if( $packet !~ m/sip/ && $packet !~ m/sdp/ ) {
     print "packet is RTP\n";
}
elsif( $packet =~ m/sip/ ) {
    if( $packet =~ m/sdp/ ) {
         print "packet is sdp\n";
    }
    else {
         print "packet is sip!\n";
    }
}