Final

Question 1

Put the steps in the most correct order for closing TCP Connection.

Answer 1

1. Client Sends segment with FIN bit set; 2. Server sends ACK of received segment; 3. Server sends segment with FIN bit set; 4. Client sends ACK of received segment.

Question 2

How might authentication work with public key encryption?

Answer 2

Sender encrypts a signature with a registered private key and distributes public key. If public key decrypts signature, we know the sender is who they say they are

Question 3

Use the RSA algorithm discussed in lecture to develop a public key and a private key for public-key encryption. Let p = 5, q = 11, e = 7, m is the original message, c is the encrypted message.

f. Kprivate(Kpublic(m)) = ?

Answer 3

m

Question 4

Use the RSA algorithm discussed in lecture to develop a public key and a private key for public-key encryption. Let p = 5, q = 11, e = 7, m is the original message, c is the encrypted message.

e. Kprivate(c) = ?

Answer 4

(c^d) % n

Question 5

Use the RSA algorithm discussed in lecture to develop a public key and a private key for public-key encryption. Let p = 5, q = 11, e = 7, m is the original message, c is the encrypted message.

d. c = Kpublic(m) = ?

Answer 5

(m^e) % n

Question 6

Use the RSA algorithm discussed in lecture to develop a public key and a private key for public-key encryption. Let p = 5, q = 11, e = 7, m is the original message, c is the encrypted message.

c) d = ?

Answer 6

23

There are several possibilities. Choose d so that ed-1 is exactly divisible by z. If we choose d = 23, ed-1 = 7 x 23 - 1 = 160, which is divisible by 40.

Question 7

Use the RSA algorithm discussed in lecture to develop a public key and a private key for public-key encryption. Let p = 5, q = 11, e = 7, m is the original message, c is the encrypted message.

b) z = ?

Answer 7

(p-1)(q-1) = 4 x 10 = 40

Question 8

Use the RSA algorithm discussed in lecture to develop a public key and a private key for public-key encryption. Let p = 5, q = 11, e = 7, m is the original message, c is the encrypted message.

a) n = ?

Answer 8

pq = 5 x 11 = 55

Question 9

What are some of the major components of networking security?

Answer 9

Confidentiality: Intruders should not be able to understand the contents of a message.

Integrity: Intruders should not be able to change the contents of a message, without the end users being aware of it.

Authentication: End users should be able to verify they are actually speaking to whom they think they are speaking to. Availability: Services should be accessible, and not interrupted by attacks (resilience to DDoS, etc…)

Question 10

What are some considerations which might be made before instituting a security policy at a company?

Answer 10

Cost vs. Benefit - How much are we willing to spend to achieve a certain level of security?

Will we secure stored information and transmitted information, or just one or the other?

How will we educate our users so that our policy is not breached from the inside? Each computer is attached to a shared medium, with a terminator on each end to absorb signal and prevent reflections. A ring topology does not have a terminator. Rather, its “ends” are connected to each other to form a ring

Question 11

What are some of the performance issues involved with mobility?

Answer 11

Data loss due to transmission medium and hand-offs - interpreted as congestion.

Additional overhead

Overall lower throughput

Question 12

In ____________ routing, the correspondent contacts the home agent to get the mobile device’s care-of address, and sends packets to the visited network

Answer 12

Direct

Question 13

DevRTT formula

Answer 13

(1 - β) ⋅ DevRTT + β ⋅ | SampleRtt - EstimatedRtt |

Question 14

What is network congestion? What causes it?

Answer 14

In the simplest terms, network congestion is the increase in end-to-end delay due to high bandwidth utilization at some point in the network. It is caused by end systems sending data faster than the network is capable of handling (those darned routers!).

Question 15

What are some consequences of a congested network?

Answer 15

Packets are dropped, delayed, or routed through a non-optimal path. Because of this, sending hosts will attempt to retransmit (if using TCP), which causes an increase in network congestion. If this were to continue, there would be a congestion collapse (essentially a network super-slow-down).

Question 16

Given a nodal delay of 3 ms when there is no traffic on the network (i.e., when usage = 0%).

What is the effective delay when network usage is 25%? 75%? 99%?

