gnrc_ipv6_auto_subnets: auto-configuration for nested subnets

If we get a large (e.g. /62) prefix from e.g. DHCPv6, we can split it
into subnets automatically to configure downstream interfaces.

This allows for automatic configuration of daisy-chained nodes or
nodes connected in a tree topology.

To enable the feature, a new pseudo-module `gnrc_ipv6_auto_subnets` is
provided.

doc/doxygen/src/gnrc_ipv6_auto_subnets_simple.puml

@@ -0,0 +1,35 @@
+' to generate SVG run plantuml -tsvg gnrc_ipv6_auto_subnets_simple.puml
+@startuml
+nwdiag {
+
+	network level1 {
+		address = "2001:db8::/60";
+
+		router_a [address = "2001:db8::<color:#8a8a8a>c8f4:13ff:fece:3f43", description = "1st level router"];
+		leaf_a   [address = "2001:db8::<color:#8a8a8a>804b:fcff:feb6:43fb", description = "1st level leaf node"];
+	}
+
+	network level2 {
+		address = "2001:db8:0:8::/61";
+
+		router_a [address = "2001:db8:0:8:<color:#8a8a8a>3c27:6dff:fe25:e95d"];
+		router_b [address = "2001:db8:0:8:<color:#8a8a8a>5075:35ff:fefa:30bc", description = "2nd level router"];
+	}
+
+	network level3 {
+		address = "2001:db8:0:c::/62";
+
+		router_b [address = "2001:db8:0:c:<color:#8a8a8a>2ca3:9eff:fea9:68f7"];
+		router_c [address = "2001:db8:0:c:<color:#8a8a8a>fc33:13ff:fe93:5ae4", description = "3rd level router"];
+		leaf_b1  [address = "2001:db8:0:c:<color:#8a8a8a>209e:deff:fea9:fd1b", description = "3rd level leaf node"];
+		leaf_b2  [address = "2001:db8:0:c:<color:#8a8a8a>5491:a2ff:fe98:61a2", description = "3rd level leaf node"];
+	}
+
+	network level4 {
+		address = "2001:db8:0:e::/63";
+
+		router_c [address = "2001:db8:0:e:<color:#8a8a8a>a8d9:e1ff:feab:d544"];
+		leaf_c   [address = "2001:db8:0:e:<color:#8a8a8a>1cf5:33ff:fe7c:c70c", description = "4th level leaf node"];
+	}
+}
+@enduml

sys/net/gnrc/Makefile

@@ -88,6 +88,9 @@ endif
 ifneq (,$(filter gnrc_rpl_p2p,$(USEMODULE)))
   DIRS += routing/rpl/p2p
 endif
+ifneq (,$(filter gnrc_ipv6_auto_subnets,$(USEMODULE)))
+  DIRS += routing/ipv6_auto_subnets
+endif
 ifneq (,$(filter gnrc_sixlowpan,$(USEMODULE)))
   DIRS += network_layer/sixlowpan
 endif

sys/net/gnrc/Makefile.dep

@@ -113,6 +113,12 @@ ifneq (,$(filter gnrc_rpl,$(USEMODULE)))
   USEMODULE += evtimer
 endif
 
+ifneq (,$(filter gnrc_ipv6_auto_subnets,$(USEMODULE)))
+  USEMODULE += gnrc_ipv6_nib_rtr_adv_pio_cb
+  CFLAGS += -DCONFIG_GNRC_IPV6_NIB_ADV_ROUTER=0
+  CFLAGS += -DCONFIG_GNRC_IPV6_NIB_ADD_RIO_IN_LAST_RA=1
+endif
+
 ifneq (,$(filter gnrc_netif,$(USEMODULE)))
   USEMODULE += netif
   USEMODULE += l2util

sys/net/gnrc/routing/ipv6_auto_subnets/Makefile

@@ -0,0 +1,3 @@
+MODULE = gnrc_ipv6_auto_subnets
+
+include $(RIOTBASE)/Makefile.base

