FINAL - EVERYTHING Flashcards
1
Q
what is multimedia?
A
two or more continuous media
2
Q
what are the three broad categories of audio/video services?
A
streaming stored audio/video, streaming live audio/video, interactive audio/video
3
Q
how does a client stream stored audio/video service?
A
downloads through the internet
4
Q
how are audio/video services stored?
A
files are compressed and stored on server
5
Q
what is digitization?
A
conversion of data into a stream of numbers
6
Q
what is compression?
A
a coding process that will reduce the overall number of bits needed to represent information
7
Q
what is sampling?
A
measuring analog signal at regular time intervals
8
Q
what is it called when you sample amplitude (voltage)?
A
quantization
9
Q
what is the nyquist theorum?
A
the idea that we need to sample the signal 2f times per second given the highest signal frequency ‘f’
10
Q
what is pulse code modulation?
A
converting analog audio to compressed binary from sampling, quantization and encoding
11
Q
what 3 questions do you need to decide when digitizing audio data?
A
sampling rate? how finely is the data quantized and is it uniform? file format?
12
Q
what is the difference between predictive encoding and perceptual encoding?
A
predictive: difference in samples are encoded instead of the actual sampled values (typically used for speech)
perceptual: tries to mimic how people perceive sound (mp3)
13
Q
what is the difference between predictive encoding and perceptual encoding?
A
predictive: difference in samples are encoded instead of the actual sampled values (typically used for speech)
perceptual: tries to mimic how people perceive sound (mp3)
14
Q
what is the value of DC value in jpeg compression?
A
the average pixel value multiplied by a constant
15
Q
what are AC values in jpeg compression?
A
changes in the pixel values - no change means AC = 0
16
Q
why is jpeg called lossy compression? and what phase is responsible for this?
A
because some data is irreversibly lost, happens during quantization phase
17
Q
what do you call a time ordered sequence of frames?
A
a video
18
Q
what is spatial compression?
A
using JPEG or something close to compress each frame of a video
19
Q
what is temporal compression?
A
removal of redundant frames
20
Q
MPEG divides frames into what 3 categories?
A
i-frames, p-frames and b-frames
21
Q
what is an i-frame?
A
independent frame not related to the previous frame or following frame, meant to represent an interval
22
Q
what is the point of iframes?
A
they can stop cross origin domain policy, you can also load lots of different resources in an i-frame
23
Q
what is a p-frame?
A
predicted frame - related to the previous i-frame or p-frame
24
Q
what is a b-frame?
A
bidirectional frame - relates to previous or following frame
25
does the first approach for streaming stored audio/video involve streaming? whynot?
no because the entire file must be downloaded before the client sees any of it
26
what is the problem with the 2nd approach (web server + metafile)
both the browser and media player use HTTP services which run over TCP, this is totally fine for retrieving the metafile but TCP is not designed for audio/video file retrieval
27
why is TCP bad for audio/video files?
it re transmits lost or damaged segments which fucks up your stream
28
what protocol does a media server use and why?
UDP so lost and damaged segments are not retransmit
29
what is RTSP?
an out-of-band control protocol that can control the playing of audio/server
30
what is RTSP similar to?
second connection in FTP
31
what does RTSP not do?
define compression schemes, define encapsulation for transmission, restrict transportation method (udp tcp), restrict how media player buffers
32
what does RTSP do?
allows media player to control transmission, play pause fast forward rewind, messages use a different port number than media stream
33
what is an out of band protocol?
sends control data from a different connection then main data
34
what are the similarities between streaming stored audio/video and streaming live audio/video?
both sensitive to delay, both do not accept retransmission
35
what are the differences between streaming live and stored?
stored communication is unicast and on demand, live is multicast and live
36
what is the "jitter" phenomenon?
when packets of audio/video are transmitted and have different network delays --> you start playing packet 1 and it finished before packet 2 arrives
37
what is the purpose of a timestamp in real-time audio/video?
