gh-148 computer networks models and TCP/UDP protocols overview added

Author: raa-dev

docs/images/el-architecture/osi-tcpip-models.png

@@ -5,8 +5,31 @@ First, we will provide some background on computer networks and then we will div
 specifics of the EL's networking protocol.
 
 ## Computer networks background
-* Distributed network of nodes
-* OSI vs TCP/IP model
+This section covers a brief overview of the differences and similarities between the OSI (Open Systems Interconnection) and TCP/IP (Transmission Control Protocol/Internet Protocol) models,
+as well as the protocols involved in the transport layer used in DevP2P: TCP and UDP.
+
+In terms of networking, both models refer to the same process of communication between layers.
+Just as Kurose and Ross explain (2020), the computer networks are divided into different layers, and each one of them has a specific responsibility. The OSI model has seven layers, while the TCP/IP model has four layers. The OSI model is more theoretical and the TCP/IP model is more practical. The OSI model is a reference model created by the International Organization for Standardization (ISO) to provide a framework for understanding networks. The TCP/IP model was created by the Department of Defense (DoD) to ensure that messages could be transmitted between computers regardless of the types of computers involved. The TCP/IP model is a concise version of the OSI model:
+
+![alt text](../../images/el-architecture/osi-tcpip-models.png)
+
+In summary, the OSI model layers are:
+1. Physical layer: responsible for the transmission and reception of raw data between devices.
+2. Data link layer: responsible for the node-to-node delivery of the message.
+3. Network layer: responsible for the delivery of packets from the source to the destination.
+4. Transport layer: responsible for the delivery of data between the source and the destination.
+5. Session layer: responsible for the establishment, management, and termination of connections between applications.
+6. Presentation layer: responsible for the translation, compression, and encryption of data.
+7. Application layer: responsible for providing network services directly to the end-user.
+
+Assuming the communication schema proposed by Claude Shannon (1948), every communication implies both a sender and a receiver, a message to be exchanged between them, a transmission medium, and a protocol to be followed.
+This is important to mention because regardless of the computer architecture, it could be part of a network if it follows the communication and protocol specifications of the models mentioned above.
+
+Focusing on the transport layer, the two protocols used by DevP2P are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
+Both protocols are used to send data over the internet, but they have different characteristics. Just as Tanenbaum points it out (2021),TCP is a connection-oriented protocol, which means that it establishes a connection between the sender and the receiver before sending data. It is reliable because it ensures that the data is delivered in the correct order and without errors. UDP is a connectionless protocol, which means that it does not establish a connection before sending data. It is faster than TCP because it does not have to establish a connection before sending data, but it is less reliable because it does not ensure that the data is delivered in the correct order or without errors.
+
+![img.png](../../images/el-architecture/tcpudp.png)
+
 ## EL's networking specs
 * peer-to-peer networking protocol
 * gossiping data (1:N) and request/response (1:1) communication
@@ -16,6 +39,8 @@ specifics of the EL's networking protocol.
   * UDP-based transport layer (discovery and finding peers)
   * TCP-based transport layer (devp2p, exchange information) 
 * Discovery protocol
+
+How nodes find each other
   * bootstrap nodes
   * Kademlia
   * Hash tables
@@ -25,14 +50,15 @@ specifics of the EL's networking protocol.
 start client --> connect to bootnode --> bond to bootnode --> find neighbours --> bond to neighbours
 
 ## ENR: Ethereum Node Records
+* Standard format for connectivity for nodes
 * Node's identity
 * Object
   * signature (hash)
   * sequence number
   * arbitrary key-value pairs
 
 ## DevP2P specs
-* RLPx: encrypted and authenticated transport protocol
+* RLPx: encrypted and authenticated transport protocol (TCP based transport system for nodes communication)
   * Handshake messaging
   * Encryption
   * Authentication
@@ -48,3 +74,6 @@ start client --> connect to bootnode --> bond to bootnode --> find neighbours --
 ### Further Reading
 * [Geth devp2p docs](https://geth.ethereum.org/docs/tools/devp2p)
 * [Ethereum devp2p GitHub](https://github.com/ethereum/devp2p)
+* Andrew S. Tanenbaum, Nick Feamster, David J. Wetherall (2021). *Computer Networks*. 6th edition. Pearson. London.
+* Jim Kurose and Keith Ross (2020). *Computer Networking: A Top-Down Approach*. 8th edition. Pearson.
+* Clause E. Shannon (1948). "A Mathematical Theory of Communication". *Bell System Technical Journal*. Vol. 27.