Internet DRAFT - draft-trammell-quic-spin
draft-trammell-quic-spin
QUIC B. Trammell, Ed.
Internet-Draft P. De Vaere
Intended status: Informational ETH Zurich
Expires: November 15, 2018 R. Even
Huawei
G. Fioccola
Telecom Italia
T. Fossati
Nokia
M. Ihlar
Ericsson
A. Morton
AT&T Labs
E. Stephan
Orange
May 14, 2018
Adding Explicit Passive Measurability of Two-Way Latency to the QUIC
Transport Protocol
draft-trammell-quic-spin-03
Abstract
This document describes the addition of a "spin bit", intended for
explicit measurability of end-to-end RTT, to the QUIC transport
protocol. It proposes a detailed mechanism for the spin bit, as well
as an additional mechanism, called the valid edge counter, to
increase the fidelity of the latency signal in less than ideal
network conditions. It describes how to use the latency spin signal
to measure end-to-end latency, discusses corner cases and their
workarounds in the measurement, describes experimental evaluation of
the mechanism done to date, and examines the utility and privacy
implications of the spin bit.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 15, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. About This Document . . . . . . . . . . . . . . . . . . . 4
2. The Spin Bit Mechanism . . . . . . . . . . . . . . . . . . . 4
3. Using the Spin Bit for Passive RTT Measurement . . . . . . . 5
3.1. Limitations and Workarounds . . . . . . . . . . . . . . . 5
3.2. Illustration . . . . . . . . . . . . . . . . . . . . . . 6
4. The Valid Edge Counter . . . . . . . . . . . . . . . . . . . 8
4.1. Proposed Short Header Format Including Spin Bit and VEC . 8
4.2. Setting the Valid Edge Counter (VEC) . . . . . . . . . . 9
4.3. Use of the VEC by a passive observer . . . . . . . . . . 10
5. Privacy and Security Considerations . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Experimental Evaluation . . . . . . . . . . . . . . 15
Appendix B. Use Cases for Passive RTT Measurement . . . . . . . 16
B.1. Inter-domain Troubleshooting . . . . . . . . . . . . . . 17
B.2. Two-Point Intradomain Measurement . . . . . . . . . . . . 18
B.3. Bufferbloat Mitigation in Cellular Networks . . . . . . . 19
B.4. Locating WiFi Problems in Home Networks . . . . . . . . . 19
B.5. Internet Measurement Research . . . . . . . . . . . . . . 20
Appendix C. Alternate RTT Measurement Approaches for Diagnosing
QUIC flows . . . . . . . . . . . . . . . . . . . . . 20
C.1. Handshake RTT measurement . . . . . . . . . . . . . . . . 20
C.2. Parallel active measurement . . . . . . . . . . . . . . . 21
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C.3. Frequency Analysis . . . . . . . . . . . . . . . . . . . 21
Appendix D. Greasing . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
The QUIC transport protocol [QUIC-TRANS] is a UDP-encapsulated
protocol integrated with Transport Layer Security (TLS) [TLS] to
encrypt most of its protocol internals, beyond those handshake
packets needed to establish or resume a TLS session, and information
required to reassemble QUIC streams (the packet number) and to route
QUIC packets to the correct machine in a load-balancing situation
(the connection ID). In contrast to TCP, QUIC's wire image (see
[WIRE-IMAGE]) exposes much less information about transport protocol
state than TCP's wire image. Specifically, the fact that sequence
and acknowledgement numbers and timestamps (available in TCP) cannot
be seen by on-path observers in QUIC means that passive TCP loss and
latency measurement techniques that rely on this information (e.g.
[CACM-TCP], [TMA-QOF]) cannot be easily ported to work with QUIC.
This document proposes a solution to this problem by adding a
"latency spin bit" to the QUIC short header. This bit is designed
solely for explicit passive measurability of the protocol. It
provides one RTT sample per RTT to passive observers of QUIC traffic.
This document describes the mechanism, how it can be added to QUIC,
and how it can be used by passive measurement facilities to generate
RTT samples. It explores potential corner cases and shortcomings of
the mechanism, and proposes an extention called the Valid Edge
Counter (VEC) to mitigate them. It further details findings on
privacy risk researched by the QUIC RTT Design Team, which was tasked
by the IETF QUIC Working Group to determine the risk/utility tradeoff
for the spin bit.
Appendices summarize experimental results to date with an
implementation of the spin bit built atop a recent QUIC
implementation, describe use cases for passive RTT measurement at the
resolution provided by the spin bit, explore alternatives to the spin
bit for passive latency measurement of QUIC flows, and discuss the
necessity of "greasing" the spin bit.
The spin bit has low overhead, presents negligible privacy risk, and
has clear utility in providing passive RTT measurability of QUIC that
is far superior to QUIC's measurability without the spin bit, and
equivalent to or better than TCP passive measurability.
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1.1. About This Document
[QUIC-SPIN-EXP] specifies the addition of the spin bit to the QUIC
transport protocol for experimental purposes. This document provides
background for that specification, documents work done in the
development of the spin bit proposal, and extends it with the VEC
signal for loss, reordering, and delay compensation without relying
on the QUIC packet number.
