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[Didier Barvaux <question679313@xxxxxxxxxxxxxxxxxxxxx>]

Question #679313 on rohc changed:
https://answers.launchpad.net/rohc/+question/679313

    Status: Open => Answered

Didier Barvaux proposed the following answer:
Hello,

> 1\as for the formal notation field =:= lsb(K,P) ,the lsb(K,P) represents
>  the WLSB procedure or it represents results of calculating with the
> WLSB, from the many statements,I think it represent the results of
> calculating with the WLSB, it is a packet type fomat, is it right?

The notation field =:= lsb(K,P) means that the compressed field is
composed of the k least significant bits of the uncompressed field.


> 2\if the lsb(K,P) represents the results of calculating with the WLSB,
> e.g. seq_number =:= lsb(16, 32767),K = 16 is calculated from the
> WLSB(P = 32767),is it right?

The W-LSB algorithm is performed on the last N seq_number values with p
= 32767. The result is the minimum number of bits that must be
transmitted by the compressor to the decompressor, let's call that value
k_min. The compressor then chooses a ROHC packet format that is able to
transmit at k_min bits of seq_number. If k_min >= 16, then the seq_1
packet format is possible.


> 3\The rohc_lsb_shift_t enum in lib 2.2.0 lists the different P values
> that are used by the several ROHC RFCs. but I can not find the
> definitions about P values in RFCs,for example RFC4996, So where is it?

RFC 4996 is obsolete. RFC 6846 replaces it. The p values are defined by
all the notations field := lsb(k, p) that you may find in §8.2 of RFC
6846 : https://tools.ietf.org/html/rfc6846#section-8.2


> 4\ in the lib 2.2.0, as for seq_number =:= lsb(16, 32767),I can not
> find the WLSB initialization with P = 32767, if so, How can we get
> the k = 16bits? thank you .

The value k for seq_number and p = 32767 is computed :
https://github.com/didier-
barvaux/rohc/blob/rohc-2.2.0/src/comp/c_tcp.c#L4666

The seq_1 packet is chosen only if value k <= 16 : https://github.com
/didier-barvaux/rohc/blob/rohc-2.2.0/src/comp/c_tcp.c#L4744

The seq_1 packet is filled with 16 LSB of seq_number :
https://github.com/didier-
barvaux/rohc/blob/rohc-2.2.0/src/comp/c_tcp.c#L2438

Regards,
Didier

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