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CoRE Working Group                                            C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                          Z. Shelby, Ed.
Expires: April 24, 2014                                        Sensinode
                                                        October 21, 2013


                      Blockwise transfers in CoAP
                        draft-ietf-core-block-14

Abstract

   CoAP is a RESTful transfer protocol for constrained nodes and
   networks.  Basic CoAP messages work well for the small payloads we
   expect from temperature sensors, light switches, and similar
   building-automation devices.  Occasionally, however, applications
   will need to transfer larger payloads -- for instance, for
   firmware updates.  With HTTP, TCP does the grunt work of slicing
   large payloads up into multiple packets and ensuring that they all
   arrive and are handled in the right order.

   CoAP is based on datagram transports such as UDP or DTLS, which
   limits the maximum size of resource representations that can be
   transferred without too much fragmentation.  Although UDP supports
   larger payloads through IP fragmentation, it is limited to 64 KiB
   and, more importantly, doesn't really work well for constrained
   applications and networks.

   Instead of relying on IP fragmentation, this specification extends
   basic CoAP with a pair of "Block" options, for transferring multiple
   blocks of information from a resource representation in multiple
   request-response pairs.  In many important cases, the Block options
   enable a server to be truly stateless: the server can handle each
   block transfer separately, with no need for a connection setup or
   other server-side memory of previous block transfers.

   In summary, the Block options provide a minimal way to transfer
   larger representations in a block-wise fashion.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.



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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 24, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Block-wise transfers  . . . . . . . . . . . . . . . . . . . .   4
     2.1.  The Block2 and Block1 Options . . . . . . . . . . . . . .   5
     2.2.  Structure of a Block Option . . . . . . . . . . . . . . .   6
     2.3.  Block Options in Requests and Responses . . . . . . . . .   8
     2.4.  Using the Block2 Option . . . . . . . . . . . . . . . . .  10
     2.5.  Using the Block1 Option . . . . . . . . . . . . . . . . .  11
     2.6.  Combining Blockwise Transfers with the Observe Option . .  12
     2.7.  Combining Block1 and Block2 . . . . . . . . . . . . . . .  13
     2.8.  Combining Block2 with Multicast . . . . . . . . . . . . .  13
     2.9.  Response Codes  . . . . . . . . . . . . . . . . . . . . .  14
       2.9.1.  2.31 Continue . . . . . . . . . . . . . . . . . . . .  14
       2.9.2.  4.08 Request Entity Incomplete  . . . . . . . . . . .  14
       2.9.3.  4.13 Request Entity Too Large . . . . . . . . . . . .  14
   3.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     3.1.  Block2 Examples . . . . . . . . . . . . . . . . . . . . .  15
     3.2.  Block1 Examples . . . . . . . . . . . . . . . . . . . . .  18
     3.3.  Combining Block1 and Block2 . . . . . . . . . . . . . . .  20
     3.4.  Combining Observe and Block2  . . . . . . . . . . . . . .  21
   4.  The Size2 and Size1 Options . . . . . . . . . . . . . . . . .  24
   5.  HTTP Mapping Considerations . . . . . . . . . . . . . . . . .  25
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     7.1.  Mitigating Resource Exhaustion Attacks  . . . . . . . . .  28



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     7.2.  Mitigating Amplification Attacks  . . . . . . . . . . . .  29
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  30
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   The CoRE WG is tasked with standardizing an Application Protocol for
   Constrained Networks/Nodes, CoAP.  This protocol is intended to
   provide RESTful [REST] services not unlike HTTP [RFC2616], while
   reducing the complexity of implementation as well as the size of
   packets exchanged in order to make these services useful in a highly
   constrained network of themselves highly constrained nodes.

   This objective requires restraint in a number of sometimes
   conflicting ways:

   o  reducing implementation complexity in order to minimize code size,

   o  reducing message sizes in order to minimize the number of
      fragments needed for each message (in turn to maximize the
      probability of delivery of the message), the amount of
      transmission power needed and the loading of the limited-bandwidth
      channel,

   o  reducing requirements on the environment such as stable storage,
      good sources of randomness or user interaction capabilities.

   CoAP is based on datagram transports such as UDP, which limit the
   maximum size of resource representations that can be transferred
   without creating unreasonable levels of IP fragmentation.  In
   addition, not all resource representations will fit into a single
   link layer packet of a constrained network, which may cause
   adaptation layer fragmentation even if IP layer fragmentation is not
   required.  Using fragmentation (either at the adaptation layer or at
   the IP layer) to enable the transport of larger representations is
   possible up to the maximum size of the underlying datagram protocol
   (such as UDP), but the fragmentation/reassembly process burdens the
   lower layers with conversation state that is better managed in the
   application layer.

   The present specification defines a pair of CoAP options to enable
   _block-wise_ access to resource representations.  The Block options
   provide a minimal way to transfer larger resource representations in
   a block-wise fashion.  The overriding objective is to avoid the need
   for creating conversation state at the server for block-wise GET



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   requests.  (It is impossible to fully avoid creating conversation
   state for POST/PUT, if the creation/replacement of resources is to be
   atomic; where that property is not needed, there is no need to create
   server conversation state in this case, either.)

   In summary, this specification adds a pair of Block options to CoAP
   that can be used for block-wise transfers.  Benefits of using these
   options include:

   o  Transfers larger than what can be accommodated in constrained-
      network link-layer packets can be performed in smaller blocks.

   o  No hard-to-manage conversation state is created at the adaptation
      layer or IP layer for fragmentation.

   o  The transfer of each block is acknowledged, enabling
      retransmission if required.

   o  Both sides have a say in the block size that actually will be
      used.

   o  The resulting exchanges are easy to understand using packet
      analyzer tools and thus quite accessible to debugging.

   o  If needed, the Block options can also be used (without changes) to
      provide random access to power-of-two sized blocks within a
      resource representation.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119, BCP 14
   [RFC2119] and indicate requirement levels for compliant CoAP
   implementations.

