Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture
draft-ietf-tsvwg-l4s-arch-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9330.
|
|
|---|---|---|---|
| Authors | Bob Briscoe , Koen De Schepper , Marcelo Bagnulo | ||
| Last updated | 2017-05-05 | ||
| Replaces | draft-briscoe-tsvwg-l4s-arch | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Wesley Eddy | ||
| IESG | IESG state | Became RFC 9330 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | Wesley Eddy <wes@mti-systems.com> |
draft-ietf-tsvwg-l4s-arch-00
Transport Area Working Group B. Briscoe, Ed.
Internet-Draft Simula Research Lab
Intended status: Informational K. De Schepper
Expires: November 6, 2017 Nokia Bell Labs
M. Bagnulo Braun
Universidad Carlos III de Madrid
May 5, 2017
Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture
draft-ietf-tsvwg-l4s-arch-00
Abstract
This document describes the L4S architecture for the provision of a
new Internet service that could eventually replace best efforts for
all traffic: Low Latency, Low Loss, Scalable throughput (L4S). It is
becoming common for _all_ (or most) applications being run by a user
at any one time to require low latency. However, the only solution
the IETF can offer for ultra-low queuing delay is Diffserv, which
only favours a minority of packets at the expense of others. In
extensive testing the new L4S service keeps average queuing delay
under a millisecond for _all_ applications even under very heavy
load, without sacrificing utilization; and it keeps congestion loss
to zero. It is becoming widely recognized that adding more access
capacity gives diminishing returns, because latency is becoming the
critical problem. Even with a high capacity broadband access, the
reduced latency of L4S remarkably and consistently improves
performance under load for applications such as interactive video,
conversational video, voice, Web, gaming, instant messaging, remote
desktop and cloud-based apps (even when all being used at once over
the same access link). The insight is that the root cause of queuing
delay is in TCP, not in the queue. By fixing the sending TCP (and
other transports) queuing latency becomes so much better than today
that operators will want to deploy the network part of L4S to enable
new products and services. Further, the network part is simple to
deploy - incrementally with zero-config. Both parts, sender and
network, ensure coexistence with other legacy traffic. At the same
time L4S solves the long-recognized problem with the future
scalability of TCP throughput.
This document describes the L4S architecture, briefly describing the
different components and how the work together to provide the
aforementioned enhanced Internet service.
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Status of This Memo
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This Internet-Draft will expire on November 6, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. L4S Architecture Overview . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. L4S Architecture Components . . . . . . . . . . . . . . . . . 7
5. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Why These Primary Components? . . . . . . . . . . . . . . 9
5.2. Why Not Alternative Approaches? . . . . . . . . . . . . . 11
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Applications . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 14
6.3. Deployment Considerations . . . . . . . . . . . . . . . . 15
6.3.1. Deployment Topology . . . . . . . . . . . . . . . . . 16
6.3.2. Deployment Sequences . . . . . . . . . . . . . . . . 17
6.3.3. L4S Flow but Non-L4S Bottleneck . . . . . . . . . . . 19
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6.3.4. Other Potential Deployment Issues . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8.1. Traffic (Non-)Policing . . . . . . . . . . . . . . . . . 21
8.2. 'Latency Friendliness' . . . . . . . . . . . . . . . . . 22
8.3. Policing Prioritized L4S Bandwidth . . . . . . . . . . . 22
8.4. ECN Integrity . . . . . . . . . . . . . . . . . . . . . . 23
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Standardization items . . . . . . . . . . . . . . . 28
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction
It is increasingly common for _all_ of a user's applications at any
one time to require low delay: interactive Web, Web services, voice,
conversational video, interactive video, interactive remote presence,
instant messaging, online gaming, remote desktop, cloud-based
applications and video-assisted remote control of machinery and
industrial processes. In the last decade or so, much has been done
to reduce propagation delay by placing caches or servers closer to
users. However, queuing remains a major, albeit intermittent,
component of latency. For instance spikes of hundreds of
milliseconds are common. During a long-running flow, even with
state-of-the-art active queue management (AQM), the base speed-of-
light path delay roughly doubles. Low loss is also important
because, for interactive applications, losses translate into even
longer retransmission delays.
It has been demonstrated that, once access network bit rates reach
levels now common in the developed world, increasing capacity offers
diminishing returns if latency (delay) is not addressed.
Differentiated services (Diffserv) offers Expedited Forwarding
[RFC3246] for some packets at the expense of others, but this is not
applicable when all (or most) of a user's applications require low
latency.
Therefore, the goal is an Internet service with ultra-Low queueing
Latency, ultra-Low Loss and Scalable throughput (L4S) - for _all_
traffic. A service for all traffic will need none of the
configuration or management baggage (traffic policing, traffic
contracts) associated with favouring some packets over others. This
document describes the L4S architecture for achieving that goal.
It must be said that queuing delay only degrades performance
infrequently [Hohlfeld14]. It only occurs when a large enough
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capacity-seeking (e.g. TCP) flow is running alongside the user's
traffic in the bottleneck link, which is typically in the access
network. Or when the low latency application is itself a large
capacity-seeking flow (e.g. interactive video). At these times, the
performance improvement from L4S must be so remarkable that network
operators will be motivated to deploy it.
Active Queue Management (AQM) is part of the solution to queuing
under load. AQM improves performance for all traffic, but there is a
limit to how much queuing delay can be reduced by solely changing the
network; without addressing the root of the problem.
The root of the problem is the presence of standard TCP congestion
control (Reno [RFC5681]) or compatible variants (e.g. TCP Cubic
[I-D.ietf-tcpm-cubic]). We shall call this family of congestion
controls 'Classic' TCP. It has been demonstrated that if the sending
host replaces Classic TCP with a 'Scalable' alternative, when a
suitable AQM is deployed in the network the performance under load of
all the above interactive applications can be stunningly improved.
For instance, queuing delay under heavy load with the example DCTCP/
DualQ solution cited below is roughly 1 millisecond (1 ms) at the
99th percentile without losing link utilization. This compares with
5 to 20 ms on _average_ with a Classic TCP and current state-of-the-
art AQMs such as fq_CoDel [I-D.ietf-aqm-fq-codel] or PIE [RFC8033].
Also, with a Classic TCP, 5 ms of queuing is usually only possible by
losing some utilization.
It has been convincingly demonstrated [DCttH15] that it is possible
to deploy such an L4S service alongside the existing best efforts
service so that all of a user's applications can shift to it when
their stack is updated. Access networks are typically designed with
one link as the bottleneck for each site (which might be a home,
small enterprise or mobile device), so deployment at a single node
should give nearly all the benefit. The L4S approach requires
component mechanisms in different parts of an Internet path to
fulfill its goal. This document presents the L4S architecture, by
describing the different components and how they interact to provide
the scalable low-latency, low-loss, Internet service.
