Transport Services (tsv) K. De Schepper
Internet-Draft Bell Labs
Intended status: Experimental B. Briscoe, Ed.
Expires: April 21, 2016 Simula Research Lab
I. Tsang
Bell Labs
October 19, 2015

Identifying Modified Explicit Congestion Notification (ECN) Semantics for Ultra-Low Queuing Delay


This specification defines the identifier to be used on IP packets for a new network service called low latency, low loss and scalable throughput (L4S). It is similar to the original (or 'Classic') Explicit Congestion Notification (ECN). 'Classic' ECN marking was required to be equivalent to a drop, both when applied in the network and when responded to by a transport. Unlike 'Classic' ECN marking, the network applies the L4S identifier more immediately and more aggressively than drop, and the transport response to each mark is reduced and smoothed relative to that for drop. The two changes counterbalance each other so that the bit-rate of an L4S flow will be roughly the same as a 'Classic' flow under the same conditions. However, the much more frequent control signals and the finer responses to them result in ultra-low queuing delay without compromising link utilization, even during high load. Examples of new active queue management (AQM) marking algorithms and examples of new transports (whether TCP-like or real-time) are specified separately. The new L4S identifier is the key piece that enables them to interwork and distinguishes them from 'Classic' traffic. It gives an incremental migration path so that existing 'Classic' TCP traffic will be no worse off, but it can be prevented from degrading the ultra-low delay and loss of the new scalable transports.

Status of This Memo

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Table of Contents

1. Introduction

This specification defines the identifier to be used on IP packets for a new network service called low latency, low loss and scalable throughput (L4S). It is similar to the original (or 'Classic') Explicit Congestion Notification (ECN). 'Classic' ECN marking was required to be equivalent to a drop, both when applied in the network and when responded to by a transport. Unlike 'Classic' ECN marking, the network applies the L4S identifier more immediately and more aggressively than drop, and the transport response to each mark is reduced and smoothed relative to that for drop. The two changes counterbalance each other so that the bit-rate of an L4S flow will be roughly the same as a 'Classic' flow under the same conditions. However, the much more frequent control signals and the finer responses to them result in ultra-low queuing delay without compromising link utilization, even during high load.

An example of an active queue management (AQM) marking algorithm that enables the L4S service is the DualQ Coupled AQM defined in a complementary specification [I-D.briscoe-aqm-dualq-coupled]. An example of a scalable transport that would enable the L4S service is Data Centre TCP (DCTCP), which until now has been applicable solely to controlled environments like data centres [I-D.bensley-tcpm-dctcp], because it is too aggressive to co-exist with existing TCP. However, AQMs like DualQ Coupled enable scalable transports like DCTCP to co-exist with existing traffic, each getting roughly the same flow rate when they compete under similar conditions.

The new L4S identifier is the key piece that enables these two parts to interwork and distinguishes them from 'Classic' traffic. It gives an incremental migration path so that existing 'Classic' TCP traffic will be no worse off, but it can be prevented from degrading the ultra-low delay and loss of the new scalable transports. The performance improvement is so great that it is hoped it will motivate initial deployment of the separate parts of this system.

1.1. Problem

Latency is becoming the critical performance factor for many (most?) applications on the public Internet, e.g. Web, voice, conversational video, gaming and finance apps. In the developed world, further increases in access network bit-rate offer diminishing returns, whereas latency is still a multi-faceted problem. In the last decade or so, much has been done to reduce propagation time by placing caches or servers closer to users. However, queuing remains a major component of latency.

The Diffserv architecture provides Expedited Forwarding [RFC3246], so that low latency traffic can jump the queue of other traffic. However, on access links dedicated to individual sites (homes, small enterprises or mobile devices), often all traffic at any one time will be latency-sensitive. Then Diffserv is of little use. Instead, we need to remove the causes of any unnecessary delay.

The bufferbloat project has shown that excessively-large buffering (`bufferbloat') has been introducing significantly more delay than the underlying propagation time. These delays appear only intermittently—only when a capacity-seeking (e.g. TCP) flow is long enough for the queue to fill the buffer, making every packet in other flows sharing the buffer sit through the queue.

