RFC 8201 / START HERE
IPv6 Path MTU, before the details
MTU means Maximum Transmission Unit. Path MTU Discovery, or PMTUD, learns the largest IPv6 packet size that can pass along a route without IPv6 fragmentation.
A sender knows the size limit of its first link, but a later link can be smaller. Routers discard packets that do not fit and send ICMPv6 Packet Too Big feedback. The sender checks that feedback and reduces its packet-size estimate.
The idea in one picture
First-link estimate
Smaller hop
Source adapts
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
RFC 8201 specifies this feedback-based process for IPv6, replacing RFC 1981. Unlike IPv4, IPv6 routers do not fragment packets and there is no Don’t Fragment bit. The guide covers valid decreases, header budgets, shared path state and cautious probing for larger limits.
A familiar example
Imagine a parcel passing through several doorways: the narrowest doorway limits the parcel size. A fictional 1500-byte packet meets a 1280-byte link and is dropped. Packet Too Big can tell the source to send smaller packets—but if that feedback is filtered, small setup packets can work while large data stalls. The doorway analogy does not authenticate feedback.
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 linkA link MTU is the largest IPv6 packet one link can carry in one piece. The path MTU is the smallest of those limits along the route. A sender learns about a smaller limit through ICMPv6 Packet Too Big feedback.→
- Check Packet Too Big feedbackA Packet Too Big message includes a reported MTU and a quote of the packet that triggered it. The sender needs to relate the quote to traffic it actually sent and to the relevant local path before changing state.→
- Budget headers and dataPMTU limits the complete IPv6 packet. Its headers consume part of that budget. A packetization layer—TCP or an application protocol above UDP—chooses message sizes; a router does not silently resize application messages.→
- Share the right path stateA host caches an estimate under a local representation of a path. Multiple transports can share that path, while another interface or a different group of routes may need a different estimate.→
- Age estimates and spot black holesPaths change. An estimate that is too large can trigger another PTB, but an estimate that is too small needs occasional probing. Blocking PTB feedback can instead leave large packets disappearing without a useful update.→
This introduction explains RFC 8201 (2017). Read the original RFC ↗