GATE CSE 2023 | Question: 42

Question

Suppose in a web browser, you click on the $\textsf{www. gate-2023.in}\; \text{URL}$. The browser cache is empty. The $\text{IP address}$ for this $\text{URL}$ is not cached in your local host, so a $\text{DNS}$ lookup is triggered (by the local $\text{DNS}$ server deployed on your local host) over the $3$-tier $\text{DNS}$ hierarchy in an iterative mode. No resource records are cached anywhere across all $\text{DNS}$ servers.

Let $\text{RTT}$ denote the round trip time between your local host and $\text{DNS}$ servers in the $\text{DNS}$ hierarchy. The round trip time between the local host and the web server hosting $\textsf{www.gate-2023.in}$ is also equal to $\text{RTT}$. The $\text{HTML}$ file associated with the $\text{URL}$ is small enough to have negligible transmission time and negligible rendering time by your web browser, which references $10$ equally small objects on the same web server.

Which of the following statements is/are $\text{CORRECT}$ about the minimum elapsed time between clicking on the $\text{URL}$ and your browser fully rendering it?

• A. $7\; \text{RTT}$s, in case of non-persistent $\text{HTTP}$ with $5$ parallel $\text{TCP}$ connections.
• B. $5\; \text{RTT}$s, in case of persistent $\text{HTTP}$ with pipelining.
• C. $9\; \text{RTT}$s, in case of non-persistent $\text{HTTP}$ with $5$ parallel $\text{TCP}$ connections.
• D. $6\; \text{RTT}$s, in case of persistent $\text{HTTP}$ with pipelining.

Comments

for non persistence connection without any parrellel connection , ans will be 25 RTT right?

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Answer 1

********************explanation behind 6 and 9 rtts***************************                           

                                           For persistent HTTP with pipelining

Persistent HTTP –  TCP Connection is Established only once and multiple files are transmitted in that single connection.

Pipelined – Pipelined means that a new HTTP request could be sent to server without receiving the Acknowledgment of previous request.

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1. The host will contact the DNS servers to Resolve the Domain Name and getting the IP address. This will take   3RTTs.

 

2. Now, client Establishes TCP connection with the gate-2023.in server that will take 1RTT 

                                 *****The last Ack is sent on HTTP Request Packet (piggybakced) *****

 

3. Client sends HTTP request to get the Base HTML file and receives it in 1RTT.                                                                            

4. Finally Client sends HTTP requests( PIPELINED ) for objects and receives all 10 objects in 1RTT . 

Everything together :

 

3RTTs (dns) + 1RTT(TCP connection establishment) + 1RTT(Html Base file) + 1RTT(objects) = 6RTTs

                                               

 

                                     For non-persistent HTTP with 5 Parallel connections

non-Persistent – TCP connection is made for each HTTP request and then closed.

 

5 parallel connections – 5 objects could be sent parallelly at same time.

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1. 3RTTs for DNS resolution.                                                                                                                                                                                            
2.  1RTT for TCP connection establishment.                                                                                                                                       
3. 1RTT for getting HTML base file.                                                                                                                                                     
4. 1RTT for establishing TCP Connection again.                                                                                                                                       
5. 1RTT for getting 5 objects parallelly ( 5 left ).                                                                                                                                       
6. 1RTT for establishing TCP connection again.                                                                                                                                       
7. 1RTT for getting remaining 5 objects.

Everything together:

                                                                                                                                                                                                                 3RTTs (dns) + 3RTTs (TCP connection establishment) + 1RTT (HTML Base file) + 2RTTs ( objects ) = 9RTTs

Comments

@S_Sandeep what are u yapping about bro ? 

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@Dev_Ops i was explaning why the RTT for tcp connection teardown phase was not considered in the question bro

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@Dev_Ops There will be an RTT for the terdown phase of tcp connection establishment right? why it is not included? do you have better explanation for it? give me.

Answer 2

Answer: C, D

Persistent HTTP –  Multiple objects can be sent over a single TCP connection.

Non-Persistent – There can be at most one object that can be sent over a single TCP connection.

Persistent with pipeline

1. 3 RTT for DNS Resolution (Given in Question 1st para)
2. 1 RTT for TCP connection
3. 1 RTT for HTTP Request & Reply (main HTML Page)
4. 1 RTT for 10 small objects on the same web server( because of a pipeline)

The minimum elapsed time: 6 RTT

Non-persistent with 5 parallel TCP connections.

1. 3 RTT for DNS Resolution (Given in Question 1st para)
2. 1 RTT for TCP connection
3. 1 RTT for HTTP Request & Reply (main HTML Page)
4. 1 RTT for TCP connection
5. 1 RTT for 5 small objects (parallel TCP connections).
6. 1 RTT for TCP connection
7. 1 RTT for other 5 small objects (parallel TCP connections).

