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In order for a client to successfully talk to a broker, it needs to know what version of protocol does the broker support. This in turn leads to following requirements.

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Informing client of unknown protocol request or unsupported version

As mentioned before, broker on receiving a protocol request it does not support will simply close the TCP connection to the client. From the client's point of view it has no way of knowing that the broker did not support the request, it could just as well be a network or software problem causing the connection reset.

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This is a nice to have part of this KIP and not a blocker.

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Informing client of versions supported by a broker

The information can be provided in various ways. However after considering many alternatives, mentioned in rejected alternatives, the KIP suggests the following.

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