draft-ietf-core-block-10.txt		draft-ietf-core-block-11.txt
	CoRE Working Group C. Bormann		CoRE Working Group C. Bormann
	Internet-Draft Universitaet Bremen TZI		Internet-Draft Universitaet Bremen TZI
	Intended status: Standards Track Z. Shelby, Ed.		Intended status: Standards Track Z. Shelby, Ed.
	Expires: April 24, 2013 Sensinode		Expires: October 01, 2013 Sensinode
	October 21, 2012		March 30, 2013
	Blockwise transfers in CoAP		Blockwise transfers in CoAP
	draft-ietf-core-block-10		draft-ietf-core-block-11
	Abstract		Abstract
	CoAP is a RESTful transfer protocol for constrained nodes and		CoAP is a RESTful transfer protocol for constrained nodes and
	networks. Basic CoAP messages work well for the small payloads we		networks. Basic CoAP messages work well for the small payloads we
	expect from temperature sensors, light switches, and similar		expect from temperature sensors, light switches, and similar
	building-automation devices. Occasionally, however, applications		building-automation devices. Occasionally, however, applications
	will need to transfer larger payloads -- for instance, for firmware		will need to transfer larger payloads -\u002D for instance, for
	updates. With HTTP, TCP does the grunt work of slicing large		firmware updates. With HTTP, TCP does the grunt work of slicing
	payloads up into multiple packets and ensuring that they all arrive		large payloads up into multiple packets and ensuring that they all
	and are handled in the right order.		arrive and are handled in the right order.
	CoAP is based on datagram transports such as UDP or DTLS, which		CoAP is based on datagram transports such as UDP or DTLS, which
	limits the maximum size of resource representations that can be		limits the maximum size of resource representations that can be
	transferred without too much fragmentation. Although UDP supports		transferred without too much fragmentation. Although UDP supports
	larger payloads through IP fragmentation, it is limited to 64 KiB		larger payloads through IP fragmentation, it is limited to 64 KiB
	and, more importantly, doesn't really work well for constrained		and, more importantly, doesn't really work well for constrained
	applications and networks.		applications and networks.
	Instead of relying on IP fragmentation, this specification extends		Instead of relying on IP fragmentation, this specification extends
	basic CoAP with a pair of "Block" options, for transferring multiple		basic CoAP with a pair of "Block" options, for transferring multiple
	blocks of information from a resource representation in multiple		blocks of information from a resource representation in multiple
	request-response pairs. In many important cases, the Block options		request-response pairs. In many important cases, the Block options
	enable a server to be truly stateless: the server can handle each		enable a server to be truly stateless: the server can handle each
	block transfer separately, with no need for a connection setup or		block transfer separately, with no need for a connection setup or
	other server-side memory of previous block transfers.		other server-side memory of previous block transfers.
	In summary, the Block options provide a minimal way to transfer		In summary, the Block options provide a minimal way to transfer
	larger representations in a block-wise fashion.		larger representations in a block-wise fashion.
	Status of this Memo		The present revision -11 fixes one example and adds the text and
			examples about the Block/Observe interaction, taken from -observe.
			It also adds a couple of formatting bugs from the new xml2rfc. The
			"grand rewrite" is next.
			Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on April 24, 2013.		This Internet-Draft will expire on October 01, 2013.
	Copyright Notice		Copyright Notice
	Copyright (c) 2012 IETF Trust and the persons identified as the		Copyright (c) 2013 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Block-wise transfers . . . . . . . . . . . . . . . . . . . . . 6		2. Block-wise transfers . . . . . . . . . . . . . . . . . . . . 5
	2.1. The Block Options . . . . . . . . . . . . . . . . . . . . 6		2.1. The Block Options . . . . . . . . . . . . . . . . . . . . 5
	2.2. Structure of a Block Option . . . . . . . . . . . . . . . 7		2.2. Structure of a Block Option . . . . . . . . . . . . . . . 6
	2.3. Block Options in Requests and Responses . . . . . . . . . 9		2.3. Block Options in Requests and Responses . . . . . . . . . 8
	2.4. Using the Block Options . . . . . . . . . . . . . . . . . 11		2.4. Using the Block2 Option . . . . . . . . . . . . . . . . . 10
	3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 14		2.5. Using the Block1 Option . . . . . . . . . . . . . . . . . 11
	4. The Size Option . . . . . . . . . . . . . . . . . . . . . . . 20		2.6. Combining Blockwise Transfers with the Observe Option . . 12
	5. HTTP Mapping Considerations . . . . . . . . . . . . . . . . . 22		2.7. Block2 and Initiative . . . . . . . . . . . . . . . . . . 13
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24		3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 13
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 25		3.1. Block2 Examples . . . . . . . . . . . . . . . . . . . . . 13
	7.1. Mitigating Resource Exhaustion Attacks . . . . . . . . . . 25		3.2. Block1 Examples . . . . . . . . . . . . . . . . . . . . . 16
	7.2. Mitigating Amplification Attacks . . . . . . . . . . . . . 26		3.3. Combining Block1 and Block2 . . . . . . . . . . . . . . . 18
	8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27		3.4. Combining Observe and Block2 . . . . . . . . . . . . . . 20
	9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28		4. The Size Option . . . . . . . . . . . . . . . . . . . . . . . 23
	9.1. Normative References . . . . . . . . . . . . . . . . . . . 28		5. HTTP Mapping Considerations . . . . . . . . . . . . . . . . . 24
	9.2. Informative References . . . . . . . . . . . . . . . . . . 28		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29		7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
			7.1. Mitigating Resource Exhaustion Attacks . . . . . . . . . 26
			7.2. Mitigating Amplification Attacks . . . . . . . . . . . . 27
			8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 27
			9. References . . . . . . . . . . . . . . . . . . . . . . . . . 28
			9.1. Normative References . . . . . . . . . . . . . . . . . . 28
			9.2. Informative References . . . . . . . . . . . . . . . . . 28
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
	1. Introduction		1. Introduction
	The CoRE WG is tasked with standardizing an Application Protocol for		The CoRE WG is tasked with standardizing an Application Protocol for
	Constrained Networks/Nodes, CoAP. This protocol is intended to		Constrained Networks/Nodes, CoAP. This protocol is intended to
	provide RESTful [REST] services not unlike HTTP [RFC2616], while		provide RESTful [REST] services not unlike HTTP [RFC2616], while
	reducing the complexity of implementation as well as the size of		reducing the complexity of implementation as well as the size of
	packets exchanged in order to make these services useful in a highly		packets exchanged in order to make these services useful in a highly
	constrained network of themselves highly constrained nodes.		constrained network of themselves highly constrained nodes.
	skipping to change at page 6, line 24		skipping to change at page 5, line 24
	o by the desire to avoid adaptation layer fragmentation (60-80 bytes		o by the desire to avoid adaptation layer fragmentation (60-80 bytes
	for 6LoWPAN [RFC4919])		for 6LoWPAN [RFC4919])
	When a resource representation is larger than can be comfortably		When a resource representation is larger than can be comfortably
	transferred in the payload of a single CoAP datagram, a Block option		transferred in the payload of a single CoAP datagram, a Block option
	can be used to indicate a block-wise transfer. As payloads can be		can be used to indicate a block-wise transfer. As payloads can be
	sent both with requests and with responses, this specification		sent both with requests and with responses, this specification
	provides two separate options for each direction of payload transfer.		provides two separate options for each direction of payload transfer.
	In the following, the term "payload" will be used for the actual		In the following, the term "payload" will be used for the actual
	content of a single CoAP message, i.e. a single block being		content of a single CoAP message, i.e. a single block being
	transferred, while the term "body" will be used for the entire		transferred, while the term "body" will be used for the entire
	resource representation that is being transferred in a block-wise		resource representation that is being transferred in a block-wise
	fashion.		fashion. The Content-Format option applies to the body, not to the
			payload, in particular the boundaries between the blocks may in
			places that are not whole units in terms of the structure, encoding,
			or content-coding used by the Content-Format.
