A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-15
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 | 2023-02-22 (Latest revision 2023-01-11) | ||
| 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 | David L. Black | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | David Black <david.black@dell.com> |
draft-ietf-tsvwg-nqb-15
Transport Area Working Group G. White
Internet-Draft CableLabs
Updates: rfc8325 (if approved) T. Fossati
Intended status: Standards Track ARM
Expires: 15 July 2023 11 January 2023
A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
Services
draft-ietf-tsvwg-nqb-15
Abstract
This document specifies properties and characteristics of a Non-
Queue-Building Per-Hop Behavior (NQB PHB). The purpose of this NQB
PHB is to provide a separate queue that enables smooth, 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.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 15 July 2023.
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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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 . . . . . . . . . . . . . . . . . . . . 4
3. Context . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Non-Queue-Building Behavior . . . . . . . . . . . . . . . 4
3.2. Relationship to the Diffserv Architecture . . . . . . . . 5
3.3. Relationship to L4S . . . . . . . . . . . . . . . . . . . 6
4. DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . . 6
4.1. Non-Queue-Building Sender Requirements . . . . . . . . . 7
4.2. Aggregation of the NQB DSCP with other Diffserv PHBs . . 8
4.3. End-to-end usage and DSCP Re-marking . . . . . . . . . . 9
4.3.1. Unmanaged Networks . . . . . . . . . . . . . . . . . 10
4.4. The NQB DSCP and Tunnels . . . . . . . . . . . . . . . . 11
5. Non-Queue-Building PHB Requirements . . . . . . . . . . . . . 11
5.1. Primary Requirements . . . . . . . . . . . . . . . . . . 12
5.2. Traffic Protection . . . . . . . . . . . . . . . . . . . 13
5.3. Guidance for Very Low-Rate Links . . . . . . . . . . . . 14
6. Impact on Higher Layer Protocols . . . . . . . . . . . . . . 14
7. Configuration and Management . . . . . . . . . . . . . . . . 15
8. Example Use Cases . . . . . . . . . . . . . . . . . . . . . . 15
8.1. DOCSIS Access Networks . . . . . . . . . . . . . . . . . 15
8.2. Mobile Networks . . . . . . . . . . . . . . . . . . . . . 15
8.3. WiFi Networks . . . . . . . . . . . . . . . . . . . . . . 16
8.3.1. Interoperability with Existing WiFi Networks . . . . 16
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 19
12. Security Considerations . . . . . . . . . . . . . . . . . . . 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. DSCP Re-marking Policies . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
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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, low-data-rate online gaming, machine-to-machine applications)
are application limited flows that are distinguished from the high-
data-rate traffic flows that are typically managed by an end-to-end
congestion control algorithm.
Most packets carried by broadband access networks are managed by an
end-to-end congestion control algorithm, such as Reno, Cubic or BBR.
These congestion control algorithms attempt 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 queuing delay (variable
latency) and possibly 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 traditional congestion-controlled applications to
effectively use the link, which exacerbates the latency and latency
variation experienced.
In contrast to traditional congestion-controlled applications, 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.
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.
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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.
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. 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. 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.
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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 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 proven problematic to enable its use for
these purposes end-to-end across the Internet.
This difficulty 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.
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
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.
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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. They also
significantly simplify access control and admission control
functions, reducing them to simple verification of behavior.
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
[I-D.ietf-tsvwg-l4s-arch]. Nonetheless, the NQB-marked traffic flows
are intended to be compatible with [I-D.ietf-tsvwg-l4s-arch], with
the result being that NQB traffic and L4S traffic can share the low-
latency queue in an L4S DualQ node
[I-D.ietf-tsvwg-aqm-dualq-coupled]. Compliance with the DualQ
Coupled AQM requirements (Section 2.5 of
[I-D.ietf-tsvwg-aqm-dualq-coupled]) 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 [I-D.ietf-tsvwg-ecn-l4s-id].
Applications that comply with both the NQB sender requirements in
Section 4.1 and the L4S "Prague" requirements in Section 4 of
[I-D.ietf-tsvwg-ecn-l4s-id] could mark their packets both with the
NQB DSCP and with the ECT(1) value. Packets marked with both
codepoints SHOULD NOT be subject to less stringent policing than they
would with either codepoint alone.
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. Current examples include many online games,
voice chat, DNS lookups, and real-time IoT analytics data. Here the
data rate is limited by the application itself rather than by network
capacity - these applications send at most a few packets per RTT or 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 100 Mbps, this implies 1 Mbps as an
upper limit. In addition, these applications 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.
Note that, while such flows ordinarily don't implement a traditional
congestion control mechanism, they nonetheless are expected to comply
with existing guidance for safe deployment on the Internet, 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]).
To be clear, the description of NQB-marked flows in this document
should not be interpreted as suggesting that such flows are in any
way exempt from this responsibility.
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 value 45 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) codepoint 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 is not 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
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mechanism as described in [I-D.ietf-tsvwg-ecn-l4s-id]. At the time
of writing, it is believed that 1 Mbps is a reasonable upper bound on
instantaneous traffic rate for an NQB-marked application, 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 but happens to exceed
the available path capacity (even on an instantaneous basis) runs the
risk of being subjected to a traffic protection algorithm (see
Section 5.2), which 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 NQB-marked traffic the same as traffic
marked "Default". It is additionally RECOMMENDED that such networks
and nodes simply classify the NQB DSCP into the same treatment
aggregate as Default traffic, or encapsulate the NQB-marked packet,
rather than re-marking NQB traffic as Default. This preservation of
the NQB marking enables hops further along the path to provide the
NQB PHB successfully.
In backbone and core network switches (particularly if shallow-
buffered), as well as in nodes that do not typically experience
congestion, treating NQB-marked traffic the same as Default might be
sufficient to preserve loss/latency/jitter performance for NQB
traffic. In other nodes, treating NQB-marked traffic as Default
could result in degradation of loss/latency/jitter performance but is
recommended nonetheless in order to preserve the incentives described
in Section 5. An alternative, in controlled environments where there
is no risk of mismarking of traffic, would be to aggregate NQB-marked
traffic with real-time, latency sensitive traffic. Similarly,
networks and nodes that aggregate service classes as discussed in
[RFC5127] and [RFC8100] might not be able to provide a PDB/PHB that
meets the requirements of this document. In these cases it is
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RECOMMENDED that NQB-marked traffic be aggregated into the Elastic
Treatment Aggregate (for [RFC5127] networks) or the Default / Elastic
Treatment Aggregate (for [RFC8100] networks), although in some cases
a network operator might instead choose to aggregate NQB traffic into
the (Bulk) Real-Time Treatment Aggregate. Either approach comes with
trade-offs: when the aggregated traffic encounters a bottleneck,
aggregating with Default/Elastic traffic could result in a
degradation of loss/latency/jitter performance for NQB traffic, while
aggregating with Real-Time (assuming such traffic is provided a
prioritized PHB) risks creating an incentive for mismarking of non-
compliant traffic as NQB (except in controlled environments). In
either case, the NQB DSCP SHOULD be preserved (possibly via
encapsulation) in order to limit the negative impact that such
networks would have on end-to-end performance for NQB traffic. This
aligns with recommendations in [RFC5127].
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 inelastic traffic that has low to very-low
tolerance for loss, latency and/or jitter as discussed in [RFC4594].
These could include Telephony (EF/VA), Signaling (CS5), Real-Time
Interactive (CS4) and Broadcast Video (CS3). Or, in some networks,
equipment limitations may necessitate aggregating all traffic marked
with DSCPs 40-47 (i.e., whose three MSBs are 0b101). As noted in
Section 4.1, the choice of the value 45 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.
4.3. End-to-end usage and DSCP Re-marking
In contrast to some existing standard PHBs, many of which are
typically only meaningful 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 by all operators and mapped across their 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 value
45 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 for internal usage. To
ensure reliable end-to-end NQB PHB treatment, the appropriate NQB
DSCP should be restored when forwarding to another network.
In order to enable interoperability with WiFi equipment as described
in Section 8.3.1, networks SHOULD ensure NQB traffic is marked DSCP
45 prior to a customer access link, subject to the safeguards
described below and in that section.
4.3.1. Unmanaged Networks
As discussed in Section 4 of [RFC2475], there may be cases where a
network operator is delivering traffic into a network outside of
their control, where there is no knowledge of the traffic management
capabilities of the downstream domain, and no agreement in place
(e.g., a residential ISP delivering traffic to a customer's home
network that may contain a legacy WiFi AP). In such cases, the
network operator should presume that the existing network equipment
in the downstream domain does not support the NQB PHB and might
instead prioritize traffic marked with the NQB DSCP. In these cases,
the network operator SHOULD take precautions to prevent issues that
could be caused by excessive NQB traffic and/or traffic mismarked as
NQB.
Network equipment that is intended to deliver traffic into such
unmanaged networks (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). Such equipment SHOULD
support 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, the operator could deploy a traffic
protection (see Section 5.2) or a shaping/policing function on NQB-
marked traffic that minimizes the potential for negative impacts on
Default traffic. It should be noted that a traffic protection
function as defined in this document might 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%
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of the interconnection data rate, or 5% of the typical rate for such
interconnections, whichever is greater, with excess traffic being
either dropped or re-marked and classified for Default forwarding. 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 approximately 50 ms of
buffering.
The recommendation to limit NQB traffic to 5% in these situations is
based on an expectation of support for at least 5 simultaneous NQB
streams, and SHOULD be adjusted according to local network policy.
4.4. 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 NQB-marked traffic 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.
5. Non-Queue-Building PHB Requirements
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
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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 any queue used for Queue-Building
traffic.
A node supporting the NQB PHB SHOULD NOT rate limit or rate police
the aggregate of NQB traffic separately from Queue-Building traffic
of equivalent importance. An exception to this recommendation is
discussed in Section 4.3.1.
The NQB queue SHOULD be given equivalent forwarding preference
compared to Queue-Building traffic of equivalent importance. The
node SHOULD provide a scheduler that allows QB and NQB traffic of
equivalent importance 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 treat traffic marked as Default
(DSCP=0) as QB traffic having equivalent importance to the NQB-marked
traffic. A node supporting the NQB DSCP MUST support the ability to
configure the classification criteria that are used to identify QB
and NQB traffic of equivalent importance.
