Path: utzoo!utgpu!watserv1!watmath!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!think.com!hsdndev!cmcl2!phri!roy
From: roy@phri.nyu.edu (Roy Smith)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: LAT
Message-ID: <1991Jan11.213512.2671@phri.nyu.edu>
Date: 11 Jan 91 21:35:12 GMT
References: <12578@hubcap.clemson.edu>
Sender: news@phri.nyu.edu (News System)
Organization: Public Health Research Institute, New York City
Lines: 30

hubcap@hubcap.clemson.edu (System Janitor) writes:
> Would anyone in this forum care to comment on these claims? [...]
> LAT causes less of a burden on the CPU and the network 

	How much of this is inherent in the protocols and how much depends
on the implementations?  Most Unix telnet servers are implemented as user
processes which use pseudo terminals to talk to login shell processes, which
is a very inefficient way to do things.  For a single character typed by the
user, you get:


	TCP/IP           pty driver           TCP/IP      kernel process
           \           /       \             /
    - - - - \ - - - - / - - - - \ - - - - - / - - - - - - - - - - - -
             \       /           \         /
	      telnetd              telnetd                user process

as the character comes in off the network, gets handed to telnetd which
turns it around to give to the pty driver which generates the echo and the
echo'ed character is handed back to telnetd which sends it back out onto the
network again; 4 context switches for each character typed.  Some time ago I
remember reading about somebody (Rick Ace?) who had a neat hack which did
most of telnet entirely in the kernal, only invoking the user-space telnetd
when a IAC character was encounted.  I wonder how that implementation would
stack up against LAT for CPU utilization?
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy
"Arcane?  Did you say arcane?  It wouldn't be Unix if it wasn't arcane!"