Subject: label encoding in case of MPLS-ATM

From: "Ashish Vyas" <[email-masked]>
Date: Tue, 23 Jan 2001 14:03:31

hi all,

I have some doubts regarding label encoding. In case of
MPLS-ATM we use VPI/VCI fields to encode labels
and in RFC 3031 (MPLS Architecture) there are three different
techniques have been specified for label encoding (SVC encoding,
SVP Encoding and SVP Multipoint encoding).

Since the width of VPI field is 12-bits (max) and width of VCI
is only 16-bits then

1. How can a label be encoded using only one field either VPI or
   VCI? (since the label field is of 20-bits in SHIM header)

2. How can label stack be maintained by using only 12+16=28 bits?

3. How is SVP Multipoint encoding done? and why label merging is
   possible only if SVP multipoint encoding is used?

I might be missing something very basic..

Regards
-ashish

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From: David Charlap <[email-masked]>
Date: Tue, 23 Jan 2001 10:49:52 -0500

>    VCI? (since the label field is of 20-bits in SHIM header)

The 20-bit limit is only for the generalized label format.  That is,
when shim headers are used.  When labels are encoded in an ATM VCID, any
valid combination of VPI/VCI is valid as a label.

> 2. How can label stack be maintained by using only 12+16=28 bits?

SVP and SVP multipoint encodings use 12-bit labels.  Since label space
on ATM switches is per-interface, this has no global consequences.

>    possible only if SVP multipoint encoding is used?

An ATM switch can never map two different inbound VCIDs to a single
outbound VCID.  If it does, there is the very real possibility that
cells from two different packets will get interleaved together.  If this
happens, then both packets will be corrupted and irrecoverable.

One expensive solution to this is to have the switch perform sufficient
buffering and keep track of where packets begin and end, in order to
prevent cell interleaving.  But this is likely to be impossible on
legacy ATM hardware.

Now, if you're switching based on VPI alone, and there are dont-care
bits in the VCI field, you can use those bits to produce a label merging
effect.  Two inbound LSPs both go out on the same MPLS label (VPI
field), but with different values in the unused VCI bits.  Since ATM
switch hardware will see these as two different VCIDs, cell interleave
won't happen.

If, however, all the bits in the VCI field are significant, you can't do
this - there are simply no bits available for use.

-- David