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RE: [PWE3] IETF Last Call comment on draft-ietf-pwe3-gal-in-pw
2011-08-18 10:34:10
Luca and all,
I have not found the statement you've proposed in draft-ietf-pwe3-fat-pw-06.
Instead, it contans the following text in Section 8.5 " Applicability to
MPLS-TP":
<quote>
The flow aware transport of a PW reorders packets, therefore MUST NOT be
deployed in a network conforming to the MPLS-TP unless these integrity
requirements
specified in the SLA can be satisfied.
<end quote>
(In the -07 version this text is repeated but followed by an incomplete
statement " In a" immediately followed by the heading of Section 8.6. Since
this addition is difficult to parse, I will ignore it for the moment.)
IMHO and FWIW this means that prohibition on using flow aware PW in MPLS-TP
environments is conditional on meeting specific SLA requirements for the
service. So I think that the use case I've presented still holds.
Please note also that, regardless of the restriction in draft-ietf-pwe3-fat-pw,
be it conditional or absolute, usage of flow labels in an MPLS-TP domain would
be perfectly safe if ECMP (i.e., hashing of the label stack and taking one of
multiple NHLFEs for the given incoming label in the ILM) were not used in this
domain, e.g., by associating exactly one ILM entry with each incoming label in
the ILM. And since MPLS-TP is supposed to carry not just PW clients but also IP
ones, I would expect that this would be the case in any MPLS-TP deployment.
I also think that releasing one restriction (on using GAL in PWs) at the
expense of making another, conditional one (on usage of flow labels in MPLS-TP
environments) absolute is not the most appropriate method for resolving
technical issues. IMHO and FWIW better way to resolve the problem would be by:
- releasing the bottom-of-stack requirement on GAL
- making use of the statement in RFC 5586 that if GAL is encountered in a
packet then G-ACh header MUST be present immediately after the bottom of the
label stack (and not immediately after GAL)
- specifying that ECMP on labeled packets MUST ignore reserved labels.
I think that these considerations have been presented already in the discussion
on draft-nadeau-pwe-vccv-2.
Of course it would be even better if we could agree on transition to universal
usage of the CW and VCCV Type 1 in PWs. But this is a different story.
Regards,
Sasha
____________________________________
From: Luca Martini [lmartini(_at_)cisco(_dot_)com]
Sent: Wednesday, August 17, 2011 9:58 PM
To: Alexander Vainshtein
Cc: Pablo Frank; mpls(_at_)ietf(_dot_)org; ietf(_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
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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