Chapter 4: The TCP/IP networking model
The chapter introduces networking models as a framework for understanding the multiple protocols and functions needed to move data across a network.
After this chapter you will be able to
- Need for multiple protocols. Explain why Ethernet connections alone are not enough for computers to communicate across a network.
- Purpose of networking models. Explain how the OSI and TCP/IP models help organize the functions involved in network communication.
- Vendor-neutral protocol interoperability. Explain what a protocol is and why vendor-neutral protocols make communication between different vendors’ devices easier than vendor-proprietary protocols.
- Layered model modularity. Describe how a networking model divides the functions needed to move data into layers, and explain why different protocols can fill a layer’s role without an application needing to handle their differences.
- OSI model and layers. Explain that the OSI model is a conceptual model of networking developed by ISO, with seven layers whose functions contribute to network communication.
- TCP/IP model layers and variants. Explain why the book uses the five-layer TCP/IP model rather than the four-layer version, and describe how its layers relate to the OSI model.
- Hop and end-to-end delivery. Distinguish Layer 2's MAC-addressed, hop-to-hop delivery from Layer 3's IP-addressed, end-to-end delivery. Describe how the destination MAC address changes at each hop while the destination IP address remains the same on the journey described.
- Transport addressing of applications. Explain why reaching the destination host is not enough and how Layer 4 port numbers direct a message to a particular application process on that host.
- Application protocols and applications. Explain that the Application Layer provides the interface between applications and the network, and distinguish a Layer 7 protocol such as HTTPS from the user application that uses it.
- Web request across layers. Trace the section's PC1-to-SRV1 web request at a high level: HTTPS forms the request, port 443 identifies the destination service, SRV1's IP identifies the destination host, and successive MAC addresses identify the next hop.
- Outbound data encapsulation. Trace how a sending host prepares application data, adds Layer 4, Layer 3, and Layer 2 information, and transmits the resulting bits.
- Inbound data de-encapsulation. Trace how a receiving host inspects and removes layer information to deliver data to an application.
- Message names and payloads. Distinguish segments, packets, and frames by what each contains, and explain what a PDU’s payload is.
What it builds on
- IEEE 802.3 standards (copper) (chapter 3) · Introduces physical connections defined by Ethernet standards.
- Copper UTP connections (chapter 3) · Provides prior exposure to Ethernet connections used in the example.
- Fiber-optic connections (chapter 3) · Fiber-optic connections provide another example.
- Routers (chapter 2) · Routers are introduced as network devices.
Section by section
Introduction
The chapter introduces networking models as a framework for understanding the multiple protocols and functions needed to move data across a network.
- Need for multiple protocols: Explain why Ethernet connections alone are not enough for computers to communicate across a network.
Why it matters: This explains why the chapter looks beyond physical connections to a broader model of communication. - Purpose of networking models: Explain how the OSI and TCP/IP models help organize the functions involved in network communication.
Why it matters: The models give you a way to place the protocols discussed later in the book into a larger picture.
Key terms: Ethernet, protocols, OSI model, TCP/IP model, networking models
4.1 Conceptual models of networking
Explains why vendor-neutral protocols and layered networking models make communication between different devices possible, and introduces the roles of OSI and TCP/IP.
- Vendor-neutral protocol interoperability: Explain what a protocol is and why vendor-neutral protocols make communication between different vendors’ devices easier than vendor-proprietary protocols.
Why it matters: Shared communication rules let devices work together regardless of who made them. - Layered model modularity: Describe how a networking model divides the functions needed to move data into layers, and explain why different protocols can fill a layer’s role without an application needing to handle their differences.
Why it matters: Layers make it easier to reason about networking without treating every combination of application and connection type as a separate system. - OSI and TCP/IP roles: Distinguish the modern use of the TCP/IP model from the continuing value of the OSI model as a way to think and talk about networks.
Why it matters: The distinction explains why the book discusses both models.
Key terms: protocol, vendor-proprietary, vendor neutral, networking models, layers, modular design, Ethernet, Wi-Fi, OSI, TCP/IP model
4.2 The OSI reference model
Introduces the OSI model as a seven-layer conceptual model and explains why its layer terminology remains useful even though the chapter focuses on TCP/IP.
- OSI model and layers: Explain that the OSI model is a conceptual model of networking developed by ISO, with seven layers whose functions contribute to network communication.
Why it matters: This gives you a framework for understanding references to OSI layers without requiring you to learn each layer’s function here. - Continuing OSI terminology: Explain why the seven OSI layer names are worth remembering even though this chapter focuses on the TCP/IP model.
Why it matters: You may encounter OSI layer names even when the discussion centers on TCP/IP.
Key terms: Open Systems Interconnection reference model, OSI model, seven layers, International Organization for Standardization (ISO), TCP/IP model
4.3 The TCP/IP model
Introduces the TCP/IP model, its five-layer version, its relationship to the OSI model, and the standards documents behind it.
- TCP/IP model layers and variants: Explain why the book uses the five-layer TCP/IP model rather than the four-layer version, and describe how its layers relate to the OSI model.
Why it matters: This distinction helps you use the book’s five-layer model without treating it as an exact copy of OSI. - TCP/IP layer number conventions: State the layer numbers used for the five TCP/IP layers and explain why the Application Layer is commonly called Layer 7.
Why it matters: These names and numbers let you recognize which TCP/IP layer a networking discussion means. - TCP/IP standards and origins: Describe the model’s origins and distinguish RFCs generally from RFCs recognized as Internet Standards.
Why it matters: It clarifies where the model comes from and why an RFC is not automatically an Internet Standard.
Key terms: TCP/IP model, five-layer model, Link Layer, Application Layer, OSI model, Physical Layer, Data Link Layer, Network Layer, Transport Layer, Layer 7
4.3.1 The layers of the TCP/IP model
This section explains how the TCP/IP layers work together to carry a message from an application on one computer to an application on another.
- Physical transmission of bits: Explain that the Physical Layer defines how bits travel between nodes over a physical medium, such as electrical signals on copper, light on fiber, or radio waves.
Why it matters: It distinguishes moving bits across a medium from addressing and delivering a message. - Hop and end-to-end delivery: Distinguish Layer 2's MAC-addressed, hop-to-hop delivery from Layer 3's IP-addressed, end-to-end delivery. Describe how the destination MAC address changes at each hop while the destination IP address remains the same on the journey described.
Why it matters: This distinction explains how a message can cross several hops while still being addressed to the same destination host. - Transport addressing of applications: Explain why reaching the destination host is not enough and how Layer 4 port numbers direct a message to a particular application process on that host.
Why it matters: A host may run many applications at once, so delivery to the right computer alone does not finish the job. - TCP and UDP delivery checks: Distinguish the basic delivery-check behavior described for TCP and UDP, while recognizing that both are Layer 4 protocols that can address application services.
Why it matters: It gives a first distinction between two common protocols that perform Layer 4's role. - Application protocols and applications: Explain that the Application Layer provides the interface between applications and the network, and distinguish a Layer 7 protocol such as HTTPS from the user application that uses it.
Why it matters: It clarifies what a browser does with HTTPS before the other layers deliver its request. - Web request across layers: Trace the section's PC1-to-SRV1 web request at a high level: HTTPS forms the request, port 443 identifies the destination service, SRV1's IP identifies the destination host, and successive MAC addresses identify the next hop.
Why it matters: Putting the layers together shows how one request reaches both the right host and the right service.
Key terms: Physical Layer, bits, physical medium, Data Link Layer, Network Layer, hop, MAC address, IP address, hop-to-hop delivery, Transport Layer
4.3.2 Data encapsulation and de-encapsulation
This section explains how TCP/IP layers add and remove information as data travels between hosts, how the resulting messages are named, and how layers interact.
- Outbound data encapsulation: Trace how a sending host prepares application data, adds Layer 4, Layer 3, and Layer 2 information, and transmits the resulting bits.
Why it matters: This sequence helps you explain what each layer contributes to a message before it leaves a host. - Inbound data de-encapsulation: Trace how a receiving host inspects and removes layer information to deliver data to an application.
Why it matters: It shows how a received message becomes data an application can use. - Message names and payloads: Distinguish segments, packets, and frames by what each contains, and explain what a PDU’s payload is.
Why it matters: These names let you identify which layer’s message is being discussed. - Adjacent and same-layer interactions: Distinguish services between adjacent layers on one computer from communication between corresponding layers on different devices.
Why it matters: The distinction clarifies whether you are describing cooperation inside a host or communication between devices.
Key terms: encapsulation, header, trailer, port number, IP address, MAC address, de-encapsulation, segment, packet, frame
4.4 Summary
Where this is used later
- Preamble and SFD (chapter 6) · The distinction between the Physical and Data Link layers.
- Address Resolution Protocol (chapter 6) · The distinction between Layer 2 and Layer 3 addresses.
- The role of Layer 4 (chapter 22) · The roles of the layers in the TCP/IP model.
- The life of a packet from PC1 to PC3 (chapter 10) · Tracing a packet from PC1 to PC3 uses the distinction between successive hops and the final destination.
- Port numbers (chapter 22) · Port numbers are examined further as part of Layer 4's role.
- Comparing TCP and UDP (chapter 22) · The later comparison develops the differences between TCP and UDP.
- PC1 to R1 (chapter 10) · Tracing a packet’s first hop uses the distinction between destination-host and next-hop addressing.
- (chapter intro) (chapter 6) · The earlier treatment of PDUs and encapsulation.
Concepts explained in this chapter
- TCP/IP protocol suiteThe TCP/IP model is a layered conceptual framework that describes how network communication functions across application, transport, network, data-link, and physical layers.
- data link layerThe data link layer is the network-model layer that provides delivery between adjacent nodes over a shared physical medium.
- physical layerThe Physical Layer is a networking model layer that defines the transmission of raw signals and bits over a physical medium.
- transport layerThe transport layer is the network-model layer that delivers data to a specific application process on a destination host.
- IP addressAn IP address is a network-layer identifier assigned to a host or interface for packet delivery across interconnected networks.
- mac addressA MAC address is a 48-bit link-layer address assigned to a network-device port, normally by the device manufacturer, for local hop-to-hop delivery.
- port numberA port number is a transport-layer identifier ranging from 0 to 65535 that directs network data to a particular application process on a host.
- network hopA hop is one journey of a message from one network node to the next node along its path to a destination.
- HTTPSHTTPS is a protocol for transferring web resources over HTTP with protection provided by TLS.
- OSI network modelThe OSI model is a seven-layer conceptual framework for describing the functions involved in network communication.
- de-encapsulationDe-encapsulation is the process of inspecting and removing protocol headers and trailers as received data moves up a networking stack.
- adjacent-layer interactionAn adjacent-layer interaction is communication in which one layer of the TCP/IP model provides a service to the layer directly above it on the same computer.
- frameA frame is a data-link-layer protocol data unit consisting of a packet combined with a Layer 2 header and trailer.
- network packetA packet is a network-layer protocol data unit consisting of a segment combined with a Layer 3 header.
- payloadA payload is the data carried inside a protocol data unit, excluding that unit's protocol control information.
- protocol data unitA protocol data unit is a unit of data consisting of a protocol payload together with the control information added at a networking layer.
- same-layer interactionA same-layer interaction is communication between corresponding layers of the TCP/IP model on different computers.
- networking modelA networking model is a framework that organizes the functions required for data to travel from a source to a destination over a network.
- headerAn IPv4 header is the network-layer protocol header that carries IPv4 addressing and control fields for delivering an IPv4 packet.
- trailerAn Ethernet trailer is supplemental data placed at the end of an Ethernet frame, including the FCS field for error detection.
- IPv4An IPv4 address is a 32-bit network-layer address used to identify a source or destination in IPv4 communication.
- IPv6IPv6 is the sixth version of the Internet Protocol, using 128-bit network addresses written in hexadecimal notation.
- TCPTransmission Control Protocol is a transport-layer protocol that provides reliable, connection-oriented delivery with mechanisms for detecting and recovering from lost data.
- UDPUser Datagram Protocol is a transport-layer protocol that sends datagrams without guaranteeing delivery of every message.
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