Computer Networking Knowledge Checks 1-3

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Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps. Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations for the server links (with transmission capacity RS)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx).

(50 + 50 + 50 + 50) / (50 + 50 + 50 + 50) = 200 / 200 = 1.00

Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps. Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations of the client links (with transmission capacity RC)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx).

(50 + 50 + 50 + 50) / (90 + 90 + 90 + 90) = 200 / 360 = 0.56

Which of the following human scenarios involve a protocol (recall: "Protocols define the format, order of messages sent and received among network entities, and actions taken on message transmission, receipt")?

1) Two people introducing themselves to each other. 2) A student raising her/his hand to ask a really insightful question, followed by the teaching acknowledging the student, listening carefully to the question, and responding with a clear, insightful answer. And then thanking the student for the question, since teachers love to get questions. 3) One person asking, and getting, the time to/from another person.

Which of the following descriptions below correspond to a "nuts-and-bolts" view of the Internet?

1) a "network of networks" 2) A collection of hardware and software components executing protocols that define the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event. 3) A collection of billions of computing devices, and packet switches interconnected by links.

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

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

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

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

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? Check all that apply.

A hostname and an IP address.

Which of the following physical layer technologies has the highest transmission rate and lowest bit error rate in practice?

Fiber optic cable

Choose one the following two definitions that makes the correct distinction between routing versus forwarding.

Forwarding is the local action of moving arriving packets from router's input link to appropriate router output link, while routing is the global action of determining the source-destination paths taken by packets.

Consider the figure below, showing a link-layer frame heading from a host to a router. There are three header fields shown. Match the name of a header with a header label shown in the figure. H1 - H2 - H3 - M

H1 - Link layer H2 - Network layer H3 - Transport layer

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.

Which of the definitions below describe what is meant by the term "encapsulation"?

Taking data from the layer above, adding header fields appropriate for this layer, and then placing the data in the payload field of the "packet" for that layer.

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=3

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

The request at t=10.

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.

Suppose an HTTP server sends the following HTTP response message a client: 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 +0000E Tag: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, because this is HTTP 1.0

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.

Which of the characteristics below are associated with the technique of packet switching?

1) Resources are used on demand, not reserved in advance. 2) This technique is used in the Internet. 3) Congestion loss and variable end-end delays are possible with this technique. 4) Data may be queued before being transmitted due to other user's data that's also queueing for transmission.

Which of the following descriptions below correspond to a "services" view of the Internet?

1) A place I go for information, entertainment, and to communicate with people. 2) A platform for building network applications.

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

1) A response phrase associated with a response code 2) A response code

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.

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

Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps. Assuming that the servers are all sending at their maximum rate possible, what are the link utilizations of the shared link (with transmission capacity R)? Enter your answer in a decimal form of 1.00 (if the utilization is 1) or 0.xx (if the utilization is less than 1, rounded to the closest xx).

(50 + 50 + 50 + 50) / 300 = 200 / 300 = 0.67

Match the name of an Internet layer with unit of data that is exchanged among protocol entities at that layer, using the pulldown menu.

1) Application layer - Message 2) Transport layer - Segment 3) Network layer - Datagram 4) Link layer - Frame 5) Physical layer - Bit

Match the functionality of a protocol with the name of a the email protocol (if any) that implements that functionality.

1) Pushes email from a mail client to a mail server. - SMTP 2) Pulls mail from one mail server to another mail server. - Neither SMTP nor IMAP does this. 3) Pulls email to a mail client from a mail server. - IMAP

Consider the circuit-switched network shown in the figure below, with four circuit switches A, B, C, and D. Suppose there are 20 circuits between A and B, 19 circuits between B and C, 15 circuits between C and D, and 16 circuits between D and A. What is the maximum number of connections that can be ongoing in the network at any one time?

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

1) 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.

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.

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

1) Congestion control. The service will control senders so that the senders do not collectively send more data than links in the network can handle. 2) 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. 3) Flow Control. The provided service will ensure that the sender does not send so fast as to overflow receiver buffers.

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

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

Consider a scenario in which 5 users are being multiplexed over a channel of 10 Mbps. Under the various scenarios below, match the scenario to whether circuit switching or packet switching is better.

1) Each user generates traffic at an average rate of 2.1 Mbps, generating traffic at a rate of 15 Mbps when transmitting - Neither works well in this overload scenario 2) Each user generates traffic at an average rate of 2 Mbps, generating traffic at a rate of 2 Mbps when transmitting - Circuit switching 3) Each user generates traffic at an average rate of 0.21 Mbps, generating traffic at a rate of 15 Mbps when transmitting - Packet switching

Match the access network with the approximate speeds that a subscriber might experience.

1) Ethernet - Wired. Up to 100's Gbps per link. 2) 802.11 WiFi - Wireless. 10's to 100's of Mbps per device. 3) Cable access network - Wired. Up to 10's to 100's of Mbps downstream per user 4) Digital Subscriber Line - Wired. Up to 10's of Mbps downstream per user. 5) 4G Cellular LTE - Wireless. Up to 10's Mbps per device.

