draft-ietf-tsvwg-datagram-plpmtud-01.txt   draft-ietf-tsvwg-datagram-plpmtud-02.txt 
Internet Engineering Task Force G. Fairhurst Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones Internet-Draft T. Jones
Intended status: Standards Track University of Aberdeen Updates: 4821 (if approved) University of Aberdeen
Expires: September 6, 2018 M. Tuexen Intended status: Standards Track M. Tuexen
I. Ruengeler Expires: December 08, 2018 I. Ruengeler
Muenster University of Applied Sciences Muenster University of Applied Sciences
March 05, 2018 June 08, 2018
Packetization Layer Path MTU Discovery for Datagram Transports Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-01 draft-ietf-tsvwg-datagram-plpmtud-02
Abstract Abstract
This document describes a robust method for Path MTU Discovery This document describes a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization layers. The method allows a (PMTUD) for datagram Packetization layers. The document describes an
Packetization Layer (PL), or a datagram application that uses a PL, extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path
to probe an network path with progressively larger packets to MTU Discovery for IPv4 and IPv6. The method allows a Packetization
determine a maximum packet size. The document describes an extension Layer (PL), or a datagram application that uses a PL, to discover
to RFC 1191 and RFC 8201, which specify ICMP-based Path MTU Discovery whether a network path can support the current size of datagram and
for IPv4 and IPv6. This provides functionally for datagram to probe a network path with progressively larger packets to find
transports that is equivalent to the Packetization layer PMTUD whether the maxium packet size can be increased. This allows a
specification for TCP, specified in RFC4821. sender to determine an appropriate packet size. This provides
functionally for datagram transports that is equivalent to the
Packetization layer PMTUD specification for TCP, specified in
RFC4821.
The document also provides implementation notes for incorporating
Datagram PMTUD into IETF Datagram transports or applications that use
transports.
When published, this specification updates RFC4821. When published, this specification updates RFC4821.
Status of This Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 08, 2018.
This Internet-Draft will expire on September 6, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . 3 1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . . 3
1.2. Packetization Layer Path MTU Discovery . . . . . . . . . 4 1.2. Packetization Layer Path MTU Discovery . . . . . . . . . . 4
1.3. Path MTU Discovery for Datagram Services . . . . . . . . 5 1.3. Path MTU Discovery for Datagram Services . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 7 3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 8
3.1. PMTU Probe Packets . . . . . . . . . . . . . . . . . . . 10 3.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . . 10
3.2. Validation of the Current Effective PMTU . . . . . . . . 11 3.2. Validation of Probe Packet Size . . . . . . . . . . . . . 11
3.3. Reduction of the Effective PMTU . . . . . . . . . . . . . 11 3.3. Reducing the PLPMTU: Confirming Path Characteristics . . . 12
4. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 12 3.4. Increasing the PLPMTU: Supporting Path Changes . . . . . . 12
4.1. Probing . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.5. Robustness to inconsistent Path information . . . . . . . 12
4.2. Verification and Use of PTB Messages . . . . . . . . . . 13 4. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . . 13
4.3. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.1. PROBE_SEARCH: Probing for a larger PLPMTU . . . . . . . . 13
4.4. Constants . . . . . . . . . . . . . . . . . . . . . . . . 14 4.2. The PROBE_DONE state . . . . . . . . . . . . . . . . . . . 14
4.5. Variables . . . . . . . . . . . . . . . . . . . . . . . . 14 4.3. Verification and Use of PTB Messages . . . . . . . . . . . 14
4.6. Selecting PROBED_SIZE . . . . . . . . . . . . . . . . . . 15 4.4. Timers . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.7. State Machine . . . . . . . . . . . . . . . . . . . . . . 15 4.5. Constants . . . . . . . . . . . . . . . . . . . . . . . . 15
5. Specification of Protocol-Specific Methods . . . . . . . . . 18 4.6. Variables . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. DPLPMTUD for UDP and UDP-Lite . . . . . . . . . . . . . . 18 4.7. Selecting PROBED_SIZE . . . . . . . . . . . . . . . . . . 16
5.1.1. UDP Options . . . . . . . . . . . . . . . . . . . . . 18 4.8. Black Hole Detection . . . . . . . . . . . . . . . . . . . 17
5.1.2. UDP Options Required for PLPMTUD . . . . . . . . . . 18 4.9. State Machine . . . . . . . . . . . . . . . . . . . . . . 17
5.1.2.1. Echo Request Option . . . . . . . . . . . . . . . 19 5. Specification of Protocol-Specific Methods . . . . . . . . . . 20
5.1.2.2. Echo Response Option . . . . . . . . . . . . . . 19 5.1. Application support for DPLPMTUD with UDP or UDP-Lite . . 20
5.1.3. Sending UDP-Option Probe Packets . . . . . . . . . . 19 5.1.1. Application Request . . . . . . . . . . . . . . . . . 20
5.1.4. Validating the Path with UDP Options . . . . . . . . 20 5.1.2. Application Response . . . . . . . . . . . . . . . . . 20
5.1.5. Handling of PTB Messages by UDP . . . . . . . . . . . 20 5.1.3. Sending Application Probe Packets . . . . . . . . . . 21
5.2. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 20 5.1.4. Validating the Path . . . . . . . . . . . . . . . . . 21
5.2.1. SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . 20 5.1.5. Handling of PTB Messages . . . . . . . . . . . . . . . 21
5.2.1.1. Sending SCTP Probe Packets . . . . . . . . . . . 20 5.2. DPLPMTUD with UDP Options . . . . . . . . . . . . . . . . 21
5.2.1.2. Validating the Path with SCTP . . . . . . . . . . 21 5.2.1. UDP Request Option . . . . . . . . . . . . . . . . . . 22
5.2.1.3. PTB Message Handling by SCTP . . . . . . . . . . 21 5.2.2. UDP Response Option . . . . . . . . . . . . . . . . . 22
5.2.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 21 5.3. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 22
5.2.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . 21 5.3.1. SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . . 22
5.2.2.2. Validating the Path with SCTP/UDP . . . . . . . . 21 5.3.1.1. Sending SCTP Probe Packets . . . . . . . . . . . . 22
5.2.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . 21 5.3.1.2. Validating the Path with SCTP . . . . . . . . . . 23
5.2.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 22 5.3.1.3. PTB Message Handling by SCTP . . . . . . . . . . . 23
5.2.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 22
5.2.3.2. Validating the Path with SCTP/DTLS . . . . . . . 22 5.3.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 23
5.2.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 22 5.3.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . . 23
5.3. PMTUD for QUIC . . . . . . . . . . . . . . . . . . . . . 22 5.3.2.2. Validating the Path with SCTP/UDP . . . . . . . . 23
5.3.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 22 5.3.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . . 24
5.3.2. Validating the Path with QUIC . . . . . . . . . . . . 23 5.3.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . . 24
5.3.3. Handling of PTB Messages by QUIC . . . . . . . . . . 23 5.3.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 24
5.4. Other IETF Transports . . . . . . . . . . . . . . . . . . 23 5.3.3.2. Validating the Path with SCTP/DTLS . . . . . . . . 24
5.5. DPLPMTUD by Applications . . . . . . . . . . . . . . . . 23 5.3.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 24
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24 5.4. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 24
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 5.4.1. Sending QUIC Probe Packets . . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24 5.4.2. Validating the Path with QUIC . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 5.4.3. Handling of PTB Messages by QUIC . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 24 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . 26 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
Appendix A. Event-driven state changes . . . . . . . . . . . . . 26 8. Security Considerations . . . . . . . . . . . . . . . . . . . 26
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . 29 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30 9.1. Normative References . . . . . . . . . . . . . . . . . . . 26
9.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. Event-driven state changes . . . . . . . . . . . . . . 28
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction 1. Introduction
The IETF has specified datagram transport using UDP, SCTP, and DCCP, The IETF has specified datagram transport using UDP, SCTP, and DCCP,
as well as protocols layered on top of these transports (e.g., SCTP/ as well as protocols layered on top of these transports (e.g., SCTP/
UDP, DCCP/UDP). UDP, DCCP/UDP) and directly 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 1.1. Classical Path MTU Discovery
Classical Path Maximum Transmission Unit Discovery (PMTUD) can be Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
used with any transport that is able to process ICMP Packet Too Big used with any transport that is able to process ICMP Packet Too Big
(PTB) messages (e.g., [RFC1191] and [RFC8201]). The term PTB message (PTB) messages (e.g., [RFC1191] and [RFC8201]). The term PTB message
is applied to both IPv4 ICMP Unreachable messages (type 3) that carry is applied to both IPv4 ICMP Unreachable messages (type 3) that carry
the error Fragmentation Needed (Type 3, Code 4) and ICMPv6 packet too the error Fragmentation Needed (Type 3, Code 4) and ICMPv6 packet too
big messages (Type 2). When a sender receives a PTB message, it big messages (Type 2). When a sender receives a PTB message, it
reduces the effective Path MTU (PMTU) to the value reported as the reduces the effective MTU to the value reported as the Link MTU in
Link MTU in the PTB message, and a method that from time-to-time the PTB message, and a method that from time-to-time increases the
increases the packet size in attempt to discover an increase in the packet size in attempt to discover an increase in the supported PMTU.
supported PMTU. The packets sent with a size larger than the current The packets sent with a size larger than the current effective PMTU
effective PMTU are known as probe packets. are known as probe packets.
Packets not intended as probe packets are either fragmented to the Packets not intended as probe packets are either fragmented to the
current effective PMTU, or the attempt to send fails with an error current effective PMTU, or the attempt to send fails with an error
code. Applications are sometimes provided with a primitive to let code. Applications are sometimes provided with a primitive to let
them read the maximum packet size, derived from the current effective them read the maximum packet size, derived from the current effective
PMTU. PMTU.
Classical PMTUD is subject to protocol failures. One failure arises Classical PMTUD is subject to protocol failures. One failure arises
when traffic using a packet size larger than the actual supported when traffic using a packet size larger than the actual PMTU is
PMTU is black-holed (all datagrams sent with this size are silently black-holed (all datagrams sent with this size, or larger, are
discarded without the sender receiving ICMP PTB messages. This could silently discarded without the sender receiving ICMP PTB messages).
arise when the ICMP messages are not delivered back to the sender for This could arise when the PTB messages are not delivered back to the
some reason [RFC2923]). For example, ICMP messages are increasingly sender for some reason [RFC2923]). For example, ICMP messages are
filtered by middleboxes (including firewalls) [RFC4890]. Also, in increasingly filtered by middleboxes (including firewalls) [RFC4890].
some cases are not correctly processed by tunnel endpoints. 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. Other examples include cases
where PTB messages are not correctly processed/generated by tunnel
endpoints.
Another failure could result if a node not on the network path sends Another failure could result if a node that is not on the network
a PTB that attempts to force the sender to change the effective PMTU path sends a PTB message that attempts to force the sender to change
[RFC8201]. A sender can protect itself from reacting to such the effective PMTU [RFC8201]. A sender can protect itself from
messages by utilising the quoted packet within the PTB message reacting to such messages by utilising the quoted packet within a PTB
payload to verify that the received PTB message was generated in message payload to verify that the received PTB message was generated
response to a packet that had actually been sent. However, there are in response to a packet that had actually originated from the sender.
situations where a sender would be unable to provide this However, there are situations where a sender would be unable to
verification. provide this verification.
Examples where verification is not possible include: Examples where verification is not possible include:
o When the router issuing the ICMP message is acting on a tunneled o When the router issuing the ICMP message is acting on a tunneled
packet the ICMP message is directed to the tunnel endpoint. This packet, the ICMP message will be directed to the tunnel endpoint.
endpoint is responsible for processed in the quoted packet in the This tunnel endpoint is responsible for forwardiung the ICMP
payload field to remove the effect of the tunnel, and return the message and also processing the quoted packet within the payload
ICMP message to the sender. Failure to do this results in black- field to remove the effect of the tunnel, and return a correctly
holing. fromatted ICMP message to the sender. Failure to do this results
in black-holing.
o When the router issuing the ICMP message implements RFC792 o When a router issuing the ICMP message implements RFC792
[RFC0792], which only requires the quoted payload to include the [RFC0792], it is only required the to include the first 64 bits of
first 64 bits of the IP payload of the packet, and the ICMP the IP payload of the packet within the quoted payload.This may be
message occurs within a tunnel. Even if the decpasulated message insufficient to perfom the tunnel processing described in the
is processed by the tunnel endpoint, there could be insufficient previous bullet. Even if the decapsulated message is processed by
bytes remaining for the sender to read the quoted transport the tunnel endpoint, there could be insufficient bytes remaining
information. RFC1812 [RFC1812] requires routers to return the for the sender to interpret the quoted transport information.
full packet if possible, often the case for IPv4 when used the RFC1812 [RFC1812] requires routers to return the full packet if
path includes tunnels; or where the packet has been encapsulated/ possible, often the case for IPv4 when used the path includes
tunneled over an encrypted transport and it is not possible to tunnels; or where the packet has been encapsulated/tunneled over
determine the original transport header ). an encrypted transport and it is not possible to determine the
original transport header ).
o Even when the PTB message includes sufficient bytes of the quoted o Even when the PTB message includes sufficient bytes of the quoted
packet, the network layer could lack sufficient context to perform packet, the network layer could lack sufficient context to perform
verification, because this depends on information about the active verification, because this depends on information about the active
transport flows at an endpoint node (e.g., the socket/address transport flows at an endpoint node (e.g., the socket/address
pairs being used, and other protocol header information). pairs being used, and other protocol header information).
