INFO: Interpreting 802.2 DLC Outage Code from SNA Server Level 2 Trace (170777)

SUMMARY

The SNA Server 802.2 DLC link service communicates over the Windows NT DLC transport protocol to support DLC connections over Token Ring, Ethernet, or FDDI. If a 802.2 DLC connection is lost for any reason, the Windows NT DLC interface returns a DLC status code in a DLC READ response to the SNA Server 802.2 link service. This status code is interpreted by the link service and mapped to the actual "qualifier" that is reported in the Windows NT application event log as follows:

Source: SNA Server
Event ID: 23
Description:
Connection <name> using Link Service <link> failed.
Qualifier = 00xx

The qualifier codes are documented in the SNA Server Reference Guide. For 802.2 links, the following qualifiers can occur:

   29 = Remote node not active
   AE = DISC/DM received
   AF = Link lost
   AB = SABME received while connection active
   AC = FRMR (frame reject) sent
   AD = FRMR (frame reject) received

If a FRMR is causing the connection failure, SNA Server will indicate additional information about the FRMR in one of the following events:

   Event 227  FRMR received
   (Windows NT DLC received a FRMR from the remote system)

   Event 228  FRMR sent
  (Windows NT DLC sent a FRMR to the remote system)

These events include a "FRMR code" returned by DLC to indicate the cause of the FRMR.

This article describes how to interpret the DLC status code returned by the DLC READ response, which can be captured within an SNA Server 802.2 link service "Level 2" trace.

NOTE: It is not necessary to decode the READ response from a Level 2 trace to determine the cause of a failure, since the Event 23, 227, and 228 events already indicate the relevant information.

MORE INFORMATION

The Windows NT DLC programming interface (DLCAPI.DLL), supports the CCB2 DLC interface, as documented by the IBM Local Area Network Technical Reference (IBM document #SC30-3383). This interface is used by the SNA Server 802.2 link service to support SNA communication over Token Ring, Ethernet, and FDDI.

After an 802.2 DLC connection is established, the DLC interface indicates a connection failure to SNA Server by responding to a DLC READ command and indicating a DLC Status code. Here is the format of the READ structure returned by DLC:

   CCB2 Structure
   ==============

   Parameter       Byte Len  Type  Description

   CCB_ADAPTER        1      DB    Adapter number (0 or 1)
   CCB_COMMAND        1      DB    Command field (31 = READ)
   CCB_RETCODE        1      DB    Completion code
   CCB_WORK           1      DB    Adapter support software work area
   CCB_POINTER        4      DW    Queue pointer
   CCB_CMPL_FLAG      4      DD    Command completion flag
   CCB_PARM_OFFSET    2      DW    Offset to CCB2 parameter table
                                 (Pointer to READ Parameter Table below)

   [ other parameters ]

   CCB2 Parameter Table for READ
   =============================

   Parameter       Byte Len  Type  Description

   STATION_ID         2      DW   SAP station ID
   OPTION_INDICATOR   1      DB   READ option indicator
   EVENT_SET          1      DB   Set of events for notification
   EVENT              1      DB   Posting event (*)
   CRITICAL_SUBSET    1      DB   Critical event subset identifier (*)
   <reserved>         2      DB   <unused > (**)
   NOTIFICATION_FLAG  4      DD   Event user notification flag (*)
   STATION_ID         2      DW   Station ID
   DLC_STATUS_CODE    2      DW   DLC status code
   FRMR_DATA          5      DB   Five byte FRMR "reason code"
   ACCESS_PRIORITY    1      DB   New access priority if status bit 5 is "on".
   REMOTE_NODE        6      DB   6-byte remote node address
   REMOTE_SAP         1      DB   Remote SAP address
   <reserved>         1      DB   <unused>
   USER_STAT_VALUE    2      DW   User value as defined in DLC.OPEN.SAP command

(*)  Indicates a returned value  
(**) In Windows NT DLC, there are 2 unused bytes resulting from the
     use of a non-packed CCB2 structure

Using the SNA Server Trace program (SNATRACE), it is possible to capture all DLC calls made by the 802.2 link service to the Windows NT DLC interface by enabling "Level 2" message tracing. If a DLC connection failure occurs, a DLC READ command will complete, and may appear as follows:

   DLC   --------------------------------------------------
   DLC   READ               OK
   DLC   ---- DLC API control block at address 7c5e0----
   DLC    00310000 00000000 00000000 FCC50700
   DLC    D4000000 00000000 00002000
   DLC
   DLC    0004010C 08000000 01000000 01040040
   DLC    00000000 00004201 02998122 04041FC0
   DLC   --------------------------------------------------

The first two lines correspond to the CCB2 structure passed in the DLC API control block, while the last two lines correspond to the CCB parameter table for the READ command. Using the structure definition above, the parameter table can be interpreted as follows:

   0004 = station ID 
   01   = option indicator
   0C   = event set
   08   = event
   00   = critical subset
   0000 = padding (unused)
   01000000 = notification flag 
   0104 = station ID (non-Intel format = 0401)
   0040 = DLC status code (non-Intel format = 4000)
   00000000 00 = FRMR data 
   00   = access priority
   42 01 02 99 81 22 04 = remote node address
   04   = remote SAP
   1F   = reserved
   C0   = user status value

Note that the DLC status code (DW = two byte "word" value) is stored in memory in Intel format, and needs to be "flipped" to show the real value as interpreted by the 802.2 link service.

Using the DLC status code definitions below (as documented in Appendix B of the IBM LAN Technical Reference), a status code of 0x4000 is interpreted as follows:

   0x4000 (hex) = 0100 0000 0000 0000 (binary)

   Bit 14 on = DM or DISC received


   DLC Status Codes
   =================
   BIT    Function                
 
   15     Link lost
   14     DM or DISC received
   13     FRMR received  (See 5-byte FRMR data "reason code")
   12     FRMR sent (See 5-byte FRMR data "reason code")
   11     SABME received for an open link station
   10     SABME received, link station opened
   9      Remote station has entered "local busy" condition
   8      Remote station has left a "local busy" condition
   7      Ti has expired
   6      DLC counter overflow (DLC.STATISTICS should be called)
   5      Access priority lowered
   4-1    reserved
   0      Local station has entered a "local busy" condition