Europe's diplomatic row over who should take in a rescue boat with hundreds of migrants onboard has drawn fresh attention to glaring gaps in the European Union's asylum rules.

The so-called Dublin Regulation, a key component keeping Europe's passport-free travel area open, is under review, but the 28 EU nations cannot agree on who among them should take responsibility for arriving migrants and how much to help those countries facing the biggest burden.

After two years of deadlock, EU leaders could be forced to draw the line at a June 28-29 summit.

A look at Europe's asylum rules and response to the migration challenge:

DUBLIN REGULATION

People arriving in Europe can apply for asylum or protection if they are fleeing conflict and fear violence or death. People searching for better lives — economic migrants — are generally not eligible and likely to be sent home.

The Dublin rules govern which EU country should examine an application and how it should be handled. In practice, it's the first country they arrive in — that's often Greece or Italy. The system collapsed in 2015 when Greece was swamped by refugees, and nations have been unable to agree on a way to reform it ever since. Conditions are still bad in Greek migrant reception centers. EU leaders will debate the rules at the upcoming summit, but no breakthrough is likely.

EU-TURKEY MIGRANT DEAL

Under a 2016 EU-Turkey deal, Europeans offered visa-free travel for Turkish citizens, fast-track EU membership talks and up to six billion euros ($7 billion) in aid for Syrian refugees in Turkey to get Ankara to stop migrants leaving and to take back thousands of others in the Greek islands.

Two years later, unauthorized arrivals were 97 percent lower than before it came into effect. Deaths in the Aegean Sea dropped massively, but have not entirely stopped.

Lauded by the EU as "a game changer," the deal was blamed for human rights abuses against migrants by NGOs and rights groups. The EU is trying to duplicate the scheme with transit countries in Africa.

REFUGEE SHARING

A two-year mandatory plan launched in 2015 originally foresaw EU members sharing almost 100,000 refugees. But as the deal with Turkey slashed numbers arriving in Greece, and with the majority of migrants heading to Italy not eligible for relocation, the number to share out was far lower than predicted

By the end of May, EU states had shared some 34,600 refugees in Greece and Italy under the plan.

The EU considers the scheme a resounding success — despite Hungary and Poland refusing to take anyone in. One proposal to reform Dublin involves a permanent quota plan, but it faces strong opposition and is unlikely to be adopted.