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Asked: 2026-05-25T21:35:49+00:00

My intent is to write a app. layer process on top of libnids. The

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My intent is to write a app. layer process on top of libnids. The reason for using libnids API is because it can emulate Linux kernel TCP functionality. Libnids would return hlf->count_new which the number of bytes from the last invocation of TCP callback function. However the tcp_callback is called every time a new packet comes in, therefore hlf->count_new contains a single TCP segment.

However, the app. layer is supposed to receive the TCP window buffer, not separate TCP segments.

Is there any way to get the data of the TCP window (and not the TCP segment)? In other words, to make libnids deliver the TCP window buffer data.

thanks in advance!

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  1. Editorial Team

    You have a misunderstanding. The TCP window is designed to control the amount of data in flight. Application reads do not always trigger TCP window changes. So the information you seek is not available in the place you are looking.

    Consider, for example, if the window is 128KB and eight bytes have been sent. The receiving TCP stack must acknowledge those eight bytes regardless of whether the application reads them or not, otherwise the TCP connection will time out. Now imagine the application reads a single byte. It would be pointless for the TCP stack to enlarge the window by one byte — and if window scaling is in use, it can’t do that even if it wants to.

    And then what? If four seconds later the application reads another single byte, adjust the window again? What would be the point?

    The purpose of the window is to control data flow between the two TCP stacks, prevent the buffers from growing infinitely, and control the amount of data ‘in flight’. It only indirectly reflects what the application has read from the TCP stack.

    It is also strange that you would even want this. Even if you could tell what had been read by the application, of what possible use would that be to you?

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