- * Device address (e.g., bus ID) of the source of the
- * transaction. The source is not responsible for setting this
- * field; it is set implicitly by the interconnect when the packet
- * is first sent.
+ * Source port identifier set on a request packet to enable
+ * appropriate routing of the responses. The source port
+ * identifier is set by any multiplexing component, e.g. a bus, as
+ * the timing responses need this information to be routed back to
+ * the appropriate port at a later point in time. The field can be
+ * updated (over-written) as the request packet passes through
+ * additional multiplexing components, and it is their
+ * responsibility to remember the original source port identifier,
+ * for example by using an appropriate sender state. The latter is
+ * done in the cache and bridge.