More Questions of Exchange Points

    > 1, Some of the exchange points are layer 2 facilities, then why do they need
    > register IP addresses? Furthermore, those IP addresses do appears in the
    > traceroute traces (from the skitter data of caida). Does this mean that these
    > IP addresses are actually in use?

Nearly all exchange points are layer 2. This means that they consist of a
layer 2 switch, normally Ethernet, on rare occasions ATM or frame relay,
or even something more exotic. The participating ISPs bring routers,
which they all connect to the switch. Each of those routers must have an
IP address in order to communicate with the others, and the IP addresses
must all be within the same subnet. That being the case, the correct
procedure is for a block of addresses to be allocated to the exchange,
rather than through any one of the ISPs, so that the rest of the
participants aren't dependent upon any one ISP which might be providing
the address space. Also, that way no ISP is forced to provide transit for
the exchange-point addresses, which theoretically don't need it.

  Er, -ALL- exchanges have a layer2 component. Some institute
  policy at layer 3. Since, in general, we are talking about
  INTERNET exchanges it makes sense that IP comes into play.
  If there is a shared medium that is used as a single broadcast
  domain, then a common subnet makes life easier for everyone.
  It is possible to use divergent networks (see Sleepy Bills (woodcock)
  list) on the same shared media. His list argues that these
  distinct subnets are unique exchanges. My take is that they
  are not but that is a nit argument. There are lots of ways to
  slice the exchange point.

So yes, those addresses are very much in use, but in somewhat the same way
that the /30 on a point-to-point link would be. As a means for the two
adjacent routers to communicate, and pass on traffic which is coming from
and going to points much more distant.

  What he said. It is the Internet after all.

    > 2, How do you categorize the exchange points into large/local IXs,
    > transit/peering IXs (besides look into the peering policies)? From the number
    > of participants? Or from who are the participants?

The difference between a local and a regional exchange is typically one of
size of participants. It isn't a technical difference, so it might be a
little hard to arrive at complete consensus on, with respect to any
particular exchanges. It's probably safe to say that in the U.S., PAIX in
Palo Alto, MAE-East ATM, and Equinix Ashburn are regional exchanges, that
in Europe the LINX and AMS-IX are regional exchanges, and that in Asia
JPIX, NXP-ISP2, and HKIX are regional exchanges. They're where large
regional ISPs would go to peer with ISPs from outside the region. A
regional exchange would often be thought of as one that an ISP from
outside the region would go to first.

  only the very brave or very foolish will attempt such
  catagorization. Posh Bill (norton) clearly points out that
  the value of an exchange, like beauty, lies in the eyes/network
  of the beholder. local/regional - peering/transit.... the
  key thing is "whats in it for me?"
  

The difference between a peering exchange and a transit exchange is a much
more easily technically-defined difference: a peering exchange is one
across which, by and large, the participants just exchange peering routes.
A transit exchange is one across which many of the participants are
exchange full transit. The consequences of this distinction are pretty
far-reaching, and generally mean that only one large peering exchange can
exist in a region, and it'll be inexpensive, whereas several smaller, more
expensive transit exchanges can coexist in the same region. Phil Smith,
Keith Mitchell, and I will be presenting a paper on the topic at the next
NANOG in Toronto.

  Humph. Difference w/o (significant) distinction.
  If -ANY- isp provides transit off the exchange fabric,
  does that make it a transit exchange? If not, why not?

  And what about exchanges that have -NO- routing protocol
  at all? (can you say ARP.... sure you can.) Not peering
  or transit. Or are they?

  For me, the key point is that an exchange acts as an aggregation
  point for the participants. Generating value off aggregation
  can take many forms. Peering and Transit are but two vectors
  that are effected by aggregation.

--grumpy bill (manning)

Ruomei Gao
Email: gte489q@prism.gatech.edu

Sorry about the empty mail sent by mistake just now.

  list) on the same shared media. His list argues that these
  distinct subnets are unique exchanges. My take is that they
  are not but that is a nit argument. There are lots of ways to
  slice the exchange point.

I did observe 2 exchange points have direct connection between them, does it mean
they belong to the same switch fabiric?

> The difference between a peering exchange and a transit exchange is a much
> more easily technically-defined difference: a peering exchange is one
> across which, by and large, the participants just exchange peering routes.

Do you mean the participants just exchange BGP routing information? So the
traceroute data will only discover the peering point they exchange traffic?

ISPs exchange their traffic at IXs or private peering points, so which
is more important to the ISPs (in term of traffic volume or other
measures)? Maybe I should also mention co-locators, then what's the
difference between co-locator and the "carrier hotel"? Are they like
"physical layer exchange points" (if there is such a concept)? Are
there any other methods to exchange traffic between peers?

  Humph. Difference w/o (significant) distinction.
  If -ANY- isp provides transit off the exchange fabric,
  does that make it a transit exchange? If not, why not?

Are those private peering points?

Regards,

Ruomei

list argues that these distinct subnets are unique exchanges.

Don't anthromophize lists, they don't like it.

No argument is being made. The property of principal interest to me in
the list is the subnets, so that's what the list is of. Subnets of
machines that can talk to each other at layer 2. Many of them overlap in
different ways, but by definition, they don't overlap in subnet terms,
which is what governs layer 3 reachability over layer 2 media.

    > only the very brave or very foolish will attempt such
    > catagorization. Posh Bill (norton) clearly points out that
    > the value of an exchange, like beauty, lies in the eyes/network
    > of the beholder. local/regional - peering/transit.... the
    > key thing is "whats in it for me?"

Right, but everyone categorizes exchanges _for their own set of criteria_.
It's feckless to attempt to argue that anyone's set of criteria are
generally applicable, but it's the intersections of different folks
utility-sets which govern the formation and endurance of exchanges.

    > Difference w/o (significant) distinction.

Not at all. Read the paper before making pronouncements about it.

    > If -ANY- isp provides transit off the exchange fabric,
    > does that make it a transit exchange?

Of course, for that ISP.

                                -Bill

The difference between a peering exchange and a transit exchange is a much

    >> more easily technically-defined difference: a peering exchange is one
    >> across which, by and large, the participants just exchange peering routes.
    >
    > Do you mean the participants just exchange BGP routing information? So the
    > traceroute data will only discover the peering point they exchange traffic?

The assumtion is that all ISPs exchange routes via BGP. What's at issue
is the degree of redundancy in the routes which they're exchanging. If
they're purchasing transit at or through a facility, it's to provide
reachability to things that they couldn't otherwise reach, either normally
or under conditions of failed peering. That makes the service much more
critical than peering, which is, by definition, an economic optimization
over transit. Thus, people are willing to spend much more money on a
facility through which they're putting transit, and they're willing to
tolerate a divided marketplace, as long as each facility is able to
maintain at least three sellers.

    > ISPs exchange their traffic at IXs or private peering points, so which
    > is more important to the ISPs (in term of traffic volume or other
    > measures)? Maybe I should also mention co-locators, then what's the
    > difference between co-locator and the "carrier hotel"? Are they like
    > "physical layer exchange points" (if there is such a concept)?

These aren't necessarily useful distinctions you're making. They're
distinctions of marketing positioning. What matters economically and
technically is how people use the facilities, not what they're called.

                                -Bill

Before someone else finds a need to argue with this, I'll clarify it
myself:

The assumption is that all ISPs WHICH exchange routes DO SO via BGP.

                                -Bill