draft-ietf-tsvwg-datagram-plpmtud-02.txt		draft-ietf-tsvwg-datagram-plpmtud-03.txt
	Internet Engineering Task Force G. Fairhurst		Internet Engineering Task Force G. Fairhurst
	Internet-Draft T. Jones		Internet-Draft T. Jones
	Updates: 4821 (if approved) University of Aberdeen		Updates: 4821 (if approved) University of Aberdeen
	Intended status: Standards Track M. Tuexen		Intended status: Standards Track M. Tuexen
	Expires: December 08, 2018 I. Ruengeler		Expires: January 3, 2019 I. Ruengeler
	Muenster University of Applied Sciences		Muenster University of Applied Sciences
	June 08, 2018		July 02, 2018
	Packetization Layer Path MTU Discovery for Datagram Transports		Packetization Layer Path MTU Discovery for Datagram Transports
	draft-ietf-tsvwg-datagram-plpmtud-02		draft-ietf-tsvwg-datagram-plpmtud-03
	Abstract		Abstract
	This document describes a robust method for Path MTU Discovery		This document describes a robust method for Path MTU Discovery
	(PMTUD) for datagram Packetization layers. The document describes an		(PMTUD) for datagram Packetization layers. The document describes an
	extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path		extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path
	MTU Discovery for IPv4 and IPv6. The method allows a Packetization		MTU Discovery for IPv4 and IPv6. The method allows a Packetization
	Layer (PL), or a datagram application that uses a PL, to discover		Layer (PL), or a datagram application that uses a PL, to discover
	whether a network path can support the current size of datagram and		whether a network path can support the current size of datagram.
	to probe a network path with progressively larger packets to find		This can be used to detect and reduce the message size when a sender
	whether the maxium packet size can be increased. This allows a		encounters a network black hole (where packets are discarded, and no
	sender to determine an appropriate packet size. This provides		ICMP message is received). The method can also probe a network path
	functionally for datagram transports that is equivalent to the		with progressively larger packets to find whether the maximum packet
	Packetization layer PMTUD specification for TCP, specified in		size can be increased. This allows a sender to determine an
	RFC4821.		appropriate packet size, providing functionally for datagram
			transports that is equivalent to the Packetization layer PMTUD
			specification for TCP, specified in RFC4821.
	The document also provides implementation notes for incorporating		The document also provides implementation notes for incorporating
	Datagram PMTUD into IETF Datagram transports or applications that use		Datagram PMTUD into IETF Datagram transports or applications that use
	transports.		transports.
	When published, this specification updates RFC4821.		When published, this specification updates RFC4821.
	Status of this Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at https://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on December 08, 2018.
			This Internet-Draft will expire on January 3, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2018 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents (http://trustee.ietf.org/		Provisions Relating to IETF Documents
	license-info) in effect on the date of publication of this document.		(https://trustee.ietf.org/license-info) in effect on the date of
	Please review these documents carefully, as they describe your rights		publication of this document. Please review these documents
	and restrictions with respect to this document. Code Components		carefully, as they describe your rights and restrictions with respect
	extracted from this document must include Simplified BSD License text		to this document. Code Components extracted from this document must
	as described in Section 4.e of the Trust Legal Provisions and are		include Simplified BSD License text as described in Section 4.e of
	provided without warranty as described in the Simplified BSD License.		the Trust Legal Provisions and are provided without warranty as
			described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . . 3		1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . 3
	1.2. Packetization Layer Path MTU Discovery . . . . . . . . . . 4		1.2. Packetization Layer Path MTU Discovery . . . . . . . . . 5
	1.3. Path MTU Discovery for Datagram Services . . . . . . . . . 5		1.3. Path MTU Discovery for Datagram Services . . . . . . . . 6
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
	3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 8		3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 8
	3.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . . 10		3.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . 10
	3.2. Validation of Probe Packet Size . . . . . . . . . . . . . 11		3.2. Validation of Probe Packet Size . . . . . . . . . . . . . 12
	3.3. Reducing the PLPMTU: Confirming Path Characteristics . . . 12		3.3. Reducing the PLPMTU: Confirming Path Characteristics . . 12
	3.4. Increasing the PLPMTU: Supporting Path Changes . . . . . . 12		3.4. Increasing the PLPMTU: Supporting Path Changes . . . . . 13
	3.5. Robustness to inconsistent Path information . . . . . . . 12		3.5. Robustness to inconsistent Path information . . . . . . . 13
	4. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . . 13		4. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 13
	4.1. PROBE_SEARCH: Probing for a larger PLPMTU . . . . . . . . 13		4.1. PROBE_SEARCH: Probing for a larger PLPMTU . . . . . . . . 14
	4.2. The PROBE_DONE state . . . . . . . . . . . . . . . . . . . 14		4.2. The PROBE_DONE state . . . . . . . . . . . . . . . . . . 15
	4.3. Verification and Use of PTB Messages . . . . . . . . . . . 14		4.3. Validation and Use of PTB Messages . . . . . . . . . . . 15
	4.4. Timers . . . . . . . . . . . . . . . . . . . . . . . . . . 14		4.4. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 16
	4.5. Constants . . . . . . . . . . . . . . . . . . . . . . . . 15		4.5. Constants . . . . . . . . . . . . . . . . . . . . . . . . 16
	4.6. Variables . . . . . . . . . . . . . . . . . . . . . . . . 16		4.6. Variables . . . . . . . . . . . . . . . . . . . . . . . . 17
	4.7. Selecting PROBED_SIZE . . . . . . . . . . . . . . . . . . 16		4.7. Selecting PROBED_SIZE . . . . . . . . . . . . . . . . . . 18
	4.8. Black Hole Detection . . . . . . . . . . . . . . . . . . . 17		4.8. Simple Black Hole Detection . . . . . . . . . . . . . . . 18
	4.9. State Machine . . . . . . . . . . . . . . . . . . . . . . 17		4.8.1. Simple Black Hole Detection State Machine . . . . . . 19
	5. Specification of Protocol-Specific Methods . . . . . . . . . . 20		4.9. Full State Machine . . . . . . . . . . . . . . . . . . . 20
	5.1. Application support for DPLPMTUD with UDP or UDP-Lite . . 20		5. Specification of Protocol-Specific Methods . . . . . . . . . 23
	5.1.1. Application Request . . . . . . . . . . . . . . . . . 20		5.1. Application support for DPLPMTUD with UDP or UDP-Lite . . 23
	5.1.2. Application Response . . . . . . . . . . . . . . . . . 20		5.1.1. Application Request . . . . . . . . . . . . . . . . . 24
	5.1.3. Sending Application Probe Packets . . . . . . . . . . 21		5.1.2. Application Response . . . . . . . . . . . . . . . . 24
	5.1.4. Validating the Path . . . . . . . . . . . . . . . . . 21		5.1.3. Sending Application Probe Packets . . . . . . . . . . 24
	5.1.5. Handling of PTB Messages . . . . . . . . . . . . . . . 21		5.1.4. Validating the Path . . . . . . . . . . . . . . . . . 24
	5.2. DPLPMTUD with UDP Options . . . . . . . . . . . . . . . . 21		5.1.5. Handling of PTB Messages . . . . . . . . . . . . . . 24
	5.2.1. UDP Request Option . . . . . . . . . . . . . . . . . . 22		5.2. DPLPMTUD with UDP Options . . . . . . . . . . . . . . . . 24
	5.2.2. UDP Response Option . . . . . . . . . . . . . . . . . 22		5.2.1. UDP Request Option . . . . . . . . . . . . . . . . . 25
	5.3. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 22		5.2.2. UDP Response Option . . . . . . . . . . . . . . . . . 25
	5.3.1. SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . . 22		5.3. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 26
	5.3.1.1. Sending SCTP Probe Packets . . . . . . . . . . . . 22		5.3.1. SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . 26
	5.3.1.2. Validating the Path with SCTP . . . . . . . . . . 23		5.3.1.1. Sending SCTP Probe Packets . . . . . . . . . . . 26
	5.3.1.3. PTB Message Handling by SCTP . . . . . . . . . . . 23		5.3.1.2. Validating the Path with SCTP . . . . . . . . . . 27
			5.3.1.3. PTB Message Handling by SCTP . . . . . . . . . . 27
	5.3.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 23		5.3.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 27
	5.3.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . . 23		5.3.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . 27
	5.3.2.2. Validating the Path with SCTP/UDP . . . . . . . . 23		5.3.2.2. Validating the Path with SCTP/UDP . . . . . . . . 27
	5.3.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . . 24		5.3.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . 27
	5.3.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . . 24		5.3.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 28
	5.3.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 24		5.3.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 28
	5.3.3.2. Validating the Path with SCTP/DTLS . . . . . . . . 24		5.3.3.2. Validating the Path with SCTP/DTLS . . . . . . . 28
	5.3.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 24		5.3.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 28
	5.4. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 24		5.4. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 28
	5.4.1. Sending QUIC Probe Packets . . . . . . . . . . . . . . 24		5.4.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 28
	5.4.2. Validating the Path with QUIC . . . . . . . . . . . . 25		5.4.2. Validating the Path with QUIC . . . . . . . . . . . . 29
	5.4.3. Handling of PTB Messages by QUIC . . . . . . . . . . . 25		5.4.3. Handling of PTB Messages by QUIC . . . . . . . . . . 29
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25		6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 26		8. Security Considerations . . . . . . . . . . . . . . . . . . . 30
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
	9.1. Normative References . . . . . . . . . . . . . . . . . . . 26		9.1. Normative References . . . . . . . . . . . . . . . . . . 30
	9.2. Informative References . . . . . . . . . . . . . . . . . . 28		9.2. Informative References . . . . . . . . . . . . . . . . . 32
	Appendix A. Event-driven state changes . . . . . . . . . . . . . . 28		Appendix A. Event-driven state changes . . . . . . . . . . . . . 32
	Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . . 31		Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . 35
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
	1. Introduction		1. Introduction
	The IETF has specified datagram transport using UDP, SCTP, and DCCP,		The IETF has specified datagram transport using UDP, SCTP, and DCCP,
	as well as protocols layered on top of these transports (e.g., SCTP/		as well as protocols layered on top of these transports (e.g., SCTP/
	UDP, DCCP/UDP) and directly over the IP network layer. This document		UDP, DCCP/UDP) and directly over the IP network layer. This document
	describes a robust method for Path MTU Discovery (PMTUD) that may be		describes a robust method for Path MTU Discovery (PMTUD) that may be
	used with these transport protocols (or the applications that use		used with these transport protocols (or the applications that use
	their transport service) to discover an appropriate size of packet to		their transport service) to discover an appropriate size of packet to
	use across an Internet path.		use across an Internet path.
	1.1. Classical Path MTU Discovery		1.1. Classical Path MTU Discovery
	Classical Path Maximum Transmission Unit Discovery (PMTUD) can be		Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
	used with any transport that is able to process ICMP Packet Too Big		used with any transport that is able to process ICMP Packet Too Big
	(PTB) messages (e.g., [RFC1191] and [RFC8201]). The term PTB message		(PTB) messages (e.g., [RFC1191] and [RFC8201]). The term PTB message
	is applied to both IPv4 ICMP Unreachable messages (type 3) that carry		is applied to both IPv4 ICMP Unreachable messages (type 3) that carry
	the error Fragmentation Needed (Type 3, Code 4) and ICMPv6 packet too		the error Fragmentation Needed (Type 3, Code 4) and ICMPv6 packet too
	big messages (Type 2). When a sender receives a PTB message, it		big messages (Type 2). When a sender receives a PTB message, it
	reduces the effective MTU to the value reported as the Link MTU in		reduces the effective MTU to the value reported as the Link MTU in
	the PTB message, and a method that from time-to-time increases the		the PTB message, and a method that from time-to-time increases the
	packet size in attempt to discover an increase in the supported PMTU.		packet size in attempt to discover an increase in the supported PMTU.
	The packets sent with a size larger than the current effective PMTU		The packets sent with a size larger than the current effective PMTU
	are known as probe packets.		are known as probe packets.
	Packets not intended as probe packets are either fragmented to the		Packets not intended as probe packets are either fragmented to the
	current effective PMTU, or the attempt to send fails with an error		current effective PMTU, or the attempt to send fails with an error
	code. Applications are sometimes provided with a primitive to let		code. Applications are sometimes provided with a primitive to let
	them read the maximum packet size, derived from the current effective		them read the maximum packet size, derived from the current effective
	PMTU.		PMTU.
	Classical PMTUD is subject to protocol failures. One failure arises		Classical PMTUD is subject to protocol failures. One failure arises
	when traffic using a packet size larger than the actual PMTU is		when traffic using a packet size larger than the actual PMTU is
	black-holed (all datagrams sent with this size, or larger, are		black-holed (all datagrams sent with this size, or larger, are
	silently discarded without the sender receiving ICMP PTB messages).		silently discarded without the sender receiving ICMP PTB messages).
	This could arise when the PTB messages are not delivered back to the		This could arise when the PTB messages are not delivered back to the
	sender for some reason [RFC2923]). For example, ICMP messages are		sender for some reason [RFC2923]). For example, ICMP messages are
	increasingly filtered by middleboxes (including firewalls) [RFC4890].		increasingly filtered by middleboxes (including firewalls) [RFC4890].
	A stateful firewall could be configured with a policy to block		A stateful firewall could be configured with a policy to block
	incoming ICMP messages, which would prevent reception of PTB messages		incoming ICMP messages, which would prevent reception of PTB messages
	to endpoints behind this firewall. Other examples include cases		to endpoints behind this firewall. Other examples include cases
	where PTB messages are not correctly processed/generated by tunnel		where PTB messages are not correctly processed/generated by tunnel
	endpoints.		endpoints.
	Another failure could result if a node that is not on the network		Another failure could result if a node that is not on the network
	path sends a PTB message that attempts to force the sender to change		path sends a PTB message that attempts to force the sender to change
	the effective PMTU [RFC8201]. A sender can protect itself from		the effective PMTU [RFC8201]. A sender can protect itself from
	reacting to such messages by utilising the quoted packet within a PTB		reacting to such messages by utilising the quoted packet within a PTB
	message payload to verify that the received PTB message was generated		message payload to validate that the received PTB message was
	in response to a packet that had actually originated from the sender.		generated in response to a packet that had actually originated from
	However, there are situations where a sender would be unable to		the sender. However, there are situations where a sender would be
	provide this verification.		unable to provide this validation.
	Examples where verification is not possible include:		Examples where validation of the PTB message is not possible include:
	o When the router issuing the ICMP message is acting on a tunneled		o When the router issuing the ICMP message is acting on a tunneled
	packet, the ICMP message will be directed to the tunnel endpoint.		packet, the ICMP message will be directed to the tunnel endpoint.
