Packetization Layer Path MTU Discovery for Datagram TransportsUniversity of AberdeenSchool of EngineeringFraser Noble BuildingAberdeenAB24 3UEUKgorry@erg.abdn.ac.ukUniversity of AberdeenSchool of EngineeringFraser Noble BuildingAberdeenAB24 3UEUKtom@erg.abdn.ac.ukMuenster University of Applied SciencesStegerwaldstrasse 3948565SteinfurtDEtuexen@fh-muenster.deMuenster University of Applied SciencesStegerwaldstrasse 3948565SteinfurtDEi.ruengeler@fh-muenster.deMuenster University of Applied SciencesStegerwaldstrasse 3948565SteinfurtDEtimo.voelker@fh-muenster.de
Transport
Internet Engineering Task ForceUDP SCTP Transport PMTUD PLPMTUDThis document describes a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization Layers (PLs). It describes an
extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path MTU
Discovery for IPv4 and IPv6. The method allows a PL, or a datagram
application that uses a PL, to discover whether a network path can
support the current size of datagram. This can be used to detect and
reduce the message size when a sender encounters a network black hole
(where packets are discarded). The method can probe a network path with
progressively larger packets to discover whether the maximum packet size
can be increased. This allows a sender to determine an appropriate
packet size, providing functionally for datagram transports that is
equivalent to the Packetization Layer PMTUD specification for TCP,
specified in RFC 4821.The document also provides implementation notes for incorporating
Datagram PMTUD into IETF datagram transports or applications that use
datagram transports.When published, this specification updates RFC 4821.Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 23 May 2020.
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Table of Contents
. Introduction
. Classical Path MTU Discovery
. Packetization Layer Path MTU Discovery
. Path MTU Discovery for Datagram Services
. Terminology
. Features Required to Provide Datagram PLPMTUD
. DPLPMTUD Mechanisms
. PLPMTU Probe Packets
. Confirmation of Probed Packet Size
. Detection of Unsupported PLPMTU Size, aka Black Hole Detection
. Disabling the Effect of PMTUD
. Response to PTB Messages
. Validation of PTB Messages
. Use of PTB Messages
. Datagram Packetization Layer PMTUD
. DPLPMTUD Components
. Timers
. Constants
. Variables
. Overview of DPLPMTUD Phases
. State Machine
. Search to Increase the PLPMTU
. Probing for a larger PLPMTU
. Selection of Probe Sizes
. Resilience to Inconsistent Path Information
. Robustness to Inconsistent Paths
. Specification of Protocol-Specific Methods
. Application support for DPLPMTUD with UDP or UDP-Lite
. Application Request
. Application Response
. Sending Application Probe Packets
. Initial Connectivity
. Validating the Path
. Handling of PTB Messages
. DPLPMTUD for SCTP
. SCTP/IPv4 and SCTP/IPv6
. DPLPMTUD for SCTP/UDP
. DPLPMTUD for SCTP/DTLS
. DPLPMTUD for QUIC
. Initial Connectivity
. Sending QUIC Probe Packets
. Validating the Path with QUIC
. Handling of PTB Messages by QUIC
. Acknowledgements
. IANA Considerations
. Security Considerations
. References
. Normative References
. Informative References
. Revision Notes
Authors' Addresses
IntroductionThe IETF has specified datagram transport using UDP, SCTP, and DCCP,
as well as protocols layered on top of these transports (e.g., SCTP/UDP,
DCCP/UDP, QUIC/UDP), and direct datagram transport over the IP network
layer. This document describes a robust method for Path MTU Discovery
(PMTUD) that may be used with these transport protocols (or the
applications that use their transport service) to discover an
appropriate size of packet to use across an Internet path.Classical Path MTU DiscoveryClassical 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., and ). 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) and ICMPv6 Packet Too Big messages (Type 2)
. When a sender receives a PTB message,
it reduces the effective MTU to the value reported as the Link MTU in
the PTB message, and a method that from time-to-time increases the
packet size in attempt to discover an increase in the supported PMTU.
The packets sent with a size larger than the current effective PMTU
are known as probe packets.Packets not intended as probe packets are either fragmented to the
current effective PMTU, or the attempt to send fails with an error
code. Applications are sometimes provided with a primitive to let them
read the Maximum Packet Size (MPS), derived from the current effective
PMTU.Classical PMTUD is subject to protocol failures. One failure arises
when traffic using a packet size larger than the actual PMTU is
black-holed (all datagrams sent with this size, or larger, are
discarded). This could arise when the PTB messages are not delivered
back to the sender for some reason (see for example ).Examples where PTB messages are not delivered include:
The generation of ICMP messages is usually rate limited. This
could result in no PTB messages being generated to the sender (see
section 2.4 of )
ICMP messages can be filtered by middleboxes (including
firewalls) . A stateful firewall
could be configured with a policy to block incoming ICMP messages,
which would prevent reception of PTB messages to a sending
endpoint behind this firewall.
When the router issuing the ICMP message drops a tunneled
packet, the resulting ICMP message will be directed to the tunnel
ingress. This tunnel endpoint is responsible for forwarding the
ICMP message and also processing the quoted packet within the
payload field to remove the effect of the tunnel, and return a
correctly formatted ICMP message to the sender . Failure to do this
prevents the PTB message reaching the original sender.
Asymmetry in forwarding can result in there being no return
route to the original sender, which would prevent an ICMP message
being delivered to the sender. This issue can also arise when
policy-based routing is used, Equal Cost Multipath (ECMP) routing
is used, or a middlebox acts as an application load balancer. An
example is where the path towards the server is chosen by ECMP
routing depending on bytes in the IP payload. In this case, when a
packet sent by the server encounters a problem after the ECMP
router, then any resulting ICMP message needs to also be directed
by the ECMP router towards the original sender.
There are additional cases where the next hop destination fails
to receive a packet because of its size. This could be due to
misconfiguration of the layer 2 path between nodes, for instance
the MTU configured in a layer 2 switch, or misconfiguration of the
Maximum Receive Unit (MRU). If the packet is dropped by the link,
this will not cause a PTB message to be sent to the original
sender.
Another failure could result if a node that is not on the network
path sends a PTB message that attempts to force a sender to change the
effective PMTU . A sender can protect
itself from reacting to such messages by utilising the quoted packet
within a PTB message payload to validate that the received PTB message
was generated in response to a packet that had actually originated
from the sender. However, there are situations where a sender would be
unable to provide this validation. Examples where validation of the
PTB message is not possible include:
When a router issuing the ICMP message implements RFC792 , it is only required to include the first
64 bits of the IP payload of the packet within the quoted payload.
There could be insufficient bytes remaining for the sender to
interpret the quoted transport information. Note: The
recommendation in RFC1812 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
.
The use of tunnels/encryption can reduce the size of the quoted
packet returned to the original source address, increasing the
risk that there could be insufficient bytes remaining for the
sender to interpret the quoted transport information.
Even when the PTB message includes sufficient bytes of the
quoted packet, the network layer could lack sufficient context to
validate the message, because validation depends on information
about the active transport flows at an endpoint node (e.g., the
socket/address pairs being used, and other protocol header
information).
When a packet is encapsulated/tunneled over an encrypted
transport, the tunnel/encapsulation ingress might have
insufficient context, or computational power, to reconstruct the
transport header that would be needed to perform validation.
