Recently I ran into a snafu with Ethernet jumbo frames that was quite annoying while trying to speed up network performance. This particular LAN has a mix of operating systems with a server running Ubuntu Linux offering Samba file sharing and NX desktop sessions to Windows users. The client PCs are running various versions of Windows (Vista and 7, both 32-bit and 64-bit versions) all with Gigabit Ethernet adapters that have drivers which claim to support jumbo frames.
Gigabit Ethernet Usually A Nice Performance Boost
Gigabit Ethernet offers 10 times the performance of Fast Ethernet. But you can often get an additional 10% or more performance boost by turning on jumbo frames when your network hardware supports its.
So I turned on jumbo frame support for the NICs on each computer as the switches on the network all support jumbo frames. On most of the computers I got a nice performance boost for big file transfers. Fast Ethernet tops out around 10 megabytes per second transfer for Samba file copies. File copying via Gigabit Ethernet, at least on this network and server, runs from about seven to twelve times faster depending upon the particular client computer, driver settings, etc.
But one of the computers running Windows Vista 32-bit with a Realtek Gigabit Ethernet PCI network interface card no longer worked well on the network. Trying to access network shares produced strange pauses and very slow performance.
Since there’s another Windows Vista 32-bit computer on the same network with the same NIC that did not have these problems, my first inclination was that it must not be a hardware incompatibility or driver problem.
Weirdly, the Samba file sharing seemed to be messed up much worse for some shares than others even though all the shares were on the same Linux server. Certain shares could not be accessed at all. The PC’s file explorer would virtually hang for more than a minute. Other shares would work so that browsing directories was fine, fooling me into thinking this was a security issue such as wrong permissions or passwords somewhere.
After examining the server settings, I could not find any explanation for why the permissions or passwords would be a problem. Further experimenting, I noticed shares which were a problem on this PC worked OK from other PCs with the same user and password for the network access.
Next, I noticed when accessing the Linux server for Linux desktop sessions via Free NX Server and NX Client on this PC, the NX logins timed out and failed. However, it worked just the same as usual for Internet access via a web browser.
When I tried to verify the shares on which directory browsing was working on this PC were in fact functional, I found that even file copies of small files were very slow, taking half a minute or more to copy files around a megabyte in size.
Clearly something was really wrong as even Fast Ethernet should be far quicker than this.
Jumbo Frame Network Driver Problems
As it turns out, the security settings such as ownership of files, permissions, and passwords were not the problem. The trouble came from how this particular combination of network card and driver apparently do not work well when jumbo frames are enabled. The network card is a TrendNET model TEG-PCITXR hardware version 3.0 using a Realtek PCI 32-bit Gigabit Ethernet Controller model RTL8169SC. The driver is the Realtek PCI GBE Family Controller version 6.241.623.2010 dated June 23, 2010.
After some searching, I found there’s a newer version of the Vista driver version 6.25 dated December 1, 2011. I downloaded it from the Realtek website as Microsoft’s automatic driver update didn’t find it. After installation, in the network adapter properties it identifies itself as version 6.250.908.2011 dated September 8, 2011. I turned jumbo frames 7K back on (this NIC only supports up to 7K, the rest of the network hardware support 9K frames) and it seemed to be working OK without the weird pauses, hangs, and slow performance. To be sure, I rebooted the computer and checked the settings again. Still fine and still using 7K jumbo frames.
Why Did The Other Computer’s Networking Not Hang and Slow Down?
I went back to the other Vista 32-bit computer to check its driver. It had the old version and its jumbo frames were set to 7K. So why was it working? Trying some file copying from the server, I noticed that it was not running as fast as it should be copying large test files. It was acting more like it was running Fast Ethernet rather than Gigabit Ethernet. Sure enough, for some reason the link negotiation for this NIC was setting it to run at 100 Mbps rather than 1 Gbps. That would turn off jumbo frames, too, as jumbo frames were not supported on Fast Ethernet.
So I updated the driver and proceeded to try to force the link negotation to 1 Gbps. Still it runs only at Fast Ethernet speed. Very strange as the switch port is gigabit and the cable is category 6 wire, the same as another cable for a nearby computer running Windows 7 that is running gigabit speed just fine. This computer isn’t heavily used with large files on the network so I’ll get around to troubleshooting this network problem later as nobody is likely to notice the slower speed for now.
The lesson to this experience is that turning on jumbo frames on your network may require a bit of testing and troubleshooting to verify that everything is indeed operating correctly. Don’t expect it to be a slam-dunk change even if you have all jumbo frames capable network hardware. If you see weird behavior, try backing out the jumbo frames on the computers with the problems to see if that helps. If it does, try a newer network interface driver and don’t depend upon Windows Update to find it for you. If that doesn’t help, you’re probably best just sticking with regular size frames on those particular computers.
