CST 311 - Week 2

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Studied this week was the application layer.  It covered topics of network applications, Web and HTTP, electronic mail, and DNS.  Network applications can be e-mail, text messaging, VOIP remote login, peer-to-peer file sharing, video streaming, etc.  There are two types of application architectures, client-server and peer-to-peer.  Sockets are used to send/receive processes to/from the client and server.  Two types of protocols are used at the app layer which are open protocols such as HTTP and proprietary protocols such as Skype.

HTTP is the Web's application layer protocol that uses the client-server model.  HTTP uses TCP to connect to a socketed server through port 80 and can either be persistent or non-persistent.  There are two types of HTTP messages, a request message, and a response message.  Cookies are used to assist in maintaining an HTTP state.  Cookies are helpful in situations of authorization, shopping cart updates, recommendations, user session states, etc.  A browser will also use a cache that stores objects that were previously requested.  This mitigates the involvement of the origin server and can improve access link utilization.

Email uses the SMTP protocol to reliably transfer messages from the client to the server using port 25.  In the email system, there are user agents, mail servers, and SMTP.  HTTP is referred to as a pull server because the client is pulling data.  SMTP is referred to as a push server because the client is pushing data to the server.  Along with SMTP, there are other protocols such as POP3, IMAP, and HTTP web-based email.

The DNS is an application-layer protocol where a host can enquire the DNS server with a hostname and the DNS will provide the IP address.  This is used when a URL is entered into a browser, the hostname will be sent to the DNS and the IP address will be returned because computers better use IP addresses rather than hostnames.  There are three tiers in the DNS server hierarchy, root DNS servers, top level domain (TLD) DNS servers, and authoritative DNS servers.  There is also a local DNS name server apart of the local network but is not part of the hierarchy.

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