draft-ietf-lpwan-coap-static-context-hc-13.txt		draft-ietf-lpwan-coap-static-context-hc-14.txt
	lpwan Working Group A. Minaburo		lpwan Working Group A. Minaburo
	Internet-Draft Acklio		Internet-Draft Acklio
	Intended status: Standards Track L. Toutain		Intended status: Standards Track L. Toutain
	Expires: September 6, 2020 Institut MINES TELECOM; IMT Atlantique		Expires: November 27, 2020 Institut MINES TELECOM; IMT Atlantique
	R. Andreasen		R. Andreasen
	Universidad de Buenos Aires		Universidad de Buenos Aires
	March 05, 2020		May 26, 2020
	LPWAN Static Context Header Compression (SCHC) for CoAP		LPWAN Static Context Header Compression (SCHC) for CoAP
	draft-ietf-lpwan-coap-static-context-hc-13		draft-ietf-lpwan-coap-static-context-hc-14
	Abstract		Abstract
	This draft defines the way SCHC (Static Context Header Compression)		This draft defines the way Static Context Header Compression (SCHC)
	header compression can be applied to the CoAP protocol. SCHC is a		header compression can be applied to the Constrained Application
	header compression mechanism adapted for constrained devices. SCHC		Protocol (CoAP). SCHC is a header compression mechanism adapted for
	uses a static description of the header to reduce the redundancy and		constrained devices. SCHC uses a static description of the header to
	the size of the information in the header. While		reduce the redundancy and the size of the information in the header.
	[I-D.ietf-lpwan-ipv6-static-context-hc] describes the SCHC		While [rfc8724] describes the SCHC compression and fragmentation
	compression and fragmentation framework, and its application for		framework, and its application for IPv6/UDP headers, this document
	IPv6/UDP headers, this document applies the use of SCHC for CoAP		applies the use of SCHC for CoAP headers. The CoAP header structure
	headers. The CoAP header structure differs from IPv6 and UDP since		differs from IPv6 and UDP since CoAP uses a flexible header with a
	CoAP uses a flexible header with a variable number of options,		variable number of options, themselves of variable length. The CoAP
	themselves of variable length. The CoAP protocol messages format is		protocol messages format is asymmetric: the request messages have a
	asymmetric: the request messages have a header format different from		header format different from the one in the response messages. This
	the one in the response messages. This specification gives guidance		specification gives guidance on how to apply SCHC to flexible headers
	on how to apply SCHC to flexible headers and how to leverage the		and how to leverage the asymmetry for more efficient compression
	asymmetry for more efficient compression Rules.		Rules.
	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 https://datatracker.ietf.org/drafts/current/.		Drafts is at https://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 September 6, 2020.		This Internet-Draft will expire on November 27, 2020.
	Copyright Notice		Copyright Notice
	Copyright (c) 2020 IETF Trust and the persons identified as the		Copyright (c) 2020 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4		1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
	2. Applying SCHC to CoAP . . . . . . . . . . . . . . . . . . . . 4		2. Applying SCHC to CoAP headers . . . . . . . . . . . . . . . . 4
	3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 5		3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 5
	3.1. Differences between CoAP and UDP/IP . . . . . . . . . . . 5		3.1. Differences between CoAP and UDP/IP Compression . . . . . 5
	4. Compression of CoAP header fields . . . . . . . . . . . . . . 6		4. Compression of CoAP header fields . . . . . . . . . . . . . . 6
	4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7		4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 7
	4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7		4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 7
	4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7		4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 7
	4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7		4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 7
	4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7		4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7
	5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8		5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 8
	5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8		5.1. CoAP Content and Accept options. . . . . . . . . . . . . 8
	5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 8		5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 8
	5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8		5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 9
	5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9		5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 9
	5.3.2. Variable number of path or query elements . . . . . . 10		5.3.2. Variable number of path or query elements . . . . . . 10
	5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme		5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
	fields . . . . . . . . . . . . . . . . . . . . . . . . . 10		fields . . . . . . . . . . . . . . . . . . . . . . . . . 10
	5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path		5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path
	and Location-Query fields . . . . . . . . . . . . . . . . 10		and Location-Query fields . . . . . . . . . . . . . . . . 10
	6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 10		6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 11
	6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 10		6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 11
	6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11		6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 11
	6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11		6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 11
	6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11		6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 11
	7. Examples of CoAP header compression . . . . . . . . . . . . . 12		7. Examples of CoAP header compression . . . . . . . . . . . . . 13
	7.1. Mandatory header with CON message . . . . . . . . . . . . 12		7.1. Mandatory header with CON message . . . . . . . . . . . . 13
	7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 13		7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14
	7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17		7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 17
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
	9. Security considerations . . . . . . . . . . . . . . . . . . . 27		9. Security considerations . . . . . . . . . . . . . . . . . . . 27
	10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28		10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
	11. Normative References . . . . . . . . . . . . . . . . . . . . 28		11. Normative References . . . . . . . . . . . . . . . . . . . . 28
			Appendix A. Extension to the RFC8724 Annex D. . . . . . . . . . 29
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
	1. Introduction		1. Introduction
	CoAP [rfc7252] is a transfer protocol that implements a subset of		CoAP [rfc7252] is designed to easily interop with HTTP (Hypertext
	HTTP (Hypertext Transfer Protocol) and is optimized for REST-based		Transfer Protocol) and is optimized for REST-based (Representational
	(Representational state transfer) services. Although CoAP was		state transfer) services. Although CoAP was designed for constrained
	designed for constrained devices, the size of a CoAP header still is		devices, the size of a CoAP header still is too large for the
	too large for the constraints of LPWAN (Low Power Wide Area Networks)		constraints of LPWAN (Low Power Wide Area Networks) and some
	and some compression is needed to reduce the header size.		compression is needed to reduce the header size.
	The [I-D.ietf-lpwan-ipv6-static-context-hc] defines SCHC, a header		The [rfc8724] defines SCHC, a header compression mechanism for LPWAN
	compression mechanism for LPWAN network based on a static context.		network based on a static context. The section 5 of the [rfc8724]
	The section 5 of the [I-D.ietf-lpwan-ipv6-static-context-hc] explains		explains the architecture where compression and decompression are
	the architecture where compression and decompression are done. The		done. The context is known by both ends before transmission. The
	context is known by both ends before transmission. The way the		way the context is configured, provisioned or exchanged is out of the
	context is configured or exchanged is out of the scope for this		scope of this document.
	document.
	SCHC compresses and decompresses headers based on shared contexts		SCHC compresses and decompresses headers based on shared contexts
	between devices. Each context consists of multiple Rules. Each rule		between devices. Each context consists of multiple Rules. Each Rule
	can match header fields and specific values or ranges of values. If		can match header fields and specific values or ranges of values. If
	a rule matches, the matched header fields are substituted by the rule		a Rule matches, the matched header fields are substituted by the
	ID and optionally some residual bits. Thus, different Rules may		RuleID and optionally some residual bits. Thus, different Rules may
	correspond to different types of packets that a device expects to		correspond to different types of packets that a device expects to
	send or receive.		send or receive.
	A Rule describes the complete header of the packet with an ordered		A Rule describes the complete header of the packet with an ordered
	list of fields descriptions, see section 7 of the		list of fields descriptions, see section 7 of [rfc8724], thereby each
	[I-D.ietf-lpwan-ipv6-static-context-hc], thereby each description		description contains the field ID (FID), its length (FL) and its
	contains the field ID (FID), its length (FL) and its position (FP), a		position (FP), a direction indicator (DI) (upstream, downstream and
	direction indicator (DI) (upstream, downstream and bidirectional) and		bidirectional) and some associated Target Values (TV).
	some associated Target Values (TV).
	A Matching Operator (MO) is associated to each header field		A Matching Operator (MO) is associated to each header field
	description. The rule is selected if all the MOs fit the TVs for all		description. The Rule is selected if all the MOs fit the TVs for all
	fields of the incoming packet.		fields of the incoming header.
	In that case, a Compression/Decompression Action (CDA) associated to		In that case, a Compression/Decompression Action (CDA) associated to
	each field defines how the compressed and the decompressed values are		each field defines how the compressed and the decompressed values are
	computed out of each other, for each of the header fields.		computed. Compression mainly results in one of 4 actions:
	Compression mainly results in one of 4 actions: * send the field
	value, * send nothing, * send some least significant bits of the
	field or * send an index. After applying the compression there may
	be some bits to be sent, these values are called Compression
	Residues.
