tcp: provide tcp_rttupdated() and call it periodically

[glebius]

Summary

On every 4-th rtt update (or session close) push connection data into the
TCP hostcache. The "every 4-th" is arbitrarily defined following previous
logic for a closed connection.

This change makes the TCP stack to add to the TCP hostcache during
a connection lifetime. Modern browsers usually open multiple connections
to a site and connections that were created later now can benefit from
host cache data that was gathered by earlier (but still open) connection.

[glebius]

• No local variable.

[tuexen]

sys/netinet/tcp_subr.c

2479

What about this comment? Don't you need to deal with different stacks measure the RTT in different units?

2588

As commented earlier, what about the time unit for srtt and rttvar? Adding entries during the lifetime of a connection changes the semantic.
Wouldn't it make more sense to keep the old behavior, but store updated buffersize when they are changed? Wouldn't that give you what you want with less impact?

[glebius]

sys/netinet/tcp_subr.c

2479

I think Randall was thinking of a potential thing rather than existing. Right now rack_fini() doesn't do any manipulations with t_rttupdated. Also, there is no signed contract, but I would dare to say that alternative stacks shall treat struct tcpcb members exactly the same way the default stack does. If a stack wants to do something special it has its own control block in t_fb_ptr.

2588

I really don't know and looking for your advice here.

My naive logic is the following. Right now, before this change, if a connection A has went through 4 (or more) RTT updates and is closed, then a host cache entry is updated(or created) and a new connection B presumably would benefit from this. However, if connection A went through the same number of updates and stays idle, the new connection B will not benefit. If we presume that such host cache update is beneficial for all members of host cache structure, then let's update all of them. My personal testing was of course mostly focused on the socket buffer limits.