Fan, I am a bit confused by your statement below.
the Detnet architecture, and the sublayer split therein, is not specific
to SR-MPLS. It applies as much to SRv6. It would be very unusual, and
need strong justification, for the SPRING WG to decide to change the
published Detnet architecture.
Yours,
Joel (speaking as a participant)
On 6/4/2021 5:44 AM, Yangfan (IP Standard) wrote:
Hi Jeffrey,
You are right about how redundancy protection is used in SR-MPLS. In
DetNet definition, S-label is purely functional not routable as it is in
service sub-layer. In the draft, we separate the redundancy segment
specification in two subsections, SR-MPLS and SRv6. For SR-MPLS,
redundancy segment obeys RFC8964. We are aligned on this point.
Regarding SRv6, DetNet doesn’t have SRv6 data plane specification, so
redundancy protection has no guide to follow. In the latest version of
draft, redundancy protection is defined to extend the capabilities in SR
paradigm to support the redundancy protection in a DetNet environment.
It should be allowed to define redundancy segment as a routable segment
in SRv6.
I will set aside time to reply to your other emails today. Thanks a lot! J
Fan
*发件人:*Jeffrey (Zhaohui) Zhang [mailto:[email protected]]
*发送时间:*2021年5月27日1:42
*收件人:*Yangfan (IP Standard) <[email protected]>; 'Rishabh
Parekh' <[email protected]>
*抄送:*Gengxuesong (Geng Xuesong) <[email protected]>; 'Rishabh
Parekh (riparekh)' <[email protected]>; 'Arvind Venkateswaran
(arvvenka)' <[email protected]>; '[email protected]' <[email protected]>
*主题:*RE: [spring] RE: Comments on
draft-geng-spring-sr-redundancy-protection
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]
<mailto:[email protected]>>
*Sent:* Thursday, May 20, 2021 8:59 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:* 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]
<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 questionsfor clarifications or open issueswith 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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