Packetization Layer Path MTU DiscoveryThe term Packetization Layer (PL) has been introduced to describe
the layer that is responsible for placing data blocks into the payload
of IP packets and selecting an appropriate MPS. This function is often
performed by a transport protocol, but can also be performed by other
encapsulation methods working above the transport layer.In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) does not rely upon reception
and validation of PTB messages. It is therefore more robust than
Classical PMTUD. This has become the recommended approach for
implementing PMTU discovery.It uses a general strategy where the PL sends probe packets to
search for the largest size of unfragmented datagram that can be sent
over a network path. Probe packets are sent with a progressively
larger packet size. If a probe packet is successfully delivered (as
determined by the PL), then the PLPMTU is raised to the size of the
successful probe. If no response is received to a probe packet, the
method reduces the probe size. The result of probing with the PLPMTU
is used to set the application MPS.PLPMTUD introduces flexibility in the implementation of PMTU
discovery. At one extreme, it can be configured to only perform ICMP
Black Hole Detection and recovery to increase the robustness of
Classical PMTUD, or at the other extreme, all PTB processing can be
disabled and PLPMTUD can completely replace Classical PMTUD (see ).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.Path MTU Discovery for Datagram Services of this document presents a set of
algorithms for datagram protocols to discover the largest size of
unfragmented datagram that can be sent over a network path. The method
described relies on features of the PL described in and applies to transport protocols
operating over IPv4 and IPv6. It does not require cooperation from the
lower layers, although it can utilize PTB messages when these received
messages are made available to the PL.The UDP Usage Guidelines state "an
application SHOULD either use the Path MTU information provided by the
IP layer or implement Path MTU Discovery (PMTUD)", but does not
provide a mechanism for discovering the largest size of unfragmented
datagram that can be used on a network path. Prior to this document,
PLPMTUD had not been specified for UDP.Section 10.2 of recommends a PLPMTUD
probing method for the Stream Control Transport Protocol (SCTP). SCTP
utilizes probe packets consisting of a minimal sized HEARTBEAT chunk
bundled with a PAD chunk as defined in ,
but RFC4821 does not provide a complete specification. The present
document provides the details to complete that specification.The Datagram Congestion Control Protocol (DCCP) requires implementations to support Classical
PMTUD and states that a DCCP sender "MUST maintain the MPS allowed for
each active DCCP session". It also defines the current congestion
control MPS (CCMPS) supported by a network path. This recommends use
of PMTUD, and suggests use of control packets (DCCP-Sync) as path
probe packets, because they do not risk application data loss. The
method defined in this specification could be used with DCCP. specifies the
method for a set of transports, and provides information to enable the
implementation of PLPMTUD with other datagram transports and
applications that use datagram transports.TerminologyThe key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when, and
only when, they appear in all capitals, as shown here.Other terminology is directly copied from , and the definitions in .
Actual PMTU:
The Actual PMTU is the PMTU of a network path between a sender
PL and a destination PL, which the DPLPMTUD algorithm seeks to
determine.
Black Hole:
A Black Hole is encountered when a sender is unaware that
packets are not being delivered to the destination end point. Two
types of Black Hole are relevant to DPLPMTUD:
Packet Black Hole:
Packets encounter a Packet Black Hole when packets are not
delivered to the destination endpoint (e.g., when the sender
transmits packets of a particular size with a previously known
effective PMTU and they are discarded by the network).
ICMP Black Hole
An ICMP Black Hole is encountered when the sender is unaware
that packets are not delivered to the destination endpoint
because PTB messages are not received by the originating PL
sender.
Black holed :
Traffic is black-holed when the sender is unaware that packets
are not being delivered. This could be due to a Packet Black Hole or
an ICMP Black Hole.
Classical Path MTU Discovery:
Classical PMTUD is a process described in and , in which nodes
rely on PTB messages to learn the largest size of unfragmented
datagram that can be used across a network path.
Datagram:
A datagram is a transport-layer protocol data unit, transmitted
in the payload of an IP packet.
Effective PMTU:
The Effective PMTU is the current estimated value for PMTU that
is used by a PMTUD. This is equivalent to the PLPMTU derived by
PLPMTUD.
EMTU_S:
The Effective MTU for sending (EMTU_S) is defined in as "the maximum IP datagram size that may be
sent, for a particular combination of IP source and destination
addresses...".
EMTU_R:
The Effective MTU for receiving (EMTU_R) is designated in as the largest datagram size that can be
reassembled by EMTU_R (Effective MTU to receive).
Link:
A Link is a communication facility or medium over which nodes
can communicate at the link layer, i.e., a layer below the IP layer.
Examples are Ethernet LANs and Internet (or higher) layer and
tunnels.
Link MTU:
The Link Maximum Transmission Unit (MTU) is the size in bytes of
the largest IP packet, including the IP header and payload, that can
be transmitted over a link. Note that this could more properly be
called the IP MTU, to be consistent with how other standards
organizations use the acronym. This includes the IP header, but
excludes link layer headers and other framing that is not part of IP
or the IP payload. Other standards organizations generally define
the link MTU to include the link layer headers.
MAX_PMTU:
The MAX_PMTU is the largest size of PLPMTU that DPLPMTUD will
attempt to use.
MPS:
The Maximum Packet Size (MPS) is the largest size of application
data block that can be sent across a network path by a PL. In
DPLPMTUD this quantity is derived from the PLPMTU by taking into
consideration the size of the lower protocol layer headers. Probe
packets generated by DPLPMTUD can have a size larger than the
MPS.
MIN_PMTU:
The MIN_PMTU is the smallest size of PLPMTU that DPLPMTUD will
attempt to use.
Packet:
A Packet is the IP header plus the IP payload.
Packetization Layer (PL):
The Packetization Layer (PL) is the layer of the network stack
that places data into packets and performs transport protocol
functions.
Path:
The Path is the set of links and routers traversed by a packet
between a source node and a destination node by a particular
flow.
Path MTU (PMTU):
The Path MTU (PMTU) is the minimum of the Link MTU of all the
links forming a network path between a source node and a destination
node.
PTB_SIZE:
The PTB_SIZE is a value reported in a validated PTB message that
indicates next hop link MTU of a router along the path.
PLPMTU:
The Packetization Layer PMTU is an estimate of the actual PMTU
provided by the DPLPMTUD algorithm.
PLPMTUD:
Packetization Layer Path MTU Discovery (PLPMTUD), the method
described in this document for datagram PLs, which is an extension
to Classical PMTU Discovery.
Probe packet:
A probe packet is a datagram sent with a purposely chosen size
(typically the current PLPMTU or larger) to detect if packets of
this size can be successfully sent end-to-end across the network
path.
Features Required to Provide Datagram PLPMTUDTCP PLPMTUD has been defined using standard TCP protocol mechanisms.
All of the requirements in also apply to
the use of the technique with a datagram PL. Unlike TCP, some datagram
PLs require additional mechanisms to implement PLPMTUD.There are eight requirements for performing the datagram PLPMTUD
method described in this specification:
PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide
information about the maximum size of packet that can be transmitted
by the sender on the local link (the local Link MTU). It MAY utilize
similar information about the receiver when this is supplied (note
this could be less than EMTU_R). This avoids implementations trying
to send probe packets that can not be transmitted by the local link.
Too high of a value could reduce the efficiency of the search
algorithm. Some applications also have a maximum transport protocol
data unit (PDU) size, in which case there is no benefit from probing
for a size larger than this (unless a transport allows multiplexing
multiple applications PDUs into the same datagram).
PLPMTU: A datagram application using a PL not supporting
fragmentation is REQUIRED to be able to choose the size of datagrams
sent to the network, up to the PLPMTU, or a smaller value (such as
the MPS) derived from this. This value is managed by the DPLPMTUD
method. The PLPMTU (specified as the effective PMTU in Section 1 of
) is equivalent to the EMTU_S
(specified in ).
Probe packets: On request, a DPLPMTUD sender is REQUIRED to be
able to transmit a packet larger than the PLMPMTU. This is used to
send a probe packet. In IPv4, a probe packet MUST be sent with the
Don't Fragment (DF) bit set in the IP header, and without network
layer endpoint fragmentation. In IPv6, a probe packet is always sent
without source fragmentation (as specified in section 5.4 of ).
