Comcast pitches IPv6 strategy to standards body

Dual-Stack Lite is backward-compatible with IPv4 and can be deployed incrementally

Comcast Corp. has developed an innovative approach for gradually migrating its customers to the next-generation Internet, and the ISP is promoting this approach to the Internet's leading standards body.

Comcast is the largest cable operator in the U.S., with 24.7 million cable customers, 14.1 million broadband customers and 5.2 million voice customers.

Comcast is upgrading its networks from IPv4, the Internet's main communications protocol, to the standard known as IPv6. IPv4 uses 32-bit addresses and can support 4.3 billion devices connected directly to the Internet. IPv6 uses 128-bit addresses and supports a virtually unlimited number of devices.

At issue is how Comcast will support new customers when IPv4 addresses run out, which is expected in 2011. Comcast can give these customers IPv6 addresses, but their home computers, printers, gaming systems and other Internet-connected devices are likely to support only IPv4.

Comcast engineers have come up with a solution to this problem, dubbed Dual-Stack Lite, which it says is backward-compatible with IPv4 and can be deployed incrementally.

Comcast outlined Dual-Stack Lite in a draft document published by the Internet Engineering Task Force on July 7. Dual-Stack Lite will be discussed at an IETF meeting in Dublin scheduled for later this month.

"This is about making IPv6 deployable incrementally," says Alain Durand, director of Internet governance and IPv6 architecture in the office of the CTO at Comcast. Durand, a longtime IETF participant and IPv6 developer, chairs the IETF's Softwires working group, which is looking at IPv4-to-IPv6 transition issues.

"If you look at all the technologies deployed on the Internet in the last 15 years, all the successful ones have been deployed incrementally," Durand says. "You can deploy [Dual-Stack Lite] in your own network and get some benefits immediately regardless of whether your neighbors are doing it."

Durand points out that Comcast has not yet committed to using Dual-Stack Lite internally.

"This is a technology that we are looking at, but we have not committed to deploy it," Durand says. "It seems promising, but we have to make sure that it actually works and that it actually scales to the size of our network before we put it in our network."

Deploying IPv6

Comcast has been deploying IPv6 internally since 2005.

"Our backbone has been operational with IPv6 since 2006, and our original network has been progressively migrating to IPv6," Durand says. "What we are doing is moving to edge management of cable modems. This is the part that's in a trial phase."

Durand says it's critical for Comcast to be able to manage its cable modems and set-top boxes after all of the IPv4 addresses are used up.

"IPv6 enables us to have global visibility of all of our networks," Durand says. "It allows us to manage tens of millions of devices in one single view."

Currently, Comcast gives one global IPv4 address to each of its customers. The customers are given home gateways that use private IPv4 addresses for each of their devices, such as PCs or gaming consoles. The global IPv4 address is matched to multiple private IPv4 addresses through a process known as network address translation (NAT), which is done by the gateway.

When IPv4 addresses are used up, Comcast will need to find a strategy for allowing a customer's IPv4-only devices to use an IPv6 address to communicate over an IPv4-driven Internet.

"We cannot force our customers to replace every single device in their homes. This is a nonstarter," Durand says. "Also, if you look at the content on the Internet, the majority is reachable with IPv4. That may change in the future, but this is going to take many, many years."

The question is how Comcast can give its customers access to IPv4 content when there are no IPv4 addresses available. Unless customers upgrade their PCs to Microsoft Vista, which is IPv6-enabled, they won't be able to reach IPv4 content without a new mechanism such as NATs for IPv6.

Comcast's idea is to allow many broadband customers to share one global IPv4 address instead of providing one global IPv4 address per customer.

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