< draft-thubert-6man-flow-label-for-rpl-00.txt		draft-thubert-6man-flow-label-for-rpl-01.txt >
	ROLL P. Thubert, Ed.		6MAN P. Thubert, Ed.
	Internet-Draft Cisco		Internet-Draft Cisco
	Intended status: Standards Track April 17, 2014		Intended status: Standards Track May 13, 2014
	Expires: October 17, 2014		Expires: November 14, 2014
	The IPv6 Flow Label within a RPL domain		The IPv6 Flow Label within a RPL domain
	draft-thubert-6man-flow-label-for-rpl-00		draft-thubert-6man-flow-label-for-rpl-01
	Abstract		Abstract
	This document present how the Flow Label can be used inside a RPL		This document present how the Flow Label can be used inside a RPL
	domain as a replacement to the RPL option and provides rules for the		domain as a replacement to the RPL option and provides rules for the
	root to set and reset the Flow Label when forwarding between the		root to set and reset the Flow Label when forwarding between the
	inside of RPL domain and the larger Internet, in both direction.		inside of RPL domain and the larger Internet, in both direction.
	This new operation saves 44 bits in each frame, and an eventual IP-		This new operation saves 44 bits in each frame, and an eventual IP-
	in-IP encapsulation within the RPL domain that is required for all		in-IP encapsulation within the RPL domain that is required for all
	packets that reach outside of the RPL domain.		packets that reach outside of the RPL domain.
	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/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	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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	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on October 17, 2014.		This Internet-Draft will expire on November 14, 2014.
	Copyright Notice		Copyright Notice
	Copyright (c) 2014 IETF Trust and the persons identified as the		Copyright (c) 2014 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. On LLN flows . . . . . . . . . . . . . . . . . . . . . . . 3		1.1. On Wasted Energy . . . . . . . . . . . . . . . . . . . . 3
	1.2. On Wasted Resources . . . . . . . . . . . . . . . . . . . 4		1.2. LLN flows . . . . . . . . . . . . . . . . . . . . . . . . 5
	1.3. On Compatibility With Existing Standards . . . . . . . . . 5		1.3. On Compatibility With Existing Standards . . . . . . . . 6
	2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
	3. Flow Label Format Within the RPL Domain . . . . . . . . . . . 6		3. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 7
	4. Root Operation . . . . . . . . . . . . . . . . . . . . . . . . 7		4. Flow Label Format Within the RPL Domain . . . . . . . . . . . 8
	4.1. Incoming Packets . . . . . . . . . . . . . . . . . . . . . 7		5. Root Operation . . . . . . . . . . . . . . . . . . . . . . . 8
	4.2. Outgoing Packets . . . . . . . . . . . . . . . . . . . . . 7		5.1. Incoming Packets . . . . . . . . . . . . . . . . . . . . 9
	5. RPL node Operation . . . . . . . . . . . . . . . . . . . . . . 8		5.2. Outgoing Packets . . . . . . . . . . . . . . . . . . . . 9
	6. Security Considerations . . . . . . . . . . . . . . . . . . . 8		6. RPL node Operation . . . . . . . . . . . . . . . . . . . . . 9
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
	8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8		9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
	9.1. Normative References . . . . . . . . . . . . . . . . . . . 8		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
	9.2. Informative References . . . . . . . . . . . . . . . . . . 9		10.1. Normative References . . . . . . . . . . . . . . . . . . 10
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9		10.2. Informative References . . . . . . . . . . . . . . . . . 10
			Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11
	1. Introduction		1. Introduction
	The emergence of radio technology enabled a large variety of new		The emergence of radio technology enabled a large variety of new
	types of devices to be interconnected, at a very low marginal cost		types of devices to be interconnected, at a very low marginal cost
	compared to wire, at any range from Near Field to interplanetary		compared to wire, at any range from Near Field to interplanetary
	distances, and in circumstances where wiring would be less than		distances, and in circumstances where wiring would be less than
	practical, for instance rotating devices.		practical, for instance rotating devices.
	In particular, IEEE802.14.5 [IEEE802154] that is chartered to specify		In particular, IEEE802.14.5 [IEEE802154] that is chartered to specify
	PHY and MAC layers for radio Lowpower Lossy Networks (LLNs), defined		PHY and MAC layers for radio Lowpower Lossy Networks (LLNs), defined
	the TimeSlotted Channel Hopping [I-D.ietf-6tisch-tsch] (TSCH) mode of		the TimeSlotted Channel Hopping [I-D.ietf-6tisch-tsch] (TSCH) mode of
	operation as part of the IEEE802.15.4e MAC specification in order to		operation as part of the IEEE802.15.4e MAC specification in order to
	address Time Sensitive applications.		address Time Sensitive applications.