This document is maintained in the GitHub repository
https://github.com/britram/draft-trammell-quic-spin, and the editor's
copy is available online at https://britram.github.io/draft-trammell-
quic-spin. Current open issues on the document can be seen at
https://github.com/britram/draft-trammell-quic-spin/issues. Comments
and suggestions on this document can be made by filing an issue
there, or by contacting the editor.
2. The Spin Bit Mechanism
The latency spin bit enables latency monitoring from observation
points on the network path. Each endpoint, client and server,
maintains a spin value, 0 or 1, for each QUIC connection, and sets
the spin bit on packets it sends for that connection to the
appropriate value (below). It also maintains the highest packet
number seen from its peer on the connection. The value is then
determined at each endpoint as follows:
o The server initializes its spin value to 0. When it receives a
packet from the client, if that packet has a short header and if
it increments the highest packet number seen by the server from
the client, it sets the spin value to the spin bit in the received
packet.
o The client initializes its spin value to 0. When it receives a
packet from the server, if the packet has a short header and if it
increments the highest packet number seen by the client from the
server, it sets the spin value to the opposite of the spin bit in
the received packet.
This procedure will cause the spin bit to change value in each
direction once per round trip. Observation points can estimate the
network latency by observing these changes in the latency spin bit,
as described in Section 3. See Section 3.2 for an illustration of
this mechanism in action.
The defails of the addition of the spin bit to the QUIC short header
are given in [QUIC-SPIN-EXP].
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3. Using the Spin Bit for Passive RTT Measurement
When a QUIC flow is sending at full rate (i.e., neither application
nor flow control limited), the latency spin bit in each direction
changes value once per round-trip time (RTT). An on-path observer
can observe the time difference between edges in the spin bit signal
in a single direction to measure one sample of end-to-end RTT. Note
that this measurement, as with passive RTT measurement for TCP,
includes any transport protocol delay (e.g., delayed sending of
acknowledgements) and/or application layer delay (e.g., waiting for a
request to complete). It therefore provides devices on path a good
instantaneous estimate of the RTT as experienced by the application.
A simple linear smoothing or moving minimum filter can be applied to
the stream of RTT information to get a more stable estimate.
An on-path observer that can see traffic in both directions (from
client to server and from server to client) can also use the spin bit
to measure "upstream" and "downstream" component RTT; i.e, the
component of the end-to-end RTT attributable to the paths between the
observer and the server and the observer and the client,
respectively. It does this by measuring the delay between a spin
edge observed in the upstream direction and that observed in the
downstream direction, and vice versa.
3.1. Limitations and Workarounds
Application-limited and flow-control-limited senders can have
application and transport layer delay, respectively, that are much
greater than network RTT. Therefore, the spin bit provides network
latency information only when the sender is neither application nor
flow control limited. When the sender is application-limited by
periodic application traffic, where that period is longer than the
RTT, measuring the spin bit provides information about the
application period, not the RTT. Simple heuristics based on the
observed data rate per flow or changes in the RTT series can be used
to reject bad RTT samples due to application or flow control
limitation.
Since the spin bit logic at each endpoint considers only samples on
packets that advance the largest packet number seen, signal
generation itself is resistant to reordering. However, reordering
can cause problems at an observer by causing spurious edge detection
and therefore low RTT estimates, if reordering occurs across a spin
bit flip in the stream. This can be probabilistically mitigated by
the observer also tracking the low-order bits of the packet number,
and rejecting edges that appear out-of-order [RFC4737].
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All of these limitations are addressed by an enhancement to the spin
bit, the Valid Edge Counter, described in detail in Section 4.
3.2. Illustration
To illustrate the operation of the spin bit, we consider a simplified
model of a single path between client and server as a queue with
slots for five packets, and assume that both client and server sent
packets at a constant rate. If each packet moves one slot in the
queue per clock tick, note that this network has a RTT of 10 ticks.
Initially, during connection establishment, no packets with a spin
bit are in flight, as shown in Figure 1.
+--------+ - - - - - +--------+
| | --------> | |
| Client | | Server |
| | <-------- | |
+--------+ - - - - - +--------+
Figure 1: Initial state, no spin bit between client and server
Either the server, the client, or both can begin sending packets with
short headers after connection establishment, as shown in Figure 2;
here, no spin edges are yet in transit.
+--------+ 0 0 - - - +--------+
| | --------> | |
| Client | | Server |
| | <-------- | |
+--------+ - - 0 0 0 +--------+
Figure 2: Client and server begin sending packets with spin 0
Once the server's first 0-marked packet arrives at the client, the
client sets its spin value to 1, and begins sending packets with the
spin bit set, as shown in Figure 3. The spin edge is now in transit
toward the server.
+--------+ 1 0 0 0 0 +--------+
| | --------> | |
| Client | | Server |
| | <-------- | |
+--------+ 0 0 0 0 0 +--------+
Figure 3: The bit begins spinning
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Five ticks later, this packet arrives at the server, which takes its
spin value from it and reflects that value back on the next packet it
sends, as shown in Figure 4. The spin edge is now in transit toward
the client.
