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Considerations for Assigning a new Recommended DiffServ Codepoint (DSCP)
draft-ietf-tsvwg-dscp-considerations-05

Document Type Active Internet-Draft (tsvwg WG)
Authors Ana Custura , Gorry Fairhurst , Raffaello Secchi
Last updated 2022-08-11 (Latest revision 2022-08-10)
Replaces draft-custura-tsvwg-dscp-considerations
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Dec 2022
Submit "Considerations for Assigning a new Recommended DSCP" as an Informational RFC
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draft-ietf-tsvwg-dscp-considerations-05
TSVWG                                                         A. Custura
Internet-Draft                                              G. Fairhurst
Intended status: Informational                                 R. Secchi
Expires: 11 February 2023                         University of Aberdeen
                                                          10 August 2022

Considerations for Assigning a new Recommended DiffServ Codepoint (DSCP)
                draft-ietf-tsvwg-dscp-considerations-05

Abstract

   This document discusses considerations for assigning a new
   recommended DiffServ Code Point (DSCP) for a new standard Per Hop
   Behaviour (PHB).  It considers the common observed remarking
   behaviours that the DiffServ field might be subjected to along an
   Internet path.  It also notes some implications of using a specific
   DSCP.

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
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   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 11 February 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Current usage of DSCPs  . . . . . . . . . . . . . . . . . . .   4
     3.1.  IP-Layer Semantics  . . . . . . . . . . . . . . . . . . .   4
     3.2.  Network Control Traffic . . . . . . . . . . . . . . . . .   6
   4.  Remarking the DSCP  . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Bleaching the DSCP Field  . . . . . . . . . . . . . . . .   8
     4.2.  IP Type of Service manipulations  . . . . . . . . . . . .   8
       4.2.1.  Impact of ToS Precedence Bleaching  . . . . . . . . .   9
       4.2.2.  Impact of ToS Precedence Remarking  . . . . . . . . .  10
     4.3.  Remarking to a Particular DSCP  . . . . . . . . . . . . .  11
   5.  Interpretation of the IP DSCP at Lower Layers . . . . . . . .  11
     5.1.  Mapping Specified for IEEE 802  . . . . . . . . . . . . .  11
       5.1.1.  Mapping Specified for IEEE 802.1  . . . . . . . . . .  11
       5.1.2.  Mapping Specified for IEEE 802.11 . . . . . . . . . .  12
     5.2.  DiffServ and MPLS . . . . . . . . . . . . . . . . . . . .  13
       5.2.1.  Mapping Specified for MPLS  . . . . . . . . . . . . .  13
       5.2.2.  Mapping Specified for MPLS Short Pipe . . . . . . . .  14
     5.3.  Mapping Specified for Mobile Networks . . . . . . . . . .  15
     5.4.  Mapping Specified for Carrier Ethernet  . . . . . . . . .  16
     5.5.  Remarking as a Side-effect of Another Policy  . . . . . .  16
     5.6.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Considerations for DSCP Selection . . . . . . . . . . . . . .  16
     6.1.  Effect of Bleaching . . . . . . . . . . . . . . . . . . .  17
     6.2.  Where the proposed DSCP > 0x07 (7)  . . . . . . . . . . .  17
     6.3.  Where the proposed DSCP < 0x07 (7)  . . . . . . . . . . .  17
       6.3.1.  Where the proposed DSCP&0x07=0x01 . . . . . . . . . .  17
     6.4.  Impact on deployed infrastructure . . . . . . . . . . . .  18
     6.5.  Questions to guide discussion of a proposed new DSCP  . .  18
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     10.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Revision Notes . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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1.  Introduction

   The Differentiated Services (DiffServ) architecture has been deployed
   in many networks.  It provides differentiated traffic forwarding
   based on the DiffServ Code Point (DSCP) [RFC2474] carried in the
   DiffServ field [RFC2474] of the IP packet header.

   A DiffServ node associates traffic with a service class [RFC4594],
   and categorises it into Behavior Aggregates [RFC4594].  Configuration
   guidelines for service classes are provided in RFC4594 [RFC4594].  In
   IP networks, behaviour aggregates are associated with a DiffServ Code
   Point (DSCP), which in turn maps to a Per Hop Behaviour (PHB).  A
   Treatment Aggregate (TA) is concerned only with the forwarding
   treatment of the traffic forming a behaviour aggregate, which could
   be mapped from a set of DSCP values [RFC5127].  Treatment
   differentiation can be realised using a variety of schedulers and
   queues, and also by algorithms that implement access to the physical
   media.

   Within a DiffServ domain, operators can set service level
   specifications [RFC3086], each of which maps to a particular Per
   Domain Behaviour (PDB).  The PDB defines which DSCP (or set of DSCPs)
   will be associated with specific TAs as the packets pass through a
   DiffServ domain, and whether the packets are remarked as they are
   forwarded.

   Application -> Service
   Traffic        Class
                    |
                  Behavior  -> DiffServ -> Per Hop
                  Aggregate    Codepoint   Behavior
                                             |
                                           Treatment -> Schedule
                                           Aggregate    Queue, Drop

   Figure showing the role of DSCPs in classifying IP traffic for
   differential network treatment by a DiffServ Node.

   This document discusses considerations for assigning a new DSCP for a
   standard PHB.  It considers some commonly observed DSCP remarking
   behaviours that might be experienced along an Internet path.  It also
   describes some packet forwarding treatments that a packet with a
   specific DSCP can expect to receive when forwarded across a link or
   subnetwork.

   The document is motivated by new opportunities to use DiffServ end-
   to-end across multiple domains, such as [I-D.ietf-tsvwg-nqb],
   proposals to build mechanisms using DSCPs in other standards-setting

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   organisations, and the desire to use a common set of DSCPs across a
   range of infrastructure (e.g., [RFC8622], [I-D.ietf-tsvwg-nqb],
   [I-D.learmonth-rfc1226-bis]).

