< draft-ietf-mpls-spring-entropy-label-01.txt		draft-ietf-mpls-spring-entropy-label-02.txt >
	Network Working Group S. Kini, Ed.		Network Working Group S. Kini, Ed.
	Internet-Draft Ericsson		Internet-Draft Ericsson
	Intended status: Informational K. Kompella		Intended status: Standards Track K. Kompella
	Expires: March 9, 2016 Juniper		Expires: July 29, 2016 Juniper
	S. Sivabalan		S. Sivabalan
	Cisco		Cisco
	S. Litkowski		S. Litkowski
	Orange		Orange
	R. Shakir		R. Shakir
	X. Xu		X. Xu
	Huawei		Huawei
	W. Hendrickx		W. Hendrickx
	Alcatel-Lucent		Alcatel-Lucent
	J. Tantsura		J. Tantsura
	Ericsson		Ericsson
	September 6, 2015		January 26, 2016
	Entropy labels for source routed stacked tunnels		Entropy labels for source routed tunnels with label stacks
	draft-ietf-mpls-spring-entropy-label-01		draft-ietf-mpls-spring-entropy-label-02
	Abstract		Abstract
	Source routed tunnel stacking is a technique that can be leveraged to		Source routed tunnels with label stacking is a technique that can be
	provide a method to steer a packet through a controlled set of		leveraged to provide a method to steer a packet through a controlled
	segments. This can be applied to the Multi Protocol Label Switching		set of segments. This can be applied to the Multi Protocol Label
	(MPLS) data plane. Entropy label (EL) is a technique used in MPLS to		Switching (MPLS) data plane. Entropy label (EL) is a technique used
	improve load balancing. This document examines and describes how ELs		in MPLS to improve load balancing. This document examines and
	are to be applied to source routed stacked tunnels.		describes how ELs are to be applied to source routed tunnels with
			label stacks.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
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	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
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	This Internet-Draft will expire on March 9, 2016.		This Internet-Draft will expire on July 29, 2016.
	Copyright Notice		Copyright Notice
	Copyright (c) 2015 IETF Trust and the persons identified as the		Copyright (c) 2016 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
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	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3		1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
	2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 3		2. Abbreviations and Terminology . . . . . . . . . . . . . . . . 3
	3. Use-case requiring multipath load balancing in source stacked		3. Use-case requiring multipath load balancing . . . . . . . . . 4
	tunnels . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
	4. Recommended EL solution for SPRING . . . . . . . . . . . . . 5		4. Recommended EL solution for SPRING . . . . . . . . . . . . . 5
	5. Options considered . . . . . . . . . . . . . . . . . . . . . 6		5. Options considered . . . . . . . . . . . . . . . . . . . . . 6
	5.1. Single EL at the bottom of the stack of tunnels . . . . . 6		5.1. Single EL at the bottom of the stack of tunnels . . . . . 6
	5.2. An EL per tunnel in the stack . . . . . . . . . . . . . . 7		5.2. An EL per tunnel in the stack . . . . . . . . . . . . . . 7
	5.3. A re-usable EL for a stack of tunnels . . . . . . . . . . 7		5.3. A re-usable EL for a stack of tunnels . . . . . . . . . . 8
	5.3.1. EL at top of stack . . . . . . . . . . . . . . . . . 8		5.3.1. EL at top of stack . . . . . . . . . . . . . . . . . 8
	5.4. ELs at readable label stack depths . . . . . . . . . . . 8		5.4. ELs at readable label stack depths . . . . . . . . . . . 8
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9		6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 9		8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
	9.1. Normative References . . . . . . . . . . . . . . . . . . 9		9.1. Normative References . . . . . . . . . . . . . . . . . . 9
	9.2. Informative References . . . . . . . . . . . . . . . . . 10		9.2. Informative References . . . . . . . . . . . . . . . . . 10
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
	1. Introduction		1. Introduction
	The source routed stacked tunnels paradigm is leveraged by techniques		The source routed tunnels with label stacking paradigm is leveraged
	such as Segment Routing (SR) [I-D.filsfils-spring-segment-routing] to		by techniques such as Segment Routing (SR)
	steer a packet through a set of segments. This can be directly		[I-D.ietf-spring-segment-routing] to steer a packet through a set of
	applied to the MPLS data plane, but it has implications on label		segments. This can be directly applied to the MPLS data plane, but
	stack depth.		it has implications on the label stack depth.
