A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-27
| Document | Type | Active Internet-Draft (tsvwg WG) | |
|---|---|---|---|
| Authors | Greg White , Thomas Fossati , Ruediger Geib | ||
| Last updated | 2024-12-17 (Latest revision 2024-11-08) | ||
| Replaces | draft-white-tsvwg-nqb | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
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| Additional resources |
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| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
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| Document shepherd | Gorry Fairhurst | ||
| Shepherd write-up | Show Last changed 2024-11-13 | ||
| IESG | IESG state | Publication Requested | |
| Action Holder | |||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Zaheduzzaman Sarker | ||
| Send notices to | gorry@erg.abdn.ac.uk |
draft-ietf-tsvwg-nqb-27
Transport Area Working Group G. White
Internet-Draft CableLabs
Updates: 8325 (if approved) T. Fossati
Intended status: Standards Track Linaro
Expires: 12 May 2025 R. Geib
Deutsche Telekom
8 November 2024
A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
Services
draft-ietf-tsvwg-nqb-27
Abstract
This document specifies characteristics of a Non-Queue-Building Per-
Hop Behavior (NQB PHB). The NQB PHB provides a shallow-buffered,
best-effort service as a complement to a Default deep-buffered best-
effort service for Internet services. The purpose of this NQB PHB is
to provide a separate queue that enables smooth (i.e. non-bursty),
low-data-rate, application-limited traffic microflows, which would
ordinarily share a queue with bursty and capacity-seeking traffic, to
avoid the latency, latency variation and loss caused by such traffic.
This PHB is implemented without prioritization and can be implemented
without rate policing, making it suitable for environments where the
use of these features is restricted. The NQB PHB has been developed
primarily for use by access network segments, where queuing delays
and queuing loss caused by Queue-Building protocols are manifested,
but its use is not limited to such segments. In particular,
applications to cable broadband links, Wi-Fi links, and mobile
network radio and core segments are discussed. This document
recommends a specific Differentiated Services Code Point (DSCP) to
identify Non-Queue-Building microflows, and updates the RFC8325
guidance on mapping Diffserv to IEEE 802.11 for this codepoint.
[NOTE (to be removed by RFC-Editor): This document references an ISE
submission draft (I-D.briscoe-docsis-q-protection) that is approved
for publication as an RFC. This draft should be held for publication
until the queue protection RFC can be referenced.]
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 12 May 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Non-Queue-Building Behavior . . . . . . . . . . . . . . . 5
3.2. Relationship to the Diffserv Architecture . . . . . . . . 6
3.3. Relationship to L4S . . . . . . . . . . . . . . . . . . . 8
3.4. Applicability . . . . . . . . . . . . . . . . . . . . . . 8
4. DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . . 9
4.1. Non-Queue-Building Sender Requirements . . . . . . . . . 9
4.2. Aggregation of the NQB DSCP into another Diffserv PHB . . 10
4.3. Aggregation of other DSCPs into the NQB PHB . . . . . . . 11
4.4. Cross-domain usage and DSCP Re-marking . . . . . . . . . 12
4.4.1. Interoperability with Non-DS-Capable Domains . . . . 12
4.5. The NQB DSCP and Tunnels . . . . . . . . . . . . . . . . 14
5. Non-Queue-Building PHB Requirements . . . . . . . . . . . . . 15
5.1. Primary Requirements . . . . . . . . . . . . . . . . . . 15
5.2. Traffic Protection . . . . . . . . . . . . . . . . . . . 18
5.3. Limiting Packet Bursts from Links . . . . . . . . . . . . 21
6. Configuration and Management . . . . . . . . . . . . . . . . 21
6.1. Guidance for Lower-Rate Links . . . . . . . . . . . . . . 22
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7. Mapping NQB to standards of other SDOs . . . . . . . . . . . 22
7.1. DOCSIS Access Networks . . . . . . . . . . . . . . . . . 23
7.2. Mobile Networks . . . . . . . . . . . . . . . . . . . . . 23
7.3. Wi-Fi Networks . . . . . . . . . . . . . . . . . . . . . 24
7.3.1. Interoperability with Existing Wi-Fi Networks . . . . 24
7.3.2. The Updates to RFC 8325 . . . . . . . . . . . . . . . 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
9. Implementation Status . . . . . . . . . . . . . . . . . . . . 28
10. Security Considerations . . . . . . . . . . . . . . . . . . . 29
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. DSCP Re-marking Policies . . . . . . . . . . . . . . 35
Appendix B. Comparison with Expedited Forwarding . . . . . . . . 36
Appendix C. Impact on Higher Layer Protocols . . . . . . . . . . 37
Appendix D. Alternative Diffserv Code Points . . . . . . . . . . 38
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
This document defines a Differentiated Services per-hop behavior
(PHB) called the "Non-Queue-Building Per-Hop Behavior" (NQB PHB),
which isolates traffic microflows (application-to-application flows,
see [RFC2475]) that are relatively low data rate and that do not
themselves materially contribute to queuing delay and loss, allowing
them to avoid the queuing delays and losses caused by other traffic.
Such Non-Queue-Building microflows (for example: interactive voice,
game sync packets, certain machine-to-machine applications, DNS
lookups, and some real-time IoT analytics data) are low-data-rate
application-limited microflows that are distinguished from bursty
traffic microflows and high-data-rate traffic microflows managed by a
classic congestion control algorithm, both of which cause queuing
delay and loss. The term "classic congestion control" is defined in
[RFC9330] to mean one that coexists with standard Reno congestion
control [RFC5681].
In accordance with IETF guidance in [RFC2914] and [RFC8085], most
packets carried by broadband access networks are managed by an end-
to-end congestion control algorithm. Many of the commonly-deployed
congestion control algorithms, such as Reno [RFC5681], Cubic
[RFC9438] or BBR [I-D.ietf-ccwg-bbr], are designed to seek the
available capacity of the path from sender to receiver (which can
frequently be the access network link capacity), and in doing so
generally overshoot the available capacity, causing a queue to build
up at the bottleneck link. This queue build-up results in delay
(variable latency) and packet loss that can affect all the
applications that are sharing the bottleneck link. Moreover, many
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bottleneck links implement a relatively deep buffer (100 ms or more)
in order to enable these congestion control algorithms to use the
link efficiently, which exacerbates the latency and latency variation
experienced.
In contrast to applications that frequently cause queuing delay,
there are a variety of relatively low data rate applications that do
not materially contribute to queuing delay and loss but are
nonetheless subjected to it by sharing the same bottleneck link in
the access network. Many of these applications can be sensitive to
latency or latency variation, as well as packet loss, and thus
produce a poor quality of experience in such conditions.
Active Queue Management (AQM) mechanisms intended for single queues
(such as PIE [RFC8033], DOCSIS-PIE [RFC8034], PI2 [RFC9332], or CoDel
[RFC8289]) can improve the quality of experience for latency
sensitive applications, but there are practical limits to the amount
of improvement that can be achieved without impacting the throughput
of capacity-seeking applications. For example, AQMs generally allow
a significant amount of queue depth variation to accommodate the
behaviors of congestion control algorithms such as Reno and Cubic.
If the AQM attempted to control the queue depth much more tightly,
applications using those algorithms would not fully utilize the link.
Alternatively, flow queuing systems, such as fq_codel [RFC8290] can
be employed to isolate microflows from one another, but they are not
appropriate for all bottleneck links due to reasons that include
complexity.
The NQB PHB supports differentiating between these two classes of
traffic in bottleneck links and queuing them separately so that both
classes can deliver satisfactory quality of experience for their
applications. In particular, the NQB PHB provides a shallow-
buffered, best-effort service as a complement to a Default (see
[RFC2474]) deep-buffered best-effort service. This PHB is primarily
applicable for high-speed broadband access network links, where there
is minimal aggregation of traffic, and deep buffers are common. The
applicability of this PHB to lower-speed links is discussed in
Section 5.
To be clear, a network implementing the NQB PHB solely provides
isolation for traffic classified as behaving in conformance with the
NQB DSCP. A node supporting the NQB PHB makes no guarantees on
latency or data rate for NQB-marked microflows (beyond the latency
bound provided by the shallow buffer), it is the NQB senders'
behavior itself which results in low latency and low loss.
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Section 4 and Section 7 of this document provide guidance for network
operators that might forward traffic marked with the NQB DSCP.
Networks that do not currently implement support for the NQB PHB are
described in Section 4 regarding treatment of the recommended DSCP
for NQB traffic, and in particular Section 4.2, which provides
specific recommendations for this case. In addition, Section 4.4
provides recommendations around interconnection with networks that do
support the NQB PHB.
2. Requirements Language
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.
3. Context
3.1. Non-Queue-Building Behavior
There are applications that send traffic at relatively low data rates
and/or in a fairly smooth and consistent manner such that they are
unlikely to exceed the available capacity of the network path between
sender and receiver even at an inter-packet timescale. Some of these
applications are transactional in nature, and might only send one
packet (or a few packets) per RTT. These applications might
themselves only cause very small, transient queues to form in network
buffers, but nonetheless they can be subjected to packet delay and
delay variation as a result of sharing a network buffer with
applications that tend to cause large and/or standing queues to form.
These applications typically implement a response to network
congestion that consists of discontinuing (or significantly reducing)
transmissions. These applications can be negatively affected by
excessive packet delay and delay variation. Such applications are
ideal candidates to be queued separately from the applications that
are the cause of queue build-up, latency and loss.
In contrast, Queue-Building (QB) microflows include those that probe
for the link capacity and induce latency and loss as a result, for
example microflows that use Cubic, Reno or other TCP/QUIC congestion
control algorithms in a capacity-seeking manner. Other types of QB
microflows include those that send at a high burst rate even if the
long-term average data rate is much lower.
