rfc9435v3.txt		rfc9435.txt
	Internet Engineering Task Force (IETF) A. Custura		Internet Engineering Task Force (IETF) A. Custura
	Request for Comments: 9435 G. Fairhurst		Request for Comments: 9435 G. Fairhurst
	Category: Informational R. Secchi		Category: Informational R. Secchi
	ISSN: 2070-1721 University of Aberdeen		ISSN: 2070-1721 University of Aberdeen
	May 2023		July 2023
	Considerations for Assigning a New Recommended Differentiated Services		Considerations for Assigning a New Recommended Differentiated Services
	Code Point (DSCP)		Code Point (DSCP)
	Abstract		Abstract
	This document discusses considerations for assigning a new		This document discusses considerations for assigning a new
	recommended Differentiated Services Code Point (DSCP) for a standard		recommended Differentiated Services Code Point (DSCP) for a standard
	Per-Hop Behavior (PHB). It considers the common observed re-marking		Per-Hop Behavior (PHB). It considers the common observed re-marking
	behaviors that the Diffserv field might be subjected to along an		behaviors that the Diffserv field might be subjected to along an
	skipping to change at line 203 ¶		skipping to change at line 203 ¶
	assignments (replacing the previous availability for experimental		assignments (replacing the previous availability for experimental
	or local use)" [RFC8436]. [RFC8622] assigned 0x01 from this pool		or local use)" [RFC8436]. [RFC8622] assigned 0x01 from this pool
	and consequentially updated [RFC4594]. Any future request to		and consequentially updated [RFC4594]. Any future request to
	assign 0x05 would be expected to similarly update [RFC4594].		assign 0x05 would be expected to similarly update [RFC4594].
	Note that [RFC4594] previously recommended a Local Use of DSCP values		Note that [RFC4594] previously recommended a Local Use of DSCP values
	0x01, 0x03, 0x05, and 0x07 (codepoints with the format of 0b000xx1),		0x01, 0x03, 0x05, and 0x07 (codepoints with the format of 0b000xx1),
	until this was updated by [RFC8436].		until this was updated by [RFC8436].
	The DSCP space is shown in the following table. Each row corresponds		The DSCP space is shown in the following table. Each row corresponds
	to one setting of the the first three bits of the DSCP field, and		to one setting of the first three bits of the DSCP field, and each
	each column to one setting of the the last three bits of the DSCP		column to one setting of the last three bits of the DSCP field.
	field.
	+--------+------+---------+----+---------+----+---------+----+		+--------+------+---------+----+---------+----+---------+----+
	| 56/CS7 | 57 | 58 | 59 | 60 | 61 | 62 | 63 |		| 56/CS7 | 57 | 58 | 59 | 60 | 61 | 62 | 63 |
	+--------+------+---------+----+---------+----+---------+----+		+--------+------+---------+----+---------+----+---------+----+
	| 48/CS6 | 49 | 50 | 51 | 52 | 53 | 54 | 55 |		| 48/CS6 | 49 | 50 | 51 | 52 | 53 | 54 | 55 |
	+--------+------+---------+----+---------+----+---------+----+		+--------+------+---------+----+---------+----+---------+----+
	| 40/CS5 | 41 | 42 | 43 | 44/VA | 45 | 46/EF | 47 |		| 40/CS5 | 41 | 42 | 43 | 44/VA | 45 | 46/EF | 47 |
	+--------+------+---------+----+---------+----+---------+----+		+--------+------+---------+----+---------+----+---------+----+
	| 32/CS4 | 33 | 34/AF41 | 35 | 36/AF42 | 37 | 38/AF43 | 39 |		| 32/CS4 | 33 | 34/AF41 | 35 | 36/AF42 | 37 | 38/AF43 | 39 |
	+--------+------+---------+----+---------+----+---------+----+		+--------+------+---------+----+---------+----+---------+----+
	skipping to change at line 392 ¶		skipping to change at line 391 ¶
	4.2. IP Type of Service Manipulations		4.2. IP Type of Service Manipulations
	The IETF first defined ToS precedence for IP packets in [RFC0791] and		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,		updated it to be part of the ToS field in [RFC1349]. Since 1998,
	this practice has been deprecated by [RFC2474]. [RFC2474] defines		this practice has been deprecated by [RFC2474]. [RFC2474] defines
	DSCPs 0bxxx000 as the Class Selector codepoints, where PHBs selected		DSCPs 0bxxx000 as the Class Selector codepoints, where PHBs selected
	by these codepoints MUST meet the "Class Selector PHB Requirements"		by these codepoints MUST meet the "Class Selector PHB Requirements"
	described in Section 4.2.2.2 of [RFC2474].		described in Section 4.2.2.2 of [RFC2474].
