Request for Comments on a topological address block for N. Calif.

   From: (Andrew Partan)

   Lets assume that everyone on the Boston area has a geographic address
   and that AlterNet and some other ISP (SmallGuy) have some customers in
   this area, and that AlterNet and SmallGuy peer at some Boston area
   exchange point. ... Now if I peer with some other ISP in some other area
   (say someone in San Francisco) ... I have now suddenly offered transit for
   SmallGuy between San Francisco and Boston.

Well, there is a "fix", but this whole topic of traffic is a major can of

There's two kinds of traffic that Alternet can provide to SmallGuy:

a) non-transit peering, meaning Alternet announces only routes it
originates to SmallGuy (and vice-versa, though that's for SmallGuy to
ensure!) and takes SmallGuy's route announcements and floods them
through Alternet's ASes (but no others!), or

b) transit peering, meaning Alternet announces to SmallGuy all routes
that SmallGuy is paying to get (Alternet's plus everyone else's
presumably, though transit-to-some-but-not-al ASes can also be done, and
where Alternet propagates all of SmallGuys route announcements to the
ASes SmallGuy buys access to.

[Buying access to some ASes but not the whole Internet as seen by
Alternet can be hard since some of these ASes may not be directly
connected to Alternet, in which case SmallGuy must also pay for transit
to the ASes between Alternet and the ones he desires. In practice
SmallGuy will buy transit to the whole Internet as heard of by

[Non-transit peering between Sprint and Alternet means that Alternet and
Sprint announce their own routes plus their transit customers' to each
other. This is so because in effect, Alternet's routes to its transit
customers can be considered to be Alternet's for the sake of the
definition I gave above in (a).]

This should make the can of worms vanish.

(This, of course, does lead to asmmetric routes. [...]

Assymetric routes can be avoided.

One fix to do this is for a provider not to accept traffic from another
provider unless that traffic is destined to an "appropriate" customer (exact
definition varies) of the first provider (or a customer of a peer provider of
the first provider, one which has a transit agreement with the first

[I'll assume permission to use the following names for the scenario
below: Alternet, Sprint, MCI, SmallGuy. May I? :slight_smile: Any resemblance to
reality is pure coincidence. :)]

If Alternet and Sprint have a non-transit peering agreement, then
Alternet announces all of its routes, including those of its transit
customers, to Sprint and vice-versa. Now if MCI also peers with Sprint
and Alternet and f everyone sticks to the definition of transit and
non-transit peering (i.e. MCI, Alternet and Sprint configure their
routers appropriately) then there's no way that Sprint will transit
SmallGuy's traffic to MCI, because MCI won't hear any routes to SmallGuy
through Sprint and Alternet will only hear MCI's routes where it peers
with MCI (if Alternet heard MCI routes elsewhere, such as through Sprint
due to a configuration error, it would still use the route through MCI
as it would have a shorter AS-path). There is then no route assymetry in
this system.

In practice this means that everyone has to be careful to filter
*outbound* announcements so that peers hear only what they are meant to
hear. So Alternet will have outbound filters that allow only routes it
originates plus routes originated by its transit customers to make it to
Sprint, and vice-versa. Also, in practice, a new provider that is trying
to get access to the whole net will have to arrange to buy transit
through someone else until it has enough peering arrangements in place
with other providers that it can offer decent reachability to its
transit customers. Large providers like Sprint and Alternet end up using
large configurations to put this sort of routing policy together.

The definition of "appropriate" is tricky; does this mean any customer (which
still leaves you vulnerable to "dumping" of long-haul load to customers of the
dumpee), or just "local" customers? If the latter, would you rather the
customer not get the traffic at all (if the traffic source is attached to a
provider which doesn't have an attachment to the destination's provider, near
the destination - we'll call this source's provider a minimal-connect
provider)? If not, how do you prevent "dumping"?

By carefully controlling outbound announcements, see above. It's true
that a bad player can rig the system so that a non-transit peer will
transit one side of the traffic to some other destination (e.g. MCI
could put static routes in its routers for SmallGuy pointing to Sprint;
Sprint will pick up traffic for SmallGuy from MCI because Sprint has a
route to SmallGuy, but traffic between SmallGuy and MCI cannot go
through Sprint without Alternet's help). In practice noone cheats like

In geographic addressing this means that you don't accept traffic for another
metro from a provider unless you have a peering arrangement with them,
otherwise you wind up doing the long-haul. This stops "dumping", true, but
still leaves your customer not getting traffic from minimal-connect customer.
You can do more or less the same thing in connecvity-based addressing, and you
have the same cut-off-minimal-connect-guys/allow-long-haul-dumping choice...

If geographic addresses don't match the underlying topology, then
dumping is hard to prevent without fragmenting the net or allowing CIDR
holes or dropping aggregation, which is to say abandon geographically
assigned addresses.

   If SmallGuy is not paying me for transit, then I am not going to do

Ah, there's the rub! What you really want, it seems, is traffic settlements
(since I don't think we can adopt/enforce a simplistic you-always-carry-your-
customers-traffic-pretty-much-to-the-destination model, which would get rid of
any need for settlements) but right now the Internet doesn't have them, for a
whole host of good reasons.

Finely detailed traffic summaries cannot be kept on the net the way
telephone calls made can be logged by phone companies, at least not
right now. If detailed summaries are ever introduced, the logging will
be done at the interface between carriers and their customers, not at
exchange points (IMHO), and the summaries will be in the form of packets
transited to a certain AS. Or will the sort of router used at exchange
points ever have enough oomph to do this sort of traffic logging
(source AS, destination AS, packet count).

There's an interesting analogy here between traffic settlements and routing
setllements; the Internet will work fine with no administrative overhead if
everyone is reasonable. However, if people try and take advantage of the
system, to a point where administrative controls are needed, the overhead
of those controls will be substanial, to the point where the cure might be
as bad as the disease. Hmm....

Sure, but if some players collude to have others transit what they
shouldn't and this is discovered, then it's all over for the rogue
players. You see, even without a central authority it is possible to
have some justice in the system.