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Internet Engineering Task Force G. Fairhurst
Internet-Draft T. Jones
Intended status: Standards Track University of Aberdeen
Expires: April 4, 2020 October 2, 2019
Datagram PLPMTUD for UDP Options
draft-fairhurst-tsvwg-udp-options-dplpmtud-01
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.
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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Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on April 4, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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 . . . . . . . . . . 6
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 . . . . . . . . . . . . . . . . . . . 8
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. Applications using UDP frequently have to implement basic
transport services such as Path Maximum Transmission Unit Discovery
(PMTUD) themselves. 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] offers calls for applications to receive ICMP
Packet Too Big (PTB) messages and to control the size of messages
that are sent, but does not offer any automatic mechanisms for an
application to discover the maximum packet size supported by a path.
Applications and upper layer protocols are left to implement robust
PMTUD mechanisms of their own.
Packetization Layer PMTUD (PLPMTUD) [RFC4821] describes a method for
a Packetization Layer (such as UDP with options) to search for the
largest MTU supported on a path in the absence of ICMP PTB messages.
Datagram PLPMTUD [I-D.ietf-tsvwg-datagram-plpmtud] describes PMTUD
probing and search algorithms for datagram transports that do not
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solely rely on ICMP PTB messages. This allows the Packetization
Layer (PL) to offer a probing mechanism which works in the presence
of lost probes. However, UDP is unable itself to offer the required
probing mechanisms to implement DPLPMTUD without some additional
transport services.
This document specifies the additional functionality required to
perform DPLPMTUD with UDP Options [I-D.ietf-tsvwg-udp-options]as a
service to upper-layer protocols. UDP Options supplies additional
functionality that can be used to implement DPLPMTUD within the UDP
transport service. Implementing DPLPMTUD using UDP Options avoids
the need for each upper layer protocol or application to implement
the DPLPMTUD method.
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
[I-D.ietf-tsvwg-datagram-plpmtud].
3. DPLPMTUD for UDP Options
The DPLPMTUD PL endpoint implements the method specified in
[I-D.ietf-tsvwg-datagram-plpmtud].
3.1. Confirmation of Connectivity across a Path
The DPLPMTUD method requires that the PL is able to confirm
connectivity on the path (Section 5.2.1 of
[I-D.ietf-tsvwg-datagram-plpmtud]).
The UDP API does not offer a mechanism for a sender to confirm
connectivity, a UDP Options sender SHOULD use reception of an option
that elicits a positive confirmation (i.e. Timestamps, ECHO Request/
Response) to confirm connectivity of the path.
3.2. Sending UDP-Options Probe Packets
This method relies upon the sender Packetization Layer to be able to
generate probe packets with a specific size. UDP Options enables
padding to be added to a UDP datagram that is used as these Probe
Packets.
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A PL needs to determine that the current path continues to support
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 (Section 4.2 of [I-D.ietf-tsvwg-datagram-plpmtud]). UDP Options
can achieve this by sending probe packets padded to the size of the
PLPMTU.
A PL also needs, from time to time, to determine whether the current
path can support a larger size of datagram that the current PLPMTU.
UDP Options can achieve this by sending probe packets padded to the
size required for the Packet Probe.
A UDP options sender MUST be able to send probes up to the maximum
for the size the local interface supports, and not constrained by the
maximum PMTU set by network layer mechanisms (such as PMTUD
[RFC1063][RFC8201]). There are various options described in DPLPMTUD
to send a Packet Probe to test the size of packet supported by a path
(see Section 4.1 of [I-D.ietf-tsvwg-datagram-plpmtud]). This
secification recommends "Probing using padding data".
3.2.1. Sending Packet Probes using the Echo Request Option Request
Option
The RECOMMENDED method sends a Packet Probe with the Echo Request
Option (RES) together with any padding needed to be inflated to the
size required for the Packet Probe. The reception of this option
generates an Echo Response Option that confirms reception of each
received Packet Probe.
Implementations ought to only send a Packet Probe with a Request
Probe Option when required by their local DPLPMTUD state machine,
i.e., when probing to grow the PLPMTU or to confirm the current
PLPMTU.
Packet Probes consume network capacity and incur endpoint processing
(see Section 4.1 of [I-D.ietf-tsvwg-datagram-plpmtud]).
Implementations MAY track multiple requests and respond to them with
a single packet.
The UDP Options used in this method are described in section 6 of
[I-D.ietf-tsvwg-udp-options]:
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.
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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 Packet Probes and acknowledgements
confirming receipt of subsequent Packet Probes (e.g., travelling
along alternate paths with a larger round trip time). Each Packet
Probe needs to be uniquely identifiable by the UDP Options sender
within the Maximum Segment Lifetime (MSL). The UDP Options sender
therefore needs to not recycle token values until they have expired
or have been acknowledged. A 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 generate Packet Probes is to send a probe
that contains only control information.
A sender could include application data in Packet Probes (Section 4.1
of [I-D.ietf-tsvwg-datagram-plpmtud] discusses the merits and
demerits of this approach). A Packet Probe then contains a data
block supplied by an application that is combined with padding to
inflate the length of the datagram to the size required for the probe
and additionally include an Echo Request Option or Timestamp Option
(Section 5.9 [I-D.ietf-tsvwg-udp-options]).
If the application/transport needs protection from the loss of data
in the Packet Probe 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].
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3.3. Validating the Path with UDP Options
A PL 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 [I-D.ietf-tsvwg-datagram-plpmtud]). This could be
provided by an upper layer protocol confirming correct reception of
data by the remote PL, but there is no generic mechanism to provide
this in UDP Options, and therefore requires generation of a Packet
Probe of size PLPMTU to confirm the path. This Packet Probe could
use either the ECHO Response Option or the TimeStamp option to elicit
a response from the remote PL.
3.3.1. Sending Packet Probes using Timestamps
Reception of a valid Timestamps Option echoed by the remote endpoint
can be used to infer connectivity and 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 very asymmetric datagram rates, because an
echo of the most recent timestamp still indicates reception of at
least one packet of the transmitted size. This is sufficient to
confirm there is no black hole (see Section 2.1 of [RFC2923]).
When sending a probe to increase the PLPMTU, a Timestamp might be
unable to unambiguously identify that a specific Packet Probe 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. Packet Probes
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 MAY ignore received ICMP PTB messages.
A UDP Options sender that utilises ICMP PTB messages received to a
Packet Probe 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 (Section 4.4.1 of [I-D.ietf-tsvwg-datagram-plpmtud]).
An implementation unable to support this validation needs to ignore
received ICMP PTB messages.
As in other implementations of DPLPMTUD, a PL implementing this
specification MUST suspend processing of ICMP PTB by the network
layer (as specified in PMTUD [RFC1191] [RFC8201]) for each flow
utilising DPLPMTUD.
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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 security considerations for using DPLPMTUD are described in
[I-D.ietf-tsvwg-datagram-plpmtud]. The proposed new method does not
change the ICMP PTB message validation method described DPLPMTUD.
7. References
7.1. Normative References
[I-D.ietf-tsvwg-datagram-plpmtud]
Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
T. Voelker, "Packetization Layer Path MTU Discovery for
Datagram Transports", draft-ietf-tsvwg-datagram-plpmtud-07
(work in progress), February 2019.
[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-07 (work in progress), March 2019.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[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>.
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>.
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
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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 Packet Probe generation
Authors' Addresses
Godred Fairhurst
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: gorry@erg.abdn.ac.uk
Tom Jones
University of Aberdeen
School of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
UK
Email: tom@erg.abdn.ac.uk
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