draft-moncaster-pcn-baseline-encoding-01.txt		draft-moncaster-pcn-baseline-encoding-02.txt
	Congestion and Pre Congestion T. Moncaster		Congestion and Pre Congestion T. Moncaster
	Internet-Draft BT		Internet-Draft BT
	Intended status: Standards Track B. Briscoe		Intended status: Standards Track B. Briscoe
	Expires: December 25, 2008 BT & UCL		Expires: January 12, 2009 BT & UCL
	M. Menth		M. Menth
	University of Wuerzburg		University of Wuerzburg
	June 23, 2008		July 11, 2008
	Baseline Encoding and Transport of Pre-Congestion Information		Baseline Encoding and Transport of Pre-Congestion Information
	draft-moncaster-pcn-baseline-encoding-01		draft-moncaster-pcn-baseline-encoding-02
	Status of this Memo		Status of this Memo
	By submitting this Internet-Draft, each author represents that any		By submitting this Internet-Draft, each author represents that any
	applicable patent or other IPR claims of which he or she is aware		applicable patent or other IPR claims of which he or she is aware
	have been or will be disclosed, and any of which he or she becomes		have been or will be disclosed, and any of which he or she becomes
	aware will be disclosed, in accordance with Section 6 of BCP 79.		aware will be disclosed, in accordance with Section 6 of BCP 79.
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	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	skipping to change at page 1, line 37		skipping to change at page 1, line 37
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	This Internet-Draft will expire on December 25, 2008.		This Internet-Draft will expire on January 12, 2009.
	Copyright Notice		Copyright Notice
	Copyright (C) The IETF Trust (2008).		Copyright (C) The IETF Trust (2008).
	Abstract		Abstract
	Pre-congestion notification (PCN) provides information to support		Pre-congestion notification (PCN) provides information to support
	admission control and flow termination in order to protect the		admission control and flow termination in order to protect the
	Quality of Service of inelastic flows. It does this by marking		Quality of Service of inelastic flows. It does this by marking
	packets when traffic load on a link is approaching or has exceeded a		packets when traffic load on a link is approaching or has exceeded a
	rate threshold below the physical link rate. This document specifies		threshold below the physical link rate. This document specifies how
	how such marks are to be encoded into the IP header. The baseline		such marks are to be encoded into the IP header. The baseline
	encoding described here provides for only two PCN encoding states.		encoding described here provides for only two PCN encoding states.
	Another document describes an extended encoding scheme that allows		Other documents describe extended encoding schemes that allow for
	for three encoding states.		three encoding states.
	Status		Status
	This memo is posted as an Internet-Draft with an intent to eventually		This memo is posted as an Internet-Draft with an intent to eventually
	progress to standards track.		progress to standards track.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3		2. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
	3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3		3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
	4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 4		4. Encoding two PCN States in IP . . . . . . . . . . . . . . . . 4
	4.1. Rationale for Encoding . . . . . . . . . . . . . . . . . . 5		4.1. Rationale for Encoding . . . . . . . . . . . . . . . . . . 5
	4.2. PCN-Enabled DiffServ Codepoints . . . . . . . . . . . . . 5		4.2. PCN-Enabled DiffServ Codepoints . . . . . . . . . . . . . 5
	4.2.1. Implications of re-using a DiffServ Codepoint . . . . 5		4.2.1. Implications of re-using a DiffServ Codepoint . . . . 6
	4.3. Valid and Invalid Encoding Transitions at a PCN Node . . . 6		4.3. Valid and Invalid Encoding Transitions at a PCN Node . . . 6
	5. Backwards Compatability . . . . . . . . . . . . . . . . . . . 7		5. Backwards Compatability . . . . . . . . . . . . . . . . . . . 7
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 7		7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
	8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 8		8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 8
	9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8		9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
	10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 8		10. Comments Solicited . . . . . . . . . . . . . . . . . . . . . . 8
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8		11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
