Assignment # 3 Re-implement the HTTP client


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Data Communications & Computer Networks
Lab Assignment # 3

In this assignment, you will re-implement the HTTP client and the HTTP remote file
manager of Assignments #1 and #2 respectively using UDP protocol. In the previous
assignments, you leverage TCP protocol for implementation to guarantee packet
transmission over unreliable network links. Because you are going to use UDP protocol that
does not guarantee the transfer, you need to insure reliability by implementing a specific
instance of the Automatic-Repeat-Request (ARQ) protocol called: Selective Repeat ARQ /
Selective Reject ARQ. Before starting on this Lab, we encourage you to review the
programming samples and the associated course materials.
The following is a summary of the main tasks of the Assignment:
1. Study the simulation network infrastructure and message structure.
2. Replace TCP by UDP in your HTTP library in both the client and server.
3. Implement `Selective-Repeat` flow control to make a reliable transport.
4. (Optional) Support multiple clients at the server.
The primary objective of this assignment is to use the unreliable UDP instead of TCP
transport protocol in the implementation of your both HTTP Client and HTTP file manager.
To this end, you should manually ensure the reliability of the transport on top of UDP
protocol by implementing Selective-Repeat ARQ technique.
Selective Repeat is part of the automatic repeat-request (ARQ). With selective repeat, the sender sends a number
of frames specified by a window size even without the need to wait for individual ACK from the receiver as in GoBack-N ARQ. The receiver may selectively reject a single frame, which may be retransmitted alone; this contrasts
with other forms of ARQ, which must send every frame from that point again. The receiver accepts out-of-order
frames and buffers them. The sender individually retransmits frames that have timed out.[1]
It is important to emphasize that both the HTTP Client and the file manager should keep
the same specifications as given in the previous assignments. The apps should be updated
to be executed in the simulation environment as will be described in the following sections.
Comp6461 – Lab Assignment # 3 Page 2
In addition to adjusting your previous apps (HTTP client and file manager server) to work
with UDP and the provided simulation environment, you are required to implement the
core functionalities of TCP protocol such as Mimicking the TCP three-way handshaking.
Important Note:
You must use only the bare-minimum socket APIs provided by the chosen programming
language. Therefore, you must not leverage any library that could abstract the socket
Important Considerations
In the following, we present a set of notes that you should take into account in the
development of your HTTP client and HTTP file manager using UDP protocol.
Endianness is the order of bytes in a multiple-bytes representation such as word, integer,
float. When sending multi-bytes data type via a network, we must convert them from the
host byte order to the network byte order (e.g., Big-Endian). And when receiving multibytes data types from the network, we must convert them from the network byte order to
the host order. Most of the languages provide APIs for this conversion.
UDP methods
You need to read UDP Socket API for your selected language. Here are some quick notes.
1. There is no accept in UDP, it is connectionless.
2. The recvfrom method returns not only the data but also the host:port of the sender. You
should use the received_addr to reply.
3. The sendto method requires two arguments: data to be sent, and the host:port of the
An address of a network application includes two parts: the IP address of the host and the
port number. To simplify the complexity of the assignment, we will use IPv4 addresses
only. An IPv4 address is encoded using 4 bytes. For example, the address is
represented by an array of 4 bytes 172, 16, 254 and 1. Port number is represented by 2
bytes in Big-Endian order.
Comp6461 – Lab Assignment # 3 Page 3
Development Environment
Local networks are more reliable than remote networks in the Internet. We could use UDP
protocol in local networks with a negligible error if not zero. For this reason, you are
requested to develop your apps in the simulation environment that we developed. The
latter is a complete communication system that allows you to experiment the unreliability
factor of the Internet such as packet delay and packet drop. Specifically, we provide a
router app that simulates the previous factors. To make your app communicate with the
router, you need to use its own custom packet structure. We urge you to consult the
provided example to have a deeper understanding of the programming model in the
simulation environments. The simulation environment is described the in the following.
Simulated network infrastructure
You are provided an unreliable UDP channel, in which packets can be dropped or delayed
randomly. Instead of sending/receiving network packets directly between a client and a
server, both of them have to send and receive packets via the router. The purpose of this
infrastructure is to simulate and control the unreliable characteristic of UDP protocol. In
the figure, both application A and B send and receive packets to/from the router.
Figure 1: The Communication Model
We provide you the executable of the router on Windows, MacOS, and Linux. This is the
manpage of the router application.
Router is a logical router that dispatches UDP packets between applications.
It receives UDP packets, then dispatches them to the associated destination
Comp6461 – Lab Assignment # 3 Page 4
of the packets. During the routing, the value of a peer address will be
modified from ‘destination’ to ‘source’.
router –port int –drop-rate float –max-delay duration –seed int

