Follow-up: Added CoAP socket #4334

[polybassa]

Description

This PR implements a CoAP socket, pretty similar on how ISOTPSoftSocket works.
I implemented the basic message exchange, mostly based on the RFC-7252.

• Congestion control
• Retransmission mechanism
• Separate responses
• Message duplication detection

Known-limitations

• No POST and DELETE methods
• No DTLS
• No discovery via multicast/broadcast, although you can still bind to one of these interfaces
• No observer
• The SR/SR1 functions cannot handle separate responses.

General comments

It has a dependency for from scapy.contrib.isotp.isotp_soft_socket import TimeoutScheduler, I found nice how this is implemented, so I just used it, I didn't want to copy/paste again.

Also I added some unit tests for the basic cases.

Quick usage

Client example:
    >>> with CoAPSocket("127.0.0.1", 1234) as coap_client:
    >>>     req = CoAPSocket.make_coap_req_packet(method=GET, uri="endpoint-uri", payload=b"")
    >>>     coap_client.send("127.0.0.1", 5683, req)
    >>>     res = coap_client.recv() # Careful, this will block until the coap_client receives something

Server without specifying resources:
    >>> with CoAPSocket("127.0.0.1", 5683) as coap_server:
    >>>     while True:
    >>>         pkg = coap_server.recv()
    >>>         handle_package(pkg)

Server with custom resources:
    >>> class DummyResource(CoAPResource):
    >>>     def get(self, payload, options, token, sa_ll):
    >>>         return {"type": ACK, "code": CONTENT_205, "options": [(CONTENT_FORMAT, CF_TEXT_PLAIN)], "payload": b'dummy response'}
    >>>
    >>> class DelayedResource(CoAPResource):
    >>>     def __init__(self, url):
    >>>         CoAPResource.__init__(self, url=url)
    >>>         self.delayed_tokens = []
    >>>
    >>>     def delayed_message(self):
    >>>         token, address = self.delayed_tokens.pop(0)
    >>>         pkt = CoAPSocket.make_delayed_resp_packet(token, [(CONTENT_FORMAT, CF_TEXT_PLAIN)], b"delayed payload")
    >>>         self._send_separate_response(pkt, address)
    >>>
    >>>     def get(self, payload, options, token, sa_ll):
    >>>         # We know that this can take a while, so we return an empty ACK now and wait for whatever resource to be available.
    >>>         TimeoutScheduler.schedule(1, self.delayed_message)
    >>>         self.delayed_tokens.append((token, sa_ll))
    >>>         return CoAPSocket.empty_ack_params()
    >>>
    >>> # Doesn't matter if it starts with "/dummy" or "dummy", but it is an error if it is in the end
    >>> lst_resources = [DummyResource("dummy"), DelayedResource("/delayed")].
    >>> with CoAPSocket("127.0.0.1", 5683, lst_resources=lst_resources) as coap_socket:
    >>>     while True:
    >>>         pkg = coap_socket.recv()
    >>>         # You can handle the packages inside your resources, here will only be the "unhandled" ones.

This follow-up reintegrates master and adds additional unit tests

[polybassa]

@gpotter2 How should I proceed with the trailer check for "old" commits?

[polybassa]

this branch has a dirty history.