< draft-ietf-lpwan-coap-static-context-hc-03.txt		draft-ietf-lpwan-coap-static-context-hc-04.txt >
	lpwan Working Group A. Minaburo		lpwan Working Group A. Minaburo
	Internet-Draft Acklio		Internet-Draft Acklio
	Intended status: Informational L. Toutain		Intended status: Informational L. Toutain
	Expires: September 5, 2018 Institut MINES TELECOM; IMT Atlantique		Expires: January 3, 2019 Institut MINES TELECOM; IMT Atlantique
	March 04, 2018		R. Andreasen
			Universidad de Buenos Aires
			July 02, 2018
	LPWAN Static Context Header Compression (SCHC) for CoAP		LPWAN Static Context Header Compression (SCHC) for CoAP
	draft-ietf-lpwan-coap-static-context-hc-03		draft-ietf-lpwan-coap-static-context-hc-04
	Abstract		Abstract
	This draft defines the way SCHC header compression can be applied to		This draft defines the way SCHC header compression can be applied to
	CoAP headers. CoAP header structure differs from IPv6 and UDP		CoAP headers. CoAP header structure differs from IPv6 and UDP
	protocols since the CoAP Header is flexible header with a variable		protocols since the CoAP
	number of options themself of a variable length. Another important		use a flexible header with a variable number of options themself of a
	difference is the asymmetry in the header information used for		variable length. Another important difference is the asymmetry in
	request and response messages. This draft takes into account the		the header format used in request and response messages. Most of the
	fact that a thing can play the role of a CoAP client, a CoAP client		compression mechanisms have been introduced in
	or both roles.		[I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how
			to use the SCHC compression for CoAP.
	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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	This Internet-Draft will expire on September 5, 2018.		This Internet-Draft will expire on January 3, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2018 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. CoAP Compressing . . . . . . . . . . . . . . . . . . . . . . 3		2. SCHC Compression Process . . . . . . . . . . . . . . . . . . 3
	3. Compression of CoAP header fields . . . . . . . . . . . . . . 4		3. CoAP Compression with SCHC . . . . . . . . . . . . . . . . . 4
	3.1. CoAP version field (2 bits) . . . . . . . . . . . . . . . 4		4. Compression of CoAP header fields . . . . . . . . . . . . . . 6
	3.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 5		4.1. CoAP version field . . . . . . . . . . . . . . . . . . . 6
	3.3. CoAP token length field . . . . . . . . . . . . . . . . . 5		4.2. CoAP type field . . . . . . . . . . . . . . . . . . . . . 6
	3.4. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6		4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 6
	3.5. CoAP Message ID field . . . . . . . . . . . . . . . . . . 8		4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . . 6
	3.6. CoAP Token field . . . . . . . . . . . . . . . . . . . . 9		4.5. CoAP Token fields . . . . . . . . . . . . . . . . . . . . 7
	4. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 9		5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . . 7
	4.1. CoAP option Content-format field. . . . . . . . . . . . . 9		5.1. CoAP Content and Accept options. . . . . . . . . . . . . 7
	4.2. CoAP option Accept field . . . . . . . . . . . . . . . . 10		5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri-
	4.3. CoAP option Max-Age field, CoAP option Uri-Host and Uri-		Port fields . . . . . . . . . . . . . . . . . . . . . . . 7
	Port fields . . . . . . . . . . . . . . . . . . . . . . . 11		5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8
	5. CoAP option Uri-Path and Uri-Query fields . . . . . . . . . . 11		5.3.1. Variable length Uri-Path and Uri-Query . . . . . . . 8
	5.1. CoAP option Proxy-URI and Proxy-Scheme fields . . . . . . 12		5.3.2. Variable number of path or query elements . . . . . . 9
	5.2. CoAP option ETag, If-Match, If-None-Match, Location-Path		5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
	and Location-Query fields . . . . . . . . . . . . . . . . 13		fields . . . . . . . . . . . . . . . . . . . . . . . . . 9
	6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 13		5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path
	6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 13		and Location-Query fields . . . . . . . . . . . . . . . . 9
	6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 13		6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 9
	6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 13		6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . . 9
	7. Protocol analysis . . . . . . . . . . . . . . . . . . . . . . 13		6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . . 10
	8. Examples of CoAP header compression . . . . . . . . . . . . . 14		6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . . 10
	8.1. Mandatory header with CON message . . . . . . . . . . . . 14		6.4. Time Scale . . . . . . . . . . . . . . . . . . . . . . . 10
	8.2. Complete exchange . . . . . . . . . . . . . . . . . . . . 16		6.5. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 10
	9. Normative References . . . . . . . . . . . . . . . . . . . . 17		7. Examples of CoAP header compression . . . . . . . . . . . . . 12
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18		7.1. Mandatory header with CON message . . . . . . . . . . . . 12
			7.2. Complete exchange . . . . . . . . . . . . . . . . . . . . 13
			7.3. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14
			7.4. Example OSCORE Compression . . . . . . . . . . . . . . . 17
			8. Normative References . . . . . . . . . . . . . . . . . . . . 22
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
	1. Introduction		1. Introduction
	CoAP [rfc7252] is an implementation of the REST architecture for		CoAP [rfc7252] is an implementation of the REST architecture for
	constrained devices. A Gateway between CoAP and HTTP can be easily		constrained devices. Nevertheless, if limited, the size of a CoAP
	built since both protocols uses the same address space (URL), caching		header may be too large for LPWAN constraints and some compression
	mechanisms and methods.		may be needed to reduce the header size.
	Nevertheless, if limited, the size of a CoAP header may be too large
	for LPWAN constraints and some compression may be needed to reduce
	the header size.
	[I-D.toutain-lpwan-ipv6-static-context-hc] defines a header		[I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
	compression mechanism for LPWAN network based on a static context.		mechanism for LPWAN network based on a static context. The context
	The context is said static since the field description composing the		is said static since the field description composing the Rules and
	Rules and the context are not learned during the packet exchanges but		the context are not learned during the packet exchanges but are
	are previously defined. The context(s) is(are) known by both ends		previously defined. The context(s) is(are) known by both ends before
	before transmission.		transmission.
	A context is composed of a set of rules that are referenced by Rule		A context is composed of a set of rules that are referenced by Rule
	IDs (identifiers). A rule contains an ordered list of the fields		IDs (identifiers). A rule contains an ordered list of the fields
	descriptions containing a field ID (FID) and its position when		descriptions containing a field ID (FID), its length (FL) and its
	repeated, a direction indicator (DI) (upstream, downstream and		position (FP), a direction indicator (DI) (upstream, downstream and
	bidirectional) and some associated Target Values (TV) which are		bidirectional) and some associated Target Values (TV). Target Value
	expected in the message header. A Matching Operator (MO) is		indicates the value that can be expected. TV can also be a list of
	associated to each header field description. The rule is selected if		values. A Matching Operator (MO) is associated to each header field
	all the MOs fit the TVs. In that case, a Compression/Decompression		description. The rule is selected if all the MOs fit the TVs for all
	Action (CDA) associated to each field defines the link between the		fields. In that case, a Compression/Decompression Action (CDA)
	compressed and decompressed value for each of the header fields.		associated to each field defines the link between the compressed and
			decompressed value for each of the header fields. Compression
			results mainly in 4 actions: send the field value, send nothing, send
			less significant bits of a field, send an index. Values sent are
			called Compression Residues and follows the rule ID.
