Chapter 2/Lecture 2 Application Layer

Question 1

Which ports and protocols are used by HTTP(S), FTP, DHCP, DNS and ICMP?

Answer 1

HTTP(S):80(443) TCP, FTP:20(data), 21(control) TCP, DHCP: 67(server)/68(client) UDP, DNS:53 UDP, ICMP: None None(Layer 3 protocol, not Layer 4).

Question 2

What does an app-layer protocol define?

Answer 2

Type of message(request/response/etc), message-syntax(fields and how they are delineated), semantics(what does information mean) and rules for when/how send/recieve.

Question 3

What’s the api from L5 to L4?

Answer 3

Sockets.

Question 4

What does UDP offer?

Answer 4

Lower overhead because of unreliable data transfer(no setup).

Question 5

What does TCP offer?

Answer 5

Reliable transport(sequence number and acks), flow control(sliding window, receive window telling how many bytes), congestion control(slow-start, congestion avoidance(?), fast retransmit), connection-oriented(3-way).

Question 6

What’s the difference between stateless and statefull?

Answer 6

Stateless; server maintains no information about past requests. If either server or client crashes, no biggie. Example: HTTP
Statefull; server maintains information about past requests. If server crashes, must rebuild state. Example: FTP

Question 7

What’s the difference between non-persistent and persistent HTTP?

Answer 7

Non-persistent sends at most one object, then shuts down the connection. Persistent leaves the connection ope, so that several objects can be sent over one connection.

Question 8

What does RTT stand for, and what is the definition?

Answer 8

Round Trip Time, time for a small packet to travel from client to server and back(or ack back).

Question 9

What are the different method types, and how’s the HTTP message request/response defined?

Answer 9

Methods; GET/POST/HEAD(1.0), PUT/DELETE(1.1)
Message:
"method" "url" "version" or "version" "status_code" "status_phrase"\r\n
 "header" "value"\r\n
....
"header" "value"\r\n
\r\n
body

Question 10

What are the three major components in e-mail?

Answer 10

User agent, mail server and SMTP.

Question 11

Explain the SMTP protocol(port, phases, what does it do etc).

Answer 11

SMTP goes over TCP on port 25. It’s a protocol for delivery of message from sender to receiver. It has three phases, handshake, transfer and closure. Commands are ASCII text, and response is a status code and phrase.

Question 12

Which protocols are used for retrieval?

Answer 12

POP(Post Office Protocol), IMAP(Internet Message Access Protocol) and HTTP

Question 13

What’s the different between POP and IMAP?

Answer 13

POP is stateless(mails are downloaded to the client, but no status change is recorded on server. Download and delete(on server) or keep(on server)), while IMAP is statefull(mails are changed on server, so changes are synced between user agent(s) and server).

Question 14

What does DNS stand for, and how is it organized?

Answer 14

Domain name system, distributed database to avoid single-point-of-failure(due to traffic volume, load distribution etc). Organized with root servers at the top, followed by TLD(top level domains) and authoritative DNS server(organizations own). Also has local DNS servers, which is operated by company, university, residential isp etc. Local DNS does not belong in the hierarchy.

Question 15

How does DNS (queries) work?

Answer 15

Local DNS(default name server) stores a cache of recent name-to-address translations. Cache entries has a timeout(TTL), after which the record disappears, but entry may still be out of date because new records takes time to spread(best-effort). If lookup misses, local dns queries hierarchy. Either via recursive query(if dns miss, asked server asks higher up) or iterative query(server responds with new server to contact if dns mss).

Question 16

Explain a DNS record.

Answer 16

Resource records is on the following format: RR(name, value, type, ttl)
Type A/AAA => name is hostname, value is IPv4/IPv6 address
Type CNAME => name is alias for some other hostname
Type NS => name is domain, value is authoritative server
Type MX => name is host, value is mailserver

Question 17

Explain the DNS protocol

Answer 17

Query and reply has same format.
Possible flags: (query or reply) | (recursion desired) | (recursion available) | (reply is authoritative)
"identification"(2) "flags"(2)
"# questions" "# answer RRs"(4)
"# authority RRs" "# additional RRs"(4)
"questions(variable # of questions)"(4)
"answer(variable # of RRs)"(4)
"authority(variable # of RRs)"(4)
"additional info(variable # of RRs"(4)

Question 18

What does DHCP stand for, and what does it do?

Answer 18

Dynamic Host Configuration Protocol. It allows hosts to dynamically obtain IP address. It also (usually) provides first-hop router(default gateway), name/ip address of DNS server and network mask. The addresses are “rented”, which means a host can renew its address and an address can be reused by other hosts.

Question 19

Which port does DHCP go over, and what are the messages?

Answer 19

Port 68 on client, port 67 on server, UDP for both. Messages are the following(the first two are optional, and broadcast address is FFx6 on link layer);

• DHCP Discover - src 0.0.0.0, dest 255.255.255.255
• DHCP Offer - src DHCP-server IP, dest 255.255.255.255
• DHCP Request - src 0.0.0.0, dest 255.255.255.255
• DHCP Ack - src DHCP-server IP, dest 255.255.255.255

Question 20

How’s the message format for DHCP?

Answer 20

"op"(1) "htype"(1) "hlen"(1) "hops"(1)
"xid"(4)
"secs"(2) "flags"(2)
"ciaddr"(4)
"yiaddr"(4)
"siaddr"(4)
"giaddr"(4)
"chaddr"(16)
"sname"(64)
"file"(128)
"options"(312)