Difference between Ethernet 2 and 802.3 Frame per the Ethernet FAQ

Difference between Ethernet 2 and 802.3 Frame per the Ethernet FAQ
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Subject Author Date
Difference between Ethernet 2 and 802.3 Frame per the Ethernet FAQ newbie123 07-28-2006
Posted by newbie123 on July 28, 2006, 9:02 am
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Hi All,

This question is probably trivial to everyone else but I am unable to
understand so please don't flame just in case.

Per the Ethernet FAQ it states that the difference between the frame
types is:

"In the 802.3 frame type, this two-byte field after the source address
is a length field specifying the number of bytes in the LLC and data
fields. If these two bytes are greater than 05DC hex (1500 decimal),
the frame is a Version 2 frame. Since all type fields are greater than
1500 decimal (the maximum Ethernet frame size), both frame types can
easily coexist on the same network"

Now in the Ethernet 2 Frame the type field is 2 bytes and since I
believe that is the most common format on most LAN's we see eg: 0800
(Hex) for ip. But per the statement quoted above it says that in the
802.3 header the length field specifies the number of bytes in the LLC
+ in the Data field. Now the LLC header I think is the SSAP and the
DSAP which are 1 byte each but adding the Data portion would be well
over the 2 bytes. Confused as to how it would store this.

Another question related to the same frame type is that in the FAQ and
other places it gives 4 frame formats. Now when it says difference
between Ethernet 2 and 802.3 is it referring to the 802.3 novell 'raw'
or to the combination of the 802.2 LLC and the 802.2 SNAP frame type.

Thank you for your help


NMFall 20%
Posted by Walter Roberson on July 28, 2006, 9:32 am
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>"In the 802.3 frame type, this two-byte field after the source address
>is a length field specifying the number of bytes in the LLC and data
>fields. If these two bytes are greater than 05DC hex (1500 decimal),
>the frame is a Version 2 frame.

In other words, if the field is up to 1500 then this frame is
a 802.3 frame, and the field is a length for the rest of the frame.
If the filed is more than 1500 then the frame is an 802.2 frame and
the field has frame type information instead of a length.


>Now in the Ethernet 2 Frame the type field is 2 bytes and since I
>believe that is the most common format on most LAN's we see eg: 0800
>(Hex) for ip. But per the statement quoted above it says that in the
>802.3 header the length field specifies the number of bytes in the LLC
>+ in the Data field. Now the LLC header I think is the SSAP and the
>DSAP which are 1 byte each but adding the Data portion would be well
>over the 2 bytes. Confused as to how it would store this.

It's a total size. The LLC header is either constant length or
contains additional size information (I don't recall which at the
moment.)

Posted by newbie123 on August 1, 2006, 8:54 am
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Hi Walter,

can you please elaborate on what you mean when you say " the field is a
length for the rest of the frame". Isn't it 1500 bytes and isn't that
the max size of the ethernet frame including data etc. Data can be
about 1460 + TCP Header + IP Header = 1500. How can the total frame
ever be above 1500. Thx

Walter Roberson wrote:
>
> >"In the 802.3 frame type, this two-byte field after the source address
> >is a length field specifying the number of bytes in the LLC and data
> >fields. If these two bytes are greater than 05DC hex (1500 decimal),
> >the frame is a Version 2 frame.
>
> In other words, if the field is up to 1500 then this frame is
> a 802.3 frame, and the field is a length for the rest of the frame.
> If the filed is more than 1500 then the frame is an 802.2 frame and
> the field has frame type information instead of a length.
>
>
> >Now in the Ethernet 2 Frame the type field is 2 bytes and since I
> >believe that is the most common format on most LAN's we see eg: 0800
> >(Hex) for ip. But per the statement quoted above it says that in the
> >802.3 header the length field specifies the number of bytes in the LLC
> >+ in the Data field. Now the LLC header I think is the SSAP and the
> >DSAP which are 1 byte each but adding the Data portion would be well
> >over the 2 bytes. Confused as to how it would store this.
>
> It's a total size. The LLC header is either constant length or
> contains additional size information (I don't recall which at the
> moment.)


Posted by anoop on July 28, 2006, 2:24 pm
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newbie123 wrote:
>
> This question is probably trivial to everyone else but I am unable to
> understand so please don't flame just in case.
>
> Per the Ethernet FAQ it states that the difference between the frame
> types is:
>
> "In the 802.3 frame type, ...

Perhaps this page will answer your questions:
http://www.geocities.com/billalexander/ethernet.html.
It shows all of the frame formats very clearly.

> Another question related to the same frame type is that in the FAQ and
> other places it gives 4 frame formats. Now when it says difference
> between Ethernet 2 and 802.3 is it referring to the 802.3 novell 'raw'
> or to the combination of the 802.2 LLC and the 802.2 SNAP frame type.

I'm not sure what the FAQ is saying but if it's calling differences
between Ethernet 2 and 802.3, it's probably referring to 802.3 as
a combination of the all of them (since they are all 802.3 frames).
If you have a specific context, post that - it would be easier
to point out what they mean in that instance.

Anoop


Posted by newbie123 on August 1, 2006, 9:13 am
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Hi Anoop,

Thanks for your reply. I was referring to the answer given in general
to the question "what is the difference between Ethernet 2 and 802.3".
Are they referring to just the 802.3 format or are they referring to
802.3 SNAP or 802.3 novell raw.

Also when reading through the FAQ again and also looking at the
different frame types specified in your link just had another question

What is the 802.2 LLC? I mean that isn't a frame type. The Faq says
that the 802.3 frame type was developed to be used with 802.2 LLC. The
difference between the ethernet 2 and 802.3 Frame type with 802.2 LLC
is the addition of the DSAP and SSAP. Just wondering why introduce the
same field split into 2. The type field specifies the same information
in the ethernet 2 format. The 802.3 SNAP makes more sense as it allows
for future expansion since now you can specify the OUI and the protocol
type and than have 256 protocols under each category and OUI.

I know I guess I am asking way too many questions. Just trying to get a
better understanding of the fundamentals since it is a grey area for
me. Thanks for your help.


anoop wrote:
> newbie123 wrote:
> >
> > This question is probably trivial to everyone else but I am unable to
> > understand so please don't flame just in case.
> >
> > Per the Ethernet FAQ it states that the difference between the frame
> > types is:
> >
> > "In the 802.3 frame type, ...
>
> Perhaps this page will answer your questions:
> http://www.geocities.com/billalexander/ethernet.html.
> It shows all of the frame formats very clearly.
>
> > Another question related to the same frame type is that in the FAQ and
> > other places it gives 4 frame formats. Now when it says difference
> > between Ethernet 2 and 802.3 is it referring to the 802.3 novell 'raw'
> > or to the combination of the 802.2 LLC and the 802.2 SNAP frame type.
>
> I'm not sure what the FAQ is saying but if it's calling differences
> between Ethernet 2 and 802.3, it's probably referring to 802.3 as
> a combination of the all of them (since they are all 802.3 frames).
> If you have a specific context, post that - it would be easier
> to point out what they mean in that instance.
>
> Anoop


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