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In the "Waiting For Request From Client" state, the server listens on UDP port 1434 for an incoming request. If the request is valid and understood, the server immediately sends an SVR_RESP response back to the client. The data content of the response depends on the request type.
For CLNT_BCAST_EX and CLNT_UCAST_EX, the server returns information about all available instances. The information about all available instances is provided by the higher layer.
For CLNT_UCAST_INST, the server returns information about the specified instance only (if available). The information about the specified instance is provided by the higher layer.
For CLNT_UCAST_DAC, the server returns information about the dedicated administrator connection (DAC) only.
The response SHOULD include information for a particular protocol as long as the aggregate information for the instance fits within the 1,024 bytes limit. If the information for a protocol would cause the total information for all protocols to exceed 1,024 bytes—for example, trying to add a 500-byte pipe name when 800 bytes of response data have already been collected—the information for this protocol SHOULD not be sent. The information for the next protocol (if any) SHOULD be included in the response (assuming that it does not cause the response to exceed the 1,024 bytes limit). Furthermore, the server SHOULD NOT include a protocol and its information if no valid information is available. For example, if the TCP port is invalid, TCP would not be included in the response. The SQL Server Resolution Protocol SHOULD NOT verify the length or content of the PIPENAME field, which is provided by the higher layer. It is the upper layer's responsibility to ensure that PIPENAME conforms to the specification of a valid pipe name [MSDN-NP].
If the request is received on an IPv4 socket, the response provides the instance's port that is associated with an IPv4 address, and likewise for IPv6.
If the request is not valid, not understood, or if there is no instance for which it can send back endpoint information, the server MUST ignore the request. The server MUST then enter the "Waiting For Request From Client" state.