Processing PTB messages: A DPLPMTUD sender MAY optionally utilize
PTB messages received from the network layer to help identify when a
network path does not support the current size of probe packet. Any
received PTB message MUST be validated before it is used to update
the PLPMTU discovery information .
This validation confirms that the PTB message was sent in response
to a packet originating by the sender, and needs to be performed
before the PLPMTU discovery method reacts to the PTB message. A PTB
message MUST NOT be used to increase the PLPMTU .
Reception feedback: The destination PL endpoint is REQUIRED to
provide a feedback method that indicates to the DPLPMTUD sender when
a probe packet has been received by the destination PL endpoint. The
mechanism needs to be robust to the possibility that packets could
be significantly delayed along a network path. The local PL endpoint
at the sending node is REQUIRED to pass this feedback to the sender
DPLPMTUD method.
Probe loss recovery: It is RECOMMENDED 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) are REQUIRED to arrange any retransmission/repair of any
resulting loss. DPLPMTUD is REQUIRED to be robust in the case where
probe packets are lost due to other reasons (including link
transmission error, congestion).
Probing and congestion control: The DPLPMTUD sender treats
isolated loss of a probe packet (with or without a corresponding PTB
message) as a potential indication of a PMTU limit for the path.
Loss of a probe packet SHOULD NOT be treated as an indication of
congestion. The loss of a probe packet SHOULD NOT directly trigger a
congestion control reaction because
this could result in unecessary reduction of the sending rate. The
interval between probe packets MUST be at least one RTT.
Shared PLPMTU state: The PLPMTU value MAY also be stored with the
corresponding entry in the destination cache and used by other PL
instances. The specification of PLPMTUD states: "If PLPMTUD updates the MTU for a
particular path, all Packetization Layer sessions that share the path
representation (as described in Section 5.2 of ) SHOULD be notified to make use of the new
MTU". Such methods MUST be robust to the wide variety of underlying
network forwarding behaviors. Section 5.2 of provides guidance on the caching of PMTU
information and also the relation to IPv6 flow labels.
In addition, the following principles are stated for design of a
DPLPMTUD method:
MPS: A method is REQUIRED to signal an appropriate MPS to the
higher layer using the PL. The value of the MPS can change following
a change to the path. It is RECOMMENDED that methods avoid forcing
an application to use an arbitrary small MPS (PLPMTU) for
transmission while the method is searching for the currently
supported PLPMTU. Datagram PLs do not necessarily support
fragmentation of PDUs larger than the PLPMTU. A reduced MPS can
adversely impact the performance of a datagram application.
Path validation: It is RECOMMENDED that methods are robust to
path changes that could have occurred since the path characteristics
were last confirmed, and to the possibility of inconsistent path
information being received.
Datagram reordering: A method is REQUIRED to be robust to the
possibility that a flow encounters reordering, or the traffic
(including probe packets) is divided over more than one network
path.
When to probe: It is RECOMMENDED that methods determine whether
the path has changed since it last measured the path. This can help
determine when to probe the path again.
DPLPMTUD MechanismsThis section lists the protocol mechanisms used in this
specification.PLPMTU Probe PacketsThe 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 . In contrast, a datagram PL that needs to
construct a probe packet has to either request an application to send a
data block that is larger than that generated by an application, or to
utilize padding functions to extend a datagram beyond the size of the
application data block. Protocols that permit exchange of control
messages (without an application data block) MAY prefer to generate a
probe packet by extending a control message with padding data.A receiver is REQUIRED to be able to distinguish an in-band data
block from any added padding. This is needed to ensure that any added
padding is not passed on to an application at the receiver.This results in three possible ways that a sender can create a
probe packet:
Probing using padding data:
A probe packet that contains only control information together
with any padding, which is needed to be inflated to the size
required for the probe packet. Since these probe packets do not
carry an application-supplied data block, they do not typically
require retransmission, although they do still consume network
capacity and incur endpoint processing.
Probing using application data and padding data:
A probe packet that contains a data block supplied by an
application that is combined with padding to inflate the length of
the datagram to the size required for the probe packet. If the
application/transport needs protection from the loss of this probe
packet, the application/transport could perform transport-layer
retransmission/repair of the data block (e.g., by retransmission
after loss is detected or by duplicating the data block in a
datagram without the padding data).
Probing using application data:
A probe packet that contains a data block supplied by an
application that matches the size required for the probe packet.
This method requests the application to issue a data block of the
desired probe size. If the application/transport needs protection
from the loss of an unsuccessful probe packet, the
application/transport needs then to perform transport-layer
retransmission/repair of the data block (e.g., by retransmission
after loss is detected).
A PL that uses a probe packet carrying an application data
block, could need to retransmit this application data block if the
probe fails.
This could need the PL to re-fragment the data block to a
smaller packet size that is expected to traverse the end-to-end path
(which could utilize endpoint network-layer or PL fragmentation when
these are available).DPLPMTUD MAY choose to use only one of these methods to simplify
the implementation.Probe messages sent by a PL MUST contain enough information to
uniquely identify the probe within Maximum Segment Lifetime, while
being robust to reordering and replay of probe response and PTB
messages.Confirmation of Probed Packet SizeThe PL needs a method to determine (confirm) when probe packets
have been successfully received end-to-end across a network path.Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP and
SCTP provide keep-alive/heartbeat features). When supported, this
mechanism SHOULD also be used by DPLPMTUD to acknowledge reception of
a probe packet.A PL that does not acknowledge data reception (e.g., UDP and
UDP-Lite) is unable itself to detect when the packets that it sends
are discarded because their size is greater than the actual PMTU.
These PLs need to either rely on an application protocol to detect
this loss. specifies this
function for a set of IETF-specified protocols.Detection of Unsupported PLPMTU Size, aka Black Hole DetectionA PL sender needs to reduce the PLPMTU when it discovers the actual
PMTU supported by a network path is less than the PLPMTU. This can be
triggered when a validated PTB message is received, or by another
event that indicates the network path no longer sustains the current
packet size, such as a loss report from the PL, or repeated lack of
response to probe packets sent to confirm the PLPMTU. Detection is
followed by a reduction of the PLPMTU.This is performed by sending packet probes of size PLPMTU to verify
that a network path still supports the last acknowledged PLPMTU size.
There are two alternative mechanism:
A PL can rely upon a mechanism implemented within the PL to
detect excessive loss of data sent with a specific packet size and
then conclude that this excessive loss could be a result of an
invalid PMTU (as in PLPMTUD for TCP ).
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 ). A
timer tracks whether acknowledgments are received. Successive loss
of probes is an indication that the current path no longer
supports the PLPMTU (e.g., when the number of probe packets sent
without receiving an acknowledgement, PROBE_COUNT, becomes greater
than MAX_PROBES).
A PL MAY inhibit sending probe packets when no application data has
been sent since the previous probe packet. A PL preferring to use an
up-to-data PLPMTU once user data is sent again, MAY choose to continue
PLPMTU discovery for each path. However, this may result in additional
packets being sent.When the method detects the current PLPMTU is not supported,
DPLPMTUD sets a lower MPS. The PL then confirms that the updated
PLPMTU can be successfully used across the path. The PL could need to
send a probe packet with a size less than the size of the data block
generated by an application. In this case, the PL could provide a way
to fragment a datagram at the PL, or use a control packet as the
packet probe.Disabling the Effect of PMTUDA PL implementing this specification MUST suspend network layer
processing of outgoing packets that enforces a PMTU for each flow
utilising 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 fragment sent packets that have a size
greater than the PMTU.Response to PTB MessagesThis method requires the DPLPMTUD sender to validate any received
PTB message before using the PTB information. The response to a PTB
message depends on the PTB_SIZE indicated in the PTB message, the
state of the PLPMTUD state machine, and the IP protocol being
used. first describes validation for both IPv4
ICMP Unreachable messages (type 3) and ICMPv6 Packet Too Big messages,
both of which are referred to as PTB messages in this document.Validation of PTB MessagesThis section specifies utilization of PTB messages.
