312-50V11 · Question #579
A circuit level gateway works at which of the following layers of the OSI Model?
The correct answer is B. Layer 4 - TCP. A circuit-level gateway operates at Layer 4 (Transport layer) of the OSI model by inspecting TCP handshake state rather than packet content.
Question
A circuit level gateway works at which of the following layers of the OSI Model?
Options
- ALayer 5 - Application
- BLayer 4 - TCP
- CLayer 3 - Internet protocol
- DLayer 2 - Data link
How the community answered
(39 responses)- A3% (1)
- B90% (35)
- C5% (2)
- D3% (1)
Why each option
A circuit-level gateway operates at Layer 4 (Transport layer) of the OSI model by inspecting TCP handshake state rather than packet content.
Layer 5 in the OSI model is the Session layer, not the Application layer (Layer 7); circuit-level gateways do not perform the deep content inspection that characterizes application-layer firewalls.
Circuit-level gateways function at the Transport layer (Layer 4) by monitoring and validating the TCP three-way handshake before permitting a session to proceed. They do not inspect payload content beyond connection-state information, which distinguishes them from application-layer proxy firewalls that operate at Layer 7.
Layer 3 is the Network layer responsible for IP routing and addressing; circuit-level gateways operate above this layer and are concerned with TCP session state, not packet routing.
Layer 2 is the Data Link layer responsible for MAC addressing and frame delivery; circuit-level gateways have no operational function at this layer.
Concept tested: Circuit-level gateway OSI Transport layer operation
Source: https://csrc.nist.gov/publications/detail/sp/800-41/rev1/final
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