mind-boggling discussion :-) Hello SR experts.
Mustapha wrote > I have the impression the authors are > trying to address SID conflict resolution outside of the natural per prefix > resolution on the router. that was my thought too. From the routing protocol I deal with a prefix. Then I need to find the SID for the prefix to program my ingress/egress labels. If the mapping prefix -> SID has conflicting results _then_ I have a problem. Or in other words: 1. (150, 1.1.1.1/32 100 range 100) 2. (150, 1.1.1.1/32 500 range 2) would result in no mapping, following the new proposal. All due to a typo of 1.1.1.1 instead of 1.1.2.1 in the 2nd mapping? While I understand the algorithm I don't want to be the poor operator having my OSS screen full with alarms for 1.1.1.3..100 . Btw, the "preference" field, what purpose does is serve? Has this been introduced solely to tie-break conflicts? So we have one more parameter that can be typo-d :-) Or is there actually an application for it? Robert wrote > After mental reset I conclude that perhaps even the introduction of the > draft is questionable That goes much further than what I had in mind ;-) but I wonder if we go too far. I still think it is useful to describe what is a conflict - and this way also saying what should not be mistaken for a conflict. The discussion about conflict-with-range vs. conflict-with-prefix seems useful to me. My problem with the earlier drafts was more about the conflict resolution/reaction, I found it complex and too specific to generally agree on. My personal opinion is when you have a conflict then _drop_ the prefix traffic. But quite frankly "dropping", "not programming", "strip to IP" or whatever else, they are all simple operations. Or simple code path, as Stewart put it, as long as we stick to _one_ of them. The draft should demand one particular operation to be a MUST for interoperability. Regards, Marc On Fri, 23 Dec 2016 15:24:55 +0000, Aissaoui, Mustapha (Nokia - CA) wrote: > Hi Robert, > In fact you are touching on the point I am trying to make in my comment (1) > on the slides. Reading this draft, I have the impression the authors are > trying to address SID conflict resolution outside of the natural per prefix > resolution on the router. While in theory this can be done but it makes the > algorithm much more complex trying to compare overlapping SRMS SID entries > with different range sizes. > > There are two sources of advertisement of the SID information: > a. Per-prefix SID entries received in the prefix SID sub-TLV within a > Prefix TLV (IS-IS IP Reach TLV or OSPF Extended Prefix TLV). > b. SRMS entries which advertise SID information for a range of prefixes. > > The range size of the SRMS entry entirely depends on how the user wants to > advertise the information and has no meaning for the resolution. From a > router’s perspective, the SID information is associated with a prefix > regardless how it is advertised. > > The per-prefix SID information is preferred to the SRMS SID information. > And thus a simple algorithm which at the top level selects the SID among > set (a) based on the source advertising router and if empty selects the SID > among set (b) based on the SRMS preference and then based on the SRMS > router-id if same preference will work. > > Regards, > Mustapha. > > From: [email protected] [mailto:[email protected]] On Behalf Of Robert > Raszuk > Sent: Thursday, December 22, 2016 5:29 PM > To: Les Ginsberg (ginsberg) <[email protected]> > Cc: Aissaoui, Mustapha (Nokia - CA) <[email protected]>; > [email protected] > Subject: Re: [spring] SID Conflict Resolution: A Simpler Proposal > > Les, > > After mental reset I conclude that perhaps even the introduction of the > draft is questionable as in real life it applies to be quite an unrealistic > scenario to have a situation where more then one protocol is used *in the > same time* as active in forwarding for an exact same IP prefix. > > Last time I checked SIDs are meaningless without a prefix they are attached > to and it is a prefix they accompany to indicate which SID should be used > on a given node. > > Therefor if you consider that today most implementations pretty well can > handle overlapping prefixes from multiple sources what real problem is this > draft solving ? > > Are you trying to create a forwarding graph by SIDs only detaching them > from IP prefixes all together ? > > Cheers, > R. > > > On Thu, Dec 22, 2016 at 10:32 PM, Les Ginsberg (ginsberg) > <[email protected]> wrote: >> Robert – >> >> You have a complete