1. Internet: Quality of Service Mechanisms at Application Level
Special Topics in Communications Networks: QOS
Hanoch Levy
Computer Science
Tel-Aviv University
1999
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.Levy, Advanced Comm, CS, TAU, Application QOS

2. .Levy, Advanced Comm, CS, TAU, Application QOS
References
E. Cohen B. Krishnamurthy, J. Rexford, “Efficient Algorithms for Predicting Requests to Web Servers”,
E. Cohen B. Krishnamurthy, J. Rexford, “Evaluating Server-Assisted Cache Replacement in the Web”,
E. Cohen, H. Kaplan, “Reducing User-Perceived Latency by Prefetching Connections and Pre-warning Servers”,
others in cs-
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3. .Levy, Advanced Comm, CS, TAU, Application QOS
Internet and Web
Data traffic (internet mostly) -- grows at exponential rates.
Expected to dominate market (take considerable fraction of all traffic).
Web traffic -- dominant fraction of internet traffic.
Elementary Web operation: “get page” (httpget).
Translates into a TCP connection and a stream of IP packets.
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4. Application/Transport QOS
Control at network level: controls located at network switches (routers) and discriminate the packets to achieve performance objectives.
Control at application level: Controls located at special network points and discriminate high level objects (web documents) to achieve performance objectives.
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Web transfer chain
User computer
ISP (proxy)
server
network
User computer
Load balancing
servers
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.Levy, Advanced Comm, CS, TAU, Application QOS

6. Load Balancing (sharing)
Used for large volume web sites.
Site data is served by multiple mirrored servers (could be multiple locations).
Load sharing device: - Aware of relative load of the various servers - Directs new request to the lowly loaded server. - Desired: keep certain user on certain server (due to session continuity and management).
May operate on geographically distributed environment (LA + NY)
Make geography based decisions
Example:Cisco DistributedDirector
Q: what is session? How is controlled?
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Load balancing (cont.)
Example:Cisco DistributedDirector
Example: Radware
Q: what is session? How is controlled?
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8. Network Control (load balance)
Cannot operate at application level.
Also: network efficiency and routing “supposedly” handled at lower layers.
Yet: one can try optimize at application level:
Radware:
Multihoming environment: enterprise connection to internet is via multiple links (multiple ISP’s)
Done typically for redundancy
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.Levy, Advanced Comm, CS, TAU, Application QOS

9. Network Control (load balance)
internet
Isp2
(zahav)
Isp1
(netvision)
TAU
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.Levy, Advanced Comm, CS, TAU, Application QOS

10. Network Control (load balance)
Problems faced by TAU:
Balance the load between 2 isp’s (as to reduce delay)
Optimize: put requests on their specific faster link)
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.Levy, Advanced Comm, CS, TAU, Application QOS

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ISP (Proxy)
Proxy sits at ISP premises.
Receives requests from clients.
Sends request to Net and receives replies.
Sends replies to clients.
Intervene in between.
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Proxy Intervention
Security (not in our scope)
Prioritization and discrimination (Comai’s talk) between users.
Caching: - A large disk is used to store web pages that requested in the past.
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13. Caching and Prefetching
Caching: keep a copy of a document you think you might need in future
Prefetching: Bring and keep a copy of page you think you might need in future
Refreshing: For a page residing in cache, browser/proxy must check whether copy is still fresh enough or stale.
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Examples
Dynacache
CacheFlow
Inktome
Others
Squid - Most popular. Public domain.
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Sky Caching
Serves a network of ISP proxies.
Consists of a central server, connected via satellite (and network) to N ISP proxies.
Proxies report central on elements requested.
Central broadcasts the “freshly popular” documents to other proxies.
Example: Starr report retrieved from Net by first 3 proxies. All rest already got it due to skycache.
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.Levy, Advanced Comm, CS, TAU, Application QOS

16. Server Side Prefetching
Basic idea: Server knows the best site “next document” statistics.
Use Java code installed on site web pages (in addition to the other text/graphics).
The code starts running at user client.
This code retrieves the proper pages from the site and stores them at client’s local cache.
Ref: (Ra’anana)
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Example
Server knows that following page A users are likely to request page B.
On page A server attaches get_url(B) Java code.
Once A is retrieved, and while user reads it, Java code retrieves B (seamlessly).
When user retrieves B -- page comes from cache.
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18. Server Side Prefetching (cont.)
Advantage: server has good knowledge of public statistics.
Disadvantage: - In many sites “fan-out” is very large ==> requires site to transmit large volume (3x) to gain significant reduction in expected delay.
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Client (Browser)
Caching - done at browsers (Netscape, Explorer) User can control parameters (e.g. cache size, and freshness check frequency).
Prefetching - Add-on packages that reside on PC. - At idle times program runs in background and brings user popular pages. - Good mainly for sites often visited by user - Good mainly for relative static sites.
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Client (cont.)
Can utilize information on user patterns (e.g., time of coming to office).
Examples: Netaccelerator (search in TUCOWS).
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21. Connection Prefetching
Significant fraction of perceived delay is due to connection setup (1-2 secs).
Includes: - Domain name lookup - Opening TCP connection
Idea: - Once retrieved page A, browser (or proxy) identifies links to external sites. - Browser initiates domain name lookup and TCP connection to those sites. - Upon user requests to those sites -- latency reduction
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Distributed Cache
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WAN of caches (Akamai)
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