Thanks for the feedback
See inline
Ahmed
On 11/28/2017 8:54 AM, Muthu Arul Mozhi Perumal wrote:
On Tue, Nov 28, 2017 at 5:34 PM, Ahmed Bashandy (bashandy)
<[email protected] <mailto:[email protected]>>wrote:
Hi,
The behavior described in section 5.3 is clear:
- The top label of incoming packet to node "S" is either a prefix
SID owned by node "F" or an adjacency SID for (S,F)
- If the link from node "S" to node "F" is up, then the normal
behavior for node "S" is to apply penultimate hop popping (PHP).
HEnce node "S" *pops* the top label and sends the packet to node "F"
- But if the link (S,F) is down and "S" is configured to do node
protection, then node "S" will still pop the top label. This will
promote the label right underneath the incoming label to become
the *top* label. Hence there is no need to peek into the label stack
What if the new top label is a BSID assigned from the SRLB of node F
or a BGP-LU or a VPN label assigned by node F?
#Ahmed: I just replied to Robert. Let me put it here
The node "S" knows the SRGB and the adj-SIDs of the neighboring node
"F". Hence if the new top label is not within these two sets, then the
node "S" will always be able to know that the node that failed is NOT a
midpoint.
I will add a statement in the document to explain how a node can
determine that a failure is a midpoint failure. I will also add a
statement to indicate that if the node determines that the failure is
not a midpoint failure then it may apply other protection techniques
that are beyond the scope of this document or simply drop the packet and
wait for normal protocol conversion.
- In a link-state envirnoment, node "S" knows the SRGB of node "F"
as well as all adjacency SIDs of node "F". Hence it can now
compare the new top label against the SRGB or the list of adj-SIDs
of the node "F"
What does "it" stand for in "it can now compare"?
"Ahmed: "It" refers to the node "S"
For the control plane to be able to compare it also needs to be
imposing the SR policy as I said earlier.
#Ahmed: There is no control plane comparison.
Or is the MPLS data plane expected to do such a comparison on the fly?
#Ahmed: data plane is expected to do such comparison. It is not that
difficult. Just make sure you have a good forwarding ASIC :)
- If the new top label is within the SRGB of node "F" or an
adj-SID of node "F", then node "S" applies the behavior described
in section 5.3.1 or section 5.3.2, respectively
The bottom line is that there is no need for any peeking into the
label stack. Just inspect the new top label
How is the MPLS data plane in a transit node expected to be
programmed to make this work?
#Ahmed: Implementation details that should become big problems for good
forwarding ASICs :)
Regards,
Muthu
Thanks
Ahmed
On 11/23/2017 5:04 AM, Muthu Arul Mozhi Perumal wrote:
My understanding is that draft wants to provide a solution for
the problem where the active segment is a prefix/adjacency
segment of the neighbor and the neighbor fails. A solution to
this is possible only at a node that is enforcing the SR policy
(consisting of the segment list). For a transit node, its data
plane would have to peek into the label stack and determine the
type of the segment/label following the active segment and act
accordingly, which is not inline with the SR architecture which
requires SR to work 'as is' on traditional MPLS data plane
Muthu
On Wed, Nov 22, 2017 at 8:22 PM, Alexander Vainshtein
<[email protected] <mailto:[email protected]>> wrote:
Muthu and all,
I do not see how the draft in quesrion us related to "SR Policy".
From my POV its scope is a SR LSP comprised of multiple Node
SIDs within a single IGP domain, and it provides local fast
protection against failure of a node that terminates one of
the segments comprising this LSP. Pritection action is
performed by the penultimate node.
My 2c.
Sent from Yahoo Mail on Android
<https://overview.mail.yahoo.com/mobile/?.src=Android>
On Wed, Nov 22, 2017 at 3:27, Muthu Arul Mozhi Perumal
<[email protected] <mailto:[email protected]>> wrote:
Section 5.3
of draft-bashandy-rtgwg-segment-routing-ti-lfa describes
protecting SR policy midpoints against node failure for
the case where the active segment is the prefix or
adjacency segment of a neighbor.
I believe the steps described in the procedure is
applicable only for a node steering packets into the SR
policy. This could be an ingress PE steering IP packets
into a SR-TE tunnel or an intermediate node steering
labeled packets received with a BSID into a SR-TE tunnel
identified by that BSID.
A transit node that has no idea about the SR policy
itself is not expected to perform the procedure described
in that section.
Is my understanding correct?
Regards,
Muthu
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