The user plane for the PDP type PPP consists of a PPP protocol stack above SNDCP for A/Gb mode or above PDCP for Iu mode in the MS, and above GTP in the GGSN. The GGSN may either terminate the PPP protocol and access the packet data network at the IP level, or further tunnel PPP PDUs via e.g. L2TP.
In case the application above PPP uses a different protocol than IP (e.g. IPX or AppleTalk), the interconnection to the packet data network is outside the scope of this specification.
The PPP peers at the MS and the GGSN handle the PPP protocol as specified in RFC 1661
. PPP requires in-sequence packet delivery by the underlying protocols. Concerning GTP, this shall be achieved by negotiation of the delivery order attribute in the QoS profile upon PDP context activation. In A/Gb mode, concerning SNDCP, out-of-sequence packets, that may be present if LLC operates in unacknowledged mode, shall be discarded. SNDCP for A/Gb mode, and PDCP for Iu mode, shall not use TCP/IP header compression because PPP may not carry IP packets at all, or because PPP may carry IP packets with already compressed TCP/IP headers. These PPP options are negotiated during the RFC 1661
Network Control Protocol establishment phase.