   In this document, the term "byte" is used in its now customary sense
   as a synonym for "octet".

   Where bit arithmetic is explained, this document uses the notation
   familiar from the programming language C, except that the operator
   "**" stands for exponentiation.

2.  Block-wise transfers

   As discussed in the introduction, there are good reasons to limit the
   size of datagrams in constrained networks:

   o  by the maximum datagram size (~ 64 KiB for UDP)




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   o  by the desire to avoid IP fragmentation (MTU of 1280 for IPv6)

   o  by the desire to avoid adaptation layer fragmentation (60-80 bytes
      for 6LoWPAN [RFC4919])

   When a resource representation is larger than can be comfortably
   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
   sent both with requests and with responses, this specification
   provides two separate options for each direction of payload transfer.
   In identifying these options, we use the number 1 to refer to the
   transfer of the resource representation that pertains to the request,
   and the number 2 to refer to the transfer of the resource
   representation for the response.

   In the following, the term "payload" will be used for the actual
   content of a single CoAP message, i.e. a single block being
   transferred, while the term "body" will be used for the entire
   resource representation that is being transferred in a block-wise
   fashion.  The Content-Format option applies to the body, not to the
   payload, in particular the boundaries between the blocks may be in
   places that are not separating 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
   same size.  The block size is not fixed by the protocol.  To keep the
   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
   (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
   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).

2.1.  The Block2 and Block1 Options

       +------+---+---+---+---+--------+--------+--------+---------+
       | Type | C | U | N | R | Name   | Format | Length | Default |
       +------+---+---+---+---+--------+--------+--------+---------+
       |   23 | C | U | - | - | Block2 | uint   | 0-3 B  | (none)  |
       |      |   |   |   |   |        |        |        |         |
       |   27 | C | U | - | - | Block1 | uint   | 0-3 B  | (none)  |
       +------+---+---+---+---+--------+--------+--------+---------+

                       Table 1: Block Option Numbers







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   Both Block1 and Block2 options can be present both in request and
   response messages.  In either case, the Block1 Option pertains to the
   request payload, and the Block2 Option pertains to the response
   payload.

   Hence, for the methods defined in [I-D.ietf-core-coap], Block1 is
   useful with the payload-bearing POST and PUT requests and their
   responses.  Block2 is useful with GET, POST, and PUT requests and
   their payload-bearing responses (2.01, 2.02, 2.04, 2.05 -- see
   section "Payload" of [I-D.ietf-core-coap]).

   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
   block-wise transfer and describes how this block-wise payload forms
   part of the entire body being transferred ("descriptive usage").
   Where it is present in the opposite direction, it provides additional
   control on how that payload will be formed or was processed ("control
   usage").

   Implementation of either Block option is intended to be optional.
   However, when it is present in a CoAP message, it MUST be processed
   (or the message rejected); therefore it is identified as a critical
   option.  It MUST NOT occur more than once.

2.2.  Structure of a Block Option

   Three items of information may need to be transferred in a Block
   (Block1 or Block2) option:

   o  The size of the block (SZX);

   o  whether more blocks are following (M);

   o  the relative number of the block (NUM) within a sequence of blocks
      with the given size.

   The value of the Block Option is a variable-size (0 to 3 byte)
   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 1 2 3 4 5 6 7
          +-+-+-+-+-+-+-+-+
          |  NUM  |M| SZX |
          +-+-+-+-+-+-+-+-+




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           0                   1
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |          NUM          |M| SZX |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           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
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                   NUM                 |M| SZX |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 1: Block option value

   The block size is encoded using a three-bit unsigned integer (0 for
   2**4 to 6 for 2**10 bytes), which we call the "SZX" ("size
   exponent"); the actual block size is then "2**(SZX + 4)".  SZX is
   transferred in the three least significant bits of the option value
   (i.e., "val & 7" where "val" is the value of the option).

   The fourth least significant bit, the M or "more" bit ("val & 8"),
   indicates whether more blocks are following or the current block-wise
   transfer is the last block being transferred.

   The option value divided by sixteen (the NUM field) is the sequence
   number of the block currently being transferred, starting from zero.
   The current transfer is therefore about the "size" bytes starting at
   byte "NUM << (SZX + 4)".

   Implementation note:  As an implementation convenience, "(val & ~0xF)
      << (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
      position of the block being transferred.

   More specifically, within the option value of a Block1 or Block2
   Option, the meaning of the option fields is defined as follows:

   NUM:  Block Number, indicating the block number being requested or
      provided.  Block number 0 indicates the first block of a body
      (i.e., starting with the first byte of the body).











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   M: More Flag (not last block).  For descriptive usage, this flag, if
      unset, indicates that the payload in this message is the last
      block in the body; when set it indicates that there are one or
      more additional blocks available.  When a Block2 Option is used in
      a request to retrieve a specific block number ("control usage"),
      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
      atomicity, see below.)

   SZX:  Block Size.  The block size is represented as three-bit
      unsigned integer indicating the size of a block to the power of
      two.  Thus block size = 2**(SZX + 4).  The allowed values of SZX
      are 0 to 6, i.e., the minimum block size is 2**(0+4) = 16 and the
      maximum is 2**(6+4) = 1024.  The value 7 for SZX (which would
      indicate a block size of 2048) is reserved, i.e. MUST NOT be sent
      and MUST lead to a 4.00 Bad Request response code upon reception
      in a request.

   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
   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
   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
   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.  (As for any uint, the explicit value 0 is efficiently
   indicated by a zero-length option; this, therefore, is different in
   semantics from the absence of the option.)

2.3.  Block Options in Requests and Responses

   The Block options are used in one of three roles:

   o  In descriptive usage, i.e., a Block2 Option in a response (such as
      a 2.05 response for GET), or a Block1 Option in a request (a PUT
      or POST):

      *  The NUM field in the option value describes what block number
         is contained in the payload of this message.

