Computer Networks Final

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

This is answer

Network Edge

hosts: clients and servers servers often in data centers

access networks, physical media

wired, wireless communication links

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

Fiber optic cable

Transmission Delay

L: packet length (bits) R: link bandwidth (bps) dtrans= L/R

Where is TCP defined?

RFC

What transport-layer protocol has congestion control?

TCP

Propagation delay

Time need for bits to physically propagate through the transmission medium from end one of a link to the other.

queuing delay

Time spent waiting in packet buffers for link transmission.

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.

Physical:

bits "on the wire"

An IPv4 router receives a datagram and it is larger than the MTU on the outgoing link. The IPv4 router:

fragments the datagram

Network Core

interconnected routers network of networks

transport:

process-process data transfer •TCP, UDP

network:

routing of datagrams from source to destination •IP, routing protocols

List and describe the four types of DNS resource records discussed in class

Type=A: A is the standard and most common resource record. The name is the host name, and the value is the IP address. Type=CNAME: CNAME stands for canonical name. The value of this record is the canonical name and the name is the host name. The canonical name points to the host name. Type=NS: NS stands for nameserver. The name of this record is a domain and the value is the hostname of an authoritative DNS server that can optain the IP addresses of hosts in the domain. It tells the internet where to find the IP address of the domain. Page 2 of 3 Type=MX: The value is the canonical name of a mail server and the name is an alias hostname. This lets the hostnames have simple aliases.

(tcp) When sending to a client, this is how a specific client is identified

As the result of an accept(), a new socket is created, which binds the client that made the corresponding connect() and server together via this new socket without the need to to explicitly specify the destination IP address and port # when sending

Which of the following protocols is used for assigning IP addresses?

DHCP

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

What layer(s) in the Internet Protocol Stack do not have to be processed by a router?

application and transport

An organization has a computer named acme.uark.edu that runs the mail server. Which DNS resource record is required so that this server can serve email addressed to [email protected]?

MX

Which of the following is a false statement about packet switching?

Routers must maintain state information for each connection

(tcp) Send to server, using this socket.

Send using a socket not explicitly created via a call to socket()

(UDP) Send to server, using this socket.

Send using the socket created using socket(AF_INET, SOCK_DGRAM)

List both the connection-oriented and connectionless transport protocols used in the Internet. Identify at least two differences between them.

The connection-oriented protocols on the internet are the Transmission Control Protocol (TCP), and the Internet Protocol (IP). The Connectionless transport protocol is the User Datagram Protocol (UDP). One difference between TCP and UDP is that TCP guarantees delivery of application-layer messages to the destination while UDP does not guarantee it. TCP also uses handshake protocols while UDP does not. TCP has flow control while UDP does not.

Processing delay

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.

Transport services and protocols

provide logical communication between app processes running on different hosts

What kind of delay happens in a router and is a function of the intensity and nature of the traffic arriving at the buffer?

queueing delay

SMTP uses 7-bit ASCII

True

Application:

supporting network applications •DNS, SMTP, HTTP

Queuing Delay

- time waiting at output link for transmission - depends on congestion level of router

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")?

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. One person asking, and getting, the time to/from another person. Two people introducing themselves to each other.

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

Application layer E. Message Transport layer C. Segment Network layer B. Datagram Link layer A. Frame Physical layer D. Bit

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.

Compare and contrast the POP3 and IMAP email protocols

Both protocols are mail access protocols. POP3 is extremely simple, short, and readable while IMAP is complex and long. IMAP keeps user state across sessions while POP3 is stateless across sessions. With IMAP being more complex it consequently has more features than POP3, messages can be accessed on more devices on IMAP, mail is kept on the server in IMAP, and before downloaded IMAP mail can be viewed partially while with POP3 it has to be downloaded before.

List and describe differences between iterative and recursive DNS queries

In a recursive query the DNS server who received it will obtain the mapping itself and return it. In an iterative query the main DNS server returns back mapping to a different DNS server that might have the answer. One difference between the two is that recursive queries are at risk of a DOS attack. Usually with queries the query from the requesting host to the local DNS server is recursive and the rest of the queries are iterative.

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.

Physical layer

Transfer of a bit into and out of a transmission media.

Link layer

Transfer of data between neighboring network devices.

Transport layer

Transfer of data between one process and another process (typically on different hosts)

In TCP socket programming, the server has two different sockets, one for handling the initial request for a connection and another one for the remainder of the connection.

True

An IPv4 address is ____ bytes long.

4

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

A hostname and an IP address.

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.

The web browser can decide to reject cookies from the web server.

True

What is the title of RFC 968?

Twas the night before start-up

Which of the following pairs of IP addresses are on the same IP subnetwork if the subnetwork mask is 255.255.255.224

192.168.76.225, 192.168.76.254

Expand the acronym DASH used for streaming multimedia and describe the main service that it provides with respect to bandwidth.

Dash stands for Dynamic Adaptive Streaming over HTTP. DASH encodes the video file in multiple chunks, encoded at different rates. The client then requests chunks each a few seconds long and depending on the amount of bandwidth, the client selects different versions of the chunks. If bandwidth is high, high-rate version of chunk is selected, and if the bandwidth is low, low-rate version of chunk is selected.

Network layer

Delivery of datagrams from a source host to a destination host (typically).

What are used to create shopping carts over the stateless HTTP protocol? Explain the required steps.

HTTP uses cookies to create shopping carts over the stateless HTTP protocol. First when the client host requests the site in which the cart is an identification number is created for the client and stored in the backend. Then each time the client adds an item to the shopping cart, the browser consults her cookie file, extracts her identification number for this site, and puts a cookie header line that includes the identification number in the HTTP request, essentially that item is then stored in the cookie file under the ID number. Once the client then goes to checkout the items under the cookie in that ID are compiled to purchase.

Four sources of packet delay

Nodal Processing Queueing Delay Transmission Delay Propagation Delay

Application layer

Protocols that are part of a distributed network application.

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

Protocols that are part of a distributed network application. A. Application Layer Transfer of data between one process and another process (typically on different hosts). B. Transport layer Delivery of datagrams from a source host to a destination host (typically). C. Network layer Transfer of data between neighboring network devices. D. Link layer Transfer of a bit into and out of a transmission media. E. Physical layer

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

transmission delay

Time spent transmitting packets bits into the link

What transport-layer protocol is connectionless?

UDP

Nodal Processing

check bit errors determine output link typically < msec

propogation delay

d: length of physical link s: propagation speed (~2x108m/sec) dprop=d/s

Link

data transfer between neighboring network elements •Ethernet, 802.111 (WiFi), PPP

bottleneck link

link on end-end path that constrains end-end throughput

List the three layers a router must implement to operate.

link-layer, physical layer, and network layer.

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

(UDP)Create a socket

Use the call socket(AF_INET, SOCK_DGRAM)

(tcp) Create a socket.

Use the call socket(AF_INET, SOCK_STREAM)

Internet Protocol Stack

1. Application 2. Transport 3. Network 4. Link 5. Physical

Which of the following datagram and segment header fields are used, when demultiplexing data up to a TCP socket?

Source and destination IP addresses, and source and destination port numbers.

(UDP) When sending to the server, this is how a specific server is identified.

The client explicitly includes the destination IP address and port #, when sending

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

What is a unique number assigned to each process and is used to keep track of transport layer sessions when the process communicates?

port number

What kind of delay is a function of the congestion level in a router

queueing