From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dan Cross <cross@math.psu.edu>
Message-Id: <200111261922.OAA12040@augusta.math.psu.edu>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] Nagle algorithm
In-Reply-To: <ytu1vhu6xb.fsf@cesium.clock.org>
References: <20011123115816.4863819A7E@mail.cse.psu.edu>
Cc:
Date: Mon, 26 Nov 2001 14:22:13 -0500
Topicbox-Message-UUID: 2a336362-eaca-11e9-9e20-41e7f4b1d025

In article <ytu1vhu6xb.fsf@cesium.clock.org> you write:
>forsyth@caldo.demon.co.uk writes:
>> ...  Nagle's algorithm for coalescing short segments,
>> which is the one that seems ill-advised to me
>
>So, how many packets should the following code segment generate?
>
>    #include <unistd.h>
>    /* for int usleep(useconds_t microseconds); */
>
>    for(i=0; i < 200; i++) {
>             write(tcpfd, "a", 1);
>             usleep(100);
>    }

Err, well, 200.  Or less if the TCP implementation can coallese packets
without incurring any additional latency.  But that's what you wanted,
right?  It's what you wrote.

If you wanted to buffer 200 ``a's'' before sending them, why not do that
explicitly?

	#include <stdio.h>
	#include <unistd.h>

	FILE	*fd = fdopen(tcpfd, "w");
	for (i = 0; i < 200; i++) {
		fwrite("a", 1, 1, fd);
		usleep(100);
	}
	fflush(fd);

If I'm not mistaken, that achieves the same effect, with far less
overhead.  The Plan 9 way might be to write a file system that overlays
/net/tcp and introduces a buffering layer, kind of like what ssl(3)
does.

>> i do find that quite a lot of this smacks of trying to
>> compensate for inadequate data provided by the protocol or the network.
>
>Abstraction and information-hiding are usually considered helpful.

Right!  So why do I want my TCP trying to figure out what my application
is doing?  All of a sudden, I'm no longer hiding information from the TCP;
my abstraction is gone.

>Who wants to muck around with RTT and pMTU in an application
>which just might happen to write to a TCP connection?

Why does the application care?  I can pick a reasonable default; say,
8 or 16 or 32 or even 64 KB.

>Perhaps cat(1) isn't complicated enough yet?

cat(1) picks a reasonable default.  :-)

	- Dan C.