[e2e] TCP Loss Differentiation

Fred Baker fred at cisco.com
Thu Feb 19 20:32:03 PST 2009

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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