< draft-ietf-pcn-3-state-encoding-00.txt		draft-ietf-pcn-3-state-encoding-01.txt >
	Congestion and Pre Congestion T. Moncaster		Congestion and Pre Congestion B. Briscoe
	Internet-Draft BT		Internet-Draft BT & UCL
	Intended status: Experimental B. Briscoe		Intended status: Experimental T. Moncaster
	Expires: October 10, 2009 BT & UCL		Expires: August 14, 2010 BT
	M. Menth		M. Menth
	University of Wuerzburg		University of Wuerzburg
	April 8, 2009		February 10, 2010
	A PCN encoding using 2 DSCPs to provide 3 or more states		A PCN encoding using 2 DSCPs to provide 3 or more states
	draft-ietf-pcn-3-state-encoding-00		draft-ietf-pcn-3-state-encoding-01
			Abstract
			Pre-congestion notification (PCN) is a mechanism designed to protect
			the Quality of Service of inelastic flows within a controlled domain.
			It does this by marking packets when traffic load on a link is
			approaching or has exceeded a threshold below the physical link rate.
			This experimental encoding scheme specifies how three encoding states
			can be carried in the IP header using a combination of two DSCPs and
			the ECN bits. The Basic scheme only allows for three encoding
			states. The Full scheme provides 6 states, enough for limited end-
			to-end support for ECN as well.
	Status of this Memo		Status of this Memo
	This Internet-Draft is submitted to IETF in full conformance with the		This Internet-Draft is submitted to IETF in full conformance with the
	provisions of BCP 78 and BCP 79. This document may contain material		provisions of BCP 78 and BCP 79.
	from IETF Documents or IETF Contributions published or made publicly
	available before November 10, 2008. The person(s) controlling the
	copyright in some of this material may not have granted the IETF
	Trust the right to allow modifications of such material outside the
	IETF Standards Process. Without obtaining an adequate license from
	the person(s) controlling the copyright in such materials, this
	document may not be modified outside the IETF Standards Process, and
	derivative works of it may not be created outside the IETF Standards
	Process, except to format it for publication as an RFC or to
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	Copyright Notice		Copyright Notice
			Copyright (c) 2010 IETF Trust and the persons identified as the
	Copyright (c) 2009 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	Abstract		include Simplified BSD License text as described in Section 4.e of
			the Trust Legal Provisions and are provided without warranty as
	Pre-congestion notification (PCN) is a mechanism designed to protect		described in the BSD License.
	the Quality of Service of inelastic flows within a controlled domain.
	It does this by marking packets when traffic load on a link is
	approaching or has exceeded a threshold below the physical link rate.
	This experimental encoding scheme specifies how three encoding states
	can be carried in the IP header using a combination of two DSCPs and
	the ECN bits. The Basic scheme only allows for three encoding
	states. The Full scheme additionally provides limited end-to-end
	support for ECN.
	Status (to be removed by RFC Editor)
	This memo is posted as an Internet-Draft with an intent to eventually
	be published as an experimental RFC. The PCN Working Group will be
	asked to adopt this memo as a Working Group document describing one
	of several possible experimental PCN encoding schemes. The intention
	is that the title of this document will change to avoid confusion
	with the three state marking scheme.
	Changes from previous drafts
	From draft-moncaster-pcn-3-state-encoding-01:
	o Changed to WG draft. Title changed from "A three state extended
	PCN encoding scheme"
	o Imposed new structure on document. This structure is intended to
	be followed by all extensions to the baseline PCN encoding scheme.
	o Extensive changes throughout to ensure consistency with the
	baseline PCN encoding scheme.
	From 00 to 01:
	o Checked terminology for consistency with
	[I-D.ietf-pcn-baseline-encoding]
	o Minor editorial changes.		This document may contain material from IETF Documents or IETF
			Contributions published or made publicly available before November
			10, 2008. The person(s) controlling the copyright in some of this
			material may not have granted the IETF Trust the right to allow
			modifications of such material outside the IETF Standards Process.