to solve the jitter problem by showing how a packet delay is relative to the previous packet
38
why do you need a playback buffer in real time interactive audio/video?
to store the data until they are played back based on the what the timestamps dictate
39
what else is needed other than a timestamp for real time traffic?
packet numbering for ordering purposes
40
real time traffic needs the support of what due to the fact that it used multimedia?
multi casting
41
what is "mixing"?
combining data from multiple streams into one
42
why is TCP bad for real time interactive traffic?
because it re transmits lost or damaged packets
43
is UDP suitable for real time interactive traffic?
not really because it lacks the ability to timestamp and order packets or mix them
44
what is RTP for?
real time traffic
45
what does RTP provide?
timestamping, sequencing, and mixing
46
what does RTP need to be used with?
UDP
47
how does encapsulation of packets work with RTP?
first encapsulated by RTP packet and then these packets are encapsulated in a UDP segment
48
what network layer does RTP operate in?
can be transport layer OR application layer
49
how is RTP used in the transport layer?
RTP libraries are used with languages in that layer to make RTP into a sublayer of the transport layer
50
how is RTP used in the application layer?
RTP packets are sent into a UDP socket interface
51
what is included in a RTP packet header to be encapsulated by a UDP segment?
data encoding, sequence number, timestamp
52
what does it mean if the P. and X. header fields are set in an RTP packet?
P - there is padding
| X - extra extension header
53
what is the point of the CC and M field in RTP packets?
CC - how many contributors (mixing)
| M - end of data
54
how many bits is the PT header field in RTP and what is it for?
8 bits - defines payload type
55
for the following RTP header fields, how many bits and how is it set?
- sequence number
- timestamp
- ssrc
- csrc
sequence number - 16 bits, first is random then incremented by 1 each time
timestamp - 32 bits, first is random then sum of past timestamps plus clock time for first bye produced
ssrc - 32 bits, defines identity of source (random # chosen by source)
csrc - 32 bits, defines # of sources (max 15)
56
what is RTCP
control protocol for RTP
57
when is RTCP used
when networking applications multicast to multiple receivers from one or more senders
58
how are RTP and RTCP packets distinguished from each other?
distinct port numbers
59
what do RTCP packets include?
sender and receiver ports
60
what are in RTCP sender report packets and what are they used for?
timestamp, allows receiver to synchronize RTP messages --> think audio and video synchronization
61
what are RTCP receiver reception packets for and what do they do?
passive participants, informs sender and other receivers of service quality
62
what are in RTCP source description packets?
email address, name of sender and application that generates RTP stream
63
what is the point of RTCP bye message?
shut down stream
64
what does an RTCP application specific message allow?
new message type definitions --> for when packet wants to use new applications
65
what is the restriction on RTP port numbers?
they must be an even number
66
where must the UDP port be that corresponds to an RTP packet?
immediately after the RTP port
67
what are the flow characteristics?
reliability, delay, jitter and bandwidth
68
what is the consequence of low reliability?
losing a packet or ack
69
what is responsible for lost packets in multimedia network apps?
network loss (congestion), end to end delay loss (packets arrive too late)
70
for multimedia application what range of delay makes a packet effectively useless?
100ms to 5 seconds
71
what causes jitter?
differing delay time between successive packets
72
what determines if jitter is high or low?
the difference between delays --> larger difference means higher jitter
73
what is packet marking need ed for?
for routers to distinguish between different classes; and treat packets accordingly
74
what are the tradeoffs when you change the size of the chunks in a group (n) for FEC simple scheme?
increase: less bandwidth waste, higher probability that 2 or more chunks are lost, longer playout delay
decrease: more lost packets can be recovered, more bandwidth waster
75
what is the disadvantage of interleaving for packet loss?
increases latency (need to sort out all the chunks in each packet)
76
what are the 3 different types of queuing?
