In this assignment, you will build a virtual
network and implement routing for this network. Each node in this network is a process, and links in this network
are emulated by UDP sockets.
You can use C or C++ to implement this assignment.
This is a group assignment. Each group should have up to two students. You can choose to work by yourself
on this assignment, though no extra credit will be given for working individually.
Once you have formed a group, at least one member in the group must send the instructor an email listing
the names and email addresses of both members of your group
we do not receive your email by this deadline, we will record that you will be working individually. No groups are
allowed to be formed after the October 24th deadline.
1 Virtual Network Topology
In your virtual network, each node/host is a process running on a machine. Each process is identified by the
hostname of the machine it runs on and a port number on the host. We then use a configuration file to specify the
virtual network topology.
The following figure shows an example network topology.
We can create a configuration file as follows:
//Node ID hostname control port data port neighbor 1 neighbor 2 neighbor 3 …
1 remote00.cs.binghamton.edu 5000 5001 2 4 5
2 remote00.cs.binghamton.edu 5002 5003 1 3
3 remote01.cs.binghamton.edu 5000 5001 2 5
4 remote02.cs.binghamton.edu 5002 5003 1 5
5 remote03.cs.binghamton.edu 5004 5005 1 3 4
Each line of the configuration files provides information about the node’s ID, which is a unique integer, hostname, port numbers, and ID of its directly connected neighbors. Each node runs on two ports. One port is the
control port, used for exchanging routing information (control packets) with directly connected neighbors.
other port is the data port, used for data packet sending, receiving, and forwarding.
At virtual network start time, each node will read this configuration file to determine the hostnames and port
numbers of its directly connected neighbors. For example, node 1 will learn its directly connected neighbors are
nodes 2, 4, and 5.
Nodes cannot use any information about nodes other than their direct neighbors. For
example, from the configuration above, node 1 should not learn anything about node 3.
2 Distance Vector Routing
Nodes on this virtual network use distance vector routing for determining the shortest path to other nodes in the
Each node periodically sends its distance vector to all its directly connected neighbors. For example, node 1
will send its distance vector to nodes 2, 4, and 5. Each distance vector entry contains (Destination, Next
hop, Distance) – to send a packet to destination Destination, the packet should first be forwarded to next
hop Next hop, and the total number of hops to the destination is Distance.
Each node also listens for incoming distance vectors sent by its neighbors. Whenever a new distance vector
is received, the node will follow the algorithm we discussed in class (lecture 8, slides 10 and 11) and update its
routing table accordingly.
Routing messages, i.e., distance vectors, should be exchanged using the node’s control port (e.g., node 1’s
control port is port 5000). Within each node, you can use a dedicated “control thread” to handle the sending and
receiving of all routing messages through a select() call. (We have used the select() call in assignment 1).
Routing messages will be transmitted between nodes using UDP. To create a UDP socket, pass SOCK_DGRAM
as the second parameter to the socket() call. However, since UDP is connectionless, a UDP socket does not need
to listen() or accept() a connection.
To receive UDP messages, we use the recvfrom() call:
ssize_t recvfrom (int socket, void *buffer, size_t size, int flags,
struct sockaddr *addr, socklen_t *length-ptr)
According to the libc manual1
“The recvfrom function reads one packet from the socket socket into the buffer buffer. The size
argument specifies the maximum number of bytes to be read.
If the packet is longer than size bytes, then you get the first size bytes of the packet and the rest of the
packet is lost. There’s no way to read the rest of the packet. Thus, when you use a packet protocol, you must
always know how long a packet to expect.
The addr and length-ptr arguments are used to return the address where the packet came from. ”
The receiver can determine the sender of the UDP packet by inspecting the addr field. For this assignment,
you should use the addr field to determine which source node sent a received distance vector.
To send UDP messages, the sendto call is used.
ssize_t sendto (int socket, const void *buffer, size_t size, int flags,
struct sockaddr *addr, socklen_t length)
Accordng to the libc manual2
“The sendto function transmits the data in the buffer through the socket socket to the destination address specified by the addr and length arguments. The size argument specifies the number of bytes to be
Each node should use a separate thread, the “data thread”, for sending, receiving, and forwarding non-control
packets. Such “data traffic” goes through the data port, e.g., node 1’s data port is port 5001.
For the purpose of this assignment, the only data packet you need to implement is routetrace packet. routetrace
works similar to the traceroute3
tool for displaying the path across an IP network.
You are free to design your routetrace packet format. But your packet should contain at least the following
8-bit Source node id
8-bit Destination node id
8-bit Packet id
Source node id and Destination node id are the sender and receiver’s node ids. Packet id
should be generated by the sender in increasing order. TTL stands for time to live. Every time a packet is forwarded
by an intermediate node, the TTL should be decremented by 1.
To perform routetrace to a destination node dst:
1. The sender src set the TTL set to 0.
2. It constructs a routetrace packet with source node id set to src, destination node id set to dst, and TTL set
3. Send the routetrace packet.
4. Once it gets a response, print out the responder’s node id.
5. If the responder’s node id is not dst, increment TTL, and go to step 2.
Upon receiving a routetrace packet at node recv with source src and destination dst:
1. If TTL is 0, modify the received routetrace packet as follows: set the TTL to 15 (the default TTL value in this
virtual network), set the source node id to recv, set the destination node id to src, and retain the original
packet id. Send the modified routetrace packet.
