Gyan, thank you for your review and thank you all for the following discussion. I have entered a No Objection ballot for this document based on the current status of the discussion. Lars > On 2021-4-29, at 8:46, Gyan Mishra via Datatracker <noreply@xxxxxxxx> wrote: > > Reviewer: Gyan Mishra > Review result: Not Ready > > I am the assigned Gen-ART reviewer for this draft. The General Area > Review Team (Gen-ART) reviews all IETF documents being processed > by the IESG for the IETF Chair. Please treat these comments just > like any other last call comments. > > For more information, please see the FAQ at > > <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>. > > Document: draft-ietf-bess-datacenter-gateway-?? > Reviewer: Gyan Mishra > Review Date: 2021-04-28 > IETF LC End Date: 2021-04-29 > IESG Telechat date: Not scheduled for a telechat > > Summary: > This document defines a mechanism using the BGP Tunnel Encapsulation > attribute to allow each gateway router to advertise the routes to the > prefixes in the Segment Routing domains to which it provides access, > and also to advertise on behalf of each other gateway to the same > Segment Routing domain. > > This draft needs to provide some more clarity as far as the use case and where > this would as well as how it would be used and implemented. From reading the > specification it appears there are some technical gaps that exist. There are > some major issues with this draft. I don’t think this draft is ready yet. > > Major issues: > > Abstract comments: > It is mentioned that the use of Segment Routing within the Data Center. Is > that a requirement for this specification to work as this is mentioned > throughout the draft? Technically I would think the concept of the discovery > of the gateways is feasible without the requirement of SR within the Data > Center. > > The concept of load balancing is a bigger issue brought up in this draft as the > problem statement and what this draft is trying to solve which I will address > in the introduction comments. > > Introduction comments: > In the introduction the use case is expanded much further to any functional > edge AS verbiage below. > > OLD > > “SR may also be operated in other domains, such as access networks. > Those domains also need to be connected across backbone networks > through gateways. For illustrative purposes, consider the Ingress > and Egress SR Domains shown in Figure 1 as separate ASes. The > various ASes that provide connectivity between the Ingress and Egress > Domains could each be constructed differently and use different > technologies such as IP, MPLS with global table routing native BGP to > the edge, MPLS IP VPN, SR-MPLS IP VPN, or SRv6 IP VPN” > > This paragraph expands the use case to any ingress or egress stub domain Data > Center, Access or any. If that is the case should the draft name change to > maybe a “stub edge domain services discovery”. As this draft can be used for > any I would not preclude any use case and make the GW discovery open to be used > for any service GW edge function and change the draft name to something more > appropriate. > > This paragraph also states for illustrative purposes which is fine but then it > expands the overlay/underlay use cases. I believe this use case can only be > used for any technology that has an overlay/underlay which would preclude any > use case with just an underlay global table routing such as what is mentioned > “IP, MPLS with global table routing native BGP to the edge. The IP or global > table routing would be an issue as this specification requires setting a RT and > an export/import RT policy for the discover of routes advertised by the GWs. > As I don’t think this solution from what I can tell would work technically for > global table routing I will update the above paragraph to preclude global table > routing. We can add back in we can figure that out but I don’t think any > public or private operator would change from global table carrying all BGP > prefixes in the underlay now drastic change to VPN overlay pushing all the > any-any prefixes into the overlay as that would be a prerequisite to be able to > use this draft. > >> From this point forward I am going to assume we are using VPN overlay > technology such as SR or MPLS. > > NEW > > “SR may also be operated in other domains, such as access networks. > Those domains also need to be connected across backbone networks > through gateways. For illustrative purposes, consider the Ingress > and Egress SR Domains shown in Figure 1 as separate ASes. The > various ASs that provide connectivity between the Ingress and Egress > Domains could be two as shown in Figure-1 or could be many more as exists > with the public internet use case, and each may be constructed differently > and use different technologies such as MPLS IP VPN, SR-MPLS IP VPN, or SRv6 > IP VPN” with a “BGP Free” Core. > > This may work without “BGP Free” core but I think to simplify the design > complexity I think constraining to “BGP Free” core transport layer. SR-TE path > steering as well gets much more complicated if all P routers are running BGP as > well. I think in this example we can even explicitly say this example shows the > public internet as that would be one of the primary use