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3.2. Relationship to the Diffserv Architecture
The IETF has defined the Differentiated Services architecture
[RFC2475] with the intention that it allows traffic to be marked in a
manner that conveys the performance requirements of that traffic
either qualitatively or in a relative sense (e.g. priority). The
architecture defines the use of the Diffserv field [RFC2474] for this
purpose, and numerous RFCs have been written that describe
recommended interpretations of the values (Diffserv Code Points
[RFC2474]) of the field, and standardized treatments (traffic
conditioning and per-hop-behaviors [RFC2475]) that can be implemented
to satisfy the performance requirements of traffic so marked.
While this architecture is powerful and flexible enough to be
configured to meet the performance requirements of a variety of
applications and traffic categories, or to achieve differentiated
service offerings, meeting the performance requirements of an
application across the entire sender-to-receiver path involves all
the networks in the path agreeing on what those requirements are and
sharing an interest in meeting them. In many cases this is made more
difficult since the performance "requirements" are not strict ones
(e.g., applications will degrade in some manner as loss/latency/
jitter increase), so the importance of meeting them for any
particular application in some cases involves a judgment as to the
value of avoiding some amount of degradation in quality for that
application in exchange for an increase in the degradation of another
application.
Further, in many cases the implementation of Diffserv PHBs has
historically involved prioritization of service classes with respect
to one another, which sets up the zero-sum game alluded to in the
previous paragraph, and results in the need to limit access to higher
priority classes via mechanisms such as access control, admission
control, traffic conditioning and rate policing, and/or to meter and
bill for carriage of such traffic. These mechanisms can be difficult
or impossible to implement in the Internet.
Finally, some jurisdictions impose regulations that limit the ability
of networks to provide differentiation of services, in large part
this seems to be based on the belief that doing so necessarily
involves prioritization or privileged access to network capacity, and
thus a benefit to one class of traffic always comes at the expense of
another.
In contrast, the NQB PHB has been designed with the goal that it
avoids many of these issues, and thus could conceivably be deployed
across the Internet. The intent of the NQB DSCP is that it signals
verifiable behavior that permits the sender to request differentiated
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treatment. Also, the NQB traffic is to be given a separate queue
with forwarding preference equal to Default traffic and given no
reserved capacity other than any minimum capacity that it shares with
Default traffic. As a result, the NQB PHB does not aim to meet
specific application performance requirements. Instead, the sole
goal of the NQB PHB is to isolate NQB traffic from other traffic that
degrades loss, latency, and jitter performance, given that the NQB
traffic is itself only an insignificant contributor to those
degradations. The PHB is also designed to reduce the incentives for
a sender to mismark its traffic, since neither higher capacity nor
reserved capacity are being offered. These attributes eliminate many
of the trade-offs that underlie the handling of differentiated
service classes in the Diffserv architecture as it has traditionally
been defined. These attributes also significantly simplify access
control and admission control functions, reducing them to simple
verification of behavior. This aspect is discussed further in
Section 4.1 and Section 5.2.
The NQB PHB is therefore intended for the situation where the
performance requirements of applications cannot be assured across the
whole sender-to-receiver path, and as a result, applications cannot
feasibly place requirements on the network. Instead, many
applications have evolved to make the best out of the network
environment that they find themselves in. In this context, the NQB
PHB provides a better network environment for applications that send
data at relatively low and non-bursty data rates.
In regards to comparison between the NQB PHB and other standardized
PHBs in the Diffserv series, the closest similarity is to the
Expedited Forwarding (EF) PHB [RFC3246], which also intends to enable
low loss, low delay, and low jitter services. Unlike EF, NQB has no
requirement for a guaranteed minimum rate, nor to police incoming
traffic to such a rate, and NQB is expected to be given the same
forwarding preference as Default (see Appendix B for details).
In nodes that support multiple DiffServ Service Classes, NQB traffic
is intended to be treated as a part of the Default treatment.
Traffic assigned to this class does not receive better forwarding
treatment (e.g., prioritization) with respect to other classes, AFxx,
EF, etc. Of course, traffic marked as NQB could(like other Default
traffic) could receive better forwarding treatment with respect to
Lower-Effort (LE) [RFC8622] (e.g. the NQB queue could be emptied in a
priority sequence before the LE queue).
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3.3. Relationship to L4S
The NQB DSCP and PHB described in this document have been defined to
operate independently of the experimental L4S Architecture [RFC9330].
Nonetheless, traffic marked with the NQB DSCP is intended to be
compatible with L4S [RFC9330], with the result being that NQB traffic
and L4S traffic can share the low-latency queue in an L4S DualQ node
[RFC9332]. Compliance, by a network node, with the DualQ Coupled AQM
requirements (Section 2.5 of [RFC9332]) is considered sufficient to
support the NQB PHB requirement of fair allocation of capacity
between the QB and NQB queues (Section 5). Note that these
requirements in turn require compliance with all the requirements in
Section 5 of [RFC9331].
Applications that comply with both the NQB sender requirements in
Section 4.1 and the L4S "Prague" requirements in Section 4 of
[RFC9331] could mark their packets both with the NQB DSCP and with
the ECT(1) value.
NQB network functions SHOULD treat packets marked with the NQB DSCP
uniformly, regardless of the value of the ECN field. Here, NQB
network functions refers to the traffic protection function (defined
in Section 5.2) and any re-marking/traffic policing function designed
to protect unmanaged networks (as described in Section 4.4.1).
In nodes that support both the NQB PHB and L4S, the L4S network
functions SHOULD treat packets marked with the NQB DSCP and ECT(1) or
CE the same as packets marked with the Default DSCP and the same ECN
value. Here, L4S network functions refers to the L4S Network Node
functions (Section 5 of [RFC9331]), and any mechanisms designed to
protect the L4S queue (such as those discussed in Section 8.2 of
[RFC9330]). The processing by an L4S node of an ECT(0) packet that
is classified to the L queue (e.g. as a result of being marked with a
NQB DSCP) is specified in Section 5.4.1.1 of [RFC9331] and
Section 2.5.1.1 of [RFC9332].
3.4. Applicability
This PHB is primarily applicable for high-speed broadband access
network links, where there is minimal aggregation of traffic, and
deep buffers are common.
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In many other links, forwarding NQB-marked packets using the Default
treatment might be sufficient to preserve loss/latency/jitter
performance for NQB traffic. This is generally true in links that do
not typically experience congestion (for example, many backbone and
core network links), and in highly aggregated links (links designed
to carry a large number of simultaneous microflows) where individual
microflow burstiness is averaged out and thus is unlikely to cause
much actual delay.
4. DSCP Marking of NQB Traffic
4.1. Non-Queue-Building Sender Requirements
Microflows that are eligible to be marked with the NQB DSCP are ones
that send traffic at a low data rate relative to typical network path
capacities. Here the data rate is limited by the application itself
rather than by network capacity - these microflows send at a data
rate of no more than about 1 percent of the "typical" network path
capacity. For example, in today's Internet, where access network
data rates are typically on the order of 50 Mbps or more (and see
Section 6.1 for a discussion of cases where this isn't true), this
implies 500 kbps as an upper limit. In addition, these microflows
are required to be sent in a smooth (i.e. paced) manner, where the
number of IP bytes sent in any time interval "T" is less than or
equal to R * T + 1 MTU, where "R" is the maximum rate described
above.
Microflows marked with the NQB DSCP are expected to comply with
existing guidance for safe deployment on the Internet, including the
guidance around response to network congestion, for example the
requirements in [RFC8085] and Section 2 of [RFC3551] (also see the
circuit breaker limits in Section 4.3 of [RFC8083] and the
description of inelastic pseudowires in Section 4 of [RFC7893]). The
fact that a microflow's data rate is low relative to typical network
capacities is no guarantee that sufficient capacity exists in any
particular network, and it is the responsibility of the application
to detect and react appropriately if the network capacity is
insufficient. To be clear, the description of NQB-marked microflows
in this document is not to be interpreted as suggesting that
applications generating such microflows are in any way exempt from
this responsibility. One way that an application marking its traffic
as NQB can handle this is to implement a low latency congestion
control mechanism as described in [RFC9331].
Microflows that are marked with the NQB DSCP align with the
description of behavior in the preceding paragraphs in this section.
Applications are RECOMMENDED to use the Diffserv Code Point (DSCP) 45
(decimal) to mark microflows as NQB. The choice of the DSCP value 45
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(decimal) is motivated in part by the desire to achieve separate
queuing in existing Wi-Fi networks (see Section 7.3) and by the
desire to make implementation of the PHB simpler in network gear that
has the ability to classify traffic based on ranges of DSCP values
(see Section 4.3 for further discussion).
The two primary considerations for whether an application chooses to
mark its traffic as NQB involve the risks of being subjected to a
traffic protection algorithm (see Section 5.2) and/or to the
consequences of overrunning the NQB shallow buffer if (in either
case) the traffic contributes to the formation of a queue in a node
that supports the PHB. In both cases, the result could be that
excess traffic is discarded or queued separately as default traffic
(and thus potentially delivered out of order). To avoid these risks,
if a microflow's traffic exceeds the rate equation provided in the
first paragraph of this section, the application SHOULD NOT mark this
traffic with the NQB DSCP. In such a case, the application could
instead consider implementing a low latency congestion control
mechanism as described in [RFC9331].
The sender requirements outlined in this section are all related to
observable attributes of the packet stream, which makes it possible
for network elements (including nodes implementing the PHB) to
monitor for inappropriate usage of the DSCP, and take action (such as
discarding or re-marking) on traffic that does not comply. This
functionality, when implemented as part of the PHB is described in
Section 5.2.
