TCP RTT

TCP RTT

Agent Check Agent Check

Overview

The TCP RTT check reports on roundtrip times between the Agent’s host and any host it is communicating with. This check is passive and only reports RTT times for packets being sent and received from outside the check. The check itself does not send any packets.

This check is only shipped with the 64-bit DEB and RPM Datadog Agent v5 packages. For other versions of the Agent, see the Datadog/go-metro usage for instructions on how to build the go-metro binary.

Setup

Installation

The check uses timestamps provided by the PCAP library to compute the time between any outgoing packet and the corresponding TCP acknowledgement. As such, PCAP must be installed and configured.

Debian-based systems should use one of the following:

$ sudo apt-get install libcap
$ sudo apt-get install libcap2-bin

Redhat-based systems should use one of these:

$ sudo yum install libcap
$ sudo yum install compat-libcap1

Finally, configure PCAP:

$ sudo setcap cap_net_raw+ep /opt/datadog-agent/bin/go-metro

Configuration

Edit go-metro.d/conf.yaml, in the conf.d/ folder at the root of your Agent’s configuration directory. See the sample go-metro.d/conf.yaml for all available configuration options.

The following example retrieves the TCP RTT times for app.datadoghq.com and 192.168.0.22:

init_config:
    snaplen: 512
    idle_ttl: 300
    exp_ttl: 60
    statsd_ip: 127.0.0.1
    statsd_port: 8125
    log_to_file: true
    log_level: info

instances:
    - interface: eth0
      tags:
          - env:prod
      ips:
          - 45.33.125.153
      hosts:
          - app.datadoghq.com

Validation

To validate that the check is running correctly, you should see system.net.tcp.rtt metrics showing in the Datadog interface. Also, if you run sudo /etc/init.d/datadog-agent status, you should see something similar to the following:

datadog-agent.service - "Datadog Agent"
  Loaded: loaded (/lib/...datadog-agent.service; enabled; vendor preset: enabled)
  Active: active (running) since Thu 2016-03-31 20:35:27 UTC; 42min ago
 Process: 10016 ExecStop=/opt/.../supervisorctl -c /etc/dd-....conf shutdown (code=exited, status=0/SUCCESS)
 Process: 10021 ExecStart=/opt/.../start_agent.sh (code=exited, status=0/SUCCESS)
Main PID: 10025 (supervisord)
  CGroup: /system.slice/datadog-agent.service
          ├─10025 /opt/datadog-...python /opt/datadog-agent/bin/supervisord -c /etc/dd-agent/supervisor.conf
          ├─10043 /opt/datadog-...python /opt/datadog-agent/agent/dogstatsd.py --use-local-forwarder
          ├─10044 /opt/datadog-agent/bin/go-metro -cfg=/etc/dd-agent/conf.d/go-metro.yaml
          ├─10046 /opt/datadog-.../python /opt/datadog-agent/agent/ddagent.py
          └─10047 /opt/datadog-.../python /opt/datadog-agent/agent/agent.py foreground --use-local-forwarder

If the TCP RTT check has started you should see something similar to the go-metro line above.

This is a passive check, so unless there are packets actively being sent to the hosts mentioned in the yaml file, the metrics will not be reported.

Data Collected

Metrics

system.net.tcp.rtt
(gauge)
The TCP round trip time.
Shown as millisecond
system.net.tcp.rtt.avg
(gauge)
The average TCP round trip time as typically computed by the TCP stack.
Shown as millisecond
system.net.tcp.rtt.jitter
(gauge)
The TCP round trip time jitter.
Shown as millisecond