comments on draft-ietf-shim6-failure-detection-06.txt {3}

[Sébastien Barré <sbarre@info.ucl.ac.be>]

Hi,

I have noticed a problem with the Hdr Ext Len field of REAP packets :
The draft specifies this field like follows :
-------------
Next Header, Hdr Ext Len, 0, 0, Checksum

     These are as specified in Section 5.3 of the SHIM6 protocol
     description [I-D.ietf-shim6-proto].
--------------
So, looking at section 5.3 in the shim6 proto draft :
--------------
Hdr Ext Len:   8-bit unsigned integer.  Length of the shim6 header in
                 8-octet units, not including the first 8 octets.
--------------
This means any shim6 or reap packet must have a length multiple of 8 
bytes. AFAIK, this is the case  for all shim6 messages defined in the 
proto draft. But that's not true for the failure detection draft in the 
case of the keepalive message : the keepalive message as defined is 12 
bytes long, which cannot be encoded in the Hdr Ext Len field. I suggest 
extending it with a 4 bytes padding. In the current version of the draft 
I don't see other occurences of this problem.

Nevertheless, when designing options in the future, we should take care 
of also enforcing a length multiple of 8 bytes.

regards,

Sébastien.

--
Sébastien Barré
Researcher,
CSE department, UCL, Belgium