Internet DRAFT - draft-fairhurst-tsvwg-udp-options-dplpmtud
draft-fairhurst-tsvwg-udp-options-dplpmtud
Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Intended status: Standards Track University of Aberdeen
Expires: February 13, 2022 August 12, 2021
Datagram PLPMTUD for UDP Options
draft-fairhurst-tsvwg-udp-options-dplpmtud-05
Abstract
This document specifies how a UDP Options sender implements Datagram
Packetization Layer Path Maximum Transmission Unit Discovery
(DPLPMTUD) as a robust method for Path Maximum Transmission Unit
Discovery. This is a robust method for Path MTU Discovery (PMTUD)
that uses the UDP Options Packetization Layer (PL). It allows a
datagram application that uses this PL, to discover the largest size
of datagram that can be sent across a network path.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on February 13, 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DPLPMTUD for UDP Options . . . . . . . . . . . . . . . . . . 3
3.1. Confirmation of Connectivity across a Path . . . . . . . 3
3.2. Sending UDP-Options Probe Packets . . . . . . . . . . . . 3
3.2.1. Sending Packet Probes using the Echo Request Option
Request Option . . . . . . . . . . . . . . . . . . . 4
3.2.2. Sending Packet Probes that include Application Data . 5
3.3. Validating the Path with UDP Options . . . . . . . . . . 5
3.3.1. Sending Packet Probes using Timestamps . . . . . . . 6
3.4. PTB Message Handling for this Method . . . . . . . . . . 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Revision Notes . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
The User Datagram Protocol [RFC0768] offers a minimal transport
service on top of IP and is frequently used as a substrate for other
protocols. Section 3.5 of UDP Guidelines [RFC8085] recommends that
applications implement some form of Path MTU Discovery to avoid the
generation of IP fragments:
"Consequently, an application SHOULD either use the path MTU
information provided by the IP layer or implement Path MTU Discovery
(PMTUD)".
The UDP API [RFC8304] provides calls for applications to receive ICMP
Packet Too Big (PTB) messages and to control the maximum size of
datagrams that are sent, but does not offer any automated mechanisms
for an application to discover the maximum packet size supported by a
path. Applications and upper layer protocols implement mechanisms
for path MTU discovery above the UDP API.
Packetization Layer PMTUD (PLPMTUD) [RFC4821] describes a method for
a Packetization Layer (PL) (such as UDP with options) to search for
the largest Packetization Layer PMTU (PLPMTU) supported on a path.
Datagram PLPMTUD (DPLPMTUD) [RFC8899] specifies this support for
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datagram transports. PLPMTUD and DPLPMTUD use a probing mechanism
that does not solely rely on ICMP PTB messages and works in the
presence of lost probes.
UDP Options [I-D.ietf-tsvwg-udp-options] supplies functionality that
can be used to implement DPLPMTUD within the UDP transport service.
This document specifies this additional functionality. Implementing
DPLPMTUD using UDP Options avoids the need for each upper layer
protocol or application to implement the DPLPMTUD method. This
provides a standard method for applications to discover the current
maximum packet size for a path and to detect when this changes.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD 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.
The structure of the present document follows the structure used to
describe DPLPMTUD for other transports [RFC8899].
3. DPLPMTUD for UDP Options
The DPLPMTUD PL endpoint implements the method specified in
[RFC8899].
3.1. Confirmation of Connectivity across a Path
The DPLPMTUD method requires a PL to be able to confirm connectivity
on the path (see Section 5.1.4 of [RFC8899]), but UDP does not offer
a mechanism for this.
UDP Options can provide this required functionality. A UDP Options
sender implementing this specification SHOULD elicit a positive
confirmation of connectivity of the path, using a suitable confirmed
UDP Option (i.e., Timestamps, ECHO Request/Response).
3.2. Sending UDP-Options Probe Packets
DPLPMTUD relies upon the ability of a sender PL to generate probe
packets with a specific size, and to confirm when these are delivered
across the path. Therefore, a UDP Options sender needs to be able to
send probes up to the maximum for the size the local interface
supports, which MUST NOT be further constrained by the maximum PMTU
set by network layer mechanisms (such as PMTUD [RFC1063][RFC8201]).