	In most cases, all blocks being transferred for a body will be of the		In most cases, all blocks being transferred for a body will be of the
	same size. The block size is not fixed by the protocol. To keep the		same size. The block size is not fixed by the protocol. To keep the
	implementation as simple as possible, the Block options support only		implementation as simple as possible, the Block options support only
	a small range of power-of-two block sizes, from 2**4 (16) to 2**10		a small range of power-of-two block sizes, from 2**4 (16) to 2**10
	(1024) bytes. As bodies often will not evenly divide into the power-		(1024) bytes. As bodies often will not evenly divide into the power-
	of-two block size chosen, the size need not be reached in the final		of-two block size chosen, the size need not be reached in the final
	block (but even for the final block, the chosen power-of-two size		block (but even for the final block, the chosen power-of-two size
	will still be indicated in the block size field of the Block option).		will still be indicated in the block size field of the Block option).
	skipping to change at page 7, line 9		skipping to change at page 6, line 13
	Table 1: Block Option Numbers		Table 1: Block Option Numbers
	Both Block1 and Block2 options can be present both in request and		Both Block1 and Block2 options can be present both in request and
	response messages. In either case, the Block1 Option pertains to the		response messages. In either case, the Block1 Option pertains to the
	request payload, and the Block2 Option pertains to the response		request payload, and the Block2 Option pertains to the response
	payload.		payload.
	Hence, for the methods defined in [I-D.ietf-core-coap], Block1 is		Hence, for the methods defined in [I-D.ietf-core-coap], Block1 is
	useful with the payload-bearing POST and PUT requests and their		useful with the payload-bearing POST and PUT requests and their
	responses. Block2 is useful with GET, POST, and PUT requests and		responses. Block2 is useful with GET, POST, and PUT requests and
	their payload-bearing responses (2.01, 2.02, 2.04, 2.05 -- see		their payload-bearing responses (2.01, 2.02, 2.04, 2.05 -\u002D see
	section "Payload" of [I-D.ietf-core-coap]).		section "Payload" of [I-D.ietf-core-coap]).
	(As a memory aid: Block_1_ pertains to the payload of the _1st_ part		(As a memory aid: Block_1_ pertains to the payload of the _1st_ part
	of the request-response exchange, i.e. the request, and Block_2_		of the request-response exchange, i.e. the request, and Block_2_
	pertains to the payload of the _2nd_ part of the request-response		pertains to the payload of the _2nd_ part of the request-response
	exchange, i.e. the response.)		exchange, i.e. the response.)
	Where Block1 is present in a request or Block2 in a response (i.e.,		Where Block1 is present in a request or Block2 in a response (i.e.,
	in that message to the payload of which it pertains) it indicates a		in that message to the payload of which it pertains) it indicates a
	block-wise transfer and describes how this block-wise payload forms		block-wise transfer and describes how this block-wise payload forms
	part of the entire body being transferred ("descriptive usage").		part of the entire body being transferred ("descriptive usage").
	Where it is present in the opposite direction, it provides additional		Where it is present in the opposite direction, it provides additional
	control on how that payload will be formed or was processed ("control		control on how that payload will be formed or was processed ("control
	usage").		usage").
	Implementation of either Block option is intended to be optional.		Implementation of either Block option is intended to be optional.
	However, when it is present in a CoAP message, it MUST be processed		However, when it is present in a CoAP message, it MUST be processed
	(or the message rejected); therefore it is identified as a critical		(or the message rejected); therefore it is identified as a critical
	option. It MUST NOT occur more than once.		option. It MUST NOT occur more than once.
	2.2. Structure of a Block Option		2.2. Structure of a Block Option
	Three items of information may need to be transferred in a Block		Three items of information may need to be transferred in a Block
	option:		(Block1 or Block2) option:
	o The size of the block (SZX);		o The size of the block (SZX);
	o whether more blocks are following (M);		o whether more blocks are following (M);
	o the relative number of the block (NUM) within a sequence of blocks		o the relative number of the block (NUM) within a sequence of blocks
	with the given size.		with the given size.
	The value of the option is a 1-, 2- or 3-byte integer which encodes		The value of the Block Option is a variable-size (0 to 3 byte)
	these three fields, see Figure 1.		unsigned integer (uint, see Appendix A of [I-D.ietf-core-coap]).
			This integer value encodes these three fields, see Figure 1. (Due to
			the CoAP uint encoding rules, when all of NUM, M, and SZX happen to
			be zero, a zero-byte integer will be sent.)
	0		0
	0 1 2 3 4 5 6 7		0 1 2 3 4 5 6 7
	+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+
	| NUM |M| SZX |		| NUM |M| SZX |
	+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+
	0 1		0 1
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| NUM |M| SZX |		| NUM |M| SZX |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	0 1 2		0 1 2
	0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3		0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| NUM |M| SZX |		| NUM |M| SZX |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Figure 1: Block option value		Figure 1: Block option value
	The block size is encoded as a three-bit unsigned integer (0 for 2**4		The block size is encoded using a three-bit unsigned integer (0 for
	to 6 for 2**10 bytes), which we call the "SZX" (size exponent); the		2**4 to 6 for 2**10 bytes), which we call the "SZX" ("size
	actual block size is then "2**(SZX + 4)". SZX is transferred in the		exponent"); the actual block size is then "2**(SZX + 4)". SZX is
	three least significant bits of the option value (i.e., "val & 7"		transferred in the three least significant bits of the option value
	where "val" is the value of the option).		(i.e., "val & 7" where "val" is the value of the option).
	The fourth least significant bit, the M or "more" bit ("val & 8"),		The fourth least significant bit, the M or "more" bit ("val & 8"),
	indicates whether more blocks are following or the current block-wise		indicates whether more blocks are following or the current block-wise
	transfer is the last block being transferred.		transfer is the last block being transferred.
	The option value divided by sixteen (the NUM field) is the sequence		The option value divided by sixteen (the NUM field) is the sequence
	number of the block currently being transferred, starting from zero.		number of the block currently being transferred, starting from zero.
	The current transfer is therefore about the "size" bytes starting at		The current transfer is therefore about the "size" bytes starting at
	byte "NUM << (SZX + 4)".		byte "NUM << (SZX + 4)".
	Implementation note: As an implementation convenience, "(val & ~0xF)		Implementation note: As an implementation convenience, "(val & ~0xF)
	<< (val & 7)", i.e. the option value with the last 4 bits masked		<< (val & 7)", i.e., the option value with the last 4 bits masked
	out, shifted to the left by the value of SZX, gives the byte		out, shifted to the left by the value of SZX, gives the byte
	position of the block being transferred.		position of the block being transferred.
	More specifically, within the option value of a Block1 or Block2		More specifically, within the option value of a Block1 or Block2
	Option, the meaning of the option fields is defined as follows:		Option, the meaning of the option fields is defined as follows:
	NUM: Block Number. The block number is a variable-size (4, 12, or		NUM: Block Number, indicating the block number being requested or
	20 bit) unsigned integer (uint, see Appendix A of		provided. Block number 0 indicates the first block of a body
	[I-D.ietf-core-coap]) indicating the block number being requested		(i.e., starting with the first byte of the body).
	or provided. Block number 0 indicates the first block of a body.
	M: More Flag (not last block). For descriptive usage, this flag, if		M: More Flag (not last block). For descriptive usage, this flag, if
	unset, indicates that the payload in this message is the last		unset, indicates that the payload in this message is the last
	block in the body; when set it indicates that there are one or		block in the body; when set it indicates that there are one or
	more additional blocks available. When a Block2 Option is used in		more additional blocks available. When a Block2 Option is used in
	a request to retrieve a specific block number ("control usage"),		a request to retrieve a specific block number ("control usage"),
	the M bit MUST be sent as zero and ignored on reception. (In a		the M bit MUST be sent as zero and ignored on reception. (In a
	Block1 Option in a response, the M flag is used to indicate		Block1 Option in a response, the M flag is used to indicate
	atomicity, see below.)		atomicity, see below.)
