Date: 1 Jul 1996 22:10:41 +0200 From: "Stefan Bethke" <stefan@Promo.DE> To: "FreeBSD Hackers" <freebsd-hackers@freebsd.org> Subject: TCP time-outs on PPP line Message-ID: <n1375863387.78047@quick.Promo.DE>
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Hi, we are connected through an analog leased line with two USR Courier, using 2.1.0R and pppd. Since upgrading from Linux we experience some TCP "connection timed out" failures. It seems that at those moments someone is downloading from our HTTP or FTP server (looking at the modem leds and ftpwho/xferlog/http-log). Trying to ping the terminal server of our ISP doesn't give a response; after 5 or so pings, ping says "No buffer space available". My understanding is that the already established TCP connection quickly fills the output buffer and saturates the line; at this point, a packet inserted into the queue will be sent after (worst case) mtu / bandwith * queue length (ifq_maxlen) = 1500 / 2880 * 50 = 27 seconds. (Or, taking fastq into account, 54 secs). What's the solution? Possibilities are - reduce ifq_maxlen, so ENOBUF will be returned earlier, so TCP will (hopefully) send slower earlier; - reduce the mtu, so a full queue run will be faster, thereby allowing a new packet to be sent faster; - devise a mechanism to promote those connections we want to precede our external users' connections to the fastq; possibly by promoting all packets smaller than xxx bytes to fastq; - upgrade to a 2 Mbit line :-) I didn't test any of these yet, because I rather like to have a slow but working line... Any comments will be greatly appreciated. Stefan Bethke -- Promo Datentechnik | Tel. 040/431360-0 + Systemberatung GmbH | Fax. 040/431360-60 Waterloohain 6-8 | e-mail: stefan@Promo.DE D-22769 Hamburg | http://www.Promo.DE/
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