SE3314 Final Flashcards
1
Q
What are the 3 categories of a/v services?
A
Streaming stored, streaming live, interactive
2
Q
What is digitization?
A
The conversion of analog signal into a stream of numbers
3
Q
What is pulse code modulation?
A
The process of converting from analog audio to compressed binary form sampling, quantization, and encoding
4
Q
What is compression?
A
the process of coding that will effectively reduce the total number of bits needed to represent certain information
5
Q
What is quantization?
A
Sampling in the amplitude/voltage dimension
6
Q
What is the Nyquist theorem?
A
If the highest frequency of a signal is F, sampling frequency should be 2F times per second
7
Q
How is data rate calculated?
A
sampling frequency * quantization rate (generally 8 or 16 bits/sample)
8
Q
How many bits are required for a pixel of a color image?
A
24 (8 for black and white)
9
Q
What are the 2 methods of audio compression?
A
- predictive encoding
- perceptual encoding
10
Q
How large are the blocks in greyscale JPEG compression?
A
8x8 pixels (64)
11
Q
What are the steps for JPEG compressoin?
A
- DCT
- Quantization
- Compression
12
Q
What is the DC value in a discrete cosine transform?
A
The value in cell (0,0) of the T table
13
Q
What is the process of JPEG quantization?
A
Values in the T table are divided by a constant, and the fraction is dropped
14
Q
Why is JPEG considered lossy?
A
Because of quantization, which is not completely reversible
15
Q
What direction is the table read during JPEG compression?
A
Diagonally in a zigzag from the top left
16
Q
How many 1’s did I give Ouda on his eval for this course
A
All of them
17
Q
What is the penalty for showing up late to the exam?
A
-4% for every 35 seconds that you’re late
18
Q
What are the types of video compression?
A
- spatial compression
- temporal compression
19
Q
What is spatial compression?
A
Compress each frame individually (ie using JPEG)
20
Q
What are the categories of frames in a temporal compression?
A
- I frame (independent)
- P frame (predicated
- B frame (bidirectional)
21
Q
What is an I-frame?
A
- not related to any other frame
- must appear periodically at regular intervale
22
Q
Why do we need I-frames
A
They serve as a starting point for further encoding
(if you have a better answer let me know)-Dylan
23
Q
What is a B-frame?
A
Related to preceding AND following I/P frames
24
Q
What is a P-frame?
A
- Related to preceding I/P frame
- changes cannot cover big segment
- carry much less info than other frames
25
What are the 4 methods for streaming stored/live AV?
1. Web server
2. Web server w/ metafile
3. Media server
4. Media server & RTSP
26
Describe how the web server approach works
- Client uses GET to download text file
- Client uses help application (media player) to play
- No streaming, file must be downloaded completely
27
Describe how the web server + metafile approach works
- media player is directly connected to web server
- server stores AV file and metafile (has info about AV file)
- browser and media player both use HTTP (TCP)
PROBLEM: TCP is not appropriate for streaming
28
Why is TCP not appropriate for streaming AV?
Because of its retransmission procedure
29
Describe how the media server approach works
- client uses GET for metafile, passes to media player
| - media player uses URL in metafile to download AV file from media server (using any protocol using UDP)
30
What does RTSP do?
Real-time streaming protocol:
- allows player to control transmission (pause, rewind)
- messages sent out-of-band
- different port (544) than the stream, can use UDP or TCP
31
Describe how media server & RTSP approach works
- Setup like media server approach
- media players send SETUP to connect, then PLAY to begin download
- can send other messages to pause, rewind, etc.
- sends TEARDOWN to break connection
32
What causes jitter?
Introduced by delay between packets
33
When is time relationship preserved?
When packets have equal delay
34
What are some solution to jitter?
- timestamp
| - playback buffer + playback delay
35
What is multicasting?
When data is distributed to several destination (like online radio broadcast)
36
What is mixing in the context of interactive AV?
Combining several streams into 1 stream (like conference call)
37
Why is UDP not suitable for interactive AV?
Does not provide timestamping, sequencing or mixing
38
How does RTP work?
- data encapsulated in an RTP packet
| - RTP packet encapsulated in a UDP segment
39
What are the 2 approaches to RTP?
