## chapter 12
### DSL
* Uses existing telephone infrastructure
* up to 8Mbps downstream, 1Mbps upstream
* **dedicated** physical line to telephone central office
* multiplexing by spliting the frequency
### Cable Modem
* Uses cable TV infrastructure
* HFC: hybrid fiber coax
* asymmetric: up to 30Mbps downstream, 2 Mbps upstream
* net work of cable and fiber attaches homes to ISP router
* homes **share** access to router
* FDM: frequency devision multiplexing
### FTTH
* Optical links from CO(central office) to the home
* Two competing optical technologies:
* Passive Optical network (PON)
* Active Optical network (AON): signal amplification
* Much higher Internet rates; fiber also carries television and phone services
### Ethernet Internet access
* Typically used in companies, universities, etc
* 10 Mbps, 100Mbps, 1Gbps, 1Gbps, 10Gbps Ethernet
* Today, end systems typically connect into Ethernet switch
### Wireless access networks
* shared wireless access network connects end system to router
* via base station aka "access point"
* wireless LANs:
* 802.11/g (WIFI): 11 or 54 Mbps
* wider area wireless access
* provide by telco operator
* ~1Mbps over cellular system(EVDO, HSDPA)
## chapter 13

### circuit switching
defdicated circuit per call: telephone net

End-end resources reserved for "call"
* link bandwidth, switch capacity
* **dedicated** resources: no sharing
* circuit-like (**low delay and no loss**) (guaranteed) performance
* call setup required (缺點)
network resources divided into calls
* resource piece idle if not used by owning call (no sharing) (缺點，資源閒置)
* devided link bandwidth into "pieces"
* frequency division
* time division

### packet switching
data sent thru net in discrete "chunks" (packet)

each end-end data stream divided into packets
* user A, B packets share network resources
* each packet uses full link backwidth
* resources used as needed
resource contention (缺點，資源爭奪)
* aggregate resource demand can exceed amount available
* congestion: packets queue, wait for link use **might loss** (缺點，會掉包)
* store and forward: packets move one hop at a time (缺點，會等到封包完整抵達之後才會傳送)
* Node receivescomplete packet before forwarding
* Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps
* bps: bits per second
* Entire packet must arrive at router before it can be transmitted on next link: **store and forward**
* **"store and forward"** delay = 3L/R
* So larger packet takes more delay, because of **store and forward**
Message Segment
* piplining: each link works in parallel
* the reason why keep small packets，越小store and forward造成的延遲越低
* 

Statistical Multiplexing
Why store and forward?
* Two key network-core functions
* routing: determines source-destination route taken by packets
* routing algorithms
* forwarding: move packets from router's input to appropriate router output
* 
* Because of the forwarding functionality, so we need to store the entire packet
forwarding
* Goad: move packets through routers from source to destination
* datagram network:
* destination address in packet determines next hop
* routes may change during session
* analogy: driving, asking directions
* virtual circuit network:
* each packet carries tag (virtual circuit ID), tag determines next hop
* fixed path determined at call setup time, remains fixed thru call
* **routers maintain per-call state**
### Packet switching vs. Circuit switching
> Packet switching allows more users to use network!
* 1Mb/s link
* each user:
* 100kb/s when "active"
* active 10% of time

* circuit-switching:
* 10 users
* packet switching:
* with 35 users, probability > 10 active at same time is less than 0.0004

> Is packet switching a "slam duck winner" ?
* Great for bursty data
* resource sharing
* simpler, no call setup
* Excessive congestion: packet delay and loss
* protocols needed for reliable data transfer, congestion control
* Q: How to probide circui-like behavior?
* bandwidth guarantees needed for audio/video apps
* still an unsolved problem (chapter 9)

## chapter 14

### Four sources of packet delay
1. nodal processing
* check bit errors
* determine output link
2. queueing
* time waiting at output link for transmission
* depends on congestion level of router
3. Transmission delay): (傳輸延遲
* R = link bandwidth (bps)
* L = packet length (bits)
* time to send bots into link = L/R
4. Propagation delay: (物理傳播延遲)
* d = length of physical link
* s = propagation speed in medium (~${2*10^8 m/sec}$)
* propagation delay = d/s
### Nodal delay
${d_{nodal} = d_{proc} + d_{queue} + d_{trans} + d_{prop}}$
* ${d_{proc}}$ = processing delay
* typically a few microsecs or less
* ${d_{queue}}$ = queuing delay
* depends on congestion
* ${d_{trans}}$ = transmission delay
* = L/R, significant for low-speed links
* ${d_{prop}}$ = propagation delay
* a few microsecs to hundreds of msecs
### Queueing delay
* R = link bandwidth (bps)
* L = packet length (bits)
* a = average packet arrival rate
traffic intensity = La/R
* La/R ~0: average queueing delay small
* La/R ->1: delays become large
* La/R > 1: more "work" arriving than can be serviced, average delay infinite!
### Packet loss
* queue (aka buffer) preceding link in buffer jas finite capacity
* packet arribing to full queue dropped (aka lost)
* lost packet may be retransmitted by previous node, by source end system, or not at all
### Throughput
* throughput: rate (bits/time unit) at which bits transferred between sender/receiver

