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

"Adrian Farrel" <adrian@olddog.co.uk> Sat, 05 January 2019 21:03 UTC

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Reply-To: <adrian@olddog.co.uk>
From: "Adrian Farrel" <adrian@olddog.co.uk>
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>
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Date: Sat, 5 Jan 2019 21:03:05 -0000
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Subject: Re: [mpls] RtgDir review: draft-ietf-mpls-sfc-04.txt
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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).

> 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.

> 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.

Best,
Adrian