Week 39 September CSCI 4431 / 6431

Lab 1

Run a client and a server. Then watch them talk.

Office hours

Tue/Thu — 12–1 pm

SEH, 4th floor couches — or remotely, link on Discord.

Today

Today, in three parts

1 Some recap, Classroom 50, prerequisites
2 Project 1, which starts next week
3 The lab — Parts A, B, C

Everything you need to hear comes first. Once the lab starts, leave whenever you're done — but don't forget to submit.

PrerequisitesA quick check

Before we start

QR code linking to the course setup form

Setup form do this first

QR code linking to the course Discord server

Discord #help lives here

Course website schedule & syllabus

On your laptop Wireshark· Python 3· Git

You have done this already

Classroom 50

classroom50.org — every assignment, all semester.

Hello, Classroom was the rehearsal. Similar three steps every time.

1 Accept → you get a private repo
2 Clone it, do the work, push
3 Tag it — that's what submits

Follow the README step by step. New to git? It's explained in there.

Next week 100 points

Project 1 — Design your own
application layer protocol

Project 14 milestones

Four phases

Phase	What	When	Weight
1	Spec, written with your teammate	Week 5	25%
2	Merged spec, with the other group	Week 5, in class	15%
3	Client + server	Week 8	20%
4	Reflection after interop	Week 8, in class	30%

The reflection is worth more than the code.

Project 1one repo per group

One of you accepts. Then adds the other.

1One of you accepts. The repo is named after them.
2Go to classroom50.org → Manage Group.
3Add your teammate's username → Save collaborators.
4Both of you clone the same repo.

The Classroom 50 Manage collaborators dialog: a list of
group members with the first marked Owner, a field to add a GitHub username, and a
Save collaborators button.

Two or three people per group. The one who accepts, plus one or two teammates.

Project 1this week

Claim your protocol on the board

PROTOCOL GROUP 1 GROUP 2 Key–value store @you + @teammate @someone + @else

One row per protocol. Two groups per row.

Joining a row that already has a group guarantees you a partner group.

Pick your protocol and form your groups by next week — Chapter 3, and Project 1 begins.

Today 25 points

Lab 1

Recaplast week

Where sockets sit

Application HTTP · SMTP · DNS — and your program today

Sockets the door between the two

Transport TCP or UDP — you chose between these last week

Network · Link later in the semester

Today: a tiny client and server, in Python.

Context

What is Wireshark?

It records every packet your laptop sends or receives.

The Wireshark window showing six UDP packets on the loopback
interface. The packet list is at the top, the packet detail tree at the lower left,
and the raw bytes at the lower right.

1 2 3

1Packet list — one line each.
2Packet detail — split into layers.
3Packet bytes — and the text inside.

udp.port == 9000 ← the filter box, at the top. ==, not =.

Contextwhat you see when you open it

Starting a capture

The Wireshark welcome screen, listing the machine's network
interfaces with a small traffic sparkline beside each, above a capture filter box.

2 1

1Pick your interface — then double-click to start recording.
2Leave this box empty. It's the capture filter, not the display filter.

While it's running, nothing is saved. Stop the capture, then File → Save As → captures/part-a.pcapng, inside your repository.

Contextthe same tool, no window

tshark — Wireshark from the terminal

tshark -D list interfaces tshark -i lo -w captures/part-a.pcapng capture — ctrl-c to stop tshark -r captures/part-a.pcapng read it back tshark -r captures/part-a.pcapng -Y "udp.port == 9000" filter, with -Y

-w writes straight to that path as it captures. No Save As step — it's already where you need it.

Context

Lab 1

Part	What	Pts
A	Your client ↔ your server	9
B	Your client ↔ my server	9
C	Questions about a real HTTP trace	7

We're going to finish all three in class today.

Step 1

Accept the assignment

Open the link. Sign in. Click the blue button.

