Explicit Congestion Notification (ECN) is an extension to the Internet Protocol and to the Transmission Control Protocol. [RFC3186] ECN allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature that may be used between two ECN-enabled endpoints when the underlying network infrastructure also supports it.
Conventionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a mark in the IP header instead of dropping a packet in order to signal impending congestion. The receiver of the packet echoes the congestion indication to the sender, which reduces its transmission rate as if it detected a dropped packet.
Rather than responding properly or ignoring the bits, some outdated or faulty network equipment has historically dropped or mangled packets that have ECN bits set. As of 2015, measurements suggested that the fraction of web servers on the public Internet for which setting ECN prevents network connections had been reduced to less than 1%. [Trammell15]
The ECN plugin for PATHspider aims to detect breakage in the Internet due to the use of ECN.
To use the ECN plugin, specify
ecn as the plugin to use on the command-line:
pathspider ecn </usr/share/doc/pathspider/examples/webtest.csv >results.txt
This will run two TCP connections for each job input, one with ECN disabled in the kernel TCP/IP stack and one with ECN enabled in the kernel TCP/IP stack.
In addition to the default output fields, the ECN plugin also provides the following fields for each flow:
|fwd_ez||ECT(0) was observed in the forward direction.|
|rev_ez||ECT(0) was observed in the reverse direction.|
|fwd_eo||ECT(1) was observed in the forward direction.|
|rev_eo||ECT(1) was observed in the reverse direction.|
|fwd_ce||CE was observed in the forward direction.|
|rev_ce||CE was observed in the reverse direction.|
|fwd_syn_flags||The SYN flags observed in the forward direction.|
|rev_syn_flags||The SYN flags observed in the reverse direction.|
|fwd_fin||A FIN flag was observed in the forward direction.|
|rev_fin||A FIN flag was observed in the reverse direction.|
|fwd_rst||A RST flag was observed in the forward direction.|
|rev_rst||A RST flag was observed in the reverse direction.|
|tcp_completed||A complete 3WHS for TCP was observed to be successful.|
- ECN behaviour is implemented by the host kernel for PATHspider, and is
switched by a
sysctlcall. PATHspider makes no guarantees the the configuration state is consistent once it has been set, though you can use the forward SYN flags in the output to validate the results within a reasonably high level of certainty that everything behaved correctly.