ICMPv6 Destination Unreachable Messages
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Version 6 of the Internet Protocol (IPv6) includes some important enhancements over the older version 4, but the basic operation of the two protocols is still fundamentally the same. Like IPv4, IPv6 is an unreliable network protocol that makes a best effort to deliver datagrams, but offers no guarantees that they will always get there. Just as was the case in IPv4, devices on an IPv6 network must not assume that datagrams sent to a destination will always be received.
When a datagram cannot be delivered, recovery from this condition normally falls to higher-layer protocols like TCP, which will detect the miscommunication and re-send the lost datagrams. In some situations, such as a datagram dropped due to congestion of a router, this is sufficient, but in other cases a datagram may not be delivered due to an inherent problem with how it is being sent. For example, the source may have specified an invalid destination address, which means even if resent many times, the datagram will never get to its intended recipient.
In general, having the source just re-send undelivered datagrams while having no idea why they were lost is inefficient. It is better to have a feedback mechanism that can tell a source device about undeliverable datagrams and provide some information about why the datagram delivery failed. As in ICMPv4, in ICMPv6 this is done with Destination Unreachable messages. Each message includes a code that indicates the basic nature of the problem that caused the datagram not to be delivered, and includes all or part of the datagram that was undelivered, to help the source device diagnose the problem.
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