FIFO, priority, weighted fair
77
In FIFO scheduling what happens if the arrival rate at the queue is higher than the processing rate?
the queue will fill up and new packets discarded
78
what is the condition of starvation for Priority scheduling?
when low priority queues never have the chance to be processed
79
how does the system process packet in weighted average scheduling?
round robin fashion with higher priority queues getting more processing relative to their priority weight
80
what is traffic shaping?
mechanism to control the amount and the rate of the traffic sent to the network
81
on what scheduling queue is the leaky bucket approach implemented on?
FIFO
82
in a token bucket approach if n = 100 and the host is idle for 100 tickets how many tokens are collected?
10000
83
what is the problem with the leaky bucket approach to traffic shaping
it limits the output rate to the average of bursty traffic
84
what is the difference between an access link and a trunk link?
access links are assigned to one specific vlan whereas trunk links carry multiple vlans
85
what are some reasons for grouping devices into vlans?
service need, protocol, physical proximity
86
each switch port can be assinged to _______ vlan(s), each switch port can be assigned to _____ vlan(s)
multiple, only one
87
where are dynamically assigned Vlan ports stored?
in virtual membership policy servers
88
what is the area between switches called?
switch fabric
89
what is the area between switches called?
switch fabric
90
what is frame tagging in Vlans?
when the switch appended a Vlan ID to each frame
91
which switch appends the Vlan ID? which switch removes it?
first switch in the path, last switch in the path
92
what happens is a tag is not removed from a frame before it is sent to non-vlan-capable devices?
the device won't be able to understand the frame
93
what is cisco's proprietary protocol called for tag formatting?
inter switch link (ISL) protocol
94
what are the benefits of using switches to create Vlans over using routers to create distinct networks?
easier to administer, less expensive, higher performance (less latency)
95
what are the disadvantages of using switches to create Vlans over using routers to create distinct networks?
Vlan may be tied to a specific vendor
96
what are routes needed for despite advances in switch technology?
filter WAN traffic, route traffic between distinct networks, route packets between Vlans
97
what is trunking?
when you connect two switches together
98
when is trunking useful?
when trying to configure Vlans that span multiple switches
99
what is the vendor for inter switch link?
cisco
100
what trunking protocol is IEEEE standard?
802.1Q
101
what do switches use to dynamically configure trunk ports?
dynamical trunking protocol
102
what is VTP?
virtual trunking protocol
103
what is the purpose of VTP?
to maintain Vlan configuration consistency throughout the network
104
how does VTP execute its purpose?
by synchronizing VTP configurations among switches in VTP domain
105
what are the 3 VTP modes?
server, client, transparent
106
what does a server mode VTP switch allow you to do ?
change the configuration information and broadcast it to other VTP devices
107
what does a client mode vtp switch allow you to do?
receive changes from a VTP server and pass to other VTP switches but you cannot modify the VLAN configuration
108
what does a transparent mode VTP switch allow you to do?
you don't receive configuration information from other switches, you simply pass it back to other switches, you can alos modify VLAN config but only apply the changes to local switch (not sent)
109
why does transparent mode exist?
to prevent issues when moving a switch between environment due to the fact that it may have a higher revision number then existing switches in new network
110
what is the default mode for switches?
server
111
what command allows you to change the mode of switches?
vtp mode command
112
what command allows you to view the current mode of switch?
show vtp status command
113
what are the STP (spanning tree protocol) requirements?
one root bridge / network
one root port / non root bridge
one designated port / segment
114
why are spanning trees necessary?
to solve the problems that arise when networks implement redundant paths between devices using multiple switches
115
what are the problems associated with redundant paths between segments?
broadcast storms, multiple frame transmission, MAC address database instabilitiy
116
what does the STA (spanning tree algorithm) do?
automatically discovers the network topology and creates single optimum path through network
117
what are the different types of bridges in STP
root bridge, designated bridge, backup bridge
118
what does a backup bridge do?
it listens to network traffic and build a bridge database but does not forward packets. it can also take over for root bridge if it fails
119
what special packets do bridges send?
bridge protocol data units
120
what are the 5 states a port can being in during a STA configuration process?
disabled, blocking, listening, learning, forwarding
121
what is the command for disabling a spanning tree on a selected VLAN?
