tcp: Use segment size excluding any options for all cwnd calculations

[rscheff]

Summary

Avoid sending small segments by making sure that cwnd is usually
calculated in full (data) segment sizes. Especially during loss
recovery and retransmission scenarios.

An alternative approach, and possibly more effective, could be to
ensure in tcp_output, that small segments are only sent, when ACKNOW
is set, or cwnd would allow sending the entire send buffer (but the
final segment being smaller than MSS).

[cc]

sys/netinet/cc/cc_newreno.c

218

This is fast path and re-calculate MSS for every input draws my efficiency concern. I remember Lawrence once discussed about this, if I remember correctly.

Why not cache the correct MSS in tcpcb?

[rscheff]

Well, I had the same thought - the full MSS (including options) is less frequently used, that the mss without options...

So, it would certainly be more efficient to store the MSS exkluding options in tcpcb, and calculating the MSS inkl. option space only where that is needed...

But that should a different Diff IMHO, as this one is more in the break/fix category.

[cc]

In D47474#1084985, @rscheff wrote:

Well, I had the same thought - the full MSS (including options) is less frequently used, that the mss without options...

So, it would certainly be more efficient to store the MSS exkluding options in tcpcb, and calculating the MSS inkl. option space only where that is needed...

But that should a different Diff IMHO, as this one is more in the break/fix category.

I want to test this patch to see if any segmentation being improved during loss recovery, as I found in https://wiki.freebsd.org/chengcui/testD43470.

[cc]

update:
Looks this patch has some significant reduction on fragment (data_size % MSS) > 0 out of TSO data chunks: testD47474
TSO not enabled:

version	fragment_cnt
before patch D47474	31
after patch D47474	1

TSO enabled:

version	fragment count	total TSO data chunks	fragment ratio
before patch D47474	2135	210786	0.010
after patch D47474	1313	206474	0.006 (-40.0%)