	This document describes how the rules can be applied to CoAP flows.		2. SCHC Compression Process
			The SCHC Compression rules can be applied to CoAP flows. 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
	above layers or independantly.		above layers (IPv6/UDP) or independently. The SCHC adaptation layers
			as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used
			as as shown in the Figure 1.
	2. CoAP Compressing		^ +------------+ ^ +------------+ ^ +------------+
			| | CoAP | | | CoAP | inner | | CoAP |
			| +------------+ v +------------+ x | OSCORE |
			| | UDP | | DTLS | outer | +------------+
			| +------------+ +------------+ | | UDP |
			| | IPv6 | | UDP | | +------------+
			v +------------+ +------------+ | | IPv6 |
			| IPv6 | v +------------+
			+------------+
			Figure 1: rule scope for CoAP
			Figure 1 shows some examples for CoAP architecture and the SCHC
			rule's scope. A rule can covers all headers from IPv6 to CoAP, SCHC
			C/D is done in the device and at the LPWAN boundary. If an end-to-
			end encryption mechanisms is used between the device and the
			application. CoAP must be compressed independently of the other
			layers. The rule ID and the compression residue are encrypted using
			a mechanism such as DTLS. Only the other end can decipher the
			information.
			Layers below may also be compressed using other SCHC rules (this is
			out of the scope of this document). OSCORE
			[I-D.ietf-core-object-security] can also define 2 rules to compress
			the CoAP message. A first rule focuses on the inner header and is
			end to end, a second rule may compress the outer header and the layer
			above. SCHC C/D for inner header is done by both ends, SCHC C/D for
			outer header and other headers is done between the device and the
			LPWAN boundary.
			3. CoAP Compression with SCHC
	CoAP differs from IPv6 and UDP protocols on the following aspects:		CoAP differs from IPv6 and UDP protocols on the following aspects:
	o IPv6 and UDP are symmetrical protocols. The same fields are found		o IPv6 and UDP are symmetrical protocols. The same fields are found
	in the request and in the response, only the location in the		in the request and in the response, only the location in the
	header may vary (e.g. source and destination fields). A CoAP		header may vary (e.g. source and destination fields). A CoAP
	request is different from a response. For example, the URI-path		request is different from a response. For example, the URI-path
	option is mandatory in the request and is not found in the		option is mandatory in the request and is not found in the
	response, a request may contain an Accept option and the response		response, a request may contain an Accept option and the response
	a Content-format option.		a Content option.
	Even when a field is "symmetric" (i.e. found in both directions)		[I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a
	the values carried are different. For instance the Type field		message direction (DI) when processing the rule which allows the
	will contain a CON value in the request and a ACK or RST value in		description of message header format in both directions.
	the response. Exploiting the asymmetry in compression will allow
	to send no bit in the compressed request and a single bit in the		o Even when a field is "symmetric" (i.e. found in both directions)
	answer. Same behavior can be applied to the CoAP Code field (O.OX		the values carried in each direction are different. Combined with
	code are present in the request and Y.ZZ in the answer).		a matching list in the TV, this will allow to reduce the range of
			expected values in a particular direction and therefore reduce the
			size of a compression residue. For instance, if a client sends
			only CON request, the type can be elided by compression and the
			answer may use one bit to carry either the ACK or RST type. Same
			behavior can be applied to the CoAP Code field (0.0X code are
			present in the request and Y.ZZ in the answer). The direction
			allows to split in two parts the possible values for each
			direction.
			o In IPv6 and UDP header fields have a fixed size. In CoAP, Token
			size may vary from 0 to 8 bytes, length is given by a field in the
			header. More systematically, the CoAP options are described using
			the Type-Length-Value.
			[I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to
			define a function for the Field Length in the Field Description.
			o In CoAP headers, a field can be duplicated several times, for
			instances, elements of an URI (path or queries). The position
			defined in a rule, associated to a Field ID, can be used to
			identify the proper element.
			[I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field id to
			appears several times in the rule, the Field Position (FP) removes
			ambiguities for the matching operation.
			o Field size defined in the CoAP protocol can be to large regarding
			LPWAN traffic constraints. This is particularly true for the
			message ID field or Token field. The use of MSB MO can be used to
			reduce the information carried on LPWANs.
	o CoAP also obeys to the client/server paradigm and the compression		o CoAP also obeys to the client/server paradigm and the compression
	rate can be different if the request is issued from an LPWAN node		rate can be different if the request is issued from an LPWAN node
	or from an non LPWAN device. For instance a Thing (ES) aware of		or from an non LPWAN device. For instance a Device (Dev) aware of
	LPWAN constraints can generate a 1 byte token, but a regular CoAP		LPWAN constraints can generate a 1 byte token, but a regular CoAP
	client will certainly send a larger token to the Thing. SCHC		client will certainly send a larger token to the Thing. SCHC
	compression will not modify the values to offer a better		compression will not modify the values to offer a better
	compression rate. Nevertheless a proxy placed before the		compression rate. Nevertheless a proxy placed before the
	compressor may change some field values to offer a better		compressor may change some field values to offer a better
	compression rate and maintain the necessary context for		compression rate and maintain the necessary context for
	interoperability with existing CoAP implementations.		interoperability with existing CoAP implementations.
	o In IPv6 and UDP header fields have a fixed size. In CoAP, Token		4. Compression of CoAP header fields
	size may vary from 0 to 8 bytes, length is given by a field in the
	header. More systematically, the CoAP options are described using
	the Type-Length-Value. When applying SCHC header compression.
	By sending compressed field information following the rule order,
	SCHC offers a serialization/deserialization mechanism. Since a
	field exists to indicate the token length there is no ambiguity.
	For options, the rule indicates also the expected options found
	the int CoAP header. Therefore only the length is needed to
	recognize an option. The length will be sent using the same CoAP
	encoding (size less than 12 are directly sent, higher values use
	the escape mechanisms defined by [rfc7252]). Delta Type is
	omitted, the value will be recovered by the decompressor. This
	reduces the option length of 4, 12 or 20 bits regarding the
	original size of the delta type encoding in the option.
	o In CoAP headers a field can be duplicated several times, for
	instances, elements of an URI (path or queries) or accepted
	formats. The position defined in a rule, associated to a Field
	ID, can be used to identify the proper element.
	3. Compression of CoAP header fields
	This section discusses of the compression of the different CoAP		This section discusses of the compression of the different CoAP
	header fields. These are just examples. The compression should take		header fields.
	into account the nature of the traffic and not only the field values.
	Next chapter will define some compression rules for some common
	exchanges.
	3.1. CoAP version field (2 bits)
	This field is bidirectional and can be elided during the SCHC
	compression, since it always contains the same value. It appears
	only in first position.
	FID FL FP DI Value MO CDA Sent
	ver 2 1 bi 1 equal not-sent
	3.2. CoAP type field
	This field can be managed bidirectionally or unidirectionally.Several
	strategies can be applied to this field regarding the values used:
	o If the ES is a client or a Server and non confirmable message are
	used, the transmission of the Type field can be avoided:
	* Pos is always 1,
	* DI can either be "uplink" if the ES is a CoAP client or
	"downlink" if the ES is a CoAP server, or "bidirectional"
	* TV is set to the value,
	* MO is set to "equal"
	* CDA is set to "not-sent".
	FID FL FP DI Target Value MO CDA Sent
	type 2 1 bi NON equal not-sent
	o If the ES is either a client or a Server and confirmable message
	are used, the DI can be used to elide the type on the request and
	compress it to 1 bit on the response. The example above shows the
	rule for a ES acting as a client, directions need to be reversed
	for a ES acting as a server.
	FID FL FP DI TV MO CDA Sent
	type 2 1 up CON equal not-sent
	type 2 1 dw [ACK,RST] match-mapping mapping-sent [1]
	o Otherwise if the ES is acting simultaneously as a client and a
	server and the rule handle these two traffics, Type field must be
	sent uncompressed.
	FID FL FP DI TV MO CDA Sent
	type 2 1 bi ignore send-value [2]
	3.3. CoAP token length field
	This field is bi-directional.
	Several strategies can be applied to this field regarding the values:
	o no token or a wellknown length, the transmission can be avoided.
	A special care must be taken, if CON messages are acknowledged
	with an empty ACK message. In that case the token is not always
	present.
	FID FL FP DI TV MO CDA Sent
	TKL 4 1 bi value ignore send-value [4]
	o If the length is changing from one message to an other, the Token
	Length field must be sent. If the Token length can be limited,
	then only the least significant bits have to be sent. The example
	below allows values between 0 and 3.
	FID FL FP DI TV MO CDA Sent
	TKL 4 1 bi 0x0 MSB(2) LSB(2) [2]
	o otherwise the field value has to be sent.
	FID FL FP DI TV MO CDA Sent
	TKL 4 1 bi ignore value-sent [4]
	3.4. CoAP code field
	This field is bidirectional, but compression can be enhanced using
	DI.
	The CoAP Code field defines a tricky way to ensure compatibility with
	HTTP values. Nevertheless only 21 values are defined by [rfc7252]
	compared to the 255 possible values.
	+------+------------------------------+-----------+
	| Code | Description | Mapping |
	+------+------------------------------+-----------+
	| 0.00 | | 0x00 |
	| 0.01 | GET | 0x01 |
	| 0.02 | POST | 0x02 |
	| 0.03 | PUT | 0x03 |
	| 0.04 | DELETE | 0x04 |
	| 0.05 | FETCH | 0x05 |
	| 0.06 | PATCH | 0x06 |
	| 0.07 | iPATCH | 0x07 |
	| 2.01 | Created | 0x08 |
	| 2.02 | Deleted | 0x09 |
	| 2.03 | Valid | 0x0A |
	| 2.04 | Changed | 0x0B |
	| 2.05 | Content | 0x0C |
	| 4.00 | Bad Request | 0x0D |
	| 4.01 | Unauthorized | 0x0E |
	| 4.02 | Bad Option | 0x0F |
	| 4.03 | Forbidden | 0x10 |
	| 4.04 | Not Found | 0x11 |
	| 4.05 | Method Not Allowed | 0x12 |
	| 4.06 | Not Acceptable | 0x13 |
	| 4.12 | Precondition Failed | 0x14 |
	| 4.13 | Request Entity Too Large | 0x15 |
	| 4.15 | Unsupported Content-Format | 0x16 |
	| 5.00 | Internal Server Error | 0x17 |
	| 5.01 | Not Implemented | 0x18 |
	| 5.02 | Bad Gateway | 0x19 |
	| 5.03 | Service Unavailable | 0x1A |
	| 5.04 | Gateway Timeout | 0x1B |
	| 5.05 | Proxying Not Supported | 0x1C |
	+------+------------------------------+-----------+
	Figure 1: Example of CoAP code mapping
	Figure 1 gives a possible mapping, it can be changed to add new codes
	or reduced if some values are never used by both ends. It could
	efficiently be coded on 5 bits.
	Even if the number of code can be increase with other RFC,
	implementations may use a limited number of values, which can help to
	reduce the number of bits sent on the LPWAN.
	The number of code may vary over time, some new codes may be
	introduced or some applications use a limited number of values.
	The client and the server do not use the same values. This asymmetry
	can be exploited to reduce the size sent on the LPWAN.
	The field can be treated differently in upstream than in downstream.
	If the Thing is a client an entry can be set on the uplink message
	with a code matching for 0.0X values and another for downlink values
	for Y.ZZ codes. It is the opposite if the thing is a server.
	If the ES always sends or receives requests with the same method, the
	Code field can be elided. The entry below shows a rule for a client
	sending only GET request.
	FID FL FP DI TV MO CDA Sent
	code 8 1 up GET equal not-sent
	If the client may send different methods, a matching-list can be
	applied. For table Figure 1, 3 bits are necessary, but it could be
	less if fewer methods are used. Example below gives an example where
	the ES is a server and receives only GET and POST requests.
	FID FL FP DI Target Value MO CDA Sent
	code 8 1 dw [0.01, 0.02] match-mapping mapping-sent [1]
	The same approach can be applied to responses.
	3.5. CoAP Message ID field
	This field is bidirectional.
	Message ID is used for two purposes:
	o To acknowledge a CON message with an ACK.
	o To avoid duplicate messages.
	In LPWAN, since a message can be received by several radio gateway,
	some LPWAN technologies include a sequence number in L2 to avoid
	duplicate frames. Therefore if the message does not need to be
	acknowledged (NON or RST message), the Message ID field can be
	avoided.
	FID FL FP DI TV MO CDA Sent
	Mid 8 1 bi ignore not-sent
	The decompressor must generate a value.
	[[Note; check id this field is not used by OSCOAP .]]
	To optimize information sent on the LPWAN, shorter values may be used
	during the exchange, but Message ID values generated a common CoAP
	implementation will not take into account this limitation. Before
	the compression, a proxy may be needed to reduce the size.
	FID FL FP DI TV MO CDA Sent
	Mid 8 1 bi 0x0000 MSB(12) LSB(4) [4]
	Otherwise if no compression is possible, the field has to be sent
	FID FL FP DI TV MO CDA Sent
	Mid 8 1 bi ignore value-sent [8]
	3.6. CoAP Token field
	This field is bi-directional.
	Token is used to identify transactions and varies from one
	transaction to another. Therefore, it is usually necessary to send
	the value of the token field on the LPWAN network. The optimization
	will occur by using small values.
	Common CoAP implementations may generate large tokens, even if
	shorter tokens could be used regarding the LPWAN characteristics. A
	proxy may be needed to reduce the size of the token before
	compression.
	The size of the compress token sent is known by a combination of the
	Token Length field and the rule entry. For instance, with the entry
	below:
	FID FL FP DI TV MO CDA Sent
	tkl 4 1 bi 2 equal not-sent
	token 8 1 bi 0x00 MSB(12) LSB(4) [4]
	The uncompressed token is 2 bytes long, but the compressed size will		4.1. CoAP version field
	be 4 bits.
	4. CoAP options		This field is bidirectional and must be elided during the SCHC
			compression, since it always contains the same value. In the future,
			if new version of CoAP are defined, new rules ID will be defined
			avoiding ambiguities between versions.
	4.1. CoAP option Content-format field.		4.2. CoAP type field
	This field is unidirectional and must not be set to bidirectional in		[rfc7252] defines 4 types of messages: CON, NON, ACK and RST. The
	a rule entry. It is used only by the server to inform the client		latter two ones are a response of the two first ones. If the device
	about of the payload type and is never found in client requests.		plays a specific role, a rule can exploit these property with the
			mapping list: [CON, NON] for one direction and [ACK, RST] for the
			other direction. Compression residue is reduced to 1 bit.
	If single value is expected by the client, the TV contains that value		The field must be elided if for instance a client is sending only NON
	and MO is set to "equal" and the CDF is set to "not-sent". The		or CON messages.
	examples below describe the rules for an ES acting as a server.
	FID FL FP DI TV MO CDA Sent		In any case, a rule must be defined to carry RST to a client.
	content 16 1 up value equal not-sent
	If several possible value are expected by the client, a matching-list		4.3. CoAP code field
	can be used.
