rfc8899xml2.original.xml   rfc8899.xml 
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<!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent">
<rfc
xmlns:xi="http://www.w3.org/2001/XInclude"
category="std"
consensus="true"
number="8899"
docName="draft-ietf-tsvwg-datagram-plpmtud-22"
indexInclude="true"
ipr="trust200902"
sortRefs="true"
submissionType="IETF"
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<front>
<title abbrev="DPLPMTUD">Packetization Layer Path MTU Discovery for Datagram
Transports</title>
<seriesInfo name="RFC" value="8899"/>
<author fullname="Godred Fairhurst" initials="G" surname="Fairhurst">
<organization>University of Aberdeen</organization>
<address>
<postal>
<extaddr>School of Engineering</extaddr>
<street>Fraser Noble Building</street>
<city>Aberdeen</city>
<code>AB24 3UE</code>
<country>United Kingdom</country>
</postal>
<email>gorry@erg.abdn.ac.uk</email>
</address>
</author>
<author fullname="Tom Jones" initials="T" surname="Jones">
<organization>University of Aberdeen</organization>
<address>
<postal>
<extaddr>School of Engineering</extaddr>
<street>Fraser Noble Building</street>
<city>Aberdeen</city>
<code>AB24 3UE</code>
<country>United Kingdom</country>
</postal>
<email>tom@erg.abdn.ac.uk</email>
</address>
</author>
<author fullname="Michael Tüxen" initials="M" surname="Tüxen">
<organization>Münster University of Applied Sciences</organization>
<address>
<postal>
<street>Stegerwaldstrasse 39</street>
<code>48565</code>
<city>Steinfurt</city>
<country>Germany</country>
</postal>
<email>tuexen@fh-muenster.de</email>
</address>
</author>
<author fullname="Irene Rüngeler" initials="I" surname="Rüngeler">
<organization>Münster University of Applied Sciences</organization>
<address>
<postal>
<street>Stegerwaldstrasse 39</street>
<code>48565</code>
<city>Steinfurt</city>
<country>Germany</country>
</postal>
<email>i.ruengeler@fh-muenster.de</email>
</address>
</author>
<author fullname="Timo Völker" initials="T" surname="Völker">
<organization>Münster University of Applied Sciences</organization>
<address>
<postal>
<street>Stegerwaldstrasse 39</street>
<code>48565</code>
<city>Steinfurt</city>
<country>Germany</country>
</postal>
<email>timo.voelker@fh-muenster.de</email>
</address>
</author>
<date month="September" year="2020"/>
<area>Transport</area>
<workgroup>TSVWG</workgroup>
<keyword>UDP</keyword>
<keyword>SCTP</keyword>
<keyword>Transport</keyword>
<keyword>PMTUD</keyword>
<keyword>PLPMTUD</keyword>
<abstract>
<t>
This document specifies Datagram Packetization Layer Path MTU Discovery
(DPLPMTUD). This is a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization Layers (PLs).
It allows a PL, or a datagram
application that uses a PL, to discover whether a network path can
support the current size of datagram. This can be used to detect and
reduce the message size when a sender encounters a packet black hole.
It can also probe a network path to discover whether the maximum
packet size can be increased. This provides functionality for datagram
transports that is equivalent to the PLPMTUD
specification for TCP, specified in RFC 4821, which it updates.
It also updates the UDP Usage Guidelines to refer to this method
for use with UDP datagrams and updates SCTP.</t>
<t>
The document provides implementation notes for incorporating
Datagram PMTUD into IETF datagram transports or applications that use
datagram transports.</t>
<t>
This specification updates RFC 4960, RFC 4821, RFC 6951, RFC 8085, and RFC
8261.</t>
</abstract>
</front>
<middle>
<section>
<name>Introduction</name>
<t>The IETF has specified datagram transport using UDP,
Stream Control Transmission Protocol (SCTP), and
Datagram Congestion Control Protocol (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 can be used with these transport protocols (or the
applications that use their transport service) to discover an
appropriate size of packet to use across an Internet path.</t>
<section anchor="Classic-PMTUD">
<name>Classical Path MTU Discovery</name>
<t>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., <xref target="RFC1191"/> and <xref target="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) <xref target="RFC0792"/> and
ICMPv6 Packet Too Big messages (Type 2)
<xref target="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. Classical PMTUD specifies a method of periodically incr
easing
the packet size in an attempt to discover an increase in the
supported PMTU.
The packets sent with a size larger than the current effective PMTU
are known as probe packets.</t>
<t>Packets not intended as probe packets are either fragmented to the
current effective PMTU, or the attempt to send fails with an error
code. Applications can be provided with a primitive to let them
read the Maximum Packet Size (MPS), which is derived from the current ef
fective
PMTU.</t>
<t>Classical PMTUD is subject to protocol failures. One failure arises
when traffic using a packet size larger than the actual PMTU is
black-holed (all datagrams larger than the actual PMTU are discarded).
This could arise when the PTB messages are not sent back to the
sender for some reason (for example, see <xref target="RFC2923"/>).</t>
<t>Examples of where PTB messages are not delivered include the followin
g: </t>
<ul>
<li>The generation of ICMP messages is usually rate limited. This
could result in no PTB messages being generated to the sender (see
<xref target="RFC4443" section="2.4" sectionFormat="of" format="defa
ult"/>).</li>
<li>ICMP messages can be filtered by middleboxes, including
firewalls <xref target="RFC4890"/>. A firewall
could be configured with a policy to block incoming ICMP messages,
which would prevent reception of PTB messages by a sending
endpoint behind this firewall.</li>
<li>When the router issuing the ICMP message drops a tunneled
packet, the resulting ICMP message is directed to the tunnel
ingress. This tunnel endpoint is responsible for forwarding the
ICMP message, processing the quoted packet within the
payload field to remove the effect of the tunnel and returning a
correctly formatted ICMP message to the sender <xref target="I-D.iet
f-intarea-tunnels"/>. Failure to do this
prevents the PTB message from reaching the original sender.</li>
<li>Asymmetry in forwarding can result in there being no return
route to the original sender, which would prevent an ICMP message
from being delivered to the sender. This issue can also arise when
either policy-based or Equal-Cost Multipath (ECMP) routing
is used or when a middlebox acts as an application load balancer.
An example of which is an ECMP router choosing a path toward the server
based on the bytes in the IP payload. In this case, if a packet
sent by the server encounters a problem after the ECMP router, then
the ECMP router needs to direct any resulting ICMP message toward
the original sender.</li>
<li>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 a packet is dropped by the link,
this will not cause a PTB message to be sent to the original
sender.</li>
</ul>
<t>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 <xref target="RFC8201"/>. A sender can protect
itself from reacting to such messages by utilizing the quoted packet
within a PTB message payload to validate that the received PTB message
was generated in response to a packet that had actually originated
from the sender. However, there are situations where a sender would be
unable to provide this validation. Examples where the validation of the
PTB message is not possible include the following: </t>
<ul>
<li>
<t>When a router issuing the ICMP message implements RFC 792 <xref t
arget="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. </t>
<t> Note: The
recommendation in RFC 1812 <xref target="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
<xref target="RFC4443"/>.</t>
</li>
<li>The use of tunnels and/or encryption can reduce the size of the qu
oted
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.</li>
<li>Even when the PTB message includes sufficient bytes of the
quoted packet, the network layer could lack sufficient context to
validate the message because validation depends on information
about the active transport flows at an endpoint node (e.g., the
socket/address pairs being used and other protocol header
information).</li>
<li>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.</li>
<li> When an ICMP message is generated by a router in a network
segment that has inserted a header into a packet, the quoted packet
could contain additional protocol header information that was not
included in the original sent packet and that the PL sender does
not process or may not know how to process. This could disrupt the
ability of the sender to validate this PTB message. </li>
<li> A Network Address Translation (NAT) device that translates a
packet header ought to also translate ICMP messages and update
the ICMP-quoted packet <xref target="RFC5508"/> in
that message. If this is not correctly translated,
then the sender would not be able to associate the message
with the PL that originated the packet, and hence this
ICMP message cannot be validated.</li>
</ul>
</section>
<section>
<name>Packetization Layer Path MTU Discovery</name>
<t>The term Packetization Layer (PL) has been introduced to describe
the layer that is responsible for placing data blocks into the payload
of IP packets and selecting an appropriate MPS. This function is often
performed by a transport protocol (e.g., DCCP, RTP, SCTP, QUIC)
but can also be performed by other encapsulation methods working
above the transport layer.</t>
<t>In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) <xref target="RFC4821"/> introduces a method
that does not rely upon reception
and validation of PTB messages. It is therefore more robust than
Classical PMTUD. This has become the recommended approach for
implementing discovery of the PMTU <xref target="BCP145"/>.</t>
<t>This document updates <xref target="RFC4821"/> to specify the PLPMTUD
method for
datagram PLs and also updates <xref target="BCP145"/> to refer to the me
thod
specified in this document for use
with UDP datagrams instead of the method in <xref target="RFC4821"/>. </
t>
<t>It uses a general strategy in which the PL sends probe packets to
search for the largest size of unfragmented datagram that can be sent
over a network path. Probe packets are sent to explore using a larger
packet size. If a probe packet is successfully delivered (as determined
by the PL), then the PLPMTU is raised to the size of the successful
probe. If a black hole is detected (e.g., where packets of size PLPMTU
are consistently not received), the method reduces the PLPMTU. </t>
<t>Datagram PLPMTUD introduces flexibility in implementation.
At one extreme, it can be configured to only perform
black hole detection and recovery with increased robustness compared to
Classical PMTUD. At the other extreme, all PTB processing can
be disabled, and PLPMTUD replaces Classical PMTUD.</t>
<t>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.</t>
</section>
<section>
<name>Path MTU Discovery for Datagram Services</name>
<t><xref target="Spec"/> of this document presents a set of
algorithms for datagram protocols to discover the largest size of
unfragmented datagram that can be sent over a network path. The method
relies upon features of the PL described in <xref target="Requirements"/
> 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.</t>
<t>The message size guidelines in Section
<xref target="BCP145" section="3.2" sectionFormat="bare" format="default
"/>
of the UDP Usage Guidelines <xref target="BCP145"/> state that "an appli
cation <bcp14>SHOULD</bcp14>
either use the Path MTU information provided by the IP layer or
implement Path MTU Discovery (PMTUD)" but do not provide a mechanism
for discovering the largest size of unfragmented datagram that can be
used on a network path. The present document updates RFC 8085 to
specify this method in place of PLPMTUD <xref target="RFC4821"/>
and provides a mechanism for sharing the discovered largest size as the
MPS (see <xref target="mps"/>).</t>
<t><xref target="RFC4821" section="10.2" sectionFormat="of" format="defa
ult"/> recommended 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 <xref target="RFC4820"/>.
