# CSE350期中筆記
###### tags: `NSYSU`
## Chapter1
1. Packet switching vs Circuit switching
2. Internet Delay & packet lost
3. Protocal stack & OSI model
4. Encapsulation
## Chapter2
1. Client & Server
+ 伺服器端有固定的 IP 位址且永遠開啟並等候要求，通常單一伺服器無法容納所有用戶端的請求，所以一個資料中心 (Data Center) 有許多主機來建立一個虛擬伺服器。
2. P2P(Peer to Peer) architecture:
+ 利用一對對間歇性連線主機 (Peer) 之間進行的直接通訊。
+ Peer 不屬於服務提供者，所以不會透過專門的伺服器進行通訊。
+ Self-scalability: Peer 之間會透過散佈檔案傳給其他 Peer 而增加系統的服務容量。
+ Advantage: 通常不需要大量的伺服器基礎設施和伺服器頻寬。
+ Challenge:
+ 對 ISP 友善：P2P 會將視訊串流與檔案傳輸應用等上傳資料流從伺服器移向家用 ISP而帶給 ISP 壓力。
+ 安全性:因高度分散與開放的本質。
+ 誘因:仰賴使用者貢獻。
+ 由於每個peer不一定會在線上，所以會有管理上的困難
3. Transport service requirements: Common apps(舉例)
+ data integrity
+ security
+ timing
+ throughput
4. TCP VS UDP (PPT 2-15)
+ TCP
+ Properties:
+ reliable data transfer
+ flow control
+ congestion control
+ connection-oriented
+ Defact: Don't provide
+ timing guarantee
+ minimum throughput guarantee
+ security
+ UDP
+ Property:
+ unreliable data transfer
+ Defact: Don't provide
+ reliability
+ flow control
+ congestion control
+ timing, throughput guarantee
+ security
+ connection setup
5. HTTP is "stateless"
+ server maintains no information about past client requests
+ state壞處:程式會變得很複雜
6. HTTP non-persistent figure & persistent
+ non-persistent HTTP
+ at most one object sent over TCP connection
+ connection then closed
+ downloading multiple objects required multiple
connections
+ Round-Trip Time(RTT) def: time for a small
packet to travel from client to server and
back.
+ Response time(downloading an object)
+ one RTT to initiate TCP connection
+ one RTT for HTTP request and first few bytes
of HTTP response to return
+ file transmission time
+ Total: 2RTT + file transmission time
+ Defact:
+ 2 RTT/object
+ OS overhead for each TCP connection
+ persistent HTTP
+ multiple objects can be sent over single TCP
connection between client, server.
+ one RTT for all the referenced objects.
7. Proxy sever: soft state
+ Proxy server(web cache)會保存最近被請求過的物件副本，
如果使用者請求的物件沒有在proxy server裡的話，proxy
server會向原本的server請求物件
+ soft state: 保存最近被請求過的物件副本，過一段時間就要
重新跟原本server請求物件

8. Pull & Push
+ Pull: Client主動向Server發出request(如:HTTP)
+ Push: Server主動向Client傳輸資料(如:SMTP)
9. DNS records
+ RR format:(name, value, type, ttl)
+ type A
+ name = hostname
+ value = IP address
+ type NS
+ name = domain
+ value = hostname of authoritative name server
for this domain.
+ type CNAME
+ name = alias name for some canonical(the real)
name
+ value = canonical name
+ type MX
+ value = name of mailserver associated with name
## Chapter3
1. Multiplexing & Demultiplexing(PPT 3-11)
+ Multiplexing at sender
+ Handle data from multiple sockets, add transport
header.
+ Demultiplexing at receiver
+ Use header info to deliver received segments to
correct socket.
+ How works
+ Host receives IP datagrams and uses IP addresses
& port numbers to direct segment to appropriate
socket.
2. Checksum: 1's complement
+ Treat segment contents, including header fields, as
sequence of 16-bit integers.
3. Reliable Data Transfer(RDT)

