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.
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.
> 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.
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.