Commit 01ff367e authored by David S. Miller's avatar David S. Miller

[TCP]: Revert 6b251858

But retain the comment fix.

Alexey Kuznetsov has explained the situation as follows:


I think the fix is incorrect. Look, the RFC function init_cwnd(mss) is
not continuous: f.e. for mss=1095 it needs initial window 1095*4, but
for mss=1096 it is 1096*3. We do not know exactly what mss sender used
for calculations. If we advertised 1096 (and calculate initial window
3*1096), the sender could limit it to some value < 1096 and then it
will need window his_mss*4 > 3*1096 to send initial burst.


So, the honest function for inital rcv_wnd derived from
tcp_init_cwnd() is:

	  min { init_cwnd(mss1)*mss1 for mss1 <= mss }

It is something sort of:

	if (mss < 1096)
		return mss*4;
	if (mss < 1096*2)
		return 1096*4;
	return mss*2;

(I just scrablled a graph of piece of paper, it is difficult to see or
to explain without this)

I selected it differently giving more window than it is strictly
required.  Initial receive window must be large enough to allow sender
following to the rfc (or just setting initial cwnd to 2) to send
initial burst.  But besides that it is arbitrary, so I decided to give
slack space of one segment.

Actually, the logic was:

If mss is low/normal (<=ethernet), set window to receive more than
initial burst allowed by rfc under the worst conditions
i.e. mss*4. This gives slack space of 1 segment for ethernet frames.

For msses slighlty more than ethernet frame, take 3. Try to give slack
space of 1 frame again.

If mss is huge, force 2*mss. No slack space.

Value 1460*3 is really confusing. Minimal one is 1096*2, but besides
that it is an arbitrary value. It was meant to be ~4096. 1460*3 is
just the magic number from RFC, 1460*3 = 1095*4 is the magic :-), so
that I guess hands typed this themselves.

Signed-off-by: default avatarDavid S. Miller <>
parent aa55a086
......@@ -194,12 +194,11 @@ void tcp_select_initial_window(int __space, __u32 mss,
* will be satisfied with 2.
if (mss > (1<<*rcv_wscale)) {
int init_cwnd;
if (mss > 1460)
int init_cwnd = 4;
if (mss > 1460*3)
init_cwnd = 2;
init_cwnd = (mss > 1095) ? 3 : 4;
else if (mss > 1460)
init_cwnd = 3;
if (*rcv_wnd > init_cwnd*mss)
*rcv_wnd = init_cwnd*mss;
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