Computer Networks Chapter 2

In DASH (Dynamic, Adaptive Streaming over HTTP), a server divides a video file into chunks that ... (pick best completion from below)

... are stored, each encoded at multiple rates (video quality). The client plays the video chunk-by-chunk, with each chunk requested at encoding rate that fits the available bandwidth at the time.

DNS AND HTTP DELAYS Before doing this question, you might want to review sections 2.2.1 and 2.2.2 on HTTP (in particular the text surrounding Figure 2.7) and the operation of the DNS (in particular the text surrounding Figure 2.19). 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 two DNS servers are visited before your host receives the IP address from DNS. The first DNS server visited is the local DNS cache, with an RTT delay of RTT0 = 1 msecs. The second DNS server contacted has an RTT of 21 msecs. Initially, let's suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Suppose the RTT between the local host and the Web server containing the object is RTTHTTP = 70 msecs. 1. Assuming zero transmission time for the HTML object, how much time (in msec) elapses from when the client clicks on the link until the client receives the object? 2. Now suppose the HTML object references 8 very small objects on the same server. Neglecting transmission times, how much time (in msec) elapses from when the client clicks on the link until the base object and all 8 additional objects are received from web server at the client, assuming non-persistent HTTP and no parallel TCP connections? 3. Suppose the HTML object references 8 very small objects on the same server, but assume that the client is configured to support a maximum of 5 parallel TCP connections, with non-persistent HTTP. 4. Suppose the HTML object references 8 very small objects on the same server, but assume that the client is configured to support a maximum of 5 parallel TCP connections, with persistent HTTP. 5. What's the fastest method we've explored: Nonpersistent-serial, Nonpersistent-parallel, or Persistent-parallel?

1. 162 2. 1282 3. 442 4. 302 5. Persistant-parallel

What information does the type "A" resource record hold in the DNS database? Check all that apply.

A hostname and an IP address.

ELECTRONIC MAIL AND SMTP Look at the scenario below, where Alice sends an email to Bob. For the questions below, assume both Bob's and Alice's user agents use the POP3 protocol. 1. At point 2 in the diagram, what protocol is being used? 2. At point 4 in the diagram, what protocol is being used? 3. At point 6 in the diagram, what protocol is being used? 4. Does SMTP use TCP or UDP? 5. Is SMTP a 'push' or 'pull' protocol? 6. Is POP3 a 'push' or 'pull' protocol? 7. What port does SMTP use? 8. What port does POP3 use?

1. SMTP 2. SMTP 3. POP3 4. TCP 5. Push 6. Pull 7. 25 8. 110

Suppose a client is sending an HTTP GET request message to a web server, gaia.cs.umass.edu. Suppose the client-to-server HTTP GET message is the following: GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11 What version of HTTP is the client using?

1.1

How many RTTs are there from when a client first contacts an email server (by initiating a TCP session) to when the client can begin sending the email message itself - that is following all initial TCP or SMTP handshaking required? Recall the figure below from our class notes:

3

Which of the following pieces of information will appear in a server's application-level HTTP reply message? (Check all that apply.)

A response code A response phrase associated with a response code

Suppose a Web server has five ongoing connections that use TCP receiver port 80, and assume there are no other TCP connections (open or being opened or closed) at that server. How many TCP sockets are in use at this server?

6

What is an HTTP cookie used for?

A cookie is a code used by a server, carried on a client's HTTP request, to access information the server had earlier stored about an earlier interaction with this Web browser. [Think about the distinction between a browser and a person.]

What do we mean when we say "HTTP is stateless"? In answering this question, assume that cookies are not used. Check all answers that apply.

An HTTP server does not remember anything about what happened during earlier steps in interacting with this HTTP client.

When an application uses a UDP socket, what transport services are provided to the application by UDP? Check all that apply.

Best effort service. The service will make a best effort to deliver data to the destination but makes no guarantees that any particular segment of data will actually get there.

Which of the following are advantages of using a web cache? Sselect one or more answers.

Caching uses less bandwidth coming into an institutional network where the client is located, if the cache is also located in that institutional network. Caching generally provides for a faster page load time at the client, if the web cache is in the client's institutional network, because the page is loaded from the nearby cache rather than from the distant server.

What is the value of caching in the local DNS name server? Check all that apply.

DNS caching provides for faster replies, if the reply to the query is found in the cache. DNS caching results in less load elsewhere in DNS, when the reply to a query is found in the local cache

Which of the following are changes between HTTP 1.1 and HTTP/2? Note: select one or more answers.

HTTP/2 allows a large object to be broken down into smaller pieces, and the transmission of those pieces to be interleaved with transmission other smaller objects, thus preventing a large object from forcing many smaller objects to wait their turn for transmission. HTTP/2 allows objects in a persistent connection to be sent in a client-specified priority order.

Which of the following characteristics apply to both HTTP and SMTP? Note: check one or more of the characteristics below.

Is able to use a persistent TCP connection to transfer multiple objects. Has ASCII command/response interaction, status codes.

What is the role of an authoritative name server in the DNS? (Check all that apply)

It provides the definitive answer to the query with respect to a name in the authoritative name server's domain.

We learned that in HTTP web browser caching, HTTP local web server caching, and in local DNS caching, that a user benefits (e.g., shorter delays over the case of no caching) from finding a local/nearby copy of a requested item. In which of the following forms of caching does a user benefit from its not only from its own recent requests (and cached replies) but also from recent requests made from other users?

Local DNS server caching HTTP local web caching

When an application uses a TCP socket, what transport services are provided to the application by TCP? Check all that apply.

Loss-free data transfer. The service will reliably transfer all data to the receiver, recovering from packets dropped in the network due to router buffer overflow. Congestion control. The service will control senders so that the senders do not collectively send more data than links in the network can handle. Flow Control. The provided service will ensure that the sender does not send so fast as to overflow receiver buffers.

Which of the characteristics below are associated with a client-server approach to structuring network applications (as opposed to a P2P approach)?

There is a server that is always on. HTTP uses this application structure. There is a server with a well known server IP address.

Provides authoritative hostname to IP mappings for organization's named hosts. ANSWER LIST: DNS root servers Top Level Domain (TLD) servers Local DNS server Authoritative DNS server QUESTION LIST: Provides authoritative hostname to IP mappings for organization's named hosts. Replies to DNS query by local host, by contacting other DNS servers to answer the query. Responsible for a domain (e.g., *.com, *.edu); knows how to contact authoritative name servers. Highest level of the DNS hierarchy, knows how to reach servers responsible for a given domain (e.g., *.com, *.edu).

Provides authoritative hostname to IP mappings for organization's named hosts.: Authoritative DNS server Replies to DNS query by local host, by contacting other DNS servers to answer the query.: Local DNS server Responsible for a domain (e.g., *.com, *.edu); knows how to contact authoritative name servers.: Top Level Domain (TLD) servers Highest level of the DNS hierarchy, knows how to reach servers responsible for a given domain (e.g., *.com, *.edu).: DNS root servers

Match the functionality of a protocol with the name of a the email protocol (if any) that implements that functionality. ANSWER LIST: IMAP SMTP Neither SMTP nor IMAP does this. QUESTION LIST: Pushes email from a mail client to a mail server. Pulls mail from one mail server to another mail server. Pulls email to a mail client from a mail server.

Pushes email from a mail client to a mail server.: SMTP Pulls mail from one mail server to another mail server.: Neither Pulls email to a mail client from a mail server.: IMAP

What approach is taken by a CDN to stream content to hundreds of thousands of simultaneous users?

Store/serve multiple copies of videos at multiple geographically distributed sites.

What is the purpose of the HTTP GET message?

The HTTP GET request message is used by a web client to request a web server to send the requested object from the server to the client.

What is the purpose of a cookie value in the HTTP GET request?

The cookie value itself doesn't mean anything. It is just a value that was returned by a web server to this client during an earlier interaction.

Check all of the phrases below that state a true property of a local DNS server.

The local DNS server can decrease the name-to-IP-address resolution time experienced by a querying local host over the case when a DNS is resolved via querying into the DNS hierarchy. The local DNS server record for a remote host is sometimes different from that of the authoritative server for that host.

