FID

FID stands for Flow Identifier.

In WebRTC SDP, FID is used in a=ssrc-group:FID lines to associate a primary media SSRC with its retransmission (RTX) SSRC. This grouping tells the receiver which RTX stream corresponds to which original media stream, enabling proper handling of retransmitted packets.

How FID works

When a sender transmits video, it uses two SSRCs per stream:

1. A primary SSRC for the original RTP media packets
2. An RTX SSRC for retransmitted packets requested via NACK

The FID group in SDP declares this pairing. For example:

a=ssrc-group:FID 12345 67890

This means SSRC 12345 is the primary video stream and SSRC 67890 is its retransmission stream.

See also: RID, MID, RRID