Computer Networking Midterm

What's the Internet? (Nuts and bolts view)

"Network of networks" with billions of connected devices through interconnected ISPs

What are Hosts? Give 4 examples.

End systems running network apps
Ex: PC, Server, Wireless Laptop, Smartphone

Bandwidth

Transmission rate/link capacity

2 types of Communication Links & 4 examples

Wired & wireless links
Ex: Fiber, Copper, Radio, Satellite

What do Packet Switches do? Give 2 examples.

Forward packets (chunks of data)
Ex: Router, Switches

What do protocols control? What are 5 examples of protocols?

Sending and receiving messages (e.g. TCP, IP, HTTP, Skype, 802.11)

What are two internet standards?

1. RFC (Requests for communication)
2. IETF (Internet engineering task force)

What's the Internet? (Service View)

Infrastructure that provides services & a programming interface to applications (web, VoIP, email) (API hooks)

Network protocols are \______ rather than \_____

machines, humans

What is all Internet communication activity governed by?

Protocols

What do protocols define?

Message format, order sent/received, action taken on transmission, and receipt

What is the Network Edge made of?

Hosts: clients and servers often in data centers

Access Networks & Physical Media represent what kind of links?

Wired & wireless communication links

What is the Network Core?

Interconnected routers - network of networks

What are three types of access nets end systems use to connect to an edge router?

1. Residential access nets
2. Institutional access nets (school/company)
3. Mobile access nets
*Keep in mind bandwidth & if it's shared or dedicated*

Access Network: Digital Subscriber Line (DSL)

Uses an existing, *dedicated* telephone line to a central office DSLAM. Over a DSL phone line, data goes to Internet & voice goes to telephone net.

Frequency Division Multiplexing (FDM)

Different channels transmitted in different frequency bands

Hybrid Fiber Coast (HFC) is what type of connection?

Asymmetric wired connection

Access Network: Cable

A network of *shared* cable and fibre that attaches homes to an ISP router at a cable headend (CMTS)

Access Network: Home

Wired and wireless communication with a *dedicated* line that connects to an ISP via cable or DSL

Access Network: Enterprise (Ethernet)

Used in companies & universities with end systems connected to an Ethernet switch --\> institution router --\> ISP

Access Network: Wireless

*Shared* wireless access network connects end systems to a router via an access point "base station"

What are 2 types of Wireless Access Networks?

1. Wireless LANs (within a building - 100ft. - 802.11 b/g/n)
2. Wide-area Wireless Access (provided by cell operator - 10s of kms - 3G/4G/LTE)

How do hosts send packets of data?

1. Take an application message
2. Break message into small chunks (packets) of L bits
3. Transmit packets into an access network at transmission rate R

Formula for Packet Transmission Delay

L (bits) / R (bits/sec)

What is Packet Transmission Delay?

The time needed to transmit an L-bit packet into the link

What does a Bit do?

Propogate between transmitter/receiver pairs

What is the physical link?

What lies between a transmitter and receiver

How do signals propagate in Guided Media?

Solid media, e.g. copper, fiber, and coax

How do signals propagate in Unguided Media?

Freely, e.g. radio

Coax Cable

Bidirectional, broadband (multiple channel/HFC), two concentric copper conductors

Fiber Optic Cable

Glass fibre carrying light pulses (ea. pulse is a bit), high speed ppp transmission, low error rate

Physical media: Radio

Bidirectional signal carried in electromagnetic spectrum vulnerable to reflection, obstruction by objects, and interference

Radio Link Types

Microwave, LAN (WiFi), Wide-Area (Cellular), Satellite

Packet-Switching

Hosts break application-layer messages into packets, forwarding across routers from source to destination (transmitted at full link capacity)

What is Store-and-Forward in packet switching?

Entire packet must arrive at router before it can be transmitted on next link

End-End Delay Formula (assuming zero propagation delay)

2L/R

When does queuing delay and loss occur?

If arrival rate exceeds transmission rate, packets will queue and wait to be transmitted or dropped (lost) if memory (buffer) fills up

What are 2 key network-core functions?

Routing & Forwarding

Routing

Determine source-destination route taken by packets using a routing algorithm

Forwarding

Move packets from router's input to output

Circuit Switching

A *dedicated* connection formed between two points that remains active for the duration of transmission, segments will be idle if not in use

What type of network-core is commonly used in traditional telephone networks?

Circuit Switching

Greater length between source & destination results in greater \_______ delay.

Propogation

Greater packet size results in greater \________ delay.

Transmission

What's the difference between FDM vs TDM?

FDM divides the channel into two or more frequency ranges that don't overlap & each signal uses a small portion of bandwidth all the time. (horizontal lines)
TDM alternates dividing and allocating time to each channel and each signal uses all the bandwidth some of the time. (vertical lines)

Does packet or circuit switching allow more users to use the network?

Packet switching

Advantages & Disadvantages of Packet Switching

Adv:
1. Great for bursty data & resource sharing
2. Simple with no call setup

Disadv:
1. Excessive congestion (packet delay/loss)
2. Need protocols for reliable transfer

How do end systems connect to the Internet?

End system --\> Access ISPs
--\> Other Access ISPs

What was the evolution of the Internet driven by?

Economics & national policies

What's the problem with connecting each Access ISP to every other Access ISP?

It doesn't scale - too many connections

What's the problem with connecting each Access ISP to one global transit ISP?

Customer-Provider ISPs have economic agreements so Global ISPs compete for business & still need to be connected

What is the solution to connecting millions of access ISPs together?

A small \# of well connected large networks at the centre - Tier 1 ISPs with national and international coverage

What does the tiered Internet architecture look like (top-down)?

\> Tier 1 ISPs (Sprint/AT&T) & Content Provider Networks (Google)
\> IXP
\> Regional ISP
\> Access ISP

What is a Content Provider Network?

Private network that connects its data centres to the Internet, often bypassing tier-1 regional ISPs

What happens to cause packets to queue in router buffers?

Arrival rate to the link temporarily exceeds output link, so they must wait for a turn

What are four sources of packet delay?

1. Nodal processing delay
2. Queuing delay
3. Transmission delay
4. Propogation delay

Nodal Processing Delay

Time it takes an intermediate device to check a packet for errors and decide where to forward it (typically less than a millisecond)

Queuing Delay

Time between when the packet arrives at the router until it leaves (varies based on router congestion)

Formula for Queuing delay

L (packet length in bits)
a (average packet arrival rate
R (link bandwidth in bps)
Queueing delay \= La/R
(0 is small,

Transmission Delay

Time it takes a packet to leave its host device (from the first bit sent to the last bit sent)

Formula for Transmission Delay

L (packet length in bits
R (link bandwidth in bits/sec)
Transmission delay \= L/R

Propogation Delay

Time it takes a bit to go from Host A to Host B

Formula for Propogation Delay

d (length of physical link)
s (propagation speed)
Propogation delay \= d/s

What does the traceroute program do?

Provides delay measurement from source to router along Internet path to destination by sending 3 packets to each router on the path & timing them

What happens to lost packets?

They may be retransmitted by the previous node, source end system, or not at all

Throughput

Rate (bits/time) at which bits transfer between sender/receiver

Instantaneous vs Average Throughput

Instantaneous: rate at given point in time
Average: rate over longer period of time

Bottleneck Link

Link on end-end path that constrains end-end throughput

Why use layering in networks?

- Structure allows identification & relationship of complex system's pieces
-Modularization eases maintenance/updates

Internet Protocol Stack Layers (Top-down)

Application (data)
Transport (segments)
Network (packets)
Link (frames)
Physical (bits)

Internet Protocol Stack - Application

Supports network applications
Ex: FTP, SMTP, HTTP

Internet Protocol Stack - Transport

Process-process data transfer
Ex: TCP, UDP

Internet Protocol Stack - Network

Route datagram from source to destination
Ex: IP, routing protocols

Internet Protocol Stack - Link

Data transfer between neighbouring network elements
Ex: Ethernet, 802.11 (Wifi), PPP

Internet Protocol Stack - Physical

bits "on the wire"

ISO/OSI Model - Presentation

Allows apps to interpret data
Ex: Encryption, compression

ISO/OSI Model - Session

Synchronisation, checkpointing, recovery of data exchange

The Internet wasn't originally designed with much security in mind. What was the original vision?

A group of mutually trusting users attached to a transparent network

What are two ways malware can get in a host?

1. Virus: requires user to receive/execute (Email attachments)
2. Worm: self-replicates

What is an example of packet-sniffing software?

Wireshark

Packet sniffing

Broadcast media (shared ethernet/wireless)
- Promiscuous network interface reads/records all packets

IP spoofing

Send packet with false source address

Whose internetworking principles define today's Internet structure? What are they?

Cerf & Kahn
- minimalism & autonomy (no internal change required to interconnect networks)
- best effort service model
- stateless routers
- decentralised control

When creating a network app, you write programs that should do what 2 things?

1. Run on different end systems
2. Communicate over network
Ex: Web server software communicates with browser

Do you need to write software for network core devices?

No, they don't run user apps.

Give 5 examples of network apps

1. Email
2. Web
3. Text messaging
4. Remote login
5. Streaming stored video (Netflix)

What are two possible structures of applications?

1. Client-server
2. Peer-to-peer

Role of the server in Client-server architecture

Always-on
Permanent IP
Data centres for scaling

Role of clients in Client-server architecture

Communicate with server
Intermittently connected Dynamic IPs
Don't communicate directly with each other

P2P architecture

No always-on server
Arbitrary end-systems directly communicate
Peers request & provide services to other peers
Peers are intermittently connected & change IPs

Self Scalability

New peers bring new service capacity & demands

Process

program running within a host

How do two processes communicate within the same host?

Inter-process communication

How do processes communicate in different hosts?

By exchanging messages

What process initiates communication?

Client process

What process waits to be contacted?

Server process

Do applications with P2P architectures have client or server processes?

Both

What are Sockets?

Interface between application & transport layer - Process sends/receives messages to/from its socket

What must the Addressing Process have to receive messages?

An identifier- IP address & port number associated with host process