Pour rice and water into medium-sized saucepan. Turn heat to high and bring to boil. Reduce heat to low, and simmer until rice is very tender and puffed up, about 25 minutes. Meanwhile, thinly slice ginger into matchsticks.
Add chicken stock, fish sauce, salt, and sugar to saucepan. Simmer over low heat for 10 minutes. Add half of ginger, and simmer for further 10 minutes.
Ladle soup into bowl and garnish with chicken, sliced scallion, chopped cilantro, and remaining ginger. Season with salt to taste.