	The 6TISCH architecture [I-D.ietf-6tisch-architecture] specifies		The 6TISCH architecture [I-D.ietf-6tisch-architecture] specifies the
	the operation IPv6 over the IEEE802.15.4e TimeSlotted Channel Hopping		operation IPv6 over TSCH wireless networks attached and synchronized
	[I-D.ietf-6tisch-tsch] (TSCH) wireless networks attached and		by backbone routers. In that model, route Computation may be
	synchronized by backbone routers. In that model, route Computation		achieved in a centralized fashion by a Path Computation Element
	may be achieved in a centralized fashion by a Path Computation		(PCE), in a distributed fashion using the Routing Protocol for Low
	Element (PCE), in a distributed fashion using the Routing Protocol		Power and Lossy Networks [RFC6550] (RPL), or in a mixed mode. The
	for Low Power and Lossy Networks [RFC6550] (RPL), or in a mixed		Backbone Routers may typically serve as roots for the RPL domain.
	mode. The Backbone Routers may typically serve as roots for the RPL
	domain.
	6TiSCH was created to simplify the adoption of IETF technology by		6TiSCH was created to simplify the adoption of IETF technology by
	other Standard Defining Organizations (SDOs), in particular in the		other Standard Defining Organizations (SDOs), in particular in the
	Industrial Automation space, which already relies on variations of		Industrial Automation space, which already relies on variations of
	IEEE802.15.4e TSCH for Wireless Sensor Networking. ISA100.11a		IEEE802.15.4e TSCH for Wireless Sensor Networking. ISA100.11a
	[ISA100.11a] is an example of such industrial WSN standard, using		[ISA100.11a] is an example of such industrial WSN standard, using
	IEEE802.15.4e over the classical IEEE802.14.5 PHY. In that case,		IEEE802.15.4e over the classical IEEE802.14.5 PHY. In that case,
	after security is applied, roughly 80 octets are available per frame		after security is applied, roughly 80 octets are available per frame
	for IP and Payload. In order to 1) avoid fragmentation and 2)		for IP and Payload. In order to 1) avoid fragmentation and 2)
	conserve energy, the SDO will scrutinize any bit in the frame and		conserve energy, the SDO will scrutinize any bit in the frame and
	reject any waste.		reject any waste.
	The challenge to obtain the adoption of IPv6 in the original standard		The challenge to obtain the adoption of IPv6 in the original standard
	was really to save any possible bit in the frames, including the UDP		was really to save any possible bit in the frames, including the UDP
	checksum which was an interesting discussion on its own. This work		checksum which was an interesting discussion on its own. This work
	was actually one of the roots for the 6LoWPAN Header Compression		was actually one of the roots for the 6LoWPAN Header Compression
	[RFC6282] work, which goes down to the individual bits to save space		[RFC6282] work, which goes down to the individual bits to save space
	in the frames for actual data, and allowed ISA100.11a to adopt IPv6.		in the frames for actual data, and allowed ISA100.11a to adopt IPv6.
	1.1. On LLN flows		1.1. On Wasted Energy
	In industrial applications such as control systems [RFC5673], a
	packet loss is usually acceptable but jitter and latency must be
	strictly controlled as they can play a critical role in the
	interpretation of the measured information. Sensory systems are
	often distributed, and the control information can in fact be
	originated from multiple sources and aggregated. As a result, it can
	be a requirement for related measurements from multiple sources to be
	treated as a single flow following a same path over the Internet in
	order to experience similar jitter and latency. The traditional
	tuple of source, destination and ports might then not be the proper
	indication to isolate a meaningful flow.
	In a typical LLN application, the bulk of the traffic consists of
	small chunks of data (in the order few bytes to a few tens of bytes)
	at a time. In the industrial case, a typical frequency is 4Hz but it
	can be a lot slower than that for, say, environmental monitoring.
	The granularity of traffic from a single source is too small to make
	a lot of sense in load balancing application.
	In such cases, related packets from multiple sources should not be
	load-balanced along their path in the Internet; load-balancing can be
	discouraged by tagging those packets with a same Flow Label in the
	IPv6 [RFC2460] header. This can be achieved if the Flow Label in
	packets outgoing a RPL domain are set by the root of the RPL
	structure as opposed to the actual source. It derives that the Flow
	Label could be reused inside the RPL domain.
