Network Simulator 2, Protocol Implementation Assignment for Telecommunications Laboratory Course

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1 Network Simulator 2, Protocol Implementation Assignment for Telecommunications Laboratory Course

2 1 Introduction This document describes the Major NS2 Assignment for the Simulation Part of the Telecommunications Laboratory Course. This assignment is personal (meaning it is not done in groups). The protocol to be implemented using Network Simulator 2 is Go-Back-N ARQ and is described later in this document as a state machine. Also a couple of MSCs (Message Sequence Charts) of the most typical situations are provided. The basic idea of the protocol should be familiar at least to some extent to the reader, but if it is not, many books dealing with protocols describe Go-Back-N ARQ (for example Data and Computer Communications, 6th edition by William Stallings). 2 Pre-requisites The Minor NS2 assignment must be done before this assignment. In addition to that, the student should have a basic knowledge about object-oriented programming and C++ (the course Olio-ohjelmointi/Object-oriented Programming would be of great help). 3 The assignment The first thing to do when you start doing the assignment is to go through Marc Greis' tutorial's Chapter VII A new protocol for ns. It gives you the needed background for implementing new protocol agents using NS2. You should download a tar-file which includes a Makefile and an implementation for the timer to be used with Go-Back-N. In Makefile and in GoBackNTimer.h you should replace strings XXXXXX with the name of the file that contains your implementation of Go-Back-N ARQ. The name of the class that contains your implementation of Go- Back-N should be named GoBackNAgent. Executing make ns produces a binary file 'ns' into the directory in which Makefile is located. Your implementation of Go-Back-N should be capable of 1) reading data from a given input file (text file), 2) transmitting this data to the peer using Go-Back-N ARQ and 3) writing the correctly received data into an output file. The protocol agent that you implement should work according to the state machine given in the next chapter. In addition to that, it should be possible to configure, through Tcl, the maximum size of the sending window and the maximum sequence number for the agent. Also, it should be possible to specify the name of the file from which the agent reads the data to be transmitted, as well as the name of the file into which the agent writes the received data. The agent should print out some key information during the simulation. From the printing it should be possible to determine what the agent is doing. Also visualization of the simulation using nam is required.

3 You can make some assumptions, for example that all the parameters described in the preceding paragraph are given in Tcl script before the simulation is started. It is sufficient to test the agent with simulations in which one agent is sending all the data and the other one is only receiving data (of course it must and REJ PDUs as needed but not data). To make your task easier, some modifications to the ns-2 source files packet.h and tcl/lib/ns-packet.tcl in /export/local/src/ns-allinone-2.1b6/ns-2.1b6, have already been done for you. These include: In packet.h Addition of line PT_GO_BACK_N to type enum_packet_t Addition of line name_[pt_go_back_n] = "go-back-n"; to p_info() member function and in tcl/lib/ns-packet.tcl Addition of line { GoBackN off_go_back_n_ } Modification that is recommended in the tutorial to be made to file ns-default.tcl has not been done. You should set the default packet size at the start of your Tcl script with the line Agent/GoBackN set packetsize_ 10. Later on in the simulation you can set the size of the packet to be created using allocpkt() by setting member variable size_ to the preferred value just before the call of allocpkt(). The functionality of the agent will be tested using Error Model provided by NS2 that drops some of the sent packets. The agent should recover from random packet loss. Agent is not however required to implement a retansmission counter. Also it will be tested that the agent correctly handles the situation where the sequence number space is wrapped, i.e sequence numbers of the sent packets return to zero like in the followingexample(maximumsequencenumberis7)0,1,2,3,4,5,6,7,0,1,2,...

4 4 State machine, packet header and some MSCs RR AND (RR.flag == POLL) AND RR.seqno == next_to_receive (RR.seqno = next_to_receive - 1, RR.flag = NONE) RR AND (RR.flag == POLL) AND (RR.seqno!= next_to_receive - 1) send REJ (REJ.seqno = next_to_receive, REJ.flag = NONE) RR AND (RR.flag == NONE) remove acked from sent queue DATA AND (DATA.seqno!= next_to_receive) send REJ (REJ.seqno = next_to_receive, REJ.flag = NONE) IDLE REJ resend sent DATA packets starting from REJ.seqno timeout (RR.seqno = last_sent, RR.flag = POLL), restart timer DATA AND (DATA.seqno == next_to_receive) data to output, update next_to_receive, (RR.seqno = next_to_receive, RR.flag = NONE) call from Tcl to send data if(data_to_send AND window not full) send DATA, put sent data to sent_queue, restart timer The packet header should contain fields for the PDU type, sequence number of PDU and flag (for polling). type seqno flag type ::= DATA RR REJ seqno ::= max_seqno flag ::= NONE POLL

5 5 What you should return You should return a modified Makefile, well commented C++ code files and at least one Tcl script file according to the instructions given on the assistant's page. The script should produce a simulation in which one agent sends the contents of a file to another agent which writes the correctly received data to an output file. In the script you must also use Error Model provided by NS2 in order to demonstrate that your agent recovers from occasional packet loss. 6 Hints 1. In order to have the agent to send data at fixed interval (of course limited by sending window) you should write a procedure in Tcl that schedules calling of itself after a given interval. 2. You should use PacketQueue in the sender for buffering the sent data that has not been acknowledged yet by the receiver (see ns manual Chapter for details). 3. You should note that in sending a data packet you should construct two matching packets from which one is sent to peer and the other one is put to the PacketQueue. You must not put the same instance of the packet to the queue that you send - otherwise when the receiver receives the packet and deletes it also the packet in the sender's PacketQueue (same instance) is deleted and the result is undefined. 4. You should use the timer class provided in the tar-file that you downloaded for timeouts caused by occasional packet loss. Timer can be scheduled by calling timer's member function resched(double delay). 5. You might want to include header files agent.h and queue.h in your C++ code. 6. Before you call allocpkt() in your code you must set the variable size_ to the value matching the number of data bytes in the Packet you are about to build. Actual user data is contained in the instance of class PacketData, which is then set on the actual packet. Class PacketData has a function data() that returns a pointer to the start of the data area and function size(), which returns the length of the data. Below is an example illustrating the handling of user data. In sender : data_bytes = 20; Packet* pkt = allocpkt(); PacketData* data = new PacketData(data_bytes); memcpy(data->data(), buffer, data_bytes); pkt->setdata(data); In receiver - pkt is the received Packet : PacketData* data; data = (PacketData*)pkt->userdata(); memcpy(buffer, data->data(), data->size());

VANET-Skeleton for ns2

VANET-Skeleton for ns2 Daniel Jungels Laboratory for computer Communications and Applications (LCA) EPFL December 23, 2005 This document quickly describes how to install ns2, and how to implement a VANET