Transport Area Working Group B. Briscoe, Ed.
Internet-Draft Simula Research Lab
Intended status: Informational K. De Schepper
Expires: September 14, 2017 Nokia Bell Labs
M. Bagnulo Braun
Universidad Carlos III de Madrid
March 13, 2017

Low Latency, Low Loss, Scalable Throughput (L4S) Internet Service: Architecture


This document describes the L4S architecture for the provision of a new service that the Internet could provide to eventually replace best efforts for all traffic: Low Latency, Low Loss, Scalable throughput (L4S). It is becoming common for all (or most) applications being run by a user at any one time to require low latency. However, the only solution the IETF can offer for ultra-low queuing delay is Diffserv, which only favours a minority of packets at the expense of others. In extensive testing the new L4S service keeps average queuing delay under a millisecond for all applications even under very heavy load, without sacrificing utilization; and it keeps congestion loss to zero. It is becoming widely recognized that adding more access capacity gives diminishing returns, because latency is becoming the critical problem. Even with a high capacity broadband access, the reduced latency of L4S remarkably and consistently improves performance under load for applications such as interactive video, conversational video, voice, Web, gaming, instant messaging, remote desktop and cloud-based apps (even when all being used at once over the same access link). The insight is that the root cause of queuing delay is in TCP, not in the queue. By fixing the sending TCP (and other transports) queuing latency becomes so much better than today that operators will want to deploy the network part of L4S to enable new products and services. Further, the network part is simple to deploy - incrementally with zero-config. Both parts, sender and network, ensure coexistence with other legacy traffic. At the same time L4S solves the long-recognized problem with the future scalability of TCP throughput.

This document describes the L4S architecture, briefly describing the different components and how the work together to provide the aforementioned enhanced Internet service.

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Table of Contents

1. Introduction

It is increasingly common for all of a user's applications at any one time to require low delay: interactive Web, Web services, voice, conversational video, interactive video, instant messaging, online gaming, remote desktop and cloud-based applications. 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. When present it typically doubles the path delay from that due to the base speed-of-light. Low loss is also important because, for interactive applications, losses translate into even longer retransmission delays.

It has been demonstrated that, once access network bit rates reach levels now common in the developed world, increasing capacity offers diminishing returns if latency (delay) is not addressed. Differentiated services (Diffserv) offers Expedited Forwarding [RFC3246] for some packets at the expense of others, but this is not applicable when all (or most) of a user's applications require low latency.

Therefore, the goal is an Internet service with ultra-Low queueing Latency, ultra-Low Loss and Scalable throughput (L4S) - for all traffic. A service for all traffic will need none of the configuration or management baggage (traffic policing, traffic contracts) associated with favouring some packets over others. This document describes the L4S architecture for achieving that goal.

It must be said that queuing delay only degrades performance infrequently [Hohlfeld14]. It only occurs when a large enough 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 must be so remarkable that network operators will be motivated to deploy it.

Active Queue Management (AQM) is part of the solution to queuing under load. AQM improves performance for all traffic, but there is a limit to how much queuing delay can be reduced by solely changing the network; without addressing the root of the problem.

The root of the problem is the presence of standard TCP congestion control (Reno [RFC5681]) or compatible variants (e.g. TCP Cubic [I-D.ietf-tcpm-cubic]). We shall call this family of congestion controls 'Classic' TCP. It has been demonstrated that if the sending host replaces Classic TCP with a 'Scalable' alternative, when a suitable AQM is deployed in the network the performance under load of all the above interactive applications can be stunningly improved. For instance, queuing delay under heavy load with the example DCTCP/DualQ solution cited below is roughly 1 millisecond (1 ms) at the 99th percentile without losing link utilization. This compares with 5 to 20 ms on average with a Classic TCP and current state-of-the-art AQMs such as fq_CoDel [I-D.ietf-aqm-fq-codel] or PIE [RFC8033]. Also, with a Classic TCP, 5 ms of queuing is usually only possible by losing some utilization.

It has been convincingly demonstrated [DCttH15] that it is possible to deploy such an L4S service alongside the existing best efforts service so that all of a user's applications can shift to it when their stack is updated. Access networks are typically designed with one link as the bottleneck for each site (which might be a home, small enterprise or mobile device), so deployment at a single node should give nearly all the benefit. The L4S approach requires a number of mechanisms in different parts of the Internet to fulfill its goal. This document presents the L4S architecture, by describing the different components and how they interact to provide the scalable low-latency, low-loss, Internet service.

2. L4S architecture overview

There are three main components to the L4S architecture (illustrated in Figure 1):

1) Network:
The L4S service traffic needs to be isolated from the queuing latency of the Classic service traffic. However, the two should be able to freely share a common pool of capacity. This is because there is no way to predict how many flows at any one time might use each service and capacity in access networks is too scarce to partition into two. So a 'semi-permeable' membrane is needed that partitions latency but not bandwidth. The Dual Queue Coupled AQM [I-D.briscoe-aqm-dualq-coupled] is an example of such a semi-permeable membrane.

