Chapter 10

Peer-to-Peer Applications

A P2P application allows a device to act as both a client and a server within the same communication, as shown in the figure. In this model, every client is a server and every server a client. P2P applications require that each end device provide a user interface and run a background service.

The Session Layer

As the name implies, functions at the session layer create and maintain dialogs between source and destination applications. The session layer handles the exchange of information to initiate dialogs, keep them active, and to restart sessions that are disrupted or idle for a long period of time.

HTTPS

For secure communication across the Internet, the HTTP Secure (HTTPS) protocol is used. HTTPS uses authentication and encryption to secure data as it travels between the client and server. HTTPS uses the same client request-server response process as HTTP, but the data stream is encrypted with Secure Socket Layer (SSL) before being transported across the network.

HTTP

HTTP is a request/response protocol. When a client, typically a web browser, sends a request to a web server, HTTP specifies the message types used for that communication. The three common message types are GET, POST, and PUT (see the figure): GET - A client request for data. A client (web browser) sends the GET message to the web server to request HTML pages. POST - Uploads data files to the web server such as form data. PUT - Uploads resources or content to the web server such as an image. Although HTTP is remarkably flexible, it is not a secure protocol. The request messages send information to the server in plain text that can be intercepted and read. The server responses, typically HTML pages, are also unencrypted.

Client-Server Model

In the client-server model, the device requesting the information is called a client and the device responding to the request is called a server. Client and server processes are considered to be in the application layer. The client begins the exchange by requesting data from the server, which responds by sending one or more streams of data to the client

Peer-to-Peer Networks

In the peer-to-peer (P2P) networking model, the data is accessed from a peer device without the use of a dedicated server. The P2P network model involves two parts: P2P networks and P2P applications. Both parts have similar features, but in practice work quite differently. In a P2P network, two or more computers are connected via a network and can share resources (such as printers and files) without having a dedicated server. Every connected end device (known as a peer) can function as both a server and a client.

DNS Info

The DNS Client service on Windows PCs also stores previously resolved names in memory. The ipconfig /displaydns command displays all of the cached DNS entries.

DNS message Format

The DNS server stores different types of resource records used to resolve names. These records contain the name, address, and type of record. Some of these record types are: A - An end device IPv4 address NS - An authoritative name server AAAA - An end device IPv6 address (pronounced quad-A) MX - A mail exchange record

The Application Layer

The application layer is closest to the end user. It is the layer that provides the interface between the applications used to communicate and the underlying network over which messages are transmitted. Application layer protocols are used to exchange data between programs running on the source and destination hosts. The upper three layers of the OSI model (application, presentation, and session) define functions of the single TCP/IP application layer.

The Presentation Layer

The presentation layer has three primary functions: Formatting, or presenting, data at the source device into a compatible form for receipt by the destination device Compressing data in a way that can be decompressed by the destination device Encrypting data for transmission and decrypting data upon receipt The presentation layer formats data for the application layer, and it sets standards for file formats.

DHCP Operation

when an IPv4, DHCP-configured device boots up or connects to the network, the client broadcasts a DHCP discover (DHCPDISCOVER) message to identify any available DHCP servers on the network. A DHCP server replies with a DHCP offer (DHCPOFFER) message, which offers a lease to the client. The offer message contains the IPv4 address and subnet mask to be assigned, the IPv4 address of the DNS server, and the IPv4 address of the default gateway. The lease offer also includes the duration of the lease.