cases. > > This paragraph is confusing to the reader > > As a precursor to this paragraph I think it maybe a good idea to state that we > are talking global table IP only routing or VPN overlay technology with SR/MPLS > underlay transport. That will make this section much easier to understand. > > Figure 1 drawing you should give a AS number to both the ingress domain and > egress domain so the reader does not have to make assumptions if it iBGP or > eBGP connected to the egress or ingress domain and state eBGP in the text > below. Lets also call the intermediate ASNs in the middle as depicted in the > diagram could be 2 as shown illustratively but could be many operator domains > such as in the case of traversing the public internet. In the drawing I would > replace ASBR for PE as per this solution as I am stating it has to be a VPN > overlay paradigm and not global routing. Also in the VPN overlay scenario when > you are doing any type of inter-as peering the inter-AS peering is almost > always between PE’s and not a separate dedicated device serving a special > “ASBR-ASBR” function as the PE is acting as the border node providing the > “ASBR” type function. So in the re-write I am assuming the drawing has been > updated changing ASBR to PE. Lets give each node a number so that we can be > clear in the text exactly what node we are referring to. In the drawing please > update that GW1 peers to PE1 and GW2 peers to PE2 and GW3 peers to PE3. GW3 > also peers to GW4 and GW2 peers to GW5 which GW4 and GW5 are part of AS3. In > the AS1-AS2 peering top peer would be PE6 peers to PE8 and bottom peer PE7 > peers to PE9. So PE6 and PE7 are in AS1 and PE8 and PE9 are in AS2. I made > the bottom to ASBRs in AS3 for the selective deterministic load balancing now > calling them GW4 and GW5 used later in the problem statement. > > One major problem with this problem statement description is that it is > incorrect as far as GW load balancing that it does not work today in the > topology given in Figure-1. The function of edge GW load balancing is based on > the iBGP path tie breaker lowest common denominator in the BGP path selection > which is lowest IGP underlay metric and as long as the metric is equal and you > have iBGP multipath enabled you now can load balance to egress PE1 and PE2 > endpoints. So in this case flows coming from AS1 into AS2 hit a P intermediate > router which has iBGP multipath enabled and has lets say equal cost for route > to the next hop attribute assuming next-hop-self is set so the cost to > loopback0 on PE1 and cost to loopback0 on PE2 is lets say 10, so now you have a > BGP multipath. What is required though is the RD has to be unique in a “BGP > Free” core RR environment where all PE’s route-reflector-clients peer to the RR > and for all the paths that are advertised to the RR to be reflected to all the > egress PE edges the RD must be unique for the RR to reflect all paths. BGP > add-paths is only used if you have Primary and Backup routing setup where > PE1-GW1 has a 0x prepend and PE2-GW2 has 1x prepend so now with BGP add-paths > along with BGP PIC Edge you now have a edge pre-programmed backup path. So the > add-paths is not necessarily something that helps for load balancing and is in > fact orthogonal to load balancing as it for Primary / Backup routing and not > Active/Active load balancing routing where load balancing with VPN overlay is > simply achieved with unique RD per PE and iBGP multipath and equal cost paths > to the underlay recursive IGP learned next-hop-attribute in this case the PE > loopback 0 per the next hop rewrite via “next-hop-sellf” done on the PE-RR > peering in a standard VPN overlay topology. As far as load balancing being > accomplished in the underlay what I have stated is independent of SR-TE however > with SR-TE candidate path the load balancing ECMP spray to egress PE egress GW > AS can also happen as well with prefix-sid. > > OLD > Suppose that there are two gateways, GW1 and GW2 as shown in > Figure 1, for a given egress SR domain and that they each advertise a > route to prefix X which is located within the egress SR domain with > each setting itself as next hop. One might think that the GWs for X > could be inferred from the routes' next hop fields, but typically it > is not the case that both routes get distributed across the backbone: > rather only the best route, as selected by BGP, is distributed. This > precludes load balancing flows across both GWs. > > I am rewriting the text in the NEW as there is some discrepancy in the routes > being distributed across the backbone and what gets distributed. So I am > completely re-writing to make it more clear what we are trying to state here as > the text appears technically to be incorrect. To help state the flow will use > the BGP route flow to help depict the routing and try to get to the problem > statement we are trying to portray. > > NEW > > Suppose that there are two gateways, GW1 and GW2 as shown in > Figure 1, for a given egress SR domain and each gateway advertises via EBGP > a VPN prefix X to AS2 core domain via EBGP with underlay next hop set to GW1 > or GW2. In this case we are Active / Active load balancing with PE1 and PE2 > receives the VPN prefix and advertised the VPN prefix X