16.2.1 DCE-CIOP RPC

   DCE-CIOP requires an RPC, which is interoperable with the DCE connection-oriented and/or connectionless protocols as specified in the X/Open CAE Specification C309 and the OSF AES/Distributed Computing RPC Volume. Some of the features of the DCE-RPC are as follows:

• Defines connection-oriented and connectionless protocols for establishing the communication between a client and server.

• Supports multiple underlying transport protocols including TCP/IP.

• Supports multiple outstanding requests to multiple CORBA objects over the same connection.

• invoke - for invoking CORBA operation requests, and

• locate - for locating server processes.

• Supports fragmentation of messages. This provides for buffer management by ORBs of CORBA requests, which contain a large amount of marshaled data.

All interactions between ORBs take the form of remote procedure calls on one of two well-known DCE-RPC interfaces. Two DCE operations are provided in each interface:

   Each DCE operation is a synchronous remote procedure call1,2, consisting of a request message being transmitted from the client to the server, followed by a response message being transmitted from the server to the client.

   1.DCE maybe operation semantics cannot be used for CORBA oneway operations because they are idempotent as opposed to at-most-once.

   2. The deferred synchronous DII API can be implemented on top of synchronous RPCs by using threads.

   Using one of the DCE-RPC interfaces, the messages are transmitted as pipes of uninterpreted bytes. By transmitting messages via DCE pipes, the following characteristics are achieved:

• Large amounts of data can be transmitted efficiently.

• Buffering of complete messages is not required.

• Marshaling and demarshaling can take place concurrently with message transmission.

• Encoding of messages and marshaling of data is completely under the control of the ORB.

• DCE client and server stubs can be used to implement DCE-CIOP.

   Using the other DCE-RPC interface, the messages are transmitted as conformant arrays of uninterpreted bytes. This interface does not offer all the advantages of the pipe-based interface, but is provided to enable interoperability with ORBs using DCE-RPC implementations that do not adequately support pipes.