[ih] TCP PUSH flag

[David Finnigan]

Following is something of a short essay that I researched and wrote in 
March. Posting here because it may invoke some comment or interesting 
discussion.

Correction and clarifications always welcome, too! :-)

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What to do with the TCP PUSH flag?

Yesterday I spent a lot of time researching the TCP PUSH flag, its 
purpose, and whether it has any relevance today. The PUSH flag evolved 
out of the End of Letter (EOL) flag during the development of TCP in the 
late 1970s and early 1980s. RFC 793, dated September 1981 tells us that 
"A push causes the TCPs to promptly forward and deliver data up to that 
point to the receiver. The exact push point might not be visible to the 
receiving user and the push function does not supply a record boundary 
marker." And further summarizes that "The purpose of push function and 
the PUSH flag is to push data through from the sending user to the 
receiving user.  It does not provide a record service."

Later on, RFC 793 describes the TCP Send function as containing a PUSH 
flag argument. When this argument is set, "the data must be transmitted 
promptly to the receiver, and the PUSH bit will be set in the last TCP 
segment created from the buffer.  If the PUSH flag is not set, the data 
may be combined with data from subsequent SENDs for transmission 
efficiency." In its description of a TCP Receive call, the document 
states that the PUSH flag may be returned as part of this call.

That was the specification in 1981, but the varying implementations 
differed. Some implementations of TCP would not return a buffer of 
received data to the reading application if the buffer were not 
completely full. The TCP would only send a partial buffer of received 
data if the PUSH flag accompanied it. The TCP and SMTP implementations 
on TOPS-20 were known to have this behavior in the early 1980s.


-- Berkeley's Implementation of PUSH --

The Berkeley TCP/IP implementation took a different approach. Received 
data that fits in the receive window is always made available to a 
reading socket, regardless of whether the PUSH flag is set or not. In 
fact, for many years, the BSD TCP did nothing at all with an incoming 
PUSH flag, other than to clear it when trimming a segment to fit the 
receive window.

The rationale, so far as I can surmise, is because Berkeley's TCP and 
sockets interface do not withhold any received, valid, in-window and 
in-order data from a receiving application.

Upon output, BSD only set the PSH bit in the segment header if it 
emptied the socket's send buffer. There was no user-accessible mechanism 
to set or clear the PUSH flag upon sending data, nor was the application 
provided any means to detect whether the PUSH flag had been set for 
incoming data.


-- RFC 1122 makes user-control of the PUSH flag optional --

RFC 1122, issued October 1989, clarified the PUSH flag in its section 
4.2.2.2, stating that "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 PSH bit in the last 
buffered segment (i.e., when there is no more queued data to be sent)."

Just as RFC1122 made a user-accessible PUSH flag an optional part of the 
Send call, this RFC also made signaling a PUSH flag to a receiving 
application optional, saying "Passing a received PSH flag to the 
application layer is now OPTIONAL."

BSD's implementation and use of TCP's PUSH function was thus 
standardized.

Thirty years later, the PUSH flag is still largely ignored by 
BSD-derived TCP implementations. With just one exception of which I am 
aware. The TCP in Mac OS X has a congestion control module that uses the 
PUSH flag as part of a heuristic to control delayed ACKs. The subroutine 
tcp_cc_delay_ack() in file tcp_cc.c has a comment reading "If TH_PUSH is 
set, take this as a clue that we need to ACK with no delay. This helps 
higher level protocols who won't send us more data even if the window is 
open because their last "segment" hasn't been ACKed."  NetBSD has a 
similar ACK-on-PUSH option which will refrain from delaying an ACK to an 
incoming segment which has the PSH bit set.

Those, so far as I know, are the only practical uses of the PUSH flag on 
incoming TCP segments in BSD-derived TCP implementations.

So, for my implementation of TCP, what should I do with the PUSH flag? 
Is it, like the URGENT flag, effectively deprecated? Is it just a relic 
of the early 1980s? Or should I just model Berkeley TCP's use of it, 
setting the PSH bit automatically when transmitting segments?


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-David Finnigan