diff --git a/src/core/tcp.c b/src/core/tcp.c
index 8049c263..7f15c04c 100644
--- a/src/core/tcp.c
+++ b/src/core/tcp.c
@@ -85,7 +85,7 @@
 #define INITIAL_MSS TCP_MSS
 #endif
 
-const char * const tcp_state_str[] = {
+static const char * const tcp_state_str[] = {
   "CLOSED",
   "LISTEN",
   "SYN_SENT",
@@ -104,10 +104,10 @@ static u16_t tcp_port = TCP_LOCAL_PORT_RANGE_START;
 
 /* Incremented every coarse grained timer shot (typically every 500 ms). */
 u32_t tcp_ticks;
-const u8_t tcp_backoff[13] =
+static const u8_t tcp_backoff[13] =
     { 1, 2, 3, 4, 5, 6, 7, 7, 7, 7, 7, 7, 7};
  /* Times per slowtmr hits */
-const u8_t tcp_persist_backoff[7] = { 3, 6, 12, 24, 48, 96, 120 };
+static const u8_t tcp_persist_backoff[7] = { 3, 6, 12, 24, 48, 96, 120 };
 
 /* The TCP PCB lists. */