Which of the characteristics below are associated with a P2P approach to structuring network applications (as opposed to a client-server approach)?

There is not a server that is always on. A process requests service from those it contacts and will provide service to processes that contact it.

What happens when a socket connect() procedure is called/invoked?

This procedure creates a new socket at the client, and connects that socket to the specified server.

What is the purpose of the conditional HTTP GET request message?

To allow a server to only send the requested object to the client if this object has changed since the server last sent this object to the client.

What is the purpose of the If-Modified-Since field in a HTTP GET request message

To indicate to the server that the client has cached this object from a previous GET, and the time it was cached.

Manifest file. What is the purpose of a manifest file in a streaming multimedia setting?

To let a client know where it can retrieve different video segments, encoded at different rates

Again, suppose a client is sending an HTTP GET request message to a web server, gaia.cs.umass.edu. The client-to-server HTTP GET message is the following (same as in previous problem): GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11 What is the language in which the client would least prefer to get a response? [You may have to search around the Web a bit to answer this.]

United Kingdom English

Which of the following characteristics apply to SMTP only (and do not apply to HTTP)? Note: check one or more of the characteristics below.

Uses CRLF.CRLF to indicate end of message. Operates mostly as a "client push" protocol. Uses server port 25.

Which of the following characteristics apply to HTTP only (and do not apply to SMTP)? Note: check one or more of the characteristics below.

Uses a blank line (CRLF) to indicate end of request header. Operates mostly as a "client pull" protocol. Uses server port 80.

Suppose an HTTP server sends the following HTTP response message a client:HTTP/1.0 200 OKDate: Wed, 09 Sep 2020 23:46:21 +0000Server: Apache/2.2.3 (CentOS)Last-Modified: Wed, 09 Sep 2020 23:51:41 +0000ETag:17dc6-a5c-bf716880.Content-Length: 418Connection: CloseContent-type: image/htmlWill the web server close the TCP connection after sending this message?

Yes, because this is HTTP 1.0

Again, suppose a client is sending an HTTP GET request message to a web server, gaia.cs.umass.edu. Suppose the client-to-server HTTP GET message is the following (same as in previous problem): GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11 Does the client have a cached copy of the object being requested? [Note: you can find additional questions similar to this here.]

Yes, because this is a conditional GET, as evidenced by the If-Modified-Since field.

Suppose a client is sending an HTTP GET message to a web server, gaia.cs.umass.edu. Suppose the client-to-server HTTP GET message is the following: GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, text/xml, image/jpeg, image/gif, audio/mpeg, audio/mp4, video/wmv, video/mp4,Accept-Language: en-us, en-gb;q=0.1, en;q=0.7, fr, fr-ch, da, de, fiIf-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11 Does the client have a cached copy of the object being requested?

Yes, because this is a conditional GET.

Which of the following characteristics below are associated with a TCP socket? Check one or more that apply.

socket(AF_INET, SOCK_STREAM) creates this type of socket a server can perform an accept() on this type of socket when contacted, the server will create a new server-side socket to communicate with that client provides reliable, in-order byte-stream transfer (a "pipe"), from client to server

Which of the following characteristics below are associated with a UDP socket? Check one or more that apply.

the application must explicitly specify the IP destination address and port number for each group of bytes written into a socket data from different clients can be received on the same socket socket(AF_INET, SOCK_DGRAM) creates this type of socket provides unreliable transfer of a groups of bytes ("a datagram"), from client to server

BROWSER CACHING Consider an HTTP server and client as shown in the figure below. Suppose that the RTT delay between the client and server is 20 msecs; the time a server needs to transmit an object into its outgoing link is 1 msecs; and any other HTTP message not containing an object has a negligible (zero) transmission time. Suppose the client again makes 80 requests, one after the other, waiting for a reply to a request before sending the next request. Assume the client is using HTTP 1.1 and the IF-MODIFIED-SINCE header line. Assume 50% of the objects requested have NOT changed since the client downloaded them (before these 80 downloads are performed) 1. How much time elapses (in milliseconds) between the client transmitting the first request, and the completion of the last request?

