Ruomei Gao Email: gte489q@prism.gatech.edu On Sat, 6 Apr 2002 bmanning@karoshi.com wrote:
> 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)