< SERVER
> CLIENT

< PING :tmi.twitch.tv
> PONG :tmi.twitch.tv
OR: > PONG
(server doesn't care)

> PING
< :tmi.twitch.tv PONG

> PING a b c
< :tmi.twitch.tv PONG :a

> PING :a b c
< :tmi.twitch.tv PONG :a b c

Rule of thumb: The first IRC message "argument" ("a"/"a b c" is what the server responds with.)
Server source (":tmi.twitch.tv") should be ignored, client responses never have sources.