1. Performance of RDT 3.0 (timeout)

• Throughput is the same at any point of time, because when there are 2 packets send, the x (times) 2 will be canceled out on the fraction, making it the same as the throughput of 1 packet being sent.
  
• The percentage of time being used by the sender to actually send data.
• RDT 3.0 performance is pretty bad!
  
• Because of the stop-and-wait protocol, sender must wait additional 30ms (in the example), making the utilisation low.

1-1. Better Solution than Wait-and-Stop

• Sending packets back to back can increase the number of packets being sent during a given time, making the throughput higher.
• Window size refers to the number of packets that can be sent back to back.

2. Pipelined Protocol

• In order to keep track of the packet bit errors and losses, each packet needs to be numbered - can no longer use 2 digits..

2-1. Go-back-N in Action

• The packets that came after a lost packet will not be accepted to keep the order of packets right.
• Go-back-N is the same as RDP 3.0, just with multiple back-to-back packets. If the window size is 1, then they are exactly the same.
• The sender ignores duplicate ACK and waits for the timeout.
• pkt4 is sent after receiving ACK0, because the window is moved.
  • window is moved after every ACK received
    
• Cumulative ACK signals that the packets previous to the sequence number is well received.
  • The window will move according to the cumulative ACK.
  • The new packets added to the window will be attempted to be sent.
• Cumulative ACK would not increase if a packet is not received in order.
• The receiver will keep track of the packets being received irrelevant to the cumulative ACK values.
  • In other words, the cumulative ACK value does not represent the status of packets in order on the receiver.
  • Ex) Receiver receives pkt4 even when the sender receives ACK2.
• If the cumulative ACK value is not the highest packet number, the packets with numbers bigger than the cACK will be timed and be re-sent.
• When time-out, resend everything that is in the window.

Go-back-N Improvement

• Instead of discarding packets that are not in order maybe we can store them somewhere else.
  
• retrasmitting only the unACKED packet is called selective repeat.

Selective Repeat

• packet 2 is resent because it is lost. Only a single packet is asked to resend.
  
• Packet 10 is lost, because packet 11 and 12 are already received and they should have been sent in order.
• Packet 6 and 7 are also lost because packet 8 and 9 are already received and sent to application layer (that's why the packets are no longer usable).

2. Connection Oriented Transport: TCP

• Point-to-point
• Connection-oriented
• Full duplex service
• Reliable, in order byte-stream

Connection Orientedd De-mux

• (srcIPAddr, srcPort, destIPAddr, destPort)
  

• Maximum segment size (MSS) does not include header size.
  • It only includes the size of the application layer data.
  • If MSS is 1460 bytes, then the total payload could be 1600+ bytes, for example.

• A: Data packet or feedback packet?
  • A = 0: Data packet
  • A = 1: Feedback packet
• S: SYNbit = 1: attempt to connect a TCP connection.
• F: Finbit = 1: on closing the socket.
  
• The sequence number represents the first byte of the segment.
• The next sequence # would then be + MSS.
  
  
• Initial Sequence Number (ISN): chosen randomly at the start.
• In bi-directional communication, both sides choose ISN.
• ACK can piggyback data packet as well.
  

• The second example shows timeout being premature.
• The ACK value would be 105, because it is cumulative ACK.

• Timeout interval must be longer than RTT, but RTT varies:
  • When there is high congestion, RTT is higher.
    
• Safety margin is added so that premature timeout is prevented.