wdiff rfc8899.original rfc8899.txt

Internet Engineering Task Force (IETF) G. Fairhurst
Internet-Draft
Request for Comments: 8899 T. Jones
Updates: 4821, 4960, 6951, 8085, 8261 (if University of Aberdeen
approved) M. Tuexen
Intended status:
Category: Standards Track M. Tüxen
ISSN: 2070-1721 I. Ruengeler
Expires: 12 December 2020 Rüngeler
T. Voelker
Muenster Völker
Münster University of Applied Sciences
10 June
September 2020

Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-22

Abstract

This document describes specifies Datagram Packetization Layer Path MTU
Discovery (DPLPMTUD). This is a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization Layers (PLs). It describes an
extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path
MTU Discovery for IPv4 and IPv6. The method allows a PL, or
a datagram application that uses a PL, to discover whether a network
path can support the current size of datagram. This can be used to
detect and reduce the message size when a sender encounters a packet
black hole (where packets are discarded). The method hole. It can also probe a network path with progressively larger packets to discover whether the
maximum packet size can be increased. This allows a sender to
determine an appropriate packet size, providing provides functionality
for datagram transports that is equivalent to the Packetization Layer
PMTUD PLPMTUD
specification for TCP, specified in RFC 4821.

This document updates RFC 4821 to specify the PLPMTUD method for
datagram PLs. 4821, which it updates. It
also updates RFC 8085 the UDP Usage Guidelines to refer to the method
specified in this document instead of the method in RFC 4821 for use
with UDP datagrams. Section 7.3 of RFC 4960 recommends an endpoint
apply the techniques in RFC 4821 on a per-destination-address basis.
RFC 4960, RFC 6951, and RFC 8261 are updated to recommend that SCTP,
SCTP encapsulated in UDP datagrams and SCTP encapsulated in DTLS use the method
specified in this document instead of the method in RFC 4821. updates SCTP.

The document also provides implementation notes for incorporating Datagram
PMTUD into IETF datagram transports or applications that use datagram
transports.

When published, this

This specification updates RFC 4960, RFC 4821, RFC
8085 6951, RFC 8085,
and RFC 8261.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on 12 December 2020.
https://www.rfc-editor.org/info/rfc8899.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . 4
1.2. Packetization Layer Path MTU Discovery . . . . . . . . . 6
1.3. Path MTU Discovery for Datagram Services . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 11
4. DPLPMTUD Mechanisms . . . . . . . . . . . . . . . . . . . . . 14
4.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . 14
4.2. Confirmation of Probed Packet Size . . . . . . . . . . . 15
4.3. Black Hole Detection and Reducing the PLPMTU . . . . . . 15
4.4. The Maximum Packet Size (MPS) . . . . . . . . . . . . . . 17
4.5. Disabling the Effect of PMTUD . . . . . . . . . . . . . . 18
4.6. Response to PTB Messages . . . . . . . . . . . . . . . . 18
4.6.1. Validation of PTB Messages . . . . . . . . . . . . . 18
4.6.2. Use of PTB Messages . . . . . . . . . . . . . . . . . 19
5. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 20
5.1. DPLPMTUD Components . . . . . . . . . . . . . . . . . . . 21
5.1.1. Timers . . . . . . . . . . . . . . . . . . . . . . . 21
5.1.2. Constants . . . . . . . . . . . . . . . . . . . . . . 22
5.1.3. Variables . . . . . . . . . . . . . . . . . . . . . . 23
5.1.4. Overview of DPLPMTUD Phases . . . . . . . . . . . . . 24
5.2. State Machine . . . . . . . . . . . . . . . . . . . . . . 26
5.3. Search to Increase the PLPMTU . . . . . . . . . . . . . . 29
5.3.1. Probing for a larger Larger PLPMTU . . . . . . . . . . . . . 29
5.3.2. Selection of Probe Sizes . . . . . . . . . . . . . . 30
5.3.3. Resilience to Inconsistent Path Information . . . . . 30
5.4. Robustness to Inconsistent Paths . . . . . . . . . . . . 31
6. Specification of Protocol-Specific Methods . . . . . . . . . 31
6.1. Application support Support for DPLPMTUD with UDP or UDP-Lite . . 31
6.1.1. Application Request . . . . . . . . . . . . . . . . . 32
6.1.2. Application Response . . . . . . . . . . . . . . . . 32
6.1.3. Sending Application Probe Packets . . . . . . . . . . 32
6.1.4. Initial Connectivity . . . . . . . . . . . . . . . . 32
6.1.5. Validating the Path . . . . . . . . . . . . . . . . . 32
6.1.6. Handling of PTB Messages . . . . . . . . . . . . . . 32
6.2. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 33
6.2.1. SCTP/IPv4 and SCTP/IPv6 . . . . . . . . . . . . . . . 33
6.2.1.1. Initial Connectivity . . . . . . . . . . . . . . 33
6.2.1.2. Sending SCTP Probe Packets . . . . . . . . . . . 33
6.2.1.3. Validating the Path with SCTP . . . . . . . . . . 34
6.2.1.4. PTB Message Handling by SCTP . . . . . . . . . . 34
6.2.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 34
6.2.2.1. Initial Connectivity . . . . . . . . . . . . . . 35
6.2.2.2. Sending SCTP/UDP Probe Packets . . . . . . . . . 35
6.2.2.3. Validating the Path with SCTP/UDP . . . . . . . . 35
6.2.2.4. Handling of PTB Messages by SCTP/UDP . . . . . . 35
6.2.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 35
6.2.3.1. Initial Connectivity . . . . . . . . . . . . . . 35
6.2.3.2. Sending SCTP/DTLS Probe Packets . . . . . . . . . 36
6.2.3.3. Validating the Path with SCTP/DTLS . . . . . . . 36
6.2.3.4. Handling of PTB Messages by SCTP/DTLS . . . . . . 36
6.3. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 36
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
9.
8. Security Considerations . . . . . . . . . . . . . . . . . . . 37
10.
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1.
9.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2.
9.2. Informative References . . . . . . . . . . . . . . . . . 39
Appendix A. Revision Notes . . . . . . . . . . . . . . . . . . . 41
Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46

1. Introduction

The IETF has specified datagram transport using UDP, SCTP, Stream Control
Transmission Protocol (SCTP), and DCCP, Datagram Congestion Control
Protocol (DCCP), as well as protocols layered on top of these
transports (e.g., SCTP/
UDP, SCTP/UDP, DCCP/UDP, QUIC/UDP), QUIC/UDP) and direct datagram
transport over the IP network layer. This document describes a
robust method for Path MTU Discovery (PMTUD) that can be used with
these transport protocols (or the applications that use their
transport service) to discover an appropriate size of packet to use
across an Internet path.

1.1. Classical Path MTU Discovery

Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
used with any transport that is able to process ICMP Packet Too Big
(PTB) messages (e.g., [RFC1191] and [RFC8201]). In this document,
the term PTB message is applied to both IPv4 ICMP Unreachable
messages (type (Type 3) that carry the error Fragmentation Needed (Type 3,
Code 4) [RFC0792] and ICMPv6 Packet Too Big messages (Type 2)
[RFC4443]. When a sender receives a PTB message, it reduces the
effective MTU to the value reported as the Link link MTU in the PTB
message. A Classical PMTUD specifies a method from time-to-time increases of periodically
increasing the packet size in an attempt to discover an increase in
the supported PMTU. The packets sent with a size larger than the
current effective PMTU are known as probe packets.

Packets not intended as probe packets are either fragmented to the
current effective PMTU, or the attempt to send fails with an error
code. Applications can be provided with a primitive to let them read
the Maximum Packet Size (MPS), which is derived from the current
effective PMTU.

Classical PMTUD is subject to protocol failures. One failure arises
when traffic using a packet size larger than the actual PMTU is
black-holed (all datagrams larger than the actual PMTU, PMTU are
discarded). This could arise when the PTB messages are not delivered sent back
to the sender for some reason (see for example (for example, see [RFC2923]).

Examples of where PTB messages are not delivered include: include the
following:

* The generation of ICMP messages is usually rate limited. This
could result in no PTB messages being generated to the sender (see
section
Section 2.4 of [RFC4443]) [RFC4443]).

* ICMP messages can be filtered by middleboxes (including firewalls) middleboxes, including firewalls
[RFC4890]. A firewall could be configured with a policy to block
incoming ICMP messages, which would prevent reception of PTB
messages to by a sending endpoint behind this firewall.

* When the router issuing the ICMP message drops a tunneled packet,
the resulting ICMP message will be is directed to the tunnel ingress.
This tunnel endpoint is responsible for forwarding the ICMP
message and also
message, processing the quoted packet within the payload field to
remove the effect of the tunnel, tunnel and return returning a correctly
formatted ICMP message to the sender [I-D.ietf-intarea-tunnels]. [TUNNELS]. Failure to do
this prevents the PTB message from reaching the original sender.

* Asymmetry in forwarding can result in there being no return route
to the original sender, which would prevent an ICMP message from
being delivered to the sender. This issue can also arise when policy-
based routing is used, Equal Cost
either policy-based or Equal-Cost Multipath (ECMP) routing is
used, used
or when a middlebox acts as an application load balancer. An
example of which is where the an ECMP router choosing a path towards toward the
server is chosen by ECMP
routing depending based on the bytes in the IP payload. In this case, when if a
packet sent by the server encounters a problem after the ECMP
router, then the ECMP router needs to direct any resulting ICMP
message also needs to be directed
by the ECMP router towards toward the original sender.

* There are additional cases where the next hop next-hop destination fails to
receive a packet because of its size. This could be due to
misconfiguration of the layer 2 path between nodes, for instance
the MTU configured in a layer 2 switch, or misconfiguration of the
Maximum Receive Unit (MRU). If a packet is dropped by the link,
this will not cause a PTB message to be sent to the original
sender.

