code: func parseHeader(b *bytes.Reader, shortHeaderConnIDLen int) (*Header, error) {
  	startLen := b.Len()
  	h, err := parseHeaderImpl(b, shortHeaderConnIDLen)
  	if err != nil {
  		return h, err
  	}
  	h.parsedLen = protocol.ByteCount(startLen - b.Len())
  	return h, err
  }
docs: // ParseHeader parses the header.
  // For short header packets: up to the packet number.
  // For long header packets:
  // * if we understand the version: up to the packet number
  // * if not, only the invariant part of the header