application layer

Question 1

application layer messages

Answer 1

often split over multiple packets or may be aggregated in a packet

Question 2

application communication needs

Answer 2

vary widely on the app
must build on Transport services
examples( Web, DNS, Skype)

Question 3

OSI session/presentation layers

Answer 3

part of app layer
presentation: converts diffrent representations
session: manages task dialogs

Question 4

session concept

Answer 4

a session is a series of related network interactions in support of an application task
often informal, not explicit
examples:
- web page fetches multiple resources
- zoom call involves audio, video, chat

Question 5

presentation concept

Answer 5

apps need to identify the type of content, and encode it for transfer
- presentation functions
- examples:
Media (MIME) types, e.g., image/jpeg, identify the type of content
transfer encodings, e.g., gzip, identify the encoding of the content
application headers are often simple and readable versus packed for efficiency

Question 6

DNS

Answer 6

Domain Name System
machines on the internet identified by their IP addresses
DNS translates human readable names to IP addresses

Question 7

DNS name space

Answer 7

hierarchical structure
- top level domains controlled by Internet Corporation for Assigned Names and Numbers (ICANN)
- organisations can request second-level domains from registrars (com-cisco-eng…)
- countries have their own second-level domains
- .edu and .gov are generic domains, but mostly used by organisations in the US
- if you own a domain, you can specify arbitrary subdomains

Question 8

name servers

Answer 8

to translate a domain name to an IP address, you ask a name server

Question 9

location of name servers

Answer 9

• hosts learn about the location of name servers via DHCP
• the operating system keeps track of name servers and dynamically selects which one to use

Question 10

DHCP

Answer 10

Dynamic Host Configuration Protocol
- MAC addresses are built into NICs, but network addresses are not
- used to configure other settings such as: DNS name servers, addresses of default gateway, time servers, etc.

Question 11

recursive and iterative queries

Answer 11

local name server-recursive
others-iterative

Question 12

DNS Resource Record (RR) Types

Answer 12

1. IPv4 address (record type A)
2. IPv6 address (record type AAAA)
3. domain that accepts email (record type MX)
4. name server for this domain (record type NS)
5. alias to canonical name (record type CNAME)
6. …

Question 13

Content Delivery Networks

Answer 13

a type of caching to increase system capability
front end forwards requests and distributed load
DNS can be used for load balancing

Question 14

Metcalfe’s Law

Answer 14

• the value of a network is proportional to the square of the number of users
• (i.e., value is proportional to the number of possible connections)
• as networks get larger, there is more value on joining them, making them larger

Question 15

email message formats

Answer 15

envelope (used to get the message to the correct recipient)
header
body

Question 16

helpful email headers

Answer 16

message-id, in-reply-to, reply-to

Question 17

email protocols

Answer 17

POP3 or IMAP for user interaction with mailbox
users and Message Transfer Message Agents use SMPT to send an email from src do dst

Question 18

How does email work?

Answer 18

mail submission uses SMPT+Extensions(e.g. AUTH)
message transfer between mail servers uses SMTP
final delivery uses IMAP/POP3 or a propriety protocol

Question 19

IMAP

Answer 19

Internet message access protocol
sends commands to mail server to manipulate mailboxes
uses mostly plain text (security through TLS)
replaced POP3

Question 20

common IMAP commands

Answer 20

LOGIN: login into server
FETCH: fetch messages from a folder
CREATE/DELETE: create or delete a folder
EXPUNGE: remove messages marked for deletion

Question 21

SMTP

Answer 21

Simple mail transfer protocol
uses ASCII
you can use TELNET to talk to a mail server
basic SMTP does not support binary data
basic SMPT does not include authentication
many extensions exist to address these issues

Question 22

MIME

Answer 22

Multipurpose Internet Mail extensions
used to create messages with multiple data types (e.g., an email with attachment)
developed for email but used broader

if MIME-Version in header → check Content-Type; else → plain text

Question 23

MIME content-types

Answer 23

text: text/plain, text/html
images: image/jpeg, image/gif
video: video/mp4, video/mpeg
multipart: multipart/mixed, multipart/alternative

Question 24

MIME headers

Answer 24

• MIME-Version
• Content-Description
• Content-Id
• Content-Transfer-Encoding
• Content-Type

Question 25

sending binary data via ASCII

Answer 25

only SMTP base64 encoding converts binary data into ASCII when MIME was introduced, servers were not expecting non-ASCII data modern SMTP supports binary data

Question 26

base64 encoding

Answer 26

- used to convert binary data to and from ASCII - alphabet: [A-Za-z0-9+/] - overhead: ?? - 6 bits are translated into 1 character - = sign indicates 2 padded bits

Question 27

HTTP Request/Response

Answer 27

- HTML documents hosted by servers - clients send request for document - from server

Question 28

HTTP Protocol

Answer 28

originally a simple text-based protocol; many options added over time HTTP Request Methods: GET, POST, PUT, HEAD…

Question 29

HTTP sequential requests

Answer 29

connection setup each time

Question 30

HTTP persistent connection

Answer 30

increases performance by reducing connection setup overhead allows browsers to issue multiple requests over the same TCP connection increase performance over the same TCP connection increase performance by reducing connection setup overhead

Question 31

Head of Line Blocking (HOL)

Answer 31

each request has to wait for the previous one to complete HTTP1.1 : pipelines requests HTTP/2: out-of-order responses

Question 32

HTTP1.1 pipelined requests

Answer 32

- reduces HOL - increase performance by pipelining request (hiding latency) - performance problem: responses need to arrive in same order as requests

Question 33

HTTP/2

Answer 33

- binary instead of plaintext - easier for machines to parse - more difficult for humans to read - multiplexed streams over a single TCP connection - supports out-of-order responses - server push allows the server to send resources before the client asks for it explicitly

Question 34

HTTP/3

Answer 34

= (HTTP+QUIC) - http3 uses QUIC protocol - QUIC orders data per stream - each HTTP request can use a separate stream; within a stream, data is delivered in order; across streams no such guarantee is made - QUIC performs multiplexing, uses UDP - UDP does not enforce in-order delivery

Question 35

Web sockets

Answer 35

a socket-like interface on the application layer full-duplex connection between server and client → the application layer can contain protocols use case: increasingly complex ‘apps on the Web that need to send data continuously

Question 36

stacking

Answer 36

application layer can continue stacking protocols protocols may provide services the ones below/above do not

Question 37

new narrow waist

Answer 37

HTTP

Question 38

advantages of using HTTP over TCP directly

Answer 38

provides a set of methods provides security provides naming

Question 39

multimedia applications

Answer 39

streaming video requires compression without compression, only possible over wired fibre-optic channels compression reduced bandwidth requirement by an order of magnitude

Question 40

digital audio compression

Answer 40

large compression rates > x10 audio typically compressed before sending lossy compression achieves higher compression rates than lossless compression, but loses data lossy encoders based on how humans perceive sound

Question 41

digital image compression

Answer 41

- changes RGB to Y Cb Cr - Y is luminance - Cb Cr are chrominances - eyes are less sensitive to chrominance than to luminance - JPEG reduces size of Cb and Cr - total compression rate x2

Question 42

digital video compression

Answer 42

large compression rates > x50 MPEG compresses over a sequence of frames, further using motion tracking to remove temporal redundancy

Question 43

types of frames in compression

Answer 43

I (Intra-coded) frames are self-contained P (Predictive) looks for comparable macro blocks in previous frames -How long to search is up to the implementation B (Bidirectional) frames may base prediction on previous frames and future frames

Question 44

run-length encoding

Answer 44

part of JPEG compression lossless compression technique

Question 45

Huffman encoding

Answer 45

- prefix code: no code word is prefix of other code word - less than half of the original size - part of JPEG compression -from a tree by select two smallest nodes, and combining them into a new node, until only the root is left

Question 46

challenges in streaming stored media

Answer 46

How to handle transmission errors? - use reliable transport (e.g., TCP) → increase jitter significantly - use forward error correction (error correction in the application layer) → increase jitter, decoding complexity, and overhead - interleave media → slightly increase jitter and decoding complexity Masking errors by interleaving media - low-water mark prevents stalls in playback - high-water mark gives time to prevent running out of buffer space

Question 47

challenges in streaming live media

Answer 47

stored + can’t stream faster than live rate to get ahead → usually need larger buffer to absorb jitter - often have many users viewing at the same time → UDP with multicast greatly improves efficiency; it is rarely available, so **many TCP connections are used**

Question 48

challenges in streaming interactive media

Answer 48

real-time conferencing has two or more connected live media streams, e.g., voice over IP, Skype video call requires low jitter and low latency - benefits from network support (Quality of Service) - large bandwidth (no congestion) difficult to provide across long distances/multiple networks