Another failure could result if a node that is not on the network
path sends a PTB message that attempts to force a sender to change
the effective PMTU [RFC8201]. A sender can protect itself from
reacting to such messages by utilizing the quoted packet within a PTB
message payload to validate that the received PTB message was
generated in response to a packet that had actually originated from
the sender. However, there are situations where a sender would be
unable to provide this validation. Examples where the validation of
the PTB message is not possible include: include the following:

* When a router issuing the ICMP message implements RFC792 RFC 792
[RFC0792], it is only required to include the first 64 bits of the
IP payload of the packet within the quoted payload. There could
be insufficient bytes remaining for the sender to interpret the
quoted transport information.

Note: The recommendation in RFC1812 RFC 1812 [RFC1812] is that IPv4
routers return a quoted packet with as much of the original
datagram as possible without the length of the ICMP datagram
exceeding 576 bytes. IPv6 routers include as much of the invoking
packet as possible without the ICMPv6 packet exceeding 1280 bytes
[RFC4443].

* The use of tunnels/encryption tunnels and/or encryption can reduce the size of the
quoted packet returned to the original source address, increasing
the risk that there could be insufficient bytes remaining for the
sender to interpret the quoted transport information.

* Even when the PTB message includes sufficient bytes of the quoted
packet, the network layer could lack sufficient context to
validate the message, message because validation depends on information
about the active transport flows at an endpoint node (e.g., the
socket/address pairs being used, used and other protocol header
information).

* When a packet is encapsulated/tunneled over an encrypted
transport, the tunnel/encapsulation ingress might have
insufficient context, or computational power, to reconstruct the
transport header that would be needed to perform validation.

* When an ICMP message is generated by a router in a network segment
that has inserted a header into a packet, the quoted packet could
contain additional protocol header information that was not
included in the original sent packet, packet and which that the PL sender does
not process or may not know how to process. This could disrupt
the ability of the sender to validate this PTB message.

* A Network Address Translation (NAT) device that translates a
packet header, header ought to also translate ICMP messages and update the ICMP quoted
ICMP-quoted packet [RFC5508] in that message. If this is not
correctly translated translated, then the sender would not be able to
associate the message with the PL that originated the packet, and
hence this ICMP message cannot be validated.

1.2. Packetization Layer Path MTU Discovery

The term Packetization Layer (PL) has been introduced to describe the
layer that is responsible for placing data blocks into the payload of
IP packets and selecting an appropriate MPS. This function is often
performed by a transport protocol (e.g., DCCP, RTP, SCTP, QUIC), QUIC) but
can also be performed by other encapsulation methods working above
the transport layer.

In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) [RFC4821] introduced introduces a method that does not rely upon
reception and validation of PTB messages. It is therefore more
robust than Classical PMTUD. This has become the recommended
approach for implementing discovery of the PMTU [BCP145].

This document updates [RFC4821] to specify the PLPMTUD method for
datagram PLs and also updates [BCP145] to refer to the method
specified in this document for use with UDP datagrams instead of the
method in [RFC4821].

It uses a general strategy where in which the PL sends probe packets to
search for the largest size of unfragmented datagram that can be sent
over a network path. Probe packets are sent to explore using a
larger packet size. If a probe packet is successfully delivered (as
determined by the PL), then the PLPMTU is raised to the size of the
successful probe. If a black hole is detected (e.g., where packets
of size PLPMTU are consistently not received), the method reduces the
PLPMTU.

Datagram PLPMTUD introduces flexibility in implementation. At one
extreme, it can be configured to only perform Black Hole Detection black hole detection
and recovery with increased robustness compared to Classical PMTUD.
At the other extreme, all PTB processing can be disabled, and PLPMTUD
replaces Classical PMTUD.

PLPMTUD can also include additional consistency checks without
increasing the risk that data is lost when probing to discover the
Path MTU. For example, information available at the PL, or higher
layers, enables received PTB messages to be validated before being
utilized.

1.3. Path MTU Discovery for Datagram Services

Section 5 of this document presents a set of algorithms for datagram
protocols to discover the largest size of unfragmented datagram that
can be sent over a network path. The method relies upon features of
the PL described in Section 3 and applies to transport protocols
operating over IPv4 and IPv6. It does not require cooperation from
the lower layers, although it can utilize PTB messages when these
received messages are made available to the PL.

The message size guidelines in section Section 3.2 of the UDP Usage
Guidelines [BCP145] state that "an application SHOULD either use the
Path MTU information provided by the IP layer or implement Path MTU
Discovery (PMTUD)", (PMTUD)" but does do not provide a mechanism for discovering the
largest size of unfragmented datagram that can be used on a network
path. The present document updates RFC 8085 to specify this method
in place of PLPMTUD [RFC4821] and provides a mechanism for sharing
the discovered largest size as the MPS (see Section 4.4).

Section 10.2 of [RFC4821] recommended a PLPMTUD probing method for
the Stream Control Transport Protocol (SCTP). SCTP utilizes probe
packets consisting of a minimal sized minimal-sized HEARTBEAT chunk bundled with a
PAD chunk as defined in [RFC4820]. However, RFC 4821 did not provide
a complete specification. The present document replaces that
description by providing a complete specification.

The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
implementations to support Classical PMTUD and states that a DCCP
sender "MUST maintain the MPS allowed for each active DCCP session".
It also defines the current congestion control MPS (CCMPS) supported
by a network path. This recommends use of PMTUD, PMTUD and suggests use of
control packets (DCCP-Sync) as path probe packets, packets because they do not
risk application data loss. The method defined in this specification
can be used with DCCP.

Section 4 and Section 5 define the protocol mechanisms and
specification for Datagram Packetization Layer Path MTU Discovery
(DPLPMTUD).

Section 6 specifies the method for datagram transports and provides
information to enable the implementation of PLPMTUD with other
datagram transports and applications that use datagram transports.

Section 6 also provides updated recommendations for [RFC6951] SCTP endpoints, updating
[RFC4960], [RFC6951], and
[RFC8261]. [RFC8261] to use the method specified in
this document instead of the method in [RFC4821].

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

The following terminology is defined. Relevant terms are directly
copied from [RFC4821], and the definitions in [RFC1122]. [RFC1122] apply.

Acknowledged PL: A PL that includes a mechanism that can confirm
successful delivery of datagrams to the remote PL endpoint (e.g.,
SCTP). Typically, the PL receiver returns acknowledgments
corresponding to the received datagrams, which can be utilised utilized to
detect black-holing of packets (c.f., Unacknowledged PL).

Actual PMTU: The Actual actual PMTU is the PMTU of a network path between a
sender PL and a destination PL, which the DPLPMTUD algorithm seeks
to determine.

Black Hole: A Black Hole black hole is encountered when a sender is unaware
that packets are not being delivered to the destination end point. endpoint.
Two types of Black Hole black hole are relevant to DPLPMTUD:

* Packets encounter a packet Black Hole black hole when packets are not
delivered to the destination endpoint (e.g., when the sender
transmits packets of a particular size with a previously known
effective PMTU PMTU, and they are discarded by the network).

* An ICMP Black Hole black hole is encountered when the sender is unaware
that packets are not delivered to the destination endpoint
because PTB messages are not received by the originating PL
sender.

Classical Path MTU Discovery: Classical PMTUD is a process described
in [RFC1191] and [RFC8201], [RFC8201] in which nodes rely on PTB messages to
learn the largest size of unfragmented packet that can be used
across a network path.

Datagram: A datagram is a transport-layer protocol data unit,
transmitted in the payload of an IP packet.

DPLPMTUD: Datagram Packetization Layer Path MTU Discovery
(DPLPMTUD), PLPMTUD performed using a datagram transport protocol.

Effective PMTU: The Effective effective PMTU is the current estimated value
for PMTU that is used by a PMTUD. This is equivalent to the
PLPMTU derived by PLPMTUD plus the size of any headers added below
the PL, including the IP layer headers.

EMTU_S: The Effective effective MTU for sending (EMTU_S) is defined in
[RFC1122] as "the maximum IP datagram size that may be sent, for a
particular combination of IP source and destination addresses...".

EMTU_R: The Effective effective MTU for receiving (EMTU_R) is designated in
[RFC1122] as "the largest datagram size that can be reassembled".

Link: A Link link is a communication facility or medium over which nodes
can communicate at the link layer, i.e., a layer below the IP
layer. Examples are Ethernet LANs and Internet (or higher) layer
tunnels.

Link MTU: The Link link Maximum Transmission Unit (MTU) is the size in
bytes of the largest IP packet, including the IP header and
payload, that can be transmitted over a link. Note that this
could more properly be called the IP MTU, to be consistent with
how other standards organizations use the acronym. This includes
the IP header, header but excludes link layer headers and other framing
that is not part of IP or the IP payload. Other standards
organizations generally define the link MTU to include the link
layer headers. This specification continues the requirement in
[RFC4821],
[RFC4821] that states states, "All links MUST enforce their MTU: links
that might non- deterministically non-deterministically deliver packets that are larger
than their rated MTU MUST consistently discard such packets."

MAX_PLPMTU: The MAX_PLPMTU is the largest size of PLPMTU that
DPLPMTUD will attempt to use (see the constants defined in
Section 5.1.2).

MIN_PLPMTU: The MIN_PLPMTU is the smallest size of PLPMTU that
DPLPMTUD will attempt to use (see the constants defined in
Section 5.1.2).

MPS: The Maximum Packet Size (MPS) is the largest size of
application data block that can be sent across a network path by a
PL using a single Datagram datagram (see Section 4.4).

MSL: The Maximum Segment Lifetime (MSL) The is the maximum delay a
packet is expected to experience across a path, taken as 2 minutes
[BCP145].

Packet: A Packet packet is the IP header(s) and any extension headers/
options plus the IP payload.

Packetization Layer (PL): The PL is a layer of the network stack
that places data into packets and performs transport protocol
functions. Examples of a PL include: include TCP, SCTP, SCTP over UDP,
SCTP over DTLS, or QUIC.

Path: The Path path is the set of links and routers traversed by a packet
between a source node and a destination node by a particular flow.

