Re: [mpls] [Bier] Encapsulation first nibble

I believe this should be applicable to the NSH as well (i.e., set the nibble to zero) if it's assumed that the NSH could be encapsulated within an MPLS packet. 

Best regards,
Xiaohu

-----邮件原件-----
发件人: BIER [mailto:bier-bounces@ietf.org] 代表 Stewart Bryant
发送时间: 2015年3月14日 3:54
收件人: Eric C Rosen; BIER
主题: Re: [Bier] Encapsulation first nibble

On 13/03/2015 19:46, Eric C Rosen wrote:
> Here's a small issue for the WG to consider.
>
> The mpls-bier-encapsulation draft specifies that the first nibble of 
> the encapsulation is to be a BIER encaps version number, initially 0.
> The values 4 and 6 are excluded from the "version number" space, as 
> there are various heuristic procedures deployed that interpret those 
> values of the first nibble following the MPLS label stack as 
> identifying the payload to be an IP packet.
>
> However, there are also various deployed heuristics that may interpret 
> the values 0 and 1 as identifying a pseudowire payload, either data or 
> OAM.
>
> This raises the issue of whether we might be better off setting this 
> nibble to a fixed value, rather than trying to use it as a version 
> number.  If we really need a version number in the encaps, perhaps it 
> should be someplace where it definitely won't impact any existing
> heuristics.   The safest value for the first nibble might be one of 
> the values 5-9, which are already assigned as IP Version Numbers, but 
> are assigned to things that (I think) don't actually exist.
>
>
>
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I agree with Eric on this.

I would set the nibble to zero which tells any parser that the payload is only understandable only by a node that knows the actions required by the BOS label.

I think the ACH RFC says something like the above, but would have to go find the specific text.

- Stewart

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