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.