Question

Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that \(n\) DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an RTT of RTT \(_{1}, \ldots\), RTT \(_{n^{*}}\). Further suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Let RTT \(_{0}\) denote the RTT between the local host and the server containing the object. Assuming zero transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?

Short answer

Total elapsed time is the sum of all DNS RTTs and the RTT to the server: \( \sum_{i=1}^{n} \text{RTT}_i + \text{RTT}_0 \).

Step-by-step solution

Understanding the Scenario

When a user clicks a link, the system needs to resolve the domain name into an IP address via DNS if the IP is not cached. This process involves several Round-Trip Times (RTTs) as the DNS query propagates from one server to another.

Recognizing RTT Components

The total time delay includes multiple RTTs for DNS query propagation and an additional RTT for the actual data request and response after the IP is obtained. Each RTT is a round trip between the local client and a DNS server or between the client and the web server.

Summarizing Total Elapsed Time

Calculate the total time by summing up all individual RTTs. The total elapsed time is the sum of all DNS-related RTTs and the final RTT between the client and the server hosting the requested object.

Calculating Total Elapsed Time

Given that there are \( n \) DNS RTTs plus one additional RTT to the server, the total elapsed time can be calculated using the formula: \( \text{Total Time} = \sum_{i=1}^{n} \text{RTT}_i + \text{RTT}_0 \).

Conceptualizing Zero Transmission Time

Since transmission time is negligible, the only delay factors are the RTTs for DNS lookup and web page request. Therefore, the entire elapsed time is the sum of the computed RTTs only.

Key concepts

Round-Trip Time (RTT)

Round-Trip Time, commonly abbreviated as RTT, is a critical concept in networking that refers to the time it takes for a data packet to travel from a source to a destination and back again. It essentially measures the latency within a network.

• RTT is composed of the time taken for the data to reach its destination plus the time it takes for the acknowledgment to return to the source.
• This metric is crucial in assessing the performance of a network, as lower RTT values signify better efficiency and faster responses.

Understanding RTT can help diagnose issues like high latency and packet loss. Monitoring RTT is essential to ensuring a smooth user experience, especially in applications that demand real-time interactions, such as video calls and online gaming. However, in the context of DNS lookups, it helps determine how quickly a domain name is resolved to an IP address.

Domain Name System (DNS)

The Domain Name System (DNS) acts as the internet's phonebook. It translates human-friendly domain names, like www.example.com, into numerical IP addresses that computers use to identify each other on the network. When you enter a URL in your browser, the DNS is responsible for resolving that name to an IP address so your browser can load the web page. This process involves several steps:

• Your device contacts a DNS server and queries for the IP address associated with the domain name.
• If the DNS server doesn't have the record, it will forward the query to another server in a hierarchical manner, until the IP address is found.

Once resolved, the IP address is often stored temporarily in a cache to speed up future lookups.

Web page retrieval

Web page retrieval is the process of fetching a web page from a server once the DNS has resolved the URL to an IP address. This step involves connecting to the web server and requesting the specific page or object. Here's how it works:

• After DNS resolution, the client establishes a connection to the server hosting the web page.
• The client sends an HTTP request for the page or object required.
• The server processes this request, and if everything is correct, it responds with the requested data.

The time taken for this process depends largely on the RTT between the client and the server. With an optimized connection and server setup, web page retrieval can be quite fast, providing a seamless user experience.

IP address resolution

IP address resolution is vital for connecting devices over a network. It is the process of translating a domain name like www.example.com into an IP address that routers can understand to direct the data. Key steps involved include:

• DNS requests are initiated to find the IP address corresponding to a given domain name.
• The DNS uses a hierarchical system of servers to eventually resolve the domain to an IP.

Once the IP is determined, communication can occur. Efficient IP resolution helps in accelerated page loads and reduced latency. Implementing techniques like DNS caching can further optimize this process by holding previously resolved names locally, reducing the need for repeated queries across the internet.