Internet DRAFT - draft-ietf-tsvwg-ecn-l4s-id
draft-ietf-tsvwg-ecn-l4s-id
Transport Services (tsv) K. De Schepper
Internet-Draft Nokia Bell Labs
Intended status: Experimental B. Briscoe, Ed.
Expires: 2 March 2023 Independent
29 August 2022
Explicit Congestion Notification (ECN) Protocol for Very Low Queuing
Delay (L4S)
draft-ietf-tsvwg-ecn-l4s-id-29
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 degrading the low queuing delay and low loss of L4S traffic. This
experimental track 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 examples of new transports (whether TCP-
like or real-time) are specified separately.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on 2 March 2023.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Latency, Loss and Scaling Problems . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 7
1.3. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2. L4S Packet Identification: Document Roadmap . . . . . . . . . 10
3. Choice of L4S Packet Identifier: Requirements . . . . . . . . 11
4. Transport Layer Behaviour (the 'Prague Requirements') . . . . 12
4.1. Codepoint Setting . . . . . . . . . . . . . . . . . . . . 12
4.2. Prerequisite Transport Feedback . . . . . . . . . . . . . 13
4.3. Prerequisite Congestion Response . . . . . . . . . . . . 14
4.3.1. Guidance on Congestion Response in the RFC Series . . 17
4.4. Filtering or Smoothing of ECN Feedback . . . . . . . . . 20
5. Network Node Behaviour . . . . . . . . . . . . . . . . . . . 20
5.1. Classification and Re-Marking Behaviour . . . . . . . . . 20
5.2. The Strength of L4S CE Marking Relative to Drop . . . . . 22
5.3. Exception for L4S Packet Identification by Network Nodes
with Transport-Layer Awareness . . . . . . . . . . . . . 23
5.4. Interaction of the L4S Identifier with other
Identifiers . . . . . . . . . . . . . . . . . . . . . . . 23
5.4.1. DualQ Examples of Other Identifiers Complementing L4S
Identifiers . . . . . . . . . . . . . . . . . . . . . 23
5.4.1.1. Inclusion of Additional Traffic with L4S . . . . 23
5.4.1.2. Exclusion of Traffic From L4S Treatment . . . . . 26
5.4.1.3. Generalized Combination of L4S and Other
Identifiers . . . . . . . . . . . . . . . . . . . . 26
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5.4.2. Per-Flow Queuing Examples of Other Identifiers
Complementing L4S Identifiers . . . . . . . . . . . . 28
5.5. Limiting Packet Bursts from Links . . . . . . . . . . . . 28
5.5.1. Limiting Packet Bursts from Links Fed by an L4S
AQM . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.5.2. Limiting Packet Bursts from Links Upstream of an L4S
AQM . . . . . . . . . . . . . . . . . . . . . . . . . 29
6. Behaviour of Tunnels and Encapsulations . . . . . . . . . . . 29
6.1. No Change to ECN Tunnels and Encapsulations in General . 29
6.2. VPN Behaviour to Avoid Limitations of Anti-Replay . . . . 30
7. L4S Experiments . . . . . . . . . . . . . . . . . . . . . . . 31
7.1. Open Questions . . . . . . . . . . . . . . . . . . . . . 32
7.2. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 33
7.3. Future Potential . . . . . . . . . . . . . . . . . . . . 33
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
9. Security Considerations . . . . . . . . . . . . . . . . . . . 35
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
10.1. Normative References . . . . . . . . . . . . . . . . . . 35
10.2. Informative References . . . . . . . . . . . . . . . . . 36
Appendix A. Rationale for the 'Prague L4S Requirements' . . . . 46
A.1. Rationale for the Requirements for Scalable Transport
Protocols . . . . . . . . . . . . . . . . . . . . . . . . 47
A.1.1. Use of L4S Packet Identifier . . . . . . . . . . . . 47
A.1.2. Accurate ECN Feedback . . . . . . . . . . . . . . . . 47
A.1.3. Capable of Replacement by Classic Congestion
Control . . . . . . . . . . . . . . . . . . . . . . . 47
A.1.4. Fall back to Classic Congestion Control on Packet
Loss . . . . . . . . . . . . . . . . . . . . . . . . 48
A.1.5. Coexistence with Classic Congestion Control at Classic
ECN bottlenecks . . . . . . . . . . . . . . . . . . . 49
A.1.6. Reduce RTT dependence . . . . . . . . . . . . . . . . 52
A.1.7. Scaling down to fractional congestion windows . . . . 53
A.1.8. Measuring Reordering Tolerance in Time Units . . . . 54
A.2. Scalable Transport Protocol Optimizations . . . . . . . . 57
A.2.1. Setting ECT in Control Packets and Retransmissions . 57
A.2.2. Faster than Additive Increase . . . . . . . . . . . . 58
A.2.3. Faster Convergence at Flow Start . . . . . . . . . . 58
Appendix B. Compromises in the Choice of L4S Identifier . . . . 59
Appendix C. Potential Competing Uses for the ECT(1) Codepoint . 64
C.1. Integrity of Congestion Feedback . . . . . . . . . . . . 64
C.2. Notification of Less Severe Congestion than CE . . . . . 65
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 66
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 66
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1. Introduction
This experimental track 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') Explicit Congestion Notification
(ECN [RFC3168]). RFC 3168 required 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 [DualPI2Linux],
[DCttH19]. 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 [I-D.ietf-tsvwg-l4s-arch]).