Answer 16

(Nodal Delay / (1-usage) = 3ms/(1-.25) = 4ms
(Nodal Delay / (1-usage) = 3ms/(1-.75) = 12ms
(Nodal Delay / (1-usage) = 3ms/(1-.99) = 300ms

Question 17

What is the goal of congestion control? What, in general, is used to do this?

Answer 17

To optimizing network utilization, such that a high throughput is ensured, with the restriction that utilization is not pushed so high that delay and packet loss become major factors. This is accomplished by attempting to detect and avoid congestion, and if congestion is un-avoidable, the sender should reduce their data output.

Question 18

How is TCP’s timeout interval set?

Answer 18

TimeoutInterval = EstimatedRTT + (4 * DevRTT)

EstimatedRTT is an exponential weighted moving average based on recent and past sampled round trip time values.

EstimatedRTT(n) = (1 - α)EstimatedRTT(n-1) + (α)SampleRTT(new)

DevRTT is a factor which increases when the most recent sampled round trip time is different from the most recent estimated round trip time. It is another exponential weighted moving average, strongly weighted toward the most recent deviation value.

Question 19

What are the two major approaches to network congestion control?

Answer 19

Network core assistance: The network core senses when congestion is an issue - directly - and sends messages (either to the destination host, which bounces it back to the source host, or directly back to the source host) indicating congestion in the core, and possibly advising on a course of action.

Host inference: A source host attempts to infer congestion in the network core based on observables (e.g. round trip time, dropped packets)

Question 20

In basic terms, how is congestion control implemented in TCP? What is this method called?

Answer 20

When no congestion is perceived, the sliding window size gradually increases (additive increase).

When there is perceived congestion, the sliding window size is drastically reduced (multiplicative decrease).

Together this makes up an additive-increase multiplicative-decrease (AIMD) scheme.

Question 21

What is an MSS?

Answer 21

Maximum Segment Size. This is the maximum amount of transport-layer data which can be sent, such that it will fit within one link-layer data frame. This ranges from the 500’s for a dial-up modem to around 1460 for cable internet.

Question 22

Describe TCP’s “Slow Start” mode. What is its purpose?

Answer 22

The purpose of TCP Slow Start is that it allows for a very slow start, but a rapid increase, in the size of the congestion window. By doubling the CongWin every time the transmitted packets ACK within the timeout interval, there is an exponential increase in the rate of transmission (until a threshold is reached).

Question 23

Describe TCP’s “Fast Retransmit” technique. What is its purpose? What affect does this have on congestion control?

Answer 23

TCP’s fast retransmit (from RDT) is intended to more quickly infer the loss of a packet, and retransmit it, to increase data throughput. If a host receives three duplicate (so four total) ACKs “requesting” the same segment number, it assumes the segment starting with that sequence number was dropped, and retransmits the segment, even if the timer for that segment has not expired.

In terms of congestion control, this assumed loss is perceived as congestion, but given less weight than a full timeout. The CongWin is cut in half, rather than set to 1 MSS (as in a timeout).

Question 24

Describe the steps of the TCP connection setup.

Answer 24

1) SYN: Client host sends a segment to the Server host with the SYN bit set, the ACK bit cleared, and some pre-generated sequence number.

2) SYN ACK: Server host sends a segment to the Client host with the SYN and ACK bits set, a locally generated sequence number, and an ACK number equal to the first segment’s sequence number plus one.

3) ACK: Client host sends a segment to the Server host with the ACK bit set, the SYN bit cleared, and the ACK number is the previous segment’s sequence number plus one. Note that this segment may contain actual application data in the payload.

Question 25

Describe the steps of the TCP connection takedown

Answer 25

1) FIN: Client host sends a segment to the Server host with the FIN bit set, the ACK bit cleared, and its next sequence number. 2) FIN ACK: Server host sends a segment to the Client host with the ACK bit set and Fin bit cleared. The ACK number is the next expected sequence number. (This is a normal TCP ACK segment). 3) FIN: Server host sends a segment to the Client host with the FIN bit set, the ACK bit cleared, and its next sequence number. 4) FIN ACK: Client host sends a segment to the Server host with the ACK bit set and the FIN bit cleared, and the ACK number is the previous segment's sequence number plus one. (This is a normal TCP ACK segment)

Question 26

What is fairness in TCP?

