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IETF RFC 9331
Last modified on Friday, January 20th, 2023
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Internet Engineering Task Force (IETF) K. De Schepper
Request for Comments: 9331 Nokia Bell Labs
Category: Experimental B. Briscoe, Ed.
ISSN: 2070-1721 Independent
January 2023
The Explicit Congestion Notification (ECN) Protocol for Low Latency, Low
Loss, and Scalable Throughput (L4S)
 Abstract
This specification defines the protocol to be used for a new network
service called Low Latency, Low Loss, and Scalable throughput (L4S).
L4S uses an Explicit Congestion Notification (ECN) scheme at the IP
layer that is similar to the original (or 'Classic') ECN approach,
except as specified within. L4S uses 'Scalable' congestion control,
which induces much more frequent control signals from the network,
and it responds to them with much more fine-grained adjustments so
that very low (typically sub-millisecond on average) and consistently
low queuing delay becomes possible for L4S traffic without
compromising link utilization. Thus, even capacity-seeking (TCP-
like) traffic can have high bandwidth and very low delay at the same
time, even during periods of high traffic load.
The L4S identifier defined in this document distinguishes L4S from
'Classic' (e.g., TCP-Reno-friendly) traffic. Then, network
bottlenecks can be incrementally modified to distinguish and isolate
existing traffic that still follows the Classic behaviour, to prevent
it from degrading the low queuing delay and low loss of L4S traffic.
This Experimental specification defines the rules that L4S transports
and network elements need to follow, with the intention that L4S
flows neither harm each other's performance nor that of Classic
traffic. It also suggests open questions to be investigated during
experimentation. Examples of new Active Queue Management (AQM)
marking algorithms and new transports (whether TCP-like or real time)
are specified separately.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are candidates for any level of
Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9331.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Latency, Loss, and Scaling Problems
1.2. Terminology
1.3. Scope
2. L4S Packet Identification: Document Roadmap
3. Choice of L4S Packet Identifier: Requirements
4. Transport-Layer Behaviour (the 'Prague L4S Requirements')
4.1. Codepoint Setting
4.2. Prerequisite Transport Feedback
4.3. Prerequisite Congestion Response
4.3.1. Guidance on Congestion Response in the RFC Series
4.4. Filtering or Smoothing of ECN Feedback
5. Network Node Behaviour
5.1. Classification and Re-Marking Behaviour
5.2. The Strength of L4S CE Marking Relative to Drop
5.3. Exception for L4S Packet Identification by Network Nodes
with Transport-Layer Awareness
5.4. Interaction of the L4S Identifier with Other Identifiers
5.4.1. DualQ Examples of Other Identifiers Complementing L4S
Identifiers
5.4.1.1. Inclusion of Additional Traffic with L4S
5.4.1.2. Exclusion of Traffic from L4S Treatment
5.4.1.3. Generalized Combination of L4S and Other
Identifiers
5.4.2. Per-flow Queuing Examples of Other Identifiers
Complementing L4S Identifiers
5.5. Limiting Packet Bursts from Links
5.5.1. Limiting Packet Bursts from Links Fed by an L4S AQM
5.5.2. Limiting Packet Bursts from Links Upstream of an L4S
AQM
6. Behaviour of Tunnels and Encapsulations
6.1. No Change to ECN Tunnels and Encapsulations in General
6.2. VPN Behaviour to Avoid Limitations of Anti-Replay
7. L4S Experiments
7.1. Open Questions
7.2. Open Issues
7.3. Future Potential
8. IANA Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Rationale for the 'Prague L4S Requirements'
A.1. Rationale for the Requirements for Scalable Transport
Protocols
A.1.1. Use of L4S Packet Identifier
A.1.2. Accurate ECN Feedback
A.1.3. Capable of Replacement by Classic Congestion Control
A.1.4. Fall Back to Classic Congestion Control on Packet Loss
A.1.5. Coexistence with Classic Congestion Control at Classic
ECN Bottlenecks
A.1.6. Reduce RTT Dependence
A.1.7. Scaling Down to Fractional Congestion Windows
A.1.8. Measuring Reordering Tolerance in Time Units
A.2. Scalable Transport Protocol Optimizations
A.2.1. Setting ECT in Control Packets and Retransmissions
A.2.2. Faster than Additive Increase
A.2.3. Faster Convergence at Flow Start
Appendix B. Compromises in the Choice of L4S Identifier
Appendix C. Potential Competing Uses for the ECT(1) Codepoint
C.1. Integrity of Congestion Feedback
C.2. Notification of Less Severe Congestion than CE
Acknowledgements
Authors' Addresses
1. Introduction
This Experimental specification defines the protocol to be used for a
new network service called Low Latency, Low Loss, and Scalable
throughput (L4S). L4S uses an Explicit Congestion Notification (ECN)
scheme at the IP layer with the same set of codepoint transitions as
the original (or 'Classic') ECN [RFC 3168]. [RFC 3168] requires an ECN
mark to be equivalent to a drop, both when applied in the network and
when responded to by a transport. Unlike Classic ECN marking, i) the
network applies L4S marking more immediately and more frequently than
drop and ii) the transport response to each mark is reduced and
smoothed relative to that for drop. The two changes counterbalance
each other so that the throughput of an L4S flow will be roughly the
same as a comparable non-L4S flow under the same conditions.
Nonetheless, the much more frequent ECN control signals and the finer
responses to these signals result in very low queuing delay without
compromising link utilization, and this low delay can be maintained
during high load. For instance, queuing delay under heavy and highly
varying load with the example DCTCP/DualQ solution described below on
a DSL or Ethernet link is sub-millisecond on average and roughly 1 to
2 milliseconds at the 99th percentile without losing link utilization
[L4Seval22] [DualPI2Linux]. Note that the queuing delay while
waiting to acquire a shared medium such as wireless has to be added
to the above. It is a different issue that needs to be addressed,
but separately (see Section 6.3 of the L4S architecture [RFC 9330]).
L4S relies on 'Scalable' congestion controls for these delay
properties and for preserving low delay as flow rate scales, hence
the name. The congestion control used in Data Center TCP (DCTCP) is
an example of a Scalable congestion control, but DCTCP is applicable
solely to controlled environments like data centres [RFC 8257],
because it is too aggressive to coexist with existing TCP-Reno-
friendly traffic. Dual-Queue Coupled AQM, which is defined in a
complementary Experimental specification [RFC 9332], is an AQM
framework that enables Scalable congestion controls derived from
DCTCP to coexist with existing traffic, each getting roughly the same
flow rate when they compete under similar conditions. Note that a
Scalable congestion control is still not safe to deploy on the
Internet unless it satisfies the requirements listed in Section 4.
L4S is not only for elastic (TCP-like) traffic -- there are Scalable
congestion controls for real-time media, such as the L4S variant
[SCReAM-L4S] of the SCReAM [RFC 8298] RTP Media Congestion Avoidance
Techniques (RMCAT). The factor that distinguishes L4S from Classic
traffic is its behaviour in response to congestion. The transport
wire protocol, e.g., TCP, QUIC, the Stream Control Transmission
Protocol (SCTP), the Datagram Congestion Control Protocol (DCCP), or
RTP/RTCP, is orthogonal (and therefore not suitable for
distinguishing L4S from Classic packets).
The L4S identifier defined in this document is the key piece that
distinguishes L4S from 'Classic' (e.g., Reno-friendly) traffic.
Then, network bottlenecks can be incrementally modified to
distinguish and isolate existing Classic traffic from L4S traffic, to
prevent the former from degrading the very low queuing delay and loss
of the new Scalable transports, without harming Classic performance
at these bottlenecks. Although both sender and network deployment
are required before any benefit, initial implementations of the
separate parts of the system have been motivated by the potential
performance benefits.
1.1. Latency, Loss, and Scaling Problems
Latency is becoming the critical performance factor for many (perhaps
most) Internet applications, e.g., interactive web, web services,
voice, conversational video, interactive video, interactive remote
presence, instant messaging, online gaming, remote desktop, cloud-
based applications & services, and remote control of machinery and
industrial processes. In many parts of the world, further increases
in access network bitrate offer diminishing returns [Dukkipati06],
whereas latency is still a multi-faceted problem. As a result, much
has been done to reduce propagation time by placing caches or servers
closer to users. However, queuing remains a major, albeit
intermittent, component of latency.
The Diffserv architecture provides Expedited Forwarding (EF)
[RFC 3246] so that low-latency traffic can jump the queue of other
traffic. If growth in latency-sensitive applications continues,
periods with solely latency-sensitive traffic will become
increasingly common on links where traffic aggregation is low.
During these periods, if all the traffic were marked for the same
treatment, Diffserv would make no difference. The links with low
aggregation also tend to become the path bottleneck under load, for
instance, the access links dedicated to individual sites (homes,
small enterprises, or mobile devices). So, instead of
differentiation, it becomes imperative to remove the underlying
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 [Bufferbloat]. These delays appear
only intermittently -- only when a capacity-seeking (e.g., TCP) flow
is long enough for the queue to fill the buffer, causing every packet
in other flows sharing the buffer to have to work its way through the
queue.
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, AQM methods 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 (a.k.a. greedy)
flows to keep the buffer empty for its intended purpose: absorbing
bursts. However, Random Early Detection (RED) and other algorithms
from the 1990s were sensitive to their configuration and hard to set
correctly [RFC 7567]. So this form of AQM was not widely deployed.
More recent state-of-the-art AQM methods, such as Flow Queue CoDel
[RFC 8290], Proportional Integral controller Enhanced (PIE) [RFC 8033],
or Adaptive RED [ARED01], are easier to configure, because they
define the queuing threshold in time not bytes, so configuration is
invariant whatever the link rate. However, the sawtoothing window of
a Classic congestion control creates a dilemma for the operator:
either i) configure a shallow AQM operating point so the tips of the
sawteeth cause minimal queue delay, but then the troughs underutilize
the link, or ii) configure the operating point deeper into the buffer
so the troughs utilize the link better, but then the tips cause more
delay variation. Even with a perfectly tuned AQM, the additional
queuing delay at the tips of the sawteeth will be of the same order
as the underlying base round-trip time (RTT), thereby roughly
doubling the total RTT.
If a sender's own behaviour is introducing queuing delay variation,
no AQM in the network can 'un-vary' the delay without significantly
compromising link utilization. Even flow queuing (e.g., [RFC 8290]),
which isolates one flow from another, cannot isolate a flow from the
delay variations it inflicts on itself. Therefore, those
applications that need to seek out high bandwidth but also need low
latency will have to migrate to Scalable congestion control, which
uses much smaller sawtooth variations.
Altering host behaviour is not enough on its own though. Even if
hosts adopt low-latency Scalable congestion controls, they need to be
isolated from the large queue variations induced by existing Classic
congestion controls. L4S AQMs provide that latency isolation in the
network, and the L4S identifier enables the AQMs to distinguish the
two types of packets that need to be isolated: L4S and Classic. L4S
isolation can be achieved with a queue per flow (e.g., [RFC 8290]),
but a DualQ [RFC 9332] is sufficient and actually gives better tail
latency [DCttH19]. Both approaches are addressed in this document.
The DualQ solution was developed to make very low latency available
without requiring per-flow queues at every bottleneck. This was
useful because per-flow queuing (FQ) has well-known downsides -- not
least the need to inspect transport-layer headers in the network,
which makes it incompatible with privacy approaches such as IPsec
Virtual Private Network (VPN) tunnels and incompatible with link-
layer queue management, where transport-layer headers can be hidden,
e.g., 5G.
Latency is not the only concern addressed by L4S. It was known when
TCP congestion avoidance was first developed that it would not scale
to high bandwidth-delay products (see footnote 6 of Jacobson and
Karels [TCP-CA]). Given that Reno congestion control is already
beyond its scaling range at regular broadband bitrates over WAN
distances [RFC 3649], 'less unscalable' CUBIC [RFC 8312] and Compound
[CTCP] variants of TCP have been successfully deployed. However,
these are now approaching their scaling limits. Unfortunately, fully
Scalable congestion controls such as DCTCP [RFC 8257] outcompete
Classic ECN congestion controls sharing the same queue, which is why
they have been confined to private data centres or research testbeds.
It turns out that these Scalable congestion control algorithms that
solve the latency problem can also solve the scalability problem of
Classic congestion controls. The finer sawteeth in the congestion
window (cwnd) have low amplitude, so they cause very little queuing
delay variation, and the average time to recover from one congestion
signal to the next (the average duration of each sawtooth) remains
invariant, which maintains constant tight control as flow rate
scales. A background paper [L4Seval22] 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 summarized without the mathematics in Section 4 of the
L4S architecture [RFC 9330].
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
Classic Congestion Control: A congestion control behaviour that can
coexist with standard Reno [RFC 5681] without causing significantly
negative impact on its flow rate [RFC 5033]. With Classic
congestion controls, such as Reno or CUBIC, because flow rate has
scaled since TCP congestion control was first designed in 1988, it
now takes hundreds of round trips (and growing) to recover after a
congestion signal (whether a loss or an ECN mark) as shown in the
examples in Section 5.1 of the L4S architecture [RFC 9330] and in
[RFC 3649]. Therefore, control of queuing and utilization becomes
very slack, and the slightest disturbances (e.g., from new flows
starting) prevent a high rate from being attained.
Scalable Congestion Control: A congestion control where the average
time from one congestion signal to the next (the recovery time)
remains invariant as flow rate scales, all other factors being
equal. This maintains the same degree of control over queuing and
utilization whatever the flow rate, as well as ensuring that high
throughput is robust to disturbances. For instance, DCTCP
averages 2 congestion signals per round trip, whatever the flow
rate, as do other recently developed Scalable congestion controls,
e.g., Relentless TCP [RELENTLESS], Prague for TCP and QUIC
[PRAGUE-CC] [PragueLinux], the L4S ECN part of Bottleneck
Bandwidth and Round-trip propagation time (BBRv2) [BBRv2]
[BBR-CC], and the L4S variant of SCReAM for real-time media
[SCReAM-L4S] [RFC 8298]. See Section 4.3 for more explanation.
Classic Service: The Classic service is intended for all the
congestion control behaviours that coexist with Reno [RFC 5681]
(e.g., Reno itself, CUBIC [RFC 8312], Compound [CTCP], and TFRC
[RFC 5348]). The term 'Classic queue' means a queue providing the
Classic service.
Low Latency, Low Loss, and Scalable throughput (L4S) service: The
'L4S' service is intended for traffic from Scalable congestion
control algorithms, such as the Prague congestion control
[PRAGUE-CC], which was derived from DCTCP [RFC 8257]. The L4S
service is for more general traffic than just Prague -- it allows
the set of congestion controls with similar scaling properties to
Prague to evolve, such as the examples listed above (Relentless,
SCReAM, etc.). The term 'L4S queue' means a queue providing the
L4S service.
The terms Classic or L4S can also qualify other nouns, such as
'queue', 'codepoint', 'identifier', 'classification', 'packet',
and 'flow'. For example, an L4S packet means a packet with an L4S
identifier sent from an L4S congestion control.
Both Classic and L4S services can cope with a proportion of
unresponsive or less-responsive traffic as well but, in the L4S
case, its rate has to be smooth enough or low enough to not build
a queue (e.g., DNS, Voice over IP (VoIP), game sync datagrams,
etc.).
Reno-friendly: The subset of Classic traffic that is friendly to the
standard Reno congestion control defined for TCP in [RFC 5681].
The TFRC spec [RFC 5348] indirectly implies that 'friendly' is
defined as "generally within a factor of two of the sending rate
of a TCP flow under the same conditions". Reno-friendly is used
here in place of 'TCP-friendly', given the latter has become
imprecise, because the TCP protocol is now used with so many
different congestion control behaviours, and Reno is used in non-
TCP transports, such as QUIC [RFC 9000].
