You can’t build a system without some idea of what you want to build

Obvious, ha? strangely enough, it is not.

This is one of a few technical posts I am writing about distributed systems,cryptography and Blockchain.

In this post I will oversimplify a few concepts around networked applications in general, messaging techniques and share some code snippets along the way.

Protocols

Distributed systems are interesting. for example there are multiple devices are connected
and have to exchange information in someway.
Programs and applications have to run on each device and cooperate in
someway to get some task done.

A complex problem usually split into smaller parts that have a
defined function and are able to communicate with each other through a well defined messages.
These messages are the protocol.

Protocols are important because they encapsulate a lot of complexity so
that application developers focus on solving business problem or improving the user interface.

OSI Layers

Although it has never been properly implemented, the *OSI Layers* have
a lot of influence when talking about distributed systems.

| OSI Layers |

| Application Layer |
| Presentation Layer |
| Session Layer |
| Transport Layer |
| Network Layer |
| Data link Layer |
| Physical Layer |

read more https://en.wikipedia.org/wiki/OSI_model.

– A repeater operates at the physical level and copies information from one subnet to the other.
– A bridge operates at the data link layer level and copies frames between networks.
– A router operates on the network level, not only moves information between networks but decides on the route.

Packet Encapsulation

The communication between the layers are done by sending packets from
one layer to the other, each layer has administrative data that it has
to keep about its layer. It adds this header information to the packet it receives as it moves along to the next layer.

Connection Modes

Connection Oriented

A single full duplex connection is established for the session as we will see later.
Think about a phone call and TCP.

Connectionless

Messages are sent independent of each other. Think about mail and UDP.

Communication Models

Message Passing

Message passing is a primitive mechanism for distributed systems communication.
Simply setup a connection and pump some data in there. The full torrent client code can be found here.

*Lets dig here more since similar techniques are used in open blockchains.*

func NewHandshake(infoHash [20]byte, peerID []byte) *Handshake {
handShake := Handshake{}
handShake.Pstrlen = byte(19)
handShake.Pstr = "BitTorrent protocol"
handShake.Reserved = [8]byte{0, 0, 0, 0, 0, 0, 0, 0}
copy(handShake.InfoHash[:], infoHash[:])
copy(handShake.PeerId[:], peerID[:])
return &handShake
}

Here is a hex of the handshake message of the torrent protocol

13426974546f7272656e742070726f746f636f6c0000000000000000e2467cbf021192c241367b892230dc1e05c0580e1400000000000000000000000000000000000000

func main(){
// define a peer
// var peer
//...
handShake := message.NewHandshake(t.TorrentFile.InfoHash, PeerId)
handshakeBytes := handShake.Serialize()
c, err := net.DialTimeout("tcp4",peer.GetAddr().String(),3*time.Second)
_, err := c.Conn.Write(handShakeBytes)
}

here is the over wire bytes

If the remote peers understand the protocol, they will respond
according to the specification.In this case its another handshake
message followed by a bitfield of the available pieces they have.

Remote Procedural Calls

This is when a computer or a device causes a procedure or subroutine to execute on another address space, machine.

Conclusion

You learnt about some fundamental terms and techniques used in protocol implementations and peer to peer to messaging.

If you wondering how it ties up to blockchain you will have to wait until the blog series complete.

You can find more at https://youssefnotes.net.

Disclaimer

Downloading copyrighted material is illegal.

The Torrent protocol is not illegal.

This guide and the accompanied code snippets are for educational purposes only.

Don’t use it to download illegal/copyrighted material.

Source: https://blogs.sap.com/2020/02/18/light-introduction-to-network-protocols/