Network Working Group Nabil Bitar (Editor)
Internet Draft Verizon
Intended status: Informational
Expiration Date: May 2008
Luca Martini (Editor) Matthew Bocci (Editor)
Cisco Systems Inc. Alcatel-Lucent
November 2007
Requirements for Multi-Segment Pseudowire Emulation Edge-to-Edge (PWE3)
draft-ietf-pwe3-ms-pw-requirements-06.txt
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Abstract
This document describes the necessary requirements to allow a service
provider to extend the reach of pseudowires across multiple domains.
These domains can be autonomous systems under one provider
administrative control, IGP areas in one autonomous system, different
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autonomous systems under the administrative control of two or more
service providers, or administratively established pseudowire
domains.
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Table of Contents
1 Specification of Requirements ........................ 4
2 Acknowledgments ...................................... 4
3 Introduction ......................................... 4
3.1 Scope ................................................ 4
3.2 Architecture ......................................... 4
4 Terminology .......................................... 7
5 Use Cases ............................................ 8
5.1 Multi-Segment Pseudowire Placement Mechanisms ........ 11
6 Multi-Segment Pseudowire Requirements ................ 11
6.1 All Mechanisms ....................................... 11
6.1.1 Architecture ......................................... 11
6.1.2 Resiliency ........................................... 12
6.1.3 Quality Of Service ................................... 13
6.1.4 Congestion Control ................................... 14
6.2 Generic Requirements for MS-PW Setup Mechanisms ...... 15
6.2.1 Routing .............................................. 16
6.3 Statically configured MS-PWs ......................... 16
6.3.1 Architecture ......................................... 17
6.3.2 MPLS-PWs ............................................. 17
6.3.3 Resiliency ........................................... 17
6.3.4 Quality of service ................................... 17
6.4 Signaled PW-segments ................................. 18
6.4.1 Architecture ......................................... 18
6.4.2 Resiliency ........................................... 18
6.4.3 Quality of Service ................................... 19
6.4.4 Routing .............................................. 19
6.5 Signaled PW / Dynamic Route .......................... 20
6.5.1 Architecture ......................................... 20
6.5.2 Resiliency ........................................... 20
6.5.3 Quality of Service ................................... 20
6.5.4 Routing .............................................. 20
7 Operations and Maintenance (OAM) ..................... 21
8 Management of Multi-Segment Pseudowires .............. 22
8.1 MIB Requirements ..................................... 23
8.2 Management Interface Requirements .................... 23
9 Security Considerations .............................. 23
9.1 Data-plane Security Requirements ..................... 23
9.2 Control-plane Security Requirements .................. 24
10 Intellectual Property Statement ...................... 26
11 Full Copyright Statement ............................. 26
12 IANA Considerations .................................. 27
13 Normative References ................................. 27
14 Informative References ............................... 27
15 Author Information ................................... 27
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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.
2. Acknowledgments
The editors gratefully acknowledge the following contributors:
Dimitri Papadimitriou (Alcatel-Lucent), Peter Busschbach (Alcatel-
Lucent), Sasha Vainshtein (Axerra), Richard Spencer (British
Telecom), Simon Delord (France Telecom), Deborah Brungard (AT&T),
David McDyson, Rahul Aggawal (Juniper), Du Ke (ZTE), Cagatay Buyukkoc
(ZTE).
3. Introduction
3.1. Scope
This document specifies requirements for extending pseudowires across
more than one packet switched network (PSN) domain and/or more than
one PSN tunnel. These pseudowires are called multi-segment
pseudowires (MS-PW). Requirements for single-segment pseudowires
(SS-PW) that extend edge to edge across only one PSN domain are
specified in [RFC3916]. This document is not intended to invalidate
any part of [RFC3985].
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 [RFC3985] to support PW emulated services.
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|<-------------- Emulated Service ---------------->|
| |
| |<------- Pseudowire ------->| |
| | | |
| | |<-- 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 [RFC3985]. 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 |<--------Multi-Segment Pseudowire----->| Native
Service | PSN PSN | Service
(AC) | |<-Tunnel->| |<-Tunnel->| | (AC)
| V V 1 V V 2 V V |
| +-----+ +-----+ +---- + |
+---+ | |T-PE1|==========|S-PE1|==========|T-PE2| | +---+
| |---------|........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.
