Internet DRAFT - draft-petithuguenin-tsvwg-stun-pmtud
draft-petithuguenin-tsvwg-stun-pmtud
Network Working Group M. Petit-Huguenin
Internet-Draft Impedance Mismatch
Intended status: Standards Track G. Salgueiro
Expires: 6 April 2023 Cisco Systems, Inc.
3 October 2022
Packetization Layer Path MTU Discovery (PLMTUD) For UDP Transports Using
Session Traversal Utilities for NAT (STUN)
draft-petithuguenin-tsvwg-stun-pmtud-01
Abstract
The datagram exchanged between two Internet endpoints have to go
through a series of physical and virtual links that may have
different limits on the upper size of the datagram they can transmit
without fragmentation. Because fragmentation is considered harmful,
most transports and protocols are designed with a mechanism that
permits dynamic measurement of the maximum size of a datagram. This
mechanism is called Packetization Layer Path MTU Discovery (PLPMTUD).
But the UDP transport and some of the protocols that use UDP were
designed without that feature. The Session Traversal Utilities for
NAT (STUN) Usage described in this document permits retrofitting an
existing UDP-based protocol with such a feature. Similarly, a new
UDP-based protocol could simply reuse the mechanism described in this
document.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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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 6 April 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Operations . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Probing Mechanisms . . . . . . . . . . . . . . . . . . . . . 5
4.1. Simple Probing Mechanism . . . . . . . . . . . . . . . . 6
4.1.1. Sending a Probe Request . . . . . . . . . . . . . . . 6
4.1.2. Receiving a Probe Request . . . . . . . . . . . . . . 7
4.1.3. Receiving a Probe Response . . . . . . . . . . . . . 7
4.2. Complete Probing Mechanism . . . . . . . . . . . . . . . 7
4.2.1. Sending a Probe Indications and Report Request . . . 8
4.2.2. Receiving an ICMP Packet . . . . . . . . . . . . . . 9
4.2.3. Receiving a Probe Indication and Report Request . . . 9
4.2.4. Receiving a Report Response . . . . . . . . . . . . . 10
4.2.5. Using Checksums as Packet Identifiers . . . . . . . . 10
4.2.6. Using Sequence Numbers as Packet Identifiers . . . . 10
5. Probe Support Signaling Mechanisms . . . . . . . . . . . . . 11
5.1. Explicit Probe Support Signaling Mechanism . . . . . . . 12
5.2. Implicit Probe Support Signaling Mechanism . . . . . . . 12
6. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. IDENTIFIERS . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. PMTUD-SUPPORTED . . . . . . . . . . . . . . . . . . . . . 13
6.3. PADDING . . . . . . . . . . . . . . . . . . . . . . . . . 13
7. DPLPMTUD Considerations . . . . . . . . . . . . . . . . . . . 13
7.1. Features Required to provide Datagram PLPMTUD . . . . . . 13
7.2. Application Support for DPLPMTUD with UDP . . . . . . . . 14
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9.1. New STUN Methods . . . . . . . . . . . . . . . . . . . . 16
9.2. New STUN Attributes . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. Normative References . . . . . . . . . . . . . . . . . . 16
10.2. Informative References . . . . . . . . . . . . . . . . . 17
Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
The Packetization Layer Path MTU Discovery (PMTUD) specification
[RFC4821] describes a method to discover the Path MTU, but does not
describe a practical protocol to do so with UDP. Many application
layer protocols based on the transport layer protocol UDP do not
implement the Path MTU discovery mechanism described in [RFC4821].
These application layer protocols can make use of the probing
mechanisms described in this document instead of designing their own
adhoc extension. These probing mechanisms are implemented with
Session Traversal Utilities for NAT (STUN), but their usage is not
limited to STUN-based protocols.
The STUN usage defined in this document for Packetization Layer Path
MTU Discovery (PLPMTUD) between a client and a server permits proper
measurement of the Path MTU for application layer protocols based on
the transport layer protocol UDP in the network. It also simplifies
troubleshooting and has multiple other applications across a wide
variety of technologies.
