diff --git a/lib/lwip/lwip/src/apps/tftp/tftp.c b/lib/lwip/lwip/src/apps/tftp/tftp.c
index 74fc1fbe586..56aeabc4d73 100644
--- a/lib/lwip/lwip/src/apps/tftp/tftp.c
+++ b/lib/lwip/lwip/src/apps/tftp/tftp.c
@@ -57,7 +57,7 @@
 #include "lwip/timeouts.h"
 #include "lwip/debug.h"
 
-#define TFTP_MAX_PAYLOAD_SIZE 512
+#define TFTP_DEFAULT_BLOCK_SIZE 512
 #define TFTP_HEADER_LENGTH    4
 
 #define TFTP_RRQ   1
@@ -65,6 +65,7 @@
 #define TFTP_DATA  3
 #define TFTP_ACK   4
 #define TFTP_ERROR 5
+#define TFTP_OACK  6
 
 enum tftp_error {
   TFTP_ERROR_FILE_NOT_FOUND    = 1,
@@ -88,9 +89,11 @@ struct tftp_state {
   int timer;
   int last_pkt;
   u16_t blknum;
+  u16_t blksize;
   u8_t retries;
   u8_t mode_write;
   u8_t tftp_mode;
+  bool wait_oack;
 };
 
 static struct tftp_state tftp_state;
@@ -137,10 +140,22 @@ send_request(const ip_addr_t *addr, u16_t port, u16_t opcode, const char* fname,
 {
   size_t fname_length = strlen(fname)+1;
   size_t mode_length = strlen(mode)+1;
-  struct pbuf* p = init_packet(opcode, 0, fname_length + mode_length - 2);
+  size_t blksize_length = 0;
+  int blksize = tftp_state.blksize;
+  struct pbuf* p;
   char* payload;
   err_t ret;
 
+  if (blksize) {
+    /* 'blksize\0'.\0" with . = 1 digit */
+    blksize_length = strlen("blksize") + 1 + 1 + 1;
+    while (blksize >= 10) {
+      blksize /= 10;
+      blksize_length++;
+    }
+  }
+
+  p = init_packet(opcode, 0, fname_length + mode_length + blksize_length - 2);
   if (p == NULL) {
     return ERR_MEM;
   }
@@ -148,7 +163,10 @@ send_request(const ip_addr_t *addr, u16_t port, u16_t opcode, const char* fname,
   payload = (char*) p->payload;
   MEMCPY(payload+2,              fname, fname_length);
   MEMCPY(payload+2+fname_length, mode,  mode_length);
+  if (tftp_state.blksize)
+    sprintf(payload+2+fname_length+mode_length, "blksize%c%d%c", 0, tftp_state.blksize, 0);
 
+  tftp_state.wait_oack = true;
   ret = udp_sendto(tftp_state.upcb, p, addr, port);
   pbuf_free(p);
   return ret;
@@ -221,14 +239,14 @@ send_data(const ip_addr_t *addr, u16_t port)
     pbuf_free(tftp_state.last_data);
   }
 
-  tftp_state.last_data = init_packet(TFTP_DATA, tftp_state.blknum, TFTP_MAX_PAYLOAD_SIZE);
+  tftp_state.last_data = init_packet(TFTP_DATA, tftp_state.blknum, TFTP_DEFAULT_BLOCK_SIZE);
   if (tftp_state.last_data == NULL) {
     return;
   }
 
   payload = (u16_t *) tftp_state.last_data->payload;
 
-  ret = tftp_state.ctx->read(tftp_state.handle, &payload[2], TFTP_MAX_PAYLOAD_SIZE);
+  ret = tftp_state.ctx->read(tftp_state.handle, &payload[2], TFTP_DEFAULT_BLOCK_SIZE);
   if (ret < 0) {
     send_error(addr, port, TFTP_ERROR_ACCESS_VIOLATION, "Error occurred while reading the file.");
     close_handle();
@@ -239,6 +257,28 @@ send_data(const ip_addr_t *addr, u16_t port)
   resend_data(addr, port);
 }
 
