Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Updates: 4821 (if approved) University of Aberdeen
Intended status: Standards Track M. Tuexen
Expires: May 24, 2019 I. Ruengeler
Muenster University of Applied Sciences
November 20, 2018
Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-06
Abstract
This document describes a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization Layers (PLs). The document
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, and no ICMP message
is received). The method can also probe a network path with
progressively larger packets to find whether the maximum packet size
can be increased. This allows a sender to determine an appropriate
packet size, providing functionally for datagram transports that is
equivalent to the Packetization layer PMTUD specification for TCP,
specified in RFC 4821.
The document also provides implementation notes for incorporating
Datagram PMTUD into IETF datagram transports or applications that use
datagram transports.
When published, this specification updates RFC 4821.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 24, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Classical Path MTU Discovery . . . . . . . . . . . . . . 4
1.2. Packetization Layer Path MTU Discovery . . . . . . . . . 6
1.3. Path MTU Discovery for Datagram Services . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 9
4. DPLPMTUD Mechanisms . . . . . . . . . . . . . . . . . . . . . 12
4.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . 12
4.2. Confirmation of Probed Packet Size . . . . . . . . . . . 13
4.3. Detection of Black Holes . . . . . . . . . . . . . . . . 14
4.4. Response to PTB Messages . . . . . . . . . . . . . . . . 14
4.4.1. Validation of PTB Messages . . . . . . . . . . . . . 15
4.4.2. Use of PTB Messages . . . . . . . . . . . . . . . . . 15
5. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 16
5.1. DPLPMTUD Components . . . . . . . . . . . . . . . . . . . 17
5.1.1. Timers . . . . . . . . . . . . . . . . . . . . . . . 17
5.1.2. Constants . . . . . . . . . . . . . . . . . . . . . . 18
5.1.3. Variables . . . . . . . . . . . . . . . . . . . . . . 19
5.2. DPLPMTUD Phases . . . . . . . . . . . . . . . . . . . . . 19
5.2.1. Path Confirmation Phase . . . . . . . . . . . . . . . 21
5.2.2. Search Phase . . . . . . . . . . . . . . . . . . . . 21
5.2.2.1. Resilience to inconsistent path information . . . 22
5.2.3. Search Complete Phase . . . . . . . . . . . . . . . . 22
5.2.4. PROBE_BASE Phase . . . . . . . . . . . . . . . . . . 23
5.2.5. ERROR Phase . . . . . . . . . . . . . . . . . . . . . 23
5.2.5.1. Robustness to inconsistent path . . . . . . . . . 23
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5.2.6. DISABLED Phase . . . . . . . . . . . . . . . . . . . 24
5.3. State Machine . . . . . . . . . . . . . . . . . . . . . . 24
5.4. Search to Increase the PLPMTU . . . . . . . . . . . . . . 27
5.4.1. Probing for a larger PLPMTU . . . . . . . . . . . . . 27
5.4.2. Selection of Probe Sizes . . . . . . . . . . . . . . 28
5.4.3. Resilience to inconsistent Path information . . . . . 29
6. Specification of Protocol-Specific Methods . . . . . . . . . 29
6.1. Application support for DPLPMTUD with UDP or UDP-Lite . . 29
6.1.1. Application Request . . . . . . . . . . . . . . . . . 30
6.1.2. Application Response . . . . . . . . . . . . . . . . 30
6.1.3. Sending Application Probe Packets . . . . . . . . . . 30
6.1.4. Validating the Path . . . . . . . . . . . . . . . . . 30
6.1.5. Handling of PTB Messages . . . . . . . . . . . . . . 30
6.2. DPLPMTUD with UDP Options . . . . . . . . . . . . . . . . 31
6.2.1. UDP Probe Request Option . . . . . . . . . . . . . . 32
6.2.2. UDP Probe Response Option . . . . . . . . . . . . . . 33
6.3. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 33
6.3.1. SCTP/IPv4 and SCTP/IPv6 . . . . . . . . . . . . . . . 33
6.3.1.1. Sending SCTP Probe Packets . . . . . . . . . . . 33
6.3.1.2. Validating the Path with SCTP . . . . . . . . . . 34
6.3.1.3. PTB Message Handling by SCTP . . . . . . . . . . 34
6.3.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 34
6.3.2.1. Sending SCTP/UDP Probe Packets . . . . . . . . . 34
6.3.2.2. Validating the Path with SCTP/UDP . . . . . . . . 35
6.3.2.3. Handling of PTB Messages by SCTP/UDP . . . . . . 35
6.3.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 35
6.3.3.1. Sending SCTP/DTLS Probe Packets . . . . . . . . . 35
6.3.3.2. Validating the Path with SCTP/DTLS . . . . . . . 35
6.3.3.3. Handling of PTB Messages by SCTP/DTLS . . . . . . 35
6.4. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 35
6.4.1. Sending QUIC Probe Packets . . . . . . . . . . . . . 36
6.4.2. Validating the Path with QUIC . . . . . . . . . . . . 36
6.4.3. Handling of PTB Messages by QUIC . . . . . . . . . . 36
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
9. Security Considerations . . . . . . . . . . . . . . . . . . . 37
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2. Informative References . . . . . . . . . . . . . . . . . 39
Appendix A. Event-driven state changes . . . . . . . . . . . . . 40
Appendix B. Revision Notes . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction
The IETF has specified datagram transport using UDP, SCTP, and DCCP,
as well as protocols layered on top of these transports (e.g., SCTP/
UDP, DCCP/UDP, QUIC/UDP), and direct datagram transport over the IP
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network layer. This document describes a robust method for Path MTU
Discovery (PMTUD) that may be used with these transport protocols (or
the applications that use their transport service) to discover an
appropriate size of packet to use across an Internet path.
1.1. Classical Path MTU Discovery
Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
used with any transport that is able to process ICMP Packet Too Big
(PTB) messages (e.g., [RFC1191] and [RFC8201]). The term PTB message
is applied to both IPv4 ICMP Unreachable messages (type 3) that carry
the error Fragmentation Needed (Type 3, Code 4) [RFC0792] and ICMPv6
packet too big messages (Type 2) [RFC4443]. When a sender receives a
PTB message, it reduces the effective MTU to the value reported as
the Link MTU in the PTB message, and a method that from time-to-time
increases the packet size in attempt to discover an increase in the
supported PMTU. The packets sent with a size larger than the current
effective PMTU are known as probe packets.
Packets not intended as probe packets are either fragmented to the
current effective PMTU, or the attempt to send fails with an error
code. Applications are sometimes provided with a primitive to let
them read the Maximum Packet Size (MPS), derived from the current
effective PMTU.
Classical PMTUD is subject to protocol failures. One failure arises
when traffic using a packet size larger than the actual PMTU is
black-holed (all datagrams sent with this size, or larger, are
silently discarded without the sender receiving ICMP PTB messages).
This could arise when the PTB messages are not delivered back to the
sender for some reason [RFC2923]).
Examples where PTB messages are not delivered include:
o The generation of ICMP messages is usually rate limited. This may
result in no PTB messages being sent to the sender (see section
2.4 of [RFC4443]
o ICMP messages are increasingly filtered by middleboxes (including
firewalls) [RFC4890]. A stateful firewall could be configured
with a policy to block incoming ICMP messages, which would prevent
reception of PTB messages to endpoints behind this firewall.
o 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
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formatted ICMP message to the sender [I-D.ietf-intarea-tunnels].
Failure to do this results in black-holing.
o Asymmetry in forwarding can result in there being no route back to
the original sender, which would prevent an ICMP message being
delivered to the sender. This can be also be an issue 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 same server (i.e., ICMP messages
need to follow the same path as the flows to which they
correspond). Failure to do this results in black-holing.
o There are 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 in a PTB to be sent, and result in consequent black-holing.
Another failure could result if a node that is not on the network
path sends a PTB message that attempts to force the sender to change
the effective PMTU [RFC8201]. A sender can protect itself from
reacting to such messages by utilising the quoted packet within 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:
o When a router issuing the ICMP message implements RFC792
[RFC0792], it is only required to include the first 64 bits of the
IP payload of the packet within the quoted payload. This may be
insufficient to perform the tunnel processing described in the
previous bullet. There could be insufficient bytes remaining for
the sender to interpret the quoted transport information. The
recommendation in RFC1812 [RFC1812] is that IPv4 routers return a
quoted packet with as much of the original datagram as possible
without the length of the ICMP datagram exceeding 576 bytes.