      *  The M bit indicates whether further blocks need to be
         transferred to complete the transfer of that body.

      *  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
         payload size if M is unset).  For Block2, if the request
         suggested a larger value of SZX, the next request MUST move SZX



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         down to the size given in the response.  (The effect is that,
         if the server uses the smaller of (1) its preferred 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):

      *  The NUM field in the Block2 Option gives the block number of
         the payload that is being requested to be returned in the
         response.

      *  In this case, the M bit has no function and MUST be set to
         zero.

      *  The block size given (SZX) suggests a block size (in the case
         of block number 0) or repeats the block size of previous blocks
         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
      response for a PUT or POST request):

      *  The NUM field of the Block1 Option indicates what block number
         is being acknowledged.

      *  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 handle the request for the entire body atomically,
         or any mix of the two.

         +  If the M bit is also set in the response, it indicates that
            this response does not carry the final response code to the
            request, i.e. the server collects further blocks from the
            same endpoint and plans to implement the request atomically
            (e.g., acts only upon reception of the last block of
            payload).  In this case, the response MUST NOT carry a
            Block2 option.

         +  Conversely, if the M bit is unset even though it was set in
            the request, it indicates the block-wise request was enacted
            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
         indicates the largest block size preferred by the server for



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         transfers toward the resource that is the same or smaller than
         the one used in the initial exchange; the client SHOULD use
         this block size or a smaller one in all further requests in the
         transfer sequence, even if that means changing the block size
         (and possibly scaling the block number accordingly) from now
         on.

   Using one or both Block options, a single REST operation can be split
   into multiple CoAP message exchanges.  As specified in
   [I-D.ietf-core-coap], each of these message exchanges uses their own
   CoAP Message ID.

2.4.  Using the 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
   the resource representation by sending further requests with the same
   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
   Option to zero and the server MUST ignore it on reception.

   To influence the block size used in a response, the requester also
   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
   indicated or a smaller size.  Any further block-wise requests for
   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
   gave a desired size using a Block2 Option.

   Once the Block2 Option is used by the requester and a first response
   has been received with a possibly adjusted block size, all further
   requests in a single block-wise transfer SHOULD ultimately use 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
   client may start using the Block2 Option in a second request after a
   first request without a Block2 Option resulted in a Block2 option in
   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
   the actual block size used in the response it receives to its initial
   request and proceed to use it in subsequent requests.  The server
   behavior MUST ensure that this client behavior results in the same
   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
   request did not contain a Block2 Option).

   Block-wise transfers can be used to GET resources the representations
   of which are entirely static (not changing over time at all, such as
   in a schema describing a device), or for dynamically changing



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   resources.  In the latter case, the Block2 Option SHOULD be used in
   conjunction with the ETag Option, to ensure that the blocks being
   reassembled are from the same version of the representation: The
   server SHOULD include an ETag option in each response.  If an ETag
   option is available, the client's reassembler, when reassembling the
   representation from the blocks being exchanged, MUST compare ETag
   Options.  If the ETag Options do not match in a GET transfer, the
   requester has the option of attempting to retrieve fresh values for
   the blocks it retrieved first.  To minimize the resulting
   inefficiency, the server MAY cache the current value of a
   representation for an ongoing sequence of requests.  (The server may
   identify the sequence by the combination of the requesting end-point
   and the URI being the same in each block-wise request.)  Note well
   that this specification makes no requirement for the server to
   establish any state; however, servers that offer quickly changing
   resources may thereby make it impossible for a client to ever
   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
   Option refers to the payload in the request (descriptive usage).

   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
   block size preference of the server for this resource (control
   usage).  Obviously, at this point the first block has already been
   transferred by the client without benefit of this knowledge.  Still,
   the client SHOULD heed the preference indicated and, for all further
   blocks, use the block size preferred by the server or a smaller one.
   Note that any reduction in the block size may mean that the second
   request starts with a block number larger than one, as the first
   request already transferred multiple blocks as counted in the smaller
   size.

   To counter the effects of adaptation layer fragmentation on packet
   delivery probability, a client may want to give up retransmitting a
   request with a relatively large payload even before MAX_RETRANSMIT
   has been reached, and try restating the request as a block-wise
   transfer with a smaller payload.  Note that this new attempt is then
   a new message-layer transaction and requires a new Message ID.
   (Because of the uncertainty whether the request or the
   acknowledgement was lost, this strategy is useful mostly for
   idempotent requests.)

   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
   server, the actual creation/replacement takes place at the time the



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   final block, i.e. a block with the M bit unset in the Block1 Option,
   is received.  In this case, all success responses to non-final blocks
   carry the response code 2.31 (Continue, Section 2.9.1).  If not all
   previous blocks are available at the server at the time of processing
   the final block, the transfer fails and error code 4.08 (Request
   Entity Incomplete, Section 2.9.2) MUST be returned.  A server MAY
   also return a 4.08 error code for any (final or non-final) Block1
   transfer that is not in sequence; clients that do not have specific
   mechanisms to handle this case therefore SHOULD always start with
   block zero and send the following blocks in order.

   The error code 4.13 (Request Entity 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 payload transfer that it would intend
   to implement in an atomic 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 4.13 response with a smaller SZX in its Block1
   option than requested is a hint to try a smaller SZX.)

   The Block1 option provides no way for a single endpoint to perform
   multiple concurrently proceeding block-wise request payload transfer
   (e.g., PUT or POST) operations to the same resource.  Starting a new
   block-wise sequence of requests to the same resource (before an old
   sequence from the same endpoint was finished) simply overwrites the
   context the server may still be keeping.  (This is probably exactly
   what one wants in this case - the client may simply have restarted
   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.

   Observation relationships always apply to an entire resource; the
   Block2 option does not provide a way to observe a single block of a
   resource.

   As with basic GET transfers, the client can indicate its desired
   block size in a Block2 Option in the GET request establishing or
   renewing the observation relationship.  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 entry in that list is updated by the server
   receiving a new GET request for the resource from the client).



