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Transport Protocols

So-called real-time delivery of traffic requires little in the way of transport protocol. In particular, real-time traffic that is sent over more than trivial distances is not re-transmittable. In fact, a number of facets of an end-to-end protocol need to be re-designed or refined including:

Separate Flows for each Media Stream

With packet multimedia data there is no need for the different media comprising a session to be carried in the same packets. In fact it simplifies receivers if different media streams are carried in separate flows (i.e., separate transport ports and/or separate multicast groups). This also allows the different media to be given different quality of service. For example, under congestion, a router might preferentially drop video packets over audio packets. In addition, some sites may not wish to receive all the media flows. For example, a site with a slow access link may be able to participate in a session using only audio and a white-board whereas other sites in the same session may also send and receiver video.

Receiver Adaptation

Best-effort traffic is delayed by queues in routers between the sender and the receivers. Even reserved priority traffic may see small transient queues in routers, and so packets comprising a flow will be delayed for different times. Such delay variance is known as jitter.

Real-time applications such as audio and video need to be able to buffer real-time data at the receiver for sufficient time to remove the jitter added by the network and recover the original timing relationships between the media data. In order to know how long to buffer for, each packet must carry a timestamp which gives the time at the sender when the data was captured. Note that for audio and video data timing recovery, it is not necessary to know the absolute time that the data was captured at the sender, only the time relative to the other data packets.


As audio and video flows will receive differing jitter and possibly differing quality of service, audio and video that were grabbed at the same time at the sender may not arrive at the receiver at the same time. At the receiver, each flow will need a play-out buffer to remove network jitter. Inter-flow synchronisation can be performed by adapting these play-out buffers so that samples/frames that originated at the same time are play-out out at the same time. This requires that the times that different flows from the same sender were captured are available at the receivers.

next up previous contents
Next: The Realtime Transport Protocol, Up: Introduction - A Brief Previous: A Brief History of