Internet Engineering Task Force P. S. Kim
Internet-Draft TU Korea
Intended status: Informational
Expires: 6 January 2023 7 July 2022
Passive Probing for Path MTU Discovery with QUIC
draft-pskim-passive-probing-pmtud-00
Abstract
This draft consider an alternative PMTUD for QUIC. To discover the
best PMTU, the passive probing approach is adopted. The process of
discovering the best PMTU is not carried out separately, but is
carried out simultaneously in the actual application data
communication. A probe packet is defined newly using 1-RTT packet
which includes actual application data as well as a short packet
header and a PING_EXT frame. The PING_EXT frame is also defined
newly. Until the best PMTU is discovered, the size of the probe
packet is changed according to the size of the PMTU candidate. A
simple discovery algorithm using only the PMTU candidate sequence
with linear upward is described in this draft. Other rather complex
discovery algorithms that consider various PMTU candidate sequences
will be dealt with in the future.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Active Probing for PMTUD with QUIC . . . . . . . . . . . . . 3
3. Passive Probing for PMTUD with QUIC . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The maximum transmission unit (MTU) is the largest size frame or
packet - in bytes or octets - that can be transmitted across a data
link. It is most used in reference to packet size on an Ethernet
network using the Internet Protocol (IP). The Path MTU (PMTU) is the
smallest MTU of all involved network interfaces for a network path
and limits the size of IP packets.
A PMTU Discovery (PMTUD) is a standardized technique in computer
networking for determining the PMTU size on the network path between
two IP hosts, usually with the goal of avoiding IP fragmentation for
IPv4[RFC1191] and for IPv6[RFC8201]. When a packet too large for the
path was sent, the PMTUD expects to receive a Packet Too Big (PTB)
message. However, there are multiple reasons why a PTB message might
not arrive at the sender.
Therefore, the PMTUD for the Packetization Layer (PL) that selects
the size of IP packets is specified recently in [RFC8899]. RFC8899
works without a signal from the network and covers generic PL
protocols such as QUIC of [RFC9000]. However, RFC8899 does not
contain details about how to discovery for the best PMTU.
Recently, therefore, [Q-PMTUD] complements RFC8899 by presenting a
discovery algorithm with QUIC. Using the discovery algorithm with a
set of possible PMTU candidates and their possible probing sequences,
the best PMTU is obtained. However, to discover the best PMTU, some
probe packets which have no semantic value might be injecting into
network, which is called active probing or active measurement. The
active probing approach can increase a network load and perturb the
network. In addition, [UDP-PMTUD] also complements RFC8899 by
specifying how a UDP Options sender implements Datagram PL PMTUD
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(DPLPMTUD). It allows a datagram application to discover the largest
size of datagram that can be sent across a specific network path.
Based on [Q-PMTUD] and [UDP-PMTUD], this draft consider an
alternative PMTUD for QUIC. To discover the best PMTU, the passive
probing approach is adopted. The process of discovering the best PMTU
is not carried out separately, but is carried out simultaneously in
the actual application data communication. A probe packet is defined
newly using 1-RTT packet which includes actual application data as
well as a short packet header and a PING_EXT frame. The PING_EXT
frame is also defined newly. Until the best PMTU is discovered, the
size of the probe packet is changed according to the size of the PMTU
candidate. A simple discovery algorithm using only the PMTU candidate
sequence with linear upward is described in this draft. Other rather
complex discovery algorithms that consider various PMTU candidate
sequences will be dealt with in the future.
1.1. Requirements Language
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.
2. Active Probing for PMTUD with QUIC[Q-PMTUD]
The specification of QUIC in RFC9000 recommends to use the PMTUD
framework of RFC8899. However, RFC8899 does not contain details about
how to discovery for the best PMTU.
Therefore, [Q-PMTUD] complements the specification, RFC8899, by
presenting a discovery algorithm with QUIC. From a practical point of
view, it might be a good choice to consider only a set of common PMTU
values. However, the PMTU value may usually change over time. Thus,
[Q-PMTUD] considers a set of possible PMTU candidates. Then, a
discovery algorithm is proposed, which probes one PMTU candidate
after the other. This means, it starts the probe for the next
candidate not before the probe for the current candidate either
succeeded or failed. Then endpoint uses this discovery algorithm that
repeatedly chooses PMTU candidates to probe.
The candidate sequence is required to specify the order in which the
discovery algorithm probes PMTU candidates. The endpoint must choose
a PMTU candidate larger than the largest successfully probed
candidate and smaller than any other probed candidate with a lost
probe packet. Seven candidate sequences are considered, evaluated,
and compared in [Q-PMTUD].
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To probe one PMTU candidate, according to RFC9000, the endpoint
builds a probe packet with a short packet header, a PING frame and
PADDING frames. The endpoint controls the size of the probe packet by
the number of PADDING frames, whose size is one byte each. The PING
frame makes the packet ack-eliciting.
However, to discover the best PMTU, some probe packets which have no
semantic value might be injecting into network, which is thus called
active measurement or active probing. This active probing approach
can increase a network load and perturb the network.
