Web Cache Communication Protocol
To ease pressure on congested WAN links in networks with many hosts, Cisco developed WCCP to coordinate the work of edge routers and content engines (also known as cache engines). Content engines collect frequently accessed data, usually HTTP traffic, locally, so that when hosts access the same pages the content can be delivered from the cache engine rather than crossing the WAN. WCCP differs from web proxy operation in that the hosts accessing the content have no knowledge that the content engine is involved in a given transaction.
WCCP works by allowing edge routers to communicate with content engines to make each aware of the other's presence and to permit the router to redirect traffic to the content engine as appropriate. Figure 5-3 shows how WCCP functions between a router and a content engine when a user requests a web object using HTTP.
Figure 5-3 WCCP Operations Between a Router and a Content Engine \ Topic
The figure shows the following steps, with the main decision point on the content engine coming at Step 4:
Step 1 The client sends an HTTP Get request with a destination address of the web server, as normal.
Step 2 The router's WCCP function notices the HTTP Get request and redirects the packet to the content engine.
Step 3 The content engine looks at its disk storage cache to discover if the requested object is cached.
Step 4A If the object is cached, the content engine sends an HTTP response, which includes the object, back to the client.
Step 4B If the object is not cached, the content engine sends the original HTTP Get request on to the original server.
Step 5 If Step 4B was taken, the server replies to the client, with no knowledge that the packet was ever redirected to a content engine.
Using WCCP, which uses UDP port 2048, a router and a content engine, or a pool of content engines (known as a cluster), become aware of each other. In a cluster of content engines, the content engines also communicate with each other using WCCP. Up to 32 content engines can communicate with a single router using WCCPv1. If more than one content engine is present, the one with the lowest IP address is elected as the lead engine.
WCCP also provides a means for content engines within a cluster to become aware of each other. content engines request information on the cluster members from the WCCP router, which replies with a list. This permits the lead content engine to determine how traffic should be distributed to the cluster.
..■• In WCCPv1, only one router can redirect traffic to a content engine or a cluster of content engines.
/ Key i Topic In WCCPv2, multiple routers and multiple content engines can be configured as a WCCP service group. This expansion permits much better scalability in content caching. Furthermore, WCCPv1 supports only HTTP traffic (TCP port 80, specifically). WCCPv2 supports several other traffic types and has other benefits compared to WCCPv1:
Supports TCP and UDP traffic other than TCP port 80, including FTP caching, FTP proxy handling, web caching for ports other than 80, Real Audio, video, and telephony.
Permits segmenting caching services provided by a caching cluster to a particular protocol or protocols, and uses a priority system for deciding which cluster to use for a particular cached protocol.
Supports multicast to simplify configuration.
Supports multiple routers (up to 32 per cluster) for redundancy and load distribution. (All content engines in a cluster must be configured to communicate with all routers in that cluster.)
Provides for MD5 security in WCCP communication using the global configuration command ip wccp password password.
Provides load distribution.
Supports transparent error handling.
When you enable WCCP globally on a router, the default version used is WCCPv2. Because the WCCP version is configured globally for a router, the version number affects all interfaces. However, multiple services can run on a router at the same time. Routers and content engines can also simultaneously participate in more than one service group. These WCCP settings are configured on a per-interface basis.
Configuring WCCP on a router is not difficult because a lot of the configuration in a caching scenario takes place on the content engines; the routers need only minimal configuration. Example 5-4 shows a WCCPv2 configuration using MD5 authentication and multicast for WCCP communication.
Example 5-4 WCCP Configuration Example
! First we enable WCCP globally on the router, ! specifying a service (web caching), a multicast address for ! the WCCP communication, and an MD5 password: ip wccp web-cache group-address 239.128.1.100 password cisco ! Next we configure an interface to redirect WCCP web-cache ! traffic outbound to a content engine: int fa0/0 ip wccp web-cache redirect out
! Finally, inbound traffic on interface fa0/1 is excluded from redirection:
int fa0/1 ip wccp redirect exclude in
Finally, WCCP can make use of access lists to filter traffic only for certain clients (or to exclude WCCP use for certain clients) using the ip wccp web-cache redirect-list access-list global command. WCCP can also use ACLs to determine which types of redirected traffic the router should accept from content engines, using the global command ip wccp web-cache group-list access-list.
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