Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Updates: 4821, 4960, 6951, 8085, 8261 (if University of Aberdeen
approved) M. Tuexen
Intended status: Standards Track I. Ruengeler
Expires: 27 August 2020 T. Voelker
Muenster University of Applied Sciences
24 February 2020
Packetization Layer Path MTU Discovery for Datagram Transports
draft-ietf-tsvwg-datagram-plpmtud-15
Abstract
This document describes a robust method for Path MTU Discovery
(PMTUD) for datagram Packetization Layers (PLs). It describes an
extension to RFC 1191 and RFC 8201, which specifies ICMP-based Path
MTU Discovery for IPv4 and IPv6. The method allows a PL, or a
datagram application that uses a PL, to discover whether a network
path can support the current size of datagram. This can be used to
detect and reduce the message size when a sender encounters a packet
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 functionality for
datagram transports that is equivalent to the Packetization Layer
PMTUD specification for TCP, specified in RFC 4821.
The document updates RFC 4821 to specify the method for datagram PLs,
and updates RFC 8085 as the method to use in place of RFC 4821 with
UDP datagrams. Section 7.3 of RFC4960 recommends an endpoint apply
the techniques in RFC 4821 on a per-destination-address basis. RFC
4960, RFC 6951 and RFC 8261 are updated to recommend that SCTP, SCTP
encapsulated in UDP and SCTP encapsulated in DTLS use the method
specified in this document instead of the method 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 4960, RFC 4821, RFC
8085 and RFC 8261.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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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
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 27 August 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Features Required to Provide Datagram PLPMTUD . . . . . . . . 10
4. DPLPMTUD Mechanisms . . . . . . . . . . . . . . . . . . . . . 13
4.1. PLPMTU Probe Packets . . . . . . . . . . . . . . . . . . 13
4.2. Confirmation of Probed Packet Size . . . . . . . . . . . 14
4.3. Black Hole Detection . . . . . . . . . . . . . . . . . . 15
4.4. The Maximum Packet Size (MPS) . . . . . . . . . . . . . . 16
4.5. Disabling the Effect of PMTUD . . . . . . . . . . . . . . 17
4.6. Response to PTB Messages . . . . . . . . . . . . . . . . 17
4.6.1. Validation of PTB Messages . . . . . . . . . . . . . 17
4.6.2. Use of PTB Messages . . . . . . . . . . . . . . . . . 18
5. Datagram Packetization Layer PMTUD . . . . . . . . . . . . . 19
5.1. DPLPMTUD Components . . . . . . . . . . . . . . . . . . . 20
5.1.1. Timers . . . . . . . . . . . . . . . . . . . . . . . 20
5.1.2. Constants . . . . . . . . . . . . . . . . . . . . . . 21
5.1.3. Variables . . . . . . . . . . . . . . . . . . . . . . 22
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5.1.4. Overview of DPLPMTUD Phases . . . . . . . . . . . . . 23
5.2. State Machine . . . . . . . . . . . . . . . . . . . . . . 24
5.3. Search to Increase the PLPMTU . . . . . . . . . . . . . . 27
5.3.1. Probing for a larger PLPMTU . . . . . . . . . . . . . 27
5.3.2. Selection of Probe Sizes . . . . . . . . . . . . . . 28
5.3.3. Resilience to Inconsistent Path Information . . . . . 28
5.4. Robustness to Inconsistent Paths . . . . . . . . . . . . 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. Initial Connectivity . . . . . . . . . . . . . . . . 30
6.1.5. Validating the Path . . . . . . . . . . . . . . . . . 30
6.1.6. Handling of PTB Messages . . . . . . . . . . . . . . 30
6.2. DPLPMTUD for SCTP . . . . . . . . . . . . . . . . . . . . 31
6.2.1. SCTP/IPv4 and SCTP/IPv6 . . . . . . . . . . . . . . . 31
6.2.1.1. Initial Connectivity . . . . . . . . . . . . . . 31
6.2.1.2. Sending SCTP Probe Packets . . . . . . . . . . . 31
6.2.1.3. Validating the Path with SCTP . . . . . . . . . . 32
6.2.1.4. PTB Message Handling by SCTP . . . . . . . . . . 32
6.2.2. DPLPMTUD for SCTP/UDP . . . . . . . . . . . . . . . . 32
6.2.2.1. Initial Connectivity . . . . . . . . . . . . . . 32
6.2.2.2. Sending SCTP/UDP Probe Packets . . . . . . . . . 33
6.2.2.3. Validating the Path with SCTP/UDP . . . . . . . . 33
6.2.2.4. Handling of PTB Messages by SCTP/UDP . . . . . . 33
6.2.3. DPLPMTUD for SCTP/DTLS . . . . . . . . . . . . . . . 33
6.2.3.1. Initial Connectivity . . . . . . . . . . . . . . 33
6.2.3.2. Sending SCTP/DTLS Probe Packets . . . . . . . . . 33
6.2.3.3. Validating the Path with SCTP/DTLS . . . . . . . 33
6.2.3.4. Handling of PTB Messages by SCTP/DTLS . . . . . . 34
6.3. DPLPMTUD for QUIC . . . . . . . . . . . . . . . . . . . . 34
6.3.1. Initial Connectivity . . . . . . . . . . . . . . . . 34
6.3.2. Sending QUIC Probe Packets . . . . . . . . . . . . . 34
6.3.3. Validating the Path with QUIC . . . . . . . . . . . . 35
6.3.4. Handling of PTB Messages by QUIC . . . . . . . . . . 35
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
9. Security Considerations . . . . . . . . . . . . . . . . . . . 35
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 36
10.1. Normative References . . . . . . . . . . . . . . . . . . 36
10.2. Informative References . . . . . . . . . . . . . . . . . 38
Appendix A. Revision Notes . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
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1. Introduction
The IETF has specified datagram transport using UDP, SCTP, and DCCP,
as well as protocols layered on top of these transports (e.g., SCTP/
UDP, DCCP/UDP, QUIC/UDP), and direct datagram transport over the IP
network layer. This document describes a robust method for Path MTU
Discovery (PMTUD) that can be used with these transport protocols (or
the applications that use their transport service) to discover an
appropriate size of packet to use across an Internet path.
