Network Working Group Luca Martini (Editor)
Internet Draft Cisco Systems Inc.
Expiration Date: August 2005
Matthew Bocci (Editor) Nabil Bitar (Editor)
Alcatel Verizon
February 2005
Requirements for inter domain Pseudo-Wires
draft-martini-pwe3-mh-pw-requirements-01.txt
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Abstract
This document describes the necessary requirements to allow a service
provider to extend the reach of pseudo-wires across multiple domains.
These domains can be autonomous systems under one provider
administrative control, IGP areas in one autonomous system, different
autonomous systems under the administrative control of two or more
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service providers, or administratively established pseudo-wire
domains.
Table of Contents
1 Specification of Requirements .......................... 2
2 Acknowledgments ........................................ 3
3 Introduction ........................................... 3
3.1 Scope .................................................. 3
3.2 Architecture ........................................... 3
4 Terminology ............................................ 6
5 Use Cases .............................................. 6
6 Multi-Segment Pseudo-Wire Requirements ................. 9
6.1 Architecture ........................................... 9
6.2 Resiliency ............................................. 10
6.3 Quality of Service and Class of Service ................ 11
6.3.1 Traffic Engineering .................................... 12
6.4 MS-PW Setup Mechanisms. ................................ 12
6.4.1 Routing ................................................ 14
7 Operations and Maintenance (OAM) ....................... 14
8 Security Considerations ................................ 16
9 Full Copyright Statement ............................... 16
10 Intellectual Property Statement ........................ 16
11 IANA Considerations .................................... 17
12 Normative References ................................... 17
13 Informative References ................................. 17
14 Author Information ..................................... 17
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119
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2. Acknowledgments
The editors gratefully acknowledge the following contributors:
Dimitri Papadimitriou (Alcatel), Peter Busschbach (Lucent), Sasha
Vainshtein (Axerra), Richard Spencer (British Telecom), Simon Delord
(France Telecom), Deborah Brungard (AT&T), Rahul Aggawal (Juniper),
Du Ke (ZTE), Cagatay Buyukkoc (ZTE).
3. Introduction
3.1. Scope
This document specifies requirements for extending pseudo-wires
across more than one packet switched network (PSN) domain and more
than one PSN tunnel. These pseudo-wires are called multi-segment
pseudo-wires (MS-PW). Requirements for single-segment pseudo wires
(SS-PW) that extend edge to edge across only one PSN domain are
specified in [PWE3-REQ].
This document specifies additional requirements that apply to MS-PWs.
These requirements do not apply to PSNs that only support SS-PWs.
3.2. Architecture
The following three figures describe the reference models which are
derived from [PWE3-ARCH] to support PW emulated services.
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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudo Wire ------>| |
| | | |
| | |<-- PSN Tunnel -->| | |
| PW End V V V V PW End |
V Service +----+ +----+ Service V
+-----+ | | PE1|==================| PE2| | +-----+
| |----------|............PW1.............|----------| |
| CE1 | | | | | | | | CE2 |
| |----------|............PW2.............|----------| |
+-----+ ^ | | |==================| | | ^ +-----+
^ | +----+ +----+ | | ^
| | Provider Edge 1 Provider Edge 2 | |
| | | |
Customer | | Customer
Edge 1 | | Edge 2
| |
| |
Attachment Circuit (AC) Attachment Circuit (AC)
Native service Native service
Figure 1: PWE3 Reference Configuration
Figure 1 shows the PWE3 reference architecture [PWE3-ARCH]. This
architecture applies to the case where a PSN tunnel extends between
two edges of a single PSN domain to transport a PW with endpoints at
these edges.
