qpack | Web Quality/Metric Development Essentials Extension Pack | Monitoring library

Duplication of the old entry makes a new copy of that table entry; it does not take a reference to the old entry. This allows for the old entry to age out (be evicted).

The Duplicate instruction achieves three goals:

1. it allows the old entry to be evicted (covered above).
2. it allows for the continuous use of the dynamic table rather than the inserting a new entry (copy of the old entry) again and potentially risking a blocked hearer; and
3. duplicating an old entry is cheaper -- from the perspective of number of bytes needed to send over the network -- than insertion of a new entry; this improves compression performance.

The QPACK Internet Draft touches upon when issuing Duplicate instruction may be profitable. Choosing when and whether to issue a Duplicate instruction is an important decision which is likely to affect compression performance greatly. Each encoder will select its own strategy.

For example, ls-qpack contains this code: