A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services
draft-ietf-tsvwg-nqb-07
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Greg White
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Thomas Fossati
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Transport Area Working Group G. White
Internet-Draft CableLabs
Intended status: Standards Track T. Fossati
Expires: 29 January 2022 ARM
28 July 2021
A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated
Services
draft-ietf-tsvwg-nqb-07
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 without rate policing,
making it suitable for environments where the use of either these
features may be 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
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/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 29 January 2022.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified 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 . . . . . . . . 4
3.3. Relationship to L4S . . . . . . . . . . . . . . . . . . . 6
4. DSCP Marking of NQB Traffic . . . . . . . . . . . . . . . . . 6
4.1. Non-Queue-Building Sender Requirements . . . . . . . . . 6
4.2. Aggregation of the NQB DSCP with other Diffserv PHBs . . 7
4.3. End-to-end usage and DSCP Re-marking . . . . . . . . . . 8
4.4. The NQB DSCP and Tunnels . . . . . . . . . . . . . . . . 9
5. Non-Queue-Building PHB Requirements . . . . . . . . . . . . . 9
5.1. Primary Requirements . . . . . . . . . . . . . . . . . . 10
5.2. Traffic Protection . . . . . . . . . . . . . . . . . . . 11
6. Impact on Higher Layer Protocols . . . . . . . . . . . . . . 12
7. Configuration and Management . . . . . . . . . . . . . . . . 12
8. Example Use Cases . . . . . . . . . . . . . . . . . . . . . . 12
8.1. DOCSIS Access Networks . . . . . . . . . . . . . . . . . 12
8.2. Mobile Networks . . . . . . . . . . . . . . . . . . . . . 13
8.3. WiFi Networks . . . . . . . . . . . . . . . . . . . . . . 13
8.3.1. Interoperability with Existing WiFi Networks . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
11. Security Considerations . . . . . . . . . . . . . . . . . . . 15
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . 17
Appendix A. DSCP Remarking Pathologies . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
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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, gaming, machine-to-machine applications) are application
limited flows that are distinguished from traffic flows managed by an
end-to-end congestion control algorithm.
The vast majority of packets that are 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
of the applications that are sharing the bottleneck link.
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 may 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 in order 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 queueuing 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 in order that
both classes can deliver satisfactory quality of experience for their
applications.
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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 may 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 buildup,
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 frequently send
at a high burst rate (e.g. several consecutive packets sent well in
excess of 1 Mbps) 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-
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hop-behaviors) that can be implemented to satisfy the performance
requirements of traffic so marked.
While this architecture is powerful, and can 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 (in an end-
to-end context) the performance requirements of an application
involves all of 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 due to the fact that 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.
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 rather than simply a desire for
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
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minimize any incentives for a sender to mismark its traffic, since
neither higher priority nor reserved bandwith are being offered.
These attributes eliminate many of the tradeoffs 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 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).
4. DSCP Marking of NQB Traffic
4.1. Non-Queue-Building Sender Requirements
Non-queue-building (NQB) flows are typically UDP flows that don't
seek the maximum capacity of the link (examples: online games, voice
chat, DNS lookups, 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, the equivalent of a few well-
spaced packets per RTT, even if the packets are not actually RTT-
clocked. In today's network this corresponds to an instantaneous
data rate (packet size divided by packet inter-arrival time) of no
more than about 1 Mbps (e.g. no more than one 1250 B packet every 10
ms), but there is no precise bound since it depends on the conditions
in which the application is operating.
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 flows in this document should not
be interpreted as suggesting that such flows are in any way exempt
from this responsibility.
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Applications that align with the description of NQB behavior in the
preceding paragraphs 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). 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
may be preferred to use another DSCP.
If the application's traffic exceeds more than a few packets per RTT,
or exceeds approximately 1 Mbps on an instantaneous (inter-packet)
basis, the application SHOULD NOT mark its traffic with the NQB DSCP.
In such a case, the application has to instead implement a relevant
congestion control mechanism, for example as described in Section 3.1
of [RFC8085] or [I-D.ietf-tsvwg-ecn-l4s-id].
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), and nodes that do not typically experience congestion,
treating NQB marked traffic the same as Default may 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] may not be able to provide a PDB/PHB that
meets the requirements of this document. In these cases it is
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 may 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,
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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. 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).
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.
To support NQB, networks MUST preserve a DSCP marking distinction
between NQB traffic and Default traffic when forwarding via an
interconnect from or to another network. To facilitate the default
treatment of NQB traffic in backbones and core networks discussed in
the previous section (where IP Precedence may be deployed), networks
that support NQB SHOULD remap NQB traffic (DSCP 45) to DSCP 5 prior
to interconnection, unless agreed otherwise between the
interconnecting partners. The fact that this PHB is intended for
end-to-end usage does not preclude networks from mapping the NQB DSCP
to a value other than 45 or 5 for internal usage, as long as the
appropriate NQB DSCP is restored when forwarding to another network.
Additionally, interconnecting networks are not precluded from
negotiating (via an SLA or some other agreement) a different DSCP to
use to signal NQB across the interconnect.
Furthermore, in other network environments where IP Precedence is
deployed, it is RECOMMENDED that the network operator re-mark NQB
traffic to DSCP 5 in order to ensure that it is aggregated with
Default traffic.
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In order to enable interoperability with WiFi equipment as described
in Section 8.3.1, networks SHOULD re-mark NQB traffic (e.g. DSCP 5)
to DSCP 45 prior to a customer access link, subject to the safeguards
described in that section.
Thus, this document recommends two DSCPs to designate NQB, the value
45 for use by hosts and in WiFi networks, and the value 5 for use
across network interconnections.
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 worthwhile to note again that the NQB designation and marking
is intended to convey verifiable traffic behavior, as opposed to
simply a desire for differentiated treatment. Also, 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 NQB flows, the NQB queue
provides better performance than the QB queue; and for QB flows, the
QB queue provides better performance than the NQB queue (this is
discussed further in this section and Section 11). By adhering to
these principles, there is no incentive for senders to mismark their
traffic as NQB, and further, any mismarking can be identified by the
network.
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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 a bound on
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.
NQB traffic, in aggregate, SHOULD NOT be rate limited or rate policed
separately from queue-building traffic of equivalent importance.
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. In environments where
mismarking is not a potential issue (e.g. a network where a marking
policy is enforced by other means), these requirements may not be
necessary.
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 (e.g. single-digit
milliseconds). 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.
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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 an NQB flow that isn't marked as NQB and
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 of the other flows that are sharing the NQB queue.
To prevent this situation from harming the performance of the real
NQB flows, network elements that support differentiating NQB traffic
SHOULD support a "traffic protection" function that can identify QB
flows that are mismarked as NQB, and either reclassify those flows/
packets 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. It may be advantageous for a traffic
protection function to employ hysteresis to prevent borderline flows
from being reclassified capriciously.
One example traffic protection algorithm can be found in
[I-D.briscoe-docsis-q-protection].
There are some situations where such function may not be necessary.
For example, a network element designed for use in controlled
environments (e.g. enterprise LAN) may not require a traffic
protection function. Additionally, some networks may prefer to
police the application of the NQB DSCP at the ingress edge, so that
in-network traffic protection is not needed.
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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
mis-marking of traffic.
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.
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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" may 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.
To support the NQB PHB, the mobile network SHOULD be configured to
give UEs 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 addition to the requirements provided in other sections of this
document, to support the NQB PHB, WiFi equipment 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).
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8.3.1. Interoperability with Existing WiFi Networks
While some existing WiFi equipment may 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.
In order to increase the likelihood that NQB traffic is provided a
separate queue from QB traffic in existing WiFi equipment that uses
the default mapping, the 45 code point is recommended for NQB. This
maps NQB to UP_5 which is in the "Video" Access Category. While this
DSCP to User Priority mapping enables these WiFi systems to support
the NQB PHB requirement for segregated queuing, it does not support
the remaining NQB PHB requirements in Section 5. The ramifications
of, and remedies for this are discussed further below.
Existing WiFi devices are unlikely to support a traffic protection
algorithm, so traffic mismarked as NQB is not likely to be detected
and remedied by such devices.
Furthermore, in their default configuration, existing WiFi devices
utilize EDCA parameters that result in statistical prioritization of
the "Video" Access Category above the "Best Effort" Access Category.
If left unchanged, this would violate the NQB PHB requirement for
equal prioritization, and could erode the principle of alignment of
incentives. In order to preserve the incentives principle for NQB,
WiFi systems SHOULD configure the EDCA parameters for the Video
Access Category to match those of the Best Effort Access Category.
In cases where a network operator is delivering traffic into an
unmanaged WiFi network outside of their control (e.g. a residential
ISP delivering traffic to a customer's home network), the network
operator should presume that the existing WiFi equipment does not
support the safeguards that are provided by the NQB PHB requirements,
and thus should take precautions to prevent issues. When the data
rate of the access network segment is less than the expected data
rate of the WiFi network, this is unlikely to be an issue. However,
if the access network rate exceeds the expected rate of the WiFi
network, the operator SHOULD deploy a policing function on NQB marked
traffic that minimizes the potential for negative impacts on traffic
marked Default, for example by limiting the rate of such traffic to a
set fraction of the customer's service rate, with excess traffic
either dropped or re-marked as Default.
