Constant MAX_READ_BYTES 

const MAX_READ_BYTES: i64 = _; // 4_194_304i64

Most bytes consumed in one tick, and the most read when first catching up on a block. A block seen from its first byte is exact; one that already had more than this when we first saw it (srv restarted mid-session) is seeded from its tail and flagged — see BlockState::partial.