Complementary techniques can be used to discover additional network
characteristics, such as the network path (using the STUN Traceroute
mechanism described in [I-D.martinsen-tram-stuntrace]) and bandwidth
availability (using the mechanism described in
[I-D.martinsen-tram-turnbandwidthprobe]). In addition, [RFC8899]
provides a robust method for Path MTU Discovery for a broader range
of protocols and applications.
2. Overview of Operations
This section is meant to be informative only and is not intended as a
substitute for [RFC4821].
A UDP endpoint that uses this specification to discover the Path MTU
over UDP and knows that the endpoint it is communicating with also
supports this specification can choose to use either the Simple
Probing mechanism (as described in Section 4.1) or the Complete
Probing mechanism (as described in Section 4.2). The selection of
which Probing Mechanism to use is dependent on performance and
security and complexity trade-offs.
If the Simple Probing mechanism is chosen, then the client initiates
Probe transactions, as shown in Figure 1, which decrease in size
until transactions succeed, indicating that the Path MTU has been
discovered. It then uses that information to update the Path MTU.
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Client Server
| |
| Probe Request |
|---------------->|
| |
| Probe Response |
|<----------------|
| |
Figure 1: Simple Probing Example
If the Complete Probing mechanism (as described in Section 4.2) is
chosen, then the client sends Probe Indications of various sizes (as
specified in [RFC4821]) interleaved with UDP packets sent by the UDP
protocol. The client then sends a Report Request for the ordered
list of identifiers for the UDP packets and Probe Indications
received by the server. The client then compares the list returned
in the Report Response with its own list of identifiers for the UDP
packets and Probe Indications it sent. The client examines the
received reports to determine which probes were successful. When a
probe succeeds with a larger size than the current PMTU, the PMTU is
increased. When the probes indicate the current PMTU is not
supported the size is decreased. This mechanism acts to detect that
traffic is being back holed.
Because of the possibility of amplification attack, the Complete
Probing mechanism must be authenticated as specified in Section 5.1.
Particular care must be taken to prevent amplification when an
external mechanism is used to trigger the Complete Probing mechanism.
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Client Server
| UDP Packet |
|------------------>|
| |
| UDP Packet |
|------------------>|
| |
| Probe Indication |
|------------------>|
| |
| UDP Packet |
|------------------>|
| |
| Probe Indication |
|------------------>|
| |
| Report Request |
|------------------>|
| Report Response |
|<------------------|
| |
Figure 2: Complete Probing Example
3. 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.
4. Probing Mechanisms
The Probing mechanism is used to discover the Path MTU in one
direction only: from the client to the server. Both endpoints MAY
behave as a client and a server to achieve bi-directional path
discovery.
Two Probing mechanisms are described: a Simple Probing mechanism and
a more complete mechanism that can converge more quickly and find an
appropriate Path MTU in the presence of congestion. Additionally,
the Simple Probing mechanism does not require authentication except
where used as an implicit signaling mechanism, whereas the complete
mechanism does.
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Implementations supporting this specification MUST implement the
server side of both the Simple Probing mechanism (Section 4.1) and
the Complete Probing mechanism (Section 4.2).
Implementations supporting this specification MUST implement the
client side of the Complete Probing mechanism. They MAY implement
the client side of the Simple Probing mechanism.
The FINGERPRINT mechanism described in section 7 of [RFC8489] MUST be
used for both probing mechanisms.
4.1. Simple Probing Mechanism
The Simple Probing mechanism is implemented by sending a Probe
Request with a PADDING attribute over UDP with the DF bit set in the
IP header for IPv4 packets and IPv6 packets without the Fragment
Header included.
| NOTE: Routers might be configured to clear the DF bit or ignore
| the DF bit which can be difficult or impossible to detect if
| reassembly occurs prior to receiving the packet.
The Simple Probing Mechanism uses only STUN Requests/Responses, which
are subject to the congestion control mechanism in [RFC8489] section
6.2.1. The default Rc and Rm values may be defined differently for a
combination of the Simple Probing Mechanism and the protocol running
on the same port.
4.1.1. Sending a Probe Request
A client forms a Probe Request by using the Probe Method and
following the rules in Section 6.1 of [RFC8489].
