[patch] sysctls for TCP timers

[Andrey Zonov]

On 9/26/12 10:55 PM, Andre Oppermann wrote:
> On 23.09.2012 17:35, Andrey Zonov wrote:
>> On 9/20/12 11:35 AM, Eggert, Lars wrote:
>>> Hi,
>>>
>>> On Sep 20, 2012, at 9:25, Andrey Zonov <zont at freebsd.org> wrote:
>>>> Some of them may be read google's article about tuning TCP parameters
>>>> [1].  I convert most of TCP timers to sysctls [2] and we are using this
>>>> patch for few months.  We tuned net.inet.tcp.rtobase and
>>>> net.inet.tcp.syncache.rexmttime and it gives good results
>>>> (especially in
>>>> conjunction with cc_htcp(4)).
>>>
>>> can you share some measurements that quantify the results?
>>>
>>
>> When we set net.inet.tcp.syncache.rexmttime=200 and
>> net.inet.tcp.syncache.rexmtlimit=7 for our external web service, the
>> number of duplicated SYN was reduced in four times.
> 
> This isn't surprising.  You're simply trading retransmits by the client
> with retransmits by the server.  Whether this is within the overall packet
> conservation principle is not clear.  On the timeline it may be an
> advantage.
> 

This is great improvement for us.  2% of our users don't wait for 3
seconds any more.

> I'm not comfortable with the rather low retransmit time you've chosen
> here.  Considering higher RTT's (e.g. Hawaii or JP/CN) and the bufferbloat
> problem this may be too low.  When it is to be tuned, then something in the
> range of 500-1000ms may be more realistic to avoid spurious retransmits.
> 

For example Linux some time ago set RTO to 1 sec.  OSX for a long time
has RTO 1 sec.

> When a SYN or SYN/ACK retransmit happens, the initial CWND should be
> reduced
> per the applicable RFC's as this indicates packet loss on the downstream.
> 


-- 
Andrey Zonov

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 535 bytes
Desc: OpenPGP digital signature
Url : http://lists.freebsd.org/pipermail/freebsd-net/attachments/20120928/aebcaaf8/signature.pgp