Hi, I also agree that treating L2 broadcast domain and L3 subnet as identical is not accurate, but it is only the minor issue for the whole text. Through these years development, NVO3 is no longer a concept and is becoming pervasive in data center networks. However, a clear use case draft is still needed to summarize the typical applications for NVO3, I think this draft is clear and ready for publishing. Thanks, weiguo
-----邮件原件----- 发件人: Joel M. Halpern [mailto:[email protected]] 发送时间: 2016年8月16日 8:57 收件人: Joe Touch; Reith, Lothar; David Allan I 抄送: Bocci, Matthew (Nokia - GB); [email protected] 主题: Re: [nvo3] FW: Call for interest on NVO3 use case draft Joe, you are citing two different definitions. And while they are usually congruent, the IP Prefix (the definition as seen by L3) and the domain of L2 forwarding (the definition as seen by L2) are not always the same. There have been more than a few designs and deployments over the years where the L3-visible subnet definition is actually provided by L3 /32 IPv4 forwarding (over a constrained scope). There have also been hybrids. I believe Lothar is quite correct in noting that there are multiple closely related but not identical definitions, and that we often get ourselves in trouble by treating them as identical. Yours, Joel On 8/15/16 8:19 PM, Joe Touch wrote: > > > On 8/15/2016 4:42 PM, Reith, Lothar wrote: >> >> Hi Joe, >> >> >> >> I must admit I am puzzled a bit because you provided the reference to >> RFC3819 which states: “this document defines a subnetwork >> > i.e., subnet > >> as a layer 2 network, >> > > That's exactly what I state below... > >> which is a >> >> network that does not rely upon the services of IP routers to forward >> packets between parts of the subnetwork.” >> > > A L2 network - by definition - doesn't require IP forwarding inside. > > I'm not sure what the issue is. This is consistent with what I say below. > > Joe > >> >> >> Lothar >> >> >> >> *Von:*Joe Touch [mailto:[email protected]] >> *Gesendet:* Dienstag, 16. August 2016 01:07 >> *An:* Reith, Lothar <[email protected]>; David Allan I >> <[email protected]> >> *Cc:* Bocci, Matthew (Nokia - GB) <[email protected]>; >> [email protected] >> *Betreff:* Re: AW: AW: [nvo3] FW: Call for interest on NVO3 use case >> draft >> >> >> >> >> >> >> >> On 8/15/2016 3:54 PM, Reith, Lothar wrote: >> >> Hi Joe, >> >> >> >> thanks for the references to these RFCs, in particular to RFC 3819 >> which I was not aware of. >> >> >> >> I think the references make very clear, that the term “subnet” is >> overloaded and therefore context dependent in meaning. >> >> >> It's almost always more useful to be clear about the way in which the >> term is used, but IMO "subnet" is an L3 term that refers to >> addressing. Everything else derives from that. >> >> >> I do not want to bother you with a detailed analysis, but my >> observation is as follows: >> >> >> >> The meaning of the term subnet is context dependent >> >> - Layer-2 subnet in the sense of an object that forwards packets >> >> - Layer-3 subnet in the sense of an object that forwards packets >> >> - network prefix in the sense of an object that is a portion of an >> address plan, more precisely a subtree of a binary tree. >> >> >> L2 doesn't have subnets; a specific L2 *is* a subnet to L3. >> >> L3 subnets are defined by having a common bit-aligned address prefix. >> >> >> Adding to this ambiguity is the fact that subnet number (IPv4) >> and subnet ID (IPv6) relate to the Layer-3 subnet context of >> “subnet” only >> >> That's because subnet is an L3 term. >> >> >> (not to the layer 2 subnet context) >> >> There is no L2 subnet. An L3 subnet is often - but not always - >> mapped to a single L2 *network*. >> >> >> but do not identify a layer-3 subnet as a global identifier, >> rather play the role of a locator in the binary subtree with local >> significance only. This appears to be similar to the locator ID >> separation problem addressed by LISP. >> >> That's exactly because LISP turns the encapsulation layer into what >> is effectively an L2 network. The problem of mapping the inner and >> outer addresses in LISP is the same as ARP, except that it requires >> determining LISP egress rather than destination. >> >> That's why we called the protocol to solve this "BARP" - it combines >> BGP and ARP - when we developed it for what we called "recursive >> routers" in the late 90s in our X-Bone system. >> >> >> Given this and Dave’s additional