Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Updates4821 (if approved) University of Aberdeen
Intended status: Standards Track M. Tuexen
Expires: 7 December 2019 I. Ruengeler
T. Voelker
Muenster University of Applied Sciences
5 June 2019
Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-08
Abstract
This document describes a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization Layers (PLs). It describes an
extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path
MTU Discovery for IPv4 and IPv6. The method allows a PL, or a
datagram application that uses a PL, to discover whether a network
path can support the current size of datagram. This can be used to
detect and reduce the message size when a sender encounters a network
black hole (where packets are discarded). The method can probe a
network path with progressively larger packets to discover whether
the maximum packet size can be increased. This allows a sender to
determine an appropriate packet size, providing functionally for
datagram transports that is equivalent to the Packetization Layer
PMTUD specification for TCP, specified in RFC 4821.
The document also provides implementation notes for incorporating
Datagram PMTUD into IETF datagram transports or applications that use
datagram transports.
When published, this specification updates RFC 4821.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 7 December 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . 4
1.2. Packetization Layer Path MTU Discovery . . . . . . . . . 6
1.3. Path MTU Discovery for Datagram Services . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 10
4. DPLPMTUD Mechanisms . . . . . . . . . . . . . . . . . . . . . 12
4.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . 12
4.2. Confirmation of Probed Packet Size . . . . . . . . . . . 14
4.3. Detection of Unsupported PLPMTU Size, aka Black Hole
Detection . . . . . . . . . . . . . . . . . . . . . . . . 14
4.4. Response to PTB Messages . . . . . . . . . . . . . . . . 15
4.4.1. Validation of PTB Messages . . . . . . . . . . . . . 15
4.4.2. Use of PTB Messages . . . . . . . . . . . . . . . . . 16
5. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 17
5.1. DPLPMTUD Components . . . . . . . . . . . . . . . . . . . 18
5.1.1. Timers . . . . . . . . . . . . . . . . . . . . . . . 18
5.1.2. Constants . . . . . . . . . . . . . . . . . . . . . . 19
5.1.3. Variables . . . . . . . . . . . . . . . . . . . . . . 20
5.1.4. Overview of DPLPMTUD Phases . . . . . . . . . . . . . 21
5.2. State Machine . . . . . . . . . . . . . . . . . . . . . . 23
5.3. Search to Increase the PLPMTU . . . . . . . . . . . . . . 26
5.3.1. Probing for a larger PLPMTU . . . . . . . . . . . . . 26
5.3.2. Selection of Probe Sizes . . . . . . . . . . . . . . 27
5.3.3. Resilience to Inconsistent Path Information . . . . . 27
5.4. Robustness to Inconsistent Paths . . . . . . . . . . . . 28
6. Specification of Protocol-Specific Methods . . . . . . . . . 28
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6.1. Application support for DPLPMTUD with UDP or
UDP-Lite . . . . . . . . . . . . . . . . . . . . . . . . 28
6.1.1. Application Request . . . . . . . . . . . . . . . . . 29
6.1.2. Application Response . . . . . . . . . . . . . . . . 29
6.1.3. Sending Application Probe Packets . . . . . . . . . . 29
6.1.4. Validating the Path . . . . . . . . . . . . . . . . . 29
6.1.5. Handling of PTB Messages . . . . . . . . . . . . . . 29
6.2. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 30
6.2.1. SCTP/IPv4 and SCTP/IPv6 . . . . . . . . . . . . . . . 30
6.2.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 31
6.2.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 31
6.3. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 32
6.3.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 32
6.3.2. Validating the Path with QUIC . . . . . . . . . . . . 33
6.3.3. Handling of PTB Messages by QUIC . . . . . . . . . . 33
6.4. DPLPMTUD for UDP-Options . . . . . . . . . . . . . . . . 33
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 33
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
9. Security Considerations . . . . . . . . . . . . . . . . . . . 33
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
10.1. Normative References . . . . . . . . . . . . . . . . . . 34
10.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Revision Notes . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
The IETF has specified datagram transport using UDP, SCTP, and DCCP,
as well as protocols layered on top of these transports (e.g., SCTP/
UDP, DCCP/UDP, QUIC/UDP), and direct datagram transport over the IP
network layer. This document describes a robust method for Path MTU
Discovery (PMTUD) that may be used with these transport protocols (or
the applications that use their transport service) to discover an
appropriate size of packet to use across an Internet path.
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1.1. Classical Path MTU Discovery
Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
used with any transport that is able to process ICMP Packet Too Big
(PTB) messages (e.g., [RFC1191] and [RFC8201]). In this document,
the term PTB message is applied to both IPv4 ICMP Unreachable
messages (type 3) that carry the error Fragmentation Needed (Type 3,
Code 4) [RFC0792] and ICMPv6 packet too big messages (Type 2)
[RFC4443]. When a sender receives a PTB message, it reduces the
effective MTU to the value reported as the Link MTU in the PTB
message, and a method that from time-to-time increases the packet
size in attempt to discover an increase in the supported PMTU. The
packets sent with a size larger than the current effective PMTU are
known as probe packets.
Packets not intended as probe packets are either fragmented to the
current effective PMTU, or the attempt to send fails with an error
code. Applications are sometimes provided with a primitive to let
them read the Maximum Packet Size (MPS), derived from the current
effective PMTU.
Classical PMTUD is subject to protocol failures. One failure arises
when traffic using a packet size larger than the actual PMTU is
black-holed (all datagrams sent with this size, or larger, are
discarded). This could arise when the PTB messages are not delivered
back to the sender for some reason (see for example [RFC2923]).
Examples where PTB messages are not delivered include:
* The generation of ICMP messages is usually rate limited. This
could result in no PTB messages being generated to the sender (see
section 2.4 of [RFC4443])
* ICMP messages can be filtered by middleboxes (including firewalls)
[RFC4890]. A stateful firewall could be configured with a policy
to block incoming ICMP messages, which would prevent reception of
PTB messages to a sending endpoint behind this firewall.
* When the router issuing the ICMP message drops a tunneled packet,
the resulting ICMP message will be directed to the tunnel ingress.
This tunnel endpoint is responsible for forwarding the ICMP
message and also processing the quoted packet within the payload
field to remove the effect of the tunnel, and return a correctly
formatted ICMP message to the sender [I-D.ietf-intarea-tunnels].
Failure to do this prevents the PTB message reaching the original
sender.
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* Asymmetry in forwarding can result in there being no return route
to the original sender, which would prevent an ICMP message being
delivered to the sender. This issue can also arise when policy-
based routing is used, Equal Cost Multipath (ECMP) routing is
used, or a middlebox acts as an application load balancer. An
example is where the path towards the server is chosen by ECMP
routing depending on bytes in the IP payload. In this case, when
a packet sent by the server encounters a problem after the ECMP
router, then any resulting ICMP message needs to also be directed
by the ECMP router towards the original sender.
* There are additional cases where the next hop destination fails to
receive a packet because of its size. This could be due to
misconfiguration of the layer 2 path between nodes, for instance
the MTU configured in a layer 2 switch, or misconfiguration of the
Maximum Receive Unit (MRU). If the packet is dropped by the link,
this will not cause a PTB message to be sent to the original
sender.
Another failure could result if a node that is not on the network
path sends a PTB message that attempts to force a sender to change
the effective PMTU [RFC8201]. A sender can protect itself from
reacting to such messages by utilising the quoted packet within a PTB
message payload to validate that the received PTB message was
generated in response to a packet that had actually originated from
the sender. However, there are situations where a sender would be
unable to provide this validation. Examples where validation of the
PTB message is not possible include:
* When a router issuing the ICMP message implements RFC792
[RFC0792], it is only required to include the first 64 bits of the
IP payload of the packet within the quoted payload. There could
be insufficient bytes remaining for the sender to interpret the
quoted transport information.
