// -1 means we want to send from the end of the backlog (the normal case),
// although only at a keyframe.
// -2 means we want to send from the _beginning_ of the backlog.
// -1 means we want to send from the end of the backlog (the normal case),
// although only at a keyframe.
// -2 means we want to send from the _beginning_ of the backlog.