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Prefetching Hyperlinks Prefetching Methods Prefetching Uncacheable/Dynamic Data Effect on Network Load Prefetching and the Wireless World Wide Web Privacy Concerns Additional References February 22, 2000 1

SLIDE 1

February 22, 2000 1

Prefetching Hyperlinks

Prefetching Methods
Prefetching Uncacheable/Dynamic Data
Effect on Network Load
Prefetching and the Wireless World Wide Web
Privacy Concerns
Additional References

SLIDE 2

February 22, 2000 2

Prefetching Methods

History-based - prefetch a link only if you’ve accessed it

before.

Link-based - prefetch all embedded hyperlinks and their

images.

This implementation is a variant of link-based prefetching.

– Prefetch some links, according to usage statistics. – Limit prefetching based on estimated bandwidth usage and probability that the prefetched page will be accessed.

SLIDE 3

February 22, 2000 3

Prefetching Methods

Client side prefetching implementations (local).
Server side prefetching implementations (server-hint).
Previous work concludes that using both client & server

statistics resulted in higher hit rates (Section 2.1).

– This implementation is a hybrid using both client and server side statistics.

Should a server trust a client to correctly/accurately report

usage, referring links, etc?

Should a client trust a server to correctly/accurately report

usage, referring links, etc?

SLIDE 4

February 22, 2000 4

Prefetching Uncacheable/Dynamic Data

Prefetching advertisements

– Overstated hit rates since the document was fetched, but never accessed. – Section 5.1 indicates 42.6% of prefetched pages were eventually accessed (so 57.4% never accessed, for the math impaired). – Is this an important fact? Do we care?

Prefetching dynamic data

– Their implementation specifically ignores zero expiration value. – How to indicate that data expires immediately? Do we need non- prefetchable data (similar to uncacheable data)? – Do people like/dislike this implementation idea? What would you do differently?

SLIDE 5

February 22, 2000 5

Effect on Network Load

Prefetching leads to increased network load.

Client specifies a bandwidth limit to prefetching efforts.
Their implementation stopped prefetching when

probability of accessing page was < 25%, or bandwidth limit was exceeded.

Overall increase to network traffic was 24% (Section 5.1).

– Top 10 Approach [21] reports similar results - 40% of requests were prefetched, with 10-20% increase to network traffic. – Top 10 Approach also implements a trace driven simulation. – Top 10 Approach also suggests prefetching during slow periods (i.e. overnight).

SLIDE 6

February 22, 2000 6

Effect on Network Load

Prefetching can lead to bursty network traffic.

Rate controlled prefetching [9, 10]
It wasn’t clear to me if they addressed this issue or not…

SLIDE 7

February 22, 2000 7

Prefetching & Wireless WWW

Does prefetching make sense in a wireless environment?

+ Latencies are longer, so prefetching could (potentially) bring greater benefits. – Bandwidth is constrained, prefetching increases network load, so perhaps prefetching isn’t justified for this application? – Some wireless devices have limited memory

For example, wireless phones (not really true for laptops)
May not be able to store prefetched pages

? The author indicates that 5 KBps (40Kbps) is median bandwidth for prefetching to be effective. (Section 3.3, Figure 3)

Within range of dialup modem...
What about wireless devices?

SLIDE 8

February 22, 2000 8

Prefetching & Wireless WWW

SLIDE 9

February 22, 2000 9

Current Wireless Technologies

Example - WaveLAN

– Maximum rate of 11 Mbps – Rate varies from <1 Mbps to 11 Mbps depending on environment (1 Mbps at 50 m in a closed office, etc.)

Example - Qualcomm HDR (High Data Rate)

– Maximum rate of 2.4 Mbps on forward link, 307 Kbps on reverse link with 1.25 MHz channel – Average throughput of 600 Kbps on forward link, 220 Kbps on reverse link in a loaded sector

SLIDE 10

February 22, 2000 10

Prefetching & Wireless WWW

Does prefetching make sense in this environment?

The bandwidth of current wireless devices is > the

median required bandwidth indicated by these experiments.

Reduced client latency by 52.3%. Is this applicable to

wireless clients?

We haven’t addressed the issue of available memory on

the wireless device.

SLIDE 11

February 22, 2000 11

Prefetching & Wireless WWW

Does prefetching make sense in this environment?

Yes - but don’t prefetch to the wireless device/mobile,

prefetch to the proxy (WaveLAN access point, base station, etc.)?

Yes - but require more accurate prefetching. Perhaps

use a higher threshold below which you will no longer prefetch? (They used 25% in their paper.)

Maybe - because...?
No - because...?

SLIDE 12

February 22, 2000 12

Privacy Concerns

HTML must be available to prefetching mechanism

– End-to-end protocols, encryption, tunnels etc. make proxy implementation impossible.