Note: The recommendation in RFC1812 [RFC1812] is that IPv4 routers
return a quoted packet with as much of the original datagram as
possible without the length of the ICMP datagram exceeding 576
bytes. IPv6 routers include as much of the invoking packet as
possible without the ICMPv6 packet exceeding 1280 bytes [RFC4443].
* The use of tunnels/encryption can reduce the size of the quoted
packet returned to the original source address, increasing the
risk that there could be insufficient bytes remaining for the
sender to interpret the quoted transport information.
* Even when the PTB message includes sufficient bytes of the quoted
packet, the network layer could lack sufficient context to
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validate the message, because validation depends on information
about the active transport flows at an endpoint node (e.g., the
socket/address pairs being used, and other protocol header
information).
* When a packet is encapsulated/tunneled over an encrypted
transport, the tunnel/encapsulation ingress might have
insufficient context, or computational power, to reconstruct the
transport header that would be needed to perform validation.
1.2. Packetization Layer Path MTU Discovery
The term Packetization Layer (PL) has been introduced to describe the
layer that is responsible for placing data blocks into the payload of
IP packets and selecting an appropriate MPS. This function is often
performed by a transport protocol, but can also be performed by other
encapsulation methods working above the transport layer.
In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) [RFC4821] does not rely upon reception and validation of
PTB messages. It is therefore more robust than Classical PMTUD.
This has become the recommended approach for implementing PMTU
discovery with TCP.
It uses a general strategy where the PL sends probe packets to search
for the largest size of unfragmented datagram that can be sent over a
network path. Probe packets are sent with a progressively larger
packet size. If a probe packet is successfully delivered (as
determined by the PL), then the PLPMTU is raised to the size of the
successful probe. If no response is received to a probe packet, the
method reduces the probe size. The result of probing with the PLPMTU
is used to set the application MPS.
PLPMTUD introduces flexibility in the implementation of PMTU
discovery. At one extreme, it can be configured to only perform ICMP
black Hole Detection and recovery to increase the robustness of
Classical PMTUD, or at the other extreme, all PTB processing can be
disabled and PLPMTUD can completely replace Classical PMTUD.
PLPMTUD can also include additional consistency checks without
increasing the risk that data is lost when probing to discover the
path MTU. For example, information available at the PL, or higher
layers, enables received PTB messages to be validated before being
utilized.
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1.3. Path MTU Discovery for Datagram Services
Section 5 of this document presents a set of algorithms for datagram
protocols to discover the largest size of unfragmented datagram that
can be sent over a network path. The method described relies on
features of the PL described in Section 3 and applies to transport
protocols operating over IPv4 and IPv6. It does not require
cooperation from the lower layers, although it can utilize PTB
messages when these received messages are made available to the PL.
The UDP Usage Guidelines [RFC8085] state "an application SHOULD
either use the Path MTU information provided by the IP layer or
implement Path MTU Discovery (PMTUD)", but does not provide a
mechanism for discovering the largest size of unfragmented datagram
that can be used on a network path. Prior to this document, PLPMTUD
had not been specified for UDP.
Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the
Stream Control Transport Protocol (SCTP). SCTP utilizes probe
packets consisting of a minimal sized HEARTBEAT chunk bundled with a
PAD chunk as defined in [RFC4820], but RFC4821 does not provide a
complete specification. The present document provides the details to
complete that specification.
The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
implementations to support Classical PMTUD and states that a DCCP
sender "MUST maintain the MPS allowed for each active DCCP session".
It also defines the current congestion control MPS (CCMPS) supported
by a network path. This recommends use of PMTUD, and suggests use of
control packets (DCCP-Sync) as path probe packets, because they do
not risk application data loss. The method defined in this
specification could be used with DCCP.
Section 6 specifies the method for a set of transports, and provides
information to enable the implementation of PLPMTUD with other
datagram transports and applications that use datagram transports.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Other terminology is directly copied from [RFC4821], and the
definitions in [RFC1122].
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Actual PMTU: The Actual PMTU is the PMTU of a network path between a
sender PL and a destination PL, which the DPLPMTUD algorithm seeks
to determine.
Black Hole: A Black Hole is encountered when a sender is unaware
that packets are not being delivered to the destination end point.
Two types of Black Hole are relevant to DPLPMTUD:
Packet Black Hole: Packets encounter a Packet Black Hole when
packets are not delivered to the destination
endpoint (e.g., when the sender transmits
packets of a particular size with a previously
known effective PMTU and they are discarded by
the network).
ICMP Black Hole An ICMP Black Hole is encountered when the
sender is unaware that packets are not
delivered to the destination endpoint because
PTB messages are not received by the
originating PL sender.
Black holed : Traffic is black-holed when the sender is unaware that
packets are not being delivered. This could be due to a Packet
Black Hole or an ICMP Black Hole.
Classical Path MTU Discovery: Classical PMTUD is a process described
in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
learn the largest size of unfragmented datagram that can be used
across a network path.
Datagram: A datagram is a transport-layer protocol data unit,
transmitted in the payload of an IP packet.
Effective PMTU: The Effective PMTU is the current estimated value
for PMTU that is used by a PMTUD. This is equivalent to the
PLPMTU derived by PLPMTUD.
EMTU_S: The Effective MTU for sending (EMTU_S) is defined in
[RFC1122] as "the maximum IP datagram size that may be sent, for a
particular combination of IP source and destination addresses...".
EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in
[RFC1122] as the largest datagram size that can be reassembled by
EMTU_R (Effective MTU to receive).
Link: A Link is a communication facility or medium over which nodes
can communicate at the link layer, i.e., a layer below the IP
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layer. Examples are Ethernet LANs and Internet (or higher) layer
and tunnels.
Link MTU: The Link Maximum Transmission Unit (MTU) is the size in
bytes of the largest IP packet, including the IP header and
payload, that can be transmitted over a link. Note that this
could more properly be called the IP MTU, to be consistent with
how other standards organizations use the acronym. This includes
the IP header, but excludes link layer headers and other framing
that is not part of IP or the IP payload. Other standards
organizations generally define the link MTU to include the link
layer headers.
MAX_PMTU: The MAX_PMTU is the largest size of PLPMTU that DPLPMTUD
will attempt to use.
MPS: The Maximum Packet Size (MPS) is the largest size of
application data block that can be sent across a network path by a
PL. In DPLPMTUD this quantity is derived from the PLPMTU by
taking into consideration the size of the lower protocol layer
headers. Probe packets generated by DPLPMTUD can have a size
larger than the MPS.
MIN_PMTU: The MIN_PMTU is the smallest size of PLPMTU that DPLPMTUD
will attempt to use.
Packet: A Packet is the IP header plus the IP payload.
Packetization Layer (PL): The Packetization Layer (PL) is the layer
of the network stack that places data into packets and performs
transport protocol functions.
Path: The Path is the set of links and routers traversed by a packet
between a source node and a destination node by a particular flow.
Path MTU (PMTU): The Path MTU (PMTU) is the minimum of the Link MTU
of all the links forming a network path between a source node and
a destination node.
PTB_SIZE: The PTB_SIZE is a value reported in a validated PTB
message that indicates next hop link MTU of a router along the
path.
PLPMTU: The Packetization Layer PMTU is an estimate of the actual
PMTU provided by the DPLPMTUD algorithm.
PLPMTUD: Packetization Layer Path MTU Discovery (PLPMTUD), the
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method described in this document for datagram PLs, which is an
extension to Classical PMTU Discovery.
Probe packet: A probe packet is a datagram sent with a purposely
chosen size (typically the current PLPMTU or larger) to detect if
packets of this size can be successfully sent end-to-end across
the network path.
3. Features Required to Provide Datagram PLPMTUD
TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
All of the requirements in [RFC4821] also apply to the use of the
technique with a datagram PL. Unlike TCP, some datagram PLs require
additional mechanisms to implement PLPMTUD.
