Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture
draft-ietf-tsvwg-l4s-arch-04
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.
Expired & archived
|
|
|---|---|---|---|
| Authors | Bob Briscoe , Koen De Schepper , Marcelo Bagnulo , Greg White | ||
| Last updated | 2020-01-09 (Latest revision 2019-07-08) | ||
| Replaces | draft-briscoe-tsvwg-l4s-arch | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
ARTART Last Call review
(of
-18)
by Marco Tiloca
Ready w/nits
|
||
| 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-04
Transport Area Working Group B. Briscoe, Ed.
Internet-Draft CableLabs
Intended status: Informational K. De Schepper
Expires: January 9, 2020 Nokia Bell Labs
M. Bagnulo Braun
Universidad Carlos III de Madrid
G. White
CableLabs
July 8, 2019
Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture
draft-ietf-tsvwg-l4s-arch-04
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.
Briscoe, et al. Expires January 9, 2020 [Page 1]
Internet-Draft L4S Architecture July 2019
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
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 January 9, 2020.
Copyright Notice
Copyright (c) 2019 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
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Why These Primary Components? . . . . . . . . . . . . . . 10
5.2. Why Not Alternative Approaches? . . . . . . . . . . . . . 12
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Applications . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 16
6.3. Deployment Considerations . . . . . . . . . . . . . . . . 17
6.3.1. Deployment Topology . . . . . . . . . . . . . . . . . 18
6.3.2. Deployment Sequences . . . . . . . . . . . . . . . . 19
6.3.3. L4S Flow but Non-L4S Bottleneck . . . . . . . . . . . 21
Briscoe, et al. Expires January 9, 2020 [Page 2]
Internet-Draft L4S Architecture July 2019
6.3.4. Other Potential Deployment Issues . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8.1. Traffic (Non-)Policing . . . . . . . . . . . . . . . . . 23
8.2. 'Latency Friendliness' . . . . . . . . . . . . . . . . . 24
8.3. Interaction between Rate Policing and L4S . . . . . . . . 24
8.4. ECN Integrity . . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Standardization items . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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 (EF
[RFC3246]) for some packets at the expense of others, but this is not
sufficient 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
Briscoe, et al. Expires January 9, 2020 [Page 3]
Internet-Draft L4S Architecture July 2019
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
[RFC8312]). 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 to 2 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 [RFC8290] or PIE [RFC8033] and about 20-30 ms
at the 99th percentile. 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 network
node should give nearly all the benefit. The L4S approach also
requires component mechanisms at the endpoints 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: L4S traffic needs to be isolated from the queuing
latency of Classic 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 might use each
Briscoe, et al. Expires January 9, 2020 [Page 4]
Internet-Draft L4S Architecture July 2019
service and capacity in access networks is too scarce to partition
into two. The Dual Queue Coupled AQM
[I-D.ietf-tsvwg-aqm-dualq-coupled] was developed as a minimal
complexity solution to this problem. The two queues appear to be
separated by a 'semi-permeable' membrane that partitions latency
but not bandwidth (explained later).
Per-flow queuing such as in [RFC8290] could be used (see
Section 4), 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 large number 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) and CE codepoints
of the ECN field represent a workable solution.
3) Host: Scalable congestion controls already exist. They solve the
scaling problem with TCP that was first pointed out in [RFC3649].
The one used most widely (in controlled environments) is Data
Center TCP (DCTCP [RFC8257]), 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 (QUIC, SCTP, RTP/RTCP, RMCAT, etc.) Indeed, between the
present document being drafted and published, the following
scalable congestion controls were implemented: TCP Prague
[PragueLinux], QUIC Prague and an L4S variant of the RMCAT SCReAM
controller [RFC8298].
Briscoe, et al. Expires January 9, 2020 [Page 5]
Internet-Draft L4S Architecture July 2019
(2) (1)
.-------^------. .--------------^-------------------.
,-(3)-----. ______
; ________ : L4S --------. | |
:|Scalable| : _\ ||___\_| mark |
:| sender | : __________ / / || / |______|\ _________
:|________|\; | |/ --------' ^ \1|condit'nl|
`---------'\_| 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
Center 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,
Briscoe, et al. Expires January 9, 2020 [Page 6]
Internet-Draft L4S Architecture July 2019
DCTCP averages 2 congestion signals per round-trip whatever the
flow rate.