Classic ECN: The original Explicit Congestion Notification (ECN)
protocol [RFC 3168] that requires ECN signals to be treated as
equivalent to drops, both when generated in the network and when
responded to by the sender.
For L4S, the names used for the four codepoints of the 2-bit IP-
ECN field are unchanged from those defined in the ECN spec
[RFC 3168], i.e., Not-ECT, ECT(0), ECT(1), and CE, where ECT stands
for ECN-Capable Transport and CE stands for Congestion
Experienced. A packet marked with the CE codepoint is termed
'ECN-marked' or sometimes just 'marked' where the context makes
ECN obvious.
Site: A home, mobile device, small enterprise, or campus where the
network bottleneck is typically the access link to the site. Not
all network arrangements fit this model, but it is a useful,
widely applicable generalization.
1.3. Scope
The new L4S identifier defined in this specification is applicable
for IPv4 and IPv6 packets (as is Classic ECN [RFC 3168]). It is
applicable for the unicast, multicast, and anycast forwarding modes.
The L4S identifier is an orthogonal packet classification to the
Differentiated Services Code Point (DSCP) [RFC 2474]. Section 5.4
explains what this means in practice.
This document is Experimental, so it does not update any Standards
Track RFCs. Therefore, it depends on [RFC 8311], which is a Standards
Track specification that:
* updates the ECN Proposed Standard [RFC 3168] to allow Experimental
RFCs to relax the requirement that an ECN mark must be equivalent
to a drop (when the network applies markings and/or when the
sender responds to them). For instance, in the Alternative
Backoff with ECN (ABE) experiment [RFC 8511], this relaxation
permits a sender to respond less to ECN marks than to drops;
* changes the status of the Experimental ECN nonce spec [RFC 3540] to
Historic; and
* makes consequent updates to the following additional Proposed
Standard RFCs to reflect the above two bullets:
- ECN for RTP [RFC 6679] and
- the congestion control specifications of various DCCP
Congestion Control Identifier (CCID) profiles [RFC 4341]
[RFC 4342] [RFC 5622].
This document is about identifiers that are used for interoperation
between hosts and networks. So the audience is broad, covering
developers of host transports and network AQMs, as well as covering
how operators might wish to combine various identifiers, which would
require flexibility from equipment developers.
2. L4S Packet Identification: Document Roadmap
The L4S ECN marking treatment is an experimental alternative to the
Classic ECN treatment in [RFC 3168], which has been updated by
[RFC 8311] to allow experiments such as the one defined in the present
specification. [RFC 4774] discusses some of the issues and evaluation
criteria when defining alternative ECN semantics, which are further
discussed in Section 4.3.1.
The L4S architecture [RFC 9330] describes the three main components of
L4S: the sending host behaviour, the marking behaviour in the
network, and the L4S ECN protocol that identifies L4S packets as they
flow between the two.
Section 3 of this document records the requirements that informed the
choice of L4S identifier. Then, subsequent sections specify the L4S
ECN protocol, which i) identifies packets that have been sent from
hosts that are expected to comply with a broad type of sending
behaviour and ii) identifies the marking treatment that network nodes
are expected to apply to L4S packets.
For a packet to receive L4S treatment as it is forwarded, the sender
sets the ECN field in the IP header to the ECT(1) codepoint. See
Section 4 for full transport-layer behaviour requirements, including
feedback and congestion response.
A network node that implements the L4S service always classifies
arriving ECT(1) packets for L4S treatment and by default classifies
CE packets for L4S treatment unless the heuristics described in
Section 5.3 are employed. See Section 5 for full network element
behaviour requirements, including classification, ECN marking, and
interaction of the L4S identifier with other identifiers and per-hop
behaviours.
L4S ECN works with ECN tunnelling and encapsulation behaviour as is,
except there is one known case where careful attention to
configuration is required, which is detailed in Section 6.
This specification of L4S ECN currently has Experimental status. So
Section 7 collects the general questions and issues that remain open
for investigation during L4S experimentation. Open issues or
questions specific to particular components are called out in the
specifications of each component part, such as the DualQ [RFC 9332].
The IANA assignment of the L4S identifier is specified in Section 8.
And Section 9 covers security considerations specific to the L4S
identifier. System security aspects, such as policing and privacy,
are covered in the L4S architecture [RFC 9330].
3. Choice of L4S Packet Identifier: Requirements
This subsection briefly records the process that led to the chosen
L4S identifier.
The identifier for packets using the L4S service needs to meet the
following requirements:
* it SHOULD survive end to end between source and destination
endpoints: across the boundary between host and network, between
interconnected networks, and through middleboxes;
* it SHOULD be visible at the IP layer;
* it SHOULD be common to IPv4 and IPv6 and be transport-agnostic;
* it SHOULD be incrementally deployable;
* it SHOULD enable an AQM to classify packets encapsulated by outer
IP or lower-layer headers;
* it SHOULD consume minimal extra codepoints; and
* it SHOULD be consistent on all the packets of a transport-layer
flow, so that some packets of a flow are not served by a different
queue to others.
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 more likely to succeed.
It is recognized that any choice of identifier is unlikely to satisfy
all these requirements, particularly given the limited space left in
the IP header. Therefore, a compromise will always be necessary,
which is why all the above requirements are expressed with the word
"SHOULD" not "MUST".
After extensive assessment of alternative schemes, "ECT(1) and CE
codepoints" was chosen as the best compromise. Therefore, this
scheme is defined in detail in the following sections, while
Appendix B records its pros and cons against the above requirements.
4. Transport-Layer Behaviour (the 'Prague L4S Requirements')
This section defines L4S behaviour at the transport layer, also known
as the 'Prague L4S Requirements' (see Appendix A for the origin of
the name).
4.1. Codepoint Setting
A sender that wishes a packet to receive L4S treatment as it is
forwarded MUST set the ECN field in the IP header (v4 or v6) to the
ECT(1) codepoint.
4.2. Prerequisite Transport Feedback
For a transport protocol to provide Scalable congestion control
(Section 4.3), it MUST provide feedback of the extent of CE marking
on the forward path. When ECN was added to TCP [RFC 3168], the
feedback method reported no more than one CE mark per round trip.
Some transport protocols derived from TCP mimic this behaviour while
others report the accurate extent of ECN marking. This means that
some transport protocols will need to be updated as a prerequisite
for Scalable congestion control. The position for a few well-known
transport protocols is given below.
TCP: Support for the accurate ECN feedback requirements [RFC 7560]
(such as that provided by AccECN [ACCECN]) by both ends is a
prerequisite for Scalable congestion control in TCP. Therefore,
the presence of ECT(1) in the IP headers even in one direction of
a TCP connection will imply that both ends support accurate ECN
feedback. However, the converse does not apply. So even if both
ends support AccECN, either of the two ends can choose not to use
a Scalable congestion control, whatever the other end's choice is.
SCTP: A suitable ECN feedback mechanism for SCTP could add a chunk
to report the number of received CE marks (as described in a long-
expired document [SCTP-ECN] or as sketched out in Appendix A of
the now-obsolete second Standards Track specification of SCTP
[RFC 4960]).
RTP over UDP: A prerequisite for Scalable congestion control is for
both (all) ends of one media-level hop to signal ECN support
[RFC 6679] and use the new generic RTCP feedback format of
[RFC 8888]. The presence of ECT(1) implies that both (all) ends of
that media-level hop support ECN. However, the converse does not
apply. So each end of a media-level hop can independently choose
not to use a Scalable congestion control, even if both ends
support ECN.
QUIC: Support for sufficiently fine-grained ECN feedback is provided
by IETF QUIC transport v1 [RFC 9000].
DCCP: The Acknowledgement (ACK) vector in DCCP [RFC 4340] is already
sufficient to report the extent of CE marking as needed by a
Scalable congestion control.
4.3. Prerequisite Congestion Response
As a condition for a host to send packets with the L4S identifier
(ECT(1)), it SHOULD implement a congestion control behaviour that
ensures that, in steady state, the average duration between induced
ECN marks does not increase as flow rate scales up, all other factors
being equal. This is termed a Scalable congestion control. This
invariant duration ensures that, as flow rate scales, the average
period with no feedback information about capacity does not become
excessive. It also ensures that queue variations remain small,
without having to sacrifice utilization.
With a congestion control that sawtooths to probe capacity, this
duration is called the recovery time, because each time the sawtooth
yields, on average it takes this time to recover to its previous high
point. A Scalable congestion control does not have to sawtooth, but
it has to coexist with Scalable congestion controls that do.
For instance, for DCTCP [RFC 8257], TCP Prague [PRAGUE-CC]
[PragueLinux], and the L4S variant of SCReAM [SCReAM-L4S] [RFC 8298],
the average recovery time is always half a round trip (or half a
reference round trip), whatever the flow rate.
As with all transport behaviours, a detailed specification (probably
an Experimental RFC) is expected for each congestion control,
following the guidelines for specifying new congestion control
algorithms in [RFC 5033]. In addition, it is expected that these L4S-
specific matters will be documented, specifically the timescale over
which the proportionality is averaged and the control of burstiness.
The recovery time requirement above is worded as a "SHOULD" rather
than a "MUST" to allow reasonable flexibility for such
implementations.
The condition 'all other factors being equal' allows the recovery
time to be different for different round-trip times, as long as it
does not increase with flow rate for any particular RTT.
Saying that the recovery time remains roughly invariant is equivalent
to saying that the number of ECN CE marks per round trip remains
invariant as flow rate scales, all other factors being equal. For
instance, an average recovery time of half of 1 RTT is equivalent to
2 ECN marks per round trip. For those familiar with steady-state
congestion response functions, it is also equivalent to say that the
congestion window is inversely proportional to the proportion of
bytes in packets marked with the CE codepoint (see Section 2 of
[PI2]).
In order to coexist safely with other Internet traffic, a Scalable
congestion control is not allowed to tag its packets with the ECT(1)
codepoint unless it complies with the following numbered requirements
and recommendations:
1. A Scalable congestion control MUST be capable of being replaced
by a Classic congestion control (by application and/or by
administrative control). If a Classic congestion control is
activated, it will not tag its packets with the ECT(1) codepoint
(see Appendix A.1.3 for rationale).
2. As well as responding to ECN markings, a Scalable congestion
control MUST react to packet loss in a way that will coexist
safely with Classic congestion controls such as standard Reno
[RFC 5681], as required by [RFC 5033] (see Appendix A.1.4 for
rationale).
3. In uncontrolled environments, monitoring MUST be implemented to
support detection of problems with an ECN-capable AQM at the path
bottleneck that appears not to support L4S and that might be in a
shared queue. Such monitoring SHOULD be applied to live traffic
that is using Scalable congestion control. Alternatively,
monitoring need not be applied to live traffic, if monitoring
with test traffic has been arranged to cover the paths that live
traffic takes through uncontrolled environments.
A function to detect the above problems with an ECN-capable AQM
MUST also be implemented and used. The detection function SHOULD
be capable of making the congestion control adapt its ECN-marking
response in real time to coexist safely with Classic congestion
controls such as standard Reno [RFC 5681], as required by
[RFC 5033]. This could be complemented by more detailed offline
detection of potential problems. If only offline detection is
used and potential problems with such an AQM are detected on
certain paths, the Scalable congestion control MUST be replaced
by a Classic congestion control, at least for the problem paths.
See Section 4.3.1, Appendix A.1.5, and the L4S operational
guidance [L4SOPS] for rationale and explanation.
Note that a Scalable congestion control is not expected to
change to setting ECT(0) while it transiently adapts to
coexist with Classic congestion controls, whereas a
replacement congestion control that solely behaves in the
Classic way will set ECT(0).
4. In the range between the minimum likely RTT and typical RTTs
expected in the intended deployment scenario, a Scalable
congestion control MUST converge towards a rate that is as
independent of RTT as is possible without compromising stability
or utilization (see Appendix A.1.6 for rationale).
5. A Scalable congestion control SHOULD remain responsive to
congestion when typical RTTs over the public Internet are
significantly smaller because they are no longer inflated by
queuing delay. It would be preferable for the minimum window of
a Scalable congestion control to be lower than 1 segment rather
than use the timeout approach described for TCP in Section 6.1.2
of the ECN spec [RFC 3168] (or an equivalent for other
transports). However, a lower minimum is not set as a formal
requirement for L4S experiments (see Appendix A.1.7 for
rationale).
6. A Scalable congestion control's loss detection SHOULD be
resilient to reordering over an adaptive time interval that
scales with throughput and adapts to reordering (as in Recent ACK
(RACK) [RFC 8985]), as opposed to counting only in fixed units of
packets (as in the 3 Duplicate ACK (DupACK) rule of NewReno
[RFC 5681] [RFC 6675], which is not scalable). As data rates
increase (e.g., due to new and/or improved technology),
congestion controls that detect loss by counting in units of
packets become more likely to incorrectly treat reordering events
as congestion-caused loss events (see Appendix A.1.8 for further
rationale). This requirement does not apply to congestion
controls that are solely used in controlled environments where
the network introduces hardly any reordering.
7. A Scalable congestion control is expected to limit the queue
caused by bursts of packets. It would not seem necessary to set
the limit any lower than 10% of the minimum RTT expected in a
typical deployment (e.g., additional queuing of roughly 250 us
for the public Internet). This would be converted to a number of
packets by multiplying by the current average packet rate. Then,
the queue caused by each burst at the bottleneck link would not
exceed 250 us (under the worst-case assumption that the flow is
filling the capacity). No normative requirement to limit bursts
is given here, and until there is more industry experience from
the L4S experiment, it is not even known whether one is needed --
it seems to be in an L4S sender's self-interest to limit bursts.
Each sender in a session can use a Scalable congestion control
independently of the congestion control used by the receiver(s) when
they send data. Therefore, there might be ECT(1) packets in one
direction and ECT(0) or Not-ECT in the other.
Later, this document discusses the conditions for mixing other
"'Safe' Unresponsive Traffic" (e.g., DNS, Lightweight Directory
Access Protocol (LDAP), NTP, voice, and game sync packets) with L4S
traffic; see Section 5.4.1.1. To be clear, although such traffic can
share the same queue as L4S traffic, it is not appropriate for the
sender to tag it as ECT(1), except in the (unlikely) case that it
satisfies the above conditions.
4.3.1. Guidance on Congestion Response in the RFC Series
[RFC 3168] requires the congestion responses to a CE-marked packet and
a dropped packet to be the same. [RFC 8311] is a Standards Track
update to [RFC 3168] that is intended to enable experimentation with
ECN, including the L4S experiment. [RFC 8311] allows an experimental
congestion control's response to a CE-marked packet to differ from
the response to a dropped packet, provided that the differences are
documented in an Experimental RFC, such as the present document.
BCP 124 [RFC 4774] gives guidance to protocol designers, when
specifying alternative semantics for the IP-ECN field. [RFC 8311]
explained that it did not need to update the best current practice in
BCP 124 in order to relax the 'equivalence with drop' requirement
because, although BCP 124 quotes the same requirement from [RFC 3168],
the BCP does not impose requirements based on it. BCP 124 [RFC 4774]
describes three options for incremental deployment, with Option 3 (in
Section 4.3 of BCP 124 [RFC 4774]) best matching the L4S case. Option
3's requirement for end-nodes is that they respond to CE marks "in a
way that is friendly to flows using IETF-conformant congestion
control." This echoes other general congestion control requirements
in the RFC Series, for example, [RFC 5033] states that "...congestion
controllers that have a significantly negative impact on traffic
using standard congestion control may be suspect" and [RFC 8085],
which concerns UDP congestion control, states that "Bulk-transfer
applications that choose not to implement TFRC or TCP-like windowing
SHOULD implement a congestion control scheme that results in
bandwidth (capacity) use that competes fairly with TCP within an
order of magnitude."
