Re: [Detnet] [spring] 答复: RE: Comments on draft-geng-spring-sr-redundancy-protection
"Joel M. Halpern" <jmh@joelhalpern.com> Fri, 04 June 2021 12:57 UTC
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To: "Yangfan (IP Standard)" <shirley.yangfan@huawei.com>, "Jeffrey (Zhaohui) Zhang" <zzhang@juniper.net>, 'Rishabh Parekh' <rishabhp@gmail.com>
Cc: "'spring@ietf.org'" <spring@ietf.org>, DetNet WG <detnet@ietf.org>
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From: "Joel M. Halpern" <jmh@joelhalpern.com>
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Date: Fri, 04 Jun 2021 08:56:44 -0400
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Subject: Re: [Detnet] [spring] 答复: RE: Comments on draft-geng-spring-sr-redundancy-protection
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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:zzhang@juniper.net] > *发送时间:*2021年5月27日1:42 > *收件人:*Yangfan (IP Standard) <shirley.yangfan@huawei.com>; 'Rishabh > Parekh' <rishabhp@gmail.com> > *抄送:*Gengxuesong (Geng Xuesong) <gengxuesong@huawei.com>; 'Rishabh > Parekh (riparekh)' <riparekh@cisco.com>; 'Arvind Venkateswaran > (arvvenka)' <arvvenka@cisco.com>; 'spring@ietf.org' <spring@ietf.org> > *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) <shirley.yangfan@huawei.com > <mailto:shirley.yangfan@huawei.com>> > *Sent:* Thursday, May 20, 2021 8:59 AM > *To:* Jeffrey (Zhaohui) Zhang <zzhang@juniper.net > <mailto:zzhang@juniper.net>>; 'Rishabh Parekh' <rishabhp@gmail.com > <mailto:rishabhp@gmail.com>> > *Cc:* Gengxuesong (Geng Xuesong) <gengxuesong@huawei.com > <mailto:gengxuesong@huawei.com>>; 'Rishabh Parekh (riparekh)' > <riparekh@cisco.com <mailto:riparekh@cisco.com>>; 'Arvind Venkateswaran > (arvvenka)' <arvvenka@cisco.com <mailto:arvvenka@cisco.com>>; > 'spring@ietf.org' <spring@ietf.org <mailto:spring@ietf.org>> > *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:zzhang@juniper.net > <mailto:zzhang@juniper.net>] > *发送时间:*2021年5月20日1:43 > *收件人:*Yangfan (IP Standard) <shirley.yangfan@huawei.com > <mailto:shirley.yangfan@huawei.com>>; 'Rishabh Parekh' > <rishabhp@gmail.com <mailto:rishabhp@gmail.com>> > *抄送:*Gengxuesong (Geng Xuesong) <gengxuesong@huawei.com > <mailto:gengxuesong@huawei.com>>; 'Rishabh Parekh (riparekh)' > <riparekh@cisco.com <mailto:riparekh@cisco.com>>; 'Arvind Venkateswaran > (arvvenka)' <arvvenka@cisco.com <mailto:arvvenka@cisco.com>>; > 'spring@ietf.org' <spring@ietf.org <mailto:spring@ietf.org>> > *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) <shirley.yangfan@huawei.com > <mailto:shirley.yangfan@huawei.com>> > *Sent:* Monday, May 17, 2021 9:20 AM > *To:* Jeffrey (Zhaohui) Zhang <zzhang@juniper.net > <mailto:zzhang@juniper.net>>; 'Rishabh Parekh' <rishabhp@gmail.com > <mailto:rishabhp@gmail.com>> > *Cc:* Gengxuesong (Geng Xuesong) <gengxuesong@huawei.com > <mailto:gengxuesong@huawei.com>>; 'Rishabh Parekh (riparekh)' > <riparekh@cisco.com <mailto:riparekh@cisco.com>>; 'Arvind Venkateswaran > (arvvenka)' <arvvenka@cisco.com <mailto:arvvenka@cisco.com>>; > 'spring@ietf.org' <spring@ietf.org <mailto:spring@ietf.org>> > *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 > > > _______________________________________________ > spring mailing list > spring@ietf.org > https://www.ietf.org/mailman/listinfo/spring >
- Re: [Detnet] [spring] 答复: RE: Comments on draft-g… Joel M. Halpern
- [Detnet] 答复: [spring] 答复: RE: Comments on draft-g… Yangfan (IP Standard)
- Re: [Detnet] 答复: [spring] 答复: RE: Comments on dra… Gyan Mishra