However, RFC 4821 did not provide a complete specification. The present
document replaces that description by providing a complete
specification.</t>
<t>The Datagram Congestion Control Protocol (DCCP) <xref target="RFC4340
"/> requires implementations to support Classical
PMTUD and states that a DCCP sender "<bcp14>MUST</bcp14> maintain the MP
S 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 can be used with DCCP.</t>
<t><xref target="dplpmtud-mechanisms"/> and <xref target="Spec"/> define
the protocol mechanisms and specification
for Datagram Packetization Layer Path MTU Discovery (DPLPMTUD). </t>
<t><xref target="protocol_specific_methods"/> specifies the
method for datagram transports and provides information to enable the
implementation of PLPMTUD with other datagram transports and
applications that use datagram transports.</t>
<t><xref target="protocol_specific_methods"/>
also provides recommendations for SCTP endpoints,
updating <xref target="RFC4960"/>, <xref target="RFC6951"/>, and <xref t
arget="RFC8261"/> to use the
method specified in this document instead of the method in <xref target=
"RFC4821"/>.
</t>
</section>
</section>
<section>
<name>Terminology</name>
<t>
The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQU
IRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>
RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to
be interpreted as
described in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/>
when, and only when, they appear in all capitals, as shown here.
</t>
<t>The following terminology is defined. Relevant terms are directly
copied from <xref target="RFC4821"/>, and the definitions in <xref target=
"RFC1122"/> apply.</t>
<dl newline="false">
<dt>Acknowledged PL:</dt>
<dd> A PL that includes a mechanism that can confirm successful
delivery of datagrams to the remote PL endpoint (e.g., SCTP).
Typically, the PL receiver returns acknowledgments corresponding to
the received datagrams, which can be utilized to detect black-holing
of packets (c.f., Unacknowledged PL). </dd>
<dt>Actual PMTU:</dt>
<dd>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.</dd>
<dt>Black Hole:</dt>
<dd>
<t>A black hole is encountered when a sender is unaware that
packets are not being delivered to the destination endpoint. Two
types of black hole are relevant to DPLPMTUD:</t>
<ul>
<li>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).</li>
<li>
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.</li>
</ul>
</dd>
<dt>Classical Path MTU Discovery:</dt>
<dd>Classical PMTUD is a process described in <xref target="RFC1191"/> a
nd <xref target="RFC8201"/> in which nodes
rely on PTB messages to learn the largest size of unfragmented
packet that can be used across a network path.</dd>
<dt>Datagram:</dt>
<dd>A datagram is a transport-layer protocol data unit, transmitted
in the payload of an IP packet.</dd>
<dt>DPLPMTUD:</dt><dd>Datagram Packetization Layer Path MTU Discovery (D
PLPMTUD),
PLPMTUD performed using a datagram transport protocol.</dd>
<dt>Effective PMTU:</dt>
<dd>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
plus the size of any headers added below the PL, including the IP
layer headers.</dd>
<dt>EMTU_S:</dt>
<dd>The effective MTU for sending (EMTU_S) is defined in <xref target="R
FC1122"/> as "the maximum IP datagram size that may be
sent, for a particular combination of IP source and destination
addresses...".</dd>
<dt>EMTU_R:</dt>
<dd>The effective MTU for receiving (EMTU_R) is designated in <xref targ
et="RFC1122"/> as "the largest datagram size that can be
reassembled".</dd>
<dt>Link:</dt>
<dd>A link is a communication facility or medium over which nodes can
communicate at the link layer, i.e., a layer below the IP layer.
Examples are Ethernet LANs and Internet (or higher) layer
tunnels.</dd>
<dt>Link MTU:</dt>
<dd>
<t>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. This specification
continues the requirement in <xref target="RFC4821"/>
that states,
"All links <bcp14>MUST</bcp14> enforce their MTU: links that might
non-deterministically deliver packets that are larger than their rated
MTU <bcp14>MUST</bcp14> consistently discard such packets."</t>
</dd>
<dt>MAX_PLPMTU:</dt>
<dd>The MAX_PLPMTU is the largest size of PLPMTU that DPLPMTUD will
attempt to use (see the constants defined in <xref target="Constants"/>
).</dd>
<dt>MIN_PLPMTU:</dt>
<dd>The MIN_PLPMTU is the smallest size of PLPMTU that DPLPMTUD will
attempt to use (see the constants defined in <xref target="Constants"/>
). </dd>
<dt>MPS:</dt>
<dd>The Maximum Packet Size (MPS) is the largest size of
application data block that can be sent across a network path by a PL
using a single datagram (see <xref target="mps"/>).</dd>
<dt>MSL:</dt>
<dd>The Maximum Segment Lifetime (MSL) is the maximum delay a packet is
expected to experience across a path, taken as 2 minutes <xref target=
"BCP145"/>. </dd>
<dt>Packet:</dt>
<dd>A packet is the IP header(s) and any extension headers/options
plus the IP payload.</dd>
<dt>Packetization Layer (PL):</dt>
<dd>The PL is a layer of the network stack that places data into
packets and performs transport protocol functions. Examples of a PL
include TCP, SCTP, SCTP over UDP, SCTP over DTLS, or QUIC. </dd>
<dt>Path:</dt>
<dd>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.</dd>
<dt>Path MTU (PMTU):</dt>
<dd>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, as used by PMTUD.</dd>
<dt>PTB:</dt>
<dd> 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) <xref target="RFC0792"/> and ICMPv6
Packet Too Big messages (Type 2) <xref target="RFC4443"/>.
</dd>
<dt>PTB_SIZE:</dt>
<dd>The PTB_SIZE is a value reported in a validated PTB message that
indicates next-hop link MTU of a router along the path.</dd>
<dt>PL_PTB_SIZE:</dt>
<dd>The size reported in a validated PTB message, reduced by the size
of all headers added by layers below the PL.
</dd>
<dt>PLPMTU:</dt>
<dd>The Packetization Layer PMTU is an estimate of the largest size
of PL datagram that can be sent by a path, controlled by PLPMTUD.</dd
>
<dt>PLPMTUD:</dt>
<dd>Packetization Layer Path MTU Discovery (PLPMTUD), the method
described in this document for datagram PLs, which is an extension
to Classical PMTU Discovery.</dd>
<dt>Probe packet:</dt>
<dd>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.</dd>
<dt>Unacknowledged PL:</dt>
<dd>A PL that does not itself provide a mechanism to confirm delivery
of datagrams to the remote PL endpoint (e.g., UDP), and therefore
requires DPLPMTUD to provide a mechanism to detect black-holing of
packets (c.f., Acknowledged PL).</dd>
</dl>
</section>
<section anchor="Requirements">
<name>Features Required to Provide Datagram PLPMTUD</name>
<t>The principles expressed in <xref target="RFC4821"/> apply to
the use of the technique with any PL.
TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
Unlike TCP, a datagram PL requires additional mechanisms and
considerations to implement PLPMTUD. </t>
<t>The requirements for datagram PLPMTUD are: </t>
<ol>
<li derivedCounter="1."> Managing the PLPMTU: For datagram PLs, the PLPM
TU is managed by
DPLPMTUD. A PL <bcp14>MUST NOT</bcp14> send a datagram (other than a p
robe packet)
with a size at the PL that is larger than the current
PLPMTU.</li>
<li derivedCounter="2."> Probe packets: The network interface below the
PL is <bcp14>REQUIRED</bcp14> to
provide a way to transmit a probe packet that is larger than the
PLPMTU. In IPv4, a probe packet <bcp14>MUST</bcp14> be sent with the D
on'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
<xref target="RFC8201" section="5.4" sectionFormat="of" format="defaul
t"/>).</li>
<li derivedCounter="3.">Reception feedback: The destination PL endpoint
is <bcp14>REQUIRED</bcp14> to
provide a feedback method that indicates to the DPLPMTUD sender when
a probe packet has been received by the destination PL endpoint.
<xref target="protocol_specific_methods"/> provides examples of
how a PL can provide this acknowledgment of received probe packets.
</li>
<li derivedCounter="4.">Probe loss recovery: It is <bcp14>RECOMMENDED</b
cp14> to use probe packets that
do not carry any user data that would require retransmission if lost.
Most datagram transports permit this. If a probe packet contains user
data requiring retransmission in case of loss, the PL (or layers
above) is <bcp14>REQUIRED</bcp14> to arrange any retransmission and/or
repair of any
resulting loss. The PL is <bcp14>REQUIRED</bcp14> to be robust in the
case where
probe packets are lost due to other reasons (including link
transmission error, congestion).</li>
<li derivedCounter="5.">PMTU parameters: A DPLPMTUD sender is <bcp14>REC
OMMENDED</bcp14> to utilize
information about the maximum size of packet that can be transmitted
by the sender on the local link (e.g., the local link MTU). A PL
sender <bcp14>MAY</bcp14> utilize similar information about the maximum
size of
network-layer packet that a receiver can accept when this is supplied
(note this could be less than EMTU_R). This avoids implementations
trying to send probe packets that cannot be transferred by the local
link. Too high of a value could reduce the efficiency of the search
algorithm. Some applications also have a maximum transport protocol
data unit (PDU) size, in which case there is no benefit from probing
for a size larger than this (unless a transport allows multiplexing
multiple applications' PDUs into the same datagram).</li>
<li derivedCounter="6.">Processing PTB messages: A DPLPMTUD sender <bcp1
4>MAY</bcp14> 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 <bcp14>MUST</bcp14> be validated before it is use
d to update
the PLPMTU discovery information <xref target="RFC8201"/>.
This validation confirms that the PTB message was sent in response to
a packet originated by the sender and needs to be performed before
the PLPMTU discovery method reacts to the PTB message. A PTB message
<bcp14>MUST NOT</bcp14> be used to increase the PLPMTU <xref target="R
FC8201"/>
but could trigger a probe to test for a larger PLPMTU.
A valid PTB_SIZE is converted to a PL_PTB_SIZE before it is to be
used in the DPLPMTUD state machine. A PL_PTB_SIZE that is greater
than that currently probed <bcp14>SHOULD</bcp14> be ignored. (This PTB
message ought
to be discarded without further processing but could be utilized as
an input that enables a resilience mode).
</li>
<li derivedCounter="7.">Probing and congestion control: A PL <bcp14>MAY<
/bcp14> use a congestion
controller to decide when to send a probe packet. If transmission of
probe packets is limited by the congestion controller, this could
result in transmission of probe packets being delayed or suspended
during congestion. When the transmission of probe packets is not
controlled by the congestion controller, the interval between probe
packets <bcp14>MUST</bcp14> be at least one RTT.
Loss of a probe packet <bcp14>SHOULD NOT</bcp14> be treated as an indi
cation of
congestion and <bcp14>SHOULD NOT</bcp14> trigger a congestion control
reaction <xref target="RFC4821"/>
because this could result in unnecessary reduction of the sending rate
.
