[rohc] Query regarding the LSB encoding in TCP Profile

[Anil Maguluri <[e-mail]>]

HI All,

Please clarify me the below queries regarding the LSB encoding in TCP Profile.

The below statements in RFC 4996 are confusing, please clarify me.

Statement-1
In RFC 4996, Page 43, under TCP Sequence Number:
" Also, for ROHC-TCP to be robust to losses, two additional bits are added to the LSB encoding of the sequence number. This
means that the base headers should contain at least 14 bits of LSB-encoded sequence number when present. According to the
logic above, the LSB offset value is set to be as large as the positive offset, i.e., p = 2^(k-1)-1. "

Statement-2
In RFC 4996, Page 81, in the compressed packet formats are represented as below:
// Send LSBs of sequence number
COMPRESSED rnd_1 {
Discriminator    =:= '101110' [ 6 ];
seq_number      =:= lsb(18, 65535) [ 18 ];
msn                       =:= lsb(4, 4) [ 4 ];
psh_flag               =:= irregular(1) [ 1 ];
header_crc         =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
}

As per my understanding, we have to use offset param value (RFC 4997, page 27) as 65535 to compress the seq_number to select
The rnd_1 packet. Is it right? If my understanding is right, then what is the use of p=2^(k-1)-1?

Also please explain me the below statement which is present in RFC 4997, page 27.
For example, the TCP sequence number:
tcp_sequence_number =:= lsb(14, 8192);
This takes up 14 bits, and can communicate any value that is between 8192 lower than the value of the field stored in context and 8191
above it.

Thanks for your support.

Regards,
Anil Kumar Maguluri

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