A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-18
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
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|---|---|---|---|
| Authors | Greg White , Thomas Fossati | ||
| Last updated | 2023-07-10 (Latest revision 2023-03-25) | ||
| Replaces | draft-white-tsvwg-nqb | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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| Document shepherd | Gorry Fairhurst | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | David Black <david.black@dell.com>, gorry@erg.abdn.ac.uk |
draft-ietf-tsvwg-nqb-18
Transport Area Working Group G. White
Internet-Draft CableLabs
Updates: rfc8325 (if approved) T. Fossati
Intended status: Standards Track ARM
Expires: 11 January 2024 10 July 2023
A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
Services
draft-ietf-tsvwg-nqb-18
Abstract
This document specifies properties and 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 flows,
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 flows.
[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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on 11 January 2024.
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Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Non-Queue-Building Behavior . . . . . . . . . . . . . . . 4
3.2. Relationship to the Diffserv Architecture . . . . . . . . 5
3.3. Relationship to L4S . . . . . . . . . . . . . . . . . . . 7
4. DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . . 7
4.1. Non-Queue-Building Sender Requirements . . . . . . . . . 8
4.2. Aggregation of the NQB DSCP with other Diffserv PHBs . . 9
4.3. Aggregation of other DSCPs in the NQB PHB . . . . . . . . 10
4.4. End-to-end usage and DSCP Re-marking . . . . . . . . . . 10
4.4.1. Interoperability with Non-DS-Capable Domains . . . . 11
4.5. The NQB DSCP and Tunnels . . . . . . . . . . . . . . . . 12
5. Non-Queue-Building PHB Requirements . . . . . . . . . . . . . 13
5.1. Primary Requirements . . . . . . . . . . . . . . . . . . 13
5.2. Traffic Protection . . . . . . . . . . . . . . . . . . . 14
6. Impact on Higher Layer Protocols . . . . . . . . . . . . . . 16
7. Configuration and Management . . . . . . . . . . . . . . . . 16
7.1. Guidance for Lower-Rate Links . . . . . . . . . . . . . . 16
8. Example Use Cases . . . . . . . . . . . . . . . . . . . . . . 17
8.1. DOCSIS Access Networks . . . . . . . . . . . . . . . . . 17
8.2. Mobile Networks . . . . . . . . . . . . . . . . . . . . . 17
8.3. WiFi Networks . . . . . . . . . . . . . . . . . . . . . . 18
8.3.1. Interoperability with Existing WiFi Networks . . . . 18
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
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10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 21
12. Security Considerations . . . . . . . . . . . . . . . . . . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. DSCP Re-marking Policies . . . . . . . . . . . . . . 26
Appendix B. Comparison to Expedited Forwarding . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
This document defines a Differentiated Services per-hop behavior
(PHB) called "Non-Queue-Building Per-Hop Behavior" (NQB PHB), which
isolates traffic flows that are relatively low data rate and that do
not themselves materially contribute to queueing delay and loss,
allowing them to avoid the queuing delays and losses caused by other
traffic. Such Non-Queue-Building flows (for example: interactive
voice, game sync packets, machine-to-machine applications, DNS
lookups, and real-time IoT analytics data) are low-data-rate
application-limited flows that are distinguished from bursty traffic
flows and high-data-rate traffic flows managed by a capacity-seeking
congestion control algorithm, both of which cause queuing delay and
loss.
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, Cubic or BBR, are
designed to seek the available capacity of the end-to-end path (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
bottleneck links implement a relatively deep buffer (100 ms or more)
in order to enable these congestion control algorithms to effectively
use the link, which exacerbates the latency and latency variation
experienced.
In contrast to applications that frequently cause queueing delay,
there are a variety of relatively low data rate applications that do
not materially contribute to queueing 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.
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Active Queue Management (AQM) mechanisms (such as PIE [RFC8033],
DOCSIS-PIE [RFC8034], 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 much more tightly, applications using those algorithms
would not perform well. Alternatively, flow queueing systems, such
as fq_codel [RFC8290] can be employed to isolate flows from one
another, but these are not appropriate for all bottleneck links, due
to complexity or other reasons.