	In a LLN, each transmitted bit represents energy and every saving
	counts dearly. Considering that the value for which the Flow Label
	is used in the IPv6 Flow Label Specification [RFC6437] is to serve
	load balancing in the core, it is unlikely that LLN devices will
	consume energy to generate and then transmit a Flow Label to serve
	interests in some other place. On the other hand, it makes sense to
	recommend the computation of a stateless Flow Label at the root of
	the LLN towards the Internet.
	Reciprocally, [RFC6437] requires that once set, a non-zero flow label
	value is left unchanged. The value for that setting is consumed once
	the packet has traversed the core and reaches the LLN. Then again,
	there is little value but a high cost for the LLN in spending 20 bits
	to transport a Flow Label from the Internet over the constrained
	network to the destination node. It results that the MUST in
	[RFC6437] should be alleviated for packets coming from the outside on
	the LLN, and that it should be acceptable that the compression over
	the LLN erases the original flow label. It should also be acceptable
	that the Flow Label field is reused in the LLN as proposed in this
	draft.
	1.2. On Wasted Resources		The design of Lowpower Lossy Networks is generally focussed on saving
			energy, which is the most constrained resource of all. The other
			constraints, such as the memory capacity and the duty cycling of the
			LLN devices, derive from that primary concern. Energy is typically
			available from batteries that are expected to last for years, or
			scavenged from the environment in very limited quantities. Any
			protocol that is intended for use in LLNs must be designed with the
			primary concern of saving energy as a strict requirement.
	The Routing Protocol for Low Power and Lossy Networks (RPL)		The Routing Protocol for Low Power and Lossy Networks (RPL) [RFC6550]
	[RFC6550] specification defines a generic Distance Vector protocol		specification defines a generic Distance Vector protocol that is
	that is adapted to a variety of LLNs. RPL forms Destination Oriented		indeed designed for very low energy consumption and adapted to a
	Directed Acyclic Graphs (DODAGs) which root often acts as the Border		variety of LLNs. RPL forms Destination Oriented Directed Acyclic
	Router to connect the RPL domain to the Internet. The root is		Graphs (DODAGs) which root often acts as the Border Router to connect
	responsible to select the RPL Instance that is used to forward a		the RPL domain to the Internet. The root is responsible to select
	packet coming from the Internet into the RPL domain.		the RPL Instance that is used to forward a packet coming from the
			Internet into the RPL domain and set the related RPL information in
			the packets.
	A classical RPL implementation will use the RPL Option for Carrying		A classical RPL implementation will use the RPL Option for Carrying
	RPL Information in Data-Plane Datagrams [RFC6553] to tag a packet		RPL Information in Data-Plane Datagrams [RFC6553] to tag a packet
	with the Instance ID and other information that RPL requires for its		with the Instance ID and other information that RPL requires for its
	operation within the RPL domain. In particular, the Rank, which is		operation within the RPL domain. In particular, the Rank, which is
	the scalar metric computed by an specialized Objective Function such		the scalar metric computed by an specialized Objective Function such
	as [RFC6552], is modified at each hop and allows to validate that the		as [RFC6552], is modified at each hop and allows to validate that the
	packet progresses in the execpted direction each upwards or downwards		packet progresses in the expected direction each upwards or downwards
	in along the DODAG.		in along the DODAG.
	With [RFC6553] the RPL option is encoded as 6 Octets; it must be		With [RFC6553] the RPL option is encoded as 6 Octets; it must be
	placed in a Hop-by-Hop header that represents 2 additional octets for		placed in a Hop-by-Hop header that represents 2 additional octets for
	a total of 8. In order to limit its range to the inside the RPL		a total of 8. In order to limit its range to the inside the RPL
	domain, the Hop-by-Hop header must be added to (or removed from)		domain, the Hop-by-Hop header must be added to (or removed from)
	packets that cross the border of the RPL domain. For reasons such as		packets that cross the border of the RPL domain. For reasons such as
	the capability to send ICMP errors back to the source, this operation		the capability to send ICMP errors back to the source, this operation
	involves an extra IP-in-IP encapsulation inside the RPL domain for		involves an extra IP-in-IP encapsulation inside the RPL domain for
	all the packets which path is not contained within the RPL domain.		all the packets which path is not contained within the RPL domain.
