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- Layer 4 Protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are both Layer 4 protocols in the OSI model, responsible for the transport of data between devices on a network.
- Exam Focus: The CCNA exam expects you to compare TCP and UDP, understanding their differences and the services they provide to applications.
- Transparent Data Transfer: Layer 4 protocols provide transparent data transfer between end hosts, meaning the transfer of data is seamless and hidden from the end users.
- Services Provided by TCP (Not by UDP):
- Reliable Data Transfer: Ensures all data is delivered correctly to the destination.
- Error Recovery: Resends data if errors are detected during transmission.
- Data Sequencing: Ensures that data arrives in the correct order.
- Flow Control: Manages the pace of data transmission to prevent overwhelming the destination host.
- Port Numbers: Layer 4 protocols use port numbers for addressing, which help identify the application layer protocol and manage multiple communication sessions.
- Session Multiplexing: Allows a single device to handle multiple communication sessions simultaneously.
- IANA Port Ranges:
- Well-known Ports: 0 - 1023 (e.g., HTTP, FTP).
- Registered Ports: 1024 - 49151 (require registration).
- Ephemeral Ports: 49152 - 65535 (used for dynamic port assignments).
- Connection-Oriented: TCP establishes a connection before data is transmitted, ensuring that both the sender and receiver are ready for communication.
- Reliable Communication: TCP guarantees that data is delivered without errors and in the correct sequence.
- Flow Control: Uses a sliding window mechanism to control the flow of data and prevent overwhelming the destination.
- TCP Header Fields:
- Source & Destination Ports: Identify the sending and receiving applications.
- Sequence Number & Acknowledgment Number: Used for sequencing and ensuring reliable communication.
- Flags (e.g., SYN, ACK, FIN): Control the establishment, maintenance, and termination of connections.
- Window Size: Manages the flow of data.
- TCP Three-Way Handshake: Establishes a connection using SYN, SYN-ACK, and ACK messages.
- TCP Four-Way Handshake: Terminates a connection using FIN and ACK messages.
- Sequencing & Acknowledgment: Ensures data is delivered in the correct order and retransmits if segments are lost.
- Connectionless: UDP does not establish a connection before sending data. Data is simply sent without any handshake process.
- Best-Effort Delivery: UDP provides no guarantee of data delivery, order, or error recovery.
- UDP Header Fields:
- Source & Destination Ports: Identify the sending and receiving applications.
- Length & Checksum: Simple fields indicating segment length and basic error checking.
- Header Size:
- TCP: Larger header with more fields for advanced services.
- UDP: Smaller header with minimal fields, resulting in lower overhead.
- Use Cases:
- TCP: Used for applications where reliable communication is essential (e.g., file transfers, web browsing).
- UDP: Preferred for real-time applications where speed is more critical than reliability (e.g., video streaming, VoIP).
- Examples:
- TCP: HTTP (Port 80), HTTPS (Port 443), FTP (Ports 20, 21).
- UDP: DNS (Port 53), DHCP (Ports 67, 68), TFTP (Port 69).
TCP:
- FTP (Data): 20
- FTP (Control): 21
- SSH: 22
- Telnet: 23
- SMTP: 25
- HTTP: 80
- POP3: 110
- HTTPS: 443
UDP:
- DHCP (server): 67
- DHCP (client): 68
- TFTP (Trivial FTP): 69
- SNMP (agent): 161
- SNMP (manager): 162
- Syslog: 514
- NTP: 123
- RADIUS: 1812, 1813
Both TCP and UDP:
- DNS: 53