Privacy groups request ability to allow clients not to send

the Referer field. This conflicts with prefetching goals.

Usage reports contain sensitive information, more so than

Referer field information.

User discomfort with prefetching

– Authors claim prefetching collects information that is already available via proxies, firewalls, server access, etc.

Server administrators reluctant to share information on

how documents on the server are accessed.

SLIDE 13

February 22, 2000 13

What I Liked...

Thorough discussion of previous work & how it related or

differed from their work.

Use of client & server information for predicting what

pages to prefetch.

Ability to set bounds on amount of data to be prefetched.
Ideas have actually been implemented (including proxy

versions).

SLIDE 14

February 22, 2000 14

What I Didn’t Like...

Extended httpd protocol, but no reference to detailed

information is given, you couldn’t go implement this yourself if you wanted to.

I’m not sure all dynamic data can be prefetched (probably

most of it can be, but what about the rest?).

SLIDE 15

February 22, 2000 15

Additional References

Product Data Sheet for WaveLAN/IEEE Turbo 11 Mb

Network Interface Cards FTP://FTP.WAVELAN.COM/PUB/DOCS/IEEE/brochure/e nglish/fs_pc11.pdf

Qualcomm, Inc. http://www.qualcomm.com/hdr/
E.P. Markatos and C. E. Chronaki. A Top-10 Approach to

Prefetching Hyperlinks

Prefetching Methods

before.

images.

– Prefetch some links, according to usage statistics. – Limit prefetching based on estimated bandwidth usage and probability that the prefetched page will be accessed.

Prefetching Methods

statistics resulted in higher hit rates (Section 2.1).

– This implementation is a hybrid using both client and server side statistics.

usage, referring links, etc?

usage, referring links, etc?

Prefetching Uncacheable/Dynamic Data

– Overstated hit rates since the document was fetched, but never accessed. – Section 5.1 indicates 42.6% of prefetched pages were eventually accessed (so 57.4% never accessed, for the math impaired). – Is this an important fact? Do we care?

– Their implementation specifically ignores zero expiration value. – How to indicate that data expires immediately? Do we need non- prefetchable data (similar to uncacheable data)? – Do people like/dislike this implementation idea? What would you do differently?

Effect on Network Load

Prefetching leads to increased network load.

probability of accessing page was < 25%, or bandwidth limit was exceeded.

– Top 10 Approach [21] reports similar results - 40% of requests were prefetched, with 10-20% increase to network traffic. – Top 10 Approach also implements a trace driven simulation. – Top 10 Approach also suggests prefetching during slow periods (i.e. overnight).

Effect on Network Load

Prefetching can lead to bursty network traffic.

Prefetching & Wireless WWW

Does prefetching make sense in a wireless environment?

+ Latencies are longer, so prefetching could (potentially) bring greater benefits. – Bandwidth is constrained, prefetching increases network load, so perhaps prefetching isn’t justified for this application? – Some wireless devices have limited memory

? The author indicates that 5 KBps (40Kbps) is median bandwidth for prefetching to be effective. (Section 3.3, Figure 3)

Prefetching & Wireless WWW

Current Wireless Technologies

– Maximum rate of 11 Mbps – Rate varies from <1 Mbps to 11 Mbps depending on environment (1 Mbps at 50 m in a closed office, etc.)

– Maximum rate of 2.4 Mbps on forward link, 307 Kbps on reverse link with 1.25 MHz channel – Average throughput of 600 Kbps on forward link, 220 Kbps on reverse link in a loaded sector

Prefetching & Wireless WWW

Does prefetching make sense in this environment?

median required bandwidth indicated by these experiments.

wireless clients?

the wireless device.

Prefetching & Wireless WWW

Does prefetching make sense in this environment?

prefetch to the proxy (WaveLAN access point, base station, etc.)?

use a higher threshold below which you will no longer prefetch? (They used 25% in their paper.)

Privacy Concerns

– End-to-end protocols, encryption, tunnels etc. make proxy implementation impossible.

the Referer field. This conflicts with prefetching goals.

Referer field information.

– Authors claim prefetching collects information that is already available via proxies, firewalls, server access, etc.

how documents on the server are accessed.

What I Liked...

differed from their work.

pages to prefetch.

versions).

What I Didn’t Like...

information is given, you couldn’t go implement this yourself if you wanted to.

most of it can be, but what about the rest?).

Additional References

Network Interface Cards FTP://FTP.WAVELAN.COM/PUB/DOCS/IEEE/brochure/e nglish/fs_pc11.pdf

Prefetching on the Web. In Proc. INET 98. July 1998. http://www.ics.forth.gr/proj/arch- vlsi/html_papers/inet98_prefetch/paper.html