	Clarifying statements on label stack depth have been provided in		Clarifying statements on label stack depth have been provided in
	[RFC7325] but they do not address the case of source routed stacked		[RFC7325] but the RFC does not address the case of source routed
	MPLS tunnels as described in [I-D.filsfils-spring-segment-routing]		stacked MPLS tunnels as described in
	where deeper label stacks are more prevalent.
			[I-D.ietf-spring-segment-routing] where deeper label stacks are more
			prevalent.
	Entropy label (EL) [RFC6790] is a technique used in the MPLS data		Entropy label (EL) [RFC6790] is a technique used in the MPLS data
	plane to provide entropy for load balancing. When using LSP		plane to provide entropy for load balancing. When using LSP
	hierarchies there are implications on how [RFC6790] should be		hierarchies there are implications on how [RFC6790] should be
	applied. One such issue is addressed by		applied. The current document addresses the case where the hierarchy
	[I-D.ravisingh-mpls-el-for-seamless-mpls] but that is when different		is created at a single LSR as required by source routed tunnels with
	levels of the hierarchy are created at different LSRs. The current		label stacks.
	document addresses the case where the hierarchy is created at a
	single LSR as required by source stacked tunnels.
	A use-case requiring load balancing with source stacked tunnels is		A use-case requiring load balancing with source routed tunnels with
	given in Section 3. A recommended solution is described in Section 4		label stacks is given in Section 3. A recommended solution is
	keeping in consideration the limitations of implementations when		described in Section 4 keeping in consideration the limitations of
	applying [RFC6790] to deeper label stacks. Options that were		implementations when applying [RFC6790] to deeper label stacks.
	considered to arrive at the recommended solution are documented for		Options that were considered to arrive at the recommended solution
	historical purposes in Section 5.		are documented for historical purposes in Section 5.
	1.1. Requirements Language		1.1. Requirements Language
	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 RFC 2119 [RFC2119].		document are to be interpreted as described in RFC 2119 [RFC2119].
	Although this document is not a protocol specification, the use of		Although this document is not a protocol specification, the use of
	this language clarifies the instructions to protocol designers		this language clarifies the instructions to protocol designers
	producing solutions that satisfy the requirements set out in this		producing solutions that satisfy the requirements set out in this
	skipping to change at page 4, line 4 ¶		skipping to change at page 3, line 50 ¶
	SR - Segment Routing		SR - Segment Routing
	ECMP - Equal Cost Multi Paths		ECMP - Equal Cost Multi Paths
	MPLS - Multiprotocol Label Switching		MPLS - Multiprotocol Label Switching
	SID - Segment Identifier		SID - Segment Identifier
	RLD - Readable Label Depth		RLD - Readable Label Depth
	OAM - Operation, Administration and Maintenance		OAM - Operation, Administration and Maintenance
	3. Use-case requiring multipath load balancing in source stacked		3. Use-case requiring multipath load balancing
	tunnels
	+------+		+------+
	| |		| |
	+-------| P3 |-----+		+-------| P3 |-----+
	| +-----| |---+ |		| +-----| |---+ |
	L3| |L4 +------+ L1| |L2 +----+		L3| |L4 +------+ L1| |L2 +----+
	| | | | +--| P4 |--+		| | | | +--| P4 |--+
	+-----+ +-----+ +-----+ | +----+ | +-----+		+-----+ +-----+ +-----+ | +----+ | +-----+
	| S |-----| P1 |------------| P2 |--+ +--| D |		| S |-----| P1 |------------| P2 |--+ +--| D |
	| | | | | |--+ +--| |		| | | | | |--+ +--| |
	+-----+ +-----+ +-----+ | +----+ | +-----+		+-----+ +-----+ +-----+ | +----+ | +-----+
	+--| P5 |--+		+--| P5 |--+
	+----+		+----+
	S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs,		S=Source LSR, D=Destination LSR, P1,P2,P3,P4,P5=Transit LSRs,
	L1,L2,L3,L4=Links		L1,L2,L3,L4=Links
	Figure 1: Traffic engineering use-case		Figure 1: Traffic engineering use-case
	Traffic-engineering (TE) is one of the applications of MPLS and is		Traffic-engineering (TE) is one of the applications of MPLS and is
	also a requirement for source stacked tunnels. Consider the topology		also a requirement for source routed tunnels with label stacks.
