Concepts

1. An XML Protocol message may be delivered through zero or more intermediaries.
   
2. The XML Protocol message specifies a set of intermediaries to visit, and the order in which they will be visited. The list of intermediaries, as specified by the sender, is called the initial path.
   
3. An XML Protocol intermediary may modify the path before forwarding the message to the next intermediary in the chain, and hence possibly change this next intermediary. This permits to cater for cases where the full path is not known in advance, for example where there exists a proxy or caching server along the way.
   
4. An intermediary may only add to the path. More precisely, it may only add one or more nodes to visit before the original next hop.
   
5. An intermediary must record its identity/address within the message, before forwarding it to the next intermediary in the path. This permits to record the actual route followed by the message.
   
6. An XML Protocol response follows the same path as the corresponding request, but in reverse order.
   
7. The recorded route is used to compute the reverse path.
   
8. When a fault occurs at an intermediary, further message forwarding is cancelled, and the fault is delivered as a response through the reverse path.