PMTU discovery

[agrover]

https://quicwg.org/base-drafts/draft-ietf-quic-transport.html#path-maximum-transmission-unit-pmtu

An endpoint SHOULD use Datagram Packetization Layer PMTU Discovery ([DPLPMTUD]) or implement Path MTU Discovery (PMTUD) [RFC1191] [RFC8201] to determine whether the path to a destination will support a desired message size without fragmentation.

In the absence of these mechanisms, QUIC endpoints SHOULD NOT send IP packets larger than 1280 bytes. Assuming the minimum IP header size, this results in a QUIC maximum packet size of 1232 bytes for IPv6 and 1252 bytes for IPv4. A QUIC implementation MAY be more conservative in computing the QUIC maximum packet size to allow for unknown tunnel overheads or IP header options/extensions.