< draft-ietf-lpwan-coap-static-context-hc-15.txt		draft-ietf-lpwan-coap-static-context-hc-16.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: January 4, 2021 Institut MINES TELECOM; IMT Atlantique		Expires: April 23, 2021 Institut MINES TELECOM; IMT Atlantique
	R. Andreasen		R. Andreasen
	Universidad de Buenos Aires		Universidad de Buenos Aires
	July 03, 2020		October 20, 2020
	LPWAN Static Context Header Compression (SCHC) for CoAP		LPWAN Static Context Header Compression (SCHC) for CoAP
	draft-ietf-lpwan-coap-static-context-hc-15		draft-ietf-lpwan-coap-static-context-hc-16
	Abstract		Abstract
	This draft defines the way Static Context Header Compression (SCHC)		This draft defines how Static Context Header Compression (SCHC) can
	header compression can be applied to the Constrained Application		be applied to the Constrained Application Protocol (CoAP). SCHC is a
	Protocol (CoAP). SCHC is a header compression mechanism adapted for		header compression mechanism adapted for constrained devices. SCHC
	constrained devices. SCHC uses a static description of the header to		uses a static description of the header to reduce the redundancy and
	reduce the redundancy and the size of the information in the header.		size of the header's information. While RFC 8724 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 SCHC for CoAP headers. The
	applies the use of SCHC for CoAP headers. The CoAP header structure		CoAP header structure differs from IPv6 and UDP since CoAP uses a
	differs from IPv6 and UDP since CoAP uses a flexible header with a		flexible header with a variable number of options, themselves of
	variable number of options, themselves of variable length. The CoAP		variable length. The CoAP protocol messages format is asymmetric:
	protocol messages format is asymmetric: the request messages have a		the request messages have a header format different from the one in
	header format different from the one in the response messages. This		the response messages. This specification gives guidance on applying
	specification gives guidance on how to apply SCHC to flexible headers		SCHC to flexible headers and how to leverage the asymmetry for more
	and how to leverage the asymmetry for more efficient compression		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/.
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	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
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	This Internet-Draft will expire on January 4, 2021.		This Internet-Draft will expire on April 23, 2021.
	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 headers . . . . . . . . . . . . . . . . 4		2. SCHC Applicability to CoAP . . . . . . . . . . . . . . . . . 4
	3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 6		3. CoAP Headers compressed with SCHC . . . . . . . . . . . . . . 7
	3.1. Differences between CoAP and UDP/IP Compression . . . . . 7		3.1. Differences between CoAP and UDP/IP Compression . . . . . 8
	4. Compression of CoAP header fields . . . . . . . . . . . . . . 8		4. Compression of CoAP header fields . . . . . . . . . . . . . . 9
	4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 8		4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 9
	4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 8		4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 9
	4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 8		4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 9
	4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 9		4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 10
	4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 9		4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 10
	5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 9		5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 10
	5.1. CoAP Content and Accept options. . . . . . . . . . . . . 10		5.1. CoAP Content and Accept options. . . . . . . . . . . . . 11
	5.2. CoAP option Max-Age, Uri-Host and Uri-Port fields . . . . 10		5.2. CoAP option Max-Age, Uri-Host, and Uri-Port fields . . . 11
	5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 10		5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 11
	5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 11		5.3.1. Variable-length Uri-Path and Uri-Query . . . . . . . 12
	5.3.2. Variable number of path or query elements . . . . . . 11		5.3.2. Variable number of Path or Query elements . . . . . . 12
	5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme		5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
	fields . . . . . . . . . . . . . . . . . . . . . . . . . 11		fields . . . . . . . . . . . . . . . . . . . . . . . . . 13
	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 . . . . . . . . . . . . . . . . 12		and Location-Query fields . . . . . . . . . . . . . . . . 13
	6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 12		6. SCHC compression of CoAP extension RFCs . . . . . . . . . . . 13
	6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 12		6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13
	6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 12		6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13
	6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 12		6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 13
	6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 12		6.4. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 14
	7. Examples of CoAP header compression . . . . . . . . . . . . . 14		7. Examples of CoAP header compression . . . . . . . . . . . . . 15
	7.1. Mandatory header with CON message . . . . . . . . . . . . 14		7.1. Mandatory header with CON message . . . . . . . . . . . . 15
	7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 15		7.2. OSCORE Compression . . . . . . . . . . . . . . . . . . . 16
	7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 18		7.3. Example OSCORE Compression . . . . . . . . . . . . . . . 19
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
	9. Security considerations . . . . . . . . . . . . . . . . . . . 28		9. Security considerations . . . . . . . . . . . . . . . . . . . 29
	10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29		10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
	11. Normative References . . . . . . . . . . . . . . . . . . . . 29		11. Normative References . . . . . . . . . . . . . . . . . . . . 30
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
	1. Introduction		1. Introduction
	CoAP [rfc7252] is designed to easily interop with HTTP (Hypertext		CoAP [rfc7252] is a command/response protocol designed for micro-
	Transfer Protocol) and is optimized for REST-based (Representational		controllers with a small amount of RAM and ROM and is optimized for
	state transfer) services. Although CoAP was designed for constrained		REST-based (Representational state transfer) services. Although CoAP
	devices, the size of a CoAP header still is too large for the		was designed for Low-Power Wireless Personal Area Networks (6LoWPAN),
	constraints of LPWAN (Low Power Wide Area Networks) and some		a CoAP header's size is still too large for LPWAN (Low Power Wide
	compression is needed to reduce the header size.		Area Networks) and some compression of the CoAP header is required
			either to increase performances or allow CoAP other some LPWAN
			technologies.
	The [rfc8724] defines SCHC, a header compression mechanism for LPWAN		The [rfc8724] defines SCHC, a header compression mechanism for the
	network based on a static context. Section 5 of the [rfc8724]		LPWAN network based on a static context. Section 5 of the [rfc8724]
	explains the architecture where compression and decompression are		explains the architecture where compression and decompression are
	done. The context is known by both ends before transmission. The		done. The SCHC compression scheme assumes as a prerequisite that the
	way the context is configured, provisioned or exchanged is out of the		static context is known to both endpoints before transmission. The
	scope of this document.		way the context is configured, provisioned or exchanged is out of
			this document's scope.
	CoAP is an End-to-End protocol at the application level, so CoAP		CoAP is an application protocol, so CoAP compression requires
	compression requires to install common Rules between two hosts and IP		installing common rules between the two SCHC instances. SCHC
	Routing may be needed to allow End-to-End communication. Therefore,		compression may apply at two different levels: one to compress IP and
	SCHC compression may apply at two different levels, one to compress		UDP in the LPWAN network and another at the application level for
	IP and UDP as described in [rfc8724] in the LPWAN network and another		CoAP. These two compressions may be independent. Both follow the
	at the application level. These two compressions may be independent.		same principle described in RFC8724. SCHC rules driving the
	The Compression Rules can be set up by two independent entities and		compression/decompression are different and may be managed by
	are out of the scope of this document. In both cases, SCHC mechanism		different entities. The [rfc8724] describes how the IP and UDP
	remains the same.		headers may be compressed. This document specifies how the SCHC
			compression rules can be applied to CoAP traffic.
