Distributed Systems Lab
=======================

In this assignment, I implemented an electronic mail service. The core features are:
- a message transfer protocol (DMTP)
- a message access protocol (DMAP)
- transfer servers (responsible for forwarding messages to mailbox servers)
- mailbox servers (responsible for storing messages and making them available to users)
- monitoring server (receives usage statistics from transfer servers via UDP)
- a decentralized naming service (for managing addresses and locations of mailbox servers)
- opportunistic encryption for enabling secure DMAP channels
- allowing users to verify the integrity of messages
- a message client (making it easy for users to use the electronic mail service)

Technologies used:
- Java Cryptography API (JCA)
- Multithreading
- Java Remote Method Invocation (RMI)
- TCP and UDP sockets
- Java Dynamic Proxies

![Electronic Mail Service](https://i.postimg.cc/JhBPcnrc/electronic-mail-service.png)

DMTP
----

The DSLab Message Transfer Protocol (DMTP) is a plaintext application-layer protocol that specifies a
way to exchange messages in a set of instructions:

- **begin** indicates the start of a message
- **to address@foo.com,another@bar.com** sets recipient(s)
- **from me@univer.ze** sets the sender
- **subject How are you** sets the subject
- **data** Want about eat lunch tonight at 6? sets the content of the message
- **send** finalizes the message and instructs the server to send it
- **quit** closes the socket connection

DMAP
----

DMAP is a plain text protocol that allows users
to access stored messages via a set of instructions:

- **login trillian secret123** attempts to authenticate the user
- **list** lists all emails of the current user
- **show 3** shows the message with id 3
- **delete 3** deletes the message with the id 3
- **logout** ends the current session (but does not close the connection)
- **quit** closes the socket connection

Domain Name Service
-------------------

Similar to DNS, our nameservers span a distributed namespace. The namespace
network is hierarchically divided into a collection of domains. There is a single top-level domain, hosted
by the root nameserver. Using nameservers, each domain can be divided into smaller subdomains (zones).
Servers can dynamically enter the network of name servers.

![Domain Name Service](https://i.postimg.cc/YC3z6smR/dns.png)

Message Client
--------------

The message client is a command-line application that provides the functionality
for
- communicating with DMAP/DMTP servers
- encryption and decryption
- message signing and verification

![Message Client](https://i.postimg.cc/NjgzvT6N/client.png)

Using gradle
------------

### Compile & Test

Gradle is the build tool we are using. Here are some instructions:

Compile the project using the gradle wrapper:

    ./gradlew assemble

Compile and run the tests:

    ./gradlew build

### Run the applications

The gradle config config contains several tasks that start application components for you.
You can list them with

    ./gradlew tasks --all

And search for 'Other tasks' starting with `run-`. For example, to run the monitoring server, execute:
(the `--console=plain` flag disables CLI features, like color output, that may break the console output when running a interactive application)

    ./gradlew --console=plain run-monitoring