In a recent Network World “High-speed LANs” newsletter, editor Jeff Caruso posed the question: “How relevant are jumbo frames today?” He noted that, although they were never officially sanctioned by the IEEE, a significant number of switch vendors support them. I’ll answer his question by saying that they are more relevant than ever.
Foremost is that ever-faster LAN versions of Ethernet exacerbate the problem caused by Ethernet’s “official” maximum frame size being pinned at a tad more than 1,500 bytes – as it was when Ethernet’s top speed was 10M bit/sec.
The argument is all about efficiency – specifically, when transporting large files, as would be likely with applications such as desktop backup and storage-over-IP. The shorter the maximum frame size (frames and packets being used synonymously by the industry), the more frames-per-second have to be handled to fill the pipe – that is, to utilize the Ethernet transport at or near wire-speed.
Imagine getting a letter in the mail that is delivered as a stack of envelopes with each envelope containing a single word and you start to get the idea.
At 10M bit/sec it takes only about 800 frame/sec to fill the pipe. Every time we ratchet up Ethernet’s speed this number goes up accordingly. With Gigabit Ethernet it is in excess of 80,000 frame/sec – and, remember, that is using the largest frames possible. While vendors have developed ways to “mitigate” this load (using offload schemes and approaches such as interrupt coalescence) it remains a problematic workload. With jumbo frames, it takes less than 1,400 frame/sec to fill the pipe.
With 10G Ethernet, the problem gets 10 times worse.
So why aren’t vendors embracing jumbo frames? Well, they are – they are just doing it very quietly.
The newsletter cites The Tolly Group’s 2002 Switch Interoperability study as evidence of the support. In our recently completed 2003 study, interest in jumbo frames continued to grow. And multiple vendors, Dell for example, have passed our stand-alone “Tolly Verified” certification for jumbo frames support.
Tests, formal and informal, run by The Tolly Group and others have shown repeatedly the benefits of jumbo frames: higher throughput and lower CPU utilization.
While those benefits have always been desirable, it seems as though it is the storage-over-IP trend that has given religion to a lot of vendors. For the past two years on our Network World “State of the LAN” tour, switching vendors have been unanimous in their support for jumbos – some even go so far as calling it a sine qua non for running IP storage networks.
My view on why it is not standard is that it is political. Jumbos were introduced and promoted primarily by Alteon WebSystems (acquired by Nortel).
Because of that, Alteon had an early lead in implementing the feature and competitors fought back by denying it “standard” status.
Jumbo frames works, use it. Maybe now that the market has declared it viable, the IEEE will decide to do the right thing and make it a standard.
