What is the Transport Layer and TCP? - Part 1

A mini lecture, examining the role of the Transport Layer in the Internet Protocol Suite, including TCP, UDP, and source and destination port numbers. I also cover the characteristics of the TCP protocol including reliability, flow control, window size, and the three-way handshake. Subscribe to danscourses!

TCP/IP Model
------------
Application Layer
Transport Layer
Internet Layer
Link Layer Layer

=====================
Transport Layer
=====================

Transport Layer offers:
- support for end-to-end services (gives end-to-end support to application layer services)
- support for multiplexing (multiple conversations happening at the same time over the wire)
- services to port numbers mapping (port 80=http, port 21=ftp, port 53=dns, port 25=smtp, port 22=ssh)
- services that need reliabiity use TCP (http, smtp, ftp, ssh)
- services that don't need reliability or need speed use UDP (dns, dhcp, VoIP)

When we send data over the network, the Transport Layer breaks the data into pieces:
- the pieces that data is broken into are called "segments" for TCP, and "datagrams" for UDP,
- each piece is given dst and src addressing at layers 2 and 3 so the pieces can reach their dest and then return
- each piece is given dst and src port numbers at layer 4 (Transport) so each conversation/service can be identified

Servers "listen" and "reply" using port numbers, examples:
- a web server is listening on port 80,
- a DNS server is listening on port 53,
- a telnet server is listening on port 23
Clients "connect" and "receive" using port numbers
Packet
(Client web browser connects using http GET) from -------[\/]--------to (server listens on port 80)
src port=50132
dst port=80
(Client web browser receives http REPLY) to----------[\/]-----------from (server replies with http REPLY)
src port=80
dst port=50132

TCP (transport control protocol)
===========================
- reliable (sequence numbers, acknowlegements ACKs, selective acknowledgements SACKs,
flow control, window size, retransmit lost packets),
- connection oriented (uses a 3-way handshake to start the session),
- slow and reliable,
- as such, the TCP protocol has many settings (header fields, flags)
- uses source and destination port numbers to identify service and host

TCP 3-way handshake
=================================
Client Server
-------------------------------------------------------------------
1 Syn(seq=0) -----)
2 (----- Ack(ack=1),Syn(seq=0)
3 Ack(ack=1) -----)

Two 2-way handshake (end transmission)
----------------------------------------------
1 Fin, Ack -----)
2 (----- Ack
3 (----- Fin
4 Ack ------)

UDP (user datagram protocol)
=============================
- unreliable - no seq numbers, no acknowledgement, no retransmission
- connectionless - no 3-way handshake
- faster than tcp but not reliable
- header is smaller with fewer fields
- uses source and destination port numbers to identify service and host

Ports
================================================
Well Known 1--1023 (whois, icmp, http, ssh, telnet, rip, dhcp)
Ephemeral
- Registered 1024--49151 (can be registered yearly from icann, for consumer services/servers)
- Dynamic 49152--65536 (dynamically assigned ephemeral)

[jboyce]

This is the absolute best set of explanations on these technologies anywhere!! THANK YOU!!!

[memyselfimemyselfi]

Why does he not have more viewers?!?!?! Thank you, sir!!!

[codepractices]

Thank u so much sir! Crystal clear explanation! Please make more tutorials on congestion control, Application layer etc.

[eligroff]

Helpful for my google Cert i'm working on. Having them teach is great but sometimes the way they do it doesn't make sense. I'm happy your out here doing what your doing cuz YouTube videos are my best bet for learning.

[maritomesquita]

such a great video, u sir just won a subscriber

[tg]

That you sir. You are always great in your tutorials. I learned more from you than school setting. Do you have any videos on sims or simlets for CCNA exam practice? Thanks again.