DNS and CDN - KAISTina.kaist.ac.kr/ee324/F13/lectures/16-CDN.pdf · 12 Typical Resolution Steps for resolving Application calls gethostbyname() (RESOLVER) Resolver contacts local

DNS AND CDN

What we have learned so far…

Socket programming, Internet

Process and Thread

Concurrent programming

RPC

Logical Time

(Distributed) Transactions and concurrency control

What we will learn…

ACID vs BASE.

We will learn how real systems work:

DNS, CDN

Distributed file system (NFS, AFS, Dropbox)

MapReduce/Hadoop

SSH, PKI

Midterm

0

1

2

3

4

100 95 90 85 80 75 70 65 60 55 50

AVG=77

PA1+PA2(part I)+Midterm

0

1

2

3

4

5

100 95 90 85 80 75 70 65 60 55 50

Total

Today's Lecture6

DNS: one of world’s largest distributed system"

CDNs: More than 60% of Internet traffic is video. More than half is

served by CDNs.

What is DNS?

Domain Name Service

Translates human readable names into IP addresses.

Challenge

How do we scale these to the wide area and to many users?

How do we make this efficient?

How do we support many updates?

This is what DNS gives you (dig)

dongsuh@ina:~/public_html$ dig ina.kaist.ac.kr

; <<>> DiG 9.8.1-P1 <<>> ina.kaist.ac.kr

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 3211

;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:

;ina.kaist.ac.kr. IN A

;; ANSWER SECTION:

ina.kaist.ac.kr. 7200 IN A 143.248.56.236

;; AUTHORITY SECTION:

kaist.ac.kr. 7200 IN NS ns1.kaist.ac.kr.

kaist.ac.kr. 7200 IN NS ns.kaist.ac.kr.

;; ADDITIONAL SECTION:

ns.kaist.ac.kr. 7200 IN A 143.248.1.177

ns1.kaist.ac.kr. 7200 IN A 143.248.2.177

;; Query time: 2 msec

;; SERVER: 127.0.0.1#53(127.0.0.1)

;; WHEN: Thu Nov 7 08:14:11 2013

;; MSG SIZE rcvd: 116

; <<>> DiG 9.8.1-P1 <<>> www.google.com


;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 45520

;; flags: qr rd ra; QUERY: 1, ANSWER: 6, AUTHORITY: 4, ADDITIONAL: 4

;; QUESTION SECTION:

;www.google.com. IN A

;; ANSWER SECTION:

www.google.com. 44 IN A 74.125.128.103






;; AUTHORITY SECTION:

google.com. 268411 IN NS ns3.google.com.




;; ADDITIONAL SECTION:

ns2.google.com. 265844 IN A 216.239.34.10

ns1.google.com. 265844 IN A 216.239.32.10

ns3.google.com. 265844 IN A 216.239.36.10

ns4.google.com. 265844 IN A 216.239.38.10

;; Query time: 0 msec

;; SERVER: 127.0.0.1#53(127.0.0.1)

;; WHEN: Thu Nov 7 08:14:32 2013

;; MSG SIZE rcvd: 264

Why is DNS distributed system?

Why can’t it be centralized?

Single point of failure

Large amount of requests

Must handle large updates!

Does not scale.

Why not use distributed transactions?

ACID approach.

11

Domain Name System Goals

Basically a wide-area distributed database

Scalability

Decentralized maintenance

Robustness

Global scope

Names mean the same thing everywhere

Don’t need

Atomicity

Strong consistency

12

Typical Resolution

Steps for resolving www.google.com

Application calls gethostbyname() (RESOLVER)

Resolver contacts local name server (S1)

S1 queries root server (S2) for (www.google.com)

S2 returns NS record for google.com. (S3) ns1.google.com

What about A record for S3?

This is what the additional information section is for (PREFETCHING)

S1 queries S3 (ns1.google.com) for www.google.com

S3 returns A record for www.google.com

Can return multiple A records what does this mean?

13

Lookup Methods

Recursive query:

Server goes out and searches for more info (recursive)

Only returns final answer or “not found”

Iterative query:

Server responds with as much as it knows (iterative)

“I don’t know this name, but ask this server”

Workload impact on choice?

Local server typically does recursive

Root/distant server does iterative

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

2

34

5 6authoritative name s

erver

dns.cs.umass.edu

intermediate name server

dns.umass.edu

7

8

iterated query

14

Workload and Caching

Are all servers/names likely to be equally popular?

Why might this be a problem? How can we solve this problem?

