Summary
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The RMI Multiplexing Protocol implementation is being removed.  It is unused and has been disabled since JDK 9.

Problem
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The RMI Multiplex protocol was introduced in JDK 1.0.2 but longer is used and the implementation is no longer needed.
The unused code will be removed to simplify documentation, support, and testing.
The multiplex protocol does not appear in the public API and there is no client side support
for the multiplexing protocol.

The RMI Specification "Java Remote Method Invocation: 10 - RMI Wire Protocol"
 includes the multiplex protocol but its support is not mandated.
There are no compatibility tests in the JCK for multiplexing.

Solution
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Replace the implementation so that it responds to a request for the `MultiplexProtocol` with the response `ProtocolNack` to indicate it is not supported.

The system property `sun.rmi.transport.tcp.enableMultiplexProtocol` is no longer needed and will be ignored.

Specification
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There are no changes to the RMI API or specification.