An update to the PLPMTU (or MPS) <bcp14>MUST NOT</bcp14> increase the
congestion
window measured in bytes <xref target="RFC4821"/>. Therefore,
an increase in the packet size does not cause an increase in the data
rate in bytes per second.
A PL that maintains the congestion window in terms of a limit to
the number of outstanding fixed-size packets <bcp14>SHOULD</bcp14> ada
pt this limit
to compensate for the size of the actual packets.
The transmission of probe packets can interact with the operation
of a PL that performs burst mitigation or pacing, and the PL could need
transmission of probe packets to be regulated by these methods.</li>
<li derivedCounter="8.">Probing and flow control: Flow control at the P
L concerns the
end-to-end flow of data using the PL service. Flow control <bcp14>SHOUL
D
NOT</bcp14> apply to DPLPMTU when probe packets use a design that does
not
carry user data to the remote application.</li>
<li derivedCounter="9.">Shared PLPMTU state: The PMTU value
calculated from the PLPMTU <bcp14>MAY</bcp14> also be stored with the
corresponding entry associated with the destination in the IP
layer cache and used by other PL instances. The specification of
PLPMTUD <xref target="RFC4821"/> states, "If PLPMTUD updates
the MTU for a particular path, all Packetization Layer sessions that
share the path representation (as described in Section
<xref target="RFC4821" section="5.2" sectionFormat="bare" format="defa
ult"/>)
<bcp14>SHOULD</bcp14> be notified to make use of the new
MTU". Such methods <bcp14>MUST</bcp14> be robust to the wide variety o
f underlying
network forwarding behaviors.
<xref target="RFC8201" section="5.2" sectionFormat="of" format="defaul
t"/>
provides guidance on the caching of PMTU
information and also the relation to IPv6 flow labels.</li>
</ol>
<t>In addition, the following principles are stated for design of a
DPLPMTUD method: </t>
<ul>
<li>A PL <bcp14>MAY</bcp14> be designed to segment data blocks larger th
an the MPS
into multiple datagrams. However, not all datagram PLs support
segmentation of data blocks. It is <bcp14>RECOMMENDED</bcp14> that met
hods avoid
forcing an application to use an arbitrary small MPS for transmission
while the method is searching for the currently supported PLPMTU. A
reduced MPS can adversely impact the performance of an
application.</li>
<li> To assist applications in choosing a suitable data block size, the
PL is <bcp14>RECOMMENDED</bcp14> to provide a primitive that
returns the MPS derived from the PLPMTU to the
higher layer using the PL. The value of the MPS can change following
a change in the path or loss of probe packets. </li>
<li>Path validation: It is <bcp14>RECOMMENDED</bcp14> that methods are r
obust to
path changes that could have occurred since the path characteristics
were last confirmed and to the possibility of inconsistent path
information being received.</li>
<li>Datagram reordering: A method is <bcp14>REQUIRED</bcp14> to be robus
t to the
possibility that a flow encounters reordering or that the traffic
(including probe packets) is divided over more than one network
path.</li>
<li>Datagram delay and duplication: The feedback
mechanism is <bcp14>REQUIRED</bcp14> to be robust to the possibility t
hat packets
could be significantly delayed or duplicated along a network path.</li
>
<li>When to probe: It is <bcp14>RECOMMENDED</bcp14> that methods determi
ne whether
the path has changed since it last measured the path. This can help
determine when to probe the path again.</li>
</ul>
</section>
<section anchor="dplpmtud-mechanisms">
<name>DPLPMTUD Mechanisms</name>
<t>This section lists the protocol mechanisms used in this
specification.</t>
<section anchor="Probe">
<name>PLPMTU Probe Packets</name>
<t>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 <xref targe
t="RFC4821"/>. In contrast, a datagram PL that constructs a
probe packet has to either request an application to send a data block
that is larger than that generated by an application, or to utilize
padding functions to extend a datagram beyond the size of the
application data block. Protocols that permit exchange of control
messages (without an application data block) can generate a probe
packet by extending a control message with padding data. The total size
of a probe packet includes all headers and padding added to the payload
data being sent (e.g., including protocol option fields,
security-related fields such as an Authenticated Encryption with
Associated Data (AEAD) tag, and TLS record layer padding). </t>
<t>A receiver is <bcp14>REQUIRED</bcp14> to be able to distinguish an in
-band data
block from any added padding. This is needed to ensure that any added
padding is not passed on to an application at the receiver.</t>
<t>This results in three possible ways that a sender can create a
probe packet: </t>
<dl>
<dt>Probing using padding data:</dt>
<dd>A probe packet that contains only control information together
with any padding, which is needed to inflate to the size
of the probe packet. Since these probe packets do not
carry an application-supplied data block, they do not typically
require retransmission, although they do still consume network
capacity and incur endpoint processing.</dd>
<dt>Probing using application data and padding data:</dt>
<dd>A probe packet that contains a data block supplied by an
application that is combined with padding to inflate the length of
the datagram to the size of the probe packet. </dd>
<dt>Probing using application data:</dt>
<dd>A probe packet that contains a data block supplied by an
application that matches the size of the probe packet.
This method requests the application to issue a data block of the
desired probe size. </dd>
</dl>
<t>A PL that uses a probe packet carrying application data and that need
s
protection from the loss of this probe packet could perform
transport-layer retransmission/repair of the data block (e.g., by
retransmitting after loss is detected or by duplicating the data block
in a datagram without the padding data). This retransmitted data block
might possibly need to be sent using a smaller PLPMTU, which could
force the PL to use a smaller packet size to traverse the end-to-end
path. (This could utilize endpoint network-layer fragmentation or a PL
that can resegment the data block into multiple datagrams).
</t>
<t>DPLPMTUD <bcp14>MAY</bcp14> choose to use only one of these methods t
o simplify
the implementation.</t>
<t>Probe messages sent by a PL <bcp14>MUST</bcp14> contain enough inform
ation to
uniquely identify the probe within the Maximum Segment Lifetime (e.g.,
including a unique identifier from the PL or the DPLPMTUD
implementation), while being robust to reordering and replay of probe
response and PTB messages.</t>
</section>
<section anchor="Valid">
<name>Confirmation of Probed Packet Size</name>
<t>The PL needs a method to determine (confirm) when probe packets
have been successfully received end-to-end across a network path.</t>
<t>Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP,
SCTP, and QUIC provide keep-alive/heartbeat features). When supported,
this mechanism <bcp14>MAY</bcp14> also be used by DPLPMTUD to acknowledge
reception of
a probe packet.</t>
<t>A PL that does not acknowledge data reception (e.g., UDP and
UDP-Lite) is unable itself to detect when the packets that it sends
are discarded because their size is greater than the actual PMTU.
These PLs need to rely on an application protocol to detect
this loss.</t>
<t><xref target="protocol_specific_methods"/> specifies this
function for a set of IETF-specified protocols.</t>
</section>
<section anchor="mechanism-bhd">
<name>Black Hole Detection and Reducing the PLPMTU</name>
<t> The description that follows uses the set of constants defined in
<xref target="Constants"/> and variables defined in <xref target="Variab
les"/>.</t>
<t>Black hole detection is
triggered by an indication that the network path could be unable to
support the current PLPMTU size. </t>
<t>There are three indicators that can be used to detect black holes:</t
>
<ul>
<li>A validated PTB message can be received that indicates a PL_PTB_SI
ZE
less than the current PLPMTU. A DPLPMTUD method <bcp14>MUST NOT</bcp14>
rely solely on
this method.</li>
<li>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, <xref target="Timers"/>). A
timer tracks whether acknowledgments are received. Successive loss
of probes is an indication that the current path no longer
supports the PLPMTU (e.g., when the number of probe packets sent
without receiving an acknowledgment, PROBE_COUNT, becomes greater
than MAX_PROBES).</li>
<li>A PL can utilize an event that indicates the network path no
longer sustains the sender's PLPMTU size. This could use a
mechanism implemented within the PL to detect excessive loss of
data sent with a specific packet size and then conclude that this
excessive loss could be a result of an invalid PLPMTU (as in
PLPMTUD for TCP <xref target="RFC4821"/>).</li>
</ul>
<t>The three methods can result in different transmission patterns for
packet probes and are expected to result in different responsiveness
following a change in the actual PMTU.</t>
<t>A PL <bcp14>MAY</bcp14> inhibit sending probe packets when no applica
tion data has
been sent since the previous probe packet. A PL that resumes sending
user data <bcp14>MAY</bcp14> continue PLPMTU discovery for each path. Th
is allows it
to use an up-to-date PLPMTU. However, this could result in additional
packets being sent.</t>
<t>When the method detects that the current PLPMTU is not supported,
DPLPMTUD sets a lower PLPMTU and a lower MPS. The PL then
confirms that the new PLPMTU can be successfully used across the path.
A probe packet could need to be smaller than the size of the data
block generated by the application. </t>
</section>
<section anchor="mps">
<name>The Maximum Packet Size (MPS)</name>
<t> The result of probing determines a usable PLPMTU, which is used to
set the MPS used by the application. The MPS is smaller than the
PLPMTU because it is reduced by the size of PL headers (including the
overhead of security-related fields such as an AEAD tag and TLS record
layer padding). The relationship between the MPS and the PLPMTUD is
illustrated in <xref target="fig-mps-relationship"/>. </t>
<figure anchor="fig-mps-relationship">
<name>Relationship between MPS and PLPMTU</name>
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Any additional
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| | |
v v v
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| IP | ** | PL | protocol data |
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</figure>
<t>A PL is unable to send a packet (other than a probe packet) with a
size larger than the current PLPMTU at the network layer. To avoid this,
a PL <bcp14>MAY</bcp14> be designed to segment data blocks larger than t
he MPS into
multiple datagrams.</t>
<t> DPLPMTUD seeks to avoid IP fragmentation. An attempt to send a data
block larger than the MPS will therefore fail if a PL is unable to
segment data. To determine the largest data block that can be sent, a
PL <bcp14>SHOULD</bcp14> provide applications with a primitive that retu
rns the MPS,
derived from the current PLPMTU.</t>
<t> If DPLPMTUD results in a change to the MPS, the application needs
to adapt to the new MPS. A particular case can arise when packets have
been sent with a size less than the MPS and the PLPMTU was subsequently
reduced. If these packets are lost, the PL <bcp14>MAY</bcp14> segment the
data using
the new MPS. If a PL is unable to resegment a previously sent datagram
(e.g., <xref target="RFC4960"/>), then the sender either discards
the datagram or could perform retransmission using network-layer
fragmentation to form multiple IP packets not larger than the PLPMTU.
For IPv4, the use of endpoint fragmentation by the sender is preferred
over clearing the DF bit in the IPv4 header. Operational experience
reveals that IP fragmentation can reduce the reliability of Internet
communication <xref target="RFC8900"/>,
which may reduce the probability of successful retransmission.