misunderstanding of roles here. >> >> How the owner of a route may be represented in the RIB isn’t impacted by >> SRMS or conflict resolution. Nor is the determination of which route is >> the best route. We are only determining whether to use or not use a SID >> which was associated with the prefix by some advertisement. >> >> The Introduction to the draft states: >> >> “The problem to be addressed is protocol independent i.e., segment >> related advertisements may be originated by multiple nodes using >> different protocols and yet the conflict resolution MUST be the same >> on all nodes regardless of the protocol used to transport the >> advertisements.” >> >> Please do a mental reset. J >> >> Les >> >> >> From: [email protected] [mailto:[email protected]] On Behalf Of Robert >> Raszuk >> Sent: Thursday, December 22, 2016 11:52 AM >> To: Les Ginsberg (ginsberg) >> Cc: Aissaoui, Mustapha (Nokia - CA); [email protected] >> >> Subject: Re: [spring] SID Conflict Resolution: A Simpler Proposal >> >> Hi Les, >> >> On #1 I am also with Mustapha here. For clarity of this discussion can you >> enumerate when from RIB to FIB/LFIB you are installing the exact same >> active prefix from more then one producer ? Is SRMS sort of zombie here >> and not treated as real route producer hence we have an issue ? And the >> issue is only with conflicts of SRMS + real route producer ? >> >> On #3 you said that "with redistribution/route leaking the source of an >> advertisement may appear to be different on different routers" that is >> very true. In fact with simple static route or static label configured on >> a router the RIB and FIB on that router will be different then RIB and FIB >> on the other routers without such static route. And the point is that such >> static route or label is there for a reason you may not know about. So >> struggling for data plane consistency deliberately excluding source when >> operational needs require otherwise is not really that much helpful I am >> afraid. >> >> Greetings, >> Robert. >> >> >> On Thu, Dec 22, 2016 at 8:37 PM, Les Ginsberg (ginsberg) >> <[email protected]> wrote: >> Mustapha - >> >> From: spring [mailto:[email protected]] On Behalf Of Aissaoui, >> Mustapha (Nokia - CA) >> Sent: Thursday, December 22, 2016 8:10 AM >> To: Les Ginsberg (ginsberg); [email protected] >> Subject: Re: [spring] SID Conflict Resolution: A Simpler Proposal >> >> Hi Les, >> I read slides 17-21 of the document you referenced below and I have the >> following comments: >> >> 1. Page 17, “Order Matters - Prefix vs SID Conflict”. >> When you perform resolution on a per prefix basis, prefix conflicts are >> naturally processed first followed by SID conflicts across different >> prefixes. So the ordering issue described is only specific if you decided >> to resolve conflicting SID entries outside of the natural prefix >> resolution by a router. >> >> [Les:] What may seem “natural” to you might not to someone else. I don’ >> t care to debate that point. What is being illustrated here is that in >> order to provide a normative specification that – if followed – >> guarantees interoperability we have to specify the order in which >> conflicts are processed otherwise different results may be obtained. >> >> 2. Page 18, “Order Matters: Derived vs non-derived– prefix conflict” >> . >> It seems to me this issue is an artifact of the specific algorithm used to >> resolve conflicts. Because the algorithm uses parameters such as “Range >> (start w smallest)” then obviously derived SRMS entries will lend a >> different result than original SRMS entries. >> With the pre-prefix resolution, the only information kept from the >> original SRMS entry is the preference and the advertising or owner router. >> Anything else is not used. It seems to me a simple algorithm based on >> preference first then followed by some rule on selecting the advertising >> router if conflicts exist within the same preference would work. >> >> [Les:] You have implemented things in a certain way. Someone else might >> choose to implement in a different way. A normative specification does not >> (and should not) constrain an implementation. What is being illustrated >> here is that if the implementation does not retain the underived entry (in >> whatever internal form it chooses) different results will be obtained >> because the preference algorithm depends on comparing the underived ranges. >> >> 3. Finally, there is something which has not been addressed in the >> slides. There is still a possibility of conflicting