path: root/doc/rfc/rfc9435.txt

Internet Engineering Task Force (IETF)                        A. Custura
Request for Comments: 9435                                  G. Fairhurst
Category: Informational                                        R. Secchi
ISSN: 2070-1721                                   University of Aberdeen
                                                               July 2023


 Considerations for Assigning a New Recommended Differentiated Services
                           Code Point (DSCP)

Abstract

   This document discusses considerations for assigning a new
   recommended Differentiated Services Code Point (DSCP) for a standard
   Per-Hop Behavior (PHB).  It considers the common observed re-marking
   behaviors 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 document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see 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/rfc9435.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Current Usage of DSCPs
     3.1.  IP-Layer Semantics
     3.2.  DSCPs Used for Network Control Traffic
   4.  Re-marking the DSCP
     4.1.  Bleaching the DSCP Field
     4.2.  IP Type of Service Manipulations
       4.2.1.  Impact of ToS Precedence Bleaching
       4.2.2.  Impact of ToS Precedence Re-marking
     4.3.  Re-marking to a Particular DSCP
   5.  Interpretation of the IP DSCP at Lower Layers
     5.1.  Mapping Specified for IEEE 802
       5.1.1.  Mapping Specified for IEEE 802.1
       5.1.2.  Mapping Specified for IEEE 802.11
     5.2.  Diffserv and MPLS
       5.2.1.  Mapping Specified for MPLS
       5.2.2.  Mapping Specified for MPLS Short Pipe
     5.3.  Mapping Specified for Mobile Networks
     5.4.  Mapping Specified for Carrier Ethernet
     5.5.  Re-marking as a Side Effect of Another Policy
     5.6.  Summary
   6.  Considerations for DSCP Selection
     6.1.  Effect of Bleaching and Re-marking to a Single DSCP
     6.2.  Where the Proposed DSCP > 0x07 (7)
       6.2.1.  Where the Proposed DSCP&0x07=0x01
     6.3.  Where the Proposed DSCP <= 0x07 (7)
     6.4.  Impact on Deployed Infrastructure
     6.5.  Considerations to Guide the Discussion of a Proposed New
           DSCP
   7.  IANA Considerations
   8.  Security Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Differentiated Services (Diffserv) architecture has been deployed
   in many networks.  It provides differentiated traffic forwarding
   based on the DSCP carried in the Diffserv field of the IP packet
   header [RFC2474].  A common set of DSCPs are defined for both IPv4
   and IPv6, and both network protocols use a common IANA registry
   [DSCP-registry].

   Diffserv associates traffic with a service class and categorizes it
   into Behavior Aggregates (BAs) [RFC4594].  Configuration guidelines
   for service classes are provided in [RFC4594].  BAs are associated
   with a DSCP, which in turn maps to a Per-Hop Behavior (PHB).
   Treatment differentiation can be achieved by using a variety of
   schedulers and queues and also 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 Behavior (PDB) that is based on one or more PHBs.  The PDB
   defines which PHB (or set of PHBs) and, hence, for a specific
   operator, which DSCP (or set of DSCPs) will be associated with
   specific BAs as the packets pass through a Diffserv domain.  It also
   defines whether the packets are re-marked as they are forwarded
   (i.e., changing the DSCP of a packet [RFC2475]).

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

         Figure 1: 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 re-marking
   behaviors 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 [NQB-PHB], proposals to build
   mechanisms using DSCPs in other standards-setting organizations, and
   the desire to use a common set of DSCPs across a range of
   infrastructure (e.g., [RFC8622], [NQB-PHB], [AX.25-over-IP]).

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] [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 are represented in text using prefix
   "0x".  Binary values use prefix "0b".

   In this document, the symbol "&" denotes a bitwise AND of two
   unsigned values.

3.  Current Usage of DSCPs

   This section describes the 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 specified,
   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].  A summary of the pools is
   provided in a table (where 'x' refers to a bit position with value
   either '0' or '1').

   DSCP Pool 1:  A pool of 32 codepoints with a format of 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.
      This was initially available for Experimental (EXP) Use or Local
      Use (LU) but was originally specified to be "preferentially
      utilized for standardized assignments if Pool 1 is ever exhausted"
      [RFC2474].  Pool 3 codepoints are now "utilized for standardized
      assignments (replacing the previous availability for 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].

