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MTU (Maximum Transmission Unit)

MTU is the largest packet size a network link can carry without fragmentation. Ethernet MTU is 1500 bytes. If a packet exceeds the path MTU, it is fragmented (IPv4) or dropped with ICMP Packet Too Big (IPv6). Path MTU Discovery (PMTUD) finds the lowest MTU along a route.

Definition

MTU defines the maximum IP packet size a link can transmit in a single frame. Standard Ethernet has 1500-byte MTU. The effective payload after headers is smaller: 1500 - 20 (IP) - 20 (TCP) = 1460 bytes (TCP MSS). Jumbo frames use 9000-byte MTU within data centers for higher throughput on bulk transfers. VPN tunnels reduce effective MTU by their encapsulation overhead – IPsec adds 50-80 bytes, WireGuard adds 60 bytes. Path MTU Discovery (PMTUD) sends packets with the Don't Fragment (DF) bit set. If a router along the path has a smaller MTU, it drops the packet and returns ICMP Fragmentation Needed. The sender then reduces packet size. Firewalls that block ICMP break PMTUD, causing silent packet loss for larger payloads – this is called a PMTU black hole.

Examples

  • –ip link set dev eth0 mtu 9000 – set jumbo frames
  • –ping -M do -s 1472 host – test PMTUD (1472 + 28 headers = 1500)
  • –tracepath host – discovers path MTU hop by hop

Related Protocols

iptcp

Related Terms

Frame

A frame is a Layer 2 (data link) unit of data that includes a header with MAC addresses, the payload (an IP packet), and a trailer with error-checking CRC. Ethernet frames have a maximum size of 1518 bytes. Frames are addressed by MAC, not IP – they do not cross router boundaries.

MSS (Maximum Segment Size)

MSS is the largest TCP payload (excluding headers) that can be sent in a single segment. MSS is negotiated during TCP handshake and derived from MTU: MSS = MTU - IP header (20) - TCP header (20). For 1500-byte Ethernet MTU, MSS is 1460 bytes. MSS avoids IP fragmentation by keeping segments within link MTU.

Fragmentation

IP fragmentation splits packets larger than the link MTU into smaller fragments for transmission, reassembled at the destination. Fragmentation hurts performance – any single lost fragment requires retransmitting the entire original packet. IPv6 does not allow router fragmentation; only endpoints can fragment.