+--------+ 1 1 1 1 1 +--------+
| | --------> | |
| Client | | Server |
| | <-------- | |
+--------+ 0 0 0 0 1 +--------+
Figure 4: Server reflects the spin edge
Five ticks later, the 1-marked packet arrives at the client, which
inverts its spin value and sends the inverted value on the next
packet it sends, as shown in Figure 5.
obs. points X Y
+--------+ 0 1 1 1 1 +--------+
| | --------> | |
| Client | | Server |
| | <-------- | |
+--------+ 1 1 1 1 1 +--------+
Y
Figure 5: Client inverts the spin edge
Here we can also see how measurement works. An observer watching the
signal at single observation point X in Figure 5 will see an edge
every 10 ticks, i.e. once per RTT. An observer watching the signal
at a symmetric observation point Y in Figure 5 will see a server-
client edge 4 ticks after the client-server edge, and a client-server
edge 6 ticks after the server-client edge, allowing it to compute
component RTT.
Figure 6 shows how this mechanism works in the presence of
reordering. Here, packet C carries the spin edge, and packet B is
reordered on the way to the client. In this case, the client will
begin sending spin 1 after the arrival of C, and ignore the spin bit
flip to 1 on packet B, since B < C; i.e. it does not increment the
highest packet number seen.
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+--------+ 0 0 0 0 0 +--------+
| | --------> | |
| Client | | Server |
| | <-------- | |
+--------+ 1 0 1 0 0 +--------+
PN= A C B D E
Figure 6: Handling reordering
4. The Valid Edge Counter
This mechanism is indented to provide additional information about
the validity of the passively observed spin edges without using
information from a cleartext packet number.
A one-bit spin signal is resistent to reordering during signal
generation, since the spin value is only updated at each endpoint on
a packet that advances the packet counter. However, without using
the packet number, a passive observer can neither detect reordered
nor lost edges, and it must use heuristics to reject delayed edges.
The Valid Edge Counter (VEC) addresses these issues with two
additional bits added to each packet, encoding values from 0 to 3,
indicating that an edge was considered to be valid when send out by
the sender, and providing a possibility to detect invalid edges due
to reordering and edge loss.
4.1. Proposed Short Header Format Including Spin Bit and VEC
As of the current editor's version of [QUIC-TRANS], this proposal
specifies using bit 0x04 of the first octet in the short header for
the spin bit, and the bits 0x03 for the valid edge counter. Note
that these values are subject to change as the layout of the first
octet is finalized.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
|0|K|1|1|0|S|VEC|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Connection ID (0..144) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packet Number (8/16/32) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protected Payload (*) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Short Header Format Spin Bit and VEC
S: The Spin bit is set 0 or 1 depending on the stored spin value that
is updated on packet reception as explained in Section 2.
VEC: The Valid Edge Counter is set as defined in Section 4.2. If the
spin bit field does not contain an edge, the VEC is set to 0.
4.2. Setting the Valid Edge Counter (VEC)
The VEC is set by each endpoint as follows; unlike the spin bit, note
that there is no difference between client and server handling of the
VEC:
o By default, the VEC is set to 0.
o If a packet contains an edge (transition 0->1 or 1->0) in the spin
signal, and that edge is delayed (sent more than a configured
delay since the edge was received, defaulting to 1ms), the VEC is
set to 1.
o If a packet contains an edge in the spin signal, and that edge is
not delayed, the VEC is set to the value of the VEC that
accompanied the last incoming spin bit transition plus one. This
counter holds at 3, instead of cycling around. In other words, an
edge received with a VEC of 0 will be reflected as an edge with a
VEC of 1; with a VEC of 1 as VEC of 2, and a VEC of 2 or 3 as a
VEC of 3.
This mechanism allows observers to recognize spurious edges due to
reordering and delayed edges due to loss, since these packets will
have been sent with VEC 0: they were not edges when they were sent.
In addition, it allows senders to signal that a valid edge was
delayed because the sender was application-limited: these edges are
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sent with the VEC set to 1 by the sender, prompting the VEC to count
back up over the next RTT.
4.3. Use of the VEC by a passive observer
The VEC can be used by observers to determine whether an edge in the
spin bit signal is valid or not, as follows:
o A packet containing an apparent edge in the spin signal with a VEC
of 0 is not a valid edge, but may be have been caused by
reordering or loss, or was marked as delayed by the sender. It
should therefore be ignored.
o A packet containing an apparent edge in the spin signal with a VEC
of 1 can be used as a left edge (i.e., to start measuring an RTT
sample), but not as a right edge (i.e., to take an RTT sample
since the last edge).
o A packet containing an apparent edge in the spin signal with a VEC
of 2 can be used as a left edge, but not as a right edge. If the
observation point is symmetric (i.e, it can see both upstream and
downstream packets in the flow), the packet can also be used to
take a component RTT sample on the segment of the path between the
observation point and the direction in which the previous VEC 1
edge was seen.
o A packet containing an apparent edge in the spin signal with a VEC
of 3 can be used as a left edge or right edge, and can be used to
compute component RTT in either direction.
5. Privacy and Security Considerations
The privacy considerations for the latency spin bit are essentially
the same as those for passive RTT measurement in general.