	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.
	can match header fields and specific values or ranges of values. If		Each context consists of multiple Rules. Each Rule can match header
	a Rule matches, the matched header fields are substituted by the		fields and specific values or ranges of values.
	RuleID and optionally some residual bits. Thus, different Rules may		If a Rule matches, the matched header fields are replaced by the
	correspond to different types of packets that a device expects to		RuleID and some residual bits. Thus, different Rules may correspond
	send or receive.		to divers protocols packets that a device expects to send or receive.
	A Rule describes the complete header of the packet with an ordered		A Rule describes the packets's entire header with an ordered list of
	list of fields descriptions, see section 7 of [rfc8724], thereby each		fields descriptions; see section 7 of [rfc8724]. Thereby
	description contains the field ID (FID), its length (FL) and its		each description contains the field ID (FID), its length (FL), and
	position (FP), a direction indicator (DI) (upstream, downstream and		its position (FP), a direction indicator (DI) (upstream, downstream,
	bidirectional) and some associated Target Values (TV).		and bidirectional), and some associated Target Values (TV). The
			direction indicator is used for compression to give the best TV to
			the FID when these values differ in the transmission direction. So a
			field may be described several times depending on the asymmetry of
			its possible TVs.
	A Matching Operator (MO) is associated to each header field		A Matching Operator (MO) is associated with each header field
	description. The Rule is selected if all the MOs fit the TVs for all		description.
	fields of the incoming header.		The Rule is selected if all the MOs fit the TVs for all fields of the
			incoming header. A rule cannot be selected if the message contains a
			field unknown to the SCHC compressor.
	In that case, a Compression/Decompression Action (CDA) associated to		In that case, a Compression/Decompression Action (CDA) associated
	each field defines how the compressed and the decompressed values are		with each field give the method to compress and decompress each
	computed. Compression mainly results in one of 4 actions:		field. Compression mainly results in one of 4 actions:
	o send the field value,		o send the field value,
	o send nothing,		o send nothing,
	o send some least significant bits of the field or		o send some least significant bits of the field or
	o send an index.		o send an index.
	After applying the compression there may be some bits to be sent,		After applying the compression, there may be some bits to be sent.
	these values are called Compression Residues.		These values are called Compression Residues.
	SCHC is a general mechanism that can be applied to different		SCHC is a general mechanism applied to different protocols, the exact
	protocols, the exact Rules to be used depend on the protocol and the		Rules to be used depending on the protocol and the application.
	application. The section 10 of the [rfc8724] describes the		Section 10 of the [rfc8724] describes the compression scheme for IPv6
	compression scheme for IPv6 and UDP headers. This document targets		and UDP headers.
	the CoAP header compression using SCHC.		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 headers		2. SCHC Applicability to CoAP
	The SCHC Compression Rules can be applied to CoAP headers. 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
	as described in Section 5 of [rfc8724] and may be used as shown in		layers, described in Section 5 of [rfc8724], may be used, as shown in
	Figure 1, Figure 2 and Figure 3.		Figure 1,Figure 2 and Figure 3
			In the first example, Figure 1, a Rule compresses the complete header
	In the first example Figure 1, a Rule compresses the complete header
	stack from IPv6 to CoAP. In this case, SCHC C/D (Static Context		stack from IPv6 to CoAP. In this case, SCHC C/D (Static Context
	Header Compression Compressor/Decompressor) is performed at the		Header Compression Compressor/Decompressor) is performed at the
	Sender and at the Receiver. The host communicating with the device		device and the application. The host communicating with the device
	do not implement SCHC C/D.		does not implement SCHC C/D.
	(device) (NGW) (internet) (App)		(device) (NGW) (App)
	+--------+ +--------+		+--------+ +--------+
	| CoAP | | CoAP |		| CoAP | | CoAP |
	+--------+ +--------+		+--------+ +--------+
	| UDP | | UDP |		| UDP | | UDP |
	+--------+ +----------------+ +--------+		+--------+ +----------------+ +--------+
	| IPv6 | | IPv6 | | IPv6 |		| IPv6 | | IPv6 | | IPv6 |
	+--------+ +--------+-------+ +--------+		+--------+ +--------+-------+ +--------+
	| SCHC | | SCHC | | | |		| SCHC | | SCHC | | | |
	+--------+ +--------+ + + +		+--------+ +--------+ + + +
	| LPWAN | | LPWAN | | | |		| LPWAN | | LPWAN | | | |
	+--------+ +--------+-------+ +--------+		+--------+ +--------+-------+ +--------+
	((((((())))))) ----- ------ ------ -----		((((LPWAN)))) ------ Internet ------
	Figure 1: Compression/decompression at the LPWAN bondary		Figure 1: Compression/decompression at the LPWAN boundary
			The SCHC can be viewed as a layer above layer 2. This layer received
			non-encrypted packets and can apply compression rule to all the
			headers. On the other end, the NGW receives the SCHC packet and
			reconstructs the headers from the rule, identified by its ID and the
			header residues. The result is a regular IPv6 packet that can be
			forwarded toward the destination. The same process applies in the
			other direction. A not encrypted packet arrived at the NGW, thanks
			to IP forwarding based on the IPv6 prefix. The NGW identifies the
			device and compresses headers using the device's rules.
	In the second example, Figure 2, the SCHC compression is applied in		In the second example, Figure 2, the SCHC compression is applied in
	the CoAP layer, compressing the CoAP header independently of the		the CoAP layer, compressing the CoAP header independently of the
	other layers. The RuleID and the Compression Residue are encrypted		other layers. The RuleID, the Compression Residue, and CoAP payload
	using a mechanism such as DTLS. Only the other end can decipher the		are encrypted using a mechanism such as DTLS. Only the other end
	information. If needed, layers below use SCHC to compress the header		(App) can decipher the information. If needed, layers below use SCHC
	as defined in [rfc8724] document. This use case realizes an End-to-		to compress the header as defined in [rfc8724] document (represented
	End context initialization between the sender and the receiver and is		in dotted lines).
	out-of-scope of this document.
	(device) (NGW) (internet) (App)		This use case needs an end-to-end context initialization between the
			device and the application and is out-of-scope of this document.
	+--------+ +--------+		(device) (NGW) (App)
	| CoAP | | CoAP |
	+--------+ +--------+
	| SCHC | | SCHC |
	+--------+ +--------+
	| DTLS | | DTLS |
	+--------+ +--------+
	. udp . . udp .
	.......... .................. ..........
	. ipv6 . . ipv6 . . ipv6 .
	.......... .................. ..........
	. schc . . schc . . . .