Classic Congestion Control: A congestion control with a flow rate
that can co-exist with standard TCP Reno [RFC5681] without
starvation. 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 identifier changes
(an unassignment and an assignment) and optional further identifiers:
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. '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 a
different meaning for ECN:
Briscoe, et al. Expires January 9, 2020 [Page 7]
Internet-Draft L4S Architecture July 2019
* much more frequent signals--too often to use drops;
* immediately tracking every fluctuation of the queue--too soon
to commit to dropping packets.
So the standards track [RFC3168] has had to be updated to allow
L4S packets to depart from the 'same as drop' constraint.
[RFC8311] is 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. [RFC8311] also reclassifies the original experimental
assignment of the ECT(1) codepoint as an ECN nonce [RFC3540] as
historic.
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].
The CE codepoint is used to indicate Congestion Experienced by
both L4S and Classic treatments. This raises the concern that a
Classic AQM earlier on the path might have marked some ECT(0)
packets as CE. Then these packets will be erroneously classified
into the L4S queue. [I-D.ietf-tsvwg-ecn-l4s-id] explains why 5
unlikely eventualities all have to coincide for this to have any
detrimental effect, which even then would only involve a
vanishingly small likelihood of a spurious retransmission.
c. A network operator might wish to include certain unresponsive,
non-L4S traffic in the L4S queue if it is deemed to be smoothly
enough paced and low enough rate not to build a queue. For
instance, VoIP, low rate datagrams to sync online games,
relatively low rate application-limited traffic, DNS, LDAP, etc.
This traffic would need to be tagged with specific identifiers,
e.g. a low latency Diffserv Codepoint such as Expedited
Forwarding (EF [RFC3246]), Non-Queue-Building (NQB
[I-D.white-tsvwg-nqb]), or operator-specific identifiers.
Network components: The L4S architecture encompasses either dual-
queue or per-flow queue solutions:
a. 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. Informational appendices of the draft are provided for
pseudocode examples of different possible AQM approaches. The
aim is for designers to be free to implement diverse ideas. So
the brief normative body of the draft only specifies the minimum
Briscoe, et al. Expires January 9, 2020 [Page 8]
Internet-Draft L4S Architecture July 2019
constraints an AQM needs to comply with to ensure that the L4S
and Classic services will coexist. The core idea is the tension
between the scheduler's prioritization of L4S over Classic and
the coupling from the Classic to the L4S AQM. The L4S AQM
derives its level of ECN marking from the maximum of the
congestion levels in both queues. So L4S flows leave enough
space between their packets for Classic flows, as if they were
all the same type of TCP, all sharing one FIFO queue.
Initially a zero-config variant of RED called Curvy RED was
implemented, tested and documented. Then, a variant of PIE
called DualPI2 (pronounced Dual PI Squared) [PI2] was implemented
and found to perform better than Curvy RED over a wide range of
conditions, so it was documented in another appendix of
[I-D.ietf-tsvwg-aqm-dualq-coupled].
b. A scheduler with per-flow queues can be used for L4S. It would
be simple to modify an existing design such as FQ-CoDel or FQ-
PIE, although this has not been implemented and evaluated because
the goal of the original proponents of L4S was to avoid per-flow
scheduling.
The idea would be to implement two AQMs (Classic and Scalable)
and switch each per-flow queue to use an instance of the
appropriate AQM for the flow, based on the ECN codepoints of the
packets. Flows of non-ECN or ECT(0) packets would use a Classic
AQM such as CoDel or PIE, while flows of ECT(1) packets without
any ECT(0) packets would use a simple shallow threshold AQM with
immediate (unsmoothed) marking. The FQ scheduler might work as
is, because it is likely that L4S flows would be continually
categorized as 'new' flows. However, this presumption has not
been tested under a wide range of conditions. A variant of FQ-
CoDel already exists that adapts to a shallower threshold AQM for
ECN-capable packets.