Terminating PE1 (T-PE1) and Terminating PE3 (T-PE3) provide PWE3
service to CE1 and CE2. These PEs terminate different PSN tunnels,
PSN Tunnel 1 and PSN Tunnel 2, and may reside in different PSN or
pseudowire domains. One PSN tunnel extends from T-PE1 to S-PE1 across
PSN1, and a second PSN tunnel extends from S-PE1 to T-PE2 across
PSN2.
PWs are used to connect the Attachment circuits (ACs) attached to T-
PE1 to the corresponding ACs attached to T-PE2. Each PW on PSN tunnel
1 is switched to a PW in the tunnel across PSN2 at S-PE2 to complete
the multi-segment PW (MS-PW) between T-PE1 and T-PE2. S-PE1 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 pseudowire. PW
segments of the same MS-PW (e.g., PW1 and PW3) MAY be of the same PW
type or different types, and PSN tunnels (e.g., PSN Tunnel 1 and PSN
Tunnel 2) can be the same or different technology. This document
requires support for MS-PWs with segments of the same PW type only.
An S-PE switches a MS-PW from one segment to another based on the PW
identifiers (e.g., PW label in case of MPLS PWs). In Figure 2, the
domains that PSN Tunnel 1 and PSN Tunnel 2 traverse could be IGP
areas in the same IGP network, or simply PWE3 domains in a single
flat IGP network for instance.
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|<------Multi-Segment Pseudowire------>|
| AS AS |
AC | |<----1---->| |<----2--->| | AC
| V V V V V V |
| +----+ +-----+ +----+ +----+ |
+----+ | | |=====| |=====| |=====| | | +----+
| |-------|.....PW1..........PW2.........PW3.....|-------| |
| CE1| | | | | | | | | | | |CE2 |
+----+ | | |=====| |=====| |=====| | | +----+
^ +----+ +-----+ +----+ +----+ ^
| T-PE1 S-PE2 S-PE3 T-PE4 |
| ^ ^ |
| | | |
| PW switching points |
| |
| |
|<------------------- Emulated Service --------------->|
Figure 3: PW switching inter provider Reference Model
Note that although Figure 2 only shows a single S-PE, a PW may
transit more than one S-PE along its path. For instance, in the
multi-AS case shown in Figure 3, there can be an S-PE (S-PE2), at the
border of one AS (AS1) and another S-PE (S-PE3) at the border of the
other AS (AS2). A MS-PW that extends from the edge of one AS (T-PE1)
to the edge of the other AS (T-PE4) is composed of three segments:
(1) PW1, a segment in AS1, (2) PW2, a segment between the two border
routers (S-PE2 and S-PE3) that are switching PEs, and (3) PWE3, a
segment in AS2. AS1 and AS2 could be belong to the same provider
(e.g., AS1 could be an access network or metro transport network, and
AS2 could be an MPLS core network) or to two different providers
(e.g., AS1 for Provider 1 and AS2 for Provider2).
4. Terminology
RFC 3985 [RFC3985] provides terminology for PWE3. The following
additional terminology is defined for multi-segment pseudowires:
- PW Terminating Provider Edge (T-PE). A PE where the customer-
facing attachment circuits (ACs) are bound to a PW forwarder. A
Terminating PE is present in the first and last segments of a
MS-PW. This incorporates the functionality of a PE as defined in
RFC 3985.
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- Single-Segment Pseudowire (SS-PW). A PW setup directly between
two PE devices. Each direction of a SS-PW traverses one PSN
tunnel that connects the two PEs.
- Multi-Segment Pseudowire (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 T-PE.
- PW Segment. A part of a single-segment or multi-segment PW, which
is set up between two PE devices, T-PEs and/or S-PEs.
- 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. The S-PE terminates the PSN tunnels
transporting the preceding and succeeding segments of the MS-PW.
It is therefore a PW switching point for a MS-PW. A PW Switching
Point is never the S-PE and the T-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 terminating PEs.
5. Use Cases
PWE3 defines the signaling and encapsulation techniques for
establishing SS-PWs between a pair of terminating PEs (T-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 T-PE in one provider domain to a T-PE in
another provider domain.
-ii. It may not be possible, desirable or feasible to establish a
direct PW control channel between the T-PEs, residing in
different provider networks, to setup and maintain PWs. At a
minimum, a direct PW control channel establishment (e.g.,
targeted LDP session) requires knowledge of and reachability
to the remote T-PE IP address. The local T-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 T-PE and the remote T-PE.
It may not be feasible or desirable to extend single,
contiguous PSN tunnels between T-PEs in one domain and T-PEs
in another domain for security and/or scalability reasons or
because the two domains may be using different PSN
technologies.
