Microsoft KB Archive/123749

= How the TCP Push Bit Was Changed for Windows NT 3.5 =

Article ID: 123749

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Workstation 3.5
 * Microsoft Windows NT Workstation 4.0 Developer Edition
 * Microsoft Windows NT Server 3.5
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q123749



SUMMARY
The TCP/IP stack for Windows NT version 3.5 was completely redesigned. This article covers one of the many performance enhancements implemented; the improved handling of the TCP PUSH bit on the receive end.



MORE INFORMATION
The TCP protocol has a bit in every header called the PUSH bit. RFC 1122 section 4.2.2.2 contains the following text on the PUSH bit.

  4.2.2.2  Use of Push: RFC-793 Section 2.8

When an application issues a series of SEND calls without setting the PUSH flag, the TCP MAY aggregate the data internally without sending it. Similarly, when a series of     segments is received without the PUSH bit, a TCP MAY queue the data internally without passing it to the receiving application.

The PUSH bit is not a record marker and is independent of     segment boundaries. The transmitter SHOULD collapse successive PUSH bits when it packetizes data, to send the largest possible segment.

A TCP MAY implement PUSH flags on SEND calls. If PUSH flags are not implemented, then the sending TCP: (1) must not buffer data indefinitely, and (2) MUST set the PUSH bit in     the last buffered segment (i.e., when there is no more      queued data to be sent).

The discussion in RFC-793 on pages 48, 50, and 74 erroneously implies that a received PUSH flag must be passed to the application layer. Passing a received PUSH flag to     the application layer is now OPTIONAL.

An application program is logically required to set the PUSH flag in a SEND call whenever it needs to force delivery of     the data to avoid a communication deadlock. However, a TCP SHOULD send a maximum-sized segment whenever possible, to     improve performance (see Section 4.2.3.4).

DISCUSSION: When the PUSH flag is not implemented on SEND calls, i.e., when the application/TCP interface uses a pure streaming model, responsibility for aggregating any tiny data fragments to form reasonable sized segments is partially borne by the application layer.

Generally, an interactive application protocol must set the PUSH flag at least in the last SEND call in each command or response sequence. A bulk transfer protocol like FTP should set the PUSH flag on the last segment of a file or when necessary to prevent buffer deadlock.

At the receiver, the PUSH bit forces buffered data to be        delivered to the application (even if less than a full         buffer has been received). Conversely, the lack of a        PUSH bit can be used to avoid unnecessary wakeup calls to the application process; this can be an important performance optimization for large timesharing hosts. Passing the PUSH bit to the receiving application allows an analogous optimization within the application.

The receive-side use of the PUSH bit was changed in Windows NT version 3.5. In Windows NT version 3.1, TCP/IP behaved as if the PUSH bit was always set by the sender. Consequently, it completed application recv calls as soon as any data was available. In Windows NT version 3.5, the receive logic was optimized to use the PUSH bit. This reduces the number of times an application has to wake up for incoming data. However, when NT is communicating with a TCP/IP implementation (or application) that does not set the PUSH bit at appropriate times, performance can suffer.

Performance degrades because, when data arrives without the PUSH bit set, TCP holds onto the application's recv while waiting for the rest of the data. Windows NT version 3.5 TCP/IP completes a recv call when:


 * Data arrives with the PUSH bit set.

-or-
 * The user recv buffer is full.

-or-
 * 0.5 seconds have elapsed since any data arrived.

When the third complete test above is required to complete a recv call, performance degrades.

The following example shows how the improved handling of the TCP PUSH bit can improve performance:

  A client-server pair is going to transfer 4096 bytes between them on   Ethernet. The client application executes a send for 4096 bytes. Windows NT TCP/IP breaks the 4096 bytes into three Ethernet packets:

1460 bytes, no PUSH 1460 bytes, no PUSH 1176 bytes, PUSH is set

At the same time, the server executes recv, for 8192 bytes. When the first packet arrives, TCP puts the data into the user buffer. The same thing happens for the second packet since PUSH is still not set. However, when the third packet arrives, TCP completes the recv because PUSH was set.

In this scenario, holding on to the recv boosts performance because of the overhead implicit in each recv call; instead of making three calls to recv, the application makes only one, returning all 4096 bytes. If the client did not set the PUSH flag on the third packet, TCP would not send recv for another 0.5 seconds. Because some TCP/IP implementations (and applications) never set PUSH, they tend to perform poorly with Windows NT version 3.5 TCP/IP.

Additional query words: RFC1122 sec prodnt performance application sockets winsock

Keywords: kbnetwork KB123749

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