2. L4S Architecture Overview
There are three main components to the L4S architecture (illustrated
in Figure 1):
1) Network: The L4S service traffic needs to be isolated from the
queuing latency of the Classic service traffic. However, the two
should be able to freely share a common pool of capacity. This is
because there is no way to predict how many flows at any one time
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might use each service and capacity in access networks is too
scarce to partition into two. So a 'semi-permeable' membrane is
needed that partitions latency but not bandwidth. The Dual Queue
Coupled AQM [I-D.ietf-tsvwg-aqm-dualq-coupled] is an example of
such a semi-permeable membrane.
Per-flow queuing such as in [I-D.ietf-aqm-fq-codel] could be used,
but it partitions both latency and bandwidth between every end-to-
end flow. So it is rather overkill, which brings disadvantages
(see Section 5.2), not least that thousands of queues are needed
when two are sufficient.
2) Protocol: A host needs to distinguish L4S and Classic packets
with an identifier so that the network can classify them into
their separate treatments. [I-D.ietf-tsvwg-ecn-l4s-id] considers
various alternative identifiers, and concludes that all
alternatives involve compromises, but the ECT(1) codepoint of the
ECN field is a workable solution.
3) Host: Scalable congestion controls already exist. They solve the
scaling problem with TCP first pointed out in [RFC3649]. The one
used most widely (in controlled environments) is Data Centre TCP
(DCTCP [I-D.ietf-tcpm-dctcp]), which has been implemented and
deployed in Windows Server Editions (since 2012), in Linux and in
FreeBSD. Although DCTCP as-is 'works' well over the public
Internet, most implementations lack certain safety features that
will be necessary once it is used outside controlled environments
like data centres (see later). A similar scalable congestion
control will also need to be transplanted into protocols other
than TCP (SCTP, RTP/RTCP, RMCAT, etc.)
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(2) (1)
.-------^------. .--------------^-------------------.
,-(3)-----. ______
; ________ : L4S --------. | |
:|Scalable| : _\ ||___\_| mark |
:| sender | : __________ / / || / |______|\ _________
:|________|\; | |/ --------' ^ \1| |
`---------'\_| IP-ECN | Coupling : \|priority |_\
________ / |Classifier| : /|scheduler| /
|Classic |/ |__________|\ --------. ___:__ / |_________|
| sender | \_\ || | |||___\_| mark/|/
|________| / || | ||| / | drop |
Classic --------' |______|
Figure 1: Components of an L4S Solution: 1) Isolation in separate
network queues; 2) Packet Identification Protocol; and 3) Scalable
Sending Host
3. 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 [RFC2119]. In this
document, these words will appear with that interpretation only when
in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance. COMMENT: Since this
will be an information document, This should be removed.
Classic service: The 'Classic' service is intended for all the
congestion control behaviours that currently co-exist with TCP
Reno (e.g. TCP Cubic, Compound, SCTP, etc).
Low-Latency, Low-Loss and Scalable (L4S) service: The 'L4S' service
is intended for traffic from scalable TCP algorithms such as Data
Centre TCP. But it is also more general--it will allow a set of
congestion controls with similar scaling properties to DCTCP (e.g.
Relentless [Mathis09]) to evolve.
Both Classic and L4S services can cope with a proportion of
unresponsive or less-responsive traffic as well (e.g. DNS, VoIP,
etc).
Scalable Congestion Control: A congestion control where the packet
flow rate per round trip (the window) is inversely proportional to
the level (probability) of congestion signals. Then, as flow rate
scales, the number of congestion signals per round trip remains
invariant, maintaining the same degree of control. For instance,
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DCTCP averages 2 congestion signals per round-trip whatever the
flow rate.
Classic Congestion Control: A congestion control with a flow rate
compatible with standard TCP Reno [RFC5681]. With Classic
congestion controls, as capacity increases enabling higher flow
rates, the number of round trips between congestion signals
(losses or ECN marks) rises in proportion to the flow rate. So
control of queuing and/or utilization becomes very slack. For
instance, with 1500 B packets and an RTT of 18 ms, as TCP Reno
flow rate increases from 2 to 100 Mb/s the number of round trips
between congestion signals rises proportionately, from 2 to 100.
The default congestion control in Linux (TCP Cubic) is Reno-
compatible for most Internet access scenarios expected for some
years. For instance, with a typical domestic round-trip time
(RTT) of 18ms, TCP Cubic only switches out of Reno-compatibility
mode once the flow rate approaches 1 Gb/s. For a typical data
centre RTT of 1 ms, the switch-over point is theoretically 1.3 Tb/
s. However, with a less common transcontinental RTT of 100 ms, it
only remains Reno-compatible up to 13 Mb/s. All examples assume
1,500 B packets.
Classic ECN: The original proposed standard Explicit Congestion
Notification (ECN) protocol [RFC3168], which requires ECN signals
to be treated the same as drops, both when generated in the
network and when responded to by the sender.
Site: A home, mobile device, small enterprise or campus, where the
network bottleneck is typically the access link to the site. Not
all network arrangements fit this model but it is a useful, widely
applicable generalisation.
4. L4S Architecture Components
The L4S architecture is composed of the following elements.
Protocols:The L4S architecture encompasses the two protocol changes
(an unassignment and an assignment) that we describe next:
a. An essential aspect of a scalable congestion control is the use
of explicit congestion signals rather than losses, because the
signals need to be sent immediately and frequently--too often to
use drops. 'Classic' ECN [RFC3168] requires an ECN signal to be
treated the same as a drop, both when it is generated in the
network and when it is responded to by hosts. L4S needs networks
and hosts to support two separate meanings for ECN. So the
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standards track [RFC3168] needs to be updated to allow L4S
packets to depart from the 'same as drop' constraint.
[I-D.ietf-tsvwg-ecn-experimentation] has been prepared as a
standards track update to relax specific requirements in RFC 3168
(and certain other standards track RFCs), which clears the way
for the experimental changes proposed for L4S.
[I-D.ietf-tsvwg-ecn-experimentation] also explains why the
original experimental assignment of the ECT(1) codepoint as an
ECN nonce [RFC3540] is being reclassified as historic (it was
never deployed, and it offers no security benefit now that
deployment is optional).
b. [I-D.ietf-tsvwg-ecn-l4s-id] recommends ECT(1) is used as the
identifier to classify L4S packets into a separate treatment from
Classic packets. This satisfies the requirements for identifying
an alternative ECN treatment in [RFC4774].
