Preventing SNA Server Keepalives on Idle LAN Connections

• Microsoft SNA Server for Windows NT, versions 2.11 and 2.11 Service Pack 1

SYMPTOMS

SNA Server Specific TCP/IP Keepalives

The WatchDogTimeOut parameter affects the TCP/IP timeout interval with all idle SNA client-server connections which are configured to use the TCP/IP sockets interface. These connections include sponsor, application, and Distributed Link Service (though not TN3270). The WatchDogTimeOut parameter can be configured for SNA Server, SNA distributed/remote link service, SnaBase, and for each SNA client platform (Windows NT, Windows 95, Windows 3.x).

After the WatchDogTimeOut expires, the SNA Server client or server component sends an SNA Server specific TCP/IP message directed at the remote connection end pointBoth the Push and Acknowledgment bits are set, designated by A(cknowledgment)P(ush) bits. The following is an example SNA Server specific TCP/IP keepalive frame:

1605 24.920 SNA_CLIENT snaserver TCP .AP..., len:   36, seq:    224002,

Here is the data portion of the frame:

                          24 00 00 06 CA FE 00 00 00 00   .4L...$.........
00040:  00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00   ................
00050:  00 00 00 00 00 00 1E 00 00 00                     ..........

This frame is then repeated by the TCP/IP protocol at the following times in this example:

26.781, 30.581, 38.182, 53.382, 83.802, 87.541 (time reflects seconds from beginning of capture). The delta in between each successive frame is approximately doubled.

The SNA Server WatchdogTimeOut presently has an upper limit of 3600 seconds (1 hour).

TCP/IP Specific Keepalives

TCP specific keepalives (initiated by TCP/IP itself; not initiated by a sockets application such as SNA Server) can be sent once every KeepAliveTime (defaults to 7,200,000 milliseconds or two hours), if 1) no data or 2) no SNA Server keepalives have been carried over the TCP connection. If there is no response to a TCP/IP specific keepalive, it is repeated once every KeepAliveInterval seconds. KeepAliveInterval defaults to 1 second. KeepAliveTime and KeepAliveInterval can be changed in the following subkey:

   Hkey_local_machine\system\CurrentControlSet\Services\Tcpip\Parameters

The Problem with Keepalives in certain WAN Environments

However, there are certain cases when either SNA Server specific or TCP/IP specific keepalives are not desired. One such case is when SNA Server connections are going over WAN links where link uptime is charged by unit of time, or by packet (ISDN links). In these environments, it is optimal to reduce the uptime or number of packets by reducing the overhead of keepalive packets when there is no information (user data) to be sent.

RESOLUTION

SNA Server Specific TCP/IP Keepalives

A fix was created to remove the one (two in SNA Server Service Pack 1) hour upper limit from the SNA Server WatchDogTimeOut registry/Win.ini parameter.

TCP/IP Specific Keepalives:

A KeepAlive parameter was created to disable TCP/IPs own keepalives. By default, TCP/IP will send its own keepalives on an idle connection every 2 hours when an SNA Server connection is bound to TCP/IP. A fix was made to the Windows 3.x IP transport DLL (Ipcli.dll). The Windows NT and Windows 95 IP transport DLLs already support this parameter.

The following registry entries/Win.ini entries are used to disable TCP/IP specific keepalives. The default is Yes.

Windows NT

HKLM\

  System\
    CurrentControlSet\
      Services\
        SnaBase\
          Parameters\
            SnaTcp\
              KeepAlive:REG_SZ:YES | NO

Windows 95

HKLM\

  Software\
    Microsoft\
      SnaBase\
        Parameters\
          SnaTcp\
            KeepAlive:REG_SZ:YES | NO

Windows 3.x

Win.ini file [WNAP] KeepAlive=YES | NO

Configuring the SNA Server WatchDogTimeout Parameter

The following registry entires/WIN.INI entries are used to adjust these SNA Server specific keepalives.

Windows NT

HKLM\

  System\
    CurrentControlSet\
      Services\
        SnaBase\
          Parameters\
            WatchDogTimeOut: REG_DWORD: < decimal value, in seconds >

Windows 95

HKLM\

  Software\
    Microsoft\
      SnaBase\
        Parameters\
          WatchDogTimeOut: REG_DWORD: <decimal value, in seconds>

Windows 3.x

Win.ini file [WNAP] WatchDogTimeOut= <seconds >

WARNING: Increasing the WatchDogTimeout above a few minutes on the server side may cause resource leakage if TCP/IP clients are not disconnected correctly. Disabling TCPs KeepAlive messages may also cause resource leakage, especially if the WatchDogTimeout is increased at the same time.

STATUS

Microsoft has confirmed this to be a problem in Microsoft SNA Server version 2.11 and 2.11.sp1. This problem was corrected in the latest Microsoft SNA Server 2.11 U.S. Service Pack. For information on obtaining the service pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

   S E R V P A C K