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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.
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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.
In practice, an IPv6 implementation does not store the transmitted packets to be able to retransmit them if needed. However, since TCP (and SCTP) buffer the segments that they transmit, a similar approach can be used in transport protocols to detect the largest MTU on a path towards a given destination. This technique is called PathMTU Discovery :rfc:`1981`.
When a TCP segment is transported in an IP packet that is fragmented in the network, the loss of a single fragment forces TCP to retransmit the entire segment (and thus all the fragments). If TCP was able to send only packets that do not require fragmentation in the network, it could retransmit only the information that was lost in the network. In addition, IP reassembly causes several challenges at high speed as discussed in :rfc:`4963`. Using IP fragmentation to allow UDP applications to exchange large messages raises several security issues [KPS2003]_.
ICMPv6 is used by TCP implementations to discover the largest MTU size that is allowed to reach a destination host without causing network fragmentation. A TCP implementation parses the `Packets Too Big` ICMP messages that it receives. These ICMP messages contain the MTU of the router's outgoing link in their `Data` field. Upon reception of such an ICMP message, the source TCP implementation adjusts its Maximum Segment Size (MSS) so that the packets containing the segments that it sends can be forwarded by this router without requiring fragmentation.
Two types of informational ICMPv6 messages are defined in :rfc:`4443` : `echo request` and `echo reply`, which are used to test the reachability of a destination by using :manpage:`ping6(8)`. Each host is supposed [#fpingproblems]_ to reply with an ICMP `Echo reply` message when it receives an ICMP `Echo request` message. A sample usage of :manpage:`ping6(8)` is shown below.
Another very useful debugging tool is :manpage:`traceroute6(8)`. The traceroute man page describes this tool as `"print the route packets take to network host"`. traceroute uses the `Time exceeded` ICMP messages to discover the intermediate routers on the path towards a destination. The principle behind traceroute is very simple. When a router receives an IP packet whose `Hop Limit` is set to ``1`` it is forced to return to the sending host a `Time exceeded` ICMP message containing the header and the first bytes of the discarded packet. To discover all routers on a network path, a simple solution is to first send a packet whose `Hop Limit` is set to `1`, then a packet whose `Hop Limit` is set to `2`, etc. A sample traceroute6 output is shown below.
Rate limitation of ICMP messages
High-end hardware based routers use special purpose chips on their interfaces to forward IPv6 packets at line rate. These chips are optimized to process `correct` IP packets. They are not able to create ICMP messages at line rate. When such a chip receives an IP packet that triggers an ICMP message, it interrupts the main CPU of the router and the software running on this CPU processes the packet. This CPU is much slower than the hardware acceleration found on the interfaces [Gill2004]_. It would be overloaded if it had to process IP packets at line rate and generate one ICMP message for each received packet. To protect this CPU, high-end routers limit the rate at which the hardware can interrupt the main CPU and thus the rate at which ICMP messages can be generated. This implies that not all erroneous IP packets cause the transmission of an ICMP message. The risk of overloading the main CPU of the router is also the reason why using hop-by-hop IPv6 options, including the router alert option is discouraged [#falert]_.
The IPv6 subnet
Until now, we have focused our discussion on the utilization of IPv6 on point-to-point links. Although there are point-to-point links in the Internet, mainly between routers and sometimes hosts, most of the hosts are attached to datalink layer networks such as Ethernet LANs or WiFi networks. These datalink layer networks play an important role in today's Internet and have heavily influenced the design of the operation of IPv6. To understand IPv6 and ICMPv6 completely, we first need to correctly understand the key principles behind these datalink layer technologies.
As explained earlier, devices attached to a Local Area Network can directly exchange frames among themselves. For this, each datalink layer interface on a device (host, router, ...) attached to such a network is identified by a MAC address. Each datalink layer interface includes a unique hardwired MAC address. MAC addresses are allocated to manufacturers in blocks and interface is numbered with a unique address. Thanks to the global unicity of the MAC addresses, the datalink layer service can assume that two hosts attached to a LAN have different addresses. Most LANs provide an unreliable connectionless service and a datalink layer frame has a header containing :
the source MAC address
the destination MAC address
some multiplexing information to indicate the network layer protocol that is responsible for the payload of the frame
LANs also provide a broadcast and a multicast service. The broadcast service enables a device to send a single frame to all the devices attached to the same LAN. This is done by reserving a special broadcast MAC address (typically all bits of the address are set to one). To broadcast a frame, a device simply needs to send a frame whose destination is the broadcast address. All devices attached to the datalink network will receive the frame.

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../../protocols/ipv6.rst:548
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locale/fr/LC_MESSAGES/protocols/ipv6.po, string 170