	SCHC is a general concept mechanism that can be applied to different		o send the field value,
			o send nothing,
			o send some least significant bits of the field or
			o send an index.
			After applying the compression there may be some bits to be sent,
			these values are called Compression Residues.
			SCHC is a general mechanism that can be applied to different
	protocols, the exact Rules to be used depend on the protocol and the		protocols, the exact Rules to be used depend on the protocol and the
	application, and CoAP differs from UDP and IPv6, see Section 3.		application. The section 10 of the [rfc8724] describes the
			compression scheme for IPv6 and UDP headers. This document targets
			the CoAP header compression using SCHC.
	1.1. Terminology		1.1. 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", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in BCP		"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [rfc2119][rfc8174] when, and only when, they appear in all		14 [RFC2119][rfc8174] when, and only when, they appear in all
	capitals, as shown here.		capitals, as shown here.
	2. Applying SCHC to CoAP		2. Applying SCHC to CoAP headers
	The SCHC Compression rules can be applied to CoAP flows. SCHC		The SCHC Compression Rules can be applied to CoAP headers. SCHC
	Compression of the CoAP header MAY be done in conjunction with the		Compression of the CoAP header MAY be done in conjunction with the
	lower layers (IPv6/UDP) or independently. The SCHC adaptation layers		lower layers (IPv6/UDP) or independently. The SCHC adaptation layers
	as described in section 5 of [I-D.ietf-lpwan-ipv6-static-context-hc]		as described in Section 5 of [rfc8724] and may be used as shown in
	may be used as shown in Figure 1.		Figure 1.
	^ +------------+ ^ +------------+ ^ +------------+		^ +------------+ ^ +------------+ ^ +------------+
	| | CoAP | | | CoAP | inner | | CoAP |		| | CoAP | | | CoAP | inner | | CoAP |
	| +------------+ v +------------+ x | OSCORE |		| +------------+ v +------------+ x | OSCORE |
	| | UDP | | DTLS | outer | +------------+		| | UDP | | DTLS | outer | +------------+
	| +------------+ +------------+ | | UDP |		| +------------+ +------------+ | | UDP |
	| | IPv6 | | UDP | | +------------+		| | IPv6 | | UDP | | +------------+
	v +------------+ +------------+ | | IPv6 |		v +------------+ +------------+ | | IPv6 |
	| IPv6 | v +------------+		| IPv6 | v +------------+
	+------------+		+------------+
	Figure 1: rule scope for CoAP		Figure 1: Rule scope for CoAP
	Figure 1 shows some examples for CoAP architecture and the SCHC		Figure 1 shows some examples for CoAP protocol stacks and the SCHC
	rule's scope.		Rule's scope.
	In the first example, a rule compresses the complete header stack		In the first example, a Rule compresses the complete header stack
	from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header		from IPv6 to CoAP. In this case, SCHC C/D (Static Context Header
	Compression Compressor/Decompressor) is performed at the Sender and		Compression Compressor/Decompressor) is performed at the Sender and
	at the Receiver.		at the Receiver.
	In the second example, an end-to-end encryption mechanisms is used		In the second example, the SCHC compression is applied in the CoAP
	between the Sender and the Receiver. The SCHC compression is applied		layer, compressing the CoAP header independently of the other layers.
	in the CoAP layer compressing the CoAP header independently of the		The RuleID and the Compression Residue are encrypted using a
	other layers. The rule ID and the compression residue are encrypted		mechanism such as DTLS. Only the other end can decipher the
	using a mechanism such as DTLS. Only the other end can decipher the		information. If needed, layers below use SCHC to compress the header
	information. Layers below may also be compressed using other SCHC		as defined in [rfc8724] document. This use case realizes an End-to-
	rules (this is out of the scope of this document) as defined in the		End context initialization between the sender and the receiver, see
	SCHC [I-D.ietf-lpwan-ipv6-static-context-hc] document.		Appendix A.
	In the third example, OSCORE [rfc8613] is used. In this case, two		In the third example, the Object Security for Constrained RESTful
	rulesets are used to compress the CoAP message. A first ruleset		Environments (OSCORE) [rfc8613] is used. In this case, two rulesets
	focused on the inner header and is applied end to end by both ends.		are used to compress the CoAP message. A first ruleset focused on
	A second ruleset compresses the outer header and the layers below and		the inner header and is applied end to end by both ends. A second
	is done between the Sender and the Receiver.		ruleset compresses the outer header and the layers below and is done
			between the Sender and the Receiver.
	3. CoAP Compression with SCHC		3. CoAP Headers compressed with SCHC
	SCHC with CoAP will be used exactly the same way as it is applied in		The use of SCHC over the CoAP header uses the same description and
	any protocol as IP or UDP with the difference that the fields		compression/decompression techniques as the one for IP and UDP
	description needs to be defined based on both headers and target		explained in the [rfc8724]. For CoAP, SCHC Rules description uses
	values of the request and the responses. SCHC Rules description use		the direction information to optimize the compression by reducing the
	the direction information to optmize the compression by reducing the		number of Rules needed to compress headers. The field description
	number of Rules needed to compress traffic. CoAP compression follows		MAY define both request/response headers and target values in the
	the [I-D.ietf-lpwan-ipv6-static-context-hc] scheme and as for other		same Rule, using the DI (direction indicator) to make the difference.
	protocols, if no valid Rule was found, then the packet MUST be sent		As for other protocols, when the compressor does not find a correct
	uncompressed using the RuleID dedicated to this purpose and the		Rule to compress the header, the packet MUST be sent uncompressed
	Compression Residue is the complete header of the packet. See		using the RuleID dedicated to this purpose, and the Compression
	section 6 of [I-D.ietf-lpwan-ipv6-static-context-hc].		Residue is the complete header of the packet. See section 6 of
			[rfc8724].
	3.1. Differences between CoAP and UDP/IP		3.1. Differences between CoAP and UDP/IP Compression
	CoAP differs from IPv6 and UDP protocols on the following aspects:		CoAP compression differs from IPv6 and UDP compression on the
			following aspects:
	o IPv6 and UDP are not request and response protocols as CoAP, and		o The CoAP protocol is asymmetric, the headers are different for a
	so the same header fields are used in all packets for all		request or a response. For example, the URI-path option is
	directions, with the value of some fields being swapped on the		mandatory in the request, and it is not present in the response, a
	return path (e.g. source and destination addresses fields). The		request may contain an Accept option, and the response may include
	CoAP headers instead are asymmetric, the headers are different for		a Content option. In comparison, IPv6 and UDP returning path swap
	a request or a response. For example, the URI-path option is		the value of some fields in the header.
	mandatory in the request and is not found in the response, a		But all the directions have the same fields (e.g., source and
	request may contain an Accept option and the response may contain		destination addresses fields).
	a Content option.
	The [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a		The [rfc8724] defines the use of a Direction Indicator (DI) in the
	Direction Indicator (DI) in the Field Description, which allows a		Field Description, which allows a single Rule to process message
	single Rule to process message headers differently depending on		headers differently depending on the direction.
	the direction.
	o Even when a field is "symmetric" (i.e. found in both directions)		o Even when a field is "symmetric" (i.e., found in both directions),
	the values carried in each direction are different. To performs		the values carried in each direction are different.
	the compression a matching list in the TV might be use because		The compression may use a matching list in the TV to limit the
	this allows reducing the range of expected values in a particular		range of expected values in a particular direction and therefore
	direction and therefore reduces the size of the		reduces the size of the Compression Residue. Through the
	compression residue. For instance, if a client sends only CON		Direction Indicator (DI), a field description in the Rules splits
	requests, the type can be elided by compression and the answer may		the possible field value into two parts, one for each direction.
	use one single bit to carry either the ACK or RST type. In CoAP		For instance, if a client sends only CON requests, the type can be
	some fields have the same behavior, for example the field Code can		elided by compression, and the answer may use one single bit to
	have 0.0X code format value in the request and Y.ZZ code format in		carry either the ACK or RST type. The field Code have as well the
	the response. Through the direction indicator, a field		same behavior, the 0.0X code format value in the request and Y.ZZ
	description in the Rules splits the possible field value in two		code format in the response.
	parts. Resulting in a smaller compression residue.
	o In IPv6 and UDP, header fields have a fixed size, defined in the		o Headers in IPv6 and UDP have a fixed size. The size is not sent
	Rule, which is not sent. In CoAP, some fields in the header have		as part of the Compression Residue, but is defined in the Rule.
	a variable length, for example the Token size may vary from 0 to 8		Some CoAP header fields have variable lengths, so the length is
	bytes, the length is given by a field in the header. The CoAP		also specified in the Field Description. For example, the Token
	options are described using the Type-Length-Value encoding format.		size may vary from 0 to 8 bytes. And the CoAP options have a
			variable length since they use the Type-Length-Value encoding
			format, as URI-path or URI-query.
