DSCP Plugin

Differentiated services or DiffServ [RFC2474] is a networking architecture that specifies a simple, scalable and coarse-grained mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks. DiffServ can, for example, be used to provide low-latency to critical network traffic such as voice or streaming media while providing simple best-effort service to non-critical services such as web traffic or file transfers.

DiffServ uses a 6-bit differentiated services code point (DSCP) in the 8-bit differentiated services field (DS field) in the IP header for packet classification purposes. The DS field and ECN field replace the outdated IPv4 TOS field. [RFC3260]

The DSCP plugin for PATHspider aims to detect breakage in the Internet due to the use of a non-zero DSCP codepoint.

Usage Example


The path given to the example list of web servers is taken from a Debian GNU/Linux installation and may differ on your computer. These are the same examples that can be found in the examples/ directory of the source distribution.

To use the DSCP plugin, specify dscp as the plugin to use on the command-line:

pspdr measure -i eth0 dscp </usr/share/doc/pathspider/examples/webtest.ndjson >results.ndjson

This will run two HTTP GET request connections over TCP for each job input, one with the DSCP set to zero (best-effort) and one with the DSCP set to 46 (expedited forwarding). If you would like to specify the code point for use on the experimental flow, you may do this with the --codepoint option. For example, to use 42:

pspdr measure -i eth0 dscp --codepoint 42 </usr/share/doc/pathspider/examples/webtest.ndjson >results.ndjson

Supported Connection Modes

This plugin supports the following connection modes:

  • http - Performs a GET request
  • tcp - Performs only a TCP 3WHS
  • dnsudp - Performs a DNS query using UDP
  • dnstcp - Performs a DNS query using TCP

To use an alternative connection mode, add the --connect argument to the invocation of PATHspider:

pspdr measure -i eth0 dscp --connect tcp </usr/share/doc/pathspider/examples/webtest.ndjson >results.ndjson

Output Conditions

The following conditions are generated for the DSCP plugin:


For each connection that was observed by PATHspider, a connectivity condition will be generated to indicate whether or not connectivity was successful using codepoint X validated against a connection using codepoint 0 (zero).

Y may have the following values:

  • works - Both connections succeeded
  • broken - Baseline connection succeeded where experimental connection failed
  • offline - Both connections failed
  • transient - Baseline connection failed where experimental connection succeeded (this can be used to give an indication of transient failure rates included in the “broken” set)


For each connection that was observed to have a response by PATHspider, a condition is generated to show values of codepoints set on response packets when codepoint X was set.


  • DSCP marking is performed using the mangle table in iptables. The config_zero function will flush this table. PATHspider makes no guarantees the the configuration state is consistent once it has been set, though you can use the forward path markings in the output to validate the results within a reasonably high level of certainty that everything behaved correctly.