---
# System prepended metadata

title: Computer Network Before Mid-Term

---

# 
1. flow/ conjction control
2. 5 layers of network
3. SMTP
4. GBN
5. 


# Computer Network Before Mid-Term

:::info
:bulb:  
:::

:::danger
not finished yet 😕
:::

:::spoiler
trash talk
:::

:::success
abstract
:::

:::warning
confusion 
:::

## 2023108 W6

### Chapter 3

#### Demultiplexing 
* receiving host receiving UDP segment
    1. check dest. port num. in segment
    2. directs UDP segment to socket with that port
    3. use source port num. to discriminate the connection

#### connection oriented demultiplexing
* how does TCP recognize 
    1. source IP
    2. dest. IP
    3. source port num.
    4. dest. port num.

#### UDP
* cons
    * lost
    * deliver out of order
    * "best effort" service

* pros 
    * no connection establishment (-1 RTT delay)
    * no connection state: smaller header(8 bytes)
    * no congestion control

* application 
    * streaming multimedia 
    * DNS
    * HTTP3

* checksum
:::success
* goal: detect error
:::

:::danger
* second class failed

:::

## 20231004 W4

### Chapter 2

#### Conditional GET
![](https://hackmd.io/_uploads/H1iSQIqea.jpg)

#### SMTP
* simple mail transfer protocol
* ASCII

#### DNS, Domain Name System
* IP address 
:::danger
* sleep 💤😪😴😴
:::






## 20231002 W4

### Chapter 2

:::info
* RTT: round- trip delay time, time for small package to travel from client to server
:::

#### *HTTP1.0*: Non- presistent HTTP
* 2RTT+ file trasnmittion time: load page
* (2RTT+ file trasnmittion time) * number of object
* TCP connection down once server send data
* browsers open multiple parallel TCP connections to load a webpage 

![](https://hackmd.io/_uploads/Sk6qZoPgp.jpg)


#### *HTTP1.1*: Presistent HTTP
* 1RTT+ (1RTT+ file trasnmittion time) * number of object
* (2RTT+ file trasnmittion time) * number of object
* implement with the concept of pipeline, send multiple request and recieve mutiple data at a time
* faster than 1.0
* server response in order, FCFS
* small object have to wait for transmission (FCFS) (HOL, head of line, blocking)

#### *HTTTP2*: increase flexibility in server sending data to client
* slice the object into frame, and transmitt interleaved
![](https://hackmd.io/_uploads/ry78489la.jpg)

#### *HTTP3* 
* add security, error, congestion control over UDP
:::spoiler
* while professor confess he is not familiar with that
:::

#### HTTP request method (coud'n)
| Method | describe |
| ------ | -------- |
| GET    | recieve message from server |
| POST   | sent entity body            |
| HEAD   | only return header          |
| PUT    | upload message to server    |

#### HTTP response code

#### cookies
* since HTTP is **stateless**
* all request are independent of each other

#### proxy
* caches server
* server+ client at the same time
* if some of the request is satisfied in proxy server, then the end-to-end delay will be reduce since less daya transfer from **access network**, bottle neck doesn't exist anymore 


## 20230925 W3

### Chapter 1

#### Network Module 

#### Network Protocol 

#### Encapsulation/ Decapsulation

| layer | unit | 
| -------- | -------- | 
| application layer    | data       | 
| transport layer      | ??         |
| network layer        | package    |
| link layer           | frame      |
| physical layer       |    x       |

:::warning
* haders will change while transferring between source - switch - router - destination
:::




## 20230920 W2

### Chapter 1

:::spoiler
* reviewing last time
* feel free to sleep 
:::

#### Circuit Switching: FDM& TDM

* FDM: Frequency Division Multiplexing
	* 以頻率分割頻寬
	* 廣播

* TDM: Time Division Multiplexing
	* 以時間分割頻寬
	* 極短的時間 (periodic slot) 裡面每個使用者擁有完整的頻寬，但其他時間無法傳輸
	* 網路

:::success
* it's hard to say whether **Circuit Switching** or **Package Switching** is the final winner, since
* Circuit Switching required  to **wait**
* Package Switching is much more **economic**
:::


#### Performance: Loss
![](https://hackmd.io/_uploads/rkb3ZydJa.jpg)

* processing: ckeck error, less than microseconds 
* queue: **unpredictable**
* transmission: *L/R*
* propagation: *d/s = distance/ 2 * 10^8 (m/sec)*

:::warning
* **high-speed network**, namely **wideband network** imply that *R*, the transition rate, is larger. Thus more data can be transferred within the same time compared to general network. 
* it WASN'T about the physical speed 
:::

#### Queuing Theory (cont'd)
:::info
* a = average package arrival rate
* traffic intensity: *La/ R*
:::

* traffic intensity ~= 0
	* small
* traffic intensity < 1
	* small
* traffic intensity -> 1
	* large 
* traffic intensity > 1
	* infinity 

#### Performance: Throughput
:::success
* throughput: rate at which bits are being sent from sender to receiver 
* basically, the smallest transmission rate 
:::

* instantaneous: in a specific time
* average: in a period of time 

#### Network Security




## 20230918 W2

### Chapter 1

#### the network core
* package is the minimum unit when data transfer
* packing- switch
	* application- layer message will be break into packages 

#### host: sends package of data
* take application message
* breaks into small package of length *L* bits
* transmits package into access network at transmission rate *R* (bps)
	* aka link capacity, link bandwidth

:::info
* L: package length, bits
* R: transmission rate, bps
:::

#### package switching: store and forward ⏩
![](https://hackmd.io/_uploads/B1Wi9mH1a.jpg)
* transmission delay
* store and forward
* end to end delay *(2L/ R) +*

* queuing delay 
	* package lost: 
	* too much data delivered into switch when queuing, than lost data
 
* routing 
	* global action:
	* determine source- destination paths 
	* routing algorithm 

* forwarding
	* local action:
	* 
:::danger

:::

#### alternative for package switching: circuit switching
end to end resources allocated to, reserved to *call* between source and destination

:::success
pros:
* exclusively for this *call*
* no queuing delay 

cons:
* resources waste, others can't use the resources even this *call* doesn't send any data 
:::




## 20230913 W1

### General
* this course is more focus on the **abstracts** of computer network theories, progress, but not the actual numbers

### Chapter 1

#### Internet
#### Protocol 
#### Network Edge: host, access network, physical media 
* host: 
  * client and **server**
* access network
  * wired, wireless **communication** from network edge and network core
  * aka, **first mile network**, in the view of *deliver data* OR 
  * **last mile network**, in the view of *receive data*

#### Physical Media
