Weblog entry #5 for bacula
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Maximum Transmission Unit, the largest physical packet size, measured in bytes, that a network can transmit. Any messages larger than the MTU are divided into smaller packets before being sent.
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man ip /mtu
Really, `ifconfig' does not work with kernel >2.2 at all! It is obsolete not!
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If you're smart enough to ask this question, you're smart enough to RTFM and find out yourself.
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s/obsolete not/obsolete now/gSorry for that stupid mistake. :)
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How often do people find setting the MTU useful? I've tried it a few times for network troubleshooting (it never helped - and I've done a lot of network troubleshooting, but never needed to dig this deep in the layers - ignoring the ethernet cards that emitted backward IP addresses a.b.c.d destination octets became d.c.b.a), but generally find that the MTU defaults to the maximum value for the appropriate hardware layer, and PMTU (discovery of end to end MTU) is enabled by default.
"ip link set eth0 mtu 1500"
Having just replaced the last 10 Mbps hub in our racks today (too many collisions), we are now 100Mbps as soon as you get away from the appallingly slow Internet connection. But the newest 100Mbps firewall "just worked" on an old PC that was lying around.
The only recent network tuning I've seen to be necessary was Gigabit tuning. Here the handling of interrupts is still crucial, as modern CPUs don't like as many interrupts as the cards generate, unless you enable one of the many features to tune this down. I also looked at Jumboframes. Anyone written up gigabit ethernet (and faster) tuning. Don't think we'll be there any time soon, but it would be good to know.
Of course the limiting factor on our Gigabit network is the fact that it is using SMB protocol on some proprietary OS, which has to be the most awful protocol for network performance.
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Only once. Ever.
Once upon a time I was responsible for setting up an OpenVPN connection between two seperate offices. The end-point of the remote VPN connection was behind some fancy hardware router.
Hardware firewall was modified to allow incoming/outgoing connections on the OpenVPN port and our tunnel was successfully made.
After a few weeks we noticed that the tunnel/VPN link would randomly die when lots of traffic had passed over it. (I could force this by doing an scp of a gig of data across the link).
After numerous phone calls back and forther between my office and the remote office we were getting nowhere. Eventually it was decided that this link was "critical" and I was ordered onto a train to go down and fix it once and for all.
I went down, looked at the console logs for the hardware router and saw numerous "packet fragmentation detected - alert" messsages on the console.
Seeing this the problem was obvious. (I'd never been led to believe the firewall box doing the forwarding to the VPN client was doing anything fancy, or had accessible logs. sigh.)
I dropped the MTU on both sides of the VPN and the problem was fixed.
Three hours on a train. Five minutes to fix the problem. An hour to convince everybody it was fixed by sending/receiving a lot of data. Two hours in the pub. Then back to sunny Edinburgh.
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If you're smart enough to ask this question, you're smart enough to RTFM and find out yourself.
[ Parent | Reply to this comment ]