Path MTU (PMTU): The Path MTU (PMTU) is the minimum of the Link link MTU
of all the links forming a network path between a source node and
a destination node, as used by PMTUD.

PTB: In this document, the term PTB message is applied to both IPv4
ICMP Unreachable messages (type (Type 3) that carry the error
Fragmentation Needed (Type 3, Code 4) [RFC0792] and ICMPv6 Packet
Too Big messages (Type 2) [RFC4443].

PTB_SIZE: The PTB_SIZE is a value reported in a validated PTB
message that indicates next hop next-hop link MTU of a router along the
path.

PL_PTB_SIZE: The size reported in a validated PTB message, reduced
by the size of all headers added by layers below the PL.

PLPMTU: The Packetization Layer PMTU is an estimate of the largest
size of PL datagram that can be sent by a path, controled controlled by
PLPMTUD.

PLPMTUD: Packetization Layer Path MTU Discovery (PLPMTUD), the
method described in this document for datagram PLs, which is an
extension to Classical PMTU Discovery.

Probe packet: A probe packet is a datagram sent with a purposely
chosen size (typically the current PLPMTU or larger) to detect if
packets of this size can be successfully sent end-to-end across
the network path.

Unacknowledged PL: A PL that does not itself provide a mechanism to
confirm delivery of datagrams to the remote PL endpoint (e.g.,
UDP), and therefore requires DPLPMTUD to provide a mechanism to
detect black-holing of packets (c.f., Acknowledged PL).

3. Features Required to Provide Datagram PLPMTUD

The principles expressed in [RFC4821] apply to the use of the
technique with any PL. TCP PLPMTUD has been defined using standard
TCP protocol mechanisms. Unlike TCP, a datagram PL requires
additional mechanisms and considerations to implement PLPMTUD.

The requirements for datagram PLPMTUD are:

1. Managing the PLPMTU: For datagram PLs, the PLPMTU is managed by
DPLPMTUD. A PL MUST NOT send a datagram (other than a probe
packet) with a size at the PL that is larger than the current
PLPMTU.

2. Probe packets: The network interface below the PL is REQUIRED to
provide a way to transmit a probe packet that is larger than the
PLPMTU. In IPv4, a probe packet MUST be sent with the Don't
Fragment (DF) bit set in the IP header, header and without network layer
endpoint fragmentation. In IPv6, a probe packet is always sent
without source fragmentation (as specified in section Section 5.4 of
[RFC8201]).

3. Reception feedback: The destination PL endpoint is REQUIRED to
provide a feedback method that indicates to the DPLPMTUD sender
when a probe packet has been received by the destination PL
endpoint. Section 6 provides examples of how a PL can provide
this acknowledgment of received probe packets.

4. Probe loss recovery: It is RECOMMENDED to use probe packets that
do not carry any user data that would require retransmission if
lost. Most datagram transports permit this. If a probe packet
contains user data requiring retransmission in case of loss, the
PL (or layers above) are is REQUIRED to arrange any retransmission/ retransmission
and/or repair of any resulting loss. The PL is REQUIRED to be
robust in the case where probe packets are lost due to other
reasons (including link transmission error, congestion).

5. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to utilize
information about the maximum size of packet that can be
transmitted by the sender on the local link (e.g., the local Link link
MTU). A PL sender MAY utilize similar information about the
maximum size of network layer network-layer packet that a receiver can accept
when this is supplied (note this could be less than EMTU_R).
This avoids implementations trying to send probe packets that can
not
cannot be transferred by the local link. Too high of a value
could reduce the efficiency of the search algorithm. Some
applications also have a maximum transport protocol data unit
(PDU) size, in which case there is no benefit from probing for a
size larger than this (unless a transport allows multiplexing
multiple
applications applications' PDUs into the same datagram).

6. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize
PTB messages received from the network layer to help identify
when a network path does not support the current size of probe
packet. Any received PTB message MUST be validated before it is
used to update the PLPMTU discovery information [RFC8201]. This
validation confirms that the PTB message was sent in response to
a packet originating originated by the sender, sender and needs to be performed
before the PLPMTU discovery method reacts to the PTB message. A
PTB message MUST NOT be used to increase the PLPMTU [RFC8201], [RFC8201] but
could trigger a probe to test for a larger PLPMTU. A valid
PTB_SIZE is converted to a PL_PTB_SIZE before it is to be used in
the DPLPMTUD state machine. A PL_PTB_SIZE that is greater than
that currently probed SHOULD be ignored. (This PTB message ought
to be discarded without further processing, processing but could be utilized
as an input that enables a resilience mode).

7. Probing and congestion control: A PL MAY use a congestion
controller to decide when to send a probe packet. If
transmission of probe packets is limited by the congestion
controller, this could result in transmission of probe packets
being delayed or suspended during congestion. When the
transmission of probe packets is not controlled by the congestion
controller, the interval between probe packets MUST be at least
one RTT. Loss of a probe packet SHOULD NOT be treated as an
indication of congestion and SHOULD NOT trigger a congestion
control reaction [RFC4821], [RFC4821] because this could result in
unnecessary reduction of the sending rate. An update to the
PLPMTU (or MPS) MUST NOT increase the congestion window measured
in bytes [RFC4821]. Therefore, an increase in the packet size
does not cause an increase in the data rate in bytes per second.
A PL that maintains the congestion window in terms of a limit to
the number of outstanding fixed size fixed-size packets SHOULD adapt this
limit to compensate for the size of the actual packets. The
transmission of probe packets can interact with the operation of
a PL that performs burst mitigation or pacing pacing, and the PL could
need transmission of probe packets to be regulated by these
methods.

8. Probing and flow control: Flow control at the PL concerns the
end-to-end flow of data using the PL service. Flow control
SHOULD NOT apply to DPLPMTU when probe packets use a design that
does not carry user data to the remote application.

9. Shared PLPMTU state: The PMTU value calculated from the PLPMTU
MAY also be stored with the corresponding entry associated with
the destination in the IP layer cache, cache and used by other PL
instances. The specification of PLPMTUD [RFC4821] states: states, "If
PLPMTUD updates the MTU for a particular path, all Packetization
Layer sessions that share the path representation (as described
in Section 5.2 of [RFC4821]) 5.2) SHOULD be notified to make use of the new MTU".
Such methods MUST be robust to the wide variety of underlying
network forwarding behaviors. Section 5.2 of [RFC8201] provides
guidance on the caching of PMTU information and also the relation
to IPv6 flow labels.

In addition, the following principles are stated for design of a
DPLPMTUD method:

* A PL MAY be designed to segment data blocks larger than the MPS
into multiple datagrams. However, not all datagram PLs support
segmentation of data blocks. It is RECOMMENDED that methods avoid
forcing an application to use an arbitrary small MPS for
transmission while the method is searching for the currently
supported PLPMTU. A reduced MPS can adversely impact the
performance of an application.

* To assist applications in choosing a suitable data block size, the
PL is RECOMMENDED to provide a primitive that returns the MPS
derived from the PLPMTU to the higher layer using the PL. The
value of the MPS can change following a change in the path, path or loss
of probe packets.

* Path validation: It is RECOMMENDED that methods are robust to path
changes that could have occurred since the path characteristics
were last confirmed, confirmed and to the possibility of inconsistent path
information being received.

* Datagram reordering: A method is REQUIRED to be robust to the
possibility that a flow encounters reordering, reordering or that the traffic
(including probe packets) is divided over more than one network
path.

* Datagram delay and duplication: The feedback mechanism is REQUIRED
to be robust to the possibility that packets could be
significantly delayed or duplicated along a network path.

* When to probe: It is RECOMMENDED that methods determine whether
the path has changed since it last measured the path. This can
help determine when to probe the path again.

4. DPLPMTUD Mechanisms

This section lists the protocol mechanisms used in this
specification.

4.1. PLPMTU Probe Packets

The DPLPMTUD method relies upon the PL sender being able to generate
probe packets with a specific size. TCP is able to generate these
probe packets by choosing to appropriately segment data being sent
[RFC4821]. In contrast, a datagram PL that constructs a probe packet
has to either request an application to send a data block that is
larger than that generated by an application, or to utilize padding
functions to extend a datagram beyond the size of the application
data block. Protocols that permit exchange of control messages
(without an application data block) can generate a probe packet by
extending a control message with padding data. The total size of a
probe packet includes all headers and padding added to the payload
data being sent (e.g., including protocol option fields, security-
related fields such as an Authenticated Encryption with Associated
Data (AEAD) tag tag, and TLS record layer padding).

A receiver is REQUIRED to be able to distinguish an in-band data
block from any added padding. This is needed to ensure that any
added padding is not passed on to an application at the receiver.

This results in three possible ways that a sender can create a probe
packet:

Probing using padding data: A probe packet that contains only
control information together with any padding, which is needed to
be inflated
inflate to the size of the probe packet. Since these probe
packets do not carry an application-supplied data block, they do
not typically require retransmission, although they do still
consume network capacity and incur endpoint processing.

Probing using application data and padding data: A probe packet that
contains a data block supplied by an application that is combined
with padding to inflate the length of the datagram to the size of
the probe packet.

Probing using application data: A probe packet that contains a data
block supplied by an application that matches the size of the
probe packet. This method requests the application to issue a
data block of the desired probe size.

A PL that uses a probe packet carrying application data and that
needs protection from the loss of this probe packet could perform
transport-layer retransmission/repair of the data block (e.g., by
retransmission
retransmitting after loss is detected or by duplicating the data
block in a datagram without the padding data). This retransmitted
data block might possibly need to be sent using a smaller PLPMTU,
which could force the PL to to use a smaller packet size to traverse the
end-to-end path. (This could utilize endpoint network-layer
fragmentation or a PL that can re-segment resegment the data block into multiple
datagrams).

DPLPMTUD MAY choose to use only one of these methods to simplify the
implementation.

Probe messages sent by a PL MUST contain enough information to
uniquely identify the probe within the Maximum Segment Lifetime
(e.g., including a unique identifier from the PL or the DPLPMTUD
implementation), while being robust to reordering and replay of probe
response and PTB messages.

