\documentclass[border=3mm]{standalone}
\usepackage[english]{babel}
\usepackage{tikz-uml}

\begin{document}

\begin{tikzpicture}
 \tikzumlset{fill object = white, fill call = gray!20} 
\begin{umlseqdiag}
\umlobject[class=B]{b}
\umlobject[class=C]{c}
\umlobject[class=D]{d}


\begin{umlfragment}[type=loop]
\begin{umlcallself}[op=run]{b} 

\begin{umlcall}[op=send, type=asynchron]{b}{c}
    \begin{umlfragment}[type=alt, label=condition, inner xsep=8]
        \begin{umlcall}[op=send, type=synchron, return=send update]{c}{d}
            \begin{umlcallself}[padding=1.5, op=run]{d} 
            \end{umlcallself}
        \end{umlcall}
        \begin{umlcallself}[op=assess update]{c} 
            \begin{umlcall}[op=send update, type=synchron, return=acknowledge]{c}{b}
            \end{umlcall}
            \begin{umlcall}[op=allow continue, padding=-2.5, type=asynchron]{c}{b}
            \end{umlcall}
        \end{umlcallself}
        \umlfpart[default]
        \begin{umlcall}[op=allow continue, type=asynchron]{c}{b}
        \end{umlcall}
    \end{umlfragment}
\end{umlcall}

\end{umlcallself}

\end{umlfragment}
\umlsdnode[dt=1.5]{b}
\umlsdnode[dt=5]{c}
\umlsdnode[dt=32]{d}
\end{umlseqdiag}
\end{tikzpicture}
\end{document}