	The 8-octets overhead is detrimental to the LLN operation, in		The 8-octets overhead is detrimental to the LLN operation, in
	particular with regards to bandwidth and battery constraints. The		particular with regards to bandwidth and battery constraints. The
	extra encapsulation may cause a containing frame to grow above		extra encapsulation may cause a containing frame to grow above
	maximum frame size, leading to Layer 2 or 6LoWPAN [RFC4944]		maximum frame size, leading to Layer 2 or 6LoWPAN [RFC4944]
	fragmentation, which in turn cause even more energy spending and		fragmentation, which in turn cause even more energy spending and
	issues discussed in the LLN Fragment Forwarding and Recovery [I-D		issues discussed in the LLN Fragment Forwarding and Recovery
	.thubert-6lo-forwarding-fragments].		[I-D.thubert-6lo-forwarding-fragments].
	------+--------- ^		------+--------- ^
	| Internet |		| Internet |
	| | Native IPv6		| | Native IPv6
	+-----+ |		+-----+ |
	| | Border Router (RPL Root) ^ | ^		| | Border Router (RPL Root) ^ | ^
	| | | | |		| | | | |
	+-----+ | | | IPv6 +		+-----+ | | | IPv6 +
	| | | | HbH		| | | | HbH
	o o o o | | | headers		o o o o | | | headers
	o o o o o o o o o | | |		o o o o o o o o o | | |
	o o o o o o o o o o | | |		o o o o o o o o o o | | |
	o o o o o o o o o | | |		o o o o o o o o o | | |
	o o o o o o o o v v v		o o o o o o o o v v v
	o o o o o o		o o o o o o
	o o o o		o o o o
	LLN		LLN
			Figure 1: IP-in-IP Encapsulation within the LLN
	Considering that, in the classical IEEE802.14.5 PHY that is used by		Considering that, in the classical IEEE802.14.5 PHY that is used by
	ISA100.11a, roughly 80 octets are available per frame after security		ISA100.11a, roughly 80 octets are available per frame after security
	is applied, and , any additional transmitted octet weights in the		is applied, and any additional transmitted bit weights in the energy
	energy consumption and drains the batteriesBut [RFC6282] does not		consumption and drains the batteries.
	provide an efficient compression for the RPL option so the cost in
	current implementations can not be alleviated in any fashion. So		Regrettably, [RFC6282] does not provide an efficient compression for
	even for packets that are confined within the RPL domain and do not		the RPL option so the cost in current implementations can not be
	need the 6in6 encapsulation, the use of the flow label instead of the		alleviated in any fashion. So even for packets that are confined
	RPL option would be a valuable saving.		within the RPL domain and do not need the IP-in-IP encapsulation, the
			use of the flow label instead of the RPL option would be a valuable
			saving.
			1.2. LLN flows
			In Industrial Automation and Control Systems (IACS) [RFC5673], a
			packet loss is usually acceptable but jitter and latency must be
			strictly controlled as they can play a critical role in the
			interpretation of the measured information. Sensory systems are
			often distributed, and the control information can in fact be
			originated from multiple sources and aggregated. In such cases,
			related packets from multiple sources should not be load-balanced
			along their path in the Internet.
			In a typical LLN application, the bulk of the traffic consists of
			small chunks of data (in the order few bytes to a few tens of bytes)
			at a time. 4Hz is a typical loop frequency in Process Control,
			though it can be a lot slower than that in, say, environmental
			monitoring. The granularity of traffic from a single source is too
			small to make a lot of sense in load balancing application.
			As a result, it can be a requirement for related measurements from
			multiple sources to be treated as a single flow following a same path
			over the Internet so as to experience similar jitter and latency.
			The traditional tuple of source, destination and ports might then not
			be the proper indication to isolate a consistent flow. On the other
			hand, the flow integrity can be preserved in a simple manner if the
			setting of the Flow Label in the IPv6 header of packets outgoing a
			RPL domain, is centralized to the root of the RPL DODAG structure, as
			opposed to distributed across the actual sources.
			Considering that the goal for setting the Flow Label as prescribed in
			the IPv6 Flow Label Specification [RFC6437] is to improve load
			balancing in the core of the Internet, it is unlikely that LLN
			devices will consume energy to generate and then transmit a Flow
			Label to serve outside interests and the Flow Label is generally left
			to zero so as to be elided in the 6LoWPAN [RFC6282] compression. So
			in a general manner the interests of the core are better served if
			the RPL roots systematically rewrite the flow label rather than if
			they never do.
			For packets coming into the RPL domain from the Internet, the value
			for setting the Flow Label as prescribed in [RFC6437] is consumed
			once the packet has traversed the core and reaches the LLN. Then
			again, there is little value but a high cost for the LLN in spending
			20 bits to transport a Flow Label from the Internet over the
			constrained network to a destination node that has no use of it.
