TCP configuration (tcp)

Description

The tcp vmod contains functions to control TCP congestion control algorithms, set pacing (rate limiting) and perform logging of protocol-related information.

Examples

TCP rate-limiting

import std;
import tcp;

sub vcl_recv
{
  # Limit all clients to 1000 KB/s.
  tcp.set_socket_pace(1000);
}

TCP congestion control algorithm

import std;
import tcp;

sub vcl_recv
{
  set req.http.X-Tcp = tcp.congestion_algorithm("bbr");
}

Here, the X-Tcp header field will be set to 0 when changing the congestion control algorithm succeeded. Otherwise, it will be -1, indicating an error.

See the tcp.congestion_algorithm() function for more information about congestion control algorithms.

API

congestion_algorithm

INT congestion_algorithm(STRING algorithm)

Set the client socket congestion control algorithm to algorithm. Returns 0 on success, and -1 on error.

sub vcl_recv {
  set req.http.x-tcp = tcp.congestion_algorithm("cubic");
}

To see your available algorithms:

# sysctl net.ipv4.tcp_available_congestion_control
net.ipv4.tcp_available_congestion_control = reno cubic bbr

The bbr congestion control algorithm is fairly new requires kernel version 4.9.0 or later. See: https://www.vultr.com/docs/how-to-deploy-google-bbr-on-centos-7

Arguments:

algorithm accepts type STRING

Type: Function

Returns: Int

dump_info

VOID dump_info()

Write the contents of the TCP_INFO data structure into varnishlog.

sub vcl_recv {
  tcp.dump_info();
}

The varnishlog output could look like this:

VCL_Log    tcpi: snd_mss=1448 rcv_mss=536 lost=0 retrans=0
VCL_Log    tcpi2: pmtu=1500 rtt=12042 rttvar=6021 snd_cwnd=10 advmss=1448 reordering=3
VCL_Log    getsockopt() returned: bbr

Arguments: None

Type: Function

Returns: None

get_estimated_rtt

REAL get_estimated_rtt()

Get the estimated round-trip-time for the client socket, measured in milliseconds.

sub vcl_recv
{
  if (tcp.get_estimated_rtt() > 300) {
    std.log("Client is far away!");
  }
}

Arguments: None

Type: Function

Returns: Real

set_socket_pace

VOID set_socket_pace(INT)

Socket pacing is a Linux method for rate limiting TCP connections in a network friendly way.

Controls TCP rate limiting for the client connection, where pace is measured in KB/s. The outgoing network interface used must be configured with a supported scheduler, such as fq.

sub vcl_recv
{
  # Set client max bandwidth to 1000kb/s for this client,
  # as long as the current network scheduler supports it:
  if (tcp.set_socket_pace(1000) != 0) {
    std.log("Failed to set pacing for client socket!");
  }
}

Servers utilizing rate limiting must change their network scheduler. This can be changed with a sysctl setting:

net.core.default_qdisc=fq

See: https://wiki.mikejung.biz/Sysctl_tweaks

Arguments: None

Type: Function

Returns: None

Availability

The tcp VMOD is available in Varnish Enterprise version 6.0.0r0 and later.