1. technology and computing
2. internet technology
3. email

Spring 2015
CS2520/TelCom2321-WANs
Project Description
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Introduction
The purpose of a file transfer protocol is to enable the transfer of files between machines, typically under the
command of a user. Several issues need to be addressed in the design of a file transfer protocol, including
dealing with differences in file name conventions, text and data representation, and directory structure.
Furthermore, the protocol much ensure reliable transfer of files from one system to another. The main
objective of this project is to design and implement a Simple File Transfer Protocol (SFTP) on top of an
unreliable transport channel.
The project is intended to expose you to several aspects of computer and network programming, including
the client/server paradigm, the concept of layering and interfaces, and a better understanding of flow and
error control protocol implementation and performance.
The basic requirement of this project include two main tasks:
1. SFTP Architecture Design and Implementation. This task entails:
• The design of a file transfer architecture composed of two main layers:
– A session layer focused on handling files and directory management, and
– A transport layer to transmit data segments reliably between client and server.
• The design of an interface between the session and transport to support connection establishment
and data transfer between the two layers, and
• The design of a name server to support name resolution between clients and servers;
2. SFTP Implementation and Analysis. This task entails:
• The implementation of SFTP based on a client-server communication model using the Socket
API;
• The design of a set of experiments to study the performance of selected window flow control
protocols, for different packet errors and different levels of traffic congestion.
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SFTP Architecture and Design Issues
SFTP is a client-server protocol that supports the transfer of text and binary files from one machine to
another. The file being transferred can be of arbitrary size. To achieve reliable file transfer, the SFTP client
has three components, as depicted in Figure 1.
• A user interface (UI) component to handle requests from an interactive user,
• A client control and file management layer (CFM) to interpret commands and handle file and directory
requests, and
• A client Transport Channel (TC) layer to handle information transfer, data or control, between the
SFTP client and server.
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The SFTP server has two components:
• A server control and file management (CFM) layer to interpret commands and handle file and directory
requests, and
• A server Transport Channel (TC) layer to handle information transfer, data or control, between the
SFTP server and client.
SFTP Client
SFTP Server
User Interface
UI-CFM
Control Connection
Control and File
Management
CFM-DD
Control and File
Management
Internet
Data Delivery
CFM-DD
Data Delivery
Data Connection
Figure 1: SFTP Architecture
2.1
SFTP Client-Server Interaction
The interaction between the SFTP client and server is achieved through two connections: a control connection and a data connection. The control connection is established between the client and server control
processes. Communication across the control connection is achieved through commands and responses. Only
one command at a time can be sent. A new command can be issued only after a response to the pending
command is received by the client.
When a server receives a command for a file transfer, the server opens a data connection to the client.
The data connection is used to transfer files between SFTP client and server. The connection can also be
used to transfer directory content. The connection remains open until a close command is issued by the the
client.
2.2
SFTP Control and File Management
Through its User Interface, an SFTP client supports two types of commands:
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1. Access Commands:
• SFTP: to start SFTP; if a machine is specified a connection to that machine is immediately made;
otherwise, the SFTP prompt is displayed.
• OPEN: to open connection to the specified machine.
• CLOSE: to close a connection to the specified machine.
• ABORT: to abort the previous SFTP command and any data transfer.
• QUIT: to terminate an SFTP session.
2. File Management Commands:
• RCD: to change directory on the remote machine.
• LCD: to change directory on the local machine.
• PWD: to list working directory on the remote machine.
• RLS: to list directory content on the remote machine.
• LLS: to list directory content on the local machine.
• GET: to copy one file from the remote machine to the local machine.
• MGET: to copy multiple files from the remote machine to the local machine.
• PUT: to copy one file from the local machine to the remote machine.
• MPUT: to copy multiple files from the remote machine to the local machine.
The Control and File Management (CFM) layer running on the SFTP client interprets user commands
and interfaces with the Transport Channel (CC) to establish appropriate connections with the SFTP server
and send requests containing the user command and associated parameters to the SFTP server. The Interface
between the CFM and the TC is defined by the following primitives:
• TranspInitCntrl(): This primitive is invoked to establish a control connection between the SFTP client
and server.
• TranspInitData(): This primitive is invoked to establish a data connections between the SFTP client
and server.
• DataTranspSend(): This primitive is invoked by the client or the server to send data over the data
connection.
• DataTranspRecv(): This primitive is invoked by the client or the server to receive data over the data
connection.
• CtrlTranspSend(): This primitive is invoked by the client to send a command over the control connection.
• CtrlTranspRecv(): This primitive is invoked by the server to receive a command over the control
connection.
• TranspCloseCntrl(): This primitive is invoked to close the control connection between the SFTP client
and server.
• TranspClose(): This primitive is invoked to close the data connection between the SFTP client and
server.
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3
SFTP Configuration and Initialization
The first step toward initialization the system is to activate the name server. The name server obtains an
IP address and port number and stores them in a well-known file, Name Server Info. The server is then
activated and initialized. During the initialization, the server is provided with the path to Name Server Info.
