cse 3461 quiz 1

lectures 1-7 inclusive

computer

common operating machine purposely used for technological and educational research

networking

computers connected together

internet

collection of computer networks

3 parts of infrastructure

1. end devices (aka end hosts)

2. communication links

3. packet switches

end devices (end hosts)

1. devices from which communication data originates/terminates

2. pc, servers, smartphones

communication link

1. carry data between 2 devices/switches

2. fiber, copper wire, radio

packet switches

1. routers & switches

protocols

define format, order of messages sent & received among network entities, and actions taken on message transmission, receipt

→ a set of rules

3 parts of network infrastructure

1. network edge

2. access networks

3. network core

network edge

→ comprises of end hosts

ie: clients & servers

access networks

→ local wired or wireless communication networks connecting end hosts to the Internet

ie: home WiFi, mobile network

→ network that physically connects end hosts to 1st router

network core

→ global network connecting access networks w/ other access networks

→ includes networks owned by diff Internet service providers (ISP)

ie: ATT, Sprint

3 different types of access networks

1. residential

2. institutional

3. mobile

what are the access network technologies?

digital subscriber line (DSL), cable, Ethernet, wireless LANs, cellular

digital subscriber line (DSL)

→ use existing telephone line to central office DSLAM

• data over DSL phone line goes to Internet

• voice over DSL phone line goes to telephone net

wireless access networks

→ shared wireless access network connects end system to router

• via base station, “access point”

bit

propagates between transmitter/receiver pairs

physical link

what lies between transmitter & receiver

guided vs unguided media

guided: signals propagate in solid media (copper, fiber)

unguided: signals propagate freely (radio)

coaxial cable

2 concentric copper conductors, broadband

fiber optic cable

glass fiber carrying light pulses

→ 1 pulse = 1 bit

→ high speed operation (10’s-100’s gps transmission rate)

→ low error rate (repeaters spaced far apart, immune to electromagnetic noise)

physical media: radio

signal carried in electromagnetic spectrum

→ bidirectional

→ propagation environment effects (reflection, obstruction by objects, interference)

packet switching

• hosts break application-layer messages into packets

• forwards packets (data) from one router to the next, across links on path from source → destination

• each packet transmitted at full link capacity

• packet must arrive in its entirety before transmitted on next link

forwarding

local action

• move arriving packets from router’s input link to appropriate router output link

routing

global action: determine source-destination paths taken by packets

• routing algorithms

circuit switching

end-to-end resources are reserved for “call” between source and destination

→ dedicated resources (no sharing)

→ guaranteed performance

→ very costly, high security

frequency division multiplexing

transmit multiple, independent signals simultaneously, divides total available bandwidth into distinct, non-overlapping frequency bands

time division multiplexing

combines data streams into 1 unique recurring time slot for each signal

→ allows for sharing of single transmission line

how do loss and delay occur

• packets queue in router buffers (delay)

• packet arrival rate to link (temporarily) exceeds output link capacity

• packets queue, wait for turn

• arriving packets dropped (loss) if no free buffers

queuing delay

time waiting at output link for transmission

• depends on congestion level of router

nodal processing delay

checking for bit errors and determining output link

propagation delay

d/s

d- length of physical link

s- speed of propagation

throughput

rate at which bits are being sent from sender to receiver (bits/time)

bottleneck link

link on end-end path that constrains end-end throughput

what does each layer do (in terms of internet, not the specific layer)

implements a service

relies on services provided by layer below

why layers for the internet?

to deal with complex systems

• explicit structure allows identification, relationship of complex system’s pieces

• modularization eases maintenance, updating of system

internet protocol (IP) stack (5 layers)

1. application

2. transport

3. network

4. link

5. physical

application layer

supporting network applications

• FTP, SMTP (email), HTTP

transport layer

process-process data transfer

• TCP, UDP

network layer

routing of datagrams from source to destination

• IP address, routing protocols

• local forwarding, global routing

link layer

data transfer between neighboring network elements

• Ethernet, bluetooth (MAC addresses), 802.11 (WiFi)

physical layer

bits “on the wire” / “over the air”, modulation, digital-to-analog sampling

2 missing layers from OSI (open systems interconnection) reference model

1. presentation

2. session

presentation layer

allow applications to interpret meaning of data (eg. encryption, compression, machine-specific conversion)

session layer

synchronization, checkpointing, recovery of data exchange

end-to-end principle

implement only on end-hosts

• to ease deployment effort, requirements for routers kept at bare minimum

packet sniffing

• broadcast media (shared Ethernet, wireless)

• network interface reads/records all packets passing by

• passively observe packets to see what’s going on

network applications

• programs that run on different end systems, communicate over network

• software isn’t written for network-core devices (keep simple as possible)

• applications on end systems allows for rapid app development

• eg. web, email, videos on demand, social networking…

what does the application layer protocol define

• types of messages exchanged

• message syntax

• message semantics

• rules

• open protocols

• proprietary protocols

what transport services does an app need

• data integrity

• latency

• throughput

• security

TCP services (transmission control protocol)

• data integrity

  • reliable, in-order delivery, flow control, congestion control

  • slower than UDP, setup required between client and server

UDP services (user datagram protocols)

• “no frills” transport protocol (bare minimum)

• best effort service

application architectures

1. client-server

2. peer-to-peer (P2P)

client-server

server: always on, permanent IP address

client: comm. w/ server, intermittently connected, don’t communicate directly w/ each other

peer-to-peer

• no “always on” server

• end systems directly comm. w/ each other

• peers intermittently connected, may change IP address

http

web’s application layer protocol

1. http client initiates TCP connection (creates socket) to server

2. server accepts TCP connection from client

3. HTTP messages exchanged between browser (client) and web server (HTTP server)

4. TCP connection closed

non-persistent http vs persistent http

non-persistent: 1 object sent over TCP connection. downloading multiple objs requires multiple connections

• requires 2 RTT per object

• OS overhead for each TCP connection

• browsers often open parallel TCP connections to fetch ref. objs.

persistent: multiple objects can be sent over single TCP connection between client & server

• 1 RTT for all referenced objects

• server leaves connection open after sending response

round-trip-time (RTT)

time for packet to travel from client to server & back

2 types of HTTP messages

1. request

2. response

post method

• web page often includes form input

• user input sent from client → server in entity body of HTTP post request message

• eg. submitting forms, uploading files

get method

• include user data in URL field of HTTP get request

head method

requests headers only that would be returned if specified URL were requested with GET method

put method

• uploads new file (obj) to server

• completely replaces file that exists at specified URL w/ content in entity body of POST http request message

4 components of cookies

1. cookie header line of HTTP response message

2. cookie header line in next HTTP request message

3. cookie file kept on user’s host, managed by user’s browser

4. back-end database at web site

what can cookies be used for

• authorization

• shopping carts

• recommendations

• user session state (web email)

how to keep “state”

protocol endpoints maintain state at sender/receiver over multiple transactions

web cache

satisfy client request w/o involving og. server, aimed to improve user perceived performance

• reduce response time for client request

• reduce traffic on institution’s access link

DNS

distributed database implemented in hierarchy of many name servers

• application layer protocol

• host applications communicate w/ name servers to resolve names

• complexity at network’s edge

DNS services

• hostname to IP address translation

• host aliasing (same name but diff IP in back-end)

• mail server aliasing

• load distribution

DNS hierarchy

1. root

2. top level domain

3. authoritative

4. local name server (not in any category)

DNS root name servers

• contact-of-last-resort

• internet can’t function w/o it

• ICANN manages root DNS domain

TLD

responsible for .com, .org, .net…

authoritative DNS servers

• organization’s own DNS server(s), provide authoritative hostname to IP mappings for organization’s named hosts

local DNS name servers

• doesn’t strictly belong to hierarchy

• when hosts make DNS query, query sent to local DNS server

iterated query

• contacted server replies w/ name of server to contact (instead of themselves figuring out name resolving)

recursive query

• burden of name resolution on contacted name server

• heavy load at upper levels of hierarchy

does DNS use UDP or TCP for name, and queries (regular or reverse)

UDP