## chapter 35
### TCP: Overview
* point-to-point:
* one sender, one receiver
* reliable, in-order byte stream:
* no "message boundaries"
* pipelined:
* TCP congestion and flow control set window size
* full duplex data:
* bi-directional data flow in same connection
* MSS: maximum segment size
* connection-oriented:
* handshaking (exchange of control msgs) inits sender, receiver state before data exchange
* flow controlled:
* sender will not overwhelm receiver

### TCP segment structure
head len: header length, UDP has the hole file length
A，ACK: 當A flag on，代表acknowledgement number有效，傳送者想要確定這些byte有收到
#### connection managment
表示封包有被做connection management
R，RST: reset connection，重設連接
S，SYN: setup connections，設定連接
F，FIN: tear down a connection，解除連接

P，PSH: push data now, 馬上回推資料 (不太用)
U，URG: urgent data，緊急資料 (也不太用)

#### 其實也有在用的bits，在not used裡面
not really for rdt, not for connection
E、C bit: 工程師預留做condition controll的

Urg data pointer: 當URG flag on，會從Urg data pointer這邊去指向要尋找的資料
receive window: window size for flow controll，要變慢receive window數字會變小，要變快receive window數字就會變大

sequence number、acknowledgement number: rdt用的，注意TCP是用byte為單位傳輸，not packet

## chapter 41
### Network layer
* transport segment from sending to receiving host
* network layer protocols in every host, router
* sending side: encapsulates segments into datagrams
* receiving side: delivers segments to transport layer
* router examines header fields in all IP datagrams passing through it
### Two key network-layer functions
* network-layer functions:
* forwarding: move packets from router's input to appropriate router output
* routing: determine route taken by packets from source to destination
* routing algorithms
* 比喻: 旅行
* forwarding: 處理每一個中繼站的交換
* routing: 預處理旅行的起點和目的地
### Network layer: data plane, control plane
* Data plane
* local, per-router funciton
* determins how datagram arriving on router input port is forwarded to router output port
* 處理 forwarding function
* Control plane
* network-wide logic
* determines how datagram is routed among routers along end-end path from source host to destination host
* 處理 routing funciton
* two control-plane approaches:
* traditional routing演算法
* distribute, locally
* implemented in routers
* software-defined networking(SDN)
* centralized
* implemented in (remote) servers
### Per-router data plane
Forwarding table to allow fast output interface lookup

### Network-wide logic in control plane
Traditional: Individual routing algorithm components in each and every router interact in the control plane

### Logically centralized control plane
SDN: A distinct (typically remote) controller interacts with local control agents (CAs)


## chapter 42

### Router 架構

### Input port functions
* decentralized switching:
* using header field values,利用記憶體裡面的 forwarding table 查找 output port
* 目標: 在line speed下，完成input port處理
* queuing: if datagrams arrive faster than forwarding rate into switch fabric
* destination-based forwarding: forward based only on destination IP address (traditional) (比較快、便宜)
* generalized forwarding: forward based on any set of header field values (SDN) (比較慢，所以要更快的lookup處理，所以比較貴)

### Switching fabrics
* transfer packet from input link to appropriate output link
* switching rate: rate at which packets can be transfer from inputs to outputs
* often measured as multiple of input/output line rate
* N inputs: switching rate N times line rate desirable
* 

* three major types of switching fabrics

### Switching via memory
first generation routers:
* 速度最慢，成本最低

### Swtiching via a bus
* 多人傳送時有競爭
* 交換速度取決於bus的頻寬

### Switching via interconnection network
* 每個人一個bus

### Input port queuing

HOL問題: 第一個封包送不出去，所以影響到後面的封包

### Output port queuing
* 進來速度 > 出去速度就會有queue問題

### Buffer Management
* drop
* 剛進來的先丟
* 有優先權的話，低的先丟


### Packet Scheduling: FCFS
* packet schefuling: 決定誰先傳
* 先進先服務(FCFS)
* 優先權
* 輪流
* 優先權決定服務權重
* 
* FCFS: 就是First-in-first-out(FIFO)

