[e2e] two questions about the Internet

Constantinos Dovrolis dovrolis at mail.eecis.udel.edu
Thu Mar 15 14:33:18 PST 2001


Sally,

I may have a (very partial) answer to the first question, i.e.,
what is the distribution of round-trip times (RTTs) for the
packets on a certain link.

A couple of initial "disclaimers":
a) an RTT can be only associated to a packet of a closed-loop
protocol, and so our measurements only looked at TCP packets
b) our measurements do not refer to per-packet RTTs, but to
per-connection RTTs. It is likely that there are important
differences in these two distributions (short-RTT flows may
tend to carry more data, causing the per-packet RTT distribution to
be heavier on lower RTT values).

So, the measurements that I refer to were done in the summer
of 99 at CAIDA. We were processing traffic traces captured from
passive monitors on certain links (note that we get two different
traces, one for each direction of the link). In a certain trace,
we were estimating the RTT of each TCP connection using the
following two rules:

a) if we observe the flow from the caller to the callee, the
RTT is estimated as the time interval from the SYN to the SYN-ACK.

b) if we observe the flow from the callee to the caller (which
is usually the traffic from the server to the client), the RTT
is estimated from the time spacing of the first 2 or 3 slow-start
bursts. The code to do this is tricky (I can send to you, or to
anyone else, the code if you want to play with this).

So, using these tricks we were measuring the distribution
of RTTs in the TCP connections that were present in each
(unidirectional) trace. Just as an example of the distributions
that we were getting, take a look at:

http://www.cis.udel.edu/~dovrolis/rtt-sdsc.eps

The graph shows two RTT distributions, one for each direction
of the OC-3 link that used to connect UCSD with CERFnet.

A few major points from the graph:
- About 35% of the connections have RTT < 50ms
- About 60% of the connections have RTT < 100ms
- There is a significant fraction of connections (20-30%)
  with RTT>200ms (which is probably close to the upper bound
  for any type of interactive applications).
- About 10% of the RTTs are quite large (some of them in the
  order of multiple seconds), which may indicate errors in our
  measurement methodology. This is why I did not include that
  fraction of RTTs in the graph.

Some very interesting measurements on this subject also appear
at Mark Allman's "A Web server's view of the transport layer"
published at CCR Oct-2000. Mark's measurements originate from
traces of the server's traffic (instead of a passive monitor in the
the network). Also, he could measure the RTTs more accurately
based on the time distance between a non-retransmitted packet
and the corresponding ACK. Obviously we cannot do the same,
because we don't have the flow of ACKs in the trace.

It is interesting that Mark's measurements (see Figure 9) are not *very*
different from the graph that I mentioned before. Specifically,
his graph shows:
- About 35% of the RTTs < 100msec
- About 60-70% of the RTTs < 200msec
- About 85% of the RTTs < 500msec.
Of course Mark's measurements/analysis were much more methodically
done (my measurements were only done to get some reasonable values
for simulations about other stuff).

I hope that this helps. I am also very interested in answers
to the rest of your questions.


Constantinos

Computer and Information Sciences - University of Delaware

http://www.cis.udel.edu/~dovrolis/

On Thu, 15 Mar 2001, Sally Floyd wrote:

> I maintain a web page
>   http://www.aciri.org/floyd/questions.html
> of (mostly unanswered) questions about the Internet.
> I just posted two new questions to that page, and I thought I would
> also mention them here, in case anyone on this list knows any
> (partial) answers to any of them.
>
> The new questions:
>
> ROUND-TRIP TIMES (HOPS, NUMBER OF ASes) OF PACKETS?
> For packets on a particular link, each packet could be assigned an
> estimated round-trip time, a number of ASes for the end-to-end
> path, etc, based on the IP source and destination addresses for
> that packet.  For packets on a particular link, what can we say
> about the distribution of round-trip times, or of the number of hops
> traversed, or the number of ASes traversed, or number of continents
> traversed, or (this is harder) the number of congested links traversed?
>
> [Example:  For link X, can we say that most packets/bytes stay on
> that continent?  Or that most packets have a minimum round-trip
> time of at least S seconds?  Or that most packets on that link
> during this period of time traverse more than one congested link
> on their path from source to destination?]
>
> PERIODS OF EXTREME CONGESTION AT A ROUTER?
> For those routers in the network that do occationally experience
> congestion, how can we characterize their rare periods of *extreme*
> congestion (defining extreme congestion, say, as packet drop rates
> above 5%)?  How frequently to these periods of extreme congestion
> occur, and how long do they last?  What fraction can be attributed
> to flash crowds? to Denial of Service attacks? to fiber cuts or
> other routing changes?
>
> Many thanks,
> - Sally
> --------------------------------
> http://www.aciri.org/floyd/
> --------------------------------
>




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