	SZX: Block Size. The block size is a three-bit unsigned integer		SZX: Block Size. The block size is represented as three-bit
	indicating the size of a block to the power of two. Thus block		unsigned integer indicating the size of a block to the power of
	size = 2**(SZX + 4). The allowed values of SZX are 0 to 6, i.e.,		two. Thus block size = 2**(SZX + 4). The allowed values of SZX
	the minimum block size is 2**(0+4) = 16 and the maximum is		are 0 to 6, i.e., the minimum block size is 2**(0+4) = 16 and the
	2**(6+4) = 1024. The value 7 for SZX (which would indicate a		maximum is 2**(6+4) = 1024. The value 7 for SZX (which would
	block size of 2048) is reserved, i.e. MUST NOT be sent and MUST		indicate a block size of 2048) is reserved, i.e. MUST NOT be sent
	lead to a 4.00 Bad Request response code upon reception in a		and MUST lead to a 4.00 Bad Request response code upon reception
	request.		in a request.
	There is no default value for the Block1 and Block2 Options. Absence		There is no default value for the Block1 and Block2 Options. Absence
	of one of these options is equivalent to an option value of 0 with		of one of these options is equivalent to an option value of 0 with
	respect to the value of NUM and M that could be given in the option,		respect to the value of NUM and M that could be given in the option,
	i.e. it indicates that the current block is the first and only block		i.e. it indicates that the current block is the first and only block
	of the transfer (block number 0, M bit not set). However, in		of the transfer (block number 0, M bit not set). However, in
	contrast to the explicit value 0, which would indicate an SZX of 0		contrast to the explicit value 0, which would indicate an SZX of 0
	and thus a size value of 16 bytes, there is no specific explicit size		and thus a size value of 16 bytes, there is no specific explicit size
	implied by the absence of the option -- the size is left unspecified.		implied by the absence of the option -\u002D the size is left
	(Note that, as for any uint, the value 0 is efficiently indicated by		unspecified. (As for any uint, the explicit value 0 is efficiently
	a zero-length option; this, therefore, is different in semantics from		indicated by a zero-length option; this, therefore, is different in
	the absence of the option.)		semantics from the absence of the option.)
	2.3. Block Options in Requests and Responses		2.3. Block Options in Requests and Responses
	The Block options are used in one of three roles:		The Block options are used in one of three roles:
	o In descriptive usage, i.e. a Block2 Option in a response (e.g., a		o In descriptive usage, i.e., a Block2 Option in a response (such as
	2.05 response for GET), or a Block1 Option in a request (e.g., PUT		a 2.05 response for GET), or a Block1 Option in a request (a PUT
	or POST):		or POST):
	* The NUM field in the option value describes what block number		* The NUM field in the option value describes what block number
	is contained in the payload of this message.		is contained in the payload of this message.
	* The M bit indicates whether further blocks are required to		* The M bit indicates whether further blocks need to be
	complete the transfer of that body.		transferred to complete the transfer of that body.
	* The block size given by SZX MUST match the size of the payload		* The block size given by SZX MUST match the size of the payload
	in bytes, if the M bit is set. (SZX does not govern the		in bytes, if the M bit is set. (SZX does not govern the
	payload size if M is unset). For Block2, if the request		payload size if M is unset). For Block2, if the request
	suggested a larger value of SZX, the next request MUST move SZX		suggested a larger value of SZX, the next request MUST move SZX
	down to the size given here. (The effect is that, if the		down to the size given in the response. (The effect is that,
	server uses the smaller of its preferred block size and the one		if the server uses the smaller of (1) its preferred block size
	requested, all blocks for a body use the same block size.)		and (2) the block size requested, all blocks for a body use the
			same block size.)
	o A Block2 Option in control usage in a request (e.g., GET):		o A Block2 Option in control usage in a request (e.g., GET):
	* The NUM field in the Block2 Option gives the block number of		* The NUM field in the Block2 Option gives the block number of
	the payload that is being requested to be returned in the		the payload that is being requested to be returned in the
	response.		response.
	* In this case, the M bit has no function and MUST be set to		* In this case, the M bit has no function and MUST be set to
	zero.		zero.
	* The block size given (SZX) suggests a block size (in the case		* The block size given (SZX) suggests a block size (in the case
	of block number 0) or repeats the block size of previous blocks		of block number 0) or repeats the block size of previous blocks
	received (in the case of block numbers other than 0).		received (in the case of a non-zero block number).
	o A Block1 Option in control usage in a response (e.g., a 2.xx		o A Block1 Option in control usage in a response (e.g., a 2.xx
	response for a PUT or POST request):		response for a PUT or POST request):
	* The NUM field of the Block1 Option indicates what block number		* The NUM field of the Block1 Option indicates what block number
	is being acknowledged.		is being acknowledged.
	* If the M bit was set in the request, the server can choose		* If the M bit was set in the request, the server can choose
	whether to act on each block separately, with no memory, or		whether to act on each block separately, with no memory, or
	whether to handle the request for the entire body atomically,		whether to handle the request for the entire body atomically,
	or any mix of the two. If the M bit is also set in the		or any mix of the two.
	response, it indicates that this response does not carry the
	final response code to the request, i.e. the server collects		+ If the M bit is also set in the response, it indicates that
	further blocks and plans to implement the request atomically		this response does not carry the final response code to the
	(e.g., acts only upon reception of the last block of payload).		request, i.e. the server collects further blocks from the
	Conversely, if the M bit is unset even though it was set in the		same endpoint and plans to implement the request atomically
	request, it indicates the block-wise request was enacted now		(e.g., acts only upon reception of the last block of
	specifically for this block, and the response carries the final		payload). In this case, the response MUST NOT carry a
	response to this request (and to any previous ones with the M		Block2 option.
	bit set in the response's Block1 Option in this sequence of
	block-wise transfers); the client is still expected to continue		+ Conversely, if the M bit is unset even though it was set in
	sending further blocks, the request method for which may or may		the request, it indicates the block-wise request was enacted
	not also be enacted per-block.		now specifically for this block, and the response carries
			the final response to this request (and to any previous ones
			with the M bit set in the response's Block1 Option in this
			sequence of block-wise transfers); the client is still
			expected to continue sending further blocks, the request
			method for which may or may not also be enacted per-block.
	* Finally, the SZX block size given in a control Block1 Option		* Finally, the SZX block size given in a control Block1 Option
	indicates the largest block size preferred by the server for		indicates the largest block size preferred by the server for
	transfers toward the resource that is the same or smaller than		transfers toward the resource that is the same or smaller than
	the one used in the initial exchange; the client SHOULD use		the one used in the initial exchange; the client SHOULD use
	this block size or a smaller one in all further requests in the		this block size or a smaller one in all further requests in the
	transfer sequence, even if that means changing the block size		transfer sequence, even if that means changing the block size
	(and possibly scaling the block number accordingly) from now		(and possibly scaling the block number accordingly) from now
	on.		on.
	2.4. Using the Block Options
	Using one or both Block options, a single REST operation can be split		Using one or both Block options, a single REST operation can be split
	into multiple CoAP message exchanges. As specified in		into multiple CoAP message exchanges. As specified in
	[I-D.ietf-core-coap], each of these message exchanges uses their own		[I-D.ietf-core-coap], each of these message exchanges uses their own
	CoAP Message ID.		CoAP Message ID.
			2.4. Using the Block2 Option
	When a request is answered with a response carrying a Block2 Option		When a request is answered with a response carrying a Block2 Option
	with the M bit set, the requester may retrieve additional blocks of		with the M bit set, the requester may retrieve additional blocks of
	the resource representation by sending further requests with the same		the resource representation by sending further requests with the same
	options and a Block2 Option giving the block number and block size		options and a Block2 Option giving the block number and block size
	desired. In a request, the client MUST set the M bit of a Block2		desired. In a request, the client MUST set the M bit of a Block2
	Option to zero and the server MUST ignore it on reception.		Option to zero and the server MUST ignore it on reception.