1.1. Classical Path MTU Discovery
Classical Path Maximum Transmission Unit Discovery (PMTUD) can be
used with any transport that is able to process ICMP Packet Too Big
(PTB) messages (e.g., [RFC1191] and [RFC8201]). In this document,
the term PTB message is applied to both IPv4 ICMP Unreachable
messages (type 3) that carry the error Fragmentation Needed (Type 3,
Code 4) [RFC0792] and ICMPv6 Packet Too Big messages (Type 2)
[RFC4443]. When a sender receives a PTB message, it reduces the
effective MTU to the value reported as the Link MTU in the PTB
message. A method 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 can be provided with a primitive to let them read
the Maximum Packet Size (MPS), derived from the current effective
PMTU.
Classical PMTUD is subject to protocol failures. One failure arises
when traffic using a packet size larger than the actual PMTU is
black-holed (all datagrams sent with this size, or larger, are
discarded). This could arise when the PTB messages are not delivered
back to the sender for some reason (see for example [RFC2923]).
Examples where PTB messages are not delivered include:
* The generation of ICMP messages is usually rate limited. This
could result in no PTB messages being generated to the sender (see
section 2.4 of [RFC4443])
* ICMP messages can be filtered by middleboxes (including firewalls)
[RFC4890]. A stateful firewall could be configured with a policy
to block incoming ICMP messages, which would prevent reception of
PTB messages to a sending endpoint behind this firewall.
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* When the router issuing the ICMP message drops a tunneled packet,
the resulting ICMP message will be directed to the tunnel ingress.
This tunnel endpoint is responsible for forwarding the ICMP
message and also processing the quoted packet within the payload
field to remove the effect of the tunnel, and return a correctly
formatted ICMP message to the sender [I-D.ietf-intarea-tunnels].
Failure to do this prevents the PTB message reaching the original
sender.
* 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 also needs to 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 a packet is dropped by the link,
this will not cause a PTB message to be sent to the original
sender.
Another failure could result if a node that is not on the network
path sends a PTB message that attempts to force a sender to change
the effective PMTU [RFC8201]. A sender can protect itself from
reacting to such messages by utilising the quoted packet within a PTB
message payload to validate that the received PTB message was
generated in response to a packet that had actually originated from
the sender. However, there are situations where a sender would be
unable to provide this validation. Examples where validation of the
PTB message is not possible include:
* When a router issuing the ICMP message implements RFC792
[RFC0792], it is only required to include the first 64 bits of the
IP payload of the packet within the quoted payload. There could
be insufficient bytes remaining for the sender to interpret the
quoted transport information.
Note: The recommendation in RFC1812 [RFC1812] is that IPv4 routers
return a quoted packet with as much of the original datagram as
possible without the length of the ICMP datagram exceeding 576
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bytes. IPv6 routers include as much of the invoking packet as
possible without the ICMPv6 packet exceeding 1280 bytes [RFC4443].
* The use of tunnels/encryption can reduce the size of the quoted
packet returned to the original source address, increasing the
risk that there could be insufficient bytes remaining for the
sender to interpret the quoted transport information.
* Even when the PTB message includes sufficient bytes of the quoted
packet, the network layer could lack sufficient context to
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.
* A Network Addres Translation (NAT) device that translates a packet
header, ought to also translate ICMP messages and update the ICMP
quoted packet [RFC5508] in that message. If this is not correctly
translated then the sender would not be able to associate the
message with the PL that originated the packet, and hence this
ICMP message cannot be validated.
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 (e.g., DCCP, RTP, SCTP, QUIC), 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] introduced a method that does not rely upon
reception and validation of PTB messages. It is therefore more
robust than Classical PMTUD. This has become the recommended
approach for implementing discovery of the PMTU [RFC8085].
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 to explore using a larger
packet size. If a probe packet is successfully delivered (as
determined by the PL), then the PLPMTU is raised to the size of the
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successful probe. If no response is received to a probe packet, the
method then reduces the PLPMTU.
Datagram PLPMTUD introduces flexibility in implementation. At one
extreme, it can be configured to only perform Black Hole Detection
and recovery with increased robustness compared to Classical PMTUD.
At the other extreme, all PTB processing can be disabled, and PLPMTUD
replaces Classical PMTUD.
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.
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 relies upon features of
the PL described in Section 3 and applies to transport protocols
operating over IPv4 and IPv6. It does not require cooperation from
the lower layers, although it can utilize PTB messages when these
received messages are made available to the PL.
The message size guidelines in section 3.2 of 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. The present document updates RFC 8085 to specify this
method in place of PLPMTUD [RFC4821] and provides a mechanism for
sharing the discovered largest size as the Maximum Packet Size (MPS)
(see Section 4.4).
Section 10.2 of [RFC4821] recommended a PLPMTUD probing method for
the Stream Control Transport Protocol (SCTP). SCTP utilizes probe
packets consisting of a minimal sized HEARTBEAT chunk bundled with a
PAD chunk as defined in [RFC4820]. However, RFC 4821 did not provide
a complete specification. The present document replaces this by
providing a complete 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
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not risk application data loss. The method defined in this
specification can be used with DCCP.
Section 6 specifies the method for datagram transports and provides
information to enable the implementation of PLPMTUD with other
datagram transports and applications that use datagram transports.
Section 6 also provides updated recommendations for [RFC6951] and
[RFC8261].
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].
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:
* 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).
* 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.
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 packet 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.
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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. This specification continues the requirement in
[RFC4821], that states "All links MUST enforce their MTU: links
that might non- deterministically deliver packets that are larger
than their rated MTU MUST consistently discard such packets."
