wdiff draft-ietf-tsvwg-datagram-plpmtud-07.txt draft-ietf-tsvwg-datagram-plpmtud-08.txt

Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Updates: 4821
Updates4821 (if approved) University of Aberdeen
Intended status: Standards Track M. Tuexen
Expires: August 22, 7 December 2019 I. Ruengeler
T. Voelker
Muenster University of Applied Sciences
February 18,
5 June 2019

Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-07
draft-ietf-tsvwg-datagram-plpmtud-08

Abstract

This document describes a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization Layers (PLs). The document 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 network
black hole (where packets are discarded, and no ICMP message
is received). discarded). The method can also probe a
network path with progressively larger packets to find discover whether
the maximum packet size can be increased. This allows a sender to
determine an appropriate packet size, providing functionally for
datagram transports that is equivalent to the Packetization Layer
PMTUD specification for TCP, specified in RFC 4821.

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

When published, this specification updates RFC 4821.

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

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents 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 August 22, 7 December 2019.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Please review these documents carefully, as they describe your rights
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
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 . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 9 10
4. DPLPMTUD Mechanisms . . . . . . . . . . . . . . . . . . . . . 12
4.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . 12
4.2. Confirmation of Probed Packet Size . . . . . . . . . . . 13 14
4.3. Detection of Unsupported PLPMTU Size, aka Black Holes Hole
Detection . . . . . . . . . . . . . . . . . . . . . . . . 14
4.4. Response to PTB Messages . . . . . . . . . . . . . . . . 15
4.4.1. Validation of PTB Messages . . . . . . . . . . . . . 15
4.4.2. Use of PTB Messages . . . . . . . . . . . . . . . . . 16
5. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 17
5.1. DPLPMTUD Components . . . . . . . . . . . . . . . . . . . 18
5.1.1. Timers . . . . . . . . . . . . . . . . . . . . . . . 18
5.1.2. Constants . . . . . . . . . . . . . . . . . . . . . . 19
5.1.3. Variables . . . . . . . . . . . . . . . . . . . . . . 19
5.2. 20
5.1.4. Overview of DPLPMTUD Phases . . . . . . . . . . . . . 21
5.2. State Machine . . . . . . . . . . 20
5.2.1. BASE_PMTU Confirmation Phase . . . . . . . . . . . . 22
5.2.2. 23
5.3. Search Phase to Increase the PLPMTU . . . . . . . . . . . . . . 26
5.3.1. Probing for a larger PLPMTU . . . . . . 22
5.2.2.1. Resilience to Inconsistent Path Information . . . 22
5.2.3. Search Complete Phase . . . . 26
5.3.2. Selection of Probe Sizes . . . . . . . . . . . . 23
5.2.4. PROBE_BASE Phase . . 27
5.3.3. Resilience to Inconsistent Path Information . . . . . 27
5.4. Robustness to Inconsistent Paths . . . . . . . . . . . 23
5.2.5. ERROR Phase . 28
6. Specification of Protocol-Specific Methods . . . . . . . . . 28
6.1. Application support for DPLPMTUD with UDP or
UDP-Lite . . . . . . . . . . . 24
5.2.5.1. Robustness to Inconsistent Path . . . . . . . . . 24

5.2.6. DISABLED Phase . . . . 28
6.1.1. Application Request . . . . . . . . . . . . . . . 24
5.3. State Machine . . 29
6.1.2. Application Response . . . . . . . . . . . . . . . . 29
6.1.3. Sending Application Probe Packets . . . . 24
5.4. Search to Increase the PLPMTU . . . . . . 29
6.1.4. Validating the Path . . . . . . . . 27
5.4.1. Probing for a Larger PLPMTU . . . . . . . . . 29
6.1.5. Handling of PTB Messages . . . . 27
5.4.2. Selection of Probe Sizes . . . . . . . . . . 29
6.2. DPLPMTUD for SCTP . . . . 28
5.4.3. Resilience to Inconsistent Path Information . . . . . 28
6. Specification of Protocol-Specific Methods . . . . . . . . . 28
6.1. Application support for DPLPMTUD with UDP or UDP-Lite . . 29
6.1.1. Application Request 30
6.2.1. SCTP/IPv4 and SCTP/IPv6 . . . . . . . . . . . . . . . 30
6.2.2. DPLPMTUD for SCTP/UDP . . 29
6.1.2. Application Response . . . . . . . . . . . . . . 31
6.2.3. DPLPMTUD for SCTP/DTLS . . 29
6.1.3. Sending Application Probe Packets . . . . . . . . . . 30
6.1.4. Validating the Path . . . 31
6.3. DPLPMTUD for QUIC . . . . . . . . . . . . . . 30
6.1.5. Handling of PTB Messages . . . . . . 32
6.3.1. Sending QUIC Probe Packets . . . . . . . . 30
6.2. DPLPMTUD with UDP Options . . . . . 32
6.3.2. Validating the Path with QUIC . . . . . . . . . . . 30
6.2.1. UDP Probe Request Option . 33
6.3.3. Handling of PTB Messages by QUIC . . . . . . . . . . . . . 32
6.2.2. UDP Probe Response Option 33
6.4. DPLPMTUD for UDP-Options . . . . . . . . . . . . . . 32
6.3. DPLPMTUD for SCTP . . . 33
7. Acknowledgements . . . . . . . . . . . . . . . . . 33
6.3.1. SCTP/IPv4 and SCTP/IPv6 . . . . . 33
8. IANA Considerations . . . . . . . . . . 33
6.3.1.1. Sending SCTP Probe Packets . . . . . . . . . . . 33
6.3.1.2. Validating the Path with SCTP .
9. Security Considerations . . . . . . . . . 34
6.3.1.3. PTB Message Handling by SCTP . . . . . . . . . . 34
6.3.2. DPLPMTUD for SCTP/UDP 33
10. References . . . . . . . . . . . . . . . . 34
6.3.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . 34
6.3.2.2. Validating the Path with SCTP/UDP . . . . . . .
10.1. Normative References . 34
6.3.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . 34
6.3.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . 34
10.2. Informative References . . . . 34
6.3.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 35
6.3.3.2. Validating the Path with SCTP/DTLS . . . . 36
Appendix A. Revision Notes . . . 35
6.3.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 35
6.4. DPLPMTUD for QUIC . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . 35
6.4.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 35
6.4.2. Validating 40

1. Introduction

The IETF has specified datagram transport using UDP, SCTP, and DCCP,
as well as protocols layered on top of these transports (e.g., SCTP/
UDP, DCCP/UDP, QUIC/UDP), and direct datagram transport over the IP
network layer. This document describes a robust method for Path MTU
Discovery (PMTUD) that may be used with QUIC . . . . . . . . . . . . 36
6.4.3. Handling 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 Messages by QUIC . . . . . . . . . . 36
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36
9. Security Considerations . . . . . . . . . . . . . . . . . . . 36
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2. Informative References . . . . . . . . . . . . . . . . . 39
Appendix A. Event-driven state changes . . . . . . . . . . . . . 40
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45

1. Introduction

The IETF has specified datagram transport using UDP, SCTP, and DCCP,
as well as protocols layered on top of these transports (e.g., SCTP/
UDP, DCCP/UDP, QUIC/UDP), and direct datagram transport over the IP
network layer. This document describes a robust method for Path MTU
Discovery (PMTUD) that may 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]). The term PTB message
is applied to both IPv4 ICMP Unreachable messages (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 MTU in the PTB message, and a method that from time-to-time
increases the packet size in attempt to discover an increase in the
supported PMTU. 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 are sometimes provided with a primitive to let
them read the Maximum Packet Size (MPS), 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 sent with this size, or larger, are
silently discarded without the sender receiving PTB messages). This
could arise when the PTB messages are not delivered back to the
sender for some reason (see for example [RFC2923]).

Examples where PTB messages are not delivered include:

o The generation of ICMP messages is usually rate limited. This may
result in no PTB messages being sent to the sender (see section
2.4 of [RFC4443])

o ICMP messages are increasingly filtered by middleboxes (including
firewalls) [RFC4890]. A stateful firewall could be configured
with a policy to block incoming ICMP messages, which would prevent
reception of PTB messages to endpoints behind this firewall.

o When the router issuing the ICMP message drops a tunneled packet,
the resulting ICMP message will be directed to the tunnel ingress.
This tunnel endpoint is responsible for forwarding the ICMP
message and also processing the quoted packet within the payload
field to remove the effect of the tunnel, and return a correctly
formatted ICMP message to the sender [I-D.ietf-intarea-tunnels].
Failure to do this results in black-holing.

o Asymmetry in forwarding can result in there being no route back to
the original sender, which would prevent an ICMP message being
delivered to the sender. This can be also be an issue when
policy-based routing is used, Equal Cost Multipath (ECMP) routing
is used, or a middlebox acts as an application load balancer. An
example is where the path towards the server is chosen by ECMP
routing depending on bytes in the IP payload. In this case, when
a packet sent by the server encounters a problem after the ECMP
router, then any resulting ICMP message needs to also be directed
by the ECMP router towards the same server (i.e., ICMP messages
need to follow the same path as the flows to which they
correspond). Failure to do this results in black-holing.

o There are cases where the 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 the packet is dropped by the link, this will not
cause a PTB message to be sent, and result in consequent black-
holing.

Another failure could result if a node that is not on the network
path sends a PTB message that attempts to force the sender to change
the effective PMTU [RFC8201]. A sender can protect itself from
reacting to such messages by utilising 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 validation of the PTB message is not possible include:

o When a router issuing the ICMP message implements RFC792
[RFC0792], it is only required to include the first 64 bits of the
IP payload of the packet within the quoted payload. This may be
insufficient to perform the tunnel processing described in the
previous bullet. There could be insufficient bytes remaining for
the sender to interpret the quoted transport information. The
recommendation in RFC1812 [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 invoking packet as possible
without the ICMPv6 packet exceeding 1280 bytes [RFC4443].)

o The use of tunnels/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.

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

o 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.

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, 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] 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 PMTU
discovery with TCP.

It uses a general strategy where the PL sends probe packets to search
for the largest size of unfragmented datagram that can be sent over a
network path. The probe packets are sent with a progressively 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 no response is received to a probe packet, the
method reduces the probe size. This PLPMTU is used to set the
application MPS.

PLPMTUD introduces flexibility in the implementation of PMTU
discovery. At one extreme, it can be configured to only perform PTB
black hole detection and recovery to increase the robustness of
Classical PMTUD, or at the other extreme, all PTB processing can be
disabled and PLPMTUD can completely replace Classical PMTUD.

PLPMTUD can also include additional consistency checks without
increasing the risk of increased black-holing. For instance,the
information available at the PL, or higher layers, makes PTB message
validation more straight forward.

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 described relies on
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 utilise PTB
messages when these received messages are made available to the PL.

The UDP Usage Guidelines [RFC8085] state "an application SHOULD
either use the Path MTU information provided by the IP layer or
implement Path MTU Discovery (PMTUD)", but does not provide a
mechanism for discovering the largest size of unfragmented datagram
that can be used on a network path. Prior to this document, PLPMTUD
had not been specified for UDP.

Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the
Stream Control Transport Protocol (SCTP). SCTP utilises probe
packets consisting of a minimal sized HEARTBEAT chunk bundled with a
PAD chunk as defined in [RFC4820], but RFC4821 does not provide a
complete specification. The present document provides the details to
complete that 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, and suggests use of
control packets (DCCP-Sync) as path probe packets, because they do
not risk application data loss. The method defined in this
specification could be used with DCCP.

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

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.

Other terminology is directly copied from [RFC4821], and the
definitions in [RFC1122].

Actual PMTU: The 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 Holed: Packets are Black holed when the sender is unaware that
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 and they are silently discarded by
the network, but is not made aware of a change to the path that
resulted in a smaller PLPMTU by ICMP messages).

Classical Path MTU Discovery: Classical PMTUD is a process described
in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
learn the largest size of unfragmented datagram 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.

Effective PMTU: The Effective PMTU is the current estimated value
for PMTU that is used by a PMTUD. This is equivalent to the
PLPMTU derived by PLPMTUD.

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

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

Link: A 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
and tunnels.

Link MTU: The 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, 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.

MAX_PMTU: The MAX_PMTU is the largest size of PLPMTU that DPLPMTUD
will attempt to use.

MPS: The Maximum Packet Size (MPS) is the largest size of
application data block that can be sent across a network path. In
DPLPMTUD this quantity is derived from the PLPMTU by taking into
consideration the size of the lower protocol layer headers.

MIN_PMTU: The MIN_PMTU is the smallest size of PLPMTU that DPLPMTUD
will attempt to use.

Packet: A Packet is the IP header plus the IP payload.

Packetization Layer (PL): The Packetization Layer (PL) is the layer
of the network stack that places data into packets and performs
transport protocol functions.

Path: The 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 MTU
of all the links forming a network path between a source node and
a destination node.

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

PLPMTU: The Packetization Layer PMTU is an estimate of the actual
PMTU provided by the DPLPMTUD algorithm.

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.

3. Features Required to Provide Datagram PLPMTUD

TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
All of the requirements in [RFC4821] also apply to the use of the
technique with a datagram PL. Unlike TCP, some datagram PLs require
additional mechanisms to implement PLPMTUD.

There are eight requirements for performing the datagram PLPMTUD
method described in this specification:

1. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide
information about the maximum size of packet that can be
transmitted by the sender on the local link (the local Link MTU).
It MAY utilize similar information about the receiver when this
is supplied (note this could be less than EMTU_R). This avoids
implementations trying to send probe packets that can not be
transmitted 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
PDUs into the same datagram).

