The Transport Layer Header, TCP and UDP

Function

flow control: process of adjusting the flow of data from the sender such that the receiving host can handle all of it
- metaphor: you speak slower if someone does not understand the language that well (etc.)
- if sender sends too fast (e.g., because of faster connection than sender), the sender can signal back to slow it down
session multiplexing: process that enables a host
- to support multiple sessions simultaneously
- to manage the individual traffic of streams over a single link
ports: used to identify the upper port layer protocol
- example:
  - HTTP: port 80,
  - SMTP email: port 25, etc.
- sender also adds a source port number to the Layer 4 header
- source & destination port number can be used to track sessions

example:

sender sends web traffic to destination port 80 from source port 1500
if the receiver wants to send traffic back, it will flip the port numbers for source and destination
stateful firewalls use this feature to keep track of connections

Sender —– DST: 80 SRC: 1500 ——> Receiver

Sender <—- SRC: 80 DST: 1500 ——- Receiver

most common protocols

TCP	UDP
Transport Control Protocol	User Datagram Protocol
connection oriented (once connection is established, data can be sent bidirectionally over that connection)	not connection oriented (no handshake connection setup between hosts)
sequencing (ensures that segments are processed in the correct order and none are missing)	no sequencing
reliable (receiver sends acknowledgements back to sender; lost segments are resent based on missing acknowledgements)	not reliable (receiver does not send back acknowledgements to sender)
performs flow control	no flow control
allows for error detection and recovery	error detection and recovery has to be provided by upper layers

To initiate connection,

Sender ———SYN———> Receiver

Sender <——SYN-ACK——– Receiver

Sender ———ACK———> Receiver

at least 20 Bytes (5 x 32 Bits)

at least 8 Bytes (2 x 32 Bits)