Administrator Guide

1564 Class-of-Service
Explicit Congestion Notification
Explicit Congestion Notification (ECN) is defined in RFC 3168.
Conventional TCP networks signal congestion by dropping packets. A
Random Early Discard scheme provides earlier notification than tail drop.
ECN marks congested packets that would otherwise have been dropped and
expects a ECN capable receiver to signal congestion back to the transmitter
without the need to retransmit the packet that would have been dropped. For
TCP, this means that the TCP receiver signals a reduced window size to the
transmitter but does not request retransmission of the CE marked packet.
ECN uses the two least significant bits of Diffserv field (TOS octet in IPv4 /
Traffic Class octet in IPv6) and codes them as follows:
00: Non ECN-Capable Transport – Non-ECT
10: ECN Capable Transport – ECT(0)
01: ECN Capable Transport – ECT(1)
11: Congestion Encountered – CE
ECN capable hosts communicate support for ECN via two flags in the TCP
header:
• ECN-Echo (ECE)
• Congestion Window Reduced (CWR)
Dell EMC Networking WRED considers packets for early discard only when
the number of packets queued for transmission on a port exceeds the relevant
minimum WRED threshold. Four thresholds are available for configuration.
The green, yellow, and red thresholds operate on TCP packets. The fourth
threshold operates on non-TCP packets.
When ECN is enabled and congestion is experienced, packets that are
marked ECN-capable, are queued for transmission, and are randomly selected
for discard by WRED are instead marked CE and are transmitted rather than
dropped. This includes packets that exceed the WRED upper threshold. If
the switch experiences severe congestion (no buffers available), then packets
are discarded.