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Internet Engineering Task Force (IETF) T. Szigeti
Request for Comments: 8325 J. Henry
Category: Standards Track Cisco Systems
ISSN: 2070-1721 F. Baker
February 2018
Mapping Diffserv to IEEE 802.11
Abstract
As Internet traffic is increasingly sourced from and destined to
wireless endpoints, it is crucial that Quality of Service (QoS) be
aligned between wired and wireless networks; however, this is not
always the case by default. This document specifies a set of
mappings from Differentiated Services Code Point (DSCP) to IEEE
802.11 User Priority (UP) to reconcile the marking recommendations
offered by the IETF and the IEEE so as to maintain consistent QoS
treatment between wired and IEEE 802.11 wireless networks.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8325.
Copyright Notice
Copyright (c) 2018 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Related Work . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Interaction with RFC 7561 . . . . . . . . . . . . . . . . 4
1.3. Applicability Statement . . . . . . . . . . . . . . . . . 4
1.4. Document Organization . . . . . . . . . . . . . . . . . . 5
1.5. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.6. Terminology Used in This Document . . . . . . . . . . . . 6
2. Service Comparison and Default Interoperation of Diffserv and
IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1. Diffserv Domain Boundaries . . . . . . . . . . . . . . . 9
2.2. EDCF Queuing . . . . . . . . . . . . . . . . . . . . . . 10
2.3. Default DSCP-to-UP Mappings and Conflicts . . . . . . . . 10
2.4. Default UP-to-DSCP Mappings and Conflicts . . . . . . . . 11
3. Recommendations for Capabilities of Wireless Device Marking
and Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 13
4. Recommendations for DSCP-to-UP Mapping . . . . . . . . . . . 13
4.1. Network Control Traffic . . . . . . . . . . . . . . . . . 14
4.1.1. Network Control Protocols . . . . . . . . . . . . . . 14
4.1.2. Operations, Administration, and Maintenance (OAM) . 15
4.2. User Traffic . . . . . . . . . . . . . . . . . . . . . . 15
4.2.1. Telephony . . . . . . . . . . . . . . . . . . . . . . 15
4.2.2. Signaling . . . . . . . . . . . . . . . . . . . . . . 16
4.2.3. Multimedia Conferencing . . . . . . . . . . . . . . . 17
4.2.4. Real-Time Interactive . . . . . . . . . . . . . . . . 17
4.2.5. Multimedia Streaming . . . . . . . . . . . . . . . . 17
4.2.6. Broadcast Video . . . . . . . . . . . . . . . . . . . 18
4.2.7. Low-Latency Data . . . . . . . . . . . . . . . . . . 18
4.2.8. High-Throughput Data . . . . . . . . . . . . . . . . 18
4.2.9. Standard . . . . . . . . . . . . . . . . . . . . . . 19
4.2.10. Low-Priority Data . . . . . . . . . . . . . . . . . . 20
4.3. Summary of Recommendations for DSCP-to-UP Mapping . . . . 20
5. Recommendations for Upstream Mapping and Marking . . . . . . 21
5.1. Upstream DSCP-to-UP Mapping within the Wireless Client
Operating System . . . . . . . . . . . . . . . . . . . . 22
5.2. Upstream UP-to-DSCP Mapping at the Wireless AP . . . . . 22
5.3. Upstream DSCP-Passthrough at the Wireless AP . . . . . . 23
5.4. Upstream DSCP Marking at the Wireless AP . . . . . . . . 24
6. Overview of IEEE 802.11 QoS . . . . . . . . . . . . . . . . . 24
6.1. Distributed Coordination Function (DCF) . . . . . . . . . 25
6.1.1. Slot Time . . . . . . . . . . . . . . . . . . . . . . 25
6.1.2. Interframe Space (IFS) . . . . . . . . . . . . . . . 26
6.1.3. Contention Window (CW) . . . . . . . . . . . . . . . 26
6.2. Hybrid Coordination Function (HCF) . . . . . . . . . . . 27
6.2.1. User Priority (UP) . . . . . . . . . . . . . . . . . 27
6.2.2. Access Category (AC) . . . . . . . . . . . . . . . . 28
6.2.3. Arbitration Interframe Space (AIFS) . . . . . . . . . 29
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6.2.4. Access Category CWs . . . . . . . . . . . . . . . . . 29
6.3. IEEE 802.11u QoS Map Set . . . . . . . . . . . . . . . . 30
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
8. Security Considerations . . . . . . . . . . . . . . . . . . . 31
8.1. Security Recommendations for General QoS . . . . . . . . 31
8.2. Security Recommendations for WLAN QoS . . . . . . . . . . 32
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.1. Normative References . . . . . . . . . . . . . . . . . . 34
9.2. Informative References . . . . . . . . . . . . . . . . . 35
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
The wireless medium defined by IEEE 802.11 [IEEE.802.11-2016] has
become the preferred medium for endpoints connecting to business and
private networks. However, it presents several design challenges for
ensuring end-to-end QoS. Some of these challenges relate to the
nature of the IEEE 802.11 Radio Frequency (RF) medium itself, being a
half-duplex and shared medium, while other challenges relate to the
fact that the IEEE 802.11 standard is not administered by the same
standards body as IP networking standards. While the IEEE has
developed tools to enable QoS over wireless networks, little guidance
exists on how to maintain consistent QoS treatment between wired IP
networks and wireless IEEE 802.11 networks. The purpose of this
document is to provide such guidance.
1.1. Related Work
Several RFCs outline Diffserv QoS recommendations over IP networks,
including:
RFC 2474 Specifies the Diffserv Codepoint Field. This RFC also
details Class Selectors, as well as the Default
Forwarding (DF) PHB for best effort traffic. The Default
Forwarding PHB is referred to as the Default PHB in RFC
2474.
RFC 2475 Defines a Diffserv architecture.
RFC 3246 Specifies the Expedited Forwarding (EF) Per-Hop Behavior
(PHB).
RFC 2597 Specifies the Assured Forwarding (AF) PHB.
RFC 3662 Specifies a Lower-Effort Per-Domain Behavior (PDB).
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RFC 4594 Presents configuration guidelines for Diffserv service
classes.
RFC 5127 Presents the aggregation of Diffserv service classes.
RFC 5865 Specifies a DSCP for capacity-admitted traffic.
Note: [RFC4594] is intended to be viewed as a framework for
supporting Diffserv in any network, including wireless networks;
thus, it describes different types of traffic expected in IP networks
and provides guidance as to what DSCP marking(s) should be associated
with each traffic type. As such, this document draws heavily on
[RFC4594], as well as [RFC5127], and [RFC8100].
In turn, the relevant standard for wireless QoS is IEEE 802.11, which
is being progressively updated; at the time of writing, the current
version of which is [IEEE.802.11-2016].
1.2. Interaction with RFC 7561
There is also a recommendation from the Global System for Mobile
Communications Association (GSMA) on DSCP-to-UP Mapping for IP Packet
eXchange (IPX), specifically their Guidelines for IPX Provider
networks [GSMA-IPX_Guidelines]. These GSMA Guidelines were developed
without reference to existing IETF specifications for various
services, referenced in Section 1.1. In turn, [RFC7561] was written
based on these GSMA Guidelines, as explicitly called out in
[RFC7561], Section 4.2. Thus, [RFC7561] conflicts with the overall
Diffserv traffic-conditioning service plan, both in the services
specified and the codepoints specified for them. As such, these two
plans cannot be normalized. Rather, as discussed in [RFC2474],
Section 2, the two domains (IEEE 802.11 and GSMA) are different
Differentiated Services Domains separated by a Differentiated
Services Boundary. At that boundary, codepoints from one domain are
translated to codepoints for the other, and maybe to Default (zero)
if there is no corresponding service to translate to.
1.3. Applicability Statement
This document is applicable to the use of Differentiated Services
that interconnect with IEEE 802.11 wireless LANs (referred to as
Wi-Fi, throughout this document, for simplicity). These guidelines
are applicable whether the wireless access points (APs) are deployed
in an autonomous manner, managed by (centralized or distributed) WLAN
controllers, or some hybrid deployment option. This is because, in
all these cases, the wireless AP is the bridge between wired and
wireless media.
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This document applies to IP networks using Wi-Fi infrastructure at
the link layer. Such networks typically include wired LANs with
wireless APs at their edges; however, such networks can also include
Wi-Fi backhaul, wireless mesh solutions, or any other type of AP-to-
AP wireless network that extends the wired-network infrastructure.
1.4. Document Organization
This document is organized as follows:
Section 1 introduces the wired-to-wireless QoS challenge, references
related work, outlines the organization of the document, and
specifies both the requirements language and the terminology used in
this document.
