I created a table, where I wanted a double horizontal line, so I used \hhline
\begin{table}[ht]
\begin{tabular}{|l|l|}
\hline
\textbf{Message} & \multicolumn{1}{c|}{\textbf{Description}} \\ \hline
ACK & Confirms an INVITE request \\ \hline
BYE & Ends a session \\ \hline
CANCEL & Cancels establishing of a session \\ \hline
INVITE & Establishes a session \\ \hline
OPTIONS & Cpabilities of the calling and receiving SIP phones \\ \hline
REGISTER & Registers the user and updates the location table \\ \hhline{|=|=|}
NOTIFY & Sends information after subscribing \\ \hline
PUBLISH & Uploads the status information to the server \\ \hline
SUBSCRIBE & Establishes a session to receive future updates \\ \hline
\end{tabular}
\caption{\label{tab:tablename}SIP methods.}
\end{table}
what caused vertical lines on top of my table to disconnect from very top horizontal line:
is there any way to fix this, please? Thank you in advance for your help.

\hlineis probably\topruleor\midrule(I guessing). – Zarko Nov 18 '20 at 03:20\hhlineas mentioned in the title. You do not need\hhlineat all here your double rule could more simply be coded as\hline\hline. Please edit the question to make a complete small document that generates the image shown. – David Carlisle Nov 18 '20 at 07:46