Both good points, thanks. For the timeout case, I don't think we care too much about what happens once the original transaction times out - returning a 400 to any subsequent transactions initiated by ASs seems fine. And for the other, I think I can finesse it by clarifying the comment a little. We only use the AsChain when processing an initial request - it's not used at all beyond that (that's why they're iFCs - sFCs were dropped IIRC). Do you think it would be reasonable for an AS to end the upstream transaction, and then start a new downstream transaction? (serious question - if you do think that's reasonable, we need to put the AsChain lifetime on a timer or something, rather than tying it to the upstream transaction lifetime).