2.  Terminology

   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] [RFC2119] [RFC8174] [RFC8174] when, and only when, they
   appear in all capitals, as shown here.

   DSCPs are specified in the IANA registry [DSCP-registry] where a
   variety of different formats are described.  A DSCP can sometimes be
   referred to by name, such as "CS1", and sometimes by a decimal, hex,
   or binary value.  Hex values will be represented in text using prefix
   0x.  Binary values will use prefix 0b.

3.  Current usage of DSCPs

   This section describes current usage of DSCPs.

3.1.  IP-Layer Semantics

   The DiffServ architecture specifies the use of the DiffServ field in
   the IPv4 and IPv6 packet headers to carry one of 64 distinct DSCP
   values.  Within a given administrative boundary, each DSCP value can
   be mapped to a distinct PHB [RFC2474].  When a new PHB is
   standardized, a recommended DSCP value among those 64 values is
   normally reserved for that PHB, and is assigned by IANA.  An operator
   is not formally required to use the recommended value; indeed
   [RFC2474] states that "the mapping of codepoints to PHBs MUST be
   configurable."  However, use of the recommended value is usually
   convenient and avoids confusion.

   The DSCP space is divided into three pools for the purpose of
   assignment and management [DSCP-registry].  The pools can be
   summarised in a table (where 'x' refers to either a bit position with
   value '0' or '1').

   DSCP Pool 1:  A pool of 32 codepoints with a format 0bxxxxx0, to be
      assigned by IANA Standards Action [RFC8126].

   DSCP Pool 2:  A pool of 16 codepoints with a format of 0bxxxx11,
      reserved for experimental or local (private) use by network
      operators (see sections 4.1 and 4.2 of [RFC8126].

   DSCP Pool 3:  A pool of 16 codepoints with a format of 0bxxxx01.

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      This was initially available for experimental or local use, but
      was originally specified to be preferentially utilised for
      standardized assignments if Pool 1 is ever exhausted.  [RFC4594]
      had recommended a local use of DSCP values 0x01, 0x03, 0x05 and
      0x07 (codepoints with the format of 0b000xx1).  Pool 3 codepoints
      are now utilised for standardized assignments and are no longer
      available for assignment to experimental or local use [RFC8436].
      [RFC8622] assigned 0x01 from this pool and consequentially updated
      [RFC4594].  Any future request to assign 0x05 would be expected to
      similarly update [RFC4594].

   The DSCP space is shown in the following Figure.

  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 56/CS7  | 57    | 58       | 59  | 60       | 61  | 62       | 63  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 48/CS6  | 49    | 50       | 51  | 52       | 53  | 54       | 55  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 40/CS5  | 41    | 42       | 43  | 44/VA    | 45  | 46/EF    | 47  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 32/CS4  | 33    | 34/AF41  | 35  | 36/AF42  | 37  | 38/AF43  | 39  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 24/CS3  | 25    | 26/AF31  | 27  | 28/AF32  | 29  | 30/AF33  | 31  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 16/CS2  | 17    | 18/AF21  | 19  | 20/AF22  | 21  | 22/AF23  | 23  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 8/CS1   | 9     | 10/AF11  | 11  | 12/AF12  | 13  | 14/AF13  | 15  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 0/CS0   | 1/LE  | 2        | 3   | 4        | 5   | 6        | 7   |
  +---------+-------+----------+-----+----------+-----+----------+-----+

   Figure showing the current list of assigned DSCPs and their assigned
   PHBs.

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   +-----+-----------------------+-----------+
   | CS  | Class Selector        | RFC 2474  |
   +-----+-----------------------+-----------+
   | BE  | Best Effort (CS0)     | RFC 2474  |
   +-----+-----------------------+-----------+
   | AF  | Assured Forwarding    | RFC 2597  |
   +-----+-----------------------+-----------+
   | EF  | Expedited Forwarding  | RFC 3246  |
   +-----+-----------------------+-----------+
   | VA  | Voice Admit           | RFC 5865  |
   +-----+-----------------------+-----------+
   | LE  | Lower Effort          | RFC 8622  |
   +-----+-----------------------+-----------+

   Figure showing the summary of the DSCP abbreviations used in previous
   RFCs [RFC2474] [RFC2597] [RFC3246] [RFC5865] [RFC8622], as described
   in the IANA registry [DSCP-registry].  BE, also known as CS0,
   describes the default forwarding treatment.

   The DiffServ architecture allows forwarding treatments to be
   associated with each DSCP, and the RFC series describes some of these
   as PHBs.  Although DSCPs are intended to identify specific treatment
   requirements, multiple DSCPs might also be mapped (aggregated) to the
   same forwarding treatment.  DSCPs can be mapped to treatment
   aggregates that might result in remarking (e.g., RFC5160 [RFC5160]
   suggests Meta-QoS-Classes to help enable deployment of standardized
   end-to-end QoS classes)

3.2.  Network Control Traffic

   Network Control Traffic is defined as packet flows that are essential
   for stable operation of the administered network (see [RFC4594],
   Section 3).  This traffic is marked with a value from a set of Class
   Selector (CS) DSCPs.  This traffic is often a special case within a
   provider network, and ingress traffic with these DSCP markings can be
   remarked.

   DSCP CS2 is recommended for the OAM (Operations, Administration, and
   Maintenance) service class (see [RFC4594], Section 3.3).

   DSCP CS6 is recommended for local network control traffic.  This
   includes routing protocols and OAM traffic that are essential to
   network operation administration, control and management.
   Section 3.2 of RFC4594 [RFC4594] recommends that "CS6 marked packet
   flows from untrusted sources (for example, end-user devices) SHOULD
   be dropped or remarked at ingress to the DiffServ network".

