Subject: [PWE3] PWE3 Liaison Response: T-MPLS

From: Stewart Bryant <[email-masked]>
Date: Sat, 03 Jun 2006 11:04:39 +0100

To: ITU-T Study Group 15
       Greg Jones [email-masked]
       [email-masked]

Cc: IETF [email-masked]
       Stephen Trowbridge [email-masked]
       Ghani Abbas [email-masked]
       Mark Jones [email-masked]
       Malcolm Betts [email-masked]
       Yoichi Maeda [email-masked]
       Scott Bradner <[email-masked]>
       Adrian Farrel <[email-masked]>
       Jari Arkko <[email-masked]>
       Mark Townsley <[email-masked]>
       Danny McPherson <[email-masked]>
       PWE3 mailing list <[email-masked]>

From: IETF PWE3 Working Group

Response contact: Stewart Bryant [email-masked]

Technical contact: Stewart Bryant [email-masked]

Purpose: Information

Subject: Response to your liaison on T-MPLS of May 4 and April 2 2006

Thank you for your liaisons of May 4 and April 2 and for the kind
invitation to the interim meeting that 19-23 June in Ottawa.

We have studied the consented documents from the April liaison
and the further clarifications in the May liaisons.

We have ensured that IETF technical experts will participate
in the Ottawa meeting.

You are already in receipt of comments from the IETF MPLS-WG
with which we fully concur.

These comments refer to G.8110.1/Y.1370.1 Appendix 1:
Functional Model for describing PWE3 and MPLS Network
Interworking

G.8110.1 states:
"The IETF PWE3 work group has developed an MPLS transport concept,
know as pseudowires."

Later G.8110.1 states:
"The PSN can be implemented by utilizing a MPLS label switched
network."

PWE3 has developed methods of transporting services over many
different PSNs, one of which is MPLS.

G.8110.1 states:
"PWE3 is intended to provide only the minimum necessary
functionality to emulate a wire with the required degree
of faithfulness for the given service definition."

The purpose behind the wording "provide only the minimum"
was to prevent over engineering. In many cases, PWE3
provides more than just the minimum necessary functionality.
We think that to put this quote from RFC3985 in it's proper
context you should note that in our view an applicability
statement MUST always be provided to clearly describe the
extent of the emulation, and that in some cases more
than one emulation method may be needed.

G.8110.1 states:
"A four octet Control Word is added to the MPLS payload field for
some encapsulations."

The control word is DEFINED for all payload types. Use is
optional for HDLC/PPP, Ethernet and some ATM modes, but
is REQUIRED in all other cases.

G.8110.1 states:
"these processes are called "Native Signal Processing (NSP)"

NSP stands for Native Service Processing

G.8110.1 states:
"The NSP function process the Control Word."

The NSP does handle the flags (when there are flags
defined). However the PW-layer processing handles the
sequencing, FRG, etc.

G.8110.1 states:
"IETF has defined pseudowire encapsulations for Ethernet, Frame
Relay, ATM, PPP, PDH, and SDH."

The PPP emulation also emulates HDLC, and the SDH design
also supports SONET. We also have a design for Fibre-
Channel and the IETF AVT WG has a design for header
compressed voice samples.

G.8110.1 states:
"The Forwarder function is called an Interworking Function
(IWF)"

It is not clear that IWF and forwarder are identical in scope
and purpose.

G.8110.1 states:
In Figure 9 Pseudowire Model G.8110.1 shows the PW as a trail
with a T-MPLS trail termination function. This termination
is defined here as injecting Y.1710 OAM.

The IETF PWE3 specifications use VCCV as the PW OAM.

G.8110.1 states:
"Pseudowires can be switched by a Subnetwork Connection
(SNC).  This creates in PWE3 terminology Multi-Segment
Pseudowires (MS-PW) as defined in the bibliography
reference [1]."

Bibliography reference [1] is RFC 3985, which was
published prior to our work on MS-PWs. The correct
reference is reference [2].

G.8110.1 states:
"When the MPLS server layer is an SDH layer network,
an SDH path layer may be used instead of the
pseudowire PSN transport tunnel. This approach has
been called "dry martini" as defined in the
bibliography reference [2]."

The correct reference is reference [3]. We must however
caution you that this work has not been adopted as an
IETF PWE3 working group item. The draft was an
individual submission which has now expired under the
IETF policy of expiring such drafts after six months.

Within the scope material you state:

"a T-MPLS network will not peer directly with an
IP/MPLS network".

The data plane behavior of the one currently
supported adaptation ("EVC") is intended to be
that of an Ethernet pseudowire as specified in
Y.1415 and RFC4448.  Thus it is possible for
"IP/MPLS" and T-MPLS networks to interoperate
at the level of the pseudowire, e.g. a multi-segment
PW could span adjacent "IP/MPLS" and T-MPLS networks
connected by an S-PE.

G.8110.1 states:

"From an OAM perspective, pseudowires provide
service-specific status and alarm management".

However PW status signaling and VCCV are
agnostic to the native service type.

PWE3 members who are also SG13 experts make the following
additional observations:

G.8110.1 states:
"ITU SG 13 has developed a series of recommendations for MPLS /
Client Network Interworking  for Ethernet (Y.1415), ATM
(Cell Mode - Y.1411), ATM (Frame Mode - Y.1412), and TDM (Y.1413).
These recommendations are similar in concept to IETF PWE3."

You omit X.84 (frame relay PW), which, although developed by
SG17, it is now in SG13.

These recommendations are are identical in concept, and
to the best of the ability of those concerned are identical
in detail.

We note that in Figure 8 you pick the semi-formal diagram
from Y.1415 when the same Recommendation has a G.805
diagram as well. We recommend that you show Figure 6-2
of Y.1415 in its place.

G.8110.1 states:
"Pseudowires can be used as a packet transport mechanism
for emulating LAN Services over a Transport MPLS network.
Like in Virtual Private LAN Service (VPLS) a full mesh of
T-MPLS trails (PW trails) is used to interconnect
split-horizon ETH Flow Domains."

We suggest that you reference Appendix I of Y.1415.

Stewart Bryant
IETF PWE3 Working Group Co-Chair

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