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Old 07-31-2013, 11:12 PM   #1
psycroptic
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iperf hangs when jumbo frames are enabled


i've enabled jumbo frames on 2 linux machines by using "ifconfig ethX mtu 9000". Running "iperf -s" seems to work normally, but when I try to do an iperf test via "iperf -c x.x.x.x" from the other jumbo-enabled Linux machine, it just hangs forever; no command output, and of course nothing in any logs. Trying this:

Code:
iperf -c x.x.x.x -M 9000
yields this:

Code:
WARNING: attempt to set TCP maximum segment size to 9000, but got 536
all other network transfers of any kind work fine, it's just iperf that doesn't. any ideas?

thanks
 
Old 08-01-2013, 05:50 PM   #2
Ser Olmy
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Quote:
Originally Posted by psycroptic View Post
Code:
iperf -c x.x.x.x -M 9000
The -M parameter specifies the TCP Maximum Segment Size. This does not include the IP header, so you've just told iperf to generate 9040 byte packets, which are larger than the MTU.

That would explain this:
Quote:
Originally Posted by psycroptic View Post
Code:
WARNING: attempt to set TCP maximum segment size to 9000, but got 536
Not sure why it defaults to 536, but you should try iperf -c x.x.x.x -M 8960 and see what happens.
 
Old 08-01-2013, 06:00 PM   #3
psycroptic
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Doing that produces the same warning. Actually, I can't seem to set the -M value to anything BUT 536. Anything below or above gives the error. strange... maybe this version of iperf isn't built to handle non-default segment sizes?
 
Old 08-01-2013, 06:14 PM   #4
Ser Olmy
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That is entirely possible, but I haven't seen any other complaints about this. Are you sure the link speed on the NIC is actually 1 Gbps?

I just noticed that you're running iperf and not the newer iperf3. I just downloaded the latest version, and the -M 8960 parameter didn't return an error.
 
Old 08-01-2013, 06:23 PM   #5
psycroptic
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Quote:
Originally Posted by Ser Olmy View Post
Are you sure the link speed on the NIC is actually 1 Gbps?
Absolutely, here's a relevant line from lshw:

Code:
configuration: autonegotiation=on broadcast=yes driver=e1000e driverversion=2.3.2-k duplex=full firmware=1.8-0 ip=x.x.x.x latency=0 link=yes multicast=yes port=twisted pair speed=1Gbit/s
i'll check out iperf3, thanks
 
  


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