	shown in Figure 1. Lets say the LSR P1 has a limitation that it can		Consider the topology shown in Figure 1. Lets say the LSR P1 has a
	only look four labels deep in the stack to do multipath decisions.		limitation that it can only look four labels deep in the stack to do
	All other transit LSRs in the figure can read deep label stacks and		multipath decisions. All other transit LSRs in the figure can read
	the LSR S can insert as many <ELI, EL> pairs as needed. The LSR S		deep label stacks and the LSR S can insert as many <ELI, EL> pairs as
	requires data to be sent to LSR D along a traffic-engineered path		needed. The LSR S requires data to be sent to LSR D along a traffic-
	that goes over the link L1. Good load balancing is also required		engineered path that goes over the link L1. Good load balancing is
	across equal cost paths (including parallel links). To engineer		also required across equal cost paths (including parallel links). To
	traffic along a path that takes link L1, the label stack that LSR S		engineer traffic along a path that takes link L1, the label stack
	creates consists of a label to the node SID of LSR P3, stacked over		that LSR S creates consists of a label to the node SID of LSR P3,
	the label for the adjacency SID of link L1 and that in turn is		stacked over the label for the adjacency SID of link L1 and that in
	stacked over the label to the node SID of LSR D. For simplicity lets		turn is stacked over the label to the node SID of LSR D. For
	assume that all LSRs use the same label space for source stacked		simplicity lets assume that all LSRs use the same label space for
	tunnels. Lets L_N-P denote the label to be used to reach the node		source routed label stacks. Lets L_N-P denote the label to be used
	SID of LSR P. Let L_A-Ln denote the label used for the adjacency SID		to reach the node SID of LSR P. Let L_A-Ln denote the label used for
	for link Ln. The LSR S must use the label stack <L_N-P3, L_A-L1,		the adjacency SID for link Ln. The LSR S must use the label stack
	L_N-D> for traffic-engineering. However to achieve good load		<L_N-P3, L_A-L1, L_N-D> for traffic-engineering. However to achieve
	balancing over the equal cost paths P2-P4-D, P2-P5-D and the parallel		good load balancing over the equal cost paths P2-P4-D, P2-P5-D and
	links L3, L4, a mechanism such as Entropy labels [RFC6790] should be		the parallel links L3, L4, a mechanism such as Entropy labels
	adapted for source stacked tunnels. Multiple ways to apply entropy		[RFC6790] should be adapted for source routed label stacks. Multiple
	labels were considered and are documented in Section 5 along with		ways to apply entropy labels were considered and are documented in
	their tradeoffs. A recommended solution is described in Section 4.		Section 5 along with their tradeoffs. A recommended solution is
			described in Section 4.
	4. Recommended EL solution for SPRING		4. Recommended EL solution for SPRING
	The solution described in this section follows [RFC6790].		The solution described in this section follows [RFC6790].