	11.1. Normative References . . . . . . . . . . . . . . . . . . . 8		11.1. Normative References . . . . . . . . . . . . . . . . . . . 8
	11.2. Informative References . . . . . . . . . . . . . . . . . . 8		11.2. Informative References . . . . . . . . . . . . . . . . . . 9
	Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 9		Appendix A. Tunnelling Constraints . . . . . . . . . . . . . . . 9
	Appendix B. Deployment Scenarios for PCN Using Baseline		Appendix B. Deployment Scenarios for PCN Using Baseline
	Encoding . . . . . . . . . . . . . . . . . . . . . . 10		Encoding . . . . . . . . . . . . . . . . . . . . . . 10
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
	Intellectual Property and Copyright Statements . . . . . . . . . . 12		Intellectual Property and Copyright Statements . . . . . . . . . . 13
	1. Introduction		1. Introduction
	Pre-congestion notification (PCN) provides information to support		Pre-congestion notification (PCN) provides information to support
	admission control and flow termination in order to protect the		admission control and flow termination in order to protect the
	quality of service (QoS) of inelastic flows. This is achieved by		quality of service (QoS) of inelastic flows. This is achieved by
	marking packets according to the level of pre-congestion at nodes		marking packets according to the level of pre-congestion at nodes
	within the PCN-domain. Two algorithms exist for that purpose.		within the PCN-domain. Two algorithms exist for that purpose.
	Excess traffic marking marks all PCN packets exceeding a certain		Excess traffic marking marks all PCN packets exceeding a certain
	reference rate on a link while threshold marking marks all PCN		reference rate on a link while threshold marking marks all PCN
	skipping to change at page 3, line 28		skipping to change at page 3, line 28
	This document specifies how these PCN marks are encoded into the IP		This document specifies how these PCN marks are encoded into the IP
	header. It also describes how packets are identified as belonging to		header. It also describes how packets are identified as belonging to
	a PCN flow. Some deployment models require two PCN encoding states,		a PCN flow. Some deployment models require two PCN encoding states,
	others require three. The baseline encoding described here only		others require three. The baseline encoding described here only
	provides for two PCN encoding states. An extended encoding described		provides for two PCN encoding states. An extended encoding described
	in [PCN-3-enc-state] provides for three PCN encoding states.		in [PCN-3-enc-state] provides for three PCN encoding states.
	Changes from previous drafts (to be removed by the RFC Editor)		Changes from previous drafts (to be removed by the RFC Editor)
			From -01 to -02:
			Minor changes throughout including tightening up language to
			remain consistent with the PCN Architecture terminology
	From -00 to -01:		From -00 to -01:
	Change of title from "Encoding and Transport of (Pre-)Congestion		Change of title from "Encoding and Transport of (Pre-)Congestion
	Information from within a DiffServ Domain to the Egress"		Information from within a DiffServ Domain to the Egress"
	Extensive changes to Introduction and abstract.		Extensive changes to Introduction and abstract.
	Added a section on the implications of re-using a DSCP.		Added a section on the implications of re-using a DSCP.
	Added appendix listing possible operator scenarios for using this		Added appendix listing possible operator scenarios for using this
	skipping to change at page 4, line 5		skipping to change at page 4, line 9
	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
	The following terms are used in this document:		The following terms are used in this document:
	o Not PCN - packets that are not PCN capable.		o not-PCN - packets that are not PCN capable.
	o PCN-marked - codepoint indicating packets that have been marked at		o PCN-marked - codepoint indicating packets that have been marked at
	a PCN interior node using some PCN marking behaviour. Also PM.		a PCN-interior-node using some PCN marking behaviour. Also PM.
	o Not-Marked - codepoint indicating packets that are PCN capable but		o not-Marked - codepoint indicating packets that are PCN capable but
	are not PCN-marked. Also NM.		are not PCN-marked. Also NM.
	o PCN-Capable codepoints - collective term for all the NM and PM		o PCN-Capable codepoints - collective term for all the NM and PM
	codepoints.		codepoints.
	o PCN enabled Diffserv codepoint - a Diffserv codepoint for which		o PCN-enabled Diffserv codepoint - a Diffserv codepoint for which
	PCN has been enabled on a particular machine.		PCN has been enabled on a particular machine.