4.2. Aggregation of the NQB DSCP into another Diffserv PHB
It is RECOMMENDED that networks and nodes that do not support the NQB
PHB be configured to treat traffic marked with the NQB DSCP the same
as traffic with the "Default" DSCP. This includes networks and nodes
that aggregate service classes as discussed in [RFC5127] and
[RFC8100], in which case this recommendation would result in traffic
marked with the NQB DSCP being aggregated into the Elastic Treatment
Aggregate (for [RFC5127] networks) or the Default / Elastic Treatment
Aggregate (for [RFC8100] networks).
Networks and nodes that do not support the NQB PHB ought to only
classify packets with the NQB DSCP value into the appropriate
treatment aggregate, or encapsulate such packets for purposes of
aggregation, and SHOULD NOT re-mark them with a different DSCP. This
preservation of the NQB DSCP value enables hops further along the
path to provide the NQB PHB successfully. This aligns with
recommendations in [RFC5127].
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In nodes that do not typically experience congestion (for example,
many backbone and core network switches), forwarding packets with the
NQB DSCP using the Default treatment might be sufficient to preserve
loss/latency/jitter performance for NQB traffic.
In nodes that do experience congestion, forwarding packets with the
NQB DSCP using the Default treatment could result in degradation of
loss/latency/jitter performance but nonetheless preserves the
incentives described in Section 5.
Aggregating traffic marked with the NQB DSCP into a PHB designed for
real-time, latency sensitive traffic (e.g. the (Bulk) Real-Time
Treatment Aggregate), might better preserve loss/latency/jitter
performance in the presence of congestion, but would need to be done
with consideration of the risk of creating an incentive for non-
compliant traffic to be mis-marked as NQB.
4.3. Aggregation of other DSCPs into the NQB PHB
The Differentiated Services model provides flexibility for operators
to control the way they choose to aggregate traffic marked with a
specific DSCP. Operators of nodes that support the NQB PHB could
choose to aggregate other service classes into the NQB queue. This
is particularly useful in cases where specialized PHBs for these
other service classes had not been provided at a potential
bottleneck, perhaps because it was too complex to manage traffic
contracts and conditioning. Candidate service classes for this
aggregation would include those that carry low-data-rate inelastic
traffic that has low to very-low tolerance for loss, latency and/or
jitter. Operators would need to use their own judgment based on the
actual traffic characteristics in their networks in deciding whether
or not to aggregate other service classes / DSCPs with NQB. For
networks that use the [RFC4594] service class definitions, this could
include Telephony (EF/VA), Signaling (CS5), and possibly Real-Time
Interactive (CS4) (depending on data rate). In some networks,
equipment limitations may necessitate aggregating a range of DSCPs
(e.g. traffic marked with DSCPs 40-47 (decimal), i.e., those whose
three MSBs are 0b101). As noted in Section 4.1, the choice of the
DSCP value 45 (decimal) is motivated in part by the desire to make
this aggregation simpler in network equipment that can classify
packets via comparing the DSCP value to a range of configured values.
A node providing only a NQB queue and a Default queue may obtain an
NQB performance similar to that of EF, for example as described by
Appendix A.3.1 of [RFC2598]. Some caveats and differences are
discussed in Appendix B.
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[NOTE: this section references the obsoleted RFC2598 instead of its
replacement RFC3246, because the former contains the description of
EF performance.]
4.4. Cross-domain usage and DSCP Re-marking
In contrast to some existing standard PHBs, which are typically only
used within a Diffserv Domain (e.g., an AS or an enterprise network),
this PHB is expected to be used across the Internet, wherever
suitable operator agreements apply. Under the [RFC2474] model, this
requires that the corresponding DSCP is recognized and mapped across
network boundaries accordingly.
If NQB support is extended across a DiffServ domain boundary, the
interconnected networks agreeing to support NQB SHOULD use the DSCP
value 45 (decimal) for NQB at network interconnection, unless a
different DSCP is explicitly documented in the TCA (Traffic
Conditioning Agreement, see [RFC2475]) for that interconnection. If
[RFC8100] is operational between interconnected domains, the
receiving domain may prefer a different DSCP for NQB traffic that
allows for a DSCP range-based classification for the Default /
Elastic Treatment Aggregate. Similar to the handling of DSCPs for
other PHBs (and as discussed in [RFC2475]), networks can re-mark NQB
traffic to a DSCP other than 45 (decimal) for internal usage. To
ensure reliable NQB PHB treatment on the entire path, the appropriate
NQB DSCP would need to be restored when forwarding to another
network.
4.4.1. Interoperability with Non-DS-Capable Domains
As discussed in Section 4 of [RFC2475], there may be cases where a
network operator that supports Diffserv is delivering traffic to
another network domain (e.g. a network outside of their
administrative control), where there is an understanding that the
downstream domain does not support Diffserv or there is no knowledge
of the traffic management capabilities of the downstream domain, and
no agreement in place. In such cases, Section 4 of [RFC2475]
suggests that the upstream domain opportunistically re-mark traffic
with a Class Selector codepoint or DSCP 0 (Default) under the
assumption that traffic so marked would be handled in a predictable
way by the downstream domain.
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In the case of a network that supports the NQB PHB (and carries
traffic marked with the recommended NQB DSCP value) the same concerns
apply. In particular, since the recommended NQB DSCP value could be
given high priority in some non-DS-compliant network gear (e.g.,
legacy Wi-Fi APs as described in Section 7.3.1), it is RECOMMENDED
that the operator of the upstream domain implement the following
safeguards before delivering traffic into a non-DS-capable domain.
1. One option for such a safeguard is to re-mark NQB traffic to DSCP
0 (Default) (or another Class Selector DSCP) before delivering
traffic into a non-DS-capable domain, in accordance with the
suggestion in Section 4 of [RFC2475]. Network equipment designed
for such environments, SHOULD by default re-mark NQB traffic to
DSCP 0, and SHOULD support the ability to change and disable this
re-marking. Re-marking NQB traffic to Default could be
considered the "safest" approach since the upstream domain can
thereby ensure that NQB traffic is not given inappropriate
treatment in the non-DS-capable domain. That said, it comes with
the downside that the re-marking ruins any possibility of NQB
isolation in any further downstream domain (not just the
immediate neighbor).
2. As an alternative to re-marking all NQB traffic, such an operator
could deploy a traffic protection (see Section 5.2) or a shaping/
policing function on traffic marked with the NQB DSCP that
minimizes the potential for negative impacts on Default traffic,
should the downstream domain treat traffic with the NQB DSCP as
high priority.
In the case that a traffic protection function is used, it SHOULD
either re-mark offending traffic to DSCP 0 or discard it. Note
that a traffic protection function as defined in this document
might only provide protection from issues occurring in subsequent
network hops if the device implementing the traffic protection
function is the bottleneck link on the path, so it might not be a
solution for all situations.
In the case that a traffic policing function or a rate shaping
function is applied to the aggregate of NQB traffic destined to
such a downstream domain, the policer/shaper rate SHOULD be set
to either 5% of the interconnection data rate, or 5% of the
typical rate for such interconnections, whichever is greater,
with excess traffic being re-marked and classified for Default
forwarding (or dropped, as a last resort). A traffic policing
function SHOULD allow approximately 100 ms of burst tolerance
(e.g. a token bucket depth equal to 100 ms multiplied by the
policer rate). A traffic shaping function SHOULD allow
approximately 10 ms of burst tolerance, and no more than 50 ms of
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buffering. The burst tolerance values recommended here are
intended to reduce the degradation that could be introduced to
latency and loss sensitive traffic marked NQB without
significantly degrading Default traffic.
The recommendation to limit NQB traffic to 5% is based on an
assumption that internal links in the downstream domain could
have data rates as low as one tenth of the interconnect rate, in
which case if the entire aggregate of NQB traffic traversed a
single instance of such a link, the aggregate would consume no
more than 50% of that link's capacity. The limit for NQB traffic
SHOULD be adjusted based on any knowledge of the local network
environment that is available.
4.5. The NQB DSCP and Tunnels
[RFC2983] discusses tunnel models that support Diffserv. It
describes a "uniform model" in which the inner DSCP is copied to the
outer header at encapsulation, and the outer DSCP is copied to the
inner header at decapsulation. It also describes a "pipe model" in
which the outer DSCP is not copied to the inner header at
decapsulation. Both models can be used in conjunction with the NQB
PHB. In the case of the pipe model, any DSCP manipulation (re-
marking) of the outer header by intermediate nodes would be discarded
at tunnel egress. In some cases, this could improve the possibility
of achieving NQB treatment in subsequent nodes, but in other cases it
could degrade that possibility (e.g. if the re-marking was designed
specifically to preserve NQB treatment in downstream domains).
As is discussed in [RFC2983], tunnel protocols that are sensitive to
reordering (such as IPSec [RFC4301] or L2TP [RFC2661]) can result in
undesirable interactions if multiple DSCP PHBs are signaled for
traffic within a tunnel instance. This is true for tunnels
containing a mix of QB and NQB traffic as well. Additionally, since
networks supporting the NQB PHB could implement a traffic protection
mechanism (see Section 5.2) and/or responses to NQB buffer overrun
that result in out-of-order delivery for traffic marked with the NQB
DSCP, even tunnels solely containing NQB traffic could have issues if
they are sensitive to reordering and the outer header retains the NQB
DSCP. As a result, the use of a reordering-sensitive tunnel protocol
to carry NQB traffic, or a mix of QB and NQB traffic, necessitates
that the outer tunnel header be re-marked with a non-NQB DSCP (e.g.
Default), in which case the “pipe” model is preferable because it
preserves the marking differentiation at tunnel decapsulation.
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5. Non-Queue-Building PHB Requirements
For the NQB PHB to become widely deployed, it is important that
incentives are aligned correctly, i.e., that there is a benefit to
the application in marking its packets correctly, and a disadvantage
(or at least no benefit) to an application in intentionally
mismarking its traffic. Thus, a useful property of nodes (i.e.
network switches and routers) that support separate queues for NQB
and QB microflows is that for microflows consistent with the NQB
sender requirements in Section 4.1, the NQB queue would likely be a
better choice than the QB queue; and for microflows inconsistent with
those requirements, the QB queue would likely be a better choice than
the NQB queue. By adhering to these principles, there is little
incentive for senders to mismark their traffic as NQB.
This principle of incentive alignment ensures a system is robust to
the behavior of the large majority of individuals and organizations
who can be expected to act in their own interests (including
application developers and service providers who act in the interests
of their users). Malicious behavior is not necessarily based on
rational self-interest, so incentive alignment is not a sufficient
defense, but the large majority of users do not act out of malice.
Protection against malicious attacks (and accidents) is addressed in
Section 5.2 and summarized in Section 10. As mentioned previously,
the NQB designation and marking is intended to convey verifiable
traffic behavior, as opposed to simply a desire for differentiated
treatment. As a result, any mismarking can be identified by the
network.
5.1. Primary Requirements
A node supporting the NQB PHB MUST provide a queue for Non-Queue-
Building traffic separate from the queue used for Default traffic.
A node supporting the NQB PHB SHOULD NOT rate limit or rate police
the aggregate of NQB traffic separately from Default traffic. An
exception to this recommendation for traffic sent towards a non-DS-
capable domain is discussed in Section 4.4.1. Note also that
Section 5.2 discusses potential uses of per-microflow (rather than
aggregate) rate policing.
The NQB queue SHOULD be given equivalent forwarding preference
compared to Default. The node SHOULD provide a scheduler that allows
NQB and Default traffic to share the link in a manner that treats the
two classes equally, e.g., a deficit round-robin (DRR) scheduler with
equal weights, or two Wireless Multimedia Access Categories with the
same channel access (EDCA) parameters. The use of equal weights for
DRR is given as a reasonable example, and is not intended to preclude
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other scheduling weights (see below for details). A node that
provides rate limits or rate guarantees for Default traffic SHOULD
ensure that such limits and/or guarantees are shared with NQB traffic
in a manner that treats the two classes equally. This could be
supported using a hierarchical scheduler where the rate limits and
guarantees are configured on a parent class, and the two queues
(Default and NQB) are arranged as the children of the parent class
and given equal access to the capacity configured for the parent
class (e.g. with equal DRR scheduling). Compliance with these
recommendations reduces the incentives for QB traffic to be mismarked
as NQB.
In the DRR example above, equal scheduling weights was only an
example. Ideally the DRR weight would be chosen to match the highest
fraction of capacity that NQB compliant flows are likely to use on a
particular network segment. Given that NQB compliant flows are not
capacity-seeking (in contrast to QB flows, which can be), and since
DRR allows unused capacity in one class to be used by traffic in the
other, providing a higher-than-necessary NQB scheduler weight could
be considered less problematic than the reverse. That said,
providing a higher-than-needed NQB scheduler weight does increase the
likelihood that a non-compliant microflow mismarked as NQB is able to
use more than its fair share of network capacity. NQB microflows are
expected to each consume no more than 1% of the link capacity, and in
low stat-mux environments (such as at the edge of the network) would
be unlikely in aggregate to consume 50% of the link capacity. Thus,
50% seems a reasonable upper bound on the weight for the NQB PHB in
these environments.
A node supporting the NQB PHB SHOULD by default classify packets
marked with the NQB DSCP 45 (decimal) into the queue for Non-Queue-
Building traffic. A node supporting the NQB PHB MUST support the
ability to configure the DSCP that is used to classify packets into
the queue for Non-Queue-Building traffic. A node supporting the NQB
PHB SHOULD support the ability to configure multiple DSCPs that are
used to classify packets into the queue for Non-Queue-Building
traffic.
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Support for the NQB PHB is advantageous at bottleneck nodes. Many
bottleneck nodes have a relatively deep buffer for Default traffic
(e.g., roughly equal to the base RTT of the expected connections,
which could be tens or hundreds of ms). Providing a similarly deep
buffer for the NQB queue would be at cross purposes to providing very
low queueing delay and would erode the incentives for QB traffic to
be marked correctly at such a bottleneck node. The NQB queue MUST
have a buffer size that is significantly smaller than the buffer
provided for Default traffic. It is RECOMMENDED to configure an NQB
buffer size less than or equal to 10 ms at the shared NQB/Default
egress rate.
In order to enable network operators to monitor the usage of the NQB
PHB, and in particular to monitor for potential mis-marking of QB
traffic, a node supporting the NQB PHB MUST provide statistics that
can be used by the network operator to detect whether abuse is
occurring (e.g. packet and drop counters). Support for such counters
ensures that operators who configure the NQB PHB have the ability to
track the amount of packet drop that is occurring due to traffic
overrunning the shallow buffer, and then take action if they believe
the PHB is causing more issues than it is solving in their
environment. Those actions could include disabling the PHB,
identifying and dealing with the sources of malicious traffic
directly, enabling traffic protection (Section 5.2) if it is
available, or pursuing a feature request with the equipment
manufacturer to add a traffic protection function if it isn't
currently available.
To prevent propagation of degradation of service for NQB traffic
caused by potential mis-marking of QB traffic, network equipment that
supports this PHB and handles traffic for multiple users (e.g.,
subscribers) SHOULD support provisioning of capacity and related
forwarding resources on a per-user (e.g., subscriber) basis and
SHOULD support enforcement of the resulting per-user limits on the
aggregate of NQB and QB traffic for each user. This functionality is
commonly available in the class of network equipment for which this
PHB is primarily applicable (see Section 3.4). Provisioning
methodology as well as decisions on whether and how to enforce the
resulting limits may vary by network operator.
While not fully described in this document, it may be possible for
network equipment to implement a separate QB/NQB pair of queues for
additional service classes beyond the Default PHB / NQB PHB pair.
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In some cases, existing network gear has been deployed that cannot
readily be upgraded or configured to support the PHB requirements.
This equipment might however be capable of loosely supporting an NQB
service – see Section 7.3.1 for details and an example where this is
particularly important. A similar approach might prove to be useful
in other network environments.
5.2. Traffic Protection
It is possible that, due to an implementation error or
misconfiguration, a QB microflow could end up being mismarked as NQB,
or vice versa. It is also possible that a malicious actor could
introduce a QB microflow marked as NQB with the intention of causing
disruptions. In the case of a low data rate microflow that isn't
marked as NQB and therefore ends up in the QB queue, it would only
impact its own quality of service, and so it seems to be of lesser
concern. However, a QB microflow that is mismarked as NQB is able to
contribute to NQB queue formation at a network node which would cause
queuing delays and/or loss for all the other microflows that are
sharing the NQB queue.
To prevent this situation from harming the performance of the
microflows that comply with the requirements in Section 4.1, network
elements that support the NQB PHB SHOULD support a "traffic
protection" function that can identify microflows or packets that are
inconsistent with the sender requirements in Section 4.1, and either
reclassify those microflows/packets to the QB queue or discard the
offending traffic. In the case of a traffic protection algorithm
that reclassifies offending traffic, the implementation MAY
additionally re-mark such traffic to Default (or possibly to another
local use code point) so that the result of the traffic protection
decision can be used by further hops. This sort of re-marking could
provide a limited layer of protection in situations where downstream
network nodes support separate queuing for NQB marked packets but
lack support for traffic protection.
If traffic protection is not supported or is not effective in
preventing queue formation and growth in the NQB queue, then QB
traffic that is mismarked as NQB is able to form a queue that
overflows the shallow buffer provided for NQB traffic, which is
expected to result in redirecting the excess packets to the QB queue
or discarding them. Both actions degrade service for not only the
mismarked QB traffic, but also for any correctly marked NQB traffic,
likely causing a significant degradation of service for NQB traffic.
Even if mismarked QB traffic does not cause buffer overflow, the
queue that forms results in QB traffic obtaining the reduced loss and
delay benefits of the NQB service while causing queuing delays for
all the other microflows that are sharing the queue. These increased
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abilities of QB traffic to damage NQB service in the absence of a
traffic protection function strongly support the "SHOULD" requirement
to support traffic protection (in the previous paragraph). An NQB
PHB implementation that does not support traffic protection risks
being limited to deployment situations where traffic protection is
potentially not necessary. One example of such a situation could be
a controlled environment (e.g., enterprise LAN) where a network
administrator is expected to manage the usage of DSCPs.
Traffic protection as it is defined here differs from Traffic
Conditioning implemented in other Diffserv contexts. Traffic
Conditioning is commonly performed at the edge of a Diffserv domain
(either ingress or egress, depending on Traffic Conditioning
Agreements in place). In contrast, traffic protection is intended to
be implemented in the nodes that implement the PHB. By placing the
traffic protection at the PHB node, an implementation can monitor the
actual NQB queue and take action only if a queue begins to form.
Implementation of traffic protection at PHB nodes that are most
likely to be a bottleneck is particularly important because these are
the nodes that would be expected to show the most queue build-up in
the presence of QB traffic mismarked as NQB.
This specification does not mandate a particular algorithm for
traffic protection. This is intentional, since this will probably be
an area where implementers innovate, and the specifics of traffic
protection could need to be different in different network equipment
and in different network contexts. Instead this specification
provides guidelines and some examples of traffic protection
algorithms which could be employed.
The traffic protection function SHOULD NOT base its decisions upon
application-layer constructs (such as the port number used by the
application or the source/destination IP address). Instead, it ought
to base its decisions on the actual behavior of each microflow (i.e.
the pattern of packet arrivals).
A conventional implementation of such a traffic protection algorithm
is a per-microflow rate policer, designed to identify microflows that
exceed the bound provided in Section 4.1, where the value R is set to
1 percent of the egress link capacity available for NQB traffic. An
alternative is to use a traffic protection algorithm that bases its
decisions on the detection of actual queuing (i.e. by monitoring the
queuing delay experienced by packets in the NQB queue) in correlation
with the arrival of packets for each microflow. While a per-
microflow rate policer is conceptually simpler (and is based directly
on the NQB sender requirements), it could often end up being more
strict than is necessary (for example by policing a flow that exceeds
the rate equation even when the link is underutilized). One example
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traffic protection algorithm based on the detection of actual queuing
can be found in [I-D.briscoe-docsis-q-protection]. This algorithm
maintains per-microflow state for a certain number of simultaneous
"queue-building" microflows (e.g. 32), and shared state for any
additional microflows above that number.
In the case of a traffic protection algorithm that reclassifies
offending traffic, different levels of hysteresis could be
considered. For example, the reclassify decision could be made on a
packet-by-packet basis, which could result in significant out-of-
order delivery for offending microflows as some portion of the
microflow's packets remain in the NQB queue and some are reclassified
to the Default queue. Alternatively, a traffic protection function
could employ a certain level of hysteresis to prevent borderline
microflows from being reclassified capriciously, thus causing less
potential for out-of-order delivery. As a third option, the decision
could be made to take action on all the future packets of the
microflow, though sufficient logic would be needed to ensure that a
future microflow (e.g. with the same 5-tuple) isn't misidentified as
the current offending microflow.
In the case of a traffic protection algorithm that discards offending
traffic, similar levels of hysteresis could be considered. In this
case, it is RECOMMENDED that the decision thresholds be set higher
than in the case of designs that reclassify, since the degradation of
communications caused by packet discard are likely to be greater than
the degradation caused by out-of-order delivery.
The traffic protection function described here might require that the
network element maintain microflow state. The traffic protection
function MUST be designed such that the node implementing the NQB PHB
does not fail (e.g. crash) in the case that the microflow state is
exhausted.
Some networks might prefer to implement a more traditional Traffic
Conditioning approach, and police the application of the NQB DSCP at
the ingress edge so that per-hop traffic protection is not needed.
This could be accomplished via the use of a per-microflow rate
policer that polices microflows at 1 percent of the minimum link
capacity of the network. This approach would generally be expected
to be inferior to per-hop traffic protection, because on one hand it
would be difficult for edge nodes to guarantee that there would never
be more than 100 NQB flows that would share a single internal
bottleneck, and on the other hand there could be internal links that
have much greater capacity than the minimum. So, Traffic
Conditioning at the edge could simultaneously be too lenient and too
strict.
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5.3. Limiting Packet Bursts from Links
Some link technologies introduce burstiness by briefly storing
packets prior to forwarding them. A common cause of this burstiness
is link discontinuity (i.e. where the link is not continuously
available for transmission by the device), for example time-division-
duplex links or time-division-multiple-access (TDMA) links. Some
link technologies that fall into this category are passive optical
networks (PON), Wi-Fi, LTE/5G and DOCSIS.
As well as NQB senders needing to limit packet bursts (see
Section 4.1), traffic designated for the NQB PHB would benefit from
configuring these link technologies to limit the burstiness
introduced. This is for three reasons. The first reason is that
burstiness, whether caused by the sender or by a link on the path,
could cause queuing delays at downstream bottlenecks and thus degrade
Quality of Experience. The second reason is that burstiness in links
typically means that packets have been delayed by a variable amount,
i.e. for packets that are being aggregated awaiting a transmission
opportunity, some packets would generally have arrived just after the
last transmission opportunity, and thus have to wait the longest,
while others would generally arrive just in time for the next
transmission opportunity, and thus would wait the least. This
manifests as latency variation (jitter) which can also degrade
Quality of Experience for applications that desire NQB treatment.
The third reason is that a downstream bottleneck that implements the
NQB PHB could have implemented a traffic protection mechanism
(Section 5.2) that responds to queuing delays by re-
marking/reclassifying/dropping packets, and bursty arrivals caused by
an upstream link could introduce queuing delays in the NQB queue and
thus be more likely to be subjected to traffic protection effects.
This document does not set any quantified requirements for links to
limit burst delay, primarily because link technologies are outside
the remit of Diffserv specifications. However, 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). This observation
aligns with a similar one in Section 5.5 of [RFC9331].
6. Configuration and Management
As required in Section 5, nodes supporting the NQB PHB provide for
the configuration of classifiers that can be used to differentiate
between QB and NQB traffic of equivalent importance. The default
classifier to distinguish NQB traffic from traffic classified as
Default (DSCP 0) is recommended to be the assigned NQB DSCP (45
decimal).
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Additionally, Section 4.2 contains configuration recommendations for
nodes that do not support the NQB PHB, and Section 4.4.1 contains
configuration recommentations for networks that interconnect with
non-DS-capable domains.
6.1. Guidance for Lower-Rate Links
The NQB sender requirements in Section 4.1 place responsibility in
the hands of the application developer to determine the likelihood
that the application's sending behavior could result in a queue
forming along the path. These requirements rely on application
developers having a reasonable sense for the network context in which
their application is to be deployed. Even so, there will undoubtedly
be networks that contain links having a data rate that is below the
lower end of what is considered "typical", and some of these links
could even be below the instantaneous sending rate of some NQB-marked
applications.
To limit the consequences of this scenario, operators of networks
with lower rate links SHOULD consider utilizing a traffic protection
function on those links that is more tolerant of burstiness (i.e., a
temporary queue). This will have the effect of allowing a larger set
of NQB-marked microflows to remain in the NQB queue, but will come at
the expense of a greater potential for latency variation. In
implementations that support [I-D.briscoe-docsis-q-protection], the
burst tolerance can be configured via the CRITICALqLSCORE_us input
parameter.
Alternatively, operators of networks with lower rate links MAY choose
to disable NQB support (and thus aggregate traffic marked with the
NQB DSCP with Default traffic) on these lower rate links. For links
that have a data rate that is less than ten percent of "typical" path
rates, it is RECOMMENDED that the NQB PHB be disabled and for traffic
marked with the NQB DSCP to thus be carried using the Default PHB.
However, the NQB DSCP SHOULD NOT be re-marked to the Default DSCP
(0).
7. Mapping NQB to standards of other SDOs
This section provide recommendations for the support of the NQB PHB
in certain use cases. This section is not exhaustive.
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7.1. DOCSIS Access Networks
Residential cable broadband Internet services are commonly configured
with a single bottleneck link (the access network link) upon which
the service definition is applied. The service definition, typically
an upstream/downstream data rate tuple, is implemented as a
configured pair of rate shapers that are applied to the user's
traffic. In such networks, the quality of service that each
application receives, and as a result, the quality of experience that
it generates for the user is influenced by the characteristics of the
access network link.
To support the NQB PHB, cable broadband services MUST be configured
to provide a separate queue for traffic marked with the NQB DSCP.
The NQB queue MUST be configured to share the service's rate shaped
capacity with the queue for QB traffic. Further discussion about
support of the NQB PHB in DOCSIS networks can be found in
[LOW_LATENCY_DOCSIS].
7.2. Mobile Networks
Historically, 3GPP mobile networks have utilized "bearers" to
encapsulate each user's user plane traffic through the radio and core
networks. A "dedicated bearer" can be allocated a Quality of Service
(QoS) to apply any prioritisation to its microflows at queues and
radio schedulers. Typically, an LTE operator provides a dedicated
bearer for IMS VoLTE (Voice over LTE) traffic, which is prioritized
in order to meet regulatory obligations for call completion rates;
and a "best effort" default bearer, for Internet traffic. The "best
effort" bearer provides no guarantees, and hence its buffering
characteristics are not compatible with low-latency traffic. The 5G
radio and core systems offer more flexibility over bearer allocation,
meaning bearers can be allocated per traffic type (e.g., loss-
tolerant, low-latency etc.) and hence support more suitable treatment
of Internet real-time microflows.
To support the NQB PHB, the mobile network SHOULD be configured to
give User Equipment a dedicated, low-latency, non-GBR, EPS bearer,
e.g., one with QCI 7, in addition to the default EPS bearer; or a
Data Radio Bearer with 5QI 7 in a 5G system (see Table 5.7.4-1:
Standardized 5QI to QoS characteristics mapping in [SA-5G]).
A packet carrying the NQB DSCP SHOULD be routed through the dedicated
low-latency EPS bearer. A packet that has no associated NQB marking
SHOULD NOT be routed through the dedicated low-latency EPS bearer.
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7.3. Wi-Fi Networks
Wi-Fi networking equipment compliant with 802.11e/n/ac/ax
[IEEE802-11] generally supports either four or eight transmit queues
and four sets of associated Enhanced Multimedia Distributed Control
Access (EDCA) parameters (corresponding to the four Wi-Fi Multimedia
(WMM) Access Categories) that are used to enable differentiated media
access characteristics. As discussed in [RFC8325], it has been a
common practice for Wi-Fi implementations to use a default DSCP to
User Priority mapping that utilizes the most significant three bits
of the Diffserv Field to select "User Priority" which is then mapped
to the four WMM Access Categories. [RFC8325] also provides an
alternative mapping that more closely aligns with the DSCP
recommendations provided by the IETF. In the case of some managed
Wi-Fi gear, this mapping can be controlled by the network operator,
e.g., via TR-369 [TR-369].
