WCCP Overview

WCCP is a transparent interception protocol first developed by Cisco Systems, Inc. in 1997. WCCP is a control plane protocol that runs between devices running Cisco IOS and WCCP "clients" such as WAAS. The protocol enables the network infrastructure to selectively intercept traffic based on IP protocol and port numbers, and redirect that traffic to a WCCP client. WCCP is considered transparent, because it allows for local interception and redirection of traffic without any configuration changes to the clients or servers. WCCP has built-in load-balancing, scalability, fault-tolerance, and service assurance (fail open) mechanisms

The current version, WCCPv2, is used by Cisco WAAS to transparently intercept and redirect all TCP traffic, regardless of port. The following section describes the basic WCCPv2 concepts and how they are specifically used by Cisco WAAS

Service Groups

The routers and WAEs participating in the same service constitute a service group. A service group defines a set of characteristics about what types of traffic should be intercepted, as well as how the intercepted traffic should be handled. There are two types of service groups:

• Well-known services
• Dynamic services

Well-known services, also referred to as static services, have a fixed set of characteristics that are known by both IOS and WCCPv2 client devices. There is currently a single well-known service called web-cache. This service redirects all TCP traffic with a destination port of 80. The characteristics of a dynamic service are initially only known to the WCCPv2 clients within the service group. The characteristics of the service group are communicated to the IOS devices by the first WCCPv2 client device to join the service group.

A unique service ID identifies service groups, which is a number from 0 to 255. Service IDs 0 to 50 are reserved for well-known services.

The WCCPv2 implementation in WAAS supports a single dynamic WCCPv2 service, the tcp-promiscuous service. Although referred to in WAAS as a single service, the tcp-promiscuous service is in fact two different services. The two service IDs enabled with the tcp-promiscuous service are 61 and 62. These are the two service group IDs that are configured in IOS when using WCCPv2 with WAAS. Two different service groups are used because both directions (client-to-server and server-to-client) of a TCP connection must be transparently intercepted. To optimize a connection, WAAS must see both directions of the connection on the same WAE. Not only does WAAS intercept the connection in both directions, but it also intercepts the connection on both sides of the WAN link. Because the packet Layer 3 and Layer 4 headers are preserved, transparent interception is used on both sides of the WAN in both directions to redirect connections to the WAAS infrastructure for optimization.

What is the difference between services 61 and 62? 

WCCP Service Group Attributes

RTR-02# show ip wccp 61 service
WCCP service information definition:
        Type:          Dynamic
        Id:            61
        Priority:      34
        Protocol:      6
        Options:       0x00000501
        --------
            Hash:      SrcIP
            Alt Hash:  SrcIP SrcPort
            Ports:     -none-

RTR-02#
RTR-02# show ip wccp 62 service
WCCP service information definition:
        Type:          Dynamic
        Id:            62
        Priority:      34
        Protocol:      6
        Options:       0x00000502
        --------
            Hash:      DstIP
            Alt Hash:  SrcIP SrcPort
            Ports:     -none-

The tcp-promiscuous services define TCP as the protocol and do not define any ports. By not defining any ports as part of the service groups, this causes interception and redirection of all TCP traffic. When traffic passes through an interface in the IOS device with WCCPv2 redirection configured, it is evaluated against the protocol and port combination defined by the service to determine whether or not the packet should be redirected. By default this is the only criteria that is used to determine whether or not a packet is redirected. It is important to note that the IOS WCCPv2 implementation is not stateful. This means that IOS WCCPv2 is only dealing with redirected traffic on a packet-by-packet basis. It does not keep track of TCP connection state for redirected traffic. On the other hand, the WCCPv2 implementation in WAAS is stateful. WAAS tracks each connection as a flow throughout the life of the connection.The command output shows that the only difference between services 61 and 62 is the value from the packet used as the hash key. By default, service group 61 hashes on the source IP address and service group 62 hashes on the destination IP address. . By default, the spoof-client-ip feature is enabled for both services. This is the WCCPv2 feature that allows WAAS to handle optimized traffic transparently. Traffic forwarded to the WAE uses the same source and destination IP addresses and TCP ports as when it entered the WAE.