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   DSCP CS7 is reserved for future use by network control traffic.  "CS7
   marked packets SHOULD NOT be sent across peering points" [RFC4594].

   RFC2474 [RFC2474] recommends PHBs selected by CS6 and CS7 "MUST give
   packets preferential forwarding treatment by comparison to the PHB
   selected by codepoint '000000'".

   At the time of writing, there is evidence to suggest CS6 is actively
   used by network operators for control traffic.  A study of traffic at
   a large Internet Exchange showed around 40% of ICMP traffic carried
   this mark [IETF113DSCP].  Similarly, another study found many routers
   remark all traffic except those packets with a DSCP that sets the
   higher order bits to 0b11 (see Section 4 of this document).

4.  Remarking the DSCP

   It is a feature of the DiffServ architecture that the DiffServ field
   of packets can be remarked at domain boundaries (see section 2.3.4.2
   of [RFC2475]).  A DSCP can be remarked at the ingress of a DiffServ
   domain.  This remarking can change the DSCP value used on the
   remainder of an IP path, or the network can restore the initial DSCP
   marking at the egress of the domain.  The DiffServ field can also be
   remarked based on common semantics and agreements between providers
   at an exchange point.  Furthermore, [RFC2474] states that remarking
   must occur when there is a possibility of theft/denial-of-service
   attack.

   If packets are received that are marked with an unknown or an
   unexpected DSCP, [RFC2474] recommends forwarding the packet using a
   default (best effort) treatment, but without changing the DSCP.  This
   seeks to support incremental DiffServ deployment in existing networks
   as well as preserve DSCP markings by routers that have not been
   configured to support DiffServ.  (See also Section 4.3).  [RFC3260]
   clarifies that this remarking specified by RFC2474 is intended for
   interior nodes within a DiffServ domain.  For DiffServ ingress nodes
   the traffic conditioning required by RFC 2475 applies first.

   Reports measuring existing deployments have categorised DSCP
   remarking [Custura] [Barik] into the following seven observed
   remarking behaviours:

   Bleach:  bleaches all traffic (sets the DSCP to zero);

   Bleach-ToS-Precedence:  set the first three bits of the DSCP field to
      0b000 (reset the 3 bits of the former ToS Precedence field,
      defined in [RFC0791], and clarified in [RFC1122]);

   Bleach-some-ToS:  set the first three bits of the DSCP field to 0b000

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      (reset the 3 bits of the former ToS Precedence field), unless the
      first two bits of the DSCP field are 0b11;

   Remark-ToS:  set the first three bits of the DSCP field to any value
      different than 0b000 (replace the 3 bits of the former ToS
      Precedence field);

   Bleach-low:  set the last three bits of the DSCP field to 0b000;

   Bleach-some-low:  set the last three bits of the DSCP field to 0b000,
      unless the first two bits of the DSCP field are 0b11;

   Remark:  remarks all traffic to one or more particular (non-zero)
      DSCP values.

   NOTE: More than one mechanism could result in the same DSCP remarking
   (see below).  It is not generally possible for an external observer
   to determine which mechanism results in a specific remarking solely
   from the change in an observed DSCP value.

4.1.  Bleaching the DSCP Field

   A specific form of remarking occurs when the DiffServ field is re-
   assigned to the default treatment, CS0 (0x00).  This results in
   traffic being forwarded using the BE PHB.  For example, AF31 (0x1a)
   would be bleached to CS0.

   A survey reported that resetting all the bits of the DiffServ field
   to 0 was seen to be more prevalent at the edge of the network, and
   rather less common in core networks [Custura].

4.2.  IP Type of Service manipulations

   The IETF first defined ToS precedence for IP packets in [RFC0791],
   and updated it to be part of the ToS Field in [RFC1349].  Since 1998,
   this practice has been deprecated by [RFC2474].  RFC 2474 defines
   DSCPs 0bxxx000 as the Class Selector codepoints, where PHBs selected
   by these codepoints MUST meet the Class Selector PHB Requirements"
   described in Sec. 4.2.2.2 of that RFC.

   However, a recent survey reports practices based on ToS semantics
   have not yet been eliminated from the Internet, and need to still be
   considered when making new DSCP assignments [Custura].

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4.2.1.  Impact of ToS Precedence Bleaching

   ToS Precedence Bleaching (/Bleach-ToS-Precedence/) is a practice that
   resets the first three bits of the DSCP field to zero (the former ToS
   Precedence field), leaving the last three bits unchanged (see section
   4.2.1 of [RFC2474]).  A DiffServ node can be configured in a way that
   results in this remarking.  This remarking can also occur when
   packets are processed by a router that is not configured with
   DiffServ (e.g., configured to operate on the former ToS precedence
   field [RFC0791]).  At the time of writing, this is a common
   manipulation of the DiffServ field.  The following Figure depicts
   this remarking.

   +-+-+-+-+-+-+
   |0 0 0|x x x|
   +-+-+-+-+-+-+

   Figure showing the ToS Precedence Bleaching (/Bleach-ToS-Precedence/)
   observed remarking behaviour, based on Section 3 of [RFC1349].  The
   bit positions marked "x" are not changed.

