Stretch tasks Protocols and Layers

Round 1: Quiz Blitz

1. SMTP SMTP sends, IMAP retrieves.
2. imap Leaving mail on the server keeps every device in sync.
3. Checks every packet arrives and puts them back in order TCP numbers, acknowledges and resends packets.
4. A resent packet would arrive too late to be useful Waiting for resends would add delay to the whole call.
5. Transport The transport layer splits data into packets and manages the connection.
6. Network IP works at the network layer.
7. True Intercepted HTTPS traffic cannot be read.
8. False Layering makes networking manageable and lets products from different makers work together.
9. Ethernet; Wi-Fi The link layer sends data over the physical medium.
10. Only the link layer HTTP, TCP and IP carry on exactly as before.
11. False IP addresses and routes packets. TCP checks delivery.
12. protocol Both ends must follow the same rules.

Round 2: Quick Fire

1. • SMTP sends email
   • IMAP retrieves email, leaving it on the server
   Model answer

   SMTP is used to send email from a client to a mail server. IMAP is used to retrieve email, leaving it on the server so it stays in sync across devices.

2. • TCP acknowledges every packet and resends lost ones
   • A missing part would leave the page incomplete or corrupt
   Model answer

   TCP acknowledges every packet and resends any that are lost, then reassembles them in order. A web page needs every byte to arrive, because a missing part would leave the page incomplete or corrupt.

3. • Each layer wraps the data in its own header
   • The receiver unwraps the headers in reverse order
   Model answer

   Each layer wraps the data it receives from the layer above in its own header before passing it down. The receiving computer unwraps the headers in the reverse order, so each layer only deals with the header its counterpart added.

4. • HTTPS encrypts the data in transit
   • So an intercepted password cannot be read
   Model answer

   HTTPS encrypts the data travelling between the browser and the server, so if the username and password are intercepted, they cannot be read without the key.

Round 3: Big Think

1. • Live matches should use UDP
   • A lost packet would arrive too late, and resending adds delay
   • A few lost packets only cause a brief glitch
   • UDP sends no acknowledgements, so it has less overhead
   • Downloaded films should use TCP
   • TCP acknowledges packets, resends lost ones and reassembles them in order
   • Every byte must arrive, or the file would be corrupt
   • A download is not time critical, so waiting for a resend is fine
   Model answer

   The live matches should use UDP. A lost packet would arrive too late to be useful, and waiting for it to be resent would add delay to the whole stream, whereas a few lost packets only cause a brief glitch. UDP also sends no acknowledgements, so it has less overhead. The downloaded films should use TCP, which acknowledges every packet, resends any that are lost and reassembles them in order. Every byte must arrive, or the file would be corrupt, and since nobody is watching it live, waiting a few seconds for a resend does not matter.

2. • It divides a huge problem into manageable parts
   • Each layer handles one part of the job
   • Layers can be changed independently
   • For example, Ethernet to Wi-Fi changes only the link layer
   • Different manufacturers can specialise in one layer
   • Products work together if they follow the standard
   • Faults are easier to isolate
   • For example, a site loading by IP address but not by name
   Model answer

   Layering divides the huge problem of networking into manageable parts, with each layer handling one part of the job. Each layer can be changed independently: moving a laptop from Ethernet to Wi-Fi changes only the link layer, while HTTP, TCP and IP carry on exactly as before. Different manufacturers can specialise in one layer, and as long as each follows the standard for its layer, their products work together. Faults are also easier to isolate: if a site loads by its IP address but not by its name, the problem is name resolution, not the cable.

This or That

1. SMTP SMTP sends. IMAP retrieves, leaving the mail on the server.
2. IMAP IMAP leaves the mail on the server, so every device sees the same inbox.
3. UDP A late position is useless, so resending it would only add lag.
4. TCP Every byte must arrive, so lost packets must be resent.
5. Application HTTP provides a service to the browser, so it is an application layer protocol.
6. No: TCP does the checking IP addresses and routes packets. TCP acknowledges them and resends any that are lost.
7. No: it is HTTP with encryption HTTPS does the same job as HTTP, but intercepted traffic cannot be read.
8. FTP FTP is the File Transfer Protocol. SMTP sends email.

Memory Match

• A 2 TCP: Checks delivery and reassembles packets in order
• B 5 UDP: Sends packets with no checking or resending
• C 1 IP: Addresses packets and routes them between networks
• D 6 SMTP: Sends email
• E 3 IMAP: Retrieves email, leaving it on the server
• F 4 HTTPS: Web pages, encrypted in transit

Sort It

1. Transport TCP TCP splits data into packets and checks delivery: transport layer.
2. Link Wi-Fi Wi-Fi carries frames over radio: link layer.
3. Application HTTP HTTP delivers web pages to the browser: an application layer service.
4. Link Ethernet Ethernet carries frames over cable: link layer.
5. Transport Splits data into packets and manages the connection That is the transport layer's job.
6. Link Sends the data over the physical medium That is the link layer's job.
7. Application Provides services to the user's software That is the application layer's job.
8. Network IP IP addresses and routes packets: network layer.
9. Application IMAP IMAP retrieves email for the user's mail program: application layer.
10. Application SMTP Email protocols serve the user's software, so they are application layer.
11. Network Adds addresses and routes packets between networks That is the network layer's job.
12. Transport UDP UDP is the transport layer's fast, unchecked option.