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Client and a server application- Assignment

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COSC 264 Assignment

1 Administrivia
This assignment is part of the COSC 264 assessment process. It is worth 10% of the final
marks. It centers around socket programming using the Python programming language.
Your program should be a text mode program that can run from the command line.
You will need to find out some details about socket programming on your own. The
Internet offers gazillions of examples about socket programming in all sorts of languages,
including Python, and it is important for you to get some practice in using that resource.
For Python, a good starting point is to familiarize yourself with the socket module,
which offers many low-level operations on sockets.
Please use the “Question and Answer Forum” on the Learn platform for raising and
discussing any unclear technical issues. Please do not send emails with technical questions directly to me or the tutors, instead use the learn forum. This way other people
can benefit from the question (and the answer).
Another piece of advice: socket programming is systems programming, and there can
be sometimes subtle differences in the precise operation of socket calls between different
(versions of) operating systems. I strongly suggest that you develop the assignment
under Linux, as this is what we are most familiar with.

3 Problem Description
You will write a client and a server application which allows a client to download a file of
its choosing from the server. The client and the server will communicate through stream
/ TCP sockets, exchanging both control and actual file data.
3.1 Server
The server is a command line application and will operate as a TCP server. It will accept
one parameter, to be read from the command line. This parameter is the port number to
which the server will bind the socket on which it accepts incoming file transfer requests.
The port number should be between 1,024 and 64,000 (including). If it is not, then the
server should print an error message and exit. Otherwise, the server will perform the
following steps:
ˆ The server creates a socket and attempts to bind it to the port number given on
the command line. If this does not work, then the server should print an error
message and exit.
ˆ The server then calls (the Python equivalent of) listen() on the socket. If this
does not work, then server should print an error message, close the socket and exit.
ˆ The server enters an infinite loop, in which the server performs the following steps:
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Introduction to Computer Networks and the Internet, June 11, 2021
– It first accept()s a new incoming connection. Recall that accept() returns
a new socket for that connection. For logging purposes, the server prints a
message which indicates the current time and the IP address and port number
of the client from which the incoming connection originated. You will have
to find out how to obtain that data from the socket returned by accept().
– It then tries to read a FileRequest record from the connection. The record
includes the filename to be read (see Section 3.3 for a detailed description
of the record format). More precisely, the server attempts to read as many
bytes from the socket as are needed for the FileRequest, stores them in a
byte array and then checks the validity of the FileRequest record (see Section
3.3). If the FileRequest record is not valid, or if the required number of bytes
for the complete FileRequest record cannot be read from the socket within
a maximum of one second after accept() has returned with a socket (see
Section 3.4), then the server prints an appropriate error message, closes the
socket obtained from accept() and goes back to the start of the loop.
– If the FileRequest record is correct and contains a filename, the server tries
to open the file for reading.
* If the file does not exist or cannot be opened, the server sends an appropriate FileResponse message back to the requesting client, closes the
socket obtained from accept(), prints an informational message and goes
back to the start of the loop.
* If the file can be opened for reading, the server sends an appropriate
FileResponse message back to the client, followed by the actual contents of the file (the server should count the actual number of bytes
transferred). Once the file transfer has been completed, the server closes
the file, it closes the socket obtained from accept(), prints an informational message which includes the actual number of bytes transferred,
and goes back to the start of the loop.
Note that the file can be of any size and it needs to be transferred completely and without
errors, and without any un-necessary gaps. The file is to be treated as a binary file (see
Section 3.5). To quickly check whether the received file on the client side is completely
identical to the file on the server side, you can use the md5sum command line tool (see
the man page for this command).
3.2 Client
The client is a command line application and will operate as a TCP client. The client
will accept three parameters, all to be read from the command line:
ˆ The first parameter is either a string giving an IP address in dotted-decimal notation (e.g. “130.66.22.212”), or it is the hostname of the computer running the server
(e.g. “fileserver.mydomain.nz”). The client will attempt to convert this parameter
to an IP address using the Python equivalent of the getaddrinfo() function. If
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this conversion fails (e.g. because the hostname does not exist or an IP address
given in dotted-decimal notation is not well-formed) then the client should print
an error message and exit.