1. 1640

A COMPARISON OF CLIENT-SERVER AND P2P FILE DISTRIBUTION DELAYS In this problem, you'll compare the time needed to distribute a file that is initially located at a server to clients via either client-server download or peer-to-peer download. Before beginning, you might want to first review Section 2.5 and the discussion surrounding Figure 2.22 in the text. The problem is to distribute a file of size F = 6 Gbits to each of these 6 peers. Suppose the server has an upload rate of u = 68 Mbps. The 6 peers have upload rates of: u1 = 29 Mbps, u2 = 26 Mbps, u3 = 21 Mbps, u4 = 15 Mbps, u5 = 26 Mbps, and u6 = 14 Mbps The 6 peers have download rates of: d1 = 34 Mbps, d2 = 31 Mbps, d3 = 30 Mbps, d4 = 19 Mbps, d5 = 28 Mbps, and d6 = 17 Mbps 1. What is the minimum time needed to distribute this file from the central server to the 6 peers using the client-server model? 2. For the previous question, what is the root cause of this specific minimum time? Answer as 's' or 'ci' where 'i' is the client's number 3. What is the minimum time needed to distribute this file using peer-to-peer download? 4. For question 3, what is the root case of this specific minimum time: the server (s), client (c), or the combined upload of the clients and the server (cu)

1. 529.41 2. s 3. 352.94 4. c

DNS - BASICS Imagine that you are trying to visit www.enterprise.com, but you don't remember the IP address the web-server is running on. Assume the following records are on the TLD DNS server: (www.enterprise.com, dns.enterprise.com, NS) (dns.enterprise.com, 146.54.199.236, A) Assume the following records are on the enterprise.com DNS server: (www.enterprise.com, east2.enterprise.com, CNAME) (east2.enterprise.com, 142.81.17.206, A) (enterprise.com, mail.enterprise.com, MX) (mail.enterprise.com, 247.29.180.85, A) Assume your local DNS server only has the TLD DNS server cached. 1. What transport protocol(s) does DNS use: TCP, UDP, or Both? 2. What well-known port does DNS use? 3. In the above example, how many unique type of Resource Records (RR) are there at the authoritative enterprise.com DNS server? 4. Can you send multiple DNS questions and get multiple RR answers in one message? Answer with Yes or No 5. To which DNS server does a host send their requests to? Answer with the full name 6. Which type of DNS server holds a company's DNS records? Answer with the full name 7. In the example given in the problem, what is the name of the DNS server for enterprise.com? 8. When you make the request for www.enterprise.com, your local DNS requests the IP on your behalf. When it contacts the TLD server, how many answers (RR) are returned? 9. In the previous question, there were two responses, one was a NS record and the other an A record. What was the content of the A record? Answer with the format: "name, value" 10. Assume that the enterprise.com website is actually hosted on east2.enterprise.com, what type of record is needed for this? 11. Now imagine we are trying to send an email to [email protected], and their mail server has the name mail.enterprise.com. What type of record will contain the name of the enterprise.com domain and the name of its mailserver(s)? 12. In that MX record, what are the contents? Answer with the format: "name, value" 13. Does your local DNS server take advantage of caching similar to web requests? Answer with Yes or No

1. Both 2. 53 3. 3 4. Yes 5. Local DNS server 6. Authoratative DNS Server 7. dns.enterprise.com 8. 2 9. dns.enterprise.com, 146.54.22.76 10. CNAME 11. MX 12. enterprise.com, mail.enterprise.com 13. Yes

DNS - ITERATIVE QUERY Assume that a user is trying to visit gaia.cs.umass.edu, but his browser doesn't know the IP address of the website. In this example, examine the difference between an iterative and recursive DNS query. 1. Between steps 1 and 2, where does the Local DNS server check first? Answer with 'User', 'DNS Local', 'DNS Root', 'DNS TLD', or 'DNS Authoritative'. 2. Between steps 2 and 3, assuming the root DNS server doesn't have the IP we want, where does the response link? Answer with 'DNS Local', 'DNS Root', 'DNS TLD', or 'DNS Authoritative'. 3. Between steps 4 and 5, assuming the TLD DNS server doesn't have the IP we want, where does the response link? Answer with 'DNS Local', 'DNS Root', 'DNS TLD', or 'DNS Authoritative'. 4. Between steps 6 and 7, the authoritative DNS server responds with the IP we want. What type of DNS record is returned? 5. Which type of query is considered best practice: iterative or recursive?

