screen_cast/.agents/skills/rtp-networking/SKILL.md

name, description, disable-model-invocation

name	description	disable-model-invocation
rtp-networking	Guidelines for RTP/UDP transport, signaling, discovery, NAT traversal, and encryption in the screen_cast project.	false

rtp-networking

Guidelines for RTP/UDP transport, signaling, discovery, NAT traversal, and encryption in the screen_cast project.

Scope

Use this skill for:

• RTP packetization, depacketization, and sequencing.
• UDP socket I/O and async event loops.
• Peer discovery (mDNS/DNS-SD).
• Session signaling and control channels.
• Optional SRTP/DTLS or ICE-lite/NAT traversal.

Transport

• Default transport is RTP over IPv4/IPv6 UDP.
• Use a small, project-specific RTP header parser/serializer (12-byte base header plus extensions if needed).
• Fragment large NAL units across RTP packets using FU-A fragmentation for H.264.

Resilience

• Start without retransmission; add NACK / PLI once the baseline path works.
• Keep a jitter buffer on the receiver to absorb network jitter without adding excessive latency.
• Sequence numbers drive depacketization; detect loss and request key frames on large gaps.

Discovery

• Use Avahi (avahi-client) for mDNS/DNS-SD service announcements.
• Service name example: _screencast._tcp for signaling, _screencast-rtp._udp for media.

Signaling

• Control channel is JSON over WebSocket or plain TCP.
• Negotiate at least: codec, resolution, frame rate, UDP endpoints, session ID.
• Keep signaling independent of media transport so either can evolve.

Security (future)

• SRTP key exchange via DTLS or pre-shared keys over the signaling channel.
• Do not hardcode keys; rotate per session.

Async I/O

• Prefer an existing event loop library (e.g. asio, libuv) over raw threads and select().
• Keep network and capture threads separated with lock-free queues where practical.

References

• RFC 3550 (RTP), RFC 6184 (H.264 RTP payload)
• Avahi client documentation
• docs/ARCHITECTURE.md
• docs/PHASES.md