Am 20.08.19 um 13:09 schrieb Wietse Venema:
Why does the kernel return this nonsensical mss value which is
1/3 of the MTU? 40kB of packet the framing overhead for loopback?
Good question, i have the same problem on different ubuntu machines,
there is no tunneling or vpn, just plain network.
I also get TCP_MAXSEG 21845 on a newly launched VPS on a cloud provider
(loopback mtu 65536).
net.ipv4.tcp_congestion_control = cubic
but i have tried "reno" too, no difference. I have no idea where the mtu
/ 3 happens.
Thanks
Matthias Schneider
Are you on PPPoE or some other tunnel? I would expect mss=1460
for TCP/IP over ethernet.
Since this is strange i prefer to switch to unixsocket.
Don't bother fighting with a hostile network stack :-)
Wietse
I also researched where the delay happened in my milter: its while
waiting for the 64k packet
https://protection.retarus.com/v1?u=https%3A%2F%2Fgithub.com%2Fmschneider82%2Fmilter%2Fblob%2Fb4a6d7c78ac39b8a1d71b53b74bc08d95652d310%2Fsession.go%23L101&c=3ii9Gwp&r=6kteDpYm1BWnNiGBD8pxt3&k=7s1&s=Qa2GzZMyAcLdZiwoBnfkEaAvdOpcAbRSJXPoLZJBjYB
Best regards
Matthias Schneider
Am 19.08.19 um 20:05 schrieb Wietse Venema:
Matthias Schneider:
Hi Wietse,
I suspect that the bottleneck is on the receiving side.
- Maybe the loopback stack does not like the 65535 block size. Try
using an ethernet interface address instead.
I tried switching from interface lo to eno1 - same issue.
What's the en01 MTU size? Postfix has code (in vstream_tweak.c
which is called by the Milter client and other Postfix code) to
automatically increase the VSTREAM buffer from 4096 to the maximal
segment size, to avoid Nagle delays, though that would not help if
the MTU is > 65535 (the Milter's 'packet' size).
I would therefore not expect Nagle delays on en01.
I compared some tcpdumps with postfix as client and my milterclient
(which is fast) as client.
It seems that we run into nagle timeout (the ACK is always delayed).
Setting TCP_QUICKACK on my milter socket doesnt help, is there a way to
set TCP_NODELAY on the milter connection in postfix?
You've got the source.
Wietse