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 a layer of
the network stack that places data into packets and performs
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transport protocol functions. Examples of a PL include: TCP,
SCTP, SCTP over DTLS or QUIC.
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
The principles expressed in [RFC4821] apply to the use of the
technique with any PL. TCP PLPMTUD has been defined using standard
TCP protocol mechanisms. Unlike TCP, datagram PLs require additional
mechanisms and considerations to implement PLPMTUD.
The requirements for datagram PLPMTUD are:
1. PLPMTU: The PLPMTU (specified as the effective PMTU in Section 1
of [RFC1191]) is equivalent to the EMTU_S (specified in
[RFC1122]). For datagram PLs,] the PLPMTU is managed by
DPLPMTUD. A PL MUST NOT send a packet (other than a probe
packet) with a size larger than the current PLPMTU at the
network layer.
2. 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
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packet is always sent without source fragmentation (as specified
in section 5.4 of [RFC8201]).
3. 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.
4. 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. The PL is REQUIRED to be robust
in the case where probe packets are lost due to other reasons
(including link transmission error, congestion).
5. PMTU parameters: A DPLPMTUD sender is RECOMMENDED to utilise
information about the maximum size of packet that can be
transmitted by the sender on the local link (e.g., 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).
6. Processing PTB messages: A DPLPMTUD sender MAY optionally
utilize PTB messages received from the network layer to help
identify when a network path does not support the current size
of probe packet. Any received PTB message MUST be validated
before it is used to update the PLPMTU discovery information
[RFC8201]. This validation confirms that the PTB message was
sent in response to a packet originating by the sender, and
needs to be performed before the PLPMTU discovery method reacts
to the PTB message. A PTB message MUST NOT be used to increase
the PLPMTU [RFC8201], but could trigger a probe to test for a
larger PLPMTU. A PTB_SIZE greater than the currently probed
MUST be ignored.
7. Probing and congestion control: The decision about when to send
a probe packet does not need to be limited by the congestion
controller. When not controlled by the congestion controller,
the interval between probe packets MUST be at least one RTT. If
transmission of probe packets is limited by the congestion
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controller, this could result in transmission of probe packets
being delayed.
8. Loss of a probe packet SHOULD NOT be treated as an indication of
congestion and SHOULD NOT trigger a congestion control reaction
[RFC4821], because this could result in unnecessary reduction of
the sending rate.
9. An update to the PLPMTU (or MPS) MUST NOT modify the congestion
window measured in bytes [RFC4821]. Therefore, an increase in
the packet size does not cause an increase the data rate in
bytes per second.
10. Probing and flow control: Flow control at the PL concerns the
end-to-end flow of data using the PL service. This does not
apply to DPLPMTU when probe packets use a design that does not
carry user data to the remote application.
11. Shared PLPMTU state: The PLPMTU value MAY also be stored with
the corresponding entry associated with the destination in the
IP layer cache, and used by other PL instances. The
specification of PLPMTUD [RFC4821] states: "If PLPMTUD updates
the MTU for a particular path, all Packetization Layer sessions
that share the path representation (as described in Section 5.2
of [RFC4821]) SHOULD be notified to make use of the new MTU".
Such methods MUST be robust to the wide variety of underlying
network forwarding behaviors. 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:
* Maximum Packet Size (MPS): A PL MAY be designed to segment data
blocks larger than the MPS into multiple datagrams. However, not
all datagram PLs support segmentation of data blocks. It is
RECOMMENDED that methods avoid forcing an application to use an
arbitrary small MPS for transmission while the method is searching
for the currently supported PLPMTU. A reduced MPS can adversely
impact the performance of an application.
* To assist applications in choosing a suitable data block size, the
PL is RECOMMENDED to provide a primitive that returns the MPS
derived from the PLPMTU to the higher layer using the PL. The
value of the MPS can change following a change in the path, or
loss of probe packets.
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* 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.
* Datagram delay and duplication: The feedback mechanism is REQUIRED
to be robust to the possibility that packets could be
significantly delayed or duplicated along a network path.
* When to probe: It is RECOMMENDED that methods determine whether
the path has changed since it last measured the path. This can
help determine when to probe the path again.
4. DPLPMTUD Mechanisms
This section lists the protocol mechanisms used in this
specification.
4.1. PLPMTU Probe Packets
The DPLPMTUD method relies upon the PL sender being able to generate
probe packets with a specific size. TCP is able to generate these
probe packets by choosing to appropriately segment data being sent
[RFC4821]. In contrast, a datagram PL that constructs a probe packet
has to either request an application to send a data block that is
larger than that generated by an application, or to utilize padding
functions to extend a datagram beyond the size of the application
data block. Protocols that permit exchange of control messages
(without an application data block) can generate a probe packet by
extending a control message with padding data.
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 of the probe packet. Since these probe
packets do not carry an application-supplied data block, they do
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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 of
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 of 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, possibly using a smaller PLPMTU. This could need the PL to to
use a smaller packet size to traverse the end-to-end path (which
could utilize endpoint network-layer or a PL that can re-segment the
data block into multiple datagrams).
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.
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 MAY also be used by DPLPMTUD to acknowledge reception of a
probe packet.
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A PL that does not acknowledge data reception (e.g., UDP and UDP-
Lite) is unable itself to detect when the packets that it sends are
discarded because their size is greater than the actual PMTU. These
PLs need to rely on an application protocol to detect this loss.
Section 6 specifies this function for a set of IETF-specified
protocols.
4.3. Black Hole Detection
Black Hole Detection is triggered by an indication that the network
path could be unable to support the current PLPMTU size.
There are three ways to detect black holes:
* A validated PTB message can be received that indicates a PTB_SIZE
less than the current PLPMTU. A DPLPMTUD method MUST NOT rely
soley on this method.
* A PL can use the DPLPMTUD probing mechanism to periodically
generate probe packets of the size of the current PLPMTU (e.g.,
using the confirmation timer Section 5.1.1). A timer tracks
whether acknowledgments are received. Successive loss of probes
is an indication that the current path no longer supports the
PLPMTU (e.g., when the number of probe packets sent without
receiving an acknowledgement, PROBE_COUNT, becomes greater than
MAX_PROBES).
