User guide

Canopy System User Guide Configuring an SM for the Destination
pmp-0229 (Mar 2013)
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MTU
• Use MTU Received from PPPoE Server causes the SM to use the MRU of the PPPoE
server received in LCP as the MTU for the PPPoE link.
• 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 will be 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 will be taken down and a reconnection attempt will be
started. For marginal links, the keep alive timer can be useful so that the session will stay
alive over periodic dropouts. The keepalive timer should 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 should be in sync so that one side does not drop the
session prematurely.
• Idle Timeout enables an idle timer that will check 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
will be dropped. Once data starts flowing from the customer again, the session will be 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 will be 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 will be 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.
PPPoE Session Status
This field indicates the current PPPoE session status, for easy reference.