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

  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 ?

The simple answer is that it does not.

If you don't specify a session timeout, the session will not timeout.

This is how we run our PPPoE services.

The protocol and BAS designer does not set the parameter - the network operator does.

Because PPPOE comes from PPP, which was designed for dialup. You typically don't want to leave a dialup connection up forever.

every few days means that if you ever have to roll out a change, you don't end
up with *That* *One* *User* who stays connected for weeks or even months with
the old parameters.

It also probably has something to do with oversubscription. Providers
generally allocated trunks (most dial up providers I knew used Livingston
Port Masters), however their subscriber base was much larger than the
number of phone lines available to take incoming calls. If you time out
idle users, you have more phone lines available to take calls. Even in the
BBS days, we still had to wait our turn.. Usually it was a redial until we
got a carrier.

I'll agree with the configuration aspect, but I really think it has much
more to do with resource allocation in a telco environment (n:1 oversub).


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.