[e2e] performance of BIC-TCP, High-Speed-TCP, H-TCP etc

Injong Rhee rhee at eos.ncsu.edu
Fri Sep 22 18:45:26 PDT 2006

Hi Doug,

Thanks for sharing your paper. Also congratulations to the acceptance  
of your journal paper to TONs. But I am wondering what's new in this  
paper. At first glance, I did not find many new things that are  
different from your previously publicized reports. How much is this  
different from the ones you put out in this mail list a year or two ago  
and also the one publicized in PFLDnet  February this year  
http://www.hpcc.jp/pfldnet2006/? In that same workshop, we also  
presented our experimental results that shows significant discrepancy  
from yours but i am not sure why you forgot to reference our  
experimental work presented in that same PFLDnet. Here is a link to a  
more detailed version of that report accepted to COMNET  

The main point of contention [that we talked about in that PFLDnet  
workshop] is the presence of background traffic and the method to add  
them. Your report mostly ignores the effect of background traffic.   
Some texts in this paper state that you added some web traffic (10%),  
but the paper shows only the results from NO background traffic  
scenarios. But our results differ from yours  in many aspects. Below  
are the links to our results (the links to them have been available in  
our BIC web site for a long time and also mentioned in our PFLDnet  
paper; this result is with the patch that corrects HTCP bugs).

[Convergence and intra protocol fairness]
without background traffic:  
with background traffic:   

[RTT fairness]:
w/o background traffic:  
with background  

[TCP friendliness]
without background  
with background  

After our discussion in that PFLDnet, I puzzled why we get different  
results. My guess is that the main difference between your experiment  
and ours is the inclusion of mid-sized flows with various RTTs -- our  
experience tells that the RTT variations of mid size flows play a very  
important role in creating significant dynamics in testing  
environments. The same point about the importance of mid size flows  
with RTT variations has been raised in several occasions by Sally Floyd  
as well, including in this year's E2E research group meeting. You can  
find some reference to the importance of RTT variations in her paper  
too [http://www.icir.org/models/hotnetsFinal.pdf ]. Just having  
web-traffic (all with the same RTTs) does not create a realistic  
environment as it does not do anything about RTTs and also flow sizes  
tend to be highly skewed with the Pareto distribution-- but I don't  
know exactly how you  create your testing environment with web-traffic  
-- I can only guess from the description you have about the web traffic  
in your paper.

Another puzzle in this difference seems that even under no background  
traffic, we also get different results from yours..hmm...especially  
with FAST because under no background traffic, FAST seems to work  
fairly well with good RTT fairness in our experiment. But your results  
show FAST has huge RTT-unfairness. That is very strange. Is that  
because we have different bandwidth and buffer sizes in the setup? I  
think we need to compare our notes more. Also in the journal paper of  
FAST experimental results  
], FAST seems to work very well under no background traffic.  We will  
verify our results again in the exact same environment as you have in  
your report, to make sure we can reproduce your results....but here are  
some samples of our results for FAST.


In this experiment, FAST flows are just perfect. Also the same result  
is confirmed inthe  FAST journal paper  
-- please look at Section IV.B and C.  But your results show really bad  
RTT fairness.]

Best regards,

Injong Rhee

On Sep 22, 2006, at 10:22 AM, Douglas Leith wrote:

> For those interested in TCP for high-speed environments, and perhaps  
> also people interested in TCP evaluation generally, I'd like to point  
> you towards the results of a detailed experimental study which are now  
> available at:
> http://www.hamilton.ie/net/eval/ToNfinal.pdf
> This study consistently compares Scalable-TCP, HS-TCP, BIC-TCP,  
> FAST-TCP and H-TCP performance under a wide range of conditions  
> including with mixes of long and short-lived flows.  This study has  
> now been subject to peer review (to hopefully give it some legitimacy)  
> and is due to appear in the Transactions on Networking.
> The conclusions (see summary below) seem especially topical as BIC-TCP  
> is currently widely deployed as the default algorithm in Linux.
> Comments appreciated.  Our measurements are publicly available - on  
> the web or drop me a line if you'd like a copy.
> Summary:
> In this paper we present experimental results evaluating the
> performance of the Scalable-TCP, HS-TCP, BIC-TCP, FAST-TCP and
> H-TCP proposals in a series of benchmark tests.
> We find that many recent proposals perform surprisingly poorly in
> even the most simple test, namely achieving fairness between two
> competing flows in a dumbbell topology with the same round-trip
> times and shared bottleneck link. Specifically, both Scalable-TCP
> and FAST TCP exhibit very substantial unfairness in this test.
> We also find that Scalable-TCP, HS-TCP and BIC-TCP induce  
> significantly greater RTT unfairness between competing flows with  
> different round-trip times.  The unfairness can be an order of  
> magnitude greater than that with standard TCP and is such that flows  
> with longer round-trip times
> can be completely starved of bandwidth.
> While the TCP proposals studied are all successful at improving
> the link utilisation in a relatively static environment with
> long-lived flows, in our tests many of the proposals exhibit poor
> responsiveness to changing network conditions.  We observe that
> Scalable-TCP, HS-TCP and BIC-TCP can all suffer from extremely
> slow (>100s) convergence times following the startup of a new
> flow. We also observe that while FAST-TCP flows typically converge
> quickly initially, flows may later diverge again to create
> significant and sustained unfairness.
> --Doug
> Hamilton Institute
> www.hamilton.ie
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