What is MTU?
Also known as: Maximum Transmission Unit
The Maximum Transmission Unit (MTU) is the largest size of a single protocol data unit (packet or frame) that a network link can forward without requiring fragmentation or dropping.
The Maximum Transmission Unit (MTU) is a fundamental property of a network link, defining the largest packet payload (including headers) that the link can deliver end-to-end without needing to fragment the packet into smaller pieces. Each link-layer technology has a native MTU: Ethernet typically uses 1500 bytes, PPPoE links often use 1492 bytes, and some WAN backbones use jumbo frames of 9000 bytes or more. The IP layer relies on the MTU to decide how large each datagram can be.
When a host sends a packet larger than the MTU of a link along the path, the router connected to that link must either fragment the IP packet into smaller chunks (IP fragmentation) or, if the Don't Fragment (DF) flag is set, drop the packet and send an ICMP Fragmentation Needed message back to the sender. Path MTU Discovery (PMTUD, defined in RFC 1191 for IPv4 and RFC 8201 for IPv6) is the standard mechanism by which hosts probe the smallest MTU across a network path, allowing them to send packets that will fit without fragmentation.
MTU mismatches are a common source of connectivity problems. For instance, if a VPN or tunnel encapsulation adds overhead (e.g., 8 or 20 bytes), the effective payload MTU shrinks. If a sending host does not adjust for this, large packets can silently vanish due to dropped fragments or blocked ICMP messages. Network operators often explicitly set the MTU on router interfaces and use tools like ping with the DF flag and varying payload sizes to measure the actual path MTU.
Key facts
- Ethernet's standard MTU is 1500 bytes, covering the entire IP packet including headers.
- Jumbo frames on Gigabit Ethernet typically use an MTU of 9000 bytes to reduce overhead.
- Path MTU Discovery (RFC 1191, RFC 8201) lets hosts find the smallest MTU along a route.
- ICMP Fragmentation Needed messages are required for PMTUD to work; they are often blocked by firewalls.
- VPNs and tunneling protocols reduce the effective MTU by adding encapsulation headers.
How it works in practice
Related terms
References
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