	To influence the block size used in a response, the requester also		To influence the block size used in a response, the requester also
	uses the Block2 Option, giving the desired size, a block number of		uses the Block2 Option, giving the desired size, a block number of
	zero and an M bit of zero. A server MUST use the block size		zero and an M bit of zero. A server MUST use the block size
	skipping to change at page 11, line 33		skipping to change at page 10, line 39
	blocks beyond the first one MUST indicate the same block size that		blocks beyond the first one MUST indicate the same block size that
	was used by the server in the response for the first request that		was used by the server in the response for the first request that
	gave a desired size using a Block2 Option.		gave a desired size using a Block2 Option.
	Once the Block2 Option is used by the requester and a first response		Once the Block2 Option is used by the requester and a first response
	has been received with a possibly adjusted block size, all further		has been received with a possibly adjusted block size, all further
	requests in a single block-wise transfer SHOULD ultimately use the		requests in a single block-wise transfer SHOULD ultimately use the
	same size, except that there may not be enough content to fill the		same size, except that there may not be enough content to fill the
	last block (the one returned with the M bit not set). (Note that the		last block (the one returned with the M bit not set). (Note that the
	client may start using the Block2 Option in a second request after a		client may start using the Block2 Option in a second request after a
	first request without a Block2 Option resulted in a Block option in		first request without a Block2 Option resulted in a Block2 option in
	the response.) The server SHOULD use the block size indicated in the		the response.) The server SHOULD use the block size indicated in the
	request option or a smaller size, but the requester MUST take note of		request option or a smaller size, but the requester MUST take note of
	the actual block size used in the response it receives to its initial		the actual block size used in the response it receives to its initial
	request and proceed to use it in subsequent requests. The server		request and proceed to use it in subsequent requests. The server
	behavior MUST ensure that this client behavior results in the same		behavior MUST ensure that this client behavior results in the same
	block size for all responses in a sequence (except for the last one		block size for all responses in a sequence (except for the last one
	with the M bit not set, and possibly the first one if the initial		with the M bit not set, and possibly the first one if the initial
	request did not contain a Block2 Option).		request did not contain a Block2 Option).
	Block-wise transfers can be used to GET resources the representations		Block-wise transfers can be used to GET resources the representations
	skipping to change at page 12, line 15		skipping to change at page 11, line 22
	the blocks it retrieved first. To minimize the resulting		the blocks it retrieved first. To minimize the resulting
	inefficiency, the server MAY cache the current value of a		inefficiency, the server MAY cache the current value of a
	representation for an ongoing sequence of requests. (The server may		representation for an ongoing sequence of requests. (The server may
	identify the sequence by the combination of the requesting end-point		identify the sequence by the combination of the requesting end-point
	and the URI being the same in each block-wise request.) Note well		and the URI being the same in each block-wise request.) Note well
	that this specification makes no requirement for the server to		that this specification makes no requirement for the server to
	establish any state; however, servers that offer quickly changing		establish any state; however, servers that offer quickly changing
	resources may thereby make it impossible for a client to ever		resources may thereby make it impossible for a client to ever
	retrieve a consistent set of blocks.		retrieve a consistent set of blocks.
			2.5. Using the Block1 Option
	In a request with a request payload (e.g., PUT or POST), the Block1		In a request with a request payload (e.g., PUT or POST), the Block1
	Option refers to the payload in the request (descriptive usage).		Option refers to the payload in the request (descriptive usage).
	In response to a request with a payload (e.g., a PUT or POST		In response to a request with a payload (e.g., a PUT or POST
	transfer), the block size given in the Block1 Option indicates the		transfer), the block size given in the Block1 Option indicates the
	block size preference of the server for this resource (control		block size preference of the server for this resource (control
	usage). Obviously, at this point the first block has already been		usage). Obviously, at this point the first block has already been
	transferred by the client without benefit of this knowledge. Still,		transferred by the client without benefit of this knowledge. Still,
	the client SHOULD heed the preference and, for all further blocks,		the client SHOULD heed the preference and, for all further blocks,
	use the block size preferred by the server or a smaller one. Note		use the block size preferred by the server or a smaller one. Note
	skipping to change at page 12, line 42		skipping to change at page 11, line 51
	has been reached, and try restating the request as a block-wise		has been reached, and try restating the request as a block-wise
	transfer with a smaller payload. Note that this new attempt is then		transfer with a smaller payload. Note that this new attempt is then
	a new message-layer transaction and requires a new Message ID.		a new message-layer transaction and requires a new Message ID.
	(Because of the uncertainty whether the request or the		(Because of the uncertainty whether the request or the
	acknowledgement was lost, this strategy is useful mostly for		acknowledgement was lost, this strategy is useful mostly for
	idempotent requests.)		idempotent requests.)
	In a blockwise transfer of a request payload (e.g., a PUT or POST)		In a blockwise transfer of a request payload (e.g., a PUT or POST)
	that is intended to be implemented in an atomic fashion at the		that is intended to be implemented in an atomic fashion at the
	server, the actual creation/replacement takes place at the time the		server, the actual creation/replacement takes place at the time the
	final block, i.e. a block with the M bit unset in the Block1 Option,		final block, i.e. a block with the M bit unset in the Block1 Option,
	is received. If not all previous blocks are available at the server		is received. If not all previous blocks are available at the server
	at this time, the transfer fails and error code 4.08 (Request Entity		at this time, the transfer fails and error code 4.08 (Request Entity
	Incomplete) MUST be returned. The error code 4.13 (Request Entity		Incomplete) MUST be returned. The error code 4.13 (Request Entity
	Too Large) can be returned at any time by a server that does not		Too Large) can be returned at any time by a server that does not
	currently have the resources to store blocks for a block-wise request		currently have the resources to store blocks for a block-wise request
	payload transfer that it would intend to implement in an atomic		payload transfer that it would intend to implement in an atomic
	fashion. (Note that a 4.13 response to a request that does not		fashion. (Note that a 4.13 response to a request that does not
	employ Block1 is a hint for the client to try sending Block1, and a		employ Block1 is a hint for the client to try sending Block1, and a
	4.13 response with a smaller SZX in the Block1 than requested is a		4.13 response with a smaller SZX in its Block1 option than requested
	hint to try a smaller SZX.)		is a hint to try a smaller SZX.)
	Note that there is no way to perform multiple concurrently proceeding		The Block1 option provides no way for a single endpoint to perform
	block-wise request payload transfer (e.g., PUT or POST) operations to		multiple concurrently proceeding block-wise request payload transfer
	the same URI. Starting a new block-wise sequence of requests to the		(e.g., PUT or POST) operations to the same resource. Starting a new
	same URI (before an old sequence from the same endpoint was finished)		block-wise sequence of requests to the same resource (before an old
	simply overwrites the context the server may still be keeping. (This		sequence from the same endpoint was finished) simply overwrites the
	is probably exactly what one wants in this case - the client may		context the server may still be keeping. (This is probably exactly
	simply have restarted and lost its knowledge of the previous		what one wants in this case - the client may simply have restarted
	sequence.)		and lost its knowledge of the previous sequence.)
			2.6. Combining Blockwise Transfers with the Observe Option
			The Observe Option provides a way for a client to be notified about
			changes over time of a resource [I-D.ietf-core-observe]. Resources
			observed by clients may be larger than can be comfortably processed
			or transferred in one CoAP message. The following rules apply to the
			combination of blockwise transfers with notifications.
			As with basic GET transfers, the client can indicate its desired
			block size in a Block2 Option in the GET request. If the server
			supports blockwise transfers, it SHOULD take note of the block size
			and apply it as a maximum size to all notifications/responses
			resulting from the GET request (until the client is removed from the
			list of observers or the server receives a new GET request for the
			resource from the client).