Network components:The Dual Queue Coupled AQM has been specified as
generically as possible [I-D.ietf-tsvwg-aqm-dualq-coupled] as a
'semi-permeable' membrane without specifying the particular AQMs to
use in the two queues. An informational appendix of the draft is
provided for pseudocode examples of different possible AQM
approaches. Initially a zero-config variant of RED called Curvy RED
was implemented, tested and documented. The aim is for designers to
be free to implement diverse ideas. So the brief normative body of
the draft only specifies the minimum constraints an AQM needs to
comply with to ensure that the L4S and Classic services will coexist.
For instance, a variant of PIE called Dual PI Squared [PI2] has been
implemented and found to perform better than Curvy RED over a wide
range of conditions, so it has been documented in a second appendix
of [I-D.ietf-tsvwg-aqm-dualq-coupled].
Host mechanisms: The L4S architecture includes a number of mechanisms
in the end host that we enumerate next:
a. Data Centre TCP is the most widely used example of a scalable
congestion control. It is being documented in the TCPM WG as an
informational record of the protocol currently in use
[I-D.ietf-tcpm-dctcp]. It will be necessary to define a number
of safety features for a variant usable on the public Internet.
A draft list of these, known as the TCP Prague requirements, has
been drawn up (see Appendix A of [I-D.ietf-tsvwg-ecn-l4s-id]).
The list also includes some optional performance improvements.
b. Transport protocols other than TCP use various congestion
controls designed to be friendly with Classic TCP. Before they
can use the L4S service, it will be necessary to implement
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scalable variants of each of these congestion control behaviours.
The following standards track RFCs currently define these
protocols: ECN in TCP [RFC3168], in SCTP [RFC4960], in RTP
[RFC6679], and in DCCP [RFC4340]. Not all are in widespread use,
but those that are will eventually need to be updated to allow a
different congestion response, which they will have to indicate
by using the ECT(1) codepoint. Scalable variants are under
consideration for some new transport protocols that are
themselves under development, e.g. QUIC [I-D.johansson-quic-ecn]
and certain real-time media congestion avoidance techniques
(RMCAT) protocols.
c. ECN feedback is sufficient for L4S in some transport protocols
(RTCP, DCCP) but not others:
* For the case of TCP, the feedback protocol for ECN embeds the
assumption from Classic ECN that an ECN mark is the same as a
drop, making it unusable for a scalable TCP. Therefore, the
implementation of TCP receivers will have to be upgraded
[RFC7560]. Work to standardize more accurate ECN feedback for
TCP (AccECN [I-D.ietf-tcpm-accurate-ecn]) is in progress.
* ECN feedback is only roughly sketched in an appendix of the
SCTP specification. A fuller specification has been proposed
[I-D.stewart-tsvwg-sctpecn], which would need to be
implemented and deployed before SCTCP could support L4S.
5. Rationale
5.1. Why These Primary Components?
Explicit congestion signalling (protocol): Explicit congestion
signalling is a key part of the L4S approach. In contrast, use of
drop as a congestion signal creates a tension because drop is both
a useful signal (more would reduce delay) and an impairment (less
would reduce delay). Explicit congestion signals can be used many
times per round trip, to keep tight control, without any
impairment. Under heavy load, even more explicit signals can be
applied so the queue can be kept short whatever the load. Whereas
state-of-the-art AQMs have to introduce very high packet drop at
high load to keep the queue short. Further, when using ECN TCP's
sawtooth reduction can be smaller, and therefore return to the
operating point more often, without worrying that this causes more
signals (one at the top of each smaller sawtooth). The consequent
smaller amplitude sawteeth fit between a very shallow marking
threshold and an empty queue, so delay variation can be very low,
without risk of under-utilization.
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All the above makes it clear that explicit congestion signalling
is only advantageous for latency if it does not have to be
considered 'the same as' drop (as required with Classic ECN
[RFC3168]). Therefore, in a DualQ AQM, the L4S queue uses a new
L4S variant of ECN that is not equivalent to drop
[I-D.ietf-tsvwg-ecn-l4s-id], while the Classic queue uses either
classic ECN [RFC3168] or drop, which are equivalent.
Before Classic ECN was standardized, there were various proposals
to give an ECN mark a different meaning from drop. However, there
was no particular reason to agree on any one of the alternative
meanings, so 'the same as drop' was the only compromise that could
be reached. RFC 3168 contains a statement that:
"An environment where all end nodes were ECN-Capable could
allow new criteria to be developed for setting the CE
codepoint, and new congestion control mechanisms for end-node
reaction to CE packets. However, this is a research issue, and
as such is not addressed in this document."
Latency isolation with coupled congestion notification (network):
Using just two queues is not essential to L4S (more would be
possible), but it is the simplest way to isolate all the L4S
traffic that keeps latency low from all the legacy Classic traffic
that does not.
Similarly, coupling the congestion notification between the queues
is not necessarily essential, but it is a clever and simple way to
allow senders to determine their rate, packet-by-packet, rather
than be overridden by a network scheduler. Because otherwise a
network scheduler would have to inspect at least transport layer
headers, and it would have to continually assign a rate to each
flow without any easy way to understand application intent.
L4S packet identifier (protocol): Once there are at least two
separate treatments in the network, hosts need an identifier at
the IP layer to distinguish which treatment they intend to use.
Scalable congestion notification (host): A scalable congestion
control keeps the signalling frequency high so that rate
variations can be small when signalling is stable, and rate can
track variations in available capacity as rapidly as possible
otherwise.
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5.2. Why Not Alternative Approaches?
All the following approaches address some part of the same problem
space as L4S. In each case, it is shown that L4S complements them or
improves on them, rather than being a mutually exclusive alternative:
Diffserv: Diffserv addresses the problem of bandwidth apportionment
for important traffic as well as queuing latency for delay-
sensitive traffic. L4S solely addresses the problem of queuing
latency (as well as loss and throughput scaling). Diffserv will
still be necessary where important traffic requires priority (e.g.
for commercial reasons, or for protection of critical
infrastructure traffic). Nonetheless, if there are Diffserv
classes for important traffic, the L4S approach can provide low
latency for _all_ traffic within each Diffserv class (including
the case where there is only one Diffserv class).
Also, as already explained, Diffserv only works for a small subset
of the traffic on a link. It is not applicable when all the
applications in use at one time at a single site (home, small
business or mobile device) require low latency. Also, because L4S
is for all traffic, it needs none of the management baggage
(traffic policing, traffic contracts) associated with favouring
some packets over others. This baggage has held Diffserv back
from widespread end-to-end deployment.
State-of-the-art AQMs: AQMs such as PIE and fq_CoDel give a
significant reduction in queuing delay relative to no AQM at all.
