gnrc/nib: consolidate prefix removal code in _nib_offl_remove_prefix()

[benpicco]

Contribution description

There are three places in which a prefix gets removed:

• lifetime timeout in _handle_pfx_timeout()
• RA handling with PIO with lifetime = 0 in _remove_prefix()
• manually calling the nib prefix del shell command resulting in gnrc_ipv6_nib_pl_del()

However, they all do slightly different things. The code in _handle_pfx_timeout() appears to be the most complete, so move it to a common function that can be used by the other cases as well.

The _handle_snd_mc_ra() function in gnrc_ipv6_nib_pl_del() did not do much in informing downstream nodes that the prefix was removed - it would only schedule sending a RA without the prefix, but omission does not mean deprecation.
I replaced it with a new _snd_rtr_advs_drop_pfx() function that sends a RA that includes a PIO with a lifetime of 0 to indicate that the prefix must be removed.
I'm not entirely happy about this as I would rather have the removed prefix (with ltime = 0) included in normal RAs for a grace period (in case this RA gets lost) but that would require some more elaborate changes.

Testing procedure

Issues/PRs references

[miri64]

Did you test this with your setup?

[benpicco]

Yes I ran the test steps from #16750

[miri64]

Please squash. I trust your testing.

[benpicco]

All green here 😉

[miri64]

Then lets gooooo 🚀

[benpicco]

@miri64 as @chrysn mentioned prefix deprecation, how would this be done in this context?
We have _nib_offl_remove_prefix() but no _nib_offl_deprecate_prefix().
There is no deprecation flag in the _nib_offl_entry_t struct, so how would a prefix be marked as deprecated?

[miri64]

There is no deprecation flag in the _nib_offl_entry_t struct, so how would a prefix be marked as deprecated?

A prefix is not deprecated, the associated addresses are (see also RFC 4862, section 5.5.4). This happens here

RIOT/sys/net/gnrc/network_layer/ipv6/nib/nib.c

Lines 1334 to 1355 in abeb9c8

	static void _handle_pfx_timeout(_nib_offl_entry_t *pfx)
	{
	gnrc_netif_t *netif = gnrc_netif_get_by_pid(_nib_onl_get_if(pfx->next_hop));
	uint32_t now = evtimer_now_msec();
	gnrc_netif_acquire(netif);
	if (now >= pfx->valid_until) {
	_nib_offl_remove_prefix(pfx);
	}
	else if (now >= pfx->pref_until) {
	for (int i = 0; i < CONFIG_GNRC_NETIF_IPV6_ADDRS_NUMOF; i++) {
	if (ipv6_addr_match_prefix(&netif->ipv6.addrs[i],
	&pfx->pfx) >= pfx->pfx_len) {
	netif->ipv6.addrs_flags[i] &= ~GNRC_NETIF_IPV6_ADDRS_FLAGS_STATE_MASK;
	netif->ipv6.addrs_flags[i] |= GNRC_NETIF_IPV6_ADDRS_FLAGS_STATE_DEPRECATED;
	}
	}
	_evtimer_add(pfx, GNRC_IPV6_NIB_PFX_TIMEOUT, &pfx->pfx_timeout,
	pfx->valid_until - now);
	}
	gnrc_netif_release(netif);
	}

The timer for that is armed here

RIOT/sys/net/gnrc/network_layer/ipv6/nib/_nib-internal.c

Lines 830 to 841 in abeb9c8

	if (pref_ltime != UINT32_MAX) {
	_evtimer_add(dst, GNRC_IPV6_NIB_PFX_TIMEOUT, &dst->pfx_timeout,
	pref_ltime);
	/* ignore capped of preferred lifetimes from sec to ms conversion */
	if (pref_ltime < (UINT32_MAX - 1)) {
	/* prevent pref_ltime from becoming UINT32_MAX */
	if (((pref_ltime + now) == UINT32_MAX) && (now != 0)) {
	pref_ltime++;
	}
	pref_ltime += now;
	}
	}

Sure, we could also add a respective flag and deprecation timer for prefixes to implement RFC 4861, section 6.2.7 to the letter. However, deprecation is in the end only relevant for addresses, so why have another mechanism for prefixes (see the Terminology section of RFC 4862 (section 2))?

 deprecated address -  An address assigned to an interface whose use
   is discouraged, but not forbidden.  A deprecated address should no
   longer be used as a source address in new communications, but
   packets sent from or to deprecated addresses are delivered as
   expected.  A deprecated address may continue to be used as a
   source address in communications where switching to a preferred
   address causes hardship to a specific upper-layer activity (e.g.,
   an existing TCP connection).