嵌入式linux开发，pptp失败，ConfReq无回复，LCP: timeout sending Config-Requests，Connection terminated，Modem Modem

嵌入式linux开发，PPTP Client。在进行拨号时失败，LCP连接失败。使用调试命令，打印如下：

root@ms:~# pon vpn_pptp debug dump logfd 2 nodetach
pppd options in effect:
debug           # (from command line)
nodetach                # (from command line)
logfd 2         # (from command line)
dump            # (from command line)
noauth          # (from /etc/ppp/options.pptp)
name moshui              # (from /etc/ppp/peers/vpn_pptp)
remotename vpn_pptp             # (from /etc/ppp/peers/vpn_pptp)
                # (from /etc/ppp/options.pptp)
pty /usr/sbin/pptp xxx.xxx.xxx.xxx --nolaunchpppd               # (from /etc/ppp/peers/vpn_pptp)
crtscts         # (from /etc/ppp/options)
                # (from /etc/ppp/options)
asyncmap 0              # (from /etc/ppp/options)
lcp-echo-failure 4              # (from /etc/ppp/options)
lcp-echo-interval 30            # (from /etc/ppp/options)
hide-password           # (from /etc/ppp/options)
ipparam vpn_pptp