Re: [mpls] [Bier] WGLC: draft-ietf-bier-mpls-encapsulation-06

Eric C Rosen <erosen@juniper.net> Mon, 08 May 2017 15:29 UTC

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To: Stewart Bryant <stewart.bryant@gmail.com>, <bier@ietf.org>, "mpls@ietf.org" <mpls@ietf.org>, "mpls-chairs@ietf.org" <mpls-chairs@ietf.org>
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From: Eric C Rosen <erosen@juniper.net>
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Date: Mon, 8 May 2017 11:29:43 -0400
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Subject: Re: [mpls] [Bier] WGLC: draft-ietf-bier-mpls-encapsulation-06
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Hi Stewart,

Thanks for your review!

On 5/8/2017 10:52 AM, Stewart Bryant wrote:
>
> Please could you clarify whether this document is being reviewed by 
> the MPLS WG as part of this process?
>

I'll let the WG chairs answer that ;-)

> Please can you explain how the entropy system works in MPLS?
>
> Are you expecting the P router to look into the BIER header, or are 
> you expecting BIER capable nodes to construct an entropy structure in 
> the outer label stack.
>

A router that is BIER-capable uses the entropy value from the BIER 
header as part of its BIER forwarding process.  This is discussed in the 
BIER architecture document.

If a packet is being tunneled from one BIER-capable router to another, 
and the tunnel passes through a sequence of intermediate routers, then 
the tunnel encapsulation probably has some entropy mechanism of its 
own.  It would make sense to ensure that if two BIER packets have the 
same entropy value in their respective BIER headers, then they have the 
same entropy in the tunnel encapsulation.  However, the tunnel 
encapsulation is not the BIER encapsulation. and how the tunnel 
encapsulation header is formed is outside the scope of this document.

The BIER entropy field does happen to be 20 bits long, so if the tunnel 
encapsulation is MPLS, the BIER entropy could be copied into the EL.

> If the latter, you have two methods available to you the EL system, 
> and the FAT PW system. The former is becoming the method of choice 
> where available, but you have the option of using the later which will 
> work in any network.
>

This seems like a general issue about MPLS tunnels, not a BIER-specific 
issue.

> I find the title a little strange since the union of MPLS and non-MPLS 
> is all networks and thus you ought to cut the title after "Replication".
>
>

Originally the draft only discussed MPLS networks.  When a non-MPLS 
encaps was added, the title was augmented with "and non-MPLS networks".  
I don't really see a problem with it, as it emphasizes that two 
encapsulation mechanisms are provided.

Eric