			Without obtaining an adequate license from the person(s) controlling
			the copyright in such materials, this document may not be modified
			outside the IETF Standards Process, and derivative works of it may
			not be created outside the IETF Standards Process, except to format
			it for publication as an RFC or to translate it into languages other
			than English.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
			1.1. Changes from Previous Drafts (to be removed by the RFC
			Editor) . . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4		2. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
	3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5		3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
	4. The Requirement for Three PCN Encoding States . . . . . . . . 5		4. The Requirement for Three PCN Encoding States . . . . . . . . 5
	5. Adding Limited End-to-End ECN Support to PCN . . . . . . . . . 6		5. Adding Limited End-to-End ECN Support to PCN . . . . . . . . . 5
	6. Encoding Three PCN States in IP . . . . . . . . . . . . . . . 6		6. Encoding Three PCN States in IP . . . . . . . . . . . . . . . 6
	6.1. Basic Three State Encoding . . . . . . . . . . . . . . . . 7		6.1. Basic Three State Encoding . . . . . . . . . . . . . . . . 6
	6.2. Full Three State Encoding . . . . . . . . . . . . . . . . 7		6.2. Full Three State Encoding . . . . . . . . . . . . . . . . 6
	6.3. Valid and invalid codepoint transitions at		6.3. Common Diffserv Per-Hop Behaviour . . . . . . . . . . . . 7
	PCN-ingress-nodes . . . . . . . . . . . . . . . . . . . . 8
	6.4. Valid and invalid codepoint transitions at		6.4. Valid and invalid codepoint transitions at
			PCN-ingress-nodes . . . . . . . . . . . . . . . . . . . . 8
			6.5. Valid and invalid codepoint transitions at
	PCN-interior-nodes . . . . . . . . . . . . . . . . . . . . 8		PCN-interior-nodes . . . . . . . . . . . . . . . . . . . . 8
	6.5. Forwarding traffic out of the PCN-domain . . . . . . . . . 9		6.6. Forwarding traffic out of the PCN-domain . . . . . . . . . 9
	7. PCN-domain support for the PCN extension encoding . . . . . . 10		7. PCN-domain support for the PCN extension encoding . . . . . . 9
	7.1. End-to-End transport behaviour compliant with the PCN		7.1. End-to-End transport behaviour compliant with the PCN
	extension encoding . . . . . . . . . . . . . . . . . . . . 10		extension encoding . . . . . . . . . . . . . . . . . . . . 10
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
	9. Security Considerations . . . . . . . . . . . . . . . . . . . 11		9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
	10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 11		10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 11
	11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11		11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
	12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11		12. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 11
	13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12		13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
	13.1. Normative References . . . . . . . . . . . . . . . . . . . 12		13.1. Normative References . . . . . . . . . . . . . . . . . . . 11
	13.2. Informative References . . . . . . . . . . . . . . . . . . 12		13.2. Informative References . . . . . . . . . . . . . . . . . . 12
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
	1. Introduction		1. Introduction
	The objective of Pre-Congestion Notification (PCN)		The objective of Pre-Congestion Notification (PCN) [RFC5559] is to
	[I-D.ietf-pcn-architecture] is to protect the quality of service		protect the quality of service (QoS) of inelastic flows within a
	(QoS) of inelastic flows within a Diffserv domain, in a simple,		Diffserv domain, in a simple, scalable and robust fashion. The
	scalable and robust fashion. The overall rate of the PCN-traffic is		overall rate of the PCN-traffic is metered on every link in the PCN-
	metered on every link in the PCN-domain, and PCN-packets are		domain, and PCN-packets are appropriately marked when certain
	appropriately marked when certain configured rates are exceeded.		configured rates are exceeded. These configured rates are below the
	These configured rates are below the rate of the link thus providing		rate of the link thus providing notification before any congestion
	notification before any congestion occurs (hence "pre-congestion		occurs (hence "pre-congestion notification"). The level of marking
	notification"). The level of marking allows the boundary nodes to		allows the boundary nodes to make decisions about whether to admit or
	make decisions about whether to admit or block a new flow request,		block a new flow request, and (in abnormal circumstances) whether to
	and (in abnormal circumstances) whether to terminate some of the		terminate some of the existing flows, thereby protecting the QoS of
	existing flows, thereby protecting the QoS of previously admitted		previously admitted flows.
	flows.
	The baseline encoding described in [I-D.ietf-pcn-baseline-encoding]		The baseline encoding described in [RFC5696] provides for deployment
	provides for deployment scenarios that only require two PCN encoding		scenarios that only require two PCN encoding states. This document
	states. This document describes an experimental extension to the		describes an experimental extension to the base-encoding in the IP
	base-encoding in the IP header that adds two capabilities:		header that adds two capabilities:
	o the addition of a third PCN encoding state in the IP header		o the addition of a third PCN encoding state in the IP header
	o preservation of the end-to-end semantics of the ECN field even		o preservation of the end-to-end semantics of the ECN field even
	though PCN uses the field within a PCN-region that interrupts the		though PCN uses the field within a PCN-region that interrupts the
	end-to-end path		end-to-end path
	The second of these capabilities is optional and the reasons for		The second of these capabilities is optional and the reasons for
	doing it are discussed in Section 5.		doing it are discussed in Section 5.
	As in the baseline encoding, this extension encoding re-uses the ECN		As in the baseline encoding, this extension encoding re-uses the ECN
	bits within the IP header within a controlled PCN-domain. This		bits within the IP header within a controlled PCN-domain. This
	extension requires the use of two DSCPs as described later in this		extension requires the use of two DSCPs as described later in this
	document. This experimental scheme is one of three that are being		document. This experimental scheme is one of three that are being
	proposed within the PCN working group. The aim is to allow		proposed within the PCN working group. The aim is to allow
	implementors to decide which scheme is most suitable for possible		implementors to decide which scheme is most suitable for possible
	future standardisation.		future standardisation.