	An LSR may have a limitation in its ability to read and process the		An LSR may have a limitation in its ability to read and process the
	label stack in order to do multipath load balancing. This limitation		label stack in order to do multipath load balancing. This limitation
	expressed in terms of the number of label stack entries that the LSR		expressed in terms of the number of label stack entries that the LSR
	can read is henceforth referred to as the Readable Label Depth (RLD)		can read is henceforth referred to as the Readable Label Depth (RLD)
	capability of that LSR. If an EL does not occur within the RLD of an		capability of that LSR. If an EL does not occur within the RLD of an
	skipping to change at page 5, line 30 ¶		skipping to change at page 5, line 30 ¶
	corresponding to a label stack, multiple <ELI, EL> pairs MAY be		corresponding to a label stack, multiple <ELI, EL> pairs MAY be
	inserted in the label stack as long as the tunnel's label below which		inserted in the label stack as long as the tunnel's label below which
	they are inserted are advertised with entropy label capability		they are inserted are advertised with entropy label capability
	enabled. The LSR that inserts <ELI, EL> pairs MAY have limitations		enabled. The LSR that inserts <ELI, EL> pairs MAY have limitations
	on the number of such pairs that it can insert and also the depth at		on the number of such pairs that it can insert and also the depth at
	which it can insert them. If due to any limitation, the inserted ELs		which it can insert them. If due to any limitation, the inserted ELs
	are at positions such that an LSR along the path receives an MPLS		are at positions such that an LSR along the path receives an MPLS
	packet without an EL in the label stack within that LSR's RLD, then		packet without an EL in the label stack within that LSR's RLD, then
	the load balancing performed by that LSR would be poor. Special		the load balancing performed by that LSR would be poor. Special
	attention should be paid when a forwarding adjacency LSP (FA-LSP)		attention should be paid when a forwarding adjacency LSP (FA-LSP)
	[RFC4206] is used as a link along the path of a source stacked LSP,		[RFC4206] is used as a link along the path of a source routed LSP's
	since the labels of the FA-LSP would additionally count towards the		label stack, since the labels of the FA-LSP would additionally count
	depth of the label stack when calculating the appropriate positions		towards the depth of the label stack when calculating the appropriate
	to insert the ELs. The recommendations for inserting <ELI, EL> pairs		positions to insert the ELs. The recommendations for inserting <ELI,
	are:		EL> pairs are:
	o An LSR that is limited in the number of <ELI, EL> pairs that it		o An LSR that is limited in the number of <ELI, EL> pairs that it
	can insert SHOULD insert such pairs deeper in the stack.		can insert SHOULD insert such pairs deeper in the stack.
	o An LSR SHOULD try to insert <ELI, EL> pairs at positions so that		o An LSR SHOULD try to insert <ELI, EL> pairs at positions so that
	for the maximum number of transit LSRs, the EL occurs within the		for the maximum number of transit LSRs, the EL occurs within the
	RLD of the incoming packet to that LSR.		RLD of the incoming packet to that LSR.
	o An LSR SHOULD try to insert the minimum number of such pairs while		o An LSR SHOULD try to insert the minimum number of such pairs while
	trying to satisfy the above criteria.		trying to satisfy the above criteria.
	skipping to change at page 6, line 24 ¶		skipping to change at page 6, line 24 ¶
	set current insertion point to new insertion point		set current insertion point to new insertion point
	}		}
	Figure 2: Algorithm to insert <ELI, EL> pairs in a label stack		Figure 2: Algorithm to insert <ELI, EL> pairs in a label stack
	When this algorithm is applied to the example described in Section 3		When this algorithm is applied to the example described in Section 3
	it will result in ELs being inserted in two positions, one below the		it will result in ELs being inserted in two positions, one below the
	label L_N-D and another below L_N-P3. Thus the resulting label stack		label L_N-D and another below L_N-P3. Thus the resulting label stack
	would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL>		would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL>
	The RLD can be advertised via protocols and those extensions would be		The RLD can be advertised via protocols and those extensions are
	described in separate documents [I-D.xu-isis-mpls-elc] and		described in separate documents [I-D.xu-isis-mpls-elc] and
	[I-D.xu-ospf-mpls-elc].		[I-D.xu-ospf-mpls-elc].
	The recommendations above are not expected to bring any additional		The recommendations above are not expected to bring any additional
	OAM considerations beyond those described in section 6 of [RFC6790].		OAM considerations beyond those described in section 6 of [RFC6790].
	However, the OAM requirements and solutions for source stacked		However, the OAM requirements and solutions for source routed tunnels
	tunnels are still under discussion and future revisions of this		formed by label stacking are still under discussion and future
	document will address those if needed.		revisions of this document will address those if needed.
	5. Options considered		5. Options considered
			Different options that were considered to arrive at the recommended
			solution are documented in this section.
