Programming Assignment:
Implementing a Reliable Transport Protocol

Overview

In this programming assignment, you will be writing the sending and receiving transport-level code for implementing a simple reliable data transfer protocol. The basic assignment will be to implement the Alternating Bit Protocol; the extra credit assignment will be to implement a Go-Back-N protocol. This should be FUN since your implementation will differ very little from what would be required in a real-world situation.

Since we do not have standalone machines (with an OS that you can modify), your code will have to execute in a simulated hardware/software environment. However, the programming interface provided to your routines (i.e., the code that would call your entities from above (i.e., from layer 5) and from below (i.e., from layer 3)) is very close to what is done in an actual UNIX environment. (Indeed, the software interfaces described in this programming assignment are much more realistic that the infinite loop senders and receivers that the text describes). Stopping/starting of timers are also simulated, and timer interrupts will cause your timer handling routine to be activated.

The desciption of the routines below is given in C.  See text at end regarding the Java version of this assignment. Note that you do not need a network connection to run this assignment, so you can do it pretty much on any machine you would like.

The routines you will write The procedures you will write are for the sending entity (A) and the receiving entity (B). Only unidirectional transfer of data (from A to B) is required. Of course, the B side will have to send packets to A to acknowledge (positively or negatively) receipt of data. Your routines are to be implemented in the form of the procedures described below. These procedures will be called by (and will call) procedures that I have written which emulate a network environment. The overall structure of the environment is shown in Figure 1:

The unit of data passed between the upper layers and your protocols is a message, which is declared as:

struct msg {
  char data[20];
  };

The unit of data passed between your routines and the network layer is the packet, which is declared as:

struct pkt {
   int seqnum;
   int acknum;
   int checksum;
   char payload[20];
    };

The routines you will write are detailed below. As noted above, such procedures in real-life would be part of the operating system, and would be called by other procedures in the operating system.

A_output(message),

where message is a structure of type msg, containing data to be sent to the B-side.

This routine will be called whenever the upper layer at the sending side (A) has a message to send. It is the job of your protocol to insure that the data in such a message is delivered in-order, and correctly, to the receiving side upper layer.

A_input(packet),

where packet is a structure of type pkt.

This routine will be called whenever a packet sent from the B-side (i.e., as a result of a tolayer3() being done by a B-side procedure) arrives at the A-side. packet is the (possibly corrupted) packet sent from the B-side.

A_timerinterrupt()

This routine will be called when A's timer expires (thus generating a timer interrupt). You'll probably want to use this routine to control the retransmission of packets. See starttimer() and stoptimer() below for how the timer is started and stopped.

A_init()

This routine will be called once, before any of your other A-side routines are called. It can be used to do any required initialization.

B_input(packet),

where packet is a structure of type pkt.

This routine will be called whenever a packet sent from the A-side (i.e., as a result of a tolayer3() being done by a A-side procedure) arrives at the B-side. packet is the (possibly corrupted) packet sent from the A-side.

B_init()

This routine will be called once, before any of your other B-side routines are called. It can be used to do any required initialization.

Software Interfaces

The procedures described above are the ones that you will write. I have written the following routines which can be called by your routines:

starttimer(calling_entity,increment),

where calling_entity is either 0 (for starting the A-side timer) or 1 (for starting the B side timer), and increment is a float value indicating the amount of time that will pass before the timer interrupts. A's timer should only be started (or stopped) by A-side routines, and similarly for the B-side timer.

To give you an idea of the appropriate increment value to use: a packet sent into the network takes an average of 5 time units to arrive at the other side when there are no other messages in the medium.

stoptimer(calling_entity),

where calling_entity is either 0 (for stopping the A-side timer) or 1 (for stopping the B side timer).

tolayer3(calling_entity,packet),

where calling_entity is either 0 (for the A-side send) or 1 (for the B side send), and packet is a structure of type pkt.

Calling this routine will cause the packet to be sent into the network, destined for the other entity.

tolayer5(calling_entity,message),

where calling_entity is either 0 (for A-side delivery to layer 5) or 1 (for B-side delivery to layer 5), and message is a structure of type msg. With unidirectional data transfer, you would only be calling this with calling_entity equal to 1 (delivery to the B-side).

