Which mechanism uses ICMPv6 to report the maximum packet size that can be forwarded without fragmentation?

• A. Hop-by-hop discovery
• B. Neighbor discovery
• C. Path MTU Discovery
• D. Router discovery

Answer: (C)

Path MTU Discovery is the mechanism that uses ICMPv6 to learn the largest packet size that can be forwarded across the entire path without fragmentation. In IPv6, routers do not fragment packets; fragmentation is done only by the source. When a packet is too large for a link along the route, a router along the path drops it and sends back an ICMPv6 Packet Too Big message that includes the MTU of the next hop. The sender then lowers the packet size accordingly and retransmits, continuing until the path MTU is found. This process ensures packets can reach the destination without fragmentation. The other options do not fit because they handle neighbor resolution, router discovery, or per-hop options rather than reporting the path’s maximum forwardable size.