Subject: RE: MPLS and fragmentation...

From: "Krishnan, Vijay G." <[email-masked]>
Date: Wed, 13 Dec 2000 20:37:44 -0500

Ben,

The LSR can calculate the MTU based on the number of labels it processes,
but there could be more labels in stack.

 ingressLSR ------>LSR1------>LSR2------>LSR3------->LSR4------>egressLSR
             GigE        POS        POS        POS
                     |-----------Tunnel 1 ------------|
                                |---T2----|

Consider a case where the LSP goes through multiple (hierarchical) LSP
tunnels. One from LSR1 to LSR4 and another from LSR2 to LSR3. Each LSR would
know only that it is going to push  a label and wouldn't know how many
labels the incoming packet has. It may process only the top two. LSR2
calculates its MTU as 9K-2labels. But the packets through LSR2 to LSR3 would
have three labels in stack (due to Tunnel 2). So packets fragmented on LSR1
may need fragmentation at LSR2 also.

Just a thought... Can we have a field in the PATH message for label stack
depth initialised to 1. It would be incremented by the LSRs when it is
switched to LSP tunnels (label push). Maybe there are much better ways for
finding the stack depth.

thanks
Vijay

-----Original Message-----
From: Ben Black [mailto:[email-masked]]
Sent: Wednesday, December 13, 2000 6:08 PM
To: Krishnan, Vijay G.
Cc: 'Serge Maskalik'; Steve Elias; [email-masked]
Subject: Re: MPLS and fragmentation...

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.

ben

---

From: Ben Black <[email-masked]>
Date: Wed, 13 Dec 2000 17:36:40 -0800

> may need fragmentation at LSR2 also.

This indicates the need for signalling of the MTU to head LSRs, which is what
I am asking be added to LDP.  Upon establishment of Tunnel 1 (or Tunnel 2)
a message could be generated back to head end LSRs for any LSPs crossing
the tunnels notifying them that the MTU had changed (this is not the same
as simply determining the MTU for an LSP at the time it is created).

> switched to LSP tunnels (label push). Maybe there are much better ways for

An LDP Query, perhaps?

ben

---

From: "James R. Leu" <[email-masked]>
Date: Thu, 14 Dec 2000 08:14:50 -0600

On Wed, Dec 13, 2000 at 05:36:40PM -0800, Ben Black wrote:

<snip>

> An LDP Query, perhaps?

You mean along the lines of draft-ietf-mpls-lsp-query-01.txt?

Jim
-- 
James R. Leu

---

From: Ben Black <[email-masked]>
Date: Thu, 14 Dec 2000 08:06:15 -0800

> You mean along the lines of draft-ietf-mpls-lsp-query-01.txt?

yes.

ben