Networks Midterm 1

List names for different access networks.

Lan (at home via ethernet)

- Connects the end systems to the edge router

DSL (Digital Subscriber Line)

- use existing telephone line to central office DSLAM

Cable-based (DSL and this both at home)

- network of cable, fiber attaches homes to ISP router

Cellular (HFC)

Wireless

FTTH (Fiber to the home)

5G

Packet Switching: How it works?

- Host breaks application-layer messages into packets

- Data is broken into packets, host sends packet to access network through communication links

- packets are forwarded from one router to the next, each transmitted at full link capacity

Method within Packet Switching

- Store and forward

Drawback:

- Queueing delay

Transmission Delay:

Time needed to transmit L-bit packet into link

L (bits) / R (bps)

Explain Queuing Delay. When is it large?

Queueing Delay is when the arrival rate to link exceeds transmission rate of link for a period of time

When does packet loss happen in networks?

Can be dropped (lost) if memory (buffer) in router fills up

Depends on quality of local priority

Why is store and forward technique useful?

Can check for corrections, if packet is received properly, and can see destination IP

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

Difference between Routing and Forwarding.

- routing: global action that action that determine source-destination paths taken by packets

(uses routing algorithm, generates forwarding table to tell which switch to forward which switch)

- forwarding: local action that moves arriving packets from router's input link to appropriate output link

(Uses packet header for destination address and forwarding table to send it)

each router has their own forwarding table

Disadvantage and Advantage of FDM.

Adv: Guaranteed Resources (dedicated frequency)

DisAdv: Limited bandwidth (Divided Resources), Under-Utilized resources (Not used to full capacity, like not using a frequency)

Disadvantage and Advantage of TDM.

Adv: Max capacity when used,

DisAdv: Can be dropped

Can use as much as you want in a shorter period of time

Explain:

PoP

Multihome

Peer

IXP

PoP (Point of Presence):

- A group of 1 or more router (same location) in provider's network where customers ISP can connect to provider ISP

- Not in access ISP

Multi-Home:

- Customer ISP Connect to 2 or more provider ISP

- Any except tier-1 ISP

Peer:

- Pair of nearby ISPs at same level that directly connect their networks together

IXP:

- a third-party company can create an Internet Exchange Point (IXP), which is a meeting point where multiple ISPs can peer together. An IXP is typically in a stand-alone building with its own switches.

Transmission vs Propagation Delay.

Transmission: Time to transmit all the packet's bits into link

L: Packet length (bits) **MAKE SURE IT IS BITS AND NOT BYTES (if bytes then convert 1 byte = 8 bits) aka multiply it by 8 silly

R: Link transmission rate (bps)

d trans = L / R

Propagation: From the beginning of the link to router (point a to point b)

d: length of physical link

s: propagation speed (around 2 x 10^8 to 3 x 10^8 m /sec)

d prop = d / s

Processing Delay

Time it takes to determine which output link you are sending to

Explain Traffic Intensity.

It's the measurement of how busy a system is (like incoming traffic ex with a highway)

a = average packet arrival rate

[ L (bits) * a] / R (bps)

La/R ~ 0: avg. queueing delay small

La/R -> 1: avg. queueing delay large

La/R > 1: more "work" arriving is more than can be serviced - average delay infinite!

How does "tracert" work?

- Provides delay measurements from source to router along end-end Internet path towards destination

- Sends 3 packets/probes that will reach router i on path towards destination

- Delay is highest when over continents

(T/F) Network edge always have bottleneck link.

False, not necessarily all the time (divided by how many users actually use it)

The network core can be the bottleneck if all clients running at max speed

Bottleneck is distributed amongst the clients

What are the five layers? and what do they make up

Application

Transport

Network

Link

Physical

5 layers make up Internet Protocol Stack (IPS)

What are the functions of the layers, packet, and their protocols?

Application:

- Supporting network applications (App. deployed)

- Packet: Message

- SMTP HTTP, FTP

Transport

- Process to process communication (data transfer)

- Packet: Segment

- TCP, UDP

Network

- Routing datagrams from source to destination (host-host)

- Packet: Datagrams

- IP, routing protocols

Link

- Data transfer between Neighboring networks

- Packet: Frames

- Different protocol based on links (Ethernet)

Physical

- Bits "on the wire"

- Packet: Bit

- Different physical media different protocol (DSL, Fiber, Hybrid)

Explain encapsulation.

The process of adding or removing additional information, called headers and trailers, to data as it travels through different layers

Top has least info (application), bottom has most (physical)

Router is from network down, switch is from link down

Network Architecture vs Network Application Architecture.

Network arch.

- Fixed w/ 5 layers (Application, transport, network, link, and physical)

Network app. arch

- Depends on developer for specific app. Could be client-server or peer to peer

Explain Client-Server Architecture.

Client: Devices initiates communication with dynamic IP addresses, DO NOT communicate with each other

Server: Provide resources, Permanent IP address

Ex: Web Browser

Explain P2P architecture

Peers request service from other peers, provide service in return to other peers

Peer can be client or server

Problems: Security, Less reliable (Peers can drop whenever)

Network application processes and assign them as a part of Network Architecture.

Clear distinction

P2P depends on how peers are communicating

Processes: Client process (initiates communication) and Server Process (waits to be connected)

Explain Socket and Address Information related to network applications.

Sockets allow sending/receiving apps to "connect" to, use Internet transport service

3 things to set in relation to socket:

1) the type of transport layer protocol

2) buffer size

3) maximum segment size

Process send/receives messages to/form its socket

IP and Port # is the address info it needs to route properly

Port #

Unique ID

What are the services provided by the transport layer to the application layer?

Reliability: No packet loss/corruption

Time

Throughput:

- Elastic (Not bandwidth sensitive) = Email, File transfer

- Non-Elastic = Social Media, streaming

Security

What are the services provided by TCP?

Congestion control: throttle sender when network overloaded

Flow control: Receiver capacity (think of catch analysis with how many objects you can hold)

Connection-Oriented: setup required between client and server processes

Reliable transport

Does NOT provide security, time, throughput

TLS extra to TCP, provides security (not automatically added)

What are the services provided by UDP?

Unreliable data transfer: between sending and receiving process

Does not provide anything else

HTTP: How does it work?

HTTP (Hypertext transfer protocol)

- client/server model

a client (like your web browser) sends a request to a server for specific information, and the server then responds with the requested data

(1) TCP client initiates connection w/ TCP server

(2) HTTP request message get generated -> Socket -> Transport -> Segment

(3) Close TCP Connection

What is protocol?

Defines the format, order of messages sent and received among network entities, and actions taken on message transmission

Decides order, structure, and actions to take for a message

Persistent HTTP (1.1) vs Non Persistent HTTP (1.0)

Persistent:

- Lower response time because can send multiple objects over a single TCP connection

Non Persistent:

- Higher response time because requires 2 RTT per object. Only one object sent over TCP connection and requires multiple connection (separate TCP connections) to download multiple objects

What is pipelining in HTTP 1.1?

Client sends request as soon as it encounters a referenced object

HTTP is a stateful protocol. Justify.

False, Server maintains NO info about past client requests

HTTP GET/response interaction is stateless

Adv: Faster

Stateful uses cookie to maintain the state between the client and server

Explain HTTP cookies.

Used to maintain state between client and server, so transaction and interactions

Infrastructure:

- Set-Cookie: Server -> Client (locally)

- Cookie: Client -> Server

- Backend database (server) side

- Cookie file (client)

Advantage and Disadvantage of HTTP cookies.

Adv: Customization for the user (via cookie value)

DisAdv: Less secure

Conditional GET

Proxy server stores certain objects in the server

Goal: don't send object if cache has up-to-date cached version

Last modified and created date stored

Conditional: It sends back 200 ok message (proxy does NOT have it), and 304 if it does it have (304 is not modified)

Request vs Response

Request:

Request line (GET, POST, HEAD, PUT, DELETE commands)

header lines

body

Response:

Status line (protocol status code like 200 status phrase)

header lines

data (HTML file)

Data in first line = Request

Data in last line = Response

HTTP response status codes

200 OK

request succeeded, requested object later in this message

301 Moved Permanently

requested object moved, new location specified later in this message (in Location: header field)

400 Bad Request

request msg not understood by server

404 Not Found

requested document not found on this server

505 HTTP Version Not Supported

History

Important dates:

- 1961: Kleinrock(MIT) - queueing theory shows effectiveness of packet-switching

- 1969: on Labor Day first ARPAnet node operational in UCLA under Kleinrock supervision

§Internet was 4 node large!!

- 1974: Cerf and Kahn - architecture for interconnecting networks

(TCP, IP, UDP conceptually in place)

- 1983: deployment of TCP/IP

- 1988: TCP congestion control

Packet vs Circuit switching

Circuit:

- Path is made before hand (effort to make path)

- Guaranteed resources (like a tunnel that never changes)

- Static (can't be changed)

- TDM

- FDM

Packet:

- Router to router, determines route (more dynamic)

- Think of google maps (makes the route)

- Store and forwarding

- Packet loss

HTTP/2

transmission order of requested objects based on client-specified object priority (not necessarily FCFS) – Request Message Prioritization

push unrequested objects to client- server push

divide objects into frames, schedule frames to mitigate HOL blocking

How does HTTP 2 solve the HOL blocking problem?

-Client request large object

HOL blocking is solved by:

- HTTP 2 divides objects into frames

- Frames transmission them interleaved (can be mixed and matched)

- Data is received from smaller to larger

What are the different types of physical media and their categories?

Types of cables:

Twisted pair, fiber, coax, copper, wireless

Guided vs Unguided:

- Guided: cable based (all cables)

- Unguided: Wireless

HTTP 3 runs over UDP. (True or False)

True

Web Cache

Goal: satisfy client request without involving origin server

browser sends all HTTP requests to cache:

if object in cache: cache returns object to client

else cache requests object from origin server, caches received object, then returns object to client

ISO/OSI reference model

Seven layer:

Application

Presentation

Session

Transport

Network

Link

Physical

Presentation: Allow applications to interpret meaning of data, encryption, compression, machine-specific conventions

Session: Synchronization, checkpointing, recovery of data exchange

HTTP Types (Persistent vs Non-Persistent)

HTTP 1.0 (Non-Persistent)

1) TCP connection opened

2) at most one object sent over TCP connection

3) TCP connection closed

HTTP 1.1 (Persistent)

1) TCP connection opened to a server

2) multiple objects can be sent over single TCP connection between client, and that server (pipelining)

3) TCP connection closed

HTTP Types (2 vs 3)

HTTP/2

1) transmission order of requested objects based on client-specified object priority (not necessarily FCFS) – Request Message Prioritization

2) push unrequested objects to client- server push

3)divide objects into frames, schedule frames to mitigate HOL blocking (HEAD OF LINE)

HTTP/3

1) Implemented in application layer over UDP

2) adds security , per object error- and congestion-control (more pipelining) over UDP

Which switching does the internet use and why does it use this?

Internet use packet switching

Because it's easier to set up (dynamic) and not wasting resources