< draft-ietf-lpwan-ipv6-static-context-hc-00.txt		draft-ietf-lpwan-ipv6-static-context-hc-01.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: June 8, 2017 Institut MINES TELECOM ; TELECOM Bretagne		Expires: September 3, 2017 IMT-Atlantique
	December 5, 2016		C. Gomez
			Universitat Politecnica de Catalunya
			March 02, 2017
	LPWAN Static Context Header Compression (SCHC) for IPv6 and UDP		LPWAN Static Context Header Compression (SCHC) and fragmentation for
	draft-ietf-lpwan-ipv6-static-context-hc-00		IPv6 and UDP
			draft-ietf-lpwan-ipv6-static-context-hc-01
	Abstract		Abstract
	This document describes a header compression scheme for IPv6, IPv6/		This document describes a header compression scheme and fragmentation
	UDP based on static contexts. This technique is especially tailored		functionality for IPv6/UDP protocols. These techniques are
	for LPWA networks and could be extended to other protocol stacks.		especially tailored for LPWAN (Low Power Wide Area Network) networks
			and could be extended to other protocol stacks.
	During the IETF history several compression mechanisms have been
	proposed. First mechanisms, such as RoHC, are using a context to
	store header field values and send smaller incremental differences on
	the link. Values in the context evolve dynamically with information
	contained in the compressed header. The challenge is to maintain
	sender's and receiver's contexts synchronized even with packet
	losses. Based on the fact that IPv6 contains only static fields,
	6LoWPAN developed an efficient context-free compression mechanisms,
	allowing better flexibility and performance.
	The Static Context Header Compression (SCHC) combines the advantages		The Static Context Header Compression (SCHC) offers a great level of
	of RoHC context which offers a great level of flexibility in the		flexibility when processing the header fields. Static context means
	processing of fields, and 6LoWPAN behavior to elide fields that are		that information stored in the context which, describes field values,
	known from the other side. Static context means that values in the		does not change during the packet transmission, avoiding complex
	context field do not change during the transmission, avoiding complex		resynchronization mechanisms, incompatible with LPWAN
	resynchronization mechanisms, incompatible with LPWA characteristics.		characteristics. In most of the cases, IPv6/UDP headers are reduced
	In most of the cases, IPv6/UDP headers are reduced to a small		to a small identifier.
	identifier.
	This document focuses on IPv6/UDP headers compression, but the		This document describes the generic compression/decompression process
	mechanism can be applied to other protocols such as CoAP. It will be		and applies it to IPv6/UDP headers. Similar mechanisms for other
	described in a separate document.		protocols such as CoAP will be described in a separate document.
			Moreover, this document specifies fragmentation and reassembly
			mechanims for SCHC compressed packets exceeding the L2 pdu size and
			for the case where the SCHC compression is not possible then the
			IPv6/UDP packet is sent.
	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.
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	Copyright Notice		Copyright Notice
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	document authors. All rights reserved.		document authors. All rights reserved.
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	1. Introduction		1. Introduction
	Headers compression is mandatory to bring the internet protocols to		Header compression is mandatory to efficiently bring Internet
	the node within a LPWA network [I-D.minaburo-lp-wan-gap-analysis].		connectivity to the node within a LPWAN network
			[I-D.minaburo-lp-wan-gap-analysis].
	Nevertheless, LPWA networks offer good properties for an efficient		Some LPWAN networks properties can be exploited for an efficient
	header compression:		header compression:
	o Topology is star oriented, therefore all the packets follows the		o Topology is star oriented, therefore all the packets follow the
	same path. For the needs of this draft, the architecture can be		same path. For the needs of this draft, the architecture can be
	summarized to End-Systems (ES) exchanging information with LPWAN		summarized to Things or End-Systems (ES) exchanging information
	Application Server (LA). The exchange goes trhough a single LPWA		with LPWAN Application Server (LA) through a Network Gateway (NG).
	Compressor (LC). In most of the cases, End Systems and LC form a
	star topology. ESs and LC maintain a static context for
	compression. Static context means that context information is not
	learned during the exchange.
	o Traffic flows are mostly deterministic, since End-Systems embed		o Traffic flows are mostly known in advanced, since End-Systems
	built-in applications. Contrary to computers or smartphones, new		embed built-in applications. Contrary to computers or
	applications cannot be easily installed.		smartphones, new applications cannot be easily installed.
	First mechanisms such as RoHC use a context to store header field		The Static Context Header Compression (SCHC) is defined for this
	values and send smaller incremental differences on the link. The		environment. SCHC uses a context where header information is kept in
	first version of RoHC targeted IP/UDP/RTP stack. RoHCv2 extends the		order, this context is static the values on the header fields do not
	principle to any protocol and introduces a formal notation [RFC4997]		change during time, avoiding complex resynchronization mechanisms,
	describing the header and associating compression functions to each		incompatible with LPWAN characteristics. In most of the cases, IPv6/
	field. To be efficient the sender and the receiver must check that		UDP headers are reduced to a small context identifier.
	the context remains synchronized (i.e. contains the same values).
	Context synchronization imposes to periodically send a full header or
	at least dynamic fields. If fully compressed, the header can be
	compatible with LPWA constraints. However, the first exchanges or
	context resynchronisations impose to send uncompressed headers, which
	may be bigger than the original one. This will force the use of
	inefficient fragmentation mechanisms. For some LPWA technologies,
	duty cycle limits can also delay the resynchronization. Figure 1
	illustrates this behavior.
	sync		The SCHC header compression is indedependent of the specific LPWAN
	^ +-+ sync sync ^		technology over which it will be used.
	| IPv6 | | +-+ +-+ | IPv6
	v | | | | | | v
	+------------+ | +-+-+ | | | | +------------+
	| +--+ | | | | | | | | | | +--+ |
	| | c| | | | | +-+-+-+ +-+-+-+-+ | | | c| |
	| | t| | | | | | | | | | | | | | | | | t| |
	| | x| | +-+-+-+-+-+-+-+-+-+-+-+-+ | | x| |
	| | t| | <----------------------------> | | t| |
	| +--+ | header size of sent packets | +--+ |
	+------------+ +------------+
	Figure 1: RoHC Compressed Header size evolution.		On the other hand, LPWAN technologies are characterized, among
			others, by a very reduced data unit and/or payload size
			[I-D.ietf-lpwan-overview]. However, some of these technologies do
			not support layer two fragmentation, therefore the only option for
			these to support IPv6 when header compression is not possible (and,
			in particular, its MTU requirement of 1280 bytes [RFC2460]) is the
			use of fragmentation mechanism at the adaptation layer below IPv6.
			This specification defines fragmentation functionality to support the
			IPv6 MTU requirements over LPWAN technologies.
	On the other hand, 6LoWPAN [RFC4944] is context-free based on the		2. Vocabulary
	fact that IPv6, its extensions or UDP headers do not contain
	incremental fields. The compression mechanism described in [RFC6282]
	is based on sending a 2-byte bitmap, which describes how the header
	should be decompressed, either using some standard values or sending
	information after this bitmap. [RFC6282] also allows for UDP
	compression.
	In the best case, when Hop limit is a standard value, flow label,		This section defines the terminology and aconyms used in this
	DiffServ fields are set to 0 and Link Local addresses are used over a		document.
	single hop network, the 6LoWPAN compressed header is reduced to 4
	bytes. This compression ratio is possible because the IID are
	derived from the MAC addresses and the link local prefix is known
	from both sides. In that case, the IPv6 compression is 4 bytes and
	UDP compression is 2 bytes, which fills half of the payload of a
	SIGFOX frame, or more than 10% of a LoRaWAN payload (with spreading
	factor 12).
	The Static Context Header Compression (SCHC) combines the advantages		o CDF: Compression/Decompression Function. A function that is used
	of RoHC context, which offers a great level of flexibility in the		for both functionnalities to compress a header field or to recover
	processing of fields, and 6LoWPAN behavior to elide fields that are		its original value in the decompression phase.
	known from the other side. Static context means that values in the
	context field do not change during the transmission, avoiding complex
	resynchronization mechanisms, incompatible with LPWA characteristics.
	In most of the cases, IPv6/UDP headers are reduced to a small context
	identifier.