into the domain with > next-hop-self set on the PE-RR peering to the PE’s loopback0. The P routers > within the domain have ECMP path with IGP metric tie to the egress PE1 and > egress PE2 for VPN Prefix X learned from GW1 and GW2. SR-TE path can now be > stitched from GW3 to PE3 SR-TE Segment-1 to PE3 to PE6 and PE7 Segment-2 to > PE8 and PE9 to Egress Domain via PE1 and PE2 to GW1 and GW2. In this case > however we don’t want the traffic to be steered via SR-TE Load balanced via > ingress GW3 and want to take GW3 out of rotation and load balance traffic to > GW4 and GW5 instead. > > **Text above provides the updated selective deterministic gateway steering > described below to achieve the goal. I think that may have been the intent of > the authors and I am just making it more clear** > > As for problem statement as GW load balancing can occur in the underlay as > stated easily that is not the problem. > > In my mind I am thinking the problem statement that we want to describe in both > the Abstract and Introduction is not vanilla simple gateway load balancing but > rather a predictable deterministic method of selecting gateways to be used that > is each VPN prefix now has a descriptor attached - tunnel encapsulation > attribute which contains multiple TLVs one or more for each “selected gateway” > with each tunnel TLV contains an egress tunnel endpoint sub-tlv that identifies > the gateway for the tunnel. Maybe we can have in the sub-tlv a priority field > for pecking order preference of which GWs are pushed up into the GW hash > selected for the SR-ERO path to be stitched end to end. So lets say you had > 10 GWs and you break them up into 2 tiers or multi tiers and have maybe gateway > 1-5 are primary and 6-10 are backup and that could be do to various reasons so > you can basically pick and choose based on priority which GW that gets added to > the GW hash. > > I have some feedback and comments on the solution and how best to write the > verbiage to make it more clear to the reader. > > I think in the solution as far s the RT to attach for the GW auto discovery. > So with this new RT we are essentially creating a new VPN RIB that has prefixes > from all the selected gateways that are discovered from the tunnel > encapsulation attribute TLV. > > In the text here what is really confusing is if the tunnel encapsulation > attribute is being attached to the underlay recursive route to next hop > attribute or the VPN overlay prefix. So the reason I am thinking it is being > attached to the VPN overlay prefix and not the underlay next hop attribute is > how would you now create another transport RIB and if you are creating a new > transport RIB there is already a draft defined by Kaliraj Vairavakkalai or > BGP-LU SAFI 4 labeled unicast that exits today to advertise next hops between > domains for an end to end LSP load balanced path. > > https://tools.ietf.org/html/draft-kaliraj-idr-bgp-classful-transport-planes-07 > > IANA code point below > 76 Classful-Transport SAFI > [draft-kaliraj-idr-bgp-classful-transport-planes-00] > > Also in line with CT another option is BGP-LU SAFI 4 to import the loopbacks > between domains which is the next hop attribute to be advertised into the core > end to end LSP. So the BGP-LU SAFI RIB could be used for the next GW next hop > advertisement between domains so that there is visibility of all the egress PE > loopback0 between domains. So you can either stitch the LSP segmented LSP > like inter-as option-b SR-TE stitched and use nex-hop self PE-RR next-hop > rewrite on each of the PEs within the internet domain or you could import all > the PE loopback from all ingress and egress domains into the internet domain > similar to inter-as opt-c create end to end LSP instantiate an end to end SR-TE > path. > > Maybe you could attach the RT tunnel encapsulation attribute tunnel tlv > endpoint tlv to the VPN overlay prefix. Not sure how that would be beneficial > the underlay steers the VPN overlay. > > So maybe you could couple the VPN overlay new GW RIB RT to the transport > Underlay CT CLAS RIB or BGP-LU RIB coupling may have some benefit but that > would have to be investigated but I think is out of scope of the goals of this > draft. > > I think we first have to figure out the goal and purpose of this draft by the > authors and how the GW discovery should work in light of the CT class CT RIB > AFI/SAFI codepoint draft that exists today as well as the BGP-LU option for > next hop advertisement within the internet domain. > > Section 3 comments > > “Each GW is configured with an identifier for the SR domain. That > identifier is common across all GWs to the domain (i.e., the same > identifier is used by all GWs to the same SR domain), and unique > across all SR domains that are connected (i.e., across all GWs to > all SR domains that are interconnected). > > **No issues with the above** > > A route target ([RFC4360]) is attached to each GW's auto-discovery > route and has its value set to the SR domain identifier. > > **So here if the RT is attached to the GW auto-discovery route we need to state > is that the underlay route and that the PE does a next-hop-self rewrite of the > eBGP link to the BGP egress domain next hop to the loopback0 so the GW next hop > that we are tracking of all the ingress and egress PE domains is the egress and > ingress PE loopback0.