Robert, Inband is one way -- it simplifies things at the cost of packet growth. Out-of-band (i.e., signaling) is another way – it adds complexity but doesn’t grow the data plane packets. Congruent out-of-band is a variation on out-of-band that may partially reduce out-of-band complexity. I think many opinions on the thread have been that all are in scope, architecturally. Draft-zhang-sfc-sch (I am a co-author), for example, defines inband metadata explicitly, but states in text that out-of-band is possible, too. I’m sure this will be a topic of discussion when we start discussing control plane requirements.
Ron From: sfc [mailto:[email protected]] On Behalf Of Robert Raszuk Sent: Friday, July 18, 2014 3:27 AM To: Xuxiaohu; [email protected] Cc: [email protected]; <[email protected]> Subject: Re: [sfc] [spring] How to carry metadata/context in an MPLS packet All, Is the idea of using data plane to carry complete metadata is "the way" or "a way" of approaching the problem ? Has this been already discussed ? I would rather consider to carry metadata in control plane and only attach a reference_id (and only when it is needed) to the data plane. Rgs, R. On Fri, Jul 18, 2014 at 3:58 AM, Xuxiaohu <[email protected]<mailto:[email protected]>> wrote: 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 _______________________________________________ spring mailing list [email protected]<mailto:[email protected]> https://www.ietf.org/mailman/listinfo/spring
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