gnrc_sixlowpan_frag: add asynchronous rbuf GC

While the current approach for garbage collection in the 6Lo reassembly
buffer is good for best-effort handling of
*fragmented* packets and nicely RAM saving, it has the problem that
incomplete, huge datagrams can basically DoS a node, if no further
fragmented datagram is received for a while (since the packet buffer is
full and GC is not triggered).

This change adds a asynchronous GC (utilizing the existing
functionality) to the reassembly buffer, so that even if there is no new
fragmented packet received, fragments older than `RBUF_TIMEOUT` will be
removed from the reassembly buffer, freeing up the otherwise wasted
packet buffer space.

sys/include/net/gnrc/sixlowpan/frag.h

@@ -38,10 +38,20 @@
 extern "C" {
 #endif
 
+/**
+ * @name    Message types
+ * @{
+ */
 /**
  * @brief   Message type for passing one 6LoWPAN fragment down the network stack
  */
-#define GNRC_SIXLOWPAN_MSG_FRAG_SND    (0x0225)
+#define GNRC_SIXLOWPAN_MSG_FRAG_SND         (0x0225)
+
+/**
+ * @brief   Message type for triggering garbage collection reassembly buffer
+ */
+#define GNRC_SIXLOWPAN_MSG_FRAG_GC_RBUF     (0x0226)
+/** @} */
 
 /**
  * @brief   An entry in the 6LoWPAN reassembly buffer.
@@ -114,6 +124,11 @@ void gnrc_sixlowpan_frag_send(gnrc_pktsnip_t *pkt, void *ctx, unsigned page);
  */
 void gnrc_sixlowpan_frag_recv(gnrc_pktsnip_t *pkt, void *ctx, unsigned page);
 
+/**
+ * @brief   Garbage collect reassembly buffer.
+ */
+void gnrc_sixlowpan_frag_gc_rbuf(void);
+
 #ifdef __cplusplus
 }
 #endif

sys/net/gnrc/network_layer/sixlowpan/frag/gnrc_sixlowpan_frag.c

@@ -320,4 +320,9 @@ void gnrc_sixlowpan_frag_recv(gnrc_pktsnip_t *pkt, void *ctx, unsigned page)
     gnrc_pktbuf_release(pkt);
 }
 
+void gnrc_sixlowpan_frag_gc_rbuf(void)
+{
+    rbuf_gc();
+}
+
 /** @} */

sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.c

@@ -48,6 +48,9 @@ static rbuf_t rbuf[RBUF_SIZE];
 
 static char l2addr_str[3 * IEEE802154_LONG_ADDRESS_LEN];
 
+static xtimer_t _gc_timer;
+static msg_t _gc_timer_msg = { .type = GNRC_SIXLOWPAN_MSG_FRAG_GC_RBUF };
+
 /* ------------------------------------
  * internal function definitions
  * ------------------------------------*/
@@ -59,8 +62,6 @@ static rbuf_int_t *_rbuf_int_get_free(void);
 static void _rbuf_rem(rbuf_t *entry);
 /* update interval buffer of entry */
 static bool _rbuf_update_ints(rbuf_t *entry, uint16_t offset, size_t frag_size);
-/* checks timeouts and removes entries if necessary (oldest if full) */
-static void _rbuf_gc(void);
 /* gets an entry identified by its tupel */
 static rbuf_t *_rbuf_get(const void *src, size_t src_len,
                          const void *dst, size_t dst_len,
@@ -78,7 +79,7 @@ void rbuf_add(gnrc_netif_hdr_t *netif_hdr, gnrc_pktsnip_t *pkt,
     rbuf_int_t *ptr;
     uint8_t *data = ((uint8_t *)pkt->data) + sizeof(sixlowpan_frag_t);
 
-    _rbuf_gc();
+    rbuf_gc();
     entry = _rbuf_get(gnrc_netif_hdr_get_src_addr(netif_hdr), netif_hdr->src_l2addr_len,
                       gnrc_netif_hdr_get_dst_addr(netif_hdr), netif_hdr->dst_l2addr_len,
                       byteorder_ntohs(frag->disp_size) & SIXLOWPAN_FRAG_SIZE_MASK,
@@ -246,7 +247,7 @@ static bool _rbuf_update_ints(rbuf_t *entry, uint16_t offset, size_t frag_size)
     return true;
 }
 
-static void _rbuf_gc(void)
+void rbuf_gc(void)
 {
     uint32_t now_usec = xtimer_now_usec();
     unsigned int i;
@@ -271,6 +272,11 @@ static void _rbuf_gc(void)
     }
 }
 
+static inline void _set_rbuf_timeout(void)
+{
+    xtimer_set_msg(&_gc_timer, RBUF_TIMEOUT, &_gc_timer_msg, sched_active_pid);
+}
+
 static rbuf_t *_rbuf_get(const void *src, size_t src_len,
                          const void *dst, size_t dst_len,
                          size_t size, uint16_t tag)
@@ -295,6 +301,7 @@ static rbuf_t *_rbuf_get(const void *src, size_t src_len,
                                          l2addr_str),
                   (unsigned)rbuf[i].super.pkt->size, rbuf[i].super.tag);
             rbuf[i].arrival = now_usec;
+            _set_rbuf_timeout();
             return &(rbuf[i]);
         }
 
@@ -347,6 +354,8 @@ static rbuf_t *_rbuf_get(const void *src, size_t src_len,
                                  l2addr_str), (unsigned)res->super.pkt->size,
           res->super.tag);
 
+    _set_rbuf_timeout();
+
     return res;
 }
 

sys/net/gnrc/network_layer/sixlowpan/frag/rbuf.h

@@ -85,6 +85,11 @@ typedef struct {
 void rbuf_add(gnrc_netif_hdr_t *netif_hdr, gnrc_pktsnip_t *frag,
               size_t frag_size, size_t offset);
 
+/**
+ * @brief   Checks timeouts and removes entries if necessary
+ */
+void rbuf_gc(void);
+
 #ifdef __cplusplus
 }
 #endif

sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c

@@ -375,6 +375,10 @@ static void *_event_loop(void *args)
                 DEBUG("6lo: send fragmented event received\n");
                 gnrc_sixlowpan_frag_send(NULL, msg.content.ptr, 0);
                 break;
+            case GNRC_SIXLOWPAN_MSG_FRAG_GC_RBUF:
+                DEBUG("6lo: garbage collect reassembly buffer event received\n");
+                gnrc_sixlowpan_frag_gc_rbuf();
+                break;
 #endif
 
             default: