[spring] 答复: Comments on draft-geng-spring-sr-redundancy-protection

["Yangfan (IP Standard)" <[e-mail]>]

Hi Jeffrey,
Apology for being so late to reply. Please see inline starts with Fan>>.

Cheers,
Fan

发件人: spring [mailto:[e-mail]] 代表 Jeffrey (Zhaohui) Zhang
发送时间: 2021年3月30日 5:06
收件人: Yangfan (IP Standard) <[e-mail]>; Gengxuesong (Geng Xuesong) <[e-mail]>; 'Rishabh Parekh (riparekh)' <[e-mail]>; 'Arvind Venkateswaran (arvvenka)' <[e-mail]>; [e-mail]
主题: Re: [spring] Comments on draft-geng-spring-sr-redundancy-protection

Hi Fan,

Please see zzh> below.

From: Yangfan (IP Standard) <[e-mail]<mailto:[e-mail]>>
Sent: Friday, March 26, 2021 11:58 PM
To: Jeffrey (Zhaohui) Zhang <zzhang=[e-mail]<mailto:zzhang=[e-mail]>>; Gengxuesong (Geng Xuesong) <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>; Rishabh Parekh (riparekh) <[e-mail]<mailto:[e-mail]>>; Arvind Venkateswaran (arvvenka) <[e-mail]<mailto:[e-mail]>>
Subject: 答复: Comments on draft-geng-spring-sr-redundancy-protection


Hi Jeffrey,



Thank you for the comments, please see the reply inline starts with [FY#].



Cheers,

Fan





-----邮件原件-----
发件人: spring [mailto:[e-mail]] 代表 Jeffrey (Zhaohui) Zhang
发送时间: 2021年3月26日 3:19
收件人: Gengxuesong (Geng Xuesong) <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>; Rishabh Parekh (riparekh) <[e-mail]<mailto:[e-mail]>>; Arvind Venkateswaran (arvvenka) <[e-mail]<mailto:[e-mail]>>
主题: [spring] Comments on draft-geng-spring-sr-redundancy-protection



Hi Xuesong, Mach, Fan,



Some comments/questions on the proposal.



1. We don't need an additional "redundancy segment" for the replication semantics. Existing "replication segment" (draft-ietf-spring-sr-replication-segment) can be used as is, especially for the scenario where the original header already carries (FI, SN) information.

------[FY1]: three considerations here:

a). For the scenario you mentioned, that is correct, redundancy segment and replication segment share a common behavior of "packet duplication". The significant difference between two segments is the behavior of adding FI and SN. Unfortunately, there is no application in SRv6 required to carry (FI,SN) information in IPv6 header, which results in a more common scenario is where the original packet doesn't carry (FI, SN). So the current design of redundancy segment is based on this scenario.



Zzh> Since the presentation talked about scenario where the (FI, SN) information is already carried, it is fair to discuss that in my initial comments; I understand that you want to focus on the other scenario, and that’s fine – see later comments below.



Fan>> I read the draft of replication segment, and have two questions if replication segment is used in redundancy protection.

1) I believe merging node should be as the downstream node, since the nodes in precedence of merging node should not be redundancy protection aware. In this case, there will be at least two identical downstream nodes. In replication segment, there is no definition of such a situation.

2) The draft states replication SID MUST only appear as the ultimate SID in a SID list. What if the merging node is not the last node of the SRv6 E2E path?



b). Even though IPv6 flow label could be encapsulated in header, it is used for ECMP or fragmentation, redundancy protection cannot simply reuse it since flow ID allocation has dependency on the merging node capability.



Zzh> IPv6 flow label is irrelevant here – it’s not discussed in either your draft/presentation or in my comments – so we can ignore this.

Fan>> I mentioned IPv6 flow label coz we had this discussion in DetNet WG. I agree we can come back to this thread when it is needed.

https://mailarchive.ietf.org/arch/browse/detnet/?q=flow%20identification%20in%20ipv6



c). In protocol design, it is important to maximally reuse the existing implementation. However, instantiation of a segment is a different story. In RFC8986, there are 14 End behaviors and 4 headend behaviors defined. We understand the principle here is to keep the semantics of a segment and further functions definition neat to make the segment routing forwarding clear and efficient. To enhance the replication segment to support redundancy segment seems quite an opposite methodology.



Zzh> RFC 8986 does specify additional flavors of End and End.X function with USP, PSP and USD behaviors which are modifications to base End and End.X function; exactly what we are proposing here – enhancing Replication Segment to add (FI,SN) when required.

Fan>> can you explain more? I don’t see correlation between flavors and adding (FI,SN).



2. Even for the scenario where the (FI, SN) information needs to be added by the redundancy node, the existing "replication segment" can be enhanced to add the (FI, SN) information.

