Refer chapter 3.4 (Principles of Reliable Data Transfer) - Computer Networking, Kurose to understand these two protocols. The code needs to include implementation of the Alternating Bit and Selective Repeat protocols.

The programs should handle UDP packet losses to make sure that the entire file is correctly received by the client program. The client program takes a file name as input and requests the file from the server. It breaks the requested file into segments and sends them to the client over UDP. The server program hosts files and responds to requests for files. The application should consist a client program and a server program. All the action performed during this process are saved on a file and also are displayed in the console – i.e. Such as, if a packet is lost, or if a checksum is not validated, or if some sequence is missed –selective repeat case. Retransmission can occur in various scenarios. The client, upon receiving the packet, verifies it, re-computing the hash, and performs a necessary action which may be saving the packet or requesting for retransmission. A packet is created for each split, which contains checksum as SHA1(20-byte hash of data), data length, sequence number, and/or alternating bit. If the file exists, server splits the file into many segments of specific UDP length (say, 1024bytes). Server checks if the requested file exists or not. In regard to the project, a client makes an HTTP request to the server requesting a file to download. This project has implemented both Selective Repeat protocol and Alternating Bit protocol along with UDP. Otherwise, it requests for the fresh set of packets from the server. The client sends a request for those missing packets and repeats the process until all the packets for a particular request are received. Upon verifying, the client can know about the missing packets – if there are any. Once receiving the packets from the server, clients somehow verify the sequence of the received packets. But unlike ABP, server parallelly sends a certain number of packets (default 10 in our case) for each new request from the client. Selective Repeat Protocol(SRP) is also connectionless protocol. Thus, looking at the AB sent by the client, the server decides whether to send a new packet or to retransmitted the old one.

If the desired response is not received from the server, a request is again sent by the client without flipping the AB. In ABP, a request or ACK message is sent from a client to the server with AB 1 or 0, and upon receiving a response from the server, AB is flipped (1-0 or 0-1) and again a new request is sent with new AB info. It is, in fact, the simplest form of a sliding window protocol where sequence number alternates between 1 and 0, and the window size is of one. either SRP or ABP.Īlternating Bit Protocol(ABP) is a connection-less protocol used to transfer one-directional the message between client and server. The primary purpose of this project is to download a big file from a server residing in different IP location using one of these protocols i.e. UDP, unlike TCP (Transmission Control Protocol), is an unreliable, connectionless data transfer protocol existing in the transport layer of TCP/IP architecture. The reliability over UDP is achieved using Alternating Bit(AB) or Selective Repeat(SR) protocol. “Reliable data transfer using UDP (User Datagram Protocol – RFC 768) for file transfer” is a client-server application.

Selective repeat protocol