Answer 26

The goal is to share bandwidth evenly amongst all connections through a router. TCP works toward fairness due entirely to its congestion window size algorithm. This is on a per-connection basis, so for example a web browser which spins off a connection for every image it has to download (in parallel) would not be "fair" to a single-connection FTP transfer.

Question 27

Is the UDP protocol fair?

Answer 27

No, UDP does not utilize any fairness-allowing schemes.

Question 28

What does TCP do better than UDP?

Answer 28

While there are quite a number of answers to this question, the basics are that TCP's reliable data transfer is the most endearing aspect it has to an end user, and congestion control for the network core.

Question 29

What does UDP do better than TCP?

Answer 29

Throughput

Question 30

Where do Network-layer protocols run?

Answer 30

Everywhere in the internet! At the edge (host systems) and in the core (routers).

Question 31

What is the purpose of the network layer?

Answer 31

At a very high level, the purpose is to ensure logical communication from host to host. In a more nuts-and-bolts view, the purpose is to determine a path from source to destination that a datagram can take, and to move it on that path (within the network layer)

Question 32

What is the difference between routing and forwarding?

Answer 32

Routing is the process of determining the path a datagram will take from source to destination in the internet, whereas forwarding is the process within a router of moving a datagram from an input port to the appropriate output port, so that it may take the next step in its journey from source to destination.

Question 33

What is a "first hop" router?

Answer 33

The "first hop" router is the first router a datagram is passed to from the source host on the path to the destination host

Question 34

What is a virtual-circuit network? What are some benefits of this type of network?

Answer 34

A connection-oriented network layer implies a virtual-circuit network. This type of network has a call setup at the beginning of a host-to-host connection, and from that point the state of the connection is preserved in all routers from source to destination, until the call is taken down. Some primary benefits of VC networks are guaranteed bandwidth and timing (jitter), which makes them ideal for streaming audio/video.

Question 35

What is a datagram network? What are some benefits of this type of network?

Answer 35

A connectionless network layer implies a datagram network. The internet is a datagram network. The advantages are similar to those of UDP - there is FAR less overhead. Each network need not preserve the state for every host-to-host communication passing through it. With the billions of devices on the internet, this would be terribly implausible regardless of the rapid development of storage space and access times.

Question 36

How does a router know which output port each one of the billions of IP addresses are located at?

Answer 36

The forwarding table in a datagram network's router matches ranges of addresses to output ports, rather than matching each address to its own output port. By doing this, it saves tremendously on time and required storage/access capability.

Question 37

What does a routing algorithm do? Where is the output of a routing algorithm stored?

Answer 37

Finds a path from a router to destination, and selecting it appropriately. The result of the algorithm is used to construct the router forwarding table (or routing table), which is stored in the router.

Question 38

What is switching fabric in terms of networking? How is it controlled?

Answer 38

The hardware switching circuitry used, within routers, to switch datagrams from an input port to an output port (or queue) without any collisions occurring. This is controlled by the routing processor, which utilizes the stored routing table. The IP address of incoming datagrams are used to determine the output port.

Question 39

Is it possible to have a queueing delay at the output port of a router? How? What about packet loss?

Answer 39

Yes, due to transmission rate limitations on the output port link: if several datagrams were switched to the same output port, they will have to wait for access to the transmission medium. Packet loss can also occur, if the buffer overflows.

Question 40

Is it possible to have a queueing delay at the input port of a router? How? What about packet loss?

Answer 40

Yes, due to head-of-line blocking or output port contention: if the datagram at the front of the line in the input port queue cannot be transferred to the output port because there is already a transfer occurring to, or a full queue at, the desired port. This would cause a delay in transferring the HOL datagram to its output port. Packet loss can occur here as well, if the input buffer overflows

Question 41

A key aspect of a virtual network is ___________________________.

Answer 41

a single, uniform address format

Question 42

What reliability services does the IP network-layer protocol implement?

Answer 42

Header Checksum! But no real reliability services.

Question 43

What are the minimum and maximum sizes of the IP header, in bytes?

Answer 43

Minimum: No options, Length = 5. 5x4 = 20 bytes. Maximum: Max options, Length = 15. 15x4 = 60 bytes.

Question 44

Theoretically, what is the maximum number of bytes of "payload" (actual application data) that can be transmitted in an IP datagram carrying a TCP segment?