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 [RFC8257],
because it is too aggressive to co-exist with existing TCP-Reno-
friendly traffic. The DualQ Coupled AQM, which is defined in a
complementary experimental
specification [I-D.ietf-tsvwg-aqm-dualq-coupled], is an AQM framework
that enables scalable congestion controls derived from DCTCP to co-
exist 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.
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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 [RFC8298] real-time media congestion
avoidance technique (RMCAT). The factor that distinguishes L4S from
Classic traffic is its behaviour in response to congestion. The
transport wire protocol, e.g. TCP, QUIC, SCTP, DCCP, 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 (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 bit rate 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 [RFC3246], 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.
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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.
Active queue management (AQM) was originally developed to solve this
problem (and others). Unlike Diffserv, which gives low latency to
some traffic at the expense of others, AQM controls latency for _all_
traffic in a class. In general, 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 (aka. greedy) flows to keep the buffer empty for its intended
purpose: absorbing bursts. However, RED [RFC2309] and other
algorithms from the 1990s were sensitive to their configuration and
hard to set correctly. So, this form of AQM was not widely deployed.
More recent state-of-the-art AQM methods, such as FQ-CoDel [RFC8290],
PIE [RFC8033] 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: i) either configure a shallow AQM operating point,
so the tips of the sawteeth cause minimal queue delay, but 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 round-trip time.
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. [RFC8290]),
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 packet that need to be isolated: L4S and Classic. L4S
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isolation can be achieved with a queue per flow (e.g. [RFC8290]) but
a DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled] 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 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 (footnote 6 of Jacobson and
Karels [TCP-CA]). Given regular broadband bit-rates over WAN
distances are already [RFC3649] beyond the scaling range of Reno
congestion control, 'less unscalable' Cubic [RFC8312] and
Compound [I-D.sridharan-tcpm-ctcp] variants of TCP have been
successfully deployed. However, these are now approaching their
scaling limits. Unfortunately, fully scalable congestion controls
such as DCTCP [RFC8257] 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 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 [DCttH19] 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 [I-D.ietf-tsvwg-l4s-arch].
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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[Note to the RFC Editor (to be removed before publication as an RFC):
The L4S architecture [I-D.ietf-tsvwg-l4s-arch] repeats the following
definitions, which should be identical, except in the architecture
Classic CC and Scalable CC are condensed because they refer to a
section later in the architecture.]
Classic Congestion Control: A congestion control behaviour that can
co-exist with standard Reno [RFC5681] without causing
significantly negative impact on its flow rate [RFC5033]. 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 [I-D.ietf-tsvwg-l4s-arch] and in [RFC3649].
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 the flow rate scales, all other factors being
equal. This maintains the same degree of control over queueing
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 [I-D.mathis-iccrg-relentless-tcp], TCP Prague
[I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux],
BBRv2 [BBRv2], [I-D.cardwell-iccrg-bbr-congestion-control] and the
L4S variant of SCREAM for real-time media [SCReAM-L4S], [RFC8298].
See Section 4.3 for more explanation.
Classic service: The Classic service is intended for all the
congestion control behaviours that co-exist with Reno [RFC5681]
(e.g. Reno itself, Cubic [RFC8312],
Compound [I-D.sridharan-tcpm-ctcp], TFRC [RFC5348]). The term
'Classic queue' means a queue providing the Classic service.
Low-Latency, Low-Loss Scalable throughput (L4S) service: The 'L4S'
service is intended for traffic from scalable congestion control
algorithms, such as the Prague congestion control
[I-D.briscoe-iccrg-prague-congestion-control], which was derived
from DCTCP [RFC8257]. 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.
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The terms Classic or L4S can also qualify other nouns, such as
'queue', 'codepoint', 'identifier', 'classification', 'packet',
'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 not to build a
queue (e.g. DNS, 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 [RFC5681].
The TFRC spec [RFC5348] 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 can be used in
non-TCP transports such as QUIC [RFC9000].
Classic ECN: The original Explicit Congestion Notification (ECN)
protocol [RFC3168], which requires ECN signals to be treated the
same as drops, both when generated in the network and when
responded to by the sender. For L4S, the names used for the four
codepoints of the 2-bit IP-ECN field are unchanged from those
defined in [RFC3168]: 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 for Classic ECN [RFC3168]). 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) [RFC2474]. Section 5.4
explains what this means in practice.
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This document is intended for experimental status, so it does not
update any standards track RFCs. Therefore, it depends on [RFC8311],
which is a standards track specification that:
* updates the ECN proposed standard [RFC3168] to allow experimental
track 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 ABE
experiment [RFC8511] this permits a sender to respond less to ECN
marks than to drops;
* changes the status of the experimental ECN nonce [RFC3540] to
historic;
* makes consequent updates to the following additional proposed
standard RFCs to reflect the above two bullets:
- ECN for RTP [RFC6679];
- the congestion control specifications of various DCCP
congestion control identifier (CCID) profiles [RFC4341],
[RFC4342], [RFC5622].
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 treatment is an experimental track alternative packet marking
treatment to the Classic ECN treatment in [RFC3168], which has been
updated by [RFC8311] to allow experiments such as the one defined in
the present specification. [RFC4774] 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 [I-D.ietf-tsvwg-l4s-arch] 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.
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The next section of the present document (Section 3) 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.
L4S ECN is currently on the experimental track. So Section 7
collects together 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
[I-D.ietf-tsvwg-aqm-dualq-coupled].
