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RFC 7959

Block-Wise Transfers in the Constrained Application Protocol (CoAP)

Pages: 37
Proposed Standard
Errata
Updates:  7252
Updated by:  8323
Part 2 of 2 – Pages 18 to 37
First   Prev   None

Top   ToC   RFC7959 - Page 18   prevText

3. Examples

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.
Top   ToC   RFC7959 - Page 19
   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.  For example, a Block2 Option
   value of 33 would be shown as 2:2/0/32) and a Block1 Option value of
   59 would be shown as 1:3/1/128.

   As in [RFC7252], "MID" is used as an abbreviation for "Message ID".

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 a payload of 128 bytes each, and the third ACK contains a payload 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 Block-Wise GET
Top   ToC   RFC7959 - Page 20
   In the second example (Figure 3), the client anticipates the block-
   wise transfer (e.g., because of a size indication in the link-format
   description [RFC6690]) and sends a block 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         |
     |                                                          |
     | 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: Block-Wise GET with Early Negotiation
Top   ToC   RFC7959 - Page 21
   In the third example (Figure 4), the client is surprised by the need
   for a block-wise 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.

   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: Block-Wise 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 (Figures 5 and 6).
Top   ToC   RFC7959 - Page 22
   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: Block-Wise 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: Block-Wise GET with Late Negotiation and Lost ACK
Top   ToC   RFC7959 - Page 23

3.2. Block1 Examples

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 succeeded. 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 Block-Wise 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.
Top   ToC   RFC7959 - Page 24
   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 Block-Wise PUT

   Finally, a server receiving a block-wise 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], 2.31 Continue, 1:0/1/32        |
     |                                                          |
     | CON [MID=1235], PUT, /options, 1:4/1/32     ------>      |
     |                                                          |
     | <------   ACK [MID=1235], 2.31 Continue, 1:4/1/32        |
     |                                                          |
     | CON [MID=1236], PUT, /options, 1:5/1/32     ------>      |
     |                                                          |
     | <------   ACK [MID=1235], 2.31 Continue, 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 Block-Wise PUT with Negotiation
Top   ToC   RFC7959 - Page 25

3.3. Combining Block1 and Block2

Block options may be used in both directions of a single exchange. The following example demonstrates a block-wise POST request, resulting in a separate block-wise 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) | | | | <------ 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 Block-Wise POST with Block-Wise Response
Top   ToC   RFC7959 - Page 26
   This model does provide for early negotiation input to the Block2
   block-wise 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 Block-Wise POST with Block-Wise Response,
                             Early Negotiation

3.4. Combining Observe and Block2

In the following example, the server first sends a direct response (Observe sequence number 62350) to the initial GET request (the resulting block-wise 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.
Top   ToC   RFC7959 - Page 27
       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
         | GET  |      Token: 0xfc
         |      |   Uri-Path: status-icon
         |      |     Block2: 2/0/128
         |      |
Top   ToC   RFC7959 - Page 28
         |<-----+     Header: 2.05 0x61451638
         | 2.05 |      Token: 0xfc
         |      |     Block2: 2/0/128
         |      |       ETag: 6f00f392
         |      |    Payload: [53 bytes]

           Figure 12: Observe Sequence with Block-Wise Response

   (Note that the choice of token 0xfc in this example is arbitrary;
   tokens are just shown in this example to illustrate that the requests
   for additional blocks cannot make use of the token of the Observation
   relationship.  As a general comment on tokens, there is no other
   mention of tokens in this document, as block-wise transfers handle
   tokens like any other CoAP exchange.  As usual, the client is free to
   choose tokens for each exchange as it likes.)

   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]
         |      |
Top   ToC   RFC7959 - Page 29
         +- - ->|     Header: 0x6000af9c
         |      |
         |      |  (Retrieval of remaining blocks)
         |      |
         +----->|     Header: GET 0x41011637
         | 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. (Size1 has already been defined in Section 5.10.9 of [RFC7252] to provide "size information about the resource representation in a request"; however, that section only details the narrow case of indicating in 4.13 responses the maximum size of request payload that the server is able and willing to handle. The present specification provides details about its use as a request option as well.)
Top   ToC   RFC7959 - Page 30
   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:

   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 [RFC7252], 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.  Either Size option
   MUST NOT occur more than once in a single message.