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Native |<-----------Pseudo Wire----------->| Native
Service | | Service
(AC) | |<-PSN1-->| |<--PSN2->| | (AC)
| V V V V V V |
| +----+ +-----+ +----+ |
+----+ | | PE1|=========| PE2 |=========| PE3| | +----+
| |----------|........PW1.........|...PW3........|----------| |
| CE1| | | | | | | | | |CE2 |
| |----------|........PW2.........|...PW4........|----------| |
+----+ | | |=========| |=========| | | +----+
^ +----+ +-----+ +----+ | ^
| Provider Edge 1 ^ Provider Edge 3 |
| | |
| | |
| PW switching point |
| |
| |
|<------------------- Emulated Service ------------------>|
Figure 2: PW switching Reference Model
Figure 2 extends this architecture to show a multi-segment case. PE1
and PE3 provide PWE3 to CE1 and CE2. These PEs reside in different
PSNs. A PSN tunnel extends from PE1 to PE2 across PSN1, and a second
PSN tunnel extends from PE2 to PE3 across PSN2. PWs are used to
connect the Attachment circuits (ACs) attached to PE1 to the
corresponding ACs attached to PE3. Each PW on the tunnel across PSN1
is stitched to a PW in the tunnel across PSN2 at PE2 to complete the
multi-segment PW (MS-PW) between PE1 and PE3. PE2 is therefore the PW
switching point and will be referred to as the PW switching provider
edge (S-PE). PW1 and PW3 are segments of the same MS-PW while PW2 and
PW4 are segments of another pseudo-wire. PW segments of the same MS-
PW (e.g., PW1 and PW3) MAY be of the same PW type or different type,
and PSN tunnels (e.g., PSN1 and PSN2) can be the same or different
technology. This document requires support for MS-PWs with segments
of the same type. An S-PE switches an MS-PW from one segment to
another based on the PW identifiers (e.g., PW label in case of MPLS
PWs).
Note that although Figure 2 only shows a single S-PE, a PW may
transit more one S-PE along its path. For instance, in the multi-
provider case shown in Figure 3, there can be an S-PE at the border
of one provider domain and another S-PE at the border of the other
provider domain. A MS-PW that extends from the edge of one provider
(PE1) to the edge of the other provider (PE4) is composed of three
segments: (1) PW1, a segment in provider1 network, (2) PW2, a segment
between the two borderrouters that are S-PEs, and (3) PWE3, a segment
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in the provider2 network.
4. Terminology
[PWE3-REQ] provides terminology for PWE3. This document defines the
following additional terms:
- Ultimate PE (U-PE). A PE where the customer-facing ACs
(attachment circuits) are bound to a PW forwarder. An ultimate PE
is present in the first and last segments of a MS-PW.
- Single-Segment PW (SS-PW). A PW setup directly between two U-PE
devices. Each LSP in one direction of a SS-PW traverses one PSN
tunnel that connects the two U-PEs.
- Multi-Segment PW (MS-PW). A static or dynamically configured set
of two or more contiguous PW segments
that behave and function as a single point-to-point PW. Each end
of a MS-PW by definition MUST terminate on a U-PE.
- PW Switching Provider Edge (S-PE). A PE capable of switching the
control and data planes of the preceding and succeeding PW
segments in a MS-PW. It is therefore a PW switching point for a
MS-PW. A PW Switching Point is never the S-PE and the U-PE for
the same MS-PW. A PW switching point runs necessary protocols to
setup and manage PW segments with other PW switching points and
ultimate PEs.
- PW Segment. A part of a single-segment or multi-segment PW, which
is set up between two PE devices, U-PEs and/or S-PEs.
5. Use Cases
PWE3 defines the signaling and encapsulation techniques for
establishing SS-PWs between a pair of ultimate PEs and in the vast
majority of cases this will be sufficient. MS-PWs may be useful in
the following situations:
-i. Inter-Provider PWs: An Inter-Provider PW is a PW that
extends from a U-PE in one provider domain to a U-PE in
another provider domain.
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-ii. It may not be possible, desirable or feasible to establish a
direct PW control channel between the ultimate source and
destination PEs to setup and maintain PWs. At minimum, a
direct PW control channel establishment (e.g., targeted LDP
session) requires knowledge of and reachability to the
remote U-PE IP address. The local U-PE may not have access
to this information due to operational or security
constraints. Moreover, a SS-PW would require the existence
of a PSN tunnel between the local U-PE and the remote U-PE.
