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What is Packet?

A unit of data for transmission: data split into pieces no larger than a set size, each with a header carrying control information such as the destination. Packets let many conversations share one line (packet switching) and let only the damaged part be resent after an error. Depending on the layer, the units are often called packets in IP, segments in TCP and frames in Ethernet. PCI Express and networks-on-chip also send data as packets.

Japanese name: パケット

Packet appears in 24 practice questions across 4 topics, including Computer Networking Basics, I/O Interfaces & Buses, Web Technology Basics.

Field: Computer Networking Basics

Try it: questions about Packet

In PCI Express, sending many packets that each write 64 bytes yields effective bandwidth well below the theoretical figure. What is the main reason?

  1. A64-byte payloads cannot be sent over PCI Express
  2. BEach packet carries headers, error-detection codes and other overhead, which are large relative to the data
  3. CSmall packets are always corrupted and resent every time
Show answer and explanation

Answer: B. Each packet carries headers, error-detection codes and other overhead, which are large relative to the data

Besides data, a PCI Express packet carries a header with address and type, a data-link sequence number and CRC, physical-layer framing and so on, about 20 to 30 bytes in total. With 64 bytes of data, roughly 30% of the bits sent are not data. Transfers of 256 bytes or more shrink this share. This is another reason to watch packet size, not just raw bandwidth.

In the OSI reference model, which layer uses logical addresses such as IP addresses to relay packets between different networks until they reach their destination?

  1. AData link layer (layer 2)
  2. BNetwork layer (layer 3)
  3. CTransport layer (layer 4)
Show answer and explanation

Answer: B. Network layer (layer 3)

The OSI reference model organizes communication functions into seven layers, from the physical layer up to the application layer. The network layer (layer 3) uses logical addresses such as IP addresses to choose a route across multiple networks and relay packets to the destination. The data link layer below it only delivers frames to a neighboring device within the same network, using MAC addresses. The transport layer handles communication between applications on the end hosts; TCP, for example, manages retransmission and ordering. In terms of equipment, routers operate at layer 3 and L2 switches at layer 2.

Packet in the explanations

Questions that mention Packet (24)

  1. In PCI Express, sending many packets that each write 64 bytes yields effective bandwidth well below the theoretical figure. What is the main reason?
  2. In the OSI reference model, which layer uses logical addresses such as IP addresses to relay packets between different networks until they reach their destination?
  3. Host A (192.168.1.10), inside a router performing NAPT whose outside global address is 198.51.100.7, connects from source port 50000 to port 443 on web server 203.0.113.5. The router translates the source to port 61000 on 198.51.100.7. What is the destination of the response packets the web server sends back?
  4. A host (192.168.1.10/24, default gateway 192.168.1.1) sends a packet to server 203.0.113.80 on a different network. With an empty ARP cache, whose MAC address does the host look up with ARP?
  5. A router's routing table has four routes: 0.0.0.0/0 (next hop router A), 10.0.0.0/8 (router B), 10.1.0.0/16 (router C) and 10.1.2.0/24 (router D). To which router is a packet for 10.1.5.9 sent?
  6. In the three-way handshake that establishes a TCP connection, what is the correct order of the packets exchanged?
  7. On 1 Gbps Ethernet, TCP data is sent continuously with IP packets filling the 1500-byte MTU. Taking the IPv4 header as 20 bytes, the TCP header as 20 bytes, the Ethernet header and FCS together as 18 bytes, and the preamble and inter-frame gap together as 20 bytes, which is closest to the upper limit on application-data throughput?
  8. Which correctly describes the basic function of a (packet-filtering) firewall?
  9. What is closest to the effective one-direction bandwidth of a PCI Express 3.0 x16 link (8 GT/s per lane, 128b/130b encoding)?
  10. How does communication in PCI Express work?
  11. What does the PCI Express data link layer do about bit errors during transmission?
  12. What is a characteristic of AXI4-Stream?
  13. Why have large SoCs adopted a network-on-chip (NoC) instead of a single shared bus?
  14. Which describes the credit-based flow control used in PCI Express?
  15. In the four-layer TCP/IP model, which layer do TCP and UDP belong to?

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Related terms

TCP (Transmission Control Protocol) Router IP (Internet Protocol) PCI Express IP address Ethernet CRC (cyclic redundancy check) OSI reference model ARQ (automatic repeat request) ARP

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