	This tunnel endpoint is responsible for forwardiung the ICMP		This tunnel endpoint is responsible for forwardiung the ICMP
	message and also processing the quoted packet within the payload		message and also processing the quoted packet within the payload
	field to remove the effect of the tunnel, and return a correctly		field to remove the effect of the tunnel, and return a correctly
	fromatted ICMP message to the sender. Failure to do this results		fromatted ICMP message to the sender. Failure to do this results
	in black-holing.		in black-holing.
	o When a router issuing the ICMP message implements RFC792		o When a router issuing the ICMP message implements RFC792
	skipping to change at page 4, line 51 ¶		skipping to change at page 5, line 19 ¶
	previous bullet. Even if the decapsulated message is processed by		previous bullet. Even if the decapsulated message is processed by
	the tunnel endpoint, there could be insufficient bytes remaining		the tunnel endpoint, there could be insufficient bytes remaining
	for the sender to interpret the quoted transport information.		for the sender to interpret the quoted transport information.
	RFC1812 [RFC1812] requires routers to return the full packet if		RFC1812 [RFC1812] requires routers to return the full packet if
	possible, often the case for IPv4 when used the path includes		possible, often the case for IPv4 when used the path includes
	tunnels; or where the packet has been encapsulated/tunneled over		tunnels; or where the packet has been encapsulated/tunneled over
	an encrypted transport and it is not possible to determine the		an encrypted transport and it is not possible to determine the
	original transport header ).		original transport header ).
	o Even when the PTB message includes sufficient bytes of the quoted		o Even when the PTB message includes sufficient bytes of the quoted
	packet, the network layer could lack sufficient context to perform		packet, the network layer could lack sufficient context to
	verification, because this depends on information about the active		validate the message, because this depends on information about
	transport flows at an endpoint node (e.g., the socket/address		the active transport flows at an endpoint node (e.g., the socket/
	pairs being used, and other protocol header information).		address pairs being used, and other protocol header information).
	1.2. Packetization Layer Path MTU Discovery		1.2. Packetization Layer Path MTU Discovery
	The term Packetization Layer (PL) has been introduced to describe the		The term Packetization Layer (PL) has been introduced to describe the
	layer that is responsible for placing data blocks into the payload of		layer that is responsible for placing data blocks into the payload of
	IP packets and selecting an appropriate Maximum Packet Size (MPS).		IP packets and selecting an appropriate Maximum Packet Size (MPS).
	This function is often performed by a transport protocol, but can		This function is often performed by a transport protocol, but can
	also be performed by other encapsulation methods working above the		also be performed by other encapsulation methods working above the
	transport.		transport.
	In contrast to PMTUD, Packetization Layer Path MTU Discovery		In contrast to PMTUD, Packetization Layer Path MTU Discovery
	(PLPMTUD) [RFC4821] does not rely upon reception and verification of		(PLPMTUD) [RFC4821] does not rely upon reception and validation of
	PTB messages. It is therefore more robust than Classical PMTUD. This		PTB messages. It is therefore more robust than Classical PMTUD.
	has become the recommended approach for implementing PMTU discovery		This has become the recommended approach for implementing PMTU
	with TCP.		discovery with TCP.
	It uses a general strategy where the PL sends probe packet to search		It uses a general strategy where the PL sends probe packet to search
	for the largest size of unfragmented datagram that can be sent over a		for the largest size of unfragmented datagram that can be sent over a
	path. The probe packets are sent with a progressively larger packet		path. The probe packets are sent with a progressively larger packet
	size. If a probe packet is successfully delivered (as determined by		size. If a probe packet is successfully delivered (as determined by
	the PL), then the PLPMTU is raised to the size of the successful		the PL), then the PLPMTU is raised to the size of the successful
	probe. If no response is received to a probe packet, the method		probe. If no response is received to a probe packet, the method
	reduces the probe size. This PLPMTU is used to set the application		reduces the probe size. This PLPMTU is used to set the application
	MPS.		MPS.
	PLPMTUD introduces flexibility in the implementation of PMTU		PLPMTUD introduces flexibility in the implementation of PMTU
	discovery. At one extreme, it can be configured to only perform PTB		discovery. At one extreme, it can be configured to only perform PTB
	black hole detection and recovery to increase the robustness of		black hole detection and recovery to increase the robustness of
	Classical PMTUD, or at the other extreme, all PTB processing can be		Classical PMTUD, or at the other extreme, all PTB processing can be
	disabled and PLPMTUD can completely replace Classical PMTUD.		disabled and PLPMTUD can completely replace Classical PMTUD.
	PLPMTUD can also include additional consistency checks without		PLPMTUD can also include additional consistency checks without
	increasing the risk of increased black-holing. For instance,the		increasing the risk of increased black-holing. For instance,the
	information available at the PL, or higher layers, makes PTB		information available at the PL, or higher layers, makes PTB
	verification more straight forward.		validation more straight forward.
	1.3. Path MTU Discovery for Datagram Services		1.3. Path MTU Discovery for Datagram Services
	Section 4 of this document presents a set of algorithms for datagram		Section 4 of this document presents a set of algorithms for datagram
	protocols to discover the largest size of unfragmented datagram that		protocols to discover the largest size of unfragmented datagram that
	can be sent over a path. The method described relies on features of		can be sent over a path. The method described relies on features of
	the PL Section 3 and apply to transport protocols operating over IPv4		the PL Section 3 and apply to transport protocols operating over IPv4
	and IPv6. It does not require cooperation from the lower layers,		and IPv6. It does not require cooperation from the lower layers,
	although it can utilise ICMP PTB messages when these received		although it can utilise ICMP PTB messages when these received
	messages are made available to the PL.		messages are made available to the PL.
	The UDP Usage Guidelines [RFC8085] state "an application SHOULD		The UDP Usage Guidelines [RFC8085] state "an application SHOULD
	either use the Path MTU information provided by the IP layer or		either use the Path MTU information provided by the IP layer or
	implement Path MTU Discovery (PMTUD)", but does not provide a		implement Path MTU Discovery (PMTUD)", but does not provide a
	mechanism for discovering the largest size of unfragmented datagram		mechanism for discovering the largest size of unfragmented datagram
	than can be used on a path. Prior to this document, PLPMTUD had not		than can be used on a path. Prior to this document, PLPMTUD had not
	been specified for UDP.		been specified for UDP.
	Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the		Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the
	Stream Control Transport Protocol (SCTP). SCTP utilises heartbeat		Stream Control Transport Protocol (SCTP). SCTP utilises heartbeat
	messages as probe packets, but RFC4821 does not provide a complete		messages as probe packets, but RFC4821 does not provide a complete
	specification. This document provides the details to complete that		specification. This document provides the details to complete that
	specification.		specification.
	The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires		The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
	implementations to support Classical PMTUD and states that a DCCP		implementations to support Classical PMTUD and states that a DCCP
	sender "MUST maintain the MPS allowed for each active DCCP session".		sender "MUST maintain the MPS allowed for each active DCCP session".
	It also defines the current congestion control MPS (CCMPS) supported		It also defines the current congestion control MPS (CCMPS) supported
	by a path. This recommends use of PMTUD, and suggests use of control		by a path. This recommends use of PMTUD, and suggests use of control
	packets (DCCP-Sync) as path probe packets, because they do not risk		packets (DCCP-Sync) as path probe packets, because they do not risk
	skipping to change at page 6, line 33 ¶		skipping to change at page 7, line 8 ¶
	2. Terminology		2. Terminology
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119].		document are to be interpreted as described in [RFC2119].
	Other terminology is directly copied from [RFC4821], and the		Other terminology is directly copied from [RFC4821], and the
	definitions in [RFC1122].		definitions in [RFC1122].
	Black-Holed: When the sender is unaware that packets are not		Black-Holed: When the sender is unaware that packets are not
	delivered to the destination endpoint (e.g., when the sender		delivered to the destination endpoint (e.g., when the sender
	transmits packets of a particular size with a previously known		transmits packets of a particular size with a previously known
	effective PMTU (also refered to as the PLPMTU), but is unaware of		effective PMTU (also refered to as the PLPMTU), but is unaware of
	a change to the path that resulted in a smaller PLPMTU).		a change to the path that resulted in a smaller PLPMTU).
	Classical Path MTU Discovery: Classical PMTUD is a process described		Classical Path MTU Discovery: Classical PMTUD is a process described
	in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to		in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
	learn the largest size of unfragmented datagram than can be used		learn the largest size of unfragmented datagram than can be used
	across a path.		across a path.
	Datagram: A datagram is a transport-layer protocol data unit,		Datagram: A datagram is a transport-layer protocol data unit,
	transmitted in the payload of an IP packet.		transmitted in the payload of an IP packet.
	Effective PMTU: The current estimated value for PMTU that is used by		Effective PMTU: The current estimated value for PMTU that is used by
	a PMTUD. This is equivalent to the PLPMTU derived by PLPMTUD.		a PMTUD. This is equivalent to the PLPMTU derived by PLPMTUD.
	EMTU_S: The Effective MTU for sending (EMTU_S) is defined in		EMTU_S: The Effective MTU for sending (EMTU_S) is defined in
	[RFC1122] as "the maximum IP datagram size that may be sent, for a		[RFC1122] as "the maximum IP datagram size that may be sent, for a
	particular combination of IP source and destination addresses...".		particular combination of IP source and destination addresses...".
	EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in		EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in
	[RFC1122] as the largest datagram size that can be reassembled by		[RFC1122] as the largest datagram size that can be reassembled by
	EMTU_R ("Effective MTU to receive").		EMTU_R ("Effective MTU to receive").
	Link: A communication facility or medium over which nodes can		Link: A communication facility or medium over which nodes can
	communicate at the link layer, i.e., a layer below the IP layer.		communicate at the link layer, i.e., a layer below the IP layer.
	Examples are Ethernet LANs and Internet (or higher) layer and		Examples are Ethernet LANs and Internet (or higher) layer and
	tunnels.		tunnels.
	Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes of		Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes
	the largest IP packet, including the IP header and payload, that		of the largest IP packet, including the IP header and payload,
	can be transmitted over a link. Note that this could more		that can be transmitted over a link. Note that this could more
	properly be called the IP MTU, to be consistent with how other		properly be called the IP MTU, to be consistent with how other
	standards organizations use the acronym MT. This includes the IP		standards organizations use the acronym MT. This includes the IP
	header, but excludes link layer headers and other framing that is		header, but excludes link layer headers and other framing that is
	not part of IP or the IP payload. Other standards organizations		not part of IP or the IP payload. Other standards organizations
	generally define link MTU to include the link layer headers.		generally define link MTU to include the link layer headers.
	MPS: The Maximum Packet Size (MPS) is the largest size of application		MPS: The Maximum Packet Size (MPS) is the largest size of
	data block that can be sent unfragmented across a path. In		application data block that can be sent unfragmented across a
	DPLPMTUD this quantity is derived from PLPMTU by taking into		path. In DPLPMTUD this quantity is derived from PLPMTU by taking
	consideration the size of the application and lower protocol layer		into consideration the size of the application and lower protocol
	headers.		layer headers.
	Packet: An IP header plus the IP payload.		Packet: An IP header plus the IP payload.
	Packetization Layer (PL): The layer of the network stack that places		Packetization Layer (PL): The layer of the network stack that places
	data into packets and performs transport protocol functions.		data into packets and performs transport protocol functions.
	Path: The set of link and routers traversed by a packet between a		Path: The set of link and routers traversed by a packet between a
	source node and a destination node by a particular flow.		source node and a destination node by a particular flow.
	Path MTU (PMTU): The minimum of the Link MTU of all the links forming		Path MTU (PMTU): The minimum of the Link MTU of all the links
	a path between a source node and a destination node.		forming a path between a source node and a destination node.
	PLPMTU: The estimate of the actual PMTU provided by the DPLPMTUD		PLPMTU: The estimate of the actual PMTU provided by the DPLPMTUD
	algorithm.		algorithm.
	PLPMTUD: Packetization Layer Path MTU Discovery, the method described		PLPMTUD: Packetization Layer Path MTU Discovery, the method
	in this document for datagram PLs, which is an extension to		described in this document for datagram PLs, which is an extension
	Classical PMTU Discovery.		to Classical PMTU Discovery.
	Probe packet: A datagram sent with a purposely chosen size (typically		Probe packet: A datagram sent with a purposely chosen size
	larger than the current PLPMTU) to detect if packets of this size		(typically larger than the current PLPMTU) to detect if packets of
	can be successfully sent end-toend across the network path.		this size can be successfully sent end-toend across the network
			path.
	3. Features Required to Provide Datagram PLPMTUD		3. Features Required to Provide Datagram PLPMTUD
	TCP PLPMTUD has been defined using standard TCP protocol mechanisms.		TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
	All of the requirements in [RFC4821] also apply to use of the		All of the requirements in [RFC4821] also apply to use of the
	technique with a datagram PL. Unlike TCP, some datagram PLs require		technique with a datagram PL. Unlike TCP, some datagram PLs require
	additional mechanisms to implement PLPMTUD.		additional mechanisms to implement PLPMTUD.
	There are eight requirements for performing the datagram PLPMTUD		There are eight requirements for performing the datagram PLPMTUD
	method described in this specification:		method described in this specification:
	1. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide		1. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide
	information about the maximum size of packet that can be		information about the maximum size of packet that can be
	transmitted by the sender on the local link (the local Link MTU).		transmitted by the sender on the local link (the local Link MTU).
	It MAY utilize similar information about the receiver when this		It MAY utilize similar information about the receiver when this
	is supplied (note this could be less than EMTU_R). This avoids		is supplied (note this could be less than EMTU_R). This avoids
	implementations trying to send probe packets that can not be		implementations trying to send probe packets that can not be
	transmited by the local link. Too high a value may reduce the		transmited by the local link. Too high a value may reduce the
	efficiency of the search algorithm. Some applications also have		efficiency of the search algorithm. Some applications also have
	a maximum transport protocol data unit (PDU) size, in which case		a maximum transport protocol data unit (PDU) size, in which case
	there is no benefit from probing for a size larger than this		there is no benefit from probing for a size larger than this
	(unless a transport allows multiplexing multiple applications		(unless a transport allows multiplexing multiple applications
	PDUs into the same datagram).		PDUs into the same datagram).
	2. PLPMTU: A datagram application MUST be able to choose the size of		2. PLPMTU: A datagram application MUST be able to choose the size of
	datagrams sent to the network, up to the PLPMTU, or a smaller		datagrams sent to the network, up to the PLPMTU, or a smaller
	value (such as the MPS) derived from this. This value is managed		value (such as the MPS) derived from this. This value is managed
	by the DPLPMTUD method. The PLPMTU (specified as the effective		by the DPLPMTUD method. The PLPMTU (specified as the effective
	PMTU in Section 1 of [RFC1191]) is equivalent to the EMTU_S		PMTU in Section 1 of [RFC1191]) is equivalent to the EMTU_S
	(specified in [RFC1122]).		(specified in [RFC1122]).