The normative Item 3 in Section 4.3 above (which concerns L4S
response to congestion from a Classic ECN AQM) aims to ensure that
these 'coexistence' requirements are satisfied, but it makes some
compromises. This subsection highlights and justifies those
compromises, and Appendix A.1.5 and the L4S operational guidance
[L4SOPS] give detailed analysis, examples, and references (the
normative text in that bullet takes precedence if any informative
elaboration leads to ambiguity). The approach is based on an
assessment of the risk of harm, which is a combination of the
prevalence of the conditions necessary for harm to occur, and the
potential severity of the harm if they do.
Prevalence: There are three cases:
* Drop Tail: Coexistence between L4S and Classic flows is not in
doubt where the bottleneck does not support any form of ECN,
which has remained by far the most prevalent case since the ECN
spec [RFC 3168] was published in 2001.
* L4S: Coexistence is not in doubt if the bottleneck supports
L4S.
* Classic ECN: The compromises centre around cases where the
bottleneck supports Classic ECN [RFC 3168] but not L4S. But it
depends on which sub-case:
- Shared Queue with Classic ECN: At the time of writing, the
members of the Transport Working Group are not aware of any
current deployments of single-queue Classic ECN bottlenecks
in the Internet. Nonetheless, at the scale of the Internet,
rarity need not imply small numbers nor that there will be
rarity in the future.
- Per-Flow Queues with Classic ECN: Most AQMs with per-flow
queuing deployed from 2012 onwards had Classic ECN enabled
by default, specifically FQ-CoDel [RFC 8290] and COBALT
[COBALT]. But the compromises only apply to the second of
two further sub-cases:
o With per-flow queuing, coexistence between Classic and
L4S flows is not normally a problem, because different
flows are not meant to be in the same queue (BCP 124
[RFC 4774] did not foresee the introduction of per-flow
queuing, which appeared as a potential isolation
technique some eight years after the BCP was published).
o However, the isolation between L4S and Classic flows is
not perfect in cases where the hashes of flow identifiers
(IDs) collide or where multiple flows within a Layer 3
VPN are encapsulated within one flow ID.
To summarize, the coexistence problem is confined to cases of
imperfect flow isolation in an FQ or in potential cases where a
Classic ECN AQM has been deployed in a shared queue (see the L4S
operational guidance [L4SOPS] for further details including recent
surveys attempting to quantify prevalence). Further, if one of
these cases does occur, the coexistence problem does not arise
unless sources of Classic and L4S flows are simultaneously sharing
the same bottleneck queue (e.g., different applications in the
same household), and flows of each type have to be large enough to
coincide for long enough for any throughput imbalance to have
developed. Therefore, how often the coexistence problem arises in
practice is listed in Section 7 as an open question that L4S
experiments will need to answer.
Severity: Where long-running L4S and Classic flows coincide in a
shared queue, testing of one L4S congestion control (TCP Prague)
has found that the imbalance in average throughput between an L4S
and a Classic flow can reach 25:1 in favour of L4S in the worst
case [ecn-fallback]. However, when capacity is most scarce, the
Classic flow gets a higher proportion of the link, for instance,
over a 4 Mb/s link the throughput ratio is below ~10:1 over paths
with a base RTT below 100 ms, and it falls below ~5:1 for base
RTTs below 20 ms.
These throughput ratios can clearly fall well outside current RFC
guidance on coexistence. However, the tendency towards leaving a
greater share for Classic flows at lower link rate and the very
limited prevalence of the conditions necessary for harm to occur led
to the possibility of allowing the RFC requirements to be
compromised, albeit briefly:
* The recommended approach is still to detect and adapt to a Classic
ECN AQM in real time, which is fully consistent with all the RFCs
on coexistence. In other words, the "SHOULD"s in Item 3 of
Section 4.3 above expect the sender to implement something similar
to the proof-of-concept code that detects the presence of a
Classic ECN AQM and falls back to a Classic congestion response
within a few round trips [ecn-fallback]. However, although this
code reliably detects a Classic ECN AQM, the current code can also
wrongly categorize an L4S AQM as Classic, most often in cases when
link rate is low or RTT is high. Although this is the safe way
round, and although implementers are expected to be able to
improve on this proof of concept, concerns have been raised that
implementers might lose faith in such detection and disable it.
* Item 3 in Section 4.3 above therefore allows a compromise where
coexistence could briefly diverge from the requirements in the RFC
Series, but mandatory monitoring is required in order to detect
such cases and trigger remedial action. This approach tolerates a
brief divergence from the RFCs given the likely low prevalence and
given harm here means a flow progresses more slowly than it would
otherwise, but it does progress. The L4S operational guidance
[L4SOPS] outlines a range of example remedial actions that include
alterations to either the sender or the network. However, the
final normative requirement in Item 3 of Section 4.3 above places
ultimate responsibility for remedial action on the sender. If
coexistence problems with a Classic ECN AQM are detected (implying
they have not been resolved by the network), it states that the
sender "MUST" revert to a Classic congestion control.
[L4SOPS] also gives example ways in which L4S congestion controls can
be rolled out initially in lower-risk scenarios.
4.4. Filtering or Smoothing of ECN Feedback
Section 5.2 below specifies that an L4S AQM is expected to signal L4S
ECN immediately, to avoid introducing delay due to filtering or
smoothing. This contrasts with a Classic AQM, which filters out
variations in the queue before signalling ECN marking or drop. In
the L4S architecture [RFC 9330], responsibility for smoothing out
these variations shifts to the sender's congestion control.
This shift of responsibility has the advantage that each sender can
smooth variations over a timescale proportionate to its own RTT.
Whereas, in the Classic approach, the network doesn't know the RTTs
of any of the flows, so it has to smooth out variations for a worst-
case RTT to ensure stability. For all the typical flows with shorter
RTTs than the worst-case, this makes congestion control unnecessarily
sluggish.
This also gives an L4S sender the choice not to smooth, depending on
its context (start-up, congestion avoidance, etc.). Therefore, this
document places no requirement on an L4S congestion control to smooth
out variations in any particular way. Implementers are encouraged to
openly publish the approach they take to smoothing as well as results
and experience they gain during the L4S experiment.
5. Network Node Behaviour
5.1. Classification and Re-Marking Behaviour
A network node that implements the L4S service:
* MUST classify arriving ECT(1) packets for L4S treatment, unless
overridden by another classifier (e.g., see Section 5.4.1.2).
* MUST classify arriving CE packets for L4S treatment as well,
unless overridden by another classifier or unless the exception
referred to next applies.
CE packets might have originated as ECT(1) or ECT(0), but the
above rule to classify them as if they originated as ECT(1) is the
safe choice (see Appendix B for rationale). The exception is
where some flow-aware in-network mechanism happens to be available
for distinguishing CE packets that originated as ECT(0), as
described in Section 5.3, but there is no implication that such a
mechanism is necessary.
An L4S AQM treatment follows similar codepoint transition rules to
those in [RFC 3168]. Specifically, the ECT(1) codepoint MUST NOT be
changed to any codepoint other 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
increasingly mark the IP-ECN field as CE, otherwise forwarding
packets unchanged as ECT(1). Necessary conditions for an L4S marking
treatment are defined in Section 5.2.
Under persistent overload, an L4S marking treatment MUST begin
applying drop to L4S traffic until the overload episode has subsided,
as recommended for all AQM methods in Section 4.2.1 of [RFC 7567],
which follows the similar advice in Section 7 of [RFC 3168]. During
overload, it MUST apply the same drop probability to L4S traffic as
it would to Classic traffic.
Where an L4S AQM is transport-aware, this requirement could be
satisfied by using drop in only the most overloaded individual per-
flow AQMs. In a DualQ with flow-aware queue protection (e.g.,
[DOCSIS-QPROT]), this could be achieved by redirecting packets in
those flows contributing most to the overload out of the L4S queue so
that they are subjected to drop in the Classic queue.
For backward compatibility in uncontrolled environments, a network
node that implements the L4S treatment MUST also implement an AQM
treatment for the Classic service as defined in Section 1.2. This
Classic AQM treatment need not mark ECT(0) packets, but if it does,
see Section 5.2 for the strengths of the markings relative to drop.
It MUST classify arriving ECT(0) and Not-ECT packets for treatment by
this Classic AQM (for the DualQ Coupled AQM; see the extensive
discussion on classification in Sections 2.3 and 2.5.1.1 of
[RFC 9332]).
In case unforeseen problems arise with the L4S experiment, it MUST be
possible to configure an L4S implementation to disable the L4S
treatment. Once disabled, ECT(1) packets MUST be treated as if they
were Not-ECT.
5.2. The Strength of L4S CE Marking Relative to Drop
The relative strengths of L4S CE and drop are irrelevant where AQMs
are implemented in separate queues per application-flow, which are
then explicitly scheduled (e.g., with an FQ scheduler as in FQ-CoDel
[RFC 8290]). Nonetheless, the relationship between them needs to be
defined for the coupling between L4S and Classic congestion signals
in a DualQ Coupled AQM [RFC 9332], as indicated below.
Unless an AQM node schedules application flows explicitly, the
likelihood that the AQM drops a Not-ECT Classic packet (p_C) MUST be
roughly proportional to the square of the likelihood that it would
have marked it if it had been an L4S packet (p_L). That is:
p_C ~= (p_L / k)^2
The constant of proportionality (k) does not have to be standardized
for interoperability, but a value of 2 is RECOMMENDED. The term
'likelihood' is used above to allow for marking and dropping to be
either probabilistic or deterministic.
This formula ensures that Scalable and Classic flows will converge to
roughly equal congestion windows, for the worst case of Reno
congestion control. This is because the congestion windows of
Scalable and Classic congestion controls are inversely proportional
to p_L and sqrt(p_C), respectively. So squaring p_C in the above
formula counterbalances the square root that characterizes Reno-
friendly flows.
| 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, as
| allowed by [RFC 8311], which updates [RFC 3168]. However, if it
| marks ECT(0) packets, it does so under the same conditions that
| it would have dropped a Not-ECT packet [RFC 3168].
Also, in the L4S architecture [RFC 9330], the sender, not the network,
is responsible for smoothing out variations in the queue. So an L4S
AQM MUST signal congestion as soon as possible. Then, an L4S sender
generally interprets CE marking as an unsmoothed signal.
This requirement does not prevent an L4S AQM from mixing in
additional congestion signals that are smoothed, such as the signals
from a Classic smoothed AQM that are coupled with unsmoothed L4S
signals in the coupled DualQ [RFC 9332], but only as long as the onset
of congestion can be signalled immediately and can be interpreted by
the sender as if it has been signalled immediately, which is
important for interoperability
5.3. Exception for L4S Packet Identification by Network Nodes with
Transport-Layer Awareness
To implement L4S packet classification, a network node does not need
to identify transport-layer flows. Nonetheless, if an L4S network
node classifies packets by their transport-layer flow ID and their
ECN field, and if all the ECT packets in a flow have been ECT(0), the
node MAY classify any CE packets in the same flow as if they were
Classic ECT(0) packets. In all other cases, a network node MUST
classify all CE packets as if they were ECT(1) packets. Examples of
such other cases are: i) if no ECT packets have yet been identified
in a flow; ii) if it is not desirable for a network node to identify
transport-layer flows; or iii) if some ECT packets in a flow have
been ECT(1) (this advice will need to be verified as part of L4S
experiments).
5.4. Interaction of the L4S Identifier with Other Identifiers
The examples in this section concern how additional identifiers might
complement the L4S identifier to classify packets between class-based
queues. Firstly, Section 5.4.1 considers two queues, L4S and
Classic, as in the DualQ Coupled AQM [RFC 9332], either alone
(Section 5.4.1.1) or within a larger queuing hierarchy
(Section 5.4.1.2). Then, Section 5.4.2 considers schemes that might
combine per-flow 5-tuples with other identifiers.
5.4.1. DualQ Examples of Other Identifiers Complementing L4S
Identifiers
5.4.1.1. Inclusion of Additional Traffic with L4S
In a typical case for the public Internet, a network element that
implements L4S in a shared queue might want to classify some low-rate
but unresponsive traffic (e.g., DNS, LDAP, NTP, voice, and game sync
packets) into the low-latency queue to mix with L4S traffic. In this
case, it would not be appropriate to call the queue an L4S queue,
because it is shared by L4S and non-L4S traffic. Instead, it will be
called the low-latency or L queue. The L queue then offers two
different treatments:
* the L4S treatment, which is a combination of the L4S AQM treatment
and a priority scheduling treatment, and
* the low-latency treatment, which is solely the priority scheduling
treatment, without ECN marking by the AQM.
To identify packets for just the scheduling treatment, it would be
inappropriate to use the L4S ECT(1) identifier, because such traffic
is unresponsive to ECN marking. Examples of relevant non-ECN
identifiers are:
* address ranges of specific applications or hosts configured to be,
or known to be, safe, e.g., hard-coded Internet of Things (IoT)
devices sending low-intensity traffic;
* certain low data-volume applications or protocols (e.g., ARP and
DNS); and
* specific Diffserv codepoints that indicate traffic with limited
burstiness such as the EF [RFC 3246], VOICE-ADMIT [RFC 5865], or
proposed Non-Queue-Building (NQB) [NQB-PHB] service classes or
equivalent Local-use DSCPs (see [L4S-DIFFSERV]).
To be clear, classifying into the L queue based on application-layer
identification (e.g., DNS) is an example of a local optimization, not
a recommendation. Applications will not be able to rely on such
unsolicited optimization. A more reliable approach would be for the
sender to set an appropriate IP-layer identifier, such as one of the
above Diffserv codepoints.
In summary, a network element that implements L4S in a shared queue
MAY classify additional types of packets into the L queue based on
identifiers other than the IP-ECN field, but the types SHOULD be
'safe' to mix with L4S traffic, where 'safe' is explained in
Section 5.4.1.1.1.
A packet that carries one of these non-ECN identifiers to classify it
into the L queue would not be subject to the L4S ECN-marking
treatment, unless it also carried an ECT(1) or CE codepoint. The
specification of an L4S AQM MUST define the behaviour for packets
with unexpected combinations of codepoints, e.g., a non-ECN-based
classifier for the L queue but with ECT(0) in the IP-ECN field (for
examples with appropriate behaviours, see Section 2.5.1.1 of the
DualQ spec [RFC 9332]).
For clarity, non-ECN identifiers, such as the examples itemized
above, might be used by some network operators who believe they
identify non-L4S traffic that would be safe to mix with L4S traffic.
They are not alternative ways for a host to indicate that it is
sending L4S packets. Only the ECT(1) ECN codepoint indicates to a
network element that a host is sending L4S packets (and CE indicates
that it could have originated as ECT(1)). Specifically, ECT(1)
indicates that the host claims its behaviour satisfies the
prerequisite transport requirements in Section 4.
In order to include non-L4S packets in the L queue, a network node
MUST NOT change Not-ECT or ECT(0) in the IP-ECN field into an L4S
identifier. This ensures that these codepoints survive for any
potential use later on the network path. If a non-compliant or
malicious network node did swap ECT(0) to ECT(1), the packet could
subsequently be ECN-marked by a downstream L4S AQM, but the sender
would respond to congestion indications thinking it had sent a
Classic packet. This could result in the flow yielding excessively
to other L4S flows sharing the downstream bottleneck.
5.4.1.1.1. 'Safe' Unresponsive Traffic
The above section requires unresponsive traffic to be 'safe' to mix
with L4S traffic. Ideally, this means that the sender never sends
any sequence of packets at a rate that exceeds the available capacity
of the bottleneck link. However, typically an unresponsive transport
does not even know the bottleneck capacity of the path, let alone its
available capacity. Nonetheless, an application can be considered
safe enough if it paces packets out (not necessarily with absolute
regularity) such that its maximum instantaneous rate from packet to
packet stays well below a typical broadband access rate.