1. RTP in transport layer (use existing libraries, RTP is a sublayer of transport layer)
2. RTP in application layer: application creates RTP packets, sends into a UDP socket interface
40
What are the RTP header fields?
- payload type (type of encoding)
- sequence no
- timestamp
- synchronization source identifier (SSRC defines source)
- contributing source identifier (if more than 1 source, the SSRC is the mixer, remaining sources are contributors)
- padding, extension, contributor count
41
What was Napster?
Like uber, but for naps
42
What is perceptual encoding?
Uses psychoacoustics and limitations in our auditory system to compress audio (used for music)
43
What is predictive encoding?
Audio compression based on differences between samples (used for speech)
44
What message is sent to the media server to end the connection?
TEARDOWN
45
In what situation is RTCP used?
(Real time control protocol): used with RTP for multicasting
46
What are the different types of RTCP control packets?
- sender report
- receiver report
- source description message
- bye message
- application-specific message
47
What is a receiver report?
An RTCP control packet sent by passive participants, informs others of quality of service
48
What is included in a source description message?
Email and name of sender, the app that generates the RTP stream
49
What is a sender report?
An RTCP control packet sent by sender, includes transmission/reception stats, wall clock timestamp (used for synchronization)
50
What port(s) is selected for RTP?
Any even numbered UDP port
51
What port(s) is selected for RTSP?
Uses a temporary port, must be one higher than RTP (odd number)
52
What are the different flow characteristics?
- reliability
- delay
- jitter
- bandwidth
53
Define delay (as a flow characteristic)
total time from packet sent to received
54
Define jitter (in the context of quality of service)
Variation in delay for packets in the same flow
55
Define reliability (in the context of quality of service)
Amount of lost packets (due to network loss, end-end delay loss)
56
What is bandwidth?
Number of bits / second
57
What is the goal of implementing flow classes?
Partition traffic on the same link into groups, giving some groups higher priority
58
What are the 3 techniques to improve quality of service?
- recover from packet loss
- scheduling
- traffic shaping
59
What are the two schemes for forward error correction?
- add redundant chunks
| - send redundant lower quality stream
60
What are the two methods for recovery from packet loss
- forward error correction (FEC)
| - interleaving
61
How many chunk can be lost using redundant chunks so that the original block can be reconstructed?
At most 1 chunk
62
What are the cons of adding a redundant chunk?
Increased bandwidth by 1/n
63
How would you determine the size of n when implementing the redundant chunks scheme for FEC?
Tradeoff:
- Large n: less bandwidth waste
- Small n : lower probability of packet loss, shorter playout delay
64
What are the advantages of interleaving?
- If packet lost, still have most of every chunk
| - no redundancy overhead
65
What is the disadvantage of interleaving?
- increases latency (via playout delay)
66
What are the different scheduling approaches?
- FIFO queuing
- Priority queuing
- Weighted fair queuing
67
Can overflow occur using a FIFO queuing approach?
YES, if arrival rate> processing rate
68
How many queues are there in a priority queuing system?
One for each prioirty class
69
What is the drawback of priority queiung?
Starvation: a continuous flow of high-priority packets means low priority queue will never be served
70
How does weighted fair queuing work?
- queues weighted on priority
- process # of packets based on priority (ie if priority is 3, process 3 packets from queue)
- switch queues in round robin fashion
71
What are the different approaches to traffic shaping?
- leaky bucket
- token bucket
- hybrid
72
What is the goal of traffic shaping?
- control amount and rate of traffic to a network
| - network knows what to expect, determine when to send, monitor flow of traffic
73
Describe leaky bucket
Remove packets from FIFO queue at constant rate
74
What is the advantage of leaky bucket?
No jitter
75
What are the disadvantages of leaky bucket?
- May drop packets if queue overflows
| - Host is not compensated for idle time
76
Describe token bucket
- host accumulates n token every clock tick when not transmitting
- host pays 1 token for every byte transmitted
77
What is the advantage of token bucket?
Allows bursty traffic at regulated maximum rate (max rate = rate of tokens earned )
78
How would you combine leaky and token buckets? Why would you?
