From bfd9deffd74b849dccb825a1dbf3adda0423e46f Mon Sep 17 00:00:00 2001
From: Dan Root <dan.n.root@gmail.com>
Date: Mon, 8 Jun 2026 09:07:08 -0500
Subject: [PATCH] expose TCPFallbackRTTThreshold and AllowUDPUnstableFallback
 via config (#4556)

---
 config-sample.yaml         |  6 ++++
 pkg/config/config.go       | 12 ++++++++
 pkg/service/roommanager.go | 58 ++++++++++++++++++++------------------
 3 files changed, 48 insertions(+), 28 deletions(-)

diff --git a/config-sample.yaml b/config-sample.yaml
index 7a1eece37..f264e721e 100644
--- a/config-sample.yaml
+++ b/config-sample.yaml
@@ -114,6 +114,12 @@ rtc:
   #   allow_pause: true
   # # allows automatic connection fallback to TCP and TURN/TLS (if configured) when UDP has been unstable, default true
   # allow_tcp_fallback: true
+  # # signaling RTT (in milliseconds) below which ICE/TCP is attempted on a UDP failure; at or above it,
+  # # supporting clients fall back directly to TURN/TLS. 0 (default) disables the check (ICE/TCP always tried).
+  # # a positive value also enables allow_udp_unstable_fallback.
+  # tcp_fallback_rtt_threshold: 0
+  # # migrate an established-but-lossy UDP connection to ICE/TCP or TURN/TLS. requires tcp_fallback_rtt_threshold > 0, default false
+  # allow_udp_unstable_fallback: false
   # # number of packets to buffer in the SFU for video, defaults to 500
   # packet_buffer_size_video: 500
   # # number of packets to buffer in the SFU for audio, defaults to 200
diff --git a/pkg/config/config.go b/pkg/config/config.go
index 6d65973f9..68d3e8a2c 100644
--- a/pkg/config/config.go
+++ b/pkg/config/config.go
@@ -116,6 +116,18 @@ type RTCConfig struct {
 	// allow TCP and TURN/TLS fallback
 	AllowTCPFallback *bool `yaml:"allow_tcp_fallback,omitempty"`
 
+	// Signaling RTT threshold (in milliseconds) governing ICE/TCP fallback. On a UDP
+	// failure, ICE/TCP is attempted only while the measured signaling RTT is below this
+	// value; at or above it, supporting clients fall back directly to TURN/TLS. When 0
+	// (the default), the RTT check is disabled and ICE/TCP is always attempted (for
+	// clients that support it). A positive value also gates allow_udp_unstable_fallback.
+	TCPFallbackRTTThreshold int `yaml:"tcp_fallback_rtt_threshold,omitempty"`
+
+	// When enabled, an established UDP connection reporting sustained high packet loss is
+	// migrated to ICE/TCP or TURN/TLS. Requires tcp_fallback_rtt_threshold to be set
+	// (> 0). Disabled by default.
+	AllowUDPUnstableFallback bool `yaml:"allow_udp_unstable_fallback,omitempty"`
+
 	// force a reconnect on a publication error
 	ReconnectOnPublicationError *bool `yaml:"reconnect_on_publication_error,omitempty"`
 
diff --git a/pkg/service/roommanager.go b/pkg/service/roommanager.go
index 65eeb5eda..9419e282e 100644
--- a/pkg/service/roommanager.go
+++ b/pkg/service/roommanager.go
@@ -471,34 +471,36 @@ func (r *RoomManager) StartSession(
 	}
 
 	participant, err = rtc.NewParticipant(rtc.ParticipantParams{
-		Identity:                pi.Identity,
-		Name:                    pi.Name,
-		SID:                     sid,
-		Config:                  &rtcConf,
-		Sink:                    responseSink,
-		AudioConfig:             r.config.Audio,
-		VideoConfig:             r.config.Video,
-		LimitConfig:             r.config.Limit,
-		ProtocolVersion:         pv,
-		SessionStartTime:        sessionStartTime,
-		SessionTimer:            observability.NewSessionTimer(sessionStartTime),
-		TelemetryListener:       room.ParticipantTelemetryListener(),
-		Trailer:                 room.Trailer(),
-		PLIThrottleConfig:       r.config.RTC.PLIThrottle,
-		CongestionControlConfig: r.config.RTC.CongestionControl,
-		PublishEnabledCodecs:    enabledCodecs,
-		SubscribeEnabledCodecs:  enabledCodecs,
-		Grants:                  pi.Grants,
-		Reconnect:               pi.Reconnect,
-		Logger:                  pLogger,
-		Reporter:                roomobs.NewNoopParticipantSessionReporter(),
-		ClientConf:              clientConf,
-		ClientInfo:              rtc.ClientInfo{ClientInfo: pi.Client},
-		Region:                  pi.Region,
-		AdaptiveStream:          pi.AdaptiveStream,
-		AllowTCPFallback:        allowFallback,
-		TURNSEnabled:            r.config.IsTURNSEnabled(),
-		ParticipantListener:     room.LocalParticipantListener(),
+		Identity:                 pi.Identity,
+		Name:                     pi.Name,
+		SID:                      sid,
+		Config:                   &rtcConf,
+		Sink:                     responseSink,
+		AudioConfig:              r.config.Audio,
+		VideoConfig:              r.config.Video,
+		LimitConfig:              r.config.Limit,
+		ProtocolVersion:          pv,
+		SessionStartTime:         sessionStartTime,
+		SessionTimer:             observability.NewSessionTimer(sessionStartTime),
+		TelemetryListener:        room.ParticipantTelemetryListener(),
+		Trailer:                  room.Trailer(),
+		PLIThrottleConfig:        r.config.RTC.PLIThrottle,
+		CongestionControlConfig:  r.config.RTC.CongestionControl,
+		PublishEnabledCodecs:     enabledCodecs,
+		SubscribeEnabledCodecs:   enabledCodecs,
+		Grants:                   pi.Grants,
+		Reconnect:                pi.Reconnect,
+		Logger:                   pLogger,
+		Reporter:                 roomobs.NewNoopParticipantSessionReporter(),
+		ClientConf:               clientConf,
+		ClientInfo:               rtc.ClientInfo{ClientInfo: pi.Client},
+		Region:                   pi.Region,
+		AdaptiveStream:           pi.AdaptiveStream,
+		AllowTCPFallback:         allowFallback,
+		TCPFallbackRTTThreshold:  r.config.RTC.TCPFallbackRTTThreshold,
+		AllowUDPUnstableFallback: r.config.RTC.AllowUDPUnstableFallback,
+		TURNSEnabled:             r.config.IsTURNSEnabled(),
+		ParticipantListener:      room.LocalParticipantListener(),
 		ParticipantHelper: &roomManagerParticipantHelper{
 			room:                     room,
 			codecRegressionThreshold: r.config.Video.CodecRegressionThreshold,