The Probe transaction MUST be authenticated if the Simple Probing
mechanism is used in conjunction with the Implicit Probing Support
mechanism described in Section 5.2. If not, the Probe transaction
MAY be authenticated.
The client adds a PADDING attribute with a length that, when added to
the IP and UDP headers and the other STUN components, is equal to the
Selected Probe Size, as defined in [RFC4821] Section 7.3. The
PADDING bits MUST be set to zero. The client MUST add the
FINGERPRINT attribute so the STUN messages are disambiguated from the
other protocol packets as specified in Section 7 of [RFC8489].
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Then the client sends the Probe Request to the server over UDP with
the DF bit set for IPv4 packets and IPv6 packets without the Fragment
Header included. For the purpose of this transaction, the Rc
parameter is set to 3 and the initial value for RTO stays at 500 ms
as specified in Section 6.2.1 of [RFC8489].
To be able to determine the reason STUN messages may be blocked, a
client MUST NOT send a probe if it does not have knowledge that the
server supports this specification. This is done either by external
signalling or by a mechanism specific to the UDP protocol to which
PMTUD capabilities are added or by one of the mechanisms specified in
Section 5.
4.1.2. Receiving a Probe Request
A server receiving a Probe Request MUST process it as specified in
[RFC8489].
The server then creates a Probe Response. The server MUST add the
FINGERPRINT attribute so the STUN messages are disambiguated from the
other protocol packets as specified in Section 7 of [RFC8489]. The
server then sends the response to the client.
4.1.3. Receiving a Probe Response
A client receiving a Probe Response MUST process it as specified in
section 6.3.3 of [RFC8489] and MUST ignore the PADDING attribute. If
a response is received this is interpreted as a Probe Success, as
defined in [RFC4821] Section 7.6.1. If an ICMP packet "Fragmentation
needed" or "Packet Too Big" is received then this is interpreted as a
Probe Failure, as defined in [RFC4821] Section 7.6.2. If the Probe
transaction times out, then this is interpreted as a Probe
Inconclusive, as defined in [RFC4821] Section 7.6.4. Validation MUST
be performed on the ICMP packet as specified in [RFC8899].
4.2. Complete Probing Mechanism
The Complete Probing mechanism is implemented by sending one or more
Probe Indications with a PADDING attribute over UDP with the DF bit
set in the IP header for IPv4 packets and IPv6 packets without the
Fragment Header included followed by a Report Request to the same
server. A router on the path to the server can reject this
Indication with an ICMP message or drop it. The server keeps a
chronologically ordered list of identifiers for all packets received
(including retransmitted packets) and sends this list back to the
client in the Report Response. The client analyzes this list to find
which packets were not received. Because UDP packets do not contain
an identifier, the Complete Probing mechanism needs a way to identify
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each packet received.
Some application layer protocols may already have a way of
identifying each individual UDP packet, in which case these
identifiers SHOULD be used in the IDENTIFIERS attribute of the Report
Response. While there are other possible packet identification
schemes, this document describes two different ways to identify a
specific packet when no application layer protocol-specific
identification mechanism is available.
In the first packet identification mechanism, the server computes a
checksum over each packet received and sends back to the sender the
list of checksums ordered chronologically. The client compares this
list to its own list of checksums.
In the second packet identification mechanism, the client prepends
the UDP data with a header that provides a sequence number. The
server sends back the chronologically ordered list of sequence
numbers received that the client then compares with its own list.
The Simple Probing Mechanism uses STUN indications, which are not
subject to the congestion control mechanism in [RFC8489] section
6.2.1. As it will have to be intricately related to the protocol
that runs on the same port, each implementation of the Complete
Probing Mechanism in association MUST define the congestion control
that will be applied to the STUN Indications. The default Rc and Rm
values for the STUN Requests/Responses may be defined differently for
a combination of the Simple Probing Mechanism and the protocol
running on the same port.
4.2.1. Sending a Probe Indications and Report Request
A client forms a Probe Indication by using the Probe Method and
following the rules in [RFC8489] Section 6.1. The client adds to a
Probe Indication a PADDING attribute with a size that, when added to
the IP and UDP headers and the other STUN components, is equal to the
Selected Probe Size, as defined in [RFC4821] Section 7.3. The
PADDING bits MUST be set to zero. If the authentication mechanism
permits it, then the Indication MUST be authenticated. The client
MUST add the FINGERPRINT attribute so the STUN messages are
disambiguated from the other protocol packets.
