Bug 813944

We can add new command line option. I think -C switch is unused so far,thus we can use it for this purpose. This RFE is easy to implement, however here is one concern we should take into the count. There is at least 5 or 6 congestion control algorithms I'm aware of and there is a mechanism in kernel to register and use new or customized algorithm, but only few of them are usable out-of-box, in my case only 2 (cubic, reno). I will implement this, without any restrictions on -C option value and I will assume that user is aware about choices he has available. We will try to set selected algorithm in everytime. If he picks algorithm which is not available on target system, setsockopt() will return failure and we will exit program with error code. I will add comment to man page how to put in use more algorithms by loading appropriate kernel modules.

By default, only cubic and reno modules are loaded and allowed.

$ cat /proc/sys/net/ipv4/tcp_available_congestion_control 
cubic reno
$ cat /proc/sys/net/ipv4/tcp_allowed_congestion_control 
cubic reno

However, there are a bunch more available in the kernel. For example.

$ sudo modprobe tcp_htcp
$ cat /proc/sys/net/ipv4/tcp_available_congestion_control 
cubic reno htcp

Of course, it still can't be used until you authorize it like this.

$ sudo sh -c 'echo cubic reno htcp >>/proc/sys/net/ipv4/tcp_allowed_congestion_control'
$ cat /proc/sys/net/ipv4/tcp_allowed_congestion_control
cubic reno htcp

Tada! Now--I think--you should be able to setsockopt() with TCP_CONGESTION.

Not sure if there are different error conditions for "algorithm not loaded or unknown" and "algorithm not allowed."

Created attachment 588020 [details]
Added implementation of -C command line option

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