< draft-ietf-tsvwg-l4s-arch-03.txt   draft-ietf-tsvwg-l4s-arch-04.txt >
Transport Area Working Group B. Briscoe, Ed. Transport Area Working Group B. Briscoe, Ed.
Internet-Draft CableLabs Internet-Draft CableLabs
Intended status: Informational K. De Schepper Intended status: Informational K. De Schepper
Expires: April 25, 2019 Nokia Bell Labs Expires: January 9, 2020 Nokia Bell Labs
M. Bagnulo Braun M. Bagnulo Braun
Universidad Carlos III de Madrid Universidad Carlos III de Madrid
October 22, 2018 G. White
CableLabs
July 8, 2019
Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service:
Architecture Architecture
draft-ietf-tsvwg-l4s-arch-03 draft-ietf-tsvwg-l4s-arch-04
Abstract Abstract
This document describes the L4S architecture for the provision of a This document describes the L4S architecture for the provision of a
new Internet service that could eventually replace best efforts for new Internet service that could eventually replace best efforts for
all traffic: Low Latency, Low Loss, Scalable throughput (L4S). It is all traffic: Low Latency, Low Loss, Scalable throughput (L4S). It is
becoming common for _all_ (or most) applications being run by a user becoming common for _all_ (or most) applications being run by a user
at any one time to require low latency. However, the only solution at any one time to require low latency. However, the only solution
the IETF can offer for ultra-low queuing delay is Diffserv, which the IETF can offer for ultra-low queuing delay is Diffserv, which
only favours a minority of packets at the expense of others. In only favours a minority of packets at the expense of others. In
skipping to change at page 2, line 20 skipping to change at page 2, line 20
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This Internet-Draft will expire on April 25, 2019. This Internet-Draft will expire on January 9, 2020.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. L4S Architecture Overview . . . . . . . . . . . . . . . . . . 4 2. L4S Architecture Overview . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. L4S Architecture Components . . . . . . . . . . . . . . . . . 7 4. L4S Architecture Components . . . . . . . . . . . . . . . . . 7
5. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Why These Primary Components? . . . . . . . . . . . . . . 9 5.1. Why These Primary Components? . . . . . . . . . . . . . . 10
5.2. Why Not Alternative Approaches? . . . . . . . . . . . . . 10 5.2. Why Not Alternative Approaches? . . . . . . . . . . . . . 12
6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 13 6. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Applications . . . . . . . . . . . . . . . . . . . . . . 13 6.1. Applications . . . . . . . . . . . . . . . . . . . . . . 15
6.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 14 6.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 16
6.3. Deployment Considerations . . . . . . . . . . . . . . . . 15 6.3. Deployment Considerations . . . . . . . . . . . . . . . . 17
6.3.1. Deployment Topology . . . . . . . . . . . . . . . . . 16 6.3.1. Deployment Topology . . . . . . . . . . . . . . . . . 18
6.3.2. Deployment Sequences . . . . . . . . . . . . . . . . 17 6.3.2. Deployment Sequences . . . . . . . . . . . . . . . . 19
6.3.3. L4S Flow but Non-L4S Bottleneck . . . . . . . . . . . 19 6.3.3. L4S Flow but Non-L4S Bottleneck . . . . . . . . . . . 21
6.3.4. Other Potential Deployment Issues . . . . . . . . . . 20 6.3.4. Other Potential Deployment Issues . . . . . . . . . . 23
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21 8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8.1. Traffic (Non-)Policing . . . . . . . . . . . . . . . . . 21 8.1. Traffic (Non-)Policing . . . . . . . . . . . . . . . . . 23
8.2. 'Latency Friendliness' . . . . . . . . . . . . . . . . . 22 8.2. 'Latency Friendliness' . . . . . . . . . . . . . . . . . 24
8.3. Interaction between Rate Policing and L4S . . . . . . . . 22 8.3. Interaction between Rate Policing and L4S . . . . . . . . 24
8.4. ECN Integrity . . . . . . . . . . . . . . . . . . . . . . 23 8.4. ECN Integrity . . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 23 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 24 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 24 10.1. Normative References . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . 24 10.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Standardization items . . . . . . . . . . . . . . . 28 Appendix A. Standardization items . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction 1. Introduction
It is increasingly common for _all_ of a user's applications at any It is increasingly common for _all_ of a user's applications at any
one time to require low delay: interactive Web, Web services, voice, one time to require low delay: interactive Web, Web services, voice,
conversational video, interactive video, interactive remote presence, conversational video, interactive video, interactive remote presence,
instant messaging, online gaming, remote desktop, cloud-based instant messaging, online gaming, remote desktop, cloud-based
applications and video-assisted remote control of machinery and applications and video-assisted remote control of machinery and
industrial processes. In the last decade or so, much has been done industrial processes. In the last decade or so, much has been done
to reduce propagation delay by placing caches or servers closer to to reduce propagation delay by placing caches or servers closer to
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component of latency. For instance spikes of hundreds of component of latency. For instance spikes of hundreds of
milliseconds are common. During a long-running flow, even with milliseconds are common. During a long-running flow, even with
state-of-the-art active queue management (AQM), the base speed-of- state-of-the-art active queue management (AQM), the base speed-of-
light path delay roughly doubles. Low loss is also important light path delay roughly doubles. Low loss is also important
because, for interactive applications, losses translate into even because, for interactive applications, losses translate into even
longer retransmission delays. longer retransmission delays.
It has been demonstrated that, once access network bit rates reach It has been demonstrated that, once access network bit rates reach
levels now common in the developed world, increasing capacity offers levels now common in the developed world, increasing capacity offers
diminishing returns if latency (delay) is not addressed. diminishing returns if latency (delay) is not addressed.
Differentiated services (Diffserv) offers Expedited Forwarding Differentiated services (Diffserv) offers Expedited Forwarding (EF
[RFC3246] for some packets at the expense of others, but this is not [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 sufficient when all (or most) of a user's applications require low
latency. latency.
Therefore, the goal is an Internet service with ultra-Low queueing Therefore, the goal is an Internet service with ultra-Low queueing
Latency, ultra-Low Loss and Scalable throughput (L4S) - for _all_ Latency, ultra-Low Loss and Scalable throughput (L4S) - for _all_
traffic. A service for all traffic will need none of the traffic. A service for all traffic will need none of the
configuration or management baggage (traffic policing, traffic configuration or management baggage (traffic policing, traffic
contracts) associated with favouring some packets over others. This contracts) associated with favouring some packets over others. This
document describes the L4S architecture for achieving that goal. document describes the L4S architecture for achieving that goal.
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network; without addressing the root of the problem. network; without addressing the root of the problem.