	In addition the document uses the terminology described in		In addition the document uses the terminology defined in [PCN-arch].
	[PCN-arch].
	4. Encoding two PCN States in IP		4. Encoding two PCN States in IP
	The PCN encoding states are defined using a combination of the DSCP		The PCN encoding states are defined using the Type of Service field
	field and ECN field in the IP header. The baseline PCN encoding		of the IP header which is a combination of the DSCP field and ECN
	closely follows the semantics of ECN [RFC3168]. It allows the		field. The baseline PCN encoding closely follows the semantics of
	encoding of two PCN states: Not Marked and PCN-Marked. It also		ECN [RFC3168]. It allows the encoding of two PCN states: Not Marked
	allows for traffic that is not PCN capable to be marked as such (not-		and PCN-Marked. It also allows for traffic that is not PCN capable
	PCN). The following table defines how to encode these states in IP:		to be marked as such (not-PCN). The following table defines how to
			encode these states in IP:
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	| 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 n | not-PCN | NM | NM | PM |		| DSCP n | not-PCN | NM | NM | PM |
	+--------+--------------+-------------+-------------+---------+		+--------+--------------+-------------+-------------+---------+
	Where DSCP n is a PCN-enabled DiffServ codepoint (see Section 4.2)		Where DSCP n is a PCN-enabled DiffServ codepoint (see Section 4.2)
	Table 1: Encoding PCN in IP		Table 1: Encoding PCN in IP
	The following rules apply to all PCN traffic:		The following rules apply to all PCN traffic:
	o PCN traffic MUST be marked with a DiffServ codepoint that		o PCN traffic MUST be marked with a PCN-enabled DiffServ Codepoint.
	indicates PCN is enabled. To conserve DSCPs, DiffServ Codepoints		That is a DiffServ codepoint that indicates that PCN is enabled.
	SHOULD be chosen that are already defined for use with admission		To conserve DSCPs, DiffServ Codepoints SHOULD be chosen that are
	controlled traffic, such as the Voice-Admit codepoint defined in		already defined for use with admission controlled traffic, such as
	[voice-admit].		the Voice-Admit codepoint defined in [voice-admit].
	o Any packet that is not PCN capable (not-PCN) but which shares the		o Any packet that is not PCN capable (not-PCN) but which shares the
	same DiffServ codepoint as PCN capable traffic MUST have the ECN		same DiffServ codepoint as PCN capable traffic MUST have the ECN
	field set to 00.		field set to 00.
	o Any packet that belongs to a PCN capable flow MUST have the ECN		o Any packet that belongs to a PCN capable flow MUST have the ECN
	field set to one of the two ECT codepoints 10 or 01 at the PCN-		field set to one of the two ECT codepoints 10 or 01 at the PCN-
	ingress-node.		ingress-node.
	o Any packet that is PCN capable and has been PCN-marked by a PCN-		o Any packet that is PCN capable and has been PCN-marked by a PCN-
	interior-node MUST have the ECN field set to 11.		interior-node MUST have the ECN field set to 11.
	4.1. Rationale for Encoding		4.1. Rationale for Encoding
	The exact choice of encoding was dictated by the constraints imposed		The exact choice of encoding was dictated by the constraints imposed
	by existing IETF RFCs, in particular [RFC3168] and [RFC4774]. Full		by existing IETF RFCs, in particular [RFC3168] and [RFC4774]. Full
	details are contained in [pcn-enc-compare]. One of the tightest		details are contained in [pcn-enc-compare]. One of the tightest
	constraints was the need for any PCN encoding to survive being		constraints was the need for any PCN encoding to survive being
	tunnelled through either an IP in IP tunnel or an IPSec Tunnel.		tunnelled through either an IP in IP tunnel or an IPSec Tunnel.
