GPRS PPP connect failture.

Hello,everyone

I did some work to transplant TCP/IP stack to MCU. Through GPRS, MCU can connect internet.
However, I met with some trouble during LCP negotiation.The following is my solution referring networking material

1.In order to make MCU PPP simpler.The LCP option request are respond with rejection.

Server:7e ff 03 c0 21 01 01 00 14 02 06 00 00 00 00 05 06 b0 70 9c c3 07 02 08 02 54 06 7e

MCU:7e ff 03 c0 21 04 01 00 04 02 06 00 00 00 00 05 06 b0 70 9c c3 07 02 08 02 69 78 7e

2.A authentication request is expected in the second step. But i get a none LCP option request.

Server:7e ff 03 c0 21 01 02 00 04 b5 5a 7e

This means server forces MCU to start LCP negotiation.I am missing.

I send a authentication option request to server so that forcing server to start authentication option negotiation.Server respond me two packets.One is a ACK packet for authentication request.The another still is the none LCP option.

MCU:7e ff 03 c0 21 01 05 00 08 03 04 C0 23 ac 56 7e

Server:7e ff 03 c0 21 02 05 00 08 03 04 C0 23 bd 34 7e
Server:7e ff 03 c0 21 01 02 00 04 b5 5a 7e

After several "none option" request, The negotiation over. What should I do?

AT command:
AT+CGCLASS="b"
AT+CGDCONT=1,"IP","CMNET"
AT+CGACT=1,1
AT+CGREG?
AT+IPR=115200;&W

(My english is not good .I hope you can understand my means.)