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Problem 1-5 Homework #3 Solution

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Problem 1   Suppose you have a new computer just set up. dig is one of the most useful DNS lookup tool. You can check out the manual of dig at http://linux.die.net/man/1/dig. A typical invocation of dig looks like: dig @server name type.   Suppose that on April 19, 2017 at 15:35:21, you have issued…

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Problem 1

 

Suppose you have a new computer just set up. dig is one of the most useful DNS lookup tool. You can check out the manual of dig at http://linux.die.net/man/1/dig. A typical invocation of dig looks like: dig @server name type.

 

Suppose that on April 19, 2017 at 15:35:21, you have issued \dig google.com a” to get an IPv4 address for google.com domain from your caching resolver and got the following result: (If a user just types “dig google.com” the default is type=A)

 

  • <<>> DiG 9.8.3-P1 <<>> google.com ;; global options: +cmd

 

;; Got answer:

 

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17779

 

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 4, ADDITIONAL: 4

 

;; QUESTION SECTION:        
;google.com.   IN A  
;; ANSWER SECTION:        
google.com. 239 IN A 172.217.4.142
;; AUTHORITY SECTION:        
google.com. 55414 IN NS ns4.google.com.
google.com. 55414 IN NS ns2.google.com.
google.com. 55414 IN NS ns1.google.com.
google.com. 55414 IN NS ns3.google.com.
;; ADDITIONAL SECTION:      
ns1.google.com. 145521 IN A 216.239.32.10
ns2.google.com. 215983 IN A 216.239.34.10
ns3.google.com. 215983 IN A 216.239.36.10
ns4.google.com. 215983 IN A 216.239.38.10

 

  • Query time: 81 msec

 

  • SERVER: 128.97.128.1#53(128.97.128.1)

 

  • WHEN: Wed Apr 19 15:35:21 2017

 

  • MSG SIZE rcvd: 180

 

  1. What is the discovered IPv4 address of google.com domain?

 

  1. If you issue the same command 1 minute later, how would \ANSWER SECTION” look like?

 

  1. When would be the earliest (absolute) time the caching resolver would contact one of the google.com name servers again? (for issuing the same command “dig google.com a”)

 

  1. When would be the earliest (absolute) time the caching resolver would contact one of the .com name servers? (for issuing the same command “dig google.com a”)

 

 

 

 

 

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Problem 2

 

Suppose that you walked into Boelter Hall and get connected to CSD WiFi network, which automatically gave you IP address of the local caching resolver. However, initially, it doesn’t allow you to do anything unless you type your username and password in a popup window (or if you try to go to any website in your browser).

 

 

  1. Explain a mechanism of how does the \CSD” network achieve this / which features of DNS/HTTP make it possible.

 

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Problem 3

 

Same context as Problem 2. After you successfully logged in, you can start using the Internet. Suppose the caching resolver has just rebooted and its cache is completely empty; RTT between your computer and the caching resolver is 10ms and RTT between the caching resolver and any authoritative name server is 100ms; all responses have TTL 12 hours.

 

  1. If you try to go to ucla.edu, what would be minimum amount of time you will need to wait before your web browser will be able to initiate connect to the UCLA’s web server?

 

  1. What would be the time, if a minute later you will decide to go to ccle.ucla.edu?

 

  1. What would be the time, if another minute later you will decide to go to piazza.com?

 

  1. What would be the time, if another minute later you will decide to go to gradescope.com?

 

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Problem 4

 

How does SMTP mark the end of a message body? How about HTTP? Can HTTP use the same method as SMTP to mark the end of a message body? Explain.

 

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Problem 5

 

Consider the following environment with a local DNS caching resolver and a set of authoritative DNS name servers.

 

Assume that initially,

 

the caching resolver cache is empty, TTL values for all records is 1 hour,

 

RTT between stub resolvers (hosts A, B, and C) and the caching resolver is 20 ms,

 

RTT between the caching resolver and any of the authoritative name servers is 150 ms, There are no packet losses,

 

All processing delays are 0 ms

 

 

 

 

 

root domain

 

 

 

edu.                                                                                 com.

 

 

 

google.com.                                                               amazon.com.

 

caching resolver

 

A

B

C

 

 

 

 

  1. At T=0 min, Host-A sends a query for A record for amazon.com, and after receiving the answer sends a query for A record for www.amazon.com. How long did it take to receive all the answers?

 

  1. At T=40 min, Host-B sends a query for MX record for google.com that returns

 

google.com. 3600 IN MX 10 primary.google.com.
google.com. 3600 IN MX 30 backup.google.com.
primary.google.com. 3600 IN A 74.125.28.27
backup.google.com. 3600 IN A 173.194.211.27
           

 

(Similar to NS records, the DNS server may return glue A/AAAA records in addition to the requested MX records.) How long did it take to get the answer?

 

  1. At T=70 min, Host-C sends a query for AAAA (IPv6) record for mail.google.com, following at T=75 mins with a query for AAAA (IPv6) record for hangout.google.com. How long did it take for Host-C to receive each of the answers (i.e., relative to T=70min for the rst, and relative to T=75 mins for the second)?

 

  1. List DNS records that the caching resolver has at T=90 minutes

 

 

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