CPSC 3600 - DAY 5
JANUARY 23, 2017

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NETWORK SOFTWARE: THE PROTOCOL HIERARCHY
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	.To reduce design complexity, most networks are organized as a series
	or hierarchy of layers.
	.Depending on its functionality, a layer may be implemented in software
	or hardware or both.
	.Layer n on one machine communicates with layer n on another machine on
	the network using an agreed upon protocol.
	.The entities comprising the corresponding layers on two communicating
	machines over the network are called peers.
	.A protocol is an agreement between the communicating layers on how the
	communication is to proceed:
		.A formal description of message formats and rules the two
		layers must follow to exchange those messages.
		.Protocol definitions range from how bits are placed on a wire
		to the format of an email message.
		.Standard protocols allow different manufacturer's computers to
		communicate.  These computers can user completely different
		software/hardware, provided each computer's software can agree 
		upon the meaning of data.

	
	HOST 1						HOST 2
	+---------------+	LAYER 5 PROTOCOL	+---------------+	
	| LAYER 5	|<----------------------------->| LAYER 5	|
	+---------------+				+---------------+
L 4/5 INTERFACE
	+---------------+	LAYER 4 PROTOCOL	+---------------+
	| LAYER 4	|<----------------------------->| LAYER 4	|
	+---------------+				+---------------+
L 3/4 INTERFACE
	+---------------+	LAYER 3 PROTOCOL	+---------------+
	| LAYER 3	|<----------------------------->| LAYER 3	|
	+---------------+				+---------------+
L 2/3 INTERFACE
	+---------------+	LAYER 2 PROTOCOL	+---------------+
	| LAYER 2	|<----------------------------->| LAYER 2	|
	+---------------+				+---------------+
L 1/2 INTEFACE
	+---------------+				+---------------+
	| LAYER 1	|<----------------------------->| LAYER 1	|
	+---------------+				+---------------+
	+---------------------------------------------------------------+
	|			PHYSICAL MEDIUM				|
	+---------------------------------------------------------------+

	
	.In reality, no data is transferred from layer n on any two machines
	Data and control information is passed to the layer below.
	.Additional information including protocol control information may
	be appended by each layer to data as it travels from higher to lower
	layers in the form of layer headers.
	.Below layer 1 is the physical medium where the actual communication
	occurs (copper wire, optical fibers, wireless channel, etc.)
	.Between adjacent layers, an interface defines which primitave
	operations and services the lower layer offers to the upper layer.
	.The set of layers and associated protocols is called a network
	architecture.

DESIGN ISSUES FOR THE LAYERS
----------------------------

	.Reliability
		.Error detection: retransmission is needed when information is
		received incorrectl.
		.Error correction: message recovered from incorrect bits.
		.Routing: automatically finds an alternative route from a
		source to a destination when a link is broken.
	.Scalability
		.Protocol layering: divides the overall problem and hides
		implementation details.
		.Addressing/naming: identifies senders and recievers.
	.Resource allocation
		.Statistical multiplexing: allocating bandwidth dynamically
		according to the short-term needs of hosts.
		.Flow control: feedback from the receiver to the sender is
		used to overcome the fast sender - slow reciever problem.
		.Congestion: too many computers want to send too much traffic
		at the same time.
		.Real-time: timeliness of delivery.
		.Quality of service: high throughput, low latency.
	.Security: defending the network against threats.
		.Confidentiality: prevents eavesdropping on communications.
		.Authentication: prevents someone from impersonating someone 
		else.
		.Integerity: prevents changes to messages.

TYPES OF NETWORK LAYER SERVICES
-------------------------------

	.Connection-oriented Service:
		.a connection is established.  Information expected to be 
		received in the order sent.  The connection is released when
		the data transmission is completed. Ex. TCP
	.Connectionless Service:
		.Each message carries the full destination address and routed
		through the system independent of other messages.  The messages
		may arrive out of order Ex. UDP.

THE OSI REFERENCE MODEL
-----------------------

	1. APPLICATION LAYER
	2. PRESENTATION LAYER
	3. SESSION LAYER
	4. TRANSPORT LAYER
	5. NETWORK LAYER
	6. DATA LINK LAYER
	7. PHYSICAL LAYER