14.1 Protocols

- Application Layer
- Transport Layer
- Internet Layer
- Link Layer

Purpose of Application Layer
- To provide services / interface with the user // access to applications, for example login, file transfer, network file access, email, etc
- To provide mechanisms for securing communication e.g. encryption/authentication
- To define/provide protocols used to allow the exchange of data/communication // to contain programs that exchange data
- Error detection and recovery mechanisms to handle application specific errors

Purpose of Transport Layer
- To provide logical communication between applications running on different hosts // To ensure that data is delivered to the correct application process on the destination machine
- To ensure error-free, end-to-end delivery of data between a source and a destination, in sequence // to provide error recovery techniques such as error detection codes and automatic repeat request
- To break data into segments when sent and to reconstruct when received // To reassemble segments at destination
- To regulate network connections // to provide flow control mechanisms to prevent data loss.

Purpose of Internet layer
- To identify the intended network and host.
- To add header containing IP addresses. // To address packets with their source and destination IP Addresses.
- To prepare packets for delivery by formatting them into datagrams.
- To route datagrams through the optimum route over a network.

Purpose of Link layer
- To prepare the next hop by managing the link between two directly connected devices. // To identify and move traffic across local segments.
- To format datagrams into frames for transmission.
- To identify network protocols in the packet header. // To ensure correct network protocols are/is followed.
- To deliver frames to the receiving network. // To receive frames from the sending network. // To map IP addresses to MAC physical addresses.
- To provide error checking/error correction/recovery/reconstruction
- including resend requests.

Describe how the TCP/IP protocol suite is applied when a message is sent through the internet from one host to another. Do not describe the function of individual layers of the TCP/IP protocol suite.
- The TCP/IP suite can be viewed as layers within a stack
- Each layer can only accept input from the next higher or the next lower layer/adjacent layer
- The user/sender/computer interfaces with the top layer/application layer to send/receive a message
- The message when sent passes from the top layer/application layer to the bottom layer/link layer
- The message when received passes from bottom layer/link layer to the top layer/application layer
- The link layer interfaces directly with the network and sends/receives the message.

Secure Socket Layer (SSL) and Transport Layer Security (TLS) are two protocols.
- Ensure security/privacy when using the internet
- Data encryption
- Identification / authentication of client and server

Give two examples of situations where the use of SSL/TLS would be appropriate.
- When transmitting authentication data e.g. passwords, session cookies
- When transmitting data that must be protected from modification on its way to or from a server e.g. user input, or results from the server
- When transmitting data classified as non-public.

how SSL/TLS is initiated
- An SSL/TLS connection is initiated by an application/client.
- Every new session begins with a handshake as defined by the SSL/TLS protocols.
- The client requests the digital certificate from the server // The server sends the digital certificate to the client.
- The client verifies the server’s digital certificate and obtains the server’s public key.
- The encryption algorithms are agreed. // The symmetric session keys are generated/defined.
- A secure session is established between client and server.