	However, a recent survey reports practices based on ToS semantics		A recent survey reports practices based on ToS semantics have not yet
	have not yet been eliminated from the Internet and need to still be		been eliminated from the Internet and need to still be considered
	considered when making new DSCP assignments [Cus17].		when making new DSCP assignments [Cus17].
	4.2.1. Impact of ToS Precedence Bleaching		4.2.1. Impact of ToS Precedence Bleaching
	Bleaching of the ToS Precedence field (see Section 4) resets the		Bleaching of the ToS Precedence field (see Section 4) resets the
	first three bits of the DSCP field to zero (the former ToS Precedence		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		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		[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		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.,		processed by a router that is not configured with Diffserv (e.g.,
	configured to operate on the former ToS Precedence field [RFC0791]).		configured to operate on the former ToS Precedence field [RFC0791]).
	skipping to change at line 597 ¶		skipping to change at line 596 ¶
	both PCP1 (Background) and PCP2 (Spare) to indicate lower priority		both PCP1 (Background) and PCP2 (Spare) to indicate lower priority
	than PCP0, and some other devices do not assign a lower priority to		than PCP0, and some other devices do not assign a lower priority to
	PCP1.		PCP1.
	5.1.2. Mapping Specified for IEEE 802.11		5.1.2. Mapping Specified for IEEE 802.11
	Section 6 of [RFC8325] provides a brief overview of IEEE 802.11 QoS.		Section 6 of [RFC8325] provides a brief overview of IEEE 802.11 QoS.
	The IEEE 802.11 standards [IEEE-802.11] provide Media Access Control		The IEEE 802.11 standards [IEEE-802.11] provide Media Access Control
	(MAC) functions to support QoS in WLANs using Access Categories		(MAC) functions to support QoS in WLANs using Access Categories
	(ACs). The upstream UP in the 802.11 header has a 3-bit QoS value.		(ACs). The upstream UP in the 802.11 header has a 3-bit QoS value.
	A DSCP can be mapped to the UP. [RFC8622] described the adding of		A DSCP can be mapped to the UP. [RFC8622] added a mapping for the LE
	mapping for the LE DSCP, mapping this to AC_BK (Background)		DSCP to AC_BK (Background).
	Most current Wi-Fi implementations use a default mapping that maps		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		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		example of equipment still classifying on ToS Precedence (which could
	be seen as a simple method to map IP layer Diffserv to layers		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		offering only 3-bit QoS codepoints). Then, in turn, this is mapped
	to the four Wi-Fi Multimedia (WMM) Access Categories. The Wi-Fi		to the four Wi-Fi Multimedia (WMM) Access Categories. The Wi-Fi
	Alliance has also specified a more flexible mapping that follows		Alliance has also specified a more flexible mapping that follows
	[RFC8325] and provides functions at an Access Point (AP) to re-mark		[RFC8325] and provides functions at an Access Point (AP) to re-mark
	packets as well as a QoS Map that maps each DSCP to an AC		packets as well as a QoS Map that maps each DSCP to an AC
	skipping to change at line 634 ¶		skipping to change at line 633 ¶
	dependent on the direction in which a packet is forwarded. It		dependent on the direction in which a packet is forwarded. It
	provides recommendations for mapping a DSCP to an IEEE 802.11 UP for		provides recommendations for mapping a DSCP to an IEEE 802.11 UP for
	interconnection between wired and wireless networks. The		interconnection between wired and wireless networks. The
	recommendation in Section 5.1.2 maps network control traffic, CS6 and		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		CS7, as well as unassigned DSCPs, to UP 0 when forwarding in the
	upstream direction (wireless-to-wired). It also recommends mapping		upstream direction (wireless-to-wired). It also recommends mapping
	CS6 and CS7 traffic to UP 7 when forwarding in the downstream		CS6 and CS7 traffic to UP 7 when forwarding in the downstream
	direction (Section 4.1 of [RFC8325]).		direction (Section 4.1 of [RFC8325]).
