Fix multicast TTL when updating SDP destinations

rust/gst-nmos-rs/src/sdp.rs

@@ -160,7 +160,7 @@ pub(crate) mod defaults {
     /// for unicast destinations (pinned by
     /// `gst_sdp_strips_ttl_for_unicast_c_lines`), so this
     /// constant only takes effect for multicast `c=` lines.
-    pub(crate) const MULTICAST_TTL: u32 = 64;
+    pub(crate) const MULTICAST_TTL: u32 = 32;
 
     /// RTP clock rate for RFC 4175 video (§6.1 `rate`).
     /// Fixed at 90 kHz regardless of frame rate, depth, or
@@ -946,7 +946,7 @@ pub(crate) fn resource_name_from_transport(text: &str) -> Result<Option<String>,
 ///                                                             when no `t=` block was added
 /// a=x-nvnmos-name:<session.name>                              ← if Some (session-level)
 /// m=<media> <destination_port> RTP/AVP <pt>
-/// c=IN IP4 <destination_ip>/64
+/// c=IN IP4 <destination_ip>/32
 /// a=rtpmap:<pt> ...      ← from `set_media_from_caps`
 /// a=fmtp:<pt> ...        ← from `set_media_from_caps`
 /// a=ts-refclk:ptp=IEEE1588-2008:traceable                     ← if session.emit_ptp_ts_refclk
@@ -2843,8 +2843,8 @@ mod tests {
     }
 
     #[test]
-    fn defaults_multicast_ttl_is_64() {
-        assert_eq!(defaults::MULTICAST_TTL, 64);
+    fn defaults_multicast_ttl_is_32() {
+        assert_eq!(defaults::MULTICAST_TTL, 32);
     }
 
     /// Pins `gst-sdp`'s `SDPMedia::add_connection` behaviour so

rust/gst-nmos-rs/src/sdp_passthrough.rs

@@ -267,10 +267,10 @@ fn replace_connection_address(m: &mut SDPMediaRef, address: &str) -> Result<(),
     let conn = m.connection(0).ok_or(SdpError::MissingConnection)?;
     let nettype = conn.nettype().unwrap_or("IN");
     let addrtype = conn.addrtype().unwrap_or("IP4");
-    let ttl = if is_multicast_address(address) {
-        defaults::MULTICAST_TTL
-    } else {
-        conn.ttl()
+    let ttl = match (is_multicast_address(address), conn.ttl()) {
+        (true, 0) => defaults::MULTICAST_TTL,
+        (true, ttl) => ttl,
+        (false, _) => 0,
     };
     let new_conn = SDPConnection::new(nettype, addrtype, address, ttl, conn.addr_number());
     m.replace_connection(0, new_conn)
@@ -448,4 +448,57 @@ mod tests {
             "unresolvable override must drop stale iface: {out}",
         );
     }
+
+    #[test]
+    fn multicast_destination_override_preserves_nonzero_ttl() {
+        init_gst();
+        let input = VIDEO_WITH_STALE_IFACE.replace("239.1.1.1/64", "239.1.1.1/127");
+        let overrides = SdpOverrides {
+            destination_ip: Some("239.1.1.2"),
+            ..Default::default()
+        };
+        let out =
+            passthrough_with_overrides(&input, &overrides, DualLegPassthroughPolicy::RejectDualLeg)
+                .expect("splice");
+        assert!(
+            out.contains("c=IN IP4 239.1.1.2/127"),
+            "multicast override must preserve a non-zero TTL: {out}",
+        );
+    }
+
+    #[test]
+    fn unicast_to_multicast_destination_override_uses_default_ttl() {
+        init_gst();
+        let input = VIDEO_WITH_STALE_IFACE.replace("239.1.1.1/64", "192.0.2.20");
+        let overrides = SdpOverrides {
+            destination_ip: Some("239.1.1.2"),
+            ..Default::default()
+        };
+        let out =
+            passthrough_with_overrides(&input, &overrides, DualLegPassthroughPolicy::RejectDualLeg)
+                .expect("splice");
+        assert!(
+            out.contains(&format!("c=IN IP4 239.1.1.2/{}", defaults::MULTICAST_TTL)),
+            "multicast override must supply the default TTL: {out}",
+        );
+    }
+
+    #[test]
+    fn multicast_to_unicast_destination_override_omits_ttl() {
+        init_gst();
+        let overrides = SdpOverrides {
+            destination_ip: Some("192.0.2.20"),
+            ..Default::default()
+        };
+        let out = passthrough_with_overrides(
+            VIDEO_WITH_STALE_IFACE,
+            &overrides,
+            DualLegPassthroughPolicy::RejectDualLeg,
+        )
+        .expect("splice");
+        assert!(
+            out.contains("c=IN IP4 192.0.2.20\r\n"),
+            "unicast override must omit the TTL suffix: {out}",
+        );
+    }
 }

src/nvnmos_impl.cpp

@@ -1224,6 +1224,12 @@ namespace nvnmos
                 sdp_params.origin.session_version = utility::ostringstreamed(sdp::ntp_now() >> 32);
 
                 auto& transport_params = nmos::fields::transport_params(nmos::fields::endpoint_active(connection_sender.data));
+                if (0 == sdp_params.connection_data.ttl)
+                {
+                    // match the nmos::sdp_parameters value-constructor default; ignored by nmos::make_session_description
+                    // for IPv4 unicast addresses
+                    sdp_params.connection_data.ttl = 32;
+                }
 
                 // use nmos::make_session_description rather than impl::make_session_description for /transportfile
                 // because e.g. the custom SDP attributes in nvnmos::attributes are only for 'internal' use