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Jumbo Frames

May 11th, 2010
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I got a question late last night about the applicability of Baby Giants and Jumbo frames in an environment; the use of Ethernet frames above 1600 bytes and up to 9000 bytes. This had me reaching into the deep dark corners of my memory to respond. So I thought that I’d put it up here for posterity.

What are they:

Lets start with the basic Ethernet Frame. In short, and Ethernet frame is made up of a source address, a destination address, a type field, some data (the payload) and a checksum.

As you can see in the diagram below, you have 18 bytes of header and checksum with a variable payload component that can range from 46-1500 bytes; giving you a total frame size of 1518 bytes.

image source Wikipedia

Ethernet has had this frame format and payload size from about 1980 (please check the history if you want exact details) .

With the creation of Gigabit Ethernet came the ability to have bigger frames (well not really that simple, see previous link). A Baby Giant frame is any frame greater than 1600 bytes and a Jumbo frame is any Ethernet frame up to 9216 (plus header and checksum).

So why are we limited to ~9000 bytes? Part of the issue is that Ethernet uses a 32 bit CRC that loses its effectiveness above about 12000 bytes, see “32-Bit Cyclic Redundancy Codes for Internet Applications and 9000 is large enough to carry an 8 KB application datagram (e.g. NFS) plus frame header and CRC overhead.

When to use oversized frames:

There are a number of reasons people want to use Baby Giant or Jumbo frames, here are a few examples and calculations to show that they are not needed.

The real use for Jumbo frames comes in to play when looking at a couple of things.

Firstly, applications with a large datagram can take advantage of the underlying frame payload size. Applications like NFS are a great example (anything that moves large quantities of data is a contender, CIFS, ISCSI, etc). This increases your overall throughput.

Second is reducing the CPU on your network equipment. Lots of frames means lots of decisions on what to do with them. Studies have shown that CPU can be reduced by up to 50% with the use of Jumbo frames. Though now a days  modern network equipment have dedicated ASICs (Application Specific Integrated Circuit) to handle a lot of the grunt work.

Caveats

In short there are a few checks that need to be reviewed before using Jumbo Frames.

  • All devices on the same segment MUST be configured for Baby Giant or Jumbo frames.
  • Check the max MTU (payload) all devices, it will vary depending on the manufacturer and even model.
  • Have a very clear understanding why you are doing it!

References:

  1. http://en.wikipedia.org/wiki/Ethernet
  2. http://sd.wareonearth.com/~phil/jumbo.htm
  3. http://en.wikipedia.org/wiki/Jumbo_frame
  4. http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a00801350c8.shtml
  5. http://www.networkworld.com/forum/0223jumboyes.html