Taking up where we left off from the previous article (Part I), let us examine the other “techniques in the Collection” that make for WAN optimization.
Throughput Optimization is a factor of the maximum bandwidth of the slowest link in the network path.
Round Trip Time (RTT): This is the time taken by a transmitter to send a packet and receive acknowledgement from the receiver. The time delay defines the transmission time between the two end points. The RTT is also known as the ping time and can be determined by using the ping command over a network. High bandwidth networks have high RTT and can have a large bandwidth delay-product for data in flight. However, the protocol requires a specialized design that accommodates a “long fat pipe” and a windows scale option.
Window Size: This defines the amount of data that can be received by a computer without having to send out an acknowledgement to the sender. If the window size is set to one byte, the sender will wait to receive the acknowledgement from the receiver before sending out the next byte. Failure to receive an acknowledgement may trigger a retransmit. This will cause latency. If the window size is large or the number of bytes that can be received without acknowledging is large, the latency will be lower. This will speed up data transfer. The full bandwidth will be optimally used.
Packet Loss: This is a congestion avoidance algorithm that is built into the TCP. This algorithm kicks in to action if there is a light to moderate packet loss. The packet is treated as lost if acknowledgement is not received by the sender and a retransmit action is initiated. The window size will be increased by one at every round trip till the acknowledgement is received. Multiple acknowledgements may be treated as timeout and the packet may be transmitted again after the system returns to the slow start state.
TCP performance has been improved with the provision of Long Fat Networks (LFNs). TCP timestamps have been developed to allow a more precise RTT estimation and windows scaling options.
Selective Acknowledgement options allow users to define when acknowledgements will be issued to senders. Additionally, Path MTU discovery technique is being used to prevent in-network fragmentation and improve performance.
Many Cloud services use the local WAN optimizer to answer client requests locally. They leverage the write-behind and read-ahead mechanisms for reducing WAN latency.
This article is Part II in a four part series on WAN Optimization.