msc {
  hscale = 2;
  UE,
  sswan [label="strongSwan"],
  ePDG [label="osmo-epdg/erlang"],
  PGW,
  HSS;

  UE <=> sswan [label="IKE: SA_INIT"];
  UE => sswan [label="IKE: AUTH_REQ (IDi=NAI, IDr=APN, CFG Req)"];
  sswan => ePDG [label="GSUP: Send Auth Information Req (NAI, APN)"];
  ePDG => HSS [label="SWx MAR (Multimedia Authentication Request)"];
  ePDG <= HSS [label="SWx MAA (Multimedia Authentication Answer)"];
  sswan <= ePDG [label="GSUP: Send Auth Information Res (Quintlet)"];
  UE <= sswan [label="IKE: AUTH_RESP (EAP-Request/AKA-Challenge)"];
  UE => sswan [label="IKE: AUTH_REQ (EAP-Request/AKA-Response)"];
  sswan => ePDG [label="GSUP: Location Update Req"];
  ePDG => HSS [label="SWx SAR (Server Assignment Request)"];
  ePDG <= HSS [label="SWx SAA (Server Assignment Answer)"];
  sswan <= ePDG [label="GSUP: Location Update Res"];
  UE <= sswan [label="IKE: AUTH_RESP (EAP Success)"];
  UE => sswan [label="IKE: AUTH_REQ"];
    sswan => ePDG [label="GSUP: Tunnel Req"];
    ePDG => PGW [label="S2b: Create Session Request"];
      ePDG => HSS [label="SWx SAR (Server Assignment Request)"];
      ePDG <= HSS [label="SWx SAA (Server Assignment Answer)"];
    ePDG <= PGW [label="S2b: Create Session Answer"];
    sswan <= ePDG [label="GSUP: Tunnel Res"];
  UE <= sswan [label="IKE: AUTH_RESP (CFG Reply)"];
  UE <=> sswan [label="IPSEC SA"];
}