WCCP Multihome Router Support

WCCP Multihome Router Support

As previously mentioned, the Cisco network caching solution enables a cache engine cluster to home to multiple WCCP-enabled routers for added redundancy. Thus, Web traffic from all of the WCCP home routers will be redirected to the cache cluster. For example, a cache engine cluster that is homing to both routers in a MHSRP router pair creates a fully redundant caching system, eliminating any single points of failure (Figure below)

Overload Bypass

With a sudden Web traffic surge, a cache engine cluster could become overloaded. To gracefully handle

this overload situation, each cache engine detects when it is overloaded, refuses additional requests, and

forwards them to the origin Web servers. The origin Web servers respond directly to the clients because

the bypassed requests were not handled by a cache engine.

The overloaded cache engine will resume accepting requests when it determines that it has the resources

to do so without retriggering overload bypass in the near future. The overload bypass on/off triggers are

automatically determined by CPU and file system load. In the extreme situation that the cache engine

becomes so overloaded that it is unable to respond to the basic WCCP status check messages from its

home router, the WCCP home router will remove the cache engine from the cluster and reallocate its

buckets.

Thus, overload bypass ensures that a cache engine cluster does not introduce abnormal latencies and

maintains network availability even under unusually high traffic conditions.

Dynamic Client Bypass

Some Web sites require clients to be authenticated using the client's IP address. However, when a network cache is inserted between a client and a Web server, the Web server only sees the cache's IP

address and not the client's IP address.

To overcome this issue and similar situations, the Cisco Cache Engine has a dynamic client bypass

feature that effectively allows clients, under certain conditions, to bypass cache engines and directly

connect to origin Web servers. The result is that a Cisco Cache Engine can preserve existing source IP

authentication models and pass through server error messages to clients. Because the cache engine dynamically adapts to these situations, less management is required to ensure cache transparency.

Dynamic Client Bypass Function

In Figure below, a client issues a Web request, which is redirected to a cache engine. If the cache engine

does not have the content, it will try to fetch the content from the origin Web server.

If the server responds to the cache engine with certain HTTP error return codes (such as 401-Unauthorized request, 403-Forbidden, or 503-Service Unavailable), the cache engine will invoke the dynamic client bypass feature. The cache engine will dynamically store a client IP-destination IP address bypass pair, so that future packets with this IP address pair will bypass the cache engine. The cache engine sends an automatic HTTP retry message to the client's browser.

when the client's browser automatically issues a reload, the request will be redirected to the cache engine. However, when the bypass table is checked and the request matches one of the table entries, the cache engine will refuse the request and send it directly to the origin Web server. Thus, the origin Web server will see the client's IP address, authenticate the client, and respond directly to the client.