As an additional safeguard, and to prevent the inadvertent
introduction of problematic traffic into unmanaged WiFi networks,
network equipment that is intended to deliver traffic into unmanaged
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WiFi networks (e.g. an access network gateway for a residential ISP)
MUST by default ensure that NQB traffic is marked with a DSCP that
selects the "Best Effort" Access Category. Such equipment MUST
support the ability to configure the remapping, so that (when
appropriate safeguards are in place) traffic can be delivered as NQB-
marked.
Similarly, systems that utilize [RFC8325] but that are unable to
fully support the PHB requirements, SHOULD map the recommended NQB
code point 45 (or the locally determined alternative) to UP_5 in the
"Video" Access Category.
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 Codepoints (DSCP) 5 ('0b000101', 0x05) and 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 codepoints for Non-Queue-Building behavior.
11. Security Considerations
When the NQB PHB is fully supported in bottleneck links, there is no
incentive for an application to mismark its packets as NQB (or vice
versa). If a queue-building flow were to mark its packets as NQB, it
would be unlikely to receive a benefit by doing so, and it could
experience excessive packet loss, excessive latency variation and/or
excessive out-of-order delivery (depending on the nature of the
traffic protection function). 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.
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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. This document recommends the implementation
of a traffic protection mechanism to achieve this goal, but
recognizes that other options may be more desirable in certain
situations.
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 does open up the NQB marking to
potential abuse on these WiFi links, but since these existing WiFi
networks already give one quarter of the DSCP space this same
treatment, and further they give another quarter of the DSCP space
even higher priority, the NQB DSCP does not seem to be of any greater
risk for abuse than these others.
The NQB signal is not integrity protected and could be flipped by an
on-path attacker. This might negatively affect the QoS of the
tampered flow.
12. References
12.1. 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>.
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[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>.
12.2. Informative References
[Barik] Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
Study", ITC 30, September 2018.
[Custura] Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
modification pathologies in mobile edge networks", TMA ,
2017.
[I-D.briscoe-docsis-q-protection]
Briscoe, B. and G. White, "Queue Protection to Preserve
Low Latency", Work in Progress, Internet-Draft, draft-
briscoe-docsis-q-protection-00, 8 July 2019,
<https://datatracker.ietf.org/doc/html/draft-briscoe-
docsis-q-protection-00>.
[I-D.ietf-tsvwg-aqm-dualq-coupled]
Schepper, K. D., Briscoe, B., and G. White, "DualQ Coupled
AQMs for Low Latency, Low Loss and Scalable Throughput
(L4S)", Work in Progress, Internet-Draft, draft-ietf-
tsvwg-aqm-dualq-coupled-16, 7 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
aqm-dualq-coupled-16>.
[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-00, 26 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
dscp-considerations-00>.
[I-D.ietf-tsvwg-ecn-l4s-id]
Schepper, K. D. 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-19, 26 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
ecn-l4s-id-19>.
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[I-D.ietf-tsvwg-l4s-arch]
Briscoe, B., Schepper, K. D., Bagnulo, M., and G. White,
"Low Latency, Low Loss, Scalable Throughput (L4S) Internet
Service: Architecture", Work in Progress, Internet-Draft,
draft-ietf-tsvwg-l4s-arch-10, 1 July 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
l4s-arch-10>.
[IEEE802-11]
IEEE-SA, "IEEE 802.11-2020", IEEE 802, December 2020,
<https://standards.ieee.org/standard/802_11-2020.html>.
[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(J)., 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>.
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[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>.
[SA-5G] 3GPP, "System Architecture for 5G", TS 23.501, 2019.
Appendix A. DSCP Remarking Pathologies
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 remark all traffic to
a particular (non-zero) DSCP value.
Regarding the DSCP values of 5 & 45, there were no observations of
DSCP manipulation reported in which traffic was marked 5 or 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 remarked 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.
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In the policies where the top three bits are overwritten, both NQB
values (5 & 45) would receive the same marking as would the currently
unassigned Pool 3 DSCPs 13,21,29,37,53,61, with all of these code
points getting mapped 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 set aside for Standards
Action, it is believed that they are not widely used currently, but
this may vary based on local-usage.
For the policy in which the low three bits are set to 0b000, the NQB
(45) value would be mapped 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 may 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. For this same re-marking policy, the NQB (5)
value would be mapped to CS0/default and would be indistinguishable
from CS0, LE (and the unassigned DSCPs 2,3,4,6,7). In this case, NQB
traffic is likely to be given default treatment in all subsequent
nodes, which would eliminate the ability to provide NQB treatment in
those nodes, but would be relatively harmless otherwise.
Authors' Addresses
Greg White
CableLabs
Email: g.white@cablelabs.com
Thomas Fossati
ARM
Email: Thomas.Fossati@arm.com
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