Re: [sfc] [mpls] Opsdir last call review of draft-ietf-mpls-sfc-encapsulation-02

["Joel M. Halpern" <[e-mail]>]

First, you have a good point that the expected behavior of the MPLS TTL 
shoudl be specified.  Given that we managed to confuse you, it needs to 
be fixed.

<speaking as co-author, not as WG chair>

As I understand it, the MPLS TTL used for transport between SFF and the 
NSH TTL used for the SFC SFP are unrelated.  The NSH TTL has a value 
that exceeds the number of expected SFF (with a default of 63.)  It is 
derement at each SFF.

The MPLS TTL needs to be set to a value sufficient to get the NSH packet 
from one SFF to the next.  That might be one MPLS hop.  It might be 
several.  Presumably therefore the default is whatever the default is 
for the MPLS network in which it is being used.

the important point is that there is no relationship between decrements 
to the MPLS TTL and decrements to the NSH TTL.  As in tunnel mode for IP 
in IP tunnels, the NSH TTL is only decrement by one for each SFF-SFF hop.

Yours,
Joel

On 2/24/19 11:54 PM, Carlos Pignataro (cpignata) wrote:
> Hi, Joel,
> 
> Maybe I am misunderstanding something. Is the SFF Label, in the context 
> of MPLS transport encapsulation for the SFC NSH, ever expected to be 
> used for Forwarding? Is its TTL ever expected to be decremented and the 
> packet sent? If so, I did not understand that from the existing text, 
> since it is equivalent to a PW Label. If not, we can take advantage of TTL.
> 
> Either way is good, I’m just asking to make this explicit.
> 
> Thanks,
> 
> — Carlos Pignataro
> 
>> On Feb 23, 2019, at 1:43 AM, Joel M. Halpern <[e-mail] 
>> <mailto:[e-mail]>> wrote:
>>
>> My comment on this may not have made it to everyone.  If you receive a 
>> duplicate, I apologize  (I received DMARC errors from something about 
>> translations from the draft.all address to gmail addresses.)
>>
>> Speaking as a co-author...
>>
>> It is not at all clear to me that GTSM applies or how it would apply. 
>> There is no requirement that successive SFF be one MPLS hop apart.
>>
>> Yours,
>> Joel
>>
>> On 2/22/19 9:27 AM, Andrew G. Malis wrote:
>>> Carlos,
>>> Looks good on all but one point - I think I see why you're 
>>> referencing GTSM, since packets at the SFC layer would generally be 
>>> one hop away from each other at that layer. Is that correct? However, 
>>> I really don't have sufficient experience with GTSM to craft 
>>> specific text. If you think it's important enough to include, could 
>>> you propose some text for me to include?
>>> Thanks again,
>>> Andy
>>> On Thu, Feb 21, 2019 at 8:41 PM Carlos Pignataro (cpignata) 
>>> <[e-mail] <mailto:[e-mail]> 
>>> <mailto:[e-mail]>> wrote:
>>>    Hi, Andy,
>>>>    On Feb 21, 2019, at 1:06 PM, Andrew G. Malis <[e-mail] 
>>>> <mailto:[e-mail]>
>>>>    <mailto:[e-mail]>> wrote:
>>>>
>>>>    Carlos,
>>>>
>>>>    Many thanks for your review! I'm also including the SFC WG on my
>>>>    reply.
>>>    Thanks for the quick response, and for considering the comments!
>>>    I enjoyed reading this document — please see below.
>>>>
>>>>    Comments inline.
>>>>
>>>>    On Wed, Feb 20, 2019 at 10:58 PM Carlos Pignataro
>>>>    <[e-mail] <mailto:[e-mail]> 
>>>> <mailto:[e-mail]>> wrote:
>>>>
>>>>        Reviewer: Carlos Pignataro
>>>>        Review result: Has Issues
>>>>
>>>>        Reviewer: Carlos Pignataro
>>>>        Review Result: Has Issues
>>>>
>>>>        I have reviewed this document as part of the Operational
>>>>        directorate's
>>>>        ongoing effort to review all IETF documents being processed by
>>>>        the IESG.  These
>>>>        comments were written with the intent of improving the
>>>>        operational aspects of
>>>>        the IETF drafts. Comments that are not addressed in last call
>>>>        may be included
>>>>        in AD reviews during the IESG review.  Document editors and WG
>>>>        chairs should
>>>>        treat these comments just like any other last call comments.
>>>>
>>>>        This document is highly readable, includes very clear textual
>>>>        descriptions, and
>>>>        is very well organized. Easy to read in its simplicity.
>>>>        However, it would
>>>>        benefit from a more explicit connection to the transport encap
>>>>        mechanics from
>>>>        RFC 8300 (e.g., S4, S6.1). Specifically, I'd recommend adding
>>>>        a Figure or an
>>>>        SFF NSH Mapping Table example, to depict and/or exemplify the
>>>>        SFF function.
>>>>
>>>>
>>>>    I'm trying to envision what would make a good figure here. We
>>>>    could add an additional line to Table 1 of RFC 8300 and reference