			1.1. Changes from Previous Drafts (to be removed by the RFC Editor)
			From draft-ietf-pcn-3-state-encoding-00 to 01:
			o Removed text implying the two DSCPs have different priority and
			added Section 6.3 specifying they must both have the same PHB.
			o Made IANA considerations text more precise.
			o Changed variable names for DSCP 1 & DSCP 2 to DSCP n & DSCP m to
			be consistent with baseline encoding.
			o Updated refs
			From draft-moncaster-pcn-3-state-encoding-01 to
			draft-ietf-pcn-3-state-encoding-00:
			o Changed to WG draft. Title changed from "A three state extended
			PCN encoding scheme"
			o Imposed new structure on document. This structure is intended to
			be followed by all extensions to the baseline PCN encoding scheme.
			o Extensive changes throughout to ensure consistency with the
			baseline PCN encoding scheme.
			From draft-moncaster-pcn-3-state-encoding-00 to 01:
			o Checked terminology for consistency with [RFC5696]
			o Minor editorial changes.
	2. Requirements notation		2. Requirements notation
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119].		document are to be interpreted as described in [RFC2119].
	3. Terminology		3. Terminology
	Most of the terminology used in this document is defined either in		Most of the terminology used in this document is defined either in
	[I-D.ietf-pcn-architecture] or in [I-D.ietf-pcn-baseline-encoding].		[RFC5559] or in [RFC5696]. The following additional terms are
	The following additional terms are defined in this document:		defined in this document:
	o PCN-flow - a flow covered by a reservation but which hasn't		o PCN-flow - a flow covered by a reservation but which hasn't
	signalled that it requires end-to-end ECN support.		signalled that it requires end-to-end ECN support.
	o PCN-enabled-ECN-flow - a flow covered by reservation and for which		o PCN-enabled-ECN-flow - a flow covered by reservation and for which
	the end-to-end transport has explicitly negotiated ECN support		the end-to-end transport has explicitly negotiated ECN support
	from the PCN-boundary-nodes.		from the PCN-boundary-nodes.
	o Not-marked (xxx), where xxx represents a standard ECN codepoint -		o Not-marked (xxx), where xxx represents a standard ECN codepoint -
	packets that are PCN capable but carry no PCN mark. Abbreviated		packets that are PCN capable but carry no PCN mark. Abbreviated
	as NM(xxx). The (xxx) represents the ECN codepoint that the		as NM(xxx). The (xxx) represents the ECN codepoint that the
	packet arrived with at the PCN-ingress-node e.g. NM(CE)		packet arrived with at the PCN-ingress-node e.g. NM(CE)
	represents a PCN capable packet that has no PCN marking but which		represents a PCN capable packet that has no PCN marking but which
	arrived with the ECN bits set to congestion experienced.		arrived with the ECN bits set to congestion experienced.
	4. The Requirement for Three PCN Encoding States		4. The Requirement for Three PCN Encoding States
	The PCN Marking Behaviours document [I-D.ietf-pcn-marking-behaviour]		The PCN Marking Behaviours document [RFC5670] describes proposed PCN
	describes proposed PCN schemes that require traffic to be metered and		schemes that require traffic to be metered and marked using both
	marked using both Threshold and Excess Traffic schemes. In order to		Threshold and Excess Traffic schemes. In order to achieve this it is
	achieve this it is necessary to allow for three PCN encoding states.		necessary to allow for three PCN encoding states. The constraints
	The constraints imposed by the way tunnels process the ECN field		imposed by the way tunnels process the ECN field severely limit how
	severely limit how to encode these states as explained in		to encode these states as explained in [RFC5696] and
	[I-D.ietf-pcn-baseline-encoding] and [I-D.ietf-tsvwg-ecn-tunnel].		[I-D.ietf-tsvwg-ecn-tunnel]. The obvious way to provide one more
	The obvious way to provide one more encoding state than the base		encoding state than the base encoding is through the use of an
	encoding is through the use of an additional PCN-compatible DiffServ		additional PCN-compatible DiffServ codepoint.
	codepoint.
	One aim of this document is to allow for experiments to show whether		One aim of this document is to allow for experiments to show whether
	such schemes are better than those that only employ two PCN encoding		such schemes are better than those that only employ two PCN encoding
	states. As such, the additional DSCP will be taken from the EXP/LU		states. As such, the additional DSCP will be taken from the EXP/LU
	pools defined in [RFC2474]. If the experiments demonstrate that PCN		pools defined in [RFC2474]. If the experiments demonstrate that PCN
	schemes employing three encoding states are significantly better than		schemes employing three encoding states are significantly better than
	those only employing two then at a later date IANA might be asked to		those only employing two, then at a later date IANA might be asked to
	assign a new PCN enabled DSCP from pool 1. Note that there are other		assign a new PCN enabled DSCP from pool 1. Note that there are other
	experimental encoding schemes being considered which only use one		experimental encoding schemes being considered which only use one
	DSCP but require either alternative tunnel semantics		DSCP but require either alternative tunnel semantics
	([I-D.briscoe-pcn-3-in-1-encoding]) or additional signalling		([I-D.ietf-pcn-3-in-1-encoding]) or additional signalling
	([I-D.menth-pcn-psdm-encoding])in order to work.		([I-D.ietf-pcn-psdm-encoding])in order to work.
