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Thank you for the clarification and detailed response. Here is an excerpt from wikipedia (article on 'ethernet frame'), regarding the type/length field: 'Thus if the field's value is greater than or equal to 1536, the frame must be an Ethernet v2 frame, with that field being a type field. If it's less than or equal to 1500, it must be an IEEE 802.3 frame, with that field being a length field. Values between 1500 and 1536, exclusive, are undefined.'
This seems to be in addition of the preamble, which has to do with ethernet ii (aka DIX) not making use of an SOF (start of frame) byte right after the 8-byte preamble; whereas ethernet 802.3 has a 7-byte preamble and 1 byte SOF. Is that correct? Thanks again.
Ethernet ii (aka DIX) not making use of an SOF (start of frame) byte right after the 8-byte preamble; whereas ethernet 802.3 has a 7-byte preamble and 1 byte SOF. The DIX version 1.0 spec says, in section 7.5.1.3 'Preamble Generation', that the 8-byte preamble has 7 bytes of 10101010 and 1 byte of 10101011. The 2000 version of 802.3 says in section 4.2.5 'Preamble generation' that the 7-byte preamble has 7 bytes of 10101010, followed by the Start Frame Delimiter, and says in section 3.2.2 'Start Frame Delimiter (SFD) field' that the SFD is 1 byte of 10101011.
I.e., the only difference in what they say goes on the wire is that DIX calls the 8th byte, the one with 10101011, part of the preamble, while 802.3 calls it the SFD; what actually goes on the wire is the same in both specs. It's not as if the preamble/SOF will tell you whether there's a type field or a length field - both types of frame begin with 7 bytes of 10101010, followed by 1 byte of 10101011, followed by the destination field, followed by the source field, followed by the 2-byte type/length field, followed by the payload. (I don't have a copy of DIX 2.0, so I don't know whether it speaks of an 8-byte preamble or a 7-byte preamble and 1 byte of SFD.) And the also says 'Many years later, the 802.3x-1997 standard, and later versions of the 802.3 standard, formally approved of both types of framing.' , so, as of 1997, frames with a type field and frames with a length field are both '802.3 frames'; a better terminology might be 'frames with a type field' vs.
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'frames with a length field'. The different frame formats are indicated by the preceding start frame delimiter (Preamble) which ends in '11' instead of '10' for 802.3 formatted frames. No, they're not, unless DIX version 2.0 had 10101010 as the last byte of the 8-byte sequence at the beginning of the frame.
Don't be confused by DIX 1.0 (and maybe 2.0) calling the entire 8-byte sequence the preamble and 802.3 calling the first 7 bytes the preamble and the 8th byte the SOF delimiter - what goes on the wire for DIX and 802.3 is exactly the same. I wasn't confused by preamble and SOF, I was just looking at a bad/unclear ethernet documentation as it seems. For DIX it said that the preamble is '10101010.' , so I guess the guy writing that document was just too lazy too mention that it ends with '11', not '10' as the '.' For 802.3 he mentioned the '11' at the end, so I guess that was the difference. You are probably right about the preamble being the same pattern.
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In that case the length/type field is the only way to decide what kind of frame format it is.