	2. Static Context Header Compression		o Context: A set of rules used to compress/decompress headers
			o ES: End System. Node connected to the LPWAN. An ES may implement
			SCHC.
			o LA: LPWAN Application. An application sending/consuming IPv6
			packets to/from the End System.
			o LC: LPWAN Compressor/Decompressor. A process in the network to
			achieve compression/decompressing headers. LC uses SCHC rules to
			perform compression and decompression.
			o MO: Matching Operator. An operator used to compare a value
			contained in a header field with a value contained in a rule.
			o Rule: A set of header field values.
			o Rule ID: An identifier for a rule, LC and ES share the same rule
			ID for a specific flow. Rule ID is sent on the LPWAN.
			o TV: Target value. A value contained in the rule that will be
			matched with the value of a header field.
			3. Static Context Header Compression
	Static Context Header Compression (SCHC) avoids context		Static Context Header Compression (SCHC) avoids context
	synchronization, which is the most bandwidth-consuming operation in		synchronization, which is the most bandwidth-consuming operation in
	RoHC. Based on the fact that the nature of data flows is highly		other header compression mechanisms such as RoHC. Based on the fact
	predictable in LPWA networks, a static context may be stored on the		that the nature of data flows is highly predictable in LPWAN
	End-System (ES). The other end, the LPWA Compressor (LC) can learn		networks, a static context may be stored on the End-System (ES). The
	the context through a provisioning protocol during the identification		context must be stored in both ends. It can also be learned by using
	phase (for instance, as it learns the encryption key).		a provisionning protocol that is out of the scope of this draft.
	The context contains a list of rules (cf. Figure 2). Each rule		End-System Appl Servers
	contains itself a list of field descriptions composed of a target		+-----------------+ +---------------+
	value (TV), a matching operator (MO) and a Compression/Decompression		| APP1 APP2 APP3 | |APP1 APP2 APP3|
	Function (CDF).		| | | |
			| UDP | | UDP |
			| IPv6 | | IPv6 |
			| | | |
			| LC (contxt)| | |
			+--------+--------+ +-------+-------+
			| +--+ +--+ +-----------+ .
			+~~ |RG| === |NG| === |LC (contxt)| ... Internet ...
			+--+ +--+ +-----+-----+
	+------------------------------------------------------------------+		Figure 1: Architecture
	| Rule N |
	+-----------------------------------------------------------------+ |
	| Rule i | |
	+----------------------------------------------------------------+ | |
	| Rule 1 | | |
	| +--------------+-------------------+-----------------+ | | |
	| Field 1 | Target Value | Matching Operator | Comp/Decomp Fct | | | |
	| +--------------+-------------------+-----------------+ | | |
	| Field 2 | Target Value | Matching Operator | Comp/Decomp Fct | | | |
	| +--------------+-------------------+-----------------+ | | |
	| ... | ... | ... | ... | | | |
	| +--------------+-------------------+-----------------+ | |-+
	| Field N | Target Value | Matching Operator | Comp/Decomp Fct | | |
	| +--------------+-------------------+-----------------+ |-+
	| |
	+----------------------------------------------------------------+
	Figure 2: Compression Decompression Context		Figure 1 based on [I-D.ietf-lpwan-overview] terminology represents
			the architecture for compression/decompression. The Thing or End-
			System is running applications which produce IPv6 or IPv6/UDP flows.
			These flows are compressed by a LPWAN Compressor (LC) to reduce the
			headers size. Resulting information is sent on a layer two (L2)
			frame to the LPWAN Radio Network to a Radio Gateway (RG) which
			forwards the frame to a Network Gateway. The Network Gateway sends
			the data to a LC for decompression which shares the same rules with
			the ES. The LC can be located on the Network Gateway or in another
			places if a tunnel is established between the NG and the LC. This
			architecture forms a star topology. After decompression, the packet
			can be sent on the Internet to one or several LPWAN Application
			Servers (LA).
	The rule does not describe the compressed/decompressed packet format		The principle is exactly the same in the other direction.
	which must be known from the compressor/decompressor. The rule just
	describes the compression/decompression behavior for a field.
	The main idea of the compression scheme is to send the rule number		The context contains a list of rules (cf. Figure 2). Each rule
	(or rule id) to the other end instead of known field values.		contains itself a list of fields descriptions composed of a field
			identifier (FID), a target value (TV), a matching operator (MO) and a
			Compression/Decompression Function (CDF).
	Matching a field with a value and header compression are related		+-----------------------------------------------------------------+
	operations; If a field matches a rule containing the value, it is not		| Rule N |
	necessary to send it on the link. Since contexts are synchronized,		+----------------------------------------------------------------+ |
	reading the rule's value is enough to reconstruct the field's value		| Rule i | |
	at the other end.		+---------------------------------------------------------------+ | |
			| Rule 1 | | |
			|+--------+--------------+-------------------+-----------------+| | |
			||Field 1 | Target Value | Matching Operator | Comp/Decomp Fct || | |
			|+--------+--------------+-------------------+-----------------+| | |
			||Field 2 | Target Value | Matching Operator | Comp/Decomp Fct || | |
			|+--------+--------------+-------------------+-----------------+| | |
			||... | ... | ... | ... || | |
			|+--------+--------------+-------------------+-----------------+| |-+
			||Field N | Target Value | Matching Operator | Comp/Decomp Fct || |
			|+--------+--------------+-------------------+-----------------+|-+
			| |
			+---------------------------------------------------------------+
	On some other cases, the value need to be sent on the link to inform		Figure 2: Compression Decompression Context
	the other end. The field value may vary from one packet to another,
	therefore the field cannot be used to select the rule id.
	2.1. Simple Example		The rule does not describe the original packet format which must be
			known from the compressor/decompressor. The rule just describes the
			compression/decompression behavior for the header fields. In the
			rule, it is recommended to describe the header field in the same
			order they appear in the packet.
	A simple header is composed of 3 fields (F1, F2, F3). The compressor		The main idea of the compression scheme is to send the rule id to the
	receives a packet containing respectively [F1:0x00, F2:0x1230,		other end instead of known field values. When a value is known by
	F3:0xABC0] in those fields. The Matching Operators (as defined in		both ends, it is not necessary to send it on the LPWAN network.
	Section 3) allow to select Rule 5 as represented in Figure 3; F1
	value is ignored and F2 and F3 packet field values are matched with
	those stored in the rule Target Values.
	Rule 5		The field description is composed of different entries:
	Target Value Matching Operator Comp/Decomp Fct
	+--------------+-------------------+-----------------+
	F1 | 0x00 | Ignore | not-sent |
	+--------------+-------------------+-----------------+
	F2 | 0x1230 | Equal | not-sent |
	+--------------+-------------------+-----------------+
	F3 | 0xABC0 | Equal | not-sent |
	+--------------+-------------------+-----------------+
	Figure 3: Matching Rule		o A Field ID (FID) is a unique value to define the field.
	The Compression/Decompression Function (as defined in Section 4		o A Target Value (TV) is the value used to make the comparison with
	describes how the fields are compressed. In this example, all the		the packet header field. The Target Value can be of any type
	fields are elided and only the rule number has to be sent to the		(integer, strings,...). It can be a single value or a more
	other end.		complex structure (array, list,...). It can be considered as a
			CBOR structure.
	The decompressor receives the rule number and reconstructs the header		o A Matching Operator (MO) is the operator used to make the
	using the values stored in the Target Value column.		comparison between the field value and the Target Value. The
			Matching Operator may require some parameters, which can be
			considered as a CBOR structure. MO is only used during the
			compression phase.
	Note that F1 value will be set to 0x00 by the decompressor, even if		o A Compression Decompression Function (CDF) is used to describe the
	the original header field was carrying a different value.		compression and the decompression process. The CDF may require
			some parameters, which can be considered as a CBOR structure.
	To allow a range of values for field F2 and F3, the MSB() Matching		3.1. Rule ID
	Operator and LSB() Compression/Decompression Function can be used (as
	defined in Section 3 and Section 4). In that case the rule will be
	rewritten as defined in Figure 4.
	Rule 5		Rule IDs are sent between both compression/decompression elements.
	Target Value Matching Operator Comp/Decomp Fct		The size of the rule ID is not specified in this document and can
	+--------------+-------------------+-----------------+		vary regarding the LPWAN technology, the number of flows,...