* A PL can utilise an event that indicates the network path no
longer sustains the sender's PLPMTU size. This could use a
mechanism implemented within the PL to detect excessive loss of
data sent with a specific packet size and then conclude that this
excessive loss could be a result of an invalid PLPMTU (as in
PLPMTUD for TCP [RFC4821]).
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 could result
in additional packets being sent.
When the method detects the current PLPMTU is not supported, DPLPMTUD
sets a lower PLPMTU, and sets a lower MPS. The PL then confirms that
the new PLPMTU can be successfully used across the path. A probe
packet could need to have a size less than the size of the data block
generated by the application.
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4.4. The Maximum Packet Size (MPS)
The result of probing determines a usable PLPMTU, which is used to
set the MPS used by the application. The MPS is smaller than the
PLPMTU because of the presence of PL headers and any IP options or
extensions added to the PL packet. The relationship between the MPS
and the PLPMTUD is illustrated in Figure 1.
any additional
headers .--- MPS -----.
| | |
v v v
+------------------------------+
| IP | ** | PL | protocol data |
+------------------------------+
<---------- PLPMTU ------------>
Figure 1: Relationship between MPS and PLPMTU
A PL is unable to send a packet (other than a probe packet) with a
size larger than the current PLPMTU at the network layer. To avoid
this, a PL MAY be designed to segment data blocks larger than the MPS
into multiple datagrams.
DPLPMTUD seeks to avoid IP fragmentation. An attempt to send a data
block larger than the MPS will therefore fail if a PL is unable to
segment data. To determine the largest data block that can be sent,
a PL SHOULD provide applications with a primitive that returns the
Maximum Packet Size (MPS), derived from the current PLPMTU.
If DPLPMTUD results in a change to the MPS, the application needs to
adapt to the new MPS. A particular case can arise when packets have
been sent with a size less than the MPS and the PLPMTU was
subsequently reduced. If these packets are lost, the PL MAY segment
the data using the new MPS. If a PL is unable to re-segment a
previously sent datagram (e.g., [RFC4960]), then the sender either
discards the datagram or could perform retransmission using network-
layer fragmentation to form multiple IP packets not larger than the
PLPMTU. For IPv4, the use of endpoint fragmentation by the sender is
preferred over clearing the DF-bit in the IPv4 header. Operational
experience reveals that IP fragmentation can reduce the reliability
of Internet communication [I-D.ietf-intarea-frag-fragile], which may
reduce the success of retransmission.
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4.5. Disabling the Effect of PMTUD
A PL implementing this specification MUST suspend network layer
processing of outgoing packets that enforces a PMTU
[RFC1191][RFC8201] 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.
4.6. Response to PTB Messages
This method requires the DPLPMTUD sender to validate any received PTB
message before using the PTB information. The response to a PTB
message depends on the PTB_SIZE indicated in the PTB message, the
state of the PLPMTUD state machine, and the IP protocol being used.
Section 4.6.1 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.
4.6.1. Validation of PTB Messages
This section specifies utilization of PTB messages.
* A simple implementation MAY ignore received PTB messages and in
this case the PLPMTU is not updated when a PTB message is
received.
* An implementation that supports PTB messages MUST validate
messages before they are further processed.
A PL that receives a PTB message from a router or middlebox, performs
ICMP validation as specified in Section 5.2 of [RFC8085][RFC8201].
Because DPLPMTUD operates at the PL, the PL needs to check that each
received PTB message is received in response to a packet transmitted
by the endpoint PL performing DPLPMTUD.
The PL MUST check the protocol information in the quoted packet
carried in an ICMP PTB message payload to validate the message
originated from the sending node. This validation includes
determining that the combination of the IP addresses, the protocol,
the source port and destination port match those returned in the
quoted packet - this is also necessary for the PTB message to be
passed to the corresponding PL.
The validation SHOULD utilize information that it is not simple for
an off-path attacker to determine [RFC8085]. For example, by
checking the value of a protocol header field known only to the two
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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 by triggering an
immediate probe for the PTB_SIZE, compared to one that relies solely
on probing using a timer-based search algorithm. Section 4.6.2
describes this processing.
4.6.2. Use of PTB Messages
A set of checks are intended to provide protection from a router that
reports an unexpected PTB_SIZE. The PL also needs to check that the
indicated PTB_SIZE is less than the size used by probe packets and at
least the 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_PMTU
* Invalid PTB_SIZE see Section 4.6.1.
* 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 Section 5.2) for an
IPv4 path when the PTB_SIZE reported in the PTB message is
larger than or equal to 68 bytes [RFC0791] and when this is
less than the BASE_PMTU.
* A robust PL MAY enter an error state (see Section 5.2) for an
IPv6 path when the PTB_SIZE reported in the PTB message is
larger than or equal to 1280 bytes [RFC8200] and when this is
less than the BASE_PMTU.
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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
* This could be an indication of a black hole. The PLPMTU SHOULD
be set to BASE_PMTU (the PLPMTU is reduced to the BASE_PMTU to
avoid unnecessary packet loss when a black hole is
encountered).
* The PL ought to start a search to quickly discover the new
PLPMTU. The PTB_SIZE reported in the PTB message can be used
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.
5. Datagram Packetization Layer PMTUD
This section specifies Datagram PLPMTUD (DPLPMTUD). The method can
be introduced at various points (as indicated with * in the figure
below) in the IP protocol stack to discover the PLPMTU so that an
application can utilize an appropriate MPS for the current network
path.
DPLPMTUD SHOULD NOT be used by an upper PL or application if it is
already used in a lower layer, DPLPMTUD SHOULD only be performed once
between a pair of endpoints. A PL MUST adjust the MPS indicated by
DPLPMTUD to account for any additional overhead introduced by the PL.
