Subject: Re: Any disadvantage about MPLS?

From: [email-masked]
Date: Tue, 5 Jun 2001 18:03:26 +0530

Hi ewliu!

one disadvantage of mpls is that a four byte shim header is inserted to every
network layer packet before being forwarded to data link layer, which
effectively increases the payload for data link layer.

Regards
Mohit

[email-masked] on 06/05/2001 02:56:17 AM

To:   [email-masked]
cc:   [email-masked] (bcc: Mohit Goel/HSS)

Subject:  Any disadvantage about MPLS?

It seems that MPLS is good for almost everything,from QoS to traffic engineering
and VPN.

I want to get some information about its disadvantages,any information will be
appreciated.

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From: "Akshat Kapoor" <[email-masked]>
Date: Tue, 5 Jun 2001 19:47:39 +0530

Hi Mohit,
    That case would only hold true
- if the native Layer 2 header doesn't have a field for label (like ATM has
VPI-VCI fields, FR has DLCI), or
- the label stack (if present) can't be adjusted in the layer 2 header.

Akshat

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From: Eric Gray <[email-masked]>
Date: Tue, 05 Jun 2001 11:06:54 -0400

Akshat,

    Even in the case where there is a representation of the label in
the L2 header, there will most often be a corresponding label stack
entry in the MPLS shim header.  If you read the specifications for
ATM and Frame Relay, you will find that inclusion of the MPLS
shim header is at least the normal case.  For this reason, it is likely
that nobody will use the abnormal case - i.e. - omitting the MPLS
shim header when it is known that ALL packets will use exactly one
MPLS label when carried on an ATM (or FR*) VC.

    However, the 'extra' space consumed using MPLS labels is a minor
cost in using the protocol particularly in TE or VPN application (where
an alternative choice might be to IP-in-IP encapsulation, for instance).

    Most of the costs associated with MPLS could be grouped under the
general headings of additonal protocol complexity and interworking with
an existing networking installed base.

*    While RFC 3034 implies that there is a non-default case where a
label stack may be omitted, it does not describe when and how.  RFC
3035 explicitly states the conditions under which it is possible to use
a VC corresponding to (a portion of) an LSP, omitting the label stack.
See section 9, RFC 3035 for ATM encaps and section 4 of RFC 3034
for Frame Relay...

--
Eric Gray