Re: [mpls] RtgDir review: draft-ietf-mpls-sfc-04.txt

Mach Chen <mach.chen@huawei.com> Thu, 10 January 2019 07:12 UTC

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From: Mach Chen <mach.chen@huawei.com>
To: "adrian@olddog.co.uk" <adrian@olddog.co.uk>, "rtg-ads@ietf.org" <rtg-ads@ietf.org>
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Thread-Topic: RtgDir review: draft-ietf-mpls-sfc-04.txt
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Date: Thu, 10 Jan 2019 07:10:19 +0000
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Subject: Re: [mpls] RtgDir review: draft-ietf-mpls-sfc-04.txt
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Hi Adrian,

I am still thinking that this may not just be an implementation problem. Looking though the draft again, I feel that the draft will benefit by adding more clarification text. 

For example, I found the following text (Section 6). This paragraph seems the only "formal" processing rule on the SPI/SI label for the swapping case. 

"  
 o  When a component of the SFC system processes a packet it uses the
      SPI Label to identify the SFP and the SI Label to determine to
      which SFF or instance of an SF (an SFI) to deliver the packet.
      Under normal circumstances (with the exception of branching and
      reclassification - see [I-D.ietf-bess-nsh-bgp-control-plane]) the
      SPI Label value is preserved on all packets.  The SI Label value
      is modified by SFFs and through reclassification to indicate the
      next hop along the SFP.
".
What does the component refer to(classifier, SFF  or SF) ? Can it be an SF? . If so, what is the processing rule?  It seems imply that the SF should be SPI/SI aware, the SPI/SI cannot be swapped, popped off. Otherwise the mechanisms defined in this document will not work. 

The last sentence says that "The SI Label value is modified by SFFs", when does a SFF modify the SI Label value?  For example, before sending the packet to an SF or after being received from an SF?  This is not clear in the current draft but. The example section gives some hint on this, that's one of the reasons that I think some of the content should be defined as formal procedures.

In addition, section 8:
"
When an SFF receives a packet containing an MPLS label stack, it
   checks whether it is processing an {SFP, SI} label pair for label
   swapping or a {context label, SFI index} label pair for label
   stacking.  It then selects the appropriate SFI to which to send the
   packet.  When it receives the packet back from the SFI, it has four
   cases to consider.

   o  If the current hop requires an {SFP, SI} and the next hop requires
      an {SFP, SI}, it sets the SI label to the SI value of the current
      hop, selects an instance of the SF to be executed at the next hop,
      and tunnels the packet to the SFF for that SFI.

   o  If the current hop requires an {SFP, SI} and the next hop requires
      a {context label, SFI label}, it pops the {SFP, SI} from the top
      of the MPLS label stack and tunnels the packet to the SFF
      indicated by the context label.
"

How does an SFF determine "the current hop requires an {SFP, SI} and the next hop requires an {SFP, SI}" or "the current hop requires an {SFP, SI} and the next hop requires a {context label, SFI label} ?

Should the "it sets the SI label to the SI value of the current hop, selects an instance of the SF to be executed at the next hop" be changed to " it selects an instance of the SF to be executed at the next hop, sets the SI label to the SI value of the next hop"? The SI label should be set to next hop, right?

