path: root/net/ipv4/tcp_output.c
diff options
authorDavid S. Miller <>2005-09-29 17:07:20 -0700
committerDavid S. Miller <>2005-09-29 17:07:20 -0700
commit01ff367e62f0474e4d39aa5812cbe2a30d96e1e9 (patch)
treecbe69160bfd0dd0d84b25d60405d511120e8001b /net/ipv4/tcp_output.c
parentaa55a08687059aa169d10a313c41f238c2070488 (diff)
[TCP]: Revert 6b251858d377196b8cea20e65cae60f584a42735
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. See? So, the honest function for inital rcv_wnd derived from tcp_init_cwnd() is: init_rcv_wnd(mss)= 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: David S. Miller <>
Diffstat (limited to 'net/ipv4/tcp_output.c')
1 files changed, 4 insertions, 5 deletions
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index caf2e2cff293..c5b911f9b662 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -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;
- else
- init_cwnd = (mss > 1095) ? 3 : 4;
+ else if (mss > 1460)
+ init_cwnd = 3;
if (*rcv_wnd > init_cwnd*mss)
*rcv_wnd = init_cwnd*mss;