Eduardo Escudero Sánchez wrote:
I have been using etehreal and i have received LLC frames.One of them
was a TEST frame (its control field was 0xf3) But ethereal says that
TEST comes with SNAP!!!!! isn't that impossible?.
So it was a test frame with SSAP=DSAP=0xAA?
Only when you receive
frames which have SSAP=DSAP=0xAA and the control byte = 0x03 (UI Frame
then ) you can have a SNAP field of 5 bytes (3 for organization code and
2 for protocol identification).
I'm wrong maybe? someone can explain me?
At least according to IEEE Std 802-2001, section "10.3.2 SNAP PDU format":
The LLC control field (CTL) is shown for PDU type UI, Unnumbered
Information, which is the most commonly used PDU type in this context;
however, other information-carrying LLC PDU types may also be used with
SNAP.
The other question i have is about analizing llc frames. how can one
knows whether the controll field of the frame is two bytes long or only
one byte?.You can know it if you know it is an unnumbered frame (1
control byte then) or a supervisory or information frame (2 control
bytes in that case), but in order to discover the type of the frame you
must see the value of the bits of the least significant byte of the
control field
No.
If the low-order bit of the *first* byte is 0, it's an I frame.
Otherwise, if the bit above that is 0, it's an S frame.
Otherwise, it's a U frame.
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