Why not use a bloom filter?
A bloom filter would be much more complex to specify and implement (it’s not the sort of thing that can be done in a few lines of PHP). SUP is already very compact, especially when using standard HTTP gzip encoding, so the savings would not be significant in most cases. That said, it would be straightforward to create a bloom-filter variant of SUP where the SUP-IDs are keys into the bloom filter. This and other potential SUP extensions could be discovered via the standard SUP feeds, so these variants would not be incompatible with the existing SUP standard.
