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Windows Server 2012 : IPv6 Introduction (part 4) - IPv6 Transition Technologies

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3/15/2015 5:32:12 AM

Today, the Internet (and the world) is mostly running on IPv4 networks. As more and more operating system and devices natively support IPv6, and even require it, making IPv6 work globally will quickly become necessary. Because IPv4 and IPv6 devices cannot natively communicate with one another, protocols have been developed to bridge the gap, and these are known as the IPv6 transition technologies.

Before discussing transition technologies, or how we can make IPv6 devices communicate with IPv4 devices, we need to examine the different types of nodes on the networks, as defined in RFC 2893:

IPv4-only node—This type of nodes uses IPv4 only and most likely does not even have the IPv6 protocol installed. This is Windows XP and Windows Server 2003 and earlier, by default, but IPv6 can be installed and configured on these operating systems.

IPv6/IPv4 node—This is the typical node today, which has both IPv4 and IPv6 protocols installed and uses both protocols. Windows Vista, Windows Server 2008 and later client and server operating systems are IPv6/IPv4 nodes.

IPv6-only node—A node that only has the IPv6 protocol installed and in use. In today’s world, finding a node that fits this description is nearly impossible.

IPv6 node—A node that uses IPv6, regardless of whether IPv4 is also used. This is a more generic term

IPv4 node—A node that uses IPv4, regardless of whether IPv6 is also used. This is a more generic term.

Communication between IPv4 and IPv6 devices does not directly occur. Devices need to communicate using the same protocol. You can enable IPv6 devices to communicate over IPv4 networks by using some form of tunneling, but when IPv6/IPv4 nodes are in use on the same network and properly addressed, they use transition technologies built in to the protocol stack architectures. Windows XP and Windows Server 2003 use the dual stack, and later operating systems (Windows Vista and Windows Server 2008 and later) leverage the dual layer. With the dual stack, when data is prepared for the network, it is prepared for both protocols separately, requiring more overhead within the operating system. With the dual layer, the upper-stack layers are shared, thus reducing overhead, and that is the architecture in use with the more recent operating systems. Future releases will most likely do away with the dual layer and move toward an IPv6-only stack. Figure 2 compares the dual stack and dual layer architectures.


Figure 2. Dual IP layer and dual stack architectures.

IPv6 Tunneling

When IPv6 nodes are separated between IPv4 networks, they cannot directly communicate. To bridge the gap, the IPv6 nodes can tunnel through the IPv4 network. This tunneling can occur on the host itself or through a designated IPv6 router tunneling router. When IPv6 is tunneled through an IPv4 network, the IPv6 packet is encapsulated within an IPv4 packet. Figure 3 shows an example of an encapsulated header.


Figure 3. IPv6 packet encapsulated in IPv4.

There are two different IPv6 tunnel configurations. The first is a configured tunnel, in which the endpoints and static routes for IPv6 traffic through an IPv4 network are defined. The second is an automatic tunnel that is created based on the IPv4 address of the device. The automatic tunnel can be leveraged only on devices that are properly configured to use both IPv6 and the IPv4. Configured tunnels, although these can be created on local windows hosts, can also be configured on designated routers to bridge the gap between IPv6-only devices on different networks separated by IPv4 networks.

The ISATAP Tunneling Protocol

The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) is an IPv6 transition technology used to allow administrators to deploy IPv6 nodes on IPv4 networks. For ISATAP to be used, an ISATAP router must be deployed and have a DNS record on the local network. ISATAP is not intended for use across the Internet (hence the user of intra-site as part of its name), but ISATAP traffic must traverse an ISATAP router to allow an IPv6-only host to communicate with IPv4 devices. The two main requirements for an ISATAP network to function are an ISATAP router and, of course, ISATAP hosts:

ISATAP router—An ISATAP router advertises subnet prefixes assigned to the ISATAP network, to ISATAP hosts, and the router forwards packets between the IPv4 and IPv6 network.

ISATAP hosts—ISATAP hosts can communicate directly with ISATAP and IPv6 hosts on the local network and with IPv4 hosts through the ISATAP router.

