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Protocol Analysis Techniques for Type II LLC

July 12, 2019

Type 2 LLC Troubleshooting

The analysis of Type 2 LLC is simply a matter of following the conversation from SABME to DM (Disconnect Mode; the last frame in a normal LLC termination sequence). You can track the sequence numbers and acknowledgments. The sequence numbers number the frames.

There are two error commands that might occur. These are:

  • REJ Reject; the other station sent the wrong sequence number
  • FRMR Frame Reject; the other station sent an invalid frame or sequence number

REJ’s are normal; they simply ask for a retransmission when a frame is lost.

FRMR’s are bad; it probably indicates a bug in a device driver or other catastrophic problem with system memory or processing in the device to which the FRMR is being sent. It could be sent in error by a device that was, itself, experiencing a catastrophic problem.

You should not see more than one REJ per minute or more than one per megabyte of data (if the data rate is greater than 1MB/minute).

LLC connections cross bridges and repeaters but they terminate at routers. A frame could be lost between bridges in a network. Station #1 could send frames in order: 0,1,2,3,4,5 but Station #2 might receive them as: 0,1,2,4,5. Frame number 3 was corrupted in transit. Station #2 would then send:

REJ NR=3 P

This is asking for a retransmission of all data starting with frame #3. After a REJ we normally see the two stations having a ‘little talk’. They exchange NR numbers once or twice after which we expect to see the recovery operation take place.

When neither side of a connection has any data to send then the LLC connection enters a ‘keep-alive’ state in which the stations periodically report ‘RR’ NR=, and they exchange their NR values.

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