fix(datachannel): send DataChannelOpen for pre-negotiated channels (closes #61)

rtc-datachannel/src/data_channel/mod.rs

@@ -71,23 +71,28 @@ impl DataChannel {
     ) -> Result<Self> {
         let mut data_channel = DataChannel::new(config.clone(), association_handle, stream_id);
 
-        if !config.negotiated {
-            let msg = Message::DataChannelOpen(DataChannelOpen {
-                channel_type: config.channel_type,
-                priority: config.priority,
-                reliability_parameter: config.reliability_parameter,
-                label: config.label.bytes().collect(),
-                protocol: config.protocol.bytes().collect(),
-            })
-            .marshal()?;
-
-            data_channel.write_outs.push_back(DataChannelMessage {
-                association_handle,
-                stream_id,
-                ppi: PayloadProtocolIdentifier::Dcep,
-                payload: msg,
-            });
-        }
+        // Send DataChannelOpen for all channels — including out-of-band negotiated ones.
+        //
+        // For non-negotiated channels this initiates the DCEP handshake per RFC 8832 §3.
+        // For pre-negotiated channels (negotiated=true) the DCEP exchange also opens the
+        // underlying SCTP stream on both sides.  Without it the SCTP association never
+        // registers the stream, causing every subsequent write to fail with
+        // "Stream not existed" (issue webrtc-rs/rtc#61).
+        let msg = Message::DataChannelOpen(DataChannelOpen {
+            channel_type: config.channel_type,
+            priority: config.priority,
+            reliability_parameter: config.reliability_parameter,
+            label: config.label.bytes().collect(),
+            protocol: config.protocol.bytes().collect(),
+        })
+        .marshal()?;
+
+        data_channel.write_outs.push_back(DataChannelMessage {
+            association_handle,
+            stream_id,
+            ppi: PayloadProtocolIdentifier::Dcep,
+            payload: msg,
+        });
 
         Ok(data_channel)
     }

rtc/src/peer_connection/handler/sctp.rs

@@ -281,7 +281,20 @@ impl<'a> sansio::Protocol<TaggedRTCMessageInternal, TaggedRTCMessageInternal, RT
                                 ::datachannel::data_channel::DataChannel::get_reliability_params(
                                     data_channel_open.channel_type,
                                 );
-                            let mut stream = conn.open_stream(message.stream_id, message.ppi)?;
+                            // For pre-negotiated (out-of-band) channels both peers send
+                            // DataChannelOpen simultaneously.  The remote's message may arrive
+                            // before we process our own outbound one, causing get_or_create_stream
+                            // to auto-create the stream first.  Treat ErrStreamAlreadyExist as
+                            // non-fatal: the stream is open, just update its reliability params.
+                            let mut stream = match conn
+                                .open_stream(message.stream_id, message.ppi)
+                            {
+                                Ok(s) => s,
+                                Err(Error::ErrStreamAlreadyExist) => {
+                                    conn.stream(message.stream_id)?
+                                }
+                                Err(e) => return Err(e),
+                            };
                             stream.set_reliability_params(
                                 unordered,
                                 reliability_type,