4.2. Confirmation of Probed Packet Size

The PL needs a method to determine (confirm) when probe packets have
been successfully received end-to-end across a network path.

Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP,
SCTP, and QUIC provide keep-alive/heartbeat features). When
supported, this mechanism MAY also be used by DPLPMTUD to acknowledge
reception of a probe packet.

A PL that does not acknowledge data reception (e.g., UDP and UDP-
Lite) is unable itself to detect when the packets that it sends are
discarded because their size is greater than the actual PMTU. These
PLs need to rely on an application protocol to detect this loss.

Section 6 specifies this function for a set of IETF-specified
protocols.

4.3. Black Hole Detection and Reducing the PLPMTU

The description that follows uses the set of constants defined in
Section 5.1.2 and variables defined in Section 5.1.3.

Black Hole Detection hole detection is triggered by an indication that the network
path could be unable to support the current PLPMTU size.

There are three indicators that can be used to detect black holes:

* A validated PTB message can be received that indicates a
PL_PTB_SIZE less than the current PLPMTU. A DPLPMTUD method MUST
NOT rely solely on this method.

* A PL can use the DPLPMTUD probing mechanism to periodically
generate probe packets of the size of the current PLPMTU (e.g.,
using the confirmation timer CONFIRMATION_TIMER, Section 5.1.1). A timer tracks
whether acknowledgments are received. Successive loss of probes
is an indication that the current path no longer supports the
PLPMTU (e.g., when the number of probe packets sent without
receiving an acknowledgment, PROBE_COUNT, becomes greater than
MAX_PROBES).

* A PL can utilize an event that indicates the network path no
longer sustains the sender's PLPMTU size. This could use a
mechanism implemented within the PL to detect excessive loss of
data sent with a specific packet size and then conclude that this
excessive loss could be a result of an invalid PLPMTU (as in
PLPMTUD for TCP [RFC4821]).

The three methods can result in different transmission patterns for
packet probes and are expected to result in different responsiveness
following a change in the actual PMTU.

A PL MAY inhibit sending probe packets when no application data has
been sent since the previous probe packet. A PL that resumes sending
user data MAY continue PLPMTU discovery for each path. This allows
it to use an up-to-date PLPMTU. However, this could result in
additional packets being sent.

When the method detects that the current PLPMTU is not supported,
DPLPMTUD sets a lower PLPMTU, PLPMTU and sets a lower MPS. The PL then confirms
that the new PLPMTU can be successfully used across the path. A
probe packet could need to have a size less be smaller than the size of the data block
generated by the application.

4.4. The Maximum Packet Size (MPS)

The result of probing determines a usable PLPMTU, which is used to
set the MPS used by the application. The MPS is smaller than the
PLPMTU because it is reduced by the size of PL headers (including the
overhead of security-related fields such as an AEAD tag and TLS
record layer padding). The relationship between the MPS and the
PLPMTUD is illustrated in Figure 1.

any

Any additional
headers .--- MPS -----.
| | |
v v v
+------------------------------+
| IP | ** | PL | protocol data |
+------------------------------+

<----- PLPMTU ----->
<---------- PMTU -------------->

Figure 1: Relationship between MPS and PLPMTU

A PL is unable to send a packet (other than a probe packet) with a
size larger than the current PLPMTU at the network layer. To avoid
this, a PL MAY be designed to segment data blocks larger than the MPS
into multiple datagrams.

DPLPMTUD seeks to avoid IP fragmentation. An attempt to send a data
block larger than the MPS will therefore fail if a PL is unable to
segment data. To determine the largest data block that can be sent,
a PL SHOULD provide applications with a primitive that returns the
MPS, derived from the current PLPMTU.

If DPLPMTUD results in a change to the MPS, the application needs to
adapt to the new MPS. A particular case can arise when packets have
been sent with a size less than the MPS and the PLPMTU was
subsequently reduced. If these packets are lost, the PL MAY segment
the data using the new MPS. If a PL is unable to re-segment resegment a
previously sent datagram (e.g., [RFC4960]), then the sender either
discards the datagram or could perform retransmission using network-
layer fragmentation to form multiple IP packets not larger than the
PLPMTU. For IPv4, the use of endpoint fragmentation by the sender is
preferred over clearing the DF bit in the IPv4 header. Operational
experience reveals that IP fragmentation can reduce the reliability
of Internet communication [I-D.ietf-intarea-frag-fragile], [RFC8900], which may reduce the probability
of successful retransmission.

4.5. Disabling the Effect of PMTUD

A PL implementing this specification MUST suspend network layer
processing of outgoing packets that enforces a PMTU
[RFC1191][RFC8201] for each flow utilizing DPLPMTUD, DPLPMTUD and instead use
DPLPMTUD to control the size of packets that are sent by a flow.
This removes the need for the network layer to drop or to fragment
sent packets that have a size greater than the PMTU.

4.6. Response to PTB Messages

This method requires the DPLPMTUD sender to validate any received PTB
message before using the PTB information. The response to a PTB
message depends on the PL_PTB_SIZE calculated from the PTB_SIZE in
the PTB message, the state of the PLPMTUD state machine, and the IP
protocol being used.

Section 4.6.1 first describes validation for both IPv4 ICMP Unreachable
messages (type (Type 3) and ICMPv6 Packet Too Big messages, both of which
are referred to as PTB messages in this document.

4.6.1. Validation of PTB Messages

This section specifies utilization and validation of PTB messages.

* A simple implementation MAY ignore received PTB messages messages, and in
this case case, the PLPMTU is not updated when a PTB message is
received.

* A PL that supports PTB messages MUST validate these messages
before they are further processed.

A PL that receives a PTB message from a router or middlebox performs
ICMP validation (see Section 4 of [RFC8201] and Section 5.2 of
[BCP145]). Because DPLPMTUD operates at the PL, the PL needs to
check that each received PTB message is received in response to a
packet transmitted by the endpoint PL performing DPLPMTUD.

The PL MUST check the protocol information in the quoted packet
carried in an ICMP PTB message payload to validate the message
originated from the sending node. This validation includes
determining that the combination of the IP addresses, the protocol,
the source port port, and destination port match those returned in the
quoted packet - -- this is also necessary for the PTB message to be
passed to the corresponding PL.

The validation SHOULD utilize information that it is not simple for an
off-path attacker to determine [BCP145]. For example, it could check
the value of a protocol header field known only to the two PL
endpoints. A datagram application that uses well-known source and
destination ports ought to also rely on other information to complete
this validation.

These checks are intended to provide protection from packets that
originate from a node that is not on the network path. A PTB message
that does not complete the validation MUST NOT be further utilized by
the DPLPMTUD method, as discussed in the Security Considerations
section.
section (Section 8).

Section 4.6.2 describes this processing of PTB messages.

4.6.2. Use of PTB Messages

PTB messages that have been validated MAY be utilized by the DPLPMTUD
algorithm,
algorithm but MUST NOT be used directly to set the PLPMTU.

Before using the size reported in the PTB message message, it must first be
converted to a PL_PTB_SIZE. The PL_PTB_SIZE is smaller than the
PTB_SIZE because it is reduced by headers below the PL PL, including any
IP options or extensions added to the PL packet.

A method that utilizes these PTB messages can improve the speed at
which the algorithm detects an appropriate PLPMTU by triggering an
immediate probe for the PL_PTB_SIZE (resulting in a network-layer
packet of size PTB_SIZE), compared to one that relies solely on
probing using a timer-based search algorithm.

A set of checks are intended to provide protection from a router that
reports an unexpected PTB_SIZE. The PL also needs to check that the
indicated PL_PTB_SIZE is less than the size used by probe packets and
at least the minimum size accepted.

This section provides a summary of how PTB messages can be utilized.
(This uses utilized,
using the set of constants defined in Section 5.1.2). 5.1.2. This processing
depends on the PL_PTB_SIZE and the current value of a set of
variables:

PL_PTB_SIZE < MIN_PLPMTU
* Invalid PL_PTB_SIZE PL_PTB_SIZE, see Section 4.6.1.

* PTB message ought to be discarded without further processing
(i.e., PLPMTU is not modified).

* The information could be utilized as an input that triggers the
enabling of a resilience mode (see Section 5.3.3).

MIN_PLPMTU < PL_PTB_SIZE < BASE_PLPMTU
* A robust PL MAY enter an error state (see Section 5.2) for an
IPv4 path when the PL_PTB_SIZE reported in the PTB message is
larger than or equal to 68 bytes [RFC0791] and when this is
less than the BASE_PLPMTU.

* A robust PL MAY enter an error state (see Section 5.2) for an
IPv6 path when the PL_PTB_SIZE reported in the PTB message is
larger than or equal to 1280 bytes [RFC8200] and when this is
less than the BASE_PLPMTU.

BASE_PLPMTU <= PL_PTB_SIZE < PLPMTU
* This could be an indication of a black hole. The PLPMTU SHOULD
be set to BASE_PLPMTU (the PLPMTU is reduced to the BASE_PLPMTU
to avoid unnecessary packet loss when a black hole is
encountered).

* The PL ought to start a search to quickly discover the new
PLPMTU. The PL_PTB_SIZE reported in the PTB message can be
used to initialize a search algorithm.

PLPMTU < PL_PTB_SIZE < PROBED_SIZE
* The PLPMTU continues to be valid, but the size of a packet used
to search (PROBED_SIZE) was larger than the actual PMTU.

* The PLPMTU is not updated.

* The PL can use the reported PL_PTB_SIZE from the PTB message as
the next search point when it resumes the search algorithm.

PL_PTB_SIZE >= PROBED_SIZE
* Inconsistent network signal.

* PTB message ought to be discarded without further processing
(i.e., PLPMTU is not modified).

* The information could be utilized as an input to trigger the
enabling of a resilience mode.

5. Datagram Packetization Layer PMTUD

This section specifies Datagram PLPMTUD (DPLPMTUD). The method can
be introduced at various points (as indicated with * in the figure
below) Figure 2) in
the IP protocol stack to discover the PLPMTU so that an application
can utilize an appropriate MPS for the current network path.

