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When debugging a network, it can be interesting to capture packets using tcpdump_ on specific links to check that they follow the expect. If you use tcpdump_ without any filter, you will capture the packets generated by xterm. To capture packets, you need to specify precise filters that will match the packets of interest. For traceroute6, you need to match the IPv6 packets that contain UDP segments and some ICMPv6 packets. The script below provides a simple filter that you can reuse. It takes one argument: the name of the interface on which tcpdump_ needs to run.
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When :manpage:`traceroute6(8)` sends UDP segments, it uses the UDP source port as a way to remember the target hop for this specific UDP segment.
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When the :manpage:`ping6(8)` is executed, it sends ICMPv6 echo request messages with increasing sequence numbers.
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With the previous exercises, you have learned how to reason about IPv6 networks "on paper". Given the availability of IPv6 implementations, it is also possible to carry out experiments in real and virtual labs. Several virtual environments are possible. In this section, we focus on mininet_. mininet_ is an emulation framework developed at Stanford University that leverages the namespaces features of recent Linux kernels. With those namespaces, a single Linux kernel can support a variety of routers and hosts interconnected by virtual links. mininet_ has been used by several universities as an educational tool, but unfortunately it was designed without IPv6 support.
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With these IP prefixes and the network topology, we can now use IPMininet_ to create the topology and assign the addresses.
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With this ``build`` method, we can now launch the network by using the python code below.
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