</t>
</section>
<section anchor="outgoingpmtu">
<name>Disabling the Effect of PMTUD</name>
<t>A PL implementing this specification <bcp14>MUST</bcp14> suspend netw
ork layer
processing of outgoing packets that enforces a PMTU <xref target="RFC1
191"/><xref target="RFC8201"/> for each flow
utilizing DPLPMTUD and instead use DPLPMTUD to control the size of
packets that are sent by a flow. This removes the need for the
network layer to drop or to fragment sent packets that have a size
greater than the PMTU.</t>
</section>
<section anchor="mechanism-ptb">
<name>Response to PTB Messages</name>
<t>This method requires the DPLPMTUD sender to validate any received
PTB message before using the PTB information. The response to a PTB
message depends on the PL_PTB_SIZE calculated from the PTB_SIZE in the
PTB message, the state of the PLPMTUD state machine, and the IP
protocol being used.</t>
<t><xref target="PTB"/> 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.</t>
<section anchor="PTB">
<name>Validation of PTB Messages</name>
<t>This section specifies utilization and validation of PTB messages.<
/t>
<ul>
<li>A simple implementation <bcp14>MAY</bcp14> ignore received PTB m
essages, and
in this case, the PLPMTU is not updated when a PTB message is
received.</li>
<li>A PL that supports PTB messages <bcp14>MUST</bcp14> validate the
se messages
before they are further processed.</li>
</ul>
<t>A PL that receives a PTB message from a router or middlebox
performs ICMP validation (see
<xref target="RFC8201" section="4" sectionFormat="of" format="default"
/> and
<xref target="BCP145" section="5.2" sectionFormat="of" format="default
"/>).
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.</t>
<t>The PL <bcp14>MUST</bcp14> check the protocol information in the qu
oted 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, th
e
source port, and destination port match those returned in the
quoted packet -- this is also necessary for the PTB message to be
passed to the corresponding PL.</t>
<t>The validation <bcp14>SHOULD</bcp14> utilize information that is no
t simple
for an off-path attacker to determine <xref target="BCP145"/>. For exa
mple, it could check the value of a
protocol header field known only to the two PL endpoints. A datagram
application that uses well-known source and destination ports ought
to also rely on other information to complete this validation.</t>
<t>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 <bcp14>MUST NOT</bcp14> be furth
er utilized by
the DPLPMTUD method, as discussed in the Security Considerations
section (<xref target="Security"/>).</t>
<t><xref target="validPTB_SIZE"/> describes this processing of
PTB messages.</t>
</section>
<section anchor="validPTB_SIZE">
<name>Use of PTB Messages</name>
<t>PTB messages that have been validated <bcp14>MAY</bcp14> be utilize
d by the
DPLPMTUD algorithm but <bcp14>MUST NOT</bcp14> be used directly to set
the PLPMTU.</t>
<t>Before using the size reported in the PTB message, it must first be
converted to a PL_PTB_SIZE. The PL_PTB_SIZE is smaller than the
PTB_SIZE because it is reduced by headers below the PL, including any
IP options or extensions added to the PL packet. </t>
<t>A method that utilizes these PTB messages can improve the speed at
which the algorithm detects an appropriate PLPMTU by triggering an
immediate probe for the PL_PTB_SIZE (resulting in a network-layer
packet of size PTB_SIZE), compared to one that relies solely on
probing using a timer-based search algorithm. </t>
<t>A set of checks are intended to provide protection from a router
that reports an unexpected PTB_SIZE. The PL also needs to check that
the indicated PL_PTB_SIZE is less than the size used by probe packets
and at least the minimum size accepted.</t>
<t> This section provides a summary of how PTB messages can be
utilized, using the set of constants defined in <xref target="Constants
"/>.
This processing depends on the PL_PTB_SIZE and the current value of a
set of variables: </t>
<dl newline="true">
<dt>PL_PTB_SIZE &lt; MIN_PLPMTU</dt>
<dd>
<ul>
<li>Invalid PL_PTB_SIZE, see <xref target="PTB"/>.</li>
<li>PTB message ought to be discarded without further
processing (i.e., PLPMTU is not modified).</li>
<li>The information could be utilized as an input that
triggers the enabling of a resilience mode (see <xref target="
Resilience"/>).</li>
</ul>
</dd>
<dt>MIN_PLPMTU &lt; PL_PTB_SIZE &lt; BASE_PLPMTU</dt>
<dd>
<ul>
<li>A robust PL <bcp14>MAY</bcp14> enter an error state (see <xr
ef target="States"/>) for an IPv4 path when the PL_PTB_SIZE
reported in the PTB message is larger than or equal to 68
bytes <xref target="RFC0791"/> and when this is less
than the BASE_PLPMTU.</li>
<li>A robust PL <bcp14>MAY</bcp14> enter an error state (see <xr
ef target="States"/>) for an IPv6 path when the PL_PTB_SIZE
reported in the PTB message is larger than or equal to 1280
bytes <xref target="RFC8200"/> and when this is less
than the BASE_PLPMTU.</li>
</ul>
</dd>
<dt>BASE_PLPMTU &lt;= PL_PTB_SIZE &lt; PLPMTU</dt>
<dd>
<ul>
<li>This could be an indication of a black hole. The PLPMTU
<bcp14>SHOULD</bcp14> be set to BASE_PLPMTU (the PLPMTU is red
uced to the
BASE_PLPMTU to avoid unnecessary packet loss when a black hole
is encountered).</li>
<li> The PL ought to start a search to quickly discover the
new PLPMTU. The PL_PTB_SIZE reported in the PTB message can be
used to initialize a search algorithm.</li>
</ul>
</dd>
<dt>PLPMTU &lt; PL_PTB_SIZE &lt; PROBED_SIZE</dt>
<dd>
<ul>
<li>The PLPMTU continues to be valid, but the size of a
packet used to search (PROBED_SIZE) was larger than the
actual PMTU.</li>
<li>The PLPMTU is not updated.</li>
<li>The PL can use the reported PL_PTB_SIZE from the PTB
message as the next search point when it resumes the search
algorithm.</li>
</ul>
</dd>
<dt>PL_PTB_SIZE &gt;= PROBED_SIZE</dt>
<dd>
<ul>
<li>Inconsistent network signal.</li>
<li>PTB message ought to be discarded without further
processing (i.e., PLPMTU is not modified).</li>
<li>The information could be utilized as an input to trigger
the enabling of a resilience mode.</li>
</ul>
</dd>
</dl>
</section>
</section>
</section>
<section anchor="Spec">
<name>Datagram Packetization Layer PMTUD</name>
<t>This section specifies Datagram PLPMTUD (DPLPMTUD). The method can be
introduced at various points (as indicated with * in <xref target="fig-plp
mtudimplement" format="default"/>)
in the IP protocol stack to discover the PLPMTU so that an application
can utilize an appropriate MPS for the current network path. </t>
<t>DPLPMTUD <bcp14>SHOULD</bcp14> only be performed at one layer between a
pair of
endpoints. Therefore, an upper PL or application should avoid using
DPLPMTUD when this is already enabled in a lower layer. A PL <bcp14>MUST</
bcp14> adjust
the MPS indicated by DPLPMTUD to account for any additional overhead
introduced by the PL.</t>
<figure anchor="fig-plpmtudimplement">
<name>Examples Where DPLPMTUD Can Be Implemented</name>
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<artwork type="ascii-art" name="" alt="" originalSrc="diagrams/dplpmtu
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+----------------------+
| Application* |
+-----+------------+---+
| |
+---+--+ +--+--+
| QUIC*| |SCTP*|
+---+--+ +-+-+-+
| | |
+---+ +----+ |
| | |
+-+--+-+ |
| UDP | |
+---+--+ |
| |
+-----------+-------+--+
| Network Interface |
+----------------------+
]]></artwork>
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</figure>
<t>The central idea of DPLPMTUD is probing by a sender. Probe packets
are sent to find the maximum size of user message that can be
completely transferred across the network path from the sender to the
destination.</t>
<t>The following sections identify the components needed for
implementation, provide an overview of the phases of operation, and
specify the state machine and search algorithm.</t>
<section anchor="dplpmtud-components">
<name>DPLPMTUD Components</name>
<t>This section describes the timers, constants, and variables of
DPLPMTUD.</t>
<section anchor="Timers">
<name>Timers</name>
<t>The method utilizes up to three timers: </t>
<dl>
<dt>PROBE_TIMER:</dt>
<dd>
<t>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 <bcp14>MUST NOT</bcp14> be smaller than
1 second
and <bcp14>SHOULD</bcp14> be larger than 15 seconds. Guidance on
the selection of
the timer value is provided in Section
<xref target="BCP145" section="3.1.1" sectionFormat="bare" forma
t="default"/>
of the UDP Usage Guidelines <xref target="BCP145"/>.</t>
</dd>
<dt>PMTU_RAISE_TIMER:</dt>
<dd>
<t>The PMTU_RAISE_TIMER is configured to the period a sender
will continue to use the current PLPMTU, after which it
reenters the Search Phase. This timer has a period of 600
seconds, as recommended by PLPMTUD <xref target="RFC4821"/>.</t>
<t>DPLPMTUD <bcp14>MAY</bcp14> inhibit sending probe packets when
no
application data has been sent since the previous probe
packet. A PL preferring to use an up-to-date PMTU once user
data is sent again can choose to continue PMTU discovery for
each path. However, this will result in sending additional
packets.</t>
</dd>
<dt>CONFIRMATION_TIMER:</dt>
<dd>
<t>When an acknowledged PL is used, this timer <bcp14>MUST NOT</bc
p14> 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
<xref target="BCP145" section="3.1.1" sectionFormat="bare" forma
t="default"/>
of the UDP Usage Guidelines <xref target="BCP145"/>.</t>
<t>DPLPMTUD <bcp14>MAY</bcp14> inhibit sending probe packets when
no
application data has been sent since the previous probe
packet. A PL preferring to use an up-to-date PMTU once user
data is sent again, can choose to continue PMTU discovery for
each path. However, this could result in sending additional
packets.</t>
</dd>
</dl>
<t>DPLPMTUD specifies various timers; however, an implementation could
choose to realize these timer functions using a single timer.</t>
</section>
<section anchor="Constants">
<name>Constants</name>
<t>The following constants are defined: </t>
<dl>
<dt>MAX_PROBES:</dt>
<dd>The MAX_PROBES is the maximum value of the PROBE_COUNT
counter (see <xref target="Variables"/>). MAX_PROBES represents
the limit for the number of consecutive probe attempts of any
size. Search algorithms benefit from a MAX_PROBES value greater
than 1 because this can provide robustness to isolated packet
loss. The default value of MAX_PROBES is 3.</dd>
<dt>MIN_PLPMTU:</dt>
<dd>The MIN_PLPMTU is the smallest size of PLPMTU that DPLPMTUD
will attempt to use. An endpoint could need to configure the
MIN_PLPMTU to provide space for extension headers and other
encapsulations at layers below the PL. This value can be
interface and path dependent. For IPv6, this size is greater
than or equal to the size at the PL that results in an 1280-byte
IPv6 packet, as specified in <xref target="RFC8200"/>. For
IPv4, this size is greater than or equal to the size at the PL
that results in an 68-byte IPv4 packet. Note: An IPv4 router is
required to be able to forward a datagram of 68 bytes without
further fragmentation. This is the combined size of an IPv4
header and the minimum fragment size of 8 bytes. In addition,
receivers are required to be able to reassemble fragmented
datagrams at least up to 576 bytes, as stated in
<xref target="RFC1122" section="3.3.3" sectionFormat="of" format="d
efault"/>. </dd>
<dt>MAX_PLPMTU:</dt>
<dd>The MAX_PLPMTU is the largest size of PLPMTU. This has to be
less than or equal to the maximum size of the PL packet that can
be sent on the outgoing interface (constrained by the local
interface MTU). When known, this also ought to be less than the
maximum size of PL packet that can be received by the remote
endpoint (constrained by EMTU_R). It can be limited by the design
or configuration of the PL being used. An application, or PL,
<bcp14>MAY</bcp14> choose a smaller MAX_PLPMTU when there is no ne
ed to send
packets larger than a specific size.</dd>
<dt>BASE_PLPMTU:</dt>
<dd>The BASE_PLPMTU is a configured size expected to work for
most paths. The size is equal to or larger than the MIN_PLPMTU
and smaller than the MAX_PLPMTU. For most PLs, a suitable
BASE_PLPMTU will be larger than 1200 bytes. When using IPv4,
there is no currently equivalent size specified, and a default
BASE_PLPMTU of 1200 bytes is <bcp14>RECOMMENDED</bcp14>.</dd>
</dl>
</section>
<section anchor="Variables">
<name>Variables</name>
<t>This method utilizes a set of variables: </t>
<dl>
<dt>PROBED_SIZE:</dt>
<dd>The PROBED_SIZE is the size of the current probe packet as
determined at the PL. This is a tentative value for the PLPMTU,
which is awaiting confirmation by an acknowledgment.</dd>
<dt>PROBE_COUNT:</dt>
<dd>The PROBE_COUNT is a count of the number of successive
unsuccessful probe packets that have been sent. Each time a probe
packet is acknowledged, the value is set to zero. (Some probe
loss is expected while searching, therefore loss of a single
probe is not an indication of a PMTU problem.) </dd>
</dl>
<t><xref target="fig-mps" format="default"/> illustrates the relations
hip 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
DPLPMTUD algorithm to further increase PROBED_SIZE toward sending a pr
obe
with the size of the actual PMTU.</t>
<figure anchor="fig-mps">
<name>Relationships between Packet Size Constants and Variables</nam
e>
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<artwork type="ascii-art" name="" alt="" originalSrc="diagrams/pac
ket-sizes-relationships.txt"><![CDATA[
MIN_PLPMTU MAX_PLPMTU
<------------------------------------------->
| | |
v | |
BASE_PLPMTU | v
| PROBED_SIZE
v
PLPMTU
