TSVWG A. Ferrieux, Ed.
Internet-Draft I. Hamchaoui, Ed.
Intended status: Informational Orange Labs
Expires: May 6, 2020 I. Lubashev, Ed.
Akamai Technologies
November 3, 2019
Packet Loss Signaling for Encrypted Protocols
draft-ferrieuxhamchaoui-quic-lossbits-02
Abstract
This draft adapts the general technique described in draft-
ferrieuxhamchaoui-tsvwg-lossbits draft-ferrieuxhamchaoui-tsvwg-
lossbits for QUIC using reserved bits in QUIC v1 header. It
describes a method that employs two bits to allow endpoints to signal
packet loss in a way that can be used by network devices to measure
and locate the source of the loss.
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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This Internet-Draft will expire on May 6, 2020.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 3
3. Loss Bits . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Setting the sQuare Bit on Outgoing Packets . . . . . . . 3
3.1.1. Q Period Selection . . . . . . . . . . . . . . . . . 4
3.2. Setting the Loss Event Bit on Outgoing Packets . . . . . 4
4. Using the Loss Bits for Passive Loss Measurement . . . . . . 4
4.1. End-To-End Loss . . . . . . . . . . . . . . . . . . . . . 5
4.2. Upstream Loss . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Correlating End-to-End and Upstream Loss . . . . . . . . 6
4.4. Downstream Loss . . . . . . . . . . . . . . . . . . . . . 6
4.5. Observer Loss . . . . . . . . . . . . . . . . . . . . . . 6
5. Ossification Considerations . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8.1. Since version 01 . . . . . . . . . . . . . . . . . . . . 8
8.2. Since version 00 . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
Packet loss is a hard and pervasive problem of day-to-day network
operation, and proactively detecting, measuring, and locating it is
crucial to maintaining high QoS and timely resolution of crippling
end-to-end throughput issues. To this effect, in a TCP-dominated
world, network operators have been heavily relying on information
present in the clear in TCP headers: sequence and acknowledgment
numbers, and SACK when enabled. These allow for quantitative
estimation of packet loss by passive on-path observation.
Additionally, the lossy segment (upstream or downstream from the
observation point) can be quickly identified by moving the passive
observer around.
With QUIC, the equivalent transport headers are encrypted and passive
packet loss observation is not possible, as described in
[TRANSPORT-ENCRYPT].
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QUIC could be routed by the network differently and the fraction of
Internet traffic delivered using QUIC is increasing every year.
Therefore, is it imperative to measure packet loss experienced by
QUIC users directly instead of relying on measuring TCP loss between
similar endpoints.
Since explicit path signals are preferred by [RFC8558], this document
proposes adding two explicit loss bits to the clear portion of short
headers to restore network operators' ability to maintain high QoS
for QUIC users.
2. Notational Conventions
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 [RFC2119].
3. Loss Bits
The proposal introduces two bits that are to be present in packets
with a short header. Therefore, only loss of short header packets is
reported using loss bits. Whenever this specification refers to
packets, it is referring only to packets with short headers.
- Q: The "sQuare signal" bit is toggled every N outgoing packets as
explained below in Section 3.1.
- L: The "Loss event" bit is set to 0 or 1 according to the
Unreported Loss counter, as explained below in Section 3.2.
Each endpoint maintains appropriate counters independently and
separately for each connection 4-tuple and destination Connection ID.
3.1. Setting the sQuare Bit on Outgoing Packets
The sQuare Value is initialized to the Initial Q Value (0 or 1) and
is reflected in the Q bit of every outgoing packet. The sQuare value
is inverted after sending every N packets (Q Period is 2*N), where N
is a parameter of the method, discussed below.
Observation points can estimate the upstream losses by counting the
number of packets during a half period of the square signal, as
described in Section 4.
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3.1.1. Q Period Selection
The sender is expected to choose Q Period based on the expected
amount of loss and reordering on the path (see Section 4.2). The Q
Period value MUST be at least 128 and be a power of 2. This
requirement allows an Observer to infer the Q Period by obsering one
period of the square signal. It also allows the Observer to identify
flows that set the loss bits to arbitrary values (see Section 5).
If the sender does not have sufficient information to make an
informed decision about Q Period, the sender SHOULD use Q Period of
128.