   Note that [RFC4594] previously recommended a Local Use of DSCP values
   0x01, 0x03, 0x05, and 0x07 (codepoints with the format of 0b000xx1),
   until this was updated by [RFC8436].

   The DSCP space is shown in the following table.  Each row corresponds
   to one setting of the first three bits of the DSCP field, and each
   column to one setting of the last three bits of the DSCP field.

      +--------+------+---------+----+---------+----+---------+----+
      | 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  |
      +========+======+=========+====+=========+====+=========+====+

        Table 1: Currently Assigned DSCPs and Their Assigned PHBs

                 +----+----------------------+-----------+
                 | CS | Class Selector       | [RFC2474] |
                 +----+----------------------+-----------+
                 | BE | Best Effort (CS0)    | [RFC2474] |
                 +----+----------------------+-----------+
                 | AF | Assured Forwarding   | [RFC2597] |
                 +----+----------------------+-----------+
                 | EF | Expedited Forwarding | [RFC3246] |
                 +----+----------------------+-----------+
                 | VA | Voice Admit          | [RFC5865] |
                 +----+----------------------+-----------+
                 | LE | Lower Effort         | [RFC8622] |
                 +----+----------------------+-----------+

                    Table 2: Abbreviations for DSCPs and
                                 PHB Groups

   Table 2 summarizes the DSCP abbreviations used in currently published
   RFCs, [RFC2474] [RFC2597] [RFC3246] [RFC5865] [RFC8622], as described
   in the IANA registry [DSCP-registry].  The Default PHB is defined in
   Section 4.1 of [RFC2474].  This provides Best Effort (BE) forwarding,
   and the recommended DSCP of '000000' (Section 4.2.2.1 of [RFC2474]).
   This is the lowest value in the set of Class Selector (CS) DSCPs, and
   hence is also known as "CS0" [DSCP-registry].

   NOTE: [RFC4594] specified a now deprecated use of Class Selector 1
   (CS1) as the codepoint for the Lower Effort PHB.  [RFC8622] updated
   [RFC4594] and [RFC8325] and obsoleted [RFC3662], assigning the LE
   DSCP codepoint to the Lower Effort PHB.

   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 (TAs) that might result in re-marking (e.g., [RFC5160]
   suggests Meta-QoS-Classes to help enable deployment of standard end-
   to-end QoS classes).

3.2.  DSCPs Used for 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).  The traffic consists of the network control service
   class and the OAM service class.  This traffic is marked with a value
   from a set of CS DSCPs.  This traffic is often a special case within
   a provider network, and ingress traffic with these DSCP markings can
   be re-marked.

   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] 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".

   DSCP CS7 is reserved for future use by network control traffic.  "CS7
   marked packets SHOULD NOT be sent across peering points" [RFC4594],
   Section 3.1.

   Section 4.2.2.2 of [RFC2474] recommends PHBs selected by CS6 and CS7
   "MUST give packets a 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 [IETF115-IEPG].  Similarly, another study found many
   routers re-mark all traffic, except for packets carrying a DSCP with
   the format 0b11xxxx (i.e., setting the higher order bits to 0b11, see
   Section 4.2.1 of this document).

4.  Re-marking the DSCP

   It is a feature of the Diffserv architecture that the Diffserv field
   of packets can be re-marked at the Diffserv domain boundaries (see
   Section 2.3.4.2 of [RFC2475]).  A DSCP can be re-marked at the
   ingress of a domain.  This re-marking 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 re-marked based on common semantics and agreements
   between providers at a Diffserv domain boundary.  Furthermore,
   [RFC2474] states that re-marking must occur when there is a
   possibility of theft or denial-of-service attack.

   A packet that arrives with a DSCP that is not associated with a PHB,
   results in an "unknown DSCP."  A node could receive a packet with an
   "unexpected DSCP" due to misconfiguration, or because there is no
   consistent policy in place.  The treatment of packets that are marked
   with an unknown or an unexpected DSCP at Diffserv domain boundaries
   is determined by the policy for a Diffserv domain.  If packets are
   received that are marked with an unknown or an unexpected DSCP by a
   Diffserv domain interior node, [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 re-marking specified by [RFC2474] is
   intended for interior nodes within a Diffserv domain.  For Diffserv
   ingress nodes, the traffic conditioning required by [RFC2475] applies
   first.

