Hi Fan,
More discussions on whether the redundancy/merging segment is a
topological/routable segment.
In case of SR-MPLS, draft-geng says:
In SR over MPLS, Redundancy Segment acts as DetNet S-Label to
explicitly identify the replication function on redundancy node.
Redundancy segment is allocated from redundancy node, and is
provisioned to the ingress node of SR domain by the controller plane
via PCEP, BGP, or NetConf protocols.
According to RFC 8964:
S-Label A DetNet "service" label that is used between DetNet
nodes that implement the DetNet service sub-layer
functions. An S-Label is used to identify a DetNet
flow at the DetNet service sub-layer at a receiving
DetNet node.
The S-label identifies the flow only. PRF/PEF/POF are not indicated by the
S-label, though the <S-label, SN> provides information needed for PEF/POF.
Additionally, an S-Label certainly is not routable. If you consider the
redundancy node A and merging node D a pair of DetNet source/destination node,
adding PEF/POF semantics to an S-label may be reasonable (A imposes the S-label
and D knows that the S-label has merging semantics). However, for the label
that the ingress node imposes, which has both the “route to A” and “Replicate
by A” semantics, calling it an S-Label just does not conform to RFC 8964.
In SRv6 case, while you could say that the redundancy segment is a routable
one, but that is true for an replication segment as well. An SRv6 SID is a
<locator, function, argument> tuple. A replication SID is really just the
“function” part, and the locator is used to get to the node – whether it is a
root, leaf or intermediate replication node.
Jeffrey
From: Yangfan (IP Standard) <[email protected]>
Sent: Thursday, May 20, 2021 8:59 AM
To: Jeffrey (Zhaohui) Zhang <[email protected]>; 'Rishabh Parekh'
<[email protected]>
Cc: Gengxuesong (Geng Xuesong) <[email protected]>; 'Rishabh Parekh
(riparekh)' <[email protected]>; 'Arvind Venkateswaran (arvvenka)'
<[email protected]>; '[email protected]' <[email protected]>
Subject: 答复: [spring] RE: Comments on draft-geng-spring-sr-redundancy-protection
[External Email. Be cautious of content]
Hi Jeff,
Thank you for your explanations about the SID list encapsulation and SID
process on each node. It did help a lot to understand how Replication SID can
be used in redundancy protection.
I move one of your comments from the other email here to focus on the packet
forwarding discussion in this thread.
As for the 3 deferred “homework”, feel free to come back when you are available.
Much appreciated with these deep discussions. Some feedbacks please see inline.
Regards,
Fan
发件人: Jeffrey (Zhaohui) Zhang [mailto:[email protected]]
发送时间: 2021年5月20日 1:43
收件人: Yangfan (IP Standard)
<[email protected]<mailto:[email protected]>>; 'Rishabh
Parekh' <[email protected]<mailto:[email protected]>>
抄送: Gengxuesong (Geng Xuesong)
<[email protected]<mailto:[email protected]>>; 'Rishabh Parekh
(riparekh)' <[email protected]<mailto:[email protected]>>; 'Arvind
Venkateswaran (arvvenka)' <[email protected]<mailto:[email protected]>>;
'[email protected]' <[email protected]<mailto:[email protected]>>
主题: RE: [spring] RE: Comments on draft-geng-spring-sr-redundancy-protection
Hi Fan,
Sorry for the late response. Please see zzh> below.
From: Yangfan (IP Standard)
<[email protected]<mailto:[email protected]>>
Sent: Monday, May 17, 2021 9:20 AM
To: Jeffrey (Zhaohui) Zhang <[email protected]<mailto:[email protected]>>;
'Rishabh Parekh' <[email protected]<mailto:[email protected]>>
Cc: Gengxuesong (Geng Xuesong)
<[email protected]<mailto:[email protected]>>; 'Rishabh Parekh
(riparekh)' <[email protected]<mailto:[email protected]>>; 'Arvind
Venkateswaran (arvvenka)' <[email protected]<mailto:[email protected]>>;
'[email protected]' <[email protected]<mailto:[email protected]>>
Subject: RE: [spring] RE: Comments on draft-geng-spring-sr-redundancy-protection
[External Email. Be cautious of content]
Hi Jeffrey,
To summarize the discussions a bit, I have two questions and hope to hear your
comments and clarifications.
