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