	5.1. Single EL at the bottom of the stack of tunnels		5.1. Single EL at the bottom of the stack of tunnels
	In this option a single EL is used for the entire label stack. The		In this option a single EL is used for the entire label stack. The
	source LSR S encodes the entropy label (EL) below the labels of all		source LSR S encodes the entropy label (EL) at the bottom of the
	the stacked tunnels. In the example described in Section 3 it will		label stack. In the example described in Section 3 it will result in
	result in the label stack at LSR S to look like <L_N-P3, L_A-L1, L_N-		the label stack at LSR S to look like <L_N-P3, L_A-L1, L_N-D, ELI,
	D, ELI, EL> <remaining packet header>. Note that the notation in		EL> <remaining packet header>. Note that the notation in [RFC6790]
	[RFC6790] is used to describe the label stack. An issue with this		is used to describe the label stack. An issue with this approach is
	approach is that as the label stack grows due an increase in the		that as the label stack grows due an increase in the number of SIDs,
	number of SIDs, the EL goes correspondingly deeper in the label		the EL goes correspondingly deeper in the label stack. Hence transit
	stack. Hence transit LSRs have to access a larger number of bytes in		LSRs have to access a larger number of bytes in the packet header
	the packet header when making forwarding decisions. In the example		when making forwarding decisions. In the example described in
	described in Section 3 the LSR P1 would poorly load-balance traffic		Section 3 the LSR P1 would poorly load-balance traffic on the
	on the parallel links L3, L4 since the EL is below the RLD of the		parallel links L3, L4 since the EL is below the RLD of the packet
	packet received by P1. A load balanced network design using this		received by P1. A load balanced network design using this approach
	approach must ensure that all intermediate LSRs have the capability		must ensure that all intermediate LSRs have the capability to
	to traverse the maximum label stack depth as required for that		traverse the maximum label stack depth as required for that
	application that uses source routed stacking.		application that uses source routed stacking.
	In the case where the hardware is capable of pushing a single <ELI,		In the case where the hardware is capable of pushing a single <ELI,
	EL> pair at any depth, this option is the same as the recommended		EL> pair at any depth, this option is the same as the recommended
	solution in Section 4.		solution in Section 4.
	This option was discounted since there exist a number of hardware		This option was discounted since there exist a number of hardware
	implementations which have a low maximum readable label depth.		implementations which have a low maximum readable label depth.
	Choosing this option can lead to a loss of load-balancing using EL in		Choosing this option can lead to a loss of load-balancing using EL in
	a significant part of the network but that is a critical requirement		a significant part of the network but that is a critical requirement
	skipping to change at page 7, line 26 ¶		skipping to change at page 7, line 28 ¶
	5.2. An EL per tunnel in the stack		5.2. An EL per tunnel in the stack
	In this option each tunnel in the stack can be given its own EL. The		In this option each tunnel in the stack can be given its own EL. The
	source LSR pushes an <ELI, EL> before pushing a tunnel label when		source LSR pushes an <ELI, EL> before pushing a tunnel label when
	load balancing is required to direct traffic on that tunnel. In the		load balancing is required to direct traffic on that tunnel. In the
	example described in Section 3, the source LSR S encoded label stack		example described in Section 3, the source LSR S encoded label stack
	would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL> where all the ELs		would be <L_N-P3, ELI, EL, L_A-L1, L_N-D, ELI, EL> where all the ELs
	can be the same. Accessing the EL at an intermediate LSR is		can be the same. Accessing the EL at an intermediate LSR is
	independent of the depth of the label stack and hence independent of		independent of the depth of the label stack and hence independent of
	the specific application that uses source stacking on that network.		the specific application that uses source routed tunnels with label
	A drawback is that the depth of the label stack grows significantly,		stacking in that network. A drawback is that the depth of the label
	almost 3 times as the number of labels in the label stack. The		stack grows significantly, almost 3 times as the number of labels in
	network design should ensure that source LSRs should have the		the label stack. The network design should ensure that source LSRs
	capability to push such a deep label stack. Also, the bandwidth		should have the capability to push such a deep label stack. Also,
	overhead and potential MTU issues of deep label stacks should be		the bandwidth overhead and potential MTU issues of deep label stacks
	accounted for in the network design.		should be accounted for in the network design.
	In the case where the RLD is the minimum value (3) for all LSRs, all		In the case where the RLD is the minimum value (3) for all LSRs, all
	LSRs are EL capable and the LSR that is inserting <ELI, EL> pairs has		LSRs are EL capable and the LSR that is inserting <ELI, EL> pairs has
	no limit on how many it can insert then this option is the same as		no limit on how many it can insert then this option is the same as
	the recommended solution in Section 4.		the recommended solution in Section 4.
	This option was discounted due to the existence of hardware		This option was discounted due to the existence of hardware
	implementations that can push a limited number of labels on the label		implementations that can push a limited number of labels on the label
	stack. Choosing this option would result in a hardware requirement		stack. Choosing this option would result in a hardware requirement
	to push two additional labels per tunnel label. Hence it would		to push two additional labels per tunnel label. Hence it would
	restrict the number of tunnels that can form a LSP and constrain the		restrict the number of tunnels that can form a LSP and constrain the
	types of LSPs that can be created. This was considered unacceptable.		types of LSPs that can be created. This was considered unacceptable.
