ECN

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft
/122t/122t8/ftwrdecn.htm#1031751

How Packets Are Treated When ECN Is Enabled

* If 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 between the minimum
threshold and the maximum threshold, one of the following three
scenarios can occur:
o 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.
o 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.
o 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.
* 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.

For More Information

For more information about implementing ECN and about the changes
required at the routers and end hosts, refer to the following RFCs:

* RFC 2309, Internet Performance Recommendations
* RFC 2884, Performance Evaluation of Explicit Congestion
Notification (ECN) in IP Networks
* RFC 3168, The Addition of Explicit Congestion Notification (ECN)
to IP

R4(config)#ip tcp ecn
R4(config)#^Z
*Jan  7 05:18:53.319: %SYS-5-CONFIG_I: Configured from console by
console
R4#deb ip tcp ecn
TCP ECN debugging is on
R4#telnet 150.1.4.4
Trying 150.1.4.4 … Open

Password required, but none set

*Jan  7 05:19:02.327: 150.1.4.4:41657 <—> 150.1.4.4:23   out ECN-setup
SYN
*Jan  7 05:19:02.327: 150.1.4.4:23 <—> 150.1.4.4:41657   in ECN-setup
SYN
*Jan  7 05:19:02.327: 150.1.4.4:23 <—> 150.1.4.4:41657   out ECN-setup
SYN-ACK
*Jan  7 05:19:02.327: 150.1.4.4:41657 <—> 150.1.4.4:23   in ECN-setup
SYN-ACK
[Connection to 150.1.4.4 closed by foreign host]

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