	3. Probe packets: On request, a PLPMTUD sender is REQUIRED to be		3. Probe packets: On request, a PLPMTUD sender is REQUIRED to be
	able to transmit a packet larger than the PLMPMTU. This can be		able to transmit a packet larger than the PLMPMTU. This can be
	uses to send a probe packet. In IPv4, a probe packet MUST be		uses to send a probe packet. In IPv4, a probe packet MUST be
	sent with the Don't Fragment (DF) bit set in the IP header, and		sent with the Don't Fragment (DF) bit set in the IP header, and
	without network layer endpoint fragmentation. In IPv6, a probe		without network layer endpoint fragmentation. In IPv6, a probe
	packet is always sent without source fragmentation (as specified		packet is always sent without source fragmentation (as specified
	in section 5.4 of [RFC8201]).		in section 5.4 of [RFC8201]).
	4. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize		4. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize
	PTB messages received from the network layer to help identify		PTB messages received from the network layer to help identify
	when a path does not support the current size of packet probe.		when a path does not support the current size of packet probe.
	Any received PTB message MUST be verified before it is used to		Any received PTB message MUST be validated before it is used to
	update the PLPMTU discovery information [RFC8201]. This		update the PLPMTU discovery information [RFC8201]. This
	verification confirms that the PTB message was sent in response		validation confirms that the PTB message was sent in response to
	to a packet originating by the sender, and needs to be performed		a packet originating by the sender, and needs to be performed
	before the PLPMTU discovery method reacts to the PTB message.		before the PLPMTU discovery method reacts to the PTB message.
	When the router link MTU is indicated in the PTB message this MAY		When the router link MTU is indicated in the PTB message this MAY
	be used by DPLPMTUD to reduce the probe size but MUST NOT be used		be used by DPLPMTUD to reduce the probe size but MUST NOT be used
	to increase the PLPMTU ([RFC8201]). Verification SHOULD utilise		to increase the PLPMTU ([RFC8201]). This validation SHOULD
	information that can not be simply determined by an off-path		utilise information that can not be simply determined by an off-
	attacker, for example, by checking the value of a protocol header		path attacker, for example, by checking the value of a protocol
	field known only to the two PL endpoints. (Some datagram		header field known only to the two PL endpoints. (Some datagram
	applications use well-known source and destination ports and		applications use well-known source and destination ports and
	therefore this check needs to rely on other information.)		therefore this check needs to rely on other information.)
	5. Reception feedback: The destination PL endpoint is REQUIRED to		5. Reception feedback: The destination PL endpoint is REQUIRED to
	provide a feedback method that indicates to the DPLPMTUD sender		provide a feedback method that indicates to the DPLPMTUD sender
	when a probe packet has been received by the destination PL		when a probe packet has been received by the destination PL
	endpoint. The local PL endpoint at the sending node is REQUIRED		endpoint. The local PL endpoint at the sending node is REQUIRED
	to pass this feedback to the sender-side DPLPMTUD method.		to pass this feedback to the sender-side DPLPMTUD method.
	6. Probing and congestion control: The isolated loss of a probe		6. Probing and congestion control: The isolated loss of a probe
	packet SHOULD NOT be treated as an indication of congestion and		packet SHOULD NOT be treated as an indication of congestion and
	its loss SHOULD NOT directly trigger a congestion control		its loss SHOULD NOT directly trigger a congestion control
	reaction [RFC4821].		reaction [RFC4821].
	7. Probe loss recovery: If the data block carried by a probe message		7. Probe loss recovery: If the data block carried by a probe packet
	needs to be sent reliably, the PL (or layers above) MUST arrange		needs to be sent reliably, the PL (or layers above) MUST arrange
	retransmission/repair of any resulting loss. This method MUST be		retransmission/repair of any resulting loss. This method MUST be
	robust in the case where probe packets are lost due to other		robust in the case where probe packets are lost due to other
	reasons (including link transmission error, congestion). The		reasons (including link transmission error, congestion). The
	DPLPMTUD method treats isolated loss of a probe packet (with or		DPLPMTUD method treats isolated loss of a probe packet (with or
	without an PTB message) as a potential indication of a PMTU limit		without an PTB message) as a potential indication of a PMTU limit
	on the path, but not as an indictaion of congestion [CC].		on the path, but not as an indictaion of congestion Paragraph 6.
	8. Shared PLPMTU state: The PLPMTU value could also be stored with		8. Shared PLPMTU state: The PLPMTU value could also be stored with
	the corresponding entry in the destination cache and used by		the corresponding entry in the destination cache and used by
	other PL instances. The specification of PLPMTUD [RFC4821]		other PL instances. The specification of PLPMTUD [RFC4821]
	states: "If PLPMTUD updates the MTU for a particular path, all		states: "If PLPMTUD updates the MTU for a particular path, all
	Packetization Layer sessions that share the path representation		Packetization Layer sessions that share the path representation
	(as described in Section 5.2 of [RFC4821]) SHOULD be notified to		(as described in Section 5.2 of [RFC4821]) SHOULD be notified to
	make use of the new MTU and make the required congestion control		make use of the new MTU and make the required congestion control
	adjustments". Such methods need to be robust to the wide variety		adjustments". Such methods need to be robust to the wide variety
	of underlying network forwarding behaviours, PLPMTU adjustments		of underlying network forwarding behaviours, PLPMTU adjustments
	based on shared PLPMTU values should be incorporated in the		based on shared PLPMTU values should be incorporated in the
	search algorithms. Section 5.2 of [RFC8201] provides guidance on		search algorithms. Section 5.2 of [RFC8201] provides guidance on
	the caching of PMTU information and also the relation to IPv6		the caching of PMTU information and also the relation to IPv6
	flow labels.		flow labels.
	In addition, the following principles are stated for design of a		In addition, the following principles are stated for design of a
	DPLPMTUD method:		DPLPMTUD method:
	o MPS: A method MUST signal appropriate MPS to the higher layer		o MPS: A method MUST signal appropriate MPS to the higher layer
	using the PL. This may change following a change to the path. The		using the PL. This may change following a change to the path.
	method SHOULD avoid forcing an application to use an arbitrary		The method SHOULD avoid forcing an application to use an arbitrary
	small MPS (PLPMTU) for transmission while the method is searching		small MPS (PLPMTU) for transmission while the method is searching
	for the currently supported PLPMTU. Datagram PLs do not		for the currently supported PLPMTU. Datagram PLs do not
	necessarily support fragmentation of PDUs larger than the PLPMTU.		necessarily support fragmentation of PDUs larger than the PLPMTU.
	A reduced MPS can adversely impact the performance of a datagram		A reduced MPS can adversely impact the performance of a datagram
	application.		application.
	o Path validation: A method MUST be robust to path changes that		o Path validation: A method MUST be robust to path changes that
	could have occurred since the path characteristics were last		could have occurred since the path characteristics were last
	confirmed, and to the possibility of inconsistent path information		confirmed, and to the possibility of inconsistent path information
	being received.		being received.
	o Datagram reordering: A method MUST be robust to the possibility		o Datagram reordering: A method MUST be robust to the possibility
	that a flow encounters reordering, or has the traffic (including		that a flow encounters reordering, or has the traffic (including
	probe packets) is divided over more than one network path.		probe packets) is divided over more than one network path.
	o When to probe: A method SHOULD determine whether the path capacity		o When to probe: A method SHOULD determine whether the path capacity
	has increased since it last measured the path. This determines		has increased since it last measured the path. This determines
	when the path should again be probed.		when the path should again be probed.
	3.1. PLPMTU Probe Packets		3.1. PLPMTU Probe Packets
	The DPLPMTUD method relies upon the PL sender being able to generate		The DPLPMTUD method relies upon the PL sender being able to generate
	probe messages with a specific size. TCP is able to generate these		probe packets with a specific size. TCP is able to generate these
	probe packets by choosing to appropriately segment data being sent		probe packets by choosing to appropriately segment data being sent
	[RFC4821].		[RFC4821].
	In contrast, a datagram PL that needs to construct a probe packet has		In contrast, a datagram PL that needs to construct a probe packet has
	to either request an application to send a data block that is larger		to either request an application to send a data block that is larger
	than that generated by an application, or to utilise padding		than that generated by an application, or to utilise padding
	functions to extend a datagram beyond the size of the application		functions to extend a datagram beyond the size of the application
	data block. Protocols that permit exchange of control messages		data block. Protocols that permit exchange of control messages
	(without an application data block) could alternatively prefer to		(without an application data block) could alternatively prefer to
	generate a probe packet by extending a control message with padding		generate a probe packet by extending a control message with padding
	skipping to change at page 11, line 10 ¶		skipping to change at page 11, line 27 ¶
	way to fragment a datagram at the PL, or could instead utilise a		way to fragment a datagram at the PL, or could instead utilise a
	control packet with padding.		control packet with padding.
	A receiver needs to be able to distinguish an in-band data block from		A receiver needs to be able to distinguish an in-band data block from
	any added padding. This is needed to ensure that any added padding		any added padding. This is needed to ensure that any added padding
	is not passed on to an application at the receiver.		is not passed on to an application at the receiver.
	This results in three possible ways that a sender can create a probe		This results in three possible ways that a sender can create a probe
	packet listed in order of preference:		packet listed in order of preference:
	Probing using padding data: A probe packet that contains only control		Probing using padding data: A probe packet that contains only
	information together with any padding needed to inflate the packet		control information together with any padding needed to inflate
	to the size required for the probe packet. Since these probe		the packet to the size required for the probe packet. Since these
	packets do not carry an application-supplied data block,they do		probe packets do not carry an application-supplied data block,they
	not typically require retransmission, although they do still		do not typically require retransmission, although they do still
	consume network capacity and incur endpoint processing.		consume network capacity and incur endpoint processing.
	Probing using appication data and padding data: A probe packet that		Probing using appication data and padding data: A probe packet that
	contains a data block supplied by an application that is combined		contains a data block supplied by an application that is combined
	with padding to inflate the length of the datagram to the size		with padding to inflate the length of the datagram to the size
	required for the probe packet. If the application/transport needs		required for the probe packet. If the application/transport needs
	protection from the loss of this probe packet, the application/		protection from the loss of this probe packet, the application/
	transport may perform transport-layer retransmission/repair of the		transport may perform transport-layer retransmission/repair of the
	data block (e.g., by retransmission after loss is detected or by		data block (e.g., by retransmission after loss is detected or by
	duplicating the data block in a datagram without the padding		duplicating the data block in a datagram without the padding
	data).		data).
	Probing using appication data: A probe packet that contains a data		Probing using appication data: A probe packet that contains a data
	block supplied by an application that matches the size required		block supplied by an application that matches the size required
	for the probe packet. This method requests the application to		for the probe packet. This method requests the application to
	issue a data block of the desired probe size. If the application/		issue a data block of the desired probe size. If the application/
	transport needs protection from the loss of an unsuccessful probe		transport needs protection from the loss of an unsuccessful probe
	packet, the application/transport needs then to perform transport-		packet, the application/transport needs then to perform transport-
	layer retransmission/repair of the data block (e.g., by		layer retransmission/repair of the data block (e.g., by
	retransmission after loss is detected).		retransmission after loss is detected).
	A PL that uses a probe packet carrying an application data block,		A PL that uses a probe packet carrying an application data block,
	could need to retransmit this application data block if the probe		could need to retransmit this application data block if the probe
	skipping to change at page 11, line 53 ¶		skipping to change at page 12, line 22 ¶
	DLPMTUD MAY choose to use only one of these methods to simplify the		DLPMTUD MAY choose to use only one of these methods to simplify the
	implementation.		implementation.
	3.2. Validation of Probe Packet Size		3.2. Validation of Probe Packet Size
	The PL needs a method to determine when probe packets have been		The PL needs a method to determine when probe packets have been
	successfully received end-to-end across a network path.		successfully received end-to-end across a network path.
	Transport protocols can include end-to-end methods that detect and		Transport protocols can include end-to-end methods that detect and
	report reception of specific datagrams that they send (e.g., DCCP and		report reception of specific datagrams that they send (e.g., DCCP and
	SCTP provide keep-alive/heartbeat features). When supported, this		SCTP provide keep-alive/heartbeat features). When supported, this
	mechanism SHOULD also be used by DPLPMTUD to acknowledge reception of		mechanism SHOULD also be used by DPLPMTUD to acknowledge reception of
	a probe packet.		a probe packet.
	A PL that does not acknowledge data reception (e.g., UDP and UDP-		A PL that does not acknowledge data reception (e.g., UDP and UDP-
	Lite) is unable to detect when the packets that it sends are		Lite) is unable to detect when the packets that it sends are
	discarded because their size is greater than the actual PMTU. These		discarded because their size is greater than the actual PMTU. These
	PLs need to either rely on an application protocol to detect this		PLs need to either rely on an application protocol to detect this
	loss, or make use of an additional transport method such as UDP-		loss, or make use of an additional transport method such as UDP-
	Options [I-D.ietf-tsvwg-udp-options]. In addition, they might need		Options [I-D.ietf-tsvwg-udp-options]. In addition, they might need
	to send reachability probes (e.g., periodically solicit a response		to send reachability probes (e.g., periodically solicit a response
	from the destination) to determine whether the last successfully		from the destination) to determine whether the last successfully
	probed PLPMTU is still supported by the network path.		probed PLPMTU is still supported by the network path.
	Section Section 4 specifies this function for a set of IETF-specified		Section Section 4 specifies this function for a set of IETF-specified
	protocols.		protocols.
	3.3. Reducing the PLPMTU: Confirming Path Characteristics		3.3. Reducing the PLPMTU: Confirming Path Characteristics
	If the DPLPMTUD method detects that a packet with the PLPMTU size is		If the DPLPMTUD method detects that a packet with the PLPMTU size is
	no supported by the network path, then the DLPMTUD method needs to		no supported by the network path, then the DLPMTUD method needs to
	validate the PLPMTU. This can happen when a validated PTB message is		validate the PLPMTU. This can happen when a validated PTB message is
	received, or another event that indicates the network path no longer		received, or another event that indicates the network path no longer
	sustains this packet size, such as a loss report from the PL		sustains this packet size, such as a loss report from the PL
	All implementations of DPLPMTUD are REQUIRED to provide support that		All implementations of DPLPMTUD are REQUIRED to provide support that
	reduces the PLPMTU when the actual PMTU supported by a network path		reduces the PLPMTU when the actual PMTU supported by a network path
	is less than the PLPMTU.		is less than the PLPMTU.
