Drivers for Pacific 9 Transportation, a trucking company that serves the ports of Los Angeles and Long Beach, California, last went on strike in April, pictured here. Reuters/Bob Riha Jr.

Truck drivers at the nation’s busiest port complex walked off their jobs Tuesday morning. The workers accused their bosses at Pacific 9 Transportation -- one of several companies that haul cargo from the ports of Los Angeles and Long Beach, California, to shipping warehouses and major retail distribution centers -- of misclassifying them as independent contractors instead of employees, thereby depriving them of benefits and higher pay.

The walkout is the latest salvo from truck drivers in a labor dispute that spans nearly two years. Drivers for Pacific 9 and other short-haul companies have gone on strike several times since 2013, most recently in late April, alleging misclassification and retaliation for speaking out about labor conditions. The striking truckers are backed by the International Brotherhood of Teamsters union, which is seeking contracts with the companies -- something that's possible only if drivers are classified as employees.

Picket lines went up early Tuesday.

Union officials say the strike is "indefinite."

At stake is the same issue that has sparked high-profile class-action lawsuits against ride-hailing companies Uber and Lyft. As in those cases, the debate in the trucking yards is about whether or not drivers currently classified as contractors should be considered employees -- and thus be eligible for standard labor protections like the right to a minimum wage, overtime pay, workers’ compensation and ability to unionize.

The contractor status is common for truck drivers working at ports -- and not just in Southern California. A report last year by the Change to Win labor federation argued that nearly 50,000 of the nation’s 75,200 port truckers are wrongly classified as independent contractors. That designation comes with a cost for workers, researchers found. According to the study, contracted drivers make about $6,000 less per year than the $35,000 employed drivers earn on average.

Pacific 9 faces outstanding claims of wage theft by the California Labor Commission that tally up to $6 million, according to the Teamsters. Last December, the commission ordered the company to pay $255,000 in back wages and violations. Pacific 9 did not respond to requests for comment.

Unlike the costly dispute involving West Coast dockworkers earlier this year, the trucking strikes have caused little disruption in port traffic so far. And businesses likely have little to fear: American labor law makes it very challenging for dockworkers to respect the truckers' picket lines.