	F1 | 0x00 | Ignore | not-sent |
	+--------------+-------------------+-----------------+
	F2 | 0x1230 | MSB(12) | LSB(4) |
	+--------------+-------------------+-----------------+
	F3 | 0xABC0 | MSB(12) | LSB(4) |
	+--------------+-------------------+-----------------+
	Figure 4: Matching Rule		Some values in the rule ID space may be reserved for goals other than
			header compression, for example fragmentation.
	In that case, if a packet with the following header fields [F1:0x00,		Rule IDs are specific to an ES. Two ESs may use the same rule ID for
	F2:0x1234, F3:0xABCD] arrives to the compressor, the new rule 5 will		different header compression. The LC needs to combine the rule ID
	be selected and sent to the other end. The compressed header will be		with the ES L2 address to find the appropriate rule.
	composed of the single byte [0x4D]. The decompressor receives the
	compressed header and follows the rule to reconstruct [0x00, 0x1234,
	0xABCD] applying a OR operator between the target value stored in the
	rule and the compressed field value sent.
	2.2. Packet processing		3.2. Packet processing
	The compression/decompression process follows several steps:		The compression/decompression process follows several steps:
	o compression rule selection: the goal is to identify which rule		o compression rule selection: the goal is to identify which rule(s)
	will be used to compress the headers. To each field is associated		will be used to compress the headers. Each field is associated to
	a matching rule for compression. Each header field's value is		a matching operator for compression. Each header field's value is
	compared to the corresponding target value stored in the rule for		compared to the corresponding target value stored in the rule for
	that field using the matching operator. If all the fields		that field using the matching operator. If all the fields in the
	satisfied the matching operator, the packet is processed using		packet's header satisfied all the matching operators of a rule,
	this Compression Decompression Function functions. Otherwise the		the packet is processed using Compression Decompression Function
	next rule is tested. If no eligible rule is found, then the		associated with the fields. Otherwise the next rule is tested.
	packet is dropped.		If no eligible rule is found, then the packet is sent without
			compression, which may require using the fragmentation procedure.
	o sending: The rule number is sent to the other end followed by data		o sending: The rule ID is sent to the other end followed by
	resulting from the field compression. The way the rule number is		information resulting from the compression of header fields. This
	sent depends of the layer two technology and will be specified in		information is sent in the order expressed in the rule for the
	a specific document. For exemple, it can either be included in a		matching fields. The way the rule ID is sent depends on the layer
	Layer 2 header or sent in the first byte of the L2 payload.		two technology and will be specified in a specific document. For
			example, it can either be included in a Layer 2 header or sent in
			the first byte of the L2 payload.
	o decompression: The receiver identifies the sender through its		o decompression: The receiver identifies the sender through its
	device-id (e.g. MAC address) and select the appropriate rule		device-id (e.g. MAC address) and selects the appropriate rule
	through the rule number. It applies the compression decompression		through the rule ID. This rule gives the compressed header format
	function to reconstruct the original header fields.		and associates these values to header fields. It applies the CDF
			function to reconstruct the original header fields. CDF of
	3. Matching operators		Compute-* must be applied after the other CDFs.
	It may exist some intermediary cases, where part of the value may be		4. Matching operators
	used to select a field and a variable part has to be sent on the
	link. This is true for Least Significant Bits (LSB) where the most
	significant bit can be used to select a rule id and the least
	significant bits have to be sent on the link.
	Several matching operators are defined:		This document describes basic matching operators (MO)s which must be
			known by both LC, endpoints involved in the header compression/
			decompression. They are not typed and can be applied indifferently
			to integer, string or any other type. The MOs and their definition
			are provided next:
	o equal: a field value in a packet matches with a field value in a		o equal: a field value in a packet matches with a field value in a
	rule if they are equal.		rule if they are equal.
	o ignore: no check is done between a field value in a packet and a		o ignore: no check is done between a field value in a packet and a
	field value in the rule. The result is always true.		field value in the rule. The result of the matching is always
			true.
	o MSB(length): a field value of length T in a packet matches with a		o MSB(length): a field value of a size equal to "length" bits in a
	field value in a rule if the most significant "length" bits are		packet matches with a field value in a rule if the most
	equal.		significant "length" bits are equal.
	4. Compression Decompression Functions (CDF)		o match-mapping: The goal of mapping-sent is to reduce the size of a
			field by allocating a shorter value. The Target Value contains a
			list of pairs. Each pair is composed of a value and a short ID.
			This operator matches if a field value is equal to one of the
			pairs' values.
	The Compression Decompression Functions (CDF) describe the action		Matching Operators may need a list of parameters to proceed to the
	taken during the compression and inversely the action taken by the		matching. For instance MSB requires an integer indicating the number
	decompressor to restore the original value.		of bits to test.
	/--------------------+-------------+--------------------------\		5. Compression Decompression Functions (CDF)
	| Function | Compression | Decompression |
	| | | |
	+--------------------+-------------+--------------------------+
	|not-sent |elided |use value stored in ctxt |
	|value-sent |send |build from received value |
	|LSB(length) |send LSB |ctxt value OR rcvd value |
	|compute-IPv6-length |elided |compute IPv6 length |
	|compute-UDP-length |elided |compute UDP length |
	|compute-UDP-checksum|elided |compute UDP checksum |
	|ESiid-DID |elided |build IID from L2 ES addr |
	|LAiid-DID |elided |build IID from L2 LA addr |
	\--------------------+-------------+--------------------------/
	Figure 5: Compression and Decompression Functions		The Compression Decompression Functions (CDF) describes the action
			taken during the compression of headers fields, and inversely, the
			action taken by the decompressor to restore the original value.
	Figure 5 lists all the functions defined to compress and decompress a		/--------------------+-------------+---------------------------\
			| Function | Compression | Decompression |
			| | | |
			+--------------------+-------------+---------------------------+
			|not-sent |elided |use value stored in ctxt |
			|value-sent |send |build from received value |
			|LSB(length) |send LSB |ctxt value OR rcvd value |
			|compute-length |elided |compute length |
			|compute-UDP-checksum|elided |compute UDP checksum |
			|ESiid-DID |elided |build IID from L2 ES addr |
			|LAiid-DID |elided |build IID from L2 LA addr |
			|mapping-sent |send index |value from index on a table|
			\--------------------+-------------+---------------------------/
			Figure 3: Compression and Decompression Functions
			Figure 3 sumarizes the functions defined to compress and decompress a
	field. The first column gives the function's name. The second and		field. The first column gives the function's name. The second and
	third columns outlines the compression/decompression process.		third columns outlines the compression/decompression behavior.
	As with 6LoWPAN, the compression process may produce some data, where		Compression is done in the rule order and compressed values are sent
	fields that were not compressed (or were partially compressed) will		in that order in the compressed message. The receiver must be able
	be sent in the order of the original packet. Information added by		to find the size of each compressed field which can be given by the
	the compression phase must be aligned on byte boundaries, but each		rule or may be sent with the compressed header.
	individual compression function may generate any size.