+ RDT1.0: reliable transfer over a reliable channel
+ underlying channel perfectly reliable
+ no bit errors
+ no loss of packets
+ RDT2.0: channel with bit errors
+ underlying channel may flip bits in packet, but won't loss packets
+ checksum to detect bit errors
+ Recover from errors
+ ACKs
+ NAKs
+ New features
+ error detection
+ receiver feedback
+ RDT2.1:
+ sender
+ sender retransmits current pkt if ACK/NAK corrupted
+ sender adds sequence number to each pkt
+ receiver
+ receiver discards (doesn’t deliver up) duplicate pkt
+ RDT2.2: A NAK-free protocol
+ using ACKs only
+ receiver sends ACK for last pkt received OK
+ receiver must explicitly include seq # of pkt being ACKed
+ duplicate ACK at sender results in same action as NAK: retransmit current pkt
+ RDT3.0:
+ Reason
+ underlying channel can also lose packets.
+ Approach
+ sender waits reasonable amount of time of ACK
+ retransmits if no ACK received in this
time
+ if packet (or ACK) just delayed:
+ retransmission will be duplicate, but
seq #'s already handles this.
+ receiver must specify seq# of pkt
being ACKed.
+ requires countdown timer
+ Sender

4. (PPT 3-45) 為何$U_{sender} = \frac{3L/R}{RTT + L/R}$中$RTT$不用乘3
ANS: $lim_{n\to\infty} \frac{n(3L/R)}{n(RTT+L/R)+2L/R} = \frac{3L/R}{RTT+L/R}$
5. Go-Back-N
+ sender
+ ACK(n):ACKs all pkts up to, including seq#n ->
cumulative ACK
+ may receive duplicate ACKs(see receiver)
+ timer for oldest in-flight pkt
+ timeout(n): retransmit packet n and all higher
seq# pkts in window
+ receiver
+ always send ACK for correctly-received pkt with
highest in-order seq#
+ may generate duplicate ACKs
+ need only remember expectedseqnum
+ out-of-order pkt
+ discard(don't buffer)
+ re-ACK pkt with highest in-order seq#

6. (PPT 3-52 下圖有錯)
圖中紅框部分: 上圖尚未送出封包下圖卻已經ACK
7. selective repeat sender & receiver
+ sender
+ case1: 收到上層的資料
+ 當收到上層的資料，sender檢查下一個可以傳的封包序號，
如果序號在窗格裡則把資料封裝成封包並送出。
+ 如果序號沒有在窗格裡則存入緩衝區或者退回上層，稍後再傳。
+ case2: Timeout
+ 每個傳出去的封包都會有一個計時器
+ 如果該封包的計時器超過一定時間，則重傳該封包並重新開始
計時器
+ case3: 收到ACK(n)
+ 收到序號n的ACK則把序號n的封包標記為已收到
+ 如果序號n在窗格內且序號等於send_base則將
send_base設為序號最小的未確認封包上。
+ receiver
+ case1: 接收到的封包序號位於\[rcv_base, rcv_base+N-1\]
+ 回傳該封包的ACK給sender
+ 若該封包之前沒收到過則存入緩衝區
+ 若該封包的序號等於rcv_base，則之前存入緩衝區的封包
都往上層傳(與rcv_base連續的封包)
+ case2: 接收到的封包序號位於\[rcv_base-N, rcv_base-1\]
+ 回傳該封包的ACK給sender且不存入緩衝區
+ Default: 不做任何事
8. TCP sender & receiver
+ sender
+ events
+ data rcvd from app
+ create segment with seq#
+ seq# is byte-stream number of first data
byte in segment
+ start timer if not already running
+ timeout
+ retransmit segment that caused timeout
+ restart timer
+ ack rcvd
+ if ack acknowledges previously unacked
segments
+ update what is known to be ACKed
+ start timer if there are still unacked
segments

9. TCP Ack generation

| event | TCP receiver action|
| -------- | -------- |
| 按預期seq編號到達，所有預期編號之前的區段皆已ACK | 等待下一區段到達再ACK，至多等待500ms |
| 按預期seq編號到達，有區段尚未ACK | 立刻送出一筆ACK，一次確認兩筆區段 |
| 比預期seq編號大的區段到達，出現Gap | 立刻送出duplicate ACK，指出預期seq編號 |
| 抵達區段可填補Gap | 立刻送出ACK，指出Gap底部 |

10. TCP fast retransmit:網路不算差時可以快速重傳
+ 逾時重送的機制，等待timeout會拖延重送遺失封包
+ 利用"Duplicate ACK"來偵測封包的遺失
+ sender通常一次送出大量區段，當區段遺失時便會收到許多相同的duplicate ACK
+ 當sender收到3個以上相同data的ACK，就會認為區段已遺失，不需等待timeout即重送

11. TCP 3-way handshake

12. TCP slow-start 變化圖

13. TCP Congestion control FSM diagram

14. TCP 不公平的原因
+ 假設只有TCP連線會通過瓶頸連結且所有連線都有相同的RTT
+ 每對主機與目的端之間都只存在單筆TCP連線
+ RTT較短會有更高的產出率
15. TCP close connection