sys/net/nanocoap: align request handling with spec

[maribu]

Contribution description

• Do not reply with a reset message to a reset or an ACK message
• Reply with a reset message when not able to process a CON/NON message (not even a suitable error reply)

Testing procedure

$ sudo ip tuntap add tap0 mode tap user $(whoami)
$ sudo ip link set tap0 up
$ make BOARD=native64 -C examples/nanocoap_server -j flash term

$ cat test.py                     
import socket, aiocoap
m = aiocoap.Message()
m.code = 0
m.mid = 0
m.mtype = aiocoap.RST
s = socket.socket(socket.AF_INET6, socket.SOCK_DGRAM)
s.connect(('fe80::9826:30ff:feb8:31f4', 5683, 0, socket.if_nametoindex('tap0')))
s.send(m.encode())
s.recv(1024)
$ python3 test.py

With master:

$ tshark -i tap0 -Y "coap"
Capturing on 'tap0'
    3 0.000397706 fe80::9826:30ff:feb8:31f3 56273 5683 fe80::9826:30ff:feb8:31f4 CoAP 66 RST, MID:0, Empty Message
    4 0.000675868 fe80::9826:30ff:feb8:31f4 5683 56273 fe80::9826:30ff:feb8:31f3 CoAP 66 RST, MID:0, Empty Message

Note the RST "reply" to the RST

With this PR:

$ tshark -i tap0 -Y "coap"
Capturing on 'tap0'
    7 3.994693734 fe80::9826:30ff:feb8:31f3 54684 5683 fe80::9826:30ff:feb8:31f4 CoAP 66 RST, MID:0, Empty Message

Note that no RST "reply" is send to the RST

Issues/PRs references

None

[maribu]

Relevant parts of the RFC:

When a recipient is not at all able to process a Confirmable message
(i.e., not even able to provide a suitable error response), it
replies with a Reset message (RST) instead of an Acknowledgement
(ACK).

and

When a recipient is not able to process a
Non-confirmable message, it may reply with a Reset message (RST).

and

To avoid an implosion of error responses, when a server is aware that
a request arrived via multicast, it MUST NOT return a Reset message
in reply to a Non-confirmable message.

(The check for non-confirmable multicast message is simplified to a check for multicast, as only NON may be send as multicats anyway.)

[maribu]

CoAP Pings still work:

$  tshark -i tap0 -Y "coap"
Capturing on 'tap0'
   21 416.775256861 fe80::9826:30ff:feb8:31f3 8061 5683 fe80::9826:30ff:feb8:31f4 CoAP 66 CON, MID:24336, Empty Message
   22 416.775519804 fe80::9826:30ff:feb8:31f4 5683 8061 fe80::9826:30ff:feb8:31f3 CoAP 66 RST, MID:24336, Empty Message
   23 417.775511480 fe80::9826:30ff:feb8:31f3 8061 5683 fe80::9826:30ff:feb8:31f4 CoAP 66 CON, MID:24337, Empty Message
   24 417.775839404 fe80::9826:30ff:feb8:31f4 5683 8061 fe80::9826:30ff:feb8:31f3 CoAP 66 RST, MID:24337, Empty Message
   25 418.776392388 fe80::9826:30ff:feb8:31f3 8061 5683 fe80::9826:30ff:feb8:31f4 CoAP 66 CON, MID:24338, Empty Message
   26 418.776805843 fe80::9826:30ff:feb8:31f4 5683 8061 fe80::9826:30ff:feb8:31f3 CoAP 66 RST, MID:24338, Empty Message
   27 419.777272963 fe80::9826:30ff:feb8:31f3 8061 5683 fe80::9826:30ff:feb8:31f4 CoAP 66 CON, MID:24339, Empty Message
   28 419.777660970 fe80::9826:30ff:feb8:31f4 5683 8061 fe80::9826:30ff:feb8:31f3 CoAP 66 RST, MID:24339, Empty Message

$ coap_ping -i tap0 '[fe80::9826:30ff:feb8:31f4]'
CoAP PING: [fe80::9826:30ff:feb8:31f3]:8061 ==> [fe80::9826:30ff:feb8:31f4]:5683
RST from [fe80::9826:30ff:feb8:31f4]:5683 seq=0 rt=0 time=0.732ms (0.732ms/0.732ms/0.732ms)
RST from [fe80::9826:30ff:feb8:31f4]:5683 seq=1 rt=0 time=0.447ms (0.447ms/0.589ms/0.732ms)
RST from [fe80::9826:30ff:feb8:31f4]:5683 seq=2 rt=0 time=0.576ms (0.447ms/0.585ms/0.732ms)
RST from [fe80::9826:30ff:feb8:31f4]:5683 seq=3 rt=0 time=0.530ms (0.447ms/0.571ms/0.732ms)

[riot-ci]

Murdock results

✔️ PASSED

9fa2661 sys/net/nanocoap: align request handling with spec

Success	Failures	Total	Runtime
10271	0	10271	08m:57s

Artifacts

• Documentation preview

[benpicco]

Looks like a straightforward fix, nicely done!

[maribu]

Thx :)