Re: TCP

Dear Rich;

On Dec 18, 2007, at 9:18 AM, Richard Carlson wrote:

> Marshall;
>
> As always, it really depends on what the scientist needs to  
> accomplish.
> The VLBI community is moving raw data from multiple radio  
> telescopes to a single coorelator machine to generate a new image.   
> All the data is time stamped and those multiple streams must be  
> time sync'ed to get the coorelator to function.  As with most real  
> time data streams, timeouts and retransmissions are bad because the  
> timing gets hosed.  So it's better to drop some packets instead of  
> retransmitting them.  In addition, the application itself has  
> multiple data channels, so it is in the best position to know what  
> to drop if congestion becomes a problem.  This means that plain old  
> UDP is the best protocol for this community.
I agree that UDP is the best for VLBI and have been saying that for  
some time. The most fundamental thing
to consider is that the VLBI correlation coefficients are typically  
very low
(that is indeed why such high data rates are needed) and the antennae  
need to be widely separated.
Because the correlation coefficients are low, the value of any data  
packet is also low, it is literally better to use any excess  
bandwidth to send a new data packet instead of retransmitting an old  
one. So, retransmissions are not helpful in this case.
In the new 2010 VLBI program, the locations of antennae will be  
limited primarily by the need for fiber optic access. (Why, oh why,  
doesn't French Polynesia have undersea fiber access ?) It will not be  
possible to provide anything like a dedicated data network for this  
effort, sites will be at remote locations with hosts without a lot of  
resources, and the need for a UDP scavenger service is strong.
People interested in more details here can look at the proceedings of  
the 6th International e-VLBI Workshop :
http://www.mpifr-bonn.mpg.de/div/vlbi/6th_evlbi/

(Disclosure - I used to be chief scientist for the US Naval  
Observatory VLBI observing program.)
> The LHC community is trying to move large amounts of stored data  
> between compute sites.  The data flows out of the detector(s) run  
> through several triggers that filter out the uninteresting stuff so  
> only a fraction of events get recorded to local disks.  This raw  
> data then gets moved from CERN Switzerland to one of a dozen sites  
> scattered around the globe.  These remote sites provide long term  
> storage of the raw data, provide some compute resources to post  
> process the data, and support a 2nd level of distributed compute  
> sites to post process the data.  This is really a classic bulk  
> transport task with tons of data being moved around the globe.  It  
> must be done in a reliable manner (raw data backup and post  
> processed data).  TCP is the protocol of choice for this community.
I know some particle physicists who feel that UDP would serve here  
too, for similar reasons, but the community have gone in a different  
direction. It is true that particle physics computation tends to  
occur mostly in places with good access to network resources, which I  
think may have influenced their thinking.
Regards
Marshall

> The LHC community has used DSCP scavenger service in the past, they  
> are moving to a new model where they lease their own infrastructure  
> so they can meet the science demands.
> Rich
>
> At 07:29 PM 12/17/2007, Marshall Eubanks wrote:
>
> > On Dec 17, 2007, at 3:05 PM, Matthew J Zekauskas wrote:
> >
> > > On 12/17/2007 2:30 PM, Fred Baker wrote:
> > >
> > > > It is probably worth looking into the so-called "LAN Speed
> > > > Records" and
> > > > talking with those who have achieved them. An example of a news
> > > > report
> > > Internet2 has sponsored some, and as part of the award the
> > > contestant is
> > > required to say exactly how they did it so the experiment can be
> > > reproduced... the history list with pointers to contestant sites is
> > > here:
> > >
> > > <http://www.internet2.edu/lsr/history.html>
> > >
> > >
> > > > Operationally, the guys who worry about this sort of thing the
> > > > most are probably the astronomers, who routinely move sensor data
> > > > from radio-telescopes across the research backbones for data
> > > > reduction. In their cases, the sensors routinely generate in
> > > > excess of 1
> > > Actually, I believe the physicists actually worry more (or at  
> > > least as
> > > much); there's lots of data to be moved around as part of the Large
> > > Hadron Collider that is starting up at CERN.
> >
> > Note that for VLBI for sure, and particle particle physics IMO,
> > fairly high packet loss rates could easily be
> > accommodated with no need for retransmission, and so there is no
> > reason to use TCP for these applications.
> >
> > This situation cries out for some sort
> > of "worst than best effort" scavenger service. If anyone else feels
> > the same way, we should try and arrange a Bar BOF in Philadelphia.
> >
> > Regards
> > Marshall
> >
> > > --Matt
> > >
> > >
> > >
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> ------------------------------------
>
>
>
> Richard A. Carlson                              e-mail:  
> RCarlson(_at_)internet2(_dot_)edu
> Network Engineer                                phone:  (734) 352-7043
> Internet2                                       fax:    (734) 913-4255
> 1000 Oakbrook Dr; Suite 300
> Ann Arbor,  MI  48104

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