Host mechanisms: The L4S architecture includes a number of mechanisms
in the end host that we enumerate next:
a. Data Center TCP is the most widely used example of a scalable
congestion control. It has been documented as an informational
record of the protocol currently in use [RFC8257]. 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.
Briscoe, et al. Expires January 9, 2020 [Page 9]
Internet-Draft L4S Architecture July 2019
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
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.ietf-quic-transport] 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 and implement more accurate
ECN feedback for TCP (AccECN) is in progress
[I-D.ietf-tcpm-accurate-ecn], [PragueLinux].
* 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
Briscoe, et al. Expires January 9, 2020 [Page 10]
Internet-Draft L4S Architecture July 2019
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.
* Explicit congestion signals can be sent immediately to track
fluctuations of the queue. L4S shifts smoothing from the
network (which doesn't know the round trip times of all the
flows) to the host (which knows its own round trip time).
Previously, the network had to smooth to keep a worst-case
round trip stable, delaying congestion signals by 100-200ms.
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 was 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.
Briscoe, et al. Expires January 9, 2020 [Page 11]
Internet-Draft L4S Architecture July 2019
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.
Low loss: Latency is not the only concern of L4S. The 'Low Loss"
part of the name denotes that L4S generally achieves zero
congestion loss due to its use of ECN. Otherwise, loss would
itself cause delay, particularly for short flows, due to
retransmission delay [RFC2884].
Scalable throughput: The "Scalable throughput" part of the name
denotes that the per-flow throughput of scalable congestion
controls should scale indefinitely, avoiding the imminent scaling
problems with TCP-Friendly congestion control algorithms
[RFC3649]. It was known when TCP was first developed that it
would not scale to high bandwidth-delay products (see footnote 6
in [TCP-CA]). Today, regular broadband bit-rates over WAN
distances are already beyond the scaling range of `classic' TCP
Reno. So `less unscalable' Cubic [RFC8312] and
Compound [I-D.sridharan-tcpm-ctcp] variants of TCP have been
successfully deployed. However, these are now approaching their
scaling limits. For instance, at 800Mb/s with a 20ms round trip,
Cubic induces a congestion signal only every 500 round trips or 10
seconds, which makes its dynamic control very sloppy. In contrast
on average a scalable congestion control like DCTCP or TCP Prague
induces 2 congestion signals per round trip, which remains
invariant for any flow rate, keeping dynamic control very tight.
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) - see [I-D.briscoe-tsvwg-l4s-diffserv].
Nonetheless, if there are Diffserv classes for important traffic,
Briscoe, et al. Expires January 9, 2020 [Page 12]
Internet-Draft L4S Architecture July 2019
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;
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 existing FQ implementations still needs to have
support for scalable congestion control added.
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
Briscoe, et al. Expires January 9, 2020 [Page 13]
Internet-Draft L4S Architecture July 2019
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 [RFC8511] alters the host behaviour in response to ECN marking
to utilize a link better and give ECN flows faster throughput. It
uses ECT(0) and assumes the network still treats ECN and drop the
same. Therefore ABE exploits any lower queuing delay that AQMs
can provide. But as explained above, AQMs still cannot reduce
queuing delay too far without losing link utilization (to allow
for other, non-ABE, flows).
BBRv1: v1 of Bottleneck Bandwidth and Round-trip propagation time
(BBR [I-D.cardwell-iccrg-bbr-congestion-control]) controls queuing
delay end-to-end without needing any special logic in the network,
such as an AQM - so it works pretty-much on any path. Setting
some problems with capacity sharing aside, queuing delay is good
with BBRv1, but perhaps not quite as low as with state-of-the-art
AQMs such as PIE or fq_CoDel, and certainly nowhere near as low as
with L4S. Queuing delay is also not consistently low, due to its
regular bandwidth probes and the aggressive flow start-up phase.
L4S is a complement to BBRv1. Indeed BBRv2 (not released at the
time of writing) is likely to use L4S ECN and a TCP-Prague-like
Briscoe, et al. Expires January 9, 2020 [Page 14]
Internet-Draft L4S Architecture July 2019
behaviour if it discovers a compatible path. Otherwise it will
use an evolution of BBRv1.