entries among SIDs >> advertised using the prefix SID sub-TLV within a Prefix TLV (IS-IS IP >> Reach TLV or OSPF Extended Prefix TLV). A simple rule selecting the >> advertising router should also work fine here. >> >> [Les:] No – source of an advertisement has been quite >> >> deliberately excluded from the preference algorithm. With >> redistribution/route leaking the source of an advertisement may appear to >> be different on different routers- this would result in different results >> on different routers. >> >> Les >> >> Regards, >> Mustapha. >> >> From: Les Ginsberg (ginsberg) [mailto:[email protected]] >> Sent: Friday, December 09, 2016 1:49 PM >> To: Aissaoui, Mustapha (Nokia - CA) <[email protected]>; >> [email protected] >> Subject: RE: SID Conflict Resolution: A Simpler Proposal >> >> Mustapha - >> >> From: Aissaoui, Mustapha (Nokia - CA) [mailto:[email protected]] >> Sent: Friday, December 09, 2016 7:44 AM >> To: Les Ginsberg (ginsberg); [email protected] >> Subject: RE: SID Conflict Resolution: A Simpler Proposal >> >> I have a couple of comments on the below proposal. >> >> 1. Regarding the SRMS Preference Sub-TLV in section 3.1 of the draft. >> In many cases, a configuration on the resolving router can assign a >> preference to each SRMS in case there is no advertisement of this sub-TLV >> or to override an advertised value. I propose that this option be allowed. >> Here is a proposed update to the relevant paragraph: >> “ >> Advertisement of a preference value is optional. Nodes which >> do not >> advertise a preference value are assigned a preference value of >> 128. >> A resolving router can override the default or the advertised >> value by local policy. >> “ >> [Les:] In order to get consistent conflict resolution on all nodes in the >> network, it is necessary that they all have the same database – which >> includes the preference value. If you allow local policy to modify the >> preference you no longer have consistent databases on all nodes and we can >> no longer guarantee consistent conflict resolution. So your proposal is >> not viable IMO. >> >> 2. I am trying to understand the concept of sorting SRMS entries which >> have different prefixes and different range sizes. >> Since a SID advertised in a prefix SID sub-TLV within a Prefix TLV (IS-IS >> IP Reach TLV or OSPF Extended Prefix TLV) has higher priority over a SID >> for the same prefix advertised from a SRMS, then you have to add to the >> below sorting an entry for each individual prefix which advertised a >> prefix SID sub-TLV within a prefix TLV. >> At this point, the concept of an entry with multiple prefixes is moot and >> you may as well just sort on a per prefix basis which is the most natural >> thing to do given that the prefix resolution and then the SID resolution >> are performed on a per prefix basis. SID conflicts can be resolved on a >> per prefix basis using the below proposed preference algorithm without >> having to ignore SID values for unrelated prefixes just because it happens >> that they were advertised in the same SRMS entry. >> >> [Les:] The simpler proposal does not require sorting on anything other >> than preference. After that, you can process entries in any order you want >> and you will get the same answer. >> With “Ignore Overlap Only” one of the issues with trying to use the >> non-conflicting portions of a mapping entry which has a range > 1 is that >> the order in which you process entries influences the result. Please see >> slides 17 – 20 from the IETF97 presentation: >> https://www.ietf.org/proceedings/97/slides/slides-97-spring-1_ietf97_draft-ietf-spring-conflict-resolution-02-00.pptx >> for some simple examples of this. >> >> Les >> >> >> Regards, >> Mustapha. >> >> From: spring [mailto:[email protected]] On Behalf Of Les Ginsberg >> (ginsberg) >> Sent: Sunday, December 04, 2016 7:04 PM >> To: [email protected] >> Subject: [spring] SID Conflict Resolution: A Simpler Proposal >> >> When the problem addressed by draft-ietf-spring-conflict-resolution was >> first >> presented at IETF 94, the authors defined the following priorities: >> >> 1)Detect the problem >> 2)Report the problem >> This alerts the network operator to the existence of a conflict so that >> the configuration error can be corrected. >> 3)Define consistent behavior >> This avoids mis-forwarding while the conflict exists. >> 4)Don’t overengineer the solution >> Given that it is impossible to know which of the conflicting entries >> is the correct one, we should apply a simple algorithm to resolve