	5. Adding Limited End-to-End ECN Support to PCN		5. Adding Limited End-to-End ECN Support to PCN
	[I-D.sarker-pcn-ecn-pcn-usecases] suggests a number of use-cases		[I-D.sarker-pcn-ecn-pcn-usecases] suggests a number of use-cases
	where explicit preservation of end-to-end ECN semantics might be		where explicit preservation of end-to-end ECN semantics might be
	needed across a PCN domain. One of the use-cases suggests that the		needed across a PCN domain. One of the use-cases suggests that the
	end-nodes might be running rate-adaptive codecs that would respond to		end-nodes might be running rate-adaptive codecs that would respond to
	ECN marks by reducing their transmission rate. If the sending		ECN marks by reducing their transmission rate. If the sending
	transport sets the ECT codepoint, the setting of the ECN field as it		transport sets the ECT codepoint, the setting of the ECN field as it
	arrives at the PCN ingress node will need to be re-instated as it		arrives at the PCN ingress node will need to be re-instated as it
	skipping to change at page 6, line 40		skipping to change at page 7, line 16
	marks to those receivers that will correctly interpret them as a		marks to those receivers that will correctly interpret them as a
	notification of congestion. The end-points may indicate they are		notification of congestion. The end-points may indicate they are
	ECN-capable through some higher-layer signalling process that sets up		ECN-capable through some higher-layer signalling process that sets up
	their reservation with the PCN boundary nodes. The exact process of		their reservation with the PCN boundary nodes. The exact process of
	negotiation is beyond the scope of this document but is likely to		negotiation is beyond the scope of this document but is likely to
	involve explicit two way signalling between the end-host and the PCN-		involve explicit two way signalling between the end-host and the PCN-
	domain.		domain.
	In the absence of such signalling the default behaviour of the PCN		In the absence of such signalling the default behaviour of the PCN
	egress node will be to clear the ECN field to 00 as in the baseline		egress node will be to clear the ECN field to 00 as in the baseline
	PCN encoding [I-D.ietf-pcn-baseline-encoding].		PCN encoding [RFC5696].
	6. Encoding Three PCN States in IP		6. Encoding Three PCN States in IP
	The three state PCN encoding scheme is based closely on that defined		The three state PCN encoding scheme is based closely on that defined
	in [I-D.ietf-pcn-baseline-encoding] so that there will be no		in [RFC5696] so that there will be no compatibility issues if a PCN-
	compatibility issues if a PCN-domain changes from using the baseline		domain changes from using the baseline encoding scheme to the
	encoding scheme to the experimental scheme described here. There are		experimental scheme described here. There are two versions of the
	two versions of the scheme. The basic three state scheme allows for		scheme. The basic three state scheme allows for carrying both
	carrying both Threshold-marked (ThM) and Excess-traffic-marked (ETM)		Threshold-marked (ThM) and Excess-traffic-marked (ETM) traffic. The
	traffic. The full scheme additionally allows end-to-end ECN to be		full scheme additionally allows end-to-end ECN to be carried across
	carried across the PCN-domain.		the PCN-domain.
	6.1. Basic Three State Encoding		6.1. Basic Three State Encoding
	The following table shows how to encode the three PCN states in IP.		Table 1 below shows how to encode the three PCN states in IP.
	The authors spent some time trying to establish which way round to
	put the two marked states before settling on this. Because it is
	envisaged that DSCP 2 will be of lower priority than DSCP 1 the
	change in marking from Threshold to Excess Traffic involves
	downgrading the traffic which seems to be consistent with the
	requirement that such changes should not be reversed.
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |		| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	| DSCP 1 | Not-PCN | NM | CU | ThM |		| DSCP n | Not-PCN | NM | CU | ThM |
	| DSCP 2 | Not-PCN | CU | CU | ETM |		| DSCP m | Not-PCN | CU | CU | ETM |
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	(where DSCP 1 is a PCN-compatible DiffServ codepoint (see		(where DSCP n is a PCN-compatible DiffServ codepoint (see [RFC5696])
	[I-D.ietf-pcn-baseline-encoding]) and DSCP 2 is a PCN-compatible DSCP		and DSCP m is a PCN-compatible DSCP from the EXP/LU pools as defined
	from the EXP/LU pools as defined in [RFC2474])		in [RFC2474])
	Table 1: Encoding three PCN states in IP		Table 1: Encoding three PCN states in IP
	6.2. Full Three State Encoding		6.2. Full Three State Encoding
	Table 2 shows how to additionally carry the end-to-end ECN state in		Table 2 shows how to additionally carry the end-to-end ECN state in
	the IP header.		the IP header.
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |		| DSCP | Not-ECT (00) | ECT(0) (10) | ECT(1) (01) | CE (11) |
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	| DSCP 1 | Not-PCN | NM(Not-ECT) | NM(CE) | ThM |		| DSCP n | Not-PCN | NM(Not-ECT) | NM(CE) | ThM |
	| DSCP 2 | Not-PCN | NM(ECT(0)) | NM(ECT(1)) | ETM |		| DSCP m | Not-PCN | NM(ECT(0)) | NM(ECT(1)) | ETM |
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	(where DSCP 1 is a PCN-compatible DiffServ codepoint (see		(where DSCP n is a PCN-compatible DiffServ codepoint (see [RFC5696])
	[I-D.ietf-pcn-baseline-encoding]) and DSCP 2 is a PCN-compatible DSCP		and DSCP m is a PCN-compatible DSCP from the EXP/LU pools as defined
	from the EXP/LU pools as defined in [RFC2474])		in [RFC2474])
	Table 2: Encoding three PCN states in IP		Table 2: Encoding three PCN states in IP
	The four different Not-marked (NM) states allow for the addition of		The four different Not-marked (NM) states allow for the addition of
	limited end-to-end ECN support as explained in the previous section.		limited end-to-end ECN support as explained in the previous section.
