RTSP over UDP unicast, with fallback to TCP #67

Split out of #7.

RtspSession always sends Transport: RTP/AVP/TCP;unicast;interleaved=0-1 (video) and interleaved=2-3 (audio), see RtspSession.java:235 and :246. RTP over UDP gives lower latency on a clean LAN and avoids TCP head-of-line blocking on a lossy one.

Majestic already serves it (checked on 172.17.32.133 and .126, Lite SigmaStar, master+012cd47):

SETUP .../video  Transport: RTP/AVP;unicast;client_port=50000-50001
RTSP/1.0 200 OK
Transport: RTP/AVP/UDP;unicast;client_port=50000-50001;server_port=46666-46667

Scope

• Per-slot transport setting: TCP (default) / UDP / Auto (UDP, fall back to TCP).
• Ephemeral ports: one DatagramSocket pair (even RTP, odd RTCP) per track, video and audio. Never fixed ports (the bug from basic: track URLs, payload types and UDP ports are hard-coded #40 on basic).
• Parse server_port (and source if present) from the SETUP reply; ignore datagrams from other sources.
• Receive threads hand packets to the existing onPacket(CHANNEL_VIDEO/AUDIO, ...), so RtspClient / QuadCell and the depacketizer stay unchanged.
• RTSP TCP connection stays open for keepalive (GET_PARAMETER / OPTIONS) and TEARDOWN.
• UDP can reorder and drop: at least a small reorder window by RTP seq, or drop late packets and let NalAssembler discard broken access units. Raise SO_RCVBUF (1–2 MB).
• Fallback to TCP: on 461 Unsupported Transport, or when no video datagram arrives within N s after PLAY (NAT/firewall). TEARDOWN, reconnect with interleaved TCP, remember the choice for that slot until the URL changes.
• Optional: send an empty RTP/RTCP datagram to server_port after SETUP to open NAT/firewall pinholes.
• Clean socket shutdown on stop / reconnect / slot change (Majestic had a UDP fd leak on its side, rtsp udp connection close error majestic#262; make sure we do not have one on ours).

Tests

• Unit: parsing the Transport reply header (client_port, server_port, source, ssrc, a server that answers TCP to a UDP request).
• Device (methodology of Device testing: v2 and open PRs (report + checklist) #53): group A cameras, H.264 + Opus and H.265 + Opus, single view and 3×3; fallback forced by blocking UDP; latency compared with TCP.