Per-flow queuing such as in [I-D.ietf-aqm-fq-codel] could be used, but it partitions both latency and bandwidth between every end-to-end flow. So it is rather overkill, which brings disadvantages (see Section 5.2), not least that thousands of queues are needed when two are sufficient.
2) Protocol:
A host needs to distinguish L4S and Classic packets with an identifier so that the network can classify them into their separate treatments. [I-D.briscoe-tsvwg-ecn-l4s-id] considers various alternative identifiers, and concludes that all alternatives involve compromises, but the ECT(1) codepoint of the ECN field is a workable solution.
3) Host:
Scalable congestion controls already exist. They solve the scaling problem with TCP first pointed out in [RFC3649]. The one used most widely (in controlled environments) is Data Centre TCP (DCTCP [I-D.ietf-tcpm-dctcp]), which has been implemented and deployed in Windows Server Editions (since 2012), in Linux and in FreeBSD. Although DCTCP as-is 'works' well over the public Internet, most implementations lack certain safety features that will be necessary once it is used outside controlled environments like data centres (see later). A similar scalable congestion control will also need to be transplanted into protocols other than TCP (SCTP, RTP/RTCP, RMCAT, etc.)
                     (2)                     (1)
              .-------^------. .--------------^-------------------.
 ,-(3)-----.                                  ______
; ________  :            L4S   --------.     |      |
:|Scalable| :               _\        ||___\_| mark |
:| sender | :  __________  / /        ||   / |______|\   _________
:|________|\; |          |/    --------'         ^    \1|         |
 `---------'\_|  IP-ECN  |              Coupling :     \|priority |_\
  ________  / |Classifier|                       :     /|scheduler| /
 |Classic |/  |__________|\    --------.      ___:__  / |_________|
 | sender |                \_\  || | |||___\_| mark/|/
 |________|                  /  || | |||   / | drop |
                      Classic  --------'     |______|

Figure 1: Components of an L4S Solution: 1) Isolation in separate network queues; 2) Packet Identification Protocol; and 3) Scalable Sending Host

3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying RFC-2119 significance. COMMENT: Since this will be an information document, This should be removed.

Classic service:
The 'Classic' service is intended for all the congestion control behaviours that currently co-exist with TCP Reno (e.g. TCP Cubic, Compound, SCTP, etc).
Low-Latency, Low-Loss and Scalable (L4S) service:
The 'L4S' service is intended for traffic from scalable TCP algorithms such as Data Centre TCP. But it is also more general—it will allow a set of congestion controls with similar scaling properties to DCTCP (e.g. Relentless [Mathis09]) to evolve.

Both Classic and L4S services can cope with a proportion of unresponsive or less-responsive traffic as well (e.g. DNS, VoIP, etc).
Scalable Congestion Control:
A congestion control where flow rate is inversely proportional to the level 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.
Classic Congestion Control:
A congestion control with a flow rate compatible with standard TCP Reno [RFC5681]. With Classic congestion controls, as capacity increases enabling higher flow rates, the number of round trips between congestion signals (losses or ECN marks) rises in proportion to the flow rate. So control of queuing and/or utilization becomes very slack. For instance, with 1500 B packets and an RTT of 18 ms, as TCP Reno flow rate increases from 2 to 100 Mb/s the number of round trips between congestion signals rises proportionately, from 2 to 100.

The default congestion control in Linux (TCP Cubic) is Reno-compatible for most scenarios expected for some years. For instance, with a typical domestic round-trip time (RTT) of 18ms, TCP Cubic only switches out of Reno-compatibility mode once the flow rate approaches 1 Gb/s. For a typical data centre RTT of 1 ms, the switch-over point is theoretically 1.3 Tb/s. However, with a less common transcontinental RTT of 100 ms, it only remains Reno-compatible up to 13 Mb/s. All examples assume 1,500 B packets.
Classic ECN:
The original proposed standard Explicit Congestion Notification (ECN) protocol [RFC3168], which requires ECN signals to be treated the same as drops, both when generated in the network and when responded to by the sender.
A home, mobile device, small enterprise or campus, where the network bottleneck is typically the access link to the site. Not all network arrangements fit this model but it is a useful, widely applicable generalisation.

4. L4S architecture components

The L4S architecture is composed by the following elements.

Protocols:The L4S architecture encompass the two protocol changes that we describe next:

  1. [I-D.briscoe-tsvwg-ecn-l4s-id] recommends ECT(1) is used as the identifier to classify L4S and Classic packets into their separate treatments, as required by [RFC4774].
  2. An essential aspect of a scalable congestion control is the use of explicit congestion signals rather than losses, because the signals need to be sent immediately and frequently—too often to use drops. 'Classic' ECN [RFC3168] requires an ECN signal to be treated the same as a drop, both when it is generated in the network and when it is responded to by hosts. L4S allows networks and hosts to support two separate meanings for ECN. So the standards track [RFC3168] will need to be updated to allow ECT(1) packets to depart from the 'same as drop' constraint.