	Warning		Warning
	In order to comply with this encoding all the nodes within the PCN-		In order to comply with this encoding all the nodes within the PCN-
	domain MUST be configured with this encoding scheme. However there		domain MUST be configured with this encoding scheme. However there
	may be operators who choose not to be fully compliant with the		may be operators who choose not to be fully compliant with the
	scheme. If an operator chooses to leave some PCN-interior-nodes that		scheme. If an operator chooses to leave some PCN-interior-nodes that
	only support two marking states (the base encoding), then they must		only support two marking states (the baseline encoding [RFC5696]),
	be aware of the following: Ideally such nodes would be configured to		then they must be aware of the following: Ideally such nodes would be
	indicate pre-congestion or congestion using the ETM state since this		configured to indicate pre-congestion or congestion using the ETM
	would ensure they could notify worst-case congestion, however this is		state since this would ensure they could notify worst-case
	not possible since it requires the packets to be re-marked to DSCP 2		congestion, however this is not possible since it requires the
	(hence altering the baseline encoding). This means that such nodes		packets to be re-marked to DSCP m (hence altering the baseline
	will only be able to indicate ThM traffic.		encoding). This means that such nodes will only be able to indicate
			ThM traffic.
	6.3. Valid and invalid codepoint transitions at PCN-ingress-nodes		6.3. Common Diffserv Per-Hop Behaviour
			Packets carrying Diffserv codepoint 'DSCP n' or 'DSCP m' MUST all be
			treated with the same Diffserv PHB [RFC2474]. The choice of PHB is
			discussed in [RFC5559] and [RFC5696].
			Two DSCPs are merely used to provide sufficient PCN encoding states,
			there is no need or intention to provide different scheduling or drop
			preference for each row in the table of PCN codepoints.
			Specifically:
			o Both DSCPs MUST be served in the same queue to prevent reordering
			within an application flow.
			o Both DSCPs MUST be assigned the same drop preference. Note that
			[RFC5670] already provides for preferential drop of excess-rate-
			marked packets, so assigning additional drop preference at the
			coarser granularity of each DSCP would be incorrect.
			6.4. Valid and invalid codepoint transitions at PCN-ingress-nodes
	A PCN-ingress-node operating the Basic version of the 3-State		A PCN-ingress-node operating the Basic version of the 3-State
	Encoding scheme MUST set the Not-marked codepoint on any arriving		Encoding scheme MUST set the Not-marked codepoint on any arriving
	packet that belongs to a PCN-flow. It MUST set the not-PCN codepoint		packet that belongs to a PCN-flow. It MUST set the not-PCN codepoint
	on any other packet.		on any other packet.
	A PCN-ingress-node operating the Full version of the 3-State Encoding		A PCN-ingress-node operating the Full version of the 3-State Encoding
	scheme MUST establish whether a packet is a member of a PCN-enabled-		scheme MUST establish whether a packet is a member of a PCN-enabled-
	ECN-flow. If it is, the PCN-ingress-node MUST set the appropriate		ECN-flow. If it is, the PCN-ingress-node MUST set the appropriate
	NM(xxx) codepoint depending on the value carried in the ECN field of		NM(xxx) codepoint depending on the value carried in the ECN field of
	skipping to change at page 8, line 42		skipping to change at page 9, line 34
	o A packet carrying the ECT(1) codepoint in the ECN field MUST be		o A packet carrying the ECT(1) codepoint in the ECN field MUST be
	assigned the NM(ECT(1)) codepoint		assigned the NM(ECT(1)) codepoint
	o A packet carrying the CE codepoint in the ECN field MUST be		o A packet carrying the CE codepoint in the ECN field MUST be
	assigned the NM(CE) codepoint		assigned the NM(CE) codepoint
	If it is not a member of such a flow then the behaviour MUST be the		If it is not a member of such a flow then the behaviour MUST be the
	same as for the Basic version of the Encoding scheme.		same as for the Basic version of the Encoding scheme.
	6.4. Valid and invalid codepoint transitions at PCN-interior-nodes		6.5. Valid and invalid codepoint transitions at PCN-interior-nodes
	A PCN-interior-node MUST obey the following rules:		A PCN-interior-node MUST obey the following rules:
	o It MUST NOT change the not-PCN codepoint to any other codepoint.		o It MUST NOT change the not-PCN codepoint to any other codepoint.
	o It MAY change any Not-marked codepoint to either the Threshold-		o It MAY change any Not-marked codepoint to either the Threshold-
	marked or Excess-traffic-marked codepoints.		marked or Excess-traffic-marked codepoints.
	o It MUST NOT change a Not-marked codepoint to the not-PCN		o It MUST NOT change a Not-marked codepoint to the not-PCN
	codepoint.		codepoint.
	skipping to change at page 9, line 18		skipping to change at page 10, line 9
	o A Not-marked codepoint MUST NOT be changed to any other Not-marked		o A Not-marked codepoint MUST NOT be changed to any other Not-marked
	codepoint.		codepoint.
	o It MAY change the ThM codepoint to the ETM codepoint but it MUST		o It MAY change the ThM codepoint to the ETM codepoint but it MUST
	NOT change the ThM codepoint to any other codepoint.		NOT change the ThM codepoint to any other codepoint.
	o It MUST NOT change the ETM codepoint to any other codepoint.		o It MUST NOT change the ETM codepoint to any other codepoint.