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?

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

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

1) 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. 2) 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?

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

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

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

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

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

Match the function of a layer in the Internet protocol stack to its its name in the pulldown menu.

1) Protocols that are part of a distributed network application. - Application Layer 2) Transfer of data between one process and another process (typically on different hosts). - Transport Layer 3) Delivery of datagrams from a source host to a destination host (typically). - Network Layer 4) Transfer of data between neighboring network devices. - Link Layer 5) Transfer of a bit into and out of a transmission media. - Physical Layer

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. - Authoritative DNS Server 2) Replies to DNS query by local host, by contacting other DNS servers to answer the query. - Local DNS Server 3) Responsible for a domain (e.g., *.com, *.edu); knows how to contact authoritative name servers. - Top Level Domain (TLD) servers 4) 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 description of a security defense with its name.

1) Specialized "middleboxes" filtering or blocking traffic, inspecting packet contents inspections - Firewall 2) Provides confidentiality by encoding contents - Encryption 3) Used to detect tampering/changing of message contents, and to identify the originator of a message. - Digital Signatures 4) Limiting use of resources or capabilities to given users. - Access control 5) Proving you are who you say you are. - Authentication

When we say that the Internet is a "network of networks," we mean?

1) The Internet is made up of a lot of different networks that are interconnected to each other. 2) The Internet is made up of access networks at the edge, tier-1 networks at the core, and interconnected regional and content provider networks as well.

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

Which of the characteristics below are associated with 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) HTTP uses this application structure. 3) There is a server that is always on.

Which of the characteristics below are associated with 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 of the characteristics below are associated with the technique of circuit switching?

1) This technique was the basis for the telephone call switching during the 20th century and into the beginning of this current century. 2) Reserves resources needed for a call from source to destination. 3) Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) are two approaches for implementing this technique.

Match the description of each component of packet delay to its name in the pull down list.

1) Time needed to perform an integrity check, lookup packet information in a local table and move the packet from an input link to an output link in a router. - Processing delay 2) Time spent waiting in packet buffers for link transmission. - Queueing delay 3) Time spent transmitting packets bits into the link. - Transmission delay 4) Time need for bits to physically propagate through the transmission medium from end one of a link to the other. - Propagation delay

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: What version of HTTP is the client using?

1.1

What is the maximum throughput achievable between sender and receiver in the scenario shown below? Link capacity from sender to router: 1.5 Mbits/sec Link capacity from router to receiver: 10 Mbits/sec

1.5 Mbps (the maximum achievable end-end throughput is the capacity of the link with the minimum capacity)

Consider the network shown in the figure below, with three links, each with the specified transmission rate and link length. Assume the length of a packet is 8000 bits. The speed of light propagation delay on each link is 3x10^8 m/sec. What is the propagation delay at (along) link 2? Link 1: transmission rate 10 Mbps, link length 2km Link 2: transmission rate 100 Mbps, link length 1000km Link 3: transmission rate 100 Mbps, link length 1km

1000 km to m = 1,000,000 m 1,000,000 m / 3x10^8 m/sec = .0033 secs

Suppose a packet is L = 1200 bytes long (one byte = 8 bits), and link transmits at R = 100 Mbps (i.e., a link can transmit bits 100,000,000 bits per second). What is the transmission delay for this packet?

1200 * 8 = 9,600 bits 9,600 bits / 100,000,000 bps = .000096 secs

Suppose a packet is L = 1500 bytes long (one byte = 8 bits), and link transmits at R = 1 Gbps (i.e., a link can transmit bits 1,000,000,000 bits per second). What is the transmission delay for this packet?

1500 * 8 = 12,000 bits 12,000 bits / 1,000,000,000 bps = .000012 secs

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:

Consider the scenario shown below, with four different servers connected to four different clients over four three-hop paths. The four pairs share a common middle hop with a transmission capacity of R = 300 Mbps. The four links from the servers to the shared link have a transmission capacity of RS = 50 Mbps. Each of the four links from the shared middle link to a client has a transmission capacity of RC = 90 Mbps. What is the maximum achievable end-end throughput (an integer value, in Mbps) for each of four client-to-server pairs, assuming that the middle link is fairly shared (divides its transmission rate equally) and all servers are trying to send at their maximum rate?

50 Mbps (the maximum achievable end-end throughput is the capacity of the link with the minimum capacity)

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?

Consider the network shown in the figure below, with three links, each with the specified transmission rate and link length. Assume the length of a packet is 8000 bits. What is the transmission delay at link 2? Link 1: transmission rate 10 Mbps, link length 2km Link 2: transmission rate 100 Mbps, link length 1000km Link 3: transmission rate 100 Mbps, link length 1km

8000 bits / 100 Mbps = 8000 bits / 100,000,000 bps = 8 x 10^(-5) secs

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

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/1.1 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.

Suppose now the server sends the following HTTP response message the client: 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 +0000E Tag: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.

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