DCN UNIT 2- FTP, Out-of-band, In-band, Application Layer Protocol, File Transfer, Access, and Management (FTAM), Email Services, Remote Login, Web Services, Directory Services, Network Management, DNS, Persistent HTTP, Non-persistent HTTP, HTTP, FTP, Cookies, POP3 Protocol.

Data Communication and Networking - Unit II

1. How FTP works? What do you mean by term 'out-of-band' and 'in-band'?


FTP (File Transfer Protocol) is a standard network protocol used to transfer files from one host to another over a TCP-based network, such as the Internet. FTP operates on a client-server model where the client initiates a connection to the server to download or upload files. FTP uses two separate channels for communication:

  • Control Channel: Used for sending commands and receiving responses between the client and server.
  • Data Channel: Used for transferring the actual file data between the client and server.

The terms 'out-of-band' and 'in-band' refer to the methods of communication between the client and server:

  • Out-of-band: Refers to the use of a separate channel for control information. In FTP, the control and data channels are separate, making it an out-of-band protocol.
  • In-band: Refers to the use of the same channel for both control and data information. This is not used in FTP but is common in other protocols like HTTP.

2. What are the various standard services extended by Application Layer Protocol?

The Application Layer in the OSI model provides various standard services to applications, including:

  • File Transfer, Access, and Management (FTAM): Services like FTP and NFS (Network File System) allow users to transfer files between systems, access files remotely, and manage file systems across networks.
  • Email Services: Protocols like SMTP (Simple Mail Transfer Protocol), IMAP (Internet Message Access Protocol), and POP3 (Post Office Protocol) enable email transmission, retrieval, and storage.
  • Remote Login: Services such as Telnet and SSH (Secure Shell) allow users to log into remote systems and execute commands as if they were physically present.
  • Web Services: HTTP (Hypertext Transfer Protocol) and HTTPS (HTTP Secure) facilitate the transfer of web pages and web-based content over the Internet.
  • Directory Services: LDAP (Lightweight Directory Access Protocol) provides access to distributed directory services that store information about users, services, and applications.
  • Network Management: SNMP (Simple Network Management Protocol) allows for the monitoring and management of network devices.

3. With the help of neat diagram, explain the working of DNS?

The Domain Name System (DNS) is a hierarchical and decentralized naming system for computers, services, or any resource connected to the Internet or a private network. It translates human-readable domain names (like www.example.com) into IP addresses (like 192.0.2.1). Here's how DNS works:

  1. The user types a URL into their web browser.
  2. The browser sends a query to the local DNS resolver.
  3. The resolver checks its cache. If the IP address is found, it returns it to the browser.
  4. If not found, the resolver queries a root DNS server, which responds with the address of a top-level domain (TLD) server.
  5. The resolver then queries the TLD server (e.g., .com), which responds with the address of the authoritative DNS server for the domain.
  6. The resolver queries the authoritative DNS server, which responds with the IP address of the domain.
  7. The resolver caches the IP address and returns it to the browser.
  8. The browser uses the IP address to request the web page from the web server.

4. Differentiate the following: i. Persistent and non-persistent HTTP ii. HTTP and FTP

i. Persistent and Non-persistent HTTP:

  • Persistent HTTP: The TCP connection remains open for multiple requests and responses between the client and server. It reduces the overhead of establishing a new connection for each request, leading to faster load times.
  • Non-persistent HTTP: A new TCP connection is established for each HTTP request/response pair. This can lead to increased latency due to the overhead of setting up and tearing down connections repeatedly.

ii. HTTP and FTP:

  • HTTP (Hypertext Transfer Protocol): Designed for transferring hypertext (web pages) over the Internet. It operates mainly on port 80 (or 443 for HTTPS) and uses a single connection for both control and data (in-band).
  • FTP (File Transfer Protocol): Designed specifically for transferring files. It operates on ports 20 and 21, using separate connections for control (command) and data transfer (out-of-band).

5. Explain: i. Cookies ii. POP3 Protocol

i. Cookies:

Cookies are small pieces of data stored on the user's computer by the web browser while browsing a website. They are used to remember information about the user, such as login status, preferences, and tracking user behavior. There are two main types of cookies:

  • Session Cookies: Temporary cookies that are deleted when the browser is closed.
  • Persistent Cookies: Remain on the user's device for a set period or until deleted by the user.

ii. POP3 Protocol (Post Office Protocol version 3):

POP3 is an application-layer protocol used by email clients to retrieve emails from a mail server. It operates over a TCP/IP connection and is defined by RFC 1939. POP3 allows users to download their emails from the server to their local device, after which they can read them offline. It typically uses port 110 (or 995 for POP3S). Unlike IMAP, POP3 does not keep emails on the server after downloading, making it less suitable for accessing emails from multiple devices.

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