IPv4 used ICMP for many things, such as error messages like destination unreachable, and troubleshooting functions like Ping and Traceroute. ICMPv6 still does those things for us, but unlike its predecessor, the v6 flavor isn’t implemented as a separate layer 4 protocol. It’s an integrated part of IPv6 and is carried after the basic IPv6 header information as an extension header. And ICMPv6 adds another cool feature—it prevents IPv6 from doing any fragmentation through an ICMPv6 process called path MTU discovery.
This is how it works: The source node of a connection will send a packet that’s equal to the MTU size of its local link’s MTU. As this packet traverses the path toward its destination, any link that has an MTU smaller than the size of the current packet will force the intermediate router to send a “packet too big” message back to the source machine. This message tells the source node what the maximum size is that the restrictive link will allow and asks the source to send a new scaled-down packet that can pass through. This process will continue until the destination is finally reached, with the source node now sporting the new path’s MTU. So now, when the rest of the data packets are transmitted, they’ll be protected from fragmentation.
ICMPv6 now takes over the task of finding the address of other devices on the local link. Address Resolution Protocol used to perform this function for IPv4, but that’s been renamed Neighbor Discovery in ICMPv6. This process is accomplished by using a multicast address called the solicited node address, and all hosts join this multicast group when they connect to the network. Part of their IPv6 address (the 24 bits farthest to the right) is added to the end of the multicast address FF02:0:0:0:0:1:FF/104. When this address is queried, the corresponding host will send back its layer 2 address. Devices can find and keep track of other neighbor devices on the network in pretty much the same way. When I talked about RA and RS messages earlier and told you that they use multicast traffic to request and send address information, that too was a function of ICMPv6—specifically, neighbor discovery.
In IPv4, the protocol IGMP was used to allow a host device to tell its local router that it was joining a multicast group and would like to receive the traffic for that group. This IGMP function has been replaced by ICMPv6, and the process has been renamed multicast listener discovery.
This is how it works: The source node of a connection will send a packet that’s equal to the MTU size of its local link’s MTU. As this packet traverses the path toward its destination, any link that has an MTU smaller than the size of the current packet will force the intermediate router to send a “packet too big” message back to the source machine. This message tells the source node what the maximum size is that the restrictive link will allow and asks the source to send a new scaled-down packet that can pass through. This process will continue until the destination is finally reached, with the source node now sporting the new path’s MTU. So now, when the rest of the data packets are transmitted, they’ll be protected from fragmentation.
ICMPv6 now takes over the task of finding the address of other devices on the local link. Address Resolution Protocol used to perform this function for IPv4, but that’s been renamed Neighbor Discovery in ICMPv6. This process is accomplished by using a multicast address called the solicited node address, and all hosts join this multicast group when they connect to the network. Part of their IPv6 address (the 24 bits farthest to the right) is added to the end of the multicast address FF02:0:0:0:0:1:FF/104. When this address is queried, the corresponding host will send back its layer 2 address. Devices can find and keep track of other neighbor devices on the network in pretty much the same way. When I talked about RA and RS messages earlier and told you that they use multicast traffic to request and send address information, that too was a function of ICMPv6—specifically, neighbor discovery.
In IPv4, the protocol IGMP was used to allow a host device to tell its local router that it was joining a multicast group and would like to receive the traffic for that group. This IGMP function has been replaced by ICMPv6, and the process has been renamed multicast listener discovery.
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