Internet in General

Billions of connected computing devices

- Hosts/End Systems
e.g servers, personal computer, laptops, smartphones

- Running network apps

Why do we call these hosts? 
They are the hosts of  network application running. 
Why do we call them end systems?
One of the primary objective is to enable the connectivity between any two hosts and to find a path to connect these entities. These entities are at the end of thee path thus they are the neding point.

Communication links

- Fiber,Copper, Radio, Satellite
- Transmission rate: Bandwidth
Bandwidth is the transmission rate per unit time. How many bits that can be transmitted
Number of bits per unit time.
Regardless of the medium

The bandwidth for wireless is dependent as the medium is the air. So it depends on the distance and interference nearby

Packet Switches

Forward packet (chunks of data)
  - routers and switches
They will deliver the information piece by piece through packet.
It is enabled by these intermediate devices. 
Packet switches choose which output link is send the packet to.
(More details later)

Network structure

Network edge

Hosts: Client and servers
Servers often in data centre

THere are other modes such as peer to peer (Other mod)
Servers are often hosted in cloud. 

Access networkws, physical media

Wired, wireless communication links
They are connected through access networks which are a small part of the entire network. They itself is another network.

Network core

Interconnected router
Network of networks

How to connect end system to edge router

- Residential access nets
- Institiutional access netowrks
- Mobile accesss networks
- Bandwidth
- Shared/ dedicated
Different isp use different tech as communicational links. Users can use different bandwidth (Bits oer second) and it also dependent on the medium. The bandwidth of wireless depends on how many other users are using the same network as you.

The network core

Mesh of interconnected routers
Packet switching:
Hosts break applications-layer message into packets
- Forwards packets from one router to the next, accross link on path from source to destination
- Each packet transmitted at full link capacity
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Packet transmission delay = time needed to transmit length (bits) packet into link = L / R (bits/sec)

Packing switching: Store and forward

- It takes L/R seconds to transmit L bit packet into link at R bps
- Store and Forward: Entire packet must arrive at router before being forward to next hop
(it is not working like pipeline)
- End- end delay = 2L/R (Assume zero probagation delay)

We do not forward the entire packet incase we need to do secondary check

Packet switching: Queuing delay, loss

Queuing and loss:
- IF arrival rate (bits) to link eexceeds transmission rate of link for a period of time, it will queue
- Packets can be dropped if memory (buffer) fills up

Two key network core functions

Routing:

Determine sources destination route taken by packets

Forwarding

Move packets from routers input to appropriate router output.

Each router have its own forwarding table, each packet have a header value at the front which corresponds to the forwarding table. This allow the router to decide which output link to forward.
The table is written by the routing algorithm which is like the brain of the router. Routers have to talk to its neighbour and to build this forwarding table. 

Alternative core: Circuit switching

Before talking, the router try to find a path and reserve certain resources. The resources is not shared but there are certain trade offs.

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- Each link has four circuits
- Call gets 2nd circuit in top link, 1st circuit in right link
- Dedicated resources no sharing
- Circuit segment idle if not use by call (wasted)
- Commonly use in traditional telephone networks

The trade offs for not sharing is that there will be idling. This will cause queueing delay if a resource is oversubscribed. The performance will thus be affected

Internet structure: Netowrk of networks

- End system connect to internet via access ISP
- Access ISP in turn must be interconnected
- Resulting network of networks is complex

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There will definitely be competitors who want to be the centralised isp to help in the local access net. If there are multiple competitors, we need to have an internet exchange point through peering link. 
Its cheaper to have one link to connect to all the different ISP but the quality might not be guaranteed.
There's also regional networks to connect access nets to ISP. 
Content provider networks

Problems with ISP
- Delay
- Performance

Insert slide 24

How do loss and delay Occur?

Packets queue in router buffers
- Packet arrival rate to link exceeds output link capacity
- Packets queue, wait for turn
Insert slide 26

Four source of packet delay

1. Nodal Processing
Happens when the packet is first arrived
the router needs to run RIP
To check if its complete and to match it to the forwarding table

2. Queuing delay
When the link is busy, the packet have to wait for its turn
The delay varies.
Dependent on:
-Buffer size
Too high, drop rate increase
Too low, queue delay increase

3. Transmission delay
- Packet length
- Link bandwidth
-L/R

4. Propagation delay
- Medium
- Length of link
- Speed
- d/s

Throughput

Throughput(bits /timeunit) at which bits is transferred between sender/reciever
- Average: Rate over longer period of tine
- Instantaneous: rate at given point in time

Insert slide 31

Average throughput from a source to the ending over a relatively long periods of times. The file being send , the avg is calculated through F/Rs + F/Rc
We use a fluid approximation, the bits are like water and links are like pipes so the water go through the pies. Throughput is how much time does the water take to go through the pipes. The minimal cap is called the bottleneck

Service view of the internet

-Infrastructures that provides services to application:
Web, email, games
-Provide programming interface to apps
Hooks that allow sendiing and recieving app programs to connect to internet
Provices service options 

This acts like postal services where there are different types of services, different alternatives. These api allow users to write requirement. 
In the networks, all these network are implemented by protocols.

What is a protocols

Is communication between different hosts/ servers and clients.
- Transfer of request, website, file, hosts, routers, applications


Each layer implements a services by:
- Performing actions within that layer
- Relying on the services provided the layer below it

Why layering?

Explicit structure allows identification

Relationship of complex system;s pieces

Modularization eases maintenance, updating of system

change of implementation of layers service transparent to rest of system
e.g changing of the gate procedure does not affect the rest of the system

Insert slide 39

Encapsulation

At each layer, we add more things to the packet.

Source
Application: Message
Transport: Segment
Network: Datagram
Link: Frame

Switch
Run only inside access network, does not do routing nor broadcast

Router