A concern was raised during the discussion of this feature within the
QUIC working group and the QUIC RTT Design Team that high-resolution
RTT information might be usable for geolocation. However, an
evaluation based on RTT samples taken over 13,780 paths in the
Internet from RIPE Atlas anchoring measurements [TRILAT] shows that
the magnitude and uncertainty of RTT data limit the resolution of
geolocation information that can be derived from Internet RTT to
national- or continental-scale; i.e., less resolution than is
generally available from free, open IP geolocation databases.
One reason for the inaccuracy of geolocation from network RTT is that
Internet backbone transmission facilities do not follow the great-
circle path between major nodes. Instead, major geographic features
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and the efficiency of connecting adjacent major cities both influence
the facility routing. An evaluation of ~3500 measurements on a mesh
of 25 backbone nodes in the continental United States shows that 85%
had RTT to great-circle error of 3ms or more, making location within
US State boundaries ambiguous [CONUS].
Therefore, in the general case, when an endpoint's IP address is
known, RTT information provides negligible additional information.
RTT information may be used to infer the occupancy of queues along a
path; indeed, this is part of its utility for performance measurement
and diagnostics. When a link on a given path has excessive buffering
(on the order of hundreds of milliseconds or more), such that the
difference in delay between an empty queue and a full queue dwarfs
normal variance and RTT along the path, RTT variance during the
lifetime of a flow can be used to infer the presence of traffic on
the bottleneck link. In practice, however, this is not a concern for
passive measurement of congestion-controlled traffic, since any
observer in a situation to observe RTT passively need not infer the
presence of the traffic, as it can observe it directly.
In addition, since RTT information contains application as well as
network delay, patterns in RTT variance from minimum, and therefore
application delay, can be used to infer or fingerprint application-
layer behavior. However, as with the case above, this is not a
concern with passive measurement, since the packet size and
interarrival time sequence, which is also directly observable,
carries more information than RTT variance sequence.
We therefore conclude that the high-resolution, per-flow exposure of
RTT for passive measurement as provided by the spin bit poses
negligible marginal risk to privacy.
As shown in Section 2, the spin bit can be implemented separately
from the rest of the mechanisms of the QUIC transport protocol, as it
requires no access to any state other than that observable in the
QUIC packet header itself. We recommend that implementations take
advantage of this property, to reduce the risk that errors in the
implementation could leak private transport protocol state through
the spin bit.
Since the spin bit is disconnected from transport mechanics, a QUIC
endpoint implementing the spin bit that has a model of the actual
network RTT and a target RTT to expose can "lie" about its spin bit
transitions, by anticipating or delaying observed transitions, even
without coordination with and the collusion of the other endpoint.
This is not the case with TCP, which requires coordination and
collusion to expose false information via its sequence and
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acknowledgment numbers and its timestamp option. When passive
measurement is used for purposes where one endpoint might gain a
material advantage by representing a false RTT, e.g. SLA
verification or enforcement of telecommunications regulations, this
situation raises a question about the trustworthiness of spin bit RTT
measurements.
This issue must be appreciated by users of spin bit information, but
mitigation is simple, as QUIC implementations designed to lie about
RTT through spin bit modification can easily be detected. A lying
server can be contacted by an honest client under the control of a
verifying party, and the client's RTT estimate compared with the
spin-bit exposed estimate. Though in the general case, it is
impossible to verify explicit path signals with two complicit
endpoints (see [WIRE-IMAGE]), a lying server/client pair may be
subject to dynamic analysis along paths with known RTTs. We consider
the ease of verification of lying in situations where this would be
prohibited by regulation or contract, combined with the consequences
of violation of said regulation or contract, to be a sufficient
incentive in the general case not to do it.
6. Acknowledgments
Many thanks to Christian Huitema, who originally proposed the spin
bit as pull request 609 on [QUIC-TRANS]. Thanks to Tobias Buehler
for feedback on the draft, and for Alexandre Ferrieux for input on
the Valid Edge Counter. Special thanks to the QUIC RTT Design Team
for discussions leading especially to the privacy and security
considerations section.
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
for Education, Research, and Innovation under contract no. 15.0268.
This support does not imply endorsement.
7. References
7.1. Normative References
[QUIC-SPIN-EXP]
Trammell, B. and M. Kuehlewind, "The QUIC Latency Spin
Bit", draft-ietf-quic-spin-exp (work in progress).
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7.2. Informative References
[ALT-MARK]
Fioccola, G., Capello, A., Cociglio, M., Castaldelli, L.,
Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
"Alternate Marking method for passive and hybrid
performance monitoring", draft-ietf-ippm-alt-mark-14 (work
in progress), December 2017.
[CACM-TCP]
Strowes, S., "Passively Measuring TCP Round-Trip Times (in
Communications of the ACM)", October 2013.
[CARRA-RTT]
Carra, D., Avrachenkov, K., Alouf, S., Blanc, A., Nain,
P., and G. Post, "Passive Online RTT Estimation for Flow-
Aware Routers Using One-Way Traffic (NETWORKING 2010, LNCS
6091, pp. 109-121)", 2010.
[CONUS] Morton, A., "Comparison of Backbone Node RTT and Great
Circle Distances (https://github.com/acmacm/CONUS-RTT)",
September 2017.
