The chapter 1 to 3 topics of introductory computer networking courses built on the top-down approach (Kurose and Ross). Every formula links to the guide that derives it, and every number in the mixed practice was recomputed in code.
Read the map first, then the formula sheet. Use the seven-day plan if you have a week, or start straight at the mixed practice if you have one evening. Every wrong answer points you to the guide to re-read.
What a chapters 1 to 3 midterm covers#
| Chapter | Topics | Guide |
|---|---|---|
| 1. Computer networks and the Internet | Edge and core, access networks, packet vs circuit switching (FDM, TDM), the network of networks | Packet switching vs circuit switching |
| The four delays, traffic intensity, loss, traceroute, throughput and the bottleneck | The four network delays, loss and throughput | |
| Protocols, the 5-layer stack, OSI, encapsulation, history | Protocol layers and encapsulation | |
| 2. The application layer | Client-server vs P2P, sockets, ports, what apps need from transport | How network applications talk |
| HTTP request and response messages, methods, status codes | HTTP request and response messages | |
| Persistent vs non-persistent HTTP, RTT arithmetic, web caches, conditional GET | Persistent vs non-persistent HTTP | |
| Cookies and state | Cookies and HTTP state | |
| Email: SMTP, POP3, IMAP | Email: SMTP, POP3 and IMAP | |
| DNS hierarchy, queries, caching, resource records | DNS: hierarchy and resource records | |
| Capturing and reading HTTP and DNS in Wireshark | Capturing HTTP with Wireshark | |
| 3. The transport layer | Multiplexing, demultiplexing, UDP, the checksum | Multiplexing, demultiplexing and UDP |
| Principles of reliable data transfer, rdt1.0 to rdt3.0 | Reliable data transfer principles | |
| TCP segment, sequence and ACK numbers, retransmission, fast retransmit | TCP sequence numbers, ACKs and retransmission | |
| Flow control, three-way handshake, closing a connection | TCP flow control and connection management |
Courses trim chapter 3 differently. Many cover Go-Back-N, Selective Repeat, RTT estimation and congestion control; some leave them for later. Your syllabus, module objectives and lecture slides are the final word on scope. If a topic is not in them, it is unlikely to be on your exam.
The five kinds of question you will see#
| Type | Example | How to answer |
|---|---|---|
| Define or compare | “What is the difference between FDM and TDM?” | Give both definitions, then the one difference that matters, in one sentence. |
| Calculate | Delays, circuit transfer time, throughput, HTTP response time, stop-and-wait utilization, rwnd | Formula, numbers with units, answer. Convert bytes to bits and km to m first. |
| Trace | Checksum in binary, sequence and ACK numbers, handshake segments | Write every line. Most marks are for the method. |
| Which one? | “Which layer?”, “which protocol?”, “TCP or UDP?” | Name it and give the one fact that decides it. |
| Read a capture | A Wireshark screenshot of HTTP or DNS | Find the header field asked for; state what it means. |
Every formula, on one sheet#
| What | Formula | Notes |
|---|---|---|
| Transmission delay | dtrans = L / R | L in bits, R in bits per second |
| Propagation delay | dprop = d / s | d in meters, s about 2 to 3 × 108 m/s |
| Nodal delay | dproc + dqueue + dtrans + dprop | At every router, per packet |
| Store-and-forward, N links | N × L / R | Ignoring propagation and queuing |
| Traffic intensity | La / R | Keep below 1 |
| Throughput | min(Rs, Rc, R / n) | Slowest link; shared link R split n ways |
| Circuit rate | R / n | n slots (TDM) or bands (FDM) |
| Circuit transfer time | file size / (R / n) + setup time | |
| Non-persistent HTTP, per object | 2 RTT + transmission time | Count the base HTML file too |
| Stop-and-wait utilization | (L / R) / (RTT + L / R) | |
| TCP ACK number | last byte received + 1 | Cumulative |
| Receive window | RcvBuffer − (LastByteRcvd − LastByteRead) | Sender keeps LastByteSent − LastByteAcked ≤ rwnd |
| Handshake ACK | x + 1 for a SYN or FIN with sequence number x | SYN and FIN each use one sequence number |
Units: 1 byte = 8 bits. 1 Mbps = 1,000,000 bits per second, not 1,048,576. 1 ms = 0.001 s.
