draft-ietf-tsvwg-datagram-plpmtud-00.txt   draft-ietf-tsvwg-datagram-plpmtud-01.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 Intended status: Standards Track University of Aberdeen
Expires: July 24, 2018 M. Tuexen Expires: September 6, 2018 M. Tuexen
I. Ruengeler I. Ruengeler
Muenster University of Applied Sciences Muenster University of Applied Sciences
January 22, 2018 March 05, 2018
Packetization Layer Path MTU Discovery for Datagram Transports Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-00 draft-ietf-tsvwg-datagram-plpmtud-01
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 method allows a
Packetization layer (or a datagram application that uses it) to probe Packetization Layer (PL), or a datagram application that uses a PL,
an network path with progressively larger packets to determine a to probe an network path with progressively larger packets to
maximum packet size. The document describes as an extension to RFC determine a maximum packet size. The document describes an extension
1191 and RFC 8201, which specify ICMP-based Path MTU Discovery for to RFC 1191 and RFC 8201, which specify ICMP-based Path MTU Discovery
IPv4 and IPv6. This provides functionally for datagram transports for IPv4 and IPv6. This provides functionally for datagram
that is equivalent to the Packetization layer PMTUD specification for transports that is equivalent to the Packetization layer PMTUD
TCP, specified in RFC4821. specification for TCP, specified in RFC4821.
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
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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 July 24, 2018. This Internet-Draft will expire on September 6, 2018.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . 3
3. Features required to provide Datagram PLPMTUD . . . . . . . . 6 1.2. Packetization Layer Path MTU Discovery . . . . . . . . . 4
3.1. PMTU Probe Packets . . . . . . . . . . . . . . . . . . . . 8 1.3. Path MTU Discovery for Datagram Services . . . . . . . . 5
3.2. Validation of the current effective PMTU . . . . . . . . . 9 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Reduction of the effective PMTU . . . . . . . . . . . . . 10 3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 7
4. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . . 10 3.1. PMTU Probe Packets . . . . . . . . . . . . . . . . . . . 10
4.1. Probing . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2. Validation of the Current Effective PMTU . . . . . . . . 11
4.2. Selecting the Size of a Probe Message . . . . . . . . . . 11 3.3. Reduction of the Effective PMTU . . . . . . . . . . . . . 11
4.3. Verification and use of PTB messages . . . . . . . . . . . 11 4. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 12
4.4. Timers . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.1. Probing . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.5. Constants . . . . . . . . . . . . . . . . . . . . . . . . 12 4.2. Verification and Use of PTB Messages . . . . . . . . . . 13
4.6. Variables . . . . . . . . . . . . . . . . . . . . . . . . 13 4.3. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.7. Selecting Probe Size . . . . . . . . . . . . . . . . . . . 13 4.4. Constants . . . . . . . . . . . . . . . . . . . . . . . . 14
4.8. State Machine . . . . . . . . . . . . . . . . . . . . . . 14 4.5. Variables . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Specification of Protocol-Specific Methods . . . . . . . . . . 16 4.6. Selecting PROBED_SIZE . . . . . . . . . . . . . . . . . . 15
5.1. DPLPMTUD for UDP and UDP-Lite . . . . . . . . . . . . . . 16 4.7. State Machine . . . . . . . . . . . . . . . . . . . . . . 15
5.1.1. UDP Options . . . . . . . . . . . . . . . . . . . . . 16 5. Specification of Protocol-Specific Methods . . . . . . . . . 18
5.1.2. UDP Options required for PLPMTUD . . . . . . . . . . . 16 5.1. DPLPMTUD for UDP and UDP-Lite . . . . . . . . . . . . . . 18
5.1.2.1. Echo Request Option . . . . . . . . . . . . . . . 16 5.1.1. UDP Options . . . . . . . . . . . . . . . . . . . . . 18
5.1.2.2. Echo Response Option . . . . . . . . . . . . . . . 17 5.1.2. UDP Options Required for PLPMTUD . . . . . . . . . . 18
5.1.3. Sending UDP-Option Probe Packets . . . . . . . . . . . 17 5.1.2.1. Echo Request Option . . . . . . . . . . . . . . . 19
5.1.4. Validating the Path with UDP Options . . . . . . . . . 17 5.1.2.2. Echo Response Option . . . . . . . . . . . . . . 19
5.1.5. Handling of PTB Messages by UDP . . . . . . . . . . . 17 5.1.3. Sending UDP-Option Probe Packets . . . . . . . . . . 19
5.2. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 17 5.1.4. Validating the Path with UDP Options . . . . . . . . 20
5.2.1. SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . . 18 5.1.5. Handling of PTB Messages by UDP . . . . . . . . . . . 20
5.2.1.1. Sending SCTP Probe Packets . . . . . . . . . . . . 18 5.2. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 20
5.2.1.2. Validating the Path with SCTP . . . . . . . . . . 18 5.2.1. SCTP/IP4 and SCTP/IPv6 . . . . . . . . . . . . . . . 20
5.2.1.3. PTB Message Handling by SCTP . . . . . . . . . . . 18 5.2.1.1. Sending SCTP Probe Packets . . . . . . . . . . . 20
5.2.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 19 5.2.1.2. Validating the Path with SCTP . . . . . . . . . . 21
5.2.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . . 19 5.2.1.3. PTB Message Handling by SCTP . . . . . . . . . . 21
5.2.2.2. Validating the Path with SCTP/UDP . . . . . . . . 19 5.2.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 21
5.2.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . . 19 5.2.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . 21
5.2.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . . 19 5.2.2.2. Validating the Path with SCTP/UDP . . . . . . . . 21
5.2.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 19 5.2.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . 21
5.2.3.2. Validating the Path with SCTP/DTLS . . . . . . . . 19 5.2.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 22
5.2.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 19 5.2.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 22
5.3. Other IETF Transports . . . . . . . . . . . . . . . . . . 20 5.2.3.2. Validating the Path with SCTP/DTLS . . . . . . . 22
5.4. DPLPMTUD by Applications . . . . . . . . . . . . . . . . . 20 5.2.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 22
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20 5.3. PMTUD for QUIC . . . . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 5.3.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20 5.3.2. Validating the Path with QUIC . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.3.3. Handling of PTB Messages by QUIC . . . . . . . . . . 23
9.1. Normative References . . . . . . . . . . . . . . . . . . . 21 5.4. Other IETF Transports . . . . . . . . . . . . . . . . . . 23
9.2. Informative References . . . . . . . . . . . . . . . . . . 22 5.5. DPLPMTUD by Applications . . . . . . . . . . . . . . . . 23
Appendix A. Event-driven state changes . . . . . . . . . . . . . . 23 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 24
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . . 25 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Event-driven state changes . . . . . . . . . . . . . 26
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
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).
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). The sender adjusts the effective Path MTU big messages (Type 2). When a sender receives a PTB message, it
(PMTU), based on reception of ICMP PTB messages to decrease the PMTU reduces the effective Path MTU (PMTU) to the value reported as the
when a packet is sent with a size larger than the value reported as Link MTU in the PTB message, and a method that from time-to-time
the Link MTU in the PTB message, and a method that from time-to-time
increases the packet size in attempt to discover an increase in the increases 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
effective PMTU are known as probe packets.
However, Classical PMTUD is subject to protocol failures. One Packets not intended as probe packets are either fragmented to the
failure arises when traffic using a packet size larger than the current effective PMTU, or the attempt to send fails with an error
actual supported PMTU is black-holed (all datagrams sent with this code. Applications are sometimes provided with a primitive to let
size are silently discarded). This could continue to happen when ICMP them read the maximum packet size, derived from the current effective
PTB messages are not delivered back to the sender for some reason PMTU.
[RFC2923]). For example, ICMP messages are increasingly filtered by
middleboxes (including firewalls) [RFC4890], and in some cases are
not correctly processed by tunnel endpoints.
