Subject: [6tisch] CoAP block option vs Application layer fragment

From: Qin Wang <[email-masked]>
Date: Tue, 27 May 2014 06:58:16 -0700 (PDT)

Hi all,

Thanks Carsten and Thomas, we now understood how CoAP block option works. See thread link http://www.ietf.org/mail-archive/web/6tisch/current/msg02268.html . 

Here is the cost estimation for the two schemes of fragmentation in 6top-to-6top communication. 

Assume the size of MAC payload is 81 Byte, then, 
nIE header : 2B
nCoAPheader: 7B (assume length of Token is 2)
nBlock option: 4B
nUri-path: 1+xB
(assume x=3)
nPayload : 64B
Thus, the estimated cost of fragmentation with CoAP block option is about 17/64, and the cost of application layer fragmentation is about 13/68.

I think the difference is not significant. In addition, according to the IEs and messages design in the 6top sublayer draft, there is not much possibility to send very big data in 6top-to-6top communication. Thus, I prefer to use CoAP block option, so as to take advantage of its entire solution of fragmentation, including retry, security....

What do you think?

Thanks
Qin