	Appendix A explains this in detail. The main effect of this		Appendix A explains this in detail. The main effect of this
	constraint was that any PCN marking has to use the ECN field set to		constraint is that any PCN marking has to use the ECN field set to 11
	11 (CE codepoint). If the packet is being tunneled then only the CE		(CE codepoint). If the packet is being tunneled then only the CE
	codepoint gets copied into the inner header upon decapsulation. An		codepoint gets copied into the inner header upon decapsulation. An
	additional constraint was the need to minimise the use of DiffServ		additional constraint is the need to minimise the use of DiffServ
	codepoints as these are in increasingly short supply. Section 4.2		codepoints as these are in increasingly short supply. Section 4.2
	explains how we have minimised this still further by reusing pre-		explains how we have minimised this still further by reusing pre-
	existing Diffserv codepoint(s) such that non-PCN traffic can still be		existing Diffserv codepoint(s) such that non-PCN traffic can still be
	distinguished from PCN traffic.		distinguished from PCN traffic.
	The encoding scheme (Table 1) that best addresses the above		The encoding scheme (Table 1) that best addresses the above
	constraints ends up looking very similar to ECN. This is perhaps not		constraints ends up looking very similar to ECN. This is perhaps not
	surprising given the similarity in architectural intent between PCN		surprising given the similarity in architectural intent between PCN
	and ECN.		and ECN.
	4.2. PCN-Enabled DiffServ Codepoints		4.2. PCN-Enabled DiffServ Codepoints
	Equipment complying with the baseline PCN encoding MUST allow PCN to		Equipment complying with the baseline PCN encoding MUST allow PCN to
	be enabled for a certain Diffserv codepoint or codepoints. This		be enabled for a certain Diffserv codepoint or codepoints. This
	document defines the term 'PCN-Enabled Diffserv Codepoint' for such a		document defines the term "PCN-Enabled Diffserv Codepoint" for such a
	DSCP. Enabling PCN for a DSCP switches on PCN marking behaviour for		DSCP. Enabling PCN for a DSCP switches on PCN marking behaviour for
	packets with that DSCP, but only if those packets also have their ECN		packets with that DSCP, but only if those packets also have their ECN
	field set to a codepoint other than not-PCN.		field set to indicate a codepoint other than not-PCN.
	Enabling PCN marking behaviour disables any other marking behaviour		Enabling PCN marking behaviour disables any other marking behaviour
	(e.g. enabling PCN also disables the default ECN marking behaviour		(e.g. enabling PCN disables the default ECN marking behaviour
	introduced in [RFC3168]). The scheduling behaviour used for a packet		introduced in [RFC3168]). The scheduling behaviour used for a packet
	does not change whether PCN is enabled for a DSCP or not and whatever		does not change whether PCN is enabled for a DSCP or not and whatever
	the setting of the ECN field.		the setting of the ECN field.
	4.2.1. Implications of re-using a DiffServ Codepoint		4.2.1. Implications of re-using a DiffServ Codepoint
	[RFC4774] requires that packets for which alternate ECN semantics		[RFC4774] requires that packets for which alternate ECN semantics
	(PCN semantics) are used are clearly distinguished from packets to		(PCN semantics) are used are clearly distinguished from packets to
	which the semantics according to [RFC3168] apply. This is done by		which the default ECN semantics [RFC3168] apply. One means of doing
	using a DSCP to indicate that the ECN field is to be interpreted in		this is using a DSCP to indicate that the ECN field is to be
	the PCN context instead of the ECN context by PCN-enabled nodes.		interpreted in a different manner. We have chosen to use this
	Non-PCN-enabled forwarding nodes outside or inside the PCN domain		approach for PCN. Non-PCN-enabled forwarding nodes treat packets
	treat packets with a PCN-enabled DSCP like ECN traffic if appropriate		with a PCN-enabled DSCP like ECN traffic if appropriate ECN
	ECN codepoints are set in the IP header. This has several		codepoints are set in the IP header. This has several consequences.