This is a vague but useful definition, because many low-latency
applications of interest, such as DNS, voice, game sync packets, RPC,
ACKs, and keep-alives, could match this description.
Low-rate streams, such as voice and game sync packets, might not use
continuously adapting ECN-based congestion control, but they ought to
at least use a 'circuit-breaker' style of congestion response
[RFC 8083]. If the volume of traffic from unresponsive applications
is high enough to overload the link, this will at least protect the
capacity available to responsive applications. However, queuing
delay in the L queue would probably then rise to the typically higher
level targeted by a Classic (drop-based) AQM. If a network operator
considers that such self-restraint is not enough, it might want to
police the L queue (see Section 8.2 of the L4S architecture
[RFC 9330]).
5.4.1.2. Exclusion of Traffic from L4S Treatment
To extend the above example, an operator might want to exclude some
traffic from the L4S treatment for a policy reason, e.g., security
(traffic from malicious sources) or commercial (e.g., the operator
may wish to initially confine the benefits of L4S to business
customers).
In this exclusion case, the classifier MUST classify on the relevant
locally used identifiers (e.g., source addresses) before classifying
the non-matching traffic on the end-to-end L4S ECN identifier.
A network node MUST NOT alter the end-to-end L4S ECN identifier from
L4S to Classic, because an operator decision to exclude certain
traffic from L4S treatment is local-only. The end-to-end L4S
identifier then survives for other operators to use, or indeed, they
can apply their own policy, independently based on their own choice
of locally used identifiers. This approach also allows any operator
to remove its locally applied exclusions in future, e.g., if it
wishes to widen the benefit of the L4S treatment to all its
customers. If a non-compliant or malicious network node did swap
ECT(1) to ECT(0), the packet could subsequently be ECN-marked by a
downstream Classic ECN AQM. L4S senders are required to detect and
handle such treatment (see Item 3 in Section 4.3), but that does not
make this swap OK, because such detection is not known to be perfect
or immediate.
A network node that supports L4S but excludes certain packets
carrying the L4S identifier from L4S treatment MUST still apply
marking or dropping that is compatible with an L4S congestion
response. For instance, it could either drop such packets with the
same likelihood as Classic packets or ECN-mark them with a likelihood
appropriate to L4S traffic (e.g., the coupled probability in a DualQ
Coupled AQM) but aiming for the Classic delay target. It MUST NOT
ECN-mark such packets with a Classic marking probability, which could
confuse the sender.
5.4.1.3. Generalized Combination of L4S and Other Identifiers
L4S concerns low latency, which it can provide for all traffic
without differentiation and without _necessarily_ affecting bandwidth
allocation. Diffserv provides for differentiation of both bandwidth
and low latency, but its control of latency depends on its control of
bandwidth. L4S and Diffserv can be combined if a network operator
wants to control bandwidth allocation but also wants to provide low
latency, i.e., for any amount of traffic within one of these
allocations of bandwidth (rather than only providing low latency by
limiting bandwidth) [L4S-DIFFSERV].
The DualQ examples so far have been framed in the context of
providing the default Best Effort Per-Hop Behaviour (PHB) using two
queues -- a low-latency (L) queue and a Classic (C) queue. This
single DualQ structure is expected to be the most common and useful
arrangement. But, more generally, an operator might choose to
control bandwidth allocation through a hierarchy of Diffserv PHBs at
a node and to offer one (or more) of these PHBs using a pair of
queues for a low latency and a Classic variant of the PHB.
In the first case, if we assume that a network element provides no
PHBs except the DualQ, if a packet carries ECT(1) or CE, the network
element would classify it for the L4S treatment irrespective of its
DSCP. And, if a packet carried (for example) the EF DSCP, the
network element could classify it into the L queue irrespective of
its ECN codepoint. However, where the DualQ is in a hierarchy of
other PHBs, the classifier would classify some traffic into other
PHBs based on DSCP before classifying between the low-latency and
Classic queues (based on ECT(1), CE, and perhaps also the EF DSCP or
other identifiers as in the above example). [L4S-DIFFSERV] gives a
number of examples of such arrangements to address various
requirements.
[L4S-DIFFSERV] describes how an operator might use L4S to offer low
latency as well as Diffserv for bandwidth differentiation. It
identifies two main types of approach, which can be combined: the
operator might split certain Diffserv PHBs between L4S and a
corresponding Classic service. Or it might split the L4S and/or the
Classic service into multiple Diffserv PHBs. In either of these
cases, a packet would have to be classified on its Diffserv and ECN
codepoints.
In summary, there are numerous ways in which the L4S ECN identifier
(ECT(1) and CE) could be combined with other identifiers to achieve
particular objectives. The following categorization articulates
those that are valid, but it is not necessarily exhaustive. Those
tagged as 'Recommended-standard-use' could be set by the sending host
or a network. Those tagged as 'Local-use' would only be set by a
network:
1. Identifiers Complementing the L4S Identifier
a. Including More Traffic in the L Queue
(could use Recommended-standard-use or Local-use identifiers)
b. Excluding Certain Traffic from the L Queue
(Local-use only)
2. Identifiers to Place L4S Classification in a PHB Hierarchy
(could use Recommended-standard-use or Local-use identifiers)
a. PHBs before L4S ECN Classification
b. PHBs after L4S ECN Classification
5.4.2. Per-flow Queuing Examples of Other Identifiers Complementing L4S
Identifiers
At a node with per-flow queuing (e.g., FQ-CoDel [RFC 8290]), the L4S
identifier could complement the transport-layer flow ID as a further
level of flow granularity (i.e., Not-ECT and ECT(0) queued separately
from ECT(1) and CE packets). In Appendix B, the "Risk of reordering
Classic CE packets within a flow" discusses the resulting ambiguity
if packets originally set to ECT(0) are marked CE by an upstream AQM
before they arrive at a node that classifies CE as L4S. It argues
that the risk of reordering is vanishingly small, and the consequence
of such a low level of reordering is minimal.
Alternatively, it could be assumed that it is not in a flow's own
interest to mix Classic and L4S identifiers. Then, the AQM could use
the IP-ECN field to switch itself between a Classic and an L4S AQM
behaviour within one per-flow queue. For instance, for ECN-capable
packets, the AQM might consist of a simple marking threshold, and an
L4S ECN identifier might simply select a shallower threshold than a
Classic ECN identifier would.
5.5. Limiting Packet Bursts from Links
As well as senders needing to limit packet bursts (Section 4.3),
links need to limit the degree of burstiness they introduce. In both
cases (senders and links), this is a trade-off, because batch-
handling of packets is done for good reason, e.g., for processing
efficiency or to make efficient use of medium acquisition delay.
Some take the attitude that there is no point reducing burst delay at
the sender below that introduced by links (or vice versa). However,
delay reduction proceeds by cutting down 'the longest pole in the
tent', which turns the spotlight on the next longest, and so on.
This document does not set any quantified requirements for links to
limit burst delay, primarily because link technologies are outside
the remit of L4S specifications. Nonetheless, the following two
subsections outline opportunities for addressing bursty links in the
process of L4S implementation and deployment.
5.5.1. Limiting Packet Bursts from Links Fed by an L4S AQM
It would not make sense to implement an L4S AQM that feeds into a
particular link technology without also reviewing opportunities to
reduce any form of burst delay introduced by that link technology.
This would at least limit the bursts that the link would otherwise
introduce into the onward traffic, which would cause jumpy feedback
to the sender as well as potential extra queuing delay downstream.
This document does not presume to even give guidance on an
appropriate target for such burst delay until there is more industry
experience of L4S. However, as suggested in Section 4.3, it would
not seem necessary to limit bursts lower than roughly 10% of the
minimum base RTT expected in the typical deployment scenario (e.g.,
250 us burst duration for links within the public Internet).
5.5.2. Limiting Packet Bursts from Links Upstream of an L4S AQM
The initial scope of the L4S experiment is to deploy L4S AQMs at
bottlenecks and L4S congestion controls at senders. This is expected
to highlight interactions with the most bursty upstream links and
lead operators to tune down the burstiness of those links in their
networks that are configurable or, failing that, to have to
compromise on the delay target of some L4S AQMs. It might also
require specific redesign work relevant to the most problematic link
types. Such knock-on effects of initial L4S deployment would all be
a part of the learning from the L4S experiment.
The details of such link changes are beyond the scope of the present
document. Nonetheless, where L4S technology is being implemented on
an outgoing interface of a device, it would make sense to consider
opportunities for reducing bursts arriving at other incoming
interfaces. For instance, where an L4S AQM is implemented to feed
into the upstream WAN interface of a home gateway, there would be
opportunities to alter the Wi-Fi profiles sent out of any Wi-Fi
interfaces from the same device, in order to mitigate incoming bursts
of aggregated Wi-Fi frames from other Wi-Fi stations.
6. Behaviour of Tunnels and Encapsulations
6.1. No Change to ECN Tunnels and Encapsulations in General
The L4S identifier is expected to work through and within any tunnel
without modification, as long as the tunnel propagates the ECN field
in any of the ways that have been defined since the first variant in
the year 2001 [RFC 3168]. L4S will also work with (but does not rely
on) any of the more recent updates to ECN propagation in [RFC 4301],
[RFC 6040], or [ECN-SHIM]. However, it is likely that some tunnels
still do not implement ECN propagation at all. In these cases, L4S
will work through such tunnels, but within them the outer header of
L4S traffic will appear as Classic.
AQMs are typically implemented where an IP-layer buffer feeds into a
lower layer, so they are agnostic to link-layer encapsulations.
Where a bottleneck link is not IP-aware, the L4S identifier is still
expected to work within any lower-layer encapsulation without
modification, as long it propagates the IP-ECN field as defined for
the link technology, for example, for MPLS [RFC 5129] or Transparent
Interconnection of Lots of Links (TRILL) [TRILL-ECN-SUPPORT]. In
some of these cases, e.g., Layer 3 Ethernet switches, the AQM
accesses the IP-layer header within the outer encapsulation, so again
the L4S identifier is expected to work without modification.
Nonetheless, the programme to define ECN for other lower layers is
still in progress [ECN-ENCAP].
6.2. VPN Behaviour to Avoid Limitations of Anti-Replay
If a mix of L4S and Classic packets is sent into the same security
association (SA) of a VPN, and if the VPN egress is employing the
optional anti-replay feature, it could inappropriately discard
Classic packets (or discard the records in Classic packets) by
mistaking their greater queuing delay for a replay attack (see
"Dropped Packets for Tunnels with Replay Protection Enabled" in
[Heist21] for the potential performance impact). This known problem
is common to both IPsec [RFC 4301] and DTLS [RFC 9147] VPNs, given they
use similar anti-replay window mechanisms. The mechanism used can
only check for replay within its window, so if the window is smaller
than the degree of reordering, it can only assume there might be a
replay attack and discard all the packets behind the trailing edge of
the window. The specifications of IPsec Authentication Header (AH)
[RFC 4302] and Encapsulating Security Payload (ESP) [RFC 4303] suggest
that an implementer scales the size of the anti-replay window with
interface speed, and DTLS v1.3 [RFC 9147] states that "The receiver
SHOULD pick a window large enough to handle any plausible reordering,
which depends on the data rate." However, in practice, the size of a
VPN's anti-replay window is not always scaled appropriately.
If a VPN carrying traffic participating in the L4S experiment
experiences inappropriate replay detection, the foremost remedy would
be to ensure that the egress is configured to comply with the above
window-sizing requirements.
If an implementation of a VPN egress does not support a sufficiently
large anti-replay window, e.g., due to hardware limitations, one of
the temporary alternatives listed in order of preference below might
be feasible instead:
* If the VPN can be configured to classify packets into different
SAs indexed by DSCP, apply the appropriate locally defined DSCPs
to Classic and L4S packets. The DSCPs could be applied by the
network (based on the least-significant bit of the IP-ECN field),
or by the sending host. Such DSCPs would only need to survive as
far as the VPN ingress.
* If the above is not possible and it is necessary to use L4S,
either of the following might be appropriate as a last resort:
- disable anti-replay protection at the VPN egress, after
considering the security implications (it is mandatory to allow
the anti-replay facility to be disabled in both IPsec and
DTLS), or
- configure the tunnel ingress not to propagate ECN to the outer,
which would lose the benefits of L4S and Classic ECN over the
VPN.
Modification to VPN implementations is outside the present scope,
which is why this section has so far focused on reconfiguration.
Although this document does not define any requirements for VPN
implementations, determining whether there is a need for such
requirements could be one aspect of L4S experimentation.
7. L4S Experiments
This section describes open questions that L4S experiments ought to
focus on. This section also documents outstanding open issues that
will need to be investigated as part of L4S experimentation, given
they could not be fully resolved during the working group phase. It
also lists metrics that will need to be monitored during experiments
(summarizing text elsewhere in L4S documents) and finally lists some
potential future directions that researchers might wish to
investigate.
In addition to this section, i) the DualQ spec [RFC 9332] sets
operational and management requirements for experiments with DualQ
Coupled AQMs, and ii) general operational and management requirements
for experiments with L4S congestion controls are given in Sections 4
and 5 above, e.g., coexistence and scaling requirements and
incremental deployment arrangements.
The specification of each Scalable congestion control will need to
include protocol-specific requirements for configuration and
monitoring performance during experiments. Appendix A of [RFC 5706]
provides a helpful checklist.
7.1. Open Questions
L4S experiments would be expected to answer the following questions:
* Have all the parts of L4S been deployed, and if so, what
proportion of paths support it?
- What types of L4S AQMs were deployed, e.g., FQ, coupled DualQ,
uncoupled DualQ, other? And how prevalent was each?
- Are the signalling patterns emitted by the deployed AQMs in any
way different from those expected when the Prague requirements
for endpoints were written?
* Does use of L4S over the Internet result in a significantly
improved user experience?
* Has L4S enabled novel interactive applications?
* Did use of L4S over the Internet result in improvements to the
following metrics:
- queue delay (mean and 99th percentile) under various loads;
- utilization;
- starvation / fairness; and
- scaling range of flow rates and RTTs?
* How dependent was the performance of L4S service on the bottleneck
bandwidth or the path RTT?
* How much do bursty links in the Internet affect L4S performance
(see "Underutilization with Bursty Links" in [Heist21]) and how
prevalent are they? How much limitation of burstiness from
upstream links was needed and/or was realized -- both at senders
and at links, especially radio links -- or how much did L4S target
delay have to be increased to accommodate the bursts (see Item 7
in Section 4.3 and see Section 5.5.2)?
* Is the initial experiment with mis-identified bursty traffic at
high RTT (see "Underutilization with Bursty Traffic" in [Heist21])
indicative of similar problems at lower RTTs, and if so, how
effective is the suggested remedy in Appendix A.1 of the DualQ
spec [RFC 9332] (or possible other remedies)?
* Was per-flow queue protection typically (un)necessary?
- How well did overload protection or queue protection work?
* How well did L4S flows coexist with Classic flows when sharing a
bottleneck?
- How frequently did problems arise?
- What caused any coexistence problems, and were any problems due
to single-queue Classic ECN AQMs (this assumes single-queue
Classic ECN AQMs can be distinguished from FQ ones)?
* How prevalent were problems with the L4S service due to tunnels/
encapsulations that do not support ECN decapsulation?
* How easy was it to implement a fully compliant L4S congestion
control, over various different transport protocols (TCP, QUIC,
RMCAT, etc.)?
Monitoring for harm to other traffic, specifically bandwidth
starvation or excess queuing delay, will need to be conducted
alongside all early L4S experiments. It is hard, if not impossible,
for an individual flow to measure its impact on other traffic. So
such monitoring will need to be conducted using bespoke monitoring
across flows and/or across classes of traffic.
