Our setup:
	Application Layer (us)
	Transport Layer (TCP)
	Internet Layer (IPv4)
	Data Link Layer (802.3 Ethernet)
		Some of you have 802.11 (wifi) instead

First:  A simple demo

Homework 1:  Easy at this point
Active Learning:  Connect to the server we make and give it a message

Network layers and protocols
	layers can be replaced
	lowest layer is physical layer
	headers

Making the network work
	Error correction
	routing
	scalability
	connections
	acknowledgement
	lower levels are unreliable
	services provided by layers

OSI Network Layers

Physical Layer:
	Transmits 0's and 1's
	This must happen reliably
	More than one method is used here
	Not in TCP/IP, although this function must be performed

Data link layer:
	Form 0's and 1's into frames
	Send acks
	Hide errors from network layer
	For broadcast:  Manage shared channel
	Link layer in TCP/IP

Network layer:
	routing
	For broadcast:  Do nothing
	Internet layer in TCP/IP
	Packet format in TCP/IP is called IP (Internet Protocol)
	IP routing is not always effective

Transport layer:
	Separate messages into packets and put them back together
	TCP and UDP handle this in TCP/IP
	TCP is reliable, UDP is not
	
Session layer:
	maintain sessions
	Sessions involve keep track of which computer should send data, checkpointing long transmissions, etc.
	Not in TCP/IP, TCP handles some of these functions

Presentation layer:
	Convert internal information to a representation that can be transmitted
	Allows high-level structures to be defined and transmitted (objects, etc)
	Not in TCP/IP

Application layer:
	Contains protocols like HTTP, SMTP, SSH, etc.
	Servers, etc. are here

We will use the TCP/IP model, because it exists
	And we're programming this semester, not philosophizing

Why TCP/IP?
	It put existing protocols into a model
	OSI is complex and includes layers nobody needs
	Initial implementations were slow compared to BSD UNIX
	Politics:  Friendly old UNIX had TCP/IP, OSI standard was from the government

Was that bad?
	TCP/IP isn't that generic
	No physical layer definition

Simple client and server in C