Hop-by-hop
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As a hop-by-hop header, Upgrade is negotiated between each pair of
HTTP counterparties. If a User Agent ...
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These scenarios also complicate diagnosis of a 426 response. Since
Upgrade is a hop-by-hop header, a proxy that does not recognize 426
might remove ...
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This hop-by-hop definition of Upgrade was a deliberate choice. It
allows for incremental deployment on either side of proxies ...
... Henrik Frystyk Nielsen, whose work on the Mandatory extension
mechanism pointed out a hop-by-hop Upgrade still requires
tunneling. ...