Transport Layer

Multiplexing/Duplexing

Multiplexing is when nodes on a network is directing traffic to multiple recievers on single connection

think of it as sending multiple letters to a single mailbox

Duplexing is when you are taking the data stream and traffic recieved and sending it to the correction application/service

so think of this as a mail carrier sorting the letters they recieved in the mail room and delivering the letters to the right destination

port

a 16 bit number that is directing traffic to specific services on a network

Port number determines which service or program you want to use

what is a server(services)

a program that is waiting for a client to ask for data

where is the port in this notation:
10.1.1.100:80

Port 80

encaspulates

packaging one data from one protocol to be used inside another

What is the TCP data packet header made up of

the source and destination port number

the segment number
- this is because most times the data packets that came from the lower level(network layer) fragmented their data packets so the segment number keeps track of which segment is which

acknowledgement number
this tells which segment follows the current packet

data offset
determines how long a tcp header is and helps see where the header begins and ends and then the payload portion

TCP window
controls the amount of data that can be sent before an acknowledgement number is required. So with big files the TCP has to break it down and when sending, the

6 control flags of TCP (SPURAF)

1. URG (urgent)
   if it is 1 → the data is urgent

2. ACK (Acknowledge)
   if it is 1 →confirms the receiver received and acknowledge data & examine acknowledgment number

3. PSH (Push)

   this means push all data that is on the buffer to the application ASAP
   (the buffer is a place where data that is not needed immediately is waiting to send)

4. RST (Reset)

   this happens when the TCP connection is unable to recover data due to missing or malformed segments

5. SYNc (Syncrhnized)

   happens in the begninning when first establishing TCP connection

6. FIN (Finished)

   when flag is 1 → that means no more data needs to be sent a connection is closed

three way handshake

Proccess used to establish TCP connection

client (computer) A sends a SYN flag to server B (Computer B), establishing a TCP connection and sending its sequence number and telling the server where it wants to go

server B then send a SYN-ACK flag that essentially is acknowledge the clients SYN and then sending their own SYN flag to the server with its own sequence number it wants to synchronize

and then client A sends a ACK flag acknowledging the servers SYN-ACK

Once the TCP connection is established data is now to be sent freely.

Then the FIN flag can be set across the devices

four way handshake

Client A sends FIN flag to server B
Server B sends a ACK flag and then a FIN flag
and then Client A sends a acknowledge flag

FIN →

← ACK

← FIN

ACK →

SYN packet contents

header:
source and destinaton IP address
payload:
the contents of the data

The importance of socket for a port

To get a response from a port number there needs to be a socket instantiated.

A port number is a virtual address think of traffic being sent to an address, but there needs to be someone home( the socket needs to be open and listening) for it to be received

A socket is a port that TCP segment activates to listen for data request

TCP VS UDP

TCP → needs to establish a connection between the client and server before sending data. The delivery is more gaurenteeed because of the consistent communication of ACK flag and retransmission (if nescary). retransmission is activated if a node doesnt recieve an ACK flag in a certain time frame

UDP → does not need an establish connection. It just sends data to the destination port. It is less reliable but faster. Normally used with watching videos

firewalls

a device that can block traffic that meets certain criteria

on the transport level, firewalls can block traffic from specific ports