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-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 T-PEs reside in different provider
networks (ASs) and it is the current practice to MD5-key all
control traffic exchanged between two networks. A MS-PW
allows the providers to confine MD5 key administration for
the LDP session to just the PW switching points connecting
the two domains.
-iv. PSN Interworking: PWE3 signaling protocols and PSN types may
differ in different provider networks. The terminating 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 terminating 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 pseudowire 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
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.
-vi. Pseudowires in access/metro metworks: Service providers wish
to extend PW technology to access and metro networks in
order to reduce maintenance complexity and operational
costs. Today's access and metro networks are either legacy
(TDM, SONET/SDH, or Frame Relay/ATM), Ethernet or IP based.
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Due to these architectures, circuits (e.g., Ethernet VCs,
ATM VCs, TDM circuits) in the access/metro are traditionally
handled as attachment circuits, in their native format, to
the edge of the IP-MPLS network where the PW starts. This
combination requires multiple separate access networks and
complicates end-to-end control, provisioning, and
maintenance. In addition, when a TDM or SONET/SDH access
network is replaced with a packet-based infrastructure,
expenses may be lowered due to moving statistical
multiplexing closer to the end-user and converging multiple
services onto a single access network.
Access networks have a number of properties that impact the
application of PWs. For example, there exist access
mechanisms where the PSN is not of an IETF specified type,
but uses mechanisms compatible with those of PWE3 at the PW
layer. Here, use case (iv) may apply. In addition, many
networks consist of hundreds or thousands of access devices.
There is therefore a desire to support a sparse mesh of PW
signaling adjacencies and PSN tunnels. Use case (v) may
therefore apply. Finally, Access networks also tend to
differ from core networks in that the access PW setup and
maintenance mechanism may only be a subset of that used in
the core.
Using the MS-PWs, 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 T-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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5.1. Multi-Segment Pseudowire Placement Mechanisms
This requirements document assumes that the above use cases are
realized using one or more of the following mechanisms:
-i. Static Configuration: The switching points (S-PEs), in
addition to the T-PEs, are manually provisioned for each
segment.
-ii. Pre-determined Route: The PW is established along an
administratively determined route using an end-to-end
signaling protocol with automated stitching at the S-PEs.
-iii. Signaled Dynamic Route: The PW is established along a
dynamically determined route using an end-to-end signaling
protocol with automated stitching at the S-PEs. The route is
selected with the aid of one or more dynamic routing
protocols.
Note that we define the PW route to be the set of S-PEs through which
a MS-PW will pass between a given pair of T-PEs. PSN tunnels along
that route can be explicitly specified or locally selected at the S-
PEs and T-PEs. The routing of the PSN tunnels themselves is outside
the scope of the requirements specified in this document.
6. Multi-Segment Pseudowire Requirements
The following sections detail the requirements that the above use
cases put on the MS-PW setup mechanisms.
6.1. All Mechanisms
The following generic requirements apply to the three MS-PW setup
mechanisms defined in the previous section.
6.1.1. Architecture
-i. If MS-PWs are tunneled, across a PSN that only supports SS-
PWs, then only the requirements of [RFC3916] apply to that
PSN. The fact that the overlay is carrying MS-PWs MUST be
transparent to the routers in the PSN.
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-ii. The PWs MUST remain transparent to the P-routers. A P-router
is not an S-PE or an T-PE from the MS-PW architecture
viewpoint. P-routers provide transparent PSN transport for
PWs and MUST not have any knowledge of the PWs traversing
them.
-iii. The MS-PWs MUST use the same encapsulation modes specified
for SS-PWs.
-iv. The MS-PWs MUST be composed of SS-PWs.
-v. A MS-PW MUST be able to pass across PSNs of all technologies
supported 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.
-vi. Both directions of a PW segment MUST terminate on the same
S-PE/T-PE.
-vii. 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.
-viii. Solutions MAY provide a way to prioritize the setup and
maintenance process among PWs.
6.1.2. Resiliency
Mechanisms to protect a MS-PW when an element on the existing path of
a MS-PW fails MUST be provided. These mechanisms will depend on the
MS-PW setup. Following are the generic resiliency requirements that
apply to all MS-PW setup mechanisms:
-i. Configuration and establishment of a backup PW to a primary
PW SHOULD be supported. Mechanisms to perform a switchover
from a primary PW to a backup PW upon failure detection
SHOULD be provided.
-ii. The ability to configure an end-to-end backup PW path for a
primary PW path SHOULD be supported. The primary and backup
paths may be statically configured, statically specified for
signaling or dynamically selected via dynamic routing
depending on the MS-PW establishment mechanism. 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.
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-iii. The ability to configure a primary PW and a backup PW with a
different T-PE from the primary SHOULD be supported.
-iv. Automatic Mechanisms to perform a fast switchover from a
primary PW to a backup PW upon failure detection SHOULD be
provided.
-v. A mechanism to automatically revert to a primary PW from a
backup PW MAY be provided. When provided, it MUST be
configurable.
6.1.3. Quality Of Service
Pseudowires 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. bandwidth),
and by providing appropriate per-packet treatment (e.g. scheduling
priority and drop precedence) throughout the network.
For 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, T-PEs MUST
have the ability to automatically request the PSN tunnel resources in
the direction of traffic (e.g. admission control of PWs onto the PSN
tunnel and accounting for reserved bandwidth and available bandwidth
on the tunnel). In cases where the tunnel supports multiple classes
of service (CoS) (e.g., E-LSP), bandwidth management is required per
CoS.
For MS-PWs, each S-PE maps a PW segment to a PSN tunnel. Solutions
Must enable S-PEs and T-PEs to automatically bind a PW segment to a
PSN-tunnel based on CoS and bandwidth requirements when these
attributes are specified for a PW. Solutions SHOULD also provide the
capability of binding a PW segment to a tunnel as a matter of policy
configuration. S-PEs and T-PEs must be capable of automatically
requesting PSN-tunnel resources per CoS.
S-PEs and T-PEs MUST be able to associate a CoS marking (e.g., EXP
field value for MPLS PWs) with PW PDUs. CoS marking in the PW PDUs
affects packet treatment. The CoS marking depends on the PSN
technology. Thus, solutions must enable the configuration of
necessary mapping for CoS marking when the MS-PW crosses from one PSN
technology to another. Similarly, different administrative domains
may use different CoS values to imply the same CoS treatment.
Solutions MUST provide the ability to define CoS marking maps on S-
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PEs at administrative domain boundaries to translate from one CoS
value to another as a a PW PDU crosses from one domain to the next.
[RFC3985] requires PWs to respond to path congestion by reducing
their transmission rate. Alternatively, RFC3985 permits PWs that do
not have a congestion control mechanism to transmit using explicitly
reserved capacity along a provisioned path. Because MS-PWs are a type
of PW, this requirement extends to them as well. RFC3985 applied to
MS-PWs consequently requires that MS-PWs employ a congestion control
mechanism that is effective across a MS path, or requires an explicit
provisioning action that reserves sufficient capacity in all domains
along the MS path before the MS-PW begins transmission. S-PEs are
therefore REQUIRED to reject attempts to establish MS-PW segments for
PW types that either do not utilize an appropriate congestion control
scheme or when resources that are sufficient to support the
transmission rate of the PW cannot be reserved along the path.
6.1.4. Congestion Control
[RFC3985] requires all PWs to respond to congestion, in order to
conform to [RFC2914]. In the absence of a well-defined congestion
control mechanism, [RFC3985] permits PWs to be carried across paths
that have been provisioned such that the traffic caused by PWs has no
harmful effect on concurrent traffic that shares the path, even under
congestion. These requirements extend to the MS-PWs defined in this
document.
Path provisioning is frequently performed through QoS reservation
protocols or network management protocols. In the case of SS-PWs,
which remain within a single administrative domain, a number of
existing protocols can provide this provisioning functionality. MS-
PWs, however, may transmit across network domains that are under the
control of multiple entities. QoS provisioning across such paths is
inherently more difficult, due to the required inter-domain
interactions. It is important to note that these difficulties do not
invalidate the requirement to provision path capacity for MS-PW use.
Each domain MUST individually implement a method to control
congestion. This can be by QoS reservation, or other congestion
control method. MS-PWs MUST NOT transmit across unprovisioned, best
effort, paths in the absence of other congestion control schemes, as
required by [RFC3985].
Solutions MUST enable S-PEs and T-PEs on the path of a MS-PW to
notify other S-PEs and T-PEs on that path of congestion when it
occurs. Congestion may be indicated by queue length, packet loss
rate, or bandwidth measurement (among others) crossing a respective
threshold. The action taken by a T-PE that receives a notification of
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congestion along the path of one of its PWs could be to reroute the
MS-PW to an alternative path, including an alternative T-PE if
available. If a PE, or an S-PE has knowledge that a particular link
or tunnel is experiencing congestion , it MUST not setup any new MS-
PW that utilize that link or tunnel. Some PW types, such as TDM PWs,
are more sensitive to congestion than others. The reaction to a
congestion notification MAY vary per PW type.
6.2. Generic Requirements for MS-PW Setup Mechanisms
The MS-PW setup mechanisms MUST accommodate Service Provider's
practices, especially in relation to security, confidentiality of SP
information and traffic engineering. Security and confidentiality are
especially important when the MS-PWs are setup across Autonomous
Systems in different administrative domains. Following are generic
requirements that apply to the three MS-PW setup mechanisms defined
earlier:
-i. The ability to statically select S-PEs and PSN tunnels on a
PW path MUST be provided. Static selection of S-PEs is by
definition a requirement for the static configuration and
signaled/static route setup mechanisms. This requirement
satisfies the need for forcing a MS-PW to traverse specific
S-PEs to enforce service provider security and
administrative policies.
-ii. Solutions SHOULD minimize the amount of configuration needed
to setup a MS-PW.
-iii. Solutions should support different PW setup mechanisms on
the same T-PE, S-PE and PSN network.
-iv. Solutions MUST allow T-PEs to simultaneously support use of
SS-PW signaling mechanisms as specified in [RFC4447] as well
as MS-PW signaling mechanisms.
-v. Solutions MUST ensure that a MS-PW will be setup when a path
that satisfies the PW constraints for bandwidth, CoS and
other possible attributes does exist in the network.
-vi. Solutions must clearly define the setup procedures for each
mechanism so that a MS-PW setup on T-PEs can be interpreted
as successful only when all PW segments are successfully
setup.
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-vii. Admission control to the PSN tunnel needs to be performed
against available resources, when applicable. This process
MUST be performed at each PW segment comprising the MS-PW.
PW admission control into a PSN tunnel MUST be configurable.
-viii. In case the PSN tunnel lacks the resources necessary to
accommodate the new PW, an attempt to signal a new PSN
tunnel, or increase the capacity of the existing PSN tunnel
MAY be made. If the expanded PSN tunnel fails to setup the
PW MUST fail to setup.
-ix. The setup mechanisms must allow the setup of a PW segment
between two directly connected S-PEs without the existence
of a PSN tunnel. This requirement allows a PW segment to be
setup between two (Autonomous System Border Routers (ASBRs)
when the MS-PW crosses AS boundaries without the need for
configuring and setting up a PSN tunnel. In this case,
admission control must be done, when enabled, on the link
between the S-PEs.
6.2.1. Routing
An objective of MS-PWs is to provide support for the following
connectivity:
-i. MS-PWs MUST be able to traverse multiple Service Provider
administrative domains.
-ii. MS-PWs MUST be able to traverse multiple Autonomous Systems
within the same administrative domain.
-iii. MS-PWs MUST be able to traverse multiple Autonomous Systems
belonging to different administrative domains.
-iv. MS-PWs MUST be able to support any hybrid combination of the
aforementioned connectivity scenarios, including both PW
transit, and termination in a domain.
6.3. Statically configured MS-PWs
When the MS-PW segments are statically configured the following
requirements apply in addition to the generic requirements previously
defined.
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6.3.1. Architecture
There are no additional requirements on the architecture.
6.3.2. MPLS-PWs
Solutions should allow for the static configuration of MPLS labels
for MPLS-PW segments and the cross-connection of these labels to
preceding and succeeding segments. This is especially useful when a
MS-PW crosses provider boundaries and two providers do not want to
run any PW signaling protocol between them. A T-PE or S-PE that
allows the configuration of static labels for MS-PW segments should
also simultaneously allow for dynamic label assignments for other
MS-PW segments. It should be noted that when two interconnected S-PEs
do not have signaling peering for the purpose of setting up MS-PW
segments, they should have in-band PW OAM capabilities that relay PW
or attachment circuit defect notifications between the adjacent S-
PEs.
6.3.3. Resiliency
The solution should allow for the protection of a PW-segment, a
contiguous set of PW-segments, as well as the end-end path. The
primary and protection segments paths must share the same segments
endpoints. Solutions should allow for having the backup paths setup
prior to the failure or as a result of failure. The choice should be
made by configuration. When resources are limited and cannot satisfy
all PWs, the PWs with the higher setup priorities should be given
preference when compared with the setup priorities of other PWs being
setup or the holding priorities of existing PWs.
Solutions should strive to minimize traffic loss between T-PEs.
6.3.4. Quality of service
The CoS and bandwidth of the MS-PW must be configurable at T-PEs and
S-PEs.
