Re: [sfc] How to carry metadata/context in an MPLS packet

> -----Original Message-----
> From: l.wood@surrey.ac.uk [mailto:l.wood@surrey.ac.uk]
> Sent: Friday, July 18, 2014 11:22 AM
> To: Xuxiaohu; mpls@ietf.org
> Cc: spring@ietf.org; sfc@ietf.org
> Subject: RE: How to carry metadata/context in an MPLS packet
> 
> No, my "why" is "why have metadata in the MPLS header at all"?

You could choose to carry the metadata context instead of the metadata itself in the data packet (See section 3.5 of the following draft (http://tools.ietf.org/html/draft-rijsman-sfc-metadata-considerations-00#page-8)).

> Why does this have to be done inband?

It doesn't have to be done inband. However, since In-band marking (see section 3.1 of the following draft (http://tools.ietf.org/html/draft-rijsman-sfc-metadata-considerations-00#page-8)) ) is one of the approaches of metadata sharing, I'm just considering how to realize that in-band approach in the MPLS case.

Best regards,
Xiaohu

> I am questioning the basis for doing this. There is no "must" here.
> 
> Lloyd Wood
> http://about.me/lloydwood
> ________________________________________
> From: Xuxiaohu <xuxiaohu@huawei.com>
> Sent: Friday, 18 July 2014 1:17 PM
> To: Wood L  Dr (Electronic Eng); mpls@ietf.org
> Cc: spring@ietf.org; sfc@ietf.org
> Subject: RE: How to carry metadata/context in an MPLS packet
> 
> The presence of metadata within an MPLS packet must be indicated in the
> encapsulation. I think that's the "why" you may want to know.
> 
> Best regards,
> Xiaohu
> 
> > -----Original Message-----
> > From: l.wood@surrey.ac.uk [mailto:l.wood@surrey.ac.uk]
> > Sent: Friday, July 18, 2014 11:11 AM
> > To: Xuxiaohu; mpls@ietf.org
> > Cc: spring@ietf.org; sfc@ietf.org
> > Subject: RE: How to carry metadata/context in an MPLS packet
> >
> > How? A better question is "why?"
> >
> > What has to be done in MPLS that cannot be done outside it?
> >
> > Lloyd Wood
> > http://about.me/lloydwood
> > ________________________________________
> > From: mpls <mpls-bounces@ietf.org> on behalf of Xuxiaohu
> > <xuxiaohu@huawei.com>
> > Sent: Friday, 18 July 2014 11:58 AM
> > To: mpls@ietf.org
> > Cc: <spring@ietf.org>; sfc@ietf.org
> > Subject: [mpls] How to carry metadata/context in an MPLS packet
> >
> > Hi all,
> >
> > I'm now considering how to carry metadata/context in an MPLS packet. I
> > just noticed that draft-guichard-mpls-metadata-00
> > (http://tools.ietf.org/html/draft-guichard-mpls-metadata-00#page-6)
> > proposes a way to carry metadata/context in an MPLS packet (see below):
> >
> > "3.  Metadata Channel Header Format
> >
> >    The presence of metadata within an MPLS packet must be indicated in
> >    the encapsulation.  This document defines that the G-ACh Generic
> >    Associated Channel Label (GAL) [RFC5586] with label value 13 is
> >    utilized for this purpose.  The GAL label provides a method to
> >    identify that a packet contains an "Associated Channel Header (ACH)"
> >    followed by a non-service payload.
> >
> >    [RFC5586] identifies the G-ACh Generic Associated Channel by setting
> >    the first nibble of the ACH that immediately follows the bottom label
> >    in the stack if the GAL label is present, to 0001b.  Further
> >    [RFC5586] expects that the ACH not be used to carry user data
> >    traffic.  This document proposes an extension to allow the first
> >    nibble of the ACH to be set to 0000b and, when following the GAL, be
> >    interpreted using the semantics defined in
> >    [I-D.guichard-metadata-header] to allow metadata to be carried
> >    through the G-ACh channel."
> >
> > However, it seems that the special usage of the GAL as mentioned above
> > still conflicts with the following statement quoted from [RFC5586]:
> >
> > "  The GAL MUST NOT appear in the label stack when transporting normal
> >    user-plane packets.  Furthermore, when present, the GAL MUST NOT
> >    appear more than once in the label stack."
> >
> > I wonder whether the special usage of the GAL as proposed in the above
> > draft would result in any backward compatibility issue. In addition, I
> > wonder whether it's worthwhile to reconsider the possibility of
> > introducing a Protocol Type (PT) field immediately after the bottom of
> > the MPLS label stack. With such PT field, any kind of future MPLS
> > payload (e.g., metadata header or NSH) can be easily identified.
> >
> > Best regards,
> > Xiaohu
> >
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