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 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 (and optionally enforces that behavior). It is the NQB
senders' behavior itself which results in low latency and low loss.
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 many applications that send traffic at relatively low data
rates and/or in a fairly smooth and consistent manner such that they
are highly unlikely to exceed the available capacity of the network
path between source and sink. 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
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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. Many of
these applications are 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) flows include those that use TCP or
QUIC, with Cubic, Reno or other TCP congestion control algorithms
that probe for the link capacity and induce latency and loss as a
result. Other types of QB flows include those that send at a high
burst rate even if the long-term average data rate is much lower.
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 quantitatively or in a relative sense (i.e. 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) of
the field, and standardized treatments (traffic conditioning and per-
hop-behaviors) 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, it has not been used for these purposes end-to-end
across the Internet.
This is in part due to the fact that meeting the performance
requirements of an application in an end-to-end context 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.
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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 an end-to-end context.
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 bandwidth, 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
end-to-end across the Internet. The intent of the NQB DSCP is that
it signals verifiable behavior that permits the sender to request
differentiated treatment. Also, the NQB traffic is to be given a
separate queue with priority equal to Default traffic and given no
reserved bandwidth other than the bandwidth that it shares with
Default traffic. As a result, the NQB PHB does not aim to meet
specific application performance requirements. Instead, the goal of
the NQB PHB is to provide statistically better loss, latency, and
jitter performance for traffic that is itself only an insignificant
contributor to those degradations. The PHB is also designed to
minimize any incentives for a sender to mismark its traffic, since
neither higher priority nor reserved bandwidth 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.
The NQB PHB is therefore intended for the prevalent situation where
the performance requirements of applications cannot be assured end-
to-end, 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 many applications that send data at
relatively low data rates.
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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. The main distinctions
between NQB and EF are discussed in Appendix B.
In nodes that support multiple DiffServ Service Classes, NQB traffic
is to be treated as a part of the Default treatment. Capacity
assigned to this class is not prioritised with respect to other
classes, AFxx, EF, etc. Of course, traffic marked as NQB could (like
other Default traffic) be prioritised with respect to LE (i.e. the
NQB queue would be emptied in a priority sequence before the LE
queue).
3.3. Relationship to L4S
The NQB DSCP and PHB described in this draft 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 [RFC9330], with the result being that NQB traffic and
L4S traffic can share the low-latency queue in an L4S DualQ node
[RFC9332]. Compliance with the DualQ Coupled AQM requirements
(Section 2.5 of [RFC9332]) is considered sufficient to support the
NQB PHB requirement of fair allocation of bandwidth 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 means 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). L4S network functions SHOULD treat packets marked
with the NQB DSCP and ECT(1) the same as packets marked with the
Default DSCP and ECT(1). Here, L4S network functions means 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]).
4. DSCP Marking of NQB Traffic
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4.1. Non-Queue-Building Sender Requirements
Flows that are eligible to be marked with the NQP DSCP are typically
UDP flows 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
applications send at a data rate of no more than about 1 percent of
the "typical" network path capacity. In today's network, where
access network data rates are typically on the order of 50 Mbps or
more (and see Section 7.1 for a discussion of cases where this isn't
true), this implies 500 kbps as an upper limit. In addition, these
applications are required to send their traffic in a smooth (i.e.
paced) manner, where the number of bytes sent in any time interval
"T" is less than or equal to R * T + 1500 bytes, where "R" is the
maximum rate described above.
Flows 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 an
application'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 flows in this document is not
to be interpreted as suggesting that such flows are in any way exempt
from this responsibility. One way that an application marking its
flows as NQB can handle this is to implement a low latency congestion
control mechanism as described in [RFC9331].
