from the rfc: (rfc1661)

The maximum length for the Information field, including Padding,
      but not including the Protocol field, is termed the Maximum
      Receive Unit (MRU), which defaults to 1500 octets.  By
      negotiation, consenting PPP implementations may use other values
      for the MRU.

The Protocol field is max 16 bits, so total length should not be higher than 1502 bytes...


also check the MRU info of LCP:

  Maximum-Receive-Unit (MRU)

   Description

      This Configuration Option may be sent to inform the peer that the
      implementation can receive larger packets, or to request that the
      peer send smaller packets.

      The default value is 1500 octets.  If smaller packets are
      requested, an implementation MUST still be able to receive the
      full 1500 octet information field in case link synchronization is
      lost.

Besides this, there is also RFC 1332, dealing with IPCP.

How it all fits together can be seen in

http://www.networksorcery.com/enp/proto...l/IPCP.htm

and

http://www.networksorcery.com/enp/protocol/ip.htm

and
http://www.networksorcery.com/enp/protocol/udp.htm


IPCP is (probably) not needed if you assign static ip-addresses to your hosts.
I would also suggest to use *nix to test and evaluate your PPP stack. You will get more (free) tools bundled with BSD or Linux that can help you troubleshoot.

If you want to go the UDP/IP/PPP route you will have to troubleshoot each layer before going to the next.

regards

Patrick