HTTP/2 enables a more efficient use of network resources and a reduced perception of latency by introducing header field compression and allowing multiple concurrent exchanges on the same connection. It also introduces unsolicited push of representations from servers to clients. This specification is an alternative to, but does not obsolete, the HTTP/1.1 message syntax.
HPACK is a compression format for efficiently representing HTTP header fields, to be used in HTTP/2.
Read more here - https://www.mnot.net/blog/2015/02/18/http2
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