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:
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.
Briscoe, et al. Expires January 9, 2020 [Page 15]
Internet-Draft L4S Architecture July 2019
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;
Briscoe, et al. Expires January 9, 2020 [Page 16]
Internet-Draft L4S Architecture July 2019
* 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.smith-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.
Briscoe, et al. Expires January 9, 2020 [Page 17]
Internet-Draft L4S Architecture July 2019
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 applicable 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.
Briscoe, et al. Expires January 9, 2020 [Page 18]
Internet-Draft L4S Architecture July 2019
______
( )
__ __ ( )
|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 data-centre 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 data-centres.
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
[RFC8170] 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
Briscoe, et al. Expires January 9, 2020 [Page 19]
Internet-Draft L4S Architecture July 2019
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 initially 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:
Briscoe, et al. Expires January 9, 2020 [Page 20]
Internet-Draft L4S Architecture July 2019
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 [I-D.cardwell-iccrg-bbr-congestion-control]. 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 the
performance improvements of TCP Prague relative to DCTCP (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]).
Briscoe, et al. Expires January 9, 2020 [Page 21]
Internet-Draft L4S Architecture July 2019
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
[I-D.cardwell-iccrg-bbr-congestion-control] 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 [UnorderedLTE],
[I-D.ietf-tsvwg-ecn-l4s-id];
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 could continually widen L4S
applicability.
Classic ECN support is starting to materialize on the Internet as an
increased level of CE marking. Given some of this Classic ECN might
be due to single-queue ECN deployment, 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]). It is hard to detect whether this is
all due to the addition of support for ECN in the Linux
implementation of FQ-CoDel, which would not require fall-back to
Classic behaviour, because FQ inherently forces the throughput of
each flow to be equal irrespective of its aggressiveness.
Briscoe, et al. Expires January 9, 2020 [Page 22]
Internet-Draft L4S Architecture July 2019
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 [RFC6040],
[I-D.ietf-tsvwg-ecn-encap-guidelines].
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 selected premium customers as a value-added
service. 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-premium customers) into the Classic queue. Clearly explaining
how operators can use an additional local classifiers (see
[I-D.ietf-tsvwg-ecn-l4s-id]) is intended to remove any tendency to
bleach the L4S identifier. Then at least the L4S ECN identifier will
Briscoe, et al. Expires January 9, 2020 [Page 23]
Internet-Draft L4S Architecture July 2019
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 uses.
8.2. 'Latency Friendliness'
The L4S service does rely on self-constraint - not in terms of
limiting rate, but in terms of limiting latency (burstiness). It is
hoped that self-interest and standardisation of dynamic behaviour
(cf. TCP slow-start) 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.
A per-flow (5-tuple) queue protection function
[I-D.briscoe-docsis-q-protection] has been developed for the low
latency queue in DOCSIS, which has adopted the DualQ L4S
architecture. It protects the low latency service from any queue-
building flows that accidentally or maliciously classify themselves
into the low latency queue. It is designed to score flows based
solely on their contribution to queuing (not flow rate in itself).
Then, if the shared low latency queue is at risk of exceeding a
threshold, the function redirects enough packets of the highest
scoring flow(s) into the Classic queue to preserve low latency.
Such a queue protection function is not considered a necessary part
of the L4S architecture, which works without it (in a similar way to
how the Internet works without per-flow rate policing). Indeed,
under normal circumstances, DOCSIS queue protection does not
intervene, and if operators find it is not necessary they can disable
it. Part of the L4S experiment will be to see whether such a
function is necessary.
8.3. Interaction between Rate Policing and L4S
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 in certain scenarios (e.g. corporate networks). Then,
within such Diffserv classes, L4S would often be applicable to give
traffic low latency and low loss as well. Within such a Diffserv
Briscoe, et al. Expires January 9, 2020 [Page 24]
Internet-Draft L4S Architecture July 2019
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).
Whenever L4S traffic encounters one of these rate policers, it will
experience drops and the source has to fall back to a Classic
congestion control, thus losing the benefits of L4S. So, in networks
that already use rate policers and plan to deploy L4S, it will be
preferable to redesign these rate policers 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]. It might also be
possible to design scalable congestion controls to respond less
catastrophically to loss that has not been preceded by a period of
increasing delay.
