[nycbug-talk] Cogent and Sprint - a signal of things getting Oldschool?

[Alex Pilosov]

On Sat, 1 Nov 2008, Isaac Levy wrote:

> This thread, and Miles' earlier email about 'hot potato' routing,  
> makes me ask perhaps a stupid question:
> 
> Why is routing not synchronous?  Why is sending more expensive than  
> receiving packets- from a transit perspective?
You meant 'why is it not symmetric'. It's not supposed to be. Just look
how BGP works - each AS "announces" a set of networks, and determines from
a set of available AS-PATHs (based on some policy, involving "money" or
as-path-distance), where the outgoing packet will go. If a packet is going
from AS A to AS B, the set of AS-PATHs and associated policies will be
very different and unlikely to be symmetric vs packets going from AS A to
AS B.

There's no such thing as 'sending vs receiving' - in any TCP
conversations, packets always flow both ways. What you mean, and what is
being sort-of-used-as-justification-of-higher-expense-for-eyeball-network 
is the fact that 

a) Almost everyone uses "hot potato" routing - that means, you get it out
of your network as soon as you can to pass it off to peer/transit/etc.  
Assuming you have multiple points of interconnection with it, for example,
if Pilosoft had nationwide network and connection to AT&T in San Jose and
NY, and I have a packet from NYC customer going to AT&T, I *should* still
hand it to AT&T in NY, regardless of where it is going to end up. [1] 

In fact, hot potato is the *preferred* way - since you don't know how your
peer/transit network works, you should optimize *your* path, and let
*them* worry how to carry it inside their network.

b) As result, assuming AT&T customer is downloading something hosted on my 
network, AT&T will carry large packets with content on the path from 
NYC->SJC, and I would carry ACKs SJC->NYC. Since ACKs are much smaller, 
the load on their network will be more than mine.

[1] There's such a thing as "MED" (multiple exit discriminators) to avoid
"hot potato" routing, but it has potential to fuck things up more - so it
is rarely used except in situations where your peers *require* you to obey
the MEDs