What are the 6 dots in template parameter packs ?

While looking at this question I found myself in the cpp reference site where I noticed a strange and new to me syntax :

template<class Ret, class... Args>
struct is_function<Ret(Args......)volatile &&> : std::true_type {};

Yep, 6 dots ! Initially I thought this was a typo, but after checking the libstdc++ source again there it was eg at line 444 :

    template<typename _Res, typename... _ArgTypes>
    struct is_function<_Res(_ArgTypes......) volatile &&> : public true_type { };

Is this a valid syntax ? Dot dot dot, are used to pack and unpack parameter packs ? What do 6 dots do ?