[Eric] Do you have an example in mind where it is useful to advertise
an Originator SRGB when the prefix in the NLRI is not a host
address?
[Stefano] in fact I don’t have any good example where a /32 (/128) must be
enforced…
Well, that's not the question I asked ;-)
Given that the SRGB is a property of a node, it seems to make sense to
associate an advertised SRGB with the address of a node. As I
explained, one can use this by pushing on a label that is computed by
combining a domain-wide unique SID with the node's SRGB, and then
pushing on a label that causes the packet to be delivered to the node in
question.
However, I can see Acee's point that (if I understand it correctly) that
all the nodes on a given subnet might use the same SRGB.
Suppose we modify my suggested text as follows:
---------------------------
OLD
When a BGP speaker attaches a Prefix-SID attribute to a given route,
the Originator SRGB TLV MUST NOT be included in the attribute unless
the following conditions hold:
- The prefix field of the route's NLRI contains a host address
(i.e., a /32 IPv4 address or a /128 IPv6 address).
- The value of the Originator SRGB TLV specifies the SRGB of the node
that is identified by the prefix field of the NLRI.
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 aPrefix-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.
NEW
If a BGP speaker attaches a Prefix-SID attribute to a given route, and
if the Prefix-SID attribute includes the Originator SRGB TLV, then:
- If the prefix field of the route's NLRI contains a host address
(i.e., a /32 IPv4 address or a /128 IPv6 address), the Originator SRGB
TLV specifies the SRGB of the node to whom the host address belongs
- If the prefix field of the route's NLRI does not contain a host
address, the Originator SRGB TLV specifies the SRGB that is used by the
set of nodes whose host addresses match the prefix, but for which there
is no "more specific" match that specifies a different Originator SRGB.
When a route carrying the Prefix-SID attribute is propagated, the
Originator SRGB TLV (if present) MUST NOT be changed.
--------------------
This new text omits the enforcement, allows an SRGB to be advertised for
an entire subnet (as suggested by Acee), but still explains how to
figure out which nodes are using the specified SRGB.
Does this modification satisfy your objection?
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