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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.