Implement a protocol stack.
- a transport protocol based on UDP
- a session-level security protocol inspired by SSL/TLS
- an application-level protocol
The trasport protocol is built on UDP sockets.
In order to communicate, the client has to connect to an address and the server has to listen for incoming clients and accept them.
When a client has requested to connect, a new port is opened for the connection.
The information is splitted into packets and every packet has a index in order to reassemble the message when read.
Each packet has the following structure: stream_id | packet_type | payload | hash | index
- stream_id - used to identify the stream the packet is part of
- packet_type - cn be DATA, CONTROL, LENGTH based on the purpose of the packet
- payload - the data the packet is carrying
- hash - the md5 hash of the payload to verify that it was not corrupted
- index - the index of the packet in the stream
To ensure congestion control used Slow Start method. Window variable denotes how many packets can be sent without getting an acknowledgement. The initial value of window is 1. For every ACK if window is smaller than ssthresh(an average nb of packets to be sent) than we increase window rapidly, if is greater than increase slow. (in the repository increase with 1 and 0.5 respectively). For every NACK window is decreased by 1/3. So is the network is congested and we receive NACKs, we send fewer packets, if everything is fine we increase the value.
while True:
payload, addr = sender.recvfrom(1024, socket.MSG_DONTWAIT)
packet = get_packet(payload)
if packet["type"] == "CONTROL" and packet["data"] == "NACK":
index = packet["index"]
sender.sendto(data2send[index], receiver)
window *= 2 / 3
elif packet["type"] == "CONTROL" and packet["data"] == "ACK":
index = packet["index"]
if index in waiting:
waiting.remove(index)
window = window + 1 if window < ssthresh else window + 0.5
Implemented Diffie Hellman in order to compute the private key for encryption (use data sent on network together with some private values and some smart modulo math).
def calculate_key(public_g, public_p, private):
sympy_exp = parse_expr('(a ** b) % c')
return int(sympy_exp.evalf(subs={a:public_g, b:private, c:public_p}))
A protocol based on the state machine of a stationary telephone.
TODO: draw state machine
TODO: demo
Use Docker in order to emulate a network with packet loss and packet delay. In real world there are many corner cases that can crash the application. For more info see branch "docker"
Improvements and/or Known bugs:
- Vulnerable to SYN flood
- Use crc instead of md5