-----[FY2]: Replication segment provides P2MP replication with target of supporting multicast service, and redundancy segment aims to provide redundant flow protection to URLLC services. Adding (FI, SN) doesn’t bring value to multicast services, and having the stitching capability of replication on redundancy node seems a waste and unpractical to URLLC service. Twisting them together in one segment results in a complicated function, where maybe only one type of service is required on the node.



Zzh> Adding (FI, SN) information is only to replication segments that are used for replication for unicast redundancy purpose. It does not mean all replication segments will be added with (FI, SN) semantics.



Fan>> How would you write the Boolean switch to differentiate the purpose of multicast replication and redundancy protection in one segment? And currently we don’t exclude the redundancy protection for multicast traffic.



Zzh> I don’t follow your argument about “seems a waste and unpractical to URLLC service”.



Zzh> I don’t follow your argument about “Twisting them together in one segment results in a complicated function where maybe only one type of service is required on the node” either. If you only need regular multicast service, the replication segment does not need the semantics to add (FI, SN) information. If you need redundancy protection then the replication segment does have the semantics to add (FI, SN) information). If you need both, then some will have that semantics and some will not; and if you have a scenario where you don’t need to add (FI, SN) information for redundancy protection then the existing replication segment w/o the additional semantics to add (FI, SN) information can be used for both. All can be achieved with a simple Boolean switch added to the replication segment.

Fan>> after seeing all these “if, then” shown above, I even feel more strongly to support separating two segments. ☺ In RFC8986, there is no single Endpoint behavior having such “if, then” structure to specify different functions.



Zzh> Note that Replication Segment is not tied to multicast either (the draft only mentioned multicast once as one use case):



   We define a new type of segment for Segment Routing [RFC8402<https://tools.ietf.org/html/rfc8402>] called

   Replication segment, which allows a node (henceforth called as

   Replication Node) to replicate packets to a set of other nodes

   (called Downstream Nodes) in a Segment Routing Domain. Replication

   segments provide building blocks for Point-to-Multipoint Service

   delivery …



Zzh> It is about replicating packets to a set of other nodes – quite applicable here as a building block.

Fan>> I do think replication segment has a very elegant design, however identical downstream nodes, design of P2MP SR policy (indirectly involves Tree-ID) may seem burden too much on redundancy segment. But it is still very welcome to have further discussion on replication segment and redundancy segment.



3. I wonder why (FI, SN) information is added as a TLV in the SRH. Would it be better to use DOH?

-----[FY3]: If the (FI,SN) is encapsulated in type of TLV, both SRH and DOH are feasible. Actually (FI,SN) information is only meaningful to merging node, putting them in the arg part of replication segment doesn't help.



Zzh> While I do think it is better to put the actual (FI, SN) information in the DOH, I did not talk about adding (FI, SN) information to the arg part of an SRv6 SID. I was saying that the argument of an SRv6 replication SID can serve as that Boolean switch to indicate if (FI, SN) information needs to be added.

Fan>> so far, this approach works for me.



For #1, and #2, reusing/enhancing existing replication segment has the following benefits:



a. Reduce protocol/implementation work

b. Reduce the amount of state in the network (the same P2MP tunnel can be used for both multicast traffic and unicast redundancy)



b) can be achieved even with #2 (redundancy node needs to add (FI, SN) information): for SRv6, the semantics of adding (FI, SN) can be indicated by the arg part of the replication SID and for SR-MPLS it can be indicated by an additional label in front of the replication sid label. If using an addition label is a concern, then indeed a single label can be used to indicate both "add FI/SN information" and "replicate", but still the replication semantics can still be set up using the replication segment infrastructure.



For SR-MPLS, where would you put the (FI, SN) information? Seems that GDFH (draft-zzhang-intarea-generic-delivery-functions) is a good option and that can be used for SRv6 as well (anything in DOH that is actually independent of IP could be extracted out to GDFH).

-----[FY4]: For SR-MPLS, currently the authors plan to keep consistent with specification in RFC8964. The original intention of this draft is to provide a PREOF solution in SR data plane to DetNet. What's why the draft is discussed first in DetNet then comes to SPRING. And FYI, DetNet MPLS data plane uses a separate service label (S-Label) and PW MPLS Control Word [RFC4385] to carry FI and SN.



Zzh> I forgot that DETnet mpls data plane already reuses PW CW for SN information. That’s fine and no need to introduce GDFH for MPLS.

Zzh> Thanks.

Fan>> thanks for bring up this topic to a deeper discussion. Redundancy protection should be taken into consideration for both SP and vendor if URLLC services should be guaranteed.



Zzh> Jeffrey



Thanks.



Jeffrey



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