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+----------------------+
| Application* |
+-+-------+----+----+--+
| | | |
+---+--+ +--+--+ | +-+---+
| QUIC*| |UDPO*| | |SCTP*|
+---+--+ +--+--+ | +--+--+
| | | | |
+-------+--+ | | |
| | | |
+-+-+--+ |
| UDP | |
+---+--+ |
| |
+--------------+-----+-+
| Network Interface |
+----------------------+
Figure 2: 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 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.
5.1. DPLPMTUD Components
This section describes the timers, constants, and variables of
DPLPMTUD.
5.1.1. Timers
The method utilizes up to three timers:
PROBE_TIMER: The PROBE_TIMER is configured to expire after a period
longer than the maximum time to receive an acknowledgment to a
probe packet. This value MUST NOT be smaller than 1 second, and
SHOULD be larger than 15 seconds. Guidance on selection of the
timer value are provided in section 3.1.1 of the UDP Usage
Guidelines [RFC8085].
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,
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as recommended by PLPMTUD [RFC4821].
DPLPMTUD MAY inhibit sending probe packets when no application
data has been sent since the previous probe packet. A PL
preferring to use an up-to-data PMTU once user data is sent again,
can choose to continue PMTU discovery for each path. However,
this could result in sending additional packets.
CONFIRMATION_TIMER: When an acknowledged PL is used, this timer MUST
NOT be used. For other PLs, the CONFIRMATION_TIMER is configured
to the period a PL sender waits before confirming the current
PLPMTU is still supported. This is less than the PMTU_RAISE_TIMER
and used to decrease the PLPMTU (e.g., when a black hole is
encountered). Confirmation needs to be frequent enough when data
is flowing that the sending PL does not black hole extensive
amounts of traffic. Guidance on selection of the timer value are
provided in section 3.1.1 of the UDP Usage Guidelines [RFC8085].
DPLPMTUD MAY inhibit sending probe packets when no application
data has been sent since the previous probe packet. A PL
preferring to use an up-to-data PMTU once user data is sent again,
can choose to continue PMTU discovery for each path. However,
this could result in sending additional packets.
An implementation could implement the various timers using a single
timer.
5.1.2. Constants
The following constants are defined:
MAX_PROBES: The MAX_PROBES is the maximum value of the PROBE_COUNT
counter (see Section 5.1.3). MAX_PROBES represents the limit for
the number of consecutive probe attempts of any size. Search
algorithms benefit from a MAX_PROBES value greater than 1 because
this can provide robustness to isolated packet loss. The default
value of MAX_PROBES is 3.
MIN_PMTU: The MIN_PMTU is the smallest allowed probe packet size.
For IPv6, this value is 1280 bytes, as specified in [RFC8200].
For IPv4, the minimum value is 68 bytes.
Note: An IPv4 router is required to be able to forward a datagram
of 68 bytes without further fragmentation. This is the combined
size of an IPv4 header and the minimum fragment size of 8 bytes.
In addition, receivers are required to be able to reassemble
fragmented datagrams at least up to 576 bytes, as stated in
section 3.3.3 of [RFC1122].
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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 [RFC8200]. When using IPv4, a size of 1200 bytes is
RECOMMENDED.
5.1.3. Variables
This method utilizes a set of variables:
PROBED_SIZE: The PROBED_SIZE is the size of the current probe
packet. This is a tentative value for the PLPMTU, which is
awaiting confirmation by an acknowledgment.
PROBE_COUNT: The PROBE_COUNT is a count of the number of successive
unsuccessful probe packets that have been sent. Each time a probe
packet is acknowledged, the value is set to zero. (Some probe
loss is expected while searching, therefore loss of a single probe
is not an indication of a PMTU problem.)
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.
MIN_PMTU MAX_PMTU
<-------------------------------------------------->
| | | |
v | | v
BASE_PMTU | v Actual PMTU
| PROBED_SIZE
v
PLPMTU
Figure 3: Relationships between packet size constants and variables
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5.1.4. Overview of DPLPMTUD Phases
This section provides a high-level informative view of the DPLPMTUD
method, by describing the movement of the method through several
phases of operation. More detail is available in the state machine
Section 5.2.
+------+
+------->| Base |----------------+ Connectivity
| +------+ | or BASE_PMTU
| | | confirmation failed
| | v
| | Connectivity +-------+
| | and BASE_PMTU | Error |
| | confirmed +-------+
| | | Consistent
| v | connectivity
PLPMTU | +--------+ | and BASE_PMTU
confirmation | | Search |<--------------+ confirmed
failed | +--------+
| ^ |
| | |
| Raise | | Search
| timer | | algorithm
| expired | | completed
| | |
| | v
| +-----------------+
+---| Search Complete |
+-----------------+
Figure 4: DPLPMTUD Phases
Base: The Base Phase confirms connectivity to the remote peer using
packets of the BASE_PMTU. This phase is implicit for a
connection-oriented PL (where it can be performed in a PL
connection handshake). A connectionless PL sends 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.
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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 Section 4.6.
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 causes 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).
5.2. State Machine
A state machine for DPLPMTUD is depicted in Figure 5. If multipath
or multihoming is supported, a state machine is needed for each path.
Note: Not all changes are shown to simplify the diagram.
