ICMPv6 Packet Too Big Messages
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Applications of Packet Too Big Messages
While Packet Too Big is obviously an error message, it also has another use: the implementation of Path MTU Discovery. This process, described in RFC 1981, defines a method for a device to determine the minimum MTU for a path to a destination. To perform path MTU discovery, the source device sends a series of test messages, decreasing the size of the datagram until it no longer receives Packet Too Big messages back in response to its tests. See the topic on IPv6 MTU and fragmentation issues for a bit more detail on this.
Incidentally, Packet Too Big is an exception to the rule that ICMP messages are sent only in response to unicast datagrams; it may be sent in reply to an oversized multicast datagram. If this occurs, it is important to realize that some of the intended targets of the multicast may still have received it, if the path the multicast took to them did not go through the link with the small MTU that caused the error.
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