The root of the problem is the presence of standard TCP congestion The root of the problem is the presence of standard TCP congestion
control (Reno [RFC5681]) or compatible variants (e.g. TCP Cubic control (Reno [RFC5681]) or compatible variants (e.g. TCP Cubic
[RFC8312]). We shall call this family of congestion controls [RFC8312]). We shall call this family of congestion controls
'Classic' TCP. It has been demonstrated that if the sending host 'Classic' TCP. It has been demonstrated that if the sending host
replaces Classic TCP with a 'Scalable' alternative, when a suitable replaces Classic TCP with a 'Scalable' alternative, when a suitable
AQM is deployed in the network the performance under load of all the AQM is deployed in the network the performance under load of all the
above interactive applications can be stunningly improved. For above interactive applications can be stunningly improved. For
instance, queuing delay under heavy load with the example DCTCP/DualQ instance, queuing delay under heavy load with the example DCTCP/DualQ
solution cited below is roughly 1 millisecond (1 ms) at the 99th solution cited below is roughly 1 millisecond (1 to 2 ms) at the 99th
percentile without losing link utilization. This compares with 5 to percentile without losing link utilization. This compares with 5 to
20 ms on _average_ with a Classic TCP and current state-of-the-art 20 ms on _average_ with a Classic TCP and current state-of-the-art
AQMs such as fq_CoDel [RFC8290] or PIE [RFC8033]. Also, with a AQMs such as fq_CoDel [RFC8290] or PIE [RFC8033] and about 20-30 ms
Classic TCP, 5 ms of queuing is usually only possible by losing some at the 99th percentile. Also, with a Classic TCP, 5 ms of queuing is
utilization. usually only possible by losing some utilization.
It has been convincingly demonstrated [DCttH15] that it is possible It has been convincingly demonstrated [DCttH15] that it is possible
to deploy such an L4S service alongside the existing best efforts 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 service so that all of a user's applications can shift to it when
their stack is updated. Access networks are typically designed with their stack is updated. Access networks are typically designed with
one link as the bottleneck for each site (which might be a home, one link as the bottleneck for each site (which might be a home,
small enterprise or mobile device), so deployment at a single network small enterprise or mobile device), so deployment at a single network
node should give nearly all the benefit. The L4S approach also node should give nearly all the benefit. The L4S approach also
requires component mechanisms at the endpoints to fulfill its goal. requires component mechanisms at the endpoints to fulfill its goal.
This document presents the L4S architecture, by describing the This document presents the L4S architecture, by describing the
different components and how they interact to provide the scalable different components and how they interact to provide the scalable
low-latency, low-loss, Internet service. low-latency, low-loss, Internet service.
2. L4S Architecture Overview 2. L4S Architecture Overview
There are three main components to the L4S architecture (illustrated There are three main components to the L4S architecture (illustrated
in Figure 1): in Figure 1):
1) Network: The L4S service traffic needs to be isolated from the 1) Network: L4S traffic needs to be isolated from the queuing
queuing latency of the Classic service traffic. However, the two latency of Classic traffic. However, the two should be able to
should be able to freely share a common pool of capacity. This is freely share a common pool of capacity. This is because there is
because there is no way to predict how many flows at any one time no way to predict how many flows at any one time might use each
might use each service and capacity in access networks is too service and capacity in access networks is too scarce to partition
scarce to partition into two. So a 'semi-permeable' membrane is into two. The Dual Queue Coupled AQM
needed that partitions latency but not bandwidth. The Dual Queue [I-D.ietf-tsvwg-aqm-dualq-coupled] was developed as a minimal
Coupled AQM [I-D.ietf-tsvwg-aqm-dualq-coupled] is an example of complexity solution to this problem. The two queues appear to be
such a semi-permeable membrane. separated by a 'semi-permeable' membrane that partitions latency
but not bandwidth (explained later).
Per-flow queuing such as in [RFC8290] could be used, but it Per-flow queuing such as in [RFC8290] could be used (see
partitions both latency and bandwidth between every end-to-end Section 4), but it partitions both latency and bandwidth between
flow. So it is rather overkill, which brings disadvantages (see every end-to-end flow. So it is rather overkill, which brings
Section 5.2), not least that thousands of queues are needed when disadvantages (see Section 5.2), not least that large number of
two are sufficient. queues are needed when two are sufficient.
2) Protocol: A host needs to distinguish L4S and Classic packets 2) Protocol: A host needs to distinguish L4S and Classic packets
with an identifier so that the network can classify them into with an identifier so that the network can classify them into
their separate treatments. [I-D.ietf-tsvwg-ecn-l4s-id] considers their separate treatments. [I-D.ietf-tsvwg-ecn-l4s-id] considers
various alternative identifiers, and concludes that all various alternative identifiers, and concludes that all
alternatives involve compromises, but the ECT(1) codepoint of the alternatives involve compromises, but the ECT(1) and CE codepoints
ECN field is a workable solution. of the ECN field represent a workable solution.
3) Host: Scalable congestion controls already exist. They solve the 3) Host: Scalable congestion controls already exist. They solve the
scaling problem with TCP that was first pointed out in [RFC3649]. scaling problem with TCP that was first pointed out in [RFC3649].
The one used most widely (in controlled environments) is Data The one used most widely (in controlled environments) is Data
Centre TCP (DCTCP [RFC8257]), which has been implemented and Center TCP (DCTCP [RFC8257]), which has been implemented and
deployed in Windows Server Editions (since 2012), in Linux and in deployed in Windows Server Editions (since 2012), in Linux and in
FreeBSD. Although DCTCP as-is 'works' well over the public FreeBSD. Although DCTCP as-is 'works' well over the public
Internet, most implementations lack certain safety features that Internet, most implementations lack certain safety features that
will be necessary once it is used outside controlled environments will be necessary once it is used outside controlled environments
like data centres (see later). A similar scalable congestion like data centres (see later). A similar scalable congestion
control will also need to be transplanted into protocols other control will also need to be transplanted into protocols other
than TCP (SCTP, RTP/RTCP, RMCAT, etc.) 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].
(2) (1) (2) (1)
.-------^------. .--------------^-------------------. .-------^------. .--------------^-------------------.
,-(3)-----. ______ ,-(3)-----. ______
; ________ : L4S --------. | | ; ________ : L4S --------. | |
:|Scalable| : _\ ||___\_| mark | :|Scalable| : _\ ||___\_| mark |
:| sender | : __________ / / || / |______|\ _________ :| sender | : __________ / / || / |______|\ _________
:|________|\; | |/ --------' ^ \1| | :|________|\; | |/ --------' ^ \1|condit'nl|
`---------'\_| IP-ECN | Coupling : \|priority |_\ `---------'\_| IP-ECN | Coupling : \|priority |_\
________ / |Classifier| : /|scheduler| / ________ / |Classifier| : /|scheduler| /
|Classic |/ |__________|\ --------. ___:__ / |_________| |Classic |/ |__________|\ --------. ___:__ / |_________|
| sender | \_\ || | |||___\_| mark/|/ | sender | \_\ || | |||___\_| mark/|/
|________| / || | ||| / | drop | |________| / || | ||| / | drop |
Classic --------' |______| Classic --------' |______|
Figure 1: Components of an L4S Solution: 1) Isolation in separate Figure 1: Components of an L4S Solution: 1) Isolation in separate
network queues; 2) Packet Identification Protocol; and 3) Scalable network queues; 2) Packet Identification Protocol; and 3) Scalable
Sending Host Sending Host
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in ALL CAPS. Lower case uses of these words are not to be in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance. COMMENT: Since this interpreted as carrying RFC-2119 significance. COMMENT: Since this
will be an information document, This should be removed. will be an information document, This should be removed.
