Rishabh,
Lots of thanks for a prompt and detailed response.
Please see more inline below.
Regards,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email: [email protected]
From: Rishabh Parekh (riparekh) <[email protected]>
Sent: Thursday, October 17, 2019 1:24 AM
To: Alexander Vainshtein <[email protected]>;
[email protected]; Clarence Filsfils (cfilsfil) <[email protected]>;
[email protected]; [email protected]
Cc: [email protected]
Subject: RE: Some questions regarding Replication SID
Alexander,
Responses to your queries are prefaced with [RP].
1. The draft says that "each branch is abstracted to a <Downstream Node,
Downstream Replication-SID>". Does that mean that the Downstream
Replication-SID is one of the SIDs defined in Sections 3, 4 and 5 of
RFC8402<https://clicktime.symantec.com/3XUuhGHPMpikj1KmCoXg6zn6H2?u=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Frfc8402>?
[RP] No, Replication SID not a topological SID as defined the sections you
point to in RFC 8402. Instead, it is a separate SID (label in SR-MPLS) that
represents the Replication segment in data plane.
[[Sasha]] Oops! This question got mangled in the process of writing. Actually I
wanted to ask whether the SIDs in the list that represent a specific
Replication Branch are the SIDs defined in RFC 8204. The text I see in the SR
P2MP Policy<https://tools.ietf.org/html/draft-voyer-pim-sr-p2mp-policy-00>
draft seems to suggest that it is not necessarily so because a Tree-SID is
considered also as the Replication SID. This looks inconsistent to me because a
Replication SID as defined in this draft is instantiated both in the
Replication node and in the Downstream nodes while the Tree-SID does not
require instantiation in the Leaf nodes (this is optional).
2. The draft also says that "A Replication branch to a particular Downstream
Node could be represented by the node's Node SID". Does this mean that the
Replication Node sends the packets it receives with the Replication SID as the
active segment with the labels representing the downstream Node SID as the
active segment across such a replication branch?
[RP] No, Replication SID relevant at a downstream node would be the bottom
label with other SIDs stacked on top which would guide the packet to the
downstream node. Of course, if the Downstream node is adjacent to the
Replication node, only the Replication SID would be present in the outgoing
packet.[[Sasha]] OK, thanks.
3. The draft also says that "Replication segment is instantiated at Downstream
nodes and at the Replication node". Does that mean that the list of SIDs
associated with the specific replication Branch is pushed by the Replication
Node on top of the label representing the Replication SID in the Downstream
node of this branch?
[RP] Yes. See response to 2 above.[[Sasha]] OK, thanks again.
4. Are the labels that represent the Replication SID at the Downstream nodes
downstream-allocated by these nodes or upstream-allocated by the replication
node?
[RP] Since the Replication SID is locally relevant at a node, the Replication
SID would be downstream-allocated. However, it may also be allocated by PCE;
see response to 5, 6 below.[[Sasha]] OK, thanks.
5. The draft also says that "A Replication segment can be either provisioned
locally on a node or programmed by a PCE". These two options look exactly the
same to me from the POV of the node on which the Replication segment is
programmed - what, if anything, did I miss?
[RP] You are right in that it does not matter how Replication segment is
instantiated at a node. The use of PCE is relevant for SR P2MP
Policy<https://clicktime.symantec.com/3BarcmSMhmQoNnLWirtL2F6H2?u=https%3A%2F%2Fdatatracker.ietf.org%2Fdoc%2Fdraft-voyer-pim-sr-p2mp-policy%2F>
draft where PCE instantiates Replication segments.[[Sasha]] OK, I will look
up the second draft. BTW, it does not appear as a reference in this one - is it
intentional?
6. Did you consider a possibility of advertising the Replication Segment from
the Downstream nodes to the Replication one using some multicast routing
protocol (e.g., creating a SR-MPLS replacement for mLDP)? Or is such a
possibility strictly precluded?
