Re: [OSPF] Comments draft-ietf-ospf-segment-routing-extensions-00.txt

[Anton Smirnov <[e-mail]>]

    Hi Peter,
    in current text ranges are not required to be processed and I think 
this kills their potential usage:

    If multiple Prefix-SIDs are advertised for the same prefix, the
    receiving router MUST use the first encoded SID and MAY use the
    subsequent ones.

(and similar text for inter-area LSA origination).
    Subsequent prefixes may be used or may be ignored. And if they are 
ignored then it is originator's fault, receiver has all rights to ignore 
them. If the mapping server wants to ensure every router installs prefix 
attributes then it has no choice but to break the range and advertise 
each prefix individually.
    So IMO ranges processing must be required on every step, otherwise 
they will never be used.




 > high level TLV advertise a single prefix/mask. It's the sub-TLV which
 > may extend the applicability to the range if required, so the scope is
 > defined by each sub-TLV.

    That what makes ranges counter intuitive. Intuitive hierarchy is 
object on the top and then subordinate items describing properties of 
the object:

Object ---
          |
          |- attribute1
          |- attribute2


But the current scheme does not define object at the top of the 
hierarchy, real object is defined together with property:

Anchor ---
          |
          |- range-of-objects-and-attribute1
          |- range-of-objects-and-attribute2

There are three problems with this representation:
1. Anchor (i.e. top level TLV) is not enough to determine which objects 
(i.e. prefixes) are affected and which are not
2. Set of objects (i.e. the range) may be different in each sub-TLV thus 
raising concerns of ambiguity
3. Text requires that anchor is advertised only once but there is no 
requirement that ranges advertised for different anchors cannot overlap

It would have been more logical to define range in the "OSPF Extended 
Prefix TLV" header itself, not in each sub-TLV. This specifies the 
object (i.e. the range) in the topmost TLV and resolves problems 
mentioned above.

Anton


On 07/03/2014 10:09 AM, Peter Psenak wrote:
> Hi Acee,
>
> please see inline:
>
> On 7/2/14 19:17 , Acee Lindem wrote:
>> Hi Peter,
>> It seems there are two distinct deployment scenarios - one where SR
>> routers are given a range and policy and allocate their own SIDs and
>> another where a mapping server does it for the routing domain.
>
> yes, that is correct. The latter is used mainly during migration from
> LDP to SR.
>
>
>
>>>>         6. Section 4.2 - I really don’t like having his sub-TLV
>>>> redefine the subsuming top-level TLV as a range of prefixes of the
>>>> same length rather than a single prefix. Although this is my first
>>>> serious reading of the draft, this encoding seems really unwieldy
>>>> and, in practice, I’d expect the range size always advertised as 1.
>>>> We can discuss this more in Toronto.
>>>
>>> an example where the range would be more then 1 is a mapping server
>>> case. This helps you to advertise SIDs for loopback addresses of all
>>> routers in a non-SR capable part of the network, assuming they are
>>> allocated from the contiguous address space. Instead of advertising
>>> hundreds of mappings for each /32 address, you can compact it to a
>>> single advertisement.
>>
>> I’ve seen loopbacks allocated sequentially in a few networks but many
>> more where there weren’t.
>
> still, having a mechanism to compact the advertisements if possible
> looks appealing.
>
>>
>>>
>>> What you need in such case is component prefix/length plus the number
>>> of components - OSPF Extended Prefix TLV gives you the component info
>>> and Prefix SID Sub-TLV "Range Size' gives you the number of components.
>>
>> I understood it but I don’t like the sub-TLV extending the
>> specification of the higher level TLV. I especially don’t like it
>> since the top-level TLV is a generic mechanism to advertise
>> attributes.  When additional attributes are defined, it begs the of
>> whether or not they apply solely to the prefix or to the range.
>
> high level TLV advertise a single prefix/mask. It's the sub-TLV which
> may extend the applicability to the range if required, so the scope is
> defined by each sub-TLV.
>
>>
>>>
>>> We could have defined a separate top level TLV in OSPF Extended
>>> Prefix LSA for the advertisement of range of components, but it looks
>>> to me that would be an overkill.
>>
>> I would have preferred that. When the SID attributes are embedded
>> (OSPFv3 and ISIS), I think the semantics are even more unnatural since
>> reachability MAY be advertised for the prefix while SID mapping is
>> being advertised for the range.
>
> I had the same reservations at the beginning :)
> But there is no problem really. Prefix-SID sub-TLV never advertises any
> reachability, whether it advertises a single SID or a range of SIDs. For
> Prefix-SID sub-TLV, prefix from the higher level TLV has a meaning of
> "start" and Prefix-SID sub-TLV always carry its own "size" - just a
> different interpretation of the data from the higher level TLV.
>
> Please note that SID range is quite different from the address range we
> are used to from summarization. SID range requires three parameters
> (address/mask and count), compared to two parameter (address/mask) that
> traditional address range uses. As a result, Extended prefix TLV as such
> can not cover the SID range, because it only has address/mask. Defining
> a top-level TLV for a SID range itself does not really fit into Extended
> Prefix LSA and having a new LSA for it is not an option I would say. So
> the current encoding looks like a good compromise to me.
>
> thanks,
> Peter
>
>>
>>> Current encoding of Prefix SID Sub-TLV gives us all the flexibility
>>> we need. In addition it matches what ISIS has done.
>>
>> I haven’t seen any discussion of the draft on the ISIS list other than
>> the revision updates.
>>
>> Thanks,
>> Acee
>>
>>
>>
>>>
>>>
>>>>
>>>>         7. Section 4.2 - Shouldn’t the reference for the mapping
>>>> server be the “Segment Routing Architecture” rather than the
>>>> “Segment Routing Use Cases”? In general, the usage of a mapping
>>>> server and the scope of assignment needs to be described better
>>>> somewhere (not in the OSPF encoding document).
>>>
>>> will fix the reference
>>>
>>>>
>>>>         8. Section 6 - It would seem that an entity calculating a
>>>> multi-area SR path would need access to the topology for all the
>>>> areas and the SID would need to be globally assigned? Right?
>>>
>>> correct.
>>>
>>>> So rules are primarily for the population of the forwarding plane.
>>>> Right?
>>>
>>> for advertisement/propagation of SIDs and for forwarding plane
>>> programming.
>>>
>>>>
>>>>         9. Section 6.2 - In standard OSPF, inter-area summary
>>>> propagation only applies to inter-area routes learned over the
>>>> backbone. Is this any different?
>>>
>>> no, the mechanism is the same as for type-3s.
>>>
>>> thanks,
>>> Peter
>>>
>>>>
>>>> Thanks,
>>>> Acee
>>>>
>>>>
>>>>
>>>> .
>>>>
>>>
>>
>> .
>>
>
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