	3.4. Increasing the PLPMTU: Supporting Path Changes		3.4. Increasing the PLPMTU: Supporting Path Changes
	An implementation that only reduces the PLPMTU to a suitable size is		An implementation that only reduces the PLPMTU to a suitable size is
	sufficient to ensure reliable operation, but may be very inefficient		sufficient to ensure reliable operation, but may be very inefficient
	when the actual PMTU changes or when the method (for whatever reason)		when the actual PMTU changes or when the method (for whatever reason)
	makes a suboptimal choice for the PLPMTU.		makes a suboptimal choice for the PLPMTU.
	A full implementation of the DPLPMTUD method is RECOMMENDED to		A full implementation of the DPLPMTUD method is RECOMMENDED to
	provide a way for the sending PL endpoint to detect when the PLPMTU		provide a way for the sending PL endpoint to detect when the PLPMTU
	is smaller than the actual PMTU size. This allows the sender to		is smaller than the actual PMTU size. This allows the sender to
	increase the PLPMTU following a change in the characteristics of the		increase the PLPMTU following a change in the characteristics of the
	path, such as when a link is reconfigured with a larger MTU, or when		path, such as when a link is reconfigured with a larger MTU, or when
	there is a change in the set of links traversed by an end-to-end flow		there is a change in the set of links traversed by an end-to-end flow
	(e.g. after a routing or fail-over decision).		(e.g. after a routing or fail-over decision).
	3.5. Robustness to inconsistent Path information		3.5. Robustness to inconsistent Path information
	The decision to increase the PLPMTU needs to be robust to the		The decision to increase the PLPMTU needs to be robust to the
	possibility that information learned about the path is inconsistent		possibility that information learned about the path is inconsistent
	(this could happen when probe packets are lost due to other reasons,		(this could happen when probe packets are lost due to other reasons,
	or some of the packets in a flow are forwarded along a portion of the		or some of the packets in a flow are forwarded along a portion of the
	path that supports a different PMTU).		path that supports a different PMTU).
	Frequent path changes could occur due to unexpected "flapping" -		Frequent path changes could occur due to unexpected "flapping" -
	where some packets from a flow pass along one path, but other packets		where some packets from a flow pass along one path, but other packets
	follow a different path with different properties. DPLPMTUD can be		follow a different path with different properties. DPLPMTUD can be
	made robust to these anomalies by introducing hysteresis into the		made robust to these anomalies by introducing hysteresis into the
	decision to increase the Maximum Packet Size.		decision to increase the Maximum Packet Size.
	XXX A future revision of this section will include recommend		XXX A future revision of this section will include recommend
	appropriate methods to provide robustness. XXX		appropriate methods to provide robustness. XXX
	4. Datagram Packetization Layer PMTUD		4. Datagram Packetization Layer PMTUD
	This section specifies Datagram PLPMTUD (DPLPMTUD). This method can		This section specifies Datagram PLPMTUD (DPLPMTUD). This method can
	be introduced at various points in the IP protocol stack, to discover		be introduced at various points in the IP protocol stack, to discover
	the PLPMTU so that the application can use an MPS appropriate to the		the PLPMTU so that the application can use an MPS appropriate to the
	current network path.		current network path.
	(preamble)		+----------------------+
			| APP* |
	+-----------+		+-+-------+----+---+---+
	| APP* |		| | | |
	+-----------+		+---+--+ +--+--+ | +-+---+
	__|| | | |___		| QUIC*| |UDPO*| | |SCTP*|
	___/ | | | \		+---+--+ +--+--+ | ++--+-+
	__/ | | | \__		| | | | |
	+------++-----+ | +------+ |		+-------++ | | |
	| QUIC*||UDPO*| | | SCTP*| |		| | | |
	+------++-----+ | +-+-----+ |		++-+--++ |
	+-----+ +------+		| UDP | |
	| UDP | | SCTP*|		+---+--+ |
	+-----+ +------+		| |
	| |		+--------------+-----+-+
	+----------------------+		| Network Interface |
	| Network Interface |		+----------------------+
	+----------------------+
	(postamble)		Figure 1: Examples where DPLPMTUD can be implemented
	The central idea of DPLPMTUD is probing by a sender. Probe packets		The central idea of DPLPMTUD is probing by a sender. Probe packets
	of increasing size are sent to find out the maximum size of user		are sent to find out the maximum size of user message that is
	message that is completely transferred across the network path from		completely transferred across the network path from the sender to the
	the sender to the destination.		destination.
			The are various functions performed by the algorithm:
	4.1. PROBE_SEARCH: Probing for a larger PLPMTU		4.1. PROBE_SEARCH: Probing for a larger PLPMTU
	The DPLPMTUD method utilises probe packets to confirm that a packet		The DPLPMTUD method utilises probe packets to confirm that a packet
	of size PROBE_SIZE can travere the network path. The PROBE_COUNT is		of size PROBED_SIZE can traverse the network path. The PROBE_COUNT
	initialised to zero when a probe packet is first sent with a		is initialised to zero when a probe packet is first sent with a
	particular size.		particular size.
	A timer is used to trigger the generation of probe packets. The		A timer is used to trigger the generation of probe packets. The
	probe_timer is started each time a probe packet is sent to the		probe_timer is started each time a probe packet is sent to the
	destination and is cancelled when receipt of the probe packet is		destination and is cancelled when receipt of the probe packet is
	acknowledged. THE PROBE_SIZE is confirmed, and this value is then		acknowledged. The PROBED_SIZE is confirmed, and this value is then
	assignmed to PLPMTU. The DPLPMTUD method may send subsequent probes		assignmed to PLPMTU. The DPLPMTUD method may send subsequent probes
	of an increasing size. Increasing probes follows a search strategy		of an increasing size. Increasing probes follow a search strategy as
	as discussed in Section 4.7.		discussed in Section 4.7.
	Each time the probe_timer expires, the PROBE_COUNT is incremented,		Each time the probe_timer expires, the PROBE_COUNT is incremented,
	teh probe_timer is reinitialised, and a probe packet of the same size		the probe_timer is reinitialised, and a probe packet of the same size
	is retransmitted.		is retransmitted.
	The maximum number of retransmissions for a PROBE_SIZE is configured		The maximum number of retransmissions for a PROBED_SIZE is configured
	(MAX_PROBES). If the value of the PROBE_COUNT reaches MAX_PROBES,		(MAX_PROBES). If the value of the PROBE_COUNT reaches MAX_PROBES,
	probing will stop.		probing will stop and enters the PROBE_DONE state.
	4.2. The PROBE_DONE state		4.2. The PROBE_DONE state
	When the PL sender complete probing for a larger PLPMTU, it enters		When the PL sender completes probing for a larger PLPMTU, it enters
	the PROBE_DONE state. This starts the PMTU_RAISE_TIMER. While this		the PROBE_DONE state. This starts the PMTU_RAISE_TIMER. While this
	running, the PLPMTU remains at the value set in the last succesful		running, the PLPMTU remains at the value set in the last succesful
	probe packet.		probe packet.
	If the PL is designed in a way that is unable to verify reachability		If the PL is designed in a way that is unable to validate
	to the destination endpoint after probing has completed, the method		reachability to the destination endpoint after probing has completed,
	uses a REACHABILITY_TIMER to periodically repeat a probe packet for		the method uses a REACHABILITY_TIMER to periodically repeat a probe
	the current PLPMTU size, while the PMTU_RAISE_TIMER is running. If		packet for the current PLPMTU size, while the PMTU_RAISE_TIMER is
	the REACHABILITY_TIMER expires, the method exits the PROBE_DONE		running. If the REACHABILITY_TIMER expires, the method exits the
	state. The done state is also exited when a verified PTB message is		PROBE_DONE state. The done state is also exited when a validated PTB
	received.		message is received.
	If the PMTU_RAISE_TIMER expires, the PL sender also exits the		If the PMTU_RAISE_TIMER expires, the PL sender also exits the
	PROBE_DONE state, but in this case resumes probing from the size of		PROBE_DONE state, but in this case resumes probing from the size of
	the PLPMTU.		the PLPMTU.
	4.3. Verification and Use of PTB Messages		4.3. Validation and Use of PTB Messages
	This section describes processing for both IPv4 ICMP Unreachable		This section describes processing for both IPv4 ICMP Unreachable
	messages (type 3) and ICMPv6 packet too big messages.		messages (type 3) and ICMPv6 packet too big messages.
	A node that receives a PTB message from a router or middlebox, MUST		A PL that receives a PTB message from a router or middlebox, MUST
	verify the PTB message. The node checks the protocol information in		validate the PTB message. The PL checks the protocol information in
	the quoted payload to verify that the message originated from the		the quoted payload to validate the message originated from the
	sending node. The node also checks that the reported MTU size is		sending node. The node also checks that the reported link MTU size
	less than the size used by packet probes. PTB messages are discarded		is less than the size used by packet probes. PTB messages are
	if they fail to pass these checks, or where there is insufficient		discarded if they fail to pass these checks, or where there is
	ICMP payload to perform these checks. The checks are intended to		insufficient ICMP payload to perform these checks. The checks are
	provide protection from packets that originate from a node that is		intended to provide protection from packets that originate from a
	not on the network path or a node that attempts to report a larger		node that is not on the network path or a node that attempts to
	MTU than the current probe size.		report a larger link MTU than the current probe size.
	PTB messages that have been verified can be utilised by the DPLPMTUD		PTB messages that have been validated can be utilised by the DPLPMTUD
	algorithm. A method that utilises these PTB messages can improve		algorithm. A method that utilises these PTB messages can improve the
	performance compared to one that relies solely on probing.		speed at the which the algorithm detects an appropriate PLPMTU
			compared to one that relies solely on probing.
	4.4. Timers		4.4. Timers
	The method in the previous subsections utilises three timers:		The method in the previous subsections utilises three timers:
	PROBE_TIMER: Configured to expire after a period longer than the		PROBE_TIMER: Configured to expire after a period longer than the
	maximum time to receive an acknowledgment to a probe packet. This		maximum time to receive an acknowledgment to a probe packet. This
	value MUST be larger than 1 second, and SHOULD be larger than 15		value MUST be larger than 1 second, and SHOULD be larger than 15
	seconds. Guidance on selection of the timer value are provide in		seconds. Guidance on selection of the timer value are provide in
	section 3.1.1 of the UDP Usage Guidelines [RFC8085].		section 3.1.1 of the UDP Usage Guidelines [RFC8085].
	If the PL has an RTT estimate and timely acknowedgements the		If the PL has an RTT estimate and timely acknowedgements the
	PROBE_TIMER can be derrived from the PL RTT estimate.		PROBE_TIMER can be derrived from the PL RTT estimate.
	PMTU_RAISE_TIMER: Configured to the period a sender ought to continue		PMTU_RAISE_TIMER: Configured to the period a sender ought to
	use the current PLPMTU, after which it re-commences probing for a		continue use the current PLPMTU, after which it re-commences
	higher PMTU. This timer has a period of 600 secs, as recommended		probing for a higher PMTU. This timer has a period of 600 secs,
	by DPLPMTUD [RFC4821].		as recommended by DPLPMTUD [RFC4821].
	REACHABILITY_TIMER: Configured to the period a sender ought to wait		REACHABILITY_TIMER: Configured to the period a sender ought to wait
	before confirming the current PLPMTU is still supported. This is		before confirming the current PLPMTU is still supported. This is
	less than the PMTU_RAISE_TIMER and used to decrease the PLPMTU		less than the PMTU_RAISE_TIMER and used to decrease the PLPMTU
	(e.g. when a black hole is encountered).		(e.g. when a black hole is encountered).
	DPLPMTUD ought to suspend reachability probes when no application		DPLPMTUD ought to suspend reachability probes when no application
	data has been sent since the previous probe packet. Guidance on		data has been sent since the previous probe packet. Guidance on
	selection of the timer value are provide in section 3.1.1 of the		selection of the timer value are provide in section 3.1.1 of the
	UDP Usage Guidelines[RFC8085]. DPLPMTUD ought to be suspended or		UDP Usage Guidelines[RFC8085]. DPLPMTUD ought to be suspended or
	only sent in conjuction with out traffic during periods of		only sent in conjuction with out traffic during periods of
	dormancy. This verification needs to be frequent enough when data		dormancy. This PLPMTU validation needs to be frequent enough when
	is flowing that you do not black hole extensive amounts of traffic		data is flowing that the sending PL does not black hole extensive
			amounts of traffic
	An implementation could implement the various timers using a single		An implementation could implement the various timers using a single
	timer process.		timer process.
	4.5. Constants		4.5. Constants
	The following constants are defined:		The following constants are defined:
	MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER. The default		MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER. The
	value of MAX_PROBES is 10.		default value of MAX_PROBES is 10.
	MIN_PMTU: The smallest allowed probe packet size. For IPv6, this		MIN_PMTU: The smallest allowed probe packet size. For IPv6, this
	value is 1280 bytes, as specified in [RFC2460]. For IPv4, the		value is 1280 bytes, as specified in [RFC2460]. For IPv4, the
	minimum value is 68 bytes. (An IPv4 routed is required to be able		minimum value is 68 bytes. (An IPv4 routed is required to be able
	to forward a datagram of 68 octets without further fragmentation.		to forward a datagram of 68 octets without further fragmentation.
	This is the combined size of an IPv4 header and the minimum		This is the combined size of an IPv4 header and the minimum
	fragment size of 8 octets.)		fragment size of 8 octets.)
	BASE_PMTU: The BASE_PMTU is a considered a size that ought to work in		BASE_PMTU: The BASE_PMTU is a considered a size that ought to work
	most cases. The size is equal to or larger than the minimum		in most cases. The size is equal to or larger than the minimum
	permitted and smaller than the maximum allowed. In the case of		permitted and smaller than the maximum allowed. In the case of
	IPv6, this value is 1280 bytes [RFC2460]. When using IPv4, a size		IPv6, this value is 1280 bytes [RFC2460]. When using IPv4, a size
	of 1200 bytes is RECOMMENDED.		of 1200 bytes is RECOMMENDED.
	MAX_PMTU: The MAX_PMTU is the largest size of PLPMTU that is probed.		MAX_PMTU: The MAX_PMTU is the largest size of PLPMTU that is probed.
	This has to be less than or equal to the minimum of the local MTU		This has to be less than or equal to the minimum of the local MTU
	of the outgoing interface and the destination PLMTU for receiving.		of the outgoing interface and the destination PLMTU for receiving.
	An application or PL may reduce this when it knows there is no		An application or PL may reduce this when it knows there is no
	need to send packets above a specific size.		need to send packets above a specific size.
			The figure below illustrates the relationship between some of these
			variables, in this case when the DPLPMTUD algorithm performs path
			probing to increase the size of the PLPMTU. The MPS is less than the
			PLPMTU. A probe packet has been sent of size PROBED_SIZE. When this
			is acknowledged, the PLPMTU will be raised to PROBED_SIZE allowing
			the PROBED_SIZE to be increased towards the actual PMTU.
