A problem on TCP in High RTT Environment.

[Michael Tuexen]

On 09 Aug 2014, at 22:45, John-Mark Gurney <jmg at funkthat.com> wrote:

> 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...
That is correct. Niu: Can you provide this?

Best regards
Michael
> 
> -- 
>  John-Mark Gurney				Voice: +1 415 225 5579
> 
>     "All that I will do, has been done, All that I have, has not."
>