cnp3-ebook/protocols/ipv6 — English @ Weblate

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157 ../../protocols/ipv6.rst:530

	English ICMP version 6 packet format 28/10000 · 28
158 ../../protocols/ipv6.rst:533

	English 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` : 188/10000 · 188
159 ../../protocols/ipv6.rst:539

	English ``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` 273/10000 · 273
160 ../../protocols/ipv6.rst:538

	English 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 258/10000 · 258
161 ../../protocols/ipv6.rst:536

	English ``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 146/10000 · 146
162 ../../protocols/ipv6.rst:537

	English ``1`` : Communication with destination administratively prohibited. This indicates that a firewall has refused to forward the packet towards its final destination. 163/10000 · 163
163 ../../protocols/ipv6.rst:538

	English ``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. 162/10000 · 162
164 ../../protocols/ipv6.rst:539

	English ``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. 173/10000 · 173
165 ../../protocols/ipv6.rst:540

	English ``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. 164/10000 · 164
166 ../../protocols/ipv6.rst:541

	English ``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` 276/10000 · 276
167 ../../protocols/ipv6.rst:542

	English ``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. 285/10000 · 285
168 ../../protocols/ipv6.rst:543

	English ``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. 347/10000 · 347
169 ../../protocols/ipv6.rst:546

	English 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. 314/10000 · 314
170 ../../protocols/ipv6.rst:548

	English 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. 322/10000 · 322
171 ../../protocols/ipv6.rst:570

	English 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. 411/10000 · 411
172 ../../protocols/ipv6.rst:572

	English 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`. 360/10000 · 360
173 ../../protocols/ipv6.rst:576

	English 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]_. 561/10000 · 561
174 ../../protocols/ipv6.rst:578

	English 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. 556/10000 · 556
175 ../../protocols/ipv6.rst:583

	English 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. 378/10000 · 378
176 ../../protocols/ipv6.rst:605

	English 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. 758/10000 · 758

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