CS4514 Computer Networks
Program 3 C01
Programming Assignment 3 (60 points)
Due: Tuesday, February 27, 2001
Concurrent Server Using Go Back N
This assignment builds on the experiences of Program 2 that implemented a simple client-server protocol on top
of an emulated physical layer. This program is to be completed in two person teams or by individuals. At this
stage of the course, the only single person projects that are encouraged are situations where your partner has
dropped out of the course.
The assignment is to build a concurrent server that handles requests from two or more clients. [Unix socket
calls are used with TCP as the physical layer for transmitting between clients and a concurrent
server]. Both the clients and the server will have a small application layer protocol that defines the interaction
between client and server. Furthermore, the architecture includes a small network layer to deal with the
conversion from messages-to-packets-to-frames. Note: Some of the data link layer issues from Program 2 can
be simplified in Program 3.
The command line for the client program is: client scriptnum
scriptnum is an input parameter that indicates (indirectly) which client this is.
Hence the command client script1 starts up client 1 and client script2 starts up client2.
Application Layer
The client application process read its "scripted actions" from the file scripti.txt. That is, client1 reads from
script1.txt and client2 read from script2.txt
Each client application process sends requests (one at a time) to the server application process of the
command number message
command = r
indicates read a message from the server
indicates quit the conversation and close the client connection
indicates write a message to the server
{r, w and q are ASCII characters}
is an integer between 1 and 10 indicating the location of the message in the server database
CS4514 Computer Networks
Program 3 C01
is a text message. The maximum size of a message is 400 bytes.
The server application process begins by reading into memory the original database of 10 messages from the
input file serverbase.txt. The server application process handles client application requests to read or write a
message from/to the database.
When a new client connects to the concurrent server, the server will fork a child process to handle all
interactions with that client. The server child process begins with a fresh copy of the original database. The
server child process responds to the client’s requests using the child’s copy of the database.
When the client application issues a read request, the server child process sends a copy of the requested
message back to the client.
Read Example
client sends message: r 6
server child extracts message 6 from its copy of the database and sends it back to the client.
When the client application issues a write request, the server child process overwrites the received message in
the correct place in the child’s copy of the database. The server must send back a response
message to the client to indicate that the write request has been completed.
Write Example
client sends the message: w 4 The Blue Devils will be number 1!
server child overwrites message 4 in the child’s database with the text
The Blue Devils will be number 1!
Thus, each server child process maintains a separate copy of the database for the client it is serving.
Quit Example
When the server receives a q message from the client, the server child process prints out the database to
the appropriate serveri.log file (where i is the number of the client). See the physical layer below for how
the server knows which file to write out. The server child then sends a response to the client before
terminating the child process. Note: You need a “clean” way to terminate the server.
The client application process sends a new command to the server application process after it receives a
message back from the server or after it receives a response message from a write or a quit command.
Network Layer
CS4514 Computer Networks
Program 3 C01
The network layer receives the messages from the application layer and converts the message into packets. The
maximum size of a packet for this assignment is 64 bytes. Packets are sent to the data link layer to be
converted to frames for transmission.
The network layer also receives packets from the data link layer. It reassembles the packets into a message to
send to the application layer. Note: the network layer will need to have a mechanism to determine the
last packet in a message.
Data Link Layer
The data link layer receives packets from the network layer, creates frames, and sends frames to the physical
layer for transmission. The data link layer also receives transmitted frames from the physical layer, extracts the
payload, reassembles packets, and forwards packets to the network layer. The maximum size for the frame
payload is 40 bytes. You must design the “overhead” bytes of the frame to implement a Go Back N sliding
window protocol. If it simplifies your task, framing bytes and byte stuffing are not necessary for assignment 3.
However, if it easier to keep these functions in this assignment that is fine. As in program 2, your
design needs to include an error-detection byte. Your design will need to include sequence numbers in the
frames and a mechanism for handling ACKs. The minimum frame size is your choice, and it is your design
decision whether to piggyback ACKs or send separate ACK frames. Because of the request/response
nature of the application layer ACK timers are not necessary for this assignment.
The goal of this assignment is to implement a Go Back N sliding window scheme at the data link layer. For
full credit you must implement a sending window size of three frames or higher. This requires timers on both
the client and server side. If you are short on time or run into problems, you should fall back to implementing a
one-bit sliding window with a single timer on both sides of the connection.
Client Process Flow
Each client will call the physical layer to establish a connection to the concurrent server. The data link layer will
then get a packet from the network layer, put together a frame and give it to the physical layer to send. The
data link layer flow will then depend on events coming from the network layer, the current availability within the
sliding window, and events coming from the physical layer. The client process will terminate when the response
message to the quit message is received at the client application layer.
Flow of Server Child Process
Each data link layer server child process waits for frames from the physical layer and passes packets up to the
network layer. Similar to the client side, the flow of the server data link layer depends on whether there is traffic
from the server to be sent back to the client (either frames with packets or ack frames).
Each data link client records significant events in a log file clienti.log. Each data link layer server child records
significant events in a log file serveri.log. Significant events include: packet sent, frame sent, frame resent, ACK
CS4514 Computer Networks
Program 3 C01
sent, ACK resent, frame received successfully, frame received in error, ACK received successfully, ACK
received in error, duplicate frame, and timer expires. For logging purposes identify the packet and the sequence
number of each frame for each event. Begin counting packets and frames at 1.
Physical Layer
The physical layer uses Unix sockets to send the constructed frames as actual TCP messages between the
clients and the concurrent server. When the client physical layer first establishes a connection to the concurrent
server, it must send one TCP message to the server child process to identify itself (i.e., the client with input
parameter script1 sends a message containing client1 to the server child process). This tells the server child
where to print out the final database.
Simulating Errors
Force every 6th frame sent by each client to be in error by flipping any single bit in the error-detection byte prior
to transmission. Force every 9th frame sent by each server child to be in error using the same flipping
mechanism. (i.e., frames 6, 12, 18, … sent by each client will have a transmission error and frames 9, 18, 27,
… sent by each server child will be in error.) Each frame with a forced error should be resent correctly on the
second try.
[Design] Plan your design in a modular fashion such that if everything is not totally working, you can turn in
an output that shows exactly what is working. Relaxing the sliding window scheme is one option.
[Documentation] Your commented program must have a special section to explain the details of your
specific design decisions. Remember: This a team project and all routines must include specify the
author as part of the documentation!!
[DEBUG] Include print statements in the various layers while debugging. You should consider some type
of verbose debugging flag that can be turned on and off.
[Performance Timing] You must measure the total execution time of the complete emulated transfer per
client and print this out in file clienti.log.
port numbers - Your clients should have unique port numbers and the clients should treat the server port
number like a “well-known” port number. {Conceivably, you could register your database server with
the oracle from program 1 and the client processes could get IP address and port number from the
What to turn in for Assignment 3
CS4514 Computer Networks
Program 3 C01
The three official tests file (script1.txt, script2.txt, serverbase.txt) will be made available a few days before
the due date. Use turnin to turn in the two source programs client.c and server.c, and the client and server
output files corresponding to running the programs using the TA's test files. The README file must indicate
the working and the non-working parts of program 3.
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