2. PLPMTU: A datagram application using a transport layer not
supporting fragmentation message is REQUIRED to be able applied to choose both IPv4 ICMP Unreachable
messages (type 3) that carry the
size of datagrams sent to 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 network, up
effective MTU to the PLPMTU, or a
smaller value (such reported as the MPS) derived from this. This value is
managed by Link MTU in the DPLPMTUD method. The PLPMTU (specified as PTB
message, and a method that from time-to-time increases the
effective PMTU packet
size in Section 1 of [RFC1191]) is equivalent attempt to the
EMTU_S (specified discover an increase in [RFC1122]).

3. Probe packets: On request, a DPLPMTUD sender is REQUIRED to be
able to transmit a packet larger than the PLMPMTU. This is used
to send a probe packet. In IPv4, a probe packet MUST be supported PMTU. The
packets sent with the Don't Fragment (DF) bit set in the IP header, and
without network layer endpoint fragmentation. In IPv6, a probe
packet is always sent without source fragmentation (as specified
in section 5.4 of [RFC8201]).

4. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize
PTB messages received from size larger than the network layer to help identify
when a network path does current effective PMTU are
known as probe packets.

Packets not support the current size of intended as probe
packet. Any received PTB message MUST be validated before it is
used packets are either fragmented to update the PLPMTU discovery information [RFC8201]. This
validation confirms that
current effective PMTU, or the PTB message was sent in response attempt to send fails with an error
code. Applications are sometimes provided with a packet originating by the sender, and needs primitive to be performed
before let
them read the PLPMTU discovery method reacts Maximum Packet Size (MPS), 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 sent with this size, or larger, are
discarded). This could arise when the PTB message. A
PTB message MUST NOT be used messages are not delivered
back to increase the PLPMTU [RFC8201].

5. Reception feedback: sender for some reason (see for example [RFC2923]).

Examples where PTB messages are not delivered include:

* The destination PL endpoint generation of ICMP messages is REQUIRED to
provide a feedback method that indicates usually rate limited. This
could result in no PTB messages being generated to the DPLPMTUD sender
when a probe packet has been received (see
section 2.4 of [RFC4443])

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

* When the possibility
that packets could be significantly delayed along router issuing the ICMP message drops a network path.

The local PL endpoint at tunneled packet,
the sending node is REQUIRED to pass
this feedback resulting ICMP message will be directed to the sender-side DPLPMTUD method.

6. Probe loss recovery: It tunnel ingress.
This tunnel endpoint is RECOMMENDED to use probe packets that
do not carry any user data. Most datagram transports permit
this. If a probe responsible for forwarding the ICMP
message and also processing the quoted packet contains user data requiring
retransmission in case of loss, within the PL (or layers above) are
REQUIRED payload
field to arrange any retransmission/repair remove the effect of any resulting
loss. DPLPMTUD is REQUIRED the tunnel, and return a correctly
formatted ICMP message to be robust the sender [I-D.ietf-intarea-tunnels].
Failure to do this prevents the PTB message reaching the original
sender.

* Asymmetry in forwarding can result in there being no return route
to the case where probe
packets are lost due original sender, which would prevent an ICMP message being
delivered to other reasons (including link
transmission error, congestion).

7. Probing and congestion control: The DPLPMTUD sender treats
isolated loss of a probe packet (with the sender. This issue can also arise when policy-
based routing is used, Equal Cost Multipath (ECMP) routing is
used, or without a corresponding
PTB message) middlebox acts as a potential indication of a PMTU limit for an application load balancer. An
example is where the
path. Loss of path towards the server is chosen by ECMP
routing depending on bytes in the IP payload. In this case, when
a probe packet SHOULD NOT be treated as an
indication of congestion and sent by the loss SHOULD NOT directly trigger server encounters a congestion control reaction [RFC4821].

8. Shared PLPMTU state: The PLPMTU value could problem after the ECMP
router, then any resulting ICMP message needs to also be stored with directed
by the corresponding entry in ECMP router towards the destination cache and used by
other PL instances. The specification of PLPMTUD [RFC4821]
states: "If PLPMTUD updates original sender.

* There are additional cases where the MTU for next hop destination fails to
receive a particular path, all
Packetization Layer sessions that share the path representation
(as described in Section 5.2 packet because of [RFC4821]) SHOULD its size. This could be notified due to
make use
misconfiguration of the new MTU". Such methods MUST be robust to layer 2 path between nodes, for instance
the
wide variety of underlying network forwarding behaviours, PLPMTU
adjustments based on shared PLPMTU values should be incorporated MTU configured in the search algorithms. Section 5.2 a layer 2 switch, or misconfiguration of [RFC8201] provides
guidance on the caching of PMTU information and also
Maximum Receive Unit (MRU). If the relation
to IPv6 flow labels.

In addition, packet is dropped by the following principles are stated for design of link,
this will not cause a
DPLPMTUD method:

o MPS: A method is REQUIRED PTB message to signal an appropriate MPS be sent to the
higher layer using the PL. The value of the MPS can change
following original
sender.

Another failure could result if a change to the path. It is RECOMMENDED node that methods
avoid forcing an application to use an arbitrary small MPS
(PLPMTU) for transmission while the method is searching for the
currently supported PLPMTU. Datagram PLs do not necessarily
support fragmentation of PDUs larger than on the network
path sends a PTB message that attempts to force a sender to change
the PLPMTU. effective PMTU [RFC8201]. A reduced
MPS sender can adversely impact protect itself from
reacting to such messages by utilising the performance of quoted packet within a datagram
application.

o Path validation: It is RECOMMENDED that methods are robust PTB
message payload to path
changes validate that could have occurred since the path characteristics
were last confirmed, and received PTB message was
generated in response to a packet that had actually originated from
the possibility of inconsistent path
information being received.

o Datagram reordering: A method is REQUIRED to sender. However, there are situations where a sender would be robust
unable to provide this validation. Examples where validation of the
possibility that
PTB message is not possible include:

* When a flow encounters reordering, or router issuing the traffic
(including probe packets) ICMP message implements RFC792
[RFC0792], it is divided over more than one network
path.

o When only required to probe: It is RECOMMENDED that methods determine whether include the path capacity has increased since it last measured first 64 bits of the path.
This determines when
IP payload of the path should again be probed.

4. DPLPMTUD Mechanisms

This section lists packet within the protocol mechanisms used in this
specification.

4.1. PLPMTU Probe Packets

The DPLPMTUD method relies upon quoted payload. There could
be insufficient bytes remaining for 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 needs to construct a
probe packet has to either request an application to send a data
block that interpret the
quoted transport information.

Note: The recommendation in RFC1812 [RFC1812] is larger than that generated by an application, or to
utilise padding functions to extend IPv4 routers
return a quoted packet with as much of the original datagram beyond as
possible without the size length of the
application data block. Protocols that permit exchange ICMP datagram exceeding 576
bytes. IPv6 routers include as much of control
messages (without an application data block) could alternatively
prefer to generate a probe packet by extending a control message with
padding data.

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

This results in three invoking packet as
possible ways that a sender can create a probe without the ICMPv6 packet listed in order exceeding 1280 bytes [RFC4443].

* The use of preference:

Probing using padding data: A probe tunnels/encryption can reduce the size of the quoted
packet that contains only
control information together with any padding, which is needed returned to the original source address, increasing the
risk that there could be inflated insufficient bytes remaining for the
sender to interpret the size required for quoted transport information.

* Even when the PTB message includes sufficient bytes of the quoted
packet, 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 layer could lack sufficient context to
validate the message, because validation depends on information
about the active transport flows at an endpoint processing.

Probing using application data node (e.g., the
socket/address pairs being used, and padding data: A probe packet that
contains other protocol header
information).

* When a data block supplied by an application that packet is combined
with padding to inflate encapsulated/tunneled over an encrypted
transport, the length of tunnel/encapsulation ingress might have
insufficient context, or computational power, to reconstruct the datagram
transport header that would be needed to perform validation.

1.2. Packetization Layer Path MTU Discovery

The term Packetization Layer (PL) has been introduced to describe the size
required
layer that is responsible for placing data blocks into the probe packet. If the application/transport needs
protection from the loss payload of this probe packet,
IP packets and selecting an appropriate MPS. This function is often
performed by a transport protocol, but can also be performed by other
encapsulation methods working above the application/ transport could perform transport-layer retransmission/repair layer.

In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) [RFC4821] does not rely upon reception and validation of
the data block (e.g., by retransmission after loss
PTB messages. It is detected or
by duplicating therefore more robust than Classical PMTUD.
This has become the data block in recommended approach for implementing PMTU
discovery with TCP.

It uses a datagram without general strategy where the padding
data).

Probing using application data: A PL sends probe packet packets to search
for the largest size of unfragmented datagram that contains can be sent over a data
block supplied by an application that matches
network path. Probe packets are sent with a progressively larger
packet size. If a probe packet is successfully delivered (as
determined by the size required
for PL), then the probe packet. This method requests PLPMTU is raised to the application size of the
successful probe. If no response is received to
issue a data block of probe packet, the
method reduces the desired probe size. If The result of probing with the application/
transport needs protection from PLPMTU
is used to set the loss of an unsuccessful probe
packet, application MPS.

PLPMTUD introduces flexibility in the application/transport needs then implementation of PMTU
discovery. At one extreme, it can be configured to only perform transport-
layer retransmission/repair ICMP
black Hole Detection and recovery to increase the robustness of
Classical PMTUD, or at the data block (e.g., by
retransmission after loss is detected).

A PL other extreme, all PTB processing can be
disabled and PLPMTUD can completely replace Classical PMTUD.

PLPMTUD can also include additional consistency checks without
increasing the risk that uses a probe packet carrying an application data block,
could need is lost when probing to retransmit this application data block if the probe
fails. This could need discover the PL to re-fragment
path MTU. For example, information available at the data block 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
smaller packet set of algorithms for datagram
protocols to discover the largest size of unfragmented datagram that is expected to traverse
can be sent over a network path. The method described relies on
features of the end-to-end path
(which could utilise endpoint network-layer or PL fragmentation 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 available).

DPLPMTUD MAY choose made available to the PL.

The UDP Usage Guidelines [RFC8085] state "an application SHOULD
either use only one of these methods to simplify the
implementation.

Probe messages sent Path MTU information provided by the IP layer or
implement Path MTU Discovery (PMTUD)", but does not provide a PL MUST contain enough information to
uniquely identify
mechanism for discovering the probe within Maximum Segment Lifetime, while
being robust to reordering and replay of probe response and PTB
messages.

4.2. Confirmation largest size of Probed Packet Size

The PL needs unfragmented datagram
that can be used on a method network path. Prior to determine (confirm) when probe packets have this document, PLPMTUD
had not been successfully received end-to-end across specified for UDP.

Section 10.2 of [RFC4821] recommends a network path. PLPMTUD probing method for the
Stream Control Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP and Protocol (SCTP). SCTP provide keep-alive/heartbeat features). When supported, this
mechanism SHOULD also be used by DPLPMTUD to acknowledge reception utilizes probe
packets consisting of a probe packet.

A PL that minimal sized HEARTBEAT chunk bundled with a
PAD chunk as defined in [RFC4820], but RFC4821 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 provide a
complete specification. The present document provides the actual PMTU. These
PLs need details to either rely on an application protocol
complete that specification.

The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
implementations to detect this
loss, or make use of an additional transport method such as UDP-
Options [I-D.ietf-tsvwg-udp-options].

Section 5 specifies this function for support Classical PMTUD and states that a set of IETF-specified
protocols.

4.3. Detection of Black Holes

A PL DCCP
sender needs to reduce "MUST maintain the PLPMTU when it discovers MPS allowed for each active DCCP session".
It also defines the actual
PMTU current congestion control MPS (CCMPS) supported
by a network path is less than the PLPMTU (i.e. to
detect that traffic is being black holed). path. This can be triggered
when a validated PTB message is received, or by another event that
indicates the network recommends use of PMTUD, and suggests use of
control packets (DCCP-Sync) as path no longer sustains probe packets, because they do
not risk application data loss. The method defined in this
specification could be used with DCCP.

Section 6 specifies the current packet
size, such as method for a loss report from the PL or repeated lack set of response
to probe packets sent transports, and provides
information to confirm enable the PLPMTU. Detection is followed
by a reduction implementation of the PLPMTU.

Black Hole detection PLPMTUD with other
datagram transports and applications that use datagram transports.

2. Terminology

Other terminology is performed by periodically sending packet
probes directly copied from [RFC4821], and the
definitions in [RFC1122].

Actual PMTU: The Actual PMTU is the PMTU of size PLPMTU to verify that a network path still supports
the last acknowledged PLPMTU size. There are two ways between a DPLPMTUD
sender detect that the current PLPMTU is not sustained by the path
(i.e., to detect a black hole):

o A PL can rely upon and a mechanisms implemented within destination PL, which the PL protocol DPLPMTUD algorithm seeks
to detect excessive loss of data sent with determine.

