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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.