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</figure>
</section>
<section anchor="phases">
<name>Overview of DPLPMTUD Phases</name>
<t>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, <xref target="States"/>.</t>
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<name>DPLPMTUD Phases</name>
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<artwork type="ascii-art" name="" alt="" originalSrc="diagrams/dpl
pmtud-phases.txt"><![CDATA[
+------+
+------->| Base |-----------------+ Connectivity
| +------+ | or BASE_PLPMTU
| | | confirmation failed
| | v
| | Connectivity +-------+
| | and BASE_PLPMTU | Error |
| | confirmed +-------+
| | | Consistent
| v | connectivity
Black Hole | +--------+ | and BASE_PLPMTU
detected | | Search |<---------------+ confirmed
| +--------+
| ^ |
| | |
| Raise | | Search
| timer | | algorithm
| expired | | completed
| | |
| | v
| +-----------------+
+---| Search Complete |
+-----------------+
]]></artwork>
</artset>
</figure>
<dl>
<dt>Base:</dt>
<dd>
<t>The Base Phase confirms connectivity to the remote peer
using packets of the BASE_PLPMTU. The confirmation of
connectivity is implicit for a connection-oriented PL (where it
can be performed in a PL connection handshake). A
connectionless PL sends a probe packet and uses acknowledgment
of this probe packet to confirm that the remote peer is
reachable. </t>
<t>The sender also confirms that BASE_PLPMTU is supported across
the network path. This may be achieved by using a PL mechanism
(e.g., using a handshake packet of size BASE_PLPMTU) or by
sending a probe packet of size BASE_PLPMTU and confirming that
this is received.</t>
<t>A probe packet of size BASE_PLPMTU can be sent immediately
on the initial entry to the Base Phase (following a
connectivity check). A PL that does not wish to support a path
with a PLPMTU less than BASE_PLPMTU can simplify the phase into
a single step by performing the connectivity checks with a
probe of the BASE_PLPMTU size.</t>
<t>Once confirmed, DPLPMTUD enters the Search Phase. If the
Base Phase fails to confirm the BASE_PLPMTU, DPLPMTUD enters
the Error Phase.</t>
</dd>
<dt>Search:</dt>
<dd>
<t>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.</t>
<t>A PL could respond to PTB messages using the PTB to advance
or terminate the search, see <xref target="mechanism-ptb"/>.</t>
</dd>
<dt>Search Complete:</dt>
<dd>
<t>The Search Complete Phase is entered when the PLPMTU is
supported across the network path. A PL can use a
CONFIRMATION_TIMER to periodically repeat a probe packet for
the current PLPMTU size. If the sender is unable to confirm
reachability (e.g., if the CONFIRMATION_TIMER expires) or the
PL signals a lack of reachability, a black hole has been
detected and DPLPMTUD enters the Base Phase. </t>
<t>The PMTU_RAISE_TIMER is used to periodically resume the
Search Phase to discover if the PLPMTU can be raised. Black
hole detection causes the sender to enter the Base Phase.</t>
</dd>
<dt>Error:</dt>
<dd>
<t>The Error Phase is entered when there is conflicting or
invalid PLPMTU information for the path (e.g., a failure to
support the BASE_PLPMTU) that causes DPLPMTUD to be unable to
progress, and the PLPMTU is lowered.</t>
<t>DPLPMTUD remains in the Error Phase until a consistent view
of the path can be discovered and it has also been confirmed
that the path supports the BASE_PLPMTU (or DPLPMTUD is
suspended).</t>
</dd>
</dl>
<t> A method 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.</t>
<t>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
failover decision).</t>
</section>
</section>
<section anchor="States">
<name>State Machine</name>
<t>A state machine for DPLPMTUD is depicted in <xref target="fig-states"
/>. If multipath or multihoming is supported,
a state machine is needed for each path.</t>
<t>Note: Not all changes are shown to simplify the
diagram.</t>
<figure anchor="fig-states">
<name>State Machine for Datagram PLPMTUD</name>
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36" fill="black" font-size="1em">E</text>
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532" fill="black" font-size="1em">C</text>
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116" fill="black" font-size="1em">U</text>
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308" fill="black" font-size="1em">T</text>
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612" fill="black" font-size="1em">I</text>
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596" fill="black" font-size="1em">P</text>
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100" fill="black" font-size="1em">O</text>
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100" fill="black" font-size="1em">I</text>
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116" fill="black" font-size="1em">o</text>
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132" fill="black" font-size="1em">A</text>
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180" fill="black" font-size="1em">L</text>
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180" fill="black" font-size="1em">o</text>
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580" fill="black" font-size="1em">E</text>
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132" fill="black" font-size="1em">I</text>
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260" fill="black" font-size="1em">e</text>
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116" fill="black" font-size="1em">d</text>
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308" fill="black" font-size="1em">B</text>
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324" fill="black" font-size="1em">N</text>
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324" fill="black" font-size="1em">T</text>
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436" fill="black" font-size="1em">G</text>
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596" fill="black" font-size="1em">P</text>
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116" fill="black" font-size="1em">i</text>
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132" fill="black" font-size="1em">P</text>
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260" fill="black" font-size="1em">t</text>
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564" fill="black" font-size="1em">E</text>
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580" fill="black" font-size="1em">o</text>
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612" fill="black" font-size="1em">I</text>
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436" fill="black" font-size="1em">T</text>
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532" fill="black" font-size="1em">r</text>
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548" fill="black" font-size="1em">U</text>
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260" fill="black" font-size="1em">k</text>
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324" fill="black" font-size="1em">B</text>
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356" fill="black" font-size="1em">E</text>
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564" fill="black" font-size="1em">_</text>
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580" fill="black" font-size="1em">R</text>
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100" fill="black" font-size="1em">P</text>
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532" fill="black" font-size="1em">o</text>
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20" fill="black" font-size="1em">o</text>
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100" fill="black" font-size="1em">E</text>
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132" fill="black" font-size="1em">i</text>
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132" fill="black" font-size="1em">B</text>
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212" fill="black" font-size="1em">E</text>
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532" fill="black" font-size="1em">=</text>
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132" fill="black" font-size="1em">S</text>
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596" fill="black" font-size="1em">o</text>
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580" fill="black" font-size="1em">_</text>
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132" fill="black" font-size="1em">P</text>
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132" fill="black" font-size="1em">P</text>
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532" fill="black" font-size="1em">B</text>
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548" fill="black" font-size="1em">L</text>
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116" fill="black" font-size="1em">e</text>
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60" fill="black" font-size="1em">l</text>
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436" fill="black" font-size="1em">E</text>
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532" fill="black" font-size="1em">i</text>
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580" fill="black" font-size="1em">O</text>
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564" fill="black" font-size="1em">p</text>
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36" fill="black" font-size="1em">v</text>
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132" fill="black" font-size="1em">L</text>
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308" fill="black" font-size="1em">R</text>
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532" fill="black" font-size="1em">B</text>
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548" fill="black" font-size="1em">M</text>
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84" fill="black" font-size="1em">O</text>
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436" fill="black" font-size="1em">A</text>
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564" fill="black" font-size="1em">R</text>
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116" fill="black" font-size="1em">C</text>
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532" fill="black" font-size="1em">M</text>
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516" fill="black" font-size="1em">k</text>
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564" fill="black" font-size="1em">R</text>
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548" fill="black" font-size="1em">P</text>
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564" fill="black" font-size="1em">E</text>
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612" fill="black" font-size="1em">_</text>
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132" fill="black" font-size="1em">y</text>
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20" fill="black" font-size="1em">i</text>
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212" fill="black" font-size="1em">B</text>
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436" fill="black" font-size="1em">S</text>
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196" fill="black" font-size="1em">a</text>
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356" fill="black" font-size="1em">I</text>
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100" fill="black" font-size="1em">B</text>
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564" fill="black" font-size="1em">T</text>
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564" fill="black" font-size="1em">:</text>
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116" fill="black" font-size="1em">c</text>
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532" fill="black" font-size="1em">S</text>
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612" fill="black" font-size="1em">Z</text>
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100" fill="black" font-size="1em">e</text>
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132" fill="black" font-size="1em">E</text>
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580" fill="black" font-size="1em">E</text>
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596" fill="black" font-size="1em">a</text>
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596" fill="black" font-size="1em">T</text>
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132" fill="black" font-size="1em">v</text>
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532" fill="black" font-size="1em">O</text>
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580" fill="black" font-size="1em">E</text>
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36" fill="black" font-size="1em">n</text>
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100" fill="black" font-size="1em">y</text>
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132" fill="black" font-size="1em">Z</text>
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564" fill="black" font-size="1em">x</text>
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580" fill="black" font-size="1em">P</text>
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260" fill="black" font-size="1em">B</text>
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308" fill="black" font-size="1em">p</text>
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516" fill="black" font-size="1em">X</text>
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548" fill="black" font-size="1em">S</text>
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564" fill="black" font-size="1em">I</text>
