// true if this contains an ack for given (remote) seq
bool Acks(std::uint16_t) const noexcept;
std::uint16_t AckBegin() const noexcept { return ack; }
// true if this contains an ack for given (remote) seq
bool Acks(std::uint16_t) const noexcept;
std::uint16_t AckBegin() const noexcept { return ack; }