Calling this routine will cause data to be passed up to layer 5.

A call to procedure tolayer3() sends packets into the medium (i.e., into the network layer). Your procedures A_input() and B_input() are called when a packet is to be delivered from the medium to your protocol layer.

The medium is capable of corrupting and losing packets. It will not reorder packets. When you compile your procedures and my procedures together and run the resulting program, you will be asked to specify values regarding the simulated network environment:

• Number of messages to simulate. My emulator (and your routines) will stop as soon as this number of messages have been passed down from layer 5, regardless of whether or not all of the messages have been correctly delivered. Thus, you need not worry about undelivered or unACK'ed messages still in your sender when the emulator stops.

Note that if you set this value to 1, your program will terminate immediately, before the message is delivered to the other side. Thus, this value should always be greater than 1.
• Loss. You are asked to specify a packet loss probability. A value of 0.1 would mean that one in ten packets (on average) are lost.
• Corruption. You are asked to specify a packet loss probability. A value of 0.2 would mean that one in five packets (on average) are corrupted.

Note that the contents of payload, sequence, ack, or checksum fields can be corrupted. Your checksum should thus include the data, sequence, and ack fields.
• Tracing. Setting a tracing value of 1 or 2 will print out useful information about what is going on inside the emulation (e.g., what's happening to packets and timers). A tracing value of 0 will turn this off. A tracing value greater than 2 will display all sorts of odd messages that are for my own emulator-debugging purposes.

A tracing value of 2 may be helpful to you in debugging your code. You should keep in mind that real implementors do not have underlying networks that provide such nice information about what is going to happen to their packets!
• Average time between messages from sender's layer5. You can set this value to any non-zero, positive value. Note that the smaller the value you choose, the faster packets will be be arriving to your sender.

You are to write the procedures, A_output(),A_input(),A_timerinterrupt(),A_init(),B_input(), and B_init() which together will implement a stop-and-wait (i.e., the alternating bit protocol, which we referred to as rdt3.0 on page 6-13 of the class notes) unidirectional transfer of data from the A-side to the B-side. Your protocol should use both ACK and NACK messages..

You should choose a very large value for the average time between messages from sender's layer5, so that your sender is never called while it still has an outstanding, unacknowledged message it is trying to send to the receiver. I'd suggest you choose a value of 1000. You should also perform a check in your sender to make sure that when A_output() is called, there is no message currently in transit. If there is, you can simply ignore (drop) the data being passed to the A_output() routine.

You should put your procedures in a file called prog2.c. You will need the initial version of this file, containing my emulation routines, and the stubs for your procedures. Students should copy the version of prog2.c from the class web site.

This assignment can be completed on any machine supporting C. It makes no use of UNIX features. (You can simply grab the file from the class sites and copy the file to whatever machine and OS you choose).

As always, you should hand in a code listing, a design document (as described in the handout accompanying the first programming assignment), and sample output.

For your sample output, your procedures should print out a message whenever an event occurs at your sender or receiver (a message/packet arrival, or a timer interrupt) as well as any action taken in response. You should hand in output for a run up to the point (approximately) when 10 messages have been ACK'ed correctly at the receiver, a loss probability of 0.1, and a corruption probability of 0.3, and a trace level of 2. You should annotate your printout with a colored pen showing how your protocol correctly recovered from packet loss and corruption.

Make sure you read the ``helpful hints'' for this assignment following the description of the extra credit assignment.

Extra Credit Assignment

You are to write the procedures, A_output(),A_input(),A_timerinterrupt(),A_init(),B_input(), and B_init() which together will implement a Go-Back-N unidirectional transfer of data from the A-side to the B-side, with a window size of 8. Your protocol should use both ACK and NACK messages. Consult the basic assignment above for information about how to obtain the network emulator.

I would STRONGLY recommend that you first implement the basic assignment (Alternating Bit) and then extend your code to implement the extra-credit assignment (Go-Back-N). Believe me - it will not be time wasted! However, some new considerations for your Go-Back-N code (which do not apply to the Alternating Bit protocol) are:

A_output(message),

where message is a structure of type msg, containing data to be sent to the B-side.