	Section 7.5.2 from [I-D.ietf-lpwan-ipv6-static-context-hc] offers		Section 7.5.2 from [rfc8724] offers the possibility to define a
	the possibility to define a function for the Field Length in the		function for the Field length in the Field Description to have
	Field Description to have knwoledge of the length before		knowledge of the length before compression. When doing SCHC
	compression. When doing SCHC compression of a variable length		compression of a variable-length field,
	field two cases may be raised after applying the CDA: * The result		if the field size is not known, the Field Length in the Rule is
	of the compression is of fixed length and the compressed value is		set as variable and the size is sent with the Compression Residue.
	sent in the residue. * Or the result of the compression is of
	variable-length and in this case, the size is sent with the
	compressed value in the residue.
	o In CoAP headers, a field can appear several times. This is		o A field can appear several time in the CoAP headers. This is
	typical for elements of a URI (path or queries). The SCHC		typical for elements of a URI (path or queries). The SCHC
	specification [I-D.ietf-lpwan-ipv6-static-context-hc] allows a		specification [rfc8724] allows a Field ID to appear several times
	Field ID to appears several times in the rule, and uses the Field		in the Rule, and uses the Field Position (FP) to identify the
	Position (FP) to identify the correct instance, and thereby		correct instance, and thereby removing the ambiguity of the
	removing the ambiguity of the matching operation.		matching operation.
	o Field sizes defined in the CoAP protocol can be too large		o Field sizes defined in the CoAP protocol can be too large
	regarding LPWAN traffic constraints. This is particularly true		regarding LPWAN traffic constraints. This is particularly true
	for the Message ID field and the Token field. SCHC uses different		for the Message ID field and the Token field. SCHC uses different
	Matching operators (MO) to performs the compression, see section		Matching operators (MO) to perform the compression, see section
	7.4 of [I-D.ietf-lpwan-ipv6-static-context-hc]. In this case the		7.4 of [rfc8724]. In this case the Most Significant Bits (MSB) MO
	Most Significant Bits (MSB) MO can be applied to reduce the		can be applied to reduce the information carried on LPWANs.
	information carried on LP
	4. Compression of CoAP header fields		4. Compression of CoAP header fields
	This section discusses the compression of the different CoAP header		This section discusses the compression of the different CoAP header
	fields. The CoAP compression with SCHC follows the Section 7.1 of		fields. The CoAP compression with SCHC follows the Section 7.1 of
	[I-D.ietf-lpwan-ipv6-static-context-hc].		[rfc8724].
	4.1. CoAP version field		4.1. CoAP version field
	CoAP version is bidirectional and MUST be elided during the SCHC		CoAP version is bidirectional and MUST be elided during the SCHC
	compression, since it always contains the same value. In the future,		compression, since it always contains the same value. In the future,
	if new versions of CoAP are defined, new rules will be needed to		if new versions of CoAP are defined, new Rules will be needed to
	avoid ambiguities between versions.		avoid ambiguities between versions.
	4.2. CoAP type field		4.2. CoAP type field
	The CoAP Protocol [rfc7252] has four type of messages: two request		The CoAP Protocol [rfc7252] has four type of messages: two request
	(CON, NON); one response (ACK) and one empty message (RST).		(CON, NON); one response (ACK) and one empty message (RST).
	The field SHOULD be elided if for instance a client is sending only		The field SHOULD be elided if for instance a client is sending only
	NON or only CON messages. For the RST message a dedicated Rule may		NON or only CON messages. For the RST message a dedicated Rule may
	be needed. For other usages a mapping list can be used.		be needed. For other usages a mapping list can be used.
	4.3. CoAP code field		4.3. CoAP code field
	The code field indicates the Request Method used in CoAP, a registry		The code field indicates the Request Method used in CoAP, a IANA
	is given in section 12.1 of [rfc7252]. The compression of the CoAP		registry [rfc7252]. The compression of the CoAP code field follows
	code field follows the same principle as that of the CoAP type field.		the same principle as that of the CoAP type field. If the device
	If the device plays a specific role, the set of code values can be		plays a specific role, the set of code values can be split in two
	split in two parts, the request codes with the 0 class and the		parts, the request codes with the 0 class and the response values.
	response values.
	If the device only implements a CoAP client, the request code can be		If the device only implements a CoAP client, the request code can be
	reduced to the set of requests the client is able to process.		reduced to the set of requests the client is able to process.
	A mapping list can be used for known values, for other values the		A mapping list can be used for known values. For other values the
	field cannot be compressed an the value needs to be sent in the		field cannot be compressed an the value needs to be sent in the
	residue.		Compression Residue.
	4.4. CoAP Message ID field		4.4. CoAP Message ID field
	The Message ID field can be compressed with the MSB(x) MO and the		The Message ID field can be compressed with the MSB(x) MO and the
	Least Significant Bits (LSB) CDA, see section 7.4 of		Least Significant Bits (LSB) CDA, see section 7.4 of [rfc8724].
	[I-D.ietf-lpwan-ipv6-static-context-hc].
	4.5. CoAP Token fields		4.5. CoAP Token fields
	Token is defined through two CoAP fields, Token Length in the		Token is defined through two CoAP fields, Token Length in the
	mandatory header and Token Value directly following the mandatory		mandatory header and Token Value directly following the mandatory
	CoAP header.		CoAP header.
	Token Length is processed as any protocol field. If the value does		Token Length is processed as any protocol field. If the value does
	not change, the size can be stored in the TV and elided during the		not change, the size can be stored in the TV and elided during the
	transmission. Otherwise, it will have to be sent in the compression		transmission. Otherwise, it will have to be sent in the Compression
	residue.		Residue.
	Token Value MUST not be sent as a variable length residue to avoid		Token Value MUST NOT be sent as a variable length residue to avoid
	ambiguity with Token Length. Therefore, Token Length value MUST be		ambiguity with Token Length. Therefore, Token Length value MUST be
	used to define the size of the residue. A specific function		used to define the size of the Compression Residue. A specific
	designated as "TKL" MUST be used in the Rule. During the		function designated as "TKL" MUST be used in the Rule. During the
	decompression, this function returns the value contained in the Token		decompression, this function returns the value contained in the Token
	Length field.		Length field.
	5. CoAP options		5. CoAP options
			CoAP defines options that are placed after the based header in Option
			Numbers order, see [rfc7252]. Each Option instance in a message uses
			the format Delta-Type (D-T), Length (L), Value (V). When applying
			SCHC compression to the Option, the D-T, L, and V format serves to
			make the Rule description of the Option. The SCHC compression builds
			the description of the Option by using in the Field ID the Option
			Number built from D-T; in TV, the Option Value; and the Option Length
			uses section 7.4 of RFC8724. When the Option Length has a wellknown
			size it can be stored in the Rule. Therefore, SCHC compression does
			not send it. Otherwise, SCHC Compression carries the length of the
			Compression Residue in addition to the Compression Residue value.
			Note that length coding differs between CoAP options and SCHC
			variable size Compression Residue.
			The following sections present how SCHC compresses some specific CoAP
			Options.
	5.1. CoAP Content and Accept options.		5.1. CoAP Content and Accept options.
	These fields are both unidirectional and MUST NOT be set to		These fields are both unidirectional and MUST NOT be set to
	bidirectional in a rule entry.		bidirectional in a Rule entry.
	If a single value is expected by the client, it can be stored in the		If a single value is expected by the client, it can be stored in the
	TV and elided during the transmission. Otherwise, if several		TV and elided during the transmission. Otherwise, if several
	possible values are expected by the client, a matching-list SHOULD be		possible values are expected by the client, a matching-list SHOULD be
	used to limit the size of the residue. Otherwise, the value has to		used to limit the size of the Compression Residue. Otherwise, the
	be sent as a residue (fixed or variable length).		value has to be sent as a Compression Residue (fixed or variable
			length).
