MTU Explained: Fragmentation, PMTUD, and Practical Troubleshooting

Quick primer: MTU (Maximum Transmission Unit) is the largest link-layer packet a network segment will carry without IP fragmentation. Misaligned MTU values or encapsulation overhead (tunnels, VPNs) commonly cause subtle packet drops, poor performance, and “blackhole” connections where large flows fail silently. Why this matters MTU affects TCP performance and whether packets are fragmented. Fragmentation can hurt latency and reliability; IPv6 drops oversized packets instead of fragmenting. MSS (Maximum Segment Size) is the TCP-side payload limit derived from MTU minus IP/TCP headers. If MSS isn't adjusted when MTU changes (encapsulation, tunnels), TCP may send packets that get dropped. Common blackhole MTU symptoms Large downloads or SSH sessions hang while small packets succeed. Intermittent connectivity for specific paths or through tunnels. ICMP “fragmentation needed” messages absent or blocked. Quick troubleshooting checklist Confirm link MTU (Ethernet commonly 1500; jumbo frames &