The L4S work is intended to complement these AQMs, and we
definitely do not want to distract from the need to deploy them as
widely as possible. Nonetheless, without addressing the large
saw-toothing rate variations of Classic congestion controls, AQMs
alone cannot reduce queuing delay too far without significantly
reducing link utilization. The L4S approach resolves this tension
by ensuring hosts can minimize the size of their sawteeth without
appearing so aggressive to legacy flows that they starve them.
Per-flow queuing: Similarly per-flow queuing is not incompatible
with the L4S approach. However, one queue for every flow can be
thought of as overkill compared to the minimum of two queues for
all traffic needed for the L4S approach. The overkill of per-flow
queuing has side-effects:
A. fq makes high performance networking equipment costly
(processing and memory) - in contrast dual queue code can be
very simple;
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B. fq requires packet inspection into the end-to-end transport
layer, which doesn't sit well alongside encryption for privacy
- in contrast the use of ECN as the classifier for L4S
requires no deeper inspection than the IP layer;
C. fq isolates the queuing of each flow from the others but not
from itself so, unlike L4S, it does not support applications
that need both capacity-seeking behaviour and very low
latency.
It might seem that self-inflicted queuing delay should not
count, because if the delay wasn't in the network it would
just shift to the sender. However, modern adaptive
applications, e.g. HTTP/2 [RFC7540] or the interactive media
applications described in Section 6, can keep low latency
objects at the front of their local send queue by shuffling
priorities of other objects dependent on the progress of other
transfers. They cannot shuffle packets once they have
released them into the network.
D. fq prevents any one flow from consuming more than 1/N of the
capacity at any instant, where N is the number of flows. This
is fine if all flows are elastic, but it does not sit well
with a variable bit rate real-time multimedia flow, which
requires wriggle room to sometimes take more and other times
less than a 1/N share.
It might seem that an fq scheduler offers the benefit that it
prevents individual flows from hogging all the bandwidth.
However, L4S has been deliberately designed so that policing
of individual flows can be added as a policy choice, rather
than requiring one specific policy choice as the mechanism
itself. A scheduler (like fq) has to decide packet-by-packet
which flow to schedule without knowing application intent.
Whereas a separate policing function can be configured less
strictly, so that senders can still control the instantaneous
rate of each flow dependent on the needs of each application
(e.g. variable rate video), giving more wriggle-room before a
flow is deemed non-compliant. Also policing of queuing and of
flow-rates can be applied independently.
Alternative Back-off ECN (ABE): Yet again, L4S is not an alternative
to ABE but a complement that introduces much lower queuing delay.
ABE [I-D.ietf-tcpm-alternativebackoff-ecn] alters the host
behaviour in response to ECN marking to utilize a link better and
give ECN flows a faster throughput, but it assumes the network
still treats ECN and drop the same. Therefore ABE exploits any
lower queuing delay that AQMs can provide. But as explained
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above, AQMs still cannot reduce queuing delay too far without
losing link utilization (to allow for other, non-ABE, flows).
6. Applicability
6.1. Applications
A transport layer that solves the current latency issues will provide
new service, product and application opportunities.
With the L4S approach, the following existing applications will
immediately experience significantly better quality of experience
under load in the best effort class:
o Gaming;
o VoIP;
o Video conferencing;
o Web browsing;
o (Adaptive) video streaming;
o Instant messaging.
The significantly lower queuing latency also enables some interactive
application functions to be offloaded to the cloud that would hardly
even be usable today:
o Cloud based interactive video;
o Cloud based virtual and augmented reality.
The above two applications have been successfully demonstrated with
L4S, both running together over a 40 Mb/s broadband access link
loaded up with the numerous other latency sensitive applications in
the previous list as well as numerous downloads - all sharing the
same bottleneck queue simultaneously [L4Sdemo16]. For the former, a
panoramic video of a football stadium could be swiped and pinched so
that, on the fly, a proxy in the cloud could generate a sub-window of
the match video under the finger-gesture control of each user. For
the latter, a virtual reality headset displayed a viewport taken from
a 360 degree camera in a racing car. The user's head movements
controlled the viewport extracted by a cloud-based proxy. In both
cases, with 7 ms end-to-end base delay, the additional queuing delay
of roughly 1 ms was so low that it seemed the video was generated
locally.
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Using a swiping finger gesture or head movement to pan a video are
extremely latency-demanding actions--far more demanding than VoIP.
Because human vision can detect extremely low delays of the order of
single milliseconds when delay is translated into a visual lag
between a video and a reference point (the finger or the orientation
of the head sensed by the balance system in the inner ear (the
vestibular system).
Without the low queuing delay of L4S, cloud-based applications like
these would not be credible without significantly more access
bandwidth (to deliver all possible video that might be viewed) and
more local processing, which would increase the weight and power
consumption of head-mounted displays. When all interactive
processing can be done in the cloud, only the data to be rendered for
the end user needs to be sent.
Other low latency high bandwidth applications such as:
o Interactive remote presence;
o Video-assisted remote control of machinery or industrial
processes.
are not credible at all without very low queuing delay. No amount of
extra access bandwidth or local processing can make up for lost time.
6.2. Use Cases
The following use-cases for L4S are being considered by various
interested parties:
o Where the bottleneck is one of various types of access network:
DSL, cable, mobile, satellite
* Radio links (cellular, WiFi, satellite) that are distant from
the source are particularly challenging. The radio link
capacity can vary rapidly by orders of magnitude, so it is
often desirable to hold a buffer to utilise sudden increases of
capacity;
* cellular networks are further complicated by a perceived need
to buffer in order to make hand-overs imperceptible;
* Satellite networks generally have a very large base RTT, so
even with minimal queuing, overall delay can never be extremely
low;
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* Nonetheless, it is certainly desirable not to hold a buffer
purely because of the sawteeth of Classic TCP, when it is more
than is needed for all the above reasons.
o Private networks of heterogeneous data centres, where there is no
single administrator that can arrange for all the simultaneous
changes to senders, receivers and network needed to deploy DCTCP:
* a set of private data centres interconnected over a wide area
with separate administrations, but within the same company
* a set of data centres operated by separate companies
interconnected by a community of interest network (e.g. for the
finance sector)
* multi-tenant (cloud) data centres where tenants choose their
operating system stack (Infrastructure as a Service - IaaS)
o Different types of transport (or application) congestion control:
* elastic (TCP/SCTP);
* real-time (RTP, RMCAT);
* query (DNS/LDAP).
o Where low delay quality of service is required, but without
inspecting or intervening above the IP layer
[I-D.you-encrypted-traffic-management]:
* mobile and other networks have tended to inspect higher layers
in order to guess application QoS requirements. However, with
growing demand for support of privacy and encryption, L4S
offers an alternative. There is no need to select which
traffic to favour for queuing, when L4S gives favourable
queuing to all traffic.
o If queuing delay is minimized, applications with a fixed delay
budget can communicate over longer distances, or via a longer
chain of service functions [RFC7665] or onion routers.
