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diff --git a/doc/sphinx/uml/requestLease4.uml b/doc/sphinx/uml/requestLease4.uml
new file mode 100644
index 0000000..4cb55b5
--- /dev/null
+++ b/doc/sphinx/uml/requestLease4.uml
@@ -0,0 +1,150 @@
+@startuml
+
+title requestLease4 algorithm (Kea 1.8.0)
+
+start
+:get lease for the client;
+
+if (reserved address) then (yes)
+  if (requested address) then (no)
+    :requested address = reserved address;
+  else (yes)
+    if (requested address is reserved for another client) then (yes)
+      :return no lease;
+      stop
+    else (no)
+    endif
+  endif
+  if (lease for requested address) then (yes)
+    if (active and owned by another client) then (yes)
+      :return no lease;
+      stop
+    else (no)
+    endif
+  else (no)
+  endif
+  if (requested address == reserved address) then (no)
+    if (lease for requested address) then (yes)
+      if (active) then (yes)
+        :return no lease;
+        stop
+      else (no)
+      endif
+    else (no)
+    endif
+    if (requested address in allowed pool) then (no)
+      :return no lease;
+      stop
+    else (yes)
+    endif
+  else (yes)
+  endif
+
+else (no)
+
+  if (requested address) then (yes)
+    if (requested address is reserved for another client) then (yes)
+      :return no lease;
+      stop
+    else (no)
+    endif
+    if (lease for requested address) then (yes)
+      if (active and owned by another client) then (yes)
+        :return no lease;
+        stop
+      else (no)
+      endif
+    else (no)
+    endif
+    if (requested address in allowed pool) then (no)
+      :return no lease;
+      stop
+    else (yes)
+    endif
+  else (no)
+    if (client lease and lease address in allowed pool) then (no)
+      while (iterate over pools and subnets)
+        :pick candidate address;
+        if (candidate is reserved for another client) then (no)
+          if (candidate is used by another thread) then (no)
+            if (lease for candidate) then (no)
+              :create and return new lease;
+              stop
+            else (yes)
+              if (expired) then (yes)
+                :reclaim expired lease;
+                :update lease information;
+                :callout lease4_select;
+                if (callout return) then (SKIP)
+                  :return no lease;
+                  stop
+                else (CONTINUE)
+                  :update lease;
+                  :return reused lease;
+                  stop
+                endif
+              else (no)
+              endif
+            endif
+          else (yes)
+          endif
+        else (yes)
+        endif
+      endwhile
+      :maximum attempts;
+      :return no lease;
+      stop
+    else (yes)
+    endif
+  endif
+endif
+
+' after check
+if (client lease) then (yes)
+  if (no requested address or requested address == client lease address) then (yes)
+    if (has reserved address or client lease address in allowed pool) then (yes)
+      :update lease information;
+      if (old lease expired) then (yes)
+        :reclaim expired lease;
+      else (no)
+      endif
+      :callout lease4_renew;
+      if (callout return) then (SKIP)
+        :return old client lease;
+        stop
+      else (CONTINUE)
+        :update lease;
+        :return renewed client lease;
+        stop
+      endif
+    else (no)
+    endif
+  else (no)
+  endif
+else (no)
+endif
+
+:get lease for requested address;
+if (requested lease) then (yes)
+  if (expired) then (no)
+    :return no lease;
+    stop
+  else (yes)
+    :reclaim expired lease;
+    :update lease information;
+    :callout lease4_select;
+    if (callout return) then (SKIP)
+      :return no lease;
+      stop
+    else (CONTINUE)
+      :update lease;
+      :return reused lease;
+      stop
+    endif
+  endif
+else (no)
+ :create and return new lease;
+ stop
+endif
+
+@enduml