Subject: Re: label encoding in case of MPLS-ATM

From: David Charlap <[email-masked]>
Date: Tue, 23 Jan 2001 15:17:53 -0500

Saurabh Shrivastava wrote:
> David Charlap wrote:
>>>
>>> 3. How is SVP Multipoint encoding done? and why label merging is
>>>    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.
>
> But somebody down the line will have to make packets out of the cells
> and does need to do buffering isnt it - please correct me if i am
> wrong.

But they'll have unique VCIDs.  The hardware SAR chips will queue the
cells separately, and there will be no interleaving.

Even though forwarding decisions are being made with the VPI field, both
fields still exist and are used by the SAR chips when reassembling cells
into packets.

> Also, if say 2 streams are merged based on the VPI field, (and can
> be distinguished by their VCI field) - the merged stream will have
> cells interleaved - so the LSR which makes packets out of this stream
> cant assume that the consecutive cells belong to the same packet.
> - in other words cell interleave problem still happens - please
> correct me if i am wrong

Cells with different VPI/VCI values are never combined when reassembling
cells into packets.

Remember, on the hardware level there is no difference between an MPLS
signalled VC and an ATM-sigalled VC.

In this case, the hardware treats labeled packets just like it would
treat a bunch of ATM VCs aggregated for VP-switching.  They're still
separate channels, even though they are all forwarded using the same
hardware cross-connect table entry.

-- David