msc {
  hscale="1";
  upf[label="User Plane function"],cpf[label="Control Plane function"];

  cpf rbox cpf [label="Look up UPF,\npick any one of the available\nIP addrs for the UPF"];

  ...;
  upf abox cpf [label="not yet associated"];
  upf rbox cpf [label="reject any session related msgs"];
  ...;

  upf <<= cpf [label="PFCP Association Setup Request\nCP function Node Id, features"];
  upf =>> cpf [label="PFCP Association Setup Response\nUP function Node Id, features"];
  upf abox cpf [label="associated"];
  upf rbox cpf [label="start Heartbeat checking"];
  ...;
  upf <<= cpf [label="Heartbeat Request"];
  upf =>> cpf [label="Heartbeat Response"];
  ...;
  upf =>> cpf [label="Heartbeat Request"];
  upf <<= cpf [label="Heartbeat Response"];
  ...;

  upf <<= cpf [label="Session Establishment Request\nCP Node-Id\nCP F-SEID\n1+ Packet Detection Rule(s)\n1+ Forward Action Rule(s)"];
  upf =>> cpf [label="Session Establishment Response"];

  upf <<= cpf [label="Session Modification Request"];
  upf =>> cpf [label="Session Modification Response"];

  upf <<= cpf [label="Session Deletion Request"];
  upf =>> cpf [label="Session Deletion Response"];

  ...;
  upf rbox cpf [label="F-SEID: accept any other IP addrs than peer's Node Id"];
  ...;

  --- [label="Graceful release initiated by CP"];

  upf <<= cpf [label="Association Update Request\nwith PFCP Association Release Preparation Start = 1"];
  upf =>> cpf [label="Association Update Response"];
  upf =>> cpf [label="Session Report Request\nto report non-zero usage reports,\nat least one message per PFCP Session"];
  upf <<= cpf [label="Association Release Request"];
  upf =>> cpf [label="Association Release Response"];

  --- [label="Graceful release initiated by UP"];

  upf =>> cpf [label="Association Update Request\nwith PFCP Association Release Preparation = 1"];
  cpf abox cpf [label="refrain from establishing sessions"];
  upf <<= cpf [label="Association Update Response"];
  upf <<= cpf [label="Session Deletion Request(s)\nto collect usage reports\nper session"];
  upf =>> cpf [label="Session Deletion Response(s)"];
  cpf rbox cpf [label="wait Graceful Release Period"];
  upf <<= cpf [label="Association Release Request"];
  upf =>> cpf [label="Association Release Response"];

  --- [label="Release (immediate)"];

  upf <<= cpf [label="Association Release Request"];
  upf =>> cpf [label="Association Release Response"];
}