OSI vs TCP/IP: Which Layer(s) Are Added? (Presentation, Session, or Both?)

To compare the OSI model with the TCP/IP (Internet) model, focus on how the upper-layer responsibilities are grouped. In OSI, the Presentation layer and Session layer are distinct layers, but in the classic TCP/IP model these responsibilities are not separated into two independent layers; instead, they are typically folded into the Application layer. Therefore, the OSI layers that are additional relative to TCP/IP are the Presentation layer and Session layer—i.e., (iv) Both session layer and presentation layer.

A useful way to remember this mapping is: OSI splits “application-side” concerns into finer-grained layers, while TCP/IP combines them.

Key keyword: OSI layer that handles data representation (e.g., encoding/format) and often encryption/compression.
Key keyword: OSI layer that manages establishment, maintenance, and termination of communication sessions.
Key keyword: Internet protocol stack that groups application-side functions mostly under the Application layer.

Note: Different textbooks sometimes draw the mapping slightly differently, but the standard conceptual conclusion remains: OSI’s extra separation at the top corresponds to Presentation + Session being “combined” into Application in TCP/IP.

OSI upper layers (what they do)

In the OSI model, the Presentation layer and Session layer provide services beyond “pure application logic”:

• keyword: converts between formats/encodings and can perform encryption/decryption or data compression.
• keyword: establishes sessions, coordinates dialog control, and handles session synchronization and orderly termination.
• keyword: provides network services that applications use (e.g., email, web, file transfer).

This separation is why OSI has more conceptual layers than TCP/IP.

TCP/IP upper layers (how it groups responsibilities)

In the TCP/IP model, the top is commonly represented as fewer layers:

• Application layer: includes functions that OSI distributes among Application + Presentation + Session.
• TCP/IP emphasizes transport and internetworking layers (Transport and Internet) while bundling many application-side concerns together.

So, OSI’s Presentation and Session layers are effectively “additional” because TCP/IP does not present them as separate layers.