Security API/TCPM Framing

When multiplexing multiple security streams over the same operating-system stream (e.g., a socket or pipe), Amanda uses TCPM Framing, which presumably stands for "TCP Multiplexing".

Framing

Each stream is assigned a distinct, numeric handle. In practice, all streams are initiated by the same end of the connection, so as far as I (Dustin) know there is no provision for ensuring that handles invented by the two ends do no collide. Each frame of data for each stream is transmitted with the following format:

 32-bit signed network-order payload length
 32-bit unsigned network-order handle
 payload

Amanda considers a negative length or a length greater than 4MB to be invalid and will close the connection with an error. As another implementation detail, Amanda will display any printable bytes in the error message, so if e.g., shell or ssh error messages are accidentally injected into the operating-system stream, the user will see at least part of the messages.

A zero-length frame indicates EOF for that stream.

For random-access streams, data boundaries are not relevant and the API does not guarantee they will be maintained - much like with ordinary pipes or sockets.

Protocol

The first frame sent over a new connection is assumed to be a protocol packet, and the handle from the frame is retained and reserved for protocol communication. Each protocol packet is contained in a single, distinct frame; that is, the framing boundaries are relevant to the protocol.