Classic service: The 'Classic' service is intended for all the Classic service: The 'Classic' service is intended for all the
congestion control behaviours that currently co-exist with TCP congestion control behaviours that currently co-exist with TCP
Reno (e.g. TCP Cubic, Compound, SCTP, etc). Reno (e.g. TCP Cubic, Compound, SCTP, etc).
Low-Latency, Low-Loss and Scalable (L4S) service: The 'L4S' service Low-Latency, Low-Loss and Scalable (L4S) service: The 'L4S' service
is intended for traffic from scalable TCP algorithms such as Data 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 Center TCP. But it is also more general--it will allow a set of
congestion controls with similar scaling properties to DCTCP (e.g. congestion controls with similar scaling properties to DCTCP (e.g.
Relentless [Mathis09]) to evolve. Relentless [Mathis09]) to evolve.
Both Classic and L4S services can cope with a proportion of Both Classic and L4S services can cope with a proportion of
unresponsive or less-responsive traffic as well (e.g. DNS, VoIP, unresponsive or less-responsive traffic as well (e.g. DNS, VoIP,
etc). etc).
Scalable Congestion Control: A congestion control where the packet Scalable Congestion Control: A congestion control where the packet
flow rate per round trip (the window) is inversely proportional to flow rate per round trip (the window) is inversely proportional to
the level (probability) of congestion signals. Then, as flow rate the level (probability) of congestion signals. Then, as flow rate
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Site: A home, mobile device, small enterprise or campus, where the Site: A home, mobile device, small enterprise or campus, where the
network bottleneck is typically the access link to the site. Not network bottleneck is typically the access link to the site. Not
all network arrangements fit this model but it is a useful, widely all network arrangements fit this model but it is a useful, widely
applicable generalisation. applicable generalisation.
4. L4S Architecture Components 4. L4S Architecture Components
The L4S architecture is composed of the following elements. The L4S architecture is composed of the following elements.
Protocols:The L4S architecture encompasses the two protocol changes Protocols:The L4S architecture encompasses the two identifier changes
(an unassignment and an assignment) that we describe next: (an unassignment and an assignment) and optional further identifiers:
a. An essential aspect of a scalable congestion control is the use a. An essential aspect of a scalable congestion control is the use
of explicit congestion signals rather than losses, because the of explicit congestion signals rather than losses, because the
signals need to be sent immediately and frequently--too often to signals need to be sent immediately and frequently. 'Classic'
use drops. 'Classic' ECN [RFC3168] requires an ECN signal to be ECN [RFC3168] requires an ECN signal to be treated the same as a
treated the same as a drop, both when it is generated in the drop, both when it is generated in the network and when it is
network and when it is responded to by hosts. L4S needs networks responded to by hosts. L4S needs networks and hosts to support a
and hosts to support a different meaning for ECN. So the different meaning for ECN:
standards track [RFC3168] needs to be updated to allow L4S
packets to depart from the 'same as drop' constraint. * 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 [RFC8311] is a standards track update to relax specific
requirements in RFC 3168 (and certain other standards track requirements in RFC 3168 (and certain other standards track
RFCs), which clears the way for the experimental changes proposed RFCs), which clears the way for the experimental changes proposed
for L4S. [RFC8311] also reclassifies the original experimental for L4S. [RFC8311] also reclassifies the original experimental
assignment of the ECT(1) codepoint as an ECN nonce [RFC3540] as assignment of the ECT(1) codepoint as an ECN nonce [RFC3540] as
historic. historic.
b. [I-D.ietf-tsvwg-ecn-l4s-id] recommends ECT(1) is used as the 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 identifier to classify L4S packets into a separate treatment from
Classic packets. This satisfies the requirements for identifying Classic packets. This satisfies the requirements for identifying
an alternative ECN treatment in [RFC4774]. an alternative ECN treatment in [RFC4774].
Network components:The Dual Queue Coupled AQM has been specified as The CE codepoint is used to indicate Congestion Experienced by
generically as possible [I-D.ietf-tsvwg-aqm-dualq-coupled] as a both L4S and Classic treatments. This raises the concern that a
'semi-permeable' membrane without specifying the particular AQMs to Classic AQM earlier on the path might have marked some ECT(0)
use in the two queues. An informational appendix of the draft is packets as CE. Then these packets will be erroneously classified
provided for pseudocode examples of different possible AQM into the L4S queue. [I-D.ietf-tsvwg-ecn-l4s-id] explains why 5
approaches. Initially a zero-config variant of RED called Curvy RED unlikely eventualities all have to coincide for this to have any
was implemented, tested and documented. The aim is for designers to detrimental effect, which even then would only involve a
be free to implement diverse ideas. So the brief normative body of vanishingly small likelihood of a spurious retransmission.
the draft only specifies the minimum constraints an AQM needs to
comply with to ensure that the L4S and Classic services will coexist. c. A network operator might wish to include certain unresponsive,
For instance, a variant of PIE called Dual PI Squared [PI2] has been non-L4S traffic in the L4S queue if it is deemed to be smoothly
implemented and found to perform better than Curvy RED over a wide enough paced and low enough rate not to build a queue. For
range of conditions, so it has been documented in another appendix of instance, VoIP, low rate datagrams to sync online games,
[I-D.ietf-tsvwg-aqm-dualq-coupled]. 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
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 Host mechanisms: The L4S architecture includes a number of mechanisms
in the end host that we enumerate next: in the end host that we enumerate next:
a. Data Centre TCP is the most widely used example of a scalable a. Data Center TCP is the most widely used example of a scalable
congestion control. It has been documented as an informational congestion control. It has been documented as an informational
record of the protocol currently in use [RFC8257]. It will be record of the protocol currently in use [RFC8257]. It will be
necessary to define a number of safety features for a variant necessary to define a number of safety features for a variant
usable on the public Internet. A draft list of these, known as 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 TCP Prague requirements, has been drawn up (see Appendix A of
[I-D.ietf-tsvwg-ecn-l4s-id]). The list also includes some [I-D.ietf-tsvwg-ecn-l4s-id]). The list also includes some
optional performance improvements. optional performance improvements.
b. Transport protocols other than TCP use various congestion b. Transport protocols other than TCP use various congestion
controls designed to be friendly with Classic TCP. Before they controls designed to be friendly with Classic TCP. Before they
skipping to change at page 9, line 21 skipping to change at page 10, line 27
[I-D.ietf-quic-transport] and certain real-time media congestion [I-D.ietf-quic-transport] and certain real-time media congestion
avoidance techniques (RMCAT) protocols. avoidance techniques (RMCAT) protocols.