	FID FL FP DI TV MO CDA Sent		The compression of the CoAP code field follows the same principle as
	content 16 1 up [50, 41] match-mapping mapping-sent [1]		for the CoAP type field. If the device plays a specific role, the
			set of code values can be split in two parts, the request codes with
			the 0 class and the response values.
	Otherwise the value can be sent.The value-sent CDF in the compressor		If the device implement only a CoAP client, the request code can be
	do not send the option type and the decompressor reconstruct it		reduced to the set of request the client is able to process.
	regarding the position in the rule.
	FID FL FP DI TV MO CDA Sent		All the response codes should be compressed with a SCHC rule.
	content 16 1 up ignore value-sent [0-16]
	4.2. CoAP option Accept field		4.4. CoAP Message ID field
	This field is unidirectional and must not be set to bidirectional in		This field is bidirectional and is used to manage acknowledgments.
	a rule entry. It is used only by the client to inform of the		Server memorizes the value for a EXCHANGE_LIFETIME period (by default
	possible payload type and is never found in server response.		247 seconds) for CON messages and a NON_LIFETIME period (by default
			145 seconds) for NON messages. During that period, a server
			receiving the same Message ID value will process the message has a
			retransmission. After this period, it will be processed as a new
			messages.
	The number of accept options is not limited and can vary regarding		In case the Device is a client, the size of the message ID field may
	the usage. To be selected a rule must contain the exact number about		the too large regarding the number of messages sent. Client may use
	accept options with their positions. Since the order in which the		only small message ID values, for instance 4 bit long. Therefore a
	Accept value are sent, the position order can be modified. The rule		MSB can be used to limit the size of the compression residue.
	below
	FID FL FP DI TV MO CDA Sent		In case the Device is a server, client may be located outside of the
	accept 16 1 up 41 egal not-sent		LPWAN area and view the device as a regular device connected to the
	accept 16 2 up 50 egal not-sent		internet. The client will generate Message ID using the 16 bits
			space offered by this field. A CoAP proxy can be set before the SCHC
			C/D to reduce the value of the Message ID, to allow its compression
			with the MSB matching operator and LSB CDA.
	will be selected only if two accept options are in the CoAP header if		4.5. CoAP Token fields
	this order.
	The rule below:		Token is defined through two CoAP fields, Token Length in the
			mandatory header and Token Value directly following the mandatory
			CoAP header.
	FID FL FP DI TV MO CDA Sent		Token Length is processed as a tradition protocol field. If the
	accept 16 0 up 41 egal not-sent		value remains the same during all the transaction, the size can be
	accept 16 0 up 50 egal not-sent		stored in the context and elided during the transmission. Otherwise
			it will have to the send as a compression residue.
	will accept a-only CoAP messages with 2 accept options, but the order		Token Value size should not be defined directly in the rule in the
	will not influence the rule selection. The decompression will		Field Length (FL). Instead a specific function designed as "TKL"
	reconstruct the header regarding the rule order.		must be used. This function informs the SCHC C/D that the length of
			this field has to be read from the Token Length field.
	Otherwise a matching-list can be applied to the different values, in		5. CoAP options
	that case the order is important to recover the appropriate value and
	the position must be clearly indicate.
	FID FL FP DI TV MO CDA Sent		5.1. CoAP Content and Accept options.
	accept 16 1 up [50,41] match-mapping mapping-sent [1]
	accept 16 2 up [50,61] match-mapping mapping-sent [1]
	accept 16 3 up [61,71] match-mapping mapping-sent [1]
	Finally, the option can be explicitly sent.		These field are both unidirectional and must not be set to
			bidirectional in a rule entry.
	FID FL FP DI TV MO CDA Sent		If single value is expected by the client, it can be stored in the TV
	accept 1 up ignore value-sent		and elided during the transmission. Otherwise, if several possible
			values are expected by the client, a matching-list should be used to
			limit the size of the residue. If not the possible, the value as to
			be sent as a residue (fixed or variable length).
	4.3. CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port		5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port
	fields		fields
	This field is unidirectional and must not be set to bidirectional in		This field is unidirectional and must not be set to bidirectional in
	a rule entry. It is used only by the server to inform of the caching		a rule entry. It is used only by the server to inform of the caching
	duration and is never found in client requests.		duration and is never found in client requests.
	If the duration is known by both ends, value can be elided on the		If the duration is known by both ends, value can be elided on the
	LPWAN.		LPWAN.
	A matching list can be used if some wellknown values are defined.		A matching list can be used if some well-known values are defined.
	Otherwise the option length and value can be sent on the LPWAN.
	[[note: we can reduce (or create a new option) the unit to minute,		Otherwise these options should be sent as a residue (fixed or
	second is small for LPWAN ]]		variable length).
	5. CoAP option Uri-Path and Uri-Query fields		5.3. CoAP option Uri-Path and Uri-Query fields
	This fields are unidirectional and must not be set to bidirectional		This fields are unidirectional and must not be set to bidirectional
	in a rule entry. They are used only by the client to access to a		in a rule entry. They are used only by the client to access to a
	specific resource and are never found in server response.		specific resource and are never found in server responses.
	The Matching Operator behavior has not changed, but the value must		Uri-Path and Uri-Query elements are a repeatable options, the Field
	take a position value, if the entry is repeated :		Position (FP) gives the position in the path.
	FID FL FP DI TV MO CDA Sent		A Mapping list can be used to reduce size of variable Paths or
	URI-Path 1 up foo equal not-sent		Queries. In that case, to optimize the compression, several elements
	URI-Path 2 up bar equal not-sent		can be regrouped into a single entry. Numbering of elements do not
			change, MO comparison is set with the first element of the matching.
	Figure 2: Position entry.		FID FL FP DI TV MO CDA
			URI-Path 1 up ["/a/b", equal not-sent
			"/c/d"]
			URI-Path 3 up ignore value-sent
	For instance, the rule Figure 2 matches with /foo/bar, but not /bar/		Figure 2: complex path example
	foo.
	When the length is not clearly indicated in the rule, the value		In Figure 2 a single bit residue can be used to code one of the 2
	length must be sent with the field data, which means for CoAP to send		paths. If regrouping was not allowed, a 2 bits residue whould have
	directly the CoAP option with length and value.		been needed.
			5.3.1. Variable length Uri-Path and Uri-Query
			When the length is known at the rule creation, the Field Length must
			be set to variable, and the unit is set to bytes.
			The MSB MO can be apply to a Uri-Path or Uri-Query element. Since
			MSB value is given in bit, the size must always be a multiple of 8
			bits and the LSB CDA must not carry any value.
			The length sent at the beginning of a variable length residue
			indicates the size of the LSB in bytes.
	For instance for a CoMi path /c/X6?k="eth0" the rule can be set to:		For instance for a CoMi path /c/X6?k="eth0" the rule can be set to:
	FID FL FP DI TV MO CDA Sent		FID FL FP DI TV MO CDA
	URI-Path 1 up c equal not-sent		URI-Path 1 up "c" equal not-sent
	URI-Path 2 up ignore value-sent		URI-Path 2 up ignore value-sent
	URI-Query 1 up k= MSB (16) LSB		URI-Query 1 up "k=" MSB (16) LSB
	Figure 3: CoMi URI compression		Figure 3: CoMi 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").
	A Mapping list can be used to reduce size of variable Paths or		5.3.2. Variable number of path or query elements
	Queries. In that case, to optimize the compression, several elements
	can be regrouped into a single entry. Numbering of elements do not
	change, MO comparison is set with the first element of the matching.
