The Cisco WAAS uses redirect method and assignment method to move packets between router to waas and to load balance packets among WAAS respectively. These methods are described in detail in following sections.
The main benefit of transparent redirection is that users need not configure their browsers to use a web proxy. Instead, they can use the target URL to request content, and have their requests automatically redirected to a cache engine. The word "transparent" is this case means that the end user does not know that a requested file (such as a web page) came from the cache engine instead of from the originally specified server. When a cache engine receives a request, it attempts to service it from its own local cache. If the requested information is not present, the cache engine issues its own request to the originally targeted server to get the required information. When the cache engine retrieves the requested information, it forwards it to the requesting client and caches it to fulfill future requests, thus maximizing download performance and substantially reducing transmission costs.
WCCP v1 and v2
With WCCP-Version 1, only a single router services a cluster. In this scenario, this router is the device that performs all the IP packet redirection. Content is not duplicated on the cache engines. The benefit of using multiple caches is that you can scale a caching solution by clustering multiple physical caches to appear as one logical cache. Multiple routers can use WCCPv2 to service a cache cluster. This is in contrast to WCCPv1, in which only one router could redirect content requests to a cluster. Using WCCPv1, the cache engines were configured with the address of the single router. WCCPv2 requires that each cache engine be aware of all the routers in the service group.
The Redirect Method, also known as the Forwarding Method, is the method by which redirected packets are transported from router to WAAS device. This method is negotiated between the router and the WAAS device.
Cisco WAAS supports two different Forwarding Methods:
1. WCCP GRE
2. WCCP Layer 2 (L2)
WCCP GRE, also known as Layer 3 Generic Routing Encapsulation (GRE), allows packets to reach the WAAS device even if there are other routers in the path between the forwarding router and the WAAS device. The connection between the router and the WAAS device is also known as a GRE Tunnel. Packet redirection is handled entirely by the router software. GRE encapsulates the selected datagram with the GRE header containing the routing information to the selected WAAS device. The WAAS device de-encapsulates the datagram, evaluates the payload using the static bypass rules and WAAS Policy specification, and either accepts or rejects the packet. If the packet is accepted for optimization, standard TCP connection setup occurs between the client and the WAAS device and between the WAAS device and the destination server. If the packet is rejected because of a static bypass rule, it is re-encapsulated and returned to the router. The router understands that the WAE is not interested in this connection and forwards the packet to its original destination. All other packets, pass-through or optimized, are returned to the router using the configured packet egress method.
WCCP L2 (Layer-2 ) redirection takes advantage of internal switching hardware that either partially or fully implements the WCCP traffic interception and redirection functions at Layer 2. Redirection occurs by overwriting the original MAC header of the IP packet with the MAC header of one of the WAAS devices in the Service Group. With L2 Redirection, the first redirected traffic packet is handled by either the router software or router hardware, depending on the platform and/or software version. The rest of the traffic may be handled by the router hardware on supported routers and switches making L2 redirection more efficient than Layer 3 GRE. Using L2 Redirection as a forwarding method allows direct forwarding to the WAAS device without further lookup. Layer-2 redirection requires that WAAS devices be directly connected to an interface on each WCCP router.
The Assignment Method is the method by which redirected packets are distributed between the WAAS devices in a Service Group effectively providing load balancing among the WAAS devices. This method is negotiated between a router and all cache engines on a per Service Group basis. Cache engines participating in multiple Service Group may have different assignment methods for each Service Group but all cache engines within a single Service Group will use the same Assignment Method. A router may advertise the supported assignment methods for a Service Group using the optional Capabilities Info component of the WCCP2_I_SEE_YOU message. The absence of such an advertisement implies the router supports the default Hash assignment method only.
There are two types of assignment methods:
1. Hash Table Assignment
2. Mask/Value Sets Assignment
The default Assignment Method uses Hash Tables to load balance and select a particular WAAS device from those registered in the Service Group. With Hash Assignment, each router in the Service Group uses a 256-bucket Redirection Hash Table to distribute traffic for a Service Group across the member WAAS devices. The hash key may be based on any combination of the source and destination IP and port of the packet. For WAAS, load-balancing hashing is based on a source IP address (default), a destination IP address, or both.
When using mask assignment, each router uses masks and a table of values to distribute traffic for a Service Group across the member WAAS devices. It is the responsibility of the Service Group's designated cache engine to assign each router's mask/value sets. For WAAS, the default mask value is 0x1741 and is applied to the source IP address for service 61 and the destination IP address for service 62. The Mask Value can be specified with a maximum of 7 bits and like the hash key, can be configured to cover both the source as well as the destination address space.
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