DNS responses are cached

Quick response for repeated translations

Other queries may reuse some parts of lookup

NS records for domains

DNS negative queries are cached

Don’t have to repeat past mistakes

E.g. misspellings, search strings in resolv.conf

Cached data periodically times out

Lifetime (TTL) of data controlled by owner of data

TTL passed with every record

15

Typical Resolution

ClientLocal

DNS server

root & edu

DNS server

ns1.cmu.edu

DNS server

www.cs.cmu.edu

ns1.cs.cmu.edu

DNS

server

16

Subsequent Lookup Example

ClientLocal

DNS server

root & edu

DNS server

cmu.edu

DNS server

cs.cmu.edu

DNS

server

ftp.cs.cmu.edu

digdongsuh@ina:~/Documents/sdn-cdn-doc/nsdi$ dig www.google.com

+trace

; <<>> DiG 9.8.1-P1 <<>> www.google.com +trace


. 436533 IN NS k.root-servers.net.

. 436533 IN NS d.root-servers.net.

. 436533 IN NS m.root-servers.net.

. 436533 IN NS f.root-servers.net.

. 436533 IN NS l.root-servers.net.

. 436533 IN NS j.root-servers.net.

. 436533 IN NS h.root-servers.net.

. 436533 IN NS b.root-servers.net.

. 436533 IN NS c.root-servers.net.

. 436533 IN NS e.root-servers.net.

. 436533 IN NS a.root-servers.net.

. 436533 IN NS i.root-servers.net.

. 436533 IN NS g.root-servers.net.

;; Received 420 bytes from 127.0.0.1#53(127.0.0.1) in 11 ms

com. 172800 IN NS d.gtld-servers.net.

com. 172800 IN NS m.gtld-servers.net.

com. 172800 IN NS g.gtld-servers.net.

com. 172800 IN NS j.gtld-servers.net.

com. 172800 IN NS i.gtld-servers.net.

com. 172800 IN NS a.gtld-servers.net.

com. 172800 IN NS f.gtld-servers.net.

com. 172800 IN NS e.gtld-servers.net.

com. 172800 IN NS k.gtld-servers.net.

com. 172800 IN NS b.gtld-servers.net.

com. 172800 IN NS l.gtld-servers.net.

com. 172800 IN NS h.gtld-servers.net.

com. 172800 IN NS c.gtld-servers.net.

;; Received 492 bytes from 192.112.36.4#53(192.112.36.4) in

1275 ms





;; Received 168 bytes from 192.35.51.30#53(192.35.51.30) in

210 ms




Switch gTCL and ccTLD (incorrect figure)

But you get the idea

Root name servers (13 servers)

Actually many physical root name servers

Several root servers have multiple physical servers

They have

Packets routed to “nearest” server by “Anycast” protocol

http://www.root-servers.org/

http://www.root-servers.org/

22

Reverse DNS

Task

Given IP address, find its name

Method

Maintain separate hierarchy based on IP names

Write 128.2.194.242 as 242.194.128.2.in-addr.arpa

Why is the address reversed?

Managing

Authority manages IP addresses assigned to it

E.g., CMU manages name space 128.2.in-addr.arpa

edu

cmu

cs

kittyhawk128.2.194.242

cmcl

unnamed root

arpa

in-addr

128

2

194

242

23

.arpa Name Server Hierarchy

At each level of hierarchy, have group of s

ervers that are authorized to handle that re

gion of hierarchy

128

2

194

kittyhawk128.2.194.242

in-addr.arpa a.root-servers.net • • • m.root-servers.net

chia.arin.net

(dill, henna, indigo, epazote, figwort, ginseng)

cucumber.srv.cs.cmu.edu,

t-ns1.net.cmu.edu

t-ns2.net.cmu.edu

mango.srv.cs.cmu.edu

(peach, banana, blueberry)

24

Prefetching

Name servers can add additional data to response

Typically used for prefetching

CNAME/MX/NS typically point to another host name

Responses include address of host referred to in “additional section”

25

Mail Addresses

MX records point to mail exchanger for a name

E.g. mail.acm.org is MX for acm.org

Addition of MX record type proved to be a challenge

How to get mail programs to lookup MX record for mail delivery?

Needed critical mass of such mailers

26

DNS (Summary)

Motivations large distributed database

Scalability

Independent update

Robustness

Hierarchical database structure

Zones

How is a lookup done

Caching/prefetching and TTLs

Reverse name lookup

HTTP Caching27

Clients often cache documents

Challenge: update of documents

If-Modified-Since requests to check

HTTP 0.9/1.0 used just date

HTTP 1.1 has an opaque “entity tag (ETAG)” (could be a file signature, etc.) as well

When/how often should the original be checked for changes?

Check every time?

Check each session? Day? Etc?

Use Expires header

If no Expires, often use Last-Modified as estimate

Example Cache Check Request28

GET / HTTP/1.1

Accept: */*

Accept-Language: en-us

Accept-Encoding: gzip, deflate

If-Modified-Since: Mon, 29 Jan 2001 17:54:18 GMT

If-None-Match: "7a11f-10ed-3a75ae4a"

User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)

Host: www.intel-iris.net

Connection: Keep-Alive

Example Cache Check Response29

HTTP/1.1 304 Not Modified

Date: Tue, 27 Mar 2001 03:50:51 GMT

Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1 OpenSSL

/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24

Connection: Keep-Alive

Keep-Alive: timeout=15, max=100

ETag: "7a11f-10ed-3a75ae4a"

Content Distribution Networks (CDNs)

The content providers are the CDN customers.

Content replication

CDN company installs hundreds of CDN servers throughout Internet

Close to users

CDN replicates its customers’ content in CDN servers. When provider updates content, CDN updates servers

30

origin server

in North America

CDN distribution node

CDN server

in S. America CDN server

in Europe

CDN server

in Asia

Server Selection31

Service is replicated in many places in network

How do direct clients to a particular server?

As part of routing anycast, cluster load balancing

As part of application HTTP redirect

As part of naming DNS

Which server?

Lowest load to balance load on servers

Best performance to improve client performance

Based on Geography? RTT? Throughput? Load?

Any alive node to provide fault tolerance

Routing Based 32

Anycast

Give service a single IP address

Each node implementing service advertises route to address

Packets get routed routed from client to “closest” service node

Closest is defined by routing metrics

May not mirror performance/application needs

What about the stability of routes?

Application Based33

HTTP support simple way to indicate that Web page has moved

Server gets Get request from client

Decides which server is best suited for particular client and object

Returns HTTP redirect to that server

Can make informed application specific decision

May introduce additional overhead multiple connection setup, name lookups, etc.

While good solution in general HTTP Redirect has some design flaws –especially with current browsers

Naming Based34

Client does name lookup for service

Name server chooses appropriate server address

A-record returned is “best” one for the client

What information can name server base decision on?

Server load/location must be collected

Information in the name lookup request

Name service client typically the local name server for client

How Akamai Works35

Clients fetch html document from primary server

E.g. fetch index.html from cnn.com

URLs for replicated content are replaced in html

E.g. <img src=“http://cnn.com/af/x.gif”> replaced with <img src=“http://a73.g.

akamaitech.net/7/23/cnn.com/af/x.gif”>

Client is forced to resolve aXYZ.g.akamaitech.net hostname, where XY

Z is the hash of the URL

How Akamai Works36

How is content replicated?

Akamai only replicates static content (*)

Modified name contains original file name

Akamai server is asked for content

First checks local cache

If not in cache, requests file from primary server and caches file

* (At least, the version we’re talking about today. Akamai actually lets sites write code that can run on Akamai’s servers, but that’s a pretty different beast)

How Akamai Works37

Root server gives NS record for akamai.net

Akamai.net name server returns NS record for g.akamaitech.net

Name server chosen to be in region of client’s name server

TTL is large

G.akamaitech.net nameserver chooses server in region

Should try to chose server that has file in cache - How to choose?

Uses aXYZ name and hash

TTL is small why?

Simple Hashing38

Given document XYZ, we need to choose a server to use

Suppose we use modulo

Number servers from 1…n

Place document XYZ on server (XYZ mod n)

What happens when a servers fails? n n-1

Same if different people have different measures of n

Why might this be bad?

Simple Hashing39

Given document XYZ, we need to choose a server to use

Suppose we use modulo

Number servers from 1…n

Place document XYZ on server (XYZ mod n)

What happens when a servers fails? n n-1

Same if different people have different measures of n

Why might this be bad?

Consistent Hash40

“view” = subset of all hash buckets that are visible

Desired features

Balanced – in any one view, load is equal across buckets

Smoothness – little impact on hash bucket contents when buckets are added/

removed

Spread – small set of hash buckets that may hold an object regardless of vi

ews

Load – across all views # of objects assigned to hash bucket is small

Consistent Hash – Example41

Smoothness addition of bucket does not cause movement between existing buckets

Spread & Load small set of buckets that lie near object

Balance no bucket is responsible for large number of objects

• Construction• Assign each of C hash buckets to random points on mod

2n circle, where, hash key size = n.

• Map object to random position on circle

• Hash of object = closest clockwise bucket

0

4

8

12Bucket

14

How Akamai Works42

End-user

cnn.com (content provider) DNS root server Akamai server

1 2 3

4

Akamai high-level DN

S server

Akamai low-level DNS

server

Nearby matching

Akamai server

11

6

7

8

9

10

Get in

dex.ht

ml

Get /cnn.com/foo.jpg

12

Get foo.jpg

5

Akamai – Subsequent Requests43

End-user

cnn.com (content provider) DNS root server Akamai server

1 2 Akamai high-level DN

S server

Akamai low-level DNS

server7

8

9

10

Get in

dex.ht

ml

Get /cnn.com/foo.jp

g

Nearby matching

Akamai server

Important Lessons44

Akamai CDN illustrate range of ideas

BASE (not ACID design)

Weak consistency

Naming of objects location translation

Consistent hashing

Why are these the right design choices for this application?