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

Ted Hardie hardie at qualcomm.com
Thu Aug 16 21:44:55 UTC 2007

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.  

To put this another way, the end user sees the campus case as a win primarily
because the campus is not resource constrained (at least as compared to its
upstream links).  You can only really expect their cooperation when this is
true.  In cases where their performance is degraded by this strategy, their
interests will run counter to the backbone's interest in removing congestive
flows.  Accounting for that is a good thing.


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