#no spanning-tree vlan "number"
122
what is the command for forcing a switch to be the root of spanning tree
#spanning-tree vlan "number" root primary
123
what is the command to show spanning tree configuration formation?
#show spanning-tree
124
a computer that can act as both a server and/or client is called a?
peer
125
a p2p system with no dedicated server is known as?
a pure p2p system
126
what makes peer to peer networks more reliable then other networks?
no central point of failure
127
what does having a ton of peers do for resource management?
workload can be spread out and the peers themselves provide a ton of resources and power
128
how is system utilization maximized in p2p systems?
computer that are connected but idle can still provide their resources
129
how is a p2p network scalable?
peers can be freely added to the network
130
what assumptions are ring systems generally built on?
that machines are all on a nearby network and owned by a single organization
131
what is the best known hierarchical system on the internet?
domain name service
132
what is the NTP protocol and what is its topology?
protocol for synchronizing clocks of computer systems over networks - hierarchical
133
what is the most "pure" decentralized system used in practice?
gnutella
134
list the criteria for evaluating topologies:
manageability, information coherence, extensibility , fault tolerance, resistant to legal/public intervention, security, scalability
135
what is the common architecture for web applications?
centralized + ring
136
what is the best architecture for p2p networks?
centralized + decentralized
137
explain the napster model
members (client) downloads software package and installs locally, napster central computer maintains directories of music files of members who are currently logged in to the system, when a user requersts a file the central computer connects them with a members computer that has that file in their directory, the target file is downloaded directly between member computers bypassing central computer
138
why was napster shut down?
because it maintained a central directory of users
139
what will a gnutella network structure have at its core after a significant amount of time has passed?
a high speed computer
140
what is a freeloader in p2p networks?
someone who only downloads and doesn't share any files (ME!)
141
what is a torrent?
a collaborative file sharing process?
142
what do you call the set of all peers that take part in a torrent?
a swarm
143
what is a seed?
a peer in a swarm that has the complete content file
144
what is a leech?
a peer that has a part of the file and wants to download the rest
145
what is a tracker?
a central node that tracks the operation of the swarm
146
what are the contents of a torrent file?
file name, # of chunks (size), checksum, IP address of tracker and peers and such
147
what are a peers neighbors?
other peers whose addresses the new peer receives when it accesses the tracker
148
what does bit torrent protocol and policies around fairness try and prevent?
overloading a peer with requests from other peers
149
what peers are in the unchoked group?
the peers that the current peer has concurrently connected to and is continuously downloading/uploading too
150
what peers are in the chocked group?
the peers that he current peer is not currently connected to but may connect to in the future
151
how does bittorrent allow new peers to join which do not have a piece to share? what is this action called?
every 30 seconds the system randomly promotes a peer out of the choked group and into the unchocked group regardless of its uploading rate - optimistic unchoking
152
what is the rarest-first strategy in piece sharing between peers?
peers try to first download the pieces with fewest repeated copies among the neighbors so that they are circulated faster
153
what layer are p2p protocols constructed in?
the application layer
154
what do p2p networks always support?
some type of message-routing capability
155
what is the collection of peer connections in a p2p network called?
a p2p overlay
156
a network that is built on top of another network is called what?
an overlay network
157
what is the most common technique for structuring p2p overaly networks?
distributed hash tables
158
most DHT implementations have what address space size?
2^160
159
What is a cryptographic hash function that DHT uses and why does it use these types of hash functions?
