Place chicken broth in a medium sauce along with lime juice, lime peel, fish sauce, soy sauce, lemongrass and dried chili pepper. Bring to a boil then reduce to low. Add tomatoes and mushrooms and simmeer for 5 minutes.
Stir in shrimp and return to a boil then simmer until shrimp are bright pink and cooked through, about 4 minutes. Divide between four bowls and serve with cilantro if you wish.