	1.3. On Compatibility With Existing Standards		1.3. On Compatibility With Existing Standards
	All the packets from all the nodes in a same DODAG that are leaving a		All the packets from all the nodes in a same DODAG that are leaving a
	RPL domain towards the Internet will transit via a same RPL root.		RPL domain towards the Internet will transit via a same RPL root.
	The RPL root segregates the Internet and the RPL domain, which		The RPL root segregates the Internet and the RPL domain, which
	enables the capability to reuse the Flow Label within the RPL domain.		enables the capability to reuse the Flow Label within the RPL domain.
	On the other hand, the operation of writing/rewriting the IPv6 Flow		On the other hand, the operation of resetting or reusing the IPv6
	Label at the root of a RPL domain may seem in contradiction with the		Flow Label at the root of a RPL domain is a deviation from the IPv6
	IPv6 Flow Label Specification [RFC6437], in that it is neither the		Flow Label Specification [RFC6437], in that it is neither the source
	source nor the first hop router that sets the final Flow Label for		nor the first hop router that sets the final Flow Label for use
	use outside the RPL domain.		outside the RPL domain.
	Additionally, using the Flow Label to transport the information that		Additionally, using the Flow Label to transport the information that
	is classically present in the RPL option implies that the Flow Label		is classically present in the RPL option implies that the Flow Label
	is modified at each hop inside the RPL domain, which again		is modified at each hop inside the RPL domain, which again is a
	contradicts [RFC6437], which explicitly requires that the flow label		limited deviation from [RFC6437], which explicitly requires that the
	cannot be modified once set.		flow label cannot be modified once set.
	But if we consider the whole RPL domain as a large virtual host from		But if we consider the whole RPL domain as a large virtual host from
	the standpoint of the rest of the Internet, the interests that lead		the standpoint of the rest of the Internet, the interests that lead
	to [RFC6437], and in particular load balancing in the core of the		to [RFC6437], and in particular load balancing in the core of the
	Internet, are probably better served if the root guarantees that the		Internet, are probably better served if the root guarantees that the
	Flow Label is set in a compliant fashion than if we rely on each		Flow Label is set in a compliant fashion than if we rely on each
	individual sensor that may not use it at all, or use it slightly		individual sensor that may not use it at all, or use it slightly
	differently such as done in ISA100.11a.		differently such as done in ISA100.11a.
	Additionally, LLN flows can be compound flows aggregating information		Additionally, LLN flows can be compound flows aggregating information
	from multiple sources. The root is an ideal place to rewrite the		from multiple sources. The root is an ideal place to rewrite the
	Flow Label to a same value for a same flow across multiple sources,		Flow Label to a same value for a same flow across multiple sources,
	ensuring compliance with the rules defined by [RFC6437] for use		ensuring compliance with the rules defined by [RFC6437] for use
	outside of the RPL domain and in particular in the core of the		outside of the RPL domain and in particular in the core of the
	Internet.		Internet.
	It can be noted that [RFC6282] provides an efficient header		It can be noted that [RFC6282] provides an efficient header
	compression for packets that do have the Flow Label set in the IPv6		compression for packets that do have the Flow Label set in the IPv6
	header. It results overhead for transporting the RPL information can		header. It results that the overhead for transporting the RPL
	be down from 64 to 20 bits, alleviating at the same time the need for		information can be down from 64 to 20 bits, alleviating at the same
	IP-in-IP encapsulation. This optimization cannot be ignored, and is		time the need for IP-in-IP encapsulation. This optimization cannot
	required for the adoption of the 6TiSCH architecture by external		be ignored, and can make the difference for the adoption of RPL and
	standard bodies.		6TiSCH by external standard bodies.
	This document specifies how the Flow Label can be reused within the		This document specifies how the Flow Label can be reused within the
	RPL domain as a replacement to the RPL option. The use of the Flow		RPL domain as a replacement to the RPL option. The use of the Flow
	Label within a RPL domain is an instance of the stateful scenarios as		Label within a RPL domain is an instance of the stateful scenarios as
	discussed in [RFC6437]where the states include the rank of a node and		discussed in [RFC6437] where the states include the Rank of a node
	the RPLInstanceID that identifies the routing topology.		and the RPLInstanceID that identifies the routing topology.
	2. Terminology		2. Terminology
	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].
	The Terminology used in this document is consistent with and		The Terminology used in this document is consistent with and
	incorporates that described in `Terminology in Low power And Lossy		incorporates that described in `Terminology in Low power And Lossy
	Networks' [RFC7102] and [RFC6550].		Networks' [RFC7102] and [RFC6550].