    [I-D.ietf-tsvwg-ecn-experimentation] has been prepared as a standards track update to relax specific requirements in RFC 3168 (and certain other standards track RFCs), which clears the way for the above experimental changes proposed for L4S. [I-D.ietf-tsvwg-ecn-experimentation] also obsoletes the original experimental assignment of the ECT(1) codepoint as an ECN nonce [RFC3540] (it was never deployed, and it offers no security benefit now that deployment is optional).

Network components:The Dual Queue Coupled AQM has been specified as generically as possible [I-D.briscoe-aqm-dualq-coupled] as a 'semi-permeable' membrane without specifying the particular AQMs to use in the two queues. An informational appendix of the draft is provided for pseudocode examples of different possible AQM approaches. Initially a zero-config variant of RED called Curvy RED was implemented, tested and documented. The aim is for designers to be free to implement diverse ideas. So the brief normative body of the draft only specifies the minimum constraints an AQM needs to comply with to ensure that the L4S and Classic services will coexist. For instance, a variant of PIE called Dual PI Squared [PI2] has been implemented and found to perform better over a wide range of conditions, so it has been documented in a second appendix of [I-D.briscoe-aqm-dualq-coupled].

Host mechanisms: The L4S architecture includes a number of mechanisms in the end host that we enumerate next:

  1. Data Centre TCP is the most widely used example of a scalable congestion control. It is being documented in the TCPM WG as an informational record of the protocol currently in use [I-D.ietf-tcpm-dctcp]. It will be necessary to define a number of safety features for a variant usable on the public Internet. A draft list of these, known as the TCP Prague requirements, has been drawn up (see Appendix A). The list also includes some optional performance improvements.
  2. 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 transport behaviours. The following standards track RFCs currently define these protocols: ECN in TCP [RFC3168], in SCTP [RFC4960], in RTP [RFC6679], and in DCCP [RFC4340]. Not all are in widespread use, but those that are will eventually need to be updated to allow a different congestion response, which they will have to indicate by using the ECT(1) codepoint. Scalable variants are under consideration for some new transport protocols that are themselves under development, e.g. QUIC [I-D.johansson-quic-ecn] and certain real-time media congestion avoidandance techniques (RMCAT) protocols.
  3. 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 it is the same as drop, making it unusable for a scalable TCP. Therefore, the implementation of TCP receivers will have to be upgraded [RFC7560]. Work to standardize more accurate ECN feedback for TCP (AccECN [I-D.ietf-tcpm-accurate-ecn]) is already in progress.
    • ECN feedback is only roughly sketched in an appendix of the SCTP specification. A fuller specification has been proposed [I-D.stewart-tsvwg-sctpecn], which would need to be implemented and deployed before SCTCP could support L4S.

5. Rationale

5.1. Why These Primary Components?

Explicit congestion signalling (protocol):
Explicit congestion signalling is a key part of the L4S approach. In contrast, use of drop as a congestion signal creates a tension because drop is both a useful signal (more would reduce delay) and an impairment (less would reduce delay). Explicit congestion signals can be used many times per round trip, to keep tight control, without any impairment. Under heavy load, even more explicit signals can be applied so the queue can be kept short whatever the load. Whereas state-of-the-art AQMs have to introduce very high packet drop at high load to keep the queue short. Further, 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 [RFC3168]). Therefore, in a DualQ AQM, the L4S queue uses a new L4S variant of ECN that is not equivalent to drop [I-D.briscoe-tsvwg-ecn-l4s-id], while the Classic queue uses either classic ECN [RFC3168] or drop, which are equivalent.

Before Classic ECN was standardized, there were various proposals to give an ECN mark a different meaning from drop. However, there was no particular reason to agree on any one of the alternative meanings, so 'the same as drop' was the only compromise that could be reached. RFC 3168 contains a statement that:
  • "An environment where all end nodes were ECN-Capable could allow new criteria to be developed for setting the CE codepoint, and new congestion control mechanisms for end-node reaction to CE packets. However, this is a research issue, and as such is not addressed in this document."
Latency isolation with coupled congestion notification (network):
Using just two queues is not essential to L4S (more would be possible), but it is the simplest way to isolate all the L4S traffic that keeps latency low from all the legacy Classic traffic that does not.

Similarly, coupling the congestion notification between the queues is not necessarily essential, but it is a clever and simple way to allow senders to determine their rate, packet-by-packet, rather than be overridden by a network scheduler. Because otherwise a network scheduler would have to inspect at least transport layer headers, and it would have to continually assign a rate to each flow without any easy way to understand application intent.
L4S packet identifier (protocol):
Once there are at least two separate treatments in the network, hosts need an identifier at the IP layer to distinguish which treatment they intend to use.
Scalable congestion notification (host):
A scalable congestion control keeps the signalling frequency high so that rate variations can be small when signalling is stable, and rate can track variations in available capacity as rapidly as possible otherwise.