6.3. Deployment Considerations
The DualQ is, in itself, an incremental deployment framework for L4S
AQMs so that L4S traffic can coexist with existing Classic "TCP-
friendly" traffic. Section 6.3.1 explains why only deploying a DualQ
AQM [I-D.ietf-tsvwg-aqm-dualq-coupled] in one node at each end of the
access link will realize nearly all the benefit of L4S.
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L4S involves both end systems and the network, so Section 6.3.2
suggests some typical sequences to deploy each part, and why there
will be an immediate and significant benefit after deploying just one
part.
If an ECN-enabled DualQ AQM has not been deployed at a bottleneck, an
L4S flow is required to include a fall-back strategy to Classic
behaviour. Section 6.3.3 describes how an L4S flow detects this, and
how to minimize the effect of false negative detection.
6.3.1. Deployment Topology
DualQ AQMs will not have to be deployed throughout the Internet
before L4S will work for anyone. Operators of public Internet access
networks typically design their networks so that the bottleneck will
nearly always occur at one known (logical) link. This confines the
cost of queue management technology to one place.
The case of mesh networks is different and will be discussed later.
But the known bottleneck case is generally true for Internet access
to all sorts of different 'sites', where the word 'site' includes
home networks, small-to-medium sized campus or enterprise networks
and even cellular devices (Figure 2). Also, this known-bottleneck
case tends to be true whatever the access link technology; whether
xDSL, cable, cellular, line-of-sight wireless or satellite.
Therefore, the full benefit of the L4S service should be available in
the downstream direction when the DualQ AQM is deployed at the
ingress to this bottleneck link (or links for multihomed sites). And
similarly, the full upstream service will be available once the DualQ
is deployed at the upstream ingress.
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______
( )
__ __ ( )
|DQ\________/DQ|( enterprise )
___ |__/ \__| ( /campus )
( ) (______)
( ) ___||_
+----+ ( ) __ __ / \
| DC |-----( Core )|DQ\_______________/DQ|| home |
+----+ ( ) |__/ \__||______|
(_____) __
|DQ\__/\ __ ,===.
|__/ \ ____/DQ||| ||mobile
\/ \__|||_||device
| o |
`---'
Figure 2: Likely location of DualQ (DQ) Deployments in common access
topologies
Deployment in mesh topologies depends on how over-booked the core is.
If the core is non-blocking, or at least generously provisioned so
that the edges are nearly always the bottlenecks, it would only be
necessary to deploy the DualQ AQM at the edge bottlenecks. For
example, some datacentre networks are designed with the bottleneck in
the hypervisor or host NICs, while others bottleneck at the top-of-
rack switch (both the output ports facing hosts and those facing the
core).
The DualQ would eventually also need to be deployed at any other
persistent bottlenecks such as network interconnections, e.g. some
public Internet exchange points and the ingress and egress to WAN
links interconnecting datacentres.
6.3.2. Deployment Sequences
For any one L4S flow to work, it requires 3 parts to have been
deployed. This was the same deployment problem that ECN faced
[I-D.iab-protocol-transitions] so we have learned from this.
Firstly, L4S deployment exploits the fact that DCTCP already exists
on many Internet hosts (Windows, FreeBSD and Linux); both servers and
clients. Therefore, just deploying DualQ AQM at a network bottleneck
immediately gives a working deployment of all the L4S parts. DCTCP
needs some safety concerns to be fixed for general use over the
public Internet (see Section 2.3 of [I-D.ietf-tsvwg-ecn-l4s-id]), but
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DCTCP is not on by default, so these issues can be managed within
controlled deployments or controlled trials.
Secondly, the performance improvement with L4S is so significant that
it enables new interactive services and products that were not
previously possible. It is much easier for companies to initiate new
work on deployment if there is budget for a new product trial. If,
in contrast, there were only an incremental performance improvement
(as with Classic ECN), spending on deployment tends to be much harder
to justify.
Thirdly, the L4S identifier is defined so that intially network
operators can enable L4S exclusively for certain customers or certain
applications. But this is carefully defined so that it does not
compromise future evolution towards L4S as an Internet-wide service.
This is because the L4S identifier is defined not only as the end-to-
end ECN field, but it can also optionally be combined with any other
packet header or some status of a customer or their access link
[I-D.ietf-tsvwg-ecn-l4s-id]. Operators could do this anyway, even if
it were not blessed by the IETF. However, it is best for the IETF to
specify that they must use their own local identifier in combination
with the IETF's identifier. Then, if an operator enables the
optional local-use approach, they only have to remove this extra rule
to make the service work Internet-wide - it will already traverse
middleboxes, peerings, etc.
+-+--------------------+----------------------+---------------------+
| | Servers or proxies | Access link | Clients |
+-+--------------------+----------------------+---------------------+
|1| DCTCP (existing) | | DCTCP (existing) |
| | | DualQ AQM downstream | |
| | WORKS DOWNSTREAM FOR CONTROLLED DEPLOYMENTS/TRIALS |
+-+--------------------+----------------------+---------------------+
|2| TCP Prague | | AccECN (already in |
| | | | progress:DCTCP/BBR) |
| | FULLY WORKS DOWNSTREAM |
+-+--------------------+----------------------+---------------------+
|3| | DualQ AQM upstream | TCP Prague |
| | | | |
| | FULLY WORKS UPSTREAM AND DOWNSTREAM |
+-+--------------------+----------------------+---------------------+
Figure 3: Example L4S Deployment Sequences
Figure 3 illustrates some example sequences in which the parts of L4S
might be deployed. It consists of the following stages:
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1. Here, the immediate benefit of a single AQM deployment can be
seen, but limited to a controlled trial or controlled deployment.
In this example downstream deployment is first, but in other
scenarios the upstream might be deployed first. If no AQM at all
was previously deployed for the downstream access, the DualQ AQM
greatly improves the Classic service (as well as adding the L4S
service). If an AQM was already deployed, the Classic service
will be unchanged (and L4S will still be added).
2. In this stage, the name 'TCP Prague' is used to represent a
variant of DCTCP that is safe to use in a production environment.
