CS 436 Winter 2010 - Assignment 2

Due Date: Wednesday, Mar 3, 2010, 22:00

In this assignment, you are provided with a channel emulator for an unreliable channel. You must implement the selective repeat versions of reliable pipelined data transfer and a simple file transfer application. The reliable transfer protocol must be able to handle network errors such as packet loss, duplication, and reordering. For simplicity, the protocol only needs to be unidirectional, i.e. data flows in one direction only (from sender to receiver) and acknowledgements (ACKs) in the opposite direction. To implement this protocol, you need to write two programs: a sender and a receiver with the specifications given below. Communication to and from the channel emulator uses UDP. You can implement your solution in any programming language. The overall setup is shown in the schematic diagram below

and uses the following addressing scheme (see Addressing for further information):

S: address/port learned automatically by channel emulator using 'recvfrom'
B: address/port written into 'channelInfo' by channel emulator, read by sender
C: address/port learned automatically by receiver using 'recvfrom'
E: address/port written in 'recvInfo' by receiver, read by channel emulator

When the sender needs to send packets to the receiver, it sends them to the channel emulator at 'B' instead of sending them directly to the receiver. The channel emulator then forwards the received packets to the receiver at 'E'. However, it may randomly discard or delay packets. The receiver sends ACKs to the sender via the channel at 'C', which may also randomly discard or delay ACKs.

Details

Packet Format

The Packet Type field indicates the type of the packet. It is set to

• 0 if it is a data packet,
• 1 if it is an ACK, and
• 2 if it is an end-of-transfer (EOT) packet (see below for details).

For data packets, the Sequence Number is the modulo 32 sequence number of the packet, i.e. the sequence number range is [0,31]. For ACK packets, Sequence Number is the sequence number of the packet being acknowledged.

The Packet Length field specifies the total length of the packet in bytes, including the packet header. For ACK and EOT packets, the size of the packet is just the size of the header.

Sender Program

You must implement a sender program that takes two arguments: the value of a timeout in milliseconds and a filename. The sender then transfers the file reliably to the receiver program. The timeout is used as the timeout period for the reliable data transfer. During the transfer, the sender program should create packets as big as possible, i.e. containing 500 bytes payload, if enough data is available. After all contents of the file have been transmitted successfully to the receiver and the corresponding ACKs have been received, the sender should send an EOT packet to the receiver. The sender can close its connection and exit, after the receiver has responded with an EOT packet. You can assume that EOT packets are never lost. The sender must be robust in the presence of a faulty receiver. For example, when receiving unsolicited ACKs or EOT, the sender must report this as error to standard error, but otherwise continue.

Receiver Program

You must implement a receiver program. The receiver program takes one argument: the filename to which the transferred file is written. When the receiver program receives the EOT packet, it sends an EOT packet back and exits. The receiver must be robust in the presence of a faulty sender. For example, when receiving data packets out of range or an EOT while packets are still missing, the receiver must report this as error, but otherwise continue.

Output

For both testing and marking purposes, your sender and receiver program must print log messages to standard output - a line for each data packet being sent and received (including duplicates) in the following format:

PKT <SEND|RECV> {DAT,ACK,EOT} <sequence number> <total length>

PKT SEND DAT 17 512
PKT RECV ACK 17 12

Further, whenever your program executes a potentially blocking function call (read, recv, select, sleep, etc.), it must print a message to standard output, describing the call that is being made. The format of this message is not important, but it should not contain the string "PKT". If the program ever hangs, this output will help to determine why.

Selective Repeat

Channel Emulator

channel <max delay> <discard probability> <random seed> <verbose>

• All data and ACK packets are subject to a random delay, uniformly distributed between 0 and <max delay> milliseconds.
• All data and ACK packets are subject to random discard with a probability of <discard probability>.
• If <random seed> is set to a non-zero value, this seed is being used to initialize the random number generator. Multiple runs with the same seed produce the same channel behaviour. If <random seed> is set to zero, the random number generator is seeded with the current system time.
• If <verbose> is set to a non-zero value, the channel emulator outputs information about its internal processing.

Addressing

cpu06.student.cs 38548

Additional Comments/Hints

1. Since UDP is used for data transmission, delivery to and from the channel emulator is not guaranteed. This should not matter for data or ACK packets. You can ignore the residual probability that this could result in a loss of an EOT packet.
2. Be aware that the channel emulator only forwards two EOT packets (one in each direction) and then exits!
3. To implement the operating system timer using C/C++, you need to use either 'select' or 'ualarm' . Read the respective man pages to learn about these mechanisms.
4. In general, carefully read the man pages of all system calls that you are using.

Download

• channel executable for Solaris (student.cs environment).
• channel executable for Linux/x86 - unofficial.

Procedures

What to hand in

• All source code files, including appropriate comments must be stored in the same directory. There should be no directory hierarchy or package definitions. Your assignment must come with a Makefile or clear compilation instructions in a 'README' file.
• The following executables (or start scripts) must be built or exist in the current directory. The sender program must accept a filename and a timeout in milliseconds as arguments. The receiver program must accept a filename as argument. The file specified at the sender must be transferred to the receiver and stored under the name given to the receiver. The <timeout> parameter is passed to the reliable data transfer protocol.

  sender <sendfilename> <timeout>
  receiver <recvfilename>
  

Evaluation

• Functionality and Correctness: 80%
• Code Quality and Documentation: 20%

Submission Instructions