Re: [mpls] Question about 0x8848 Ethertype

[Nitin Bahadur <nitinb at juniper.net>]

I'm not 100% sure if your interpretation is correct.

The RFC I think tries to say that the context-label must be unique. In other words both P1 and P2 cannot use the same label to
denote their context . In other words, upstream distributed labels must be unique when using ether-type 0x8848.

I don't think the intention of the RFC was that both upstream and downstream allocated labels must be unique.

Nitin

On 10/22/09 11:17 AM, "Shahram Davari" <davari at broadcom.com> wrote:

Hi Nitin,

Thanks for your answer. But RFC 5332 requires the use of unique "context label" at the top of the label stack which can't be used as downstream-assigned label on the multi-access media. So the mere presence of a "context label" should be enough and we don't need a dedicated Ethertype to tell us that the context label is upstream assigned.

Do you agree?

Regards,
Shahram

-----Original Message-----
From: Nitin Bahadur [mailto:nitinb at juniper.net]
Sent: Thursday, October 22, 2009 11:10 AM
To: Shahram Davari; mpls at ietf.org
Subject: Re: [mpls] Question about 0x8848 Ethertype


See NB> below...

On 10/22/09 10:37 AM, "Shahram Davari" <davari at broadcom.com> wrote:

Hi,

RFC 5332 says:

"Ethertype 0x8848, formerly known as the "MPLS multicast codepoint", is to be used only when an MPLS packet whose top label is upstream- assigned is carried in a multicast Ethernet frame."

It also says:

"It is expected that the LSR will follow the procedures of [RFC5331], pushing on two labels, with the topmost label being a "context label" that is the same for all MPLS packets being transmitted by the LSR onto the Ethernet, but with the second label being different for different LSPs"

And RFC 5331 defines context label as:

"A "context label" is one that identifies a label table in which the label immediately below the context label should be looked up. A context label carried as an outermost label over a particular multi-access subnet/tunnel MUST be unique within the scope of that subnet/tunnel."

Considering the above statements we can conclude that:

"Multicast MPLS packets with upstream assigned labels encapsulated in Multicast Ethernet frames in a Multi-access media (LAN) require to use 0x8848 as Ethertype and require to use a context label that is unique over that media to identify the label space for the Multicast MPLS packet"

The question is since a context label is unique in a LAN and can be immediately identified as an upstream label, then why do we need 0x8848 Ethertype? Looks like we could just use the context label.

Am I missing something?

NB> The question is "how to identify that this is an upstream label". Consider label 100. This label could have been allocated by upstream P1 router as an upstream label and distributed to downstream router PE. The same label could have been allocated by PE router and distributed to router P1. So now, when PE
receives a packet over the LAN (from P1) with label 100, it does not whether to look it up as an upstream label or as a downstream label..

Ethertype 0x8848 allows PE to perform lookups in context of P1 and not in context of labels distributed by PE.

Thanks
Nitin