Answer 44

65495 bytes. The "length" field of the IP datagram header is 16 bits, so the maximum datagram size is 65,535 bytes. The TCP header and the IP datagram header are each at least 20 bytes. 65535 - 40 = 65495

Question 45

Why can't we use hardware addresses for our network address?

Answer 45

IP addresses are hierarchically organized, allowing for streamlined routing throughout the internet. If we used hardware addresses, we would not be able to use longest-prefix matching. Indeed, some routers would have to keep lookup tables for every single hardware address connected to the internet. To make it worse, some hardware types use different addressing schemes than others.

Question 46

Convert the following 32-bit binary number to a dotted-decimal IP address format. 10000000 11000001 00000100 01110000

Answer 46

128.193.4.112

Question 47

First 20 bits of an IPv4 address represent what?

Answer 47

Network Address

Question 48

Last 12 bits of an IPv4 address represent what?

Answer 48

Host number

Question 49

Network address + host number = ?

Answer 49

Host Address

Question 50

Bits 21-24 of an IPv4 address are for:

Answer 50

ISP subnets

Question 51

How many hosts can be supported in /28 subnet?

Answer 51

2^4 - 2 = 14 (32 - 28) = 4 bits available 2^4 = total numbers available Minus 2 because all 0's and all 1's (broadcast) is restricted

Question 52

What is a subnet?

Answer 52

Most generally, a subnet is a part of a network which shares a common address prefix component. More specifically, it is often a discrete portion of a network, separated by a router from all other portions of the network.

Question 53

What are the two addresses in a subnet which are reserved, and what are they reserved for?

Answer 53

The network address cannot be assigned to a host. To find the network address, set the host part of the IP address to all zeros. (mask it with the netmask) The broadcast address is also reserved. To find the broadcast address, set the host part of the IP address to all ones.

Question 54

What is a next-hop router?

Answer 54

Once a prefix match is made within a routing table, a packet is forwarded to its appropriate output link. The router at the other end of that link is the next-hop router.

Question 55

What is DHCP?

Answer 55

Dynamic Host Configuration Protocol: - application layer protocol, DHCP client-server - dynamically "lease" IP address from a server when joining the network - IP address can be reused by other hosts if released - IP lease can be renewed while still connected

Question 56

What is the purpose of DHCP?

Answer 56

DHCP is intended to streamline the connection of new devices to a network. Rather than having a sysadmin hard-code a relationship between hardware (MAC) addresses and IP addresses, a new device can enter a network and automatically obtain an IP address valid in the network.

Question 57

Give the steps undergone when a new device enters a DHCP-enabled network. (very long)

Answer 57

1. It broadcasts a "DHCP discover" via UDP to port 67, sending the message to IP address 255.255.255.255 with a 'source IP address' of 0.0.0.0. It passes this datagram to the Link Layer which will broadcast to all nodes on the subnet. 2. A DHCP-enabled server will pick up this message and send back a "DHCP offer" message on the broadcast IP address (255.255.255.255). This message will contain the proposed IP address, lease time, network mask, etc. NOTE: The link-layer frame is addressed to the MAC address of the new device. 3. The new device sends a "DHCP request" message, echoing the configuration parameters, again to the IP broadcast address from source 0.0.0.0, and again to the MAC broadcast address, requesting an IP address. 4. The DHCP-enabled server, in response, sends a "DHCP acknowledgement" message, confirming the parameters, to the Broadcast IP address (and the device's hardware address). It also records the relationship between this IP address, and the device's hardware address, in a table.

Question 58

Why does an arriving DHCP client have to broadcast its hardware address?

Answer 58

An arriving DHCP client has no IP address. By broadcasting its hardware address to the entire network, it is received by the DHCP server. The DHCP server can then offer an IP address lease to the client, by sending to its (unique) hardware address

Question 59

Is IP address space exhaustion a problem?

Answer 59

This depends on your outlook. There are some mechanisms that are proving a stopgap, but really we've already run out.

Question 60

A ____________ is used to find a datagram's path through a network

Answer 60

routing algorithm

Question 61

In a network graph... * "Nodes" represent ________________.

Answer 61

Routers

Question 62

In a network graph... * Edges represent ________________________.

Answer 62

direct connections between routers

Question 63

In a network graph... * Weights represent ________________________.

Answer 63

costs (speed, traffic, $$$, distance, etc...)