It may not be feasible or desirable to extend single,
contiguous PSN tunnels between U-PEs in one domain and U-PEs
in another domain for security and/or scalability reasons or
for the fact that the two domains may be using different PSN
technologies.
-iii. MS-PW setup, maintenance and forwarding procedures must
satisfy requirements placed by the constraints of a multi-
provider environment. An example is the inter-AS L2VPN
scenario where the ultimate PEs reside in different provider
networks (ASs) and it is the practice to MD5-key all control
traffic exchanged between two networks. An MS-PW allows the
providers to confine MD5 key administration to just the PW
switching points connecting the two domains.
-iv. PSN Internetworking: PWE3 signaling protocols and PSN types
may differ in different provider networks. The ultimate PEs
may be connected to networks employing different PW
signaling and /or PSN protocols. In this case it is not
possible to use a SS-PW. A MS-PW with the appropriate
interworking performed at the PW switching points can enable
PW connectivity between the ultimate PEs in this scenario.
-v. Traffic Engineered PSN Tunnels and bandwidth-managed PWs:
There is a requirement to deploy PWs edge to edge in large
service provider networks. Such networks typically encompass
hundreds or thousands of aggregation devices at the edge,
each of which would be a PE. Furthermore, there is a
requirement That these PWs have explicit bandwidth
guarantees. To satisfy these requirements, the PWs will be
tunneled over PSN TE-tunnels with bandwidth constraints. A
single segment pseudo-wire architecture would require that a
full mesh of PSN TE-tunnels be provisioned to allow PWs to
be established between all PEs. Inter provider PWs riding
traffic engineered tunnels further add to the number of
tunnels that would have to be supported by the PEs and the
core network as the total number of PEs increases.
In this environment, there is a requirement either to
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support a sparse mesh of PSN TE-tunnels and PW signaling
adjacencies, or to partition the network into a number of
smaller PWE3 domains. In either case, a PW would have to
pass through more than one PSN tunnel hop along its path. An
objective is to reduce the number of tunnels that must be
supported, and thus the complexity and scalability problem
that may arise. The following use case would benefit from
this solution.
-vi. Pseudo-Wires in Access and Metro Networks: Service providers
are looking to extend PWs to access and metro networks. The
prime motive is cost reduction in capital and operation
expenses. For instance, in metro networks, providers are
looking into extending PWs to next generation SONET ADMs
using MPLS mechanisms. The objective is to achieve
statistical multiplexing of packets closer to the edge of
the network, reducing the recurring costs of SONET circuits
or maximizing the utilization of existing SONET
infrastructure. In access and metro Ethernet networks,
providers are looking to take advantage of MPLS mechanisms
to setup point to point Ethernet virtual circuits with
endpoints in the same or different access/metro networks.
Using the MS-PW solution, access and metro network elements
need only maintain PW signaling adjacencies with the PEs to
which they directly connect. They do not need PW signaling
adjacencies with every other access and metro network
device. PEs in the PSN backbone in turn maintain PW
signaling adjacencies among each other. In addition, a PSN
tunnel is setup between an access element and the PE to
which it connects. Another PSN tunnel needs to be
established between every PE pair in the PSN backbone. A
MS-PW may be setup from one access network element to
another another access element with three segments: (1)
access-element - PSN PE, (2) PSN-PE to PSN-PE, and (3) PSN-
PE to access element. In this MS-PW setup, access elements
are U-PEs while PSN-PEs are S-PEs. It should be noted that
the PSN backbone can be also segmented into PWE3 domains
resulting in more segments per PW.
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6. Multi-Segment Pseudo-Wire Requirements
|<--------------Pseudo Wire----------->|
| Provider Provider |
AC | |<----1---->| |<----2--->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+----+ | | |=====| |=====| |=====| | | +----+
| |-------|.....PW1..........PW2.........PW3.....|-------| |
| CE1| | | | | | | | | | | |CE2 |
+----+ | | |=====| |=====| |=====| | | +----+
^ +----+ +-----+ +----+ +----+ ^
| PE1 PE2 PE3 PE4 |
| ^ ^ |
| | | |
| PW switching points |
| |
| |
|<------------------- Emulated Service --------------->|
Figure 3: PW switching inter provider Reference Model
6.1. Architecture
The following requirements apply to the architecture for MS-PWs:
-i. S-PEs MAY only support switching PWs of the same PW type. In
this case, the PW type is transparent to the S-PE in the
forwarding plane, except for functions needed to provide for
interworking between different PSN technologies.
