Network Programg代写：COMP3331 Reliable Transport Protocol

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Introduction

基于TCP和UDP写一套可靠的网络通信协议，比较考察Linux网络编程的基本功底。
Linux网络编程，虽然网上例子很多，但是基本上没有对各类异常和各种信号的处理，对于编写应用程序来说并不靠谱。

Goal and learning objectives

For this assignment, you will be asked to implement a reliable transport protocol over the UDP protocol. We will refer to the reliable transport protocol that you will be programming in this assignment as Simple Transport Protocol (STP). STP will include most (but not all) of the features that are described in Sections 3.5.4 and 3.5.6 of the text Computer Networking (6th ed.). Examples of these features include timeout, ACK, sequence number etc. Note that these features are commonly found in many transport protocols. Therefore, this assignment will give you an opportunity to implement some of these basic features of a transport protocol. In addition, you may have wondered why the designer of the TCP/IP protocol stack includes such feature-less transport protocol as UDP. You will find in this assignment that you can design your own transport protocol and run it over UDP. This is the case for some existing multimedia delivery services in the Internet, where they have implemented their own proprietary transport protocol over UDP.
Note that it is mandatory that you implement STP over UDP. Do not use TCP sockets. You will not receive any mark for this assignment if you use TCP socket.

Learning Objectives

On completing this assignment you will gain sufficient expertise in the following skills:

  1. Detailed understanding of how reliable transport protocols such as TCP function.
  2. Socket programming for UDP transport protocol.
  3. Protocol and message design.

Overview

As part of this assignment, you will have to implement Simple Transport Protocol (STP), a piece of software that consists of a sender and receiver component that allows reliable unidirectional data transfer. STP includes some of the features of the TCP protocols that are described in sections 3.5.4 and 3.5.6 of the textbook (6th edition). You will use your STP protocol to transfer simple text (ASCII) files (examples provided on the assignment webpage) from the sender to the receiver. You should implement STP as two separate programs: Sender and Receiver. You only have to implement unidirectional transfer of data from the Sender to the Receiver. As illustrated in Figure 1, data segments will flow from Sender to Receiver while ACK segments will flow from Receiver to Sender. Let us reiterate this, STP must be implemented on top of UDP. Do not use TCP sockets. If you use TCP you will not receive any marks for your assignment.
You will find it useful to review sections 3.5.4 and 3.5.6 of the text. It may also be useful to review the basic concepts of reliable data transfer from section 3.4.

Assignment Specifications

This section gives detailed specifications of the assignment. There are two versions of this assignment, a standard version (with a total of 15 marks) and an extended version (with a total of 17 marks of which 2 marks are bonus marks). The specifications for the extended version can be found in Section 5 of the specification. Note that the bonus marks may not be proportional to the amount of extra work that you will have to do. They are there to encourage you to go beyond the standard assignment. The bonus marks can be used to make up for lost marks in the lab exercises and the second assignment but NOT for any of the exams (mid-session and final).

File Names

The main code for the sender and receiver should be contained in the following files: sender.c, or Sender.java or sender.py, and receiver.c or Receiver.java or receiver.py. You are free to create additional files such as header files or other class files and name them as you wish.

List of features provided by the Sender and Receiver

You are required to implement the following features in the Sender and Receiver:

  1. A three-way handshake (SYN, SYN+ACK, ACK) for the connection establishment. The ACK sent by the sender to conclude the three-way handshake should not contain any payload (i.e. data). See Section 3.5.6 for further details.
  2. The four-segment connection termination (FIN, ACK, FIN, ACK). The Sender will initiate the connection close once the entire file has been successfully transmitted. See Section 3.5.6 for further details.
  3. Sender must maintain a single-timer for timeout operation (Section 3.5.4 of the text).
  4. Sender should implement all the features mentioned in Section 3.5.4 of the text, with the exception of doubling the timeout. The STP protocol must include the simplified TCP sender (Figure 3.33 of the text) and fast retransmit (pages 247-248). You will need to use a number of concepts that we have discussed in class, e.g., sequence numbers, cumulative acknowledgements, timers, buffers, etc. for implementing your protocol.
  5. Receiver should implement the features mentioned in Section 3.5.4 of the text. However, you do not need to follow Table 3.2 for ACK generation. All packets should be immediately acknowledged, i.e. you do not have to implement delayed ACKs.
  6. STP is a byte-stream oriented protocol. You will need to include sequence number and acknowledgement number fields in the STP header for each segment. The meaning of sequence number and acknowledgment number are the same as TCP.
  7. MSS (Maximum segment size) is the maximum number of bytes of data that your STP segment can contain. In other words, MSS counts data ONLY and does NOT include header. Sender must be able to deal with different values of MSS. The value of MSS will be supplied to Sender as an input argument.
  8. Another input argument for Sender is Maximum Window Size (MWS). MWS is the maximum number of un-acknowledged bytes that the Sender can have at any time. MWS counts ONLY data. Header length should NOT be counted as part of MWS.
    Remarks: Note that TCP does not explicitly define a maximum window size. In TCP, the maximum number of un-acknowledged bytes is limited by the smaller of receive window and the congestion control window. Since you will not be implementing flow or congestion control, you will be limiting the number of un-acknowledged bytes by using the MWS parameter. In other words, you will need to ensure that during the lifetime of the connection, the following condition is satisfied.
  9. Even though you will use UDP since the sender and receiver will mostly be running on machines that are within close proximity of each other (e.g.: on the same Ethernet LAN or even on the same physical machine), there will be no real possibility of datagrams being dropped. In order to test the reliability of your protocol, it is imperative to introduce artificially induced packet loss and delays. For this purpose you must also implement a Packet Loss and Delay (PLD) Module as part of the Sender program. The details for this module are explained later in the specification.
    Remarks: For the standard version of the assignment, the PLD module will only need to drop packets while for extended version, the PLD module will need to drop and delay packets. For simplicity, I have chosen to call both of them the PLD module, even though the PLD module for the standard version does not delay packets.
  10. You must use a constant timeout in your program. The value of the timeout will be supplied to Sender as an input argument. Note that, this requirement applies to the standard version of the assignment. The extended version has a different requirement.

