Smart mailserver selection based on round trip time

[adambabik]

As a user, I would like to have a mailserver selected automatically from a list of available mailservers based on the round trip time, so that I use the closest.

Acceptance criteria

• Based on a list of mailservers, the algorithm should select the closest mailserver based on round trip time
• It should fallback to the next mailserver if the previously selected was disconnected
• Verify selection periodically and provide event-based interface
• (optional) Observe connected mailservers and if one is gone signalise another candidate

Next

• Measure additional things like throughput and adjust selection based on that

[adambabik]

RTT calculation strategy can be split into two:

1. Initial probing,
2. Continuous calculation from a TCP flow.

We definitely need to implement (1) in order to select the closest mailserver when the app is started. (2) might be interesting in case of a long term session like on a desktop but I would say it's optional for the time being.

(1) can be done using active probing in the form of ICMP ping or application-specific measurement (there is an initial devp2p handshake which not necessarily needs to succeed to calculate RTT).