Another failure could result if a system not on the network path Classical PMTUD is subject to protocol failures. One failure arises
sends a PTB that attempts to force the sender to change the effective when traffic using a packet size larger than the actual supported
PMTU [RFC8201]. A sender can protect itself from reacting to such PMTU is black-holed (all datagrams sent with this size are silently
discarded without the sender receiving ICMP PTB messages. This could
arise when the ICMP messages are not delivered back to the sender for
some reason [RFC2923]). For example, ICMP messages are increasingly
filtered by middleboxes (including firewalls) [RFC4890]. Also, in
some cases are not correctly processed by tunnel endpoints.
Another failure could result if a node not on the network path sends
a PTB that attempts to force the sender to change the effective PMTU
[RFC8201]. A sender can protect itself from reacting to such
messages by utilising the quoted packet within the PTB message messages by utilising the quoted packet within the PTB message
payload to verify that the received PTB message was generated in payload to verify that the received PTB message was generated in
response to a packet that had actually been sent. However, there are response to a packet that had actually been sent. However, there are
situations where a sender is unable to provide this verification situations where a sender would be unable to provide this
(e.g., when the PTB message does not include sufficient information, verification.
often the case for IPv4; or where the information corresponds to an
encrypted packet). Most routers implement RFC792 [RFC0792], which
requires them to return only the first 64 bits of the IP payload of
the packet, whereas RFC1812 [RFC1812] requires routers to return the
full packet if possible.
Even when the PTB message includes sufficient bytes of the quoted Examples where verification is not possible include:
packet, the network layer could lack sufficient context to perform
verification, because this depends on information about the active o When the router issuing the ICMP message is acting on a tunneled
transport flows at an endpoint node (e.g., the socket/address pairs packet the ICMP message is directed to the tunnel endpoint. This
being used, and other protocol header information). endpoint is responsible for processed in the quoted packet in the
payload field to remove the effect of the tunnel, and return the
ICMP message to the sender. Failure to do this results in black-
holing.
o When the router issuing the ICMP message implements RFC792
[RFC0792], which only requires the quoted payload to include the
first 64 bits of the IP payload of the packet, and the ICMP
message occurs within a tunnel. Even if the decpasulated message
is processed by the tunnel endpoint, there could be insufficient
bytes remaining for the sender to read the quoted transport
information. RFC1812 [RFC1812] requires routers to return the
full packet if possible, often the case for IPv4 when used the
path includes tunnels; or where the packet has been encapsulated/
tunneled over 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
packet, the network layer could lack sufficient context to perform
verification, because this depends on information about the active
transport flows at an endpoint node (e.g., the socket/address
pairs being used, and other protocol header information).
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 packets and selecting an appropriate maximum packet size. This
function is often performed by a transport protocol, but can also be function is often performed by a transport protocol, but can also be
performed by other encapsulation methods working above the transport. performed by other encapsulation methods working above the transport.
PTB verification is more straight forward at the PL or at a higher PTB verification is more straight forward at the PL or at a higher
layer. 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. This PTB messages. It is therefore more robust than Classical PMTUD.
has become the recommended approach for implementing PMTU discovery This has become the recommended approach for implementing PMTU
with TCP. It uses a general strategy where the PL searches for an discovery with TCP.
appropriate PMTU by sending probe packets along the network path with
a progressively larger packet size. If a probe packet is It uses a general strategy where the PL sends probe packet to search
successfully delivered (as determined by the PL), then the effective for an appropriate PMTU. The probe packets are sent a progressively
Path MTU is raised to the size of the successful probe. larger packet size. If a probe packet is successfully delivered (as
determined by the PL), then the effective Path MTU is raised to the
size of the successful probe. If no response is received to a probe
packet, the method reduces the probe size.
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. PLPMTUD
can also include additional consistency checks without increasing the can also include additional consistency checks without increasing the
risk of increased blackholing. risk of increased black-holing.
The UDP-Guidelines [RFC8085] state "an application SHOULD either use 1.3. Path MTU Discovery for Datagram Services
the path MTU information provided by the IP layer or implement Path
MTU Discovery (PMTUD)", but does not provide a mechanism for
discovering the largest size of unfragmented datagram than can be
used on a path. PLPMTUD has not currently been specified for UDP,
while Section 10.2 of [RFC4821] recommends a PLPMTUD probing method
for SCTP that utilises heartbeat messages as probe packets, but does
not provide a complete specification. This document provides the
details to complete that specification. Similarly, the method
defined in this specification could be used with the Datagram
Congestion Control Protocol (DCCP) [RFC4340] requires implementations
to support Classical PMTUD and states that a DCCP sender "MUST
maintain the maximum packet size (MPS) allowed for each active DCCP
session". It also defines the current congestion control maximum
packet size (CCMPS) supported by a path. This recommends use of
PMTUD, and suggests use of control packets (DCCP-Sync) as path probe
packets, because they do not risk application data loss.
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 a maximum size for the effective PMTU across a
path. The methods described rely on features of the PL Section 3 and path. The methods described rely on features of the PL Section 3 and
apply to transport protocols over IPv4 and IPv6. It does not require apply to transport protocols over IPv4 and IPv6. It does not require
cooperation from the lower layers (except that they are consistent cooperation from the lower layers (except that they are consistent
about which packet sizes are acceptable). A method can utilise ICMP about which packet sizes are acceptable). A method can utilise ICMP
PTB messages when received messages are made available to the PL. PTB messages when these received messages are made available to the
PL.
Finally, Section 5 specifies the method for a set of transports, and The UDP-Guidelines [RFC8085] state "an application SHOULD either use
provides information to enables the implementation of PLPMTUD with the Path MTU information provided by the IP layer or implement Path
other datagram transports and applications that use datagram MTU Discovery (PMTUD)", but does not provide a mechanism for
transports. discovering the largest size of unfragmented datagram than can be
used on a path. Prior to this document, PLPMTUD had not been
specified for UDP.
Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the
Stream Control Transport Protocol (SCTP). SCTP utilises heartbeat
messages as probe packets, but RFC4821 does not provide a complete
specification. This document provides the details to complete that
specification.
The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
implementations to support Classical PMTUD and states that a DCCP
sender "MUST maintain the maximum packet size (MPS) allowed for each
active DCCP session". It also defines the current congestion control
maximum packet size (CCMPS) supported by a path. This recommends use
of PMTUD, and suggests use of control packets (DCCP-Sync) as path
probe packets, because they do not risk application data loss. The
method defined in this specification could be used with DCCP.
Section 5 specifies the method for a set of transports, and provides
information to enables the implementation of PLPMTUD with other
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 PMTU, but is unaware of a change to the path that resulted in a
smaller PMTU). smaller PMTU).
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 Packetization Layer.
EMTU_S: The Effective MTU for sending (EMTU_S) is defined in EMTU_S: The Effective MTU for sending (EMTU_S) is defined in
[RFC1122] as "the maximum IP datagram size that may be sent, for a [RFC1122] as "the maximum IP datagram size that may be sent, for a
particular combination of IP source and destination addresses...". particular combination of IP source and destination addresses...".
EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in
[RFC1122] as the largest datagram size that can be reassembled by [RFC1122] as the largest datagram size that can be reassembled by
EMTU_R ("Effective MTU to receive"). EMTU_R ("Effective MTU to receive").
Link: A communication facility or medium over which nodes can Link: A communication facility or medium over which nodes can
communicate at the link layer, i.e., a layer below the IP layer. communicate at the link layer, i.e., a layer below the IP layer.
Examples are Ethernet LANs and Internet (or higher) layer and Examples are Ethernet LANs and Internet (or higher) layer and
tunnels. tunnels.
Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes of Link MTU: The Maximum Transmission Unit (MTU) is the size in bytes
the largest IP packet, including the IP header and payload, that of the largest IP packet, including the IP header and payload,
can be transmitted over a link. Note that this could more that can be transmitted over a link. Note that this could more
properly be called the IP MTU, to be consistent with how other properly be called the IP MT, to be consistent with how other
standards organizations use the acronym MTU. 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), 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 PLPMTUD this quantity is derived from Effective PMTU by taking
into consideration the size of the application and lower protocol into consideration the size of the application and lower protocol
layer headers, and can be limited by the application protocol. layer headers, and can be limited by the application protocol.
