Right. There is definitely a tradeoff between requiring retention of the identifiers, which requires keeping more state, and re-specifying them, which requires

Right. There is definitely a tradeoff between requiring retention of the identifiers, which requires keeping more state, and re-specifying them, which requires sending more data. There are definitely more sophisticated ways to handle this (see: your HTTP2 header example, which I think even includes value caching), but an easy way to chose a point on that tradeoff spectrum is to simply keep a fixed-sized ringbuffer and retain, say, the last 256 keys.