Data Communications and Networking > The Application Layer > Flashcards

The Application Layer Flashcards

(102 cards)

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1
Q

What does the application layer do?

A

“Defines message types

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2
Q

What distinguishes a client from a server?

A

“Client initiates communication; server waits to be contacted”

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3
Q

What is the main property of client-server architecture?

A

“Server is always on and typically has a static IP address”

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4
Q

What is a disadvantage of client-server?

A

“Scaling requires adding more servers”

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5
Q

What is the defining property of P2P architecture?

A

“Peers act as both clients and servers”

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6
Q

Why do P2P systems self-scale?

A

“New peers add both load and capacity”

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7
Q

Why do P2P systems need peer discovery?

A

“Peers join/leave dynamically and change IP addresses”

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8
Q

What do open protocols allow?

A

“Interoperability and multiple independent implementations”

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9
Q

What is a proprietary protocol?

A

“A protocol controlled by a single organisation (e.g.

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10
Q
A
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11
Q

What model does HTTP use?

A

“Client-server model”

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12
Q

What transport protocol does HTTP use?

A

“TCP”

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13
Q

What ports does HTTP(S) use?

A

“Port 80 for HTTP and 443 for HTTPS”

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14
Q

What does stateless mean in HTTP?

A

“Server keeps no session information between requests”

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15
Q

Why is statelessness beneficial?

A

“Simpler design and recovery if client/server crashes”

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16
Q

What is a URL?

A

“protocol://hostname/path”

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17
Q

What does https:// mean?

A

“HTTP carried over TLS”

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18
Q

What are the three parts of an HTTP request line?

A

“Method, Request Target, HTTP version”

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19
Q

What separates HTTP headers from the body?

A

“A blank line (CRLF CRLF)”

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20
Q

What are the two major HTTP request methods?

A

“GET and POST”

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21
Q

What does GET do?

A

“Sends form data in the request URL”

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22
Q

What does POST do?

A

“Sends form data in the message body”

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23
Q
A
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24
Q

What is included in an HTTP response line?

A

“HTTP version, Status Code, Reason Phrase”

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25
What class of status codes indicate success?
"2xx"
26
What class indicates redirect?
"3xx"
27
What class indicates client error?
"4xx"
28
What class indicates server error?
"5xx"
29
What does HTTP 404 mean?
"Not Found"
30
What does HTTP 200 mean?
"Request succeeded"
31
32
What is the response time for non-persistent HTTP?
"2 RTTs + file transmission time"
33
What is persistent HTTP?
"Connection stays open for multiple requests"
34
What advantage does persistent HTTP give?
"Saves one RTT per object"
35
What is pipelining in HTTP?
"Sending multiple requests without waiting for previous responses"
36
37
Why did HTTP beat Gopher?
"Open standard
38
39
What is FTP used for?
"Transfer of files between hosts"
40
What control port does FTP use?
"TCP port 21"
41
What does FTP use for data transfer?
"A separate TCP data connection"
42
What is the problem with FTP's active mode?
"Server opens data connection to client — broken by NAT"
43
What is passive FTP?
"Client opens both control and data connections (PASV command)"
44
What are typical FTP commands?
"USER, PASS QUIT, SYST"
45
What does RETR do?
"Download a file"
46
What does STOR do?
"Upload a file"
47
48
What problem does DNS solve?
"Mapping human-readable names to IP addresses"
49
What did ARPANET use before DNS?
"hosts.txt file from a central server"
50
What type of system is DNS?
"Distributed
51
Why not a single DNS server?
"Single point of failure
52
What are DNS domains?
"Nested hierarchical names (e.g.
53
54
What do root DNS servers do?
"Provide referrals to TLD servers"
55
How many root server operators exist?
"12 operators with ~1400 anycast servers globally"
56
What do TLD servers do?
"Handle domains like .com
57
What do authoritative DNS servers do?
"Provide hostname-to-IP mappings for an organisation"
58
59
What is the local DNS resolver?
"ISP-configured DNS server used by clients"
60
What two key functions does the local resolver perform?
"Recursive query forwarding and caching"
61
What is caching controlled by?
"TTL values in DNS responses"
62
What happens if DNS records change before TTL expires?
"Clients still receive old data until TTL expires"
63
64
What is an iterated DNS query?
"Each server returns the next server to contact until the authoritative server is reached"
65
66
What is an A record?
"Hostname → IPv4 address"
67
What is AAAA (not in slides but may appear)?
"Hostname → IPv6 address"
68
What is an NS record?
"Domain → authoritative DNS server"
69
What is a CNAME record?
"Alias name → canonical hostname"
70
What is an MX record?
"Domain → mail exchange server"
71
72
What protocol does email submission use?
"SMTP"
73
What port is used for modern message submission?
"587"
74
What entities relay email between domains?
"Mail Transfer Agents (MTAs)"
75
What record does SMTP use to find destination mail servers?
"DNS MX record"
76
What command begins sending the message body?
"DATA"
77
What terminates the DATA body?
"A single dot (.) on its own line"
78
What is the main weakness of SMTP authentication?
"Sender can claim to be anyone"
79
80
What protocols retrieve email?
"IMAP and POP3"
81
What is the key difference between IMAP and POP3?
"POP downloads+deletes by default; IMAP stores emails on server"
82
What benefit does IMAP give?
"Remote folder management and multi-device access"
83
84
What is SNMP used for?
"Network management: monitoring devices
85
What are managed devices?
"Routers
86
What does SNMP use to store management data?
"MIB (Management Information Base)"
87
What defines the structure of MIB objects?
"SMI (Structure of Management Information)"
88
What transport does SNMP use?
"UDP or TCP"
89
90
What is the ISO object identifier tree used for?
"Naming every SNMP-managed object uniquely"
91
What two modes does SNMP operate in?
"Request/response and trap mode"
92
What are traps?
"Unsolicited notifications from agents to managers"
93
94
What is the weakness of SNMPv1/v2 security?
"Passwords (community strings) sent in cleartext"
95
What new modes does SNMPv3 add?
"noAuth
96
Why are early SNMP versions insecure?
"Cleartext authentication and weak cryptography"
97
98
What does TLS provide?
"Privacy
99
At what layer does TLS operate?
"Application layer (via libraries like OpenSSL)"
100
What attacks does TLS protect against?
"Eavesdropping
101
What does HTTPS mean?
"HTTP carried over TLS"
102
What other protocols commonly use TLS?
"FTPS
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