3.2. Setting the Loss Event Bit on Outgoing Packets
The Unreported Loss counter is initialized to 0, and the L bit of
every outgoing packet indicates whether the Unreported Loss counter
is positive (L=1 if the counter is positive, and L=0 otherwise). The
value of the Unreported Loss counter is decremented every time a
packet with L=1 is sent.
The value of the Unreported Loss counter is incremented for every
packet that the protocol declares lost, using QUIC's existing loss
detection machinery.
This loss signaling is similar to loss signaling in [RFC7713], except
the Loss Event bit is reporting the exact number of lost packets,
whereas Echo Loss bit in [RFC7713] is reporting an approximate number
of lost bytes.
Observation points can estimate the end-to-end loss, as determined by
the upstream endpoint's loss detection machinery, by counting packets
in this direction with a L bit equal to 1, as described in Section 4.
4. Using the Loss Bits for Passive Loss Measurement
There are three sources of observable loss:
- _upstream loss_ - loss between the sender and the observation
point (Section 4.2)
- _downstream loss_ - loss between the observation point and the
destination (Section 4.4)
- _observer loss_ - loss by the observer itself that does not cause
downstream loss (Section 4.5)
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The upstream and downstream loss together constitute _end-to-end
loss_ (Section 4.1).
The Q and L bits allow detection and measurement of the types of loss
listed above.
4.1. End-To-End Loss
The Loss Event bit allows an observer to calculate the end-to-end
loss rate by counting packets with L bit value of 0 and 1 for a given
connection. The end-to-end loss rate is the fraction of packets with
L=1.
The simplifying assumption here is that upstream loss affects packets
with L=0 and L=1 equally. This may be a simplification, if some loss
is caused by tail-drop in a network device. If the sender congestion
controller reduces the packet send rate after loss, there may be a
sufficient delay before sending packets with L=1 that they have a
greater chance of arriving at the observer.
4.2. Upstream Loss
Blocks of N (half of Q Period) consecutive packets are sent with the
same value of the Q bit, followed by another block of N packets with
inverted value of the Q bit. Hence, knowing the value of N, an on-
path observer can estimate the amount of loss after observing at
least N packets. The upstream loss rate is one minus the average
number of packets in a block of packets with the same Q value divided
by N.
The observer needs to be able to tolerate packet reordering that can
blur the edges of the square signal.
The observer also needs to differentiate packets as belonging to
different connections, since they use independent counters.
The choice of N strikes a compromise: the observation could become
too unreliable in case of packet reordering and loss if N is too
small; and when N is too large, short connections may not yield a
useful upstream loss measurement.
To leave some room for adaptation, we only constrain the sender to
select an N that is (1) constant for a given connection and (2) equal
to a power of two. The latter allows on-path observers to derive N
after a few periods. It is thus also acceptable for a simple
implementation to choose a global constant; N=64 has been extensively
tried in large-scale field tests and yielded good results.
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4.3. Correlating End-to-End and Upstream Loss
Upstream loss is calculated by observing the actual packets that did
not suffer the upstream loss. End-to-end loss, however, is
calculated by observing subsequent packets after the sender's
protocol detected the loss. Hence, end-to-end loss is generally
observed with a delay of between 1 RTT (loss declared due to multiple
duplicate acknowledgments) and 1 RTO (loss declared due to a timeout)
relative to the upstream loss.
The connection RTT can sometimes be estimated by timing protocol
handshake messages. This RTT estimate can be greatly improved by
observing a dedicated protocol mechanism for conveying RTT
information, such as the Latency Spin bit of [QUIC-TRANSPORT].
Whenever the observer needs to perform a computation that uses both
upstream and end-to-end loss rate measurements, it SHOULD use
upstream loss rate leading the end-to-end loss rate by approximately
1 RTT. If the observer is unable to estimate RTT of the connection,
it should accumulate loss measurements over time periods of at least
4 times the typical RTT for the observed connections.
If the calculated upstream loss rate exceeds the end-to-end loss rate
calculated in Section 4.1, then either the Q Period is too short for
the amount of packet reordering or there is observer loss, described
in Section 4.5. If this happens, the observer SHOULD adjust the
calculated upstream loss rate to match end-to-end loss rate.
4.4. Downstream Loss
Because downstream loss affects only those packets that did not
suffer upstream loss, the end-to-end loss rate ("e") relates to the
upstream loss rate ("u") and downstream loss rate ("d") as
"(1-u)(1-d)=1-e". Hence, "d=(e-u)/(1-u)".
