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why does dail-up or pppoe access always has session-timeout ?

On 13-05-30 05:10, Joe wrote:
>   a question  obsessed me for a long time.  "why my  pppoe connection to internet has a max session time, even if every thing goes ok? "  
>   In our DSL access network , max session timeout is set to 4 days, this parameter is sent to BAS by radius server after finishing authenticating procedure.  As I know,   beside us some other service providers also applied this parameter to pppoe session, the parameter varies from   48 hours to 96 hours.   Reading  documents of BAS, we found this is default value for session-time on BAS, that means even if    radius server does not response BAS with  session-timeout attribute  BAS will cut pppoe session after sometime.    so , why does those BAS designer or  protocol designer  set such a parameter for  pppoe access ?
> should anyone do me a favor on explaining  this ? 

Under normal circumstances, usage accounting data is only sent when the
PPPoE connection ends.  So an ISP might want to force PPPoE sessions to
end every couple of days to capture usage data for accounting/billing

Note that there are techniques to get the BAS (or router) to send these
accounting updates to the radius server at regular intervals, allowing
uninterruoted PPPoE sessions. Not all ISPs may have implemented this.

In Canada, when threathened with UBB and now morphed into CBB billing
regime for wholesale, many ISPs read their router's manuals and found
the way to get the forced sending of accounting data at regular
intervals. This has allowed them for instance to provide for unlimited
usage during the wee hours of morning. They get the routers to send out
accounting info at 02:00 in morning and at 12:00 (noon) and the ISP can
then count only the usage between noon and 02:00.