Link, path and payload sizes
A route can cross links with different packet limits. Its path MTU reflects the constraining link, so the local interface’s MTU alone does not determine the entire route’s capacity.
Application payload must allow for protocol headers, with encapsulation adding overhead. TCP’s MSS concerns its data portion and is not identical to an MTU that includes network-layer headers.
Adjusting size from feedback
Classical discovery uses control feedback when a packet is too large to forward. IPv6 routers do not fragment for the source; Packet Too Big reports a relevant limit so the sender can adjust.
Paths can change, invalidating an earlier estimate. Discovery must handle decreases and possible increases without continuously losing oversized traffic.
Missing control messages and probing
Filtered or lost size feedback can leave a sender repeatedly using packets that cannot pass, creating a black hole. Small requests succeeding while larger responses stall can indicate size-dependent path behavior.
RFC 8899 defines datagram packetization-layer probing based on actual probe delivery rather than relying entirely on ICMP. It still needs suitable acknowledgment and loss handling.
References: [3]