TOPICS IN INTERNET TECHNOLOGY Multicasting Streaming Media to Mobile Users Ashutosh Dutta, Jasmine Chennikara, and Wai Chen, Telcordia Technologies Inc. Onur Altintas, Toyota InfoTechnology Center Henning Schulzrinne, Columbia University ABSTRACT over a wide area network, such as Protocol Inde- pendent Multicast (PIM), Multicast over Open Content distribution in general, and multicas- Shortest Path First (MOSPF), Distance Vector ting in particular, over a wired network to static Multicast Routing Protocol (DVMRP), Core hosts can be realized by placing proxies and Based Tree (CBT), and Border Gateway Multi- gateways at several parts of the network. Howev- cast Protocol (BGMP). There can be several er, if the end hosts are mobile over heteroge- types of multicast models, such as one-to-many, neous wireless access networks, one needs to many-to-many, and many-to-one. Examples of consider many operational issues such as net- one-to-many applications include scheduled work detection, handoff, join and leave latency, audio/video distribution, push media, file distri- and desired level of quality of service, as well as bution, caching, and monitoring of stock prices. caching and load balancing. This article surveys Multimedia conferencing, synchronized a set of protocols and technologies that offer resources, concurrent processing, collaboration, multicast-based services for streaming multime- distance learning, chat groups, distributed inter- dia in a mobile environment. It also brings forth active simulations, multiplayer games, and jam some of the issues related to mobile content dis- sessions fall into the many-to-many category. tribution in the wireless Internet that may be Some of the many-to-one applications include helpful during its deployment by application ser- resource discovery, data collection, auctions, vice providers. polling, and accounting. Currently multicast is not widely deployed since there are many issues INTRODUCTION such as pricing, security, QoS, and maintenance of the router states in the core of the network Lately, streaming real-time multimedia content (for a detailed discussion of these issues peculiar over the Internet is gaining momentum in the to wide area networking see [2]). On the other communications, entertainment, music, automo- hand, local multicasting within a subnet becomes tive, and interactive game industries. Streaming more attractive for mobile users experiencing applications include broadcasting multimedia intradomain handoffs because of its ease of content, multiparty conferences, collaborations, deployment and ability to provide more flexible and multiplayer games. All of these applications services such as localized advertisements, news also find use in a military context, including broadcast, and location specific information. coordination, education, situation awareness, This article is organized as follows. We pro- distributed simulation, and battlefield communi- vide some alternate proposals based on network cation. Real-time streaming content (audio and and application layers that can build a multicast- video) is mostly an Real-Time Transport Proto- ing content distribution network for both non- col (RTP) [1] based application that has strin- mobile and mobile users. We discuss some gent delay and loss requirements. Mobility, on mobility components and highlight various issues the other hand, affects the delay and transient involved in multicasting content distribution in loss for multimedia stream delivery to a great mobile networks. We then conclude the article. extent because of associated repeated handoffs. Thus, it becomes more challenging to maintain session continuity and provide proper quality of MULTICASTING STREAMING service (QoS). ONTENT OVER THE In order to make efficient use of network C bandwidth within the core of the network, IP MOBILE INTERNET multicasting is used in wide area networking. There are several proposed network layer Content distribution from a single source follows schemes that provide native IP multicast routing the one-to-many model. Most broadcasting 2 0163-6804/03/$17.00 © 2003 IEEE IEEE Communications Magazine • October 2003 By virtue of IP Global IAL multicasting, IP Inter- content Global satellite providers/ station Internet packets are links radio/TV in the sky Broadband LEOs IP I/F delivered from a single source to a Uplink Uplink group of receivers that are part Downlink IP I/F with of the same Terrestrial spot beam multicast group. Internet Individual broadcaster Joining and Access network advertisement of Local Local Local server A server B server C multicast groups Local ad Local ad server Local ad server server is handled Local subnet Local subnet Local subnet through IGMP Access network Access network and SAP, respectively I Figure 1. A typical mobile content distribution network. sources such as radio and TV networks follow membership in each group is small, unlike tradi- this kind of model. Multicasting streaming con- tional multicast that supports a limited number tent to end users over the Internet may include of large multicast sessions. In Xcast, the sending both mobile and nonmobile clients over wired node includes the IP addresses of all the mem- and wireless media. Figure 1 illustrates a content bers of the multicast group in the packet header. distribution network, with multiple proxy servers, Intermediate routers use the header information different kinds of sources, and several types of to create unicast packets, encapsulating the mul- core and access networks, that offers flexible ser- ticast packet and forwarding it to the group vices to mobile users. members who are at the next hop. The routers The subsections below discuss some of the then modify the packet header of the original related work applicable to both mobile and non- multicast packet and remove the members who mobile users. have been sent a copy of the multicast packet before relaying it to the next router. Implemen- MULTICASTING TO NONMOBILE USERS tation of this scheme requires modification to By virtue of IP multicasting, IP packets are the sending and receiving nodes as well as to the delivered from a single source to a group of intermediate routers. receivers that are part of the same multicast An IETF developed protocol, Source Specific group. Joining and advertisement of multicast Multicast (SSM), addresses issues such as multi- groups is handled through Internet Group Man- cast address allocation, destination unawareness, agement Protocol (IGMP) and Session interdomain routing, source advertisement, and Announcement Protocol (SAP), respectively, connection state that creates a huge multicast although some alternative application layer tech- forwarding table. In the SSM model, each multi- niques are described in [3]. Multicast packets are cast group is not only defined by a multicast generally routed along a single shared tree or address, but also by a sending or source node IP multiple source-based spanning trees for effi- address. SSM does not require IP multicast cient distribution. To support IP multicast, the address management since it does not need a network must maintain knowledge of its routing unique multicast address for each group. There- tables for the multicast routes as well as for the fore, SSM is ideal for Internet broadcast applica- unicast routes. Traditional multicasting tech- tions, allowing content providers to support niques do not handle large numbers of distinct services without requiring a unique IP multicast multicast groups and do not provide a means to address. However, this approach requires router handle multicast when some routers may not be modifications to handle multicast group identifi- multicast-capable. cation based on both the source IP address and Explicit multicast (Xcast) being developed multicast group address. within the Internet Engineering Task Force (IETF) supports multicast groups when the IEEE Communications Magazine • October 2003 3 There have been some advances in research with Internet Multicast Home agent respect to source supporting mobility for Border router multicast users, specifically through Mobile Access Access router Enterprise DFA router IP. One such domain Multicast method is the stream bi-directional tunneling solution, which puts the multicasting Mobile host burden on the home agent. I Figure 2. Mobility support for multicast in Mobicast. MULTICASTING TO MOBILE USERS updates due to mobility. Figure 2 shows a practi- cal application of how Mobicast can be used to There have been some advances in research on distribute content to mobile users using the supporting mobility for multicast users, specifi- DFA approach. cally through Mobile IP. One such method is the Mobile Multicast (MOM) [5] provides a bidirectional tunneling solution, which puts the mobility scheme for multicast multimedia ses- multicasting burden on the home agent (HA). In sions for wide area networking and adopts a this case, a user wanting to join a certain multi- Mobile IP-based approach. It proposes to reduce cast group joins the group through the user’s the problem in bidirectional tunneling of deliver- HA using IGMP. When the user moves to a for- ing multiple copies of the same multicast packet eign network, the HA is responsible for tunnel- to a foreign network. In this solution, one HA ing multicast packets to the user. However, when will be elected to tunnel multicast packets to a a single HA or multiple HAs have users in the foreign network. Range-based MOM [6] takes same multicast group visiting the same foreign