communication on the old access network in order to better control the handover procedure. Soft handovers (i.e. where the two active connections can be exploited in the same moment to send the session data) could be exploited. The switch of the "active" interface during a SIP transaction may be supported. As an example the terminal should be able to send (receive) an INVITE on the currenlty active interface, switch to another interface and receive (send) the 200 OK on the other interface. Decoupling of "user level" registration and mobility and "terminal level" mobility may be allowed. As an example a user with AOR "sip:user@domain.com" should be allowed to use different terminals (i.e. Mobile Hosts supporting the handover solutions as well as normal SIP terminals). These terminals can be used in sequence or at the same time depending on the capability of his own "home" registrar/proxy server, and this is independent of the vertical handover solution which takes care of the mobility of only one specific Mobile Host. A concrete example for this requirement is to support a user "sip:user@domain.com" who owns three Mobile Hosts (one could be his phone, one his PDA, one his laptop) and two fixed terminals (his desktop and his home VoIP phone). The vertical handover solution takes care of the mobility of the phone, PDA and laptop as three separate Mobile Hosts (which can also be all active in the same time). Providing privacy with respect to user location and user movements would be preferable. Existing user agents for the Mobile Host may inter-work with the handover procedure without the need to be updated. It would be desirable to reuse existing SIP clients (User Agents) without updating them to support the terminal mobility. It is desirable that the service quality is adjusted to optimal levels according to the access network after every handover. Non-RTP media such as IM or file transfer using MSRP may be supported.

The application level terminal mobility solutions based on SIP can be classified in "Correspondent host based" or "Intermediate Element based". In addition, a session (service) mobility is introduced just for reference. Correspondent Host based terminal mobility solutions RFC 3261 has a built in mechanism for mobility management. The "off-call" mobility management consists in the Registration process. The "on-call" handover is performed using RE-INVITE messages towards the Corresponding Node . No intermediate entities are directly involved in the handover process. This has the advantage that no additional procedure for the handover needs to be implemented in network elements, and that there is no additional load in the networks due to the handovers. On the other hand, the procedure requirese that the Corresponding Node (which in general is not a mobile host) supports the RE-INVITE mechanism. A second drawback is that the handover delay is directly proportional to the end-to-end delay, and this could be higher with respect to the delay occurring between a mobile node and an intermediate entity. Intermediate Element based terminal mobility solutions In order to overcome the drawbacks of the Correspondent Host based solutions, "intermediate" entities that take an active role in the handover can be introduced. Several proposals can be found in the literature, but to our knowledge no internet draft has been proposed in this respect. Hereafter we mention some of the existing proposals. In , intermediate entities are used only to speed up the handover process, but the handover procedure still involves the Corresponding Node as well. A similar approach is followed in , which also deals with location based selection of the "optimal" intermediate entity and of wireless access points. In the intermediate entities fully handle the user mobility, hiding the mobility to the Corresponding Nodes. In , the intermediate entities are used to support MH's mobility as well as adjusting service quality to the MH's target access network. Session (service) mobility solutions Concerning session (service) mobility using SIP, has examined through the use of both Third-Party Call Control (3PCC) as well as SIP's REFER method as possible solutions. In addition, more recently, has expanded upon this work and demonstrated the suitability of employing either 3PCC or SIP's REFER request as suitable mechanisms for session (service) mobility between mobile devices. Although such session (service) mobility can maintain a media session even while changing terminal through the use of 3PCC or SIP's REFER method, there can be a service interruption during the hard swithing (handover). Therefore, it is necessary to look into a fast/seamless handover for SIP-based terminal mobility where the existing media session seamlessly continues on the same device after a handover.

As a concluding remark, we believe that it is important to consider a new solution for vertical handover that meets the set of requirements that has been analysed. This solution will help providing seamless handover to SIP based application with a better performance and overcoming some shortcomings of the current solution based on .

The security considerations should be taken into account in the design of the handover solution, so that no new additional security issues will be introduced.

This memo includes no request to IANA.