Description

1 Introduction
In this assignment, we will implement a multi-client file transfer application called MYFTP. MYFTP is a simplified version of the File Transfer Protocol (FTP). It includes the basic functionalities of FTP, such as file upload/download and file listing. It also enables multiple clients to simultaneously connect to the server at the same time. Our objective is to develop hands-on skills of socket programming and multi-threaded programming in C/C++.
2 Protocol Description
2.1 Overview
MYFTP is mainly composed of two entities: client and server. As their names suggest, the server hosts a file repository where the client can request to upload or download files.
MYFTP supports three main functions: (i) list the files that are stored on the server, (ii) upload files from the client to the server, or (iii) download files from the server to the client.
2.2 Protocol Messages
The client and the server will exchange protocol messages in order to perform the functions. All protocol messages are preceded with the protocol header shown below:
struct message_s {
unsigned char protocol[5]; /* protocol string (5 bytes) *
unsigned char type; /* type (1 byte) */
unsigned int length;
} __attribute__ ((packed)); /* length (header + payload) (4 bytes) */
The protocol field is always set to be the string that starts with the string “fubar”. The type field has different possible values as specified in Section 2.3. The length field is the length of the message (including the header and payload) to be sent over the network. The size of the payload is a variable depending on the message type. You can safely assume that the length is no larger than 232−1.
We add attribute ((packed)) to avoid data structure alignment and pack all variables together without any gap . Defining a packed structure makes our life easier when we calculate the size of a message to be sent over the network.
2.3 Protocol Details
We now explain the functions that MYFTP supports. We also introduce the protocol messages that accompany each function, as well as the formats of the protocol messages. For now, let us assume that there is only one single client. Table 1 lists all protocol messages that are used.
LIST REQUEST protocol fubar
type 0xA1
length header length
LIST REPLY protocol fubar
type 0xA2
length header length + payload length
payload file names, in null-terminated string
GET REQUEST protocol fubar
type 0xB1
length header length + payload length
payload file name, in null-terminated string
GET REPLY protocol fubar
type 0xB2 or 0xB3
length header length
PUT REQUEST protocol fubar
type 0xC1
length header length + payload length
payload file name, in null-terminated string
PUT REPLY protocol fubar
type 0xC2
length header length
FILE DATA protocol fubar
type 0xFF
length header length + file size
payload file payload
Table 1: Protocol messages
2.3.1 List Files
Suppose that the client wants to list all files that are stored in the server. The client will issue a command list. Then it will send a protocol message LIST REQUEST to the server. The server will reply a protocol message LIST REPLY, together with the list of available files.
2.3.2 File Download
Suppose that the client wants to download a file from the server. The client will issue a command get FILE, where FILE is the name of the file to be downloaded. It first sends a protocol message GET REQUEST to the server. The server will reply a protocol message GET REPLY with the type field set to 0xB2 if the file exists or 0xB3 otherwise. If the file exists, the server will send a FILE DATA message that contains the file content following the GET REPLY message.
Note that a file can be in either ASCII or binary mode. You should make sure that both ASCII and binary files are supported. Here, we assume that the file size is at most 1GiB (which is equal to 230 bytes).
2.3.3 File Upload
Suppose that the client wants to upload a file to the server. The client will issue a command put FILE, where FILE is the name of the file to be uploaded. First, the client will check whether the file exists locally. If not, the client will display an error message saying the file doesn’t exist. If the file exists, then the client will send a protocol message PUT REQUEST to the server. The server will reply a protocol message PUT REPLY and awaits the file. Then the client will send a FILE DATA message that contains the file content (we use the same FILE DATA message as in file download).
Here, we assume that the file to be uploaded resides in the same working directory as the client program. If the server has already stored the file before, the file will be overwritten. Note again that all files are in ASCII or binary mode. We again assume that the file size is at most 1GiB.
2.4 Interfaces
The server uses the following command-line interface:
sparc1> ./myftpserver PORT_NUMBER
client> ./myftpclient <server ip addr> <server port> <list|get|put> <file>
2.5 Implementation Issues
The client and the server can reside in different machines with different operating systems (e.g., Linux vs. Unix), and all functions should remain correct. Make sure all byte ordering issues are properly addressed (see Tutorial notes).
3 Milestones
Your implementation should have the following functions:
• The client can list files in the repository directory of the server.
• The client can download a file (either an ASCII or binary file) from the server. If the file doesn’t exist, the server replies with an error message.
• The client can upload a file (either an ASCII or binary file) to the server. If the file doesn’t exist locally, the client displays an error message.
• The client and the server can reside in different machines with different operating systems.
• Now the server needs to support a general number N of active connections (assume that N ≤10). You will have to implement a multi-client version using multi-threading based on the pthread library.
4 Submission Guidelines
• myftp.h: the header file that includes all definitions shared by the client and the server (e.g., the message header)
• myftp.c: the implementation of common functions shared by both the client and the server
• myftpclient.c: the client program
• myftpserver.c: the server program
• Makefile: a makefile to compile both client and server programs
• Declaration form for group projects:
(http://www.cuhk.edu.hk/policy/academichonesty/Eng_htm_files_(2013-14) /p10.htm)