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Friday, July 12, 2013

Sliding Window Protocols? – Part 1

- There are many types of data transmission protocols of which one type is the packet based data transmission protocols. 
- These protocols have a feature called the sliding window protocol.
- The sliding window protocols are a great help wherever the in-order delivery of the data packets demand reliability. 
- For example, the Data link layer of the TCP (transmission control protocol) model and OSI model demand such reliability and thus use window sliding protocol. 
- According to the concept of the sliding window protocols, a consecutive number which is unique is assigned to each and every portion of the transmission i.e., the packets.
- These numbers are used by the receiver for placing the packets it will receive in their correct order. 
- Also, with the help of these numbers, the missing packets can be identified and the duplicate packets can be removed. 
- One problem regarding the sliding window protocols is that it has kept no limits for the size of these numbers that are required. 

- An unlimited number of data packets can be allowed to be communicated at any instant of time if limits are placed on the number of packets involved in transmission or reception. 
- By this, we mean using the sequence numbers of fixed size. 
- By term window we refer to the transmission side. 
- It actually represents the logical boundary or limit of the number of packets that the receiver has to acknowledge. 
- The transmitter has to be informed by the receiver for each ACK (acknowledgement) packet regarding the maximum size or the window boundary of the current receiver buffer. 
- For reporting the window size of the received buffer, a 16 bit field is used in the TCP header. 
- The maximum limit or boundary of the window that we can have is 216 i.e., 64 KB. 
- When operating in the slow start mode, the counting of the transmitter begins with a low packet count.
- Gradually, the number of packets involved increases in every transmission after the ACK packet has been received. 
- Whenever it receives an ACK packet, the window slides logically by one packet for the transmission of a new packet. 
- On reaching the window threshold, one packet is sent by the transmitter for every one packet of ACK received. 
- Suppose the limit of the window is 10 packets and the transmitter is in slow start mode. 
- Then, first one packet will be transmitted followed by another two. 
- Between these two transmissions, it will send an ACK packet also. 
- This process will continue until the limit of 10 has reached. 
- After crossing the limit, the transmission is restricted to one i.e., for every ACK packet received only one data packet is transmitted. 
- When viewed during simulation, it seems as if the window is shifting by distance of one packet whenever an ACK packet is received. 
- For avoiding the traffic congestion, the sliding window protocol works up a great deal.
- In this way the application layer would not have to worry about transmission the next set of data packets. 
- It can continue to do so since the sliding windows of the packet buffer will be implemented on both the sides i.e., the receiver’s and the sender’s side by the TCP. 
-However, the network traffic influences the window size dynamically to a great extent. 
- In order to achieve the highest possible throughput, care should be taken for not forcing the transmitter to stop the transmission before one RTT or round trip delay time by the sliding window protocol. 
- The bandwidth delay product of the links in the communication should be less than the limit of the data amount that can be sent before sending ACK packet. - If this condition is not met, the links’ effective bandwidth will be limited by the protocol. 

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