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I am currently researching jumbo ethernet frames.

There are a lot of articles, etc. out there that explain what jumboframes are and how to enable them, but there seem to be no statistics on how widely they are used. Personally I have never seen them used in a real setting so I am wondering if I am just watching the wrong networks, or if jumboframes are generally not used (and then: why? Are devices unable to handle jumbo frames still an issue today?)

Why are jumbo frames limited to 9000 Bytes? I think i read something about this being related to limitations of the 32 Bit CRC sum (not sure where I read that), but the way I see it one is already changing the frame format to use jumboframes and could just as well change the length of the checksum too, to overcome this limitation.

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Personaly i have never seen them used in a real setting so i am wondering if i am just watching the wrong networks, or if jumboframes are generally not used (and then: why? Are devices unable to handle jubo frames still an issue today?).

Jumbo frames are practically non-existent on the internet at large; however, many companies make heavy use of jumbo frames in private LAN and WANs. The most common application is for database / storage traffic to make the links / servers more efficient.

The max link efficiency at 1500 byte MTUs is about 95% (assuming TCP traffic) vs 99% at 9000 byte MTUs:

  • Ethernet includes 20 bytes of framing overhead (Preamble + SFD + IPG)
  • The ethernet header is 18 bytes
  • IP and TCP transmit an additional 40+ bytes of headers (20 Bytes for IP header and at least 20 bytes for TCP header)
  • CRC is 4 bytes

Ethernet payload efficiency at 1500-byte MTUs

The efficiency at 1500 bytes (assuming no Vlan tag) is: 94.93%

(data_size) / (ethernet_frame_size_on_wire)
(1500 - 40) / (18 + 1500 + 4 + 20) = 1460 / 1538 = 0.949284

Ethernet payload efficiency at 9000-byte MTUs

The efficiency at 9000 bytes (assuming no Vlan tag) is: 99.14%

(data_size) / (ethernet_frame_size_on_wire)
(9000 - 40) / (18 + 9000 + 4 + 20) = 8960 / 9038 = 0.99136977

At GigabitEthernet rates you're getting about 42Mbps more with Jumbo frames (assuming the link operates at max capacity).

Why are jumbo frames limited to 9000 Bytes? I think i read something about this being related to limitations of the 32 Bit CRC sum

As you asserted in your question, it's mostly due to the math of a 32-bit CRC. Changing the CRC is pretty radical, since your switches and all NICs would need to use the same CRC calculation.

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It's hard to research because jumbo frames don't exist on the internet. (Internet2, maybe, but that's not a global, public network.) The primary industry use for jumbo frames is large bulk data transfers -- i.e. SAN infrastructure. Use of jumbo on a network connected to the internet is asking for problems -- PMTUd should allow it to work, but there are plenty of networks that will break it.

The frame size isn't limited to 9000. In fact, there is no standard defining jumbo frames. (for backward compatibility reasons, it will never be part of any 802 standard) I have gear that allows 16k frames. And I have some gear that will only do 5000 -- it's limitation is buffer space.

Longer frames statistically increase the probability of bit errors going undetected. The length of the checksum cannot be altered. It is a fundamental part of ethernet.If you change the FCS algorithm, it will no longer be ethernet.

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