	/--------------+-------------------+-----------------------------------\		5.1. not-sent CDF
	| Field |Comp Decomp Fct | Behavior |
	+--------------+-------------------+-----------------------------------+
	|IPv6 version |not-sent |The value is not sent, but each |
	|IPv6 DiffServ | |end agrees on a value, which can |
	|IPv6 FL | |be different from 0. |
	|IPv6 NH |value-sent |Depending on the matching operator,|
	| | |the entire field value is sent or |
	| | |an adjustment to the context value |
	+--------------+-------------------+-----------------------------------+
	|IPv6 Length |compute-IPv6-length|Dedicated fct to reconstruct value |
	+--------------+-------------------+-----------------------------------+
	|IPv6 Hop Limit|not-sent+MO=ignore |The receiver takes the value stored|
	| | |in the context. It may be different|
	| | |from one originally sent, but in a |
	| | |star topology, there is no risk of |
	| | |loops |
	| |not-sent+matching |Receiver and sender agree on a |
	| | |specific value. |
	| |value-sent |Explicitly sent |
	+--------------+-------------------+-----------------------------------+
	|IPv6 ESPrefix |not-sent |The 64 bit prefix is stored on |
	|IPv6 LAPrefix | |the context |
	| |value-sent |Explicitly send 64 bits on the link|
	+--------------+-------------------+-----------------------------------+
	|IPv6 ESiid |not-sent |IID is not sent, but stored in the |
	|IPv6 LAiid | |context |
	| |ESiid-DID|LAiid-DID|IID is built from the ES/LA Dev. ID|
	| |value-sent |IID is explicitly sent on the link.|
	| | |Size depends of the L2 technology |
	+--------------+-------------------+-----------------------------------+
	|UDP ESport |not-sent |In the context |
	|UDP LAport |value-sent |Send the 2 bytes of the port number|
	| |LSB(length) |or least significant bits if MSB |
	| | |matching is specified in the |
	| | |matching operator. |
	+--------------+-------------------+-----------------------------------+
	|UDP length |compute-UDP-length |Dedicated fct to reconstruct value |
	+--------------+-------------------+-----------------------------------+
	|UDP Checksum |compute-UDP-checksum|Dedicated fct to reconstruct value|
	+--------------+-------------------+-----------------------------------+
	Figure 6: SCHC functions' example assignment for IPv6 and UDP		Not-sent function is generally used when the field value is specified
			in the rule and therefore known by the both Compressor and
			Decompressor. This function is generally used with the "equal" MO.
			If MO is "ignore", there is a risk to have a decompressed field value
			different from the compressed field.
	Figure 6 gives an example of function assignment to IPv6/UDP fields.		The compressor does not send any value on the compressed header for
			that field on which compression is applied.
	4.1. Compression Decompression Functions (CDF)		The decompressor restores the field value with the target value
			stored in the matched rule.
	4.1.1. not-sent		5.2. value-sent CDF
	The compressor do not sent the field value on the link. The		The value-sent function is generally used when the field value is not
	decompressor restore the field value with the one stored in the		known by both Compressor and Decompressor. The value is sent in the
	matched rule.		compressed message header. Both Compressor and Decompressor must
			know the size of the field, either implicitely (the size is known by
			both sides) or explicitely in the compressed header field by
			indicating the length. This function is generally used with the
			"ignore" MO.
	4.1.2. value-sent		The compressor sends the Target Value stored on the rule in the
			compressed header message. The decompressor restores the field value
			with the one received from the LPWAN
	The compressor send the field value on the link, if the matching		5.3. LSB CDF
	operator is "=". Otherwise the matching operator indicates the
	information that will be sent on the link. For a LSB operator only
	the Least Significant Bits are sent.
	4.1.3. LSB(length)		LSB function is used to send a fixed part of the packet field header
			to the other end. This function is used together with the "MSB" MO
	The compressor sends the "length" Least Significant Bits. The		The compressor sends the "length" Least Significant Bits. The
	decompressor combines with a OR operator the value received with the		decompressor combines with an OR operator the value received with the
	Target Value.		Target Value.
	4.1.4. ESiid-DID, LAiid-DID		5.4. ESiid-DID, LAiid-DID CDF
	These functions are used to process respectively the End System and		These functions are used to process respectively the End System and
	the LA Device Identifier (DID). The IID value is computed from		the LA Device Identifier (DID).
	device ID present in the Layer 2 header. The computation depends on
	the technology and the device ID size.
	4.1.5. Compute-*		The IID value is computed from the device ID present in the Layer 2
			header. The computation depends on the technology and the device ID
			size.
	These functions compute the field value based on received		5.5. mapping-sent
	information. They are elided during the compression and
	reconstructed during the decompression.
	o compute-ipv6-length: compute the IPv6 length field as described in		mapping-sent is used to send a smaller index associated to the field
	[RFC2460].		value in the Target Value. This function is used together with the
			"match-mapping" MO.
	o compute-udp-length: compute the IPv6 length field as described in		The compressor looks in the TV to find the field value and send the
	[RFC0768].		corresponding index. The decompressor uses this index to restore the
			field value.
	o compute-udp-checksum: compute the IPv6 length field as described		5.6. Compute-*
	in [RFC0768].
	5. Examples		These functions are used by the decompressor to compute the
			compressed field value based on received information. Compressed
			fields are elided during the compression and reconstructed during the
			decompression.
			o compute-length: compute the length assigned to this field. For
			instance, regarding the field ID, this CDF may be used to compute
			IPv6 length or UDP length.
			o compute-checksum: compute a checksum from the information already
			received by the LC. This field may be used to compute UDP
			checksum.
			6. Application to IPv6 and UDP headers
			This section lists the different IPv6 and UDP header fields and how
			they can be compressed.
			6.1. IPv6 version field
			This field always holds the same value, therefore the TV is 6, the MO
			is "equal" and the CDF "not-sent".
			6.2. IPv6 Traffic class field
			If the DiffServ field identified by the rest of the rule do not vary
			and is known by both sides, the TV should contain this wellknown
			value, the MO should be "equal" and the CDF must be "not-sent.
			If the DiffServ field identified by the rest of the rule varies over
			time or is not known by both sides, then there are two possibilities
			depending on the variability of the value, the first one there is
			without compression and the original value is sent, or the sencond
			where the values can be computed by sending only the LSB bits:
			o TV is not set, MO is set to "ignore" and CDF is set to "value-
			sent"
			o TV contains a stable value, MO is MSB(X) and CDF is set to
			LSB(8-X)
			6.3. Flow label field
			If the Flow Label field identified by the rest of the rule does not
			vary and is known by both sides, the TV should contain this well-
			known value, the MO should be "equal" and the CDF should be "not-
			sent".
			If the Flow Label field identified by the rest of the rule varies
			during time or is not known by both sides, there are two
			possibilities dpending on the variability of the value, the first one
			is without compression and then the value is sent and the second
			where only part of the value is sent and the decompressor needs to
			compute the original value:
			o TV is not set, MO is set to "ignore" and CDF is set to "value-
			sent"
			o TV contains a stable value, MO is MSB(X) and CDF is set to
			LSB(20-X)
			6.4. Payload Length field
			If the LPWAN technology does not add padding, this field can be
			elided for the transmission on the LPWAN network. The LC recompute
			the original payload length value. The TV is not set, the MO is set
			to "ignore" and the CDF is "compute-IPv6-length".
			If the payload is small, the TV can be set to 0x0000, the MO set to
			"MSB (16-s)" and the CDF to "LSB (s)". The 's' parameter depends on
			the maximum packet length.
			On other cases, the payload length field must be sent and the CDF is
			replaced by "value-sent".
			6.5. Next Header field
			If the Next Header field identified by the rest of the rule does not
			vary and is known by both sides, the TV should contain this Next
			Header value, the MO should be "equal" and the CDF should be "not-
			sent".
			If the Next header field identified by the rest of the rule varies
			during time or is not known by both sides, then TV is not set, MO is
			set to "ignore" and CDF is set to "value-sent".
			6.6. Hop Limit field
			The End System is generally a host and does not forward packets,
			therefore the Hop Limit value is constant. So the TV is set with a
			default value, the MO is set to "equal" and the CDF is set to "not-
			sent".
			Otherwise the value is sent on the LPWAN: TV is not set, MO is set to
			ignore and CDF is set to "value-sent".
			6.7. IPv6 addresses fields
			As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
			long fields; one for the prefix and one for the Interface Identifier
			(IID). These fields should be compressed. To allow a single rule,
			these values are identified by their role (ES or LA) and not by their
			position in the frame (source or destination). The LC must be aware
			of the traffic direction (upstream, downstream) to select the
			appropriate field.
			6.7.1. IPv6 source and destination prefixes
			Both ends must be synchronized with the appropriate prefixes. For a
			specific flow, the source and destination prefix can be unique and
			stored in the context. It can be either a link-local prefix or a
			global prefix. In that case, the TV for the source and destination
			prefixes contains the values, the MO is set to "equal" and the CDF is
			set to "not-sent".
			In case the rule allows several prefixes, static mapping must be
			used. The different prefixes are listed in the TV associated with a
			short ID. The MO is set to "match-mapping" and the CDF is set to
			"mapping-sent".
			Otherwise the TV contains the prefix, the MO is set to "equal" and
			the CDF is set to value-sent.
			6.7.2. IPv6 source and destination IID
			If the ES or LA IID are based on an LPWAN address, then the IID can
			be reconstructed with information coming from the LPWAN header. In
			that case, the TV is not set, the MO is set to "ignore" and the CDF
			is set to "ESiid-DID" or "LAiid-DID". Note that the LPWAN technology
			is generally carrying a single device identifier corresponding to the
			ES. The LC may also not be aware of these values.
			For privacy reasons or if the ES address is changing over time, it
			maybe better to use a static value. In that case, the TV contains
			the value, the MO operator is set to "equal" and the CDF is set to
			"not-sent".
			If several IIDs are possible, then the TV contains the list of
			possible IID, the MO is set to "match-mapping" and the CDF is set to
			"mapping-sent".
			Otherwise the value variation of the IID may be reduced to few bytes.
			In that case, the TV is set to the stable part of the IID, the MO is
			set to MSB and the CDF is set to LSB.
			Finally, the IID can be sent on the LPWAN. In that case, the TV is
			not set, the MO is set to "ignore" and the CDF is set to "value-
			sent".
			6.8. IPv6 extensions
			No extension rules are currently defined. They can be based on the
			MOs and CDFs described above.
			6.9. UDP source and destination port
			To allow a single rule, the UDP port values are identified by their
			role (ES or LA) and not by their position in the frame (source or
			destination). The LC must be aware of the traffic direction
			(upstream, downstream) to select the appropriate field. The
			following rules apply for ES and LA port numbers.
			If both ends knows the port number, it can be elided. The TV
			contains the port number, the MO is set to "equal" and the CDF is set
			to "not-sent".
			If the port variation is on few bits, the TV contains the stable part
			of the port number, the MO is set to "MSB" and the CDF is set to
			"LSB".
			If some well-known values are used, the TV can contain the list of
			this values, the MO is set to "match-mapping" and the CDF is set to
			"mapping-sent".
			Otherwise the port numbers are sent on the LPWAN. The TV is not set,
			the MO is set to "ignore" and the CDF is set to "value-sent".
			6.10. UDP length field
			If the LPWAN technology does not introduce padding, the UDP length
			can be computed from the received data. In that case the TV is not
			set, the MO is set to "ignore" and the CDF is set to "compute-UDP-
			length".
			If the payload is small, the TV can be set to 0x0000, the MO set to
			"MSB" and the CDF to "LSB".
			On other cases, the length must be sent and the CDF is replaced by
			"value-sent".
			6.11. UDP Checksum field
			IPv6 mandates a checksum in the protocol above IP. Nevertheless, if
			a more efficient mechanism such as L2 CRC or MIC is carried by or
			over the L2 (such as in the LPWAN fragmentation process (see XXXX)),
			the UDP checksum transmission can be avoided. In that case, the TV
			is not set, the MO is set to "ignore" and the CDF is set to "compute-
			UDP-checksum".
			In other cases the checksum must be explicitly sent. The TV is not
			set, the MO is set to "ignore" and the CDF is set to "value-sent".
			7. Examples
	This section gives some scenarios of the compression mechanism for		This section gives some scenarios of the compression mechanism for
	IPv6/UDP. The goal is to illustrate the SCHC behavior.		IPv6/UDP. The goal is to illustrate the SCHC behavior.
	5.1. IPv6/UDP compression in a star topology		7.1. IPv6/UDP compression
	The most common case will be a LPWA end-system embeds some		The most common case using the mechanisms defined in this document
	applications running over CoAP. In this example, the first flow is		will be a LPWAN end-system that embeds some applications running over
	for the device management based on CoAP using Link Local addresses		CoAP. In this example, three flows are considered. The first flow
	and UDP ports 123 and 124. The second flow will be a CoAP server for		is for the device management based on CoAP using Link Local IPv6
	measurements done by the end-system (using ports 5683) and Global		addresses and UDP ports 123 and 124 for ES and LA, respectively. The
	Addresses alpha::IID/64 to beta::1/64. The last flow is for legacy		second flow will be a CoAP server for measurements done by the end-
	applications using different ports numbers, the destination is		system (using ports 5683) and Global IPv6 Address prefixes
	gamma::1/64.		alpha::IID/64 to beta::1/64. The last flow is for legacy
			applications using different ports numbers, the destination IPv6
			address prefix is gamma::1/64.
	Figure 7 presents the protocol stack for this end-system. IPv6 and		Figure 4 presents the protocol stack for this End-System. IPv6 and
	UDP are represented with dotted lines since these protocols are		UDP are represented with dotted lines since these protocols are
	compressed on the radio link.		compressed on the radio link.
	Managment Data		Managment Data
	+----------+---------+---------+		+----------+---------+---------+
	| CoAP | CoAP | legacy |		| CoAP | CoAP | legacy |
	+----||----+---||----+---||----+		+----||----+---||----+---||----+
	. UDP . UDP | UDP |		. UDP . UDP | UDP |
	................................		................................
	. IPv6 . IPv6 . IPv6 .		. IPv6 . IPv6 . IPv6 .
	+------------------------------+		+------------------------------+
			| SCHC Header compression |
			| and fragmentation |
			+------------------------------+
	| 6LPWA L2 technologies |		| 6LPWA L2 technologies |
	+------------------------------+		+------------------------------+
	End System or LPWA GW		End System or LPWA GW
	Figure 7: Simplified Protocol Stack for LP-WAN		Figure 4: Simplified Protocol Stack for LP-WAN
	Note that in some LPWA technologies, only End Systems have a device		Note that in some LPWAN technologies, only the End Systems have a
	ID . Therefore it is necessary to define statically an IID for the		device ID. Therefore, when such technologie are used, it is
	Link Local address for the LPWA Compressor.		necessary to define statically an IID for the Link Local address for
			the LPWAN compressor.
	Rule 0		Rule 0
	+----------------+---------+--------+-------------++------+		+----------------+---------+--------+-------------++------+
	| Field | Value | Match | Function || Sent |		| Field | Value | Match | Function || Sent |
	+----------------+---------+----------------------++------+		+----------------+---------+----------------------++------+
	|IPv6 version |6 | equal | not-sent || |		|IPv6 version |6 | equal | not-sent || |
	|IPv6 DiffServ |0 | equal | not-sent || |		|IPv6 DiffServ |0 | equal | not-sent || |
	|IPv6 Flow Label |0 | equal | not-sent || |		|IPv6 Flow Label |0 | equal | not-sent || |
	|IPv6 Length | | ignore | comp-IPv6-l || |		|IPv6 Length | | ignore | comp-IPv6-l || |
	|IPv6 Next Header|17 | equal | not-sent || |		|IPv6 Next Header|17 | equal | not-sent || |
	|IPv6 Hop Limit |255 | ignore | not-sent || |		|IPv6 Hop Limit |255 | ignore | not-sent || |
	|IPv6 ESprefix |FE80::/64| equal | not-sent || |		|IPv6 ESprefix |FE80::/64| equal | not-sent || |
	|IPv6 ESiid | | ignore | ESiid-DID || |		|IPv6 ESiid | | ignore | ESiid-DID || |
	|IPv6 LCprefix |FE80::/64| equal | not-sent || |		|IPv6 LCprefix |FE80::/64| equal | not-sent || |
	|IPv6 LAiid |::1 | equal | not-sent || |		|IPv6 LAiid |::1 | equal | not-sent || |
	+================+=========+========+=============++======+		+================+=========+========+=============++======+
	|UDP ESport |123 | equal | not-sent || |		|UDP ESport |123 | equal | not-sent || |
	|UDP LAport |124 | equal | not-sent || |		|UDP LAport |124 | equal | not-sent || |
	|UDP Length | | ignore | comp-UDP-l || |		|UDP Length | | ignore | comp-length || |
	|UDP checksum | | ignore | comp-UDP-c || |		|UDP checksum | | ignore | comp-chk || |
	+================+=========+========+=============++======+		+================+=========+========+=============++======+
	Rule 1		Rule 1
	+----------------+---------+--------+-------------++------+		+----------------+---------+--------+-------------++------+
	| Field | Value | Match | Function || Sent |		| Field | Value | Match | Function || Sent |