The design of L4S-friendly rate policers will require a separate
dedicated document. For further discussion of the interaction
between L4S and Diffserv, see [I-D.briscoe-tsvwg-l4s-diffserv].
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 (see 2nd para of
Section 20.2 of [RFC3168]).
o A network can enforce a congestion response to its ECN markings
(or packet losses) by auditing congestion exposure (ConEx)
[RFC7713].
Briscoe, et al. Expires January 9, 2020 [Page 25]
Internet-Draft L4S Architecture July 2019
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 has been reclassified as historic
[RFC8311].
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 Richard Scheffenegger, Wes Eddy, Karen Nielsen and David
Black for their useful review comments.
Bob Briscoe and Koen De Schepper were part-funded by the European
Community under its Seventh Framework Programme through the Reducing
Internet Transport Latency (RITE) project (ICT-317700). Bob Briscoe
was also part-funded by the Research Council of Norway through the
TimeIn project. The views expressed here are solely those of the
authors.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
10.2. Informative References
[DCttH15] De Schepper, K., Bondarenko, O., Briscoe, B., and I.
Tsang, "`Data Centre to the Home': Ultra-Low Latency for
All", RITE project Technical Report , 2015,
<http://riteproject.eu/publications/>.
[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.briscoe-conex-policing]
Briscoe, B., "Network Performance Isolation using
Congestion Policing", draft-briscoe-conex-policing-01
(work in progress), February 2014.
Briscoe, et al. Expires January 9, 2020 [Page 26]
Internet-Draft L4S Architecture July 2019
[I-D.briscoe-docsis-q-protection]
Briscoe, B. and G. White, "Queue Protection to Preserve
Low Latency", draft-briscoe-docsis-q-protection-00 (work
in progress), July 2019.
[I-D.briscoe-tsvwg-l4s-diffserv]
Briscoe, B., "Interactions between Low Latency, Low Loss,
Scalable Throughput (L4S) and Differentiated Services",
draft-briscoe-tsvwg-l4s-diffserv-02 (work in progress),
November 2018.
[I-D.cardwell-iccrg-bbr-congestion-control]
Cardwell, N., Cheng, Y., Yeganeh, S., and V. Jacobson,
"BBR Congestion Control", draft-cardwell-iccrg-bbr-
congestion-control-00 (work in progress), July 2017.
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-20 (work
in progress), April 2019.
[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
ecn-08 (work in progress), March 2019.
[I-D.ietf-tcpm-generalized-ecn]
Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
draft-ietf-tcpm-generalized-ecn-03 (work in progress),
October 2018.
[I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
AQMs for Low Latency, Low Loss and Scalable Throughput
(L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-09 (work in
progress), July 2019.
[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-13 (work in progress), May 2019.
Briscoe, et al. Expires January 9, 2020 [Page 27]
Internet-Draft L4S Architecture July 2019
[I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. and B. Briscoe, "Identifying Modified
Explicit Congestion Notification (ECN) Semantics for
Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s-
id-06 (work in progress), March 2019.
[I-D.smith-encrypted-traffic-management]
Smith, K., "Network management of encrypted traffic",
draft-smith-encrypted-traffic-management-05 (work in
progress), May 2016.
[I-D.sridharan-tcpm-ctcp]
Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
"Compound TCP: A New TCP Congestion Control for High-Speed
and Long Distance Networks", draft-sridharan-tcpm-ctcp-02
(work in progress), November 2008.
[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.
[I-D.white-tsvwg-nqb]
White, G. and T. Fossati, "Identifying and Handling Non
Queue Building Flows in a Bottleneck Link", draft-white-
tsvwg-nqb-02 (work in progress), June 2019.
[L4Sdemo16]
Bondarenko, O., De Schepper, K., Tsang, I., and B.
Briscoe, "orderedUltra-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, <https://www.gdt.id.au/~gdt/
presentations/2010-07-06-questnet-tcp/reference-
materials/papers/
mathis-relentless-congestion-control.pdf>.
