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Re: [Question #201484]: In what situation will the rand change when decompressing


Question #201484 on rohc changed:

Didier Barvaux posted a new comment:

> Thanks for the excellent explanation. However i have a doubt
> regarding the 3rd bullet.
> 3/ Scenario "RND=0 and change to RND=1":
>       - at compressor: IPv4 + context(RND): 0 -> 1 => UOR-2-RTP
>          + extension 3 with packet(RND)=1
>       - at decompressor: IPv4 + context(RND)=0 => UOR-2-ID
>       - when parsing extension 3, packet(RND)=1 is found, and
>         packet is re-parsed as UOR-2-RTP
> Can you please highlight or share the reference in the source code where we compute the ip_id correctly?

a/ When parsing extension 3, the library checks for changed RND: http://bazaar.launchpad.net/~didier-barvaux/rohc/main/view/head:/src/decomp/d_generic.c#L7041
b/ In such case, the parsing of the UOR-2* packet is stopped because it has to be reparsed with different asumptions: http://bazaar.launchpad.net/~didier-barvaux/rohc/main/view/head:/src/decomp/d_generic.c#L5712
c/ The correct type of UOR-2* packet is detected here, then packet is parsed again (please note the 2 calls to parse_uor2(): http://bazaar.launchpad.net/~didier-barvaux/rohc/main/view/head:/src/decomp/d_generic.c#L5938

The Offset IP-ID is always decoded the same way from bits extracted from
the packet: http://bazaar.launchpad.net/~didier-

> Here is my understanding:
> a. when 1st packet received with RND bit 1, then we have to
>     compute and store the id_offset_ref as curr_ip_id - curr_SN
> b. when 2nd packet is received with RND bit toggled to 0, then
>     we hv to use the id_ref computed at step 1 and compute the
>     current ip_id as id_offset_ref + curr_SN.
> pls correct my algorithm if you think there are any mistakes.

I do not understand your algorithm. Scenario 3 is about RND
changing from 0 to 1, not from 1 to 0. Are you talking about
scenario 4 instead?

For a/, the random IP-ID value is recorded as the new reference
by a call to ip_id_offset_set_ref() in the update_context() function
that is called once a packet is successfully decompressed.

For b/, if RND=0, then some bits of IP-ID offset are transmitted.
They must be used to compute the new IP-ID offset (see the
description of the W-LSB algorithm in RFC 3095). Once the new
IP-ID Offset is decoded, the new IP-ID value can be decoded from
the IP-ID offset and the decoded SN (see the description of the
Offset IP-ID algorithm in RFC 3095). The Offset IP-ID decoded at
step a/ is not used.

I hope that my explanations were clear enough. Do not hesitate to
ask additional questions if you need to.


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