(IPv6 routers include as much of invoking packet as possible
without the ICMPv6 packet exceeding 1280 bytes [RFC4443].)
o The use of tunnels/encryption can reduce the size of the quoted
packet returned to the original source address, increasing the
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risk that there could be insufficient bytes remaining for the
sender to interpret the quoted transport information.
o 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).
o When a packet is encapsulated/tunneled over an encrypted
transport, the tunnel/encapsulation ingress might have
insufficient context, or computational power, to reconstruct the
transport header that would be needed to perform validation.
1.2. Packetization Layer Path MTU Discovery
The term Packetization Layer (PL) has been introduced to describe the
layer that is responsible for placing data blocks into the payload of
IP packets and selecting an appropriate MPS. This function is often
performed by a transport protocol, but can also be performed by other
encapsulation methods working above the transport layer.
In contrast to PMTUD, Packetization Layer Path MTU Discovery
(PLPMTUD) [RFC4821] does not rely upon reception and validation of
PTB messages. It is therefore more robust than Classical PMTUD.
This has become the recommended approach for implementing PMTU
discovery with TCP.
It uses a general strategy where the PL sends probe packets to search
for the largest size of unfragmented datagram that can be sent over a
network path. The 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. This 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 PTB
black hole detection and recovery to increase the robustness of
Classical PMTUD, or at the other extreme, all PTB processing can be
disabled and PLPMTUD can completely replace Classical PMTUD.
PLPMTUD can also include additional consistency checks without
increasing the risk of increased black-holing. For instance,the
information available at the PL, or higher layers, makes PTB
validation more straight forward.
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1.3. Path MTU Discovery for Datagram Services
Section 5 of this document presents a set of algorithms for datagram
protocols to discover the largest size of unfragmented datagram that
can be sent over a network path. The method described relies on
features of the PL described in Section 3 and applies to transport
protocols operating over IPv4 and IPv6. It does not require
cooperation from the lower layers, although it can utilise ICMP PTB
messages when these received messages are made available to the PL.
The UDP Usage Guidelines [RFC8085] state "an application SHOULD
either use the Path MTU information provided by the IP layer or
implement Path MTU Discovery (PMTUD)", but does not provide a
mechanism for discovering the largest size of unfragmented datagram
that can be used on a network path. Prior to this document, PLPMTUD
had not been specified for UDP.
Section 10.2 of [RFC4821] recommends a PLPMTUD probing method for the
Stream Control Transport Protocol (SCTP). SCTP utilises heartbeat
messages as probe packets, but RFC4821 does not provide a complete
specification. The present document provides the details to complete
that specification.
The Datagram Congestion Control Protocol (DCCP) [RFC4340] requires
implementations to support Classical PMTUD and states that a DCCP
sender "MUST maintain the MPS allowed for each active DCCP session".
It also defines the current congestion control MPS (CCMPS) supported
by a network path. This recommends use of PMTUD, and suggests use of
control packets (DCCP-Sync) as path probe packets, because they do
not risk application data loss. The method defined in this
specification could be used with DCCP.
Section 6 specifies the method for a set of transports, and provides
information to enable the implementation of PLPMTUD with other
datagram transports and applications that use datagram transports.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [[RFC8174]] when, and only when, they appear in all
capitals, as shown here.
Other terminology is directly copied from [RFC4821], and the
definitions in [RFC1122].
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Actual PMTU: The Actual PMTU is the PMTU of a network path between a
sender PL and a destination PL, which the DPLPMTUD algorithm seeks
to determine.
Black Holed: Packets are Black holed when the sender is unaware that
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 silently discarded by
the network, but is not made aware of a change to the path that
resulted in a smaller PLPMTU by ICMP messages).
Classical Path MTU Discovery: Classical PMTUD is a process described
in [RFC1191] and [RFC8201], in which nodes rely on PTB messages to
learn the largest size of unfragmented datagram that can be used
across a network path.
Datagram: A datagram is a transport-layer protocol data unit,
transmitted in the payload of an IP packet.
Effective PMTU: The Effective PMTU is the current estimated value
for PMTU that is used by a PMTUD. This is equivalent to the
PLPMTU derived by PLPMTUD.
EMTU_S: The Effective MTU for sending (EMTU_S) is defined in
[RFC1122] as "the maximum IP datagram size that may be sent, for a
particular combination of IP source and destination addresses...".
EMTU_R: The Effective MTU for receiving (EMTU_R) is designated in
[RFC1122] as the largest datagram size that can be reassembled by
EMTU_R ("Effective MTU to receive").
Link: A Link is a communication facility or medium over which nodes
can communicate at the link layer, i.e., a layer below the IP
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.
MPS: The Maximum Packet Size (MPS) is the largest size of
application data block that can be sent across a network path. In
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DPLPMTUD this quantity is derived from the PLPMTU by taking into
consideration the size of the lower protocol layer headers.
MIN_PMTU: The MIN_PMTU is the smallest size of PLPMTU that DPLPTMUD
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.
3. Features Required to Provide Datagram PLPMTUD
TCP PLPMTUD has been defined using standard TCP protocol mechanisms.
All of the requirements in [RFC4821] also apply to the use of the
technique with a datagram PL. Unlike TCP, some datagram PLs require
additional mechanisms to implement PLPMTUD.
There are eight requirements for performing the datagram PLPMTUD
method described in this specification:
1. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to provide
information about the maximum size of packet that can be
transmitted by the sender on the local link (the local Link MTU).
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It MAY utilize similar information about the receiver when this
is supplied (note this could be less than EMTU_R). This avoids
implementations trying to send probe packets that can not be
transmitted by the local link. Too high of a value could reduce
the efficiency of the search algorithm. Some applications also
have a maximum transport protocol data unit (PDU) size, in which
case there is no benefit from probing for a size larger than this
(unless a transport allows multiplexing multiple applications
PDUs into the same datagram).
2. PLPMTU: A datagram application is REQUIRED to be able to choose
the size of datagrams sent to the network, up to the PLPMTU, or a
smaller value (such as the MPS) derived from this. This value is
managed by the DPLPMTUD method. The PLPMTU (specified as the
effective PMTU in Section 1 of [RFC1191]) is equivalent to the
EMTU_S (specified in [RFC1122]).
3. Probe packets: On request, a DPLPMTUD sender is REQUIRED to be
able to transmit a packet larger than the PLMPMTU. This is used
to send a probe packet. In IPv4, a probe packet MUST be sent
with the Don't Fragment (DF) bit set in the IP header, and
without network layer endpoint fragmentation. In IPv6, a probe
packet is always sent without source fragmentation (as specified
in section 5.4 of [RFC8201]).
4. Processing PTB messages: A DPLPMTUD sender MAY optionally utilize
PTB messages received from the network layer to help identify
when a network path does not support the current size of probe
packet. Any received PTB message MUST be validated before it is
used to update the PLPMTU discovery information [RFC8201]. This
validation confirms that the PTB message was sent in response to
a packet originating by the sender, and needs to be performed
before the PLPMTU discovery method reacts to the PTB message.
When the PTB_SIZE is indicated in the PTB message, this MAY be
used by DPLPMTUD to reduce the probe size but MUST NOT be used to
increase the PLPMTU ([RFC8201]). This validation SHOULD utilise
information that can not be simply determined by an off-path
attacker, for example, by checking the value of a protocol header
field known only to the two PL endpoints. (Some datagram
applications use well-known source and destination ports and
therefore this check needs to rely on other information.)
5. Reception feedback: The destination PL endpoint is REQUIRED to
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.
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The local PL endpoint at the sending node is REQUIRED to pass
this feedback to the sender-side DPLPMTUD method.
6. Probing and congestion control: The isolated loss of a probe
packet SHOULD NOT be treated as an indication of congestion and
its loss SHOULD NOT directly trigger a congestion control
reaction [RFC4821].
7. Probe loss recovery: If the data block carried by a probe packet
needs to be sent reliably, the PL (or layers above) are REQUIRED
to arrange any retransmission/repair of any resulting loss. This
method is REQUIRED to be robust in the case where probe packets
are lost due to other reasons (including link transmission error,
congestion). The DPLPMTUD sender treats isolated loss of a probe
packet (with or without an PTB message) as a potential indication
of a PMTU limit for the path, but not as an indication of
congestion, see Paragraph 6.