>>>>    that table:
>>>>
>>>>           +------+------+---------------------+-------------------------+
>>>>           | SPI  | SI   | Next Hop(s)         | Transport 
>>>> Encapsulation |
>>>>           +------+------+---------------------+-------------------------+
>>>>           | 25   | 220  | Label 5467          | MPLS 
>>>>                    |
>>>>           +------+------+---------------------+-------------------------+
>>>>
>>>>    Is that what you had in mind? If not, I'm open to other suggestions.
>>>    If you think it helps, this would be a good addition.
>>>>
>>>>        >From an Operational standpoint, the document seems largely
>>>>        appropriate in terms
>>>>        of dataplane considerations. Some key considerations are
>>>>        explicitly out of
>>>>        scope:
>>>>           The method used by the downstream receiving node to
>>>>        advertise SFF
>>>>           Labels to the upstream sending node is out of scope of this
>>>>        document.
>>>>
>>>>        This really seems to mean that, with the simple definition in 
>>>> this
>>>>        Informational document, interoperable implementations cannot
>>>>        yet exist. If
>>>>        there is no mechanism to advertise the SFF Label or to manage
>>>>        the semantics of
>>>>        this particular label, how will it know? Static configuration,
>>>>        which is not
>>>>        covered anyway, is not in my humble opinion a manageable
>>>>        scalable approach.
>>>>
>>>>
>>>>    Actually, while it is outside the scope of this document, it is
>>>>    within the scope of draft-ietf-bess-nsh-bgp-control-plane, and
>>>>    text is being added to the next revision of that draft to show how
>>>>    it can be used to signal the encapsulation defined here. This was
>>>>    worked out after this draft was forwarded to the IESG, but we can
>>>>    now add a reference to that draft seeing as we'll be doing a
>>>>    post-last-call update.
>>>    I think that will help, as an Informative “one embodiment” type of 
>>> link.
>>>>
>>>>        Title: MPLS Encapsulation For The SFC NSH
>>>>
>>>>        RFC 8300 makes an explicit distinction between the terms
>>>>        'encapsulation' and
>>>>        'transport encapsulation' (see e.g., Figure 1, Section 1.5 5.,
>>>>        and Section 4 of
>>>>        RFC 8300).
>>>>
>>>>        It seems to me that this is the "MPLS Transport Encapsulation
>>>>        for the SFC NSH"
>>>>
>>>>
>>>>    Thanks, we'll fix that.
>>>>
>>>>
>>>>        2.  MPLS Encapsulation Using an SFF Label
>>>>
>>>>        Similarly, "2. MPLS Transport Encapsulation Using an SFF Label"
>>>>
>>>>           The encapsulation is a standard MPLS label stack [RFC3032]
>>>>        with an
>>>>           SFF Label at the bottom of the stack, followed by a NSH as
>>>>        defined by
>>>>           [RFC8300] and the NSH payload.
>>>>
>>>>        Insteadf of "NSH payload" I think "orignal packet" is meant.
>>>>
>>>>
>>>>    RFC 8300 uses both "payload" and "original packet/frame", but the
>>>>    latter more than the former. So we can change "payload" to
>>>>    "original packet/frame".
>>>>
>>>>
>>>>        Also, this encapsulation is Underdefined: What is the value of
>>>>        TTL? TC?
>>>>
>>>>
>>>>    I've been looking back at other related RFCs (such as PW and IP
>>>>    VPN label definitions) and they're also mostly silent on these
>>>>    values. I did find the following in RFC 6073:
>>>>
>>>>        The setting of the TTL of the PW MPLS
>>>>        label is a matter of local policy on the originating PE, but 
>>>> SHOULD
>>>>        be set to 255.
>>>>
>>>>    Regarding the TC, we can follow the example of RFC 6391:
>>>>
>>>>        This document does not define a use for the Traffic Class 
>>>> (TC) field
>>>>        [RFC5462  <https://tools.ietf.org/html/rfc5462>] (formerly 
>>>> known as the Experimental Use (EXP) bits
>>>>        [RFC3032  <https://tools.ietf.org/html/rfc3032>]) in the flow 
>>>> label.  Future documents may define a use for
>>>>        these bits; therefore, implementations conforming to this
>>>>        specification MUST set the TC field to zero at the ingress 
>>>> and MUST
>>>>        ignore them at the egress.
>>>>
>>>>    Do you have any alternative suggestions?
>>>    These two approaches sounds good to me. And Ack to the other
>>>    previous responses.
>>>>
>>>>           Much like a pseudowire label, an SFF Label is allocated by the
>>>>           downstream receiver of the NSH from its per-platform label
>>>>        space.
>>>>
>>>>        A PW Label is more restrictive. RFC 8077 says it MUST be
>>>>        allocated as
>>>>        per-platform:
