From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Fri Aug 13 2004 - 22:11:18 GMT-3
C comes from "if the packet comes already marked as having experienced
congestion, then let it go". meaning, some other router upstream is also
congested.
ccie2be wrote:
> OK, here's my understanding of this feature based on my reading of the
> feature description at the link you posted.
>
> This feature only has an affect on the router on which the command
> random-detect ecn is configured. And, then it only affects a packet that
> might have been dropped based on wred settings where the packet is in
> between the minimum drop and max drop thresholds.
>
> If the packet is below the min threshold, it is always transmited. If it's
> above max threshold, it's always dropped.
>
> So, the only situation where this command has any affect is when the packet
> falls between the min and max thresholds.
>
> First consider what happens when a packet falls between these thresholds
> without this feature enabled. In this case, the packet faces a probability
> of being dropped that based on the mark probability denominator which, by
> default, is 10. This means that a packet faces a 1 in 10 chance of being
> dropped.
>
> Now, consider what happens when this feature is enabled. There are 3
> possibilities:
>
> a) the endpoints aren't ecn capable result: packet is treated by wred the
> same as if ecn weren't enabled.
>
> b) the endpoints are ecn capable result: if wred would have dropped
> without ecn, it now marks both fields with 1 and transmits the packet.
>
> c) the network is congested result: the packet is transmit
>
>
> Now, as far as I'm concerned, this explanation isn't complete and I don't
> understand why "C" is true, but that's what it says.
>
> HTH a little, Tim
>
> ----- Original Message -----
> From: <gladston@br.ibm.com>
> To: <ccielab@groupstudy.com>
> Sent: Friday, August 13, 2004 4:30 PM
> Subject: ECN
>
>
>
>>Any example of using ECN between 3 routes?
>>
>>I could not make a logical map of how it works reading Cisco Doc.
>>
>>If R1 sends a packet marked with ECT and CE, then would R2 learn that R1
>
> is ECN capable and would consider this information when sending packets back
> to R1?
>
>>=====================================
>>from Cisco
>>
>>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:
>
>>If the ECN field on the packet indicates that the endpoints are
>
> ECN-capable and the WRED algorithm determines that the packet should have
> been dropped based on the drop probabilitythe 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 R2 receives a packet from R1 with ECN field saying that it is a ECN
>
> capable, which device R2 considers ECN capable, the routers that sends this
> packet or the device to which this packet will be send?
>
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-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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