1.2. Packetization Layer Path MTU Discovery 1.2. Packetization Layer Path MTU Discovery
The term Packetization Layer (PL) has been introduced to describe the The term Packetization Layer (PL) has been introduced to describe the
layer that is responsible for placing data blocks into the payload of layer that is responsible for placing data blocks into the payload of
packets and selecting an appropriate maximum packet size. This IP packets and selecting an appropriate Maximum Packet Size (MPS).
function is often performed by a transport protocol, but can also be This function is often performed by a transport protocol, but can
performed by other encapsulation methods working above the transport. also be performed by other encapsulation methods working above the
PTB verification is more straight forward at the PL or at a higher transport.
layer.
In contrast to PMTUD, Packetization Layer Path MTU Discovery In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) [RFC4821] does not rely upon reception and verification of (PLPMTUD) [RFC4821] does not rely upon reception and verification of
PTB messages. It is therefore more robust than Classical PMTUD. PTB messages. It is therefore more robust than Classical PMTUD. This
This has become the recommended approach for implementing PMTU has become the recommended approach for implementing PMTU discovery
discovery with TCP. with TCP.
It uses a general strategy where the PL sends probe packet to search It uses a general strategy where the PL sends probe packet to search
for an appropriate PMTU. The probe packets are sent a progressively for the largest size of unfragmented datagram that can be sent over a
larger packet size. If a probe packet is successfully delivered (as path. The probe packets are sent with a progressively larger packet
determined by the PL), then the effective Path MTU is raised to the size. If a probe packet is successfully delivered (as determined by
size of the successful probe. If no response is received to a probe the PL), then the PLPMTU is raised to the size of the successful
packet, the method reduces the probe size. 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 PLPMTUD introduces flexibility in the implementation of PMTU
discovery. At one extreme, it can be configured to only perform PTB discovery. At one extreme, it can be configured to only perform PTB
black hole detection and recovery to increase the robustness of black hole detection and recovery to increase the robustness of
Classical PMTUD, or at the other extreme, all PTB processing can be Classical PMTUD, or at the other extreme, all PTB processing can be
disabled and PLPMTUD can completely replace Classical PMTUD. PLPMTUD disabled and PLPMTUD can completely replace Classical PMTUD.
can also include additional consistency checks without increasing the
risk of increased black-holing. 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
verification more straight forward.
1.3. Path MTU Discovery for Datagram Services 1.3. Path MTU Discovery for Datagram Services
Section 4 of this document presents a set of algorithms for datagram Section 4 of this document presents a set of algorithms for datagram
protocols to discover a maximum size for the effective PMTU across a protocols to discover the largest size of unfragmented datagram that
path. The methods described rely on features of the PL Section 3 and can be sent over a path. The method described relies on features of
apply to transport protocols over IPv4 and IPv6. It does not require the PL Section 3 and apply to transport protocols operating over IPv4
cooperation from the lower layers (except that they are consistent and IPv6. It does not require cooperation from the lower layers,
about which packet sizes are acceptable). A method can utilise ICMP although it can utilise ICMP PTB messages when these received
PTB messages when these received messages are made available to the messages are made available to the PL.
PL.
The UDP-Guidelines [RFC8085] state "an application SHOULD either use The UDP Usage Guidelines [RFC8085] state "an application SHOULD
the Path MTU information provided by the IP layer or implement Path either use the Path MTU information provided by the IP layer or
MTU Discovery (PMTUD)", but does not provide a mechanism for implement Path MTU Discovery (PMTUD)", but does not provide a
discovering the largest size of unfragmented datagram than can be mechanism for discovering the largest size of unfragmented datagram
used on a path. Prior to this document, PLPMTUD had not been than can be used on a path. Prior to this document, PLPMTUD had not
specified for UDP. been specified for UDP.
Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the
Stream Control Transport Protocol (SCTP). SCTP utilises heartbeat Stream Control Transport Protocol (SCTP). SCTP utilises heartbeat
messages as probe packets, but RFC4821 does not provide a complete messages as probe packets, but RFC4821 does not provide a complete
specification. This document provides the details to complete that specification. This document provides the details to complete that
specification. specification.
The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
implementations to support Classical PMTUD and states that a DCCP implementations to support Classical PMTUD and states that a DCCP
sender "MUST maintain the maximum packet size (MPS) allowed for each sender "MUST maintain the MPS allowed for each active DCCP session".
active DCCP session". It also defines the current congestion control It also defines the current congestion control MPS (CCMPS) supported
maximum packet size (CCMPS) supported by a path. This recommends use by a path. This recommends use of PMTUD, and suggests use of control
of PMTUD, and suggests use of control packets (DCCP-Sync) as path packets (DCCP-Sync) as path probe packets, because they do not risk
probe packets, because they do not risk application data loss. The application data loss. The method defined in this specification
method defined in this specification could be used with DCCP. could be used with DCCP.
Section 5 specifies the method for a set of transports, and provides Section 5 specifies the method for a set of transports, and provides
information to enables the implementation of PLPMTUD with other information to enables the implementation of PLPMTUD with other
datagram transports and applications that use datagram transports. datagram transports and applications that use datagram transports.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
Other terminology is directly copied from [RFC4821], and the Other terminology is directly copied from [RFC4821], and the
definitions in [RFC1122]. definitions in [RFC1122].
Black-Holed: When the sender is unaware that packets are not Black-Holed: When the sender is unaware that packets are not
delivered to the destination endpoint (e.g., when the sender delivered to the destination endpoint (e.g., when the sender
transmits packets of a particular size with a previously known transmits packets of a particular size with a previously known
PMTU, but is unaware of a change to the path that resulted in a effective PMTU (also refered to as the PLPMTU), but is unaware of
smaller PMTU). a change to the path that resulted in a smaller PLPMTU).
Classical Path MTU Discovery: Classical PMTUD is a process described Classical Path MTU Discovery: Classical PMTUD is a process described
in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
learn the largest size of unfragmented datagram than can be used learn the largest size of unfragmented datagram than can be used
across a path. across a path.
Datagram: A datagram is a transport-layer protocol data unit, Datagram: A datagram is a transport-layer protocol data unit,
transmitted in the payload of an IP packet. transmitted in the payload of an IP packet.
Effective PMTU: The current estimated value for PMTU that is used by Effective PMTU: The current estimated value for PMTU that is used by
a Packetization Layer. a PMTUD. This is equivalent to the PLPMTU derived by PLPMTUD.
EMTU_S: The Effective MTU for sending (EMTU_S) is defined in EMTU_S: The Effective MTU for sending (EMTU_S) is defined in
[RFC1122] as "the maximum IP datagram size that may be sent, for a [RFC1122] as "the maximum IP datagram size that may be sent, for a
particular combination of IP source and destination addresses...". particular combination of IP source and destination addresses...".
EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in
[RFC1122] as the largest datagram size that can be reassembled by [RFC1122] as the largest datagram size that can be reassembled by
EMTU_R ("Effective MTU to receive"). EMTU_R ("Effective MTU to receive").
Link: A communication facility or medium over which nodes can Link: A communication facility or medium over which nodes can
communicate at the link layer, i.e., a layer below the IP layer. communicate at the link layer, i.e., a layer below the IP layer.
Examples are Ethernet LANs and Internet (or higher) layer and Examples are Ethernet LANs and Internet (or higher) layer and
tunnels. tunnels.
Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes of
of the largest IP packet, including the IP header and payload, the largest IP packet, including the IP header and payload, that
that can be transmitted over a link. Note that this could more can be transmitted over a link. Note that this could more
properly be called the IP MT, to be consistent with how other properly be called the IP MTU, to be consistent with how other
standards organizations use the acronym MT. This includes the IP standards organizations use the acronym MT. This includes the IP
header, but excludes link layer headers and other framing that is header, but excludes link layer headers and other framing that is
not part of IP or the IP payload. Other standards organizations not part of IP or the IP payload. Other standards organizations
generally define link MTU to include the link layer headers. generally define link MTU to include the link layer headers.
MPS: The Maximum Packet Size (MPS), the largest size of application MPS: The Maximum Packet Size (MPS) is the largest size of application
data block that can be sent unfragmented across a path. In data block that can be sent unfragmented across a path. In
PLPMTUD this quantity is derived from Effective PMTU by taking DPLPMTUD this quantity is derived from PLPMTU by taking into
into consideration the size of the application and lower protocol consideration the size of the application and lower protocol layer
layer headers, and can be limited by the application protocol. headers.
Packet: An IP header plus the IP payload. Packet: An IP header plus the IP payload.
Packetization Layer (PL): The layer of the network stack that places Packetization Layer (PL): The layer of the network stack that places
data into packets and performs transport protocol functions. data into packets and performs transport protocol functions.
Path: The set of link and routers traversed by a packet between a Path: The set of link and routers traversed by a packet between a
source node and a destination node. source node and a destination node by a particular flow.
Path MTU (PMTU): The minimum of the link MTU of all the links Path MTU (PMTU): The minimum of the Link MTU of all the links forming
forming a path between a source node and a destination node. a path between a source node and a destination node.
PLPMTUD: Packetization Layer Path MTU Discovery, the method PLPMTU: The estimate of the actual PMTU provided by the DPLPMTUD
described in this document for datagram PLs, which is an extension algorithm.
to Classical PMTU Discovery.
Probe packet: A datagram sent with a purposely chosen size PLPMTUD: Packetization Layer Path MTU Discovery, the method described
(typically larger than the current Effective PMTU or MPS) to in this document for datagram PLs, which is an extension to
detect if messages of this size can be successfully sent along the Classical PMTU Discovery.
end-to-end path.
Probe packet: A datagram sent with a purposely chosen size (typically
larger than the current PLPMTU) to detect if packets of this size
can be successfully sent end-toend across the network path.
3. Features Required to Provide Datagram PLPMTUD 3. Features Required to Provide Datagram PLPMTUD
TCP PLPMTUD has been defined using standard TCP protocol mechanisms. TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
All of the requirements in [RFC4821] also apply to use of the All of the requirements in [RFC4821] also apply to use of the
technique with a datagram PL. Unlike TCP, some datagram PLs require technique with a datagram PL. Unlike TCP, some datagram PLs require
additional mechanisms to implement PLPMTUD. additional mechanisms to implement PLPMTUD.
There are nine requirements for performing the datagram PLPMTUD There are eight requirements for performing the datagram PLPMTUD
method described in this specification: method described in this specification:
1. PMTU parameters: A PLPMTUD sender is REQUIRED to provide 1. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide
information about the maximum size of packet that can be information about the maximum size of packet that can be
transmitted by the sender on the local link (the link MTU and MAY transmitted by the sender on the local link (the local Link MTU).
utilize similar information about the receiver when this is It MAY utilize similar information about the receiver when this
supplied (note this could be less than EMTU_R). Some is supplied (note this could be less than EMTU_R). This avoids
applications also have a maximum transport protocol data unit implementations trying to send probe packets that can not be
(PDU) size, in which case there is no benefit from probing for a transmited by the local link. Too high a value may reduce the
size larger than this (unless a transport allows multiplexing efficiency of the search algorithm. Some applications also have
multiple applications PDUs into the same datagram). 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. Effective PMTU: A datagram application MUST be able to choose the 2. PLPMTU: A datagram application MUST be able to choose the size of
size of datagrams sent to the network, up to the effective PMTU, datagrams sent to the network, up to the PLPMTU, or a smaller
or a smaller value (such as the MPS) derived from this. This value (such as the MPS) derived from this. This value is managed
value is managed by the PMTUD method. The effective PMTU by the DPLPMTUD method. The PLPMTU (specified as the effective
(specified in Section 1 of [RFC1191]) is equivalent to the EMTU_S PMTU in Section 1 of [RFC1191]) is equivalent to the EMTU_S
(specified in [RFC1122]). (specified in [RFC1122]).
