Subscriber ids: from reading the cookbook, I assumed the subscriber id was generated by the syndicator, and that the same subscriber would have different ids for each syndicator.
GMT: The cookbook doesn't mention that all time stamps should be GMT. I had to go to the spec.
ice-payload location attribute used in cookbook but not in spec. Many places; in cookbook; for example p. 9, Section 2.2.2, Syndication Implementation Guidelines, item 4.