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RFC 1191 / CLASSIC IPv4 PATH MTU DISCOVERY

Probe slowly and distrust false feedback

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Routes can improve, but probing larger sizes costs packets. False or missing ICMP feedback can mislead or stall classic discovery.

DIAGRAMRead RFC section 3 ↗
RFC 1191 · § 3EXAMPLE
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Probe slowly and distrust false feedbackToo Big at time0 lowers estimate to1280. An upward probe at4minutes is blocked before the5-minute minimum. At10minutes a changed path delivers1500 with upper-layer confirmation. Another increase at10:30 is blocked before the1-minute minimum since success. A separate false9000 Too Big cannot raise cache1500. A separate blocked-ICMP case leaves source estimate1500 after its oversized DF packet is dropped at1280; no delivery or successful discovery is shown.Probe increases slowly · errors cannot raise PMTUSourceRouterDestination1500 · DF=1link1280: dropICMP filterICMP 3/4feedback blockedSource estimate stays 1500 · no size feedbackClassic black hole · see RFC 2923 §2.1
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See a classic PMTU black hole.

Separate path: source assumes 1500, router limit 1280 drops the DF packet, and a filter blocks the returning ICMP. The source gets no size feedback and does not discover 1280 from that missing report.

Source for this drawing: RFC 1191 · Read RFC section 3 ↗

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RFC 791 IPv4 RFC 792 ICMPv4 RFC 9293 TCP

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KEY POINT

A Too Big report can lower an estimate; it cannot announce a safe increase.

  • Hosts MAY detect increases, but MUST do so infrequently: no attempt less than 5 minutes after a Too Big message for the destination, nor less than 1 minute after a previous successful increase. RFC 1191 recommends twice these minima:10 and 2 minutes.
  • Hosts MUST NOT increase PMTU based on a Too Big message, and MUST never reduce the estimate below 68 octets. False smaller values can still degrade performance; these bounds do not authenticate ICMP.
  • A larger estimate alone is not evidence of a dropped packet: §6.3 says an upper layer MUST NOT retransmit merely because the estimate increased. If necessary ICMP is blocked, classic discovery can black-hole larger DF packets; RFC 2923 §2.1 documents the TCP failure. Packetization-layer alternatives and IPv6 discovery have separate specifications.
DETAILS

The timeline’s 10-minute probe follows the recommendation;5 and 1 are the normative minimum waits. The successful probe assumes a route change and evidence of delivery from the upper-layer exchange, not merely an absence of ICMP. The half-minute retry, false 9000 report and blocked-ICMP scene are separate cases. No wall-clock waiting or real probing occurs in this guide.

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