Black Hole: A Black Hole is encountered when a specific packet size
and then conclude sender is unaware
that this excessive loss could be a result of an
invalid PMTU (as in PLPMTUD for TCP [RFC4821]).

o A PL can use the probing mechanism to send confirmation probe packets of the size of the current PLPMTU and a timer track
whether acknowledgments are received (e.g., not being delivered to the number destination end point.
Two types of probe Black Hole are relevant to DPLPMTUD:

Packet Black Hole: Packets encounter a Packet Black Hole when
packets sent without receiving an acknowledgement, PROBE_COUNT,
becomes greater than the MAX_PROBES). These messages need are not delivered to be
generated periodically (e.g., using the confirmation timer
Section 5.1.1), and MAY inhibit sending probe packets destination
endpoint (e.g., when no
application data has been sent since the previous probe packet. A
PL preferring to use an up-to-data PMTU once user data is sent
again, MAY choose to continue PMTU discovery for each path.
However, this may result in additional sender transmits
packets being sent.
Successive loss of probes a particular size with a previously
known effective PMTU and they are discarded by
the network).

ICMP Black Hole An ICMP Black Hole is an indication that encountered when the current path
no longer supports
sender is unaware that packets are not
delivered to the PLPMTU.

When destination endpoint because
PTB messages are not received by the method detects
originating PL sender.

Black holed : Traffic is black-holed when the current PLPMTU sender is unaware that
packets are not supported (a black
hole being delivered. This could be due to a Packet
Black Hole or an ICMP Black Hole.

Classical Path MTU Discovery: Classical PMTUD is found), DPLPMTUD sets a lower MPS. The PL then confirms that process described
in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
learn the updated PLPMTU largest size of unfragmented datagram that can be successfully used
across the path. This
can need the PL to send a probe packet with network path.

Datagram: A datagram is a size less than transport-layer protocol data unit,
transmitted in the size payload of the data block generated by an application. In this case, IP packet.

Effective PMTU: The Effective PMTU is the PL
could provide current estimated value
for PMTU that is used by a way PMTUD. This is equivalent to fragment the
PLPMTU derived by PLPMTUD.

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

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

Link: A Link is a communication facility or medium over which nodes
can communicate at the PL, or could
instead utilise link layer, i.e., a control packet with padding.

4.4. Response to PTB Messages

This method requires the DPLPMTUD sender to validate any received PTB
message before using layer below the PTB information. IP
layer. Examples are Ethernet LANs and Internet (or higher) layer
and tunnels.

Link MTU: The response to a PTB
message depends on Link Maximum Transmission Unit (MTU) is the PTB_SIZE indicated size in the PTB message, the
state
bytes of the PLPMTUD state machine, and largest IP packet, including the IP protocol being used.

Section 4.4.1 first describes validation for both IPv4 ICMP
Unreachable messages (type 3) header and ICMPv6 packet too big messages,
both of which are referred to as PTB messages in
payload, that can be transmitted over a link. Note that this document.

4.4.1. Validation of PTB Messages
could more properly be called the IP MTU, to be consistent with
how other standards organizations use the acronym. This section specifies utlisation of PTB messages.

o A simple implementation MAY ignore received PTB messages and in
this case includes
the PLPMTU IP header, but excludes link layer headers and other framing
that is not updated when a PTB message part of IP or the IP payload. Other standards
organizations generally define the link MTU to include the link
layer headers.

MAX_PMTU: The MAX_PMTU is
received.

o An implementation the largest size of PLPMTU that supports PTB messages MUST validate
messages before they are further processed.

A PL DPLPMTUD
will attempt to use.

MPS: The Maximum Packet Size (MPS) is the largest size of
application data block that receives can be sent across a PTB message from network path by a router or middlebox, performs
ICMP validation as specified in Section 5.2 of [RFC8085][RFC8201].
Because
PL. In DPLPMTUD operates at the PL, the PL needs to check that each
received PTB message this quantity is received in response to a packet transmitted derived from the PLPMTU by
taking into consideration the endpoint PL performing DPLPMTUD.

The PL MUST check size of the lower protocol information in layer
headers. Probe packets generated by DPLPMTUD can have a size
larger than the quoted packet
carried in MPS.

MIN_PMTU: The MIN_PMTU is the ICMP PTB message payload smallest size of PLPMTU that DPLPMTUD
will attempt to validate use.

Packet: A Packet is the message
originated from IP header plus the sending node. This validation includes
determining that IP payload.

Packetization Layer (PL): The Packetization Layer (PL) is the combination layer
of the IP addresses, the protocol, network stack that places data into packets and performs
transport protocol functions.

Path: The Path is the set of links and routers traversed by a packet
between a source port node and a destination port match those returned in the
quoted packet - this is also necessary for the PTB message to be
passed to the corresponding PL. node by a particular flow.

Path MTU (PMTU): The validation SHOULD utilise information that it Path MTU (PMTU) is not simple for
an off-path attacker to determine. For example, by checking the
value minimum of a protocol header field known only to the two PL endpoints.
A datagram application that uses well-known Link MTU
of all the links forming a network path between a source node and
a 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 node.

PTB_SIZE: The PTB_SIZE is a node value reported in a validated PTB
message that is not on indicates next hop link MTU of a router along the network
path.

A PTB message that does not complete the validation MUST NOT be
further utilised by

PLPMTU: The Packetization Layer PMTU is an estimate of the DPLPMTUD method.

PTB messages that have been validated MAY be utilised actual
PMTU provided by the DPLPMTUD
algorithm, but MUST NOT be used directly to set algorithm.

PLPMTUD: Packetization Layer Path MTU Discovery (PLPMTUD), the PLPMTU. A
method
that utilises these PTB messages can improve the speed at the described in this document for datagram PLs, which
the algorithm detects is an appropriate PLPMTU, compared
extension to one that
relies solely on probing. Section 4.4.2 describes this processing.

4.4.2. Use of PTB Messages Classical PMTU Discovery.

Probe packet: A set of checks are intended to provide protection from a router that
reports an unexpected PTB_SIZE. The PL needs to check that the
indicated PTB_SIZE probe packet is less than the a datagram sent with a purposely
chosen size used by probe (typically the current PLPMTU or larger) to detect if
packets and
larger than minimum size accepted.

This section provides a summary of how PTB messages this size can be utilised.
This processing depends on the PTB_SIZE and successfully sent end-to-end across
the current value network path.

3. Features Required to Provide Datagram PLPMTUD

TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
All of a
set the requirements in [RFC4821] also apply to the use of variables:

MIN_PMTU < PTB_SIZE < BASE_PMTU

* A robust PL MAY enter the PROBE_ERROR state
technique with a datagram PL. Unlike TCP, some datagram PLs require
additional mechanisms to implement PLPMTUD.

There are eight requirements for an IPv4 path
when performing the PTB_SIZE reported datagram PLPMTUD
method described in the PTB message >= 68 bytes and
when this specification:

1. PMTU parameters: A DPLPMTUD sender is less than RECOMMENDED to provide
information about the BASE_PMTU.

* A robust PL MAY enter maximum size of packet that can be
transmitted by the PROBE_ERROR state for an IPv6 path
when sender on the PTB_SIZE reported in local link (the local Link MTU).
It MAY utilize similar information about the PTB message >= 1280 bytes and receiver when this
is supplied (note this could be less than the BASE_PMTU.

PTB_SIZE = PLPMTU

* Transition to SEARCH_COMPLETE.

PTB_SIZE > PROBED_SIZE

* The PTB_SIZE > PROBED_SIZE, inconsistent network signal. These
PTB messages ought EMTU_R). This avoids
implementations trying to be discarded without further processing
(the PLPMTU send probe packets that can not updated).

* The information could be utilised as an input to trigger
enabling a resilience mode.

BASE_PMTU <= PTB_SIZE < PLPMTU

* Black hole detection is triggered and
transmitted by the PLPMTU ought to be
set to BASE_PMTU.

* The PL local link. Too high of a value could use PTB_SIZE reported in reduce
the efficiency of the PTB message to
initialise a search algorithm.

PLPMTU < PTB_SIZE < PROBED_SIZE

* The PLPMTU continues to be valid, but the last PROBED_SIZE
searched was 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
PDUs into the actual PMTU.

* The PLPMTU is not updated.

* The same datagram).

2. PLPMTU: A datagram application using a PL can use not supporting
fragmentation is REQUIRED to be able to choose the size of
datagrams sent to the reported PTB_SIZE network, up to the PLPMTU, or a smaller
value (such as the MPS) derived from this. This value is managed
by the PTB message DPLPMTUD method. The PLPMTU (specified as the next search point when it resumes effective
PMTU in Section 1 of [RFC1191]) is equivalent to the search algorithm.

xxx Author Note: Do we want EMTU_S
(specified in [RFC1122]).

3. Probe packets: On request, a DPLPMTUD sender is REQUIRED to specify how be
able to handle PTB Message with
PTB_SIZE = 0? xxx

5. Datagram Packetization Layer PMTUD transmit a packet larger than the PLMPMTU. This section specifies Datagram PLPMTUD (DPLPMTUD). The method can is used
to send a probe packet. In IPv4, a probe packet MUST be introduced at various points (as indicated sent
with * in the figure
below) Don't Fragment (DF) bit set in the IP protocol stack to discover the PLPMTU so that an
application can utilise an appropriate MPS for the current header, and
without network
path. DPLPMTUD SHOULD NOT be used by an application if it layer endpoint fragmentation. In IPv6, a probe
packet is already
used always sent without source fragmentation (as specified
in a lower layer.

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

Figure 1: Examples where DPLPMTUD can be implemented

The central idea section 5.4 of [RFC8201]).

4. Processing PTB messages: A DPLPMTUD is probing by a sender. Probe packets
are sent sender MAY optionally utilize
PTB messages received from the network layer to find help identify
when a network path does not support the maximum current size of a user probe
packet. Any received PTB message MUST be validated before it is
used to update the PLPMTU discovery information [RFC8201]. This
validation confirms that can the PTB message was sent in response to
a packet originating by the sender, and needs to be
completely transferred across performed
before the network path from PLPMTU discovery method reacts to the PTB message. A
PTB message MUST NOT be used to increase the PLPMTU [RFC8201].

5. 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. The mechanism needs to be robust to the
destination.

This section identifies possibility
that packets could be significantly delayed along a network path.
The local PL endpoint at the components needed for implementation, sending node is REQUIRED to pass
this feedback to the
phases sender DPLPMTUD method.

6. Probe loss recovery: It is RECOMMENDED to use probe packets that
do not carry any user data. Most datagram transports permit
this. If a probe packet contains user data requiring
retransmission in case of operation, loss, the state machine and search algorithm.

5.1. DPLPMTUD Components

This section describes components PL (or layers above) are
REQUIRED to arrange any retransmission/repair of DPLPMTUD.

5.1.1. Timers

The method utilises up any resulting
loss. DPLPMTUD is REQUIRED to three timers:

PROBE_TIMER: be robust in the case where probe
packets are lost due to other reasons (including link
transmission error, congestion).

7. Probing and congestion control: The PROBE_TIMER is configured to expire after DPLPMTUD sender treats
isolated loss of a period
longer than probe packet (with or without a corresponding
PTB message) as a potential indication of a PMTU limit for the maximum time to receive an acknowledgment to
path. Loss of a probe packet. This value MUST packet SHOULD NOT be smaller than 1 second, treated as an
indication of congestion and the loss SHOULD NOT directly trigger
a congestion control reaction [RFC4821].

8. Shared PLPMTU state: The PLPMTU value could also be larger than 15 seconds. Guidance on selection of stored with
the
timer value are provided corresponding entry in section 3.1.1 of the UDP Usage
Guidelines [RFC8085].

If the PL has a path Round Trip Time (RTT) estimate destination cache and timely
acknowledgements the PROBE_TIMER can be derived from the used by
other PL RTT
estimate.

PMTU_RAISE_TIMER: instances. The PMTU_RAISE_TIMER is configured to specification of PLPMTUD [RFC4821]
states: "If PLPMTUD updates the period MTU for a
sender will continue particular path, all
Packetization Layer sessions that share the path representation
(as described in Section 5.2 of [RFC4821]) SHOULD be notified to
make use of the current PLPMTU, after which it re-
enters new MTU". Such methods MUST be robust to the Search phase. This timer has a period
wide variety of 600 secs, as
recommended by PLPMTUD [RFC4821].

DPLPMTUD MAY inhibit sending probe packets when no application
data has been sent since underlying network forwarding behaviors, PLPMTU
adjustments based on shared PLPMTU values should be incorporated
in the search algorithms. Section 5.2 of [RFC8201] provides
guidance on the caching of PMTU information and also the relation
to IPv6 flow labels.

In addition, the previous probe packet. following principles are stated for design of a
DPLPMTUD method:

* MPS: A PL
preferring to use an up-to-data PMTU once user data method is sent again,
can choose REQUIRED to continue PMTU discovery for each path. However,
this could in sending additional packets.

CONFIRMATION_TIMER: When signal an acknowledged PL is used, this timer MUST
NOT be used. For other PLs, the CONFIRMATION_TIMER is configured appropriate MPS to the period
higher layer using the PL. The value of the MPS can change
following a PL sender waits before confirming change to the current
PLPMTU path. It is still supported. This RECOMMENDED that methods
avoid forcing an application to use an arbitrary small MPS
(PLPMTU) for transmission while the method is less searching for the
currently supported PLPMTU. Datagram PLs do not necessarily
support fragmentation of PDUs larger than the PMTU_RAISE_TIMER
and used to decrease PLPMTU. A reduced
MPS can adversely impact the PLPMTU (e.g., when performance of a black hole datagram
application.