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   When sending a 2.05 (Content) notification, the server only sends the
   first block of the representation.  The client retrieves the rest of
   the representation as if it had caused this first response by a GET
   request, i.e., by using additional GET requests with Block2 options
   containing NUM values greater than zero.  (This results in the
   transfer of the entire representation, even if only some of the
   blocks have changed with respect to a previous notification.)

   As with other dynamically changing resources, to ensure that the
   blocks being reassembled are from the same version of the
   representation, the server SHOULD include an ETag option in each
   response, and the reassembling client MUST compare the ETag options
   (Section 2.4).

   See Section 3.4 for examples.

2.7.  Combining Block1 and Block2

   In PUT and particularly in POST exchanges, both the request body and
   the response body may be large enough to require the use of block-
   wise transfers.  First, the Block1 transfer of the request body
   proceeds as usual.  In the exchange of the last slice of this block-
   wise transfer, the response carries the first slice of the Block2
   transfer (NUM is zero).  To continue this Block2 transfer, the client
   continues to send requests similar to the requests in the Block1
   phase, but leaves out the Block1 options and includes a Block2
   request option with non-zero NUM.

   Block2 transfers that retrieve the response body for a request that
   used Block1 MUST be performed in sequential order.

2.8.  Combining Block2 with Multicast

   A client can use the Block2 option in a multicast GET request with
   NUM = 0 to aid in limiting the size of the response.

   Similarly, a response to a multicast GET request can use a Block2
   option with NUM = 0 if the representation is large, or to further
   limit the size of the response.

   In both cases, the client retrieves any further blocks using unicast
   exchanges; in the unicast requests, the client SHOULD heed any block
   size preferences indicated by the server in the response to the
   multicast request.

   Other uses of the Block options in conjunction with multicast
   messages are for further study.




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2.9.  Response Codes

   Two response codes are defined by this specification beyond those
   already defined in [I-D.ietf-core-coap], and another response code is
   extended in its meaning.

   TODO: Add or appropriate useful response codes for Block2 problems
   (overshoot beyond end of representation, unsupported out-of-order
   access).

2.9.1.  2.31 Continue

   This new success status code indicates that the transfer of this
   block of the request body was successful and that the serve
   encourages sending further blocks, but that a final outcome of the
   whole block-wise request cannot yet be determined.  No payload is
   returned with this response code.

2.9.2.  4.08 Request Entity Incomplete

   This new client error status code indicates that the server has not
   received the blocks of the request body that it needs to proceed.
   The client has not sent all blocks, not sent them in the order
   required by the server, or has sent them long enough ago that the
   server has already discarded them.

2.9.3.  4.13 Request Entity Too Large

   In [I-D.ietf-core-coap], section 5.9.2.9, the response code 4.13
   (Request Entity Too Large) is defined to be like HTTP 413 "Request
   Entity Too Large".  [I-D.ietf-core-coap] also recommends that this
   response SHOULD include a Size1 Option Section 4 to indicate the
   maximum size of request entity the server is able and willing to
   handle, unless the server is not in a position to make this
   information available.

   The present specification allows the server to return this response
   code at any time during a Block1 transfer to indicate that it does
   not currently have the resources to store blocks for a transfer that
   it would intend to implement in an atomic fashion.  It also allows
   the server to return a 4.13 response to a request that does not
   employ Block1 as a hint for the client to try sending Block1.
   Finally, a 4.13 response to a request with a Block1 option (control
   usage, see Section 2.3) where the response carries a smaller SZX in
   its Block1 option is a hint to try that smaller SZX.

3.  Examples




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   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
   the basic operation, the operation in the presence of
   retransmissions, and examples for the operation of the block size
   negotiation.

   In all these examples, a Block option is shown in a decomposed way
   indicating the kind of Block option (1 or 2) followed by a colon, and
   then the block number (NUM), more bit (M), and block size exponent
   (2**(SZX+4)) separated by slashes.  E.g., a Block2 Option value of 33
   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
   three blocks.  The server proposes a block size of 128, and the
   client agrees.  The first two ACKs contain 128 bytes of payload each,
   and third ACK contains between 1 and 128 bytes.

   CLIENT                                                     SERVER
     |                                                            |
     | CON [MID=1234], GET, /status                       ------> |
     |                                                            |
     | <------   ACK [MID=1234], 2.05 Content, 2:0/1/128          |
     |                                                            |
     | CON [MID=1235], GET, /status, 2:1/0/128            ------> |
     |                                                            |
     | <------   ACK [MID=1235], 2.05 Content, 2:1/1/128          |
     |                                                            |
     | CON [MID=1236], GET, /status, 2:2/0/128            ------> |
     |                                                            |
     | <------   ACK [MID=1236], 2.05 Content, 2:2/0/128          |

                      Figure 2: Simple blockwise GET

   In the second example (Figure 3), the client anticipates the
   blockwise transfer (e.g., because of a size indication in the link-
   format description [RFC6690]) and sends a size proposal.  All ACK
   messages except for the last carry 64 bytes of payload; the last one
   carries between 1 and 64 bytes.

   CLIENT                                                     SERVER
     |                                                          |
     | CON [MID=1234], GET, /status, 2:0/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1234], 2.05 Content, 2:0/1/64         |
     |                                                          |



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     | CON [MID=1235], GET, /status, 2:1/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1235], 2.05 Content, 2:1/1/64         |
     :                                                          :
     :                          ...                             :
     :                                                          :
     | CON [MID=1238], GET, /status, 2:4/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1238], 2.05 Content, 2:4/1/64         |
     |                                                          |
     | CON [MID=1239], GET, /status, 2:5/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1239], 2.05 Content, 2:5/0/64         |

              Figure 3: Blockwise GET with early negotiation

   In the third example (Figure 4), the client is surprised by the need
   for a blockwise transfer, and unhappy with the size chosen
   unilaterally by the server.  As it did not send a size proposal
   initially, the negotiation only influences the size from the second
   message exchange onward.  Since the client already obtained both the
   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
   is (or needs to be) understood by the server, which simply responds
   to the requests as it best can.


























