Subject: [AVT] Possible issues in RTP/RTCP (RFC 3550)

From: "Bhattacharya, Abhijit \(Abhijit\)" <[email-masked]>
Date: Wed, 7 Feb 2007 20:38:44 +0530

Hi all,

I feel the following issues exist in RFC 3550 (RFC for RTP and RTCP).

Problem 1:
If an RTP packet arrives after the jitter tolerance period of that
network node on which the packet is received, the packet becomes useless
because the frame which was supposed to contain this packet has already
been played out to avoid delay. So this packet, which arrived after the
stipulated jitter tolearnce period, is as good as lost for the receiving
node, although it has arrived at its destination. At present this packet
is not considered as a lost packet.

Example:

If an RTP packet of sequence number n arrives at time t1 at a network
node N with ptime of p and jitter tolerance of J, then the expected time
e of arrival of RTP packet of sequence number n-1 at the same node is
(t1 - p). Hence the RTP packet with sequence number n-1 is a delayed
packet if it arrived at t2 and (t2 - e) > J. This is because, if the RTP
packet of sequence number n-1 comes after that, its likely that the next
RTP packet would have been played out thus making the RTP packet with
sequence number n-1 useless.

Effect:
It gives a wrong impression of loss of packets as the quality of the
media (audio/video) played out is definitely reduced due to the absence
of the packet which arrived late. If this happens more frequently, the
quality of the media will be significantly degraded although it will not
be reflected in the report showing loss of packets.

Problem 2: 

Now if a duplicate RTP packet is received, it is not handled separately.
It is considered for all the feedback report generation. Since loss of
packets is equal to expected packets - the received number of packets,
let us consider the following example.

Example: 

Let us consider the expected packet count to be c and received packets
to be r which includes d number of duplicate packets.  Now lost packet
count p is equal to c-r, but actually the count MUST be c-r-d.  Thus the
lost packet count gives a wrong impression.

I need some advice how to go about with this issue. 

Thanks and regards,

Abhijit Bhattacharya

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt