8 - Content Distribution

Question 1

An internet wide tool that enables websites and network operators to deliver data quickly and efficiently

Answer 1

Content Distribution

Question 2

HTTP (Hypertext Transfer Protocol)

Answer 2

An application layer protocol to transfer web content. Layered over a byte stream protocoal (almost always TCP)

Question 3

Contents of HTTP request

Answer 3

*Requestline: Method-> GET, POST, HEAD
URL -> /index.html
version number
*Headers: Referrer-> what url caused the page to be requested
user agent -> client software being used to fetch the page

Question 4

Which HTTP header field indicates client software making request

Answer 4

The user-agent field.

Question 5

HTTP Response includes…

Answer 5

A Status line: HTTP Version
Response code
Location: used for redirection
Server: indicates server software
Allow: http methods allowed (get, head, etc)
Content-encoding: compressed or not
content-length: how long is content is in bytes
expires: how long can it be cached
last-modified: When was page last modified

Question 6

Response codes

Answer 6

100 - informational
200 - success
300 - redirection
     301 - moved permanently
      304 - not modified response
400 - errors 
     404 - not found
500 - server errors

Question 7

Early HTTP

Answer 7

Simple to implement but needed a TCP connection for every request. Solution? Persistent connections

Question 8

Multiple request/response are multiplexed on a single TCP connection

Answer 8

Persistent Connections

Question 9

A client sends a request as soon as it encounters a referenced object

Answer 9

Pipelining

Question 10

Persistent connection with pipelining is the

Answer 10

default behavior in HTTP 1.1

Question 11

Clients can cache documents in …

Answer 11

browser(local)
in network (i.e. content distribution, networks)

Question 12

Clients can be directed to a cache in multiple ways

Answer 12

1) Browser configuration

2) Server directed

Question 13

Benefits of HTTP caching

Answer 13

1) Reduced transit costs for local ISP

2) Improved performance for local clients

Question 14

What is a CDN (Content Distribution Networks)

Answer 14

#An Overlay network of web caches designed to deliver content to a client from the optimal location.
*Geographically disperate servers

Question 15

Who owns CDNs?

Answer 15

• Content providers

* Network/ISPs

Question 16

Main goal of a CDN?

Answer 16

To replicate content on many servers

Question 17

CDN problem: Server Selection

Answer 17

Which server? Lowest latency

Question 18

CDN problem: Content routing

Answer 18

How to route? 1) Routing (i.e anycast) coarse

        2) Application-based (i.e. HTTP redirect) delays
3) Naming-based (i.e DNS) (best way) flexible and fast

Question 19

CDNs peer with ISPS because

Answer 19

1) where a customer is located, better throughput (lower latency)
2) Increased reliability and redundancy
3) burstiness -> lower transit costs

Question 20

ISPs per with CDNs because

Answer 20

1) Good performance for customers when close (i.e. Ga tech has a google cache node in its own network resulting in low latency for customers connecting to google.. making the customers happy)
2) lower transit costs (hosting a local cache node reduces cost)

Question 21

Why do ISPs want to peer with CDNs?

Answer 21

1) Lower transit cost

2) Provide better performance

Question 22

Peer to peer content distribution network

Answer 22

Bit Torrent (file sharing, large file distribution)
Fetch the content from peers instead of one place.)

clients swap pieces of the file

Question 23

Bit Torrent steps for publishing

Answer 23

1) Peer creates a “torrent” which has tracker and pieces of file
2) “Seeders” creates initial copy
3) “Leechers” clients that contain incomplete copies of the files.

Question 24

Bit Torrent solved this problem:

Answer 24

“Freeloading”… called “Choking” or “tit-for-tat”

Question 25

Choking: temporary refusal to upload

Answer 25

If a peer cant download from a client, don’t upload to it
eliminates Free rider problem.

Tit for tat strategy ensures cooperation

Question 26

Getting chunks to swap (the right chunk)

Answer 26

Bit torrent uses “rarest piece first”

Question 27

Enable a form of content overlay (content overlay)

Answer 27

Distributed Hash Tables

Question 28

Chord

Answer 28

A Distributed Hash table that is a scalable, distributed “lookup service” that maps keys to values (i.e. DNS, directory services)

Plus: Scalable, provable correctness and performance

Question 29

Chord motivation

Answer 29

Scalable location of data in a large distribute system

Question 30

Main Idea: Keys and the nodes map to the same ID space

Answer 30

Consistent Hashing

Question 31

Chord - Consistent Hashing

Answer 31

The Key is stored at the successor…The node with the next highest ID. (Counter clockwise in the example)

consistent hashing offers: Load balancing and Flexibility

Question 32

Option: every node knows location of every other node

Answer 32

Lookups have good performance at the expense of larger tables

Question 33

Option: Every Node knows only successor

Answer 33

Routing tables can be small but every lookup operation is o(n) - not the best performance

Question 34

Best option that provides the best of both worlds: “Finger Tables:

Answer 34

Every node knows m other nodes in the ring

Question 35

Distributed Hash Table (DHT)

Answer 35

Consistent Hashing - Keys and Nodes map to same ID space

Question 36

Finger tables

Answer 36

O(N) time

Question 37

Finger table with O(log(N)) entries

Answer 37

O(log(N)) time