Active queue management (AQM) was originally developed to solve this problem (and others). Unlike Diffserv, which gives low latency to some traffic at the expense of others, AQM controls latency for all traffic in a class. In general, AQMs introduce an increasing level of discard from the buffer the longer the queue persists above a shallow threshold. This gives sufficient signals to capacity-seeking (aka. greedy) flows to keep the buffer empty for its intended purpose: absorbing bursts. However, RED [RFC2309] and other algorithms from the 1990s were sensitive to their configuration and hard to set correctly. So, AQM was not widely deployed. More recent state-of-the-art AQMs, e.g. fq_CoDel [I-D.ietf-aqm-fq-codel], PIE [I-D.ietf-aqm-pie], Adaptive RED [ARED01], define the threshold in time not bytes, so it is invariant for different link rates.

Latency is not our only concern: It was known when TCP was first developed that it would not scale to high bandwidth-delay products. Given regular broadband bit-rates over WAN distances are already [RFC3649] beyond the scaling range of `classic' TCP Reno, `less unscalable' Cubic [I-D.zimmermann-tcpm-cubic] and Compound [I-D.sridharan-tcpm-ctcp] variants of TCP have been successfully deployed. However, these are now approaching their scaling limits. Unfortunately, fully scalable TCPs such as DCTCP [I-D.bensley-tcpm-dctcp] cause `classic' TCP to starve itself, which is why they have been confined to private data centres or research testbeds (until now).

It turns out that a TCP algorithm like DCTCP that solves TCP's scalability problem also solves the latency problem, because the finer sawteeth cause very little queuing delay. A supporting paper [DCttH15] gives the full explanation of why the design solves both the latency and the scaling problems, both in plain English and in more precise mathematical form. THe explanation is summarised without the maths in [I-D.briscoe-aqm-dualq-coupled].

1.2. 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.

Classic service:
The `Classic' service is intended for all the behaviours that currently co-exist with TCP Reno (TCP Cubic, Compound, SCTP, etc).
Low-Latency, Low-Loss and Scalable (L4S):
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).
Classic ECN:
The original Explicit Congestion Notification (ECN) protocol [RFC3168].

1.3. Scope

.The new L4S identifier defined in this specification is applicable for IPv4 and IPv6 packets (as for classic ECN [RFC3168]). It is applicable for the unicast, multicast and anycast forwarding modes. It is an orthogonal packet classification to Differentiated Services (Diffserv [RFC2474]), therefore it can be applied to any packet in any Diffserv traffic class. However, as with classic ECN, any particular forwarding node might not implement an active queue management algorithm in all its DIffserv queues.

This document is intended for experimental status, so it does not update any standards track RFCs. If the experiment is successful and this document proceeds to the standards track, it would be expected to update the specification of ECN in IP and in TCP [RFC3168]. For packets carrying the L4S identifier, it would update both the network's ECN marking behaviour and the TCP response to ECN feedback, making them distinct from the behaviours for drop. It would also update the specification of ECN in RTP over UDP [RFC6679] {ToDo: DCCP and SCTP refs}. Finally, it would also obsolete the experimental ECN nonce [RFC3540].

2. L4S Packet Identifier

2.1. L4S Packet Identification Requirements

Ideally, the identifier for packets using the Low Latency, Low Loss, Scalable throughput (L4S) service ought to meet the following requirements:

It is recognised that the chosen identifier is unlikely to satisfy all these requirements, particularly given the limited space left in the IP header. Therefore a compromise will be necessary, which is why all the requirements are expressed with the word 'SHOULD' not 'MUST'. Appendix A discusses the pros and cons of the compromises made in various competing identification schemes. The chosen scheme is defined in Section 2.2 below.

Whether the identifier would be recoverable if the experiment failed is a factor that could be taken into account. However, this has not been made a requirement, because that would favour schemes that would be easier to fail, rather than those more likely to succeed.

2.2. L4S Packet Identification

The L4S treatment is an alternative packet marking treatment [RFC4774] to the classic ECN treatment [RFC3168]. Like classic ECN, it identifies the marking treatment that network nodes are expected to apply to L4S packets, and it identifies packets that are expected to have been sent from hosts applying a broad type of behaviour, termed L4S congestion control.

For a packet to receive L4S treatment as it is forwarded, the sender MUST set the ECN field in the IP header (v4 or v6) to the ECT(1) codepoint.