1. DNS Root 2. DNS TLD 3. DNS Authoritative 4. A 5. Iterative

DNS - RECURSIVE QUERY Assume that a user is trying to visit gaia.cs.umass.edu, but his browser doesn't know the IP address of the website. In this example, examine the difference between an iterative and recursive DNS query. 1. Between steps 1 and 2, where does the Local DNS server check first? Answer with 'User', 'DNS Local', 'DNS Root', 'DNS TLD', or 'DNS Authoritative'. 2. Between steps 2 and 3, assuming the root DNS server doesn't have the IP we want, where does the response link? Answer with 'DNS Local', 'DNS Root', 'DNS TLD', or 'DNS Authoritative'. 3. Between steps 4 and 5, assuming the TLD DNS server doesn't have the IP we want, where does the response link? Answer with 'DNS Local', 'DNS Root', 'DNS TLD', or 'DNS Authoritative'. 4. Between steps 6 and 7, the authoritative DNS server responds with the IP we want. What type of DNS record is returned? 5. Which type of query is considered best practice: iterative or recursive?

1. DNS Root 2. DNS TLD 3. DNS TLD 4. A 5. Iterative

THE HTTP RESPONSE MESSAGE Consider the figure below, where the server is sending a HTTP RESPONSE message back the client. Suppose the server-to-client HTTP RESPONSE message is the following: HTTP/1.0 404 Not FoundDate: Wed, 05 Oct 2022 15:45:42 +0000Server: Apache/2.2.3 (CentOS)Content-Length: 522Connection: CloseContent-type: image/html 1. Is the response message using HTTP 1.0 or HTTP 1.1? 2. Was the server able to send the document successfully? Yes or No 3. How big is the document in bytes? 4. Is the connection persistent or nonpersistent? 5. What is the type of file being sent by the server in response? 6. What is the name of the server and its version? Write your answer as server/x.y.z 7. Will the ETag change if the resource content at this particular resource location changes? Yes or No

1. HTTP/1.0 2. No 3. 522 4. Non-persistant 5. image-hmtl 6. Apache/2.2.3 7. Yes

THE HTTP GET MESSAGE Consider the figure below, where a client is sending an HTTP GET message to a web server, gaia.cs.umass.edu Suppose the client-to-server HTTP GET message is the following: GET /kurose_ross_sandbox/interactive/quotation7.htm HTTP/1.1Host: gaia.cs.umass.eduAccept: text/plain, text/html, image/gif, image/jpeg, audio/basic, audio/vnf.wave, video/mp4, video/mpeg,Accept-Language: en-us, en-gb;q=0.3, en;q=0.2, fr, fr-ch, de, fi, ar, csIf-Modified-Since: Wed, 05 Oct 2022 08:28:17 -0700User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.56 Safari/535.11 1. What is the name of the file that is being retrieved in this GET message? 2. What version of HTTP is the client running? 3. True or False: The client will accept html files 4. True or False: The client will accept jpeg images 5. What is the client's preferred version of English? 6. What is the client's least preferred version of English? 7. True or False: The client will accept the German language 8. True or False: The client already has a cached copy of the file

1. quotation7.htm 2. HTTP/1.1 3. True 4. True 5. American English 6. English 7. True 8. True

Match the definition/function of an element or approach in a networked streaming video system, with its name. QUESTION LIST: A unit of video, each of which may be encoded at multiple different rates, stored in different files. A file containing the location and encoding rate of files corresponding to video segments in a video. An approach that allows a client to adapt the encoding rate of retrieved video to network congestion conditions. A CDN approach that stores content in access networks, close to clients.

A unit of video, each of which may be encoded at multiple different rates, stored in different files.: Chunk A file containing the location and encoding rate of files corresponding to video segments in a video.: Manifest An approach that allows a client to adapt the encoding rate of retrieved video to network congestion conditions.: DASH A CDN approach that stores content in access networks, close to clients.: Enter deep

How does the networked application running on a server know the client IP address and the port number to reply to in response to a received datagram?