	Obviously in every case a codepoint can remain unchanged. The		Obviously in every case a codepoint can remain unchanged. The
	precise rules governing which valid transition to use are set out in		precise rules governing which valid transition to use are set out in
	[I-D.ietf-pcn-marking-behaviour]		[RFC5670]
	6.5. Forwarding traffic out of the PCN-domain		6.6. Forwarding traffic out of the PCN-domain
	As each packet exits the PCN-domain, the PCN-egress-node MUST check		As each packet exits the PCN-domain, the PCN-egress-node MUST check
	whether it belongs to a PCN-enabled-ECN-flow. If it belongs to such		whether it belongs to a PCN-enabled-ECN-flow. If it belongs to such
	a flow then the following rules dictate how the ECN field should be		a flow then the following rules dictate how the ECN field should be
	reset:		reset:
	o A packet carrying the not-PCN codepoint MUST be given the not-ECT		o A packet carrying the not-PCN codepoint MUST be given the not-ECT
	codepoint.		codepoint.
	o A packet carrying the NM(not-ECT) codepoint MUST be assigned the		o A packet carrying the NM(not-ECT) codepoint MUST be assigned the
	skipping to change at page 10, line 10		skipping to change at page 10, line 50
	In addition all packets should have their DSCP reset to the		In addition all packets should have their DSCP reset to the
	appropriate DSCP for the next hop. If the next hop is not another		appropriate DSCP for the next hop. If the next hop is not another
	PCN region this will not be a PCN-compatible DSCP, and by default		PCN region this will not be a PCN-compatible DSCP, and by default
	will be the best-efforts DSCP. Alterntively, higher layer signalling		will be the best-efforts DSCP. Alterntively, higher layer signalling
	mechanisms may allow the DSCP that packets entered the PCN-domain		mechanisms may allow the DSCP that packets entered the PCN-domain
	with to be reinstated.		with to be reinstated.
	7. PCN-domain support for the PCN extension encoding		7. PCN-domain support for the PCN extension encoding
	PCN traffic MUST be marked with a DiffServ codepoint that indicates		PCN traffic MUST be marked with a DiffServ codepoint that indicates
	PCN is enabled. To comply with the PCN extension encoding, this		PCN is enabled. To comply with the PCN extension encoding, codepoint
	codepoint is either a PCN-compatible DSCP assigned by IANA for use		'DSCP n' MUST be a PCN-compatible DSCP assigned by IANA for use with
	with the baseline PCN encoding [I-D.ietf-pcn-baseline-encoding] or a		the baseline PCN encoding [RFC5696] while 'DSCP m' can be a DSCP from
	DSCP from pools 2 or 3 for experimental and local use [RFC2474]. The		pools 2 or 3 for experimental and local use [RFC2474]. The exact
	exact choice of DSCP may vary between PCN-domains but MUST be fixed		choice of DSCP may vary between PCN-domains but MUST be fixed within
	within each PCN-domain.		each PCN-domain.
	7.1. End-to-End transport behaviour compliant with the PCN extension		7.1. End-to-End transport behaviour compliant with the PCN extension
	encoding		encoding
	Transports wishing to use both PCN and end-to-end ECN MUST establish		Transports wishing to use both PCN and end-to-end ECN MUST establish
	that their path supports this combination. Support of end-to-end ECN		that their path supports this combination. Support of end-to-end ECN
	by PCN-boundary-nodes is OPTIONAL. Therefore transports MUST check		by PCN-boundary-nodes is OPTIONAL. Therefore transports MUST check
	with both the PCN-ingress-node and PCN-egress-node for each flow.		with both the PCN-ingress-node and PCN-egress-node for each flow.
	The sending of such a request MUST NOT be taken to mean the request		The sending of such a request MUST NOT be taken to mean the request
	has been granted. The PCN-boundary-nodes MAY choose to inform the		has been granted. The PCN-boundary-nodes MAY choose to inform the
	skipping to change at page 10, line 45		skipping to change at page 11, line 37
	scheme.		scheme.
	If either of a PCN ingress-egress pair does not support end-to-end		If either of a PCN ingress-egress pair does not support end-to-end
	ECN or if the end-to-end transport does not request support for end-		ECN or if the end-to-end transport does not request support for end-
	to-end ECN then the PCN-boundary-nodes MUST assume the packet belongs		to-end ECN then the PCN-boundary-nodes MUST assume the packet belongs
	to a PCN-flow.		to a PCN-flow.
