Application Layer Practice Questions

The Dynamic Host Configuration Protocol (\text{DHCP}$) is used to automatically assign IP addresses to devices on a network. When a client connects to a network, it initially lacks an IP address and is unaware of the$\text{DHCP}$server's location. Therefore, it must send a$\text{DHCPDISCOVER}$message as a broadcast (destination IP$255.255.255.255$) to reach any available servers. In contrast,$\text{SMTP}$,$\text{FTP}$, and$\text{HTTP}$operate via unicast communication, requiring a direct, point-to-point connection between a client and a known server address. Thus,$\text{DHCP}$ is unique among these options for its reliance on broadcasting to initiate communication.

HTTP $1.1$ implements persistent connections, which allow multiple HTTP requests and responses to be sent over a single TCP connection, provided the resources reside on the same server (A). This mechanism also facilitates fetching multiple webpages from the same host without renegotiating the connection (D). Additionally, HTTP $1.1$ supports pipelining, allowing clients to send multiple requests without waiting for the completion of previous responses (C). Option B is false because a TCP connection is bound to a specific host, preventing it from serving resources from different servers.

[CORRECT_OPTION: A, C, D]

The chronological order of packets for browsing a webpage from an empty cache:

1. DNS Request: Before connecting to the server, the browser needs to resolve the web server's hostname to an IP address. This is done via a DNS request (ii).
2. TCP SYN: Once the IP address is known, a TCP connection must be established with the web server. This begins with a TCP SYN packet (iv).
3. HTTP GET for Index Page: After the TCP connection is established, the browser sends an HTTP GET request for the top-level index page (i).
4. HTTP GET for Image Objects: The index page typically contains references to embedded objects like images. After parsing the index page, the browser sends HTTP GET requests for these image objects (iii).

Therefore, the correct chronological order is (ii), (iv), (i), (iii).

The total elapsed time is the sum of DNS lookup time and HTTP request/response time.
DNS lookup: 3-tier DNS hierarchy in iterative mode, with no caching. Each tier requires one RTT. So, DNS lookup time = 3 RTTs.
HTTP request/response:
The HTML file is small and references 10 equally small objects on the same web server.
Case A: Non-persistent HTTP with 5 parallel TCP connections.

• Initial TCP connection setup for HTML: 1 RTT.
• HTTP request for HTML + response: 1 RTT.
• For 10 objects, with 5 parallel connections, it takes 2 rounds of requests (5 objects per round). Each round requires 1 RTT for request/response.
• Total RTTs for objects = 2 RTTs.
• Total HTTP time = 1 RTT (setup) + 1 RTT (HTML) + 2 RTTs (objects) = 4 RTTs.
• Total elapsed time = 3 RTTs (DNS) + 4 RTTs (HTTP) = 7 RTTs. Statement (A) is TRUE.

Case B: Persistent HTTP with pipelining.

• Initial TCP connection setup for HTML: 1 RTT.
• HTTP request for HTML + response: 1 RTT.
• For 10 objects, with persistent HTTP and pipelining, all 10 requests can be sent in one RTT after the HTML is received, and all responses can be received in another RTT.
• Total RTTs for objects = 1 RTT.
• Total HTTP time = 1 RTT (setup) + 1 RTT (HTML) + 1 RTT (objects) = 3 RTTs.
• Total elapsed time = 3 RTTs (DNS) + 3 RTTs (HTTP) = 6 RTTs. Statement (B) is FALSE.

Case C: Non-persistent HTTP with 5 parallel TCP connections. (Same as A)

• Total elapsed time = 7 RTTs. Statement (C) is FALSE.

Case D: Persistent HTTP with pipelining. (Same as B)

• Total elapsed time = 6 RTTs. Statement (D) is TRUE.

In an iterative query for DNS resolution, the local DNS resolver contacts the root server, then the TLD server, then the authoritative server.
For www.gate.org.in:

1. The local DNS resolver queries a root DNS server for www.gate.org.in. The root server responds with the IP address of the .in TLD server. (1 query-response pair)
2. The local DNS resolver queries the .in TLD server for www.gate.org.in. The TLD server responds with the IP address of the org.in authoritative DNS server. (1 query-response pair)
3. The local DNS resolver queries the org.in authoritative DNS server for www.gate.org.in. The authoritative server responds with the IP address of the gate.org.in authoritative DNS server. (1 query-response pair)
4. The local DNS resolver queries the gate.org.in authoritative DNS server for www.gate.org.in. The authoritative server responds with the IP address of www.gate.org.in. (1 query-response pair)

In total, there are 4 query-response pairs involved in completely resolving the domain name www.gate.org.in using an iterative query mechanism with no caching.

In non-persistent HTTP, a separate TCP connection is established for each requested object. The web page consists of one text object (the HTML file itself) and five image objects.

Therefore, to display the entire web page:

1. One TCP connection is needed for the text (HTML) content.
2. Five separate TCP connections are needed, one for each of the five small images.

The total number of minimum TCP connections required is the sum of connections for the text and images: $1 + 5 = 6$.

[CORRECT_OPTION: N/A]

Remote Procedure Calls (RPC) are a protocol used by a program to request a service from a program located in another address space.
Option A is logically incoherent, describing "remotely different" processes on the same system.
Option B refers to general Inter-Process Communication (IPC) mechanisms rather than RPC.
Option C, while commonly associated with networked systems, is technically restrictive; RPC is defined by the requirement of separate address spaces, which can exist on the same system as well as on separate systems.
Since none of the provided choices comprehensively or accurately define the scope and nature of RPC, the most logical selection is "none of the above".

The Simple Mail Transfer Protocol (SMTP) is the standard internet protocol for sending (pushing) electronic mail. The Post Office Protocol 3 (POP3) is a standard protocol used by email clients to retrieve (pull) email from a remote server. Therefore, to send and then retrieve e-mails, the correct protocol pair in that order is SMTP, POP3.

The Internet layer is responsible for logical addressing and packet routing within the TCP/IP model. Among the given options, the Internet Protocol Security ($IPSec$) suite is the standardized IETF framework explicitly designed to provide authentication, integrity, and confidentiality for data packets at the Internet layer (Layer 3). In contrast, SSL/TLS operates primarily at the Transport layer to secure end-to-end connections, while "NetSec" and "PacketSec" are not recognized IETF protocol suites. Therefore, $IPSec$ is the only valid technology listed that fulfills the requirements for securing packets at the Internet layer.

To determine the correct protocol for transferring electronic mail, we evaluate the function of each option:

• TELNET is designed for remote login and terminal emulation, not for email delivery.
• FTP (File Transfer Protocol) is specifically used for the transfer of files between systems.
• SNMP (Simple Network Management Protocol) is used for network management and monitoring devices.
• SMTP (Simple Mail Transfer Protocol) is the standard application-layer protocol dedicated to sending and relaying electronic mail messages between servers.
  Therefore, SMTP is the correct protocol.

WPA stands for Wi-Fi Protected Access, a security protocol and certification program developed by the Wi-Fi Alliance to secure wireless computer networks. It was introduced to address the significant security vulnerabilities inherent in the older WEP (Wired Equivalent Privacy) standard. Given that the protocol is designed specifically to authenticate and secure wireless connectivity, option B is the only accurate expansion of the acronym.

When a host requests a webpage, it must first resolve the server's domain name (e.g., www.example.com) into an IP address. This is done via a DNS query. Once the IP address is obtained, a reliable connection must be established with the server using the TCP protocol, which starts with a TCP SYN packet (the first step of the three-way handshake). Only after the TCP connection is established can the browser send the HTTP GET request to fetch the webpage content.

1. DNS lookup (A) refers to the legitimate resolution process, not the acceptance of invalid data.
2. DNS hijacking (B) typically involves modifying a user's DNS settings, rather than injecting data into a server's cache.
3. DNS spoofing (C), often called cache poisoning, occurs when a resolver is tricked into accepting forged, unauthoritative information.
4. Consequently, the DNS server uses this fake $IP$ mapping to direct traffic to incorrect sites.
5. Thus, the specific act of accepting incorrect, unauthoritative information is defined as DNS spoofing.

The solution evaluates which of the listed protocols are both stateful and operate at the application layer. HTTP is stateless. TCP is stateful but is a transport layer protocol, not an application layer one. FTP is an application layer protocol that is stateful, as it maintains state for control and data connections, including user authorization. POP3 is also a stateful application layer protocol because it maintains state across a session, for example, to handle user authentication and track retrieved messages. Therefore, FTP (ii) and POP3 (iv) are the correct examples.

HTTP can use multiple TCP connections, especially in its non-persistent mode, where a separate connection is established for each object (like an image or script) on a webpage. FTP inherently uses two parallel TCP connections: one for control commands (like user authentication) and another for the actual data transfer. Protocols like TELNET and SMTP typically maintain a single TCP connection for the duration of a session.

The PING utility operates using the Internet Control Message Protocol (ICMP) to test network connectivity. When a PING request encounters an error, such as a destination being unreachable or a time-to-live (TTL) expiry, the network infrastructure or the target host explicitly generates and returns an ICMP error message (e.g., Type 3: Destination Unreachable or Type 11: Time Exceeded) to the sender. Option C is false because these ICMP error messages are fundamentally required by the protocol to inform the source about the specific nature of the failure. Consequently, the claim that an ICMP message is not returned during an error condition is incorrect.

The Domain Name System (DNS) functions as a distributed directory service for the Internet. While commonly used for forward lookups (mapping domain names to IP addresses), it also supports reverse lookups. A reverse DNS lookup translates an IP address back into its associated domain name. Because the Internet's naming structure is organized as a tree-like system, these names represent a structured hierarchy (such as top-level domains and subdomains). Therefore, DNS maps the IP address to a hierarchy of domain names.

An HTTP cookie is not a piece of executable code. It is a small piece of data, typically a string in a key-value format, stored in a text file on the user's computer. A web server sends this data to the user's browser, which then sends it back with subsequent requests to the same server. While cookies can be used for tracking and session management, they are passive data and do not have the ability to execute code or directly compromise a user's system security like a virus or malware could.

To interact with a web server, a browser follows a specific sequence of network operations. First, the browser must determine the server's IP address from its domain name using the Domain Name System (DNS). Once the IP address is known, a reliable connection is established through the Transmission Control Protocol (TCP). After the TCP connection is set up, the browser can then send an HTTP request for the desired webpage. Finally, the web server responds by sending the requested webpage back to the browser via HTTP.

Browsing the World Wide Web primarily relies on the $HTTP$ (Hypertext Transfer Protocol) to retrieve web pages. Because web data transmission requires high reliability and ordered delivery, it utilizes the connection-oriented transport protocol $TCP$ rather than the connectionless $UDP$. The Internet Assigned Numbers Authority (IANA) has reserved port $80$ as the well-known port for $HTTP$ traffic. Consequently, the correct configuration for web browsing is $TCP$ and port $80$.