	5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields		5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields
	These fields are unidirectional and MUST NOT be set to bidirectional		These fields are unidirectional and MUST NOT be set to bidirectional
	in a rule DI entry. see section 7.1 of		in a Rule DI entry, see section 7.1 of [rfc8724]. They are used only
	[I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the		by the server to inform of the caching duration and is never found in
	server to inform of the caching duration and is never found in client		client requests.
	requests.
	If the duration is known by both ends, the value can be elided on the		If the duration is known by both ends, the value can be elided.
	LPWAN.
	A matching list can be used if some well-known values are defined.		A matching list can be used if some well-known values are defined.
	Otherwise these options can be sent as a residue (fixed or variable		Otherwise these options can be sent as a Compression Residue (fixed
	length).		or variable length).
	5.3. CoAP option Uri-Path and Uri-Query fields		5.3. CoAP option Uri-Path and Uri-Query fields
	These fields are unidirectional and MUST NOT be set to bidirectional		These fields are unidirectional and MUST NOT be set to bidirectional
	in a rule entry. They are used only by the client to access a		in a Rule entry. They are used only by the client to access a
	specific resource and are never found in server responses.		specific resource and are never found in server responses.
	Uri-Path and Uri-Query elements are a repeatable options, the Field		Uri-Path and Uri-Query elements are a repeatable options, the Field
	Position (FP) gives the position in the path.		Position (FP) gives the position in the path.
	A Mapping list can be used to reduce the size of variable Paths or		A Mapping list can be used to reduce the size of variable Paths or
	Queries. In that case, to optimize the compression, several elements		Queries. In that case, to optimize the compression, several elements
	can be regrouped into a single entry. Numbering of elements do not		can be regrouped into a single entry. Numbering of elements do not
	change, MO comparison is set with the first element of the matching.		change, MO comparison is set with the first element of the matching.
	+-------------+--+--+--+--------+---------+-------------+		+-------------+---+--+--+--------+---------+-------------+
	| Field |FL|FP|DI| Target | Match | CDA |		| Field |FL |FP|DI| Target | Match | CDA |
	| | | | | Value | Opera. | |		| | | | | Value | Opera. | |
	+-------------+--+--+--+--------+---------+-------------+		+-------------+---+--+--+--------+---------+-------------+
	|URI-Path | | 1|up|["/a/b",|equal |not-sent |		|URI-Path | | 1|up|["/a/b",|equal |not-sent |
	| | | | |"/c/d"] | | |		| | | | |"/c/d"] | | |
	|URI-Path | | 3|up| |ignore |value-sent |		|URI-Path |var| 3|up| |ignore |value-sent |
	+-------------+--+--+--+--------+---------+-------------+		+-------------+---+--+--+--------+---------+-------------+
	Figure 2: complex path example		Figure 2: complex path example
	In Figure 2 a single bit residue can be used to code one of the 2		In Figure 2 a single bit residue can be used to code one of the 2
	paths. If regrouping were not allowed, a 2 bits residue would be		paths. If regrouping were not allowed, a 2 bits residue would be
	needed.		needed. The third path element is sent as a variable size residue.
	5.3.1. Variable length Uri-Path and Uri-Query		5.3.1. Variable length Uri-Path and Uri-Query
	When the length is not known at the rule creation, the Field Length		When the length is not known at the Rule creation, the Field Length
	MUST be set to variable, and the unit is set to bytes.		MUST be set to variable, and the unit is set to bytes.
	The MSB MO can be applied to a Uri-Path or Uri-Query element. Since		The MSB MO can be applied to a Uri-Path or Uri-Query element. Since
	MSB value is given in bit, the size MUST always be a multiple of 8		MSB value is given in bit, the size MUST always be a multiple of 8
	bits.		bits.
	The length sent at the beginning of a variable length residue		The length sent at the beginning of a variable length residue
	indicates the size of the LSB in bytes.		indicates the size of the LSB in bytes.
	For instance for a CORECONF path /c/X6?k="eth0" the rule can be set		For instance for a CORECONF path /c/X6?k="eth0" the Rule can be set
	to:		to:
	+-------------+---+--+--+--------+---------+-------------+		+-------------+---+--+--+--------+---------+-------------+
	| Field |FL |FP|DI| Target | Match | CDA |		| Field |FL |FP|DI| Target | Match | CDA |
	| | | | | Value | Opera. | |		| | | | | Value | Opera. | |
	+-------------+---+--+--+--------+---------+-------------+		+-------------+---+--+--+--------+---------+-------------+
	|URI-Path | 8| 1|up|"c" |equal |not-sent |		|URI-Path | 8| 1|up|"c" |equal |not-sent |
	|URI-Path |var| 2|up| |ignore |value-sent |		|URI-Path |var| 2|up| |ignore |value-sent |
	|URI-Query |var| 1|up|"k=" |MSB(16) |LSB |		|URI-Query |var| 1|up|"k=" |MSB(16) |LSB |
	+-------------+---+--+--+--------+---------+-------------+		+-------------+---+--+--+--------+---------+-------------+
	Figure 3: CORECONF URI compression		Figure 3: CORECONF URI compression
	Figure 3 shows the parsing and the compression of the URI, where c is		Figure 3 shows the parsing and the compression of the URI, where c is
	not sent. The second element is sent with the length (i.e. 0x2 X 6)		not sent. The second element is sent with the length (i.e. 0x2 X 6)
	followed by the query option (i.e. 0x05 "eth0").		followed by the query option (i.e. 0x05 "eth0").
	5.3.2. Variable number of path or query elements		5.3.2. Variable number of path or query elements
	The number of Uri-path or Uri-Query elements in a rule is fixed at		The number of Uri-path or Uri-Query elements in a Rule is fixed at
	the rule creation time. If the number varies, several rules SHOULD		the Rule creation time. If the number varies, several Rules SHOULD
	be created to cover all the possibilities. Another possibility is to		be created to cover all the possibilities. Another possibility is to
	define the length of Uri-Path to variable and send a compression		define the length of Uri-Path to variable and send a Compression
	residue with a length of 0 to indicate that this Uri-Path is empty.		Residue with a length of 0 to indicate that this Uri-Path is empty.
	This adds the 4 bits of the variable residue size. See section 7.5.2		This adds 4 bits to the variable Residue size. See section 7.5.2
	[I-D.ietf-lpwan-ipv6-static-context-hc]		[rfc8724]
	5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields		5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields
	These fields are unidirectional and MUST NOT be set to bidirectional		These fields are unidirectional and MUST NOT be set to bidirectional
	in a rule DI entry, see section 7.1 of the		in a Rule DI entry, see section 7.1 of the [rfc8724]. They are used
	[I-D.ietf-lpwan-ipv6-static-context-hc]. They are used only by the		only by the client to access a specific resource and are never found
	client to access a specific resource and are never found in server		in server response.
	response.
	If the field value has to be sent, TV is not set, MO is set to		If the field value has to be sent, TV is not set, MO is set to
	"ignore" and CDA is set to "value-sent". A mapping MAY also be used.		"ignore" and CDA is set to "value-sent". A mapping MAY also be used.
	Otherwise, the TV is set to the value, MO is set to "equal" and CDA		Otherwise, the TV is set to the value, MO is set to "equal" and CDA
	is set to "not-sent".		is set to "not-sent".
	5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and		5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and
	Location-Query fields		Location-Query fields
	These fields are unidirectional.		These fields are unidirectional.
	These fields values cannot be stored in a rule entry. They MUST		These fields values cannot be stored in a Rule entry. They MUST
	always be sent with the compression residues.		always be sent with the Compression Residues.
	6. SCHC compression of CoAP extension RFCs		6. SCHC compression of CoAP extension RFCs
	6.1. Block		6.1. Block
	Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also		Block [rfc7959] allows a fragmentation at the CoAP level. SCHC also
	includes a fragmentation protocol. They are compatible. If a block		includes a fragmentation protocol. They can be both used. If a
	option is used, its content MUST be sent as a compression residue.		block option is used, its content MUST be sent as a Compression
			Residue.
