Hi, after some time running a custom 2.6.15 kernel, yesterday, I decided to upgrade to stock kernel26-2.6.17.11-1.
I rebooted and all seemed to work fine until I opened a browser and noticed I couldn't connect to the internet. Nothing in the system logs nor in dmesg catched my attention and, according to ifconfig, the network interface was up. So, at first, I thought my crappy DSL router had died. I tried to connect to its web interface with no luck... but I could ping it and I could ping internet sites, even using DNS names. In some moment I realized that there is one fundamental difference between ping, dns and http or ftp: ping uses ICMP and dns uses UDP, while http and ftp use TCP. After booting with an older kernel and doing some googling (without any network issue) I found the source of the problem [1]. A change in the way the kernel calculates TCP buffer sizes has caused it to use TCP window scaling by default under some circumstances. Window scaling is an old standard, but anyway it seems some modern equipment (such as my router) still can't handle it. There are several posts in the mailing list [2] and in the forums which seem to be related to this [3,4,5,6] (some regarding kernel 2.6.8, as that version introduced a similar change that was reverted or somehow fixed later). TCP window scaling can be disabled by setting the kernel parameter net.ipv4.tcp_window_scaling to 0 (I am sending this email from a 2.6.17, after adding that setting to /etc/sysctl.conf). Disabling window scaling will only slightly affect performance over high speed, long distance network links, while having it enabled will greatly affect performance much more often. So I think it would be wise to disable it in the default sysctl.conf. If someone wants window scaling (s)he can comment out the line. [1] http://kerneltrap.org/node/6723 [2] http://www.archlinux.org/pipermail/arch/2006-June/011250.html [3] http://bbs.archlinux.org/viewtopic.php?t=24248 [4] http://bbs.archlinux.org/viewtopic.php?t=22818 [5] http://bbs.archlinux.org/viewtopic.php?t=22743 [6] http://bbs.archlinux.org/viewtopic.php?t=6188 _______________________________________________ arch mailing list [email protected] http://www.archlinux.org/mailman/listinfo/arch