2. If TTL is not 0, decrement TTL and forward the routetrace packet to the next hop according to the routing
Because both the data thread and the control thread read/write the routing table, accesses of the routing table
should be protected by an exclusive lock.
4 A Control and Testing Client Program
You will have to show that your routing and data forwarding operates according to the specification described in
the previous sections.
To do so, you will also write a client program to control nodes’ network links and to instruct nodes to send new
data packets. This client will accept command line inputs and send corresponding control commands to each node
via their “control port” individually.
These control commands include:
1. ask a node to trace route to a destination node. For example, if the command line argument says “routetrace
1 3”, then your control client should send a message to node 1 to create routetrace packets destined to node 3. As
responses from intermediate nodes and the response from the destination node are received, node 1 prints out ids
of all nodes on the path on its concole.
2. add a new link. For example, if the command line argument says “create-link 1 2”, then your control
client should send messages to node 1 and node 2, respectively, indicating a new link has been established. These
nodes will then exchange their distance vectors and update their routing tables correspondingly.
3. remove an existing link. For example, if the command line argument says “remove-link 1 2”, then
your control client should send messages to node 1 and node 2, respectively, indicating a link has been removed.
These nodes will immediately update their routing tables and send them to their neighbors.
Because the “control thread” has to handle these different types of messages as well as distance vector messages,
you need to define these message formats and include a special “type” field to allow the receiver node to understand
how to interpret these client control packets.
Once the network topology has been changed, e.g., via create-link or remove-link, you should show that your
virtual network sends distance vector messages appropriately, nodes’ routing tables are changed correspondingly,
and packets follow new paths to their destinations. For simplicity, you can assume that the set of nodes in the
virtual network remain unchanged.
After the submission deadline, every group will sign up for a demonstration time slot with the TA. Both group
members have to be present during the demonstration. You will show the capabilities of the network nodes that
your group has implemented, such as routing, routetrace, and how your virtual network adapts to network topology
6 Other Helpful Information
Each node will create at least two threads: the control thread and the data thread. You will use the pthread4
for creating and managing these threads.
If you are not familiar the pthread library, you can read the following link to learn about how to use it:
7 Github classroom
Once you have formed your group, you can access the assignment by first logging into your Github account created
with your BU email. Then go to: https://classroom.github.com/g/oK0t462d.
Github classroom will ask you to create a team name. Please use a team name with both students’ BU email IDs.
For example, if a group’s two students’ email IDs are email@example.com and firstname.lastname@example.org,
you should create a team name called jdoe-jsmith.
Github classroom will automatically create a repository (e.g., if your team name is jdoe-jsmith, the repository
will be named cs428-cs528-pa2-jdoe-jsmith). This is the repository you will push your code to.
If your partner has already created a repository, you can select your team from a list of existing teams and join
The repository created by Github classroom is a private repository. Only the group members, course instructor,
and teaching assistant are able to see this repository. Follow the instruction on the Github page to create a new
repository on your local directory and link it to the Github repository.
Note that git is already installed on CS
department computers. To use it on your own computer, you must install it first.
To add a file to the next commit, use the git add command. To commit your code, use the git commit
command. Be sure to also push your commit to the Github repository after every commit using the git push
command (e.g., git push origin master).
We expect each repository to have at least three commits, with the first one and the last one more than 48
hours apart. Submissions that do not meet the three commits / 48 hours requirement will not be accepted or
8 How to submit
To submit, make a final commit with the message “final commit”, and push your local repository to the private
Github repository Github classroom created.
Your final commit should contain the following files:
1. Your source code.
2. A Makefile to compile your source code into executables.
3. A Readme file describing:
• the tasks both group members worked on in this assignment,
• completion status of the assignment, e.g., what has been implemented and tested, what has not,
• anything else you want the TA to be aware of while grading your assignment.
4. Two STATEMENT files, signed by each group member individually, containing the following statement:
“I have done this assignment completely on my own and in collaboration with my partner. I have not copied
my portion of the assignment, nor have I given the project solution to anyone else. I understand that if I
am involved in plagiarism or cheating I will have to sign an official form that I have cheated and that this
form will be stored in my official university record.
I also understand that I will receive a grade of 0 for the
involved assignment and my grade will be reduced by one level (e.g., from A to A- or from B+ to B) for my
first offense, and that I will receive a grade of “F” for the course for any additional offense of any kind.”
After pushing your final commit to the Github repository, please let us know by submitting your commit hash
There are two ways to locate your commit hash: You can type git log in your local repository, which will
output the commit history. Locate the commit you want to use as your final submission, record the hash associated
with the commit. The SHA1 hash code used by git should be 40 characters long. For example, the commit in the
starter code has a hash value of “baf077737a2101994c50f28ff76fd56222f68d4a3”. Alternatively, you can also go
to the Github page of your repository and locate it on the webpage.
It is important that you submit your commit hash to myCourses. This helps us know your submission is
ready for grading and which of your commits we should grade. We will not grade your assignment unless
you have submitted the commit hash to myCourses before the assignment submission deadline
Your assignment will be graded on the CS Department computers remote.cs.binghamton.edu. It is
your responsibility to make sure that your code compiles and runs correctly on CS Department computers.