The maximum amount of bytes that can be put in a single [[Ethernet Frame]] (for [[Ethernet]], 1500 bytes + 18 bytes of overhead = 1518 bytes). A bigger MTU means: - Each frame carries more info, less overhead spent on packaging - Processing fewer packets = time savings - Corruption and Communication Errors - Increased network delay ## How to find the MTU One method is to just ask: *Simple Version* - Sender sends packet with the `DF` flag - 'dont fragment' - "If too big, don't fragment, drop it & tell me", - Router with smaller MTU responds with ICMP response: - "fragmentation needed" - MTU limit of next hop - Continue untilsuccess *Complicated Version* - [[ICMP]] Packets often dropped for security. reasons - Shoul you trust the responder? - [[PLMTUD]] - Packetization Layer Path MTU Discovery