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ICMPv6 messages are carried inside IPv6 packets (the `Next Header` field for ICMPv6 is ``58``). Each ICMP message contains a 32 bits header with an 8 bits `type` field, a `code` field and a 16 bits checksum computed over the entire ICMPv6 message. The message body contains a copy of the IPv6 packet in error.
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Some IPv6 implementations send the fragments of a packet in increasing fragment offset order, starting from the first fragment. Others send the fragments in reverse order, starting from the last fragment. The latter solution can be advantageous for the host that needs to reassemble the fragments, as it can easily allocate the buffer required to reassemble all fragments of the packet upon reception of the last fragment. When a host receives the first fragment of an IPv6 packet, it cannot know a priori the length of the entire IPv6 packet.
The figure below provides an example of a fragmented IPv6 packet containing a UDP segment. The `Next Header` type reserved for the IPv6 fragmentation option is 44.
IPv6 fragmentation example
The following pseudo-code details the IPv6 fragmentation, assuming that the packet does not contain options.
In the above pseudocode, we maintain a single 32 bits counter that is incremented for each packet that needs to be fragmented. Other implementations to compute the packet identification are possible. :rfc:`2460` only requires that two fragmented packets that are sent within the MSL between the same pair of hosts have different identifications.
The fragments of an IPv6 packet may arrive at the destination in any order, as each fragment is forwarded independently in the network and may follow different paths. Furthermore, some fragments may be lost and never reach the destination.
The reassembly algorithm used by the destination host is roughly as follows. First, the destination can verify whether a received IPv6 packet is a fragment or not by checking whether it contains a fragment header. If so, all fragments with the some identification must be reassembled together. The reassembly algorithm relies on the `Identification` field of the received fragments to associate a fragment with the corresponding packet being reassembled. Furthermore, the `Fragment Offset` field indicates the position of the fragment payload in the original non-fragmented packet. Finally, the packet with the `M` flag reset allows the destination to determine the total length of the original non-fragmented packet.
Note that the reassembly algorithm must deal with the unreliability of the IP network. This implies that a fragment may be duplicated or a fragment may never reach the destination. The destination can easily detect fragment duplication thanks to the `Fragment Offset`. To deal with fragment losses, the reassembly algorithm must bind the time during which the fragments of a packet are stored in its buffer while the packet is being reassembled. This can be implemented by starting a timer when the first fragment of a packet is received. If the packet has not been reassembled upon expiration of the timer, all fragments are discarded and the packet is considered to be lost.
Header compression on low bandwidth links
Given the size of the IPv6 header, it can cause huge overhead on low bandwidth links, especially when small packets are exchanged such as for Voice over IP applications. In such environments, several techniques can be used to reduce the overhead. A first solution is to use data compression in the datalink layer to compress all the information exchanged [Thomborson1992]_. These techniques are similar to the data compression algorithms used in tools such as :manpage:`compress(1)` or :manpage:`gzip(1)` :rfc:`1951`. They compress streams of bits without taking advantage of the fact that these streams contain IP packets with a known structure. A second solution is to compress the IP and TCP header. These header compression techniques, such as the one defined in :rfc:`5795` take advantage of the redundancy found in successive packets from the same flow to significantly reduce the size of the protocol headers. Another solution is to define a compressed encoding of the IPv6 header that matches the capabilities of the underlying datalink layer :rfc:`4944`.
The last type of `IPv6 header extension` is the `Routing` header. The ``type 0`` routing header defined in :rfc:`2460` is an example of an IPv6 option that must be processed by some routers. This option is encoded as shown below.
The Type 0 routing header (:rfc:`2460`)
The type 0 routing option was intended to allow a host to indicate a loose source route that should be followed by a packet by specifying the addresses of some of the routers that must forward this packet. Unfortunately, further work with this routing header, including an entertaining demonstration with scapy_ [BE2007]_ , revealed severe security problems with this routing header. For this reason, loose source routing with the type 0 routing header has been removed from the IPv6 specification :rfc:`5095`.
ICMP version 6
It is sometimes necessary for intermediate routers or the destination host to inform the sender of the packet of a problem that occurred while processing a packet. In the TCP/IP protocol suite, this reporting is done by the Internet Control Message Protocol (ICMP). ICMPv6 is defined in :rfc:`4443`. It is used both to report problems that occurred while processing an IPv6 packet, but also to distribute addresses.
ICMPv6 messages are carried inside IPv6 packets (the `Next Header` field for ICMPv6 is ``58``). Each ICMP message contains a 32 bits header with an 8 bits `type` field, a `code` field and a 16 bits checksum computed over the entire ICMPv6 message. The message body contains a copy of the IPv6 packet in error.
ICMP version 6 packet format
ICMPv6 specifies two classes of messages : error messages that indicate a problem in handling a packet and informational messages. Four types of error messages are defined in :rfc:`4443` :
``1`` : `Destination Unreachable`. Such an ICMPv6 message is sent when the destination address of a packet is unreachable. The `code` field of the ICMP header contains additional information about the type of unreachability. The following codes are specified in :rfc:`4443`
Destination Unreachable. Such an ICMPv6 message is sent when the destination address of a packet is unreachable. The code field of the ICMP header contains additional information about the type of unreachability. The following codes are specified in RFC 4443
``0`` : No route to destination. This indicates that the router that sent the ICMPv6 message did not have a route towards the packet's destination
``1`` : Communication with destination administratively prohibited. This indicates that a firewall has refused to forward the packet towards its final destination.
``2`` : Beyond scope of source address. This message can be sent if the source is using link-local addresses to reach a global unicast address outside its subnet.
``3`` : Address unreachable. This message indicates that the packet reached the subnet of the destination, but the host that owns this destination address cannot be reached.
``4`` : Port unreachable. This message indicates that the IPv6 packet was received by the destination, but there was no application listening to the specified port.
``2`` : Packet Too Big. The router that was to send the ICMPv6 message received an IPv6 packet that is larger than the MTU of the outgoing link. The ICMPv6 message contains the MTU of this link in bytes. This allows the sending host to implement Path MTU discovery :rfc:`1981`
``3`` : Time Exceeded. This error message can be sent either by a router or by a host. A router would set `code` to `0` to report the reception of a packet whose `Hop Limit` reached `0`. A host would set `code` to `1` to report that it was unable to reassemble received IPv6 fragments.
``4`` : Parameter Problem. This ICMPv6 message is used to report either the reception of an IPv6 packet with an erroneous header field (code `0`) or an unknown `Next Header` or IP option (codes `1` and `2`). In this case, the message body contains the erroneous IPv6 packet and the first 32 bits of the message body contain a pointer to the error.
The `Destination Unreachable` ICMP error message is returned when a packet cannot be forwarded to its final destination. The first four ICMPv6 error messages (type ``1``, codes ``0-3``) are generated by routers while hosts may return code ``4`` when there is no application bound to the corresponding port number.
The `Packet Too Big` ICMP messages enable the source host to discover the MTU size that it can safely use to reach a given destination. To understand its operation, consider the (academic) scenario shown in the figure below. In this figure, the labels on each link represent the maximum packet size supported by this link.
If ``A`` sends a 1500 bytes packet, ``R1`` will return an ICMPv6 error message indicating a maximum packet length of 1400 bytes. ``A`` would then fragment the packet before retransmitting it. The small fragment would go through, but the large fragment will be refused by ``R2`` that would return an ICMPv6 error message. ``A`` can fragment again the packet and send it to the final destination as two fragments.

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../../protocols/ipv6.rst:524
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locale/fr/LC_MESSAGES/protocols/ipv6.po, string 156