TCP window scale option

The TCP window scale option is an option to increase the receive window size allowed in Transmission Control Protocol above its former maximum value of 65,535 bytes. This TCP option, along with several others, is defined in IETF RFC 1323 which deals with long fat networks (LFNs). The TCP window scale option is an option to increase the receive window size allowed in Transmission Control Protocol above its former maximum value of 65,535 bytes. This TCP option, along with several others, is defined in IETF RFC 1323 which deals with long fat networks (LFNs). The throughput of a communication is limited by two windows: the congestion window and the receive window. The congestion window tries not to exceed the capacity of the network (congestion control); the receive window tries not to exceed the capacity of the receiver to process data (flow control). The receiver may be overwhelmed by data if for example it is very busy (such as a Web server). Each TCP segment contains the current value of the receive window. If, for example, a sender receives an ack which acknowledges byte 4000 and specifies a receive window of 10000 (bytes), the sender will not send packets after byte 14000, even if the congestion window allows it. TCP window scale option is needed for efficient transfer of data when the bandwidth-delay product (BDP) is greater than 64K. For instance, if a T1 transmission line of 1.5 Mbit/second was used over a satellite link with a 513 millisecond round trip time (RTT), the bandwidth-delay product is ( 1 , 500 , 000 ∗ 0.513 ) = 769 , 500 {displaystyle scriptstyle (1,500,000*0.513)=769,500}  bits or about 96,187 bytes. Using a maximum buffer size of 64 KiB only allows the buffer to be filled to (65,535 / 96,187) = 68% of the theoretical maximum speed of 1.5 Mbits/second, or 1.02 Mbit/s. For example if the receive window is 2 15 = 32 , 768 {displaystyle scriptstyle 2^{15}=32,768} bytes(receive window is the 2 bytes in the TCP header). If the window scale factor(options in TCP) is 3, calculation is as follows: 2 15 ∗ 2 3 = 262 , 144 {displaystyle scriptstyle 2^{15}*2^{3}=262,144} bytes.In essence, this is 2 18 = 262 , 144 {displaystyle scriptstyle 2^{18}=262,144} . (Shifting 3 bits to the left)Note that in the options field, window scale factor is sent in the SYN packet.Also, the value of the Window scale factor can go until 255 ( 2 8 = 1 {displaystyle scriptstyle 2^{8}=1} byte) but the maximum allowed is 14.The reason why maximum value is 14 is because 2 16 ∗ 2 14 = 1 , 073 , 741 , 824 {displaystyle scriptstyle 2^{16}*2^{14}=1,073,741,824} . If it increases beyond this then it will surpass the Sequence no#. Sequence no# is 4 bytes ( 2 32 {displaystyle scriptstyle 2^{32}} ) and the size of the window can't go beyond the maximum value of the sequence no#.