	consequences.
	o Care must be taken that the PCN encoding of packets is not falsely		o Care must be taken to ensure that forwarding nodes do not
	interpreted by forwarding nodes as ECN encoding, and that no harm		interpret PCN encodings as ECN encodings, and that no harm is done
	is done if this were to happen. To that end, appropriate marking		if this were to happen. To that end, appropriate marking and re-
	and re-marking is performed at the ingress and the egress of a PCN		marking is performed at the ingress and the egress of a PCN-
	domain.		domain.
	o The re-used DSCP should be able to serve its original purpose		o The re-used DSCP should be able to serve its original purpose
	which was not PCN support. This is achieved by marking the		which was not PCN support. This is achieved by marking the
	packets of such flows with a not-PCN codepoint.		packets of such flows with a not-PCN codepoint.
	o The scheduling behaviour is coupled with the DSCP only.		o The scheduling behaviour is coupled with the DSCP only.
	Therefore, the same scheduling and buffer management rules are		Therefore, the same scheduling and buffer management rules are
	applied for non-PCN-capable and PCN-capable traffic using the same		applied for non-PCN-capable and PCN-capable traffic using the same
	PCN-enabled DSCP.		PCN-enabled DSCP.
	o Once the ECN field of a packet is used for PCN encoding, it has		o Once the ECN field of a packet is used for PCN encoding, it has
	lost its previous information unless this information was		lost its previous information unless this information is tunnelled
	tunnelled through the PCN domain. Therefore, the baseline PCN		through the PCN domain. Therefore, the baseline PCN encoding
	encoding disables ECN for PCN-enabled DSCPs. [PCN-3-enc-state]		disables ECN for PCN-enabled DSCPs. [PCN-3-enc-state] provides
	provides end-to-end ECN support where this is needed.		end-to-end ECN support where this is needed.
	4.3. Valid and Invalid Encoding Transitions at a PCN Node		4.3. Valid and Invalid Encoding Transitions at a PCN Node
	PCN edge node behaviour compliant with the PCN baseline encoding:		PCN-boundary-node behaviour compliant with the PCN baseline encoding:
	o Any packets with the ECN field already marked as CE or ECT		o Any packet with the ECN field already marked as CE or ECT arriving
	arriving at a PCN ingress node SHOULD be dropped or alternatively		at a PCN-ingress-node SHOULD be dropped or downgraded to a lower
	MAY be tunnelled through the PCN-domain. They MUST NOT be		class of service. Alternatively it MAY be tunnelled through the
	admitted to the PCN-domain directly.		PCN-domain. It MUST NOT be admitted to the PCN-domain directly.
	o On leaving the PCN-domain the ECN bits MUST be set to 00 (Not		o On leaving the PCN-domain the ECN bits of every PCN-packet MUST be
	ECT).		set to 00 (not-ECT).
	PCN interior node behaviour compliant with the PCN baseline encoding:		PCN-interior-node behaviour compliant with the PCN baseline encoding:
	o PCN Interior nodes MUST NOT change not-PCN to another codepoint		o PCN-interior-nodes MUST NOT change not-PCN to another codepoint
	and they MUST NOT change a PCN-Capable codepoint to not-PCN.		and they MUST NOT change a PCN-Capable codepoint to not-PCN.
	o PCN interior nodes that are in a pre-congestion state above the		o PCN-interior-nodes that are in a pre-congestion state above the
	configured level MUST set the PM codepoint by changing the ECN		configured level MUST set the PM codepoint by changing the ECN
	bits of NM marked packets to 11.		bits of NM marked packets to 11.
	o The PM codepoint MUST NOT be changed to NM.		o The PM codepoint MUST NOT be changed to NM.