[NewCC_Proc]
Eggert, L., "Experimental Specification of New Congestion
Control Algorithms", IETF Operational Note ion-tsv-alt-cc,
July 2007.
Briscoe, et al. Expires January 9, 2020 [Page 28]
Internet-Draft L4S Architecture July 2019
[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>.
[PragueLinux]
Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
Tilmans, O., Kuehlewind, M., and A. Ahmed, "Implementing
the `TCP Prague' Requirements for Low Latency Low Loss
Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,
March 2019, <https://www.netdevconf.org/0x13/
session.html?talk-tcp-prague-l4s>.
[RFC2697] Heinanen, J. and R. Guerin, "A Single Rate Three Color
Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999,
<https://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,
<https://www.rfc-editor.org/info/rfc2698>.
[RFC2884] Hadi Salim, J. and U. Ahmed, "Performance Evaluation of
Explicit Congestion Notification (ECN) in IP Networks",
RFC 2884, DOI 10.17487/RFC2884, July 2000,
<https://www.rfc-editor.org/info/rfc2884>.
[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,
<https://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,
<https://www.rfc-editor.org/info/rfc3246>.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, DOI 10.17487/RFC3540, June 2003,
<https://www.rfc-editor.org/info/rfc3540>.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC3649, December 2003,
<https://www.rfc-editor.org/info/rfc3649>.
Briscoe, et al. Expires January 9, 2020 [Page 29]
Internet-Draft L4S Architecture July 2019
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://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,
<https://www.rfc-editor.org/info/rfc4774>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://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,
<https://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, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <https://www.rfc-editor.org/info/rfc6040>.
[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, <https://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,
<https://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,
<https://www.rfc-editor.org/info/rfc7560>.
[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.
Briscoe, et al. Expires January 9, 2020 [Page 30]
Internet-Draft L4S Architecture July 2019
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC7713, December 2015,
<https://www.rfc-editor.org/info/rfc7713>.
[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,
<https://www.rfc-editor.org/info/rfc8033>.
[RFC8170] Thaler, D., Ed., "Planning for Protocol Adoption and
Subsequent Transitions", RFC 8170, DOI 10.17487/RFC8170,
May 2017, <https://www.rfc-editor.org/info/rfc8170>.
[RFC8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>.
[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>.
[RFC8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
2017, <https://www.rfc-editor.org/info/rfc8298>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>.
[RFC8511] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", RFC 8511,
DOI 10.17487/RFC8511, December 2018,
<https://www.rfc-editor.org/info/rfc8511>.
[TCP-CA] Jacobson, V. and M. Karels, "Congestion Avoidance and
Control", Laurence Berkeley Labs Technical Report ,
November 1988, <http://ee.lbl.gov/papers/congavoid.pdf>.
Briscoe, et al. Expires January 9, 2020 [Page 31]
Internet-Draft L4S Architecture July 2019
[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>.
[UnorderedLTE]
Austrheim, M., "Implementing immediate forwarding for 4G
in a network simulator", Masters Thesis, Uni Oslo , June
2019.
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 Center TCP as currently used (in
controlled environments);
DCTCP bis: Applicable to an future Data Center TCP congestion
control intended for controlled environments;
XXX Prague: Applicable to a Scalable variant of XXX (TCP/SCTP/RMCAT)
congestion control.
Briscoe, et al. Expires January 9, 2020 [Page 32]
Internet-Draft L4S Architecture July 2019
+-----+------------------------+------------------------------------+
| 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 | [RFC8257] |
| 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.ietf-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) |
+-----+------------------------+------------------------------------+
Briscoe, et al. Expires January 9, 2020 [Page 33]
Internet-Draft L4S Architecture July 2019
+-----+--------+-----+-------+-----------+--------+--------+--------+
| # | 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)
CableLabs
UK
Email: ietf@bobbriscoe.net
URI: http://bobbriscoe.net/
Briscoe, et al. Expires January 9, 2020 [Page 34]
Internet-Draft L4S Architecture July 2019
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
Greg White
CableLabs
US
Email: G.White@CableLabs.com
Briscoe, et al. Expires January 9, 2020 [Page 35]