Revisiting Jumbo Frames

While Jumbo Frames technology remains in standards limbo, those who have used the technology for years continue to extol the virtues of stretching out the standard Ethernet frame size to boost Gigabit Ethernet throughput.

Extending the size of Ethernet frames can increase Gigabit Ethernet throughput by as much as 50 per cent to 300 per cent in large server-to-server database transactions or IP storage applications. And while some are still calling for an official standardization of Jumbo Frames, others say it works fine as a proprietary niche technology and would be impractical for widespread adoption if standardized.

“[Jumbo Frames] is a very useful technology, and it certainly has its place,” says David Newman, president of Network Test Inc., a network consulting and equipment testing firm. “If you have boxes in a data centre and they’re made by the same vendors [with support for Jumbo Frames], I’ll design that in any day.”

Alteon WebSystems Inc. originally developed Jumbo Frames in 1997 for its network interface cards (NIC) and switches. (Nortel Networks Corp. acquired the firm in June 2000 and sold it to 3Com Corp. in the fall of that year.) The technology involves changing an Ethernet frame’s maximum transmission unit (MTU) size from the IEEE standard of 1.5KB up to 9KB. By increasing the payload of each packet, the amount of packet processing work switches and servers would have to do would decrease while throughput would go up, because more data was being passed with less headers.

Since Alteon first put the technology in its gear, several vendors have adopted their own proprietary versions of Jumbo Frames. But for Jumbo Frames to be effective, the equipment used must be from the same NIC and switch vendors. Also, the Jumbo Frame traffic cannot touch nonsupporting gear.

This reality forces users to pick and choose carefully where they run Jumbo Frames, says John Savage, chief systems engineer at Christopher Newport University in Newport News, Va.

“Right now, you can only use it in segments of your network that are only dedicated to Jumbo Frames. Any machine that doesn’t support it won’t understand it,” he says.

Savage uses Jumbo Frames with Foundry Networks Inc. switches and Alteon NICs on four of his network servers. He says the technology has improved the amount of data the database and e-mail servers can process by 50 per cent, but the switches must reside on their own subnet and virtual LAN where Jumbo Frame traffic does not touch other servers or switches.

“It definitely would be a benefit if [Jumbo Frames] were approved as a standard,” Savage says. “Until a standard is in place, it will be a lot harder to use it.”

Other users are fine with using Jumbo Frames in certain network areas with homogeneous gear, such as storage-area networks.

Jumbo Frames is used on storage devices and for moving large blocks of data between servers at Lawrence Livermore National Labs (LLNL), a U.S. Department of Energy research lab located at the University of California, Berkeley.

The school started using Jumbo Frames several years ago when the network staff first implemented Gigabit Ethernet. While large switch-to-switch throughput ran fine, they discovered that the servers could only process a trickle of the available bandwidth, says Dave Wiltzius, network division leader at the lab.

“On any of our machines, such as high-end Intel, or even Alphas, they would tap out at a couple 100Mbps,” when using standard frame sizes over Gigabit Ethernet, Wiltzius says. “By increasing the MTU size, we found that we could get anywhere between two and three times the throughput and cut down on CPU utilization.”

While the extra throughput of Jumbo Frames is good, the relief on a server’s processor and NIC are key because less processing of packets means more resources are available for the server to do what it’s supposed to – run applications and store files – instead of processing network traffic, he adds.

LLNL uses Jumbo Frames technology on its Cisco Catalyst switches and NICs from several vendors on its servers. While the proprietary nature of Jumbo Frames limits Wiltzius to using products from a few vendors end-to-end, he says this is not a problem because Jumbo Frames are only effective for a specific few applications.

“It’s not a campuswide technology,” Wiltzius says of Jumbo Frames. “It’s probably limited to 100 NICs and switch ports total.”

Because the lab only uses Jumbo Frames on its high-end machines, Wiltzius says he contains those 100 or so Jumbo Frame-enabled ports in their own Layer 2 network, physically separated from the rest of the university.

The Institute of Electrical and Electronics Engineers Inc. has mulled changing the standard MTU size for Ethernet for several years, but no formal proposals are being considered to change the standard anytime soon. Opponents of Jumbo Frame standardization say issues with backwards compatibly with Jumbo Frames is the major reason why the idea has been shelved.

Part of what makes Ethernet such a simple-to-use and relatively inexpensive technology, standards makers say, is that it can interoperate easily among equipment with different speeds, and the small frame size is part of that. Other technologies, such as ATM and SONET, let the sizes of their transmission units vary, but this adds complexity and, as a result, greater cost.

While Network Test’s Newman, who is also a Network World columnist and Test Alliance member, is not one of those opponents, he says there is a valid argument there.

“Realistically, I can see us sticking to the 1,500 [byte frame size] for the foreseeable future,” Newman says. “There is a huge installed base of legacy Ethernet equipment out there” – estimated by some to be around 200 million devices from NICs, to routers and switches. “Are we going to go throw all those away because we want to have new frame size? I don’t think so.”

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