Computer Networking, Fall 2021
Assignment 2: A Simple Reliable Transport Protocol
2021.10.25
1 Introduction
In this assignment, you will build a simple reliable transport protocol, RTP, on top
of UDP. Your RTP implementation must provide in order, reliable delivery of UDP
datagrams in the presence of events like packet loss, delay, corruption, duplication, and
re ordering. There are a variety of ways to ensure a message is reliably delivered from a
sender to a receiver. You are to implement a sender and a receiver that follows the
following RTP specification.
2 RTP Specification
RTP sends data in the format of a header, followed by a chunk of data. RTP has four
header types: ST ART, END, DAT A, and ACK, all following the same format:
1 typedef s t r u c t RTP_header {
2 uint8_t type ; // 0 : START; 1 : END; 2 : DATA; 3 : ACK
3 uint16_t length ; // Length of data ; 0 f o r ACK, START and END packets
4 uint32_t seq_num ;
5 uint32_t checksum ; // 32−b i t CRC
6 } rtp_header_t ;
Establish connection. To initiate a connection, sender starts with a ST ART message
along with a random seq_num value, and wait for an ACK for this ST ART
message.
Data transmission. After establishing the connection, additional packets in the same
connection are sent using the DAT A message type, adjusting seq_num appropriately.
sender will use 0 as the initial sequence number for data packets in that connection.
Terminate connection. After everything has been transferred, the connection should
be terminated with sender sending an END message, and waiting for the corresponding
ACK for this message.
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ACK for START & END. The ACK seq_num values for ST ART and END messages
should both be set to whatever the seq_num values are that were sent by sender.
Packet Size. An important limitation is the maximum size of your packets. The UDP
protocol has an 8 byte header, and the IP protocol underneath it has a header of 20
bytes. Because we will be using Ethernet networks, which have a maximum frame size
of 1500 bytes, this leaves 1472 bytes for your entire packet structure (including both
the header and the chunk of data).
3 Assignment components
3.1 Part 1: Implement sender
sender should read an input message and transmit it to a specified receiver using
UDP sockets following the RTP protocol. It needs to split the input message into
appropriately sized chunks of data, and append a checksum to each packet. seq_num
should increment by one for each additional packet in a connection. Please use the
32-bit CRC header we provide in src/util.c, in order to add a checksum to your packet.
You will implement reliable transport using a sliding window mechanism. The
size of the window (window_size) will be specified in the command line. sender must
accept cumulative ACK packets from receiver.
After transferring the entire message, you should send an END packet to mark the
end of connection.
sender must ensure reliable data transfer under the following network conditions:
• Loss of arbitrary levels
• Re ordering of ACK messages
• Duplication of any amount for any packet
• Delay in the arrivals of ACKs
• Packet corruption
To handle cases where ACK packets are lost, you should implement a 500 milliseconds
retransmission timer to automatically retransmit packets that were never
acknowledged. Whenever the window moves forward (i.e., some ACK(s) are received
and some new packets are sent out), you reset the timer. If after 500 ms the window
still has not advanced, you retransmit all packets in the window because they are all
never acknowledged.
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3.1.1 Running sender
sender should be invoked as follows:
1 . / sender [ Receiver IP ] [ Receiver Port ] [ Window Size ] [ Message ]
• Receiver IP: The IP address of the host that receiver is running on.
• Receiver Port: The port number on which receiver is listening.
• Window Size: Maximum number of outstanding packets.
• Message: The message to be transferred. It can be a text string as well as a
filename. sender will try to open the file with filename [Message] at first. If the
file exists, it sends the content of this file to receiver; otherwise, it directly sends
the [Message] content.
3.2 Part 2: Implement receiver
receiver needs to handle only one sender at a time and should ignore ST ART messages
while in the middle of an existing connection. It must receive and store the
message sent by the sender on disk completely and correctly.
receiver should also calculate the checksum value for the data in each packet it
receives using the header mentioned in Part 1. If the calculated checksum value does
not match the checksum provided in the header, it should drop the packet (i.e. not
send an ACK back to the sender).
For each packet received, it sends a cumulative ACK with the seq_num it expects to
receive next. If it expects a packet of sequence number N, the following two scenarios
may occur:
1. If it receives a packet with seq_num not equal to N, it will send back an ACK
with seq_num=N. Note that this is slightly different from the Go-Back-N (GBN)
mechanism discussed in class. GBN totally discards out-of-order packets, while
here receiver buffers out-of-order packets. The mechanism here is more efficient
than GBN.