comments regarding: “please >> provide a use case please” may I kindly ask you to provide a use >> case with “context tag” or perhaps using the term network prefix >> instead of subnet where appropriate. >> >> >> >> In particular I would be interested in a use case describing your >> stated requirement for “revisitation, where a single node >> participates multiple times in an overlay”. Could you please >> clarify the cardinality relations in this use case, e.g. do you >> mean >> >> - One physical interface to appear like multiple physical >> interfaces (such as a single physical NIC to appear as multiple >> VNICs)? >> >> - One physical interface (typically Ethernet Interface or >> station) to be associated with one MAC addresses at Layer 2, >> where multiple IP addresses are associated with that MAC address, >> where said IP addresses belong to multiple Layer3 subnets., thus >> multiple Layer3 subnets associated with the same Layer2 subnet? >> >> - One physical interface (typically Ethernet Interface or >> station) to be associated with multiple MAC addresses at Layer 2, >> where each of that MAC addresses is assigned one IP address (one >> to one relation between Layer3 subnet and Layer2 subnet, but >> multiple layer2 subnets on the same physical interface (Ethernet >> station i.e. PNIC or VNIC)? >> >> First, if you're thinking about physical anything, you're limiting >> yourself to one layer of virtualization. That's unnecessary. >> >> So let's assume that when you say "physical" you really mean "in the >> lowest layer of virtualization" (i.e., the base case of what is >> ultimately a recursive structure - e.g., see www.isi.edu/rna >> <http://www.isi.edu/rna>, which is a generalization of the X-Bone >> architecture). >> >> When I say "revisitation", I mean one base L3 interface that acts >> like multiple NVO3 L3 interfaces. >> >> That is accomplished by associating it with multiple virtual L2 (L2 >> over L3), each virtual-L2 of which is associated with one virtual L3. >> >> None if this is what you're describing above; you're stuck at the >> base layer L2, which is below where NVO3 operates. >> >> Joe >> >> >> >> >> >> Best Regards, Lothar >> >> PS: Given that we live in times of emerging software defined >> networks, it is increasingly important WHAT an orchestration >> software defines when it defines a “network” or a “subnet”, and >> rapidly becoming irrelevant what context tag was not explicitly >> spelled out in an IETF RFC. So I recommend to analyse WHAT it is, >> that the OpenStack method “create network” creates. It may be an >> empty binding between a L2-subnet and a L3-subnet with all details >> and sizes yet undefined, but with a handle in form of a globally >> unique identifier in the form of a UUID, and the problem solved is >> the lack of a global identifier that is not a locator or address. >> >> >> >> >> >> *Von:*Joe Touch [mailto:[email protected]] >> *Gesendet:* Sonntag, 14. August 2016 01:45 >> *An:* Reith, Lothar <[email protected]> >> <mailto:[email protected]>; David Allan I >> <[email protected]> <mailto:[email protected]> >> *Cc:* Bocci, Matthew (Nokia - GB) <[email protected]> >> <mailto:[email protected]>; [email protected] <mailto:[email protected]> >> *Betreff:* Re: AW: [nvo3] FW: Call for interest on NVO3 use case >> draft >> >> >> >> >> >> >> >> On 8/13/2016 4:20 PM, Reith, Lothar wrote: >> >> Hi Joe, Dave, dear all, >> >> >> >> I think that the term “subnet” is indeed only defined in a context >> sensitive way. >> >> >> >> Unfortunately there is no distinction made between a L2-subnet >> (aka a broadcast domain, or a MEF EVC, or o broadcast domain >> equivalent achieved without broadcast such as EVPN) and a >> L3-subnet (the object that does not appear in any information >> model that I am aware of, but which has a size which is defined >> by a subnet-mask and which supports the use case of “change the >> IP-address range assigned to that subnet”. >> >> The reason that L2 broadcast domain and L3 subnet (as per RFC1812) >> coincide is that is the basis of most L3:L2 mapping mechanisms, >> e.g., ARP (RFC826) or ARP emulation (RFC1577). >> >> And the concept of an L3 subnet is pervasive in many RFCs, being >> the basis of hierarchical IP forwarding since its inception. >> There's even a specific RFC advising on the design of L2 subnets >> to support L3 subnets (3819). >> >> >> >> Part of the reason for complexity is the lack of a proper >> definition of this object