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 [I-D.ietf-tsvwg-l4s-arch].
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 Low Latency, Low Loss, Scalable
throughput (L4S) service needs to meet the following requirements:
* it SHOULD survive end-to-end between source and destination end-
points: across the boundary between host and network, between
interconnected networks, and through middleboxes;
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* it SHOULD be visible at the IP layer;
* it SHOULD be common to IPv4 and IPv6 and 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;
* 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 those 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 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.
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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 [RFC3168], 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 [RFC7560]
(such as that provided by AccECN [I-D.ietf-tcpm-accurate-ecn]) 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.
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 draft [I-D.stewart-tsvwg-sctpecn] or as sketched out in
Appendix A of the now obsolete second standards track
specification of SCTP [RFC4960]).
RTP over UDP: A prerequisite for scalable congestion control is for
both (all) ends of one media-level hop to signal ECN
support [RFC6679] and use the new generic RTCP feedback format of
[RFC8888]. 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 the v1 IETF QUIC transport [RFC9000].
DCCP: The ACK vector in DCCP [RFC4340] is already sufficient to
report the extent of CE marking as needed by a scalable congestion
control.
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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 [RFC8257], TCP Prague
[I-D.briscoe-iccrg-prague-congestion-control], [PragueLinux] and the
L4S variant of SCReAM [SCReAM-L4S], [RFC8298], 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 [RFC5033]. In addition, it is expected to document
these L4S-specific matters, specifically the timescale over which the
proportionality is averaged, and 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]).
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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 [RFC5681], as required by [RFC5033] (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 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 [RFC5681], as required by
[RFC5033]. 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 [I-D.ietf-tsvwg-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).
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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 S.6.1.2 of the
ECN spec [RFC3168] (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
RACK [RFC8985]), as opposed to counting only in fixed units of
packets (as in the 3 DupACK rule of New Reno [RFC5681] and
[RFC6675], 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 250us (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.
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Later (Section 5.4.1.1) this document discusses the conditions for
mixing other "'Safe' Unresponsive Traffic" (e.g. DNS, LDAP, NTP,
voice, game sync packets) with L4S traffic. 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 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 [RFC4774] gives guidance to protocol designers, when
specifying alternative semantics for the 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 describes
three options for incremental deployment, with Option 3 (in
Section 4.3 of BCP 124) 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 [RFC5033], which says "...congestion controllers
that have a significantly negative impact on traffic using standard
congestion control may be suspect", or [RFC8085] concerning UDP
congestion control says "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 third normative bullet 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 [I-D.ietf-tsvwg-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.
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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
RFC was published in 2001.
* L4S: Coexistence is not in doubt if the bottleneck supports
L4S.
* Classic ECN [RFC3168]: The compromises centre around cases
where the bottleneck supports Classic ECN 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 (FQ) deployed from 2012 onwards had Classic ECN
enabled by default, specifically FQ-CoDel [RFC8290] and
COBALT [COBALT]. But the compromises only apply to the
second of two further sub-cases:
o With per-flow-queuing, co-existence between Classic and
L4S flows is not normally a problem, because different
flows are not meant to be in the same queue
(BCP 124 [RFC4774] 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 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 [I-D.ietf-tsvwg-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
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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 falls below ~5:1 for base RTTs
below 20ms.
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 the third bullet
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.
* Therefore the third bullet in Section 4.3 above allows a
compromise where coexistence could diverge from the requirements
in the RFC Series briefly, 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 otherwise, but it does progress. The L4S
operational guidance [I-D.ietf-tsvwg-l4sops] outlines a range of
example remedial actions that include alterations either to the
sender or to the network. However, the final normative
requirement in the third bullet of Section 4.3 above places
ultimate responsibility for remedial action on the sender. If
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coexistence problems with a Classic ECN AQM are detected (implying
they have not been resolved by the network), it says the sender
"MUST" revert to a Classic congestion control."
[I-D.ietf-tsvwg-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 [I-D.ietf-tsvwg-l4s-arch], 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
RTT 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, and the 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;
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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 other codepoint than CE, and CE MUST NOT be changed to
any other codepoint. An ECT(1) packet is classified as ECN-capable
and, if congestion increases, an L4S AQM algorithm will increasingly
mark the 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 [RFC7567] (Section 4.2.1),
which follows the similar advice in RFC 3168 (Section 7). 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. [I-D.briscoe-docsis-q-protection]), 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
[I-D.ietf-tsvwg-aqm-dualq-coupled]).
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, all packets of all ECN codepoints will
receive Classic treatment and ECT(1) packets MUST be treated as if
they were Not-ECT.
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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 [RFC8290]). Nonetheless, the relationship between them needs
to be defined for the coupling between L4S and Classic congestion
signals in a DualQ Coupled AQM [I-D.ietf-tsvwg-aqm-dualq-coupled], as
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
[RFC8311], 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 [RFC3168].
Also, In the L4S architecture [I-D.ietf-tsvwg-l4s-arch], 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.