Then the client sends a Probe Indication to the server over UDP with
the DF bit set for IPv4 packets and IPv6 packets without the Fragment
Header included.
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Then the client forms a Report Request by following the rules in
[RFC8489] Section 6.1. The Report transaction MUST be authenticated
to prevent amplification attacks. The client MUST add the
FINGERPRINT attribute so the STUN messages are disambiguated from the
other protocol packets.
Then the client waits half the RTO after sending the last Probe
Indication and then sends the Report Request to the server over UDP.
4.2.2. Receiving an ICMP Packet
If an ICMP packet "Fragmentation needed" or "Packet Too Big" is
received then this is interpreted as a Probe Failure, as defined in
[RFC4821] Section 7.5. Validation MUST be performed on the ICMP
packet as specified in [RFC8899].
4.2.3. Receiving a Probe Indication and Report Request
A server supporting this specification will keep the identifiers of
all packets received in a chronologically ordered list. The packets
that are to be associated to a given flow's identifier are selected
according to Section 5.2 of [RFC4821]. The same identifier can
appear multiple times in the list because of retransmissions. The
maximum size of this list is calculated such that when the list is
added to the Report Response, the total size of the packet does not
exceed the unknown Path MTU, as defined in [RFC8489] Section 6.1.
Older identifiers are removed when new identifiers are added to a
list that is already full.
A server receiving a Report Request MUST process it as specified in
[RFC8489] and MUST ignore the PADDING attribute.
The server creates a Report Response and adds an IDENTIFIERS
attribute that contains the chronologically ordered list of all
identifiers received so far. The server MUST add the FINGERPRINT
attribute. The server then sends the response to the client.
The exact content of the IDENTIFIERS attribute depends on what type
of identifiers have been chosen for the protocol. Each protocol
adding PMTUD capabilities as specified by this specification MUST
describe the format of the contents of the IDENTIFIERS attribute,
unless it is using one of the formats described in this
specification. See Section 6.1 for details about the IDENTIFIERS
attribute.
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4.2.4. Receiving a Report Response
A client receiving a Report Response processes it as specified in
[RFC8489]. If the response IDENTIFIERS attribute contains the
identifier of a Probe Indication, then this is interpreted as a Probe
Success for this probe, as defined in [RFC4821] Section 7.5. If a
Probe Indication identifier cannot be found in the Report Response,
this is interpreted as a Probe Failure, as defined in [RFC4821]
Section 7.5. If a Probe Indication identifier cannot be found in the
Report Response but identifiers for other packets sent before or
after the Probe Indication can all be found, this is interpreted as a
Probe Failure as defined in [RFC4821] Section 7.5. If the Report
Transaction times out, this is interpreted as a Full-Stop Timeout, as
defined in [RFC4821] Section 3.
4.2.5. Using Checksums as Packet Identifiers
When using a checksum as a packet identifier, the client keeps a
chronologically ordered list of the packets it transmits, along with
an associated checksum value. For STUN Probe Indication or Request
packets, the associated checksum value is the FINGERPRINT value from
the packet; for other packets a checksum value is computed. The
value of the checksum is computed as the CRC-32 of the UDP payload,
as defined by the Length field of the UDP datagram [RFC4821], XOR'ed
with the 32-bit value 0x5354554e. The 32-bit CRC is the one defined
in ITU V.42 [ITU.V42.2002], which has a generator polynomial of x^32
+ x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 + x^7 + x^5 +
x^4 + x^2 + x + 1.
For each STUN Probe Indication or Request, the server retrieves the
STUN FINGERPRINT value. For all other packets, the server calculates
the checksum as described above. It puts these FINGERPRINT and
checksum values in a chronologically ordered list that is sent back
in the Report Response.
The contents of the IDENTIFIERS attribute is a list of 4 byte
numbers, each using the same encoding that is used for the contents
of the FINGERPRINT attribute.