7.2. Open Issues
* What is the best way forward to deal with L4S over single-queue
Classic ECN AQM bottlenecks, given current problems with
misdetecting L4S AQMs as Classic ECN AQMs? See the L4S
operational guidance [L4SOPS].
* Fixing the poor interaction between current L4S congestion
controls and CoDel with only Classic ECN support during flow
startup. Originally, this was due to a bug in the initialization
of the congestion average in the Linux implementation of TCP
Prague. That was quickly fixed, which removed the main
performance impact, but further improvement would be useful (by
modifying either CoDel or Scalable congestion controls, or both).
7.3. Future Potential
Researchers might find that L4S opens up the following interesting
areas for investigation:
* potential for faster convergence time and tracking of available
capacity;
* potential for improvements to particular link technologies and
cross-layer interactions with them;
* potential for using virtual queues, e.g., to further reduce
latency jitter or to leave headroom for capacity variation in
radio networks;
* development and specification of reverse path congestion control
using L4S building blocks (e.g., AccECN or QUIC);
* once queuing delay is cut down, what becomes the 'second-longest
pole in the tent' (other than the speed of light)?
* novel alternatives to the existing set of L4S AQMs; and
* novel applications enabled by L4S.
8. IANA Considerations
The semantics of the 01 codepoint of the ECN field of the IP header
are specified by this Experimental RFC. The process for an
Experimental RFC to assign this codepoint in the IP header (v4 and
v6) is documented in Proposed Standard [RFC 8311], which updates the
Proposed Standard [RFC 3168].
IANA has updated the 01 entry in the "ECN Field (Bits 6-7)" registry
(see <https://www.iana.org/assignments/dscp-registry/>) as follows:
+========+=====================+=======================+
| Binary | Keyword | Reference |
+========+=====================+=======================+
| 01 | ECT(1) (ECN-Capable | [RFC 8311] [RFC Errata |
| | Transport(1))[1] | 5399] RFC 9331 |
+--------+---------------------+-----------------------+
Table 1: ECN Field (Bits 6-7) Registry
[1] ECT(1) is for experimental use only [RFC 8311], Section 4.2
9. Security Considerations
Approaches to assure the integrity of signals using the new
identifier are introduced in Appendix C.1. See the security
considerations in the L4S architecture [RFC 9330] for further
discussion of misuse of the identifier, as well as extensive
discussion of policing rate and latency in regard to L4S.
Defining ECT(1) as the L4S identifier introduces a difference between
the effects of ECT(0) and ECT(1) that [RFC 3168] previously defined as
distinct but with equivalent effect. For L4S ECN, a network node is
still required not to swap one to the other, even if the network
operator chooses to locally apply the treatment associated with the
opposite codepoint (see Sections 5.4.1.1 and 5.4.1.2). These
sections also describe the potential effects if a non-compliant or
malicious network node does swap one to the other. The present
specification does not change the effects of other unexpected
transitions of the IP-ECN field, which are still as described in
Section 18 of [RFC 3168].
If the anti-replay window of a VPN egress is too small, it will
mistake deliberate delay differences as a replay attack and discard
higher-delay packets (e.g., Classic) carried within the same security
association (SA) as low-delay packets (e.g., L4S). Section 6.2
recommends that VPNs used in L4S experiments are configured with a
sufficiently large anti-replay window, as required by the relevant
specifications. It also discusses other alternatives.
If a user taking part in the L4S experiment sets up a VPN without
being aware of the above advice, and if the user allows anyone to
send traffic into their VPN, they would open up a DoS vulnerability
in which an attacker could induce the VPN's anti-replay mechanism to
discard enough of the user's Classic (C) traffic (if they are
receiving any) to cause a significant rate reduction. While the user
is actively downloading C traffic, the attacker sends C traffic into
the VPN to fill the remainder of the bottleneck link, then sends
intermittent L4S packets to maximize the chance of exceeding the
VPN's replay window. The user can prevent this attack by following
the recommendations in Section 6.2.
The recommendation to detect loss in time units prevents the ACK-
splitting attacks described in [Savage-TCP].
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC 3168, September 2001,
<https://www.rfc-editor.org/info/RFC 3168>.
[RFC 4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, DOI 10.17487/RFC 4774, November 2006,
<https://www.rfc-editor.org/info/RFC 4774>.
[RFC 6679] 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/RFC 6679, August
2012, <https://www.rfc-editor.org/info/RFC 6679>.
10.2. Informative References
[A2DTCP] Zhang, T., Wang, J., Huang, J., Huang, Y., Chen, J., and
Y. Pan, "Adaptive-Acceleration Data Center TCP", IEEE
Transactions on Computers, Volume 64, Issue 6, pp.
1522-1533, DOI 10.1109/TC.2014.2345393, June 2015,
<https://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=6871352>.
[ACCECN] Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", Work in Progress, Internet-
Draft, draft-ietf-tcpm-accurate-ecn-22, 9 November 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
accurate-ecn-22>.
[Ahmed19] Ahmed, A.S., "Extending TCP for Low Round Trip Delay",
Master's Thesis, University of Oslo, August 2019,
<https://www.duo.uio.no/handle/10852/70966>.
[Alizadeh-stability]
Alizadeh, M., Javanmard, A., and B. Prabhakar, "Analysis
of DCTCP: Stability, Convergence, and Fairness",
SIGMETRICS '11: Proceedings of the ACM SIGMETRICS Joint
International Conference on Measurement and Modeling of
Computer Systems, pp. 73-84, DOI 10.1145/1993744.1993753,
June 2011,
<https://dl.acm.org/doi/10.1145/1993744.1993753>.
[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 301, August
2001, <https://www.icsi.berkeley.edu/icsi/node/2032>.
[BBR-CC] Cardwell, N., Cheng, Y., Hassas Yeganeh, S., Swett, I.,
and V. Jacobson, "BBR Congestion Control", Work in
Progress, Internet-Draft, draft-cardwell-iccrg-bbr-
congestion-control-02, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-cardwell-
iccrg-bbr-congestion-control-02>.
[BBRv2] "TCP BBR v2 Alpha/Preview Release", commit 17700ca, June
2022, <https://github.com/google/bbr>.
[Bufferbloat]
The Bufferbloat community, "Bufferbloat",
<https://bufferbloat.net/>.
[COBALT] Palmei, J., Gupta, S., Imputato, P., Morton, J.,
Tahiliani, M. P., Avallone, S., and D. Täht, "Design and
Evaluation of COBALT Queue Discipline", IEEE International
Symposium on Local and Metropolitan Area Networks
(LANMAN), DOI 10.1109/LANMAN.2019.8847054, July 2019,
<https://ieeexplore.ieee.org/abstract/document/8847054>.
[CTCP] Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
"Compound TCP: A New TCP Congestion Control for High-Speed
and Long Distance Networks", Work in Progress, Internet-
Draft, draft-sridharan-tcpm-ctcp-02, 3 November 2008,
<https://datatracker.ietf.org/doc/html/draft-sridharan-
tcpm-ctcp-02>.
[DCttH19] De Schepper, K., Bondarenko, O., Tilmans, O., and B.
Briscoe, "'Data Centre to the Home': Ultra-Low Latency for
All", Updated RITE project Technical Report, July 2019,
<https://bobbriscoe.net/projects/latency/
dctth_journal_draft20190726.pdf>.
[DOCSIS-QPROT]
Briscoe, B., Ed. and G. White, "The DOCSIS® Queue
Protection Algorithm to Preserve Low Latency", Work in
Progress, Internet-Draft, draft-briscoe-docsis-q-
protection-06, 13 May 2022,
<https://datatracker.ietf.org/doc/html/draft-briscoe-
docsis-q-protection-06>.
[DualPI2Linux]
Albisser, O., De Schepper, K., Briscoe, B., Tilmans, O.,
and H. Steen, "DUALPI2 - Low Latency, Low Loss and
Scalable (L4S) AQM", Proceedings of Linux Netdev 0x13,
March 2019, <https://www.netdevconf.org/0x13/
session.html?talk-DUALPI2-AQM>.
[Dukkipati06]
Dukkipati, N. and N. McKeown, "Why Flow-Completion Time is
the Right Metric for Congestion Control", ACM SIGCOMM
Computer Communication Review, Volume 36, Issue 1, pp.
59-62, DOI 10.1145/1111322.1111336, January 2006,
<https://dl.acm.org/doi/10.1145/1111322.1111336>.
[ECN++] Bagnulo, M. and B. Briscoe, "ECN++: Adding Explicit
Congestion Notification (ECN) to TCP Control Packets",
Work in Progress, Internet-Draft, draft-ietf-tcpm-
generalized-ecn-10, 27 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-
generalized-ecn-10>.
[ECN-ENCAP]
Briscoe, B. and J. Kaippallimalil, "Guidelines for Adding
Congestion Notification to Protocols that Encapsulate IP",
Work in Progress, Internet-Draft, draft-ietf-tsvwg-ecn-
encap-guidelines-17, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
ecn-encap-guidelines-17>.
[ecn-fallback]
Briscoe, B. and A. Ahmed, "TCP Prague Fall-back on
Detection of a Classic ECN AQM", Technical Report: TR-BB-
2019-002, DOI 10.48550/arXiv.1911.00710, February 2021,
<https://arxiv.org/abs/1911.00710>.
[ECN-SHIM] Briscoe, B., "Propagating Explicit Congestion Notification
Across IP Tunnel Headers Separated by a Shim", Work in
Progress, Internet-Draft, draft-ietf-tsvwg-rfc6040update-
shim-15, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
rfc6040update-shim-15>.
[Heist21] "L4S Tests", commit e21cd91, August 2021,
<https://github.com/heistp/l4s-tests>.
[L4S-DIFFSERV]
Briscoe, B., "Interactions between Low Latency, Low Loss,
Scalable Throughput (L4S) and Differentiated Services",
Work in Progress, Internet-Draft, draft-briscoe-tsvwg-l4s-
diffserv-02, 1 November 2018,
<https://datatracker.ietf.org/doc/html/draft-briscoe-
tsvwg-l4s-diffserv-02>.
[L4Seval22]
De Schepper, K., Albisser, O., Tilmans, O., and B.
Briscoe, "Dual Queue Coupled AQM: Deployable Very Low
Queuing Delay for All", Preprint submitted to IEEE/ACM
Transactions on Networking, DOI 10.48550/arXiv.2209.01078,
September 2022, <https://arxiv.org/abs/2209.01078>.
[L4SOPS] White, G., Ed., "Operational Guidance for Deployment of
L4S in the Internet", Work in Progress, Internet-Draft,
draft-ietf-tsvwg-l4sops-03, 28 April 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
l4sops-03>.
[LinuxPacedChirping]
Misund, J. and B. Briscoe, "Paced Chirping - Rethinking
TCP start-up", Proceedings of Linux Netdev 0x13, March
2019, <https://legacy.netdevconf.info/0x13/
session.html?talk-chirp>.
[NQB-PHB] White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
Behavior (NQB PHB) for Differentiated Services", Work in
Progress, Internet-Draft, draft-ietf-tsvwg-nqb-15, 11
January 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-tsvwg-nqb-15>.
[PI2] De Schepper, K., Bondarenko, O., Tsang, I., and B.
Briscoe, "PI^2: A Linearized AQM for both Classic and
Scalable TCP", Proceedings of ACM CoNEXT 2016, pp.
105-119, DOI 10.1145/2999572.2999578, December 2016,
<https://dl.acm.org/citation.cfm?doid=2999572.2999578>.
[PRAGUE-CC]
De Schepper, K., Tilmans, O., and B. Briscoe, Ed., "Prague
Congestion Control", Work in Progress, Internet-Draft,
draft-briscoe-iccrg-prague-congestion-control-01, 11 July
2022, <https://datatracker.ietf.org/doc/html/draft-
briscoe-iccrg-prague-congestion-control-01>.
[PragueLinux]
Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
Tilmans, O., Kühlewind, M., and A. Ahmed, "Implementing
the 'TCP Prague' Requirements for L4S", Proceedings of
Linux Netdev 0x13, March 2019,
<https://www.netdevconf.org/0x13/session.html?talk-tcp-
prague-l4s>.
[QV] Briscoe, B. and P. Hurtig, "Report on Prototype
Development and Evaluation of Network and Interaction
Techniques", RITE Technical Report, Deliverable 2.3,
Appendix C.2: "Up to Speed with Queue View", September
2015, <https://riteproject.files.wordpress.com/2015/12/
rite-deliverable-2-3.pdf>.
[RELENTLESS]
Mathis, M., "Relentless Congestion Control", Work in
Progress, Internet-Draft, draft-mathis-iccrg-relentless-
tcp-00, 4 March 2009,
<https://datatracker.ietf.org/doc/html/draft-mathis-iccrg-
relentless-tcp-00>.
[RFC 2474] 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/RFC 2474, December 1998,
<https://www.rfc-editor.org/info/RFC 2474>.
[RFC 3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC 3246, March 2002,
<https://www.rfc-editor.org/info/RFC 3246>.
[RFC 3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, DOI 10.17487/RFC 3540, June 2003,
<https://www.rfc-editor.org/info/RFC 3540>.
[RFC 3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC 3649, December 2003,
<https://www.rfc-editor.org/info/RFC 3649>.
[RFC 4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC 4301,
December 2005, <https://www.rfc-editor.org/info/RFC 4301>.
[RFC 4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC 4302, December 2005,
<https://www.rfc-editor.org/info/RFC 4302>.
[RFC 4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC 4303, December 2005,
<https://www.rfc-editor.org/info/RFC 4303>.
[RFC 4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC 4340, March 2006,
<https://www.rfc-editor.org/info/RFC 4340>.
[RFC 4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, DOI 10.17487/RFC 4341, March
2006, <https://www.rfc-editor.org/info/RFC 4341>.
[RFC 4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
DOI 10.17487/RFC 4342, March 2006,
<https://www.rfc-editor.org/info/RFC 4342>.
[RFC 4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC 4960, September 2007,
<https://www.rfc-editor.org/info/RFC 4960>.
[RFC 5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC 5033, August 2007,
<https://www.rfc-editor.org/info/RFC 5033>.
[RFC 5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, DOI 10.17487/RFC 5129, January
2008, <https://www.rfc-editor.org/info/RFC 5129>.
[RFC 5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC 5348, September 2008,
<https://www.rfc-editor.org/info/RFC 5348>.
[RFC 5562] 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/RFC 5562, June 2009,
<https://www.rfc-editor.org/info/RFC 5562>.
[RFC 5622] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion ID 4: TCP-Friendly Rate
Control for Small Packets (TFRC-SP)", RFC 5622,
DOI 10.17487/RFC 5622, August 2009,
<https://www.rfc-editor.org/info/RFC 5622>.
[RFC 5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC 5681, September 2009,
<https://www.rfc-editor.org/info/RFC 5681>.
[RFC 5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, DOI 10.17487/RFC 5706, November 2009,
<https://www.rfc-editor.org/info/RFC 5706>.
[RFC 5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC 5865, May 2010,
<https://www.rfc-editor.org/info/RFC 5865>.
[RFC 5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC 5925,
June 2010, <https://www.rfc-editor.org/info/RFC 5925>.
[RFC 6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC 6040, November
2010, <https://www.rfc-editor.org/info/RFC 6040>.
[RFC 6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
Briscoe, "Open Research Issues in Internet Congestion
Control", RFC 6077, DOI 10.17487/RFC 6077, February 2011,
<https://www.rfc-editor.org/info/RFC 6077>.
[RFC 6660] 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/RFC 6660, July 2012,
<https://www.rfc-editor.org/info/RFC 6660>.