8. Shared PLPMTU state: The PLPMTU value could also be stored with
the corresponding entry in the destination cache and used by
other PL instances. The specification of PLPMTUD [RFC4821]
states: "If PLPMTUD updates the MTU for a particular path, all
Packetization Layer sessions that share the path representation
(as described in Section 5.2 of [RFC4821]) SHOULD be notified to
make use of the new MTU and make the required congestion control
adjustments". Such methods MUST be robust to the wide variety of
underlying network forwarding behaviours, PLPMTU adjustments
based on shared PLPMTU values should be incorporated in the
search algorithms. Section 5.2 of [RFC8201] provides guidance on
the caching of PMTU information and also the relation to IPv6
flow labels.
In addition, the following principles are stated for design of a
DPLPMTUD method:
o 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.
o Path validation: It is RECOMMENDED that methods are robust to path
changes that could have occurred since the path characteristics
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were last confirmed, and to the possibility of inconsistent path
information being received.
o 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.
o When to probe: It is RECOMMENDED that methods determine whether
the path capacity has increased since it last measured the path.
This determines when the path should again be probed.
4. DPLPMTUD Mechanisms
This section lists the protocol mechanisms used in this
specification.
4.1. PLPMTU Probe Packets
The DPLPMTUD method relies upon the PL sender being able to generate
probe packets with a specific size. TCP is able to generate these
probe packets by choosing to appropriately segment data being sent
[RFC4821]. In contrast, a datagram PL that needs to construct a
probe packet has to either request an application to send a data
block that is larger than that generated by an application, or to
utilise padding functions to extend a datagram beyond the size of the
application data block. Protocols that permit exchange of control
messages (without an application data block) could alternatively
prefer to generate a probe packet by extending a control message with
padding data.
A receiver needs to be able to distinguish an in-band data block from
any added padding. This is needed to ensure that any added padding
is not passed on to an application at the receiver.
This results in three possible ways that a sender can create a probe
packet listed in order of preference:
Probing using padding data: A probe packet that contains only
control information together with any padding, which is needed to
be inflated to the size required for the probe packet. Since
these probe packets do not carry an application-supplied data
block, they do not typically require retransmission, although they
do still consume network capacity and incur endpoint processing.
Probing using application data and padding data: A probe packet that
contains a data block supplied by an application that is combined
with padding to inflate the length of the datagram to the size
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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 utilise 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 ICMP PTB
messages.
4.2. Confirmation of Probed Packet Size
The PL needs a method to determine (confirm) when probe packets have
been successfully received end-to-end across a network path.
Transport protocols can include end-to-end methods that detect and
report reception of specific datagrams that they send (e.g., DCCP and
SCTP provide keep-alive/heartbeat features). When supported, this
mechanism SHOULD also be used by DPLPMTUD to acknowledge reception of
a probe packet.
A PL that does not acknowledge data reception (e.g., UDP and UDP-
Lite) is unable itself to detect when the packets that it sends are
discarded because their size is greater than the actual PMTU. These
PLs need to either rely on an application protocol to detect this
loss, or make use of an additional transport method such as UDP-
Options [I-D.ietf-tsvwg-udp-options].
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Section Section 5 specifies this function for a set of IETF-specified
protocols.
4.3. Detection of Black Holes
A PL sender needs to reduce the PLPMTU when it discovers the actual
PMTU supported by a network path is less than the PLPMTU (i.e. to
detect that traffic is being black holed). 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.
Black Hole detection is performed by periodically sending packet
probes of size PLPMTU to verify that a network path still supports
the last acknowledged PLPMTU size. There are two ways a DPLPMTUD
sender detect that the current PLPMTU is not sustained by the path
(i.e., to detect a black hole):
o A PL can rely upon a mechanisms implemented within the PL protocol
to detect excessive loss of data sent with a specific packet size
and then conclude that this excessive loss could be a result of an
invalid PMTU (as in PLPMTUD for TCP [RFC4821]).
o A PL can use the probing mechanism to send confirmation probe
packets of the size of the current PLPMTU and a timer track
whether acknowledgments are received (e.g., The number of probe
packets sent without receiving an acknowledgement, PROBE_COUNT,
becomes greater than the MAX_PROBES). These messages need to be
generated periodically (e.g., using the confirmation timer
Section 5.1.1), and should be suppressed when the PL is not
actively sending data. Successive loss of probes is an indication
that the current path no longer supports the PLPMTU.
When the method detects the current PLPMTU is not supported (a black
hole is found), DPLPMTUD sets a lower MPS. The PL then confirms that
the updated PLPMTU can be successfully used across the path. This
can need the PL 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 could
instead utilise a control packet with padding.
4.4. Response to PTB Messages
This method requires the DPLPMTUD sender to validate any received PTB
message before using the PTB information. The response to a PTB
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message depends on the PTB_SIZE indicated in the PTB message, the
state of the PLPMTUD state machine, and the IP protocol being used.
Section 4.4.1 first describes validation for both IPv4 ICMP
Unreachable messages (type 3) and ICMPv6 packet too big messages,
both of which are referred to as PTB messages in this document.
4.4.1. Validation of PTB Messages
A PL that receives a PTB message from a router or middlebox, MUST
perform ICMP validation as specified in Section 5.2 of [RFC8085].
This needs the PL to check the protocol information in the quoted
payload to validate the message originated from the sending node.
This check includes determining the appropriate port and IP
information - necessary for the PTB message to be passed to the PL.
In addition, the PL SHOULD validate information from the ICMP payload
to determine that the quoted packet was sent by the PL. These checks
are intended to provide protection from packets that originate from a
node that is not on the network path. PTB messages are discarded if
they fail to pass these checks, or where there is insufficient ICMP
payload to perform the checks
PTB messages that have been validated can be utilised by the DPLPMTUD
algorithm. A method that utilises these PTB messages can improve the
speed at the which the algorithm detects an appropriate PLPMTU,
compared to one that relies solely on probing.
4.4.2. Use of PTB Messages
A set of checks are intended to provide protection from a router that
reports an unexpected PTB_SIZE. The PL 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 an informative summary of how PTB messages can
be utilised.
Validating PTB Messages:
* A simple implementation is permitted to ignore received PTB
messages and therefore the PLPMTU is not updated when a PTB
message is received.
* An implementation that supports PTB messages MUST validate
messages before they are processed.
MIN_PMTU < PTB_SIZE < BASE_MTU
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* A robust PL MAY enter the PROBE_ERROR state for an IPv4 path
when the PTB_SIZE reported in the PTB message >= 576B and when
this is less than the BASE_MTU.
* A robust PL MAY enter the PROBE_ERROR state for an IPv6 path
when the PTB_SIZE reported in the PTB message >= 1280B and when
this is less than the BASE_MTU.
PTB_SIZE = PLPMTU
* Transition to SEARCH_COMPLETE.
PTB_SIZE > PROBED_SIZE
* The PTB_SIZE > PROBED_SIZE, inconsistent network signal. These
PTB messages ought to be discarded without further processing
(the PLPMTU not updated).
* The information could be utilised 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 PTB_SIZE reported in the PTB message to
initialise 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.
5. Datagram Packetization Layer PMTUD
This section specifies Datagram PLPMTUD (DPLPMTUD). The method can
be introduced at various points in the IP protocol stack to discover
the PLPMTU so that an application can utilise an appropriate MPS for
the current network path.
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+----------------------+
| APP* |
+-+-------+----+---+---+
| | | |
+---+--+ +--+--+ | +-+---+
| QUIC*| |UDPO*| | |SCTP*|
+---+--+ +--+--+ | ++--+-+
| | | | |
+-------+-+ | | |
| | | |
++-+--++ |
| UDP | |
+---+--+ |
| |
+--------------+-----+-+
| Network Interface |
+----------------------+
Figure 1: Examples where DPLPMTUD can be implemented
The central idea of DPLPMTUD is probing by a sender. Probe packets
are sent to find the maximum size of user message that is completely
transferred across the network path from the sender to the
destination.
This section identifies the components needed for implementation, the
phases of operation, the state machine and search algorithm.
5.1. DPLPMTUD Components
This section describes components of DPLPMTUD.
5.1.1. Timers
The method utilises 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 be larger than 1 second, and SHOULD
be larger than 15 seconds. Guidance on selection of the timer
value are provided in section 3.1.1 of the UDP Usage Guidelines
[RFC8085].
If the PL has a path Round Trip Time (RTT) estimate and timely
acknowledgements the PROBE_TIMER can be derived from the PL RTT
estimate.
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PMTU_RAISE_TIMER: The PMTU_RAISE_TIMER is configured to the period a
sender will continue to use the current PLPMTU, after which it re-
enters the Search phase. This timer has a period of 600 secs, as
recommended by PLPMTUD [RFC4821].
DPLPMTUD SHOULD inhibit sending probe packets when no application
data has been sent since the previous probe packet.
CONFIRMATION_TIMER: The CONFIRMATION_TIMER is configured to the
period a PL sender waits before confirming the current PLPMTU is
still supported. This is less than the PMTU_RAISE_TIMER and used
to decrease the PLPMTU (e.g., when a black hole is encountered).
Confirmation needs to be frequent enough when data is flowing that
the sending PL does not black hole extensive amounts of traffic.
Guidance on selection of the timer value are provided in section
3.1.1 of the UDP Usage Guidelines[RFC8085].
DPLPMTUD SHOULD inhibit sending probe packets when no application
data has been sent since the previous probe packet.
An implementation could implement the various timers using a single
timer process.
5.1.2. Constants
The following constants are defined:
MAX_PROBES: MAX_PROBES is the maximum value of the
PROBE_ERROR_COUNTER. The default value of MAX_PROBES is 10.
MIN_PMTU: The MIN_PMTU is smallest allowed probe packet size. For
IPv6, this value is 1280 bytes, as specified in [RFC2460]. For
IPv4, the minimum value is 68 bytes. (An IPv4 router is required
to be able to forward a datagram of 68 octets without further
fragmentation. This is the combined size of an IPv4 header and
the minimum fragment size of 8 octets. In addition, receivers are
required to be able to reassemble fragmented datagrams at least up
to 576B, as stated in section 3.3.3 of [RFC1122]))
MAX_PMTU: The MAX_PMTU is the largest size of PLPMTU. This has to
be less than or equal to the minimum of the local MTU of the
outgoing interface and the destination PMTU for receiving. An
application or PL MAY reduce the MAX_PMTU when there is no need to
send packets larger than a specific size.
BASE_PMTU: The BASE_PMTU is a configured size expected to work for
most paths. The size is equal to or larger than the MIN_PMTU and
smaller than the MAX_PMTU. In the case of IPv6, this value is
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1280 bytes [RFC2460]. When using IPv4, a size of 1200 bytes is
RECOMMENDED.
5.1.3. Variables
This method utilises a set of variables:
PROBED_SIZE: The PROBED_SIZE is the size of the current probe
packet. This is a tentative value for the PLPMTU, which is
awaiting confirmation by an acknowledgment.
PROBE_COUNT: The PROBE_COUNT is a count of the number of
unsuccessful probe packets that have been sent with a size of
PROBED_SIZE. The value is initialised to zero when a particular
size of PROBED_SIZE is first attempted.
The figure below illustrates the relationship between the packet size
constants and variables, in this case when the DPLPMTUD algorithm
performs path probing to increase the size of the PLPMTU. The MPS is
less than the PLPMTU. A probe packet has been sent of size
PROBED_SIZE. When this is acknowledged, the PLPMTU will be raised to
PROBED_SIZE allowing the PROBED_SIZE to be increased towards the
actual PMTU.
MIN_PMTU PMTU_MAX
<------------------------------------------------------>
| | | | |
V | | | V
BASE_PMTU V | V Actual PMTU
MPS | PROBED_SIZE
V
PLPMTU
Figure 2: Relationships between probe and packet sizes
5.2. DPLPMTUD Phases
The Datagram PLPMTUD algorithm moves through several phases of
operation.
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
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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).
Black hole detection, see Section 4.3 and PTB processing Section 4.4
proceed in parallel with these phases of operation.
+-------------------+
| Path Confirmation +-- Connectivity
+--------+----------+ \----- or BASE_PMTU
| /\ \/ Confirmation Fails
Connectivity and | | +-------+
BASE_PMTU confirmed | ---------+ Error |
| +-------+
| CONFIRMATION_TIMER
| Fires
\/
+----------------+ +--------------+
| Search Complete|<---------+ Search |
+----------------+ +--------------+
Search Algorithm
Completes
Figure 3: DPLPMTUD Phases
Path Confirmation
* Connectivity is confirmed.
* DPLPMTUD confirms the BASE_PMTU is supported across the network
path.
* DPLPMTUD then enters the search phase.
Search
* DPLPMTUD performs probing to increase the PLPMTU.
* DPLPMTUD then enters the search complete or an error phase.
Search Complete
* DPLPMTUD has found a suitable PLPMTU that is supported across
the network path.
* Black hole detection will confirm this PLPMTU continues to be
supported.
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* On a longer time-frame, DPLPMTUD will re-enter the search phase
to discover if the PLPMTU can be raised.
Error
* Inconsistent or invalid network signals cause DPLPMTUD to be
unable to progress.
* This causes the algorithm to lower the MPS until the path is
shown to support the BASE_PMTU, or to suspend DPLPMTUD.
5.2.1. Path Confirmation Phase
DPLPMTUD starts in the Path confirmation phase. Path confirmation is
performed in two stages:
1. Connectivity to the remote peer is first confirmed. When a
connection-oriented PL is used, this stage is implicit. It is
performed as part of the normal PL connection handshake. In
contrast, an connectionless PL MUST send an acknowledged probe
packet to confirm that the remote peer is reachable.
2. In the second stage, the PL confirms it can successfully send a
datagram of the BASE_PMTU size across the current path.
A PL that does not wish to support a network path with a PLPMTU less
than BASE_PMTU can simplify the phase into a single step by
performing connectivity checks with probes of the BASE_PMTU size.
A PL MAY respond to PTB messages while in this phase, see
Section 4.4.
Once path confirmation has completed, DPLPMTUD can advertise an MPS
to an upper layer.
If DPLPMTUD fails to complete these tests it enters the
PROBE_DISABLED phase, see Section 5.2.6, and ceases using DPLPTMUD.
5.2.2. Search Phase
The search phase utilises a search algorithm in attempt to increase
the PLPMTU (see Section 5.4.1). The PL sender increases the MPS each
time a packet probe confirms a larger PLPMTU is supported by the
path. The algorithm concludes by entering the SEARCH_COMPLETE phase,
see Section 5.2.3.
A PL MAY respond to PTB messages while in this phase, using the PTB
to advance or terminate the search, see Section 4.4. Similarly black
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hole detection can terminate the search by entering the PROBE_BASE
phase, see Section 5.2.4.
5.2.2.1. Resilience to inconsistent path information
Sometimes a PL sender is able to detect inconsistent results from the
sequence of PLPMTU probes that it sends or the sequence of PTB
messages that it receives. This could be manifested as excessive
fluctuation of the MPS.
When inconsistent path information is detected, a PL sender can
enable an alternate search mode that clamps the offered MPS to a
smaller value for a period of time. This avoids unnecessary black-
holing of packets.
5.2.3. Search Complete Phase
On entry to the search complete phase, the DPLPMTUD sender starts the
PMTU_RAISE_TIMER. In this phase, the PLPMTU remains at the value
confirmed by the last successful probe packet.
In this phase, the PL MUST periodically confirm that the PLPMTU is
still supported by the path. If the PL is designed in a way that is
unable to confirm reachability to the destination endpoint after
probing has completed, the method uses a CONFIRMATION_TIMER to
periodically repeat a probe packet for the current PLPMTU size.
If the DPLPMTUD sender is unable to confirm reachability for packets
with a size of the current PLPMTU (e.g., if the CONFIRMATION_TIMER
expires) or the PL signals a lack of reachability, the method exits
the phase and enters the PROBE_BASE phase, see Section 5.2.4.
If the PMTU_RAISE_TIMER expires, the DPLPMTUD sender re-enters the
Search phase, see Section 5.2.2, and resumes probing for a larger
PLPMTU.
