Winsock App's Reject Connection Requests with Reset Frames

• Microsoft Windows NT operating system version 3.1
• Microsoft Windows NT Advanced Server version 3.1

SYMPTOM

If a Windows NT TCP/IP Winsock server application (for example, Microsoft NT SQL Server) is bombarded with multiple simultaneous TCP\IP connection requests, it quickly begins to reject requests by replying with a TCP\IP Reset Frame.

The symptom may be difficult to detect. If you trace this problem with a protocol analyzer you will see that the 7th or 8th TCP SYN connection request frames are replied to with TCP Reset bit set, which tells the sending station that the frame was received but the server did not have the resources to process the connection request.

This might show up as a delayed or slower connection. The client should attempt the connection again after waiting for a timeout (so you don't see an error) but this can slow the connection in certain types of systems.

CAUSE

A Winsock application accepts connections on a port by calling a Listen() function, which has a Backlog parameter specifying the maximum length of the pending-connection queue. The Winsock specification defines the maximum Listen() backlog at 5 and when this is exceeded, TCP/IP issues a Reset.

RESOLUTION

In Windows NT 3.1 Service Pack 3 the backlog limit has been increased from 5 to 60, Socket applications now can accept 60 simultaneous connection requests without issuing a reset.

In Windows NT 3.5x the backlog limit has been increased to 100. In Windows NT Server version 4.0 the backlog limit has been increased to 200. In Windows NT Workstation version 4.0 the backlog limit has been kept at the Winsock specification of 5.

NOTE: This fix applies only to the MAXIMUM allowed Listen() Backlog. Each Winsock application must request its own Listen() Backlog queue length.

STATUS

Microsoft has confirmed this to be a problem in Windows NT and Windows NT Advanced Server version 3.1. This problem has been corrected in the latest U.S. Service Pack for Windows NT and Windows NT Advanced Server version 3.1. 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

REFERENCES

The Winsock specification (available on ftp://ftp.microsoft.com/bussys/winsock), has more information about Winsock and the listen() function. See Visual C++ documentation as well.

"Internetworking with TCP\IP" by Comer, has more information about TCP\IP connections.