** > > Each GW constructs an import filtering rule to import any route > that carries a route target with the same SR domain identifier > that the GW itself uses. This means that only these GWs will > import those routes, and that all GWs to the same SR domain will > import each other's routes and will learn (auto-discover) the > current set of active GWs for the SR domain.” > > **So if this is the case and we are tracking the underlay RIB and attach a > route target to all the ingress PE & P next hops which is loopback0 = this is > literally identical to BGP-LU importing all the loopbacks between domains or > using CT class** There is no need for this feature to use the tunnel > encapsulation attribute. I am not following why you would not use BGP-LU or CT > clas RIB.** > > “To avoid the side effect of applying the Tunnel Encapsulation > attribute to any packet that is addressed to the GW itself, the GW > SHOULD use a different loopback address for packets intended for it.” > > **I don’t understand this statement as the next hop is the ingress and egress > PE loopback0 that is the next hop being tracked for the gateway load balancing. > The GW device subnet between the GW and PE is not advertised into the internet > domain as we do next-hop-self on the PE PE-RR iBGP peering and so the GW to PE > subnet is not advertised.** Looking at it a second time I think we are > thinking here BGP-LU inter-as opt c style import of loops between domains and > so instead of importing the loop0 which carries all packets on the GW device > use a different loopback GW1 so it does not carry the FEC of all BAU packets > similar concept utilized in RSVP-TE to VPN mapping "per-vrf TE" concept. > > “As described in Section 1, each GW will include a Tunnel > Encapsulation attribute with the GW encapsulation information for > each of the SR domain's active GWs (including itself) in every route > advertised externally to that SR domain. As the current set of > active GWs changes (due to the addition of a new GW or the failure/ > removal of an existing GW) each externally advertised route will be > re-advertised with a new Tunnel Encapsulation attribute which > reflects current set of active GWs.” > > **What is the route being advertised externally from the GW. So the routes > advertised would be all the PE loopback would be advertised from both ingress > and egress domains into the internet domain and all loopback from the internet > domain into the ingress and egress domain which could be done via BGP-LU or CT > RIB – no need do reinvent the wheel and create a new RIB. So BGP-LU or CT RIB > track the current set of active next hop GWs loopbacks between domains**If you > do SR-TE stitching then you can do the next-hop self on each PE PE-RR for the > load balancing and that would work and the load balancing would be to the PE > loopbacks or if its an end to end SR-TE path using BGP-LU or CT RIB via > importing all the PE loopbacks between domains the current set of active GWs > would be tracked via the BGP-LU or CT RIB. So if the active GWs change due to > GW failures they would be withdrawn from the BGP-LU or CT underlay RIB. No > need now for the tunnel encapsulation attribute at least for the GW auto > discovery load balancing** > > I think it still maybe possible to retrofit this draft to utilize the CT RIB or > BGP-LU for the GW load balancing so nothing new has to be designed as far as > the underlay goes, however maybe the idea of providing some visibility into the > VPN overlay route to the underlay – maybe their maybe some benefit of using the > tunnel encapsulation attribute RT import policy to attach to the VPN overlay > prefixes. > > As CT draft provides a complete solution of providing the VPN overlay per VPN > or per prefix underpinning of the VPN overlay to underlay CT RIB the problem > statement is completely solved with either the CT draft or BGP-LU. > > Minor issues: > None > > Nits/editorial comments: > > Please add normative and informative references below. > > I would reference as normative and maybe even informative the CT Class draft > which creates a new transport class and I think this draft can really work well > in conjunction with use of the CT class to couple the GW RIB created to the CT > class transport RIB and provide the end to end inter-AS stitching via the PCE > CC controller. I am one of the co-authors of this draft and I think this draft > could be coupled with this GW draft to provide the overall goals of selective > GW load balancing. > > https://tools.ietf.org/html/draft-kaliraj-idr-bgp-classful-transport-planes-07 > > I would also reference this draft for CT class PCEP coloring extension. > > https://tools.ietf.org/html/draft-rajagopalan-pcep-rsvp-color-00 > > As this solution would utilize a centralized controller PCE CC for inter as > path instantiation for the GW load balancing, I think it would be a good idea > to reference the PCE CC, H-PCE and Inter-AS PCE and PCE SR extension as > informative and maybe even normative reference. > > > > -- > last-call mailing list > last-call@xxxxxxxx > https://www.ietf.org/mailman/listinfo/last-call
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