Link is in #labs on Discord — and on the course website.
Your repos live at classroom50.org.

you get: csci4431-lab1-wireshark-<you>

The Classroom 50 accept page: assignment name, the GitHub
account you are signed in as, the repository that will be created, and a blue
Accept Assignment and Create Repository button.

Step 2

Clone your repository

git clone https://github.com/gw-computer-networks-f26/csci4431-lab1-wireshark-<you> cd csci4431-lab1-wireshark-<you>

src/server.py · src/client.py — for Part A and B
answers.txt — where written answers go
README.md — everything I say today, in writing, and more

The codetwo files, ten lines that matter

What the programs actually do

src/server.py

sock = socket.socket(AF_INET, SOCK_DGRAM)
sock.bind((host, 9000))

while True:
    data, addr = sock.recvfrom(65535)
    sock.sendto(data.upper(), addr)

src/client.py

sock = socket.socket(AF_INET, SOCK_DGRAM)

sock.sendto(payload, (host, 9000))
reply, _ = sock.recvfrom(65535)

SOCK_DGRAM is the only word that says UDP.
Only the server binds — the client's port is whatever the OS spares.
No connection, so every datagram carries its own address.

Part A9 points

Your own client and server

# 1 python3 src/server.py leave it running # 2 capture on loopback lo / lo0 / "Adapter for loopback" filter udp.port == 9000 # 3 python3 src/client.py --student your_github_username

Capture before you run the client. Not after.

Part Acheck yourself

Six packets

sent 'octocat:hello' -> 'OCTOCAT:HELLO'

Three requests, three replies. That's the whole protocol.

Stop the capture → Save As → captures/part-a.pcapng

Part B9 points

Now talk to my server

My server’s IP 161.253.78.154

Same client. Same code. My server does not speak your protocol.

Part BSupposed to fail

Run it anyway

capture on Wi-Fi this time python3 src/client.py --student your_github_username --host <my address>

-> 'ERR ······························'

The reply is a sentence. It tells you what this server wants.

I've hidden the rest on purpose. Go read it.

Part B

Read it. Change it. Send it again.

1 Read the reply — bottom pane, or Follow → UDP Stream
2 Send what it asked for, with --raw

python3 src/client.py --student your_github_username --host <my address> \ --raw --message "?"

You'll know. The reply says OK, not ERR.

Part Bhand in

Save both

captures/part-b.pcapng — the failure and the fix
B1 and B2 in answers.txt

You were never given the specification, but you figured it out.

Part C7 points

A trace from the textbook's authors

Real HTTP, captured by Kurose & Ross.

traces/http-wireshark-trace1-1.pcapng ← open this answers.txt ← C1 … C7

Exact match. Write 200, not status code 200.

Step 4

Submit your work

git add -A git commit -m "Lab 1" git push git tag submit/1 ← this is what hands it in git push origin submit/1

The tag is the one that hands your work in — a git push on its own does not submit.

Submit before you leave. That's one of your three — the next two are submit/2 and submit/3.

Before you go#help on Discord

If it's broken, say so

No interfaces

ChmodBPF on macOS. Npcap on Windows. Log out and back in on Linux.

No packets in A

Wrong interface, or a typo in the filter.

Nothing at all in B

Not even an ERR? UDP may be blocked. Tell me.

Not finished? Office hours Thursday. Then it's due Friday 11 Sept, 11:59 pm.

Lab 1

Office hours

Today, in three parts

Before we start

Classroom 50

Project 1 — Design your ownapplication layer protocol

Four phases

One of you accepts. Then adds the other.

Claim your protocol on the board

Lab 1

Where sockets sit

What is Wireshark?

Starting a capture

tshark — Wireshark from the terminal

Lab 1

Accept the assignment

Clone your repository

What the programs actually do

Your own client and server

Six packets

Now talk to my server

Run it anyway

Read it. Change it. Send it again.

Save both

A trace from the textbook's authors

Submit your work

If it's broken, say so

No interfaces

No packets in A

Nothing at all in B

Project 1 — Design your own
application layer protocol