[Live-devel] live555 question

[Ross Finlayson]

> Given that the data flow is across a one-way link (and there is no return path) the sender will not receive Receiver Reports from the receiver.  Off the top of your head, do you know whether this will cause any problems?

No.  (Of course, you’ll need a TCP back channel for the RTSP connection - but once RTP/RTCP packets start flowing from the sender, it doesn’t matter if the sender doesn’t receive RTCP Reception Report (“RR”) packets back from the receiver(s).)

Once a stream has been RTCP-synchronized, then a receiver can compute the end-to-end latency *only if* its clock is synchronized with that of the sender (e.g., using NTP), and then this is trivial to do: Just look at the presentation time of each received media frame, and compare this to the (receiver’s) current ‘wall clock’ time (by calling “gettimeofday()”).

(The *sender* can compute the end-to-end latency to each receiver even without synchronized clocks, but *only if* it receives RTCP “RR” packets back from the receiver.  This is described in RFC 3550.)

Applications that use our library do not need to look at (or care about) ‘RTP timestamps’.  Our software automatically converts these to/from ‘presentation times’, which is all that you should need to care about.


Ross Finlayson
Live Networks, Inc.
http://www.live555.com/