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| |
| Start | PL indicates loss
| | of connectivity
v v
+---------------+ +---------------+
| DISABLED | | ERROR |
+---------------+ PROBE_TIMER expiry: +---------------+
| PL indicates PROBE_COUNT = MAX_PROBES or ^ |
| connectivity PTB: PTB_SIZE < BASE_PMTU | |
+--------------------+ +---------------+ |
| | |
v | BASE_PMTU Probe |
+---------------+ acked |
| BASE |----------------------+
+---------------+ |
^ | ^ ^ |
Black hole detected | | | | Black hole detected |
+--------------------+ | | +--------------------+ |
| +----+ | |
| PROBE_TIMER expiry: | |
| PROBE_COUNT < MAX_PROBES | |
| | |
| PMTU_RAISE_TIMER expiry | |
| +-----------------------------------------+ | |
| | | | |
| | v | v
+---------------+ +---------------+
|SEARCH_COMPLETE| | SEARCHING |
+---------------+ +---------------+
| ^ ^ | | ^
| | | | | |
| | +-----------------------------------------+ | |
| | MAX_PMTU Probe acked or | |
| | PROBE_TIMER expiry: PROBE_COUNT = MAX_PROBES or | |
+----+ PTB: PTB_SIZE = PLPMTU +----+
CONFIRMATION_TIMER expiry: PROBE_TIMER expiry:
PROBE_COUNT < MAX_PROBES or PROBE_COUNT < MAX_PROBES or
PLPMTU Probe acked Probe acked or PTB:
PLPMTU < PTB_SIZE < PROBED_SIZE
Figure 5: State machine for Datagram PLPMTUD
The following states are defined:
DISABLED: The DISABLED state is the initial state before probing has
started. It is also entered from any other state, when the PL
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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 when a sender searching for a larger PLPMTU is unaware
that packets are not being delivered due to a packet or ICMP Black
Hole.
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 to enter SEARCH_COMPLETE when the PROBE_COUNT
reaches MAX_PROBES, a validated PTB is received that corresponds
to the last successfully probed size (PTB_SIZE = PLPMTU), or a
probe of size MAX_PMTU is acknowledged (PLPMTU = MAX_PMTU).
When a black hole is detected in the SEARCHING state, this causes
the PL sender to enter the BASE state.
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 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
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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.
The PL sender then enters the SEARCHING state.
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 will transition to the DISABLED state.
Note: MIN_PMTU could be identical to BASE_PMTU, simplifying the
actions in this state.
5.3. Search to Increase the PLPMTU
This section describes the algorithms used by DPLPMTUD to search for
a larger PLPMTU.
5.3.1. Probing for a larger PLPMTU
Implementations use a search algorithm across the search range to
determine whether a larger PLPMTU can be supported across a network
path.
The method discovers the search range by confirming the minimum
PLPMTU and then using the probe method to select a PROBED_SIZE less
than or equal to MAX_PMTU. MAX_PMTU is the minimum of the local MTU
and EMTU_R (learned from the remote endpoint). The MAX_PMTU MAY be
reduced by an application that sets a maximum to the size of
datagrams it will send.
The PROBE_COUNT is initialized to zero when the first probe with a
size greater than or equal to PLPMTUD is sent. A timer is used 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.
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Each time a probe packet is sent to the destination, the PROBE_TIMER
is started. The timer is canceled when the PL receives
acknowledgment that the probe packet has been successfully sent
across the path Section 4.1. This confirms that the PROBED_SIZE is
supported, and the PROBED_SIZE value is then assigned to the PLPMTU.
The search algorithm can continue to send subsequent probe packets of
an increasing size.
If the timer expires before a probe packet is acknowledged, the probe
has failed to confirm the PROBED_SIZE. Each time the PROBE_TIMER
expires, the PROBE_COUNT is incremented, the PROBE_TIMER is
reinitialized, and a 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.
5.3.2. Selection of Probe Sizes
The search algorithm determines a minimum useful gain in PLPMTU. It
would not be constructive for a PL sender to attempt to probe for all
sizes. This would incur unnecessary load on the path.
Implementations 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.
5.3.3. Resilience to Inconsistent Path Information
A decision to increase the PLPMTU needs to be resilient to the
possibility that information learned about the network path is
inconsistent. A path is inconsistent, when, for example, probe
packets are lost due to other reasons (i.e., not packet size) or due
to frequent path changes. Frequent path changes could occur by
unexpected "flapping" - where some packets from a flow pass along one
path, but other packets follow a different path with different
properties.
A PL sender is able to detect inconsistency from the sequence of
PLPMTU probes that are acknowledged 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
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MPS to a smaller value for a period of time. This avoids unnecessary
loss of packets.
5.4. Robustness to Inconsistent Paths
Some paths could be unable to sustain packets of the BASE_PMTU size.
To be robust to these paths an implementation could implement the
Error State. This allows fallback to a smaller than desired PLPMTU,
rather than suffer connectivity failure. This could utilize methods
such as endpoint IP fragmentation to enable the PL sender to
communicate using packets smaller than the BASE_PMTU.
6. Specification of Protocol-Specific Methods
DPLPMTUD 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 when using
SCTP and QUIC.
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 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 from the IP
layer 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.
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6.1.1. Application Request
An application needs an application-layer protocol mechanism (such as
a message acknowledgement method) that solicits a response from a
destination endpoint. The method SHOULD allow the sender to check
the value returned in the response to provide additional protection
from off-path insertion of data [RFC8085], suitable methods include a
parameter known only to the two endpoints, such as a session ID or
initialized sequence number.
6.1.2. Application Response
An application needs an application-layer protocol mechanism to
communicate the response from the destination endpoint. This
response could indicate successful reception of the probe across the
path, but could also indicate that some (or all packets) have failed
to reach the destination.
6.1.3. Sending Application Probe Packets
A probe packet that could carry an application data block, but the
successful transmission of this data is at risk when used for
probing. Some applications might prefer to use a probe packet that
does not carry an application data block to avoid disruption to data
transfer.
6.1.4. Initial Connectivity
An 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.
6.1.5. 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.
6.1.6. Handling of PTB Messages
An application that is able and wishes to receive PTB messages MUST
perform ICMP validation as specified in Section 5.2 of [RFC8085].
This requires that the application to check each received PTB
messages to validate it is received in response to transmitted
traffic and that the reported PTB_SIZE is less than the current
probed size (see Section 4.6.2). A validated PTB message MAY be used
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as input to the DPLPMTUD algorithm, but MUST NOT be used directly to
set the PLPMTU.
