SSRC rewrite?

My understanding is that mediasoup “forwards” RTP packets, and doesn’t touch the payload or the headers, but I seem to be getting different SSRC integers on the receiving end than what is being sent out by the producer. Does mediasoup touch the SSRC from the producer’s packets before forwarding them?

The ssrc, the RTP header extensions, the seq number, the timestamp and even the payload (when video simulcast).