Content Distribution Networks

Costin RaiciuAdvanced Topics in Distributed Systems

Fall 2012

Problem: making the web go faster

• Use case: type ebay.com in your browser– The site takes a while to load– How long do you wait before you give up and try

another site?40% of customers will wait no more than 3 seconds for a webpage to load [forrester consulting]

Why would a site take long to load?

• Let’s assume the content is generated quickly by the servers

• A webpage will have many small objects and perhaps a few larger ones

• What network conditions will affect download time?

The Internet

www.site.com

Autonomous System (AS) AS

AS ASAS

Origin server

Authoritative DNS

Lookup www.site.comDownload webpage

How do we fix the web?

• Change the Internet architecture – rather difficult to deploy

• More pragmatic choice: target static web content– Basic idea: bring content closer to the clients

• Design principles– Design for reliability and scalability (Akamai has

120K servers today)– Limit the need for human management

CDNs are really popular

• Quick check

The Basic Idea of CDNs: Cache static content near the user

www.site.com

Autonomous System (AS) AS

AS ASAS

Origin server

CDN Operation Summary

www.site.com

Autonomous System (AS) AS

AS ASAS

Origin server

Authoritative DNS

CDN Nameserver

Lookup www.site.com

1. www.site.com delegates zone to CDN namerserver

DNS Resolver

2. CDN nameserver uses resolver’s IP to find edge server that is nearest to customer (geo-location)

E3. Edge server E will serve

file from local storage if it has it

4. Otherwise will fetch file from origin server

Overview of the Akamai CDN [Akamai]

Edge server functionality

• Located at ISPs worldwide - close to customers• Maintain metadata for each served file• Metadata specified by:– XML files delivered using Akamai infrastructure– Akamai specific HTTP Response Headers

Metadata stored for each file

• Origin server location• Content path• Cache control – how long should we store replica, how

is it invalidated?• Cache indexing – how should an URL be used to create

a key for that object?• Access control – who is allowed to view this file?• Response to origin server failure• HTTP Header alteration – rewrite headers including

cookies to deal with different browsers etc.

Mapping System

• A scoring system creates an up-to-date topological map of Internet– Divides IP addresses into equivalence classes– Computes connectivity between these classes

• Implementation:– Run and parse ping and traceroutes in real time– Parse BGP data and logs

Mapping System

• The Real Time Mapping system creates the actual maps used to direct users to the best Akamai edge servers

• Runs in two steps:a) Map to cluster: select a preferred edge server

cluster for each equivalence class of users b) Map to server: a low level map sends the user to

a specific server within the clustergoal: maintain locality within clusters

Implementing the mapping system using DNS

1. The first request goes to generic TLD servers, which return Akamai Top Level Name Servers (TLNS) as authorities, generally with long DNS TTLs. The Akamai TLNS are globally distributed, using a mixture of IP Anycast and large clusters.

2. The next query, to an Akamai TLNS, returns delegations with shorter DNS TTLs to a number of Akamai Low Level Name Servers (LLNS). The Akamai LLNS are typically located in close network proximity to the resolving name server.

3. The final query, to an Akamai LLNS, returns edge server IP addresses based on both the cluster assignment and the low level map described above. These answers have very short TTLs so that changes to the mapping assignments (such as in response to failures or shifts in demand) can be rapidly distributed to end users.

Akamai’s Transport protocol

• The communications between any two Akamai servers can be optimized to overcome the inefficiencies of BGP routing

• Goals:– Accelerate non-cacheable content– Accelerate apps that check origin server for freshness

• Techniques:– Path optimizations– Protocol enhancements

Path Optimization

• Build and Internet overlay– Use end-to-end path quality between servers

maintained by the mapping system– Move traffic onto the best performing path

according to measurements or use multiple paths.• 30-50% performance improvements in Asia

Akamai Transport Protocol

• Proprietary (modified TCP)• Use pools of persistent connections to avoid

3WHS• Play with TCP Window size based on path

conditions– E.g. increase initial cwnd when path is know to be

good• Set aggressive timeouts based on known path

information

Coral CDN(slides adapted from Mike Freedman)

A problem…

• Feb 3: Google linked banner to “julia fractals”• Users clicking directed to Australian University web site• …University’s network link overloaded, web server taken

down temporarily…

The problem strikes again!

• Feb 4: Slashdot ran the story about Google• …Site taken down temporarily…again

The response from down under…

• Feb 4, later…Paul Bourke asks:

“They have hundreds (thousands?) of servers

worldwide that distribute their traffic load. If even a

small percentage of that traffic is directed to a single

server … what chance does it have?”

→ Help the little guy ←

Coral’s solution…

• Implement an open CDN• Allow anybody to contribute• Works with unmodified clients• CDN only fetches once from origin server

OriginServer

Coralhttpprxdnssrv

CoralhttpprxdnssrvCoral

httpprxdnssrv

Coralhttpprxdnssrv

Coralhttpprxdnssrv

Coralhttpprxdnssrv

Browser

Browser

Browser

Browser

Pool resources to dissipate flash crowds

Using CoralCDN• Rewrite URLs into “Coralized” URLs

www.x.com → www.x.com.nyud.net:8090

– Directs clients to Coral, which absorbs load

• Who might “Coralize” URLs?– Web server operators Coralize URLs

– Coralized URLs posted to portals, mailing lists

– Users explicitly Coralize URLs

httpprx dnssrv

Browser

Resolver

DNS RedirectionReturn proxy, preferably one near client

CooperativeWeb Caching

CoralCDN components

httpprx

www.x.com.nyud.net216.165.108.10

Fetch datafrom nearby

?

?

OriginServer

Functionality needed DNS: Given network location of resolver, return a

proxy near the client

put (network info, self)

get (resolver info) → {proxies}

HTTP: Given URL, find proxy caching object, preferably one nearby

put (URL, self)

get (URL) → {proxies}

Key Idea

• Use a distributed hash table – but locality is poor– So use multiple DHTs (called clusters)! – Each peer takes part in 3 clusters based on

network proximity (<20ms, <60ms, all others)– Insertions are done in all DHTs– Lookups prefer “nearest” DHT

• A lot more details in the paper.

• Coral lacks…– Central management

– A priori knowledge of network topology• Anybody can join system

– Any special tools (e.g., BGP feeds)

• Coral has…– Large # of vantage points to probe topology

– Distributed index in which to store network hints

– Each Coral node maps nearby networks to self

Challenges for DNS Redirection

• Coral DNS server probes resolver

• Once local, stay local

When serving requests from nearby DNS resolver– Respond with nearby Coral proxies

– Respond with nearby Coral DNS servers

→ Ensures future requests remain local

• Else, help resolver find local Coral DNS server

Coral’s DNS Redirection

• Return servers within appropriate cluster– e.g., for resolver RTT = 19 ms, return from cluster < 20 ms

• Use network hints to find nearby servers– i.e., client and server on same subnet

• Otherwise, take random walk within cluster

DNS measurement mechanism

Resolver

BrowserCoralhttpprxdnssrv

Server probes client (2 RTTs)

Coralhttpprxdnssrv

References

• Forrester Consulting. eCommerce Web Site Performance Today: An Updated Look At Consumer Reaction To A Poor Online Shopping Experience. Aug. 17, 2009.

• The Akamai Network: A Platform for High-Performance Internet Applications – Erik Nygren et al. - http://www.akamai.com/dl/technical_publications/network_overview_osr.pdf

• Democratizing Content Publication with Coral.Michael J. Freedman, Eric Freudenthal, and David Mazières. In Proc. 1st USENIX/ACM Symposium on Networked Systems Design and Implementation