rtx for VP8 Codec

[Fatih Odaci]

Hi all,

I'm newbie about WebRTC. In these days I'm trying to understand how to use VP8 codec for video transferring. 

In SDP file, Generally I can see  something like this:

...

a=ssrc:3117563476 label:ARDAMSa0

m=video 9 RTP/SAVPF 100 116 117 96

a=rtcp:9 IN IP4 0.0.0.0

a=ice-ufrag:qyLPnHtshN4pFdnR

a=ice-pwd:4Ia9DTQCuEgBHE29nusLjABc

a=ice-options:google-ice

a=mid:video

a=extmap:2 urn:ietf:params:rtp-hdrext:toffset

a=extmap:3 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time

a=sendrecv

a=rtcp-mux

a=crypto:1 AES_CM_256_HMAC_SHA1_80 inline:Gjxv7JvfTBtzwpSFd6R/Hl4GuyFY9qvosy7D+uTJ

a=rtpmap:100 VP8/90000

a=rtcp-fb:100 ccm fir

a=rtcp-fb:100 nack

a=rtcp-fb:100 nack pli

a=rtcp-fb:100 goog-remb

a=rtpmap:116 red/90000

a=rtpmap:117 ulpfec/90000

a=rtpmap:96 rtx/90000

a=fmtp:96 apt=100

a=ssrc-group:FID 229172185 2616243815

....

All of the video packets are the same rtp-dynamic-payload-type 116 and same SSRC value, but RTX packets are different.

At this point I don't understand what is RTX? and how to WebRTC uses these packets for VP8.

Thanks in Advance,

[jpu...@tokbox.com]

Hi!

The RTX means re-transmission. Those packets are the response to NACK packets from the decoder to the encoder, meaning "send me the packet X, because I did not receive it". 

The RTX format is explained in https://tools.ietf.org/html/rfc4588

Cheers

José Carlos