From owner-freebsd-hackers Fri Jun 19 23:46:08 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id XAA19403 for freebsd-hackers-outgoing; Fri, 19 Jun 1998 23:46:08 -0700 (PDT) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from stennis.ca.sandia.gov (stennis.ca.sandia.gov [146.246.243.44]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id XAA19395 for ; Fri, 19 Jun 1998 23:46:05 -0700 (PDT) (envelope-from bmah@stennis.ca.sandia.gov) Received: (from bmah@localhost) by stennis.ca.sandia.gov (8.9.0/8.9.0) id XAA24111; Fri, 19 Jun 1998 23:44:51 -0700 (PDT) Message-Id: <199806200644.XAA24111@stennis.ca.sandia.gov> X-Mailer: exmh version 2.0.2 2/24/98 To: spork Cc: Joel Ray Holveck , opsys@mail.webspan.net, root@bmccane.maxbaud.net, hackers@FreeBSD.ORG Subject: Re: TweakDUN In-Reply-To: Your message of "Fri, 19 Jun 1998 22:17:09 EDT." From: bmah@CA.Sandia.GOV (Bruce A. Mah) Reply-To: bmah@CA.Sandia.GOV X-Face: g~c`.{#4q0"(V*b#g[i~rXgm*w;:nMfz%_RZLma)UgGN&=j`5vXoU^@n5v4:OO)c["!w)nD/!!~e4Sj7LiT'6*wZ83454H""lb{CC%T37O!!'S$S&D}sem7I[A 2V%N&+ X-Url: http://www.ca.sandia.gov/~bmah/ Mime-Version: 1.0 Content-Type: multipart/signed; boundary="==_Exmh_-450885312P"; micalg=pgp-md5; protocol="application/pgp-signature" Content-Transfer-Encoding: 7bit Date: Fri, 19 Jun 1998 23:44:51 -0700 Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG --==_Exmh_-450885312P Content-Type: text/plain; charset=us-ascii If memory serves me right, spork wrote: > modem-> dialup PPP 1500 -> term server -> ethernet 1500 -> router -> T1(s) > HDLC 1500 -> core router -> fast ethernet 1500 -> upstream's border > router -> FDDI 40?? -> upstream core router -> ATM/SONET/whatever ? FDDI: 4192? ATM: 9188 (usually) I don't use dialups very often nowadays, but I dimly remember trying to negotiate a *smaller* MTU on a downlink, in order to try to get better interactive performance (mumble mumble, use IP TOS bits and smarter queueing, mumble mumble). > Generally, one avoids small MTUs on big links, I beleive. ATM's small > cell size makes *every* packet get fragmented at layer 2, but I'm not sure > that's even relevant. You're right, it's not relevent. :-) The IP layer doesn't see the segmentation-and-reassembly to/from ATM cells (thank goodness...think of having a 48-byte MTU). The ATM MTU of ~9KB was picked to be the same as that of IP over SMDS. > Anyone else? I've never heard of the oft quoted "Internet standard MTU of > 576"... RFC 1122 says that Internet hosts need to be able to *reassemble* a packet of *at least* 576 octets, but that if you're sending data and you don't have any other MTU information, you should assume an MTU of at most 576. ("Other MTU information" includes knowing the link MTU if you're sending to a directly-connected host, Path MTU Discovery results, and so on.) This discussion reminds me of an ISP that I once consulted for. The users kept doing various tweaks to Trumpet WinSock's PPP driver, including setting odd-valued MTUs and TCP MSSs that had no relation to the MTUs. Unfortunately, a lot of these folks didn't really know what they were doing. The results of these tunings were pretty bizarre (TCP/IP Illustrated, Volume 3 has a chapter entitled "Packets Arriving at a Web Server", or some such thing, which gives some similar results). Cheers, Bruce. --==_Exmh_-450885312P Content-Type: application/pgp-signature -----BEGIN PGP MESSAGE----- Version: 2.6.2 iQCVAwUBNYtaYqjOOi0j7CY9AQEl9wP+PEncErdUExEVwtMAx2xhg08y931MvAkd FlgndVg+pdPmck3+4kHipaSAXHTcJSHi8nxZ0967CQgo9y1HUFgh3k0XQcbPbNvk jW+OV8mn2CBrzxJoa6bw9JS50M2Gg3hzWdyXs8FZfju9v+IHQ9J1/2m4h+rUpN8J xKvDxvJXx+k= =Pkaw -----END PGP MESSAGE----- --==_Exmh_-450885312P-- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message