Section 2 begins the discussion with a comparison of IETF Diffserv
QoS and Wi-Fi QoS standards and highlights discrepancies between
these that require reconciliation.
Section 3 presents the marking and mapping capabilities that wireless
APs and wireless endpoint devices are recommended to support.
Section 4 presents DSCP-to-UP mapping recommendations for each of the
[RFC4594] service classes, which are primarily applicable in the
downstream (wired-to-wireless) direction.
Section 5, in turn, considers upstream (wireless-to-wired) QoS
options, their respective merits and recommendations.
Section 6 (in the form of an Appendix) presents a brief overview of
how QoS is achieved over IEEE 802.11 wireless networks, given the
shared, half-duplex nature of the wireless medium.
Section 7 contains IANA considerations.
Section 8 presents security considerations relative to DSCP-to-UP
mapping, UP-to-DSCP mapping, and re-marking.
1.5. 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.
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1.6. Terminology Used in This Document
Key terminology used in this document includes:
AC: Access Category. A label for the common set of enhanced
distributed channel access (EDCA) parameters that are used by a
QoS station (STA) to contend for the channel in order to transmit
medium access control (MAC) service data units (MSDUs) with
certain priorities; see [IEEE.802.11-2016], Section 3.2.
AIFS: Arbitration Interframe Space. Interframe space used by QoS
stations before transmission of data and other frame types defined
by [IEEE.802.11-2016], Section 10.3.2.3.6.
AP: Access Point. An entity that contains one station (STA) and
provides access to the distribution services, via the wireless
medium (WM) for associated STAs. An AP comprises a STA and a
distribution system access function (DSAF); see
[IEEE.802.11-2016], Section 3.1.
BSS: Basic Service Set. Informally, a wireless cell; formally, a set
of stations that have successfully synchronized using the JOIN
service primitives and one STA that has used the START primitive.
Alternatively, a set of STAs that have used the START primitive
specifying matching mesh profiles where the match of the mesh
profiles has been verified via the scanning procedure. Membership
in a BSS does not imply that wireless communication with all other
members of the BSS is possible. See the definition in
[IEEE.802.11-2016], Section 3.1.
Contention Window: See CW.
CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance. A
MAC method in which carrier sensing is used, but nodes attempt to
avoid collisions by transmitting only when the channel is sensed
to be "idle". When these do transmit, nodes transmit their packet
data in its entirety.
CSMA/CD: Carrier Sense Multiple Access with Collision Detection. A
MAC method (used most notably in early Ethernet technology) for
local area networking. It uses a carrier-sensing scheme in which
a transmitting station detects collisions by sensing transmissions
from other stations while transmitting a frame. When this
collision condition is detected, the station stops transmitting
that frame, transmits a jam signal, and then waits for a random
time interval before trying to resend the frame.
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CW: Contention Window. Limits a CWMin and CWMax, from which a
random backoff is computed.
CWMax: Contention Window Maximum. The maximum value (in units of
Slot Time) that a CW can take.
CWMin: Contention Window Minimum. The minimum value that a CW can
take.
DCF: Distributed Coordinated Function. A class of coordination
function where the same coordination function logic is active in
every station (STA) in the BSS whenever the network is in
operation.
DIFS: Distributed (Coordination Function) Interframe Space. A unit
of time during which the medium has to be detected as idle before
a station should attempt to send frames, as per
[IEEE.802.11-2016], Section 10.3.2.3.5.
DSCP: Differentiated Service Code Point [RFC2474] and [RFC2475].
The DSCP is carried in the first 6 bits of the IPv4 Type of
Service (TOS) field and the IPv6 Traffic Class field (the
remaining 2 bits are used for IP Explicit Congestion Notification
(ECN) [RFC3168]).
EIFS: Extended Interframe Space. A unit of time that a station has
to defer before transmitting a frame if the previous frame
contained an error, as per [IEEE.802.11-2016], Section 10.3.2.3.7.
HCF: Hybrid Coordination Function. A coordination function that
combines and enhances aspects of the contention-based and
contention-free access methods to provide QoS stations (STAs) with
prioritized and parameterized QoS access to the WM, while
continuing to support non-QoS STAs for best-effort transfer; see
[IEEE.802.11-2016], Section 3.1.
IFS: Interframe Space. Period of silence between transmissions over
IEEE 802.11 networks. [IEEE.802.11-2016] describes several types
of Interframe Spaces.
Random Backoff Timer: A pseudorandom integer period of time (in
units of Slot Time) over the interval (0,CW), where CWmin is less
than or equal to CW, which in turn is less than or equal to CWMax.
Stations desiring to initiate transfer of data frames and/or
management frames using the DCF shall invoke the carrier sense
mechanism to determine the busy-or-idle state of the medium. If
the medium is busy, the STA shall defer until the medium is
determined to be idle without interruption for a period of time
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equal to DIFS when the last frame detected on the medium was
received correctly or after the medium is determined to be idle
without interruption for a period of time equal to EIFS when the
last frame detected on the medium was not received correctly.
After this DIFS or EIFS medium idle time, the STA shall then
generate a random backoff period for an additional deferral time
before transmitting. See [IEEE.802.11-2016], Section 10.3.3.
RF: Radio Frequency.
SIFS: Short Interframe Space. An IFS used before transmission of
specific frames as defined in [IEEE.802.11-2016],
Section 10.3.2.3.3.
Slot Time: A unit of time used to count time intervals in IEEE
802.11 networks; it is defined in [IEEE.802.11-2016],
Section 10.3.2.13.
Trust: From a QoS-perspective, "trust" refers to the accepting of
the QoS markings of a packet by a network device. Trust is
typically extended at Layer 3 (by accepting the DSCP), but may
also be extended at lower layers, such as at Layer 2 by accepting
UP markings. For example, if an AP is configured to trust DSCP
markings and it receives a packet marked EF, then it would treat
the packet with the Expedite Forwarding PHB and propagate the EF
marking value (DSCP 46) as it transmits the packet.
Alternatively, if a network device is configured to operate in an
untrusted manner, then it would re-mark packets as these entered
the device, typically to DF (or to a different marking value at
the network administrator's preference). Note: The terms
"trusted" and "untrusted" are used extensively in [RFC4594].
UP: User Priority. A value associated with an MSDU that indicates
how the MSDU is to be handled. The UP is assigned to an MSDU in
the layers above the MAC; see [IEEE.802.11-2016], Section 3.1.
The UP defines a level of priority for the associated frame, on a
scale of 0 to 7.
Wi-Fi: An interoperability certification defined by the Wi-Fi
Alliance. However, this term is commonly used, including in the
present document, to be the equivalent of IEEE 802.11.
Wireless: In the context of this document, "wireless" refers to the
media defined in IEEE 802.11 [IEEE.802.11-2016], and not 3G/4G LTE
or any other radio telecommunications specification.
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2. Service Comparison and Default Interoperation of Diffserv and
IEEE 802.11
(Section 6 provides a brief overview of IEEE 802.11 QoS.)
The following comparisons between IEEE 802.11 and Diffserv services
should be noted:
[IEEE.802.11-2016] does not support an EF PHB service [RFC3246],
as it is not possible to assure that a given access category will
be serviced with strict priority over another (due to the random
element within the contention process)
[IEEE.802.11-2016] does not support an AF PHB service [RFC2597],
again because it is not possible to assure that a given access
category will be serviced with a minimum amount of assured
bandwidth (due to the non-deterministic nature of the contention
process)
[IEEE.802.11-2016] loosely supports a Default PHB ([RFC2474]) via
the Best Effort Access Category (AC_BE)
[IEEE.802.11-2016] loosely supports a Lower Effort PDB service
([RFC3662]) via the Background Access Category (AC_BK)
As such, these high-level considerations should be kept in mind when
mapping from Diffserv to [IEEE.802.11-2016] (and vice versa);
however, APs may or may not always be positioned at Diffserv domain
boundaries, as will be discussed next.
2.1. Diffserv Domain Boundaries
It is important to recognize that the wired-to-wireless edge may or
may not function as an edge of a Diffserv domain or a domain
boundary.
In most commonly deployed WLAN models, the wireless AP represents not
only the edge of the Diffserv domain, but also the edge of the
network infrastructure itself. As such, only client endpoint devices
(and no network infrastructure devices) are downstream from the
access points in these deployment models. Note: security
considerations and recommendations for hardening such Wi-Fi-at-the-
edge deployment models are detailed in Section 8; these
recommendations include mapping network control protocols (which are
not used downstream from the AP in this deployment model) to UP 0.
