RFC 1191 / START HERE
IPv4 Path MTU, before the details
Path MTU Discovery discovers the Path Maximum Transmission Unit: the largest complete IP datagram that can cross a path without fragmentation. RFC 1191 describes the classic IPv4 mechanism.
A sender starts with its first link’s size limit and sets Don’t Fragment (DF). If a router cannot forward a datagram without splitting it, the router drops it and returns an ICMP (Internet Control Message Protocol) error with a smaller next-hop limit. The sender adjusts the size it sends.
The idea in one picture
Sender: DF packet
Router: size check
Smaller packet
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
The smallest link on a route limits every complete datagram, even when the first link allows larger ones. Learning this limit avoids needless fragmentation while allowing larger packets when possible. This 1990 Draft Standard obsoletes RFC 1063; IPv6 PMTU Discovery and packetization-layer techniques are separate mechanisms.
A familiar example
A download can open a TCP connection yet stall when larger packets hit a smaller link and the needed ICMP feedback is blocked. This guide follows a fictional 1500→1280 example, separates packet size from TCP data size, and explains timers, older routers and trust limits. MTU discovery does not measure bandwidth or guarantee delivery.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Find the smallest link limitA path’s MTU is the smallest link MTU along the path. The sender initially knows its first-hop MTU, not every later link’s limit.→
- Read the ICMP feedbackThe router reports the MTU of its constricting next hop and quotes enough of the original datagram to identify the affected traffic.→
- Fit TCP data inside the new limitA new path estimate limits the complete datagram. TCP must also respect the peer’s MSS (Maximum Segment Size: its advertised data limit) and subtract the headers actually sent.→
- Handle a router that reports zeroAn old router can return a Too Big message with zero in the newly defined MTU field. The host still has to reduce its estimate.→
- Probe slowly and distrust false feedbackRoutes can improve, but probing larger sizes costs packets. False or missing ICMP feedback can mislead or stall classic discovery.→
This introduction explains RFC 1191 (1990). Read the original RFC ↗