5.2. Why Not Alternative Approaches?

All the following approaches address some part of the same problem space as L4S. In each case, it is shown that L4S complements them or improves on them, rather than being a mutually exclusive alternative:

Diffserv addresses the problem of bandwidth apportionment for important traffic as well as queuing latency for delay-sensitive traffic. L4S solely addresses the problem of queuing latency (as well as loss and throughput scaling). Diffserv will still be necessary where important traffic requires priority (e.g. for commercial reasons, or for protection of critical infrastructure traffic). Nonetheless, if there are Diffserv classes for important traffic, the L4S approach can provide low latency for all traffic within each Diffserv class (including the case where there is only one Diffserv class).

Also, as already explained, Diffserv only works for a small subset of the traffic on a link. It is not applicable when all the applications in use at one time at a single site (home, small business or mobile device) require low latency. Also, because L4S is for all traffic, it needs none of the management baggage (traffic policing, traffic contracts) associated with favouring some packets over others. This baggage has held Diffserv back from widespread end-to-end deployment.
State-of-the-art AQMs:
AQMs such as PIE and fq_CoDel give a significant reduction in queuing delay relative to no AQM at all. The L4S work is intended to complement these AQMs, and we definitely do not want to distract from the need to deploy them as widely as possible. Nonetheless, without addressing the large saw-toothing rate variations of Classic congestion controls, AQMs alone cannot reduce queuing delay too far without significantly reducing link utilization. The L4S approach resolves this tension by ensuring hosts can minimize the size of their sawteeth without appearing so aggressive to legacy flows that they starve.
Per-flow queuing:
Similarly per-flow queuing is not incompatible with the L4S approach. However, one queue for every flow can be thought of as overkill compared to the minimum of two queues for all traffic needed for the L4S approach. The overkill of per-flow queuing has side-effects:
  1. fq makes high performance networking equipment costly (processing and memory) - in contrast dual queue code can be very simple;
  2. fq requires packet inspection into the end-to-end transport layer, which doesn't sit well alongside encryption for privacy - in contrast a dual queue only operates at the IP layer;
  3. fq isolates the queuing of each flow from the others and it prevents any one flow from consuming more than 1/N of the capacity. In contrast, all L4S flows are expected to keep the queue shallow, and policing of individual flows to enforce this may be applied separately, as a policy choice.

    An fq scheduler has to decide packet-by-packet which flow to schedule without knowing application intent. Whereas a separate policing function can be configured less strictly, so that senders can still control the instantaneous rate of each flow dependent on the needs of each application (e.g. variable rate video), giving more wriggle-room before a flow is deemed non-compliant. Also policing of queuing and of flow-rates can be applied independently.
Alternative Back-off ECN (ABE):
Yet again, L4S is not an alternative to ABE but a complement that introduces much lower queuing delay. ABE [I-D.khademi-tcpm-alternativebackoff-ecn] alters the host behaviour in response to ECN marking to utilize a link better and give ECN flows a faster throughput, but it assumes the network still treats ECN and drop the same. Therefore ABE exploits any lower queuing delay that AQMs can provide. But as explained above, AQMs still cannot reduce queuing delay too far without losing link utilization (for other non-ABE flows).

6. Applicability

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:

The significantly lower queuing latency also enables some interactive application functions to be offloaded to the cloud that would hardly even be usable today:

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. A panoramic video of a football stadium can be swiped and pinched so that on the fly a proxy in the cloud generates a sub-window of the match video under the finger-gesture control of each user. At the same time, a virtual reality headset fed from a 360 degree camera in a racing car has been demonstrated, where the user's head movements control the scene generated in the cloud. In both cases, with 7 ms end-to-end base delay, the additional queuing delay of roughly 1 ms is so low that it seems the video is generated locally. See for videos of these demonstrations.

Using a swiping finger gesture or head movement to pan a video are extremely demanding applications—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).

If low network delay is not available, all fine interaction has to be done locally and therefore much more redundant data has to be downloaded. When all interactive processing can be done in the cloud, only the data to be rendered for the end user needs to be sent. Whereas, once applications can rely on minimal queues in the network, they can focus on reducing their own latency by only minimizing the application send queue.

6.1. Use Cases

The following use-cases for L4S are being considered by various interested parties:

6.2. Deployment Considerations

{ToDo: This section TBA - currently, bullet points only.}

Incremental deployment parts.

Possible deployment sequences.

Prioritizing the most-likely bottlenecks in the various use-cases (access links, downstream and upstream, broadband, mobile, DC, etc).

Deployment incentives: Immediate vs. deferred benefits.

7. IANA Considerations

This specification contains no IANA considerations.

8. Security Considerations

8.1. Traffic (Non-)Policing

Because the L4S service can serve all traffic that is using the capacity of a link, it should not be necessary to police access to the L4S service. In contrast, Diffserv only works if some packets get less favourable treatement than others. So it 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 would 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 (see item 3-1 in Appendix A). 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 restict access to the L4S class, perhaps only to customers who have paid a premium. Their packet classifer (item 2 in Figure 1) could identify such customers against some other field (e.g. source address range) as well as ECN. If only the ECN L4S identifier matched, but not the source address (say), the classifier could direct these packets (from non-paying customers) into the Classic queue. Allowing operators to use an additional local classifier is intended to remove any incentive to bleach the L4S identifier. Then at least the L4S ECN identifier will be more likely to survive end-to-end even though the service may not be supported at every hop. Such arrangements would only require simple registered/not-registered packet classification, rather than the managed application-specific traffic policing against customer-specific traffic contracts that Diffserv requires.

