gnrc/ipv6/nib: automatically create 6ctx for downstream networks

[benpicco]

Contribution description

When gnrc_ipv6_auto_subnets announces the creation of a downstream subnet to the upstream router, if that router is a 6lbr, it might as well create a compression context for that network.

To also inform the downstream router about this compression context, send another RA to the downstream router, containing only the 6ctx information (to avoid creating a packet ping-pong loop if it also contained a prefix information option).

Testing procedure

An instance of examples/gnrc_border_router and one of examples/gnrc_networking with both ZEP and TAP enabled (I used two instances of the border router with #21081 to only enable the ABR functionality of one at run-time).

start 6LBR on-demand

static gnrc_netif_t *_get_6lo_interface(void)
{
    gnrc_netif_t *netif = NULL;
    while ((netif = gnrc_netif_iter(netif))) {
        if (gnrc_netif_is_6ln(netif)) {
            return netif;
        }
    }

    return NULL;
}

static int cmd_start_network(int argc, char **argv)
{
    (void)argc;
    (void)argv;

    gnrc_netif_t *downstream = _get_6lo_interface();
    if (!downstream) {
        puts("can't find 6lo interface");
        return;
    }

    ipv6_addr_t prefix;
    int len = ipv6_prefix_from_str(&prefix, "fd12::/16");
    if (len <= 0) {
        puts("Can't parse prefix string");
        return;
    }

    /* configure subnet on downstream interface */
    int idx = gnrc_netif_ipv6_add_prefix(downstream, &prefix, len,
                                         UINT32_MAX, UINT32_MAX);
    if (idx < 0) {
        DEBUG("adding prefix to %u failed\n", downstream->pid);
        return;
    }

    netopt_enable_t enable = NETOPT_ENABLE;
    gnrc_netapi_set(downstream->pid, NETOPT_6LO_ABR, 0, &enable, sizeof(enable));
    gnrc_netapi_set(downstream->pid, NETOPT_IPV6_SND_RTR_ADV, 0, &enable, sizeof(enable));

    return 0;
}
SHELL_COMMAND(leader, "Make the node a leader", cmd_start_network);

Downstream node receives the fd12::/16 prefix and creates a fd12:1284:c87:1fb7::/64 prefix from it, sends this to the upstream router. Upstream router creates a compression context and response with yet another RA that contains the updated list of compression contexts.

6ctx on both nodes shows the same information:

cid|prefix                                     |C|ltime
-----------------------------------------------------------
  0|                                 fd12::/16 |1| 6046min
  1|                   fd12:1284:c87:1fb7::/64 |1| 6046min

Issues/PRs references

[miri64]

Shouldn't there already be a function like this somewhere in the code base?

[benpicco]

Only a function that adds all options

[miri64]

To avoid code duplication, does it make sense to call the function at hand there as well?