[vlc-devel] Re: Proposed changes for SAP module

Marian Durkovic md at bts.sk
Wed Jun 29 11:13:31 CEST 2005

On Wed, Jun 29, 2005 at 12:46:08AM +0200, Derk-Jan Hartman wrote:
> On 28 jun 2005, at 23:08, Ross Finlayson wrote:
> >IMHO, it would be foolish to try to duplicate existing functionality 
> >(SDP parsing, RTP receiving, RTCP) that's already present in the 
> >LIVE.COM libraries.
> Well, not necessarily. Something like basic SDP parsing from SAP 
> announces is used by a lot of ppl, but RTSP isn't used by that many 
> people at all (in the VLC world of MPEG2 broadcasting). Many might 
> prefer to have something that just receives UDP TS SAP announces and 
> play just those and save a couple of KB on their binaries. (think 
> embedded devices).

Yes indeed, the TS was a very good choice from the beginning so people don't
need to look for alternatives. Anyway, RAW UDP as transport is becoming
more and more problematic, since vendors rushing for high-speed interfaces
often introduce some parallelism into their switches/routers which results
in packet reordering becoming a problem even on university LAN - and being
the default behaviour on larger networks. Several reordered packets per
minute is not uncommon, and if PIM ASSERT comes to play, it means you
loose about 5 seconds of video every 3 minutes, which is hardly acceptable.

Support for RTP (which solves all those problems) was introduced into
VideoLAN's server side 3 years ago - and we're using it here by principle
for all network streaming. But to benefit from it, also the client side 
must able to restore the correct packet order using RTP sequence numbers. 
Forwarding RTP to livedotcom library could immediately provide this 
functionality for large part of users - and the special needs of embedded
devices could still be honoured by using #ifdefs around this code to make
sure it only gets compilled when livedotcom is available.

As mentioned in other email, my personal opinion is that VideoLAN would
definitely gain more robustness if TS over RTP is made the default
transport for network streaming - and TS over RAW UDP just an option
in case some special devices (e.g. STBs) don't understand RTP and are
not able to throw away the 12 extra bytes of RTP header. This would
probably only require to change the GUI and Docs, since RTP support for
server side is present also in standard vlc modules for quite a long time.

	With kind regards,


----                                                                  ----
----   Marian Durkovic                       network  manager         ----
----                                                                  ----
----   Slovak Technical University           Tel: +421 2 524 51 301   ----
----   Computer Centre, Nam. Slobody 17      Fax: +421 2 524 94 351   ----
----   812 43 Bratislava, Slovak Republic    E-mail/sip: md at bts.sk    ----
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