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Slide 1
Sipat Triukose, Zhihua Wen, Michael Rabinovich WWW 2011
Presented by Ye Tian for Course CS05112
Slide 2
Overview Background and Motivation Methodology Performance of
Akamai CDN Performance of Consolidated Akamai CDN Review
Slide 3
Background A CDN operator deploys a large number of edge
servers throughout the Internet. outsources certain hostnames to
the CDNs domain name service
Slide 4
HTTP End-user cnn.com (content provider) DNS root server 12
Nearby Akamai cluster GET index. html 4
http://cache.cnn.com/cnn.com/foo.jpg HTTP Akamai cluster Akamai
global DNS server Akamai regional DNS server
Slide 5
HTTP End-user cnn.com (content provider) DNS root server 12
Nearby Akamai cluster 5 DNS lookup cache.cnn.com Akamai cluster 3 4
ALIAS: g.akamai.net Akamai global DNS server Akamai regional DNS
server
Slide 6
HTTP End-user cnn.com (content provider) DNS root server 12
Akamai global DNS server Akamai regional DNS server Nearby Akamai
cluster 6 Akamai cluster 3 4 6 5 ALIAS a73.g.akamai.net DNS lookup
g.akamai.net
Slide 7
HTTP End-user cnn.com (content provider) DNS root server 12
Akamai global DNS server Akamai regional DNS server Nearby Akamai
cluster 7 Akamai cluster 3 4 6 5 8 7 DNS a73.g.akamai.net Address
1.2.3.4
Slide 8
HTTP End-user cnn.com (content provider) DNS root server 12
Akamai global DNS server Akamai regional DNS server Nearby Akamai
cluster 8 Akamai cluster 3 4 6 5 8 7 9 GET /foo.jpg Host:
cache.cnn.com
Slide 9
HTTP End-user cnn.com (content provider) DNS root server 12
Akamai global DNS server Akamai regional DNS server Nearby Akamai
cluster 9 Akamai cluster 3 4 6 5 8 7 9 GET /foo.jpg Host:
cache.cnn.com 12 11 GET foo.jpg
Slide 10
HTTP End-user cnn.com (content provider) DNS root server 12
Akamai global DNS server Akamai regional DNS server Nearby Akamai
cluster 10 Akamai cluster 3 4 6 5 8 7 9 12 11 10
Slide 11
Background Example: outsourcing images.firm-x.com Step 1-2:
firm-xs DNS server redirects DNS queries for outsourced hostnames
to CDN by returning a CNAME; e.g., images.firm-x.com.CDNname.net
Step 3: The user resolves the canonical name, with a query that
will arrive at the DNS system for the CDN-name.net domain, operated
by the CDN Step 4-5: selects an appropriate edge server (typically
close to the client as long as server) and responds to the query
with the selected server IP address, enacting the content download
from the selected server
Slide 12
Example nslookup www.bestbuy.com Example AExample B
Sourceplanetlab9.millennium.be rkeley.edu planetlab2.dit.upm.es
Source locationBerkeley, CA, USAMadrid, Spain Local
DNS169.229.48.155138.4.2.10 CNAMEa1105.b.akamai.net Edge Server
IP173.223.232.160 173.223.232.112 130.206.192.47 130.206.192.49
Edge Server LocationSan Francisco, CA, USAMadrid, Spain
Slide 13
From Berkeley From Madrid
Slide 14
Motivation Two CDN design approaches Akamai-like design: Akamai
platform in 2007 spanned more than 3,000 locations in over 750
cities in over 70 countries, with each location having on average
less than ten servers Digital Island-like design: utilizes massive
data centers, comprising thousands of servers, but in many fewer
locations. Limelight currently lists 20 data centers on its web
site Question: How many locations is enough from the client-
observed performance perspective?
Slide 15
Overview Background and Motivation Methodology Performance of
Akamai CDN Performance of Consolidated Akamai CDN Review
Slide 16
Edge Server Discovery Needs to harvest a large collection of
edge servers since this requires hostname lookups resolutions from
different geographical locations. Utilize DipZoom: Typically more
then 400 active MPs, mostly on PlanetLab nodes but also on some
academic and residential hosts.
Slide 17
Edge Server Discovery Compiled canonical names outsourced to
Akamai of the 95 Akamai customers listed on the Akamais Web site.
Performed DNS resolution of these names from a large number of
network locations over a period of 13 weeks. By clustering these
edge servers by city and autonomous system, we conservatively
estimate we discovered at least 308 locations.
Slide 18
Youkus CDN Huajun He, Yang Zhao, Jinfu Wu and Ye Tian,
Cost-Aware Capacity Provisioning for Internet Video Streaming CDNs,
The Computer Journal (Oxford), 2015.