	FID FL FP DI TV MO CDA Sent
	URI-Path 1 up {0:"/c/c", equal not-sent
	1:"/c/d"
	URI-Path 3 up ignore value-sent
	URI-Query 1 up k= MSB (16) LSB
	Figure 4: complex path example
	For instance, the following Path /foo/bar/variable/stable can leads		The number of Uri-path or Uri-Query element in a rule is fixed at the
	to the rule defined Figure 4.		rule creation time. If the number varies, several rules should be
			created to cover all the possibilities. Another possibilities is to
			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.
			This add 4 bits to the compression residue.
	5.1. CoAP option 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 entry. They are used only by the client to access to a		in a rule entry. They are used only by the client to access to a
	specific resource and are never found in server response.		specific resource and are never found in server response.
	If the field value must be sent, TV is not set, MO is set to "ignore"		If the field value must be sent, TV is not set, MO is set to "ignore"
	and CDF is set to "value-sent. A mapping can also be used.		and CDF is set to "value-sent. A mapping can also be used.
	Otherwise the TV is set to the value, MO is set to "equal" and CDF is		Otherwise the TV is set to the value, MO is set to "equal" and CDF is
	set to "not-sent"		set to "not-sent"
	5.2. 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 request.		always be sent with the compression residues.
	[[Can include OSCOAP Object security in that category ]]
	6. Other RFCs		6. Other RFCs
	6.1. Block		6.1. Block
	Block option should be avoided in LPWAN. The minimum size of 16		Block [rfc7959] allows a fragmentation at the CoAP level. SCHC
	bytes can be incompatible with some LPWAN technologies.		includes also a fragmentation protocol. They are compatible. If a
			block option is used, its content must be sent as a compression
	[[Note: do we recommand LPWAN fragmentation since the smallest value		residue.
	of 16 is too big?]]
	6.2. Observe		6.2. Observe
	[rfc7641] defines the Observe option. The TV is not set, MO is set		[rfc7641] defines the Observe option. The TV is not set, MO is set
	to "ignore" and the CDF is set to "value-sent". SCHC does not limit		to "ignore" and the CDF is set to "value-sent". SCHC does not limit
	the maximum size for this option (3 bytes). To reduce the		the maximum size for this option (3 bytes). To reduce the
	transmission size either the Thing implementation should limit the		transmission size either the device implementation should limit the
	value increase or a proxy can be used limit the increase.		value increase or a proxy can modify the incrementation.
	Since RST message may be sent to inform a server that the client do		Since RST message may be sent to inform a server that the client do
	not require Observe response, a rule must allow the transmission of		not require Observe response, a rule must allow the transmission of
	this message.		this message.
	6.3. No-Response		6.3. No-Response
	[rfc7967] defines an No-Response option limiting the responses made		[rfc7967] defines an No-Response option limiting the responses made
	by a server to a request. If the value is not by both ends, then TV		by a server to a request. If the value is not known by both ends,
	is set to this value, MO is set to "equal" and CDF is set to "not-		then TV is set to this value, MO is set to "equal" and CDF is set to
	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 CDF 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.
	7. Protocol analysis		6.4. Time Scale
	8. Examples of CoAP header compression
	8.1. Mandatory header with CON message		Time scale [I-D.toutain-core-time-scale] option allows a client to
			inform the server that it is in a slow network and that message ID
			should be kept for a duration given by the option.
			If the value is not known by both ends, then TV is set to this value,
			MO is set to "equal" and CDA is set to "not-sent".
			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
			used to reduce the size.
			6.5. OSCORE
			OSCORE [I-D.ietf-core-object-security] defines end-to-end protection
			for CoAP messages. This section describes how SCHC rules can be
			applied to compress OSCORE-protected messages.
			0 1 2 3 4 5 6 7 <--------- n bytes ------------->
			+-+-+-+-+-+-+-+-+---------------------------------
			|0 0 0|h|k| n | Partial IV (if any) ...
			+-+-+-+-+-+-+-+-+---------------------------------
			| |
			| <--------- CoAP OSCORE_piv ------------------> |
			<- 1 byte -> <------ s bytes ----->
			+------------+----------------------+-----------------------+
			| s (if any) | kid context (if any) | kid (if any) ... |
			+------------+----------------------+-----------------------+
			| | |
			| <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->|
			Figure 4: OSCORE Option
			The encoding of the OSCORE Option Value defined in Section 6.1 of
			[I-D.ietf-core-object-security] is repeated in Figure 4.
			The first byte is used for flags that specify the contents of the
			OSCORE option. The 3 most significant bits are reserved and 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 of a
			kid field. The 3 least significant bits n indicate to length of the
			piv field in bytes, n = 0 taken to mean that no piv is present.
			After the flag byte follow the piv field, kid context field and kid
			field in order and if present; the length of the kid context field is
			encoded in the first byte denoting by s the length of the kid context
			in bytes.
			This draft recommends to implement a parser that is able to identify
			the OSCORE Option and the fields it contains - this makes it possible
			to do a preliminary processing of the message in preparation for
			regular SCHC compression.
			Conceptually, the OSCORE option can transmit up to 3 distinct pieces
			of information at a time: the piv, the kid context, and the kid.
			This draft proposes that the SCHC Parser split the contents of this
			option into 3 SCHC fields:
			o CoAP OSCORE_piv,
			o CoAP OSCORE_ctxt,
			o CoAP OSCORE_kid.
			These fields are superposed on the OSCORE Option format in Figure 4,
			and include the corresponding flag and size bits for each part of the
			option. Both the flag and size bits can be omitted by use of the MSB
			matching operator on each field.
			7. Examples of CoAP header compression
			7.1. Mandatory header with CON message
	In this first scenario, the LPWAN compressor receives from outside		In this first scenario, the LPWAN compressor receives from outside
	client a POST message, which is immediately acknowledged by the		client a POST message, which is immediately acknowledged by the
	Thing. For this simple scenario, the rules are described Figure 5.		Device. For this simple scenario, the rules are described Figure 5.
	Rule ID 1		Rule ID 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 version | | |bi| 01 |equal |not-sent || |		|CoAP version | | |bi| 01 |equal |not-sent || |
	|CoAP Type | | |bi| |ignore |value-sent ||TT |		|CoAP Type | | |dw| CON |equal |not-sent || |
			|CoAP Type | | |up|[ACK, | | || |
			| | | | | RST] |match-map|matching-sent|| T |
	|CoAP TKL | | |bi| 0 |equal |not-sent || |		|CoAP TKL | | |bi| 0 |equal |not-sent || |
	|CoAP Code | | |bi| ML1 |match-map|matching-sent|| CC CCC |		|CoAP Code | | |bi| ML1 |match-map|matching-sent|| CC CCC |
	|CoAP MID | | |bi| 0000 |MSB(7 ) |LSB(9) || M-ID|		|CoAP MID | | |bi| 0000 |MSB(7 ) |LSB(9) || M-ID|
	|CoAP Uri-Path| | |dw| path |equal 1 |not-sent || |		|CoAP Uri-Path| | |dw| path |equal 1 |not-sent || |
	+-------------+--+--+--+------+---------+-------------++------------+		+-------------+--+--+--+------+---------+-------------++------------+
	Figure 5: CoAP Context to compress header without token		Figure 5: CoAP Context to compress header without token
	The version and Token Length fields are elided. Code has shrunk to 5		The version and Token Length fields are elided. Code has shrunk to 5
	bits using the matching list (as the one given Figure 1: 0.01 is		bits using a matching list. Uri-Path contains a single element
	value 0x01 and 2.05 is value 0x0c) Message-ID has shrunk to 9 bits to
	preserve alignment on byte boundary. The most significant bit must
	be set to 0 through a CoAP proxy. Uri-Path contains a single element
	indicated in the matching operator.		indicated in the matching operator.
