6RD Tunnel

Basic Concepts

• 6RD domain

  It is a special IPv6 network. The IPv6 prefixes of devices or hosts in the 6RD domain share the same 6RD delegated prefix. A 6RD domain consists of 6RD Customer Edge (CE) routers and 6RD Border Relays (BRs). Each 6RD domain uses a unique 6RD prefix.

• 6RD CE

  It is a border device between the 6RD network and the IPv4 network. The interface connected to the IPv4 network has an IPv4 address, and the interface connected to the 6RD network requires an IPv6 address with the 6RD delegated prefix.

• 6RD BR

  It connects to a native IPv6 network outside a 6RD domain and requires an IPv4 address. Each 6RD domain houses only one 6RD BR.

• 6RD prefix

  It is the IPv6 address prefix used by the carrier and serves as a part of the 6RD delegated prefix.

• IPv4 prefix length

  The IPv4 prefix length indicates the bits to be deleted from the source IPv4 address of the tunnel in high-bit order. The rest bits become a part of the 6RD delegated prefix.

• 6RD delegated prefix

  The 6RD delegated prefix is used in the 6RD domain for assigning IPv6 address prefixes to devices or hosts. It consists of a 6RD prefix and a part of or entire IPv4 address. Its value is obtained on the basis of the two.

6RD Address Format

The 6RD address consists of a 6RD prefix (an IPv6 prefix assigned by the carrier for the 6RD address), an IPv4 address, a subnet ID, and an interface identifier, as shown in Figure 1.

Figure 1 6RD address format

The 6RD address prefix has 64 bits, including the 6RD delegated prefix and the user-defined subnet. The 6RD delegated prefix value is calculated by the 6RD prefix and a part of or entire IPv4 address. The length of the IPv4 address in the 6RD delegated prefix depends on the IPv4 prefix length specified for the 6RD tunnel.

6RD Tunnel

The 6RD tunnel applies to the communication between 6RD domains or the communication between a 6RD domain and a native IPv6 network. The 6RD tunnel is implemented through the tunnel interface. The source IPv4 address of the tunnel is specified manually, and its destination IP address is determined by the packet forwarded by the tunnel. If the destination IP address of an IPv6 packet is a 6RD address, the embedded IPv4 address serves as the destination address of the tunnel.

Figure 2 6RD tunnel

As shown in Figure 2, when host A connects to host B, the processing procedures are as follows:

1. The carrier allocates a 6RD prefix, and an IPv4 address for CE_A. CE_A itself generates a 6RD prefix and delivers it to host A.
2. After the IPv6 packet sent by host A arrives at CE_A, CE_A searches the IPv6 forwarding table based on the destination IP address of the IPv6 packet and discovers that the outbound interface is the tunnel interface of the 6RD tunnel and the destination IP address is a 6RD address. Then CE_A encapsulates the IPv6 packet into an IPv4 packet, whose destination IP address is the IPv4 address obtained from the 6RD address and source IP address is the source IPv4 address specified on the local tunnel interface.
3. The encapsulated packet is forwarded by CE_A from the tunnel interface and routed to CE_B over the IPv4 network.
4. After receiving the IPv4 packet, CE_B decapsulates it, searches for the destination address in the decapsulated IPv6 packet header, and forwards the packet to host B based on the routing table.
5. Host B receives and responds to the packet. The response packet is processed in the same way.

6RD Relay

The preceding procedures enable the communication between 6RD networks. To enable the communication between a 6RD domain and a native IPV6 network, deploy a 6RD BR. The 6RD BR connects to a native IPv6 network and an IPv4 network and establishes a 6RD tunnel with the 6RD CE.

Figure 3 6RD relay

As shown in Figure 3, when host A connects to host B, the processing procedures are as follows:

1. The carrier allocates a 6RD prefix and an IPv4 address for the 6RD CE and specify the IPv4 address of the 6RD BR on the 6RD CE. The 6RD CE itself generates a 6RD prefix and delivers it to host A.
2. After the IPv6 packet sent by host A arrives at the 6RD CE, the 6RD CE searches the IPv6 forwarding table based on the destination IP address of the IPv6 packet and discovers that the outbound interface is the tunnel interface of the 6RD tunnel and the 6RD address is not the destination IP address but the next hop. Then the 6RD CE encapsulates the IPv6 packet into an IPv4 packet, whose destination IP address is the IPv4 address obtained from the 6RD address and source IP address is the source IPv4 address specified on the local tunnel interface.
3. The encapsulated packet is forwarded by the 6RD CE from the tunnel interface and routed to the 6RD BR over the IPv4 network.
4. After receiving the IPv4 packet, the 6RD BR decapsulates it, searches for the destination address in the decapsulated IPv6 packet header, and forwards the packet to host B based on the routing table.
5. Host B receives and responds to the packet. The response packet is processed in the same way.