The traps that cost the most marks#
- Mixing up transmission and propagation. L/R ignores distance; d/s ignores packet size.
- Forgetting bytes to bits. Multiply by 8 before dividing by a rate.
- Counting routers instead of links. Two routers between two hosts means three links.
- Using the full link rate for one circuit. Divide by the number of slots or bands first.
- Forgetting the base HTML file when counting objects on a web page.
- One RTT per object for non-persistent HTTP. It is two: one for the TCP handshake, one for the request.
- Wrong PDU names. Message, segment, datagram, frame, from the top down.
- UDP vs TCP sockets. UDP: 2 values (destination IP and port). TCP: 4 values.
- Dropping the checksum carry. Wrap it around and add it back in.
- ACK = last byte received. It is the next byte expected, one more.
- Fast retransmit on the first duplicate. It takes three duplicates.
- Handshake ACK = x. It is x + 1.
A seven-day plan#
| Day | Cover | Finish the day able to |
|---|---|---|
| Day 1 | Chapter 1 edge, core and switching; chapter 1 layers | Say the five layers and four PDU names from memory. |
| Day 2 | Delays and throughput | Do every practice question on paper, with units. |
| Day 3 | Chapter 2: applications, HTTP messages, persistent HTTP, cookies | Work the HTTP timing example without looking. |
| Day 4 | Chapter 2: email, DNS, Wireshark | Explain push vs pull for email and trace one DNS lookup aloud. |
| Day 5 | Chapter 3: UDP and the checksum; reliable data transfer | Do two checksums by hand. |
| Day 6 | Chapter 3: TCP numbering, retransmission, flow control, handshake, close | Draw the handshake and close from memory, states included. |
| Day 7 | Mixed practice below, then re-read only the guides you got wrong | Stop learning new material. Sleep. |
Practice from memory, then check. Re-reading notes feels productive but builds far less recall than closing the book and writing the answer. Spread the work over several days rather than one long night: spaced practice lasts much better.
Acronyms to know on sight#
| Acronym | Stands for |
|---|---|
| ISP, IXP | Internet service provider; Internet exchange point |
| FDM, TDM | Frequency-division and time-division multiplexing |
| DSL, FTTH | Digital subscriber line; fiber to the home |
| OSI | Open Systems Interconnection, the seven-layer reference model |
| PDU | Protocol data unit: the packet at a given layer |
| RTT | Round-trip time |
| HTTP, SMTP, POP3, IMAP, DNS | Web, mail sending, two mail access protocols, the name system |
| TCP, UDP | Transmission Control Protocol; User Datagram Protocol |
| ACK, NAK | Acknowledgment; negative acknowledgment |
| MSS, rwnd | Maximum segment size; receive window |
| SYN, FIN, RST | Open, finish, and reset flags in the TCP header |
Practice#
Mixed on purpose: the exam will not tell you which chapter a question comes from. Time yourself at about two minutes per question.
- A 1,000-byte packet crosses one 2 Mbps link that is 400 km long, with signal speed 2 × 108 m/s. Give the transmission and propagation delays.
- A file of 3,200,000 bits goes over a circuit on 1.536 Mbps links with TDM and 24 slots. Setup is 500 ms. Total time?
- Which layer's packet is called a datagram, and which device reads it to forward it?
- A page has a base HTML file and 4 images. RTT is 40 ms, transmission negligible. Give the non-persistent (one at a time) and persistent (back-to-back requests) response times.
