Re: [mpls] Opsdir last call review of draft-ietf-mpls-sfc-encapsulation-02
"Joel M. Halpern" <jmh@joelhalpern.com> Fri, 22 February 2019 16:43 UTC
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To: "Andrew G. Malis" <agmalis@gmail.com>, "Carlos Pignataro (cpignata)" <cpignata@cisco.com>
Cc: mpls <mpls@ietf.org>, "ops-dir@ietf.org" <ops-dir@ietf.org>, "draft-ietf-mpls-sfc-encapsulation.all@ietf.org" <draft-ietf-mpls-sfc-encapsulation.all@ietf.org>, IETF Discussion <ietf@ietf.org>, "sfc@ietf.org" <sfc@ietf.org>
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From: "Joel M. Halpern" <jmh@joelhalpern.com>
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Subject: Re: [mpls] Opsdir last call review of draft-ietf-mpls-sfc-encapsulation-02
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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) > <cpignata@cisco.com <mailto:cpignata@cisco.com>> wrote: > > Hi, Andy, > >> On Feb 21, 2019, at 1:06 PM, Andrew G. Malis <agmalis@gmail.com >> <mailto:agmalis@gmail.com>> 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 >> <cpignata@cisco.com <mailto:cpignata@cisco.com>> 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 >
- [mpls] Opsdir last call review of draft-ietf-mpls… Carlos Pignataro
- Re: [mpls] Opsdir last call review of draft-ietf-… Andrew G. Malis
- Re: [mpls] Opsdir last call review of draft-ietf-… Carlos Pignataro (cpignata)
- Re: [mpls] Opsdir last call review of draft-ietf-… Andrew G. Malis
- Re: [mpls] Opsdir last call review of draft-ietf-… Joel Halpern
- Re: [mpls] Opsdir last call review of draft-ietf-… Joel M. Halpern
- Re: [mpls] Opsdir last call review of draft-ietf-… Carlos Pignataro (cpignata)
- Re: [mpls] Opsdir last call review of draft-ietf-… Carlos Pignataro (cpignata)
- Re: [mpls] Opsdir last call review of draft-ietf-… Joel M. Halpern