Applications that align with the description of behavior in the
preceding paragraphs in this section SHOULD identify themselves to
the network using a Diffserv Code Point (DSCP) of 45 (decimal) so
that their packets can be queued separately from QB flows. The
choice of the DSCP value 45 (decimal) is motivated in part by the
desire to achieve separate queuing in existing WiFi networks (see
Section 8.3) 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 value (see Section 4.2 for further
discussion).
In networks where another (e.g., a local-use) DSCP is designated for
NQB traffic, 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
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available to the application, the CS5 DSCP (40 decimal) could be used
as a fallback. See Section 4.2 for rationale as to why this choice
could be fruitful.
If the application's traffic exceeds the rate equation provided in
the first paragraph of this section, the application SHOULD NOT mark
its 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]. At the time of writing, it is
believed that 500 kbps is a reasonable upper bound on instantaneous
traffic rate for an application marking its traffic with the NQB
DSCP, but this value is of course subject to the context in which the
application is expected to be deployed.
An application that marks its traffic as NQB runs the risk of being
subjected to a traffic protection algorithm (see Section 5.2) if it
contributes to the formation of a queue in a node that supports the
PHB. This could result in the excess traffic being discarded or
queued separately as default traffic (and thus potentially delivered
out of order). As a result, applications that aren't clearly beneath
the threshold described above would need to weigh the risk of
additional loss or out-of-order delivery against the expected latency
benefits of NQB treatment in determining whether to mark their
packets as NQB.
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 re-mark 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 with other Diffserv PHBs
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).
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.
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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.
An alternative would be to aggregate traffic marked with the NQB DSCP
with real-time, latency sensitive traffic (e.g. the (Bulk) Real-Time
Treatment Aggregate), although this risks creating an incentive for
mismarking of non-compliant traffic as NQB and so is NOT RECOMMENDED
in general.
Networks and nodes that do not support the NQB PHB should simply
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].
4.3. Aggregation of other DSCPs in the NQB PHB
Operators of nodes that support the NQB PHB may choose to aggregate
other service classes into the NQB queue. This is particularly
useful in cases where specialized PHBs for these other service
classes are not provided. 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. An operator would need to use their own judgement based on
the actual traffic characteristics in their network 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.
4.4. End-to-end usage and DSCP Re-marking
In contrast to some existing standard PHBs, many of which are
typically only used within a Diffserv Domain (e.g., an AS or an
enterprise network), this PHB is expected to be used end-to-end
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.
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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.
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 end-to-end NQB PHB
treatment, the appropriate NQB DSCP should 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.
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 WiFi APs as described in Section 8.3.1), it is RECOMMENDED
that the operator of the upstream domain re-mark NQB traffic to DSCP
0 (Default) before delivering traffic into a non-DS-capable domain.
Network equipment that is intended to deliver traffic into networks
that are expected to be non-DS-compliant (e.g., an access network
gateway for a residential ISP) SHOULD by default ensure that NQB
traffic is re-marked with a DSCP that is unlikely to result in
prioritized treatment in the downstream domain, such as DSCP 0
(Default). It is RECOMMENDED that this equipment supports the
ability to configure the re-marking, so that (when it is appropriate)
traffic can be delivered as NQB-marked.
As an alternative to re-marking, 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. It should be noted
that a traffic protection function as defined in this document might
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only provide protection from issues occuring 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 either re-marked and classified for Default
forwarding or dropped. 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 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. This 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, potentially improving the possibility of achieving
NQB treatment in subsequent nodes.
As is discussed in [RFC2983], tunnel protocols that are sensitive to
reordering can result in undesirable interactions if multiple DSCP
PHBs are signaled for traffic within a tunnel instance. This is true
for traffic marked with the NQB DSCP as well. If a tunnel contains a
mix of QB and NQB traffic, and this is reflected in the outer DSCP in
a network that supports the NQB PHB, it would be necessary to avoid a
reordering-sensitive tunnel protocol.