DPLPMTUD SHOULD only be performed at one layer between a pair of
endpoints. Therefore, an upper PL or application should avoid using
DPLPMTUD when this is already enabled in a lower layer. A PL MUST
adjust the MPS indicated by DPLPMTUD to account for any additional
overhead introduced by the PL.

+----------------------+
| Application* |
+-----+------------+---+
| |
+---+--+ +--+--+
| QUIC*| |SCTP*|
+---+--+ +-+-+-+
| | |
+---+ +----+ |
| | |
+-+--+-+ |
| UDP | |
+---+--+ |
| |
+-----------+-------+--+
| Network Interface |
+----------------------+

Figure 2: Examples where Where DPLPMTUD can be implemented Can Be Implemented

The central idea of DPLPMTUD is probing by a sender. Probe packets
are sent to find the maximum size of user message that can be
completely transferred across the network path from the sender to the
destination.

The following sections identify the components needed for
implementation, provides provide an overview of the phases of operation, and
specifies
specify the state machine and search algorithm.

5.1. DPLPMTUD Components

This section describes the timers, constants, and variables of
DPLPMTUD.

5.1.1. Timers

The method utilizes up to three timers:

PROBE_TIMER: The PROBE_TIMER is configured to expire after a period
longer than the maximum time to receive an acknowledgment to a
probe packet. This value MUST NOT be smaller than 1 second, second and
SHOULD be larger than 15 seconds. Guidance on the selection of
the timer value are is provided in Section 3.1.1 of the UDP Usage
Guidelines [BCP145].

PMTU_RAISE_TIMER: The PMTU_RAISE_TIMER is configured to the period a
sender will continue to use the current PLPMTU, after which it re-
enters
reenters the Search phase. Phase. This timer has a period of 600
seconds, as recommended by PLPMTUD [RFC4821].

DPLPMTUD MAY inhibit sending probe packets when no application
data has been sent since the previous probe packet. A PL
preferring to use an up-to-date PMTU once user data is sent again, again
can choose to continue PMTU discovery for each path. However,
this will result in sending additional packets.

CONFIRMATION_TIMER: When an acknowledged PL is used, this timer MUST
NOT be used. For other PLs, the CONFIRMATION_TIMER is configured
to the period a PL sender waits before confirming the current
PLPMTU is still supported. This is less than the PMTU_RAISE_TIMER
and used to decrease the PLPMTU (e.g., when a black hole is
encountered). Confirmation needs to be frequent enough when data
is flowing that the sending PL does not black hole extensive
amounts of traffic. Guidance on selection of the timer value are
provided in Section 3.1.1 of the UDP Usage Guidelines [BCP145].

DPLPMTUD MAY inhibit sending probe packets when no application
data has been sent since the previous probe packet. A PL
preferring to use an up-to-date PMTU once user data is sent again,
can choose to continue PMTU discovery for each path. However,
this could result in sending additional packets.

DPLPMTD

DPLPMTUD specifies various timers, however timers; however, an implementation could
choose to realise realize these timer functions using a single timer.

5.1.2. Constants

The following constants are defined:

MAX_PROBES: The MAX_PROBES is the maximum value of the PROBE_COUNT
counter (see Section 5.1.3). MAX_PROBES represents the limit for
the number of consecutive probe attempts of any size. Search
algorithms benefit from a MAX_PROBES value greater than 1 because
this can provide robustness to isolated packet loss. The default
value of MAX_PROBES is 3.

MIN_PLPMTU: The MIN_PLPMTU is the smallest size of PLPMTU that
DPLPMTUD will attempt to use. An endpoint could need to be configure
the MIN_PLPMTU to provide space for extension headers and other
encapsulations at layers below the PL. This value can be
interface and path dependent. For IPv6, this size is greater than
or equal to the size at the PL that results in an 1280 byte 1280-byte IPv6
packet, as specified in [RFC8200]. For IPv4, this size is greater
than or equal to the size at the PL that results in an 68
byte 68-byte
IPv4 packet. Note: An IPv4 router is required to be able to
forward a datagram of 68 bytes without further fragmentation.
This is the combined size of an IPv4 header and the minimum
fragment size of 8 bytes. In addition, receivers are required to
be able to reassemble fragmented datagrams at least up to 576
bytes, as stated in section Section 3.3.3 of [RFC1122].

MAX_PLPMTU: The MAX_PLPMTU is the largest size of PLPMTU. This has
to be less than or equal to the maximum size of the PL packet that
can be sent on the outgoing interface (constrained by the local
interface MTU). When known, this also ought to be less than the
maximum size of PL packet that can be received by the remote
endpoint (constrained by EMTU_R). It can be limited by the design
or configuration of the PL being used. An application, or PL, MAY
choose a smaller MAX_PLPMTU when there is no need to send packets
larger than a specific size.

BASE_PLPMTU: The BASE_PLPMTU is a configured size expected to work
for most paths. The size is equal to or larger than the
MIN_PLPMTU and smaller than the MAX_PLPMTU. For most PLs PLs, a
suitable BASE_PLPMTU will be larger than 1200 bytes. When using
IPv4, there is no currently equivalent size specified specified, and a
default BASE_PLPMTU of 1200 bytes is RECOMMENDED.

5.1.3. Variables

This method utilizes a set of variables:

PROBED_SIZE: The PROBED_SIZE is the size of the current probe packet
as determined at the PL. This is a tentative value for the
PLPMTU, which is awaiting confirmation by an acknowledgment.

PROBE_COUNT: The PROBE_COUNT is a count of the number of successive
unsuccessful probe packets that have been sent. Each time a probe
packet is acknowledged, the value is set to zero. (Some probe
loss is expected while searching, therefore loss of a single probe
is not an indication of a PMTU problem.)

The figure below

Figure 3 illustrates the relationship between the packet size
constants and variables at a point of time when the DPLPMTUD
algorithm performs path probing to increase the size of the PLPMTU.
A probe packet has been sent of size PROBED_SIZE. Once this is
acknowledged, the PLPMTU will raise to PROBED_SIZE PROBED_SIZE, allowing the
DPLPMTUD algorithm to further increase PROBED_SIZE toward sending a
probe with the size of the actual PMTU.

Figure 3: Relationships between packet size constants Packet Size Constants and variables Variables

5.1.4. Overview of DPLPMTUD Phases

This section provides a high-level high-level, informative view of the DPLPMTUD
method, by describing the movement of the method through several
phases of operation. More detail is available in the state machine machine,
Section 5.2.

+------+
+------->| Base |-----------------+ Connectivity
| +------+ | or BASE_PLPMTU
| | | confirmation failed
| | v
| | Connectivity +-------+
| | and BASE_PLPMTU | Error |
| | confirmed +-------+
| | | Consistent
| v | connectivity
Black Hole | +--------+ | and BASE_PLPMTU
detected | | Search |<---------------+ confirmed
| +--------+
| ^ |
| | |
| Raise | | Search
| timer | | algorithm
| expired | | completed
| | |
| | v
| +-----------------+
+---| Search Complete |
+-----------------+

Figure 4: DPLPMTUD Phases

Base: The Base Phase confirms connectivity to the remote peer using
packets of the BASE_PLPMTU. The confirmation of connectivity is
implicit for a connection-oriented PL (where it can be performed
in a PL connection handshake). A connectionless PL sends a probe
packet and uses acknowledgment of this probe packet to confirm
that the remote peer is reachable.

The sender also confirms that BASE_PLPMTU is supported across the
network path. This may be achieved by using a PL mechanism (e.g.,
using a handshake packet of size BASE_PLPMTU), BASE_PLPMTU) or by sending a
probe packet of size BASE_PLPMTU and confirming that this is
received.

A probe packet of size BASE_PLPMTU can be sent immediately on the
initial entry to the Base Phase (following a connectivity check).
A PL that does not wish to support a path with a PLPMTU less than
BASE_PLPMTU can simplify the phase into a single step by
performing the connectivity checks with a probe of the BASE_PLPMTU
size.

Once confirmed, DPLPMTUD enters the Search Phase. If the Base
Phase fails to confirm the BASE_PLPMTU, DPLPMTUD enters the Error
Phase.

Search: The Search Phase utilizes a search algorithm to send probe
packets to seek to increase the PLPMTU. The algorithm concludes
when it has found a suitable PLPMTU, PLPMTU by entering the Search
Complete Phase.

A PL could respond to PTB messages using the PTB to advance or
terminate the search, see Section 4.6.

Search Complete: The Search Complete Phase is entered when the
PLPMTU is supported across the network path. A PL can use a
CONFIRMATION_TIMER to periodically repeat a probe packet for the
current PLPMTU size. If the sender is unable to confirm
reachability (e.g., if the CONFIRMATION_TIMER expires) or the PL
signals a lack of reachability, a black hole has been detected and
DPLPMTUD enters the Base phase. Phase.

The PMTU_RAISE_TIMER is used to periodically resume the search
phase Search
Phase to discover if the PLPMTU can be raised. Black Hole
Detection hole
detection causes the sender to enter the Base Phase.

Error: The Error Phase is entered when there is conflicting or
invalid PLPMTU information for the path (e.g., a failure to
support the BASE_PLPMTU) that cause causes DPLPMTUD to be unable to
progress
progress, and the PLPMTU is lowered.

DPLPMTUD remains in the Error Phase until a consistent view of the
path can be discovered and it has also been confirmed that the
path supports the BASE_PLPMTU (or DPLPMTUD is suspended).

A method that only reduces the PLPMTU to a suitable size would be
sufficient to ensure reliable operation, operation but can be very inefficient
when the actual PMTU changes or when the method (for whatever reason)
makes a suboptimal choice for the PLPMTU.

A full implementation of DPLPMTUD provides an algorithm enabling the
DPLPMTUD sender to 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 there is a change in the set of links traversed
by an end-to-end flow (e.g., after a routing or path fail-over failover
decision).

