Re: [Idr] [spring] Comments on draft-ietf-idr-bgp-prefix-sid-01

["Acee Lindem (acee)" <[e-mail]>]

Hi Eric, 

On 11/9/15, 10:22 AM, "spring on behalf of Eric C Rosen"
<[e-mail] on behalf of [e-mail]> wrote:

>Hi Stefano,
>
>>>    If a BGP route is received that contains a Prefix-SID attribute
>>>with an
>>>    Originator SRGB TLV, but the prefix field of the NLRI does not
>>>contain a
>>>    host address, the attribute SHOULD be regarded as malformed. If a
>>>    Prefix-SID attribute contains more than one SRGB TLV, it SHOULD be
>>>    regarded as malformed.  See section 7 for the treatment of a
>>>malformed
>>>    Prefix-SID attribute.
>>>
>>>    When a route carrying the Prefix-SID attribute is propagated, the
>>>    Originator SRGB TLV (if present) MUST NOT be changed.
>
>> why would you need such limitation ? A prefix may have a shorter mask
>> than 32 (or 128) and still be ok for the Originator SRGB to be there.
>
>The SRGB is a property of a node, not a property of a prefix.  To make
>use of the "Originator SRGB", you have to know the node whose property
>it is.  And you have to be able to tunnel packets to that node.  In the
>text I wrote above, the prefix field in the NLRI identifies the node to
>which the "Originator SRGB" belongs, and the prefix-SID field
>essentially gives you a node-SID that you can use to tunnel to the node
>in question.

I agree the predominant use case will be advertisement of a loopback.
However, independent of whether or not the Originator-SRGB TLV is
included, I see no reason why a BGP Speaker could not associate a
label-index with a locally attached subnet.

Thanks,
Acee 




>
>> The Originator-SRGB may only be inserted by the originator of the
>> prefix, maybe we should emphasize that, but the masklength is mostly
>> irrelevant here.
>
>I don't see that the Originator-SRGB TLV is useful without an explicit
>identification of the node whose SRGB it describes.  Certainly if you
>are trying to set up an explicitly routed path (perhaps as a loose
>source route) what you need are the node-SIDs are of the hops you want
>to specify, and the SRGB of each hop.
>
>When you talk about "the originator of the prefix", I think what you
>really mean is "the last node of the BGP prefix segment".  But I don't
>think that that term necessarily denotes a unique node, as there might
>be multiple ECMP paths to the prefix, and the prefix-SID does not
>distinguish among them.  E.g., the prefix itself might be multi-homed,
>or there might be multiple exit points from the SR domain, all of which
>are equidistant from the prefix.  In cases like that, you have no way of
>knowing whether a label computed from the Originator-SRGB is actually
>going to be correctly interpreted, because you don't really know the
>path a packet will take when it is labeled with the prefix-SID.
>
>Eric
>
>
>
>
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