        +-----+---+---+---+---+-------+--------+--------+---------+
        | No. | C | U | N | R | Name  | Format | Length | Default |
        +-----+---+---+---+---+-------+--------+--------+---------+
        |  60 |   |   | x |   | Size1 | uint   |    0-4 | (none)  |
        |     |   |   |   |   |       |        |        |         |
        |  28 |   |   | x |   | Size2 | uint   |    0-4 | (none)  |
        +-----+---+---+---+---+-------+--------+--------+---------+

                       Table 2: Size Option Numbers

   Implementation Notes:

   o  As a quality of implementation consideration, block-wise 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.
Top   ToC   RFC7959 - Page 31
   o  The end of a block-wise 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 for
      either Size option.

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

5. HTTP-Mapping Considerations

In this subsection, we give some brief examples of the influence that 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. For example, 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, historically there was more variation in how HTTP servers might implement ranges; recently, [RFC7233] has defined that Range header fields received with a request method other than GET are not to be interpreted. So, in general, the CoAP-to-HTTP intermediary will have to try sending the payload of all the blocks of a block-wise transfer for these other methods 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 that cover the range.
Top   ToC   RFC7959 - Page 32
   (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 an 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 document adds the following option numbers to the "CoAP Option Numbers" registry defined by [RFC7252]: +--------+--------+-----------+ | Number | Name | Reference | +--------+--------+-----------+ | 23 | Block2 | RFC 7959 | | | | | | 27 | Block1 | RFC 7959 | | | | | | 28 | Size2 | RFC 7959 | +--------+--------+-----------+ Table 3: CoAP Option Numbers This document adds the following response codes to the "CoAP Response Codes" registry defined by [RFC7252]: +------+---------------------------+-----------+ | Code | Description | Reference | +------+---------------------------+-----------+ | 2.31 | Continue | RFC 7959 | | | | | | 4.08 | Request Entity Incomplete | RFC 7959 | +------+---------------------------+-----------+ 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 (IDSs); it is RECOMMENDED that an 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 specific security associations, 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. Future end-to- end security mechanisms that may be added to CoAP itself may have related security considerations, this includes considerations about caching of blocks in clients and in proxies (see Sections 2.10 and 5 for different strategies in performing this caching); these security considerations will need to be described in the specifications of those mechanisms. 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 block-wise requests may induce the server to create state, e.g., to create a snapshot for the block-wise 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
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   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
   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), for instance, because
   there are 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 block-wise
   transfers in an expedient way to minimize the likelihood of running
   into such a timeout.

7.2. Mitigating Amplification Attacks

[RFC7252] discusses the susceptibility of CoAP endpoints 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. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <http://www.rfc-editor.org/info/rfc7252>.
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   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://www.rfc-editor.org/info/rfc7641>.

8.2. 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, DOI 10.17487/RFC4919, August 2007, <http://www.rfc-editor.org/info/rfc4919>. [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, <http://www.rfc-editor.org/info/rfc4944>. [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, <http://www.rfc-editor.org/info/rfc6690>. [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014, <http://www.rfc-editor.org/info/rfc7228>. [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>. [RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Range Requests", RFC 7233, DOI 10.17487/RFC7233, June 2014, <http://www.rfc-editor.org/info/rfc7233>.
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Acknowledgements

Much of the content of this document is the result of discussions with the [RFC7252] authors, and via many CoRE WG discussions. Charles Palmer provided extensive editorial comments to a previous draft version of this document, some of which have been covered in this document. 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 (Section 2.8). 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. Qin Wu provided a review for the IETF Operations directorate, and Goeran Selander commented on the security considerations. Kepeng Li, Linyi Tian, and Barry Leiba wrote up an early version of the Size option, which is described in this document. Klaus Hartke wrote some of the text describing the interaction of Block2 with Observe. Matthias Kovatsch provided a number of significant simplifications of the protocol. The IESG reviewers provided very useful comments. Spencer Dawkins even suggested new text. He and Mirja Kuehlewind insisted on more explicit information about the layering of block-wise transfers on top of the base protocol. Ben Campbell helped untangle some MUST/ SHOULD soup. Comments by Alexey Melnikov, as well as the Gen-ART review by Jouni Korhonen, resulted in further improvements to the text.
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Authors' Addresses

Carsten Bormann Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63921 Email: cabo@tzi.org Zach Shelby (editor) ARM 150 Rose Orchard San Jose, CA 95134 United States of America Phone: +1-408-203-9434 Email: zach.shelby@arm.com