TCP/IP Clients May Intermittently Lose 3270 Sessions

• Microsoft SNA Server, versions 3.0, 3.0 SP1, 3.0 SP2, 3.0 SP3, 3.0 SP4, 4.0, 4.0 SP1, 4.0 SP2, 4.0 SP3

SYMPTOMS

SNA Clients connecting to the SNA Server computer through TCP/IP may have intermittent problems with 3270 sessions dropping. The user's emulator may go to a blank screen, or it may be returned to the mainframe sign-on screen. In either case, the session must be restarted.

The following event may be logged in the Application event log on the SNA Server computer:

Event 21
(1124) Negative Response Sent on Connection value (SENSE = 831)
SUBCODE INFORMATION:

X'1124' A negative response was sent and logged. This error normally follows a more specific error message, depending on the type of negative response sent and its cause. The SNA sense data provides details about the type of error.

ACTION: See IBM Systems Network Architecture: Formats for an explanation of the sense data.

CAUSE

When the SNA Client is configured to use the TCP/IP sockets interface to communicate with the SNA Server computer, the TCP/IP messages flowing between the computers have the "Don't Fragment" bit set. A TCP/IP router may not return a "destination unreachable" message to the host when it needs to fragment a TCP segment with the "Don't Fragment" bit set. Instead, it drops the segment, and the host gets a "destination not receiving data" message. Consequently, the host retransmits the segment until the retransmission limit is reached (the default is 5). After reaching this limit, the session is reset.

Note: This is just one cause of the Event 21 error. There may be other issues that cause this error to be logged.

RESOLUTION

On the SNA Server computer, set the following registry value to 1. The server must be restarted for the change to take affect.

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).

1. Start Registry Editor (Regedt32.exe).



2. Locate the following key in the registry:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\tcpip\parameters




3. On the Edit menu, click Add Value, and then add the following registry value:

   Value Name: EnablePMTUBDetect
   Data Type: REG_DWORD
   Value: 1




4. Quit Registry Editor.

MORE INFORMATION

When a TCP/IP segment is sent from the host to the router with the "Don't Fragment" bit set, and the segment is too large for the router to handle, the router should notify the host with a "Destination Unreachable" message. However, sometimes the router does not send the notification to the host, and instead, it drops the segment. A router that simply drops packets in this manner is called a "Black Hole" router.

Setting the registry parameter to 1 (True) enables TCP to try to detect "Black Hole" routers. With this feature enabled, TCP attempts to send segments without the "Don't Fragment" bit set if several retransmissions of a segment are unacknowledged. If the segment is acknowledged as a result, the TCP Maximum Segment Size (MSS) is decreased and the "Don't Fragment" bit is set in future packets on the connection.

For additional information on this topic, click the article numbers below to view the articles in the Microsoft Knowledge Base:

Q120642 TCP/IP and NBT Configuration Parameters for Windows NT

Q136970 PMTU Black Hole Detection Algorithm Change for Windows NT 3.51

Q159211 Diagnoses and Treatment of Black Hole Routers

Additional query words:

Keywords : sna3 sna3sp1 sna3sp2 sna3sp3 sna3sp4 sna4 sna4sp1 sna4sp2 sna4sp3
Version : WINDOWS:3.0,3.0 SP1,3.0 SP2,3.0 SP3,3.0 SP4,4.0,4.0 SP1,4.0 SP2,4.0 SP3
Platform : WINDOWS
Issue type : kbprb