COMP2190 - Net Centric Computing - Chapter 2

Which of the requests require 6 time units to be resolved?

The request at t = 5, .edu domain has already been looked up and is in the cache of the local DNS server. The request at t = 12, same as above

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

In DASH (Dynamic, Adaptive Streaming over HTTP), a server divides a video file into chunks that ...

... 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.

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.

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.

What is the formula for time taken to distribute a file using client-server approach?

max{ ((number of users)*(file size))/(server upload speed); (file size)/(minimum download speed of all users) }

Match the function of a server to a given type of DNS server in the DNS server hierarchy. 1. Provides authoritative hostname to IP mappings for organization's named hosts. 2. Replies to DNS query by local host, by contacting other DNS servers to answer the query. 3. Responsible for a domain (e.g., *.com, *.edu); knows how to contact authoritative name servers. 4. Highest level of the DNS hierarchy, knows how to reach servers responsible for a given domain (e.g., *.com, *.edu).

1. Authorative DNS server 2. Local DNS server 3. Top Level Domain (TLD) server 4. DNS root server

What are some advantages of using a web cache?

1. Caching uses less bandwidth coming into an institutional network where the client is located, if the cache is also located in that institutional network. 2. 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.

Match the definition/function of an element or approach in a networked streaming video system, with its name. 1. A unit of video, each of which may be encoded at multiple different rates, stored in different files. 2. A file containing the location and encoding rate of files corresponding to video segments in a video. 3. An approach that allows a client to adapt the encoding rate of retrieved video to network congestion conditions. 4. A CDN approach that stores content in access networks, close to clients.

1. Chunk 2. Manifest 3. DASH 4. Enter deep

What is the value of caching in the local DNS name server?

1. DNS caching provides for faster replies, if the reply to the query is found in the cache. 2. DNS caching results in less load elsewhere in DNS, when the reply to a query is found in the local cache.

Which characteristics below are associated with a UDP socket?

1. Data from different clients can be received on the same socket. 2. The application must explicitly specify the IP destination address and port number for each group of bytes written into a socket. 3. Socket(AF_INET, SOCK_DGRAM) creates this type of socket. 4. Provides unreliable transfer of a groups of bytes ("a datagram"), from client to server.

What are some changes between HTTP 1.1 and HTTP/2?

1. HTTP/2 allows objects in a persistent connection to be sent in a client-specified priority order. 2. 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.

Which characteristics apply to both HTTP and SMTP?

1. Is able to use a persistent TCP connection to transfer multiple objects. 2. Has ASCII command/response interaction, status codes.

Which forms of caching does a user benefit from, not only from its recent requests (and cached replies) but also from recent requests made from other users?

1. Local DNS server caching 2. HTTP local web caching

Which of the following functionalities use either SMTP or IMAP: 1. Pushes email from a mail client to a mail server. 2. Pulls mail from one mail server to another mail server. 3. Pulls email to a mail client from a mail server.

1. SMTP 2. Neither 3. IMAP

What are properties of a local DNS server?

1. 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. 2. The local DNS server record for a remote host is sometimes different from that of the authoritative server for that host.

What are the characteristics of a client-server approach to structuring network applications (as opposed to a P2P approach)?

1. There is a server with a well known server IP address. 2. There is a server that is always on. 3. HTTP uses this application structure

What are the characteristics of a P2P approach to structuring network applications (as opposed to a client-server approach)?

1. There is not a server that is always on. 2. A process requests service from those it contacts and will provide service to processes that contact it.

Which characteristics apply to SMTP only (and do not apply to HTTP)?

1. Uses CRLF.CRLF to indicate end of message. 2. Operates mostly as a "client push" protocol. 3. Uses server port 25.

Which characteristics apply to HTTP only (and do not apply to SMTP)?

1. Uses server port 80. 2. Uses a blank line (CRLF) to indicate end of request header. 3. Operates mostly as a "client pull" protocol.

Which of the following characteristics below are associated with a TCP socket?

1. When contacted, the server will create a new server-side socket to communicate with that client. 2. A server can perform accept() on this type of socket. 3. Socket(AF_INET, SOCK_STREAM) creates this type of socket. 4. Provides reliable, in-order byte-stream transfer (a "pipe"), from client to server.

GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1 Host: gaia.cs.umass.edu Accept: 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, fi If-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700 User 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 HTTP version?

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?

3 RTTs: 1st RTT: Initiate TCP Connection 2nd and 3rd RTT: SMTP Handshaking

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 information does the type "A" resource record hold in the DNS database?

A hostname and an IP address.

What will appear in a server's application-level HTTP reply message?

A response code A response phrase associated with a response code

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

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?

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.

What is the role of an authoritative name server in the DNS?

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

When an application uses a TCP socket, what transport services are provided to the application by TCP?

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.

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.

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.

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.

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, because the cache is empty. The request at t = 1, domain .org hasn't been contacted yet The request at t = 30, the chache as been emptied after 20 time unites since the request at t = 10 has been sent.

Which of the requests require 2 time units to be resolved?

The request at t = 10, all components are still in the local DNS cache

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.

GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1 Host: gaia.cs.umass.edu Accept: 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, fi If-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700 User 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?

United Kingdom English.

GET /kurose_ross_sandbox/interactive/quotation2.htm HTTP/1.1 Host: gaia.cs.umass.edu Accept: 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, fi If-Modified-Since: Wed, 09 Sep 2020 16:06:01 -0700 User 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, as evidenced by the If-Modified-Since field.

HTTP/1.0 200 OK Date: Wed, 09 Sep 2020 23:46:21 +0000 Server: Apache/2.2.3 (CentOS) Last-Modified: Wed, 09 Sep 2020 23:51:41 +0000 ETag:17dc6-a5c-bf716880. Content-Length: 418 Connection: Close Content-type: image/html Will the web server close the TCP connection after sending this message?

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

What is the formula for time taken to distribute a file using peer-to-peer approach?

max{ (file size)/(server upload speed); (file size)/(minimum download speed of all users); ((number of users)*(file size))/((server upload speed)+(sum of upload speed of all users)) }