			When sending a 2.05 (Content) notification, the server always sends
			all blocks of the representation, suitably sequenced by its
			congestion control mechanism, even if only some of the blocks have
			changed with respect to a previous notification. The server performs
			the blockwise transfer by making use of the Block2 Option in each
			block. When reassembling representations that are transmitted in
			multiple blocks, the client MUST NOT combine blocks carrying
			different Observe Option values.
			Blockwise transfers of notifications MUST use Confirmable messages
			and MUST NOT use Non-confirmable messages.
			See Section 3.4 for examples.
			2.7. Block2 and Initiative
			In a basic block-wise GET request, it is the job of the client to
			initiate each further block transfer. We say that the "initiative"
			is with the client. If no buffering of a snapshot of the resource is
			required, the server can stay entirely stateless. This is
			particularly useful for very simple servers for which all resources
			that are big enough to merit block-wise transfer are static (such as
			the links in "/.well-known/core").
			However, when Block2 is combined with Observe or Block1, this simple
			approach no longer works very well. Therefore, the presence of an
			Observe or Block1 option in combination with a Block2 option is said
			to reverse the initiative: From then on, it is the job of the server
			to provide additional responses that complete the blockwise transfer
			of the notification (Observe) or response to a block-wise PUT or POST
			transfer (Block1). As all these additional responses are in response
			to the single request that caused them, they all carry the token of
			this request: The GET with an Observe option, or the PUT/POST with a
			Block1 option.
			(For the request side of block-wise transfers that use the Block1
			option, it is of course always the initiative of the client to send
			the next block - which is quite natural, as the client has to
			generate them and therefore knows when it is time to send the next
			block.)
	3. Examples		3. Examples
	This section gives a number of short examples with message flows for		This section gives a number of short examples with message flows for
	a block-wise GET, and for a PUT or POST. These examples demonstrate		a block-wise GET, and for a PUT or POST. These examples demonstrate
	the basic operation, the operation in the presence of		the basic operation, the operation in the presence of
	retransmissions, and examples for the operation of the block size		retransmissions, and examples for the operation of the block size
	negotiation.		negotiation.
	In all these examples, a Block option is shown in a decomposed way		In all these examples, a Block option is shown in a decomposed way
	separating the kind of Block option (1 or 2), block number (NUM),		indicating the kind of Block option (1 or 2) followed by a colon, and
	more bit (M), and block size exponent (2**(SZX+4)) by slashes. E.g.,		then the block number (NUM), more bit (M), and block size exponent
	a Block2 Option value of 33 would be shown as 2/2/0/32), or a Block1		(2**(SZX+4)) separated by slashes. E.g., a Block2 Option value of 33
	Option value of 59 would be shown as 1/3/1/128.		would be shown as 2:2/0/32), or a Block1 Option value of 59 would be
			shown as 1:3/1/128.
			3.1. Block2 Examples
	The first example (Figure 2) shows a GET request that is split into		The first example (Figure 2) shows a GET request that is split into
	three blocks. The server proposes a block size of 128, and the		three blocks. The server proposes a block size of 128, and the
	client agrees. The first two ACKs contain 128 bytes of payload each,		client agrees. The first two ACKs contain 128 bytes of payload each,
	and third ACK contains between 1 and 128 bytes.		and third ACK contains between 1 and 128 bytes.
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], GET, /status ------> |		| CON [MID=1234], GET, /status ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |		| <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
	| |		| |
	| CON [MID=1235], GET, /status, 2/1/0/128 ------> |		| CON [MID=1235], GET, /status, 2:1/0/128 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.05 Content, 2/1/1/128 |		| <------ ACK [MID=1235], 2.05 Content, 2:1/1/128 |
	| |		| |
	| CON [MID=1236], GET, /status, 2/2/0/128 ------> |		| CON [MID=1236], GET, /status, 2:2/0/128 ------> |
	| |		| |
	| <------ ACK [MID=1236], 2.05 Content, 2/2/0/128 |		| <------ ACK [MID=1236], 2.05 Content, 2:2/0/128 |
	Figure 2: Simple blockwise GET		Figure 2: Simple blockwise GET
	In the second example (Figure 3), the client anticipates the		In the second example (Figure 3), the client anticipates the
	blockwise transfer (e.g., because of a size indication in the link-		blockwise transfer (e.g., because of a size indication in the link-
	format description [RFC6690]) and sends a size proposal. All ACK		format description [RFC6690]) and sends a size proposal. All ACK
	messages except for the last carry 64 bytes of payload; the last one		messages except for the last carry 64 bytes of payload; the last one
	carries between 1 and 64 bytes.		carries between 1 and 64 bytes.
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], GET, /status, 2/0/0/64 ------> |		| CON [MID=1234], GET, /status, 2:0/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.05 Content, 2/0/1/64 |		| <------ ACK [MID=1234], 2.05 Content, 2:0/1/64 |
	| |		| |
	| CON [MID=1235], GET, /status, 2/1/0/64 ------> |		| CON [MID=1235], GET, /status, 2:1/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.05 Content, 2/1/1/64 |		| <------ ACK [MID=1235], 2.05 Content, 2:1/1/64 |
	: :		: :
	: ... :		: ... :
	: :		: :
	| CON [MID=1238], GET, /status, 2/4/0/64 ------> |		| CON [MID=1238], GET, /status, 2:4/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1238], 2.05 Content, 2/4/1/64 |		| <------ ACK [MID=1238], 2.05 Content, 2:4/1/64 |
	| |		| |
	| CON [MID=1239], GET, /status, 2/5/0/64 ------> |		| CON [MID=1239], GET, /status, 2:5/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1239], 2.05 Content, 2/5/0/64 |		| <------ ACK [MID=1239], 2.05 Content, 2:5/0/64 |
	Figure 3: Blockwise GET with early negotiation		Figure 3: Blockwise GET with early negotiation
	In the third example (Figure 4), the client is surprised by the need		In the third example (Figure 4), the client is surprised by the need
	for a blockwise transfer, and unhappy with the size chosen		for a blockwise transfer, and unhappy with the size chosen
	unilaterally by the server. As it did not send a size proposal		unilaterally by the server. As it did not send a size proposal
	initially, the negotiation only influences the size from the second		initially, the negotiation only influences the size from the second
	message exchange onward. Since the client already obtained both the		message exchange onward. Since the client already obtained both the
	first and second 64-byte block in the first 128-byte exchange, it		first and second 64-byte block in the first 128-byte exchange, it
	goes on requesting the third 64-byte block ("2/0/64"). None of this		goes on requesting the third 64-byte block ("2/0/64"). None of this
	is (or needs to be) understood by the server, which simply responds		is (or needs to be) understood by the server, which simply responds
	to the requests as it best can.		to the requests as it best can.
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], GET, /status ------> |		| CON [MID=1234], GET, /status ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |		| <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
	| |		| |
	| CON [MID=1235], GET, /status, 2/2/0/64 ------> |		| CON [MID=1235], GET, /status, 2:2/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.05 Content, 2/2/1/64 |		| <------ ACK [MID=1235], 2.05 Content, 2:2/1/64 |
	| |		| |
	| CON [MID=1236], GET, /status, 2/3/0/64 ------> |		| CON [MID=1236], GET, /status, 2:3/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1236], 2.05 Content, 2/3/1/64 |		| <------ ACK [MID=1236], 2.05 Content, 2:3/1/64 |
	| |		| |
	| CON [MID=1237], GET, /status, 2/4/0/64 ------> |		| CON [MID=1237], GET, /status, 2:4/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1237], 2.05 Content, 2/4/1/64 |		| <------ ACK [MID=1237], 2.05 Content, 2:4/1/64 |
	| |		| |
	| CON [MID=1238], GET, /status, 2/5/0/64 ------> |		| CON [MID=1238], GET, /status, 2:5/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1238], 2.05 Content, 2/5/0/64 |		| <------ ACK [MID=1238], 2.05 Content, 2:5/0/64 |
	Figure 4: Blockwise GET with late negotiation		Figure 4: Blockwise GET with late negotiation
	In all these (and the following) cases, retransmissions are handled		In all these (and the following) cases, retransmissions are handled
	by the CoAP message exchange layer, so they don't influence the block		by the CoAP message exchange layer, so they don't influence the block
	operations (Figure 5, Figure 6).		operations (Figure 5, Figure 6).