-ii. If MS-PWs are tunneled, using a PSN tunnel overlay, across a
PSN that only supports SS-PWs, then only the requirements of
[PWE3-REQ] apply to that PSN. The fact that the overlay is
carrying MS-PWs MUST be transparent to the routers in the
PSN.
-iii. The PWs MUST remain transparent to the P-routers. A P-router
is not an S-PE or an U-PE from the MS-PW architecture
viewpoint. P-routers provide transparent PSN transport for
PWs and MUST not have any knowledge of PW traversing them.
-iv. The MS-PWs MUST use the same encapsulation modes specified
for SS-PWs.
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-v. S-PEs MAY change the type or encapsulation mode of a PW in
the NSP function. This enables the establishment of a MS-PW
between two PEs with different attachment circuit
encapsulation but with the same PW type.
-vi. A MS-PW SHOULD be able to pass across PSNs of all
technologies specified by PWE3 for SS-PWs. When crossing
from one PSN technology to another, an S-PE must provide the
necessary PSN interworking functions in that case.
-vii. MS-PWs are composed of SS-PW, and SS-PW are bi-directional,
therefore both directions of a PW segment MUST terminate on
the same S-PE/U-PE.
6.2. Resiliency
Mechanisms to protect an MS-PW when the existing path of a MS-PW
fails (including S-PE failure or any segment failure) MUST be
provided. These mechanisms will depend on the methods of a MS-PW
setup. The following are the resiliency requirements:
-i. The ability to configure an end-end backup PW path for a
primary PW path MUST be supported. The backup and primary
paths should have the ability to traverse separate S-PEs.
The backup path MAY be signaled at configuration time or
after failure detection.
-ii. The ability to configure a backup PW for a primary PW. The
backup and primary PWs should have the ability to traverse
separate S-PEs.
-iii. When a MS-PW segment is tunneled over PSN tunnels with fast
reroute capability fast re-route events SHOULD be
transparent to the MS-PW.
-iv. Automatic Mechanisms to perform a fast switchover from a
primary PW to a backup PW upon failure detection SHOULD be
provided to minimize packet loss.
-v. Configuration Mechanisms to perform a switchover from a
primary PW to a backup PW upon failure detection SHOULD be
provided.
-vi. A mechanism to automatically revert to a primary PW from a
backup PW MAY be provided. When provided, it SHOULD be
enabled/disabled by configuration.
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-vii. Mechanisms for PW segment failure detection and notification
to other segments of a MS-PW MUST be provided.
6.3. Quality of Service and Class of Service
Pseudo-wires are intended to support emulated services (e.g., TDM and
ATM) which may have strict per-connection quality of service
requirements. This may include either absolute or relative guarantees
on packet loss, delay, and jitter. These guarantees are in part
delivered by reserving sufficient network resources (e.g. BW), and by
providing appropriate per-packet treatment (e.g. scheduling priority
and drop precedence) throughout the network.
In SS-PWs, a traffic engineered PSN tunnel (i.e., MPLS-TE) may be
used to ensure that sufficient resources are reserved in the P-
routers to provide QoS to PWs on the tunnel. In this case, the
ability to automatically manage the PSN tunnel resources (e.g.
admission control of PWs onto the PSN tunnel) is a requirement at
each PW segment. The ability to associate the appropriate QoS class
with each PW PDU is a strict requirement at each router transited in
the network.
For MS-PWs, each S-PE maps a PW to a PSN tunnel. That is, an S-PE
decides what PW to bind to which PSN tunnel. Control over binding a
PW to a specific PSN tunnel SHOULD be provided as a matter of policy
configuration.
When the U-PE attempts to signal a PW the following capability is
required:
-i. Admission control to the PSN tunnel needs to be performed
against available resources. PW admission control into a PSN
tunnel MUST be configurable.
-ii. A per PW/QoS class setup priority should be provided.
-iii. In case the PSN tunnel lacks the resources necessary to
accommodate the new PW, a PW setup failure message MUST be
returned and the PW MUST fail to setup. Alternatively: In
case the PSN tunnel lacks the resources necessary to
accommodate the new PW, an attempt to signal an new PSN
tunnel, or increase the capacity of the existing PSN tunnel
MUST be made. If the expanded PSN tunnels fails to setup the
PW MUST fail to setup.
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-iv. PW traffic parameter representations MUST be the same for
all types of PW.
-v. The PW signaling MUST enable separate traffic parameter
values to be specified for the forward and reverse
directions of the PW.
6.3.1. Traffic Engineering
The following requirements apply to the traffic engineering of MS-
PWs:
-i. When setting up a MS-PW, S-PEs and U-PEs MUST be able to
select a tunnel across the PSN in such a way as to satisfy
the MS-PW QoS and bandwidth requirements. Restrictions may
apply depending on the method used for setting up a PW
segment and on PSN tunnel types.
-ii. Upon setting up a MS-PW for which QoS/bandwidth commitments
are made over the PSN, S-PEs and U-PEs SHOULD be able to
perform admission control for each PW segment over a PSN
tunnel to ensure that PW bandwidth and QoS requirements can
be satisfied.
6.4. MS-PW Setup Mechanisms.
The MS-PW setup mechanisms MUST accommodate Service Provider's
practices, especially in relation to security and confidentiality and
traffic engineering. Security and confidentiality are especially
important when the MS-PW's are setup across ASs in different
administrative domains.
There are four different SS-PW signaling protocols that are defined
to signal PWs:
-i. Static configuration of the SS-PW (MPLS or L2TPv3).
-ii. LDP using PWid FEC 128
-iii. LDP using the generalized PW FEC 129
-iv. L2TPv3
Any combination of these signaling protocols MUST be supported.
Following are further requirements on MS-PW setup mechanisms:
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-i. Static S-PE selection and PSN tunnel selection MUST be
provided.
-ii. The MS-PW path MUST have the ability to be dynamically setup
between the U-PEs by provisioning only the U-PEs.
-iii. Dynamic MS-PW pseudowire setup requires that a unique
identifier be associated with a PW and be carried in the
signaling message. That identifier must contain sufficient
information to determine the path to the remote U-PE through
intermediate S-PEs.
-iv. In a single-provider domain it is natural to have the U-PE
identified by one of its IP addresses. This may also apply
when a MS-PW is setup across multiple domains operated by
the same provider. However, some service providers have
security and confidentiality policies that prevent them from
advertising reachability to routers in their networks to
other providers (reachability to an ASBR is an exception).
Thus, procedures MUST be provided to allow dynamic setup of
MS-PWs under these conditions.
-v. Addressing of MS-PW end point at the U-PE MUST be
independent of the IP address of the U-PEs themselves.
-vi. Solutions MUST strive to minimize the amount of
configuration needed to setup an MS-PW.
-vii. MS-PW signaling procedures MUST define clear rules for
triggering the setup of segments of a MS-PW.
-viii. The signaling procedures MUST be defined such that the setup
of a MS-PW is considered successful if and only if all
segments of the MS-PW are successfully setup.
-ix. Mechanisms MUST be developed to propagate setup explicit
failure indications to the S-PEs and U-PEs associated with
the failed MS-PW.
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6.4.1. Routing
An objective of MS-PWs is to provide support for the following
connectivity:
-i. MS-PW MUST be able to traverse multiple IGP domains.
-ii. MS-PW MUST be able to traverse multiple autonomous systems
within the same administrative domain.
-iii. MS-PW MUST be able to traverse multiple autonomous systems
belonging to different administrative domains.
-iv. MS-PW MUST be able to terminate, as well, in any of above
mentioned domains.
-v. MS-PWs MUST be able to support any hybrid combination of
the aforementioned connectivity scenarios.