	8. IANA Considerations		8. IANA Considerations
	This document asks IANA to assign one DiffServ codepoint from Pool 2		This document asks IANA to assign one DiffServ codepoint from Pool 2
	or Pool 3 (for experimental/local use)[RFC2474]. Should any of the		or Pool 3 (for experimental/local use)[RFC2474]. Should this
	three encoding state experimental PCN schemes prove sufficiently		experimental PCN scheme prove sufficiently successful then IANA will
	successful then IANA will be requested in a later document to assign		be requested in a later document to assign a dedicated DiffServ
	a dedicated DiffServ codepoint from pool 1 for standards use.		codepoint from pool 1 for standards use and the experimental
			codepoint will be returned to its IANA pool.
	9. Security Considerations		9. Security Considerations
	The security concerns relating to this extended PCN encoding are		The security concerns relating to this extended PCN encoding are
	essentially the same as those in [I-D.ietf-pcn-baseline-encoding].		essentially the same as those in [RFC5696].
	This extension coding gives end-to-end support for the ECN nonce		This extension coding gives end-to-end support for the ECN nonce
	[RFC3540], which is intended to protect the sender against the		[RFC3540], which is intended to protect the sender against the
	receiver or against network elements concealing a congestion		receiver or against network elements concealing a congestion
	experienced marking or a lost packet. PCN-based reservations		experienced marking or a lost packet. PCN-based reservations
	combined with end-to-end ECN are intended for partially inelastic		combined with end-to-end ECN are intended for partially inelastic
	traffic using rate-adaptive codecs. Therefore the end-to-end		traffic using rate-adaptive codecs. Therefore the end-to-end
	transport is unlikely to be TCP, but at this time the nonce has only		transport is unlikely to be TCP, but at this time the nonce has only
	been defined for TCP transports.		been defined for TCP transports.
	10. Conclusions		10. Conclusions
	This document describes an extended encoding scheme for PCN that		This document describes an extended encoding scheme for PCN that
	provides for three encoding states as well as optional support for		provides for three encoding states as well as optional support for
	end-to-end ECN. The encoding scheme builds on the baseline encoding		end-to-end ECN. The encoding scheme builds on the baseline encoding
	described in [I-D.ietf-pcn-baseline-encoding]. Using this encoding		described in [RFC5696]. Using this encoding scheme it is possible
	scheme it is possible for operators to conduct experiments to check		for operators to conduct experiments to check whether the addition of
	whether the addition of an extra encoding state will significantly		an extra encoding state will significantly improve the performance of
	improve the performance of PCN. It will also allow experiments to		PCN. It will also allow experiments to determine whether there is a
	determine whether there is a need for end-to-end ECN support within		need for end-to-end ECN support within the PCN-domain (as against
	the PCN-domain (as against end-to-end ECN support through the use of		end-to-end ECN support through the use of IP-in-IP tunnelling or by
	IP-in-IP tunnelling or by downgrading the traffic to a lower service		downgrading the traffic to a lower service class).
	class).
	11. Acknowledgements		11. Acknowledgements
	This document builds extensively on work done in the PCN working		This document builds extensively on work done in the PCN working
	group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Joe		group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Joe
	Babiarz and others. Full details of alternative schemes that were		Babiarz and others. Full details of alternative schemes that were
	considered for adoption can be found in the document		considered for adoption can be found in the document
	[I-D.chan-pcn-encoding-comparison].		[I-D.ietf-pcn-encoding-comparison].
	12. Comments Solicited		12. Comments Solicited
	(Section to be removed by RFC_Editor) Comments and questions are		(Section to be removed by RFC_Editor) Comments and questions are
	encouraged and very welcome. They can be addressed to the IETF		encouraged and very welcome. They can be addressed to the IETF
	Transport Area working group mailing list <tsvwg@ietf.org>, and/or to		Transport Area working group mailing list <tsvwg@ietf.org>, and/or to
	the authors.		the authors.
	13. References		13. References
	13.1. Normative References
	[I-D.ietf-pcn-marking-behaviour]		13.1. Normative References
	Eardley, P., "Marking behaviour of PCN-nodes",
	draft-ietf-pcn-marking-behaviour-02 (work in progress),
	March 2009.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC4774] Floyd, S., "Specifying Alternate Semantics for the		[RFC4774] Floyd, S., "Specifying Alternate Semantics for the
	Explicit Congestion Notification (ECN) Field", BCP 124,		Explicit Congestion Notification (ECN) Field", BCP 124,
	RFC 4774, November 2006.		RFC 4774, November 2006.
			[RFC5670] Eardley, P., "Metering and Marking Behaviour of PCN-
			Nodes", RFC 5670, November 2009.
			[RFC5696] Moncaster, T., Briscoe, B., and M. Menth, "Baseline
			Encoding and Transport of Pre-Congestion Information",
			RFC 5696, November 2009.
	13.2. Informative References		13.2. Informative References
	[I-D.briscoe-pcn-3-in-1-encoding]		[I-D.ietf-pcn-3-in-1-encoding]
	Briscoe, B., "PCN 3-State Encoding Extension in a single		Briscoe, B. and T. Moncaster, "PCN 3-State Encoding
	DSCP", draft-briscoe-pcn-3-in-1-encoding-00 (work in		Extension in a single DSCP",
	progress), October 2008.		draft-ietf-pcn-3-in-1-encoding-01 (work in progress),
			February 2010.
