We know how to navigate, how do we communicate now
➡️ We need the 4th layer of the OSI model
Layer 4¶
specification of how to send data
from server A to client B and vice versa.
Different established protocols :
TCP
UDP
...
⚠️ Attention ⚠️
The transport layer only defines the way two applications communicate
but does not specify the content of these communications
A server == an application?¶
Knowing the server’s IP doesn’t allow you to communicate with the application on that server yet
❓ By the way, can there be only one network application on a server or can we put several ❓
We can have several applications on the same server, and fortunately 🥳
The choice of the application we will talk to involves the notion of port
port = service entry door 🚪
(but we don’t run that many applications on a server)
Network packet anatomy¶
and here is what a packet looks like
vocabulary
to be precise, in general we call packet level 3 and beyond
when as here we include level 2 we generally speak of “frame”
Standard ports¶
On a machine we have 216 = 65,536 ports
Some standardized ports:
| service | port |
|---|---|
| SSH | 22 |
| SMTP | 25 |
| DNS | 53 |
| HTTP | 80 |
| HTTPS | 443 |
| ... | ... |
TCP/IP¶
Principle¶
Transmission Control Protocol
is the historical protocol (Bob Kahn and Vinton Cerf, September 1973), which owes its longevity to its robustness and reliability.
Today when you browse the web
most exchanges that take place between your browser and websites are based on TCP
The principle of TCP is very simple and breaks down into three steps:
connection establishment
data transfer
connection termination
TCP/IP: open¶
The connection of a client to a TCP server breaks down into three steps
three way handshake
in the following way:
1️⃣ Client: Hello server can you hear me ?
2️⃣ Server: Yes I hear you and you?
3️⃣ Client: Yes it’s good I hear you
TCP/IP: close¶
Closure in 4 steps
1️⃣ Client: I’m done
2️⃣ Server: Ok noted
3️⃣ Server: me too I have nothing more to tell you
4️⃣ Client: Ok see you next time
Let’s see how it really works¶
let’s go see the python/tcp folder of the course
TCP a rich thing 🤑¶
You can therefore see that with this approach
✅ the connection is extremely reliable and there is little chance of having misses
However this reliability is not free 💰️
❌ it is accompanied by a relatively high cost in terms of exchanges
That’s why there’s an alternative to TCP 😯
UDP¶
The UDP protocol (User Datagram Protocol) is complementary to the TCP protocol. Created by David Reed in 1980.
Use cases:
Fast data transmission and reception of the whole not imperative
TCP = very reliable but slow
vs
UDP = fast but unreliable
UDP applications are numerous, for example:
