Consider a network with end to end window flow control applied to each virtual c

Question

Consider a network with end to end window flow control applied to each virtual circuit Assume that the data link control operates perfectly and that packets are never thrown away inside the network. Thus packets always arrive at the destination in the order sent, and all packets eventually arrive. a) Suppose that the destination sends permits in packets returning to the source, if no return packet is available for some time out period, a special permit packet is sent back to the source. These permits consist of the number modulo m of the next packet awaited by the destination. What is the restriction on the window size w in terms of the modulus and why?

Explanation / Answer

Here there is no loss of packets in the network and we know that packet that has been sent will be received sooner or later by the receiver and the packets are always received in the same order as sent by the sender. so if we try to maintain a large window size in order to receive out of order packets that it would add no benefit, as packet which is sent first will be recived first. So we maintain a window size of one, that is, we only keep track of the current packet being sent.

The window size w in this situation is modulus 2. That is we just need to maintain 1 frame window only and the sequence number can take on value either 0 or 1, because here all the packets are arrived in the same order as sent. No packets are lost inside the network. This is just a stop and wait protocol.

Suppose first packet indexed by zero is sent, now the receiver window is single unit and wants to receive packet 0, sender window moves and goes to packet 1. Whenever the receiver receives a packet 0, it would send a ack 1 that is now it wants first packet and the process goes on.

One more thing is the receiver process the packets at the same rate as sender sends them then it would just keep sending the packets.

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