XCF provides protocols intended to help clients and servers function compatibly with mixed levels of support and functionality. In particular, these protocols provide the criteria that XCF uses to determine whether a server instance is suitable for processing a given request. A request is not presented to a particular server instance if the instance is not suitable for the request.

When you start a server through IXCSRVR, you can specify the range of server levels it supports, the range of client levels whose requests it is willing to process, and the features that it supports.

When you use IXCSEND to send a request to a server, the client can either target a specific server instance, or it can have XCF select the server instance that is to process the request. If the client targets a particular server instance, XCF targets the client request to the specified server instance without comparing client specified server selection criteria to the server defined supported client levels, server levels and features.

The client is responsible for determining the suitability of the specified servers. If XCF is to select the server, the client must specify the client level, the desired range of server levels, and the features that a suitable server must support. The client and server mutually determine the content and interpretation of the levels and features.

When the request arrives on the server system, XCF compares the range of server levels requested by the client to the range of levels supported by the server. If the requested range does not intersect the supported range, the server is not eligible to process the request.

XCF also compares the client level to the range of client levels whose requests the server is willing to accept. If the client level is not in the indicated range, the server is not eligible to process the request. Finally XCF compares the features requested by the client with the features supported by the server. If the server does not support the required features, it is not eligible to process the request. If no suitable server remains, the request is cancelled with a "no receiver" response code. Otherwise, XCF selects one of the suitable server instances to process the request.