3. Probe packets: On request, a PLPMTUD sender is REQUIRED to be 3. Probe packets: On request, a PLPMTUD sender is REQUIRED to be
able to transmit a packet larger than the current effective PMTU able to transmit a packet larger than the PLMPMTU. This can be
(but always with a total size less than the link MTU). The uses to send a probe packet. In IPv4, a probe packet MUST be
method can use this as a probe packet. In IPv4, a probe packet sent with the Don't Fragment (DF) bit set in the IP header, and
is always sent with the Don't Fragment (DF) bit set in the IP without network layer endpoint fragmentation. In IPv6, a probe
header, and without network layer endpoint fragmentation. In packet is always sent without source fragmentation (as specified
IPv6, a probe packet is always sent without source fragmentation in section 5.4 of [RFC8201]).
(as specified in section 5.4 of [RFC8201]).
4. Processing PTB messages: A PLPMTUD sender MAY optionally utilize 4. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize
PTB messages received from the network layer to help identify PTB messages received from the network layer to help identify
when a path does not support the current size of packet probe. when a path does not support the current size of packet probe.
Any received PTB message SHOULD/MUST be verified before it is Any received PTB message MUST be verified before it is used to
used to update the PMTU discovery information [RFC8201]. This update the PLPMTU discovery information [RFC8201]. This
verification confirms that the PTB message was sent in response verification confirms that the PTB message was sent in response
to a packet originating by the sender, and needs to be performed to a packet originating by the sender, and needs to be performed
before the PMTU discovery method reacts to the PTB message. When before the PLPMTU discovery method reacts to the PTB message.
the router link MTU is indicated in the PTB message this MAY be When the router link MTU is indicated in the PTB message this MAY
used by datagram PLPMTUD to reduce the size of a probe, but MUST be used by DPLPMTUD to reduce the probe size but MUST NOT be used
NOT be used to increase the effective PMTU ([RFC8201]). to increase the PLPMTU ([RFC8201]). Verification SHOULD utilise
information that can not be simply determined by an off-path
attacker, for example, by checking the value of a protocol header
field known only to the two PL endpoints. (Some datagram
applications use well-known source and destination ports and
therefore this check needs to rely on other information.)
5. Reception feedback: The destination PL endpoint is REQUIRED to 5. Reception feedback: The destination PL endpoint is REQUIRED to
provide a feedback method that indicates to the PLPMTUD sender provide a feedback method that indicates to the DPLPMTUD sender
when a probe packet has been received by the destination when a probe packet has been received by the destination PL
endpoint. The local PL endpoint at the sending node is REQUIRED endpoint. The local PL endpoint at the sending node is REQUIRED
to pass this feedback to the sender-side PLPMTUD method. to pass this feedback to the sender-side DPLPMTUD method.
6. Probing and congestion control: The isolated loss of a probe 6. Probing and congestion control: The isolated loss of a probe
packet SHOULD NOT be treated as an indication of congestion and packet SHOULD NOT be treated as an indication of congestion and
its loss does not directly trigger a congestion control reaction its loss SHOULD NOT directly trigger a congestion control
[RFC4821]. reaction [RFC4821].
7. Probe loss recovery: If the data block carried by a probe message 7. Probe loss recovery: If the data block carried by a probe message
needs to be sent reliably, the PL (or layers above) MUST arrange needs to be sent reliably, the PL (or layers above) MUST arrange
retransmission/repair of any resulting loss. This method MUST be retransmission/repair of any resulting loss. This method MUST be
robust in the case where probe packets are lost due to other robust in the case where probe packets are lost due to other
reasons (including link transmission error, congestion). The reasons (including link transmission error, congestion). The
PLPMTUD method treats isolated loss of a probe packet (with or DPLPMTUD method treats isolated loss of a probe packet (with or
without an PTB message) as a potential indication of a PMTU limit without an PTB message) as a potential indication of a PMTU limit
on the path. The PL MAY retransmit any data included in a lost on the path, but not as an indictaion of congestion [CC].
probe packet without adjusting its congestion window [RFC4821].
8. Cached effective PMTU: The sender MUST cache the effective PMTU
value used by an instance of the PL between probes and needs also
to consider the disruption that could be incurred by an
unsuccessful probe - both upon the flow that incurs a probe loss,
and other flows that experience the effect of additional probe
traffic.
9. Shared effective PMTU state: The PMTU value could also be stored 8. Shared PLPMTU state: The PLPMTU value could also be stored with
with the corresponding entry in the destination cache and used by the corresponding entry in the destination cache and used by
other PL instances. The specification of PLPMTUD [RFC4821] other PL instances. The specification of PLPMTUD [RFC4821]
states: "If PLPMTUD updates the MTU for a particular path, all states: "If PLPMTUD updates the MTU for a particular path, all
Packetization Layer sessions that share the path representation Packetization Layer sessions that share the path representation
(as described in Section 5.2 of [RFC4821]) SHOULD be notified to (as described in Section 5.2 of [RFC4821]) SHOULD be notified to
make use of the new MTU and make the required congestion control make use of the new MTU and make the required congestion control
adjustments". Such methods need to robust to the wide variety of adjustments". Such methods need to be robust to the wide variety
underlying network forwarding behaviours. Section 5.2 of of underlying network forwarding behaviours, PLPMTU adjustments
[RFC8201] provides guidance on the caching of PMTU information based on shared PLPMTU values should be incorporated in the
and also the relation to IPv6 flow labels. 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 following design principles are stated: In addition, the following principles are stated for design of a
DPLPMTUD method:
o Suitable MPS: The PLPMTUD method SHOULD avoid forcing an o MPS: A method MUST signal appropriate MPS to the higher layer
application to use an arbitrary small MPS (effective PMTU) for using the PL. This may change following a change to the path. The
transmission while the method is searching for the currently method SHOULD avoid forcing an application to use an arbitrary
supported PMTU. Datagram PLs do not necessarily support small MPS (PLPMTU) for transmission while the method is searching
fragmentation of PDUs larger than the PMTU. A reduced MPS can for the currently supported PLPMTU. Datagram PLs do not
adversely impact the performance of a datagram application. necessarily support fragmentation of PDUs larger than the PLPMTU.
A reduced MPS can adversely impact the performance of a datagram
application.
o Path validation: The PLPMTUD method MUST be robust to path changes o Path validation: A method MUST be robust to path changes that
that could have occurred since the path characteristics were last could have occurred since the path characteristics were last
confirmed. confirmed, and to the possibility of inconsistent path information
being received.
o Datagram reordering: A method MUST be robust to the possibility o Datagram reordering: A method MUST be robust to the possibility
that a flow encounters reordering, or has the traffic (including that a flow encounters reordering, or has the traffic (including
probe packets) is divided over more than one network path. probe packets) is divided over more than one network path.
o When to probe: The PLPMTUD method SHOULD determine whether the o When to probe: A method SHOULD determine whether the path capacity
path capacity has increased since it last measured the path. This has increased since it last measured the path. This determines
determines when the path should again be probed. when the path should again be probed.
3.1. PMTU Probe Packets 3.1. PLPMTU Probe Packets
PMTU discovery relies upon the sender being able to generate probe The DPLPMTUD method relies upon the PL sender being able to generate
messages with a specific size. TCP is able to generate probe packets probe messages with a specific size. TCP is able to generate these
by choosing to appropriately segment data being sent [RFC4821]. probe packets by choosing to appropriately segment data being sent
[RFC4821].
In contrast, a datagram PL that needs to construct a probe packet has In contrast, a datagram PL that needs to construct a probe packet has
to either request an application to send a data block that is larger to either request an application to send a data block that is larger
than that generated by an application, or to utilise padding than that generated by an application, or to utilise padding
functions to extend a datagram beyond the size of the application functions to extend a datagram beyond the size of the application
data block. Protocols that permit exchange of control messages data block. Protocols that permit exchange of control messages
(without an application data block) could alternatively prefer to (without an application data block) could alternatively prefer to
generate a probe packet by extending a control message with padding generate a probe packet by extending a control message with padding
data. data.
When the method fails to validate the PMTU for the path, it may be When the method fails to validate the PLPMTU, it may be required to
required to send a probe packet with a size less than the size of the send a probe packet with a size less than the size of the data block
data block generated by an application. In this case, the PL could generated by an application. In this case, the PL could provide a
provide a way to fragment a datagram at the PL, or could instead way to fragment a datagram at the PL, or could instead utilise a
utilise a control packet with padding. control packet with padding.
A receiver needs to be able to distinguish an in-band data block from A receiver needs to be able to distinguish an in-band data block from
any added padding. This is needed to ensure that any added padding any added padding. This is needed to ensure that any added padding
is not passed on to an application at the receiver. is not passed on to an application at the receiver.
This results in three possible ways that a sender can create a probe This results in three possible ways that a sender can create a probe
packet: packet listed in order of preference:
Probing using appication data: A probe packet that contains a data Probing using padding data: A probe packet that contains only control
block supplied by an application that matches the size required information together with any padding needed to inflate the packet
for the probe. This method requests the application to issue a to the size required for the probe packet. Since these probe
data block of the desired probe size. If the application/ packets do not carry an application-supplied data block,they do
transport needs protection from the loss of an unsuccessful probe not typically require retransmission, although they do still
packet, the application/transport needs then to perform transport- consume network capacity and incur endpoint processing.
layer retransmission/repair of the data block (e.g., by
retransmission after loss is detected or by duplicating the data
block in a datagram without the padding).
Probing using appication data and padding data: A probe packet that Probing using appication data and padding data: A probe packet that
contains a data block supplied by an application that is combined contains a data block supplied by an application that is combined
with padding to inflate the length of the datagram to the size with padding to inflate the length of the datagram to the size
required for the probe. If the application/transport needs required for the probe packet. If the application/transport needs
protection from the loss of this probe packet, the application/ protection from the loss of this probe packet, the application/
transport may perform transport-layer retransmission/repair of the transport may perform transport-layer retransmission/repair of the
data block (e.g., by retransmission after loss is detected or by data block (e.g., by retransmission after loss is detected or by
duplicating the data block in a datagram without the padding duplicating the data block in a datagram without the padding
data). data).
Probing using padding data: A probe packet that contains only Probing using appication data: A probe packet that contains a data
control information together with any padding needed to inflate block supplied by an application that matches the size required
the packet to the size required for the probe. Since these probe for the probe packet. This method requests the application to
packets do not carry an application-supplied data block,they do issue a data block of the desired probe size. If the application/
not typically require retransmission, although they do still transport needs protection from the loss of an unsuccessful probe
consume network capacity and incur endpoint processing. packet, the application/transport needs then to perform transport-
layer retransmission/repair of the data block (e.g., by
retransmission after loss is detected).
A datagram PLPMTUD MAY choose to use only one of these methods to A PL that uses a probe packet carrying an application data block,
simplify the implementation. could need to retransmit this application data block if the probe
fails. This could need the PL to re-fragment the data block to a
smaller packet size that is expected to traverse the end-to-end path
(which could utilise network-layer or PL fragmentation when these are
available).
3.2. Validation of the Current Effective PMTU DLPMTUD MAY choose to use only one of these methods to simplify the
implementation.
3.2. Validation of Probe Packet Size
The PL needs a method to determine when probe packets have been The PL needs a method to determine when probe packets have been
successfully received end-to-end across a network path. successfully received end-to-end across a network path.
Transport protocols can include end-to-end methods that detect and Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP and report reception of specific datagrams that they send (e.g., DCCP and
SCTP provide keep-alive/heartbeat features). When supported, this SCTP provide keep-alive/heartbeat features). When supported, this
mechanism SHOULD also be used by PLPMTUD to acknowledge reception of mechanism SHOULD also be used by DPLPMTUD to acknowledge reception of
a probe packet. a probe packet.
A PL that does not acknowledge data reception (e.g., UDP and UDP- A PL that does not acknowledge data reception (e.g., UDP and UDP-
Lite) is unable to detect when the packets it sends are discarded Lite) is unable to detect when the packets that it sends are
because their size is greater than the actual PMTUD. These PLs need discarded because their size is greater than the actual PMTU. These
to either rely on an application protocol to detect this, or make use PLs need to either rely on an application protocol to detect this
of an additional transport method such as UDP-Options loss, or make use of an additional transport method such as UDP-
[I-D.ietf-tsvwg-udp-options]. In addition, they might need to send Options [I-D.ietf-tsvwg-udp-options]. In addition, they might need
reachability probes (e.g., periodically solicit a response from the to send reachability probes (e.g., periodically solicit a response
destination) to determine whether the current effective PMTU is still from the destination) to determine whether the last successfully
supported by the network path. probed PLPMTU is still supported by the network path.
Section Section 4 specifies this function for a set of IETF-specified Section Section 4 specifies this function for a set of IETF-specified
protocols. protocols.