* Path validation: It is
encountered). Confirmation needs RECOMMENDED that methods are robust to be frequent enough when data
is flowing path
changes that could have occurred since the sending PL does not black hole extensive
amounts of traffic. Guidance on selection of path characteristics
were last confirmed, and to the timer value are
provided in section 3.1.1 possibility of inconsistent path
information being received.

* Datagram reordering: A method is REQUIRED to be robust to the UDP Usage Guidelines [RFC8085].

DPLPMTUD MAY inhibit sending
possibility that a flow encounters reordering, or the traffic
(including probe packets when no application
data packets) is divided over more than one network
path.

* When to probe: It is RECOMMENDED that methods determine whether
the path has been sent changed since it last measured the previous probe packet. A PL
preferring to use an up-to-data PMTU once user data is sent again, path. This can choose
help determine when to continue PMTU discovery for each path. However,
this may result in sending additional packets.

An implementation could implement the various timers using a single
timer.

5.1.2. Constants

The following constants are defined:

MAX_PROBES: MAX_PROBES is probe the maximum value of path again.

4. DPLPMTUD Mechanisms

This section lists the PROBE_COUNT
counter. The default value of MAX_PROBES is 10.

MIN_PMTU: protocol mechanisms used in this
specification.

4.1. PLPMTU Probe Packets

The MIN_PMTU is smallest allowed DPLPMTUD method relies upon the PL sender being able to generate
probe packet packets with a specific size. For
IPv6, this value is 1280 bytes, as specified in [RFC2460]. For
IPv4, the minimum value is 68 bytes. (An IPv4 router TCP is required
to be able to forward generate these
probe packets by choosing to appropriately segment data being sent
[RFC4821]. In contrast, 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 PL that needs to be able construct a
probe packet has to reassemble fragmented datagrams at least up either request an application to 576 bytes, as stated in section 3.3.3 of [RFC1122]))

MAX_PMTU: The MAX_PMTU send a data
block that is the largest size of PLPMTU. This has to
be less larger than that generated by an application, or equal to
utilize padding functions to extend a datagram beyond the minimum size of the local MTU
application data block. Protocols that permit exchange of the
outgoing interface and the destination PMTU for receiving. An control
messages (without an application or PL MAY reduce the MAX_PMTU when there is no need data block) could alternatively
prefer to
send packets larger than generate a specific size.

BASE_PMTU: The BASE_PMTU is probe packet by extending a configured size expected control message with
padding data.

A receiver needs to work for
most paths. The size is equal be able to or larger than the MIN_PMTU and
smaller than the MAX_PMTU. In the case of IPv6, this value distinguish an in-band data block from
any added padding. This is
1280 bytes [RFC2460]. When using IPv4, a size of 1200 bytes needed to ensure that any added padding
is
RECOMMENDED.

5.1.3. Variables not passed on to an application at the receiver.

This method utilises results in three possible ways that a set sender can create a probe
packet listed in order of variables:

PROBED_SIZE: The PROBED_SIZE preference:

Probing using padding data: A probe packet that contains only
control information together with any padding, which is needed to
be inflated to the size of required for the current probe packet. 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
unsuccessful 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 have been sent with
contains a size of
PROBED_SIZE. The value data block supplied by an application that is initialised combined
with padding to zero when a particular
size inflate the length of PROBED_SIZE is first attempted.

The figure below illustrates the relationship between datagram to the packet size
constants and variables, in this case when
required for the DPLPMTUD algorithm
performs path probing to increase probe packet. If the size application/transport needs
protection from the loss of this probe packet, the PLPMTU. The MPS application/
transport could perform transport-layer retransmission/repair of
the data block (e.g., by retransmission after loss is
less than detected or
by duplicating the PLPMTU. data block in a datagram without the padding
data).

Probing using application data: A probe packet has been sent of that contains a data
block supplied by an application that matches the size
PROBED_SIZE. When this is acknowledged, required
for the PLPMTU will be raised to
PROBED_SIZE allowing probe packet. This method requests the PROBED_SIZE application to be increased towards
issue a data block of the
actual PMTU.

Figure 2: Relationships between desired probe and packet sizes

5.2. DPLPMTUD Phases

The Datagram PLPMTUD algorithm moves through several phases of
operation.

An implementation that only reduces size. If the PLPMTU to a suitable size
would be sufficient to ensure reliable operation, but can be very
inefficient when application/
transport needs protection from the actual PMTU changes or when loss of an unsuccessful probe
packet, the method (for
whatever reason) makes a suboptimal choice for application/transport needs then to perform transport-
layer retransmission/repair of the PLPMTU. data block (e.g., by
retransmission after loss is detected).

A full implementation of DPLPMTUD provides PL that uses a probe packet carrying an algorithm enabling the
DPLPMTUD sender application data block,
could need to increase retransmit this application data block if the PLPMTU following a change in probe
fails. This could need the
characteristics of PL to re-fragment the path, such as when data block to a link
smaller packet size that is reconfigured with
a larger MTU, expected to traverse the end-to-end path
(which could utilize endpoint network-layer or PL fragmentation when there is
these are available).

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

Probe messages sent by a change in PL MUST contain enough information to
uniquely identify the set probe within Maximum Segment Lifetime, while
being robust to reordering and replay of links traversed
by an 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 flow (e.g., after across a routing or path fail-over
decision).

Black hole detection (Section 4.3) network path.

Transport protocols can include end-to-end methods that detect and PTB processing (Section 4.4)
proceed in parallel with these phases
report reception of operation.

Figure 3: DPLPMTUD Phases

BASE_PMTU Confirmation

* Connectivity is confirmed.

*
SCTP provide keep-alive/heartbeat features). When supported, this
mechanism SHOULD also be used by DPLPMTUD confirms the BASE_PMTU to acknowledge reception of
a probe packet.

A PL that does not acknowledge data reception (e.g., UDP and UDP-
Lite) is supported across the network
path.

* DPLPMTUD then enters the search phase.

* DPLPMTUD performs probing unable itself to increase detect when the PLPMTU.

* DPLPMTUD then enters packets that it sends are
discarded because their size is greater than the search complete actual PMTU. These
PLs need to either rely on an application protocol to detect this
loss, or make use of an error phase.

Search Complete

* DPLPMTUD has found additional transport method such as UDP-
Options [I-D.ietf-tsvwg-udp-options].

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

4.3. Detection of Unsupported PLPMTU that is supported across
the network path.

* Size, aka Black hole detection will confirm this PLPMTU continues to be
supported.

* On a longer time-frame, DPLPMTUD will re-enter the search phase Hole Detection

A PL sender needs to discover if reduce the PLPMTU can be raised.

Error

* Inconsistent or invalid network signals cause DPLPMTUD to be
unable to progress.

* This causes the algorithm to lower the MPS until when it discovers the actual
PMTU supported by a network path is
shown to support less than the BASE_PMTU, PLPMTU. This can
be triggered when a validated PTB message is received, or to suspend DPLPMTUD.

5.2.1. BASE_PMTU Confirmation Phase

DPLPMTUD starts in by another
event that indicates the BASE_PMTU confirmation phase. BASE_PMTU
confirmation is performed in two stages:

1. Connectivity to network path no longer sustains the remote peer is first confirmed. When a
connection-oriented PL is used, this stage is implicit. It is
performed current
packet size, such as part of a loss report from the normal PL connection handshake. In
contrast, an connectionless PL MUST send an acknowledged PL, or repeated lack of
response to probe
packet packets sent to confirm that the remote peer PLPMTU. Detection is reachable.

2. In the second stage, the PL confirms it can successfully send
followed by a
datagram reduction of the BASE_PMTU PLPMTU.

This is performed by sending packet probes of size across the current path.

A PL that does not wish PLPMTU to support verify
that a network path with a PLPMTU less
than BASE_PMTU can simplify the phase into a single step by
performing connectivity checks with probes of still supports the BASE_PMTU last acknowledged PLPMTU size.
There are two alternative mechanism:

* A PL MAY respond to PTB messages while in this phase, see
Section 4.4.

Once BASE_PMTU confirmation has completed, DPLPMTUD can advertise an
MPS to an upper layer.

If DPLPMTUD fails to complete these tests it enters the
PROBE_DISABLED phase, see Section 5.2.6, and ceases using DPLPTMUD.

5.2.2. Search Phase

The search phase utilises a search algorithm in attempt to increase
the PLPMTU (see Section 5.4.1). The PL sender increases the MPS each
time a packet probe confirms rely upon a larger PLPMTU is supported by the
path. The algorithm concludes by entering the SEARCH_COMPLETE phase,
see Section 5.2.3.

A PL MAY respond to PTB messages while in this phase, using the PTB
to advance or terminate the search, see Section 4.4. Similarly black
hole detection can terminate the search by entering mechanism implemented within the PROBE_BASE
phase, see Section 5.2.4.

5.2.2.1. Resilience to Inconsistent Path Information

Sometimes a PL sender is able to detect inconsistent results from the
sequence of PLPMTU probes that it sends or the sequence
excessive loss of PTB
messages data sent with a specific packet size and then
conclude that it receives. This this excessive loss could be manifested as excessive
fluctuation of the MPS.

When inconsistent path information is detected, a result of an invalid
PMTU (as in PLPMTUD for TCP [RFC4821]).

* A PL sender can
enable an alternate search mode that clamps use the offered MPS DPLPMTUD probing mechanism to a
smaller value for a period periodically
generate probe packets of time. This avoids unnecessary black-
holing the size of packets.

5.2.3. Search Complete Phase

On entry to the search complete phase, current PLPMTU (e.g.,
using the DPLPMTUD sender starts confirmation timer Section 5.1.1). A timer tracks
whether acknowledgments are received. Successive loss of probes
is an indication that the
PMTU_RAISE_TIMER. In this phase, current path no longer supports the
PLPMTU remains at (e.g., when the value
confirmed by number of probe packets sent without
receiving an acknowledgement, PROBE_COUNT, becomes greater than
MAX_PROBES).

A PL MAY inhibit sending probe packets when no application data has
been sent since the last successful previous probe packet.

In A PL preferring to use an
up-to-data PLPMTU once user data is sent again, MAY choose to
continue PLPMTU discovery for each path. However, this phase, may result in
additional packets being sent.

When the method detects the current PLPMTU is not supported, DPLPMTUD
sets a lower MPS. The PL MUST periodically confirm then confirms that the updated PLPMTU is
still supported by can
be successfully used across the path. If the The PL is designed in a way that is
unable to confirm reachability could need to send a
probe packet with a size less than the destination endpoint after
probing has completed, size of the method uses data block
generated by an application. In this case, the PL could provide a CONFIRMATION_TIMER
way to
periodically repeat fragment a probe datagram at the PL, or use a control packet for as the current PLPMTU size.

If
packet probe.

4.4. Response to PTB Messages

This method requires the DPLPMTUD sender is unable to confirm reachability for packets
with validate any received PTB
message before using the PTB information. The response to a size of PTB
message depends on the current PLPMTU (e.g., if PTB_SIZE indicated in the CONFIRMATION_TIMER
expires) or PTB message, the PL signals a lack
state of reachability, the method exits
the phase PLPMTUD state machine, and enters the PROBE_BASE phase, see Section 5.2.4.

If the PMTU_RAISE_TIMER expires, the DPLPMTUD sender re-enters the
Search phase, see IP protocol being used.

Section 5.2.2, and resumes probing 4.4.1 first describes validation for a larger
PLPMTU.

Back hole detection can be used in parallel both IPv4 ICMP
Unreachable messages (type 3) and ICMPv6 packet too big messages,
both of which are referred to check that as PTB messages in this document.

4.4.1. Validation of PTB Messages

This section specifies utilization of PTB messages.

* A simple implementation MAY ignore received PTB messages and in
this case the PLPMTU is not updated when a network
path continues to support PTB message is
received.

* An implementation that supports PTB messages MUST validate
messages before they are further processed.

A PL that receives a previously confirmed PLPMTU. If PTB message from a black
hole is detected router or middlebox, performs
ICMP validation as specified in Section 5.2 of [RFC8085][RFC8201].
Because DPLPMTUD operates at the algorithm moves to PL, the PROBE_BASE phase, see
Section 5.2.4.

The phase can also exited when a validated PL needs to check that each
received PTB message is received
(see Section 4.4.1).

5.2.4. PROBE_BASE Phase

This phase is entered when black hole detection or 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
indicates payload to validate the message
originated from the sending node. This validation includes
determining that the PLPMTU is not supported by combination of the path.

On entry to this phase, IP addresses, the PLPMTU protocol,
the source port and destination port match those returned in the
quoted packet - this is set also necessary for the PTB message to be
passed to the BASE_PMTU, and a corresponding reduced MPS is advertised.

PROBED_SIZE PL.

The validation SHOULD utilize information that it is then set not simple for
an off-path attacker to determine [RFC8085]. For example, by
checking the PLPMTU (i.e., value of a protocol header field known only to the BASE_PMTU), two
PL endpoints. A datagram application that uses well-known source and
destination ports ought to
confirm also rely on other information to complete
this size validation.

These checks are intended to provide protection from packets that
originate from a node that is supported across not on the network path. If confirmed, A PTB message
that does not complete the validation MUST NOT be further utilized by
the DPLPMTUD enters method.

PTB messages that have been validated MAY be utilized by the Search Phase DPLPMTUD
algorithm, but MUST NOT be used directly to determine whether set the PL sender
can use a larger PLPMTU.

