msc {
  width="400";
  "Client",  "Server";
  "Server" -> "Client" [ label = "Connect" ];  
  "Server" <-> "Client" [ label = "TLS Upgrade" ];  
  "Client" <- "Server" [ label = "authorize(...)" ];
  "Client" -> "Server" [ label = "authorize(...)" ];
  "Client" abox "Server" [ label = "Race Condition" ] ;
}