3.3. Reduction of the Effective PMTU 3.3. Reducing the PLPMTU: Confirming Path Characteristics
When the current effective PMTU is no longer supported by the network If the DPLPMTUD method detects that a packet with the PLPMTU size is
path, the transport needs to detect this and reduce the effective no supported by the network path, then the DLPMTUD method needs to
PMTU. validate the PLPMTU. This can happen when a validated PTB message is
received, or another event that indicates the network path no longer
sustains this packet size, such as a loss report from the PL
o A PL that sends a datagram larger than the actual PMTU that All implementations of DPLPMTUD are REQUIRED to provide support that
includes no application data block, or one that does not attempt reduces the PLPMTU when the actual PMTU supported by a network path
to provide any retransmission, can send a new probe packet with an is less than the PLPMTU.
updated probe size.
o A PL that wishes to resend the application data block, could then 3.4. Increasing the PLPMTU: Supporting Path Changes
need to re-fragment the data block to a smaller packet size that
is expected to traverse the end-to-end path. This could utilise
network-layer or PL fragmentation when these are available. A
fragmented datagram MUST NOT be used as a probe packet (see
[RFC8201]).
A method can additionally utilise PTB messages to detect when the An implementation that only reduces the PLPMTU to a suitable size is
actual PMTU supported by a network path is less than the current size sufficient to ensure reliable operation, but may be very inefficient
of datagrams (or probe messages) that are being sent. when the actual PMTU changes or when the method (for whatever reason)
makes a suboptimal choice for the PLPMTU.
A full implementation of the DPLPMTUD method is RECOMMENDED to
provide a way for the sending PL endpoint to detect when the PLPMTU
is smaller than the actual PMTU size. This allows the sender to
increase the PLPMTU following a change in the characteristics of the
path, such as when a link is reconfigured with a larger MTU, or when
there is a change in the set of links traversed by an end-to-end flow
(e.g. after a routing or fail-over decision).
3.5. Robustness to inconsistent Path information
The decision to increase the PLPMTU needs to be robust to the
possibility that information learned about the path is inconsistent
(this could happen when probe packets are lost due to other reasons,
or some of the packets in a flow are forwarded along a portion of the
path that supports a different 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 robust to these anomalies by introducing hysteresis into the
decision to increase the Maximum Packet Size.
XXX A future revision of this section will include recommend
appropriate methods to provide robustness. XXX
4. Datagram Packetization Layer PMTUD 4. Datagram Packetization Layer PMTUD
This section specifies Datagram PLPMTUD. This section specifies Datagram PLPMTUD (DPLPMTUD). This method can
be introduced at various points in the IP protocol stack, to discover
the PLPMTU so that the application can use an MPS appropriate to the
current network path.
The central idea of PLPMTU discovery is probing by a sender. Probe (preamble)
packets of increasing size are sent to find out the maximum size of a
user message that is completely transferred across the network path
from the sender to the destination.
4.1. Probing +-----------+
| APP* |
+-----------+
__|| | | |___
___/ | | | \
__/ | | | \__
+------++-----+ | +------+ |
| QUIC*||UDPO*| | | SCTP*| |
+------++-----+ | +-+-----+ |
+-----+ +------+
| UDP | | SCTP*|
+-----+ +------+
| |
+----------------------+
| Network Interface |
+----------------------+
The PLPMTUD method utilises a timer to trigger the generation of (postamble)
probe packets. The probe_timer is started each time a probe packet
is sent to the destination and is cancelled when receipt of the probe
packet is acknowledged.
The PROBE_COUNT is initialised to zero when a probe packet is first The central idea of DPLPMTUD is probing by a sender. Probe packets
sent with a particular size. Each time the probe_timer expires, the of increasing size are sent to find out the maximum size of user
PROBE_COUNT is incremented, and a probe packet of the same size is message that is completely transferred across the network path from
retransmitted. The maximum number of retransmissions per probing the sender to the destination.
size is configured (MAX_PROBES). If the value of the PROBE_COUNT
reaches MAX_PROBES, probing will be stopped and the last successfully
probed PMTU is set as the effective PMTU.
Once probing is completed, the sender continues to use the effective 4.1. PROBE_SEARCH: Probing for a larger PLPMTU
PMTU until either a PTB message is received or the PMTU_RAISE_TIMER
expires. If the PL is unable to verify reachability to the
destination endpoint after probing has completed, the method uses a
REACHABILITY_TIMER to periodically repeat a probe packet for the
current effective PMTU size, while the PMTU_RAISE_TIMER is running.
If the resulting probe packet is not acknowledged (i.e. the
PROBE_TIMER expires), the method re-starts probing for the PMTU.
4.2. Verification and Use of PTB Messages The DPLPMTUD method utilises probe packets to confirm that a packet
of size PROBE_SIZE can travere the network path. The PROBE_COUNT is
initialised to zero when a probe packet is first sent with a
particular size.
A timer is used to trigger the generation of probe packets. The
probe_timer is started each time a probe packet is sent to the
destination and is cancelled when receipt of the probe packet is
acknowledged. THE PROBE_SIZE is confirmed, and this value is then
assignmed to PLPMTU. The DPLPMTUD method may send subsequent probes
of an increasing size. Increasing probes follows a search strategy
as discussed in Section 4.7.
Each time the probe_timer expires, the PROBE_COUNT is incremented,
teh probe_timer is reinitialised, and a probe packet of the same size
is retransmitted.
The maximum number of retransmissions for a PROBE_SIZE is configured
(MAX_PROBES). If the value of the PROBE_COUNT reaches MAX_PROBES,
probing will stop.
4.2. The PROBE_DONE state
When the PL sender complete probing for a larger PLPMTU, it enters
the PROBE_DONE state. This starts the PMTU_RAISE_TIMER. While this
running, the PLPMTU remains at the value set in the last succesful
probe packet.
If the PL is designed in a way that is unable to verify reachability
to the destination endpoint after probing has completed, the method
uses a REACHABILITY_TIMER to periodically repeat a probe packet for
the current PLPMTU size, while the PMTU_RAISE_TIMER is running. If
the REACHABILITY_TIMER expires, the method exits the PROBE_DONE
state. The done state is also exited when a verified PTB message is
received.
If the PMTU_RAISE_TIMER expires, the PL sender also exits the
PROBE_DONE state, but in this case resumes probing from the size of
the PLPMTU.
4.3. Verification and Use of PTB Messages
This section describes processing for both IPv4 ICMP Unreachable This section describes processing for both IPv4 ICMP Unreachable
messages (type 3) and ICMPv6 packet too big messages. messages (type 3) and ICMPv6 packet too big messages.
A node that receives a PTB message from a router or middlebox, MUST A node that receives a PTB message from a router or middlebox, MUST
verify the PTB message. The node checks the protocol information in verify the PTB message. The node checks the protocol information in
the quoted payload to verify that the message originated from the the quoted payload to verify that the message originated from the
sending node. The node also checks that the reported MTU size is sending node. The node also checks that the reported MTU size is
less than the size used by packet probes. PTB messages are discarded less than the size used by packet probes. PTB messages are discarded
if they fail to pass these checks, or where there is insufficient if they fail to pass these checks, or where there is insufficient
ICMP payload to perform these checks. The checks are intended to ICMP payload to perform these checks. The checks are intended to
provide protection from packets that originate from a node that is provide protection from packets that originate from a node that is
not on the network path or a node that attempts to report a larger not on the network path or a node that attempts to report a larger
MTU than the current probe size. MTU than the current probe size.
PTB messages that have been verified can be utilised by the DPLPMTUD PTB messages that have been verified can be utilised by the DPLPMTUD
algorithm. A method that utilises these PTB messages can improve algorithm. A method that utilises these PTB messages can improve
performance compared to one that relies solely on probing. performance compared to one that relies solely on probing.
4.3. Timers 4.4. Timers
This method utilises three timers: The method in the previous subsections utilises three timers:
PROBE_TIMER: Configured to expire after a period longer than the PROBE_TIMER: Configured to expire after a period longer than the
maximum time to receive an acknowledgment to a probe packet. This maximum time to receive an acknowledgment to a probe packet. This
value MUST be larger than 1 second, and SHOULD be larger than 15 value MUST be larger than 1 second, and SHOULD be larger than 15
seconds. Guidance on selection of the timer value are provide in seconds. Guidance on selection of the timer value are provide in
section 3.1.1 of the UDP Usage Guidelines [RFC8085]. section 3.1.1 of the UDP Usage Guidelines [RFC8085].
PMTU_RAISE_TIMER: Configured to the period a sender ought to If the PL has an RTT estimate and timely acknowedgements the
continue use the current effective PMTU, after which it re- PROBE_TIMER can be derrived from the PL RTT estimate.
commences probing for a higher PMTU. This timer has a period of
600 secs, as recommended by PLPMTUD [RFC4821].
REACHABILITY_TIMER: Configured to the period a sender ought to wait PMTU_RAISE_TIMER: Configured to the period a sender ought to continue
before confirming the current effective PMTU is still supported. use the current PLPMTU, after which it re-commences probing for a
This is less than the PMTU_RAISE_TIMER. higher PMTU. This timer has a period of 600 secs, as recommended
by DPLPMTUD [RFC4821].
An application that needs to employ keep-alive messages to deliver REACHABILITY_TIMER: Configured to the period a sender ought to wait
useful service over UDP SHOULD NOT transmit them more frequently before confirming the current PLPMTU is still supported. This is
than once every 15 seconds and SHOULD use longer intervals when less than the PMTU_RAISE_TIMER and used to decrease the PLPMTU
possible. DPLPMTUD ought to suspend reachability probes when no (e.g. when a black hole is encountered).
application data has been sent since the previous probe packet.
Guidance on selection of the timer value are provide in section DPLPMTUD ought to suspend reachability probes when no application
3.1.1 of the UDP Usage Guidelines[RFC8085]. data has been sent since the previous probe packet. Guidance on
selection of the timer value are provide in section 3.1.1 of the
UDP Usage Guidelines[RFC8085]. DPLPMTUD ought to be suspended or
only sent in conjuction with out traffic during periods of
dormancy. This verification needs to be frequent enough when data
is flowing that you do not black hole extensive amounts of traffic
An implementation could implement the various timers using a single An implementation could implement the various timers using a single
timer process. timer process.
4.4. Constants 4.5. Constants
The following constants are defined: The following constants are defined:
MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER. The MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER. The default
default value of MAX_PROBES is 10. value of MAX_PROBES is 10.
MIN_PMTU: The smallest allowed probe packet size. This value is MIN_PMTU: The smallest allowed probe packet size. For IPv6, this
1280 bytes, as specified in [RFC2460]. For IPv4, the minimum value is 1280 bytes, as specified in [RFC2460]. For IPv4, the
value is 68 bytes. (An IPv4 routed is required to be able to minimum value is 68 bytes. (An IPv4 routed is required to be able
forward a datagram of 68 octets without further fragmentation. to forward a datagram of 68 octets without further fragmentation.
This is the combined size of an IPv4 header and the minimum This is the combined size of an IPv4 header and the minimum
fragment size of 8 octets.) fragment size of 8 octets.)
BASE_PMTU: The BASE_PMTU is a considered a size that ought to work BASE_PMTU: The BASE_PMTU is a considered a size that ought to work in
in most cases. The size is equal to or larger than the minimum most cases. The size is equal to or larger than the minimum
permitted and smaller than the maximum allowed. In the case of permitted and smaller than the maximum allowed. In the case of
IPv6, this value is 1280 bytes [RFC2460]. When using IPv4, a size IPv6, this value is 1280 bytes [RFC2460]. When using IPv4, a size
of 1200 is RECOMMENDED. of 1200 bytes is RECOMMENDED.
MAX_PMTU: The MAX_PMTU is the largest size of PMTU that is probed. MAX_PMTU: The MAX_PMTU is the largest size of PLPMTU that is probed.
This has to be less than or equal to the minimum of the local MTU This has to be less than or equal to the minimum of the local MTU
of the outgoing interface and the destination effective MTU for of the outgoing interface and the destination PLMTU for receiving.
receiving. An application or PL may reduce this when it knows An application or PL may reduce this when it knows there is no
there is no need to send packets above a specific size. need to send packets above a specific size.
4.5. Variables 4.6. Variables
This method utilises a set of variables: This method utilises a set of variables:
effective PMTU: The effective PMTU is the maximum size of datagram PROBE_TIMER: Configured to expire after a period longer than the
that the method has currently determined can be supported along maximum time to receive an acknowledgment to a probe packet. This
the entire path. value MUST be larger than 1 second, and SHOULD be larger than 15
seconds. Guidance on selection of the timer value are provide in
section 3.1.1 of the UDP Usage Guidelines [RFC8085].
