Summary

  • RFC 1063 defined four-byte Probe MTU and Reply MTU options, numbered 11 and 12, to measure and return the smallest usable datagram size along an IPv4 path.
  • RFC 1191 replaced this success-path accumulation with a failure signal: DF-marked packets that were too large elicited ICMP Type 3, Code 4 carrying the next-hop MTU.

The Probe MTU option began with a modest physical fact: the MTU of the sender's first-hop network. Its two-octet value could then move in only one direction. A gateway compared it with the MTUs of both the inbound and outbound links and lowered it whenever either link was narrower. The destination performed its own check and could lower the value again if it could not accept a datagram that large.

The format was almost austere. Option 11 contained a type byte, a length byte set to four, and the two-octet value. Option 12 used the same shape to carry the discovered result back. In RFC 1063's preferred IP-layer design, receiving one probe caused one reply option to be attached to a return datagram. The temporary value was then discarded.

This made the path itself participate in measurement. The option did not merely record where a packet had gone; every gateway was expected to compute a running minimum. RFC 1063 even warned that routers following the architecture of RFC 1009 might not know both interfaces when options were processed, so support could require major software changes.

The authors tried to contain the cost. A host was not supposed to place the probe in every packet, but roughly once per round-trip interval. A lower reply could immediately reduce the cached estimate. A higher reply deserved suspicion: routing might be flapping, so the smaller estimate should be allowed to age before it was raised.

Two years later, RFC 1191 formally obsoleted RFC 1063 and changed where the useful evidence appeared. A source sent datagrams with Don't Fragment set. If a router met a next hop too narrow for the datagram, it discarded the packet and returned ICMP Destination Unreachable, Type 3 Code 4. The low 16 bits of a formerly unused field carried the largest datagram that router could forward without fragmentation. That field was the only router-side change the new RFC required.

The contrast is architectural. RFC 1063 asked routers to inspect and rewrite successful probes. RFC 1191 let successful traffic pass without that computation and produced information at the bottleneck only when forwarding failed. The latter did not make discovery effortless: hosts had to converge, tolerate older zero-valued ICMP replies, detect decreases quickly, and test increases sparingly.

By 2014, RFC 7126 treated both options as obsolete and advised routers, gateways and firewalls to drop packets containing them. It also noted that forged values could distort a host's estimate. That later judgement should not be projected backward as proof of how widely the options were deployed. The primary record establishes a design, its replacement and its eventual status—not a deployment census.

Sources