Citrix NetScaler EE, Руководство по установке и настройке

Страница: 536 / 1026

© 1999-2017 Citrix Systems, Inc. All rights reserved. p.536 https://docs.citrix.com
performed at the hardware layer, giving it the maximum performance. Because of its performance advantage, L2
forwarding is the preferred mode, but not all WCCP-capable devices support it.
GRE Mode— Generic Routing Encapsulation (GRE) is a routed protocol and the appliance can in theory be placed
anywhere, but for performance it should be placed close to the router, on a fast, uncongested path that traverses as
few switches and routers as possible. GRE is the original WCCP mode. A GRE header is created and the data packet is
appended to it. The receiving device removes the GRE header. With encapsulation, the appliance can be on a subnet that
is not directly attached to the router. However, both the encapsulation process and the subsequent routing add CPU
overhead to the router, and the addition of the 28-byte GRE header can lead to packet fragmentation, which adds
additional overhead.
WCCP mode supports multiple routers and both GRE vs. L2 forwarding. Each router can have multiple WAN links. Each link
can have its own WCCP service group.
Traffic shaping is not effective unless the appliance manages UDP traffic as well as TCP traffic. A second service group, with
a UDP service group for each WAN link, is recommended if traffic shaping is desired.
A WCCP client (an appliance) uses UDP port 2048 to register itself with the router and to negotiate which traffic should be
sent to it, and also which WCCP features should be used for this traffic. The appliance operates on this traffic and
forwards the resulting traffic to the original endpoint. The status of an appliance is tracked through the WCCP registration
process and a heartbeat protocol. The appliance first contacts the router over the WCCP control channel (UDP port 2048),
and the appliance and router exchange information with packets named “Here_I_Am” and “I_See_You,” respectively. By
default, this process is repeated every ten seconds. If the router fails to receive a message from the appliance for three of
these intervals, it considers the appliance to have failed and stops forwarding traffic to it until contact is reestablished.
Different appliances using the same router can provide different services. To keep track of which services are assigned to
which appliances, the WCCP protocol uses a service group identifier, a one-byte integer. When an appliance registers itself
with a router, it includes service group numbers as well.
A single appliance can support more than one service group.
A single router can support more than one service group.
A single appliance can use the same service group with more than one router.
A single router can use the same service group with more than one appliance. For SD-WAN appliances, multiple
appliances are supported in WCCP cluster mode, and a single appliance is supported in WCCP mode.
Each appliance specifies a “return type” (L2 or GRE) independently for each direction and each service group. SD-WAN
WANOP 4000/5000 appliances always specify the same return type for both directions. Other SD-WAN appliances allow
the return type to be different.
Figure 2. Using different WCCP service groups for different services
Mult iple service groups Mult iple service groups can be used with WCCP on the same appliance. For example, the appliance can receive service-
group 51 traffic from one WAN link and service-group 62 traffic from another WAN link. The appliance also supports
multiple routers. It is indifferent to whether all the routers use the same service group or different routers use different
service groups.
Service Group Tracking Service Group Tracking. If a packet arrives on one service group, output packets for the same connection are sent on