Syndication

License statements may be included in various synication formats, generally at the feed and item level, where the latter overrides the former.

RSS 1.0

http://web.resource.org/rss/1.0/modules/cc/

RSS 2.0

http://backend.userland.com/creativeCommonsRssModule

Atom 1.0

http://ietfreport.isoc.org/idref/draft-snell-atompub-feed-license/

Human-visible notice

Some podcast players display the contents of a copyright element found in a feed. Example usage, not really intended to be machine-parsable, customize for communication with human readers, but do include URL of the relevant CC license and most importantly URL where users can get back to your site:

<copyright>All the songs in this podcast, and the podcast itself, are licensed to the public under http://creativecommons.org/licenses/by-nc-sa/1.0/ verify at http://magnatune.com/info/openmusic</copyright>

Licensed Content Discovery

Not all feeds include license metadata. In psuedocode, here are heuristics for discovering licensed content via feeds, perhaps useful in particular for aggregators.

function feedItemLicense(feedItem) {
    # license found for feed item URL (e.g., rel="license")?
    if (feedItem.license() != null) {
        return feedItem.license();
    }
    # license found at feed top level?
    if (feedItem.feed().license() != null) {
        return feedItem.feed().license();
    }
    # no explicit license in feed, fetch feed item URL
    feedItemContent = httpGet(feedItem.itemURL());
    if (parseLicenseFromHTML(feedItemContent) != null) {
        return parseLicenseFromHTML(feedItemContent);
    }
    # no license found in content at feed item URL, fetch
    # item referred to in enclosure if there is one,
    # look for embedded metadata
    if (feedItem.enclosure() != null) {
        enclosureContent = httpGet(feedItem.enclosure());
        return parseLicenseFromMedia(enclosureContent);
    }

This is obviously just a sketch. Aggregators may wish to use different heuristics for different feeds. Different aggregators will want to be more or less aggressive -- e.g., many will not want to go through the step of downloading and attempting to parse enclosure content. See Content Curators for some sites with feeds of CC licensed content.