There are eight requirements for performing the datagram PLPMTUD
method described in this specification:
1. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide
information about the maximum size of packet that can be
transmitted by the sender on the local link (the local Link MTU).
It MAY utilize similar information about the receiver when this
is supplied (note this could be less than EMTU_R). This avoids
implementations trying to send probe packets that can not be
transmitted by the local link. Too high of a value could reduce
the efficiency of the search algorithm. Some applications also
have a maximum transport protocol data unit (PDU) size, in which
case there is no benefit from probing for a size larger than this
(unless a transport allows multiplexing multiple applications
PDUs into the same datagram).
2. PLPMTU: A datagram application using a PL not supporting
fragmentation is REQUIRED to be able to choose the size of
datagrams sent to the network, up to the PLPMTU, or a smaller
value (such as the MPS) derived from this. This value is managed
by the DPLPMTUD method. The PLPMTU (specified as the effective
PMTU in Section 1 of [RFC1191]) is equivalent to the EMTU_S
(specified in [RFC1122]).
3. Probe packets: On request, a DPLPMTUD sender is REQUIRED to be
able to transmit a packet larger than the PLMPMTU. This is used
to send a probe packet. In IPv4, a probe packet MUST be sent
with the Don't Fragment (DF) bit set in the IP header, and
without network layer endpoint fragmentation. In IPv6, a probe
packet is always sent without source fragmentation (as specified
in section 5.4 of [RFC8201]).
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4. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize
PTB messages received from the network layer to help identify
when a network path does not support the current size of probe
packet. Any received PTB message MUST be validated before it is
used to update the PLPMTU discovery information [RFC8201]. This
validation confirms that the PTB message was sent in response to
a packet originating by the sender, and needs to be performed
before the PLPMTU discovery method reacts to the PTB message. A
PTB message MUST NOT be used to increase the PLPMTU [RFC8201].
5. Reception feedback: The destination PL endpoint is REQUIRED to
provide a feedback method that indicates to the DPLPMTUD sender
when a probe packet has been received by the destination PL
endpoint. The mechanism needs to be robust to the possibility
that packets could be significantly delayed along a network path.
The local PL endpoint at the sending node is REQUIRED to pass
this feedback to the sender DPLPMTUD method.
6. Probe loss recovery: It is RECOMMENDED to use probe packets that
do not carry any user data. Most datagram transports permit
this. If a probe packet contains user data requiring
retransmission in case of loss, the PL (or layers above) are
REQUIRED to arrange any retransmission/repair of any resulting
loss. DPLPMTUD is REQUIRED to be robust in the case where probe
packets are lost due to other reasons (including link
transmission error, congestion).
7. Probing and congestion control: The DPLPMTUD sender treats
isolated loss of a probe packet (with or without a corresponding
PTB message) as a potential indication of a PMTU limit for the
path. Loss of a probe packet SHOULD NOT be treated as an
indication of congestion and the loss SHOULD NOT directly trigger
a congestion control reaction [RFC4821].
8. Shared PLPMTU state: The PLPMTU value could also be stored with
the corresponding entry in the destination cache and used by
other PL instances. The specification of PLPMTUD [RFC4821]
states: "If PLPMTUD updates the MTU for a particular path, all
Packetization Layer sessions that share the path representation
(as described in Section 5.2 of [RFC4821]) SHOULD be notified to
make use of the new MTU". Such methods MUST be robust to the
wide variety of underlying network forwarding behaviors, PLPMTU
adjustments based on shared PLPMTU values should be incorporated
in the search algorithms. Section 5.2 of [RFC8201] provides
guidance on the caching of PMTU information and also the relation
to IPv6 flow labels.
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In addition, the following principles are stated for design of a
DPLPMTUD method:
* MPS: A method is REQUIRED to signal an appropriate MPS to the
higher layer using the PL. The value of the MPS can change
following a change to the path. It is RECOMMENDED that methods
avoid forcing an application to use an arbitrary small MPS
(PLPMTU) for transmission while the method is searching for the
currently supported PLPMTU. Datagram PLs do not necessarily
support fragmentation of PDUs larger than the PLPMTU. A reduced
MPS can adversely impact the performance of a datagram
application.
* Path validation: It is RECOMMENDED that methods are robust to path
changes that could have occurred since the path characteristics
were last confirmed, and to the possibility of inconsistent path
information being received.
* Datagram reordering: A method is REQUIRED to be robust to the
possibility that a flow encounters reordering, or the traffic
(including probe packets) is divided over more than one network
path.
* When to probe: It is RECOMMENDED that methods determine whether
the path has changed since it last measured the path. This can
help determine when to probe the path again.
4. DPLPMTUD Mechanisms
This section lists the protocol mechanisms used in this
specification.
4.1. PLPMTU Probe Packets
The DPLPMTUD method relies upon the PL sender being able to generate
probe packets with a specific size. TCP is able to generate these
probe packets by choosing to appropriately segment data being sent
[RFC4821]. In contrast, a datagram PL that needs to construct a
probe packet has to either request an application to send a data
block that is larger than that generated by an application, or to
utilize padding functions to extend a datagram beyond the size of the
application data block. Protocols that permit exchange of control
messages (without an application data block) could alternatively
prefer to generate a probe packet by extending a control message with
padding data.
A receiver needs to be able to distinguish an in-band data block from
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any added padding. This is needed to ensure that any added padding
is not passed on to an application at the receiver.
This results in three possible ways that a sender can create a probe
packet listed in order of preference:
Probing using padding data: A probe packet that contains only
control information together with any padding, which is needed to
be inflated to the size required for the probe packet. Since
these probe packets do not carry an application-supplied data
block, they do not typically require retransmission, although they
do still consume network capacity and incur endpoint processing.
Probing using application data and padding
data: A probe packet that
contains a data block supplied by an application that is combined
with padding to inflate the length of the datagram to the size
required for the probe packet. If the application/transport needs
protection from the loss of this probe packet, the application/
transport could perform transport-layer retransmission/repair of
the data block (e.g., by retransmission after loss is detected or
by duplicating the data block in a datagram without the padding
data).
Probing using application data: A probe packet that contains a data
block supplied by an application that matches the size required
for the probe packet. This method requests the application to
issue a data block of the desired probe size. If the application/
transport needs protection from the loss of an unsuccessful probe
packet, the application/transport needs then to perform transport-
layer retransmission/repair of the data block (e.g., by
retransmission after loss is detected).
A PL that uses a probe packet carrying an application data block,
could need to retransmit this application data block if the probe
fails. This could need the PL to re-fragment the data block to a
smaller packet size that is expected to traverse the end-to-end path
(which could utilize endpoint network-layer or PL fragmentation when
these are available).
DPLPMTUD MAY choose to use only one of these methods to simplify the
implementation.
Probe messages sent by a PL MUST contain enough information to
uniquely identify the probe within Maximum Segment Lifetime, while
being robust to reordering and replay of probe response and PTB
messages.
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4.2. Confirmation of Probed Packet Size
The PL needs a method to determine (confirm) when probe packets have
been successfully received end-to-end across a network path.
Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP and
SCTP provide keep-alive/heartbeat features). When supported, this
mechanism SHOULD also be used by DPLPMTUD to acknowledge reception of
a probe packet.
A PL that does not acknowledge data reception (e.g., UDP and UDP-
Lite) is unable itself to detect when the packets that it sends are
discarded because their size is greater than the actual PMTU. These
PLs need to either rely on an application protocol to detect this
loss, or make use of an additional transport method such as UDP-
Options [I-D.ietf-tsvwg-udp-options].
Section 6 specifies this function for a set of IETF-specified
protocols.
4.3. Detection of Unsupported PLPMTU Size, aka Black Hole Detection
A PL sender needs to reduce the PLPMTU when it discovers the actual
PMTU supported by a network path is less than the PLPMTU. This can
be triggered when a validated PTB message is received, or by another
event that indicates the network path no longer sustains the current
packet size, such as a loss report from the PL, or repeated lack of
response to probe packets sent to confirm the PLPMTU. Detection is
followed by a reduction of the PLPMTU.
This is performed by sending packet probes of size PLPMTU to verify
that a network path still supports the last acknowledged PLPMTU size.
There are two alternative mechanism:
* A PL can rely upon a mechanism implemented within the PL to detect
excessive loss of data sent with a specific packet size and then
conclude that this excessive loss could be a result of an invalid
PMTU (as in PLPMTUD for TCP [RFC4821]).
* A PL can use the DPLPMTUD probing mechanism to periodically
generate probe packets of the size of the current PLPMTU (e.g.,
using the confirmation timer Section 5.1.1). A timer tracks
whether acknowledgments are received. Successive loss of probes
is an indication that the current path no longer supports the
PLPMTU (e.g., when the number of probe packets sent without
receiving an acknowledgement, PROBE_COUNT, becomes greater than
MAX_PROBES).
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A PL MAY inhibit sending probe packets when no application data has
been sent since the previous probe packet. A PL preferring to use an
up-to-data PLPMTU once user data is sent again, MAY choose to
continue PLPMTU discovery for each path. However, this may result in
additional packets being sent.
When the method detects the current PLPMTU is not supported, DPLPMTUD
sets a lower MPS. The PL then confirms that the updated PLPMTU can
be successfully used across the path. The PL could need to send a
probe packet with a size less than the size of the data block
generated by an application. In this case, the PL could provide a
way to fragment a datagram at the PL, or use a control packet as the
packet probe.
4.4. Response to PTB Messages
This method requires the DPLPMTUD sender to validate any received PTB
message before using the PTB information. The response to a PTB
message depends on the PTB_SIZE indicated in the PTB message, the
state of the PLPMTUD state machine, and the IP protocol being used.
Section 4.4.1 first describes validation for both IPv4 ICMP
Unreachable messages (type 3) and ICMPv6 packet too big messages,
both of which are referred to as PTB messages in this document.
4.4.1. Validation of PTB Messages
This section specifies utilization of PTB messages.
* A simple implementation MAY ignore received PTB messages and in
this case the PLPMTU is not updated when a PTB message is
received.
* An implementation that supports PTB messages MUST validate
messages before they are further processed.
A PL that receives a PTB message from a router or middlebox, performs
ICMP validation as specified in Section 5.2 of [RFC8085][RFC8201].
Because DPLPMTUD operates at the PL, the PL needs to check that each
received PTB message is received in response to a packet transmitted
by the endpoint PL performing DPLPMTUD.
The PL MUST check the protocol information in the quoted packet
carried in an ICMP PTB message payload to validate the message
originated from the sending node. This validation includes
determining that the combination of the IP addresses, the protocol,
the source port and destination port match those returned in the
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quoted packet - this is also necessary for the PTB message to be
passed to the corresponding PL.
The validation SHOULD utilize information that it is not simple for
an off-path attacker to determine [RFC8085]. For example, by
checking the value of a protocol header field known only to the two
PL endpoints. A datagram application that uses well-known source and
destination ports ought to also rely on other information to complete
this validation.
These checks are intended to provide protection from packets that
originate from a node that is not on the network path. A PTB message
that does not complete the validation MUST NOT be further utilized by
the DPLPMTUD method.
PTB messages that have been validated MAY be utilized by the DPLPMTUD
algorithm, but MUST NOT be used directly to set the PLPMTU. A method
that utilizes these PTB messages can improve the speed at the which
the algorithm detects an appropriate PLPMTU, compared to one that
relies solely on probing. Section 4.4.2 describes this processing.
4.4.2. Use of PTB Messages
A set of checks are intended to provide protection from a router that
reports an unexpected PTB_SIZE. The PL also needs to check that the
indicated PTB_SIZE is less than the size used by probe packets and
larger than minimum size accepted.
This section provides a summary of how PTB messages can be utilized.
This processing depends on the PTB_SIZE and the current value of a
set of variables:
MIN_PMTU < PTB_SIZE < BASE_PMTU
* A robust PL MAY enter an error state (see Section 5.2) for an
IPv4 path when the PTB_SIZE reported in the PTB message is
larger than or equal to 68 bytes and when this is less than the
BASE_PMTU.
* A robust PL MAY enter an error state (see Section 5.2) for an
IPv6 path when the PTB_SIZE reported in the PTB message is
larger than or equal to 1280 bytes and when this is less than
the BASE_PMTU.
PTB_SIZE = PLPMTU
* Completes the search for a larger PLPMTU.
PTB_SIZE > PROBED_SIZE
* Inconsistent network signal.
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* PTB message ought to be discarded without further processing
(e. g. PLPMTU not modified).
* The information could be utilized as an input to trigger
enabling a resilience mode.
BASE_PMTU <= PTB_SIZE < PLPMTU
* Black Hole Detection is triggered and the PLPMTU ought to be
set to BASE_PMTU.
* The PL could use the PTB_SIZE reported in the PTB message to
initialize a search algorithm.
PLPMTU < PTB_SIZE < PROBED_SIZE
* The PLPMTU continues to be valid, but the last PROBED_SIZE
searched was larger than the actual PMTU.
* The PLPMTU is not updated.
* The PL can use the reported PTB_SIZE from the PTB message as
the next search point when it resumes the search algorithm.
xxx Author Note: Do we want to specify how to handle PTB Message with
PTB_SIZE = 0? xxx
5. Datagram Packetization Layer PMTUD
This section specifies Datagram PLPMTUD (DPLPMTUD). The method can
be introduced at various points (as indicated with * in the figure
below) in the IP protocol stack to discover the PLPMTU so that an
application can utilize an appropriate MPS for the current network
path. DPLPMTUD SHOULD NOT be used by an application if it is already
used in a lower layer.
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+----------------------+
| Application* |
+-+-------+----+----+--+
| | | |
+---+--+ +--+--+ | +-+---+
| QUIC*| |UDPO*| | |SCTP*|
+---+--+ +--+--+ | +--+--+
| | | | |
+-------+--+ | | |
| | | |
+-+-+--+ |
| UDP | |
+---+--+ |
| |
+--------------+-----+-+
| Network Interface |
+----------------------+
Figure 1: Examples where DPLPMTUD can be implemented
The central idea of DPLPMTUD is probing by a sender. Probe packets
are sent to find the maximum size of a user message that can be
completely transferred across the network path from the sender to the
destination.
The folloowing sections identify the components needed for
implementation, provides an overvoew of the phases of operation, and
specifies the state machine and search algorithm.
5.1. DPLPMTUD Components
This section describes the timers, constants, and variables of
DPLPMTUD.
5.1.1. Timers
The method utilizes up to three timers:
PROBE_TIMER: The PROBE_TIMER is configured to expire after a
period longer than the maximum time to receive
an acknowledgment to a probe packet. This value
MUST NOT be smaller than 1 second, and SHOULD be
larger than 15 seconds. Guidance on selection
of the timer value are provided in section 3.1.1
of the UDP Usage Guidelines [RFC8085].
If the PL has a path Round Trip Time (RTT)
estimate and timely acknowledgements the
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PROBE_TIMER can be derived from the PL RTT
estimate.
PMTU_RAISE_TIMER: The PMTU_RAISE_TIMER is configured to the period
a sender will continue to use the current
PLPMTU, after which it re-enters the Search
phase. This timer has a period of 600 secs, as
recommended by PLPMTUD [RFC4821].
DPLPMTUD MAY inhibit sending probe packets when
no application data has been sent since the
previous probe packet. A PL preferring to use
an up-to-data PMTU once user data is sent again,
can choose to continue PMTU discovery for each
path. However, this may result in sending
additional packets.
CONFIRMATION_TIMER: When an acknowledged PL is used, this timer MUST
NOT be used. For other PLs, the
CONFIRMATION_TIMER is configured to the period a
PL sender waits before confirming the current
PLPMTU is still supported. This is less than
the PMTU_RAISE_TIMER and used to decrease the
PLPMTU (e.g., when a black hole is encountered).
Confirmation needs to be frequent enough when
data is flowing that the sending PL does not
black hole extensive amounts of traffic.
Guidance on selection of the timer value are
provided in section 3.1.1 of the UDP Usage
Guidelines [RFC8085].
DPLPMTUD MAY inhibit sending probe packets when
no application data has been sent since the
previous probe packet. A PL preferring to use
an up-to-data PMTU once user data is sent again,
can choose to continue PMTU discovery for each
path. However, this may result in sending
additional packets.
An implementation could implement the various timers using a single
timer.
5.1.2. Constants
The following constants are defined:
MAX_PROBES: The MAX_PROBES is the maximum value of the PROBE_COUNT
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counter (see Section 5.1.3). The default value of
MAX_PROBES is 10.
MIN_PMTU: The MIN_PMTU is the smallest allowed probe packet size.
For IPv6, this value is 1280 bytes, as specified in
[RFC2460]. For IPv4, the minimum value is 68 bytes.
Note: An IPv4 router is required to be able to forward a
datagram of 68 bytes without further fragmentation.
This is the combined size of an IPv4 header and the
minimum fragment size of 8 bytes. In addition,
receivers are required to be able to reassemble
fragmented datagrams at least up to 576 bytes, as stated
in section 3.3.3 of [RFC1122].
MAX_PMTU: The MAX_PMTU is the largest size of PLPMTU. This has to
be less than or equal to the minimum of the local MTU of
the outgoing interface and the destination PMTU for
receiving. An application, or PL, MAY reduce the
MAX_PMTU when there is no need to send packets larger
than a specific size.
BASE_PMTU: The BASE_PMTU is a configured size expected to work for
most paths. The size is equal to or larger than the
MIN_PMTU and smaller than the MAX_PMTU. In the case of
IPv6, this value is 1280 bytes [RFC2460]. When using
IPv4, a size of 1200 bytes is RECOMMENDED.
5.1.3. Variables
This method utilizes a set of variables:
PROBED_SIZE: The PROBED_SIZE is the size of the current probe
packet. This is a tentative value for the PLPMTU,
which is awaiting confirmation by an acknowledgment.
PROBE_COUNT: The PROBE_COUNT is a count of the number of
unsuccessful probe packets that have been sent with a
size of PROBED_SIZE. The value is initialized to zero
when a particular size of PROBED_SIZE is first
attempted.
The figure below illustrates the relationship between the packet size
constants and variables at a point of time when the DPLPMTUD
algorithm performs path probing to increase the size of the PLPMTU.
A probe packet has been sent of size PROBED_SIZE. Once this is
acknowledged, the PLPMTU will raise to PROBED_SIZE allowing the
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DPLPMTUD algorithm to further increase PROBED_SIZE towards the actual
PMTU.
MIN_PMTU MAX_PMTU
<-------------------------------------------------->
| | | |
v | | v
BASE_PMTU | v Actual PMTU
| PROBED_SIZE
v
PLPMTU
Figure 2: Relationships between packet size constants and variables
5.1.4. Overview of DPLPMTUD Phases
This section provides a high-level informative view of the DPLPMTUD
method, by describing the movement of the method through several
phases of operation. More detail is available in the state machine
Section 5.2.
+------+
+------->| Base |----------------+ Connectivity
| +------+ | or BASE_PMTU
| | | confirmation failed
| | v
| | Connectivity +-------+
| | and BASE_PMTU | Error |
| | confirmed +-------+
| | |
| v | Consistent connectivity
PLPMTU | +--------+ | and BASE_PMTU
confirmation | | Search |<--------------+ confirmed
failed | +--------+
| ^ |
| | |
| Raise | | Search
| timer | | algorithm
| expired | | completed
| | |
| | v
| +-----------------+
+---| Search Complete |
+-----------------+
Figure 3: DPLPMTUD Phases
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BASE_PMTU Confirmation Phase
* The BASE_PMTU Confirmation Phase confirms connectivity to the
remote peer. This phase is implicit for a connection-oriented
PL (where it can be performed in a PL connection handshake). A
connectionless PL needs to send an acknowledged probe packet to
confirm that the remote peer is reachable.
* The sender also confirms that BASE_PMTU is supported across the
network path.
* A PL that does not wish to support a path with a PLPMTU less
than BASE_PMTU can simplify the phase into a single step by
performing the connectivity checks with a probe of the
BASE_PMTU size.
* Once confirmed, DPLPMTUD enters the Search Phase. If this
phase fails to confirm, DPLPMTUD enters the Error Phase.
Search Phase
* The Search Phase utilizes a search algorithm to send probe
packets to seek to increase the PLPMTU.
* The algorithm concludes when it has found a suitable PLPMTU, by
entering the Search Complete Phase.
* A PL could respond to PTB messages using the PTB to advance or
terminate the search, see Section 4.4.
* Black Hole Detection can also terminate the search by entering
the BASE_PMTU Confirmation phase.
Search Complete Phase
* The Search Complete Phase is entered when the PLPMTU is
supported across the network path.
* A PL can use a CONFIRMATION_TIMER to periodically repeat a
probe packet for the current PLPMTU size. If the sender is
unable to confirm reachability (e.g., if the CONFIRMATION_TIMER
expires) or the PL signals a lack of reachability, DPLPMTUD
enters the BASE_PMTU Confirmation phase.
* The PMTU_RAISE_TIMER is used to periodically resume the search
phase to discover if the PLPMTU can be raised.
* Black Hole Detection or receipt of a validated PTB message
Section 4.4.1) can cause the sender to enter the BASE_PMTU
Confirmation Phase.
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Error Phase
* The Error Phase is entered when there is conflicting or invalid
PLPMTU information for the path (e.g. a failure to support the
BASE_PMTU) that cause DPLPMTUD to be unable to progress and the
PLPMTU is lowered
* DPLPMTUD remains in the Error Phase until a consistent view of
the path can be discovered and it has also been confirmed that
the path supports the BASE_PMTU (or DPLPMTUD is suspended).
* Note: MIN_PMTU may be identical to BASE_PMTU, simplifying the
actions in this phase.
An implementation that only reduces the PLPMTU to a suitable size
would be sufficient to ensure reliable operation, but can be very
inefficient when the actual PMTU changes or when the method (for
whatever reason) makes a suboptimal choice for the PLPMTU.
A full implementation of DPLPMTUD provides an algorithm enabling the
DPLPMTUD sender to increase the PLPMTU following a change in the
characteristics of the path, such as when a link is reconfigured with
a larger MTU, or when there is a change in the set of links traversed
by an end-to-end flow (e.g., after a routing or path fail-over
decision).
5.2. State Machine
A state machine for DPLPMTUD is depicted in Figure 4. If multipath
or multihoming is supported, a state machine is needed for each path.
Note: Some state changes are not shown to simplify the diagram.
