\documentclass[ border={60pt 0pt 55pt 0pt} 
% left bottom right top] real order is left bottom right top
]{standalone}
\usepackage{float}
\usepackage{tikz}
\usetikzlibrary{positioning, fit, calc, shapes, arrows}
\usepackage[underline=false]{pgf-umlsd}
% message between threads
% Example:
% \bloodymess[delay]{sender}{message content}{receiver}{DIR}{start note}{end note}
\newcommand{\bloodymess}[7][0]{
  \stepcounter{seqlevel}
  \path
  (#2)+(0,-\theseqlevel*\unitfactor-0.7*\unitfactor) node (mess from) {};
  \addtocounter{seqlevel}{#1}
  \path
  (#4)+(0,-\theseqlevel*\unitfactor-0.7*\unitfactor) node (mess to) {};
  \draw[->,>=angle 60] (mess from) -- (mess to) node[midway, above]
  {#3};

  \if R#5
    \node (#3 from) at (mess from) {\llap{#6~}};
    \node (#3 to) at (mess to) {\rlap{~#7}};
  \else\if L#5
         \node (#3 from) at (mess from) {\rlap{~#6}};
         \node (#3 to) at (mess to) {\llap{#7~}};
       \else
         \node (#3 from) at (mess from) {#6};
         \node (#3 to) at (mess to) {#7};
       \fi
  \fi
}
\begin{document}

%\begin{figure}[H]
%    \centering
    \begin{sequencediagram}
        \newinst{c}{Client}
        \newinst[6]{s}{Server}

        \bloodymess[1]{c}{Requesting Access to Server}{s}{R}{Start}{Server receives}
        \bloodymess[1]{s}{Access Granted}{c}{L}{Server responds}{}
        \bloodymess[1]{c}{Acknowledgement}{s}{R}{2nd handshake begins}{Server receives}
        \bloodymess[1]{s}{Send Data and Acknowledge}{c}{L}{}{End}
    \end{sequencediagram}
    %\caption{Client-Server messaging example}
%\end{figure}
\end{document}