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5. Non-Queue-Building PHB Requirements
For the NQB PHB to succeed, 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 flows is that for flows
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
flows inconsistent with those requirements, the QB queue would likely
be a better choice than the NQB queue (this is discussed further in
this section and Section 12). By adhering to these principles, there
is no incentive for senders to mismark their traffic as NQB. 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 makes no guarantees on latency or data
rate for NQB-marked flows, but instead aims to provide an upper-bound
to queuing delay for as many such marked flows as it can and shed
load when needed.
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 is discussed in Section 4.4.1.
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 fair manner, e.g., a
deficit round-robin scheduler with equal weights. Compliance with
these recommendations helps to ensure that there are no incentives
for QB traffic to be mismarked as NQB.
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 classify packets into the queue for Non-Queue-Building traffic.
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The NQB queue SHOULD have a buffer size that is significantly smaller
than the buffer provided for Default traffic. It is expected that
most Default traffic is engineered to work well when the network
provides a relatively deep buffer (e.g., on the order of tens or
hundreds of ms) in nodes where support for the NQB PHB is
advantageous (i.e., bottleneck nodes). 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. An NQB buffer size less than or equal to 10 ms
is RECOMMENDED.
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.
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 8.3.1 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 flow could end up being mismarked as NQB, or
vice versa. It is also possible that a malicious actor could
introduce a QB flow marked as NQB with the intention of causing
disruptions. In the case of a low data rate flow 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 flow that is mismarked as NQB would cause queuing
delays and/or loss for all the other flows that are sharing the NQB
queue.
To prevent this situation from harming the performance of the flows
that are in compliance with the requirements in Section 4.1, network
elements that support the NQB PHB SHOULD support a "traffic
protection" function that can identify flows that are inconsistent
with the sender requirements in Section 4.1, and either re-mark those
flows/packets as Default and reclassify them to the QB queue or
discard the offending traffic. Such a function SHOULD base its
decisions upon the behavior of each flow rather than on application-
layer constructs (such as the port number used by the application or
the source/destination IP address).
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This specification does not mandate a particular algorithm for
traffic protection. This is intentional, since 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.
One potential implementation of such a traffic protection algorithm
is a per-flow rate policer, designed to identify flows 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 flow. One example traffic
protection algorithm along these lines can be found in
[I-D.briscoe-docsis-q-protection]. This algorithm maintains per-flow
state for up to 32 simultaneous "queue-building" flows, and shared
state for any additional flows in excess of that number.
In the case of a traffic protection algorithm that re-marks and
reclassifies offending traffic, different levels of hysteresis could
be considered. For example, the re-mark/reclassify decision could be
made on a packet-by-packet basis, which could result in significant
out-of-order delivery for offending flows as some portion of the
flow's packets remain in the NQB queue and some are re-marked and
reclassified to the Default queue. Alternatively, a traffic
protection function could employ a certain level of hysteresis to
prevent borderline flows 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 flow, though sufficient logic would be needed to ensure that a
future flow (e.g. with the same 5-tuple) isn't misidentified as the
current offending flow.
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 use re-mark/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 requires that the
network element maintain some sort of flow 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 flow
state is exhausted.
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There are some situations where such a function is potentially not
necessary. For example, a network element designed for use in
controlled environments (e.g., enterprise LAN) where a network
administrator is expected to manage the usage of DSCPs.
Additionally, some networks might prefer to police the application of
the NQB DSCP at the ingress edge, so that per-hop traffic protection
is not needed.
6. Impact on Higher Layer Protocols
Network elements that support the NQB PHB and that support traffic
protection as discussed in the previous section introduce the
possibility that flows 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
flow 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
queue. In the case of redirection, depending on the queueing 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.
7. Configuration and Management
As required above, 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 for
such classifiers is recommended to be the assigned NQB DSCP (to
identify NQB traffic) and the Default (0) DSCP (to identify QB
traffic).
7.1. 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
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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 flows 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.
8. Example Use Cases
8.1. 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
bandwidth with the queue for QB traffic.