If the application is primarily unidirectional, 'TCP Prague' at
one end will provide all the benefit needed. Accurate ECN
feedback (AccECN) [I-D.ietf-tcpm-accurate-ecn] is needed at the
other end, but it is a generic ECN feedback facility that is
already planned to be deployed for other purposes, e.g. DCTCP,
BBR [BBR]. The two ends can be deployed in either order, because
TCP Prague only enables itself if it has negotiated the use of
AccECN feedback with the other end during the connection
handshake. Thus, deployment of TCP Prague on a server enables
L4S trials to move to a production service in one direction,
wherever AccECN is deployed at the other end. This stage might
be further motivated by performance improvements between DCTCP
and TCP Prague (see Appendix A.2 of [I-D.ietf-tsvwg-ecn-l4s-id]).
3. This is a two-move stage to enable L4S upstream. The DualQ or
TCP Prague can be deployed in either order as already explained.
To motivate the first of two independent moves, the deferred
benefit of enabling new services after the second move has to be
worth it to cover the first mover's investment risk. As
explained already, the potential for new interactive services
provides this motivation. The DualQ AQM also greatly improves
the upstream Classic service, assuming no other AQM has already
been deployed.
Note that other deployment sequences might occur. For instance: the
upstream might be deployed first; a non-TCP protocol might be used
end-to-end, e.g. QUIC, RMCAT; a body such as the 3GPP might require
L4S to be implemented in 5G user equipment, or other random acts of
kindness.
6.3.3. L4S Flow but Non-L4S Bottleneck
If L4S is enabled between two hosts but there is no L4S AQM at the
bottleneck, any drop from the bottleneck will trigger the L4S sender
to fall back to a classic ('TCP-Friendly') behaviour (see
Appendix A.1.3 of [I-D.ietf-tsvwg-ecn-l4s-id]).
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Unfortunately, as well as protecting legacy traffic, this rule
degrades the L4S service whenever there is a loss, even if the loss
was not from a non-DualQ bottleneck (false negative). And
unfortunately, prevalent drop can be due to other causes, e.g.:
o congestion loss at other transient bottlenecks, e.g. due to bursts
in shallower queues;
o transmission errors, e.g. due to electrical interference;
o rate policing.
Three complementary approaches are in progress to address this issue,
but they are all currently research:
o In TCP Prague, ignore certain losses deemed unlikely to be due to
congestion (using some ideas from BBR [BBR] but with no need to
ignore nearly all losses). This could mask any of the above types
of loss (requires consensus on how to safely interoperate with
drop-based congestion controls).
o A combination of RACK, reconfigured link retransmission and L4S
could address transmission errors (no reference yet);
o Hybrid ECN/drop policers (see Section 8.3).
L4S deployment scenarios that minimize these issues (e.g. over
wireline networks) can proceed in parallel to this research, in the
expectation that research success will continually widen L4S
applicability.
Classic ECN support is starting to materialize (in the upstream of
some home routers as of early 2017), so an L4S sender will have to
fall back to a classic ('TCP-Friendly') behaviour if it detects that
ECN marking is accompanied by greater queuing delay or greater delay
variation than would be expected with L4S (see Appendix A.1.4 of
[I-D.ietf-tsvwg-ecn-l4s-id]).
6.3.4. Other Potential Deployment Issues
An L4S AQM uses the ECN field to signal congestion. So, in common
with Classic ECN, if the AQM is within a tunnel or at a lower layer,
correct functioning of ECN signalling requires correct propagation of
the ECN field up the layers [I-D.ietf-tsvwg-ecn-encap-guidelines].
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7. IANA Considerations
This specification contains no IANA considerations.
8. Security Considerations
8.1. Traffic (Non-)Policing
Because the L4S service can serve all traffic that is using the
capacity of a link, it should not be necessary to police access to
the L4S service. In contrast, Diffserv only works if some packets
get less favourable treatment than others. So Diffserv has to use
traffic policers to limit how much traffic can be favoured, In turn,
traffic policers require traffic contracts between users and networks
as well as pairwise between networks. Because L4S will lack all this
management complexity, it is more likely to work end-to-end.
During early deployment (and perhaps always), some networks will not
offer the L4S service. These networks do not need to police or re-
mark L4S traffic - they just forward it unchanged as best efforts
traffic, as they already forward traffic with ECT(1) today. At a
bottleneck, such networks will introduce some queuing and dropping.
When a scalable congestion control detects a drop it will have to
respond as if it is a Classic congestion control (as required in
Section 2.3 of [I-D.ietf-tsvwg-ecn-l4s-id]). This will ensure safe
interworking with other traffic at the 'legacy' bottleneck, but it
will degrade the L4S service to no better (but never worse) than
classic best efforts, whenever a legacy (non-L4S) bottleneck is
encountered on a path.
Certain network operators might choose to restrict access to the L4S
class, perhaps only to customers who have paid a premium. Their
packet classifier (item 2 in Figure 1) could identify such customers
against some other field (e.g. source address range) as well as ECN.
If only the ECN L4S identifier matched, but not the source address
(say), the classifier could direct these packets (from non-paying
customers) into the Classic queue. Allowing operators to use an
additional local classifier is intended to remove any incentive to
bleach the L4S identifier. Then at least the L4S ECN identifier will
be more likely to survive end-to-end even though the service may not
be supported at every hop. Such arrangements would only require
simple registered/not-registered packet classification, rather than
the managed application-specific traffic policing against customer-
specific traffic contracts that Diffserv requires.
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8.2. 'Latency Friendliness'
The L4S service does rely on self-constraint - not in terms of
limiting capacity usage, but in terms of limiting burstiness. It is
hoped that standardisation of dynamic behaviour (cf. TCP slow-start)
and self-interest will be sufficient to prevent transports from
sending excessive bursts of L4S traffic, given the application's own
latency will suffer most from such behaviour.
Whether burst policing becomes necessary remains to be seen. Without
it, there will be potential for attacks on the low latency of the L4S
service. However it may only be necessary to apply such policing
reactively, e.g. punitively targeted at any deployments of new bursty
malware.
8.3. Policing Prioritized L4S Bandwidth
As mentioned in Section 5.2, L4S should remove the need for low
latency Diffserv classes. However, those Diffserv classes that give
certain applications or users priority over capacity, would still be
applicable. Then, within such Diffserv classes, L4S would often be
applicable to give traffic low latency and low loss. WIthin such a
class, the bandwidth available to a user or application is often
limited by a rate policer. Similarly, in the default Diffserv class,
rate policers are used to partition shared capacity.
A classic rate policer drops any packets exceeding a set rate,
usually also giving a burst allowance (variants exist where the
policer re-marks non-compliant traffic to a discard-eligible Diffserv
codepoint, so they may be dropped elsewhere during contention). In
networks that deploy L4S and use rate policers, it will be preferable
to deploy a policer designed to be more friendly to the L4S service,
This is currently a research area. It might be achieved by setting a
threshold where ECN marking is introduced, such that it is just under
the policed rate or just under the burst allowance where drop is
introduced. This could be applied to various types of policer, e.g.
[RFC2697], [RFC2698] or the 'local' (non-ConEx) variant of the ConEx
congestion policer [I-D.briscoe-conex-policing]. Otherwise, whenever
L4S traffic encounters a rate policer, it will experience drops and
the source will fall back to a Classic congestion control, thus
losing the benefits of L4S.
Further discussion of the applicability of L4S to the various
Diffserv classes, and the design of suitable L4S rate policers will
require a separate dedicated document.
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8.4. ECN Integrity
Receiving hosts can fool a sender into downloading faster by
suppressing feedback of ECN marks (or of losses if retransmissions
are not necessary or available otherwise). Various ways to protect
TCP feedback integrity have been developed. For instance:
o The sender can test the integrity of the receiver's feedback by
occasionally setting the IP-ECN field to the congestion
experienced (CE) codepoint, which is normally only set by a
congested link. Then the sender can test whether the receiver's
feedback faithfully reports what it expects
[I-D.moncaster-tcpm-rcv-cheat].
o A network can enforce a congestion response to its ECN markings
(or packet losses) by auditing congestion exposure (ConEx)
[RFC7713].
o The TCP authentication option (TCP-AO [RFC5925]) can be used to
detect tampering with TCP congestion feedback.
o The ECN Nonce [RFC3540] was proposed to detect tampering with
congestion feedback, but it is being reclassified as historic.
Appendix C.1 of [I-D.ietf-tsvwg-ecn-l4s-id] gives more details of
these techniques including their applicability and pros and cons.
9. Acknowledgements
Thanks to Wes Eddy, Karen Nielsen and David Black for their useful
review comments.
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,
<http://www.rfc-editor.org/info/rfc2119>.
10.2. Informative References
[BBR] Cardwell, N., Cheng, Y., Gunn, C., Yeganeh, S., and V.
Jacobson, "BBR: Congestion-Based Congestion Control;
Measuring bottleneck bandwidth and round-trip propagation
time", ACM Queue (14)5, December 2016.
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[DCttH15] De Schepper, K., Bondarenko, O., Tsang, I., and B.
Briscoe, "'Data Centre to the Home': Ultra-Low Latency for
All", 2015, <http://www.bobbriscoe.net/projects/latency/
dctth_preprint.pdf>.
(Under submission)
[Hohlfeld14]
Hohlfeld , O., Pujol, E., Ciucu, F., Feldmann, A., and P.
Barford, "A QoE Perspective on Sizing Network Buffers",
Proc. ACM Internet Measurement Conf (IMC'14) hmm, November
2014.
[I-D.bagnulo-tcpm-generalized-ecn]
Bagnulo, M. and B. Briscoe, "Adding Explicit Congestion
Notification (ECN) to TCP control packets and TCP
retransmissions", draft-bagnulo-tcpm-generalized-ecn-03
(work in progress), April 2017.
[I-D.briscoe-conex-policing]
Briscoe, B., "Network Performance Isolation using
Congestion Policing", draft-briscoe-conex-policing-01
(work in progress), February 2014.
[I-D.iab-protocol-transitions]
Thaler, D., "Planning for Protocol Adoption and Subsequent
Transitions", draft-iab-protocol-transitions-08 (work in
progress), March 2017.
[I-D.ietf-aqm-fq-codel]
Hoeiland-Joergensen, T., McKenney, P.,
dave.taht@gmail.com, d., Gettys, J., and E. Dumazet, "The
FlowQueue-CoDel Packet Scheduler and Active Queue
Management Algorithm", draft-ietf-aqm-fq-codel-06 (work in
progress), March 2016.
[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
ecn-02 (work in progress), October 2016.
[I-D.ietf-tcpm-alternativebackoff-ecn]
Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", draft-ietf-tcpm-
alternativebackoff-ecn-01 (work in progress), May 2017.
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[I-D.ietf-tcpm-cubic]
Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
draft-ietf-tcpm-cubic-04 (work in progress), February
2017.
[I-D.ietf-tcpm-dctcp]
Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion
Control for Datacenters", draft-ietf-tcpm-dctcp-05 (work
in progress), March 2017.
[I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K., Briscoe, B., Bondarenko, O., and I. Tsang,
"DualQ Coupled AQM for Low Latency, Low Loss and Scalable
Throughput", draft-ietf-tsvwg-aqm-dualq-coupled-00 (work
in progress), April 2017.
[I-D.ietf-tsvwg-ecn-encap-guidelines]
Briscoe, B., Kaippallimalil, J., and P. Thaler,
"Guidelines for Adding Congestion Notification to
Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn-
encap-guidelines-08 (work in progress), March 2017.
[I-D.ietf-tsvwg-ecn-experimentation]
Black, D., "Explicit Congestion Notification (ECN)
Experimentation", draft-ietf-tsvwg-ecn-experimentation-02
(work in progress), April 2017.
[I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K., Briscoe, B., and I. Tsang, "Identifying
Modified Explicit Congestion Notification (ECN) Semantics
for Ultra-Low Queuing Delay", draft-ietf-tsvwg-ecn-l4s-
id-00 (work in progress), April 2017.
[I-D.johansson-quic-ecn]
Johansson, I., "ECN support in QUIC", draft-johansson-
quic-ecn-02 (work in progress), April 2017.
[I-D.moncaster-tcpm-rcv-cheat]
Moncaster, T., Briscoe, B., and A. Jacquet, "A TCP Test to
Allow Senders to Identify Receiver Non-Compliance", draft-
moncaster-tcpm-rcv-cheat-03 (work in progress), July 2014.
[I-D.stewart-tsvwg-sctpecn]
Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", draft-stewart-
tsvwg-sctpecn-05 (work in progress), January 2014.
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[I-D.you-encrypted-traffic-management]
You, J. and C. Xiong, "The Effect of Encrypted Traffic on
the QoS Mechanisms in Cellular Networks", draft-you-
encrypted-traffic-management-00 (work in progress),
October 2015.
[L4Sdemo16]
Bondarenko, O., De Schepper, K., Tsang, I., and B.
Briscoe, "Ultra-Low Delay for All: Live Experience, Live
Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016,
<http://dl.acm.org/citation.cfm?doid=2910017.2910633
(videos of demos: https://riteproject.eu/
dctth/#1511dispatchwg )>.
[Mathis09]
Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
May 2009, <http://www.hpcc.jp/pfldnet2009/
Program_files/1569198525.pdf>.
