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Embedded Systems September 2000 Vol13_10

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In reality, "dual-stack" is a misnomer. Actually, only one IP module handles both 1Pv4 and 1Pv6. an IPv4 address between binary and text form. The analogous IPv6 fun c- tions are call ed inet_ptonO and inet_ntopO. inet_ptonO conver ts a text address to its binary equivalent; i net_ntop() does the reverse, convert- 6bone The 6bone is an 1Pv6 testbed. It's run as a world-wide informal collaborative project with oversight from the NGtrans (1Pv6 Transition) working group of the IETF. There are 462 sites in 42 countries. You can use the 6bone for testing purposes, or to gain experience with I Pv6 before releasing your product commercially. To join the 6bone, you will need the following: • A router that supports 1Pv6 • A 6bone site you can connect to • A nameserver that support 1Pv6 AAAA records You can find more information about the 6bone at www.6bone.net. The easiest way to connect a single workstation to the 6bone is to use Freenet6 by Viagenie. Freenet6 is a server that will generate a configuration for you to download for your specific operating system. After downloading and installing the configuration, you're on the 6bone. See www.freenet6.net for more information. ing a binary address to printable text. Since both of these functions have an parameter for specifyi ng the address family, they can be used to convert both IPv4 and IPv6 addresses. The state of 1Pv6 today When the IETF started working on IPv6, one of the requirements was to allow for a slow migration from IPv4 to IPv6. Obviously, an abrupt cutover from one protocol to another would be impossible. To make IPv6 success- ful , it would have to work on the In te rn et simultaneously wi th IPv4. RFC 1933, "Transition Mechanisms for IPv6 Hosts and Routers," defin es sev- e ral methods for deploying IPv6 devices.'3 Dual-stack nodes A dual-stack node supports both IPv4 and IPv6 simultaneously. When com- municating with an IPv6 node, or another dual-stack node, the dual- stack node will use IPv6. When com- municating with an IPv4 node, the dual-stack node uses IPv4. Both proto- cols are handled gracefull y, unbe- knownst to the applications. In reali ty, "dual-stack" is a mis- o . 1Pv6 Client -~ I Pv6 Packets 1Pv4 Internet 1Pv4 Dual-Stack Router Tunnel Dual-Stack Router ======- 0 0 [J 0 1Pv4 Tunnel 1Pv6 Server 102 SEPTEMBER 2000 Embedded Systems Programming nomer. Actually, only one IP module handles both IPv4 and IPv6. The sock- et layer, based on the AF _INET or AF_INET6 parame ter, knows which protocol is being used. IfiPv4 is being used, it constructs an 1Pv4-mapped address, where the 80 high-order bits are zero, the next 16 bits are Oxffff, and the 32 low-order bi ts are the IPv4 address, for example, ::FFFF:123.45.67.89. Thus, the IP mod- ule always receives a 128-bi t address from the upper layer. When the IP module receives an IPv4-mapped address, it assembles the IPv4 heade· rs . When it receives a non- IPv4-mapped address, it assembles the IPv6 headers. Either way, the same module does the work of both proto- cols, so there really is only one stack. Tunnels What happens if two IPv6 nodes are separated by an 1Pv4 network? How do they communicate? By using d ual- stack routers, a tunnel can be dug across the IPv4 network. Tunnels are dug by encapsulating an IP packet within the payload of another packet.

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