ˆ The second parameter is the port number to use on the server. The port number
should be between 1,024 and 64,000 (including). If it is not, then the client should
print an error message and exit.
ˆ The third parameter is the name of the file that the client wishes to retrieve from
the server. The client should check whether the indicated file actually exists and
can be opened locally. If this is the case, then the client should print an error
message and exit, to avoid over-writing local files of the same name.
If there are fewer or more than three parameters on the command line, the client should
print an error message and exit. If the parameters are all ok, the client will go through
the following steps:
ˆ The client creates a socket. If this does not succeed, then the client prints an error
message and exits.
ˆ Next, the client calls connect() to connect with the server, using the IP address
and port number inferred from the command line parameters. If the connect()
call does not succeed, then the client closes the socket, prints an error message and
exits.
ˆ Next, the client prepares a FileRequest record and sends this to the server over
the socket.
ˆ Next, the client will read a FileResponse record from the server. More precisely, the client attempts to read as many bytes from the socket as are needed
for the FileResponse, stores them in a byte array, and checks the validity of the
FileResponse record. If it is not valid or if there is a gap of more than one second
while reading the FileResponse record (see Section 3.4), then the client prints an
error message, closes the socket and exits.
ˆ If the FileResponse record is valid, its contents are checked:
– If the FileResponse record indicates that no file data is following (e.g. because the file does not exist on server side), then the client prints an appropriate informational message, closes the socket and exits.
– Otherwise, the client performs the following steps:
* It opens the file with the indicated filename locally for writing. If that
does not work, the client prints an error message, closes the socket and
exits.
* The client then processes the file data. The file data is read in blocks
of up to 4,096 bytes size, such that each block is first transferred into a
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Introduction to Computer Networks and the Internet, June 11, 2021
byte array, and from there written to the file. While doing this, the client
counts the number of bytes received. This continues until no further data
is received from the socket (how will you know?). If an error occurs when
reading the data from the socket or when writing it to a file, or if there
is a gap in the received data of more than one second, then the client
prints an error message, closes the socket, closes the file and exits. It is
also important to check that there are exactly as many data bytes as
indicated in the ’DataLength’ field. If there are more or fewer data bytes,
error processing shall be carried out.
* At the end of the file transfer the client prints an informational message
(which includes the number of bytes received), closes the file, closes the
socket and then exits.
3.3 Message Formats
3.3.1 The FileRequest Record
The format of the FileRequest record is shown in the following figure:
MagicNo (16 bit)
1 16 32
Type (8 bit)=1 FilenameLen
(16 bit)
FilenameLen
(16 bit)
Filename
(n*8 bit)
Filename
(n*8 bit)
Filename
(n*8 bit)
Filename
(n*8 bit)
It consists of the following fields:
ˆ The first 16 bit field ’MagicNo’ is taken up by a magic number, which needs to
have the value 0x497E in network byte order. This is a simple safeguard to check
whether the received data could actually be a FileRequest record.
ˆ The next 8 bit field ’Type’ needs to contain the fixed value ’1’ for the FileRequest
record.
ˆ The next 16 bit field ’FilenameLen’ contains the length of the following filename
as a number of bytes. The length value is given in network byte order, and for a
valid FileRequest record the length field should at least be one and should be no
larger than 1,024. Denote by n the value stored in the ’FilenameLen’ field.
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Introduction to Computer Networks and the Internet, June 11, 2021
ˆ Finally, the FileRequest record contains n bytes for the actual filename.
We refer to the first five bytes of the record (comprising the ’MagicNo’, ’Type’ and
’FilenameLen’ fields) as the fixed header. The FileRequest record is being sent from
the client to the server. To receive and process a FileRequest record the server performs
the following steps:
ˆ First the server attempts to read the five bytes of the fixed header into a byte array.
