Thanks for the new paper, there is a lot of nice data here. The presented graphs and analysis indeed suggest that the BitTorrent protocol's choices achieve near-optimal data distribution in its paradigm (single file, block-based content distribution). I agree with your suggestions for fairness criteria, and my own suggestions are similar (my suggestion is for a reputation-based system to model leechers and free-riders).
However, I disagree with several conclusions you make along the way. A brief overview:
- Byte-for-byte fairness is not appropriate
- Network Coding is not justified
Fairness:
I am unconvinced by the arguments against byte-for-byte fairness. Your evidence to support this is that tit-for-tat does not take into account the excess capacity of some torrents (ie: more seeders than leechers). Yet this situation is equivalent under byte-for-byte and choking rules. Leechers are upholding the byte-for-byte policy (seeders cannot, as you mention) and thus only leechers are restricting their upload capacities to free-riders, and rightly so. Seeds (who hold the excess capacity) are distributing this capacity fairly to all peers. Thus, choking and byte-for-byte are equivalent in this regard - free riders receive a share of the bandwidth equal to the fair distribution given by seeders in both cases.
The more important case, when excess capacity is not available, free riders should be eliminated or diminished. Because leecher participation cannot be proven, the only other method of measurement is via equal exchange of services. Byte-for-byte is one such measurement (reputation systems are another). In this scenario, free riders will only receive a small credit from the leechers in the swarm before having to resort to hand-outs from the seeds. With the seeds giving fair capacity to the swarm, the free riders will receive very little of the swarm capacity, while the participating leechers will receive fair resources.
Free riders always receive more resources under choking than under exchange measurement models.
Network Coding:
On the topic of Network Coding [1], I did not see any evidence in your paper refuting the network coding literature. Please correct me if I missed it, but it appears your only rebuttal is in the poor modeling of BitTorrent, not in the application of network coding techniques. From this I don't see how you conclude that network coding is not justified? I agree that most of the simulation literature does not model BitTorrent correctly, but this doesn't negate the positive results of the network coding trials, only their comparisons to BitTorrent.
In section 4-A, pp 6-7, you suggest that source or network coding may not propose interesting pieces to peers during the startup phase. I counter that this is exactly what network coding solves - every packet has an innovation rate based on the network flows from the local node to the source(s). Assuming a densely connected graph with fair sharing from the seeder, it is unlikely that any packet is not innovative during the startup phase. Unless the local node's bandwidth is many multiples of the seeder, a problem which is not specific to network coding.
Network Coding is superior to Rarest First and other BitTorrent-like protocols for several reasons:
1) The innovation rate is the min-cut of all flows from the local node to the source(s), which is likely to be much larger than BitTorrent's default of 4 active connections.
2) BitTorrent transfers pieces in blocks-at-a-time, but only advertises in piece-at-a-time. This injects a significant delay between downloading of an innovative byte and sharing of that byte (on average, one half the download time of an entire piece). Network coding imposes no such delay.
3) There is no first-blocks or last-blocks problem with network coding, they are entirely non-issues. Large portions of your paper are devoted to these issues which simply don't occur under network coding.
4) The meta-network utilization is higher for network coding. That is, since BitTorrent cannot align to the underlying physical connections of the network elements, it is impossible to maximally utilize them. Network coding (using an innovation rate tracking algorithm) can rapidly align to the structure of the physical network, eliminating cross-core retransmissions. This saves a significant amount of ISP bandwidth and reduces the overall burden of supporting filesharing services on the internet.
Of course, network coding does have cons also:
- Typically larger memory footprint
- Typically larger CPU requirements (falling rapidly with new research)
- Typically more complex supporting protocols (innovation tracking, security checks, etc)
Is the full implementation of Network Coding superior enough to BitTorrent that it offers compelling reasons for migration to it as a solution? Your paper argues that BitTorrent is "good enough", and I tend to agree.
Please discuss! :-)
--Bryan
[EMAIL PROTECTED]
[1] Network Coding: An Instant Primer - http://infoscience.epfl.ch/search.py?recid=64508
On 5/3/06, Arnaud Legout <[EMAIL PROTECTED]> wrote:
Hello,
we have a new paper called "rarest first and choke algorithms are enough"
that is about an experimental evaluation of the two core BitTorrent
algorithms
on real torrents.
A. Legout, G. Urvoy-Keller, and P. Michiardi. Rarest First and Choke
Algorithms Are Enough. Technical Report
(inria-00001111, version 1 - 13 February 2006), INRIA, Sophia Antipolis,
February 2006.
http://hal.inria.fr/inria-00001111/en
Comments are welcomed.
Regards,
Arnaud Legout.
--
Arnaud Legout, Ph.D.
INRIA Sophia Antipolis - Planète Phone : 00.33.4.92.38.78.15
2004 route des lucioles - BP 93 Fax : 00.33.4.92.38.79.78
06902 Sophia Antipolis CEDEX E-mail: [EMAIL PROTECTED]
FRANCE Web : http://www-sop.inria.fr/planete/Arnaud.Legout/index.html
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