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.
Path MTU (PMTU): The minimum of the link MTU of all the links forming Path MTU (PMTU): The minimum of the link MTU of all the links
a path between a source node and a destination node. forming a path between a source node and a destination node.
PLPMTUD: Packetization Layer Path MTU Discovery, the method described PLPMTUD: Packetization Layer Path MTU Discovery, the method
in this document for datagram PLs, which is an extension to described in this document for datagram PLs, which is an extension
Classical PMTU Discovery. to Classical PMTU Discovery.
Probe packet: A datagram sent with a purposely chosen size (typically Probe packet: A datagram sent with a purposely chosen size
larger than the current Effective PMTU or MPS) to detect if (typically larger than the current Effective PMTU or MPS) to
messages of this size can be successfully sent along the end-to- detect if messages of this size can be successfully sent along the
end path. end-to-end 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 nine 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 PLPMTUD sender is REQUIRED 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 link MTU and MAY
utilize similar information about the receiver when this is utilize similar information about the receiver when this is
supplied (note this could be less than EMTU_R). Some applications supplied (note this could be less than EMTU_R). Some
also have a maximum transport protocol data unit (PDU) size, in applications also have a maximum transport protocol data unit
which case there is no benefit from probing for a size larger (PDU) size, in which case there is no benefit from probing for a
than this (unless a transport allows multiplexing multiple size larger than this (unless a transport allows multiplexing
applications PDUs into the same datagram). multiple applications PDUs into the same datagram).
2. Effective PMTU: A datagram application MUST be able to choose the 2. Effective PMTU: A datagram application MUST be able to choose the
size of datagrams sent to the network, up to the effective PMTU, size of datagrams sent to the network, up to the effective PMTU,
or a smaller value (such as the MPS) derived from this. This or a smaller value (such as the MPS) derived from this. This
value is managed by the PMTUD method. The effective PMTU value is managed by the PMTUD method. The effective PMTU
(specified in Section 1 of [RFC1191]) is equivalent to the EMTU_S (specified 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 current effective PMTU
(but always with a total size less than the link MTU). The method (but always with a total size less than the link MTU). The
can use this as a probe packet. In IPv4, a probe packet is method can use this as a probe packet. In IPv4, a probe packet
always sent with the Don't Fragment (DF) bit set and without is always sent with the Don't Fragment (DF) bit set in the IP
network layer endpoint fragmentation. In IPv6, a probe packet is header, and without network layer endpoint fragmentation. In
always sent without source fragmentation (as specified in section IPv6, a probe packet is always sent without source fragmentation
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 PLPMTUD 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 SHOULD/MUST be verified before it is
used to update the PMTU discovery information [RFC8201]. This used to update the PMTU 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 PMTU discovery method reacts to the PTB message. When
the router link MTU is indicated in the PTB message this MAY be the router link MTU is indicated in the PTB message this MAY be
used by datagram PLPMTUD to reduce the size of a probe, but MUST used by datagram PLPMTUD to reduce the size of a probe, but MUST
NOT be used increase the effective PMTU ([RFC8201]). NOT be used to increase the effective PMTU ([RFC8201]).
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 when a probe packet has provide a feedback method that indicates to the PLPMTUD sender
been received by the destination endpoint. The local PL endpoint when a probe packet has been received by the destination
at the sending node is REQUIRED to pass this feedback to the endpoint. The local PL endpoint at the sending node is REQUIRED
sender-side PLPMTUD method. to pass this feedback to the sender-side PLPMTUD 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 does not directly trigger a congestion control reaction
[RFC4821]. [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 PLPMTUD 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. The PL MAY retransmit any data included in a lost
probe packet without adjusting its congestion window [RFC4821]. probe packet without adjusting its congestion window [RFC4821].
8. Cached effective PMTU: The sender MUST cache the effective PMTU 8. Cached effective PMTU: The sender MUST cache the effective PMTU
value used by an instance of the PL between probes and needs also value used by an instance of the PL between probes and needs also
to consider the disruption that could be incurred by an to consider the disruption that could be incurred by an
unsuccessful probe - both upon the flow that incurs a probe loss, unsuccessful probe - both upon the flow that incurs a probe loss,
and other flows that experience the effect of additional probe and other flows that experience the effect of additional probe
skipping to change at page 8, line 26 skipping to change at page 9, line 44
adjustments". Such methods need to robust to the wide variety of adjustments". Such methods need to robust to the wide variety of
underlying network forwarding behaviours. Section 5.2 of underlying network forwarding behaviours. Section 5.2 of
[RFC8201] provides guidance on the caching of PMTU information [RFC8201] provides guidance on the caching of PMTU information
and also the relation to IPv6 flow labels. and also the relation to IPv6 flow labels.
In addition the following design principles are stated: In addition the following design principles are stated:
o Suitable MPS: The PLPMTUD method SHOULD avoid forcing an o Suitable MPS: The PLPMTUD method SHOULD avoid forcing an
application to use an arbitrary small MPS (effective PMTU) for application to use an arbitrary small MPS (effective PMTU) for
transmission while the method is searching for the currently transmission while the method is searching for the currently
supported PMTU. Datagram PLs do not necessarily support supported PMTU. Datagram PLs do not necessarily support
fragmentation of PDUs larger than the PMTU. A reduced MPS can fragmentation of PDUs larger than the PMTU. A reduced MPS can
adversely impact the performance of a datagram application. adversely impact the performance of a datagram application.
o Path validation: The PLPMTUD method MUST be robust to path changes o Path validation: The PLPMTUD method MUST be robust to path changes
that could have occurred since the path characteristics were last that could have occurred since the path characteristics were last
confirmed. confirmed.
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.
skipping to change at page 9, line 18 skipping to change at page 10, line 41
provide a way to fragment a datagram at the PL, or could instead provide a way to fragment a datagram at the PL, or could instead
utilise a control packet with padding. utilise a 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:
Probing using appication data: A probe packet that contains a data Probing using appication data: A probe packet that contains a data
block supplied by an application that matches the size required block supplied by an application that matches the size required
for the probe. This method requests the application to issue a for the probe. This method requests the application to issue a
data block of the desired probe size. If the application/ data block of the desired probe size. If the application/
transport needs protection from the loss of an unsuccessful probe transport needs protection from the loss of an unsuccessful probe
packet, the application/transport needs then to perform transport- packet, the application/transport needs then to perform transport-
layer retransmission/repair of the data block (e.g., by layer retransmission/repair of the data block (e.g., by
retransmission after loss is detected or by duplicating the data retransmission after loss is detected or by duplicating the data
block in a datagram without the padding). 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. 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 control Probing using padding data: A probe packet that contains only
information together with any padding needed to inflate the packet control information together with any padding needed to inflate
to the size required for the probe. Since these probe packets do the packet to the size required for the probe. Since these probe
not carry an application-supplied data block,they do not typically packets do not carry an application-supplied data block,they do
require retransmission, although they do still consume network not typically require retransmission, although they do still
capacity and incur endpoint processing. consume network capacity and incur endpoint processing.
A datagram PLPMTUD MAY choose to use only one of these methods to A datagram PLPMTUD MAY choose to use only one of these methods to
simplify the implementation. simplify the implementation.
