QTECH SmartEdge 100 PPP and PPPoE - Manual (Page 8)

Configuring PPP and PPPoE
1.4 PPP Keepalive Checks
Keepalive checks are LCP echo messages sent over PPP sessions in the
context to determine if sessions are still active (alive). Normally, when a PPP
session is ending, the peer sends the SmartEdge OS an LCP termination
request (TERMREQ) message to indicate that it is ending. Keepalive checks
detect abnormal disconnects that the SmartEdge OS would not otherwise know
about. In addition to facilitating accurate timing of accounting information, it
is important to detect these abnormal terminations so that allocated system
resources can be reallocated to new sessions.
The keepalive checks feature can be used with or without a data check
option. The data check option is recommended when it is preferred to limit
the overhead for PPP keepalive processing. However, using the data check
option to determine that a session is no longer active can take longer than using
the PPP keepalive feature without the data check option, by a length of one
check interval. This condition occurs because with the data check enabled, the
check interval timer is reset as long as data has been received since the last
successful keepalive check.
If a session sends data and then abnormally terminates between keepalive
checks, the SmartEdge OS has no indication that the session has terminated
until the following check interval timer expires with no data being received. At
that point, the SmartEdge OS begins sending LCP echo requests. Without a
data check, the SmartEdge OS begins sending LCP echo requests, regardless
of whether data has been received since the last check.
Table 1 compares the two scenarios. In both cases, the following configuration
applies:
• Keepalive check interval is set to 60 seconds
• Response timer is set to 10 seconds
• Number of retries is set to 2
Table 1 Time Elapsed Before an Abnormally Terminated Session Is Torn Down
PPP Keepalives Without Data Check
Enabled
PPP Keepalives with Data Check Enabled
Step in the
Process
Seconds
Elapsed
Since
Previous
Step
Cumu
lative
Seconds
Elapsed
Step in the
Process
Seconds
Elapsed
Since
Previous
Step
Cumu
lative
Seconds
Elapsed
Successful
keepalive
check—check
interval timer reset
to zero 0 Successful keepa
live check—check
interval timer reset
to zero 0
4 64/1543-CRA 119 1170/1 Uen K | 2012-12-04