[rohc] RoHCv2 : Problems in decoding sequentially late packets

Hi,
 I am facing problems in decoding sequentially late packets if link
reordering (between compressor and the decompressor) situation is being
simulated. I am explaining the situation with
an example:

Late packet

Suppose initiallly the encoder has 0x0004 in its window and encoded value of
0x0004 is reached to the
decompressor.Assuming sequential increment of the field encoder then encodes
0x0005 and 0x0006.

Suppose in the link the value 0x0005 and 0x0006 is lost and these value
didn't reach the decoder,
so the decoder still uses 0x0004 as its reference value (assuming successful
decoding of encoded
value of 0x0004).

Now encoder encodes 0x0007 using 0x0004,0x0005,0x0006 having in its
reference window and it will send
0x0003 as its encoded value.
Decompressor receives 0x0003 (sequentially early packet) and it can
reconstruct, the reconstructed value
will be 0x0007 which is offcourse equal to the original value. Decoder will
make its reference value as 0x0007.

Now suppose the encoded value of 0x0005 (sequentially late packet) is
reached to the decompressor. Using
reference value of 0x0004 the encoded value of 0x0005 will be 0x0001.The
decoder has 0x0007 in its reference
and this time the decoded value will be 0x0008 which is not equal to the
original value (0x0005)

I am not finding any clue how to handle sequentially late packets and how to
identify sequentially late packet.
According to RFC 5225 "How the decompressor detects a sequentially late
packet is outside the scope of this specification,
but it can for example use the MSN for this purpose", but as per my
knowledge MSN has to be decoded first then it can be used to
identify sequentially late packet.
So, I would, therefore request you to suggest some idea so that the decoder
can identify sequentially late packet before decoding the
encoded value and it can decode it successfully.

Thanks & Regards
Sudip Kumar Mandal
Tata Elxsi Ltd
Bangalore, India

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