If A method
that utilizes these PTB messages can improve the path cannot be confirmed to support speed at the BASE_PMTU after
sending MAX_PROBES, DPLPMTUD moves to which
the Error phase, see algorithm detects an appropriate PLPMTU, compared to one that
relies solely on probing. Section 5.2.5.

5.2.5. ERROR Phase

The ERROR phase is entered when there is conflicting or invalid
PLPMTU information for the path (e.g. 4.4.2 describes this processing.

4.4.2. Use of PTB Messages

A set of checks are intended to provide protection from a failure router that
reports an unexpected PTB_SIZE. The PL also needs to support the
BASE_PMTU). In this phase, check that the MPS
indicated PTB_SIZE is set to a value less than the
BASE_PMTU, but at least the size of the MIN_PMTU.

DPLPMTUD remains in the ERROR phase until used by probe packets and
larger than minimum size accepted.

This section provides a consistent view summary of the
path how PTB messages can be discovered and it has also been confirmed that utilized.
This processing depends on the path
supports PTB_SIZE and the BASE_PMTU.

Note: current value of a
set of variables:

MIN_PMTU may be identical to BASE_PMTU, simplifying < PTB_SIZE < BASE_PMTU
* A robust PL MAY enter an error state (see Section 5.2) for an
IPv4 path when the actions PTB_SIZE reported in the PTB message is
larger than or equal to 68 bytes and when this phase.

If no acknowledgement is received for PROBE_COUNT probes of size
MIN_PMTU, less than the method suspends DPLPMTUD, see
BASE_PMTU.

* A robust PL MAY enter an error state (see Section 5.2.5.

5.2.5.1. Robustness to Inconsistent Path

Robustness to paths unable 5.2) for an
IPv6 path when the PTB_SIZE reported in the PTB message is
larger than or equal to sustain 1280 bytes and when this is less than
the BASE_PMTU. Some paths

PTB_SIZE = PLPMTU
* Completes the search for a larger PLPMTU.

PTB_SIZE > PROBED_SIZE
* Inconsistent network signal.

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

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

BASE_PMTU size. These
paths <= PTB_SIZE < PLPMTU
* Black Hole Detection is triggered and the PLPMTU ought to be
set to BASE_PMTU.

* The PL could use an alternate algorithm to implement the PROBE_ERROR
phase that allows fallback PTB_SIZE reported in the PTB message to
initialize a smaller than desired PLPMTU, rather
than suffer connectivity failure.

This could also utilise methods such as endpoint IP fragmentation search algorithm.

PLPMTU < PTB_SIZE < PROBED_SIZE
* The PLPMTU continues to
enable be valid, but the PL sender to communicate using packets smaller last PROBED_SIZE
searched was larger than the
BASE_PMTU.

5.2.6. DISABLED Phase

This phase suspends operation of DPLPMTUD. It disables probing for
the actual PMTU.

* The PLPMTU until action is taken by the not updated.

* The PL or application using can use the
PL.

5.3. State Machine

A state machine for DPLPMTUD is depicted in Figure 4. If multihoming
is supported, a state machine is needed for each path.

xxx Author Note: Do we want to specify how to handle PTB Message with
PTB_SIZE < = 0? xxx

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) 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 NOT be used by an application if it is already
used in a lower layer.

+----------------------+
|
+--------------------+ | | +--------------------+ |
| +----+ | |
| PROBE_TIMER expiry: | |
| PROBE_COUNT < MAX_PROBES | |
| | |
| PMTU_RAISE_TIMER expiry | |
| +-----------------------------------------+ | |
| Application* |
+-+-------+----+----+--+
| | | |
+---+--+ +--+--+ | V +-+---+
| V
+---------------+ +---------------+
|SEARCH_COMPLETE| QUIC*| |UDPO*| | SEARCHING |SCTP*|
+---+--+ +--+--+ |
+---------------+ +---------------+ +--+--+
| ^ ^ | | ^ | |
+-------+--+ | | |
| | | +-----------------------------------------+ |
+-+-+--+ |
| UDP | MAX_PMTU Probe acked or |
+---+--+ |
| | PTB (BASE_PMTU <= PTB_SIZE < PROBED_SIZE) or
+--------------+-----+-+
| Network Interface |
+----+ PROBE_COUNT = MAX_PROBES +----+
CONFIRMATION_TIMER expiry: PROBE_TIMER expiry:
PROBE_COUNT < MAX_PROBES or PROBE_COUNT < MAX_PROBES or
PLPMTU Probe acked Probe acked
+----------------------+

Figure 4: State machine for Datagram PLPMTUD. Note: Some state
changes are not show to simplify the diagram.

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.

BASE: The BASE state is used to confirm that the BASE_PMTU 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 1: Examples where traffic is black holed while searching for a larger
PLPMTU.

On entry, the PROBED_SIZE DPLPMTUD can be implemented

The central idea of DPLPMTUD is set probing by a sender. Probe packets
are sent to find the BASE_PMTU maximum size and the
PROBE_COUNT is set to zero.

Each time a probe packet is sent, and 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; of a
PTB user message is validated. This causes that can be
completely transferred across the network path from 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_PMTU was successful.
destination.

The PROBE_COUNT is set to zero when folloowing sections identify the first probe packet is sent components needed for each probe size. Each time a probe packet is acknowledged,
the PLPMTU is set to
implementation, provides an overvoew of the PROBED_SIZE, phases of operation, and then the PROBED_SIZE is
increased using
specifies the state machine and search algorithm.

When a probe packet is sent and not acknowledged within the period
of the PROBE_TIMER,

5.1. DPLPMTUD Components

This section describes the PROBE_COUNT is incremented timers, constants, and the probe
packet is retransmitted. variables of
DPLPMTUD.

5.1.1. Timers

The state method utilizes up to three timers:

PROBE_TIMER: The PROBE_TIMER is exited when the PROBE_COUNT
reaches MAX_PROBES; configured to expire after a PTB message is validated;
period longer than the maximum time to receive
an acknowledgment to a probe of size
MAX_PMTU is acknowledged or black hole detection is triggered.

SEARCH_COMPLETE: The SEARCH_COMPLETE state indicates a successful
end to packet. This value
MUST NOT be smaller than 1 second, and SHOULD be
larger than 15 seconds. Guidance on selection
of the PROBE_SEARCH state. DPLPMTUD remains timer value are provided in this state
until either section 3.1.1
of the UDP Usage Guidelines [RFC8085].

If the PMTU_RAISE_TIMER expires; a received PTB message
is validated; or black hole detection is triggered.

When DPLPMTUD uses an unacknowledged PL has a path Round Trip Time (RTT)
estimate and timely acknowledgements the
PROBE_TIMER can be derived from the PL RTT
estimate.

PMTU_RAISE_TIMER: The PMTU_RAISE_TIMER is in configured to the
SEARCH_COMPLETE state, period
a CONFIRMATION_TIMER periodically resets sender will continue to use the PROBE_COUNT and schedules a probe packet with current
PLPMTU, after which it re-enters the size Search
phase. This timer has a period of 600 secs, as
recommended by PLPMTUD [RFC4821].

DPLPMTUD MAY inhibit sending probe packets when
no application data has been sent since the
PLPMTU. If the
previous probe packet fails packet. A PL preferring to be acknowledged after
MAX_PROBES attempts, the method enters the BASE state. use
an up-to-data PMTU once user data is sent again,
can choose to continue PMTU discovery for each
path. However, this may result in sending
additional packets.

CONFIRMATION_TIMER: When used
with an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue
to generate PLPMTU probes in is used, this state.

ERROR: The ERROR state represents the case where either timer MUST
NOT be used. For other PLs, the network
path
CONFIRMATION_TIMER is not known configured to support the period a PLPMTU of at least
PL sender waits before confirming the BASE_PMTU
size or when there current
PLPMTU is contradictory information about the network
path that would otherwise result in excessive variation in still supported. This is less than
the MPS
signalled PMTU_RAISE_TIMER and used to decrease the higher layer. The state implements
PLPMTU (e.g., when a method black hole is encountered).
Confirmation needs to
mitigate oscillation in be frequent enough when
data is flowing that the state-event engine. It signals a
conservative value sending PL does not
black hole extensive amounts of traffic.
Guidance on selection of the MPS to the higher layer by timer value are
provided in section 3.1.1 of the PL. The
state is exited UDP Usage
Guidelines [RFC8085].

DPLPMTUD MAY inhibit sending probe packets when Packet Probes
no longer detect the error or
when the PL indicates that connectivity application data has been lost.

Implementations are permitted to enable endpoint fragmentation if sent since the DPLPMTUD is unable
previous probe packet. A PL preferring to validate MIN_PMTU within PROBE_COUNT
probes. If DPLPMTUD use
an up-to-data PMTU once user data is unable sent again,
can choose to validate MIN_PMTU the continue PMTU discovery for each
path. However, this may result in sending
additional packets.

An implementation should transition to PROBE_DISABLED.

Appendix A contains an informative description could implement the various timers using a single
timer.

5.1.2. Constants

The following constants are defined:

MAX_PROBES: The MAX_PROBES is the maximum value of key events.

5.4. Search to Increase the PLPMTU

This section describes PROBE_COUNT
counter (see Section 5.1.3). The default value of
MAX_PROBES is 10.

MIN_PMTU: The MIN_PMTU is the algorithms used by DPLPMTUD to search for
a larger PLPMTU.

5.4.1. Probing for a Larger PLPMTU

Implementations use a search algorithm across smallest allowed probe packet size.
For IPv6, this value is 1280 bytes, as specified in
[RFC2460]. For IPv4, the search range minimum value is 68 bytes.

Note: An IPv4 router is required to
determine whether a larger PLPMTU can be supported across able to forward a network
path.

The method discovers
datagram of 68 bytes without further fragmentation.
This is the search range by confirming combined size of an IPv4 header and the
minimum
PLPMTU and then using 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 3.3.3 of [RFC1122].

MAX_PMTU: The MAX_PMTU is the probe method largest size of PLPMTU. This has to select a PROBED_SIZE
be less than or equal to MAX_PMTU. MAX_PMTU is the minimum of the local MTU of
the outgoing interface and EMTU_R (learned from the remote endpoint). The MAX_PMTU destination PMTU for
receiving. An application, or PL, MAY be
reduced by an application that sets a maximum to reduce the size of
datagrams it will send.

The PROBE_COUNT is initialised to zero
MAX_PMTU when a probe packet is first
sent with a particular size. A timer there is used by the search algorithm no need to trigger the sending of probe send packets of size PROBED_SIZE, larger
than the PLPMTU. Each probe packet successfully sent to the remote
peer is confirmed by acknowledgement at the PL, see Section 4.1.

Each time a probe packet specific size.

BASE_PMTU: The BASE_PMTU is sent a configured size expected to the destination, the PROBE_TIMER
is started. work for
most paths. The timer size is cancelled when the PL receives
acknowledgment that equal to or larger than the probe packet has been successfully sent
across
MIN_PMTU and smaller than the path Section 4.1. This confirms that MAX_PMTU. In the PROBED_SIZE case of
IPv6, this value is
supported, and the 1280 bytes [RFC2460]. When using
IPv4, a size of 1200 bytes is RECOMMENDED.

5.1.3. Variables

This method utilizes a set of variables:

PROBED_SIZE: The PROBED_SIZE value is then assigned to the PLPMTU.
The search algorithm can continue to send subsequent probe packets size of
an increasing size.

If the timer expires before a current probe packet
packet. This is acknowledged, the probe
has failed to confirm the PROBED_SIZE. Each time the PROBE_TIMER
expires, a tentative value for the PROBE_COUNT PLPMTU,
which is incremented, the PROBE_TIMER awaiting confirmation by an acknowledgment.

PROBE_COUNT: The PROBE_COUNT is
reinitialised, and a probe packet count of the same number of
unsuccessful probe packets that have been sent with a
size of PROBED_SIZE. The value is retransmitted
(the replicated probe improve the resilience initialized to loss). The maximum
number of retransmissions for zero
when a particular size of PROBED_SIZE is configured
(MAX_PROBES). If first
attempted.

The figure below illustrates the value of relationship between the PROBE_COUNT reaches MAX_PROBES,
probing will stop, packet size
constants and the PL sender enters the SEARCH_COMPLETE
state.

5.4.2. Selection of Probe Sizes

The search algorithm needs to determine a minimum useful gain in
PLPMTU. It would not be constructive for variables at a PL sender to attempt to
probe for all sizes - this would incur unnecessary load on the path
and has the undesirable effect point of slowing the time to reach a more
optimal MPS. Implementations SHOULD select when the set of probe packet
sizes DPLPMTUD
algorithm performs path probing to maximise 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 increase the
appropriate next size to search, an implementor ought to also
consider that there can be common sizes of MPS that applications seek
to use.

xxx Author Note: the PLPMTU.
A future version probe packet has been sent of size PROBED_SIZE. Once this section is
acknowledged, the PLPMTU will detail example
methods for selecting probe size values, but does not plan to mandate
a single method. xxx

5.4.3. Resilience raise to Inconsistent Path Information

A decision PROBED_SIZE allowing the
DPLPMTUD algorithm to further increase PROBED_SIZE towards the actual
PMTU.

MIN_PMTU MAX_PMTU
<-------------------------------------------------->
| | | |
v | | v
BASE_PMTU | v Actual PMTU
| PROBED_SIZE
v
PLPMTU needs to be resilient to the
possibility that information learned about the network path is
inconsistent (this could happen when probe packets are lost due to
other reasons, or some

Figure 2: Relationships between packet size constants and variables

5.1.4. Overview of the packets in a flow are forwarded along DPLPMTUD Phases

This section provides a
portion high-level informative view of the path that supports a different actual PMTU).

