Computer Network Unit 1 q what are the topologies in computer n/w ?

Sliding Window Protocol :
Sliding window algorithms are a method of flow control for network data transfers.
TCP uses a sliding window algorithm, which allows a sender to have more than one unacknowledged packet "in flight" at a time, which improves network throughput. 

Key concepts of the Sliding Window

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  Both the sender and receiver maintain a finite size buffer to hold outgoing and incoming packets from the other side.
  
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  Every packet sent by the sender, must be acknowledged by the receiver. The sender maintains a timer for every packet sent, and any packet unacknowledged in a certain time, is resent.
  
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  The sender may send a whole window of packets before receiving an acknowledgement for the first packet in the window.
  This results in higher transfer rates, as the sender may send multiple packets without waiting for each packet's acknowledgement.
  
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  The Receiver advertises a window size that tells the sender how much data it can receive, in order for the sender not to fill up the receivers buffers.
  

Example
Figure shows an unrealistic example of a sliding window. The sender has sent bytes up to 202. We assume that cwnd is 20 (in reality this value is thousands of bytes). The receiver has sent an acknowledgment number of 200 with an rwnd of 9 bytes (in reality this value is thousands of bytes). The size of the sender window is the minimum of rwnd and cwnd or 9 bytes. Bytes 200 to 202 are sent, but not acknowledged. Bytes 203 to 208 can be sent without worrying about acknowledgment. Bytes 209 and above cannot be sent.

In Figure the server receives a packet with an acknowledgment value of 202 and an rwnd of 9. The host has already sent bytes 203, 204, and 205. The value of cwnd is still 20. Show the new window.

Go Back-N :
Go-Back-N ARQ is a specific instance of the Automatic Repeat-reQuest (ARQ) Protocol, in which the sending process continues to send a number of frames specified by a window size even without receiving an ACK packet from the receiver.
The receiver process keeps track of sequence number of the next frame it expects to receive, and sends that number with every ACK it sends. The receiver will ignore any frame that does not have the exact sequence number it expects -- whether that frame is a "past" duplicate of a frame it has already ACK'ed, or whether that frame is a "future" frame past the lost packet it is waiting for. Once the sender has sent all of the frames in its window, it will detect that all of the frames since the first lost frame are outstanding, and will go back to sequence number of the last ACK it received from the receiver process and fill its window starting with that frame and continue the process over again.