Hi all,
I would like to raise the following issue with regard to
draft-ietf-pwe3-gal-in-pw: controversy vs.
draft-ietf-pwe3-fat-pw with regard to bottom-of-stack position.
As stated in the Introduction, this draft removes the restriction imposed by RFC 5586 on usage of Generic Associated Channel Label (GAL) in PWs. The corresponding text Section 4.2 of RFC 5586 states:
In MPLS-TP, the GAL MUST be used with packets on a G-ACh on LSPs, Concatenated Segments of LSPs, and with Sections, and MUST NOT be
used with PWs. It MUST always be at the bottom of the label stack (i.e., S bit set to 1).
draft-ietf-pwe3-gal-in-pw proposed to replace the original text in RFC 5586 with the following
In MPLS-TP, the GAL MUST be used with packets on a G-ACh on LSPs, Concatenated Segments of LSPs, and with Sections, and MAY be used
with PWs. It MUST always be at the bottom of the label stack (i.e., S bit set to 1).
I.e., while removing this restriction of 5586, it does not modify its requirement for the GAL being always at the bottom of the label stack.
At the same draft-ietf-pwe3-fat-pw (currently also in the IESG review) reserves the bottom of the PW stack for the PW flow labels, e.g., in Section 1.1:
This document describes a method of adding an additional label stack entry (LSE) at the bottom of stack in order to facilitate the
load balancing of the flows within a PW over the available ECMPs.
One could argue that draft-ietf-pwe3-gal-in-pw only applies to MPLS-TP pseudowires, and that MPLS-TP does not use ECMP. IMHO and FWIW,
such an argument, were it presented, would be highly problematic, because:
1.
RFC 5960 (which defines the MPLS-TP data plane) did not define any differences between the PW data plane in IP/MPLS and MPLS-TP.
2.
One of the most popular scenarios for using multi-segment pseudowires is the case when an edge-to-edge service emulation crosses multiple IP/MPLS and MPLS-TP domains. In these scenarios, the flow label of draft-ietf-pwe3-fat-pw
(inserted by a flow-aware T-PE at the edge of an IP/MPLS domain) would potentially compete with GAL (inserted by a T-PE at the edge of an MPLS-TP domain, e.g., for relying a PW status message that it has received over a Targeted LDP session from the IP/MPLS
domain to a static PW status message to cross the MPLS-TP domain) for the bottom-of-stack position.
The issue I am raising Is not new. It has been actively discussed on the PWE3 mailing list with regard to adoption of draft-nadeau-pwe3-vccv-2 as a WG document, with arguments for both the flow label and GAL
taking the bottom-of-the-stack position. But, to the best of my understanding, consensus on this issue has not been reached.
Hopefully this comment will be useful.
Regards,
Sasha
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