Rework initial congestion window calculation.

[hiren]

Summary

Rework initial congestion window calculation.

The patch is my first (barely compiling) attempt at:

1. add ability to specify initcwnd in number of bytes
2. move entire calculation into a separate function
3. add a new sysctl to enable RFC6928 which is ON by default
4. D5124's fix included.

Approach here is, user can specify initcwnd of choice in number of segments or bytes. If none is present, we default to RFC6928 and have initcwnd as 10 segments.

AFAIK, this is not changing the default behavior of what we have in FreeBSD-head right now. OR at least, that is the plan.
This change also makes enabling RFC3390 (to get initcwnd at 4 segments) more obvious/easier. (You just have to disable RFC6928.)

Suggestions?

(I need to remember to do any manpage updates with this.)

Test Plan

I'll work on it once we have a general consensus on the approach.

[hiren]

Bump!
I'd appreciate any feedback on the approach taken so I can move this forward.

[hiren]

Talked to Lawrence on IRC and need to make a bunch of changes. bbib

[hiren]

1. • Rework initial congestion window calculation.
   • Rework the rework after talking to lstewart@. :-) Let's see if this is something people like, if so, I'd do more testing.
     
     An obvious problem I am seeing is,
2. sysctl net.inet.tcp.rfc3390=1

net.inet.tcp.rfc3390: 0 -> 0

instead of what I want: 0 -> 1.

[lstewart]

You're a bit "warmer" with the revised changes but still a fair ways off the mark. Apologies to anyone watching but I'm too time poor at the moment to engage in the proper but protracted back-and-forth public Phabricator discussion to resolve all the problems with this work. Perhaps another brief sync on IRC is in order and you can always summarise the chat logs here as context for others.

[hiren]

• Rework initial congestion window calculation.
• Rework the rework after talking to lstewart@. :-)
• Sysctl handlers now work as expected.
  
  Lawrence and I need to talk before making further changes here.

[rrs]

Should there not be a check to the experimental setting to allow a IW of 10?