	5. Backwards Compatability		5. Backwards Compatability
	BCP 124 [RFC4774] gives guidelines for specifying alternative		BCP 124 [RFC4774] gives guidelines for specifying alternative
	semantics for the ECN field. It sets out a number of factors that		semantics for the ECN field. It sets out a number of factors that
	must be taken into consideration. It also suggests various		must be taken into consideration. It also suggests various
	techniques to allow the co-existence of default ECN and alternative		techniques to allow the co-existence of default ECN and alternative
	ECN semantics. The alternative semantics specified here are		ECN semantics. The alternative semantics specified here are
	compliant with this BCP:		compliant with this BCP:
	o they use a DSCP to allow routers to distinguish that traffic uses		o they use a DSCP to allow routers to distinguish that traffic uses
	the alternate ECN semantics;		the alternate ECN semantics;
	o these semantics are defined for use within a controlled domain;		o these semantics are defined for use within a controlled domain;
	o ECN marked traffic is blocked from entering the PCN domain		o ECN marked traffic is blocked from entering the PCN-domain
	directly (though it might be tunnelled through the domain).		directly (though it might be tunnelled through the PCN-domain).
			o All traffic leaving the controlled domain is re-marked as not-ECT.
	6. IANA Considerations		6. IANA Considerations
	This document makes no request to IANA. It does however suggest a		This document makes no request to IANA. It does however suggest a
	change to the default ([RFC3168]) behaviour for the ECN field for the		change to the default ([RFC3168]) behaviour for the ECN field for the
	Voice-Admit [voice-admit] DSCP.		Voice-Admit [voice-admit] DSCP.
	7. Security Considerations		7. Security Considerations
	Packets claim entitlement to be PCN marked by carrying a PCN-enabled		Packets claim entitlement to be PCN marked by carrying a PCN-enabled
	skipping to change at page 7, line 47		skipping to change at page 8, line 6
	be described in a future separate document on PCN edge-node		be described in a future separate document on PCN edge-node
	behaviour. The PCN working group has initially been chartered to		behaviour. The PCN working group has initially been chartered to
	only consider a PCN-domain to be entirely under the control of one		only consider a PCN-domain to be entirely under the control of one
	operator, or a set of operators who trust each other [PCN-charter].		operator, or a set of operators who trust each other [PCN-charter].
	However there is a requirement to keep inter-domain scenarios in mind		However there is a requirement to keep inter-domain scenarios in mind
	when defining the PCN encoding. One way to extend to multiple		when defining the PCN encoding. One way to extend to multiple
	domains would be to concatenate PCN-domains and use PCN-boundary-		domains would be to concatenate PCN-domains and use PCN-boundary-
	nodes back to back at borders. Then any one domain's security		nodes back to back at borders. Then any one domain's security
	against its neighbours would be described as part of the edge-node		against its neighbours would be described as part of the edge-node
	behaviour document as above. One proposal on the table allows one to		behaviour document as above. One proposal on the table allows one to
	extend PCN across multiple domains without PCN edge nodes back-to-		extend PCN across multiple domains without PCN-boundary-nodes back-
	back at borders [re-PCN]. It is believed that the encoding described		to-back at borders [re-PCN]. It is believed that the encoding
	here would not be incompatible with the security framework described		described here would be compatible with the security framework
	there.		described there.
	8. Conclusions		8. Conclusions
	This document defines the baseline PCN encoding utilising a		This document defines the baseline PCN encoding utilising a
	combination of a PCN-enabled DSCP and the ECN field in the IP header.		combination of a PCN-enabled DSCP and the ECN field in the IP header.
