Re: [Lsr] LSR WG Adoption Poll for "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints" - draft-hegde-lsr-flex-algo-bw-con-02

Peter Psenak <> Mon, 24 May 2021 18:58 UTC

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To: Tony Li <>, Ketan Jivan Talaulikar <>
Cc: "Acee Lindem (acee)" <>, "" <>, "" <>
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From: Peter Psenak <>
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Date: Mon, 24 May 2021 20:58:12 +0200
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Subject: Re: [Lsr] LSR WG Adoption Poll for "Flexible Algorithms: Bandwidth, Delay, Metrics and Constraints" - draft-hegde-lsr-flex-algo-bw-con-02
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Hi Tony,

On 24/05/2021 20:44, Tony Li wrote:
> Hi Ketan,
>> In general, I support the adoption of this document. There is, 
>> however, one specific point which is not clear to me (8) below that I 
>> would appreciate some clarity on before adoption.
> As the chairs have noted, adoption is binary and not contingent upon 
> rough consensus on the content, just on rough consensus on the interest.
>>  1. Why is the Generic Metric type in ISIS limited to 3 byte size?
>>     OSPF allows 4 byte size and so why not the same for ISIS?
>>     Elsewhere in the document, I do see MAX METRIC being referred to
>>     as 4,261,412,864.
> Because I’m a lazy sod.
> It’s far easier to detect metric overflow on three byte values than four 
> byte values. True, four byte is not impossible, but it’s just quick and 
> easy with three byte values.  Adding a fourth byte would add range to 
> the metric space, but in practice, this seemed like it was not really 
> relevant. Most areas are not a very high diameter and the need for 
> detailed metric distinctions has not been that high.  Thus, we went with 
> a 3 byte metric in RFC 5305 (sec 3.7) and that seems to work.
>> 1.
>>  2. Would be good to cover the max-metric considerations for the
>>     Generic Metric. Similar
>>     to
> Fair
>> 2.
>>  3. Since the draft is covering FlexAlgo, I would have expected that
>>     Generic Metric is carried only in the ASLA and this document
>>     specifies usage only for the FA application. Later this can be
>>     also used/extended for other applications but still within ASLA.
>>     Keeping an option of advertising both outside and within the ASLA
>>     is problematic – we will need precedence rules and such. I prefer
>>     we avoid this complication.
> We preferred avoiding ASLA.

we are not avoiding ASLA. We allow the ISIS Generic Metric sub-TLV to be 
sent inside or outside ASLA.

For flex-algo purposes we mandate it to be in ASLA. That's all what we 
need for the purpose of this draft. The rest is for future.

>> 3.
>>  4. For the newly proposed FAD b/w constraints, I would suggest the
>>     following names for the constraint sub-TLVs where the b/w value
>>     signalled by all is compared with the Max Link B/w attribute. This
>>     is just to make the meaning, at least IMHO, more clear.
>>      1. Exclude Higher Bandwidth Links
>>      2. Exclude Lower Bandwidth Links
>>      3. Include-Only Higher Bandwidth Links
>>      4. Include-Only Lower Bandwidth Links
>>  5. Similar naming for the FAD delay constraints as well would help.
>>     Though I can only think of the use of “exclude” for links above a
>>     certain delay threshold to be more practical but perhaps others
>>     might eventually be required as well?
> Thank you for the suggestions.
>> 5.
>>  6. For the Max B/w Link attribute and its comparison with the FAD b/w
>>     constraints, I see the reference to ASLA. While in OSPF
>>     max-bandwidth is not allowed in ASLA
>>     -, in case
>>     of ISIS also, it is not really appropriate for use within ASLA
>>     -
> I’m sorry, I don’t understand this comment.
>> 6.
>>  7. Document should cover the FAPM aspects for the Generic Metric and
>>     especially the Bandwidth metric.
> Nor this one.
>> 7.
>>  8. The document introduces a new Generic Metric type called Bandwidth
>>     metric. I’ve been trying to follow some of the discussion related
>>     to this on the mailing list – about it being cumulative or not. I
>>     am perhaps somewhat confused by those discussions. The OSPF/ISIS
>>     SPT computation has always worked with cumulative link (and
>>     prefix) metrics. If the computation for the Generic Metric of this
>>     new type b/w is not going to be cumulative (I thought it is – but
>>     not very clear anymore), then the document needs to describe the
>>     computation algorithm. Is it then hop count based? Perhaps I am
>>     missing something very basic here and if so, please point me to
>>     the text in the draft.
> I’m sorry if this has been confusing. My understanding is that the 
> metric is cumulative. Others had other expectations.

my expectation is to be cumulative as well.


> When there are multiple links with the same bandwidth, and thus the same 
> metric, then the total path metric becomes (link metric) * (number of 
> links).
> Regards,
> Tony