Frequent path changes could occur due to unexpected "flapping" -
where some packets from a flow pass along one path, but other packets
follow a different path with different properties. DPLPMTUD can be
made resilient to these anomalies
method, by introducing hysteresis into describing the
search decision movement of the method through several
phases of operation. More detail is available in the state machine
Section 5.2.

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

Figure 3: DPLPMTUD Phases

BASE_PMTU Confirmation Phase
* The BASE_PMTU Confirmation Phase confirms connectivity to increase the MPS.

6. Specification of Protocol-Specific Methods
remote peer. This section specifies protocol-specific details for datagram PLPMTUD phase is implicit for IETF-specified transports.

The first subsection provides guidance on how to implement the
DPLPMTUD method as a part of connection-oriented
PL (where it can be performed in a PL connection handshake). A
connectionless PL needs to send an application using UDP or UDP-Lite. acknowledged probe packet to
confirm that the remote peer is reachable.

* The guidance sender also applies to other datagram services confirms that do BASE_PMTU is supported across the
network path.

* A PL that does not
include wish to support a specific transport protocol (such as path with a tunnel
encapsulation). The following subsections describe how DPLPMTUD PLPMTU less
than BASE_PMTU can
be implemented as a part of the transport service, allowing
applications using simplify the service to benefit from discovery of phase into a single step by
performing the
PLPMTU without themselves needing to implement this method.

6.1. Application support for DPLPMTUD connectivity checks with UDP or UDP-Lite

The current specifications of UDP [RFC0768] and UDP-Lite [RFC3828] do
not define a method in the RFC-series that supports PLPMTUD. In
particular, probe of the UDP transport does not provide
BASE_PMTU size.

* Once confirmed, DPLPMTUD enters the transport layer
features needed Search Phase. If this
phase fails to implement datagram PLPMTUD.

The confirm, DPLPMTUD method can be implemented as enters the Error Phase.

Search Phase
* The Search Phase utilizes a part of an application
built directly or indirectly on UDP or UDP-Lite, but relies on
higher-layer protocol features search algorithm to implement the method [RFC8085].

Some primitives used by DPLPMTUD might not be available via send probe
packets to seek to increase the
Datagram API (e.g., PLPMTU.

* The algorithm concludes when it has found a suitable PLPMTU, by
entering the ability Search Complete Phase.

* A PL could respond to access PTB messages using the PLPMTU cache, or
interpret received PTB messages).

In addition, it is desirable that PMTU discovery is not performed to advance or
terminate the search, see Section 4.4.

* Black Hole Detection can also terminate the search by
multiple protocol layers. An application SHOULD avoid implementing
DPLPMTUD entering
the BASE_PMTU Confirmation phase.

Search Complete Phase
* The Search Complete Phase is entered when the underlying transport system provides this
capability. Using PLPMTU is
supported across the network path.

* A PL can use a common method CONFIRMATION_TIMER to periodically repeat a
probe packet for managing the current PLPMTU has
benefits, both in size. If the ability to share state between different
processes and opportunities sender is
unable to coordinate probing.

6.1.1. Application Request

An application needs an application-layer protocol mechanism (such as
a message acknowledgement method) that solicits a response from confirm reachability (e.g., if the CONFIRMATION_TIMER
expires) or the PL signals a
destination endpoint. lack of reachability, DPLPMTUD
enters the BASE_PMTU Confirmation phase.

* The method SHOULD allow PMTU_RAISE_TIMER is used to periodically resume the sender search
phase to check discover if the value returned in PLPMTU can be raised.

* Black Hole Detection or receipt of a validated PTB message
Section 4.4.1) can cause the response sender to provide additional protection
from off-path insertion of data [RFC8085], suitable methods include enter the BASE_PMTU
Confirmation Phase.

Error Phase
* The Error Phase is entered when there is conflicting or invalid
PLPMTU information for the path (e.g. a
parameter known only failure to support the two endpoints, such as a session ID or
initialised sequence number.

6.1.2. Application Response

An application needs an application-layer protocol mechanism
BASE_PMTU) that cause DPLPMTUD to
communicate be unable to progress and the response from
PLPMTU is lowered

* DPLPMTUD remains in the destination endpoint. This
response may indicate successful reception Error Phase until a consistent view of
the probe across the
path, but could path can be discovered and it has also indicate been confirmed that some (or all packets) have failed
to reach
the destination.

6.1.3. Sending Application Probe Packets

A probe packet that may carry an application data block, but path supports the
successful transmission of this data BASE_PMTU (or DPLPMTUD is at risk when used for
probing. Some applications suspended).

* Note: MIN_PMTU may prefer be identical to use a probe packet BASE_PMTU, simplifying the
actions in this phase.

An implementation that
does not carry an application data block only reduces the PLPMTU to avoid disruption a suitable size
would be sufficient to
normal data transfer.

6.1.4. Validating ensure reliable operation, but can be very
inefficient when the Path

An application that does not have other higher-layer information
confirming correct delivery actual PMTU changes or when the method (for
whatever reason) makes a suboptimal choice for the PLPMTU.

A full implementation of datagrams SHOULD implement DPLPMTUD provides an algorithm enabling the
CONFIRMATION_TIMER
DPLPMTUD sender to periodically send probe packets while increase the PLPMTU following a change in the
SEARCH_COMPLETE state.

6.1.5. Handling
characteristics of PTB Messages

An application that is able and wishes to receive PTB messages MUST
perform ICMP validation the path, such as specified when a link is reconfigured with
a larger MTU, or when there is a change in Section 5.2 of [RFC8085].
This requires that the application to check each received PTB
messages to validate it set of links traversed
by an end-to-end flow (e.g., after a routing or path fail-over
decision).

5.2. State Machine

A state machine for DPLPMTUD is received depicted in response Figure 4. If multipath
or multihoming is supported, a state machine is needed for each path.

Note: Some state changes are not shown to transmitted
traffic and that simplify the reported diagram.

6.2. DPLPMTUD with UDP Options

UDP Options[I-D.ietf-tsvwg-udp-options] can supply the additional
functionality required to implement DPLPMTUD within the UDP transport
service. Implementing DPLPMTUD using UDP Options avoids the need < PROBED_SIZE

Figure 4: State machine for
each application to implement the DPLPMTUD method.

Section 5.6 of[I-D.ietf-tsvwg-udp-options] defines Datagram PLPMTUD

The following states are defined:

DISABLED: The DISABLED state is the Maximum
Segment Size (MSS) option, which allows initial state before
probing has started. It is also entered from any
other state, when the local sender to indicate PL indicates loss of
connectivity. This state is left, once the EMTU_R PL
indicates connectivity to the peer. remote PL.

BASE: The value received in this option can be BASE state is used to initialise MAX_PMTU.

UDP Options enables padding to be added to UDP datagrams confirm that are
used as Probe Packets. Feedback confirming reception of each Probe
Packet the
BASE_PMTU size is provided supported by two new UDP Options:

o The Probe Request Option (Section 6.2.1) the network path and
is set by a sending PL designed to
solicit a response from a remote endpoint. A four-byte token
identifies each request.

o The Probe Response Option (Section 6.2.2 is generated by the UDP
Options receiver in response allow an application to reception of a previously received
Probe Request Option. Each Probe Response Option echoes a
previously received four-byte token.

The token value allows implementations continue
working when there are transient reductions in the
actual PMTU. It also seeks to be distinguish between
acknowledgements avoid long periods
where traffic is black holed while searching for initial probe packets and acknowledgements
confirming receipt of subsequent probe packets (e.g., travelling
along alternate paths with a
larger RTT). PLPMTU.

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

Each time a probe packet needs to
be uniquely identifiable by is sent, the UDP Options PROBE_TIMER
is started. The state is exited when the probe
packet is acknowledged, and the PL sender within enters
the Maximum
Segment Lifetime (MSL). SEARCHING state.

The UDP Options sender therefore needs to
not recycle token values until they have expired state is also left when the PROBE_COUNT reaches
MAX_PROBES or have been
acknowledged. A 4 byte value for a received PTB message is validated.
This causes the token field provides sufficient
space for multiple unique probes PL sender to be made within enter the MSL. ERROR state.

SEARCHING: The initial value of SEARCHING state is the four byte token field SHOULD be assigned to
a randomised value, as described in section 5.1 of [RFC8085]) to
enhance protection from off-path attacks.

Implementations ought to only send a probe packet with a Request
Probe Option when required by their local main probing state.
This state machine, i.e., is entered when probing to grow the PLPMTU or to confirm for the current PLPMTU.
BASE_PMTU was successful.

The
procedure PROBE_COUNT is set to handle zero when the loss of first probe
packet is sent for each probe size. Each time a response
probe packet is acknowledged, the
responsibility of the sender of the request. Implementations are
allowed PLPMTU is set to track multiple requests
the PROBED_SIZE, and respond to them with a single
packet.

A PL needs to determine that then the path can still support PROBED_SIZE is
increased using the search algorithm.

When a probe packet is sent and not acknowledged
within the size period of
datagram that the application PROBE_TIMER, the
PROBE_COUNT is currently sending in incremented and the DPLPMTUD
search_done probe packet is
retransmitted. The state (i.e., to detect black-holing of data). One way to
achieve this is to send exited when the
PROBE_COUNT reaches MAX_PROBES, a received PTB
message is validated, a probe packets of size PLPMTU MAX_PMTU is
acknowledged, or to utilise a
higher-layer method that provides explicit feedback indicating any
packet loss. Another possibility black hole is to utilise data packets that
carry a Timestamp Option. Reception of detected.

SEARCH_COMPLETE: The SEARCH_COMPLETE state indicates a valid timestamp that was
echoed by the remote endpoint can be used successful
end to infer connectivity.
This can provide useful feedback even over paths with asymmetric
capacity and/or that carry UDP Option flows that have very asymmetric
datagram rates, because an echo of the most recent timestamp still
indicates reception of at least one packet of SEARCHING state. DPLPMTUD remains in
this state until either the transmitted size.
This is sufficient to confirm there PMTU_RAISE_TIMER
expires, a received PTB message is no black hole.

In contrast, when sending validated, or a probe to increase
black hole is detected.

When DPLPMTUD uses an unacknowledged PL and is in
the PLPMTU, SEARCH_COMPLETE state, a timestamp
might be unable to unambiguously identify that CONFIRMATION_TIMER
periodically resets the PROBE_COUNT and schedules a specific
probe packet has been received. Timestamp mechanisms cannot be used to
confirm with the reception size of individual the PLPMTU. If the
probe messages and cannot be used packet fails to stimulate a response from be acknowledged after
MAX_PROBES attempts, the remote peer.

6.2.1. UDP Probe Request Option

The Probe Request Option allows a sending endpoint method enters the BASE
state. When used with an acknowledged PL (e.g.,
SCTP), DPLPMTUD SHOULD NOT continue to solicit a
response from a destination endpoint. generate
PLPMTU probes in this state.

ERROR: The Probe Request Option carries a four byte token set by ERROR state represents the sender.
This token can be set to a value that case where either
the network path is likely to be not known only to support a PLPMTU
of at least the sender (and BASE_PMTU size or when there is sent along the end-to-end path). The initial
value of
contradictory information about the token SHOULD be assigned to a randomised value, as
described network path
that would otherwise result in section 5.1 of [RFC8085]) excessive variation
in the MPS signalled to enhance protection from
off-path attacks. the higher layer. The sender needs
state implements a method to then check the value returned mitigate oscillation
in the UDP Probe
Response Option. The state-event engine. It signals a
conservative value of the Token field, uniquely identifies a
probe within the maximum segment lifetime.

+----------+--------+-----------------+
| Kind=9* | Len=6 | Token |
+----------+--------+-----------------+
1 byte 1 byte 4 bytes

* To be confirmed by IANA.

Figure 5: UDP Probe REQ Option Format

6.2.2. UDP Probe Response Option

The Probe Response Option is generated in response MPS to reception of a
previously received Probe Request Option. This response is generated the higher layer
by the UDP Option processing.

The Probe Response Option carries a four byte token field. PL. The Token
field associates state is exited when packet probes
no longer detect the response with error or when the Token value carried in PL indicates
that connectivity has been lost.

Implementations are permitted to enable endpoint
fragmentation if the
most recently-received Echo Request. The rate of generation of UDP
packets carrying a Probe Response Option DPLPMTUD is expected unable to be less than
once per RTT and SHOULD be rate-limited (see Section 9).

+----------+--------+-----------------+
| Kind=10* | Len=6 | Token |
+----------+--------+-----------------+
1 byte 1 byte 4 bytes

* To be confirmed validate
MIN_PMTU within PROBE_COUNT probes. If DPLPMTUD is
unable to validate MIN_PMTU the implementation
should transition to the DISABLED state.

5.3. Search to Increase the PLPMTU

This section describes the algorithms used by IANA.

Figure 6: UDP Probe RES Option Format

6.3. DPLPMTUD to search for SCTP

Section 10.2 of [RFC4821] specifies
a recommended PLPMTUD probing
method larger PLPMTU.

5.3.1. Probing for SCTP. It recommends the a larger PLPMTU

Implementations use of a search algorithm across the PAD chunk, defined in
[RFC4820] to be attached search range to
determine whether a minimum length HEARTBEAT chunk to build larger PLPMTU can be supported across a probe packet. This enables probing without affecting network
path.

