cnp3-ebook/exercises/ipv6 — French @ Weblate: A second problem is when there is a forwarding loop …

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	We start the network topology shown above with the ``sudo python script.py`` command. It launches the mininet_ interactive shell that provides several useful commands:
	Some of the standard mininet commands assume the utilisation of IPv4 and do not have a direct IPv6 equivalent. Here are some useful commands.
	The ``nodes`` command lists the routers and hosts that have been created in the mininet topology.
	The ``links`` command lists the links that have been instantiated and shows that mapping between the named interfaces on each node.
	It is possible to execute any of the standard Linux commands to configure the network stack on any of the hosts by prefixing the command with the corresponding host. Remember to always specify ``inet6`` as the address family to retrieve the IPv6 information.
	Host ``a`` has two interfaces: the standard loopback interface and a network interface named ``a-eth0`` that is attached to router ``r1``. We can also verify how the IPv6 addresses have been configured:
	On its ``a-eth0`` interface, host ``a`` uses IPv6 address ``2001:db8:1341:1::a/64``. The link local address (``fe80::c44e:26ff:fed9:de6d/64``) will be described in another chapter. Finally, we can check the forwarding table of host ``a``.
	There are three routes in this table. The first two correspond to the two prefixes that are used over the ``a-eth0`` interface. These routes are automatically created when an IPv6 address is configured on an interface. The last route is the default route (``::/0``) which points towards ``2001:db8:1341:1::1``, i.e. router ``r1``.
	Another useful command is ``xterm`` 'node' that allows to launch a terminal on the specified node. This gives you a interactive shell on any node. You can use it to capture packets with tcpdump_. As an example, let us use :manpage:`traceroute6(8)` to trace the path followed by packets from host ``a`` towards the IPv6 address of host ``b`` i.e. ``2001:db8:1341:3::b``. The output of this command shows that the path passes through routers ``r1``, ``r2`` and ``r3``.
	Another interesting mininet_ command is ``pingall`` it allows to check that any host can reach any other host inside the network. It executes a ping from any host to any other host inside the network topology.
	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.
	Starting from the :download:`/exercises/ipmininet_scripts/static-1.py` IPMininet_ script, we can explore classical problems when networks are configured with static routes. A first problem is when a router has an incomplete forwarding table. We configure the static routes as shown below. The entire script is available from :download:`/exercises/ipmininet_scripts/static-1-hole.py`.
	We first check with ``pingall`` whether the network works correctly.
	The problem can be detected by using :manpage:`traceroute6(8)`.
	In the output of :manpage:`traceroute6(8)`, a ``!N`` indicates that host ``a`` received from ``2001:db8:1341:12::2``, i.e. router ``r2``, a Network unreachable ICMPv6 message. The forwarding table of ``r2`` confirms the root cause of this problem.
	A second problem is when there is a forwarding loop inside the network, i.e. packets sent to a specific destination loop through several routers. With the static routes shown below, router ``r2`` forwards the packets towards ``2001:db8:1341:3::b`` via router ``r1``. The entire script is available from :download:`/exercises/ipmininet_scripts/static-1-loop.py`.
	The ``pingall`` command reveals that there is a problem in this network.
	We can analyze this configuration problem in more details by using ``traceroute6``. The loop appears clearly.
	On host ``b``, the problem is different. The packets that it sends towards host ``a`` do not seem to go beyond router ``r3``.
	To debug this problem, let us look at the forwarding table of ``r3``. This router forwards the packets sent to host ``a`` to router ``r1`` that is directly connected to host ``a``.
	Unfortunately, when router ``r1`` sends its ICMP HopLimit exceeded message, the destination of this IP packet is ``2001:db8:1341:3::b``. This packet is forward to router ``r2`` that returns the packet back to router ``r1``. The packet loops between the two routers until their HopLimit reaches zero.
	IPv6 packets
	To correctly understand the operation of IPv6, it is sometimes important to remember the packet format and how the different fields are used.
	The `Next Header` of the IPv6 packet indicates the type of the header that follows the IPv6 packet. IANA_ maintains a list of all the assigned values of this header at https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
	When an IPv6 router receives a packet that is larger than the Maximum Transmission Unit (MTU) on its outgoing interface, it drops the packet and returns an ICMPv6 message back to the source. Upon reception of this ICMPv6 message, the source will either adjust the size of the packets that it transmits or use IPv6 packet fragmentation. The exercises below show a few examples of the utilization of IPv6 fragmentation.
	Network engineers often rely on :manpage:`ping6(8)` to verify the reachability of a remote host or router. :manpage:`ping6(8)` sends ICMPv6 echo request messages and analyzes the received ICMPv6 echo responses. Each echo request message contains an identifier and a sequence number that is returned in the response.
	When the :manpage:`ping6(8)` is executed, it sends ICMPv6 echo request messages with increasing sequence numbers.
	The :manpage:`traceroute6(8)` software is very useful to debug network problems. It sends a series of UDP segments encapsulated inside IP packets with increasing values of the HopLimit. The first packet has a HotLimit and the first router on the path returns an ICMPv6 HopLimit exceeded message.
	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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