[e2e] TCP Loss Differentiation

Fred Baker fred at cisco.com
Wed Feb 11 14:07:19 PST 2009

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  

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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