SHA-1 because they are collision resistant
160
which method of storing objects do DHT typically use and why is it superior?
indirect method, efficiency
161
is it guaranteed that an object can be found if it exists in an unstructured overlay?
no
162
how are nodes organized in unstructured overlay networks?
in random graphs
163
what is the impact of increasing the degree of a peer in a graph
the peer will require more storage, but the diameter of the overlay will be reduced (more connection = more density = less spread out = smaller diameter)
164
how is endless message circulation prevented when flooding is used?
message identifiers and TLL values
165
what is the iterative deepening or expanding ring?
when the TTL starts small for a flooding search and is increased by a small amount if the search stops until the query returns successfully or the entire network has been queried
166
what is the benefit of random walk when compared with flooding?
greatly reduces the message overhead
167
structured overlay lay networks have routing mechanisms that are?
deterministic and guarantee the ability to locate objects in the overaly
168
what are the challenges associated with having an adaptive (peers can join and leave) structured overlay network?
distributing responsibility to nodes that are joining, redistributing responsibility of nodes that are leaving
169
what is the format of the key based routing that structured overlays use?
object id's are mapped to peer address and object request are routed to nearest peer in peer address space
170
what are P2P systems using key based routing called?
distributed object location and routing systems
171
what are three DOLR DHT protocols?
pastry, kademlia, chords
172
in pastry keys are store in the node whose ID is closest in terms of what?
number (numerical)
173
what is the common "m" value for pastry?
128
174
a new node in pastry ring needs what?
to know at least on node that should be close to it
175
if a node is not responding to probe messages what is assumed?
that is has failed or departed
176
given a node X what nodes will test it for liveliness?
the nodes in X's leaf set
177
what is the key difference between pastry and kademlia?
in pastry nodes are routed based on how numerically close nodes are to each other whereas kademlia routes nodes based on the physical distance (bitwise exclusive or) between nodes
178
what information does kademlia keep for each node?
id and routing table - no leaf set
179
what is the point of k-buckets in kademlia?
allows nodes to use alternative nodes when a node leaves or the network fails
180
what is a row in a kademlia routing table referred to as?
a k-bucket
181
what does the network security model 1 require us to do?
1. design algorithm for security transformation
2. generate secret key information (session keys)
3. develop methods to distribute secret inro
4. specify a protocol
182
what does the network security model 2 require us to do?
select gatekeeper function to identify users, implement security controls for users
183
if a protocol is not flawed then its implementations are also not flawed?
false
184
what is cryptography about?
controlling access to information
185
key generation, encryption and decryption are examples of what?
access control algorithms, computations
186
what is a cipher?
something that is used to encrypt plaintext
187
what is the result of encryption?
ciphertext
188
how can you recover plaintext from cipher text?
decrypt
189
what is used to configure a cryptosystem (cipher)
a key
190
what is the difference between a symmetric key cryptosystem and a public key cryptosystem?
symmetric - uses the same key to encrypt and decrypt
| public - uses a public key to encrypt and a private key to decrypt
191
computational complexity grows with what?
input size
192
what complexity is considered infeasible?
super polynomial time
193
what is polynomial time?
O(n)
194
what is in super polynomial time?
O(2^n), O(2^squrt(n))
195
what is kreckhoff's principle?
that crypto algorithms are not secret
196
why do we assume that crypto algorithms are not secret?
experience --> they are weak when exposed, the never remain secret, better to find weakness before hand
197
what is the general idea for IFF (identify friend or foe)?
a challenge (random number) is sent to unknown entity, if the entity can encrypt the challenge using the expected key then it was a friend if not then its a foe
198
what is the main concern of authentication on a standalone computer?
an attack on the authentication software
199
on a network what can attackers do?
passively observers message, replay messages, perform active attacks
200
what are active attacks?
inserting, deleting or changing messages
201
what is a replay attack?
when an attacker uses information that it read passively from an exchange between entities to pose as one of the entities
202
what is a nonce?
a number used once
203
why is a nonce used in the context of challenge response?
for "freshness"
204
what is a man in the middle attack?
when "trudy" knows the thing you are trying to encrypt and what it looks like when it is encrypted she can figure out the key used for encryption and its game over
205
is one way authentication with a symmetric key secure?
yes
206
is mutual authentication (challenger response) secure?
