prioritization, but they are not widely utilized, and IPv6 purports prioritization but has yet to find widespread deployment. It should be noted that in any network there is no such thing as truly guaranteed delivery. The traditional "best-effort" service of the Internet offers no guarantee of deliv- ery or delivery in the order the packets were sent. Expected rates over the Internet can range from zero in times of congestion to tens of kilobits, although the rates are strictly governed by the network topology. High bandwidth might be possible across a few hops or within a single institution, but as the topological distance increases the available band- width generally decreases. Real-time traffic needs to be able to cope with potentially serious loss. Video Transport over the Internet There are various higher-level protocols for dealing with real-time media and for reserv- ing resources to guarantee certain QoS. The most popular of these are the Real-Time Trans- port Protocol (RTP), currently implemented at the application layer for IPv4 and the Reservation Protocol (RSVP), which are expected to be provided at the transport layer within IPv6. The RTP protocol provides facilities such as sequence numbering and timestamps, which are required by real-time media for synchronization, loss detection, and SO forth. However, RTP itself does not provide any mechanisms for QoS or real-time delivery. The design and implementation of robust media coders warrants a detailed understanding of the characteristics of the network used. If an Internet-type network becomes congested, packets are queued at the routers. IP packets contain a TTL stamp that is effectively a count of the number of routers through which the packet has passed. If the TTL figure is exceeded, the packet will be discarded; this mechanism prevents packets from circulating the network indefinitely. Another implica- tion of queueing is the interarrival time; that is, the source-destination journey time is vari- able. If a packet is delayed, the client will need