From 478f6937c38624cffb971668af0a2e76639feb62 Mon Sep 17 00:00:00 2001
From: David Chen <chenosaurus@gmail.com>
Date: Sun, 30 Aug 2026 17:43:57 -0700
Subject: [PATCH] Fix FlexFEC negotiation with libwebrtc publishers

---
 pkg/rtc/mediaengine.go            | 5 +++--
 pkg/rtc/transport_fec_test.go     | 6 +++---
 pkg/sfu/buffer/buffer_fec_test.go | 2 +-
 test/flexfec_upstream_test.go     | 2 +-
 4 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/pkg/rtc/mediaengine.go b/pkg/rtc/mediaengine.go
index 57016344c..2f5156643 100644
--- a/pkg/rtc/mediaengine.go
+++ b/pkg/rtc/mediaengine.go
@@ -26,8 +26,9 @@ import (
 )
 
 // flexFECRepairWindow is the flexfec-03 "repair-window" fmtp value in
-// microseconds (2 s).
-const flexFECRepairWindow = 2_000_000
+// microseconds (10 s), matching libwebrtc and pion's fixed advertised value.
+// A different value prevents those publishers from negotiating FlexFEC.
+const flexFECRepairWindow = 10_000_000
 
 // flexFECCodecParameters returns the flexfec-03 codec registered/offered when
 // FlexFEC is enabled for a direction. Mirrors pion's ConfigureFlexFEC03 codec
diff --git a/pkg/rtc/transport_fec_test.go b/pkg/rtc/transport_fec_test.go
index fb33b66f0..4c0e9a194 100644
--- a/pkg/rtc/transport_fec_test.go
+++ b/pkg/rtc/transport_fec_test.go
@@ -40,7 +40,7 @@ a=rtpmap:96 VP8/90000
 a=rtpmap:97 rtx/90000
 a=fmtp:97 apt=96
 a=rtpmap:115 flexfec-03/90000
-a=fmtp:115 repair-window=2000000
+a=fmtp:115 repair-window=10000000
 a=ssrc-group:FID 1111 2222
 a=ssrc-group:FEC-FR 1111 3333
 a=ssrc:1111 cname:test
@@ -156,7 +156,7 @@ func TestMediaEngineRegistersFlexFEC(t *testing.T) {
 			require.NoError(t, err)
 
 			flexFECParams := flexFECCodecParameters(115)
-			assert.Equal(t, "repair-window=2000000", flexFECParams.SDPFmtpLine)
+			assert.Equal(t, "repair-window=10000000", flexFECParams.SDPFmtpLine)
 			filtered := filterCodecs(
 				[]webrtc.RTPCodecParameters{flexFECParams},
 				enabledCodecs,
@@ -168,7 +168,7 @@ func TestMediaEngineRegistersFlexFEC(t *testing.T) {
 				require.Len(t, filtered, 1)
 				assert.Equal(t, webrtc.MimeTypeFlexFEC03, filtered[0].MimeType)
 				assert.Contains(t, offer.SDP, "a=rtpmap:115 flexfec-03/90000")
-				assert.Contains(t, offer.SDP, "a=fmtp:115 repair-window=2000000")
+				assert.Contains(t, offer.SDP, "a=fmtp:115 repair-window=10000000")
 			} else {
 				assert.Empty(t, filtered)
 				assert.NotContains(t, offer.SDP, "flexfec-03")
diff --git a/pkg/sfu/buffer/buffer_fec_test.go b/pkg/sfu/buffer/buffer_fec_test.go
index da20c38b3..876e58dbb 100644
--- a/pkg/sfu/buffer/buffer_fec_test.go
+++ b/pkg/sfu/buffer/buffer_fec_test.go
@@ -42,7 +42,7 @@ var flexFECCodec = webrtc.RTPCodecParameters{
 	RTPCodecCapability: webrtc.RTPCodecCapability{
 		MimeType:    webrtc.MimeTypeFlexFEC03,
 		ClockRate:   90000,
-		SDPFmtpLine: "repair-window=2000000",
+		SDPFmtpLine: "repair-window=10000000",
 	},
 	PayloadType: webrtc.PayloadType(fecTestFECPT),
 }
diff --git a/test/flexfec_upstream_test.go b/test/flexfec_upstream_test.go
index a5f8f4b1c..95e39f923 100644
--- a/test/flexfec_upstream_test.go
+++ b/test/flexfec_upstream_test.go
@@ -78,7 +78,7 @@ func TestFlexFECUpstreamNegotiation(t *testing.T) {
 		if !strings.Contains(sd.SDP, "flexfec-03") {
 			return "SFU answer does not contain flexfec-03"
 		}
-		if !strings.Contains(sd.SDP, "repair-window=2000000") {
+		if !strings.Contains(sd.SDP, "repair-window=10000000") {
 			return "SFU answer does not contain the configured FlexFEC repair window"
 		}
 		return ""