4.5. Observer Loss
A typical deployment of a passive observation system includes a
network tap device that mirrors network packets of interest to a
device that performs analysis and measurement on the mirrored
packets. The observer loss is the loss that occurs on the mirror
path.
Observer loss affects upstream loss rate measurement since it causes
the observer to account for fewer packets in a block of identical Q
bit values (see {{upstreamloss)}). The end-to-end loss rate
measurement, however, is unaffected by the observer loss, since it is
a measurement of the fraction of packets with the set L bit value,
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and the observer loss would affect all packets equally (see
Section 4.1).
The need to adjust the upstream loss rate down to match end-to-end
loss rate as described in Section 4.3 is a strong indication of the
observer loss, whose magnitude is between the amount of such
adjustment and the entirety of the upstream loss measured in
Section 4.2.
5. Ossification Considerations
Accurate loss information is not critical to the operation of any
protocol, though its presence for a sufficient number of connections
is important for the operation of the networks.
The loss bits are amenable to "greasing" described in [GREASE], if
the protocol designers are not ready to dedicate (and ossify) bits
used for loss reporting to this function. The greasing could be
accomplished similarly to the Latency Spin bit greasing in
[QUIC-TRANSPORT]. Namely, implementations could decide that a
fraction of connections should not encode loss information in the
loss bits and, instead, the bits would be set to arbitrary values.
The observers would need to be ready to ignore connections with loss
information more resembling noise than the expected signal.
6. Security Considerations
Passive loss observation has been a part of the network operations
for a long time, so exposing loss information to the network does not
add new security concerns.
7. Privacy Considerations
Guarding user's privacy is an important goal for modern protocols and
protocol extensions per [RFC7258]. While an explicit loss signal - a
preferred way to share loss information per [RFC8558] - helps to
minimize unintentional exposure of additional information,
implementations of loss reporting must ensure that loss information
does not compromise protocol's privacy goals.
For example, [QUIC-TRANSPORT] allows changing Connection IDs in the
middle of a connection to reduce the likelihood of a passive observer
linking old and new subflows to the same device. A QUIC
implementation would need to reset all counters when it changes
Connection ID used for outgoing packets. It would also need to avoid
incrementing Unreported Loss counter for loss of packets sent with a
different Connection ID.
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8. IANA Considerations
This document makes no request of IANA.
8.1. Since version 01
- Add reference to RFC7713
8.2. Since version 00
- Rewrote to base this draft on [LOSSBITS]
9. Acknowledgments
The sQuare Bit was originally specified by Kazuho Oku in early
proposals for loss measurement and is an instance of the "alternate
marking" as defined in [RFC8321].
10. References
10.1. Normative References
[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>.
[RFC8321] Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
"Alternate-Marking Method for Passive and Hybrid
Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
January 2018, <https://www.rfc-editor.org/info/rfc8321>.
[RFC8558] Hardie, T., Ed., "Transport Protocol Path Signals",
RFC 8558, DOI 10.17487/RFC8558, April 2019,
<https://www.rfc-editor.org/info/rfc8558>.
10.2. Informative References
[GREASE] Benjamin, D., "Applying GREASE to TLS Extensibility",
draft-ietf-tls-grease-04 (work in progress), August 2019.
[LOSSBITS]
Ferrieux, A., Hamchaoui, I., and I. Lubashev, "Packet Loss
Signaling for Encrypted Protocols", draft-
ferrieuxhamchaoui-tsvwg-lossbits-01 (work in progress),
July 2019.
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[QUIC-TRANSPORT]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-24 (work
in progress), September 2019.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC7713, December 2015,
<https://www.rfc-editor.org/info/rfc7713>.
[TRANSPORT-ENCRYPT]
Fairhurst, G. and C. Perkins, "The Impact of Transport
Header Confidentiality on Network Operation and Evolution
of the Internet", draft-ietf-tsvwg-transport-encrypt-09
(work in progress), August 2019.
Authors' Addresses
Alexandre Ferrieux (editor)
Orange Labs
EMail: alexandre.ferrieux@orange.com
Isabelle Hamchaoui (editor)
Orange Labs
EMail: isabelle.hamchaoui@orange.com
Igor Lubashev (editor)
Akamai Technologies
150 Broadway
Cambridge, MA 1122
USA
EMail: ilubashe@akamai.com
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