The NQB queue SHOULD have a buffer size that is significantly smaller
than the buffer provided for QB traffic. It is expected that most QB
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.
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
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service - see Section 8.3.1 for details and an example where this is
particularly important. A similar approach might prove necessary in
other network environments.
5.2. Traffic Protection
It is possible that due to an implementation error or
misconfiguration, a QB flow would end up getting mismarked as NQB, or
vice versa. 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 be implemented
in an objective and verifiable manner, basing its decisions upon the
behavior of the flow rather than on application-layer constructs.
While it is possible to utilize a per-flow rate policer to perform
this function, it is RECOMMENDED that traffic protection algorithms
base their decisions on the detection of actual queuing, as opposed
to simply packet arrival rate or data rate. It could be advantageous
for a traffic protection function to employ hysteresis to prevent
borderline flows from being reclassified capriciously.
The traffic protection function described here requires that the
network element maintain some sort of flow state. The traffic
protection function MUST be designed to fail gracefully in the case
that the flow state is exhausted.
One example traffic protection algorithm 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. [NOTE (to
be removed by RFC-Editor): This ISE submission draft is approved for
publication as an RFC, the NQB draft should be held for publication
until the queue protection RFC can be referenced.]
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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). 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.
5.3. Guidance for Very Low-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 such
networks SHOULD utilize a traffic protection function that is more
tolerant of burstiness (i.e., a temporary queue). Alternatively,
operators of such networks MAY choose to disable NQB support (and
thus aggregate NQB-marked traffic with Default traffic) on these low-
speed links. For links that are less than ten percent of "typical"
path rates, it is RECOMMENDED that NQB support be disabled and for
NQB-marked traffic to thus be carried using the default PHB.
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 NQB. This is particularly true 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
mismarking of traffic.
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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).
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 NQB-marked traffic. 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"
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.
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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], most existing WiFi
implementations 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 codepoint 45 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 NQB-marked traffic
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 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 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
NQB-marked traffic.
* All known WiFi gear 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.
* NQB-compliant traffic 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 45 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 upstream network components to police
the usage of the NQB codepoint and potentially re-mark traffic
that is considered non-compliant, as is recommended in
Section 4.3.1. 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 Video Access Category
traffic (particularly in the case of traffic originating outside of
the WiFi network as mentioned above) could erode the principle of
alignment of incentives. 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 4.3.1 would apply.
Similarly, systems that utilize [RFC8325] but that are unable to
fully support the PHB requirements, SHOULD map the recommended NQB
codepoint 45 (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 NQB-marked traffic 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 marking could be abused in order to gain priority on
such links, the potential presence of traffic protection functions
along the path (which apply to the NQB marking 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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The NQB signal (DSCP) is not integrity protected and could be changed
by an on-path attacker. 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. and RFC Publisher, "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. and RFC Publisher, "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://www.ietf.org/archive/id/draft-briscoe-
docsis-q-protection-06.txt>.
[I-D.ietf-tsvwg-aqm-dualq-coupled]
De Schepper, K., Briscoe, B., and G. White, "DualQ Coupled
AQMs for Low Latency, Low Loss and Scalable Throughput
(L4S)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-aqm-dualq-coupled-25, 29 August 2022,
<https://www.ietf.org/archive/id/draft-ietf-tsvwg-aqm-
dualq-coupled-25.txt>.
[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-08, 13 December 2022,
<https://www.ietf.org/archive/id/draft-ietf-tsvwg-dscp-
considerations-08.txt>.
[I-D.ietf-tsvwg-ecn-l4s-id]
De Schepper, K. and B. Briscoe, "Explicit Congestion
Notification (ECN) Protocol for Very Low Queuing Delay
(L4S)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-ecn-l4s-id-29, 29 August 2022,
<https://www.ietf.org/archive/id/draft-ietf-tsvwg-ecn-l4s-
id-29.txt>.
[I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., De Schepper, K., Bagnulo, M., and G. White,
"Low Latency, Low Loss, Scalable Throughput (L4S) Internet
Service: Architecture", Work in Progress, Internet-Draft,
draft-ietf-tsvwg-l4s-arch-20, 29 August 2022,
<https://www.ietf.org/archive/id/draft-ietf-tsvwg-l4s-
arch-20.txt>.
[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>.
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[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>.
[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>.
[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>.
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[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>.
[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, there were no observations of DSCP
manipulation reported in which traffic was marked 45 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 NQB-marked traffic that is subjected to one of these
policies, the result would be that NQB-marked traffic 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 value
(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 codepoints
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
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Internet-Draft Non-Queue-Building PHB January 2023
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 codepoint (i.e. 5, 13, or 21) to
the NQB PHB, or re-mark such traffic as 45, 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 codepoints 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.
Authors' Addresses
Greg White
CableLabs
Email: g.white@cablelabs.com
Thomas Fossati
ARM
Email: Thomas.Fossati@arm.com
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