The Flow Manager library (reflow)

reflow sends and receives media for recon, and handles the NAT traversal and media security for it.

It is built on asio and the reTurn client library.

Main Classes

• flowmanager::FlowManager : Creates media streams, and owns the thread that runs all of their asynchronous socket operations. For DTLS-SRTP, initializeDtlsFactory() creates the client certificate that every stream uses.
• flowmanager::MediaStream : One media stream, created with flowmanager::FlowManager::createMediaStream(). An RTP stream has two components, one for RTP and one for RTCP (unless RTCP is disabled), each carried by its own flow. The application learns that the stream is ready, or has failed, through flowmanager::MediaStreamHandler.
• flowmanager::Flow : One component of a media stream: a UDP, TCP or TLS socket, possibly relayed through a TURN allocation. Its send() and receive() functions send and wait for packets, and getLocalTuple(), getReflexiveTuple() and getRelayTuple() give the addresses to put in SDP.

NAT Traversal

Each media stream uses one of the flowmanager::MediaStream::NatTraversalMode modes:

• NoNatTraversal : use the local address and port as they are;
• StunBindDiscovery : learn the public address and port with a STUN binding request to the configured server;
• TurnAllocation : relay media through a TURN server, over the transport of the stream's local binding (UDP, TCP or TLS).

reflow does not implement ICE.

Media Security

• SRTP keyed through SDP (SDES) : the application gives each stream its keys with flowmanager::MediaStream::createOutboundSRTPSession() and flowmanager::MediaStream::createInboundSRTPSession().
• DTLS-SRTP : each flow runs a DTLS handshake on its media port (see reflow/dtls_wrapper) and checks the peer's certificate against the fingerprint from SDP, set with flowmanager::Flow::setRemoteSDPFingerprint(). SRTP keys come from the handshake.

SRTP itself is provided by libsrtp: version 2, or version 1 with the CMake option USE_SRTP1.