   Reports measuring existing deployments have defined a set of
   categories for DSCP re-marking [Cus17] [Bar18] in the following seven
   observed re-marking behaviors:

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

   Bleach-ToS-Precedence:  set the first three bits of the DSCP field to
      0b000 (reset the three 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
      (reset the three bits of the former ToS Precedence field), unless
      the first two bits of the DSCP field are 0b11

   Re-mark-ToS:  set the first three bits of the DSCP field to any value
      different from 0b000 (replace the three 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

   Re-mark-DSCP:  re-marks all traffic to one or more particular (non-
      zero) DSCP values

   These behaviors are explained in the following subsections and cross-
   referenced in the remainder of the document.

   The network nodes forming a particular path might or might not have
   supported Diffserv.  It is not generally possible for an external
   observer to determine which mechanism results in a specific re-
   marking solely from the change in an observed DSCP value.

   NOTE: More than one mechanism could result in the same DSCP re-
   marking (see below).  These behaviors were measured on both IPv4 and
   IPv6 Internet paths between 2017 and 2021 [Cus17].  IPv6 routers were
   found to perform all the types of re-marking described above to a
   lesser extent than IPv4 ones.

4.1.  Bleaching the DSCP Field

   A specific form of re-marking 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 [Cus17].

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].  [RFC2474] defines
   DSCPs 0bxxx000 as the Class Selector codepoints, where PHBs selected
   by these codepoints MUST meet the "Class Selector PHB Requirements"
   described in Section 4.2.2.2 of [RFC2474].

   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 [Cus17].

4.2.1.  Impact of ToS Precedence Bleaching

   Bleaching of the ToS Precedence field (see Section 4) 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 re-marking.  This re-marking 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 re-marking.

   +-+-+-+-+-+-+
   |5|4|3|2|1|0|
   +-+-+-+-+-+-+
   |0 0 0|x x x|
   +-+-+-+-+-+-+

   Figure 2: Bits in the Diffserv Field Modified by Bleaching of the ToS
                                 Precedence

   Figure 2 shows bleaching of the ToS Precedence (see Section 4), 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  |
      +========+======+=========+====+=========+====+=========+====+

                           Table 3: DSCP Values

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

   Data on the observed re-marking at the time of writing was presented
   in [IETF115-IEPG].

   +========+=======+===============+======+===+===+===========+======+
   | 0/CS0  | 1/LE  | 2             | 3    | 4 | 5 | 6         | 7    |
   +========+=======+===============+======+===+===+===========+======+
   | Assigned       | Re-marked     | EXP/ | * |   | Re-marked | EXP/ |
   |                | from AF11..41 | LU   |   |   | from      | LU   |
   |                |               |      |   |   | AF13..EF  |      |
   +----------------+---------------+------+---+---+-----------+------+

                         Table 4: 0b000xxx DSCPs

   *  DSCP 4 has been historically used by the SSH application [Kol10]

   Table 4 shows 0b000xxx DSCPs.  This highlights any current
   assignments and whether they are affected by any known re-marking
   behaviors, such as ToS Precedence Bleaching.

   DSCPs of the form 0b000xxx can be impacted by known re-marking
   behaviors of other assigned DSCPs.  For example, ToS Precedence
   Bleaching of popular DSCPs AF11, AF21, AF31, and AF41 would result in
   traffic being re-marked with DSCP 2 in the Internet core.  The Lower
   Effort (LE) Per-Hop Behavior PHB uses a DSCP of 1.  The DSCP value of
   4 has been historically used by the SSH application, following
   semantics that precede Diffserv [Kol10].

   Bleach-ToS-Precedence (see Section 4) of packets with a DSCP 'x'
   results in the DSCP being re-marked to 'x' & 0x07 and then forwarded
   using the PHB specified for the resulting DSCP in that Diffserv
   domain.  In subsequent networks, the packet will receive treatment as
   specified by the domain's operator corresponding to the re-marked
   codepoint.

   A variation of this observed re-marking behavior 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 Re-marking

   [RFC2474] states:

   |  Implementors should note that the DSCP field is six bits wide.
   |  DS-compliant nodes MUST select PHBs by matching against the entire
   |  6-bit DSCP field, e.g., by treating the value of the field as a
   |  table index which is used to select a particular packet handling
   |  mechanism which has been implemented in that device.

   This replaced re-marking according to ToS precedence (see Section 4)
   [RFC1349].  These practices based on ToS semantics have not yet been
   eliminated from deployed networks.