The method discovers the transfer
of user messages search range by confirming the minimum
PLPMTU and without interfering with congestion control.
This is preferred to then using DATA chunks (with padding as required) as
path probes.

XXX Author Note: Future versions of this document might define a
parameter contained in the INIT and INIT ACK chunk probe method to indicate the
remote peer MTU select a PROBED_SIZE less
than or equal to MAX_PMTU. MAX_PMTU is the minimum of the local peer. However, multihoming makes this a
bit complex, so it might not be worth doing. XXX

6.3.1. SCTP/IPv4 MTU
and SCTP/IPv6 EMTU_R (learned from the remote endpoint). The base protocol is specified in [RFC4960]. This provides MAX_PMTU MAY be
reduced by an
acknowledged PL. A sender can therefore enter application that sets a maximum to the PROBE_BASE state
as soon as connectivity has been confirmed.

6.3.1.1. Sending SCTP Probe Packets

Probe packets consist size of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk.
datagrams it will send.

The PAD chunk PROBE_COUNT is used initialized to control
the length of the zero when a probe packet. The HEARTBEAT chunk packet is first
sent with a particular size. A timer is used by the search algorithm
to trigger the sending of a HEARTBEAT ACK chunk. The reception probe packets of size PROBED_SIZE, larger
than the
HEARTBEAT ACK chunk acknowledges reception of PLPMTU. Each probe packet successfully sent to the remote
peer is confirmed by acknowledgement at the PL, see Section 4.1.

Each time a successful probe. probe packet is sent to the destination, the PROBE_TIMER
is started. The HEARTBEAT chunk carries a Heartbeat Information parameter which
should include, besides timer is canceled when the information suggested in [RFC4960], PL receives
acknowledgment that the probe size, which packet has been successfully sent
across the path Section 4.1. This confirms that the PROBED_SIZE is
supported, and the size of PROBED_SIZE value is then assigned to the complete datagram. PLPMTU.
The size search algorithm can continue to send subsequent probe packets of
an increasing size.

If the PAD chunk timer expires before a probe packet is therefore computed by reducing acknowledged, the probing size by probe
has failed to confirm the IPv4 or IPv6 header size, PROBED_SIZE. Each time the SCTP common header, PROBE_TIMER
expires, the HEARTBEAT
request PROBE_COUNT is incremented, the PROBE_TIMER is
reinitialized, and a probe packet of the PAD chunk header. same size is retransmitted
(the replicated probe improve the resilience to loss). The payload maximum
number of retransmissions for a particular size 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 needs to determine 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 PAD chunk
contains arbitrary data.

To avoid fragmentation of retransmitted data, probing starts right
after path
and has the handshake, before data is sent. Assuming normal behaviour
(i.e., undesirable effect of slowing the PMTU is smaller than or equal time to the interface MTU), this
process will take reach a few round trip time periods depending on more
optimal MPS. Implementations SHOULD select the
number set of PMTU probe packet
sizes probed. The Heartbeat timer can be used to
implement the PROBE_TIMER.

6.3.1.2. Validating the Path with SCTP

Since SCTP provides an acknowledged PL, a sender MUST NOT implement maximize the CONFIRMATION_TIMER while gain in PLPMTU from each search step.

Implementations could optimize the SEARCH_COMPLETE state.

6.3.1.3. PTB Message Handling search procedure by SCTP

Normal ICMP validation MUST selecting step
sizes from a table of common PMTU sizes. When selecting the
appropriate next size to search, an implementor ought to also
consider that there can be performed as specified in Appendix C common sizes of [RFC4960]. This requires MPS that the first 8 bytes applications seek
to use, and their could be common sizes of MTU used within the SCTP
common header
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 quoted in 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 from the payload sequence of
PLPMTU probes that it sends or the sequence of PTB message, which can
be the case for ICMPv4 and messages that it
receives. When inconsistent path information is normally detected, a PL
sender could use an alternate search mode that clamps the case offered MPS
to a smaller value for ICMPv6.

When a PTB message has been validated, the PTB_SIZE reported in period of time. This avoids unnecessary
loss of packets due to MTU limitation.

5.4. Robustness to Inconsistent Paths

Some paths could be unable to sustain packets of the
PTB message SHOULD BASE_PMTU size.
To be used with robust to these paths an implementation could implement the DPLPMTUD algorithm, providing
that
Error State. This allows fallback to a smaller than desired PLPMTU,
rather than suffer connectivity failure. This could utilize methods
such as endpoint IP fragmentation to enable the reported PTB_SIZE is less PL sender to
communicate using packets smaller than the current probe size.

6.3.2. DPLPMTUD for SCTP/UDP

The UDP encapsulation BASE_PMTU.

6. Specification of SCTP is specified in [RFC6951].

6.3.2.1. Sending SCTP/UDP Probe Packets

Packet probing can be performed as specified in Section 6.3.1.1. Protocol-Specific Methods

This section specifies protocol-specific details for datagram PLPMTUD
for IETF-specified transports.

The
maximum payload is reduced by 8 bytes, which has first subsection provides guidance on how to be considered
when filling the PAD chunk.

6.3.2.2. Validating implement the Path with SCTP/UDP

Since SCTP provides
DPLPMTUD method as a part of an acknowledged PL, 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 sender MUST NOT implement part of the CONFIRMATION_TIMER while in transport service, allowing
applications using the SEARCH_COMPLETE state.

6.3.2.3. Handling service to benefit from discovery of PTB Messages by SCTP/UDP

Normal ICMP validation MUST be performed the
PLPMTU without themselves needing to implement this method.

6.1. Application support for PTB messages as
specified in Appendix C DPLPMTUD with UDP or UDP-Lite

The current specifications of [RFC4960]. This requires that UDP [RFC0768] and UDP-Lite [RFC3828] do
not define a method in the first 8
bytes of RFC-series that supports PLPMTUD. In
particular, the SCTP common header are contained in UDP transport does not provide the PTB message,
which transport layer
features needed to implement datagram PLPMTUD.

The DPLPMTUD method can be the case for ICMPv4 (but note the UDP header also
consumes implemented as a part of an application
built directly or indirectly on UDP or UDP-Lite, but relies on
higher-layer protocol features to implement the quoted packet header) and is normally the case
for ICMPv6. When the validation is completed, the PTB_SIZE indicated
in the PTB message SHOULD be method [RFC8085].

Some primitives used with the by DPLPMTUD providing that
the reported PTB_SIZE is less than might not be available via the current probe size.

6.3.3. DPLPMTUD for SCTP/DTLS

The
Datagram Transport Layer Security (DTLS) encapsulation of SCTP API (e.g., the ability to access the PLPMTU cache, or
interpret received PTB messages).

In addition, it is
specified in [RFC8261]. It desirable that PMTU discovery is used for data channels in WebRTC
implementations.

6.3.3.1. Sending SCTP/DTLS Probe Packets

Packet probing can be done as specified in Section 6.3.1.1.

6.3.3.2. Validating not performed by
multiple protocol layers. An application SHOULD avoid using DPLPMTUD
when the Path with SCTP/DTLS

Since SCTP underlying transport system provides this capability. To
use common method for managing the PLPMTU has benefits, both in the
ability to share state between different processes and opportunities
to coordinate probing.

6.1.1. Application Request

An application needs an acknowledged PL, application-layer protocol mechanism (such as
a message acknowledgement method) that solicits a response from a
destination endpoint. The method SHOULD allow the sender MUST NOT implement to check
the CONFIRMATION_TIMER while value returned in the SEARCH_COMPLETE state.

6.3.3.3. Handling response to provide additional protection
from off-path insertion of PTB Messages by SCTP/DTLS

It is not possible data [RFC8085], suitable methods include a
parameter known only to perform normal ICMP validation the two endpoints, such as specified in
[RFC4960], since even if a session ID or
initialized sequence number.

6.1.2. Application Response

An application needs an application-layer protocol mechanism to
communicate the ICMP message payload contains sufficient
information, response from the reflected SCTP common header would be encrypted.
Therefore it is not possible to process PTB messages at destination endpoint. This
response may indicate successful reception of the PL.

6.4. DPLPMTUD for QUIC

Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a
UDP-based transport probe across the
path, but could also indicate that provides reception feedback. The UDP
payload includes some (or all packets) have failed
to reach the QUIC destination.

6.1.3. Sending Application Probe Packets

A probe packet header, protected payload, and any
authentication fields. QUIC depends on a PMTU of at least 1280
bytes.

Section 9.2 of [I-D.ietf-quic-transport] describes the path
considerations when sending QUIC packets. It recommends that may carry an application data block, but the use
successful transmission of
PADDING frames to build the probe packet. Pure probe-only packets
are constructed with PADDING frames and PING frames this data is at risk when used for
probing. Some applications may prefer to create use a
padding only probe packet that will elicit
does not carry an acknowledgement. Padding
only frames enable probing application data block to avoid disruption to data
transfer.

6.1.4. Validating the without affecting Path

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

6.1.5. Handling of
other QUIC frames.

The recommendation for QUIC endpoints implementing DPLPMTUD is
therefore PTB Messages

An application that a MPS is maintained for each combination of local able and
remote IP addresses [I-D.ietf-quic-transport]. If a QUIC endpoint
determines wishes to receive PTB messages MUST
perform ICMP validation as specified in Section 5.2 of [RFC8085].
This requires that the PMTU between any pair of local and remote IP
addresses has fallen below an acceptable MPS, application to check each received PTB
messages to validate it needs is received in response to immediately
cease sending QUIC packets on transmitted
traffic and that the affected path. This could result
in termination of reported PTB_SIZE is less than the connection if an alternative path cannot be
found [I-D.ietf-quic-transport].

6.4.1. Sending QUIC Probe Packets current
probed size (see Section 4.4.2). A probe packet consists validated PTB message MAY be used
as input to the DPLPMTUD algorithm, but MUST NOT be used directly to
set the PLPMTU.

6.2. DPLPMTUD for SCTP

Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing
method for SCTP. It recommends the use of the PAD chunk, defined in
[RFC4820] to be attached to a QUIC Header and minimum length HEARTBEAT chunk to build
a payload containing
PADDING Frames probe packet. This enables probing without affecting the transfer
of user messages and without interfering with congestion control.
This is preferred to using DATA chunks (with padding as required) as
path probes.

XXX Author Note: Future versions of this document might define a PING Frame. PADDING Frames are a single octet
(0x00)
parameter contained in the INIT and several of these can be used INIT ACK chunk to create indicate the
remote peer MTU to the local peer. However, multihoming makes this a probe packet of
size PROBED_SIZE. QUIC
bit complex, so it might not be worth doing. XXX

6.2.1. SCTP/IPv4 and SCTP/IPv6

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

6.2.1.1. 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.

The HEARTBEAT chunk carries a Heartbeat Information parameter which
should include, besides the information suggested in [RFC4960], the
probe size, which is the size of the complete datagram. The size of
the PAD chunk is therefore computed by reducing the probing size by
the IPv4 or IPv6 header size, the SCTP common header, the HEARTBEAT
request and the PAD chunk header. The current specification payload of QUIC sets the following:

o BASE_PMTU: 1200. A QUIC sender needs to pad initial packets to
1200 bytes PAD chunk
contains arbitrary data.

To avoid fragmentation of retransmitted data, probing starts right
after the PL handshake, before data is sent. Assuming this behavior
(i.e., the PMTU is smaller than or equal to confirm the path can support packets of interface MTU), this
process will take a useful
size.

o MIN_PMTU: 1200 bytes. A QUIC sender that determines few round trip time periods depending on the
number of PMTU has
fallen below 1200 bytes MUST immediately stop sending on sizes probed. The Heartbeat timer can be used to
implement the
affected path.

6.4.2. PROBE_TIMER.

6.2.1.2. Validating the Path with QUIC

QUIC SCTP

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

6.4.3. Handling of

6.2.1.3. PTB Messages by QUIC

QUIC operates over the UDP transport, and the guidelines on ICMP
validation as specified in Section 5.2 of [RFC8085] therefore apply.
In addition to UDP Port validation QUIC can validate an ICMP message
by looking for valid Connection IDs in the quoted packet.

7. Acknowledgements

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).

8. IANA Considerations

This memo includes no request to IANA.

XXX If new UDP Options are specified in this document, a request to
IANA will be included here. XXX

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

9. Security Considerations

The security considerations for Message Handling by SCTP

Normal ICMP validation MUST be performed as specified in Appendix C
of [RFC4960]. This requires that the use first 8 bytes of UDP and the SCTP
common header are provided quoted in the references RFCs. Security guidance for applications using UDP
is provided in payload of the UDP Usage Guidelines [RFC8085], specifically PTB message, which can
be the
generation of probe packets case for ICMPv4 and is regarded as normally the case for ICMPv6.

When a "Low Data-Volume
Application", described PTB message has been validated, the PTB_SIZE reported in section 3.1.3 of this document. This
recommends the
PTB message SHOULD be used with the DPLPMTUD algorithm, providing
that sender limits generation of probe packets to an
average rate lower the reported PTB_SIZE is less than one probe per 3 seconds.

A PL sender needs to ensure that the method used to confirm reception
of current probe packets offers protection from off-path attackers injecting
packets into the path. This protection if provided in IETF-defined
protocols (e.g., TCP, SCTP) using a randomly-initialised sequence
number. A description of one way to do this when using size (see
Section 4.4).

6.2.2. DPLPMTUD for SCTP/UDP

The UDP encapsulation of SCTP is
provided specified in section 5.1 of [RFC8085]).

