diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index 2d54172..fd01aac 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -57,6 +57,16 @@
     MBEDTLS_SSL_CONTEXT_FLAG_HOSTNAME_SET = 1,
 } mbedtls_ssl_context_flags_t;
 
+/** Flags from ::mbedtls_ssl_context_flags_t to keep in
+ * mbedtls_ssl_session_reset().
+ *
+ * The flags that are in this list are kept until explicitly updated or
+ * until mbedtls_ssl_free(). The flags that are not listed here are
+ * reset to 0 in mbedtls_ssl_session_reset().
+ */
+#define MBEDTLS_SSL_CONTEXT_FLAGS_KEEP_AT_SESSION       \
+    (MBEDTLS_SSL_CONTEXT_FLAG_HOSTNAME_SET)
+
 #define MBEDTLS_SSL_INITIAL_HANDSHAKE           0
 #define MBEDTLS_SSL_RENEGOTIATION_IN_PROGRESS   1   /* In progress */
 #define MBEDTLS_SSL_RENEGOTIATION_DONE          2   /* Done or aborted */