5.2. State Machine

A state machine for DPLPMTUD is depicted in Figure 5. If multipath
or multihoming is supported, a state machine is needed for each path.

Note: Not all changes are shown to simplify the diagram.

| |
| Start | PL indicates loss
| | of connectivity
v v
+---------------+ +---------------+
| DISABLED | | ERROR |
+---------------+ PROBE_TIMER expiry: +---------------+
| PL indicates PROBE_COUNT = MAX_PROBES or ^ |
| connectivity PTB: PL_PTB_SIZE < BASE_PLPMTU | |
+--------------------+ +---------------+ +------------------+ |
| | |
v | BASE_PLPMTU Probe |
+---------------+ acked |
| BASE |--------------------->+
+---------------+ |
^ | ^ ^ |
Black hole detected | | | | Black hole detected |
+--------------------+ | | +--------------------+ |
| +----+ | |
| PROBE_TIMER expiry: | |
| PROBE_COUNT < MAX_PROBES | |
| | |
| PMTU_RAISE_TIMER expiry | |
| +-----------------------------------------+ | |
| | | | |
| | v | v
+---------------+ +---------------+
|SEARCH_COMPLETE| | SEARCHING |
+---------------+ +---------------+
| ^ ^ | | ^
| | | | | |
| | +-----------------------------------------+ | |
| | MAX_PLPMTU Probe acked or | |
| | PROBE_TIMER expiry: PROBE_COUNT = MAX_PROBES or | |
+----+ PTB: PL_PTB_SIZE = PLPMTU +----+
CONFIRMATION_TIMER expiry: PROBE_TIMER expiry:
PROBE_COUNT < MAX_PROBES or PROBE_COUNT < MAX_PROBES or
PLPMTU Probe acked Probe acked or PTB:
PLPMTU < PL_PTB_SIZE < PROBED_SIZE

Figure 5: State machine Machine for Datagram PLPMTUD

The following states are defined:

DISABLED: The DISABLED state is the initial state before probing has
started. It is also entered from any other state, when the PL
indicates loss of connectivity. This state is left once the PL
indicates connectivity to the remote PL. When transitioning to
the BASE state, a probe packet of size BASE_PLPMTU can be sent
immediately.

BASE: The BASE state is used to confirm that the BASE_PLPMTU size is
supported by the network path and is designed to allow an
application to continue working when there are transient
reductions in the actual PMTU. It also seeks to avoid long
periods when a sender searching for a larger PLPMTU is unaware
that packets are not being delivered due to a packet or ICMP Black
Hole. black
hole.

On entry, the PROBED_SIZE is set to the BASE_PLPMTU size size, and the
PROBE_COUNT is set to zero.

Each time a probe packet is sent, the PROBE_TIMER is started. The
state is exited when the probe packet is acknowledged, and the PL
sender enters the SEARCHING state.

The state is also left when the PROBE_COUNT reaches MAX_PROBES or
a received PTB message is validated. This causes the PL sender to
enter the ERROR state.

SEARCHING: The SEARCHING state is the main probing state. This
state is entered when probing for the BASE_PLPMTU completes.

Each time a probe packet is acknowledged, the PROBE_COUNT is set
to zero, the PLPMTU is set to the PROBED_SIZE PROBED_SIZE, and then the
PROBED_SIZE is increased using the search algorithm (as described
in Section 5.3. 5.3).

When a probe packet is sent and not acknowledged within the period
of the PROBE_TIMER, the PROBE_COUNT is incremented incremented, and a new
probe packet is transmitted.

The state is exited to enter SEARCH_COMPLETE when the PROBE_COUNT
reaches MAX_PROBES, a validated PTB is received that corresponds
to the last successfully probed size (PL_PTB_SIZE = PLPMTU), or a
probe of size MAX_PLPMTU is acknowledged (PLPMTU = MAX_PLPMTU).

When a black hole is detected in the SEARCHING state, this causes
the PL sender to enter the BASE state.

SEARCH_COMPLETE: The SEARCH_COMPLETE state indicates that a search
has completed. This is the normal maintenance state, where the PL
is not probing to update the PLPMTU. DPLPMTUD remains in this
state until either the PMTU_RAISE_TIMER expires or a black hole is
detected.

When DPLPMTUD uses an unacknowledged PL and is in the
SEARCH_COMPLETE state, a CONFIRMATION_TIMER periodically resets
the PROBE_COUNT and schedules a probe packet with the size of the
PLPMTU. If MAX_PROBES successive PLPMTUD sized PLPMTUD-sized probes fail to be
acknowledged
acknowledged, the method enters the BASE state. When used with an
acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to
generate PLPMTU probes in this state.

ERROR: The ERROR state represents the case where either the network
path is not known to support a PLPMTU of at least the BASE_PLPMTU
size or when there is contradictory information about the network
path that would otherwise result in excessive variation in the MPS
signaled to the higher layer. The state implements a method to
mitigate oscillation in the state-event engine. It signals a
conservative value of the MPS to the higher layer by the PL. The
state is exited when packet probes no longer detect the error.
The PL sender then enters the SEARCHING state.

Implementations are permitted to enable endpoint fragmentation if
the DPLPMTUD is unable to validate MIN_PLPMTU within PROBE_COUNT
probes. If DPLPMTUD is unable to validate MIN_PLPMTU MIN_PLPMTU, the
implementation will transition to the DISABLED state.

Note: MIN_PLPMTU could be identical to BASE_PLPMTU, simplifying
the actions in this state.

5.3. Search to Increase the PLPMTU

This section describes the algorithms used by DPLPMTUD to search for
a larger PLPMTU.

5.3.1. Probing for a larger Larger PLPMTU

Implementations use a search algorithm across the search range to
determine whether a larger PLPMTU can be supported across a network
path.

The method discovers the search range by confirming the minimum
PLPMTU and then using the probe method to select a PROBED_SIZE less
than or equal to MAX_PLPMTU. MAX_PLPMTU is the minimum of the local
MTU and EMTU_R (when this is learned from the remote endpoint). The
MAX_PLPMTU MAY be reduced by an application that sets a maximum to
the size of datagrams it will send.

The PROBE_COUNT is initialized to zero when the first probe with a
size greater than or equal to PLPMTUD PLPMTU is sent. Each probe packet
successfully sent to the remote peer is confirmed by acknowledgment
at the PL, see PL (see Section 4.1. 4.1).

Each time a probe packet is sent to the destination, the PROBE_TIMER
is started. The timer is canceled when the PL receives
acknowledgment that the probe packet has been successfully sent
across the path Section 4.1. (Section 4.1). This confirms that the PROBED_SIZE is
supported, and the PROBED_SIZE value is then assigned to the PLPMTU.
The search algorithm can continue to send subsequent probe packets of
an increasing size.

If the timer expires before a probe packet is acknowledged, the probe
has failed to confirm the PROBED_SIZE. Each time the PROBE_TIMER
expires, the PROBE_COUNT is incremented, the PROBE_TIMER is
reinitialized, and a new probe of the same size or any other size
(determined by the search algorithm) can be sent. The maximum number
of consecutive failed probes is configured (MAX_PROBES). If the
value of the PROBE_COUNT reaches MAX_PROBES, probing will stop, and
the PL sender enters the SEARCH_COMPLETE state.

5.3.2. Selection of Probe Sizes

The search algorithm determines a minimum useful gain in PLPMTU. It
would not be constructive for a PL sender to attempt to probe for all
sizes. This would incur unnecessary load on the path.
Implementations SHOULD select the set of probe packet sizes to
maximize the gain in PLPMTU from each search step.

Implementations could optimize the search procedure by selecting step
sizes from a table of common PMTU sizes. When selecting the
appropriate next size to search, an implementer ought to also
consider that there can be common sizes of MPS that applications seek
to use, and their there could be common sizes of MTU used within the
network.

5.3.3. Resilience to Inconsistent Path Information

A decision to increase the PLPMTU needs to be resilient to the
possibility that information learned about the network path is
inconsistent. A path is inconsistent when, for example, probe
packets are lost due to other reasons (i.e., not packet size) or due
to frequent path changes. Frequent path changes could occur by
unexpected "flapping" - -- where some packets from a flow pass along
one path, but other packets follow a different path with different
properties.

A PL sender is able to detect inconsistency either from the sequence
of PLPMTU probes that are acknowledged or from the sequence of PTB
messages that it receives. When inconsistent path information is
detected, a PL sender could use an alternate search mode that clamps
the offered MPS to a smaller value for a period of time. This avoids
unnecessary loss of packets.

5.4. Robustness to Inconsistent Paths

Some paths could be unable to sustain packets of the BASE_PLPMTU
size. The Error State could be implemented to provide rubustness robustness to
such paths. This allows fallback to a smaller than desired PLPMTU, PLPMTU
rather than suffer connectivity failure. This could utilize methods
such as endpoint IP fragmentation to enable the PL sender to
communicate using packets smaller than the BASE_PLPMTU.

6. Specification of Protocol-Specific Methods

DPLPMTUD requires protocol-specific details to be specified for each
PL that is used.

The first subsection provides guidance on how to implement the
DPLPMTUD method as a part of an application using UDP or UDP-Lite.
The guidance also applies to other datagram services that do not
include a specific transport protocol (such as a tunnel
encapsulation). The following subsections describe how DPLPMTUD can
be implemented as a part of the transport service, allowing
applications using the service to benefit from discovery of the
PLPMTU without themselves needing to implement this method when using
SCTP and QUIC.

6.1. Application support Support for DPLPMTUD with UDP or UDP-Lite

The current specifications of UDP [RFC0768] and UDP-Lite [RFC3828] do
not define a method in the RFC-series RFC series that supports PLPMTUD. In
particular, the UDP transport does not provide the transport features
needed to implement datagram PLPMTUD.

The DPLPMTUD method can be implemented as a part of an application
built directly or indirectly on UDP or UDP-Lite, UDP-Lite but relies on
higher-layer higher-
layer protocol features to implement the method [BCP145].

