TEL-682: use even RTP port

rtp/listen.go

@@ -80,3 +80,55 @@ func ListenUDPPortRangeWithConfig(portMin, portMax int, ip netip.Addr, lc *net.L
 		return conn.(*net.UDPConn), nil
 	})
 }
+
+// bindEvenRange binds an even UDP port within the given range.
+func bindEvenRange(portMin, portMax int, ip netip.Addr, create bindFunc) (*net.UDPConn, error) {
+	if portMin <= 0 {
+		portMin = 1
+	}
+	if portMax <= 0 || portMax > 0xFFFF {
+		portMax = 0xFFFF
+	}
+	evenMin := (portMin + 1) &^ 1
+	evenMax := portMax &^ 1
+	if evenMin > evenMax {
+		return nil, ErrListenFailed
+	}
+	ports := (evenMax-evenMin)/2 + 1
+	portCurrent := evenMin + 2*rand.Intn(ports)
+
+	for try := 0; try < ports; try++ {
+		c, err := create(portCurrent, ip)
+		if err == nil {
+			return c, nil
+		} else if !errors.Is(err, syscall.EADDRINUSE) {
+			return c, err
+		}
+		portCurrent += 2
+		if portCurrent > evenMax {
+			portCurrent = evenMin
+		}
+	}
+	return nil, ErrListenFailed
+}
+
+// ListenUDPEvenPortRange binds an even UDP port for RTP, per RFC 3550 convention.
+func ListenUDPEvenPortRange(portMin, portMax int, ip netip.Addr) (*net.UDPConn, error) {
+	return bindEvenRange(portMin, portMax, ip, func(port int, ip netip.Addr) (*net.UDPConn, error) {
+		addr := &net.UDPAddr{IP: ip.AsSlice(), Port: port}
+		return net.ListenUDP("udp", addr)
+	})
+}
+
+// ListenUDPEvenPortRangeWithConfig is ListenUDPEvenPortRange with a custom net.ListenConfig.
+func ListenUDPEvenPortRangeWithConfig(portMin, portMax int, ip netip.Addr, lc *net.ListenConfig) (*net.UDPConn, error) {
+	ctx := context.Background() // ctx is only used to resolve domain names, which we don't use here.
+	ipStr := ip.String()
+	return bindEvenRange(portMin, portMax, ip, func(port int, ip netip.Addr) (*net.UDPConn, error) {
+		conn, err := lc.ListenPacket(ctx, "udp", fmt.Sprintf("%s:%d", ipStr, port))
+		if err != nil {
+			return nil, err
+		}
+		return conn.(*net.UDPConn), nil
+	})
+}

rtp/listen_test.go

@@ -0,0 +1,42 @@
+// Copyright 2026 LiveKit, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package rtp
+
+import (
+	"net"
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestListenUDPEvenPortRange(t *testing.T) {
+	ip := netip.AddrFrom4([4]byte{127, 0, 0, 1})
+
+	// block the first even port; allocation must skip to the next one
+	const portMin, portMax = 34200, 34203
+	blocker, err := net.ListenUDP("udp", &net.UDPAddr{IP: ip.AsSlice(), Port: 34200})
+	require.NoError(t, err)
+	defer blocker.Close()
+
+	c, err := ListenUDPEvenPortRange(portMin, portMax, ip)
+	require.NoError(t, err)
+	defer c.Close()
+	require.Equal(t, 34202, c.LocalAddr().(*net.UDPAddr).Port)
+
+	// even ports exhausted
+	_, err = ListenUDPEvenPortRange(portMin, portMax, ip)
+	require.ErrorIs(t, err, ErrListenFailed)
+}