Subject: Question related to shim header...

From: "G.B.Naidu" <[email-masked]>
Date: Fri, 5 Nov 1999 18:54:58 +0530 (IST)

Hi

I have a question regarding shim header.

When L2 media is ATM & Frame relay, is shim header must or optional?

If L2 media is ethernet, what will be the case? Shim header is a must or
not?

If it is optional, how we will find out whether shim header is present or
not?

that's all

thanks
-- gbnaidu.

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From: "Peter Ashwood-Smith" <[email-masked]>
Date: Fri, 05 Nov 1999 09:47:06 -0500

> When L2 media is ATM & Frame relay, is shim header must or optional?

  Must, it carries the TTL.

> If it is optional, how we will find out whether shim header is present or

  The Ethertype tells you if it is MPLS or not. If the Ethertype says its
MPLS then there follows a shim header otherwise its whatever the Ethertype says.
Two Ethertypes 0x8847 and 0x8848 indicate MPLS unicast and MPLS multicast.

  Cheers,
 
  Peter

---

From: "PHILIP MATTHEWS" <[email-masked]>
Date: Fri, 05 Nov 1999 12:42:12 -0500

Actually, section 9 of draft-ietf-mpls-atm-02.txt specifies that the
shim header can be omitted in a couple of special situations.
To quote from the draft:

  The only ways to eliminate this extra overhead are:

     - through apriori knowledge that packets have only a single label
       (e.g., perhaps the network only supports one level of label)

     - by using two VCs per FEC, one for those packets which have only a
       single label, and one for those packets which have more than one
       label

   The second technique would require that there be some way of
   signalling via LDP that the VC is carrying only packets with a single
   label, and is not carrying a shim. When supporting VC merge, one
   would also have to take care not to merge a VC on which the shim  is
   not used into a VC on which it is used, or vice versa.

   While either of these techniques is permitted, it is doubtful that
   they have any practical utility.  Note that if the shim header is not
   present, the outgoing TTL is carried in the TTL field of the network
   layer header.

- Philip Matthews, Newbridge Networks Corporation

---

From: "G.B.Naidu" <[email-masked]>
Date: Sat, 6 Nov 1999 10:47:18 +0530 (IST)

Thanks a lot for the replies. See my comments below.

>   Must, it carries the TTL.

According to the MPLS-ATM draft, you can avoid the shim in some
exceptional cases.

> Two Ethertypes 0x8847 and 0x8848 indicate MPLS unicast and MPLS multicast.

Coming to the Ehternet type, could you please let me know where this is
mentioned. Where are the above ethertypes are mentioned? So you say that
in ethernet type, shim header is a must.

Even I guess so, the reason being that to use TTL facility, some draft
mentiones that ingress LSR has to copy TTL from IP header to shim header
and the egress LSR has to copy back the current TTL from shim header to IP
header. Any comments on these. Please be specific whether it is a must or
optional.

Any info where MPLS on ehternet type discussed, could you send me some
pointers.

cheeers
-- gbnaidu.