A problem on TCP in High RTT Environment.

[Michael Tuexen]

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...

Best regards
Michael
> 
> If you use netstat -a, you should be able to see the send-q on the
> sender grow as necessary...
> 
> -- 
>  John-Mark Gurney				Voice: +1 415 225 5579
> 
>     "All that I will do, has been done, All that I have, has not."
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