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[Didier Barvaux <didier.barvaux@xxxxxxxxxxxxxxxxxxxx>]

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