In addition to the requirements provided in other sections of this
document, to support the NQB PHB, Wi-Fi equipment (including
equipment compliant with [RFC8325]) SHOULD map the NQB DSCP 45
(decimal) into a separate queue in the same Access Category as the
queue that carries Default traffic (i.e. the Best Effort Access
Category). It is RECOMMENDED that Wi-Fi equipment provide a separate
queue in UP 0, and map the NQB DSCP 45 (decimal) to that queue. If a
separate queue in UP 0 cannot be provided (due to hardware
limitations, etc.) a Wi-Fi device MAY map the NQB DSCP 45 (decimal)
to UP 3.
7.3.1. Interoperability with Existing Wi-Fi Networks
While some existing Wi-Fi equipment might be capable (in some cases
via firmware update) of supporting the NQB PHB requirements, many
currently deployed devices cannot be configured in this way. As a
result, the remainder of this section discusses interoperability with
these existing Wi-Fi networks, as opposed to PHB compliance.
Since this equipment is widely deployed, and the Wi-Fi link can
become a bottleneck link, the performance of traffic marked with the
NQB DSCP across such links could have a significant impact on the
viability and adoption of the NQB DSCP and PHB. Depending on the
DSCP used to mark NQB traffic, existing Wi-Fi equipment that uses the
default mapping of DSCPs to Access Categories and the default EDCA
parameters will support either the NQB PHB requirement for separate
queuing of NQB traffic from Default, or the recommendation to treat
NQB traffic with forwarding preference equal to Default traffic, but
not both.
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The DSCP value 45 (decimal) is recommended for NQB. This maps NQB to
UP_5 using the default mapping, which is in the "Video" Access
Category. While this choice of DSCP enables these Wi-Fi systems to
support the NQB PHB requirement for separate queuing, existing Wi-Fi
devices generally utilize EDCA parameters that result in statistical
prioritization of the "Video" Access Category above the "Best Effort"
Access Category. In addition this equipment does not support the
remaining NQB PHB recommendations in Section 5. The rationale for
the choice of DSCP 45 (decimal) as well as its ramifications, and
remedies for its limitations are discussed further below.
The choice of separated queuing rather than equal forwarding
preference in existing Wi-Fi networks was motivated by the following:
* Separate queuing is necessary in order to provide a benefit for
traffic marked with the NQB DSCP.
* The arrangement of queues in Wi-Fi gear is typically fixed,
whereas the relative priority of the Access Category queues is
configurable. Most Wi-Fi gear has hardware support (albeit
generally not exposed for user control) which could be used to
adjust the EDCA parameters in order to meet the equal forwarding
preference recommendation. This is discussed further below.
* Traffic that is compliant with the NQB sender requirements
Section 4.1 is expected to cause minimal degradation to traffic in
lower priority Access Categories, and in any case would be
unlikely to cause more degradation to lower priority Access
Categories than the existing recommended Video Access Category
traffic types: Broadcast Video, Multimedia Streaming, Multimedia
Conferencing from [RFC8325], and AudioVideo, ExcellentEffort from
[QOS_TRAFFIC_TYPE].
* Several existing client applications that are compatible with the
NQB sender requirements already select the Video Access Category,
and thus would not see a degradation in performance by
transitioning to the NQB DSCP, regardless of whether the network
supported the PHB.
* Application instances on Wi-Fi client devices are already free to
choose any Access Category that they wish, regardless of their
sending behavior, without any policing of usage. So, the choice
of using DSCP 45 (decimal) for NQB creates no new avenues for non-
NQB-compliant client applications to exploit the prioritization
function in Wi-Fi.
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* For application traffic that originates outside of the Wi-Fi
network, and thus is transmitted by the Access Point, the choice
of DSCP 45 does create a potential for abuse by non-compliant
applications. But, opportunities exist in the network components
upstream of the Wi-Fi Access Point to police the usage of the NQB
DSCP and potentially re-mark traffic that is considered non-
compliant, as is recommended in Section 4.4.1. Furthermore, it is
reasonable to expect that ISPs currently manage the DSCPs on
traffic destined to their customers' networks, and will continue
to do so whether they support NQB or not. This includes the
practice in residential broadband networks of re-marking the
Diffserv field to zero on all traffic. Any change to these
practices done to enable the NQB DSCP to pass through could be
done alongside the implementation of the recommendations in
Section 4.4.1.
The choice of Video Access Category rather than the Voice Access
Category was motivated by the desire to minimize the potential for
degradation of Best Effort Access Category traffic. The choice of
Video Access Category rather than the Background Access Category was
motivated by the much greater potential of degradation to NQB traffic
that would be caused by the vast majority of traffic in most Wi-Fi
networks, which utilizes the Best Effort Access Category.
As stated above, the use of DSCP 45 (decimal) for NQB is not expected
to create incentives for abuse by non-compliant applications in the
Wi-Fi uplink direction. The fact that the NQB DSCP brings with it
the potential for degradation of non-compliant applications (traffic
protection and/or a shallow queue resulting in reordering and/or
packet loss) plus the existence of multiple other DSCP values that
don't carry the risk of degradation, and which could be readily used
to obtain prioritization (AC_VI or even AC_VO), leads to the
conclusion that NQB non-compliant applications that are seeking
prioritization in the Wi-Fi uplink would be better off selecting one
of those other DSCPs. This conclusion is not expected to be
disturbed by network support for NQB increasing the likelihood of
DSCP 45 traffic traversing network boundaries without change to the
DSCP, as that likelihood of increased network boundary traversal is
balanced by a likelihood of NQB traffic encountering the traffic-
limiting aspects of NQB support, traffic protection and shallow
buffers, which limit the potential for abuse.
In the case of traffic originating outside of the Wi-Fi network, the
prioritization of traffic marked with the NQB DSCP via the Video
Access Category (if left unchanged) could potentially erode the
principle of alignment of incentives discussed in Section 5. In
order to preserve the incentives principle for NQB, Wi-Fi systems MAY
be configured such that the EDCA parameters for the Video Access
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Category match those of the Best Effort Access Category, which will
mean AC_VI is at the same priority level as AC_BE. These changes
might not be possible on all Access Points, and in any case the
requirements and recommendations in Section 4.4.1 would apply in this
situation.
Systems that utilize [RFC8325] but cannot provide a separate AC_BE
queue for NQB traffic, SHOULD map the recommended NQB DSCP 45
(decimal) (or the locally determined alternative) to UP_5 in the
"Video" Access Category as well (see Section 7.3.2).
7.3.2. The Updates to RFC 8325
[XXX RFC Editor please replace RFCXXXX with this RFC number when
published XXX]
Section 4.2.9 of [RFC8325] describes the recommendation for the
handling of Standard service class traffic that carries the Default
DSCP. This update to [RFC8325] changes the title of Section 4.2.9 of
[RFC8325] from "Standard" to "Standard and Non-Queue-Building". This
update additionally adds a paragraph at the end of Section 4.2.9 of
[RFC8325] as follows:
RFCXXXX defines a shallow-buffered best-effort service for
traffic marked with the NQB DSCP (45 decimal) and recommends the
following treatment in Wi-Fi equipment. It is RECOMMENDED that
Wi-Fi equipment map the NQB DSCP 45 (decimal) into a separate
queue in the same Access Category as the queue that carries
Default traffic (i.e. the Best Effort Access Category). It is
RECOMMENDED that Wi-Fi equipment provide a separate queue in UP
0, and map the NQB DSCP 45 (decimal) to that queue. If a
separate queue in UP 0 cannot be provided (due to hardware
limitations, etc.) a Wi-Fi device MAY map the NQB DSCP 45
(decimal) to UP 3. If neither of these options provides a
separate queue from Default traffic, it is RECOMMENDED that Wi-Fi
equipment map the NQB DSCP 45 (decimal) to UP 5 (which
corresponds to the default mapping described in Section 2.3).
RFCXXXX provides additional recommendations and requirements for
support of the NQB PHB that aren't available in the QoS model
described in Section 6, but nonetheless could be supported in
implementations.
This update to [RFC8325] inserts a new row for "Non-Queue-Building"
traffic between the existing "Low-Latency Data" and "OAM" rows in its
Figure 1 as follows:
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+---------------+------+----------+-------------+--------------------+
| Low- | AF21 | | | |
| Latency | AF22 | RFC 2597 | 3 | AC_BE (Best Effort)|
| Data | AF23 | | | |
+---------------+------+----------+-------------+--------------------+
| Non- | | | 0, 3 | AC_BE (Best Effort)|
| Queue- | NQB | RFC XXXX | OR |
| Building | | | 5 | AC_VI (Video) |
| | | | See Section 4.2.9 |
+---------------+------+----------+-------------+--------------------+
| OAM | CS2 | RFC 2474 | 0 | AC_BE (Best Effort)|
+---------------+------+----------+-------------+--------------------+
This update adds the following sentence to the end of the first
paragraph in Section 5.3 of [RFC8325]: "An exception to this is the
NQB DSCP 45 (decimal) which encodes for best-effort service."
8. IANA Considerations
This document requests that IANA assign the Differentiated Services
Field Codepoint (DSCP) 45 ('0b101101', 0x2D) from the "Differentiated
Services Field Codepoints (DSCP)" registry
(https://www.iana.org/assignments/dscp-registry/) ("DSCP Pool 3
Codepoints", Codepoint Space xxxx01, Standards Action) as the
RECOMMENDED codepoint for Non-Queue-Building behavior.
