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This page describes some [Linux] SysControls and what they do. SysControls can be viewed/edited via the [sysctl(8)] command, or via <tt>/proc/sys/</tt>. Therefore, <tt>sysctl foo.bar</tt> is equivalent to <tt>cat /proc/sys/foo/bar</tt>. ---- __kernel__:: __kernel.panic__:: This is the number of seconds to wait after a kernel has panicked before the machine will reboot itself automatically. Very useful for unattended servers, or machines that are difficult to get physical access to. __net__:: These SysControls affect networking. See also http://bec.at/support/ipsysctl-tutorial/tcpvariables.html __net.ipv4__:: The IPv4 specific networking SysControls. __net.ipv4.ip_forwarding__:: Enable global IP forwarding. Very important. __net.ipv4.tcp_vegas_cong_avoid__ (0):: [TCP] Vegas congestion avoidance is a sender side congestion control algorithm (ie only used by the machine initiating the [TCP] connection) that causes [TCP] to back off when it detects the [RTT] changing (ie, queuing is occuring). It works well when there is only a few flows using the bottleneck link. [TCP] Vegas is is no longer considered to be particularly useful. See [TCP] Westwood, below. __net.ipv4.tcp_westwood__ (0):: [TCP] Westwood+ is a sender side congestion control algorithm (like [TCP] Vegas) that keeps estimates of throughput to try and make sure that the stack uses the optimum amount of bandwidth at all times. It is very useful and should probably be enabled for many sites. __net.ipv4.tcp_sack__ (1):: __S__elective __Ack__nowledgement reduces the number of segments that need to be retransmitted when packet loss occurs. Good to have on, needed to be enabled by both the sender and the reciever. __net.ipv4.tcp_fack__:: Enable __F__orward __Ack__nowledgement congestion avoidance and fast retransmission. Only has an effect if __<tt>net.ipv4.tcp_sack</tt>__ is enabled. See http://www.psc.edu/networking/papers/fack_abstract.html for details on how this option works. Basically it seems to assume that missing sequence ranges are dropped (ie, implies no reordering). [Linux] will disable Forward Acknowledgement on a per connection basis if it detects reordering. __net.ipv4.tcp_low_latency__ (0):: "If set, the [TCP] stack makes decisions that prefer lower latency as opposed to higher throughput." This seems to disable a function called __<tt>tcp_prequeue</tt>__, no idea what it does. __net.ipv4.tcp_reordering__ (3):: How many duplicate [ACK]s you need before you enter fast retransmit. If you are on a network with lots of reordering then this will need to be raised. [Linux] can dynamically tune this on a per [TCP] flow basis, so changing it is normally not that necessary. __net.ipv4.tcp_ecn__ (0):: Explicit Congestion Notification can be used by routers on the internet to signal that congestion is imminent and to therefore to slow down sending before packet loss actually occurs. However many firewalls on the internet incorrectly detect the [ECN] data as an attack and drop all packets using [ECN]. Sigh. __net.ipv4.tcp_retries1__ (3):: How many times to send a [SYN]/[ACK] packet before giving up on a connection. __net.ipv4.tcp_retries2__ (15):: How many times to send a [TCP] data packet before giving up on a connection. __inet.ipv4.tcp_syn_retrans__ (5):: How many times to send a [SYN] packet before giving up on a connection. __inet.ipv4.tcp_retrans_collapse__ (1):: Whether retransmissions should be sent as full sized packets. Presumably works around some [TCP] implementation bugs. ---- See also SysctlNotes
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SysControls
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KernelTweaking
SysctlNotes
