[FFmpeg-devel] [RFC] rtpdec: Reordering RTP packets

Luca Barbato lu_zero
Mon May 24 05:35:50 CEST 2010

On 05/24/2010 01:46 AM, Michael Niedermayer wrote:
> this might work with playing a video but for lowdelay communication this
> design fails

Let me make a summary:

In rtp you have timestamp and sequence number. rtp mandates that every
packets is within the mtu since lower protocols might not be adequate to
handle fragmentation. That makes you have a frame spanning multiple rtp

Sometimes for some reasons you might get misordered packets, the simple
solution in those case is just discard them, the decoder gets garbage at
best and does the concealment. Another solution applicable if you can
tolerate some delay is keep fetching some more packets and once you get
all you need you feed them to the demuxer so the codec gets sane data
with a bit of delay (say 5-10 packets at most)

> There normally is some buffer, to compensate variable network delay
> and there is one (possible the same) buffer to compensate bitrate
> variation between frames (and all video since mpeg1 has variable sized
> frames, keyframes are much larger) thats the vbv buffer, its mandatory.
> currently these buffers are in the application and thus cannot be used
> by rtpdec.
> if rtpdec would return a packet if it has one when the application
> requests one then these buffers could be inside rtpdec and rtpdec
> could use them for free for reordering packets until the packets are
> needed by the application.

The behavior for the application that is accounting for network glitches
won't change at all. If you don't have misordering you get the frame as
soon you get the last packet containing it; if you have misordering the
frame might be ready a bit later or never.

That said _probably_ the current network protocol implementation might
be improved, right now we have a separate select loop in each _read
function and the application has no way to know which is the network
status (e.g. accessing rtcp statistics).

Right now ffplay works between fairly well and really well, even in
situations in which mplayer or vlc (with either live555 or libnemesi)
have problems.



Luca Barbato

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