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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 [I-D.ietf-tsvwg-aqm-dualq-coupled]. 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 Coupled DualQ AQM [I-D.ietf-tsvwg-aqm-dualq-coupled],
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, 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:
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* The L4S treatment, which is a combination of the L4S AQM treatment
and a priority scheduling treatment;
* 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 IoT devices sending low
intensity traffic;
* certain low data-volume applications or protocols (e.g. ARP, DNS);
* specific Diffserv codepoints that indicate traffic with limited
burstiness such as the EF (Expedited Forwarding [RFC3246]), Voice-
Admit [RFC5865] or proposed NQB (Non-Queue-
Building [I-D.ietf-tsvwg-nqb]) service classes or equivalent
local-use DSCPs (see [I-D.briscoe-tsvwg-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 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 ECT(0) in the ECN field (for examples
see section 2.5.1.1 of the DualQ
spec [I-D.ietf-tsvwg-aqm-dualq-coupled]).
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.
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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 alter Not-ECT or ECT(0) in the IP-ECN field to 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 completely
regularly) 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, 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 [RFC8083]. 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 will probably rise to that controlled by
the 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 [I-D.ietf-tsvwg-l4s-arch]).
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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. initially the
operator may wish to 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 (Section 4.3 item 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 it could 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. The two can be combined if a network operator wants to
control bandwidth allocation but it also wants to provide low latency
- for any amount of traffic within one of these allocations of
bandwidth (rather than only providing low latency by limiting
bandwidth) [I-D.briscoe-tsvwg-l4s-diffserv].
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The DualQ examples so far have been framed in the context of
providing the default Best Efforts 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 (say) 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).
[I-D.briscoe-tsvwg-l4s-diffserv] gives a number of examples of such
arrangements to address various requirements.
[I-D.briscoe-tsvwg-l4s-diffserv] describes how an operator might use
L4S to offer low latency as well as using 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 'Recommended-standard-use' could be set by the sending host or
a network. Those tagged '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)
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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 queueing (e.g. FQ-CoDel [RFC8290]), the L4S
identifier could complement the Layer-4 flow ID as a further level of
flow granularity (i.e. Not-ECT and ECT(0) queued separately from
ECT(1) and CE packets). "Risk of reordering Classic CE packets" in
Appendix B discusses the resulting ambiguity if packets originally
marked 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 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 tradeoff, because batch-handling
of packets is done for good reason, e.g. 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.
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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
network 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 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
interface(s). 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 [RFC3168]. L4S will also work with (but does not rely
on) any of the more recent updates to ECN propagation in [RFC4301],
[RFC6040] or [I-D.ietf-tsvwg-rfc6040update-shim]. However, it is
likely that some tunnels still do not implement ECN propagation at
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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 ECN field as defined for the
link technology, for example for MPLS [RFC5129] or
TRILL [I-D.ietf-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 [I-D.ietf-tsvwg-ecn-encap-guidelines].
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 virtual private network (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 [RFC4301] and
DTLS [RFC9147] 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 AH [RFC4302] and ESP [RFC4303] suggest that an implementer
scales the size of the anti-replay window with interface speed, and
DTLS v1.3 [RFC9147] says "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:
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* 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 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);
- 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 WG 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, the DualQ
spec [I-D.ietf-tsvwg-aqm-dualq-coupled] sets operational and
management requirements for experiments with DualQ Coupled AQMs; and
General operational and management requirements for experiments with
L4S congestion controls are given in Section 4 and Section 5 above,
e.g. co-existence and scaling requirements, 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 the
guidelines in [RFC5706] provides a helpful checklist.
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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 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;
- 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 bullet
#7 in Section 4.3 and Section 5.5.2)?
* Is the initial experiment with mis-marked 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 [I-D.ietf-tsvwg-aqm-dualq-coupled] (or possible other
remedies)?
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* 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 [I-D.ietf-tsvwg-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 EWMA in the Linux implementation of TCP Prague.
That was quickly fixed, which removed the main performance impact,
but further improvement would be useful (either by modifying
CoDel, Scalable congestion controls, or both).
7.3. Future Potential
Researchers might find that L4S opens up the following interesting
areas for investigation:
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* 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, 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;
* Novel applications enabled by L4S.
8. IANA Considerations
The 01 codepoint of the ECN Field of the IP header is specified by
the present Experimental RFC. The process for an experimental RFC to
assign this codepoint in the IP header (v4 and v6) is documented in
Proposed Standard [RFC8311], which updates the Proposed Standard
[RFC3168].
When the present document is published as an RFC, IANA is asked to
update the 01 entry in the registry, "ECN Field (Bits 6-7)" to the
following (see https://www.iana.org/assignments/dscp-registry/dscp-
registry.xhtml#ecn-field ):
+========+=====================+=============================+
| Binary | Keyword | References |
+========+=====================+=============================+
| 01 | ECT(1) (ECN-Capable | [RFC8311] [RFC Errata 5399] |
| | Transport(1))[1] | [RFCXXXX] |
+--------+---------------------+-----------------------------+
Table 1
[XXXX is the number that the RFC Editor assigns to the present
document (this sentence to be removed by the RFC Editor)].
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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 [I-D.ietf-tsvwg-l4s-arch] for
further discussion of mis-use 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 ECT0 and ECT1 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 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
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
Explicit Congestion Notification (ECN) Field", BCP 124,
RFC 4774, DOI 10.17487/RFC4774, November 2006,
<https://www.rfc-editor.org/info/rfc4774>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <https://www.rfc-editor.org/info/rfc6679>.
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 64(6):1522-1533, June 2015,
<https://ieeexplore.ieee.org/xpl/
articleDetails.jsp?arnumber=6871352>.
