12-07-2011 10:46 AM
I have a couple of Brocade 7500 talking to each other using FCIP tunnels. We are seeing retransmissions but networking is not seeing any packet drops. There is a parameter called Min Retransmit Time with a default value of 100ms. We are thinking, maybe we are running over this limit which is causing retransmissions. Has anybody every played with this parameter or if what we are thinking could be a probable cause of retransmissions or if we are just looking in the wrong direction.
Any help is appreciated.
12-07-2011 11:00 AM
How many tcp retransmits do you have ?
FYI here is a drawing of retransmits on ge interface on one of my 7500 :
I don't have any issues with FCIP flows
Hope this helps
12-07-2011 11:08 AM
Here is latest stats.
8691577231 output packets
5671 pkt/s 30s avg, 15772 pkt/s lifetime avg
8414301449756 output Bytes
5015530 Bps 30s avg, 15269246 Bps lifetime avg
1111952660 packets lost (retransmits)
4.30% loss rate 30s avg
6515082828 input packets
4081 pkt/s 30s avg, 11822 pkt/s lifetime avg
1558970946184 input Bytes
539726 Bps 30s avg, 2829030 Bps lifetime avg
Tunnel has been up for 6 days and 9 hours. The loss rate goes up to as much as 16% at times..
By the way, what are you using for the graph ?
12-07-2011 10:58 PM
here stats from mine :
25411149126 output packets
14885 pkt/s 30s avg, 23556 pkt/s lifetime avg
28504168177784 output Bytes
12255456 Bps 30s avg, 26423355 Bps lifetime avg
321625 packets lost (retransmits)
0.00% loss rate 30s avg
24231790657 input packets
15404 pkt/s 30s avg, 22462 pkt/s lifetime avg
23101403295932 input Bytes
14352944 Bps 30s avg, 21414993 Bps lifetime avg
very few retransmits compared to yours ... Min Retransmit Time is default value (100).
As said by Dion, I use rrdtool (a GREAT tool, thanks Tobi !) and my data collection tool is .... as simple as snmpwalk on FCIP MIB.
One thing you may consider : how did you configure MTU size on IP interfaces ?
12-09-2011 08:32 AM
You rate is 12% of retransmissions. This is too much. Actually retransmissions on the TCP protocol are normal and expected but in your case retransmissions are too high.
This is because poor quality of the link. Network issue. Talk to your IP network administrator to solve the issue.