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

[Zhang Miao]

First, I would like to say sorry. 
I have read the paper by Sally Floyd you mentioned before. 
But I forgot the related content when I wrote down my question.
After re-read that paper, I still can't get the answer to the question.
The author listed 2 disadvantages of Source Quench, and listed several
advantages of source quench over ECN. There were no explicit answer to my
question in it.

The disadvantages of Source Quench mentioned:
(1) the overhead of Source Quench traffic in times of congestion.
(2) addtional load on the network in times of congestion.

The author also admitted that: "the use of intelligent gateway mechanisms such as
those in RED gateways would limit the overhead of the Source Quench traffic."
Also, "Although we have not explored the dynamics of backward ECN mechanism such as
Source Quench in our simulation, ..."

Now, let us see the disadvantages of Source Quench mentioned in that paper.
(1) Is the overhead of Source Quench traffic a significant problem?
    I guess this arguement was based on router of single processor architecture.
The CPU in the router does everything, routing, forwarding, etc. But the architecture
of router has changed a lot. We now have router with distributed architecture, with
ASIC. The processing of Source Quench may be not a big problem now.

(2) About the additional load
    If Source Quench introduces additional load to the network, ECN also does. 
Perhaps in ECN, the congestion will influence more broad than in Source Quench.
(For the congestion will be "echoed" by the congested router with Source Quench). 
Addtionally, if the link is congested in one direction, can we deduce that the link
is also congested in the reverse direction. The more often scenario is links
with heterogenous load in the two directions.


>please refer to "TCP and Explicit Congestion Notification" for more
>information, especially Section 9.1
>
>-----Original Message-----
>From: Zhang Miao [mailto:zm at csnet1.cs.tsinghua.edu.cn]
>Sent: Monday, February 12, 2001 9:37 AM
>To: end2end-interest at isi.edu
>Subject: [e2e] Can feedback be generated more fast in ECN?
>
>
>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                        *
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