A network node that implements the L4S service MUST classify arriving ECT(1) packets for L4S treatment and it SHOULD classify arriving CE packets for L4S treatment as well. Section 2.3 describes an exception to this latter rule.

The L4S AQM treatment follows similar codepoint transition rules to those in RFC 3168. Specifically, the ECT(1) codepoint MUST NOT be changed to any other codepoint than CE, and CE MUST NOT be changed to any other codepoint. An ECT(1) packet is classified as ECN-capable and, if congestion increases, an L4S AQM algorithm will set the ECN marking of an increasing proportion of packets to CE, otherwise forwarding packets unchanged as ECT(1). The L4S marking treatment is defined in Section 2.4. Under persistent overload conditions, the AQM will follow RFC 3168 and turn off ECN marking, using drop as a congestion signal until the overload episode has subsided.

The L4S treatment is the default for ECT(1) packets in all Diffserv Classes [RFC4774].

For backward compatibility, a network node that implements the L4S treatment MUST also implement a classic AQM treatment. It MUST classify arriving ECT(0) and Not-ECT packets for treatment by the Classic AQM. Classic treatment means that the AQM will mark ECT(0) packets under the same conditions as it would drop Not-ECT packets [RFC3168].

2.3. L4S Packet Identification with Transport-Layer Awareness

To implement the L4S treatment, a network node does not need to identify transport-layer flows. Nonetheless, if a network node is capable of identifying transport-layer flows, it SHOULD classify CE packets for classic ECN [RFC3168] treatment if the most recent ECT packet in the same flow was ECT(0). If a network node does not identify transport-layer flows, or if the most recent ECT packet was ECT(1), it MUST classify CE packets for L4S treatment.

Only the most recent ECT packet of a flow is used to classify a CE packet, because a sender might have to switch from sending ECT(1) (L4S) packets to sending ECT(0) (Classic) packets, or back again, in the middle of a transport-layer flow. Such a switch-over is likely to be very rare, but It could be necessary if the path bottleneck moves from a network node that supports L4S to one that only supports Classic ECN. Such a change ought to be detectable from the change in RTT variation.

2.4. The Meaning of CE Relative to Drop

The likelihood that an AQM drops a Not-ECT Classic packet MUST be proportional to the square of the likelihood that it would have marked it if it had been an L4S packet. The constant of proportionality does not have to be standardised for interoperability, but a value of 1 is RECOMMENDED.

[I-D.briscoe-aqm-dualq-coupled].specifies the essential aspects of an L4S AQM, as well as recommending other aspects. It gives an example implementation in an appendix.

The term 'likelihood' is used above to allow for marking and dropping to be either probabilistic or deterministic. This example AQM in [I-D.briscoe-aqm-dualq-coupled] drops and marks probabilistically, so the drop probability is arranged to be the square of the marking probability. Nonetheless, an alternative AQM that dropped and marked deterministically would be valid, as long as the dropping frequency was proportional to the square of the marking frequency.

Note that, contrary to RFC 3168, an AQM implementing the L4S and Classic treatments does not mark an ECT(1) packet under the same conditions that it would have dropped a Not-ECT packet. However, it does mark and ECT(0) packet under the same conditions that it would have dropped a Not-ECT packet.

3. IANA Considerations

This specification contains no IANA considerations.

{ToDo: If this specification becomes and experimental RFC, should IANA be asked to update <> so that the reference for the specification of ECT(1) points to this document, and CE points to both RFC3168 and this document? I think not, because this experimental specification will not update RFC3168, which is standards track.}

4. Security Considerations

Two approaches to assure the integrity of signals using the new identifer are introduced in Appendix B.1.

5. Acknowledgements

Thanks to Richard Scheffenegger, John Leslie, David Täht, Jonathan Morton, Gorry Fairhurst, Michael Welzl, Mikael Abrahamsson and Andrew McGregor for the discussions that led to this specification.

The authors' contributions are part-funded by the European Community under its Seventh Framework Programme through the Reducing Internet Transport Latency (RITE) project (ICT-317700). The views expressed here are solely those of the authors.

6. References

6.1. 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.
[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.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field", BCP 124, RFC 4774, DOI 10.17487/RFC4774, November 2006.
[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.