[RFC 6675] Blanton, E., Allman, M., Wang, L., Jarvinen, I., Kojo, M.,
and Y. Nishida, "A Conservative Loss Recovery Algorithm
Based on Selective Acknowledgment (SACK) for TCP",
RFC 6675, DOI 10.17487/RFC 6675, August 2012,
<https://www.rfc-editor.org/info/RFC 6675>.
[RFC 7560] Kuehlewind, M., Ed., Scheffenegger, R., and B. Briscoe,
"Problem Statement and Requirements for Increased Accuracy
in Explicit Congestion Notification (ECN) Feedback",
RFC 7560, DOI 10.17487/RFC 7560, August 2015,
<https://www.rfc-editor.org/info/RFC 7560>.
[RFC 7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC 7567, July 2015,
<https://www.rfc-editor.org/info/RFC 7567>.
[RFC 7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC 7713, December 2015,
<https://www.rfc-editor.org/info/RFC 7713>.
[RFC 8033] 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/RFC 8033, February 2017,
<https://www.rfc-editor.org/info/RFC 8033>.
[RFC 8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", RFC 8083,
DOI 10.17487/RFC 8083, March 2017,
<https://www.rfc-editor.org/info/RFC 8083>.
[RFC 8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC 8085,
March 2017, <https://www.rfc-editor.org/info/RFC 8085>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8257] Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
Control for Data Centers", RFC 8257, DOI 10.17487/RFC 8257,
October 2017, <https://www.rfc-editor.org/info/RFC 8257>.
[RFC 8290] Hoeiland-Joergensen, T., McKenney, P., Taht, D., Gettys,
J., and E. Dumazet, "The Flow Queue CoDel Packet Scheduler
and Active Queue Management Algorithm", RFC 8290,
DOI 10.17487/RFC 8290, January 2018,
<https://www.rfc-editor.org/info/RFC 8290>.
[RFC 8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", RFC 8298, DOI 10.17487/RFC 8298, December
2017, <https://www.rfc-editor.org/info/RFC 8298>.
[RFC 8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC 8311, January 2018,
<https://www.rfc-editor.org/info/RFC 8311>.
[RFC 8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC 8312, February 2018,
<https://www.rfc-editor.org/info/RFC 8312>.
[RFC 8511] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", RFC 8511,
DOI 10.17487/RFC 8511, December 2018,
<https://www.rfc-editor.org/info/RFC 8511>.
[RFC 8888] Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
Control Protocol (RTCP) Feedback for Congestion Control",
RFC 8888, DOI 10.17487/RFC 8888, January 2021,
<https://www.rfc-editor.org/info/RFC 8888>.
[RFC 8985] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
RACK-TLP Loss Detection Algorithm for TCP", RFC 8985,
DOI 10.17487/RFC 8985, February 2021,
<https://www.rfc-editor.org/info/RFC 8985>.
[RFC 9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC 9000, May 2021,
<https://www.rfc-editor.org/info/RFC 9000>.
[RFC 9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC 9001, May 2021,
<https://www.rfc-editor.org/info/RFC 9001>.
[RFC 9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC 9147, April 2022,
<https://www.rfc-editor.org/info/RFC 9147>.
[RFC 9330] Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
White, "Low Latency, Low Loss, and Scalable Throughput
(L4S) Internet Service: Architecture", RFC 9330,
DOI 10.17487/RFC 9330, January 2023,
<https://www.rfc-editor.org/info/RFC 9330>.
[RFC 9332] De Schepper, K., Briscoe, B., Ed., and G. White, "Dual-
Queue Coupled Active Queue Management (AQM) for Low
Latency, Low Loss, and Scalable Throughput (L4S)",
RFC 9332, DOI 10.17487/RFC 9332, January 2023,
<https://www.rfc-editor.org/info/RFC 9332>.
[Savage-TCP]
Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
"TCP Congestion Control with a Misbehaving Receiver", ACM
SIGCOMM Computer Communication Review, Volume 29, Issue 5,
pp. 71–78, DOI 10.1145/505696.505704, October 1999,
<https://dl.acm.org/doi/abs/10.1145/505696.505704>.
[SCReAM-L4S]
"SCReAM", commit 140e292, November 2022,
<https://github.com/EricssonResearch/scream>.
[SCTP-ECN] Stewart, R., Tüxen, M., and X. Dong, "ECN for Stream
Control Transmission Protocol (SCTP)", Work in Progress,
Internet-Draft, draft-stewart-tsvwg-sctpecn-05, 15 January
2014, <https://datatracker.ietf.org/doc/html/draft-
stewart-tsvwg-sctpecn-05>.
[sub-mss-prob]
Briscoe, B. and K. De Schepper, "Scaling TCP's Congestion
Window for Small Round Trip Times", BT Technical Report:
TR-TUB8-2015-002, DOI 10.48550/arXiv.1904.07598, May 2015,
<https://arxiv.org/abs/1904.07598>.
[TCP-CA] Jacobson, V. and M. J. Karels, "Congestion Avoidance and
Control", Laurence Berkeley Labs Technical Report,
November 1988, <https://ee.lbl.gov/papers/congavoid.pdf>.
[TCPPrague]
Briscoe, B., "Notes: DCTCP evolution 'bar BoF': Tue 21 Jul
2015, 17:40, Prague", message to the tcpPrague mailing
list, July 2015, <https://www.ietf.org/mail-
archive/web/tcpprague/current/msg00001.html>.
[TRILL-ECN-SUPPORT]
Eastlake 3rd, D. and B. Briscoe, "TRILL (TRansparent
Interconnection of Lots of Links): ECN (Explicit
Congestion Notification) Support", Work in Progress,
Internet-Draft, draft-ietf-trill-ecn-support-07, 25
February 2018, <https://datatracker.ietf.org/doc/html/
draft-ietf-trill-ecn-support-07>.
[VCP] Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,
"One more bit is enough", SIGCOMM '05: Proceedings of the
2005 conference on Applications, technologies,
architectures, and protocols for computer communications,
pp. 37-48, DOI 10.1145/1080091.1080098, August 2005,
<https://doi.acm.org/10.1145/1080091.1080098>.
Appendix A. Rationale for the 'Prague L4S Requirements'
This appendix is informative, not normative. It gives a list of
modifications to current Scalable congestion controls so that they
can be deployed over the public Internet and coexist safely with
existing traffic. The list complements the normative requirements in
Section 4 that a sender has to comply with before it can set the L4S
identifier in packets it sends into the Internet. As well as
rationale for safety improvements (the requirements in Section 4),
this appendix also includes preferable performance improvements
(optimizations).
The requirements and recommendations in Section 4 have become known
as the 'Prague L4S Requirements', because they were originally
identified at an ad hoc meeting during IETF 94 in Prague [TCPPrague].
They were originally called the 'TCP Prague Requirements', but they
are not solely applicable to TCP, so the name and wording has been
generalized for all transport protocols, and the name 'TCP Prague' is
now used for a specific implementation of the requirements.
At the time of writing, DCTCP [RFC 8257] is the most widely used
Scalable transport protocol. In its current form, DCTCP is specified
to be deployable only in controlled environments. Deploying it in
the public Internet would lead to a number of issues, from both the
safety and the performance perspective. The modifications and
additional mechanisms listed in this section will be necessary for
its deployment over the global Internet. Where an example is needed,
DCTCP is used as a base, but the requirements in Section 4 apply
equally to other Scalable congestion controls, covering adaptive
real-time media, etc., not just capacity-seeking behaviours.
A.1. Rationale for the Requirements for Scalable Transport Protocols
A.1.1. Use of L4S Packet Identifier
Description: A Scalable congestion control needs to distinguish the
packets it sends from those sent by Classic congestion controls (see
the precise normative requirement wording in Section 4.1).
Motivation: It needs to be possible for a network node to classify
L4S packets without flow state into a queue that applies an L4S ECN-
marking behaviour and isolates L4S packets from the queuing delay of
Classic packets.
A.1.2. Accurate ECN Feedback
Description: The transport protocol for a Scalable congestion control
needs to provide timely, accurate feedback about the extent of ECN
marking experienced by all packets (see the precise normative
requirement wording in Section 4.2).
Motivation: Classic congestion controls only need feedback about the
existence of a congestion episode within a round trip, not precisely
how many packets were ECN-marked or dropped. Therefore, in 2001,
when ECN feedback was added to TCP [RFC 3168], it could not inform the
sender of more than one ECN mark per RTT. Since then, requirements
for more accurate ECN feedback in TCP have been defined in [RFC 7560],
and [ACCECN] specifies a change to the TCP protocol to satisfy these
requirements. Most other transport protocols already satisfy this
requirement (see Section 4.2).
A.1.3. Capable of Replacement by Classic Congestion Control
Description: It needs to be possible to replace the implementation of
a Scalable congestion control with a Classic control (see the precise
normative requirement wording in Section 4.3, Paragraph 8, Item 1).
Motivation: L4S is an experimental protocol; therefore, it seems
prudent to be able to disable it at source in case of insurmountable
problems, perhaps due to some unexpected interaction on a particular
sender; over a particular path or network; or with a particular
receiver, or even ultimately an insurmountable problem with the
experiment as a whole.
A.1.4. Fall Back to Classic Congestion Control on Packet Loss
Description: As well as responding to ECN markings in a scalable way,
a Scalable congestion control needs to react to packet loss in a way
that will coexist safely with a Reno congestion control [RFC 5681]
(see the precise normative requirement wording in Section 4.3,
Paragraph 8, Item 2).
Motivation: Part of the safety conditions for deploying a Scalable
congestion control on the public Internet is to make sure that it
behaves properly when it builds a queue at a network bottleneck that
has not been upgraded to support L4S. Packet loss can have many
causes, but it usually has to be conservatively assumed that it is a
sign of congestion. Therefore, on detecting packet loss, a Scalable
congestion control will need to fall back to Classic congestion
control behaviour. If it does not comply, it could starve Classic
traffic.
A Scalable congestion control can be used for different types of
transport, e.g., for real-time media or for reliable transport like
TCP. Therefore, the particular Classic congestion control behaviour
to fall back on will need to be dependent on the specific congestion
control implementation. In the particular case of DCTCP, the DCTCP
specification [RFC 8257] states that "A DCTCP sender MUST react to
loss episodes in the same way as conventional TCP,...". To ensure
any Scalable congestion control is safe to deploy over the public
Internet, Item 2 of Section 4.3 in the present spec does not require
precisely the same response as Reno TCP, but it does require a
response that will coexist safely with Classic congestion controls
like Reno.
Even though a bottleneck is L4S capable, it might still become
overloaded and have to drop packets. In this case, the sender may
receive a high proportion of packets marked with the CE codepoint and
also experience loss. Current DCTCP implementations each react
differently to this situation. One approach is to react only to the
drop signal (e.g., by halving the cwnd); another approach is to react
to both signals, which reduces cwnd by more than half. A compromise
between these two has been proposed where the loss response is
adjusted to result in a halving when combined with any ECN response
earlier in the same round. We believe that further experimentation
is needed to understand what is the best behaviour for the public
Internet, which may or may not be one of these existing approaches.
A.1.5. Coexistence with Classic Congestion Control at Classic ECN
Bottlenecks
Description: Monitoring has to be in place so that a non-L4S but ECN-
capable AQM can be detected at path bottlenecks. This is in case
such an AQM has been implemented in a shared queue, in which case any
long-running Scalable flow would predominate over any simultaneous
long-running Classic flow sharing the queue. The precise requirement
wording in Section 4.3, Paragraph 8, Item 3 is written so that such a
problem could be resolved either in real time or via administrative
intervention.
Motivation: Similarly to the discussion in Appendix A.1.4, this
requirement in Section 4.3 is a safety condition to ensure an L4S
congestion control coexists well with Classic flows when it builds a
queue at a shared network bottleneck that has not been upgraded to
support L4S. Nonetheless, if necessary, it is considered reasonable
to resolve such problems over management timescales (possibly
involving human intervention) because:
* although a Classic flow can considerably reduce its throughput in
the face of a competing Scalable flow, it still makes progress and
does not starve;
* implementations of a Classic ECN AQM in a queue that is intended
to be shared are believed to be rare; and
* detection of such AQMs is not always clear-cut; so focused out-of-
band testing (or even contacting the relevant network operator)
would improve certainty.
The relevant normative requirement (Section 4.3) is therefore divided
into three stages: monitoring, detection, and action:
Monitoring: Monitoring involves collection of the measurement data
to be analysed. Monitoring is expressed as a "MUST" for
uncontrolled environments, although the placement of the
monitoring function is left open. Whether monitoring has to be
applied in real time is expressed as a "SHOULD". This allows for
the possibility that the operator of an L4S sender (e.g., a
Content Distribution Network (CDN)) might prefer to test out-of-
band for signs of Classic ECN AQMs, perhaps to avoid continually
consuming resources to monitor live traffic.
Detection: Detection involves analysis of the monitored data to
detect the likelihood of a Classic ECN AQM. Detection can either
directly detect actual coexistence problems between flows or aim
to identify AQM technologies that are likely to present
coexistence problems, based on knowledge of AQMs deployed at the
time. The requirements recommend that detection occurs live in
real time. However, detection is allowed to be deferred (e.g., it
might involve further testing targeted at candidate AQMs).
Action: This involves the act of switching the sender to a Classic
congestion control. This might occur in real time within the
congestion control for the subsequent duration of a flow, or it
might involve administrative action to switch to Classic
congestion control for a specific interface or for a certain set
of destination addresses.
Instead of the sender taking action itself, the operator of the
sender (e.g., a CDN) might prefer to ask the network operator to
modify the Classic AQM's treatment of L4S packets; ensure L4S
packets bypass the AQM; or upgrade the AQM to support L4S (see the
L4S operational guidance [L4SOPS]). If L4S flows then no longer
shared the Classic ECN AQM, they would obviously no longer detect
it, and the requirement to act on it would no longer apply.
The whole set of normative requirements concerning Classic ECN AQMs
in Section 4.3 is worded so that it does not apply in controlled
environments, such as private networks or data-centre networks. CDN
servers placed within an access ISP's network can be considered as a
single controlled environment, but any onward networks served by the
access network, including all the attached customer networks, would
be unlikely to fall under the same degree of coordinated control.
Monitoring is expressed as a "MUST" for these uncontrolled segments
of paths (e.g., beyond the access ISP in a home network), because
there is a possibility that there might be a shared queue Classic ECN
AQM in that segment. Nonetheless, the intent of the wording is to
only require occasional monitoring of these uncontrolled regions and
not to burden CDN operators if monitoring never uncovers any
potential problems.
More detailed discussion of all the above options and alternatives
can be found in the L4S operational guidance [L4SOPS].
Having said all the above, the approach recommended in Section 4.3 is
to monitor, detect, and act in real time on live traffic. A passive
monitoring algorithm to detect a Classic ECN AQM at the bottleneck
and fall back to Classic congestion control is described in an
extensive technical report [ecn-fallback], which also provides a link
to Linux source code and a large online visualization of its
evaluation results. Very briefly, the algorithm primarily monitors
RTT variation using the same algorithm that maintains the mean
deviation of TCP's smoothed RTT, but it smooths over a duration of
the order of a Classic sawtooth. The outcome is also conditioned on
other metrics such as the presence of CE marking and congestion
avoidance phase having stabilized. The report also identifies
further work to improve the approach, for instance, improvements with
low-capacity links and combining the measurements with a cache of
what had been learned about a path in previous connections. The
report also suggests alternative approaches.
Although using passive measurements within live traffic (as above)
can detect a Classic ECN AQM, it is much harder (perhaps impossible)
to determine whether or not the AQM is in a shared queue.
Nonetheless, this is much easier using active test traffic out-of-
band because two flows can be used. Section 4 of the same report
[ecn-fallback] describes a simple technique to detect a Classic ECN
AQM and determine whether it is in a shared queue, which is
summarized here.