			MIN_PMTU PMTU_MAX
			<------------------------------------------------------>
			| | | | |
			V | | | V
			BASE_PMTU V | V Actual PMTU
			MPS | PROBED_SIZE
			V
			PLPMTU
			Figure 2: Relationships between probe and packet sizes
	4.6. Variables		4.6. Variables
	This method utilises a set of variables:		This method utilises a set of variables:
	PROBE_TIMER: Configured to expire after a period longer than the		PROBE_TIMER: Configured to expire after a period longer than the
	maximum time to receive an acknowledgment to a probe packet. This		maximum time to receive an acknowledgment to a probe packet. This
	value MUST be larger than 1 second, and SHOULD be larger than 15		value MUST be larger than 1 second, and SHOULD be larger than 15
	seconds. Guidance on selection of the timer value are provide in		seconds. Guidance on selection of the timer value are provide in
	section 3.1.1 of the UDP Usage Guidelines [RFC8085].		section 3.1.1 of the UDP Usage Guidelines [RFC8085].
	PL with RTT estimates may use values smaller than 1 seconded		PL with RTT estimates may use values smaller than 1 seconded
	derrived from their RTT estimate to speed up detection of		derrived from their RTT estimate to speed up detection of
	connectivity issues on the path.		connectivity issues on the path.
	PROBED_SIZE: The PROBED_SIZE is the size of the current probe packet.		PROBED_SIZE: The PROBED_SIZE is the size of the current probe
	This is a tentative value for the PLPMTU, which is awaiting		packet. This is a tentative value for the PLPMTU, which is
	confirmation by an acknowledgment.		awaiting confirmation by an acknowledgment.
	PROBE_COUNT: This is a count of the number of unsuccessful probe		PROBE_COUNT: This is a count of the number of unsuccessful probe
	packets that have been sent with size PROBED_SIZE. The value is		packets that have been sent with size PROBED_SIZE. The value is
	initialised to zero when a particular size of PROBED_SIZE is first		initialised to zero when a particular size of PROBED_SIZE is first
	attempted.		attempted.
	PTB_SIZE: The PTB_Size is value returned by a verified PTB message		PTB_SIZE: The PTB_Size is value returned by a validated PTB message
	indicating the local MTU size of a router along the path.		indicating the local MTU size of a router along the path.
	4.7. Selecting PROBED_SIZE		4.7. Selecting PROBED_SIZE
	Implementations discover the search range by validating the minimum		Implementations discover the search range by validating the minimum
	path MTU and then using the probe method to select a PROBED_SIZE less		path MTU and then using the probe method to select a PROBED_SIZE less
	than or equal to the maximum PMTU_MAX. Where PMTU_MAX is the minimum		than or equal to the maximum PMTU_MAX. Where PMTU_MAX is the minimum
	of the local link MTU and EMTU_R (learned from the remote endpoint).		of the local link MTU and EMTU_R (learned from the remote endpoint).
	The PMTU_MAX MAY be constrained by an application that has a maximum		The PMTU_MAX MAY be constrained by an application that has a maximum
	to the size of datagrams it wishes to send.		to the size of datagrams it wishes to send.
	Implementations use a search algorithm to choose probe sizes within		Implementations use a search algorithm to choose probe sizes within
	the search range.		the search range.
	xxx A future version of this section will detail example methods for		xxx A future version of this section will detail example methods for
	selecting probe size values, but does not plan to mandate a single		selecting probe size values, but does not plan to mandate a single
	method. xxx		method. xxx
	Implementations MAY optimizse the search procedure by selecting step		Implementations MAY optimizse the search procedure by selecting step
	sizes from a table of common PMTU sizes.		sizes from a table of common PMTU sizes.
	Implementations SHOULD select probe sizes to maximise the gain in		Implementations SHOULD select probe sizes to maximise the gain in
	PLPMTU each search step. Implementations ought to take into		PLPMTU each search step. Implementations ought to take into
	consideration useful probe size steps and a minimum useful gain in		consideration useful probe size steps and a minimum useful gain in
	PLPMTU.		PLPMTU.
	4.8. Black Hole Detection		4.8. Simple Black Hole Detection
	The DPLPMTUD method can be used to detect paths that fail to support		The DPLPMTUD method can be used to provide black hole detection.
	a packet size, but return no PTB message. The black hole detection		This enables a reduction of the PLPMTU when a PL sender encounters a
	function detects such cases and responds by reducing the PLPMTU,		path that fails to support the current MPS and also fails to return a
	allowing the endpoint to inform the application of the reduced MPS		PTB message to the sender.
	and accordingly send smaller packets. Black Hole detection is
	triggered by the reachability function.
	4.9. State Machine		The simple method starts by setting the PLPMTU to the BASE_PMTU.
			When the method detects that communication is not possible with this
			size of packet, the PLPMTU is reduced, until an operable message size
			is reached or the PLPMTU reaches the BASE_MTU size. The method
			enables a sending PL to inform an application of the reduced MPS and
			accordingly send smaller packets.
	A state machine for DPLPMTUD is depicted in Figure 2. If multihoming		The simple black hole detetction method does not seek to increase the
	is supported, a state machine is needed for each active path.		PLPMTU. This makes it vulneable to transient reductions in the
			actual PLPMTU, which could result in a PLPMTU lower than the actual
			PMTU.
	PROBE_TIMER expiry		The full methiod is specified in Section 4.9.
	(PROBE_COUNT = MAX_PROBES)
	+-------------+ +--------------+		4.8.1. Simple Black Hole Detection State Machine
	=->| PROBE_START |--------------->|PROBE_DISABLED|
	PROBE_TIMER expiry | +-------------+ +--------------+		The PL sender starts with the PLPMTU and PROBED_SIZE set to the
	(PROBE_COUNT = | | |		BASE_PMTU.
	MAX_PROBES) ------- | Connectivity confirmed
	v		While a PL has a PLPMTU greater than the BASE_MTU, the PL needs to
	----------- +------------+ -- PROBE_TIMER expiry		send probe packets at the PROBED_SIZE to revalidate the PLPMTU.
	MAX_PMTU acked or | | PROBE_BASE | | (PROBE_COUNT <		Black hole detection is also triggered by lack of reachability at the
	PTB (>= BASE_PMTU)| -----> +------------+ <- MAX_PROBES)		PL. When the PL sender detects that multiple transmissions of
	---------------- | /\ | |		packets of PROBED_SIZE are no longer being acknowledged (e.g., When
	| | | | | PTB		the number of probe packets sent without receiving an acknowledgement
	| PMTU_RAISE_TIMER| | | | (PTB_SIZE < BASE_PMTU)		(PROBE_COUNT) becomes greater than the MAX_PROBES), the PL concludes
	| or reachability | | | | or		that it has detected a black hole and reduces PLPMTU.
	| (PROBE_COUNT | | | | PROBE_TIMER expiry
	| = MAX_PROBES) | | | | (PROBE_COUNT = MAX_PROBES)		The connectivity check may be performened by the protocol
	| ------------- | | \		implementing the PL (as in PLPMTUD for TCP [RFC4821]). When the
	| | PTB | | \		application using the PL does not regularly send packets of size
	| | (< PROBED_SIZE)| | \		PROBED_SIZE, additional probe packets need to be sent by PL using the
	| | | | ----------------		reachability timer Section 4.4.
	| | | | |
	| | | | Probe |		If method does reduces the PLPMTU to the MIN_PMTU, the method
	| | | | acked |		concludes the path does not support the MIN_PMTU.
	v | | v v
	+------------+ +--------------+ Probe +-------------+		If multihoming is supported, a state machine is needed for each
	| PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR |		active path.
	+------------+ MAX_PMTU acked +--------------+ acked +-------------+
	/\ | or /\ |		The state machine for a simple black hole detection mechanism is
	| | PROBE_TIMER expiry | |		depicted in Figure 3.
	| |(PROBE_COUNT = MAX_PROBES) | |
	| | | |		XXX a future version of the simple black hole detection state machine
	------ --------		might consider icmp PTB messages XXX
	Reachability probe acked PROBE_TIMER expiry		+------------+
	or PROBE_TIMER expiry (PROBE_COUNT < MAX_PROBES)		| PROBE_START|
	(PROBE_COUNT < MAX_PROBES) or		+-----+------+
	Probe acked		| Connectivity confirmed
			| (reachability tests start)
			PROBE_COUNT >= V
			MAX_PROBES +------------+
			+---------------| PROBE_BASE +->-+
			| +-----+------+ |
			| | ^ | PROBE_COUNT < MAX_PROBES
			| | +-----+
			| V
			| | PROBE_ACK
			| PROBE_COUNT |
			| = MAX_PROBES +------------+
			| (reduce +-<-+ PROBE_DONE +->-+
			| PLPMTU) | +------+-----+ |
			| | ^ | ^ | PROBE_COUNT < MAX_PROBES
			| | | | | | (Contine probing)
			| +-----+ | +-----+
			V V
			+------------+ |
			| PROBE_ERROR|<------------+
			+------------+
			Figure 3: State machine for detecting black holes
			4.9. Full State Machine
			A full state machine for DPLPMTUD is depicted in Figure 4. If
			multihoming is supported, a state machine is needed for each active
			path.
			PROBE_TIMER expiry
			(PROBE_COUNT = MAX_PROBES)
			+-------------+ +--------------+
			+->| PROBE_START +--------------->|PROBE_DISABLED|
			PROBE_TIMER expiry | +--+-------+--+ +--------------+
			(PROBE_COUNT = | | |
			MAX_PROBES) +-----+ | Connectivity confirmed
			v
			+---------- +------------+ -+ PROBE_TIMER expiry
			MAX_PMTU acked or | | PROBE_BASE | | (PROBE_COUNT <
			PTB (>= BASE_PMTU)| +----> +--------+---+ <+ MAX_PROBES)
			+---------------+ | /\ | |
			| | | | | PTB
			| PMTU_RAISE_TIMER| | | | (PTB_SIZE < BASE_PMTU)
			| or reachability | | | | or
			| (PROBE_COUNT | | | | PROBE_TIMER expiry
			| = MAX_PROBES) | | | | (PROBE_COUNT = MAX_PROBES)
			| +-----------+ | | \
			| | PTB | | \
			| | (< PROBED_SIZE)| | \
			| | | | ---------------+
			| | | | |
			| | | | Probe |
			| | | | acked |
			v | | v v
			+----------+-+ +----+---------+ Probe +-------------+
			| PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR |
			+------+-----+ MAX_PMTU acked +------------+-+ acked +-------------+
			/\ | or /\ |
			| | PROBE_TIMER expiry | |
			| |(PROBE_COUNT = MAX_PROBES) | |
			| | | |
			+----+ +------+
			Reachability probe acked PROBE_TIMER expiry
			or PROBE_TIMER expiry (PROBE_COUNT < MAX_PROBES)
			(PROBE_COUNT < MAX_PROBES) or
			Probe acked
			Figure 4: State machine for Datagram PLPMTUD
	XXX A future version of this document will update the state machine		XXX A future version of this document will update the state machine
	to describe handling of validated PTB messages. XXX		to describe handling of validated PTB messages. XXX
	The following states are defined to reflect the probing process:		The following states are defined to reflect the probing process:
	PROBE_START: The PROBE_START state is the initial state before		PROBE_START: The PROBE_START state is the initial state before
	probing has started. PLPMTUD is not performed in this state. The		probing has started. PLPMTUD is not performed in this state. The
	state transitions to PROBE_BASE, when a path has been confirmed,		state transitions to PROBE_BASE, when a path has been confirmed,
	i.e. when a sent packet has been acknowledged on this path. Any		i.e. when a sent packet has been acknowledged on this path. Any
	transport method may be used to exit PROBE_BASE as long as the		transport method may be used to exit PROBE_BASE as long as the
	send packet is acknowledge by the other side. The PLPMTU is set		send packet is acknowledge by the other side. The PLPMTU is set
	to the BASE_PMTU size. Probing ought to start immediately after		to the BASE_PMTU size. Probing ought to start immediately after
	connection setup to prevent the prevent the loss of user data.		connection setup to prevent the prevent the loss of user data.
	PROBE_BASE: The PROBE_BASE state is the starting point for probing		PROBE_BASE: The PROBE_BASE state is the starting point for probing
	with datagram PLPMTUD. It is used to confirm whether the BASE_PMTU		with datagram PLPMTUD. It is used to confirm whether the
	size is supported by the network path. On entry, the PROBED_SIZE		BASE_PMTU size is supported by the network path. On entry, the
	is set to the BASE_PMTU size and the PROBE_COUNT is set to zero.		PROBED_SIZE is set to the BASE_PMTU size and the PROBE_COUNT is
	A probe packet is sent, and the PROBE_TIMER is started. The state		set to zero. A probe packet is sent, and the PROBE_TIMER is
	is left when the PROBE_COUNT reaches MAX_PROBES; a PTB message is		started. The state is left when the PROBE_COUNT reaches
	verified, or a probe packet is acknowledged.		MAX_PROBES; a PTB message is validated, or a probe packet is
			acknowledged.
	PROBE_SEARCH: The PROBE_SEARCH state is the main probing state. This		PROBE_SEARCH: The PROBE_SEARCH state is the main probing state.
	state is entered either when probing for the BASE_PMTU was		This state is entered either when probing for the BASE_PMTU was
	successful or when there is a successful reachability test in the		successful or when there is a successful reachability test in the
	PROBE_ERROR state. On entry, the PLPMTU is set to the last		PROBE_ERROR state. On entry, the PLPMTU is set to the last
	acknowledged PROBED_SIZE.		acknowledged PROBED_SIZE.
	The PROBE_COUNT is set to zero when the first probe packet is sent		The PROBE_COUNT is set to zero when the first probe packet is sent
	for each probe size. Each time a probe packet is acknowledged,		for each probe size. Each time a probe packet is acknowledged,
	the PLPMTU is set to the PROBED_SIZE, and then the PROBED_SIZE is		the PLPMTU is set to the PROBED_SIZE, and then the PROBED_SIZE is
	increased.		increased.
	When a probe packet is sent and not acknowledged within the period		When a probe packet is sent and not acknowledged within the period
	of the PROBE_TIMER, the PROBE_COUNT is incremented and the probe		of the PROBE_TIMER, the PROBE_COUNT is incremented and the probe
	packet is retransmitted. The state is exited when the PROBE_COUNT		packet is retransmitted. The state is exited when the PROBE_COUNT
	reaches MAX_PROBES; a PTB message is verified; or a probe of size		reaches MAX_PROBES; a PTB message is validated; or a probe of size
	PMTU_MAX is acknowledged.		PMTU_MAX is acknowledged.