	Figure 6 shows the time diagram of the exchange. A LPWAN Application		Figure 6 shows the time diagram of the exchange. A client in the
	Server sends a CON message. Compression reduces the header sending		Application Server sends a CON request. It can go through a proxy
	only the Type, a mapped code and the least 9 significant bits of		which reduces the message ID to a smallest value, with at least the 9
	Message ID. The receiver decompresses the header. .		most significant bits equal to 0. SCHC Compression reduces the
			header sending only the Type, a mapped code and the least 9
	The CON message is a request, therefore the LC process to a dynamic		significant bits of Message ID.
	mapping. When the ES receives the ACK message, this will not
	initiate locally a message ID mapping since it is a response. The LC
	receives the ACK and uncompressed it to restore the original value.
	Dynamic Mapping context lifetime follows the same rules as message ID
	duration.
	End System LPWA LC		Device LPWAN SCHC C/D
	| |		| |
	| rule id=1 |<--------------------		| rule id=1 |<--------------------
	|<-------------------| +-+-+--+----+------+		|<-------------------| +-+-+--+----+------+
	<------------------- | TTCC CCCM MMMM MMMM| |1|0| 4|0.01|0x0034|		<------------------- | CCCCCMMMMMMMMM | |1|0| 4|0.01|0x0034|
	+-+-+--+----+-------+ | 0000 0010 0011 0100| | 0xb4 p a t|		+-+-+--+----+-------+ | 00001000110100 | | 0xb4 p a t|
	|1|0| 1|0.01|0x0034 | | | | h |		|1|0| 1|0.01|0x0034 | | | | h |
	| 0xb4 p a t | | | +------+		| 0xb4 p a t | | | +------+
	| h | | |		| h | | |
	+------+ | |		+------+ | |
	| |		| |
	| |		| |
	---------------------->| rule id=1 |		---------------------->| rule id=1 |
	+-+-+--+----+--------+ |------------------->|		+-+-+--+----+--------+ |------------------->|
	|1|2| 0|2.05| 0x0034 | | TTCC CCCM MMMM MMMM|--------------------->		|1|2| 0|2.05| 0x0034 | | TCCCCCMMMMMMMMM |--------------------->
	+-+-+--+----+--------+ | 1001 1000 0011 0100| +-+-+--+----+------+		+-+-+--+----+--------+ | 001100000110100 | +-+-+--+----+------+
	| | |1|2| 0|2.05|0x0034|		| | |1|2| 0|2.05|0x0034|
	v v +-+-+--+----+------+		v v +-+-+--+----+------+
	Figure 6: Compression with global addresses		Figure 6: Compression with global addresses
	The message can be further optimized by setting some fields		7.2. Complete exchange
	unidirectional, as described in Figure 7. Note that Type is no more
	sent in the compressed format, Compressed Code size in not changed in
	that example (8 values are needed to code all the requests and 21 to
	code all the responses in the matching list Figure 1)
	Rule ID 2
	+-------------+--+--+--+------+---------+------------++------------+
	| Field |FL|FP|DI|Target| MO | CDA || Sent |
	| | | | |Value | | || [bits] |
	+-------------+--+--+--+------+---------+------------++------------+
	|CoAP version | | |bi|01 |equal |not-sent || |
	|CoAP Type | | |dw|CON |equal |not-sent || |
	|CoAP Type | | |up| ACK |equal |not-sent || |
	|CoAP TKL | | |bi|0 |equal |not-sent || |
	|CoAP Code | | |dw|ML2 |match-map|mapping-sent||CCCC C |
	|CoAP Code | | |up|ML3 |match-map|mapping-sent||CCCC C |
	|CoAP MID | | |bi|0000 |MSB(5) |LSB(11) || M-ID |
	|CoAP Uri-Path| | |dw|path |equal 1 |not-sent || |
	+-------------+--+--+--+------+---------+------------++------------+
	ML1 = {CON : 0, ACK:1} ML2 = {POST:0, 2.04:1, 0.00:3}
	Figure 7: CoAP Context to compress header without token
	8.2. Complete exchange
	In that example, the Thing is using CoMi and sends queries for 2 SID.		In that example, the Thing is using CoMi and sends queries for 2 SID.
	CON		CON
	MID=0x0012 | |		MID=0x0012 | |
	POST | |		POST | |
	Accept X | |		Accept X | |
	/c/k=AS |------------------------>|		/c/k=AS |------------------------>|
	| |		| |
	| |		| |
	|<------------------------| ACK MID=0x0012		|<------------------------| ACK MID=0x0012
	| | 0.00		| | 0.00
	| |		| |
	| |		| |
	|<------------------------| CON		|<------------------------| CON
	| | MID=0X0034		| | MID=0X0034
	| | Content-Format X		| | Content-Format X
	ACK MID=0x0034 |------------------------>|		ACK MID=0x0034 |------------------------>|
	0.00		0.00
	Rule ID 3		7.3. OSCORE Compression
	+--------------+--+--+--+------+--------+-----------++------------+
	| Field |FL|FP|DI|Target| MO | CDA || Sent |
	| | | | |Value | | || [bits] |
	+--------------+--+--+--+------+--------+-----------++------------+
	|CoAP version | | |bi| 01 |equal |not-sent || |
	|CoAP Type | | |up| CON |equal |not-sent || |
	|CoAP Type | | |dw| ACK |equal |not-sent || |
	|CoAP TKL | | |bi| 1 |equal |not-sent || |
	|CoAP Code | | |up| POST |equal |not-sent || |
	|CoAP Code | | |dw| 0.00 |equal |not-sent || |
	|CoAP MID | | |bi| 0000 |MSB(8) |LSB ||MMMMMMMM |
	|CoAP Token | | |up| |ignore |send-value ||TTTTTTTT |
	|CoAP Uri-Path | | |dw| /c |equal 1 |not-sent || |
	|CoAP Uri-query| | |dw| ML4 |equal 1 |not-sent ||P |
	|CoAP Content | | |up| X |equal |not-sent || |
	+--------------+--+--+--+------+--------+-----------++------------+
	Rule ID 4		OSCORE aims to solve the problem of end-to-end encryption for CoAP
	+--------------+--+--+--+------+--------+-----------++------------+		messages, which are otherwise required to terminate their TLS or DTLS
	| Field |FL|FP|DI|Target| MO | CDA || Sent |		protection at the proxy, as discussed in Section 11.2 of [rfc7252].
	| | | | |Value | | || [bits] |		CoAP proxies are men-in-the-middle, but not all of the information
	+--------------+--+--+--+------+--------+-----------++------------+		they have access to is necessary for their operation. The goal,
	|CoAP version | | |bi| 01 |equal |not-sent || |		therefore, is to hide as much of the message as possible while still
	|CoAP Type | | |dw| CON |equal |not-sent || |		enabling proxy operation.
	|CoAP Type | | |up| ACK |equal |not-sent || |
	|CoAP TKL | | |bi| 1 |equal |not-sent || |
	|CoAP Code | | |dw| 2.05 |equal |not-sent || |
	|CoAP Code | | |up| 0.00 |equal |not-sent || |
	|CoAP MID | | |bi| 0000 |MSB(8) |LSB ||MMMMMMMM |
	|CoAP Token | | |dw| |ignore |send-value||TTTTTTTT |
	|COAP Accept | | |dw| X |equal |not-sent || |
	+--------------+--+--+--+------+---------+----------++------------+
	alternative rule:		Conceptually this is achieved by splitting the CoAP message into an
			Inner Plaintext and Outer OSCORE Message. The Inner Plaintext
			contains sensible information which is not necessary for proxy
			operation. This, in turn, is the part of the message which can be
			encrypted and need not be decrypted until it reaches its end
			destination. The Outer Message acts as a shell matching the format
			of a regular CoAP message, and includes all Options and information
			needed for proxy operation and caching. This decomposition is
			illustrated in Figure 7.
