Can SIP and IPv6 Co-Exist?

SIP and IPv6 will get along, but the transition from IPv4 will be a challenge, as running a dual-stack environment will not always work on devices with limited memory and operating systems that support only one version of IP.

That was the general consensus at the recent SIP Network Operators Conference, according to Dan York, senior content strategist at the Internet Society. SIP works fine in either an IPv4-only or IPv6-only environment, but issues arise in a mixed environment. Most enterprises are dealing with the transition period by using dual stacks on clients, but some devices may not be able to handle both addressing protocols.

"The biggest challenge we will see is there are a lot of IP phones out there--desktop phones, hard phones--that have limited memory and embedded operating systems, and may not have a dual-stack capability," says York.

He said carrier-grade NAT will continue to play a role in the foreseeable future as IPv4 and IPv6 co-exist. When it comes to SIP, or any other real-time communications, using these translation systems adds latency. "Certainly SIP doesn't play well across NAT, even today with just an IPv4 environment. Adding in two layers of NAT for IPv4 is not necessarily going to make SIP work any better," says York.

NAT has for many years prolonged the life of IPv4 by acting as the translator between private IPv4 address spaces on a local network and public IPv4 address spaces, but carrier-grade NAT is a short-term solution for the IPv4-to-IPv6 transition.

The challenge with SIP in general is maintaining real-time communication, says Michael Brandenburg, an analyst at Frost & Sullivan. "There's concern anytime when messing with SIP. If you drop too many packets on a SIP session, you're going to get choppy calls."

However, it's service providers who first need to start making the transition to IPv6, he says. There's plenty of time for enterprises to make the transition internally--unlike Y2K, which had a hard-and-fast deadline. "The real concern would be when enterprises start to move to IPv6 internally," says Brandenburg, noting that IP PBX systems tend to have long shelf lives and may not be upgradeable or be dual-stack capable. In that case, he says, session border controllers can handle the translation for the foreseeable future.

Jeff Doyle, president of Jeff Doyle and Associates, says there are an increasing number of VoIP products that support IPv6, including SIP over IPv6, but VoIP systems and related SIP signaling will continue to be addressed with private IPv4 space inside the enterprise while session border controllers can interface with the outside world.

"With the exception of some very large enterprises or those with a footprint in areas where only IPv6 is available--opening a branch office in China, for example--depleted IPv4 address space isn't much of a driver for IPv6 in the enterprise. Most are already designed to work just fine behind NATs."

Jonathan Zarkower, director of product marketing at Acme Packet, says that when it comes to IP real-time communications such as VoIP, videoconferencing and unified communications, interoperability between IPv6 and IPv4 endpoints becomes much trickier. That's because endpoints such as IP phones use not only an IP address, but also other upper-layer protocols such as SIP and RTP. Session border controllers are better suited to handle this translation than traditional address conversion methods such as NAT or tunneling IPv6 packets as IPv4, he adds.

Zarkower says he anticipates that IPv4 and IPv6 will co-exist for the next decade as service providers gradually upgrade their infrastructure; enterprises have plenty of time to work with their vendors to upgrade and migrate endpoints behind the firewall. "IPv6 and IPv4 are going to have to co-exist for some time," he says.

Dany Jenneve, VP terminals and mobile clients, Alcatel-Lucent Enterprise, says he expects enterprises to continue to have IPv4 addresses as far ahead as 10 years from now. Right now, he says, IPv6 capable means devices are dual-stack and have been on the market for several years, so he estimates that 50% of enterprise already have dual-stack phones. The ones that can't be upgraded are those that have limited memory and were likely deployed nearly 10 years ago. Jenneve predicts that almost all devices will be dual stack within two years, and session border controllers will continue to manage address conversion at the network edge. "The transition on the public network is crucial, but within enterprise it will be slower," he says. "There's no real sense of urgency."

Jenneve says enterprises should work with their vendors to determine what equipment should be upgraded and what equipment should be replaced

York says it's important to understand all of your options and what can run dual stack during the transition period. "If you've got a world where everything runs dual stack, you're probably pretty good. If not, identify those points where you have to look at transition issues."