diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index 2e50f2ddfa..93a9c8df2b 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -411,6 +411,13 @@ in the ancillary data:
 * ``VHOST_USER_SET_INFLIGHT_FD`` (if ``VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD``)
 * ``VHOST_USER_SET_DEVICE_STATE_FD``
 
+When sending file descriptors in ancilliary data, *front-end* should
+associate the ancilliary data with a ``sendmsg`` operation (or
+equivalent) that sends bytes starting with the first byte of the
+message header.  *back-end* can therefore expect that file descriptors
+will only be received in the first ``recvmsg`` operation for a message
+header.
+
 If *front-end* is unable to send the full message or receives a wrong
 reply it will close the connection. An optional reconnection mechanism
 can be implemented.