Low Latency, Low Loss, Scalable
Throughput (L4S) Internet Service: ArchitectureCableLabsUKietf@bobbriscoe.nethttp://bobbriscoe.net/Nokia Bell LabsAntwerpBelgiumkoen.de_schepper@nokia.comhttps://www.bell-labs.com/usr/koen.de_schepperUniversidad Carlos III de MadridAv. Universidad 30Leganes, Madrid 28911Spain34 91 6249500marcelo@it.uc3m.eshttp://www.it.uc3m.es
Transport
Transport Area Working GroupInternet-DraftI-DThis 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.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
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 . It only occurs when a large
enough 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 ) or compatible variants (e.g. TCP
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 or
PIE . Also, with a Classic TCP, 5 ms of
queuing is usually only possible by losing some utilization.It has been convincingly demonstrated 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.There are three main components to the L4S architecture (illustrated
in ):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 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 is an example of such a
semi-permeable membrane.Per-flow queuing
such as in 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 ), not least that thousands of queues are
needed when two are sufficient.A host needs to distinguish L4S and
Classic packets with an identifier so that the network can classify
them into their separate treatments. considers various alternative
identifiers, and concludes that all alternatives involve
compromises, but the ECT(1) codepoint of the ECN field is a workable
solution.Scalable congestion controls already exist.
They solve the scaling problem with TCP first pointed out in . The one used most widely (in controlled
environments) is Data Centre TCP (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.)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 .
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.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).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 ) 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).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, DCTCP averages 2 congestion signals per round-trip
whatever the flow rate.A congestion control with
a flow rate compatible with standard TCP Reno . 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.The original proposed standard Explicit
Congestion Notification (ECN) protocol ,
which requires ECN signals to be treated the same as drops, both
when generated in the network and when responded to by the
sender.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.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: 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 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
standards track needs to be updated to
allow L4S packets to depart from the 'same as drop'
constraint. 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.
also reclassifies the original experimental assignment of the ECT(1)
codepoint as an ECN nonce as historic. 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 .Network components:The Dual Queue Coupled AQM has been specified as
generically as possible 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 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 .Host mechanisms: The L4S architecture includes a number of mechanisms
in the end host that we enumerate next:Data Centre 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 . 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
). The list also includes
some optional performance improvements.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 , in SCTP ,
in RTP , and in DCCP . 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 and certain real-time
media congestion avoidance techniques (RMCAT) protocols.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 . Work to standardize more accurate ECN
feedback for TCP (AccECN ) is in progress.ECN feedback is only roughly sketched in an appendix of the
SCTP specification. A fuller specification has been proposed
, which would need to
be implemented and deployed before SCTCP could support L4S.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. 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 ).
Therefore, in a DualQ AQM, the L4S queue uses a new L4S variant of
ECN that is not equivalent to drop , while the Classic queue uses
either classic ECN 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."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.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.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.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 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 . 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.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.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:fq makes high performance networking equipment costly
(processing and memory) - in contrast dual queue code can be
very simple;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;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 or the interactive media applications
described in , 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.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.Yet again, L4S is
not an alternative to ABE but a complement that introduces much
lower queuing delay. ABE 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 above,
AQMs still cannot reduce queuing delay too far without losing link
utilization (to allow for other, non-ABE, flows).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: Gaming;VoIP;Video conferencing;Web browsing;(Adaptive) video streaming;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: Cloud based interactive video;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 . 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.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:Interactive remote presence;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.The following use-cases for L4S are being considered by various