Some primitives used by DPLPMTUD might not be available via the
Datagram API (e.g., the ability to access the PLPMTU from the IP IP-
layer cache, cache or to interpret received PTB messages).

In addition, it is recommended that PMTU discovery is not performed
by multiple protocol layers. An application SHOULD avoid using
DPLPMTUD when the underlying transport system provides this
capability. A common method for managing the PLPMTU has benefits,
both in the ability to share state between different processes and in
opportunities to coordinate probing for different PL instances.

6.1.1. Application Request

An application needs an application-layer protocol mechanism (such as
a message acknowledgment method) that solicits a response from a
destination endpoint. The method SHOULD allow the sender to check
the value returned in the response to provide additional protection
from off-path insertion of data [BCP145]. Suitable methods include a
parameter known only to the two endpoints, such as a session ID or
initialized sequence number.

6.1.2. Application Response

An application needs an application-layer protocol mechanism to
communicate the response from the destination endpoint. This
response could indicate successful reception of the probe across the
path,
path but could also indicate that some (or all packets) have failed
to reach the destination.

6.1.3. Sending Application Probe Packets

A probe packet can carry an application data block, but the
successful transmission of this data is at risk when used for
probing. Some applications might prefer to use a probe packet that
does not carry an application data block to avoid disruption to of data
transfer.

6.1.4. Initial Connectivity

An application that does not have other higher-layer information
confirming connectivity with the remote peer SHOULD implement a
connectivity mechanism using acknowledged probe packets before
entering the BASE state.

6.1.5. Validating the Path

An application that does not have other higher-layer information
confirming correct delivery of datagrams SHOULD implement the
CONFIRMATION_TIMER to periodically send probe packets while in the
SEARCH_COMPLETE state.

6.1.6. Handling of PTB Messages

An application that is able and wishes to receive PTB messages MUST
perform ICMP validation as specified in Section 5.2 of [BCP145].
This requires that the application checks each received PTB message
to validate that it was is received in response to transmitted
traffic and that the reported PL_PTB_SIZE is less than the current
probed size (see Section 4.6.2). A validated PTB message MAY be used
as input to the DPLPMTUD algorithm, algorithm but MUST NOT be used directly to
set the PLPMTU.

6.2. DPLPMTUD for SCTP

Section 10.2 of [RFC4821] specified specifies a recommended PLPMTUD probing
method for SCTP SCTP, and Section 7.3 of [RFC4960] recommended recommends an endpoint
apply the techniques in RFC4821 RFC 4821 on a per-destination-address basis.
The specification for DPLPMTUD continues the practice of using the PL
to discover the PMTU, PMTU but updates, updates RFC4960 with a recommendation to use
the method specified in this document: The RECOMMENDED method for
generating probes is to add a chunk consisting only of padding to an
SCTP message. The PAD chunk defined in [RFC4820] SHOULD be attached
to a minimum length minimum-length HEARTBEAT (HB) chunk to build a probe packet.
This enables probing without affecting the transfer of user messages
and without being limited by congestion control or flow control.
This is preferred to using DATA chunks (with padding as required) as
path probes.

Section 6.9 of [RFC4960] describes dividing the user messages into
data
DATA chunks sent by the PL when using SCTP. This notes that once an
SCTP message has been sent, it cannot be re-segmented. resegmented. [RFC4960]
describes the method to retransmit data for retransmitting DATA chunks when the MPS has
been reduced, and the use Section 6.9 of [RFC4960] describes use of IP
fragmentation for this case. This is unchanged by this document.

6.2.1. SCTP/IPv4 and SCTP/IPv6

6.2.1.1. Initial Connectivity

The base protocol is specified in [RFC4960]. This provides an
acknowledged PL. A sender can therefore enter the BASE state as soon
as connectivity has been confirmed.

6.2.1.2. Sending SCTP Probe Packets

Probe packets consist of an SCTP common header followed by a
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
trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful probe. A
successful probe updates the association and path counters, but an
unsuccessful probe is discounted (assumed to be a result of choosing
too large a PLPMTU).

The SCTP sender needs to be able to determine the total size of a
probe packet. The HEARTBEAT chunk could carry a Heartbeat
Information parameter that includes, besides the information
suggested in [RFC4960], the probe size to help an implementation
associate a HEARTBEAT-ACK HEARTBEAT ACK with the size of probe that was sent. The
sender could also use other methods, such as sending a nonce and
verifying the information returned also contains the corresponding
nonce. The length of the PAD chunk is computed by reducing the
probing size by the size of the SCTP common header and the HEARTBEAT
chunk. The payload of the PAD chunk contains arbitrary data. When
transmitted at the IP layer, the PMTU size also includes the IPv4 or
IPv6 header(s).

Probing can start directly after the PL handshake, handshake; this can be done
before data is sent. Assuming this behavior (i.e., the PMTU is
smaller than or equal to the interface MTU), this process will take
several round trip round-trip time periods, dependent on the number of DPLPMTUD
probes sent. The Heartbeat timer can be used to implement the
PROBE_TIMER.

6.2.1.3. Validating the Path with SCTP

Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.

6.2.1.4. PTB Message Handling by SCTP

Normal ICMP validation MUST be performed as specified in Appendix C
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
be the case for ICMPv4 and is normally the case for ICMPv6.

When a PTB message has been validated, the PL_PTB_SIZE calculated
from the PTB_SIZE reported in the PTB message SHOULD be used with the
DPLPMTUD algorithm, providing provided that the reported PL_PTB_SIZE is less
than the current probe size (see Section 4.6).

6.2.2. DPLPMTUD for SCTP/UDP

The UDP encapsulation of SCTP is specified in [RFC6951].

This specification updates the reference to RFC 4821 in section Section 5.6
of RFC 6951 to refer to XXXTHISRFCXXX. this document (RFC 8899). RFC 6951 is
updated by the addition of the following sentence at the end of section
Section 5.6: "The

| The RECOMMENDED method for determining the MTU of the path is
| specified in XXXTHISRFCXXX".

XXX RFC EDITOR - please replace XXXTHISRFCXXX when published XXX 8899.

6.2.2.1. Initial Connectivity

A sender can enter the BASE state as soon as SCTP connectivity has
been confirmed.

6.2.2.2. Sending SCTP/UDP Probe Packets

Packet probing can be performed as specified in Section 6.2.1.2. The
size of the probe packet includes the 8 bytes of UDP Header. header. This
has to be considered when filling the probe packet with the PAD
chunk.

6.2.2.3. Validating the Path with SCTP/UDP

SCTP provides an acknowledged PL, therefore PL; therefore, a sender does not
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.

6.2.2.4. Handling of PTB Messages by SCTP/UDP

ICMP validation MUST be performed for PTB messages as specified in
Appendix C of [RFC4960]. This requires that the first 8 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 consumes a part of the
quoted packet header) and is normally the case for ICMPv6. When the
validation is completed, the PL_PTB_SIZE calculated from the PTB_SIZE
in the PTB message SHOULD be used with the DPLPMTUD providing that
the reported PL_PTB_SIZE is less than the current probe size.

6.2.3. DPLPMTUD for SCTP/DTLS

The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
specified in [RFC8261]. This is used for data channels in WebRTC
implementations. This specification updates the reference to RFC
4821 in section Section 5 of RFC 8261 to refer to XXXTHISRFCXXX.

XXX RFC EDITOR - please replace XXXTHISRFCXXX when published XXX this document (RFC 8899).

6.2.3.1. Initial Connectivity

A sender can enter the BASE state as soon as SCTP connectivity has
been confirmed.

6.2.3.2. Sending SCTP/DTLS Probe Packets

Packet probing can be done, done as specified in Section 6.2.1.2. The
maximum payload is reduced by the size of the DTLS headers, which has
to be considered when filling the PAD chunk. The size of the probe
packet includes the DTLS PL headers. This has to be considered when
filling the probe packet with the PAD chunk.

6.2.3.3. Validating the Path with SCTP/DTLS

Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.

6.2.3.4. Handling of PTB Messages by SCTP/DTLS

[RFC4960] does not specify a way to validate SCTP/DTLS ICMP message
payload and neither does this document. This can prevent processing
of PTB messages at the PL.

6.3. DPLPMTUD for QUIC

QUIC [I-D.ietf-quic-transport] [QUIC] is a UDP-based PL that provides reception feedback. The
UDP payload includes a QUIC packet header, a protected payload, and
any authentication fields. It supports padding and packet
coalescence that can be used to construct probe packets. From the
perspective of DPLPMTUD, QUIC can function as an acknowledged PL. [I-D.ietf-quic-transport]
[QUIC] describes the method for using DPLPMTUD with QUIC packets.

7. Acknowledgments

This work was partially funded by the European Union's Horizon 2020
research and innovation programme under grant agreement No. 644334
(NEAT). The views expressed are solely those of the author(s).

Thanks to all that have commented or contributed, the TSVWG and QUIC
working groups, and Mathew Calder and Julius Flohr for providing
early implementations.

8. IANA Considerations

This memo includes no request to IANA.

If there are document has no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor.

9. IANA actions.

8. Security Considerations

The security considerations for the use of UDP and SCTP are provided
in the referenced RFCs.

To avoid excessive load, the interval between individual probe
packets MUST be at least one RTT, and the interval between rounds of
probing is determined by the PMTU_RAISE_TIMER.

A PL sender needs to ensure that the method used to confirm reception
of probe packets protects from off-path attackers injecting packets
into the path. This protection is provided in IETF-defined protocols
(e.g., TCP, SCTP) using a randomly-initialized randomly initialized sequence number. A
description of one way to do this when using UDP is provided in
section
Section 5.1 of [BCP145]).

There are cases where ICMP Packet Too Big (PTB) messages are not
delivered due to policy, configuration configuration, or equipment design (see
Section 1.1). This method therefore does not rely upon PTB messages
being received, received but is able to utilize these when they are received by
the sender. PTB messages could potentially be used to cause a node
to inappropriately reduce the PLPMTU. A node supporting DPLPMTUD
MUST therefore appropriately validate the payload of PTB messages to
ensure these are received in response to transmitted traffic (i.e., a
reported error condition that corresponds to a datagram actually sent
by the path layer, see Section 4.6.1).

