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Load Balancing Servers, Firewalls and Caches - Kopparapu C.

Kopparapu C. Load Balancing Servers, Firewalls and Caches - Wiley Computer Publishing, 2002. - 123 p.
ISBN 0-471-41550-2
Download (direct link): networkadministration2002.pdf
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For example, if we use concurrent connections as the metric to measure load and capacity, the GSLB load balancer must collect both the site’s capacity and the site’s current load in terms of concurrent connections. The GSLB load balancer can then compare the available capacity as a percentage of the total capacity for all the sites and pick a site that has the most available capacity.
It’s also worth noting that when the GSLB load balancer replies with a particular site as the best one in the DNS response, we cannot predict how many new users or requests that site will receive. The local DNS that got the DNS response may cache it and provide it to many subsequent queries. It may provide this DNS response to a dial-up user or a megaproxy resulting in drastically different load.
Geography-Based Site Selection
IP addresses are allocated in blocks to different countries and continents. A block of IP addresses is administered by the Asia Pacific Network Information Center (APNIC) for allocation in the Asia Pacific region. A block of IP addresses is reserved by the American Registery for Internet Numbers (ARIN) for regions including, but not limited to, North America and South America. Réseaux IP Européens Network Coordination Centre (RIPE NCC) administers another block for regions that include Europe and Africa. For more details, please refer to http://www.apnic.net/db/RIRs.html
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Selecting the Best Site
When the GSLB load balancer is picking the best site, it knows the IP address of the local DNS making the DNS query on behalf of the end user. The load balancer tries to match the address block of the local DNS to the address block of the different sites. If the IP address of the local DNS falls in North America, the GSLB load balancer picks the site in North America by looking at the address blocks for the VIPs.
The granularity of this mechanism may not be fine enough for some users. In general, this mechanism is probably good enough to provide a rough approximation and find a server that’s geographically close to the end user. There may be certain exceptions in which an IP address may belong to the North American block, but is used in a different continent. This policy will not be able to deal with these exceptions accurately. However, these exceptions may be tolerable for many customers.
Geographical proximity may not necessarily be the best site for an end user. A user in San Jose may get a better response time from a site located in Tokyo than in New York because of specific site load conditions in New York or due to the Internet backbone congestion in the United States.
User Response Time-Based Site Selection
We observed in Figure 5.1 that the end-user response time consists of three parts: client-side delay, Internet delay, and server-side delay. Server-side delay is independent of the end-user location and the GSLB load balancer can measure it and direct users accordingly. Client-side delay is independent of the Web site location and there is nothing we can do to control it. But the Internet delay depends on the location of the client and the location of the Web site. In order to measure Internet delay, we need to know the Web site location and the user location and then we need to find a way to measure the response time between the site and the clients. Although we know the Web site locations, we don’t know the user locations. Further, the users can be all over the world. There are several approaches to address this, but none of them is perfect. We discuss the various approaches first, and then discuss ways to combine them to get closer to an ideal solution.
All of the approaches discussed next will require a protocol that enables the GSLB load balancer and the local load balancer at each site to communicate with one another.
Ping
Ping is a program commonly used to check if an IP address is active. Ping also provides the amount of time it takes to hear back from the given IP address. When the GSLB load balancer receives the DNS query, it sends a request to the local load balancer at each site to ping the IP address of the local DNS that sent the DNS query. Each local load balancer sends a ping to the local DNS and reports the ping response time to the GSLB load balancer. The GSLB load balancer compares the response times and picks the site that has the fastest response time.
The first problem with this approach is that the DNS request is held up until the GSLB load balancer collects the ping response times from each local load balancer to the local DNS. Although each local load balancer measures this concurrently, this process does add latency to the DNS request and can potentially impact the first impression of an end user. We can address this by not holding up the DNS request. The GSLB load balancer replies to the DNS request based on other policies, and continues to collect the ping response times in the background. The GSLB load balancer uses this information for the next query from the same local DNS. This approach does not add any extra latency to the DNS response, but may not provide the best site for the first query.
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