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Alternatively, in other deployment models, such as Wi-Fi backhaul,
wireless mesh infrastructures, wireless AP-to-AP deployments, or in
cases where a Wi-Fi link connects to a device providing service via
another technology (e.g., Wi-Fi to Bluetooth or Zigbee router), the
wireless AP extends the network infrastructure and thus, typically,
the Diffserv domain. In such deployments, both client devices and
infrastructure devices may be expected downstream from the APs, and,
as such, network control protocols are RECOMMENDED to be mapped to UP
7 in this deployment model, as is discussed in Section 4.1.1.
Thus, as can be seen from these two examples, the QoS treatment of
packets at the AP will depend on the position of the AP in the
network infrastructure and on the WLAN deployment model.
However, regardless of whether or not the AP is at the Diffserv
boundary, marking-specific incompatibilities exist from Diffserv to
802.11 (and vice versa) that must be reconciled, as will be discussed
next.
2.2. EDCF Queuing
[IEEE.802.11-2016] displays a reference implementation queuing model
in Figure 10-24, which depicts four transmit queues, one per access
category.
However, in practical implementations, it is common for WLAN network
equipment vendors to implement dedicated transmit queues on a per-UP
(versus a per-AC) basis, which are then dequeued into their
associated AC in a preferred (or even in a strict priority manner).
For example, it is common for vendors to dequeue UP 5 ahead of UP 4
to the hardware performing the EDCA function (EDCAF) for the Video
Access Category (AC_VI).
Some of the recommendations made in Section 4 make reference to this
common implementation model of queuing per UP.
2.3. Default DSCP-to-UP Mappings and Conflicts
While no explicit guidance is offered in mapping (6-Bit) Layer 3 DSCP
values to (3-Bit) Layer 2 markings (such as IEEE 802.1D, 802.1p or
802.11e), a common practice in the networking industry is to map
these by what we will refer to as "default DSCP-to-UP mapping" (for
lack of a better term), wherein the three Most Significant Bits
(MSBs) of the DSCP are used as the corresponding L2 markings.
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Note: There are mappings provided in [IEEE.802.11-2016], Annex V
Tables V-1 and V2, but it bears mentioning that these mappings are
provided as examples (as opposed to explicit recommendations).
Furthermore, some of these mappings do not align with the intent and
recommendations expressed in [RFC4594], as will be discussed in this
and the following section (Section 2.4).
However, when this default DSCP-to-UP mapping method is applied to
packets marked per recommendations in [RFC4594] and destined to
802.11 WLAN clients, it will yield a number of inconsistent QoS
mappings, specifically:
o Voice (EF-101110) will be mapped to UP 5 (101), and treated in the
Video Access Category (AC_VI) rather than the Voice Access
Category (AC_VO), for which it is intended
o Multimedia Streaming (AF3-011xx0) will be mapped to UP 3 (011) and
treated in the Best Effort Access Category (AC_BE) rather than the
Video Access Category (AC_VI), for which it is intended
o Broadcast Video (CS3-011000) will be mapped to UP 3 (011) and
treated in the Best Effort Access Category (AC_BE) rather than the
Video Access Category (AC_VI), for which it is intended
o OAM traffic (CS2-010000) will be mapped to UP 2 (010) and treated
in the Background Access Category (AC_BK), which is not the intent
expressed in [RFC4594] for this service class
It should also be noted that while [IEEE.802.11-2016] defines an
intended use for each access category through the AC naming
convention (for example, UP 6 and UP 7 belong to AC_VO, the Voice
Access Category), [IEEE.802.11-2016] does not:
o define how upper-layer markings (such as DSCP) should map to UPs
(and, hence, to ACs)
o define how UPs should translate to other mediums' Layer 2 QoS
markings
o strictly restrict each access category to applications reflected
in the AC name
2.4. Default UP-to-DSCP Mappings and Conflicts
In the opposite direction of flow (the upstream direction, that is,
from wireless-to-wired), many APs use what we will refer to as
"default UP-to-DSCP mapping" (for lack of a better term), wherein
DSCP values are derived from UP values by multiplying the UP values
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by 8 (i.e., shifting the three UP bits to the left and adding three
additional zeros to generate a DSCP value). This derived DSCP value
is then used for QoS treatment between the wireless AP and the
nearest classification and marking policy enforcement point (which
may be the centralized wireless LAN controller, relatively deep
within the network). Alternatively, in the case where there is no
other classification and marking policy enforcement point, then this
derived DSCP value will be used on the remainder of the Internet
path.
It goes without saying that when six bits of marking granularity are
derived from three, then information is lost in translation.
Servicing differentiation cannot be made for 12 classes of traffic
(as recommended in [RFC4594]), but for only eight (with one of these
classes being reserved for future use (i.e., UP 7, which maps to DSCP
CS7).
Such default upstream mapping can also yield several inconsistencies
with [RFC4594], including:
o Mapping UP 6 (which would include Voice or Telephony traffic, see
[RFC4594]) to CS6, which [RFC4594] recommends for Network Control
o Mapping UP 4 (which would include Multimedia Conferencing and/or
Real-Time Interactive traffic, see [RFC4594]) to CS4, thus losing
the ability to differentiate between these two distinct service
classes, as recommended in [RFC4594], Sections 4.3 and 4.4
o Mapping UP 3 (which would include Multimedia Streaming and/or
Broadcast Video traffic, see [RFC4594]) to CS3, thus losing the
ability to differentiate between these two distinct service
classes, as recommended in [RFC4594], Sections 4.5 and 4.6
o Mapping UP 2 (which would include Low-Latency Data and/or OAM
traffic, see [RFC4594]) to CS2, thus losing the ability to
differentiate between these two distinct service classes, as
recommended in [RFC4594], Sections 4.7 and 3.3, and possibly
overwhelming the queues provisioned for OAM (which is typically
lower in capacity (being Network Control Traffic), as compared to
Low-Latency Data queues (being user traffic))
o Mapping UP 1 (which would include High-Throughput Data and/or Low-
Priority Data traffic, see [RFC4594]) to CS1, thus losing the
ability to differentiate between these two distinct service
classes, as recommended in [RFC4594], Sections 4.8 and 4.10, and
causing legitimate business-relevant High-Throughput Data to
receive a [RFC3662] Lower-Effort PDB, for which it is not intended
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The following sections address these limitations and concerns in
order to reconcile [RFC4594] and [IEEE.802.11-2016]. First
downstream (wired-to-wireless) DSCP-to-UP mappings will be aligned
and then upstream (wireless-to-wired) models will be addressed.
3. Recommendations for Capabilities of Wireless Device Marking and
Mapping
This document assumes and RECOMMENDS that all wireless APs (as the
interconnects between wired-and-wireless networks) support the
ability to:
o mark DSCP, per Diffserv standards
o mark UP, per the [IEEE.802.11-2016] standard
o support fully configurable mappings between DSCP and UP
o process DSCP markings set by wireless endpoint devices
This document further assumes and RECOMMENDS that all wireless
endpoint devices support the ability to:
o mark DSCP, per Diffserv standards
o mark UP, per the [IEEE.802.11-2016] standard
o support fully configurable mappings between DSCP (set by
applications in software) and UP (set by the operating system and/
or wireless network interface hardware drivers)
Having made the assumptions and recommendations above, it bears
mentioning that, while the mappings presented in this document are
RECOMMENDED to replace the current common default practices (as
discussed in Sections 2.3 and 2.4), these mapping recommendations are
not expected to fit every last deployment model; as such, they MAY be
overridden by network administrators, as needed.
4. Recommendations for DSCP-to-UP Mapping
The following section specifies downstream (wired-to-wireless)
mappings between [RFC4594], "Configuration Guidelines for Diffserv
Service Classes" and [IEEE.802.11-2016]. As such, this section draws
heavily from [RFC4594], including service class definitions and
recommendations.
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This section assumes [IEEE.802.11-2016] wireless APs and/or WLAN
controllers that support customizable, non-default DSCP-to-UP mapping
schemes.
This section also assumes that [IEEE.802.11-2016] APs and endpoint
devices differentiate UP markings with corresponding queuing and
dequeuing treatments, as described in Section 2.2.
4.1. Network Control Traffic
Network Control Traffic is defined as packet flows that are essential
for stable operation of the administered network [RFC4594],
Section 3. Network Control Traffic is different from user
application control (signaling) that may be generated by some
applications or services. Network Control Traffic MAY be split into
two service classes:
o Network Control, and
o Operations, Administration, and Maintenance (OAM)
4.1.1. Network Control Protocols
The Network Control service class is used for transmitting packets
between network devices (e.g., routers) that require control
(routing) information to be exchanged between nodes within the
administrative domain, as well as across a peering point between
different administrative domains.