interested parties:Where the bottleneck is one of various types of access network:
DSL, cable, mobile, satelliteRadio 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;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.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
companya 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)Different types of transport (or application) congestion
control:elastic (TCP/SCTP);real-time (RTP, RMCAT);query (DNS/LDAP).Where low delay quality of service is required, but without
inspecting or intervening above the IP layer :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.If queuing delay is minimized, applications with a fixed delay
budget can communicate over longer distances, or via a longer
chain of service functions or onion
routers.The DualQ is, in itself, an incremental deployment framework for
L4S AQMs so that L4S traffic can coexist with existing Classic
"TCP-friendly" traffic. explains
why only deploying a DualQ AQM in one node at each end of
the access link will realize nearly all the benefit of L4S.L4S involves both end systems and the network, so 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. describes how an
L4S flow detects this, and how to minimize the effect of false
negative detection.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 (). 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.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.For any one L4S flow to work, it requires 3 parts to have been
deployed. This was the same deployment problem that ECN faced 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 ), but 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 . 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. illustrates some example
sequences in which the parts of L4S might be deployed. It consists
of the following stages: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).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) 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 . 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 ).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.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 ).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.:congestion loss at other transient bottlenecks, e.g. due to
bursts in shallower queues;transmission errors, e.g. due to electrical interference;rate policing.Three complementary approaches are in progress to address this
issue, but they are all currently research:In TCP Prague, ignore certain losses deemed unlikely to be
due to congestion (using some ideas from 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).A combination of RACK, reconfigured link retransmission and
L4S could address transmission errors (no reference yet);Hybrid ECN/drop policers (see ).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 ).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 .This specification contains no IANA considerations.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 ). 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 ) 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. 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.The L4S service does rely on self-constraint - not in terms of
limiting rate, but in terms of limiting latency. 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.As mentioned in , 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 as well. Within such a Diffserv 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.
, or the 'local'
(non-ConEx) variant of the ConEx congestion policer . 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 .
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: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 ).A network can enforce a congestion response to its ECN markings
(or packet losses) by auditing congestion exposure (ConEx) .The TCP authentication option (TCP-AO )
can be used to detect tampering with TCP congestion feedback.The ECN Nonce was proposed to detect
tampering with congestion feedback, but it has been reclassified
as historic .Appendix C.1 of gives
more details of these techniques including their applicability and
pros and cons.Thanks to 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.A QoE Perspective on Sizing Network BuffersRelentless Congestion ControlPSC'Data Centre to the Home': Ultra-Low Latency for AllBell LabsSimula Research LabBell LabsBT(Under submission)PI^2 : A Linearized AQM for both Classic and Scalable
TCPBell LabsSimula Research LabBell LabsBTScaling TCP's Congestion Window for Small Round Trip
TimesBTBell LabsExperimental Specification of New Congestion Control
AlgorithmsNokia Research CentreBBR: Congestion-Based Congestion Control; Measuring
bottleneck bandwidth and round-trip propagation timeGoogleGoogleGoogleGoogleGoogleUltra-Low Delay for All: Live Experience, Live
AnalysisSimula Research LabBell LabsBell LabsBTThe 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: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 ;Applicable to all forms of TCP congestion
control;Applicable to Data Centre TCP as currently used
(in controlled environments);Applicable to an future Data Centre TCP
congestion control intended for controlled environments;Applicable to a Scalable variant of XXX
(TCP/SCTP/RMCAT) congestion control.Req #RequirementReference0ARCHITECTURE1L4S IDENTIFIER2DUAL QUEUE AQM3Suitable ECN Feedback, .SCALABLE TRANSPORT - SAFETY ADDITIONS4-1Fall back to Reno/Cubic on loss S.2.3, 4-2Fall back to Reno/Cubic if classic ECN bottleneck detected S.2.34-3Reduce RTT-dependence S.2.34-4Scaling TCP's Congestion Window for Small Round Trip Times S.2.3, SCALABLE TRANSPORT - PERFORMANCE ENHANCEMENTS5-1Setting ECT in TCP Control Packets and Retransmissions5-2Faster-than-additive increase (Appx A.2.2)5-3Faster Convergence at Flow Start (Appx A.2.2)#WGTCPDCTCPDCTCP-bisTCP PragueSCTP PragueRMCAT Prague0tsvwgYYYYYY1tsvwgYYYY2tsvwgn/an/an/an/an/an/a3tcpmYYYYn/an/a4-1tcpmYYYYY4-2tcpm/ iccrg?YY?4-3tcpm/ iccrg?YYY?4-4tcpmYYYYY?5-1tcpmYYYYn/an/a5-2tcpm/ iccrg?YYY?5-3tcpm/ iccrg?YYY?