PROBED_SIZE: The PROBED_SIZE is the size of the current probe PL with RTT estimates may use values smaller than 1 seconded
packet. This is a tentative value for the effective PMTU, which derrived from their RTT estimate to speed up detection of
is awaiting confirmation by an acknowledgment. connectivity issues on the path.
PROBE_COUNT: This is a count of the number of unsuccessful probe PROBED_SIZE: The PROBED_SIZE is the size of the current probe packet.
packets that have been sent with size PROBED_SIZE. The value is This is a tentative value for the PLPMTU, which is awaiting
confirmation by an acknowledgment.
PROBE_COUNT: This is a count of the number of unsuccessful probe
packets that have been sent with size PROBED_SIZE. The value is
initialised to zero when a particular size of PROBED_SIZE is first initialised to zero when a particular size of PROBED_SIZE is first
attempted. attempted.
PTB_SIZE: The PTB_Size is value returned by a verified PTB message PTB_SIZE: The PTB_Size is value returned by a verified PTB message
indicating the local MTU size of a router along the path. indicating the local MTU size of a router along the path.
4.6. Selecting PROBED_SIZE 4.7. Selecting PROBED_SIZE
Implementations discover the search range by validating the minimum Implementations discover the search range by validating the minimum
path MTU and then using the probe method to select a PROBED_SIZE less path MTU and then using the probe method to select a PROBED_SIZE less
than or equal to the maximum PMTU_MAX. Where PMTU_MAX is the minimum than or equal to the maximum PMTU_MAX. Where PMTU_MAX is the minimum
of the the local link MTU and EMTU_R (learned from the remote of the local link MTU and EMTU_R (learned from the remote endpoint).
endpoint). The PMTU_MAX MAY be constrained by an application that The PMTU_MAX MAY be constrained by an application that has a maximum
has a maximum to the size of datagrams it wishes to send. to the size of datagrams it wishes to send.
Implementations use a search algorithm to choose probe sizes within Implementations use a search algorithm to choose probe sizes within
the search range. XXX The current method does not specify or the search range.
recommend a specific methods for selecting a probe size. One simple
method is to increase the size of probe in increments until it fails,
other methods may use tables to select probe sizes, or search
algorithms - this part to be expanded based on experience and
consideration of methods XXX
xxx A future version of this section will detail example methods for
selecting probe size values, but does not plan to mandate a single
method. xxx
Implementations MAY optimizse the search procedure by selecting step Implementations MAY optimizse the search procedure by selecting step
sizes from a table of common MTU sizes. sizes from a table of common PMTU sizes.
Implementations SHOULD select probe sizes to maximise the gain in Implementations SHOULD select probe sizes to maximise the gain in
PMTU each search step. Implementations ought to take into PLPMTU each search step. Implementations ought to take into
consideration useful probe size steps and a minimum useful gain in consideration useful probe size steps and a minimum useful gain in
PMTU. PLPMTU.
4.7. State Machine 4.8. Black Hole Detection
A state machine for Datagram PLPMTUD is depicted in Figure 1. If The DPLPMTUD method can be used to detect paths that fail to support
multihoming is supported, a state machine is needed for each active a packet size, but return no PTB message. The black hole detection
path. function detects such cases and responds by reducing the PLPMTU,
allowing the endpoint to inform the application of the reduced MPS
and accordingly send smaller packets. Black Hole detection is
triggered by the reachability function.
PROBE_TIMER expiry 4.9. State Machine
(PROBE_COUNT = MAX_PROBES)
+-------------+ +--------------+
=->| PROBE_START |--------------->|PROBE_DISABLED|
PROBE_TIMER expiry | +-------------+ +--------------+
(PROBE_COUNT = | | |
MAX_PROBES) ------- | Connectivity confirmed
v
----------- +------------+ -- PROBE_TIMER expiry
MAX_PMTU acked or | | PROBE_BASE | | (PROBE_COUNT <
PTB (>= BASE_PMTU)| -----> +------------+ <- MAX_PROBES)
---------------- | /\ | |
| | | | | PTB
| PMTU_RAISE_TIMER| | | | (PTB_SIZE < BASE_PMTU)
| or reachability | | | | or
| (PROBE_COUNT | | | | PROBE_TIMER expiry
| = MAX_PROBES) | | | | (PROBE_COUNT = MAX_PROBES)
| ------------- | | \
| | PTB | | \
| | (< PROBED_SIZE)| | \
| | | | ----------------
| | | | |
| | | | Probe |
| | | | acked |
v | | v v
+------------+ +--------------+ Probe +-------------+
| PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR |
+------------+ MAX_PMTU acked +--------------+ acked +-------------+
/\ | or /\ |
| | PROBE_TIMER expiry | |
| |(PROBE_COUNT = MAX_PROBES) | |
| | | |
------ --------
Reachability probe acked PROBE_TIMER expiry
or PROBE_TIMER expiry (PROBE_COUNT < MAX_PROBES)
(PROBE_COUNT < MAX_PROBES) or
Probe acked
Figure 1: State machine for Datagram PLPMTUD A state machine for DPLPMTUD is depicted in Figure 2. If multihoming
is supported, a state machine is needed for each active path.
XXX State machine to be updated to describe handling of validated PTB PROBE_TIMER expiry
messages XXX (PROBE_COUNT = MAX_PROBES)
+-------------+ +--------------+
=->| PROBE_START |--------------->|PROBE_DISABLED|
PROBE_TIMER expiry | +-------------+ +--------------+
(PROBE_COUNT = | | |
MAX_PROBES) ------- | Connectivity confirmed
v
----------- +------------+ -- PROBE_TIMER expiry
MAX_PMTU acked or | | PROBE_BASE | | (PROBE_COUNT <
PTB (>= BASE_PMTU)| -----> +------------+ <- MAX_PROBES)
---------------- | /\ | |
| | | | | PTB
| PMTU_RAISE_TIMER| | | | (PTB_SIZE < BASE_PMTU)
| or reachability | | | | or
| (PROBE_COUNT | | | | PROBE_TIMER expiry
| = MAX_PROBES) | | | | (PROBE_COUNT = MAX_PROBES)
| ------------- | | \
| | PTB | | \
| | (< PROBED_SIZE)| | \
| | | | ----------------
| | | | |
| | | | Probe |
| | | | acked |
v | | v v
+------------+ +--------------+ Probe +-------------+
| PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR |
+------------+ MAX_PMTU acked +--------------+ acked +-------------+
/\ | or /\ |
| | PROBE_TIMER expiry | |
| |(PROBE_COUNT = MAX_PROBES) | |
| | | |
------ --------
Reachability probe acked PROBE_TIMER expiry
or PROBE_TIMER expiry (PROBE_COUNT < MAX_PROBES)
(PROBE_COUNT < MAX_PROBES) or
Probe acked
XXX Method may be updated to clarify how probe sizes are used during XXX A future version of this document will update the state machine
probing XXX to describe handling of validated PTB messages. XXX
The following states are defined to reflect the probing process: The following states are defined to reflect the probing process:
PROBE_START: The PROBE_START state is the initial state before PROBE_START: The PROBE_START state is the initial state before
probing has started. PLPMTUD is not performed in this state. The probing has started. PLPMTUD is not performed in this state. The
state transitions to PROBE_BASE, when a path has been confirmed, state transitions to PROBE_BASE, when a path has been confirmed,
i.e. when a sent packet has been acknowledged on this path. The i.e. when a sent packet has been acknowledged on this path. Any
effective PMTU is set to the BASE_PMTU size. Probing ought to transport method may be used to exit PROBE_BASE as long as the
start immediately after connection setup to prevent the loss of send packet is acknowledge by the other side. The PLPMTU is set
user data. to the BASE_PMTU size. Probing ought to start immediately after
connection setup to prevent the prevent the loss of user data.
PROBE_BASE: The PROBE_BASE state is the starting point for probing PROBE_BASE: The PROBE_BASE state is the starting point for probing
with datagram PLPMTUD. It is used to confirm whether the with datagram PLPMTUD. It is used to confirm whether the BASE_PMTU
BASE_PMTU size is supported by the network path. On entry, the size is supported by the network path. On entry, the PROBED_SIZE
PROBED_SIZE is set to the BASE_PMTU size and the PROBE_COUNT is is set to the BASE_PMTU size and the PROBE_COUNT is set to zero.
set to zero. A probe packet is sent, and the PROBE_TIMER is A probe packet is sent, and the PROBE_TIMER is started. The state
started. The state is left when the PROBE_COUNT reaches is left when the PROBE_COUNT reaches MAX_PROBES; a PTB message is
MAX_PROBES; a PTB message is verified, or a probe packet is verified, or a probe packet is acknowledged.
acknowledged.
PROBE_SEARCH: The PROBE_SEARCH state is the main probing state. PROBE_SEARCH: The PROBE_SEARCH state is the main probing state. This
This state is entered either when probing for the BASE_PMTU was state is entered either when probing for the BASE_PMTU was
successful or when there is a successful reachability test in the successful or when there is a successful reachability test in the
PROBE_ERROR state. On entry, the effective PMTU is set to the PROBE_ERROR state. On entry, the PLPMTU is set to the last
last acknowledged PROBED_SIZE. acknowledged PROBED_SIZE.
The PROBE_COUNT is set to zero when the first probe packet is sent The PROBE_COUNT is set to zero when the first probe packet is sent
for each probed size. Each time a probe packet is acknowledged, for each probe size. Each time a probe packet is acknowledged,
the effective PMTU is set to the PROBED_SIZE, and then the the PLPMTU is set to the PROBED_SIZE, and then the PROBED_SIZE is
PROBED_SIZE is increased. increased.
When a probe packet is sent and not acknowledged within the period When a probe packet is sent and not acknowledged within the period
of the PROBE_TIMER, the PROBE_COUNT is incremented and the probe of the PROBE_TIMER, the PROBE_COUNT is incremented and the probe
packet is retransmitted. The state is exited when the PROBE_COUNT packet is retransmitted. The state is exited when the PROBE_COUNT
reaches MAX_PROBES; a PTB message is verified; or a probe of size reaches MAX_PROBES; a PTB message is verified; or a probe of size
PMTU_MAX is acknowledged. PMTU_MAX is acknowledged.
PROBE_ERROR: The PROBE_ERROR state represents the case where the PROBE_ERROR: The PROBE_ERROR state represents the case where the
network path is not known to support an effective PMTU of at least network path is not known to support an PLPMTU of at least the
the BASE_PMTU size. It is entered when either a probe of size BASE_PMTU size. It is entered when either a probe of size
BASE_PMTU has not been acknowledged or a verified PTB message BASE_PMTU has not been acknowledged or a verified PTB message
indicates a smaller link MTU than the BASE_PMTU. On entry, the indicates a smaller link MTU than the BASE_PMTU. On entry, the
PROBE_COUNT is set to zero and the PROBED_SIZE is set to the PROBE_COUNT is set to zero and the PROBED_SIZE is set to the
MIN_PMTU size, and the effective PMTU is reset to MIN_PMTU size. MIN_PMTU size, and the PLPMTU is reset to MIN_PMTU size. In this
In this state, a probe packet is sent, and the PROBE_TIMER is state, a probe packet is sent, and the PROBE_TIMER is started.
started. The state transitions to the PROBE_SEARCH state when a The state transitions to the PROBE_SEARCH state when a probe
probe packet is acknowledged. packet is acknowledged.
PROBE_DONE: The PROBE_DONE state indicates a successful end to a PROBE_DONE: The PROBE_DONE state indicates a successful end to a
probing phase. Datagram PLPMTUD remains in this state until probing phase. DPLPMTUD remains in this state until either the
either the PMTU_RAISE_TIMER expires or a received PTB message is PMTU_RAISE_TIMER expires or a received PTB message is verified.
verified.
When PLPMTUD uses an unacknowledged PL and is in the PROBE_DONE When PLPMTUD uses an unacknowledged PL and is in the PROBE_DONE
state, a REACHABILITY_TIMER periodically resets the PROBE_COUNT state, a REACHABILITY_TIMER periodically resets the PROBE_COUNT
and schedules a probe packet with the size of the effective PMTU. and schedules a probe packet with the size of the PLPMTU. If the
If the probe packet fails to be acknowledged after MAX_PROBES probe packet fails to be acknowledged after MAX_PROBES attempts,
attempts, the method enters the PROBE_BASE state. When used with the method enters the PROBE_BASE state. When used with an
an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to
probe in this state. probe in this state.
PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity
could not be established. DPLPMTUD MUST NOT probe in this state. could not be established. DPLPMTUD MUST NOT probe in this state.
Appendix A contains an informative description of key events. Appendix Appendix A contains an informative description of key
events.