in the information model. >> >> >> >> If I am wrong �C I would be happy to be corrected by someone who >> can point me to an authoritative definition of the term subnet as >> an object that supports the use case “change address range >> assigned to subnet” without specifying how the subnet is built as >> layer 2 construct (e.g. as Ethernet yellow cable, VLAN in a >> Switched Ethernet domain, VXLAN overlay, EVPN, MEF EVC or ATM >> point to point link). >> >> >> Subnet in L3 in IPv4 is defined as per RFC 4632 >> >> Subnet in L3 in IPv6 is defined as per RFC 4291 >> >> These are very old and fundamental concepts to the Internet >> architecture. Any notion of virtualizing L3 needs to deal with them. >> >> Joe >> >> >> >> >> >> Thanks to OpenStack for finally fixing the problem by introducing >> the method “create network”, which creates �C exactly �C this >> missing object in an abstract way. >> >> >> >> So Dave’s question is very valid, simply because the term >> “subnetting” is not properly defined �C unless the authors point to >> a reference RFC where the term is defined in an authoritative way. >> >> >> >> Lothar >> >> >> >> >> >> >> >> *Von:*nvo3 [mailto:[email protected]] *Im Auftrag von *Joe Touch >> *Gesendet:* Samstag, 13. August 2016 19:30 >> *An:* David Allan I <[email protected]> >> <mailto:[email protected]> >> *Cc:* Bocci, Matthew (Nokia - GB) <[email protected]> >> <mailto:[email protected]>; [email protected] <mailto:[email protected]> >> *Betreff:* Re: [nvo3] FW: Call for interest on NVO3 use case >> draft >> >> >> >> >> >> On 8/13/2016 9:52 AM, David Allan I wrote: >> >> Hi Joe >> >> >> >> And the use case for wanting to do subnet emulation is….? >> >> >> You want the properties of a subnet and/or to emulate the behavior >> of a shared link, i.e., to limit the scope of various protocols, >> including IP routing, IPv6 automatic addressing, L2 address >> translation (virtualizing L2 underneath a virtual L3 is needed to >> support revisitation, where a single node participates multiple >> times in an overlay), and basically any subnet-based resource >> discovery. >> >> Joe >> >> >> >> >> >> >> That‘s my question >> >> Dave >> >> >> >> *From:*Joe Touch [mailto:[email protected]] >> *Sent:* Friday, August 12, 2016 8:20 PM >> *To:* David Allan I <[email protected]> >> <mailto:[email protected]> >> *Cc:* [email protected] <mailto:[email protected]>; Bocci, Matthew (Nokia >> - GB) <[email protected]> <mailto:[email protected]> >> *Subject:* Re: [nvo3] FW: Call for interest on NVO3 use case >> draft >> >> >> >> The typical use case is to support subnet emulation, e.g., a group >> of links over which broadcast is emulated as with LANE. >> >> >> On Aug 12, 2016, at 7:11 PM, David Allan I >> <[email protected] <mailto:[email protected]>> >> wrote: >> >> My point would be that introducing additional complexity in an >> overlay should have a use case associate with it. It would not be >> something you would do gratuitously…. >> >> >> >> SO I’m looking for the draft to provide a use case for this vs. >> simply mentioning subnetting without any context J >> >> >> >> Cheers >> >> Dave >> >> >> >> *From:*nvo3 [mailto:[email protected]] *On Behalf Of *Joe Touch >> *Sent:* Friday, August 12, 2016 5:07 PM >> *To:* David Allan I <[email protected] >> <mailto:[email protected]>>; [email protected] >> <mailto:[email protected]>; Bocci, Matthew (Nokia - GB) >> <[email protected] <mailto:[email protected]>> >> *Subject:* Re: [nvo3] FW: Call for interest on NVO3 use case >> draft >> >> >> >> >> >> >> >> On 8/12/2016 4:16 PM, David Allan I wrote: >> >> 4.2 Why I would subnet my overlay could use some explanation. I >> normally think of subnetting as a convenient address >> summarization technique dependent on topology, and with an overlay >> I don’t have a topology. >> >> >> The topology of an overlay is determined by its tunnels, just as >> the topology of the underlying net is determined by its links. >> >> A subnet in an overlay corresponds either to a single multipoint >> tunnel or to a set of tunnels that transparently acts as such - >> just as a subnet in the Internet base network corresponds to a >> shared access link or a set of links that transparently act as >> such (e.g., switched ethernet). >> >> Joe >> >> >> >> >> >> >> >> >> > > > > _______________________________________________ > nvo3 mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/nvo3 >
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