	.......... .......... . . .
	. lpwan . . lpwan . . . .
	.......... .................. ..........
	((((((())))))) ----- ------ ------ -----
	Figure 2: Standalone CoAP ene-to-end compression/decompression		+--------+ +--------+
			| CoAP | | CoAP |
			+--------+ +--------+
			| SCHC | | SCHC |
			+--------+ +--------+
			| DTLS | | DTLS |
			+--------+ +--------+
			. udp . . udp .
			.......... .................. ..........
			. ipv6 . . ipv6 . . ipv6 .
			.......... .................. ..........
			. schc . . schc . . . .
			.......... .......... . . .
			. lpwan . . lpwan . . . .
			.......... .................. ..........
			((((LPWAN)))) ------ Internet ------
	In the third example, Figure 3 the Object Security for Constrained		Figure 2: Standalone CoAP end-to-end compression/decompression
			In the third example, Figure 3, the Object Security for Constrained
	RESTful Environments (OSCORE) [rfc8613] is used. In this case, two		RESTful Environments (OSCORE) [rfc8613] is used. In this case, two
	rulesets are used to compress the CoAP message. A first ruleset		rulesets are used to compress the CoAP message. A first ruleset
	focused on the inner header and is applied end to end by both ends.		focused on the inner header compresses it. The result is encrypted
	A second ruleset compresses the outer header and the layers below and		using the OSCORE mechanism. A second ruleset compresses the outer
	is done between the Sender and the Receiver.		header, including the OSCORE Options.
	(device) (NGW) (internet) (App)		(device) (NGW) (App)
	+--------+ +--------+		+--------+ +--------+
	| CoAP | | CoAP |		| CoAP | | CoAP |
	| inner | | inner |		| inner | | inner |
	+--------+ +--------+		+--------+ +--------+
	| SCHC | | SCHC |		| SCHC | | SCHC |
	+--------+ +--------+		| inner | | inner |
	| CoAP | | CoAP |		+--------+ +--------+
	| outer | | outer |		| CoAP | | CoAP |
	+--------+ +--------+		| outer | | outer |
	. udp . . udp .		+--------+ +--------+
	.......... .................. ..........		| SCHC | | SCHC |
	. ipv6 . . ipv6 . . ipv6 .		| outer | | outer |
	.......... .................. ..........		+--------+ +--------+
	. schc . . schc . . . .		. udp . . udp .
	.......... .......... . . .		.......... .................. ..........
	. lpwan . . lpwan . . . .		. ipv6 . . ipv6 . . ipv6 .
	.......... .................. ..........		.......... .................. ..........
	((((((())))))) ----- ------ ------ -----		. schc . . schc . . . .
			.......... .......... . . .
			. lpwan . . lpwan . . . .
			.......... .................. ..........
			((((LPWAN)))) ------ Internet ------
	Figure 3: OSCORE compression/decompression.		Figure 3: OSCORE compression/decompression.
	In case of 2 rule-sets, as shown in Figure 2 and Figure 3, they may		In the case of several SCHC instances, as shown in Figure 3 and
	come from different provisioning domains, and that they do not		Figure 3, the rulesets may come from different provisioning domains.
	include the cryptography part that is done in between the two SCHC
	activities. This document focuses on CoAP compression represented in		This document focuses on CoAP compression represented in the dashed
	the dashed boxes in the previous figures.		boxes in the previous figures.
	3. CoAP Headers compressed with SCHC		3. CoAP Headers compressed with SCHC
	The use of SCHC over the CoAP header uses the same description and		The use of SCHC over the CoAP header uses the same description and
	compression/decompression techniques as the one for IP and UDP		compression/decompression techniques like the one for IP and UDP
	explained in the [rfc8724]. For CoAP, SCHC Rules description uses		explained in the [rfc8724]. For CoAP, SCHC Rules description uses
	the direction information to optimize the compression by reducing the		the direction information to optimize the compression by reducing the
	number of Rules needed to compress headers. The field description		number of Rules needed to compress headers. The field description
	MAY define both request/response headers and target values in the		MAY define both request/response headers and target values in the
	same Rule, using the DI (direction indicator) to make the difference.		same Rule, using the DI (direction indicator) to make the difference.
	As for other protocols, when the compressor does not find a correct
	Rule to compress the header, the packet MUST be sent uncompressed		As for other header compression protocols, when the compressor does
	using the RuleID dedicated to this purpose, and the Compression		not find a correct Rule to compress the header, the packet MUST be
	Residue is the complete header of the packet. See section 6 of		sent uncompressed using the RuleID dedicated to this purpose. Where
	[rfc8724].		the Compression Residue is the complete header of the packet. See
			section 6 of [rfc8724].
	3.1. Differences between CoAP and UDP/IP Compression		3.1. Differences between CoAP and UDP/IP Compression
	CoAP compression differs from IPv6 and UDP compression on the		CoAP compression differs from IPv6 and UDP compression on the
	following aspects:		following aspects:
	o The CoAP protocol is asymmetric, the headers are different for a		o The CoAP protocol is asymmetric; the headers are different for a
	request or a response. For example, the URI-path option is		request or a response. For example, the URI-Path option is
	mandatory in the request, and it is not present in the response, a		mandatory in the request, and it may not be present in the
	request may contain an Accept option, and the response may include		response. A request may contain an Accept option, and the
	a Content option. In comparison, IPv6 and UDP returning path swap		response may include a Content-Format option. In comparison, IPv6
	the value of some fields in the header.		and UDP returning path swap the value of some fields in the
	But all the directions have the same fields (e.g., source and		header. But all the directions have the same fields (e.g., source
	destination addresses fields).		and destination address fields).
	The [rfc8724] defines the use of a Direction Indicator (DI) in the		The [rfc8724] defines the use of a Direction Indicator (DI) in the
	Field Description, which allows a single Rule to process message		Field Descriptor, which allows a single Rule to process a message
	headers differently depending on the direction.		headers differently depending on 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.		the values carried in each direction are different.
	The compression may use a matching list in the TV to limit the		The compression may use a matching list in the TV to limit the
	range of expected values in a particular direction and therefore		range of expected values in a particular direction and therefore
	reduces the size of the Compression Residue. Through the		reduce the Compression Residue's size. Through the Direction
	Direction Indicator (DI), a field description in the Rules splits		Indicator (DI), a field description in the Rules splits the
	the possible field value into two parts, one for each direction.		possible field value into two parts, one for each direction. For
	For instance, if a client sends only CON requests, the type can be		instance, if a client sends only CON requests, the type can be
	elided by compression, and the answer may use one single bit to		elided by compression, and the answer may use one single bit to
	carry either the ACK or RST type. The field Code have as well the		carry either the ACK or RST type. The field Code has the same
	same behavior, the 0.0X code format value in the request and Y.ZZ		behavior, the 0.0X code format value in the request, and Y.ZZ code
	code format in the response.		format in the response.
	o Headers in IPv6 and UDP have a fixed size. The size is not sent		o Headers in IPv6 and UDP have a fixed size. The size is not sent
	as part of the Compression Residue, but is defined in the Rule.		as part of the Compression Residue but is defined in the Rule.
	Some CoAP header fields have variable lengths, so the length is		Some CoAP header fields have variable lengths, so the length is
	also specified in the Field Description. For example, the Token		also specified in the Field Description. For example, the Token
	size may vary from 0 to 8 bytes. And the CoAP options have a		size may vary from 0 to 8 bytes. And the CoAP options have a
	variable length since they use the Type-Length-Value encoding		variable length since they use the Type-Length-Value encoding
	format, as URI-path or URI-query.		format, as URI-path or URI-query.
	Section 7.5.2 from [rfc8724] offers the possibility to define a		Section 7.5.2 from [rfc8724] offers the possibility to define a
	function for the Field length in the Field Description to have		function for the Field length in the Field Description to know the
	knowledge of the length before compression. When doing SCHC		length before compression. When doing SCHC compression of a
	compression of a variable-length field,		variable-length field,
	if the field size is not known, the Field Length in the Rule is		if the field size is unknown, the Field Length in the Rule is set
	set as variable and the size is sent with the Compression Residue.		as variable, and the size is sent with the Compression Residue.
	o A field can appear several time in the CoAP headers. This is		o A field can appear several times 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 [rfc8724] allows a Field ID to appear several times		specification [rfc8724] allows a Field ID to appear several times
	in the Rule, and uses the Field Position (FP) to identify the		in the Rule and uses the Field Position (FP) to identify the
	correct instance, and thereby removing the ambiguity of the		correct instance, and thereby 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
	regarding LPWAN traffic constraints. This is particularly true		large regarding LPWAN traffic constraints. This is particularly
	for the Message ID field and the Token field. SCHC uses different		true for the Message-ID field and the Token field. SCHC uses
	Matching operators (MO) to perform the compression, see section		different Matching operators (MO) to perform the compression. See
	7.4 of [rfc8724]. In this case the Most Significant Bits (MSB) MO		section 7.4 of [rfc8724]. In this case, the Most Significant Bits
	can be applied to reduce the information carried on LPWANs.		(MSB) MO can be applied to reduce the information carried on
			LPWANs.
	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
	[rfc8724].		[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 types of messages: two requests
	(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 IANA		The code field indicates the Request Method used in CoAP, an IANA
	registry [rfc7252]. The compression of the CoAP code field follows		registry [rfc7252]. The compression of the CoAP code field follows
	the same principle as that of the CoAP type field. If the device		the same principle as that of the CoAP type field. If the device
	plays a specific role, the set of code values can be split in two		plays a specific role, the set of code values can be split into two
	parts, the request codes with the 0 class and the response values.		parts, the request codes with the 0 class and the 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 can to process.
	A mapping list can be used for known values. For other values the		A mapping list can be used for known values. The field cannot be
	field cannot be compressed an the value needs to be sent in the		compressed for other values, and the value needs to be sent in the
	Compression 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 [rfc8724].		Least Significant Bits (LSB) CDA. See section 7.4 of [rfc8724].
	4.5. CoAP Token fields		4.5. CoAP Token fields
	Token is defined through two CoAP fields, Token Length in the		A 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, the Token Length value MUST
	used to define the size of the Compression Residue. A specific		be used to define the size of the Compression Residue. A specific
	function 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		CoAP defines options that are placed after the based header in Option
	Numbers order, see [rfc7252]. Each Option instance in a message uses		Numbers order, see [rfc7252]. Each Option instance in a message uses
	the format Delta-Type (D-T), Length (L), Value (V). When applying		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		SCHC compression to the Option, the D-T, L, and V format serve to
	make the Rule description of the Option. The SCHC compression builds		make the Rule description of the Option. The SCHC compression builds
	the description of the Option by using in the Field ID the Option		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		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		uses section 7.4 of [rfc8724]. When the Option Length has a
	size it can be stored in the Rule. Therefore, SCHC compression does		wellknown size, it can be stored in the Rule. Therefore, SCHC
	not send it. Otherwise, SCHC Compression carries the length of the		compression does not send it. Otherwise, SCHC Compression carries
	Compression Residue in addition to the Compression Residue value.		the length of the Compression Residue, in addition to the Compression
			Residue value.
	CoAP request and response do not include the same options. So		CoAP requests and responses do not include the same options. So
	Compression Rules may reflect these assymetry by tagging the		Compression Rules may reflect this asymmetry by tagging the direction
	direction indicator.		indicator.
	Note that length coding differs between CoAP options and SCHC		Note that length coding differs between CoAP options and SCHC
	variable size Compression Residue.		variable size Compression Residue.
	The following sections present how SCHC compresses some specific CoAP		The following sections present how SCHC compresses some specific CoAP
	Options.		options.
			If a new option is introduced in CoAP, a new Field ID has to be
			assigned in the Rules to allow its compression. Otherwise, if no
			Rule describes this Option, the SCHC compression is not possible, and
			the CoAP header is sent without compression.
	5.1. CoAP Content and Accept options.		5.1. CoAP Content and Accept options.
	If a single value is expected by the client, it can be stored in the		If the client expects a single value, it can be stored in the TV and
	TV and elided during the transmission. Otherwise, if several		elided during the transmission. Otherwise, if the client expects
	possible values are expected by the client, a matching-list SHOULD be		several possible values, a matching-list SHOULD be used to limit the
	used to limit the size of the Compression Residue. Otherwise, the		Compression Residue's size. Otherwise, the value has to be sent as a
	value has to be sent as a Compression Residue (fixed or variable		Compression Residue (fixed or variable length).
	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
	If the duration is known by both ends, the value can be elided.		If both ends know the value, the value can be elided.
	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 Compression Residue (fixed		Otherwise, these options can be sent as a Compression Residue.
	or variable length).
	5.3. CoAP option Uri-Path and Uri-Query fields		5.3. CoAP option Uri-Path and Uri-Query fields
	Uri-Path and Uri-Query elements are a repeatable options, the Field		Uri-Path and Uri-Query elements are 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. The Numbering of elements do
	change, MO comparison is set with the first element of the matching.		not 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 |var| 3|up| |ignore |value-sent |		|Uri-Path |var| 3|up| |ignore |value-sent |
	+-------------+---+--+--+--------+---------+-------------+		+-------------+---+--+--+--------+---------+-------------+
	Figure 4: complex path example		Figure 4: complex path example
	In Figure 4 a single bit residue can be used to code one of the 2		In Figure 4, 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. The third path element is sent as a variable size residue.		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 5: CORECONF URI compression		Figure 5: CORECONF URI compression
	Figure 5 shows the parsing and the compression of the URI, where c is		Figure 5 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 4 bits to the variable Residue size. See section 7.5.2		This adds 4 bits to the variable Residue size. See section 7.5.2
	[rfc8724]		[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
	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 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 can be both used. If a		includes a fragmentation protocol. They can be both used. If a
	block option is used, its content MUST be sent as a Compression		block option is used, its content MUST be sent as a Compression
	Residue.		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 SHOULD exist to allow the
	of this message.		message's compression with the RST type.