[RP] . We do not strictly preclude any protocol , but one of the goals of SR is
to simplify. The idea is same here - use replication segments to realize P2MP
trees computed by PCE (without need of multicast protocols) as specified in SR
P2MP draft[[Sasha]] One of the well-known aspects that make multicast different
is that the traffic in the Service Provider domain is driven by the dynamic
customer requests. Handling these requests via a PCE looks problematic to me.
Any details regarding instantiation of the Replication Segment in SR-MPLS would
be highly appreciated.
[RP]SR P2MP policy draft lists different protocols (PCEP, BGP, etc.) that can
be used to instantiate Replication segments. SR P2MP
PCEP<https://clicktime.symantec.com/3JuFtE8qkd9Viz4Z8BMaLWb6H2?u=https%3A%2F%2Ftools.ietf..org%2Fhtml%2Fdraft-dhs-spring-pce-sr-p2mp-policy-00>
would be updated; other drafts will be published in future.
More of the same...
Pleade note that if the anser to #3 in my original message is positive, then
the statement in the draft that say the Replication Segment is similar to the
Binding segment srems to be inaccurate.
[RP] Since Replication SID is local to a Node, the Replication SID of the
Replication segment at Root (or Headend) node can be used as a (constant)
Binding SID to steer traffic into the segment.
[[Sasha]] The difference between the Binding SID as defined in 8204 and the
Replication SID as defined here is that the former is instantiated just locally
while the Replication SID is instantiated both in the Replication node and in
the Downstream nodes.
-Rishabh
From: Alexander Vainshtein
<[email protected]<mailto:[email protected]>>
Sent: Tuesday, October 15, 2019 6:31 AM
To: [email protected]<mailto:[email protected]>; Clarence Filsfils
(cfilsfil) <[email protected]<mailto:[email protected]>>; Rishabh Parekh
(riparekh) <[email protected]<mailto:[email protected]>>;
[email protected]<mailto:[email protected]>;
[email protected]<mailto:[email protected]>
Cc: [email protected]<mailto:[email protected]>
Subject: Some questions regarding Replication SID
Dear colleagues,
I have read the Replication SID
draft<https://clicktime.symantec.com/3G11JyktvUDpkKaz5dfYE9E6H2?u=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Fdraft-voyer-spring-sr-replication-segment-00>,
and I have a few questions dealing with possible instantiation of the
Replication SOD in
SR-MPLS<https://clicktime.symantec.com/3Hq42mK61kxvw5A1kBS8D1g6H2?u=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Fdraft-ietf-spring-segment-routing-mpls-22>.
1. The draft says that "each branch is abstracted to a <Downstream Node,
Downstream Replication-SID>". Does that mean that the Downstream
Replication-SID is one of the SIDs defined in Sections 3, 4 and 5 of
RFC8402<https://clicktime.symantec.com/3XUuhGHPMpikj1KmCoXg6zn6H2?u=https%3A%2F%2Ftools.ietf.org%2Fhtml%2Frfc8402>?
2. The draft also says that "A Replication branch to a particular Downstream
Node could be represented by the node's Node SID". Does this mean that the
Replication Node sends the packets it receives with the Replication SID as the
active segment with the labels representing the downstream Node SID as the
active segment across such a replication branch?
3. The draft also says that "Replication segment is instantiated at Downstream
nodes and at the Replication node". Does that mean that the list of SIDs
associated with the specific replication Branch is pushed by the Replication
Node on top of the label representing the Replication SID in the Downstream
node of this branch?
4. Are the labels that represent the Replication SID at the Downstream nodes
downstream-allocated by these nodes or upstream-allocated by the replication
node?
5. The draft also says that "A Replication segment can be either provisioned
locally on a node or programmed by a PCE". These two options look exactly the
same to me from the POV of the node on which the Replication segment is
programmed - what, if anything, did I miss?
6. Did you consider a possibility of advertising the Replication Segment from
the Downstream nodes to the Replication one using some multicast routing
protocol (e.g., creating a SR-MPLS replacement for mLDP)? Or is such a
possibility strictly precluded?
Any details regarding instantiation of the Replication Segment in SR-MPLS would
be highly appreciated.
Regards, and lots of thanks in advance,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email:
[email protected]<mailto:[email protected]>
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