An on-path attacker able to create a PTB message could forge PTB
messages that include a valid quoted IP packet. Such an attack could
be used to drive down the PLPMTU. An on-path device could similarly
force a reduction of the PLPMTU by implementing a policy that drops
packets larger than a configured size. There are two ways this
method can be mitigated against such attacks: First, first, by ensuring that
a PL sender never reduces the PLPMTU below the base size, size solely in
response to receiving a PTB message. This is achieved by first
entering the BASE state when such a message is received. Second, the
design does not require processing of PTB messages, messages; a PL sender could
therefore suspend processing of PTB messages (e.g., in a robustness
mode after detecting that subsequent probes actually confirm that a
size larger than the PTB_SIZE is supported by a path).

Parsing the quoted packet inside a PTB message can introduce addional
additional per-packet processing at the PL sender. This processing
SHOULD be limited to avoid a denial of service denial-of-service attack when arbitrary
headers are included. Rate-limiting the processing could result in
PTB messages not being received by a PL, however PL; however, the DPLPMTUD method
is robust to such loss.

The successful processing of an ICMP message can trigger a probe when
the reported PTB size is valid, but this does not directly update the
PLPMTU for the path. This prevents a message attempting to black
hole data by indicating a size larger than supported by the path.

It is possible that the information about a path is not stable. This
could be a result of forwarding across more than one path that has a
different actual PMTU or a single path presents a varying PMTU. The
design of a PLPMTUD implementation SHOULD consider how to mitigate
the effects of varying path information. One possible mitigation is
to provide robustness (see Section 5.4) in the method that avoids
oscillation in the MPS.

DPLPMTUD methods can introduce padding data to inflate the length of
the datagram to the total size required for a probe packet. The
total size of a probe packet includes all headers and padding added
to the payload data being sent (e.g., including security-related
fields such as an AEAD tag and TLS record layer padding). The value
of the padding data does not influence the DPLPMTUD search algorithm,
and therefore needs to be set consistent with the policy of the PL.

If a PL can make use of cryptographic confidentiality or data-
integrity mechanisms, then the design ought to avoid adding anything
(e.g., padding) to DPLPMTUD probe packets that is not also protected
by those cryptographic mechanisms.

10.

9. References

10.1.

9.1. Normative References

[BCP145] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, March 2017.

<https://www.rfc-editor.org/info/bcp145> 2017,
<https://www.rfc-editor.org/info/bcp145>.

[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC1191] Mogul, J.C. J. and S.E. S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
and G. Fairhurst, Ed., "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
2004, <https://www.rfc-editor.org/info/rfc3828>.

[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
<https://www.rfc-editor.org/info/rfc4820>.

[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.

[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.

[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<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>.

10.2.

9.2. Informative References

[I-D.ietf-intarea-frag-fragile]
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", Work
in Progress, Internet-Draft, draft-ietf-intarea-frag-
fragile-17, 30 September 2019, <http://www.ietf.org/
internet-drafts/draft-ietf-intarea-frag-fragile-17.txt>.

[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", Work in Progress, Internet-Draft, draft-
ietf-intarea-tunnels-10, 12 September 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-intarea-
tunnels-10.txt>.

[I-D.ietf-quic-transport]

[QUIC] Iyengar, J. J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", Work in Progress,
Internet-Draft,
draft-ietf-quic-transport-27, 21 February draft-ietf-quic-transport-29, June 10,
2020,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-27.txt>. <https://tools.ietf.org/html/draft-ietf-quic-
transport-29>.

[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.

[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>.

[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.

[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.

[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890,
DOI 10.17487/RFC4890, May 2007,
<https://www.rfc-editor.org/info/rfc4890>.

[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
DOI 10.17487/RFC5508, April 2009,
<https://www.rfc-editor.org/info/rfc5508>.

Appendix A. Revision Notes

Note to RFC-Editor: please remove this entire section prior to
publication.

Individual draft -00:

* Comments

[RFC8900] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and corrections are welcome directly to the authors or
via F. Gont, "IP Fragmentation Considered Fragile",
RFC 8900, BCP 230, September 2020,
<https://www.rfc-editor.org/info/rfc8900>.

[TUNNELS] Touch, J. and M. Townsley, "IP Tunnels in the IETF TSVWG working group mailing list.

* Internet
Architecture", Work in Progress, Internet-Draft, draft-
ietf-intarea-tunnels-10, September 12, 2019,
<https://tools.ietf.org/html/draft-ietf-intarea-tunnels-
10>.

Acknowledgments

This update is proposed work was partially funded by the European Union Horizon 2020
Research and Innovation Programme under grant agreement No. 644334,
"A New, Evolutive API and Transport-Layer Architecture for WG comments.

Individual draft -01:

* Contains the first representation
Internet" (NEAT). The views expressed are solely those of the algorithm, showing the
states and timers

* This update is proposed for WG comments.

Individual draft -02:

* Contains updated representation of the algorithm, and textual
corrections.

* The text describing when to set the effective PMTU has not yet
been validated by the authors

* To determine security to off-path-attacks: We need to decide
whether a received PTB message SHOULD/MUST be validated? The text
on how
author(s).

Thanks to handle a PTB message indicating a link MTU larger than
the probe has yet not been validated by all who have commented or contributed, the authors

* No text currently describes how to handle inconsistent results
from arbitrary re-routing along different parallel paths
* This update is proposed for WG comments.

Working Group draft -00:

* This draft follows a successful adoption call for TSVWG

* There is still work to complete, please comment on this draft.

Working Group draft -01:

* This draft includes improved introduction.

* The draft is updated to require ICMP validation prior to accepting
PTB messages - this to be confirmed by WG

* Section added to discuss Selection of Probe Size - methods to be
evaluated and recommendations to be considered

* Section added to align with work proposed in the QUIC WG.

Working Group draft -02:

* The draft was updated based on feedback from the WG, and a
detailed review by Magnus Westerlund.

* The document updates RFC 4821.

* Requirements list updated.

* Added more explicit discussion of a simpler black-hole detection
mode.

* This draft includes reorganisation of the section on IETF
protocols.

* Added more discussion of implementation within an application.

* Added text on flapping paths.

* Replaced 'effective MTU' with new term PLPMTU.

Working Group draft -03:

* Updated figures

* Added more discussion on blackhole detection

* Added figure describing just blackhole detection
* Added figure relating MPS sizes

Working Group draft -04:

* Described phases and named these consistently.

* Corrected transition from confirmation directly to the search
phase (Base has been checked).

* Redrawn state diagrams.

* Renamed BASE_MTU to BASE_PMTU (because it is a base for the PMTU).

* Clarified Error state.

* Clarified suspending DPLPMTUD.

* Verified normative text in requirements section.

* Removed duplicate text.

* Changed all text to refer to /packet probe/probe packet/
/validation/verification/ added term /Probe Confirmation/
working groups, and
clarified BlackHole detection.

Working Group draft -05:

* Updated security considerations.

* Feedback after speaking with Joe Touch helped improve UDP-Options
description.

Working Group draft -06:

* Updated description of ICMP issues in section 1.1

* Update to description of QUIC.

Working group draft -07:

* Moved description of the PTB processing method from the PTB
requirements section.

* Clarified what is performed in the PTB validation check.

* Updated security consideration to explain PTB security without
needing to read the rest of the document.

* Reformatted state machine diagram

Working group draft -08:

* Moved to rfcxml v3+

* Rendered diagrams to svg in html version.

* Removed Appendix A. Event-driven state changes.

* Removed section on DPLPMTUD with UDP Options.

* Shortened the description of phases.

Working group draft -09:

* Remove final mention of UDP Options

* Add Initial Connectivity sections to each PL

* Add to disable outgoing pmtu enforcement of packets

Working group draft -10:

* Address comments from Lars Eggert

* Reinforce that PROBE_COUNT is successive attempts to probe for any
size

* Redefine MAX_PROBES to 3

* Address PTB_SIZE of 0 or less that MIN_PLPMTU

Working group draft -11:

* Restore a sentence removed in previous rev

* De-acronymise QUIC

* Address some nits

Working group draft -12:

* Add TSVWG, QUIC Mathew Calder and implementers to acknowledgments

* Shorten a diagram line.

* Address nits from Julius and Wes.

* Be clearer when talking about IP layer caches

Working group draft -13, -14:

* Updated after WGLC.

Working group draft -15:

* Updated after AD evaluation and prepared for IETF-LC.

Working group draft -16:

* Updated text after SECDIR review.

Working group draft -17:

* Updated text after GENART and IETF-LC.

* Renamed BASE_MTU to BASE_PLPMTU, and MIN and MAX PMTU to PLPMTU
(because these are about a base for the PLPMTU), and ensured
consistent separation of PMTU and PLPMTU.

* Adopted US-style English throughout.

Working group draft -18:

* Updated text and address nits from OPSDIR, ART and IESG reviews.

* Order PTB processing based on PL_PTB_SIZE

Working group draft -19:

* Updated text and address nits based on comments from Tim Chown and
Murray S. Kucherawy.

Working group draft -20:

* Address nits and comments from IESG

* Refer to BCP 145 rather than RFC 8085 in most places.

* Update probing method text for SCTP and QUIC.

Working group draft -21:

* Update QUIC text Flohr for skipping into BASE state.

Working group draft -22:

* Add a section reference to MPS

* Clarify MIN_PLPMTU text

* Remove most QUIC text

* Make QUIC reference informative. providing
early implementations.

Authors' Addresses

Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom

Email: gorry@erg.abdn.ac.uk

Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom

Email: tom@erg.abdn.ac.uk

Michael Tuexen
Muenster Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany

Email: tuexen@fh-muenster.de

Irene Ruengeler
Muenster Rüngeler
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany

Email: i.ruengeler@fh-muenster.de

Timo Voelker
Muenster Völker
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany

Email: timo.voelker@fh-muenster.de