### Scheduling policies: priority
priority scheduling:
* 把進來的東西分等級，根據等級排順序
* 同個等級FCFS
* 

### Scheduling policies: round robin
* 分等級，每個等級輪流輸出
* 

### Scheduling policies: weighted fair queuing
Weighted Fair Queuing (WFQ):
* 每個等級都給服務，但服務權重不同

## chapter 43

### Network Layer: Internet
host, router network layer functions:

### IP Datagram format
* ver: 版本
* head len: header 有多長，新版ipv6是固定長度
* tos(type of service): 資料欄位服務形式
* length: 包含header的長度
* 如果ip要切割的話，需要這些欄位
* time to live: 為了避免有些有問題的封包，找不到目的地，但是被一直繞送，所以每次轉送這個封包，這個欄位就會-1，當值為0，router就會把這個封包丟掉，然後告訴sender這個封包被丟掉了
* upper layer: 說明上層的欄位哪個協定處理
* header checksum: 判斷收到的封包有沒有錯誤
* source ip: 就sender ip
* destination ip: 就receiver ip
* options: 選項增加的欄位

### IP fragmentation/reassembly
* 當從MTU比較大的網路進入到MTU比較小的網路，封包就需要切割，在router切割
* 重組只能在receiver執行
* 16bit identifier: 辨識哪些原本是同一個ip封包的
* flgs: 知道哪個ip分段是最後一個分段，辨識什麼時候結束，0代表後面沒有了，1代表後面還有
* fragment offset: 辨識切割前順序的編號，8bytes為一單位
* 
* 

### IP addressing: introduction
* ip address: 32-bit identifire，定義到網卡上面，是定義在介面，一台機器可能會有多個ip，所以要說是某台機器的某一個介面的ip
* interface: connection between host/router and physical link
* 通常一台router有多個介面，所以有多個ip
* host通常只有一或兩個介面(e.g., wired Ethernet, wireless 802.11)

### Subnets 子網路
* ip分為兩段，前段一樣的話代表在同一個子網路
* 互相連接的設備在同一個子網路的話不用透過router
* IP addresses have structure
* subnet part: 設備在同一個子網路前面會有同樣的ip
* host part: 後面那段ip會不一樣
* 

### IP addressing: CIDR
CIDR: Classless InterDomain Routing
* https://aws.amazon.com/tw/what-is/cidr/
* 
* 

### DHCP: Dynamic Host Configuration Protocol
動態配置ip
goal: 當host加入網路時，動態配置ip給他
* 可以租借一個ip來用，server會說可以用多久
* 允許使用以前用過的ip
* 支援加入網路的mobile用戶
實現了少量ip服務多數人

### DHCP: more than IP address
* 還會給default gateway和DNS
* first-hop router 的地址
* name and IP address of DNS server
* network mask (indicating network versus host protion of address)

### IP addresses: how to get one
ISP要跟國際組織申請

## chapter 62

### Internet checksum
https://www.cnblogs.com/shelmean/p/16091622.html

### Cyclic redundancy check CRC
除法進行時，用XOR做餘數計算

## chapter 63
multiple access protocols

### Multiple access links, protocols
兩種links:
* point-to-point 點對點
* point-to-point link between Ethernet switch, host
* PPP for dial-up access
* broadcast (shared wire or medium) 被多人共享
* old-fashioned Ethernet
* upstream HFC in cable-based access network
* 802.11 wireless LAN, 4G/4G, satellite


### Multiple access protocols
* 決定哪一個人哪一個節點何時傳送
* 決定發言的機制
* 用同一個頻道控制

### 理想的 multiple access protocols
用於判斷協定優缺
給定環境: multiple access channel of rate R bps
* 如果只有一個人在用，那他要可以用到全部rate R速度
* 當有M個節點要用，那平均速率要R/M
* fully decentralized(完全分散式):
* 沒有一個特別的node去處理排程、協調、碰撞
* 節點彼此不用同步
* simple 簡單

### Random access protocols
沒有事先規劃好哪個節點在什麼時候傳送
* when node has packet to send
* 傳送時使用完整rate R
* 沒有排程
* two or more transmitting nodes: collision 碰撞
* random access MAC protocol 著重於:
* 如何偵測碰撞
* 如何解決碰撞
* examples of random access MAC protocols:
* ALOHA, slotted ALOHA
* CSMA, CSMA/CD, CSMA/CA

### Pure ALOHA
* 幾乎沒有限制，沒有規定時間，沒有規定環境
* 很容易發生碰撞
* 發生碰撞的機率是兩個packet time，發送前、發送後
* pure Aloha 效率: 18%

### Slotted ALOPHA
假設:
* 所有封包大小一樣
* 時間切成大小一樣的time slot
* nodes只能在slot開始的時候傳送
* nodes同步了
* 如果有兩個含以上在slot裡面，全部nodes都偵測到碰撞

操作:
* node要傳送只能在下一個slot開始的時候傳送
* 沒有碰撞就成功了
* 有碰撞就在後面的slot以機率p的方法決定傳送，直到成功

效率

優點:
* 一個人要使用可以使用全部資源
* 分散式的，但是要同步
* 簡單

缺點:
* 碰撞
* idle slots
* node可能會花很多時間在處理碰撞
* 要同步
* 很多人要用不能均分

## Reference
好看影片: https://www.youtube.com/@pollyhuang
還有隔壁班老師的影片