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Jumbo Frames

May 11th, 2010 1 comment
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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.

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