Bit of confusion with your OSI Layers here -
TCP deals in streams, it chops these streams into segments and then hands them to IP which builds a packet. IP packets can go up to 65536 bytes in length.
This IP packet is then handed to the media access layer which fragments the IP packet up into frames of a maximum size depending on the physical transmission media you are using.
Ethernet has always had a maximum PAYLOAD size of 1500bytes. When you add the frame itself not including preamble or interframe gap you get 1518bytes. Anything over this is a jumbo frame, which some devices support and other don't.
The source of confusion is that it was found that passing large ip packets across the internet was wastefull as internet routers that have to fragment a packet have to do it in software not in the application specific hardware they normally use for packet switching. This was leading to excessive loading on internet routers so the IETF decided to modify TCP so that it would negotiate a Maximum Segment Size to coincide with the MTU of the transmission media., so that large segments would never get sent in the first place.
Last edited by baldy3105; 11-03-2005 at 06:48 AM.
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