Question: I am using Ubuntu 14.04 LTS with Linux Kernel version 3.13.0-34.

How can I find out what is the default or what is the current TCP congestion control algorithm being used ? 

Thanks.

Answer: 
Adding to ***@grawity*** answer, it is possible to check all TCP congestion control algorithms available with the following command:

```
sysctl net.ipv4.tcp_available_congestion_control

```

A list of some of the possible output (i.e.- availble flavours) is:

*reno*: Traditional TCP used by almost all other OSes. (default)

*cubic*: CUBIC-TCP

*bic*: BIC-TCP

*htcp*: Hamilton TCP

*vegas*: TCP Vegas

*westwood*: optimized for lossy networks

*YeAH*: delay-aware/state-enabled to keep a pipe at or below a threshold

NOTE: 
If *cubic* and/or *htcp* are not listed when you do `'sysctl net.ipv4.tcp_available_congestion_control'`, try the following, as most distributions include them as loadable kernel modules:

```
/sbin/modprobe tcp_htcp
/sbin/modprobe tcp_cubic

```

For more detailes you can have look here:

[TCP tuning Details](https://fasterdata.es.net/host-tuning/linux/expert/)

Hope it helps.

Cheers,

Guy.
If you want to find out the current congestion control algorithm used on each connection you can use the `ss` command (part of the [iproute2](https://wiki.linuxfoundation.org/networking/iproute2) package/tools):

```
ss -ti

```

Here's some example output: Below the connection details, the first item in the parameter output is the congestion control algorithm - in this case `cubic`- which is followed by various parameters related to the TCP connection.

```
ESTAB      0      0      192.168.56.102:ssh           192.168.56.1:61795
 cubic wscale:6,7 rto:201 rtt:0.218/0.038 ato:40 mss:1448 rcvmss:1392
 advmss:1448 cwnd:10 bytes_acked:8753 bytes_received:3945 segs_out:40
 segs_in:63 send 531.4Mbps lastsnd:1 lastrcv:2 lastack:1
 pacing_rate 1059.1Mbps rcv_rtt:2 rcv_space:28960

```

Note: It is possible for applications to chose the congestion control algorithm they want to use by using the [`TCP_CONGESTION`](https://www.systutorials.com/docs/linux/man/7-tcp/) setsockopt() call.