Re: [spring] WG Last Call for draft-ietf-spring-conflict-resolution

[Shraddha Hegde <[e-mail]>]

Thanks for the response.

   It is important  to keep the network functioning correctly in case of transitioning from
   one protocol to the other.

   Let us assume a case of OSPF SR network transitioning to ISIS SR network.
   Most typical transitioning technique is ships in the night where both protocols will be
   enabled in the network with OSPF having better preference and the issue in ISIS routing do
   not affect the traffic. Once the ISIS deployment is complete, the traffic will be switched
   to ISIS by changing preference.

   In case of OSPF-SR transitioning to ISIS SR, because of the conflict resolution
   rules that the conflicts are protocol independent, it is possible that config mistakes in ISIS
   will bring down the routes in OSPF. The ISIS topology and OSPF topology is not expected to be congruent
   during transition,
   so the conflicts seen on each node combining the two views will not be similar.
   This has potential to cause routing loops/ traffic drops in the network.

   I suggest to add the protocol-preference as one of the parameters in the preference algorithm
   with this being on the top of the list.
   The resultant conflict resolution will be consistent on ISIS topology and OSPF topology
  and is the best suited model for ships in the night transitions.

Pls see inline for other responses.

Rgds
Shraddha


From: Les Ginsberg (ginsberg) [mailto:[e-mail]]
Sent: Tuesday, July 25, 2017 1:57 AM
To: Shraddha Hegde <[e-mail]>; [e-mail]
Subject: RE: [spring] WG Last Call for draft-ietf-spring-conflict-resolution


Shraddha -



Thanx for the comments - responses inline.



> -----Original Message-----

> From: spring [mailto:[e-mail]] On Behalf Of Shraddha Hegde

> Sent: Thursday, July 20, 2017 11:44 PM

> To: [e-mail]<mailto:[e-mail]>

> Subject: Re: [spring] WG Last Call for draft-ietf-spring-conflict-resolution

>

> SPRING WG,

>

> Conflict resolution is an important problem to solve and it is important to

> Standardize this draft.

>

> I generally support the draft but have a few major comments which I hope

> the authors will work on.

>

>

>  1.Conflict resolution and forwarding

>

>             Section 3.4 has the statement

>             "Active Entries in the database may be used in forwarding."

>

>              This is a very loose statement which does not enforce

> implementations to program the forwarding plane

>             with the active database entries.

>             This does not ensure traffic drops are minimized.

>

[Les:] Conflict resolution is only determining which entries are eligible to be used in forwarding. This does not mean that all "active" entries will be used . The most obvious example (but not the only possible one) of this is an SRMS entry that is associated with a prefix which is not actually reachable. So the language in the draft is intentional and is correct.

<Shraddha> The language in the draft is ambiguous and it does not help achieve consistent forwarding behavior across implementations.

                      Prefixes that are not reachable may not be used in forwarding which is acceptable but the draft does not mandate that the reachable prefixes which are active

                    MUST be programmed. Normative language is necessary in the draft w.r.t using active entries in forwarding.





>             The Forwarding plane programming aspects are completely missing in

> the document.

>             A separate section is needed which describes the different aspects

> of programming the forwarding plane.



[Les:] This is NOT in scope for this draft. If you want a description of how SR MPLS forwarding works please see draft-ietf-spring-segment-routing-mpls.



<Shraddha> The point I am bringing up here is not how SR MPLS forwarding works. It is w.r.t to programming the forwarding plane when there is a conflict.



The abstract section has below statement



"In cases where the

   information advertised by a given protocol instance is either

   internally inconsistent or conflicts with advertisements from another

   protocol instance a means of achieving consistent forwarding behavior

   in the network is required."



If the draft is not going to address the forwarding plane detail w.r.t conflicting entries it is definitely not meeting the

The objective described above.



Take an example of a conflict

(192, 192.0.2.1/32, 200, 1, 0, 0)

   (192, 192.0.2.222/32, 200, 1, 0, 0)

   SID 200 has been assigned to 192.0.2.1/32 by the

   first advertisement.

   The second advertisement assigns SID 200 to 192.0.2.222/32.



In this case after applying the preference rules

(192, 192.0.2.1/32, 200, 1, 0, 0)

 Becomes active entry.



As per the text in the draft," it may be used in forwarding" so some implementations

May choose to program forwarding plane and some may not which does not give a consistent forwarding behavior

Across implementations.









>

> 2.  Protocol independent resolution and impact on network migrations

>              In case of network migration from one protocol to other for ex: OSPF-

> SR to ISIS-SR,

>              it is useful to associate protocol preferences on a local node to the SID

> advertisement

>             and feed into the conflict resolution. This would make sure the

> conflicts will always

>             have a winner which is an advertisement from protocol with

> preferred admin-distance.