   +-+-+-+-+-+-+
   |5|4|3|2|1|0|
   +-+-+-+-+-+-+
   |0 0 1|x x x|
   +-+-+-+-+-+-+

      Figure 3: Bits in the Diffserv Field Modified by ToS Precedence
                                 Re-marking

   Figure 3 shows the ToS Precedence Re-marking (see Section 4) observed
   behavior, based on Section 3 of [RFC1349].  The bit positions marked
   'x' remain unchanged.

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

   If ToS Precedence Re-marking occurs, packets are forwarded using the
   PHB specified for the resulting DSCP in that domain.  For example,
   the AF31 DSCP (0x1a) could be re-marked to either AF11 or AF21.  If
   such a re-marked packet further traverses other Diffserv domains, it
   would receive treatment as specified by each domain's operator
   corresponding to the re-marked codepoint.

4.3.  Re-marking to a Particular DSCP

   A network device might re-mark the Diffserv field of an IP packet
   based on a local policy with a specific set of DSCPs (see Section 4).

   Section 3 of [RFC2474] recommends: "Packets received with an
   unrecognized codepoint SHOULD be forwarded as if they were marked for
   the Default behavior, and their codepoints should not be changed."
   Some networks might not follow this recommendation and instead re-
   mark packets with these codepoints to the default class: CS0 (0x00).
   This re-marking is sometimes performed using a Multi-Field (MF)
   classifier [RFC2475] [RFC3290] [RFC4594].

   If re-marking occurs, packets are forwarded using the PHB specified
   for the resulting DSCP in that domain.  As an example, re-marking
   traffic AF31, AF32, and AF33 all to a single DSCP, e.g., AF11, stops
   any drop probability differentiation, which may have been expressed
   by these three DSCPs.  If such a re-marked packet further traverses
   other domains, it would receive treatment as specified by the
   domain's operator corresponding to the re-marked codepoint.
   Bleaching (see Section 4) is a specific example of this observed re-
   marking behavior that re-marks to CS0 (0x00) (see Section 4.1).

5.  Interpretation of the IP DSCP at Lower Layers

   Transmission systems and subnetworks can, and do, utilize 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 re-mark the DSCP used at the IP
   layer.  This function is constrained by designs that utilize fewer
   than 6 bits to express the service class and, therefore, infer a
   mapping to a smaller L2 QoS field, for example, the 3-bit Priority
   Code Point (PCP) field in an IEEE Ethernet 802.1Q header, the 3-bit
   User Priority (UP) field, or the 3-bit Traffic Class field of Multi-
   Protocol Label Switching (MPLS).  A Treatment Aggregate (TA)
   [RFC5127] is an optional intermediary mapping group of BAs to PHBs.

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

   IEEE 802.1Q specified a 3-bit PCP field, which includes a tag that
   allows Ethernet frames to be marked as one of eight priority values
   [IEEE-802.1Q].  Use of this field is described by various documents,
   including IEEE P802.1p and IEEE 802.1D.

   The mapping specified in [IEEE-802.1Q] revises a previous standard,
   [IEEE-802.1D], in an effort to align with Diffserv practice
   [RFC4594].  In 802.1Q, the traffic types are specified to match the
   first three bits of a suitable DSCP (e.g., the first three bits of
   the Expedited Forwarding (EF) DSCP are mapped to a PCP of 5).

   In this mapping, PCP0 is used to indicate the default Best Effort
   treatment, and PCP1 indicates a background traffic class.  The
   remaining PCP values indicate increasing priority.  Internet control
   traffic can be marked as CS6, and network control is marked as CS7.

   Other re-marking behaviors have also been implemented in Ethernet
   equipment.  Historically, a previous standard, [IEEE-802.1D], used
   both PCP1 (Background) and PCP2 (Spare) to indicate lower priority
   than PCP0, and some other devices do not assign a lower priority to
   PCP1.

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 Media Access Control
   (MAC) functions to support QoS in WLANs using Access Categories
   (ACs).  The upstream UP in the 802.11 header has a 3-bit QoS value.
   A DSCP can be mapped to the UP.  [RFC8622] added a mapping for the LE
   DSCP to AC_BK (Background).