	6.2. Observe		6.2. Observe
	The [rfc7641] defines the Observe option. The TV is not set, MO is		The [rfc7641] defines the Observe option. The TV is not set, MO is
	set to "ignore" and the CDA is set to "value-sent". SCHC does not		set to "ignore" and the CDA is set to "value-sent". SCHC does not
	limit the maximum size for this option (3 bytes). To reduce the		limit the maximum size for this option (3 bytes). To reduce the
	transmission size, either the device implementation MAY limit the		transmission size, either the device implementation MAY limit the
	delta between two consecutive values, or a proxy can modify the		delta between two consecutive values, or a proxy can modify the
	increment.		increment.
	Since an RST message may be sent to inform a server that the client		Since an RST message may be sent to inform a server that the client
	does not require Observe response, a rule MUST allow the transmission		does not require Observe response, a Rule MUST allow the transmission
	of this message.		of this message.
	6.3. No-Response		6.3. No-Response
	The [rfc7967] defines a No-Response option limiting the responses		The [rfc7967] defines a No-Response option limiting the responses
	made by a server to a request. If the value is known by both ends,		made by a server to a request. If the value is known by both ends,
	then TV is set to this value, MO is set to "equal" and CDA is set to		then TV is set to this value, MO is set to "equal" and CDA is set to
	"not-sent".		"not-sent".
	Otherwise, if the value is changing over time, TV is not set, MO is		Otherwise, if the value is changing over time, TV is not set, MO is
	set to "ignore" and CDA to "value-sent". A matching list can also be		set to "ignore" and CDA to "value-sent". A matching list can also be
	used to reduce the size.		used to reduce the size.
	6.4. OSCORE		6.4. OSCORE
	OSCORE [rfc8613] defines end-to-end protection for CoAP messages.		OSCORE [rfc8613] defines end-to-end protection for CoAP messages.
	This section describes how SCHC rules can be applied to compress		This section describes how SCHC Rules can be applied to compress
	OSCORE-protected messages.		OSCORE-protected messages.
	0 1 2 3 4 5 6 7 <--------- n bytes ------------->		0 1 2 3 4 5 6 7 <--------- n bytes ------------->
	+-+-+-+-+-+-+-+-+---------------------------------		+-+-+-+-+-+-+-+-+---------------------------------
	|0 0 0|h|k| n | Partial IV (if any) ...		|0 0 0|h|k| n | Partial IV (if any) ...
	+-+-+-+-+-+-+-+-+---------------------------------		+-+-+-+-+-+-+-+-+---------------------------------
	| | |		| | |
	|<-- CoAP -->|<------ CoAP OSCORE_piv ------> |		|<-- CoAP -->|<------ CoAP OSCORE_piv ------> |
	OSCORE_flags		OSCORE_flags
	skipping to change at page 12, line 8 ¶		skipping to change at page 12, line 25 ¶
	| s (if any) | kid context (if any) | kid (if any) ... |		| s (if any) | kid context (if any) | kid (if any) ... |
	+------------+----------------------+-----------------------+		+------------+----------------------+-----------------------+
	| | |		| | |
	| <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->|		| <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->|
	Figure 4: OSCORE Option		Figure 4: OSCORE Option
	The encoding of the OSCORE Option Value defined in Section 6.1 of		The encoding of the OSCORE Option Value defined in Section 6.1 of
	[rfc8613] is repeated in Figure 4.		[rfc8613] is repeated in Figure 4.
	The first byte is used for flags that specify the contents of the		The first byte specifies the content of the OSCORE options using
	OSCORE option. The 3 most significant bits of this byte are reserved		flags. The three most significant bits of this byte are reserved and
	and always set to 0. Bit h, when set, indicates the presence of the		always set to 0. Bit h, when set, indicates the presence of the kid
	kid context field in the option. Bit k, when set, indicates the		context field in the option. Bit k, when set, indicates the presence
	presence of a kid field. The 3 least significant bits n indicate the		of a kid field. The three least significant bits n indicate the
	length of the piv (Partial Initialization Vector) field in bytes.		length of the piv (Partial Initialization Vector) field in bytes.
	When n = 0, no piv is present.		When n = 0, no piv is present.
	The flag byte is followed by the piv field, kid context field and kid		The flag byte is followed by the piv field, kid context field, and
	field in this order and if present; the length of the kid context		kid field in this order, and if present, the length of the kid
	field is encoded in the first byte denoting by s the length of the		context field is encoded in the first byte denoting by s the length
	kid context in bytes.		of the kid context in bytes.
	This specification recommends to identify the OSCORE Option and the
	fields it contains.
	Conceptually, it discerns up to 4 distinct pieces of information		This specification recommends identifying the OSCORE Option and the
	within the OSCORE option: the flag bits, the piv, the kid context,		fields it contains Conceptually, it discerns up to 4 distinct pieces
	and the kid. It is thus recommended that the parser split the OSCORE		of information within the OSCORE option: the flag bits, the piv, the
	option into the 4 subsequent fields:		kid context, and the kid. The SCHC Rule splits into four field
			descriptions the OSCORE option to compress them:
	o CoAP OSCORE_flags,		o CoAP OSCORE_flags,
	o CoAP OSCORE_piv,		o CoAP OSCORE_piv,
	o CoAP OSCORE_kidctxt,		o CoAP OSCORE_kidctxt,
	o CoAP OSCORE_kid.		o CoAP OSCORE_kid.
	These fields are shown superimposed on the OSCORE Option format in		The OSCORE Option shows superimposed these four fields using the
	Figure 4, the CoAP OSCORE_kidctxt field including the size bits s.		format Figure 4, the CoAP OSCORE_kidctxt field includes the size bits
	Their size SHOULD be reduced using SCHC compression.		s.
	7. Examples of CoAP header compression		7. Examples of CoAP header compression
	7.1. Mandatory header with CON message		7.1. Mandatory header with CON message
	In this first scenario, the LPWAN compressor at the Network Gateway		In this first scenario, the LPWAN Compressor at the Network Gateway
	side receives from an Internet client a POST message, which is		side receives from an Internet client a POST message, which is
	immediately acknowledged by the Device. For this simple scenario,		immediately acknowledged by the Device. For this simple scenario,
	the rules are described Figure 5.		the Rules are described Figure 5.
	Rule ID 1		RuleID 1
	+-------------+--+--+--+------+---------+-------------++------------+		+-------------+--+--+--+------+---------+-------------++------------+
	| Field |FL|FP|DI|Target| Match | CDA || Sent |		| Field |FL|FP|DI|Target| Match | CDA || Sent |
	| | | | |Value | Opera. | || [bits] |		| | | | |Value | Opera. | || [bits] |
	+-------------+--+--+--+------+---------+-------------++------------+		+-------------+--+--+--+------+---------+-------------++------------+
	|CoAP version | | |bi| 01 |equal |not-sent || |		|CoAP version | | |bi| 01 |equal |not-sent || |
	|CoAP Type | | |dw| CON |equal |not-sent || |		|CoAP Type | | |dw| CON |equal |not-sent || |
	|CoAP Type | | |up|[ACK, | | || |		|CoAP Type | | |up|[ACK, | | || |
	| | | | | RST] |match-map|matching-sent|| T |		| | | | | RST] |match-map|matching-sent|| T |
	|CoAP TKL | | |bi| 0 |equal |not-sent || |		|CoAP TKL | | |bi| 0 |equal |not-sent || |
	|CoAP Code | | |bi|[0.00,| | || |		|CoAP Code | | |bi|[0.00,| | || |
	skipping to change at page 13, line 35 ¶		skipping to change at page 13, line 48 ¶
	response codes defined in [rfc7252] has been shrunk to 5 bits using a		response codes defined in [rfc7252] has been shrunk to 5 bits using a
	matching list. Uri-Path contains a single element indicated in the		matching list. Uri-Path contains a single element indicated in the
	matching operator.		matching operator.
	SCHC Compression reduces the header sending only the Type, a mapped		SCHC Compression reduces the header sending only the Type, a mapped
	code and the least significant bits of Message ID (9 bits in the		code and the least significant bits of Message ID (9 bits in the
	example above).		example above).
	Note that a request sent by a client located in an Application Server		Note that a request sent by a client located in an Application Server
	to a server located in the device, may not be compressed through this		to a server located in the device, may not be compressed through this
	rule since the MID will not start with 7 bits equal to 0. A CoAP		Rule since the MID will not start with 7 bits equal to 0. A CoAP
	proxy, before the core SCHC C/D can rewrite the message ID to a value		proxy, before the core SCHC C/D can rewrite the message ID to a value
	matched by the rule.		matched by the Rule.