6.2. DPLPMTUD for SCTP
Section 10.2 of [RFC4821] specified a recommended PLPMTUD probing
method for SCTP and Section 7.3 of [RFC4960] and recommended an
endpoint apply the techniques in RFC4821 on a per-destination-address
basis. The specification for DPLPMTUD continues the practice of
using the PL to discover the PMTU, but updates, RFC4960 with a
recommendation to use the method specified in this document: The
RECOMMENDED method for generating probes is to add a chunk consisting
only of padding to an SCTP message. The PAD chunk defined in
[RFC4820] SHOULD be attached to a minimum length HEARTBEAT (HB) chunk
to build a probe packet. This enables probing without affecting the
transfer of user messages and without being limited by congestion
control or flow control. This is preferred to using DATA chunks
(with padding as required) as path probes.
Section 6.9 of [RFC4960] describes dividing the user messages into
data chunks sent by the PL when using SCTP. This notes that once an
SCTP message has been sent, it cannot be re-segmented. [RFC4960]
describes the method to retransmit data chunks when the MPS has
reduced, and the use of IP fragmentation for this case.
6.2.1. SCTP/IPv4 and SCTP/IPv6
6.2.1.1. Initial Connectivity
The base protocol is specified in [RFC4960]. This provides an
acknowledged PL. A sender can therefore enter the BASE state as soon
as connectivity has been confirmed.
6.2.1.2. Sending SCTP Probe Packets
Probe packets consist of an SCTP common header followed by a
HEARTBEAT chunk and a PAD chunk. The PAD chunk is used to control
the length of the probe packet. The HEARTBEAT chunk is used to
trigger the sending of a HEARTBEAT ACK chunk. The reception of the
HEARTBEAT ACK chunk acknowledges reception of a successful probe. A
successful probe updates the association and path counters, but an
unsuccessful probe is discounted (assumed to be a result of choosing
too large a PLPMTU).
The HEARTBEAT chunk carries a Heartbeat Information parameter which
includes, 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
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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.
Probing starts directly 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, dependent on the number of PMTU probes sent. The Heartbeat
timer can be used to implement the PROBE_TIMER.
6.2.1.3. 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.
6.2.1.4. PTB Message Handling by SCTP
Normal ICMP validation MUST be performed as specified in Appendix C
of [RFC4960]. This requires that the first 8 bytes of the SCTP
common header are quoted in the payload of the PTB message, which can
be the case for ICMPv4 and is normally the case for ICMPv6.
When a PTB message has been validated, the PTB_SIZE reported in the
PTB message SHOULD be used with the DPLPMTUD algorithm, providing
that the reported PTB_SIZE is less than the current probe size (see
Section 4.6).
6.2.2. DPLPMTUD for SCTP/UDP
The UDP encapsulation of SCTP is specified in [RFC6951].
This specification updates the reference to RFC 4821 in section 5.6
of RFC 6951 to refer to XXXTHISRFCXXX. RFC 6951 is updated by
addition of the following sentence is to be added at the end of
section 5.6: "The RECOMMENDED method for determining the MTU of the
path is specified in XXXTHISRFCXXX".
XXX RFC EDITOR - please replace XXXTHISRFCXXX when published XXX
6.2.2.1. Initial Connectivity
A sender can enter the BASE state as soon as SCTP connectivity has
been confirmed.
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6.2.2.2. Sending SCTP/UDP Probe Packets
Packet probing can be performed as specified in Section 6.2.1.2. The
maximum payload is reduced by 8 bytes, which has to be considered
when filling the PAD chunk.
6.2.2.3. Validating the Path with SCTP/UDP
Since SCTP provides an acknowledged PL, a sender MUST NOT implement
the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.2.2.4. Handling of PTB Messages by SCTP/UDP
ICMP validation MUST be performed for PTB messages as specified in
Appendix C of [RFC4960]. This requires that the first 8 bytes of the
SCTP common header are contained in the PTB message, which can be the
case for ICMPv4 (but note the UDP header also consumes a part of the
quoted packet header) and is normally the case for ICMPv6. When the
validation is completed, the PTB_SIZE indicated in the PTB message
SHOULD be used with the DPLPMTUD providing that the reported PTB_SIZE
is less than the current probe size.
6.2.3. DPLPMTUD for SCTP/DTLS
The Datagram Transport Layer Security (DTLS) encapsulation of SCTP is
specified in [RFC8261] . This is used for data channels in WebRTC
implementations. This specification updates the reference to RFC
4821 in section 5 of RFC 8261 to refer to XXXTHISRFCXXX.
XXX RFC EDITOR - please replace XXXTHISRFCXXX when published XXX
6.2.3.1. Initial Connectivity
A sender can enter the BASE state as soon as SCTP connectivity has
been confirmed.
6.2.3.2. Sending SCTP/DTLS Probe Packets
Packet probing can be done, as specified in Section 6.2.1.2.
6.2.3.3. 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.
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6.2.3.4. Handling of PTB Messages by SCTP/DTLS
[RFC4960] does not specify a way to validate SCTP/DTLS ICMP message
payload. This can prevent processing of PTB messages at the PL.
6.3. DPLPMTUD for QUIC
QUIC [I-D.ietf-quic-transport] is a UDP-based transport that provides
reception feedback. The UDP payload includes the QUIC packet header,
protected payload, and any authentication fields. QUIC depends on a
PMTU of at least 1280 bytes.
Section 14 of [I-D.ietf-quic-transport] describes the path
considerations when sending QUIC packets. It recommends the use of
PADDING frames to build the probe packet. Pure probe-only packets
are constructed with PADDING frames and PING frames to create a
padding only packet that will elicit an acknowledgement. Such
padding only packets enable probing without affecting the transfer of
other QUIC frames.
The recommendation for QUIC endpoints implementing DPLPMTUD is that a
MPS is maintained for each combination of local and remote IP
addresses [I-D.ietf-quic-transport]. If a QUIC endpoint determines
that the PMTU between any pair of local and remote IP addresses has
fallen below an acceptable MPS, it immediately ceases to send QUIC
packets on the affected path. This could result in termination of
the connection if an alternative path cannot be found
[I-D.ietf-quic-transport].
