306 Understanding IPv6, Second Edition

6to4 Relay

131.107.0.1 Local Area Connection

6to4 Router

Local Area Connection 2

2002:836B:1:472A:/64

2002:836B:1::/48

IPv6 Router

IPv6-Capable Intranet

Figure 13-6 An example configuration for a manually configured 6to4 router

6to4 Communication Examples

This section describes the details of how 6to4 communication works on the example configuration shown in Figure 13-4 when a 6to4 host sends a packet to a 6to4 host/router and when a 6to4 host sends a packet to an IPv6 host on the IPv6 Internet.

6to4 Host to 6to4 Host/Router

In the example shown in Figure 13-4, 6to4 Host A wants to send a packet to 6to4 Host/router B. 6to4 Host A has determined 6to4 Host B’s IPv6 address through a DNS name query

or other method, such as the Windows Peer-to-Peer Networking platform’s Peer Name Resolution Protocol (PNRP). The journey of the IPv6 packets from 6to4 Host A to 6to4 Host/ router B has two parts:

■Part 1: From 6to4 Host A to the 6to4 router

■Part 2: From the 6to4 router to 6to4 Host/router B

In the first part of the journey, IPv6 on 6to4 Host A performs the route determination process and finds that the closest matching route to the destination is the default route. The default route has a next-hop address of the link-local address of the 6to4 router. 6to4 Host A performs

normal IPv6 address resolution and sends the IPv6 packet to the 6to4 router. Figure 13-7 shows the delivery of the IPv6 packet to the 6to4 router.

Figure 13-7 6to4 host to 6to4 host/router communication—Part 1

In the second part of the journey, IPv6 on the 6to4 router performs the route determination process and finds that the closest matching route to the destination is the 2002::/16 route. Because it is an on-link route, the next-hop IPv6 address is set to the destination (2002:836B:1::836B:1). The IPv6 packet and the next-hop address are handed to the 6to4 tunneling interface.

The 6to4 tunneling interface sets the destination IPv4 address in the IPv4 header to the

32 bits corresponding to the second and third blocks of the next-hop address, which in this case is 6to4 Host/router B’s IPv4 address of 131.107.0.1. IPv4 on the 6to4 router determines that the best source address to use is the public IPv4 address assigned to the 6to4 router (157.60.0.1) and then sends the packet. Figure 13-8 shows the delivery of the IPv4encapsulated IPv6 packet to 6to4 Host/router B.

On 6to4 Host/router B, IPv4 processes the IPv4 header. Because the Protocol field is set to 41, it hands the IPv6 packet to IPv6 for additional processing.

Figure 13-9 6to4 host to IPv6 host communication—Part 1

The 6to4 tunneling interface sets the destination IPv4 address in the IPv4 header to the 32 bits corresponding to the second and third blocks of the next-hop 6to4 address, which in this case is 6to4 relay’s IPv4 address of 194.207.1.5. IPv4 on the 6to4 router determines that the best source address to use is the public IPv4 address assigned to the 6to4 router (157.60.0.1) and then sends the packet. Figure 13-10 shows the delivery of the IPv4encapsulated IPv6 packet to the 6to4 relay.

In the third part of the journey, IPv4 on the 6to4 relay processes the IPv4 header. Because the Protocol field is set to 41, it hands the IPv6 packet to IPv6. IPv6 on the 6to4 relay performs the route determination process and finds that the closest matching route to the destination is the default route (::/0). The default route has a next-hop IPv6 address of the next IPv6 router on the IPv6 Internet (not shown in Figure 13-11). The IPv6 packet and the next-hop address are handed to the appropriate LAN interface for forwarding. Intermediate IPv6 routers forward the packet across the IPv6 Internet to its destination. Figure 13-11 shows the journey of the IPv6 packet from the 6to4 relay to IPv6 Host C.

Network Monitor Capture

Here is an example of an IPv4-encapsulated ICMPv6 Echo Request message from a 6to4 host/router on the IPv4 Internet to a host on the IPv6 Internet as displayed by Network Monitor 3.1 (frame 1 of capture 13_01 in the \NetworkMonitorCaptures folder on the companion CD-ROM):

Frame:

+ Ethernet: Etype = Internet IP (IPv4)

-Ipv4: Next Protocol = IPv6 over IPv4, Packet ID = 2225, Total IP Length = 100

+Versions: IPv4, Internet Protocol; Header Length = 20

+DifferentiatedServicesField: DSCP: 0, ECN: 0 TotalLength: 100 (0x64)

Identification: 2225 (0x8B1)

+FragmentFlags: 0 (0x0) TimeToLive: 128 (0x80)

NextProtocol: IPv6 over IPv4, 41(0x29) Checksum: 42468 (0xA5E4) SourceAddress: 71.112.33.18 DestinationAddress: 192.88.99.1

-Ipv6: Next Protocol = ICMPv6, Payload Length = 40

+Versions: IPv6, Internet Protocol, DSCP 0 PayloadLength: 40 (0x28)

NextProtocol: ICMPv6, 58(0x3a) HopLimit: 128 (0x80)

SourceAddress: 2002:4770:2112:0:0:0:4770:2112 DestinationAddress: 2001:770:800:3:0:0:0:1

+ Icmpv6: Echo request, ID = 0x0, Seq = 0x3

In the IPv4 header, the packet is addressed from 71.112.33.18, the public IPv4 address of the 6to4 host/router, to 192.88.99.1, an address in the 6to4 Relay anycast prefix of 192.88.99.0/24 that is defined in RFC 3068. In the IPv6 header, the packet is addressed from the derived 6to4 address of the 6to4 host/router (2002:4770:2112:0:0:0:4770:2112, in which 4770:2112 is the colon hexadecimal notation of 71.112.33.18).

The following is the corresponding IPv4-encapsulated ICMPv6 Echo Reply message from the host on the IPv6 Internet as displayed by Network Monitor 3.1 (frame 2 of capture 13_01):

Frame:

+ Ethernet: Etype = Internet IP (IPv4)

-Ipv4: Next Protocol = IPv6 over IPv4, Packet ID = 44903, Total IP Length = 100

+Versions: IPv4, Internet Protocol; Header Length = 20

+DifferentiatedServicesField: DSCP: 0, ECN: 0 TotalLength: 100 (0x64)

Identification: 44903 (0xAF67)

+FragmentFlags: 0 (0x0) TimeToLive: 244 (0xF4)

NextProtocol: IPv6 over IPv4, 41(0x29) Checksum: 10848 (0x2A60) SourceAddress: 193.1.195.37 DestinationAddress: 71.112.33.18

-Ipv6: Next Protocol = ICMPv6, Payload Length = 40

+Versions: IPv6, Internet Protocol, DSCP 0