IANA should update this registry as follows:
* Name: NQB
* Value (Binary): 101101
* Value (Decimal): 45
* Reference: this document
9. Implementation Status
Note to RFC Editor: This section should be removed prior to
publication
The NQB PHB is implemented in equipment compliant with the current
DOCSIS 3.1 specification, published by CableLabs at: CableLabs
Specifications Search (https://www.cablelabs.com/specifications/searc
h?query=&category=DOCSIS&subcat=DOCSIS%203.1&doctype=Specifications&c
ontent=false&archives=false¤tPage=1).
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CableLabs maintains a list of production cable modem devices that are
Certified as being compliant to the DOCSIS Specifications, this list
is available at https://www.cablelabs.com/wp-content/uploads/2013/10/
cert_qual.xlsx. DOCSIS 3.1 modems certified in CW 134 or greater
implement the NQB PHB. This includes products from Arcadyan
Technology Corporation, Arris, AVM, Castlenet, Commscope, Hitron,
Motorola, Netgear, Sagemcom and Vantiva. There are additional
production implementations that have not been Certified as compliant
to the specification, but which have been tested in non-public
Interoperability Events. These implementations are all proprietary,
not available as open source.
10. Security Considerations
The security considerations for the NQB PHB relate to the potential
to impact the capacity available or latency experienced by other
flows that share a bottleneck on the path with traffic that is marked
with the recommended NQB DSCP.
Full support for the NQB PHB in bottleneck links limits the
incentives for a Queue-Building application to mismark its packets as
NQB, particularly for implementations that support traffic
protection. If a Queue-Building microflow were to mismark its
packets as NQB, it would be unlikely to receive a benefit by doing
so, and it would usually experience a degradation, in contrast to
mismarking its packets for a higher-priority PHB, e.g., the EF PHB
[RFC3246]. The nature of the degradation would depend on the
specifics of the PHB implementation, including response to NQB buffer
overflow (and on the presence or absence of a traffic protection
function), but could include excessive packet loss, excessive latency
variation and/or excessive out-of-order delivery. If a Non-Queue-
Building microflow was to fail to mark its packets as NQB, it could
suffer the latency and loss typical of sharing a queue with capacity
seeking traffic.
To preserve low latency performance for NQB traffic, networks that
support the NQB PHB will need to ensure that mechanisms are in place
to prevent malicious traffic marked with the NQB DSCP from causing
excessive queue delays. Section 5.2 recommends the implementation of
a traffic protection mechanism to achieve this goal. The
recommendations on traffic protection mechanisms in this document
presume that some type of "flow" state be maintained in order to
differentiate between microflows that are causing queuing delay and
those that aren't. Since this flow state is likely finite, this
opens up the possibility of flow-state exhaustion attacks. While
this document requires that traffic protection mechanisms be designed
with this possibility in mind, the outcomes of flow-state exhaustion
would depend on the implementation.
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If traffic protection is not implemented or is not able to prevent
queue formation in the NQB shallow buffer, the limited size of that
buffer will cause a growing queue to overrun that buffer, resulting
in negative effects (e.g., reforwarding as Default, discarding) that
potentially impact multiple NQB-marked microflows, independent of
whether each affected microflow contributed to queue formation. As
discussed elsewhere in this draft, those negative effects serve to
discourage misuse and abuse of NQB by QB traffic, but the negative
side effects on NQB traffic that is using NQB (and the associated
shallow buffer) as intended motivates limiting the effects of shallow
buffer overrun via per-user provisioning limits that prevent queue
overrun effects from affecting other users (see Section 5.1).
Notwithstanding the above, the choice of DSCP for NQB does allow
existing Wi-Fi networks to readily (and by default) support some of
the PHB requirements, but without a traffic protection function, and
(when left in the default state) by giving NQB traffic higher
priority than QB traffic. This is not considered to be a compliant
implementation of the PHB. These existing Wi-Fi networks currently
provide priority to half of the DSCP space, whether or not 45 is
assigned to the NQB DSCP. While the NQB DSCP value could also be
abused to gain priority on such links, the potential presence of
traffic protection functions in other hops along the path (which
likely act on the NQB DSCP value alone) would make it less attractive
for such abuse than any of the other 31 DSCP values that are given
priority.
This document discusses the potential use of the NQB DSCP and NQB PHB
in network technologies that are standardized in other SDOs. Any
security considerations that relate to deployment and operation of
NQB solely in specific network technologies are not discussed here.
NQB uses the Diffserv field. The design of Diffserv does not include
integrity protection for the DSCP, and thus it is possible for the
DSCP to be changed by an on-path attacker. The NQB PHB and
associated DSCP don't change this. While re-marking DSCPs is
permitted for various reasons (some are discussed in this document,
others can be found in [RFC2474] and [RFC2475]), if done maliciously,
this might negatively affect the QoS of the tampered microflow.
Nonetheless, an on-path attacker can also alter other mutable fields
in the IP header (e.g. the TTL), which can wreak much more havoc than
just altering QoS treatment.
11. References
11.1. 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>.
[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>.
[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>.
[RFC8325] Szigeti, T., Henry, J., and F. Baker, "Mapping Diffserv to
IEEE 802.11", RFC 8325, DOI 10.17487/RFC8325, February
2018, <https://www.rfc-editor.org/info/rfc8325>.
11.2. Informative References
[Barik] Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
Study", ITC 30, September 2018.
[Custura] Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
modification pathologies in mobile edge networks", TMA ,
2017.
[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-07, 23
November 2023, <https://datatracker.ietf.org/doc/html/
draft-briscoe-docsis-q-protection-07>.
[I-D.ietf-ccwg-bbr]
Cardwell, N., Swett, I., and J. Beshay, "BBR Congestion
Control", Work in Progress, Internet-Draft, draft-ietf-
ccwg-bbr-01, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-ccwg-
bbr-01>.
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[IEEE802-11]
IEEE-SA, "IEEE 802.11-2020", IEEE 802, December 2020,
<https://standards.ieee.org/standard/802_11-2020.html>.
[LOW_LATENCY_DOCSIS]
CableLabs, "Low Latency DOCSIS: Technology Overview",
February 2019, <https://cablela.bs/low-latency-docsis-
technology-overview-february-2019>.
[QOS_TRAFFIC_TYPE]
Microsoft, Corporation, "QOS_TRAFFIC_TYPE enumeration",
2022, <https://learn.microsoft.com/en-
us/windows/win32/api/qos2/ne-qos2-qos_traffic_type>.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.
[RFC2598] Jacobson, V., Nichols, K., and K. Poduri, "An Expedited
Forwarding PHB", RFC 2598, DOI 10.17487/RFC2598, June
1999, <https://www.rfc-editor.org/info/rfc2598>.
[RFC2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
RFC 2661, DOI 10.17487/RFC2661, August 1999,
<https://www.rfc-editor.org/info/rfc2661>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC2983, October 2000,
<https://www.rfc-editor.org/info/rfc2983>.
[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>.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
DOI 10.17487/RFC3551, July 2003,
<https://www.rfc-editor.org/info/rfc3551>.
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[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>.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594,
DOI 10.17487/RFC4594, August 2006,
<https://www.rfc-editor.org/info/rfc4594>.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
February 2008, <https://www.rfc-editor.org/info/rfc5127>.
[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>.
[RFC7893] Stein, Y., Black, D., and B. Briscoe, "Pseudowire
Congestion Considerations", RFC 7893,
DOI 10.17487/RFC7893, June 2016,
<https://www.rfc-editor.org/info/rfc7893>.
[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>.
[RFC8034] White, G. and R. Pan, "Active Queue Management (AQM) Based
on Proportional Integral Controller Enhanced (PIE) for
Data-Over-Cable Service Interface Specifications (DOCSIS)
Cable Modems", RFC 8034, DOI 10.17487/RFC8034, February
2017, <https://www.rfc-editor.org/info/rfc8034>.
[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>.
[RFC8100] Geib, R., Ed. and D. Black, "Diffserv-Interconnection
Classes and Practice", RFC 8100, DOI 10.17487/RFC8100,
March 2017, <https://www.rfc-editor.org/info/rfc8100>.
[RFC8289] Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
Iyengar, Ed., "Controlled Delay Active Queue Management",
RFC 8289, DOI 10.17487/RFC8289, January 2018,
<https://www.rfc-editor.org/info/rfc8289>.
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[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>.
[RFC8622] Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
June 2019, <https://www.rfc-editor.org/info/rfc8622>.
[RFC9330] Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
White, "Low Latency, Low Loss, and Scalable Throughput
(L4S) Internet Service: Architecture", RFC 9330,
DOI 10.17487/RFC9330, January 2023,
<https://www.rfc-editor.org/info/rfc9330>.
[RFC9331] De Schepper, K. and B. Briscoe, Ed., "The Explicit
Congestion Notification (ECN) Protocol for Low Latency,
Low Loss, and Scalable Throughput (L4S)", RFC 9331,
DOI 10.17487/RFC9331, January 2023,
<https://www.rfc-editor.org/info/rfc9331>.
[RFC9332] De Schepper, K., Briscoe, B., Ed., and G. White, "Dual-
Queue Coupled Active Queue Management (AQM) for Low
Latency, Low Loss, and Scalable Throughput (L4S)",
RFC 9332, DOI 10.17487/RFC9332, January 2023,
<https://www.rfc-editor.org/info/rfc9332>.
[RFC9435] Custura, A., Fairhurst, G., and R. Secchi, "Considerations
for Assigning a New Recommended Differentiated Services
Code Point (DSCP)", RFC 9435, DOI 10.17487/RFC9435, July
2023, <https://www.rfc-editor.org/info/rfc9435>.