>

>             There is need for introducing another preference value specific to

> protocol preference

>             and make it the top rule in the preference rule hierarchy.

>

[Les:] "admin distance" is a locally defined preference which is not advertised. It is therefore not possible to include it as a parameter in an algorithm which requires a consistent answer on all nodes throughout the network.


<Shraddha>  In migration cases, topologies across two different protocols are not congruent causing inconsistent behavior.

                       Using admin-distance as an input parameter keeps the conflict resolution with-in the protocol and guarantees

                      Consistent behavior across all the nodes corresponding to that protocol.



                     The drafts tries to address this scenario partially between IGP and BGP by suggesting preference values. But that does not solve the

Problem between two different IGPs.





>             This would also solve the issue of MT-ID numbers being different in

> different protocols

>             as the SIDs would be compared within a protocol advertisement.



[Les:] I do not understand what relationship you see between "protocol preference" and "MT-ID".

MT-ID values are scoped by  the protocol which uses them. For example, OSPFv2 supports a 7 bit MT-ID while IS-IS supports a 12 bit MT-ID. It is therefore possible for non-matching MTIDs to be used by different protocols when advertising routes for the same physical topology. This is why the draft's use of "topology" is not as MTID but rather as a locally scoped identifier. From Section 3:



" Note: Topology is a locally scoped identifier assigned by each

   router.  Although it may have an association with Multitopology

   Identifiers (MTID) advertised by routing protocols it is NOT

   equivalent to these identifiers.  MTIDs are scoped by a given routing

   protocol.  MTID ranges are protocol specific and there may be

   standardized protocol specific MTID assignments for topologies of a

   specific type (e.g., an AFI specific topology).  As mapping entries

   can be sourced from multiple protocols it is not possible to use a

   network scoped identifier for a topology when storing mapping entries

   in the local database."



Topology is then used to detect different scopes for a mapping entry - which may result in a SID conflict if the same SID is used in different topologies, but it cannot be used as a tiebreaker since its value is local and any preference (e.g., higher value wins)  is not guaranteed to result in consistent answers on all nodes in the network. Which is why we have Section 3.3 Rule #8:



   "8.  If topology IDs are NOT identical both entries MUST be ignored"

<Shraddha> Lets keep this discussion on-hold until we decide on the protocol preference and migration issues.



>

> 3. In case of hierarchical IGP networks with multiple ISIS Levels or OSPF areas,

> It's possible that the

>      conflicts are not visible in entire domain but are visible only on the border

> router as the border routers

>      have the database of both domains.

>     The conflict resolution preference Rules should be enhanced to include the

> Level information in the preference rule.

>     A new parameter called sub-domain should be defined.

>

>                             One could propose using existing SRMS preference values

> and assigning prefixes with preference values

>             based on levels they are advertised in. This introduces more complex

> configuration requirements on the

>             network. The objective of this draft is to achieve consistent

> behaviours in case of misconfigurations and

>             introducing more configurations as a solution does not help.

>

>

>                             Based on the Advertisement originated in ISIS Level or OSPF

> area below values are defined.

>

>                             Level 1 , non-zero OSPF area =1

>                             Level 2, OSPF Area 0 = 2

>                             Non IGPs set subdomain = 0

>

>                                                             Preference algorithm is changed as

>

>     1. Higher protocol preference wins



[Les:] I have explained above why protocol preference cannot be used.



>    2. smaller sub-domain wins

>     3. Higher srms preference value wins

>     4. Smaller range wins

>     5.IPv6 entry wins over IPv4 entry

>     6.Longer prefix length wins

>     7.Smaller starting address (considered as an unsigned integer

>        value) wins

>     8.Smaller algorithm wins

>     9. Smaller topology Id wins >>>>>>>>>>..Moved above SID comparison.

> since the all these rules are applied

>                                                     within protocol it's safe to compare topology IDs

[Les:] No - it isn't - as explained above.

>     10. Smaller starting SID wins

>

[Les:] SIDs are assigned either by the node(s) originating the prefix reachability advertisement or by SRMS advertisements. The latter are level/area agnostic - and even you are agreeing that we should not change that.

There is then no reason for the SID to be altered as it is advertised into different areas.  Which leads us to the conclusion that SIDs are not level/area specific.



If your concern is that border routers who may have more entries in their SID database than intra-area routers may come to a different conclusion as regards conflicts - I agree with you - but I do not believe your proposal resolves the problem.

Consider the following simple topology:



A1-----A2------B2-----B1



All nodes run IS-IS.

