Extend PCM Ad Identifier (Source ID) field from an 8-bit value to 20-bits

[ranton256]

The 8-bit ad identifier (Source ID) field limits the number of creatives that can be identified on a publisher domain to 256. An 8-bit ad identifier limits common advertising technology use cases, including limited creative variation and testing to provide an improved customer advertising experience. Typically, an ad network server has millions of possible creatives on each domain. We propose to increase the 8-bit field to 20-bit.

[johnwilander]

PCM is designed to not allow cross-site tracking of individual users at scale. Allowing for over a million unique IDs on either side would enable cross-site tracking.

The existing 8-bit identifier was originally 6 bits. Discussion about what’s useful led to the change to 8 bits while lowering to 4 bits on the destination side to reduce cross-site tracking risk.

Note that PCM is not designed to be a drop-in replacement for cookie or navigational tracking. If we would accept cross-sure tracking we wouldn’t need PCM and could just stick with third-party cookies.

If anything, the 8-bit source ID would be reduced. Adding any extra bits is out of the question for the existing, on-device processing design.