Reliable Transport Protocol

Assigned: Friday, Apr 22, 2016

Due: Wednesday, May 4, 2016 at 10:30pm

Collaboration: Work with your assigned partner for this lab. You may use your classmates and their code as a resource, but please cite them. Sharing of complete or nearly-complete answers is not permitted.

Groups

  • David Ca. and David Ch.
  • Marcel and Moses
  • Evan and Aleksandar
  • Nick and Shaun
  • Michael and Helen
  • Reilly and Otabek
  • Mari and Fengyuan
  • Dave and Hamza
  • Bazil and Albert
  • Kumar and Uzo
  • Jerry and Alex
  • Daniel and Sarah

Overview

In this laboratory programming assignment, you will be writing the sending and receiving transport-level code for implementing a simple reliable data transfer protocol. There are two versions of this lab, the alternating-bit protocol version and the Go-Back-N version. This lab should be fun since your implementation will differ very little from what would be required in a real-world situation.

Since you don’t 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 and from below 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 many texts describe). Stopping/starting of timers are also simulated, and timer interrupts will cause your timer handling routine(s) to be activated.

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 emulating a network environment. The overall structure of the environment is shown in the figure below.

end-to-end diagram of the reliable data transfer system

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

struct msg { 
    char data[DATA_LENGTH]; 
};

This declaration, and all other data structure and emulator routines, as well as stub routines (i.e., those you are to complete) are in the file, rdt.c, described later. Your sending entity will thus receive data in DATA_LENGTH byte chunks from layer5; your receiving entity should deliver DATA_LENGTH byte chunks of correctly received data to layer5 at the receiving side.

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[DATA_LENGTH]; 
};

Your routines will fill in the payload field from the message data passed down from layer5-easy to do with memcpy. The other packet fields will be used by your protocols to insure reliable delivery, as we’ve studied.

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

The simulated network environment

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 the emulator procedures together and run the resulting program, you will be asked to specify values regarding the simulated network environment:

Number of messages to simulate.
The 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.

Assignment

Alternating Bit Protocol

Design

Write a design statement for your alternating bit protocol using the routines above. Your protocol must send both ACK and NAK messages. How you accomplish that is up to you; due to the alternating bit, you could implement NAKs explicitly or implicitly in a fashion similar to the rdt2.2 protocol. Note that receiving a NAK will allow the sender to take corrective action more quickly.

In particular, you should clarify the meaning of the fields in struct pkt as you are using them (i.e., with respect to ACK and NAK) and draw or very clearly describe the FSMs for sender and receiver. (Professor Weinman completed this lab without doing so, which caused unnecessary grief.)

Implementation

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 in the text) unidirectional transfer of data from the A-side to the B-side.

You should 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.

To prevent such dropped outgoing packet, you can 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.

Put your procedures in a file called abp.c. You will need the initial version of this file, containing the emulation routines and the stubs for your procedures. You can obtain this program from the MathLAN:

$ cp /home/curtsinger/courses/csc216/rdt/rdt.c somewhere/abp.c

Your procedures should print a message whenever an event occurs at your sender or receiver (i.e., a message/packet arrival, or a timer interrupt) indicating the procedure invoked, the event interpretation, as well as any action taken in response.

As you proceed to test your implementation, if you find you missed something in your design, be sure to update and revise your statement to reflect what you plan to do (and end up doing) in your code.

Go-Back-N Protocol

Instead of a stop-and-wait protocol, you may implement a pipelined Go-Back-N protocol. Some new considerations (which do not apply to the alternating bit protocol) for the procedures you’d write include:

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 almost certainly 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 (which is full). 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 drop the request should all 50 buffers be in use at one point (Note: using the values given 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 about how to use this single timer.

Design

Write a design statement documenting and explaining your choices in how to implement the Go-Back-N protocol (and why they are correct). Your protocol must send both ACK and NAK messages. How exactly you accomplish this is up to you. In particular, you should clarify the meaning of the fields in struct pkt as you are using them (i.e., with respect to ACK and NAK messages) and draw or very clearly describe the FSMs for sender and receiver.

Implementation

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 WINDOW_SIZE. Copy a new version of the network emulator into a file called gbn.c:

$ cp /home/curtsinger/courses/csc216/rdt/rdt.c somewhere/gbn.c

Note:

To “wrap around” to the beginning of of an array, use modular arithmetic:

index = (index + 1) % BUFFER_LENGTH; // Increment index with wrap-around to zero

Evaluation

Assuming the general guidelines for lab exercises are met, completing the alternating bit protocol will earn a maximum of 93%, while completing the Go-Back-N protocol would earn up to 100% on the assignment.

Although the grading for the two versions is not cumulative, the learning is. I therefore strongly recommend you complete the alternating bit protocol before attempting the Go-Back-N protocol.

Suggestions

Checksums
You can use whatever approach for checksumming you want. Remember that the sequence number and ack field can also be corrupted. We 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).
Globals
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. Please locate any global variable declarations together in an easy to find place (preferably just above your routines).
Time
There is a float global variable called time that you can access from within your code to help you out with your diagnostics, should you find it necessary.
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 recommend that you set the tracing level to 2 and put LOTS of printfs in your own code while your debugging your procedures. Even better, use gdb to debug your program. I would be happy help you use gdb if you run into problems.

What to turn in

Please submit the following items via email:

  1. Your abp.c or gbn.c file
  2. Your design statement
  3. A transcript of running your program
    • For abp use a loss probability of 0.1, a corruption probability of 0.3, and a trace level of 2.
    • For gbn use a loss probability of 0.2, a corruption probability of 0.2, a trace level of 2, and a mean time between arrivals of 10.
  4. A brief statement explaining (via your transcript) how your protocol correctly recovered from packet loss and corruption.

Acknowledgements

This lab was written by Jerod Weinman, who based it heavily on Programming Assignment 5: Implementing a Reliable Transport Protcol by James F. Kurose and Keith W. Ross.

This course is based in part on materials from Jerod Weinman's fall 2014 offering of CSC 364.

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