3.2. Validation of the current effective PMTU 3.2. Validation of the Current Effective PMTU
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 PLPMTUD 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 it sends are discarded
because their size is greater than the actual PMTUD. These PLs need because their size is greater than the actual PMTUD. These PLs need
to either rely on an application protocol to detect this, or make use to either rely on an application protocol to detect this, or make use
of an additional transport method such as UDP-Options [I-D.ietf- of an additional transport method such as UDP-Options
tsvwg-udp-options]. In addition, they might need to send [I-D.ietf-tsvwg-udp-options]. In addition, they might need to send
reachability probes (e.g., periodically solicit a response from the reachability probes (e.g., periodically solicit a response from the
destination) to determine whether the current effective PMTU is still destination) to determine whether the current effective PMTU is still
supported by the network path. 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. Reduction of the Effective PMTU
When the current effective PMTU is no longer supported by the network When the current effective PMTU is no longer supported by the network
path, the transport needs to detect this and reduce the effective path, the transport needs to detect this and reduce the effective
PMTU. PMTU.
o A PL that sends a datagram larger than the actual PMTU that o A PL that sends a datagram larger than the actual PMTU that
includes no application data block, or one that does not attempt includes no application data block, or one that does not attempt
to provide any retransmission, can send a new probe packet with an to provide any retransmission, can send a new probe packet with an
updated probe size. updated probe size.
skipping to change at page 11, line 9 skipping to change at page 12, line 41
The PLPMTUD method utilises a timer to trigger the generation of The PLPMTUD method utilises a timer to trigger the generation of
probe packets. The probe_timer is started each time a probe packet 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 is sent to the destination and is cancelled when receipt of the probe
packet is acknowledged. packet is acknowledged.
The PROBE_COUNT is initialised to zero when a probe packet is first The PROBE_COUNT is initialised to zero when a probe packet is first
sent with a particular size. Each time the probe_timer expires, the sent with a particular size. Each time the probe_timer expires, the
PROBE_COUNT is incremented, and a probe packet of the same size is PROBE_COUNT is incremented, and a probe packet of the same size is
retransmitted. The maximum number of retransmissions per probing retransmitted. The maximum number of retransmissions per probing
size is configured (MAX_PROBES). If the value of the PROBE_COUNT size is configured (MAX_PROBES). If the value of the PROBE_COUNT
reaches MAX_PROBES, probing will be stopped and the last successfully reaches MAX_PROBES, probing will be stopped and the last successfully
probed PMTU is set as the effective PMTU. probed PMTU is set as the effective PMTU.
Once probing is completed, the sender continues to use the effective Once probing is completed, the sender continues to use the effective
PMTU until either a PTB message is received or the PMTU_RAISE_TIMER PMTU until either a PTB message is received or the PMTU_RAISE_TIMER
expires. If the PL is unable to verify reachability to the expires. If the PL is unable to verify reachability to the
destination endpoint after probing has completed, the method uses a destination endpoint after probing has completed, the method uses a
REACHABILITY_TIMER to periodically repeat a probe packet for the REACHABILITY_TIMER to periodically repeat a probe packet for the
current effective PMTU size, while the PMTU_RAISE_TIMER is running. current effective PMTU size, while the PMTU_RAISE_TIMER is running.
If the resulting probe packet is not acknowledged (i.e. the If the resulting probe packet is not acknowledged (i.e. the
PROBE_TIMER expires), the method re-starts probing for the PMTU. PROBE_TIMER expires), the method re-starts probing for the PMTU.
4.2. Selecting the Size of a Probe Message 4.2. Verification and Use of PTB Messages
Path probing relies on generation of probe packets with a specific
size. This section decribes how the algorithm selects the size of
the next probe message.
XXX Details may be specified in later revisions of this document- the
next list is for information XXX
There are serveral things to consider, these include:
step granularity (i.e., it could be unecessary to probe for each
possible PMTU, and probes could be at a courser elvel - every 16B?
every 128B? )
There are various algorithms that could be used to arrive at a set
of suitable probe sizes. Should the value be derived from a table
of probe sizes or via a search algorithm (e.g. a binary search),
or is a hybrid approach at different times to be preferred?
Should one method be specified?.
How much overhead is present in the probe packet? to map from a
probe payload to the size of the probe on the wire (does this
matter in selection of the probe size?
4.3. Verification and use of PTB messages
XXX A decision on SHOULD/MUST needs to be made on how to verify
messages XXX
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, SHOULD A node that receives a PTB message from a router or middlebox, MUST
/MUST verify the PTB message. The node checks the protocol verify the PTB message. The node checks the protocol information in
information in the quoted payload to verify that the message the quoted payload to verify that the message originated from the
originated from the sending node. The node also checks that the sending node. The node also checks that the reported MTU size is
reported MTU size is less than the size used by packet probes. PTB less than the size used by packet probes. PTB messages are discarded
messages are discarded if they fail to pass these checks, or where if they fail to pass these checks, or where there is insufficient
there is insufficient ICMP payload to perform these checks. The ICMP payload to perform these checks. The checks are intended to
checks are intended to provide protection from packets that originate provide protection from packets that originate from a node that is
from a node that is not on the network path or a node that attempts not on the network path or a node that attempts to report a larger
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.
XXX Specification needed of how to utilise the reported Link MTU size 4.3. Timers
to generate a probe, and how to account for encapuslations that may
be present at the point where the PTB message was generated. XXX
4.4. Timers
This method utilises three timers: This method 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 continue PMTU_RAISE_TIMER: Configured to the period a sender ought to
use the current effective PMTU, after which it re-commences continue use the current effective PMTU, after which it re-
probing for a higher PMTU. This timer has a period of 600 secs, as commences probing for a higher PMTU. This timer has a period of
recommended by PLPMTUD [RFC4821]. 600 secs, as recommended by PLPMTUD [RFC4821].
REACHABILITY_TIMER: Configured to the period a sender ought to wait REACHABILITY_TIMER: Configured to the period a sender ought to wait
before confirming the current effective PMTU is still supported. before confirming the current effective PMTU is still supported.
This is less than the PMTU_RAISE_TIMER. This is less than the PMTU_RAISE_TIMER.
An application that needs to employ keep-alive messages to deliver An application that needs to employ keep-alive messages to deliver
useful service over UDP SHOULD NOT transmit them more frequently useful service over UDP SHOULD NOT transmit them more frequently
than once every 15 seconds and SHOULD use longer intervals when than once every 15 seconds and SHOULD use longer intervals when
possible. DPLPMTUD ought to suspend reachability probes when no possible. DPLPMTUD ought to suspend reachability probes when no
application data has been sent since the previous probe packet. application data has been sent since the previous probe packet.
Guidance on selection of the timer value are provide in section Guidance on selection of the timer value are provide in section
3.1.1 of the UDP Usage Guidelines[RFC8085]. 3.1.1 of the UDP Usage Guidelines[RFC8085].
An implementation could implement the various timers using a single An implementation could implement the various timers using a single
timer process. timer process.
4.5. Constants 4.4. Constants
The following constants are defined: The following constants are defined:
MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER. The default MAX_PROBES: The maximum value of the PROBE_ERROR_COUNTER. The
value of MAX_PROBES is 10. default value of MAX_PROBES is 10.
MIN_PMTU: The smallest allowed probe packet size. This value is 1280 MIN_PMTU: The smallest allowed probe packet size. This value is
bytes, as specified in [RFC2460]. For IPv4, the minimum value is 1280 bytes, as specified in [RFC2460]. For IPv4, the minimum
68 bytes. (An IPv4 routed is required to be able to forward a value is 68 bytes. (An IPv4 routed is required to be able to
datagram of 68 octets without further fragmentation. This is the forward a datagram of 68 octets without further fragmentation.
combined size of an IPv4 header and the minimum fragment size of 8 This is the combined size of an IPv4 header and the minimum
octets.) fragment size of 8 octets.)
BASE_PMTU: The BASE_PMTU is a considered a size that ought to work in BASE_PMTU: The BASE_PMTU is a considered a size that ought to work
most cases. The size is equal to or larger than the minimum in most cases. The size is equal to or larger than the minimum
permitted and smaller than the maximum allowed. In the case of permitted and smaller than the maximum allowed. In the case of
IPv6, this value is 1280 bytes [RFC2460]. When using IPv4, a size IPv6, this value is 1280 bytes [RFC2460]. When using IPv4, a size
of 1200 is RECOMMENDED. of 1200 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 PMTU 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 effective MTU for
receiving. An application or PL may reduce this when it knows receiving. An application or PL may reduce this when it knows
there is no need to send packets above a specific size. there is no need to send packets above a specific size.