Question 64

In a network graph... A "shortest path" from node A to node G is ________________________.

Answer 64

the set of edges to traverse from A to G with the smallest sum of edge weights

Question 65

Once the routing algorithm is complete, what is stored in the routing table?

Answer 65

A series of IP prefixes, each matched to a single next-hop router.

Question 66

What is the purpose of IP fragmentation?

Answer 66

It is possible for a datagram to encounter a link-layer technology which is incapable of encapsulating the entirety of the datagram into the link-layer frame, due to sizing constraints. If this happens, the datagram must be subdivided such that it will fit - this is fragmentation - or else be dropped from the network.

Question 67

What is an MTU? What is a path MTU?

Answer 67

An MTU is the maximum transmission unit, in terms of bytes of data, for a specific piece of networking hardware. A path MTU is the minimum MTU on the entire path from source to destination. If a datagram is smaller than the path MTU, there will be no fragmentation.

Question 68

Is the header of the original IP datagram included in the payload of fragmented datagrams?

Answer 68

No. Each fragmented IP datagram has its own header (which is almost identical to the original IP datagram header), but the original header is not included in the payload.

Question 69

If it is a TCP segment which has been fragmented (with header length = 20 bytes) where does the TCP header go?

Answer 69

The TCP header was originally the first 20 bytes of the un-fragmented IP datagram. IP doesn't care about the TCP header, and just sees it as IP-layer payload. As a result, it becomes the first 20 bytes of the first fragmented IP datagram. It does not, however, reappear in subsequent datagram fragments.

Question 70

How does the ID field of the IP datagram change from Fragment #1 to Fragment #N?

Answer 70

It doesn't. All fragments of the same original datagram have the same ID field value.

Question 71

Where does reassembly of fragmented datagrams take place?

Answer 71

Destination Host

Question 72

What happens if an IP datagram is fragmented into N datagrams, but the destination only receives the first 1 ... N-1 fragmented datagrams?

Answer 72

When the fragment timer expires, the destination router drops all of the fragmented datagrams.

Question 73

Can a fragmented IP datagram be re-fragmented?

Answer 73

Yes. This occurs if the fragment encounters a link with an even smaller MTU.

Question 74

ICMP allows information to be carried between what types of devices? (e.g. router to...)?

Answer 74

Router to Router Router to Source Host Source Host to Destination Host Destination Host to Source Host

Question 75

Where is the ICMP carried?

Answer 75

The ICMP message is in the payload of the IP datagram. The format of the information in the payload depends on the type of ICMP message being sent.

Question 76

If an IPv4 datagram had to traverse 15 routers to reach its destination host, and I set the IPv4 header TTL field to 10, what would happen?

Answer 76

The datagram would be dropped by the 10th router in the path, and that router would send an ICMP Type 11 (Time Exceeded), Code 0 (TTL expired in transit) message back to the sending host.

Question 77

Give common usages of ICMP.

Answer 77

* PING requests use ICMP messages with echo requested. * Traceroute (tracert) uses a series of ICMP messages, each with an incrementing TTL, to trace the path from source to destination. * Destination Network Unreachable is extremely common if your internet goes down somewhere. * Destination Port Unreachable is common if a web server goes down. * Internet Core Routing Algorithms use ICMP to establish paths and active router listings.

Question 78

What is the motivation for Network Address Translation (NAT)?

Answer 78

There are not enough IPv4 addresses to go around. NAT allows a sub-network of computers to use the same "global" IP address, but different "local" addresses (behind the NAT device).

Question 79

What are some advantages NAT?

Answer 79

* Multiple computers on the same IP address * Local addresses are inaccessible from the global network without any prior information. (good for privacy). * You can change ISPs without changing inner network structure.

Question 80

What are some disadvantages of NAT?

Answer 80

* NAT translation adds some minor delay. * Local addresses are inaccessible from the global network without prior information. (bad for hosting servers) * NAT breaks some networking conventions, such as ports being used to specify processes (in NAT they also specify hosts). They also allow routers to modify data above layer 3.

Question 81

Why does NAT have to translate port numbers?

Answer 81

Multiple devices in the local network might be attempting to access the same external address/port. These are necessary to keep the sockets straight.

