The Nokia service model uses encapsulation tunnels (also referred to as service tunnels) through the core to interconnect service routers. An SDP is a logical way of referencing the entrance to an encapsulation tunnel.
The 7705 SAR-Hm series supports Layer 2 or Layer 3 services within generic routing encapsulation (GRE-MPLS encapsulation).
An SDP has an implicit maximum transmission unit (MTU) value because services are carried in encapsulation tunnels and an SDP is an entrance to the tunnel. The MTU is configurable (in octets), where the transmitted frame can be no larger than the MTU.