[e2e] TCP Loss Differentiation

Injong Rhee rhee at ncsu.edu
Fri Feb 20 04:38:40 PST 2009

Agreed. It is rather silly to raise your window at the time of full 
utilization; it ends up filling up the buffer and increase delays. But 
unless we have some mechanism that tells explicitly how much utilization a 
flow has in all the links that flow goes through (e.g., ECN on all links), 
loss based congestion control will stay, i think. Going back to the original 
question, differentiating congestion related losses from other losses is 
important and I believe it can be done effectively. The point is that we 
don't have to differentiate all the losses -- the congestion related losses 
seem to be rather easier to distinguish from the other types of losses whose 
models or patterns are not well understood.

----- Original Message ----- 
From: "Fred Baker" <fred at cisco.com>
To: "Injong Rhee" <rhee at ncsu.edu>
Cc: "David P. Reed" <dpreed at reed.com>; "end2end-interest list" 
<end2end-interest at postel.org>
Sent: Thursday, February 19, 2009 11:32 PM
Subject: Re: [e2e] TCP Loss Differentiation

> Which begs the question - why are we tuning to loss in the first  place? 
> Once you have filled the data path enough to achieve your "fair  share" of 
> the capacity, filling the queue more doesn't improve your  speed and it 
> hurts everyone around you. As your cwnd grows, your mean  RTT grows with 
> it so that the ratio of cwnd/rtt remains equal to the  capacity of the 
> bottleneck.
> Seems pointless and selfish, the kind of thing we discipline our  children 
> if they do.
> On Feb 19, 2009, at 7:07 PM, Injong Rhee wrote:
>> Perhaps I might add on this thread. Yes. I agree that it is not so  clear 
>> that we have a model for non-congestion related losses. The  motivation 
>> for this differentiation is, I guess, to disregard non- congestion 
>> related losses for TCP window control. So the motivation  is valid. But 
>> maybe we should look at the problem from a different  perspective. 
>> Instead of trying to detect non-congestion losses, why  not try to detect 
>> congestion losses? Well..congestion signals are  definitely easy to 
>> detect because losses are typically associated  with some patterns of 
>> delays. So the scheme would be "reduce the  congestion window ONLY when 
>> it is certain with high probability that  losses are from congestion". 
>> This scheme would be different from  "reduce whenever any indication of 
>> congestion occurs". Well my view  could be too dangerous. But given that 
>> there are protocols out  there, e.g., DCCP, that react to congestion much 
>> more slowly than  TCP, this type of protocols may not be so bad...
>> ----- Original Message ----- From: "Fred Baker" <fred at cisco.com>
>> To: "David P. Reed" <dpreed at reed.com>
>> Cc: "end2end-interest list" <end2end-interest at postel.org>
>> Sent: Wednesday, February 11, 2009 5:07 PM
>> Subject: Re: [e2e] TCP Loss Differentiation
>>> Copying the specific communicants in this thread as my postings to 
>>> end2end-interest require moderator approval (I guess I'm not an 
>>> acceptable person for some reason, and the moderator has told me  that 
>>> he will not tell me what rule prevents me from posting  without 
>>> moderation).
>>> I think you're communicating just fine. I understood, and agreed  with, 
>>> your comment.
>>> I actually think that a more important model is not loss  processes, 
>>> which as you describe are both congestion-related and  related to other 
>>> underlying issues, but a combination of several  underlying and 
>>> fundamentally different kinds of processes. One is  perhaps "delay 
>>> processes" (of which loss is the extreme case and L2  retransmission is 
>>> a partially-understood and poorly modeled  contributor to). Another 
>>> might be interference processes (such as  radio interference in 
>>> 802.11/802.16 networks) that cause end to  end packet loss for other 
>>> reasons. In mobile networks, it might be  worthwhile to distinguish the 
>>> processes of network change - from  the perspective of an endpoint that 
>>> is in motion, its route, and  therefore its next hop, is constantly 
>>> changing and might at times  not exist.
>>> Looking at it from a TCP/SCTP perspective, we can only really  discuss 
>>> it as how we can best manage to use a certain share of the  capacity 
>>> the network provides, how much use is counterproductive,  when to 
>>> retransmit, and all that. But understanding the underlying  issues will 
>>> contribute heavily to that model.
>>> On Feb 11, 2009, at 7:20 AM, David P. Reed wrote:
>>>> I don't understand how what I wrote could be interpreted as "a 
>>>> congestion-based loss process cannot be modeled or predicted".
>>>> I was speaking about *non-congestion-based* "connectivity loss 
>>>> related loss process", and I *said* that it is not a single- 
>>>> parameter, memoryless loss process.
>>>> I said nothing whatsoever about congestion-based loss processes, 
>>>> having differentiated carefully the two types of loss (which 
>>>> differentiation was what Detlef started this thread with).
>>>> Clearly I am not communicating, despite using English and common 
>>>> terms from systems modeling mathematics.
>>>> Xai Xi wrote:
>>>>> are you saying that a congestion-based loss process cannot be 
>>>>> modeled or predicted? a tool, badabing, from sigcomm'05, claims  to 
>>>>> be highly accurate in measuring end-to-end loss processes.
>>>>> David wrote:
>>>>>> A "loss process" would be a mathematically more sound term,   because 
>>>>>> it
>>>>> does not confuse> the listener into thinking that there is a 
>>>>> simplistic, memoryless, one-parameter model that> can be 
>>>>> "discovered" by TCP's control algorithms.
>>>>>> That said, I was encouraging a dichotomy where the world is far  more
>>>>> complicated:
>>>>>> congestion drops vs. connectivity drops.  One *might* be
>>>>> able to make much practical
>>>>>> headway by building a model and a theory of
>>>>> "connectivity drops".
>>>>> _________________________________________________________________
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