   Most current Wi-Fi implementations use a default mapping that maps
   the first three bits of the DSCP to the 802.11 UP value.  This is an
   example of equipment still classifying on ToS Precedence (which could
   be seen as a simple method to map IP layer Diffserv to layers
   offering only 3-bit QoS codepoints).  Then, in turn, this is mapped
   to the four Wi-Fi Multimedia (WMM) Access Categories.  The Wi-Fi
   Alliance has also specified a more flexible mapping that follows
   [RFC8325] and provides functions at an Access Point (AP) to re-mark
   packets as well as a QoS Map that maps each DSCP to an AC
   [WIFI-ALLIANCE].

   +-+-+-+-+-+-+
   |5|4|3|2|1|0|
   +-+-+-+-+-+-+
   |x x x|. . .|
   +-+-+-+-+-+-+

          Figure 4: 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] notes inconsistencies that can result from such re-marking
   and recommends a different mapping to perform this re-marking,
   dependent on the direction in which a packet is forwarded.  It
   provides recommendations for mapping a DSCP to an IEEE 802.11 UP for
   interconnection between wired and wireless networks.  The
   recommendation in Section 5.1.2 maps network control traffic, CS6 and
   CS7, as well as unassigned DSCPs, to UP 0 when forwarding in the
   upstream direction (wireless-to-wired).  It also recommends mapping
   CS6 and CS7 traffic to UP 7 when forwarding in the downstream
   direction (Section 4.1 of [RFC8325]).

   For other UPs, [RFC8325] recommends a mapping in the upstream
   direction (wireless-to-wired interconnections) that derives the DSCP
   from the value of the UP multiplied by 8.  This mapping, currently
   used by some Access Points (APs), can result in a specific DSCP re-
   marking behavior:

   |  wherein DSCP values are derived from UP values by multiplying the
   |  UP values 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.

   This can result in re-marking by Bleach-low (see Section 4).

   +-+-+-+-+-+-+
   |5|4|3|2|1|0|
   +-+-+-+-+-+-+
   |x x x|0 0 0|
   +-+-+-+-+-+-+

        Figure 5: Bits in the Diffserv Field Modified by Re-marking
         Observed as a Result of 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 Wi-Fi 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].  [RFC5127] describes the aggregation of Diffserv service
   classes and introduces four Diffserv TAs.  Traffic marked with
   multiple DSCPs can be forwarded in a single MPLS TC.

   There are three Label Switching Router (LSR) models: the Pipe, the
   Short Pipe, and the Uniform Model [RFC3270].  In the Uniform and Pipe
   models, the egress MPLS router forwards traffic based on the received
   MPLS TC.  The Uniform Model includes an egress DSCP rewrite.  With
   the Short Pipe Model, the egress MPLS router forwards traffic based
   on the Diffserv DSCP as present at the egress router.  If the domain
   supports IP and MPLS QoS differentiation, controlled behavior
   requires the DSCP of an (outer) IP header to be assigned or re-
   written by all domain ingress routers to conform with the domain's
   internal Diffserv deployment.  Note that the Short Pipe Model is
   prevalent in MPLS domains.

5.2.1.  Mapping Specified for MPLS

   [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].

   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] describes
   four TAs 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 codepoint scheme [RFC8100]
   are treated as unexpected and will possibly be re-marked (a Re-mark-
   DSCP, see Section 4 behavior) or dealt with via additional agreements
   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 favors re-marking unexpected DSCP values to zero in
   the absence of additional agreements, as explained in [RFC8100].
   This can result in bleaching (see Section 4).

           +=======================================+==========+
           | Treatment Aggregate [RFC8100]         |   DSCP   |
           +=======================================+==========+
           | Telephony Service Treatment Aggregate |    EF    |
           |                                       |    VA    |
           +---------------------------------------+----------+
           | Bulk Real-Time Treatment Aggregate    |   AF41   |
           |                                       | (AF42)*  |
           |                                       | (AF43)*  |
           +---------------------------------------+----------+
           | Assured Elastic Treatment Aggregate   |   AF31   |
           |                                       |   AF32   |
           |                                       | (AF33)** |
           +---------------------------------------+----------+
           | Default / Elastic Treatment Aggregate |  BE/CS0  |
           +---------------------------------------+----------+
           | Network Control: Local Use (LU)       |   CS6    |
           +---------------------------------------+----------+

           Table 5: The Short Pipe MPLS Mapping from [RFC8100]

   *  May be added

   **  Reserved for the extension of PHBs

5.3.  Mapping Specified for Mobile Networks

   Mobile LTE and 5G standards have evolved from older Universal Mobile
   Telecommunications System (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 Data Network Gateway (P-GW) boundary is
   determined by network operators.  However, TS 23.107 (version 16.0.0,
   applies to LTE and below) mandates that Differentiated Services,
   defined by the 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 IP Packet eXchange
   (IPX) Provider networks [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 provided 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
   this situation, the IPX Provider can re-mark the DSCP value to a
   static default value.