Your A_output() routine will now sometimes be called when there are outstanding, unacknowledged messages in the medium - implying that you will have to buffer multiple messages in your sender. Also, you'll also need buffering in your sender because of the nature of Go-Back-N: sometimes your sender will be called but it won't be able to send the new message because the new message falls outside of the window.

Rather than have you worry about buffering an arbitrary number of messages, it will be OK for you to have some finite, maximum number of buffers available at your sender (say for 50 messages) and have your sender simply abort (give up and exit) should all 50 buffers be in use at one point (Note: using the values I have given you below, this should never happen!) In the ``real-world,'' of course, one would have to come up with a more elegant solution to the finite buffer problem!
 

A_timerinterrupt()

This routine will be called when A's timer expires (thus generating a timer interrupt). Remember that you've only got one timer, and may have many outstanding, unacknowledged packets in the medium, so you'll have to think a bit about how to use this single timer.

Consult the basic assignment above for a general description of what to hand in. You should hand in output for a run that was long enough so that at least 20 messages were successfully transfered from sender to receiver (i.e., the sender receives ACK for these messages) transfers, a loss probability of 0.2, and a corruption probability of 0.2, and a trace level of 2, and a mean time between arrivals of 10. You should annotate parts of your printout with a colored pen showing how your protocol correctly recovered from packet loss and corruption.
 

Helpful Hints and the like

• Checksumming. You can use whatever approach for checksumming you want. Remember that the sequence number and ack field can also be corrupted. I would suggest a TCP-like checksum, which consists of the sum of the (integer) sequence and ack field values, added to a character-by-character sum of the payload field of the packet (i.e., treat each character as if it were an 8 bit integer and just add them together).
• Note that any shared ``state'' among your routines needs to be in the form of global variables. Note also that any information that your procedures need to save from one invocation to the next must also be a global (or static) variable. For example, your routines will need to keep a copy of a packet for possible retransmission. It would probably be a good idea for such a data structure to be a global variable in your code. Note, however, that if one of your global variables is used by your sender side, that variable should NOT be accessed by the receiving side entity, since in real life, communicating entities connected only by a communication channel can not share global variables.
• There is a float global variable called time that you can access from within your code to help you out with your diagnostics msgs.
• START SIMPLE. Set the probabilities of loss and corruption to zero and test out your routines. Better yet, design and implement your procedures for the case of no loss and no corruption, and get them working first. Then handle the case of one of these probabilities being non-zero, and then finally both being non-zero.
• Debugging. I'd recommend that you set the tracing level to 2 and put LOTS of printf's in your code while your debugging your procedures.
• Random Numbers.

The emulator generates packet loss and errors using a random number generator. My past experience is that random number generators can vary widely from one machine to another. Some of you may need to modify the random number generation code in my emulator. My emulation routines have a test to see if the random number generator on your machine will work with my code. If you get an error message:

It is likely that random number generation on your machine is different from what this emulator expects. Please take a look at the routine jimsrand() in the emulator code. Sorry.

then you'll know you'll need to look at how random numbers are generated in the routine jimsrand(); see the comments in that routine.

• Event.class
• Event.java
• EventList.class
• EventList.java
• EventListImpl.class
• EventListImpl.java
• Message.class
• Message.java
• NetworkSimulator.class
• NetworkSimulator.java
• Packet.class
• Packet.java
• Project.class
• Project.java
• StudentNetworkSimulator.java

• NetworkSimulator is an abstract class that is the bulk of the simulator. StudentNetworkSimulator is the only class that you will have to modify.
• Packet, Message, Event, and EventListImpl are support classes.  EventList is an interface.  Project is the "driver" for the whole thing.
• You only need the  .class  files, and StudentNetworkSimulator.java.  The other sources are there in case you are interested.
• StudentNetworkSimulator.java contains inline comments documenting the interfaces of the other classes that you will need.  These class files will need to be in the CLASSPATH (which will happen automatically if you edit and compile StudentNetworkSimulator.java in the same directory as the other class files).