Implementing sesson tickets

[kazu-yamamoto]

warp-tls now uses Network.TLS.SessionManager. The motivation which I implemented this module is to allow early data (0RTT) of TLS 1.3. It requires that session information is used only once to prevent replay attacks. For this purpose, sessionResumeOnlyOnce ofSessionManager was introduced in tls. And in-memory manager was required to check the only-once policy.

However, memory usage of the session manager is more expensive than I imagined. So, I would like to implement the approach of session tickets which can off-load the session information to clients. With this approach, we should give up the early data. This is trade-off:

• Session manager: much memory but can use early data
• Session tickets: very little memory but cannot use early data

So, I'm planning to let warp-tls users to choose either the session manager or the session tickets. Probably, we should discuss which one should be the default.

[vdukhovni]

FWIW, I agree that early data is not worth it. Use session tickets.