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DPLPMTUD needs to be able to generate probe packets that are not
delivered to the upper layer protocol as a part of the end-to-end
transport data (i.e. to ensure any added padding data is not
delivered to the upper layer protocol at the receiver). UDP Options
provide the necessary additional support required to do this within
the transport layer.
There are various designs described in DPLPMTUD for the sending of a
Packet Probe to test the size of packet supported by a path (see
Section 4.1 of [RFC8899]). This prevents "Probing using padding
data" or "Probing using application data and padding data" (see
Section 4.1 of [RFC8899]).
A PL needs to determine whether the current path supports datagrams
used as Probe Packets. DPLPMTUD SHOULD send (or add) a UDP Option
(e.g., Timestamps, ECHO Request/Response) to a Packet Probe to elicit
a positive confirmation that the path has delivered the Probe Packet
of the corresponding size. From time to time, such probes can also
be used to determine whether the current path can support a larger
size of datagram that the current PLPMTU.
A PL also needs to determine that the current path supports the size
of datagram that the application is currently sending when in the
DPLPMTUD search_done state i.e., to detect black-holing of data (see
Section 4.2 of [RFC8899]). UDP Options can provide this by eliciting
a positive confirmation that the path has delivered a Datagram of the
corresponding size.
3.2.1. Sending Packet Probes using the Echo Request Option Request
Option
The RECOMMENDED method sends a Probe Packet with the Echo Request
Option (RES) together with any padding needed to inflate the required
size. The reception of this option generates an Echo Response Option
that confirms reception of each received Probe Packet.
Probe Packets consume network capacity and incur endpoint processing
(see Section 4.1 of [RFC8899]). Implementations ought to send a
Probe Packet with a Request Probe Option only when required by their
local DPLPMTUD state machine, i.e., when probing to grow the PLPMTU
or to confirm the current PLPMTU.
Implementations MAY track multiple requests and respond acknowledging
them with a single packet.
The UDP Options used in this method are described in section 6 of
[I-D.ietf-tsvwg-udp-options]:
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o The Echo Request Option (RES) is set by a sending PL to solicit a
response from a remote endpoint. A four-byte token identifies
each request.
o The Echo Response Option (REQ) is generated by the UDP Options
receiver in response to reception of a previously received Echo
Request Option. Each Echo Response Option echoes a previously
received four-byte token.
The token value allows implementations to distinguish between
acknowledgements for initial Probe Packets and acknowledgements
confirming receipt of subsequent Probe Packets (e.g., travelling
along alternate paths with a larger round trip time). This needs
each Probe Packet needs to be uniquely identifiable by the UDP
Options sender within the Maximum Segment Lifetime (MSL). The UDP
Options sender therefore MUST NOT recycle token values until they
have expired or have been acknowledged. A four byte value for the
token field provides sufficient space for multiple unique probes to
be made within the MSL.
The initial value of the four byte token field SHOULD be assigned to
a randomised value to enhance protection from off-path attacks, as
described in section 5.1 of [RFC8085]).
The procedure to handle the loss of a datagram is the responsibility
of the sender of the request. Implementations MAY track multiple
requests and respond to them with a single packet carrying the Echo
Response Option (REQ).
3.2.2. Sending Packet Probes that include Application Data
The RECOMMENDED approach to generating a Probe Packet is to send a
probe formed of a UDP Options datagram contains only control
information, padded to the size required for the probe. This allows
"Probing using padding data"[RFC8899], and avoids a need to
retransmit application data when a probe fails.
If an application/transport needs protection from the loss of data in
the Probe Packet payload, 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) [RFC8085].
3.3. Validating the Path with UDP Options
A PL also needs to validate that the path continues to support the
PLPMTU discovered in a previous search for a suitable PLPMTU value
(see Section 6.1.4 of [RFC8899]). This confirmation MAY be provided
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by an upper layer protocol confirming correct reception of data by
the remote PL, but there is no generic mechanism to access this upper
layer information.
This function can be implemented within UDP Options, by generating a
Probe Packet of size PLPMTU to confirm the path. This Probe Packet
MUST elicit a response from the remote PL and could use either the
ECHO Response Option or the TimeStamp option (see Section 5.9
[I-D.ietf-tsvwg-udp-options]).