	Rule ID 4		CoAP options are sorted into one of 3 classes, each granted a
	+--------------+--+--+--+------+---------+-----------++------------+		specific type of protection by the protocol:
	| Field |FL|FP|DI|Target| MO | CDA || Sent |
	| | | | |Value | | || [bits] |
	+--------------+--+--+--+------+---------+-----------++------------+
	|CoAP version | | |bi| 01 |equal |not-sent || |
	|CoAP Type | | |bi| ML1 |match-map|match-sent ||t |
	|CoAP TKL | | |bi| 1 |equal |not-sent || |
	|CoAP Code | | |up| ML2 |match-map|match-sent || cc |
	|CoAP Code | | |dw| ML3 |match-map|match-sent || cc |
	|CoAP MID | | |bi| 0000 |MSB(8) |LSB ||MMMMMMMM |
	|CoAP Token | | |dw| |ignore |send-value ||TTTTTTTT |
	|CoAP Uri-Path | | |dw| /c |equal 1 |not-sent || |
	|CoAP Uri-query| | |dw| ML4 |equal 1 |not-sent ||P |
	|CoAP Content | | |up| X |equal |not-sent || |
	|COAP Accept | | |dw| x |equal |not-sent || |
	+--------------+--+--+--+------+---------+-----------++------------+
	ML1 {CON:0, ACK:1} ML2 {POST:0, 0.00: 1} ML3 {2.05:0, 0.00:1}		o Class E: Enrypted options moved to the Inner Plaintext,
	ML4 {NULL:0, k=AS:1, K=AZE:2}
	9. Normative References		o Class I: Intergrity-protected options included in the AAD for the
			encryption of the Plaintext but otherwise left untouched in the
			Outer Message,
	[I-D.toutain-lpwan-ipv6-static-context-hc]		o Class U: Unprotected options left untouched in the Outer Message.
	Minaburo, A. and L. Toutain, "LPWAN Static Context Header
	Compression (SCHC) for IPv6 and UDP", draft-toutain-lpwan-		Additionally, the OSCORE Option is added as an Outer option,
	ipv6-static-context-hc-00 (work in progress), September		signaling that the message is OSCORE protected. This option carries
	2016.		the information necessary to retrieve the Security Context with which
			the message was encrypted so that it may be correctly decrypted at
			the other end-point.
			Orignal CoAP Message
			+-+-+---+-------+---------------+
			|v|t|tkl| code | Msg Id. |
			+-+-+---+-------+---------------+....+
			| Token |
			+-------------------------------.....+
			| Options (IEU) |
			. .
			. .
			+------+-------------------+
			| 0xFF |
			+------+------------------------+
			| |
			| Payload |
			| |
			+-------------------------------+
			/ \
			/ \
			/ \
			/ \
			Outer Header v v Plaintext
			+-+-+---+--------+---------------+ +-------+
			|v|t|tkl|new code| Msg Id. | | code |
			+-+-+---+--------+---------------+....+ +-------+-----......+
			| Token | | Options (E) |
			+--------------------------------.....+ +-------+------.....+
			| Options (IU) | | OxFF |
			. . +-------+-----------+
			. OSCORE Option . | |
			+------+-------------------+ | Payload |
			| 0xFF | | |
			+------+ +-------------------+
			Figure 7: OSCORE inner and outer header form a CoAP message
			Figure 7 shows the message format for the OSCORE Message and
			Plaintext. In the Outer Header, the original message code is hidden
			and replaced by a default value (POST or FETCH) depending on whether
			the original message was a Request or a Response. The original
			message code is put into the first byte of the Plaintext. Following
			the message code come the class E options and if present the original
			message Payload preceded by its payload marker.
			The Plaintext is now encrypted by an AEAD algorithm which integrity
			protects Security Context parameters and eventually any class I
			options from the Outer Header. Currently no CoAP options are marked
			class I. The resulting Ciphertext becomes the new Payload of the
			OSCORE message, as illustrated in Figure 8.
			Outer Header
			+-+-+---+--------+---------------+
			|v|t|tkl|new code| Msg Id. |
			+-+-+---+--------+---------------+....+
			| Token |
			+--------------------------------.....+
			| Options (IU) |
			. .
			. OSCORE Option .
			+------+-------------------+
			| 0xFF |
			+------+-------------------------+
			| |
			| Encrypted Inner Header and |
			| Payload |
			| |
			+--------------------------------+
			Figure 8: OSCORE message
			The SCHC Compression scheme consists of compressing both the
			Plaintext before encryption and the resulting OSCORE message after
			encryption, see Figure 9. This way compression reaches all fields of
			the original CoAP message.
			Outer Message OSCORE Plaintext
			+-+-+---+--------+---------------+ +-------+
			|v|t|tkl|new code| Msg Id. | | code |
			+-+-+---+--------+---------------+....+ +-------+-----......+
			| Token | | Options (E) |
			+--------------------------------.....+ +-------+------.....+
			| Options (IU) | | OxFF |
			. . +-------+-----------+
			. OSCORE Option . | |
			+------+-------------------+ | Payload |
			| 0xFF | | |
			+------+------------+ +-------------------+
			| Ciphertext |<---------\ |
			| | | v
			+-------------------+ | +-----------------+
			| | | Inner SCHC |
			v | | Compression |
			+-----------------+ | +-----------------+
			| Outer SCHC | | |
			| Compression | | v
			+-----------------+ | +-------+
			| | |Rule ID|
			v | +-------+--+
			+--------+ +------------+ | Residue |
			|Rule ID'| | Encryption | <--- +----------+--------+
			+--------+--+ +------------+ | |
			| Residue' | | Payload |
			+-----------+-------+ | |
			| Ciphertext | +-------------------+
			| |
			+-------------------+
			Figure 9: OSCORE Compression Diagram
			7.4. Example OSCORE Compression
			In what follows we present an example GET Request and consequent
			CONTENT Response and show a possible set of rules for the Inner and
			Outer SCHC Compression. We then show a dump of the results and
			contrast SCHC + OSCORE performance with SCHC + COAP performance.
			This gives an approximation to the cost of security with SCHC-OSCORE.
			Our first example CoAP message is the GET Request in Figure 10
			Original message:
			=================
			0x4101000182bb74656d7065726174757265
			Header:
			0x4101
			01 Ver
			00 CON
			0001 tkl
			00000001 Request Code 1 "GET"
			0x0001 = mid
			0x82 = token
			Options:
			0xbb74656d7065726174757265
			Option 11: URI_PATH
			Value = temperature
			Original msg length: 17 bytes.
			Figure 10: CoAP GET Request
			Its corresponding response is the CONTENT Response in Figure 11.
			Original message:
			=================
			0x6145000182ff32332043
			Header:
			0x6145
			01 Ver
			10 ACK
			0001 tkl
			01000101 Successful Response Code 69 "2.05 Content"
			0x0001 = mid
			0x82 = token
			0xFF Payload marker
			Payload:
			0x32332043
			Original msg length: 10
			Figure 11: CoAP CONTENT Response
			The SCHC Rules for the Inner Compression include all fields that are
			already present in a regular CoAP message, what matters is the order
			of appearance and inclusion of only those CoAP fields that go into
			the Plaintext, Figure 12.