Slide 19
Overriding CDN Edge Server Selection Why? To measure download
performance from a particular edge server rather than the server of
Akamais choosing. How? Connect to the desired edge server directly
using its raw IP address rather than the DNS hostname from the
URL.
Slide 20
Controlling Edge Server Caching Why? Compare the download
performance from origin server and from the edge server cache. How?
Use HTTPs Cache-Control header. Not working Modern caches use the
entire URL strings, including the search string as the cache key.
On the other hand, origin servers ignore unexpected search strings
in otherwise valid URLs. A request for foo.jpg?randomstring will
return the valid foo.jpg image from the origin server.
Slide 21
Controlling Edge Server Caching 1, 4, 9 from origin server 2,
3,5, 6, 7, 8 from edge server Precisely control the caching
behavior of Akamais edge servers by appending search strings to
image URLs.
Slide 22
Assessing Client-Side Caching Bias How HTTP download
performance from a given Akamai edge server depends on its ping
round-trip latency. Perform a set of 4 curl downloads, 5 pings, and
a traceroute from each MP to various edge servers. Spearmans rank
correlation coefficient of 0.8074 indicating a strong
dependency.
Slide 23
Overview Background and Motivation Methodology Performance of
Akamai CDN Performance of Consolidated Akamai CDN Review
Slide 24
Does a CDN Enhance Performance? Compare download performance
Cache vs. non-cache (Note: non-cache is still outsourced) Cache vs.
origin CDN delivery outperforms both non-cache and origin delivery
in 98% and 96% of cases respectively. Missing penalty can be
significant Non-cache: first from CDN edge server to cache; then
from cache to the client
Slide 25
How Good Is Akamai Server Selection? How good is the Akamai
selection algorithm from the client performance perspective. For
each measuring point, the percentage of time that the
Akamai-selected server performed better than the alternative server
CDNs rarely select the best edge server but successfully avoid the
worst ones. In roughly 75% of the MPs, the Akamai-selected server
outperformed half of the alternatives.
Slide 26
How Good Is Akamai Server Selection? The average difference,
the maximum gain, and the maximum loss of throughput of the
Akamai-selected server vs. the alternatives. Maximum gain and
maximum loss are the throughput differences of the Akamai- selected
server over, resp., the slowest and fastest alternative server.
There is a substantial room for further improvements.
Slide 27
Overview Background and Motivation Methodology Performance of
Akamai CDN Performance of Consolidated Akamai CDN Review
Slide 28
Data Center Consolidation Group edge servers that are close to
each other into hypothetical consolidated data centers Refer to as
big clusters. Place each big cluster into the location of a central
server in the cluster called the representative of the cluster. For
a given client, we replace the server selected by Akamai, S akam
with the representative of the cluster to which S akam
belongs.
Slide 29
Data Center Consolidation Estimate the pairwise network
distances between all the servers using the dynamic triangles
method Use the so-called hierarchical clustering with complete
linkage method, following by the cut-the-tree procedure
Slide 30
DipZoom Experiment From each measurement point, downloaded the
object of a given size from the Akamai-selected server and from the
center of its cluster in the consolidated platform. Group 10,231
edge servers into 150, 100, 60, 40, and 20 data centers
Slide 31
DipZoom Experiment 150, 100, and 60 data centers: consolidated
and current platforms show performance advantage over each other
with equal probability. 40 and 20 data centers: the original
platform starts outperforming the consolidated configuration
Slide 32
DipZoom Experiment Group the MPs by the maximum download
throughput
Slide 33
DipZoom Experiment The existing Akamai configuration with large
number of data centers favors well-connected users. Compare the
performance of existing vs. consolidated configurations for all
measurement points with bandwidth below 1.5Mbps A typical download
bandwidth for DSL users.
Slide 34
A Live Study Built a Web page with an AJAX application that
measures latency to the server that Akamai selects for this browser
as well as to a list of other Akamai edge servers, and reports the
results to us. Pick a CNAME (a1694.g.akamai.net) which is mapped by
Akamai to 979 edge servers. Partitioned the 979 servers into 10,
50, and 100 clusters.
Slide 35
A Live Study Consider the differences between the measured
latencies from each client to its closest consolidated data center
and the Akamai-selected server. Negative indicates that the
consolidated platform is better Positive reflects existing Akamai
platform is better Even with significant consolidation, better
server selection can make up for any performance losses from
consolidation.
Slide 36
Summary Two CDN design approaches? Example. How CDN redirects
its clients to one of its edge server? From user performance
perspective, can we consolidate the Akamai CDN platform? To what
extent?