Added CoAP socket

[eHonnef]

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

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.

[codecov]

Codecov Report

Merging #4334 (f6b86b5) into master (6294c6e) will increase coverage by 0.24%.
Report is 119 commits behind head on master.
The diff coverage is 89.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4334      +/-   ##
==========================================
+ Coverage   81.90%   82.15%   +0.24%     
==========================================
  Files         330      351      +21     
  Lines       76380    83547    +7167     
==========================================
+ Hits        62561    68640    +6079     
- Misses      13819    14907    +1088     

Files	Coverage Δ
scapy/contrib/coap.py	96.42% <100.00%> (+0.93%)	⬆️
scapy/contrib/coap_socket.py	88.60% <88.60%> (ø)

... and 114 files with indirect coverage changes

[eHonnef]

Any idea on how I can fix these failed tests?
If I try to run on my machine without root, it works, so I don't know really how to debug this.

[gpotter2]

Tests are failing on versions older than 3.9. You're using 3.9+ specific features. Scapy should work with anything starting from 3.7, so this probably needs to be updated.

[eHonnef]

@gpotter2 Thanks for the insights, it is ready for reviews :D

[polybassa]

Could you please implement a select function for your socket, so that sr, sr1 and sniff will work. Could you please also provide unit tests for these functions.