Place leaky bucket after token bucket. Rate of leaky bucket must be > rate of tokens dropped.
Why: handles bursty traffic but processes at constant rate
79
What is a VLAN?
Virtual LAN: broadcast domains defined by switch port rather than network address
80
T/F: each switch port can be assigned to multiple VLANs
F: each switch port can be assigned to only 1 VLAN at a time
81
T/F: each switch can be connected to multiple VLANs
T
82
T/F: workstations connected to the same switch but in different VLANs can communicate with each other
F
83
What do you call the area between switches?
switch fabric
84
As a frame moves from switch to switch within the switch fabric, what must each switch be able to identify?
The destination VLAN
85
How is the destination VLAN for a frame determined?
The VLAN ID of destination is appended to the frame by 1st switch in path, removed by last switch - this is called frame tagging/coloring
86
What is the disadvantage of VLANs?
Might be tied to a specific vendor
87
What are the advantages of VLANs?
```
Switches are:
- cheaper
- easier to administrate
- higher performance
than routers
```
88
If switches are so dandy, why do we still need routers?
- filter WAN traffic
- route traffic between seperate networks
- route packets between VLANs
89
What is Trunking?
connecting 2 switches together - required for VLANs spanning multiple switches
90
What do trunk ports identify?
Which ports are connected to other switches
91
T/F: Trunk ports are automatically members of the most recently created VLAN on the switch.
F: Trunk ports are automatically members of ALL VLANs on the switch.
92
What is DTP?
Dynamic Trunking Protocol
| - used by switches to detect trunk ports and select trunking protocol to use
93
What is VTP?
Virtual Trunking Protocol
| - maintains configuration consistency throughout network
94
What are the 3 VTP modes?
- server
- client
- transparent
95
Which VTP mode is the default?
server
96
Describe VTP client mode
- receives changes from server, passes to other switches
| - cannot modify configuration
97
Describe VTP server mode
- modifies VLAN configuration
98
Describe VTP transparent mode
- passes VTP info to other switches
| - can modify configuration, but applies only to local switch
99
What problems arise due to redundant paths?
Bridging loops:
- broadcast storms
- multiple frame transmission
- MAC address database instability
100
What is spanning tree protocol?
One bridge for each route is assigned as route bridge, making it the only one that can forward packets
101
What are the benefits of STP?
- eliminate bridging loops
- provide redundant paths
- enables dynamic role configuration
- recovers automatically from topology change or device failure
- identifies optimal path between 2 devices
102
What are the roles in STP?
- Root bridge: selects routes & configures roles. 1 per network
- Designated bridge - forwards packets, 1 per segment
- Backup bridge - won't forward packets
103
What are BPDUs?
Bridge Protocol Data Units (type of packet)
| - determine roles, verify neighbors functioning, recover from topology changes
104
When are BPDUs sent?
- On startup, sent out each port (to define roles)
| - then periodically (to ensure connectivity and discover topology changes)
105
What are the 5 port states in STP?
1. Disabled
2. Blocking
3. Listening
4. Learning
5. Forwarding
106
What is the default port state in STP?
Blocking
107
When would a port enter the listening state?
- only temporarily, transitioning from blocking to learning
| - On topology change, all bridges go to listening
108
T/F: a port can be in both the listening and learning states
F: it can be in both FORWARDING and learning states
109
What port state are backup bridges in?
Blocking
110
What does a blocking port do?
Receives and processes only packets sent to all bridges
111
What does a learning port do?
- receiving packets and building bridge database (associating MAC addresses with ports)
- when timer expires, goes to Forwarding
112
What are the disadvantages of P2P networks?
- peer more susceptible to hackers
- hard to enforce standards
- cannot guarantee particular resource
- difficult to prevent illegal downloads
- enormous network traffic
113
What are the advantages of P2P networks?
- distributed workload
- maximize system utilization
- no central point of failure
- scalability
114
Describe the centralized topology
Client server pattern, directly connected 1-1
115
What are the pros/cons of centralized topology?
- Manageable, coherent, secure
| - NOT extensible, fault tolerant, lawsuit-proof
116
Describe the Ring topology
- communication between nodes coordinates state-sharing (fail-over and load balancing)
- works when machines are nearby on network, owned by single organization
117
What are the pros/cons of ring topology?