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   CLIENT                                                     SERVER
     |                                                          |
     | CON [MID=1234], GET, /status                     ------> |
     |                                                          |
     | <------   ACK [MID=1234], 2.05 Content, 2:0/1/128        |
     |                                                          |
     | CON [MID=1235], GET, /status, 2:2/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1235], 2.05 Content, 2:2/1/64         |
     |                                                          |
     | CON [MID=1236], GET, /status, 2:3/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1236], 2.05 Content, 2:3/1/64         |
     |                                                          |
     | CON [MID=1237], GET, /status, 2:4/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1237], 2.05 Content, 2:4/1/64         |
     |                                                          |
     | CON [MID=1238], GET, /status, 2:5/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1238], 2.05 Content, 2:5/0/64         |

               Figure 4: Blockwise GET with late negotiation

   In all these (and the following) cases, retransmissions are handled
   by the CoAP message exchange layer, so they don't influence the block
   operations (Figure 5, Figure 6).
























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   CLIENT                                                     SERVER
     |                                                          |
     | CON [MID=1234], GET, /status                     ------> |
     |                                                          |
     | <------   ACK [MID=1234], 2.05 Content, 2:0/1/128        |
     |                                                          |
     | CON [MID=1235], GE/////////////////////////              |
     |                                                          |
     | (timeout)                                                |
     |                                                          |
     | CON [MID=1235], GET, /status, 2:2/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1235], 2.05 Content, 2:2/1/64         |
     :                                                          :
     :                          ...                             :
     :                                                          :
     | CON [MID=1238], GET, /status, 2:5/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1238], 2.05 Content, 2:5/0/64         |

        Figure 5: Blockwise GET with late negotiation and lost CON

   CLIENT                                                     SERVER
     |                                                          |
     | CON [MID=1234], GET, /status                     ------> |
     |                                                          |
     | <------   ACK [MID=1234], 2.05 Content, 2:0/1/128        |
     |                                                          |
     | CON [MID=1235], GET, /status, 2:2/0/64           ------> |
     |                                                          |
     | //////////////////////////////////tent, 2:2/1/64         |
     |                                                          |
     | (timeout)                                                |
     |                                                          |
     | CON [MID=1235], GET, /status, 2:2/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1235], 2.05 Content, 2:2/1/64         |
     :                                                          :
     :                          ...                             :
     :                                                          :
     | CON [MID=1238], GET, /status, 2:5/0/64           ------> |
     |                                                          |
     | <------   ACK [MID=1238], 2.05 Content, 2:5/0/64         |

        Figure 6: Blockwise GET with late negotiation and lost ACK

3.2.  Block1 Examples




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   The following examples demonstrate a PUT exchange; a POST exchange
   looks the same, with different requirements on atomicity/idempotence.
   Note that, similar to GET, the responses to the requests that have a
   more bit in the request Block1 Option are provisional and carry the
   response code 2.31 (Continue); only the final response tells the
   client that the PUT did succeed.

   CLIENT                                                     SERVER
     |                                                          |
     | CON [MID=1234], PUT, /options, 1:0/1/128    ------>      |
     |                                                          |
     | <------   ACK [MID=1234], 2.31 Continue, 1:0/1/128       |
     |                                                          |
     | CON [MID=1235], PUT, /options, 1:1/1/128    ------>      |
     |                                                          |
     | <------   ACK [MID=1235], 2.31 Continue, 1:1/1/128       |
     |                                                          |
     | CON [MID=1236], PUT, /options, 1:2/0/128    ------>      |
     |                                                          |
     | <------   ACK [MID=1236], 2.04 Changed, 1:2/0/128        |

                   Figure 7: Simple atomic blockwise PUT

   A stateless server that simply builds/updates the resource in place
   (statelessly) may indicate this by not setting the more bit in the
   response (Figure 8); in this case, the response codes are valid
   separately for each block being updated.  This is of course only an
   acceptable behavior of the server if the potential inconsistency
   present during the run of the message exchange sequence does not lead
   to problems, e.g. because the resource being created or changed is
   not yet or not currently in use.

   CLIENT                                                     SERVER
     |                                                          |
     | CON [MID=1234], PUT, /options, 1:0/1/128    ------>      |
     |                                                          |
     | <------   ACK [MID=1234], 2.04 Changed, 1:0/0/128        |
     |                                                          |
     | CON [MID=1235], PUT, /options, 1:1/1/128    ------>      |
     |                                                          |
     | <------   ACK [MID=1235], 2.04 Changed, 1:1/0/128        |
     |                                                          |
     | CON [MID=1236], PUT, /options, 1:2/0/128    ------>      |
     |                                                          |
     | <------   ACK [MID=1236], 2.04 Changed, 1:2/0/128        |

                 Figure 8: Simple stateless blockwise PUT




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   Finally, a server receiving a blockwise PUT or POST may want to
   indicate a smaller block size preference (Figure 9).  In this case,
   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.

   CLIENT                                                     SERVER
     |                                                          |
     | CON [MID=1234], PUT, /options, 1:0/1/128    ------>      |
     |                                                          |
     | <------   ACK [MID=1234], 2.04 Changed, 1:0/1/32         |
     |                                                          |
     | CON [MID=1235], PUT, /options, 1:4/1/32     ------>      |
     |                                                          |
     | <------   ACK [MID=1235], 2.04 Changed, 1:4/1/32         |
     |                                                          |
     | CON [MID=1236], PUT, /options, 1:5/1/32     ------>      |
     |                                                          |
     | <------   ACK [MID=1235], 2.04 Changed, 1:5/1/32         |
     |                                                          |
     | CON [MID=1237], PUT, /options, 1:6/0/32     ------>      |
     |                                                          |
     | <------   ACK [MID=1236], 2.04 Changed, 1:6/0/32         |

          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.
   The following example demonstrates a blockwise POST request,
   resulting in a separate blockwise response.

