2022 Oct/Nov · Paper 32 · Q3 (a, b, c)6 marksProtocols & TCP/IP suite
2022 May/June · Paper 32 · Q3 (a, b)8 marksProtocols & TCP/IP suite
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school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Protocols & TCP/IP suite · Core · Paper 3 style · 4 marks
(a)Explain why protocols are needed for communication between devices.[2]
(b)Name the four layers of the TCP/IP protocol suite, from the top layer down.[2]
Show answer
(a) A protocol is an agreed set of rules (e.g. for data format, addressing, error checking, timing); devices from different manufacturers and with different software can only communicate if they follow the same rules.
1 mark set of rules for data transmission; 1 mark allows different devices/systems to communicate/understand each other.
(b) Application, Transport, Internet (Network), Link (Network access).
2 marks all four in the correct order; 1 mark four correct in the wrong order or three correct in order.
Q2Protocols & TCP/IP suite · Core · Paper 3 style · 6 marks
A web-design agency in Lahore uses several protocols.
(a)For each protocol, state the TCP/IP layer at which it operates: (i) HTTP (ii) TCP (iii) IP (iv) IMAP.[4]
(b)State which protocol would be used for each task: (i) uploading the pages of a website from a developer's laptop to a web server; (ii) sending an email from a mail client to the mail server.[2]
Show answer
(a) (i) Application (ii) Transport (iii) Internet (iv) Application.
1 mark each.
(b) (i) FTP (ii) SMTP.
1 mark each.
Q3Protocols & TCP/IP suite · Core · Paper 3 style · 5 marks
Amina in Karachi sends an email to her friend Kofi in Toronto.
(a)Describe the role of SMTP when Amina sends the email.[2]
(b)Kofi reads his emails on a phone and a laptop and wants both to show the same inbox. State which protocol his devices should use to retrieve email, and explain why.[3]
Show answer
(a) SMTP is a push protocol that transfers the email from Amina's mail client to her mail server, and then from her mail server to the recipient's mail server.
1 mark sends/pushes email from client to sender's server; 1 mark relays it between mail servers to the recipient's server.
(b) IMAP. Emails stay on the server and the devices synchronise with it, so reading, deleting or moving a message on one device is shown on the other. With POP3 messages are usually downloaded to one device and deleted from the server.
1 mark IMAP; 1 mark messages kept on the server / synchronised; 1 mark contrast with POP3 downloading and deleting.
Q4Protocols & TCP/IP suite · Core · Paper 3 style · 4 marks
A photographer in Nairobi uploads large image files to a print company's server.
(a)State the purpose of FTP.[1]
(b)Describe two features of FTP.[2]
(c)Give one reason why FTP should not be used to upload confidential files without additional security.[1]
Show answer
(a) To transfer files between a client and a server over a network.
B1.
(b) Client can log in with a username and password (or anonymously); supports commands to list, upload, download, rename and delete files on the server; uses separate control and data connections; can transfer in text or binary mode.
1 mark each for any two.
(c) Standard FTP sends data, usernames and passwords as plain text, so they could be intercepted and read.
B1.
Q5Protocols & TCP/IP suite · Core · Paper 3 style · 4 marks
(a)State the most suitable protocol for each situation: (i) a browser requesting a web page; (ii) retrieving emails so they are kept on the server for access from several devices; (iii) downloading pieces of a large file from many other users at once; (iv) moving a backup file to a remote server using a dedicated file-transfer client.[4]
Show answer
(a) (i) HTTP (ii) IMAP (iii) BitTorrent (iv) FTP
1 mark each.
Q6Protocols & TCP/IP suite · Core · Paper 3 style · 4 marks
(a)State the layer of the TCP/IP suite that is responsible for each task: (i) deciding the route a packet takes between networks; (ii) providing the email or web service that the user interacts with; (iii) converting frames into signals sent over the cable; (iv) numbering segments so they can be reassembled.[4]
Show answer
(a) (i) Internet (ii) Application (iii) Link (iv) Transport
1 mark each.
