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../../protocols/ipv6.rst:524
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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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../../protocols/ipv6.rst:530
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ICMP version 6 packet format
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../../protocols/ipv6.rst:533
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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` :
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../../protocols/ipv6.rst:539
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``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`
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../../protocols/ipv6.rst:538
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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
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../../protocols/ipv6.rst:536
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``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
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../../protocols/ipv6.rst:537
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``1`` : Communication with destination administratively prohibited. This indicates that a firewall has refused to forward the packet towards its final destination.
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../../protocols/ipv6.rst:538
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``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.
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../../protocols/ipv6.rst:539
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``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.
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../../protocols/ipv6.rst:540
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``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.
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../../protocols/ipv6.rst:541
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``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`
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../../protocols/ipv6.rst:542
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``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.
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../../protocols/ipv6.rst:543
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``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.
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../../protocols/ipv6.rst:546
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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.
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../../protocols/ipv6.rst:548
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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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../../protocols/ipv6.rst:570
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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:572
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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`.
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../../protocols/ipv6.rst:576
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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]_.
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../../protocols/ipv6.rst:578
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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.
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../../protocols/ipv6.rst:583
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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.
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