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580" fill="black" font-size="1em">O</text>
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116" fill="black" font-size="1em">E</text>
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356" fill="black" font-size="1em">x</text>
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580" fill="black" font-size="1em">S</text>
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596" fill="black" font-size="1em">k</text>
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116" fill="black" font-size="1em">O</text>
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324" fill="black" font-size="1em">P</text>
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356" fill="black" font-size="1em">I</text>
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132" fill="black" font-size="1em">T</text>
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308" fill="black" font-size="1em">e</text>
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324" fill="black" font-size="1em">M</text>
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356" fill="black" font-size="1em">i</text>
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516" fill="black" font-size="1em">A</text>
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596" fill="black" font-size="1em">c</text>
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36" fill="black" font-size="1em">y</text>
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100" fill="black" font-size="1em">r</text>
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116" fill="black" font-size="1em">a</text>
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532" fill="black" font-size="1em">P</text>
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180" fill="black" font-size="1em">M</text>
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532" fill="black" font-size="1em">_</text>
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580" fill="black" font-size="1em">P</text>
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212" fill="black" font-size="1em">A</text>
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532" fill="black" font-size="1em">p</text>
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532" fill="black" font-size="1em">M</text>
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532" fill="black" font-size="1em">A</text>
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564" fill="black" font-size="1em">i</text>
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324" fill="black" font-size="1em">O</text>
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324" fill="black" font-size="1em">X</text>
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436" fill="black" font-size="1em">E</text>
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516" fill="black" font-size="1em">M</text>
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516" fill="black" font-size="1em">P</text>
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548" fill="black" font-size="1em">B</text>
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564" fill="black" font-size="1em">e</text>
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612" fill="black" font-size="1em">T</text>
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132" fill="black" font-size="1em">S</text>
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308" fill="black" font-size="1em">x</text>
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516" fill="black" font-size="1em">U</text>
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20" fill="black" font-size="1em">P</text>
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36" fill="black" font-size="1em">C</text>
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516" fill="black" font-size="1em">P</text>
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308" fill="black" font-size="1em">y</text>
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580" fill="black" font-size="1em">A</text>
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596" fill="black" font-size="1em">d</text>
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260" fill="black" font-size="1em">t</text>
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548" fill="black" font-size="1em">B</text>
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64" fill="black" font-size="1em">I</text>
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196" fill="black" font-size="1em">d</text>
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260" fill="black" font-size="1em">c</text>
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100" fill="black" font-size="1em">R</text>
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116" fill="black" font-size="1em">X</text>
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132" fill="black" font-size="1em">i</text>
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260" fill="black" font-size="1em">c</text>
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260" fill="black" font-size="1em">d</text>
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532" fill="black" font-size="1em">I</text>
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64" fill="black" font-size="1em">N</text>
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596" fill="black" font-size="1em">e</text>
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20" fill="black" font-size="1em">s</text>
<text text-anchor="middle" font-family="monospace" x="208" y="
260" fill="black" font-size="1em">e</text>
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516" fill="black" font-size="1em">T</text>
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4" fill="black" font-size="1em">L</text>
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132" fill="black" font-size="1em">U</text>
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80" fill="black" font-size="1em">O</text>
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132" fill="black" font-size="1em">&lt;</text>
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548" fill="black" font-size="1em">P</text>
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548" fill="black" font-size="1em">T</text>
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548" fill="black" font-size="1em">_</text>
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580" fill="black" font-size="1em">M</text>
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116" fill="black" font-size="1em">E</text>
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356" fill="black" font-size="1em">_</text>
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580" fill="black" font-size="1em">B</text>
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132" fill="black" font-size="1em">B</text>
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580" fill="black" font-size="1em">O</text>
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612" fill="black" font-size="1em">R</text>
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564" fill="black" font-size="1em">M</text>
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564" fill="black" font-size="1em">x</text>
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596" fill="black" font-size="1em">e</text>
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324" fill="black" font-size="1em">_</text>
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260" fill="black" font-size="1em">c</text>
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516" fill="black" font-size="1em">d</text>
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532" fill="black" font-size="1em">y</text>
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612" fill="black" font-size="1em">T</text>
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116" fill="black" font-size="1em">S</text>
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260" fill="black" font-size="1em">e</text>
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116" fill="black" font-size="1em">O</text>
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180" fill="black" font-size="1em">A</text>
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564" fill="black" font-size="1em">M</text>
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564" fill="black" font-size="1em">i</text>
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564" fill="black" font-size="1em">R</text>
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564" fill="black" font-size="1em">y</text>
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260" fill="black" font-size="1em">e</text>
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532" fill="black" font-size="1em">R</text>
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356" fill="black" font-size="1em">e</text>
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612" fill="black" font-size="1em">M</text>
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</artwork>
<artwork type="ascii-art" name="" alt="" originalSrc="diagrams/dplpm
tud-statemachine.txt"><![CDATA[
| |
| Start | PL indicates loss
| | of connectivity
v v
+---------------+ +---------------+
| DISABLED | | ERROR |
+---------------+ PROBE_TIMER expiry: +---------------+
| PL indicates PROBE_COUNT = MAX_PROBES or ^ |
| connectivity PTB: PL_PTB_SIZE < BASE_PLPMTU | |
+--------------------+ +------------------+ |
| | |
v | BASE_PLPMTU Probe |
+---------------+ acked |
| BASE |--------------------->+
+---------------+ |
^ | ^ ^ |
Black hole detected | | | | Black hole detected |
+--------------------+ | | +--------------------+ |
| +----+ | |
| PROBE_TIMER expiry: | |
| PROBE_COUNT < MAX_PROBES | |
| | |
| PMTU_RAISE_TIMER expiry | |
| +-----------------------------------------+ | |
| | | | |
| | v | v
+---------------+ +---------------+
|SEARCH_COMPLETE| | SEARCHING |
+---------------+ +---------------+
| ^ ^ | | ^
| | | | | |
| | +-----------------------------------------+ | |
| | MAX_PLPMTU Probe acked or | |
| | PROBE_TIMER expiry: PROBE_COUNT = MAX_PROBES or | |
+----+ PTB: PL_PTB_SIZE = PLPMTU +----+
CONFIRMATION_TIMER expiry: PROBE_TIMER expiry:
PROBE_COUNT < MAX_PROBES or PROBE_COUNT < MAX_PROBES or
PLPMTU Probe acked Probe acked or PTB:
PLPMTU < PL_PTB_SIZE < PROBED_SIZE
]]></artwork>
</artset>
</figure>
<t/>
<t>The following states are defined:</t>
<dl>
<dt>DISABLED:</dt>
<dd>The DISABLED state is the initial state before probing has
started. It is also entered from any other state, when the PL
indicates loss of connectivity. This state is left once the PL
indicates connectivity to the remote PL. When transitioning to the
BASE state, a probe packet of size BASE_PLPMTU can be sent
immediately.</dd>
<dt>BASE:</dt>
<dd>
<t>The BASE state is used to confirm that the BASE_PLPMTU size is
supported by the network path and is designed to allow an
application to continue working when there are transient
reductions in the actual PMTU. It also seeks to avoid long
periods when a sender searching for a larger PLPMTU is unaware
that packets are not being delivered due to a packet or ICMP
black hole.</t>
<t>On entry, the PROBED_SIZE is set to the BASE_PLPMTU size, and
the PROBE_COUNT is set to zero.</t>
<t>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.
</t>
<t>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.</t>
</dd>
<dt>SEARCHING:</dt>
<dd>
<t>The SEARCHING state is the main probing state. This state is
entered when probing for the BASE_PLPMTU completes. </t>
<t>Each time a probe packet is acknowledged, the PROBE_COUNT is
set to zero, the PLPMTU is set to the PROBED_SIZE, and then the
PROBED_SIZE is increased using the search algorithm (as described
in <xref target="searchincrease"/>).</t>
<t>When a probe packet is sent and not acknowledged within the
period of the PROBE_TIMER, the PROBE_COUNT is incremented, and
a new probe packet is transmitted. </t>
<t> The state is exited to enter SEARCH_COMPLETE when the
PROBE_COUNT reaches MAX_PROBES, a validated PTB is received that
corresponds to the last successfully probed size (PL_PTB_SIZE =
PLPMTU), or a probe of size MAX_PLPMTU is acknowledged (PLPMTU =
MAX_PLPMTU).</t>
<t> When a black hole is detected in the SEARCHING state, this cause
s
the PL sender to enter the BASE state. </t>
</dd>
<dt>SEARCH_COMPLETE:</dt>
<dd>
<t> The SEARCH_COMPLETE state indicates that a search has
completed. This is the normal maintenance state, where the PL is
not probing to update the PLPMTU. DPLPMTUD remains in this state
until either the PMTU_RAISE_TIMER expires or a black hole is
detected. </t>
<t>When DPLPMTUD uses an unacknowledged PL and is in the
SEARCH_COMPLETE state, a CONFIRMATION_TIMER periodically resets
the PROBE_COUNT and schedules a probe packet with the size of the
PLPMTU. If MAX_PROBES successive PLPMTUD-sized probes fail to be
acknowledged, the method enters the BASE state. When used with an
acknowledged PL (e.g., SCTP), DPLPMTUD <bcp14>SHOULD NOT</bcp14> c
ontinue to
generate PLPMTU probes in this state.</t>
</dd>
<dt>ERROR:</dt>
<dd>
<t>The ERROR state represents the case where either the network
path is not known to support a PLPMTU of at least the BASE_PLPMTU
size or when there is contradictory information about the network
path that would otherwise result in excessive variation in the
MPS signaled to the higher layer. The state implements a method
to mitigate oscillation in the state-event engine. It signals a
conservative value of the MPS to the higher layer by the PL. The
state is exited when packet probes no longer detect the error.