[Ahmed19] Ahmed, A.S., "Extending TCP for Low Round Trip Delay",
Master's Thesis, Uni 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", ACM
SIGMETRICS 2011 , June 2011,
<https://people.csail.mit.edu/alizadeh/papers/
dctcp_analysis-sigmetrics11.pdf>.
[ARED01] Floyd, S., Gummadi, R., and S. Shenker, "Adaptive RED: An
Algorithm for Increasing the Robustness of RED's Active
Queue Management", ACIRI Technical Report , August 2001,
<https://www.icir.org/floyd/red.html>.
[BBRv2] Cardwell, N., "BRTCP BBR v2 Alpha/Preview Release", GitHub
repository; Linux congestion control module,
<https://github.com/google/bbr/blob/v2alpha/README.md>.
De Schepper & Briscoe Expires 2 March 2023 [Page 36]
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[Bufferbloat]
"Bufferbloat", <https://bufferbloat.net/>. (last accessed
27 Aug 2022)
[COBALT] Palmei, J., Gupta, S., Imputato, P., Morton, J.,
Tahiliani, M., Avallone, S., and D. Taht, "Design and
Evaluation of COBALT Queue Discipline", In Proc. IEEE
Int'l Symp. on Local and Metropolitan Area Networks 2019,
pp1--6, 2019,
<https://doi.org/10.1109/LANMAN.2019.8847054>.
[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/pubs.html#DCttH_TR>.
[DualPI2Linux]
Albisser, O., De Schepper, K., Briscoe, B., Tilmans, O.,
and H. Steen, "DUALPI2 - Low Latency, Low Loss and
Scalable (L4S) AQM", Proc. 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 CCR
36(1):59--62, January 2006,
<https://dl.acm.org/doi/10.1145/1111322.1111336>.
[ecn-fallback]
Briscoe, B. and A.S. Ahmed, "TCP Prague Fall-back on
Detection of a Classic ECN AQM", bobbriscoe.net Technical
Report TR-BB-2019-002, April 2020,
<https://arxiv.org/abs/1911.00710>.
[Heist21] Heist, P. and J. Morton, "L4S Tests", GitHub README, May
2021, <https://github.com/heistp/l4s-tests/>.
[I-D.briscoe-docsis-q-protection]
Briscoe, B. and G. White, "The DOCSIS(r) 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/api/v1/doc/document/draft-
briscoe-docsis-q-protection/>.
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[I-D.briscoe-iccrg-prague-congestion-control]
Schepper, K. D., Tilmans, O., and B. Briscoe, "Prague
Congestion Control", Work in Progress, Internet-Draft,
draft-briscoe-iccrg-prague-congestion-control-01, 11 July
2022, <https://datatracker.ietf.org/api/v1/doc/document/
draft-briscoe-iccrg-prague-congestion-control/>.
[I-D.briscoe-tsvwg-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, 2 July 2018,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
briscoe-tsvwg-l4s-diffserv/>.
[I-D.cardwell-iccrg-bbr-congestion-control]
Cardwell, N., Cheng, Y., Yeganeh, S. H., 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/api/v1/doc/document/draft-
cardwell-iccrg-bbr-congestion-control/>.
[I-D.ietf-tcpm-accurate-ecn]
Briscoe, B., Kühlewind, M., and R. Scheffenegger, "More
Accurate ECN Feedback in TCP", Work in Progress, Internet-
Draft, draft-ietf-tcpm-accurate-ecn-20, 25 July 2022,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
ietf-tcpm-accurate-ecn/>.
[I-D.ietf-tcpm-generalized-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/api/v1/doc/document/draft-
ietf-tcpm-generalized-ecn/>.
[I-D.ietf-trill-ecn-support]
Eastlake, D. E. 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/api/v1/doc/document/draft-
ietf-trill-ecn-support/>.
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[I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K. D., Briscoe, B., and G. White, "DualQ Coupled
AQMs for Low Latency, Low Loss and Scalable Throughput
(L4S)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-aqm-dualq-coupled-24, 7 July 2022,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
ietf-tsvwg-aqm-dualq-coupled/>.
[I-D.ietf-tsvwg-ecn-encap-guidelines]
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/api/v1/doc/document/draft-
ietf-tsvwg-ecn-encap-guidelines/>.
[I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., Schepper, K. D., Bagnulo, M., and G. White,
"Low Latency, Low Loss, Scalable Throughput (L4S) Internet
Service: Architecture", Work in Progress, Internet-Draft,
draft-ietf-tsvwg-l4s-arch-19, 27 July 2022,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
ietf-tsvwg-l4s-arch/>.
[I-D.ietf-tsvwg-l4sops]
White, G., "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/api/v1/doc/document/draft-
ietf-tsvwg-l4sops/>.
[I-D.ietf-tsvwg-nqb]
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-10, 4 March
2022, <https://datatracker.ietf.org/api/v1/doc/document/
draft-ietf-tsvwg-nqb/>.
[I-D.ietf-tsvwg-rfc6040update-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/api/v1/doc/document/draft-
ietf-tsvwg-rfc6040update-shim/>.
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[I-D.mathis-iccrg-relentless-tcp]
Mathis, M., "Relentless Congestion Control", Work in
Progress, Internet-Draft, draft-mathis-iccrg-relentless-
tcp-00, 4 March 2009, <https://www.ietf.org/archive/id/
draft-mathis-iccrg-relentless-tcp-00.txt>.
