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The proxy server is an important security function provided by the Internet "circuit level gateway". Its work is mainly at the session layer of the Open Systems Interconnection (OSI) model, which acts as a firewall.

how to understand this sentence?
what's the circuit level gateway?
why the proxy server is mainly works at the session-layer?

EDIT-01

the session layer is the (5/7 of OSI)

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  • In the real world, the session layer doesn't actually exist. The OSes have grouped the session, presentation, and application layers all together in the application layer. It is up to an application to do sessions. Rather than create a session with a server, the application creates a session with a proxy that then stands in for the application.
    – Ron Maupin ♦
    Commented May 3, 2019 at 7:15

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A gateway is a device that can send traffic to a different network. The gateway operates on the same data-link LAN as the source device, which sends traffic destined to a different LAN to the gateway.

The OSI model doesn't have a "dialogue layer." I presume the quote means the application layer.

Typically, a proxy server is for an application (usually web browsing, but not limited to that). You would have your web browser (or other application) configured to use the proxy server instead of the default gateway for the host, and the application traffic is then sent to the proxy server that stands in (proxies) for the host when communicating to a web server. The proxy server can then be configured to perform security functions such as denying unsafe web sites, data inspection for malware, etc.

I'm not sure of the source of you quote, but it seems to use its own, non-standard terminology, so I would look for a different learning source.

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  • The quote could also be referring to some sort of "transparent" firewall/security device, operating at L2.
    – YLearn
    Commented May 3, 2019 at 4:49
  • Random thought, I wonder if the text was generated from a speech-to-text application? "Dialogue" could be a result of improper enunciation of "data link"? Purely hypothetical though.
    – YLearn
    Commented May 3, 2019 at 4:54
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    @YLearn, I kind of wondered the same thing, but it does have "link-level" in it. In any case, it seems a poor source (at least in English). I have recently seen a lot of things that simply make up different terminology, presumably to make it easier to understand, but it seems to confuse the situation even more.
    – Ron Maupin ♦
    Commented May 3, 2019 at 5:01
  • @YLearn, I was certain that the recent trend to use the term "packages" instead of "packets" on the SE technology sites was due to translations, but I was browsing in a bookstore a couple of weeks ago, and I ran across a book in English that used that term. I suppose it is possible the book started out in a different language and was mistranslated, but the people who buy it will not know that.
    – Ron Maupin ♦
    Commented May 3, 2019 at 5:29
  • So, do you mean the link-level gateway is the gateway we usually said? but why the proxy server is mainly works at the session-layer?
    – aircraft
    Commented May 3, 2019 at 7:15

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