   CLIENT                                                     SERVER
     |                                                              |
     | CON [MID=1234], POST, /soap, 1:0/1/128      ------>          |
     |                                                              |
     | <------   ACK [MID=1234], 2.31 Continue, 1:0/1/128           |
     |                                                              |
     | CON [MID=1235], POST, /soap, 1:1/1/128      ------>          |
     |                                                              |
     | <------   ACK [MID=1235], 2.31 Continue, 1:1/1/128           |
     |                                                              |
     | CON [MID=1236], POST, /soap, 1:2/0/128      ------>          |
     |                                                              |
     | <------   ACK [MID=1236], 2.04 Changed, 2:0/1/128, 1:2/0/128 |
     |                                                              |
     | CON [MID=1237], POST, /soap, 2:1/0/128      ------>          |
     | (no payload for requests with Block2 with NUM != 0)          |
     | (could also do late negotiation by requesting e.g. 2:2/0/64) |



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     |                                                              |
     | <------   ACK [MID=1237], 2.04 Changed, 2:1/1/128            |
     |                                                              |
     | CON [MID=1238], POST, /soap, 2:2/0/128      ------>          |
     |                                                              |
     | <------   ACK [MID=1238], 2.04 Changed, 2:2/1/128            |
     |                                                              |
     | CON [MID=1239], POST, /soap, 2:3/0/128      ------>          |
     |                                                              |
     | <------   ACK [MID=1239], 2.04 Changed, 2:3/0/128            |

         Figure 10: Atomic blockwise POST with blockwise response

   This model does provide for early negotiation input to the Block2
   blockwise transfer, as shown below.

   CLIENT                                                     SERVER
     |                                                              |
     | CON [MID=1234], POST, /soap, 1:0/1/128 ------>               |
     |                                                              |
     | <------   ACK [MID=1234], 2.31 Continue, 1:0/1/128           |
     |                                                              |
     | CON [MID=1235], POST, /soap, 1:1/1/128 ------>               |
     |                                                              |
     | <------   ACK [MID=1235], 2.31 Continue, 1:1/1/128           |
     |                                                              |
     | CON [MID=1236], POST, /soap, 1:2/0/128, 2:0/0/64 ------>     |
     |                                                              |
     | <------   ACK [MID=1236], 2.04 Changed, 1:2/0/128, 2:0/1/64 |
     |                                                              |
     | CON [MID=1237], POST, /soap, 2:1/0/64      ------>           |
     | (no payload for requests with Block2 with NUM != 0)          |
     |                                                              |
     | <------   ACK [MID=1237], 2.04 Changed, 2:1/1/64             |
     |                                                              |
     | CON [MID=1238], POST, /soap, 2:2/0/64      ------>           |
     |                                                              |
     | <------   ACK [MID=1238], 2.04 Changed, 2:2/1/64             |
     |                                                              |
     | CON [MID=1239], POST, /soap, 2:3/0/64      ------>           |
     |                                                              |
     | <------   ACK [MID=1239], 2.04 Changed, 2:3/0/64             |

      Figure 11: Atomic blockwise POST with blockwise response, early
                                negotiation

3.4.  Combining Observe and Block2




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   In the following example, the server first sends a direct response
   (Observe sequence number 62350) to the initial GET request (the
   resulting blockwise transfer is as in Figure 4 and has therefore been
   left out).  The second transfer is started by a 2.05 notification
   that contains just the first block (Observe sequence number 62354);
   the client then goes on to obtain the rest of the blocks.

       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: 62350
         |      |       ETag: 6f00f38e
         |      |    Payload: [128 bytes]
         |      |
         |      |  (Usual GET transfer left out)
           ...
         |      |  (Notification of first block:)
         |      |
         |<-----+     Header: 2.05 0x4145af9c
         | 2.05 |      Token: 0xfb
         |      |     Block2: 0/1/128
         |      |    Observe: 62354
         |      |       ETag: 6f00f392
         |      |    Payload: [128 bytes]
         |      |
         +- - ->|     Header: 0x6000af9c
         |      |
         |      |  (Retrieval of remaining blocks)
         |      |
         +----->|     Header: GET 0x41011637
         | GET  |      Token: 0xfc
         |      |   Uri-Path: status-icon
         |      |     Block2: 1/0/128
         |      |
         |<-----+     Header: 2.05 0x61451637
         | 2.05 |      Token: 0xfc
         |      |     Block2: 1/1/128
         |      |       ETag: 6f00f392
         |      |    Payload: [128 bytes]
         |      |
         +----->|     Header: GET 0x41011638



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         | GET  |      Token: 0xfc
         |      |   Uri-Path: status-icon
         |      |     Block2: 2/0/128
         |      |
         |<-----+     Header: 2.05 0x61451638
         | 2.05 |      Token: 0xfc
         |      |     Block2: 2/0/128
         |      |       ETag: 6f00f392
         |      |    Payload: [53 bytes]


            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: 62350
      |      |       ETag: 6f00f38e
      |      |    Max-Age: 60
      |      |    Payload: [64 bytes]
      |      |
      |      |  (Usual GET transfer left out)
        ...
      |      |  (Notification of first block:)
      |      |
      |<-----+     Header: 2.05 0x4145af9c
      | 2.05 |      Token: 0xfb
      |      |     Block2: 0/1/64
      |      |    Observe: 62354
      |      |       ETag: 6f00f392
      |      |    Payload: [64 bytes]
      |      |
      +- - ->|     Header: 0x6000af9c
      |      |
      |      |  (Retrieval of remaining blocks)
      |      |
      +----->|     Header: GET 0x41011637



