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networking

Packet

A packet is a unit of data transmitted over a network. Each packet contains a header (source/destination addresses, protocol info, sequence numbers) and a payload (the actual data). IP packets are limited by MTU – typically 1500 bytes on Ethernet. Larger messages are split into multiple packets.

Definition

A packet is the fundamental transmission unit in IP networking. When an application sends data, the operating system's network stack breaks it into packets that can traverse routers independently. Each packet has an IP header (20+ bytes) containing source and destination IP addresses, TTL, protocol number, and identification fields. The payload carries the transport layer segment (TCP or UDP header + application data). Packets may arrive out of order, be duplicated, or be lost – TCP handles reordering and retransmission at the transport layer while UDP leaves this to the application. Routers forward packets independently based on destination IP, making packet-switched networks more resilient than circuit-switched alternatives.

Examples

  • –tcpdump -c 10 -i eth0 captures 10 packets
  • –Wireshark shows packet headers decoded by layer
  • –ping sends ICMP echo request packets and measures RTT

Related Protocols

iptcpudp

Related Terms

Frame

A frame is a Layer 2 (data link) unit of data that includes a header with MAC addresses, the payload (an IP packet), and a trailer with error-checking CRC. Ethernet frames have a maximum size of 1518 bytes. Frames are addressed by MAC, not IP – they do not cross router boundaries.

MTU (Maximum Transmission Unit)

MTU is the largest packet size a network link can carry without fragmentation. Ethernet MTU is 1500 bytes. If a packet exceeds the path MTU, it is fragmented (IPv4) or dropped with ICMP Packet Too Big (IPv6). Path MTU Discovery (PMTUD) finds the lowest MTU along a route.

Fragmentation

IP fragmentation splits packets larger than the link MTU into smaller fragments for transmission, reassembled at the destination. Fragmentation hurts performance – any single lost fragment requires retransmitting the entire original packet. IPv6 does not allow router fragmentation; only endpoints can fragment.