8.2. 'Latency Friendliness'

The L4S service does rely on self-constraint - not in terms of limiting capacity usage, but in terms of limiting burstiness. It is hoped that standardisation of dynamic behaviour (cf. TCP slow-start) and self-interest will be sufficient to prevent transports from sending excessive bursts of L4S traffic, given the application's own latency will suffer most from such behaviour.

Whether burst policing becomes necessary remains to be seen. Without it, there will be potential for attacks on the low latency of the L4S service. However it may only be necessary to apply such policing reactively, e.g. punitively targeted at any deployments of new bursty malware.

8.3. Policing Prioritized L4S Bandwidth

As mentioned in Section 5.2, L4S should remove the need for low latency Diffserv classes. However, those Diffserv classes that give certain applications or users priority over capacity, would still be applicable. Then, within such Diffserv classes, L4S would often be applicable to give traffic low latency and low loss. WIthin such a class, the bandwidth available to a user or application is often limited by a rate policer. Similarly, in the default Diffserv class, rate policers are used to partition shared capacity.

A classic rate policer drops any packets exceeding a set rate, usually also giving a burst allowance (variant exist where the policer re-marks non-compliant traffic to a discard-eligible Diffserv codepoint, so they may be dropped elsewhere during contention). In networks that deploy L4S and use rate policers, it will be preferable to deploy a policer designed to be more friendly to the L4S service,

This might be achieved by setting a threshold where ECN marking is introduced, such that it is just under the policed rate or just under the burst allowance where drop is introduced. This could be applied to various types of policer, e.g. [RFC2697], [RFC2698] or the local (non-ConEx) variant of the ConEx congestion policer [I-D.briscoe-conex-policing]. Otherwise, whenever L4S traffic encounters a rate policer, it will experience drops and the source will fall back to a Classic congestion control, thus losing all the benefits of L4S.

Further discussion of the applicability of L4S to the various Diffserv classes, and the design of suitable L4S rate policers.

8.4. ECN Integrity

Receiving hosts can fool a sender into downloading faster by suppressing feedback of ECN marks (or of losses if retransmissions are not necessary or available otherwise). [RFC3540] proposes that a TCP sender could pseudorandomly set either of ECT(0) or ECT(1) in each packet of a flow and remember the sequence it had set, termed the ECN nonce. If the receiver supports the nonce, it can prove that it is not suppressing feedback by reflecting its knowledge of the sequence back to the sender. The nonce was proposed on the assumption that receivers might be more likely to cheat congestion control than senders (although senders also have a motive to cheat).

If L4S uses the ECT(1) codepoint of ECN for packet classification, it will have to obsolete the experimental nonce. As far as is known, the ECN Nonce has never been deployed, and it was only implemented for a couple of testbed evaluations. It would be nearly impossible to deploy now, because any misbehaving receiver can simply opt-out, which would be unremarkable given all receivers currently opt-out.

Other ways to protect TCP feedback integrity have since been developed. For instance:

9. Acknowledgements

Thanks to Wes Eddy, Karen Nielsen and David Black for their useful review comments.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

10.2. Informative References

[Alizadeh-stability] Alizadeh, M., Javanmard, A. and B. Prabhakar, "Analysis of DCTCP: Stability, Convergence, and Fairness", ACM SIGMETRICS 2011 , June 2011.
[DCttH15] De Schepper, K., Bondarenko, O., Tsang, I. and B. Briscoe, "'Data Centre to the Home': Ultra-Low Latency for All", 2015.

(Under submission)

