The Tor Browser / file revision

 This is a generic media transport system for WebRTC.

 The basic model is that you have a TransportFlow which contains a

 series of TransportLayers, each of which gets an opportunity to

 manipulate data up and down the stack (think SysV STREAMS or a

 standard networking stack). You can also address individual

 sublayers to manipulate them or to bypass reading and writing

 at an upper layer; WebRTC uses this to implement DTLS-SRTP.

 DATAFLOW MODEL

 Unlike the existing nsSocket I/O system, this is a push rather

 than a pull system. Clients of the interface do writes downward

 with SendPacket() and receive notification of incoming packets

 via callbacks registed via sigslot.h. It is the responsibility

 of the bottom layer (or any other layer which needs to reference

 external events) to arrange for that somehow; typically by

 using nsITimer or the SocketTansportService.

 This sort of push model is a much better fit for the demands

 of WebRTC, expecially because ICE contexts span multiple

 network transports.

 THREADING MODEL

 There are no thread locks. It is the responsibility of the caller to

 arrange that any given TransportLayer/TransportFlow is only

 manipulated in one thread at once. One good way to do this is to run

 everything on the STS thread. Many of the existing layer implementations

 (TransportLayerPrsock, TransportLayerIce, TransportLayerLoopback)

 already run on STS so in those cases you must run on STS, though

 you can do setup on the main thread and then activate them on the

 STS.

 EXISTING TRANSPORT LAYERS

 The following transport layers are currently implemented:

 * DTLS -- a wrapper around NSS's DTLS [RFC 6347] stack

 * ICE  -- a wrapper around the nICEr ICE [RFC 5245] stack.

 * Prsock -- a wrapper around NSPR sockets

 * Loopback -- a loopback IO mechanism

 * Logging -- a passthrough that just logs its data

 The last three are primarily for debugging.