	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 both ends know the value, then TV
	then TV is set to this value, MO is set to "equal" and CDA is set to		is set to this value, MO is set to "equal", and CDA is set to "not-
	"not-sent".		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
	used to reduce the size.		be 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
	<- 1 byte -> <------ s bytes ----->		<- 1 byte -> <------ s bytes ----->
	+------------+----------------------+-----------------------+		+------------+----------------------+-----------------------+
	| 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_kidctx ------>|<-- CoAP OSCORE_kid -->|
	Figure 6: OSCORE Option		Figure 6: 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 6.		[rfc8613] is repeated in Figure 6.
	The first byte specifies the content of the OSCORE options using		The first byte specifies the content of the OSCORE options using
	flags. The three most significant bits of this byte are reserved and		flags. The three most significant bits of this byte are reserved and
	always set to 0. Bit h, when set, indicates the presence of the kid		always set to 0. Bit h, when set, indicates the presence of the kid
	context field in the option. Bit k, when set, indicates the presence		context field in the option. Bit k, when set, indicates the presence
	of a kid field. The three 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		The flag byte is followed by the piv field, kid context field, and
	kid field in this order, and if present, the length of the kid		kid field in this order, and if present, the length of the kid
	context field is encoded in the first byte denoting by s the length		context field is encoded in the first byte denoting by s the length
	of the kid context in bytes.		of the kid context in bytes.
	This specification recommends identifying the OSCORE Option and the		This specification recommends identifying the OSCORE Option and the
	fields it contains Conceptually, it discerns up to 4 distinct pieces		fields it contains. Conceptually, it discerns up to 4 distinct
	of information within the OSCORE option: the flag bits, the piv, the		pieces of information within the OSCORE option: the flag bits, the
	kid context, and the kid. The SCHC Rule splits into four field		piv, the kid context, and the kid. The SCHC Rule splits into four
	descriptions the OSCORE option to compress them:		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_kidctx,
	o CoAP OSCORE_kid.		o CoAP OSCORE_kid.
	The OSCORE Option shows superimposed these four fields using the		Figure 6 shows the OSCORE Option format with those four fields
	format Figure 6, the CoAP OSCORE_kidctxt field includes the size bits		superimposed on it. Note that the CoAP OSCORE_kidctx field includes
	s.		directly the size octet 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 7.		the Rules are described in Figure 7.
	RuleID 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 | 2| 1|bi| 01 |equal |not-sent || |
	|CoAP Type | | |dw| CON |equal |not-sent || |		|CoAP Type | 2| 1|dw| CON |equal |not-sent || |
	|CoAP Type | | |up|[ACK, | | || |		|CoAP Type | 2| 1|up|[ACK, | | || |
	| | | | | RST] |match-map|matching-sent|| T |		| | | | | RST] |match-map|matching-sent|| T |
	|CoAP TKL | | |bi| 0 |equal |not-sent || |		|CoAP TKL | 4| 1|bi| 0 |equal |not-sent || |
	|CoAP Code | | |bi|[0.00,| | || |		|CoAP Code | 8| 1|bi|[0.00,| | || |
	| | | | | ... | | || |		| | | | | ... | | || |
	| | | | | 5.05]|match-map|matching-sent|| CC CCC |		| | | | | 5.05]|match-map|matching-sent|| CC CCC |
	|CoAP MID | | |bi| 0000 |MSB(7 ) |LSB || M-ID|		|CoAP MID |16| 1|bi| 0000 |MSB(7 ) |LSB || M-ID|
	|CoAP Uri-Path| | |dw| path |equal 1 |not-sent || |		|CoAP Uri-Path|var 1|dw| path |equal 1 |not-sent || |
	+-------------+--+--+--+------+---------+-------------++------------+		+-------------+--+--+--+------+---------+-------------++------------+
	Figure 7: CoAP Context to compress header without token		Figure 7: CoAP Context to compress header without token
	The version and Token Length fields are elided. The 26 method and		The version and Token Length fields are elided. The 26 method and
	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
	skipping to change at page 15, line 13 ¶		skipping to change at page 16, line 23 ¶
	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 that is not necessary for proxy
	operation. This, in turn, is the part of the message which can be		operation. This, in turn, is the part of the message which can be
	encrypted until it reaches its end destination. The Outer Message		encrypted until it reaches its end destination. The Outer Message
	acts as a shell matching the format of a regular CoAP message, and		acts as a shell matching the regular CoAP message format and includes
	includes all Options and information needed for proxy operation and		all Options and information needed for proxy operation and caching.
	caching. This decomposition is illustrated in Figure 8.		This decomposition is illustrated in Figure 8.
	CoAP options are sorted into one of 3 classes, each granted a		CoAP options are sorted into one of 3 classes, each granted a
	specific type of protection by the protocol:		specific type of protection by the protocol:
	o Class E: Encrypted options moved to the Inner Plaintext,		o Class E: Encrypted options moved to the Inner Plaintext,
	o Class I: Integrity-protected options included in the AAD for the		o Class I: Integrity-protected options included in the AAD for the
	encryption of the Plaintext but otherwise left untouched in the		encryption of the Plaintext but otherwise left untouched in the
	Outer Message,		Outer Message,
	o Class U: Unprotected options left untouched in the Outer Message.		o Class U: Unprotected options left untouched in the Outer Message.
	Additionally, the OSCORE Option is added as an Outer option,		Additionally, the OSCORE Option is added as an Outer option,
	signalling that the message is OSCORE protected. This option carries		signaling that the message is OSCORE protected. This option carries
	the information necessary to retrieve the Security Context with which		the information necessary to retrieve the Security Context with which
	the message was encrypted so that it may be correctly decrypted at		the message was encrypted to be correctly decrypted at the other end-
	the other end-point.		point.
	Original CoAP Message		Original CoAP Message
	+-+-+---+-------+---------------+		+-+-+---+-------+---------------+
	|v|t|tkl| code | Msg Id. |		|v|t|tkl| code | Msg Id. |
	+-+-+---+-------+---------------+....+		+-+-+---+-------+---------------+....+
	| Token |		| Token |
	+-------------------------------.....+		+-------------------------------.....+
	| Options (IEU) |		| Options (IEU) |
	. .		. .