	PROBE_ERROR: The PROBE_ERROR state represents the case where the		PROBE_ERROR: The PROBE_ERROR state represents the case where the
	network path is not known to support an PLPMTU of at least the		network path is not known to support an PLPMTU of at least the
	BASE_PMTU size. It is entered when either a probe of size		BASE_PMTU size. It is entered when either a probe of size
	BASE_PMTU has not been acknowledged or a verified PTB message		BASE_PMTU has not been acknowledged or a validated PTB message
	indicates a smaller link MTU than the BASE_PMTU. On entry, the		indicates a smaller link MTU than the BASE_PMTU. On entry, the
	PROBE_COUNT is set to zero and the PROBED_SIZE is set to the		PROBE_COUNT is set to zero and the PROBED_SIZE is set to the
	MIN_PMTU size, and the PLPMTU is reset to MIN_PMTU size. In this		MIN_PMTU size, and the PLPMTU is reset to MIN_PMTU size. In this
	state, a probe packet is sent, and the PROBE_TIMER is started.		state, a probe packet is sent, and the PROBE_TIMER is started.
	The state transitions to the PROBE_SEARCH state when a probe		The state transitions to the PROBE_SEARCH state when a probe
	packet is acknowledged.		packet is acknowledged.
	PROBE_DONE: The PROBE_DONE state indicates a successful end to a		PROBE_DONE: The PROBE_DONE state indicates a successful end to a
	probing phase. DPLPMTUD remains in this state until either the		probing phase. DPLPMTUD remains in this state until either the
	PMTU_RAISE_TIMER expires or a received PTB message is verified.		PMTU_RAISE_TIMER expires or a received PTB message is validated.
	When PLPMTUD uses an unacknowledged PL and is in the PROBE_DONE		When PLPMTUD uses an unacknowledged PL and is in the PROBE_DONE
	state, a REACHABILITY_TIMER periodically resets the PROBE_COUNT		state, a REACHABILITY_TIMER periodically resets the PROBE_COUNT
	and schedules a probe packet with the size of the PLPMTU. If the		and schedules a probe packet with the size of the PLPMTU. If the
	probe packet fails to be acknowledged after MAX_PROBES attempts,		probe packet fails to be acknowledged after MAX_PROBES attempts,
	the method enters the PROBE_BASE state. When used with an		the method enters the PROBE_BASE state. When used with an
	acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to		acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to
	probe in this state.		probe in this state.
	PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity		PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity
	could not be established. DPLPMTUD MUST NOT probe in this state.		could not be established. DPLPMTUD MUST NOT probe in this state.
	Appendix Appendix A contains an informative description of key		Appendix A contains an informative description of key events.
	events.
	5. Specification of Protocol-Specific Methods		5. Specification of Protocol-Specific Methods
	This section specifies protocol-specific details for datagram PLPMTUD		This section specifies protocol-specific details for datagram PLPMTUD
	for IETF-specified transports.		for IETF-specified transports.
	The first subsection provides guidance on how to implement the		The first subsection provides guidance on how to implement the
	DPLPMTUD method as a part of an application using UDP or UDP-Lite.		DPLPMTUD method as a part of an application using UDP or UDP-Lite.
	The guidance also applies to other datagram services that do not		The guidance also applies to other datagram services that do not
	include a specific transport protocol (such as a tunnel		include a specific transport protocol (such as a tunnel
	encapsulation). The following subsection describe how DPLPMTUD can be		encapsulation). The following subsection describe how DPLPMTUD can
	implemented as a part of the transport service, allowing applications		be implemented as a part of the transport service, allowing
	using the service to benefit from discovery of the PLPMTU without		applications using the service to benefit from discovery of the
	themselves needing to implement this method.		PLPMTU without themselves needing to implement this method.
	5.1. Application support for DPLPMTUD with UDP or UDP-Lite		5.1. Application support for DPLPMTUD with UDP or UDP-Lite
	The current specifications of UDP [RFC0768] and UDP-Lite [RFC3828] do		The current specifications of UDP [RFC0768] and UDP-Lite [RFC3828] do
	not define a method in the RFC-series that supports PLPMTUD. In		not define a method in the RFC-series that supports PLPMTUD. In
	particular, the UDP transport does not provide the transport layer		particular, the UDP transport does not provide the transport layer
	features needed to implement datagram PLPMTUD.		features needed to implement datagram PLPMTUD.
	The DPLPMTUD method can be implemented as a part of an application		The DPLPMTUD method can be implemented as a part of an application
	built directly or indirectly on UDP or UDP-Lite, but relies on		built directly or indirectly on UDP or UDP-Lite, but relies on
	higher-layer protocol features to implement the method [RFC8085].		higher-layer protocol features to implement the method [RFC8085].
	Some primitives used by DPLPMTUD might not be available via the		Some primitives used by DPLPMTUD might not be available via the
	Datagram API (e.g., the ability to access the PLPMTU cache, or		Datagram API (e.g., the ability to access the PLPMTU cache, or
	interpret received ICMP PTB messages).		interpret received ICMP PTB messages).
	skipping to change at page 21, line 28 ¶		skipping to change at page 24, line 41 ¶
	5.1.4. Validating the Path		5.1.4. Validating the Path
	An application that does not have other higher-layer information		An application that does not have other higher-layer information
	confirming correct delivery of datagrams SHOULD implement the		confirming correct delivery of datagrams SHOULD implement the
	REACHABILITY_TIMER to periodically send probe packets while in the		REACHABILITY_TIMER to periodically send probe packets while in the
	PROBE_DONE state.		PROBE_DONE state.
	5.1.5. Handling of PTB Messages		5.1.5. Handling of PTB Messages
	An application that is able and wishes to receive PTB messages MUST		An application that is able and wishes to receive PTB messages MUST
	perform ICMP verification as specified in Section 5.2 of [RFC8085].		perform ICMP validation as specified in Section 5.2 of [RFC8085].
	This requires that the application verifies each received PTB		This requires that the application to check each received PTB
	messages to verify these are received in response to transmitted		messages to validate it is received in response to transmitted
	traffic and that the reported link MTU is less than the current probe		traffic and that the reported link MTU is less than the current probe
	size. A verified PTB message MAY be used as input to the DPLPMTUD		size. A validated PTB message MAY be used as input to the DPLPMTUD
	algorithm, but MUST NOT be used directly to set the PLPMTU.		algorithm, but MUST NOT be used directly to set the PLPMTU.
	5.2. DPLPMTUD with UDP Options		5.2. DPLPMTUD with UDP Options
	UDP-Options [I-D.ietf-tsvwg-udp-options] can supply the additional		UDP-Options [I-D.ietf-tsvwg-udp-options] can supply the additional
	functionality required to implement DPLPMTUD within the UDP transport		functionality required to implement DPLPMTUD within the UDP transport
	service. This avoids the need for applications to implement the		service. This avoids the need for applications to implement the
	DPLPMTUD method.		DPLPMTUD method.
	This enables padding to be added to UDP datagrams and can be used to		This enables padding to be added to UDP datagrams and can be used to
	provide feedback acknowledgement of received probe packets.		provide feedback acknowledgement of received probe packets.
	The specification also defines two UDP Options to support DPLMTUD.		The specification also defines two UDP Options to support DPLMTUD.
	Section 5.6 of [I-D.ietf-tsvwg-udp-options] defines the MSS option		Section 5.6 of [I-D.ietf-tsvwg-udp-options] defines the MSS option
	which allows the local sender to indicate the EMTU_R to the peer.		which allows the local sender to indicate the EMTU_R to the peer.
	This option can be used to initialise PMTU_MAX. An application		This option can be used to initialise PMTU_MAX. An application
	wishing to avoid the effects of MSS-Clamping (where a middlebox		wishing to avoid the effects of MSS-Clamping (where a middlebox
	changes the advertised TCP maximum sending size) ought to use a		changes the advertised TCP maximum sending size) ought to use a
	cryptographic method to encrypt this parameter.		cryptographic method to encrypt this parameter.
	5.2.1. UDP Request Option		5.2.1. UDP Request Option
	The Request Option allows a sending endpoint to solicit a response		The Request Option allows a sending endpoint to solicit a response
	from a destination endpoint.		from a destination endpoint.
	The Request Option carries a four byte token set by the sender. This		The Request Option carries a four byte token set by the sender. This
	token can be set to a value that is likely to be known only to the		token can be set to a value that is likely to be known only to the
	sender (and becomes known to nodes along the end-to-end path). The		sender (and becomes known to nodes along the end-to-end path). The
	sender can then check the value returned in the response to provide		sender can then check the value returned in the response to provide
	additional protection from off-path insertion of data [RFC8085].		additional protection from off-path insertion of data [RFC8085].
	+---------+--------+-----------------+		+---------+--------+-----------------+
	| Kind=9 | Len=6 | Token |		| Kind=9 | Len=6 | Token |
	+---------+--------+-----------------+		+---------+--------+-----------------+
	1 byte 1 byte 4 bytes		1 byte 1 byte 4 bytes
			Figure 5: UDP REQ Option Format
	5.2.2. UDP Response Option		5.2.2. UDP Response Option
	The Response Option is generated by the PL in response to reception		The Response Option is generated by the PL in response to reception
	of a previously received Echo Request. The Token field associates		of a previously received Echo Request. The Token field associates
	the response with the Token value carried in the most recently-		the response with the Token value carried in the most recently-
	received Echo Request. The rate of generation of UDP packets		received Echo Request. The rate of generation of UDP packets
	carrying a Response Option MAY be rate-limited.		carrying a Response Option MAY be rate-limited.
	+---------+--------+-----------------+		+---------+--------+-----------------+
	| Kind=10 | Len=6 | Token |		| Kind=10 | Len=6 | Token |
	+---------+--------+-----------------+		+---------+--------+-----------------+
	1 byte 1 byte 4 bytes		1 byte 1 byte 4 bytes
			Figure 6: UDP RES Option Format
	5.3. DPLPMTUD for SCTP		5.3. DPLPMTUD for SCTP
	Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing		Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing
	method for SCTP. It recommends the use of the PAD chunk, defined in		method for SCTP. It recommends the use of the PAD chunk, defined in
	[RFC4820] to be attached to a minimum length HEARTBEAT chunk to build		[RFC4820] to be attached to a minimum length HEARTBEAT chunk to build
	a probe packet. This enables probing without affecting the transfer		a probe packet. This enables probing without affecting the transfer
	of user messages and without interfering with congestion control.		of user messages and without interfering with congestion control.
	This is preferred to using DATA chunks (with padding as required) as		This is preferred to using DATA chunks (with padding as required) as
	path probes.		path probes.
	XXX Future versions of this document might define a parameter		XXX Future versions of this document might define a parameter
	contained in the INIT and INIT ACK chunk to indicate the remote peer		contained in the INIT and INIT ACK chunk to indicate the remote peer
	MTU to the local peer. However, multihoming makes this a bit		MTU to the local peer. However, multihoming makes this a bit
	complex, so it might not be worth doing. XXX		complex, so it might not be worth doing. XXX
	5.3.1. SCTP/IP4 and SCTP/IPv6		5.3.1. SCTP/IP4 and SCTP/IPv6
	The base protocol is specified in [RFC4960]. This provides an		The base protocol is specified in [RFC4960]. This provides an
	acknowledged PL. A sender can therefore enter the PROBE_BASE state as		acknowledged PL. A sender can therefore enter the PROBE_BASE state
	soon as connectivity has been confirmed.		as soon as connectivity has been confirmed.
	5.3.1.1. Sending SCTP Probe Packets		5.3.1.1. Sending SCTP Probe Packets
	Probe packets consist of an SCTP common header followed by a		Probe packets consist of an SCTP common header followed by a
	HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control		HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
	the length of the probe packet. The HEARTBEAT chunk is used to		the length of the probe packet. The HEARTBEAT chunk is used to
	trigger the sending of a HEARTBEAT ACK chunk. The reception of the		trigger the sending of a HEARTBEAT ACK chunk. The reception of the
	HEARTBEAT ACK chunk acknowledges reception of a successful probe.		HEARTBEAT ACK chunk acknowledges reception of a successful probe.
	The HEARTBEAT chunk carries a Heartbeat Information parameter which		The HEARTBEAT chunk carries a Heartbeat Information parameter which
	should include, besides the information suggested in [RFC4960], the		should include, besides the information suggested in [RFC4960], the
	probe size, which is the size of the complete datagram. The size of		probe size, which is the size of the complete datagram. The size of
	the PAD chunk is therefore computed by reducing the probing size by		the PAD chunk is therefore computed by reducing the probing size by
	skipping to change at page 23, line 32 ¶		skipping to change at page 27, line 12 ¶
	number of PMTU sizes probed. The Heartbeat timer can be used to		number of PMTU sizes probed. The Heartbeat timer can be used to
	implement the PROBE_TIMER.		implement the PROBE_TIMER.
	5.3.1.2. Validating the Path with SCTP		5.3.1.2. Validating the Path with SCTP
	Since SCTP provides an acknowledged PL, a sender does MUST NOT		Since SCTP provides an acknowledged PL, a sender does MUST NOT
	implement the REACHABILITY_TIMER while in the PROBE_DONE state.		implement the REACHABILITY_TIMER while in the PROBE_DONE state.
	5.3.1.3. PTB Message Handling by SCTP		5.3.1.3. PTB Message Handling by SCTP
	Normal ICMP verification MUST be performed as specified in Appendix C		Normal ICMP validation MUST be performed as specified in Appendix C
	of [RFC4960]. This requires that the first 8 bytes of the SCTP		of [RFC4960]. This requires that the first 8 bytes of the SCTP
	common header are quoted in the payload of the PTB message, which can		common header are quoted in the payload of the PTB message, which can
	be the case for ICMPv4 and is normally the case for ICMPv6.		be the case for ICMPv4 and is normally the case for ICMPv6.
	When a PTB message has been verified, the router Link MTU indicated		When a PTB message has been validated, the router Link MTU indicated
	in the PTB message SHOULD be used with the DPLPMTUD algorithm,		in the PTB message SHOULD be used with the DPLPMTUD algorithm,
	providing that the reported Link MTU is less than the current probe		providing that the reported Link MTU is less than the current probe
	size.		size.
	5.3.2. DPLPMTUD for SCTP/UDP		5.3.2. DPLPMTUD for SCTP/UDP
	The UDP encapsulation of SCTP is specified in [RFC6951].		The UDP encapsulation of SCTP is specified in [RFC6951].
	5.3.2.1. Sending SCTP/UDP Probe Packets		5.3.2.1. Sending SCTP/UDP Probe Packets
	Packet probing can be performed as specified in Section 5.3.1.1. The		Packet probing can be performed as specified in Section 5.3.1.1. The
	maximum payload is reduced by 8 bytes, which has to be considered		maximum payload is reduced by 8 bytes, which has to be considered
	when filling the PAD chunk.		when filling the PAD chunk.