A simple implementation MAY ignore received PTB messages and
in this case the PLPMTU is not updated when a PTB message is
received.
An implementation that supports PTB messages MUST validate
messages before they are further processed.
A PL that receives a PTB message from a router or middlebox,
performs ICMP validation as specified in Section 5.2 of . Because
DPLPMTUD operates at the PL, the PL needs to check that each
received PTB message is received in response to a packet transmitted
by the endpoint PL performing DPLPMTUD.The PL MUST check the protocol information in the quoted packet
carried in an ICMP PTB message payload to validate the message
originated from the sending node. This validation includes
determining that the combination of the IP addresses, the protocol,
the source port and destination port match those returned in the
quoted packet - this is also necessary for the PTB message to be
passed to the corresponding PL.The validation SHOULD utilize information that it is not simple
for an off-path attacker to determine . For example, by checking the value of a
protocol header field known only to the two PL endpoints. A datagram
application that uses well-known source and destination ports ought
to also rely on other information to complete this validation.These checks are intended to provide protection from packets that
originate from a node that is not on the network path. A PTB message
that does not complete the validation MUST NOT be further utilized
by the DPLPMTUD method.PTB messages that have been validated MAY be utilized by the
DPLPMTUD algorithm, but MUST NOT be used directly to set the PLPMTU.
A method that utilizes these PTB messages can improve the speed at
the which the algorithm detects an appropriate PLPMTU, compared to
one that relies solely on probing. describes this processing.Use of PTB MessagesA set of checks are intended to provide protection from a router
that reports an unexpected PTB_SIZE. The PL also needs to check that
the indicated PTB_SIZE is less than the size used by probe packets
and larger than minimum size accepted.This section provides a summary of how PTB messages can be
utilized. This processing depends on the PTB_SIZE and the current
value of a set of variables:
PTB_SIZE < MIN_MTU
Invalid PTB_SIZE see .
PTB message ought to be discarded without further
processing (e. g. PLPMTU not modified).
The information could be utilized as an input to trigger
enabling a resilience mode.
MIN_PMTU < PTB_SIZE < BASE_PMTU
A robust PL MAY enter an error state (see ) for an IPv4 path when the PTB_SIZE
reported in the PTB message is larger than or equal to 68
bytes and when this is less than the BASE_PMTU.
A robust PL MAY enter an error state (see ) for an IPv6 path when the PTB_SIZE
reported in the PTB message is larger than or equal to 1280
bytes and when this is less than the BASE_PMTU.
PTB_SIZE = PLPMTU
Completes the search for a larger PLPMTU.
PTB_SIZE > PROBED_SIZE
Inconsistent network signal.
PTB message ought to be discarded without further
processing (e. g. PLPMTU not modified).
The information could be utilized as an input to trigger
enabling a resilience mode.
BASE_PMTU <= PTB_SIZE < PLPMTU
Black Hole Detection is triggered and the PLPMTU ought
to be set to BASE_PMTU.
The PL could use the PTB_SIZE reported in the PTB
message to initialize a search algorithm.
PLPMTU < PTB_SIZE < PROBED_SIZE
The PLPMTU continues to be valid, but the last
PROBED_SIZE searched was larger than the actual PMTU.
The PLPMTU is not updated.
The PL can use the reported PTB_SIZE from the PTB
message as the next search point when it resumes the search
algorithm.
Datagram Packetization Layer PMTUDThis section specifies Datagram PLPMTUD (DPLPMTUD). The method can be
introduced at various points (as indicated with * in the figure below)
in the IP protocol stack to discover the PLPMTU so that an application
can utilize an appropriate MPS for the current network path. DPLPMTUD
SHOULD NOT be used by an application if it is already used in a lower
layer.The central idea of DPLPMTUD is probing by a sender. Probe packets
are sent to find the maximum size of a user message that can be
completely transferred across the network path from the sender to the
destination.The following sections identify the components needed for
implementation, provides an overview of the phases of operation, and
specifies the state machine and search algorithm.DPLPMTUD ComponentsThis section describes the timers, constants, and variables of
DPLPMTUD.TimersThe method utilizes up to three timers:
PROBE_TIMER:
The PROBE_TIMER is configured to expire after a period
longer than the maximum time to receive an acknowledgment to a
probe packet. This value MUST NOT be smaller than 1 second,
and SHOULD be larger than 15 seconds. Guidance on selection of
the timer value are provided in section 3.1.1 of the UDP Usage
Guidelines .If the PL has a path Round Trip Time (RTT) estimate and
timely acknowledgements the PROBE_TIMER can be derived from
the PL RTT estimate.
PMTU_RAISE_TIMER:
The PMTU_RAISE_TIMER is configured to the period a sender
will continue to use the current PLPMTU, after which it
re-enters the Search phase. This timer has a period of 600
seconds, as recommended by PLPMTUD .DPLPMTUD MAY inhibit sending probe packets when no
application data has been sent since the previous probe
packet. A PL preferring to use an up-to-data PMTU once user
data is sent again, can choose to continue PMTU discovery for
each path. However, this may result in sending additional
packets.
CONFIRMATION_TIMER:
When an acknowledged PL is used, this timer MUST NOT be
used. For other PLs, the CONFIRMATION_TIMER is configured to
the period a PL sender waits before confirming the current
PLPMTU is still supported. This is less than the
PMTU_RAISE_TIMER and used to decrease the PLPMTU (e.g., when a
black hole is encountered). Confirmation needs to be frequent
enough when data is flowing that the sending PL does not black
hole extensive amounts of traffic. Guidance on selection of
the timer value are provided in section 3.1.1 of the UDP Usage
Guidelines .DPLPMTUD MAY inhibit sending probe packets when no
application data has been sent since the previous probe
packet. A PL preferring to use an up-to-data PMTU once user
data is sent again, can choose to continue PMTU discovery for
each path. However, this may result in sending additional
packets.
An implementation could implement the various timers using a
single timer.ConstantsThe following constants are defined:
MAX_PROBES:
The MAX_PROBES is the maximum value of the PROBE_COUNT
counter (see ). MAX_PROBES represents
the limit for the number of consecutive probe attempts of any
size. The default value of MAX_PROBES is 3. This value is greater
than 1 to provide robustness to isolated packet loss.
MIN_PMTU:
The MIN_PMTU is the smallest allowed probe packet size. For
IPv6, this value is 1280 bytes, as specified in . For IPv4, the minimum value is 68
bytes.Note: An IPv4 router is required to be able to
forward a datagram of 68 bytes without further fragmentation.
This is the combined size of an IPv4 header and the minimum
fragment size of 8 bytes. In addition, receivers are required to
be able to reassemble fragmented datagrams at least up to 576
bytes, as stated in section 3.3.3 of .
MAX_PMTU:
The MAX_PMTU is the largest size of PLPMTU. This has to be
less than or equal to the minimum of the local MTU of the
outgoing interface and the destination PMTU for receiving. An
application, or PL, MAY choose a smaller MAX_PMTU when there is no
need to send packets larger than a specific size.
BASE_PMTU:
The BASE_PMTU is a configured size expected to work for most
paths. The size is equal to or larger than the MIN_PMTU and
smaller than the MAX_PMTU. In the case of IPv6, this value is
1280 bytes . When using IPv4, a size of
1200 bytes is RECOMMENDED.