5. Specification of Protocol-Specific Methods 5. Specification of Protocol-Specific Methods
This section specifies protocol-specific details for datagram PLPMTUD This section specifies protocol-specific details for datagram PLPMTUD
for IETF-specified transports. for IETF-specified transports.
5.1. DPLPMTUD for UDP and UDP-Lite The first subsection provides guidance on how to implement the
DPLPMTUD method as a part of an application using UDP or UDP-Lite.
The guidance also applies to other datagram services that do not
include a specific transport protocol (such as a tunnel
encapsulation). The following subsection describe how DPLPMTUD can be
implemented as a part of the transport service, allowing applications
using the service to benefit from discovery of the PLPMTU without
themselves needing to implement this method.
The current specifications of UDP [RFC0768] and UDP-LIte [RFC3828] do 5.1. Application support for DPLPMTUD with UDP or UDP-Lite
not define a method in the RFC-series that supports PLPMTUD. In
particular, these transports do not provide the transport layer
features needed to implement datagram PLPMTUD, and any support for
Datagram PLPMTUD would therefore need to rely on higher-layer
protocol features [RFC8085].
5.1.1. UDP Options The current specifications of UDP [RFC0768] and UDP-Lite [RFC3828] do
not define a method in the RFC-series that supports PLPMTUD. In
particular, the UDP transport does not provide the transport layer
features needed to implement datagram PLPMTUD.
UDP-Options [I-D.ietf-tsvwg-udp-options] supply the additional The DPLPMTUD method can be implemented as a part of an application
functionality required to implement datagram PLPMTUD. This enables built directly or indirectly on UDP or UDP-Lite, but relies on
padding to be added to UDP datagrams and can be used to provide higher-layer protocol features to implement the method [RFC8085].
feedback acknowledgement of received probe packets.
5.1.2. UDP Options Required for PLPMTUD Some primitives used by DPLPMTUD might not be available via the
Datagram API (e.g., the ability to access the PLPMTU cache, or
interpret received ICMP PTB messages).
This subsection proposes two new UDP-Options that add support for In addition, it is desirable that PMTU discovery is not performed by
requesting a datagram response be sent and to mark this datagram as a multiple protocol layers. An application SHOULD avoid implementing
response to a request. DPLPMTUD when the underlying transport system provides this
capability. Using a common method for manging the PLPMTU has
benefits, both in the ability to share state between different
processes and opportunities to coordinate probing.
XXX Future versions of the spec may define a parameter in an Option 5.1.1. Application Request
to indicate the EMTU_R to the peer that can be used to initialise
PMTU_MAX. XXX
5.1.2.1. Echo Request Option An application needs an application-layer protocol mechanism (such as
a message acknowledgement method) that solicits a response from a
destination endpoint. The method SHOULD allow the sender to check
the value returned in the response to provide additional protection
from off-path insertion of data [RFC8085], suitable methods include a
parameter known only to the two endpoints, such as a session ID or
initialised sequence number.
The Echo Request Option allows a sending endpoint to solicit a 5.1.2. Application Response
response from a destination endpoint. An application needs an application-layer protocol mechanism to
communicate the response from the destination endpoint. This
response may indicate successful reception of the probe across the
path, but could also indicate that some (or all packets) have failed
to reach the destination.
The Echo Request carries a four byte token set by the sender. This 5.1.3. Sending Application Probe Packets
token can be set to a value that is likely to be known only to the
sender (and becomes known to nodes along the end-to-end path). The
sender can then check the value returned in the response to provide
additional protection from off-path insertion of data [RFC8085].
+---------+--------+-----------------+ A probe packet that may carry an application data block, but the
| Kind=9 | Len=6 | Token | successful transmission of this data is at risk when used for
+---------+--------+-----------------+ probing. Some applications may prefer to use a probe packet that
1 byte 1 byte 4 bytes does not carry an application data block to avoid disruption to
normal data transfer.
Figure 2: UDP ECHOREQ Option Format 5.1.4. Validating the Path
5.1.2.2. Echo Response Option An application that does not have other higher-layer information
confirming correct delivery of datagrams SHOULD implement the
REACHABILITY_TIMER to periodically send probe packets while in the
PROBE_DONE state.
The Echo Response Option is generated by the PL in response to 5.1.5. Handling of PTB Messages
reception of a previously received Echo Request. The Token field
associates the response with the Token value carried in the most
recently-received Echo Request. The rate of generation of UDP
packets carrying an Echo Response Option MAY be rate-limited.
+---------+--------+-----------------+ An application that is able and wishes to receive PTB messages MUST
| Kind=10 | Len=6 | Token | perform ICMP verification as specified in Section 5.2 of [RFC8085].
+---------+--------+-----------------+ This requires that the application verifies each received PTB
1 byte 1 byte 4 bytes messages to verify these are received in response to transmitted
traffic and that the reported link MTU is less than the current probe
size. A verified PTB message MAY be used as input to the DPLPMTUD
algorithm, but MUST NOT be used directly to set the PLPMTU.
Figure 3: UDP ECHORES Option Format 5.2. DPLPMTUD with UDP Options
5.1.3. Sending UDP-Option Probe Packets UDP-Options [I-D.ietf-tsvwg-udp-options] can supply the additional
functionality required to implement DPLPMTUD within the UDP transport
service. This avoids the need for applications to implement the
DPLPMTUD method.
This method specifies a probe packet that does not carry an This enables padding to be added to UDP datagrams and can be used to
application data block. The probe packet consists of a UDP datagram provide feedback acknowledgement of received probe packets.
header followed by a UDP Option containing the ECHOREQ option, which
is followed by NOP Options to pad the remainder of the datagram
payload to the probe size. NOP padding is used to control the length
of the probe packet.
A UDP Option carrying the ECHORES option is used to provide feedback The specification also defines two UDP Options to support DPLMTUD.
when a probe packet is received at the destination endpoint.
5.1.4. Validating the Path with UDP Options Section 5.6 of [I-D.ietf-tsvwg-udp-options] defines the MSS option
which allows the local sender to indicate the EMTU_R to the peer.
This option can be used to initialise PMTU_MAX. An application
wishing to avoid the effects of MSS-Clamping (where a middlebox
changes the advertised TCP maximum sending size) ought to use a
cryptographic method to encrypt this parameter.
Since UDP is an unacknowledged PL, a sender that does not have 5.2.1. UDP Request Option
higher-layer information confirming correct delivery of datagrams
SHOULD implement the REACHABILITY_TIMER to periodically send probe
packets while in the PROBE_DONE state.
5.1.5. Handling of PTB Messages by UDP The Request Option allows a sending endpoint to solicit a response
from a destination endpoint.
Normal ICMP verification MUST be performed as specified in The Request Option carries a four byte token set by the sender. This
Section 5.2 of [RFC8085]. This requires that the PL verifies each token can be set to a value that is likely to be known only to the
received PTB messages to verify these are received in response to sender (and becomes known to nodes along the end-to-end path). The
transmitted traffic and that the reported LInk MTU is less than the sender can then check the value returned in the response to provide
current probe size. A verified PTB message MAY be used as input to additional protection from off-path insertion of data [RFC8085].
the PLPMTUD algorithm.
5.2. DPLPMTUD for SCTP +---------+--------+-----------------+
| Kind=9 | Len=6 | Token |
+---------+--------+-----------------+
1 byte 1 byte 4 bytes
5.2.2. UDP Response Option
The Response Option is generated by the PL in response to reception
of a previously received Echo Request. The Token field associates
the response with the Token value carried in the most recently-
received Echo Request. The rate of generation of UDP packets
carrying a Response Option MAY be rate-limited.
+---------+--------+-----------------+
| Kind=10 | Len=6 | Token |
+---------+--------+-----------------+
1 byte 1 byte 4 bytes
5.3. DPLPMTUD for SCTP
Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing
method for SCTP. It recommends the use of the PAD chunk, defined in method for SCTP. It recommends the use of the PAD chunk, defined in
[RFC4820] to be attached to a minimum length HEARTBEAT chunk to build [RFC4820] to be attached to a minimum length HEARTBEAT chunk to build
a probe packet. This enables probing without affecting the transfer a probe packet. This enables probing without affecting the transfer
of user messages and without interfering with congestion control. of user messages and without interfering with congestion control.
This is preferred to using DATA chunks (with padding as required) as This is preferred to using DATA chunks (with padding as required) as
path probes. path probes.
XXX Future versions of this specification might define a parameter XXX Future versions of this document might define a parameter
contained in the INIT and INIT ACK chunk to indicate the MTU to the contained in the INIT and INIT ACK chunk to indicate the remote peer
peer. However, multihoming makes this a bit complex, so it might not MTU to the local peer. However, multihoming makes this a bit
be worth doing. XXX complex, so it might not be worth doing. XXX
5.2.1. SCTP/IP4 and SCTP/IPv6
The base protocol is specified in [RFC4960]. 5.3.1. SCTP/IP4 and SCTP/IPv6
5.2.1.1. Sending SCTP Probe Packets The base protocol is specified in [RFC4960]. This provides an
acknowledged PL. A sender can therefore enter the PROBE_BASE state as
soon as connectivity has been confirmed.
5.3.1.1. Sending SCTP Probe Packets
Probe packets consist of an SCTP common header followed by a Probe packets consist of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
the length of the probe packet. The HEARTBEAT chunk is used to the length of the probe packet. The HEARTBEAT chunk is used to
trigger the sending of a HEARTBEAT ACK chunk. The reception of the trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful probe. HEARTBEAT ACK chunk acknowledges reception of a successful probe.
The HEARTBEAT chunk carries a Heartbeat Information parameter which The HEARTBEAT chunk carries a Heartbeat Information parameter which
should include, besides the information suggested in [RFC4960], the should include, besides the information suggested in [RFC4960], the
probing size, which is the MTU size the complete datagram will add up probe size, which is the size of the complete datagram. The size of
to. The size of the PAD chunk is therefore computed by reducing the the PAD chunk is therefore computed by reducing the probing size by
probing size by the IPv4 or IPv6 header size, the SCTP common header, the IPv4 or IPv6 header size, the SCTP common header, the HEARTBEAT
the HEARTBEAT request and the PAD chunk header. The payload of the request and the PAD chunk header. The payload of the PAD chunk
PAD chunk contains arbitrary data. contains arbitrary data.
To avoid fragmentation of retransmitted data, probing starts right To avoid fragmentation of retransmitted data, probing starts right
after the handshake, before data is sent. Assuming normal behaviour after the handshake, before data is sent. Assuming normal behaviour
(i.e., the PMTU is smaller than or equal to the interface MTU), this (i.e., the PMTU is smaller than or equal to the interface MTU), this
process will take a few round trip time periods depending on the process will take a few round trip time periods depending on the
number of PMTU sizes probed. The Heartbeat timer can be used to number of PMTU sizes probed. The Heartbeat timer can be used to
implement the PROBE_TIMER. implement the PROBE_TIMER.
5.2.1.2. Validating the Path with SCTP 5.3.1.2. Validating the Path with SCTP
Since SCTP provides an acknowledged PL, a sender does MUST NOT Since SCTP provides an acknowledged PL, a sender does MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state. implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.2.1.3. PTB Message Handling by SCTP 5.3.1.3. PTB Message Handling by SCTP
Normal ICMP verification MUST be performed as specified in Appendix C Normal ICMP verification MUST be performed as specified in Appendix C
of [RFC4960]. This requires that the first 8 bytes of the SCTP of [RFC4960]. This requires that the first 8 bytes of the SCTP
common header are quoted in the payload of the PTB message, which can common header are quoted in the payload of the PTB message, which can
be the case for ICMPv4 and is normally the case for ICMPv6. be the case for ICMPv4 and is normally the case for ICMPv6.
When a PTB message has been verified, the router Link MTU indicated When a PTB message has been verified, the router Link MTU indicated
in the PTB message SHOULD be used with the PLPMTUD algorithm, in the PTB message SHOULD be used with the DPLPMTUD algorithm,
providing that the reported Link MTU is less than the current probe providing that the reported Link MTU is less than the current probe
size. size.
5.2.2. DPLPMTUD for SCTP/UDP 5.3.2. DPLPMTUD for SCTP/UDP
The UDP encapsulation of SCTP is specified in [RFC6951]. The UDP encapsulation of SCTP is specified in [RFC6951].
5.2.2.1. Sending SCTP/UDP Probe Packets 5.3.2.1. Sending SCTP/UDP Probe Packets
Packet probing can be performed as specified in Section 5.2.1.1. The Packet probing can be performed as specified in Section 5.3.1.1. The
maximum payload is reduced by 8 bytes, which has to be considered maximum payload is reduced by 8 bytes, which has to be considered
when filling the PAD chunk. when filling the PAD chunk.
