Determining update availability via set intersection over a sub-optimal pathway

A low bandwidth link can be used optimally for software updates, by successively transferring more information about the updates only as the likelihood of an applicable update successively increases. A many-to-one mapping function (e.g. a hash function) is applied to update identifiers on a server to generate a table of single bit entries corresponding to the updates. At a client, the same mapping function is applied to program identifiers to determine whether the server has a potential update. If a potential update is noted, a second transmission is requested for conveying additional data from the server by which hash collisions can be identified. A third transmission from the server is received conveying the actual update only after the availability of an actual update (versus a hash collision) is confirmed. The same arrangement can be employed in reverse.