The MPLS WG Archive

[Ajay Simha <asimha@cisco.com>]

On Wed, 12 Dec 2001, Neeraja Jasty wrote:

NJ>Hi,
NJ>  Hi have some doubts in MPLS fragmentation.
NJ>
NJ>Section 3.4 of RFC says the following.
NJ>
NJ>-----------------
NJ>If a labeled IPv4 datagram is "too big", and the DF
NJ>bit is not set in its IP header, then the LSR MAY
NJ>silently discard the datagram.
NJ>
NJ>Note that discarding such datagrams is a sensible
NJ>procedure only if the "Maximum Initially Labeled IP
NJ>Datagram Size" is set to a non-zero value in every LSR
NJ>in the network which is capable of adding a label
NJ>stack to an unlabeled IP datagram.
NJ>-----------------
NJ>
NJ>I didn't understand this. In general, If the DF bit is
NJ>not set and the packet is too big, we fragment the
NJ>packet.
NJ>
NJ>Can anyone explain this?

The answer to the above is in the text below (in your next question).

NJ>
NJ>
NJ>Next question
NJ>RFC says
NJ>------------
NJ>If the LSR chooses not to discard a labeled IPv4
NJ>datagram which is too big, or if the DF bit is set in
NJ>that datagram, then it MUST execute the following
NJ>algorithm:
NJ>
NJ>1. Strip off the label stack entries to obtain the IP
NJ>datagram.
NJ>
NJ>2. Let N be the number of bytes in the label stack
NJ>(i.e, 4 times the number of label stack entries).
NJ>
NJ>3. If the IP datagram does NOT have the "Don't
NJ>Fragment" bit set in its IP header:              <===Does NOT have DF set
NJ>------------------
NJ>
NJ>My question is, if the DF set, How can we fragment the
NJ>packet?

Please read the #4 below:

4. If the IP datagram has the "Don't Fragment" bit set in its IP <==HAS DF set
         header:

         a. the datagram MUST NOT be forwarded

         b. Create an ICMP Destination Unreachable Message:

             i. set its Code field [3] to "Fragmentation Required and DF
                Set",

            ii. set its Next-Hop MTU field [4] to the difference between
                the Effective Maximum Frame Payload Size and the value
                of N

         c. If possible, transmit the ICMP Destination Unreachable
            Message to the source of the of the discarded datagram.


it basically sends an ICMP unreacable to the source of the orginal message.

-ajay

NJ>
NJ>thanks
NJ>neeraja
NJ>
NJ>
NJ>
NJ>
NJ>
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NJ>

-- 
Ajay Simha
MPLS Deployment Engineer
IOS Technology Division
(919) 392-3141

"Study as if you were to live forever
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