Re: [rohc] RFC4996 IP-ID with co_common and Random behavior

[RoHC Team <[e-mail]>]

  Hello Carl,

  When using co_common compressed packet format, described page 80 of 
the RFC, ip_id is optionaly sent with the optional_ip_id_lsb{} method. Ok.

  But when the IP-ID behavior is random, the value is not present in the 
header :

      COMPRESSED not_present {
        ENFORCE((behavior == IP_ID_BEHAVIOR_RANDOM) ||
                (behavior == IP_ID_BEHAVIOR_ZERO));
      }

  So I don't understand how to send IP-ID random value ...

  Perhaps you think of the ipv4_innermost_irregular{} method, page 63, 
but I don't understand how to use this method and the compressed format 
co_common{} ...

  And I don't have any samples or reference captures file : only the RFC 
text!

  Thank you for your help and explanation.

  Best regards,

               FWX.

Le 05/03/2012 09:39, Carl Knutsson a écrit :
> Hi,
>
> Comment inline.
>
> Cheers
>
> /Calle
>
> On 02/27/2012 05:02 PM, RoHC Team wrote:
>>   Hello everybody,
>>
>>   We are coding the RFC4996 and we don't understand how to send the value
>> of the IPv4 IP-ID field, using the co_common compressed format, when the
>> IP-ID has a random behavior.
>>
>>   With the co_common format, there are :
>>   ip_id_indicator
>>   ip_id_behavior
>>   and ip_id =:= optional_ip_id_lsb( ip_id_behavior.UVALUE,
>>                                     ip_id_indicator.CVALUE)
>>
>>   with :
>>
>> optional_ip_id_lsb(behavior, indicator)
>>   {
>>     UNCOMPRESSED {
>>       ip_id [ 16 ];
>>     }
>>
>>     COMPRESSED short {
>>       ip_id =:= ip_id_lsb(behavior, 8, 3) [ 8 ];
>>       ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
>>               (behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
>>       ENFORCE(indicator == 0);
>>     }
>>
>>     COMPRESSED long {
>>       ip_id =:= irregular(16)  [ 16 ];
>>       ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
>>               (behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
>>       ENFORCE(indicator == 1);
>>     }
>>
>>     COMPRESSED not_present {
>>       ENFORCE((behavior == IP_ID_BEHAVIOR_RANDOM) ||
>>               (behavior == IP_ID_BEHAVIOR_ZERO));
>>     }
>>   }
>>
>>
>>
>>   So, if ip_id_behavior is RANDOM, ip_id is not compressed (not present)
>> ! And we can't send the value ...
>>
>>   Why not ip_id =:= irregular(16)  [ 16 ]; when behavior is RANDOM ?
>>
>
> The FN is correct. IP-ID is sent in the irregular chain.
>
>>
>>
>>   There is a contradiction with the paragraph 8.1 page 44/45 saying :
>>
>>     All of the compressed base headers transmit LSB-encoded MSN bits, the
>>     TCP Push flag, and a CRC, and in addition to this, all the base
>>     headers in the sequential packet format set contain LSB-encoded IP-ID
>>     bits.
>>
>>    o  Common header format: The common header format can be used for all
>>        kinds of IP-ID behavior and should be useful when some of the more
>>        rarely changing fields in the IP or TCP header change.  Since this
>>        header format can update control fields that decide how the
>>        decompressor interprets packets, it carries a 7-bit CRC to reduce
>>        the probability of context corruption.  This header can basically
>>        convey changes to any of the dynamic fields in the IP and TCP
>>        headers, and it uses a large set of flags to provide information
>>        about which fields are present in the header format.
>>
>
> I can't find any contradiction. Please be more specific.
>
>
>