4.6. Variables 4.5. 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 effective PMTU: The effective PMTU is the maximum size of datagram
that the method has currently determined can be supported along that the method has currently determined can be supported along
the entire path. the entire path.
PROBED_SIZE: The PROBED_SIZE is the size of the current probe packet. PROBED_SIZE: The PROBED_SIZE is the size of the current probe
This is a tentative value for the effective PMTU, which is packet. This is a tentative value for the effective PMTU, which
awaiting confirmation by an acknowledgment. is awaiting confirmation by an acknowledgment.
PROBE_COUNT: This is a count of the number of unsuccessful probe PROBE_COUNT: This is a count of the number of unsuccessful probe
packets that have been sent with size PROBED_SIZE. The value is packets that have been sent with size PROBED_SIZE. The value is
initialised to zero when a particular size of PROBED_SIZE is first initialised to zero when a particular size of PROBED_SIZE is first
attempted. attempted.
PTB_SIZE: The PTB_Size is value returned by a verified PTB message PTB_SIZE: The PTB_Size is value returned by a 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.
4.7. Selecting Probe Size 4.6. Selecting PROBED_SIZE
XXX There are serveral things to consider when selecting the size to Implementations discover the search range by validating the minimum
probe, some of which may be specified in later revisions of this path MTU and then using the probe method to select a PROBED_SIZE less
document XXX 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
endpoint). The PMTU_MAX MAY be constrained by an application that
has a maximum to the size of datagrams it wishes to send.
Issues include: Implementations use a search algorithm to choose probe sizes within
the search range. XXX The current method does not specify or
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
step granularity (i.e., it could be unecessary to probe for each Implementations MAY optimizse the search procedure by selecting step
possible PMTU, and probes could be at a courser elvel - every 16B? sizes from a table of common MTU sizes.
every 128B? )
There are various algorithms that could be used to arrive at a set
of suitable probe sizes. Should the value be derived from a table
of probe sizes or via a search algorithm, or is a hybrid approach
at different times to be preferred? Should one method be
specified?.
How much overhead is present in the probe packet? to map from a Implementations SHOULD select probe sizes to maximise the gain in
probe payload to the size of the probe on the wire (does this PMTU each search step. Implementations ought to take into
matter in selection of the probe size? consideration useful probe size steps and a minimum useful gain in
PMTU.
4.8. State Machine 4.7. State Machine
A state machine for Datagram PLPMTUD is depicted in Figure 1. If A state machine for Datagram PLPMTUD is depicted in Figure 1. If
multihoming is supported, a state machine is needed for each active multihoming is supported, a state machine is needed for each active
path. path.
PROBE_TIMER expiry PROBE_TIMER expiry
(PROBE_COUNT = MAX_PROBES) (PROBE_COUNT = MAX_PROBES)
+-------------+ +--------------+ +-------------+ +--------------+
=->| PROBE_START |--------------->|PROBE_DISABLED| =->| PROBE_START |--------------->|PROBE_DISABLED|
PROBE_TIMER expiry | +-------------+ +--------------+ PROBE_TIMER expiry | +-------------+ +--------------+
(PROBE_COUNT = | | | (PROBE_COUNT = | | |
MAX_PROBES) ------- | Connectivity confirmed MAX_PROBES) ------- | Connectivity confirmed
v v
----------- +------------+ -- PROBE_TIMER expiry ----------- +------------+ -- PROBE_TIMER expiry
MAX_PMTU acked or | | PROBE_BASE | | (PROBE_COUNT < MAX_PMTU acked or | | PROBE_BASE | | (PROBE_COUNT <
PTB (>= BASE_PMTU)| -----> +------------+ <- MAX_PROBES) PTB (>= BASE_PMTU)| -----> +------------+ <- MAX_PROBES)
---------------- | /\ | | ---------------- | /\ | |
| | | | | PTB | | | | | PTB
| PMTU_RAISE_TIMER| | | | (PTB_SIZE < BASE_PMTU) | PMTU_RAISE_TIMER| | | | (PTB_SIZE < BASE_PMTU)
| or reachability | | | | or | or reachability | | | | or
| (PROBE_COUNT | | | | PROBE_TIMER expiry | (PROBE_COUNT | | | | PROBE_TIMER expiry
| = MAX_PROBES) | | | | (PROBE_COUNT = MAX_PROBES) | = MAX_PROBES) | | | | (PROBE_COUNT = MAX_PROBES)
| ------------- | | \ | ------------- | | \
| | PTB | | \ | | PTB | | \
| | (< PROBED_SIZE)| | \ | | (< PROBED_SIZE)| | \
| | | | ---------------- | | | | ----------------
| | | | | | | | | |
| | | | Probe | | | | | Probe |
| | | | acked | | | | | acked |
v | | v v v | | v v
+------------+ +--------------+ Probe +-------------+ +------------+ +--------------+ Probe +-------------+
| PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR | | PROBE_DONE |<-------------- | PROBE_SEARCH |<-------| PROBE_ERROR |
+------------+ MAX_PMTU acked +--------------+ acked +-------------+ +------------+ MAX_PMTU acked +--------------+ acked +-------------+
/\ | or /\ | /\ | or /\ |
| | PROBE_TIMER expiry | | | | PROBE_TIMER expiry | |
| |(PROBE_COUNT = MAX_PROBES) | | | |(PROBE_COUNT = MAX_PROBES) | |
| | | | | | | |
------ -------- ------ --------
Reachability probe acked PROBE_TIMER expiry Reachability probe acked PROBE_TIMER expiry
or PROBE_TIMER expiry (PROBE_COUNT < MAX_PROBES) or PROBE_TIMER expiry (PROBE_COUNT < MAX_PROBES)
(PROBE_COUNT < MAX_PROBES) or
Probe acked
(PROBE_COUNT < MAX_PROBES) or Figure 1: State machine for Datagram PLPMTUD
Probe acked
XXX State machine to be updated for PTB messages - to probe for PTB XXX State machine to be updated to describe handling of validated PTB
size XXX messages XXX
XXX Method may be updated to clarify how probe sizes are used during
probing 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. The
effective PMTU is set to the BASE_PMTU size. Probing ought to effective PMTU is set to the BASE_PMTU size. Probing ought to
start immediately after connection setup to prevent the loss of start immediately after connection setup to prevent the loss of
user data. user data.
PROBE_BASE: The PROBE_BASE state is the starting point for probing PROBE_BASE: The PROBE_BASE state is the starting point for probing
with datagram PLPMTUD. It is used to confirm whether the BASE_PMTU with datagram PLPMTUD. It is used to confirm whether the
size is supported by the network path. On entry, the PROBED_SIZE BASE_PMTU size is supported by the network path. On entry, the
is set to the BASE_PMTU size and the PROBE_COUNT is set to zero. PROBED_SIZE is set to the BASE_PMTU size and the PROBE_COUNT is
A probe packet is sent, and the PROBE_TIMER is started. The state set to zero. A probe packet is sent, and the PROBE_TIMER is
is left when the PROBE_COUNT reaches MAX_PROBES; a PTB message is started. The state is left when the PROBE_COUNT reaches
verified, or a probe packet is acknowledged. MAX_PROBES; a PTB message is verified, or a probe packet is
acknowledged.
PROBE_SEARCH: The PROBE_SEARCH state is the main probing state. This PROBE_SEARCH: The PROBE_SEARCH state is the main probing state.
state is entered either when probing for the BASE_PMTU was This state is entered either when probing for the BASE_PMTU was
successful or when there is a successful reachability test in the successful or when there is a successful reachability test in the
PROBE_ERROR state. On entry, the effective PMTU is set to the PROBE_ERROR state. On entry, the effective PMTU is set to the
last acknowledged PROBED_SIZE. last acknowledged PROBED_SIZE.