- manageable, coherent, fault tolerant, secure, scalable
| - NOT extensible (only ring owner can add), lawsuit-proof
118
What is the Network Time Protocol?
Protocol for synchronizing the clocks of computers over networks
119
What are the pros/cons of hierarchical topology?
- very scalable
- NOT lawsuit-proof
- 1/2 on everything else?
120
Describe the decentralized topology
- all peers communicate symmetrically, have equal roles
121
What are the pros/cons of decentralized topology?
- extensible, fault-tolerant, lawsuit-proof
| - NOT manageable, coherent, secure, scalable (in practice)
122
Describe the centralized + ring topology
- server is a ring, for clients it appears centralized
| - common for web apps
123
What are the pros/cons of centralized + ring topology?
- manageable, coherent, fault tolerant, secure, scalable
| - NOT extensible or lawsuit-proof
124
Describe the centralized + decentralized topology
Peers have a centralized relationship to a "supernode", which has decentralized relationship to other supernodes
125
What are the pros/cons of centralized + decentralized topology?
Extensible, fault tolerant, lawsuit-proof
| NOT manageable, secure
126
Which topologies are lawsuit proof?
- decentralized
| - centralized + decentralized
127
Which topology is most scalable?
Hierarchical
128
Which topology is best for P2P networks?
centralized + decentralized
129
What are the challenges of P2P applications?
- peer identification
- routing protocols
- network topology
- peer discovery
- protocols
- quality of service
- security
130
What was the Napster P2P model?
-central directory connected 2 members who could share music files directly with each other
131
How do new P2P systems avoid being shut down like Napster was?
They have no central directory or even server, nothing to shut down
132
What is the Gnutella model?
Hop between peers until file is found, fastest computers at the core
133
What was the motivation behind BitTorrent?
large proportion of freeloaders
134
How does the BitTorrent model work?
each peer participating in the torrent downloads chunks from a peer that has it and uploads chunks to others that don't have them
135
What is a swarm?
Set of all peers in a torrent
136
What is a seed?
A peer w/ complete content of a file
137
What is a leech?
A peer w/ only part of a file
138
T/F: BitTorrent uses DHT
T
139
What is contained in a torrent file?
- file name
- # chunks, size
- checksum
- IP address of tracker
140
T/F: A peer cannot leave the torrent until every peer in the swarm has all the pieces
F
141
What are the different flags in BitTorrent?
- choked / unchoked (as in, "you're gonna choke on this exam")
- interested / uninterested (as in, "I am uninterested in the stupid shit")
142
What is the choked group in BitTorrent?
List of neighbors that the peer is not currently connected, but may be in the future
143
What is optimistic unchoking?
Every 30 seconds, a random unchoked but interested neighbor is promoted to choked, to allow newly joined peer to receive pieces
144
What is the rarest-first strategy in BitTorrent?
peer downloads pieces w/ fewest copies among neighbors
145
At what layer are P2P protocols constructed?
Application layer
146
What is a P2P overlay?
A collection of peer connections in P2P network
147
What are the common features of P2P applications?
- constructed at app layer
- peers have unique ID
- supports message-routing
148
Describe unstructured P2P networks
- Nodes linked randomly
| - search is inefficient, query may not be solves
149
Describe structured P2P networks
- use predefined rules to link nodes (such as DHT)
| - effective searches
150
What is the address space size in DHT?
2^m (m usually 160)
151
What is DHT?
Distributed Hash Table
152
How is a node ID calculated in DHT?
node ID = hash(Peer IP)
153
How is a key calculated in DHT?
key = hash(Object name)
154
Describe the two strategies for storing objects in DHT
1. Direct
- object stored in node whose ID is closest (defined by proximity metric) to key
2. Indirect
- peers that owns object keeps it
- reference to object stored in node whose ID is closest to key
155
Which strategy for storing objects do most DHT systems use?
Indirect
156
What is an unstructured overlay technology?
Nodes organized into random graphs with no constraints
| - cannot find rare items efficiently, no guarantee that object will be found
157
What are the different search methods in an unstructured overlay topology?
