I love sequence diagrams to explain how a client/server request is handled. UML is natively pretty weak on client/server and even more on P2P.
Here is a notation which I’m not sure is official but helps a lot (see the “Sequence Diagram Header Elements” sub-section):
http://www.sequencediagrameditor.com/uml/sequence-diagram.htm
I’m currently using manual drawing to do that, just feels weird.