On the first probe packet for each probed size, the PROBE_COUNT is The PROBE_COUNT is set to zero when the first probe packet is sent
set to zero. Each time a probe packet is acknowledged, the for each probed size. Each time a probe packet is acknowledged,
effective PMTU is set to the PROBED_SIZE, and then the PROBED_SIZE the effective PMTU is set to the PROBED_SIZE, and then the
is increased. When a probe packet is not acknowledged within the PROBED_SIZE is increased.
period of the PROBE_TIMER, the PROBE_COUNT is incremented and the
probe packet is retransmitted. The state is exited when the
PROBE_COUNT reaches MAX_PROBES; a PTB message is verified; or a
probe of size PMTU_MAX is acknowledged.
PROBE_ERROR: The PROBE_ERROR state represents the case where the When a probe packet is sent and not acknowledged within the period
of the PROBE_TIMER, the PROBE_COUNT is incremented and the probe
packet is retransmitted. The state is exited when the PROBE_COUNT
reaches MAX_PROBES; a PTB message is verified; or a probe of size
PMTU_MAX is acknowledged.
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 effective PMTU of at least
the BASE_PMTU size. It is entered when either a probe of size the 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 effective PMTU is reset to MIN_PMTU size.
In this state, a probe packet is sent, and the PROBE_TIMER is In this state, a probe packet is sent, and the PROBE_TIMER is
started. The state transitions to the PROBE_SEARCH state when a started. The state transitions to the PROBE_SEARCH state when a
probe packet is acknowledged. probe 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. Datagram PLPMTUD remains in this state until
either the PMTU_RAISE_TIMER expires or a PTB message is verified. either the PMTU_RAISE_TIMER expires or a received PTB message is
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 effective PMTU.
If the probe packet fails to be acknowledged after MAX_PROBES If the probe packet fails to be acknowledged after MAX_PROBES
attempts, the method enters the PROBE_BASE state. When used with attempts, the method enters the PROBE_BASE state. When used with
an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT continue to
probe in this state. probe in this state.
PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity
could not be established. DPLPMTUD MUST NOT probe in this state. could not be established. DPLPMTUD MUST NOT probe in this state.
Appendix Appendix A contains an informative description of key Appendix A contains an informative description of key events.
events.
5. Specification of Protocol-Specific Methods 5. Specification of Protocol-Specific Methods
This section specifies protocol-specific details for datagram PLPMTUD This section specifies protocol-specific details for datagram PLPMTUD
for IETF-specified transports. for IETF-specified transports.
5.1. DPLPMTUD for UDP and UDP-Lite 5.1. DPLPMTUD for UDP and UDP-Lite
The current specifications of UDP [RFC0768] and UDP-LIte [RFC3828] do The current specifications of UDP [RFC0768] and UDP-LIte [RFC3828] do
not define a method in the RFC-series that supports PLPMTUD. In not define a method in the RFC-series that supports PLPMTUD. In
particular, these transports do not provide the transport layer particular, these transports do not provide the transport layer
features needed to implement datagram PLPMTUD, and any support for features needed to implement datagram PLPMTUD, and any support for
Datagram PLPMTUD would therefore need to rely on higher-layer Datagram PLPMTUD would therefore need to rely on higher-layer
protocol features [RFC8085]. protocol features [RFC8085].
5.1.1. UDP Options 5.1.1. UDP Options
UDP-Options [I-D.ietf-tsvwg-udp-options] supply the additional UDP-Options [I-D.ietf-tsvwg-udp-options] supply the additional
functionality required to implement datagram PLPMTUD. This enables functionality required to implement datagram PLPMTUD. This enables
padding to be added to UDP datagrams and can be used to provide padding to be added to UDP datagrams and can be used to provide
feedback acknowledgement of received probe packets. feedback acknowledgement of received probe packets.
5.1.2. UDP Options required for PLPMTUD 5.1.2. UDP Options Required for PLPMTUD
This subsection proposes two new UDP-Options that add support for This subsection proposes two new UDP-Options that add support for
requesting a datagram response be sent and to mark this datagram as a requesting a datagram response be sent and to mark this datagram as a
response to a request. response to a request.
XXX << Future versions of the spec may define a parameter in an XXX Future versions of the spec may define a parameter in an Option
Option to indicate the EMTU_R to the peer.>> to indicate the EMTU_R to the peer that can be used to initialise
PMTU_MAX. XXX
5.1.2.1. Echo Request Option 5.1.2.1. Echo Request Option
The Echo Request Option allows a sending endpoint to solicit a The Echo Request Option allows a sending endpoint to solicit a
response from a destination endpoint. response from a destination endpoint.
The Echo Request carries a four byte token set by the sender. This The Echo Request carries a four byte token set by the sender. This
token can be set to a value that is likely to be known only to the token can be set to a value that is likely to be known only to the
sender (and becomes known to nodes along the end-to-end path). The sender (and becomes known to nodes along the end-to-end path). The
sender can then check the value returned in the response to provide sender can then check the value returned in the response to provide
additional protection from off-path insertion of data [RFC8085]. additional protection from off-path insertion of data [RFC8085].
+---------+--------+-----------------+ +---------+--------+-----------------+
| Kind=9 | Len=6 | Token | | Kind=9 | Len=6 | Token |
+---------+--------+-----------------+ +---------+--------+-----------------+
1 byte 1 byte 4 bytes 1 byte 1 byte 4 bytes
Figure 2: UDP ECHOREQ Option Format
5.1.2.2. Echo Response Option 5.1.2.2. Echo Response Option
The Echo Response Option is generated by the PL in response to The Echo Response Option is generated by the PL in response to
reception of a previously received Echo Request. The Token field reception of a previously received Echo Request. The Token field
associates the response with the Token value carried in the most associates the response with the Token value carried in the most
recently-received Echo Request. The rate of generation of UDP recently-received Echo Request. The rate of generation of UDP
packets carrying an Echo Response Option MAY be rate-limited. packets carrying an Echo Response Option MAY be rate-limited.
+---------+--------+-----------------+ +---------+--------+-----------------+
| Kind=10 | Len=6 | Token | | Kind=10 | Len=6 | Token |
+---------+--------+-----------------+ +---------+--------+-----------------+
1 byte 1 byte 4 bytes 1 byte 1 byte 4 bytes
Figure 3: UDP ECHORES Option Format
5.1.3. Sending UDP-Option Probe Packets 5.1.3. Sending UDP-Option Probe Packets
This method specifies a probe packet that does not carry an This method specifies a probe packet that does not carry an
application data block. The probe packet consists of a UDP datagram application data block. The probe packet consists of a UDP datagram
header followed by a UDP Option containing the ECHOREQ option, which header followed by a UDP Option containing the ECHOREQ option, which
is followed by NOP Options to pad the remainder of the datagram 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 payload to the probe size. NOP padding is used to control the length
of the probe packet. of the probe packet.
skipping to change at page 17, line 50 skipping to change at page 20, line 14
5.1.4. Validating the Path with UDP Options 5.1.4. Validating the Path with UDP Options
Since UDP is an unacknowledged PL, a sender that does not have Since UDP is an unacknowledged PL, a sender that does not have
higher-layer information confirming correct delivery of datagrams higher-layer information confirming correct delivery of datagrams
SHOULD implement the REACHABILITY_TIMER to periodically send probe SHOULD implement the REACHABILITY_TIMER to periodically send probe
packets while in the PROBE_DONE state. packets while in the PROBE_DONE state.