–port int-number
port number that the router is listening for the incoming packets.
default value is 3000.

–drop-rate float-number
drop rate is the probability of packets will be dropped during on the way.
use 0 to disable the drop feature.
–max-delay duration (eg. 5ms, 4s, or 1m)
max delay the maximum duration that any packet can be delayed.
any packet will be subject to a delay duration between 0 and this value.
the duration is in format 5s, 10ms. Uses 0 to deliver packets immediately.
–seed int
seed is used to initialize the random generator.
if the same seed is provided, the random behaviors are expected to repeat.
router –port=3000 –drop-rate=0.2 –max-delay=10ms –seed=1
Message structure
To interact properly with the router, all the packets (e.g. UDP message) should follow the
following structure; otherwise, the router may reject or fail to dispatch it to a proper
Figure 2: Packet Structure
Comp6461 – Lab Assignment # 3 Page 5
Packet Type: 1 byte
This value indicates the type of the packet, which can be Data or ACK or SYN or SYN-ACK or
NAK. You are free to define and manage this value.
Sequence number: 4 bytes – unsigned and big-endian
The sequence number has two meanings which depend on the value of packet type. If the
packet type is SYN, this value is the initial sequence number during the handshaking.
Otherwise, the sequence number is the accumulated sequence number of the first byte of
the payload (like TCP). You can use the sequence number as the packet number (e.g., 1, 2,
3) as it can be simpler.
Peer Address and Peer Port
The peer address of a packet also has two meanings. When you send a packet, the peer
address is the address of the destination that you want to send. Thus, you have to set the
peer address and port of the packet by the values of the receiver. On the other hands, when
you receive a packet, the peer address is the address of the original sender. The router
executes this translation. When dispatching the packet, the router modifies the peer
address from Destination to Source.
You are allowed to send a maximum of 1013 bytes’ payload for each packet. In other words,
each packet must be in range 11 (when payload is 0) to 1024 bytes.
Flow Example
In this section, we describe step by step a simple interaction between a client and a server.
1. Router is running at port 3000 at the host
2. Server is running at port 8007 at the host
3. Client is running at port 41830 (uses an ephemeral port) at the host
Comp6461 – Lab Assignment # 3 Page 6
Figure 3: Flow Example
Step 1: Client wants to send “Hi S” to the server
In order to send a payload to the server, the client has to construct a packet P1 which is 15
bytes. These bytes are broken down as follows.
• Byte 0: – Packet Type – in this example we use 0, which means this is a data packet.
• Byte 1-4: – Sequence Number – assume this packet has the sequence number 1.
Therefore, it is encoded by 4 bytes 0, 0, 0, and 1 in BigEndian order.
• Byte 5-8 – Peer Address – the server (eg. destination) is running at host “”,
thus the client has to put this value to the packet as [192, 168, 2, 3].
• Byte 9-10: – Peer Port Number – the server is listening at port 8007, thus the client has
to put this value in BigEndian order to the packet as [11, 184].
• Byte 11-14: – Payload – the client wants to send “Hi S” which is encoded as [105, 32,
Once the packet p1 is constructed, client calls sendto(p1, router_addr) to send the
packet to the router. The client is provided with the router address in advanced.
Comp6461 – Lab Assignment # 3 Page 7
Step 2: Router receives packet p1 and routes to the server
After the client invokes sendto(p1, router_addr), the router can receive the packet p1
via recvfrom function. The recvfrom function returns not only the data but also the
host:port of the sender. In particular, the router receives packet p1 and the sender’s
address is “”. The router translates the peer address for packet p1.
It creates a new packet p1′ which is almost similar to the packet p1 except that the values
of the peer address and port. The new values of peer address:port are the address:port of
the client. The router then calls sendto(p1′, packet_dest) with the packet_dest is the
address of the server which is extracted from the peer_address of the packet p1.
