Implementing Ecn; Packet Handling When Ecn Is Enabled - Cisco ASR 9000 Series Configuration Manual

Explicit Congestion Notification
For more information on the ECN feature, please refer the Cisco ASR 9000 Series Aggregation Services
Router Modular Quality of Service Configuration Guide

Implementing ECN

Implementing ECN requires an ECN-specific field that has two bits—the ECN-capable Transport (ECT) bit
and the CE (Congestion Experienced) bit—in the IP header. The ECT bit and the CE bit can be used to make
four ECN field combinations of 00 to 11. The first number is the ECT bit and the second number is the CE
bit.
ECN Bit Setting
ECT Bit
0
0
1
1
The ECN field combination 00 indicates that a packet is not using ECN. The ECN field combinations 01 and
10—called ECT(1) and ECT(0), respectively—are set by the data sender to indicate that the endpoints of the
transport protocol are ECN-capable. Routers treat these two field combinations identically. Data senders can
use either one or both of these two combinations. The ECN field combination 11 indicates congestion to the
endpoints. Packets arriving a full queue of a router will be dropped.

Packet Handling when ECN is enabled

When the number of packets in the queue is below the minimum threshold, packets are transmitted. This
happens whether or not ECN is enabled, and this treatment is identical to the treatment a packet receives when
WRED only is being used on the network. If the number of packets in the queue is above the maximum
threshold, packets are dropped based on the drop probability. This is the identical treatment a packet receives
when WRED is enabled without ECN configured on the router. Three different scenarios arise if the number
of packets in the queue is between the minimum threshold and the maximum threshold:
• If the ECN field on the packet indicates that the endpoints are ECN-capable (that is, the ECT bit is set
to 1 and the CE bit is set to 0, or the ECT bit is set to 0 and the CE bit is set to 1)—and the WRED
algorithm determines that the packet should have been dropped based on the drop probability—the ECT
and CE bits for the packet are changed to 1, and the packet is transmitted. This happens because ECN
is enabled and the packet gets marked instead of dropped.
• If the ECN field on the packet indicates that neither endpoint is ECN-capable (that is, the ECT bit is set
to 0 and the CE bit is set to 0), the packet may be dropped based on the WRED drop probability. This
is the identical treatment that a packet receives when WRED is enabled without ECN configured on the
router.
• If the ECN field on the packet indicates that the network is experiencing congestion (that is, both the
ECT bit and the CE bit are set to 1), the packet is transmitted. No further marking is required.
Cisco ASR 9000 Series Aggregation Services Router Modular Quality of Service Configuration Guide, Release
6.1.x
66
CE Bit Combination Indicates
0 Not-ECN-capable.
1 Endpoints of the transport protocol are ECN-capable.
0 Endpoints of the transport protocol are ECN-capable.
1 Congestion experienced.
Configuring Modular QoS Congestion Management