[I-D.sridharan-tcpm-ctcp]
Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
"Compound TCP: A New TCP Congestion Control for High-Speed
and Long Distance Networks", Work in Progress, Internet-
Draft, draft-sridharan-tcpm-ctcp-02, 29 October 2007,
<https://datatracker.ietf.org/api/v1/doc/document/draft-
sridharan-tcpm-ctcp/>.
[I-D.stewart-tsvwg-sctpecn]
Stewart, R. R., Tuexen, 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://www.ietf.org/archive/id/draft-stewart-
tsvwg-sctpecn-05.txt>.
[LinuxPacedChirping]
Misund, J. and B. Briscoe, "Paced Chirping - Rethinking
TCP start-up", Proc. Linux Netdev 0x13 , March 2019,
<https://www.netdevconf.org/0x13/session.html?talk-chirp>.
[Paced-Chirping]
Misund, J., "Rapid Acceleration in TCP Prague", Master's
Thesis , May 2018,
<https://riteproject.files.wordpress.com/2018/07/
misundjoakimmastersthesissubmitted180515.pdf>.
[PI2] De Schepper, K., Bondarenko, O., Tsang, I., and B.
Briscoe, "PI^2 : A Linearized AQM for both Classic and
Scalable TCP", Proc. ACM CoNEXT 2016 pp.105-119, December
2016,
<https://dl.acm.org/citation.cfm?doid=2999572.2999578>.
[PragueLinux]
Briscoe, B., De Schepper, K., Albisser, O., Misund, J.,
Tilmans, O., Kühlewind, M., and A.S. Ahmed, "Implementing
the `TCP Prague' Requirements for Low Latency Low Loss
Scalable Throughput (L4S)", Proc. Linux Netdev 0x13 ,
March 2019, <https://www.netdevconf.org/0x13/
session.html?talk-tcp-prague-l4s>.
De Schepper & Briscoe Expires 2 March 2023 [Page 40]
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[QV] Briscoe, B. and P. Hurtig, "Up to Speed with Queue View",
RITE Technical Report D2.3; Appendix C.2, August 2015,
<https://riteproject.files.wordpress.com/2015/12/rite-
deliverable-2-3.pdf>.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, DOI 10.17487/RFC2309, April 1998,
<https://www.rfc-editor.org/info/rfc2309>.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.
[RFC3246] 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/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>.
[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
Congestion Notification (ECN) Signaling with Nonces",
RFC 3540, DOI 10.17487/RFC3540, June 2003,
<https://www.rfc-editor.org/info/rfc3540>.
[RFC3649] Floyd, S., "HighSpeed TCP for Large Congestion Windows",
RFC 3649, DOI 10.17487/RFC3649, December 2003,
<https://www.rfc-editor.org/info/rfc3649>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
December 2005, <https://www.rfc-editor.org/info/rfc4301>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
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[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<https://www.rfc-editor.org/info/rfc4340>.
[RFC4341] 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/RFC4341, March
2006, <https://www.rfc-editor.org/info/rfc4341>.
[RFC4342] 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/RFC4342, March 2006,
<https://www.rfc-editor.org/info/rfc4342>.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5033] Floyd, S. and M. Allman, "Specifying New Congestion
Control Algorithms", BCP 133, RFC 5033,
DOI 10.17487/RFC5033, August 2007,
<https://www.rfc-editor.org/info/rfc5033>.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
2008, <https://www.rfc-editor.org/info/rfc5129>.
[RFC5348] Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 5348, DOI 10.17487/RFC5348, September 2008,
<https://www.rfc-editor.org/info/rfc5348>.
[RFC5562] Kuzmanovic, A., Mondal, A., Floyd, S., and K.
Ramakrishnan, "Adding Explicit Congestion Notification
(ECN) Capability to TCP's SYN/ACK Packets", RFC 5562,
DOI 10.17487/RFC5562, June 2009,
<https://www.rfc-editor.org/info/rfc5562>.
[RFC5622] 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/RFC5622, August 2009,
<https://www.rfc-editor.org/info/rfc5622>.
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[RFC5681] Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
Control", RFC 5681, DOI 10.17487/RFC5681, September 2009,
<https://www.rfc-editor.org/info/rfc5681>.
[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, DOI 10.17487/RFC5706, November 2009,
<https://www.rfc-editor.org/info/rfc5706>.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<https://www.rfc-editor.org/info/rfc5865>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6040] Briscoe, B., "Tunnelling of Explicit Congestion
Notification", RFC 6040, DOI 10.17487/RFC6040, November
2010, <https://www.rfc-editor.org/info/rfc6040>.
[RFC6077] Papadimitriou, D., Ed., Welzl, M., Scharf, M., and B.
Briscoe, "Open Research Issues in Internet Congestion
Control", RFC 6077, DOI 10.17487/RFC6077, February 2011,
<https://www.rfc-editor.org/info/rfc6077>.
[RFC6660] Briscoe, B., Moncaster, T., and M. Menth, "Encoding Three
Pre-Congestion Notification (PCN) States in the IP Header
Using a Single Diffserv Codepoint (DSCP)", RFC 6660,
DOI 10.17487/RFC6660, July 2012,
<https://www.rfc-editor.org/info/rfc6660>.
[RFC6675] 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/RFC6675, August 2012,
<https://www.rfc-editor.org/info/rfc6675>.
[RFC7560] 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/RFC7560, August 2015,
<https://www.rfc-editor.org/info/rfc7560>.
