Change:
payload an endpoint sends at any given time is referred to as the endpoint's maximum packet size. <del> QUIC depends on a PMTU of at least 1280 bytes.  This is the IPv6 minimum size RFC8200 and is also supported by most modern IPv4 networks.  All QUIC packets (except for PMTU probe packets) SHOULD be sized to fit within the maximum packet size to avoid the packet being fragmented or dropped RFC8085. An endpoint SHOULD use Datagram Packetization Layer PMTU Discovery (DPLPMTUD) or implement Path MTU Discovery (PMTUD) RFC1191 RFC8201 to </del> <ins> An endpoint SHOULD use DPLPMTUD (dplpmtud) or PMTUD (pmtud) to </ins> determine whether the path to a destination will support a desired <del> message size without fragmentation. </del> <ins> maximum packet size without fragmentation.  In the absence of these mechanisms, QUIC endpoints SHOULD NOT send IP packets larger than the smallest allowed maximum packet size. </ins> <del> 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. </del> <ins> Both DPLPMTUD and PMTUD send IP packets that are larger than the current maximum packet size.  We refer to these as PMTU probes.  All QUIC packets that are not sent in a PMTU probe SHOULD be sized to fit within the maximum packet size to avoid the packet being fragmented or dropped RFC8085. If a QUIC endpoint determines that the PMTU between any pair of local and remote IP addresses has fallen below the smallest allowed maximum packet size of 1200 bytes, it MUST immediately cease sending QUIC packets, except for those in PMTU probes or those containing CONNECTION_CLOSE frames, on the affected path.  An endpoint MAY terminate the connection if an alternative path cannot be found. </ins> Each pair of local and remote addresses could have a different PMTU. QUIC implementations that implement any kind of PMTU discovery therefore SHOULD maintain a maximum packet size for each combination of local and remote IP addresses. <del> If a QUIC endpoint determines that the PMTU between any pair of local and remote IP addresses has fallen below the size needed to support the smallest allowed maximum packet size, it MUST immediately cease sending QUIC packets, except for PMTU probe packets, on the affected path.  An endpoint MAY terminate the connection if an alternative path cannot be found. </del> <ins> A QUIC implementation MAY be more conservative in computing the maximum packet size to allow for unknown tunnel overheads or IP header options/extensions. </ins> <del> 14.2. </del> <ins> 14.1.1. </ins> <del> PMTU discovery RFC1191 RFC8201 relies on reception of ICMP messages (e.g., IPv6 Packet Too Big messages) that indicate when a packet is dropped because it is larger than the local router MTU.  DPLPMTUD can also optionally use these messages.  This use of ICMP messages is potentially vulnerable to off-path attacks that successfully guess the addresses used on the path and reduce the PMTU to a bandwidth- inefficient value. </del> <ins> Path Maximum Transmission Unit Discovery (PMTUD; RFC1191, RFC8201) relies on reception of ICMP messages (e.g., IPv6 Packet Too Big messages) that indicate when a packet is dropped because it is larger than the local router MTU.  DPLPMTUD can also optionally use these messages.  This use of ICMP messages is potentially vulnerable to off-path attacks that successfully guess the addresses used on the path and reduce the PMTU to a bandwidth-inefficient value. </ins> An endpoint MUST ignore an ICMP message that claims the PMTU has <del> decreased below 1280 bytes. </del> <ins> decreased below the minimum QUIC packet size. </ins> The requirements for generating ICMP (RFC1812, RFC4443) state that the quoted packet should contain as much of the original packet as

Discussion:
This is with some minor additional changes: Reflow. Move the odd text about using Handshake back where it belongs. Make the language about how to construct a probe less prescriptive. Mention 1200 when referring to the "smallest allowed maximum packet size" so it is clear what that means.
NAME I went with "PMTU probe packet", which is an IP packet. Let me know if that makes sense.
OK - There are already "IP packets" and "QUIC packets" - so some other term works fro me.
A top-level comment on the term "probing": In Section: Probing a New Path: A packet containing only probing frames is a "probing packet", and a packet containing any other frame is a "non-probing packet". Please be sure that "probe" can not be confused with "probing" in PLPMTUD.
In the subsection on “Packet Size {#packet-size}”, I think the blocks of text describe two things. There is some general guidance on size, and there are specific instructions for the size of the initial packet. I’d suggest we do try to separate these two, it would make it much easier for someone to understand the size constraints for forming the initial packet.
Regarding the sizing of Initial packets, see . I checked the text for probing and we have "PMTUD probe" in one case and "probing packet" elsewhere. It's not ideal, but I didn't see any cases that were clearly busted. I don't have a good idea for alternative labels, but would welcome suggestions.
It turns out that coordinating the publication of all the documents is a real challenge. Right now, DPLPMTUD depends on QUIC, which is fine, but it turns out that this is a problem for other things that depend on DPLPMTUD who now transitively depend on QUIC. QUIC could adopt the DPLPMTUD text that is specific to QUIC and loosen this dependency chain. The text changes are relatively modest, see . While this could be editorial, I think that this will require a consensus call. The nature of the changes is such that it should be treated as design. Also, I think that there are substantive changes required.

So from an AD perspective, draft-ietf-tsvwg-datagram-plpmtud is currently blocking another document (URL) from being published that has a missref on it. Except for this question of QUIC I am ready to approve draft-ietf-tsvwg-datagram-plpmtud. To me it is also clear that integration of DPLPMTUD done in the QUIC WG will result in less editorial clarity issues that potentially could make an future implementer uncertain. As can already been seen in the PR there a few question that potentially can improve the operation and not unnecessary restrict the operation of DPLPMTUD. From my perspective it looks like the core is a set of minor editorial issues to resolve that will make the DPLPMTUD be clearer and resolve the missref issue and function as well as keeping it in the TSVWG document would. If a little bit more effort is invested after the first step then there is potential for additional improvements.