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What is the Variable stack size ?

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What variable stack size is and how does it affect computer networks?

  • A variable stack depth (VSD) means that the maximum number of frames the system can store in its transmit buffer changes dynamically. 
  • When the number of free buffers nears exhaustion, the VSD decreases, freeing space in the buffer pool by discarding buffers as it receives them and so slowing down new transmissions. 
  • Conversely, when there are more free buffers than needed for simplex connections, VSD increases to save space in the buffer pool for potential retransmissions.
  • The benefits of VSD are improved resource utilization and reduced probability of overloads.

Advantages of variable stack size:

  • Better resource utilization: Frame discards limit the number of buffers and therefore the amount of bandwidth in the system. This results in a reduction in unnecessary queuing when there are too many frames, and an increase in throughput during light load periods.
  • Fractional Buffers: Showing that VSD can be used to conserve buffer space also helps show that buffers do not have to be a fixed size, as long as enough frames are saved for retransmissions. In fact, VSD could be used to produce smaller buffers than would normally be needed to preserve some transmission quality measures. This is particularly useful for wireless links because of their limited buffer space.

Disadvantages of variable stack size:

  • Inability to predict or control when variable buffer length will be needed during transmission.
  • Buffer space management can introduce an artificial delay between the receiving software and hardware buffers, making the system slow or fail. This problem can be avoided by adjusting a parameter called Window Size, which controls how many windows are built and transmitted at a time. The size of each window determines how long the packet is held in the queue before it is retransmitted. It might make more sense to use a fixed buffer space that adjusts based on current transmission conditions rather than on the number of frames buffered.
  • Some protocols require specific buffer sizes for specific purposes. MPLS labels, for example, are only 24 bits long yet can be of any length. A VSD would be required to support all possible lengths.
  • Variable Stacks depths could violate the principle of fairness, since for some connections the resources are more readily available and for others, they are more scarce.

Important points:

  • The VSD mechanism can be effective only if it is well-matched to the properties of the underlying transmission media.
  • A VSD can be used in a protocol stack that uses a variable maximum frame size, such as Frame Relay or ATM, but when using a fixed maximum frame size, such as Ethernet, it must be paired with an appropriate window size. The window size controls how many frames are allowed in the output queue at any given time.
  • In Computer Networking, Protocols are used for defining network communication procedures for two or more nodes. In this paper, these protocols work with some information transmission media but not with others like Ethernet and FDDI networks.


Variable Stacks depth in Networking can be used to make networks faster, more responsive, and more efficient. However Variable Stacks depth can cause problems such as frame loss and inefficient use of buffer space. Besides that TCP/IP protocol has some more rules that are very important in computer networking.

  • A protocol must be designed to allow operating efficiently over a wide range of network conditions while remaining useful and reliable. This prevents the need for adjustment of many parameters at each transmission, which can cause delays or congestion in the network path.
  • A protocol must have enough bandwidth on each link to accommodate variations in demand without full-duplex operation or requiring multiple buffers at each end. Buffers may be used at the receiving station to conserve space until the data is needed.
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Last Updated : 26 Jun, 2022
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