Re: [mpls] Clarification on the motivation of draft-xu-spring-islands-connection-over-ip-05

Eric C Rosen <erosen@juniper.net> Wed, 06 April 2016 20:48 UTC

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To: Xuxiaohu <xuxiaohu@huawei.com>, "spring@ietf.org" <spring@ietf.org>
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Subject: Re: [mpls] Clarification on the motivation of draft-xu-spring-islands-connection-over-ip-05
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On 4/6/2016 11:37 AM, Xuxiaohu wrote:
> The situation in MPLS-SR is a little bit complex since the outgoing label for a given /32 or /128 prefix FEC could be learnt either from the IGP next-hop of that FEC or the originator of that FEC due to the IGP flooding property. In the former case, the IGP next-hop for a given FEC is taken as the next-hop of the received MPLS packet belonging to that FEC; in the latter case, the originator of that FEC is taken as the next-hop of the MPLS packet belonging to that FEC ... the latter case belongs to the "remote label distribution peer" case as defined in RFC3031

I don't believe this is correct.  In SR, the fact that label L was 
advertised by node N does not imply that a packet with L at the top of 
the stack needs to be tunneled to N.  In the typical case, the packet 
would just follow the IGP best path, and all the intermediate nodes 
would be expected to recognize the label at the top of the stack.  If 
the intermediate nodes are expected to recognize the label,  this is not 
the "remote label distribution peer case".

You seem to be positing a case where two nodes are in the same IGP 
domain, and same SPRING domain, but there is no LSP that can be used to 
transport packets from one to the other.   It would be somewhat unusual 
to have an IGP domain in which some nodes support MPLS and some don't.  
In BIER, we do accommodate this sort of situation, where some of the 
nodes in the BIER domain do not support BIER.  But I don't know whether 
that sort of scenario needs to be supported for MPLS-SR.  Do you have a 
particular use case in mind?