6.2. Informative References

[ARED01] Floyd, S., Gummadi, R. and S. Shenker, "Adaptive RED: An Algorithm for Increasing the Robustness of RED's Active Queue Management", ACIRI Technical Report , August 2001.
[DCttH15] De Schepper, K., Bondarenko, O., Briscoe, B. and I. Tsang, "`Data Centre to the Home': Ultra-Low Latency for All", 2015.

(Under submission)

[I-D.bensley-tcpm-dctcp] Bensley, S., Eggert, L., Thaler, D., Balasubramanian, P. and G. Judd, "Microsoft's Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters", Internet-Draft draft-bensley-tcpm-dctcp-05, July 2015.
[I-D.briscoe-aqm-dualq-coupled] Schepper, K., Briscoe, B., Bondarenko, O. and i., "DualQ Coupled AQM for Low Latency, Low Loss and Scalable Throughput", Internet-Draft draft-briscoe-aqm-dualq-coupled-00, August 2015.
[I-D.ietf-aqm-fq-codel] Hoeiland-Joergensen, T., McKenney, P.,, d., Gettys, J. and E. Dumazet, "FlowQueue-Codel", Internet-Draft draft-ietf-aqm-fq-codel-02, October 2015.
[I-D.ietf-aqm-pie] Pan, R., Natarajan, P. and F. Baker, "PIE: A Lightweight Control Scheme To Address the Bufferbloat Problem", Internet-Draft draft-ietf-aqm-pie-02, August 2015.
[I-D.ietf-conex-abstract-mech] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx) Concepts, Abstract Mechanism and Requirements", Internet-Draft draft-ietf-conex-abstract-mech-13, October 2014.
[I-D.ietf-tcpm-accecn-reqs] Kuehlewind, M., Scheffenegger, R. and B. Briscoe, "Problem Statement and Requirements for a More Accurate ECN Feedback", Internet-Draft draft-ietf-tcpm-accecn-reqs-08, March 2015.
[I-D.ietf-tsvwg-ecn-encap-guidelines] Briscoe, B., Kaippallimalil, J. and P. Thaler, "Guidelines for Adding Congestion Notification to Protocols that Encapsulate IP", Internet-Draft draft-ietf-tsvwg-ecn-encap-guidelines-04, October 2015.
[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.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", Internet-Draft draft-sridharan-tcpm-ctcp-02, November 2008.
[I-D.zimmermann-tcpm-cubic] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L. and R. Scheffenegger, "CUBIC for Fast Long-Distance Networks", Internet-Draft draft-zimmermann-tcpm-cubic-01, April 2015.
[Mathis09] Mathis, M., "Relentless Congestion Control", PFLDNeT'09 , May 2009.
[QV] Briscoe, B. and P. Hurtig, "Up to Speed with Queue View", RITE Technical Report , August 2015.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering, S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G., Partridge, C., Peterson, L., Ramakrishnan, K., Shenker, S., Wroclawski, J. and L. Zhang, "Recommendations on Queue Management and Congestion Avoidance in the Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, DOI 10.17487/RFC2474, December 1998.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983, DOI 10.17487/RFC2983, October 2000.
[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.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S. and K. Ramakrishnan, "Adding Explicit Congestion Notification (ECN) Capability to TCP's SYN/ACK Packets", RFC 5562, DOI 10.17487/RFC5562, June 2009.
[RFC6077] Papadimitriou, D., Welzl, M., Scharf, M. and B. Briscoe, "Open Research Issues in Internet Congestion Control", RFC 6077, DOI 10.17487/RFC6077, February 2011.
[RFC6660] Briscoe, B., Moncaster, T. and M. Menth, "Encoding Three Pre-Congestion Notification (PCN) States in the IP Header Using a Single Diffserv Codepoint (DSCP)", RFC 6660, DOI 10.17487/RFC6660, July 2012.
[VCP] Xia, Y., Subramanian, L., Stoica, I. and S. Kalyanaraman, "One more bit is enough", Proc. SIGCOMM'05, ACM CCR 35(4)37--48, 2005.

Appendix A. Alternative Identifiers

This appendix is informative, not normative. It records the pros and cons of various alternative ways to identify L4S packets to record the rationale for the choice of ECT(1) (Appendix A.1) as the L4S identifier. At the end, Appendix A.6 summarises the distinguishing features of the leading alternatives,.It is intended to supplement, not replace the detailed text.