[NewCC_Proc]
Eggert, L., "Experimental Specification of New Congestion
Control Algorithms", IETF Operational Note ion-tsv-alt-cc,
July 2007.
[PI2] De Schepper, K., Bondarenko, O., Tsang, I., and B.
Briscoe, "PI^2 : A Linearized AQM for both Classic and
Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December
2016,
<http://dl.acm.org/citation.cfm?doid=2999572.2999578>.
[RFC2697] Heinanen, J. and R. Guerin, "A Single Rate Three Color
Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999,
<http://www.rfc-editor.org/info/rfc2697>.
[RFC2698] Heinanen, J. and R. Guerin, "A Two Rate Three Color
Marker", RFC 2698, DOI 10.17487/RFC2698, September 1999,
<http://www.rfc-editor.org/info/rfc2698>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<http://www.rfc-editor.org/info/rfc3168>.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<http://www.rfc-editor.org/info/rfc3246>.
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[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, DOI 10.17487/RFC3540, June 2003,
<http://www.rfc-editor.org/info/rfc3540>.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC3649, December 2003,
<http://www.rfc-editor.org/info/rfc3649>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, DOI 10.17487/RFC4774, November 2006,
<http://www.rfc-editor.org/info/rfc4774>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<http://www.rfc-editor.org/info/rfc4960>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<http://www.rfc-editor.org/info/rfc5681>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <http://www.rfc-editor.org/info/rfc5925>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <http://www.rfc-editor.org/info/rfc6679>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
[RFC7560] Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
"Problem Statement and Requirements for Increased Accuracy
in Explicit Congestion Notification (ECN) Feedback",
RFC 7560, DOI 10.17487/RFC7560, August 2015,
<http://www.rfc-editor.org/info/rfc7560>.
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[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<http://www.rfc-editor.org/info/rfc7665>.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC7713, December 2015,
<http://www.rfc-editor.org/info/rfc7713>.
[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<http://www.rfc-editor.org/info/rfc8033>.
[TCP-sub-mss-w]
Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion
Window for Small Round Trip Times", BT Technical Report
TR-TUB8-2015-002, May 2015,
<http://www.bobbriscoe.net/projects/latency/
sub-mss-w.pdf>.
Appendix A. Standardization items
The following table includes all the items that will need to be
standardized to provide a full L4S architecture.
The table is too wide for the ASCII draft format, so it has been
split into two, with a common column of row index numbers on the
left.
The columns in the second part of the table have the following
meanings:
WG: The IETF WG most relevant to this requirement. The "tcpm/iccrg"
combination refers to the procedure typically used for congestion
control changes, where tcpm owns the approval decision, but uses
the iccrg for expert review [NewCC_Proc];
TCP: Applicable to all forms of TCP congestion control;
DCTCP: Applicable to Data Centre TCP as currently used (in
controlled environments);
DCTCP bis: Applicable to an future Data Centre TCP congestion
control intended for controlled environments;
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XXX Prague: Applicable to a Scalable variant of XXX (TCP/SCTP/RMCAT)
congestion control.
+-----+------------------------+------------------------------------+
| Req | Requirement | Reference |
| # | | |
+-----+------------------------+------------------------------------+
| 0 | ARCHITECTURE | |
| 1 | L4S IDENTIFIER | [I-D.ietf-tsvwg-ecn-l4s-id] |
| 2 | DUAL QUEUE AQM | [I-D.ietf-tsvwg-aqm-dualq-coupled] |
| 3 | Suitable ECN Feedback | [I-D.ietf-tcpm-accurate-ecn], |
| | | [I-D.stewart-tsvwg-sctpecn]. |
| | | |
| | SCALABLE TRANSPORT - | |
| | SAFETY ADDITIONS | |
| 4-1 | Fall back to | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3, |
| | Reno/Cubic on loss | [I-D.ietf-tcpm-dctcp] |
| 4-2 | Fall back to | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3 |
| | Reno/Cubic if classic | |
| | ECN bottleneck | |
| | detected | |
| | | |
| 4-3 | Reduce RTT-dependence | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3 |
| | | |
| 4-4 | Scaling TCP's | [I-D.ietf-tsvwg-ecn-l4s-id] S.2.3, |
| | Congestion Window for | [TCP-sub-mss-w] |
| | Small Round Trip Times | |
| | SCALABLE TRANSPORT - | |
| | PERFORMANCE | |
| | ENHANCEMENTS | |
| 5-1 | Setting ECT in TCP | [I-D.bagnulo-tcpm-generalized-ecn] |
| | Control Packets and | |
| | Retransmissions | |
| 5-2 | Faster-than-additive | [I-D.ietf-tsvwg-ecn-l4s-id] (Appx |
| | increase | A.2.2) |
| 5-3 | Faster Convergence at | [I-D.ietf-tsvwg-ecn-l4s-id] (Appx |
| | Flow Start | A.2.2) |
+-----+------------------------+------------------------------------+
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+-----+--------+-----+-------+-----------+--------+--------+--------+
| # | WG | TCP | DCTCP | DCTCP-bis | TCP | SCTP | RMCAT |
| | | | | | Prague | Prague | Prague |
+-----+--------+-----+-------+-----------+--------+--------+--------+
| 0 | tsvwg | Y | Y | Y | Y | Y | Y |
| 1 | tsvwg | | | Y | Y | Y | Y |
| 2 | tsvwg | n/a | n/a | n/a | n/a | n/a | n/a |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| 3 | tcpm | Y | Y | Y | Y | n/a | n/a |
| | | | | | | | |
| 4-1 | tcpm | | Y | Y | Y | Y | Y |
| | | | | | | | |
| 4-2 | tcpm/ | | | | Y | Y | ? |
| | iccrg? | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| 4-3 | tcpm/ | | | Y | Y | Y | ? |
| | iccrg? | | | | | | |
| 4-4 | tcpm | Y | Y | Y | Y | Y | ? |
| | | | | | | | |
| | | | | | | | |
| 5-1 | tcpm | Y | Y | Y | Y | n/a | n/a |
| | | | | | | | |
| 5-2 | tcpm/ | | | Y | Y | Y | ? |
| | iccrg? | | | | | | |
| 5-3 | tcpm/ | | | Y | Y | Y | ? |
| | iccrg? | | | | | | |
+-----+--------+-----+-------+-----------+--------+--------+--------+
Authors' Addresses
Bob Briscoe (editor)
Simula Research Lab
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
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Koen De Schepper
Nokia Bell Labs
Antwerp
Belgium
Email: koen.de_schepper@nokia.com
URI: https://www.bell-labs.com/usr/koen.de_schepper
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
Spain
Phone: 34 91 6249500
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
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