  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 56/CS7  | 57    | 58       | 59  | 60       | 61  | 62       | 63  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 48/CS6  | 49    | 50       | 51  | 52       | 53  | 54       | 55  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 40/CS5  | 41    | 42       | 43  | 44/VA    | 45  | 46/EF    | 47  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 32/CS4  | 33    | 34/AF41  | 35  | 36/AF42  | 37  | 38/AF43  | 39  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 24/CS3  | 25    | 26/AF31  | 27  | 28/AF32  | 29  | 30/AF33  | 31  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 16/CS2  | 17    | 18/AF21  | 19  | 20/AF22  | 21  | 22/AF23  | 23  |
  +---------+-------+----------+-----+----------+-----+----------+-----+
  | 8/CS1   | 9     | 10/AF11  | 11  | 12/AF12  | 13  | 14/AF13  | 15  |
  +=========+=======+==========+=====+==========+=====+==========+=====+
  | 0/CS0   | 1/LE  | 2        | 3   | 4        | 5   | 6        | 7   |
  +=========+=======+==========+=====+==========+=====+==========+=====+

   As a result of ToS Precedence Bleaching, all the DSCPs in each column
   are remarked to the smallest DSCP in that column.  The DSCPs in the
   bottom row therefore have higher survivability across an end-to-end
   Internet path.

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   +=========+=======+============+====+======+======+============+====+
   | 0/CS0   | 1/LE  | 2          | 3  | 4    | 5    | 6          | 7  |
   +=========+=======+============+====+======+======+============+====+
   |Assigned         |ToS Prec Bl.|EXP/|Used  |Future|ToS Prec Bl.|Exp/|
   |                 |of AF11..41 |LU  |by SSH|NQB   |of AF13..EF |LU  |
   +=================+============+====+======+======+============+====+

   Figure showing 0b000xxx DSCPs, highlighting any current assignments
   and whether they are affected by any known remarking behaviours.  For
   example, ToS Precedence Bleaching of popular DSCPs AF11,21,31,41
   would result in traffic being remarked with DSCP 2 in the Internet
   core.  DSCP 4 has been historically used by the SSH application,
   following semantics which precede DiffServ[DSCP4].

   If ToS Precedence Bleaching occurs, packets with a DSCP 'x' would be
   remarked to 'x' & 0x07and then would be treated according to the
   corresponding PHB.

   A variation of this observed remarking behaviour clears the top three
   bits of a DSCP, unless these have values 0b110 or 0b111
   (corresponding to the CS6 and CS7 DSCPs).  As a result, a DSCP value
   greater than 48 decimal (0x30) is less likely to be impacted by ToS
   Precedence Bleaching.

4.2.2.  Impact of ToS Precedence Remarking

   Practices based on ToS Precedence Remarking (/Remark-ToS/) [RFC1349]
   were deprecated by [RFC2474].  These practices based on ToS semantics
   have not yet been eliminated from deployed networks.

   +-+-+-+-+-+-+
   |0 0 1|x x x|
   +-+-+-+-+-+-+

   Figure showing ToS Precedence Remarking (/Remark-ToS/) observed
   behaviour, based on Section 3 of [RFC1349].  The bit positions marked
   "x" remain unchanged.

   A less common remarking, ToS Precedence Remarking sets the first
   three bits of the DSCP to a non-zero value corresponding to a CS PHB.
   This remarking occurs when routers are not configured to perform
   DiffServ remarking.

   If remarking occurs, packets are forwarded using the PHB specified
   for the resulting DSCP.  For example, the AF31 DSCP (0x1a) could be
   remarked to either AF11 or AF21.

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4.3.  Remarking to a Particular DSCP

   A network device might remark the DiffServ field of an IP packet
   based on a local policy with a specific (set of) DSCPs, /Remark/.

   Both [RFC2474] and [RFC8100] recommend that DiffServ boundary nodes
   use remarking, if necessary, to avoid theft/denial of service or
   ensure that appropriate DSCPs are used within a DiffServ domain.
   Some networks therefore may not follow the earlier recommendation in
   [RFC2474] to carry unknown or unexpected DSCPs without modification,
   and instead remark packets with these codepoints to the default
   class, CS0 (0x00).

   Remarking is sometimes performed using a Multi-Field (MF) classifier
   [RFC2475] [RFC3290] [RFC4594].  For example, a common remarking is to
   remark all traffic to a single DSCP, thus removing any traffic
   differentiation (see Section 4.1).  Bleaching (/Bleach/) is a
   specific example of this observed remarking behaviour that remarks to
   CS0 (0x00).

5.  Interpretation of the IP DSCP at Lower Layers

   Transmission systems and subnetworks can, and do, utilise the
   DiffServ field in an IP packet to set a QoS-related field or function
   at the lower layer.  A lower layer could also implement a traffic
   conditioning function that could remark the DSCP used at the IP
   layer.  In many cases, this use is constrained by designs that
   utilise fewer than 6 bits to express the service class, and therefore
   infer mapping to a smaller L2 QoS field, for example, WiFi or Multi-
   Protocol Label Switching (MPLS).

5.1.  Mapping Specified for IEEE 802

   The IEEE specifies standards that include mappings for DSCPs to lower
   layer elements.

5.1.1.  Mapping Specified for IEEE 802.1

   A 3-bit Priority Code Point (PCP) is specified in the IEEE 802.1Q tag
   to mark Ethernet frames to one of eight priority values
   [IEEE-802-1Q].  The value zero indicates the default best effort
   treatment, and the value one indicates a background traffic class.
   The remaining values indicate increasing priority.  Internet control
   traffic can be marked as CS6, and network control is marked as CS7.

   The mapping specified in [IEEE-802-1Q] revises a previous standard
   [IEEE-802-1D], in an effort to align with DiffServ practice: the
   traffic types are specified to match the first three bits of a

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   suitable DSCP (e.g., the first three bits of the EF DSCP are mapped
   to a PCP of 5).  However, [IEEE-802-1D] maps both PCP1 (Background)
   and PCP2 (Spare) to indicate lower priority than PCP0, RFC8622.
   Therefore, different remarking behaviours are expected depending on
   the age of deployed devices.