>>>>
>>>>           egress LSR only.  Note that the PW label must always be at
>>>>        the bottom
>>>>           of the packet's label stack, and labels MUST be allocated
>>>>        from the
>>>>           per-platform label space.
>>>>
>>>>        Is this the case for the SFF Label as well? If so, what is the
>>>>        implication of
>>>>        the MUST? If not, why is it different than other equivalent
>>>>        similar labels?
>>>>
>>>>
>>>>    We can change the text to:
>>>>
>>>>     Much like a pseudowire label, an SFF Label MUST be allocated by
>>>>    the downstream receiver of the NSH from its per-platform label
>>>>    space, since the meaning of the label is identical independent of
>>>>    which incoming interface it is received [RFC3031].
>>>>
>>>    That’s a great improvement.
>>>>
>>>>           2.  Push the SFF Label to identify the desired SFF in the
>>>>        receiving
>>>>               MPLS node.
>>>>
>>>>        TTL value? 1? 2? 255 for GTSM? GTSM RFC 5082 could be used here.
>>>>
>>>>
>>>>    As I noted above, 255, although I used RFC 6073 as my source
>>>>    rather than 5082. We'll add that here as well.
>>>>
>>>    Sounds good.
>>>    These protocols use 5082 in one form or another:
>>> https://datatracker.ietf.org/doc/rfc5082/referencedby/
>>>>
>>>>        4.  Operations, Administration, and Maintenance (OAM)
>>>>        Considerations
>>>>
>>>>           OAM at the SFC Layer is handled by SFC-defined mechanisms
>>>>        [RFC8300].
>>>>           However, OAM may be required at the MPLS transport layer. 
>>>>         If so,
>>>>           then standard MPLS-layer OAM mechanisms such as the Generic
>>>>           Associated Channel [RFC5586] label may be used.
>>>>
>>>>        RFC 5586 is _not_ an OAM mechanism. It is an associated
>>>>        channel creation
>>>>        mechanism, over which OAM could be carried.
>>>>
>>>>        Thus, what traditional MPLS OAM can be carried here? Things
>>>>        like RFC 4379 / RFC
>>>>        8029 would need the definition of an SFF Label FEC (which does
>>>>        not exist).
>>>>        Which other one? IP/ICMP seems of very limited value.
>>>>
>>>>
>>>>    That's a good point about RFC 5586. The intention is that the MPLS
>>>>    OAM would be at the transport label layer above the SFF label, so
>>>>    most any MPLS-layer OAM would be applicable. So how about
>>>>    rewording to make that more clear:
>>>>
>>>>    OAM at the SFC Layer is handled by SFC-defined mechanisms
>>>>    [RFC8300]. However, OAM may be required at the MPLS transport
>>>>    layer.  If so, then standard MPLS-layer OAM mechanisms may be used
>>>>    at the transport label layer (the labels above the SFF label).
>>>    Looks good to me, thank you.
>>>>
>>>>
>>>>        6.  Security Considerations
>>>>
>>>>        Have you considered the use of GTSM?
>>>>
>>>>
>>>>    No, we hadn't. Can you point me to any examples of GTSM being used
>>>>    in an MPLS or PW context?
>>>    Yes, see above.
>>>>
>>>>        8.  References
>>>>
>>>>           [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service
>>>>        Function
>>>>                      Chaining (SFC) Architecture", RFC 7665,
>>>>                      DOI 10.17487/RFC7665, October 2015,
>>>>                      <https://www.rfc-editor.org/info/rfc7665
>>>>        <https://www..rfc-editor.org/info/rfc7665>>.
>>>>
>>>>        SHould RFC 7665 be Normative? It defines the "SFF" which is
>>>>        quite central to
>>>>        understanding this document.
>>>>
>>>>
>>>>    Good point. It was there because 7665 is an Informational RFC, but
>>>>    RFC 8067 does allow normative references to informational RFCs, so
>>>>    I'll move it.
>>>    Thank you.
>>>>
>>>>        Other Nits and Editorials:
>>>>
>>>>           SFF Labels are similar to other service labels at the
>>>>        bottom of an
>>>>           MPLS label stack that denote the contents of the MPLS
>>>>        payload being
>>>>           other than IP, such as a layer 2 pseudowire, an IP packet
>>>>        that is
>>>>           routed in a VPN context with a private address, or an Ethernet
>>>>           virtual private wire service.
>>>>
>>>>        This says "being other than IP, such as IP", which seems to be
>>>>        self-contradictory :-)
>>>>
>>>>    :-)
>>>>
>>>>    How about we change "other than IP," to "other than a normally
>>>>    routed IP packet”,
>>>    That would disambiguate it.
>>>    Thanks again.
>>>    To me, the control plane / advertisement was the most important
>>>    operationally-relevant comment.
>>>    Thanks,
>>>    Carlos.
>>>>
>>>>    Thanks again,
>>>>    Andy
>