Generally, throttling at an ISP level works by either crude blocking of ports right through to analysis of the types of traffic, the protocols in use, and the duration of the connections between hosts on the network. An HTTP request for example will look rather different to a torrent download. Unfortunately for ISPs, any system can be defeated or at least compromised when people come up with a new system that works against them.
Crude systems can be like this. Good systems are rather better.
Last time I tried, Plusnet's system was perfectly capable of detecting P2P stuff on odd ports, even when encrypted. It wasn't very defeatable - but things might have changed; I guess it is an ongoing battle
In any event, the assumption that BT actively throttle any protocol or port on an infinity connection is also flawed. Unfortunately it seems to be peddled relentlessly by some. My experience, as someone using the service for 17 months, suggests it simply isn't true. If one takes time to read the published BT policy on usage they'll see it specifically states that no protocols are blocked as a matter of course. Furthermore no protocols are prioritised. P2P applications "may be managed" at "peak times" which is essentially "during the day and evenings".
In simplified terms this translates as "We'll only manage P2P and we'll only manage it on the occasions when it starts having an impact on overall service".
The only problem is that you have no real idea what you are signing up for until you try it - and with 18 month contracts, you're locked in for an awfully long time, with little recourse if "things" start to happen.
Now, note that this statement of fact has 2 different consequences for 2 different groups.
For avid P2P people, they'd prefer that "management" never happens, and hope that either BT don't see P2P "having an impact" or BT find it impossible to manage effectively. But if P2P gets out of hand and it *is* possible to manage, then your 17 month period may turn out to be no more than a lucky honeymoon. Imagine a 1 month honeymoon and 17 months with dire P2P service instead!
For non-P2P people, they want precisely the opposite. A fast, stable network that prioritises interactive & time critical protocols that doesn't get any interference from the sheer volume of P2P. If BT fail to control P2P, this group run the risk of being locked into an 18 month contract with terrible behaviour - akin to the worst of Virgin's latency problems.
Which way will it go? The problem is that we just don't know yet. On one hand, BT's "unlimited" policy means it is very attractive to the very worst-case P2P 24-7 max volume crowd - so BT are more likely to get to the point of needing to manage. On the other hand, if they fail to control things, then they have access to Plusnet's expertise on traffic management.
It's why I find the "but BT traffic shape your Bit Torrents!" posters even more funny. You'd think they'd understand how foolish they look when they come out with statements such as this - with no evidence and no experience of the situation (as they are almost always users of other (more expensive) ISPs). Essentially, these types are paying more for a service that is actually more actively throttled than BT's one. Their perceptions obviously played a decision in their original ISP selection process but as explained the perceptions are faulty. I can understand this would be rather frustrating so the default response, rather than find fault with their own ISPs or their own purchasing decisions, is to find fault with the decisions and ISPs of others.
I agree with you regarding the evidence & experience. So far BT *have* been able to cope with the P2P levels thrown at it so far.
With ADSL, it took a while for congestion to start hitting throughput, and FTTC will be the same: but still, congestion will come eventually. We subscribers just don't know (yet) how the chips will fall, or when. BT probably does know, but chooses not to tell us.
I'm only a casual P2P user (Linux disks, etc), and have far stronger requirements that I can predict that my connection stays usable throughout the day, throughout my contract period, without uncontrolled (and uncontrollable) interference from others. The Plusnet package gives me the best of both worlds on this, but I think it happens to give me one very great advantage: The mere existence of peak hour limits keeps the very worst bandwidth abusers on someone else's ISP!
So yes, I am deliberately getting a connection that is limited at times (but paying less for it), but am willing to do so because there is a specification of what those limits are, and I can live with those limits.
I *could* live with BT's current offering too, but I don't know if I could live with it tomorrow.
Risk management. Not such a simple science