An L4S-enabled test server could be set up so that, when a test
client accesses it, it serves a script that gets the client to open
two parallel long-running flows. It could serve one with a Classic
congestion control (C, that sets ECT(0)) and one with a Scalable CC
(L, that sets ECT(1)). If neither flow induces any ECN marks, it can
be presumed that the path does not contain a Classic ECN AQM. If
either flow induces some ECN marks, the server could measure the
relative flow rates and round-trip times of the two flows. Table 2
shows the AQM that can be inferred for various cases (presuming no
more types of AQM behaviour than those known at the time of writing).
+========+=======+========================+
| Rate | RTT | Inferred AQM |
+========+=======+========================+
| L > C | L = C | Classic ECN AQM (FIFO) |
+--------+-------+------------------------+
| L = C | L = C | Classic ECN AQM (FQ) |
+--------+-------+------------------------+
| L = C | L < C | FQ-L4S AQM |
+--------+-------+------------------------+
| L ~= C | L < C | DualQ Coupled AQM |
+========+=======+========================+
| L = L4S; C = Classic |
+=========================================+
Table 2: Out-of-Band Testing with Two
Parallel Flows
Finally, we motivate the recommendation in Section 4.3 that a
Scalable congestion control is not expected to change to setting
ECT(0) while it adapts its behaviour to coexist with Classic flows.
This is because the sender needs to continue to check whether it made
the right decision and switch back if it was wrong, or if a different
link becomes the bottleneck:
* If, as recommended, the sender changes only its behaviour but not
its codepoint to Classic, its codepoint will still be compatible
with either an L4S or a Classic AQM. If the bottleneck does
actually support both, it will still classify ECT(1) into the same
L4S queue, where the sender can measure that switching to Classic
behaviour was wrong so that it can switch back.
* In contrast, if the sender changes both its behaviour and its
codepoint to Classic, even if the bottleneck supports both, it
will classify ECT(0) into the Classic queue, reinforcing the
sender's incorrect decision so that it never switches back.
* Also, not changing its codepoint avoids the risk of being flipped
to a different path by a load balancer or multipath routing that
hashes on the whole of the former Type-of-Service (ToS) byte
(which is unfortunately still a common pathology).
| Note that if a flow is configured to _only_ use a Classic
| congestion control, it is then entirely appropriate not to use
| ECT(1).
A.1.6. Reduce RTT Dependence
Description: A Scalable congestion control needs to reduce RTT bias
as much as possible at least over the low-to-typical range of RTTs
that will interact in the intended deployment scenario (see the
precise normative requirement wording in Section 4.3, Paragraph 8,
Item 4).
Motivation: The throughput of Classic congestion controls is known to
be inversely proportional to RTT, so one would expect flows over very
low RTT paths to nearly starve flows over larger RTTs. However,
Classic congestion controls have never allowed a very low RTT path to
exist because they induce a large queue. For instance, consider two
paths with base RTT 1 ms and 100 ms. If a Classic congestion control
induces a 100 ms queue, it turns these RTTs into 101 ms and 200 ms,
leading to a throughput ratio of about 2:1. Whereas if a Scalable
congestion control induces only a 1 ms queue, the ratio is 2:101,
leading to a throughput ratio of about 50:1.
Therefore, with very small queues, long RTT flows will essentially
starve, unless Scalable congestion controls comply with the
requirement in Section 4.3.
Over higher than typical RTTs, L4S flows can use the same RTT bias as
in current Classic congestion controls and still work satisfactorily.
So there is no additional requirement in Section 4.3 for high RTT L4S
flows to remove RTT bias -- they can, but they don't have to.
One way for a Scalable congestion control to satisfy these
requirements is to make its additive increase behave as if it were a
standard Reno flow but over a larger RTT by using a virtual RTT
(rtt_virt) that is a function of the actual RTT (rtt). Example
functions might be:
rtt_virt = max(rtt, 25 ms)
rtt_virt = rtt + 10 ms
These example functions are chosen so that, as the actual RTT reduces
from high to low, the virtual RTT reduces less (see [PRAGUE-CC] for
details).
However, short RTT flows can more rapidly respond to changes in
available capacity, whether due to other flows arriving and departing
or radio capacity varying. So it would be wrong to require short RTT
flows to be as sluggish as long RTT flows, which would unnecessarily
underutilize capacity and result in unnecessary overshoots and
undershoots (instability). Therefore, rather than requiring strict
RTT independence, the wording in Item 4 of Section 4.3 is "as
independent of RTT as possible without compromising stability or
utilization". This allows shorter RTT flows to exploit their agility
advantage.
A.1.7. Scaling Down to Fractional Congestion Windows
Description: A Scalable congestion control needs to remain responsive
to congestion when typical RTTs over the public Internet are
significantly smaller because they are no longer inflated by queuing
delay (see the precise normative requirement wording in Section 4.3,
Paragraph 8, Item 5).
Motivation: As currently specified, the minimum congestion window of
ECN-capable TCP (and its derivatives) is expected to be 2 sender
maximum segment sizes (SMSS), or 1 SMSS after a retransmission
timeout. Once the congestion window reaches this minimum, if there
is further ECN marking, TCP is meant to wait for a retransmission
timeout before sending another segment (see Section 6.1.2 of the ECN
spec [RFC 3168]). In practice, most known window-based congestion
control algorithms become unresponsive to ECN congestion signals at
this point. No matter how much ECN marking, the congestion window no
longer reduces. Instead, the sender's lack of any further congestion
response forces the queue to grow, overriding any AQM and increasing
queuing delay (making the window large enough to become responsive
again). This can result in a stable but deeper queue, or it might
drive the queue to loss, in which case the retransmission timeout
mechanism acts as a backstop.
Most window-based congestion controls for other transport protocols
have a similar minimum window, albeit when measured in bytes for
those that use smaller packets.
L4S mechanisms significantly reduce queuing delay so, over the same
path, the RTT becomes lower. Then, this problem becomes surprisingly
common [sub-mss-prob]. This is because, for the same link capacity,
smaller RTT implies a smaller window. For instance, consider a
residential setting with an upstream broadband Internet access of 8
Mb/s, assuming a max segment size of 1500 B. Two upstream flows will
each have the minimum window of 2 SMSS if the RTT is 6 ms or less,
which is quite common when accessing a nearby data centre. So any
more than two such parallel TCP flows will become unresponsive to ECN
and increase queuing delay.
Unless Scalable congestion controls address the requirement in
Section 4.3 from the start, they will frequently become unresponsive
to ECN, negating the low-latency benefit of L4S, for themselves and
for others.
That would seem to imply that Scalable congestion controllers ought
to be required to be able work with a congestion window less than 1
SMSS. For instance, if an ECN-capable TCP gets an ECN mark when it
is already sitting at a window of 1 SMSS, [RFC 3168] requires it to
defer sending for a retransmission timeout. A less drastic but more
complex mechanism can maintain a congestion window less than 1 SMSS
(significantly less if necessary), as described in [Ahmed19]. Other
approaches are likely to be feasible.
However, the requirement in Section 4.3 is worded as a "SHOULD"
because it is believed that the existence of a minimum window is not
all bad. When competing with an unresponsive flow, a minimum window
naturally protects the flow from starvation by at least keeping some
data flowing.
By stating the requirement to go lower than 1 SMSS as a "SHOULD",
while the requirement in [RFC 3168] still stands as well, we shall be
able to watch the choices of minimum window evolve in different
Scalable congestion controllers.
A.1.8. Measuring Reordering Tolerance in Time Units
Description: When detecting loss, a Scalable congestion control needs
to be tolerant to reordering over an adaptive time interval, which
scales with throughput, rather than counting only in fixed units of
packets, which does not scale (see the precise normative requirement
wording in Section 4.3, Paragraph 8, Item 6).
Motivation: A primary purpose of L4S is scalable throughput (it's in
the name). Scalability in all dimensions is, of course, also a goal
of all IETF technology. The inverse linear congestion response in
Section 4.3 is necessary, but not sufficient, to solve the congestion
control scalability problem identified in [RFC 3649]. As well as
maintaining frequent ECN signals as rate scales, it is also important
to ensure that a potentially false perception of loss does not limit
throughput scaling.
End systems cannot know whether a missing packet is due to loss or
reordering, except in hindsight -- if it appears later. So they can
only deem that there has been a loss if a gap in the sequence space
has not been filled, either after a certain number of subsequent
packets has arrived (e.g., the 3 DupACK rule of standard TCP
congestion control [RFC 5681]) or after a certain amount of time
(e.g., the RACK approach [RFC 8985]).
As we attempt to scale packet rate over the years:
* Even if only _some_ sending hosts still deem that loss has
occurred by counting reordered packets, _all_ networks will have
to keep reducing the time over which they keep packets in order.
If some link technologies keep the time within which reordering
occurs roughly unchanged, then loss over these links, as perceived
by these hosts, will appear to continually rise over the years.
* In contrast, if all senders detect loss in units of time, the time
over which the network has to keep packets in order stays roughly
invariant.
Therefore, hosts have an incentive to detect loss in time units (so
as not to fool themselves too often into detecting losses when there
are none). And for hosts that are changing their congestion control
implementation to L4S, there is no downside to including time-based
loss detection code in the change (loss recovery implemented in
hardware is an exception, which is covered later). Therefore,
requiring L4S hosts to detect loss in time-based units would not be a
burden.
If the requirement in Section 4.3 were not placed on L4S hosts, even
though it would be no burden on hosts to comply, all networks would
face unnecessary uncertainty over whether some L4S hosts might be
detecting loss by counting packets. Then, _all_ link technologies
would have to unnecessarily keep reducing the time within which
reordering occurs. That is not a problem for some link technologies,
but it becomes increasingly challenging for other link technologies
to continue to scale, particularly those relying on channel bonding
for scaling, such as LTE, 5G, and Data Over Cable Service Interface
Specification (DOCSIS).
Given Internet paths traverse many link technologies, any scaling
limit for these more challenging access link technologies would
become a scaling limit for the Internet as a whole.
It might be asked how it helps to place this loss detection
requirement only on L4S hosts, because networks will still face
uncertainty over whether non-L4S flows are detecting loss by counting
DupACKs. The answer is that those link technologies for which it is
challenging to keep squeezing the reordering time will only need to
do so for non-L4S traffic (which they can do because the L4S
identifier is visible at the IP layer). Therefore, they can focus
their processing and memory resources into scaling non-L4S (Classic)
traffic. Then, the higher the proportion of L4S traffic, the less of
a scaling challenge they will have.
To summarize, there is no reason for L4S hosts not to be part of the
solution instead of part of the problem.
Requirement ("MUST") or recommendation ("SHOULD")? As explained
above, this is a subtle interoperability issue between hosts and
networks, which seems to need a "MUST". Unless networks can be
certain that all L4S hosts follow the time-based approach, they still
have to cater for the worst case -- continually squeeze reordering
into a smaller and smaller duration -- just for hosts that might be
using the counting approach. However, it was decided to express this
as a recommendation, using "SHOULD". The main justification was that
networks can still be fairly certain that L4S hosts will follow this
recommendation, because following it offers only gain and no pain.
Details:
The time spent recovering a loss is much more significant for short
flows than long; therefore, a good compromise is to adapt the
reordering window from a small fraction of the RTT at the start of a
flow to a larger fraction of the RTT for flows that continue for many
round trips.
This is broadly the approach adopted by RACK [RFC 8985]. However,
RACK starts with the 3 DupACK approach, because the RTT estimate is
not necessarily stable. As long as the initial window is paced, such
initial use of 3 DupACK counting would amount to time-based loss
detection and therefore would satisfy the time-based loss detection
recommendation of Section 4.3. This is because pacing of the initial
window would ensure that 3 DupACKs early in the connection would be
spread over a small fraction of the round trip.
As mentioned above, hardware implementations of loss recovery using
DupACK counting exist (e.g., some implementations of Remote Direct
Memory Access over Converged Ethernet version 2 (RoCEv2)). For low
latency, these implementations can change their congestion control to
implement L4S, because the congestion control (as distinct from loss
recovery) is implemented in software. But they cannot easily satisfy
this loss recovery requirement. However, it is believed they do not
need to, because such implementations are believed to solely exist in
controlled environments, where the network technology keeps
reordering extremely low anyway. This is why controlled environments
with hardly any reordering are excluded from the scope of the
normative recommendation in Section 4.3.
Detecting loss in time units also prevents the ACK-splitting attacks
described in [Savage-TCP].
A.2. Scalable Transport Protocol Optimizations
A.2.1. Setting ECT in Control Packets and Retransmissions
Description: This item concerns TCP and its derivatives (e.g., SCTP)
as well as RTP/RTCP [RFC 6679]. The original specification of ECN for
TCP precluded the use of ECN on control packets and retransmissions.
Similarly, [RFC 6679] precludes the use of ECT on RTCP datagrams, in
case the path changes after it has been checked for ECN traversal.
To improve performance, Scalable transport protocols ought to enable
ECN at the IP layer in TCP control packets (SYN, SYN-ACK, pure ACKs,
etc.) and in retransmitted packets. The same is true for other
transports, e.g., SCTP and RTCP.
Motivation (TCP): [RFC 3168] prohibits the use of ECN on these types
of TCP packets, based on a number of arguments. This means these
packets are not protected from congestion loss by ECN, which
considerably harms performance, particularly for short flows. ECN++
[ECN++] proposes experimental use of ECN on all types of TCP packets
as long as AccECN feedback [ACCECN] is available (which itself
satisfies the accurate feedback requirement in Section 4.2 for using
a Scalable congestion control).
Motivation (RTCP): L4S experiments in general will need to observe
the rule in the RTP ECN spec [RFC 6679] that precludes ECT on RTCP
datagrams. Nonetheless, as ECN usage becomes more widespread, it
would be useful to conduct specific experiments with ECN-capable RTCP
to gather data on whether such caution is necessary.
A.2.2. Faster than Additive Increase
Description: It would improve performance if Scalable congestion
controls did not limit their congestion window increase to the
standard additive increase of 1 SMSS per round trip [RFC 5681] during
congestion avoidance. The same is true for derivatives of TCP
congestion control, including similar approaches used for real-time
media.
Motivation: As currently defined [RFC 8257], DCTCP uses the
conventional Reno additive increase in the congestion avoidance
phase. When the available capacity suddenly increases (e.g., when
another flow finishes or if radio capacity increases) it can take
very many round trips to take advantage of the new capacity. TCP
CUBIC [RFC 8312] was designed to solve this problem, but as flow rates
have continued to increase, the delay accelerating into available
capacity has become prohibitive. See, for instance, the examples in
Section 5.1 of the L4S architecture [RFC 9330]. Even when out of its
Reno-friendly mode, every 8 times scaling of CUBIC's flow rate leads
to 2 times more acceleration delay.
In the steady state, DCTCP induces about 2 ECN marks per round trip,
so it is possible to quickly detect when these signals have
disappeared and seek available capacity more rapidly, while
minimizing the impact on other flows (Classic and Scalable)
[LinuxPacedChirping]. Alternatively, approaches such as Adaptive-
Acceleration Data Center TCP (A2DTCP) [A2DTCP]) have been proposed to
address this problem in data centres, which might be deployable over
the public Internet.
A.2.3. Faster Convergence at Flow Start
Description: It would improve performance if Scalable congestion
controls converged (reached their steady-state share of the capacity)
faster than Classic congestion controls or at least no slower. This
affects the flow start behaviour of any L4S congestion control
derived from a Classic transport that uses TCP slow start, including
those for real-time media.