	3. Flow Label Format Within the RPL Domain		3. Applicability
			This specification applies to a RPL [RFC6282] domain that forms a
			stub LLN and is connected to the Internet by and only by its RPL
			root(s), which act(s) as Border Router(s) for the LLN. With RPL, a
			root is the bottleneck for all the traffic between the Internet and
			the Destination-Oriented Directed Acyclic Graph (DODAG) that it
			serves.
			In that context, the specification entitles a RPL root to rewrite the
			IPv6 [RFC2460] Flow Label of all packets entering or leaving the RPL
			domain in both directions, from and towards the Internet, regardless
			of its original setting. This may seem contradictory with the IPv6
			Flow Label Specification [RFC6437] which stipulates that once it is
			set, the Flow Label is left unchanged; but the RFC also indicates a
			violation to the rule can be accepted for compelling reasons, and
			that security is a case justifying such a violation. This
			specification suggests that energy-saving is another compelling
			reason for a violation to the aforementioned rule.
			For the compelling reason of saving energy, this specification allows
			that regardless of its original setting, a root of a RPL domain MAY
			reset the Flow Label of IPv6 packets entering the RPL domain to zero
			for an optimal Header Compression by 6LoWPAN [RFC6282]. The
			specification also allows that the root and LLN routers MAY reuse the
			Flow Label inside the LLN for LLN purposes, such as to carry the RPL
			Information as detailed hereafter.
			This specification also allows that regardless of its original
			setting, a a root of a RPL domain MAY set low Label of IPv6 packets
			that exits the RPL domain MAY be set by the RPL, in a manner that
			SHOULD conform the prescriptions in [RFC6437], and that a source in
			the RPL domain MAY NOT expect that its setting of the Flow Label be
			preserved end-to-end. From there, the capability by RPL routers
			inside the LLN to alter a non-zero Flow Label between the source and
			the root is another minor deviation to [RFC6437] that is also
			acceptable since it is transparent to the core of the Internet.
			4. Flow Label Format Within the RPL Domain
	[RFC6550] section 11.2 specifies the fields that are to be placed		[RFC6550] section 11.2 specifies the fields that are to be placed
	into the packets for the purpose of Instance Identification, as well		into the packets for the purpose of Instance Identification, as well
	as Loop Avoidance and Detection. Those fields include an 'O', and		as Loop Avoidance and Detection. Those fields include an 'O', and
	'R' and an 'F' bits, the 8-bit RPLInstanceID, and the 16-bit		'R' and an 'F' bits, the 8-bit RPLInstanceID, and the 16-bit
	SenderRank. SenderRank is the result of the DAGRank operation on the		SenderRank. SenderRank is the result of the DAGRank operation on the
	rank of the sender, where the DAGRank operation is defined in section		rank of the sender, where the DAGRank operation is defined in section
	3.5.1 as:		3.5.1 as:
	DAGRank(rank) = floor(rank/MinHopRankIncrease)		DAGRank(rank) = floor(rank/MinHopRankIncrease)
	If MinHopRankIncrease is set to a multiple of 256, it appears that		If MinHopRankIncrease is set to a multiple of 256, it appears that
	the most significant 8 bits of the SenderRank will be all zeroes and		the most significant 8 bits of the SenderRank will be all zeroes and
	could be ommitted. In that case, the Flow Label MAY be used as a		could be omitted. In that case, the Flow Label MAY be used as a
	replacement to the [RFC6553] RPL option. To achive this, the		replacement to the [RFC6553] RPL option. To achieve this, the
	SenderRank is expressed with 8 least significant bits, and the		SenderRank is expressed with 8 least significant bits, and the
	information carried within the Flow Label in a packet is constructed		information carried within the Flow Label in a packet is constructed
	follows:		follows:
	0 1 2		0 1 2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| |O|R|F| SenderRank | RPLInstanceID |		| |O|R|F| SenderRank | RPLInstanceID |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Figure 1: The RPL Flow Label
	The first (leftmost) bit of the Flow Label is reserved and should be		The first (leftmost) bit of the Flow Label is reserved and should be
	set to zero.		set to zero.
	4. Root Operation		5. Root Operation
	[RFC6437] section 3 intentionally does not consider flow label values		[RFC6437] section 3 intentionally does not consider flow label values
	in which any of the bits have semantic significance. However, the		in which any of the bits have semantic significance. However, the
	present specification assigns semantics to various bits in the flow		present specification assigns semantics to various bits in the flow
	label, destroying within the edge network that is the RPL domaina		label, destroying within the edge network that is the RPL domain the
	property of belonging to a statistically uniform distribution		property of belonging to a statistically uniform distribution that is
	that is desirable in the rest of the Internet. This property MUST be		desirable in the rest of the Internet.
	restored by the root for outgoing packets.