[IMC-CONGESTION]
Luckie, M., Dhamdhere, A., Clark, D., Huffaker, B., and k.
claffy, "Challenges in Inferring Internet Interdomain
Congestion (in Proc. ACM IMC 2014)", November 2014.
[IMC-TCPSIG]
Sundaresan, S., Dhamdhere, A., Allman, M., and . k claffy,
"TCP Congestion Signatures (in Proc. ACM IMC 2017)", n.d..
[MINQ] Rescorla, E., "MINQ, a simple Go implementation of QUIC
(https://github.com/ekr/minq)", November 2017.
[MOKUMOKUREN]
Trammell, B., "Mokumokuren, a lightweight flow meter using
gopacket (https://github.com/britram/mokumokuren)",
November 2017.
[NOSPIN] Morton, A., "Description of a tool chain to evaluate
Unidirectional Passive RTT measurement (and results)
(https://github.com/acmacm/PassiveRTT)", October 2017.
[QUIC-MGT]
Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
Transport Protocol", draft-ietf-quic-manageability-01
(work in progress), October 2017.
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[QUIC-TRANS]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-11 (work
in progress), April 2018.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.
[RFC4433] Kulkarni, M., Patel, A., and K. Leung, "Mobile IPv4
Dynamic Home Agent (HA) Assignment", RFC 4433,
DOI 10.17487/RFC4433, March 2006,
<https://www.rfc-editor.org/info/rfc4433>.
[RFC4737] Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
DOI 10.17487/RFC4737, November 2006,
<https://www.rfc-editor.org/info/rfc4737>.
[RFC5357] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, DOI 10.17487/RFC5357, October 2008,
<https://www.rfc-editor.org/info/rfc5357>.
[RFC6049] Morton, A. and E. Stephan, "Spatial Composition of
Metrics", RFC 6049, DOI 10.17487/RFC6049, January 2011,
<https://www.rfc-editor.org/info/rfc6049>.
[RFC7799] Morton, A., "Active and Passive Metrics and Methods (with
Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
May 2016, <https://www.rfc-editor.org/info/rfc7799>.
[SHBAIR] Shbair, W., Cholez, T., Francois, J., and I. Chrisment, "A
multi-level framework to identify HTTPS services (in Proc.
IEEE/IFIP NOMS)", April 2016.
[SPINBIT-REPORT]
De Vaere, P., "Latency Spinbit Implementation Experience
(https://devae.re/f/eth/quic/spinbit_report/)", November
2017.
[TLS] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[TMA-QOF] Trammell, B., Gugelmann, D., and N. Brownlee, "Inline Data
Integrity Signals for Passive Measurement (in Proc. TMA
2014)", April 2014.
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[TOKYO-PING]
Pelsser, C., Cittadini, L., Vissicchio, S., and R. Bush,
"From Paris to Tokyo - On the Suitability of ping to
Measure Latency (In Proc. ACM IMC 2014)", October 2014.
[TRILAT] Trammell, B., "On the Suitability of RTT Measurements for
Geolocation
(https://github.com/britram/trilateration/blob/paper-rev-
1/paper.ipynb)", August 2017.
[WIRE-IMAGE]
Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", draft-trammell-wire-image-04 (work in
progress), April 2018.
[WWMM-BLOAT]
Alfredsson, S., Giudice, G., Garcia, J., Brunstrom, A.,
Cicco, L., and S. Mascolo, "Impact of TCP Congestion
Control on Bufferbloat in Cellular Networks (in Proc. IEEE
WoWMoM 2013)", June 2013.
Appendix A. Experimental Evaluation
We have evaluated the effectiveness of the spin bit in an emulated
network environment. The spin bit was added to a fork of [MINQ],
using the mechanism described in Section 2, but with the spin bit
appearing in a measurement byte added to the header for passive
measurability experiments. Spin bit measurement support was added to
[MOKUMOKUREN]. Full results of these ongoing experiments are
available online in [SPINBIT-REPORT], but we summarize our findings
here.
First, we confirm that the spin bit works as advertised: it provides
one useful RTT sample per RTT to any passive observer of the flow.
This sample tracks each sender's local instantaneous estimate of RTT
as well as the expected RTT (i.e., defined by the emulation) fairly
well. One surprising implication of this is that the spin bit
provides _more_ information than is available by local estimation to
an endpoint which is mostly receiving data frames and sending mainly
ACKs, and as such can also be useful in purely endpoint-local
observations of the RTT evolution during the flow. The spin bit also
works correctly under moderate to heavy packet loss and jitter.
Second, we confirm that the spin bit can be easily implemented
without requiring deep integration into a QUIC implementation.
Indeed, it could be implemented completely independently, as a shim,
aside from the requirement that the spin bit value be integrity-
protected along with the rest of the QUIC header.
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Third, we performed experiments focused on the intermittent-sender
problem described in Section 3.1. We confirm that the spin bit does
not provide useful RTT samples after the handshake when packets are
only sent intermittently. Simple heuristics can be used to recognize
this situation, however, and to reject these RTT samples. We also
find that a simple sender-side heuristic can be used to determine
whether a sample will be useful. If a sender sends a packet more
than a specified delay (e.g. 1ms) after the last packet received by
the client, it knows that any latency spin observation of that packet
will be invalid. If a second "spin valid" bit were available, the
sender could then mark that packet "spin invalid". Our experiments
show that this simple heuristic and spin validity bit are successful
in marking all packets whose RTT samples should be rejected.
