rohc team mailing list archive

[Harpreet Singh <harpreeta@xxxxxxxxxxxxx>]

Hi Didier,

Thanks for explaining.

I understand why second stream will have 21 byte header.

I am jyst trying to figure out where exactl the code breaks.

Previously i was able to pinpoint where uint16_t access was happening.

In this crash, I am not sure which instruction from which is doing the 
problem work.

Regards,
Harpreet
On Mon, 2009-11-30 at 14:27 +0100, Didier Barvaux wrote:
> Hi,
> 
> > In the attached debug print dump.
> > If we look at the last packet being decoded.
> > for this packet the rohc compressor gives below given print finally.
> > [../rohc/rohc_comp.c:373 rohc_compress()] ROHC size = 529 (feedback =
> > 0, header = 21, payload = 508), output buffer size = 2048
> > 
> > which says that the header size is 21.
> > 
> > and in decompressor, the decoded header size for this packet is 20..
> > as printed by 
> > [../rohc/rohc_decomp.c:848 rohc_ir_packet_crc_ok()] size of IR packet
> > header : 20
> > 
> > and the point to be noted here is that this thing happens only if I
> > start the system by sending uncompressed profile packets.
> > and then, when I send the very first ip profile packet this error
> > comes up.
> > 
> > may be there is something to be initialized properly when this change
> > profile change happens at the start of application.
> > 
> > can you please give your views here ?
> 
> This behaviour is normal.
> 
> The IR header is 20-byte long + the CID if the CID is not 0. So, the
> CID may be omitted (0 byte), small (1 byte) or large (2 byte). In your
> tests, you use small CIDs, so the compressor sends 20-byte IR packets
> for the first stream (CID 0 is omitted) and 21-byte IR packets for the
> next streams (CID 1 is not omitted and is transmitted over 1 byte).
> 
> When the decompressor receives a packet, it first determines if the
> compressor uses small CIDs or large CIDs because the format of the
> ROHC header is different for small and large CIDs. The decompressor
> then checks the CRC. The function that computes the CRC (see
> rohc_ir_packet_crc_ok) also prints the size of the header, but without
> the length of the CID since it was already analysed. It thus reports
> 20 - 0 = 20 bytes for the first stream and 21 - 1 = 20 bytes for the
> next streams.
> 
> You get a different behaviour if you first compress one ICMP stream
> with the Uncompressed profile or not:
>  - When you first compress one ICMP stream with the Uncompressed
>    profile, the compressor reserves the CID 0 for that stream. The next
>    IP stream got the CID 1 that can not be omitted from the ROHC
>    header.
>  - If you do not compress one ICMP stream with the Uncompressed profile
>    before compressing your IP stream, the IP stream got the CID 0 that
>    can be omitted from the ROHC header.
> 
> I'm not sure I'm clear enough, ask me for more explanations if needed.
> 
> Regards,
> 
> Didier Barvaux
> Viveris Technologies
> 
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