no - "alice" could be "trudy"
207
are 2 one way authentications a secure method of mutual authentication?
no - man in the middle attack
208
is symmetric key mutual authentication secure?
yes apparently
209
can you use the same key pair for encryption and signing for pubic keys?
no
210
what is perfect forward secrecy?
prevents trudy from decrypting recorded ciphertext at a later time, even if Trudy get Kab or other secrets
211
what is used to ensure perfect forward secrecy with public key authentication?
sessions key
212
what is a session key?
a key that is used and then forgotten
213
what is diffie hellman?
a key exchange algorithm to establish a shared symmetric key
214
what does the diffie hellman algorithm rely on?
the difficulty in solving the discrete logarithm problem
215
explain the deffie hellman process
alice and bob chose secret values and agree on values for the prime and generator, they take each other's values and calculate g^(value)mod(p) and send the result to each other, they then use their own secret value that same formula where g = value received from other party to get a shared secret value to be used as a symmetric key
216
how does a man in the middle diffie helman attack work?
trudy switches out the secret values passed between alice and bob with her own
217
how do you prevent MIM attack with diffie hellman?
encrypt DH exchange with symmetric key or public key and then sign DH values with private key
218
what do timestamps allow in security protocols?
a reduction in the number of messages (a nonce both sides know in advance)
219
what must you allow for when issuing timestamps?
clock skew
220
is sign and encrypt with nonce secure?
yes
221
is encrypt and sign with nonce secure?
yes
222
is sign and encrypt with timestamp secure
yes
223
is encrypt and sign with timestamp secure
NO! - but trudy must ask within clock skew
224
for mutual authentication with public key is encrypt and sign with timestamp secure?
yes
225
where is the socket layer situated?
between the application and transport layers
226
what protocol is used for most secure transaction over the internet?
SSL
227
describe the simplified SSL protocol
alice asks bob to talk securely and sends him a list of ciphers she has plus a nonce, bob responds with his certificate and selects one of alices ciphers and sends it along with his nonce, alice sends "S" that she generated along with a hash that has been encrypted with "k", this hash keeps a record of all previous message along with a literal string CLNT, bob responds with a similar hash, boom auth complete
228
how many ssl keys are there? and what are they for
6
- 2 for encryption
- 2 for integrity
- 1 IV (?)
229
why is there one key for sending and one for receiving in SSL for each key type?
it helps to prevent certain types of replay attacks
230
how is mutual authentication possible in simplified ssl protocol?
the server (bob) can send a certificate request to the client (alice) in message 2
231
is an SSL connection the same an SSL session?
no, once an SSL session is established you don't need to go through the entire protocol again to establish a new connection
232
what must both sides know in order to establish a new connection given an existing SSL session?
the session ID
233
are there any public key operations when establishing a new connection given an existing SSL session?
no
234
where isIPSec located?
at the network layer
235
what are the problems with IPSec
complex as fuck, over engineered, flawed, not very interoperable
236
what are the two parts in IPSec?
Internet key exchange and ESP/AH (encapsulating security payload, authentication header)
237
how many version of IKE phase 1 are there?
8 fucking bullshit phases
238
how many key options are there in IKE
4
239
what are the two different IKE modes?
main mode and aggressive mode
240
what is the difference between the IKE digital signature main mode and aggressive mode?
in aggressive mode there is not attempt to disguise the identities of alice or bob
241
T/F: alice and bobs identities in main mode are protected from all attackers
F : only protected from passive attackers, active attackers can still find out
242
what is the problem with symmetric key in main mode?
alices ID must be IP address
243
what happens in symmetric key aggressive mode?
not trying to hide identities means IP address doesn't need to be Alice's ID --> solves issue
244
what is the problem with public key encryption for IPSec ( in both modes)
trudy can compute valid keys and proofs
245
what is unique about the public key encryption aggressive mode
it hides Id's
246
what is plausible deniability in the context of IPSec public encryption mode?
alice and bob can deny that any conversation took place since trudy can "create" conversations