A1 is a Level-1 router in Area A. It advertises: 1.1.1.1/32 SID 100

A2 is a Level-1-2 router in Area A



B1 is a Level-1 router in Area B. It advertises: 2.2.2.2/32 SID 100

B2 is a Level-1-2 router in Area B



If Level 1 routes are leaked into Level 2 but NOT down into Level 1, we have the following SID databases on the four routers:



A1


1.1.1.1/32 100


A2


1.1.1.1/32 100

2.2.2.2/32 100


B1


2.2.2.2/32 100


B2


1.1.1.1/32 100

2.2.2.2/32 100






Here are the active entries on each node comparing the two algorithms



Node


Draft Algorithm


Shraddha Algorithm


A1


1.1.1.1/32 100


1.1.1.1/32 100


A2


1.1.1.1/32 100


1.1.1.1/32 100


B1


2.2.2.2/32 100


2.2.2.2/32 100


B2


1.1.1.1/32 100


2.2.2.2/32 100




There is a tradeoff here between being able to forward some inter-area traffic entering the network via the L2 sub-domain but impacting some intra-area traffic  vs being able to forward all intra-area traffic but no inter-area traffic.

Not clear which strategy is "better" - but it is clear that neither strategy eliminates all issues. Given that the same SID database will NOT exist on all routers in multi-area deployments some risk exists and cannot be totally eliminated.



I do agree that we should try to minimize the use of conflicting SIDs for inter-area traffic. What is lacking in the draft is a statement that conflicting SIDs should not be leaked out of an area. I will work on a statement in the draft to make that point clear.



<Shraddha> Not leaking the conflicting SIDs makes sense. But Even if the conflicting SIDs are not leaked across boundaries, there is still a possibility that inter-area/intra-area traffic gets misforwarded at the area boundary. This issue can cause potential security risks as the traffic can get delivered to unintended node.The best option is to ignore both conflicting entries when they belong to different area/level



     1. Higher protocol preference wins

    2. If the entries belong to different sub-domains ignore both entries

     3. Higher srms preference value wins

     4. Smaller range wins

     5.IPv6 entry wins over IPv4 entry

     6.Longer prefix length wins

     7.Smaller starting address (considered as an unsigned integer

        value) wins

     8.Smaller algorithm wins

     9. Smaller topology Id

    10. Smaller starting SID wins







Thanx for bringing this issue up.



    Les



>

>

> Rgds

> Shraddha

>

> -----Original Message-----

> From: Martin Horneffer [mailto:[e-mail]]

> Sent: Friday, July 14, 2017 8:22 PM

> To: [e-mail]<mailto:[e-mail]>

> Subject: Re: [spring] WG Last Call for draft-ietf-spring-conflict-resolution

>

> Strong support from me, too.

>

>  From an operator's point of view this is really needed.

>

> Best regards, Martin

>

>

> Am 10.07.17 um 14:58 schrieb Martin Vigoureux:

> >

> > WG,

> >

> > We are half-way through the WG Last Call and I am very surprised to

> > only see a single answer to it.

> >

> > I am not sure I'll move this forward with only silence as support.

> >

> > -m

> >

> > Le 29/06/2017 à 15:28, Martin Vigoureux a écrit :

> >> Hello Working Group,

> >>

> >> This email starts a Working Group Last Call on

> >> draft-ietf-spring-conflict-resolution-04 [1] which is considered

> >> mature and ready for a final working group review.

> >>

> >> ¤ Please read this document if you haven't read the most recent

> >> version yet, and send your comments to the list, no later than *21st

> >> of July*.

> >> Note that this is *not only* a call for comments on the document; it

> >> is also a call for support (or not) to publish this document as a

> >> Proposed Standard RFC.

> >>

> >> ¤ *Coincidentally*, we are also polling for knowledge of any IPR that

> >> applies to draft-ietf-spring-conflict-resolution, to ensure that IPR

> >> has been disclosed in compliance with IETF IPR rules (see RFCs 3979,

> >> 4879,

> >> 3669 and 5378 for more details).

> >>

> >> If you are listed as an Author or Contributor of

> >> draft-ietf-spring-conflict-resolution-04 please respond to this email

> >> and indicate whether or not you are aware of any relevant IPR.

> >>

> >> Note that, as of today, no IPR has been disclosed against this

> >> document or its earlier versions.

> >>

> >> Thank you,

> >> Martin

> >>

> >> [1]

> >> https://datatracker.ietf.org/doc/draft-ietf-spring-conflict-resolutio

> >> n/

> >>

> >> _______________________________________________

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> >> https://www.ietf.org/mailman/listinfo/spring

> >>

> >

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> >

>

>

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