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[RFC7567] Baker, F., Ed. and G. Fairhurst, Ed., "IETF
Recommendations Regarding Active Queue Management",
BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
<https://www.rfc-editor.org/info/rfc7567>.
[RFC7713] Mathis, M. and B. Briscoe, "Congestion Exposure (ConEx)
Concepts, Abstract Mechanism, and Requirements", RFC 7713,
DOI 10.17487/RFC7713, December 2015,
<https://www.rfc-editor.org/info/rfc7713>.
[RFC8033] Pan, R., Natarajan, P., Baker, F., and G. White,
"Proportional Integral Controller Enhanced (PIE): A
Lightweight Control Scheme to Address the Bufferbloat
Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
<https://www.rfc-editor.org/info/rfc8033>.
[RFC8083] Perkins, C. and V. Singh, "Multimedia Congestion Control:
Circuit Breakers for Unicast RTP Sessions", RFC 8083,
DOI 10.17487/RFC8083, March 2017,
<https://www.rfc-editor.org/info/rfc8083>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8257] 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/RFC8257,
October 2017, <https://www.rfc-editor.org/info/rfc8257>.
[RFC8290] 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/RFC8290, January 2018,
<https://www.rfc-editor.org/info/rfc8290>.
[RFC8298] Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
2017, <https://www.rfc-editor.org/info/rfc8298>.
[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.
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[RFC8312] Rhee, I., Xu, L., Ha, S., Zimmermann, A., Eggert, L., and
R. Scheffenegger, "CUBIC for Fast Long-Distance Networks",
RFC 8312, DOI 10.17487/RFC8312, February 2018,
<https://www.rfc-editor.org/info/rfc8312>.
[RFC8511] Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
"TCP Alternative Backoff with ECN (ABE)", RFC 8511,
DOI 10.17487/RFC8511, December 2018,
<https://www.rfc-editor.org/info/rfc8511>.
[RFC8888] Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP
Control Protocol (RTCP) Feedback for Congestion Control",
RFC 8888, DOI 10.17487/RFC8888, January 2021,
<https://www.rfc-editor.org/info/rfc8888>.
[RFC8985] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "The
RACK-TLP Loss Detection Algorithm for TCP", RFC 8985,
DOI 10.17487/RFC8985, February 2021,
<https://www.rfc-editor.org/info/rfc8985>.
[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[Savage-TCP]
Savage, S., Cardwell, N., Wetherall, D., and T. Anderson,
"TCP Congestion Control with a Misbehaving Receiver", ACM
SIGCOMM Computer Communication Review 29(5):71--78,
October 1999.
[SCReAM-L4S]
Johansson, I., "SCReAM", GitHub repository; ,
<https://github.com/EricssonResearch/scream/blob/master/
README.md>.
De Schepper & Briscoe Expires 2 March 2023 [Page 45]
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[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, 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", tcpprague mailing list archive ,
July 2015, <https://www.ietf.org/mail-
archive/web/tcpprague/current/msg00001.html>.
[VCP] Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,
"One more bit is enough", Proc. SIGCOMM'05, ACM CCR
35(4)37--48, 2005,
<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 [RFC8257] 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, both from 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,
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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 marked with ECN or dropped. Therefore, in
2001, when ECN feedback was added to TCP [RFC3168], 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 [RFC7560] and [I-D.ietf-tcpm-accurate-ecn] 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).
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; with a particular receiver
or even ultimately an insurmountable problem with the experiment as a
whole.
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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 [RFC5681]
(see the precise normative requirement wording in Section 4.3).
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 [RFC8257] states that "It is RECOMMENDED that an
implementation deal with loss episodes in the same way as
conventional TCP." For safe deployment, Section 4.3 requires any
specification of a scalable congestion control for the public
Internet to define the above requirement as a "MUST".
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 bit set and
also experience loss. Current DCTCP implementations each react
differently to this situation. At least one implementation reacts
only to the drop signal (e.g. by halving the CWND) and at least
another DCTCP implementation reacts to both signals (e.g. by halving
the CWND due to the drop and also further reducing the CWND based on
the proportion of marked packet). A third approach for the public
Internet has been proposed that adjusts the loss response to result
in a halving when combined with the ECN response. We believe that
further experimentation is needed to understand what is the best
behaviour for the public Internet, which may or not be one of these
existing approaches.
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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 is written so that such a problem could either
be resolved 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;
* 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.
Therefore, the relevant normative requirement (Section 4.3) is
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 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 it
can aim to identify AQM technologies that are likely to present
coexistence problems, based on knowledge of AQMs deployed at the
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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; or to ensure
L4S packets bypass the AQM; or to upgrade the AQM to support L4S
(see the L4S operational guidance [I-D.ietf-tsvwg-l4sops]). Once
L4S flows 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 [I-D.ietf-tsvwg-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
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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,
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 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 the AQM
behaviours 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 | Coupled DualQ AQM |
+--------+-------+------------------------+
Table 2: Out-of-band testing with two
parallel flows. L:=L4S, C:=Classic.
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:
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* 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 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 ex-ToS byte (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).
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 this
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.
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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, 25ms)
rtt_virt = rtt + 10ms
These example functions are chosen so that, as the actual RTT reduces
from high to low, the virtual RTT reduces less (see
[I-D.briscoe-iccrg-prague-congestion-control] 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 wrong to require short RTT
flows to be as sluggish as long-RTT flows, which would unnecessarily
under-utilize capacity and result in unnecessary overshoots and
undershoots (instability). Therefore, rather than requiring strict
RTT-independence, the wording says "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).
