Cisco Unified Contact Center Enterprise Solution Reference ...

Chapter 11 Bandwidth Provisioning and QoS Considerations
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Unified CCE Network Architecture Overview
routers with priority queuing in a way that gives preference to TCP packets with a high-priority IP
address and to UDP heartbeats over the other traffic. When using this prioritization scheme, 90% of the
total available bandwidth should be granted to the high-priority queue
A QoS-enabled network applies prioritized processing (queuing, scheduling, and policing) to packets
based on QoS markings as opposed to IP addresses. Unified ICM Release 7.0 provides marking
capability of both Layer-3 DSCP and Layer-2 802.1p (using the Microsoft Windows Packet Scheduler)
for private and public network traffic. Traffic marking in Unified ICM implies that configuring dual IP
addresses on the Network Interface Controller (NIC) is no longer necessary because the network is
QoS-aware. If the traffic is marked at the network edge instead, however, dual-IP configuration is still
required to differentiate packets by using access control lists based on IP addresses. For details, see
Where to Mark Traffic, page 11-9.
UDP Heartbeat and TCP Keep-Alive
The primary purpose of the UDP heartbeat design is to detect if a circuit has failed. Detection can be
made from either end of the connection, based on the direction of heartbeat loss. Both ends of a
connection send heartbeats at periodic intervals (typically every 100 or 400 milliseconds) to the opposite
end, and each end looks for analogous heartbeats from the other. If either end misses 5 heartbeats in a
row (that is, if a heartbeat is not received within a period that is 5 times the period between heartbeats),
then the side detecting this condition assumes that something is wrong and the application closes the
socket connection. At that point, a TCP Reset message is typically generated from the closing side. Loss
of heartbeats can be caused by various reasons, such as: the network failed, the process sending the
heartbeats failed, the machine on which the sending process resides is shut down, the UDP packets are
not properly prioritized, and so forth.
There are several parameters associated with heartbeats. In general, you should leave these parameters
set to their system default values. Some of these values are specified when a connection is established,
while others can be specified by setting values in the Microsoft Windows 2000 registry. The two values
of most interest are:
The amount of time between heartbeats
The number of missed heartbeats (currently hard-coded as 5) that the system uses to determine
whether a circuit has apparently failed
The default value for the heartbeat interval is 100 milliseconds between the duplexed sides, meaning that
one side can detect the failure of the circuit or the other side within 500 ms. Prior to Unified ICM
Release 5.0, the default heartbeat interval between a central site and a peripheral gateway was 400 ms,
meaning that the circuit failure threshold was 2 seconds in this case.
In Unified ICM Releases 5.0 and 6.0, as a part of the Unified ICM QoS implementation, the UDP
heartbeat is replaced by a TCP keep-alive message in the public network connecting a Central Controller
to a Peripheral Gateway. (An exception is that, when a Unified ICM Release 5.0 or 6.0 Central Controller
talks to a PG that is prior to Release 5.0, the communication automatically reverts to the UDP
mechanism.) Note that the UDP heartbeat remains unchanged in the private network connecting
duplexed sites.
In Unified ICM Release 7.0, a consistent heartbeat or keep-alive mechanism is enforced for both the
public and private network interface. When QoS is enabled on the network interface, a TCP keep-alive
message is sent; otherwise UDP heartbeats are retained.
The TCP keep-alive feature, provided in the TCP stack, detects inactivity and in that case causes the
server/client side to terminate. It operates by sending probe packets (namely, keep-alive packets) across
a connection after the connection has been idle for a certain period, and the connection is considered
down if a keep-alive response from the other side is not heard. Microsoft Windows 2000/2003 allows
Cisco Unified Contact Center Enterprise 7.x SRND
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