	For other UPs, [RFC8325] recommends a mapping in the upstream		For other UPs, [RFC8325] recommends a mapping in the upstream
	direction that derives the DSCP from the value of the UP multiplied		direction (wireless-to-wired interconnections) that derives the DSCP
	by 8. This mapping can result in a specific DSCP re-marking		from the value of the UP multiplied by 8. This mapping, currently
	behavior.		used by some Access Points (APs), 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 UP values by multiplying the		| wherein DSCP values are derived from UP values by multiplying the
	| UP values by 8 (i.e., shifting the three UP bits to the left and		| UP values by 8 (i.e., shifting the three UP bits to the left and
	| adding three additional zeros to generate a DSCP value). This		| adding three additional zeros to generate a DSCP value). This
	| derived DSCP value is then used for QoS treatment between the		| derived DSCP value is then used for QoS treatment between the
	| wireless AP and the nearest classification and marking policy		| wireless AP and the nearest classification and marking policy
	| enforcement point (which may be the centralized wireless LAN		| enforcement point (which may be the centralized wireless LAN
	| controller, relatively deep within the network). Alternatively,		| controller, relatively deep within the network). Alternatively,
	| in the case where there is no other classification and marking		| in the case where there is no other classification and marking
	| policy enforcement point, then this derived DSCP value will be		| policy enforcement point, then this derived DSCP value will be
	skipping to change at line 668 ¶		skipping to change at line 663 ¶
	+-+-+-+-+-+-+		+-+-+-+-+-+-+
	|x x x|0 0 0|		|x x x|0 0 0|
	+-+-+-+-+-+-+		+-+-+-+-+-+-+
	Figure 5: Bits in the Diffserv Field Modified by Re-marking		Figure 5: Bits in the Diffserv Field Modified by Re-marking
	Observed as a Result of UP-to-DSCP Mapping in Some 802.11		Observed as a Result of UP-to-DSCP Mapping in Some 802.11
	Networks		Networks
	An alternative to UP-to-DSCP remapping uses the DSCP value of a		An alternative to UP-to-DSCP remapping uses the DSCP value of a
	downstream IP packet (e.g., the Control and Provisioning of Wireless		downstream IP packet (e.g., the Control and Provisioning of Wireless
	Access Points (CAPWAP) protocol [RFC5415] maps an IP packet Diffserv		Access Points, CAPWAP, protocol [RFC5415] maps an IP packet Diffserv
	field to the Diffserv field of the outer IP header in a CAPWAP		field to the Diffserv field of the outer IP header in a CAPWAP
	tunnel).		tunnel).
	Some current constraints of Wi-Fi mapping are discussed in Section 2		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		of [RFC8325]. A QoS profile can be used to limit the maximum DSCP
	value used for the upstream and downstream traffic.		value used for the upstream and downstream traffic.
	5.2. Diffserv and MPLS		5.2. Diffserv and MPLS
	Multi-Protocol Label Switching (MPLS) specified eight MPLS Traffic		Multi-Protocol Label Switching (MPLS) specified eight MPLS Traffic
	Classes (TCs), which restrict the number of different treatments		Classes (TCs), which restrict the number of different treatments
	[RFC5129]. [RFC5127] describes the aggregation of Diffserv TCs and		[RFC5129]. [RFC5127] describes the aggregation of Diffserv service
	introduces four Diffserv TAs. Traffic marked with multiple DSCPs can		classes and introduces four Diffserv TAs. Traffic marked with
	be forwarded in a single MPLS TC.		multiple DSCPs can be forwarded in a single MPLS TC.
