2.6 The Channel
2.6.1 Socket #
An authority that federates over PSI MUST listen on a SOCK_STREAM Unix
domain socket.
Unlike PGSS Logon's path, this one is not normative. PSI is not a
conformance requirement (§2.1), and an authority that offers it may put
it where it likes provided its sources are told. Mainline's is
/run/psi.sock.
2.6.2 Access control #
The socket SHOULD carry a security descriptor. It is DoS protection and nothing more, and an implementation MUST be written as though it were absent.
The reason is that the socket cannot be the boundary. What establishes a source's identity is the peer's token (§2.9), which is checked on every connection. A descriptor that kept casual traffic away would be a convenience; a descriptor relied upon would be a second, weaker access control that someone will eventually assume is doing the work.
An authority MUST therefore bound the number of unregistered connections it will hold open, and the time it will wait for a registration, independently of any descriptor.
2.6.3 Long-lived connections #
A source's connection persists for the life of the source and carries every logon routed to it.
An authority MUST bound the number of registered sources and the number of concurrent conversations per source. A source MUST bound the conversations it will track, and MUST NOT depend on the authority's bookkeeping to do it — a source that trusted the authority's limit would be trusting a bound it cannot verify.
2.6.4 Failure #
A framing error is fatal to the connection, not to a conversation. Once a message has failed to parse there is no way to know where the next one starts, so both parties MUST tear the connection down rather than attempt resynchronisation.
A failed write is likewise fatal. A partial write desynchronises the stream just as a bad frame does, and treating it as a per-conversation error would leave a corrupt connection in use.
Ordinary semantic failures — an unknown principal, a bad credential, a
refused logon — are not connection failures. They are Refusal
messages (§2.13) and the connection continues.