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]<mailto:[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)
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