	[I-D.chan-pcn-encoding-comparison]		[I-D.ietf-pcn-encoding-comparison]
	Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,		Chan, K., Karagiannis, G., Moncaster, T., Menth, M.,
	Eardley, P., and B. Briscoe, "Pre-Congestion Notification		Eardley, P., and B. Briscoe, "Pre-Congestion Notification
	Encoding Comparison",		Encoding Comparison",
	draft-chan-pcn-encoding-comparison-04 (work in progress),		draft-ietf-pcn-encoding-comparison-01 (work in progress),
	March 2009.		October 2009.
	[I-D.ietf-pcn-architecture]
	Eardley, P., "Pre-Congestion Notification (PCN)
	Architecture", draft-ietf-pcn-architecture-11 (work in
	progress), April 2009.
	[I-D.ietf-pcn-baseline-encoding]		[I-D.ietf-pcn-psdm-encoding]
	Moncaster, T., Briscoe, B., and M. Menth, "Baseline		Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
	Encoding and Transport of Pre-Congestion Information",		"PCN Encoding for Packet-Specific Dual Marking (PSDM)",
	draft-ietf-pcn-baseline-encoding-03 (work in progress),		draft-ietf-pcn-psdm-encoding-00 (work in progress),
	April 2009.		June 2009.
	[I-D.ietf-tsvwg-ecn-tunnel]		[I-D.ietf-tsvwg-ecn-tunnel]
	Briscoe, B., "Tunnelling of Explicit Congestion		Briscoe, B., "Tunnelling of Explicit Congestion
	Notification", draft-ietf-tsvwg-ecn-tunnel-02 (work in		Notification", draft-ietf-tsvwg-ecn-tunnel-06 (work in
	progress), March 2009.		progress), December 2009.
	[I-D.menth-pcn-psdm-encoding]
	Menth, M., Babiarz, J., Moncaster, T., and B. Briscoe,
	"PCN Encoding for Packet-Specific Dual Marking (PSDM)",
	draft-menth-pcn-psdm-encoding-00 (work in progress),
	July 2008.
	[I-D.sarker-pcn-ecn-pcn-usecases]		[I-D.sarker-pcn-ecn-pcn-usecases]
	Sarker, Z. and I. Johansson, "Usecases and Benefits of end		Sarker, Z. and I. Johansson, "Usecases and Benefits of end
	to end ECN support in PCN Domains",		to end ECN support in PCN Domains",
	draft-sarker-pcn-ecn-pcn-usecases-02 (work in progress),		draft-sarker-pcn-ecn-pcn-usecases-01 (work in progress),
	November 2008.		May 2008.
	[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,		[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
	"Definition of the Differentiated Services Field (DS		"Definition of the Differentiated Services Field (DS
	Field) in the IPv4 and IPv6 Headers", RFC 2474,		Field) in the IPv4 and IPv6 Headers", RFC 2474,
	December 1998.		December 1998.
	[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition		[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
	of Explicit Congestion Notification (ECN) to IP",		of Explicit Congestion Notification (ECN) to IP",
	RFC 3168, September 2001.		RFC 3168, September 2001.
	[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit		[RFC3540] Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
	Congestion Notification (ECN) Signaling with Nonces",		Congestion Notification (ECN) Signaling with Nonces",
	RFC 3540, June 2003.		RFC 3540, June 2003.
			[RFC5559] Eardley, P., "Pre-Congestion Notification (PCN)
			Architecture", RFC 5559, June 2009.
	Authors' Addresses		Authors' Addresses
			Bob Briscoe
			BT & UCL
			B54/77, Adastral Park
			Martlesham Heath
			Ipswich IP5 3RE
			UK
			Phone: +44 1473 645196
			Email: bob.briscoe@bt.com
	Toby Moncaster		Toby Moncaster
	BT		BT
	B54/70, Adastral Park		B54/70, Adastral Park
	Martlesham Heath		Martlesham Heath
	Ipswich IP5 3RE		Ipswich IP5 3RE
	UK		UK
	Phone: +44 1473 648734		Phone: +44 1473 648734
	Email: toby.moncaster@bt.com		Email: toby.moncaster@bt.com
	URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/		URI: http://www.cs.ucl.ac.uk/staff/B.Briscoe/
	Bob Briscoe
	BT & UCL
	B54/77, Adastral Park
	Martlesham Heath
	Ipswich IP5 3RE
	UK
	Phone: +44 1473 645196
	Email: bob.briscoe@bt.com
	Michael Menth		Michael Menth
	University of Wuerzburg		University of Wuerzburg
	room B206, Institute of Computer Science		room B206, Institute of Computer Science
	Am Hubland		Am Hubland
	Wuerzburg D-97074		Wuerzburg D-97074
	Germany		Germany
	Phone: +49 931 888 6644		Phone: +49 931 888 6644
	Email: menth@informatik.uni-wuerzburg.de		Email: menth@informatik.uni-wuerzburg.de
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