フリー問題

問題文

A public web tier is spread over many servers that share no common store. The operators want each resumption ticket to be usable once only. Which cost does the specification attach to that choice in a deployment shaped like this one?

選択肢

1. The lifetime of each ticket has to be cut to the shortest that the clients will accept, so the share of resumptions that succeed falls in step with that lifetime and the store never grows.
2. The store of outstanding tickets has to be shared between the servers, so the share of resumptions that succeed is harder to keep high than it is with tickets the server encrypts for itself.
3. The store of outstanding tickets can stay local to each server, so the share of resumptions that succeed is unaffected and the tickets the server encrypts for itself need no store at any point.
4. The key that protects each ticket has to be rotated on every use, so the share of resumptions that succeed depends on how fast the new key reaches the other servers and nothing else.