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Hi! First time posting in this forum, so please let me know if my issue contains all relevant info.

I have a use case where I want to send two (or more) video streams via webrtcbin to a web client. The video streams are produced by an appsrc-element. It is important that the different sources can be disambiguated at the client. I have not found a good way to match a particular instance of the appsrc with the received MediaTracks at the client.

Here is how I set up the pipeline:

    const VP8_PIPELINE: &str = "
        webrtcbin name=webrtcbin bundle-policy=max-bundle

        appsrc name=appsrc0 is-live=true min-latency=0 format=time block=true ! queue !
        videoconvert !
        vp8enc end-usage=vbr target-bitrate=2000000 deadline=1 threads=4 token-partitions=4 static-threshold=100 !
        rtpvp8pay pt=96 !
        webrtcbin.sink_0

        appsrc name=appsrc1 is-live=true min-latency=0 format=time block=true ! queue !
        videoconvert !
        vp8enc end-usage=vbr target-bitrate=2000000 deadline=1 threads=4 token-partitions=4 static-threshold=100 !
        rtpvp8pay pt=96 !
        webrtcbin.sink_1
        ";

Schematically I visualize it as

appsrc0 -> Video Payload -\
                           sink_0          transceiver index 'a', mid='x'
                                webrtcbin <
                           sink_1          transceiver index 'b', mid='y'
appsrc1 -> Video Payload -/

At the web client I get a callback for each MediaTrack. The MediaTrack has an id that seems connected to the transceiver index ('a' or 'b' in the above figure), as an example id webrtctransceiver2. The problem is, how can I figure out which appsrc-instance (appsrc0 or appsrc1) is mapped to which transceiver index? I can't assume that it will always be webrtctransceiver0 and webrtctransceiver1, since refreshing the page (including dropping and recreating the full pipeline on the server) will result in the transceiver indexes increasing (to 2 and 3, then 4 and 5 etc. until the server is restarted). Alternatively I can look at the transceiver instance on the client and get its mid. This is stable even between refreshes (video0 and video1), but still I need to be sure which of them map to which apprsc-instance. Is there a way to specify the mid used by webrtcbin? Can I walk the "pipeline graph" from each transceiver back to its appsrc? Is there a simpler way of gainin control of which client MediaTrack that belongs to which appsrc-instance?