c. ECN feedback is sufficient for L4S in some transport protocols c. ECN feedback is sufficient for L4S in some transport protocols
(RTCP, DCCP) but not others: (RTCP, DCCP) but not others:
* For the case of TCP, the feedback protocol for ECN embeds the * 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 assumption from Classic ECN that an ECN mark is the same as a
drop, making it unusable for a scalable TCP. Therefore, the drop, making it unusable for a scalable TCP. Therefore, the
implementation of TCP receivers will have to be upgraded implementation of TCP receivers will have to be upgraded
[RFC7560]. Work to standardize more accurate ECN feedback for [RFC7560]. Work to standardize and implement more accurate
TCP (AccECN [I-D.ietf-tcpm-accurate-ecn]) is in progress. 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 * ECN feedback is only roughly sketched in an appendix of the
SCTP specification. A fuller specification has been proposed SCTP specification. A fuller specification has been proposed
[I-D.stewart-tsvwg-sctpecn], which would need to be [I-D.stewart-tsvwg-sctpecn], which would need to be
implemented and deployed before SCTCP could support L4S. implemented and deployed before SCTCP could support L4S.
5. Rationale 5. Rationale
5.1. Why These Primary Components? 5.1. Why These Primary Components?
Explicit congestion signalling (protocol): Explicit congestion Explicit congestion signalling (protocol): Explicit congestion
signalling is a key part of the L4S approach. In contrast, use of 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 drop as a congestion signal creates a tension because drop is both
a useful signal (more would reduce delay) and an impairment (less a useful signal (more would reduce delay) and an impairment (less
would reduce delay). Explicit congestion signals can be used many would reduce delay):
times per round trip, to keep tight control, without any
impairment. Under heavy load, even more explicit signals can be * Explicit congestion signals can be used many times per round
applied so the queue can be kept short whatever the load. Whereas trip, to keep tight control, without any impairment. Under
state-of-the-art AQMs have to introduce very high packet drop at heavy load, even more explicit signals can be applied so the
high load to keep the queue short. Further, when using ECN, TCP's queue can be kept short whatever the load. Whereas state-of-
sawtooth reduction can be smaller and therefore return to the the-art AQMs have to introduce very high packet drop at high
operating point more often, without worrying that this causes more load to keep the queue short. Further, when using ECN, TCP's
signals (one at the top of each smaller sawtooth). The consequent sawtooth reduction can be smaller and therefore return to the
smaller amplitude sawteeth fit between a very shallow marking operating point more often, without worrying that this causes
threshold and an empty queue, so delay variation can be very low, more signals (one at the top of each smaller sawtooth). The
without risk of under-utilization. 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 All the above makes it clear that explicit congestion signalling
is only advantageous for latency if it does not have to be is only advantageous for latency if it does not have to be
considered 'the same as' drop (as required with Classic ECN considered 'the same as' drop (as was required with Classic ECN
[RFC3168]). Therefore, in a DualQ AQM, the L4S queue uses a new [RFC3168]). Therefore, in a DualQ AQM, the L4S queue uses a new
L4S variant of ECN that is not equivalent to drop L4S variant of ECN that is not equivalent to drop
[I-D.ietf-tsvwg-ecn-l4s-id], while the Classic queue uses either [I-D.ietf-tsvwg-ecn-l4s-id], while the Classic queue uses either
classic ECN [RFC3168] or drop, which are equivalent. classic ECN [RFC3168] or drop, which are equivalent.
Before Classic ECN was standardized, there were various proposals Before Classic ECN was standardized, there were various proposals
to give an ECN mark a different meaning from drop. However, there to give an ECN mark a different meaning from drop. However, there
was no particular reason to agree on any one of the alternative was no particular reason to agree on any one of the alternative
meanings, so 'the same as drop' was the only compromise that could meanings, so 'the same as drop' was the only compromise that could
be reached. RFC 3168 contains a statement that: be reached. RFC 3168 contains a statement that:
skipping to change at page 10, line 46 skipping to change at page 12, line 15
L4S packet identifier (protocol): Once there are at least two L4S packet identifier (protocol): Once there are at least two
separate treatments in the network, hosts need an identifier at separate treatments in the network, hosts need an identifier at
the IP layer to distinguish which treatment they intend to use. the IP layer to distinguish which treatment they intend to use.
Scalable congestion notification (host): A scalable congestion Scalable congestion notification (host): A scalable congestion
control keeps the signalling frequency high so that rate control keeps the signalling frequency high so that rate
variations can be small when signalling is stable, and rate can variations can be small when signalling is stable, and rate can
track variations in available capacity as rapidly as possible track variations in available capacity as rapidly as possible
otherwise. 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? 5.2. Why Not Alternative Approaches?
All the following approaches address some part of the same problem 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 space as L4S. In each case, it is shown that L4S complements them or
improves on them, rather than being a mutually exclusive alternative: improves on them, rather than being a mutually exclusive alternative:
Diffserv: Diffserv addresses the problem of bandwidth apportionment Diffserv: Diffserv addresses the problem of bandwidth apportionment
for important traffic as well as queuing latency for delay- for important traffic as well as queuing latency for delay-
sensitive traffic. L4S solely addresses the problem of queuing sensitive traffic. L4S solely addresses the problem of queuing
latency (as well as loss and throughput scaling). Diffserv will latency (as well as loss and throughput scaling). Diffserv will
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A. fq makes high performance networking equipment costly A. fq makes high performance networking equipment costly
(processing and memory) - in contrast dual queue code can be (processing and memory) - in contrast dual queue code can be
very simple; very simple;
B. fq requires packet inspection into the end-to-end transport B. fq requires packet inspection into the end-to-end transport
layer, which doesn't sit well alongside encryption for privacy layer, which doesn't sit well alongside encryption for privacy
- in contrast the use of ECN as the classifier for L4S - in contrast the use of ECN as the classifier for L4S
requires no deeper inspection than the IP layer; requires no deeper inspection than the IP layer;
C. fq isolates the queuing of each flow from the others but not C. fq isolates the queuing of each flow from the others but not
from itself so, unlike L4S, it does not support applications from itself so existing FQ implementations still needs to have
that need both capacity-seeking behaviour and very low support for scalable congestion control added.
latency.
It might seem that self-inflicted queuing delay should not It might seem that self-inflicted queuing delay should not
count, because if the delay wasn't in the network it would count, because if the delay wasn't in the network it would
just shift to the sender. However, modern adaptive just shift to the sender. However, modern adaptive
applications, e.g. HTTP/2 [RFC7540] or the interactive media applications, e.g. HTTP/2 [RFC7540] or the interactive media
applications described in Section 6, can keep low latency applications described in Section 6, can keep low latency
objects at the front of their local send queue by shuffling objects at the front of their local send queue by shuffling
priorities of other objects dependent on the progress of other priorities of other objects dependent on the progress of other
transfers. They cannot shuffle packets once they have transfers. They cannot shuffle packets once they have
released them into the network. released them into the network.
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which flow to schedule without knowing application intent. which flow to schedule without knowing application intent.