4.2.6. Using Sequence Numbers as Packet Identifiers
When using sequence numbers, a small header similar to the TURN
ChannelData header, as defined in Section 11.4 of [RFC5766], is added
in front of all packets that are not a STUN Probe Indication or
Request. The initial sequence number MUST be randomized and is
monotonically incremented by one for each packet sent. The most
significant bit of the sequence number is always 0. The server
collects the sequence number of the packets sent, or the 4 first
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bytes of the transaction ID if a STUN Probe Indication or Request is
sent. In that case, the most significant bit of the 4 first bytes is
set to 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Application Data /
/ /
| |
| +-------------------------------+
| |
+-------------------------------+
Figure 3
The Channel Number is always 0xFFFF. The Length field specifies the
length in bytes of the sequence number and application data fields.
The header values are encoded using network order.
The contents of the IDENTIFIERS attribute is a chronologically
ordered list of 4 byte numbers, each containing either a sequence
number, if the packet was not a STUN Probe Indication or Request, or
the 4 first bytes of the transaction ID, with the most significant
bit forced to 1, if the packet is a STUN Probe Indication or Request.
5. Probe Support Signaling Mechanisms
The PMTUD mechanism described in this document is intended to be used
by any UDP-based protocols that do not have built-in PMTUD
capabilities, irrespective of whether those UDP-based protocols are
STUN-based or not. So the manner in which a specific protocol
discovers that it is safe to send PMTUD probes is largely dependent
on the details of that specific protocol, with the exception of the
Implicit Mechanism described below, which applies to any protocol.
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5.1. Explicit Probe Support Signaling Mechanism
Some of these mechanisms can use a separate signalling mechanism (for
instance, an SDP attribute in an Offer/Answer exchange [RFC3264]), or
an optional flag that can be set in the protocol that is augmented
with PMTUD capabilities. STUN Usages that can benefit from PMTUD
capabilities can signal in-band that they support probing by
inserting a PMTUD-SUPPORTED attribute in some STUN methods. The
decision of which methods support this attribute is left to each
specific STUN Usage.
UDP-based protocols that want to use any of these mechanisms,
including the PMTUD-SUPPORTED attribute, to signal PMTUD capabilities
MUST ensure that it cannot be used to launch an amplification attack.
An amplification attack can be prevented using techniques such as:
* Authentication, where the source of the packet and the destination
share a secret.
* 3 way handshake with some form of unpredictable cookie.
* Make sure that the total size of the traffic potentially generated
is lower than the size of the request that generated it.
5.2. Implicit Probe Support Signaling Mechanism
As a result of the fact that all endpoints implementing this
specification are both clients and servers, a Probe Request or
Indication received by an endpoint acting as a server implicitly
signals that this server can now act as a client and MAY send a Probe
Request or Indication to probe the Path MTU in the reverse direction
toward the former client, that will now be acting as a server.
The Probe Request or Indication that are used to implicitly signal
probing support in the reverse direction MUST be authenticated to
prevent amplification attacks.
6. STUN Attributes
6.1. IDENTIFIERS
The IDENTIFIERS attribute carries a chronologically ordered list of
UDP packet identifiers.
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While Section 4.2.5 and Section 4.2.6 describe two possible methods
for acquiring and formatting the identifiers used for this purpose,
ultimately each protocol has to define how these identifiers are
acquired and formatted. Therefore, the contents of the IDENTIFIERS
attribute is opaque.
6.2. PMTUD-SUPPORTED
The PMTUD-SUPPORTED attribute indicates that its sender supports this
mechanism, as incorporated into the STUN usage or protocol being
used. This attribute has no value part and thus the attribute length
field is 0.
6.3. PADDING
The PADDING attribute allows for the entire message to be padded to
force the STUN message to be divided into IP fragments. The PADDING
bits MUST be set to zero. PADDING can be used in either Binding
Requests or Binding Responses.
PADDING MUST NOT be longer than the length that brings the total IP
datagram size to 64K, minus the IP and UDP headers and the other STUN
components. It SHOULD be equal in length to the MTU of the outgoing
interface, rounded up to an even multiple of four bytes and SHOULD
ensure a probe does not result in a packet larger than the MTU for
the outgoing interface. STUN messages sent with PADDING are intended
to test the behavior of UDP fragmentation, therefore they are an
exception to the usual rule that STUN messages need to be less than
the PMTU for the path.
