Google and The Trade Desk are at odds over the future of internet advertising with two competing visions of what comes after cookies: Google’s Privacy Sandbox or The Trade Desk’s universal ID. Indeed, sides are being drawn among ad tech companies betting on Google and those that are casting their claims with The Trade Desk, as both companies become more forceful in promoting their post-cookie initiatives.

Just this week, The Trade Desk CEO Jeff Green wrote a new critique of Google’s Privacy Sandbox. “Privacy Sandbox is not innovative,” Green said in his post. “It’s not good for the open internet. And I don’t even see how it’s good for Google. All those brilliant minds inside of a nearly $2 trillion company couldn’t do better than this?”

Meanwhile, a Google spokesperson responded to Green, saying Privacy Sandbox is better than “cross-site identifiers that still enable users to be tracked across sites.”

Also read: A guide to Google’s post-cookie ad tech

The Trade Desk has been vocal in its resistance to Privacy Sandbox, which is Google’s proposal to kill third-party cookies, the data lubricant that’s used to fuel advertising on the open web. The Trade Desk has been proselytizing Unified ID 2.0, an advertising identity based on a consumer’s email address, replacing the cookie. The UID 2.0, as it’s called, is most useful for publishers and connected TV apps that collect email addresses with the consent of subscribers. The Trade Desk still plans to test Privacy Sandbox, though.

“We’ll test them all. Of course, we’re considering the product carefully,” Green wrote this week. “We’ve spent a lot of time with the specs—or better said, the ever-changing specs. But I honestly leave the examination of their many docs and APIs wondering if we’re witnessing the end of an era for Google.”

Google has mostly stayed silent about The Trade Desk, but it did respond to Green’s recent statements.

“We're proud of the ongoing, collaborative work we’ve done with the industry, and are happy to hear The Trade Desk is evaluating the APIs,” a Google spokesperson said in an email to Ad Age. “We hope as they familiarize themselves more with the Privacy Sandbox building blocks, they’ll gain a better understanding of how it's possible to achieve advertiser and publisher goals without relying on new cross-site identifiers that still enable users to be tracked across sites.”

Google’s Chrome browser team is building Privacy Sandbox to control how ads get served after it phases out cookies, which it wants to do by the end of the year. Privacy Sandbox is certainly complex—a web of APIs that demand- and supply-side platforms plug into to prevent consumers’ personal information from leaking into the ad ecosystem. To do this, Google is making ad tech companies re-engineer their entire systems, at a high cost, if they want to participate in ad auctions in Privacy Sandbox. Earlier this month, to facilitate testing the new pipes, Google shut off cookies on 1% of traffic in Chrome.

Read more: How ad tech players are preparing for post-cookie world

Google’s race to eliminate cookies is contentious. There are a number of ad tech companies, including Index Exchange, Criteo, OpenX, RTB House, PubMatic, Microsoft and others, that have publicly stated they are trying out Privacy Sandbox APIs. At the same time, some companies are edging into The Trade Desk’s camp, such as Lotame, a data management and identity platform, that said this week it would adopt UID 2.0. Lotame is decidedly not testing Privacy Sandbox at the moment. In recent years, Lotame developed an alternative ID called Panorama, which it has tested on cookieless traffic on Apple’s Safari browser. Apple started phasing out third-party cookies in the name of privacy in 2017. Lotame will now also use UID 2.0 to try to expand into more connected TV ad inventory, where The Trade Desk has a growing footprint, and all traffic is cookieless, according to Eli Heath, head of identity at Lotame.

“There’s not really going to be a winner takes all in the ID space,” Heath said, “and to preserve Lotame audience activation for our buyer clients, we really wanted to take kind of a multi-pronged approach, and I’ll say hedge our bets.”

Also read: Unilever, Mindshare test Disney’s cookie alternative with The Trade Desk

Google’s Chrome browser is such a big deal for any ad tech participant because it represents more than 60% of browsers. And the bulk of internet activity comes from users who don’t log into every website, so they can’t be targeted with an ID based on their email address. Lotame is working on a hybrid approach to maintain its Panorama ID, which can advertise to non-authenticated traffic, while also working with The Trade Desk.

“We’re recognizing that buyers and publishers really embrace all the different solutions and certainly test them,” Heath said, “and then we’ll see where we all land, but it’s certainly going to be a portfolio approach.”

PubMatic, the supply-side platform that works with publishers to monetize their audiences, is working on the “portfolio” approach. PubMatic is integrated with nearly 30 IDs, including LiveRamp’s RampID and UID 2.0. Meanwhile, it also is testing Privacy Sandbox.

“We are excited participants [in Privacy Sandbox],” said Andrew Baron, PubMatic’s senior VP of marketplace and addressability. “We are part of the Google data program, and we are eagerly meeting their development milestones. We’re bringing along partners to test with us to prove that there is value here in continuing to pursue this big architectural change.”

Still, there are ad tech companies forging their own way, including 33Across, an SSP and publisher monetization platform. Like Lotame, 33Across is holding off on participating in Privacy Sandbox. But it also has not integrated with The Trade Desk’s UID 2.0. “The DSPs need to do the heavy lifting right now [in Privacy Sandbox],” said Paul Bell, president at 33Across, adding that the advertisers, the buy side, need to “make sure they can target and measure without cookies.”

33Across will eventually have to contend with Privacy Sandbox, since Chrome represents about 50% of the traffic on the open web in the U.S., Bell said. The timing is tight for Google if it wants to fully implement the new system this year, Bell said.

“It’s still sort of mid-build,” Bell said, adding that there are still a lot of “open questions.”