Whereas a separate policing function can be configured less Whereas a separate policing function can be configured less
strictly, so that senders can still control the instantaneous strictly, so that senders can still control the instantaneous
rate of each flow dependent on the needs of each application rate of each flow dependent on the needs of each application
(e.g. variable rate video), giving more wriggle-room before a (e.g. variable rate video), giving more wriggle-room before a
flow is deemed non-compliant. Also policing of queuing and of flow is deemed non-compliant. Also policing of queuing and of
flow-rates can be applied independently. flow-rates can be applied independently.
Alternative Back-off ECN (ABE): Yet again, L4S is not an alternative Alternative Back-off ECN (ABE): Yet again, L4S is not an alternative
to ABE but a complement that introduces much lower queuing delay. to ABE but a complement that introduces much lower queuing delay.
ABE [I-D.ietf-tcpm-alternativebackoff-ecn] alters the host ABE [RFC8511] alters the host behaviour in response to ECN marking
behaviour in response to ECN marking to utilize a link better and to utilize a link better and give ECN flows faster throughput. It
give ECN flows faster throughput, but it assumes the network still uses ECT(0) and assumes the network still treats ECN and drop the
treats ECN and drop the same. Therefore ABE exploits any lower same. Therefore ABE exploits any lower queuing delay that AQMs
queuing delay that AQMs can provide. But as explained above, AQMs can provide. But as explained above, AQMs still cannot reduce
still cannot reduce queuing delay too far without losing link queuing delay too far without losing link utilization (to allow
utilization (to allow for other, non-ABE, flows). 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
behaviour if it discovers a compatible path. Otherwise it will
use an evolution of BBRv1.
6. Applicability 6. Applicability
6.1. Applications 6.1. Applications
A transport layer that solves the current latency issues will provide A transport layer that solves the current latency issues will provide
new service, product and application opportunities. new service, product and application opportunities.
With the L4S approach, the following existing applications will With the L4S approach, the following existing applications will
immediately experience significantly better quality of experience immediately experience significantly better quality of experience
under load in the best effort class: under load:
o Gaming; o Gaming;
o VoIP; o VoIP;
o Video conferencing; o Video conferencing;
o Web browsing; o Web browsing;
o (Adaptive) video streaming; o (Adaptive) video streaming;
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the match video under the finger-gesture control of each user. For the match video under the finger-gesture control of each user. For
the latter, a virtual reality headset displayed a viewport taken from the latter, a virtual reality headset displayed a viewport taken from
a 360 degree camera in a racing car. The user's head movements a 360 degree camera in a racing car. The user's head movements
controlled the viewport extracted by a cloud-based proxy. In both controlled the viewport extracted by a cloud-based proxy. In both
cases, with 7 ms end-to-end base delay, the additional queuing delay 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 of roughly 1 ms was so low that it seemed the video was generated
locally. locally.
Using a swiping finger gesture or head movement to pan a video are Using a swiping finger gesture or head movement to pan a video are
extremely latency-demanding actions--far more demanding than VoIP. extremely latency-demanding actions--far more demanding than VoIP.
Because human vision can detect extremely low delays of the order of Because human vision can detect extremely low delays of the order of
single milliseconds when delay is translated into a visual lag single milliseconds when delay is translated into a visual lag
between a video and a reference point (the finger or the orientation 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 of the head sensed by the balance system in the inner ear --- the
vestibular system). vestibular system).
Without the low queuing delay of L4S, cloud-based applications like Without the low queuing delay of L4S, cloud-based applications like
these would not be credible without significantly more access these would not be credible without significantly more access
bandwidth (to deliver all possible video that might be viewed) and bandwidth (to deliver all possible video that might be viewed) and
more local processing, which would increase the weight and power more local processing, which would increase the weight and power
consumption of head-mounted displays. When all interactive consumption of head-mounted displays. When all interactive
processing can be done in the cloud, only the data to be rendered for processing can be done in the cloud, only the data to be rendered for
the end user needs to be sent. the end user needs to be sent.
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service). If an AQM was already deployed, the Classic service service). If an AQM was already deployed, the Classic service
will be unchanged (and L4S will still be added). will be unchanged (and L4S will still be added).
2. In this stage, the name 'TCP Prague' is used to represent a 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. variant of DCTCP that is safe to use in a production environment.
If the application is primarily unidirectional, 'TCP Prague' at If the application is primarily unidirectional, 'TCP Prague' at
one end will provide all the benefit needed. Accurate ECN one end will provide all the benefit needed. Accurate ECN
feedback (AccECN) [I-D.ietf-tcpm-accurate-ecn] is needed at the feedback (AccECN) [I-D.ietf-tcpm-accurate-ecn] is needed at the
other end, but it is a generic ECN feedback facility that is other end, but it is a generic ECN feedback facility that is
already planned to be deployed for other purposes, e.g. DCTCP, already planned to be deployed for other purposes, e.g. DCTCP,
BBR [BBR]. The two ends can be deployed in either order, because BBR [I-D.cardwell-iccrg-bbr-congestion-control]. The two ends
TCP Prague only enables itself if it has negotiated the use of can be deployed in either order, because TCP Prague only enables
AccECN feedback with the other end during the connection itself if it has negotiated the use of AccECN feedback with the
handshake. Thus, deployment of TCP Prague on a server enables other end during the connection handshake. Thus, deployment of
L4S trials to move to a production service in one direction, TCP Prague on a server enables L4S trials to move to a production
wherever AccECN is deployed at the other end. This stage might service in one direction, wherever AccECN is deployed at the
be further motivated by the performance improvements of TCP other end. This stage might be further motivated by the
Prague relative to DCTCP (see Appendix A.2 of performance improvements of TCP Prague relative to DCTCP (see
[I-D.ietf-tsvwg-ecn-l4s-id]). 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 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. TCP Prague can be deployed in either order as already explained.
To motivate the first of two independent moves, the deferred To motivate the first of two independent moves, the deferred
benefit of enabling new services after the second move has to be benefit of enabling new services after the second move has to be
worth it to cover the first mover's investment risk. As worth it to cover the first mover's investment risk. As
explained already, the potential for new interactive services explained already, the potential for new interactive services
provides this motivation. The DualQ AQM also greatly improves provides this motivation. The DualQ AQM also greatly improves
the upstream Classic service, assuming no other AQM has already the upstream Classic service, assuming no other AQM has already
been deployed. been deployed.
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in shallower queues; in shallower queues;
o transmission errors, e.g. due to electrical interference; o transmission errors, e.g. due to electrical interference;
o rate policing. o rate policing.