Q1: Are there any questions for clarifications or open issues with the current
solution of redundancy-SID+Merging-SID+redundancy policy? If so, we would like
to clarify them first.
Zzh> No.
Fan1>> It is very good and important if the current redundancy protection
mechanism can be understood clearly in WG.
Q2:
Figure 1 in draft-geng-spring-sr-redundnacy-protection is the typical topology
for Redundancy Protection in SR domain, as shown below.
ingress------A (R)-------B-------D(M)-------egress
+------C------------+
+----E-------+
Could you please explain how exactly Tree-SID solution is used in data plane
for redundancy protection? For example, how the SR SID List is assigned, how
the packet is encapsulated and forwarded from R1 to R2 in data plane, how
Tree-SID and Replication SID are configured on network nodes, etc.
Zzh> I sort of replied in my late response to the other email, but let me reply
here as well because that thread was indeed getting very deep.
Zzh> I changed the letters so that we can distinguish between node
representation and function representation. A is the redundancy node and D is
merging node. I also added another node E.
Zzh6> Unlike unicast where you use a segment list to encode the path that a
packet flows through, with multicast it is impractical to encode a (sub-)tree
as a segment list. Therefore, for SR-P2MP, an replication tree is not
represented by a segment list. Rather, each root/replication/leaf node on a
tree installs a replication segment to represent how replication is done on
that node for that tree, and only one SID is needed in the packet for it to
flow from the root to all the leaves through the intermediate replication
points. This is exactly like today’s P2MP tunnel in the forwarding plane. We
can put aside the argument “oh this requires per-flow state” here, as it is not
relevant to our topic here.
Fan1>> To differentiate the node representation and function representation
actually reveals a significant difference between Replication SID and
Redundancy SID. In terms of SRv6, Replication Segment is a local and
non-routed segment, and Redundancy Segment is a routed segment. And the merging
segment in redundancy protection is also a routed (topological) SID with
merging function.
As you mentioned, it is impractical to use a segment list to encode a tree for
multicast traffic. So Replication SID only represents the replication function,
moreover is designed to separate with the routed Node SID, however the benefit
is Replication SID can be identical on one tree.
Meanwhile, Redundancy Segment is usually used for unicast traffic, it is
straightforward to design it as a routed and functional segment. It is not
necessary to separate the route and function apart into different SIDs, at
least it saves one 128bit SID space.
Zzh> Let’s say that we’ll only make two copies for D to receive. A and D have
replication segment R installed. R on A simply says “make two copies, one
through B and one through C”. B/C do not need any state because A is simply
going to tunnel through B/C respectively. R on D simply says “I am a leaf so I
just pop R”.
Zzh> The ingress will send packets with SL <A, R, M>. SID A gets the packet to
A, who sees R and do the replication (R is not popped by A). D sees R in the SL
and pops R (this is the replication segment behavior on a leaf). It then sees M
and do the merging. Alternatively, A could pop R so D will see M directly and
do the merging.
Zzh> Now let’s say we want to make three copies. The ingress will still send
the same SL. R on A now says “tunnel one copy to D through B and send one copy
to C” and C now also has R installed, saying “send one copy to D directly and
another copy to D via E”. Now three copies will arrive on D (with A/C each
replicating once). D sees R SID and pops the R SID (as the behavior of
replication segment on leaf). It then sees M and do the merging.
Zzh> The replication is pure SR-P2MP behavior (assuming A does not need to add
FI/SN). M is pure non-topological merging behavior (relying on the FI/SN added
by the ingress node).
Fan1>> Thanks for your detailed explanations in this and other email, now I
think I understand how data plane works. There are still details needs further
discussion, e.g. in terms of SRv6, if B/C is downstream nodes, in this
sentence “D sees R SID and pops the R SID” pop means remove the IPv6 header, in
this case M SID will not be processed. But that’s all right, there are surely a
lot details if you want to design one unified Segment can work for P2MP
multicast and redundancy protection, also unified in SR-MPLS and SRv6. We can
surely continue the discussions.
Zzh> Thanks.
Zzh> Jeffrey
I don’t think I understand the entire forwarding flow correctly. In fact we are
very interested in Tree-SID solution as it has been adopted as a WG work. It
would be very helpful if you or anyone can provide a detailed illustration or
comparison, for instance like the way of the Appendix in replication SID/Tree
SID drafts.
Looking forward to your reply!
Best,
Fan
Juniper Business Use Only
Juniper Business Use Only
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