[Hohlfeld14] Hohlfeld , O., Pujol, E., Ciucu, F., Feldmann, A. and P. Barford, "A QoE Perspective on Sizing Network Buffers", Proc. ACM Internet Measurement Conf (IMC'14) hmm, November 2014.
[I-D.briscoe-aqm-dualq-coupled] Schepper, K., Briscoe, B., Bondarenko, O. and I. Tsang, "DualQ Coupled AQM for Low Latency, Low Loss and Scalable Throughput", Internet-Draft draft-briscoe-aqm-dualq-coupled-01, March 2016.
[I-D.briscoe-conex-policing] Briscoe, B., "Network Performance Isolation using Congestion Policing", Internet-Draft draft-briscoe-conex-policing-01, February 2014.
[I-D.briscoe-tsvwg-ecn-l4s-id] Schepper, K., Briscoe, B. and I. Tsang, "Identifying Modified Explicit Congestion Notification (ECN) Semantics for Ultra-Low Queuing Delay", Internet-Draft draft-briscoe-tsvwg-ecn-l4s-id-02, October 2016.
[I-D.ietf-aqm-fq-codel] Hoeiland-Joergensen, T., McKenney, P.,, d., Gettys, J. and E. Dumazet, "The FlowQueue-CoDel Packet Scheduler and Active Queue Management Algorithm", Internet-Draft draft-ietf-aqm-fq-codel-06, March 2016.
[I-D.ietf-tcpm-accurate-ecn] Briscoe, B., Kuehlewind, M. and R. Scheffenegger, "More Accurate ECN Feedback in TCP", Internet-Draft draft-ietf-tcpm-accurate-ecn-02, October 2016.
[I-D.ietf-tcpm-cubic] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L. and R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", Internet-Draft draft-ietf-tcpm-cubic-04, February 2017.
[I-D.ietf-tcpm-dctcp] Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P. and G. Judd, "Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters", Internet-Draft draft-ietf-tcpm-dctcp-04, February 2017.
[I-D.ietf-tsvwg-ecn-experimentation] Black, D., "Explicit Congestion Notification (ECN) Experimentation", Internet-Draft draft-ietf-tsvwg-ecn-experimentation-01, March 2017.
[I-D.johansson-quic-ecn] Johansson, I., "ECN support in QUIC", Internet-Draft draft-johansson-quic-ecn-01, February 2017.
[I-D.khademi-tcpm-alternativebackoff-ecn] Khademi, N., Welzl, M., Armitage, G. and G. Fairhurst, "TCP Alternative Backoff with ECN (ABE)", Internet-Draft draft-khademi-tcpm-alternativebackoff-ecn-01, October 2016.
[I-D.moncaster-tcpm-rcv-cheat] Moncaster, T., Briscoe, B. and A. Jacquet, "A TCP Test to Allow Senders to Identify Receiver Non-Compliance", Internet-Draft draft-moncaster-tcpm-rcv-cheat-03, July 2014.
[I-D.stewart-tsvwg-sctpecn] Stewart, R., Tuexen, M. and X. Dong, "ECN for Stream Control Transmission Protocol (SCTP)", Internet-Draft draft-stewart-tsvwg-sctpecn-05, January 2014.
[] You, J. and C. Xiong, "The Effect of Encrypted Traffic on the QoS Mechanisms in Cellular Networks", Internet-Draft draft-you-encrypted-traffic-management-00, October 2015.
[Mathis09] Mathis, M., "Relentless Congestion Control", PFLDNeT'09 , May 2009.
[NewCC_Proc] Eggert, L., "Experimental Specification of New Congestion Control Algorithms", IETF Operational Note ion-tsv-alt-cc, July 2007.
[PI2] De Schepper, K., Bondarenko, O., Tsang, I. and B. Briscoe, "PI^2 : A Linearized AQM for both Classic and Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December 2016.
[RFC2697] Heinanen, J. and R. Guerin, "A Single Rate Three Color Marker", RFC 2697, DOI 10.17487/RFC2697, September 1999.
[RFC2698] Heinanen, J. and R. Guerin, "A Two Rate Three Color Marker", RFC 2698, DOI 10.17487/RFC2698, September 1999.
[RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, DOI 10.17487/RFC3168, September 2001.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec, J., Courtney, W., Davari, S., Firoiu, V. and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002.
[RFC3540] Spring, N., Wetherall, D. and D. Ely, "Robust Explicit Congestion Notification (ECN) Signaling with Nonces", RFC 3540, DOI 10.17487/RFC3540, June 2003.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows", RFC 3649, DOI 10.17487/RFC3649, December 2003.
[RFC4340] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", RFC 4340, DOI 10.17487/RFC4340, March 2006.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field", BCP 124, RFC 4774, DOI 10.17487/RFC4774, November 2006.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, September 2007.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P. and K. Carlberg, "Explicit Congestion Notification (ECN) for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August 2012.
[RFC7560] Kuehlewind, M., Scheffenegger, R. and B. Briscoe, "Problem Statement and Requirements for Increased Accuracy in Explicit Congestion Notification (ECN) Feedback", RFC 7560, DOI 10.17487/RFC7560, August 2015.
[RFC7665] Halpern, J. and C. Pignataro, "Service Function Chaining (SFC) Architecture", RFC 7665, DOI 10.17487/RFC7665, October 2015.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx) Concepts, Abstract Mechanism, and Requirements", RFC 7713, DOI 10.17487/RFC7713, December 2015.
[RFC8033] Pan, R., Natarajan, P., Baker, F. and G. White, "Proportional Integral Controller Enhanced (PIE): A Lightweight Control Scheme to Address the Bufferbloat Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017.
[TCP-sub-mss-w] Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion Window for Small Round Trip Times", BT Technical Report TR-TUB8-2015-002, May 2015.
[TCPPrague] Briscoe, B., "Notes: DCTCP evolution 'bar BoF': Tue 21 Jul 2015, 17:40, Prague", tcpprague mailing list archive , July 2015.

Appendix A. Required features for scalable transport protocols to be safely deployable in the Internet (a.k.a. TCP Prague requirements)

This list contains a list of features, mechanisms and modifications from currently defined behaviour for scalable Transport protocols so that they can be safely deployed over the public Internet. This list of requirements was produced at an ad hoc meeting during IETF-94 in Prague [TCPPrague].