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      | GET  |      Token: 0xfc
      |      |   Uri-Path: status-icon
      |      |     Block2: 1/0/64
      |      |
      |<-----+     Header: 2.05 0x61451637
      | 2.05 |      Token: 0xfc
      |      |     Block2: 1/1/64
      |      |       ETag: 6f00f392
      |      |    Payload: [64 bytes]
        ....
      |      |
      +----->|     Header: GET 0x41011638
      | GET  |      Token: 0xfc
      |      |   Uri-Path: status-icon
      |      |     Block2: 4/0/64
      |      |
      |<-----+     Header: 2.05 0x61451638
      | 2.05 |      Token: 0xfc
      |      |     Block2: 4/0/64
      |      |       ETag: 6f00f392
      |      |    Payload: [53 bytes]

            Figure 13: Observe sequence with early negotiation

4.  The Size2 and Size1 Options

   In many cases when transferring a large resource representation block
   by block, it is advantageous to know the total size early in the
   process.  Some indication may be available from the maximum size
   estimate attribute "sz" provided in a resource description [RFC6690].
   However, the size may vary dynamically, so a more up-to-date
   indication may be useful.

   This specification defines two CoAP Options, Size1 for indicating the
   size of the representation transferred in requests, and Size2 for
   indicating the size of the representation transferred in responses.

   The Size2 Option may be used for two purposes:

   o  in a request, to ask the server to provide a size estimate along
      with the usual response ("size request").  For this usage, the
      value MUST be set to 0.

   o  in a response carrying a Block2 Option, to indicate the current
      estimate the server has of the total size of the resource
      representation, measured in bytes ("size indication").

   Similarly, the Size1 Option may be used for two purposes:



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   o  in a request carrying a Block1 Option, to indicate the current
      estimate the client has of the total size of the resource
      representation, measured in bytes ("size indication").

   o  in a 4.13 response, to indicate the maximum size that would have
      been acceptable [I-D.ietf-core-coap], measured in bytes.

   Apart from conveying/asking for size information, the Size options
   have no other effect on the processing of the request or response.
   If the client wants to minimize the size of the payload in the
   resulting response, it should add a Block2 option to the request with
   a small block size (e.g., setting SZX=0).

   The Size Options are "elective", i.e., a client MUST be prepared for
   the server to ignore the size estimate request.  The Size Options
   MUST NOT occur more than once.

       +------+---+---+---+---+-------+--------+--------+---------+
       | Type | C | U | N | R | Name  | Format | Length | Default |
       +------+---+---+---+---+-------+--------+--------+---------+
       | 60   |   |   | x |   | Size1 | uint   | 0-4 B  | (none)  |
       |      |   |   |   |   |       |        |        |         |
       | 28   |   |   | x |   | Size2 | uint   | 0-4 B  | (none)  |
       +------+---+---+---+---+-------+--------+--------+---------+

                       Table 2: Size Option Numbers

   Implementation Notes:

   o  As a quality of implementation consideration, blockwise transfers
      for which the total size considerably exceeds the size of one
      block are expected to include size indications, whenever those can
      be provided without undue effort (preferably with the first block
      exchanged).  If the size estimate does not change, the indication
      does not need to be repeated for every block.

   o  The end of a blockwise transfer is governed by the M bits in the
      Block Options, _not_ by exhausting the size estimates exchanged.

   o  As usual for an option of type uint, the value 0 is best expressed
      as an empty option (0 bytes).  There is no default value.

   o  The Size Options are neither critical nor unsafe, and are marked
      as No-Cache-Key.

5.  HTTP Mapping Considerations





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   In this subsection, we give some brief examples for the influence the
   Block options might have on intermediaries that map between CoAP and
   HTTP.

   For mapping CoAP requests to HTTP, the intermediary may want to map
   the sequence of block-wise transfers into a single HTTP transfer.
   E.g., for a GET request, the intermediary could perform the HTTP
   request once the first block has been requested and could then
   fulfill all further block requests out of its cache.  A constrained
   implementation may not be able to cache the entire object and may use
   a combination of TCP flow control and (in particular if timeouts
   occur) HTTP range requests to obtain the information necessary for
   the next block transfer at the right time.

   For PUT or POST requests, there is more variation in how HTTP servers
   might implement ranges.  Some WebDAV servers do, but in general the
   CoAP-to-HTTP intermediary will have to try sending the payload of all
   the blocks of a block-wise transfer within one HTTP request.  If
   enough buffering is available, this request can be started when the
   last CoAP block is received.  A constrained implementation may want
   to relieve its buffering by already starting to send the HTTP request
   at the time the first CoAP block is received; any HTTP 408 status
   code that indicates that the HTTP server became impatient with the
   resulting transfer can then be mapped into a CoAP 4.08 response code
   (similarly, 413 maps to 4.13).

   For mapping HTTP to CoAP, the intermediary may want to map a single
   HTTP transfer into a sequence of block-wise transfers.  If the HTTP
   client is too slow delivering a request body on a PUT or POST, the
   CoAP server might time out and return a 4.08 response code, which in
   turn maps well to an HTTP 408 status code (again, 4.13 maps to 413).
   HTTP range requests received on the HTTP side may be served out of a
   cache and/or mapped to GET requests that request a sequence of blocks
   overlapping the range.

















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   (Note that, while the semantics of CoAP 4.08 and HTTP 408 differ,
   this difference is largely due to the different way the two protocols
   are mapped to transport.  HTTP has an underlying TCP connection,
   which supplies connection state, so a HTTP 408 status code can
   immediately be used to indicate that a timeout occurred during
   transmitting a request through that active TCP connection.  The CoAP
   4.08 response code indicates one or more missing blocks, which may be
   due to timeouts or resource constraints; as there is no connection
   state, there is no way to deliver such a response immediately;
   instead, it is delivered on the next block transfer.  Still, HTTP 408
   is probably the best mapping back to HTTP, as the timeout is the most
   likely cause for a CoAP 4.08.  Note that there is no way to
   distinguish a timeout from a missing block for a server without
   creating additional state, the need for which we want to avoid.)

