Internet DRAFT - draft-ietf-tsvwg-dscp-considerations

draft-ietf-tsvwg-dscp-considerations







TSVWG                                                         A. Custura
Internet-Draft                                              G. Fairhurst
Intended status: Informational                                 R. Secchi
Expires: 4 September 2023                         University of Aberdeen
                                                            3 March 2023


Considerations for Assigning a new Recommended DiffServ Codepoint (DSCP)
                draft-ietf-tsvwg-dscp-considerations-13

Abstract

   This document discusses considerations for assigning a new
   recommended DiffServ Code Point (DSCP) for a new 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 4 September 2023.

Copyright Notice

   Copyright (c) 2023 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.  DSCPs used for Network Control Traffic  . . . . . . . . .   6
   4.  Re-marking the DSCP . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Bleaching the DSCP Field  . . . . . . . . . . . . . . . .   8
     4.2.  IP Type of Service manipulations  . . . . . . . . . . . .   9
       4.2.1.  Impact of ToS Precedence Bleaching  . . . . . . . . .   9
       4.2.2.  Impact of ToS Precedence Re-marking . . . . . . . . .  11
     4.3.  Re-marking to a Particular DSCP . . . . . . . . . . . . .  12
   5.  Interpretation of the IP DSCP at Lower Layers . . . . . . . .  12
     5.1.  Mapping Specified for IEEE 802  . . . . . . . . . . . . .  12
       5.1.1.  Mapping Specified for IEEE 802.1  . . . . . . . . . .  13
       5.1.2.  Mapping Specified for IEEE 802.11 . . . . . . . . . .  13
     5.2.  DiffServ and MPLS . . . . . . . . . . . . . . . . . . . .  15
       5.2.1.  Mapping Specified for MPLS  . . . . . . . . . . . . .  15
       5.2.2.  Mapping Specified for MPLS Short Pipe . . . . . . . .  15
     5.3.  Mapping Specified for Mobile Networks . . . . . . . . . .  17
     5.4.  Mapping Specified for Carrier Ethernet  . . . . . . . . .  18
     5.5.  Re-marking as a Side-effect of Another Policy . . . . . .  18
     5.6.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Considerations for DSCP Selection . . . . . . . . . . . . . .  19
     6.1.  Effect of Bleaching and Re-marking to a single DSCP . . .  19
     6.2.  Where the proposed DSCP > 0x07 (7)  . . . . . . . . . . .  19
       6.2.1.  Where the proposed DSCP&0x07=0x01 . . . . . . . . . .  19
     6.3.  Where the proposed DSCP <= 0x07 (7) . . . . . . . . . . .  20
     6.4.  Impact on deployed infrastructure . . . . . . . . . . . .  20
     6.5.  Considerations to guide the discussion of a proposed new
           DSCP  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Revision Notes . . . . . . . . . . . . . . . . . . .  26



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29

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 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 [RFC4594] and
   categorises it into Behavior Aggregates [RFC4594].  Configuration
   guidelines for service classes are provided in RFC4594 [RFC4594].
   Behavior aggregates are associated with a DiffServ Code Point (DSCP),
   which in turn maps to a Per Hop Behavior (PHB).  Treatment
   differentiation can be achieved 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 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
   Behavior Aggregates (BAs) as the packets pass through a DiffServ
   domain, and 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 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 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.




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   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
   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] [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.

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 0bxxxxx0, to be
      assigned by IANA Standards Action [RFC8126].

   DSCP Pool 2:  A pool of 16 codepoints with a format of 0bxxxx11,




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      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) or Local Use
      (LU), but was originally specified to be "preferentially utilized
      for standards assignments" if Pool 1 is ever exhausted.  Pool 3
      codepoints are now "utilized for standards 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].

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

   The DSCP space is shown in the following table.


   +---------+------+---------+-----+---------+-----+---------+----+
   | 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 showing the currently 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  |
   +-----+-----------------------+-----------+

   Table showing the summary of the DSCP abbreviations used in published
   RFCs.

   The above table summarises the DSCP abbreviations used in currently
   published RFCs [RFC2474] [RFC2597] [RFC3246] [RFC5865] [RFC8622], as
   described in the IANA registry [DSCP-registry].  BE, also known as
   CS0, describes the default forwarding treatment.

   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 that might result in re-marking (e.g., RFC5160 [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 Class Selector (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).



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   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 re-marked 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].

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

   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 an exchange point.  Furthermore, [RFC2474]
   states that re-marking must occur when there is a possibility of
   theft or denial-of-service attack.

   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 RFC 2475 applies first.




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   Reports measuring existing deployments have defined a set of
   categories for DSCP re-marking [Cus17] [Bar18] into 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 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
      (reset the 3 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 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;

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

   These behaviours 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.





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   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].  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 [Cus17].

4.2.1.  Impact of ToS Precedence Bleaching

   Bleaching of the ToS Precedence field (Bleach-ToS-Precedence
   (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.


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

   Figure showing bleaching of the ToS Precedence (Bleach-ToS-Precedence
   (Section 4)), based on Section 3 of [RFC1349].  The bit positions
   marked "x" are not changed.













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     +--------+------+---------+----+---------+----+---------+----+
     | 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 of 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 showing 0b000xxx DSCPs.  This highlights any current
   assignments and whether they are affected by any known re-marking
   behaviors, such as ToS Precdence bleaching.  * DSCP 4 has been
   historically used by the SSH application.  [Kol10].


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



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   Bleach-ToS-Precedence (Section 4) of packets with a DSCP 'x' result
   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 (Re-mark-ToS (Section 4))
   [RFC1349].  These practices based on ToS semantics have not yet been
   eliminated from deployed networks.


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

   Figure showing the ToS Precedence Re-marking (Re-mark-ToS
   (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.








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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 (Re-mark-DSCP
   (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 (Bleach-DSCP (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 PCP field
   in an IEEE Ethernet 802.1Q header, the 3-bit 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 groups of BAs to PHBs.

5.1.  Mapping Specified for IEEE 802

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








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5.1.1.  Mapping Specified for IEEE 802.1

   IEEE 802.1Q specified a 3-bit Priority Code Point (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 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.  This
   aligned with the now deprecated use of CS1 as the codepoint for the
   lower effort service, as previously specified in [RFC4594].  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 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.  [RFC8622] added mapping for the LE DSCP,
   mapping this 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 AP to re-mark packets as well as
   a QoS Map that maps each DSCP to an AC [WIFI-ALLIANCE].


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



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   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 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).

   For other UPs, RFC8325 recommends a mapping in the upstream direction
   that derives the DSCP from the value of the UP multiplied by 8.  This
   mapping can result in a specific DSCP re-marking behavior.

   In the upstream direction (wireless-to-wired interconnections), this
   mapping can result in a specific DSCP re-marking behavior.  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 a central 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 (Section 4).


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

   Figure showing the observed re-marking behavior 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 [RFC5415], maps an IP packet
   DiffServ field to the DiffServ field of the outer IP header in a
   CAPWAP tunnel).



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   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].  RFC 5127 describes the 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) 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 [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].




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   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 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 re-marked (a
   Re-mark-DSCP (Section 4) behavior) 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 re-marking
   unexpected DSCP values to zero in the absence of an additional
   agreement(s), as explained in [RFC8100].  This can result in
   bleaching (Bleach-DSCP (Section 4)).


   +--------------------------------------+--------+
   |  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 (LU)       |  CS6   |
   +--------------------------------------+--------+



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   Table: 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 network 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].  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 which situation the IPX Provider
   can re-mark 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   |
   +---------------+-------+

   Table showing the PHB mapping recommended in the guidelines
   recommended in [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
   [MEF23.1].

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 - MPLS - IP - Ethernet - Wi-Fi).







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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.

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 (Bleach-DSCP (Section 4)) or
   re-marking (Re-mark-DSCP (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 (Section 4) and Bleach-some-low
   (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)

   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 (Section 4) 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

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






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   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 traffic class.  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)

   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 which
   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 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 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.





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      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 described in Section 4 can arise for
   various reasons, one of which is old equipment which 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.





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   *  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.  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.

8.  IANA Considerations

   IANA is requested to append the page for the Differentiated Services
   Field Codepoints (DSCP) registry at:
   https://www.iana.org/assignments/dscp-registry/dscp-registry.xhtml.
   This request is to add the following separate paragraph to the Note
   at the top of the registry page: "See [RFC-to-be] for considerations
   when assigning a new codepoint from the DSCP registry."

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>.



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   [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>.

10.2.  Informative References

   [Bar18]    Barik, R., Welzl, M., Elmokashfi, A., Dreibholz, T., and
              S. Gjessing, "Can WebRTC QoS Work? A DSCP Measurement
              Study", ITC 30, September 2018.

   [Cus17]    Custura, A., Venne, A., and G. Fairhurst, "Exploring DSCP
              modification pathologies in mobile edge networks", TMA ,
              2017.



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   [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-15, 11
              January 2023, <https://datatracker.ietf.org/doc/html/
              draft-ietf-tsvwg-nqb-15>.

   [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://datatracker.ietf.org/doc/html/draft-learmonth-
              rfc1226-bis-03>.

   [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.

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

   [Kol10]    Kolu, A., "Bogus DSCP value for SSH", online 
              https://lists.freebsd.org/pipermail/freebsd-
              stable/2010-July/057710.html, 2010.



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   [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>.

   [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>.

   [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>.





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   [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 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.





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   *  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).

         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).




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         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 Ruediger Geib).

         Add note on understanding the process of re-marking (comments
         from Ruediger Geib).

         Improve readability.

   *  Working Group -06

         Quote RFC2474 with respect to IP precedence (comments from
         Ruediger Geib).

         Ensure it is clear that different re-marking processes may
         result in the same observed re-marking.

         Clarify Treatment Aggregates are part of methods such as MPLS
         (comments from David Black).

         Clarify implications on the rest of the path by re-marking in
         one domain.

         Include all observed re-marking behaviors in Section 6.

         Remove mentions of DSCP 5 being provisionally assigned to NQB.

         Clarify scope of network control traffic in Section 3.2.

         Improve readibility.

   *  Working Group -07

         Update Section 4 to clarify both types of paths measured.

         Revised paragraph 2 in Introduction

   *  Working Group -08

         Update after Shepherd review with additional comments from R.
         Geib.  D.  Black and B.  Carpenter provided comments on
         relationship to RFC 2474.

   *  Working Group -09



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         Updates to document structure to avoid references in artwork
         legend.

         Fix DSCP table indentation

         Update ref to nqb draft to -15

   *  Working Group -10

         Document updated after AD review

         Add clarification on former use of CS1

   *  Working Group -11

         Updated to complete response to AD review and resolved
         pathology types to xrefs.

   *  Working Group -12

         Finalize response to AD review, address comment from Brian
         Carpenter.

   *  Working Group -13

         Review by Erik Kline

         Added recommended change by IANA to cite this document from the
         registry when it is published.

         The latest DSCP contribution to IEPG was at IETF-115.

         Consistently use re-mark instead of remark.

         Improve artwork abbreviations

         Address NiTs from John Scudder

Authors' Addresses

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



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   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

































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