Merge pull request #8235 from zhuoshuguo/gnrc_gomach_beacon_length_limit

gnrc_gomach: add sender ID number limit in beacon packet.

sys/include/net/gnrc/gomach/gomach.h

@@ -276,6 +276,19 @@ extern "C" {
 #define GNRC_GOMACH_MAX_ALLOC_SLOTS_NUM           (25U)
 #endif
 
+/**
+ * @brief Maximum number of senders allowed to be allocated slots in one cycle.
+ *
+ * Exclude the static GoMacH MAC header payload in the beacon, which is 20 bytes,
+ * we have 107 bytes left for constructing the sender-ID list and the related slots-number
+ * list. A combined slots allocation information pair (sender ID with its corresponded
+ * allocate slots number) will cost 9 (8+1) bytes, thus we can hold a maximum of 11
+ * i.e., ((127 - 20) / 9), sender IDs in the beacon.
+ */
+#ifndef GNRC_GOMACH_MAX_ALLOC_SENDER_NUM
+#define GNRC_GOMACH_MAX_ALLOC_SENDER_NUM          (11U)
+#endif
+
 /**
  * @brief Maximum t2k attempts before going to t2u in GoMacH.
  *

sys/net/gnrc/link_layer/gomach/gomach_internal.c

@@ -488,7 +488,13 @@ int gnrc_gomach_send_beacon(gnrc_netif_t *netif)
                 total_tdma_slot_num -= redueced_slots_num;
                 break;
             }
+
             j++;
+
+            /* If reach the maximum sender ID number limit, stop. */
+            if (total_tdma_node_num >= GNRC_GOMACH_MAX_ALLOC_SENDER_NUM) {
+                break;
+            }
         }
     }