Packets

Fundamental unit of data transmission in packet-switched networks - data is broken into smaller chunks for efficient routing and delivery
Each packet contains both header information (addressing, control data) and payload (actual user data)
Packets travel independently through the network and may take different paths to reach the same destination
Receiving device reassembles packets in correct order using sequence numbers in headers

Packet Structure Components

Header: Contains source/destination addresses, protocol information, sequence numbers, error checking
Payload/Data: The actual information being transmitted (web page, email, file data)
Trailer (when present): Additional error checking information, end-of-frame markers

Layer-Specific Packet Names

OSI Layer	Packet Name	Example Protocols	Key Information Added
Physical (Layer 1)	Bits	Ethernet, Wi-Fi	Electrical signals, radio waves
Data Link (Layer 2)	Frames	Ethernet, PPP	MAC addresses, error detection
Network (Layer 3)	Packets	IP, ICMP	IP addresses, TTL, fragmentation
Transport (Layer 4)	Segments/Datagrams	TCP/UDP	Port numbers, sequence numbers

Encapsulation Process

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Application Data
       ↓
[Add TCP Header]
   Segment
       ↓
[Add IP Header]
    Packet
       ↓
[Add Frame Header]
     Frame
       ↓
  Physical Bits

┌─────────────────┐
│Frame│IP│TCP│Data│
└─────────────────┘
  L2   L3 L4  App

Data moves down the OSI stack, each layer adds its own header (and sometimes trailer)
Application data becomes a segment at Transport layer (TCP adds sequence numbers, port info)
Segment becomes a packet at Network layer (IP adds source/destination IP addresses)
Packet becomes a frame at Data Link layer (Ethernet adds MAC addresses, frame check sequence)
Frame becomes bits at Physical layer for transmission

Packet Switching Benefits

Efficiency: Multiple conversations can share same network links simultaneously
Reliability: If one path fails, packets can be rerouted through alternate paths
Scalability: Network can handle varying traffic loads by distributing packets
Error Recovery: Individual packets can be retransmitted if lost or corrupted

Vocabulary

MTU (Maximum Transmission Unit): Largest packet size that can be transmitted over a network segment (Ethernet standard is 1500 bytes)
Fragmentation: Process of breaking large packets into smaller pieces when crossing networks with smaller MTUs
Jitter: Variation in packet arrival times (critical for voice/video applications)
Latency: Time delay for packet to travel from source to destination
Packet Loss: Percentage of packets that fail to reach their destination

Notes

Remember the acronym “Please Do Not Throw Sausage Pizza Away” for OSI layers and corresponding packet names (Physical-bits, Data Link-frames, Network-packets, Transport-segments)
Ethernet frames have minimum size of 64 bytes - smaller packets are padded to meet this requirement
TCP segments are connection-oriented and reliable, UDP datagrams are connectionless and faster but unreliable
Use show interface to check MTU settings on Cisco devices
Packet capture tools like Wireshark show the encapsulation process in action - excellent for troubleshooting
In IPv4, packets can be fragmented by routers; in IPv6, only the source can fragment packets
QoS (Quality of Service) policies prioritize certain types of packets over others for better performance