5.1.5. Handling of PTB Messages by UDP 5.1.5. Handling of PTB Messages by UDP
Normal ICMP verification MUST be performed as specified in Section Normal ICMP verification MUST be performed as specified in
5.2 of [RFC8085]. This requires that the PL verifies each received Section 5.2 of [RFC8085]. This requires that the PL verifies each
PTB messages to verify these are received in response to transmitted received PTB messages to verify these are received in response to
traffic and that the reported LInk MTU is less than the current probe transmitted traffic and that the reported LInk MTU is less than the
size. A verified PTB message MAY be used as input to the PLPMTUD current probe size. A verified PTB message MAY be used as input to
algorithm. the PLPMTUD algorithm.
5.2. DPLPMTUD for SCTP 5.2. 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 specification 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 MTU to the
peer. However, multihoming makes this a bit complex, so it might not peer. However, multihoming makes this a bit complex, so it might not
be worth doing.>> be worth doing. XXX
5.2.1. SCTP/IP4 and SCTP/IPv6 5.2.1. SCTP/IP4 and SCTP/IPv6
The base protocol is specified in [RFC4960]. The base protocol is specified in [RFC4960].
5.2.1.1. Sending SCTP Probe Packets 5.2.1.1. 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
skipping to change at page 19, line 16 skipping to change at page 21, line 37
in the PTB message SHOULD be used with the PLPMTUD algorithm, in the PTB message SHOULD be used with the PLPMTUD 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 5.2.2. 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 5.2.2.1. 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 5.2.1.1. 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 5.2.2.2. 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 5.2.2.3. 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 PLPMTUD
algorithm providing that the reported LInk MTU is less than the algorithm providing that the reported LInk MTU is less than the
current probe size. current probe size.
5.2.3. DPLPMTUD for SCTP/DTLS 5.2.3. 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.
skipping to change at page 20, line 4 skipping to change at page 22, line 26
5.2.3.1. Sending SCTP/DTLS Probe Packets 5.2.3.1. 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 5.2.1.1.
5.2.3.2. Validating the Path with SCTP/DTLS 5.2.3.2. 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 5.2.3.3. 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. Other IETF Transports 5.3. PMTUD for QUIC
XXX New section XXX
Quick UDP Internet Connection (QUIC) is a UDP-based transport that Quick UDP Internet Connection (QUIC) is a UDP-based transport that
provides reception feedback [I-D.ietf-quic-transport]. provides reception feedback [I-D.ietf-quic-transport].
XXX << This section will be completed in a future revision of this ID Section 9.2 of [I-D.ietf-quic-transport] details the path
>> considerations when sending QUIC packets. It reccomends the use of
PADDING frames to buld the probe packet. This enables probing the
without affecting the transfer of other frames.
5.4. DPLPMTUD by Applications 5.3.1. Sending QUIC Probe Packets
Probe packets consist of a QUIC Header and a payload containing only
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
the path can support packets of a useful size. If a QUIC sender
determines the PMTU on a path has fallen below 1280 octets it MUST
immediately stop sending on the affected path.
5.3.2. Validating the Path with QUIC
Since QUIC provides an acknowledged PL, a sender does MUST NOT
implement the REACHABILITY_TIMER while in the PROBE_DONE state.
5.3.3. Handling of PTB Messages by QUIC
QUIC does not specify any methods for validating ICMP responses, but
does provide some guidlines 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
packets, so that most invalid ICMP messages arrive when there are
no DF packets outstanding, and can therefore be identified as
spurious.
o Store additional information from the IP or UDP headers from DF
packets (for example, the IP ID or UDP checksum) to further
authenticate incoming Datagram Too Big messages.
o Any reduction in PMTU due to a report contained in an ICMP packet
is provisional until QUIC's loss detection algorithm determines
that the packet is actually lost.
XXX The above list was pulled whole from quic-transport 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 Applications that use the Datagram API (e.g., applications built
directly or indirectly on UDP) can implement DPLPMTUD. Some directly or indirectly on UDP) can implement DPLPMTUD. Some
primitives used by DPLPMTUD might not be available via this interface primitives used by DPLPMTUD might not be available via this interface
(e.g., the ability to access the PMTU cache, or interpret received (e.g., the ability to access the PMTU cache, or interpret received
ICMP PTB messages). ICMP PTB messages).
In addition, it is important that PMTUD is not performed by multiple In addition, it is important that PMTUD is not performed by multiple
protocol layers. protocol layers.
XXX << This section will be completed in a future revision of this ID 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 XXX If new UDP Options are specified in this document, a request to
to IANA will be included here.>> 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-Guidelines [RFC8085].
PTB messages could potentially be used to cause a node to PTB messages could potentially be used to cause a node to
inappropriately reduce the effective PMTU. A node supporting PLPMTUD inappropriately reduce the effective PMTU. A node supporting PLPMTUD
SHOULD/MUST appropriately verify the payload of PTB messages to MUST appropriately verify the payload of PTB messages to ensure these
ensure these are received in response to transmitted traffic (i.e., a are received in response to transmitted traffic (i.e., a reported
reported error condition that corresponds to a datagram actually sent error condition that corresponds to a datagram actually sent by the
by the path layer. path layer.
XXX Determine if parallel forwarding paths needs to be considered XXX XXX Determine if parallel forwarding paths needs to be considered.
XXX
A node performing PLPMTUD could experience conflicting information A node performing PLPMTUD 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 blackholed when the effective PMTU is larger than the smallest PMTU
across the current paths. across the current paths.
9. References 9. References
9.1. Normative References 9.1. Normative References
[I-D.ietf-quic-transport] [I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Internet-Draft draft-ietf-quic- and Secure Transport", draft-ietf-quic-transport-04 (work
transport-04, June 2017. in progress), 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", Internet-Draft draft-ietf- Encapsulation of SCTP Packets", draft-ietf-tsvwg-sctp-
tsvwg-sctp-dtls-encaps-09, January 2015. dtls-encaps-09 (work in progress), January 2015.
[I-D.ietf-tsvwg-udp-options] [I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", Internet-Draft Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
draft-ietf-tsvwg-udp-options-01, June 2017. udp-options-01 (work in progress), June 2017.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
10.17487/RFC0768, August 1980, <http://www.rfc-editor.org/ DOI 10.17487/RFC0768, August 1980, <https://www.rfc-
info/rfc768>. editor.org/info/rfc768>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981, <https:// RFC 792, DOI 10.17487/RFC0792, September 1981,
www.rfc-editor.org/info/rfc792>. <https://www.rfc-editor.org/info/rfc792>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, DOI 10.17487/ Communication Layers", STD 3, RFC 1122,
RFC1122, October 1989, <http://www.rfc-editor.org/info/ DOI 10.17487/RFC1122, October 1989, <https://www.rfc-
rfc1122>. editor.org/info/rfc1122>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995, <https://www RFC 1812, DOI 10.17487/RFC1812, June 1995,
.rfc-editor.org/info/rfc1812>. <https://www.rfc-editor.org/info/rfc1812>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, <http://www.rfc-editor.org/info/ DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
rfc2119>. 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, <http://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, <http://www.rfc-editor.org/info/rfc3828>. 2004, <https://www.rfc-editor.org/info/rfc3828>.
[RFC4820] Tuexen, M., Stewart, R. and P. Lei, "Padding Chunk and [RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007, (SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
<https://www.rfc-editor.org/info/rfc4820>. <https://www.rfc-editor.org/info/rfc4820>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007, <https:// RFC 4960, DOI 10.17487/RFC4960, September 2007,
www.rfc-editor.org/info/rfc4960>. <https://www.rfc-editor.org/info/rfc4960>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream [RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951, DOI 10.17487/ to End-Host Communication", RFC 6951,
RFC6951, May 2013, <https://www.rfc-editor.org/info/ DOI 10.17487/RFC6951, May 2013, <https://www.rfc-
rfc6951>. editor.org/info/rfc6951>.
[RFC8085] Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <http://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.C. and S.E. Deering, "Path MTU discovery", RFC [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
1191, DOI 10.17487/RFC1191, November 1990, <http://www DOI 10.17487/RFC1191, November 1990, <https://www.rfc-
.rfc-editor.org/info/rfc1191>. editor.org/info/rfc1191>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery", RFC [RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
2923, DOI 10.17487/RFC2923, September 2000, <https://www RFC 2923, DOI 10.17487/RFC2923, September 2000,
.rfc-editor.org/info/rfc2923>. <https://www.rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, Congestion Control Protocol (DCCP)", RFC 4340,
March 2006, <https://www.rfc-editor.org/info/rfc4340>. 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,
<http://www.rfc-editor.org/info/rfc4821>. <https://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering [RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/ ICMPv6 Messages in Firewalls", RFC 4890,
RFC4890, May 2007, <http://www.rfc-editor.org/info/ DOI 10.17487/RFC4890, May 2007, <https://www.rfc-
rfc4890>. editor.org/info/rfc4890>.
Appendix A. Event-driven state changes Appendix A. Event-driven state changes
This appendix contains an informative description of key events: This appendix contains an informative description of key events:
Path Setup: When a new path is initiated, the state is set to Path Setup: When a new path is initiated, the state is set to
PROBE_START. As soon as the path is confirmed, the state changes PROBE_START. As soon as the path is confirmed, the state changes
to PROBE_BASE and the probing mechanism for this path is started. to PROBE_BASE and the probing mechanism for this path is started.
A probe packet with the size of the BASE_PMTU is sent. 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 4, which is just a
simplification of Figure 1 focusing on this event. simplification of Figure 1 focusing on this event.
+--------------+ +----------------+ +--------------+ +----------------+
| PROBE_START | --3------------------------------->| PROBE_DISABLED | | PROBE_START | --3------------------------------->| PROBE_DISABLED |
+--------------+ --4-----------\ +----------------+ +--------------+ --4-----------\ +----------------+
\ \
+--------------+ \ +--------------+ \
| PROBE_ERROR | --------------- \ | PROBE_ERROR | --------------- \
+--------------+ \ \ +--------------+ \ \
\ \ \ \
+--------------+ \ \ +--------------+ +--------------+ \ \ +--------------+
| PROBE_BASE | --1---------- \ ------------> | PROBE_BASE | | PROBE_BASE | --1---------- \ ------------> | PROBE_BASE |
+--------------+ --2----- \ \ +--------------+ +--------------+ --2----- \ \ +--------------+
\ \ \ \ \ \
+--------------+ \ \ ------------> +--------------+ +--------------+ \ \ ------------> +--------------+
| PROBE_SEARCH | --2--- \ -----------------> | PROBE_SEARCH | | PROBE_SEARCH | --2--- \ -----------------> | PROBE_SEARCH |
+--------------+ --1---\----\---------------------> +--------------+ +--------------+ --1---\----\---------------------> +--------------+
\ \ \ \
+--------------+ \ \ +--------------+ +--------------+ \ \ +--------------+
| PROBE_DONE | \ -------------------> | PROBE_DONE | | PROBE_DONE | \ -------------------> | PROBE_DONE |
+--------------+ -----------------------> +--------------+ +--------------+ -----------------------> +--------------+
Condition 1: The maximum PMTU size has not yet been reached. Condition 1: The maximum PMTU size has not yet been reached.
Condition 2: The maximum PMTU size has been reached. Conition 3: Condition 2: The maximum PMTU size has been reached. Conition 3:
Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4: Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4:
PROBE_ACK received. PROBE_ACK received.
Probing timeout: The PROBE_COUNT is initialised to zero each time the Figure 4: State changes at the arrival of an acknowledgment
value of PROBED_SIZE is changed. The PROBE_TIMER is started each
time a probe packet is sent. It is stopped when an acknowledgment
arrives that confirms delivery of a probe packet. If the probe
packet is not acknowledged before,the PROBE_TIMER expires, the
PROBE_ERROR_COUNTER is incremented. When the PROBE_COUNT equals
the value MAX_PROBES, the state is changed, otherwise a new probe
packet of the same size (PROBED_SIZE) is resent. The state
transitions are illustrated in Figure 5. This shows a
simplification of Figure 1 with a focus only on this event.
+--------------+ +----------------+ Probing timeout: The PROBE_COUNT is initialised to zero each time
| PROBE_START |----------------------------------->| PROBE_DISABLED | the value of PROBED_SIZE is changed. The PROBE_TIMER is started
+--------------+ +----------------+ each time a probe packet is sent. It is stopped when an
acknowledgment arrives that confirms delivery of a probe packet.
If the probe packet is not acknowledged before the PROBE_TIMER
expires, the PROBE_ERROR_COUNTER is incremented. When the
PROBE_COUNT equals the value MAX_PROBES, the state is changed,
otherwise a new probe packet of the same size (PROBED_SIZE) is
resent. The state transitions are illustrated in Figure 5. This
shows a simplification of Figure 1 with a focus only on this
event.
+--------------+ +--------------+ +--------------+ +----------------+
| PROBE_ERROR | -----------------> | PROBE_ERROR | | PROBE_START |----------------------------------->| PROBE_DISABLED |
+--------------+ / +--------------+ +--------------+ +----------------+
/
+--------------+ --2----------/ +--------------+
| PROBE_BASE | --1------------------------------> | PROBE_BASE |
+--------------+ +--------------+
+--------------+ +--------------+ +--------------+ +--------------+
| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH | | PROBE_ERROR | -----------------> | PROBE_ERROR |
+--------------+ --2--------- +--------------+ +--------------+ / +--------------+
\ /
+--------------+ \ +--------------+ +--------------+ --2----------/ +--------------+
| PROBE_DONE | -------------------> | PROBE_DONE | | PROBE_BASE | --1------------------------------> | PROBE_BASE |
+--------------+ +--------------+ +--------------+ +--------------+
+--------------+ +--------------+
| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |
+--------------+ --2--------- +--------------+
\
+--------------+ \ +--------------+
| PROBE_DONE | -------------------> | PROBE_DONE |
+--------------+ +--------------+
Condition 1: The maximum number of probe packets has not been Condition 1: The maximum number of probe packets has not been
reached. Condition 2: The maximum number of probe packets has been reached. Condition 2: The maximum number of probe packets has been
reached. reached.
PMTU raise timer timeout: The path through the network can change Figure 5: State changes at the expiration of the probe timer
PMTU raise timer timeout: The path through the network can change
over time. It impossible to discover whether a path change has over time. It impossible to discover whether a path change has
increased in the actual PMTU by exchanging packets less than or increased the actual PMTU by exchanging packets less than or equal
equal to the effective PMTU. This requires PLPMTUD to periodically to the effective PMTU. This requires PLPMTUD to periodically send
send a probe packet to detect whether a larger PMTU is possible. a probe packet to detect whether a larger PMTU is possible. This
This probe packet is generated by the PMTU_RAISE_TIMER. When the probe packet is generated by the PMTU_RAISE_TIMER. When the timer
timer expires, probing is restarted with the BASE_PMTU and the expires, probing is restarted with the BASE_PMTU and the state is
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 effective MTU and the probe that triggered
the PTB message. the PTB 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
skipping to change at page 25, line 26 skipping to change at page 29, line 22
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 is probed again, because there could be a link further along This ought to be probed again, because there could be a link
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 5.
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.
skipping to change at page 26, line 24 skipping to change at page 30, line 24
from arbitrary re-routing along different parallel paths from arbitrary re-routing along different parallel paths
o This update is proposed for WG comments. o This update is proposed for WG comments.
Working Group draft -00: Working Group draft -00:
o This draft follows a successful adoption call for TSVWG o This draft follows a successful adoption call for TSVWG
o There is still work to complete, please comment on this draft. o There is still work to complete, please comment on this draft.
o Sections marked XXX indicate areas that are expected to change in Working Group draft -01:
the next revision.
o This draft includes improved introduction.
o The draft is updated to require ICMP validation prior to accepting
PTB messages - this to be confirmed by WG
o Section added to discuss Selection of Probe Size - methods to be
evlauated and recommendations to be considered
o Section added to align with work proposed in the QUIC WG.
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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