Features excluded

There are a number of transport layer features adopted by TCP that are excluded from this assignment:

  1. You do not need to implement timeout estimation unless you want to attempt the extended version of the assignment.
  2. You do not need to double timeout interval unless you want to attempt the extended version of the assignment.
  3. You do not need to implement any flow nor congestion control.
  4. STP does not have to deal with corrupted packets. Packets will very rarely be corrupted in our test topology, if at all. In short, it is safe for you to assume that packets are only lost.

Packet header and MSS

In designing the segment header, you only need to include the fields that you think are necessary for STP. You can draw inspiration from TCP but the exact format of the STP packet header is for you to decide. The header portion can include as many fields as you think are necessary. Two important fields that will be needed are the sequence number and acknowledgement number. You will also need a number of flags for connection establishment and teardown.
The data portion must not contain more than MSS bytes of data. You must use the same STP segment format for data transfer as well as for the acknowledgements flowing back from the receiver to the sender. The only difference will be that the acknowledgement segments will not contain any data. All information that is necessary for the proper functioning of your protocol must be provided in the STP headers. You should not use any information from the header of the UDP datagram that will encapsulate the STP packets.

Sender

This section provides details on the Sender.
For the standard version of the assignment, the Sender should accept the following eight (8) arguments (note that the last two arguments are used exclusively by the PLD module):

  1. receiver_host_ip: the IP address of the host machine on which the Receiver is running.
  2. receiver_port: the port number on which Receiver is expecting to receive packets from the sender.
  3. file.txt: the name of the text file that has to be transferred from sender to receiver using your reliable transport protocol.
  4. MWS: the maximum window size used by your STP protocol in bytes.
  5. MSS: Maximum Segment Size which is the maximum amount of data (in bytes) carried in each STP segment.
  6. timeout: the value of timeout in milliseconds. The following two arguments are used exclusively by the PLD module:
  7. pdrop: the probability that a STP data segment which is ready to be transmitted will be dropped. This value must be between 0 and 1. For example if pdrop = 0.5, it means that 50% of the transmitted packets are dropped by the PLD.
  8. seed: The seed for your random number generator. The use of seed will be explained in Section 4.5.2 of the specification.

Receiver

The Receiver should accept the following two arguments:

  1. receiver_port: the port number on which the Receiver will open a UDP socket for receiving datagrams from the Sender.
  2. file.txt: the name of the text file into which the text sent by the sender should be stored (this is the file that is being transferred from sender to receiver).

The Receiver should generate an ACK immediately after receiving a data segment. This is the only ACK generation rule you need. You do not need to follow Table 3.2 of the text. In other words, you do no have to implement delayed ACKs. The format of the acknowledgement segment must be exactly similar to the STP data segment. It should however not contain any payload.
The receiver is expected to buffer out-of-order arrival packets.
The receiver should first open a UDP listening socket on receiver_port and then wait for segments to arrive from the Sender. The first segment to be sent by the Sender is a SYN segment and the receiver is expected to reply a SYNACK segment.
After the completion of the three-way handshake, the receiver should create a new text file called file.txt. All incoming data should be stored in this file. The Receiver should first extract the STP packet from the arriving UDP datagrams and then extract the data (i.e. payload) from the STP packet. Note that, the Receiver is allowed to examine the header of the UDP datagram that encapsulates the STP Packet to determine the UDP port that the Sender is using.
The data should be written into file.txt. At the end of the transfer, the Receiver should have a duplicate of the text file sent by the Sender. You can verify this by using the diff command on a Linux machine (diff file1.txt file2.txt).
The Receiver should also maintain a log file titled Receiver_log.txt where it records the information about each segment that it sends and receives. The format should be exactly similar to the sender log file as outlined in the Sender specification.