	It can be noted that the rationale for the statistically uniform		It can be noted that the rationale for the statistically uniform
	distribution does not necessarily bring a lot of value within the RPL		distribution does not necessarily bring a lot of value within the RPL
	domain. In a specific use case where it would, that value must be		domain. In a specific use case where it would, that value must be
	compared with that of the battery savings in order to decide which		compared with that of the battery savings in order to decide which
	technique the deployment will use to transport the RPL information.		technique the deployment will use to transport the RPL information.
	4.1. Incoming Packets		5.1. Incoming Packets
	When routing a packet towards the RPL domain, the root applies a		When routing a packet towards the RPL domain, the root applies a
	policy to determine whether the Flow Label is to be used to carry the		policy to determine whether the Flow Label is to be used to carry the
	RPL information. If so, the root MUST reset the Flow Label and then		RPL information. If so, the root MUST reset the Flow Label and then
	it MUST set all the fields in the Flow Label as prescribed by		it MUST set all the fields in the Flow Label as prescribed by
	[RFC6553] using the format specified in Figure 2. In particular, the		[RFC6553] using the format specified in Figure 1. In particular, the
	root selects the Instance that will be used to forward the packet		root selects the Instance that will be used to forward the packet
	within the RPL domain.		within the RPL domain.
	4.2. Outgoing Packets		5.2. Outgoing Packets
	When routing a packet outside the RPL domain, the root applies a		When routing a packet outside the RPL domain, the root applies a
	policy to determine whether the Flow Label was used to carry the RPL		policy to determine whether the Flow Label was used to carry the RPL
	information. If so, the root MUST reset the Flow Label. The root		information. If so, the root MUST reset the Flow Label. The root
	SHOULD recompute a Flow Label following the rules prescribed by		SHOULD recompute a Flow Label following the rules prescribed by
	[RFC6553]. In particular, the root MAY ignore the source address but		[RFC6553]. In particular, the root MAY ignore the source address but
	it SHOULD use the RPLInstanceID for the computation.		it SHOULD use the RPLInstanceID for the computation.
	5. RPL node Operation		6. RPL node Operation
	Depending on the policy in place, the source of a packet will decide		Depending on the policy in place, the source of a packet will decide
	whether to use this specification to transport the RPL information in		whether to use this specification to transport the RPL information in
	the IPv6 packets. If it does, the source in the LLN SHOULD set the		the IPv6 packets. If it does, the source in the LLN SHOULD set the
	Flow Label to zero and MUST NOT expect that the flow label will be		Flow Label to zero and MUST NOT expect that the flow label will be
	conserved end-to-end".		conserved end-to-end".
	6. Security Considerations		7. Security Considerations
	The process of using the Flow Label as opposed to the RPL option does		Because the flow label is not protected by IPSec, it is expected that
	not appear to create any opening for new threat compared to		Layer-2 security is deployed in the LLN where is specification is
	[RFC6553].		applied. This is the actual best practice in LLNs, which serves in
			particular to avoid forwarding of untrusted packets over the
			constrained network.
	7. IANA Considerations		If the link layer is secured adequately, using the Flow Label as
			opposed to the RPL option does not create an opening for a new threat
			compared to [RFC6553].
			8. IANA Considerations
	No IANA action is required for this specification.		No IANA action is required for this specification.
	8. Acknowledgements		9. Acknowledgements
	The author wishes to thank Brian Carpenter for his in-depth review		The author wishes to thank Brian Carpenter for his in-depth review
	and constructive approach to the problem and its resolution.		and constructive approach to the problem resolution.
	9. References		10. References
	9.1. Normative References		10.1. Normative References
	[IEEE802154]		[IEEE802154]
	IEEE standard for Information Technology, "IEEE std.		IEEE standard for Information Technology, "IEEE std.
	802.15.4, Part. 15.4: Wireless Medium Access Control		802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
	(MAC) and Physical Layer (PHY) Specifications for Low-Rate		and Physical Layer (PHY) Specifications for Low-Rate
	Wireless Personal Area Networks", June 2011.		Wireless Personal Area Networks", June 2011.
	[ISA100.11a]		[ISA100.11a]
	ISA, "ISA100, Wireless Systems for Automation", May 2008,		ISA, "ISA100, Wireless Systems for Automation", May 2008,
	< http://www.isa.org/Community/		< http://www.isa.org/Community/
	SP100WirelessSystemsforAutomation>.		SP100WirelessSystemsforAutomation>.