			Rule ID 0
			+----------------+--+--+-----------+-----------+-----------++--------+
			| Field |FP|DI| Target | MO | CDA || Sent |
			| | | | Value | | || [bits] |
			+----------------+--+--+-----------+-----------+-----------++--------+
			|CoAP Code | |up| 1 | equal |not-sent || |
			|CoAP Code | |dw|[69,132] | match-map |match-sent || c |
			|CoAP Uri-Path | |up|temperature| equal |not-sent || |
			|COAP Option-End | |dw| 0xFF | equal |not-sent || |
			+----------------+--+--+-----------+-----------+-----------++--------+
			Figure 12: Inner SCHC Rules
			The Outer SCHC Rules (Figure 13) must process the OSCORE Options
			fields. Here we mask off the repeated bits (most importantly the
			flag and size bits) with the MSB Mathing Operator.
			Rule ID 0
			+---------------+--+--+--------------+---------+-----------++------------+
			| Field |FP|DI| Target | MO | CDA || Sent |
			| | | | Value | | || [bits] |
			+---------------+--+--+--------------+---------+-----------++------------+
			|CoAP version | |bi| 01 |equal |not-sent || |
			|CoAP Type | |up| 0 |equal |not-sent || |
			|CoAP Type | |dw| 2 |equal |not-sent || |
			|CoAP TKL | |bi| 1 |equal |not-sent || |
			|CoAP Code | |up| 2 |equal |not-sent || |
			|CoAP Code | |dw| 68 |equal |not-sent || |
			|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM |
			|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT |
			|CoAP OSCORE_piv| |up| 0x0900 |MSB(12) |LSB ||PPPP |
			|COAP OSCORE_kid| |up|b'\x06client' |MSB(52) |LSB ||KKKK |
			|CoAP OSCORE_piv| |dw| b'' |equal |not-sent || |
			|COAP Option-End| |dw| 0xFF |equal |not-sent || |
			+---------------+--+--+--------------+---------+-----------++------------+
			Figure 13: Outer SCHC Rules
			Next we show a dump of the compressed message:
			Compressed message:
			==================
			0x00291287f0a5c4833760d170
			0x00 = Rule ID
			piv = 0x04
			Compression residue:
			0b0001 010 0100 0100 (15 bits -> 2 bytes with padding)
			mid tkn piv kid
			Payload
			0xa1fc297120cdd8345c
			Compressed message length: 12 bytes
			Figure 14: SCHC-OSCORE Compressed GET Request
			Compressed message:
			==================
			0x0015f4de9cb814c96aed9b1d981a3a58
			0x00 = Rule ID
			Compression residue:
			0b0001 010 (7 bits -> 1 byte with padding)
			mid tkn
			Payload
			0xfa6f4e5c0a64b576cd8ecc0d1d2c
			Compressed msg length: 16 bytes
			Figure 15: SCHC-OSCORE Compressed CONTENT Response
			For contrast, we compare these results with what would be obtained by
			SCHC compressing the original CoAP messages without protecting them
			with OSCORE. To do this, we compress the CoAP mesages according to
			the SCHC rules in Figure 16.
			Rule ID 1
			+---------------+--+--+-----------+---------+-----------++------------+
			| Field |FP|DI| Target | MO | CDA || Sent |
			| | | | Value | | || [bits] |
			+---------------+--+--+-----------+---------+-----------++------------+
			|CoAP version | |bi| 01 |equal |not-sent || |
			|CoAP Type | |up| 0 |equal |not-sent || |
			|CoAP Type | |dw| 2 |equal |not-sent || |
			|CoAP TKL | |bi| 1 |equal |not-sent || |
			|CoAP Code | |up| 2 |equal |not-sent || |
			|CoAP Code | |dw| [69,132] |equal |not-sent || |
			|CoAP MID | |bi| 0000 |MSB(12) |LSB ||MMMM |
			|CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT |
			|CoAP Uri-Path | |up|temperature|equal |not-sent || |
			|COAP Option-End| |dw| 0xFF |equal |not-sent || |
			+---------------+--+--+-----------+---------+-----------++------------+
			Figure 16: SCHC-CoAP Rules (No OSCORE)
			This yields the results in Figure 17 for the Request, and Figure 18
			for the Response.
			Compressed message:
			==================
			0x0114
			0x01 = Rule ID
			Compression residue:
			0b00010100 (1 byte)
			Compressed msg length: 2
			Figure 17: CoAP GET Compressed without OSCORE
			Compressed message:
			==================
			0x010a32332043
			0x01 = Rule ID
			Compression residue:
			0b00001010 (1 byte)
			Payload
			0x32332043
			Compressed msg length: 6
			Figure 18: CoAP CONTENT Compressed without OSCORE
			As can be seen, the difference between applying SCHC + OSCORE as
			compared to regular SCHC + COAP is about 10 bytes of cost.
			8. Normative References
			[I-D.ietf-core-object-security]
			Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
			"Object Security for Constrained RESTful Environments
			(OSCORE)", draft-ietf-core-object-security-13 (work in
			progress), June 2018.
			[I-D.ietf-lpwan-ipv6-static-context-hc]
			Minaburo, A., Toutain, L., Gomez, C., and D. Barthel,
			"LPWAN Static Context Header Compression (SCHC) and
			fragmentation for IPv6 and UDP", draft-ietf-lpwan-ipv6-
			static-context-hc-16 (work in progress), June 2018.
			[I-D.toutain-core-time-scale]
			Minaburo, A. and L. Toutain, "CoAP Time Scale Option",
			draft-toutain-core-time-scale-00 (work in progress),
			October 2017.
	[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>.
			[rfc7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
			the Constrained Application Protocol (CoAP)", RFC 7959,
			DOI 10.17487/RFC7959, August 2016,
			<https://www.rfc-editor.org/info/rfc7959>.
	[rfc7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.		[rfc7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
	Bose, "Constrained Application Protocol (CoAP) Option for		Bose, "Constrained Application Protocol (CoAP) Option for
	No Server Response", RFC 7967, DOI 10.17487/RFC7967,		No Server Response", RFC 7967, DOI 10.17487/RFC7967,
	August 2016, <https://www.rfc-editor.org/info/rfc7967>.		August 2016, <https://www.rfc-editor.org/info/rfc7967>.
	Authors' Addresses		Authors' Addresses
	Ana Minaburo		Ana Minaburo
	Acklio		Acklio
	2bis rue de la Chataigneraie		1137A avenue des Champs Blancs
	35510 Cesson-Sevigne Cedex		35510 Cesson-Sevigne Cedex
	France		France
	Email: ana@ackl.io		Email: ana@ackl.io
	Laurent Toutain		Laurent Toutain
	Institut MINES TELECOM; IMT Atlantique		Institut MINES TELECOM; IMT Atlantique
	2 rue de la Chataigneraie		2 rue de la Chataigneraie
	CS 17607		CS 17607
	35576 Cesson-Sevigne Cedex		35576 Cesson-Sevigne Cedex
	France		France
	Email: Laurent.Toutain@imt-atlantique.fr		Email: Laurent.Toutain@imt-atlantique.fr
			Ricardo Andreasen
			Universidad de Buenos Aires
			Av. Paseo Colon 850
			C1063ACV Ciudad Autonoma de Buenos Aires
			Argentina
			Email: randreasen@fi.uba.ar
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