[RFC4594], Section 3.2, recommends that Network Control Traffic be
marked CS6 DSCP. Additionally, as stated in [RFC4594], Section 3.1:
"CS7 DSCP value SHOULD be reserved for future use, potentially for
future routing or control protocols."
By default (as described in Section 2.4), packets marked DSCP CS7
will be mapped to UP 7 and serviced within the Voice Access Category
(AC_VO). This represents the RECOMMENDED mapping for CS7, that is,
packets marked to CS7 DSCP are RECOMMENDED to be mapped to UP 7.
However, by default (as described in Section 2.4), packets marked
DSCP CS6 will be mapped to UP 6 and serviced within the Voice Access
Category (AC_VO); such mapping and servicing is a contradiction to
the intent expressed in [RFC4594], Section 3.2. As such, it is
RECOMMENDED to map Network Control Traffic marked CS6 to UP 7 (per
[IEEE.802.11-2016], Section 10.2.4.2, Table 10-1), thereby admitting
it to the Voice Access Category (AC_VO), albeit with a marking
distinguishing it from (data-plane) voice traffic.
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It should be noted that encapsulated routing protocols for
encapsulated or overlay networks (e.g., VPN, Network Virtualization
Overlays, etc.) are not Network Control Traffic for any physical
network at the AP; hence, they SHOULD NOT be marked with CS6 in the
first place.
Additionally, and as previously noted, the Security Considerations
section (Section 8) contains additional recommendations for hardening
Wi-Fi-at-the-edge deployment models, where, for example, network
control protocols are not expected to be sent nor received between
APs and client endpoint devices that are downstream.
4.1.2. Operations, Administration, and Maintenance (OAM)
The OAM (Operations, Administration, and Maintenance) service class
is recommended for OAM&P (Operations, Administration, and Maintenance
and Provisioning). The OAM service class can include network
management protocols, such as SNMP, Secure Shell (SSH), TFTP, Syslog,
etc., as well as network services, such as NTP, DNS, DHCP, etc.
[RFC4594], Section 3.3, recommends that OAM traffic be marked CS2
DSCP.
By default (as described in Section 2.3), packets marked DSCP CS2
will be mapped to UP 2 and serviced with the Background Access
Category (AC_BK). Such servicing is a contradiction to the intent
expressed in [RFC4594], Section 3.3. As such, it is RECOMMENDED that
a non-default mapping be applied to OAM traffic, such that CS2 DSCP
is mapped to UP 0, thereby admitting it to the Best Effort Access
Category (AC_BE).
4.2. User Traffic
User traffic is defined as packet flows between different users or
subscribers. It is the traffic that is sent to or from end-terminals
and that supports a very wide variety of applications and services
[RFC4594], Section 4.
Network administrators can categorize their applications according to
the type of behavior that they require and MAY choose to support all
or a subset of the defined service classes.
4.2.1. Telephony
The Telephony service class is recommended for applications that
require real-time, very low delay, very low jitter, and very low
packet loss for relatively constant-rate traffic sources (inelastic
traffic sources). This service class SHOULD be used for IP telephony
service. The fundamental service offered to traffic in the Telephony
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service class is minimum jitter, delay, and packet loss service up to
a specified upper bound. [RFC4594], Section 4.1, recommends that
Telephony traffic be marked EF DSCP.
Traffic marked to DSCP EF will map by default (as described in
Section 2.3) to UP 5 and, thus, to the Video Access Category (AC_VI)
rather than to the Voice Access Category (AC_VO), for which it is
intended. Therefore, a non-default DSCP-to-UP mapping is
RECOMMENDED, such that EF DSCP is mapped to UP 6, thereby admitting
it into the Voice Access Category (AC_VO).
Similarly, the VOICE-ADMIT DSCP (44 decimal / 101100 binary)
described in [RFC5865] is RECOMMENDED to be mapped to UP 6, thereby
admitting it also into the Voice Access Category (AC_VO).
4.2.2. Signaling
The Signaling service class is recommended for delay-sensitive
client-server (e.g., traditional telephony) and peer-to-peer
application signaling. Telephony signaling includes signaling
between 1) IP phone and soft-switch, 2) soft-client and soft-switch,
and 3) media gateway and soft-switch as well as peer-to-peer using
various protocols. This service class is intended to be used for
control of sessions and applications. [RFC4594], Section 4.2,
recommends that Signaling traffic be marked CS5 DSCP.
While Signaling is recommended to receive a superior level of service
relative to the default class (i.e., AC_BE), it does not require the
highest level of service (i.e., AC_VO). This leaves only the Video
Access Category (AC_VI), which it will map to by default (as
described in Section 2.3). Therefore, it is RECOMMENDED to map
Signaling traffic marked CS5 DSCP to UP 5, thereby admitting it to
the Video Access Category (AC_VI).
Note: Signaling traffic is not control-plane traffic from the
perspective of the network (but rather is data-plane traffic); as
such, it does not merit provisioning in the Network Control service
class (marked CS6 and mapped to UP 6). However, Signaling traffic is
control-plane traffic from the perspective of the voice/video
telephony overlay-infrastructure. As such, Signaling should be
treated with preferential servicing versus other data-plane flows.
This may be achieved in common WLAN deployments by mapping Signaling
traffic marked CS5 to UP 5. On APs supporting per-UP EDCAF queuing
logic (as described in Section 2.2), this will result in preferential
treatment for Signaling traffic versus other video flows in the same
access category (AC_VI), which are marked to UP 4, as well as
preferred treatment over flows in the Best Effort (AC_BE) and
Background (AC_BK) Access Categories.
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4.2.3. Multimedia Conferencing
The Multimedia Conferencing service class is recommended for
applications that require real-time service for rate-adaptive
traffic. [RFC4594], Section 4.3, recommends Multimedia Conferencing
traffic be marked AF4x (that is, AF41, AF42, and AF43, according to
the rules defined in [RFC2475]).
The primary media type typically carried within the Multimedia
Conferencing service class is video; as such, it is RECOMMENDED to
map this class into the Video Access Category (AC_VI), which it does
by default (as described in Section 2.3). Specifically, it is
RECOMMENDED to map AF41, AF42, and AF43 to UP 4, thereby admitting
Multimedia Conferencing into the Video Access Category (AC_VI).
4.2.4. Real-Time Interactive
The Real-Time Interactive service class is recommended for
applications that require low loss and jitter and very low delay for
variable-rate inelastic traffic sources. Such applications may
include inelastic video-conferencing applications, but may also
include gaming applications (as pointed out in [RFC4594], Sections
2.1 through 2.3 and Section 4.4). [RFC4594], Section 4.4, recommends
Real-Time Interactive traffic be marked CS4 DSCP.
The primary media type typically carried within the Real-Time
Interactive service class is video; as such, it is RECOMMENDED to map
this class into the Video Access Category (AC_VI), which it does by
default (as described in Section 2.3). Specifically, it is
RECOMMENDED to map CS4 to UP 4, thereby admitting Real-Time
Interactive traffic into the Video Access Category (AC_VI).
4.2.5. Multimedia Streaming
The Multimedia Streaming service class is recommended for
applications that require near-real-time packet forwarding of
variable-rate elastic traffic sources. Typically, these flows are
unidirectional. [RFC4594], Section 4.5, recommends Multimedia
Streaming traffic be marked AF3x (that is, AF31, AF32, and AF33,
according to the rules defined in [RFC2475]).
The primary media type typically carried within the Multimedia
Streaming service class is video; as such, it is RECOMMENDED to map
this class into the Video Access Category (AC_VI), which it will by
default (as described in Section 2.3). Specifically, it is
RECOMMENDED to map AF31, AF32, and AF33 to UP 4, thereby admitting
Multimedia Streaming into the Video Access Category (AC_VI).
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4.2.6. Broadcast Video
The Broadcast Video service class is recommended for applications
that require near-real-time packet forwarding with very low packet
loss of constant rate and variable-rate inelastic traffic sources.
Typically these flows are unidirectional. [RFC4594] Section 4.6
recommends Broadcast Video traffic be marked CS3 DSCP.
As directly implied by the name, the primary media type typically
carried within the Broadcast Video service class is video; as such,
it is RECOMMENDED to map this class into the Video Access Category
(AC_VI); however, by default (as described in Section 2.3), this
service class will map to UP 3 and, thus, the Best Effort Access
Category (AC_BE). Therefore, a non-default mapping is RECOMMENDED,
such that CS4 maps to UP 4, thereby admitting Broadcast Video into
the Video Access Category (AC_VI).
4.2.7. Low-Latency Data
The Low-Latency Data service class is recommended for elastic and
time-sensitive data applications, often of a transactional nature,
where a user is waiting for a response via the network in order to
continue with a task at hand. As such, these flows are considered
foreground traffic, with delays or drops to such traffic directly
impacting user productivity. [RFC4594], Section 4.7, recommends
Low-Latency Data be marked AF2x (that is, AF21, AF22, and AF23,
according to the rules defined in [RFC2475]).
By default (as described in Section 2.3), Low-Latency Data will map
to UP 2 and, thus, to the Background Access Category (AC_BK), which
is contrary to the intent expressed in [RFC4594].
Mapping Low-Latency Data to UP 3 may allow targeted traffic to
receive a superior level of service via per-UP transmit queues
servicing the EDCAF hardware for the Best Effort Access Category
(AC_BE), as described in Section 2.2. Therefore it is RECOMMENDED to
map Low-Latency Data traffic marked AF2x DSCP to UP 3, thereby
admitting it to the Best Effort Access Category (AC_BE).
4.2.8. High-Throughput Data
The High-Throughput Data service class is recommended for elastic
applications that require timely packet forwarding of variable-rate
traffic sources and, more specifically, is configured to provide
efficient, yet constrained (when necessary) throughput for TCP
longer-lived flows. These flows are typically not user interactive.
According to [RFC4594], Section 4.8, it can be assumed that this
class will consume any available bandwidth and that packets
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traversing congested links may experience higher queuing delays or
packet loss. It is also assumed that this traffic is elastic and
responds dynamically to packet loss. [RFC4594], Section 4.8,
recommends High-Throughput Data be marked AF1x (that is, AF11, AF12,
and AF13, according to the rules defined in [RFC2475]).
By default (as described in Section 2.3), High-Throughput Data will
map to UP 1 and, thus, to the Background Access Category (AC_BK),
which is contrary to the intent expressed in [RFC4594].
Unfortunately, there really is no corresponding fit for the High-
Throughput Data service class within the constrained 4 Access
Category [IEEE.802.11-2016] model. If the High-Throughput Data
service class is assigned to the Best Effort Access Category (AC_BE),
then it would contend with Low-Latency Data (while [RFC4594]
recommends a distinction in servicing between these service classes)
as well as with the default service class; alternatively, if it is
assigned to the Background Access Category (AC_BK), then it would
receive a less-then-best-effort service and contend with Low-Priority
Data (as discussed in Section 4.2.10).
As such, since there is no directly corresponding fit for the High-
Throughout Data service class within the [IEEE.802.11-2016] model, it
is generally RECOMMENDED to map High-Throughput Data to UP 0, thereby
admitting it to the Best Effort Access Category (AC_BE).
4.2.9. Standard
The Standard service class is recommended for traffic that has not
been classified into one of the other supported forwarding service
classes in the Diffserv network domain. This service class provides
the Internet's "best-effort" forwarding behavior. [RFC4594],
Section 4.9, states that the "Standard service class MUST use the
Default Forwarding (DF) PHB".
The Standard service class loosely corresponds to the
[IEEE.802.11-2016] Best Effort Access Category (AC_BE); therefore, it
is RECOMMENDED to map Standard service class traffic marked DF DSCP
to UP 0, thereby admitting it to the Best Effort Access Category
(AC_BE). This happens to correspond to the default mapping (as
described in Section 2.3).
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4.2.10. Low-Priority Data
The Low-Priority Data service class serves applications that the user
is willing to accept without service assurances. This service class
is specified in [RFC3662] and [LE-PHB].
[RFC3662] and [RFC4594] both recommend Low-Priority Data be marked
CS1 DSCP.
Note: This marking recommendation may change in the future, as
[LE-PHB] defines a Lower Effort (LE) PHB for Low-Priority Data
traffic and recommends an additional DSCP for this traffic.
The Low-Priority Data service class loosely corresponds to the
[IEEE.802.11-2016] Background Access Category (AC_BK); therefore, it
is RECOMMENDED to map Low-Priority Data traffic marked CS1 DSCP to UP
1, thereby admitting it to the Background Access Category (AC_BK).
This happens to correspond to the default mapping (as described in
Section 2.3).
4.3. Summary of Recommendations for DSCP-to-UP Mapping
Figure 1 summarizes the [RFC4594] DSCP marking recommendations mapped
to [IEEE.802.11-2016] UP and Access Categories applied in the
downstream direction (i.e., from wired-to-wireless networks).
+-------------------------------------------------------------------+
| IETF Diffserv | PHB |Reference | IEEE 802.11 |
| Service Class | | RFC |User Priority| Access Category |
|===============+======+==========+=============+====================|
| | | | 7 | AC_VO (Voice) |
|Network Control| CS7 | RFC 2474 | OR |
|(reserved for | | | 0 | AC_BE (Best Effort)|
| future use) | | |See Security Considerations-Sec.8 |
+---------------+------+----------+-------------+--------------------+
| | | | 7 | AC_VO (Voice) |
|Network Control| CS6 | RFC 2474 | OR |
| | | | 0 | AC_BE (Best Effort)|
| | | | See Security Considerations |
+---------------+------+----------+-------------+--------------------+
| Telephony | EF | RFC 3246 | 6 | AC_VO (Voice) |
+---------------+------+----------+-------------+--------------------+
| VOICE-ADMIT | VA | RFC 5865 | 6 | AC_VO (Voice) |
| | | | | |
+---------------+------+----------+-------------+--------------------+
| Signaling | CS5 | RFC 2474 | 5 | AC_VI (Video) |
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+---------------+------+----------+-------------+--------------------+
| Multimedia | AF41 | | | |
| Conferencing | AF42 | RFC 2597 | 4 | AC_VI (Video) |
| | AF43 | | | |
+---------------+------+----------+-------------+--------------------+
| Real-Time | CS4 | RFC 2474 | 4 | AC_VI (Video) |
| Interactive | | | | |
+---------------+------+----------+-------------+--------------------+
| Multimedia | AF31 | | | |
| Streaming | AF32 | RFC 2597 | 4 | AC_VI (Video) |
| | AF33 | | | |
+---------------+------+----------+-------------+--------------------+
|Broadcast Video| CS3 | RFC 2474 | 4 | AC_VI (Video) |
+---------------+------+----------+-------------+--------------------+
| Low- | AF21 | | | |
| Latency | AF22 | RFC 2597 | 3 | AC_BE (Best Effort)|
| Data | AF23 | | | |
+---------------+------+----------+-------------+--------------------+
| OAM | CS2 | RFC 2474 | 0 | AC_BE (Best Effort)|
+---------------+------+----------+-------------+--------------------+
| High- | AF11 | | | |
| Throughput | AF12 | RFC 2597 | 0 | AC_BE (Best Effort)|
| Data | AF13 | | | |
+---------------+------+----------+-------------+--------------------+
| Standard | DF | RFC 2474 | 0 | AC_BE (Best Effort)|
+---------------+------+----------+-------------+--------------------+
| Low-Priority | CS1 | RFC 3662 | 1 | AC_BK (Background) |
| Data | | | | |
+--------------------------------------------------------------------+
Note: All unused codepoints are RECOMMENDED to be mapped to UP 0
(See Security Considerations below)
Figure 1: Summary of Mapping Recommendations from Downstream
DSCP to IEEE 802.11 UP and AC
5. Recommendations for Upstream Mapping and Marking
In the upstream direction (i.e., wireless-to-wired), there are three
types of mapping that may be implemented:
o DSCP-to-UP mapping within the wireless client operating system,
and
o UP-to-DSCP mapping at the wireless AP, or
o DSCP-Passthrough at the wireless AP (effectively a 1:1 DSCP-to-
DSCP mapping)
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As an alternative to the latter two options, the network
administrator MAY choose to use the wireless-to-wired edge as a
Diffserv boundary and explicitly set (or reset) DSCP markings
according to administrative policy, thus making the wireless edge a
Diffserv policy enforcement point; this approach is RECOMMENDED
whenever the APs support the required classification and marking
capabilities.
Each of these options will now be considered.
5.1. Upstream DSCP-to-UP Mapping within the Wireless Client Operating
System
Some operating systems on wireless client devices utilize a similar
default DSCP-to-UP mapping scheme as that described in Section 2.3.
As such, this can lead to the same conflicts as described in that
section, but in the upstream direction.
Therefore, to improve on these default mappings, and to achieve
parity and consistency with downstream QoS, it is RECOMMENDED that
wireless client operating systems instead utilize the same DSCP-to-UP
mapping recommendations presented in Section 4. Note that it is
explicitly stated that packets requesting a marking of CS6 or CS7
DSCP SHOULD be mapped to UP 0 (and not to UP 7). Furthermore, in
such cases, the wireless client operating system SHOULD re-mark such
packets to DSCP 0. This is because CS6 and CS7 DSCP, as well as UP 7
markings, are intended for network control protocols, and these
SHOULD NOT be sourced from wireless client endpoint devices. This
recommendation is detailed in the Security Considerations section
(Section 8).
5.2. Upstream UP-to-DSCP Mapping at the Wireless AP
UP-to-DSCP mapping generates a DSCP value for the IP packet (either
an unencapsulated IP packet or an IP packet encapsulated within a
tunneling protocol such as Control and Provisioning of Wireless
Access Points (CAPWAP) -- and destined towards a wireless LAN
controller for decapsulation and forwarding) from the Layer 2
[IEEE.802.11-2016] UP marking. This is typically done in the manner
described in Section 2.4.
It should be noted that any explicit re-marking policy to be
performed on such a packet generally takes place at the nearest
classification and marking policy enforcement point, which may be:
o At the wireless AP, and/or
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o At the wired network switch port, and/or
o At the wireless LAN controller
Note: Multiple classification and marking policy enforcement points
may exist, as some devices have the capability to re-mark at only
Layer 2 or Layer 3, while other devices can re-mark at either/both
layers.
As such, UP-to-DSCP mapping allows for wireless L2 markings to affect
the QoS treatment of a packet over the wired IP network (that is,
until the packet reaches the nearest classification and marking
policy enforcement point).
It should be further noted that nowhere in the [IEEE.802.11-2016]
specification is there an intent expressed for UP markings to be used
to influence QoS treatment over wired IP networks. Furthermore,
[RFC2474], [RFC2475], and [RFC8100] all allow for the host to set
DSCP markings for end-to-end QoS treatment over IP networks.
Therefore, wireless APs MUST NOT leverage Layer 2 [IEEE.802.11-2016]
UP markings as set by wireless hosts and subsequently perform a
UP-to-DSCP mapping in the upstream direction. But rather, if
wireless host markings are to be leveraged (as per business
requirements, technical constraints, and administrative policies),
then it is RECOMMENDED to pass through the Layer 3 DSCP markings set
by these wireless hosts instead, as is discussed in the next section.
5.3. Upstream DSCP-Passthrough at the Wireless AP
It is generally NOT RECOMMENDED to pass through DSCP markings from
unauthenticated and unauthorized devices, as these are typically
considered untrusted sources.
When business requirements and/or technical constraints and/or
administrative policies require QoS markings to be passed through at
the wireless edge, then it is RECOMMENDED to pass through Layer 3
DSCP markings (over Layer 2 [IEEE.802.11-2016] UP markings) in the
upstream direction, with the exception of CS6 and CS7 (as will be
discussed further), for the following reasons:
o [RFC2474], [RFC2475], and [RFC8100] all allow for hosts to set
DSCP markings to achieve an end-to-end differentiated service
o [IEEE.802.11-2016] does not specify that UP markings are to be
used to affect QoS treatment over wired IP networks
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o Most present wireless device operating systems generate UP values
by the same method as described in Section 2.3 (i.e., by using the
3 MSBs of the encapsulated 6-bit DSCP); then, at the AP, these
3-bit markings are converted back into DSCP values, typically in
the default manner described in Section 2.4; as such, information
is lost in the translation from a 6-bit marking to a 3-bit marking
(which is then subsequently translated back to a 6-bit marking);
passing through the original (encapsulated) DSCP marking prevents
such loss of information
o A practical implementation benefit is also realized by passing
through the DSCP set by wireless client devices, as enabling
applications to mark DSCP is much more prevalent and accessible to
programmers of applications running on wireless device platforms,
vis-a-vis trying to explicitly set UP values, which requires
special hooks into the wireless device operating system and/or
hardware device drivers, many of which do not support such
functionality
CS6 and CS7 are exceptions to this passthrough recommendation because
wireless hosts SHOULD NOT use them (see Section 5.1) and traffic with
those two markings poses a threat to operation of the wired network
(see Section 8.2). CS6 and CS7 SHOULD NOT be passed through to the
wired network in the upstream direction unless the AP has been
specifically configured to do that by a network administrator or
operator.
5.4. Upstream DSCP Marking at the Wireless AP
An alternative option to mapping is for the administrator to treat
the wireless edge as the edge of the Diffserv domain and explicitly
set (or reset) DSCP markings in the upstream direction according to
administrative policy. This option is RECOMMENDED over mapping, as
this typically is the most secure solution because the network
administrator directly enforces the Diffserv policy across the IP
network (versus an application developer and/or the developer of the
operating system of the wireless endpoint device, who may be
functioning completely independently of the network administrator).
6. Overview of IEEE 802.11 QoS
QoS is enabled on wireless networks by means of the Hybrid
Coordination Function (HCF). To give better context to the
enhancements in HCF that enable QoS, it may be helpful to begin with
a review of the original Distributed Coordination Function (DCF).
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6.1. Distributed Coordination Function (DCF)
As has been noted, the Wi-Fi medium is a shared medium, with each
station -- including the wireless AP -- contending for the medium on
equal terms. As such, it shares the same challenge as any other
shared medium in requiring a mechanism to prevent (or avoid)
collisions, which can occur when two (or more) stations attempt
simultaneous transmission.
The IEEE Ethernet Working Group solved this challenge by implementing
a Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
mechanism that could detect collisions over the shared physical cable
(as collisions could be detected as reflected energy pulses over the
physical wire). Once a collision was detected, then a predefined set
of rules was invoked that required stations to back off and wait
random periods of time before reattempting transmission. While CSMA/
CD improved the usage of Ethernet as a shared medium, it should be
noted the ultimate solution to solving Ethernet collisions was the
advance of switching technologies, which treated each Ethernet cable
as a dedicated collision domain.
However, unlike Ethernet (which uses physical cables), collisions
cannot be directly detected over the wireless medium, as RF energy is
radiated over the air and colliding bursts are not necessarily
reflected back to the transmitting stations. Therefore, a different
mechanism is required for this medium.
As such, the IEEE modified the CSMA/CD mechanism to adapt it to
wireless networks to provide Carrier Sense Multiple Access/Collision
Avoidance (CSMA/CA). The original CSMA/CA mechanism used in IEEE
802.11 was the Distributed Coordination Function. DCF is a timer-
based system that leverages three key sets of timers, the slot time,
interframe spaces and CWs.
6.1.1. Slot Time
The slot time is the basic unit of time measure for both DCF and HCF,
on which all other timers are based. The slot-time duration varies
with the different generations of data rates and performances
described by [IEEE.802.11-2016]. For example, [IEEE.802.11-2016]
specifies the slot time to be 20 microseconds ([IEEE.802.11-2016],
Table 15-5) for legacy implementations (such as IEEE 802.11b,
supporting 1, 2, 5.5, and 11 Mbps data rates), while newer
implementations (including IEEE 802.11g, 802.11a, 802.11n, and
802.11ac, supporting data rates from 6.5 Mbps to over 2 Gbps per
spatial stream) define a shorter slot time of 9 microseconds
([IEEE.802.11-2016], Section 17.4.4, Table 17-21).
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6.1.2. Interframe Space (IFS)
The time interval between frames that are transmitted over the air is
called the Interframe Space (IFS). Several IFSs are defined in
[IEEE.802.11-2016], with the most relevant to DCF being the Short
Interframe Space (SIFS), the DCF Interframe Space (DIFS), and the
Extended Interframe Space (EIFS).
The SIFS is the amount of time in microseconds required for a
wireless interface to process a received RF signal and its associated
frame (as specified in [IEEE.802.11-2016]) and to generate a response
frame. Like slot times, the SIFS can vary according to the
performance implementation of [IEEE.802.11-2016]. The SIFS for IEEE
802.11a, 802.11n, and 802.11ac (in 5 GHz) is 16 microseconds
([IEEE.802.11-2016], Section 17.4.4, Table 17-21).
Additionally, a station must sense the status of the wireless medium
before transmitting. If it finds that the medium is continuously
idle for the duration of a DIFS, then it is permitted to attempt
transmission of a frame (after waiting an additional random backoff
period, as will be discussed in the next section). If the channel is
found busy during the DIFS interval, the station must defer its
transmission until the medium is found to be idle for the duration of
a DIFS interval. The DIFS is calculated as:
DIFS = SIFS + (2 * Slot time)
However, if all stations waited only a fixed amount of time before
attempting transmission, then collisions would be frequent. To
offset this, each station must wait, not only a fixed amount of time
(the DIFS), but also a random amount of time (the random backoff)
prior to transmission. The range of the generated random backoff
timer is bounded by the CW.
6.1.3. Contention Window (CW)
Contention windows bound the range of the generated random backoff
timer that each station must wait (in addition to the DIFS) before
attempting transmission. The initial range is set between 0 and the
CW minimum value (CWmin), inclusive. The CWmin for DCF (in 5 GHz) is
specified as 15 slot times ([IEEE.802.11-2016], Section 17.4.4,
Table 17-21).
However, it is possible that two (or more) stations happen to pick
the exact same random value within this range. If this happens, then
a collision may occur. At this point, the stations effectively begin
the process again, waiting a DIFS and generate a new random backoff
value. However, a key difference is that for this subsequent
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attempt, the CW approximately doubles in size (thus, exponentially
increasing the range of the random value). This process repeats as
often as necessary if collisions continue to occur, until the maximum
CW size (CWmax) is reached. The CWmax for DCF is specified as 1023
slot times ([IEEE.802.11-2016], Section 17.4.4, Table 17-21).
At this point, transmission attempts may still continue (until some
other predefined limit is reached), but the CW sizes are fixed at the
CWmax value.
Incidentally it may be observed that a significant amount of jitter
can be introduced by this contention process for wireless
transmission access. For example, the incremental transmission delay
of 1023 slot times (CWmax) using 9-microsecond slot times may be as
high as 9 ms of jitter per attempt. And, as previously noted,
multiple attempts can be made at CWmax.
6.2. Hybrid Coordination Function (HCF)
Therefore, as can be seen from the preceding description of DCF,
there is no preferential treatment of one station over another when
contending for the shared wireless media; nor is there any
preferential treatment of one type of traffic over another during the
same contention process. To support the latter requirement, the IEEE
enhanced DCF in 2005 to support QoS, specifying HCF in IEEE 802.11,
which was integrated into the main IEEE 802.11 standard in 2007.
6.2.1. User Priority (UP)
One of the key changes to the frame format in [IEEE.802.11-2016] is
the inclusion of a QoS Control field, with 3 bits dedicated for QoS
markings. These bits are referred to the User Priority (UP) bits and
these support eight distinct marking values: 0-7, inclusive.
While such markings allow for frame differentiation, these alone do
not directly affect over-the-air treatment. Rather, it is the
non-configurable and standard-specified mapping of UP markings to the
Access Categories (ACs) from [IEEE.802.11-2016] that generate
differentiated treatment over wireless media.
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6.2.2. Access Category (AC)
Pairs of UP values are mapped to four defined access categories that
correspondingly specify different treatments of frames over the air.
These access categories (in order of relative priority from the top
down) and their corresponding UP mappings are shown in Figure 2
(adapted from [IEEE.802.11-2016], Section 10.2.4.2, Table 10-1).
+-----------------------------------------+
| User | Access | Designative |
| Priority | Category | (informative) |
|===========+============+================|
| 7 | AC_VO | Voice |
+-----------+------------+----------------+
| 6 | AC_VO | Voice |
+-----------+------------+----------------+
| 5 | AC_VI | Video |
+-----------+------------+----------------+
| 4 | AC_VI | Video |
+-----------+------------+----------------+
| 3 | AC_BE | Best Effort |
+-----------+------------+----------------+
| 0 | AC_BE | Best Effort |
+-----------+------------+----------------+
| 2 | AC_BK | Background |
+-----------+------------+----------------+
| 1 | AC_BK | Background |
+-----------------------------------------+
Figure 2: Mappings between IEEE 802.11
Access Categories and User Priority
The manner in which these four access categories achieve
differentiated service over-the-air is primarily by tuning the fixed
and random timers that stations have to wait before sending their
respective types of traffic, as will be discussed next.
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6.2.3. Arbitration Interframe Space (AIFS)
As previously mentioned, each station must wait a fixed amount of
time to ensure the medium is idle before attempting transmission.
With DCF, the DIFS is constant for all types of traffic. However,
with [IEEE.802.11-2016], the fixed amount of time that a station has
to wait will depend on the access category and is referred to as an
Arbitration Interframe Space (AIFS). AIFSs are defined in slot times
and the AIFSs per access category are shown in Figure 3 (adapted from
[IEEE.802.11-2016], Section 9.4.2.29, Table 9-137).
+-------------------------------------------+
| Access | Designative | AIFS |
| Category | (informative) |(slot times)|
|============+=================+============|
| AC_VO | Voice | 2 |
+------------+-----------------+------------+
| AC_VI | Video | 2 |
+------------+-----------------+------------+
| AC_BE | Best Effort | 3 |
+------------+-----------------+------------+
| AC_BK | Background | 7 |
+------------+-----------------+------------+
Figure 3: Arbitration Interframe Spaces by Access Category
6.2.4. Access Category CWs
Not only is the fixed amount of time that a station has to wait
skewed according to its [IEEE.802.11-2016] access category, but so
are the relative sizes of the CWs that bound the random backoff
timers, as shown in Figure 4 (adapted from [IEEE.802.11-2016],
Section 9.4.2.29, Table 9-137).
+-------------------------------------------------------+
| Access | Designative | CWmin | CWmax |
| Category | (informative) |(slot times)|(slot times)|
|===========+=================+============|============|
| AC_VO | Voice | 3 | 7 |
+-----------+-----------------+------------+------------+
| AC_VI | Video | 7 | 15 |
+-----------+-----------------+------------+------------+
| AC_BE | Best Effort | 15 | 1023 |
+-----------+-----------------+------------+------------+
| AC_BK | Background | 15 | 1023 |
+-----------+-----------------+------------+------------+
Figure 4: CW Sizes by Access Category
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When the fixed and randomly generated timers are added together on a
per-access-category basis, then traffic assigned to the Voice Access
Category (i.e., traffic marked to UP 6 or 7) will receive a
statistically superior service relative to traffic assigned to the
Video Access Category (i.e., traffic marked UP 5 and 4), which, in
turn, will receive a statistically superior service relative to
traffic assigned to the Best Effort Access Category traffic (i.e.,
traffic marked UP 3 and 0), which finally will receive a
statistically superior service relative to traffic assigned to the
Background Access Category traffic (i.e., traffic marked to UP 2 and
1).
6.3. IEEE 802.11u QoS Map Set
IEEE 802.11u [IEEE.802-11u-2011] is an addendum that has now been
included within the main standard ([IEEE.802.11-2016]), and which
includes, among other enhancements, a mechanism by which wireless APs
can communicate DSCP to/from UP mappings that have been configured on
the wired IP network. Specifically, a QoS Map Set information
element (described in [IEEE.802.11-2016], Section 9.4.2.95, and
commonly referred to as the "QoS Map element") is transmitted from an
AP to a wireless endpoint device in an association / re-association
Response frame (or within a special QoS Map Configure frame).
The purpose of the QoS Map element is to provide the mapping of
higher-layer QoS constructs (i.e., DSCP) to User Priorities. One
intended effect of receiving such a map is for the wireless endpoint
device (that supports this function and is administratively
configured to enable it) to perform corresponding DSCP-to-UP mapping
within the device (i.e., between applications and the operating
system / wireless network interface hardware drivers) to align with
what the APs are mapping in the downstream direction, so as to
achieve consistent end-to-end QoS in both directions.
The QoS Map element includes two key components:
1) each of the eight UP values (0-7) is associated with a range of
DSCP values, and
2) (up to 21) exceptions from these range-based DSCP to/from UP
mapping associations may be optionally and explicitly specified.
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In line with the recommendations put forward in this document, the
following recommendations apply when the QoS Map element is enabled:
1) each of the eight UP values (0-7) are RECOMMENDED to be mapped to
DSCP 0 (as a baseline, so as to meet the recommendation made in
Section 8.2, and
2) (up to 21) exceptions from this baseline mapping are RECOMMENDED
to be made in line with Section 4.3, to correspond to the
Diffserv Codepoints that are in use over the IP network.
It is important to note that the QoS Map element is intended to be
transmitted from a wireless AP to a non-AP station. As such, the
model where this element is used is that of a network where the AP is
the edge of the Diffserv domain. Networks where the AP extends the
Diffserv domain by connecting other APs and infrastructure devices
through the IEEE 802.11 medium are not included in the cases covered
by the presence of the QoS Map element, and therefore are not
included in the present recommendation.
7. IANA Considerations
This document has no IANA actions.