6.  IANA Considerations

   This draft adds the following option numbers to the CoAP Option
   Numbers registry of [I-D.ietf-core-coap]:

                      +--------+--------+-----------+
                      | Number | Name   | Reference |
                      +--------+--------+-----------+
                      | 23     | Block2 | [RFCXXXX] |
                      |        |        |           |
                      | 27     | Block1 | [RFCXXXX] |
                      |        |        |           |
                      | 28     | Size2  | [RFCXXXX] |
                      |        |        |           |
                      | 60     | Size1  | [RFCXXXX] |
                      +--------+--------+-----------+

                       Table 3: CoAP Option Numbers

   This draft adds the following response code to the CoAP Response
   Codes registry of [I-D.ietf-core-coap]:

             +------+---------------------------+-----------+
             | Code | Description               | Reference |
             +------+---------------------------+-----------+
             | 2.31 | Continue                  | [RFCXXXX] |
             |      |                           |           |
             | 4.08 | Request Entity Incomplete | [RFCXXXX] |
             +------+---------------------------+-----------+

                       Table 4: CoAP Response Codes





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7.  Security Considerations

   Providing access to blocks within a resource may lead to surprising
   vulnerabilities.  Where requests are not implemented atomically, an
   attacker may be able to exploit a race condition or confuse a server
   by inducing it to use a partially updated resource representation.
   Partial transfers may also make certain problematic data invisible to
   intrusion detection systems; it is RECOMMENDED that an intrusion
   detection system (IDS) that analyzes resource representations
   transferred by CoAP implement the Block options to gain access to
   entire resource representations.  Still, approaches such as
   transferring even-numbered blocks on one path and odd-numbered blocks
   on another path, or even transferring blocks multiple times with
   different content and obtaining a different interpretation of
   temporal order at the IDS than at the server, may prevent an IDS from
   seeing the whole picture.  These kinds of attacks are well understood
   from IP fragmentation and TCP segmentation; CoAP does not add
   fundamentally new considerations.

   Where access to a resource is only granted to clients making use of a
   specific security association, all blocks of that resource MUST be
   subject to the same security checks; it MUST NOT be possible for
   unprotected exchanges to influence blocks of an otherwise protected
   resource.  As a related consideration, where object security is
   employed, PUT/POST should be implemented in the atomic fashion,
   unless the object security operation is performed on each access and
   the creation of unusable resources can be tolerated.

   A stateless server might be susceptible to an attack where the
   adversary sends a Block1 (e.g., PUT) block with a high block number:
   A naive implementation might exhaust its resources by creating a huge
   resource representation.

   Misleading size indications may be used by an attacker to induce
   buffer overflows in poor implementations, for which the usual
   considerations apply.

7.1.  Mitigating Resource Exhaustion Attacks

   Certain blockwise requests may induce the server to create state,
   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 for all blocks, or to create temporary resource
   representations that are collected until pressed into service by a
   final PUT or POST with the more bit unset.  All mechanisms that
   induce a server to create state that cannot simply be cleaned up
   create opportunities for denial-of-service attacks.  Servers SHOULD
   avoid being subject to resource exhaustion based on state created by



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   untrusted sources.  But even if this is done, the mitigation may
   cause a denial-of-service to a legitimate request when it is drowned
   out by other state-creating requests.  Wherever possible, servers
   should therefore minimize the opportunities to create state for
   untrusted sources, e.g. by using stateless approaches.

   Performing segmentation at the application layer is almost always
   better in this respect than at the transport layer or lower (IP
   fragmentation, adaptation layer fragmentation), e.g. because there is
   application layer semantics that can be used for mitigation or
   because lower layers provide security associations that can prevent
   attacks.  However, it is less common to apply timeouts and keepalive
   mechanisms at the application layer than at lower layers.  Servers
   MAY want to clean up accumulated state by timing it out (cf. response
   code 4.08), and clients SHOULD be prepared to run blockwise transfers
   in an expedient way to minimize the likelihood of running into such a
   timeout.

7.2.  Mitigating Amplification Attacks

   [I-D.ietf-core-coap] discusses the susceptibility of CoAP end-points
   for use in amplification attacks.

   A CoAP server can reduce the amount of amplification it provides to
   an attacker by offering large resource representations only in
   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
   block) instead of a 1016-byte response, considerably reducing the
   amplification provided.

8.  Acknowledgements

   Much of the content of this draft is the result of discussions with
   the [I-D.ietf-core-coap] authors, and via many CoRE WG discussions.

   Charles Palmer provided extensive editorial comments to a previous
   version of this draft, some of which the authors hope to have covered
   in this version.  Esko Dijk reviewed a more recent version, leading
   to a number of further editorial improvements, a solution to the 4.13
   ambiguity problem, and the section about combining Block and
   multicast.  Markus Becker proposed getting rid of an ill-conceived
   default value for the Block2 and Block1 options.  Peter Bigot
   insisted on a more systematic coverage of the options and response
   code.

   Kepeng Li, Linyi Tian, and Barry Leiba wrote up an early version of
   the Size Option, which has informed this draft.  Klaus Hartke wrote
   some of the text describing the interaction of Block2 with Observe.



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   Matthias Kovatsch provided a number of significant simplifications of
   the protocol.

9.  References

9.1.  Normative References

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP", draft-ietf-
              core-observe-11 (work in progress), October 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

9.2.  Informative References

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", Ph.D. Dissertation,
              University of California, Irvine, 2000, <http://
              www.ics.uci.edu/~fielding/pubs/dissertation/
              fielding_dissertation.pdf>.

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals", RFC
              4919, August 2007.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, August 2012.

Authors' Addresses











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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org


   Zach Shelby (editor)
   Sensinode
   Kidekuja 2
   Vuokatti  88600
   Finland

   Phone: +358407796297
   Email: zach@sensinode.com

































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