	7.2. OSCORE Compression		7.2. OSCORE Compression
	OSCORE aims to solve the problem of end-to-end encryption for CoAP		OSCORE aims to solve the problem of end-to-end encryption for CoAP
	messages. The goal, therefore, is to hide as much of the message as		messages. The goal, therefore, is to hide as much of the message as
	possible while still enabling proxy operation.		possible while still enabling proxy operation.
	Conceptually this is achieved by splitting the CoAP message into an		Conceptually this is achieved by splitting the CoAP message into an
	Inner Plaintext and Outer OSCORE Message. The Inner Plaintext		Inner Plaintext and Outer OSCORE Message. The Inner Plaintext
	contains sensitive information which is not necessary for proxy		contains sensitive information which is not necessary for proxy
	skipping to change at page 15, line 44 ¶		skipping to change at page 15, line 44 ¶
	+------+-------------------+ | Payload |		+------+-------------------+ | Payload |
	| 0xFF | | |		| 0xFF | | |
	+------+ +-------------------+		+------+ +-------------------+
	Figure 6: A CoAP message is split into an OSCORE outer and plaintext		Figure 6: A CoAP message is split into an OSCORE outer and plaintext
	Figure 6 shows the message format for the OSCORE Message and		Figure 6 shows the message format for the OSCORE Message and
	Plaintext.		Plaintext.
	In the Outer Header, the original message code is hidden and replaced		In the Outer Header, the original message code is hidden and replaced
	by a default dummy value. As seen in sections 4.1.3.5 and 4.2 of the		by a default dummy value. As seen in Sections 4.1.3.5 and 4.2 of the
	[rfc8613], the message code is replaced by POST for requests and		[rfc8613], the message code is replaced by POST for requests and
	Changed for responses when Observe is not used. If Observe is used,		Changed for responses when Observe is not used. If Observe is used,
	the message code is replaced by FETCH for requests and Content for		the message code is replaced by FETCH for requests and Content for
	responses.		responses.
	The original message code is put into the first byte of the		The original message code is put into the first byte of the
	Plaintext. Following the message code, the class E options comes and		Plaintext. Following the message code, the class E options comes and
	if present the original message Payload is preceded by its payload		if present the original message Payload is preceded by its payload
	marker.		marker.
	skipping to change at page 16, line 47 ¶		skipping to change at page 16, line 47 ¶
	+----------------------------------+		+----------------------------------+
	Figure 7: OSCORE message		Figure 7: OSCORE message
	The SCHC Compression scheme consists of compressing both the		The SCHC Compression scheme consists of compressing both the
	Plaintext before encryption and the resulting OSCORE message after		Plaintext before encryption and the resulting OSCORE message after
	encryption, see Figure 8.		encryption, see Figure 8.
	This translates into a segmented process where SCHC compression is		This translates into a segmented process where SCHC compression is
	applied independently in 2 stages, each with its corresponding set of		applied independently in 2 stages, each with its corresponding set of
	rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way		Rules, with the Inner SCHC Rules and the Outer SCHC Rules. This way
	compression is applied to all fields of the original CoAP message.		compression is applied to all fields of the original CoAP message.
	Note that since the Inner part of the message can only be decrypted		Note that since the Inner part of the message can only be decrypted
	by the corresponding end-point, this end-point will also have to		by the corresponding end-point, this end-point will also have to
	implement Inner SCHC Compression/Decompression.		implement Inner SCHC Compression/Decompression.
	Outer Message OSCORE Plaintext		Outer Message OSCORE Plaintext
	+-+-+---+--------+---------------+ +-------+		+-+-+---+--------+---------------+ +-------+
	|v|t|tkl|new code| Msg Id. | | code |		|v|t|tkl|new code| Msg Id. | | code |
	+-+-+---+--------+---------------+....+ +-------+-----......+		+-+-+---+--------+---------------+....+ +-------+-----......+
	skipping to change at page 17, line 30 ¶		skipping to change at page 17, line 30 ¶
	+------+------------+ +-------------------+		+------+------------+ +-------------------+
	| Ciphertext |<---------\ |		| Ciphertext |<---------\ |
	| | | v		| | | v
	+-------------------+ | +-----------------+		+-------------------+ | +-----------------+
	| | | Inner SCHC |		| | | Inner SCHC |
	v | | Compression |		v | | Compression |
	+-----------------+ | +-----------------+		+-----------------+ | +-----------------+
	| Outer SCHC | | |		| Outer SCHC | | |
	| Compression | | v		| Compression | | v
	+-----------------+ | +-------+		+-----------------+ | +-------+
	| | |Rule ID|		| | |RuleID |
	v | +-------+--+		v | +-------+--+
	+--------+ +------------+ | Residue |		+--------+ +------------+ | Residue |
	|Rule ID'| | Encryption | <--- +----------+--------+		|RuleID' | | Encryption | <--- +----------+--------+
	+--------+--+ +------------+ | |		+--------+--+ +------------+ | |
	| Residue' | | Payload |		| Residue' | | Payload |
	+-----------+-------+ | |		+-----------+-------+ | |
	| Ciphertext | +-------------------+		| Ciphertext | +-------------------+
	| |		| |
	+-------------------+		+-------------------+
	Figure 8: OSCORE Compression Diagram		Figure 8: OSCORE Compression Diagram
	7.3. Example OSCORE Compression		7.3. Example OSCORE Compression
	An example is given with a GET Request and its consequent CONTENT		An example is given with a GET Request and its consequent Content
	Response from a device-based CoAP client to a cloud-based CoAP		Response from a device-based CoAP client to a cloud-based CoAP
	server. A possible set of rules for the Inner and Outer SCHC		server. A possible set of Rules for the Inner and Outer SCHC
	Compression is shown. A dump of the results and a contrast between		Compression is shown. A dump of the results and a contrast between
	SCHC + OSCORE performance with SCHC + COAP performance is also		SCHC + OSCORE performance with SCHC + COAP performance is also
	listed. This gives an approximation to the cost of security with		listed. This gives an approximation to the cost of security with
	SCHC-OSCORE.		SCHC-OSCORE.
	Our first example CoAP message is the GET Request in Figure 9		Our first example CoAP message is the GET Request in Figure 9
	Original message:		Original message:
	=================		=================
	0x4101000182bb74656d7065726174757265		0x4101000182bb74656d7065726174757265
	skipping to change at page 19, line 5 ¶		skipping to change at page 19, line 5 ¶
	0x82 = token		0x82 = token
	0xFF Payload marker		0xFF Payload marker
	Payload:		Payload:
	0x32332043		0x32332043
	Original msg length: 10		Original msg length: 10
	Figure 10: CoAP CONTENT Response		Figure 10: CoAP CONTENT Response
	TheSCHC Rules for the Inner Compression include all fields that are		The SCHC Rules for the Inner Compression include all fields that are
	alreadypresent in a regular CoAP message. The methods described in		already present in a regular CoAP message. The methods described in
	section Section 4 applies to these fields. As an example, see		Section 4 applies to these fields. As an example, see Figure 11.
	Figure 11.
	Rule ID 0		RuleID 0
	+---------------+--+--+-----------+-----------+-----------++------+		+---------------+--+--+-----------+-----------+-----------++------+
	| Field |FP|DI| Target | MO | CDA || Sent |		| Field |FP|DI| Target | MO | CDA || Sent |
	| | | | Value | | ||[bits]|		| | | | Value | | ||[bits]|
	+---------------+--+--+-----------+-----------+-----------++------+		+---------------+--+--+-----------+-----------+-----------++------+
	|CoAP Code | |up| 1 | equal |not-sent || |		|CoAP Code | |up| 1 | equal |not-sent || |
	|CoAP Code | |dw|[69,132] | match-map |match-sent || c |		|CoAP Code | |dw|[69,132] | match-map |match-sent || c |
	|CoAP Uri-Path | |up|temperature| equal |not-sent || |		|CoAP Uri-Path | |up|temperature| equal |not-sent || |
	|COAP Option-End| |dw| 0xFF | equal |not-sent || |		|COAP Option-End| |dw| 0xFF | equal |not-sent || |
	+---------------+--+--+-----------+-----------+-----------++------+		+---------------+--+--+-----------+-----------+-----------++------+
	Figure 11: Inner SCHC Rules		Figure 11: Inner SCHC Rules
	Figure 12 shows the Plaintext obtained for our example GET Request		Figure 12 shows the Plaintext obtained for our example GET Request
	and follows the process of Inner Compression and Encryption until we		and follows the process of Inner Compression and Encryption until we
	end up with the Payload to be added in the outer OSCORE Message.		end up with the Payload to be added in the outer OSCORE Message.
