Re: [OSPF] TTZ, my 2c
Huaimo Chen <huaimo.chen@huawei.com> Tue, 09 July 2013 20:53 UTC
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From: Huaimo Chen <huaimo.chen@huawei.com>
To: "A. Przygienda" <prz@mail.zeta2.ch>, 'OSPF List' <ospf@ietf.org>
Thread-Topic: [OSPF] TTZ, my 2c
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Date: Tue, 09 Jul 2013 20:53:26 +0000
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Subject: Re: [OSPF] TTZ, my 2c
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Hi Tony, Thanks for your comments! See my responses inline below. Best Regards, Huaimo -----Original Message----- From: ospf-bounces@ietf.org [mailto:ospf-bounces@ietf.org] On Behalf Of A. Przygienda Sent: Tuesday, July 09, 2013 1:29 PM To: 'OSPF List' Subject: [OSPF] TTZ, my 2c Somehow the list killed my email. Another attempt. Kind of agree with Hannes I guess So, looking at other stuff zipped over the TTZ drafts and it's an intriguing idea but IMHO suffers from several heavy defects. Quick incomplete list draft-chen-ospf-ttz-app-03.txt 1. Section 5.1 is bogus. A reasonable scalability comparison should not be between 1600 Rs flat OSPF and 10 TTZs but between 10 areas & 10 TTZs (which will come by my gut feeling to about the same or rather small difference) [Huaimo] The reason for comparing a network having one area with the one having 10 TTZ is that the latter has some attributes of one area. The network having 10 areas does not have these attributes. 2. Section 5.2.2.2 is probably misleading. The example completely skirts the issue how a tunnel can be computed not only through the TTZ border routers but through internals of TTZ. I assume that happends by the 'virtual links' and that leads to good amount of problems by itself since there is the need to setup labels within the TTZ, issues with link- and node-disjointness in case of protections and similiar nit-picks. All those issues are probably similar to the new 'segment-routing' drafts. [Huaimo] Can we focus on LSP setup first? For LSP setup, do you see any issue in this section? For LSP protections, we can propose some solutions later. 3. 5.2.2.3 basically assumes that there is no reachability needed within the TTZ (i.e. prefixes). Yes, then ABR summaries are not needed but how do you manage something like the routers in TTZ (loopback addresses of the routers) ? It's one thing to intentionally hide-the-transits and another to not have a possibility to reach the routers. [Huaimo] loopback address distribution will be addressed. 4. I see how section 5.5 (POP) or 5.7 will work. That maybe a possible application and simpler than NSSA. However, customer prefixes must bubble up somehow unless they are configured only on a single TTZ pointing towards \ core ? [Huaimo] This will be considered. draft-chen-ospf-ttz-05.txt In general, all the claims to 'simpler than area' seem to me based on the fact that a) all the things that an area needs are ignored here . no indication what happens on partitioning of TTZs(should be simple) [Huaimo] This will be addressed. . no indication what happens when TTZs find themselves in 2 different areas ? Do the TTZ borders start to act like border routers ? Saying 'they MUST not' is IMHO not good enough for deployable protocol, mode of failure must be described & debugging procedure. Best would be e.g. if TTZ borders check for Area ID on router LSAs (which would need to be carried in a new opaque or something like this ?) and make sure TTZ is in a single area. [Huaimo] At first, we limited a TTZ in one area. You are right. Some checks need to be done for this. . I assume that routing integrity (hop-by-hop) is guaranteed, by external routers computing 'routes using the "virtual TTZ links"'. This IMHO will be a major scalability problem (and routing loops) once the TTZs grow since the number of virtual links grows about N^2/2 of edges. This will quickly lead to TONS of virtual links which will need to be distributed & lead to transit routing loops since those 'virtual links' are topology summaries and will not reflect the 'real topology', i.e to make sure you don't have routing loops the TTZ border routers can only use the tunnels when they have been redistribugted to everyone i.e. after everyone has the same topology (how you do that?). Otherwise they must use the 'previous' topology before the summarization into virtualk links. Yes, those are transient loops but having 10 TTZ borders will generate already 50 tunnel summaries and they take time to distribute. [Huaimo] The number of edge nodes of a TTZ should be small. . no indication how internal reachability within TTZ is guaranteed & summarized and how it is guaranteed that e.g. prefixes are not being reimported as type 5 or 7 into the TTZ again through backdoor mechanisms (e.g. TTz borders in two different areas) in case TTZ ends up summarizing. [Huaimo] Can you give more details about this? b) I don't understand 8.3. What is "a TTZ is virtualized as a group of edge routers of the TTZ connected". Section 7 implies that TTZ edges are advertised as full mesh of 'virtualized links'. Contradiction ? [Huaimo] Full mesh connections of the edges is one type of connections of the edges. The above sentence indicates that a TTZ may have a type of connections including full mesh. So, my gut feeling is after everything has been solved this will be more complex or equal to areas in terms of configuration, behavior, contraints for a questionable gain of having a full-mesh-TTZ construct versus a prefix-summary ABR construct. [Huaimo] Configuring a TTZ will be simpler than configuring an area. We are working on it. The behavior should also be simpler. And it suffers from couple issues I pointed out above which would have to be addressed before anything like this is deployed in the wild ;-) --- tony _______________________________________________ OSPF mailing list OSPF@ietf.org https://www.ietf.org/mailman/listinfo/ospf
- Re: [OSPF] TTZ, my 2c Jeff Tantsura
- Re: [OSPF] TTZ, my 2c A. Przygienda
- Re: [OSPF] TTZ, my 2c Huaimo Chen
- [OSPF] TTZ, my 2c A. Przygienda
- Re: [OSPF] TTZ, my 2c Acee Lindem
- Re: [OSPF] TTZ, my 2c Alvaro Retana (aretana)
- Re: [OSPF] TTZ, my 2c Acee Lindem
- Re: [OSPF] TTZ, my 2c William McCall
- Re: [OSPF] TTZ, my 2c Huaimo Chen
- Re: [OSPF] TTZ, my 2c Huaimo Chen
- Re: [OSPF] TTZ, my 2c Huaimo Chen