6.3.1. Initial Connectivity
The base protocol is specified in [I-D.ietf-quic-transport]. This
provides an acknowledged PL. A sender can therefore enter the BASE
state as soon as connectivity has been confirmed.
6.3.2. Sending QUIC Probe Packets
A probe packet consists of a QUIC Header and a payload containing
PADDING Frames and a PING Frame. PADDING Frames are a single octet
(0x00) and several of these can be used to create a probe packet of
size PROBED_SIZE. QUIC provides an acknowledged PL, a sender can
therefore enter the BASE state as soon as connectivity has been
confirmed.
The current specification of QUIC sets the following:
* BASE_PMTU: 1280. A QUIC sender pads initial packets to confirm
the path can support packets of the required size.
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* MIN_PMTU: 1280 bytes. A QUIC sender that determines the PLPMTU
has fallen below 1280 bytes MUST immediately stop sending on the
affected path.
6.3.3. Validating the Path with QUIC
QUIC provides an acknowledged PL. A sender therefore MUST NOT
implement the CONFIRMATION_TIMER while in the SEARCH_COMPLETE state.
6.3.4. Handling of PTB Messages by QUIC
QUIC validates ICMP PTB messages. In addition to UDP Port
validation, QUIC can validate an ICMP message by using other PL
information (e.g., validation of connection IDs in the quoted packet
of any received ICMP message).
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).
Thanks to all that have commented or contributed, the TSVWG and QUIC
working groups, and Mathew Calder and Julius Flohr for providing
early implementations.
8. IANA Considerations
This memo includes no request to IANA.
If there are no requirements for IANA, the section will be removed
during conversion into an RFC by the RFC Editor.
9. Security Considerations
The security considerations for the use of UDP and SCTP are provided
in the referenced RFCs.
To avoid excessive load, 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 protects from off-path attackers injecting packets
into the path. This protection if provided in IETF-defined protocols
(e.g., TCP, SCTP) using a randomly-initialized sequence number. A
description of one way to do this when using UDP is provided in
section 5.1 of [RFC8085]).
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There are cases where ICMP Packet Too Big (PTB) messages are not
delivered due to policy, configuration or equipment design (see
Section 1.1), this method therefore does not rely upon PTB messages
being received, but is able to utilize these when they are received
by the sender. PTB messages could potentially be used to cause a
node to inappropriately reduce the PLPMTU. A node supporting
DPLPMTUD MUST therefore appropriately validate the payload of PTB
messages to ensure these are received in response to transmitted
traffic (i.e., a reported error condition that corresponds to a
datagram actually sent by the path layer, see Section 4.6.1).
An on-path attacker, able to create a PTB message could forge PTB
messages that include a valid quoted IP packet. Such an attack could
be used to drive down the PLPMTU. There are two ways this method can
be mitigated against such attacks: First, by ensuring that a PL
sender never reduces the PLPMTU below the base size, solely in
response to receiving a PTB message. This is achieved by first
entering the BASE state when such a message is received. Second, the
design does not require processing of PTB messages, a PL sender could
therefore suspend processing of PTB messages (e.g., in a robustness
mode after detecting that subsequent probes actually confirm that a
size larger than the PTB_SIZE is supported by a path).
The successful processing of an ICMP message can trigger a probe when
the reported PTB size is valid, but this does not directly update the
PLPMTU for the path. This prevents a message attempting to black
hole data by indicating a size larger than supported by the path.
Parallel forwarding paths SHOULD be considered. Section 5.4
identifies the need for robustness in the method because the path
information might 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 packets not
being delivered (black holed) when the PLPMTU is larger than the
smallest actual PMTU.
10. References
10.1. Normative References
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", Work in Progress, Internet-Draft,
draft-ietf-quic-transport-27, 21 February 2020,
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<http://www.ietf.org/internet-drafts/draft-ietf-quic-
transport-27.txt>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
[RFC1191] Mogul, J.C. and S.E. Deering, "Path MTU discovery",
RFC 1191, DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
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[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[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-frag-fragile]
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", Work
in Progress, Internet-Draft, draft-ietf-intarea-frag-
fragile-17, 30 September 2019, <http://www.ietf.org/
internet-drafts/draft-ietf-intarea-frag-fragile-17.txt>.
[I-D.ietf-intarea-tunnels]
Touch, J. and M. Townsley, "IP Tunnels in the Internet
Architecture", Work in Progress, Internet-Draft, draft-
ietf-intarea-tunnels-10, 12 September 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-intarea-
tunnels-10.txt>.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>.
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[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>.
[RFC5508] Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT
Behavioral Requirements for ICMP", BCP 148, RFC 5508,
DOI 10.17487/RFC5508, April 2009,
<https://www.rfc-editor.org/info/rfc5508>.
Appendix A. Revision Notes
Note to RFC-Editor: please remove this entire section prior to
publication.
Individual draft -00:
* Comments and corrections are welcome directly to the authors or
via the IETF TSVWG working group mailing list.
* This update is proposed for WG comments.
Individual draft -01:
* Contains the first representation of the algorithm, showing the
states and timers
* This update is proposed for WG comments.
Individual draft -02:
* Contains updated representation of the algorithm, and textual
corrections.
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* 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.
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* 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
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* 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
Working group draft -11:
* Restore a sentence removed in previous rev
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* De-acronymise QUIC
* Address some nits
Working group draft -12:
* Add TSVWG, QUIC and implementers to acknowledgements
* Shorten a diagram line.
* Address nits from Julius and Wes.
* Be clearer when talking about IP layer caches
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen
AB24 3UE
United Kingdom
Email: tom@erg.abdn.ac.uk
Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: tuexen@fh-muenster.de
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Irene Ruengeler
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: i.ruengeler@fh-muenster.de
Timo Voelker
Muenster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany
Email: timo.voelker@fh-muenster.de
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