If that is not possible without gap (see below), then the server concludes that
the received FileRequest is erroneous and performs error processing as described
in Section 3.1 (i.e. printing an error message on the terminal, closing the socket
obtained from accept() and going back to the start of the loop). If this is possible,
then the server performs the following checks:
– The contents of the ’MagicNo’ field must equal 0x497E.
– The contents of the ’Type’ field must equal 1.
– The contents of the ’FilenameLen’ field must be at least one and must not be
larger than 1,024.
If any of these conditions is not true, then the server concludes that the received
FileRequest is erroneous and performs error processing.
ˆ Then the server attempts to read exactly n further bytes from the FileRequest
record. These bytes are read into a byte array. If reading from the socket is not
possible without gap (see Section 3.4), then the server concludes that the received
FileRequest is erroneous and performs error processing as above. If the server
reads fewer than n bytes or more than n bytes, then again the server concludes
that processing failed and performs error processing.
3.3.2 The FileResponse Record
The format of the FileResponse record is shown in the following figure:
MagicNo (16 bit)
1 16 32
Type (8 bit)=2 StatusCode
(8 bit)
File Data
(n*8 bit)
File Data
(n*8 bit)
File Data
(n*8 bit)
DataLength (32 bit)
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Introduction to Computer Networks and the Internet, June 11, 2021
It consists of the following fields:
ˆ The first 16 bit field ’MagicNo’ is taken up by a magic number, which needs to
have the value 0x497E in network byte order. This is a simple safeguard to check
whether the received data could actually be a FileResponse record.
ˆ The next 8 bit field ’Type’ needs to contain the fixed value ’2’ for the FileResponse
record.
ˆ The next 8 bit field ’StatusCode’ contains information about whether the server
was able to successfully open the requested file for reading. It contains the value
’0’ if the file does not exist on the server or the server was not able to open it for
reading, and it contains a ’1’ if the server was able to open it for reading, in which
case the remaining FileResponse record contains actual file data.
ˆ The next 32 bit field ’DataLength’, stored in network byte order, contains the
length of the requested file and consequently also tells how many data bytes follow.
If the value in the ’StatusCode’ field is 0, then the client should ignore the value
of this field. The server should set it to zero in this case. Otherwise, if the
’StatusCode’ is 1, then this field can contain any unsigned integer value, including
0 (which corresponds to an existing but empty file).
ˆ Finally, the ’FileData’ field contains as many bytes as indicated in the ’DataLength’
field if the ’StatusCode’ is 1, otherwise it contains no data bytes.
We refer to the first eight bytes of the FileResponse record (comprising the ’MagicNo’,
’Type’, ’StatusCode’ and ’DataLength’ fields) as the fixed header. The FileResponse
record is being sent from the server to the client (in response to a FileRequest record).
To receive and process a FileResponse record the client performs the following steps:
ˆ First the client attempts to read the eight bytes of the fixed header into a byte array.
If that is not possible without gap (see Section 3.4), then the client concludes that
the received FileResponse is erroneous and performs error processing as described
in Section 3.2 (i.e. printing an error message on the terminal, closing the socket
and exiting). If this is possible, then the client performs the following checks:
– The contents of the ’MagicNo’ field must equal 0x497E.
– The contents of the ’Type’ field must equal 2.
– The contents of the ’StatusCode’ field must be either 0 or 1.
If any of these conditions is not true, then the client concludes that the received
FileResponse is erroneous and performs error processing.
ˆ Then, if the value of the ’StatusCode’ field is 1, the client attempts to read and
process the actual file data bytes as described in Section 3.2.
