Age estimates and spot black holes
05Paths 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.
Separate connection example: small TCP setup packets passed, but a 1500-byte data packet stops at a 1280 hop. Its PTB is blocked before the sender. The sender retains its stale estimate; PLPMTUD is separate related reading, not an unseen successful recovery.
Source for this drawing: RFC 8201 · Read RFC section 4 ↗
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A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
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These pictures are analogies to help you learn. The linked RFC explains the actual rules.
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Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
Try increases infrequently, never learn an increase from PTB, and do not treat silence as success.
- For an implementation that probes for PMTU increases, an attempt must not occur less than5 minutes after a PTB received for that path (§4). Ten minutes is the recommended timer, not a required exact probe schedule. These are source requirements in lower-case language, not an invented RFC 2119 mapping.
- §5.3 recommends aging cached information on the order of10 minutes and a configurable timeout, including infinity. An optional probe is not mandatory at exactly 300 or600 seconds. A fresh PTB restarts the relevant wait; another path’s timer need not be reset.
- A probe larger than the current estimate must not be fragmented down to that estimate (§5.4). The drawing’s 1400-byte probe is an intact test packet. Permission to probe is not proof that 1400 works, and RFC 8201 does not prescribe the complete success-confirmation algorithm.
- If the path remains 1280, the larger probe can be dropped and return PTB 1280; retain the 1280 estimate and wait again. The packetization layer handles loss/retransmission. A valid PTB must never raise the estimate (§4).
- An ICMPv6 filter can suppress useful PTB feedback, causing a black-hole connection: small handshake packets may pass while large data packets disappear (§§1,6). Silence is not proof of delivery, a dead peer or a particular cause. Do not depict a blocked report reaching the sender.
- PLPMTUD, specified separately in RFC 4821, uses packetization-layer techniques without relying on ICMPv6 delivery and is more robust to that filtering (§§1,6). It is a related mechanism, not a formal UpdatedBy edge in RFC 8201 metadata or a fully implemented fallback in this drawing. Correlated false decreases can still reduce throughput; matching a quote is not cryptographic authentication (§6).
Example last PTB at t=0. Timeline geometry uses seconds:299 is before the 300-second minimum;600 marks the recommended wait. Last state resets to a separate blocked-feedback example.
Read the RFC section ↗