S1 / 03 MODULE 3 / PROTOCOLS AND MODELS
Understand the rules.
Follow the data.
Protocols define how devices communicate. Models help explain how those protocols cooperate as data moves from an application, through a network, and to another host.
Protocols define communication rules
A protocol describes the rules used by communicating devices. Communication requires a sender and receiver, and protocols define message encoding, formatting and encapsulation, message size, timing, and delivery options. Multiple protocols cooperate to deliver one application message. A protocol suite is a group of related protocols; TCP/IP is the protocol suite used by modern networks. Open standards support interoperability between equipment and software from different vendors.
IMG-0218 · IMG-0254Compare OSI and TCP/IP reference models
The OSI model has seven layers: Application, Presentation, Session, Transport, Network, Data Link, and Physical. The TCP/IP model groups networking functions into four layers: Application, Transport, Internet, and Network Access. A useful mapping is OSI Application + Presentation + Session to TCP/IP Application; OSI Transport to TCP/IP Transport; OSI Network to TCP/IP Internet; and OSI Data Link + Physical to TCP/IP Network Access. These are conceptual models, not physical devices. They provide a shared vocabulary for understanding where a protocol operates and how responsibilities are separated.
IMG-0248 · IMG-0270Understand encapsulation and PDUs
As data moves down the protocol stack, each layer adds the control information it needs. Application data is passed to the transport layer, which adds a TCP or UDP header to form a segment. The network layer adds an IP header to form a packet. The data link layer adds a frame header and trailer, including MAC addressing. The physical layer transmits bits as signals. The common PDU names are data, segment, packet, frame, and bits. At the receiver, decapsulation removes these headers in reverse order.
IMG-0177 · IMG-0270 · IMG-0193Separate Layer 2 and Layer 3 addressing
IP addressing identifies the source and ultimate destination across networks. MAC addressing delivers a frame across the local link. When the destination is on the same IP network, the sender can address the frame to the destination host's MAC address. When the destination is remote, the sender keeps the remote host's IP address in the packet but sends the local Ethernet frame to the default gateway's MAC address. Routers remove the incoming link-layer frame and build a new frame for the next link.
IMG-0195 · IMG-0201 · IMG-0202Sequence and reassemble TCP data
Segmentation divides a data stream into smaller transport units for transmission. TCP sequence numbers identify the byte position represented by each segment. Because segments can arrive out of order, the receiver uses those byte positions to place the data in the correct order and reconstruct the original stream. For example, a segment beginning at sequence 1000 and carrying 100 bytes represents bytes 1000 through 1099; the next contiguous segment begins at 1100. Sequence numbering supports reassembly; it is distinct from the acknowledgment-number calculations studied in the Transport Layer module.
IMG-0180 · IMG-0266OSI MODEL
7 layers
- Application
- Presentation
- Session
- Transport
- Network
- Data Link
- Physical
TCP/IP MODEL
4 layers
- Application
- Transport
- Internet
- Network Access
CHECK YOUR UNDERSTANDING
Trace a message through the stack.
Practice the layer responsibilities, PDU names, and the difference between IP and MAC addressing.