Bring all ingredients to a boil in an 8- to 10-quart heavy pot. Skim froth. Reduce heat and gently simmer, uncovered, skimming froth occasionally, 3 hours. Pour stock through a fine-mesh sieve into a large bowl and discard solids. If using stock right away, skim off and discard any fat. If not, cool stock completely, uncovered, before skimming fat, then chill, covered.