One of such scalable transport protocols is DCTCP, currently specified in [I-D.ietf-tcpm-dctcp]. In its current form, DCTCP is specified to be deployable in controlled environments and deploying it in the public Internet would lead to a number of issues, both from the safety and the performance perspective. In this section, we describe the modifications and additional mechanisms that are required for its deployment over the global Internet. We use DCTCP as a base, but it is likely that most of these requirements equally apply to other scalable transport protocols.

We next provide a brief description of each required feature.

Requirement #4.1: Fall back to Reno/Cubic congestion control on packet loss.

Description: In case of packet loss, the scalable transport MUST react as classic TCP (whatever the classic version of TCP is running in the host, e.g. Reno, Cubic).

Motivation: As part of the safety conditions for deploying a scalable transport over the public Internet is to make sure that it behaves properly when some or all the network devices connecting the two endpoints that implement the scalable transport have not been upgraded. In particular, it may be the case that some of the switches along the path between the two endpoints may only react to congestion by dropping packets (i.e. no ECN marking). It is important that in these cases, the scalable transport react to the congestion signal in the form of a packet drop similarly to classic TCP.

In the particular case of DCTCP, the current DCTCP specification states that "It is RECOMMENDED that an implementation deal with loss episodes in the same way as conventional TCP." For safe deployment in the public Internet of a scalable transport, the above requirement needs to be defined as a MUST.

Packet loss, while rare, may also occur in the case that the bottleneck is L4S capable. In this case, the sender may receive a high number of packets marked with the CE bit set and also experience a loss. Current DCTCP implementations react differently to this situation. At least one implementation reacts only to the drop signal (e.g. by halving the CWND) and at least another DCTCP implementation reacts to both signals (e.g. by halving the CWND due to the drop and also further reducing the CWND based on the proportion of marked packet). We believe that further experimentation is needed to understand what is the best behaviour for the public Internet, which may or not be one of the existent implementations.

Requirement #4.2: Fall back to Reno/Cubic congestion control on classic ECN bottlenecks.

Description: The scalable transport protocol SHOULD/MAY? behave as classic TCP with classic ECN if the path contains a legacy bottleneck which marks both ect(0) and ect(1) in the same way as drop (non L4S, but ECN capable bottleneck).

Motivation: Similarly to Requirement #3.1, this requirement is a safety condition in case L4S-capable endpoints are communicating over a path that contains one or more non-L4S but ECN capable switches and one of them happens to be the bottleneck. In this case, the scalable transport will attempt to fill in the buffer of the bottleneck switch up to the marking threshold and produce a small sawtooth around that operation point. The result is that the switch will set its operation point with the buffer full and all other non-scalable transports will be starved (as they will react reducing their CWND more aggressively than the scalable transport).

Scalable transports then MUST be able to detect the presence of a classic ECN bottleneck and fall back to classic TCP/classic ECN behaviour in this case.

Discussion: It is not clear at this point if it is possible to design a mechanism that always detect the aforementioned cases. One possibility is to base the detection on an increase on top of a minimum RTT, but it is not yet clear which value should trigger this. Having a delay based fall back response on L4S may as well be beneficial for preserving low latency without legacy network nodes. Even if it possible to design such a mechanism, it may well be that it would encompass additional complexity that implementers may consider unnecessary. The need for this mechanism depends on the extent of classic ECN deployment.

Requirement #4.3: Reduce RTT dependence

Description: Scalable transport congestion control algorithms MUST reduce or eliminate the RTT bias within the range of RTTs available.

Motivation: Classic TCP's throughput is known to be inversely proportional to RTT. One would expect flows over very low RTT paths to nearly starve flows over larger RTTs. However, because Classic TCP induces a large queue, it has never allowed a very low RTT path to exist, so far. For instance, consider two paths with base RTT 1ms and 100ms. If Classic TCP induces a 20ms queue, it turns these RTTs into 21ms and 120ms leading to a throughput ratio of about 1:6. Whereas if a Scalable TCP induces only a 1ms queue, the ratio is 2:101. Therefore, with small queues, long RTT flows will essentially starve.

Scalable transport protocol MUST then accommodate flows across the range of RTTs enabled by the deployment of L4S service over the public Internet.

Requirement #4.4: Scaling down the congestion window.

Description: Scalable transports MUST be responsive to congestion when RTTs are significantly smaller than in the current public Internet.

Motivation: As currently specified, the minimum CWND of TCP (and the scalable extensions such as DCTCP), is set to 2 MSS. Once this minimum CWND is reached, the transport protocol ceases to react to congestion signals (the CWND is not further reduced beyond this minimum size).

