Independent Submission M. Welzl
Request for Comments: 8774 University of Oslo
Category: Informational 1 April 2020
ISSN: 2070-1721
The Quantum Bug
Abstract
The age of quantum networking is upon us, and with it comes
"entanglement": a procedure in which a state (i.e., a bit) can be
transferred instantly, with no measurable delay between peers. This
will lead to a perceived round-trip time of zero seconds on some
Internet paths, a capability which was not predicted and so not
included as a possibility in many protocol specifications. Worse
than the millennium bug, this unexpected value is bound to cause
serious Internet failures unless the specifications are fixed in
time.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8774.
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Table of Contents
1. Introduction
2. Protocols and Protocol Mechanisms That Will Fail
2.1. LEDBAT
2.2. Multipath TCP (MPTCP)
2.3. RTP Circuit Breakers
3. What can be done?
4. Conclusion
5. IANA Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Author's Address
1. Introduction
[RFC6921] discusses faster-than-light communication, where packets
arrive before they are sent. While it is amusing to entertain the
possibility of time travel, we have to accept the cold facts: time
travel will never work (or it would already have been used). Quantum
networking, however, is an entirely different matter -- commercial
products are already available, and quantum networks will without a
doubt become the prevalent Internet link-layer technology across the
globe within the next five to ten years.
With the help of entanglement, implemented in quantum repeaters,
quantum networks can transfer information faster than ever before: a
state can be transmitted over a long distance instantly, with no
delay. This is so cool that it is also called (and, by some,
mistaken for) teleportation. If a path between a sender and a
receiver is fully quantum-ized, the measured one-way delay (OWD) will
be zero. What's more, assuming that there are blazing fast quantum
computers involved on both ends, the processing time will be well
below anything measurable; hence, even the round-trip time (RTT) will
be zero in these scenarios.
In today's Internet, only very few protocols are prepared for such
"0-RTT" situations (e.g., TCP with "TCP Fast Open" (TFO) [RFC7413],
TLS 1.3 [RFC8446], and QUIC [QUIC-TRANS]). Many others will fail in
interesting ways; we coin the term "Quantum Bug" for such failures.
In the following section, we will discuss some examples of Quantum
Bugs.
2. Protocols and Protocol Mechanisms That Will Fail
The number of protocols and protocol mechanisms that will fail in the
face of a zero RTT is too large to report here; we are truly heading
towards something close to an Internet meltdown. We can only provide
some guidance to those who hunt for the Quantum Bug, by discussing
examples of specification mistakes that will need to be fixed.
2.1. LEDBAT
The Low Extra Delay Background Transfer (LEDBAT) congestion control
mechanism [RFC6817] is a very interesting failure case: designed to
"get out of the way" of other traffic; it will end up sending as fast
as possible. Specifically, when the algorithm described in
Section 2.4.2 of [RFC6817] obtains a delay sample, it updates a list
of base delays that will all become 0 and current delays that will
also all become 0. It calculates a queuing delay as the difference
between the current delay and the base delay (resulting in 0) and
keeps increasing the Congestion Window (cwnd) until the queuing delay
reaches a predefined parameter value TARGET (100 milliseconds or
less).
A TARGET value of 100 milliseconds will never be reached, because the
queuing delay does not grow when the sender increases its cwnd; this
means that LEDBAT would endlessly increase its cwnd, limited only by
the number of bits that are used to represent cwnd. However, given
that TARGET=0 is also allowed, this parameter choice may seem to be a
way out. Always staying at the target means that the sender would
maintain its initial cwnd, which should be set to 2. This may seem
like a small number, but remember that cwnd is the number of bytes
that can be transmitted per RTT (which is 0). Thus, irrespective of
the TARGET value, the sender will send data as fast as it can.
2.2. Multipath TCP (MPTCP)
The coupled congestion control mechanism proposed for MPTCP in
[RFC6356] requires calculating a value called "alpha". Equation 2 in
[RFC6356] contains a term where a value called "cwnd_i" is divided by
the square of the RTT, and another term where this value is divided
by the RTT. Enough said.
