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

Sam Stickland sam_mailinglists at spacething.org
Fri Aug 17 09:54:47 UTC 2007

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).


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