	This baseline encoding allows the existence of two PCN encoding		This baseline encoding allows the existence of two PCN encoding
	states, Not Marked and PCN-Marked. It also allows for the co-		states, not-Marked and PCN-Marked. It also allows for the co-
	existence of non-PCN traffic within the same DSCP. The encoding		existence of traffic that is not PCN-capable within the same DSCP so
	scheme is conformant with [RFC4774].		long as theat traffic doesn't require end-to-end ECN support. The
			encoding scheme is conformant with [RFC4774].
	9. Acknowledgements		9. 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 and		group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley and
	others. Full details of the alternative schemes that were considered		others. Full details of the alternative schemes that were considered
	for adoption can be found in the document [pcn-enc-compare]. Thanks		for adoption can be found in the document [pcn-enc-compare]. Thanks
	to Ruediger Geib for providing comments on this document.		to Ruediger Geib for providing detailed comments on this document.
	10. Comments Solicited		10. Comments Solicited
	Comments and questions are encouraged and very welcome. They can be		Comments and questions are encouraged and very welcome. They can be
	addressed to the IETF Transport Area working group mailing list		addressed to the IETF congestion and pre-congestion working group
	<tsvwg@ietf.org>, and/or to the authors.		mailing list <pcn@ietf.org>, and/or to the authors.
	11. References		11. References
	11.1. Normative References		11.1. Normative References
	[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,
	skipping to change at page 9, line 41		skipping to change at page 9, line 49
	[voice-admit]		[voice-admit]
	Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-		Baker, F., Polk, J., and M. Dolly, "DSCPs for Capacity-
	Admitted Traffic",		Admitted Traffic",
	draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in		draft-ietf-tsvwg-admitted-realtime-dscp-04 (work in
	progress), February 2008.		progress), February 2008.
	Appendix A. Tunnelling Constraints		Appendix A. Tunnelling Constraints
	The rules that govern the behaviour of the ECN field for IP-in-IP		The rules that govern the behaviour of the ECN field for IP-in-IP
	tunnels were defined in [RFC3168]. This allowed for two tunnel modes		tunnels were defined in [RFC3168]. This allowed for two tunnel
	to exist. The limited functionality mode sets the outer header to		modes. The limited functionality mode sets the outer header to not-
	Not ECT, regardless of the value of the inner header. The full		ECT, regardless of the value of the inner header, in other words
	functionality mode copies the inner ECN field into the outer header		disabling ECN within the tunnel. The full functionality mode copies
	if the inner header is Not ECT or either of the 2 ECT codepoints. If		the inner ECN field into the outer header if the inner header is not-
	the inner header is CE then the outer header is set to ECT(0). On		ECT or either of the 2 ECT codepoints. If the inner header is CE
	decapsulation, if the CE codepoint is set on the outer header then		then the outer header is set to ECT(0). On decapsulation, if the CE
	this is copied into the inner header. Otherwise the inner header is		codepoint is set on the outer header then this is copied into the
	left unchanged. The apparent reason for blocking CE from being		inner header. Otherwise the inner header is left unchanged. The
	copied to the outer header was to prevent this from being used as a		stated reason for blocking CE from being copied to the outer header
	covert channel through IPSec tunnels.		was to prevent this from being used as a covert channel through IPSec
			tunnels.
	The IPSec protocol [RFC4301] changed the ECN tunnelling rule to allow		The IPSec protocol [RFC4301] changed the ECN tunnelling rule to allow
	IPSec tunnels to simply copy the inner header into the outer header.		IPSec tunnels to simply copy the inner header into the outer header.
	On decapsulation the outer header is discarded and the ECN field is		On decapsulation the outer header is discarded and the ECN field is
	only copied down if it is set to CE. Because of the possible		only copied down if it is set to CE.
	existence of tunnels, only CE (11) can be used as a PCN marking as it
	is the only mark that will survive decapsulation.
