dms05 14:44 11 Jan 2006

Maximum Transfer Unit size on the Internet.

Microsoft detail the MTU size in a knowledge article click here;en-us;283165

MS says the DEFAULT is 1480 bytes, packets above this size will be fragmented. Apparently AOL is considerably lower.

However the Protocol adds 28 bytes to this figure and I'm unsure if this is (i)included in the 1480 bytes or is (ii)in addition to the 1480 bytes.

I read an article published elsewhere that say it's in addition, but logic tells me it should be included in the 1480 bytes otherwise the packet size will be exceeded.

Using ping ip address> -f -l 1492 is OK but with 1493 it says it has to be defragmented. So I can either use 1492+28=1520 or 1492-28=1464. I favour the latter.

Can anyone comment?

The MS MTU table is:

Network MTU (bytes)
16 Mbps Token Ring 17914
4 Mbps Token Ring 4464
FDDI 4352
Ethernet 1500
IEEE 802.3/802.2 1492
PPPoE (WAN Miniport) 1480
X.25 576

  dms05 14:52 11 Jan 2006

MS say in another knowledge article click here :

The largest buffer that can be sent unfragmented equals the MTU minus the IP and ICMP headers (MTU-28).

So don't believe everything you read in 'paid for magazines!

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