Packets

Path MTU Discovery Finds Packet Size Limits

Every network link has a maximum transmission unit, or MTU, that limits the size of an IP packet carried in one link-layer frame. A route can cross links with different limits, so the smallest relevant MTU along that route constrains packet size end to end. Path MTU Discovery, commonly shortened to PMTUD, lets an endpoint find that constraint and adjust packet sizes rather than relying on fragmentation. One route can contain several MTU limits Ethernet commonly uses an IP MTU of 1500 bytes, but tunnels, VPNs, encapsulation, and other link types can reduce the usable size. A packet that fits the sender’s local interface can therefore be too large for a later hop.