The PL sender then enters the SEARCHING state. </t>
<t>Implementations are permitted to enable endpoint
fragmentation if the DPLPMTUD is unable to validate MIN_PLPMTU
within PROBE_COUNT probes. If DPLPMTUD is unable to validate
MIN_PLPMTU, the implementation will transition to the DISABLED
state.</t>
<t>Note: MIN_PLPMTU could be identical to BASE_PLPMTU,
simplifying the actions in this state.</t>
</dd>
</dl>
</section>
<section anchor="searchincrease">
<name>Search to Increase the PLPMTU</name>
<t>This section describes the algorithms used by DPLPMTUD to search
for a larger PLPMTU.</t>
<section anchor="Increase">
<name>Probing for a Larger PLPMTU</name>
<t>Implementations use a search algorithm across the search range to
determine whether a larger PLPMTU can be supported across a network
path.</t>
<t>The method discovers the search range by confirming the minimum
PLPMTU and then using the probe method to select a PROBED_SIZE less
than or equal to MAX_PLPMTU. MAX_PLPMTU is the minimum of the local
MTU and EMTU_R (when this is learned from the remote endpoint). The
MAX_PLPMTU <bcp14>MAY</bcp14> be reduced by an application that sets a
maximum to
the size of datagrams it will send.</t>
<t> The PROBE_COUNT is initialized to zero when the first probe with
a size greater than or equal to PLPMTU is sent. Each probe packet
successfully sent to the remote peer is confirmed by acknowledgment
at the PL (see <xref target="Probe"/>).</t>
<t>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 (<xref target="Probe"/>). 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.</t>
<t>If the timer expires before a probe packet is acknowledged, the
probe has failed to confirm the PROBED_SIZE. Each time the
PROBE_TIMER expires, the PROBE_COUNT is incremented, the PROBE_TIMER
is reinitialized, and a new probe of the same size or any other size
(determined by the search algorithm) can be sent. The maximum number
of consecutive failed probes is configured (MAX_PROBES). If the
value of the PROBE_COUNT reaches MAX_PROBES, probing will stop, and
the PL sender enters the SEARCH_COMPLETE state. </t>
</section>
<section>
<name>Selection of Probe Sizes</name>
<t>The search algorithm determines a minimum useful gain in
PLPMTU. It would not be constructive for a PL sender to attempt to
probe for all sizes. This would incur unnecessary load on the path.
Implementations <bcp14>SHOULD</bcp14> select the set of probe packet
sizes to maximize the gain in PLPMTU from each search step.</t>
<t>Implementations could optimize the search procedure by selecting
step sizes from a table of common PMTU sizes. When selecting the
appropriate next size to search, an implementer ought to also
consider that there can be common sizes of MPS that applications
seek to use, and there could be common sizes of MTU used within the
network.</t>
</section>
<section anchor="Resilience">
<name>Resilience to Inconsistent Path Information</name>
<t>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.</t>
<t>A PL sender is able to detect inconsistency either from the sequenc
e of
PLPMTU probes that are acknowledged or from the sequence of PTB messag
es 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.</t>
</section>
</section>
<section anchor="robustness">
<name>Robustness to Inconsistent Paths</name>
<t>Some paths could be unable to sustain packets of the BASE_PLPMTU
size. The Error State could be implemented to provide robustness to
such paths. 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_PLPMTU.</t>
</section>
</section>
<section anchor="protocol_specific_methods">
<name>Specification of Protocol-Specific Methods</name>
<t>DPLPMTUD requires protocol-specific details to be specified for each
PL that is used.</t>
<t>The first subsection provides guidance on how to implement the
DPLPMTUD method as a part of an application using UDP or UDP-Lite. The
guidance also applies to other datagram services that do not include a
specific transport protocol (such as a tunnel encapsulation). The
following subsections describe how DPLPMTUD can be implemented as a part
of the transport service, allowing applications using the service to
benefit from discovery of the PLPMTU without themselves needing to
implement this method when using SCTP and QUIC.</t>
<section>
<name>Application Support for DPLPMTUD with UDP or UDP-Lite</name>
<t>The current specifications of UDP <xref target="RFC0768"/>
and UDP-Lite <xref target="RFC3828"/> do not define a method in
the RFC series that supports PLPMTUD. In particular, the UDP transport
does not provide the transport features needed to implement
datagram PLPMTUD.</t>
<t>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 <xref target="BCP
145"/>.</t>
<t>Some primitives used by DPLPMTUD might not be available via the
Datagram API (e.g., the ability to access the PLPMTU from the IP-layer
cache or to interpret received PTB messages).</t>
<t> In addition, it is recommended that PMTU discovery is not performed
by multiple protocol layers. An application <bcp14>SHOULD</bcp14> avoid u
sing
DPLPMTUD when the underlying transport system provides this capability.
A common method for managing the PLPMTU has benefits, both in the
ability to share state between different processes and in opportunities t
o
coordinate probing for different PL instances. </t>
<section anchor="UDP-REQ">
<name>Application Request</name>
<t>An application needs an application-layer protocol mechanism
(such as a message acknowledgment method) that solicits a response
from a destination endpoint. The method <bcp14>SHOULD</bcp14> allow th
e sender to
check the value returned in the response to provide additional
protection from off-path insertion of data <xref target="BCP145"/>. Su
itable methods include a parameter known
only to the two endpoints, such as a session ID or initialized
sequence number.</t>
</section>
<section anchor="UDP-Probe">
<name>Application Response</name>
<t>An application needs an application-layer protocol mechanism to
communicate the response from the destination endpoint. This
response could indicate successful reception of the probe across the
path but could also indicate that some (or all packets) have failed
to reach the destination.</t>
</section>
<section anchor="UDP-probing">
<name>Sending Application Probe Packets</name>
<t>A probe packet can carry an application data block, but the
successful transmission of this data is at risk when used for
probing. Some applications might prefer to use a probe packet that
does not carry an application data block to avoid disruption of data
transfer.</t>
</section>
<section>
<name>Initial Connectivity</name>
<t>An application that does not have other higher-layer information
confirming connectivity with the remote peer <bcp14>SHOULD</bcp14> imp
lement a
connectivity mechanism using acknowledged probe packets before
entering the BASE state.</t>
</section>
<section>
<name>Validating the Path</name>
<t>An application that does not have other higher-layer information
confirming correct delivery of datagrams <bcp14>SHOULD</bcp14> impleme
nt the
CONFIRMATION_TIMER to periodically send probe packets while in the
SEARCH_COMPLETE state.</t>
</section>
<section anchor="udpopt_ptb_handling">
<name>Handling of PTB Messages</name>
<t>An application that is able and wishes to receive PTB messages
<bcp14>MUST</bcp14> perform ICMP validation as specified in
<xref target="BCP145" section="5.2" sectionFormat="of" format="default
"/>. This requires that the application checks
each received PTB message to validate that it was is received in
response to transmitted traffic and that the reported PL_PTB_SIZE is
less than the current probed size (see <xref target="validPTB_SIZE"/>)
.
A validated PTB message <bcp14>MAY</bcp14> be used
as input to the DPLPMTUD algorithm but <bcp14>MUST NOT</bcp14> be used
directly to
set the PLPMTU.</t>
</section>
</section>
<section>
<name>DPLPMTUD for SCTP</name>
<t><xref target="RFC4821" section="10.2" sectionFormat="of" format="defa
ult"/>
specifies a recommended PLPMTUD probing method for SCTP, and
<xref target="RFC4960" section="7.3" sectionFormat="of" format="default"
/>
recommends an endpoint apply the
techniques in RFC 4821 on a per-destination-address basis. The
specification for DPLPMTUD continues the practice of using the PL to
discover the PMTU but updates RFC4960 with a recommendation to use
the method specified in this document: The <bcp14>RECOMMENDED</bcp14> me
thod for
generating probes is to add a chunk consisting only of padding to an
SCTP message. The PAD chunk defined in <xref target="RFC4820"/>
<bcp14>SHOULD</bcp14> be attached to a minimum-length HEARTBEAT (HB) chu
nk to build a
probe packet. This enables probing without affecting the transfer of
user messages and without being limited by congestion control or flow
control. This is preferred to using DATA chunks (with padding as
required) as path probes.</t>
<t><xref target="RFC4960" section="6.9" sectionFormat="of" format="defau
lt"/>
describes dividing the
user messages into DATA chunks sent by the PL when using SCTP. This notes
that once an SCTP message has been sent, it cannot be resegmented.
<xref target="RFC4960"/> describes the method for retransmitting DATA
chunks when the MPS has been reduced, and <xref target="RFC4960" section="6.9
" sectionFormat="of"/>
describes use of IP fragmentation for this case.
This is unchanged by this document.</t>
<section anchor="sctp_over_ip">
<name>SCTP/IPv4 and SCTP/IPv6</name>
<section>
<name>Initial Connectivity</name>
<t>The base protocol is specified in <xref target="RFC4960"/>. This
provides an acknowledged PL. A
sender can therefore enter the BASE state as soon as connectivity
has been confirmed. </t>
</section>
<section anchor="sctp_over_ip_probing">
<name>Sending SCTP Probe Packets</name>
<t>Probe packets consist of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
the length of the probe packet. The HEARTBEAT chunk is used to
trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful
probe. A successful probe updates the association and path
counters, but an unsuccessful probe is discounted (assumed
to be a result of choosing too large a PLPMTU).</t>
<t>The SCTP sender needs to be able to determine the total size of
a probe packet. The HEARTBEAT chunk could carry a Heartbeat
Information parameter that includes, besides the information
suggested in <xref target="RFC4960"/>, the probe size to help
an implementation associate a HEARTBEAT ACK with the size of probe
that was sent. The sender could also use other methods, such as
sending a nonce and verifying the information returned also
contains the corresponding nonce. The length of the PAD chunk is
computed by reducing the probing size by the size of the SCTP
common header and the HEARTBEAT chunk. The payload of the PAD
chunk contains arbitrary data. When transmitted at the IP layer,
the PMTU size also includes the IPv4 or IPv6 header(s).</t>
<t> Probing can start directly after the PL handshake; this can be
done before data is sent. Assuming this behavior (i.e., the PMTU
is smaller than or equal to the interface MTU), this process will
take several round-trip time periods, dependent on the number of
DPLPMTUD probes sent. The Heartbeat timer can be used to implement
the PROBE_TIMER.</t>
</section>
<section>
<name>Validating the Path with SCTP</name>
<t>Since SCTP provides an acknowledged PL, a sender <bcp14>MUST NOT<
/bcp14>
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
state.</t>
</section>
<section anchor="sctp_over_ip_ptb_handling">
<name>PTB Message Handling by SCTP</name>
<t>Normal ICMP validation <bcp14>MUST</bcp14> be performed as specif
ied in
<xref target="RFC4960" section="C" sectionFormat="of" format="defaul
t"/>.