Fourth, we performed experiments focused on the reordering problem
described in Section 3.1. We find that while reordering can cause
spurious samples at a naive observer, two simple approaches can be
used to reject spurious RTT samples due to reordering. First, a two-
bit spin signal that always advances in a single direction (e.g. 00
-> 01 -> 10 -> 11) successfully rejects all reordered samples,
including under amounts of reordering that render the transport
itself mostly useless. However, adding a bit is not necessary:
having the observer keep the least significant bits of the packet
number, and rejecting samples from packets that reverse the sequence
[RFC4737], as suggested in Section 3.1, is essentially as successful
as a two-bit spin signal in mitigating the effects of reordering on
RTT measurement.
Fifth, we performed parallel active measurements using ping, as
described in Appendix C.2. In our emulated network, the ICMP packets
and the QUIC packets traverse the same links with the same treatment,
and share queues at each link, which mitigates most of the issues
with ping. We find that while ping works as expected in measuring
end-to-end RTT, it does not track the sender's estimate of RTT, and
as such does not measure the RTT experienced by the application layer
as well as the spin bit does.
In summary, our experiments show that the spin bit is suitable for
purpose, can be implemented with minimal disruption, and that most of
the identified problems can be easily mitigated. See
[SPINBIT-REPORT] for more.
Appendix B. Use Cases for Passive RTT Measurement
This section describes use cases for passive RTT measurement. Most
of these are currently achieved with TCP, i.e., the matching of
packets based on sequence and acknowledgment numbers, or timestamps
and timestamp echoes, in order to generate upstream and downstream
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RTT samples which can be added to get end-to-end RTT. These use
cases could be achieved with QUIC by replacing sequence/
acknowledgement and timestamp analysis with spin bit analysis, as
described in Section 3.
In any case, the measurement methodology follows one of a few basic
variants:
o The RTT evolution of a flow or a set of flows can be compared to
baseline or expected RTT measurements for flows with the same
characteristics in order to detect or localize latency issues in a
specific network.
o The RTT evolution of a single flow can also be examined in detail
to diagnose performance issues with that flow.
o The spin bit can be used to generate a large number of samples of
RTT for a flow aggregate (e.g., all flows between two given
networks) without regard to temporal evolution of the RTT, in
order to examine the distribution of RTTs for a group of flows
that should have similar RTT (e.g., because they should share the
same path(s)).
B.1. Inter-domain Troubleshooting
Network access providers are often the first point of contact by
their customers when network problems impact the performance of
bandwidth-intensive and latency-sensitive applications such as video,
regardless of whether the root cause lies within the access
provider's network, the service provider's network, on the Internet
paths between them, or within the customer's own network.
The network performance is currently measured by points of presence
on-the-path which extract spatial delay and loss metrics measurements
[RFC6049] from fields of the transport layer (e.g. TCP) or of
application layer (e.g. RTP). The information is captured in the
upper layer because neither the IP header nor the UDP layer includes
fields allowing the measurement of upstream and downstream delay and
loss.
Local network performance problems are detected with monitoring tools
which observe the variation of upstream metrics and downstream
metrics.
Inter-domain troubleshooting relies on the same metrics but is not a
pro-active task. It is a recursive process which hones in on the
domain and link responsible for the failure. In practice, inter-
domain troubleshooting is a communication process between the Network
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Operations Center (NOC) teams of the networks on the path, because
the root cause of a problem is rarely located on a single network,
and requires cooperation and exchange of data between the NOCs.
One example is the troubleshooting performance degradation resulting
from a change of routing policy on one side of the path which
increases the burden on a defective line card of a device located
somewhere on the path. The card's misbehavior introduces an abnormal
reordered packets only in the traffic exchanged at line rate.
Other examples are similar in terms of cooperation requirements and
the need to refer to measurements. NOCs need to share the same
measurement metrics and to measure these metrics on the same fields
of the packet to enable a minimal level of technical cooperation.
Experimentation with the spinbit Appendix A has shown ability to
replace the current RTT measurement opportunities based on clear-text
transport or application header fields with a standard approach for
measuring passive upstream and downstream RTT, which are a
fundamental metric for this diagnostic process.
B.2. Two-Point Intradomain Measurement
The spin bit is also useful as a basic signal for instantaneous
measurement of the treatment of QUIC traffic within a single network.
Though the primary design goal of the spin bit signal is to enable
single-observer on-path measurement of end-to-end RTT, the spin bit
can also be used by two cooperating observers with access to traffic
flowing in the same direction as an alternate marking signal, as
described in [ALT-MARK]. The only difference from alternate marking
with a generated signal is that the size of the alternation will
change with the flight size each RTT. However, these changes do not
affect the applicability of the method that works for each marking
batch separately applied between two measurement points on the same
direction. This two point measurement is an additional feature
enabled "for free" by the spin bit signal.