VariablesThis method utilizes a set of variables:
PROBED_SIZE:
The PROBED_SIZE is the size of the current probe packet.
This is a tentative value for the PLPMTU, which is awaiting
confirmation by an acknowledgment.
PROBE_COUNT:
The PROBE_COUNT is a count of the number of successive
unsuccessful probe packets that have been sent. Each time a probe
packet is acknowledged, the value is set to zero.
The figure below illustrates the relationship between the packet
size constants and variables at a point of time when the DPLPMTUD
algorithm performs path probing to increase the size of the PLPMTU.
A probe packet has been sent of size PROBED_SIZE. Once this is
acknowledged, the PLPMTU will raise to PROBED_SIZE allowing the
DPLPMTUD algorithm to further increase PROBED_SIZE towards the
actual PMTU.Overview of DPLPMTUD PhasesThis 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 .
Base:
The Base Phase confirms connectivity to the remote peer.
This phase is implicit for a connection-oriented PL (where it
can be performed in a PL connection handshake). A
connectionless PL needs to send an acknowledged probe packet
to confirm that the remote peer is reachable. The sender also
confirms that BASE_PMTU is supported across the network
path.A PL that does not wish to support a path with a PLPMTU
less than BASE_PMTU can simplify the phase into a single step
by performing the connectivity checks with a probe of the
BASE_PMTU size.Once confirmed, DPLPMTUD enters the Search Phase. If this
phase fails to confirm, DPLPMTUD enters the Error Phase.
Search:
The Search Phase utilizes a search algorithm to send probe
packets to seek to increase the PLPMTU. The algorithm
concludes when it has found a suitable PLPMTU, by entering the
Search Complete Phase.A PL could respond to PTB messages using the PTB to advance
or terminate the search, see .
Search Complete:
The Search Complete Phase is entered when the PLPMTU is
supported across the network path. A PL can use a
CONFIRMATION_TIMER to periodically repeat a probe packet for
the current PLPMTU size. If the sender is unable to confirm
reachability (e.g., if the CONFIRMATION_TIMER expires) or the
PL signals a lack of reachability, DPLPMTUD enters the Base
phase.The PMTU_RAISE_TIMER is used to periodically resume the
search phase to discover if the PLPMTU can be raised. Black
Hole Detection or receipt of a validated PTB message (see
) can cause the sender to enter the Base
Phase.
Error:
The Error Phase is entered when there is conflicting or
invalid PLPMTU information for the path (e.g. a failure to
support the BASE_PMTU) that cause DPLPMTUD to be unable to
progress and the PLPMTU is lowered.DPLPMTUD remains in the Error Phase until a consistent view
of the path can be discovered and it has also been confirmed
that the path supports the BASE_PMTU (or DPLPMTUD is
suspended).
An implementation that only reduces the PLPMTU to a suitable size
would be sufficient to ensure reliable operation, but can be very
inefficient when the actual PMTU changes or when the method (for
whatever reason) makes a suboptimal choice for the PLPMTU.A full implementation of DPLPMTUD provides an algorithm enabling
the DPLPMTUD sender to increase the PLPMTU following a change in the
characteristics of the path, such as when a link is reconfigured
with a larger MTU, or when there is a change in the set of links
traversed by an end-to-end flow (e.g., after a routing or path
fail-over decision).State MachineA state machine for DPLPMTUD is depicted in . If multipath or multihoming is supported,
a state machine is needed for each path.Note: Not all changes are not shown to simplify the
diagram.The following states are defined:
DISABLED:
The DISABLED state is the initial state before probing has
started. It is also entered from any other state, when the PL
indicates loss of connectivity. This state is left, once the PL
indicates connectivity to the remote PL.
BASE:
The BASE state is used to confirm that the BASE_PMTU size is
supported by the network path and is designed to allow an
application to continue working when there are transient
reductions in the actual PMTU. It also seeks to avoid long
periods where traffic is black holed while searching for a
larger PLPMTU.On entry, the PROBED_SIZE is set to the BASE_PMTU size and
the PROBE_COUNT is set to zero.Each time a probe packet is sent, the PROBE_TIMER is started.
The state is exited when the probe packet is acknowledged, and
the PL sender enters the SEARCHING state.The state is also left when the PROBE_COUNT reaches
MAX_PROBES or a received PTB message is validated. This causes
the PL sender to enter the ERROR state.
SEARCHING:
The SEARCHING state is the main probing state. This state is
entered when probing for the BASE_PMTU was successful.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.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. The state is exited when the
PROBE_COUNT reaches MAX_PROBES, a received PTB message is
validated, a probe of size MAX_PMTU is acknowledged, or a black
hole is detected.
SEARCH_COMPLETE:
The SEARCH_COMPLETE state indicates a successful end to the
SEARCHING state. DPLPMTUD remains in this state until either the
PMTU_RAISE_TIMER expires, a received PTB message is validated,
or a black hole is detected.When DPLPMTUD uses an unacknowledged PL and is in 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 SHOULD NOT continue to
generate PLPMTU probes in this state.
ERROR:
The ERROR state represents the case where either the network
path is not known to support a PLPMTU of at least the BASE_PMTU
size or when there is contradictory information about the
network path that would otherwise result in excessive variation
in the MPS signalled to the higher layer. The state implements a
method to mitigate oscillation in the state-event engine. It
signals a conservative value of the MPS to the higher layer by
the PL. The state is exited when packet probes no longer detect
the error or when the PL indicates that connectivity has been
lost.Implementations are permitted to enable endpoint
fragmentation if the DPLPMTUD is unable to validate MIN_PMTU
within PROBE_COUNT probes. If DPLPMTUD is unable to validate
MIN_PMTU the implementation should transition to the DISABLED
state.Note: MIN_PMTU may be identical to BASE_PMTU, simplifying the
actions in this state.