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], GET, /status ------> |		| CON [MID=1234], GET, /status ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |		| <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
	| |		| |
	| CON [MID=1235], GE///////////////////////// |		| CON [MID=1235], GE///////////////////////// |
	| |		| |
	| (timeout) |		| (timeout) |
	| |		| |
	| CON [MID=1235], GET, /status, 2/2/0/64 ------> |		| CON [MID=1235], GET, /status, 2:2/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.05 Content, 2/2/1/64 |		| <------ ACK [MID=1235], 2.05 Content, 2:2/1/64 |
	: :		: :
	: ... :		: ... :
	: :		: :
	| CON [MID=1238], GET, /status, 2/5/0/64 ------> |		| CON [MID=1238], GET, /status, 2:5/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1238], 2.05 Content, 2/5/0/64 |		| <------ ACK [MID=1238], 2.05 Content, 2:5/0/64 |
	Figure 5: Blockwise GET with late negotiation and lost CON		Figure 5: Blockwise GET with late negotiation and lost CON
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], GET, /status ------> |		| CON [MID=1234], GET, /status ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.05 Content, 2/0/1/128 |		| <------ ACK [MID=1234], 2.05 Content, 2:0/1/128 |
	| |		| |
	| CON [MID=1235], GET, /status, 2/2/0/64 ------> |		| CON [MID=1235], GET, /status, 2:2/0/64 ------> |
	| |		| |
	| //////////////////////////////////tent, 2/2/1/64 |		| //////////////////////////////////tent, 2:2/1/64 |
	| |		| |
	| (timeout) |		| (timeout) |
	| |		| |
	| CON [MID=1235], GET, /status, 2/2/0/64 ------> |		| CON [MID=1235], GET, /status, 2:2/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.05 Content, 2/2/1/64 |		| <------ ACK [MID=1235], 2.05 Content, 2:2/1/64 |
	: :		: :
	: ... :		: ... :
	: :		: :
	| CON [MID=1238], GET, /status, 2/5/0/64 ------> |		| CON [MID=1238], GET, /status, 2:5/0/64 ------> |
	| |		| |
	| <------ ACK [MID=1238], 2.05 Content, 2/5/0/64 |		| <------ ACK [MID=1238], 2.05 Content, 2:5/0/64 |
	Figure 6: Blockwise GET with late negotiation and lost ACK		Figure 6: Blockwise GET with late negotiation and lost ACK
			3.2. Block1 Examples
	The following examples demonstrate a PUT exchange; a POST exchange		The following examples demonstrate a PUT exchange; a POST exchange
	looks the same, with different requirements on atomicity/idempotence.		looks the same, with different requirements on atomicity/idempotence.
	Note that, similar to GET, the responses to the requests that have a		Note that, similar to GET, the responses to the requests that have a
	more bit in the request Block1 Option are provisional; only the final		more bit in the request Block1 Option are provisional; only the final
	response tells the client that the PUT succeeded.		response tells the client that the PUT succeeded.
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], PUT, /options, 1/0/1/128 ------> |		| CON [MID=1234], PUT, /options, 1:0/1/128 ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.04 Changed, 1/0/1/128 |		| <------ ACK [MID=1234], 2.04 Changed, 1:0/1/128 |
	| |		| |
	| CON [MID=1235], PUT, /options, 1/1/1/128 ------> |		| CON [MID=1235], PUT, /options, 1:1/1/128 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.04 Changed, 1/1/1/128 |		| <------ ACK [MID=1235], 2.04 Changed, 1:1/1/128 |
	| |		| |
	| CON [MID=1236], PUT, /options, 1/2/0/128 ------> |		| CON [MID=1236], PUT, /options, 1:2/0/128 ------> |
	| |		| |
	| <------ ACK [MID=1236], 2.04 Changed, 1/2/0/128 |		| <------ ACK [MID=1236], 2.04 Changed, 1:2/0/128 |
	Figure 7: Simple atomic blockwise PUT		Figure 7: Simple atomic blockwise PUT
	A stateless server that simply builds/updates the resource in place		A stateless server that simply builds/updates the resource in place
	(statelessly) may indicate this by not setting the more bit in the		(statelessly) may indicate this by not setting the more bit in the
	response (Figure 8); in this case, the response codes are valid		response (Figure 8); in this case, the response codes are valid
	separately for each block being updated. This is of course only an		separately for each block being updated. This is of course only an
	acceptable behavior of the server if the potential inconsistency		acceptable behavior of the server if the potential inconsistency
	present during the run of the message exchange sequence does not lead		present during the run of the message exchange sequence does not lead
	to problems, e.g. because the resource being created or changed is		to problems, e.g. because the resource being created or changed is
	not yet or not currently in use.		not yet or not currently in use.
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], PUT, /options, 1/0/1/128 ------> |		| CON [MID=1234], PUT, /options, 1:0/1/128 ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.04 Changed, 1/0/0/128 |		| <------ ACK [MID=1234], 2.04 Changed, 1:0/0/128 |
	| |		| |
	| CON [MID=1235], PUT, /options, 1/1/1/128 ------> |		| CON [MID=1235], PUT, /options, 1:1/1/128 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.04 Changed, 1/1/0/128 |		| <------ ACK [MID=1235], 2.04 Changed, 1:1/0/128 |
	| |		| |
	| CON [MID=1236], PUT, /options, 1/2/0/128 ------> |		| CON [MID=1236], PUT, /options, 1:2/0/128 ------> |
	| |		| |
	| <------ ACK [MID=1236], 2.04 Changed, 1/2/0/128 |		| <------ ACK [MID=1236], 2.04 Changed, 1:2/0/128 |
	Figure 8: Simple stateless blockwise PUT		Figure 8: Simple stateless blockwise PUT
	Finally, a server receiving a blockwise PUT or POST may want to		Finally, a server receiving a blockwise PUT or POST may want to
	indicate a smaller block size preference (Figure 9). In this case,		indicate a smaller block size preference (Figure 9). In this case,
	the client SHOULD continue with a smaller block size; if it does, it		the client SHOULD continue with a smaller block size; if it does, it
	MUST adjust the block number to properly count in that smaller size.		MUST adjust the block number to properly count in that smaller size.
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], PUT, /options, 1/0/1/128 ------> |		| CON [MID=1234], PUT, /options, 1:0/1/128 ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.04 Changed, 1/0/1/32 |		| <------ ACK [MID=1234], 2.04 Changed, 1:0/1/32 |
	| |		| |
	| CON [MID=1235], PUT, /options, 1/4/1/32 ------> |		| CON [MID=1235], PUT, /options, 1:4/1/32 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.04 Changed, 1/4/1/32 |		| <------ ACK [MID=1235], 2.04 Changed, 1:4/1/32 |
	| |		| |
	| CON [MID=1236], PUT, /options, 1/5/1/32 ------> |		| CON [MID=1236], PUT, /options, 1:5/1/32 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.04 Changed, 1/5/1/32 |		| <------ ACK [MID=1235], 2.04 Changed, 1:5/1/32 |
	| |		| |
	| CON [MID=1237], PUT, /options, 1/6/0/32 ------> |		| CON [MID=1237], PUT, /options, 1:6/0/32 ------> |
	| |		| |
	| <------ ACK [MID=1236], 2.04 Changed, 1/6/0/32 |		| <------ ACK [MID=1236], 2.04 Changed, 1:6/0/32 |
	Figure 9: Simple atomic blockwise PUT with negotiation		Figure 9: Simple atomic blockwise PUT with negotiation
			3.3. Combining Block1 and Block2
	Block options may be used in both directions of a single exchange.		Block options may be used in both directions of a single exchange.
