Use pub/sub for user-defined policy delivery

[Zhiyi-Zhang]

In the current (6d433ac) ndn-lite, user-defined policies are distributed through signed Interest sent by the controller directly to a device.
We may want to change this direct Interest notification to pub/sub mode. That is, each device subscribe to the topics that are:

1. related to itself, e.g., /home/POLICY/TEMP/bedroom/sensor1
2. related to the service and the room, e.g. /home/POLICY/TEMP/bedroom
3. related to the service it serves, e.g., /home/POLICY/TEMP

When the IoT controller generates a new policy, it will send out a notification containing the policy name.
A device, say /home/TEMP/bedroom/sensor1, receives this notification, will check whether the policy affects itself.
If yes, the device will fetch the new content from the topic, verify the signature, and install the policy.

[yoursunny]

Given you are talking about "notification", I suppose these are considered command topics.

A major limitation is that, ps_subscribe_to_command can accommodate up to five topics.

ndn-lite/app-support/pub-sub.c

Lines 486 to 494 in d60a114

	for (int i = 0; i < 5; i++) {
	if (m_pub_sub_state.sub_topics[i].service == NDN_SD_NONE) {
	topic = &m_pub_sub_state.sub_topics[i];
	}
	}
	if (topic == NULL) {
	NDN_LOG_ERROR("[PUB/SUB] No more space for new subscription topics. Abort");
	return;
	}

This proposal would occupy 60% of available topics.

While it's possible to change this limitation, it would increase memory usage and take NDN-Lite further from being able to run on memory-constrained devices.

P.S. Is NDN-Lite still usable on constrained devices such as the nRF52840?