Re: [Linux-ATM-General] how to get the length indicator from the ATM trailer in realtime if the user does not know how many rx bytes received

[mahendra v. <mah...@gm...>]

I could able to receive all the cells upto last cell.
I do not have a register in the ATM controller indicating the length count
of the received
bytes ( the datasheet claims the length count is done internally and updated
in the last cell)

I need to remove padding and send only user data to upper layer.

I have only the last cell buffer address and entire content of the last cell
including padding
and trailer.

My doubt is Since I do not have the length indicator through some register
how to find the trailer in the entire content of the last cell ?





On Thu, Aug 5, 2010 at 8:06 PM, Ashish <wri...@gm...> wrote:

> And if that kind of register is not there then system should have max
> possible length of buffer (perhaps mtu sized) and then as we get cells we
> put there and when we received last cell of all5 (based on PTI field) we
> remove the padding and send only user data to upper layer.
>
> --Ashish
>
>
> On Thu, Aug 5, 2010 at 7:51 PM, MIke Westall <we...@cs...>wrote:
>
>> I'm not 100% sure what you are asking here.   Some NICs (that do SAR)
>> have registers that the driver can read to determine application payload
>> length.   At minimum you MUST be able (somehow) to know how many
>> cell's comprise the AAL5 PDU.   But once you can do that the rest is
>> trivial..  The CS trailer is always "right" justified in the LAST cell of
>> the
>> PDU.
>> In your second example,  80 byte payload requires two cells.  Two cells
>> have payload capacity 96.   Thus CRC is at offset 92 with UU at offset 88,
>> CPI at 89 and len at offset 90.
>> Stated another way the 2 byte length field will always be located 6 bytes
>> before the end of the last 48 byte cell.
>> mahendra varman wrote:
>>
>>> Hi
>>> In the ATM driver during reception, I have a doubt in finding the
>>> length of the received bytes.
>>>
>>> 1. If the user send me 40 bytes of data, below is the dump of the
>>> buffer with trailer
>>>
>>>  size: 56 getrsmbuffer = cf888000
>>>
>>> -------------------------------------------------------------------------------------------------------------
>>> 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69
>>> 0x69 0x69
>>> 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69
>>> 0x69 0x69
>>> 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x0 0x0 0x0 0x28 0xdf 0xe9 0x8c
>>> 0x21
>>> 0xae 0x7b 0x62 0x16 0xce 0xf5 0xf1 0x2f
>>>
>>> 0x0 0x0 0x0 0x28 0xdf 0xe9 0x8c 0x21 = trailer with 0x28 as length
>>> indicator.
>>>
>>>
>>> 2. If the user send me 80 bytes of data, below is the dump of the LAST
>>> buffer with trailer
>>>
>>>  size: 56 getrsmbuffer = cf878048
>>>
>>>  ---------------------------------------------------------------------------------------------------------------------------
>>>
>>> 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69
>>> 0x69 0x69
>>> 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x69 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0
>>> 0x0 0x0 0x0 0x50 0xdc 0x9e 0xca 0xa4 0x0 0x0 0x0 0x0 0x1e 0xd8 0xd2 0xeb
>>> 0xff 0xff 0xff 0xff 0x0 0x0 0x0 0x0
>>>
>>> 0x0 0x0 0x0 0x0 0x0 0x0 0x0 0x0         = padding data(8 bytes) after
>>> data 0x69
>>> 0x0 0x0 0x0 0x50 0xdc 0x9e 0xca 0xa4 =  trailer with 0x50 as length
>>> indicator
>>>
>>> But, In realtime I do not know how many bytes the user will send to me.
>>> The padding bytes and the trailer area in the last buffer differs
>>> depending on the number of received bytes. I need to get the length
>>> indicator field through some logic irrespective of the number of
>>> received bytes.
>>>
>>> Can you please help me on the above ?
>>>
>>> Mahendra
>>>
>>>
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>>>
>>
>