5.2.2.2. Validating the Path with SCTP/UDP 5.3.2.2. Validating the Path with SCTP/UDP
Since SCTP provides an acknowledged PL, a sender does MUST NOT Since SCTP provides an acknowledged PL, a sender does MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state. implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.2.2.3. Handling of PTB Messages by SCTP/UDP 5.3.2.3. Handling of PTB Messages by SCTP/UDP
Normal ICMP verification MUST be performed for PTB messages as Normal ICMP verification MUST be performed for PTB messages as
specified in Appendix C of [RFC4960]. This requires that the first 8 specified in Appendix C of [RFC4960]. This requires that the first 8
bytes of the SCTP common header are contained in the PTB message, bytes of the SCTP common header are contained in the PTB message,
which can be the case for ICMPv4 (but note the UDP header also which can be the case for ICMPv4 (but note the UDP header also
consumes a part of the quoted packet header) and is normally the case consumes a part of the quoted packet header) and is normally the case
for ICMPv6. When the verification is completed, the router Link MTU for ICMPv6. When the verification is completed, the router Link MTU
size indicated in the PTB message SHOULD be used with the PLPMTUD size indicated in the PTB message SHOULD be used with the DPLPMTUD
algorithm providing that the reported LInk MTU is less than the providing that the reported link MTU is less than the current probe
current probe size. size.
5.2.3. DPLPMTUD for SCTP/DTLS 5.3.3. DPLPMTUD for SCTP/DTLS
The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
specified in [I-D.ietf-tsvwg-sctp-dtls-encaps]. It is used for data specified in [I-D.ietf-tsvwg-sctp-dtls-encaps]. It is used for data
channels in WebRTC implementations. channels in WebRTC implementations.
5.2.3.1. Sending SCTP/DTLS Probe Packets 5.3.3.1. Sending SCTP/DTLS Probe Packets
Packet probing can be done as specified in Section 5.2.1.1. Packet probing can be done as specified in Section 5.3.1.1.
5.2.3.2. Validating the Path with SCTP/DTLS 5.3.3.2. Validating the Path with SCTP/DTLS
Since SCTP provides an acknowledged PL, a sender does MUST NOT Since SCTP provides an acknowledged PL, a sender does MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state. implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.2.3.3. Handling of PTB Messages by SCTP/DTLS 5.3.3.3. Handling of PTB Messages by SCTP/DTLS
It is not possible to perform normal ICMP verification as specified It is not possible to perform normal ICMP verification as specified
in [RFC4960], since even if the ICMP message payload contains in [RFC4960], since even if the ICMP message payload contains
sufficient information, the reflected SCTP common header would be sufficient information, the reflected SCTP common header would be
encrypted. Therefore it is not possible to process PTB messages at encrypted. Therefore it is not possible to process PTB messages at
the PL. the PL.
5.3. PMTUD for QUIC 5.4. DPLPMTUD for QUIC
XXX New section XXX Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a
UDP-based transport that provides reception feedback.
Quick UDP Internet Connection (QUIC) is a UDP-based transport that Section 9.2 of [I-D.ietf-quic-transport] describes the path
provides reception feedback [I-D.ietf-quic-transport]. considerations when sending QUIC packets. It recommends the use of
PADDING frames to build the probe packet. This enables probing the
without affecting the transfer of other QUIC frames.
Section 9.2 of [I-D.ietf-quic-transport] details the path This provides an acknowledged PL. A sender can therefore enter the
considerations when sending QUIC packets. It reccomends the use of PROBE_BASE state as soon as connectivity has been confirmed.
PADDING frames to buld the probe packet. This enables probing the
without affecting the transfer of other frames.
5.3.1. Sending QUIC Probe Packets 5.4.1. Sending QUIC Probe Packets
A probe packet consists of a QUIC Header and a payload containing
only PADDING Frames. PADDING Frames are a single octet (0x00) and
several of these can be used to create a probe packet of size
PROBED_SIZE. QUIC provides an acknowledged PL. A sender can therefore
enter the PROBE_BASE state as soon as connectivity has been
confirmed.
Probe packets consist of a QUIC Header and a payload containing only The current specification of QUIC sets the following:
PADDING Frames. PADDING Frames are a single octet (0x00) and
serveral of these can be used to create a probe packet of size
PROBED_SIZE.
A QUIC sender needs to pad initial packets to 1200 bytes to validate o BASE_PMTU: 1200. A QUIC sender needs to pad initial packets to
the path can support packets of a useful size. If a QUIC sender 1200 bytes to validate the path can support packets of a useful
determines the PMTU on a path has fallen below 1280 octets it MUST size.
immediately stop sending on the affected path.
5.3.2. Validating the Path with QUIC o MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU has
fallen below 1200 bytes MUST immediately stop sending on the
affected path.
Since QUIC provides an acknowledged PL, a sender does MUST NOT 5.4.2. Validating the Path with QUIC
QUIC provides an acknowledged PL. A sender therefore MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state. implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.3.3. Handling of PTB Messages by QUIC 5.4.3. Handling of PTB Messages by QUIC
QUIC does not specify any methods for validating ICMP responses, but QUIC operates over the UDP transport, and the guidelines on ICMP
does provide some guidlines to make it harder for an off path verification as specified in Section 5.2 of [RFC8085] therefore
attacker to inject ICMP messages. apply. Although QUIC does not currently specify a method for
validating ICMP responses, it does provide some guidelines to make it
harder for an off-path attacker to inject ICMP messages.
o Set the IPv4 Don't Fragment (DF) bit on a small proportion of o Set the IPv4 Don't Fragment (DF) bit on a small proportion of
packets, so that most invalid ICMP messages arrive when there are packets, so that most invalid ICMP messages arrive when there are
no DF packets outstanding, and can therefore be identified as no DF packets outstanding, and can therefore be identified as
spurious. spurious.
o Store additional information from the IP or UDP headers from DF o Store additional information from the IP or UDP headers from DF
packets (for example, the IP ID or UDP checksum) to further packets (for example, the IP ID or UDP checksum) to further
authenticate incoming Datagram Too Big messages. authenticate incoming Datagram Too Big messages.
o Any reduction in PMTU due to a report contained in an ICMP packet o Any reduction in PMTU due to a report contained in an ICMP packet
is provisional until QUIC's loss detection algorithm determines is provisional until QUIC's loss detection algorithm determines
that the packet is actually lost. that the packet is actually lost.
XXX The above list was pulled whole from quic-transport XXX XXX The above list was pulled whole from quic-transport - input is
invited from QUIC contributors. XXX
5.4. Other IETF Transports
XXX This section to be updated in a later revision. XXX
5.5. DPLPMTUD by Applications
Applications that use the Datagram API (e.g., applications built
directly or indirectly on UDP) can implement DPLPMTUD. Some
primitives used by DPLPMTUD might not be available via this interface
(e.g., the ability to access the PMTU cache, or interpret received
ICMP PTB messages).
In addition, it is important that PMTUD is not performed by multiple
protocol layers.
XXX This section will be completed in a future revision of this ID
XXX
6. Acknowledgements 6. Acknowledgements
This work was partially funded by the European Union's Horizon 2020 This work was partially funded by the European Union's Horizon 2020
research and innovation programme under grant agreement No. 644334 research and innovation programme under grant agreement No. 644334
(NEAT). The views expressed are solely those of the author(s). (NEAT). The views expressed are solely those of the author(s).
7. IANA Considerations 7. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
XXX If new UDP Options are specified in this document, a request to XXX If new UDP Options are specified in this document, a request to
IANA will be included here. XXX IANA will be included here. XXX
If there are no requirements for IANA, the section will be removed If there are no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor. during conversion into an RFC by the RFC Editor.
8. Security Considerations 8. Security Considerations
skipping to change at page 24, line 25 skipping to change at page 26, line 16
XXX If new UDP Options are specified in this document, a request to XXX If new UDP Options are specified in this document, a request to
IANA will be included here. XXX IANA will be included here. XXX
If there are no requirements for IANA, the section will be removed If there are no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor. during conversion into an RFC by the RFC Editor.
8. Security Considerations 8. Security Considerations
The security considerations for the use of UDP and SCTP are provided The security considerations for the use of UDP and SCTP are provided
in the references RFCs. Security guidance for applications using UDP in the references RFCs. Security guidance for applications using UDP
is provided in the UDP-Guidelines [RFC8085]. is provided in the UDP Usage Guidelines [RFC8085].
PTB messages could potentially be used to cause a node to There are cases where PTB messages are not delivered due to policy,
inappropriately reduce the effective PMTU. A node supporting PLPMTUD configuration or equipment design (see Section 1.1), this method
MUST appropriately verify the payload of PTB messages to ensure these therefore does not rely upon PTB messages being received, but is able
are received in response to transmitted traffic (i.e., a reported to utilise these when they are received by the sender. PTB messages
error condition that corresponds to a datagram actually sent by the could potentially be used to cause a node to inappropriately reduce
path layer. the PLPMTU. A node supporting DPLPMTUD MUST therefore appropriately
verify the payload of PTB messages to ensure these are received in
response to transmitted traffic (i.e., a reported error condition
that corresponds to a datagram actually sent by the path layer.
XXX Determine if parallel forwarding paths needs to be considered. Parallel forwarding paths may need to be considered. Section 3.5
XXX identifies the need for robustness in the method when the path
information may be inconsistent.
A node performing PLPMTUD could experience conflicting information A node performing DPLPMTUD could experience conflicting information
about the size of supported probe packets. This could occur when about the size of supported probe packets. This could occur when
there are multiple paths are concurrently in use and these exhibit a there are multiple paths are concurrently in use and these exhibit a
different PMTU. If not considered, this could result in data being different PMTU. If not considered, this could result in data being
blackholed when the effective PMTU is larger than the smallest PMTU black holed when the PLPMTU is larger than the smallest PMTU across
across the current paths. the current paths.
An on-path attacker could forge PTB messages to drive down the PLPMTU
9. References 9. References
9.1. Normative References 9.1. Normative References
[I-D.ietf-quic-transport] [I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-04 (work and Secure Transport", Internet-Draft draft-ietf-quic-
in progress), June 2017. transport-04, June 2017.
[I-D.ietf-tsvwg-sctp-dtls-encaps] [I-D.ietf-tsvwg-sctp-dtls-encaps]
Tuexen, M., Stewart, R., Jesup, R., and S. Loreto, "DTLS Tuexen, M., Stewart, R., Jesup, R. and S. Loreto, "DTLS
Encapsulation of SCTP Packets", draft-ietf-tsvwg-sctp- Encapsulation of SCTP Packets", Internet-Draft draft-ietf-
dtls-encaps-09 (work in progress), January 2015. tsvwg-sctp-dtls-encaps-09, January 2015.
[I-D.ietf-tsvwg-udp-options] [I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg- Touch, J., "Transport Options for UDP", Internet-Draft
udp-options-01 (work in progress), June 2017. draft-ietf-tsvwg-udp-options-01, June 2017.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980, <https://www.rfc- August 1980.
editor.org/info/rfc768>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981, RFC 792, DOI 10.17487/RFC0792, September 1981, <https://
<https://www.rfc-editor.org/info/rfc792>. www.rfc-editor.org/info/rfc792>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, Communication Layers", STD 3, RFC 1122, DOI 10.17487/
DOI 10.17487/RFC1122, October 1989, <https://www.rfc- RFC1122, October 1989, <https://www.rfc-editor.org/info/
editor.org/info/rfc1122>. rfc1122>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995, RFC 1812, DOI 10.17487/RFC1812, June 1995, <https://www
<https://www.rfc-editor.org/info/rfc1812>. .rfc-editor.org/info/rfc1812>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119, March 1997.
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>. December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed., [RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E.Ed.,
and G. Fairhurst, Ed., "The Lightweight User Datagram and G. Fairhurst, Ed., "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
2004, <https://www.rfc-editor.org/info/rfc3828>. 2004, <https://www.rfc-editor.org/info/rfc3828>.
[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and [RFC4820] Tuexen, M., Stewart, R. and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007, (SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
<https://www.rfc-editor.org/info/rfc4820>. <https://www.rfc-editor.org/info/rfc4820>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007, RFC 4960, DOI 10.17487/RFC4960, September 2007, <https://
<https://www.rfc-editor.org/info/rfc4960>. www.rfc-editor.org/info/rfc4960>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream [RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951, to End-Host Communication", RFC 6951, DOI 10.17487/
DOI 10.17487/RFC6951, May 2013, <https://www.rfc- RFC6951, May 2013, <https://www.rfc-editor.org/info/
editor.org/info/rfc6951>. rfc6951>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage [RFC8085] Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>. March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed., [RFC8201] McCann, J., Deering, S., Mogul, J. and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201, "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017, <https://www.rfc- DOI 10.17487/RFC8201, July 2017, <https://www.rfc-
editor.org/info/rfc8201>. editor.org/info/rfc8201>.