So, with more than one observer on the same direction, it can be
useful to segment the RTT and deduce the contribution to the RTT of
the portion of the network between two on-path observers. This can
be easily performed by calculating the delay between two or more
measurement points on a single direction by applying [ALT-MARK]. In
this way, packet loss, delay and delay variation can be measured for
each segment of the network depending on the number and distribution
of the available on-path observation points. When these observation
points are applied at network borders, the alternate-marking signal
can be used to measure the performance of QUIC traffic within a
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network operator's own domain of responsibility. own portion of the
network.
B.3. Bufferbloat Mitigation in Cellular Networks
Cellular networks consist of multiple Radio Access Networks (RAN)
where mobile devices are attached to base stations. It is common
that base stations from different vendors and different generations
are deployed in the same cellular network.
Due to the dynamic nature of RANs, base stations have typically been
provisioned with large buffers to maximize throughput despite rapid
changes in capacity. As a side effect, bufferbloat has become a
common issue in such networks [WWMM-BLOAT].
An effective way of mitigating bufferbloat without sacrificing too
much throughput is to deploy Active Queue Management (AQM) in
bottleneck routers and base stations. However, due to the variation
in deployed base-stations it is not always possible to enable AQM at
the bottlenecks, without massive infrastructure investments.
An alternative approach is to deploy AQM as a network function in a
more centralized location than the traditional bottleneck nodes.
Such an AQM monitors the RTT progression of flows and drops or marks
packets when the measured latency is indicative of congestion. Such
a function also has the possibility to detect misbehaving flows and
reduce the negative impact they have on the network.
B.4. Locating WiFi Problems in Home Networks
Many residential networks use WiFi (802.11) on the last segment, and
WiFi signal strength degradation manifests in high first-hop delay,
due to the fact that the MAC layer will retransmit packets lost at
that layer. Measuring the RTT between endpoints on the customer
network and parts of the service provider's own infrastructure (which
have predictable delay characteristics) can be used to isolate this
cause of performance problems.
The network provider can measure the RTT and packet loss in the home
gateway or an upstream point if there is no access to home gateway.
A problem in the WiFi network is identified by seeing high delay and
low packet loss.
These measurements are particularly useful for traffic which is
latency sensitive, such as interactive video applications. However,
since high latency is often correlated with other network-layer
issues such as chronic interconnect congestion [IMC-CONGESTION], it
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is useful for general troubleshooting of network layer issues in an
interdomain setting.
In this case, multiple RTT samples per flow are useful less for
observing intraflow behavior, and more for generating sufficient
samples for a given aggregate to make a high-quality measurement.
B.5. Internet Measurement Research
As a large, distributed, engineered system with no centralized
control, the Internet has emergent properties of interest to the
research community not just for purely scientific curiosity, but also
to provide applicable guidance to Internet engineering, Internet
protocol design and development, network operations, and policy
development. Latency measurements in particular are both an active
area of research as well as an important tool for certain measurement
studies (see, e.g. [IMC-TCPSIG], from the most recent Internet
Measurement Conference). While much of this work is currently done
with active measurements, the ability to generate latency samples
passively or using a hybrid measurement approach (i.e., through
passive observation of purpose-generated active measurement traffic;
see [RFC7799]) can drastically increase the efficiency and
scalability of these studies. A latency spin bit would make these
techniques applicable to QUIC, as well.
Appendix C. Alternate RTT Measurement Approaches for Diagnosing QUIC
flows
There are three broad alternatives to explicit signaling for passive
RTT measurement of the RTT experienced by QUIC flows.
C.1. Handshake RTT measurement
The first of these is handshake RTT measurement. As described in
[QUIC-MGT], the packets of the QUIC handshake are distinguishable on
the wire in such a way that they can be used for one RTT measurement
sample per flow: the delay between the client initial and the server
cleartext packet can be used to measure "upstream" RTT (between the
observer and the server), and the delay between the server cleartext
packet and the next client cleartext packet can be used to measure
"downstream" RTT (between the client and the observer). When RTT
measurements are used in large aggregates (all flows traversing a
large link, for example), a methodology based on handshake RTT could
be used to generate sufficient samples for some purposes without the
spin bit.
However, this methodology would rely on the assumption that the
difference between handshake RTT and nominal in-flow RTT is
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negligible. Specifically, (1) any additional delay required to
compute any cryptographic parameters must be negligible with respect
to network RTT; (2) any additional delay required to establish state
along the path must be negligible with respect to network RTT; and
(3) network treatment of initial packets in a flow must be identical
to that of later packets in the flow. When these assumptions cannot
be shown to hold, spin-bit based RTT measurement is preferable to
handshake RTT measurement, even for applications for which handshake
RTT measurement would otherwise be suitable.
C.2. Parallel active measurement
The second alternative is parallel active measurement: using ICMP
Echo Request and Reply [RFC0792] [RFC4433], a dedicated measurement
protocol like TWAMP [RFC5357], or a separate diagnostic QUIC flow to
measure RTT. Regardless of protocol, the active measurement must be
initiated by a client on the same network as the client of the QUIC
flow(s) of interest, or a network close by in the Internet topology,
toward the server. Note that there is no guarantee that ICMP flows
will receive the same network treatment as the flows under study,
both due to differential treatment of ICMP traffic and due to ECMP
routing (see e.g. [TOKYO-PING]). TWAMP and QUIC diagnostic flows,
though both use UDP, have similar issues regarding ECMP. However, in
situations where the entity doing the measurement can guarantee that
the active measurement traffic will traverse the subpaths of interest
(e.g. residential access network measurement under a network
architecture and business model where the network operator owns the
CPE), active measurement can be used to generate RTT samples at the
cost of at least two non-productive packets sent though the network
per sample.