	+----------------+---------+--------+-------------++------+		+----------------+---------+--------+-------------++------+
	|IPv6 version |6 | equal | not-sent || |		|IPv6 version |6 | equal | not-sent || |
	|IPv6 DiffServ |0 | equal | not-sent || |		|IPv6 DiffServ |0 | equal | not-sent || |
	|IPv6 Flow Label |0 | equal | not-sent || |		|IPv6 Flow Label |0 | equal | not-sent || |
	|IPv6 Length | | ignore | comp-IPv6-l || |		|IPv6 Length | | ignore | comp-IPv6-l || |
	|IPv6 Next Header|17 | equal | not-sent || |		|IPv6 Next Header|17 | equal | not-sent || |
	|IPv6 Hop Limit |255 | ignore | not-sent || |		|IPv6 Hop Limit |255 | ignore | not-sent || |
	|IPv6 ESprefix |alpha/64 | equal | not-sent || |		|IPv6 ESprefix |alpha/64 | equal | not-sent || |
	|IPv6 ESiid | | ignore | ESiid-DID || |		|IPv6 ESiid | | ignore | ESiid-DID || |
	|IPv6 LAprefix |beta/64 | equal | not-sent || |		|IPv6 LAprefix |beta/64 | equal | not-sent || |
	|IPv6 LAiid |::1000 | equal | not-sent || |		|IPv6 LAiid |::1000 | equal | not-sent || |
	+================+=========+========+=============++======+		+================+=========+========+=============++======+
	|UDP ESport |5683 | equal | not-sent || |		|UDP ESport |5683 | equal | not-sent || |
	|UDP LAport |5683 | equal | not-sent || |		|UDP LAport |5683 | equal | not-sent || |
	|UDP Length | | ignore | comp-UDP-l || |		|UDP Length | | ignore | comp-length || |
	|UDP checksum | | ignore | comp-UDP-c || |		|UDP checksum | | ignore | comp-chk || |
	+================+=========+========+=============++======+		+================+=========+========+=============++======+
	Rule 2		Rule 2
	+----------------+---------+--------+-------------++------+		+----------------+---------+--------+-------------++------+
	| Field | Value | Match | Function || Sent |		| Field | Value | Match | Function || Sent |
	+----------------+---------+--------+-------------++------+		+----------------+---------+--------+-------------++------+
	|IPv6 version |6 | equal | not-sent || |		|IPv6 version |6 | equal | not-sent || |
	|IPv6 DiffServ |0 | equal | not-sent || |		|IPv6 DiffServ |0 | equal | not-sent || |
	|IPv6 Flow Label |0 | equal | not-sent || |		|IPv6 Flow Label |0 | equal | not-sent || |
	|IPv6 Length | | ignore | comp-IPv6-l || |		|IPv6 Length | | ignore | comp-IPv6-l || |
	|IPv6 Next Header|17 | equal | not-sent || |		|IPv6 Next Header|17 | equal | not-sent || |
	|IPv6 Hop Limit |255 | ignore | not-sent || |		|IPv6 Hop Limit |255 | ignore | not-sent || |
	|IPv6 ESprefix |alpha/64 | equal | not-sent || |		|IPv6 ESprefix |alpha/64 | equal | not-sent || |
	|IPv6 ESiid | | ignore | ESiid-DID || |		|IPv6 ESiid | | ignore | ESiid-DID || |
	|IPv6 LAprefix |gamma/64 | equal | not-sent || |		|IPv6 LAprefix |gamma/64 | equal | not-sent || |
	|IPv6 LAiid |::1000 | equal | not-sent || |		|IPv6 LAiid |::1000 | equal | not-sent || |
	+================+=========+========+=============++======+		+================+=========+========+=============++======+
	|UDP ESport |8720 | MSB(12)| LSB(4) || lsb |		|UDP ESport |8720 | MSB(12)| LSB(4) || lsb |
	|UDP LAport |8720 | MSB(12)| LSB(4) || lsb |		|UDP LAport |8720 | MSB(12)| LSB(4) || lsb |
	|UDP Length | | ignore | comp-UDP-l || |		|UDP Length | | ignore | comp-length || |
	|UDP checksum | | ignore | comp-UDP-c || |		|UDP checksum | | ignore | comp-chk || |
	+================+=========+========+=============++======+		+================+=========+========+=============++======+
	Figure 8: Context rules		Figure 5: Context rules
	All the fields described in the three rules Figure 8 are present in		All the fields described in the three rules Figure 5 are present in
	the IPv6 and UDP headers. The ESDevice-ID value is found in the L2		the IPv6 and UDP headers. The ESDevice-ID value is found in the L2
	header.		header.
	The second and third rules use global addresses. The way the ES		The second and third rules use global addresses. The way the ES
	learn the prefix is not in the scope of the document. One possible		learns the prefix is not in the scope of the document.
	way is to use a management protocol to set up in both end rules the
	prefix used on the LPWA network.
	The third rule compresses port numbers on 4 bits. This value is		The third rule compresses port numbers to 4 bits.
	selected to maintain alignment on byte boundaries for the compressed
	header.
	6. Acknowledgements		8. Fragmentation
	Thanks to Dominique Barthel, Arunprabhu Kandasamy, Antony Markovski,		8.1. Overview
	Alexander Pelov, Juan Carlos Zuniga for useful design consideration.
	7. Normative References		Fragmentation in LPWAN is mandatory and it is used if after the SCHC
			header compression the size of the packet is larger than the L2 data
			unit maximum payload or if the SCHC header compression is not able to
			compress the packet. In LPWAN technologies the L2 data unit size
			typically varies from tens to hundreds of bytes. If the entire IPv6
			datagram fits within a single L2 data unit, the fragmentation
			mechanims is not used and the packet is sent unfragmented.
			If the datagram does not fit within a single L2 data unit, it SHALL
			be broken into fragments.
	[I-D.minaburo-lp-wan-gap-analysis]		Moreover, LPWAN technologies impose some strict limitations on
	Minaburo, A., Pelov, A., and L. Toutain, "LP-WAN GAP		traffic; therefore it is desirable to enable optional fragment
	Analysis", draft-minaburo-lp-wan-gap-analysis-01 (work in		retransmission, while a single fragment loss should not lead to
	progress), February 2016.		retransmitting the full datagram. To preserve energy, Things (End
			Systems) are sleeping most of the time and may receive data during a
			short period of time after transmission.
	[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,		This specification enables two main fragment delivery reliability
	DOI 10.17487/RFC0768, August 1980,		options, namely: Unreliable and Reliable. The same reliability
	<http://www.rfc-editor.org/info/rfc768>.		option MUST be used for all fragments of a packet. Note that the
			fragment delivery reliability option to be used is not necessarily
			tied to the particular characteristics of the underlying L2 LPWAN
			technology (e.g. Unreliable may be used on top of an L2 LPWAN
			technology with symmetric characteristics for uplink and downlink).
			In Unreliable, the receiver MUST NOT issue acknowledgments and the
			sender MUST NOT perform fragment transmission retries.
			In Reliable, there exist two possible suboptions, namely: packet mode
			and window mode. In packet mode, the receiver may transmit one
			acknowledgment (ACK) after all fragments carrying an IPv6 packet have
			been transmitted. The ACK informs the sender about received and
			missing fragments from the IPv6 packet. In window mode, an ACK may
			be transmitted by the fragment receiver after a window of fragments
			have been sent. A window of fragments is a subset of the fragments
			needed to carry an IPv6 packet. In this mode, the ACK informs the
			sender about received and missing fragments from the window of
			fragments. In either mode, upon receipt of an ACK that informs about
			any lost fragments, the sender may retransmit the lost fragments.
			The maximum number of ACK and retransmission rounds is TBD. In
			Reliable, the same reliability suboption MUST be used for all
			fragments of a packet.
			Some LPWAN deployments may benefit from conditioning the creation and
			transmission of an ACK to the detection of at least one fragment loss
			(per-packet or per-window), thus leading to negative ACK (NACK)-
			oriented behavior, while not having such condition may be preferred
			for other scenarios.
			This document does not make any decision as to whether Unreliable or
			Reliable are used, or whether in Reliable a fragment receiver
			generates ACKs per packet or per window, or whether the transmission
			of such ACKs is conditioned to the detection of fragment losses or
			not. A complete specification of the receiver and sender behaviors
			that correspond to each acknowledgment policy is also out of scope.
			Nevertheless, this document does provide examples of the different
			reliability options described.
