[e2e] [tcpm] RTTM + timestamps

Detlef Bosau detlef.bosau at web.de
Thu Jul 28 14:25:22 PDT 2011

On 07/28/2011 10:48 PM, William Allen Simpson wrote:
> A couple of years ago, when I first asked this list about TCPCT related
> work, *several* folks wanted finer grained resolution.  So, I added the
> 64-bit NTP4-format timestamps.  Then, many months later, a naysayer 
> raised
> the possibility that NTP5 will someday replace NTP4.  I had to redo the
> format to be even more extensible, rendering my extant code obsolete....
> But I don't know anybody actually writing the code for 64-bit timestamps
> though, let alone NTP5-sized.  Would folks be interested in a draft
> describing them?  Would that help move things along?
As I said, I don't think that there is an open issue with the 
granulation - and your post confirms this position. At least in that 
particular respect, that our time stamps _are_ fine enough or, if 
necessary, can be made so. One important question is, of course, the 
resolution of clocks and OS timers in practical implementations. In some 
private e-mail conservation, a timer with a resolution of 10^(-12) 
seconds was suggested - and my simple and harsh reaction was: This is 
complete nonsense. However, if our clocks and OS timers could yield a 
resolution like this, it is nice to know, that we can define time stamps 
to have this implemented.

The origin of my question is that I work with WWAN and I'm looking for a 
reasonable way to model WWAN links in some generic manner. There are 
dozens of simulations around which model particular technologies, 
topologies, scenarios and so on. However, there is nothing generic which 
is not likely to be criticized as too much technology dependent etc.. 
Therefore, I'm basically interested in an approach which discusses the 
relevant problems of timers in WWAN, where the focus shall be the 
structural ones. Not that ones which are due to the n technologies we 
have - and overcome with the n plus first.

This evening, I had a very first glance at the "peak hopper" paper - 
however, what left me alone was the simulation. It is always fine work 
to confirm an RTO design with NS2 simulations, however the everlasting 
question arises whether the simulations confirm the RTO design - or the 
RTO desin confirms the simulator.

In other words: Are the underlying models and the simulation scenario 
realistic? Are the results meaningful?

Personally, I tend to to simulations in that direction that I want to 
simulate the significant problems with WWAN - only these and not the 
stuff whether a certain simulation model works exactly like the Ultime 
Omega Brand New Wirelss WAN from Lucent or whomever.

I would be grateful to see concrete time series from real WWAN 
measurements, so one could discuss if, e.g., spurious timeouts are an 
issue or not. (This is an open depate for about 15 years now.) Or 
whether short time "disconnections" (what ever this means in WWAN) are 
an issue or not.
Or whether recovery from temporarily short throughput is an issue or not.

I think there are lots of interesting questions and I'm, as always ;-), 
in some lack of dialogue partners here, but I'm looking forward to 
turning this into some better direction here.


Detlef Bosau
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