Implement Keepalive Connection String Options

[caleblloyd]

Keep Alive, Keepalive (default: 0) For TCP connections, idle connection time measured in seconds, before the first keepalive packet is sent. A value of 0 indicates that keepalive is not used.

Should call socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, value) on TCP Socket if value > 0

[caleblloyd]

Respecting System-Wide TCP Keepalive

Operating Systems (both Windoows and Linux) have default TCP Keepalive values, but TCP Keepalive must be enabled on the socket in order for Keepalive to be used. (Windows Reference, Linux Reference)

This setting is achieved by calling socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true)

Should we always enable TCP Keepalive on the Socket? Or only if the Keepalive connection string parameter is set? I'm partial to always enabling it due to the nature of pooled connections potentially being idle.

Overriding System-Wide settings with Per-Socket settings

There are 3 settings that can be overridden, outlined here. Example:

tcp_keepalive_time = 7200
tcp_keepalive_intvl = 75
tcp_keepalive_probes = 9

The first two parameters are expressed in seconds, and the last is the pure number. This means that the keepalive routines wait for two hours (7200 secs) before sending the first keepalive probe, and then resend it every 75 seconds. If no ACK response is received for nine consecutive times, the connection is marked as broken.

Overriding the system-wide parameters on Windows is described in this stackoverflow post

Overriding the system-wide parameters on Linux needs to be done with socket options, described here. I will have to dig into .NET's socket option ENUMs and see if these are defined

[caleblloyd]

I can't get the socket-level options to work on Linux or OSX, the proper Socket Options are not available in .NET Core. Opened upstream:

https://github.com/dotnet/corefx/issues/14237

[bgrainger]

Shipped in 0.8.0.