2.3. RTP Circuit Breakers
The RTP Circuit Breakers [RFC8083] require calculation of a well-
known equation which yields the throughput of a TCP connection:
s
X = -------------------------------------------------------------
Tr*sqrt(2*b*p/3)+(t_RTO * (3*sqrt(3*b*p/8) * p * (1+32*p*p)))
where Tr is the RTT and t_RTO is the retransmission timeout of TCP
(we don't need to care about the other variables). As we will
discuss in Section 3, t_RTO is lower-bounded with 1 second;
therefore, it saves us from a division by zero. However, there is
also a simplified version of this equation:
s
X = ----------------
Tr*sqrt(2*b*p/3)
Unfortunately, [RFC8083] states: "It is RECOMMENDED that this
simplified throughput equation be used since the reduction in
accuracy is small, and it is much simpler to calculate than the full
equation." Due to this simplification, many multimedia applications
will crash.
3. What can be done?
Fear not: when everything else fails, TCP will still work. Its
retransmission timeout is lower-bounded by 1 second [RFC6298].
Moreover, while its cwnd may grow up to the maximum storable number,
data transmission is limited by the Receiver Window (rwnd). This
means that flow control will save TCP from failing.
From this, we can learn two simple rules: lower-bound any values
calculated from the RTT (and, obviously, do not divide by the RTT),
and use flow control. Specifications will need to be updated by
fixing all RTT-based calculations and introducing flow control
everywhere. For example, UDP will have to be extended with a
receiver window, e.g., as a UDP option [UDP-OPT].
4. Conclusion
We are in trouble, and there is only one way out: develop a
comprehensive list of all RFCs containing "0-RTT" mistakes (taking
[RFC2626] as a guideline), and update all code. This needs to happen
fast, the clock is ticking. Luckily, if we are too slow, we will
still be able to use TCP to access the specifications. With DNS over
TCP [RFC7766], name resolution to find the server containing the
specifications should also work.
5. IANA Considerations
This document has no IANA actions.
6. Security Considerations
Flow control must be used on 0-RTT paths, or else an attacker can
completely overwhelm a sender with data in a denial-of-service (DoS)
attack within an instant. Flow control will need to be added to
protocols that do not currently have it, such as UDP or ICMP. IPv6
will not save us.
7. References
7.1. Normative References
[RFC2626] Nesser II, P., "The Internet and the Millennium Problem
(Year 2000)", RFC 2626, DOI 10.17487/RFC2626, June 1999,
<https://www.rfc-editor.org/info/rfc2626>.
[RFC6921] Hinden, R., "Design Considerations for Faster-Than-Light
(FTL) Communication", RFC 6921, DOI 10.17487/RFC6921,
April 2013, <https://www.rfc-editor.org/info/rfc6921>.
7.2. Informative References
[QUIC-TRANS]
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,
<https://tools.ietf.org/html/draft-ietf-quic-transport-
27>.
[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.
[RFC6356] Raiciu, C., Handley, M., and D. Wischik, "Coupled
Congestion Control for Multipath Transport Protocols",
RFC 6356, DOI 10.17487/RFC6356, October 2011,
<https://www.rfc-editor.org/info/rfc6356>.
[RFC6817] Shalunov, S., Hazel, G., Iyengar, J., and M. Kuehlewind,
"Low Extra Delay Background Transport (LEDBAT)", RFC 6817,
DOI 10.17487/RFC6817, December 2012,
<https://www.rfc-editor.org/info/rfc6817>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.
[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", RFC 8083,
DOI 10.17487/RFC8083, March 2017,
<https://www.rfc-editor.org/info/rfc8083>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[UDP-OPT] Touch, J., "Transport Options for UDP", Work in Progress,
Internet-Draft, draft-ietf-tsvwg-udp-options-08, 12
September 2019, <https://tools.ietf.org/html/draft-ietf-
tsvwg-udp-options-08>.
Author's Address
Michael Welzl
University of Oslo
PO Box 1080 Blindern
N-0316 Oslo
Norway
Phone: +47 22 85 24 20
Email: michawe@ifi.uio.no