	There are three Label Switching Router (LSR) models: the Pipe, the		There are three Label Switching Router (LSR) models: the Pipe, the
	Short Pipe, and the Uniform Model [RFC3270]. In the Uniform and Pipe		Short Pipe, and the Uniform Model [RFC3270]. In the Uniform and Pipe
	models, the egress MPLS router forwards traffic based on the received		models, the egress MPLS router forwards traffic based on the received
	MPLS TC. The Uniform Model includes an egress DSCP rewrite. With		MPLS TC. The Uniform Model includes an egress DSCP rewrite. With
	the Short Pipe Model, the egress MPLS router forwards traffic based		the Short Pipe Model, the egress MPLS router forwards traffic based
	on the Diffserv DSCP as present at the egress router. If the domain		on the Diffserv DSCP as present at the egress router. If the domain
	supports IP and MPLS QoS differentiation, controlled behavior		supports IP and MPLS QoS differentiation, controlled behavior
	requires the DSCP of an (outer) IP header to be assigned or re-		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		written by all domain ingress routers to conform with the domain's
	skipping to change at line 950 ¶		skipping to change at line 945 ¶
	1. Networks containing misconfigured devices that do not comply		1. Networks containing misconfigured devices that do not comply
	with the relevant RFCs.		with the relevant RFCs.
	2. Networks containing devices that implement an obsolete		2. Networks containing devices that implement an obsolete
	specification or contain software bugs.		specification or contain software bugs.
	3. Networks containing devices that re-mark the DSCP as a result		3. Networks containing devices that re-mark the DSCP as a result
	of lower layer interactions.		of lower layer interactions.
	The DSCP re-marking corresponding to the Bleach-ToS-Precedence		The DSCP re-marking corresponding to the Bleach-ToS-Precedence
	Section 4 observed behavior can arise for various reasons, one of		(Section 4) observed behavior can arise for various reasons, one of
	which is old equipment that precedes Diffserv. The same re-marking		which is old equipment that precedes Diffserv. The same re-marking
	can also arise in some cases when traffic conditioning is provided by		can also arise in some cases when traffic conditioning is provided by
	Diffserv routers at operator boundaries or as a result of		Diffserv routers at operator boundaries or as a result of
	misconfiguration.		misconfiguration.
	6.5. Considerations to Guide the Discussion of a Proposed New DSCP		6.5. Considerations to Guide the Discussion of a Proposed New DSCP
	A series of questions emerge that need to be answered when		A series of questions emerge that need to be answered when
	considering a proposal to the IETF that requests a new assignment.		considering a proposal to the IETF that requests a new assignment.
	These questions include:		These questions include:
	skipping to change at line 1143 ¶		skipping to change at line 1138 ¶
	<https://lists.freebsd.org/pipermail/freebsd-		<https://lists.freebsd.org/pipermail/freebsd-
	stable/2010-July/057710.html>.		stable/2010-July/057710.html>.
	[MEF-23.1] MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet		[MEF-23.1] MEF, "Implementation Agreement MEF 23.1 Carrier Ethernet
	Class of Service - Phase 2", MEF 23.1, January 2012,		Class of Service - Phase 2", MEF 23.1, January 2012,
	<https://mef.net/Assets/Technical_Specifications/PDF/		<https://mef.net/Assets/Technical_Specifications/PDF/
	MEF_23.1.pdf>.		MEF_23.1.pdf>.
	[NQB-PHB] White, G. and T. Fossati, "A Non-Queue-Building Per-Hop		[NQB-PHB] White, G. and T. Fossati, "A Non-Queue-Building Per-Hop
	Behavior (NQB PHB) for Differentiated Services", Work in		Behavior (NQB PHB) for Differentiated Services", Work in
	Progress, Internet-Draft, draft-ietf-tsvwg-nqb-17, 25		Progress, Internet-Draft, draft-ietf-tsvwg-nqb-18, 10 July
	March 2023, <https://datatracker.ietf.org/doc/html/draft-		2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
	ietf-tsvwg-nqb-17>.		tsvwg-nqb-18>.
	[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,		[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
	DOI 10.17487/RFC0791, September 1981,		DOI 10.17487/RFC0791, September 1981,
	<https://www.rfc-editor.org/info/rfc791>.		<https://www.rfc-editor.org/info/rfc791>.
	[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -		[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
	Communication Layers", STD 3, RFC 1122,		Communication Layers", STD 3, RFC 1122,
	DOI 10.17487/RFC1122, October 1989,		DOI 10.17487/RFC1122, October 1989,
	<https://www.rfc-editor.org/info/rfc1122>.		<https://www.rfc-editor.org/info/rfc1122>.
End of changes. 9 change blocks.
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