Step 3: Server receives packet p1′ from the router
Once the router executed sendto(p1′, packet_dest), the server may receive packet p1′
with the carrier address (eg. the router address). It exacts the message’s payload, and got
“Hi S”.
Step 4: Server wants to reply “Hi” to the client
The server in turn wants to reply a message “Hi” to the client. It has to construct a packet
called p2. The packet p2 is shorter than p1 and p1′ as its payload is only 2 bytes while p1
and p1′ are 4 bytes. The peer address of the packet p2 is the address of the client
(extracted from the peer address of packet p1′). To reply to the packet, the server calls
sendto(p2, router_addr). The router address is the carrier address in Step 3.
Step 5: Router receives packet p2 and routes to the client
Step 5 is similar to Step 2. When the router receives the packet p2 from the server, it
modifies the peer address from “″ to ”″ and
forwards the modified packet to the client.
Step 6: Client receives packet p2′ from the router
Step 6 is similar to Step 3.
Optional Tasks (Bonus Marks)
If you have successfully completed the material above, congratulations; as you now
understand the implementation of reliable protocols over unreliable networks. For the rest
of this lab exercise, we have included the following optional tasks. These optional tasks will
help you gain a deeper understanding of the material, and if you can do so, we encourage
you to complete them as well. Bonus marks will be given for that.
Multi-Requests Support
In order to support multiple clients at the server, you are required to mimic the accept behavior
of the TCP. In particular, for each new handshaking request, you create a new UDP Socket for
Comp6461 – Lab Assignment # 3 Page 8
the requested client exclusively. Another approach to support multiple clients is to use the client
address to as its identifier.
Submission and Grading
Important Note: You can this assignment individually or in a group of at most 2 members
(i.e. you and another student). No extra marks or any special considerations will be given
for working individually.
1) Create one zip file, containing the necessary source-code files (.java, .c, etc.)
You must name your file using the following convention:
If the work is done by 1 student: Your file should be called A#_studentID, where # is the
number of the assignment. studentID is your student ID number.
If the work is done by 2 students: The zip file should be called A#_studentID1_studentID2,
where # is the number of the assignment. studentID1 and studentID2 are the student ID
numbers of each of the group members.
2) Assignments must be submitted in the right folder of the assignments. Upload your zip
file at the URL: as Programming Assignment 3. Assignments uploaded
to an incorrect folder will not be marked and result in a zero mark. No resubmissions
will be allowed.
A demo is needed for this assignment and your lab instructors will communicate the
available demo times to you, where you must register a time-slot for the demo, and you
must prepare your assignment and be ready to demo at the start of your time-slot. If the
assignment is done by 2 members, then both members must be present for the demo.
During your presentation, you are expected to demo the functionality of the application,
explain some parts of your implementation, and answer any questions that the lab
instructor may ask in relation to the assignment and your work. Different marks may be
assigned to the two members of the team if needed. Demos are mandatory. Failure to
demo your assignment will entail a mark of zero for the assignment. Failure to show
up for a scheduled demo will entail you to a zero marks as no replacement of a demo
time is/will be allowed.
Comp6461 – Lab Assignment # 3 Page 9
Grading Policy (10 Marks)
1- With reliable environment: 6 Marks
– Mimicking the TCP three-way handshaking technique when you in or start the
communication with the server: 1 Mark
– GET: 2.5 Marks (e.g. read the content of a file and listing files)
– POST: 2.5 Marks (e.g. create a new file)
2- With drop rate only: 1.5 Marks
3- With delayed only: 1.5 Marks
4- With both drop and delay: 1 Mark
Optional Tasks (2 Marks)
1. Support multiple clients at the server: 2 Marks
[1] Selective Repeat.

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