Citrix NetScaler EE, Installation And Configuration Manual

Page: 551 / 1026

© 1999-2017 Citrix Systems, Inc. All rights reserved. p.551 https://docs.citrix.com
Load-Balancing in the WCCP Cluster
Sep 12, 2014
Traffic is distributed among the appliances in the WCCP cluster. If an appliance leaves the cluster (through failure, overload,
or being manually disabled), its traffic is rebalanced by distributing it among the surviving members. If an appliance joins the
cluster, traffic is rebalanced once more to give the new appliance its fair share.
T he Address Mask T he Address Mask
Traffic is distributed on the basis of an address mask that is applied to the source and destination addresses of WAN
traffic. You must select an appropriate mask field for efficient load-balancing. An inappropriate mask can result in load-
balancing that is poor to nonexistent. For example, if the mask matches an address field that is identical at all your remote
sites, all your WAN traffic is sent to a single appliance, overloading it. For example, if all of your remote sites have an address
in the form of 10.0.x.x, and your mask bits are within the 10.0 portion of the address all traffic is sent to a single appliance.
The address bits extracted by the address mask are used as an index that is used (indirectly) to select one of the WCCP
caches (appliances). For example, an address mask with two ”one” bits results in four possible values, depending on the
address. Each of these values can be thought of as a bucket. With two mask bits, you have four buckets, numbered 0-3.
The buckets are assigned to WCCP caches. To be effective, there must be at least as many buckets as caches. If you use a
two-bit mask and have five or more caches, some caches are idle, because each bucket is assigned to only one cache, and
there are not enough buckets to cover all five caches:
Cache 1 2 3 4 5
Buckets 0 1 2 3 -
If there are more buckets than caches, some caches are assigned multiple buckets. For example, if you set three mask bits,
creating eight buckets, and you have four caches, two buckets are assigned to each cache. If you have five caches, three
caches are assigned two buckets each, and two caches are assigned just one. If each bucket represents the same number
of users, you have a 2:1 load imbalance across caches:
Cache 1 2 3 4 5
Buckets 0-1 2-3 4-5 6 7
Increasing the number of set mask bits reduces this imbalance. With four mask bits (16 index values) and five caches, four
caches receive three buckets and one cache receives four buckets, resulting in only a 4:3 imbalance. With six set mask bits
(the largest number supported), four caches receive 13 buckets and one receives 12, which is only a 13:12 load imbalance.
Cache 1 2 3 4 5
Buckets 0-12 13-25 26-38 39-51 52-63
Ideally, you would like each remote site to be directed to a single appliance in the WCCP cluster, so that all traffic to and
from a given site is stored in the same compression history. With this arrangement, any traffic from one user at the site can
be used to compress similar traffic from any other user at that site. In other words, for compressibility, load-balancing works
best if it the address mask selects the bits that differentiate one remote site from another. These are often the least-
significant bits of the subnet portion of the IP address. Using these bits tends to allocate the same number of remote sites
(not users) per local appliance. A mask that aligns with the host portion of the address instead of the subnet results in a