sys/net/gnrc/routing/ipv6_auto_subnets/gnrc_ipv6_auto_subnets.c

@@ -0,0 +1,218 @@
+/*
+ * Copyright (C) 2021 ML!PA Consulting GmbH
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup    net_gnrc_ipv6_auto_subnets Simple-Subnet auto-configuration
+ * @ingroup     net_gnrc
+ * @brief       Automatic configuration for cascading subnets
+ *
+ * About
+ * =====
+ *
+ * This module provides an automatic configuration for networks with a simple
+ * tree topology.
+ *
+ * If a sufficiently large IPv6 prefix (> /64) is provided via Router Advertisements,
+ * a routing node with this module will automatically configure subnets from it
+ * by dividing it into sub-prefixes for each downstream interface.
+ *
+ * There can only be a single routing node on each level of the network but an
+ * arbitrary number of leaf nodes.
+ *
+ * ![Example Topology](gnrc_ipv6_auto_subnets_simple.svg)
+ *
+ * The downstream network(s) receive the sub-prefix via Router Advertisements
+ * and the process repeats until the bits of the prefix are exhausted.
+ * The smallest subnet must still have a /64 prefix.
+ *
+ * The new subnet must no longer be considered on-link by the hosts in the
+ * parent network.
+ * Therefore the downstream router will send a router advertisement with only
+ * a Route Information Option included to the upstream network.
+ * The Route Information Option contains the prefix of the downstream network
+ * so that upstream routers will no longer consider hosts in this subnet on-link
+ * but instead will use the downstream router to route to the new subnet.
+ *
+ * Usage
+ * =====
+ *
+ * Simply add the `gnrc_ipv6_auto_subnets` module to the code of the nodes that
+ * should act as routers in the cascading network.
+ * The upstream network will be automatically chosen as the one that first
+ * receives a router advertisement.
+ *
+ * @{
+ *
+ * @file
+ * @author  Benjamin Valentin <benjamin.valentin@ml-pa.com>
+ */
+
+#include "net/gnrc/ipv6/nib.h"
+#include "net/gnrc/ndp.h"
+
+#define ENABLE_DEBUG 0
+#include "debug.h"
+
+static char addr_str[IPV6_ADDR_MAX_STR_LEN];
+
+static void _init_sub_prefix(ipv6_addr_t *out,
+                             const ipv6_addr_t *prefix, uint8_t bits,
+                             uint8_t idx, uint8_t idx_bits)
+{
+    uint8_t bytes = bits / 8;
+    uint8_t rem   = bits % 8;
+    int8_t shift  = 8 - rem - idx_bits;
+
+    /* first copy old prefix */
+    memset(out, 0, sizeof(*out));
+    ipv6_addr_init_prefix(out, prefix, bits);
+
+    /* if new bits are between bytes, first copy over the most significant bits */
+    if (shift < 0) {
+        out->u8[bytes] |= idx >> -shift;
+        out->u8[++bytes] = 0;
+        shift += 8;
+    }
+
+    /* shift remaining bits at the end of the prefix */
+    out->u8[bytes] |= idx << shift;
+}
+
+static bool _remove_old_prefix(gnrc_netif_t *netif,
+                               const ipv6_addr_t *pfx, uint8_t pfx_len,
+                               gnrc_pktsnip_t **ext_opts)
+{
+    gnrc_ipv6_nib_pl_t entry;
+    gnrc_pktsnip_t *tmp;
+    void *state = NULL;
+    ipv6_addr_t old_pfx;
+    uint8_t old_pfx_len = 0;
+
+    /* iterate prefix list to see if the prefix already exists */
+    while (gnrc_ipv6_nib_pl_iter(netif->pid, &state, &entry)) {
+        uint8_t match_len = ipv6_addr_match_prefix(&entry.pfx, pfx);
+
+        /* The prefix did not change - nothing to do here */
+        if (match_len >= pfx_len && pfx_len == entry.pfx_len) {
+            return true;
+        }
+
+        /* find prefix that is closest to the new prefix */
+        if (match_len > old_pfx_len) {
+            old_pfx_len = entry.pfx_len;
