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Re: [PWE3] IETF Last Call comment on draft-ietf-pwe3-gal-in-pw
2011-08-18 10:31:51
The solution is quite simple:
"Flow Labels MUST not be used in an MPLS-TP environment."
Luca
On 08/16/11 21:46, Alexander Vainshtein wrote:
Pablo,
Sorry, but I think you're wrong. Only T-PE can insert the flow label
(because only T=PE can be "flow-aware"). S-PE simply performs swap on
PW label.
Regards,
Sasha
------------------------------------------------------------------------
*From:* Pablo Frank [pabloisnot(_at_)gmail(_dot_)com]
*Sent:* Wednesday, August 17, 2011 12:17 AM
*To:* Alexander Vainshtein
*Cc:* ietf(_at_)ietf(_dot_)org; mpls(_at_)ietf(_dot_)org; Vladimir Kleiner;
Idan Kaspit;
Mishael Wexler; pwe3; Oren Gal; John Shirron; Rotem Cohen
*Subject:* Re: [PWE3] IETF Last Call comment on draft-ietf-pwe3-gal-in-pw
I think it's okay because as the PW crosses the ECMP-enabled IP/MPLS
domain in the middle segment, you're no longer in an MPLS-TP
environment and so the GAL is not required to be BOS. During that
middle segment, the PW flow label would be placed below the GAL and
above the GACh. It gets removed when it hits the S-PE that switches
you back into the MPLS-TP environment. In other words, whether you're
in an MPLS-TP environment is determined segment by segment in a MS-PW.
Pablo
On Tue, Aug 16, 2011 at 1:02 PM, Alexander Vainshtein
<Alexander(_dot_)Vainshtein(_at_)ecitele(_dot_)com
<mailto:Alexander(_dot_)Vainshtein(_at_)ecitele(_dot_)com>> wrote:
Hi all,
After having sent out my comments I've noticed that the specific
example to illustrate the need to combine GAL and "flow label" was
inaccurate.
A more relevant example would look like following (I do not
include a diagram, but it can be easily provided if necessary)
1. A MS-PW:
* Starts at an S-PE that resides at the edge of an MPLS-TP
domain (no ECMP)
* Crosses this domain and enters an IP/MPLS domain with ECMP
enabled using a T-PE that resides at the age of these two
domains
* Leaves this domain and enters a 2nd MPLS-TP domain (using
the 2nd T-PE)
* Terminates on another S-PE at the edge of the 2nd MPLS-TP
domain
2. The operator intends to improve traffic distribution in the
IP/MPLS domain, hence he enables insertion and discard of
"flow labels" at the two S-PEs. Note that:
* This does not violate the MPLS-TP restriction on ECMP:
ECMP does not happen in he MPLS-TP domains
* T-PEs do not even have to be aware of flow labels
3. The operator also intends to operate some end-to-end OAM for
this MS-PW using "GAL-in-PW". This results in a conflict since
both GAL and "flow label" are defined (in the corresponding
drafts) as bottom of stack.
IMHO this describes a realistic scenario where the two drafts are
in controversy.
Regards,
Sasha
------------------------------------------------------------------------
*From:* mpls-bounces(_at_)ietf(_dot_)org
<mailto:mpls-bounces(_at_)ietf(_dot_)org>
[mpls-bounces(_at_)ietf(_dot_)org
<mailto:mpls-bounces(_at_)ietf(_dot_)org>] On Behalf
Of Alexander Vainshtein [Alexander(_dot_)Vainshtein(_at_)ecitele(_dot_)com
<mailto:Alexander(_dot_)Vainshtein(_at_)ecitele(_dot_)com>]
*Sent:* Tuesday, August 16, 2011 4:26 PM
*To:* ietf(_at_)ietf(_dot_)org <mailto:ietf(_at_)ietf(_dot_)org>
*Cc:* mpls(_at_)ietf(_dot_)org <mailto:mpls(_at_)ietf(_dot_)org>;
Vladimir Kleiner; Idan
Kaspit; Mishael Wexler; pwe3; Oren Gal; John Shirron; Rotem Cohen
*Subject:* [mpls] IETF Last Call comment on draft-ietf-pwe3-gal-in-pw
Hi all,
I would like to raise the following issue with regard to
draft-ietf-pwe3-gal-in-pw
<http://datatracker.ietf.org/doc/draft-ietf-pwe3-mpls-tp-gal-in-pw/?include_text=1>:
controversy vs. draft-ietf-pwe3-fat-pw
<http://datatracker.ietf.org/doc/draft-ietf-pwe3-fat-pw/?include_text=1>
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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