RFC 2736 (rfc2736) - Page 2 of 10

Guidelines for Writers of RTP Payload Format Specifications

RFC 2736     Guidelines for Writers of RTP Payload Formats December 1999


   order with respect to other ADUs.  Thus the ADU is the minimum unit
   of error recovery.

   The key property of a transport protocol for ADUs is that each ADU
   contains sufficient information to be processed by the receiver
   immediately.  An example is a video stream, wherein the compressed
   video data in an ADU must be capable of being decompressed regardless
   of whether previous ADUs have been received.  Additionally the ADU
   must contain "header" information detailing its position in the video
   image and the frame from which it came.

   Although an ADU need not be a packet, there are many applications for
   which a packet is a natural ADU.  Such ALF applications have the
   great advantage that all packets that are received can be processed
   by the application immediately.

   RTP was designed around an ALF philosophy.  In the context of a
   stream of RTP data, an RTP packet header provides sufficient
   information to be able to identify and decode the packet irrespective
   of whether it was received in order, or whether preceding packets
   have been lost. However, these arguments only hold good if the RTP
   payload formats are also designed using an ALF philosophy.

   Note that this also implies smart, network aware, end-points. An
   application using RTP should be aware of the limitations of the
   underlying network, and should adapt its transmission to match those
   limitations.  Our experience is that a smart end-point implementation
   can achieve significantly better performance on real IP-based
   networks than a naive implementation.

3.  Channel Characteristics

   We identify the following channel characteristics that influence the
   best-effort transport of RTP over UDP/IP in the Internet:

   o  Packets may be lost

   o  Packets may be duplicated

   o  Packets may be reordered in transit

   o  Packets will be fragmented if they exceed the MTU of the
      underlying network

   The loss characteristics of a link may vary widely over short time
   intervals.





Handley & Perkins        Best Current Practice