that are not directly connected to the Mbone forms, IP tunnels are implemented between them and the rest of the Mbone. We will see that SIP was first designed for use in the Mbone to invite users to multicast sessions. IETF sponsors ongoing work on interdomain multicast routing protocols such as Border Gateway Multicast Protocol (BGMP) [draft-ietf-bgmp-spec], anticipating that they will provide a more scalable multicast routing infra- structure that introduces hierarchy. Traffic aggregation between domains will reduce the state information kept by multicast routers. Transport of Real-Time Data: RTP The system requirements of different types of traffic are often very differ- ent. This disparity necessitates the implementation of different protocols to transport data with different requirements. For data types that need reli- able transport across the network - an e-mail, for example - the sender wants the recipient to read exactly what he or she wrote, but is not equally invested in preventing delivery delay. For most e-mail, the difference between 10 seconds and 1 minute is not significant. A delayed correct packet is acceptable in this situation, whereas an on-time incorrect packet would not be. As previously noted, the requirements for real-time traffic look quite dif- ferent. Let's say that two parties embark on a voice conversation through the Internet. The encoded audio will have to be transmitted in IP data- grams to be played back to the recipient. Using common sense, a delayed correct packet in this scenario is worse than unacceptable; it's literally use- less. If an out-of-sequence datagram containing what the other party said five seconds ago arrives, it will be discarded. The Internet Multimedia Conferencing Architecture Jitter and Sequencing of Datagrams Beyond these general real-time requirements are some lesser issues that a protocol designed for real-time traffic must address: jitter and sequencing of datagrams. IP networks introduce some delay to every packet traversing them. The amount of delay d