5.1.2.  Mapping Specified for IEEE 802.11

   Section 6 of [RFC8325] provides a brief overview of IEEE 802.11 QoS.
   The IEEE 802.11 standards [IEEE-802-11] provide MAC functions to
   support QoS in WLANs using Access Classes (AC).  The upstream User
   Priority (UP) in the 802.11 header has a 3-bit QoS value.  A DSCP can
   be mapped to the UP.

   Most current WiFi implementations [RFC8325] use a default mapping
   that maps the first three bits of the DSCP to the 802.11 UP value.
   This is then in turn mapped to the four WiFi Multimedia (WMM) Access
   Categories.  The Wi-Fi Alliance has also specified a more flexible
   mapping that follows RFC8325 and provides functions at an AP to
   remark packets as well as a QoS Map that maps each DSCP to an AC
   [WIFI-ALLIANCE].

   +-+-+-+-+-+-+
   |x x x|. . .|
   +-+-+-+-+-+-+

   Figure showing the DSCP bits commonly mapped to the UP in 802.11.
   The bit positions marked "x" are mapped to the 3-bit UP value, while
   the ones marked "." are ignored.

   RFC8325 [RFC8325] notes inconsistencies that can result from such
   remarking, and recommends how to perform this remarking.  It proposes
   several recommendations for mapping a DSCP to an IEEE 802.11 UP for
   wired-to-wireless interconnection.  The recommendation includes
   mapping network control traffic CS6 and CS7, as well unassigned
   DSCPs, to UP 0.

   In the upstream direction (wireless-to-wired interconnections, this
   mapping can result in a specific DSCP remarking behaviour.  Some
   Access Points (APs) currently use a default UP-to-DSCP mapping
   [RFC8325], wherein "DSCP values are derived from the layer 2 UP
   values by multiplying the UP values by eight (i.e., shifting the
   three UP bits to the left and adding three additional zeros to
   generate a 6-bit DSCP value).  This derived DSCP value is used for
   QoS treatment between the wireless AP and the nearest classification
   and marking policy enforcement point (which may be the centralized

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   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."  This can result in
   remarking /Bleach-low/.

   +-+-+-+-+-+-+
   |x x x|0 0 0|
   +-+-+-+-+-+-+

   Figure showing the observed remarking behaviour resulting from
   deriving from UP-to-DSCP mapping in some 802.11 networks.

   An alternative to UP-to-DSCP remapping uses the DSCP value of a
   downstream IP packet (e.g., the Control And Provisioning of Wireless
   Access Points (CAPWAP) protocol, RFC5415, maps an IP packet DiffServ
   field to the DiffServ field of the outer IP header in a CAPWAP
   tunnel).

   Some current constraints of WiFi mapping are discussed in section 2
   of [RFC8325].  A QoS profile can be used to limit the maximum DSCP
   value used for the upstream and downstream traffic.

5.2.  DiffServ and MPLS

   Multi-Protocol Label Switching (MPLS) specified eight MPLS Traffic
   Classes (TCs), which restrict the number of different treatments
   [RFC5129].  RFC 5127 describes aggregation of DiffServ TCs [RFC5127],
   and introduces four DiffServ Treatment Aggregates.  Traffic marked
   with multiple DSCPs can be forwarded in a single MPLS TC.

   There are three Label-Switched Router (LSR) behaviours: the Pipe, the
   Short Pipe and the Uniform Model [RFC3270].  These only differ when a
   LSP performs a push or a pop.

5.2.1.  Mapping Specified for MPLS

   RFC3270 [RFC3270] defines a flexible solution for support of DiffServ
   over MPLS networks.  This allows an MPLS network administrator to
   select how BAs (marked by DSCPs) are mapped onto Label Switched Paths
   (LSPs) to best match the DiffServ, Traffic Engineering and protection
   objectives within their particular network.

   Mappings from the IP DSCP to the MPLS header are defined in
   Section 4.2 of [RFC3270].

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   The Pipe Model conveys the "LSP Diff-Serv Information" to the LSP
   Egress so that its forwarding treatment can be based on the IP DSCP.

   When Penultimate Hop Popping (PHP) is used, the Penultimate LSR needs
   to be aware of the encapsulation mapping for a PHB to the label
   corresponding to the exposed header to perform DiffServ Information
   Encoding (Section 2.5.2 of [RFC3270]).

5.2.2.  Mapping Specified for MPLS Short Pipe

   The Short Pipe Model is an optional variation of the Pipe Model
   [RFC3270].

   ITU-T Y.1566 [ITU-T-Y-1566] further defined a set of four common QoS
   classes and four auxiliary classes to which a DSCP can be mapped when
   interconnecting Ethernet, IP and MPLS networks.  [RFC8100] proposes
   four treatment aggregates for interconnection with four defined
   DSCPs.  This was motivated by the requirements of MPLS network
   operators that use Short-Pipe tunnels, but can be applicable to other
   networks, both MPLS and non-MPLS.

   RFC8100 recommends preserving the notion of end-to-end service
   classes, and recommends a set of standard DSCPs mapped to a small set
   of standard PHBs at interconnection.  The key requirement is that the
   DSCP at the network ingress is restored at the network egress.  The
   current version of RFC8100 limits the number of DSCPs to 6 and 3 more
   are suggested for extension.  RFC8100 respects the deployment of PHB
   groups for DS domain internal use, which limits the number of
   acceptable external DSCPs (and possibilities for their transparent
   transport or restoration at network boundaries).  In this design,
   packets marked with DSCPs not part of the RFC8100 codepoint scheme
   are treated as unexpected and will possibly be remarked (a /Remark/
   behaviour) or dealt with via an additional agreement(s) among the
   operators of the interconnected networks.  RFC8100 can be extended to
   support up to 32 DSCPs by future standards.  RFC8100 is operated by
   at least one Tier 1 backbone provider.  Use of the MPLS Short Pipe
   Model favours remarking unexpected DSCP values to zero in the absence
   of an additional agreement(s), as explained in [RFC8100].  This can
   result in bleaching (/Bleach/).