	5.3. A re-usable EL for a stack of tunnels		5.3. A re-usable EL for a stack of tunnels
	In this option an LSR that terminates a tunnel re-uses the EL of the		In this option an LSR that terminates a tunnel re-uses the EL of the
	terminated tunnel for the next inner tunnel. It does this by storing		terminated tunnel for the next inner tunnel. It does this by storing
	the EL from the outer tunnel when that tunnel is terminated and re-		the EL from the outer tunnel when that tunnel is terminated and re-
	inserting it below the next inner tunnel label during the label swap		inserting it below the next inner tunnel label during the label swap
	operation. The LSR that stacks tunnels SHOULD insert an EL below the		operation. The LSR that stacks tunnels should insert an EL below the
	outermost tunnel. It SHOULD NOT insert ELs for any inner tunnels.		outermost tunnel. It should not insert ELs for any inner tunnels.
	Also, the penultimate hop LSR of a segment MUST NOT pop the ELI and		Also, the penultimate hop LSR of a segment must not pop the ELI and
	EL even though they are exposed as the top labels since the		EL even though they are exposed as the top labels since the
	terminating LSR of that segment would re-use the EL for the next		terminating LSR of that segment would re-use the EL for the next
	segment.		segment.
	In Section 3 above, the source LSR S encoded label stack would be		In Section 3 above, the source LSR S encoded label stack would be
	<L_N-P3, ELI, EL, L_A-L1, L_N-D>. At P1 the outgoing label stack		<L_N-P3, ELI, EL, L_A-L1, L_N-D>. At P1 the outgoing label stack
	would be <L_N-P3, ELI, EL, L_A-L1, L_N-D> after it has load balanced		would be <L_N-P3, ELI, EL, L_A-L1, L_N-D> after it has load balanced
	to one of the links L3 or L4. At P3 the outgoing label stack would		to one of the links L3 or L4. At P3 the outgoing label stack would
	be <L_N-D, ELI, EL>. At P2 the outgoing label stack would be <L_N-D,		be <L_N-D, ELI, EL>. At P2 the outgoing label stack would be <L_N-D,
	ELI, EL> and it would load balance to one of the nexthop LSRs P4 or		ELI, EL> and it would load balance to one of the nexthop LSRs P4 or
	P5. Accessing the EL at an intermediate LSR (e.g. P1) is		P5. Accessing the EL at an intermediate LSR (e.g. P1) is
	independent of the depth of the label stack and hence independent of		independent of the depth of the label stack and hence independent of
	the specific use-case to which the stacked tunnels are applied.		the specific use-case to which the label stack is applied.
	This option was discounted due to the significant change in label		This option was discounted due to the significant change in label
	swap operations that would be required for existing hardware.		swap operations that would be required for existing hardware.
	5.3.1. EL at top of stack		5.3.1. EL at top of stack
	A slight variant of the re-usable EL option is to keep the EL at the		A slight variant of the re-usable EL option is to keep the EL at the
	top of the stack rather than below the tunnel label. In this case		top of the stack rather than below the tunnel label. In this case
	each LSR that is not terminating a segment should continue to keep		each LSR that is not terminating a segment should continue to keep
	the received EL at the top of the stack when forwarding the packet		the received EL at the top of the stack when forwarding the packet
	skipping to change at page 9, line 22 ¶		skipping to change at page 9, line 28 ¶
	labels that need to be pushed against the requirement for entropy.		labels that need to be pushed against the requirement for entropy.
	6. Acknowledgements		6. Acknowledgements
	The authors would like to thank John Drake, Loa Andersson, Curtis		The authors would like to thank John Drake, Loa Andersson, Curtis
	Villamizar, Greg Mirsky, Markus Jork, Kamran Raza and Nobo Akiya for		Villamizar, Greg Mirsky, Markus Jork, Kamran Raza and Nobo Akiya for
	their review comments and suggestions.		their review comments and suggestions.
	7. IANA Considerations		7. IANA Considerations
	This memo includes no request to IANA.		This memo includes no request to IANA. Note to RFC Editor: Remove
			this section before publication.