Search to Increase the PLPMTUThis section describes the algorithms used by DPLPMTUD to search
for a larger PLPMTU.Probing for a larger PLPMTUImplementations use a search algorithm across the search range to
determine whether a larger PLPMTU can be supported across a network
path.The method discovers the search range by confirming the minimum
PLPMTU and then using the probe method to select a PROBED_SIZE less
than or equal to MAX_PMTU. MAX_PMTU is the minimum of the local MTU
and EMTU_R (learned from the remote endpoint). The MAX_PMTU MAY be
reduced by an application that sets a maximum to the size of
datagrams it will send. The PROBE_COUNT is initialized to zero when the first probe with
a size greater than or equal to PLPMTUD is sent. A timer is used by
the search algorithm to trigger the sending of probe packets of size
PROBED_SIZE, larger than the PLPMTU. Each probe packet successfully
sent to the remote peer is confirmed by acknowledgement at the PL,
see .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 . This confirms that the
PROBED_SIZE is supported, and the PROBED_SIZE value is then assigned
to the PLPMTU. The search algorithm can continue to send subsequent
probe packets of an increasing size.If the timer expires before a probe packet is acknowledged, the
probe has failed to confirm the PROBED_SIZE. Each time the
PROBE_TIMER expires, the PROBE_COUNT is incremented, the PROBE_TIMER
is reinitialized, and a 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. Selection of Probe SizesThe search algorithm needs to determine a minimum useful gain in
PLPMTU. It would not be constructive for a PL sender to attempt to
probe for all sizes. This would incur unnecessary load on the path
and has the undesirable effect of slowing the time to reach a more
optimal MPS. Implementations SHOULD select the set of probe packet
sizes to maximize the gain in PLPMTU from each search step.Implementations could optimize the search procedure by selecting
step sizes from a table of common PMTU sizes. When selecting the
appropriate next size to search, an implementer ought to also
consider that there can be common sizes of MPS that applications
seek to use, and their could be common sizes of MTU used within the
network.Resilience to Inconsistent Path InformationA 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.A PL sender is able to detect inconsistency from the sequence of
PLPMTU probes that it sends or the sequence of PTB messages that it
receives. When inconsistent path information is detected, a PL
sender could use an alternate search mode that clamps the offered
MPS to a smaller value for a period of time. This avoids unnecessary
loss of packets due to MTU limitation.Robustness to Inconsistent PathsSome paths could be unable to sustain packets of the BASE_PMTU
size. To be robust to these paths an implementation could implement
the Error State. This allows fallback to a smaller than desired
PLPMTU, rather than suffer connectivity failure. This could utilize
methods such as endpoint IP fragmentation to enable the PL sender to
communicate using packets smaller than the BASE_PMTU.Specification of Protocol-Specific MethodsDPLPMTUD requires protocol-specific details to be specified for each
PL that is used.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.Application support for DPLPMTUD with UDP or UDP-LiteThe current specifications of UDP
and UDP-Lite do not define a method in
the RFC-series that supports PLPMTUD. In particular, the UDP transport
does not provide the transport layer features needed to implement
datagram PLPMTUD.The DPLPMTUD method can be implemented as a part of an application
built directly or indirectly on UDP or UDP-Lite, but relies on
higher-layer protocol features to implement the method .Some primitives used by DPLPMTUD might not be available via the
Datagram API (e.g., the ability to access the PLPMTU cache, or
interpret received PTB messages).In addition, it is desirable that PMTU discovery is not performed
by multiple protocol layers. An application SHOULD avoid using
DPLPMTUD when the underlying transport system provides this
capability. To use common method for managing the PLPMTU has benefits,
both in the ability to share state between different processes and
opportunities to coordinate probing.Application RequestAn application needs an application-layer protocol mechanism
(such as a message acknowledgement method) that solicits a response
from a destination endpoint. The method SHOULD allow the sender to
check the value returned in the response to provide additional
protection from off-path insertion of data , suitable methods include a parameter known
only to the two endpoints, such as a session ID or initialized
sequence number.Application ResponseAn application needs an application-layer protocol mechanism to
communicate the response from the destination endpoint. This
response may indicate successful reception of the probe across the
path, but could also indicate that some (or all packets) have failed
to reach the destination.Sending Application Probe PacketsA probe packet that may carry an application data block, but the
successful transmission of this data is at risk when used for
probing. Some applications may prefer to use a probe packet that
does not carry an application data block to avoid disruption to data
transfer.Initial ConnectivityAn application that does not have other higher-layer information
confirming connectivity with the remote peer SHOULD implement a
connectivity mechanism using acknowledged probe packets before
entering the BASE state.Validating the PathAn application that does not have other higher-layer information
confirming correct delivery of datagrams SHOULD implement the
CONFIRMATION_TIMER to periodically send probe packets while in the
SEARCH_COMPLETE state.Handling of PTB MessagesAn application that is able and wishes to receive PTB messages
MUST perform ICMP validation as specified in Section 5.2 of . This requires that the application to
check each received PTB messages to validate it is received in
response to transmitted traffic and that the reported PTB_SIZE is
less than the current probed size (see ). A validated PTB message MAY be used
as input to the DPLPMTUD algorithm, but MUST NOT be used directly to
set the PLPMTU.DPLPMTUD for SCTPSection 10.2 of specifies a
recommended PLPMTUD probing method for SCTP. It recommends the use of
the PAD chunk, defined in to be
attached to a minimum length HEARTBEAT chunk to build a probe packet.
This enables probing without affecting the transfer of user messages
and without interfering with congestion control. This is preferred to
using DATA chunks (with padding as required) as path probes.SCTP/IPv4 and SCTP/IPv6Initial ConnectivityThe base protocol is specified in . This provides an acknowledged PL. A
sender can therefore enter the BASE state as soon as connectivity
has been confirmed.Sending SCTP Probe PacketsProbe packets consist of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
the length of the probe packet. The HEARTBEAT chunk is used to
trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful
probe.The HEARTBEAT chunk carries a Heartbeat Information parameter
which should include, besides the information suggested in , the probe size, which is the size of the
complete datagram. The size of the PAD chunk is therefore computed
by reducing the probing size by the IPv4 or IPv6 header size, the
SCTP common header, the HEARTBEAT request and the PAD chunk
header. The payload of the PAD chunk contains arbitrary data.To avoid fragmentation of retransmitted data, probing starts
right after the PL handshake, before data is sent. Assuming this
behavior (i.e., the PMTU is smaller than or equal to the interface
MTU), this process will take a few round trip time periods
depending on the number of PMTU sizes probed. The Heartbeat timer
can be used to implement the PROBE_TIMER.Validating the Path with SCTPSince SCTP provides an acknowledged PL, a sender MUST NOT
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
state.PTB Message Handling by SCTPNormal ICMP validation MUST be performed as specified in
Appendix C of . This requires that
the first 8 bytes of the SCTP common header are quoted in the
payload of the PTB message, which can be the case for ICMPv4 and
is normally the case for ICMPv6.When a PTB message has been validated, the PTB_SIZE reported in
the PTB message SHOULD be used with the DPLPMTUD algorithm,
providing that the reported PTB_SIZE is less than the current
probe size (see ).DPLPMTUD for SCTP/UDPThe UDP encapsulation of SCTP is specified in .Initial ConnectivityA sender can enter the BASE state as soon as SCTP connectivity
has been confirmed.Sending SCTP/UDP Probe PacketsPacket probing can be performed as specified in . The maximum payload is
reduced by 8 bytes, which has to be considered when filling the
PAD chunk.Validating the Path with SCTP/UDPSince SCTP provides an acknowledged PL, a sender MUST NOT
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
state.Handling of PTB Messages by SCTP/UDPICMP validation MUST be performed for PTB messages as specified
in Appendix C of . This requires
that the first 8 bytes of the SCTP common header are contained in
the PTB message, which can be the case for ICMPv4 (but note the
UDP header also consumes a part of the quoted packet header) and
is normally the case for ICMPv6. When the validation is completed,
the PTB_SIZE indicated in the PTB message SHOULD be used with the
DPLPMTUD providing that the reported PTB_SIZE is less than the
current probe size.DPLPMTUD for SCTP/DTLSThe Datagram Transport Layer Security (DTLS) encapsulation of
SCTP is specified in . It is used for
data channels in WebRTC implementations.Initial ConnectivityA sender can enter the BASE state as soon as SCTP connectivity
has been confirmed.Sending SCTP/DTLS Probe PacketsPacket probing can be done as specified in .Validating the Path with SCTP/DTLSSince SCTP provides an acknowledged PL, a sender MUST NOT
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
state.Handling of PTB Messages by SCTP/DTLSIt is not possible to perform ICMP validation as specified in
, since even if the ICMP message
payload contains sufficient information, the reflected SCTP common
header would be encrypted. Therefore it is not possible to process
PTB messages at the PL.DPLPMTUD for QUICQUIC is a UDP-based transport that
provides reception feedback. The UDP payload includes the QUIC packet
header, protected payload, and any authentication fields. QUIC depends
on a PMTU of at least 1280 bytes.Section 14.1 of
describes the path considerations when sending QUIC packets. It
recommends the use of PADDING frames to build the probe packet. Pure
probe-only packets are constructed with PADDING frames and PING frames
to create a padding only packet that will elicit an acknowledgement.
