Aug 3, 2009

RCSP Study Guide - WCCP Assignment and Redirection Methods

Assignment and Redirection Methods
The assignment method refers to how a router chooses which Steelhead appliance in a WCCP service group to redirect packets to. There are two assignment methods: the Hash assignment method and the Mask assignment method. Steelhead appliances support both the Hash assignment and Mask assignment methods.

HASH
Redirection using Hash assignment is a two-stage process. In the first stage a primary key is formed from the packet which is defined by the Service Group and is hashed to yield an index. This index number will then be placed into a Redirection Hash Table.

In the Redirection Hash Table a packet has either an unflagged web-cache, unassigned bucket, or a flagged packet. In the event the packet has an unflagged web-cache, the packet is redirected to that web-cache. If the bucket is unassigned the packet is forwarded normally. However, if the bucket is flagged indicating a secondary hash then a secondary key is formed (as defined by the Service Group description). This key is hashed to yield an index number which in turn is placed into the Redirection Hash Table. If this secondary entry contains a web-cache index then the packet is directed to that web-cache. If the entry is unassigned the packet is forwarded normally.

MASK
The first phase of Mask assignment is defining the mask itself. The mask can be up to seven bits and can be applied to the SRC TCP port, DST TCP port, source IP address or DST IP address or a combination of the four attributes but may not exceed seven bits. Depending on the amount of bits selected different number of buckets are created and assigned to the different Steelhead appliances in the service group. As traffic traverses the router a bitwise AND operation is performed between the mask and the IP address/TCP port depending on the mask defined. The traffic is assigned to the different buckets based on the results of the AND operation.

Mask IP address/TCP port pairs are processed in an order they are received and in turn are compared to the seven bits.

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