Phân mảnh IP

Why Fragmentation Happens

Every network link has a Maximum Transmission Unit (MTU) — the largest IP packet it can carry without breaking it apart. When a router receives a packet larger than the outgoing link's MTU, it must either fragment the packet into smaller pieces or drop it and send an ICMP "Fragmentation Needed" message back to the sender (when the Don't Fragment bit is set). Common default MTUs are 1500 bytes on Ethernet and 576 bytes on some older WAN links.

Fragmentation in IPv4 vs. IPv6

IPv4Internet Protocol version 4. The fourth revision of IP using 32-bit addresses (e.g., 192.168.1.1), providing approximately 4.3 billion unique addresses. Still the most widely used internet protocol despite address exhaustion. routers can fragment packets in transit, adding overhead and making reassembly the responsibility of the destination host. IPv6Internet Protocol version 6. The successor to IPv4 using 128-bit addresses (e.g., 2001:0db8::1), providing a virtually unlimited address space of 3.4 x 10^38 addresses. Designed to solve IPv4 address exhaustion. eliminates in-transit fragmentation entirely: intermediate routers will never fragment IPv6 packets. Instead, hosts must perform Path MTU Discovery (PMTUD) — sending packets with the IPv6 Don't Fragment equivalent and adjusting based on ICMPv6 Packet Too Big messages — before transmitting large payloads.

Performance and Security Impact

Fragmentation degrades ThroughputThe actual rate of successful data transfer over a network, measured in bits per second. Unlike bandwidth (theoretical maximum), throughput reflects real-world performance after accounting for latency, packet loss, and protocol overhead. because fragment reassembly is CPU-intensive and fragments that arrive out of order or are lost require retransmission of the entire original TCPTransmission Control Protocol. A reliable, connection-oriented transport protocol that guarantees ordered, error-checked delivery of data through three-way handshakes, acknowledgments, and retransmission. The foundation of HTTP, SSH, and most internet services. segment in some cases. Firewalls that block ICMPInternet Control Message Protocol. A network-layer protocol used for diagnostic and error-reporting purposes. Ping (echo request/reply) and traceroute both rely on ICMP messages. messages break PMTUD, causing mysterious connection stalls (the "black hole" effect) for large transfers like file downloads or VPN tunnels. Network teams should enable PMTUD and avoid filtering ICMP type 3 code 4 messages. Ping Test