A sender MAY choose to include application data in Probe Packets (see
Section 4.1 of [RFC8899] for discussion of the merits and demerits of
this approach). For example, this might reduce the need to send an
additional datagram when confirming that the current path supports
datagrams of size PLPMTU.
3.3.1. Sending Packet Probes using Timestamps
Reception of a valid Timestamp Option echoed by the remote endpoint
can be used to infer connectivity. It can also confirm that packets
of the current size are being received by the remote PL. This can
provide useful feedback, even over paths with asymmetric capacity
and/or that carry UDP Option flows that have asymmetric datagram
rates, because an echo of the most recent timestamp still indicates
reception of at least one packet of the transmitted size. This is
sufficient to confirm there is no black hole (see Section 2.1 of
[RFC2923]).
When sending a probe to increase the PLPMTU, such a Timestamp might
be unable to unambiguously identify that a specific Probe Packet has
been received [KP87]. Timestamp mechanisms therefore cannot be used
to confirm the reception of individual probe messages and cannot be
used to stimulate a response from the remote peer.
Note: Probe Packets used to search for a larger PLPMTU MUST include
the Echo Request Option.
3.4. PTB Message Handling for this Method
A UDP Options sender can ignore received ICMP PTB messages, and this
support is OPTIONAL for use with DPLPMTUD.
A UDP Options sender that utilises ICMP PTB messages received to a
Probe Packet MUST use the quoted packet to validate the UDP port
information in combination with the token and/or timestamp value
contained in the UDP Option, before processing the packet using the
DPLPMTUD method (see Section 4.4.1 of [RFC8899]). An implementation
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unable to support this validation needs to ignore received ICMP PTB
messages.
4. Acknowledgements
Gorry Fairhurst and Tom Jones are supported by funding provided by
the University of Aberdeen.
5. IANA Considerations
This memo includes no requests to IANA.
6. Security Considerations
The security considerations for using UDP Options are described in
[I-D.ietf-tsvwg-udp-options]. The proposed new method does not
change the integrity protection offered by the UDP options method.
The specification recommends that the token in the REQ/RES message is
initialised to a randomised value to enhance protection from off-path
attacks.
The security considerations for using DPLPMTUD are described in
[RFC8899]. The proposed new method does not change the ICMP PTB
message validation method described DPLPMTUD: A UDP Options sender
that utilises ICMP PTB messages received to a Probe Packet MUST use
the quoted packet to validate the UDP port information in combination
with the token and/or timestamp value contained in the UDP Option,
before processing the packet using the DPLPMTUD method.
7. References
7.1. Normative References
[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-09 (work in progress), November 2020.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
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[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>.
[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>.
[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>.
7.2. Informative References
[KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", 1987.
[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, DOI 10.17487/RFC1063,
July 1988, <https://www.rfc-editor.org/info/rfc1063>.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, DOI 10.17487/RFC2923, September 2000,
<https://www.rfc-editor.org/info/rfc2923>.
[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>.
[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>.
[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>.
[RFC8304] Fairhurst, G. and T. Jones, "Transport Features of the
User Datagram Protocol (UDP) and Lightweight UDP (UDP-
Lite)", RFC 8304, DOI 10.17487/RFC8304, February 2018,
<https://www.rfc-editor.org/info/rfc8304>.
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Appendix A. Revision Notes
XXX Note to RFC-Editor: please remove this entire section prior to
publication. XXX
Individual draft-00.
o This version contains a description for consideration and comment
by the TSVWG.
Individual draft-01.
o Address Nits
o Change Probe Request and Probe Reponse options to Echo to align
names with draft-ietf-tsvwg-udp-options
o Remove Appendix B, Informative Description of new UDP Options
o Add additional sections around Probe Packet generation
Individual draft-02.
o Address Nits
Individual draft-03.
o Referenced DPLPMTUD RFC.
o Tidied language to clarify the method.
Individual draft-04
o Reworded text on probing with data a little.
o Removed paragraph on suspending ICMP PTB suspension.
Individual draft -05
o Fixed typos.
Authors' Addresses
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Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: tom@erg.abdn.ac.uk
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