the >> conflict. >> 5)Agree on the resolution behavior >> >> The resolution behavior was deliberately the last point because it was >> considered the least important. >> >> Input was received over the past year which emphasized the importance of >> trying to "maximize forwarding" in the presence of conflicts. Subsequent >> revisions of the draft have tried to address this concern. However the >> authors >> have repeatedly stressed that the solution being proposed >> ("ignore overlap only") was more complex than other offered alternatives >> and >> would be more difficult to guarantee interoperability because subtle >> differences in an implementation could produce different results. >> >> At IETF97 significant feedback was received preferring a simpler solution >> to >> the problem. The authors are very sympathetic to this feedback and >> therefore >> are proposing a solution based on what the draft defines as the "Ignore" >> policy - where all entries which are in conflict are ignored. We believe >> this >> is far more desirable and aligns with the priorities listed above. >> >> We outline the proposed solution below and would like to receive feedback >> from >> the WG before publishing the next revision of the draft. >> >> Les (on behalf of the authors) >> >> New Proposal >> >> In the latest revision of the draft "SRMS Preference" was introduced. This >> provides a way for a numerical preference to be explicitly associated with >> an >> SRMS advertisement. Using this an operator can indicate which >> advertisement is >> to be preferred when a conflict is present. The authors think this is a >> useful >> addition and we therefore want to include this in the new solution. >> >> The new preference rule used to resolve conflicts is defined as follows: >> >> A given mapping entry is compared against all mapping entries in the >> database >> with a preference greater than or equal to its own. If there is a >> conflict, >> the mapping entry with lower preference is ignored. If two mapping entries >> are >> in conflict and have equal preference then both entries are ignored. >> >> Implementation of this policy is defined as follows: >> >> Step 1: Within a single address-family/algorithm/topology sort entries >> based on preference >> Step 2: Starting with the lowest preference entries, resolve prefix >> conflicts >> using the above preference rule. The output is an active policy per >> topology. >> Step 3: Take the outputs from Step 2 and again sort them by preference >> Step 4: Starting with the lowest preference entries, resolve SID conflicts >> using the above preference rule >> >> The output from Step 4 is then the current Active Policy. >> >> Here are a few examples. Each mapping entry is represented by the tuple: >> (Preference, Prefix/mask Index range <#>) >> >> Example 1: >> >> 1. (150, 1.1.1.1/32 100 range 100) >> 2. (149, 1.1.1.10/32 200 range 200) >> 3. (148, 1.1.1.101/32 500 range 10) >> >> Entry 3 conflicts with entry 2, it is ignored. >> Entry 2 conflicts with entry 1, it is ignored. >> Active policy: >> >> (150, 1.1.1.1/32 100 range 100) >> >> Example 2: >> >> 1. (150, 1.1.1.1/32 100 range 100) >> 2. (150, 1.1.1.10/32 200 range 200) >> 3. (150, 1.1.1.101/32 500 range 10) >> 4. (150, 2.2.2.1/32 1000 range 1) >> >> Entry 1 conflicts with entry 2, both are marked as ignore. >> Entry 3 conflicts with entry 2. It is marked as ignore. >> Entry 4 has no conflicts with any entries >> >> Active policy: >> (150, 2.2.2.1/32 1000 range 1) >> >> Example 3: >> >> 1. (150, 1.1.1.1/32 100 range 500) >> 2. (150, 1.1.1.10/32 200 range 200) >> 3. (150, 1.1.1.101/32 500 range 10) >> 4. (150, 2.2.2.1/32 1000 range 1) >> >> Entry 1 conflicts with entries 2, 3, and 4. All entries are marked ignore. >> >> Active policy: >> Empty >> >> Example 4: >> >> 1. (150, 1.1.1.1/32 100 range 10) >> 2. (149, 1.1.1.10/32 200 range 300) >> 3. (149, 1.1.1.101/32 500 range 10) >> 4. (148, 2.2.2.1/32 1000 range 1) >> >> Entry 4 conflicts with entry 2. It is marked ignore. >> Entry 2 conflicts with entry 3. Entries 2 and 3 are marked ignore. >> >> Active policy: >> (150, 1.1.1.1/32 100 range 10) >> >> >> >> >> >> _______________________________________________ >> spring mailing list >> [email protected] >> https://www.ietf.org/mailman/listinfo/spring >> > > > > _______________________________________________ > spring mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/spring _______________________________________________ spring mailing list [email protected] https://www.ietf.org/mailman/listinfo/spring