7. DPLPMTUD Considerations
This section specifies how the PMTUD mechanism described in this
document conforms to Sections 3 and 6.1 of [RFC8899] and indicates
where each requirement is addressed. The text in this section must
be compared side-by-side with [RFC8899] to understand the
relationship between the two.
7.1. Features Required to provide Datagram PLPMTUD
This section covers Section 3 of [RFC8899] and refers back to
sections in this document covering each of the feature requirements.
1. Managing the PLMPTU: This requirement is fulfilled by the Simple
Probing and Complete Probing mechanisms as discussed in
Section 2, Section 4.1 and Section 4.2 of this document.
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2. Probe packets: This requirement is fulfilled by including a
PADDING attribute which indicates that the DF bit is set in the
IP header for IPv4 packets and not including the Don't Fragment
header in IPv6 packets as discussed in Section 4.1 and
Section 4.2 of this document.
3. Reception feedback: This requirement fulfilled by the Probe
Response and Report Response in Section 2 of this document.
4. Probe loss recovery: This requirement is fulfilled by requiring
that the PADDING bits MUST be set to zero as discussed in
Section 4.1.1 and Section 4.2.1 of this document. No
retransmission is required as no user data is being transmitted
in the probe.
5. PMTU parameters: This requirement is fulfilled by setting the
Selected Probe Size as defined in [RFC4821] and discussed in
Section 4.1 and Section 4.2 of this document.
6. Processing PTB messages: This requirement is fulfilled by the
Probe Response and Report Response in Section 4.1.3 and
Section 4.2.2 of this document.
7. Probing and congestion control: This requirement is fulfilled by
the Probe Request and Probe Indication discussed in Section 4.1.1
and Section 4.2.1 of this document. It conforms to Section 6.2.1
of [RFC8489].
8. Probing and flow control: This requirement is out of scope and is
not discussed in this document.
9. Shared PLPMTU state: An implementation follows the same
guidelines to share state than in [RFC8899].
Datagram reordering: This requirement is fulfilled by the Report
Response in Section 4.2 of this document.
Datagram delay and duplication: This requirement is fulfilled by the
Report Response in Section 4.2 of this document.
When to probe: This requirement is discussed in Section 2 of this
document.
7.2. Application Support for DPLPMTUD with UDP
This section covers Section 6.1 of [RFC8899] and refers back to which
sections in this document covers each of the feature requirements.
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6.1.1 Application Request: This requirement is fulfilled by the
Simple Probing and Complete Probing mechanisms as discussed in
Section 2, Section 4.1 and Section 4.2 of this document.
6.1.2 Application Response: This requirement is fulfilled by the
Simple Probing and Complete Probing mechanisms as discussed in
Section 4.1 and Section 4.2 of this document.
6.1.3 Sending Application Probe Packets: This requirement is
fulfilled by the requirement that the PADDING bits MUST are set to
zero as discussed in Section 4.1.1 and Section 4.2.1 of this
document.
6.1.4 Initial Connectivity: This requirement is fulfilled by the
Implicit and Explicit Probe Support Signaling mechanisms as discussed
Section 5 of this document.
6.1.5 Validating the Path: This requirement is fulfilled by the
Report Request and Report Response mechanisms as discussed in
Section 4.2 of this document.
6.1.6 Handling of PTB Messages: This requirement is fulfilled by the
Probe Response and Report Response in Section 4.1.3 and Section 4.2.2
of this document.
8. Security Considerations
The PMTUD mechanism described in this document, when used without the
signalling mechanism described in Section 5.1, does not introduce any
specific security considerations beyond those described in [RFC4821]
and [RFC8899].
The attacks described in Section 11 of [RFC4821] apply equally to the
mechanism described in this document.
The amplification attacks introduced by the signalling mechanism
described in Section 5.1 can be prevented by using one of the
techniques described in that section.
The Simple Probing mechanism may be used without authentication
because this usage by itself cannot trigger an amplification attack
as the Probe Response is smaller than the Probe Request except when
used in conjunction with the Implicit Probing Support Signaling
mechanism.