ISATAP addresses are automatically assigned or created on ISATAP hosts, but before that occurs an ISATAP router must be detected by a potential host. A router is detected when a host can resolve the name ISATAP with a DNS lookup within their primary DNS suffix, or through another form of short-name resolution. When an ISATAP router is detected, an address is constructed based on the address prefix provided by the ISATAP router, concatenated with the local IP address of the ISATAP host. The ISATAP address is constructed of the 64-bit IPv6 prefix already defined for the IPv6 network followed by a 32-bit ISATAP designation and then by the 32-bit IPv4 address. For example, with an IPv6 prefix of 2001:0dba:1234:5678::/64 for a host with an IPv4 address in a private network range, the address is as follows:

2001:0dba:1234:5678:0:5efe:w.x.y.z, where the w.x.y.z represents the IPv4 address

The 6to4 Tunneling Protocol

The 6to4 tunneling protocol provides automatic address assignment and tunneling of IPv6 traffic across the IPv4 Internet. This is mainly used when the host or client is connected directly and assigned a public IPv4 Internet address, but can also be used when a host has an IPv4 private address assigned. The 6to4 address format uses an IPv6 global prefix because it is an IPv6 prefix that is okay to route across the Internet. Figure 4 shows the 6to4 address format.


Figure 4. 6to4 IPv6 address format.

A 6to4 global address prefix is in the format of 2002:WWXX:YYZZ::/48, where WWXX:YYZZ is the hexadecimal representation of the public IP address. Each of the two letters represents one of the 32-bit IPv4 octets. As an example of a 6to4 address converted, a public IP address of is converted to 2002:4822:710b::/48.

The 6to4 network can include the following components:

6to4 host—A host that has both IPv4 and IPv6 and is configured with a 6to4 address in the IPv6 global address range of 2002::/16.

6to4 router—An IPv6/IPv4 forwards traffic between 6to4 hosts on the local network to other 6to4 routers and to 6to4 relay routers.

6to4 relay—Forwards 6to4 traffic between the IPv4 Internet and IPv6-only devices directly connected to the Internet.

The Teredo Tunneling Protocol

The Teredo IPv6 transition technology is commonly used when the client system is assigned a private IP address and a 6to4 network is not enabled or preferred. One of the biggest advantages of Teredo over 6to4 is that 6to4 is not so NAT friendly when traversing the Internet and each endpoint needs to have a public IPv4 address. Also, 6to4 traffic has a reasonably high failure rate because the packets are encapsulated and marked with protocol field 41, which is unknown to many firewalls, and unknown protocols are usually blocked by default. Also, using the public Internet, end users cannot control how many NAT traversals (NAT-T) the packets must go through from the sources to the destination, making 6to4 not so resilient to support routing through NATs. This is where Teredo (IPv6 NAT-T) becomes the preferred tunneling protocol. RFC 4380 describes the Teredo tunneling protocol. Teredo makes its way around the NAT challenge by changing the way the IPv6 packet is encapsulated. With the ISATAP and 6to4 tunneling protocols, the IPv6 packet is encapsulated within an IPv4 packet, and the header IP protocol field is set to a value of 41 to identify tunnelled traffic. Teredo tunnels Ipv6 over Ipv4 differently as the IPv6 packet are encapsulated and sent within an IPv4 User Datagram Protocol (UDP) packet that easily gets through NAT traversals. But, the Teredo protocol is considered the protocol of last resort, mainly because of the high overhead associated with the encapsulation mechanism, but also because of security concerns. Teredo basically allows hosts to directly traverse NATs to communicate across the Internet with other Teredo hosts using what is referred to as open-ended tunnels. Because of the way the Teredo protocol encapsulates Ipv6 traffic within Ipv4 UDP packets, it can essentially bypass some of the strict traffic inspection performed by network firewalls and intrusion prevention system (IPS). This leaves the burden of validating the Ipv6 traffic to the Teredo host receiving the data. This might not be the most secure or ideal scenario and in cases where Teredo must be used, network administrators should fully understand Teredo security risks and how to mitigate them.

Until a true IPv6 Internet is configured, there will be security and functionality challenges to make IPv6 work on both the internal network and across the Internet. In some ways, having the transition technologies in place slows down the larger ISPs’ adoption of an Ipv6 Internet.

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