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      • 2.4 The End-to-End Principle
      • 2.5 Error Detection:3 schemes (Checksum,CRC and MAC)
      • 2.6 Finite State Machines(有限状态机)
      • 2.7 Flow Control
      • 2.8 Sliding window
      • 2.9 Retransmission Strategies
      • 2.10 TCP Header
      • 2.11 TCP Setup and Teardown
      • 2.12 Recap
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  • Stop and wait 的缺陷
  • Sliding window
  • sliding Window VS Stop And Wait
  • sliding windows sender（发送方）
  • sliding windows receiver（接收方）
  • RWS,SWS,and Sequence Space
  • TCP Flow Control

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  1. CS144
  2. Transport Layer

2.8 Sliding window

Previous2.7 Flow ControlNext2.9 Retransmission Strategies

Last updated 4 years ago

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Stop and wait 的缺陷

带宽利用效率低

Sliding window

  • 广义的 stop and wait: 允许多个 un-acked segments。

  • Bound on number of un-acked segments, 称为窗口

  • Can keep pipe full（能保持管道满载）

sliding Window VS Stop And Wait

sliding windows sender（发送方）

  • 每个 segment 都有一个序列号(seqNo)

  • 维护三个变量

    • Send windows size(SWS)

    • Last acknowledgment recivied (LAR)

    • Last segment sent(LSS)

  • 维护一个变量关系

    • (LSS - LAR) <= SWS

    • 即 LSS <= LAR + SWS

    • 如果中间有丢失， LSS 则没法推进

  • Advance(推进) LAR on new acknowledgment

  • Buffer(缓存) up to SWS segments

sliding windows receiver（接收方）

  • 维护三个变量

    • Receive windows size(RWS) 接收的 windows size

    • Last acceptable segment (LAS)最后一个可接收的 segment

    • Last segment received(LSR) 最后一个接收的 segment

  • 维护一个变量关系

    • (LAS - LSR) <= RWS

    • 即 LAS <= LSR + RWS

  • 如果接受的 packet < LAS,则发送 acknowledge，如果不是会直接丢掉

    • 发送 cumulative(累计) acks: 如果接收 1,2,3,5，acknowledge 3 (TCP acks 是下一个期待的数据，因此发送的是 ack 4)

RWS,SWS,and Sequence Space

  • RWS >= 1, SWS >= 1, RWS <= SWS

  • 如果 RWS = SWS ，需要 2SWS sequence numbers

  • 通常需要 RWS + SWS sequence numbers

    • RWS packet 在一个未知的状态 (ACK may/may not be lost)

    • SWS packets in flight(飞行) must not overflow sequence number space(不能溢出序列号空间)

TCP Flow Control

  • Receiver advertises(通知) RWS using window field

  • Sender 只能将数据发送到 LAR + window

如果 RWS = 1, "go back N"协议, 需要 SWS + 1 sequence numbers

SR 简书
滑动窗口协议的窗口大小
slidingWindowVSstopAndWait
SR part1
TCPFlowControl
Go Back N