The leading solutions all use the ECN field, sometimes in combination with the Diffserv field. Both the ECN and Diffserv fields have the additional advantage that they are no different in either IPv4 or IPv6. A couple of alternatives that use other fields are mentioned at the end, but it is quickly explained why they are not serious contenders.

A.1. ECT(1) and CE codepoints



Consumes the last ECN codepoint:
The L4S service is intended to supersede the service provided by classic ECN, therefore using ECT(1) to identify L4S packets could ultimately mean that the ECT(0) codepoint was `wasted' purely to distinguish one form of ECN from its successor;
ECN hard in some lower layers:
It is not always possible to support ECN in an AQM acting in a buffer below the IP layer [I-D.ietf-tsvwg-ecn-encap-guidelines]. In such cases, the L4S service would have to drop rather than mark frames even though they might contain an ECN-capable packet. However, such cases would be unusual.
Risk of reordering classic CE packets:
Having to classify all CE packets as L4S risks some classic CE packets arriving early, which is a form of reordering. Reordering can cause the TCP sender to retransmit spuriously. However, one or two packets delivered early does not cause any spurious retransmissions because the subsequent packets continue to move the cumulative acknowledgement boundary forwards. Anyway, even the risk of reordering would be low, because: i) it is quite unusual to experience more than one bottleneck queue on a path; ii) even then, reordering would only occur if there was simultaneous mixing of classic and L4S traffic, which would be more unlikely in an access link, which is where most bottlenecks are located; iii) even then, spurious retransmissions would only occur if a contiguous sequence of three or more classic CE packets from one bottleneck arrived at the next, which should in itself happen very rarely with a good AQM. The risk would be completely eliminated in AQMs that were transport-aware (but they should not need to be);
Non-L4S service for control packets:
The classic ECN RFCs [RFC3168] and [RFC5562] require a sender to clear the ECN field to Not-ECT for retransmissions and certain control packets specifically pure ACKs, window probes and SYNs. When L4S packets are classified by the ECN field alone, these control packets would not be classified into an L4S queue, and could therefore be delayed relative to the other packets in the flow. This would not cause re-ordering (because retransmissions are already out of order, and the control packets carry no data). However, it would make critical control packets more vulnerable to loss and delay. {ToDo: Discuss the likelihood that all these packets might be made ECN-capable in future.}


Should work e2e:
The ECN field generally works end-to-end across the Internet. Unlike the DSCP, the setting of the ECN field is at least forwarded unchanged by networks that do not support ECN, and networks rarely clear it to zero;
Should work in tunnels:
Unlike Diffserv, ECN is defined to always work across tunnels. However, tunnels do not always implement ECN processing as they should do, particularly because IPsec tunnels were defined differently for a few years.
Could migrate to one codepoint:
If all classic ECN senders eventually evolve to use the L4S service, the ECT(0) codepoint could be reused for some future purpose, but only once use of ECT(0) packets had reduced to zero, or near-zero, which might never happen.

A.2. ECN Plus a Diffserv Codepoint (DSCP)


Use of a DSCP is the only approach for alternative ECN semantics given as an example in [RFC4774]. However, it was perhaps considered more for controlled environments than new end-to-end services;


