What they typically require is a message of some sort sent regularly from the client, and if they haven't received anything in a while, they'll send a PING to elicit a response from the client (if it's still alive). Most servers don't even check that the PONG matches the PING they sent, they're just happy to get something back before the timeout, although there are servers that send a cookie in a PING at connect time that they require to be reflected back correctly as a countermeasure for certain kinds of proxy abuse.
For example, EPIC is one of the oldest clients and it doesn't send its own PING messages unless scripted to do so, and it works fine.
What they typically require is a message of some sort sent regularly from the client, and if they haven't received anything in a while, they'll send a PING to elicit a response from the client (if it's still alive). Most servers don't even check that the PONG matches the PING they sent, they're just happy to get something back before the timeout, although there are servers that send a cookie in a PING at connect time that they require to be reflected back correctly as a countermeasure for certain kinds of proxy abuse.
For example, EPIC is one of the oldest clients and it doesn't send its own PING messages unless scripted to do so, and it works fine.