1. Redirection and Load Balancing
Herng-Yow Chen

2. Outline HTTP redirection DNS redirection Anycast routing
Policy routing
IP MAC forwarding
IP address forwarding

3. Outline (cont.) The Web Cache Coordination Protocol (WCCP)
The Intercache Communication Protocol (ICP)
The Hyper Text Caching Protocol (HTCP)
The Network Element Control Protocol (NECP)
The Cache Array Routing Protocol (CARP)
The Web Proxy Autodiscovery Protocol (WPAD)

4. Why Redirect?
Redirection is a fact of life in the modern Web because HTTP applications always want to do three things:
Perform HTTP transactions reliably
Minimize delay
Conserve network bandwidth
For these reasons, web content often is distribute in multiple locations.
Redirection can be considered as a set of techniques that help to find “best” distributed content.
Redirection vs. load balancing
Most redirection deployments include some form of load balancing.
Conversely, any form of load balancing involved redirection techniques.

5. Where to redirect
General techniques: many redirection techniques work for web servers, proxies, caches, and gateways because of their common, server-like traits.
Specialized techniques: specially designed for a particular class of endpoint.
Web servers handle requests on a per-IP basis. Distributing requests to duplicate servers means that each request for a specific URL should be sent to an optimal web server (the one nearest to the client, or the least-loaded one, or some other optimization).
Proxies tend to handle requests on a per-protocol basis, all HTTP traffic in the neighborhood of a proxy should go through the proxy.

6. Overview of redirection protocols
The goal of redirection is to send HTTP messages to available web servers as quickly as possible. Several mechanisms can be provided for redirection:
The browser application’s proxy configuration. (only for redirecting traffic to proxies)
HTTP redirections.
DNS resolvers. (can be used for redirecting traffic to any server).
TCP/IP Routing (e.g. Routers and switches)

7. HTTP redirections How it works (next slide)
Basis for rerouting (many options)
From round-robin load balancing,
to minimizing latency,
To choosing the shortest path
Limitation
Can be slow – every transaction involves the extra redirect step. Also, the first server must be able to handle the request load

8. HTTP Redirections Internet Internet HTTP/1.0 302 Redirection
Locations:
HTTP Redirections
(a)Alice sends HTTP request to
(b)Server returns 302redirect to
Alice
(c) Browser resends HTTP request,this time to
Internet
Bob
(d)Bob sends HTTP request to
Alice
(e)Server returns 302redirect to
(f)Browser resends HTTP request,this time to
Internet
Bob

9. HTTP Redirection (cont.)
Several disadvantages
A significant amount of processing power is required from the original server. (Sometime almost as much server horse-power is required to issue the redirect as would be to serve up the page itself.)
User delays are increased, because two round trips are required to access pages.
If the redirecting server is broken, the site will be broken.

10. DNS redirections How it works (next slide)
Try nslookup ccsun.ncnu.edu.tw,
Basis for rerouting (many options)
From round-robin load balancing,
to minimizing latency,
To choosing the shortest path
Limitation
Need to configure DNS server

11. DNS Redirection DNS server www.joes-hardware.com www.joes-hardware.com
Decides whether to resolve to , , ,
Server4
Server1
Backbone network
router
Switch
Edge network
Server2
Server3

12. DNS round robin for load balancing
(a)Alice asks DNS for IP address of
(b) DNS replies with
Alice
DNS Server
(c) Alice sends HTTP request to
Internet
Bob
(d)Bob asks DNS for IP address of
Alice
(eDNS replies with
DNS Server
(f) Bob sends HTTP request to
Internet
Bob

13. The impact of DNS caching
DNS address rotation spreads the load around, because each DNS lookup to a server gets a different ordering of server addresses.
However, this load balancing isn’t perfect, because the results of the DNS lookup may be memorized and reused by applications, operating systems, and some primitive child DNS servers.
Many web browsers perform a DNS lookup for a host but then use the same address over and over again, to eliminate the cost of DNS lookups and because some servers prefer to keep talking to the same client.

14. The impact of DNS caching
Furthermore, many OSs perform the DNS lookup automatically, and cache the result, but don’t rotate the addresses.
Consequently, DNS round robin generally doesn’t balance the load of a single client – one client typically will be stuck to one server for a long time.
However, it can spread the aggregate load of multiple clients. As long as there is a modestly large number of clients with similar demand, the load will be relatively well distributed across servers.

15. Other DNS-based redirection algorithms
Load-balancing algorithms
DNS servers keep track of the load on the web servers and place the least-loaded web servers at the front of the list.
Proximity-routing algorithms
DNS servers can attempt to direct users to nearby web servers, when the farm of web servers is geographically dispersed.
Fault-masking algorithms
DNS servers can monitor the health of the network and route requests away from service interruptions or other faults.

16. DNS request involving authoritative server

17. DNS-based redirection (cont.)
Typically, the DNS server that runs sophisticated server-tracking algorithm is an authoritative server that is under the control of the content provider.
Several distributed hosting services use this DNS redirection model.
One drawback
The authoritative DNS serer uses to make its decision is the IP address of the local DNS server,
Not the IP address of the client.

18. Anycast Addressing Backbone network www.joes-hardware.com 10.10.10.1
Server1
Backbone network
router
router
Switch
router
Edge network
Server2
Server3

19. IP MAC Forwarding Client MAC 1 Client MAC 2 Switch MAC4 Hub MAC3
To Internet
Gateway MAC5
Client MAC 2

20. IP MAC Forwarding Client MAC 1 Client MAC 2 Switch MAC4 Hub MAC3
To Internet
Gateway MAC5
Port 80 traffic
Client MAC 2
Caching proxy MAC6

21. IP Address Forwarding Joe’s server Joe’s edge network Backbone network
Destination proxy
Joe’s server
Backbone network
router
Switch
Joe’s edge network
Edge network

22. IP Address Forwarding HTTP data ■ HTTP data ■ Form:Client: 80
Form:NAT device: 80
To:Joe’s Server 80
To:Proxy 80
Passes through network address translation (NAT) device
HTTP data ■
─────────────
HTTP data ■
─────────────

23. Proxy Auto-configuration

24. PAC file autodiscovery

25. GRE packet encapsulation
To Proxy:
8080
Proto:GRE
Form:Client: 80
Form:Client: 80
To:Joe’s Server 80
To:Joe’s Server 80
Passes through WCCP router
HTTP data ■
─────────────
HTTP data ■
─────────────

26. Cache Array Routing Protocol

27. Cache Array Routing Protocol

28. Hyper Text Caching protocol