2. If it receives a packet with seq_num=N, it will check for the highest sequence
number (say M) of the in -order packets it has already received and send ACK
with seq_num=M + 1.
If the next expected seq_num is N, receiver will drop all packets with seq_num
greater than or equal to N+ window_size to maintain a window_size window.
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3.2.1 Running receiver.
receiver should be invoked as follows:
1 . / r e c e i v e r [ Receiver Port ] [ Window Size ] [ F i l e Name]
• Receiver Port: The port number on which receiver is listening for data.
• Window Size: Maximum number of outstanding packets.
• File Name: The name of the file used to store received messages.
3.3 Part 3: Optimizations
For this part of the assignment, you will be making a few modifications to the programs
written in the previous two parts. Consider how the programs written in the previous
parts would behave for the following case in Figure 1 where there is a window of size 3:
Figure 1: Inefficient transfer of data.
In this case receiver would send back two ACKs both with the sequence number
set to 0 (as this is the next packet it is expecting). This will result in a timeout in
sender and a retransmission of packets 0, 1 and 2. However, since receiver has already
received and buffered packets 1 and 2. Thus, there is an unnecessary retransmission of
these packets.
In order to account for situations like this, you will be modifying your receiver
and sender accordingly:
• receiver will not send cumulative ACKs anymore; instead, it will send back an
ACK with seq_num set to whatever it was in the data packet (i.e., if a sender
sends a data packet with seq_num set to 2, receiver will also send back an ACK
with seq_num set to 2). It should still drop all packets with seq_num greater than
or equal to N+ window_size, where N is the next expected seq_num.
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• sender must maintain information about all the ACKs it has received in its
current window. In this way, packet 0 having a timeout would not necessarily
result in a retransmission of packets 1 and 2.
For a more concrete example, Figure 2 and Figure 3 show how your improved sender
(opt_sender) and receiver (opt_receiver) should behave for the case described at the
beginning of this part.
Figure 2: Only ACK necessary data.
opt_receiver individually ACKs both packet 1 and 2 in Figure 2.
Figure 3: Only send unACKd data.
opt_sender receives these ACKs and denotes in its buffer that packets 1 and 2 have
been received in Figure 3. Then, it waits for the 500 ms timeout and only retransmits
packet 0 again.
You need to save your optimized program in src/opt_sender.c and src/opt_receiver.c).
Note that you should update the CMakeList.txt file to add the targets of opt_sender
and opt_receiver. The command line parameters passed to opt_sender and opt_receiver
are the same as the previous two parts.
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4 Important Notes
4.1 Development and test environment
This assignment needs to be developed and tested under the Linux system. We
strongly recommend that you use ubuntu 18.04 (i.e. the final testing environment
of TA).
4.2 Code template
We provide two code templates, which are placed in the assignment2-rtp/templates/basic
and assignment2-rtp/templates/recommended directories. Both templates demonstrate
the same basic function, i.e., how to encapsulate and transmit the RTP protocol atop
UDP. You can complete this assignment on the basis of either template. The main
difference between the two templates is that under the recommended template, you
need to encapsulate and implement the RTP protocol in a POSIX-like API. We define
the POSIX-like API of RTP in rtp.h and you need to provide your implementations
in rtp.c. We detail the differences between the semantics of the RTP API and the
standard POSIX API that you need to pay attention to in the source file.
Note that no matter which template you choose, you need to copy the files under
your chosen template directory to the src directory.
4.3 Compilation
The code templates we provide are managed using the cmake. The recommended
compilation approach is as follows. (Before you try to compile the template, you should
copy the files in your chosen template to the src directory.)
1 cd assignment2−rtp
2 mkdir build
3 cd build
4 cmake . .
5 make
All the compiled executable files (including test cases) and intermediate files of cmake
will be put in this build directory. Of course, you can compile a specified executable
file at a time, e.g. run make sender to compile sender. You only need to re-run the
cmake command to update the Makefile every time you modify the CMakeList.txt file.
5 Testing
We provide two executable files (broken_sender and broken_receiver) in assignment2-
rtp/test directory. broken_sender and broken_recevier follows the specification of
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RTP protocol in Part 1 & 2. Both files can simulate some different types of network
failures during transmission. You can test your implementation using the two test cases.