Q7Protocols & TCP/IP suite · Standard · Paper 3 style · 5 marks
(a)Compare POP3 and IMAP. Give three differences.[3]
(b)A field researcher works in remote areas of Gilgit-Baltistan with an internet connection only once a week, using a single laptop. Suggest which protocol is more suitable and justify your choice.[2]
Show answer
(a) POP3 downloads messages to the client and normally deletes them from the server; IMAP leaves them on the server. IMAP synchronises folders/read status across many devices; POP3 does not. POP3 messages can be read offline after download and use little server storage; IMAP needs a connection (or a cached copy) and uses server storage. IMAP can download headers only and fetch the body when opened.
1 mark each for any three valid differences (each must state both sides or be clearly comparative).
(b) POP3 – all messages are downloaded when connected and can then be read offline for the rest of the week on the single laptop; synchronisation across devices is not needed.
1 mark POP3; 1 mark justification linked to offline reading / one device.
Q8Protocols & TCP/IP suite · Standard · Paper 3 style · 6 marks
(a)Describe the role of the transport layer when a large file is sent across the internet using TCP.[4]
(b)Describe the role of the internet layer.[2]
Show answer
(a) Splits the data into segments (packets); adds a header with source/destination port numbers so data reaches the correct application; adds sequence numbers so the receiver can reassemble segments in order; receiver sends acknowledgements and missing segments are retransmitted, giving reliable delivery; also controls flow (rate of sending).
1 mark each for any four: segmentation; port numbers; sequence numbers / reassembly; acknowledgements; retransmission of lost segments; flow control; establishes the connection (handshake).
(b) Adds the IP header containing source and destination IP addresses, and routes each packet across networks (via routers) towards the destination.
1 mark IP addressing (source and destination IP in header); 1 mark routing packets between networks.
Q9Protocols & TCP/IP suite · Standard · Paper 3 style · 6 marks
(a)Describe the role of the link layer.[2]
(b)Explain two benefits of using a layered protocol suite such as TCP/IP.[4]
Show answer
(a) Places the data on the physical medium: puts packets into frames with MAC (hardware) addresses for the next device on the local network, and transmits them as signals over the cable or wireless link.
1 mark framing / MAC addresses of next hop on the local network; 1 mark physical transmission of bits over the medium; accept error detection on the link.
(b) Each layer has a separate job, so a layer's protocol can be changed or improved (e.g. replacing wired Ethernet with Wi-Fi at the link layer) without changing the other layers. Different manufacturers can develop hardware/software for one layer independently, and products still work together because the interfaces between layers are standard. Complex communication is broken into simpler parts that are easier to design and debug.
2 marks per benefit (identify + explain), any two.
Q10Protocols & TCP/IP suite · Standard · Paper 3 style · 6 marks
(a)A user in Islamabad types the address of a news website into a browser. Describe how the TCP/IP layers on the user's computer prepare the request for transmission.[5]
(b)State what happens to these headers at the web server.[1]
Show answer
(a) Application layer: the browser creates an HTTP GET request for the page. Transport layer: TCP opens a connection, splits the request into segments and adds a header with port numbers (e.g. destination port for HTTP) and sequence numbers. Internet layer: each segment is put in an IP packet with source and destination IP addresses. Link layer: each packet is put in a frame with MAC addresses and transmitted over the Wi-Fi/Ethernet link to the router.
1 mark HTTP request at application layer; 1 mark TCP segments with sequence numbers; 1 mark port numbers; 1 mark IP header with addresses; 1 mark link layer framing and transmission. Max 5.
(b) Each layer removes (strips) its own header and passes the data up to the layer above until the HTTP request reaches the web-server application.
B1.
Q11Protocols & TCP/IP suite · Standard · Paper 3 style · 6 marks
A university in Ghana distributes a large open-source operating-system image using BitTorrent.
(a)Define each of these BitTorrent terms: (i) swarm (ii) seed (iii) tracker (iv) leech.[4]
(b)Explain why BitTorrent is described as a peer-to-peer protocol.[2]
Show answer
(a) (i) Swarm: all the peers connected and sharing the same torrent/file. (ii) Seed: a peer that has the complete file and uploads pieces to others. (iii) Tracker: a server that keeps a list of the peers in the swarm and tells a new peer which peers to connect to. (iv) Leech: a peer that downloads much more than it uploads (has a poor share ratio), harming the swarm.
1 mark each. For leech also accept: a peer that is still downloading and does not yet have the complete file.
(b) Each computer (peer) downloads pieces of the file from other peers and also uploads pieces it has to others; there is no single central server sending the whole file.
1 mark peers both download and upload / act as client and server; 1 mark no central server supplies the file.
Q12Protocols & TCP/IP suite · Standard · Paper 3 style · 5 marks
An indie game studio in Lagos releases a free game update through BitTorrent.
(a)Describe how a new user downloads the file using BitTorrent.[5]
Show answer
(a) The user obtains the torrent file (metadata including the tracker address and the hashes of each piece) and opens it in a BitTorrent client. The client contacts the tracker, which returns a list of peers in the swarm. The client connects to several peers and requests different pieces simultaneously. Each piece is checked against its hash and re-requested if corrupt. As soon as it has pieces, the client uploads them to other peers. When all pieces are received the file is reassembled, and the user may stay as a seed.
1 mark each to max 5: torrent file with metadata; contacts tracker; receives list of peers; downloads different pieces from several peers in parallel; pieces verified with hash; uploads pieces it has to others; reassembly / becomes a seed.
Q13Protocols & TCP/IP suite · Standard · Paper 3 style · 6 marks
A college in Jeddah runs an online examination portal.
(a)Explain how the transport layer makes sure that data arriving at the server is passed to the correct application.[2]
(b)Describe how TCP makes sure all the data of an online exam submission arrives correctly, even if some packets are lost or arrive out of order.[4]
Show answer
(a) Each segment's header holds a destination port number that identifies the application/service (e.g. web server or mail server); the receiving computer uses the port number to pass the data to that application.
1 mark port numbers in transport-layer header; 1 mark port identifies the application/process on the host.
(b) Each segment has a sequence number; the receiver uses these to put segments back in the right order. The receiver sends acknowledgements for segments received. If the sender gets no acknowledgement before a timeout, it retransmits the segment. A checksum detects corrupted segments, which are discarded and resent.
1 mark each: sequence numbers; reordering at receiver; acknowledgements; timeout and retransmission; checksum for errors. Max 4.
Q14Protocols & TCP/IP suite · Standard · Paper 3 style · 5 marks
A tourism website promotes Hunza Valley.
(a)Describe what happens, in terms of HTTP, when a user clicks a link to a page on a web server.[3]
(b)State the TCP/IP layer at which HTTP operates and the transport-layer protocol it normally uses.[2]
Show answer
(a) The browser (client) sends an HTTP request (GET) with the address/resource of the page to the web server; the server processes the request and sends back an HTTP response containing a status code (e.g. 200 OK or 404 Not Found) and the HTML of the page; the browser renders the page and makes further requests for images, style sheets, etc.
1 mark HTTP request (GET) from client to server; 1 mark response with status code; 1 mark page content (HTML) returned and rendered / further requests.
(b) Application layer; TCP.
1 mark each.
Q15Protocols & TCP/IP suite · Standard · Paper 3 style · 6 marks
A packet travels from a laptop in Multan to a server in Frankfurt through 12 routers.
(a)Describe two items found in an IP packet header and explain why each is needed.[4]
(b)Explain why the destination IP address stays the same for the whole journey but the link-layer (MAC) address changes at every router.[2]
Show answer
(a) Source IP address – so a reply/error message can be returned to the sender. Destination IP address – so routers can forward the packet towards the correct host. Time to live (hop limit) – so a packet that cannot be delivered is discarded instead of circulating forever. Header checksum – to detect errors in the header. Protocol field – identifies the transport protocol (e.g. TCP) to pass the data to.
2 marks per item (name + reason), any two.
(b) The IP address identifies the final destination host, so it is not changed; the MAC address is only used on each local link, so each router re-frames the packet with the MAC address of the next device (next hop).
1 mark IP is end-to-end/final destination; 1 mark MAC addresses are local to each link, replaced at each hop.
Q16Protocols & TCP/IP suite · Challenge · Paper 3 style · 5 marks
(a)Explain the process of encapsulation in the TCP/IP suite, and explain why it means a router does not need to process the application data.[5]
Show answer
(a) Data from the application layer is passed down; each layer treats what it receives as its payload and adds its own header (TCP header, then IP header, then link-layer header and trailer). At the receiver, each layer removes its header and passes the payload up. A router works only up to the internet layer: it removes the link-layer frame, reads the IP header to choose the next hop, and re-frames the packet; it never opens the TCP segment or the application data inside, so it does not need to understand HTTP, SMTP etc.
1 mark data passed down the layers; 1 mark each layer adds a header (and trailer at link layer) to the data from the layer above; 1 mark headers removed in reverse order at the receiver; 1 mark router only uses link and internet layers / reads IP header; 1 mark payload above IP left unchanged/unread.
Q17Protocols & TCP/IP suite · Challenge · Paper 3 style · 6 marks
A 1.5 GiB video of a science lecture series is shared by BitTorrent with a piece size of 512 KiB.
(a)Calculate the number of pieces the file is split into.[2]
(b)Explain why a client requests the rarest pieces in the swarm first.[2]
(c)Explain why BitTorrent can deliver this file to 10 000 users faster than a single web server using HTTP.[2]
Show answer
(a) 3072 pieces
M1: 1.5 × 1024 × 1024 KiB ÷ 512 KiB; A1: 3072.
(b) If a rare piece is held by only one or two peers who then leave, nobody can complete the file; downloading and re-sharing rare pieces first spreads them so the swarm keeps every piece available.
1 mark rare pieces could be lost if their holders leave; 1 mark spreading them keeps the whole file available / speeds up overall distribution.
(c) With HTTP every user downloads from one server whose upload bandwidth is shared by all users; with BitTorrent every peer also uploads, so total upload capacity grows as more users join.
1 mark single server bandwidth is a bottleneck; 1 mark peers contribute upload bandwidth / load spread across many computers.
Q18Protocols & TCP/IP suite · Challenge · Paper 3 style · 6 marks
A 6000-byte message is sent using TCP/IP. Each segment can carry at most 1460 bytes of message data, and each segment has 40 bytes of TCP and IP headers in total. Ignore link-layer headers.
(a)Calculate the number of segments needed to send the message.[1]
(b)Calculate the total number of bytes transmitted, including headers.[2]
(c)Calculate the percentage of transmitted bytes that are message data. Give your answer correct to 2 decimal places.[2]
(d)Explain why the last segment is less efficient than the others.[1]
Show answer
(a) 5
B1: 6000 ÷ 1460 = 4.11, rounded up to 5.
(b) 6200 bytes
M1: 6000 + 5 × 40; A1: 6200.
(c) 96.77 %
M1: 6000 ÷ 6200 × 100; A1: 96.77 %.
(d) It carries only 160 bytes of data but still needs the full 40-byte header.
B1.
Q19Protocols & TCP/IP suite · Challenge · Paper 3 style · 6 marks
Bilal, whose email provider is based in Islamabad, sends an email to Fatima, whose email address is with a company based in London.
(a)Describe how the email travels from Bilal's computer to Fatima's phone. Refer to the protocols used and to the role of DNS.[6]
Show answer
(a) Bilal's mail client uses SMTP to send the email to his mail server (the Islamabad provider). The sending server uses DNS to look up the mail (MX) record for Fatima's domain and find the IP address of her mail server. It then uses SMTP to transfer the email to Fatima's mail server, where it is stored in her mailbox. Fatima's phone uses IMAP (or POP3) to retrieve the email from her mail server. Underneath, TCP and IP carry the data, split into packets routed across the internet.
1 mark each to max 6: SMTP client to sender's server; DNS lookup of recipient's domain (mail server record); server-to-server transfer with SMTP; stored on recipient's server/mailbox; retrieval by IMAP/POP3; reference to TCP/IP packets across the internet.
Q20Protocols & TCP/IP suite · Challenge · Paper 3 style · 6 marks
A gaming company in Riyadh must deliver a 4 GiB update to 200 000 players on the same day.
(a)The company is deciding whether to distribute the 4 GiB update with HTTP from its own servers or with BitTorrent. Discuss the advantages and disadvantages of BitTorrent for this company.[6]
Show answer
(a) Advantages: the load is shared between peers, so the company needs far less server bandwidth and cost; download speed can increase as more users join; no single point of failure. Disadvantages: needs enough seeds – early in the release, or for unpopular files, downloads may be slow; users' upload bandwidth and data allowances are used, which customers may dislike; some networks block BitTorrent traffic; the company has less control/monitoring of who downloads; peers' IP addresses are visible to the swarm (privacy).
Max 4 for one side only. 1 mark per valid point, max 6, at least one advantage and one disadvantage, related to the scenario.
Q21Circuit & packet switching · Core · Paper 3 style · 5 marks
(a)Describe circuit switching.[2]
(b)Describe packet switching.[3]
Show answer
(a) A dedicated communication path (circuit) is set up between the two ends before any data is sent; it is reserved for the whole communication and released at the end.
1 mark dedicated path set up before transmission; 1 mark reserved for the whole session and all data follows the same route.
(b) The data is split into packets, each with a header containing the destination address and sequence number. Packets are sent independently and may take different routes, decided by routers. They are reassembled in order at the destination.
1 mark split into packets with headers; 1 mark each packet routed independently / may take different routes; 1 mark reassembled using sequence numbers.
Q22Circuit & packet switching · Core · Paper 3 style · 4 marks
A telephone company in Peshawar compares switching methods.
(a)Give two advantages of circuit switching for a live telephone call.[2]
(b)Give two disadvantages of circuit switching.[2]
Show answer
(a) Data arrives in order along a dedicated route; constant bandwidth and very little delay/jitter, so speech is not broken up; no reassembly needed.
1 mark each for any two.
(b) The circuit's bandwidth is wasted when no data is being sent (e.g. silences); the circuit must be set up first, which takes time; if a link fails the connection is lost; no other user can use the reserved channel.
1 mark each for any two.
Q23Circuit & packet switching · Core · Paper 3 style · 4 marks
A farm in Punjab is choosing how its soil sensors will report to a central computer.
(a)Describe the three stages of a communication that uses circuit switching.[3]
(b)State one reason why the set-up stage makes circuit switching unsuitable for sending many short messages, such as sensor readings every few seconds.[1]
Show answer
(a) 1 Circuit establishment: a dedicated path through the switches/exchanges is set up between sender and receiver. 2 Data transfer: all data is sent along this path, in order, using the reserved bandwidth. 3 Circuit disconnection: when the communication ends the path is released so its links can be used by others.
1 mark each stage described.
(b) Setting up (and releasing) a circuit takes time and resources each time, which is large compared with the tiny amount of data sent.
B1.
Q24Circuit & packet switching · Standard · Paper 3 style · 6 marks
(a)Explain the role of a router in a packet-switched network.[3]
(b)Explain why packets from the same message may arrive out of order, and how the receiver deals with this.[3]
Show answer
(a) A router receives a packet, reads the destination IP address in its header, looks it up in its routing table and forwards the packet to the next router (next hop) on the best available route; it can choose another route if a link is busy or has failed.
1 mark reads destination address; 1 mark uses routing table; 1 mark forwards to next hop / chooses route depending on traffic or failures.
(b) Routers choose routes for each packet independently, so packets can take different paths with different lengths and congestion; the receiver uses the sequence numbers to reassemble them in the correct order, and asks for any missing packet to be resent.
1 mark different routes; 1 mark different delays/congestion; 1 mark reorder using sequence numbers (request missing packets).
Q25Circuit & packet switching · Standard · Paper 3 style · 4 marks
(a)For each use, state whether circuit switching or packet switching is more suitable and justify your choice: (i) downloading a 200 MB software update; (ii) a traditional landline call between Karachi and Quetta.[4]
Show answer
(a) (i) Packet switching – no need for a dedicated line; the network bandwidth is shared efficiently; lost or delayed packets can be resent and data reassembled; delay does not matter for a download. (ii) Circuit switching – a dedicated channel gives constant bandwidth and no delay or jitter, which real-time speech needs.
1 mark each choice; 1 mark each justification.
Q26Circuit & packet switching · Standard · Paper 3 style · 5 marks
(a)State three items of data that are found in the header of each packet in a packet-switched network.[3]
(b)Explain why packet switching uses network capacity more efficiently than circuit switching.[2]
Show answer
(a) Any three: source IP address; destination IP address; sequence number / packet number; total number of packets; packet length; time to live (hop count); protocol; checksum.
1 mark each.
(b) Links are shared – packets from many users are interleaved on the same links and no bandwidth is reserved while a user is silent or idle, so capacity is used whenever there is data to send.
1 mark links shared by many communications; 1 mark no capacity reserved/wasted during gaps.
Q27Circuit & packet switching · Standard · Paper 3 style · 4 marks
(a)A video-conference between doctors in Karachi and Kuala Lumpur uses the internet. Explain why packet switching on the internet can cause problems for real-time video, and describe one way the problem is reduced.[4]
Show answer
(a) Packets take different routes and meet different congestion, so they arrive with variable delay (jitter), out of order, or are lost; real-time video cannot wait for retransmission, so the picture freezes or breaks up. Reduced by buffering packets at the receiver before playing, by giving video packets priority (quality of service), or by lowering resolution/compression when bandwidth drops.
1 mark variable delay/jitter; 1 mark lost or out-of-order packets; 1 mark effect on real-time video; 1 mark a valid method of reducing it.
Q28Circuit & packet switching · Challenge · Paper 3 style · 6 marks
A hospital in Peshawar sends a 2 000 000-byte scan to a specialist using a packet-switched network. Each packet carries at most 1480 bytes of data and a 20-byte header.
(a)Calculate the minimum number of packets needed to send the file.[2]
(b)Calculate the total number of bytes sent, including headers.[2]
(c)One router on the route fails during the transfer. Explain why the transfer can still complete with packet switching but a circuit-switched transfer would fail.[2]
Show answer
(a) 1352 packets
M1: 2 000 000 ÷ 1480 = 1351.35…; A1: rounded up to 1352.
(b) 2 027 040 bytes
M1: 2 000 000 + 1352 × 20; A1: 2 027 040.
(c) Routers send the remaining packets by another route and missing packets are retransmitted; in circuit switching the single dedicated path is broken and the circuit must be set up again.
1 mark rerouting of packets around the failure; 1 mark circuit uses one fixed path which is lost.
Q29Circuit & packet switching · Challenge · Paper 3 style · 4 marks
(a)Calculate the value of the time-to-live (TTL) field when the packet arrives at the destination, if it started at 64 and passed through 9 routers, each of which reduces the TTL by 1.[1]
(b)Explain the purpose of the TTL field in a packet-switched network.[2]
(c)Another packet starts with TTL 8 and the route to its destination has 11 routers. State what happens to it.[1]
Show answer
(a) 55
B1: 64 − 9 = 55.
(b) It limits the number of hops a packet can make; when it reaches 0 the router discards the packet, so packets caught in a routing loop or that cannot be delivered do not circulate forever and waste bandwidth.
1 mark decremented at each router and discarded at 0; 1 mark stops packets circulating indefinitely / congestion.
(c) It is discarded by the 8th router (TTL reaches 0) and never arrives; the sender may be notified with an error message.
B1.
Q30Circuit & packet switching · Challenge · Paper 3 style · 6 marks
A circuit-switched call reserves a 64 kbps channel for its whole length of 10 minutes. The callers are speaking for only 40 % of the time.
(a)Calculate the amount of data capacity, in megabytes, that is reserved for the call. 1 MB = 1 000 000 bytes.[2]
(b)Calculate the amount of reserved capacity, in megabytes, that is wasted.[1]
(c)Explain how a packet-switched voice-over-IP call would avoid this waste, and describe one problem packet switching can cause for voice calls.[3]
(c) During silences VoIP sends few or no packets, so the shared links carry other users' packets instead. Problem: packets can be delayed by different amounts (jitter) or lost, causing gaps, echoes or out-of-sync speech; buffering adds delay.
1 mark no packets/bandwidth used in silence; 1 mark capacity shared with others; 1 mark problem: latency/jitter/lost packets with effect on speech.
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