The application code at the server determines client IP address and port # from the initial segment sent by client, and must explicitly specify these values when sending into a socket back to that client.

Suppose that the local DNS server caches all information coming in from all root, TLD, and authoritative DNS servers for 20 time units. (Thus, for example, when a root server returns the name and address of a TLD server for .com, the cache remembers that this is the TLD server to use to resolve a .com name). Assume also that the local cache is initially empty, that iterative DNS queries are always used, that DNS requests are just for name-to-IP-address translation, that 1 time unit is needed for each server-to-server or host-to-server (one way) request or response, and that there is only one authoritative name server (each) for any .edu or .com domain. Consider the following DNS requests, made by the local host at the given times: t=0, the local host requests that the name gaia.cs.umass.edu be resolved to an IP address. t=1, the local host requests that the name icann.org be resolved to an IP address. t=5, the local host requests that the name cs.umd.edu be resolved to an IP address. (Hint: be careful!) t=10, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. t=12, the local host requests that the name cs.mit.edu be resolved to an IP address. t=30, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. (Hint: be careful!) Which of the requests require 8 time units to be resolved?

The request at t=0. The request at t=1. The request at t=30.

[This question is the same as an earlier question, except for the question statement at the very end.] Suppose that the local DNS server caches all information coming in from all root, TLD, and authoritative DNS servers for 20 time units. (Thus, for example, when a root server returns the name and address of a TLD server for .com, the cache remembers that this is the TLD server to use to resolve a .com name). Assume also that the local cache is initially empty, that iterative DNS queries are always used, that DNS requests are just for name-to-IP-address translation, that 1 time unit is needed for each server-to-server or host-to-server (one way) request or response, and that there is only one authoritative name server (each) for any .edu or .com domain. Consider the following DNS requests, made by the local host at the given times: t=0, the local host requests that the name gaia.cs.umass.edu be resolved to an IP address. t=1, the local host requests that the name icann.org be resolved to an IP address. t=5, the local host requests that the name cs.umd.edu be resolved to an IP address. (Hint: be careful!) t=10, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. t=12, the local host requests that the name cs.mit.edu be resolved to an IP address. t=30, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. (Hint: be careful!) Which of the requests require 2 time units to be resolved?

The request at t=10.

[This question is the same as an earlier question, except for the question statement at the very end.] Suppose that the local DNS server caches all information coming in from all root, TLD, and authoritative DNS servers for 20 time units. (Thus, for example, when a root server returns the name and address of a TLD server for .com, the cache remembers that this is the TLD server to use to resolve a .com name). Assume also that the local cache is initially empty, that iterative DNS queries are always used, that DNS requests are just for name-to-IP-address translation, that 1 time unit is needed for each server-to-server or host-to-server (one way) request or response, and that there is only one authoritative name server (each) for any .edu or .com domain. Consider the following DNS requests, made by the local host at the given times: t=0, the local host requests that the name gaia.cs.umass.edu be resolved to an IP address. t=1, the local host requests that the name icann.org be resolved to an IP address. t=5, the local host requests that the name cs.umd.edu be resolved to an IP address. (Hint: be careful!) t=10, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. t=12, the local host requests that the name cs.mit.edu be resolved to an IP address. t=30, the local host again requests that the name gaia.cs.umass.edu be resolved to an IP address. (Hint: be careful!) Which of the requests require 6 time units to be resolved?

The request at t=5. The request at t=12.

Suppose now the server sends the following HTTP response message the client:HTTP/1.0 200 OKDate: Wed, 09 Sep 2020 23:46:21 +0000Server: Apache/2.2.3 (CentOS)Last-Modified: Wed, 09 Sep 2020 23:51:41 +0000ETag:17dc6-a5c-bf716880.Content-Length: 418Connection: CloseContent-type: image/htmlWill the web server close the TCP connection after sending this message? [Note: you can find more questions like this one here.]

Yes, the server will close this connection because version 1.0 of HTTP is being used, and TCP connections do not stay open persistently.