Extreme congestion (was Re: inter-domain link recovery)

Leigh Porter leigh.porter at ukbroadband.com
Fri Aug 17 10:14:50 UTC 2007

Sam Stickland wrote:
> Ted Hardie wrote:
>> Fred Baker writes:
>>> Hence, moving a file into a campus doesn't mean that the campus has
>>> the file and will stop  bothering you. I'm pushing an agenda in the
>>> open source world to add  some concept of locality, with the purpose
>>> of moving traffic off ISP  networks when I can. I think the user
>>> will be just as happy or  happier, and folks pushing large optics
>>> will certainly be.
>> As I mentioned to Fred in a bar once, there is at least one case
>> where you have
>> to be a bit careful with how you push locality.  In the wired campus
>> case, he's certainly
>> right:  if you have the file topologically close to other potentially
>> interested users,
>> delivering it from that "nearer" source is a win for pretty much
>> everyone.
>> This is partly the case because the local wired network is unlikely
>> to be resource
>> constrained, especially in comparison to the upstream network links.
>> In some wireless cases, though, it can be a bad thing.  Imagine for a
>> moment that
>> Fred and I are using a p2p protocol while stuck in an airport.  We're
>> both looking
>> for the same file.  The p2p network pushes it first to Fred and then
>> directs me to get
>> it from him.  If he and I are doing this while we're both connected
>> to the same resource-constrained base station, we may actually be
>> worse off, as the
>> same base station has to allocate data channels for two high data
>> traffic
>> flows while it passes from him to me.  If I/the second user gets the
>> file from outside the pool of devices connected to that base 
>> station, in other words, the base station , I, and its other users
>> may well be better off. 
> A similar (and far more common) issue exists in the UK where ISPs are
> buying their DSL 'last mile' connectivity via a BT central pipe.
> Essentially in this setup BT owns all the exchange equipment and the
> connectivity back to a central hand-off location - implemented as a
> L2TP VPDN. When the DSL customers connects, their realm is used to
> route their connection over the VPDN to the ISP. The physical hand-off
> point between BT and the ISP is what BT term a BT Central Pipe, which
> is many orders of magnitude more expensive than IP transit.
> In this scenario it's more expensive for the ISP to have a customer
> retrieve the file from another customer on their network, then it is
> to go off net for the file.
> (LLU (where the ISP has installed their own equipment in the exchange)
> changes this dynamic obviously).
> S

Also bear in mind that many wireless systems have constrained uplink
capacity and anything P2P can quite happily kill a wireless network by
using up too much uplink resource.

Leigh Porter

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