Such padding only packets enable probing without affecting the
transfer of other QUIC frames.The recommendation for QUIC endpoints implementing DPLPMTUD is that
a MPS is maintained for each combination of local and remote IP
addresses . If a QUIC
endpoint determines that the PMTU between any pair of local and remote
IP addresses has fallen below an acceptable MPS, it needs to
immediately cease sending QUIC packets on the affected path. This
could result in termination of the connection if an alternative path
cannot be found .Initial ConnectivityThe base protocol is specified in . This provides an
acknowledged PL. A sender can therefore enter the BASE state as soon
as connectivity has been confirmed.Sending QUIC Probe PacketsA probe packet consists of a QUIC Header and a payload containing
PADDING Frames and a PING Frame. PADDING Frames are a single octet
(0x00) and several of these can be used to create a probe packet of
size PROBED_SIZE. QUIC provides an acknowledged PL, a sender can
therefore enter the BASE state as soon as connectivity has been
confirmed.The current specification of QUIC sets the following:
BASE_PMTU: 1200. A QUIC sender needs to pad initial packets
to 1200 bytes to confirm the path can support packets of a
useful size.
MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU
has fallen below 1200 bytes MUST immediately stop sending on the
affected path.
Validating the Path with QUICQUIC provides an acknowledged PL. A sender therefore MUST NOT
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE
state.Handling of PTB Messages by QUICQUIC operates over the UDP transport, and the guidelines on ICMP
validation as specified in Section 5.2 of therefore apply. In addition to UDP Port
validation QUIC can validate an ICMP message by looking for valid
Connection IDs in the quoted packet.AcknowledgementsThis work was partially funded by the European Union's Horizon 2020
research and innovation programme under grant agreement No. 644334
(NEAT). The views expressed are solely those of the author(s).IANA ConsiderationsThis memo includes no request to IANA.If there are no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor.Security ConsiderationsThe security considerations for the use of UDP and SCTP are provided
in the references RFCs. The interval between individual probe packets
MUST be at least one RTT, and the interval between rounds of probing is
determined by the PMTU_RAISE_TIMER. A PL sender needs to ensure that the method used to confirm reception
of probe packets offers protection from off-path attackers injecting
packets into the path. This protection if provided in IETF-defined
protocols (e.g., TCP, SCTP) using a randomly-initialized sequence
number. A description of one way to do this when using UDP is provided
in section 5.1 of ).There are cases where ICMP Packet Too Big (PTB) messages are not
delivered due to policy, configuration or equipment design (see ), this method therefore does not rely
upon PTB messages being received, but is able to utilize these when they
are received by the sender. PTB messages could potentially be used to
cause a node to inappropriately reduce the PLPMTU. A node supporting
DPLPMTUD MUST therefore appropriately validate the payload of PTB
messages to ensure these are received in response to transmitted traffic
(i.e., a reported error condition that corresponds to a datagram
actually sent by the path layer, see ).An on-path attacker, able to create a PTB message could forge PTB
messages that include a valid quoted IP packet. Such an attack could be
used to drive down the PLPMTU. There are two ways this method can be
mitigated against such attacks: First, by ensuring that a PL sender
never reduces the PLPMTU below the base size, solely in response to
receiving a PTB message. This is achieved by first entering the BASE
state when such a message is received. Second, the design does not
require processing of PTB messages, a PL sender could therefore suspend
processing of PTB messages (e.g., in a robustness mode after detecting
that subsequent probes actually confirm that a size larger than the
PTB_SIZE is supported by a path).Parallel forwarding paths SHOULD be considered. identifies the need for robustness in the
method when the path information may be inconsistent.A node performing DPLPMTUD could experience conflicting information
about the size of supported probe packets. This could occur when there
are multiple paths are concurrently in use and these exhibit a different
PMTU. If not considered, this could result in data being black holed
when the PLPMTU is larger than the smallest PMTU across the current
paths.ReferencesNormative ReferencesQUIC: A UDP-Based Multiplexed and Secure TransportThis document defines the core of the QUIC transport protocol. Accompanying documents describe QUIC's loss detection and congestion control and the use of TLS for key negotiation. Note to Readers Discussion of this draft takes place on the QUIC working group mailing list (quic@ietf.org), which is archived at <https://mailarchive.ietf.org/arch/search/?email_list=quic>. Working Group information can be found at <https://github.com/ quicwg>; source code and issues list for this draft can be found at <https://github.com/quicwg/base-drafts/labels/-transport>.User Datagram ProtocolPath MTU discoveryThis memo describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. It specifies a small change to the way routers generate one type of ICMP message. For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice. [STANDARDS-TRACK]Key words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Internet Protocol, Version 6 (IPv6) SpecificationThis document specifies version 6 of the Internet Protocol (IPv6), also sometimes referred to as IP Next Generation or IPng. [STANDARDS-TRACK]The Lightweight User Datagram Protocol (UDP-Lite)This document describes the Lightweight User Datagram Protocol (UDP-Lite), which is similar to the User Datagram Protocol (UDP) (RFC 768), but can also serve applications in error-prone network environments that prefer to have partially damaged payloads delivered rather than discarded. If this feature is not used, UDP-Lite is semantically identical to UDP. [STANDARDS-TRACK]Padding Chunk and Parameter for the Stream Control Transmission Protocol (SCTP)This document defines a padding chunk and a padding parameter and describes the required receiver side procedures. The padding chunk is used to pad a Stream Control Transmission Protocol (SCTP) packet to an arbitrary size. The padding parameter is used to pad an SCTP INIT chunk to an arbitrary size. [STANDARDS-TRACK]Stream Control Transmission ProtocolThis document obsoletes RFC 2960 and RFC 3309. It describes the Stream Control Transmission Protocol (SCTP). SCTP is designed to transport Public Switched Telephone Network (PSTN) signaling messages over IP networks, but is capable of broader applications.SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP. It offers the following services to its users:-- acknowledged error-free non-duplicated transfer of user data,-- data fragmentation to conform to discovered path MTU size,-- sequenced delivery of user messages within multiple streams, with an option for order-of-arrival delivery of individual user messages,-- optional bundling of multiple user messages into a single SCTP packet, and-- network-level fault tolerance through supporting of multi-homing at either or both ends of an association. The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks. [STANDARDS-TRACK]UDP Encapsulation of Stream Control Transmission Protocol (SCTP) Packets for End-Host to End-Host CommunicationThis document describes a simple method of encapsulating Stream Control Transmission Protocol (SCTP) packets into UDP packets and its limitations. This allows the usage of SCTP in networks with legacy NATs that do not support SCTP. It can also be used to implement SCTP on hosts without directly accessing the IP layer, for example, implementing it as part of the application without requiring special privileges.Please note that this document only describes the functionality required within an SCTP stack to add on UDP encapsulation, providing only those mechanisms for two end-hosts to communicate with each other over UDP ports. In particular, it does not provide mechanisms to determine whether UDP encapsulation is being used by the peer, nor the mechanisms for determining which remote UDP port number can be used. These functions are out of scope for this document.This document covers only end-hosts and not tunneling (egress or ingress) endpoints.UDP Usage GuidelinesThe User Datagram Protocol (UDP) provides a minimal message-passing transport that has no inherent congestion control mechanisms. This document provides guidelines on the use of UDP for the designers of applications, tunnels, and other protocols that use UDP. Congestion control guidelines are a primary focus, but the document also provides guidance on other topics, including message sizes, reliability, checksums, middlebox traversal, the use of Explicit Congestion Notification (ECN), Differentiated Services Code Points (DSCPs), and ports.Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as an Internet transport must employ mechanisms to prevent congestion collapse and to establish some degree of fairness with concurrent traffic. They may also need to implement additional mechanisms, depending on how they use UDP.Some guidance is also applicable to the design of other protocols (e.g., protocols layered directly on IP or via IP-based tunnels), especially when these protocols do not themselves provide congestion control.This document obsoletes RFC 5405 and adds guidelines for multicast UDP usage.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Path MTU Discovery for IP version 6This document describes Path MTU Discovery (PMTUD) for IP version 6. It is largely derived from RFC 1191, which describes Path MTU Discovery for IP version 4. It obsoletes RFC 1981.Datagram Transport Layer Security (DTLS) Encapsulation of SCTP PacketsThe Stream Control Transmission Protocol (SCTP) is a transport protocol originally defined to run on top of the network protocols IPv4 or IPv6. This document specifies how SCTP can be used on top of the Datagram Transport Layer Security (DTLS) protocol. Using the encapsulation method described in this document, SCTP is unaware of the protocols being used below DTLS; hence, explicit IP addresses cannot be used in the SCTP control chunks. As a consequence, the SCTP associations carried over DTLS can only be single-homed.Informative ReferencesIP Tunnels in the Internet ArchitectureThis document discusses the role of IP tunnels in the Internet architecture. An IP tunnel transits IP datagrams as payloads in non- link layer protocols. This document explains the relationship of IP tunnels to existing protocol layers and the challenges in supporting IP tunneling, based on the equivalence of tunnels to links. The implications of this document are used to derive recommendations that update MTU and fragment issues in RFC 4459.Internet Control Message ProtocolRequirements for Internet Hosts - Communication LayersThis RFC is an official specification for the Internet community. It incorporates by reference, amends, corrects, and supplements the primary protocol standards documents relating to hosts. [STANDARDS-TRACK]Requirements for IP Version 4 RoutersThis memo defines and discusses requirements for devices that perform the network layer forwarding function of the Internet protocol suite. [STANDARDS-TRACK]TCP Problems with Path MTU DiscoveryThis memo catalogs several known Transmission Control Protocol (TCP) implementation problems dealing with Path Maximum Transmission Unit Discovery (PMTUD), including the long-standing black hole problem, stretch acknowlegements (ACKs) due to confusion between Maximum Segment Size (MSS) and segment size, and MSS advertisement based on PMTU. This memo provides information for the Internet community.Datagram Congestion Control Protocol (DCCP)The Datagram Congestion Control Protocol (DCCP) is a transport protocol that provides bidirectional unicast connections of congestion-controlled unreliable datagrams. DCCP is suitable for applications that transfer fairly large amounts of data and that can benefit from control over the tradeoff between timeliness and reliability. [STANDARDS-TRACK]Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) SpecificationThis document describes the format of a set of control messages used in ICMPv6 (Internet Control Message Protocol). ICMPv6 is the Internet Control Message Protocol for Internet Protocol version 6 (IPv6). [STANDARDS-TRACK]Packetization Layer Path MTU DiscoveryThis document describes a robust method for Path MTU Discovery (PMTUD) that relies on TCP or some other Packetization Layer to probe an Internet path with progressively larger packets. This method is described as an extension to RFC 1191 and RFC 1981, which specify ICMP-based Path MTU Discovery for IP versions 4 and 6, respectively. [STANDARDS-TRACK]Recommendations for Filtering ICMPv6 Messages in FirewallsIn networks supporting IPv6, the Internet Control Message Protocol version 6 (ICMPv6) plays a fundamental role with a large number of functions, and a correspondingly large number of message types and options. ICMPv6 is essential to the functioning of IPv6, but there are a number of security risks associated with uncontrolled forwarding of ICMPv6 messages. Filtering strategies designed for the corresponding protocol, ICMP, in IPv4 networks are not directly applicable, because these strategies are intended to accommodate a useful auxiliary protocol that may not be required for correct functioning.This document provides some recommendations for ICMPv6 firewall filter configuration that will allow propagation of ICMPv6 messages that are needed to maintain the functioning of the network but drop messages that are potential security risks. This memo provides information for the Internet community.Revision NotesNote to RFC-Editor: please remove this entire section prior to
publication.Individual draft -00:
Comments and corrections are welcome directly to the authors or
via the IETF TSVWG working group mailing list.
This update is proposed for WG comments.
Individual draft -01:
Contains the first representation of the algorithm, showing the
states and timers
This update is proposed for WG comments.
Individual draft -02:
Contains updated representation of the algorithm, and textual
corrections.
The text describing when to set the effective PMTU has not yet
been validated by the authors
To determine security to off-path-attacks: We need to decide
whether a received PTB message SHOULD/MUST be validated? The text on
how to handle a PTB message indicating a link MTU larger than the
probe has yet not been validated by the authors
No text currently describes how to handle inconsistent results
from arbitrary re-routing along different parallel paths
This update is proposed for WG comments.
Working Group draft -00:
This draft follows a successful adoption call for TSVWG
There is still work to complete, please comment on this
draft.
Working Group draft -01:
This draft includes improved introduction.
The draft is updated to require ICMP validation prior to
accepting PTB messages - this to be confirmed by WG
Section added to discuss Selection of Probe Size - methods to be
evaluated and recommendations to be considered
Section added to align with work proposed in the QUIC WG.
Working Group draft -02:
The draft was updated based on feedback from the WG, and a
detailed review by Magnus Westerlund.
The document updates RFC 4821.
Requirements list updated.
Added more explicit discussion of a simpler black-hole detection
mode.
This draft includes reorganisation of the section on IETF
protocols.
Added more discussion of implementation within an
application.
Added text on flapping paths.
Replaced 'effective MTU' with new term PLPMTU.
Working Group draft -03:
Updated figures
Added more discussion on blackhole detection
Added figure describing just blackhole detection
Added figure relating MPS sizes
Working Group draft -04:
Described phases and named these consistently.
Corrected transition from confirmation directly to the search
phase (Base has been checked).
Redrawn state diagrams.
Renamed BASE_MTU to BASE_PMTU (because it is a base for the
PMTU).
Clarified Error state.
Clarified suspending DPLPMTUD.
Verified normative text in requirements section.
Removed duplicate text.
Changed all text to refer to /packet probe/probe packet/
/validation/verification/ added term /Probe Confirmation/ and
clarified BlackHole detection.
Working Group draft -05:
Updated security considerations.
Feedback after speaking with Joe Touch helped improve UDP-Options
description.
Working Group draft -06:
Updated description of ICMP issues in section 1.1
Update to description of QUIC.
Working group draft -07:
Moved description of the PTB processing method from the PTB
requirements section.
Clarified what is performed in the PTB validation check.
Updated security consideration to explain PTB security without
needing to read the rest of the document.
Reformatted state machine diagram
Working group draft -08:
Moved to rfcxml v3+
Rendered diagrams to svg in html version.
Removed Appendix A. Event-driven state changes.
Removed section on DPLPMTUD with UDP Options.
Shortened the description of phases.
Working group draft -09:
Remove final mention of UDP Options
Add Initial Connectivity sections to each PL
Add to disable outgoing pmtu enforcement of packets
Working group draft -10:
Address comments from Lars Eggert
Reinforce that PROBE_COUNT is successive attempts to probe for any size
Redefine MAx_PROBES to 3
Address PTB_SIZE of 0 or less that MIN_PMTU
Authors' AddressesUniversity of AberdeenSchool of EngineeringFraser Noble BuildingAberdeenAB24 3UEUKgorry@erg.abdn.ac.ukUniversity of AberdeenSchool of EngineeringFraser Noble BuildingAberdeenAB24 3UEUKtom@erg.abdn.ac.ukMuenster University of Applied SciencesStegerwaldstrasse 3948565SteinfurtDEtuexen@fh-muenster.deMuenster University of Applied SciencesStegerwaldstrasse 3948565SteinfurtDEi.ruengeler@fh-muenster.deMuenster University of Applied SciencesStegerwaldstrasse 3948565SteinfurtDEtimo.voelker@fh-muenster.de