Best regards,
Mach

> -----Original Message-----
> From: Adrian Farrel [mailto:adrian@olddog.co.uk]
> Sent: Tuesday, January 08, 2019 5:21 AM
> To: Mach Chen <mach.chen@huawei.com>;; rtg-ads@ietf.org
> Cc: rtg-dir@ietf.org; mpls@ietf.org; draft-ietf-mpls-sfc.all@ietf.org
> Subject: RE: RtgDir review: draft-ietf-mpls-sfc-04.txt
> 
> Hmmm, I think we may disagree on this ☹
> 
> I'd ask the WG chairs and AD (especially as the review is for the benefit of the
> AD) to guide the authors.
> 
> IMHO it is not the place of an IETF spec to tell people how to implement
> unless the implementation changes the resultant behaviour on the wire.
> From my perspective, this document tells the implementer what black-box
> behaviour is expected: i.e. what result receiving a packet should have, and
> what packets to generate.
> 
> We could describe how to implement, but I can't see how that would be
> normative. And I think there are plenty of ways to achieve the required
> behaviour in an optimised way (for example, performing dual-label access).
> We provided section 14 to give guidance that implementation using existing
> hardware could be possible, but we didn't want to imply that this was *the*
> way to implement.
> 
> Best,
> Adrian
> 
> -----Original Message-----
> From: Mach Chen <mach.chen@huawei.com>;
> Sent: 07 January 2019 03:52
> To: adrian@olddog.co.uk; rtg-ads@ietf.org
> Cc: rtg-dir@ietf.org; mpls@ietf.org; draft-ietf-mpls-sfc.all@ietf.org
> Subject: RE: RtgDir review: draft-ietf-mpls-sfc-04.txt
> 
> Hi Adrian,
> 
> Thanks for your reply!
> 
> See some response inline...
> 
> > -----Original Message-----
> > From: Adrian Farrel [mailto:adrian@olddog.co.uk]
> > Sent: Sunday, January 06, 2019 5:03 AM
> > To: Mach Chen <mach.chen@huawei.com>;; rtg-ads@ietf.org
> > Cc: rtg-dir@ietf.org; mpls@ietf.org; draft-ietf-mpls-sfc.all@ietf.org
> > Subject: RE: RtgDir review: draft-ietf-mpls-sfc-04.txt
> >
> > Hi Mach,
> >
> > Thanks for your careful review.
> >
> > > Minor Issues:
> > > *	Section 13 says:
> > > "
> > >   b.  When the packet arrives at SFFa it strips any labels associated
> > >       with the tunnel that runs from the classifier to SFFa.  SFFa
> > >       examines the top labels and matches the SPI/SI to identify that
> > >       the packet should be forwarded to SFa.  The packet is forwarded
> > >       to SFa unmodified.
> > >
> > >   c.  SFa performs its designated function and returns the packet to
> > >       SFFa.
> > >
> > >   d.  SFFa modifies the SI in the lower label stack entry (to 254) and
> > >       uses the SPI/SI to look up the forwarding instructions.  It sends
> > >       the packet with two label stack entries:
> > >"
> > > From above text, it seems that an SFF will have different process
> > >for a
> > packet that
> > > has the same SPI/SI.
> > >  1) if a packet is received from a previous SFF, it will match the
> > > SPI/SI
> > and determine
> > >  to where the packet will be sent.
> > >  2) if a packet is received from an SF, it will directly modify the
> > > inner
> > label, and then
> > > by matching the new SPI/SI to determine to where the packet will be
> sent.
> > >
> > > Is this the intention ? If so, how does an SFF determine whether a
> > > packet
> > is from an
> > > SFF or from SF?
> >
> > You have understood the processing correctly.
> > In fact, the same situation exists with NSH processing: the SFF is
> > expected to "know" whether the packet has arrived from the previous SFF
> or from the SF.
> >
> > There are probably two answers:
> > Firstly, it is entirely possible that the SFF and SF are co-resident.
> > That is, they form part of the same implementation, possibly being
> separated by an API.
> > In this case, of course, the question does not arise.
> > Secondly, it is reasonable to expect that the SFs are attached to the
> > SFF by dedicated tunnels or ports (indeed, SFs are port-attached in
> > the legacy service function world) so that a packet arriving on a line
> > interface (from another SFF) is easily distinguished from one that
> > arrives from an SF. (You might think about port-attached L3VPNs to help
> clarity how this could work).
> 
> If so, it's better to add some text to clarify this. Given the current text, it
> implies that it could be determined just by matching the SPI/SI.
> 
> >
> > > In addition, how does an SFF apply the swap and/or pop operations here.
> > E.g., when an SFF looks up the SPI,
> > > it matches an item in a local table; what operation (swap or pop)
> > > will
> > apply to the SPI label? According to " The
> > > packet is forwarded to SFa unmodified." It seems implies that there
> > > is no
> > any operation will be applied. If so,
> > > this will not align with the MPLS.
> >
> > I think you should also look at this in the manner of a L3VPN. That
> > is, the top label is popped to derive the context, the forwarding
> > label is looked up within the context and swapped, and the context
> > label is re-imposed as the packet is forwarded.
> >
> > Of course, there are optimisations on how an SFF might implement this,
> > but we don't need to describe those. What we know is that the VPN-like
> > approach can be made to work with existing PEs.
> 
> As above, it's better to add some text to make it clearer.
> 
> >
> > > If the above issues exist, the " context/SF" case has the similar issues.
> >
> > Hope I have explained why this is not an issue.
> >
> > Now, we could debate why we have not made these answers more clear in
> > the text.
> > The first thing is that we are not trying to make any changes to the
> > architecture used for NSH, so we did not feel we should go into
> > explanations about how the SF/SFF relationship works.
> > Additionally, we did not feel that (beyond the short section 14) we
> > should tell people how to implement. The purpose of the document is to
> > describe wire formats.
> 
> IMHO, it may not be enough if just describing the wire formats, the
> operations/process on the "formats" are necessary to make it an
> implementable and interoperable protocol.
> 
> Although Section 13 is listed as a worked example, I personally think that
> they are necessary procedures. I'd suggest to change it to more "formal"
> section, e.g., Procedures.
> 
> Best regards,
> Mach
> 
> >
> > Best,
> > Adrian