Preventing SNA Server Keepalives on Idle LAN Connections (150511)

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,
ack: 463884666, win: 7476, src: 1026  dst: 1477  157.57.11.90 157.57.15.21
IP

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                     ..........
				

Bytes 5 and 6 (CA FE) designate an SNA Server specific keepalive.

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

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

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.

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

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

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

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

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

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