	In this case the original message has no payload and its resulting		In this case the original message has no payload and its resulting
	Plaintext can be compressed up to only 1 byte (size of the Rule ID).		Plaintext can be compressed up to only 1 byte (size of the RuleID).
	The AEAD algorithm preserves this length in its first output, but		The AEAD algorithm preserves this length in its first output, but
	also yields a fixed-size tag which cannot be compressed and has to be		also yields a fixed-size tag which cannot be compressed and has to be
	included in the OSCORE message. This translates into an overhead in		included in the OSCORE message. This translates into an overhead in
	total message length, which limits the amount of compression that can		total message length, which limits the amount of compression that can
	be achieved and plays into the cost of adding security to the		be achieved and plays into the cost of adding security to the
	exchange.		exchange.
	________________________________________________________		________________________________________________________
	| |		| |
	| OSCORE Plaintext |		| OSCORE Plaintext |
	skipping to change at page 20, line 28 ¶		skipping to change at page 20, line 28 ¶
	|		|
	| Inner SCHC Compression		| Inner SCHC Compression
	|		|
	v		v
	_________________________________		_________________________________
	| |		| |
	| Compressed Plaintext |		| Compressed Plaintext |
	| |		| |
	| 0x00 |		| 0x00 |
	| |		| |
	| Rule ID = 0x00 (1 byte) |		| RuleID = 0x00 (1 byte) |
	| (No residue) |		| (No residue) |
	|_________________________________|		|_________________________________|
	|		|
	| AEAD Encryption		| AEAD Encryption
	| (piv = 0x04)		| (piv = 0x04)
	v		v
	_________________________________________________		_________________________________________________
	| |		| |
	| encrypted_plaintext = 0xa2 (1 byte) |		| encrypted_plaintext = 0xa2 (1 byte) |
	skipping to change at page 21, line 32 ¶		skipping to change at page 21, line 32 ¶
	|		|
	| Inner SCHC Compression		| Inner SCHC Compression
	|		|
	v		v
	__________________________________________		__________________________________________
	| |		| |
	| Compressed Plaintext |		| Compressed Plaintext |
	| |		| |
	| 0x001919902180 (6 bytes) |		| 0x001919902180 (6 bytes) |
	| |		| |
	| 00 Rule ID |		| 00 RuleID |
	| |		| |
	| 0b0 (1 bit match-map residue) |		| 0b0 (1 bit match-map residue) |
	| 0x32332043 >> 1 (shifted payload) |		| 0x32332043 >> 1 (shifted payload) |
	| 0b0000000 Padding |		| 0b0000000 Padding |
	|__________________________________________|		|__________________________________________|
	|		|
	| AEAD Encryption		| AEAD Encryption
	| (piv = 0x04)		| (piv = 0x04)
	v		v
	_________________________________________________________		_________________________________________________________
	| |		| |
	| encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) |		| encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes) |
	| tag = 0xe9aef3f2461e0c29 (8 bytes) |		| tag = 0xe9aef3f2461e0c29 (8 bytes) |
	| |		| |
	| ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |		| ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |
	|_________________________________________________________|		|_________________________________________________________|
	Figure 13: Plaintext compression and encryption for CONTENT Response		Figure 13: Plaintext compression and encryption for CONTENT Response
	The Outer SCHC Rules (Figure 16) MUST process the OSCORE Options		The Outer SCHC Rules (Figure 16) must process the OSCORE Options
	fields. In Figure 14 and Figure 15 we show a dump of the OSCORE		fields. In Figure 14 and Figure 15 we show a dump of the OSCORE
	Messages generated from our example messages once they have been		Messages generated from our example messages once they have been
	provided with the Inner Compressed Ciphertext in the payload. These		provided with the Inner Compressed Ciphertext in the payload. These
	are the messages that have to be compressed by the Outer SCHC		are the messages that have to be compressed by the Outer SCHC
	Compression.		Compression.
	Protected message:		Protected message:
	==================		==================
	0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62		0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62
	(25 bytes)		(25 bytes)
	skipping to change at page 24, line 10 ¶		skipping to change at page 24, line 10 ¶
	established.		established.
	The size s included in the kid context field MAY be masked off with		The size s included in the kid context field MAY be masked off with
	CDA MSB. The rest of the field could have additional bits masked		CDA MSB. The rest of the field could have additional bits masked
	off, or have the whole field be fixed with MO equal and CDA not-sent.		off, or have the whole field be fixed with MO equal and CDA not-sent.
	The same holds for the kid field.		The same holds for the kid field.
	Figure 16 shows a possible set of Outer Rules to compress the Outer		Figure 16 shows a possible set of Outer Rules to compress the Outer
	Header.		Header.
	Rule ID 0		RuleID 0
	+-------------------+--+--+--------------+--------+---------++------+		+-------------------+--+--+--------------+--------+---------++------+
	| Field |FP|DI| Target | MO | CDA || Sent |		| Field |FP|DI| Target | MO | CDA || Sent |
	| | | | Value | | ||[bits]|		| | | | Value | | ||[bits]|
	+-------------------+--+--+--------------+--------+---------++------+		+-------------------+--+--+--------------+--------+---------++------+
	|CoAP version | |bi| 01 |equal |not-sent || |		|CoAP version | |bi| 01 |equal |not-sent || |
	|CoAP Type | |up| 0 |equal |not-sent || |		|CoAP Type | |up| 0 |equal |not-sent || |
	|CoAP Type | |dw| 2 |equal |not-sent || |		|CoAP Type | |dw| 2 |equal |not-sent || |
	|CoAP TKL | |bi| 1 |equal |not-sent || |		|CoAP TKL | |bi| 1 |equal |not-sent || |
	|CoAP Code | |up| 2 |equal |not-sent || |		|CoAP Code | |up| 2 |equal |not-sent || |
	|CoAP Code | |dw| 68 |equal |not-sent || |		|CoAP Code | |dw| 68 |equal |not-sent || |
	skipping to change at page 25, line 8 ¶		skipping to change at page 25, line 8 ¶
	Figure 16: Outer SCHC Rules		Figure 16: Outer SCHC Rules
	These Outer Rules are applied to the example GET Request and CONTENT		These Outer Rules are applied to the example GET Request and CONTENT
	Response. The resulting messages are shown in Figure 17 and		Response. The resulting messages are shown in Figure 17 and
	Figure 18.		Figure 18.
	Compressed message:		Compressed message:
	==================		==================
	0x001489458a9fc3686852f6c4 (12 bytes)		0x001489458a9fc3686852f6c4 (12 bytes)
	0x00 Rule ID		0x00 RuleID
	1489 Compression Residue		1489 Compression Residue
	458a9fc3686852f6c4 Padded payload		458a9fc3686852f6c4 Padded payload
	Compression residue:		Compression Residue:
	0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding)		0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding)
	mid tkn piv kid		mid tkn piv kid
	Payload		Payload
	0xa2c54fe1b434297b62 (9 bytes)		0xa2c54fe1b434297b62 (9 bytes)
	Compressed message length: 12 bytes		Compressed message length: 12 bytes
	Figure 17: SCHC-OSCORE Compressed GET Request		Figure 17: SCHC-OSCORE Compressed GET Request
	Compressed message:		Compressed message:
	==================		==================
	0x0014218daf84d983d35de7e48c3c1852 (16 bytes)		0x0014218daf84d983d35de7e48c3c1852 (16 bytes)
	0x00 Rule ID		0x00 RuleID
	14 Compression residue		14 Compression Residue
	218daf84d983d35de7e48c3c1852 Padded payload		218daf84d983d35de7e48c3c1852 Padded payload
	Compression residue:		Compression Residue:
	0b0001 010 (7 bits -> 1 byte with padding)		0b0001 010 (7 bits -> 1 byte with padding)
	mid tkn		mid tkn
	Payload		Payload
	0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)		0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
	Compressed msg length: 16 bytes		Compressed msg length: 16 bytes
	Figure 18: SCHC-OSCORE Compressed CONTENT Response		Figure 18: SCHC-OSCORE Compressed CONTENT Response
	For contrast, we compare these results with what would be obtained by		For contrast, we compare these results with what would be obtained by
	SCHC compressing the original CoAP messages without protecting them		SCHC compressing the original CoAP messages without protecting them
	with OSCORE. To do this, we compress the CoAP messages according to		with OSCORE. To do this, we compress the CoAP messages according to
	the SCHC rules in Figure 19.		the SCHC Rules in Figure 19.