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   +--------------------------------------+--------+
   |  RFC8100                             |  DSCP  |
   |  Agg. Class                          |        |
   +--------------------------------------+--------+
   |Telephony Service Treatment Aggregate |   EF   |
   |                                      |   VA   |
   +--------------------------------------+--------+
   |Bulk Real-Time Treatment Aggregate    |  AF41  |
   |                         May be added | (AF42) |
   |                         May be added | (AF43) |
   +--------------------------------------+--------+
   |Assured Elastic Treatment Aggregate   |  AF31  |
   |                                      |  AF32  |
   |    Reserved for the extension of PHBs| (AF33) |
   +--------------------------------------+--------+
   |Default / Elastic Treatment Aggregate | BE/CS0 |
   +--------------------------------------+--------+
   |Network Control: Local Use            |  CS6   |
   +--------------------------------------+--------+

   The short-pipe MPLS mapping from RFC 8100.

5.3.  Mapping Specified for Mobile Networks

   Mobile LTE and 5G standards have evolved from older UMTS standards,
   and support DiffServ.  LTE (4G) and 5G standards [SA-5G] identify
   traffic classes at the interface between User Equipment (UE) and the
   mobile Packet Core network by QCI (QoS Class Identifiers) and 5QI (5G
   QoS Identifier).  The 3GPP standards do not define or recommend any
   specific mapping between each QCI or 5QI and DiffServ (and mobile
   QCIs are based on several criteria service class definitions).  The
   way packets are mapped at the Packet Gateway (P-GW) boundary is
   determined by operators.  However, TS 23.107 (version 16.0.0, applies
   to LTE and below) mandates that Differentiated Services, defined by
   IETF, shall be used to interoperate with IP backbone networks.

   The GSM Association (GSMA) has defined four aggregated classes and
   seven associated PHBs in their guidelines for IPX Provider networks
   GSMA IR.34 [GSMA-IR-34].  This was previously specified as the Inter-
   Service Provider IP Backbone Guidelines, and provides a mobile ISP to
   ISP QoS mapping mechanism, and interconnection with other IP networks
   in the general Internet.  If realised by an IP VPN, the operator is
   free to apply its DS Domain internal codepoint scheme at outer
   headers and inner IPX DSCPs may be transported transparently.  The
   guidelines also describe a case where the DSCP marking from a Service
   Provider cannot be trusted (depending on the agreement between the
   Service Provider and its IPX Provider), in which situation the IPX
   Provider can remark the DSCP value to a static default value.

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   +---------------+-------+
   |  GSMA IR.34   |  PHB  |
   |  Agg. Class   |       |
   +---------------+-------+
   |Conversational |  EF   |
   +---------------+-------+
   | Streaming     | AF41  |
   +---------------+-------+
   | Interactive   | AF31  |
   +               +-------+
   | (ordered by   | AF32  |
   +   priority,   +-------+
   | AF3 highest)  | AF21  |
   +               +-------+
   |               | AF11  |
   +---------------+-------+
   | Background    | CS0   |
   +---------------+-------+

   Figure showing the PHB mapping recommended in the guidelines proposed
   in GSMA IR.34 [GSMA-IR-34].

5.4.  Mapping Specified for Carrier Ethernet

   Metro Ethernet Forum (MEF) provides a mapping of DSCPs at the IP
   layer to quality of service markings in the Ethernet frame headers
   MEF 23.1 [MEF23.1].

5.5.  Remarking as a Side-effect of Another Policy

   This includes any other remarking that does not happen as a result of
   traffic conditioning, such as policies and L2 procedures that result
   in remarking traffic as a side-effect of other functions (e.g., in
   response to Distributed Denial of Service, DDoS).

5.6.  Summary

   This section has discussed the various ways in which DSCP remarking
   behaviours can arise from interactions with lower layers.

6.  Considerations for DSCP Selection

   This section provides advice for the assignment of a new DSCP value.
   It is intended to aid the IETF and IESG in considering a request for
   a new DSCP.  The section identifies known issues that might influence
   the finally assigned DSCP, and provides a summary of considerations
   for assignment of a new DSCP.

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6.1.  Effect of Bleaching

   New DSCP assignments should consider the impact of bleaching, which
   can limit the ability to provide the expected treatment end-to-end.
   This is particularly important for cases where the codepoint is
   intended to result in lower than best effort treatment, as was the
   case when defining the LE PHB [RFC8622].  In this case, bleaching, or
   remarking to "CS0" would result in elevating the lower effort traffic
   (LE) to the default class (BE/CS0).  This is an example of priority
   inversion.

6.2.  Where the proposed DSCP > 0x07 (7)

   Although the IETF specifications require systems to use DSCP marking
   semantics in place of methods based on the former ToS field, the
   current recommendation is that any new assignment where the DSCP is
   greater than 0x07 should consider the semantics associated with the
   resulting DSCP when ToS Precedence Bleaching is experienced.  For
   example, it can be desirable to avoid choosing a DSCP that could be
   remarked to LE, Lower Effort [RFC8622], which could otherwise
   potentially result in a priority inversion in the treatment.

6.3.  Where the proposed DSCP < 0x07 (7)

   ToS Precedence Bleaching can unintentionally result in extra traffic
   aggregated to the same DSCP.  For example, after experiencing ToS
   Precedence Bleaching, all traffic marked AF11, AF21, AF31 and AF41
   would be aggregated with traffic marked with DSCP 2 (0x02),
   increasing the volume of traffic which receives the treatment
   associated with DSCP 2.  New DSCP assignments should consider
   unexpected consequences arising from this observed remarking
   behaviour.

6.3.1.  Where the proposed DSCP&0x07=0x01

   As a consequence of assigning the LE PHB [RFC8622], the IETF
   allocated the DSCP 0b000001 from Pool 3.