[RFC9438] Xu, L., Ha, S., Rhee, I., Goel, V., and L. Eggert, Ed.,
"CUBIC for Fast and Long-Distance Networks", RFC 9438,
DOI 10.17487/RFC9438, August 2023,
<https://www.rfc-editor.org/info/rfc9438>.
[SA-5G] 3GPP, "System Architecture for 5G", TS 23.501 V18.6.0,
June 2024.
[TR-369] Broadband Forum, "The User Services Platform", January
2022, <https://usp.technology/specification/index.html>.
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Appendix A. DSCP Re-marking Policies
Some network operators typically bleach (zero out) the Diffserv field
on ingress into their network [RFC9435][Custura][Barik], and in some
cases apply their own DSCP for internal usage. Bleaching the NQB
DSCP is not expected to cause harm to Default traffic, but it will
severely limit the ability to provide NQB treatment. Reports on
existing deployments of DSCP manipulation [Custura][Barik] categorize
the re-marking behaviors into the following six policies: bleach all
traffic (set DSCP to zero), set the top three bits (the former
Precedence bits) on all traffic to 0b000, 0b001, or 0b010, set the
low three bits on all traffic to 0b000, or re-mark all traffic to a
particular (non-zero) DSCP value.
Regarding the DSCP value 45 (decimal), there were no observations of
DSCP manipulation reported in which traffic was marked 45 (decimal)
by any of these policies. Thus it appears that these re-marking
policies would be unlikely to result in QB traffic being marked as
NQB (45). In terms of the fate of traffic marked with the NQB DSCP
that is subjected to one of these policies, it would be
indistinguishable from some subset (possibly all) of other traffic.
In the policies where all traffic is re-marked using the same (zero
or non-zero) DSCP, the ability for a subsequent network hop to
differentiate NQB traffic via DSCP would clearly be lost entirely.
In the policies where the top three bits are overwritten (see
Section 4.2 of [RFC9435]), the NQB DSCP (45) would receive the same
marking as would the currently unassigned Pool 3 DSCPs
5,13,21,29,37,53,61, with all of these DSCPs getting re-marked to
DSCP = 5, 13 or 21 (depending on the overwrite value used). Since
none of the DSCPs in the preceding lists are currently assigned by
IANA, and they all are reserved for Standards Action, it is believed
that they are not widely used currently, but this could vary based on
local-usage, and could change in the future. If networks in which
this sort of re-marking occurs (or networks downstream) classify the
resulting DSCP (i.e. 5, 13, or 21) to the NQB PHB, or re-mark such
traffic as 45 (decimal), they risk treating as NQB other traffic,
which was not originally marked as NQB. In addition, as described in
Section 6 of [RFC9435] future assignments of these 0bxxx101 DSCPs
would need to be made with consideration of the potential that they
all are treated as NQB in some networks.
For the policy in which the low three bits are set to 0b000, the NQB
(45) value would be re-marked to CS5 and would be indistinguishable
from CS5, VA, EF (and the unassigned DSCPs 41, 42, 43). Traffic
marked using the existing standardized DSCPs in this list are likely
to share the same general properties as NQB traffic (non-capacity-
seeking, very low data rate or relatively low and consistent data
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rate). Similarly, any future recommended usage for DSCPs 41, 42, 43
would likely be somewhat compatible with NQB treatment, assuming that
IP Precedence compatibility (see Section 1.5.4 of [RFC4594]) is
maintained in the future. Here there might be an opportunity for a
node to provide the NQB PHB or the CS5 PHB to CS5-marked traffic and
retain some of the benefits of NQB marking. This could be another
motivation to classify CS5-marked traffic into the NQB queue (as
discussed in Section 4.3).
Appendix B. Comparison with Expedited Forwarding
The Expedited Forwarding definition [RFC3246] provides the following
text to describe the EF PHB forwarding behavior: "This specification
defines a PHB in which EF packets are guaranteed to receive service
at or above a configured rate" and "the rate at which EF traffic is
served at a given output interface should be at least the configured
rate R, over a suitably defined interval, independent of the offered
load of non-EF traffic to that interface." Notably, this description
is true of any class of traffic that is configured with a guaranteed
minimum rate, including the Default PHB if configured per the
guidelines in Section 1.5.1 of [RFC4594]. [RFC3246] goes on to
formalize the definition of EF by requiring that an EF node be
characterizable in terms of the fidelity with which it is able to
provide a guaranteed rate.
While the NQB PHB is not required to be configured with a guaranteed
minimum rate, [RFC2474] and [RFC4594] recommend assigning some
minimum resources for the Default PHB, in particular some dedicated
capacity. If such a guaranteed minimum rate is configured for the
Default PHB, it is recommended (Section 5) that NQB traffic share and
be given equal access to that rate. In such cases, the NQB PHB could
effectively receive a rate guarantee of (e.g.) 50% of the rate
guaranteed to the combined NQB/Default PHBs, and so technically
complies with the PHB forwarding behavior defined for EF.
However, EF is intended to be a managed service, and requires that
traffic be policed such that the arriving rate of traffic into the EF
PHB doesn't exceed the guaranteed forwarding rate configured for the
PHB, thereby ensuring that low latency and low latency variation are
provided. NQB is intended as a best effort service, and hence the
aggregate of traffic arriving to the NQB PHB queue could exceed the
forwarding rate available to the PHB. Section 5.2 discusses the
recommended mechanism for handling excess traffic in NQB. While EF
relies on rate policing and dropping of excess traffic at the domain
border, this is only one option for NQB. NQB primarily recommends
traffic protection located at each potential bottleneck, where actual
queuing can be detected and where excess traffic can be reclassified
into the Default PHB rather than dropping it. Local traffic
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protection is more feasible for NQB, given the focus is on access
networks, where one node is typically designed to be the known
bottleneck where traffic control functions all reside. In contrast,
EF is presumed to follow the Diffserv architecture [RFC2475] for core
networks, where traffic conditioning is delegated to border nodes, in
order to simplify high capacity interior nodes. Further, NQB
recommends a microflow-based mechanism to limit the performance
impact of excess traffic to those microflows causing potential
congestion of the NQB queue, whereas EF ignores microflow properties.
Note that under congestion, low loss for NQB conformant flows is only
ensured if such a mechanism is operational. Note also that this
mechanism for NQB operates at the available forwarding rate for the
PHB (which could vary based on other traffic load) as opposed to a
configured guaranteed rate, as in EF.
The lack of a requirement of a guaranteed minimum rate, and the lack
of a requirement to police incoming traffic to such a rate, makes the
NQB PHB suitable for implementation in networks where link capacity
is not or cannot be guaranteed.
There are additional distinctions between EF and NQB arising from the
intended usage as described in [RFC4594] and the actual usage in
practice in the Internet. In Section 1.5.3 of [RFC4594], EF is
described as generally being used to carry voice or data that
requires "wire like" behavior through the network. The NQB PHB
similarly is useful to carry application traffic requiring wire like
performance, characterized by low packet delay and delay variation,
but places a pre-condition that each microflow be relatively low data
rate and sent in a smooth (non-bursty) manner. In actual practice,
EF traffic is oftentimes prioritized over Default traffic. This
contrasts with NQB traffic which is to be treated with the same
forwarding precedence as Default (and sometimes aggregated with
Default).
Appendix C. Impact on Higher Layer Protocols
The NQB PHB itself has no impact on higher layer protocols, because
it only isolates NQB traffic from non-NQB. However, traffic
protection of the PHB can have unintended side-effects on higher
layer protocols. Traffic protection introduces the possibility that
microflows classified into the NQB queue could experience out-of-
order delivery or packet loss if their behavior is not consistent
with the NQB sender requirements. Out-of-order delivery could be
particularly likely if the traffic protection algorithm makes
decisions on a packet-by-packet basis. In this scenario, a microflow
that is (mis)marked as NQB and that causes a queue to form in this
bottleneck link could see some of its packets forwarded by the NQB
queue, and some of them either discarded or redirected to the QB
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queue. In the case of redirection, depending on the queuing latency
and scheduling within the network element, this could result in
packets being delivered out of order. As a result, the use of the
NQB DSCP by a higher layer protocol carries some risk that an
increased amount of out-of-order delivery or packet loss will be
experienced. This characteristic provides one disincentive for
incorrectly setting the NQB DSCP on traffic that doesn't comply with
the NQB sender requirements.
Appendix D. Alternative Diffserv Code Points
In networks where the DSCP 45 (decimal) is already in use for another
(e.g., a local-use) purpose, or where specialized PHBs are available
that can meet specific application requirements (e.g., a guaranteed-
latency path for voice traffic), it could be preferred to use another
DSCP.
In end systems where the choice of using DSCP 45 (decimal) is not
available to the application, the CS5 DSCP (40 decimal) could be used
as a fallback. See Section 4.3 for rationale as to why this choice
could be fruitful.
Acknowledgements
Thanks to Gorry Fairhurst, Diego Lopez, Stuart Cheshire, Brian
Carpenter, Bob Briscoe, Greg Skinner, Toke Hoeiland-Joergensen, Luca
Muscariello, David Black, Jerome Henry, Steven Blake, Jonathan
Morton, Roland Bless, Kevin Smith, Martin Dolly and Kyle Rose for
their review comments. Thanks also to Gorry Fairhurst and Ana
Custura for their input on selection of appropriate DSCPs.
Authors' Addresses
Greg White
CableLabs
Email: g.white@cablelabs.com
Thomas Fossati
Linaro
Email: thomas.fossati@linaro.org
Rüdiger Geib
Deutsche Telekom
Email: Ruediger.Geib@telekom.de
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