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VMOD_TCP(3) VMOD_TCP(3)

NAME

vmod_tcp - TCP vmod

SYNOPSIS

import tcp [from "path"] ;

DESCRIPTION

The TCP vmod is a set of functions that accesses and changes attributes on the (client) TCP connection. It allows you to access TCP_INFO data, and also change the congestion control algorithm used.

Varnish Cache 4.1 and newer is supported.

The VMOD is Linux-specific and requires a recent kernel to be useful. (>=3.13)

CONTENTS

  • func_congestion_algorithm
  • func_dump_info
  • func_get_estimated_rtt
  • func_set_socket_pace

congestion_algorithm

INT congestion_algorithm(STRING algorithm)


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

Example:

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


dump_info

VOID dump_info()


Write the contents of the TCP_INFO data structure into varnishlog.

Example

tcp.dump_info();



get_estimated_rtt

REAL get_estimated_rtt()


Get the estimated round-trip-time for the client socket. Unit: milliseconds.

Example:

if (tcp.get_estimated_rtt() > 300) {
    std.log("client is far away.");
}


set_socket_pace

VOID set_socket_pace(INT)


Set socket pacing on client-side TCP connection to PACE KB/s. Network interface used must be using a supported scheduler. (fq)

Example:

tcp.set_socket_pace(1000);


Usage

In your VCL you could then use this vmod along the following lines: 

import tcp;
sub vcl_recv {
        tcp.dump_info();
        if (tcp.get_estimated_rtt() > 300) {
            set req.http.x-tcp = tcp.congestion_algorithm("hybla");
        }
}


Example varnishlog output from dump_info():

-   VCL_Log        tcpi: snd_mss=1448 rcv_mss=536 lost=0 retrans=0
-   VCL_Log        tcpi2: pmtu=1500 rtt=152000 rttvar=76000 snd_cwnd=10 advmss=1448 reordering=3
-   VCL_Log        getsockopt() returned: cubic