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...>]

Hi Mlke

Thanks for your Information.
My doubt is cleared and I understood the concept

Thanks a Lot

Mahendra

On Thu, Aug 5, 2010 at 8:33 PM, MIke Westall <we...@cs...> wrote:

>  The full 8 byte trailer is ALWAYS located at offset +40 from start of last
> cell.   Hence
> length is always located at offset +42 from start of last cell (as it was
> in your examples).
> Mike
>
>
> mahendra varman wrote:
>
>
> 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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>>>>
>>>>
>>>>
>>>
>>
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