Consumes DSCP pairs:
A DSCP is obviously not orthogonal to Diffserv. Therefore, wherever the L4S service is applied to multiple Diffserv scheduling behaviours, it would be necessary to replace each DSCP with a pair of DSCPs.
Uses critical lower-layer header space:
The resulting increased number of DSCPs might be hard to support for some lower layer technologies, e.g. 802.1p and MPLS both offer only 3-bits for a maximum of 8 traffic class identifiers. Although L4S should reduce and possibly remove the need for some DSCPs intended for differentiated queuing delay, it will not remove the need for Diffserv entirely, because Diffserv is also used to allocate bandwidth, e.g. by prioritising some classes of traffic over others when traffic exceeds available capacity.
Not end-to-end (host-network):
Very few networks honour a DSCP set by a host. Typically a network will zero (bleach) the Diffserv field from all hosts. Sometimes networks will attempt to identify applications by some form of packet inspection and, based on network policy, they will set the DSCP considered appropriate for the identified application. Network-based application identification might use some combination of protocol ID, port numbers(s), application layer protocol headers, IP address(es), VLAN ID(s) and even packet timing.
Not end-to-end (network-network):
Very few networks honour a DSCP received from a neighbouring network. Typically a network will zero (bleach) the Diffserv field from all neighbouring networks at an interconnection point. Sometimes bilateral arrangements are made between networks, such that the receiving network remarks some DSCPs to those it uses for roughly equivalent services. The likelihood that a DSCP will be bleached or ignored depends on the type of DSCP:
Local-use DSCP:
These tend to be used to implement application-specific network policies, but a bilateral arrangement to remark certain DSCPs is often applied to DSCPs in the local-use range simply because it is easier not to change all of a network's internal configurations when a new arrangement is made with a neighbour;
Global-use DSCP:
These do not tend to be honoured across network interconnections more than local-use DSCPs. However, if two networks decide to honour certain of each other's DSCPs, the reconfiguration is a little easier if both of their globally recognised services are already represented by the relevant global-use DSCPs.

Note that today a global-use DSCP gives little more assurance of end-to-end service than a local-use DSCP. In future the global-use range might give more assurance of end-to-end service than local-use, but it is unlikely that either assurance will be high, particularly given the hosts are included in the end-to-end path.
Not all tunnels:
Diffserv codepoints are often not propagated to the outer header when a packet is encapsulated by a tunnel header. DSCPs are propagated to the outer of uniform mode tunnels, but not pipe mode [RFC2983], and pipe mode is fairly common.
ECN hard in some lower layers::
Because this approach uses both the Diffserv and ECN fields, an AQM wil only work at a lower layer if both can be supported. If individual network operators wished to deploy an AQM at a lower layer, they would usually propagate an IP Diffserv codepoint to the lower layer, using for example IEEE 802.1p. However, the ECN capability is harder to propagate down to lower layers because few lower layers support it.


Could migrate to e2e:
If all usage of classic ECN migrates to usage of L4S, the DSCP would become redundant, and the ECN capability alone could eventually identify L4S packets without the interconnection problems of Diffserv detailed below, and without having permanently consumed more than one codepoint in the IP header. Although the DSCP does not generally function as an end-to-end identifier (see below), it could be used initially by individual ISPs to introduce the L4S service for their own locally generated traffic;

A.3. ECN capability alone



Near-infeasible deployment constraints:
The constraints for deployment above represent a highly unlikely set of circumstances, but not completely impossible. If, despite the above measures, a pair of hosts did negotiate to use classic ECN, their packets would be classified into the same queue as L4S traffic, and if they had to compete with a long-running L4S flow they would get a very small capacity share;
ECN hard in some lower layers:
See the same issue with "ECT(1) and CE codepoints" (Appendix A.1);
Non-L4S service for control packets:
See the same issue with "ECT(1) and CE codepoints" (Appendix A.1).


Consumes no additional codepoints:
The ECT(1) codepoint and all spare Diffserv codepoints would remain available for future use;
Should work e2e:
As with "ECT(1) and CE codepoints" (Appendix A.1);
Should work in tunnels:
As with "ECT(1) and CE codepoints" (Appendix A.1).

A.4. Protocol ID

It has been suggested that a new ID in the IPv4 Protocol field or the IPv6 Next Header field could identify L4S packets. However this approach is ruled out by numerous problems:

A.5. Source or destination addressing

Locally, a network operator could arrange for L4S service to be applied based on source or destination addressing, e.g. packets from its own data centre and/or CDN hosts, packets to its business customers, etc. It could use addressing at any layer, e.g. IP addresses, MAC addresses, VLAN IDs, etc. Although addressing might be a useful tactical approach for a single ISP, it would not be a feasible approach to identify an end-to-end service like L4S. Even for a single ISP, it would require packet classifiers in buffers to be dependent on changing topology and address allocation decisions elsewhere in the network. Therefore this approach is not a feasible solution.

A.6. Summary: Merits of Alternative Identifiers

Table 1 provides a very high level summary of the pros and cons detailed against the schemes described respectively in Appendix A.2, Appendix A.3 and Appendix A.1, for six issues that set them apart.

