diff --git a/include/openssl/bio.h b/include/openssl/bio.h
index a2f0d83..75afada 100644
--- a/include/openssl/bio.h
+++ b/include/openssl/bio.h
@@ -435,12 +435,18 @@
 
 /* BIO_set_fd sets the file descriptor of |bio| to |fd|. If |close_flag| is
  * non-zero then |fd| will be closed when |bio| is. It returns one on success
- * or zero on error. */
+ * or zero on error.
+ *
+ * This function may also be used with socket BIOs (see |BIO_s_socket| and
+ * |BIO_new_socket|). */
 OPENSSL_EXPORT int BIO_set_fd(BIO *bio, int fd, int close_flag);
 
 /* BIO_get_fd returns the file descriptor currently in use by |bio| or -1 if
  * |bio| does not wrap a file descriptor. If there is a file descriptor and
- * |out_fd| is not NULL, it also sets |*out_fd| to the file descriptor. */
+ * |out_fd| is not NULL, it also sets |*out_fd| to the file descriptor.
+ *
+ * This function may also be used with socket BIOs (see |BIO_s_socket| and
+ * |BIO_new_socket|). */
 OPENSSL_EXPORT int BIO_get_fd(BIO *bio, int *out_fd);
 
 
@@ -520,7 +526,17 @@
 OPENSSL_EXPORT int BIO_set_write_buffer_size(BIO *bio, int buffer_size);
 
 
-/* Socket BIOs. */
+/* Socket BIOs.
+ *
+ * Socket BIOs behave like file descriptor BIOs but wrap the system's |recv|
+ * and |send| functions. This is relevant for Windows systems where file
+ * descriptors, provided by C runtime for use with |read| and |write|, are not
+ * interchangeable with sockets.
+ *
+ * Socket BIOs may be used with |BIO_set_fd| and |BIO_get_fd|.
+ *
+ * TODO(davidben): Add separate APIs and fix the internals to use |SOCKET|s
+ * around rather than rely on int casts. */
 
 OPENSSL_EXPORT const BIO_METHOD *BIO_s_socket(void);
 
diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index be5776c..b34fa56 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -251,25 +251,39 @@
 
 /* SSL_get_rfd returns the file descriptor that |ssl| is configured to read
  * from. If |ssl|'s read |BIO| is not configured or doesn't wrap a file
- * descriptor then it returns -1. */
+ * descriptor then it returns -1.
+ *
+ * Note: On Windows, this may return either a file descriptor or a socket (cast
+ * to int), depending on whether |ssl| was configured with a file descriptor or
+ * socket |BIO|. */
 OPENSSL_EXPORT int SSL_get_rfd(const SSL *ssl);
 
 /* SSL_get_wfd returns the file descriptor that |ssl| is configured to write
  * to. If |ssl|'s write |BIO| is not configured or doesn't wrap a file
- * descriptor then it returns -1. */
+ * descriptor then it returns -1.
+ *
+ * Note: On Windows, this may return either a file descriptor or a socket (cast
+ * to int), depending on whether |ssl| was configured with a file descriptor or
+ * socket |BIO|. */
 OPENSSL_EXPORT int SSL_get_wfd(const SSL *ssl);
 
 /* SSL_set_fd configures |ssl| to read from and write to |fd|. It returns one
  * on success and zero on allocation error. The caller retains ownership of
- * |fd|. */
+ * |fd|.
+ *
+ * On Windows, |fd| is cast to a |SOCKET| and used with Winsock APIs. */
 OPENSSL_EXPORT int SSL_set_fd(SSL *ssl, int fd);
 
 /* SSL_set_rfd configures |ssl| to read from |fd|. It returns one on success and
- * zero on allocation error. The caller retains ownership of |fd|. */
+ * zero on allocation error. The caller retains ownership of |fd|.
+ *
+ * On Windows, |fd| is cast to a |SOCKET| and used with Winsock APIs. */
 OPENSSL_EXPORT int SSL_set_rfd(SSL *ssl, int fd);
 
 /* SSL_set_wfd configures |ssl| to write to |fd|. It returns one on success and
- * zero on allocation error. The caller retains ownership of |fd|. */
+ * zero on allocation error. The caller retains ownership of |fd|.
+ *
+ * On Windows, |fd| is cast to a |SOCKET| and used with Winsock APIs. */
 OPENSSL_EXPORT int SSL_set_wfd(SSL *ssl, int fd);
 
 /* SSL_do_handshake continues the current handshake. If there is none or the
diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index c4cc3d7..c1e6a00 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -999,7 +999,7 @@
 }
 
 int SSL_set_fd(SSL *ssl, int fd) {
-  BIO *bio = BIO_new(BIO_s_fd());
+  BIO *bio = BIO_new(BIO_s_socket());
   if (bio == NULL) {
     OPENSSL_PUT_ERROR(SSL, ERR_R_BUF_LIB);
     return 0;
@@ -1011,9 +1011,9 @@
 
 int SSL_set_wfd(SSL *ssl, int fd) {
   if (ssl->rbio == NULL ||
-      BIO_method_type(ssl->rbio) != BIO_TYPE_FD ||
+      BIO_method_type(ssl->rbio) != BIO_TYPE_SOCKET ||
       BIO_get_fd(ssl->rbio, NULL) != fd) {
-    BIO *bio = BIO_new(BIO_s_fd());
+    BIO *bio = BIO_new(BIO_s_socket());
     if (bio == NULL) {
       OPENSSL_PUT_ERROR(SSL, ERR_R_BUF_LIB);
       return 0;
@@ -1028,9 +1028,9 @@
 }
 
 int SSL_set_rfd(SSL *ssl, int fd) {
-  if (ssl->wbio == NULL || BIO_method_type(ssl->wbio) != BIO_TYPE_FD ||
+  if (ssl->wbio == NULL || BIO_method_type(ssl->wbio) != BIO_TYPE_SOCKET ||
       BIO_get_fd(ssl->wbio, NULL) != fd) {
-    BIO *bio = BIO_new(BIO_s_fd());
+    BIO *bio = BIO_new(BIO_s_socket());
     if (bio == NULL) {
       OPENSSL_PUT_ERROR(SSL, ERR_R_BUF_LIB);
       return 0;