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

Stephen Wilcox steve.wilcox at packetrade.com
Fri Aug 17 10:55:06 UTC 2007

On Fri, Aug 17, 2007 at 10:54:47AM +0100, 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.

Hey Sam,
 thats an excellent point made..

Altho I dont think its unique to UK/BT .. since last mile is recognised as most places as the big cost (in the UK its around 100x the cost of the backbone roughly) .. here anything traversing the last mile is not desirable, especially if it does it twice.


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