9.2. Informative References 9.2. Informative References
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [RFC1191] Mogul, J.C. and S.E. Deering, "Path MTU discovery", RFC
DOI 10.17487/RFC1191, November 1990, <https://www.rfc- 1191, DOI 10.17487/RFC1191, November 1990, <https://www
editor.org/info/rfc1191>. .rfc-editor.org/info/rfc1191>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC
RFC 2923, DOI 10.17487/RFC2923, September 2000, 2923, DOI 10.17487/RFC2923, September 2000, <https://www
<https://www.rfc-editor.org/info/rfc2923>. .rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram [RFC4340] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion
Congestion Control Protocol (DCCP)", RFC 4340, Control Protocol (DCCP)", RFC 4340, March 2006.
DOI 10.17487/RFC4340, March 2006, <https://www.rfc-
editor.org/info/rfc4340>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007, Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>. <https://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering [RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/
DOI 10.17487/RFC4890, May 2007, <https://www.rfc- RFC4890, May 2007, <https://www.rfc-editor.org/info/
editor.org/info/rfc4890>. rfc4890>.
Appendix A. Event-driven state changes Appendix A. Event-driven state changes
This appendix contains an informative description of key events: This appendix contains an informative description of key events:
Path Setup: When a new path is initiated, the state is set to Path Setup: When a new path is initiated, the state is set to
PROBE_START. As soon as the path is confirmed, the state changes PROBE_START. As soon as the path is confirmed, the state changes
to PROBE_BASE and the probing mechanism for this path is started. to PROBE_BASE and the probing mechanism for this path is started.
the first probe packet is sent with the size of the BASE_PMTU. the first probe packet is sent with the size of the BASE_PMTU.
Arrival of an Acknowledgment: Depending on the probing state, the Arrival of an Acknowledgment: Depending on the probing state, the
reaction differs according to Figure 4, which is just a reaction differs according to Figure 5, which is just a
simplification of Figure 1 focusing on this event. simplification of Figure 2 focusing on this event.
+--------------+ +----------------+ +--------------+ +----------------+
| PROBE_START | --3------------------------------->| PROBE_DISABLED | | PROBE_START | --3------------------------------->| PROBE_DISABLED |
+--------------+ --4-----------\ +----------------+ +--------------+ --4-----------\ +----------------+
\ \
+--------------+ \ +--------------+ \
| PROBE_ERROR | --------------- \ | PROBE_ERROR | --------------- \
+--------------+ \ \ +--------------+ \ \
\ \ \ \
+--------------+ \ \ +--------------+ +--------------+ \ \ +--------------+
| PROBE_BASE | --1---------- \ ------------> | PROBE_BASE | | PROBE_BASE | --1---------- \ ------------> | PROBE_BASE |
+--------------+ --2----- \ \ +--------------+ +--------------+ --2----- \ \ +--------------+
\ \ \ \ \ \
+--------------+ \ \ ------------> +--------------+ +--------------+ \ \ ------------> +--------------+
| PROBE_SEARCH | --2--- \ -----------------> | PROBE_SEARCH | | PROBE_SEARCH | --2--- \ -----------------> | PROBE_SEARCH |
+--------------+ --1---\----\---------------------> +--------------+ +--------------+ --1---\----\---------------------> +--------------+
\ \ \ \
+--------------+ \ \ +--------------+ +--------------+ \ \ +--------------+
| PROBE_DONE | \ -------------------> | PROBE_DONE | | PROBE_DONE | \ -------------------> | PROBE_DONE |
+--------------+ -----------------------> +--------------+ +--------------+ -----------------------> +--------------+
Condition 1: The maximum PMTU size has not yet been reached. Condition 1: The maximum PMTU size has not yet been reached.
Condition 2: The maximum PMTU size has been reached. Conition 3: Condition 2: The maximum PMTU size has been reached. Conition 3:
Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4: Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4:
PROBE_ACK received. PROBE_ACK received.
Figure 4: State changes at the arrival of an acknowledgment Probing timeout: The PROBE_COUNT is initialised to zero each time the
value of PROBED_SIZE is changed. The PROBE_TIMER is started each
Probing timeout: The PROBE_COUNT is initialised to zero each time time a probe packet is sent. It is stopped when an acknowledgment
the value of PROBED_SIZE is changed. The PROBE_TIMER is started arrives that confirms delivery of a probe packet. If the probe
each time a probe packet is sent. It is stopped when an packet is not acknowledged before the PROBE_TIMER expires, the
acknowledgment arrives that confirms delivery of a probe packet. PROBE_ERROR_COUNTER is incremented. When the PROBE_COUNT equals
If the probe packet is not acknowledged before the PROBE_TIMER the value MAX_PROBES, the state is changed, otherwise a new probe
expires, the PROBE_ERROR_COUNTER is incremented. When the packet of the same size (PROBED_SIZE) is resent. The state
PROBE_COUNT equals the value MAX_PROBES, the state is changed, transitions are illustrated in Figure 6. This shows a
otherwise a new probe packet of the same size (PROBED_SIZE) is simplification of Figure 2 with a focus only on this event.
resent. The state transitions are illustrated in Figure 5. This
shows a simplification of Figure 1 with a focus only on this
event.
+--------------+ +----------------+ +--------------+ +----------------+
| PROBE_START |----------------------------------->| PROBE_DISABLED | | PROBE_START |----------------------------------->| PROBE_DISABLED |
+--------------+ +----------------+ +--------------+ +----------------+
+--------------+ +--------------+ +--------------+ +--------------+
| PROBE_ERROR | -----------------> | PROBE_ERROR | | PROBE_ERROR | -----------------> | PROBE_ERROR |
+--------------+ / +--------------+ +--------------+ / +--------------+
/ /
+--------------+ --2----------/ +--------------+ +--------------+ --2----------/ +--------------+
| PROBE_BASE | --1------------------------------> | PROBE_BASE | | PROBE_BASE | --1------------------------------> | PROBE_BASE |
+--------------+ +--------------+ +--------------+ +--------------+
+--------------+ +--------------+ +--------------+ +--------------+
| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH | | PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |
+--------------+ --2--------- +--------------+ +--------------+ --2--------- +--------------+
\ \
+--------------+ \ +--------------+ +--------------+ \ +--------------+
| PROBE_DONE | -------------------> | PROBE_DONE | | PROBE_DONE | -------------------> | PROBE_DONE |
+--------------+ +--------------+ +--------------+ +--------------+
Condition 1: The maximum number of probe packets has not been Condition 1: The maximum number of probe packets has not been
reached. Condition 2: The maximum number of probe packets has been reached. Condition 2: The maximum number of probe packets has been
reached. reached.
Figure 5: State changes at the expiration of the probe timer PMTU raise timer timeout: The path through the network can change
PMTU raise timer timeout: The path through the network can change
over time. It impossible to discover whether a path change has over time. It impossible to discover whether a path change has
increased the actual PMTU by exchanging packets less than or equal increased the actual PMTU by exchanging packets less than or equal
to the effective PMTU. This requires PLPMTUD to periodically send to the PLPMTU. This requires PLPMTUD to periodically send a probe
a probe packet to detect whether a larger PMTU is possible. This packet to detect whether a larger PMTU is possible. This probe
probe packet is generated by the PMTU_RAISE_TIMER. When the timer packet is generated by the PMTU_RAISE_TIMER. When the timer
expires, probing is restarted with the BASE_PMTU and the state is expires, probing is restarted with the BASE_PMTU and the state is
changed to PROBE_BASE. changed to PROBE_BASE.
Arrival of an ICMP message: The active probing of the path can be Arrival of an ICMP message: The active probing of the path can be
supported by the arrival of PTB messages sent by routers or supported by the arrival of PTB messages sent by routers or
middleboxes with a link MTU that is smaller than the probe packet middleboxes with a link MTU that is smaller than the probe packet
size. If the PTB message includes the router link MTU, three size. If the PTB message includes the router link MTU, three
cases can be distinguished: cases can be distinguished:
1. The indicated link MTU in the PTB message is between the 1. The indicated link MTU in the PTB message is between the
already probed and effective MTU and the probe that triggered already probed and PLMTU and the probe that triggered the PTB
the PTB message. message.
2. The indicated link MTU in the PTB message is smaller than the 2. The indicated link MTU in the PTB message is smaller than the
effective PMTU. PLPMTU.
3. The indicated link MTU in the PTB message is equal to the 3. The indicated link MTU in the PTB message is equal to the
BASE_PMTU. BASE_PMTU.
In first case, the PROBE_BASE state transitions to the PROBE_ERROR In first case, the PROBE_BASE state transitions to the PROBE_ERROR
state. In the PROBE_SEARCH state, a new probe packet is sent with state. In the PROBE_SEARCH state, a new probe packet is sent with
the sized reported by the PTB message. Its result is handled the sized reported by the PTB message. Its result is handled
according to the former events. according to the former events.
The second case could be a result of a network re-configuration. The second case could be a result of a network re-configuration.
If the reported link MTU in the PTB message is greater than the If the reported link MTU in the PTB message is greater than the
BASE_MTU, the probing starts again with a value of PROBE_BASE. BASE_MTU, the probing starts again with a value of PROBE_BASE.
Otherwise, the method enters the state PROBE_ERROR. Otherwise, the method enters the state PROBE_ERROR.
In the third case, the maximum possible PMTU has been reached. In the third case, the maximum possible PMTU has been reached.
This ought to be probed again, because there could be a link This ought to be probed again, because there could be a link
further along the path with a still smaller MTU. further along the path with a still smaller MTU.
Note: Not all routers include the link MTU size when they send a Note: Not all routers include the link MTU size when they send a
PTB message. If the PTB message does not indicate the link MTU, PTB message. If the PTB message does not indicate the link MTU,
the probe is handled in the same way as condition 2 of Figure 5. the probe is handled in the same way as condition 2 of Figure 6.
Appendix B. Revision Notes Appendix B. Revision Notes
Note to RFC-Editor: please remove this entire section prior to Note to RFC-Editor: please remove this entire section prior to
publication. publication.
Individual draft -00: Individual draft -00:
o Comments and corrections are welcome directly to the authors or o Comments and corrections are welcome directly to the authors or
via the IETF TSVWG working group mailing list. via the IETF TSVWG working group mailing list.
skipping to change at page 30, line 36 skipping to change at page 32, line 25
o This draft includes improved introduction. o This draft includes improved introduction.
o The draft is updated to require ICMP validation prior to accepting o The draft is updated to require ICMP validation prior to accepting
PTB messages - this to be confirmed by WG PTB messages - this to be confirmed by WG
o Section added to discuss Selection of Probe Size - methods to be o Section added to discuss Selection of Probe Size - methods to be
evlauated and recommendations to be considered evlauated and recommendations to be considered
o Section added to align with work proposed in the QUIC WG. o Section added to align with work proposed in the QUIC WG.
Working Group draft -02:
o The draft was updated based on feedback from the WG, and a
detailed review by Magnus Westerlund.
o The document updates RFC 4821.
o Requirements list updated.
o Added more explicit discussion of a simpler black-hole detection
mode.
o This draft includes reorganisation of the section on IETF
protocols.
o Added more discussion of implementation within an application.
o Added text on flapping paths.
o Replaced 'effective MTU' with new term PLPMTU.
Authors' Addresses Authors' Addresses
Godred Fairhurst Godred Fairhurst
University of Aberdeen University of Aberdeen
School of Engineering School of Engineering
Fraser Noble Building Fraser Noble Building
Aberdeen AB24 3U Aberdeen, AB24 3U
UK UK
Email: gorry@erg.abdn.ac.uk Email: gorry@erg.abdn.ac.uk
Tom Jones Tom Jones
University of Aberdeen University of Aberdeen
School of Engineering School of Engineering
Fraser Noble Building Fraser Noble Building
Aberdeen AB24 3U Aberdeen, AB24 3U
UK UK
Email: tom@erg.abdn.ac.uk Email: tom@erg.abdn.ac.uk
Michael Tuexen Michael Tuexen
Muenster University of Applied Sciences Muenster University of Applied Sciences
Stegerwaldstrasse 39 Stegerwaldstrasse 39
Stein fart 48565 Stein fart, 48565
DE DE
Email: tuexen@fh-muenster.de Email: tuexen@fh-muenster.de
Irene Ruengeler Irene Ruengeler
Muenster University of Applied Sciences Muenster University of Applied Sciences
Stegerwaldstrasse 39 Stegerwaldstrasse 39
Stein fart 48565 Stein fart, 48565
DE DE
Email: i.ruengeler@fh-muenster.de Email: i.ruengeler@fh-muenster.de
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