Question 82

The computers in your network have addresses of the form 10.0.0.x/8, and your network uses a NAPT device as an internet gateway. The NAPT box has external address 192.205.11.1, and internal address 10.0.0.1 . The next available port number on the NAPT device is 36409. Suppose that the original sender at computer 10.0.0.25 uses port 23000 to send a query to a remote host at 209.53.77.5 on port 80.

Answer 82

When message from original sender arrives at NAPT: Sender: 10.0.0.25:23000 Destination: 209.53.77.5:80 When message forwarded from NAPT to remote host: Sender: 192.205.11.1:36409 Destination: 209.53.77.5:80 When response from remote host arrives at NAPT: Sender: 209.53.77.5:80 Destination: 192.205.11.1:36409 NAPT to Original Sender: Sender: 209.53.77.5:80 Destination: 10.0.0.25:23000

Question 83

What does a NAPT device change in an outgoing TCP/IP datagram?

Answer 83

Source host:port and checksum

Question 84

What does a NAPT device change in an incoming TCP/IP datagram?

Answer 84

Destination host:port and checksum

Question 85

What are some methods by which an outside host can contact a server running behind a NAPT device?

Answer 85

* You can setup the NAPT device to forward from a specific port number to a specific host inside the network. You can then advertise this address:port combination outside the network to devices which want to reach the internal server (UPnP). * Have the NAPT device forward ALL packets for which there isn't a current mapping to the same server (Static NAT Traversal). * Have the internal host connect to an outside relay, and the remote host also connect to the relay. The relay then will establish the connection between the end hosts (NAT Traversal relaying). * The NAPT device provides DNS services for hosts within its network. These devices can then be accessed by name (Twice NAT - very similar to UPnP).

Question 86

What is the difference between "wireless "and "mobility" in terms of networking?

Answer 86

Wireless generally refers to a link-layer protocol which enables a physical layer to be transmitted without a wired link (over the air). Mobility, on the other hand, refers to the management of a host which moves amongst a number of networks (which may not be related, hierarchically).

Question 87

Describe a scenario that is wired, but still mobile.

Answer 87

A laptop which moves between a home Ethernet network to a work Ethernet network.

Question 88

Describe a scenario that is wireless, but not mobile.

Answer 88

A home PC which is connected over Wifi, but which never moves from this network

Question 89

What multiple access control scheme does 802.11g Wifi use?

Answer 89

CSMA/CA

Question 90

What are some differences between wired and wireless links?

Answer 90

Undirected Media (broadcast in a sphere or cone, rather than on a wire) This leads to much faster signal degradation (think flashlight vs. laser) Multipath propagation (which also helps with coverage, actually) Interference (much worse than unshielded cables - more like bus network, but with all manner of communications on the bus).

Question 91

Describe the RTS/CTS channel reservation scheme.

Answer 91

A connected host transmits a broadcasts a request-to-send message into the channel. The AP receives this (and possibly several other RTS's) and makes a decision to grant exclusive access to the channel to one host. They broadcast a CTS indicating how long this one host will have access to the channel. All connected hosts receive this and halt their "random access" attempts to send data into the channel for the allotted period of time.

Question 92

Why is "letting the routers handle it" not feasible for mobility?

Answer 92

This would require routers to keep track of all mobile devices attached to the internet, resulting in massive lookup tables and very slow processing.

Question 93

In ____________ routing, the correspondent sends all packets to the home network, which then redirects them to the mobile device's visited network.

Answer 93

Indirect

Question 94

With regard to wireless mobility, please describe each of the following terms. a. Home Network: b. Home Agent: c. Permanent Address: d. Visited Network: e. Foreign Agent: f. Care-of Address g. Correspondent

Answer 94

a. The registered "home network" in which the home agent resides. b. The entity in the home network which performs mobility functions for the device c. Address in home network, which will always correspond to the device d. Network which mobile device is currently in (assuming it is not in the home network) e. Entity in the visited network which performs mobility functions for the device f. Address in the visited network which is registered to the mobile device g. Entity attempting to communicate with the mobile device

Question 95

When using RTS/CTS, are collisions still possible?

Answer 95

Yes. For example, the RTS packet can collide with other RTS or random access packets. Or, during the CTS window, another user can attempt to join the network, causing a collision.

Question 96

What are some issues with using non-printable ASCII as frame delimiters?