	8. Security Considerations		8. Security Considerations
	This document does not introduce any new security considerations		This document does not introduce any new security considerations
	beyond those already listed in [RFC6790].		beyond those already listed in [RFC6790].
	9. References		9. References
	9.1. Normative References		9.1. Normative References
	[I-D.filsfils-spring-segment-routing]		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,		Requirement Levels", BCP 14, RFC 2119,
	Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,		DOI 10.17487/RFC2119, March 1997,
	Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,		<http://www.rfc-editor.org/info/rfc2119>.
	"Segment Routing Architecture", draft-filsfils-spring-
	segment-routing-04 (work in progress), July 2014.
	[I-D.ravisingh-mpls-el-for-seamless-mpls]		[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
	Singh, R., Shen, Y., and J. Drake, "Entropy label for		L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
	seamless MPLS", draft-ravisingh-mpls-el-for-seamless-		RFC 6790, DOI 10.17487/RFC6790, November 2012,
	mpls-04 (work in progress), October 2014.		<http://www.rfc-editor.org/info/rfc6790>.
			9.2. Informative References
			[I-D.ietf-spring-segment-routing]
			Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
			and r. rjs@rob.sh, "Segment Routing Architecture", draft-
			ietf-spring-segment-routing-07 (work in progress),
			December 2015.
	[I-D.xu-isis-mpls-elc]		[I-D.xu-isis-mpls-elc]
	Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.		Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
	Litkowski, "Signaling Entropy Label Capability Using IS-		Litkowski, "Signaling Entropy Label Capability Using IS-
	IS", draft-xu-isis-mpls-elc-02 (work in progress), April		IS", draft-xu-isis-mpls-elc-02 (work in progress), April
	2015.		2015.
	[I-D.xu-ospf-mpls-elc]		[I-D.xu-ospf-mpls-elc]
	Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.		Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
	Litkowski, "Signaling Entropy Label Capability Using		Litkowski, "Signaling Entropy Label Capability Using
	OSPF", draft-xu-ospf-mpls-elc-01 (work in progress),		OSPF", draft-xu-ospf-mpls-elc-01 (work in progress),
	October 2014.		October 2014.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,
	<http://www.rfc-editor.org/info/rfc2119>.
	[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)		[RFC4206] Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
	Hierarchy with Generalized Multi-Protocol Label Switching		Hierarchy with Generalized Multi-Protocol Label Switching
	(GMPLS) Traffic Engineering (TE)", RFC 4206,		(GMPLS) Traffic Engineering (TE)", RFC 4206,
	DOI 10.17487/RFC4206, October 2005,		DOI 10.17487/RFC4206, October 2005,
	<http://www.rfc-editor.org/info/rfc4206>.		<http://www.rfc-editor.org/info/rfc4206>.
	[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
	L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
	RFC 6790, DOI 10.17487/RFC6790, November 2012,
	<http://www.rfc-editor.org/info/rfc6790>.
	9.2. Informative References
	[I-D.filsfils-spring-segment-routing-use-cases]
	Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
	Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
	Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
	Crabbe, "Segment Routing Use Cases", draft-filsfils-
	spring-segment-routing-use-cases-01 (work in progress),
	October 2014.
	[I-D.ietf-isis-segment-routing-extensions]
	Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
	Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
	Extensions for Segment Routing", draft-ietf-isis-segment-
	routing-extensions-05 (work in progress), June 2015.
	[I-D.ietf-ospf-segment-routing-extensions]
	Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
	Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
	Extensions for Segment Routing", draft-ietf-ospf-segment-
	routing-extensions-05 (work in progress), June 2015.
	[RFC7325] Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,		[RFC7325] Villamizar, C., Ed., Kompella, K., Amante, S., Malis, A.,
	and C. Pignataro, "MPLS Forwarding Compliance and		and C. Pignataro, "MPLS Forwarding Compliance and
	Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,		Performance Requirements", RFC 7325, DOI 10.17487/RFC7325,
	August 2014, <http://www.rfc-editor.org/info/rfc7325>.		August 2014, <http://www.rfc-editor.org/info/rfc7325>.
	Authors' Addresses		Authors' Addresses
	Sriganesh Kini (editor)		Sriganesh Kini (editor)
	Ericsson		Ericsson
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