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The tcp vmod contains functions to control TCP congestion control algorithms,
set pacing (rate limiting) and perform logging of protocol-related information.
import std;
import tcp;
sub vcl_recv
{
# Limit all clients to 1000 KB/s.
tcp.set_socket_pace(1000);
}
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.
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 STRINGType: Function
Returns: Int
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
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
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
INT get_quick_ack()
Get the current setting of the TCP_QUICKACK socket option of the client socket. This function may only be called from the client side.
Arguments: None
Type: Function
Returns: Int
VOID set_quick_ack(INT quickack)
Set the current setting of the TCP_QUICKACK socket option of the client socket. When ‘quickack’ is 1, TCP ACK will be sent immediately without waiting for TCP to select an appropiate time to send one. This is useful to workaround a client sending small messages without using TCP_NODELAY. TCP may override this setting at a later time or delay ACKs for other reasons. This function may only be called from the client side.
Arguments:
quickack accepts type INTType: Function
Returns: None
The tcp VMOD is available in Varnish Enterprise version 6.0.0r0 and later.