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

                  Table 6: The PHB Mapping Recommended in
                       the Guidelines Recommended in
                                [GSMA-IR.34]

5.4.  Mapping Specified for Carrier Ethernet

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

5.5.  Re-marking as a Side Effect of Another Policy

   This includes any other re-marking that does not happen as a result
   of traffic conditioning, such as policies and L2 procedures that
   result in re-marking 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 re-marking
   behaviors can arise from interactions with lower layers.

   A provider service path may consist of sections where multiple and
   changing layers use their own code points to determine differentiated
   forwarding (e.g., IP to MPLS to IP to Ethernet to Wi-Fi).

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.  This section identifies known issues that might
   influence the finally assigned DSCP and provides a summary of
   considerations for assignment of a new DSCP.

6.1.  Effect of Bleaching and Re-marking to a Single DSCP

   Section 4 describes re-marking of the DSCP.  New DSCP assignments
   should consider the impact of bleaching or re-marking (see Section 4)
   to a single DSCP, 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].
   Forwarding LE using the default PHB is in line with [RFC8622], but it
   is recommended to maintain the distinct LE DSCP codepoint end-to-end
   to allow for differentiated treatment by domains supporting LE.
   Rewriting the LE DSCP to the default class (CS0) results in an
   undesired promotion of the priority for LE traffic in such a domain.
   Bleaching the lower three bits of the DSCP (both Bleach-low and
   Bleach-some-low in Section 4), as well as re-marking to a particular
   DSCP, can result in similar changes of priority relative to traffic
   that is marked with other DSCPs.

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

   This considers a proposed DSCP with a codepoint larger than 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 the ToS Precedence is bleached (Bleach-ToS-
   Precedence and Bleach-some-ToS, Section 4) or ToS Precedence Re-
   marking (Re-mark-ToS, Section 4) is experienced.  For example, it can
   be desirable to avoid choosing a DSCP that could be re-marked to LE,
   Lower Effort [RFC8622], which could otherwise potentially result in a
   priority inversion in the treatment.

6.2.1.  Where the Proposed DSCP&0x07=0x01

   This considers a proposed DSCP where the least significant 3 bits
   together represent a value of 1 (i.e., 0b001).

   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 Bleach-ToS-Precedence (Section 4) 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 PHB.  Care should be taken
   to consider the implications of re-marking when choosing to assign a
   DSCP with this format.

6.3.  Where the Proposed DSCP <= 0x07 (7)

   This considers a proposed DSCP where the DSCP is less than or equal
   to 7.

   ToS Precedence Bleaching or ToS Precedence Re-marking 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 that
   receives the treatment associated with DSCP 2.  New DSCP assignments
   should consider unexpected consequences arising from this observed
   re-marking behavior.

6.4.  Impact on Deployed Infrastructure

   Behavior 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 re-marking behaviors described in the
   previous section can result from different configurations and
   approaches:

   Networks not re-marking Diffserv:
      A network that either does not implement PHBs or implements one or
      more PHBs while 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 Service
      Level Agreements (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]).
      Operators need to classify the received traffic, assign it to a
      corresponding PHB, and could re-mark the DSCP to a value that is
      appropriate for the domain's deployed Diffserv infrastructure.

   Networks that re-mark 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 re-mark the DSCP as a result
          of lower layer interactions.

   The DSCP re-marking corresponding to the Bleach-ToS-Precedence
   (Section 4) observed behavior can arise for various reasons, one of
   which is old equipment that precedes Diffserv.  The same re-marking
   can also arise in some cases when traffic conditioning is provided by
   Diffserv routers at operator boundaries or as a result of
   misconfiguration.

6.5.  Considerations to Guide the 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:

   *  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.

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

   *  Service classes [RFC4594] that can utilize 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 Internet
      Draft contain an appropriate request to IANA?

   *  The value selected for a new DSCP can impact the ability of an
      operator to apply logical functions (e.g., a bitwise mask) to
      related codepoints when configuring Diffserv.  A suitable value
      can simplify configurations by aggregating classification on a
      range of DSCPs.  This classification based on DSCP ranges can
      increase the comprehensibility of documenting forwarding
      differentiation.