Motivation: As an example, a new DCTCP flow takes longer than a
Classic congestion control to obtain its share of the capacity of the
bottleneck when there are already ongoing flows using the bottleneck
capacity. In a data-centre environment, DCTCP takes about 1.5 to 2
times longer to converge due to the much higher typical level of ECN
marking that DCTCP background traffic induces, which causes new flows
to exit slow start early [Alizadeh-stability]. In testing for use
over the public Internet, the convergence time of DCTCP relative to a
regular loss-based TCP slow start is even less favourable
[LinuxPacedChirping] due to the shallow ECN-marking threshold needed
for L4S. It is exacerbated by the typically greater mismatch between
the link rate of the sending host and typical Internet access
bottlenecks. This problem is detrimental in general but would
particularly harm the performance of short flows relative to Classic
congestion controls.
Appendix B. Compromises in the Choice of L4S Identifier
This appendix is informative, not normative. As explained in
Section 3, there is insufficient space in the IP header (v4 or v6) to
fully accommodate every requirement. So the choice of L4S identifier
involves trade-offs. This appendix records the pros and cons of the
choice that was made.
Non-normative recap of the chosen codepoint scheme:
Packets with ECT(1) and conditionally packets with CE signify L4S
semantics as an alternative to the semantics of Classic ECN
[RFC 3168], specifically:
- The ECT(1) codepoint signifies that the packet was sent by an
L4S-capable sender.
- Given the shortage of codepoints, both the L4S and Classic ECN
sides of an AQM have to use the same CE codepoint to indicate
that a packet has experienced congestion. If a packet that had
already been marked CE in an upstream buffer arrived at a
subsequent AQM, this AQM would then have to guess whether to
classify CE packets as L4S or Classic ECN. Choosing the L4S
treatment is a safer choice, because then a few Classic packets
might arrive early rather than a few L4S packets arriving late.
- Additional information might be available if the classifier
were transport-aware. Then, it could classify a CE packet for
Classic ECN treatment if the most recent ECT packet in the same
flow had been set to ECT(0). However, the L4S service ought
not need transport-layer awareness.
Cons:
Consumes the last ECN codepoint: The L4S service could potentially
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
the equivalent of an IP-ECN field in an AQM acting in a buffer
below the IP layer [ECN-ENCAP]. Then, depending on the lower-
layer scheme, the L4S service might have to drop rather than mark
frames even though they might encapsulate an ECN-capable packet.
Risk of reordering Classic CE packets within a flow: Classifying all
CE packets into the L4S queue risks any CE packets that were
originally ECT(0) being incorrectly classified as L4S. If there
were delay in the Classic queue, these incorrectly classified CE
packets would arrive early, which is a form of reordering.
Reordering within a microflow can cause TCP senders (and senders
of similar transports) to retransmit spuriously. However, the
risk of spurious retransmissions would be extremely low for the
following reasons:
1. It is quite unusual to experience queuing at more than one
bottleneck on the same path (the available capacities have to
be identical).
2. In only a subset of these unusual cases would the first
bottleneck support Classic ECN marking and the second L4S ECN
marking. This would be the only scenario where some ECT(0)
packets could be CE marked by an AQM supporting Classic ECN
while subsequently the remaining ECT(0) packets would
experience further delay through the Classic side of a
subsequent L4S DualQ AQM.
3. Even then, when a few packets are delivered early, it takes
very unusual conditions to cause a spurious retransmission, in
contrast to when some packets are delivered late. The first
bottleneck has to apply CE marks to at least N contiguous
packets, and the second bottleneck has to inject an
uninterrupted sequence of at least N of these packets between
two packets earlier in the stream (where N is the reordering
window that the transport protocol allows before it considers
a packet is lost).
For example, consider N=3, and consider the sequence of
packets 100, 101, 102, 103,... Imagine that packets 150,
151, 152 from later in the flow are injected as follows:
100, 150, 151, 101, 152, 102, 103,... If this were late
reordering, even one packet arriving out of sequence would
trigger a spurious retransmission, but there is no spurious
retransmission here with early reordering, because packet
101 moves the cumulative ACK counter forward before 3
packets have arrived out of order. Later, when packets
148, 149, 153,... arrive, even though there is a 3-packet
hole, there will be no problem, because the packets to fill
the hole are already in the receive buffer.
4. Even with the current TCP recommendation of N=3 [RFC 5681],
spurious retransmissions will be unlikely for all the above
reasons. As RACK [RFC 8985] is becoming widely deployed, it
tends to adapt its reordering window to a larger value of N,
which will make the chance of a contiguous sequence of N early
arrivals vanishingly small.
5. Even a run of 2 CE marks within a Classic ECN flow is
unlikely, given FQ-CoDel is the only known widely deployed AQM
that supports Classic ECN marking, and it takes great care to
separate out flows and to space any markings evenly along each
flow.
It is extremely unlikely that the above set of 5 eventualities
that are each unusual in themselves would all happen
simultaneously. But, even if they did, the consequences would
hardly be dire: the odd spurious fast retransmission. Whenever
the traffic source (a Classic congestion control) mistakes the
reordering of a string of CE marks for a loss, one might think
that it will reduce its congestion window as well as emitting a
spurious retransmission. However, it would have already reduced
its congestion window when the CE markings arrived early. If it
is using ABE [RFC 8511], it might reduce cwnd a little more for a
loss than for a CE mark. But it will revert that reduction once
it detects that the retransmission was spurious.
In conclusion, the impact of early reordering on spurious
retransmissions due to CE being ambiguous will generally be
vanishingly small.
Insufficient anti-replay window in some pre-existing VPNs: If delay
is reduced for a subset of the flows within a VPN, the anti-replay
feature of some VPNs is known to potentially mistake the
difference in delay for a replay attack. Section 6.2 recommends
that the anti-replay window at the VPN egress is sufficiently
sized, as required by the relevant specifications. However, in
some VPN implementations, the maximum anti-replay window is
insufficient to cater for a large delay difference at prevailing
packet rates. Section 6.2 suggests alternative work-rounds for
such cases, but end users using L4S over a VPN will need to be
able to recognize the symptoms of this problem, in order to seek
out these work-rounds.
Hard to distinguish Classic ECN AQM: With this scheme, when a source
receives ECN feedback, it is not explicitly clear which type of
AQM generated the CE markings. This is not a problem for Classic
ECN sources that send ECT(0) packets, because an L4S AQM will
recognize the ECT(0) packets as Classic and apply the appropriate
Classic ECN-marking behaviour.
However, in the absence of explicit disambiguation of the CE
markings, an L4S source needs to use heuristic techniques to work
out which type of congestion response to apply (see
Appendix A.1.5). Otherwise, if long-running Classic flows are
sharing a Classic ECN AQM bottleneck with long-running L4S flows,
and the L4S flows apply an L4S response to the Classic CE signals,
they would then outcompete the Classic flows. Experiments have
shown that L4S flows can take about 20 times more capacity share
than equivalent Classic flows. Nonetheless, as link capacity
reduces (e.g., to 4 Mb/s), the inequality reduces. So Classic
flows always make progress and are not starved.
When L4S was first proposed (in 2015, 14 years after the Classic
ECN spec [RFC 3168] was published), it was believed that Classic
ECN AQMs had failed to be deployed because research measurements
had found little or no evidence of CE marking. In subsequent
years, Classic ECN was included in FQ deployments; however, an FQ
scheduler stops an L4S flow outcompeting Classic, because it
enforces equality between flow rates. It is not known whether
there have been any non-FQ deployments of Classic ECN AQMs in the
subsequent years or whether there will be any in future.
An algorithm for detecting a Classic ECN AQM as soon as a flow
stabilizes after start-up has been proposed [ecn-fallback] (see
Appendix A.1.5 for a brief summary). Testbed evaluations of v2 of
the algorithm have shown detection is reasonably good for Classic
ECN AQMs, in a wide range of circumstances. However, although it
can correctly detect an L4S ECN AQM in many circumstances, it is
often incorrect at low link capacities and/or high RTTs. Although
this is the safe way round, there is a danger that it will
discourage use of the algorithm.
Non-L4S service for control packets: Solely for the case of TCP, the
Classic ECN RFCs [RFC 3168] and [RFC 5562] require a sender to clear
the IP-ECN field to Not-ECT on retransmissions and on certain
control packets, specifically pure ACKs, window probes, and SYNs.
When L4S packets are classified by the IP-ECN field, these TCP
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 reordering (because retransmissions
are already out of order, and these control packets typically
carry no data). However, it would make critical TCP control
packets more vulnerable to loss and delay. To address this
problem, ECN++ [ECN++] proposes an experiment in which all TCP
control packets and retransmissions are ECN-capable as long as
appropriate ECN feedback is available in each case.
Pros:
Should work end to end: The IP-ECN field generally propagates end to
end across the Internet without being wiped or mangled, at least
over fixed networks. Unlike the DSCP, the setting of the ECN
field is at least meant to be forwarded unchanged by networks that
do not support ECN.
Should work in tunnels: The L4S identifiers work across and within
any tunnel that propagates the IP-ECN field in any of the variant
ways it has been defined since ECN-tunneling was first specified
in the year 2001 [RFC 3168]. However, it is likely that some
tunnels still do not implement ECN propagation at all.
Should work for many link technologies: At most, but not all, path
bottlenecks there is IP awareness, so that L4S AQMs can be located
where the IP-ECN field can be manipulated. Bottlenecks at lower-
layer nodes without IP awareness have to either use drop to signal
congestion or have a specific congestion notification facility
defined for that link technology, including propagation to and
from IP-ECN. The programme to define these is progressing, and in
each case so far, the scheme already defined for ECN inherently
supports L4S as well (see Section 6.1).
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 has reduced to zero, or near zero, which might
never happen.
L4 not required: Being based on the IP-ECN field, this scheme does
not need the network to access transport-layer flow IDs.
Nonetheless, it does not preclude solutions that do.
Appendix C. Potential Competing Uses for the ECT(1) Codepoint
The ECT(1) codepoint of the IP-ECN field has already been assigned
once for the ECN nonce spec [RFC 3540], which has now been categorized
as Historic [RFC 8311]. 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 (L4S) without carefully assessing competing
potential uses. These fall into the categories described below.
C.1. Integrity of Congestion Feedback
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).
The Historic ECN nonce spec [RFC 3540] proposed that a TCP sender
could set either ECT(0) or ECT(1) in each packet of a flow and
remember the sequence it had set. If any packet was lost or
congestion marked, the receiver would miss that bit of the sequence.
An ECN nonce receiver had to feed back the least-significant bit of
the sum, so it could not suppress feedback of a loss or mark without
a 50-50 chance of guessing the sum incorrectly.
It is highly unlikely that ECT(1) will be needed as a nonce for
integrity protection of congestion notifications in future. The ECN
nonce spec [RFC 3540] has been reclassified as Historic, partly
because other ways (that do not consume a codepoint in the IP header)
have been developed to protect feedback integrity of TCP and other
transports [RFC 8311]. For instance:
* The sender can test the integrity of a small random sample of the
receiver's feedback by occasionally setting the IP-ECN field to a
value normally only set by the network. Then, it can test whether
the receiver's feedback faithfully reports what it expects (see
Paragraph 2 of Section 20.2 of the ECN spec [RFC 3168]. This works
for loss, and it will work for the accurate ECN feedback [RFC 7560]
intended for L4S. Like the (Historic) ECN nonce spec, this
technique does not protect against a misbehaving sender. But it
allows a well-behaved sender to check that each receiver is
correctly feeding back congestion notifications.
* A network can check that its ECN markings (or packet losses) have
been passed correctly around the full feedback loop by auditing
Congestion Exposure (ConEx) [RFC 7713]. This assures that the
integrity of congestion notifications and feedback messages must
have both been preserved. ConEx information is also available
anywhere along the network path, so it can be used to enforce a
congestion response. Whether the receiver or a downstream network
is suppressing congestion feedback or the sender is unresponsive
to the feedback, or both, ConEx is intended to neutralize any
advantage that any of these three parties would otherwise gain.
* Congestion feedback fields in transport-layer headers are
immutable end to end and therefore amenable to end-to-end
integrity protection. This preserves the integrity of a
receiver's feedback messages to the sender, but it does not
protect against misbehaving receivers or misbehaving senders. The
TCP Authentication Option (TCP-AO) [RFC 5925], QUIC's end-to-end
protection [RFC 9001], or end-to-end IPsec integrity protection
[RFC 4303] can be used to detect any tampering with congestion
feedback (whether malicious or accidental), respectively, in TCP,
QUIC, or any transport. TCP-AO covers the main TCP header and TCP
options by default, but it is often too brittle to use on many
end-to-end paths, where middleboxes can make verification fail in
their attempts to improve performance or security, e.g., by
resegmentation or shifting the sequence space.
At the time of writing, it is becoming common to protect the
integrity of transport feedback using QUIC. However, it is still not
common to protect the integrity of the wider congestion feedback
loop, whether based on loss or Classic ECN. If this position changes
during the L4S experiment, one or more of the above techniques might
need to be developed and deployed.
C.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., the Variable-structure
congestion Control Protocol (VCP) [VCP] and Queue View (QV) [QV].
Before assigning ECT(1) as an identifier for L4S, we must carefully
consider whether it might be better to hold ECT(1) in reserve for
future standardization of rapid flow acceleration, which is an
important and enduring problem [RFC 6077].
Pre-Congestion Notification (PCN) is another scheme that assigns
alternative semantics to the IP-ECN field. It uses ECT(1) to signify
a less severe level of pre-congestion notification than CE [RFC 6660].
However, the IP-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 the L4S service
identifier defined in Section 2.
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.
Ing-jyh (Inton) Tsang was a contributor to the early draft versions
of this document. Thanks to Mikael Abrahamsson, Lloyd Wood, Nicolas
Kuhn, Greg White, Tom Henderson, David Black, Gorry Fairhurst, Brian
Carpenter, Jake Holland, Rod Grimes, Richard Scheffenegger, Sebastian
Moeller, Neal Cardwell, Praveen Balasubramanian, Reza Marandian Hagh,
Pete Heist, Stuart Cheshire, Vidhi Goel, Mirja Kühlewind, Ermin
Sakic, and Martin Duke for providing help and reviewing this
document. And thanks to Ingemar Johansson for reviewing and
providing substantial text. Thanks also to the area reviewers:
Valery Smyslov, Maria Ines Robles, Bernard Aboba, Lars Eggert, Roman
Danyliw, and Éric Vyncke. Thanks to Sebastian Moeller for
identifying the interaction with VPN anti-replay and to Jonathan
Morton for identifying the attack based on this. Particular thanks
to tsvwg chairs Gorry Fairhurst, David Black, and Wes Eddy for
patiently helping this and the other L4S documents through the IETF
process. Appendix A, which lists the Prague L4S Requirements, is
based on text authored by Marcelo Bagnulo Braun that was originally
an appendix to [RFC 9330]. That text was in turn based on the
collective output of the attendees listed in the minutes of a 'bar
BoF' on DCTCP Evolution during IETF 94 [TCPPrague].
The authors' contributions were partly funded by the European
Community under its Seventh Framework Programme through the Reducing
Internet Transport Latency (RITE) project (ICT-317700). The
contribution of Koen De Schepper was also partly funded by the
5Growth and DAEMON EU H2020 projects. Bob Briscoe was partly funded
by the Research Council of Norway through the TimeIn project,
CableLabs, and the Comcast Innovation Fund. The views expressed here
are solely those of the authors.
Authors' Addresses
Koen De Schepper
Nokia Bell Labs
Antwerp
Belgium
Email: koen.de_schepper@nokia.com
URI: https://www.bell-labs.com/about/researcher-profiles/
koende_schepper/
Bob Briscoe (editor)
Independent
United Kingdom
Email: ietf@bobbriscoe.net
URI: https://bobbriscoe.net/
RFC TOTAL SIZE: 163281 bytes
PUBLICATION DATE: Friday, January 20th, 2023
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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