   When making assignments where the DSCP has a format: 0bxxx001, the
   case of ToS Precedence Bleaching (/Bleach-ToS-Precedence/) of a DSCP
   to a value of 0x01 needs to be considered.  ToS Precedence Bleaching
   will result in demoting the traffic to the lower effort traffic
   class.  Care should be taken to consider the implications of
   remarking when shooing to assign a DSCP with this format.

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6.4.  Impact on deployed infrastructure

   Behaviour of deployed PHBs and conditioning treatments also needs to
   be considered when assigning a new DSCP.  Network operators have
   choices when it comes to configuring DiffServ support within their
   domains, and the observed remarking behaviours described in the
   previous section can result from different configurations and
   approaches:

   Networks not remarking DiffServ:  A network that either does not
      implement PHBs, or implements one or more PHBs whilst restoring
      the DSCP field at network egress with the value at network
      ingress.  Operators in this category pass all DSCPs transparently.

   Networks that condition the DSCP:  A network that implements more
      than one PHB and enforces SLAs with its peers.  Operators in this
      category use conditioning to ensure that only traffic that matches
      a policy is permitted to use a specific DSCP (see [RFC8100]).
      This requires operators to choose to support or remark a new DSCP
      assignment.

   Networks that remark in some other way  , which includes:

 
      1.  Networks containing misconfigured devices that do not comply
          with the relevant RFCs.

      2.  Networks containing devices that implement an obsolete
          specification or contain software bugs.

      3.  Networks containing devices that remark the DSCP as a result
          of lower layer interactions.

   For example, the ToS Precedence Bleaching (/Bleach-ToS-Precedence/)
   remarking behaviour cannot be observed in the case of networks not
   remarking DiffServ, but can arise as a result of traffic conditioning
   at operator boundaries.  It can also arise in the case of
   misconfiguration or in networks using old equipment which precedes
   DiffServ.

6.5.  Questions to guide discussion of a proposed new DSCP

   A series of questions emerge that need to be answered when
   considering a proposal to the IETF that requests a new assignment.
   These questions include:

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   *  Is the request for local use within a DiffServ domain that does
      not require interconnection with other DiffServ domains?  This
      request can use DSCPs in Pool 2 for local or experimental use,
      without any IETF specification for the DSCP or associated PHB.

   *  How is the proposed service class characterised: What are the
      characteristics of the traffic to be carried?  What are the
      expectations for treatment?

   *  Service classes [RFC4594] that can utilise existing PHBs should
      use assigned DSCPs to mark their traffic: Could the request be met
      by using an existing IETF DSCP?

   *  Specification of a new recommended DSCP requires Standards Action.
      RFC2474 states: "Each standardized PHB MUST have an associated
      RECOMMENDED codepoint".  If approved, new IETF assignments are
      normally made by IANA in Pool 1, but the IETF can request
      assignments to be made from Pool 3 [RFC8436].  Does the ID contain
      an appropriate request to IANA?

   *  Section 5.2 describes examples of treatment aggregation.  What are
      the effects of treatment aggregation on the proposed DSCP?

   *  Section 5 describes some observed treatments by layers below IP.
      What are the implications of the treatments and mapping described
      in Section 5 on the proposed DSCP?

   *  DSCPs are assigned to PHBs and can be used to enable nodes along
      an end-to-end path to classify the packet for a suitable PHB.
      Section 4 describes some observed remarking behaviour, and
      Section 6.4 identifies root causes for why this remarking is
      observed.  What is the expected effect of currently-deployed
      remarking on the end-to-end service?

7.  Acknowledgments

   The authors acknowledge the helpful discussions and analysis by Greg
   White and Thomas Fossati in a draft concerning NQB.  Ruediger Geib
   and Brian Carpenter contributed comments, along with other members of
   the TSVWG.

8.  IANA Considerations

   This memo provides information to assist in considering new
   assignments to the IANA DSCP registry
   (https://www.iana.org/assignments/dscp-registry/dscp-registry.xhtml).

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   This memo includes no request to IANA, or update to the IANA
   procedures.

9.  Security Considerations

   The security considerations are discussed in the security
   considerations of each cited RFC.

10.  References

10.1.  Normative References

   [DSCP-registry]
              IANA, "Differentiated Services Field Codepoints (DSCP)
              Registry",  https://www.iana.org/assignments/dscp-
              registry/dscp-registry.xhtml, 2019.

   [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>.

   [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>.

   [RFC3260]  Grossman, D., "New Terminology and Clarifications for
              Diffserv", RFC 3260, DOI 10.17487/RFC3260, April 2002,
              <https://www.rfc-editor.org/info/rfc3260>.

   [RFC3290]  Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
              Informal Management Model for Diffserv Routers", RFC 3290,
              DOI 10.17487/RFC3290, May 2002,
              <https://www.rfc-editor.org/info/rfc3290>.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,
              <https://www.rfc-editor.org/info/rfc4594>.

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   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
              2008, <https://www.rfc-editor.org/info/rfc5129>.

   [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>.

   [RFC8436]  Fairhurst, G., "Update to IANA Registration Procedures for
              Pool 3 Values in the Differentiated Services Field
              Codepoints (DSCP) Registry", RFC 8436,
              DOI 10.17487/RFC8436, August 2018,
              <https://www.rfc-editor.org/info/rfc8436>.

10.2.  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.

   [DSCP4]    Kolu, A., "Bogus DSCP value for SSH", online 
              https://lists.freebsd.org/pipermail/freebsd-
              stable/2010-July/057710.html, 2010.

   [GSMA-IR-34]
              GSM Association, "IR.34 Guidelines for IPX Provider
              networks (Previously Inter-Service Provider IP Backbone
              Guidelines)", IR 34, 2017.

