Comcast pitches IPv6 strategy to standards body

"The exact ratio of IPv4 addresses to customers is something we are studying right now," Durand says. "We are working on some tests to see if it is 1-to-5, or 1-to-100 or 1-to-200."

This approach would be for new customers only; existing Comcast customers would keep the global IPv4 addresses they already have.

The trick with sharing public IPv4 addresses among many customers is doing it in the simplest way, Durand says.

One possibility involves two layers of NATs: one in customer gateways between private IPv4 addresses and shared public IPv4 addresses, and another inside carrier networks between shared IPv4 addresses and IPv6 addresses.

Durand says multiple layers of NATs would result in networks that are more complex and costly for carriers to operate.

"With two layers of NATs, there are two places where NATs can be tricky and create problems," Durand says. "Also, your single view of the network is fragmented," which hinders debugging and repairs.

How Dual-Stack Lite works

Instead, the Dual-Stack Lite approach would use one layer of NAT — the carrier-grade NAT — along with IPv4 to IPv6 tunneling from the customer's gateway to the carrier's NAT.

With Dual-Stack Lite, the carrier upgrades its networks to IPv6 but uses a combination of tunneling and NAT to allow customers with IPv4-only devices and IPv6 addresses to access IPv4 and IPv6 content.

New customers with IPv6 addresses would get special home gateways that do tunneling but not NAT. These gateways would take IPv4 packets and ship them over an IPv6 tunnel to the carrier-grade NAT, which handles translation in a way that's similar to today's IPv4 NATs.

"This greatly simplifies and reduces the cost of the home gateway," Durand says. He adds that the new home gateways would be dual-stack, which means they would support both IPv4 and IPv6.

The carrier-grade NAT would be a dual-stack router that terminates IPv4-to-IPv6 tunnels and performs traditional IPv4 NAT. Durand says the carrier-grade NAT could be implemented on a PC running Linux, or it could be implemented in software only.

"We are talking to some open-source developers about creating this," Durand says.

Durand says the NAT/tunneling combination is simpler and less expensive than multiple layers of NATs for carriers to maintain during the transition from IPv4 to IPv6. This approach also allows a carrier to have visibility into home gateways through IPv6.

"The beauty of this is that there is no new technology to invent," Durand says. "This is combining pieces that already exist. We have known how to use tunnels for 15 years, and IPv4 NATs are nothing new. We'll have the same thing with the carrier-grade NAT except that it does also need the capacity to decapsulate the packets from the tunnel and remember which tunnel it was coming from."

Although Comcast's focus is on serving its residential customers, Durand says the Dual-Stack Lite approach could be used in enterprise networks.

"You can apply most of this to any large-sized network that has lots of remote branches," he says. "The remote branches would do IPv4 and IPv6 internally and then have an IPv6-only address. The IPv4 traffic would be tunneled over an IPv6 connection to a carrier-grade NAT in the corporate network. This will enable you to deploy very large IPv4 networks without worrying about running out of IPv4 addresses."

Durand says that the Dual-Stack Lite approach will accelerate the Internet's transition to IPv6, not delay it.

"The alternative is all those layers of NAT, which will be all IPv4 with not IPv6 at all," Durand says. "If we go forward with multiple layers of NATs, there is no incentive ever for deploying IPv6. ...This plan can accelerate the deployment of IPv6 because it makes it incrementally deployable."

Comcast points out that it is trying to propose Dual-Stack Lite as an industrywide solution to IPv6 transition.

"This is not something that is a Comcast-only solution. This is something that we are working with the rest of the industry on," Durand says. "I have had a number of discussions with service providers around the world, especially in Japan and Europe, who are very interested in something like this."