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Response Time Computation for IP SLA
The IP SLA responder needs at least one second to open a socket and program Local Packet Transport
Note
Services (LPTS). Therefore, configure the IP SLA timeout to at least 2000 milli seconds.
The IP SLA Responder must be used with the UDP jitter operation, but it is optional for UDP echo operation.
If services that are already provided by the target router are chosen, the IP SLA Responder need not be enabled.
For devices that are not Cisco devices, the IP SLA Responder cannot be configured, and the IP SLA can send
operational packets only to services native to those devices.
Response Time Computation for IP SLA
T3 is the time the reply packet is sent at the IP SLA Responder node, and T1 is the time the request is sent at
the source node. Because of other high-priority processes, routers can take tens of milliseconds to process
incoming packets. The delay affects the response times, because the reply to test packets might be sitting in
a queue while waiting to be processed. In this situation, the response times would not accurately represent
true network delays. IP SLA minimizes these processing delays on the source router and on the target router
(if IP SLA Responder is being used) to determine true round-trip times. Some IP SLA probe packets contain
delay information that are used in the final computation to make measurements more accurate.
When enabled, the IP SLA Responder allows the target device to take two time stamps, both when the packet
arrives on the interface and again just as it is leaving, and accounts for it when calculating the statistics. This
time stamping is made with a granularity of submilliseconds. At times of high network activity, an ICMP
ping test often shows a long and inaccurate response time, while an IP SLA-based responder shows an accurate
response time.
Figure 4: IP SLA Operations, on page 201
make the calculation for round-trip time. At the target router, with the responder functionality enabled, time
stamp 2 (TS2) is subtracted from time stamp 3 (TS3) to produce the time spent processing the test packet as
represented by delta. This delta value is then subtracted from the overall round-trip time. Notice that the same
principle is applied by IP SLA on the source router on which the incoming time stamp 4 (TS4) is taken in a
high-priority path to allow for greater accuracy.
Figure 5: IP SLA Responder Time Stamping
IP SLA VRF Support
Service providers need to monitor and measure network performance from both the perspective of the core
network and a customer's network. To do so, it is necessary to use nondefault VPN routing and forwarding
(VRF) tables for IP SLA operations in addition to the default VRF table.
IP SLA, on page 202
an operation supports the use of nondefault VRF tables.
Cisco ASR 9000 Series Aggregation Services Router System Monitoring Configuration Guide, Release 4.2.x
204
shows how the responder works. Four time stamps are taken to
describes the different IP SLA operations, including information about whether or not
Implementing IP Service Level Agreements
Table 22: Types of Operations for
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