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| |
| Start | PL indicates loss
| | of connectivity
v v
+---------------+ +---------------+
| DISABLED | | ERROR |
+---------------+ PROBE_TIMER expiry: +---------------+
| PL indicates PROBE_COUNT = MAX_PROBES or ^ |
| connectivity PTB: PTB_SIZE < BASE_PMTU | |
+--------------------+ +---------------+ |
| | |
v | BASE_PMTU Probe |
+---------------+ acked |
| BASE |----------------------+
+---------------+ |
Black hole detected or ^ | ^ ^ Black hole detected or |
PTB: PTB_SIZE < PLPMTU | | | | PTB: PTB_SIZE < PLPMTU |
+--------------------+ | | +--------------------+ |
| +----+ | |
| PROBE_TIMER expiry: | |
| PROBE_COUNT < MAX_PROBES | |
| | |
| PMTU_RAISE_TIMER expiry | |
| +-----------------------------------------+ | |
| | | | |
| | v | v
+---------------+ +---------------+
|SEARCH_COMPLETE| | SEARCHING |
+---------------+ +---------------+
| ^ ^ | | ^
| | | | | |
| | +-----------------------------------------+ | |
| | MAX_PMTU Probe acked or PROBE_TIMER | |
| | expiry: PROBE_COUNT = MAX_PROBES or | |
+----+ PTB: PTB_SIZE = PLPMTU +----+
CONFIRMATION_TIMER expiry: PROBE_TIMER expiry:
PROBE_COUNT < MAX_PROBES or PROBE_COUNT < MAX_PROBES or
PLPMTU Probe acked Probe acked or PTB:
PLPMTU < PTB_SIZE < PROBED_SIZE
Figure 4: State machine for Datagram PLPMTUD
The following states are defined:
DISABLED: The DISABLED state is the initial state before
probing has started. It is also entered from any
other state, when the PL indicates loss of
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connectivity. This state is left, once the PL
indicates connectivity to the remote PL.
BASE: The BASE state is used to confirm that the
BASE_PMTU size is supported by the network path and
is designed to allow an application to continue
working when there are transient reductions in the
actual PMTU. It also seeks to avoid long periods
where traffic is black holed while searching for a
larger PLPMTU.
On entry, the PROBED_SIZE is set to the BASE_PMTU
size and the PROBE_COUNT is set to zero.
Each time a probe packet is sent, the PROBE_TIMER
is started. The state is exited when the probe
packet is acknowledged, and the PL sender enters
the SEARCHING state.
The state is also left when the PROBE_COUNT reaches
MAX_PROBES or a received PTB message is validated.
This causes the PL sender to enter the ERROR state.
SEARCHING: The SEARCHING state is the main probing state.
This state is entered when probing for the
BASE_PMTU was successful.
The PROBE_COUNT is set to zero when the first probe
packet is sent for each probe size. Each time a
probe packet is acknowledged, the PLPMTU is set to
the PROBED_SIZE, and then the PROBED_SIZE is
increased using the search algorithm.
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 received PTB
message is validated, a probe of size MAX_PMTU is
acknowledged, or a black hole is detected.
SEARCH_COMPLETE: The SEARCH_COMPLETE state indicates a successful
end to the SEARCHING state. DPLPMTUD remains in
this state until either the PMTU_RAISE_TIMER
expires, a received PTB message is validated, or a
black hole is detected.
When DPLPMTUD uses an unacknowledged PL and is in
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the SEARCH_COMPLETE state, a CONFIRMATION_TIMER
periodically resets the PROBE_COUNT and schedules a
probe packet with the size of the PLPMTU. If the
probe packet fails to be acknowledged after
MAX_PROBES attempts, the method enters the BASE
state. When used with an acknowledged PL (e.g.,
SCTP), DPLPMTUD SHOULD NOT continue to generate
PLPMTU probes in this state.
ERROR: The ERROR state represents the case where either
the network path is not known to support a PLPMTU
of at least the BASE_PMTU size or when there is
contradictory information about the network path
that would otherwise result in excessive variation
in the MPS signalled to the higher layer. The
state implements a method to mitigate oscillation
in the state-event engine. It signals a
conservative value of the MPS to the higher layer
by the PL. The state is exited when packet probes
no longer detect the error or when the PL indicates
that connectivity has been lost.
Implementations are permitted to enable endpoint
fragmentation if the DPLPMTUD is unable to validate
MIN_PMTU within PROBE_COUNT probes. If DPLPMTUD is
unable to validate MIN_PMTU the implementation
should transition to the DISABLED state.
5.3. Search to Increase the PLPMTU
This section describes the algorithms used by DPLPMTUD to search for
a larger PLPMTU.
5.3.1. Probing for a larger PLPMTU
Implementations use a search algorithm across the search range to
determine whether a larger PLPMTU can be supported across a network
path.
The method discovers the search range by confirming the minimum
PLPMTU and then using the probe method to select a PROBED_SIZE less
than or equal to MAX_PMTU. MAX_PMTU is the minimum of the local MTU
and EMTU_R (learned from the remote endpoint). The MAX_PMTU MAY be
reduced by an application that sets a maximum to the size of
datagrams it will send.
The PROBE_COUNT is initialized to zero when a probe packet is first
sent with a particular size. A timer is used by the search algorithm
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to trigger the sending of probe packets of size PROBED_SIZE, larger
than the PLPMTU. Each probe packet successfully sent to the remote
peer is confirmed by acknowledgement at the PL, see Section 4.1.
Each time a probe packet is sent to the destination, the PROBE_TIMER
is started. The timer is canceled when the PL receives
acknowledgment that the probe packet has been successfully sent
across the path Section 4.1. This confirms that the PROBED_SIZE is
supported, and the PROBED_SIZE value is then assigned to the PLPMTU.
The search algorithm can continue to send subsequent probe packets of
an increasing size.
If the timer expires before a probe packet is acknowledged, the probe
has failed to confirm the PROBED_SIZE. Each time the PROBE_TIMER
expires, the PROBE_COUNT is incremented, the PROBE_TIMER is
reinitialized, and a probe packet of the same size is retransmitted
(the replicated probe improve the resilience to loss). The maximum
number of retransmissions for a particular size is configured
(MAX_PROBES). If the value of the PROBE_COUNT reaches MAX_PROBES,
probing will stop, and the PL sender enters the SEARCH_COMPLETE
state.
5.3.2. Selection of Probe Sizes
The search algorithm needs to determine a minimum useful gain in
PLPMTU. It would not be constructive for a PL sender to attempt to
probe for all sizes. This would incur unnecessary load on the path
and has the undesirable effect of slowing the time to reach a more
optimal MPS. Implementations SHOULD select the set of probe packet
sizes to maximize the gain in PLPMTU from each search step.
Implementations could optimize the search procedure by selecting step
sizes from a table of common PMTU sizes. When selecting the
appropriate next size to search, an implementor ought to also
consider that there can be common sizes of MPS that applications seek
to use, and their could be common sizes of MTU used within the
network.
5.3.3. Resilience to Inconsistent Path Information
A decision to increase the PLPMTU needs to be resilient to the
possibility that information learned about the network path is
inconsistent. A path is inconsistent, when, for example, probe
packets are lost due to other reasons (i. e. not packet size) or due
to frequent path changes. Frequent path changes could occur by
unexpected "flapping" - where some packets from a flow pass along one
path, but other packets follow a different path with different
properties.
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A PL sender is able to detect inconsistency from the sequence of
PLPMTU probes that it sends or the sequence of PTB messages that it
receives. When inconsistent path information is detected, a PL
sender could use an alternate search mode that clamps the offered MPS
to a smaller value for a period of time. This avoids unnecessary
loss of packets due to MTU limitation.
5.4. Robustness to Inconsistent Paths
Some paths could be unable to sustain packets of the BASE_PMTU size.
To be robust to these paths an implementation could implement the
Error State. This allows fallback to a smaller than desired PLPMTU,
rather than suffer connectivity failure. This could utilize methods
such as endpoint IP fragmentation to enable the PL sender to
communicate using packets smaller than the BASE_PMTU.
6. Specification of Protocol-Specific Methods
This section specifies protocol-specific details for datagram PLPMTUD
for IETF-specified transports.
The first subsection provides guidance on how to implement the
DPLPMTUD method as a part of an application using UDP or UDP-Lite.
