Considerations for Assigning a new Recommended DiffServ Codepoint (DSCP)
draft-ietf-tsvwg-dscp-considerations-00

Document Type Active Internet-Draft (tsvwg WG)
Authors Ana Custura  , Gorry Fairhurst  , Raffaello Secchi 
Last updated 2021-07-26
Replaces draft-custura-tsvwg-dscp-considerations
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TSVWG                                                         A. Custura
Internet-Draft                                              G. Fairhurst
Intended status: Informational                                 R. Secchi
Expires: January 24, 2022                         University of Aberdeen
                                                           July 23, 2021

Considerations for Assigning a new Recommended DiffServ Codepoint (DSCP)
                draft-ietf-tsvwg-dscp-considerations-00

Abstract

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

Status of This Memo

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

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

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1.  Introduction

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

   Internet traffic can be associated with a service class, and
   categorised into Behavior Aggregates [RFC4594].  In IP networks,
   these are associated with a DiffServ Code Point (DSCP) [RFC2474].
   Each DSCP is mapped to a Per Hop Behaviour (PHB).  A treatment
   aggregate is concerned only with the forwarding treatment of the
   aggregated traffic, which could be marked with multiple DSCPs
   [RFC5127].  Treatment differentiation can be realised using a variety
   of schedulers and queues, and also by algorithms that implement
   access to the physical media.

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

   Figure showing the role of DSCPs in classifying IP traffic for
   differential network treatment.

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

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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

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3.  Current usage of DSCPs

   This section describes current usage of DSCPs.

3.1.  IP-Layer Semantics

   The DiffServ architecture specifies use of the Diffserv field in the
   IPv4 and IPv6 packet headers to carry one of 64 distinct DSCP values.
   Within a given administrative boundary, each DSCP value can be mapped
   to a distinct PHB[RFC2474].  When a new PHB is standardized, a
   recommended DSCP value among those 64 values is normally reserved for
   that PHB.

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

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

   DSCP Pool 2  A pool of 16 codepoints with a format 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 0bxxxx01.  This was
      initially available for experimental or local use, but which was
      originally specified to be preferentially utilised for
      standardized assignments if Pool 1 is ever exhausted.  Pool 3
      codepoints are now utilised for standardized assignments and are
      no longer available for experimental or local use [RFC8436].

   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.  Several IP standards map treatment
   aggregates to DSCPs, that might result in remarking: RFC5160
   [RFC5160] suggests Meta-QoS-Classes to help enable deployment of
   standardized end-to-end QoS classes.

   Note: A DSCP is sometimes referred to by name, such as "CS1", and
   sometimes by a decimal, hex, or binary value [DSCP-registry].

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3.2.  Network Control Traffic

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

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

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

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

4.  Remarking the DSCP

   It is a feature of the DiffServ architecture that the Diffserv field
   of packets can be remarked at domain boundaries (see section 2.3.4.2
   of RFC2475 [RFC2475]).  A DSCP can be remarked at the ingress of a
   DiffServ domain.  This can be with or without restoring the initial
   DSCP marking at the egress of the same domain.  The Diffserv field
   can also be re-marked based upon common semantics and agreements
   between providers at an exchange point.

   If packets are received that are marked with an unknown or an
   unexpected DSCP, RFC2474 [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 preserving DSCP markings by routers that
   have not been configured to support DiffServ.  (See also
   Section 4.3).

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

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

   Bleach-ToS-Precedence:  set the first three bits of the DSCP field to
      0b000 (reset the 3 bits of the former ToS Precedence field) ;

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

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

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

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

4.1.  Bleaching the DSCP Field

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

   Resetting all the bits of the DiffServ field to 0 is more prevalent
   at the edge of the network, and rather less common in core networks
   [Custura].

4.2.  IP Type of Service manipulations

   The IETF first defined ToS precedence for IP packets [RFC0791],
   updated by specification as a part of the ToS Field RFC1349
   [RFC1349].  Since 1998, this practice has been deprecated by RFC2474
   [RFC2474].  RFC 2474 defines codepoints 0x xxx000 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, practices based on ToS semantics have not yet been
   eliminated from Internet, and need to still be considered when making
   new DSCP assignments.

4.2.1.  Impact of ToS Precedence Bleaching

   ToS Precedence Bleaching (/Bleach-ToS-Precedence/) is a practice that
   resets to zero the upper 3 bits of the DSCP field (the former ToS
   Precedence field), leaving the lower bits unchanged (see section
   4.2.1 of RFC2474 [RFC2474]).  This behaviour is distinctive of non-
   DiffServ aware routers and one of the more common manipulations of
   the DiffServ field.

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

   Figure showing the ToS Precedence Bleaching (/Bleach-ToS-Precedence/)
   pathology, based on Section 3 of RFC1349 [RFC1349].

   If ToS Precedence Bleaching occurs, packets with a DSCP 'x' would be
   remarked and then would be treated according to the PHB specified for
   'x' & 0x07.  For example, AF31 (0x1a) would be remarked to DSCP 2
   (0x02).

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

4.2.2.  Impact of ToS Precedence Remarking

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

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

   Figure showing an example of ToS Remarking (/Remark/) pathology,
   based on Section 3 of RFC1349 [RFC1349].

   A less common behaviour, ToS precedence remarking sets the upper
   three bits of the DSCP field to a non-zero value corresponding to a
   CS PHB.  This behaviour is distinctive of non-DiffServ aware routers.

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

4.3.  Remarking to a Particular DSCP

   A network device might remark the Diffserv field of an IP packet
   based on a local policy with a specific (set of) DSCPs, /Remark/.
   This is sometimes performed using a Multi-Field (MF) classifier
   [RFC2475] [RFC3290] [RFC4594].  For example, a common behaviour is to
   mark all traffic to a single DSCP, thus removing any traffic

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   differentiation (see Section 4.1).  Bleaching (/Bleach/) is a
   specific example of this that remarks to CS0 (0x00).

   In another example, some networks do not follow the recommendation in
   RFC2475 [RFC2475], and instead remark packets with an unknown or
   unexpected DSCP to the default class, CS0 (0x00) to ensure that
   appropriate DSCPs are used within a DiffServ domain.  (e.g., see
   [RFC8100]).

5.  Interpretation of the IP DSCP at Lower Layers

   Transmission systems and subnetworks can, and do, utilise the
   Diffserv field in an IP packet to select a lower layer forwarding
   treatment.  In many cases, this use is constrained by designs that
   utilise fewer than 6 bits to express the service class, and therefore
   infer mappings to a smaller L2 QoS field, for example, WiFi or Multi-
   Protocol Label Switching (MPLS).

5.1.  Mapping Specified for IEEE 802.11

   << This section is currently seeking more input. >>

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

   The mapping specified in [IEEE-802-1Q] revises a previous standard
   [IEEE-802-1D], in an effort to align with Diffserv practice: the
   traffic types are specified to match the first three bits of a
   suitable DSCP (for example, Voice, which has PCP value 5, matches the
   first three bits of EF).  However, [IEEE-802-1D] maps both PCP1
   (Background) and PCP2 (Spare) to indicate lower priority than PCP0,
   RFC 8622.  Therefore, different behaviour is expected depending on
   the age of deployed devices.

   Section 6 of RFC 8325 [RFC8325] provides a brief overview of IEEE
   802.11 QoS.  The IEEE 802.11 standards [IEEE-802-11] provide MAC
   functions to support QoS in WLANs using Access Classes (AC).  The
   upstream User Priority (UP) in the 802.11 header has a 3-bit QoS
   value.  A DSCP can be mapped to the UP.  Most existing WiFi
   implementations [RFC8325] use a default mapping that utilises the
   three most significant bits of the DiffServ Field to the 802.11 UP.
   This is then in turn mapped to the four WiFi Multimedia (WMM) Access
   Categories.

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   RFC 8325 [RFC8325] proposes several recommendations for mapping a
   DSCP to an IEEE 802.11 UP for wired-to-wireless interconnection.  The
   DSCP value of a downstream IP packet can be used (e.g., the Control
   And Provisioning of Wireless Access Points (CAPWAP) protocol,
   RFC5415, maps an IP packet Diffserv field to the Diffserv field of
   outer IP headier in a CAPWAP tunnel).

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

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

   Figure showing the DSCP bits commonly mapped to the UP in 802.11.

   This UP mapping can result in a specific DSCP remarking pathology.
   In the upstream direction, some Access Points (APs) currently use a
   default UP-to-DSCP mapping RFC8325 [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 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 behaviour can result in remarking /Reset-low/.

   RFC8325 [RFC8325] notes inconsistencies that can result from such
   remarking, and recommends how to perform this remarking.

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

   Figure showing the DSCP bits commonly mapped to the UP in 802.11.

5.2.  Mappings Specified for MPLS

   << This section is currently seeking more input. >>

   Multi-Protocol Label Switching (MPLS) specified eight MPLS Traffic
   Classes (TCs), which restricts the number of different treatments

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   (e.g., see [RFC5129]).  RFC 5127 describes aggregation of DiffServ
   TCs [RFC5127], and introduces four DiffServ Treatment Aggregates.
   Traffic marked with multiple DSCPs can be forwarded in a single MPLS
   TC.

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

5.2.1.  Mappings 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].

   Using the Pipe Model, the "LSP Diff-Serv Information" is conveyed 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 set of PHB to encapsulation mappings for the label
   corresponding to the exposed header to perform DiffServ Information
   Encoding (Section 2.5.2 of RFC3270 [RFC3270]).

5.2.2.  Mappings Specified for MPLS Short Pipe

   << This section is currently seeking more input.  >>

   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 [RFC8100]
   proposes four treatment aggregates for interconnection with four
   defined DSCPs.  This was motivated by the requirements of MPLS
   network operators that use Short-Pipe tunnels, but can be applicable
   to other networks, both MPLS and non-MPLS.

   RFC8100 recommends preserving the notion of end-to-end service
   classes, and recommends that the original DSCP marking is not changed
   when treatment aggregates are used.  The key requirement is that the
   DSCP at the network ingress is restored at the network egress.  This

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   is only feasible in general for a small number of DSCPs.  In this
   design, packets marked with other DSCPs can be re-marked (This can
   result in the /Remark/ behaviour) or dealt with via an additional
   agreement(s) among the operators of the interconnected networks.  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 [RFC8100].  This can result in bleaching
   (/Bleach/).

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

   Figure showing the short-pipe MPLS mapping from RFC 8100.

5.3.  Mapping Specified for Mobile Networks

   << This section is currently seeking more input. >>

   Mobile LTE and 5G standards have evolved from older UMTS standards,
   and are Diffserv aware.  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 mappings between each QCI or 5QI and Diffserv (and mobile
   QCIs are based on several criteria service class definitions).  The
   way packets are mapped at the Packet Gateway (P-GW) boundary is
   determined by operators.  However, TS 23.107 (version 16.0.0, applies
   to LTE and below) mandates that Differentiated Services, defined by
   IETF, shall be used to interoperate with IP backbone networks.

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   The GSM Association (GSMA) has defined four aggregated classes and
   seven associated PHBs in their guidelines for IPX Provider networks
   GSMA IR.34 [GSMA-IR-34].  This was previously specified as the Inter-
   Service Provider IP Backbone Guidelines, and provides a mobile ISP to
   ISP QoS mapping mechanism, and interconnection with other IP networks
   in the general Internet.  This describes the case where the DSCP
   marking from a Service Provider cannot be trusted (depending on the
   agreement between the Service Provider and its IPX Provider),
   allowing the IPX Provider to correct the DSCP value to a static
   default value.

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

   Figure showing the PHB mapping from GSMA IR.34 [GSMA-IR-34].

5.4.  Mappings Specified for Carrier Ethernet

   << This section is currently seeking more input. >>

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

5.5.  Remarking as a Side-effect of Another Policy

   The result of applying a QoS policy (such as matching the IP address,
   or traffic reaching a rate limit) could also result in a packet being
   remarked to a different DSCP (/Remark/) when it is not admitted to a
   service.  Traffic marked with an EF and VA DSCP are often policed by
   such policies.

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   Policies and L2 procedures can also result in remarking traffic as a
   side-effect of other functions (e.g., in response to DDos).

5.6.  Summary

   << This section might contain a summary table >>

6.  Considerations for DSCP Selection

   This section provides advice for the assignment of a new DSCP value.
   It identifies known issues that might influence the finally assigned
   DSCP, followed by a summary of considerations for assignment of a new
   DSCP.

6.1.  Effect of Bleaching

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

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

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

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

   ToS bleaching can unintentionally result in extra traffic aggregated
   to the same DSCP.  For example, after experiencing ToS 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 DiffServ
   codepoint assignments should consider unexpected consequences arising
   from ToS bleaching.

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

   As a consequence of assigning the LE PHB [RFC8622], the IETF
   allocated the DSCP 000001b from Pool 3.

   When making assignments where the DSCP has a format: xxx 001b, the
   case of ToS Precedence Bleaching (/Bleach-ToS-Precedence/) of a DSCP
   to a value of 0x01 needs to be considered.  ToS Precedence Bleaching
   will result in demoting the traffic to the lower effort traffic
   class.  Care should be taken to consider the implications that a DSCP
   in this category could be remarked as LE.

6.4.  Impact on deployed infrastructure

   Behaviour of deployed PHBs and conditioning treatments also needs to
   be considered when assigning a new DSCP.  In some domains, a network
   operator can provide transparent transport of unknown or unassigned
   DSCPs across their domain.  In other domains, policing can ensure
   that only traffic that matches a policy is permitted to use a
   specific DSCP (e.g., as in MPLS TC).

6.5.  Questions to guide discussion of a proposed new DSCP

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

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

   o  Service Classes that can utilise existing PHBs should use assigned
      DSCPs to mark their traffic: Could the request be met by using an
      existing IETF DSCP?  How would the proposed new DSCP be used to
      map traffic to a new or existing PHB?

   o  Diffserv domains can use the codepoints in Pool 2 for local or
      experimental use, without any IETF specification for the DSCP or
      associated PHB: Could the request be met by using a DSCP chosen
      from Pool 2?

   o  This documents describes some observed marking behaviors (see also
      below): What is the expected effect of currently-deployed
      remarking on the end-to-end service?

   o  Many DiffServ domains support only a small number of treatment
      aggregates: What are the effects of treatment aggregation on the
      proposed method?

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   o  This documents describes some observed treatments by layers below
      IP: What are the implications of using the proposed DSCP on the
      expected lower layers over which the traffic may be forwarded?

   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].

7.  Acknowledgments

   The authors acknowledge the helpful discussions and analysis by Greg
   White and Thomas Fossati in a draft concerning NQB.  We look forward
   to further comments and review.

8.  IANA Considerations

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

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

9.  Security Considerations

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

10.  References

10.1.  Normative References

   [DSCP-registry]
              IANA, "Differentiated Services Field Codepoints (DSCP)
              Registry".

   [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>.

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   [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>.

   [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

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

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

   [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", draft-
              ietf-tsvwg-nqb-03 (work in progress), November 2020.

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   [I-D.learmonth-rfc1226-bis]
              Learmonth, I., "Internet Protocol Encapsulation of AX.25
              Frames", draft-learmonth-rfc1226-bis-03 (work in
              progress), May 2020.

   [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.

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

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

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

   [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>.

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

   [RFC5127]  Chan, K., Babiarz, J., and F. Baker, "Aggregation of
              Diffserv Service Classes", RFC 5127, DOI 10.17487/RFC5127,
              February 2008, <https://www.rfc-editor.org/info/rfc5127>.

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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>.

   [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>.

   [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.

Appendix A.  Revision Notes

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

   o  Individual draft -00

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

   o  Individual draft -02; Revisions to improve language consistency.

   o  Working Group -00, replacing draft-ietf-tsvwg-dscp-considerations-
      02.

Appendix B.  Table of DSCP Values

   This table may help in the discussion of DSCP remarking.  The current
   assignments are at: Section 8.

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   +-------+------+------+------+------+------+------+------+------+
   |Hi \ Lo|0b 000|0b 001|0b 010|0b 011|0b 100|0b 101|0b 110|0b 111|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 000|BE/DE |LE    |  CU  |Pool 3|  CU  |  CU  |  CU  |Pool 3|
   |       | CSO  |      |      |LU/EXP|      |      |      |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 001|CS1   |  CU  |AF11  |LU/EXP|AF12  |  CU  |AF13  |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 010|CS2   |  CU  |AF21  |LU/EXP|AF22  |  CU  |AF23  |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 011|CS3   |  CU  |AF31  |LU/EXP|AF32  |  CU  |AF33  |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 100|CS4   |  CU  |AF41  |LU/EXP|AF42  |  CU  |AF43  |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 101|CS5   |  CU  |  CU  |LU/EXP|VA    |  CU  |EF    |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 110|CS6   |  CU  |  CU  |LU/EXP|  CU  |  CU  |  CU  |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+
   | 0b 111|CS7   |  CU  |  CU  |LU/EXP|  CU  |  CU  |  CU  |LU/EXP|
   +-------+------+------+------+------+------+------+------+------+

   LU/EXP - Local or Experimental Use
   CU - Currently Unassigned (reserved for IANA allocation)

   Table: Summary of Currently Assigned DSCP Values

Appendix C.  Example of operational practice and operator requirements.

   RFC5127 backbone networks are often over provisioned under stable
   operating conditions.  Operating and maintaining a plethora of
   differentiated, DSCP based service differentiations can be
   operationally difficult.  Network Operations staff might not be happy
   to reconfigure all or a majority of the backbone routers in response
   to frequent DSCP-to-PHB mapping table configuration changes.

   Instead, operational practice might prefer a simple to configure and
   operate, comprehensible with limited expertise for monitoring and
   debugging.

   - For discussion:

   There are a set of different expectations of traffic sources and
   sinks that set DSCP markings:

   o  Is the setting behaviour intended to enable a local or sectional
      differential treatment, without expecting end-to-end service
      differentiation?

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         Example: A sender domain that sets DSCPs to influence stream
         playout priorities in receiver browsers, or any/many unknown
         intentions when setting DSCPs.

   o  Is the setting behaviour intended to enable a local or sectional
      differential treatment, without expecting end-to-end service
      differentiation?

         Example: A sender domain that sets DSCPs to influence stream
         playout priorities in receiver browsers, or any/many unknown
         intentions when setting a DSCP.

   o  Is the setting behaviour intended to enable end-to-end service
      differentiation with an expectation of interconnection agreements
      in place?

         Example: Provider interconnection agreements, e.g., for the
         public telephony service.

   o  Is the setting behaviour intended to enable end-to-end service
      differentiation without an expectation of interconnection
      agreements in place?

         Example: The Non-Queue-Building or Lower Effort PHBs.

Authors' Addresses

   Ana Custura
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE
   UK

   Email: ana@erg.abdn.ac.uk

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE
   UK

   Email: gorry@erg.abdn.ac.uk

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   Raffaello Secchi
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen  AB24 3UE
   UK

   Email: r.secchi@abdn.ac.uk

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