Constant toad_msg::opt::known::no_repeat::SIZE2

source · 
pub const SIZE2: OptNumber;
Expand description

Size1 Option

generated from RFC7252 section 5.10.9

The Size1 option provides size information about the resource representation in a request. The option value is an integer number of bytes. Its main use is with block-wise transfers [BLOCK]. In the present specification, it is used in 4.13 responses (Section 5.9.2.9) to indicate the maximum size of request entity that the server is able and willing to handle. 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.)

The Size2 Option may be used for two purposes:

  • 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.
  • 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:

  • 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”).
  • 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.