[國考-電信類] 01 計算機概論-01 電腦網路 筆記(三)

第三章. Transport Layer 傳輸層 network edge

1. UDP (User Datagram Protocol, 使用者資料包協定)

• best-effort, no connection establishment, simple, 通常用來傳送multimedia data

2. Go-Back-N vs. Selective repeat

• Go-Back-N: 只用1個timer, timeout就全部重送
• Selective repeat: 封包各用一個timer

3. Reliable data transfer

	Status	Solution
rdt 1.0	Reliable channel (理想情況)
rdt 2.0	Bit error	Check sum, ACK/NAK
rdt 2.1	ACK/NAK corrupted, duplicate	SEQ number 1/0 in packet
rdt 2.2	NAK-free	SEQ 1/0 for ACK
rdt 3.0	Errors and loss	timer

4. TCP (Transmission Control Protocol, 傳輸控制協定)

• reliable data transfer: single re-transmission timer + cumulative ACKs, (n-1)前都收到
• fast re-transmit: 收到3個duplicate ACKs，不等timeout，重送

• flow control: Recv藉由RcvWindow控制Sender傳送量
• Connection Management: Three way handshake

• Congestion control (常考計算題)
• After 3 duplicate ACKs: CongWin is cut in half, window then grows linearly
• Threshold = CongWin / 2
• After timeout event: CongWin set to 1 MSS, window then grows exponentially to threshold, then grows linearly