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6.4. Signaled PW-segments
When the MS-PW segments are dynamically signaled the following
requirements apply in addition to the generic requirements previously
defined.
These signaled MS-PW segments can be on the path of a statically
configured MS-PW, signaled/statically routed MS-PW or
signaled/dynamically routed MS-PW.
There are four different SS-PW signaling protocols that are defined
to signal PWs:
-i. Static setup of the SS-PW (MPLS or L2TPv3 forwarding).
-ii. LDP using PWid FEC 128
-iii. LDP using the generalized PW FEC 129
-iv. L2TPv3
The MS-PW setup mechanism MUST be able to support PW segments
signaled with any of the above protocols, however the specification
of which combinations of SS-PW signaling protocols is supported by a
specific implementation is outside the scope of this document.
For the signaled/statically routed and signaled/dynamically routed
MS-PW setup mechanisms the following requirements apply in addition
to the generic requirements previously defined.
6.4.1. Architecture
There are no additional requirements on the architecture.
6.4.2. Resiliency
Solutions should allow for the signaling of a protection path for a
PW segment, sequence of segments or end-to-end path. The protection
and primary paths for the protected segment(s) share the same
respective segments endpoints. When admission control is enabled,
systems must be careful not to double account for bandwidth
allocation at merged points (e.g., tunnels). Solutions should allow
for having the backup paths setup prior to the failure or as a result
of failure. The choice should be made by configuration at the
endpoints of the protected path. When resources are limited and
cannot satisfy all PWs, the PWs with the higher setup priorities
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should be given preference when compared with the setup priorities of
other PWs being setup or the holding priorities of existing PWs.
Procedures must allow for the primary and backup paths to be
diversified
6.4.3. Quality of Service
When the T-PE attempts to signal a MS-PW the following capability is
required:
-i. Signaling must be able to identify the CoS associated with a
MS-PW
-ii. Signaling must be able to carry the traffic parameters for a
MS-PW per CoS. Traffic parameters should be based on
existing INTSERV definitions and must be used for admission
control when admission control is enabled.
-iii. The PW signaling MUST enable separate traffic parameter
values to be specified for the forward and reverse
directions of the PW.
-iv. PW traffic parameter representations MUST be the same for
all types of MS-PWs.
-v. The signaling protocol must be able to accommodate a method
to prioritize the PW setup and maintenance operation among
PWs.
6.4.4. Routing
See the requirements for "Resiliency" above.
Following are further requirements on signaled MS-PW setup
mechanisms:
-i. The signaling procedures MUST be defined such that the setup
of a MS-PW is considered successful if all segments of the
MS-PW are successfully setup.
-ii. The MS-PW path MUST have the ability to be dynamically setup
between the T-PEs by provisioning only the T-PEs.
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-iii. Dynamic MS-PW 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 T-PE through
intermediate S-PEs.
-iv. In a single-provider domain it is natural to have the T-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.
6.5. Signaled PW / Dynamic Route
The following requirements apply, in addition to those in Section 6.1
and Section 6.2, when both dynamic signaling and dynamic routing are
used.
6.5.1. Architecture
There are no additional architectural requirements.
6.5.2. Resiliency
The PW Routing function MUST support dynamic re-routing around
failure points when segments are setup using the dynamic setup
method.
6.5.3. Quality of Service
There are no additional QoS requirements.
6.5.4. Routing
Following are requirements associated with dynamic route selection
for a MS-PW:
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-i. Routing must enable S-PEs and T-PEs to discover S-PEs on the
path to a destination T-PE.
-ii. The MS-PW routing function MUST have the ability to
automatically select the S-PEs along the MS-PW path. Some of
the S-PEs MAY be statically selected and carried in the
signaling to constrain the route selection process.
-iii. 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.
-iv. Routing protocols must be able to advertise reachability
information of attachment circuit (AC) endpoints. This
reachability information must be consistent with the AC
identifiers carried in signaling.
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 [i610] 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.
Note that this document uses the term OAM as Operations and
Management as per ITU-T I.610.
The interworking of OAM mechanisms for SS-PWs between ACs and PWs is
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 T-PE to T-PE, T-PE
to S-PE, or S-PE to S-PE.
The following requirements apply to OAM for MS-PWs:
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-i. Mechanisms for PW segment failure detection and notification
to other segments of a MS-PW MUST be provided.
-ii. MS-PW OAM SHOULD be supported end-to-end across the network.
-iii. Single ended monitoring SHOULD be supported for both
directions of the MS-PW.