The minimum elapsed time: 9 RTT

A similar type of question is in the standard book Computer Networking: A Top-Down Approach 6th Edition by Kurose & Rose

page no:173

Answer to this question:
https://cs.nyu.edu/~jcf/classes/CSCI-GA.2262-001/handouts/Homework2Solutions.pdf

Comments

@Devansh Maru One small query – persistent/non-persistent \(\mathrm{HTTP}\), \(\mathrm{HTTP}\) pipelining, and parallel \(\mathrm{TCP}\) connections are separate concepts, which can be combined anyways we like, right? For example – we may very well have a persistent, non-pipelined \(\mathrm{HTTP}\) connection with 4 parallel \(\mathrm{TCP}\) connections, and/or a non-persistent, pipelined \(\mathrm{HTTP}\) connection with 6 parallel \(\mathrm{TCP}\) connections (with no assumptions on the \(\mathrm{HTTP}\) request/response packets transmission time(s)), right?

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Generally, pipeline are used with persistent connections.

Non-persistent connection HTTP can be either with a parallel or without a parallel connection.

HTTP/1.1 introduced the concept of pipelining to send multiple requests over a single connection in sequence.

HTTP/2 gave us multiplexing, which allows browsers to send multiple requests in parallel over a single connection.

There are several restrictions for pipelining:

• HTTP clients should not pipeline until they are sure the connection is persistent.

• HTTP responses must be returned in the same order as the requests. HTTP messages are not tagged with sequence numbers, so there is no way to match responses with requests if the responses are received out of order.

 

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@Devansh Maru Thanks. Also, as for the restrictions for pipelining, they are restrictions for pipelining under \(\mathrm{HTTP/1.1}\), right? Because, as you pointed out –

\(\mathrm{HTTP/2}\) gave us multiplexing…

and as per https://en.wikipedia.org/wiki/HTTP_persistent_connection#See_also –

\(\mathrm{HTTP/2}\), which allows out-of-order pipelining of requests and responses, and also predictive pushing of content before it has been requested.

Answer 3

Is question mein humein total time (minimum elapsed time) nikalna hai. Question ke mutabiq sabhi RTT barabar hain.

Step 1: DNS Lookup Time

• Humein 3-tier DNS hierarchy di gayi hai aur iterative mode ka use ho raha hai.

• Kyuki cache empty hai, local DNS server ko 3 alag-alag servers (Root, TLD, Authoritative) se baat karni hogi.

• Iska total DNS time hoga: 3 RTT.

Step 2: Connection and Base HTML

• TCP connection establish karne ke liye: 1 RTT.

• Base HTML file request aur receive karne ke liye: 1 RTT.

• Ab tak total time: $3 + 1 + 1 = 5$ RTT.

Step 3: Fetching 10 Objects

Ab options ke hisaab se scenarios dekhte hain:

• Case 1: Non-persistent HTTP with 5 parallel connections (Option A & C)

  • Yahan har object ke liye naya TCP connection chahiye.

  • Pehle batch mein 5 objects jayenge: $1\text{ (TCP)} + 1\text{ (Request)} = 2$ RTT.

  • Doosre batch mein baaki 5 objects: Fir se $2$ RTT.

  • Total: $5\text{ (pichla)} + 2 + 2 = 9$ RTT. (Option C sahi hai, A galat hai).

• Case 2: Persistent HTTP with Pipelining (Option B & D)

  • Persistent mein TCP connection wahi rehta hai, naya connection nahi banana padta.

  • Pipelining ka matlab hai ki saare 10 objects ki request ek saath bheji ja sakti hai.

  • Saare 10 objects fetch karne mein sirf 1 RTT lagega.

  • Total: $5\text{ (pichla)} + 1 = 6$ RTT. (Option D sahi hai, B galat hai).

Answer 4

Answer: (C) and (D) are correct
Given:

• 3-tier DNS, iterative, no cache
• 1 HTML + 10 objects
• All RTT equal

Persistent vs Non-persistent:

Type	After fetching HTML, for more objects...
Non-persistent	Close TCP, open new one (2 RTT each)
Persistent	Keep TCP open, reuse it (1 RTT each)

Non-persistent, 5 parallel connections

Step	RTT
DNS resolution	3
TCP setup + HTML	2
10 objects (2 batches × 2 RTT)	4
Total	9 ✓

 

Persistent + pipelining

Step	RTT
DNS resolution	3
TCP setup + HTML	2
10 objects (all pipelined)	1
Total	6 ✓

Answer 5

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