Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!ucbvax!PURDUE.EDU!sbm
From: sbm@PURDUE.EDU
Newsgroups: comp.protocols.appletalk
Subject: Re: PacerLink weirdness
Message-ID: <8812232126.AA24977@merlin.cs.purdue.edu>
Date: 23 Dec 88 21:26:34 GMT
References: <132@sickkids.UUCP>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 25

The question was about round-trip times reported by ping:

> But with the Pacer software running in the Mac, there's an unexpected
> periodicity, with near-monotonicity within each period.

     The reason for this behavior is pretty simple.  It looks like the
Pacer software uses the default socket listener, which posts an event
when a packet arrives, and checks for events about every half second,
but not exactly every half second, so that some packets will arrive just
before the event queue is checked, yielding the actual packet turnaround
time of 90-103ms, and others will arrive just after the queue was
checked, yielding the longest delay (caused by waiting for the software
to check the queue again) of 466-551ms.  Ping sends packets every
second, so you see a nice progression as the phase between ping and the
Pacer software slowly changes.

     NCSA telnet, apparently, either checks for events very often
(perhaps once every 30ms, judging from the variation of the round-trip
times), or uses its own socket listener to respond immediately,
interrupt-style, when packets arrive.

					Steve Munson
					sbm@Purdue.EDU
					sbm@Purdue.CSNET
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