- SMTP is a push protocol. Which protocols pull mail from a mail server to a user's agent?
- What transport protocol and port does a DNS query normally use, and why that protocol?
- Compute the checksum of 1000000000000000 and 1000000000000001.
- A TCP segment has Seq 3,000 and carries 500 bytes. What ACK number acknowledges it?
- Three clients send UDP segments to port 53 on the same server. How many sockets receive them? What if they were TCP connections to port 80?
- The client's ISN is 700. What ACK number does the server's SYNACK carry?
Solutions#
Find the first line where your working split from the solution, and redo the problem from that line. Do not just read the answer and nod.
- 4 ms and 2 ms. 1,000 × 8 = 8,000 bits; 8,000 / 2,000,000 = 4 ms. 400,000 m / 200,000,000 m/s = 2 ms. See the delay guide.
- 50.5 s. Circuit rate: 1,536,000 / 24 = 64,000 bps. 3,200,000 / 64,000 = 50 s. Plus 0.5 s setup. See the switching guide.
- The network layer; a router. See the layers guide.
- 400 ms and 120 ms. 5 objects × 2 RTT × 40 ms = 400 ms. Persistent: handshake + HTML + all images = 3 RTT = 120 ms. See the HTTP timing guide.
- POP3 and IMAP (or HTTP, for webmail). See the email guide.
- UDP, port 53. A lookup is one small question and one small answer, so avoiding TCP's handshake saves a round trip. See the DNS guide.
- 1111111111111101. The sum is 10000000000000001 (17 bits). Wrap around: 0000000000000001 + 1 = 0000000000000010. Flip: 1111111111111101. See the UDP guide.
- 3,500. The segment holds bytes 3,000 to 3,499, so the next byte expected is 3,500. See the TCP numbering guide.
- One socket for UDP; three for TCP. UDP sockets are identified by destination IP and port only. TCP sockets use the full 4-tuple, which differs for each client.
- 701. The SYN uses one sequence number, so the server acknowledges ISN + 1. See the handshake guide.
Test-readiness checklist#
- ☐ I have checked my syllabus and slides for what my course includes in chapter 3.
- ☐ I can write the formula sheet above from memory.
- ☐ I can name the five layers and four PDU names without hesitating.
- ☐ I can do a delay, a circuit, an HTTP timing and a throughput calculation with units.
- ☐ I can do a checksum by hand and trace TCP sequence and ACK numbers.
- ☐ I can draw the handshake and the close with states.
- ☐ I have done the mixed practice under time and re-read the guides for anything I got wrong.
Show your working on every calculation and trace. A correct method with one slip earns most of the marks; a bare wrong number earns none, and a bare right number may not earn all of them.
Frequently asked questions#
What is on a computer networking midterm covering chapters 1 to 3?
Usually the Internet's structure and packet vs circuit switching, the four network delays and throughput, protocol layers and encapsulation, the application layer (HTTP, email, DNS), and the transport layer (UDP, the checksum, reliable data transfer, and TCP's numbering, retransmission, flow control and connection setup). Check your own syllabus for the exact scope.
What formulas do I need for a networking midterm?
Transmission delay L/R, propagation delay d/s, nodal delay as the sum of four delays, traffic intensity La/R, throughput as the minimum link rate, circuit rate R/n, non-persistent HTTP at 2 RTT per object plus transmission, stop-and-wait utilization, and the TCP receive window.
What are the most common mistakes in networking exams?
Mixing up transmission and propagation delay, forgetting to convert bytes to bits, counting routers instead of links, using the full link rate for one circuit, forgetting the base HTML file, and writing a TCP ACK number one too low.
How long should I study for a networking midterm?
Spread it over several days if you can. A week of short sessions, one or two topics a day with practice from memory, beats one long night of re-reading.
Something here not clear? A topic you wish we covered? Tell us. We read every message, and a request is the fastest way to get a guide written.
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