Cambium Networks PMP 450 AP, Configuration Manual

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Chapter 1: Configuration Configuring IP and Ethernet interfaces
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Use User Defined MTU allows the operator to specify an MTU value to use
to override any MTU that may be determined in the LCP phase of PPPoE
session setup. If this is selected, the user is able to enter an MTU value up to
1492. However, if the MTU determined in LCP negotiations is less than this
user-specified value, the SM will use the smaller value as its MTU for the
PPPoE link.
Timer Type Keep Alive is the default timer type. This timer will enable a keepalive that
will check the status of the link periodically. The user can set a keepalive
period. If no data is seen from the PPPoE server for that period, the link is
taken down and a reconnection attempt is started. For marginal links, the
keep alive timer can be useful so that the session will stay alive over periodic
dropouts. The keepalive timer must be set such that the session can outlast
any session drop. Some PPPoE servers will have a session check timer of
their own so that the timeouts of the server and the SM are in sync, to
ensure one side does not drop the session prematurely.
Idle Timeout enables an idle timer that checks the usage of the link from the
customer side. If there is no data seen from the customer for the idle timeout
period, the PPPoE session is dropped. Once data starts flowing from the
customer again, the session is started up again. This timer is useful for users
who may not be using the connection frequently. If the session is idle for
long periods of time, this timer will allow the resources used by the session
to be returned to the server. Once the connection is used again by the
customer, the link is reestablished automatically.
Timer Period The length in seconds of the PPPoE keepalive timer.
TCP MSS Clamping If this is enabled, then the SM will alter TCP SYN and SYN-ACK packets by
changing the Maximum Segment Size to be compatible with the current
MTU of the PPPoE link. This way, the user does not have to worry about MTU
on the client side for TCP packets. The MSS is set to the current MTU – 40
(20 bytes for IP headers and 20 bytes for TCP headers). This will cause the
application on the client side to not send any TCP packets larger than the
MTU. If the network is exhibiting large packet loss, try enabling this option.
This may not be an option on the PPPoE server itself. The SM will NOT
reassemble IP fragments, so if the MTUs are incorrect on the end stations,
then MSS clamping will solve the problem for TCP connections.