8.2. Mobile Networks
Historically, 3GPP mobile networks have utilised "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 flows at queues and radio
schedulers. Typically, an LTE operator provides a dedicated bearer
for IMS VoLTE (Voice over LTE) traffic, which is prioritised in order
to meet regulatory obligations for call completion rates; and a "best
effort" default bearer, for Internet traffic. The "best effort"
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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 flows.
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.
8.3. WiFi Networks
WiFi 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 WiFi Multimedia (WMM) Access
Categories) that are used to enable differentiated media access
characteristics. As discussed in [RFC8325], it has been a common
practice for WiFi 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 WiFi 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, WiFi 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).
8.3.1. Interoperability with Existing WiFi Networks
While some existing WiFi 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 WiFi networks, as opposed to PHB compliance.
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Since this equipment is widely deployed, and the WiFi link is
commonly 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 WiFi 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, or the recommendation to treat NQB traffic
with priority equal to Default traffic, but not both.
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 WiFi systems to
support the NQB PHB requirement for separate queuing, existing WiFi
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 priority in
existing WiFi networks was motivated by the following:
* Separate queuing is necessary in order to provide a benefit for
traffic marked with the NQB DSCP.
* WiFi gear typically has hardware support (albeit generally not
exposed for user control) for adjusting the EDCA parameters in
order to meet the equal priority recommendation. This is
discussed further below.
* Traffic that is compliant with the NQB sender requirements
Section 4.1 is 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].
* Application instances on WiFi 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 WiFi.
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* 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.
* For application traffic that originates outside of the WiFi
network, and thus is transmitted by the Access Point,
opportunities exist in the network components upstream of the WiFi
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. A residential ISP that re-marks the
Diffserv field to zero, bleaches all DSCPs and hence would not be
impacted by the introduction of traffic marked as NQB.
Furthermore, any change to this practice ought to be done
alongside the implementation of those recommendations in the
current document.
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 WiFi
networks, which utilizes the Best Effort Access Category.
If left unchanged, the prioritization of traffic marked with the NQB
DSCP via the Video Access Category (particularly in the case of
traffic originating outside of the WiFi network as mentioned above)
could erode the principle of alignment of incentives discussed in
Section 5. In order to preserve the incentives principle for NQB,
WiFi systems SHOULD be configured such that the EDCA parameters for
the Video Access Category match those of the Best Effort Access
Category. These changes can be deployed in managed WiFi systems or
those deployed by an ISP and are intended for situations when the
vast majority of traffic that would use AC_VI is NQB. In other
situations (e.g., consumer-grade WiFi gear deployed by an ISP's
customer) this configuration might not be possible, and the
requirements and recommendations in Section 4.4.1 would apply.
Similarly, systems that utilize [RFC8325] but that are unable to
fully support the PHB requirements, SHOULD map the recommended NQB
DSCP 45 (decimal) (or the locally determined alternative) to UP_5 in
the "Video" Access Category.
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9. Acknowledgements
Thanks to Diego Lopez, Stuart Cheshire, Brian Carpenter, Bob Briscoe,
Greg Skinner, Toke Hoeiland-Joergensen, Luca Muscariello, David
Black, Sebastian Moeller, Ruediger Geib, Jerome Henry, Steven Blake,
Jonathan Morton, Roland Bless, Kevin Smith, Martin Dolly, and Kyle
Rose for their review comments. Thanks also to Gorry Fairhurst, Ana
Custura, and Ruediger Geib for their input on selection of
appropriate DSCPs.
10. 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
11. 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).
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
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to the specification, but which have been tested in non-public
Interoperability Events. These implementations are all proprietary,
not available as open source.
12. Security Considerations
When the NQB PHB is fully supported in bottleneck links, there is no
incentive for a Queue-Building application to mismark its packets as
NQB (or vice versa). If a Queue-Building flow 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. The nature of the
degradation would depend on the specifics of the PHB implementation
(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 flow
were to fail to mark its packets as NQB, it could suffer the latency
and loss typical of sharing a queue with capacity seeking traffic.
