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(QCT) (Config)#exit
3.16. ECN
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 a tail drop scheme
by dropping packets already queued for transmission. ECN marks congested packets that would otherwise
have been dropped and expects an 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)
QNOS WRED considers packets for early discard only when the number of packets queued for transmission
on a port exceeds the relevant minimum WRED threshold. The green, yellow, red thresholds operate on TCP
packets. The fourth threshold operates on non-TCP packets.
When ECN is enabled and congestion is experienced, TCP packets that are marked ECN Capable that are
queued for
transmission
and are selected for discarded by WRED, are instead marked CE and
transmitted.
This
includes packets that exceed the WRED upper threshold. If the switch experiences severe congestion (no
buffers available), then packets are discarded.
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.
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