Re: [lp-wan] Fwd: I-D Action: draft-ietf-lpwan-schc-over-nbiot-08.txt

[Sergio Aguilar Romero <[e-mail]>]

Hello Ana, Edgar, 

I have reviewed the draft and I have the following comments, mostly regarding SCHC Fragmentation.

In section 4.3.2: 

There are two fragmentation modes, No-Ack (for SCHC Packets larger than 1358 bytes) and ACK-on-Error for transfer blocks of up to, and larger than, 300 bits. However, there are no configuration values for No-ACK. 

The tile size is in the order of bits, from 2 bits to 16 bits, to avoid padding in transfer blocks. The recommend values for 300 bit transfer block are M = 1, and N = 3, therefore, 2 windows of 7 tiles, for a total of 14 tiles. Considering a tile of 8 bits, the MAX_SCHC_PACKET is 114 bits, and 224 bits for an 16 bits tile, below the 300 bits. 

For more than 300 bits, and M of 3 bits, i.e., 7 windows, and 7 tiles per windows (N = 3), the MAX_SCHC_PACKET is 392 bits for a tile of 8 bits, and 784 bits for a tile if 16 bits. I wonder if these values are meant to be bytes. Also, if the 7 windows are to have more downlink opportunities i.e., more All-0? 

There no fragment figures, as there is in the LoRaWAN and Sigfox documents. 

From RFC8734 Appendix D, is missing:
- RulesIDs for the different Fragmentation modes.
- WINDOW_SIZE, M and N for No-ACK mode. 
 - How the to differentiate the All-0 from the SCHC ACK REQ, and the All-1 from the Sender-Abort, for ACK-on-Error and No-ACK.
- if the last tile must be in the All-1, or be a L2 word less.
- when to listen to ACKs (not sure if you have to send an uplink message to receive a downlink, if so, appendix F must be taken into account).
- size and method of the RCS.
- recommendation on padding bits value.

Another detail is that the L2 word is defined as 8 bits. In ACK-on-Error, the tile size MUST be at least the size of the L2 word. Therefore, 2 bits and 4 bits are not possibles tile size values. Moreover, tiles sizes are missing for No-ACK mode. 

Note that if the MTU is 1358 bytes, the optimal tile size in terms of ACK size is the largest one, 1357 or 1356 bytes depending on the header size. If fragmentation is required, for example, for a SCHC Packet of 1500 bytes, with a tile of 8 bits, 1500 tiles are required, for a tile of 4 bits, 3000 tiles, and 6000 tiles for 2 bits, without considering the number of windows and M size. The bitmap, in a 1358 bytes MTU, and using compound ACK, can be up to 10000 bits, but the device energy consumption and processing time increases with the number of tiles, at least in my experience, as every tile must be processed before building the SCHC Fragment. Moreover, with a tile of 1356 bytes you need 2 tiles and 1 window. In both cases only two SCHC Fragments are needed.

Let me know if I missed any detail in the document. I can collaborate with fragment figures if needed.

Best regards,

Sergio

> On May 20, 2022, at 7:37 AM, Carles Gomez Montenegro <[e-mail]> wrote:
> Hello Ana, Edgar,
> 
> Many thanks for taking my comments into consideration!
> 
> I just have the following remaining comments:
> 
> - Section 4.3 is entitled "End-to-End Compression". However, it also
> defines how to use fragmentation (in 4.3.2), so probably the title should
> be updated.
> 
> - Regarding the fragmentation parameters in 4.3.2, I understand that
> default values for the following parameters would need to be specified as
> well:
> 
>    - MAX_ACK_REQUESTS
> 
>    - WINDOW_SIZE
> 
>    - RCS size
> 
> 
> Thanks,
> 
> Carles
> 
> 
> 
> 
>> Hello,
>> 
>> Here you will find the last version of the draft-SCHC over NBIoT, this
>> version includes the inputs from Carles.
>> Please review the file and send comments.
>> 
>> Thank you for your inputs
>> Ana & Edgar
>> 
>> ---------- Forwarded message ---------
>> From: <[e-mail]>
>> Date: Thu, May 19, 2022 at 6:55 PM
>> Subject: [lp-wan] I-D Action: draft-ietf-lpwan-schc-over-nbiot-08.txt
>> To: <[e-mail]>
>> Cc: <[e-mail]>
>> 
>> 
>> 
>> A New Internet-Draft is available from the on-line Internet-Drafts
>> directories.
>> This draft is a work item of the IPv6 over Low Power Wide-Area Networks WG
>> of the IETF.
>> 
>>        Title           : SCHC over NBIoT
>>        Authors         : Edgar Ramos
>>                          Ana Minaburo
>>        Filename        : draft-ietf-lpwan-schc-over-nbiot-08.txt
>>        Pages           : 20
>>        Date            : 2022-05-19
>> 
>> Abstract:
>>   The Static Context Header Compression and Fragmentation (SCHC)
>>   specification describes header compression and fragmentation
>>   functionalities for LPWAN (Low Power Wide Area Networks)
>>   technologies.  The Narrowband Internet of Things (NB-IoT)
>>   architecture may adapt SCHC to improve its capacities.
>> 
>>   This document describes the use of SCHC over the NB-IoT wireless
>>   access and provides use-cases for efficient parameterization.
>> 
>> 
>> The IETF datatracker status page for this draft is:
>> https://datatracker.ietf.org/doc/draft-ietf-lpwan-schc-over-nbiot/
>> 
>> There is also an htmlized version available at:
>> https://datatracker.ietf.org/doc/html/draft-ietf-lpwan-schc-over-nbiot-08
>> 
>> A diff from the previous version is available at:
>> https://www.ietf.org/rfcdiff?url2=draft-ietf-lpwan-schc-over-nbiot-08
>> 
>> 
>> Internet-Drafts are also available by rsync at rsync.ietf.org:
>> :internet-drafts
>> 
>> 
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