   *  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 re-marking behavior, and
      Section 6.4 identifies root causes for why this re-marking is
      observed.  What is the expected effect of currently-deployed re-
      marking on the service, end-to-end or otherwise?

7.  IANA Considerations

   IANA has added the following text as a note at the top of the
   "Differentiated Services Field Codepoints (DSCP)" registry
   [DSCP-registry]: "See RFC 9435 for considerations when assigning a
   new codepoint from the DSCP registry."

8.  Security Considerations

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

9.  References

9.1.  Normative References

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

   [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>.

   [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>.

9.2.  Informative References

   [AX.25-over-IP]
              Learmonth, I. R., "Internet Protocol Encapsulation of
              AX.25 Frames", Work in Progress, Internet-Draft, draft-
              learmonth-intarea-rfc1226-bis-03, 23 May 2021,
              <https://datatracker.ietf.org/doc/html/draft-learmonth-
              intarea-rfc1226-bis-03>.

   [Bar18]    Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
              S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
              Study", 2018 30th International Teletraffic Congress (ITC
              30), DOI 10.1109/ITC30.2018.00034, September 2018,
              <https://doi.org/10.1109/ITC30.2018.00034>.

   [Cus17]    Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
              modification pathologies in mobile edge networks", 2017
              Network Traffic Measurement and Analysis Conference (TMA),
              DOI 10.23919/TMA.2017.8002923, June 2017,
              <https://doi.org/10.23919/TMA.2017.8002923>.

   [GSMA-IR.34]
              GSM Association, "Guidelines for IPX Provider networks
              (Previously Inter-Service Provider IP Backbone
              Guidelines)", Version 17.0, IR.34, May 2021,
              <https://www.gsma.com/newsroom/wp-content/uploads/
              IR.34-v17.0.pdf>.

   [IEEE-802.11]
              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",
              DOI 10.1109/IEEESTD.2021.9363693, IEEE Standard 802.11,
              February 2021,
              <https://standards.ieee.org/ieee/802.11/7028/>.

   [IEEE-802.1D]
              IEEE, "IEEE Standard for Local and metropolitan area
              network--Media Access Control (MAC) Bridges", IEEE
              Standard 802.1D-2004, DOI 10.1109/IEEESTD.2004.94569, June
              2004, <https://doi.org/10.1109/IEEESTD.2004.94569>.

   [IEEE-802.1Q]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Network--Bridges and Bridged Networks", IEEE Standard 
              802.1Q-2018, DOI 10.1109/IEEESTD.2018.8403927, July 2018,
              <https://doi.org/10.1109/IEEESTD.2018.8403927>.

   [IETF115-IEPG]
              Custura, A., "Real-world DSCP Traversal Measurements",
              November 2022,
              <https://datatracker.ietf.org/meeting/115/materials/
              slides-115-iepg-sessa-considerations-for-assigning-dscps-
              01>.

   [ITU-T-Y.1566]
              ITU-T Recommendation, "Quality of service mapping and
              interconnection between Ethernet, Internet Protocol and
              multiprotocol label switching networks", ITU-T Y.1566,
              July 2012, <https://www.itu.int/rec/T-REC-Y.1566/en>.

   [Kol10]    Kolu, A., "Subject: bogus DSCP value for ssh", message to
              the freebsd-stable mailing list, 12 July 2010,
              <https://lists.freebsd.org/pipermail/freebsd-
              stable/2010-July/057710.html>.

   [MEF-23.1] MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet
              Class of Service - Phase 2", MEF 23.1, January 2012,
              <https://mef.net/Assets/Technical_Specifications/PDF/
              MEF_23.1.pdf>.

   [NQB-PHB]  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-18, 10 July
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              tsvwg-nqb-18>.

   [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>.

   [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., Ed., 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>.

   [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>.

   [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>.

   [RFC5415]  Calhoun, P., Ed., Montemurro, M., Ed., and D. Stanley,
              Ed., "Control And Provisioning of Wireless Access Points
              (CAPWAP) Protocol Specification", RFC 5415,
              DOI 10.17487/RFC5415, March 2009,
              <https://www.rfc-editor.org/info/rfc5415>.

   [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>.

   [SA-5G]    3GPP, "System architecture for the 5G System (5GS)",
              TS 23.501, 2019.

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

Acknowledgements

   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.

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


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