	. .		. .
	skipping to change at page 16, line 44 ¶		skipping to change at page 17, line 44 ¶
	+------+-------------------+ | Payload |		+------+-------------------+ | Payload |
	| 0xFF | | |		| 0xFF | | |
	+------+ +-------------------+		+------+ +-------------------+
	Figure 8: A CoAP message is split into an OSCORE outer and plaintext		Figure 8: A CoAP message is split into an OSCORE outer and plaintext
	Figure 8 shows the message format for the OSCORE Message and		Figure 8 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
	[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 come,
	if present the original message Payload is preceded by its payload		and, if present, the original message Payload is preceded by its
	marker.		payload marker.
	The Plaintext is now encrypted by an AEAD algorithm which integrity		The Plaintext is now encrypted by an AEAD algorithm which integrity
	protects Security Context parameters and eventually any class I		protects Security Context parameters and, eventually, any class I
	options from the Outer Header. Currently no CoAP options are marked		options from the Outer Header. Currently, no CoAP options are marked
	class I. The resulting Ciphertext becomes the new Payload of the		class I. The resulting Ciphertext becomes the new Payload of the
	OSCORE message, as illustrated in Figure 9.		OSCORE message, as illustrated in Figure 9.
	This Ciphertext is, as defined in RFC 5116, the concatenation of the		As defined in [rfc5116], this Ciphertext is the concatenation of the
	encrypted Plaintext and its authentication tag. Note that Inner		encrypted Plaintext and its authentication tag. Note that Inner
	Compression only affects the Plaintext before encryption, thus we can		Compression only affects the Plaintext before encryption. Thus only
	only aim to reduce this first, variable length component of the		the first variable-length of the Ciphertext can be reduced. The
	Ciphertext. The authentication tag is fixed in length and considered		authentication tag is fixed in length and is considered part of the
	part of the cost of protection.		cost of protection.
	Outer Header		Outer Header
	+-+-+---+--------+---------------+		+-+-+---+--------+---------------+
	|v|t|tkl|new code| Msg Id. |		|v|t|tkl|new code| Msg Id. |
	+-+-+---+--------+---------------+....+		+-+-+---+--------+---------------+....+
	| Token |		| Token |
	+--------------------------------.....+		+--------------------------------.....+
	| Options (IU) |		| Options (IU) |
	. .		. .
	. OSCORE Option .		. OSCORE Option .
	skipping to change at page 17, line 47 ¶		skipping to change at page 18, line 47 ¶
	+----------------------------------+		+----------------------------------+
	Figure 9: OSCORE message		Figure 9: 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 10.		encryption, see Figure 10.
	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 corresponding end-point can only decrypt the
	by the corresponding end-point, this end-point will also have to		Inner part of the message, this end-point will also have to implement
	implement Inner SCHC Compression/Decompression.		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 |
	+-+-+---+--------+---------------+....+ +-------+-----......+		+-+-+---+--------+---------------+....+ +-------+-----......+
	| Token | | Options (E) |		| Token | | Options (E) |
	+--------------------------------.....+ +-------+------.....+		+--------------------------------.....+ +-------+------.....+
	| Options (IU) | | OxFF |		| Options (IU) | | OxFF |
	. . +-------+-----------+		. . +-------+-----------+
	. OSCORE Option . | |		. OSCORE Option . | |
	skipping to change at page 19, line 41 ¶		skipping to change at page 20, line 41 ¶
	Original message:		Original message:
	=================		=================
	0x6145000182ff32332043		0x6145000182ff32332043
	Header:		Header:
	0x6145		0x6145
	01 Ver		01 Ver
	10 ACK		10 ACK
	0001 tkl		0001 tkl
	01000101 Successful Response Code 69 "2.05 Content"		01000101 Successful Response Code 69 "2.05 Content"
	0x0001 = mid		0x0001 = mid
	0x82 = token		0x82 = token
	0xFF Payload marker		0xFF Payload marker
	Payload:		Payload:
	0x32332043		0x32332043
	Original msg length: 10		Original msg length: 10
	Figure 12: CoAP CONTENT Response		Figure 12: CoAP CONTENT Response
	The SCHC Rules for the Inner Compression include all fields that are		The SCHC Rules for the Inner Compression include all fields already
	already present in a regular CoAP message. The methods described in		present in a regular CoAP message. The methods described in
	Section 4 applies to these fields. As an example, see Figure 13.		Section 4 apply to these fields. As an example, see Figure 13.
	RuleID 0		RuleID 0
	+---------------+--+--+-----------+-----------+-----------++------+		+--------------+--+--+--+-----------+----------+----------++------+
	| Field |FP|DI| Target | MO | CDA || Sent |		| Field |FL|FP|DI| Target | MO | CDA || Sent |
	| | | | Value | | ||[bits]|		| | | | | Value | | ||[bits]|
	+---------------+--+--+-----------+-----------+-----------++------+		+--------------+--+--+--+-----------+----------+----------++------+
	|CoAP Code | |up| 1 | equal |not-sent || |		|CoAP Code | 8| 1|up| 1 | equal |not-sent || |
	|CoAP Code | |dw|[69,132] | match-map |match-sent || c |		|CoAP Code | 8| 1|dw|[69,132] | match-map|match-sent|| c |
	|CoAP Uri-Path | |up|temperature| equal |not-sent || |		|CoAP Uri-Path |88| 1|up|temperature| equal |not-sent || |
	|COAP Option-End| |dw| 0xFF | equal |not-sent || |		+--------------+--+--+--+-----------+----------+----------++------+
	+---------------+--+--+-----------+-----------+-----------++------+
	Figure 13: Inner SCHC Rules		Figure 13: Inner SCHC Rules
	Figure 14 shows the Plaintext obtained for our example GET Request		Figure 14 shows the Plaintext obtained for the example GET Request
	and follows the process of Inner Compression and Encryption until we		and follows the process of Inner Compression and Encryption until the
	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 RuleID).		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 and
	also yields a fixed-size tag which cannot be compressed and has to be		yields a fixed-size tag that 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, limiting the amount of compression that can be
	be achieved and plays into the cost of adding security to the		achieved and plays into the cost of adding security to the exchange.
	exchange.
	________________________________________________________		________________________________________________________
	| |		| |
	| OSCORE Plaintext |		| OSCORE Plaintext |
	| |		| |
	| 0x01bb74656d7065726174757265 (13 bytes) |		| 0x01bb74656d7065726174757265 (13 bytes) |
	| |		| |
	| 0x01 Request Code GET |		| 0x01 Request Code GET |
	| |		| |
	| bb74656d7065726174757265 Option 11: URI_PATH |		| bb74656d7065726174757265 Option 11: URI_PATH |
	skipping to change at page 21, line 28 ¶		skipping to change at page 22, line 28 ¶
	|		|
	| Inner SCHC Compression		| Inner SCHC Compression
	|		|
	v		v
	_________________________________		_________________________________
	| |		| |
	| Compressed Plaintext |		| Compressed Plaintext |
	| |		| |
	| 0x00 |		| 0x00 |
	| |		| |
	| RuleID = 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) |
	| tag = 0xc54fe1b434297b62 (8 bytes) |		| tag = 0xc54fe1b434297b62 (8 bytes) |
	| |		| |
	| ciphertext = 0xa2c54fe1b434297b62 (9 bytes) |		| ciphertext = 0xa2c54fe1b434297b62 (9 bytes) |
	|_________________________________________________|		|_________________________________________________|
	Figure 14: Plaintext compression and encryption for GET Request		Figure 14: Plaintext compression and encryption for GET Request
	In Figure 15 the process is repeated for the example CONTENT		In Figure 15, the process is repeated for the example CONTENT
	Response. The residue is 1 bit long. Note that since SCHC adds		Response. The residue is 1 bit long. Note that since SCHC adds
	padding after the payload, this misalignment causes the hexadecimal		padding after the payload, this misalignment causes the hexadecimal
	code from the payload to differ from the original, even though it has		code from the payload to differ from the original, even though it has
	not been compressed.		not been compressed.
