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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.
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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../../exercises/ipv6.rst:987
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locale/fr/LC_MESSAGES/exercises/ipv6.po, string 103