+static u16_t payload_size(void)
+{
+  if (tftp_state.blksize)
+    return tftp_state.blksize;
+  return TFTP_DEFAULT_BLOCK_SIZE;
+}
+
+static const char *
+find_option(struct pbuf *p, const char *option)
+{
+  int i;
+  u16_t optlen = strlen(option);
+  const char *b = p->payload;
+
+  for (i = 0; i + optlen + 1 < p->len; i++) {
+    if (lwip_strnstr(b + i, option, optlen))
+      return b + i + optlen + 2;
+  }
+
+  return NULL;
+}
+
 static void
 tftp_recv(void *arg, struct udp_pcb *upcb, struct pbuf *p, const ip_addr_t *addr, u16_t port)
 {
@@ -338,6 +378,15 @@ tftp_recv(void *arg, struct udp_pcb *upcb, struct pbuf *p, const ip_addr_t *addr
       }
 
       blknum = lwip_ntohs(sbuf[1]);
+      if (tftp_state.blksize && tftp_state.wait_oack) {
+        /*
+         * Data received while we are expecting an OACK for our blksize option.
+         * This means the server doesn't support it, let's switch back to the
+         * default block size.
+         */
+       tftp_state.blksize = 0;
+       tftp_state.wait_oack = false;
+      }
       if (blknum == tftp_state.blknum) {
         pbuf_remove_header(p, TFTP_HEADER_LENGTH);
 
@@ -349,7 +398,7 @@ tftp_recv(void *arg, struct udp_pcb *upcb, struct pbuf *p, const ip_addr_t *addr
           send_ack(addr, port, blknum);
         }
 
-        if (p->tot_len < TFTP_MAX_PAYLOAD_SIZE) {
+        if (p->tot_len < payload_size()) {
           close_handle();
         } else {
           tftp_state.blknum++;
@@ -386,7 +435,7 @@ tftp_recv(void *arg, struct udp_pcb *upcb, struct pbuf *p, const ip_addr_t *addr
       lastpkt = 0;
 
       if (tftp_state.last_data != NULL) {
-        lastpkt = tftp_state.last_data->tot_len != (TFTP_MAX_PAYLOAD_SIZE + TFTP_HEADER_LENGTH);
+        lastpkt = tftp_state.last_data->tot_len != (TFTP_DEFAULT_BLOCK_SIZE + TFTP_HEADER_LENGTH);
       }
 
       if (!lastpkt) {
@@ -405,6 +454,25 @@ tftp_recv(void *arg, struct udp_pcb *upcb, struct pbuf *p, const ip_addr_t *addr
         close_handle();
       }
       break;
+    case PP_HTONS(TFTP_OACK): {
+      const char *optval = find_option(p, "blksize");
+      u16_t srv_blksize = 0;
+      tftp_state.wait_oack = false;
+      if (optval) {
+	if (!tftp_state.blksize) {
+	  /* We did not request this option */
+          send_error(addr, port, TFTP_ERROR_ILLEGAL_OPERATION, "blksize unexpected");
+	}
+	srv_blksize = atoi(optval);
+	if (srv_blksize <= 0 || srv_blksize > tftp_state.blksize) {
+	  send_error(addr, port, TFTP_ERROR_ILLEGAL_OPERATION, "Invalid blksize");
+	}
+	LWIP_DEBUGF(TFTP_DEBUG | LWIP_DBG_STATE, ("tftp: accepting blksize=%d\n", srv_blksize));
+	tftp_state.blksize = srv_blksize;
+      }
+      send_ack(addr, port, 0);
+      break;
+    }
     default:
       send_error(addr, port, TFTP_ERROR_ILLEGAL_OPERATION, "Unknown operation");
       break;
@@ -495,6 +563,18 @@ tftp_init_client(const struct tftp_context *ctx)
   return tftp_init_common(LWIP_TFTP_MODE_CLIENT, ctx);
 }
 
+/** @ingroup tftp
+ * Set the block size to be used by the TFTP client. The server may choose to
+ * accept a lower value.
+ * @param blksize Block size in bytes
+ */
+void
+tftp_client_set_blksize(u16_t blksize)
+{
+  if (blksize != TFTP_DEFAULT_BLOCK_SIZE)
+    tftp_state.blksize = blksize;
+}
+
 /** @ingroup tftp
  * Deinitialize ("turn off") TFTP client/server.
  */
diff --git a/lib/lwip/lwip/src/include/lwip/apps/tftp_client.h b/lib/lwip/lwip/src/include/lwip/apps/tftp_client.h
index 24dbda6a8c9..e1e21d06b67 100644
--- a/lib/lwip/lwip/src/include/lwip/apps/tftp_client.h
+++ b/lib/lwip/lwip/src/include/lwip/apps/tftp_client.h
@@ -44,6 +44,7 @@ enum tftp_transfer_mode {
 };
 
 err_t tftp_init_client(const struct tftp_context* ctx);
+void tftp_client_set_blksize(u16_t blksize);
 err_t tftp_get(void* handle, const ip_addr_t *addr, u16_t port, const char* fname, enum tftp_transfer_mode mode);
 err_t tftp_put(void* handle, const ip_addr_t *addr, u16_t port, const char* fname, enum tftp_transfer_mode mode);
 