	The following example demonstrates a blockwise POST request,		The following example demonstrates a blockwise POST request,
	resulting in a separate blockwise response.		resulting in a separate blockwise response.
	CLIENT SERVER		CLIENT SERVER
	| |		| |
	| CON [MID=1234], POST, /soap, 1/0/1/128 ------> |		| CON [MID=1234], POST, /soap, 1:0/1/128 ------> |
	| |		| |
	| <------ ACK [MID=1234], 2.01 Created, 1/0/1/128 |		| <------ ACK [MID=1234], 2.01 Created, 1:0/1/128 |
	| |		| |
	| CON [MID=1235], POST, /soap, 1/1/1/128 ------> |		| CON [MID=1235], POST, /soap, 1:1/1/128 ------> |
	| |		| |
	| <------ ACK [MID=1235], 2.01 Created, 1/1/1/128 |		| <------ ACK [MID=1235], 2.01 Created, 1:1/1/128 |
	| |		| |
	| CON [MID=1236], POST, /soap, 1/2/0/128 ------> |		| CON [MID=1236], POST, /soap, 1:2/0/128 ------> |
	| |		| |
	| <------ ACK [MID=1236], 0, 1/2/0/128 |		| <------ ACK [MID=1236], 2.01 Created, 2:0/1/128, 1:2/0/128 |
	| |		| |
	| <------ CON [MID=4712], 2.01 Created, 2/0/1/128 |		| (initiative changes to server) |
	| |		| |
	| ACK [MID=4712], 0, 2/0/1/128 ------> |		| <------ CON [MID=4713], 2.01 Created, 2:1/1/128 |
	| |		| |
	| <------ CON [MID=4713], 2.01 Created, 2/1/1/128 |		| ACK [MID=4713], 0 ------> |
	| |		| |
	| ACK [MID=4713], 0, 2/1/1/128 ------> |		| <------ CON [MID=4714], 2.01 Created, 2:2/1/128 |
	| |		| |
	| <------ CON [MID=4714], 2.01 Created, 2/2/1/128 |		| ACK [MID=4714], 0 ------> |
	| |		| |
	| ACK [MID=4714], 0, 2/2/1/128 ------> |		| <------ CON [MID=4715], 2.01 Created, 2:3/0/128 |
	| |		| |
	| <------ CON [MID=4715], 2.01 Created, 2/3/0/128 |		| ACK [MID=4715], 0 ------> |
	| |
	| ACK [MID=4715], 0, 2/3/0/128 ------> |
	Figure 10: Atomic blockwise POST with separate blockwise response		Figure 10: Atomic blockwise POST with separate blockwise response
			This model does provide for early negotiation input to the Block2
			blockwise transfer, as shown below. (However, there is no way to
			provide late negotiation with server initiative.)
			CLIENT SERVER
			| |
			| CON [MID=1234], POST, /soap, 1:0/1/128 ------> |
			| |
			| <------ ACK [MID=1234], 2.01 Created, 1:0/1/128 |
			| |
			| CON [MID=1235], POST, /soap, 1:1/1/128 ------> |
			| |
			| <------ ACK [MID=1235], 2.01 Created, 1:1/1/128 |
			| |
			| CON [MID=1236], POST, /soap, 1:2/0/128, 2:0/0/64 ------> |
			| |
			| <------ ACK [MID=1236], 2.01 Created, 1:2/0/128, 2:0/1/64 |
			| |
			| (initiative changes to server) |
			| |
			| <------ CON [MID=4713], 2.01 Created, 2:1/1/64 |
			| |
			| ACK [MID=4713], 0 ------> |
			| |
			| <------ CON [MID=4714], 2.01 Created, 2:2/1/64 |
			| |
			| ACK [MID=4714], 0 ------> |
			| |
			| <------ CON [MID=4715], 2.01 Created, 2:3/0/64 |
			| |
			| ACK [MID=4715], 0 ------> |
			Figure 11: Atomic blockwise POST with separate blockwise response,
			early negotiation
			3.4. Combining Observe and Block2
			In the following example, the server sends two notifications of two
			blocks each. The first notification is a direct response to the GET
			request; the first block therefore can be sent piggy-backed in the
			ACK. The Observe Option indicates that the initiative has switched
			to the server.
			CLIENT SERVER
			| |
			+----->| Header: GET 0x41011636
			| GET | Token: 0xfb
			| | Uri-Path: status-icon
			| | Observe: (empty)
			| |
			|<-----+ Header: 2.05 0x61451636
			| 2.05 | Token: 0xfb
			| | Block2: 0/1/128
			| | Observe: 62354
			| | Max-Age: 60
			| | Payload: [128 bytes]
			| |
			| | (initiative changes to server)
			| |
			|<-----+ Header: 2.05 0x4145af9c
			| 2.05 | Token: 0xfb
			| | Block2: 1/0/128
			| | Observe: 62354
			| | Max-Age: 60
			| | Payload: [27 bytes]
			| |
			+- - ->| Header: 0x6000af9c
			| |
			|<-----+ Header: 2.05 0x4145af9d
			| 2.05 | Token: 0xfb
			| | Block2: 0/1/128
			| | Observe: 62444
			| | Max-Age: 60
			| | Payload: [128 bytes]
			| |
			+- - ->| Header: 0x6000af9d
			| |
			|<-----+ Header: 2.05 0x4145af9e
			| 2.05 | Token: 0xfb
			| | Block2: 1/0/128
			| | Observe: 62444
			| | Max-Age: 60
			| | Payload: [27 bytes]
			| |
			+- - ->| Header: 0x6000af9e
			| |
			Figure 12: Observe sequence with blockwise response
			In the following example, the client also uses early negotiation to
			limit the block size to 64 bytes.
			CLIENT SERVER
			| |
			+----->| Header: GET 0x41011636
			| GET | Token: 0xfb
			| | Uri-Path: status-icon
			| | Observe: (empty)
			| | Block2: 0/0/64
			| |
			|<-----+ Header: 2.05 0x61451636
			| 2.05 | Token: 0xfb
			| | Block2: 0/1/64
			| | Observe: 62354
			| | Max-Age: 60
			| | Payload: [64 bytes]
			| |
			| | (initiative changes to server)
			| |
			|<-----+ Header: 2.05 0x4145af9c
			| 2.05 | Token: 0xfb
			| | Block2: 1/1/64
			| | Observe: 62354
			| | Max-Age: 60
			| | Payload: [64 bytes]
			| |
			+- - ->| Header: 0x6000af9c
			| |
			|<-----+ Header: 2.05 0x4145af9d
			| 2.05 | Token: 0xfb
			| | Block2: 2/0/64
			| | Observe: 62354
			| | Max-Age: 60
			| | Payload: [27 bytes]
			| |
			+- - ->| Header: 0x6000af9d
			| |
			|<-----+ Header: 2.05 0x4145af9e
			| 2.05 | Token: 0xfb
			| | Block2: 0/1/64
			| | Observe: 62444
			| | Max-Age: 60
			| | Payload: [128 bytes]
			| |
			+- - ->| Header: 0x6000af9e
			| |
			|<-----+ Header: 2.05 0x4145af9f
			| 2.05 | Token: 0xfb
			| | Block2: 1/1/64
			| | Observe: 62444
			| | Max-Age: 60
			| | Payload: [128 bytes]
			| |
			+- - ->| Header: 0x6000af9f
			| |
			|<-----+ Header: 2.05 0x4145afa0
			| 2.05 | Token: 0xfb
			| | Block2: 2/0/64
			| | Observe: 62444
			| | Max-Age: 60
			| | Payload: [27 bytes]
			| |
			+- - ->| Header: 0x6000afa0
			| |
			Figure 13: Observe sequence with early negotiation
	4. The Size Option		4. The Size Option
	In many cases when transferring a large resource representation block		In many cases when transferring a large resource representation block
	by block, it is advantageous to know the total size early in the		by block, it is advantageous to know the total size early in the
	process. Some indication may be available from the maximum size		process. Some indication may be available from the maximum size
	estimate attribute "sz" provided in a resource description [RFC6690].		estimate attribute "sz" provided in a resource description [RFC6690].
