Comcast’s Possible IPv6 Deployment Plan

A follow up to the post named “The Future of IPv4“:

In the Network World article “Comcast pitches IPv6 strategy to standards body” the Cable ISP discusses their current plans on how to migrate their customers to IPv6.

The summary of what happens once Comcast runs out of IPv4 address to give to its customers:

Customers get a new cable modem or router device that does real routed IPv6. You get a real IPv6 address or range of IPv6 addresses and your home network is a directly connected fully functional member of the IPv6 world.

Newer customers won’t get their own IPv4 address anymore. Existing customers would retain their ability to get an IPv4 address for a while but nothing is guaranteed. The modem or home router will hand out private addresses to your home computers and devices. Any attempt to contact the outside IPv4 world will be blindly tunneled over IPv6 to a NAT router that Comcast runs in one of their data centers. Multiple customers will share the same IPv4 address and NAT will be used for all IPv4 connections. This will hurt or possibly prevent many forms of P2P over IPv4. Assuming they didn’t firewall off all incoming IPv6 connections these P2P tools would work normally over IPv6.

At a quick glance it doesn’t seem like an unworkable plan. Of course being NATed by your ISP and not by a router you control will just plain suck. I’d strongly suspect any “NAT” solution they implement on their servers will have to do some inspection of connections (similar to Linux’s conntrack modules) and do a bit more than straight NAT to work around common application problems. Unfortunately if the ISP can’t get any new IP allocations at a reasonable cost and is truly out of IPv4 address then the customer is crap out of luck. The question is exactly under which conditions the transition would start and what restrictions they would place on the IPv6 end of things. And I hope they will read their RFCs and hand out the RFC recommended number of IPv6 IPs to every customer. That is half the point of IPv6 after all.

Update: Comcast’s approach seems to be covered in the IETF document Dual-stack lite broadband deployments post IPv4 exhaustion. On the topic of NAT and IPv4 application compatibility the document says:

Most applications that today work transparently through an IPv4 home
gateway NAT should keep working the same way. However, it is not
expected that applications that requires specific port assignment or
port mapping from the NAT box will keep working. Details and
recommendations for application behavior are outside the scope of
this document and should be discussed in the behave working group.

Translation: We chose not to care.