8. Security Considerations
The recommendations in this document concern widely deployed wired
and wireless network functionality, and, for that reason, do not
present additional security concerns that do not already exist in
these networks. In fact, several of the recommendations made in this
document serve to protect wired and wireless networks from potential
abuse, as is discussed further in this section.
8.1. Security Recommendations for General QoS
It may be possible for a wired or wireless device (which could be
either a host or a network device) to mark packets (or map packet
markings) in a manner that interferes with or degrades existing QoS
policies. Such marking or mapping may be done intentionally or
unintentionally by developers and/or users and/or administrators of
such devices.
To illustrate: A gaming application designed to run on a smartphone
or tablet may request that all its packets be marked DSCP EF and/or
UP 6. However, if the traffic from such an application is forwarded
without change over a business network, then this could interfere
with QoS policies intended to provide priority services for business
voice applications.
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To mitigate such scenarios, it is RECOMMENDED to implement general
QoS security measures, including:
o Setting a traffic conditioning policy reflective of business
objectives and policy, such that traffic from authorized users
and/or applications and/or endpoints will be accepted by the
network; otherwise, packet markings will be "bleached" (i.e.,
re-marked to DSCP DF and/or UP 0). Additionally, Section 5.3 made
it clear that it is generally NOT RECOMMENDED to pass through DSCP
markings from unauthorized and/or unauthenticated devices, as
these are typically considered untrusted sources. This is
especially relevant for Internet of Things (IoT) deployments,
where tens of billions of devices are being connected to IP
networks with little or no security capabilities, leaving them
vulnerable to be utilized as agents for DDoS attacks. These
attacks can be amplified with preferential QoS treatments, should
the packet markings of such devices be trusted.
o Policing EF marked packet flows, as detailed in [RFC2474],
Section 7, and [RFC3246], Section 3.
In addition to these general QoS security recommendations, WLAN-
specific QoS security recommendations can serve to further mitigate
attacks and potential network abuse.
8.2. Security Recommendations for WLAN QoS
The wireless LAN presents a unique DoS attack vector, as endpoint
devices contend for the shared media on a completely egalitarian
basis with the network (as represented by the AP). This means that
any wireless client could potentially monopolize the air by sending
packets marked to preferred UP values (i.e., UP values 4-7) in the
upstream direction. Similarly, airtime could be monopolized if
excessive amounts of downstream traffic were marked/mapped to these
same preferred UP values. As such, the ability to mark/map to these
preferred UP values (of UP 4-7) should be controlled.
If such marking/mapping were not controlled, then, for example, a
malicious user could cause WLAN DoS by flooding traffic marked CS7
DSCP downstream. This codepoint would map by default (as described
in Section 2.3) to UP 7 and would be assigned to the Voice Access
Category (AC_VO). Such a flood could cause Denial-of-Service to not
only wireless voice applications, but also to all other traffic
classes. Similarly, an uninformed application developer may request
all traffic from his/her application be marked CS7 or CS6, thinking
this would achieve the best overall servicing of their application
traffic, while not realizing that such a marking (if honored by the
client operating system) could cause not only WLAN DoS, but also IP
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network instability, as the traffic marked CS7 or CS6 finds its way
into queues intended for servicing (relatively low-bandwidth) network
control protocols, potentially starving legitimate network control
protocols in the process.
Therefore, to mitigate such an attack, it is RECOMMENDED that all
packets marked to Diffserv Codepoints not authorized or explicitly
provisioned for use over the wireless network by the network
administrator be mapped to UP 0; this recommendation applies both at
the AP (in the downstream direction) and within the operating system
of the wireless endpoint device (in the upstream direction).
Such a policy of mapping unused codepoints to UP 0 would also prevent
an attack where non-standard codepoints were used to cause WLAN DoS.
Consider the case where codepoints are mapped to UP values using a
range function (e.g., DSCP values 48-55 all map to UP 6), then an
attacker could flood packets marked, for example, to DSCP 49, in
either the upstream or downstream direction over the WLAN, causing
DoS to all other traffic classes in the process.
In the majority of WLAN deployments, the AP represents not only the
edge of the Diffserv domain, but also the edge of the network
infrastructure itself; that is, only wireless client endpoint devices
are downstream from the AP. In such a deployment model, CS6 and CS7
also fall into the category of codepoints that are not in use over
the wireless LAN (since only wireless client endpoint devices are
downstream from the AP in this model and these devices do not
(legitimately) participate in network control protocol exchanges).
As such, it is RECOMMENDED that CS6 and CS7 DSCP be mapped to UP 0 in
these Wi-Fi-at-the-edge deployment models. Otherwise, it would be
easy for a malicious application developer, or even an inadvertently
poorly programmed IoT device, to cause WLAN DoS and even wired IP
network instability by flooding traffic marked CS6 DSCP, which would,
by default (as described in Section 2.3), be mapped to UP 6, causing
all other traffic classes on the WLAN to be starved, as well as
hijacking queues on the wired IP network that are intended for the
servicing of routing protocols. To this point, it was also
recommended in Section 5.1 that packets requesting a marking of CS6
or CS7 DSCP SHOULD be re-marked to DSCP 0 and mapped to UP 0 by the
wireless client operating system.
Finally, it should be noted that the recommendations put forward in
this document are not intended to address all attack vectors
leveraging QoS marking abuse. Mechanisms that may further help
mitigate security risks of both wired and wireless networks deploying
QoS include strong device- and/or user-authentication, access-
control, rate-limiting, control-plane policing, encryption, and other
techniques; however, the implementation recommendations for such
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mechanisms are beyond the scope of this document to address in
detail. Suffice it to say that the security of the devices and
networks implementing QoS, including QoS mapping between wired and
wireless networks, merits consideration in actual deployments.
9. References
9.1. Normative References
[IEEE.802.11-2016]
IEEE, "IEEE Standard for Information technology -
Telecommunications and information exchange between
systems - Local and metropolitan area networks - Specific
requirements - Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications",
IEEE 802.11, DOI 10.1109/IEEESTD.2016.7786995, December
2016, <https://standards.ieee.org/findstds/
standard/802.11-2016.html>.
[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>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
<https://www.rfc-editor.org/info/rfc3246>.
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[RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
Per-Domain Behavior (PDB) for Differentiated Services",
RFC 3662, DOI 10.17487/RFC3662, December 2003,
<https://www.rfc-editor.org/info/rfc3662>.
[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>.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, DOI 10.17487/RFC5865, May 2010,
<https://www.rfc-editor.org/info/rfc5865>.
[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>.
9.2. Informative References
[GSMA-IPX_Guidelines]
GSM Association, "Guidelines for IPX Provider networks
(Previously Inter-Service Provider IP Backbone Guidelines)
Version 11.0", Official Document IR.34, November 2014,
<https://www.gsma.com/newsroom/wp-content/uploads/
IR.34-v11.0.pdf>.
[IEEE.802-11u-2011]
IEEE, "IEEE Standard for Information technology -
Telecommunications and information exchange between
systems - Local and metropolitan area networks - Specific
requirements - Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) specifications: Amendment
9: Interworking with External Networks", IEEE 802.11,
DO 10.1109/IEEESTD.2011.5721908, February 2011,
<http://standards.ieee.org/getieee802/
download/802.11u-2011.pdf>.
[LE-PHB] Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
Work in Progress, draft-ietf-tsvwg-le-phb-02, June 2017.
[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>.
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[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
February 2008, <https://www.rfc-editor.org/info/rfc5127>.
[RFC7561] Kaippallimalil, J., Pazhyannur, R., and P. Yegani,
"Mapping Quality of Service (QoS) Procedures of Proxy
Mobile IPv6 (PMIPv6) and WLAN", RFC 7561,
DOI 10.17487/RFC7561, June 2015,
<https://www.rfc-editor.org/info/rfc7561>.
[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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Acknowledgements
The authors wish to thank David Black, Gorry Fairhurst, Ruediger
Geib, Vincent Roca, Brian Carpenter, David Blake, Cullen Jennings,
David Benham, and the TSVWG.
The authors also acknowledge a great many inputs, notably from David
Kloper, Mark Montanez, Glen Lavers, Michael Fingleton, Sarav
Radhakrishnan, Karthik Dakshinamoorthy, Simone Arena, Ranga Marathe,
Ramachandra Murthy, and many others.
Authors' Addresses
Tim Szigeti
Cisco Systems
Vancouver, British Columbia V6K 3L4
Canada
Email: szigeti@cisco.com
Jerome Henry
Cisco Systems
Research Triangle Park, North Carolina 27709
United States of America
Email: jerhenry@cisco.com
Fred Baker
Santa Barbara, California 93117
United States of America
Email: FredBaker.IETF@gmail.com
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