- flood
- expanding ring
- random walk
158
Describe the flood search approach
- Send query to all neighbors, who then query all their neighbors
- to prevent endless circulation: message ID or TTL
159
Describe the expanding ring search approach
same as flood, but TTL starts small and grows if query not satisfied
160
Describe the random walk search approach
- query sent to a random neighbor, TTL decrements with every hop
- can issue several queries in parallel
161
What is a structured overlay topology?
Deterministic, provides guarantee to locate any object. Particular nodes assigned to hold particular content
162
What are the challenges in building a structured overlay topology?
- Distributed: want to distribute responsibility
| - Adaptive: distribute to joining nodes, redistribute from leaving nodes
163
What is key-based routing?
- object ID is mapped to peer ID address space
| - object request is routed to nearest peer
164
Which overlay typology supports key-based routing?
Structured
165
What do you call a P2P system using key-based routing?
Distributed Object Location and Routing System (DOLR) - e.g. DHT
166
Pastry, Kademlia and Chord are examples of what?
Key-based routing protocols
167
What hashing algorithm is used by Pastry?
SHA-1
168
What is the size of m in Patry?
128
169
What is the format for an identifier in patry?
n-digit string where n = m / b in base 2 ^ b
-m = 128, b = 4 (normally)
170
How large is a pastry routing table?
n rows * 2^b collumns
171
What does the row heading in a Pastry routing table represent?
The length of the common prefix that a node shares with N
172
What does the column heading in a Pastry routing table represent?
The value of the first digit in a node after the common prefix with N
173
What happens when multiple nodes could fit into the same cell of a Pastry routing table?
Choose the node closest to N (based on proximity metric)
174
What is the proximity metric for Pastry?
It is application specific: could be number of hops, RTT, etc.
175
How large is the leaf set in Pastry?
One row, 2^b cells
176
What is contained in a Pastry leaf set?
- left half contains closest nodes with smaller IDs than N
| - right half contains closest nodes with larger IDs than N
177
Describe how a lookup is executed in Pastry
1. Check if key is in range of leaf set
- if yes, ask that node
2. Else, look in routing table, ask closest node
- look for Table[p,v] where p is length of shared prefix, v is value in key after shared prefix
178
For a pastry Join operation, what must be assume about the joining node X and known node N0?
Assume that X and N0 have no common prefix
179
Describe the steps in a Pastry join
1. N0 gives row 0 to X
2. N0 calls lookup w/ X's ID as key, forwards join to N1 whose ID is closest to key
3. N1 gives row 1 to X
4, Continue until routing table complete
5. Last node gives leaf table to X
180
What happens when a node detects a node in its leaf set is unresponsive?
- if leaf node unresponsive, assume failed/left
| - contact node in leaf set with highest ID, repair itself with that node's leaf set
181
What happens when a node detects a node in its routing table is unresponsive?
Send message to node in same row and request the ID in the corresponding cell of that node
182
How does Kademlia route messages between nodes?
Based on distance between two identifiers (XOR)
183
How are nodes and data items identified in Kademlia?
With an m-bit ID, in a binary tree with 2^m leaves (m=160)
184
T/F: A leaf set in Kademlia is the same size as 1 row of the routing table
F: there is no leaf set in Kademlia
185
How is the binary tree divided in Kademlia?
- each node divides tree in m subtrees
| - subtree i includes nodes sharing i leftmost bits
186
How large is the routing table in Kademlia?
m rows, 1 column (select closest node in each subtree)
187
Describe how a Lookup is executed in Kademlia
- Ask node in row i, where i = common prefix N+key
| - forward and repeat until found
188
What is the concept of K-buckets?
- Each row in routing table can have up to 20 nodes
- rows referred to as k-buckets
- allows for alternate nodes
189
How does a new node join in Kademlia?
- node must know another
- send ID to known node, as though it is key to be found
- use response to build routing table
190
What are the two models for network secutiry?
Model 1: transfer over insecure channel
| Model 2: control access to info on a computer system
191
What is encryption?
Combining a message and key to obtain ciphertext
192
What is decryption?
Retrieving a message from a ciphertext using a key
193
What is a cipher/cryptosystem
Encrypts plaintext --> ciphertext
194
What is cryptography?
All about controlling access to information
195
How many versions of IKE phase one are there?
8
196
What is a public key cipher?
Uses a public key to encrypt, a private key to decrypt
197
What are the two parts of IPSec
IKE (Internet Key Exchange)
| ESP/AH (Encapsulating Security Protocol/ Authentication Header
198
What is Kerckhoff's principle?
Assume the system is completely know to attacker
199
T of F: IPSec implementation requires you to change the OS and Apps
False
200
T or F: IPSec is more complex than SSL
True
201
What is required to set up a new connection with an existing session in SSL
Session ID and Key K which is based on S,Ra,Rb
202
What are the SSL keys:
6 keys:
2 for encryption
2 for integrity:
2 for IV
203
What is a session key?
A symmetric key that is only used for one session then discarded
204
T or F: in IPSec public key signature aggressive, a passive attacker cannot know the IDs of Alice and Bob
False, there is no ID hiding in aggressive for public key signature
205
What does SSL stand for?
Secure Socket Layer
206
T or F: TCP based auth is secure
FALSE
207
How can you prevent replay attacks in simple authentication protocol?
- use challenge-response with a nonce
- alice then hashes her password with nonce
Problem: bob must know Alice's password
208
Is symmetric key authentication secure for one-way authentication? What about mutual?
One-way: yes
| Mutual: yes, but must add IDs to encrypted message (otherwise vulnerable to MiM)
209
How can you prevent MiM attack with Diffie-Hellman?
- encrypt DH exchange with symmetric key (ephemeral diffie-hellman)
- encrypt DH exchange with public key
- sign DH values with private key
210
What is a session key?
A symmetric key that is only used for one session then discarded
211
T or F: SSL implementation requires you to change the OS
False
212
T of F: IPSec implementation requires you to change the OS and Apps
False
213
T or F: IPSec is more complex than SSL
True
214
What is required to set up a new connection with an existing session in SSL
Session ID and Key K which is based on S,Ra,Rb
215
What are the SSL keys:
6 keys:
2 for encryption
2 for integrity:
2 for IV
216
Why does Alice always authenticate Bob, and not vice versa, in SSL
Bob has a certificate, and we can't always expect the client to have a verified certificate.
Instead, we can challenge them to present a password
217
Why is Mig in the middle prevented in SSL
The SSL certificate must be verified by a third party
218
What does SSL stand for?
Secure Socket Layer
219
T or F: TCP based auth is secure
FALSE
220
Describe the features of each variation of public key auth:
- encrypt only
- sign only
- sign and encrypt
- encrypt and sign
- Encrypt only: protects key, no mutual auth
- Sign only: mutual auth, doesn't protect key
- Sign and encrypt: secure and mutual auth
- Encrypt and sign: attacker records ciphertext and later hacks Alice or Bob to find key. Then can decrypt message.
221
What is PFS?
Perfect forward security: cannot later decrypt recorded ciphertext
- must use session key, then forget it
222
How can a sender and receiver agree on a session key?
Using a key exchange algorithm, like Diffie Hellman
223
What is the security of DH based on?
The difficulty of discrete logarithm problem
224
What are the steps in the DH process?
-Alice selects secret a
-Bob selects secret b
-Alice sends g^a mod p
-Bob sends g ^b mod p
-Both compute symmetric key
For alice: (response from bob)^a mod p
225
How can you prevent MiM attack with Diffie-Hellman?
- encrypt DH exchange with symmetric key
- encrypt DH exchange with public key
- sign DH values with private key
226
What are the steps for SSL
- Alice Ask to talk, sends Ra and Cipher list
- Bob sends back Rb, certificate, and chosen Cipher
- Using Ra,Rb and pre master secret s, Alice computes K. Alice sends back S, encrypted with Bobs public key, and an a message encrypted with S containing K
- Bob uses S to decrypt the last message and get K
227
What is clock skew? What issue does it create?
Creates risk of replay, if intruder can act within clock skew - can be solved in K sent only once
228
Is signing, then encrypting a message which uses a timestamp secure?
Yes
229
How do you secure a timestamp based authentication message which is first encrypted, then signed?
Ensure Bob does not send K back to Alive
