+ // Number of bytes we are into the stream (ie., the end of last send).
+ // -1 means we want to send from the end of the backlog (the normal case),
+ // -2 means we want to send from the _beginning_ of the backlog.
+ // Once we go into SENDING_DATA, these negative values will be translated
+ // to real numbers.
+ size_t stream_pos;
+