Random Blue Screens Caused by Bad Packet and DLC.SYS

CAUSE

An assumption was made by DLC.SYS that a Length Field (LPDU Length) in the 802.3 Ethernet header was valid. This length field should always be 3 or greater.

An IEEE 802.3 Frame has the following format.

|<------------------ Maximum Length 1532 Bytes ------------------>|
V                                                                 V
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-//--+-+-+-+-+-+-+-+
|  PREAMBLE   |S| DEST ADDR | SRC ADDR  |LP | LPDU  |Pad  |  FCS  |
|  7 Bytes    |D|  6 Bytes  |  6 Bytes  |DU | Bytes |     |4 Bytes|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-//--+-+-+-+-+-+-+-+
                                                |
                                                |
                                 /--------------/ 
      |                          |                        |
      V                          V                        V
    <-+--------+--------+--------+--------+-------//------+>
      |  DSAP  |  SSAP  |  Control Field  | Information   |
      | 1 Byte | 1 Byte |  1 or 2 Bytes   |  Field        |
    <-+--------+--------+--------+--------+-------//------+>

                     IEEE 802.3 Frame Format

The field lengths are as follows:

     * Preamble - 7 bytes
     * Start Delimiter (SD) - 1 byte
     * Destination Address (DEST ADDR)- 6 bytes
     * Source Address (SRC ADDR) - 6 bytes
     * LPDU Length (Length of the LLC Protocol Data Unit) - 2 bytes
       (This field should always have a value of 3-1500)
     * LPDU (LLC Protocol Data Unit) - variable length (3-1500 bytes)
       - Destination SAP Address (DSAP) - 1 byte
       - Source SAP Address (SSAP) - 1 byte
       - Control Field - 1 or 2 bytes
       - Informational Field - variable length (0-1497)
     * Pad - 0 to 43 bytes
     * Frame Check Sequence (FCS) - 4 bytes
				

In the case that caused the Windows NT STOP Message, a corrupt IEEE 802.3 frame was received by DLC.SYS with a LPDU Length field of 0 (which is invalid). Because every LLC Protocol Data Unit (LPDU) consists of a 1 byte DSAP, 1 byte SSAP, and at least a 1 byte Control Field, the LPDU Length should always be at least 3.