Answer 96

You may send data that looks like an EOT, but is actually usable data. If the receiver sees an EOT it assumes this is the end of the frame - so if you see this before the end of the frame, it would effectively destroy the frame.

Question 97

Describe perimeter security.

Answer 97

This entails isolating a network from the outside world by filtering incoming and/or outgoing packets. Packets not meeting certain qualifications or requirements will be blocked by the policy (firewall)

Question 98

What is message encryption? (High-level is OK)

Answer 98

Altering the contents of a message so that it is difficult (or impossible) to ascertain the original message, unless you are the intended recipient, by means of cryptography

Question 99

How would encryption work with public key encryption?

Answer 99

Sender encrypts with recipient's registered public key. Recipient decrypts with their own private key.

Question 100

Given a router with 5 input ports and 5 output ports. If the switching fabric is 5 times as fast as the input/output line speed, queueing _______ occur at an output port and/or input port (same for both!)

Answer 100

can

Question 101

The Internet Protocol (IP) implements timing controls and/or congestion control.

Answer 101

False

Question 102

What can cause packet queueing at a router's output port? (Check all that apply)

Answer 102

- Multiple data flows requiring the same outbound link. - Slow outbound link transmission rate

Question 103

The rate of CongWin size increase (in terms of MSS) while in TCP's Congestion Avoidance phase is _____.

Answer 103

Linear

Question 104

Select the proper equation for TCP's calculation of DevRTT.

Answer 104

DevRTTn = (1-B) * DevRTTn-1 + B * |SampleRTTnew - EstimatedRTTn-1|

Question 105

The process of moving a datagram from a router's input port to output port is handled by the _____.

Answer 105

Switching Fabric

Question 106

In a datagram network, the responsibilities of the network layer include: (check all that apply).

Answer 106

packet routing, host-to-host comms, packet forwarding

Question 107

Which of the following are benefits of a virtual circuit network? (Check all that apply)

Answer 107

Guaranteed Timing, Guaranteed Bandwidth, Connection states are preserved

Question 108

The transport-layer header is encapsulated in the first fragmented IP datagram.

Answer 108

True

Question 109

It is the responsibility of a routing algorithm to determine the cost of an output link.

Answer 109

False

Question 110

The "traceroute" application (on Windows) receives ICMP messages.

Answer 110

True

Question 111

The path MTU is the smallest MTU on a path from sender to receiver.

Answer 111

True

Question 112

In a fragmented IP datagram, the "offset" IP header field value is exactly equal to the number of bytes of fragmented data preceding this fragment.

Answer 112

False - equal to sets of 8 bytes

Question 113

In a prefix-matching network, a routing table stores... (Check all that apply)

Answer 113

Next Hop link info, IP Prefixes

Question 114

It is the responsibility of a routing algorithm to correlate MAC addresses with IP addresses.

Answer 114

False - responsibility of ARP

Question 115

The "time to live" field in a modern IPv4 datagram header specifies...

Answer 115

the number of remaining hops before the datagram is dropped.

Question 116

IP datagrams fragments can not be fragmented again.

Answer 116

False - can if a smaller MTU occurs

Question 117

The "traceroute" application (on Windows) sends UDP messages by default.

Answer 117

False

Question 118

When a destination host's IP fragment timer expires, it drops all accumulated fragments corresponding to that timer.

Answer 118

True

Question 119

The "ping" application (on Windows) uses ICMP echo request/reply.

Answer 119

True

Question 120

Network address translation alters IP to add new IP addresses.

Answer 120

False

Question 121

NAPT devices translate IP address and port numbers.

Answer 121

True

Question 122

In IPv6, there is no datagram fragmentation performed in the network core..

Answer 122

True

Question 123

Select all features explicitly available in IPv6 which were already available explicitly in IPv4.

Answer 123

Source/Dest Addressing Version Hop Limit Traffic Type

Question 124

Select all features explicit in IPv6 which are NOT explicitly available in IPv4. (Check all that apply)

Answer 124

Explicit Payload Length Extension Headers Flow Labeling 128-bit Addresses

Question 125

When encountering an IPv4-only router, an IPv6 datagram is dropped.

Answer 125

False

Question 126

The transition from IPv4 to IPv6 requires that ____________. (Check all that apply)

Answer 126

IPv4 routers still in use must “tunnel” IPv6 datagrams, by fragmenting/encapsulating them in IPv4 datagrams

Question 127

In Random Access multiple access schemes, no two nodes will ever transmit at the same time.

