IPC Mapping for Scalability Protocols

This mapping should be layered directly on the top of AF_UNIX sockets of type SOCK_STREAM. On the platforms where AF_UNIX sockets are not available IPC mapping may be done in a platform-specific way and SHOULD be described in a separate RFC. There's no fixed file to use for SP communication. Instead, filenames are assigned to individual services by the user.

Before binding the AF_UNIX socket the implementation SHOULD check whether there's another process bound to the address. If not so it SHOULD try to delete the associated file, if present. This measure will prevent subsequent bind from failing if there's a leftover file from the previous runs of the application. The check can be performed in a platform-specific way, however, a generic way to implement it is to try to connect to the address and close the connection immediately if successful. After establishing underlying AF_UNIX connection, both parties MUST send the protocol header immediately. Both endpoints MUST then wait for the protocol header from the peer before proceeding on. The protocol header is 8 bytes long and looks like this:

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x00 | 0x53 | 0x50 | version | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | type | reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ First four bytes of the protocol header are used to make sure that the peer's protocol is compatible with the protocol used by the local endpoint. First four bytes of the protocol header MUST be set to 0x00, 0x53, 0x50 and 0x00 respectively. If the protocol header received from the peer differs, the TCP connection MUST be closed immediately. The fact that the first byte of the protocol header is binary zero eliminates any text-based protocols that were accidentally connected to the endpoint. Subsequent two bytes make the check even more rigorous. At the same time they can be used as a debugging hint to indicate that the connection is supposed to use one of the scalability protocols -- ASCII representation of these bytes is 'SP'. Finally, the fourth byte rules out any incompatible versions of this protocol. Fifth and sixth bytes of the header form a 16-bit unsigned integer in network byte order representing the type of SP endpoint on the layer above. The value SHOULD NOT be interpreted by the mapping, rather the interpretation should be delegated to the scalability protocol above the mapping. For informational purposes, it should be noted that the field encodes information such as SP protocol ID, protocol version and the role of endpoint within the protocol. Individual values are assigned by IANA. Finally, the last two bytes of the protocol header are reserved for future use and must be set to binary zeroes. If the protocol header from the peer contains anything else than zeroes in this field, the implementation MUST close the underlying TCP connection.

Once the protocol header is accepted, endpoint can send and receive messages. Every message starts with a message header consisting of of a single byte called 'message type'. The only value of this field that is currently allowed is 0x1, which means "in-band" message, i.e. message whose body is passed as a stream of bytes via the AF_UNIX socket. The intent of this field is to eventually allow out-of-band transfer of the message bodies, e.g. via shared memory. The in-band message type MUST be implemented.

For in-band messages, message header is immediately followed by 64-bit unsigned integer in network byte order representing the payload size, in bytes. Thus, the message payload can be from 0 to 2^64-1 bytes long. The payload of the specified size follows directly after the size field:

+-----------+------------+-----------------+ | type (8b) | size (64b) | payload | +-----------+------------+-----------------+

This memo includes no request to IANA.

The mapping isn't intended to provide any additional security in addition to what AF_UNIX does.