			8.2. Fragment format
			A fragment comprises a fragmentation header and a fragment payload,
			and conforms to the format shown in Figure 6. The fragment payload
			carries a subset of either the IPv6 packet after header compression
			or an IPv6 packet which could not be compressed. A fragment is the
			payload in the L2 protocol data unit (PDU).
			+---------------+-----------------------+
			| Fragm. Header | Fragment payload |
			+---------------+-----------------------+
			Figure 6: Fragment format.
			8.3. Fragmentation header formats
			Fragments except the last one SHALL
			contain the fragmentation header as defined in Figure 7. The total
			size of this fragmentation header is R bits.
			<----------- R ----------->
			<-- N -->
			+----- ... -----+-- ... --+
			| Rule ID | CFN |
			+----- ... -----+-- ... --+
			Figure 7: Fragmentation Header for Fragments except the Last One
			The last fragment SHALL contain a fragmentation header that conforms
			to the format shown in Figure 8. The total size of this
			fragmentation header is R+M bits.
			<----------- R ---------->
			<-- N --> <---- M ----->
			+----- ... -----+-- ... --+---- ... ----+
			| Rule ID | 11..1 | MIC |
			+----- ... -----+-- ... --+---- ... ----+
			Figure 8: Fragmentation Header for the Last Fragment
			Rule ID: this field has a size of R - N bits in all fragments. Rule
			ID may be used to signal whether Unreliable or Reliable are in use,
			and within the latter, whether window mode or packet mode are used.
			CFN: CFN stands for Compressed Fragment Number. The size of the CFN
			field is N bits. In Unreliable, N=1. For Reliable, N equal to or
			greater than 3 is recommended. This field is an unsigned integer
			that carries a non-absolute fragment number. The CFN MUST be set
			sequentially decreasing from 2^N - 2 for the first fragment, and MUST
			wrap from 0 back to 2^N - 2 (e.g. for N=3, the first fragment has
			CFN=6, subsequent CFNs are set sequentially and in decreasing order,
			and CFN will wrap from 0 back to 6). The CFN for the last fragment
			has all bits set to 1. Note that, by this definition, the CFN value
			of 2^N - 1 is only used to identify a fragment as the last fragment
			carrying a subset of the IPv6 packet being transported, and thus the
			CFN does not strictly correspond to the N least significant bits of
			the actual absolute fragment number. It is also important to note
			that, for N=1, the last fragment of the packet will carry a CFN equal
			to 1, while all previous fragments will carry a CFN of 0.
			MIC: MIC stands for Message Integrity Check. This field has a size
			of M bits. It is computed by the sender over the complete IPv6
			packet before fragmentation by using the TBD algorithm. The MIC
			allows to check for errors in the reassembled IPv6 packet, while it
			also enables compressing the UDP checksum by use of SCHC.
			8.4. ACK format
			The format of an ACK is shown in Figure 9:
			<----- R ---->
			+-+-+-+-+-+-+-+-+----- ... ---+
			| Rule ID | bitmap |
			+-+-+-+-+-+-+-+-+----- ... ---+
			Figure 9: Format of an ACK
			Rule ID: In all ACKs, Rule ID has a size of R bits and SHALL be set
			to TBD_ACK to signal that the message is an ACK.
			bitmap: size of the bitmap field of an ACK can be equal to 0 or
			Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments
			denotes the number of fragments of a window (in window mode) or the
			number of fragments that carry the IPv6 packet (in packet mode). The
			bitmap is a sequence of bits, where the n-th bit signals whether the
			n-th fragment transmitted has been correctly received (n-th bit set
			to 1) or not (n-th bit set to 0). Remaining bits with bit order
			greater than the number of fragments sent (as determined by the
			receiver) are set to 0, except for the last bit in the bitmap, which
			is set to 1 if the last fragment (carrying the MIC) has been
			correctly received, and 0 otherwise. Absence of the bitmap in an ACK
			confirms correct reception of all fragments to be acknowledged by
			means of the ACK.
			Figure 10 shows an example of an ACK in packet mode, where the bitmap
			indicates that the second and the ninth fragments have not been
			correctly received. In this example, the IPv6 packet is carried by
			eleven fragments in total, therefore the bitmap in has a size of two
			bytes.
			1
			<----- R ----> 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| Rule ID |1|0|1|1|1|1|1|1|0|1|1|0|0|0|0|1|
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Figure 10: Example of the Bitmap in an ACK
			Figure 11 shows an example of an ACK in window mode (N=3), where the
			bitmap indicates that the second and the fifth fragments have not
			been correctly received.
			<----- R ----> 0 1 2 3 4 5 6 7
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			| Rule ID |1|0|1|1|0|1|1|1|
			+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Figure 11: Example of the bitmap in an ACK (in window mode, for N=3)
			Figure 12 illustrates an ACK without bitmap.
			<----- R ---->
			+-+-+-+-+-+-+-+-+
			| Rule ID |
			+-+-+-+-+-+-+-+-+
			Figure 12: Example of an ACK without bitmap
			8.5. Baseline mechanism
			The receiver of link fragments SHALL use (1) the sender's L2 source
			address (if present), (2) the destination's L2 address (if present),
			and (3) Rule ID to identify all the fragments that belong to a given
			datagram. The fragment receiver SHALL determine the fragment
			delivery reliability option in use for the fragment based on the Rule
			ID field in that fragment.
			Upon receipt of a link fragment, the receiver starts constructing the
			original unfragmented packet. It uses the CFN and the order of
			arrival of each fragment to determine the location of the individual
			fragments within the original unfragmented packet. For example, it
			may place the data payload of the fragments within a payload datagram
			reassembly buffer at the location determined from the CFN and order
			of arrival of the fragments, and the fragment payload sizes. Note
			that the size of the original, unfragmented IPv6 packet cannot be
			determined from fragmentation headers.
			In Reliable, when a fragment with all CFN bits set to 0 is received,
			the recipient MAY transmit an ACK for the last window of fragments
			sent. Note that the first fragment of the window is the one sent
			with CFN=2^N-2. In window mode, the fragment with CFN=0 is
			considered the last fragment of its window, except for the last
			fragment (with all CFN bits set to 1). The last fragment of a packet
			is also the last fragment of the last window.
			Once the recipient has received the last fragment, it checks for the
			integrity of the reassembled IPv6 datagram, based on the MIC
			received. In Unreliable, if the integrity check indicates that the
			reassembled IPv6 datagram does not match the original IPv6 datagram
			(prior to fragmentation), the reassembled IPv6 datagram MUST be
			discarded. In Reliable, upon receipt of the last fragment (i.e. with
			all CFN bits set to 1), the recipient MAY transmit an ACK for the
			last window of fragments sent (window mode) or for the whole set of
			fragments sent that carry a complete IPv6 packet (packet mode). In
			Reliable, the sender retransmits any lost fragments reported in the
			ACK. A maximum of TBD iterations of ACK and fragment retransmission
			rounds are allowed per-window or per-IPv6-packet in window mode or in
			packet mode, respectively. A complete specification of the
			mechanisms needed to enable the above described fragment delivery
			reliability options is out of the scope of this document.
			If a fragment recipient disassociates from its L2 network, the
			recipient MUST discard all link fragments of all partially
			reassembled payload datagrams, and fragment senders MUST discard all
			not yet transmitted link fragments of all partially transmitted
			payload (e.g., IPv6) datagrams. Similarly, when a node first
			receives a fragment of a packet, it starts a reassembly timer. When
			this time expires, if the entire packet has not been reassembled, the
			existing fragments MUST be discarded and the reassembly state MUST be
			flushed. The reassembly timeout MUST be set to a maximum of TBD
			seconds).
			8.6. Examples
			This section provides examples of different fragment delivery
			reliability options possible on the basis of this specification.
			Figure 13 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Unreliable.
			Sender Receiver
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=0-------->|
			|-------CFN=1-------->|MIC checked =>
			Figure 13: Transmission of an IPv6 packet carried by 11 fragments in
			Unreliable
			Figure 14 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, for N=3, NACK-oriented packet mode, without
			losses.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=3-------->|
			|-------CFN=2-------->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=7-------->|MIC checked =>
			(no NACK)
			Figure 14: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, NACK-oriented packet mode; no losses.
			Figure 15 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, for N=3, NACK-oriented packet mode, with
			three losses.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=3-------->|
			|-------CFN=2---X---->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=7-------->|MIC checked =>
			|<-------NACK---------|Bitmap:1101011110100000
			|-------CFN=4-------->|
			|-------CFN=2-------->|
			|-------CFN=4-------->|MIC checked =>
			(no NACK)
			Figure 15: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, NACK-oriented packet mode; three losses.
			Figure 16 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, window mode, for N=3, without losses.
			Receiver feedback is NACK-oriented. Note: in window mode, an
			additional bit will be needed to number windows.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=3-------->|
			|-------CFN=2-------->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			(no NACK)
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=7-------->|MIC checked =>
			(no NACK)
			Figure 16: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, NACK-oriented window mode; without losses.
			Figure 17 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, window mode, for N=3, with three losses.
			Receiver feedback is NACK-oriented. Note: in window mode, an
			additional bit will be needed to number windows.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=3-------->|
			|-------CFN=2---X---->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			|<-------NACK---------|Bitmap:11010110
			|-------CFN=4-------->|
			|-------CFN=2-------->|
			(no NACK)
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=7-------->|MIC checked =>
			|<-------NACK---------|Bitmap:11010000
			|-------CFN=4-------->|MIC checked =>
			(no NACK)
			Figure 17: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, NACK-oriented window mode; three losses.
			Figure 18 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, packet mode, for N=3, without losses.
			Receiver feedback is positive-ACK-oriented.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=3-------->|
			|-------CFN=2-------->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=7-------->|MIC checked =>
			|<-------ACK----------|no bitmap
			(End)
			Figure 18: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, packet mode, positive-ACK-oriented; no losses.
			Figure 19 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, packet mode, for N=3, with three losses.
			Receiver feedback is positive-ACK-oriented.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=3-------->|
			|-------CFN=2---X---->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=7-------->|MIC checked =>
			|<-------ACK----------|bitmap:1101011110100000
			|-------CFN=4-------->|
			|-------CFN=2-------->|
			|-------CFN=4-------->|MIC checked =>
			|<-------ACK----------|no bitmap
			(End)
			Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, packet mode, positive-ACK-oriented; with three
			losses.
			8.6.1. Reliable, window mode, ACK-oriented
			Figure 20 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, window mode, for N=3, without losses.
			Receiver feedback is positive-ACK-oriented. Note: in window mode, an
			additional bit will be needed to number windows.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=3-------->|
			|-------CFN=2-------->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			|<-------ACK----------|no bitmap
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4-------->|
			|-------CFN=7-------->|MIC checked =>
			|<-------ACK----------|no bitmap
			(End)
			Figure 20: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, window mode, positive-ACK-oriented; no losses.
			Figure 21 illustrates the transmission of an IPv6 packet that needs
			11 fragments in Reliable, window mode, for N=3, with three losses.
			Receiver feedback is positive-ACK-oriented. Note: in window mode, an
			additional bit will be needed to number windows.
			Sender Receiver
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=3-------->|
			|-------CFN=2---X---->|
			|-------CFN=1-------->|
			|-------CFN=0-------->|
			|<-------ACK----------|bitmap:11010110
			|-------CFN=4-------->|
			|-------CFN=2-------->|
			|<-------ACK----------|no bitmap
			|-------CFN=6-------->|
			|-------CFN=5-------->|
			|-------CFN=4---X---->|
			|-------CFN=7-------->|MIC checked =>
			|<-------ACK----------|bitmap:11010000
			|-------CFN=4-------->|MIC checked =>
			|<-------ACK----------|no bitmap
			(End)
			Figure 21: Transmission of an IPv6 packet carried by 11 fragments in
			Reliable, for N=3, window mode, positive-ACK-oriented; with three
			losses.
			9. Security considerations
			9.1. Security considerations for header compression
			TBD
			9.2. Security considerations for fragmentation
			This subsection describes potential attacks to LPWAN fragmentation
			and proposes countermeasures, based on existing analysis of attacks
			to 6LoWPAN fragmentation {HHWH}.
			A node can perform a buffer reservation attack by sending a first
			fragment to a target. Then, the receiver will reserve buffer space
			for the whole packet on the basis of the datagram size announced in
			that first fragment. Other incoming fragmented packets will be
			dropped while the reassembly buffer is occupied during the reassembly
			timeout. Once that timeout expires, the attacker can repeat the same
			procedure, and iterate, thus creating a denial of service attack.
			The (low) cost to mount this attack is linear with the number of
			buffers at the target node. However, the cost for an attacker can be
			increased if individual fragments of multiple packets can be stored
			in the reassembly buffer. To further increase the attack cost, the
			reassembly buffer can be split into fragment-sized buffer slots.
			Once a packet is complete, it is processed normally. If buffer
			overload occurs, a receiver can discard packets based on the sender
			behavior, which may help identify which fragments have been sent by
			an attacker.
			In another type of attack, the malicious node is required to have
			overhearing capabilities. If an attacker can overhear a fragment, it
			can send a spoofed duplicate (e.g. with random payload) to the
			destination. A receiver cannot distinguish legitimate from spoofed
			fragments. Therefore, the original IPv6 packet will be considered
			corrupt and will be dropped. To protect resource-constrained nodes
			from this attack, it has been proposed to establish a binding among
			the fragments to be transmitted by a node, by applying content-
			chaining to the different fragments, based on cryptographic hash
			functionality. The aim of this technique is to allow a receiver to
			identify illegitimate fragments.
			Further attacks may involve sending overlapped fragments (i.e.
			comprising some overlapping parts of the original IPv6 datagram).
			Implementers should make sure that correct operation is not affected
			by such event.
			10. Acknowledgements
			Thanks to Dominique Barthel, Carsten Bormann, Arunprabhu Kandasamy,
			Antony Markovski, Alexander Pelov, Pascal Thubert, Juan Carlos Zuniga
			for useful design consideration.
			In the fragmentation section, the authors have reused parts of text
			available in section 5.3 of RFC 4944, and would like to thank the
			authors of RFC 4944.
			Carles Gomez has been funded in part by the Spanish Government
			(Ministerio de Educacion, Cultura y Deporte) through the Jose
			Castillejo grant CAS15/00336, and by the ERDF and the Spanish
			Government through project TEC2016-79988-P. Part of his contribution
			to this work has been carried out during his stay as a visiting
			scholar at the Computer Laboratory of the University of Cambridge.
			11. References
			11.1. Normative References
	[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6		[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
	(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,		(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
	December 1998, <http://www.rfc-editor.org/info/rfc2460>.		December 1998, <http://www.rfc-editor.org/info/rfc2460>.
	[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,		[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
	"Transmission of IPv6 Packets over IEEE 802.15.4		"Transmission of IPv6 Packets over IEEE 802.15.4
	Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,		Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
	<http://www.rfc-editor.org/info/rfc4944>.		<http://www.rfc-editor.org/info/rfc4944>.
	[RFC4997] Finking, R. and G. Pelletier, "Formal Notation for RObust		11.2. Informative References
	Header Compression (ROHC-FN)", RFC 4997,
	DOI 10.17487/RFC4997, July 2007,
	<http://www.rfc-editor.org/info/rfc4997>.
	[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6		[I-D.ietf-lpwan-overview]
	Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,		Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
	DOI 10.17487/RFC6282, September 2011,		overview-01 (work in progress), February 2017.
	<http://www.rfc-editor.org/info/rfc6282>.
			[I-D.minaburo-lp-wan-gap-analysis]
			Minaburo, A., Pelov, A., and L. Toutain, "LP-WAN GAP
			Analysis", draft-minaburo-lp-wan-gap-analysis-01 (work in
			progress), February 2016.
	Authors' Addresses		Authors' Addresses
	Ana Minaburo		Ana Minaburo
	Acklio		Acklio
	2bis rue de la Chataigneraie		2bis rue de la Chataigneraie
	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 ; TELECOM Bretagne		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@telecom-bretagne.eu		Email: Laurent.Toutain@imt-atlantique.fr
			Carles Gomez
			Universitat Politecnica de Catalunya
			C/Esteve Terradas, 7
			08860 Castelldefels
			Spain
			Email: carlesgo@entel.upc.edu
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