Explicit Congestion Notification | Dropping Tools

Traditionally, the only way to inform sending hosts that there was congestion on the network and that the hosts should slow their transmission rates was by dropping TCP packets.

RFC 3168, however, defined a new and more efficient way that the network could communicate congestion to sending hosts, namely the “The Addition of Explicit Congestion Notification (ECN) to IP.” By marking the final two bits of the ToS byte of the IP header, devices can communicate to each other and to endpoints that they are experiencing congestion. These two bits have been defined as follows:

• ECN-capable Transport (ECT) bit: This bit indicates whether the device and the transport protocol supports ECN.
• Congestion Experienced (CE) bit: This bit (in conjunction with the ECT bit) indicates whether congestion was experienced en route.

Figure 1 shows the location of the ECN bits in the TOS byte of an IP packet header.

 
Figure 1: IP ToS byte ECN bits

During periods of congestion, WRED/DSCP-based WRED drops packets when the average queue length exceeds a specific threshold value. ECN is an extension to WRED, such that ECN marks packets, instead of dropping them, to communicate the existence of congestion when the average queue length exceeds a specific threshold value. Routers configured with the WRED ECN feature use this marking as a signal to application endpoints that the network is congested. This way, TCP transmission rates can be adjusted by the application endpoints without dropping packets, (or at least with dropping far fewer packets).

WRED ECN is enabled with the ecn keyword with the random-detect command. WRED ECN can be enabled by itself or in conjunction with WRED or DSCP-based WRED (as shown in Example 1).

Example 1: DSCP-Based WRED ECN Policy Example

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Router(config)# policy-map WRED-ECN
Router(config-pmap)# class class-default
Router(config-pmap-c)# fair-queue
Router(config-pmap-c)# random-detect dscp-based
Router(config-pmap-c)# random-detect ecn