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9. IANA Considerations
This specification defines two new STUN methods and two new STUN
attributes.
9.1. New STUN Methods
IANA is requested to add the following methods to the STUN Method
Registry:
0xXXX : Probe
0xXXX : Report
See Sections Section 4.1 and Section 4.2 for the semantics of these
new methods.
9.2. New STUN Attributes
IANA is requested to add the following attributes to the STUN Method
Registry:
Comprehension-required range (0x0000-0x7FFF):
0xXXXX: IDENTIFIERS
Comprehension-optional range (0x8000-0xFFFF):
0xXXXX: PMTUD-SUPPORTED
IANA is requested to add a reference to RFC-to-be (in addition to RFC
5780) for the following STUN attribute:
0x0026: PADDING
The IDENTIFIERS STUN attribute is defined in Section 6.1, the PMTUD-
SUPPORTED STUN attribute is defined in Section 6.2; the PADDING STUN
attribute is redefined in Section 6.3.
10. References
10.1. Normative References
[ITU.V42.2002]
International Telecommunications Union, "Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion", ITU-T Recommendation V.42, 2002.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, BCP 14, RFC 2119,
DOI 10.17487/RFC2119, 1 March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, 1 March 2007,
<https://www.rfc-editor.org/info/rfc4821>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, BCP 14, RFC 8174,
DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.
[RFC8489] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
February 2020, <https://www.rfc-editor.org/info/rfc8489>.
[RFC8899] Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
T. Voelker, "Packetization Layer Path MTU Discovery for
Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
September 2020, <https://www.rfc-editor.org/info/rfc8899>.
10.2. Informative References
[I-D.martinsen-tram-stuntrace]
Martinsen, M. and D. Wing, "STUN Traceroute", Work in
Progress, Internet-Draft, draft-martinsen-tram-stuntrace-
01, June 2015, <https://datatracker.ietf.org/doc/html/
draft-martinsen-tram-stuntrace-01>.
[I-D.martinsen-tram-turnbandwidthprobe]
Martinsen, M., Andersen, T., Salgueiro, G., and M. Petit-
Huguenin, "Traversal Using Relays around NAT (TURN)
Bandwidth Probe", Work in Progress, Internet-Draft, draft-
martinsen-tram-turnbandwidthprobe-00, June 2015,
<https://datatracker.ietf.org/doc/html/draft-martinsen-
tram-turnbandwidthprobe-00>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, 1 June 2002,
<https://www.rfc-editor.org/info/rfc3264>.
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[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, 1 April 2010,
<https://www.rfc-editor.org/info/rfc5766>.
Changelog
This section must be removed before publication as an RFC.
draft-petithuguenin-tsvwg-stun-pmtud-01:
* Refresh.
draft-petithuguenin-tsvwg-stun-pmtud-00:
* Downgraded to an individual draft.
draft-ietf-tram-stun-pmtud-20:
* Modifications following reviews by Gorry and Magnus.
draft-ietf-tram-stun-pmtud-19:
* Nits.
* Update references, including sections number.
* Move co-editor to contributor section.
* Changelog (formerly Release Notes) is more compact.
draft-ietf-tram-stun-pmtud-18:
* Modifications to address DISCUSS and COMMENT from IESG review.
updated section 7.
draft-ietf-tram-stun-pmtud-17:
* Modifications to address DISCUSS and COMMENT from IESG review.
Added section 7.
draft-ietf-tram-stun-pmtud-16:
* Modifications to address DISCUSS and COMMENT from IESG review
draft-ietf-tram-stun-pmtud-15:
* Modifications to address DISCUSS and COMMENT from IESG review
draft-ietf-tram-stun-pmtud-14:
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* Modifications to address COMMENTS from IESG review
draft-ietf-tram-stun-pmtud-13:
* Modifications to address nits
draft-ietf-tram-stun-pmtud-12:
* Modifications following IESG review. Incorporated RFC5780 PADDING
attribute (Adam's Discuss) and added IPv6 language (Suresh's
Discuss).
draft-ietf-tram-stun-pmtud-11:
* Modifications following IESG review.
draft-ietf-tram-stun-pmtud-10:
* Modifications following reviews for gen-art (Roni Even) and secdir
(Carl Wallace).
draft-ietf-tram-stun-pmtud-09:
* Add 3 ways of preventing amplification attacks.
draft-ietf-tram-stun-pmtud-08:
* Updates following Spencer's review.
draft-ietf-tram-stun-pmtud-07:
* Updates following Shepherd review.
draft-ietf-tram-stun-pmtud-06:
* Nits.