	5.3.2.2. Validating the Path with SCTP/UDP		5.3.2.2. Validating the Path with SCTP/UDP
	Since SCTP provides an acknowledged PL, a sender does MUST NOT		Since SCTP provides an acknowledged PL, a sender does MUST NOT
	implement the REACHABILITY_TIMER while in the PROBE_DONE state.		implement the REACHABILITY_TIMER while in the PROBE_DONE state.
	5.3.2.3. Handling of PTB Messages by SCTP/UDP		5.3.2.3. Handling of PTB Messages by SCTP/UDP
	Normal ICMP verification MUST be performed for PTB messages as		Normal ICMP validation MUST be performed for PTB messages as
	specified in Appendix C of [RFC4960]. This requires that the first 8		specified in Appendix C of [RFC4960]. This requires that the first 8
	bytes of the SCTP common header are contained in the PTB message,		bytes of the SCTP common header are contained in the PTB message,
	which can be the case for ICMPv4 (but note the UDP header also		which can be the case for ICMPv4 (but note the UDP header also
	consumes a part of the quoted packet header) and is normally the case		consumes a part of the quoted packet header) and is normally the case
	for ICMPv6. When the verification is completed, the router Link MTU		for ICMPv6. When the validation is completed, the router Link MTU
	size indicated in the PTB message SHOULD be used with the DPLPMTUD		size indicated in the PTB message SHOULD be used with the DPLPMTUD
	providing that the reported link MTU is less than the current probe		providing that the reported link MTU is less than the current probe
	size.		size.
	5.3.3. DPLPMTUD for SCTP/DTLS		5.3.3. DPLPMTUD for SCTP/DTLS
	The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is		The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
	specified in [I-D.ietf-tsvwg-sctp-dtls-encaps]. It is used for data		specified in [RFC8261]. It is used for data channels in WebRTC
	channels in WebRTC implementations.		implementations.
	5.3.3.1. Sending SCTP/DTLS Probe Packets		5.3.3.1. Sending SCTP/DTLS Probe Packets
	Packet probing can be done as specified in Section 5.3.1.1.		Packet probing can be done as specified in Section 5.3.1.1.
	5.3.3.2. Validating the Path with SCTP/DTLS		5.3.3.2. Validating the Path with SCTP/DTLS
	Since SCTP provides an acknowledged PL, a sender does MUST NOT		Since SCTP provides an acknowledged PL, a sender does MUST NOT
	implement the REACHABILITY_TIMER while in the PROBE_DONE state.		implement the REACHABILITY_TIMER while in the PROBE_DONE state.
	5.3.3.3. Handling of PTB Messages by SCTP/DTLS		5.3.3.3. Handling of PTB Messages by SCTP/DTLS
	It is not possible to perform normal ICMP verification as specified		It is not possible to perform normal ICMP validation as specified in
	in [RFC4960], since even if the ICMP message payload contains		[RFC4960], since even if the ICMP message payload contains sufficient
	sufficient information, the reflected SCTP common header would be		information, the reflected SCTP common header would be encrypted.
	encrypted. Therefore it is not possible to process PTB messages at		Therefore it is not possible to process PTB messages at the PL.
	the PL.
	5.4. DPLPMTUD for QUIC		5.4. DPLPMTUD for QUIC
	Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a		Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a
	UDP-based transport that provides reception feedback.		UDP-based transport that provides reception feedback.
	Section 9.2 of [I-D.ietf-quic-transport] describes the path		Section 9.2 of [I-D.ietf-quic-transport] describes the path
	considerations when sending QUIC packets. It recommends the use of		considerations when sending QUIC packets. It recommends the use of
	PADDING frames to build the probe packet. This enables probing the		PADDING frames to build the probe packet. This enables probing the
	without affecting the transfer of other QUIC frames.		without affecting the transfer of other QUIC frames.
	This provides an acknowledged PL. A sender can therefore enter the		This provides an acknowledged PL. A sender can therefore enter the
	PROBE_BASE state as soon as connectivity has been confirmed.		PROBE_BASE state as soon as connectivity has been confirmed.
	5.4.1. Sending QUIC Probe Packets		5.4.1. Sending QUIC Probe Packets
	A probe packet consists of a QUIC Header and a payload containing		A probe packet consists of a QUIC Header and a payload containing
	only PADDING Frames. PADDING Frames are a single octet (0x00) and		only PADDING Frames. PADDING Frames are a single octet (0x00) and
	several of these can be used to create a probe packet of size		several of these can be used to create a probe packet of size
	PROBED_SIZE. QUIC provides an acknowledged PL. A sender can therefore		PROBED_SIZE. QUIC provides an acknowledged PL. A sender can
	enter the PROBE_BASE state as soon as connectivity has been		therefore enter the PROBE_BASE state as soon as connectivity has been
	confirmed.		confirmed.
	The current specification of QUIC sets the following:		The current specification of QUIC sets the following:
	o BASE_PMTU: 1200. A QUIC sender needs to pad initial packets to		o BASE_PMTU: 1200. A QUIC sender needs to pad initial packets to
	1200 bytes to validate the path can support packets of a useful		1200 bytes to validate the path can support packets of a useful
	size.		size.
	o MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU has		o MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU has
	fallen below 1200 bytes MUST immediately stop sending on the		fallen below 1200 bytes MUST immediately stop sending on the
	affected path.		affected path.
	5.4.2. Validating the Path with QUIC		5.4.2. Validating the Path with QUIC
	QUIC provides an acknowledged PL. A sender therefore MUST NOT		QUIC provides an acknowledged PL. A sender therefore MUST NOT
	implement the REACHABILITY_TIMER while in the PROBE_DONE state.		implement the REACHABILITY_TIMER while in the PROBE_DONE state.
	5.4.3. Handling of PTB Messages by QUIC		5.4.3. Handling of PTB Messages by QUIC
	QUIC operates over the UDP transport, and the guidelines on ICMP		QUIC operates over the UDP transport, and the guidelines on ICMP
	verification as specified in Section 5.2 of [RFC8085] therefore		validation as specified in Section 5.2 of [RFC8085] therefore apply.
	apply. Although QUIC does not currently specify a method for		Although QUIC does not currently specify a method for validating ICMP
	validating ICMP responses, it does provide some guidelines to make it		responses, it does provide some guidelines to make it harder for an
	harder for an off-path attacker to inject ICMP messages.		off-path attacker to inject ICMP messages.
	o Set the IPv4 Don't Fragment (DF) bit on a small proportion of		o Set the IPv4 Don't Fragment (DF) bit on a small proportion of
	packets, so that most invalid ICMP messages arrive when there are		packets, so that most invalid ICMP messages arrive when there are
	no DF packets outstanding, and can therefore be identified as		no DF packets outstanding, and can therefore be identified as
	spurious.		spurious.
	o Store additional information from the IP or UDP headers from DF		o Store additional information from the IP or UDP headers from DF
	packets (for example, the IP ID or UDP checksum) to further		packets (for example, the IP ID or UDP checksum) to further
	authenticate incoming Datagram Too Big messages.		authenticate incoming Datagram Too Big messages.
	o Any reduction in PMTU due to a report contained in an ICMP packet		o Any reduction in PMTU due to a report contained in an ICMP packet
	is provisional until QUIC's loss detection algorithm determines		is provisional until QUIC's loss detection algorithm determines
	that the packet is actually lost.		that the packet is actually lost.
	XXX The above list was pulled whole from quic-transport - input is		XXX The above list was pulled whole from quic-transport - input is
	invited from QUIC contributors. XXX		invited from QUIC contributors. XXX
	6. Acknowledgements		6. Acknowledgements
	This work was partially funded by the European Union's Horizon 2020		This work was partially funded by the European Union's Horizon 2020
	research and innovation programme under grant agreement No. 644334		research and innovation programme under grant agreement No. 644334
	(NEAT). The views expressed are solely those of the author(s).		(NEAT). The views expressed are solely those of the author(s).
	7. IANA Considerations		7. IANA Considerations
	This memo includes no request to IANA.		This memo includes no request to IANA.
	XXX If new UDP Options are specified in this document, a request to		XXX If new UDP Options are specified in this document, a request to
	IANA will be included here. XXX		IANA will be included here. XXX
	If there are no requirements for IANA, the section will be removed		If there are no requirements for IANA, the section will be removed
	during conversion into an RFC by the RFC Editor.		during conversion into an RFC by the RFC Editor.
	8. Security Considerations		8. Security Considerations
	skipping to change at page 26, line 23 ¶		skipping to change at page 30, line 22 ¶
	The security considerations for the use of UDP and SCTP are provided		The security considerations for the use of UDP and SCTP are provided
	in the references RFCs. Security guidance for applications using UDP		in the references RFCs. Security guidance for applications using UDP
	is provided in the UDP Usage Guidelines [RFC8085].		is provided in the UDP Usage Guidelines [RFC8085].
	There are cases where PTB messages are not delivered due to policy,		There are cases where PTB messages are not delivered due to policy,
	configuration or equipment design (see Section 1.1), this method		configuration or equipment design (see Section 1.1), this method
	therefore does not rely upon PTB messages being received, but is able		therefore does not rely upon PTB messages being received, but is able
	to utilise these when they are received by the sender. PTB messages		to utilise these when they are received by the sender. PTB messages
	could potentially be used to cause a node to inappropriately reduce		could potentially be used to cause a node to inappropriately reduce
	the PLPMTU. A node supporting DPLPMTUD MUST therefore appropriately		the PLPMTU. A node supporting DPLPMTUD MUST therefore appropriately
	verify the payload of PTB messages to ensure these are received in		validate the payload of PTB messages to ensure these are received in
	response to transmitted traffic (i.e., a reported error condition		response to transmitted traffic (i.e., a reported error condition
	that corresponds to a datagram actually sent by the path layer.		that corresponds to a datagram actually sent by the path layer.
	Parallel forwarding paths may need to be considered. Section 3.5		Parallel forwarding paths may need to be considered. Section 3.5
	identifies the need for robustness in the method when the path		identifies the need for robustness in the method when the path
	information may be inconsistent.		information may be inconsistent.
	A node performing DPLPMTUD could experience conflicting information		A node performing DPLPMTUD could experience conflicting information
	about the size of supported probe packets. This could occur when		about the size of supported probe packets. This could occur when
	there are multiple paths are concurrently in use and these exhibit a		there are multiple paths are concurrently in use and these exhibit a
	different PMTU. If not considered, this could result in data being		different PMTU. If not considered, this could result in data being
	black holed when the PLPMTU is larger than the smallest PMTU across		black holed when the PLPMTU is larger than the smallest PMTU across
	the current paths.		the current paths.
	An on-path attacker could forge PTB messages to drive down the PLPMTU		An on-path attacker could forge PTB messages to drive down the PLPMTU
	9. References		9. References
	9.1. Normative References		9.1. Normative References
	[I-D.ietf-quic-transport]		[I-D.ietf-quic-transport]
	Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed		Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
	and Secure Transport", Internet-Draft draft-ietf-quic-		and Secure Transport", draft-ietf-quic-transport-13 (work
	transport-04, June 2017.		in progress), June 2018.
	[I-D.ietf-tsvwg-sctp-dtls-encaps]
	Tuexen, M., Stewart, R., Jesup, R. and S. Loreto, "DTLS
	Encapsulation of SCTP Packets", Internet-Draft draft-ietf-
	tsvwg-sctp-dtls-encaps-09, January 2015.
	[I-D.ietf-tsvwg-udp-options]		[I-D.ietf-tsvwg-udp-options]
	Touch, J., "Transport Options for UDP", Internet-Draft		Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
	draft-ietf-tsvwg-udp-options-01, June 2017.		udp-options-04 (work in progress), July 2018.
	[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,		[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
	August 1980.		DOI 10.17487/RFC0768, August 1980,
			<https://www.rfc-editor.org/info/rfc768>.
	[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,		[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
	RFC 792, DOI 10.17487/RFC0792, September 1981, <https://		RFC 792, DOI 10.17487/RFC0792, September 1981,
	www.rfc-editor.org/info/rfc792>.		<https://www.rfc-editor.org/info/rfc792>.
	[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -		[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
	Communication Layers", STD 3, RFC 1122, DOI 10.17487/		Communication Layers", STD 3, RFC 1122,
	RFC1122, October 1989, <https://www.rfc-editor.org/info/		DOI 10.17487/RFC1122, October 1989,
	rfc1122>.		<https://www.rfc-editor.org/info/rfc1122>.
	[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",		[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
	RFC 1812, DOI 10.17487/RFC1812, June 1995, <https://www		RFC 1812, DOI 10.17487/RFC1812, June 1995,
	.rfc-editor.org/info/rfc1812>.		<https://www.rfc-editor.org/info/rfc1812>.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119,
			DOI 10.17487/RFC2119, March 1997,
			<https://www.rfc-editor.org/info/rfc2119>.
	[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6		[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
	(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,		(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
	December 1998, <https://www.rfc-editor.org/info/rfc2460>.		December 1998, <https://www.rfc-editor.org/info/rfc2460>.
	[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.Ed.,		[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
	and G. Fairhurst, Ed., "The Lightweight User Datagram		and G. Fairhurst, Ed., "The Lightweight User Datagram
	Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July		Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
	2004, <https://www.rfc-editor.org/info/rfc3828>.		2004, <https://www.rfc-editor.org/info/rfc3828>.
	[RFC4820] Tuexen, M., Stewart, R. and P. Lei, "Padding Chunk and		[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
	Parameter for the Stream Control Transmission Protocol		Parameter for the Stream Control Transmission Protocol
	(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,		(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
	<https://www.rfc-editor.org/info/rfc4820>.		<https://www.rfc-editor.org/info/rfc4820>.
	[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",		[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
	RFC 4960, DOI 10.17487/RFC4960, September 2007, <https://		RFC 4960, DOI 10.17487/RFC4960, September 2007,
	www.rfc-editor.org/info/rfc4960>.		<https://www.rfc-editor.org/info/rfc4960>.
	[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream		[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
	Control Transmission Protocol (SCTP) Packets for End-Host		Control Transmission Protocol (SCTP) Packets for End-Host
	to End-Host Communication", RFC 6951, DOI 10.17487/		to End-Host Communication", RFC 6951,
	RFC6951, May 2013, <https://www.rfc-editor.org/info/		DOI 10.17487/RFC6951, May 2013,
	rfc6951>.		<https://www.rfc-editor.org/info/rfc6951>.
	[RFC8085] Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage		[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
	Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,		Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
	March 2017, <https://www.rfc-editor.org/info/rfc8085>.		March 2017, <https://www.rfc-editor.org/info/rfc8085>.
