Information Technology • Communications Technology

Packet Journey

A web request, step by step

Packet Journey — your review

Packet Journey — Explanation

What happens when you open a web page? Choose a view in the title bar.

Journey. Play the five stages, or click one to replay it.

  1. DNS lookup. Your laptop knows the domain name (www.learnitwithme.com) but needs the server's IP address. It asks a DNS resolver (at your internet service provider), which answers 203.0.113.42. The answer is cached, so the next lookup is instant.
  2. Connect and request. TCP sets up a connection (the three-way handshake: SYN, SYN-ACK, ACK), HTTPS sets up encryption, then the browser sends an HTTP GET request.
  3. Split into packets. The server's TCP splits the page into packets. Each has a header: the source and destination IP addresses, and a sequence number (1 of 6, 2 of 6…), as well as the data (the payload).
  4. Routing. Each router reads a packet's destination IP address and forwards it to the next hop, choosing the best route at that moment. So packets of one message can take different routes and arrive out of order. A busy router may drop one.
  5. Reassembly. Your laptop's TCP puts the packets back in order by sequence number. It acknowledges what arrives; a packet with no acknowledgement is resent by the server. Then the browser shows the page.

Packet switching against circuit switching (the second toggle). In circuit switching (the old telephone network) a dedicated path is set up first, all the data follows it in order, and it's released at the end: no reassembly, a steady rate for live calls, but the path is reserved even when nothing is sent, and if a link fails the circuit is lost. Packet switching (the internet) shares every link: efficient, and packets route round a broken link, but they can arrive out of order or be delayed, so they need sequence numbers.

Load time. Bandwidth is how much data can be sent each second (megabits per second, Mbps). Latency is the delay before data arrives (milliseconds): a round trip takes 2 × latency. A page needs several round trips before any of it arrives (DNS, TCP, HTTPS, the request), then its transfer time: size × 8 ÷ bandwidth. So for a small page latency matters most, and for a big download bandwidth does: compare the three pages, and the "double the bandwidth" and "halve the latency" savings under each. Packet loss adds resends, each costing another round trip. The model is simplified (real TCP starts slowly and sends many packets before waiting), but the pattern is right.

Protocols are the agreed rules for communicating. The TCP/IP model has four layers: application (HTTP, HTTPS, FTP, SMTP, IMAP, POP3, DNS), transport (TCP), internet (IP) and link (Ethernet, WiFi). Click each one for what it does.

Common exam mistakes: saying the sender chooses the packets' routes (each router does, hop by hop); saying packets arrive in order (they're reordered by sequence number); mixing up bandwidth and latency, or thinking more bandwidth always speeds up a page; confusing SMTP (sending email) with IMAP and POP3 (fetching it); saying HTTPS is faster (it's more secure); forgetting that 1 byte = 8 bits in a transfer-time calculation; saying circuit switching uses bandwidth efficiently.

Objective: Cambridge International A Level Information Technology (9626), communications technology: how data is sent across networks (packets and their headers, routing, packet switching against circuit switching), bandwidth and latency, and the protocols HTTP, HTTPS, TCP/IP, FTP, SMTP, IMAP and POP3. DNS and IP addresses are also AS Level networks.

Where this fits

  • AP: AP Computer Science Principles
  • AQA: AQA A Level Computer Science (7517)
  • Cambridge: Cambridge IGCSE Computer Science (0478); Cambridge A Level Computer Science (9618); Cambridge AS Level Computer Science (9618); Cambridge A Level Information Technology (9626) Goes beyond Cambridge IGCSE Computer Science (0478): TCP, circuit switching, bandwidth and latency go beyond 0478. Goes beyond Cambridge AS Level Computer Science (9618): Circuit switching and TCP are A Level (14.1-14.2), not AS.
  • IB: IB Computer Science HL; IB Computer Science SL; IB Digital Society HL; IB Digital Society SL
  • NCEA Level 2 Digital Technologies: 91895 Use advanced techniques to develop a network; 91895 Use advanced techniques to develop a network
  • Pearson Edexcel International: Edexcel International GCSE Computer Science (4CP0); Edexcel International GCSE ICT (4IT1); Edexcel International A Level Information Technology Goes beyond Edexcel International GCSE ICT (4IT1): DNS, TCP and packet routing go beyond 4IT1; bandwidth and latency are in it.
  • USDP: USDP Computer Science

Packet Journey — Key Terms

Key concepts in English, with te reo Māori, Chinese (Simplified) and Korean.

EnglishTe reo Māori中文（简体）한국어What it means on this page
DNS (Domain Name System)no attested term域名系统 (yùmíng xìtǒng)도메인 네임 시스템 (domein neim siseutem)The system that turns a domain name, such as www.learnitwithme.com, into the IP address of the server, so that a computer can contact it.
IP Addressno attested termIP地址 (IP dìzhǐ)IP 주소 (IP juso)An address given to a device by the network it joins, used to send data to it across networks; it changes if the device moves to another network.
Packetno attested term数据包 (shùjùbāo)패킷 (paeket)A small piece of data sent across a network, with a header (source and destination IP addresses and a sequence number) and the data itself (the payload).
Sequence Numberno attested term序列号 (xùlièhào)시퀀스 번호 (sikwonseu beonho)The number in a packet's header showing its place in the message, so the receiving computer can put the packets back in order and notice any that are missing.
Routerno attested term路由器 (lùyóuqì)라우터 (rauteo)A device that joins networks (such as a LAN and the internet) and forwards data packets between them, using IP addresses.
Packet Switchingno attested term分组交换 (fēnzǔ jiāohuàn)패킷 교환 (paeket gyohwan)Sending data as packets that are each routed independently, possibly by different routes, and reassembled in order at the destination; the links are shared.
Circuit Switchingno attested term电路交换 (diànlù jiāohuàn)회선 교환 (hoeseon gyohwan)Setting up a dedicated path between two devices before any data is sent; all the data follows that path in order, and it is reserved until it is released.
Bandwidthno attested term带宽 (dàikuān)대역폭 (daeyeokpok)How much data can be sent across a connection each second, measured in bits per second (for example megabits per second, Mbps).
Latencyno attested term延迟 (yánchí)지연 시간 (jiyeon sigan)The delay before data arrives: the time a signal takes to travel across a network, in milliseconds; a round trip takes about twice as long.
Protocolno attested term协议 (xiéyì)프로토콜 (peurotokol)An agreed set of rules for how devices communicate, such as how data is formatted, addressed, sent and checked.
TCP/IPno attested termTCP/IP协议 (TCP/IP xiéyì)TCP/IP 프로토콜 (TCP/IP peurotokol)The protocols the internet is built on: TCP splits data into numbered packets, checks they all arrive and puts them in order; IP addresses and routes each packet.
HTTPSno attested term超文本传输安全协议 (chāowénběn chuánshū ānquán xiéyì)보안 하이퍼텍스트 전송 프로토콜 (boan haipeotekseuteu jeonsong peurotokol)Hypertext Transfer Protocol Secure: web pages sent over an encrypted connection, shown by https:// and a padlock in the address bar.
FTPno attested term文件传输协议 (wénjiàn chuánshū xiéyì)파일 전송 프로토콜 (pail jeonsong peurotokol)File Transfer Protocol: the protocol for transferring files between a client and a server, for example uploading a website's files to its web server.
SMTPno attested term简单邮件传输协议 (jiǎndān yóujiàn chuánshū xiéyì)간이 우편 전송 프로토콜 (gani upyeon jeonsong peurotokol)Simple Mail Transfer Protocol: the protocol for sending email, from an email program to a mail server and from one mail server to the next.
IMAPno attested term互联网消息访问协议 (hùliánwǎng xiāoxī fǎngwèn xiéyì)인터넷 메시지 액세스 프로토콜 (inteonet mesiji aekseseu peurotokol)Internet Message Access Protocol: the protocol for fetching email that leaves the messages on the mail server, so every device shows the same messages; POP3 instead downloads them to one device.

On the te reo Māori column. Terms marked as gaps have no attested equivalent in the sources checked — Karaitiana Taiuru's Dictionary of Māori Computer and Social Media Terms, Paekupu, the Reserve Bank's te reo financial glossary, NZQA and Te Aka. No coinage is printed as though it were established; where a class needs one, commission it from Te Taura Whiri i te Reo Māori and credit the translator. Te reo Māori is not italicised and takes no plural "s".