-iv. SS-PW OAM mechanisms (e.g., [VCCV]) SHOULD be extended to
support MS-PWs on both end-end basis and segment basis.
-v. All PE routers along the MS-PW MUST agree on a common PW OAM
mechanism to use for the MS-PW.
-vi. At the S-PE, defects on an PSN tunnel MUST be propagated to
all PWs that utilize that particular PSN tunnel.
-vii. The directionality of defect notifications MUST be
maintained across the S-PE.
-viii. The S-PE SHOULD be able to behave as a segment endpoint for
PW OAM mechanisms.
-ix. The S-PE MUST be able to pass T-PE to T-PE PW OAM messages
transparently.
-x. Performance OAM, is required for both MS-PWs and SS-PWs to
measure round-trip delay, one-way delay, jitter, and packet
loss ratio.
8. Management of Multi-Segment Pseudowires
Each PWE3 approach that uses MS-PWs SHOULD provide some mechanisms
for network operators to manage the emulated service. Management
mechanisms for MS-PWs MUST provide at least the same capabilities as
those for SS-PWs, as defined in [RFC3916].
It SHOULD also be possible to manage the additional attributes for
MS-PWs. Since the operator that initiates the establishment of an
MS-PW may reside in a different PSN domain from the S-PEs and one of
the T-PEs along the path of the MS-PW, mechanisms for the remote
management of the MS-PW SHOULD be provided.
The following additional requirements apply:
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8.1. MIB Requirements
-i. MIB Tables MUST be designed to facilitate configuration and
provisioning of the MS-PW at the S-PEs and T-PEs.
-ii. The MIB(s) MUST facilitate inter-PSN configuration and
monitoring of the ACs.
8.2. Management Interface Requirements
-i. Mechanisms MUST be provided to enable remote management of a
MS-PW at an S-PE or T-PE. It SHOULD be possible for these
mechanisms to operate across PSN domains. An example of a
commonly available mechanism is the command line interface
(CLI) over a telnet session.
-ii. For security or other reasons, it SHOULD be possible to
disable the remote management of a MS-PW.
9. Security Considerations
This document specifies the requirements for MS-PWs that can be setup
across domain boundaries administered by one ore more service
providers. The security requirements for MS-PW setup across domains
administered by one service provider are the same as those described
under security considerations in [RFC4447]. These requirements also
apply to interprovider MS-PWs. In addition, [RFC4111] identifies user
and provider requirements for L2 VPNs that apply to MS-PWs described
in this document. In this section, the focus is on the additional
security requirements for interprovider operation of MS-PWs in both
the control plane and data plane, and some of these requirements may
overlap with those in [RFC4111].
9.1. Data-plane Security Requirements
MS-PWs that cross SP domain boundaries may connect one T-PE in a SP
domain to a T-PE in another SP-domain. They may also transit other SP
domains even if the two T-PEs are under the control of one SP. Under
these scenarios, there is a chance that one or more PDUs could be
falsely inserted into a MS-PW at any of the originating, terminating
or transit domains. Such false injection can be the result of a
malicious attack or fault in the MS-PW cross-connects. Solutions MAY
provide mechanisms for ensuring the end-to-end authenticity of MS-PW
PDUs.
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One of the crossed domains may decide to selectively mirror the PDUs
of a MS-PW for eavesdropping purposes. It may also decide to
selectively hijack the PDUs of a MS-PW by directing the PDUs away
from their destination. In either case, the privacy of a MS-PDU can
be violated.
Some types pf PWs make assumptions about the security of the
underling PSN. The minimal security provided by an MPLS PSN might
not be sufficient to meet the security requirements expected by the
applications using the MS-PW. This document does not place any
requirements on protecting the privacy of a MS-PW PDU via encryption.
However, encryption is required at a higher layer in the protocol
stack, based on the application or network requirements."
The data plane of an S-PE at a domain boundary MUST be able to police
an incoming MS-PW traffic to the MS-PW traffic parameters or to an
administratively configured profile. The option to enable/disable
policing MUST be provided to the network administrator. This is to
ensure that a MS-PW or a group of MS-PWs do not grab more resources
than they are allocated. In addition, the data plane of an S-PE MUST
be able to police OAM messages to a pre-configured traffic profile or
to filter out these messages upon administrative configuration.
An ingress S-PE MUST ensure that a MS-PW receive the CoS treatment
configured or signaled for that MS-PW at the S-PE. Specifically, an
S-PE MUST guard against packets marked in the exp bits or IP-header
DS field (depending on the PSN) for a better CoS than they should
receive.