The guidance also applies to other datagram services that do not
include a specific transport protocol (such as a tunnel
encapsulation). The following subsections describe how DPLPMTUD can
be implemented as a part of the transport service, allowing
applications using the service to benefit from discovery of the
PLPMTU without themselves needing to implement this method.
6.1. Application support for DPLPMTUD with UDP or UDP-Lite
The current specifications of UDP [RFC0768] and UDP-Lite [RFC3828] do
not define a method in the RFC-series that supports PLPMTUD. In
particular, the UDP transport does not provide the transport layer
features needed to implement datagram PLPMTUD.
The DPLPMTUD method can be implemented as a part of an application
built directly or indirectly on UDP or UDP-Lite, but relies on
higher-layer protocol features to implement the method [RFC8085].
Some primitives used by DPLPMTUD might not be available via the
Datagram API (e.g., the ability to access the PLPMTU cache, or
interpret received PTB messages).
In addition, it is desirable that PMTU discovery is not performed by
multiple protocol layers. An application SHOULD avoid using DPLPMTUD
when the underlying transport system provides this capability. To
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use common method for managing the PLPMTU has benefits, both in the
ability to share state between different processes and opportunities
to coordinate probing.
6.1.1. Application Request
An application needs an application-layer protocol mechanism (such as
a message acknowledgement method) that solicits a response from a
destination endpoint. The method SHOULD allow the sender to check
the value returned in the response to provide additional protection
from off-path insertion of data [RFC8085], suitable methods include a
parameter known only to the two endpoints, such as a session ID or
initialized sequence number.
6.1.2. Application Response
An application needs an application-layer protocol mechanism to
communicate the response from the destination endpoint. This
response may indicate successful reception of the probe across the
path, but could also indicate that some (or all packets) have failed
to reach the destination.
6.1.3. Sending Application Probe Packets
A probe packet that may carry an application data block, but the
successful transmission of this data is at risk when used for
probing. Some applications may prefer to use a probe packet that
does not carry an application data block to avoid disruption to data
transfer.
6.1.4. Validating the Path
An application that does not have other higher-layer information
confirming correct delivery of datagrams SHOULD implement the
CONFIRMATION_TIMER to periodically send probe packets while in the
SEARCH_COMPLETE state.
6.1.5. Handling of PTB Messages
An application that is able and wishes to receive PTB messages MUST
perform ICMP validation as specified in Section 5.2 of [RFC8085].
This requires that the application to check each received PTB
messages to validate it is received in response to transmitted
traffic and that the reported PTB_SIZE is less than the current
probed size (see Section 4.4.2). A validated PTB message MAY be used
as input to the DPLPMTUD algorithm, but MUST NOT be used directly to
set the PLPMTU.
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6.2. DPLPMTUD for SCTP
Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing
method for SCTP. It recommends the use of the PAD chunk, defined in
[RFC4820] to be attached to a minimum length HEARTBEAT chunk to build
a probe packet. This enables probing without affecting the transfer
of user messages and without interfering with congestion control.
This is preferred to using DATA chunks (with padding as required) as
path probes.
XXX Author Note: Future versions of this document might define a
parameter contained in the INIT and INIT ACK chunk to indicate the
remote peer MTU to the local peer. However, multihoming makes this a
bit complex, so it might not be worth doing. XXX
6.2.1. SCTP/IPv4 and SCTP/IPv6
The base protocol is specified in [RFC4960]. This provides an
acknowledged PL. A sender can therefore enter the BASE state as soon
as connectivity has been confirmed.
6.2.1.1. Sending SCTP Probe Packets
Probe packets consist of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
the length of the probe packet. The HEARTBEAT chunk is used to
trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful probe.
The HEARTBEAT chunk carries a Heartbeat Information parameter which
should include, besides the information suggested in [RFC4960], the
probe size, which is the size of the complete datagram. The size of
the PAD chunk is therefore computed by reducing the probing size by
the IPv4 or IPv6 header size, the SCTP common header, the HEARTBEAT
request and the PAD chunk header. The payload of the PAD chunk
contains arbitrary data.
To avoid fragmentation of retransmitted data, probing starts right
after the PL handshake, before data is sent. Assuming this behavior
(i.e., the PMTU is smaller than or equal to the interface MTU), this
process will take a few round trip time periods depending on the
number of PMTU sizes probed. The Heartbeat timer can be used to
implement the PROBE_TIMER.
6.2.1.2. Validating the Path with SCTP
Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
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6.2.1.3. PTB Message Handling by SCTP
Normal ICMP validation MUST be performed as specified in Appendix C
of [RFC4960]. This requires that the first 8 bytes of the SCTP
common header are quoted in the payload of the PTB message, which can
be the case for ICMPv4 and is normally the case for ICMPv6.
When a PTB message has been validated, the PTB_SIZE reported in the
PTB message SHOULD be used with the DPLPMTUD algorithm, providing
that the reported PTB_SIZE is less than the current probe size (see
Section 4.4).
6.2.2. DPLPMTUD for SCTP/UDP
The UDP encapsulation of SCTP is specified in [RFC6951].
6.2.2.1. Sending SCTP/UDP Probe Packets
Packet probing can be performed as specified in Section 6.2.1.1. The
maximum payload is reduced by 8 bytes, which has to be considered
when filling the PAD chunk.
6.2.2.2. Validating the Path with SCTP/UDP
Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.2.2.3. Handling of PTB Messages by SCTP/UDP
ICMP validation MUST be performed for PTB messages as specified in
Appendix C of [RFC4960]. This requires that the first 8 bytes of the
SCTP common header are contained in the PTB message, which can be the
case for ICMPv4 (but note the UDP header also consumes a part of the
quoted packet header) and is normally the case for ICMPv6. When the
validation is completed, the PTB_SIZE indicated in the PTB message
SHOULD be used with the DPLPMTUD providing that the reported PTB_SIZE
is less than the current probe size.
6.2.3. DPLPMTUD for SCTP/DTLS
The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
specified in [RFC8261]. It is used for data channels in WebRTC
implementations.
6.2.3.1. Sending SCTP/DTLS Probe Packets
Packet probing can be done as specified in Section 6.2.1.1.
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6.2.3.2. Validating the Path with SCTP/DTLS
Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.2.3.3. Handling of PTB Messages by SCTP/DTLS
It is not possible to perform ICMP validation as specified in
[RFC4960], since even if the ICMP message payload contains sufficient
information, the reflected SCTP common header would be encrypted.
Therefore it is not possible to process PTB messages at the PL.
6.3. DPLPMTUD for QUIC
Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a
UDP-based transport that provides reception feedback. The UDP
payload includes the QUIC packet header, protected payload, and any
authentication fields. QUIC depends on a PMTU of at least 1280
bytes.
Section 14.1 of [I-D.ietf-quic-transport] describes the path
considerations when sending QUIC packets. It recommends the use of
PADDING frames to build the probe packet. Pure probe-only packets
are constructed with PADDING frames and PING frames to create a
padding only packet that will elicit an acknowledgement. Such
padding only packets enable probing without affecting the transfer of
other QUIC frames.
The recommendation for QUIC endpoints implementing DPLPMTUD is that a
MPS is maintained for each combination of local and remote IP
addresses [I-D.ietf-quic-transport]. If a QUIC endpoint determines
that the PMTU between any pair of local and remote IP addresses has
fallen below an acceptable MPS, it needs to immediately cease sending
QUIC packets on the affected path. This could result in termination
of the connection if an alternative path cannot be found
[I-D.ietf-quic-transport].
6.3.1. Sending QUIC Probe Packets
A probe packet consists of a QUIC Header and a payload containing
PADDING Frames and a PING Frame. PADDING Frames are a single octet
(0x00) and several of these can be used to create a probe packet of
size PROBED_SIZE. QUIC provides an acknowledged PL, a sender can
therefore enter the BASE state as soon as connectivity has been
confirmed.
