Re: About ppp_receive()

[Steven <liu-dj@neusoft.com>]

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Hi Zhenhua

Zhang, Zhenhua wrote:
> Hi Steven,
> 
> Steven wrote:
>> Hi,
>>
>> In function ppp_receive, we first check the protocol type of this
>> frame 
>> like:
>>
>>   guint16 protocol = ppp_proto(buf);
>>
>> and here we assumed the length of the protocol field is 16 bits, but
>> in 
>> RFC 1661, the protocol field should be one or two octets.
>>
>> "The Protocol field is one or two octets, and its value identifies
>> the datagram encapsulated in the Information field of the packet."
>>
>> why we given the assumption that protocol field is 16 bit length?
> 
> First I am not ppp expert. :-).
> 
> If you take look at pppd source code, main.c, get_input() also always fetch two bytes 'protocol' for struct protent as well.
> 
> Can you give a case we failed in our ppp stack?

If you interested in this topic, you can reference RFC 1661 Section 6.5, 
which said

--------------
This Configuration Option provides a method to negotiate the
compression of the PPP Protocol field. By default, all
implementations MUST transmit packets with two octet PPP Protocol
fields.
PPP Protocol field numbers are chosen such that some values may be
compressed into a single octet form which is clearly
distinguishable from the two octet form. This Configuration
Option is sent to inform the peer that the implementation can
receive such single octet Protocol fields.
-------------

In our current source code, because we only negotiate two configuration 
options - REQ_OPTION_MRU and REQ_OPTION_ACCM. so it's okey for our PPP 
stack.

But some carriers, like China Telecom or Sprint Network etc, will 
support the full configuration 
options(Magic-Number,Protocol-Field-Compression,Address-and-Control-Field-Compression),
So if PFC option is used ,our code will got wrong with ppp_receive().

We should first check if PPP protocol field is compressed or not, and 
then get the right protocol value to form a 16 bits protocol field, and 
  pass this value to the rest functions.

Because of my company's security policy ,I can't provide a patch for 
this issue. But i can provide a method for doing this. Here it is.

First byte of PPP protocol field may be compressed, if the LS bit is 1 
then this indicates that the upper protocol us compressed, because the 
upper byte should be even, the lower byte should be odd.

>> In CDMA 2000 environment, just as the Sprint Network, PPP should
>>   support a compressed protocol field. Is there anything difference
>> between GSM 
>> and CDMA?
>>
>> B.R
>>
>> Steven

B.R

Steven
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