Intro to Comp Networks Exam 1 (Ch 1 - Ch 2)

Protocols define the ________, __________ of messages sent and received among network entities, and actions taken on msg transmission receipt

format, order

Because the internet is so large, it is usually described as a network of _______.

networks

(T/F) A web server is considered a host in a network.

True

The network core of the Internet is:
- Interconnected hosts
- Interconnected routers
- Data centers
- None of the above

Interconnected routers

- A single router is transmitting packets, each of length L bits, over a single link with transmission rate R Mbps to another router at the end of the link
- Suppose that the packet length is L = 16000 bits, and that the link transmission rate along the link to router on the right is R = 1000 Mbps

What is the transmission delay?

0.000016 sec

- A single router is transmitting packets, each of length L bits, over a single link with transmission rate R Mbps to another router at the end of the link
- Suppose that the packet length is L = 16000 bits, and that the link transmission rate along the link to router on the right is R = 1000 Mbps

What is the maximum number of packets per sec that can be transmitted by this link?

62,500 packets

(T/F) One advantage in the Client-Server paradigm is that both the client and server have permanent IP addresses

False

A sender’s email server uses _________ protocol to deliver an email message to the receiver’s server.

SMTP

One example of an open application-layer protocol is:
- HTTP
- IP
- Skype
- UDP

HTTP

Since servers using HTTP protocol can not maintain information about past client requests, HTTP is described as ______.

stateless

(T/F) If Process_1 running within host_A wants to send a message to Process_2 within host_B, then Process_2 will be identified by its IP address

False

Let’s suppose that:
- The RTT delay between the client and server is 10 msecs;
- The time a server needs to transmit an object into its outgoing link is 0.25 msecs;
- Any other HTTP message not containing an object has a negligible (zero) transmission time

Suppose the client makes 90 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_SINE header line
- Assume 40% of the objects requested have NOT changed since the client downloaded them (before these 90 downloads are performed)

How much time elapses in (milliseconds) between the client transmitting the first request, and the completion of the late request?

913.5 ms

Hosts

End systems, devices connected to the Internet

Communication links

Connect end systems in a network, made up of different types of physical media

Packets

data chunks created when end system 1 sends data to end system 2
end system 1: segments data and adds header bytes to each segment
end system 2: reassembles packets into original data

Packet switch

takes arriving packets and forwards it to another communication link

Route/path

sequence of communication links and packet switches that a packet travels through

Internet Service Provider (ISP)

network of packet switches and communication links, allows end systems to access Internet

Internet Protocol (IP)

specifies format and order of packets that are sent/received from routers and end systems

At the network's edge is

hosts (clients and servers) and servers often found in data centers

At the network's core is

interconnected routers and a network of networks

How are end systems connected to the edge routers?

through residential access nets, institutional access networks (school, company), and mobile access networks (WiFi, 4G/5G)

Packet-switching

hosts break application-layer messages into packets (chunks of data)
forwards packets from router to router
entire packet must arrive at router before it can be transmitted to next link

Transmission delay

L bits/R Mbps
L: packet length (bits), R: link transmission rate (bps)

End-end delay

2L bits/R Mbps
L: packet length (bits), R: link transmission rate (bps)

If arrival rate (in bps) to link exceeds transmission rate (bps) of link for a period of time

- packets will queue, waiting to be transmitted on output link
- packets can be dropped (lost) if memory buffer in router fills up

The two key network-core functions are

forwarding function (local action) and routing function (global action)

Network-core forwarding function

local action: move arriving packets from router’s input link to appropriate router output link

Network-core forwarding function

global action: determine source-destination paths taken by packets, routing algorithms

Circuit switching

end-end resources allocated to, reserved for "call" between source and destination

Which of these graphs represents circuit switching's FDM?

Upper

Which of these graphs represents circuit switching's TDM?

Lower

Why is packet switching considered better than circuit switching?

It allows more users to use the network, is simpler with no call setup, and great for sending bursts of data

What are the downsides to using packet switching?

Excessive congestion is possible leading to packet delay and loss due to buffer overflow

Given millions of acces to ISPs, how can they be connected together?

Connect each acces ISP to one global transit ISP, where customer and provider ISPs have economic agreement

How does packet loss and delay occur?

Arrival rate to link (temporarily) exceeds output link capacity

Propagation delay

d/s
d: length of physical link, s: propagation speed

Throughput

Rate (bits/time units) at which bits are being sent from sender to receiver

If arrival rate of packets to link exceeds transmission rate of link for a period of time AND if buffer in router fills up

- packets can be dropped
- some packets will be returned to source
- packets will queue as long as needed
- buffer in router will adjust to process all packets

packets can be dropped

(T/F) Local forwarding in network-core determines source-destination paths taken by packets

true

The Internet protocol stack, as a complex system, is organized as _________________.

layers

Suppose that the packet length is L = 4000 bits, and that the link transmission rate along the link to router on the right is R = 10 Mbps.
What is the transmission delay?

0.0004 sec

Suppose that the packet length is L = 4000 bits, and that the link transmission rate along the link to router on the right is R = 10 Mbps.
What is the maxmimum number of packets per second that can be transmitted by this link?

2500 packets

Instantaneous throughput

rate at given point in time at which bits are being sent from
sender to receiver

Average throughput

rate over longer period of time at which bits are being sent from
sender to receiver

Bottleneck link

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

Layers

each one implements a service, via its own internal-layer actions relying on services provided by the one below it

Why are network protocols organized as layers?

helps deal with complex systems and allows easier identification, maintenance, and updates

Internet protocol stack

(top to bottom) application, transport, network, link, physical

A circuit-switched network with circuit switches A, B, C, and D. Suppose there are 12 circuits between A and B, 16 circuits between B and C, 15 circuits between C and D, and 10 circuits between D and A.
What is the maximum number of connections that can be ongoing in the network at any one time?

53

A circuit-switched network with circuit switches A, B, C, and D. Suppose there are 12 circuits between A and B, 16 circuits between B and C, 15 circuits between C and D, and 10 circuits between D and A.
Suppose that these maximum number of connections are all ongoing. What happens when another call connection request arrives to the network, will it be accepted?

no

Server

always-on host, permanent IP address

Clients

contact and communicate with server, may have dynamic IP addresses

Peer-peer (P2P) architecture

peers request service from other peers, provide service in return to other peers

Process

program running within a host

Client process

process that initiates communication

Server process

process that waits to be contacted

Socket

process sends/receives messages to/from this

(T/F) A socket's sending process doesn't rely on transport infrastructure on other side of door to deliver messages to the socket at receiving process.

false

Identifier includes both ___________ and ___________ associated with process on host

IP address, port numbers

Does the IP address of a host on which a process runs suffice for identifying the process?

no, many processes can be running on the same host

Application-layer protocol

types of messages exchanged, message syntax, message semantics, open protocols, proprietary protocols, rules for processes

What transport service does an app need?

data integrity, throughput, timing, security

What are the main types of Internet transport protocol services?

TCP, UDP

What is the main different between TCP and UDP?

TCP is more reliable in sending and transferring data than UDP

(T/F) In a client-server paradigm, clients can communicate directly with each other

false

(T/F) In order for two processes running on different hosts to exchange messages, two sockets must be involved: one on each side

true

The default port for processes running on an HTTP server is ______________.

80

(T/F) Generally, a file transfer application can tolerate some loss of data

false

In a non-persistent HTTP connection:

- the connection will remain open until all objects are sent
- multiple object can be sent
- at most one object is sent
- the objects that are sent will not be persisted

at most one object is sent

In order for websites and client browsers to maintain some state between transactions, they use:

- persistent TCP
- none of the others
- persistent HTTP connections
- cookies

cookies

(T/F) HTTP/2 has increased flexibility at server in sending objects to client

true

(T/F) One similarity between HTTP and SMTP is that both are pull protocols

False

When a user agent retrieves an e-mail message from its e-mail server, it uses this protocol:

- IMAP
- UDP
- none of the others
- SMTP

IMAP

What is the purpose of HTTP GET?

- 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
- The HTTP GET request message is sent by a web server to a web client to get the next request from the web client
- The HTTP GET request message is used by a web client to post an object on a web server
- The HTTP GET request message is sent by a web server to a web client to identity of the web client

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

Assume that the RTT delay between the client and server is 30 msecs and the TCP connected is already established;

- the time a server needs to transmit an object into its outgoing link is 0.5 msecs;
- any other HTTP message not containing an object has a negligible (zero) transmission time

Suppose the client again makes 50 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 50 downloads are performed)

How much time elapses (in milliseconds) between the client transmitting the first request, and the completion of the last request?

1512.5 ms

Assume an RTT of 10 msec is needed for each:
- to initiate and establish a TCP connection
- for HTTP request and first few bytes of HTTP response to return
Assume an object transmission time is 0.5 msec
Assume the HTML page being requested references 5 objects in addition to the base HTML file and that HTTP/1.0 is being used

What is the total response time for the entire page to be delivered to the browser?

123.0 ms

Assume that the RTT delay between the client and server is 30 msecs and the TCP connected is already established;

- the time a server needs to transmit an object into its outgoing link is 1 msecs;
- any other HTTP message not containing an object has a negligible (zero) transmission time

Suppose the client again makes 60 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 60% of the objects requested have NOT changed since the client downloaded them (before these 60 downloads are performed)

How much time elapses (in milliseconds) between the client transmitting the first request, and the completion of the last request?

1824 ms

(T/F) A user requests a Web page that consists of some text and three images. For this page, the client will send one request message and receive four response messages.

False

(T/F) With nonpersistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages.

False

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.
- The cookie value indicates whether the user wants to use HTTP/1, HTTP/1.1, or HTTP/2 for this GET request
- None of the others
- The cookie value encodes the format of the reply preferred by the client in the response to this GET request
- The cookie value is an encoding of a user email address associated with the 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.

A web page consists of base HTML-file which includes several referenced ___________, each addressable by a URL.

objects

HTTP

hypertext transfer protocol, web's application layer protocol

(T/F) Protocols that maintain their past history (state) are complex.

True

What is the main different between non-persistent and persistent HTTP?

- non-persistent sends at most one object over a TCP connection, persistent sends multiple objects over a single TCP connection
- non-persistent uses multiple TCP connections to send a few objects at a time, persistent uses a single TCP connection to send all objects at once
- non-persistent sends all objects through a single TCP connection, persistent uses several TCP connections to send one object at a time
- none of these

non-persistent sends at most one object over a TCP connection, persistent sends multiple objects over a single TCP connection

RTT

time for a small packet to travel from client to server and back

Non-persistent HTTP response time

2RTT + file transmission time

(T/F) It requires one RTT to initiate a TCP connection and two RTT for an HTTP request.

False

The main two types of HTTP mesages:

request, response

HTTP GET/response interaction is ____________

stateless

Web sites and client browser use ____________ to maintain some state between transactions

cookies

User configures browser to point to a ____________

web cache

What is the goal of web caches (proxy servers)?

to satisfy client request without involving origin server

Why do we use web caching?

- reduce response time for messages being sent out
- reduce internet traffic on any access link and increase the flexibility of request messages
- reduce response time for client request as well as the traffic on an institution's access link
- none of the others

reduce response time for client request as well as the traffic on an institution's access link