Three complementary approaches are in progress to address this issue, Three complementary approaches are in progress to address this issue,
but they are all currently research: but they are all currently research:
o In TCP Prague, ignore certain losses deemed unlikely to be due to o In TCP Prague, ignore certain losses deemed unlikely to be due to
congestion (using some ideas from BBR [BBR] but with no need to 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 ignore nearly all losses). This could mask any of the above types
of loss (requires consensus on how to safely interoperate with of loss (requires consensus on how to safely interoperate with
drop-based congestion controls). drop-based congestion controls).
o A combination of RACK, reconfigured link retransmission and L4S o A combination of RACK, reconfigured link retransmission and L4S
could address transmission errors [I-D.ietf-tsvwg-ecn-l4s-id]; could address transmission errors [UnorderedLTE],
[I-D.ietf-tsvwg-ecn-l4s-id];
o Hybrid ECN/drop policers (see Section 8.3). o Hybrid ECN/drop policers (see Section 8.3).
L4S deployment scenarios that minimize these issues (e.g. over L4S deployment scenarios that minimize these issues (e.g. over
wireline networks) can proceed in parallel to this research, in the wireline networks) can proceed in parallel to this research, in the
expectation that research success will continually widen L4S expectation that research success could continually widen L4S
applicability. applicability.
Classic ECN support is starting to materialize (in the upstream of Classic ECN support is starting to materialize on the Internet as an
some home routers as of early 2017), so an L4S sender will have to 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 fall back to a classic ('TCP-Friendly') behaviour if it detects that
ECN marking is accompanied by greater queuing delay or greater delay ECN marking is accompanied by greater queuing delay or greater delay
variation than would be expected with L4S (see Appendix A.1.4 of variation than would be expected with L4S (see Appendix A.1.4 of
[I-D.ietf-tsvwg-ecn-l4s-id]). [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.
6.3.4. Other Potential Deployment Issues 6.3.4. Other Potential Deployment Issues
An L4S AQM uses the ECN field to signal congestion. So, in common 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, with Classic ECN, if the AQM is within a tunnel or at a lower layer,
correct functioning of ECN signalling requires correct propagation of correct functioning of ECN signalling requires correct propagation of
the ECN field up the layers [I-D.ietf-tsvwg-ecn-encap-guidelines]. the ECN field up the layers [RFC6040],
[I-D.ietf-tsvwg-ecn-encap-guidelines].
7. IANA Considerations 7. IANA Considerations
This specification contains no IANA considerations. This specification contains no IANA considerations.
8. Security Considerations 8. Security Considerations
8.1. Traffic (Non-)Policing 8.1. Traffic (Non-)Policing
Because the L4S service can serve all traffic that is using the 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 capacity of a link, it should not be necessary to police access to
the L4S service. In contrast, Diffserv only works if some packets the L4S service. In contrast, Diffserv only works if some packets
get less favourable treatment than others. So Diffserv has to use 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 to limit how much traffic can be favoured. In turn,
traffic policers require traffic contracts between users and networks traffic policers require traffic contracts between users and networks
as well as pairwise between networks. Because L4S will lack all this as well as pairwise between networks. Because L4S will lack all this
management complexity, it is more likely to work end-to-end. management complexity, it is more likely to work end-to-end.
During early deployment (and perhaps always), some networks will not During early deployment (and perhaps always), some networks will not
offer the L4S service. These networks do not need to police or re- offer the L4S service. These networks do not need to police or re-
mark L4S traffic - they just forward it unchanged as best efforts mark L4S traffic - they just forward it unchanged as best efforts
traffic, as they already forward traffic with ECT(1) today. At a traffic, as they already forward traffic with ECT(1) today. At a
bottleneck, such networks will introduce some queuing and dropping. bottleneck, such networks will introduce some queuing and dropping.
When a scalable congestion control detects a drop it will have to When a scalable congestion control detects a drop it will have to
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will degrade the L4S service to no better (but never worse) than will degrade the L4S service to no better (but never worse) than
classic best efforts, whenever a legacy (non-L4S) bottleneck is classic best efforts, whenever a legacy (non-L4S) bottleneck is
encountered on a path. encountered on a path.
Certain network operators might choose to restrict access to the L4S Certain network operators might choose to restrict access to the L4S
class, perhaps only to selected premium customers as a value-added class, perhaps only to selected premium customers as a value-added
service. Their packet classifier (item 2 in Figure 1) could identify service. Their packet classifier (item 2 in Figure 1) could identify
such customers against some other field (e.g. source address range) 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 as well as ECN. If only the ECN L4S identifier matched, but not the
source address (say), the classifier could direct these packets (from source address (say), the classifier could direct these packets (from
non-premium customers) into the Classic queue. Allowing operators to non-premium customers) into the Classic queue. Clearly explaining
use an additional local classifier is intended to remove any how operators can use an additional local classifiers (see
incentive to bleach the L4S identifier. Then at least the L4S ECN [I-D.ietf-tsvwg-ecn-l4s-id]) is intended to remove any tendency to
identifier will be more likely to survive end-to-end even though the bleach the L4S identifier. Then at least the L4S ECN identifier will
service may not be supported at every hop. Such arrangements would be more likely to survive end-to-end even though the service may not
only require simple registered/not-registered packet classification, be supported at every hop. Such arrangements would only require
rather than the managed, application-specific traffic policing simple registered/not-registered packet classification, rather than
against customer-specific traffic contracts that Diffserv requires. the managed, application-specific traffic policing against customer-
specific traffic contracts that Diffserv uses.
8.2. 'Latency Friendliness' 8.2. 'Latency Friendliness'
The L4S service does rely on self-constraint - not in terms of The L4S service does rely on self-constraint - not in terms of
limiting rate, but in terms of limiting latency (burstiness). It is limiting rate, but in terms of limiting latency (burstiness). It is
hoped that standardisation of dynamic behaviour (cf. TCP slow-start) hoped that self-interest and standardisation of dynamic behaviour
and self-interest will be sufficient to prevent transports from (cf. TCP slow-start) will be sufficient to prevent transports from
sending excessive bursts of L4S traffic, given the application's own sending excessive bursts of L4S traffic, given the application's own
latency will suffer most from such behaviour. latency will suffer most from such behaviour.
Whether burst policing becomes necessary remains to be seen. Without Whether burst policing becomes necessary remains to be seen. Without
it, there will be potential for attacks on the low latency of the L4S 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 service. However it may only be necessary to apply such policing
reactively, e.g. punitively targeted at any deployments of new bursty reactively, e.g. punitively targeted at any deployments of new bursty
malware. 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 8.3. Interaction between Rate Policing and L4S
As mentioned in Section 5.2, L4S should remove the need for low As mentioned in Section 5.2, L4S should remove the need for low
latency Diffserv classes. However, those Diffserv classes that give latency Diffserv classes. However, those Diffserv classes that give
certain applications or users priority over capacity, would still be certain applications or users priority over capacity, would still be
applicable. Then, within such Diffserv classes, L4S would often be applicable in certain scenarios (e.g. corporate networks). Then,
applicable to give traffic low latency and low loss as well. Within within such Diffserv classes, L4S would often be applicable to give
such a Diffserv class, the bandwidth available to a user or traffic low latency and low loss as well. Within such a Diffserv
application is often limited by a rate policer. Similarly, in the class, the bandwidth available to a user or application is often
default Diffserv class, rate policers are used to partition shared limited by a rate policer. Similarly, in the default Diffserv class,
capacity. rate policers are used to partition shared capacity.
A classic rate policer drops any packets exceeding a set rate, A classic rate policer drops any packets exceeding a set rate,
usually also giving a burst allowance (variants exist where the usually also giving a burst allowance (variants exist where the
policer re-marks non-compliant traffic to a discard-eligible Diffserv policer re-marks non-compliant traffic to a discard-eligible Diffserv
codepoint, so they may be dropped elsewhere during contention). codepoint, so they may be dropped elsewhere during contention).
Whenever L4S traffic encounters one of these rate policers, it will Whenever L4S traffic encounters one of these rate policers, it will
experience drops and the source has to fall back to a Classic experience drops and the source has to fall back to a Classic
congestion control, thus losing the benefits of L4S. So, in networks congestion control, thus losing the benefits of L4S. So, in networks
that already use rate policers and plan to deploy L4S, it will be 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 preferable to redesign these rate policers to be more friendly to the
skipping to change at page 23, line 39 skipping to change at page 26, line 17
o The ECN Nonce [RFC3540] was proposed to detect tampering with o The ECN Nonce [RFC3540] was proposed to detect tampering with
congestion feedback, but it has been reclassified as historic congestion feedback, but it has been reclassified as historic
[RFC8311]. [RFC8311].
Appendix C.1 of [I-D.ietf-tsvwg-ecn-l4s-id] gives more details of Appendix C.1 of [I-D.ietf-tsvwg-ecn-l4s-id] gives more details of
these techniques including their applicability and pros and cons. these techniques including their applicability and pros and cons.
9. Acknowledgements 9. Acknowledgements
Thanks to Wes Eddy, Karen Nielsen and David Black for their useful Thanks to Richard Scheffenegger, Wes Eddy, Karen Nielsen and David
review comments. Black for their useful review comments.
Bob Briscoe and Koen De Schepper were part-funded by the European Bob Briscoe and Koen De Schepper were part-funded by the European
Community under its Seventh Framework Programme through the Reducing Community under its Seventh Framework Programme through the Reducing
Internet Transport Latency (RITE) project (ICT-317700). Bob Briscoe Internet Transport Latency (RITE) project (ICT-317700). Bob Briscoe
was also part-funded by the Research Council of Norway through the was also part-funded by the Research Council of Norway through the
TimeIn project. The views expressed here are solely those of the TimeIn project. The views expressed here are solely those of the
authors. authors.
10. References 10. References
10.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
10.2. Informative References 10.2. Informative References
[BBR] Cardwell, N., Cheng, Y., Gunn, C., Yeganeh, S., and V. [DCttH15] De Schepper, K., Bondarenko, O., Briscoe, B., and I.
Jacobson, "BBR: Congestion-Based Congestion Control; Tsang, "`Data Centre to the Home': Ultra-Low Latency for
Measuring bottleneck bandwidth and round-trip propagation All", RITE project Technical Report , 2015,
time", ACM Queue (14)5, December 2016. <http://riteproject.eu/publications/>.
[DCttH15] De Schepper, K., Bondarenko, O., Tsang, I., and B.
Briscoe, "'Data Centre to the Home': Ultra-Low Latency for
All", 2015, <http://www.bobbriscoe.net/projects/latency/
dctth_preprint.pdf>.
(Under submission)
[Hohlfeld14] [Hohlfeld14]
Hohlfeld , O., Pujol, E., Ciucu, F., Feldmann, A., and P. Hohlfeld , O., Pujol, E., Ciucu, F., Feldmann, A., and P.
Barford, "A QoE Perspective on Sizing Network Buffers", Barford, "A QoE Perspective on Sizing Network Buffers",
Proc. ACM Internet Measurement Conf (IMC'14) hmm, November Proc. ACM Internet Measurement Conf (IMC'14) hmm, November
2014. 2014.
[I-D.briscoe-conex-policing] [I-D.briscoe-conex-policing]
Briscoe, B., "Network Performance Isolation using Briscoe, B., "Network Performance Isolation using
Congestion Policing", draft-briscoe-conex-policing-01 Congestion Policing", draft-briscoe-conex-policing-01
(work in progress), February 2014. (work in progress), February 2014.
[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] [I-D.briscoe-tsvwg-l4s-diffserv]
Briscoe, B., "Interactions between Low Latency, Low Loss, Briscoe, B., "Interactions between Low Latency, Low Loss,
Scalable Throughput (L4S) and Differentiated Services", Scalable Throughput (L4S) and Differentiated Services",
draft-briscoe-tsvwg-l4s-diffserv-01 (work in progress), draft-briscoe-tsvwg-l4s-diffserv-02 (work in progress),
July 2018. 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] [I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-15 (work and Secure Transport", draft-ietf-quic-transport-20 (work
in progress), October 2018. in progress), April 2019.
[I-D.ietf-tcpm-accurate-ecn] [I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate- Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate-
ecn-07 (work in progress), July 2018. ecn-08 (work in progress), March 2019.
[I-D.ietf-tcpm-alternativebackoff-ecn]
Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", draft-ietf-tcpm-
alternativebackoff-ecn-12 (work in progress), September
2018.
[I-D.ietf-tcpm-generalized-ecn] [I-D.ietf-tcpm-generalized-ecn]
Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets", Congestion Notification (ECN) to TCP Control Packets",
draft-ietf-tcpm-generalized-ecn-03 (work in progress), draft-ietf-tcpm-generalized-ecn-03 (work in progress),
October 2018. October 2018.
[I-D.ietf-tsvwg-aqm-dualq-coupled] [I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K., Briscoe, B., Bondarenko, O., and I. Tsang, Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
"DualQ Coupled AQMs for Low Latency, Low Loss and Scalable AQMs for Low Latency, Low Loss and Scalable Throughput
Throughput (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-06 (L4S)", draft-ietf-tsvwg-aqm-dualq-coupled-09 (work in
(work in progress), July 2018. progress), July 2019.
[I-D.ietf-tsvwg-ecn-encap-guidelines] [I-D.ietf-tsvwg-ecn-encap-guidelines]
Briscoe, B., Kaippallimalil, J., and P. Thaler, Briscoe, B., Kaippallimalil, J., and P. Thaler,
"Guidelines for Adding Congestion Notification to "Guidelines for Adding Congestion Notification to
Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn- Protocols that Encapsulate IP", draft-ietf-tsvwg-ecn-
encap-guidelines-10 (work in progress), March 2018. encap-guidelines-13 (work in progress), May 2019.
[I-D.ietf-tsvwg-ecn-l4s-id] [I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. and B. Briscoe, "Identifying Modified Schepper, K. and B. Briscoe, "Identifying Modified
Explicit Congestion Notification (ECN) Semantics for Explicit Congestion Notification (ECN) Semantics for
Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s- Ultra-Low Queuing Delay (L4S)", draft-ietf-tsvwg-ecn-l4s-
id-03 (work in progress), July 2018. id-06 (work in progress), March 2019.
