[e2e] A simple scenario. (Basically the reason for the sliding window thread ; -))

Detlef Bosau detlef.bosau at web.de
Wed Jan 17 11:21:28 PST 2007

Joe Touch wrote:
>> First.
>> The only scenrios where I see a justification / necessity for doing
>> splitting or spoofing are scenarios where the TCP flow must pass the
>> split box / spoofing box / PEP anyway. These are scenarios without path
>> redundancy or path transparency.
> Why are you so confident about the path, when you cannot control whether
> there is a PEP/spoofing box in it?

Honestly, I don´t understand the question.

I wrote: "The only scenrios where I see a justification / necessity for 

splitting or spoofing are scenarios where the TCP flow must pass the
split box / spoofing box / PEP anyway. "

In other words: I restrict the use of split boxes to scenarios where there is no other path. Either the flow passes the box - or the flow passes away.

This _is_ a strong restriction.

I don´t want to advocate split boxes etc., which are hard state by nature, as an optimal solution for any problem. I´m totally with you that nearly any alternative to a split box is better then a split box. I only want to concede that there may be situations where the use of a splitter should be considered.

Practically spoken: If the word "splitter" appears in the abstract of a paper submission, please don´t reject it immediately. Please read at least the introduction ;-)

> ...
>> To be not misunderstood: I don´t want to make restrictions for the
>> benefit of a splitter. I think in scenarios where an alternative path to
>> a splitter exist, a splitter must not be used.
> Either the use of splitters is under your control or it is not.

 From my assumptions / restrictions it clearly _is_. And if you feel 
more comfortable that way we perfectly can integrate some kind of option 
or switch in a mobile network´s UNI where the user has the choise 
whether a splitter shall be allowed or shall be forbidden. So the use of 
a splitter must not be transparent but explicitely granted / requested 
by a user. We have similar options for transcoders / WWW proxies in 
mobile networks here in Germany. IIRC, E-plus offers optional 
transcoders / application level PEP.
> If it is, then there are a number of reasons to remove them, alternate
> paths are just one.
> If it is not, then you cannot make assumptions about the path.
Hm. Admittedly, I think we´re talking somewhat at cross-purposes here.

I perfectly understand why you are strongly opposed against splitters 
and the reasons are compelling. However, when in a particular situation 
a splitter is the only yet known possibility e.g. to achieve acceptable 
throughput for a flow within a settling time of 10 seconds instead of 10 
minutes ore more, then we should consider giving the user the option to 
allow splitting.
>> In my opinion splitters
>> are to be used with maximum care and only in exceptional cases where any
>> known alternative is worse than a splitter.
> It would be interesting if you could explain a sample case. 

IIRC, Mark Allman has published some interesing work where he used 
splitters for  satellite  / deep space networks.
To my understanding the major concern was the extremely large time TCP 
needs to fill the line here.

I did not deal with TCP and extremely large line capacities too much 
yet. However, actually I do. It´s just your opposition to splitting 
which made me reconsider my paper on Path Tail Emulation and to redesign 
it that way that it relies only on pacing / spacing and does not assume 
/ use splitting or spoofing.

I´m not sure whether one is interested in the results. If so, I would be 
glad to discuss this.

> IMO,
> splitters just lie - they lie about being an endpoint they are not.

When I was a child, my mother occasionally sang a song, I don´t know 
where she got it from or if anybody know ist, "It´s a sin to tell a lie"
And I don´t know (I never saw the text in a written form) whether this 
is a statement or a question. (According to the WWW, it´s a statement.)

> Either you are lying to yourself (you own the endpoint you're lying to)
> or you're lying to others. The first is silly - just install a true
> application proxy - and the second is YOU making a decision for ME about
> what's more important. If I don't want to talk to a true proxy, you have
> no business tricking me into thinking I'm not.

As I said: We can agree that splitters shall not be used transparently / 
without permission by the user.


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