RE: 6man w.g. last call for <draft-ietf-6man-segment-routing-header-11.txt>

Ron Bonica <rbonica@juniper.net> Wed, 11 April 2018 19:34 UTC

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From: Ron Bonica <rbonica@juniper.net>
To: Brian E Carpenter <brian.e.carpenter@gmail.com>, stefano previdi <stefano@previdi.net>
CC: IPv6 List <ipv6@ietf.org>, Bob Hinden <bob.hinden@gmail.com>
Subject: RE: 6man w.g. last call for <draft-ietf-6man-segment-routing-header-11.txt>
Thread-Topic: 6man w.g. last call for <draft-ietf-6man-segment-routing-header-11.txt>
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Date: Wed, 11 Apr 2018 19:34:24 +0000
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Folks,

Inline.......

> -----Original Message-----
> From: Brian Carpenter <becarpenter46@gmail.com> On Behalf Of Brian E
> Carpenter
> Sent: Tuesday, April 10, 2018 10:48 PM
> To: stefano previdi <stefano@previdi.net>; Ron Bonica
> <rbonica@juniper.net>
> Cc: IPv6 List <ipv6@ietf.org>; Bob Hinden <bob.hinden@gmail.com>
> Subject: Re: 6man w.g. last call for <draft-ietf-6man-segment-routing-
> header-11.txt>
> 
> On 09/04/2018 20:09, stefano previdi wrote:
> ...
> >> By contrast, draft-ietf-6man-segment-routing header defines the
> Segments Left as follows:
> >>
> >> "Defined in [RFC8200], it contains the index, in the Segment List, of the
> next segment to inspect.  Segments Left  is decremented at each segment.
> 
> Maybe you should say:
>  Segments Left is decremented at each segment after processing the
> segment list and  before forwarding the packet.

I don't think that this is always the intent. See Section 4.13 of draft-filsfils-spring-srv6-network-programming-04.

> 
> >> Therefore, if a packet arrives at a node and the following conditions are
> true, the node processes Segment[0]:
> >>
> >> - The packets IPv6 Destination address is local to the node
> >> - The packet contains a Routing Extension Header
> >> - The SL value in the Routing Extension Header is equal to 0
> >
> >
> > SL==0 doesn’t imply any processing of the SRH. Both END and END.x
> functions starts with “IF SegmentsLeft > 0”.
> 
> IMHO that needs to be stated normatively in the spec, even if it seems
> obvious.
> The section you are referring to only says:
> "In this section we review two functions that may be associated to a
> segment:"
> and cites an informational reference (draft-ietf-spring-segment-routing).
> So the junior programmer asked to actually write some network processor
> code doesn't know that she MUST NOT process the SRH, because it's only a
> lower case "may".
> 
>    Brian