	[RFC8201] McCann, J., Deering, S., Mogul, J. and R. Hinden, Ed.,		[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
	"Path MTU Discovery for IP version 6", STD 87, RFC 8201,		"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
	DOI 10.17487/RFC8201, July 2017, <https://www.rfc-		DOI 10.17487/RFC8201, July 2017,
	editor.org/info/rfc8201>.		<https://www.rfc-editor.org/info/rfc8201>.
			[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
			"Datagram Transport Layer Security (DTLS) Encapsulation of
			SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
			2017, <https://www.rfc-editor.org/info/rfc8261>.
	9.2. Informative References		9.2. Informative References
	[RFC1191] Mogul, J.C. and S.E. Deering, "Path MTU discovery", RFC		[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
	1191, DOI 10.17487/RFC1191, November 1990, <https://www		DOI 10.17487/RFC1191, November 1990,
	.rfc-editor.org/info/rfc1191>.		<https://www.rfc-editor.org/info/rfc1191>.
	[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC		[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
	2923, DOI 10.17487/RFC2923, September 2000, <https://www		RFC 2923, DOI 10.17487/RFC2923, September 2000,
	.rfc-editor.org/info/rfc2923>.		<https://www.rfc-editor.org/info/rfc2923>.
	[RFC4340] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion		[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
	Control Protocol (DCCP)", RFC 4340, March 2006.		Congestion Control Protocol (DCCP)", RFC 4340,
			DOI 10.17487/RFC4340, March 2006,
			<https://www.rfc-editor.org/info/rfc4340>.
	[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU		[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
	Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,		Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
	<https://www.rfc-editor.org/info/rfc4821>.		<https://www.rfc-editor.org/info/rfc4821>.
	[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering		[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
	ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/		ICMPv6 Messages in Firewalls", RFC 4890,
	RFC4890, May 2007, <https://www.rfc-editor.org/info/		DOI 10.17487/RFC4890, May 2007,
	rfc4890>.		<https://www.rfc-editor.org/info/rfc4890>.
	Appendix A. Event-driven state changes		Appendix A. Event-driven state changes
	This appendix contains an informative description of key events:		This appendix contains an informative description of key events:
	Path Setup: When a new path is initiated, the state is set to		Path Setup: When a new path is initiated, the state is set to
	PROBE_START. As soon as the path is confirmed, the state changes		PROBE_START. As soon as the path is confirmed, the state changes
	to PROBE_BASE and the probing mechanism for this path is started.		to PROBE_BASE and probing for this path is started. The first
	the first probe packet is sent with the size of the BASE_PMTU.		probe packet is sent with the size of the BASE_PMTU.
	Arrival of an Acknowledgment: Depending on the probing state, the		Arrival of an Acknowledgment: Depending on the probing state, the
	reaction differs according to Figure 5, which is just a		reaction differs according to Figure 7, which is a simplification
	simplification of Figure 2 focusing on this event.		of Figure 4 focusing on this event.
	+--------------+ +----------------+		+--------------+ +----------------+
	| PROBE_START | --3------------------------------->| PROBE_DISABLED |		| PROBE_START | --3------------------------------->| PROBE_DISABLED |
	+--------------+ --4-----------\ +----------------+		+--------------+ --4-----------\ +----------------+
	\		\
	+--------------+ \		+--------------+ \
	| PROBE_ERROR | --------------- \		| PROBE_ERROR | --------------- \
	+--------------+ \ \		+--------------+ \ \
	\ \		\ \
	+--------------+ \ \ +--------------+		+--------------+ \ \ +--------------+
	| PROBE_BASE | --1---------- \ ------------> | PROBE_BASE |		| PROBE_BASE | --1---------- \ ------------> | PROBE_BASE |
	+--------------+ --2----- \ \ +--------------+		+--------------+ --2----- \ \ +--------------+
	\ \ \		\ \ \
	+--------------+ \ \ ------------> +--------------+		+--------------+ \ \ ------------> +--------------+
	| PROBE_SEARCH | --2--- \ -----------------> | PROBE_SEARCH |		| PROBE_SEARCH | --2--- \ -----------------> | PROBE_SEARCH |
	+--------------+ --1---\----\---------------------> +--------------+		+--------------+ --1---\----\---------------------> +--------------+
	\ \		\ \
	+--------------+ \ \ +--------------+		+--------------+ \ \ +--------------+
	| PROBE_DONE | \ -------------------> | PROBE_DONE |		| PROBE_DONE | \ -------------------> | PROBE_DONE |
	+--------------+ -----------------------> +--------------+		+--------------+ -----------------------> +--------------+
	Condition 1: The maximum PMTU size has not yet been reached.		Condition 1: The maximum PMTU size has not yet been reached.
	Condition 2: The maximum PMTU size has been reached. Conition 3:		Condition 2: The maximum PMTU size has been reached. Conition 3:
	Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4:		Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4:
	PROBE_ACK received.		PROBE_ACK received.
	Probing timeout: The PROBE_COUNT is initialised to zero each time the		Figure 7: State changes at the arrival of an acknowledgment
	value of PROBED_SIZE is changed. The PROBE_TIMER is started each
	time a probe packet is sent. It is stopped when an acknowledgment
	arrives that confirms delivery of a probe packet. If the probe
	packet is not acknowledged before the PROBE_TIMER expires, the
	PROBE_ERROR_COUNTER is incremented. When the PROBE_COUNT equals
	the value MAX_PROBES, the state is changed, otherwise a new probe
	packet of the same size (PROBED_SIZE) is resent. The state
	transitions are illustrated in Figure 6. This shows a
	simplification of Figure 2 with a focus only on this event.
	+--------------+ +----------------+		Probing timeout: The PROBE_COUNT is initialised to zero each time
	| PROBE_START |----------------------------------->| PROBE_DISABLED |		the value of PROBED_SIZE is changed and when a acknowledgment
	+--------------+ +----------------+		confirming delivery of a probe packet arries. The PROBE_TIMER is
			started each time a probe packet is sent. It is stopped when an
			acknowledgment arrives that confirms delivery of a probe packet of
			PROBED_SIZE. If the probe packet is not acknowledged before the
			PROBE_TIMER expires, the PROBE_COUNT is incremented. When the
			PROBE_COUNT equals the value MAX_PROBES, the state is changed,
			otherwise a new probe packet of the same size (PROBED_SIZE) is
			resent. The state transitions are illustrated in Figure 8. This
			shows a simplification of Figure 4 with a focus only on this
			event.
	+--------------+ +--------------+		+--------------+ +----------------+
	| PROBE_ERROR | -----------------> | PROBE_ERROR |		| PROBE_START |----------------------------------->| PROBE_DISABLED |
	+--------------+ / +--------------+		+--------------+ +----------------+
	/
	+--------------+ --2----------/ +--------------+
	| PROBE_BASE | --1------------------------------> | PROBE_BASE |
	+--------------+ +--------------+
	+--------------+ +--------------+		+--------------+ +--------------+
	| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |		| PROBE_ERROR | -----------------> | PROBE_ERROR |
	+--------------+ --2--------- +--------------+		+--------------+ / +--------------+
	\		/
	+--------------+ \ +--------------+		+--------------+ --2----------/ +--------------+
	| PROBE_DONE | -------------------> | PROBE_DONE |		| PROBE_BASE | --1------------------------------> | PROBE_BASE |
	+--------------+ +--------------+		+--------------+ +--------------+
			+--------------+ +--------------+
			| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |
			+--------------+ --2--------- +--------------+
			\
			+--------------+ \ +--------------+
			| PROBE_DONE | -------------------> | PROBE_DONE |
			+--------------+ +--------------+
	Condition 1: The maximum number of probe packets has not been		Condition 1: The maximum number of probe packets has not been
	reached. Condition 2: The maximum number of probe packets has been		reached. Condition 2: The maximum number of probe packets has been
	reached.		reached.
	PMTU raise timer timeout: The path through the network can change		Figure 8: State changes at the expiration of the probe timer
			PMTU raise timer timeout: The path through the network can change
	over time. It impossible to discover whether a path change has		over time. It impossible to discover whether a path change has
	increased the actual PMTU by exchanging packets less than or equal		increased the actual PMTU by exchanging packets less than or equal
	to the PLPMTU. This requires PLPMTUD to periodically send a probe		to the PLPMTU. This requires PLPMTUD to periodically send a probe
	packet to detect whether a larger PMTU is possible. This probe		packet to detect whether a larger PMTU is possible. This probe
	packet is generated by the PMTU_RAISE_TIMER. When the timer		packet is generated by the PMTU_RAISE_TIMER. When the timer
	expires, probing is restarted with the BASE_PMTU and the state is		expires, probing is restarted with the BASE_PMTU and the state is
	changed to PROBE_BASE.		changed to PROBE_BASE.
	Arrival of an ICMP message: The active probing of the path can be		Arrival of a PTB message: The active probing of the path can be
	supported by the arrival of PTB messages sent by routers or		supported by the arrival of a PTB message sent by a router or
	middleboxes with a link MTU that is smaller than the probe packet		middleboxes indicating the router's local link MTU. Two cases can
	size. If the PTB message includes the router link MTU, three		be distinguished:
	cases can be distinguished:
	1. The indicated link MTU in the PTB message is between the		1. The indicated link MTU in the PTB message is between the
	already probed and PLMTU and the probe that triggered the PTB		already probed and PLPMTU and the probe that triggered the PTB
	message.		message.
	2. The indicated link MTU in the PTB message is smaller than the		2. The indicated link MTU in the PTB message is smaller than the
	PLPMTU.		PLPMTU.
	3. The indicated link MTU in the PTB message is equal to the
	BASE_PMTU.
	In first case, the PROBE_BASE state transitions to the PROBE_ERROR		In first case, the PROBE_BASE state transitions to the PROBE_ERROR
	state. In the PROBE_SEARCH state, a new probe packet is sent with		state. In the PROBE_SEARCH state, a new probe packet is sent with
	the sized reported by the PTB message. Its result is handled		the sized reported by the PTB message. Its result is handled
	according to the former events.		according to the former events.
	The second case could be a result of a network re-configuration.		The second case could be a result of a network re-configuration.
	If the reported link MTU in the PTB message is greater than the		If the reported link MTU in the PTB message is greater than the
	BASE_MTU, the probing starts again with a value of PROBE_BASE.		BASE_MTU, the probing starts again with a value of PROBE_BASE.
	Otherwise, the method enters the state PROBE_ERROR.		Otherwise, the method enters the state PROBE_ERROR.
	In the third case, the maximum possible PMTU has been reached.
	This ought to be probed again, because there could be a link
	further along the path with a still smaller MTU.
	Note: Not all routers include the link MTU size when they send a		Note: Not all routers include the link MTU size when they send a
	PTB message. If the PTB message does not indicate the link MTU,		PTB message. If the PTB message does not indicate the link MTU,
	the probe is handled in the same way as condition 2 of Figure 6.		the probe is handled in the same way as condition 2 of Figure 8.
	Appendix B. Revision Notes		Appendix B. Revision Notes
	Note to RFC-Editor: please remove this entire section prior to		Note to RFC-Editor: please remove this entire section prior to
	publication.		publication.
	Individual draft -00:		Individual draft -00:
	o Comments and corrections are welcome directly to the authors or		o Comments and corrections are welcome directly to the authors or
	via the IETF TSVWG working group mailing list.		via the IETF TSVWG working group mailing list.
	skipping to change at page 31, line 48 ¶		skipping to change at page 35, line 47 ¶
	states and timers		states and timers
	o This update is proposed for WG comments.		o This update is proposed for WG comments.
	Individual draft -02:		Individual draft -02:
	o Contains updated representation of the algorithm, and textual		o Contains updated representation of the algorithm, and textual
	corrections.		corrections.
	o The text describing when to set the effective PMTU has not yet		o The text describing when to set the effective PMTU has not yet
	been verified by the authors		been validated by the authors
	o To determine security to off-path-attacks: We need to decide		o To determine security to off-path-attacks: We need to decide
	whether a received PTB message SHOULD/MUST be verified? The text		whether a received PTB message SHOULD/MUST be validated? The text
	on how to handle a PTB message indicating a link MTU larger than		on how to handle a PTB message indicating a link MTU larger than
	the probe has yet not been verified by the authors		the probe has yet not been validated by the authors
	o No text currently describes how to handle inconsistent results		o No text currently describes how to handle inconsistent results
	from arbitrary re-routing along different parallel paths		from arbitrary re-routing along different parallel paths
	o This update is proposed for WG comments.		o This update is proposed for WG comments.
	Working Group draft -00:		Working Group draft -00:
	o This draft follows a successful adoption call for TSVWG		o This draft follows a successful adoption call for TSVWG
	skipping to change at page 32, line 46 ¶		skipping to change at page 36, line 49 ¶
	o This draft includes reorganisation of the section on IETF		o This draft includes reorganisation of the section on IETF
	protocols.		protocols.
	o Added more discussion of implementation within an application.		o Added more discussion of implementation within an application.
	o Added text on flapping paths.		o Added text on flapping paths.
	o Replaced 'effective MTU' with new term PLPMTU.		o Replaced 'effective MTU' with new term PLPMTU.
			Working Group draft -03:
			o Updated figures
			o Added more discussion on blackhole detection
			o Added figure describing just blackhole detection
			o Added figure relating MPS sizes
			o Updated full state machine artwork for clarity
			o Changed all text to refer to /packet probes/ /validation/ (rather
			than /verification/).
	Authors' Addresses		Authors' Addresses
	Godred Fairhurst		Godred Fairhurst
	University of Aberdeen		University of Aberdeen
	School of Engineering		School of Engineering
	Fraser Noble Building		Fraser Noble Building
	Aberdeen, AB24 3U		Aberdeen AB24 3U
	UK		UK
	Email: gorry@erg.abdn.ac.uk		Email: gorry@erg.abdn.ac.uk
	Tom Jones		Tom Jones
	University of Aberdeen		University of Aberdeen
	School of Engineering		School of Engineering
	Fraser Noble Building		Fraser Noble Building
	Aberdeen, AB24 3U		Aberdeen AB24 3U
	UK		UK
	Email: tom@erg.abdn.ac.uk		Email: tom@erg.abdn.ac.uk
	Michael Tuexen		Michael Tuexen
	Muenster University of Applied Sciences		Muenster University of Applied Sciences
	Stegerwaldstrasse 39		Stegerwaldstrasse 39
	Stein fart, 48565		Stein fart 48565
	DE		DE
	Email: tuexen@fh-muenster.de		Email: tuexen@fh-muenster.de
	Irene Ruengeler		Irene Ruengeler
	Muenster University of Applied Sciences		Muenster University of Applied Sciences
	Stegerwaldstrasse 39		Stegerwaldstrasse 39
	Stein fart, 48565		Stein fart 48565
	DE		DE
	Email: i.ruengeler@fh-muenster.de		Email: i.ruengeler@fh-muenster.de
End of changes. 173 change blocks.
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