A problem on TCP in High RTT Environment.

[John-Mark Gurney]

Michael Tuexen wrote this message on Sat, Aug 09, 2014 at 21:51 +0200:
> 
> On 09 Aug 2014, at 20:42, John-Mark Gurney <jmg at funkthat.com> wrote:
> 
> > Niu Zhixiong wrote this message on Fri, Aug 08, 2014 at 20:34 +0800:
> >> Dear all,
> >> 
> >> Last month, I send problems related to FTP/TCP in a high RTT environment.
> >> After that, I setup a simulation environment(Dummynet) to test TCP and SCTP
> >> in high delay environment. After finishing the test, I can see TCP is
> >> always slower than SCTP. But, I think it is not possible. (Plz see the
> >> figure in the attachment). When the delay is 200ms(means RTT=400ms).
> >> Besides, the TCP is extremely slow.
> >> 
> >> ALL BW=20Mbps, DELAY= 0 ~ 200MS, Packet LOSS = 0 (by dummynet)
> >> 
> >> This is my parameters:
> >> FreeBSD vfreetest0 10.0-RELEASE FreeBSD 10.0-RELEASE #0: Thu Aug  7
> >> 11:04:15 HKT 2014
> >> 
> >> sysctl net.inet.tcp
> > 
> > [...]
> > 
> >> net.inet.tcp.recvbuf_auto: 0
> > 
> > [...]
> > 
> >> net.inet.tcp.sendbuf_auto: 0
> > 
> > Try enabling this...  This should allow the buffer to grow large enough
> > to deal w/ the higher latency...
> > 
> > Also, make sure your program isn't setting the recv buffer size as that
> > will disable the auto growing...
> I think the program sets the buffer to 2MB, which it also does for SCTP.
> So having both statically at the same size makes sense for the comparison.
> I remember that there was a bug in the combination of LRO and delayed ACK,
> which was fixed, but I don't remember it was fixed before 10.0...

Sounds like disabling LRO and TSO would be a useful test to see if that
improves things...  But hiren said that the fix made it, so...

> > If you use netstat -a, you should be able to see the send-q on the
> > sender grow as necessary...

Also, getting the send-q output while it's running would let us know
if the buffer is getting to 2MB or not...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."