[e2e] Can feedback be generated more fast in ECN?

[Zhang Miao]

Hi,
   In ECN-RED, router only marks the packet and let the receiving end to "echo" the 
congestion notification to the sender. 
   I have a naive question. Can the feedback be sent directly from the congested router
to the TCP sender?
   Some schemes of action at router are listed here:
1. Only mark the packet. (ECN)
2. Mark the packet and send a feedback to TCP sender immediately.
3. Send a feedback to TCP sender and drop the packet.
   This scheme can be used for droptail.

The advantages of sending a feedback directly:
1. reduce the response time.
2. reduce the loss probability of feedback.
   consider the path of marked packet and the ACK with CN bit set.

Disadvantages:
1. add more burden the reverse direction of the link.
2. add more burden to the router.

   Some mechanisms also need to be added to TCP to cope with the duplicate feedback
in scheme 2.
   
   The designer of ECN must have thought of the scheme above. Can anyone tell me why
this scheme was not used when ECN was designed?

Thanks!

*****************************************************************
*    Zhang Miao                                                 *
*    Ph.D candidate,Department of Computer Science & Technology * 
*    Tsinghua University,Beijing,China(100084)                  *
*    Tel: (8610)-62785822                                       *
*    Email: zm at csnet1.cs.tsinghua.edu.cn                        *
*****************************************************************