This requires that the first 8 bytes of the SCTP common header are q
uoted in the
payload of the PTB message, which can be the case for ICMPv4 and
is normally the case for ICMPv6.</t>
<t>When a PTB message has been validated, the PL_PTB_SIZE
calculated from the PTB_SIZE reported in the PTB message <bcp14>SHOU
LD</bcp14> be
used with the DPLPMTUD algorithm, provided that the reported
PL_PTB_SIZE is less than the current probe size (see <xref target="m
echanism-ptb"/>).</t>
</section>
</section>
<section>
<name>DPLPMTUD for SCTP/UDP</name>
<t>The UDP encapsulation of SCTP is specified in <xref target="RFC6951
"/>.</t>
<t>This specification updates the reference to RFC 4821 in Section
<xref target="RFC6951" section="5.6" sectionFormat="bare" format="defa
ult"/>
of RFC 6951 to refer to this document (RFC 8899). RFC 6951 is updated
by the addition of the following sentence at the end of
Section <xref target="RFC6951" section="5.6" sectionFormat="bare" form
at="default"/>: </t>
<blockquote>
<t>The <bcp14>RECOMMENDED</bcp14> method for determining the MTU of
the
path is specified in RFC 8899.</t>
</blockquote>
<section>
<name>Initial Connectivity</name>
<t>A sender can enter the BASE state as soon as SCTP connectivity
has been confirmed.</t>
</section>
<section anchor="sctp_over_udp_probing">
<name>Sending SCTP/UDP Probe Packets</name>
<t>Packet probing can be performed as specified in <xref target="sct
p_over_ip_probing"/>. The size of the probe
packet includes the 8 bytes of UDP header. This has to be
considered when filling the probe packet with the PAD chunk. </t>
</section>
<section>
<name>Validating the Path with SCTP/UDP</name>
<t> SCTP provides an acknowledged PL; therefore, a sender does not
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
state. </t>
</section>
<section anchor="sctp_over_udp_ptb_handling">
<name>Handling of PTB Messages by SCTP/UDP</name>
<t>ICMP validation <bcp14>MUST</bcp14> be performed for PTB messages
as specified in
<xref target="RFC4960" section="C" sectionFormat="of" format="defaul
t"/>.
This requires that
the first 8 bytes of the SCTP common header are contained in the
PTB message, which can be the case for ICMPv4 (but note the UDP
header also consumes a part of the quoted packet header) and is
normally the case for ICMPv6. When the validation is completed, the
PL_PTB_SIZE calculated from the PTB_SIZE in the PTB message <bcp14>S
HOULD</bcp14>
be used with the DPLPMTUD providing that the reported PL_PTB_SIZE
is less than the current probe size.</t>
</section>
</section>
<section>
<name>DPLPMTUD for SCTP/DTLS</name>
<t>The Datagram Transport Layer Security (DTLS) encapsulation of SCTP
is specified in <xref target="RFC8261"/>. This is used for
data channels in WebRTC implementations. This specification updates
the reference to RFC 4821 in Section
<xref target="RFC8261" section="5" sectionFormat="bare" format="defaul
t"/>
of RFC 8261 to refer to this document (RFC 8899).
</t>
<section>
<name>Initial Connectivity</name>
<t>A sender can enter the BASE state as soon as SCTP connectivity
has been confirmed.</t>
</section>
<section anchor="sctp_over_dtls_probing">
<name>Sending SCTP/DTLS Probe Packets</name>
<t>Packet probing can be done as specified in <xref target="sctp_ove
r_ip_probing"/>. The maximum payload is
reduced by the size of the DTLS headers, which has to be considered
when filling the PAD chunk. The size of the probe packet includes
the DTLS PL headers. This has to be considered when filling the
probe packet with the PAD chunk. </t>
</section>
<section>
<name>Validating the Path with SCTP/DTLS</name>
<t>Since SCTP provides an acknowledged PL, a sender <bcp14>MUST NOT<
/bcp14>
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
state.</t>
</section>
<section anchor="sctp_over_dtls_ptb_handling">
<name>Handling of PTB Messages by SCTP/DTLS</name>
<t><xref target="RFC4960"/> does not specify a way to
validate SCTP/DTLS ICMP message payload and neither does this
document. This can prevent processing of PTB messages at the
PL.</t>
</section>
</section>
</section>
<section>
<name>DPLPMTUD for QUIC</name>
<t>QUIC <xref target="I-D.ietf-quic-transport"/> is a UDP-based
PL that provides reception feedback. The UDP payload includes a QUIC
packet header, a protected payload, and any authentication fields. It
supports padding and packet coalescence that can be used to construct
probe packets. From the perspective of DPLPMTUD, QUIC can function as
an acknowledged PL. <xref target="I-D.ietf-quic-transport"/>
describes the method for using DPLPMTUD with QUIC packets.</t>
</section>
</section>
<section anchor="IANA">
<name>IANA Considerations</name>
<t>This document has no IANA actions.</t>
</section>
<section anchor="Security">
<name>Security Considerations</name>
<t>The security considerations for the use of UDP and SCTP are provided
in the referenced RFCs.</t>
<t> To avoid excessive load, the interval between individual probe packets
<bcp14>MUST</bcp14> be at least one RTT, and the interval between rounds o
f probing is
determined by the PMTU_RAISE_TIMER. </t>
<t>A PL sender needs to ensure that the method used to confirm reception
of probe packets protects from off-path attackers injecting
packets into the path. This protection is provided in IETF-defined
protocols (e.g., TCP, SCTP) using a randomly initialized sequence
number. A description of one way to do this when using UDP is provided
in Section
<xref target="BCP145" section="5.1" sectionFormat="bare" format="default"/
>
of <xref target="BCP145"/>).</t>
<t>There are cases where ICMP Packet Too Big (PTB) messages are not
delivered due to policy, configuration, or equipment design
(see <xref target="Classic-PMTUD"/>). 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 <bcp14>MUST</bcp14> 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 <xref target="PTB"/>).</t>
<t>An on-path attacker able to create a PTB message could forge PTB
messages that include a valid quoted IP packet. Such an attack could be
used to drive down the PLPMTU. An on-path device could similarly force a
reduction of the PLPMTU by implementing a policy that drops packets
larger than a configured size. There are two ways this method can be
mitigated against such attacks: first, 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).</t>
<t>Parsing the quoted packet inside a PTB message can
introduce additional per-packet processing at the PL sender.
This processing <bcp14>SHOULD</bcp14> be limited to avoid a
denial-of-service attack when arbitrary headers are included. Rate-limitin
g
the processing could result in PTB messages not being
received by a PL; however, the DPLPMTUD method is robust to such
loss.</t>
<t>The successful processing of an ICMP message can trigger a probe
when the reported PTB size is valid, but this does not
directly update the PLPMTU for the path. This prevents a message
attempting to black hole data by indicating a size larger than
supported by the path.</t>
<t>It is possible that the information about a path is not stable. This
could be a result of forwarding across more than one path that has a
different actual PMTU or a single path presents a varying PMTU. The
design of a PLPMTUD implementation <bcp14>SHOULD</bcp14> consider how to m
itigate the
effects of varying path information. One possible mitigation is to
provide robustness (see <xref target="robustness"/>) in the method
that avoids oscillation in the MPS.</t>
<t>
DPLPMTUD methods can introduce padding data to inflate the length
of the datagram to the total size required for a probe packet. The
total size of a probe packet includes all headers and padding added
to the payload data being sent (e.g., including security-related
fields such as an AEAD tag and TLS record layer padding). The value
of the padding data does not influence the DPLPMTUD search algorithm,
and therefore needs to be set consistent with the policy of the PL.
</t>
<t> If a PL can make use of cryptographic confidentiality or
data-integrity mechanisms, then the design ought to avoid adding anything
(e.g., padding) to DPLPMTUD probe packets that is not also protected by
those cryptographic mechanisms. </t>
</section>
</middle>
<back>
<displayreference target="I-D.ietf-intarea-tunnels" to="TUNNELS"/>
<displayreference target="I-D.ietf-quic-transport" to="QUIC"/>
<references>
<name>References</name>
<references>
<name>Normative References</name>
<reference anchor='BCP145' target='https://www.rfc-editor.org/info/bcp145'>
<front>
<title>UDP Usage Guidelines</title>
<author initials='L.' surname='Eggert' fullname='L. Eggert'><organization />
</author>
<author initials='G.' surname='Fairhurst' fullname='G. Fairhurst'><organizat
ion /></author>
<author initials='G.' surname='Shepherd' fullname='G. Shepherd'><organizatio
n /></author>
<date year='2017' month='March' />
</front>
<seriesInfo name='BCP' value='145'/>
<seriesInfo name='RFC' value='8085'/>
</reference>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.0768.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.0791.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.1191.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.3828.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.4820.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.4960.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.6951.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.8174.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.8200.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.8201.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.8261.xml"/>
</references>
<references>
<name>Informative References</name>
<reference anchor='RFC8900' target='https://www.rfc-editor.org/info/rfc8900'>
<front>
<title>IP Fragmentation Considered Fragile</title>
<author initials='R' surname='Bonica' fullname='Ron Bonica'>
<organization />
</author>
<author initials='F' surname='Baker' fullname='Fred Baker'>
<organization />
</author>
<author initials='G' surname='Huston' fullname='Geoff Huston'>
<organization />
</author>
<author initials='R' surname='Hinden' fullname='Robert M. Hinden'>
<organization />
</author>
<author initials='O' surname='Troan' fullname='Ole Troan'>
<organization />
</author>
<author initials='F' surname='Gont' fullname='Fernando Gont'>
<organization />
</author>
<date month='September' year='2020' />
</front>
<seriesInfo name="RFC" value="8900"/>
<seriesInfo name="BCP" value="230"/>
</reference>
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D
.draft-ietf-intarea-tunnels.xml"/>
<reference anchor="I-D.ietf-quic-transport">
<front>
<title>QUIC: A UDP-Based Multiplexed and Secure Transport</title>
<author initials="J" surname="Iyengar" fullname="Jana Iyengar" role="editor">
<organization/>
</author>
<author initials="M" surname="Thomson" fullname="Martin Thomson" role="editor">
<organization/>
</author>
<date month="June" day="10" year="2020"/>
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-quic-transport-29"/>
</reference>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.0792.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.1122.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.1812.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.2923.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.4340.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.4443.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.4821.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.4890.xml"/>
<xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/refer
ence.RFC.5508.xml"/>
</references>
</references>
<section anchor="Acknowledgments" numbered="false">
<name>Acknowledgments</name>
<t>This work was partially funded by the European Union Horizon 2020
Research and Innovation Programme under grant agreement No. 644334,
"A New, Evolutive API and Transport-Layer Architecture for the Internet"
(NEAT). The views expressed are solely those of the author(s).</t>
<t>
Thanks to all who have commented or contributed, the TSVWG and QUIC
working groups, and <contact fullname="Mathew Calder"/> and
<contact fullname="Julius Flohr"/> for providing early
implementations.
</t>
</section>
</back>
</rfc>
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