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(Last updated: Feb. 27, 2010)

Additional thoughts on ISP delivery of IPv6

A recent IPv6 post on DSL Reports got me to thinking about how Comcast is going to deliver to the end user by means of IPv6 in their upcoming IPv6 Trials. Comcast has given us a few more details in that they will be performing these tests throughout the network, that they will begin slowly starting in the next 90 days, and that they will be providinging the end user with a new gateway. As an engineer I understand this completely including the amount of time it takes to perform these kinds of tests. A lot of thinking goes on before these kinds of things go into the network because one little mistake can bring down a network. This stuff is very complicated and if the engineer has done their job properly no one wil know about that or appreciate the complexity of the network, it will just work.

Comcast has made it very clear that they will use a dual stack technology. What that means is that the router/gateway (just gateway for the rest of these notes) will have both IPv4 (what we know and love today) and IPv6 (what we'll know and love tomorrow :-). Inside the customer's network the IPv4 will look and behave just like it always has. The gateway will perform NAT as it has for a number of years. On the IPv6 side the gateway will behave as a firewall/router. IPv6 doesn't use

Now, understand that I don't work for Comcast. I'm not a contractor for Comcast. I don't have any inside information on what Comcast is doing with their network. In fact my only relationship I have with them is that I am a customer of their TV and internet service. My only interest in this engineering exercise is that I am a network engineer and that I'm a customer who will eventually be using the service with IPv6.

ARIN Recommendation

ARIN recommendations
RIR - /23 -> ISP - /32 -> Customer - /48 -> Host /64

Now what this table says is that the RIR will be given a /23 prefix length for it region (in the Americas ARIN is the RIR ). It then delegates a /32 prefix length to the ISP who in turn gives a /48 to the customer who then redistributes that in their network where end user hosts are given a /64 prefix length.

The host

From my point of view as an engineer the ARIN recommendation works very well for business customers who have an IT budget to manage the network. But for the Comcast's general end users, consumers, this is overkill. And that becomes the problem. One thing that makes the engineers job a bit easier is that Operating Systems such as Windows, BSD (Mac OS included) and Linux, and equipment vendors are pretty much expecting to follow the RFC2462 - IPv6 Stateless Address Autoconfiguration. Additional RFCs also depend on the host getting a /64 prefix length. In my mind this pretty much makes the host /64 prefix length a given.

The gateway

Okay, now that we have the host and the /64 prefix length taken care of we can move onto the gateway. One of the first problems is does the customer even have a gateway. If we deliver a /48 prefix length to the customer (like ARIN recommends) we end up wasting a lot of address on a single host. I'm also not sure how well the host's software will behave if it suddenly gets a /48 prefix when it's expecting a /64. I'm certain that the average end user will not know how to configure their PC.

The other problem is that we probably can't deliver a /64 to the gateway as that would mean it's a bridge. Well, actually we can but I kind of doubt that we'll see this as most vendors of gateways still think in term of routers and I don't like the idea of an ISP being able to see everything on my home network. That's none of their business so I'd like to think that the idea of a router/gateway will stick around a bit longer. Oh before anyone gets too crazy, yes I undertand Firewalls, VLANs, bridging, routing and service demarcs. But your average user won't and this service will be designed with the lowest common denominator in mind (the just works crowd - which is not necessarily a bad thing).

So some form of subnetting will problably be delivered to the gateway. This means that the maximum prefix deliverd to a gateway would be /63 (two /64 networks, the ISP link and the customer's home network). I'd guess that Comcast would probably charge more to get a smaller prefix (more subnets). And why might you want more subnets? Probably for service delivery. Lets say that boiler vendor xyz is offering you a service to monitor your boiler. You could put them on thier own subnet which would separate their traffic from your home's traffic (hmm, this sounds a bit weak as there are other ways to do this - I need to think this through further).

So the outcome of all this is that the ISP will probably deliver a prefix length between /48 (overkill) to /64 (good enough for your average user). If it's a /64 prefix then they'll probably charge more for a smaller prefix (/64 -> /48, a /48 is a smaller prefix).