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

[Eric C Rosen <[e-mail]>]

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.

> 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