Where is the edge of the Internet?

Martin marty at supine.com
Tue Nov 5 10:13:16 UTC 2002

$author = "alok" ;
> so its a hardware limitation?....bigger cores needed

not necessarily. if you do the filtering in the right places you can leave
the core to do it's job of passing packets.

also, the idea of filtering at the edges is designed to reduce the distance
dud packets travel in your network, leaving your routers to worry about
passing legit packets.  

> fair enuf...... 2 schools of thought, and ur idea makes sense
> too... no denying that...but you have corner cases... which wont come up if
> it could be in the core.....

the idea behind the extended filtering capabilities in routing software /
OSes is to address the problems you describe.

> well that covers everything doesnt it ;o)... even those not in ur
> network..does it actually ping and check to see if its there?

no, a default route is a default route. it doesn't check the IP address, but
any packets to dud addresses will get dropped the second they hit a default
free zone (if there is no matching prefix) or the upstream router (addresses
covered by a prefix but not used).

> do u inject BGP into IGP? ....do all access boxes have the
> entire BGP table/or know every address/network on the internet?

i'd be running iBGP across the default free core and IGP to cover link state
of your core. i've seen BGP injected into IGP and it can end up ugly if your
not careful.

so yes, you'd have a subset of your routers with full tables. you can filter
on these routers using "reachable-via any" to address asymmetry. on routers
closer to the customer edge, you might not have a full table but you can
apply stricter filtering given that you should know what subnets are coming
in your customer facing interfaces.

> most access would be the corner cases... i have cases where tier-2
> ISPs would simply take a 3 Mb uplink from 1 service provider and a fat
> downlink from another (ISP-2) ...all the BGP routes/advertisements would be
> in the 2nd ISPs networks, ISP 1 has no idea what this guys address range is
> at the access is... this is a common mechanism lots of tier-2 ISPs would
> apply......

? ISP-1 can filter packets based on subnets known to be attached to the
customer circuit (your customer system does record IP addresses assigned to
customers or provider independent IP subnets that your customers have,
doesn't it?!?)...

ISP-2 would do the same for upstream traffic. downstream both ISPs could
apply whatever filtering is appropriate (loose / strict) given their network

> we cud start a new topic...
> "where is the core of the internet"?
> coz assymetric routing messes up everything :o) even for those scenarios
> on the core...

read up a little RPF and the difference between "strict" and "loose"...



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