	However, the size may vary dynamically, so a more up-to-date		However, the size may vary dynamically, so a more up-to-date
	indication may be useful.		indication may be useful.
	The Size Option may be used for three purposes:		The Size Option may be used for three purposes:
	skipping to change at page 25, line 43		skipping to change at page 27, line 4
	A stateless server might be susceptible to an attack where the		A stateless server might be susceptible to an attack where the
	adversary sends a Block1 (e.g., PUT) block with a high block number:		adversary sends a Block1 (e.g., PUT) block with a high block number:
	A naive implementation might exhaust its resources by creating a huge		A naive implementation might exhaust its resources by creating a huge
	resource representation.		resource representation.
	Misleading size indications may be used by an attacker to induce		Misleading size indications may be used by an attacker to induce
	buffer overflows in poor implementations, for which the usual		buffer overflows in poor implementations, for which the usual
	considerations apply.		considerations apply.
	7.1. Mitigating Resource Exhaustion Attacks		7.1. Mitigating Resource Exhaustion Attacks
	Certain blockwise requests may induce the server to create state,		Certain blockwise requests may induce the server to create state,
	e.g. to create a snapshot for the blockwise GET of a fast-changing		e.g. to create a snapshot for the blockwise GET of a fast-changing
	resource to enable consistent access to the same version of a		resource to enable consistent access to the same version of a
	resource for all blocks, or to create temporary resource		resource for all blocks, or to create temporary resource
	representations that are collected until pressed into service by a		representations that are collected until pressed into service by a
	final PUT or POST with the more bit unset. All mechanisms that		final PUT or POST with the more bit unset. All mechanisms that
	induce a server to create state that cannot simply be cleaned up		induce a server to create state that cannot simply be cleaned up
	create opportunities for denial-of-service attacks. Servers SHOULD		create opportunities for denial-of-service attacks. Servers SHOULD
	avoid being subject to resource exhaustion based on state created by		avoid being subject to resource exhaustion based on state created by
	untrusted sources. But even if this is done, the mitigation may		untrusted sources. But even if this is done, the mitigation may
	cause a denial-of-service to a legitimate request when it is drowned		cause a denial-of-service to a legitimate request when it is drowned
	out by other state-creating requests. Wherever possible, servers		out by other state-creating requests. Wherever possible, servers
	should therefore minimize the opportunities to create state for		should therefore minimize the opportunities to create state for
	untrusted sources, e.g. by using stateless approaches.		untrusted sources, e.g. by using stateless approaches.
	Performing segmentation at the application layer is almost always		Performing segmentation at the application layer is almost always
	better in this respect than at the transport layer or lower (IP		better in this respect than at the transport layer or lower (IP
	fragmentation, adaptation layer fragmentation), e.g. because there is		fragmentation, adaptation layer fragmentation), e.g. because there
	application layer semantics that can be used for mitigation or		is application layer semantics that can be used for mitigation or
	because lower layers provide security associations that can prevent		because lower layers provide security associations that can prevent
	attacks. However, it is less common to apply timeouts and keepalive		attacks. However, it is less common to apply timeouts and keepalive
	mechanisms at the application layer than at lower layers. Servers		mechanisms at the application layer than at lower layers. Servers
	MAY want to clean up accumulated state by timing it out (cf. response		MAY want to clean up accumulated state by timing it out (cf.
	code 4.08), and clients SHOULD be prepared to run blockwise transfers		response code 4.08), and clients SHOULD be prepared to run blockwise
	in an expedient way to minimize the likelihood of running into such a		transfers in an expedient way to minimize the likelihood of running
	timeout.		into such a timeout.
	7.2. Mitigating Amplification Attacks		7.2. Mitigating Amplification Attacks
	[I-D.ietf-core-coap] discusses the susceptibility of CoAP end-points		[I-D.ietf-core-coap] discusses the susceptibility of CoAP end-points
	for use in amplification attacks.		for use in amplification attacks.
	A CoAP server can reduce the amount of amplification it provides to		A CoAP server can reduce the amount of amplification it provides to
	an attacker by offering large resource representations only in		an attacker by offering large resource representations only in
	relatively small blocks. With this, e.g., for a 1000 byte resource,		relatively small blocks. With this, e.g., for a 1000 byte resource,
	a 10-byte request might result in an 80-byte response (with a 64-byte		a 10-byte request might result in an 80-byte response (with a 64-byte
	skipping to change at page 27, line 18		skipping to change at page 28, line 8
	the [I-D.ietf-core-coap] authors, and via many CoRE WG discussions.		the [I-D.ietf-core-coap] authors, and via many CoRE WG discussions.
	Charles Palmer provided extensive editorial comments to a previous		Charles Palmer provided extensive editorial comments to a previous
	version of this draft, some of which the authors hope to have covered		version of this draft, some of which the authors hope to have covered
	in this version. Esko Dijk reviewed a more recent version, leading		in this version. Esko Dijk reviewed a more recent version, leading
	to a number of further editorial improvements as well as a solution		to a number of further editorial improvements as well as a solution
	to the 4.13 ambiguity problem. Markus Becker proposed getting rid of		to the 4.13 ambiguity problem. Markus Becker proposed getting rid of
	an ill-conceived default value for the Block2 and Block1 options.		an ill-conceived default value for the Block2 and Block1 options.
	Kepeng Li, Linyi Tian, and Barry Leiba wrote up an early version of		Kepeng Li, Linyi Tian, and Barry Leiba wrote up an early version of
	the Size Option, which has informed this draft.		the Size Option, which has informed this draft. Klaus Hartke wrote
			some of the text describing the interaction of Block2 with Observe.
	9. References		9. References
	9.1. Normative References		9.1. Normative References
	[I-D.ietf-core-coap]		[I-D.ietf-core-coap]
	Shelby, Z., Hartke, K., Bormann, C., and B. Frank,		Shelby, Z., Hartke, K., and C. Bormann, "Constrained
	"Constrained Application Protocol (CoAP)",		Application Protocol (CoAP)", draft-ietf-core-coap-14
	draft-ietf-core-coap-12 (work in progress), October 2012.		(work in progress), March 2013.
			[I-D.ietf-core-observe]
			Hartke, K., "Observing Resources in CoAP", draft-ietf-
			core-observe-08 (work in progress), February 2013.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,		[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
	Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext		Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
	Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.		Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
	9.2. Informative References		9.2. Informative References
	[REST] Fielding, R., "Architectural Styles and the Design of		[REST] Fielding, R., "Architectural Styles and the Design of
	Network-based Software Architectures", Ph.D. Dissertation,		Network-based Software Architectures", Ph.D. Dissertation,
	University of California, Irvine, 2000, <http://		University of California, Irvine, 2000, <http://
	www.ics.uci.edu/~fielding/pubs/dissertation/		www.ics.uci.edu/~fielding/pubs/dissertation/
	fielding_dissertation.pdf>.		fielding_dissertation.pdf>.
	[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6		[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
	over Low-Power Wireless Personal Area Networks (6LoWPANs):		over Low-Power Wireless Personal Area Networks (6LoWPANs):
	Overview, Assumptions, Problem Statement, and Goals",		Overview, Assumptions, Problem Statement, and Goals", RFC
	RFC 4919, August 2007.		4919, August 2007.
	[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link		[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
	Format", RFC 6690, August 2012.		Format", RFC 6690, August 2012.
	Authors' Addresses		Authors' Addresses
	Carsten Bormann		Carsten Bormann
	Universitaet Bremen TZI		Universitaet Bremen TZI
	Postfach 330440		Postfach 330440
	Bremen D-28359		Bremen D-28359
End of changes. 100 change blocks.
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