	There is a further issue involving tunnelling. In RFC3168, IP in IP		Because of the possible existence of tunnels, only CE (11) can be
			used as a PCN marking as it is the only mark that will survive
			decapsulation. However there is a need for caution with all
			tunneling within the PCN-domain. RFC3168 full functionality IP in IP
	tunnels are expected to set the ECN field to ECT(0) if the inner ECN		tunnels are expected to set the ECN field to ECT(0) if the inner ECN
	field is set to CE. This leads to the possibility that some packets		field is set to CE. This leads to the possibility that some packets
	within the PCN field that have already been marked may have that mark		within the PCN-domain that have already been marked may have that
	concealed further into the domain. This is undesirable for many PCN		mark concealed further into the domain. This is undesirable for many
	schemes and thus standard IP in IP tunnels SHOULD NOT be used within		PCN schemes and thus standard IP in IP tunnels SHOULD NOT be used
	a PCN-domain.		within a PCN-domain. Further work is needed within the Transport
			Area to rationalise the behaviour of tunnels in respect to the ECN
			field.
	Appendix B. Deployment Scenarios for PCN Using Baseline Encoding		Appendix B. Deployment Scenarios for PCN Using Baseline Encoding
	We illustrate the use of PCN baseline encoding for different PCN		This appendix illustrates possible PCN deployment scenarios where the
	deployment scenarios and explain also a case for which baseline		baseline encoding can be used and also explain a case for which
	encoding is not applicable. {Note this appendix is provided for		baseline encoding is not sufficient. {Note this appendix is provided
	information only}		for information only}.
	1. An operator may wish to use PCN-based admission control only. To		1. An operator may wish to use PCN-based admission control only. To
	that end, threshold marking based on admissible rates may be used		that end, threshold marking based on admissible rates might be
	as the only PCN metering and marking algorithm. As a		used as the only PCN metering and marking algorithm. As a
	consequence, the packet marks M are interpreted as admission-stop		consequence, the PM marks on the packets are interpreted as
	(AS) marks. The admission-control algorithm is based on		admission-stop (AS) marks. The admission-control algorithm is
	"admissible-rate overload".		based on "admissible-rate overload".
	2. An operator may wish to use PCN-based flow termination only. To		2. An operator may wish to use PCN-based flow termination only. To
	that end, excess rate marking based on supportable rates may be		that end, excess rate marking based on supportable rates might be
	used as the only PCN metering and marking algorithm. As a		used as the only PCN metering and marking algorithm. As a
	consequence, the packet marks M are interpreted as excess-traffic		consequence, the PM marks on the packets are interpreted as
	(ET) marks. The flow termination algorithm is based on		excess-traffic (ET) marks. The flow termination algorithm is
	"supportable-rate overload".		based on "supportable-rate overload".
	3. An operator may wish to use both PCN-based admission control and		3. An operator may wish to use both PCN-based admission control and
	flow termination. To that end, excess rate marking based on		flow termination. To that end, excess rate marking based on
	admissible rates may be used as the only PCN metering and marking		admissible rates may be used as the only PCN metering and marking
	algorithm. As a consequence, the packet marks are interpreted as		algorithm. As a consequence, the PM marks on the packets are
	admission-stop (AS) marks. Both the admission control and the		interpreted as admission-stop (AS) marks. Both the admission
	flow termination algorithm are based on "admissible-rate		control and the flow termination algorithm are based on
	overload".		"admissible-rate overload".
	4. An operator may wish to implement admission control based on		4. An operator may wish to implement admission control based on
	threshold marking at admissible rates and flow termination based		threshold marking at admissible rates and flow termination based
	on excess rate marking at supportable rates because these methods		on excess rate marking at supportable rates because these methods
	are believed to work better with small ingress-egress aggregates.		are believed to work better with small ingress-egress aggregates.
	Then two different markings are needed that cannot be recorded by		Then two different markings are needed. Such a deployment
	the PCN baseline encoding.		scenario is not supported by the PCN baseline encoding.
	Authors' Addresses		Authors' Addresses
	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
End of changes. 48 change blocks.
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