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integer requestID = 0;
 
// Add generateKey here!!
 
default {
    touch_start(integer x) {
        llMessageLinked(
            LINK_ROOT, 
            1234, 
            "I am a request", 
            generateKey("echo", (string)requestID++)
        );
    }
 
    link_message(integer x, integer y, string msg, key id) {
        if (y == 1234) 
            llOwnerSay("Request: " + (string)id + " = " + msg);
    }
}

default {
    link_message(integer x, integer y, string msg, key id) {
        if (y == 1234) // Echo, send straight back
            llMessageLinked(x, y, msg, id);
    }
}

Implementation

key generateKey(string service, string variable) {
    return (key)llInsertString(
        llInsertString(
            llInsertString(
                llInsertString(
                    llMD5String(
                        "secondlife://" + llGetRegionName()  + "/" + 
                            (string)llGetKey() + "/" + 
                            (string)llGetLinkNumber() + "/" + 
                            llGetScriptName() + "/" + 
                            service + "/" + variable, 
                        0 // This is reserved by specification, will 
                          // be increased with new/different versions.
                    ),
                    8,
                    "-"
                ),
                13,
                "-"
            ),
            18,
            "-"
        ),
        23,
        "-"
    );
}

Optimisation

Instead of always calling llGetRegionName(), llGetKey(), llGetLinkNumber(), and llGetScriptName(), you may wish to cache their return values into a uri variable, and only dynamically add the service and variable parameters each-time. Remember to update this variable when you know it's components will have changed, using the changed(), on_rez(), and/or attach() events.