Subject: Re: [alto] ns-3/ALTO: Influencing suboptimal peer selection

From: "Vijay K. Gurbani" <[email-masked]>
Date: Mon, 02 May 2011 08:42:26 -0500

On 04/30/2011 11:10 AM, Richard Alimi wrote:
> Very interesting results!  It does look like it converges to a good
> result, but I'm wondering if it would be possible to reduce the
> convergence time further (smaller than 10m). Can you give some more
> details on how ALTO was integrated with the BitTorrent clients here?

Rich: Sure.  Please see inline.

> Was it used for ranking peers returned from a tracker, or also
> peer-exchange (if enabled)?

We have not implemented peer-exchange in our ns-3 model.  So,
the results you see are only from getting the peer list from a
tracker.

> How often were new peers discovered from a tracker or peer-exchange?

A peer in our model contacts the tracker every 30s of simulated time.

> Was it a greedy strategy in which only closest peers were chosen, or
> were a few sub-optimal peers chosen for robustness?

For starters, the peer list returned by the tracker is not modified
to remove any peers; the peer list is simply re-ordered with the
least desirable peers appearing at the bottom of the list.

The announcing peer gets this list from the ALTO server and starts
to contact each peer in the list starting from the top of the
list, subject to the constraints that:

(a) it may already have an existing connection with that peer,
  so no point in establishing a new one;
(b) the contacted peer may not accept the announcing peer's connection
  request;
(c) the contacted peer may accept the announcing peer's request, but
  choke it until further notice;

Thus, a few sub-optimal peers will sneak depending on the constraints
outlined above.  I have not done an analysis on output of the
simulation to see what is the distribution of the connections between
an announcing peer to sub-optimal peers, but intuitively, it appears
that there will be a minority of suboptimal peers.

Thanks,

- vijay
-- 
Vijay K. Gurbani, Bell Laboratories, Alcatel-Lucent
1960 Lucent Lane, Rm. 9C-533, Naperville, Illinois 60566 (USA)
Email: vkg@{bell-labs.com,acm.org} / [email-masked]
Web:   http://ect.bell-labs.com/who/vkg/

---

From: Arnaud Legout <[email-masked]>
Date: Thu, 05 May 2011 14:07:51 +0200

Hi,

On 15/04/2011 18:43, Vijay K. Gurbani wrote:
> One issue that has been talked about in ALTO is whether it can
> influence the choice of sub-optimal peers; in other words,
> can ALTO be used to purposely provide sub-optimal peers simply
> because a peering relationship exists and the peers in the
> peering network are sub-optimal (i.e., they may have less upload
> bandwidth when compared to other peers).
>
> The third simulation shows that this is not the case, and other
> publications [2] have experimentally demonstrated that this
> is not the case for BitTorrent at least.  BitTorrent exhibits
> strong clustering behaviour, where similar-bandwidth peers
> cluster together to ensure effective sharing incentives by
> rewarding peers who contribute proportionally more to the
> system.

what we have done in [2] is in the context of no constraint on the 
interconnexion graph
among peers. We studied in [3] the case of a constrained graph (used to 
enforce locality).
I believe [3] is more relevant to the discussion, as it shows that even 
with a very constrained graph,
BitTorrent is still quite efficient.
However, we didn't looked at the clustering properties with a 
constrained graph in [3]. From [2], I am convinced that
if the incoming rate of new pieces in a given ISP is lower than the 
upload rate of the fast uploading leechers in that ISP,
it is likely that clustering and sharing incentives will not hold 
anymore. The lack of sharing incentives might be a real issue.

For this reason, it is certainly a good idea to use fast uploading 
leechers as bridges among ISPs so that
each of those leechers can act as fast surrogate feeds for new pieces. 
In that case, clustering and sharing incentives
will still hold.

All my reasoning is in case of torrents that are starting (many 
leechers, one seed). As long as there are enough seeds, the
sharing incentive property is less important.

Regards,
Arnaud.

> [2] "Clustering and sharing incentives in BitTorrent systems",
>  Arnaud Legout, Nikitas Liogkas, Eddie Kohler and Lixia Zhang,
>  ACM Sigmetrics, June 2007. 

[3] Stevens Le Blond, Arnaud Legout, Walid Dabbous. Pushing BitTorrent 
Locality to the Limit. Computer Networks, 
doi:10.1016/j.comnet.2010.09.014, 2010.