Comparison of the Merits of Three Alternative Identifiers
Issue DSCP + ECN ECN ECT(1) + CE
initial   eventual initial initial   eventual
end-to-end N . .      . ? . . . Y . . Y      . . Y
tunnels . O .      . O . . . ? . . ?      . . Y
lower layers N . .      . ? . . O . . O .      . . ?
codepoints N . .      . . ? . . Y N . .      . . ?
reordering . . Y      . . Y . . Y . O .      . . ?
ctrl pkts . . Y      . . Y . O . . O .      . . ?
Note 1

Note 1: Only feasible if classic ECN is obsolete.

The schemes are scored based on both their capabilities now ('initial') and in the long term ('eventual'). The 'ECN' scheme shares the 'eventual' scores of the 'ECT(0) + CE' scheme. The scores are one of 'N, O, Y', meaning 'Poor', 'Ordinary', 'Good' respectively. The same scores are aligned vertically to aid the eye. A score of "?" in one of the positions means that this approach might optimisitically become this good, given sufficient effort. The table is not meant to be understandable without referring to the text.

Appendix B. Potential Competing Uses for the ECT(1) Codepoint

The ECT(1) codepoint of the ECN field has already been assigned once for experimental use [RFC3540]. ECN is probably the only remaining field in the Internet Protocol that is common to IPv4 and IPv6 and still has potential to work end-to-end, with tunnels and with lower layers. Therefore, ECT(1) should not be reassigned to a different experimental use without carefully assessing competing potential uses. These fall into the following categories:

B.1. Integrity of Congestion Feedback

Receiving hosts can fool a sender into downloading faster by suppressing feedback of ECN marks (or loss if retransmissions are not necessary or available otherwise). [RFC3540] proposes that a TCP sender could set either ECT(0) or ECT(1) in each packet of a flow and remember the pattern, termed the ECN nonce. If any packet is lost or congestion marked, the receiver will miss that bit of the sequence. An ECN Nonce receiver has to feed back the least significant bit of the sum, so it cannot suppress feedback of a loss or mark without a 50-50 chance of guessing the sum incorrectly.

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 that do not consume any extra codepoints. For instance:

ECN in RTP [RFC6679] is defined so that the receiver can ask the sender to send all ECT(0); all ECT(1); or both randomly. It recommends that the receiver asks for ECT(0), which is the default. The sender can choose to ignore the receiver's request. A rather complex but optional nonce mechaism was included in early drafts of RFC 6679, but it was replaced with a statement that a nonce mechanism is not specified, explaining that misbehaving receivers could opt-out anyway. RFC 6679 as published gives no rationale for why ECT(1) or 'random' might be needed, but it warns that 'random' would make header compression highly inefficient. The possibility of using ECT(1) may have been left in the RFC to allow a nonce mechanism to be added later.

Therefore, it seems unlikely that anyone has implemented the optional use of ECT(1) for RTP, it even if they have, it seems even less likely that any deployment actually uses it. However these assumptions will need to be verified.

B.2. Notification of Less Severe Congestion than CE

Various researchers have proposed to use ECT(1) as a less severe congestion notification than CE, particularly to enable flows to fill available capacity more quickly after an idle period, when another flow departs or when a flow starts, e.g. VCP [VCP], Queue View (QV) [QV] {ToDo: Jonathan Morton's ELR if relevant once the promised write-up appears}.

Before assigning ECT(1) as an identifer for L4S, we must carefully consider whether it might be better to hold ECT(1) in reserve for future standardisation of rapid flow acceleration, which is an important and enduring problem [RFC6077].

Pre-Congestion Notification (PCN) is another scheme that assigns alternative semantics to the ECN field. It uses ECT(1) to signify a less severe level of pre-congestion notification than CE [RFC6660]. However, the ECN field only takes on the PCN semantics if packets carry a Diffserv codepoint defined to indicate PCN marking within a controlled environment. PCN is required to be applied solely to the outer header of a tunnel across the controlled region in order not to interfere with any end-to-end use of the ECN field. Therefore a PCN region on the path would not interfere with any of the L4S service identifiers proposed in Appendix A.

Authors' Addresses

Koen De Schepper Bell Labs Antwerp, Belgium EMail: URI:
Bob Briscoe (editor) Simula Research Lab EMail: URI:
Ing-jyh Tsang Bell Labs Antwerp, Belgium EMail: