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Worksheet

A12.4 The TCP/IP stack and protocols

Networks and the web · A level · OCR H446 1.3.3, AQA 7517 4.9.4.1, Eduqas A500QS 2.2 · about 30 min

BugBotLab
NameClassDate

What this lesson is about

The four layers, encapsulation, ports and sockets, and HTTP, HTTPS, FTP, SMTP, POP3 and SSH.

Questions 6 marks in all

  1. [1 mark]Put the layers of the TCP/IP stack in order, from the top (nearest the application) to the bottom.

    Number the lines 1 to 4 to put them in the right order.

    1. Link
    2. Network
    3. Transport
    4. Application

  2. [1 mark]Which layer adds the source and destination port numbers?

    1. ATransport
    2. BApplication
    3. CNetwork
    4. DLink

  3. [1 mark]What is a socket?

    1. AThe combination of an IP address and a port number, one endpoint of a connection
    2. BA physical connector on a network card
    3. CA 48-bit hardware address
    4. DThe list of rules in a firewall

  4. [1 mark]On which port does a web server listen for HTTPS by default?

  5. [1 mark]Which protocol does an email client use to download messages from its mail server, usually deleting them from the server?

    1. APOP3
    2. BSMTP
    3. CFTP
    4. DSSH

  6. [1 mark]As a packet crosses several routers on the Internet, which addresses are replaced at every hop?

    1. AThe MAC addresses in the link layer header
    2. BThe IP addresses in the network layer header
    3. CThe port numbers in the transport header
    4. DNone of them change

The task: wrap it and unwrap it

The robot at 192.168.4.23 asks the Server at 192.168.4.1 for its battery level. Write checksum(text): its input is any string; it returns the sum of the character codes (ord) of every character in text, modulo 256, as two uppercase hexadecimal digits (format(n, "02X")). 1. Build and send one packet: the body <source IP>,<destination IP>,<source port>,<destination port>,<data> with source port 49152, destination port 80 and data GET /battery, then *, then the body's checksum. 2. The Server replies with a packet in the same format. Wait for it, checking messages() every 0.1 s. 3. Split off the checksum after the last *. If the body's checksum matches, print checksum ok, then from socket <source IP>:<source port>, then data: <data>. The data is everything after the fourth comma. If it does not match, print checksum wrong. The robot does not drive.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def checksum(text):
    return "00"

send("192.168.4.23,192.168.4.1,49152,80,GET /battery")

Plan your program here, then type it in and press Run.

QR code
Do it on the robot
www.bugbotlab.com/learn/a12-4-tcp-ip-and-protocols/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

  1. Swap two characters in a body. Does the checksum notice? Why not, and what does that say about this checksum?
  2. Add a TTL field to the packet and a router function that decrements it and drops the packet at 0.
  3. Look up which port IMAP uses and why many people now use it instead of POP3.