Subject: RE: Fragmentation and reassembly of MPLS packets

From: Sajith Kizhikkepurayil <[email-masked]>
Date: Tue, 15 May 2001 09:07:24 -0700

is it not possible to extend LDP(s) to exchange MTU info for an FEC,
so that each ingress LSR know the path MTU of LSPs 
originating from them ? that is each LSP has an MTU associated with it.
any thoughts ?

thanks
sajith

>-----Original Message-----
>From: Eric Gray [mailto:[email-masked]]
>Sent: Tuesday, May 15, 2001 6:30 AM
>To: [email-masked]
>Cc: [email-masked]; [email-masked]
>Subject: Re: Fragmentation and reassembly of MPLS packets
>
>
>    Fragmentation of MPLS packets is, in general, a pretty bad idea.
>
>    The main issue is knowing that the first thing after the last label
>is in fact a 'normal' IP packet.  If you cannot predictably determine
>this, then you do not know how valid the fields of the 'IP header'
>are and it could be disastrous to attempt either to 'return' an ICMP
>error message to the 'Source IP Address' (which may be random
>data if the packet has a compressed RTP/UDP/IP header or is
>encrypted) or to fragment the packet.
>
>    One might be able to infer 'reliably' that - if the FEC associated
>with an LSP is an IPv4 FEC, and there is only one label in the label
>stack - the following header is a normal IPv4 header.  This should
>always be the case if the LSP was established using LDP for hop
>by hop routing.  So, if - while forwarding the packet - you can be
>mindful of how the LSP was created and you know that there is
>only one label in the stack, you may be able to do IP fragmentation
>of an MPLS labeled packet.
>
>    Considering that the dominant application for MPLS seems to be
>based on LSP tunneling using explicit routes (as opposed to hop by
>hop routed LSPs), it does not seem likely that scenarios where it
>makes sense to attempt to fragment MPLS packets are likely to be
>the common case.
>
>--
>Eric Gray
>
>YTR wrote:
>
>> Hi Mohit,
>>     If my understanding is correct, its impossible to do 
>fragmentation without IP header.  one has to remove (preserve 
>for later use) MPLS label stack and check IP DF bit in IP 
>header. If its set generate ICMP packets otherwise, fragment 
>IP packet keep the depth of the label stack in mind and 
>reconstruct the fragmented packets with MPLS label stack and 
>forwards those based on the top label. At the egress IP module 
>will reconsruct the IP packet when it gets from MPLS module.
>>
>> If I am wrong pls correct me.
>>
>> Thanks
>> YTR
>>
>> On Tue, 15 May 2001, [email-masked] wrote:
>>
>> >
>> >
>> >
>> > Hi all,
>> >
>> > Section 3 of RFC 3032 discusses the procedure for 
>fragmenting the labeled as
>> > well as unlabeled MPLS packet. In IP there are fields in 
>the datagram header
>> > (IDENTIFICATION, FLAGS, and FRAGMENT OFFSET) which control 
>fragmentation and
>> > reassembely of datagrams. But in MPLS header there are no 
>such fields defined.
>> > So how does reassembly of the fragmented MPLS packets 
>happens at the destination
>> > host.
>> >
>> > Thanks in advance
>> > Mohit
>>
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---

From: Giles Heron <[email-masked]>
Date: Tue, 15 May 2001 17:14:39 +0100

Ben and Kireeti wrote a draft on this recently:

http://search.ietf.org/internet-drafts/draft-black-ldp-mtu-extensions-00.txt

Giles

-- 
============================================================
Giles Heron      Principal Network Architect      Gone2 Inc.
ph: +44 7880 506185         "if you build it they will yawn"
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