Block2 random access bypass for GET

[mkovatsc]

When requested out-of-order, Cf can skip the (atomic) buffering and just return the block without creating transfer state for the client.

[martinlanter]

I dealt with this issue (https://github.com/Lanterm/californium/tree/memorytest). The client still creates transfer state, though. The reason is that when receiving the response block, the client has to remember that it only asked for this single block and that it was not the server that opted for the blockwise transfer. Sure, the client could also derive this information by checking the request it previously sent, however, I favored explicitly setting a boolean that the exchange is a random access.

The user now can write code such as the following:

Request request = Request.newGet();
request.setURI("coap://localhost:5683/resource");
request.getOptions().setBlock2(BlockOption.size2Szx(64), false, 7); // access block 7
Response response = request.send().waitForResponse();

There remains one problem: Assume the code above but instead we want to access block 0 instead of 7. When the client processes the request in the BlockwiseLayer, it does not know, however, if the user wanted to

• a) Perform a random access and only retrieve the first 64 bytes of the full response payload.
• b) Just limit the block size the target is allowed to use to 64 bytes but actually wants to retrieve the full response (= early block size negotiation).

We could of course just define that the second scenario is no longer possible with Cf as scenario a) seems to be more expedient then b). Tell me what you think.