	[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.
	[RFC2460] Deering, S.E. and R.M. Hinden, "Internet Protocol, Version		[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
	6 (IPv6) Specification", RFC 2460, December 1998.		(IPv6) Specification", RFC 2460, December 1998.
	[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6		[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
	Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,		Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
	September 2011.		September 2011.
	[RFC6437] Amante, S., Carpenter, B., Jiang, S. and J. Rajahalme,		[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
	"IPv6 Flow Label Specification", RFC 6437, November 2011.		"IPv6 Flow Label Specification", RFC 6437, November 2011.
	[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,		[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
	Levis, P., Pister, K., Struik, R., Vasseur, JP. and R.		Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
	Alexander, "RPL: IPv6 Routing Protocol for Low-Power and		Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
	Lossy Networks", RFC 6550, March 2012.		Lossy Networks", RFC 6550, March 2012.
	[RFC6552] Thubert, P., "Objective Function Zero for the Routing		[RFC6552] Thubert, P., "Objective Function Zero for the Routing
	Protocol for Low-Power and Lossy Networks (RPL)", RFC		Protocol for Low-Power and Lossy Networks (RPL)", RFC
	6552, March 2012.		6552, March 2012.
	[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-		[RFC6553] Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
	Power and Lossy Networks (RPL) Option for Carrying RPL		Power and Lossy Networks (RPL) Option for Carrying RPL
	Information in Data-Plane Datagrams", RFC 6553, March		Information in Data-Plane Datagrams", RFC 6553, March
	2012.		2012.
	9.2. Informative References		10.2. Informative References
	[I-D.ietf-6tisch-architecture]		[I-D.ietf-6tisch-architecture]
	Thubert, P., Watteyne, T. and R. Assimiti, "An		Thubert, P., Watteyne, T., and R. Assimiti, "An
	Architecture for IPv6 over the TSCH mode of IEEE		Architecture for IPv6 over the TSCH mode of IEEE
	802.15.4e", Internet-Draft draft-ietf-6tisch-		802.15.4e", draft-ietf-6tisch-architecture-01 (work in
	architecture-01, February 2014.		progress), February 2014.
	[I-D.ietf-6tisch-tsch]		[I-D.ietf-6tisch-tsch]
	Watteyne, T., Palattella, M. and L. Grieco, "Using		Watteyne, T., Palattella, M., and L. Grieco, "Using
	IEEE802.15.4e TSCH in an LLN context: Overview, Problem		IEEE802.15.4e TSCH in an LLN context: Overview, Problem
	Statement and Goals", Internet-Draft draft-ietf-6tisch-		Statement and Goals", draft-ietf-6tisch-tsch-00 (work in
	tsch-00, November 2013.		progress), November 2013.
	[I-D.thubert-6lo-forwarding-fragments]		[I-D.thubert-6lo-forwarding-fragments]
	Thubert, P. and J. Hui, "LLN Fragment Forwarding and		Thubert, P. and J. Hui, "LLN Fragment Forwarding and
	Recovery", Internet-Draft draft-thubert-6lo-forwarding-		Recovery", draft-thubert-6lo-forwarding-fragments-01 (work
	fragments-01, February 2014.		in progress), February 2014.
	[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler,		[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
	"Transmission of IPv6 Packets over IEEE 802.15.4		"Transmission of IPv6 Packets over IEEE 802.15.4
	Networks", RFC 4944, September 2007.		Networks", RFC 4944, September 2007.
	[RFC5673] Pister, K., Thubert, P., Dwars, S. and T. Phinney,		[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
	"Industrial Routing Requirements in Low-Power and Lossy		"Industrial Routing Requirements in Low-Power and Lossy
	Networks", RFC 5673, October 2009.		Networks", RFC 5673, October 2009.
	[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and		[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
	Lossy Networks", RFC 7102, January 2014.		Lossy Networks", RFC 7102, January 2014.
	Author's Address		Author's Address
	Pascal Thubert, editor
			Pascal Thubert (editor)
	Cisco Systems		Cisco Systems
	Village d'Entreprises Green Side		Village d'Entreprises Green Side
	400, Avenue de Roumanille		400, Avenue de Roumanille
	Batiment T3		Batiment T3
	Biot - Sophia Antipolis, 06410		Biot - Sophia Antipolis 06410
	FRANCE		FRANCE
	Phone: +33 4 97 23 26 34		Phone: +33 4 97 23 26 34
	Email: pthubert@cisco.com		Email: pthubert@cisco.com
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