Re: CRC-16 or CRC-32

[Jan Just Keijser <jan.just.keijser@gmail.com>]

Arne Lie wrote:
>> -----Original Message-----
>> From: linux-ppp-owner@vger.kernel.org [mailto:linux-ppp-
>> owner@vger.kernel.org] On Behalf Of Milan P. Stanic
>> Sent: 20. april 2010 17:10
>> To: linux-ppp@vger.kernel.org
>> Subject: Re: CRC-16 or CRC-32
>>
>> On Tue, 2010-04-20 at 16:49, Jan Just Keijser wrote:
>>     
>>> Arne Lie wrote:
>>>       
>>>> Hm, my observations are that if there are bit errors introduced in
>>>>         
>> the PPP frame, the "ifconfig ppp0" will count such packets in the
>> "error" tab. From this I anticipated that the PPP stack included CRC
>> checking of its payload. By CRC-16 I mean 2 byte CRC, CRC-32 4 byte.
>> Our customer wants 4 byte CRC. From RFC2823 I see both are mentioned.
>> The question is, how does one select either? Perhaps it is not
>> configurable via pppd?
>>     
>>> RFC2823 is about PPP SDL on SONET/SDH ; AFAIK the pppd code does not
>>> support this *at all* so I am not surprised that I did not find any
>>> references to CRCs in the headers...
>>>
>>> So the short answer to your question would be: no, PPP v2.4.5 does
>>> not support CRC-16 or CRC-32.
>>>       
>> In Linu CRC is done in kernel ppp driver not the ppp daemon.
>> ZLIB compression uses crc-32, AFAIR.
>>     
> [Arne::] Milan, thanks for your information. Yes, the CRC is done in the kernel part of PPP. My question was how, of possible, one could select CRC-16 or CRC-32 to be activated using options when starting pppd (in a similar manner as one is able to select compression algorithm). In http://fixunix.com/ppp/62264-hdlc-like-framing.html James Carson says that the Solaris implementation of pppd make it possible to negotiate to CRC-32. I read your answer this way: by choosing the deflate compression algorithm, which I believe is using ZLIB, then we *are* using the CRC-32 since it defaults to this.
> /Arne
>   
ah I've learned something myself here :)

unpacked the linux sources (2.6.28 which I had at hand) and checked the 
ppp module code:

ppp_async includes some CRC support BUT it only supports 
'crc_ccitt_byte' which is a 16bit CRC. The problem now is, which CRC32 
needs to be suppported? according to
  http://en.wikipedia.org/wiki/Cyclic_redundancy_check

"there are three polynomials reported for CRC-12, thirteen conflicting 
definitions of CRC-16, and six of CRC-32"

the second problem is: if you want to add CRC32 support then you'd have 
to get it into the linux kernel . that will take *quite* some time 
(unless you can live with patched linux kernels).

cheers,

JJK / Jan Just Keijser