18 - TCP/IP Layer 3 (TCP and UDP)
- Fun, but important Network theory bit
- Recap
	- Networking works by layers
		- Layer 1 - Machine to Machine
		- Layer 2 - IP protocol
		- Layer 3 - Transport Layer - TODAY'S TOPIC
			- not just individual packets, but entire streams of data
				- 2 important protocols
					- TCP
						- Transmission Control Protocol
						- THE most important Protocol, such that it is part of the namesake of the whole networking stack (TCP/IP)
					- UDP
- TCP
	- problems it is trying to solve
		- layer 2 sends individual packets
			- so one might think you just do that a whole bunch of times
			- there are complications
				- 1. packets get corrupted
				- 2. When routers get busy, they just throws out packets
				- 3. Packets are treated independently by routers, so they probably will arrive at the destination in different orders
	- TCP deals with all of these
		- TCP promises that all information will arrive safely, re-request any packets that didn't arrive, and puts them all back in the right order again when it does.
	- throws in a new layer we haven't discussed deeply yet
		- Ports
			- analogy
				- each machine is an apartment building
				- each room is a port
				- when a machine wants to communicate, it has to know not only what building it's going to, but also what room
			- a packet must have a source port, source IP address, destination port, and destination IP address for the connection to be made
			- A specific Application is "listening" if a specific port
				- when traffic comes in on a port, the computer passes the data on to whatever application  was listening on the port
					- e.g.
					-  SSH listens on port 22
					- Webservers listen on port 80
			- Privileged port
				- any port under 1024
					- have to be root (or System Admin on windows) to listen on this port
- Up until now, you may think TCP is perfect.
	- However, TCP has one major tradeoff:
		- overhead
			- each TCP connection is a two way connection
				- TCP is "chattier"
			- it also requires more memory and CPU
			- thus it is slower and requires more powerful
- On the other hand, if you need to do something fast and with little overhead, and you dont care about dropped or scrambled packets, there is an alternative to TCP
	- UDP
		- limited use, but streaming almost always uses it
			- skype
- Command
	- lsof
		- list open files
			- UNIX treats everything as a file
				- even Network connections
		- lsof -i
			- i stands for internet
			- all your open TCP or UDP connections
- Bart Busschots
	- bartb.ie <http://bartb.ie/>
	- impodcast.tv <http://impodcast.tv/>
	- podfeet.com <http://podfeet.com/>