Subject: questions on MPLS fragmentation

From: Ramanjaneyulu Y T <[email-masked]>
Date: Sat, 19 Aug 2000 15:05:50 +0530 (IST)

Hi,
  I have some doubts in draft-ietf-mpls-label-encaps-08.txt . Please help
me in this regards.

I have questions in 
-----------------
  '3.4. Processing Labeled IPv4 Datagrams which are Too Big'which is  
under 'Fragmentation and PathMTU' section .
-------------------

  1. How to check DF bit in IP packet ? ( Shall i strip all MPLS labels
and check or any direct method availabel ?)

  2.  The following is the text from the draft. 

--------------------------
   If a labeled IPv4 datagram is "too big", and the DF bit is not set in
   its IP header, then the LSR MAY silently discard the datagram.

   Note that discarding such datagrams is a sensible procedure only if
   the "Maximum Initially Labeled IP Datagram Size" is set to a non-zero
   value in every LSR in the network which is capable of adding a label
   stack to an unlabeled IP datagram.
 -----------------

I didn't get the condition behind this statement ?. 'DF' bit is not set
means one can fragment the packet, but why condition (Maximum Initially
Labeled IP Datagram Size )  is required to fragment eventhough if DF is
not set ?

 3. The following is the text from draft.

------------
If the LSR chooses not to discard a labeled IPv4 datagram which is
   too big, or if the DF bit is set in that datagram, then it MUST
   execute the following algorithm:

      1. Strip off the label stack entries to obtain the IP datagram.

      2. Let N be the number of bytes in the label stack (i.e, 4 times
         the number of label stack entries).

      3. If the IP datagram does NOT have the "Don't Fragment" bit set
         in its IP header:
------------------

   In above para eventhough DF bit is set why fragmentation is required? 

Please explain clearly what this section means.

         Thanking you,

                                         Regards
                                        Ramanjaneyulu Y.T. 
 
 " A real friend is one who walks in when the rest of the world walks
 out."
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