Understanding "why TCP buffers data on receiver side"

Introduction

TCP must buffer received data because it doesn't know when the application is going to actually read the data and it has told the sender that it is willing to receive (the available "window"). All this data gets stored in the "receive window" until such time as it gets read out by the application.

Once the application reads the data, it drops the data from the receive window and increases the size it reports back to the sender with the next ACK.

Significance in high performance system

Buffering is used throughout high performance network systems to handle delays in the system. In general, buffer size will need to be scaled proportionally to the amount of data "in flight" at any time.

Impact of Buffer Management on TCP Window Size

To understand why the window size may fluctuate, we need to understand what it represents. The simplest way of considering the window size is that it indicates the size of the device's receive buffer for the particular connection. That is, window size represents how much data a device can handle from its peer at one time before it is passed to the application process. Let's consider the aforementioned example. I said that the server's window size was 360. This means the server is willing to take no more than 360 bytes at a time from the client.

When the server receives data from the client, it places it into this buffer. The server must then do two distinct things with this data:

• Acknowledgment: The server must send an acknowledgment back to the client to indicate that the data was received.

• Transfer: The server must process the data, transferring it to the destination application process.

It is critically important that we differentiate between these two activities. Unfortunately, the TCP standards don't do a great job in this regard, which makes them very difficult to understand. The key point is that in the basic sliding windows system, data is acknowledged when received, but not necessarily immediately transferred out of the buffer. This means that is possible for the buffer to fill up with received data faster than the receiving TCP can empty it. When this occurs, the receiving device may need to adjust window size to prevent the buffer from being overloaded.

Reference

http://stackoverflow.com/questions/13220956/why-does-tcp-buffer-data-on-receiver-side

https://en.wikipedia.org/wiki/TCP_tuning

http://www.tcpipguide.com/free/t_TCPWindowSizeAdjustmentandFlowControl.htm