L4S mechanisms reduce significantly the queueing delay, achieving smaller RTTs over the Internet. For the same CWND, smaller RTTs imply higher transmission rates. The result is that when scalable transport are used and small RTTs are achieved, the minimum value of the CWND currently defined in 2 MSS may still result in a high transmission rate for a large number of common scenarios. For example, as described in [TCP-sub-mss-w], consider a residential setting with an broadband Internet access of 40Mbps. Suppose now a number of equal TCP flows running in parallel with the Internet access link being the bottleneck. Suppose that for these flows, the RTT is 6ms and the MSS is 1500B. The minimum transmission rate supported by TCP in this scenario is when CWND is set to 2 MSS, which results in 4Mbps for each flow. This means that in this scenario, if the number of flows is higher than 10, the congestion control ceases to be responsive and starts to build up a queue in the network.

In order to address this issue, the congestion control mechanism for scalable transports MUST be responsive for the new range of RTT resulting from the decrease of the queueing delay.

There are several ways how this can be achieved. One possible sub-MSS window mechanism is described in [TCP-sub-mss-w].

In addition to the safety requirements described before, there are some optimizations that while not required for the safe deployment of scalable transports over the public Internet, would results in an optimized performance. We describe them next.

Optimization #5.1: Setting ECT in SYN, SYN/ACK and pure ACK packets.

Description: Scalable transport SHOULD set the ECT bit in SYN, SYN/ACK and pure ACK packets.

Motivation: Failing to set the ECT bit in SYN, SYN/ACK or ACK packets results in these packets being more likely dropped during congestion events. Dropping SYN and SYN/ACK packets is particularly bad for performance as the retransmission timers for these packets are large. [RFC3168] prevents from marking these packets due to security reasons. The arguments provided should be revisited in the the context of L4S and evaluate if avoiding marking these packets is still the best approach.

Optimization #5.2: Faster than additive increase.

Description: Scalable transport MAY support faster than additive increase in the congestion avoidance phase.

Motivation: As currently defined, DCTCP supports additive increase in congestion avoidance phase. It would be beneficial for performance to update the congestion control algorithm to increase the CWND more than 1 MSS per RTT during the congestion avoidance phase. In the context of L4S such mechanism, must also provide fairness with other classes of traffic, including classic TCP and possibly scalable TCP that uses additive increase.

Optimization #5.3: Faster convergence to fairness.

Description: Scalable transport SHOULD converge to a fair share allocation of the available capacity as fast as classic TCP or faster.

Motivation: The time required for a new flow to obtain its fair share of the capacity of the bottleneck when the there are already ongoing flows using up all the bottleneck capacity is higher in the case of DCTCP than in the case of classic TCP (about a factor of 1,5 and 2 larger according to [Alizadeh-stability]). This is detrimental in general, but it is very harmful for short flows, which performance can be worse than the one obtained with classic TCP. for this reason it is desirable that scalable transport provide convergence times no larger than classic TCP.

Appendix B. Standardization items

The following table includes all the itmes that should 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 [NewCC_Proc];
Applicable to all forms of TCP congestion control;
Applicable to Data Centre TCP as currently used (in controlled environments);
DCTCP bis:
Applicable to an future Data Centre TCP congestion control intended for controlled environments;
XXX Prague:
Applicable to a Scalable variant of XXX (TCP/SCTP/RMCAT) congestion control.
Req # Requirement Reference
1 L4S IDENTIFIER [I-D.briscoe-tsvwg-ecn-l4s-id]
2 DUAL QUEUE AQM [I-D.briscoe-aqm-dualq-coupled]
3 Suitable ECN Feedback [I-D.ietf-tcpm-accurate-ecn], [I-D.stewart-tsvwg-sctpecn].
4-1 Fall back to Reno/Cubic on loss [I-D.ietf-tcpm-dctcp]
4-2 Fall back to Reno/Cubic if classic ECN bottleneck detected
4-3 Reduce RTT-dependence
4-4 Scaling TCP's Congestion Window for Small Round Trip Times [TCP-sub-mss-w]
5-1 Setting ECT in SYN, SYN/ACK and pure ACK packets draft-bagnulo-tsvwg-generalized-ECN
5-2 Faster-than-additive increase
5-3 Less drastic exit from slow-start
# WG TCP DCTCP DCTCP-bis TCP Prague SCTP Prague RMCAT Prague
0 tsvwg? Y Y Y Y Y Y
1 tsvwg? Y Y Y Y
2 aqm? n/a n/a n/a n/a n/a n/a
3 tcpm Y Y Y Y n/a n/a
4-1 tcpm Y Y Y Y Y
4-2 tcpm/ iccrg? Y Y ?
4-3 tcpm/ iccrg? Y Y Y ?
4-4 tcpm Y Y Y Y Y ?
5-1 tsvwg Y Y Y Y n/a n/a
5-2 tcpm/ iccrg? Y Y Y ?
5-3 tcpm/ iccrg? Y Y Y ?

Authors' Addresses

Bob Briscoe (editor) Simula Research Lab EMail: URI:
Koen De Schepper Nokia Bell Labs Antwerp, Belgium EMail: URI:
Marcelo Bagnulo Universidad Carlos III de Madrid Av. Universidad 30 Leganes, Madrid 28911, Spain Phone: 34 91 6249500 EMail: URI: