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IETF RFC 8436
Update to IANA Registration Procedures for Pool 3 Values in the Differentiated Services Field Codepoints (DSCP) Registry
Last modified on Tuesday, August 21st, 2018
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Internet Engineering Task Force (IETF) G. Fairhurst
Request for Comments: 8436 University of Aberdeen
Updates: 2474 August 2018
Category: Standards Track
ISSN: 2070-1721
Update to IANA Registration Procedures for Pool 3 Values in the
Differentiated Services Field Codepoints (DSCP) Registry
 Abstract
The Differentiated Services (Diffserv) architecture specifies use of
the DS field in the IPv4 and IPv6 packet headers to carry one of 64
distinct differentiated services field codepoint (DSCP) values. The
Internet Assigned Numbers Authority (IANA) maintains a registry of
assigned DSCP values.
This update to RFC 2474 changes the IANA registration policy for Pool
3 of the registry (i.e., DSCP values of the form xxxx01) to Standards
Action, i.e., values are assigned through a Standards Track or Best
Current Practice RFC. The update also removes permission for
experimental and local use of the codepoints that form Pool 3 of the
DSCP registry; Pool 2 Codepoints (i.e., DSCP values of the form
xxxx11) remain available for these purposes.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 8436.
Fairhurst Standards Track PAGE 1 
RFC 8436 IANA Registration Procedures for DSCP Pool 3 August 2018
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. The Updates to RFC 2474 . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
The Differentiated Services (Diffserv) [RFC 2475] architecture
(updated by [RFC 3260]) provides scalable service differentiation in
the Internet. Diffserv uses the six most significant bits of the
former IPv4 Type of Service (TOS) octet or the former IPv6 Traffic
Class octet to convey the field, which is used to carry the DSCP.
This DSCP value is used to select a Diffserv per-hop behavior (PHB).
The six-bit field is capable of conveying 64 distinct codepoints, and
this codepoint space has been divided into three pools for the
purpose of codepoint assignment and management (as shown in
Figure 1). Pool 1 comprises 32 codepoints [RFC 2474]. These are
assigned by Standards Action, as defined in [RFC 8126]. Pool 2
comprises a pool of 16 codepoints reserved for Experimental or Local
Use (EXP/LU) as defined in [RFC 2474]. Pool 3 comprises 16
codepoints, which were originally specified as "initially available
for experimental or local use, but which should be preferentially
utilized for standardized assignments if Pool 1 is ever exhausted" by
[RFC 2474].
Fairhurst Standards Track PAGE 2
RFC 8436 IANA Registration Procedures for DSCP Pool 3 August 2018
+------+-----------------+
| Pool | Codepoint Space |
+------+-----------------+
| 1 | xxxxx0 |
+------+-----------------+
| 2 | xxxx11 |
+------+-----------------+
| 3 | xxxx01 |
+------+-----------------+
Figure 1: Format of the Field for Codepoints Allocated in the Three
IANA Pools (Where "x" Refers to Either "0" or "1")
At the time of writing this document, 22 of the 32 Pool 1 codepoints
have been assigned.
Although Pool 1 has not yet been completely exhausted, there is a
need to assign codepoints for particular PHBs that are unable to use
any of the unassigned values in Pool 1. This document changes the
IANA registration policy of Pool 3 to assignment by Standards Action.
(Section 4.9 of [RFC 8126] defines this as "assigned only through
Standards Track or Best Current Practice RFCs in the IETF Stream".)
An example is the need to assign a suitable recommended default
codepoint for the Lower Effort (LE) PHB [LE-PHB]. The LE PHB is
designed to protect best-effort (BE) traffic (packets forwarded with
the default PHB) from LE traffic in congestion situations (when
resources become scarce, best-effort traffic has precedence over LE
traffic and is allowed to preempt it). In deployed networks,
bleaching (i.e. intentionally setting to zero) of the IP Precedence
field continues to be used. (Setting the IP Precedence field to zero
disables any class-based flow management by routers configured with
TOS-based packet processing.) This causes the first three bits of
the former TOS byte (now the upper part of the DSCP field) to become
zero. Therefore, there is a need to avoid this remapping of the DSCP
for the LE PHB by assigning a codepoint that already has a zero value
in the first three bits [LE-PHB].
Furthermore, if the LE PHB were to have been assigned one of the
currently unused Pool 1 codepoints with a zero value in the first
three bits, any bleaching of the IP Precedence field would result in
other (higher assurance) traffic being also remapped to the assigned
DSCP. This remapping could then cause Diffserv-marked traffic to
receive an unintentional LE treatment for the remainder of the
Internet path. Therefore, it is important to avoid the resulting
priority inversion. The absence of unassigned codepoints in Pool 1
that exhibit these important properties motivates assigning a Pool 3
codepoint as the default that is recommended for use with this PHB.
Fairhurst Standards Track PAGE 3
RFC 8436 IANA Registration Procedures for DSCP Pool 3 August 2018
To allow the IETF to utilize Pool 3 codepoints, this document
requests IANA to manage Pool 3 assignments for DSCP values in Pool 3
via the Standards Action policy [RFC 8126].
2. Terminology
This document assumes familiarity with the terminology used in
[RFC 2475] updated by [RFC 3260].
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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
3. The Updates to RFC 2474
This document updates Section 6 of [RFC 2474] in the following ways.
It updates the following text concerning the assignment policy:
OLD: which are initially available for experimental or local use,
but which should be preferentially utilized for standardized
assignments if Pool 1 is ever exhausted.
NEW: which are utilized for standardized assignments (replacing the
previous availability for experimental or local use).
It removes the footnote in RFC 2474 relating to Pool 3:
DELETE: "(*) may be utilized for future Standards Action allocations
as necessary"
The new registry assignment policy is shown in Figure 2.
Pool Codepoint Space Assignment Policy
---- --------------- ------------------
1 xxxxx0 Standards Action
2 xxxx11 EXP/LU
3 xxxx01 Standards Action
Note for Pool 2: "Reserved for Experimental or Local Use"
Figure 2: Updated Assignment Policy for the DSCP Registry
Fairhurst Standards Track PAGE 4
RFC 8436 IANA Registration Procedures for DSCP Pool 3 August 2018
4. Security Considerations
Security considerations for the use of DSCP values are described in
the RFCs that define their usage. This document does not present new
security considerations.
5. IANA Considerations
IANA has changed the use of Pool 3 in the "Differentiated Services
Field Codepoints (DSCP)" registry and will manage this pool using
Standards Action, as defined as Section 4.9 of [RFC 8126].
IANA has made the following changes to the "Differentiated Services
Field Codepoints (DSCP)" registry, made available at [Registry].
IANA has referenced RFC 2474 and Section 4 of RFC 3260 for the
overall format of this registry.
IANA has referenced RFC 2474 and Section 4 of RFC 3260 for Pool 1.
This document does not modify the IANA registry text for Pool 2.
This pool continues to preserve the note shown in Figure 2.
The previous registry text for Pool 3:
3 xxxx01 Experimental or local use may be utilized for future
Standards Action allocations as necessary.
is replaced with the following registry text:
3 xxxx01 Standards Action.
To manage codepoints in Pool 3, IANA has created and will maintain
the "DSCP Pool 3 Codepoints" subregistry. Pool 3 of the registry has
been created initially empty, with a format identical to that used
for "DSCP Pool 1 Codepoints".
IANA has referenced RFC 2474, Section 4 of RFC 3260, and the current
document for Pool 3.
The registration procedure for use of Pool 3 is Standards Action, as
defined as Section 4.9 of [RFC 8126]. IANA is expected to normally
make assignments from Pool 1, until this Pool is exhausted, but it
MAY make assignments from Pool 3 when the format of the codepoint has
properties that are needed for a specific PHB. The required
characteristics for choosing a requested DSCP value MUST be explained
in the IANA Considerations section of the document that requests any
assignment from Pool 3.
Fairhurst Standards Track PAGE 5
RFC 8436 IANA Registration Procedures for DSCP Pool 3 August 2018
6. References
6.1. Normative References
[Registry]
IANA, "Differentiated Services Field Codepoints (DSCP)",
<https://www.iana.org/assignments/dscp-registry/>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 2474] 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/RFC 2474, December 1998,
<https://www.rfc-editor.org/info/RFC 2474>.
[RFC 3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC 3260, DOI 10.17487/RFC 3260, April 2002,
<https://www.rfc-editor.org/info/RFC 3260>.
[RFC 8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC 8126, June 2017,
<https://www.rfc-editor.org/info/RFC 8126>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
6.2. Informative References
[LE-PHB] Bless, R., "A Lower Effort Per-Hop Behavior (LE PHB)",
Work in Progress, draft-ietf-tsvwg-le-phb-05, July 2018.
[RFC 2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC 2475, December 1998,
<https://www.rfc-editor.org/info/RFC 2475>.
Acknowledgments
Godred Fairhurst received funding from the European Union's Horizon
2020 research and innovation program 2014-2018 under grant agreement
No. 644334 (NEAT).
Fairhurst Standards Track PAGE 6
RFC 8436 IANA Registration Procedures for DSCP Pool 3 August 2018
Author's Address
Godred Fairhurst
University of Aberdeen
Department of Engineering
Fraser Noble Building
Aberdeen AB24 3UE
United Kingdom
Email: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/
Fairhurst Standards Track PAGE 7
Update to IANA Registration Procedures for Pool 3 Values in the Differentiated Services Field Codepoints (DSCP) Registry
RFC TOTAL SIZE: 13999 bytes
PUBLICATION DATE: Tuesday, August 21st, 2018
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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