C.3. Frequency Analysis
The third alternative, proposed during the QUIC RTT design team
process, relies on the inter-packet spacing to convey information
about the RTT, and would therefore allow measurements confined to a
single direction of transmission, as described in [CARRA-RTT].
We evaluated the applicability of this work to passive RTT
measurement in QUIC, and found it wanting. We assembled a toolchain,
as described in [NOSPIN], that allowed evaluation of a critical
aspect of the [CARRA-RTT] method: extraction of inter-packet times of
real packet streams and the analysis of frequencies present in the
packet stream using the Lomb-Scargle Periodogram. Several streams
were evaluated, as summarized below:
o It seems that Carra et al. [CARRA-RTT] took the noisy and low-
confidence results of a statistical process (no RTT-related
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frequency has been detected even after using very low alpha
confidence) and added heuristics with sliding-window averaging to
infer the fundamental frequency and RTT present in a
unidirectional stream.
o There appear to be several limitations on the streams that are
applicable. Streams with long RTT (~50ms) are more likely to be
suitable (having a better match between packet rate and relatively
low frequencies to detect).
o None of the TCP streams analysed (to date) possess a sufficient
packet rate such that the measured fundamental frequency or the
multiples of the fundamental are actually within the detectable
range.
o "Ideal" interarrival time streams were simulated with uniform
sampling and period. The Lomb-Scargle Periodogram is surprisingly
unable to detect the fundamental frequency at 100 Hz from the
constant 10 ms packet spacing.
o It is not clear if IETF QUIC protocol stream will possess the same
inter-packet arrival time features as TCP streams. Also, Carra et
al. note that their process may not work if the TCP stream
encounters a bottleneck, which would be an essential circumstance
for network troubleshooting. Mobile networks with time-slot
service disciplines would likely cause similar issues as a
bottleneck, by imposing their time-slot interval on the spacing of
most packets.
o The Carra et al. [CARRA-RTT] calculation of minimum and maximum
frequencies that can be detected may not be applicable when the
inter-arrival times are (both) the signal being detected and
govern the non-uniform sampling frequency.
Appendix D. Greasing
Routes, congestion levels and therefore latency between two fixed
QUIC endpoints, as well as the shape of individual application flows,
fluctuate in ways that are not totally predictable by an on path
observer. In general, there is no a-priori pattern for the spin-bit
distribution that will always materialise on a certain flow
aggregate, even for a single user.
There has been discussion in the QUIC working group that greasing
could be a strategy to counter an evil access provider that might
gate access to its users on a valid spin bit signal. Let's accept
for a moment this threat model and consider the practical case of a
home gateway that temporarily misbehaves, for example draining its
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queues slower than it would normally do while a firmware download is
in progress. It would be ill-considered for an access provider (even
a malicious one) to block, or otherwise interfere with, QUIC flows
originating from behind that CPE solely based on the fact that RTTs
are now different from "usual". In fact, providing a numerical
assessment of what such "usual" RTT looks like would necessarily
include many paths with different length, and considerable RTT
variability within any fixed path, which is clearly beyond most ISPs'
reach. But even assuming it were, there is a simple cost-benefit
counterargument here that the same effect (i.e., gating traffic from
or to a given user based on observed traffic patterns) could be
achieved with much cheaper and effective means (e.g., [SHBAIR]).
So, the potential for ossification appears to be extremely low.
Since it depends on so much external noise, the spin-bit result
variability is self-greasing to an extent. In fact, implementing
explicit greasing around the spin-bit might even be harmful as it
would potentially erode confidence in the veracity of the signal.
However, if a greasing algorithm is really needed - for example, if
we want to reuse the bit with different semantics in the future
(i.e.: the spin-bit is not included in the header invariants), one
very simple implementation would be as follows: each server will
refuse to spin its bit on a per-flow basis with a given probability
p, instead leaving it stuck to a randomly chosen value, 0 or 1. The
client will then end up leaving its bit stuck to the opposite value,
or could detect this condition and also pick a randomly chosen stuck
value. The value chosen for p must be small enough to let the spin-
bit mechanics work and large enough not to be seen as an error
instead of an intentional protocol feature.
Authors' Addresses
Brian Trammell (editor)
ETH Zurich
Email: ietf@trammell.ch
Piet De Vaere
ETH Zurich
Email: piet@devae.re
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Roni Even
Huawei
Email: roni.even@huawei.com
Giuseppe Fioccola
Telecom Italia
Email: giuseppe.fioccola@telecomitalia.it
Thomas Fossati
Nokia
Email: thomas.fossati@nokia.com
Marcus Ihlar
Ericsson
Email: marcus.ihlar@ericsson.com
Al Morton
AT&T Labs
Email: acmorton@att.com
Emile Stephan
Orange
Email: emile.stephan@orange.com
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