The MPLS WG Archive

[David Fiore <david@micromuse.com>]

--On Wednesday, December 13, 2000 03:08:25 PM -0800 Ben Black 
<ben@layer8.net> wrote:

> On Wed, Dec 13, 2000 at 05:42:45PM -0500, Krishnan, Vijay G. wrote:
>> Hi,
>>
>> I understand that the RSVP-TE finds the path MTU of an LSP based on the
>> minimum MTU size of the router interfaces on the path. The source
>> (ingress LSR or LER) can send the packets with this size to avoid
>> fragmentation. Can the packet size overshoot the path MTU due to
>> addition of labels (hierarchical tunnels or something) on its path,
>> which might require fragmentation?
>>
>
> The MTU determined should take into account whatever the depth of the
> label stack actually is for the given LSP.  Fragmentation is not avoided,
> it is simply done, if required, outside the MPLS domain.

It would seem to me that fragmentation must occur inside of the MPLS domain 
becuase if it didn't, why would the LSR MIB we have the following object, 
instantiated on all MPLS interfaces:

mplsInterfaceOutFragments OBJECT-TYPE
   SYNTAX        Counter32
   MAX-ACCESS    read-only
   STATUS        current
   DESCRIPTION
       "This object counts the number of outgoing MPLS
        packets that required fragmentation before
        transmission on this interface. This object
        MUST count on a per-interface basis regardless of
        which label space the interface participates in."
::= { mplsInterfacePerfEntry 4 }


-dbf

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