Answer 127

False

Question 128

To retrieve an adjacent node's MAC address, ______ is used.

Answer 128

ARP

Question 129

Which of the following are used in a wired Ethernet network? (Check all that apply)

Answer 129

Exponential back-off/retry for collision resolution Carrier Sense Multi-Access (CSMA) Collision Detection (CD)

Question 130

An address table with MAC Addresses and IP Addresses would be maintained by a host, router, or switch by..

Answer 130

ARP

Question 131

Select all Random Access schemes below.

Answer 131

CSMA, ALOHA

Question 132

A multiple access scheme which listens to the channel to make sure it is empty, prior to transmitting, is called...

Answer 132

carrier sense protocol

Question 133

A MAC address was originally designed to be permanent and unique.

Answer 133

True

Question 134

A link-layer link between more than two adjacent nodes is called a/an ________.

Answer 134

broadcast link

Question 135

When sending a message to all devices on a link, you would send it to the broadcast MAC address: 00-00-00-00-00-00

Answer 135

False - all Fs

Question 136

A network with a bus topology must terminate the endpoints, but in with a ring topology they are connected so there is no endpoint.

Answer 136

Answer over there

Question 137

A link-layer link between only two adjacent nodes is called a/an point to point link.

Answer 137

point to point

Question 138

On the sending or receiving host, most of the protocol tasks "below" the application layer of the protocol stack (data encapsulation, IP addressing, etc. ) are handled by

Answer 138

Network Interface Card

Question 139

It is fairly easy to detect collisions in wired networks.

Answer 139

True

Question 140

Which are functions of the Ethernet preamble? (Check all that apply)

Answer 140

Clock synchronization Clock wakeup Start signal

Question 141

A switch is a link-layer device.

Answer 141

True

Question 142

The IPv6 address size is 120 bits.

Answer 142

False - 128 bits (16 bytes)

Question 143

IPv6 datagrams cannot be converted to IPv4 datagrams without losing any information.

Answer 143

True

Question 144

1234::a03:abcd is a valid preferred-format IPv6 address.

Answer 144

True

Question 145

In a CSMA/CD system, when a collision is detected, ...

Answer 145

the sender will cut off transmission and wait some time before retransmitting.

Question 146

The link-layer device at the center of an ethernet star is a ______________.

Answer 146

Switch

Question 147

If an Ethernet sender senses a clear channel, and begins transmission, but shortly thereafter detects a collision, it will...

Answer 147

Terminate transmission and enter exponential backoff.

Question 148

For a 10Mbps link, 1000 bit times is 0.1ms.

Answer 148

True - 1000/10,000,000 = .0001s = .1ms

Question 149

In one type of wireless network, hosts communicate directly with other hosts that are within range. This communication model forms a "grid" called a(n)

Answer 149

ad-hoc network

Question 150

The default multiple access scheme of 802.11g is RTS/CTS.

Answer 150

False - CSMA/CA

Question 151

When a mobile unit moves from a home or foreign agent to another (foreign) agent, the new agent must assign.... (Check all that apply)

Answer 151

a new “care-of” address to the mobile unit

Question 152

A device which is connected to the network through a link which does not utilize any physical connection is a _________ device.

Answer 152

Wireless

Question 153

Which of the following are major issues that must be handled in wireless networks (i.e., issues that are more significant than in wired networks). Check all that apply.

Answer 153

Hosts may frequently move from one network to another Obstacles that block radio signals Radio waves are more susceptible to interference than signals carried on cable Multi-path propagation when radio signals bounce off obstacles The "hidden node" problem

Question 154

When an organization establishes a network security policy, which of the following should be considered?

Answer 154

the cost of damage control after various types of security breaches the cost of installing "secure" systems the value of the information that is stored or transmitted by the site

Question 155

S represents a source host and D represents a destination host. Which of the following is the most typical use of public key encryption, when S sends an encrypted message to D?

Answer 155

S encrypts a message using D's public key, and D decrypts the message using D's private key.

Question 156

S represents a source host and D represents a destination host. Which of the following is the most typical use of public key encryption, when S sends an authenticated (digitally signed) message to D?

Answer 156

S encrypts a signature using S's private key, and D decrypts the signature using S's public key.