	On top of this, the overhead from the tag bytes is incurred as		On top of this, the overhead from the tag bytes is incurred as
	before.		before.
	________________________________________________________		________________________________________________________
	| |		| |
	skipping to change at page 23, line 5 ¶		skipping to change at page 24, line 5 ¶
	_________________________________________________________		_________________________________________________________
	| |		| |
	| 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 15: Plaintext compression and encryption for CONTENT Response		Figure 15: Plaintext compression and encryption for CONTENT Response
	The Outer SCHC Rules (Figure 18) must process the OSCORE Options		The Outer SCHC Rules (Figure 18) must process the OSCORE Options
	fields. In Figure 16 and Figure 17 we show a dump of the OSCORE		fields. The Figure 16 and Figure 17 show a dump of the OSCORE
	Messages generated from our example messages once they have been		Messages generated from the 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)
	Header:		Header:
	skipping to change at page 24, line 31 ¶		skipping to change at page 25, line 31 ¶
	0xd008 (2 bytes)		0xd008 (2 bytes)
	Option 21: OBJECT_SECURITY		Option 21: OBJECT_SECURITY
	Value = b''		Value = b''
	0xFF Payload marker		0xFF Payload marker
	Payload:		Payload:
	0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)		0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)
	Figure 17: Protected and Inner SCHC Compressed CONTENT Response		Figure 17: Protected and Inner SCHC Compressed CONTENT Response
	For the flag bits, a number of compression methods has been shown to		For the flag bits, some SCHC compression methods are useful,
	be useful depending on the application. The simplest alternative is		depending on the application. The simplest alternative is to provide
	to provide a fixed value for the flags, combining MO equal and CDA		a fixed value for the flags, combining MO equal and CDA not- sent.
	not- sent. This saves most bits but could prevent flexibility.		This saves most bits but could prevent flexibility. Otherwise,
	Otherwise, match-mapping could be used to choose from an interested		match-mapping could be used to choose from an interesting number of
	number of configurations to the exchange. Otherwise, MSB could be		configurations for the exchange.
	used to mask off the 3 hard-coded most significant bits.		Otherwise, MSB could be used to mask off the 3 hard-coded most
			significant bits.
	Note that fixing a flag bit will limit the choice of CoAP Options		Note that fixing a flag bit will limit CoAP Options choice that can
	that can be used in the exchange, since their values are dependent on		be used in the exchange since their values are dependent on certain
	certain options.		options.
	The piv field lends itself to having a number of bits masked off with		The piv field lends itself to having some bits masked off with MO MSB
	MO MSB and CDA LSB. This could be useful in applications where the		and CDA LSB. This could be useful in applications where the message
	message frequency is low such as that found in LPWAN technologies.		frequency is low such as LPWAN technologies. Note that compressing
	Note that compressing the sequence numbers effectively reduces the		the sequence numbers effectively reduces the maximum number of
	maximum amount of sequence numbers that can be used in an exchange.		sequence numbers used in an exchange. Once this amount is exceeded,
	Once this amount is exceeded, the OSCORE keys need to be re-		the OSCORE keys need to be re-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
	off, or have the whole field be fixed with MO equal and CDA not-sent.		or have the whole field be fixed with MO equal and CDA not-sent. The
	The same holds for the kid field.		same holds for the kid field.
	Figure 18 shows a possible set of Outer Rules to compress the Outer		Figure 18 shows a possible set of Outer Rules to compress the Outer
	Header.		Header.
	RuleID 0		RuleID 0
	+-------------------+--+--+--------------+--------+---------++------+		+------------------+--+--+--+--------------+-------+--------++------+
	| Field |FP|DI| Target | MO | CDA || Sent |		| Field |FL|FP|DI| Target | MO | CDA || Sent |
	| | | | Value | | ||[bits]|		| | | | | Value | | ||[bits]|
	+-------------------+--+--+--------------+--------+---------++------+		+------------------+--+--+--+--------------+-------+--------++------+
	|CoAP version | |bi| 01 |equal |not-sent || |		|CoAP version | 2| 1|bi| 01 |equal |not-sent|| |
	|CoAP Type | |up| 0 |equal |not-sent || |		|CoAP Type | 2| 1|up| 0 |equal |not-sent|| |
	|CoAP Type | |dw| 2 |equal |not-sent || |		|CoAP Type | 2| 1|dw| 2 |equal |not-sent|| |
	|CoAP TKL | |bi| 1 |equal |not-sent || |		|CoAP TKL | 4| 1|bi| 1 |equal |not-sent|| |
	|CoAP Code | |up| 2 |equal |not-sent || |		|CoAP Code | 8| 1|up| 2 |equal |not-sent|| |
	|CoAP Code | |dw| 68 |equal |not-sent || |		|CoAP Code | 8| 1|dw| 68 |equal |not-sent|| |
	|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM |		|CoAP MID |16| 1|bi| 0000 |MSB(12)|LSB ||MMMM |
	|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT |		|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT |
	|CoAP OSCORE_flags | |up| 0x09 |equal |not-sent || |		|CoAP OSCORE_flags | 8| 1|up| 0x09 |equal |not-sent|| |
	|CoAP OSCORE_piv | |up| 0x00 |MSB(4) |LSB ||PPPP |		|CoAP OSCORE_piv |var 1|up| 0x00 |MSB(4) |LSB ||PPPP |
	|COAP OSCORE_kid | |up|0x636c69656e70|MSB(52) |LSB ||KKKK |		|COAP OSCORE_kid |var 1|up|0x636c69656e70|MSB(52)|LSB ||KKKK |
	|COAP OSCORE_kidctxt| |bi| b'' |equal |not-sent || |		|COAP OSCORE_kidctx|var 1|bi| b'' |equal |not-sent|| |
	|CoAP OSCORE_flags | |dw| b'' |equal |not-sent || |		|CoAP OSCORE_flags | 8| 1|dw| b'' |equal |not-sent|| |
	|CoAP OSCORE_piv | |dw| b'' |equal |not-sent || |		|CoAP OSCORE_piv |var 1|dw| b'' |equal |not-sent|| |
	|CoAP OSCORE_kid | |dw| b'' |equal |not-sent || |		|CoAP OSCORE_kid |var 1|dw| b'' |equal |not-sent|| |
	|COAP Option-End | |dw| 0xFF |equal |not-sent || |		+------------------+--+--+--+--------------+-------+--------++------+
	+-------------------+--+--+--------------+--------+---------++------+
	Figure 18: Outer SCHC Rules		Figure 18: 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 19 and		Response. The resulting messages are shown in Figure 19 and
	Figure 20.		Figure 20.