   [I-D.ietf-tsvwg-nqb]
              White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
              Behavior (NQB PHB) for Differentiated Services", Work in
              Progress, Internet-Draft, draft-ietf-tsvwg-nqb-10, March
              2022, <https://www.ietf.org/archive/id/draft-ietf-tsvwg-
              nqb-10.txt>.

   [I-D.learmonth-rfc1226-bis]
              Learmonth, I. R., "Internet Protocol Encapsulation of
              AX.25 Frames", Work in Progress, Internet-Draft, draft-
              learmonth-rfc1226-bis-03, 19 May 2020,
              <https://www.ietf.org/archive/id/draft-learmonth-rfc1226-
              bis-03.txt>.

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   [IEEE-802-11]
              IEEE, "Wireless LAN Medium Access Control (MAC) and
              Physical Layer (PHY) Specifications", IEEE 802.11, 2007.

   [IEEE-802-1D]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Network-- Media Access Control (MAC) Bridges",
              IEEE 802.1D, 2004.

   [IEEE-802-1Q]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Network-- Bridges and Bridged Networks", IEEE 802.1Q,
              2018.

   [IETF113DSCP]
              Custura, A. and G. Fairhurst, "Considerations for
              assigning DSCPs", IETF 113 Proceedings 
              https://datatracker.ietf.org/meeting/113/materials/slides-
              113-tsvwg-61-dscp-considerations-07, 2022.

   [ITU-T-Y-1566]
              ITU-T, "Quality of Service Mapping and Interconnection
              Between Ethernet, Internet Protocol and Multiprotocol
              Label Switching Networks", ITU-T Y.1566, 2012.

   [MEF23.1]  MEF, "MEF Technical Specification MEF 23.1-- Carrier
              Ethernet Class of Service ? Phase 2", MEF 23.1, 2012.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC1349]  Almquist, P., "Type of Service in the Internet Protocol
              Suite", RFC 1349, DOI 10.17487/RFC1349, July 1992,
              <https://www.rfc-editor.org/info/rfc1349>.

   [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>.

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   [RFC3086]  Nichols, K. and B. Carpenter, "Definition of
              Differentiated Services Per Domain Behaviors and Rules for
              their Specification", RFC 3086, DOI 10.17487/RFC3086,
              April 2001, <https://www.rfc-editor.org/info/rfc3086>.

   [RFC3246]  Davie, B., Charny, A., Bennet, J C R., Benson, K., Le
              Boudec, J Y., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
              <https://www.rfc-editor.org/info/rfc3246>.

   [RFC3270]  Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
              P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
              Protocol Label Switching (MPLS) Support of Differentiated
              Services", RFC 3270, DOI 10.17487/RFC3270, May 2002,
              <https://www.rfc-editor.org/info/rfc3270>.

   [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>.

   [RFC5160]  Levis, P. and M. Boucadair, "Considerations of Provider-
              to-Provider Agreements for Internet-Scale Quality of
              Service (QoS)", RFC 5160, DOI 10.17487/RFC5160, March
              2008, <https://www.rfc-editor.org/info/rfc5160>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8622]  Bless, R., "A Lower-Effort Per-Hop Behavior (LE PHB) for
              Differentiated Services", RFC 8622, DOI 10.17487/RFC8622,
              June 2019, <https://www.rfc-editor.org/info/rfc8622>.

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   [SA-5G]    3GPP, "System Architecture for 5G", TS 23.501, 2019.

   [WIFI-ALLIANCE]
              Wi-Fi Alliance, "Wi-Fi QoS Management Specification
              Version 2.0", Wi-Fi QoS Management Specification
              Version 2.0, 2021.

Appendix A.  Revision Notes

   Note to RFC-Editor: please remove this entire section prior to
   publication.

   *  Individual draft -00, initial document.

   *  Individual draft -01, address comments from Martin Duke; Brian
      Carpenter; Ruediger Geib, and revisions to improve language
      consistency.

   *  Individual draft -02, revise to improve language consistency.

   *  Working Group -00, replace individual draft.

   *  Working Group -01, address feedback in preparation for IETF 113
      Vienna.

   *  Working Group -02:

         Consolidate terminology after IETF 113 Vienna.

         Add clarification to RFC2474 and RFC2475 addressed in RFC3260
         (Comments from Ruediger Geib).

         Include figures to show the full list of codepoints, their
         assigned PHBs & impact of ToS Precedence Bleaching.

         Add network categories that differentiate between network
         operator approaches to DiffServ.

         Add Terminology section to clarify representations of DSCPs.

   *  Working Group -03

         Add table to explain DSCP abbreviations (comment from Brian
         Carpenter).

         Add some refs, improve language consistency (comments from
         Brian Carpenter).

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         Clarify figure captions.

   *  Working Group -04

         Reference RFC3086 (comment from Brian Carpenter).

         Improve references (comments from Ruediger Geib).

         Clarify intended document audience and scope (comments from
         Ruediger Geib).

         Clarify terms and language around re-marking, DiffServ domains
         and nodes, RFC8100 (comments from Ruediger Geib).

         More in-depth on mappings specified for mobile networks/MPLS
         short-pipe (comments from Ruediger Geib).

   *  Working Group -05

         Clarify meaning of RFC2474 with respect to IP precedence
         (Comments from RG).

         Add note on understanding the process of remarking.  (comments
         from Ruediger Geib).

         Improve readibility.

Authors' Addresses

   Ana Custura
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen
   AB24 3UE
   United Kingdom
   Email: ana@erg.abdn.ac.uk

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen
   AB24 3UE
   United Kingdom
   Email: gorry@erg.abdn.ac.uk

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   Raffaello Secchi
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen
   AB24 3UE
   United Kingdom
   Email: r.secchi@abdn.ac.uk

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