[vlc-devel] Re: [Patch] RTP reordering - dropping of old/duplicate packets

[Marian Durkovic]

On Thu, Sep 29, 2005 at 10:12:51PM -0400, braymond at echostorm.net wrote:
> I've worked in the VTC community for a long time and although long delays
> are bad a small smount of buffering can smooth what would be an unplayable
> stream into something usable by handling some jitter and out of order
> packets. Like you say buffering can be bad, I can however see a
> configurable static buffer being useful, or even optionally the same kind
> of dynamic RTP buffer used in most H.323 stacks.
> 
> Generally a 100-200ms buffer is enough to smooth out minor troublespots
> and most people won't even notice anything like that, but it doesn't have
> to be used.

Just to add:  requirements for packet reordering are in most cases less
demanding than requirements to compensate network or OS jitter.

VLC by default implements a 300 msec buffer to compensate network jitter
(see udp-caching). This is much more than satisfactory for reordering
purposes - however it will probably require serious modifications to
be able to insert late packet somewhere into the middle of this buffer.

Thus I've prepared just a simple patch which improves the situation by
trashing duplicates and/or late packets and thus reducing the number
of discontinuities from 3 to only 1. Since it's very simple, I'd appeciate 
if this patch could be included into 0.8.4-test2.


	Thanks & kind regards,

		M.  


> 
> - Brian
> 
> > Be careful about anything reordering RTP -- the whole idea is Real Time --
> > adding buffers is bad juju for people who rely on it to be just that.
> >
> > -----Original Message-----
> > From: vlc-devel-bounce at videolan.org [mailto:vlc-devel-bounce at videolan.org]
> > On Behalf Of Marian Durkovic
> > Sent: Thursday, September 29, 2005 10:10 AM
> > To: vlc-devel at videolan.org
> > Subject: [vlc-devel] [Patch] RTP reordering - dropping of old/duplicate
> > packets
> >
> > Hi all,
> >
> >   I've added a bit of code to the UDP/RTP input, which assures that old or
> > duplicate RTP packets are trashed. When the network performs packet
> > reordering, only one discontinuity happens instead of 3 before - since the
> > old packet is trashed. If multiple copies of the same packet are received,
> > only one is used and all the others are trashed.
> >
> >   This is still not the full solution, since in case of packet reordering
> > this code is not able to restore the correct order and still creates one
> > discontinuity. But to avoid this, some kind of buffer needs to be
> > implemented, which would be able to hold the early packets until the
> > correct
> > one arrives.
> >
> >   Please find the patch in the attachment.
> >
> >
> > 	With kind regards,
> >
> > 		M.
> >
> > --------------------------------------------------------------------------
> > ----                                                                  ----
> > ----   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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