[AVTCORE] Possible Additional functionalities for draft-brandenburg-avt-rtcp-for-idms

[Fernando Boronat Seguí <[e-mail]>]

Dear Group members,

As this is our first mail to the list, we introduce ourselves. We are 
Fernando Boronat and Mario Montagud, co-authors of the Internet Draft 
(I-D) "RTCP for inter-destination media synchronization" 
(draft-ietf-avtcore-idms-01.txt).
As the I-D was accepted as a WG document, any changes will have to be 
approved by the AVTCORE group. So, we would like to know if you consider 
appropriate to introduce some additional functionalities to the proposed 
IDMS approach in the I-D.

In many distributed applications it is required that all the receivers 
start the playout of the media stream almost simultaneously (we refer 
this as Coarse Sync) and, after that, all of them play-out the media in 
a synchronous way (we refer this as Fine Sync). Those processes are 
essential for acquiring IDMS. A Global Initial Playout Instant for all 
the geographically distributed receivers (SCs) in a media session could 
prevent from an initial playout time discrepancy (asynchrony) at the 
beginning of the playout of the media stream/s (if a fixed global 
initial buffering delay is set) mainly due to the variable network 
delays from the media server to each one of the SCs.

Thus, we are thinking about including a new section in the I-D about 
possible packets for Coarse Sync for both the initial playout instant 
(at the begining of a media session) and the initial instant at each 
stage in which the session can be divided (we consider that a stage ends 
when no streams are transmitted during a fixed interval (it could be 
defined depending on the applications), and that the next stage starts 
when data is transmitted again).

This technique has been considered in some previous IDMS works (see 
reference [Boronat09] in the I-D), but using proprietary protocols. The 
methods we propose here are based on the use of already defined and 
standard RTCP packets, which would avoid the introduction of proprietary 
closed solutions.

Regarding the Initial Playout Instant, we have primarily thought about 
the use of already proposed RTCP XR packets (RFC 3611):

1. Use of RTCP XR with RRTR and DLRR report blocks (defined RFC 3611) 
for measuring the delay between MSAS and SCs, allowing MSAS to estimate 
the RTT for all the SCs, then calculate a common initial playout instant 
and then communicate it to all SCs by sending them an RTCP IDMS Packet 
(defined in the I-D). The RTCP IDMS packet must include the RTP 
Timestamp of the first MU to be sent by the source and its proper 
playout instant, which will be common for all of them.
MSAS may estimate its round trip time (RTT) to the SCs by sending them 
an RTCP XR with the Receiver Reference Time Report Block and forcing 
(according to RFC 3611) them to reply by sending an RTCP XR with a DLRR 
Report Block. Notice that both XR blocks are defined in RFC 3611 to be 
used in the opposite way we propose.

If this solution is not considered appropriate we could think about 
another solution based on the packets already defined in the I-D, but 
maybe we would need to change the format adding new field/s.

2. Once joined the session, if it is not started yet, all the SCs could 
start sending RTCP XR with IDMS block (until they receive the Initial 
Playout Instant indication, sent using the RTCP IDMS Packet) as it is 
defined in the I-D, but including only the essential timing information 
(e.g. the wallclock time at which the packet is sent by the SC) to let 
the MSAS estimate the one-way network delays (not round-trip as in the 
previous case) for each SC (assuming symmetric delays as in TCP) and 
calculate the Initial Playout Instant. For that purpose, some field of 
the RTCP XR IDMS block should be used to indicate the XR IDMS block is 
carrying out temporal information for Coarse Sync, and not for reporting 
about arrival/playout timing of a specific RTP packet.
 From the Initial Instant they will continue playing the streams and 
sending the RTCP XR with IDMS complete block (as currently defined in 
the I-D). It could be a possible solution.

The process of the calculation of the optimum Initial Playout Instant 
must take into consideration some factors such as the maximum and 
minimum estimated delays, jitter values, expected duration of the 
session, or probable clock deviations, in order to allow enough buffered 
MUs to start the playout and to maintain it continuously, thus avoiding 
underflow/overflow situations (playout discontinuities), during the 
session. This process is out of the scope of the ID.

With the introduction of the Coarse Sync functionality, the IDMS 
proposal can counteract the effect of the delay variability at the 
beginning of the playout of the media stream/s in each one of the stages 
in which a media session can be divided.

We prefer the first method because there is no need to define new IDMS 
control messages (or to include some fields to already defined packets), 
but the use of already defined RTCP reports (RFC 3611) can be adapted 
for that purpose.

Moreover, with the use of the first method, SCs do not have to send 
control messages once they join the session, which may possibly result 
on inefficient control traffic overhead. They will only have to send 
RTCP XR DLRR messages in response to a new incoming RTRR message from 
the source, which is who really knows the initial transmission time for 
the RTP media stream. Moreover, the Initial Playout Instant can be sent 
to the receivers by using the RTCP IDMS packet, already introduced in 
the I-D.

Feedback will be much appreciated.

Regards
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Dr. Fernando Boronat Seguí
IEEE Senior Member
Profesor Titular / Lecturer	
Dept. Comunicaciones           Tlf.-+34+962 849 300 Ext.-49341
Tlf. directo -+34+962 849 341  Fax.-+34+962 849 309 (Compartido)
UNIVERSIDAD POLITÉCNICA DE VALENCIA-CAMPUS DE GANDIA
Calle Pararaninfo, 1, CP. 46730, Grao de Gandia (Valencia), SPAIN
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