Z:gnu-www-ja-gpl-faq--90fcc5-Second, section 6(e) of GPLv3/en

Second, section 6(e) of GPLv3 is designed to give distributors&mdash;people who initially seed torrents&mdash;a clear and straightforward way to provide the source, by telling recipients where it is available on a public network server. This ensures that everyone who wants to get the source can do so, and it's almost no hassle for the distributor.