Re: [lp-wan] Fw: re-order header field request

["Carles Gomez Montenegro" <[e-mail]>]

Hi Gengyu,

Please find inline responses to your questions below:

> Hi Carles,
>
>> In the context of the SCHC specification, an L2 LPWAN frame may carry an
>> unfragmented IPv6 datagram with a compressed header (see Fig. 4).
>
> This doc is just a draft.
> The format in Fig.4 can be re-organized to get the suppression optimized.
>
>> The Rule ID is the first field of the compressed IPv6 datagram.
> Defining the first two bits are defined as an indications, these problems
> ara solved.
>
> If it is 00, the fragment is the first and contains a Rule ID.
>         If the Rule ID says this is an unfragment datagram, let it go;
>         If the Rule ID says this is an fragmented one, you then get the
> Dtag.
> If it is not 00, you need to get the Dtag for the fragment,
>         and get the Rule ID by the Dtag.
>
>> When a compressed IPv6 datagram needs to be fragmented, then some Rule
>> ID
>> space (of size R bits) is 'stolen' and used for fragmentation
>> functionality. So a requirement here is that whatever is defined for
>> fragmentation needs to be consistent with the format used for header
>> compression. Some special Rule ID (or set of Rule IDs in terms of R-bit
>> patterns) needs to be used to signal that a data unit being carried is a
>> fragment.
>
> It has been clarified that only one Rule ID is required for an IPv6
> packets
> if it is framennted.
> Or fragments belonged to one IPv6 packet just needs one Rule ID.
> The Rule ID can be carried in the frist fragment to the receiver.
> Only one is enough for conveying this information.
>
>> Note that we cannot risk communication failure due to e.g. loss of a
>> first
>> fragment (which in your proposal carries a Rule ID) followed by other
>> fragments (which in your proposal do not carry a Rule ID) that are
>> actually received. In such case, a receiver may process the received
>> data
>> units as e.g. unfragmented (compressed) IPv6 datagrams.
>
> Could you explain how the receiver precesses fragments without getting the
> first fragment of an fragmented IPv6 packet?
> No, it cannot by the current draft either.

The point is that with the current draft, if the first fragment is lost,
subsequent fragments cannot be confused with unfragmented IPv6 datagrams,
since all fragments have a Rule ID field that is set to a value that
indicates that the data unit being carried is a fragment.

> Can you explain how the the current draft avoids the risk of communication
> failure due to e.g. loss of a first fragment?
> It is not available to have each fragment processed independently by the
> current draft.

Please see the response above.

>> In such case, a receiver may process the received data units as e.g.
>> unfragmented (compressed) IPv6 datagrams.
> What is an unfragmented (compressed) IPv6 datagram?
> If it is an unfragmented, it is the first, it contain the Rule ID.  Do you
> suppose it lost?

An unfragmented (compressed) IPv6 datagram is an IPv6 datagram which,
after SCHC header compression, has a size that does not exceed the maximum
L2 data unit payload size currently allowed. Therefore, it does not need
fragmentation.

Greetings,

Carles


> Regards,
>
> Gengyu WEI
> Network Technology Center
> School of Computer
> Beijing University of Posts and Telecommunications