Back hole detection can be used in parallel to check that a network
path continues to support a previously confirmed PLPMTU. If a black
hole is detected the algorithm moves to the PROBE_BASE phase, see
Section 5.2.4.
The phase can also exited when a validated PTB message is received
(see Section 4.4.1).
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5.2.4. PROBE_BASE Phase
This phase is entered when black hole detection or a PTB message
indicates that the PLPMTU is not supported by the path.
On entry to this phase, the PLPMTU is set to the BASE_PMTU, and a
corresponding reduced MPS is advertised.
PROBED_SIZE is then set to the PLPMTU (i.e., the BASE_PMTU), to
confirm this size is supported across the path. If confirmed,
DPLPMTUD enters the Search Phase to determine whether the PL sender
can use a larger PLPMTU.
If the path cannot be confirmed to support the BASE_PMTU after
sending MAX_PROBES, DPLPMTUD moves to the Error phase, see
Section 5.2.5.
5.2.5. ERROR Phase
The ERROR phase is entered when there is conflicting or invalid
PLPMTU information for the path (e.g. a failure to support the
BASE_PMTU). In this phase, the MPS is set to a value less than the
BASE_PMTU, but at least the size of the MIN_PMTU.
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.
Note: MIN_PMTU may be identical to BASE_PMTU, simplifying the actions
in this phase.
If no acknowledgement is received for PROBE_COUNT probes of size
MIN_PMTU, the method suspends DPLPMTUD, see Section 5.2.5.
5.2.5.1. Robustness to inconsistent path
Robustness to paths unable to sustain the BASE_PMTU. Some paths
could be unable to sustain packets of the BASE_PMTU size. These
paths could use an alternate algorithm to implement the PROBE_ERROR
phase that allows fallback to a smaller than desired PLPMTU, rather
than suffer connectivity failure.
This could also utilise methods such as endpoint IP fragmentation to
enable the PL sender to communicate using packets smaller than the
BASE_PMTU.
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5.2.6. DISABLED Phase
This phase suspends operation of DPLPMTUD. It disables probing for
the PLPMTU until action is taken by the PL or application using the
PL.
5.3. State Machine
A state machine for DPLPMTUD is depicted in Figure 4. If multihoming
is supported, a state machine is needed for each active path.
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PROBE_TIMER expiry
(PROBE_COUNT = MAX_PROBES)
+-------------------+ +--------------+
| PROBE_START +------>|PROBE_DISABLED|
+-------------------+ +--------------+
| ^
| Path confirmed |
v |
MAX_PMTU acked or +--------------+-+ (PROBE_COUNT |
PTB (BASE_PMTU <= +---------| PROBE_SEARCH | | < MAX_PROBES) |
PTB_SIZE | +--> +--------------+<+ or Probe acked |
<PROBED_SIZE) | | | ^ | |
or | | | | | |
(PROBE_COUNT | | | | |((PTB_SIZE < |
=MAX_PROBES) | | | | | BASE_PMTU) |
+---------------+ | | | | or |
| | | | |(PLPMTU < BASE_MTU)) |
| | | | |and (PROBE_COUNT = |
| PMTU_RAISE_TIMER | | | | MAX_PROBES) |
| | | | | |
| | | | \ |
| +-----------+ | | \ Suspend DPLPDMTUD:|
| | | | \ |
| | | | \---------+ |
| | (PTB_SIZE < PLPMTU)| | | |
| | or | | BASE_PMTU | |
| | Black hole detected | | Probe acked | |
v | v | v |
+----------+----+ +--------------+ +-------------+
|SEARCH_COMPLETE|----------->| PROBE_BASE |<-------| PROBE_ERROR |
+------+--------+ +--------------+ +-------------+
/\ | Black hole detected ^ | | BASE_PMTU Probe acked: ^
| | or | | | |
| | (PTB_SIZE < PLPMTU) | | | Probe BASE_PMTU: |
| | | | | (PROBE_COUNT = MAX_PROBES)|
| | | | +---------------------------+
+----+ +--+
Confirmation: PROBE_TIMER expiry:
(PROBE_COUNT < MAX_PROBES) (PROBE_COUNT < MAX_PROBES)
or
PLPMTU Probe acked
Figure 4: State machine for Datagram PLPMTUD. Note: Some state
changes are not show to simplify the diagram.
The following states are defined:
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PROBE_START: The PROBE_START state is the initial state before
probing has started. The state confirms connectivity to the
remote PL.
The PLPMTU is set to the BASE_PMTU size. Probing ought to start
immediately after connection setup to prevent the prevent the loss
of user data. PLPMTUD is not performed in this state. The state
transitions to PROBE_SEARCH, when a network path has been
confirmed, i.e., when a sent packet has been acknowledged on this
network path and the BASE_PMTU is confirmed to be supported. If
the network path cannot be confirmed this state transitions to
PROBE_DISABLED.
PROBE_SEARCH: The PROBE_SEARCH state is the main probing state.
This state is entered when probing for the BASE_PMTU was
successful.
The PROBE_COUNT is set to zero when the first probe packet is sent
for each probe size. Each time a probe packet is acknowledged,
the PLPMTU is set to the PROBED_SIZE, and then the PROBED_SIZE is
increased using the search algorithm.
When a probe packet is sent and not acknowledged within the period
of the PROBE_TIMER, the PROBE_COUNT is incremented and the probe
packet is retransmitted. The state is exited when the PROBE_COUNT
reaches MAX_PROBES; a PTB message is validated; a probe of size
PMTU_MAX is acknowledged or black hole detection is triggered.
SEARCH_COMPLETE: The SEARCH_COMPLETE state indicates a successful
end to the PROBE_SEARCH state. DPLPMTUD remains in this state
until either the PMTU_RAISE_TIMER expires; a received PTB message
is validated; or black hole detection is triggered.
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 the probe packet fails to be acknowledged after
MAX_PROBES attempts, the method enters the PROBE_BASE state. When
used with an acknowledged PL (e.g., SCTP), DPLPMTUD SHOULD NOT
continue to generate PLPMTU probes in this state.
PROBE_BASE: The PROBE_BASE state is used to confirm whether 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.
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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, and the PROBE_TIMER is started.
The state is exited when the probe packet is acknowledged, and the
PL sender enters the PROBE_SEARCH state.
The state is also left when the PROBE_COUNT reaches MAX_PROBES; a
PTB message is validated. This causes the PL sender to enter the
PROBE_ERROR state.
PROBE_ERROR: The PROBE_ERROR state represents the case where the
network path is not known to support a PLPMTU of at least the
BASE_PMTU size. It is entered when either a probe of size
BASE_PMTU has not been acknowledged or a validated PTB message
indicates a smaller PTB_SIZE smaller than the BASE_PMTU.
On entry, the PROBE_COUNT is set to zero and the PROBED_SIZE is
set to the MIN_PMTU size, and the PLPMTU is reset to MIN_PMTU
size. In this state, a probe packet is sent, and the PROBE_TIMER
is started. The state transitions to the PROBE_SEARCH state when
a probe packet is acknowledged of at least size BASE_PMTU. Robust
implementations may validate the BASE_PMTU several times before
transition to the PROBE_SEARCH.
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 PROBE_DISABLED.
PROBE_DISABLED: The PROBE_DISABLED state indicates that connectivity
could not be established. DPLPMTUD MUST NOT probe in this state.
Appendix A contains an informative description of key events.
5.4. Search to Increase the PLPMTU
This section describes the algorithms used by DPLPMTUD to search for
a larger PLPMTU.
5.4.1. Probing for a larger PLPMTU
Implementations use a search algorithm across the search range to
determine whether a larger PLPMTU can be supported across a network
path.
The method discovers the search range by confirming the minimum
PLPMTU and then using the probe method to select a PROBED_SIZE less
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than or equal to PMTU_MAX. PMTU_MAX is the minimum of the local MTU
and EMTU_R (learned from the remote endpoint). The PMTU_MAX MAY be
reduced by an application that sets a maximum to the size of
datagrams it will send.
The PROBE_COUNT is initialised to zero when a probe packet is first
sent with a particular size. 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 Section 4.1.
Each time a probe packet is sent to the destination, the PROBE_TIMER
is started. The timer is cancelled when the PL receives
acknowledgment that the probe packet has been successfully sent
across the path Section 4.1. This confirms that the PROBED_SIZE is
supported, and the PROBED_SIZE value is then assigned to the PLPMTU.
The search algorithm can continue to send subsequent probe packets of
an increasing size.
If the timer expires before a probe packet is acknowledged, the probe
has failed to confirm the PROBED_SIZE. Each time the PROBE_TIMER
expires, the PROBE_COUNT is incremented, the PROBE_TIMER is
reinitialised, and a probe packet of the same size is retransmitted
(the replicated probe improve the resilience to loss). The maximum
number of retransmissions for a particular size is configured
(MAX_PROBES). If the value of the PROBE_COUNT reaches MAX_PROBES,
probing will stop, and the PL sender enters the SEARCH_COMPLETE
state.
5.4.2. Selection of Probe Sizes
The search algorithm needs to determine a minimum useful gain in
PLPMTU. It would not be constructive for a PL sender to attempt to
probe for all sizes - this would incur unnecessary load on the path
and has the undesirable effect of slowing the time to reach a more
optimal MPS. Implementations SHOULD select the set of probe packet
sizes to maximise the gain in PLPMTU from each search step.
Implementations could optimize the search procedure by selecting step
sizes from a table of common PMTU sizes. When selecting the
appropriate next size to search, an implementor ought to also
consider that there can be common sizes of MPS that applications seek
to use.
xxx Author Note: A future version of this section will detail example
methods for selecting probe size values, but does not plan to mandate
a single method. xxx
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5.4.3. Resilience to inconsistent Path information
A decision to increase the PLPMTU needs to be resilient to the
possibility that information learned about the network path is
inconsistent (this could happen when probe packets are lost due to
other reasons, or some of the packets in a flow are forwarded along a
portion of the path that supports a different actual PMTU).
Frequent path changes could occur due to unexpected "flapping" -
where some packets from a flow pass along one path, but other packets
follow a different path with different properties. DPLPMTUD can be
made resilient to these anomalies by introducing hysteresis into the
search decision to increase the MPS.
6. Specification of Protocol-Specific Methods
This section specifies protocol-specific details for datagram PLPMTUD
for IETF-specified transports.
The first subsection provides guidance on how to implement the
DPLPMTUD method as a part of an application using UDP or UDP-Lite.
The guidance also applies to other datagram services that do not
include a specific transport protocol (such as a tunnel
encapsulation). The following subsection describe how DPLPMTUD can
be implemented as a part of the transport service, allowing
applications using the service to benefit from discovery of the
PLPMTU without themselves needing to implement this method.
6.1. Application support for DPLPMTUD with UDP or UDP-Lite
The current specifications of UDP [RFC0768] and UDP-Lite [RFC3828] do
not define a method in the RFC-series that supports PLPMTUD. In
particular, the UDP transport does not provide the transport layer
features needed to implement datagram PLPMTUD.
The DPLPMTUD method can be implemented as a part of an application
built directly or indirectly on UDP or UDP-Lite, but relies on
higher-layer protocol features to implement the method [RFC8085].
Some primitives used by DPLPMTUD might not be available via the
Datagram API (e.g., the ability to access the PLPMTU cache, or
interpret received ICMP PTB messages).
In addition, it is desirable that PMTU discovery is not performed by
multiple protocol layers. An application SHOULD avoid implementing
DPLPMTUD when the underlying transport system provides this
capability. Using a common method for managing the PLPMTU has
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benefits, both in the ability to share state between different
processes and opportunities to coordinate probing.
6.1.1. Application Request
An application needs an application-layer protocol mechanism (such as
a message acknowledgement method) that solicits a response from a
destination endpoint. The method SHOULD allow the sender to check
the value returned in the response to provide additional protection
from off-path insertion of data [RFC8085], suitable methods include a
parameter known only to the two endpoints, such as a session ID or
initialised sequence number.
6.1.2. Application Response
An application needs an application-layer protocol mechanism to
communicate the response from the destination endpoint. This
response may indicate successful reception of the probe across the
path, but could also indicate that some (or all packets) have failed
to reach the destination.
6.1.3. Sending Application Probe Packets
A probe packet that may carry an application data block, but the
successful transmission of this data is at risk when used for
probing. Some applications may prefer to use a probe packet that
does not carry an application data block to avoid disruption to
normal data transfer.
6.1.4. Validating the Path
An application that does not have other higher-layer information
confirming correct delivery of datagrams SHOULD implement the
CONFIRMATION_TIMER to periodically send probe packets while in the
SEARCH_COMPLETE state.
6.1.5. Handling of PTB Messages
An application that is able and wishes to receive PTB messages MUST
perform ICMP validation as specified in Section 5.2 of [RFC8085].
This requires that the application to check each received PTB
messages to validate it is received in response to transmitted
traffic and that the reported PTB_SIZE is less than the current
probed size. A validated PTB message MAY be used as input to the
DPLPMTUD algorithm, but MUST NOT be used directly to set the PLPMTU.
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6.2. DPLPMTUD with UDP Options
UDP Options[I-D.ietf-tsvwg-udp-options] can supply the additional
functionality required to implement DPLPMTUD within the UDP transport
service. Implementing DPLPMTU using UDP Options avoids the need for
each application to implement the DPLPMTUD method.
Section 5.6 of[I-D.ietf-tsvwg-udp-options] defines the Maximum
Segment Size (MSS) option, which allows the local sender to indicate
the EMTU_R to the peer. The value received in this option can be
used to initialise PMTU_MAX.
UDP Options enables padding to be added to UDP datagrams that are
used as Probe Packets. Feedback confirming reception of each Probe
Packet is provided by two new UDP Options:
o The Probe Request Option (Section 6.2.1) is set by a sending PL to
solicit a response from a remote endpoint. A four-byte token
identifies each request.
o The Probe Response Option (Section 6.2.2 is generated by the UDP
Options receiver in response to reception of a previously received
Probe Request Option. Each Probe Response Option echoes a
previously received four-byte token.
The token value allows implementations to be distinguish between
acknowledgements for initial probe packets and acknowledgements
confirming receipt of subsequent probe packets (e.g., travelling
along alternate paths with a larger RTT). Each probe packet needs to
be uniquely identifiable by the UDP Options sender within the Maximum
Segment Lifetime (MSL). The UDP Options sender therefore needs to
not recycle token values until they have expired or have been
acknowledged. A 4 byte value for the token field provides sufficient
space for multiple unique probes to be made within the MSL.
The initial value of the four byte token field SHOULD be assigned to
a randomised value, as described in section 5.1 of [RFC8085]) to
enhance protection from off-path attacks.
Implementations ought to only send a probe packet with a Request
Probe Option when required by their local state machine, i.e., when
probing to grow the PLPMTU or to confirm the current PLPMTU. The
procedure to handle the loss of a response packet is the
responsibility of the sender of the request. Implementations are
allowed to track multiple requests and respond to them with a single
packet.
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A PL needs to determine that the path can still support the size of
datagram that the application is currently sending in the DPLPMTUD
search_done state (i.e., to detect black-holing of data). One way to
achieve this is to send probe packets of size PLPMTU or to utilise a
higher-layer method that provides explicit feedback indicating any
packet loss. Another possibility is to utilise data packets that
carry a Timestamp Option. Reception of a valid timestamp that was
echoed by the remote endpoint can be used to infer connectivity.
This can provide useful feedback even over paths with asymmetric
capacity and/or that carry UDP Option flows that have very asymmetric
datagram rates, because an echo of the most recent timestamp still
indicates reception of at least one packet of the transmitted size.
This is sufficient to confirm there is no black hole.
In contrast, when sending a probe to increase the PLPMTU, a timestamp
might be unable to unambiguously identify that a specific probe
packet has been received. Timestamp mechanisms cannot be used to
confirm the reception of individual probe messages and cannot be used
to stimulate a response from the remote peer.
6.2.1. UDP Probe Request Option
The Probe Request Option allows a sending endpoint to solicit a
response from a destination endpoint.
The Probe Request Option carries a four byte token set by the sender.
This token can be set to a value that is likely to be known only to
the sender (and is sent along the end-to-end path). The initial
value of the token SHOULD be assigned to a randomised value, as
described in section 5.1 of [RFC8085]) to enhance protection from
off-path attacks.
The sender needs to then check the value returned in the UDP Probe
Response Option. The value of the Token field, uniquely identifies a
probe within the maximum segment lifetime.