[I-D.smith-encrypted-traffic-management] [I-D.smith-encrypted-traffic-management]
Smith, K., "Network management of encrypted traffic", Smith, K., "Network management of encrypted traffic",
draft-smith-encrypted-traffic-management-05 (work in draft-smith-encrypted-traffic-management-05 (work in
progress), May 2016. 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] [I-D.stewart-tsvwg-sctpecn]
Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream Stewart, R., Tuexen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", draft-stewart- Control Transmission Protocol (SCTP)", draft-stewart-
tsvwg-sctpecn-05 (work in progress), January 2014. 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] [L4Sdemo16]
Bondarenko, O., De Schepper, K., Tsang, I., and B. Bondarenko, O., De Schepper, K., Tsang, I., and B.
Briscoe, "Ultra-Low Delay for All: Live Experience, Live Briscoe, "orderedUltra-Low Delay for All: Live Experience,
Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016, Live Analysis", Proc. MMSYS'16 pp33:1--33:4, May 2016,
<http://dl.acm.org/citation.cfm?doid=2910017.2910633 <http://dl.acm.org/citation.cfm?doid=2910017.2910633
(videos of demos: https://riteproject.eu/ (videos of demos: https://riteproject.eu/
dctth/#1511dispatchwg )>. dctth/#1511dispatchwg )>.
[Mathis09] [Mathis09]
Mathis, M., "Relentless Congestion Control", PFLDNeT'09 , Mathis, M., "Relentless Congestion Control", PFLDNeT'09 ,
May 2009, <http://www.hpcc.jp/pfldnet2009/ May 2009, <https://www.gdt.id.au/~gdt/
Program_files/1569198525.pdf>. presentations/2010-07-06-questnet-tcp/reference-
materials/papers/
mathis-relentless-congestion-control.pdf>.
[NewCC_Proc] [NewCC_Proc]
Eggert, L., "Experimental Specification of New Congestion Eggert, L., "Experimental Specification of New Congestion
Control Algorithms", IETF Operational Note ion-tsv-alt-cc, Control Algorithms", IETF Operational Note ion-tsv-alt-cc,
July 2007. July 2007.
[PI2] De Schepper, K., Bondarenko, O., Tsang, I., and B. [PI2] De Schepper, K., Bondarenko, O., Tsang, I., and B.
Briscoe, "PI^2 : A Linearized AQM for both Classic and Briscoe, "PI^2 : A Linearized AQM for both Classic and
Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December
2016, 2016,
<http://dl.acm.org/citation.cfm?doid=2999572.2999578>. <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 [RFC2697] Heinanen, J. and R. Guerin, "A Single Rate Three Color
Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999, Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999,
<https://www.rfc-editor.org/info/rfc2697>. <https://www.rfc-editor.org/info/rfc2697>.
[RFC2698] Heinanen, J. and R. Guerin, "A Two Rate Three Color [RFC2698] Heinanen, J. and R. Guerin, "A Two Rate Three Color
Marker", RFC 2698, DOI 10.17487/RFC2698, September 1999, Marker", RFC 2698, DOI 10.17487/RFC2698, September 1999,
<https://www.rfc-editor.org/info/rfc2698>. <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 [RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001, RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>. <https://www.rfc-editor.org/info/rfc3168>.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec, [RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D. J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002, Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>. <https://www.rfc-editor.org/info/rfc3246>.
skipping to change at page 27, line 31 skipping to change at page 30, line 27
<https://www.rfc-editor.org/info/rfc4960>. <https://www.rfc-editor.org/info/rfc4960>.
[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion [RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009, Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>. <https://www.rfc-editor.org/info/rfc5681>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925, Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/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., [RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN) and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <https://www.rfc-editor.org/info/rfc6679>. 2012, <https://www.rfc-editor.org/info/rfc6679>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>. <https://www.rfc-editor.org/info/rfc7540>.
skipping to change at page 28, line 31 skipping to change at page 31, line 31
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257, Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>. October 2017, <https://www.rfc-editor.org/info/rfc8257>.
[RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys, [RFC8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290, and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC8290, January 2018, DOI 10.17487/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>. <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 [RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311, Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018, DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>. <https://www.rfc-editor.org/info/rfc8311>.
[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and [RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018, RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>. <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>.
[TCP-sub-mss-w] [TCP-sub-mss-w]
Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion
Window for Small Round Trip Times", BT Technical Report Window for Small Round Trip Times", BT Technical Report
TR-TUB8-2015-002, May 2015, TR-TUB8-2015-002, May 2015,
<http://www.bobbriscoe.net/projects/latency/ <http://www.bobbriscoe.net/projects/latency/
sub-mss-w.pdf>. 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 Appendix A. Standardization items
The following table includes all the items that will need to be The following table includes all the items that will need to be
standardized to provide a full L4S architecture. standardized to provide a full L4S architecture.
The table is too wide for the ASCII draft format, so it has been 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 split into two, with a common column of row index numbers on the
left. left.
The columns in the second part of the table have the following The columns in the second part of the table have the following
meanings: meanings:
WG: The IETF WG most relevant to this requirement. The "tcpm/iccrg" WG: The IETF WG most relevant to this requirement. The "tcpm/iccrg"
combination refers to the procedure typically used for congestion combination refers to the procedure typically used for congestion
control changes, where tcpm owns the approval decision, but uses control changes, where tcpm owns the approval decision, but uses
the iccrg for expert review [NewCC_Proc]; the iccrg for expert review [NewCC_Proc];
TCP: Applicable to all forms of TCP congestion control; TCP: Applicable to all forms of TCP congestion control;
DCTCP: Applicable to Data Centre TCP as currently used (in DCTCP: Applicable to Data Center TCP as currently used (in
controlled environments); controlled environments);
DCTCP bis: Applicable to an future Data Centre TCP congestion DCTCP bis: Applicable to an future Data Center TCP congestion
control intended for controlled environments; control intended for controlled environments;
XXX Prague: Applicable to a Scalable variant of XXX (TCP/SCTP/RMCAT) XXX Prague: Applicable to a Scalable variant of XXX (TCP/SCTP/RMCAT)
congestion control. congestion control.
+-----+------------------------+------------------------------------+ +-----+------------------------+------------------------------------+
| Req | Requirement | Reference | | Req | Requirement | Reference |
| # | | | | # | | |
+-----+------------------------+------------------------------------+ +-----+------------------------+------------------------------------+
| 0 | ARCHITECTURE | | | 0 | ARCHITECTURE | |
skipping to change at line 1402 skipping to change at page 35, line 21
Marcelo Bagnulo Marcelo Bagnulo
Universidad Carlos III de Madrid Universidad Carlos III de Madrid
Av. Universidad 30 Av. Universidad 30
Leganes, Madrid 28911 Leganes, Madrid 28911
Spain Spain
Phone: 34 91 6249500 Phone: 34 91 6249500
Email: marcelo@it.uc3m.es Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es URI: http://www.it.uc3m.es
Greg White
CableLabs
US
Email: G.White@CableLabs.com
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