packets - dwilson2547/wiki_demo GitHub Wiki

When data is sent over a network, it is broken down into packets. Each packet is structured according to the OSI model or TCP/IP model, with each layer adding its own header (and sometimes a trailer) to the data. Here’s how a packet is constructed:

• 1. Data Breakdown by Layer
  • Application Layer (Layer 7)
  • Transport Layer (Layer 4)
  • Network Layer (Layer 3)
  • Data Link Layer (Layer 2)
  • Physical Layer (Layer 1)
• 2. Packet Structure Visualization
• 3. How Packets Travel
• 4. Example: Sending an HTTP Request
• 5. Key Terms
• 6. Why Packetization Matters

• Data: The actual payload (e.g., an HTTP request, email, or file).
• Example: The text of this message or a webpage request.

• Header Added:
  • Source Port: Identifies the sending application (e.g., 54321).
  • Destination Port: Identifies the receiving application (e.g., 80 for HTTP).
  • Sequence Number: Ensures data is reassembled in the correct order.
  • Checksum: Verifies data integrity.
• Protocols: TCP (reliable, connection-oriented) or UDP (fast, connectionless).
• Result: The data is now a segment (TCP) or datagram (UDP).

• Header Added:
  • Source IP Address: The sender’s IP (e.g., 192.168.1.10).
  • Destination IP Address: The receiver’s IP (e.g., 203.0.113.45).
  • TTL (Time To Live): Limits how long the packet can exist in the network (e.g., 64).
  • Protocol: Identifies the transport layer protocol (e.g., 6 for TCP, 17 for UDP).
• Result: The segment is now a packet.

• Header Added:
  • Source MAC Address: The sender’s hardware address (e.g., 00:1A:2B:3C:4D:5E).
  • Destination MAC Address: The next hop’s hardware address (e.g., 00:1F:2E:3D:4C:5B).
  • EtherType: Identifies the network layer protocol (e.g., 0x0800 for IPv4).
• Trailer Added:
  • FCS (Frame Check Sequence): Detects errors in the frame.
• Result: The packet is now a frame.

• Conversion: The frame is converted into bits (1s and 0s) for transmission over physical media (e.g., Ethernet cable, Wi-Fi).
• Result: The frame is transmitted as electrical signals, light pulses, or radio waves.

+---------------------+
|   Application Data  |  <--- Original data (e.g., HTTP request)
+---------------------+
|   TCP/UDP Header    |  <--- Transport Layer
+---------------------+
|      IP Header      |  <--- Network Layer
+---------------------+
|  Ethernet Header   |  <--- Data Link Layer
+---------------------+
|      Payload        |  <--- Encapsulated data from higher layers
+---------------------+
|   Ethernet Trailer  |  <--- Data Link Layer (FCS)
+---------------------+

1. Encapsulation: Each layer adds its header (and trailer) as the packet moves down the stack.
2. Transmission: The physical layer sends the bits over the network.
3. Decapsulation: At the destination, each layer strips its header and processes the data, moving up the stack until the original data is reconstructed.

1. Application Layer: Creates an HTTP request (e.g., GET /index.html).
2. Transport Layer: Adds a TCP header (e.g., source port 54321, destination port 80).
3. Network Layer: Adds an IP header (e.g., source IP 192.168.1.10, destination IP 203.0.113.45).
4. Data Link Layer: Adds an Ethernet header (e.g., source MAC 00:1A:2B:3C:4D:5E, destination MAC 00:1F:2E:3D:4C:5B).
5. Physical Layer: Transmits the bits over the network.
6. Destination: The process reverses—each layer removes its header and passes the data up.

• Payload: The actual data being transmitted.
• MTU (Maximum Transmission Unit): The largest size a packet can be (typically 1500 bytes for Ethernet).
• Fragmentation: If a packet exceeds the MTU, it is split into smaller fragments and reassembled at the destination.

• Efficiency: Breaking data into packets allows multiple devices to share the network.
• Reliability: Packets can take different paths and be reassembled, improving fault tolerance.
• Scalability: Networks can handle varying amounts of traffic by managing packets individually.