+----------+--------+-----------------+
| Kind=9* | Len=6 | Token |
+----------+--------+-----------------+
1 byte 1 byte 4 bytes
* To be confirmed by IANA.
Figure 5: UDP Probe REQ Option Format
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6.2.2. UDP Probe Response Option
The Probe Response Option is generated in response to reception of a
previously received Probe Request Option. This response is generated
by the UDP Option processing.
The Probe Response Option carries a four byte token field. The Token
field associates the response with the Token value carried in the
most recently-received Echo Request. The rate of generation of UDP
packets carrying a Probe Response Option is expected to be less than
once per RTT and SHOULD be rate-limited (see Section 9).
+----------+--------+-----------------+
| Kind=10* | Len=6 | Token |
+----------+--------+-----------------+
1 byte 1 byte 4 bytes
* To be confirmed by IANA.
Figure 6: UDP Probe RES Option Format
6.3. DPLPMTUD for SCTP
Section 10.2 of [RFC4821] specifies a recommended PLPMTUD probing
method for SCTP. It recommends the use of the PAD chunk, defined in
[RFC4820] to be attached to a minimum length HEARTBEAT chunk to build
a probe packet. This enables probing without affecting the transfer
of user messages and without interfering with congestion control.
This is preferred to using DATA chunks (with padding as required) as
path probes.
XXX Author Note: Future versions of this document might define a
parameter contained in the INIT and INIT ACK chunk to indicate the
remote peer MTU to the local peer. However, multihoming makes this a
bit complex, so it might not be worth doing. XXX
6.3.1. SCTP/IPv4 and SCTP/IPv6
The base protocol is specified in [RFC4960]. This provides an
acknowledged PL. A sender can therefore enter the PROBE_BASE state
as soon as connectivity has been confirmed.
6.3.1.1. Sending SCTP Probe Packets
Probe packets consist of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
the length of the probe packet. The HEARTBEAT chunk is used to
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trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful probe.
The HEARTBEAT chunk carries a Heartbeat Information parameter which
should include, besides the information suggested in [RFC4960], the
probe size, which is the size of the complete datagram. The size of
the PAD chunk is therefore computed by reducing the probing size by
the IPv4 or IPv6 header size, the SCTP common header, the HEARTBEAT
request and the PAD chunk header. The payload of the PAD chunk
contains arbitrary data.
To avoid fragmentation of retransmitted data, probing starts right
after the handshake, before data is sent. Assuming normal behaviour
(i.e., the PMTU is smaller than or equal to the interface MTU), this
process will take a few round trip time periods depending on the
number of PMTU sizes probed. The Heartbeat timer can be used to
implement the PROBE_TIMER.
6.3.1.2. Validating the Path with SCTP
Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.3.1.3. PTB Message Handling by SCTP
Normal ICMP validation MUST be performed as specified in Appendix C
of [RFC4960]. This requires that the first 8 bytes of the SCTP
common header are quoted in the payload of the PTB message, which can
be the case for ICMPv4 and is normally the case for ICMPv6.
When a PTB message has been validated, the PTB_SIZE reported in the
PTB message SHOULD be used with the DPLPMTUD algorithm, providing
that the reported PTB_SIZE is less than the current probe size.
6.3.2. DPLPMTUD for SCTP/UDP
The UDP encapsulation of SCTP is specified in [RFC6951].
6.3.2.1. Sending SCTP/UDP Probe Packets
Packet probing can be performed as specified in Section 6.3.1.1. The
maximum payload is reduced by 8 bytes, which has to be considered
when filling the PAD chunk.
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6.3.2.2. Validating the Path with SCTP/UDP
Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.3.2.3. Handling of PTB Messages by SCTP/UDP
Normal ICMP validation MUST be performed for PTB messages as
specified in Appendix C of [RFC4960]. This requires that the first 8
bytes of the SCTP common header are contained in the PTB message,
which can be the case for ICMPv4 (but note the UDP header also
consumes a part of the quoted packet header) and is normally the case
for ICMPv6. When the validation is completed, the PTB_SIZE indicated
in the PTB message SHOULD be used with the DPLPMTUD providing that
the reported PTB_SIZE is less than the current probe size.
6.3.3. DPLPMTUD for SCTP/DTLS
The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
specified in [RFC8261]. It is used for data channels in WebRTC
implementations.
6.3.3.1. Sending SCTP/DTLS Probe Packets
Packet probing can be done as specified in Section 6.3.1.1.
6.3.3.2. Validating the Path with SCTP/DTLS
Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.3.3.3. Handling of PTB Messages by SCTP/DTLS
It is not possible to perform normal ICMP validation as specified in
[RFC4960], since even if the ICMP message payload contains sufficient
information, the reflected SCTP common header would be encrypted.
Therefore it is not possible to process PTB messages at the PL.
6.4. DPLPMTUD for QUIC
Quick UDP Internet Connection (QUIC) [I-D.ietf-quic-transport] is a
UDP-based transport that provides reception feedback. The UDP
payload includes the QUIC packet header, protected payload, and any
authentication fields. QUIC depends on a PMTU of at least 1280
bytes.
Section 9.2 of [I-D.ietf-quic-transport] describes the path
considerations when sending QUIC packets. It recommends the use of
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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 illict an acknowledgement. Padding
only frames enable probing the without affecting the transfer of
other QUIC frames.
The recommendation for QUIC endpoints implementing DPLPMTUD is
therefore that a MPS is maintained for each combination of local and
remote IP addresses [I-D.ietf-quic-transport]. If a QUIC endpoint
determines that the PMTU between any pair of local and remote IP
addresses has fallen below an acceptable MPS, it needs to immediately
cease sending QUIC packets on the affected path. This could result
in termination of the connection if an alternative path cannot be
found [I-D.ietf-quic-transport].
6.4.1. Sending QUIC Probe Packets
A probe packet consists of a QUIC Header and a payload containing
PADDING Frames and a PING Frame. PADDING Frames are a single octet
(0x00) and several of these can be used to create a probe packet of
size PROBED_SIZE. QUIC provides an acknowledged PL, A sender can
therefore enter the PROBE_BASE state as soon as connectivity has been
confirmed.
The current specification of QUIC sets the following:
o 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.
o MIN_PMTU: 1200 bytes. A QUIC sender that determines the PMTU has
fallen below 1200 bytes MUST immediately stop sending on the
affected path.
6.4.2. Validating the Path with QUIC
QUIC provides an acknowledged PL. A sender therefore MUST NOT
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.4.3. Handling of PTB Messages by QUIC
QUIC operates over the UDP transport, and the guidelines on ICMP
validation as specified in Section 5.2 of [RFC8085] therefore apply.
In addition to UDP Port validation QUIC can validate an ICMP message
by looking for valid Connection IDs in the quoted packet.
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7. Acknowledgements
This work was partially funded by the European Union's Horizon 2020
research and innovation programme under grant agreement No. 644334
(NEAT). The views expressed are solely those of the author(s).
8. IANA Considerations
This memo includes no request to IANA.
XXX If new UDP Options are specified in this document, a request to
IANA will be included here. XXX
If there are no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor.
9. Security Considerations
The security considerations for the use of UDP and SCTP are provided
in the references RFCs. Security guidance for applications using UDP
is provided in the UDP Usage Guidelines [RFC8085], specifically the
generation of probe packets is regarded as a "Low Data-Volume
Application", described in section 3.1.3 of this document. This
recommends that sender limits generation of probe packets to an
average rate lower than one probe per 3 seconds.
A PL sender needs to ensure that the method used to confirm reception
of probe packets offers protection from off-path attackers injecting
packets into the path. This protection if provided in IETF-defined
protocols (e.g., TCP, SCTP) using a randomly-initialised sequence
number. A description of one way to do this when using UDP is
provided in section 5.1 of [RFC8085]).
There are cases where PTB messages are not delivered due to policy,
configuration or equipment design (see Section 1.1), this method
therefore does not rely upon PTB messages being received, but is able
to utilise 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).
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
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response to receiving a PTB message. This is achieved by first
entering the PROBE_BASE state when such a message is received.
Second, the design does not require processing of PTB messages, a PL
sender could therefore suspend processing of PTB messages (e.g., in a
robustness mode after detecting that subsequent probes actually
confirm that a size larger than the PTB_SIZE is supported by a path).
Parallel forwarding paths SHOULD be considered. Section 5.2.5.1
identifies the need for robustness in the method when the path
information may be inconsistent.
A node performing DPLPMTUD could experience conflicting information
about the size of supported probe packets. This could occur when
there are multiple paths are concurrently in use and these exhibit a
different PMTU. If not considered, this could result in data being
black holed when the PLPMTU is larger than the smallest PMTU across
the current paths.
10. References
10.1. Normative References
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-16 (work
in progress), October 2018.
[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-05 (work in progress), July 2018.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>.
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[RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed.,
and G. Fairhurst, Ed., "The Lightweight User Datagram
Protocol (UDP-Lite)", RFC 3828, DOI 10.17487/RFC3828, July
2004, <https://www.rfc-editor.org/info/rfc3828>.
[RFC4820] Tuexen, M., Stewart, R., and P. Lei, "Padding Chunk and
Parameter for the Stream Control Transmission Protocol
(SCTP)", RFC 4820, DOI 10.17487/RFC4820, March 2007,
<https://www.rfc-editor.org/info/rfc4820>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951,
DOI 10.17487/RFC6951, May 2013,
<https://www.rfc-editor.org/info/rfc6951>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
"Datagram Transport Layer Security (DTLS) Encapsulation of
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.
10.2. Informative References
[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", draft-ietf-intarea-tunnels-09 (work in
progress), July 2018.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
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[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
ICMPv6 Messages in Firewalls", RFC 4890,
DOI 10.17487/RFC4890, May 2007,
<https://www.rfc-editor.org/info/rfc4890>.
Appendix A. Event-driven state changes
This appendix contains an informative description of key events:
Path Setup: When a new path is initiated, the state is set to
PROBE_START. This sends a probe packet with the size of the
BASE_PMTU. As soon as the path is confirmed, the state changes to
PROBE_SEARCH.
Arrival of an Acknowledgment: Depending on the probing state, the
reaction differs according to Figure 7, which is a simplification
of Figure 4 focusing on this event.
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+--------------+ +----------------+
| PROBE_START | --3------------------------------> | PROBE_DISABLED |
+--------------+ --4---------------- ------------> +----------------+
\/
+--------------+ /\ +--------------+
| PROBE_ERROR | -------------------- \ ----------> | PROBE_BASE |
+--------------+ --4--------------/ \ +--------------+
\
+--------------+ --1 -------- \ +--------------+
| PROBE_BASE | \ --- \ ------> | PROBE_ERROR |
+--------------+ --3--------- \ -----/ \ +--------------+
\ \
+--------------+ \ -----> +--------------+
| PROBE_SEARCH | --2--- -----------------> | PROBE_SEARCH |
+--------------+ \ ------------------> +--------------+
\ ---- /
+---------------+ / \ +---------------+
|SEARCH_COMPLETE| -1--- \ |SEARCH_COMPLETE|
+---------------+ -5-- -----------------------> +---------------+
\
\ +--------------+
--------------------------> | PROBE_BASE |
+--------------+
Condition 1: The maximum PMTU size has not yet been reached.
Condition 2: The maximum PMTU size has been reached. Condition 3:
Probe Timer expires and PROBE_COUNT = MAX_PROBEs. Condition 4:
PROBE_ACK received. Condition 5: Black hole detected.
Figure 7: State changes at the arrival of an acknowledgment
Probing timeout: The PROBE_COUNT is initialised to zero each time
the value of PROBED_SIZE is changed and when a acknowledgment
confirming delivery of a probe packet. The PROBE_TIMER is started
each time a probe packet is sent. It is stopped when an
acknowledgment arrives that confirms delivery of a probe packet of
PROBED_SIZE. If the probe packet is not acknowledged before the
PROBE_TIMER expires, the PROBE_COUNT is incremented. When the
PROBE_COUNT equals the value MAX_PROBES, the state is changed,
otherwise a new probe packet of the same size (PROBED_SIZE) is
resent. The state transitions are illustrated in Figure 8. This
shows a simplification of Figure 4 with a focus only on this
event.
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+--------------+ +----------------+
| PROBE_START | --2------------------------------->| PROBE_DISABLED |
+--------------+ +----------------+
+--------------+ +--------------+
| PROBE_ERROR | -----------------> | PROBE_ERROR |
+--------------+ / +--------------+
/
+--------------+ --2----------/ +--------------+
| PROBE_BASE | --1------------------------------> | PROBE_BASE |
+--------------+ +--------------+
+--------------+ +--------------+
| PROBE_SEARCH | --1------------------------------> | PROBE_SEARCH |
+--------------+ --2--------- +--------------+
\
+---------------+ \ +---------------+
|SEARCH_COMPLETE| -------------------> |SEARCH_COMPLETE|
+---------------+ +---------------+
Condition 1: The maximum number of probe packets has not been
reached. Condition 2: The maximum number of probe packets has been
reached. XXX This diagram has not been validated.
Figure 8: State changes at the expiration of the probe timer
PMTU raise timer timeout: DPLPMTUD periodically sends a probe packet
to detect whether a larger PMTU is possible. This probe packet is
generated by the PMTU_RAISE_TIMER.
Arrival of a PTB message: The active probing of the path can be
supported by the arrival of a PTB message indicating the PTB_SIZE.
Two examples are:
1. The PTB_SIZE is between the PLPMTU and the probe that
triggered the PTB message.
2. The PTB_SIZE is smaller than the PLPMTU.
In first case, the PROBE_BASE state transitions to the PROBE_ERROR
state. In the PROBE_SEARCH state, a new probe packet is sent with
the size reported by the PTB message.
In second case, the probing starts again with a value of
PROBE_BASE.
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Appendix B. Revision Notes
Note to RFC-Editor: please remove this entire section prior to
publication.
Individual draft -00:
o Comments and corrections are welcome directly to the authors or
via the IETF TSVWG working group mailing list.
o This update is proposed for WG comments.
Individual draft -01:
o Contains the first representation of the algorithm, showing the
states and timers
o This update is proposed for WG comments.
Individual draft -02:
o Contains updated representation of the algorithm, and textual
corrections.
o The text describing when to set the effective PMTU has not yet
been validated by the authors
o 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
o No text currently describes how to handle inconsistent results
from arbitrary re-routing along different parallel paths
o This update is proposed for WG comments.
Working Group draft -00:
o This draft follows a successful adoption call for TSVWG
o There is still work to complete, please comment on this draft.
Working Group draft -01:
o This draft includes improved introduction.
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o The draft is updated to require ICMP validation prior to accepting
PTB messages - this to be confirmed by WG
o Section added to discuss Selection of Probe Size - methods to be
evlauated and recommendations to be considered
o Section added to align with work proposed in the QUIC WG.
Working Group draft -02:
o The draft was updated based on feedback from the WG, and a
detailed review by Magnus Westerlund.
o The document updates RFC 4821.
o Requirements list updated.
o Added more explicit discussion of a simpler black-hole detection
mode.
o This draft includes reorganisation of the section on IETF
protocols.
o Added more discussion of implementation within an application.
o Added text on flapping paths.
o Replaced 'effective MTU' with new term PLPMTU.
Working Group draft -03:
o Updated figures
o Added more discussion on blackhole detection
o Added figure describing just blackhole detection
o Added figure relating MPS sizes
Working Group draft -04:
o Described phases and named these consistently.
o Corrected transition from confirmation directly to the search
phase (Base has been checked).
o Redrawn state diagrams.
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o Renamed BASE_MTU to BASE_PMTU (because it is a base for the PMTU).
o Clarified Error state.
o Clarified supsending DPLPMTUD.
o Verified normative text in requirements section.
o Removed duplicate text.
o Changed all text to refer to /packet probe/probe packet/
/validation/verification/ added term /Probe Confirmation/ and
clarified BlackHole detection.
Working Group draft -05:
o Updated security considerations.
o Feedback after speaking with Joe Touch helped improve UDP-Options
description.
Working Group draft -06:
o Updated description of ICMP issues in section 1.1
o Update to description of QUIC.
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: tom@erg.abdn.ac.uk
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Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Stein fart 48565
DE
Email: tuexen@fh-muenster.de
Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Stein fart 48565
DE
Email: i.ruengeler@fh-muenster.de
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