Re: [mpls] MPLS-RT review of draft-vainshtein-mpls-gal-tc-ttl-handling-01

["Carlos Pignataro (cpignata)" <[e-mail]>]

Loa,

-----Original Message-----
From: Loa Andersson <[e-mail]>
Date: Monday, June 29, 2015 at 9:53 AM
To: Carlos Pignataro <[e-mail]>, Adrian Farrel
<[e-mail]>
Cc: "[e-mail]"
<[e-mail]>,
"[e-mail]" <[e-mail]>, "[e-mail]"
<[e-mail]>
Subject: Re: MPLS-RT review of draft-vainshtein-mpls-gal-tc-ttl-handling-01

>Carlos,
>
>On 2015-06-28 21:07, Carlos Pignataro (cpignata) wrote:
>> Hi Adrian,
><snip>
>
>>>
>
>>
>> My point is that the current text does not say what you say.
>> The text says "It is RECOMMENDED that implementations set the
>> TC field of an LSE that contains the GAL to all zero (0b000).²,
>
>> and that statement does not include any qualifiers or conditionals.
>> I understand you say that there is an ³there may be valid reasons
>> to do otherwise, with implications carefully understood before
>> doing so² implicit to the SHOULD.
>
>Maybe naive, but I thought SHOULD and RECOMMENDED have the same implicit
>conditions, RFC 2119 says:
>
>3. SHOULD   This word, or the adjective "RECOMMENDED", mean that there
>    may exist valid reasons in particular circumstances to ignore a
>    particular item, but the full implications must be understood and
>    carefully weighed before choosing a different course.

That does not mean ³Absent local policy, then it is RECOMMENDED Š². The
text shows an absolute recommendation.

Carlos.

>
>/Loa
>
>
>However, that is different
>
>than saying ³In the absence of local policy for setting those values, it
>is RECOMMENDED Š².
>>
>> I suggest that a ³Absent such local policy, ³ be prepended to the reco.
>>
>>
>>> I'm happy to be guided by WG opinion on this.
>>>
>>> I suppose a return question to you is "What damage will it do to make
>>>this recommendation"?
>>
>> To me, it is an absolute recommendation without basis on interop, and
>>somewhat contradics the previous sentence.
>>
>>>
>>>> CMP: A nit, is 0b000 the same as 000b? Not sure the notation.
>>>
>>> I looked at...
>>>     https://en.wikipedia.org/wiki/Binary_number#Representation
>>> and found a number of options including:
>>>       0b100101 (a prefix indicating binary format, common in
>>>programming languages)
>>
>> Ack.
>>
>>>
>>>> The LER that inspects an LSE that contains the GAL MUST ignore the
>>>> value of the TC field.
>>>>
>>>> CMP: Similarly, this document concerns itself with maximizing
>>>>interoperability.
>>>> Why this strongest ³MUST²? Setting ourselves up for updating this
>>>>when there
>>>> is a use?
>>>
>>> I think there is a common misunderstanding of "MUST" in this sort of
>>>case.
>>> Consider that when a specific meaning is assigned to the TC in the GAL
>>>LSE, this will be done in a new document that will update this document.
>>> It will be really, really important that when new meanings of this TC
>>>are defined, they pass safely through legacy nodes.
>>> It's also pretty important that this TC field is not used as a covert
>>>channel by implementations that might fail to interoperate correctly.
>>>
>>> So, there is an alternative to "MUST ignore on receipt" and that is
>>>"MUST set to zero on transmission".
>>> I'd be happy to go either way.
>>
>> What if the receiving LER wants to apply a specific TC treatment? That
>>is not a covert channel. Can there be different LSPs per TC?
>>
>> In other words, is the document saying ³on send you can use a local
>>policy to set a value, but on receipt there is absolutely no reason to
>>read it²?
>>
>>>
>>>> 3.2.  New Procedures for Handling the TTL Field in an LSE Containing
>>>>GAL
>>>>
>>>> CMP: I agree with this comment:
>>>> http://www.ietf.org/mail-archive/web/mpls/current/msg14300.html
>>>
>>> I'll respond to George in his thread.
>>>
>>>> Nits:
>>>>
>>>> Handling the TC and TTL fields in a Label Stack Entry when the Generic
>>>>                  Associated Channel Label is Present
>>>>
>>>> CMP: When I read this, I was not sure if the GAL was present in any
>>>>LSE.
>>>> This is likely just me, but it would be best to disambiguate potential
>>>> misreads (like mine) with "Label Stack Entry (LSE) containing the GAL²
>>>> (i.e., otherwise present where?)
>>>
>>> Yup.
>>>
>>>> I hope these are useful ‹ thanks!
>>>
>>> Thanks again.
>>
>>
>> Thank you for the quick response!
>>
>> ‹ Carlos.
>>
>>
>>>
>>> Adrian
>>>
>>
>
>-- 
>
>
>Loa Andersson                        email: [e-mail]
>Senior MPLS Expert                          [e-mail]
>Huawei Technologies (consultant)     phone: [phone]