+            old_pfx = entry.pfx;
+        }
+    }
+
+    /* no prefix found */
+    if (old_pfx_len == 0) {
+        return false;
+    }
+
+    DEBUG("auto_subnets: remove old prefix %s/%u\n",
+          ipv6_addr_to_str(addr_str, &old_pfx, sizeof(addr_str)), old_pfx_len);
+
+    /* invalidate old prefix in RIO */
+    tmp = gnrc_ndp_opt_ri_build(&old_pfx, old_pfx_len, 0,
+                                NDP_OPT_RI_FLAGS_PRF_NONE, *ext_opts);
+    if (tmp) {
+        *ext_opts = tmp;
+    }
+
+    /* remove the prefix */
+    gnrc_ipv6_nib_pl_del(netif->pid, &old_pfx, old_pfx_len);
+
+    return false;
+}
+
+static void _configure_subnets(uint8_t subnets, gnrc_netif_t *upstream,
+                               const ndp_opt_pi_t *pio)
+{
+    gnrc_netif_t *downstream = NULL;
+    gnrc_pktsnip_t *ext_opts = NULL;
+    const ipv6_addr_t *prefix = &pio->prefix;
+    uint32_t valid_ltime = byteorder_ntohl(pio->valid_ltime);
+    uint32_t pref_ltime = byteorder_ntohl(pio->pref_ltime);
+    const uint8_t prefix_len = pio->prefix_len;
+    uint8_t new_prefix_len, subnet_len;
+
+    DEBUG("auto_subnets: create %u subnets\n", subnets);
+
+    /* Calculate remaining prefix length.
+     * For n subnets we consume floor(log_2 n) + 1 bits.
+     * To calculate floor(log_2 n) quickly, find the position of the
+     * most significant set bit by counting leading zeros.
+     */
+    subnet_len = 32 - __builtin_clz(subnets);
+    new_prefix_len = prefix_len + subnet_len;
+
+    if (new_prefix_len > 64) {
+        DEBUG("auto_subnets: can't split /%u into %u subnets\n", prefix_len, subnets);
+        return;
+    }
+
+    while ((downstream = gnrc_netif_iter(downstream))) {
+        gnrc_pktsnip_t *tmp;
+        ipv6_addr_t new_prefix;
+
+        if (downstream == upstream) {
+            continue;
+        }
+
+        /* create subnet from upstream prefix */
+        _init_sub_prefix(&new_prefix, prefix, prefix_len, subnets--, subnet_len);
+
+        DEBUG("auto_subnets: configure prefix %s/%u on %u\n",
+              ipv6_addr_to_str(addr_str, &new_prefix, sizeof(addr_str)),
+              new_prefix_len, downstream->pid);
+
+        /* first remove old prefix if the prefix changed */
+        _remove_old_prefix(downstream, &new_prefix, new_prefix_len, &ext_opts);
+
+        /* configure subnet on downstream interface */
+        gnrc_netif_ipv6_add_prefix(downstream, &new_prefix, new_prefix_len,
+                                   valid_ltime, pref_ltime);
+
+        /* start advertising subnet */
+        gnrc_ipv6_nib_change_rtr_adv_iface(downstream, true);
+
+        /* add route information option with new subnet */
+        tmp = gnrc_ndp_opt_ri_build(&new_prefix, new_prefix_len, valid_ltime,
+                                    NDP_OPT_RI_FLAGS_PRF_NONE, ext_opts);
+        if (tmp == NULL) {
+            DEBUG("auto_subnets: No space left in packet buffer. Not adding RIO\n");
+        } else {
+            ext_opts = tmp;
+        }
+    }
+
+    /* immediately send an RA with RIO */
+    if (ext_opts) {
+        gnrc_ndp_rtr_adv_send(upstream, NULL,
+                              &ipv6_addr_all_nodes_link_local, true, ext_opts);
+    } else {
+        DEBUG("auto_subnets: Options empty, not sending RA\n");
+    }
+}
+
+void gnrc_ipv6_nib_rtr_adv_pio_cb(gnrc_netif_t *upstream, const ndp_opt_pi_t *pio)
+{
+    /* create a subnet for each downstream interface */
+    unsigned subnets = gnrc_netif_numof() - 1;
+
+    if (subnets == 0) {
+        return;
+    }
+
+    if (pio->valid_ltime.u32 == 0) {
+        return;
+    }
+
+    _configure_subnets(subnets, upstream, pio);
+}
+/** @} */