Knowledge Base

SNA Server Sessions Lost Over 802.2 DLC Connection.

Article ID: 196947

Article Last Modified on 6/30/2004

Microsoft SNA Server 2.0
Microsoft SNA Server 2.1
Microsoft SNA Server 3.0
Microsoft SNA Server 2.11 Service Pack 1
Microsoft SNA Server 2.11 Service Pack 2
Microsoft SNA Server 3.0 Service Pack 4
Microsoft SNA Server 4.0
Microsoft SNA Server 3.0 Service Pack 2
Microsoft SNA Server 3.0 Service Pack 3
Microsoft SNA Server 4.0
Microsoft SNA Server 4.0 Service Pack 1
Microsoft Windows NT 4.0

This article was previously published under Q196947

SYMPTOMS

If SNA Server is communicating to an IBM host or AS/400 system over an 802.2 DLC connection, session loss may occur under heavy stress load. This problem has been observed most frequently when communicating over 100 MB Ethernet connections, though the problem can occur over Token Ring and 10 MB Ethernet connections as well. When this problem occurs, SNA Server may log one of the following events:

Event ID: 4
Source: SNA Server
Description: (1133) Message Sequence Error

X'1133' The node received a request with an invalid sequence number.

-or-

Event ID: 21
Source: SNA Server
Description: (1124) Negative Response Sent On Connection <name>
(SENSE = 2001)

Where sense code 2001 = Sequence Number: The sequence number received on the normal-flow request was not 1 greater than the last sequence number.

CAUSE

The SNA Server 802.2 DLC link service receives data from the Windows NT Dlc.sys driver. Under heavy load, the link service may encounter a "local busy" condition, causing the DLC driver to issue a Receiver Not Ready (RNR) frame to the remote system. When exiting an RNR condition, the DLC driver may pass up a duplicate frame to SNA Server, which may cause that SNA session to terminate unexpectedly.

RESOLUTION

This problem is fixed in Microsoft Windows NT 4.0 Service Pack 4. The updated module is Dlc.sys.

STATUS

Microsoft has confirmed this to be a problem in Windows NT 4.0, 4.0 Service Pack 1, 4.0 Service Pack 2, and 4.0 Service Pack 3. Any version of SNA Server can encounter this problem when running on these versions of Windows NT.

This problem is fixed in Windows NT 4.0 Service Pack 4.

MORE INFORMATION

In SNA Server back-to-back testing on 100 MB Ethernet connections, SNA Server can occasionally receive a "duplicate frame" in an SNA session, which can cause the session to drop. The following illustrates the reproduction scenario:

DLC app/Box #1     Box #2
==============     =====================================
              <- sends a window of frames (say, N(S) = 01 - 08)

<- DLC passes up frame 01-06,
then encounters local busy state

DLC sends RNR with N(R)= 0x06 ->

 < DLC ends local busy state >

RR, N(R) = 0x06 ->
                   <- resends N(S) = 06

<- DLC passes 0x06 to app

This causes I-frame 0x06 to be received twice by the DLC application in error. With SNA Server, the DLC data contains the SNA message including the SNA transmission header, which has its own SNA session sequence number. When this DLC problem occurs, the SNA Server then detects a duplicate SNA message, and drops the session. SNA Server has implemented a workaround to ignore such duplicate messages, though if the SNA message happens to be a segmented message (that is, the middle of an SNA basic information unit), then it's not possible to work around the problem, causing the SNA session to be dropped.

Keywords: kbbug kbfix kbqfe KB196947