10.6.2 Connection State and Flow Control

Connections are manipulated using the various operations defined by the multiplexing protocol. The following are the names of the operations defined by the protocol: OPEN, CLOSE, CLOSEACK, REQUEST, and TRANSMIT. The exact format and rules for all the operations are detailed in Section 10.6.3, "Protocol Format."

The OPEN, CLOSE, and CLOSEACK operations control connections becoming opened and closed, while the REQUEST and TRANSMIT operations are used to transmit data across an open connection within the constraints of the flow control mechanism.

Connection States

A virtual connection is open with respect to a particular endpoint if the endpoint has sent an OPEN operation for that connection, or it has received an OPEN operation for that connection (and it had not been subsequently closed). The various protocol operations are described below.

A virtual connection is pending close with respect to a particular endpoint if the endpoint has sent a CLOSE operation for that connection, but it has not yet received a subsequent CLOSE or CLOSEACK operation for that connection.

A virtual connection is closed with respect to a particular endpoint if it has never been opened, or if it has received a CLOSE or a CLOSEACK operation for that connection (and it has not been subsequently opened).

Flow Control

The multiplexing protocol using a simple packeting flow control mechanism to allow multiple virtual connections to exist in parallel over the same concrete connection. The high level requirement of the flow control mechanism is that the state of all virtual connections is independent; the state of one connection may not affect the behavior of others. For example, if the data buffers handling data coming in from one connection become full, this cannot prevent the transmission and processing of data for any other connection. This is necessary if the proceedings of one connection is dependent on the completion of the use of another connection, such as would happen with recursive RMI calls. Therefore, the practical implication is that the implementation must always be able to consume and process all of the multiplexing protocol data ready for input on the concrete connection (assuming that it conforms to this specification).

Each endpoint has two state values associated with each connection: how many bytes of data the endpoint has requested but not received (input request count) and how many bytes the other endpoint has requested but have not been supplied by this endpoint (output request count).

An endpoint's output request count is increased when it receives a REQUEST operation from the other endpoint, and it is decreased when it sends a TRANSMIT operation. An endpoint's input request count is increased when it sends a REQUEST operation, and it is decreased when it receives a TRANSMIT operation. It is a protocol violation if either of these values becomes negative.

It is a protocol violation for an endpoint to send a REQUEST operation that would increase its input request count to more bytes that it can currently handle without blocking. It should, however, make sure that its input request count is greater than zero if the user of the connection is waiting to read data.

It is a protocol violation for an endpoint to send a TRANSMIT operation containing more bytes that its output request count. It may buffer outgoing data until the user of the connection requests that data written to the connection be explicitly flushed. If data must be sent over the connection, however, by either an explicit flush or because the implementation's output buffers are full, then the user of the connection may be blocked until sufficient TRANSMIT operations can proceed.

Beyond the rules outlined above, implementations are free to send REQUEST and TRANSMIT operations as deemed appropriate. For example, an endpoint may request more data for a connection even if its input buffer is not empty.