Activating Numerous DLC Connections Results in Inactive LUs

ID: Q216047

The information in this article applies to:

Microsoft SNA Server, versions 3.0, 3.0SP1, 3.0SP2, 3.0SP3, 4.0, 4.0SP1, 4.0SP2

IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SYMPTOMS

An SNA Server that has numerous DLC 802.2 connections may experience symptoms similar to the following if the connections are activated concurrently:

All of the connections are in an Active state, but some of the connections have Inactive 3270 Logical Units (LUs).

Some of the connections stay in an Inactive state.

The Windows NT application event log contains numerous occurrences of the following message:

Event ID: 266
Source: SNA DLC Link Service
Description: The DLC 802.2 link service could not allocate a DLC buffer for an outgoing message, message type 16.

The number of connections that can be activated before experiencing this problem will vary. In general, this may occur in environments that have 50 or more DLC 802.2 connections where each connection has many 3270 LUs defined.

CAUSE

The SNA Server DLC 802.2 link service uses an internal buffer pool to handle messages that it has to process. If this buffer pool is exhausted, the link service may discard some of the messages that is it receiving from the SNA Server service or from the DLC protocol stack. If the link service discards these messages, the symptoms described above can occur.

RESOLUTION

SNA Server 4.0

To resolve this problem, obtain the latest service pack for SNA Server version 4.0. For additional information, please see the following article in the Microsoft Knowledge Base:

Q215838 How to Obtain the Latest SNA Server Version 4.0 Service Pack

SNA Server 3.0

To resolve this problem, obtain the latest service pack for SNA Server version 3.0. For additional information, please see the following article in the Microsoft Knowledge Base:

Q184307 How to Obtain the Latest SNA Server Version 3.0 Service Pack

STATUS

Microsoft has confirmed this to be a problem in Microsoft SNA Server versions 3.0, 3.0 SP1, 3.0 SP2, 3.0 SP3, 4.0, 4.0 SP1, 4.0 SP2. This problem was first corrected in SNA Server version 3.0 Service Pack 4 and SNA Server version 4.0 Service Pack 3.

MORE INFORMATION

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys and Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

By default, the size of the internal buffer pool used by the DLC 802.2 link service is 1 MB (1,024,000 bytes). SNA Server 3.0 Service Pack 1 and later allows this buffer pool to be increased by adding the following Registry parameter:

HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/snadlcX /Parameters/ExtraParameters/LlcBufferPoolSize

snadlcX is the name of the DLC 802.2 link service (that is, snadlc1) that the buffer pool is being increased for.

This parameter may need to be increased on SNA Servers that have a large number of DLC 802.2 connections and 3270 LUs. This buffer can be exhausted by simultaneously starting a large number of these connections because of the large number of messages that have to be processed by the link service during the startup sequence. The following is a list of messages that have to be processed by the link service when connections containing 3270 LUs are started:

XID exchanges

ACTPU (Activate PU) and ACTPU Responses for each per connection

ACTLU (Activate LU) and ACTLU Responses for each LU on each connection

Notify and Notify Responses for each LU on each connection

After applying the update, the DLC 802.2 link service will allocate additional memory outside of the LLC buffer pool whenever this buffer pool is exhausted. This prevents the case where the link service discards messages when the buffer pool is exhausted. It is recommended that the LLC buffer pool is large enough to prevent the allocation of memory outside of this pool under normal operating conditions as using memory outside the LLC buffer pool for transmit buffers effects performance to some extent.

For additional information about another DLC 802.2 link service problem related to exhausting the LLC buffer pool, please see the following article in the Microsoft Knowledge Base:

Q198398 Access Violation in Snalink.exe Activating Numerous Connections

The previously referenced article also includes information on how to add and change the LLCBufferPoolSize registry parameter.

Additional query words:

Keywords : sna3sp4fix sna4sp3fix
Version : WINDOWS:3.0,3.0SP1,3.0SP2,3.0SP3,4.0,4.0SP1,4.0SP2
Platform : WINDOWS
Issue type : kbbug