Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 12 Feb 2001 21:28:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 12 Feb 2001 21:28:12 -0500 Received: from kea.grace.cri.nz ([131.203.4.51]:7697 "EHLO kea.grace.cri.nz") by vger.kernel.org with ESMTP id ; Mon, 12 Feb 2001 21:28:06 -0500 Date: Mon, 12 Feb 2001 21:27:24 -0500 (EST) Message-Id: <200102130227.VAA06389@whio.grace.cri.nz> From: roger@kea.grace.cri.nz To: linux-kernel@vger.kernel.org CC: roger@kea.grace.cri.nz, dwmw2@infradead.org, R.Bain@comnet.co.nz Subject: Problem with Netscape/Linux v.2.4.x [repeat] (MTU problem??) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Symptoms: The browser (Netscape or Lynx) will not download from remote web sites (dynamic ppp connection via external modem). This is a second post. The problem is still not resolved, but can now be described in more detail, thanks to help given by David Woodhouse (and others) and my ISP. Description: Typically Netscape/Lynx will connect to a remote site but will not download (it will hang indefinitely). When the browser is in such a hung state I am still able to ping/telnet/ftp to the URL. I have no difficulty browsing with Linux 2.2.16. The problem only occurs with the 2.4.x kernels (2.4.0, 2.4.1). My ISP operates a "transparent proxy server". According to tcpdump TX packets from my machine are passed on by the proxy server to the remote site, and the response from the latter is also logged by the server and passed on to me. However at that point there is no further traffic from the proxy server. This looks to be a problem for my PC and the 2.4.x kernel, however the proxy server is also involved for the following reason: although the browser is locked for almost all remote sites, I _am_ able to connect to (the web page of) the proxy server itself. And after I do this the browser is *unlocked*, and I can connect/download from any web address. However this only lasts for 5 minutes or so, after which time I must reconnect to the ISP proxy server. It is as though some information has been cached and then lost after a time?? Now I include a note from my ISP: >Roger, as we discussed I think the problem is to do with the MTU being = >used for TCP connections in combination with the 2.4.1 kernel and PPP. > >At any rate, what we have found from the packet dumps are when you use = >kernel 2.2.16 and you set the MTU at 552 our cache receives SYN packets = >from your host with a "mss" option set at 512 (MTU =3D 552 - IP header = >(20) - TCP header (20)) (and here is a packet dump of that): > >19:29:33.146337 131.203.xxx.yyy.1028 > www.google.com.www: S = >1878153551:1878153551(0) win 15872 614080,nop,wscale 0> (DF) > >however, when your 2.4.1 kernel also set with an MTU of 552 does the = >same thing, we find a "mss" option set at 536 (MTU =3D 576 - IP header - = >TCP header) not 552! Here is the packet trace: > >19:34:17.559674 131.203.xxx.yyy.32771 > www.google.com.www: S = >2178626299:2178626299(0) win 2144 175390,nop,wscale 0> (DF) > >There is more in the trace that indicates packets with data segment = >sizes of 536 are not getting through, and when the data segment drops to = >468 it does get through, likewise with the 2.2.16 kernel packets only = >get as big as 512 and they all get through ok. > >This indicates that although the MTU is being set to 552, this is being = >ignored by the 2.4.1 kernel and it is using 576 instead. Kernel 2.2.16 = >correctly uses the 552 as specified. This is as far as my understanding of the situation reaches. There appear to be 3 interdependent elements: (1) the web browser (2) the 2.4.x kernel (3) the ISP transparent proxy server Can anyone throw further light on this problem and/or suggest how to fix it? I'll say straight away that it has nothing to do with ECN since this has not been selected as a kernel option. Our analysis seems to suggest that with 2.4.x the MTU is being incorrectly set, but I don't know whether this is the whole explanation. Thanks for any help you can provide... Roger Young. (roger@maths.grace.cri.nz) ................................................................... Motherboard: GA-6VX7-4X with Via Apollo Pro AGP chipset CPU: P3/733 MHz Memory: 256 Mb SDRAM Modem: Dynalink 56K external modem. Serial port IRQ4, I/O 03F8-03FF Distribution: Slackware 7.1 Linux kernel(s): 2.4.1/2.4.0/2.2.16 PPP: 2.4.0. MTU 552 Netscape: 4.76 XFree86: 4.0.2 modutils: 2.4.1 binutils: 2.10.1 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://vger.kernel.org/lkml/