Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!bellcore!ka9q.bellcore.com!karn
From: karn@ka9q.bellcore.com (Phil Karn)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Keywords: x.25
Message-ID: <17713@bellcore.bellcore.com>
Date: 27 Sep 89 02:03:29 GMT
References: <727@idacom.UUCP> <17683@bellcore.bellcore.com> <486@castle.ed.ac.uk>
Sender: news@bellcore.bellcore.com
Reply-To: karn@ka9q.bellcore.com (Phil Karn)
Organization: Secular Humanists for No-Code
Lines: 23

>Throughput through a [JANET] switch will be several hundred megabytes a day.

That's only a few tens of kilobits per second. The US Internet is now
largely based on T-1 (1.536 megabit/sec) links, and the routers in at least
our portion of the Internet (JvNCNet) have no trouble keeping their links
full if enough traffic is offered. DS-3 (43 megabit) links are already being
talked about, and the router manufacturers say they'll be able to handle
them in a year or so. Can JANET's X.25 switches run at multi-megabit
throughputs?

>If two hosts have 64K  bits-per-sec links I can easily manage 32K bps
>throughput in the data transfer phase of FTP.

That's a good illustration of the main problem with X.25. Even with the
relatively small default 576 byte MSS, a FTP transfer with TCP/IP achieves
about 93% of the link speed in actual user data, assuming a full duplex path
and a sufficiently large window size to keep the pipe full. If there's a
performance problem in the Internet right now, it's that the TCP window
sizes on most hosts are now too small to fully utilize the new T1 links.
But TCP is a transport protocol, so it resides in the end systems where it's
much easier for me to change than if it were buried inside the network.

Phil