An ingress S-PE MUST be able to define per-interface or interface-
group (a group may correspond to interfaces to a peer-provider) label
space for MPLS-PWs. An S-PE MUST be configurable not to accept
labeled packets from another provider unless the bottom label is a
PW-label assigned by the S-PE on the interface on which the packet
arrived.
Data plane security considerations for SS-PWs specified in [RFC3985]
also apply to MS-PWs.
9.2. Control-plane Security Requirements
A MS-PW connects two attachment circuits. It is important to make
sure that PW connections are not arbitrarily accepted from anywhere,
or else a local attachment circuit might get connected to an
arbitrary remote attachment circuit. The fault in the connection can
start at a remote T-PE or an S-PE.
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An incoming MS-PW request/reply MUST NOT be accepted unless its IP
source address is known to be the source of an "eligible" peer. If a
peering adjacency has to be established prior to exchanging setup
requests/responses, peering MUST only be done with eligible peers.
The set of eligible peers could be pre-configured (either as a list
of IP addresses, or as a list of address/mask combinations) or
automatically generated from the local PW configuration information.
Furthermore, the restriction of peering sessions to specific
interfaces MUST also be provided. The S-PE and T-PE MUST drop the
unaccepted signaling messages in the data path to avoid a DoS attack
on the control plane.
Even if a connection request appears to come from an eligible peer,
its source address may have been spoofed. So some means of preventing
source address spoofing must be in place. For example, if eligible
peers are in the same network, source address filtering at the border
routers of that network could eliminate the possibility of source
address spoofing.
The configuration and maintenance protocol MUST provide a strong
authentication and control protocol data protection mechanism. This
option MUST be implemented , but it should be deployed according to
the specific PSN environment requirements. Furthermore authentication
using a signature for each individual MS-PW setup messages MUST be
available, in addition to an overall control protocol session
authentication, and message validation.
Peer authentication protects against IP address spoofing but does not
prevent one peer (S-PE or T-PE) from connecting to the wrong
attachment circuit. Under a single administrative authority, this may
be the result of a misconfiguration. When the MS-PW crosses multiple
provider domains, this may be the result of a malicious act by a
service provider or a security hole in that provider network. Static
manual configuration of MS-PWs at S-PEs and T-PEs provide a greater
degree of security. If an identification of both ends of a MS-PW is
configured and carried in the signaling message, an S-PE can verify
the signaling message against the configuration. To support dynamic
signaling of MS-PWs, whereby only endpoints are provisioned and S-PEs
are dynamically discovered, mechanisms SHOULD be provided to
configure such information on a server and to use that information
during a connection attempt for validation.
S-PEs that connect one provider domain to another provider domain
MUST have the capability to rate-limit signaling traffic in order to
prevent DoS attacks on the control plane. Furthermore, detection and
disposition of malformed packets and defense against various forms of
attacks that can be protocol-specific MUST be provided.
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10. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
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.
11. Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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12. IANA Considerations
This document has no IANA Actions.
13. Normative References
[RFC3916] "Requirements for Pseudo-Wire Emulation Edge-to-Edge
(PWE3)", X. Xiao, D. McPherson, P. Pate, September 2004
[RFC3985] "PWE3 Architecture" Bryant, et al., RFC3985.
14. Informative References
[VCCV] Nadeau, T., et al."Pseudo Wire Virtual Circuit Connection
Verification (VCCV)", Internet Draft
"draft-ietf-pwe3-vccv-15.txt", September 2007.(work in progress)
[RFC4447] L. Martini, Ed, et al., "Pseudowire Setup and
Maintenance Using the Label Distribution Protocol (LDP)",
RFC4447, April 2006
[RFC4111] "Security Framework for
Provider-Provisioned Virtual Private Networks (PPVPNs),"
Fang. L., et al., RFC 4111, July 2005
[PWE3-OAM] "Pseudo Wire (PW) OAM Message Mapping" Nadeau et al.,
draft-ietf-pwe3-oam-msg-map-02.txt (work in progress),
February 2005
[RFC2914] "Congestion Control Principles", S. Floyd,
September 2000
15. Author Information
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145
e-mail: nabil.bitar@verizon.com
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Matthew Bocci
Alcatel-Lucent Telecom Ltd,
Voyager Place
Shoppenhangers Road
Maidenhead
Berks, UK
e-mail: matthew.bocci@alcatel-lucent.co.uk
Luca Martini
Cisco Systems, Inc.
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
e-mail: lmartini@cisco.com
Bitar, et al. [Page 28]