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Introduction to Computer Networks and the Internet, June 11, 2021
3.4 Reading from a Stream/TCP Socket
For a stream socket, when a sending station sends 50 bytes by writing them in one go
into its socket buffer, the underlying TCP protocol implementation has full discretion
over when the data is sent, how many “packets” it sends and what the size of these
packets is. So theoretically, the TCP implementation on the sender can choose to send
the first three bytes immediately, then wait three seconds, send the next thirteen bytes,
then wait another five seconds and then send the remaining 34 bytes. In practice such
large time gaps are unlikely and point to problems in the underlying network. It is
required that you detect gaps of one second or more and abort operation of either client
or server should that occur (of course, when aborting you should not forget to print an
informative error message and clean up / return all the resources like closing sockets,
files etc).
To achieve this, as a preparation you will have to set a socket option on the socket you
want to read data from, this can be done using the function setsockopt(), with the
specific option SO RCVTIMEO, or you can use the Python library function settimeout()
on the socket. This should be done immediately after creating the socket (i.e. after
a socket() or accept() call). Then, when actually reading data from a socket, e.g.
using recv() or read(), in Python you will have to check if this operation throws an
exception. If it does, then you will need to determine the reason for the exception (a
reading timeout on the socket could be one of the reasons) and print an informational
message, then do cleanup and exit.
3.5 Data Representation
There are a few things to consider regarding data representation:
ˆ The filename that the client reads from the command line will be treated by Python
as a string. Strings in Python are represented using the UTF-8 character encoding
system, and in this system a printed character may require a variable number of
bytes to store in memory. Hence, a string which looks like having m characters
may in fact need a number of n ≥ m of bytes to represent it, and your client
program needs to transmit all n bytes. To achieve this, you can use the encode
method in Python to convert a string to a byte array. For example:

mystring = “hello world”
mybytes = mystring.encode(’utf-8’)

and then you can use the len function to find out the length n of the byte array
mybytes. You will transmit the complete byte array in the ’Filename’ field of the
FileRequest record, and the value of n will be filled into the ’FilenameLen’ field.
ˆ All 16- or 32-bit fields in the fixed headers of the FileRequest and FileResponse
records are encoded in network byte order. For a 16-bit field this means that the
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Introduction to Computer Networks and the Internet, June 11, 2021
first or leftmost eight bits are occupied by the highest-valued bits and the last or
rightmost eight bits are occupied by the lowest-valued bits. Similarly for a 32-bit
field.
ˆ The file contents should be treated as bytes throughout. In other words, when the
server reads bytes from the file, they should be stored in a byte array, and then
the byte array contents should be sent over the socket. Similarly, on the client side
the file data bytes should be storec in a byte array, and from there written to the
file.
4 Deliverables
Each student has to submit a single pdf file which includes the following items:
ˆ A cover sheet with your name and student-id.
ˆ A listing of your source code. We would appreciate if you use a pretty printer. Do
not use screenshots!
ˆ You have to print the plagiarism declaration form from the learn page of COSC
264, sign it, scan it, convert the scanned form into a pdf and include this into your
submission.
Warning:
ˆ Submissions that are not in pdf format or which contain more than
one file automatically receive 0 marks!!
ˆ Submissions which do not include the signed plagiarism declaration
form automatically receive 0 marks!!
The pdf file has to be submitted via the learn page of COSC 264 (see https://
learn.canterbury.ac.nz/course/view.php?id=542&home=1). Please submit no later
than Sunday, August 22, 2021, 11:59 pm. Late submissions are not accepted,
except through the special consideration process.
5 Marking
Marking will be based on the source code. In particular, we will mark it for its ability
to produce the right results (e.g. is packet processing correctly implemented, are all
the right socket calls there, is the gap detection implemented correctly), and for the
amount of error / consistency checking you do – if we find that you use system-/socket
calls without any error checking or that you do not check incoming data / records for
correctness, we will apply deductions. We will check whether you have returned all
resources (sockets, files) to the operating system, wherever sensible. We will also have
an eye on style, and may apply deductions if your code is particularly ugly or messy.
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Client and a server application- Assignment
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