In order to simplify testing, you can run the two processes on the same host (using the
local ip 127.0.0.1 for receiver). We detail the use of these two test cases and the types
of failures that can be simulated in §5.1 and §5.2. You need to implement your own
test case to test your code in Part 3.
Note that the test cases we provide only simulate the most simple network failure.
The test cases used by TA for grading will be more complicated (multiple types of
failures appear multiple times). You are encouraged to customize your test case to cover
more failures. You can test your implementation against your classmates’ customized
broken_sender and broken_receiver. Of course, you should exchange the compiled
executable files, NOT SOURCE CODES.
5.1 broken_receiver
To test your sender, you can run your sender with broken_receiver in two terminals
respectively as in the following example.
Terminal 1:
1 cd assignment2−rtp / build
2 . / broken_receiver 8076 10 . . / data / recv . txt [ Error Code ]
Here, the first three arguments are the same as receiver in §3.2.1. The value of Error
Code argument for broken_receiver can be as follows.
• If error code is 1, broken_receiver will drop one packet randomly during transmission
(without ACK).
• If error code is 2, broken_receiver will exchange the order of two ACK packets.
• If error code is 3, broken_receiver will select one received packet and send its
ACK twice.
Terminal 2:
1 cd assignment2−rtp / build
2 . / sender 1 2 7 . 0 . 0 . 1 8076 10 . . / data / t e s t . txt
We provide a test text file (RFC-793 of TCP protocol) in the assignment2-rtp/data
directory. You can use diff command to compare the raw file and the file saved by
broken_receiver. If the function of sender is correct, the two files should be the
same.
1 cd assignment2−rtp / data
2 d i f f t e s t . txt recv . txt
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5.2 broken_sender
To test your receiver, you should run broken_sender with your receiver together.
Terminal 1:
1 . / r e c e i v e r 8076 10 . . / data / recv . txt
Terminal 2:
1 . / broken_sender 1 2 7 . 0 . 0 . 1 8076 10 . . / data / t e s t . txt [ Error Code ]
Similarly, broken_sender will send the test file to your receiver. Here, the Error
Code argument of broken_sender indicates different types of network failure, which
can be as follows.
• If error code is 1, broken_sender will drop one packet randomly during transmission.
• If error code is 2, broken_sender will select one packet and send it twice.
• If error code is 3, broken_sender will send one packet with wrong checksum.
6 Grading
6.1 Part 1 & 2 (70%)
If your implementation (sender and receiver in Part 1 and 2) can pass one complicated
test case that mixes all the failure types (described below), you can directly get all the
scores of this part. Otherwise, your implementation will get corresponding scores based
on the passed test cases.
For your sender:
• Normal case without network failures (10%)
• Loss of arbitrary amount and types of ACK messages (5%)
• Re ordering of ACK messages (5%)
• Duplication of ACK messages (5%)
• Delay in the arrivals of ACK messages (5%)
• Connection failure (receive incorrect ACK for ST ART message) (5%)
For your receiver:
• Normal case without network failures (10%)
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• Loss of arbitrary levels (5%)
• Re ordering of DAT A messages (5%)
• Duplication of any amount for any packet (5%)
• Packet corruption (bad checksum) (5%)
• Wrong connection (receive incorrect ST ART message) (5%)
6.2 Part 3 (30%)
For Part 3, if your opt_sender and opt_receiver can pass two normal case without
network failures respectively, you can get 10% for each one. If your implementations
can pass the test cases with mixed failures above respectively, you can get 5% for each
one.
7 Submission Guidelines
You need to submit your assignment by compressing the assignment2-rtp directory,
which includes at least the following files:
• CMakeLists.txt
• src/sender.c
• src/opt_sender.c
• src/receiver.c
• src/opt_receiver.c
• src/util.h
• src/util.c
• src/rtp.h
• (optional) src/rtp.c
You may submit additional files that are needed. Your programs must be implemented
in C. Please refer to the course website for the submission instructions.
Deadline: November 28 23:59:59
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8 Recommended references
• Beej’s Guide to Network Programming
• Linux C Programming
• The Missing Semester of Your CS Education
9 Acknowledgements
This programming assignment is based on JHU’s Assignment 2 from EN.601.414/614:
Computer Networks.