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 [RFC3168]). 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, then the retransmission timeout mechanism
acts as a backstop.
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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 queueing 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).
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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 [RFC3649]. 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 [RFC5681]) or after a certain amount of time
(e.g. the RACK approach [RFC8985]).
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, 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
will 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 DOCSIS.
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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 speed of loss recovery 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 TCP RACK (Recent
ACKnowledgements) [RFC8985]. 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.
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As mentioned above, hardware implementations of loss recovery using
DupACK counting exist (e.g. some implementations of RoCEv2 for RDMA).
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 [RFC6679]. 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, RTCP.
Motivation (TCP): RFC 3168 prohibits the use of ECN on these types of
TCP packet, 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++ [I-D.ietf-tcpm-generalized-ecn] proposes experimental use of
ECN on all types of TCP packet as long as AccECN
feedback [I-D.ietf-tcpm-accurate-ecn] 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 [RFC6679] 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.
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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 [RFC5681] 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 [RFC8257], DCTCP uses the
conventional Reno additive increase in 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 [RFC8312] 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 [I-D.ietf-tsvwg-l4s-arch]. Even
when out of its Reno-compatibility mode, every 8x scaling of Cubic's
flow rate leads to 2x 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 (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 a factor of
1.5 to 2 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
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favourable [Paced-Chirping] 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 tradeoffs. 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 [RFC3168], specifically:
- The ECT(1) codepoint signifies that the packet was sent by an
L4S-capable sender.
- Given shortage of codepoints, both 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 marked ECT(0). However, the L4S service ought
not to 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.
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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 [I-D.ietf-tsvwg-ecn-encap-guidelines]. 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 while the second
supported L4S ECN marking, which would be the only scenario
where some ECT(0) packets could be CE marked by an AQM
supporting Classic ECN then the remainder experienced 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).
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For example consider N=3, and consider the sequence of
packets 100, 101, 102, 103,... and 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 [RFC5681]
spurious retransmissions will be unlikely for all the above
reasons. As RACK [RFC8985] 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 [RFC8511], 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
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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 flow(s) are
sharing a Classic ECN AQM bottleneck with long-running L4S
flow(s), which then apply an L4S response to Classic CE signals,
the L4S flows would outcompete the Classic flow(s). 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 [RFC3168] 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 per-flow-queuing (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 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, its 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.
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Non-L4S service for control packets: Solely for the case of TCP, the
Classic ECN RFCs [RFC3168] and [RFC5562] require a sender to clear
the 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 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++ [I-D.ietf-tcpm-generalized-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 e2e: The 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 ECN field in any of the variant
ways it has been defined since ECN-tunneling was first specified
in the year 2001 [RFC3168]. 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 either have to use drop to signal
congestion or a specific congestion notification facility has to
be 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 had reduced to zero, or near-zero, which might
never happen.
L4 not required: Being based on the ECN field, this scheme does not
need the network to access transport layer flow identifiers.
Nonetheless, it does not preclude solutions that do.
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Appendix C. Potential Competing Uses for the ECT(1) Codepoint
The ECT(1) codepoint of the ECN field has already been assigned once
for the ECN nonce [RFC3540], which has now been categorized as
historic [RFC8311]. 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 following categories:
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 protocol [RFC3540] proposed that a TCP sender
could set either of 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 RFC [RFC3540] 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 [RFC8311]. 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
para 2 of Section 20.2 of the ECN spec [RFC3168]. This works for
loss and it will work for the accurate ECN feedback [RFC7560]
intended for L4S. Like the (historic) ECN nonce, 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 round the full feedback loop by auditing
congestion exposure (ConEx) [RFC7713]. This assures that the
integrity of congestion notifications and feedback messages must
have both been preserved. ConEx information is also available
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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 neutralise 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 [RFC5925]), QUIC's end-to-end
protection [RFC9001] or end-to-end IPsec integrity protection
[RFC4303] 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 not common to protect the integrity of
congestion feedback, whether 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. VCP [VCP], 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 standardisation of rapid flow acceleration, which is an
important and enduring problem [RFC6077].
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Pre-Congestion Notification (PCN) is another scheme that assigns
alternative semantics to the ECN field. It uses ECT(1) to signify a
less severe level of pre-congestion notification than CE [RFC6660].
However, the ECN field only takes on the PCN semantics if packets
carry a Diffserv codepoint defined to indicate PCN marking within a
controlled environment. PCN is required to be applied solely to the
outer header of a tunnel across the controlled region in order not to
interfere with any end-to-end use of the ECN field. Therefore, a PCN
region on the path would not interfere with the L4S service
identifier defined in Section 2.
Acknowledgements
Thanks to Richard Scheffenegger, John Leslie, David Taeht, 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 drafts of this document.
And 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 Kuehlewind, Ermin Sakic and
Martin Duke for providing help and reviewing this draft, 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 Eric 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 drafts
through the IETF process. Appendix A listing the Prague L4S
Requirements is based on text authored by Marcelo Bagnulo Braun that
was originally an appendix to [I-D.ietf-tsvwg-l4s-arch]. 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 part-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 part-funded by the 5Growth and DAEMON EU
H2020 projects. Bob Briscoe was also funded partly by the Research
Council of Norway through the TimeIn project, partly by CableLabs and
partly by the Comcast Innovation Fund. The views expressed here are
solely those of the authors.
Authors' Addresses
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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/
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