2. Sliding Window

<aside> 📖 Features

• Generalization of stop-and-wait: allow multiple un-acked segments
• Bound on number of un-acked segments, called window
• Can keep pipe full </aside>

<aside> 📖 Flow Control

• Allow a “window” of unacknowledged packets in flight
• When acknowledgment arrives, advance window
• Required sequence number space size depends on window sizes </aside>

2.1 Sliding Window Sender

• Every segment has a sequence number (SeqNo)
• Maintain 3 variables
  • Send window size (SWS)
  • Last acknowledgment received (LAR)
  • Last segment sent (LSS)
• Maintain invariant: $\text{(LSS - LAR) ≤ SWS}$
• Advance LAR on new acknowledgment
• Buffer up to SWS segments

2.2 Sliding Window Receiver

• Maintain 3 variables
  • Receive window size (RWS)
  • Last acceptable segment (LAS)
  • Last segment received (LSR)
• Maintain invariant: $\text{(LAS - LSR) ≤ RWS}$
• If received packet is < LAS, send acknowledgment
  • Send cumulative acks
  • NOTE:TCP acks are next expected data

2.3 RWS, SWS, and Sequence Space

• $\text{RWS≥1, SWS ≥1, RWS≤SWS}$
• Generally need RWS+SWS sequence numbers
  • RWS packets in unknown state (ACK may/may not be lost)
  • SWS packets in flight must not overflow sequence number space

2.4 TCP Flow Control

• Receiver advertises RWS using window field
• Sender can only send data up to LAR + window