observe: Add in support for all observe responses to be non-confirmable

https://tools.ietf.org/html/rfc7641#section-4.5 requires that non-confirmable
observe responses should be sent as confirmable at least once every 24 hours.

However there is a requirement to override this periodical confirmable sending
behavior in lossy under DDoS attack type environments.

https://tools.ietf.org/html/draft-ietf-dots-signal-channel-41#section-4.4.2.1

Add in support for, and document, a new coap_resource_init() flag
COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS to handle this non RFC7641 requirement.

include/coap2/resource.h

@@ -62,15 +62,26 @@ typedef struct coap_attr_t {
 /**
  * Notifications will be sent non-confirmable by default. RFC 7641 Section 4.5
  * https://tools.ietf.org/html/rfc7641#section-4.5
+ * Libcoap will always send every fifth packet as confirmable.
  */
 #define COAP_RESOURCE_FLAGS_NOTIFY_NON  0x0
 
 /**
- * Notifications will be sent confirmable by default. RFC 7641 Section 4.5
+ * Notifications will be sent confirmable. RFC 7641 Section 4.5
  * https://tools.ietf.org/html/rfc7641#section-4.5
  */
 #define COAP_RESOURCE_FLAGS_NOTIFY_CON  0x2
 
+/**
+ * Notifications will always be sent non-confirmable. This is in
+ * violation of RFC 7641 Section 4.5
+ * https://tools.ietf.org/html/rfc7641#section-4.5
+ * but required by the DOTS signal channel protocol which needs to operate in
+ * lossy DDoS attack environments.
+ * https://tools.ietf.org/html/draft-ietf-dots-signal-channel-41#section-4.4.2.1
+ */
+#define COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS  0x4
+
 typedef struct coap_resource_t {
   unsigned int dirty:1;          /**< set to 1 if resource has changed */
   unsigned int partiallydirty:1; /**< set to 1 if some subscribers have not yet

man/coap_observe.txt.in

@@ -94,13 +94,19 @@ manage the deletion of the resource at the appropriate time.
 
 *NOTE:* The type (confirmable or non-confirmable) of the triggered observe
 GET response is determined not by the initial GET request, but independently
-by the server as per RFC 7641 3.5. Transmission.  This is controlled by the
-flags (one of COAP_RESOURCE_FLAGS_NOTIFY_NON or COAP_RESOURCE_FLAGS_NOTIFY_CON)
+by the server as per https://tools.ietf.org/html/rfc7641#section-3.5.
+This is controlled by the flags (one of COAP_RESOURCE_FLAGS_NOTIFY_NON,
+COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS or COAP_RESOURCE_FLAGS_NOTIFY_CON)
 used when creating the resource using *coap_resource_init*(3).
-Furthermore, the server must send at least one "observe" response as
-confirmable, when generally sending non-confirmable,  every 24 hours.
-libcoap handles this by sending every fifth (COAP_OBS_MAX_NON) response as a
-confirmable response for detection that the client is still responding.
+
+*NOTE:* Furthermore, the server must send at least one "observe" response as
+confirmable, when generally sending non-confirmable, at least every 24 hours
+as per https://tools.ietf.org/html/rfc7641#section-4.5.
+Libcoap automatically handles this by sending every fifth (COAP_OBS_MAX_NON)
+response as a confirmable response for detection that the client is still
+responding unless if COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS is set, which is a
+RFC7641 violation, where non-confirmable "observe" responses are always sent
+as required by some higher layer protocols.
 
 The *coap_resource_notify_observers*() function needs to be called whenever the
 server application determines that there has been a change to the state of

man/coap_resource.txt.in

@@ -84,7 +84,13 @@ _flags_ can be one of the following definitions or'ed together.
 [horizontal]
 *COAP_RESOURCE_FLAGS_NOTIFY_NON*::
 Set the notification message type to non-confirmable for any trigggered
-"observe" responses.
+"observe" responses with type set to confirmable every 5 packets as required by
+https://tools.ietf.org/html/rfc7641#section-4.5.
+//-
+*COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS*::
+Set the notification message type to always non-confirmable for any trigggered
+"observe" responses. This should only be used if a upper layer protocol
+requires it.
 //-
 *COAP_RESOURCE_FLAGS_NOTIFY_CON*::
 Set the notification message type to confirmable for any trigggered
@@ -120,7 +126,8 @@ _context_ and frees their storage.
 
 The *coap_resource_set_mode*() changes the notification message type of
 _resource_ to the given _mode_ which must be one of
-COAP_RESOURCE_FLAGS_NOTIFY_NON or COAP_RESOURCE_FLAGS_NOTIFY_CON.
+COAP_RESOURCE_FLAGS_NOTIFY_NON, COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS or
+COAP_RESOURCE_FLAGS_NOTIFY_CON.
 
 The *coap_resource_set_userdata*() function allows a pointer to user _data_
 to be associated with a _resource_ that can accessed in any callback that

src/resource.c

@@ -793,8 +793,9 @@ coap_notify_observers(coap_context_t *context, coap_resource_t *r) {
       token.s = obs->token;
 
       obs->tid = response->tid = coap_new_message_id(obs->session);
-      if ((r->flags & COAP_RESOURCE_FLAGS_NOTIFY_CON) == 0
-          && obs->non_cnt < COAP_OBS_MAX_NON) {
+      if ((r->flags & COAP_RESOURCE_FLAGS_NOTIFY_CON) == 0 &&
+          ((r->flags & COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS) ||
+           obs->non_cnt < COAP_OBS_MAX_NON)) {
         response->type = COAP_MESSAGE_NON;
       } else {
         response->type = COAP_MESSAGE_CON;
@@ -805,7 +806,8 @@ coap_notify_observers(coap_context_t *context, coap_resource_t *r) {
       /* TODO: do not send response and remove observer when
        *  COAP_RESPONSE_CLASS(response->hdr->code) > 2
        */
-      if (response->type == COAP_MESSAGE_CON) {
+      if (response->type == COAP_MESSAGE_CON ||
+          (r->flags & COAP_RESOURCE_FLAGS_NOTIFY_NON_ALWAYS)) {
         obs->non_cnt = 0;
       } else {
         obs->non_cnt++;