Re: [mpls] Question about 0x8848 Ethertype

[Mitchell Erblich <erblichs at earthlink.net>]

Nitin Bahadur,

		Are you you asking for uniqueness wrt only upstream or only  
downstream or both?

		I think it is the latter, last one.

		Mitchell Erblich
		=====================
On Oct 22, 2009, at 11:27 AM, Nitin Bahadur wrote:

> 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
>
>
>
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