Bandwidth Problems with Freebsd 5.x

Donald J. O'Neill duncan.fbsd at
Sun Feb 26 08:49:07 PST 2006

On Sunday 26 February 2006 08:10, ptitoliv wrote:
> Hello Everybody,
> I made lots of searches on the Internet and I found something about
> the option
> net.inet.tcp.inflight.enable.
> If I set this value to 0, my bandwitdh problems are resolved.
> Does anybody knows something about this options and why can it be the
> origin of this problem ?
> Regards,
> Ptitoliv
> _______________________________________________

I made some quick check, there is some information available. Most of it 
seems to be pretty recent. I suggest joining freebsd-stable@ as there 
is some questions going on about it there.

You can also google for information on:
There may be other things to look at and good reasons to have it 
enabled, or disabled.

Take from chapter 11 of the handbook:
===================== TCP Bandwidth Delay Product

The TCP Bandwidth Delay Product Limiting is similar to TCP/Vegas in 
NetBSD. It can be enabled by setting net.inet.tcp.inflight.enable 
sysctl variable to 1. The system will attempt to calculate the 
bandwidth delay product for each connection and limit the amount of 
data queued to the network to just the amount required to maintain 
optimum throughput.

This feature is useful if you are serving data over modems, Gigabit 
Ethernet, or even high speed WAN links (or any other link with a high 
bandwidth delay product), especially if you are also using window 
scaling or have configured a large send window. If you enable this 
option, you should also be sure to set net.inet.tcp.inflight.debug to 0 
(disable debugging), and for production use setting 
net.inet.tcp.inflight.min to at least 6144 may be beneficial. However, 
note that setting high minimums may effectively disable bandwidth 
limiting depending on the link. The limiting feature reduces the amount 
of data built up in intermediate route and switch packet queues as well 
as reduces the amount of data built up in the local host's interface 
queue. With fewer packets queued up, interactive connections, 
especially over slow modems, will also be able to operate with lower 
Round Trip Times. However, note that this feature only effects data 
transmission (uploading / server side). It has no effect on data 
reception (downloading).

Adjusting net.inet.tcp.inflight.stab is not recommended. This parameter 
defaults to 20, representing 2 maximal packets added to the bandwidth 
delay product window calculation. The additional window is required to 
stabilize the algorithm and improve responsiveness to changing 
conditions, but it can also result in higher ping times over slow links 
(though still much lower than you would get without the inflight 
algorithm). In such cases, you may wish to try reducing this parameter 
to 15, 10, or 5; and may also have to reduce net.inet.tcp.inflight.min 
(for example, to 3500) to get the desired effect. Reducing these 
parameters should be done as a last resort only.

    Note: In 4.X and earlier releases of FreeBSD the inflight sysctl 
variables are directly under net.inet.tcp. Their names were (in 
alphabetic order): net.inet.tcp.inflight_debug, 
net.inet.tcp.inflight_enable, net.inet.tcp.inflight_max, 
net.inet.tcp.inflight_min, net.inet.tcp.inflight_stab.


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