* Restore missing changelog for previous version.
draft-ietf-tram-stun-pmtud-05:
* Modifications following Brandon Williams review.
draft-ietf-tram-stun-pmtud-04:
* Modifications following Simon Perreault and Brandon Williams
reviews.
draft-ietf-tram-stun-pmtud-03:
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* Add new Overview of Operations section with ladder diagrams.
* Authentication is mandatory for the Complete Probing mechanism,
optional for the Simple Probing mechanism.
* All the ICE specific text moves to a separate draft to be
discussed in the ICE WG.
* The TURN usage is removed because probing between a TURN server
and TURN client is not useful.
* Any usage of PMTUD-SUPPORTED or other signaling mechanisms
(formerly knows as discovery mechanisms) must now be
authenticated.
* Both probing mechanisms are MTI in the server, the complete
probing mechanism is MTI in the client.
* Make clear that stopping after 3 retransmission is done by
changing the STUN parameter.
* Define the format of the attributes.
* Make clear that the specification is for any UDP protocol that
does not already have PMTUD capabilities, not just STUN based
protocols.
* Change the default delay to send the Report Request to 250 ms
after the last Indication if the RTO is unknown.
* Each usage of this specification must the format of the
IDENTIFIERS attribute contents.
* Better define the implicit signaling mechanism.
* Extend the Security Consideration section.
* Tons of nits.
draft-ietf-tram-stun-pmtud-02:
* Cleaned up references.
draft-ietf-tram-stun-pmtud-01:
* Added Security Considerations Section.
* Added IANA Considerations Section.
draft-ietf-tram-stun-pmtud-00:
* Adopted by WG - Text unchanged.
draft-petithuguenin-tram-stun-pmtud-01:
* Moved some Introduction text to the Probing Mechanism section.
* Added cross-reference to the other two STUN troubleshooting
mechanism drafts.
* Updated references.
* Added Gonzalo Salgueiro as co-author.
draft-petithuguenin-tram-stun-pmtud-00:
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* General refresh for republication.
draft-petithuguenin-behave-stun-pmtud-03:
* Changed author address.
* Changed the IPR to trust200902.
draft-petithuguenin-behave-stun-pmtud-02:
* Defined checksum and sequential numbers as possible packet
identifiers.
* Updated the reference to RFC 5389
* The FINGERPRINT attribute is now mandatory.
* Changed the delay between Probe indication and Report request to
be RTO/2 or 50 milliseconds.
* Added ICMP packet processing.
* Added Full-Stop Timeout detection.
* Stated that Binding request with PMTUD-SUPPORTED does not start
the PMTUD process if already started.
draft-petithuguenin-behave-stun-pmtud-01:
* Removed the use of modified STUN transaction but shorten the
retransmission for the simple probing mechanism.
* Added a complete probing mechanism.
* Removed the PADDING-RECEIVED attribute.
* Added release notes.
Acknowledgements
Thanks to Eilon Yardeni, Geir Sandbakken, Paal-Erik Martinsen,
Tirumaleswar Reddy, Ram Mohan R, Simon Perreault, Brandon Williams,
Tolga Asveren, Spencer Dawkins, Carl Wallace, Roni Even, Gorry
Fairhurst, and Magnus Westerlund for the comments, suggestions and
questions that helped improve this document.
Special thanks to Dan Wing, who supported this document since its
first publication back in 2008.
Contributors
Felipe Garrido
Email: fegarrid@cisco.com
Cisco Systems, Inc.
Research Triangle Park, NC 27709
United States
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Authors' Addresses
Marc Petit-Huguenin
Impedance Mismatch
Email: marc@petit-huguenin.org
Gonzalo Salgueiro
Cisco Systems, Inc.
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
United States
Email: gsalguei@cisco.com
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