	Rule ID 1		RuleID 1
	+---------------+--+--+-----------+---------+-----------++--------+		+---------------+--+--+-----------+---------+-----------++--------+
	| Field |FP|DI| Target | MO | CDA || Sent |		| Field |FP|DI| Target | MO | CDA || Sent |
	| | | | Value | | || [bits] |		| | | | Value | | || [bits] |
	+---------------+--+--+-----------+---------+-----------++--------+		+---------------+--+--+-----------+---------+-----------++--------+
	|CoAP version | |bi| 01 |equal |not-sent || |		|CoAP version | |bi| 01 |equal |not-sent || |
	|CoAP Type | |up| 0 |equal |not-sent || |		|CoAP Type | |up| 0 |equal |not-sent || |
	|CoAP Type | |dw| 2 |equal |not-sent || |		|CoAP Type | |dw| 2 |equal |not-sent || |
	|CoAP TKL | |bi| 1 |equal |not-sent || |		|CoAP TKL | |bi| 1 |equal |not-sent || |
	|CoAP Code | |up| 2 |equal |not-sent || |		|CoAP Code | |up| 2 |equal |not-sent || |
	|CoAP Code | |dw| [69,132] |match-map|map-sent ||C |		|CoAP Code | |dw| [69,132] |match-map|map-sent ||C |
	skipping to change at page 26, line 30 ¶		skipping to change at page 26, line 30 ¶
	+---------------+--+--+-----------+---------+-----------++--------+		+---------------+--+--+-----------+---------+-----------++--------+
	Figure 19: SCHC-CoAP Rules (No OSCORE)		Figure 19: SCHC-CoAP Rules (No OSCORE)
	This yields the results in Figure 20 for the Request, and Figure 21		This yields the results in Figure 20 for the Request, and Figure 21
	for the Response.		for the Response.
	Compressed message:		Compressed message:
	==================		==================
	0x0114		0x0114
	0x01 = Rule ID		0x01 = RuleID
	Compression residue:		Compression Residue:
	0b00010100 (1 byte)		0b00010100 (1 byte)
	Compressed msg length: 2		Compressed msg length: 2
	Figure 20: CoAP GET Compressed without OSCORE		Figure 20: CoAP GET Compressed without OSCORE
	Compressed message:		Compressed message:
	==================		==================
	0x010a32332043		0x010a32332043
	0x01 = Rule ID		0x01 = RuleID
	Compression residue:		Compression Residue:
	0b00001010 (1 byte)		0b00001010 (1 byte)
	Payload		Payload
	0x32332043		0x32332043
	Compressed msg length: 6		Compressed msg length: 6
	Figure 21: CoAP CONTENT Compressed without OSCORE		Figure 21: CoAP CONTENT Compressed without OSCORE
	As can be seen, the difference between applying SCHC + OSCORE as		As can be seen, the difference between applying SCHC + OSCORE as
	compared to regular SCHC + COAP is about 10 bytes of cost.		compared to regular SCHC + COAP is about 10 bytes of cost.
	8. IANA Considerations		8. IANA Considerations
	This document has no request to IANA.		This document has no request to IANA.
	9. Security considerations		9. Security considerations
	This document does not have any more Security consideration than the		The Security Considerations of SCHC header compression RFC8724 are
	ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc].		valid for SCHC CoAP header compression. When CoAP uses OSCORE, the
	Variable length residues may be used to compress URI elements. They		security considerations defined in RFC8613 does not change when SCHC
	cannot produce a packet expansion either on the LPWAN network or in		header compression is applied.
	the Internet network after decompression. The length send is not
	used to indicate the information that should be reconstructed at the		The definition of SCHC over CoAP header fields permits the
	other end, but on the contrary the information sent as a Residue.		compression of header information only. The SCHC header compression
	Therefore, if a length is set to a high value, but the number of bits		itself does not increase or reduce the level of security in the
	on the SCHC packet is smaller, the packet must be dropped by the		communication. When the communication does not use any security
	decompressor.		protocol as OSCORE, DTLS, or other. It is highly necessary to use a
			layer two security.
			DoS attacks are possible if an intruder can introduce a compressed
			SCHC corrupted packet onto the link and cause a compression
			efficiency reduction. However, an intruder having the ability to add
			corrupted packets at the link layer raises additional security issues
			than those related to the use of header compression.
			SCHC compression returns variable-length Residues for some CoAP
			fields. In the compressed header, the length sent is not the
			original header field length but the length of the Residue. So if a
			corrupted packet comes to the decompressor with a longer or shorter
			length than the one in the original header, SCHC decompression will
			detect an error and drops the packet.
	OSCORE compression is also based on the same compression method		OSCORE compression is also based on the same compression method
	described in [I-D.ietf-lpwan-ipv6-static-context-hc]. The size of		described in [rfc8724]. The size of the Initialisation Vector (IV)
	the Initialisation Vector residue size must be considered carefully.		residue size must be considered carefully. A too large value has an
	A too large value has a impact on the compression efficiency and a		impact on the compression efficiency and a too small value will force
	too small value will force the device to renew its key more often.		the device to renew its key more often. This operation may be long
	This operation may be long and energy consuming.		and energy consuming. The size of the compressed IV MUST be choosen
			regarding the highest expected traffic from the device.
			SCHC header and compression Rules MUST remain tightly coupled.
			Otherwise, an encrypted residue may be decompressed in a different
			way by the receiver. To avoid this situation, if the Rule is
			modified in one location, the OSCORE keys MUST be re-established.
	10. Acknowledgements		10. Acknowledgements
	The authors would like to thank Dominique Barthel, Carsten Bormann,		The authors would like to thank (in alphabetic order): Christian
	Thomas Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov,		Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
	Goran Selander.		Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran
			Selander.
	11. Normative References		11. Normative References
	[I-D.ietf-lpwan-ipv6-static-context-hc]		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
	Zuniga, "Static Context Header Compression (SCHC) and
	fragmentation for LPWAN, application to UDP/IPv6", draft-
	ietf-lpwan-ipv6-static-context-hc-24 (work in progress),
	December 2019.
	[rfc2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained		[rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
	Application Protocol (CoAP)", RFC 7252,		Application Protocol (CoAP)", RFC 7252,
	DOI 10.17487/RFC7252, June 2014,		DOI 10.17487/RFC7252, June 2014,
	<https://www.rfc-editor.org/info/rfc7252>.		<https://www.rfc-editor.org/info/rfc7252>.
	[rfc7641] Hartke, K., "Observing Resources in the Constrained		[rfc7641] Hartke, K., "Observing Resources in the Constrained
	skipping to change at page 29, line 5 ¶		skipping to change at page 29, line 19 ¶
	[rfc8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[rfc8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	[rfc8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,		[rfc8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
	"Object Security for Constrained RESTful Environments		"Object Security for Constrained RESTful Environments
	(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,		(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
	<https://www.rfc-editor.org/info/rfc8613>.		<https://www.rfc-editor.org/info/rfc8613>.
			[rfc8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
			Zuniga, "SCHC: Generic Framework for Static Context Header
			Compression and Fragmentation", RFC 8724,
			DOI 10.17487/RFC8724, April 2020,
			<https://www.rfc-editor.org/info/rfc8724>.
			Appendix A. Extension to the RFC8724 Annex D.
			This section extends the RFC8724 Annex D list.
			o How to establish the End-to-End context initialization using SCHC
			for CoAP header only.
	Authors' Addresses		Authors' Addresses
	Ana Minaburo		Ana Minaburo
	Acklio		Acklio
	1137A avenue des Champs Blancs		1137A avenue des Champs Blancs
	35510 Cesson-Sevigne Cedex		35510 Cesson-Sevigne Cedex
	France		France
	Email: ana@ackl.io		Email: ana@ackl.io
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