Sliding Window

• Sender sends a bunch of packets at once without waiting for ACKs right then
• Receiver sends an ACK for each packet it receives
• When the sender receives an ACK, it knows the packet was delivered, so it slides the window forward

Pipelining

Pipelining: sender allows multiple, “in-flight,” yet-to-be-acknowledged packets

• Range of sequence numbers must be increased
• Buffering at sender and/or receiver
• Goal: Increased utilization

Dealing with packet loss

1. Go-Back-N: Retransmit all un-ACK-ed packets
   • Send N packets at once
   • If the kth packet (where 1 ⇐ k ⇐ N) doesn’t make it to receiver, packets k..N must be resent (wasteful)
2. Selective Repeat/Selective Retransmit: Retransmit only lost packets
   • Unlike Go-Back-N, receiver may selectively reject a single packet, rather than all the packets following a packet in a particular window

How to Slide Window Forward

If a packet in the middle of the window is ACKed but not the ones before it, the end of the window moves forward but not the start

• Window size doesn’t actually grow
• I think this is actually only applicable to Selective Repeat. With Go-Back-N, you’d resend everything after the first lost/rejected packet

Throughput

Utilization with a window size of 3 (fraction of time sender busy sending): $U_{\text{sender}}=\frac{3\cdot L/R}{RTT+L/R}$

With a window size of 3, utilization is 3 times that of Stop and Wait