Sliding window

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The term sliding window ( English "sliding window") designated in the data flow control in computer networks a window that a transmitter transmitting a given amount of data allowed before an acknowledgment is expected back.

Network protocols based on sliding windows are called sliding window protocols or sliding window protocols . The term is particularly important for data packets .


The goal of the sliding window method is to use the capacities of the line and the receiver optimally, i.e. to send as many data packets ( data frames ) as possible. The delay-bandwidth product represents the maximum amount of data in the transmission that can be sent without waiting for the first confirmation.

With the sliding window method, the sender permanently keeps a list of consecutive sequence numbers that correspond to the number of frames that it is allowed to send. As soon as a data packet is successfully delivered to the recipient, the latter sends back an acknowledgment, also known as an ACK signal, which prompts the sender to transmit another frame. If, however, the sender does not receive an ACK within the timeout , it tries to transmit the frame again. Assuming that the delay bandwidth product has already been reached, no new frame can then be transmitted, i. E. H. there is a jam in the pipe . The transmission window shifts with each incoming confirmation, in that the confirmed frame falls out of the window and a new frame to be transmitted is added to the window. This means that the window only ever contains unacknowledged frames. In the event that frames are lost during transmission, the sender must keep all data packets in its memory so that they can be retransmitted.

Similar to the sender's window, the receiver also manages a sliding window. Both windows do not necessarily have to be the same size, as this can vary over time due to the sending and receiving of frames. The size of the transmission window is determined by the maximum specified by the recipient and the network load. Each confirmation for a successfully transmitted frame contains a value that indicates the number of additional data packets for which the recipient still has capacity.

Difference to the stop-and-wait algorithm

With the stop-and-wait algorithm , which, like the sliding window method , is an ARQ protocol , the sender waits for an acknowledgment after the transmission of a frame before transmitting the next frame. If there is no confirmation within the waiting time, the sender retransmits the frame. In contrast to this algorithm, which allows the sender only one outstanding frame on the connection line and is therefore inefficient, the sliding window algorithm can transmit several frames at the same time. The stop-and-wait algorithm is a special case of the sliding window algorithm. Both work identically if the size of the send window is set to 1 frame in the sliding window algorithm.


The sliding window algorithm has the following tasks in the area of ​​computer networking:

  1. the reliable delivery of data packets via an unreliable connection line,
  2. compliance with the transmission sequence of the frames,
  3. the flow control , d. H. a feedback mechanism that prevents the sender from transmitting more data than the receiver can process.

The sliding window protocol is used by the Transmission Control Protocol (TCP) to transfer data as efficiently as possible. The sliding window method is also used by the Point-to-Point Protocol (PPP).

Application example

The use of a sliding window in a radar device is described here as an example of the application of the method :

Use of the sliding window in a radar device

In some radars, this method is used to generate a digital target signal from analog target pulses. Are as in the plot extractor written a number of pulse repetition periods of all known echo signals in a memory designated radar assembly. Then, starting with the first range cell , the signals of the pulse periods are compared. If the echo signal reaches a certain threshold value in all pulse periods (i.e. in the memory: in all memory groups considered in parallel), the position of the sliding window also represents the coordinates of the object to be located.


  • Larry S. Peterson, Bruce S. Davie: Computer Networks - A Modern Textbook . 1st edition. dpunkt.verlag, Heidelberg 2000, ISBN 3-932588-69-X , pp. 99–112
  • Andrew S. Tanenbaum: Computer Networks. Pearson Studium, Munich 2003, ISBN 3-8273-7046-9 , p. 241

Web links

Individual evidence

  1. a b Set of slides on transport protocols (PDF; 59 kB) from the Swiss Federal Institute of Technology in Zurich; Retrieved November 6, 2006