The routing function MUST support the various MS-PW setup methods and
the various connectivity scenarios. Following are the consequent
requirements:
-i. MUST support the setup of a statically configured segment of
a MS-PW. ( static S-PE selection )
-ii. The MS-PW MUST have the ability to automatically select the
S-PEs along the MS-PW path. Some of the S-PEs MAY be
statically selected.
-iii. The PW Routing function MUST support dynamic re-routing
around failure points when segments are setup using the
dynamic setup method.
-iv. The PW Routing function MUST support re-routing around
failures that occur between the statically configured
segment endpoints. This may be done by choosing another PSN
tunnel between the two segment endpoints or setting up an
alternative tunnel.
-v. Routing MUST support the operation of backup protection of
primary paths.
-vi. Manual routing SHOULD be supported to allow diversity for
1:1 protected PWs.
7. Operations and Maintenance (OAM)
OAM mechanisms for the attachment circuits are defined in the
specifications for PW emulated specific technologies (e.g., ITU-T
I.610 for ATM). These mechanisms enable, among other things, defects
in the network to be detected, localized and diagnosed. They also
enable communication of PW defect states on the PW attachment
circuit.
The interworking of OAM mechanisms for SS-PWs between ACs and PWs is
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defined in [PWE3-OAM]. These enable defect states to be propagated
across a PWE3 network following the failure and recovery from faults.
OAM mechanisms for MS-PWs MUST provide at least the same capabilities
as those for SS-PWs.
In addition, it should be possible to support both segment and end-
to-end OAM mechanisms for both defect notifications and connectivity
verification in order to allow defects to be localized in a multi-
segment network. That is, PW OAM segments can be U-PE to U-PE, U-PE
to S-PE, or S-PE to S-PE.
It is desirable to use existing PW OAM mechanisms for MS-PWs or
extensions to them if needed.
The following requirements apply to OAM for MS-PWs:
-i. MS-PW OAM SHOULD be supported end-to-end across the network.
-ii. OAM activation/deactivation SHOULD be tied to MS-PW
setup/tear down.
-iii. The MS-PW SHOULD support a Forward Defect Indicator (FDI).
-iv. Single ended monitoring SHOULD be supported for both
directions of the MS-PW.
-v. MS-PW OAM SHOULD support switch over between 1:1 protected
LSPs end-to-end.
-vi. In-band monitoring SHOULD be provided both end-to-end across
the MS-PW, and on a segment (i.e. SS-PW) basis.
-vii. At the S-PE, defect notifications on the upstream segment of
PWs must be propagated to the downstream PW segment.
-viii. All PE routers along the MS-PW MUST agree on a common PW OAM
mechanism to use to the MS-PW.
-ix. At the S-PE, defects on an PSN tunnel must be propagated to
all PWs that utilize that particular PSN tunnel.
-x. The directionality of defect notifications must be
maintained across the S-PE.
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-xi. The S-PE should be able to behave as a segment endpoint for
PW OAM mechanisms.
-xii. The S-PE MUST be able to pass U-PE to U-PE PW OAM messages
transparently.
8. Security Considerations
Section to be completed later. Editor's note: This section needs
extensive work. Security requirements are needed for inter-as , and
inter -providers situations.
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The IETF invites any interested party to bring to its attention any
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Internet Draftdraft-martini-pwe3-MH-PW-requirements-01.txt February 2005
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668.
11. IANA Considerations
This document has no IANA Actions.
12. Normative References
[PWE3-ARCH] "PWE3 Architecture" Bryant, et al., RFC3985.
[PWE3-OAM] "Pseudo Wire (PW) OAM Message Mapping" Nadeau et al.,
draft-ietf-pwe3-oam-msg-map-01.txt (work in progress),
October 2004
13. Informative References
[ITUQ] ITU-T Recommendation Q.933, and Q.922 Specification for
Frame Mode Basic call control, ITU Geneva 1995
14. Author Information
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
e-mail: lmartini@cisco.com
Matthew Bocci
Alcatel Telecom Ltd,
Voyager Place
Shoppenhangers Road
Maidenhead
Berks, UK
e-mail: matthew.bocci@alcatel.co.uk
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Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145
e-mail: nabil.bitar@verizon.com
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