	Compressed message:		Compressed message:
	==================		==================
	0x001489458a9fc3686852f6c4 (12 bytes)		0x001489458a9fc3686852f6c4 (12 bytes)
	skipping to change at page 26, line 40 ¶		skipping to change at page 27, line 40 ¶
	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 20: SCHC-OSCORE Compressed CONTENT Response		Figure 20: SCHC-OSCORE Compressed CONTENT Response
	For contrast, we compare these results with what would be obtained by		In contrast, comparing 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 is done by compressing the CoAP messages according to the
	the SCHC Rules in Figure 21.		SCHC Rules in Figure 21.
	RuleID 1		RuleID 1
	+---------------+--+--+-----------+---------+-----------++--------+		+---------------+--+--+--+-----------+---------+-----------++-------+
	| Field |FP|DI| Target | MO | CDA || Sent |		| Field |FL|FP|DI| Target | MO | CDA || Sent |
	| | | | Value | | || [bits] |		| | | | | Value | | || [bits]|
	+---------------+--+--+-----------+---------+-----------++--------+		+---------------+--+--+--+-----------+---------+-----------++-------+
	|CoAP version | |bi| 01 |equal |not-sent || |		|CoAP version | 2| 1|bi| 01 |equal |not-sent || |
	|CoAP Type | |up| 0 |equal |not-sent || |		|CoAP Type | 2| 1|up| 0 |equal |not-sent || |
	|CoAP Type | |dw| 2 |equal |not-sent || |		|CoAP Type | 2| 1|dw| 2 |equal |not-sent || |
	|CoAP TKL | |bi| 1 |equal |not-sent || |		|CoAP TKL | 4| 1|bi| 1 |equal |not-sent || |
	|CoAP Code | |up| 2 |equal |not-sent || |		|CoAP Code | 8| 1|up| 2 |equal |not-sent || |
	|CoAP Code | |dw| [69,132] |match-map|map-sent ||C |		|CoAP Code | 8| 1|dw| [69,132] |match-map|map-sent ||C |
	|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM |		|CoAP MID |16| 1|bi| 0000 |MSB(12) |LSB ||MMMM |
	|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT |		|CoAP Token |tkl 1|bi| 0x80 |MSB(5) |LSB ||TTT |
	|CoAP Uri-Path | |up|temperature|equal |not-sent || |		|CoAP Uri-Path |88| 1|up|temperature|equal |not-sent || |
	|COAP Option-End| |dw| 0xFF |equal |not-sent || |		+---------------+--+--+--+-----------+---------+-----------++-------+
	+---------------+--+--+-----------+---------+-----------++--------+
	Figure 21: SCHC-CoAP Rules (No OSCORE)		Figure 21: SCHC-CoAP Rules (No OSCORE)
	This yields the results in Figure 22 for the Request, and Figure 23		This yields the results in Figure 22 for the Request, and Figure 23
	for the Response.		for the Response.
	Compressed message:		Compressed message:
	==================		==================
	0x0114		0x0114
	0x01 = RuleID		0x01 = RuleID
	skipping to change at page 28, line 21 ¶		skipping to change at page 29, line 21 ¶
	0b00001010 (1 byte)		0b00001010 (1 byte)
	Payload		Payload
	0x32332043		0x32332043
	Compressed msg length: 6		Compressed msg length: 6
	Figure 23: CoAP CONTENT Compressed without OSCORE		Figure 23: 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.
	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
	When applied to LPWAN, the Security Considerations of SCHC header		When applied to LPWAN, the Security Considerations of SCHC header
	compression [rfc8724] are valid for SCHC CoAP header compression.		compression [rfc8724] are valid for SCHC CoAP header compression.
	When CoAP uses OSCORE, the security considerations defined in		When CoAP uses OSCORE, the security considerations defined in
	skipping to change at page 29, line 10 ¶		skipping to change at page 30, line 10 ¶
	SCHC compression returns variable-length Residues for some CoAP		SCHC compression returns variable-length Residues for some CoAP
	fields. In the compressed header, the length sent is not the		fields. In the compressed header, the length sent is not the
	original header field length but the length of the Residue. So if a		original header field length but the length of the Residue. So if a
	corrupted packet comes to the decompressor with a longer or shorter		corrupted packet comes to the decompressor with a longer or shorter
	length than the one in the original header, SCHC decompression will		length than the one in the original header, SCHC decompression will
	detect an error and drops the packet.		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 [rfc8724]. The size of the Initialisation Vector (IV)		described in [rfc8724]. The size of the Initialisation Vector (IV)
	residue must be considered carefully. A residue size obtained with		residue must be considered carefully. A residue size obtained with
	LSB CDA over the IV has an impact on the compression efficiency and		LSB CDA over the IV impacts on the compression efficiency and the
	the frequency the device will renew its key. This operation requires		frequency the device will renew its key. This operation requires
	several exchanges and is energy-consuming.		several exchanges and is energy-consuming.
	SCHC header and compression Rules MUST remain tightly coupled.		SCHC header and compression Rules MUST remain tightly coupled.
	Otherwise, an encrypted residue may be decompressed differently by		Otherwise, an encrypted residue may be decompressed differently by
	the receiver. To avoid this situation, if the Rule is modified in		the receiver. To avoid this situation, if the Rule is modified in
	one location, the OSCORE keys MUST be re-established.		one location, the OSCORE keys MUST be re-established.
	10. Acknowledgements		10. Acknowledgements
	The authors would like to thank (in alphabetic order): Christian		The authors would like to thank (in alphabetic order): Christian
	skipping to change at page 29, line 33 ¶		skipping to change at page 30, line 33 ¶
	Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran		Fossati, Klaus Hartke, Francesca Palombini, Alexander Pelov and Goran
	Selander.		Selander.
	11. Normative References		11. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		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>.
			[rfc5116] McGrew, D., "An Interface and Algorithms for Authenticated
			Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
			<https://www.rfc-editor.org/info/rfc5116>.
	[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
	Application Protocol (CoAP)", RFC 7641,		Application Protocol (CoAP)", RFC 7641,
	DOI 10.17487/RFC7641, September 2015,		DOI 10.17487/RFC7641, September 2015,
	<https://www.rfc-editor.org/info/rfc7641>.		<https://www.rfc-editor.org/info/rfc7641>.
End of changes. 123 change blocks.
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