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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