The current specification of QUIC sets the following:
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* BASE_PMTU: 1200. A QUIC sender needs to pad initial packets to
1200 bytes to confirm the path can support packets of a useful
size.
* MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU has
fallen below 1200 bytes MUST immediately stop sending on the
affected path.
6.3.2. Validating the Path with QUIC
QUIC provides an acknowledged PL. A sender therefore MUST NOT
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.3.3. Handling of PTB Messages by QUIC
QUIC operates over the UDP transport, and the guidelines on ICMP
validation as specified in Section 5.2 of [RFC8085] therefore apply.
In addition to UDP Port validation QUIC can validate an ICMP message
by looking for valid Connection IDs in the quoted packet.
6.4. DPLPMTUD for UDP-Options
UDP Options [I-D.ietf-tsvwg-udp-options] provides a way to extend UDP
to provide new transport mechanisms.
Support for using DPLPMTUD with UDP-Options is defined in the UDP-
Options specification [I-D.ietf-tsvwg-udp-options].
7. Acknowledgements
This work was partially funded by the European Union's Horizon 2020
research and innovation programme under grant agreement No. 644334
(NEAT). The views expressed are solely those of the author(s).
8. IANA Considerations
This memo includes no request to IANA.
If there are no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor.
9. Security Considerations
The security considerations for the use of UDP and SCTP are provided
in the references RFCs. Security guidance for applications using UDP
is provided in the UDP Usage Guidelines [RFC8085], specifically the
generation of probe packets is regarded as a "Low Data-Volume
Application", described in section 3.1.3 of this document. This
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recommends that sender limits generation of probe packets to an
average rate lower than one probe per 3 seconds.
A PL sender needs to ensure that the method used to confirm reception
of probe packets offers protection from off-path attackers injecting
packets into the path. This protection if provided in IETF-defined
protocols (e.g., TCP, SCTP) using a randomly-initialized sequence
number. A description of one way to do this when using UDP is
provided in section 5.1 of [RFC8085]).
There are cases where ICMP Packet Too Big (PTB) messages are not
delivered due to policy, configuration or equipment design (see
Section 1.1), this method therefore does not rely upon PTB messages
being received, but is able to utilize these when they are received
by the sender. PTB messages could potentially be used to cause a
node to inappropriately reduce the PLPMTU. A node supporting
DPLPMTUD MUST therefore appropriately validate the payload of PTB
messages to ensure these are received in response to transmitted
traffic (i.e., a reported error condition that corresponds to a
datagram actually sent by the path layer, see Section 4.4.1).
An on-path attacker, able to create a PTB message could forge PTB
messages that include a valid quoted IP packet. Such an attack could
be used to drive down the PLPMTU. There are two ways this method can
be mitigated against such attacks: First, by ensuring that a PL
sender never reduces the PLPMTU below the base size, solely in
response to receiving a PTB message. This is achieved by first
entering the BASE state when such a message is received. Second, the
design does not require processing of PTB messages, a PL sender could
therefore suspend processing of PTB messages (e.g., in a robustness
mode after detecting that subsequent probes actually confirm that a
size larger than the PTB_SIZE is supported by a path).
Parallel forwarding paths SHOULD be considered. Section 5.4
identifies the need for robustness in the method when the path
information may be inconsistent.
A node performing DPLPMTUD could experience conflicting information
about the size of supported probe packets. This could occur when
there are multiple paths are concurrently in use and these exhibit a
different PMTU. If not considered, this could result in data being
black holed when the PLPMTU is larger than the smallest PMTU across
the current paths.
10. References
10.1. Normative References
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[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-20 (work
in progress), 23 April 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-20.txt>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC1191] Mogul, J.C. and S.E. Deering, "Path MTU discovery",
RFC 1191, DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
and G. Fairhurst, Ed., "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
2004, <https://www.rfc-editor.org/info/rfc3828>.
[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
<https://www.rfc-editor.org/info/rfc4820>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
"Datagram Transport Layer Security (DTLS) Encapsulation of
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.
10.2. Informative References
[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", draft-ietf-intarea-tunnels-09 (work in
progress), 19 July 2018,
<http://www.ietf.org/internet-drafts/draft-ietf-intarea-
tunnels-09.txt>.
[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-07 (work in progress), 8 March 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-tsvwg-udp-
options-07.txt>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
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DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890,
DOI 10.17487/RFC4890, May 2007,
<https://www.rfc-editor.org/info/rfc4890>.
Appendix A. Revision Notes
Note to RFC-Editor: please remove this entire section prior to
publication.
Individual draft -00:
* Comments and corrections are welcome directly to the authors or
via the IETF TSVWG working group mailing list.
* This update is proposed for WG comments.
Individual draft -01:
* Contains the first representation of the algorithm, showing the
states and timers
* This update is proposed for WG comments.
Individual draft -02:
* Contains updated representation of the algorithm, and textual
corrections.
* The text describing when to set the effective PMTU has not yet
been validated by the authors
* To determine security to off-path-attacks: We need to decide
whether a received PTB message SHOULD/MUST be validated? The text
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on how to handle a PTB message indicating a link MTU larger than
the probe has yet not been validated by the authors
* No text currently describes how to handle inconsistent results
from arbitrary re-routing along different parallel paths
* This update is proposed for WG comments.
Working Group draft -00:
* This draft follows a successful adoption call for TSVWG
* There is still work to complete, please comment on this draft.
Working Group draft -01:
* This draft includes improved introduction.
* The draft is updated to require ICMP validation prior to accepting
PTB messages - this to be confirmed by WG
* Section added to discuss Selection of Probe Size - methods to be
evlauated and recommendations to be considered
* Section added to align with work proposed in the QUIC WG.
Working Group draft -02:
* The draft was updated based on feedback from the WG, and a
detailed review by Magnus Westerlund.
* The document updates RFC 4821.
* Requirements list updated.
* Added more explicit discussion of a simpler black-hole detection
mode.
* This draft includes reorganisation of the section on IETF
protocols.
* Added more discussion of implementation within an application.
* Added text on flapping paths.
* Replaced 'effective MTU' with new term PLPMTU.
Working Group draft -03:
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* Updated figures
* Added more discussion on blackhole detection
* Added figure describing just blackhole detection
* Added figure relating MPS sizes
Working Group draft -04:
* Described phases and named these consistently.
* Corrected transition from confirmation directly to the search
phase (Base has been checked).
* Redrawn state diagrams.
* Renamed BASE_MTU to BASE_PMTU (because it is a base for the PMTU).
* Clarified Error state.
* Clarified supsending DPLPMTUD.
* Verified normative text in requirements section.
* Removed duplicate text.
* Changed all text to refer to /packet probe/probe packet/
/validation/verification/ added term /Probe Confirmation/ and
clarified BlackHole detection.
Working Group draft -05:
* Updated security considerations.
* Feedback after speaking with Joe Touch helped improve UDP-Options
description.
Working Group draft -06:
* Updated description of ICMP issues in section 1.1
* Update to description of QUIC.
Working group draft -07:
* Moved description of the PTB processing method from the PTB
requirements section.
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* Clarified what is performed in the PTB validation check.
* Updated security consideration to explain PTB security without
needing to read the rest of the document.
* Reformatted state machine diagram
Working group draft -08:
* Moved to rfcxml v3+
* Rendered diagrams to svg in html version.
* Removed Appendix A. Event-driven state changes.
* Removed section on DPLPMTUD with UDP Options.
* Shortened the dsecription of phases.
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering, Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
Tom Jones
University of Aberdeen
School of Engineering, Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: tom@erg.abdn.ac.uk
Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
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Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: i.ruengeler@fh-muenster.de
Timo Voelker
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: timo.voelker@fh-muenster.de
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