On-board network system and program

A program executable by a computer to perform processing, the processing comprising:

• for each date item retained at a first relay device, measuring a retention duration for which the date item is retained without being relayed, the first relay device being included among a plurality of relay devices, and each relay device temporarily retaining received date items and relaying the retained date items in descending order of relay priority level;
• determining whether or not the measured retention duration of each date item exceeds a predetermined threshold that is specified for each of the date items;
• and
• for a congested date item, which is a date item having a retention duration that has been determined to exceed the predetermined threshold, requesting a second relay device to raise a relay priority level of the congested date item at the second relay device and relay the congested date item, the second relay device being a different relay device from the first relay device among the plurality of relay devices.