There are cases where ICMP [RFC6951].

6.2.2.1. Sending SCTP/UDP Probe Packets

Packet Too Big (PTB) messages are not
delivered due to policy, configuration or equipment design (see probing can be performed as specified in Section 1.1), this method therefore does not rely upon PTB messages
being received, but 6.2.1.1. The
maximum payload is able to utilise these when they are received reduced by the sender. PTB messages could potentially be used to cause a
node 8 bytes, which has to inappropriately reduce be considered
when filling the PLPMTU. A node supporting
DPLPMTUD PAD chunk.

6.2.2.2. Validating the Path with SCTP/UDP

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

6.2.2.3. Handling of PTB Messages by SCTP/UDP

ICMP validation MUST be performed for PTB messages to ensure these are received as specified in response to transmitted
traffic (i.e., a reported error condition
Appendix C of [RFC4960]. This requires that corresponds to a
datagram actually sent by the path layer, see Section 4.4.1).

An on-path attacker, able to create a PTB message could forge first 8 bytes of the
SCTP common header are contained in the PTB
messages that include message, which can be the
case for ICMPv4 (but note the UDP header also consumes a valid part of the
quoted IP packet. Such an attack could packet header) and is normally the case for ICMPv6. When the
validation is completed, the PTB_SIZE indicated in the PTB message
SHOULD be used to drive down with the PLPMTU. There are two ways this method can
be mitigated against such attacks: First, by ensuring DPLPMTUD providing that a PL
sender never reduces the PLPMTU below reported PTB_SIZE
is less than the base size, solely current probe size.

6.2.3. DPLPMTUD for SCTP/DTLS

The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
specified in
response to receiving [RFC8261]. It is used for data channels in WebRTC
implementations.

6.2.3.1. Sending SCTP/DTLS Probe Packets

Packet probing can be done as specified in Section 6.2.1.1.

6.2.3.2. 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.

6.2.3.3. Handling of PTB message. This is achieved Messages by first
entering SCTP/DTLS

It is not possible to perform ICMP validation as specified in
[RFC4960], since even if the PROBE_BASE state when such a ICMP message is received.
Second, payload contains sufficient
information, the design does reflected SCTP common header would be encrypted.
Therefore it is not require processing of PTB messages, a PL
sender could therefore suspend processing of possible to process PTB messages (e.g., in at the PL.

6.3. DPLPMTUD for QUIC

Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a
robustness mode after detecting that subsequent probes actually
confirm
UDP-based transport that a size larger than provides reception feedback. The UDP
payload includes the PTB_SIZE is supported by QUIC packet header, protected payload, and any
authentication fields. QUIC depends on a path).

Parallel forwarding paths SHOULD be considered. PMTU of at least 1280
bytes.

Section 5.2.5.1
identifies the need for robustness in 14.1 of [I-D.ietf-quic-transport] describes the method path
considerations when sending QUIC packets. It recommends the path
information may be inconsistent.

A node performing use of
PADDING frames to build the probe packet. Pure probe-only packets
are constructed with PADDING frames and PING frames to create a
padding only packet that will elicit an acknowledgement. Such
padding only packets enable probing without affecting the transfer of
other QUIC frames.

The recommendation for QUIC endpoints implementing DPLPMTUD could experience conflicting information
about is that a
MPS is maintained for each combination of local and remote IP
addresses [I-D.ietf-quic-transport]. If a QUIC endpoint determines
that the PMTU between any pair of local and remote IP addresses has
fallen below an acceptable MPS, it needs to immediately cease sending
QUIC packets on the size of supported probe packets. affected path. This could occur when
there are multiple paths are concurrently in use and these exhibit a
different PMTU. If not considered, this could result in data being
black holed when the PLPMTU is larger than the smallest PMTU across termination
of the current paths.

10. References

10.1. Normative References

[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: connection if an alternative path cannot be found
[I-D.ietf-quic-transport].

6.3.1. Sending QUIC Probe Packets

A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-16 (work
in progress), October 2018.

[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-05 (work in progress), July 2018.

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

[RFC1191] Mogul, J. and 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>.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.

[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., probe packet consists of a QUIC Header and P. Lei, "Padding Chunk a payload containing
PADDING Frames 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. a PING Frame. PADDING Frames are a single octet
(0x00) and R. Stewart, "UDP Encapsulation several of Stream
Control Transmission Protocol (SCTP) Packets for End-Host these can be used to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.

[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.

[RFC8174] Leiba, B., "Ambiguity create a probe packet 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>.

[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
size PROBED_SIZE. QUIC provides an acknowledged PL, a sender can
therefore enter the BASE state as soon as connectivity has been
confirmed.

The current specification of
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.

10.2. Informative References

[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels QUIC sets the following:

* BASE_PMTU: 1200. A QUIC sender needs to pad initial packets to
1200 bytes to confirm the path can support packets of a useful
size.

* MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU has
fallen below 1200 bytes MUST immediately stop sending on the
affected path.

6.3.2. Validating the Path with QUIC

QUIC provides an acknowledged PL. A sender therefore MUST NOT
implement the CONFIRMATION_TIMER while in the Internet
Architecture", draft-ietf-intarea-tunnels-09 (work SEARCH_COMPLETE state.

6.3.3. Handling of PTB Messages by QUIC

QUIC operates over the UDP transport, and the guidelines on ICMP
validation as specified in
progress), July 2018.

[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 Section 5.2 of [RFC8085] therefore apply.
In addition to UDP Port validation QUIC can validate an ICMP message
by looking 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 valid Connection IDs in the quoted packet.

6.4. DPLPMTUD 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 UDP-Options

UDP Options [I-D.ietf-tsvwg-udp-options] provides a way to extend UDP
to provide new transport mechanisms.

Support for using DPLPMTUD 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., UDP-Options is defined in the UDP-
Options specification [I-D.ietf-tsvwg-udp-options].

7. Acknowledgements

This work was partially funded by the European Union's Horizon 2020
research and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) innovation programme under grant agreement No. 644334
(NEAT). The views expressed are solely those of the author(s).

8. IANA Considerations

This memo includes no request to IANA.

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

9. Security Considerations

The security considerations for the use of UDP and J. Mohacsi, "Recommendations SCTP are provided
in the references RFCs. Security guidance for Filtering
ICMPv6 Messages applications using UDP
is provided in Firewalls", RFC 4890,
DOI 10.17487/RFC4890, May 2007,
<https://www.rfc-editor.org/info/rfc4890>.

Appendix A. Event-driven state changes the UDP Usage Guidelines [RFC8085], specifically the
generation of probe packets is regarded as a "Low Data-Volume
Application", described in section 3.1.3 of this document. This appendix contains
recommends that sender limits generation of probe packets to an informative description
average rate lower than one probe per 3 seconds.

A PL sender needs to ensure that the method used to confirm reception
of key events:

Path Setup: When probe packets offers protection from off-path attackers injecting
packets into the path. This protection if provided in IETF-defined
protocols (e.g., TCP, SCTP) using a new path randomly-initialized sequence
number. A description of one way to do this when using UDP is initiated, the state
provided in section 5.1 of [RFC8085]).

There are cases where ICMP Packet Too Big (PTB) messages are not
delivered due to policy, configuration or equipment design (see
Section 1.1), this method therefore does not rely upon PTB messages
being received, but is set able to
PROBE_START. This sends utilize these when they are received
by the sender. PTB messages could potentially be used to cause a probe packet with
node to inappropriately reduce the size of PLPMTU. A node supporting
DPLPMTUD MUST therefore appropriately validate the
BASE_PMTU. As soon as 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 is confirmed, the state changes layer, see Section 4.4.1).

An on-path attacker, able to
PROBE_SEARCH.

Arrival of create a PTB message could forge PTB
messages that include a valid quoted IP packet. Such an Acknowledgment: Depending on attack could
be used to drive down the probing state, PLPMTU. There are two ways this method can
be mitigated against such attacks: First, by ensuring that a PL
sender never reduces the
reaction differs according PLPMTU below the base size, solely in
response to Figure 7, which is receiving a simplification
of Figure 4 focusing on this event.

+--------------+ +----------------+
| PROBE_START | --3------------------------------> | PROBE_DISABLED |
+--------------+ --4---------------- ------------> +----------------+
\/
+--------------+ /\ +--------------+
| PROBE_ERROR | -------------------- \ ----------> | PROBE_BASE |
+--------------+ --4--------------/ \ +--------------+
\
+--------------+ --1 -------- \ +--------------+
| PROBE_BASE | \ --- \ ------> | PROBE_ERROR |
+--------------+ --3--------- \ -----/ \ +--------------+
\ \
+--------------+ \ -----> +--------------+
| PROBE_SEARCH | --2--- -----------------> | PROBE_SEARCH |
+--------------+ \ ------------------> +--------------+
\ ---- /
+---------------+ / \ +---------------+
|SEARCH_COMPLETE| -1--- \ |SEARCH_COMPLETE|
+---------------+ -5-- -----------------------> +---------------+
\
\ +--------------+
--------------------------> | PROBE_BASE |
+--------------+

Condition 1: The maximum PMTU size has not yet been reached.
Condition 2: The maximum PMTU size has been reached. Condition 3:
Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4:
PROBE_ACK received. Condition 5: Black hole detected.

Figure 7: State changes at PTB message. This is achieved by first
entering the arrival of an acknowledgment

Probing timeout: The PROBE_COUNT BASE state when such a message is initialised to zero each time received. Second, the value
design does not require processing of PROBED_SIZE is changed and when PTB messages, a acknowledgment
confirming delivery PL sender could
therefore suspend processing of PTB messages (e.g., in a probe packet. The PROBE_TIMER is started
each time robustness
mode after detecting that subsequent probes actually confirm that a probe packet is sent. It
size larger than the PTB_SIZE is stopped supported by a path).

Parallel forwarding paths SHOULD be considered. Section 5.4
identifies the need for robustness in the method when an
acknowledgment arrives that confirms delivery the path
information may be inconsistent.

A node performing DPLPMTUD could experience conflicting information
about the size of supported probe packets. This could occur when
there are multiple paths are concurrently in use and these exhibit a probe packet of
PROBED_SIZE.
different PMTU. If the probe packet is not acknowledged before the
PROBE_TIMER expires, considered, this could result in data being
black holed when the PROBE_COUNT PLPMTU is incremented. When the
PROBE_COUNT equals the value MAX_PROBES, larger than the state is changed,
otherwise a new probe packet of smallest PMTU across
the same size (PROBED_SIZE) is
resent. The state transitions are illustrated current paths.

10. References

10.1. Normative References

[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-20 (work
in Figure 8. This
shows a simplification of Figure 4 with a focus only on this
event.

+--------------+ +----------------+
| PROBE_START | --2------------------------------->| PROBE_DISABLED |
+--------------+ +----------------+

+--------------+ +--------------+
| PROBE_ERROR | -----------------> | PROBE_ERROR |
+--------------+ / +--------------+
/
+--------------+ --2----------/ +--------------+
| PROBE_BASE | --1------------------------------> | PROBE_BASE |
+--------------+ +--------------+

+--------------+ +--------------+
| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |
+--------------+ --2--------- +--------------+
\
+---------------+ \ +---------------+
|SEARCH_COMPLETE| -------------------> |SEARCH_COMPLETE|
+---------------+ +---------------+

Condition 1: The maximum number of probe packets has not been
reached. Condition 2: The maximum number of probe packets has been
reached. XXX This diagram has not been validated.

Figure 8: State changes at progress), 23 April 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-20.txt>.

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

[RFC1191] Mogul, J.C. and S.E. 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>.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.

[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the expiration 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 the probe timer

PMTU raise timer timeout: DPLPMTUD periodically sends a probe packet Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to detect whether a larger PMTU is possible. This probe packet is
generated by the PMTU_RAISE_TIMER.

Arrival of a PTB message: The active probing End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.

[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.

[RFC8174] Leiba, B., "Ambiguity of the path can be
supported by the arrival 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>.

[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 a PTB message indicating the PTB_SIZE.
Two examples are:

1. The PTB_SIZE is between the PLPMTU
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.

10.2. Informative References

[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels in the probe that
triggered the PTB message.

2. The PTB_SIZE is smaller than the PLPMTU.

In first case, the PROBE_BASE state transitions to the PROBE_ERROR
state. In the PROBE_SEARCH state, a new probe packet is sent Internet
Architecture", draft-ietf-intarea-tunnels-09 (work in
progress), 19 July 2018,
<http://www.ietf.org/internet-drafts/draft-ietf-intarea-
tunnels-09.txt>.

[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-07 (work in progress), 8 March 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-udp-
options-07.txt>.

[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 size reported by the PTB message.

In second case, the probing starts again with a value of
PROBE_BASE. 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>.

Appendix B. A. Revision Notes

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

Individual draft -00:

* Comments and corrections are welcome directly to the authors or
via the IETF TSVWG working group mailing list.

* This update is proposed for WG comments.

Individual draft -01:

* Contains the first representation 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 to handle a PTB message indicating a link MTU larger than
the probe has yet not been validated by 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
evlauated 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 supsending 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/ 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 dsecription of phases.

Authors' Addresses

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

Email: gorry@erg.abdn.ac.uk

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

Email: tom@erg.abdn.ac.uk

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

Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Steinfurt
48565
DE Steinfurt
Germany

Email: i.ruengeler@fh-muenster.de

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

Email: timo.voelker@fh-muenster.de