In order 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
but recognizes that other options might be more desirable in certain
situations. The recommendations on traffic protection mechanisms in
this document presume that some type of "flow" state be maintained in
order to differentiate between flows 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.
Notwithstanding the above, the choice of DSCP for NQB does allow
existing WiFi 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 WiFi networks currently
provide priority to half of the DSCP space, including the NQB DSCP.
While the NQB DSCP value could be abused in order 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 seem to make it less attractive for such abuse than any
of the other 31 DSCP values that are provided high priority.
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NQB uses the Diffserv code point (DSCP). 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 flow.
13. References
13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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>.
[RFC2983] Black, D., "Differentiated Services and Tunnels",
RFC 2983, DOI 10.17487/RFC2983, October 2000,
<https://www.rfc-editor.org/info/rfc2983>.
[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>.
13.2. 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.
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[I-D.briscoe-docsis-q-protection]
Briscoe, B. and G. White, "The DOCSIS(r) Queue Protection
Algorithm to Preserve Low Latency", Work in Progress,
Internet-Draft, draft-briscoe-docsis-q-protection-06, 13
May 2022, <https://datatracker.ietf.org/doc/html/draft-
briscoe-docsis-q-protection-06>.
[I-D.ietf-tsvwg-dscp-considerations]
Custura, A., Fairhurst, G., and R. Secchi, "Considerations
for Assigning a new Recommended DiffServ Codepoint
(DSCP)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-dscp-considerations-13, 3 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
dscp-considerations-13>.
[IEEE802-11]
IEEE-SA, "IEEE 802.11-2020", IEEE 802, December 2020,
<https://standards.ieee.org/standard/802_11-2020.html>.
[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>.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41,
RFC 2914, DOI 10.17487/RFC2914, September 2000,
<https://www.rfc-editor.org/info/rfc2914>.
[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>.
[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>.
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[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>.
[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>.
[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>.
[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>.
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[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>.
[SA-5G] 3GPP, "System Architecture for 5G", TS 23.501, 2019.
[TR-369] Broadband Forum, "The User Services Platform", January
2022, <https://usp.technology/specification/index.html>.
Appendix A. DSCP Re-marking Policies
Some network operators typically bleach (zero out) the Diffserv field
on ingress into their network
[I-D.ietf-tsvwg-dscp-considerations][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 end-to-end.
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, the result would be that
traffic marked with the NQB DSCP 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 [I-D.ietf-tsvwg-dscp-considerations]), 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-
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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 [I-D.ietf-tsvwg-dscp-considerations] 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
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 (as discussed in Section 4.2) classify CS5-marked
traffic into NQB queue.
Appendix B. Comparison to Expedited Forwarding
The Expedited Forwarding definition [RFC3246] provides the following
intuitive description of the EF PHB: "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." This differs from the
definition of the NQB PHB, in which NQB traffic is treated as being
the same forwarding preference (and hence the same likelihood of
being deferred if other traffic is being served) as Default traffic.
As a result there is no rate R that can be configured for the NQB
PHB, and moreover the NQB PHB does not guarantee any serving rate for
NQB-marked traffic, let alone one that is independent of the offered
load of non-NQB-marked traffic. This difference additionally makes
the NQB PHB suitable for implementation in networks where link
capacity is not guaranteed.
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The Formal Definition of the EF PHB Section 2.2 of [RFC3246] requires
support for the configured rate R as well as a guarantee on departure
times of EF-marked packets within a characterizable bound (E_a), as
described by a pair of equations (eq_1 and eq_2). Since the NQB PHB
cannot be configured with a rate R, these equations cannot be
complied with, except for the degenerate cases where R equals 0 and/
or E_a equals infinity.
From a practical perspective, EF-marked traffic may be treated with
high priority in networks where the DSCP is supported, even if the EF
PHB requirements aren't fully met (e.g. as discussed in [RFC8325]).
Authors' Addresses
Greg White
CableLabs
Email: g.white@cablelabs.com
Thomas Fossati
ARM
Email: Thomas.Fossati@arm.com
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