diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 2845da8..1f5a75f 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2182,6 +2182,54 @@
 }
 #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
 
+#if defined(MBEDTLS_SSL_TRUNCATED_HMAC)
+static void ssl_write_cid_ext( mbedtls_ssl_context *ssl,
+                               unsigned char *buf,
+                               size_t *olen )
+{
+    unsigned char *p = buf;
+    size_t ext_len;
+    const unsigned char *end = ssl->out_msg + MBEDTLS_SSL_OUT_CONTENT_LEN;
+
+    *olen = 0;
+
+    /* Skip writing the extension if we don't want to use it or if
+     * the client hasn't offered it. */
+    if( ssl->handshake->cid_in_use == MBEDTLS_SSL_CID_DISABLED )
+        return;
+
+    /* ssl->own_cid_len is at most MBEDTLS_SSL_CID_IN_LEN_MAX
+     * which is at most 255, so the increment cannot overflow. */
+    if( end < p || (size_t)( end - p ) < (unsigned)( ssl->own_cid_len + 5 ) )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "buffer too small" ) );
+        return;
+    }
+
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, adding CID extension" ) );
+
+    /*
+     * Quoting
+     * https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04:
+     *
+     *   struct {
+     *      opaque cid<0..2^8-1>;
+     *   } ConnectionId;
+    */
+
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_CID >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( MBEDTLS_TLS_EXT_CID      ) & 0xFF );
+    ext_len = (size_t) ssl->own_cid_len + 1;
+    *p++ = (unsigned char)( ( ext_len >> 8 ) & 0xFF );
+    *p++ = (unsigned char)( ( ext_len      ) & 0xFF );
+
+    *p++ = (uint8_t) ssl->own_cid_len;
+    memcpy( p, ssl->own_cid, ssl->own_cid_len );
+
+    *olen = ssl->own_cid_len + 5;
+}
+#endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
 static void ssl_write_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
                                             unsigned char *buf,
@@ -2703,6 +2751,11 @@
     ext_len += olen;
 #endif
 
+#if defined(MBEDTLS_SSL_CID)
+    ssl_write_cid_ext( ssl, p + 2 + ext_len, &olen );
+    ext_len += olen;
+#endif
+
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
     ssl_write_encrypt_then_mac_ext( ssl, p + 2 + ext_len, &olen );
     ext_len += olen;