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Bài giảng Mạng máy tính nâng cao - Chương 4: Content Distribution Networks (CDNs) - Lê Ngọc Sơn

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Content Distribution Networks  
(CDNs)  
MẠNG MÁY TÍNH NÂNG CAO  
Single Server, Poor Performance  
qSingle server  
qPopular content  
§ Single point of  
§ Popular site  
failure  
§ Easily overloaded  
§ “Flash crowd” (aka  
“Slashdot effect”)  
§ Far from most clients § Denial of Service  
attack  
2
Web Caching  
3
4
Proxy Caches  
origin  
server  
qReactively replicates  
popular content  
qSmaller round-trip  
Proxy  
server  
client  
times to clients  
qReduces load on  
origin servers  
client  
qReduces network load, and bandwidth costs  
qMaintain persistent TCP connections  
Forward Proxy  
qCache close to the client  
Proxy  
server  
§ Improves client performance  
client  
§ Reduces network provider’s costs  
qExplicit proxy  
§ Requires configuring browser  
client  
qImplicit proxy  
§ Service provider deploys an “on path” proxy  
§ … that intercepts and handles Web requests  
5
Reverse Proxy  
qCache close to server  
origin  
server  
§ Improve client performance  
§ Reduce content provider cost  
Proxy  
server  
§ Load balancing, content  
assembly, transcoding, etc.  
qDirecting clients to the proxy  
§ Map the site name to the  
origin  
server  
IP address of the proxy  
6
Google Design  
.
.
.
a Centers  
Servers  
Servers  
Private  
Backbone  
Reverse  
Proxy  
Reverse  
Proxy  
Internet  
Requests  
Client  
Client Client  
Limitations of Web Caching  
qMuch content is not cacheable  
§ Dynamic data: stock prices, scores, web  
cams  
§ CGI scripts: results depend on parameters  
§ Cookies: results may depend on passed data  
§ SSL: encrypted data is not cacheable  
§ Analytics: owner wants to measure hits  
qStale data  
§ Or, overhead of refreshing the cached data  
8
Content Distribution  
Networks  
9
Content Distribution Network  
origin server  
in North America  
qProactive content replication  
§ Content provider (e.g., CNN)  
contracts with a CDN  
CDN distribution node  
qCDN replicates the content  
§ On many servers spread  
throughout the Internet  
qUpdating the replicas  
§ Updates pushed to replicas when  
CDN server  
in S. America  
the content changes  
CDN server  
CDN server  
in Europe  
in Asia  
10  
Server Selection Policy  
qLive server  
Requires continuous monitoring of  
liveness, load, and performance  
§ For availability  
qLowest load  
§ To balance load across the servers  
qClosest  
§ Nearest geographically, or in round-trip time  
qBest performance  
§ Throughput, latency, …  
qCheapest bandwidth, electricity, …  
11  
12  
Server Selection Mechanism  
qApplication  
qAdvantages  
§ HTTP redirection  
§ Fine-grain control  
§ Selection based on  
client IP address  
GET  
qDisadvantages  
§ Extra round-trips for  
Redirect  
TCP connection to  
GET  
server  
OK  
§ Overhead on the  
server  
13  
Server Selection Mechanism  
qRouting  
qAdvantages  
§ Anycast routing  
§ No extra round trips  
§ Route to nearby  
server  
qDisadvantages  
1.2.3.0/24  
§ Does not consider  
network or server load  
§ Different packets may  
go to different servers  
§ Used only for simple  
request-response apps  
1.2.3.0/24  
Server Selection Mechanism  
qNaming  
qAdvantages  
§ DNS-based server  
selection  
§ Avoid TCP set-up delay  
§ DNS caching reduces  
overhead  
§ Relatively fine control  
1.2.3.4  
qDisadvantage  
DNS  
query  
§ Based on IP address of  
local DNS server  
§ “Hidden load” effect  
1.2.3.5  
§ DNS TTL limits  
local DNS server  
adaptation  
14  
How Akamai Works  
15  
Akamai Statistics  
qDistributed servers qClient requests  
§ Servers: ~61,000  
§ Networks: ~1,000  
§ Countries: ~70  
§ Hundreds of  
billions per day  
§ Half in the top  
45 networks  
qMany customers  
§ 15-20% of all  
§ Apple, BBC, FOX, GM  
IBM, MTV, NASA,  
NBC, NFL, NPR,  
Web traffic  
worldwide  
Puma, Red Bull,  
Rutgers, SAP, …  
16  
17  
How Akamai Uses DNS  
cnn.com (content provider)  
DNS root server  
GET  
index.  
html  
http://cache.cnn.com/foo.jpg  
Akamai  
cluster  
Akamai global  
DNS server  
1 2  
HT
HTTP  
Akamai regional  
DNS server  
Nearby  
Akamai  
End user  
cluster  
18  
How Akamai Uses DNS  
cnn.com (content provider)  
DNS root server  
DNS lookup  
cache.cnn.com  
Akamai  
cluster  
Akamai global  
DNS server  
3
1 2  
HT
ALIAS:  
4
Akamai regional  
DNS server  
g.akamai.net  
Nearby  
Akamai  
End user  
cluster  
19  
How Akamai Uses DNS  
cnn.com (content provider)  
DNS root server  
DNS lookup  
g.akamai.net  
Akamai  
cluster  
Akamai global  
DNS server  
5
6
3
1 2  
HT
4
Akamai regional  
DNS server  
ALIAS  
a73.g.akamai.net  
Nearby  
Akamai  
End user  
cluster  
20  
How Akamai Uses DNS  
cnn.com (content provider)  
DNS root server  
Akamai  
cluster  
Akamai global  
DNS server  
5
3
1 2  
HTTP  
6
7
4
Akamai regional  
DNS server  
8
Address  
1.2.3.4  
Nearby  
End user  
Akamai  
cluster  
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