3. Passive Probing for PMTUD with QUIC
There are three possible ways to create a PMTU probe packet as
follows[RFC8899]:
- Probing using padding data
- Probing using application data and padding data
- Probing using application data
[UDP-PMTUD] describes "Probe Packets that include Application Data"
to implement "Probing using application data" of [RFC8899].
3.1. A new PMTU probe packet (1-RTT packet format)
(1) Probe packet format for active probing [Q-PMTUD]
IP header + UDP header + Short header(QUIC header) + PING frame +
PADDING frames
The size of the probe packet is controlled by the number of PADDING
frames.
(2) Probe packet format for passive probing
In this drfat, a probe packet is defined newly using 1-RTT packet
including actual application data as well as a PING_EXT frame as
follows:
IP header + UDP header + Short header(QUIC Header) +
PING_EXT frame + Actual application data
- PING_EXT frame (defined newly)
. Frame Type Name : PING_EXT
. Type Value : 0x20
. The PING_EXT frame makes the packet ack-eliciting. In addition,
the PING_EXT frame indicates that the current 1-RTT packet is
now discovering the best PMTU as well as transmitting actual
application data.
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- Application data
. Actual application data controls the size of the probe packet
by a multiple of four bytes.
The size of probe packet is changed according to PMTU candidates
(=1280 + incremental where, for example, incremental can be a
multiple of four as shown in [Q-PMTUD]).
3.2. Passive probing to both discover best PMTU and transmit actual
application data
Through the new probe packet, it is possible not only to discovery
the best PMTU, but also to transmit actual application data. That
is, to discover the best PMTU size and carry actual application
data, the endpoint expand the payload of all UDP datagrams.
(1) A simple algorithm for discovering the best PMTU
As specified in RFC9000, QUIC must send QUIC packets with the
smallest allowed maximum datagram size when validating a path during
connection initiation or migration. Thus, the endpoint sets the probe
packet initially to the smallest allowed maximum datagram size of
1280 bytes including actual application data as well as a short
packet header, a PING_EXT frame.
As mentioned, until the best PMTU is discovered, the size of the
probe packet is changed successively according to the size of the
PMTU candidate. The size of the probe packet is controlled with the
size of actual application data. The size of actual application data
is a multiple of four.
In the active probing approach [Q-PMTUD], the endpoint uses a simple
discovery algorithm that repeatedly chooses PMTU candidates to probe.
Thus, seven PMTU candidate sequences are considered and each
candidate sequence specifies the order in which the discovery
algorithm probes PMTU candidates. In addition, four metrics such as
number of probed PMTU candidates, time to discover the best PMTU,
network load, average PMTU estimation are defined for performance
evaluations of seven sequences.
However, because the process of discovering the best PMTU is carried
out simultaneously in the actual application data communication, only
the PMTU candidate sequence with linear upward is adopted first in
this draft. The linear upward sequence selects one candidate after
the other from a list of candidates in ascending order, starting with
the second one (the first one was probed with the smallest allowed
maximum datagram size of 1280 bytes). Other rather complex discovery
algorithms that consider various PMTU candidate sequence will be
dealt with in the future.
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Until the best PMTU is discovered, the endpoint repeats a series of
probing steps. In absence of a PTB message, the discovery algorithm
considers a probe for a PMTU candidate as failed, only if the probe
packet of the size of the candidate were detected as lost. A probe
for a PMTU candidate that fails, lets all other probes for larger
candidates fail as well. Therefore, the best PMTU is the PMTU
candidate that succeeded just before the failure.
(2) Discovery complete and PMTU cache
When the algorithm determines that it has discovered the best PMTU,
the endpoint terminates the probing. Then, the endpoint sets the
1-RTT packet finally to the best datagram size using the best PMTU
discovered. From now on, the 1-RTT packet does not include a
PING_EXT frame. QUIC can cache the best PMTU discovered and use it
for future connections to the same endpoint.
(3) Other rather complex discovery algorithms
Other rather complex discovery algorithms that consider various PMTU
candidate sequences will be dealt with in the future.
4. IANA Considerations
This memo includes no request to IANA.
5. Security Considerations
The same security considerations as those described in RFC7880 will
apply to this document.
6. References
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.
[RFC8201] McCann, J., S. Deering, J. Mogul, R. Hinden, Ed. "Path MTU
Discovery for IP version 6", RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
[RFC8899] Fairhurst, G., T. Jones, M. Tuxen, I. Rungeler, T. Volker,
"Packetization Layer Path MTU Discovery for Datagram
Transports", RFC 8899, DOI 10.17487/RFC8899, September
2020, <https://www.rfc-editor.org/info/rfc8899>.
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[RFC9000] J. Iyengar, Ed., M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[Q-PMTUD]
Timo Volker, Michael Tuxen, "The search of the path MTU
with QUIC", EPIQ '21: Proceedings of the 2021 Workshop
on Evolution, Performance and Interoperability of QUIC,
December 2021
[UDP-PMTUD]
Work in Progress, Internet-Draft,
draft-ietf-tsvwg-udp-options-dplpmtud-03, 25 February
2022, <https://www.ietf.org/archive/id/draft-ietf-tsvwg-
udp-options-dplpmtud-03.txt>.
Authors' Addresses
Pyung Soo Kim
Tech University of Korea
Siheung, Gyeonggi
Korea
Email: pskim@tukorea.ac.kr
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