CS2307 – Network Lab
PROTOCOL SIMULATION Sliding window protocols are used where reliable in-order delivery is required. Each frame is assigned a unique sequence number, and the receiver uses the numbers to place received packets in the correct order. Sequence numbers range from 0 up to some maximum, circularly. In Go-Back-N Automatic Repeat Request, several frames are sent before receiving acknowledgments and a copy of these frames is kept until the acknowledgments arrive. The maximum size of the sender's window is 2 m - 1. The size of the receive window is always 1. The receiver is always looking for the arrival of a specific frame. Any frame arriving out of order is discarded and needs to be resent. When the correct frame arrives, the receive window slides. The GO-Back-N ARQ is highly inefficient for noisy channels, since frames are more likely to be lost or corrupt and be resent. In Selective Repeat ARQ, only the damaged frame is resent. The send window size and the receive window size is 2m-1. Therefore, the frames can arrive out of order and be stored until they can be delivered. The delivery of a packet to a host or a router requires two levels of addressing, logical and physical. Therefore, it is required to map a logical address to its corresponding physical address and vice versa. A host has an IP datagram to be sent to another host has the logical address of the receiver obtained using DNS. Address Resolution Protocol (ARP) is used to know the physical address of a machine when its logical address is known. It broadcasts a request, to which the intended recipient replies with its physical address. In certain cases, a host aware of its physical address needs to know the logical address. For instance, a diskless station booted from its ROM. The station can find its physical address by checking its interface provided by the manufacturer. The system can use the physical address to get the logical address by using the Reverse Address Resolution Protocol (RARP).