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CompTIA Network+ (V9)

OSI model layers explained for Network+

The OSI reference model has seven layers: Physical, Data link, Network, Transport, Session, Presentation and Application. Each layer serves the one above it. The Internet protocol suite has four layers, and its application layer covers the work of OSI's Presentation and Application layers.

What each layer does

These descriptions follow ITU-T X.200, the OSI reference model.

Layer 1, Physical: the mechanical, electrical, functional and procedural means to activate, maintain and release physical connections for bit transmission.

Layer 2, Data link: transfers data between network entities over one or more physical connections. It detects and possibly corrects errors which may occur in the Physical layer.

Layer 3, Network: moves data between transport entities and gives them independence from routing and relaying, including when several subnetworks are crossed in tandem.

Layer 4, Transport: transparent transfer of data between end systems. Its protocols have end-to-end significance and include end-to-end error detection, error recovery, sequence control and flow control.

Layer 5, Session: lets the communicating parties organize and synchronize their dialogue, establish a connection and release it in an orderly manner.

Layer 6, Presentation: a common representation of the data being transferred, so applications do not have to agree on syntax themselves.

Layer 7, Application: the highest layer and the only way an application process reaches the OSI environment. It has no layer above it.

Hop by hop or end to end

Error handling appears at two layers, and the exam tests which is which. The Data link layer handles errors on one physical hop, before the Network layer sees the data. The Transport layer handles errors end to end, between the two communicating end systems only.

Not every transport protocol recovers errors. RFC 768 says UDP is transaction oriented and that delivery and duplicate protection are not guaranteed. If a UDP datagram arrives twice, UDP passes both up, and the application must notice and discard the duplicate. TCP, by contrast, provides a reliable, in-order byte stream (RFC 9293).

OSI layers and the Internet protocol suite

RFC 1122 lists the Internet suite's layers as application, transport, internet and link. It states that the Internet application layer essentially combines the functions of the top two OSI layers, Presentation and Application.

TCP and UDP belong to the transport layer, which lines up with OSI layer 4. IP belongs to the internet layer, which lines up with OSI layer 3.

Where the exam catches people

  • Believing the Network layer corrects transmission errors on a link: X.200 gives that job to the Data link layer, hop by hop.
  • Believing UDP drops duplicate datagrams: RFC 768 says UDP gives no duplicate protection, so the application must detect them.
  • Believing the Session layer handles data format or encoding: that is the Presentation layer. The Session layer organizes and synchronizes the dialogue.
  • Believing the Internet suite's application layer matches only OSI layer 7: RFC 1122 says it combines the Presentation and Application layers.

Written from CompTIA's documentation

Try 3 questions on this

Pick an answer to see why each option is right or wrong.

  1. A function detects transmission errors that arose on one physical hop and handles them before the Network Layer sees them, rather than recovering end to end. Which of the following OSI layers performs this?

    Right. In the game that’s Renown for your Networking Concepts building.

    Not that one: B is right. In the game your Networking Concepts building would lose a little Renown, and this question would come back later.

    Show the full explanation and doc quotes

    That is hop-by-hop error handling on one physical connection, performed below the Network Layer, which is what the stem describes.

    Written from www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-X.200-199407-I!!PDF-E&type=items

  2. The Internet protocol suite has four layers, and its top layer combines the functions of the top two layers of the OSI reference model. Which of the following OSI layers is combined with Application there?

    Right. In the game that’s Renown for your Networking Concepts building.

    Not that one: A is right. In the game your Networking Concepts building would lose a little Renown, and this question would come back later.

    Written from www.rfc-editor.org/rfc/rfc1122.html

  3. After a brief routing change, a host receives the same UDP datagram twice from one sender. Which of the following OSI layers must recognize and discard the duplicate?

    Right. In the game that’s Renown for your Networking Concepts building.

    Not that one: D is right. In the game your Networking Concepts building would lose a little Renown, and this question would come back later.

    Written from www.rfc-editor.org/rfc/rfc768.html