The server then requests an IP address and a port number from the system and register its name (SFTP)
and the assigned address and port number with the name server. Upon completion of the registration
phase, the server invokes the CtrlTranspRecv() and remains passive until it receives a request for connection
establishment from a client.
In response to TranspInit, the TC client establishes a connection with the passive SFTP server using
TCP Socket Streams. The DataTranspSend(), DataTranspRecv() and CtrlTranspSend() primitives use the
established TCP connections for all data and control transmissions between the client and the server. The
TC layer accepts data passed via DataTranspSend() and CtrlTranspSend(), places the data into segments
and sends these segments on the appropriate TCP connection. The data portion of a segment is limited by
the maximum segment size, a configurable parameter.
The actual communication between the SFTP client and server takes places using the Socket API over
TCP/IP. The design of the TC layer must include the ability to handle an error-prone transport channel.
Consequently, the TC layer must include considerations for buffering, retransmissions and a form a flow
control. You must implement a sliding window protocol of fixed size. For error control, you will experiment
with two protocols: Go-Back-N and Selective Repeat. Retransmissions will require a timer mechanism to
detect lack of acknowledgments. This can be achieved using the select() system call. When an entire file has
been transferred, the receiving side should close the file for writing. If multiple segments are outstanding at a
time, segments received by the TC layer may not be passed up to to the CFM layer if they are out-of-sequence.
Instead, they should be buffered or discarded depending on the ARQ protocol being used.
As part of introducing unreliability into the network, you are responsible for artificially introducing errors
into the segment to be transmitted; this is achieved by randomly selecting a bit and changing its value before
transmission. The error must be detected by the receiving end using a CRC-based error checking algorithm.
Depending on the protocol being implemented, a NAK must be sent to the sender. As part of dealing with
congestion control, a segment is randomly selected and dropped before transmission. Any segment can be
potentially lost. The error rate to be used by the client and the server is a variable parameter. So is the
probability of dropping a segment. The server and the client should be configured accordingly during the
initialization phase.
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SFTP Name Service Design
The SFTP server translates names (i.e., SFTPServer) into IP address and port number. The SFTP Name
Service protocol supports two primitives: RegisterName() and ResolveName(). The first primitive, RegisterName(), is used by a server to register its name, IP address and port number with the Name Server. In
response, the Name Server records this association into its database. The second primitive, ResolveName(),
is used to resolve a name (e.g., SFTPServer) into an IP address and a port number.
For robustness, it is recommended to have at least two name servers. The first server is the primary
server, while the second one is used as a backup server if the first server is not responding.
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Project Requirements
• You must design and implement a simple user interface to initialize and configure the name server,
SFTP client and SFTP server with the appropriate parameters must be provided. A graphical interface
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should be considered only if time permits. The implementation of such an interface will be awarded
extra credits (10 pts).
• You must design and implement the control and File Management layer, including its interface to the
Data Delivery layer.
• The design must accommodate several clients connected simultaneously to the SFTP server.
• You must design and implement the Data Delivery layer, using Stream Sockets.
– The TC layer protocol protocol must include a CRC-based error control protocol. It must also
implement a window-based flow control, with the possibility to select a Go-back-N or SelectRepeat ARQ protocol.
– The design must include an ACK/NAK mechanism for reliable transmission between the sender
and the receiver.
– The Design must handle out-of-sequence segments.
– The design must simulate segment errors and segment drops.
• You must design a name server for the system. As a minimum requirement, only one server is required.
Dealing with robustness, using a backup DNS server, will be awarded extra credits (10 pts).
The following tasks are to be completed as part of the project:
• Task 1.
1. A complete design of the system architecture, with clearly defined semantics for each command
and primitive, and a description of the segment format to be used for control and data transmission, ACKs and NAKs, and file management related data transfer. The design must include
a description of how the data and control connections will be established: which entity is the
passive side and which entity is the active side of the connection. The design must also describe
in sufficient detail how multiple connections to the same server are handled.
2. As part of the experimental study of the sliding window protocol performance in an error-prove,
potentially congested environment, you must design a set of experiments to collect statistics of
interest. You need to describe the set of experiments you plan to carry out, the data to be
collected, and the statistical tools you will use to validate your analysis.
3. A first progress report which discusses the design and implementation issues must be submitted. The report will be read and feedback will be returned promptly after submission, including
recommendation for improvement or changes when applicable.
4. Due Date : February 10, 2015
• Task 2.
1. Implement the SFTP model using Socket API and TCP/IP protocol, including the proposed set
of experiments.
2. Submit a final report in which you describe your final architecture and implementation design
decisions.
– As part of the final report, you are expected to discuss the design and implementation in
detail (justify whatever final design decisions you made), and provide a thorough analysis of
the performance study results. Statistical validation of your results is a must.
– You need to make available in a public directory a well documented listing of the source code,
in a compressed file (A Makefile will be highly appreciated).
– You need to schedule a time onbetween April 16 or April 17 to demo your project.
– The final grade will be based on of the implementation of the model and the quality of the
experiments carried out and their analysis.
– (Due Date : April 15, 2015)
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